Physics Nobel Prize 2010: Graphene

Andre Geim (photo courtesy: Sergeom, Wikimedia Commons) and Konstantin Novoselov (photo courtesy: University of Manchester, UK)

The Nobel Prize in Physics 2010 was awarded jointly to Andre Geim and Konstantin Novoselov of the University of Manchester "for groundbreaking experiments regarding the two-dimensional material graphene"

Graphene is a form of carbon. As a material it is completely new – not only the thinnest ever but also the strongest. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it. Carbon, the basis of all known life on earth, has surprised us once again.

Homepage of Andre Geim >> Homepage of Konstantin Novoselov >>
Link to the Mesoscopic Physics Group, University of Manchester, UK >>

Geim and Novoselov extracted the graphene from a piece of graphite such as is found in ordinary pencils. Using regular adhesive tape they managed to obtain a flake of carbon with a thickness of just one atom. This at a time when many believed it was impossible for such thin crystalline materials to be stable.

However, with graphene, physicists can now study a new class of two-dimensional materials with unique properties. Graphene makes experiments possible that give new twists to the phenomena in quantum physics. Also a vast variety of practical applications now appear possible including the creation of new materials and the manufacture of innovative electronics. Graphene transistors are predicted to be substantially faster than today’s silicon transistors and result in more efficient computers.

Since it is practically transparent and a good conductor, graphene is suitable for producing transparent touch screens, light panels, and maybe even solar cells.

When mixed into plastics, graphene can turn them into conductors of electricity while making them more heat resistant and mechanically robust. This resilience can be utilised in new super strong materials, which are also thin, elastic and lightweight. In the future, satellites, airplanes, and cars could be manufactured out of the new composite materials.

This year’s Laureates have been working together for a long time now. Konstantin Novoselov, 36, first worked with Andre Geim, 51, as a PhD-student in the Netherlands. He subsequently followed Geim to the United Kingdom. Both of them originally studied and began their careers as physicists in Russia. Now they are both professors at the University of Manchester.

Playfulness is one of their hallmarks, one always learns something in the process and, who knows, you may even hit the jackpot. Like now when they, with graphene, write themselves into the annals of science.

Solving the Superconductor Puzzle

Thomas A. Maier [photo courtesy: Oak Ridge National Laboratory]

Superconducting materials, which transmit power resistance-free, are found to perform optimally when high- and low-charge density varies on the nanoscale level, according to research performed at the US Department of Energy's Oak Ridge National Laboratory (ORNL) and Institut für Theoretische Physik, Zürich, Switzerland.

In research toward better understanding the dynamics behind high-temperature superconductivity, the ORNL scientists rewrote computational code for the numerical Hubbard model that previously assumed copper-compound superconducting materials known as cuprates to be homogenous — the same electron density — from atom to atom. The paper is published in Physical Review Letters [1].

Lead author Thomas Maier and colleagues Gonzalo Alvarez, Michael Summers and Thomas Schulthess received the Association for Computing Machinery Gordon Bell Prize two years ago for their high-performance computing application. The application has now been used to examine the nanoscale inhomogeneities in superconductors that had long been noticed but left unexplained.













Researchers have found that atom clusters with inhomogenous stripes of lower density (shown in red) raise critical temperature needed to reach superconductor state [Courtsey: ORNL]

"Cuprates and other chemical compounds used as superconductors require very cold temperatures, nearing absolute zero, to transition from a phase of resistance to no resistance," said Jack Wells, director of the Office of Institutional Planning and a former Computational Materials Sciences group leader.

Liquid nitrogen is used to cool superconductors into phase transition. The colder the conductive material has to get to reach the resistance-free superconductor phase, the less efficient and more costly are superconductor power infrastructures. Such infrastructures include those used on magnetic levitation trains, hospital Magnetic Resonance Imaging, particle accelerators and some city power utilities.

In angle-resolved photoemission experiments and transport studies on a cuprate material that exhibits striped electronic inhomogeneity, scientists for years observed that superconductivity is heavily affected by the nanoscale features and in some respect even optimized.

"The goal following the Gordon Bell Prize was to take that supercomputing application and learn whether these inhomogenous stripes increased or decreased the temperature required to reach transition," Wells said. "By discovering that striping leads to a strong increase in critical temperature, we can now ask the question: is there an optimal inhomogeneity?"

In an ideal world, a material could become superconductive at an easily achieved and maintained low temperature, eliminating much of the accompanying cost of the cooling infrastructure.

"The next step in our progress is a hard problem," Wells said. "But from our lab's point of view, all of the major tools suited for studying this phenomenon — the computational codes we've written, the neutron scattering experiments that allow us to examine nanoscale properties — are available to us here."

Reference
[1] T. A. Maier, G. Alvarez, M. Summers, T. C. Schulthess, "Dynamic Cluster Quantum Monte Carlo Simulations of a Two-Dimensional Hubbard Model with Stripelike Charge-Density-Wave Modulations: Interplay between Inhomogeneities and the Superconducting State", Phys. Rev. Lett. 104, 247001 (2010). Abstract.

[We thank Oak Ridge National Laboratory for materials used in this posting]

Transition from Superfluid to Mott Insulator

Karina Jiménez-García [photo courtesy: Joint Quantum Institute, Maryland]

Researchers studying a gas of trapped ultracold atoms have identified a set of conditions, never before observed but in excellent agreement with new theoretical predictions, that determine the onset of a critical “phase transition” in atomic arrays used to model the behavior of condensed-matter systems.

The findings provide a novel insight into the way collections of atoms suddenly cease to be a superfluid, which flows without resistance, and switch to a very different state called a “Mott insulator.” That transition and similar phenomena are of central interest to the science of solid-state materials, including superconductors.

“This work shows that the transition can be precisely controlled and confirms that it can be described by only two independent variables,” says lead researcher Karina Jiménez-García, a member of Ian Spielman’s group at the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (JQI). The group reports its findings in a forthcoming issue of Physical Review Letters [1].

In order to understand the behavior of materials on the atomic and molecular scale, researchers often cannot experiment directly with samples. In many cases, they need model systems – analogous, at microscopic dimensions, to the physical models built by engineers to test the dynamics of a planned structure – that allow them to change one or two experimental parameters at a time while holding the rest constant. That can be prohibitively difficult, if not impossible, in bulk samples of real material.

But in recent years, quantum science has made it possible to create accurate and highly illuminating models of condensed-matter systems by using ensembles of individual atoms which are confined by electrical and magnetic forces into patterns that mimic the fundamental physics of the repeating structural pattern, or “lattice,” of a solid material.

Improving these quantum-mechanical models is an important research area at JQI, and Spielman’s group has been investigating a model for the superfluid-to-Mott insulator (SF-MI) phase transition – the point at which the atoms cease to share the same quantum properties, as if each atom were spread over the entire lattice, and change into a set of individual atoms trapped at specific locations, that do not communicate with one another.

Figure 1

The group’s experimental setup at NIST’s Gaithersburg, MD facility uses a cloud of about 200,000 atoms of rubidium that have been cooled to near absolute zero and confined in a combination of magnetic and optical potentials. In those conditions, a majority of the atoms forms a Bose-Einstein condensate (BEC), an exotic condition in which all the atoms coalesce into exactly the same quantum state.

Then the team loads the BEC – which is about 10 micrometers in diameter, or about one-tenth the width of a human hair – into an “optical lattice” that forms at the intersection of three laser beams placed at right angles to one another [See Figure 1], two horizontal and one vertical. Interference patterns in the beams’ waves cause regularly spaced areas of higher and lower energy; atoms naturally tend to settle into the lowest-energy locations like eggs in an egg carton.

The depth of the lattice wells (the cavities in the egg carton) is adjusted by varying the intensity of the laser beams. [See Figure 2] In a relatively shallow lattice, atoms can easily “tunnel” from one site to another in the condensate superfluid state, whereas deep lattice wells tend to hold each atom in place, producing the non-condensate insulator state. “We can tune the depth of all the wells in the carton by adjusting the intensity of the laser beams which create it,” Jiménez-García explains. “We can go from a flat carton to a carton with very deep wells.”

Figure 2 (click to view hi resolution image)

That general lab arrangement – ultracold trapped atoms suspended in an optical lattice – is the current standard worldwide for experiments on condensed-matter models. But it has a serious problem: The mathematical theory behind the model is predicated on a completely homogenous system, whereas arrays such as the JQI group uses are only homogenous on small spatial scales. Globally, they are inhomogenous because the magnetic trapping potential is not uniform across the width of the trap. As a result, the equations used to calculate expected outcomes do not accurately predict the SF-MI transition, compromising their utility.

Last year, however, an international collaboration of theorists determined [2] that in such configurations, where there were spatially separated SF and MI phases, the quantum state of the system could be fully specified by the relationship between only two variables: the characteristic density of the system (a composite of trapping potential, total number of trapped atoms, tunneling energy, lattice spacing and dimensionality); and the strength of the interactions between neighboring atoms.

Jiménez-García and colleagues in the JQI group set out to see if they could make an experimental system that performed according to the theorists’ specifications.

They set the depth of the vertical lattice beam such that it partitioned the roughly spherical BEC into about 60 two-dimensional, pancake-shaped segments, and then used a method similar to medical MRI scanning to select and analyze just a couple of individual 2D segments at the same time. The inhomogeneity of the originally 3D atomic sample results in the selection of 2D systems with different total number of atoms, ranging from 0 (at the edges of the system) to 4000 atoms (in the center of the system), allowing the researchers to examine a broad range of total atom numbers and lattice depths.

Because the trapping potential was not homogenous across the BEC, the group’s lattices were not completely orthogonal. “What we get instead,” Jiménez-García says, “is an array of egg cartons which have a parabolic curvature. Imagine each egg carton with the overall shape of a bowl, and the whole system as a stack of egg carton bowls.”

To determine the state of the atoms in the 2D slice, the scientists abruptly turn off the trap and let the atoms begin to fly apart. After a few thousandths of a second, they take a picture of the expanding population. If the atoms were deep into the SF state, the images will show a tightly focused bunch. If they were in the MI state, the bunch will have dispersed farther and appear more diffuse. “We detect a sharp peak in the momentum distribution which we associate with the condensate fraction,” Jiménez-García says. “Wider dispersion – that is, less condensate fraction -- would mean more MI.”

After measuring about 1300 different samples, the group was able to determine that the two-variable theory completely described the state of each slice.

References
[1] K. Jimenez-Garcia, R.L. Compton, Y.-J. Lin, W.D. Phillips, J.V. Porto and I.B. Spielman, "Phases of a 2D Bose Gas in an Optical Lattice", accepted for publication in Physical Review Letters. arXiv:1003.1541.
[2] Marcos Rigol, George G. Batrouni, Valery G. Rousseau, Richard T. Scalettar, "State diagrams for harmonically trapped bosons in optical lattices", Phys. Rev. A 79, 053605 (2009). Abstract.

Glimpse of Heavy Electrons Reveals “Hidden Order”

J.C. Séamus Davis (Photo courtesy: Cornell University)

Using a microscope designed to image the arrangement and interactions of electrons in crystals, scientists have captured the first images of electrons that appear to take on extraordinary mass under certain extreme conditions. The technique reveals the origin of an unusual electronic phase transition in one particular material, and opens the door to further explorations of the properties and functions of so-called heavy fermions. Scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, McMaster University, and Los Alamos National Laboratory describe the results in the June 3, 2010, issue of Nature.

“Physicists have been interested in the ‘problem’ of heavy fermions — why these electrons act as if they are hundreds or thousands of times more massive under certain conditions — for thirty or forty years,” said study leader Séamus Davis, a physicist at Brookhaven and the J.D. White Distinguished Professor of Physical Sciences at Cornell University. Understanding heavy fermion behavior could lead to the design of new materials for high-temperature superconductors.

In the current study, the scientists were imaging electronic properties in a material composed of uranium, ruthenium, and silicon that itself has been the subject of a 25-year scientific mystery. In this material — synthesized by Graeme Luke’s group at McMaster — the effects of heavy fermions begin to appear as the material is cooled below 55 kelvin (-218 °C). Then, an even more unusual electronic phase transition occurs below 17.5K.

[Image courtesy: Los Alamos National Laboratory] Spectroscopic imaging scanning tunneling microscopy reveals a "hidden order" of electrons, seen as bright areas, within uranium ruthenium silicate as it is cooled to very low temperatures. Seeing this hidden order for the first time has unraveled a 25-year-old physics mystery.

Scientists had attributed this lower-temperature phase transition to some form of “hidden order.” They could not distinguish whether it was related to the collective behavior of electrons acting as a wave, or interactions of individual electrons with the uranium atoms. Alexander Balatsky, a Los Alamos theoretical physicist at the Center for Integrated Nanotechnologies, provided guidance on how to examine this problem.

With that guidance, Davis’ group used a technique they’d designed to visualize the behavior of electrons to “see” what the electrons were doing as they passed through the mysterious phase transition. The technique, spectroscopic imaging scanning tunneling microscopy (SI-STM), measures the wavelength of electrons on the surface of the material in relation to their energy.

“Imagine flying over a body of water where standing waves are moving up and down, but not propagating toward the shore,” said Davis. “When you pass over high points, you can touch the water; over low points, you can’t. This is similar to what our microscope does. It images how many electrons can jump to the tip of our probe at every point on the surface.”

From the wavelength and energy measurements, the scientists can calculate the effective electron mass.

[Image credit: Mohammad Hamidian/Davis Lab, Cornell University] In this schematic diagram, individual electrons (white spheres) interact with uranium atoms (shown as yellow and blue f-electron orbitals of the uranium atoms) as they move through the URu2Si2 crystal. These interactions drastically inhibit the progress of the electrons, making them appear to take on extraordinary mass – an effect imaged for the first time in this study.

“This technique reveals that we are dealing with very heavy electrons — or electrons that act as if they are extremely heavy because they are somehow being slowed down,” Davis said.

The detection of “heavy electron” characteristics below the second transition temperature provides direct experimental evidence that the electrons are interacting with the uranium atoms rather than acting as a wave.

To visualize this, imagine a team of football players running up the field after a kickoff. If each player were free to run unimpeded, the whole team would appear to operate as a wave of relatively independent “electrons.” But imagine instead that the field is strewn with an array of chairs, and each player has to sit for an instant every time he encounters a chair before continuing up the field. In this case the chairs are analogous to the uranium atoms. Those interactions between players and chairs (or electrons and uranium atoms) clearly slow the progress.

In the case of the uranium material, the electron slowdown lasts only a tiny fraction of a second at each uranium atom. But because kinetic energy and mass are mathematically related, the slowdown makes it appear as if the electrons are more massive than a free electron.

Besides revealing these interactions as the source of “hidden order” in the uranium compound, Davis’ study shows that the SI-STM technique can be used to visualize heavy electrons. That in turn opens the door to more ways to investigate and visualize this phenomenon.

The research team is continuing to probe a variety of related compounds with this new approach to further their understanding of heavy fermion systems.

“Heavy fermions remain mysterious in many ways, and it’s our job as scientists to solve the problem,” Davis said.

Reference
A. R. Schmidt,M. H. Hamidian,P. Wahl,F. Meier,A. V. Balatsky,J. D. Garrett,T. J. Williams,G. M. Luke& J. C. Davis, "Imaging the Fano lattice to ‘hidden order’ transition in URu₂Si₂", Nature, Vol 465, pp 570–576 (03 June 2010). Abstract.

[We thank Brookhaven National Laboratory for materials used in this posting]

The Mechanism behind Superinsulation

From Left to Right: Valerii Vinokur, Tatyana Baturina and Nikolai Chtchelkatchev (photo courtesy: Argonne National Laboratory)

Scientists at the U.S. Department of Energy's Argonne National Laboratory have discovered the microscopic mechanism behind the phenomenon of superinsulation, the ability of certain materials to completely block the flow of electric current at low temperatures.

The essence of the mechanism is what the authors termed "multi-stage energy relaxation" in a recent paper [1] published in Physical Review Letters. An earlier paper [2] on the discovery of superinsulation was published in Nature in April, 2008.

Traditionally, energy dissipation accompanying current flow is viewed as disadvantageous, as it transforms electricity into heat and thus results in power losses. In arrays of tunnel junctions that are the basic building units of modern electronics, this dissipation permits the generation of current.

Argonne scientist Valerii Vinokour, along with Russian scientists Nikolai Chtchelkatchev (Moscow Institute of Physics and Technology) and Tatyana Baturina (Institute of Semiconductor Physics, Novosibirsk), found that at very low temperatures the energy transfer from tunneling electrons to the thermal environment may occur in several stages.

An electron microscopy image of titanium nitride, on which the effect of superinsulation was first observed [image courtesy: Argonne National Laboratory]

“First, the passing electrons lose their energy not directly to the heat bath; they transfer their energy to electron-hole plasma, which they generate themselves,” Vinokour said. “Then this plasma 'cloud' transforms the acquired energy into the heat. Thus, tunneling current is controlled by the properties of this electron-hole cloud.”

As long as the electrons and holes in the plasma cloud are able to move freely, they can serve as a reservoir for energy—but below certain temperatures, electrons and holes become bound into pairs. This does not allow for the transfer of energy from tunneling electrons and impedes the tunneling current, sending the conductivity of the entire system to zero.

“Electron-hole plasma disappears from the game and electrons cannot generate the energy exchange necessary for tunneling,” Vinokour said. Because the current transfer in thin films and granular systems that exhibit superinsulating behavior relies on electron tunneling, the multistage relaxation explains the origin of the superinsulators.

Superinsulation is the opposite of superconductivity; instead of a material that has no resistivity, a superinsulator has a near-infinite resistance. Integration of the two materials may allow for the creation of a new class of quantum electronic devices. This discovery may one day allow researchers to create super-sensitive sensors and other electronic devices.

Reference
[1] N. M. Chtchelkatchev, V. M. Vinokur, and T. I. Baturina, "Hierarchical Energy Relaxation in Mesoscopic Tunnel Junctions: Effect of a Nonequilibrium Environment on Low-Temperature Transport", Physical Review Letters, 103, 247003 (2009). Abstract.
[2] Valerii M. Vinokur, Tatyana I. Baturina, Mikhail V. Fistul, Aleksey Yu. Mironov, Mikhail R. Baklanov and Christoph Strunk, "Superinsulator and quantum synchronization", Nature, 452, 613-615 (3 April 2008). Abstract.

[We thank Argonne National Laboratory, IL, USA for materials used in this report]

Interference-Induced Terahertz Transparency in a Semiconductor Magneto-plasma

Junichiro Kono

[This is an invited article based on recently published work by the author and his collaborators from Rice University, Texas A&M University, and Los Alamos National Laboratory -- 2Physics.com]

Author: Junichiro Kono
Affiliation: Dept of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA.

Maximum modulation of light transmission occurs when an initially opaque medium is suddenly made transparent. This dramatic phenomenon, induced transparency, indeed occurs in atomic and molecular gases through different mechanisms [1,2], while there remains much room for further studies in solids. A plasma is an illustrative system exhibiting opacity, where light is completely reflected if its frequency is smaller than the plasma frequency. Light-plasma interaction theory provides a universal framework to describe such diverse phenomena and systems as radiation in space plasmas, diagnostics of laboratory plasmas, and collective excitations in condensed matter. However, quite surprisingly, induced transparency in plasmas is a rather uncharted area of research.

In a paper published online in Nature Physics on December 6, 2009, researchers at Rice University, Texas A&M University, and Los Alamos National Laboratory reported a novel type of thermally- and magnetically-induced transparency in a semiconductor plasma, revealed by coherent terahertz (THz) magneto-spectroscopy [3]. They observed a sudden appearance and disappearance of transmission through a slab of electron-doped InSb over narrow temperature and magnetic field ranges.

To explain these striking observations, the researchers developed a theoretical model based on coherent interference between the left- and right-circularly polarized eigenmodes of the low-density magneto-plasma in InSb. Detailed simulations demonstrated how the observed THz modulation and interference effects depend sensitively on the magnetic field, as well as on the temperature through the intrinsic carrier density of narrow-gap semiconductors. Excellent agreement between experiment and theory demonstrated surprisingly long-lived coherence of magnetoplasmon excitations.

The free electrons in the conduction band of doped narrow-gap semiconductors, e.g., InSb, InAs, and HgCdTe, behave as classic solid-state plasmas and have been examined through a number of infrared spectroscopy studies [4,5]. Due to the low electron densities achievable in these materials and to the electrons’ small effective mass and high mobility, most of the important energy scales (the cyclotron energy, the plasma energy, the Fermi energy, intra-donor transition energies, etc.) can all lie within the same narrow energy range from ~1 to 10 meV, or the THz frequency range (1 THz = 4.1 meV). The interplay between these material properties, which are tunable with magnetic field, doping density, and/or temperature, make doped narrow-gap semiconductors a useful material system in which to probe and explore novel phenomena that can be exploited for future THz technology.

The Rice researchers used a time-domain THz magneto-spectroscopy system [6] with a linearly-polarized, coherent THz beam to investigate magneto-plasmonic effects in a lightly n-doped InSb sample that exhibits a sharp plasma edge at ~0.3 THz at zero magnetic field as well as sharp absorption and dispersion features around the cyclotron resonance. These spectral features can be sensitively controlled by changing the magnetic field and temperature due to the very small effective masses of electrons and low thermal excitation energy in this narrow-gap semiconductor. Furthermore, long decoherence times (< 40 ps) of electron cyclotron oscillations give rise to sharp interference fringes and coherent beating between different normal modes (coupled photon-magneto-plasmon excitations) of the semiconductor plasma.

Figure 1 (click on the image to see hi resolution version) Temperature dependence of THz transmittance spectra for lightly-doped InSb in a magnetic field. a, Transmittance versus temperature at 0.25 THz at a magnetic field of 0.9 T (corresponding to a horizontal cut in the contour map of b), showing thermally induced transparency. b, Measured and c, calculated transmittance contour as a function of temperature (2-240 K) and frequency (0.12-2.6 THz) at a fixed magnetic field of 0.9 T. d, Transmittance versus magnetic field at 0.25 THz at a temperature of 40 K (corresponding to a horizontal cut in the contour map of e), showing magnetically induced transparency. e, Measured and f, calculated transmittance contour as a function of magnetic field (0-2 T) and frequency (0.12-2.6 THz) at a fixed temperature of 40 K.

As an example, the temperature (Figs. 1a, 1b, and 1c) and magnetic field (Figs. 1d, 1e, and 1f) dependence of THz transmittance spectra are shown. A striking feature in both Figs. 1a and 1d is a narrow range of temperature (1a) and magnetic field (1d) where the transmission of THz light is high. Figure 1b shows a full contour map of the transmittance as a function of frequency and temperature at a fixed magnetic field. Figure 1c shows a calculated contour plot of the transmittance, based on their model. A horizontal cut of the contour at 0.25 THz is shown in Fig. 1a. Similarly, Figs. 1e and 1f show, respectively, measured and calculated contour plots of the transmittance as a function of frequency and magnetic field. A horizontal cut of the contour at 0.25 THz is shown in Fig. 1d.

At zero magnetic field, the only spectral feature appearing in our InSb samples is the plasma edge at the plasma frequency (0.3 THz). When a magnetic field is applied along the wave propagation direction, the incident linearly-polarized THz wave propagates in the sample as a superposition of the two transverse normal modes of the magneto-plasma: the left-circularly-polarized mode, called the ‘extraordinary’ or cyclotron resonance active (CRA) wave, and the right-circularly-polarized mode, called the ‘ordinary’ or cyclotron resonance inactive (CRI) wave. The CRA mode couples with the cyclotron motion of electrons. With increasing magnetic field, the plasma edge splits into the two magnetoplasmon frequencies for the CRA and CRI modes.

The THz response of the InSb sample was modeled through a dielectric tensor for a classical magneto-plasma for both electrons and holes, including the effect of conduction band non-parabolicity. The CRA wave experiences strong absorption and dispersion, while the transmission of the CRI mode is nearly flat and featureless everywhere except at very low frequencies. Simple addition of the two, however, does not produce any of the experimentally observed spectral features. What is measured experimentally is a superposition of the two fields, which contains the interference between the CRA and CRI modes. The interference term depends on the index difference between the two modes, and its inclusion in the simulation indeed totally modified the spectra at finite magnetic fields. The agreement between theory and experiment is outstanding. The positions and shapes of all the transmission peaks, plateaus, and dips in the spectra are accurately reproduced in great detail, confirming the accuracy of our interpretation and theoretical model and indicating the long coherence times of coupled photon-magnetoplasmon excitations reaching tens of ps.

The dominant process affecting the temperature dependence of the dielectric tensor at elevated temperatures is the thermal excitation of intrinsic carriers across the band gap given, which leads to an exponentially growing plasma frequency. The density of intrinsic carriers eventually exceeds the doping density at ~180 K. Therefore, one would expect a weakly temperature-dependent transmittance below ~180 K that would abruptly decrease above this temperature due to the exponentially growing plasma frequency. The intensities of individually-transmitted CRA and CRI modes indeed exhibit this expected temperature dependence.

However, again, one has to include the interference term in calculating the transmission. With realistic parameters for the sample and experimental conditions, this interference term is negative and almost exactly cancels the other two terms below 160 K, leading to interference-induced opacity. As the temperature increases above 160 K, the difference between the refractive indices of the two modes starts growing exponentially, causing strong oscillations in the total transmittance due to interference. These oscillations, however, are strongly damped above 200 K due to the exponentially growing absorption coefficient for both normal modes. As a result, only one strong peak remains prominent, followed by a few progressively smaller peaks, explaining the existence of the observed transparency bands. This is further illustrated by the excellent agreement between the observed and calculated temperature dependence of transmittance in Figs. 1b and 1e (experiment) and 1c and 1f (theory).

These results demonstrate that free carrier plasmas in lightly-doped narrow-gap semiconductors are promising materials systems for THz physics, exhibiting huge magnetic anisotropy effects and plasmon excitations in the THz range that are highly tunable with external fields, temperature, and doping. In particular, coherent interference phenomena, which are commonly observed and used in the visible and near-infrared range, can be extended into the THz regime. Moreover, the observed novel interference phenomena depend sensitively on plasma properties and carrier interactions, and thus, can be used to study solid-state plasmas over a vast range of external fields and temperatures from the classical limit to the ultra-quantum limit. This experimental finding may open up further new opportunities for using coherent THz methods to probe more exotic phenomena in condensed matter systems that occur due to many-body interactions and disorder.

References
[1] Harris, S. E. "Electromagnetically induced transparency". Phys. Today 50, 36-42 (1997). Abstract.
[2] McCall, S. L. & Hahn, E. L. "Self-induced transparency by pulsed coherent light", Phys. Rev. Lett. 18, 908-911 (1967). Abstract.
[3] X. Wang, A. A. Belyanin, S. A. Crooker, D. M. Mittleman, and J. Kono, “Interference-Induced Terahertz Transparency in a Semiconductor Magneto-plasma,” Nature Physics, published online on December 6, 2009. Abstract. Rice University Press Release.
[4] Palik, E. D. & Furdyna, J. K. "Infrared and microwave magnetoplasma effects in semiconductors". Rep. Prog. Phys. 33, 1193-1322 (1970). Abstract.
[5] McCombe, B. D & Wagner, R. J. in "Advances in Electronics and Electron Physics", Vol 37 (eds Marton, L.) 1-79 (Academic Press, 1975).
[6] Wang, X., Hilton, D. J., Ren, L., Mittleman, D. M., Kono, J. & Reno, J. L. "Terahertz time-domain magnetospectroscopy of a high-mobility two-dimensional electron gas". Optics Lett. 32, 1845-1847 (2007). Abstract.

Observation of Magnetic Monopoles in Spin Ice

Hiroaki Kadowaki, Yuji Aoki and Naohiro Doi of Tokyo Metropolitan University


[This is an invited article based on recently published work of the authors -- 2Physics.com]






Authors: H. Kadowaki¹, Y. Aoki¹, T. J. Sato², J. W. Lynn³

Affiliations: ¹Department of Physics, Tokyo Metropolitan University, Tokyo, Japan,
²NSL, Institute for Solid State Physics, University of Tokyo, Tokai, Japan,
³NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA

From the symmetry of Maxwell's equations of electromagnetism, magnetic charges or monopoles would be expected to exist in parallel with electric charges. About 80 years ago, a quantum mechanical hypothesis of the existence of magnetic monopoles was proposed by Dirac [1]. Since then, many experimental searches have been performed, ranging from a monopole search in rocks of the moon to experiments using high energy accelerators [2]. But none of them was successful, and the monopole is an open question in experimental physics. Theoretically, monopoles are predicted in grand unified theories as topological defects in the energy range of the order 10¹⁶ GeV [2]. However these enormous energies preclude all hope of creating them in laboratory experiments.

Taku J. Sato of University of Tokyo

Alternatively, recent theories predict that tractable analogs of the magnetic monopole might be found in condensed matter systems [3,4,5]. One prediction [4] is for an emergent elementary excitation in the spin ice compound Dy₂Ti₂O₇ [6], where the strongly competing magnetic interactions exhibit the same type of frustration as water ice [7]. In addition to macroscopically degenerate ground states [6], the excitations from these states are topological in nature and mathematically equivalent to the Dirac monopoles [1,4]. We have successfully observed [8] the signature of magnetic monopoles in the spin ice Dy₂Ti₂O₇ using neutron scattering, and find that they interact via the magnetic inverse-square Coulomb force. In addition, specific heat measurements show that the density of monopoles can be controlled by temperature and magnetic field, with the density following the expected Arrhenius law.

Jeffrey W. Lynn of NIST, USA

In Fig. 1 we illustrate creation of a magnetic monopole and antimonopole pair in spin ice under applied magnetic field along a [111] direction. This excitation is generated by flipping a spin, which results in ice-rule-breaking "3-in, 1-out" and "1-in, 3-out" tetrahedral neighbors, simulating magnetic monopoles, with net positive and negative charges sitting on the centers of tetrahedra. The monopoles can move and separate by consecutively flipping spins in the kagome lattice.

Fig. 1. Spins of Dy₂Ti₂O₇ occupy a cubic pyrochlore lattice, which is a corner -sharing network of tetrahedra, and consists of a stacking of triangular and kagome lattices. The competing magnetic interaction brings about a geometrical constraint where the lowest energy spin configurations on each tetrahedron follow the ice rule, in which two spins point inward and two point outward on each tetrahedron. (A) By applying a small magnetic field along a [111] direction, the spins on the triangular lattices are parallel to the field, while those on the kagome lattices retain disorder under the same ice rules. This is referred to as the kagome ice state [9]. (B) Creation of a magnetic monopole (blue sphere) and antimonopole (red sphere) pair in the kagome ice state.

A straightforward signature of monopole-pair creation is an Arrhenius law in the temperature (T) dependence of the specific heat (C). This Arrhenius law of C(T) is clearly seen in Fig. 2 at low temperatures, indicating that monopole-antimonopole pairs are thermally activated from the ground state, and that the number of monopoles can be tuned by changing temperature and magnetic field.

Fig. 2. Specific heat of Dy₂Ti₂O₇ under [111] magnetic fields is plotted as a function of 1/T. In intermediate temperature ranges these data are well represented by the Arrhenius law denoted by solid lines.

A microscopic experimental method of observing monopoles is to perform magnetic neutron scattering using the neutron's dipole moment as the probe. One challenge to the experiments is to distinguish the relatively weak scattering from the monopoles from the very strong magnetic scattering of the ground state. By choosing appropriate field-temperature values, we have successfully observed scattering by magnetic monopoles, diffuse scattering close to the (2,-2,0) reflections, and that by the ground state (Fig. 3) [8].

Fig. 3. Intensity maps of neutron scattering at T = T_c + 0.05 K in the scattering plane perpendicular to the [111] field are shown for H = 0.5 T and H = H_c. The kagome ice state at H = 0.5 T (A) compared with the MC simulation (C). The weakened kagome-ice state scattering plus the diffuse monopole scattering (B) at H = H_c agree with the MC simulation (D).

Typical elementary excitations in condensed matter, such as acoustic phonons and (gapless) magnons, are Nambu-Goldstone modes where a continuous symmetry is spontaneously broken when the ordered state is formed. This contrasts with the monopoles in spin ice, which are point defects that can be fractionalized in the frustrated ground states. Such excitations are unprecedented in condensed matter, and now enable conceptually new emergent phenomena to be explored experimentally [10].

References:
[1] "Quantised singularities in the electromagnetic field",
P. A. M. Dirac, Proc. R. Soc. A 133, 60 (1931). Article.
[2] "Theoretical and experimental status of magnetic monopoles",
K. A. Milton, Rep. Prog. Phys. 69, 1637 (2006). Abstract.
[3] "The anomalous Hall effect and magnetic monopoles in momentum space", Zhong Fang, Naoto Nagaosa, Kei S. Takahashi, Atsushi Asamitsu, Roland Mathieu, Takeshi Ogasawara, Hiroyuki Yamada, Masashi Kawasaki, Yoshinori Tokura, Kiyoyuki Terakura, Science 302, 92 (2003). Abstract.
[4] "Magnetic monopoles in spin ice"
C. Castelnovo, R. Moessner, S. L. Sondhi, Nature 451, 42 (2008). Abstract.
[5] "Inducing a magnetic monopole with topological surface states"
X-L. Qi, R. Li, J. Zang, S-C. Zhang, Science 323, 1184 (2009). Abstract.
[6] "Spin ice state in frustrated magnetic pyrochlore materials"
S. T. Bramwell, M. J. P. Gingras, Science 294, 1495 (2001). Abstract.
[7] "The structure and entropy of ice and of other crystals with some randomness of atomic arrangement" , L. Pauling, J. Am. Chem. Soc. 57, 2680 (1935). Abstract.
[8] "Observation of Magnetic Monopoles in Spin Ice", H. Kadowaki, N. Doi, Y. Aoki, Y. Tabata, T. J. Sato, J. W. Lynn, K. Matsuhira, Z. Hiroi, J. Phys. Soc. Jpn. 78, 103706 (2009). Abstract.
[9] "A new macroscopically degenerate ground state in the spin ice compound Dy2Ti2O7 under a magnetic field" K. Matsuhira, Z. Hiroi, T. Tayama, S. Takagi and T. Sakakibara, J. Phys. Condens. Matter 14, L559 (2002). Article; "Kagome ice State in the dipolar spin ice Dy2Ti2O7" Y. Tabata, H. Kadowaki, K. Matsuhira, Z. Hiroi, N. Aso, E. Ressouche, and B. Fåk, Phys. Rev. Lett. 97, 257205 (2006). Abstract.
[10] In Oct. 2009, in addition to [8], three experimental papers on the magnetic monopoles in spin ice have been published: "Measurement of the charge and current of magnetic monopoles in spin ice" S. T. Bramwell, S. R. Giblin, S. Calder, R. Aldus, D. Prabhakaran & T. Fennell, Nature 461, 956 (2009), Abstract; "Dirac Strings and Magnetic Monopoles in the Spin Ice Dy2Ti2O7" D. J. P. Morris, D. A. Tennant, S. A. Grigera, B. Klemke, C. Castelnovo, R. Moessner, C. Czternasty, M. Meissner, K. C. Rule, J.-U. Hoffmann, K. Kiefer, S. Gerischer, D. Slobinsky, R. S. Perry, Science 326, 411 (2009) Abstract; "Magnetic Coulomb Phase in the Spin Ice Ho2Ti2O7" T. Fennell, P. P. Deen, A. R. Wildes, K. Schmalzl, D. Prabhakaran, A. T. Boothroyd, R. J. Aldus, D. F. McMorrow, S. T. Bramwell, Science 326, 415 (2009). Abstract.

Topological Insulators : A New State of Quantum Matter

M. Zahid Hasan

[This is an invited article based on a series of recent works by the author and his collaborators -- 2Physics.com]

Author: M. Zahid Hasan

Affiliation: Joseph Henry Laboratories of Physics, Department of Physics,
Princeton University, USA

Most quantum states of condensed-matter systems or the fundamental forces are categorized by spontaneously broken symmetries. The remarkable discovery of quantum Hall effects (1980s) revealed that there exists an organizational principle of matter based not on the broken symmetry but only on the topological distinctions in the presence of time-reversal symmetry breaking [1,2]. In the past few years, theoretical developments suggest that new classes of topological states of quantum matter might exist in nature [3,4,5]. Such states are purely topological in nature in the sense that they do not break time-reversal symmetry, and hence can be realized without any applied magnetic field : "Quantum Hall-like effects without magnetic field".

Research Team at Princeton University: [L to R] David Hsieh, Dong Qian, L. Andrew Wray, YuQi Xia

This exotic phase of matter is a subject of intense research because it is predicted to give rise to dissipationless (energy saving) spin currents, quantum entanglements and novel macroscopic behavior that obeys axionic electrodynamics rather than Maxwell's equations [6]. Unlike ordinary quantum phases of matter such as superconductors, magnets or superfluids, topological insulators are not described by a local order parameter associated with a spontaneously broken symmetry but rather by a quantum entanglement of its wave function, dubbed topological order. In a topological insulator this quantum entanglement survives over the macroscopic dimensions of the crystal and leads to surface states that have unusual spin textures.

Topologically ordered phases of matter are extremely rare and are experimentally challenging to identify. The only known example was the quantum Hall effect discovered in the 1980s by von Klitzing (Nobel Prize 1985). It was identified by measuring a quantized magneto-transport in a two-dimensional electron system under a large external magnetic field at very low temperatures, which is characterized by robust conducting states localized along the one-dimensional edges of the sample. Two-dimensional topological insulators, on the other hand, are predicted to exhibit similar edge states even in the absence of a magnetic field because spin-orbit coupling can simulate its effect (Fig.1A) due to the relativistic terms added in a band insulator's Hamiltonian.

Remarkably, three-dimensional topological insulators, an entirely new state of matter with no charge quantum Hall analogue, are also postulated to exist. And its topological order or exotic quantum entanglement is predicted to give rise to unusual conducting two-dimensional surface states (Fig.1B) that have novel spin-selective energy-momentum dispersion relations. Utilizing state-of-the-art angle-resolved photoemission spectroscopy, an international collaboration led by scientists from Princeton University have studied the electronic structure of several bismuth based spin-orbit materials [7,8,9]. By systematic tuning of the incident photon energy, it was possible to isolate surface quantum states from the bulk states, which confirmed that these materials realized a three-dimensional topological insulator phase.

Figure 1. (A) Schematic of the 1D edge states in a 2D topological insulator. The red and blue curves represent the edge current with opposite spin character. (B) Schematic of the 2D surface states in a 3D topological insulator. (C) Most elemental topological Insulators exhibit odd number of Dirac cones on their surface unlike the even numbers observed in graphene. Topological insulator Dirac cones are spin polarized where as Dirac cones in graphene are not.

The remarkable property of the surface states of a 3D topological insulator is that its Fermi surface supports a geometrical quantum entanglement phase, which occurs when the spin-polarized Fermi surface encloses the Kramers' points and on the surface Brillouin zone an odd number of times in total (Fig.2B). ARPES intensity map of the (111) surface states of bulk insulating Bi1-xSbx (Fig.2A) shows that a single Fermi surface encloses . However, determination of the degeneracy of the additional Fermi surface around requires a detailed study of its energy-momentum dispersion. ARPES spectra along the - direction (Fig.2C) reveal that the Fermi surface enclosing is actually composed of two bands, therefore two Fermi surfaces enclose , leading to a total of seven and Fermi surface enclosures.

Figure 2. (A) ARPES surface state (SS) Fermi surface of insulating Bi1-xSbx showing spin polarization directions as indicated by red and blue arrows. (B) Schematic of the SS Fermi surface of a 3D topological insulator. (C) ARPES energy-momentum dispersion of the surface states. The shaded areas denote the bulk bands while the dashed white lines are guides to the eye for surface state dispersions. (D) A single Dirac cone is observed in Bi2Te3.

These results constitute the first direct experimental evidence of a topological insulator in nature which is fully quantum entangled. The observed spin-texture in BiSb is consistent with a magnetic monopole image field beneath the surface. It shows that spin-orbit materials are a new family in which exotic topological order quantum phenomena, such as dissipationless spin currents and axion-like electrodynamics, may be found without the need for an external magnetic field. The results presented in this study also demonstrate a general measurement algorithm of identifying and characterizing topological insulator materials for future research which can be utilized to discover, observe and study other forms of topological order and quantum entanglements in nature. A detailed study of topological order and quantum entanglement can potentially pave the way for fault-tolerant (topological) quantum computing [10].

Figure 3: A new type of quantum matter called a topological insulator contains only half an electron pair (represented by just one Dirac cone in schematic crystal structure at top left), which is observed in the form of a single ring (red) in the center of the electron-map (top right) with electron spin in only one direction. This highly unusual observation shows that if an electron is tagged "red" and then undergoes a full 360-degree revolution about the ring, it does not recover its initial face as an ordinary everyday object would, but instead acquires a different color "blue" (represented by the changing color of the arrows around the ring). This new quantum effect can be the basis for the realization of a rare quantum phase that had been a long-sought key ingredient for developing quantum computers that can be highly fault-tolerant.

References:
[1] K. von Klitzing, G. Dorda, M. Pepper, "New method for high-accuracy determination of the fine-structure constant based on quantized Hall resistance", Phys. Rev. Lett. 45, 494-497 (1980). Abstract.
[2] D.C. Tsui, H. Stormer, A.C. Gossard, "Two-dimensional magnetotransport in the extreme quantum limit", Phys. Rev. Lett. 48, 1559-1562 (1982). Abstract.
[3] L. Fu, C. L. Kane and E. J. Mele, "Topological insulators in three dimensions", Physical Review Letters 98, 106803 (2007). Abstract.
[4] J. E. Moore and L. Balents, "Topological invariants of time-reversal-invariant band structures", Physical Review B 75, 121306(R) (2007). Abstract.
[5] S.-C. Zhang, "Topological states of quantum matter", Physics 1, 6 (2008). Abstract.
[6] M. Franz, "High energy physics in a new guise", Physics 1, 36 (2008). Abstract.
[7] D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. S. Hor, R. J. Cava and M. Z. Hasan, "A topological Dirac insulator in a quantum spin Hall phase", Nature 452, 970 (2008). Abstract.
[8] Y. Xia, D. Qian, L. Wray, D. Hsieh, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava and M. Z. Hasan, "Observation of a large-gap topological insulator class with single surface Dirac cone”, Nature Physics 5, 398 (2009). Abstract.
[9] D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J. H. Dil, J. Osterwalder, F. Meier, G. Bihlmayer, C. L. Kane, Y. S. Hor, R. J. Cava and M. Z. Hasan, "Observation of Unconventional Quantum Spin Textures in Topological Insulators", Science 323, 919 (2009). Abstract.
[10] A. Akhmerov, J. Nilsson, C. Beenakker, “Electrically detected interferometry of Majorana fermions in a topological insulator”, Phys. Rev. Lett. 102, 216404 (2009). Abstract.

Ferrofluidic Deformable Mirrors for Adaptive Optics

Ermanno F. Borra (left) and Denis Brousseau (right)



[This is an invited article. The authors have built the first deformable liquid mirror from a magnetic liquid or “ferrofluid”, which is set to find wide range of applications including correction of aberration in the images of telescopes and many other optical devices. -- 2Physics.com]



Authors: Denis Brousseau and Ermanno F. Borra
Affiliation: Université Laval, Département de physique, de génie physique et d’optique and Centre d’Optique, Photonique et Laser (COPL), Québec, Canada.

For roughly the last 25 years, adaptive optic (AO) systems were primarily used for astronomical applications. In the last 10 years, the range of scientific applications of AO has soared and now includes vision science, medical imaging and free space optical communications to name only a few. These new applications have stimulated research for low-cost, high-stroke deformable mirrors with a large number of actuators. Most actual deformable mirrors are expensive, costing about $1000 US per actuator. Current high-stroke deformable mirrors like the Imagine Optics 52-actuator MIRAO DM can produce large deformations (50 µm peak-to-valley tilt) but having a larger number of actuators would greatly increase its cost. Micro-Electro-Mechanical Systems (MEMS) deformable mirrors having large numbers of actuators (over 1000) and fabricated by a technique similar to surface micromachining have a great potential for low cost, but are currently limited to strokes of only a few microns.

It is well known that a liquid follows an equipotential surface to a high degree of precision. For example, the surface of a rotating pool of mercury takes a parabolic shape which can be used as a the primary mirror of a low cost telescope (http://wood.phy.ulaval.ca/what.html and http://www.astro.ubc.ca/lmt/lm/index.html). During the last few years, we have developed a new type of deformable mirror made of a ferrofluid whose surface is shaped by an array of magnetic coils. A ferrofluid is a liquid that contains a suspension of small ferromagnetic particles (Ø ~ 10 nm) within a water- or oil- based carrier liquid. In the presence of an external magnetic field, the magnetic particles react with the field and the fluid surface takes a shape that is determined by the equilibrium between the magnetic, gravitational and surface tension forces. The equation that describes the shape of the surface can be derived using equations found in [1]

where µ_r and ρ are the relative permeability and density of the ferrofluid respectively, n is a unit vector perpendicular to the liquid surface and B is the external magnetic field vector at the liquid-air interface. The external magnetic field B can be produced by an array of small current carrying coils located just under the surface of the liquid. Based on this principle, we built a 37-channel deformable mirror prototype, made of a ferrofluid whose surface is actuated by a hexagonal array of small current carrying coils.

In standard modal control of deformable mirrors, the mirror surface is shaped by the linear addition of the individual response function of the actuators. We see, from the preceding equation, that in the case of a ferrofluid deformable mirror (FDM), the liquid surface deformation is non-linear with respect to the external magnetic field, and also depends on the individual orientation of the external magnetic field components. Consequently, conventional modal control of a FDM is impossible; however we have successfully developed a custom algorithm that is able to compute the currents that must be assigned to the coils for a given mirror surface shape.

Using a commonly available ferrofluid we found that a maximum deformation of over a millimeter can be achieved before reaching instability [2]. In theory, much larger deformations (several mm) could be obtained with magnetic fields having components mostly parallel to the liquid surface and/or using ferrofluids having different physical properties.

Fig. 1. A custom ferrofluid developed in our labs is shown coated with a reflective layer of MeLLF and under the influence of a magnetic field from a permanent magnet located under the container. Picture clearly shows the very large deformation amplitudes that can be obtained.

Ferrofluids have a low reflectivity similar to motor oil and for many applications must be coated with a reflective layer. This can be done using reflective liquids based on interfacial films of silver particles known as Metal Liquid-Like Films or MeLLFs [3]. MeLLFs combine the properties of metals and liquids, can be deformed and are therefore well adapted to applications in the field of liquid optics. MeLLFs are not compatible with currently available commercial ferrofluids, which are hydrophobic, and for compatibility with MeLLFs we had to developed a custom hydrophilic ferrofluid (see Fig. 1) [4]. Our team is also considering the deposition of a chemical membrane on the ferrofluid.

Our prototype consists of 37 custom made coils (actuators) closely packed in a hexagonal array 35 mm in diameter (see Fig. 2). Each coil is made of about 200 loops of AWG28 magnet wire and has an external diameter of 5 mm. A small ferrite core is placed at the center of each coil to lower the current requirement of the device. An aluminum container (not seen) filled with a one-millimetre-thick layer of ferrofluid is placed on top.

Fig. 2. Our 37-channel prototype showing the hexagonal array of 37 coils of 5-mm diameter.

Total cost of the FDM was estimated at about $100 per actuator, including materials, electronics and shop time. Costs can certainly be reduced further with improved technology.

Using our algorithm, we have computed the required currents to produce standard Zernike polynomials (http://en.wikipedia.org/wiki/Zernike_polynomials). Those currents were then fed to the FDM and the resulting wavefronts were measured using a wavefront sensor (see Fig. 3).

Fig. 3. Experimental wavefronts representing Zernike polynomials reproduced by the FDM and measured using a Shack-Hartmann wavefront sensor. Each wavefront has a PV wavefront amplitude of about 5 μm.

Because of the vector-dependent response of our device, we suspected that trying to fit real wavefronts made from combining several Zernike polynomials would result in lower wavefront residual errors than by adding the residuals of each Zernike that made up the original wavefront. We performed experiments to test this assumption. We purposely introduced optical aberrations of 0.58 µm RMS wavefront amplitude and 2.42 µm PV wavefront amplitude in our wavefront measurement setup. PSFs before and after correction can be seen in Fig. 4. The achieved Strehl ratio of the corrected wavefront is 0.84 at a wavelength of 659.5 nm.

We also introduced much greater amplitude aberrations with PV and RMS wavefront amplitudes of 11.43 and 2.58 µm respectively. The RMS residual error of the corrected wavefront was measured to be 0.15 µm. We found that this error drops to 0.05 µm if we consider only the low order aberration terms. Correction for high spatial frequency Zernike polynomials would improve if the FDM had a greater number of actuators.

Fig. 4. Experimental result showing the PSF (log scale) of an aberrated wavefront (left) corrected by using our deformable mirror (right). Strehl ratio of corrected wavefront is 0.84 (659.5 nm).

Although we got promising results, some drawbacks remain. We need to bias the surface of the liquid to allow for a push-pull effect as the amplitude varies as the square of the current applied to a given actuator (deformations can only be positive). This reduces the available stroke of the mirror and also adds a surface residual error.

A novel way to control those liquid mirrors has recently been introduced by Iqbal and Amara, and solves most of these drawbacks [5]. The technique consists of adding a constant and uniform magnetic field whose orientation is along the direction perpendicular to the surface of the liquid. The amplitude of this constant magnetic field is about 10 times greater than what is produced by the coils (~ 2.5 gauss). The magnetic field of the actuators acts as a small perturbation of the uniform field and this linearizes the response of the liquid (as shown in Fig. 5). This also has the effect of amplifying the stroke produced by the coils, reducing the required currents, so that ferrite cores in the actuators are no longer necessary, and making negative deformations possible.

Fig. 5. Measured amplitudes of the deformations produced by a single actuator in the presence of an external uniform magnetic field, as a function of current in the coil. The red and blue curves correspond to external magnetic fields of 25 and 30 gauss respectively. Negative deformations can be produced by inverting the current flow. The actuator used in the experiment has no ferrite core.

Until recently, we thought that those liquid mirrors were limited to a time response of only a few tens of hertz, because when driven at frequencies higher than about 20 Hz, we saw a rapid loss in amplitude response of the liquid and a phase lag of over 90 degrees appeared between the driving signal and the resulting liquid deformation, quite similar to the response of a low-pass RLC filter.

We demonstrated that the amplitude loss can be overcome by overdriving the coils with a very short and high amplitude current pulse launched at the beginning of each driving signal. By using this technique, a desired surface deformation is reached faster and the remaining signal stabilize the liquid shape.

We also demonstrated that the phase lag can be countered by increasing the viscosity of the ferrofluid. The critical frequency (a 90 degrees phase lag) was improved from 20 to 450 Hz by increasing the viscosity of the ferrofluid from 6 cP to 450 cP (viscosity of water is 1 cP and SAE 50 motor viscosity is about 500 cP). Since a single square wave signal sent to the liquid corresponds to two corrections (rise and fall), this actually implies a frequency response of 900 Hz. But increasing the viscosity also increases the time required for the liquid to stabilize. However, a solution to this problem is to use overdriving pulses that give an initial velocity to the liquid as discussed in the preceding paragraph.

To conclude, we have demonstrated a liquid deformable mirror prototype that can produce standard aberration terms, and we successfully corrected a 11 µm PV amplitude aberrated wavefront, yielding a residual RMS wavefront error of 0.05 µm. A new technique linearizes the response of these new deformable mirrors and allows the use of regular control algorithms. This will simplify our goal to demonstrate closed-loop operation of these new mirrors.

We have also shown the counterintuitive result that using a liquid having a sufficiently high viscosity improves their frequency response up to 900 Hz. By using both overdriving pulses and a higher viscosity ferrofluid, utilizing these mirrors in closed-loop at a running frequency of hundreds of hertz, appears to be possible. This will enable these mirrors to be used in many more applications than we previously thought. Ongoing tests on the chemical deposition of thin chemical membranes on ferrofluids could also improve the response of FDMs. We are now building a new prototype having 91 actuators of 2-mm diameter, thus reducing the footprint and allowing a higher density of actuators.

References
[1] "Interaction of a magnetic liquid with a conductor containing current and a permanent magnet"
V. V. Kiryushin and A. V. Nazarenko, Fluid Dynamics, 23, 306–311 (1988). Abstract.
[2] R. E. Rosensweig, "Ferrohydrodynamics". (Dover, 1997).
[3] "Nanoengineered astronomical optics",
E. F. Borra, A. M. Ritcey, R. Bergamasco, P. Laird, J. Gingras, M. Dallaire, L. Da Silva and H. Yockell-Lelievre, Astron. Astrophys. 419, 777-782 (2004). Abstract.
[4] "Ethylene Glycol Based Ferrofluid for the Fabrication of Magnetically Deformable Liquid Mirrors"
J. -P. Déry, E. F. Borra, and A. M. Ritcey, Chem. Mater. 20 (2008). Abstract.
[5] "Modeling of a Magnetic-Fluid Deformable Mirror for Retinal Imaging Adaptive Optics Systems"
A. Iqbal and F. B. Amara, International Journal of Optomechatronics 1, 180-208 (2007). Abstract.

Up to 400-fold Improvement in Magnetic Field Detection

W.F. Egelhoff, Jr. (photo courtesy: NIST)

A team of researchers at the National Institute of Standards and Technology (NIST) has reported
dramatically enhanced sensitivity -- a 400-fold improvement in some cases -- in a carefully built magnetic flux concentrator that draw in external magnetic field lines and concentrate them in a small region. The flux concentrator is a kind of magnetic sandwich that interleaves layers of a magnetic alloy with a few nanometers of silver “spacer”. They are used to amplify fields in compact magnetic sensors used for a wide variety of applications from weapons detection and non-destructive testing to medical devices and high-performance data storage.

Those applications and many others are based on thin films of magnetic materials in which the direction of magnetization can be switched from one orientation to another. An important characteristic of a magnetic film is its saturation field, the magnitude of the applied magnetic field that completely magnetizes the film in the same direction as the applied field—the smaller the saturation field, the more sensitive the device.

The saturation field is often determined by the amount of stress in the film—atoms under stress due to the pull of bonds with neighboring atoms are more resistant to changing their magnetic orientation. Metallic films develop not as a single monolithic crystal, like diamonds, but rather as a random mosaic of microscopic crystals called grains. Atoms on the boundaries between two different grains tend to be more stressed, so films with a lot of fine grains tend to have more internal stress than coarser grained films. Film stress also increases as the film is made thicker, which is unfortunate because thick films are often required for high magnetization applications.

Transmission electron microscope (TEM) images show sections of a continuous 400-nanometer-thick magnetic film of a nickle-iron-copper-molybdenum alloy (top) and a film of the same alloy layered with silver every 100 nanometers (bottom). By relieving strain in the film, the silver layers promote the growth of notably larger crystal grains in the layered material as compared to the monolithic film (several are highlighted for emphasis). Electron diffraction patterns (insets) tell a similar story—the material with larger crystal grains display sharper, more discrete scattering patterns. (Color added for clarity). Image credit: J. Bonevich, NIST

The NIST research team discovered that magnetic film stress could be lowered dramatically by periodically adding a layer of a metal, having a different crystal structure or lattice spacing, in between the magnetic layers. Although the mechanism isn’t completely understood, according to lead author William Egelhoff Jr., the intervening layers disrupt the magnetic film growth and induce the creation of new grains that grow to be larger than they do in the monolithic films. The researchers prepared multilayer films with layers of a nickel-iron-copper-molybdenum magnetic alloy each 100 nanometers (nm) thick, interleaved with 5-nm layers of silver. The structure reduced the tensile stress (over a monolithic film of equivalent thickness) by a factor of 200 and lowered the saturation field by a factor of 400.

Reference
"400-fold reduction in saturation field by interlayering",
W.F. Egelhoff, Jr., J. Bonevich, P. Pong, C.R. Beauchamp, G.R. Stafford, J. Unguris, and R.D. McMichael,
J. Appl. Phys. 105, 013921 (2009). Abstract.

[We thank Media relations, NIST for materials used in this report]

Field Effect Tuning of Superconductivity at Oxide Interfaces

Photo of the Geneva Group: (from Left to Right) Stefano Gariglio, Andrea Caviglia, Claudia Cancellieri, Nicolas Reyren and Jean-Marc Triscone

[This is an invited article based on recent work of this collaboration -- 2Physics.com]



Authors: A.D. Caviglia¹ , S. Gariglio¹, N. Reyren¹, C. Cancellieri¹, D. Jaccard¹, S. Thiel², G. Hammerl², J. Mannhart², J.-M. Triscone¹

Affiliation: ¹Département de Physique de la Matière Condensée, University of Geneva, Genève, Switzerland, >>Link to Group Homepage
²Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Germany, >>Link to Group Homepage

Charge transfer in semiconductors interfaces has brought about exceptional technological progress, one of the best examples being the development of the Field Effect Transistor (FET). Applying the same principle to materials with a wider spectrum of electronic properties, such as complex oxides, is an exciting opportunity both for fundamental and applied physics. These oxide compounds often exhibit strong electronic correlations and complex phase diagrams. In such systems, the electric field effect can be used to tune the ground state of the system [1]. These materials also display a broad range of functional properties, such as high dielectric permittivity, piezoelectricity and ferroelectricity, superconductivity, spin polarised current, colossal magnetoresistance and ferromagnetism. Recent advances in growth methods have allowed the fabrication of atomically abrupt interfaces between these materials where novel electronic phases are created. Indeed the emerging field of complex oxide interfaces has a high potential impact for applications [2] and has been classified as one of the 10 breakthroughs of 2007 by the journal Science [3].

Fig.1 Photo of the device (courtesy of J. Mannhart)

The LaAlO3/SrTiO3 interface
A particularly interesting system is the interface between band insulators LaAlO3 and SrTiO3, which was reported to be conducting in 2004 in a seminal publication [4]. This result is indeed amazing: by depositing on top of an insulating crystal (SrTiO3) a thin film of a good insulator (LaAlO3), a metallic interface is generated. This immediately calls to mind the two dimensional (2D) electron gas generated by modulation doping in III-V semiconductors. Correlated oxide systems are however more complex than semiconductors and in fact, in 2007 we discovered that this metallic interface undergoes a 2D superconducting transition at around 200 mK [5]. The superconducting sheet is 10 nm thick and confined between two dielectrics. What a perfect opportunity to try modulating the superconducting state by applying an external electric field!

Fig.2: Atomic view of the interface (courtesy of J. Mannhart)

A complex phase diagram uncovered
Hence a gate electrode has been deposited on the backside of the SrTiO3 crystal and the sheet resistance as a function of temperature for different applied gate voltages has been measured down to 20 mK. For large negative voltages (typically less than -200 V), corresponding to the smallest accessible electron densities, the sheet resistance increases as the temperature is decreased, indicating an insulating ground state. No traces of superconductivity are left! As the electron density is increased the system becomes a superconductor. A further increase in the electron density produces first a rise of the critical temperature to a maximum of 310 mK. For larger voltages the critical temperature decreases again. This is a beautiful example of a quantum phase transition: a change of the electronic phase of matter driven not by a variation of temperature but by the application of an electric field. These findings have been reported recently in the journal Nature [6].

A bright future
This fascinating interface offers many possibilities, among them, fundamental studies of quantum phase transitions in low dimensions. This discovery also opens the way to the fabrication of new mesoscopic devices based on the ability to switch on and off the superconducting state at the nanoscale.

References
[1] "Electric field effect in correlated oxide systems", C. H. Ahn, J.-M. Triscone and J. Mannhart,
Nature 424, 1015-1018 (2003). Abstract.
[2] "When Oxides Meet Face to Face", Elbio Dagotto, Science 318, 1076 (2007). Abstract.
[3] Science 318, 1844 (2007). Link.
[4] "A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface",
A. Ohtomo and H. Y. Hwang, Nature 427, 423-426 (2004). Abstract.
[5] "Superconducting Interfaces Between Insulating Oxides", N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Rüetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone, J. Mannhart, Science 317, 1196-1199 (2007). Abstract.
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