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.
[6] "Electric field control of the LaAlO3/SrTiO3 interface ground state", A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart & J.-M. Triscone,
Nature 456, 624-627 (2008). Abstract.

Optical Magnus Effect: Topological Monopole Deflects Spinning Light

Konstantin Y. Bliokh

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

Authors: Konstantin Y. Bliokh, Avi Niv, Vladimir Kleiner, and Erez Hasman

Affiliation: Micro and Nanooptics Laboratory, Faculty of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa, Israel
>> Link to the Group Homepage

In an article published in the December 2008 issue of Nature Photonics a team from Micro- and Nanooptical Laboratory of Technion-Israel Institute of Technology has reported the first direct observation of the topological spin transport of photons, also known as the spin Hall effect of light or the optical Magnus effect [1]. The effect represents a polarization-dependent transverse deflection of the light beam upon a bending of its trajectory, and it can be attributed to a Coriolis effect or a spin-orbit coupling of light. Remarkably, the spin-orbit interaction of light has an inherent topological origin which is described by the Berry-phase monopole in the momentum space.

Vladimir Kleiner, Erez Hasman, and Avi Niv (left to right)

Two decades ago, the Berry phase brought a geometrical beauty to the description of quantum adiabatic evolution [2,3]. Afterwards, physicists realized that seemingly ‘passive’ geometrical concepts, such as Berry curvature, also manifest themselves dynamically, producing a real action on physical objects. As a result, the geometry-induced forces appear which affect the dynamics of quantum particles with some internal properties [4]. In particular, they describe the Magnus effect of quantum vortices [5] and spin Hall effect of spinning particles [6-8]. This offers a novel type of quantum transport which is robust against the details of the system and is determined solely by the geometry and intrinsic properties of the particles.

The spin-Hall effect was invented in the context of semiconductor spintronics, where it is expected to have promising applications [6]. The same effect also occurs within the fundamental equations of high-energy physics involving such intriguing mathematical objects as topological monopoles and space non-commutativity [8]. It seems that optics provides an ideal field for exploring this striking phenomenon. First, trajectory of light propagation can be directly observed in relatively clean and simple systems, and the accuracy of modern optics allows sub-wavelength resolution at nano-scales. Second, classical light captures all basic features of relativistic spinning particles, which enables one to extrapolate results to a diversity of physical systems, where such observations are impossible.

Fig. 1. The trajectories of left- and right-handed circularly polarized light beams propagating along the reflecting surface of a glass cylinder. The spin-orbit coupling between the intrinsic angular momentum of light and the curved propagation trajectory produces opposite deflections for the two beams. This is the spin Hall effect of light described by a Lorentz-force-type term from a topological monopole in momentum space [This figure is reprinted from "The dynamics of spinning light" by Franco Nori, Nature Photonics 2, 717 (2008). Our thanks to 'Nature Photonics']

The experiment of Ref. 1 was realized by launching a laser beam at a grazing angle to the internal surface of a glass cylinder, so that the light propagated along a smooth helical trajectory due to total internal reflection, Fig. 1. Such a helical path induces a spin-orbit interaction between the geometry of the trajectory and the intrinsic spin angular momentum carried by the polarized light. The theory and experiment of Ref. 1 provide a fairly complete picture of the geometrodynamical evolution of polarized light. On the one hand, the geometry of the trajectory determines the variations of the polarization of light. On the other hand, a spin-dependent perturbation of the trajectory occurs which deflects the right- and left-handed circularly polarized beams in opposite directions tangent to the cylinder surface (see Fig. 1).

In addition to fundamental interest, the spin Hall effect of light may have promising applications in photonics. Utilizing this effect in optical devices may lead to the development of a promising new area of research – spinoptics. The hope is that we will be able to control light in all-optical nanometer scale devices in ways that were impossible before [9,10]. While tiny wavelength-scale effects were negligible a decade ago, nowadays they can be crucial for numerous nano-optical applications.

References:
[1] “Geometrodynamics of Spinning Light”, K.Y. Bliokh, A. Niv, V. Kleiner, E. Hasman, Nature Photonics, 2, 748 (2008). Abstract.
[2] “Quantal Phase Factors Accompanying Adiabatic Changes”, M.V. Berry, Proc. R. Soc. A 392, 45 (1984). Abstract.
[3] “Geometric Phases in Physics”, A. Shapere, F. Wilczek (eds) (World Scientific, Singapore, 1989).
[4] “Origin of the Geometric Forces Accompanying Berry’s Geometric Potentials”, Y. Aharonov, A. Stern, Phys. Rev. Lett. 69, 3593 (1992). Abstract.
[5] “Transverse Force on a Quantized Vortex in a Superfluid”, D.J. Thouless, P. Ao, Q. Niu, Phys. Rev. Lett. 76, 3758 (1996). Abstract.
[6] “Dissipationless Quantum Spin Current at Room Temperature”, S. Murakami, N. Nagaosa, S.C. Zhang, Science 301, 1348 (2003). Abstract.
[7] “Topological Spin Transport of Photons: The Optical Magnus Effect and Berry phase”, K.Y. Bliokh, Y.P. Bliokh, Phys. Lett. A 333, 181 (2004). Abstract.
[8] “Spin Hall Effect and Berry Phase of Spinning Particles”, A. Bérard, H. Mohrbach, Phys. Lett. A 352, 190 (2006). Abstract.
[9] “Observation of the Spin Hall Effect of Light via Weak Measurements”, O. Hosten, P. Kwiat, Science 319, 787 (2008). Abstract. Related article in 2Physics.
[10] “Coriolis Effect in Optics: Unified Geometric Phase and Spin-Hall Effect”, K.Y. Bliokh, Y. Gorodetski, V. Kleiner, E. Hasman, Phys. Rev. Lett. 101, 030404 (2008). Abstract.

Adding and Subtracting Photons for Fundamental Tests of Physics and for Optical Quantum Technologies

(from left to right) Alessandro Zavatta, Marco Bellini, and Valentina Parigi

Author: Marco Bellini

Affiliation: Istituto Nazionale di Ottica Applicata – CNR
and
European Laboratory for Non-linear Spectroscopy (LENS), Florence, Italy.
>>Link to the Group Homepage.

[This is an invited article based on recent work of the author and his colleagues -- 2Physics.com]

Imagine a magician’s hat containing some rabbits whose precise number is unknown, but whose number probability distribution is well defined, so that, for a large ensemble of identical hats with the same probability distribution, one may define an average number of rabbits.

Now, if the magician puts one more rabbit in each hat, the mean rabbit number will, quite naturally, increase by one, while it will decrease by one if he takes one away (unless, of course, the hat was initially empty, in which case he would not be able to extract anything). Moreover, whatever the initial distribution, if the magician performs the two actions in a sequence, by first adding one rabbit and then taking one away, he/she will end up with exactly the same distribution for the number of rabbits remaining in the hat.

What would happen if, instead of normal rabbits, the magician used a microscopic hat containing quantum rabbits?

According to quantum physics, an electromagnetic field is composed of photons, which are so small that even a laser pointer with a typical power of 1mW emits a few millions of billions of them each second. Pure single photons are the ideal means to carry and encode information in emerging quantum technologies, but generating and manipulating them is still a very challenging task.

If the rabbits were identical quantum particles, one could assimilate them to photons in a radiation field (the hat), and would naturally use the so-called creation and annihilation operators to perform the addition and the subtraction of quantum rabbits to/from the hat. Indeed, as undergraduate physics students know, the photon creation operator acts on a state with a well-defined number of photons (also called a Fock state) by increasing this number by one. Conversely, when the photon annihilation operator acts on the same state, it subtracts a quantum of excitation, thus reducing the number of photons in the state by exactly one.

However, the situation becomes completely different as soon as one starts dealing with general superpositions or mixtures of Fock states. If the magician were using a distribution of quantum rabbits, the operation of adding one animal to the hat by a “rabbit creation operator” and then, immediately after, subtracting another by a “rabbit annihilation operator”, would lead to a final probability distribution of rabbits in the hat completely different from the initial one. Furthermore, the reverse sequence of operations would lead to a third outcome, different from both, i.e. the two operations do not commute.

This is the manifestation of one of the most profound laws in quantum physics. Indeed, the non-commutativity of particular quantum operations leads to many of the counterintuitive and fascinating aspects of quantum mechanics, including the famous Heisenberg uncertainty principle.

In 2007 our team (A. Zavatta, V. Parigi, and M. Bellini) at the Istituto Nazionale di Ottica Applicata – CNR (Florence, Italy), in collaboration with M. S. Kim from the Queen’s University (Belfast, UK), succeeded in performing the first direct tests of this fundamental principle of quantum physics in a laboratory [1]. We chose to use photons (which are much easier to manipulate than rabbits) and applied sequences of the creation and annihilation operators to an ordinary light pulse by making use of beam-splitters [2] and non-linear crystals [3]. As non-commutativity predicts, we found that the order of the operations makes a big difference to the outcome.

Figure 2: Setups to conditionally subtract (a) and add (b) a single photon from/to a light field. BS is a low-reflectivity beam-splitter; PDC is a nonlinear crystal where parametric down-conversion takes place; the two white boxes denote on/off photodetectors that herald the success of the corresponding quantum operation on the initial field state.

During those experiments we also found that the quantum operations behave so unusually that, under particular conditions, subtracting a photon changed the quantum state of the light pulse to the extent that its mean number of photons increased instead of diminishing. Taking a quantum rabbit away from the hat could actually increase the mean number of the remaining ones!

In one of our recent works [4] we decided to verify this behavior in a systematic way for some paradigmatic states of light. By applying photon annihilation to a Fock state with a well-defined number of photons we confirmed the intuitive decrease of the photon number by exactly one unit. Surprises appeared when we subtracted a single photon from a thermal state, the most common state of light (both the sun and ordinary light bulbs emit chaotic thermal light). We found that the mean number of photons in the pulse after subtraction was the double of the initial one.

Figure 3: Experimental density matrices and Wigner functions for a thermal state (left panel) and for the same state after a single-photon subtraction (right panel). The photon-subtracted state has a broader Wigner and photon number distribution than the original one.

Finally, when we tried to subtract a photon from a coherent state (the most classical, wave-like, state of light) we found that nothing changed in the state. In other words, we performed the first experimental demonstration that coherent states are invariant under photon annihilation. Since their introduction by Nobel laureate Roy Glauber in the 60’s, coherent states have been a cornerstone in the quantum description of light, but their definition as eigenstates of the annihilation operator had never been verified so directly in an experiment.

Figure 4: Experimental density matrices and Wigner functions for a coherent state (left panel) and for the same state after a single-photon subtraction (right panel). Photon annihilation does not modify a coherent state.

Although counterintuitive, the strange behavior of quantum operations is not unphysical and does not put energy conservation at stake: most of its weirdness simply derives from the misleading implicit assumption that a deterministic addition and subtraction of particles can be represented by the creation and annihilation operators which, on the contrary, work in a probabilistic way (i.e., the probability of extracting a particle from the hat scales with the number of particles already there) [5].

Apart from providing some beautiful demonstrations of the inner working of quantum mechanics, the techniques used in these experiments could in principle be used to arbitrarily engineer light at the most accurate levels by the appropriate sequence of photon additions and subtractions. This capability will open the way to “tailor-made” quantum light for future technologies, like the secure exchange of information through quantum cryptography or the development of novel protocols for quantum-enhanced measurements and communications.

For further info, please contact: Dr. Marco Bellini, Email: bellini@inoa.it

References
[1] “Probing Quantum Commutation Rules by Addition and Subtraction of Single Photons to/from a Light Field”, V. Parigi, A. Zavatta, M.S. Kim, and M. Bellini, Science, 317, 1890-1893 (2007). Abstract.
[2] “Non-Gaussian Statistics from Individual Pulses of Squeezed Light”, J. Wenger, R. Tualle-Brouri, and P. Grangier, Phys. Rev. Lett. 92, 153601 (2004). Abstract.
[3] “Quantum-to-classical transition with single-photon-added coherent states of light”, A. Zavatta, S. Viciani and M. Bellini, Science, 306, 660-662 (2004). Abstract.
[4] “Subtracting photons from arbitrary light fields: experimental test of coherent state invariance by single-photon annihilation”, A. Zavatta, V. Parigi, M. S. Kim, and M. Bellini, New Journal of Physics, 10, 123006 (2008). Abstract.
[5] “Recent developments in photon-level operations on travelling light fields”, M. S. Kim, J. Phys. B 41, 133001 (2008). Abstract.