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2Physics Quote:
"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). As a result, they behave like photons interacting with each other and with their environment, like for instance thermal lattice vibrations."
-- Sebastian Klembt, Emilien Durupt, Sanjoy Datta, Thorsten Klein, Yoan Léger, Augustin Baas, Detlef Hommel, Carsten Kruse, Anna Minguzzi, Maxime Richard (Read full article: "Using Exciton-Polaritons as a Nonequilibrium Coolant Fluid")

Sunday, July 26, 2015

Space-borne Gravitational Wave Detector LISA/eLISA

Yan Wang

[Yan Wang is the recipient of the 2014 Stefano Braccini Thesis Prize administered by the Gravitational Wave International Committee (GWIC) for his PhD thesis “On inter-satellite laser ranging, clock synchronization and gravitational wave data analysis” (PDF). His thesis work was carried out at Leibniz University of Hannover, Germany.

The Stefano Braccini Thesis Prize was established to honor the memory of a talented gravitational wave physicist whose promising career was cut short. Stefano worked with the French-Italian Virgo project, and contributed to the superattentuator design, to the integration and commissioning of Virgo and to its data analysis efforts. -- 2Physics.com]

Author: Yan Wang

Affiliation: School of physics, University of Western Australia, Perth, Australia.

Observations of electromagnetic radiation have revolutionized our understanding of the Universe and fundamental physics during the last century. With the advent of the expected first detection of gravitational waves (GWs) in near future, a completely new window onto the Universe will soon be opened by GW astronomy. GWs are spacetime ripples, predicted by Einstein’s theory of General Relativity. Their existence has been indirectly proven by measurements of the orbital decay due to gravitational radiation of the binary pulsar PSR 1913+16 [1], for which Hulse and Taylor won the 1993 Nobel Prize, but, due to their weak coupling with matter (i.e. GWs pass through stars, galaxies, Earth, Sun, and everything), direct detection of GWs has been beyond our technological capabilities until now. This weak coupling does mean, however, that GWs carry uncorrupted physical, astrophysical and cosmological information, enabling us to probe deep into the very early Universe, to test general relativity (GR) with unprecedented precision, and to measure the masses and spins of black holes (BHs) with exquisite accuracy.

Currently, large laser interferometers are the most sensitive GW detectors. There are several existing ground-based interferometric GW detectors: LIGO (Hanford and Livingston) [2], VIRGO [3], GEO600 [4], either operating or being upgraded; and KAGRA [5] under construction. During the past few decades, scientists took all efforts to isolate or mitigate various kinds of disturbances on the Earth, in order to increase the sensitivity of the detectors. These detectors are expected to detect GWs in near future.
Figure 1: Classic LISA configuration.

There are also space-borne interferometric GW detector (planned) missions. Among them, the most mature one is the Laser Interferometer Space Antenna (LISA) [6-7] ‘family’ (e.g. classic LISA, eLISA, and variations). LISA/eLISA consists of three spacecrafts (Fig. 1), each individually following a slightly elliptical orbit around the Sun, trailing the Earth by about 20 degree. These orbits are chosen such that the three spacecrafts retain an equilateral triangular configuration with an arm length of a few million kilometers as much as possible. This is accomplished by tilting the plane of the triangle by about 60 degree out of the ecliptic. Graphically, the triangular configuration does a cartwheel motion around the Sun.

The benefits of sending an interferometric GW detector to space are mainly: (i) Less noise disturbances, (ii) More GW sources. Roughly speaking, laser interferometric GW detectors are most sensitive to celestial systems of a size comparable to the interferometers’ arm length. LISA/eLISA is sitting in a frequency band, where there are most abundant GW sources (Fig. 2). There are several known white dwarf binaries in our galaxy directly visible to LISA/eLISA. In addition, LISA/eLISA can resolve thousands of other white dwarf binaries in our galaxy. LISA/eLISA can also observe massive black hole mergers throughout the entire universe [8]. Extreme mass ratio inspirals (i.e. stellar mass compact object orbiting a massive black hole) and primordial GWs from the birth of the universe add great scientific values to the mission as well [8].
Figure 2: (Click on the image to view with higher resolution) The GW spectrum from extremely low frequency to high frequency [Image courtesy: Chris Henze]

For many years, scientists have been spending great effort -- both theoretically and experimentally -- in preparation of LISA/eLISA. Some of the key techniques required by LISA/eLISA cannot be tested on the ground. LISA pathfinder satellite is going to be launched in this November (2015) to test the drag-free altitude control system in space, laser interferometry with picometer resolution at mHz band, the reliability of the instruments in the space environment, etc.

Unlike the ground-based interferometric GW detectors, the arm lengths of LISA/eLISA are varying significantly with time due to celestial mechanics in the solar system. As a result, the arm lengths differ by about one percent (i.e. tens of kilometers), and the dominating laser-frequency noise will not cancel out. The remaining laser-frequency noise would be stronger than other noises by about 8 orders of magnitude. Fortunately, the coupling between distance variations and the laser-frequency noise is very well known and understood. Therefore, we can use time-delay interferometry (TDI) techniques [9], which combine the measurement data series with appropriate time delays, in order to cancel the laser-frequency noise to the desired level.

However, the performance of TDI depends largely on the knowledge of arm lengths and relative longitudinal velocities between the spacecrafts, which are required to determine the correct delays to be adopted in the TDI combinations. In addition, the raw data are referred to the individual spacecraft clocks, which are not physically synchronized but independently drifting and jittering. This timing mismatch would degrade the performance of TDI variables. Therefore, they need to be referred to a virtual common constellation clock which needs to be synthesized from the inter-spacecraft measurements. Simultaneously, one also needs to extract the inter-spacecraft separations and synchronize the time-stamps properly to ensure the TDI performance. This has been a long existing gap.

Recently [10-11], we have tried to bridge this gap by designing sophisticated first stage data analysis algorithms for LISA/eLISA. The following are the main steps involved in the algorithms: (i) different types of inter-spacecraft measurements (e.g. the pseudo ranging measurements, the beat-notes of the carrier frequencies of the lasers emitted from one spacecraft and a remote spacecraft, the beat-notes of the laser sidebands) are precisely formulated as functions of the system state variables; (ii) several precise and effective dynamic models are designed for the system state variables (these models basically describe how the state variables evolve with time); (iii) the measurement data are pre-processed so that they can be used by a optimal filtering algorithm; (iv) the information of the measurements and the information from the dynamic models of the system state variables are optimally combined via a Kalman-like optimal filter, in order to reduce the noise in the measurements and the clock recording time stamps.

Simulation shows that our algorithms can successfully calibrate and synchronize the phasemeter raw data, estimate the inter-spacecraft distances and the clock errors, hence making the raw measurements usable for TDI techniques and astrophysical data analysis algorithms. This result can significantly increase the robustness of the LISA/eLISA project. The flexible design structure of our algorithms also provides a general framework of first stage LISA/eLISA data preparation, which can be easily extended to deal with various emergent scenarios in the future.

[1] Joel M. Weisberg, Joseph H. Taylor, "The Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis", ASP Conference Series on 'Binary Radio Pulsars', vol. 328, p25 (2005). Article.
[2] http://www.advancedligo.mit.edu/
[3] http://www.cascina.virgo.infn.it/advirgo/
[4] http://www.geo600.org
[5] http://gwcenter.icrr.u-tokyo.ac.jp/en/
[6] https://www.elisascience.org/
[7] LISA International Science Team 2011 (European Space Agency), "LISA Unveiling a hidden universe", LISA Assessment Study Report (Yellow Book), ESA/SRE(2011) 3. Link.
[8] The eLISA Constortium, "The Gravitational Universe", Whitepaper submitted to ESA for the L2/L3 Cosmic Vision call. arXiv:1305.5720 [astro-ph.CO] (2013).
[9] Massimo Tinto, Sanjeev V. Dhurandhar, "Time-Delay Interferometry", Living Review Relativity 17 (2014), 6. Article.
[10] Y. Wang, Thesis: ‘On inter-satellite laser ranging, clock synchronization and gravitational wave data analysis’ (2014). Link.
[11] Yan Wang, Gerhard Heinzel, Karsten Danzmann, "First stage of LISA data processing: Clock synchronization and arm-length determination via a hybrid-extended Kalman filter", Physical Review D, 90, 064016 (2014). Abstract.

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Sunday, July 19, 2015

A Quantum Gas Microscope for Fermionic Atoms

The Fermi gas microscope group: (from left) graduate students Katherine Lawrence and Melih Okan, postdoc Thomas Lompe, graduate student Matt Nichols, Professor Martin Zwierlein, and graduate student Lawrence Cheuk. Photo credit: Jose-Luis Olivares/MIT.

Authors: Lawrence Cheuk and Martin Zwierlein

Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

Link to Ultracold Quantum Gases Group >>

What do electrons, protons, neutrons and even quarks have in common? They all are fermions, particles with half-integer spin. Unlike their bosonic counterparts, integer spin particles, fermions cannot occupy one and the same quantum state. This simple fact leads to the structure of our elements, where electrons have to avoid each other and occupy different orbits around the atomic nucleus, or at least differ in their spin orientation.

When many fermions interact strongly with each other, they can form complex matter with exotic properties, from atomic nuclei to solid state materials, to distant neutron stars. Their collective behavior leads to diverse phenomena such as the structure of the elements, high-temperature superconductivity and colossal magneto-resistance.

Yet our understanding of strongly-interacting Fermi systems is limited. In recent years, ultracold atomic Fermi gases have emerged as a pristine platform to study many-fermion systems. In particular, fermionic atoms trapped in an optical lattice formed by standing waves of light can simulate the physics of electrons in a crystalline solid, shedding light on novel physical phenomena in materials with strong electron correlations.

Yet our understanding of strongly-interacting Fermi systems is limited. In recent years, ultracold atomic Fermi gases have emerged as a pristine platform to study many-fermion systems. In particular, fermionic atoms trapped in an optical lattice formed by standing waves of light can simulate the physics of electrons in a crystalline solid, shedding light on novel physical phenomena in materials with strong electron correlations.

In the present work, recently published in Physical Review Letters [3], we have realized quantum gas microscope that images ultracold fermionic 40K atoms with single-lattice-site resolution. Similar results have also been achieved at about the same time by researchers at University of Strathclyde and Harvard University [4,5].
Figure Caption: Fermionic 40K atoms in a 2D optical lattice with 541nm spacing imaged using Raman sideband cooling. Image taken from [3].

In our experiment, we prepare a two-dimensional layer of 40K atoms via laser cooling and forced evaporation. The atoms are then trapped in an optical lattice formed by retro-reflected laser beams, which form a standing wave with 541nm spacing. In order to resolve atoms with single-lattice-site resolution, we utilize a novel setup that incorporates a solid immersion lens into the vacuum window. This allows an enhancement in the numerical aperture, leading to higher resolution and enhanced light collection. In addition, optical aberrations that arise from a planar window are minimized in this setup.

In order to detect the atoms, we perform fluorescence imaging while simultaneously cooling the atoms. To make the atoms fluoresce, they are illuminated with near-resonant light. However, as the atoms emit photons, they experience heating from the recoil of photons. As the atoms are heated up, they hop between lattice sites and can even hop out of the lattice. In order to faithfully measure the occupation of the lattice sites, one must therefore eliminate the heating that arises when atoms fluoresce. We accomplish this via a technique known as Raman sideband cooling.

Raman sideband cooling, a technique first demonstrated in the 1990s, selectively transfers atoms from high-energy states to lower energy states via a two-photon Raman process. Atoms that are already in the lowest energy state, however, remain “dark” to the Raman light. By collecting photons that are scattered during this cooling process, we extract the position of the atoms while cooling the atoms. Hopping and atom loss are thus avoided. Furthermore, we have found that even after imaging the atoms with Raman sideband cooling, the atoms are predominantly in the lowest energy state. This invites the possibility of assembling low-entropy many-fermion states atom by atom.

The advent of fermion microscope will allow new studies of many-fermion systems in optical lattices, such as measurement of high order correlations and detection of magnetic ordering. Such studies could shed light on the behavior of other fermions, in particular, electrons. This may one day advance our understanding of the diverse phenomena that arise in complex solid-state systems.

[1] Waseem S. Bakr, Jonathon I. Gillen, Amy Peng, Simon Fölling, Markus Greiner, "A quantum gas microscope for detecting single atoms in a Hubbard-regime optical lattice", Nature, 462, 74 (2009). Abstract.
[2] Jacob F. Sherson, Christof Weitenberg, Manuel Endres, Marc Cheneau, Immanuel Bloch, Stefan Kuhr, "Single-atom-resolved fluorescence imaging of an atomic Mott insulator", Nature, 467, 68 (2010). Abstract.
[3] Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, Martin W. Zwierlein, "Quantum-Gas Microscope for Fermionic Atoms", Physical Review Letters, 114, 193001 (2015). Abstract.
[4] Maxwell F. Parsons, Florian Huber, Anton Mazurenko, Christie S. Chiu, Widagdo Setiawan, Katherine Wooley-Brown, Sebastian Blatt, Markus Greiner, "Site-Resolved Imaging of Fermionic 6Li in an Optical Lattice", Physical Review Letters, 114, 213002 (2015). Abstract.
[5] Elmar Haller, James Hudson, Andrew Kelly, Dylan A. Cotta, Bruno Peaudecerf, Graham D. Bruce, Stefan Kuhr, "Single-atom imaging of fermions in a quantum-gas microscope", arXiv:1503.02005v2 [cond-mat.quant-gas] (2015).

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Sunday, July 12, 2015

Metamaterial Shrinks Integrated-Photonics Devices

Rajesh Menon

Author: Rajesh Menon

Affiliation: Department of Electrical & Computer Engineering, University of Utah, USA.

Integrated electronics is the driving force behind the information revolution of the last 6 decades. A similar revolution is happening in photonics, where devices that manipulate the flow of light (or photons) are being miniaturized and integrated. The main challenge for integrated photonics is that the wavelength of light is far larger than the equivalent wavelength of electrons. This is the main reason that devices fundamental to integrated electronics are significantly smaller than those used in integrated photonics. Furthermore, no one had come up with a way to design devices close to this limit for integrated photonics.

We recently solved this problem by first coming up with a new design algorithm and then experimentally verifying that our devices work as intended [1-3]. One crucial advantage of our method is that our fabrication process is completely compatible with the very mature processes already developed for silicon electronics. This means that we can exploit the vast existing manufacturing infrastructure to enable integrated photonics.

In our recent publication, we demonstrated the smallest polarization beam-splitter to date [1]. This device (shown in the figure below) has 1 input and 2 outputs. The 2 outputs correspond to the 2 linear polarization states of light. The device is designed to take either polarization of light (or both) as the input and separate the 2 polarizations into the 2 outputs. We input light into our device one polarization at a time and measured the transmission efficiency into the correct output. This allowed us to verify that the device performs as designed. This is analogous to separating two channels of communication (for example, a video stream from PBS and another from Netflix). Previously such separation would have required time and power-consuming electronics or if photonics devices were used, they would have been much larger (so much harder to integrate onto a chip).
Figure 1: (a) Scanning-electron micrograph of fabricated polarization beamsplitter. Simulated intensity distribution at (b) TE polarization and (c) TM polarization showing the separation of the beams.

In the big picture, our research has the potential to maintain Moore's law for photonics. By enabling integrated photonics devices to be much smaller (in fact, close to their theoretical limits), we allow the integration of more devices in the same area (which increases functionality) and also enable the devices to communicate faster (since they are closer together; light has to travel shorter distances). Finally, by packing more devices into the same chip, one also exploits economies of scale to reduce the cost per chip (similar to what has happened in electronics). The practical impact for customers is that one can expect to drastically reduce power consumption and enable faster communications and computing. Data centers today consume over 2% of the total global electricity. Reducing power consumption in data centers and other electronics can go a long way to reduce our CO2 emissions and stem global climate change.

Our vision is to create a library of ultra-compact devices (including beamsplitters, but also other devices) that can then be all connected together in a variety of different ways to enable both optical computing and communications. The first devices were fabricated at a University. Next, we need to fabricate these in a standard process at a company, and then provide this library of devices to designers and hopefully, unleash their creativity. We believe that these devices will usher in unpredictable, but unbelievably exciting applications.

References :
[1] Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “An integrated-nanophotonic polarization beamsplitter with 2.4 × 2.4 μm2 footprint”, Nature Photonics, 9, 378-382 (2015). Abstract.
[2] Bing Shen, Peng Wang, Randy Polson, Rajesh Menon, “Integrated metamaterials for efficient, compact free-space-to-waveguide coupling”, Optics Express, 22, 27175-27182 (2014). Abstract.
[3] Bing Shen, Randy Polson, Rajesh Menon, “Integrated digital metamaterials enables ultra-compact optical diodes”, Optics Express, 23, 10847-10855 (2015). Abstract.

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Sunday, July 05, 2015

Site-Dependent Evolution of Electrical Conductance from Tunneling to Atomic Point Contact

Howon Kim (left) and Yukio Hasegawa

Authors: Howon Kim and Yukio Hasegawa

Affiliation: The Institute for Solid State Physics, University of Tokyo, Japan.

In our recent work [1], the evolution of electrical conductance was investigated from tunneling to atomic point contact whose atomic geometry was precisely defined using scanning tunneling microscopy (STM). We found that the conductance evolution depends on the contact site; for instance, on-top, bridge, or hollow ('hexagonal close packed, hcp' and 'face centered cubic, fcc') site in the close-packed lattice of the substrate, indicating the importance of the atomic configuration in the conductance of the atomic junctions.

Electronic conduction through atomic-sized metal contacts is of fundamental interest as a transport mechanism though the ultimately squeezed conductor [2]. Several seminal phenomena, such as quantization and step-wise variation [3, 4] in the conductance, have been reported using a method called break junction in which the conductance is measured just before the breaking moment of a nanometer-width thin wire. Since atomic geometry of the point contact cannot be controlled and the measured conductance fluctuates at every breaking, therefore, the obtained conductances are usually analyzed in a statistical manner. Scanning tunneling microscopy (STM) has also been utilized for the study of atomic point contacts, in which the contact is formed by pushing the probe tip toward the sample surface. Because of plastic deformation of the tip by the contact formation, however, quantitatively reliable and reproducible measurements have been difficult.

Figure 1. schematic showing the atomic geometry of the atomic point contacts formed at an on-top site (left) and a 3-fold hollow site (right) of a close-packed surface.

Here, in our study, making most of the capability of the atomically resolved imaging of STM, we measured the conductance of the atomic point contact in an atomically controlled manner. We first positioned the probe tip on a specific site, for instance, on-top, bridge, or hollow (fcc and hcp) site, in the crystallographic lattice of the substrate surface (Fig. 1), and then measured the conductance while moving the tip toward the substrate from tunneling to contact regimes. It is found that the conductance evolution depends significantly on the contact site. When the contact is formed, the hollow site has the largest conductance, and among the two hollow sites the hcp site is more conductive than fcc. When the tip is pulled from the contact by just 20-30 pm, a crossover occurs and the conductance at on-top site becomes the largest.

Figure 2: electrical conductance measured from tunneling (Δz = -20 pm) to contact (Δz = -60 pm) regimes. The measured conductance G is normalized by the quantum conductance G0 given by 2e2/h (~77.5 μS). For each conductance trace, 10 traces taken at the corresponding sites marked in the atomically-resolved STM image (inset) are averaged.

The traces of the electrical conductance measured from tunneling to contact at on-top, bridge, fcc, and hcp sites of the Pb(111) surface are shown in Fig. 2. For each plot, 10 traces obtained from the corresponding marked sites in the inset STM image are averaged. At the tip displacement Δz of -50 ~ -60 pm from the tunneling (Δz = 0), the atomic contact is formed as the conductance shows saturation around the quantum conductance G0 given by 2e2/h (~77.5 μS). The contact conductance shows strong site dependence; the conductance at the hcp site is largest and more than 50 % larger than the one measured at the on-top site. Around Δz = -30 ~ -40 pm, that is, when the tip is located above the substrate by 20 ~ 30 pm from the contact, the plot indicates the largest conductance at on-top site.
Figure 3. Spatial mappings of the conductance at various tip displacements (upper left) topographic STM image (3.0 X 3.0 nm2) taken simultaneously with 64 X 64 conductance traces. (lower left) conductance mapping at Δz = -32pm, where the largest conductance at on-top site is enhanced (Lower right) conductance mapping at Δz = -60 pm, that is, the contact regime, where hollow site, particularly hcp site, has large conductance. (upper right) schematics explaining the site dependence of the conductance. The atoms on which the chemical interaction is exerted are marked red.

In order to spatially demonstrate the site dependence, we performed real-space mappings of the conductance in the on-top enhancement region and in the contact regime. The upper-left panel of Fig. 3 is an STM image showing the atomic contrast taken simultaneously with the conductance traces. At a tip displacement Δz of -32pm, the conductance mapping (lower-left of Fig. 3) exhibits bright contrast at the on-top site, similarly to that in the topographic image. As the conductance mapping at Δz = 0 does not have any contrast, the bright contrast indicates the conductance enhancement at the on-top site. On the other hand, the conductance mapping in the contact regime (lower right of Fig. 3, Δz = -60pm) has its contrast reversed from that of the topographic one, indicating a larger conductance at the hollow site than at the on-top site. These results clearly demonstrate that the point contact conductance is quite sensitive to the atomic configuration.

When the distance between the tip and substrate is reduced, the attractive chemical interaction is exerted between the surface and tip apex atoms. This interaction presumably opens up the conduction channel and contributes to the development of the conductance. Schematics in the upper right panel of Fig. 3 show how the chemical interaction works in the case of contacts formed at on-top and hollow sites. When the tip approaches from the tunneling regime, the attractive interaction is exerted first at the on-top site (the force-exerted atoms are marked red in the schematics) because the substrate atoms are closer at on-top site than at hollow sites, thus making the on-top conductance enhanced. In the contact regime, however, the attractive force becomes stronger at hollow sites because of the greater number of involved atoms than the on-top site. This is probably the reason why conductance becomes larger there at the contact. Obviously, theoretical studies [5], simultaneous measurements of force and conductance by atomic force microscopy [6], and/or conduction channel analysis of the atomic point contact [7] are needed to elucidate the observed conductance behaviors.

References :
[1] Howon Kim and Yukio Hasegawa, "Site-dependent evolution of electrical conductance from tunneling to atomic point contact", Physical Review Letters, 114, 206801 (2015). Abstract.
[2] Nicolás Agraı̈t, Alfredo Levy Yeyati, Jan M. van Ruitenbeek, "Quantum properties of atomic-sized conductors", Physics Reports, 377, 81 (2003). Abstract.
[3] J. M. Krans, J. M. van Ruitenbeek, V. V. Fisun, I. K. Yanson, L. J. de Jongh; "The signature of conductance quantization in metallic point contacts", Nature, 375, 767 (1995). Abstract.
[4] L. Olesen, E. Lægsgaard, I. Stensgaard, F. Besenbacher, J. Schiøtz, P. Stoltze, K. W. Jacobsen, J. K. Nørskov; "Quantized conductance in an atom-sized point contact", Physical Review Letters, 72, 2251 (1994). Abstract.
[5] Jose Manuel Blanco, Cesar González, Pavel Jelínek, José Ortega, Fernando Flores, Rubén Pérez, "First-principles simulations of STM images: From tunneling to the contact regime", Physical Review B, 70, 085405 (2004). Abstract.
[6] Yoshiaki Sugimoto, Keiichi Ueda, Masayuki Abe, Seizo Morita "Three-dimensional scanning force/tunneling spectroscopy at room temperature", Journal of Physics : Condensed Matter, 24, 084008 (2012). Abstract.
[7] Howon Kim and Yukio Hasegawa, "Site-dependent conduction channel transmission in atomic-scale superconducting junctions", arXiv:1506.05528 [cond-mat.mes-hall].

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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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