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2Physics Quote:
"The discovery of laser filamentation can be attributed to M. Hercher, who observed damage tracks along the laser path in crystals. Later, the filamentation phenomenon was shown for a laser propagating in air. For the first time, the optical power at hand could allow one to witness a new type of light propagation based on the Kerr effect, a non-linear phenomenon which acts as a focusing lens and overcomes the beam natural diffraction. As a consequence, the propagation medium is ionized, and produces a plasma filament of tens of microns wide, which can be sustained over meters in air." -- Wahb Ettoumi (Read full article: "Percolation in Laser Filamentation")

Sunday, June 28, 2015

Spiral Electric Fields Imposed on Laser Beam Creates Spiral Complex, Surface Micro-structures

Jinglie Ouyang, the Ph.D student of the Laser Group at Liverpool, who carried out this research led by Dr. Walter Perrie and Dr. Olivier Allegre. 

Author: Walter Perrie

Affiliation: Laser Group, University of Liverpool, UK.

Scientists at the University of Liverpool have generated new polarisation states of light and imposed these on an ultrafast laser beam producing 10 picosecond (1ps = 10-12s) laser pulses [1]. Linear polarisation states are familiar in physics where the electric field is uni-directional in space across the laser beam and output intensity often a “Gaussian” mode with an intensity maximum at centre. Much less familiar polarisation states are, for example, Radial and Azimuthal polarisations which are vector fields in which the electric field direction varies spatially in a fixed plane with radially pointed vectors over 0-360° (Radial polarisation) and the orthogonal state (Azimuthal polarisation) where the field vectors consist of concentric circles. Such states have an intensity and polarisation singularity at their centre and so have ring intensity distributions.

By creating superpositions of Radial and Azimuthal polarisation states, the resulting laser electric fields were logarithmic spirals, a natural spiral (first described by Descartes and admired by Bernoulli) describing, for example the spiral arms of galaxies. The electric field at a given point is given by E(r, φ) = a ek φ where a and k are constants.

Ph.D student Jinglie Ouyang and colleagues (led by Dr. W.Perrie and Dr.O.J Allegre) used these states to imprint Laser Induced Periodic Surface Structures (LIPSS) to create beautiful spiral grooved structures with 1 μm pitch on polished metals for the first time [1]. These Plasmon structures develop orthogonal to the local electric field component and so elucidate the incident electric field distribution unambiguously. The spiral states are created by rotating an incident linear polarised laser beam on a specially nano-structured waveplate which generates Radial, Azimuthal and superpostion states of polarisation resulting in the spirals.
Interestingly, these spiral fields were predicted theoretically by Professor Franco Gori of the University degli Studi Roma Tre in 2001 [2] and not observed until now. The above figure shows two of these spiral structures ablated on stainless steel with incident angles to the waveplate axis of 22.5° and 45° respectively with theoretical fits (red curves) which agree well.

The scientists also added Optical Angular momentum (OAM) to these states by twisting the wavefronts so that each photon carries a z-component of angular momentum, Lz = h/2π per photon and focusing of these beams created a near Gaussian beam intensity distribution with circular polarisation (carrying spin angular momentum Lz = h/2π per photon) at the centre. This is an example of Orbital to Spin angular momentum conversion and creates even more complex microstructures.

Relevant Papers:
[1] J. Ouyang, W. Perrie, O. J. Allegre, T. Heil, Y. Jin, E. Fearon, D. Eckford, S. P. Edwardson, G. Dearden, “Tailored Optical vector fields for ultrashort-pulse laser induced complex surface Plasmon structuring”. Optics Express, 23, 12562-12572 (2015). Abstract.
[2] Franco Gori, "Polarisation basis for vortex beams”. Journal of the Optical Society of America A 18, 1612 (2001). Abstract.
[3] Qiwen Zhan, “Cylindrical Vector Beams from mathematical concepts to applications”. Advances in Optics and Photonics, 1(1), 1-57 (2009). Abstract.
[4] Martynas Beresna, Mindaugas Gecevičius, Peter G. Kazansky, Titas Gertus, “Radially polarised optical converter created by femtosecond laser nanostructuring of glass”, Applied Physics Letters, 98, 201101 (2011). Abstract.
[5] Y.Jin, O. J. Allegre, W. Perrie, K. Abrams, J. Ouyang, E. Fearon, S. Edwardson, G.Dearden, “Dynamic modulation of spatially stuctured polarisation fields for real time control of ultrafast laser –material interactions”. Optics Express, 21, 25333 (2013). Abstract.

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Sunday, June 21, 2015

Tuning Superconductivity in a Molecular System: The Key Role of the Jahn-Teller Metallic State

Authors of the paper in Science Advances (reference [1]). Left to Right: (top row) R. H. Zadik, Y. Takabayashi, G. Klupp, R. H. Colman, A. Y. Ganin, A. Potočnik, (middle row) P. Jeglič, D. Arčon, P. Matus, K. Kamarás, Y. Kasahara, Y. Iwasa, (bottom row) A. N. Fitch, Y. Ohishi, G. Garbarino, K. Kato, M. J. Rosseinsky, K. Prassides.

Authors: Gyöngyi Klupp and Ruth H. Zadik

Affiliation: Department of Chemistry, Durham University, UK

In our recent work, published in the journal Science Advances [1], we have addressed the relationship between the parent insulating phase, the normal metallic state and the superconducting pairing mechanism, a key challenge for all unconventional superconductors, in a series of chemically-pressurized fulleride superconductors. This work has revealed a new state of matter straddling the Mott insulating and Fermi liquid states at the two extremes of the phase diagram: the Jahn-Teller metal, where localized electrons on the fullerene molecules coexist with metallicity.

Figure 1: Crystal structure of face-centred-cubic A3C60 (A = alkali metal) with C603- anions in grey, and the alkali metal cations in colour; reproduced from ref. [1].

Alkali metal intercalated fullerene compounds with the stoichiometry A3C60 (Fig. 1) are superconducting, with the highest superconducting Tc found in a molecular material being 38 K in pressurised Cs3C60 [2]. This material is an unconventional superconductor; however, its Rb analogue, Rb3C60 exhibits conventional superconductivity. The question arises how the two types of behaviours are related both in the normal and in the superconducting state. The nature of the normal state from which the highest Tc emerges is also of key importance.

In order to address these questions, mixed bulk superconducting salts of C60 were prepared with compositions RbxCs3-xC60 (0 < x < 3) [1]. In the isostructural face-centred-cubic- structured phases of this compositional series, tuning the ratio of cations with different diameters controls the distance between C603- anions, without the need to apply external pressure, thus permitting a wide range of measurements to be employed. Variable temperature high-resolution synchrotron x-ray powder diffraction, SQUID (superconducting quantum interference device) magnetometry, nuclear magnetic resonance (NMR), infrared (IR) spectroscopy and specific heat measurements were undertaken. When the intermolecular separation is large, like in Cs3C60 at ambient conditions, the electrons of the C603− anions cannot hop from one site to the other, and the material is a Mott insulator (Fig. 2). Electrons localised on the C603- anions couple to intramolecular vibrations leading to the distortion of the molecule [3]. The Jahn-Teller distortion removes electronic as well as vibrational degeneracies. The latter results in splitting of vibrational lines in the IR spectrum, which provides an excellent way of detecting the Jahn-Teller effect. Thus the parent insulating state of A3C60 superconductors is a Mott-Jahn-Teller insulator, as has previously been demonstrated through IR spectroscopy for the most expanded member Cs3C60 [3].
Figure 2: The different electronic phases encountered in RbxCs3-xC60 fullerides ranging from the conventional metallic (green) through the Jahn-Teller metallic (orange) to the Mott-Jahn-Teller insulating regime (cyan). The top two rows show schematics of the molecular geometry together with the molecular electronic structure determined by the Jahn-Teller effect and the most characteristic region of the infrared spectrum. The middle panel is the electronic phase diagram; symbols represent the insulator-to-metal transitions and the superconducting Tcs as a function of the volume occupied by a C603- anion, which is increasing as the intermolecular distance increases. The lower panel shows the variation in the superconducting gap normalised by Tc. Figure reproduced from ref. [1].

Decreasing the intermolecular distance from this state allows the hopping of the electrons, yielding metallicity. Thus an insulator-to-metal transition, or crossover, is observed on increasing the proportion of the smaller Rb+ ion in the material (Fig. 2). Signatures of this transition from the experimental techniques deployed include anomalous shrinkage of the unit cell size, cusps (or maxima) in the magnetic susceptibility and temperature-normalised spin-lattice relaxation rates as a function of temperature, and a step-wise decrease in IR spectral background transmittance.

However, the metallic state encountered close to the metal-insulator boundary is not conventional; the electrons are not forming the conventional bands of a Fermi liquid. Electron correlation results in some persisting localised features in the electron system, like the continued presence of the Jahn-Teller effect as evidenced by IR spectroscopy. The coexistence of the molecular Jahn-Teller effect with metallicity is reflected in the designation of the newly observed phase as a Jahn-Teller metal. The further decrease of intermolecular distances leads to the gradual disappearance of the localised features, like e.g. the Jahn-Teller effect, until a conventional metal is encountered in Rb2CsC60 and Rb3C60 (Fig. 2). When the electrons are delocalised over the whole crystal to provide a conventional band, they cannot induce Jahn-Teller distortion any more [4].

A similar crossover between the unconventional and the conventional behaviour is also present in the superconducting state. The superconducting gap probed by NMR spectroscopy at large intermolecular separations is much larger than that of conventional BCS (Bardeen, Cooper and Schrieffer)-type weakly-coupled superconductors (Fig. 2). As the localised character of the electronic structure fades away gradually with decreasing intermolecular distances, the size of the gap returns to the value characteristic for conventional superconductors. Decreasing the intermolecular separation in the unconventional region leads to a rise in Tc , while it leads to the long-known decrease in the conventional region. Thus the highest Tc emerges where the molecular and extended properties of the electronic structure are balanced.

The observed behaviour shows how the superconducting Tc can be tuned in fullerides, paving the way for the preparation of other molecular superconductors with enhanced Tc. Establishing the whole phase diagram of face-centered-cubic A3C60 superconductors and tracking the transition between conventional and unconventional states can provide important clues for the understanding of high-Tc superconductivity in other materials, as well. Our results are also expected to stimulate the development of improved theoretical descriptions of the A3C60 system [5], further advancing our understanding of the origins and mechanism of superconductivity in other strongly-correlated high-Tc superconductors.

[1] Ruth H. Zadik, Yasuhiro Takabayashi, Gyöngyi Klupp, Ross H. Colman, Alexey Y. Ganin, Anton Potočnik, Peter Jeglič, Denis Arčon, Péter Matus, Katalin Kamarás, Yuichi Kasahara, Yoshihiro Iwasa, Andrew N. Fitch, Yasuo Ohishi, Gaston Garbarino, Kenichi Kato, Matthew J. Rosseinsky, Kosmas Prassides, “Optimized unconventional superconductivity in a molecular Jahn-Teller metal”. Science Advances, 1, e1500059 (2015). Abstract.
[2] Alexey Y. Ganin, Yasuhiro Takabayashi, Yaroslav Z. Khimyak, Serena Margadonna, Anna Tamai, Matthew J. Rosseinsky, Kosmas Prassides, “Bulk superconductivity at 38 K in a molecular system”. Nature Materials, 7, 367 (2008). Abstract.
[3] Gyöngyi Klupp, Péter Matus, Katalin Kamarás, Alexey Y. Ganin, Alec McLennan, Matthew J. Rosseinsky, Yasuhiro Takabayashi, Martin T. McDonald, Kosmas Prassides, “Dynamic Jahn-Teller effect in the parent insulating state of the molecular superconductor Cs3C60”. Nature Communications, 3, 912 (2012). Abstract.
[4] A. Wachowiak, R. Yamachika, K. H. Khoo, Y. Wang, M. Grobis, D.-H. Lee, S. G. Louie, M. F. Crommie, “Visualization of the molecular Jahn-Teller effect in an insulating K4C60 monolayer”, Science, 310, 468 (2005). Abstract.
[5] Yusuke Nomura, Shiro Sakai, Massimo Capone, Ryotaro Arita, “Unified understanding of superconductivity and Mott transition in alkali-doped fullerides from first principles”, arXiv:1505.05849v1 [cond-mat.supr-con] (2015).

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Sunday, June 14, 2015

Evidence for Pre-formed Pairs in an Oxide Superconductor

From left to right: Mengcheng Huang, Shicheng Lu, Jeremy Levy, Guanglei Cheng, Michelle Tomczyk, Patrick Irvin. 

Authors: Guanglei Cheng, Michelle Tomczyk, Jeremy Levy

Department of Physics and Astronomy, University of Pittsburgh, USA.
Pittsburgh Quantum Institute, Pittsburgh, Pennsylvania, USA.

Link to Levy Research Group >>

Strontium titanate (SrTiO3 or simply STO) is the first and best-known superconducting semiconductor. Its many fascinating properties, especially superconductivity, motivated Georg Bednorz and Alex Müller to search for high-temperature (high-Tc) superconductors in perovskite oxides [1]. STO shares many features related to the cuprate high-Tc superconductors, including a dome-shaped phase diagram and a pseudogap phase [2]. However, STO has much lower temperature and carrier density than the high-Tc compounds. A longstanding question surrounds the nature of the pseudogap state: ‘To pair or not to pair?’ – Does the existence of a pseudogap indicate electron pairing in the absence of superconductivity?

In 1969, long before high-Tc superconductivity was discovered, D.M. Eagles predicted [3] that electrons could remain paired outside of the superconducting state in STO. Eagles predicted that electrons can form a dilute gas so that the size of pairs is very small compared to the inter-electron distance. Namely, electrons pair in real space and the superconductivity is a result of Bose-Einstein condensation (BEC), contrasting the weak pairing in momentum space described by Bardeen-Schrieffer-Cooper (BCS) theory, which is highly successful in explaining conventional superconductors. Eagles was the first to propose the concept of BEC-BCS crossover, which was later independently developed by Nobel laureate Anthony Leggett [4] and experimentally realized in an ultracold atomic gas [5]. The discovery of BEC-type superconductivity in solid state systems has been challenging.

The LaAlO3/SrTiO3 (LAO/STO) interface [6] has attracted numerous interests in the last decade. It hosts a two-dimensional conducting interface that possesses a wealth of strongly correlated phenomena including superconductivity, magnetism, metal-insulator transition (MIT) and spin-orbit interaction [7]. A few years ago, we developed a lithography technique that allows us to ‘write’ and ‘erase’ nanostructures at the LAO/STO interface by using a sharp conductive atomic force microscope (c-AFM) tip, thus effectively programming all the novel properties at the nanoscale [8,9]. The writing mechanism relies critically on the MIT, in which the interface becomes conducting above the critical 3 unit cell (uc) LAO thickness. While the 3uc LAO/STO interface is insulating, it is switchable by a voltage-biased c-AFM tip. Under the c-AFM tip, a series of nanoscale devices have been made available. The resulting nanowires are only a few nanometers wide, exhibit anomalously high mobility [10] and show potential for the development of new types of nanoelectronics.

To investigate electron pairing, we use a superconducting single-electron transistor (Figure 1a). The device consists of a main superconducting nanowire channel intersected with voltage leads. Two tunnel potential barriers are engineered through c-AFM ‘cutting’ procedures so that a nanoscale island is defined. Due to the nanoscale confinement, the energy levels inside the island are quantized, and carrier transport is only possible when the chemical levels of external electrodes (source and drain) are aligned with an island energy level, which is dependent on the side gate voltages.
Figure 1: (click on the figure to view with higher resolution) Device schematic and transport characteristics. (a), Device schematic written by c-AFM lithography. The nanowires are typically 5 nm wide, and the length between 2 barriers is 1 μm. (b) and (c) Differential conductance dependent on the source-drain bias and side gate voltages at B=0 T (b) and B=4 T (c). The number of diamonds is doubled in (c). Color scales are 0-80 µS. (d) Magnetic field dependence of the conductance peaks. The bifurcation of conductance peaks above Bp suggest electron pairing without superconductivity. Color scale: 0-40 µS.

Indeed, bias spectroscopy reveals [11] a series of conductance diamonds (Fig. 1b), reminiscent of so-called ‘Coulomb diamonds’ in conventional blockade physics. In the latter case, each sequential Coulomb diamond corresponds to the stability of one additional electron, and applying an external magnetic field merely changes the size of diamonds due to Zeeman Effect. Remarkably, when we increase the magnetic field, the diamonds initially remain insensitive to field, then bifurcate above a critical magnetic field Bp~2 T (Fig. 1b,c). Such behavior is clearly revealed when we track the magnetic field dependence of the zero-bias conductance peaks. We find that all of the peaks bifurcate above a ‘pairing field’ Bp (Fig. 1d), suggesting transport is dominated by electron pairs rather than single electrons below Bp. Electron pairing persists far above the critical temperature (Tc~0.3 K) and for magnetic fields far above the upper critical field (Hc2~0.2 T) for superconductivity in bulk STO.

The observed electron pairing without superconductivity is difficult to explain using BCS theory. Pair fluctuations in disordered BCS superconductor films may give signatures of pairing above Tc which is greatly suppressed by disorder. However, the corresponding pairing temperature will not exceed Tc in the clean limit. Here the pairing temperature we have observed is around several kelvin, one order of magnitude higher than the Tc in the bulk. These experimental signatures are captured by a phenomenological model that favors BEC pairing, consistent with D. M. Eagles’s theory proposed 46 years back.

[1] J. Georg Bednorz and K. Alex Müller, "Perovskite-type oxides - the new approach to High Tc superconductivity", Nobel Lecture (1987).
[2] C. Richter, H. Boschker, W. Dietsche, E. Fillis-Tsirakis, R. Jany, F. Loder, L. F. Kourkoutis, D. A. Muller, J. R. Kirtley, C. W. Schneider, J. Mannhart, "Interface superconductor with gap behaviour like a high-temperature superconductor", Nature, 502, 528 (2013). Abstract.
[3] D.M. Eagles, "Possible pairing without superconductivity at low carrier concentrations in bulk and thin-film superconducting semiconductors", Physical Review, 186, 456 (1969). Abstract.
[4] Anthony J. Leggett, "A theoretical description of the new phases of liquid 3He", Reviews of Modern Physics,  47, 331 (1975). Abstract.
[5] M. W. Zwierlein, J.R. Abo-Shaeer, A. Schirotzek, C.H. Schunck, W. Ketterle, "Vortices and superfluidity in a strongly interacting Fermi gas",  Nature, 435, 1047 (2005). Abstract.
[6] A. Ohtomo,  H.Y. Hwang, "A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface", Nature, 427, 423 (2004). Abstract.
[7] Joseph A. Sulpizio, Shahal Ilani, Patrick Irvin, Jeremy Levy, "Nanoscale Phenomena in Oxide Heterostructures", Annual Review of Materials Research, 44, 117 (2014). Abstract.
[8] C. Cen, S. Thiel, G. Hammerl, C. W. Schneider, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, "Nanoscale control of an interfacial metal-insulator transition at room temperature", Nature Materials, 7, 298 (2008). Abstract.
[9] Cheng Cen, Stefan Thiel, Jochen Mannhart, Jeremy Levy, "Oxide nanoelectronics on demand", Science, 323, 1026 (2009). Abstract.
[10] Patrick Irvin, Joshua P. Veazey, Guanglei Cheng, Shicheng Lu, Chung-Wung Bark, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy, "Anomalous High Mobility in LaAlO3/SrTiO3 Nanowires", Nano Letters, 13, 364 (2013). Abstract.
[11] Guanglei Cheng, Michelle Tomczyk, Shicheng Lu, Joshua P. Veazey, Mengchen Huang, Patrick Irvin, Sangwoo Ryu, Hyungwoo Lee, Chang-Beom Eom, C. Stephen Hellberg, Jeremy Levy, "Electron pairing without superconductivity", Nature 521, 196 (2015). Abstract.

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Sunday, June 07, 2015

Using Exciton-Polaritons as a Nonequilibrium Coolant Fluid

From left to right: Lesi Dang, Sebastian Klembt, Anna Minguzzi, Maxime Richard and Emilien Durupt.

Authors: Sebastian Klembt1, Emilien Durupt1, Sanjoy Datta2, Thorsten Klein3, Yoan Léger4, Augustin Baas1, Detlef Hommel3, Carsten Kruse3, Anna Minguzzi2, Maxime Richard1

1Institut Néel, Université Grenoble Alpes and CNRS, Grenoble, France,
2Lab. de physique et modélisation des milieux condensés, Université Grenoble Alpes and CNRS, Grenoble, France,
3Universität Bremen, Bremen, Germany,
4Laboratoire FOTON, CNRS and INSA-Rennes, Rennes, France.

Exciton-polaritons are short-lived hybrid excitations that are obtained in solid-state (semiconductor) microcavities in the strong coupling regime. “Hybrid” stands here for the fact that polaritons have a mixed photonic and excitonic nature (excitons are hydrogenoid bound electron-hole pair excitations in semiconductors) [1]. As a result, they behave like photons interacting with each other and with their environment, like for instance thermal lattice vibrations.

From the thermodynamics point-of view, a typical gas of polaritons does not meet any criterion of thermodynamical equilibrium [2]: as particles, their lifetime is too short to reach a proper equilibration with their solid-state thermostat (i.e. thermal lattice vibrations); as waves, they cannot be considered as a black-body radiation since at the wavelength they display (in the visible), the electromagnetic vacuum can be considered as T=0K black-body, meaning that the polariton number at equilibrium is zero.
Figure: Principle of polaritonic cooling. The black curve is a typical lower polariton dispersion ω(k||). The red spot and arrows figure the optically pumped polaritons (of energy ħω0). Some of the pumped polaritons are scattered by thermal phonons (spring-like arrow) and thus pick up an average energy ħΩf(T) (i.e. the average thermal phonons energy) from the thermal phonons bath, and finally recombine by emitting photons in vacuum (yellow arrows).

In a recent experiment, we have studied how polaritons, with this nonequilibrium character, could be used as a coolant fluid to pick up heat from the surrounding thermal lattice vibrations and release it into the electromagnetic vacuum [3]. The principle of this mechanism is shown in Fig.1: polaritons are injected by resonant optical excitation in the lowest energy momentum state k||=0 (red arrows). This state is the “ground-state” of the polaritonic subsystem, in the sense that polaritons cannot have a lower total energy.

It thus behaves as a “cold” fluid (an effective injected polariton temperature Tp=4K can be defined) [4] interacting with the much hotter gas (T= 20K to 150K) of thermal lattice vibrations (phonons). During its lifetime, a polariton can pick-up a thermal phonon’s energy by inelastic scattering, thus gaining in average an energy ħΩf(T) under the form of kinetic energy (yellow state in Fig.1), where ħΩf(T) is the average phonons energy. The most likely ensuing event is the annihilation of these excited polaritons into photons, which are emitted away from the microcavity. Thermodynamically, the net result is the transfer of local thermal vibrational energy into “escaping” electromagnetic energy. For bare photons (i.e. weakly “dressed” by the surrounding material), such an inelastic scattering with phonons is also used in the context of optical cooling [5,6] and known as anti-Stokes Raman scattering.

In our experiment we have measured the thermal flux between the microcavity thermal phonons bath in a volume and the electromagnetic vacuum by counting the number and energy of the photons emitted within this mechanism. We found some other mechanisms competing with that described above like two-photon absorption and bare excitons anti-Stokes excitation. Taking them carefully into account, we measured a net maximum positive cooling power of Pcool= 80±16µW/cm3 for a cryostat temperature T=50K.

This cooling power is still too weak to cause a sizeable temperature decrease in the present experiment which was not optimized for this purpose. However, in a better insulated environment, better optimized microcavity, and by removing every unwanted light absorbing layers (like the GaAs substrate in our case), it should be possible to increase this cooling power by orders of magnitude. This mechanism is an interesting alternative to more conventional optical cooling as for instance, it keeps on working in conditions where the latter usually turns off, like in for low temperatures. From a more fundamental point of view, this work shows that polaritons offer a unique playground to investigate the thermodynamics of nonequilibrium weakly interacting quantum fluids in contact with some environments at equilibrium (e.g. electromagnetic vacuum, phonons, and excitons).

[1] Iacopo Carusotto, Cristiano Ciuti, "Quantum fluids of light". Review of Modern Physics, 85, 299 (2013). Abstract.
[2] J. Kasprzak, D.D. Solnyshkov, R. André, Le Si Dang, G. Malpuech, "Formation of an Exciton Polariton Condensate: Thermodynamic versus Kinetic Regimes". Physical Review Letters, 101, 146404 (2008). Abstract.
[3] Sebastian Klembt, Emilien Durupt, Sanjoy Datta, Thorsten Klein, Augustin Baas, Yoan Léger, Carsten Kruse, Detlef Hommel, Anna Minguzzi, Maxime Richard, "Exciton-Polariton Gas as a Nonequilibrium Coolant". Physical Review Letters, 114, 186403 (2015). Abstract.
[4] For polaritons, the terms “hot” and “cold” are not refereeing to a proper equilibrium temperature, but rather, by analogy, to the polaritons steady-state energy distribution.
[5] Mansoor Sheik-Bahae, Richard I. Epstein, "Optical refrigeration". Nature Photonics, 1, 693 (2007). Abstract.
[6] Jun Zhang, Dehui Li, Renjie Chen, Qihua Xiong "Laser cooling of a semiconductor by 40 kelvin". Nature, 493, 504 (2013). Abstract.

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Sunday, May 31, 2015

Finding Optical Transitions for Testing a Fundamental Constant’s Constancy

From left to right: José, Alexander and Hendrik  discuss the spectral analysis -- standing next to the Heidelberg electron beam ion trap, where the results were obtained.

Authors: Alexander Windberger, Hendrik Bekker, José R. Crespo López-Urrutia

Affiliation: Max-Planck-Institut für Kernphysik, Heidelberg, Germany.

We studied the uncharted optical spectra of the highly charged ions W14+, Re15+, Os16+, Ir17+, and Pt18+, and demonstrated generally applicable methods to identify the measured spectral lines. That allowed us to infer the transition energies for proposed ultra-stable frequency standards using Hf12+ and W14+ ions. In Ir17+, optical transitions with the highest sensitivity to a potential variation of the fine structure constant α ever predicted for a stable atomic system were determined. Highly advanced atomic structure calculations were benchmarked in the extreme regime of a triple level crossing.

Fundamental constants are taken as given by Nature. Our understanding of the origin of these constants is, however, rather poor: Their values are not set within the Standard Model but have to be determined empirically. Alternative theories, such as string or coupled dark energy theories, assume that fundamental constants emerge from dynamical fields and can vary at different times or places in the universe (see [1] for a review). Therefore, probing the stability of these constants allows us to search for physics beyond the Standard Model.

Our work focusses on testing a possible variation of the fine structure constant α, which characterizes the strength of interaction between charged particles and photons. Very small variations of α would lead to a detectable shift of wavelength, or color, of light which is emitted or absorbed by atoms. Following this approach the group of Webb et al. obtained the absorption spectra of interstellar clouds billions of light years away from us and in an extensive analysis found wavelength shifts for spectra observed at different angles [2]. This was interpreted as a spatial dipole-like variation of α.

We aim to test this extraordinary claim under well-defined laboratory conditions, preventing systematic uncertainties that the astrophysical observations might suffer from. Since the Earth, the Solar System, and our galaxy all move, a spatial variation translates into an effective temporal variation which was estimated at 10-19/year [3]. Such a minuscule drift could be measured by monitoring the frequency ratio of two highly accurate optical atomic clocks. The clock transitions should be very sensitive to an α variation, but to nothing else. Highly charged ions fulfill these requirements. With an increasing ionic charge, the wavelengths of electronic transitions decrease and leave the range accessible to lasers. Systems with level crossings are an exception. When two or more configurations are almost equal in energy, optical transitions are possible. The level crossing of the 5s and 4f subshells predicted for Ir17+ should enable the highest sensitivity to the sought-after α variation in a stable atomic system [4].

No detailed knowledge of the electronic structure of these ion species existed. Most heavy highly charged ions, as Ir17+, are experimentally unexplored, and calculations are not sufficiently accurate for these complex systems. For our studies, we used the Heidelberg electron beam ion trap, which produces and traps the ions of interest. The continuously excited ions decay by emitting radiation, of which we analyzed the optical spectrum. An exemplary measurement can be seen in Fig. 1, showing the optical spectra of W14+, Re15+, Os16+, Ir17+, and Pt18+ (atomic numbers Z=74-78). These ions encompass the whole predicted 5s-4f level crossing region.
Figure 1. (Click on the figure to view with higher resolution) Typical spectral map of Ir ions measured using the Heidelberg electron beam ion trap. For this measurement we acquired spectra at 10 V intervals of the electron beam acceleration potential. New groups of fluorescence lines start to appear when the electron beam energy reaches the ionization potential of an ionic charge state. The new charge state is produced more efficiently as the electron beam energy further increases, and the fluorescence lines become stronger until the ionization threshold of the next higher charge state is reached. At this point the ion population is transferred to the next charge state, which starts to fluoresce, while the former one disappears. This dependence of the fluorescence intensity on the acceleration potential is depicted in the right graph. It is notable that this section of the optical spectrum assigned to Ir17+ already shows a dense manifold of spectral lines. In order to derive the level structure from the spectrum, sophisticated identification schemes had to be applied.

Subsequently, we assigned the measured spectral lines to their corresponding electronic transitions to establish the level scheme [5]. Given the large theoretical uncertainties, a direct comparison of calculated and measured spectra is futile. Instead, we used three alternative methods to identify the spectral lines.

First, we exploited the fact that these ions are isoelectronic, since they have the same number of electrons, and thus similar atomic structures. Over a limited range of atomic numbers, the transition energies depend on the respective nuclear charge with a simple polynomial scaling. By comparison between the measured scaling functions and theoretical predictions we were able to reliably identify all underlying transitions as can be seen in Fig. 2.
Figure 2. (Click on the figure to view with higher resolution) Identification of isoelectronic transitions using their characteristic energy scaling. (a) Measured spectra of W14+, Re15+, Os16+, Ir17+, and Pt18+ (black lines). An algorithm found nine isoelectronic transition energies that obey simple quadratic scaling laws (colored lines) as expected from theoretical considerations. (b) By comparing the experimentally determined constant offset A and the linear term B (full symbols) to the calculated ones (open symbols) an unambiguous identification of the underlying transitions could be achieved.

This method could be independently confirmed by measuring the identified transitions in Ir17+ with increased resolution and accuracy. The spectral lines revealed a characteristic line shape caused by the 8 T magnetic field present at the position of the trapped ions. The observed line shapes were individually modelled according to the Zeeman effect, leading to an independent verification in perfect agreement with the scaling method.

The identified transitions allowed us to test advanced atomic structure calculations for the first time in systems with such a complex level crossing of 4f 12 5s2, 4f13 5s, and 4f 14 configurations. We found that only relativistic multi-reference Fock-space coupled cluster calculations consistently showed a fair agreement with most of the observed lines.

A direct application is the determination of the transition energies of two proposed optical clock transitions with a potential relative frequency uncertainty of less than 10-19 in W14+ and Hf12+ [6], another isoelectronic ion. Although we did not measure the spectrum of Hf12+, we were able to extrapolate the transition energy by applying the established energy scaling. Our experimental uncertainty is at least one order of magnitude smaller than that of predictions.

These clock transitions are exceptionally stable. However, they are not sensitive to a variation of the fine structure constant. For that, Ir17+ is ideal. By searching our data, we found closed transition cycles (Rydberg-Ritz principle) combining the identified with unidentified transitions. This enabled us to find two possible, but mutually excluding, candidates for the proposed α-sensitive transitions. We are currently performing more accurate measurements to remove this ambiguity.

Our method, line assignments by isoelectronic scaling of transitions (LINE ASSIST) is a straight-forward and general tool for exploring unknown spectra of highly charged ions. With the recent successful application of sympathetic cooling to highly charged ions [7], much higher accuracy can be achieved in future work: the ion temperature was reduced by nearly six orders of magnitude and the Doppler width accordingly. The experimental values for the transition energies obtained in the present work are needed for follow-up laser spectroscopy studies, applications as optical clock transitions, and testing the constancy of fundamental constants.

[1] Jean-Philippe Uzan, "The fundamental constants and their variation: observational and theoretical status". Review of Modern Physics, 75, 403–455 (2003). Abstract.
[2] J. K. Webb, J. A. King, M. T. Murphy, V. V. Flambaum, R. F. Carswell, M. B. Bainbridge, "Indications of a Spatial Variation of the Fine Structure Constant". Physical Review Letters, 107, 191101 (2011). Abstract.
[3] J.C. Berengut, V.V. Flambaum, "Manifestations of a spatial variation of fundamental constants in atomic and nuclear clocks, Oklo, meteorites, and cosmological phenomena". Europhysics Letters, 97, 20006 (2012). Abstract.
[4] J.C. Berengut, V.A. Dzuba, V.V. Flambaum, A. Ong, "Electron-Hole Transitions in Multiply Charged Ions for Precision Laser Spectroscopy and Searching for Variations in α". Physical Review Letters, 106, 210802 (2011). Abstract.
[5] A. Windberger, J.R. Crespo López-Urrutia, H. Bekker, N.S. Oreshkina, J.C. Berengut, V. Bock, A. Borschevsky, V.A. Dzuba, E. Eliav, Z. Harman, U. Kaldor, S. Kaul, U.I. Safronova, V.V. Flambaum, C.H. Keitel, P.O. Schmidt, J. Ullrich, O.O. Versolato, "Identification of the Predicted 5s−4f  Level Crossing Optical Lines with Applications to Metrology and Searches for the Variation of Fundamental Constants". Physical Review Letters, 114, 150801 (2015). Abstract.
[6] V. A. Dzuba, A. Derevianko, V.V. Flambaum, "High-precision atomic clocks with highly charged ions: Nuclear-spin-zero f 12-shell ions". Physical Review A, 86, 054501 (2012). Abstract.
[7] L. Schmöger, O.O. Versolato, M. Schwarz, M. Kohnen, A. Windberger, B. Piest, S. Feuchtenbeiner, J. Pedregosa-Gutierrez, T. Leopold, P. Micke, A.K. Hansen, T.M. Baumann, M. Drewsen, J. Ullrich, P.O. Schmidt, J.R. Crespo López-Urrutia, "Coulomb crystallization of highly charged ions". Science, 347, 1233 (2015). Abstract.

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Sunday, May 17, 2015

A Current Out Of Fluctuations

(From left to right) Pierre Pfeffer, Fabian Hartmann, Sven Höfling, Martin Kamp, Lukas Worschech.

Authors: Pierre Pfeffer1, Fabian Hartmann1, Sven Höfling1,2, Martin Kamp1, Lukas Worschech1

1Technische Physik, Universität Würzburg, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex Material Systems, Würzburg, Germany.
2SUPA, School of Physics and Astronomy, University of St. Andrews, United Kingdom.

In a recent experiment [1] we demonstrated the conversion of voltage fluctuations into a directed current on the nanoscale in a system consisting of two Coulomb-coupled quantum dots. The use of external gates allows a control of the current magnitude and a switching of its direction. The work was inspired by theoretical proposals by Sánchez et al. [2] and Sothmann et al. [3]. Possible future applications include energy harvesting as well as local cooling on the nanoscale, both vital to the further development of autonomous and energy-saving electronics.

Since heat and fluctuations are especially challenging factors in miniaturizing electronic circuits [4], the design of heat engines and rectifiers has become a central point of research in nanoelectronics and led to concepts and devices such as Brownian- and Büttiker-Landauer-motors [5,6] phonon rectifiers [7,8] and piezoelectric nanogenerators [9-10]. Despite being diverse concepts, they have in common an interplay of non-linearity, a certain form of symmetry breaking and the presence of fluctuations. For instance, Brownian motors enable a unidirectional particle flow even when there is no net average force in one direction, if the device’s potential is asymmetric. In this context, Sánchez et al. [2] and Sothmann et al. [3] theoretically examined rectifying devices which stand out because of their conception as three terminal devices, which allows a decoupling of the directions of energy flow and charge current.

Figure 1: Illustration of the energy transfer. Coulomb-coupling transfers energy from higher-energy electrons in the lower cavity to lower-energy electrons in the upper one.

The basic operating principle of the Coulomb coupled rectifier is illustrated in Fig. 1. Energy is transferred locally between two Coulomb-coupled cavities. If the electrons in one cavity (lower sphere in Fig. 1, in the following named QDb ) have on average higher energies than the electrons in the other cavity (upper sphere, QDt ), electrons in the first cavity can change into a lower-energy state and transfer the energy difference δE to electrons in the second cavity by means of Coulomb-interaction. No particle exchange takes place between the two cavities.

If QDt is coupled to two reservoirs by means of two quantum point contacts (QPC), a rectified current through the upper cavity occurs if three essential requirements are met: The amount of fluctuations in the upper and lower cavity have to differ, the transmissions of the QPCs have to be energy-dependent, and the two QPCs have to be asymmetric (Fig. 2a). This is for instance possible by applying an external electric field and enables a switching of the current direction.
Figure 2: a) Schematic operating principle. i): QDt is connected to the outer reservoir by two transport barriers. ii): Asymmetric configuration with charge current flow to the right. iii): Asymmetric configuration with charge current flow to the left. b) Electron microscopy image of the sample (side view) together with a schematic of the applied voltages. Vgl and Vgr allow a switching between the conditions displayed in a) ii) and a) iii).

To realize this device, we grew a GaAs/AlGaAs semiconductor heterostructure with a high mobility two-dimensional electron gas by molecular beam epitaxy. Electron beam lithography and dry chemical etching techniques were used to define the structural layout. Fig. 2b shows a side view electron microscopy image of the sample and is overlain with a schematic representation of the QDs indicating the two separate parts of the system (upper subsystem where the resulting current flows in blue and lower subsystem where the fluctuations occur in red). Moreover, the connected voltages are schematically added to the picture. Vgl and Vgr are the gate voltages applied to the upper side gates which allow us to individually manipulate the transmission coefficients of the two QPC’s. For instance, applying a negative voltage to Vgl and a positive one to Vgr opens the left but closes the right QPC and vice versa. In order to keep the central electrostatic potential of QDt constant while asymmetrically controlling the transmission of the left and right QPC we varied the voltages in push-pull configuration with Vgl = -Vgr. The lower side gates remain unused in the presented experiments. The static gate voltage Vgb further allows to tune the transmission of the top current carrying system. A noise voltage Vnoise is supplied to the lower reservoir. All experiments reported here were conducted at 4.2 K in the dark by immersing the sample in liquid helium.
Figure 3: a) Dependence of output current on the voltages applied to the two laterally defined side gates for increasing noise from 7.6 to 144 mV in steps of 15 mV. The current direction and magnitude can be altered by the gate voltage configuration and noise amplitude, respectively. b) Output powers P against the counteractive voltage Vlr for different noise amplitudes increasing from 7.6 to 150 mV in steps of 15 mV. c) Maximum output powers P max versus σnoise.

Fig. 3a shows the rectified current through QDt for different noise amplitudes from 7.6 to 144 mV when changing the gate voltages Vgl and Vgr. Notably, without noise, no current could be measured. Starting with a gate voltage configuration of Vgl = − Vgr = − 2 V and increasing the push-pull voltage initially increases the current from zero to a maximal value which depends on the applied noise amplitude. Thereafter, I decreases again and vanishes completely at around Vgl = 0.01 V (independently on the noise amplitude). A further increase of Vgl changes the current direction. I therefore becomes negative, reaches a minimum and finally increases back to zero again.

In order to harvest useful work from the rectifier, it has to power a load or, equivalently, the generated current has to flow against an applied voltage difference. Consequently, to measure the output power P, a voltage difference Vlr counteracting the current was applied between the channels. The output power is the product of the current and the counteracting applied voltage. Fig. 3b shows the voltage dependent output power for different noise amplitudes. The output power has a parabolic dependency on the applied voltage and vanishes at two particular points: at Vlr = 0 V (maximal current) and at the stopping voltage Vst (no current) which depends on σnoise. Fig. 3c presents the maximum output powers, obtained at half the stopping voltage, depending on the noise amplitude. There, a quadratic dependency on the noise amplitude can be seen with a maximum output power value of Pmax = 24 pW.

In conclusion, we demonstrated a rectification mechanism via Coulomb-coupled quantum dots. The presented findings are a step towards reducing the power consumption of electronic devices and may allow a further miniaturization of electronic circuits and thus pave the way towards sustainable, efficient and autonomous electronics.

[1] F. Hartmann, P. Pfeffer, S. Höfling, M. Kamp, L. Worschech, "Voltage Fluctuation to Current Converter with Coulomb-Coupled Quantum Dots". Physical Review Letters, 114, 146805 (2015). Abstract.
[2] Rafael Sánchez, Markus Büttiker, "Optimal energy quanta to current conversion". Physical Review B, 83, 085428 (2011). Abstract.
[3] Björn Sothmann, Rafael Sánchez, Andrew N. Jordan, Markus Büttiker, "Rectification of thermal fluctuations in a chaotic cavity heat engine". Physical Review B, 85, 205301 (2012). Abstract.
[4] Mehdi Asghari, Ashok V. Krishnamoorthy, "Silicon photonics: Energy-efficient communication". Nature Photonics, 5, 268 (2011). Abstract.
[5] R. Dean Astumian, Peter Hänggi, "Brownian Motors". Physics Today, 55, 33 (2002). Link.
[6] M. Büttiker, "Transport as a consequence of state-dependent diffusion". Zeitschrift für Physik B, 68, 161 (1987). Abstract.
[7] Rolf Landauer, "Motion out of noisy states". Journal of Statistical Physics, 53, 233 (1988). Abstract.
[8] C. W. Chang, D. Okawa, A. Majumdar, A. Zettl, "Solid-State Thermal Rectifier". Science, 314, 1121 (2006). Abstract.
[9] Nan Zeng, Jian-Sheng Wang, "Mechanisms causing thermal rectification: The influence of phonon frequency, asymmetry, and nonlinear interactions". Physical Review B, 78, 024305 (2008). Abstract.
[10] Zhong Lin Wang, Jinhui Song, "Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays", Science, 312, 242 (2006). Abstract.

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Sunday, April 26, 2015

Core-shell Hybrid Nanostructure Based High Performance Supercapacitor Electrode

Ashutosh Kumar Singh (left) and Kalyan Mandal

Authors: Ashutosh Kumar Singh, Kalyan Mandal

Affiliation: Department of Condensed Matter Physics and Material Sciences,
S.N. Bose National Centre for Basic Sciences
, Kolkata, India.


Since last decade, energy crisis has been one of the vital problems in the society due to the excessive use of fossil-fuel resources and environmental pollution. Therefore, the development of very light-weight and environment-friendly proficient energy storage devices has become the priority of the researchers and scientists for satisfying the demand of modern consumer’s hybrid electric and portable electronics devices [1,2]. In this progression, researchers have developed new type of energy storage devices called supercapacitors, also known as electrochemical capacitors which offer high power and eneergy density, high rate capability as well as superb cycle stability as compared to conventional battery and capacitors [3,4].

The supercapacitors are classified in two groups based on their charge storage method. The first group is called pseudocapacitor which involves the redox reactions of the electrode materials at the interface of electrode and electrolyte, whereas the second group is known as electric double layer capacitor which holds charge separation at the interface of electrode and electrolyte [5,6]. By changing the morphology of the electrode materials, one can manipulate the performance of a supercapacitor. The performance and quality of the supercapacitors are very much dependent on the materials and morphology used in the preparation of their electrodes. Recently, many metal oxide based materials (like RuO2, NiO, Fe2O3, MnO2, Co3O4, TiO2, etc.) have been used for the fabrication of pseudocapacitor electrodes; among them NiO and Fe2O3 have been widely used as redox active materials for the fabrication of supercapacitor electrodes of different morphologies, such as Fe2O3-nanotube, Fe2O3-thinfilm, electrospun Co3O4-nanostrtuctures, porous Fe2O3-nanostrtuctures, NiO–nanobelts, NiO–nanoballs, NiO–nanoflowers, NiO–nanoflakes. The reasons behind the extensive use of NiO and Fe2O3 as supercapacitor electrode materials are: they are very stable in nature, they are non-toxic as well as environment friendly and they are very cheap and easily available.


After having so many of supportive properties for being used as electrode materials in supercapacitors, still their reported specific capacitance values are very low compared to their own theoretical specific capacitance value and other metal oxide based electrodes. The only problem restricts them to be used as an electrode material for high performance supercapaitor is their bad electrical conductivity and we all are aware of the fact that electrode material must have high electrical conductivity for high performance supercapacitor.


In the recent development process of supercapacitor performance, it has been found that the electrical conductivity could be improved by introducing impurities via doping of one metal oxide material with other metal oxide material. This doping process enhances the charge movement which affects the reactions at the interface of electrode and electrolyte. So far in the literature we have not found any work based on NiO and Fe2O3 as mixed component transition metal oxides for supercapacitor electrodes. However, keeping all the above research facts in the mind, still there exist a plenty of remarkable opportunities to enhance the electrochemical properties of NiO and Fe2O3 based electrodes.
Unique features of the approach:

Therefore, we report a simple fabrication technique and unique electrochemical properties of the electrode based on core/shell Fe-Ni/Fe2O3-NiO hybrid nanostructures (HNs). This core-shell HNs have very high aspect ratio with a porous thin nanolayer of redox active oxides which would provide a very large surface area for redox reactions at the interface of electrode and electrolyte. This would contribute to the enhancement of the ion and electron movement and performance of the supercapacitor. In addition, the core material consists of conductive FeNi nanowires (NWs) which provides the expressway for the electrons to transport to the current collector via core material.

This would automatically improve the rate capability and power density of the supercapacitor. The electrical conductivity of the electrode could be improved by introducing impurities via doping of one metal oxide (NiO) material with another metal oxide (Fe2O3) material. The unique feature of this electrode fabrication technique is that it doesn’t contain any extra binder material. As a result, there would be enhancement in the charge transfer kinetics [7]. This kind of fabrication technique could be applied in the fabrication process of electrodes of all energy storage devices in general.

Significant Results:

According to our anticipations, the core/shell Fe-Ni/Fe2O3-NiO hybrid nanostructure shows high quality supercapacitive performance in terms of specific capacitance (1415 F/g), energy density (27.6 Wh/kg), power density (10.3 kW/kg), cycling stability (remain 95% of initial specific capacitance after 3000 charge/discharge cycle) and rate capability [7]; these profound results made it a very good and unique alternative for the next generation supercapacitor electrodes.

[1] Patrice Simon, Yuri Gogotsi, "Materials for electrochemical capacitors". Nature Materials, 7, 845 (2008).  Abstract.
[2] John R. Miller, Patrice Simon, Electrochemical Capacitors for Energy Management". Science 321, 651 (2008). Abstract.
[3] Zhibin Lei, Li Lu, X.S. Zhao, "The electrocapacitive properties of graphene oxide reduced by urea". Energy & Environmental Science, 5, 6391 (2012). Abstract.
[4] Sheng Chen, Junwu Zhu, Xiaodong Wu, Qiaofeng Han, Xin Wang, "Graphene Oxide−MnO2 Nanocomposites for Supercapacitors". ACS Nano 4, 2822 (2010). Abstract.
[5] Wei Chen, R.B. Rakhi, Liangbing Hu, Xing Xie, Yi Cui, H.N. Alshareef, "High-Performance Nanostructured Supercapacitors on a Sponge". Nano Letters, 11, 5165 (2011). Abstract.
[6] Raghavan Baby Rakhi, Wei Chen, Dongkyu Cha, H. N. Alshareef, "Nanostructured Ternary Electrodes for Energy-Storage Applications". Advanced Energy Materials, 2, 381 (2012). Abstract.
[7] Ashutosh K. Singh, Kalyan Mandal, "Engineering of high performance supercapacitor electrode based on Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures". Journal of Applied Physics, 117, 105101 (2015). Abstract.

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