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Volume 42, Issue 5, May 2016

Bosonic lasers represent a new generation of coherent light sources. In contrast to conventional, fermionic, lasers they do not require inversion of electronic population and do not rely on the stimulated emission of radiation. Bosonic lasers are based on the spontaneous emission of light by condensates of bosonic quasiparticles. The first realization of bosonic lasers has been reported in semiconductor microcavities where bosonic condensates of excitonpolaritons first studied several decades ago by K. B. Tolpygo can be formed under optical or electronic pumping. In this paper we overview the recent progress in the research area of polaritonics, address the perspective of realization of polariton devices: from bosonic cascade lasers to spin transistors and switches.

Kirill Borisovich Tolpygo (1916–1994) On the centenary of his birth
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Bosonic lasers: The state of the art (Review Article)
View Description Hide DescriptionBosonic lasers represent a new generation of coherent light sources. In contrast to conventional, fermionic, lasers they do not require inversion of electronic population and do not rely on the stimulated emission of radiation. Bosonic lasers are based on the spontaneous emission of light by condensates of bosonic quasiparticles. The first realization of bosonic lasers has been reported in semiconductor microcavities where bosonic condensates of excitonpolaritons first studied several decades ago by K. B. Tolpygo can be formed under optical or electronic pumping. In this paper we overview the recent progress in the research area of polaritonics, address the perspective of realization of polariton devices: from bosonic cascade lasers to spin transistors and switches.

Excitonpolariton laser
View Description Hide DescriptionWe present a review of the investigations realized in the last decades of the phenomenon of the BoseEinstein condensation (BEC) in the system of twodimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity excitonpolariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of excitonpolaritons in semiconductor microcavities is related to the ultralow threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.

Coherence of BoseEinstein condensates of dipolar excitons in GaAs/AlGaAs heterostructures
View Description Hide DescriptionExperiments relating to studies of the coherence of Bose condensates of dipolar excitons in GaAs/AlGaAs heterostructures with a wide, single quantum well and a Schottky gate are analyzed. Dipolar excitons were excited by light in an annular trap formed along the perimeter of a window in a metal gate with an applied electric voltage. A dualbeam interference technique involving interference combination of the amplitudes of the luminescence light field, together with subsequent analysis of first order correlators, is used to study the temporal (longitudinal) and spatial (transverse) coherence of the exciton condensates. It is found that the transverse coherence length of an exciton condensate is considerably longer than its thermal De Broglie wavelength. Experimental studies of the luminescence intensity correlator also confirm the coherence of the exciton Bose condensate.

Polaritons in a nonideal array of ultracold quantum dots
View Description Hide DescriptionWe develop a numerical model for a defectcontaining square lattice of microcavities with embedded ultracold atomic clusters (quantum dots). It is assumed that certain fractions of quantum dots and cavities are absent, which leads to transformation of polariton spectrum of the overall structure. The dispersion relations for polaritonic modes are derived as functions of defect concentrations and on this basis the band gap, the effective masses of lower and upper dispersion branch polaritons as well as their densities of states are evaluated.

Rayleigh and Wood anomalies in the diffraction of acoustic waves from the periodically corrugated surface of an elastic medium
View Description Hide DescriptionBy the use of the Rayleigh method we have calculated the angular dependence of the reflectivity and the efficiencies of several other diffracted orders when the periodically corrugated surface of an isotropic elastic medium is illuminated by a volume acoustic wave of shear horizontal polarization. These dependencies display the signatures of Rayleigh and Wood anomalies, usually associated with the diffraction of light from a metallic grating. The Rayleigh anomalies occur at angles of incidence at which a diffracted order appears or disappears; the Wood anomalies here are caused by the excitation of the shear horizontal surface acoustic waves supported by the periodically corrugated surface of an isotropic elastic medium. The dispersion curves of these waves in both the nonradiative and radiative regions of the frequencywavenumber plane are calculated, and used in predicting the angles of incidence at which the Wood anomalies are expected to occur.

Superconductor digital electronics: Scalability and energy efficiency issues (Review Article)
View Description Hide DescriptionSuperconductor digital electronics using Josephson junctions as ultrafast switches and magneticflux encoding of information was proposed over 30 years ago as a subterahertz clock frequency alternative to semiconductor electronics based on complementary metaloxidesemiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to highperformance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin superconducting layer for making compact highkineticinductance bias inductors. All circuit layers are fully planarized using chemical mechanical planarization (CMP) of SiO2 interlayer dielectric. The physical limitations imposed on the circuit density by Josephson junctions, circuit inductors, shunt and bias resistors, etc., are discussed. Energy dissipation in superconducting circuits is also reviewed in order to estimate whether this technology, which requires cryogenic refrigeration, can be energy efficient. Fabrication process development required for increasing the density of superconductor digital circuits by a factor of ten and achieving densities above 10^{7} Josephson junctions per cm^{2} is described.

Formation of nanostructure in magnesium diboride based materials with high superconducting characteristics
View Description Hide DescriptionThe paper presents an analysis of the properties of bulk superconducting magnesium diboridebased materials obtained by heating at high quasihydrostatic pressures (1–2 GPa), hot pressing (30 MPa), spark plasma sintering (16–96 MPa) and loose powder sintering. It is shown that the optimization of impurity distribution in MgB2 can be achieved by varying the synthesis conditions and introducing dopants. In particular, polycrystalline MgB2 materials synthesized at 2 GPa and containing a high amount of impurity oxygen demonstrates high critical current densities (10^{6} and 10^{3} A/cm^{2} at 20 K in magnetic fields of 1 and 8.5 T, respectively). It is found that the oxygen impurities are mainly localized in nanolayers or nanoinclusions, homogeneously distributed in the matrix. They act as pinning centers, while the MgB2 matrix also contains small amounts of dissolved oxygen. Impurity or intentionally added carbon entering the magnesium diboride structure leads to an increase in the critical magnetic fields up to Bc 2 (22 K) = 15 T and B irr (18.5 K) = 15 T. The results of ab initio calculations of the electronic structure and stability of the magnesium diboride compounds with partial oxygen or carbon substitution for boron show that it is energetically favorable for carbon to distribute homogeneously in MgB2 structure, while oxygen atoms replace boron pairwise in neighboring positions or form zigzag chains.

Spin dephasing in pseudomagnetic fields: Susceptibility and geometry
View Description Hide DescriptionWe present a theory of spin dynamics caused by spinorbit coupling for twodimensional gases of cold atoms and other quasiparticles with pseudospin 1/2 moving in orbital gauge fields. Our approach is based on the gauge transformation in the form of a SU(2) rotation gauging out the spinorbit coupling. As a result, the analysis of the spin dynamics is reduced to calculation of the densityrelated susceptibility of the system without spinorbit coupling at the wavevector determined by the spinrotation length. This approach allows one to treat the spin dynamics in terms of the linear response theory for bosonic and fermionic ensembles. We study different regimes of irreversible spin relaxation and coherent spin dynamics in these systems. For bosonic gases the effects of low temperature are crucial due to accumulation of particles in the smallmomentum subspace even if the Bose–Einstein condensation does not occur due to the system low dimensionality.

Role of acoustic phonons in the negative thermal expansion of layered structures and nanotubes based on them
View Description Hide DescriptionCalculations on a microscopic level are used to explain the experimentally observed negative linear thermal expansion along some directions in a number of crystalline compounds with complicated lattices and anisotropic interactions between atoms. Anomalies in the temperature dependence of the coefficient of linear thermal expansion are analyzed in layered crystals made up of monatomic layers (graphite and graphene nanofilms) and multilayer “sandwiches” (transition metal dichalcogenides), in multilayered crystal structures such as hightemperature superconductors where the anisotropy of the interatomic interactions is not conserved in the longrange order, and in graphene nanotubes. The theoretical calculations are compared with data from xray, neutron diffraction, and dilatometric measurements.

Manybody interaction and deformation of the atomic electron shells in the lattice dynamics of compressed atomic cryocrystals
View Description Hide DescriptionThe lattice dynamics of compressed atomic cryocrystals are based on ab initio quantummechanical theories of deformable and polarizable atoms (Tolpygo model), while taking into account the manybody interaction. The parameters of the threeparticle interaction and deformation of the atomic electron shells, which are calculated in terms of the overlap integrals of atomic orbitals and their derivatives, have the same order of magnitude thus demonstrating that they must be considered in tandem. Accounting for the deformation effects of the electron shells in the dipole approximation when calculating phonon frequencies leads to a “softening” of the longitudinal modes at points L and X, for an entire series of NeXe crystals, and of the transverse modes in the directions Σ and Λ for Xe, under high compression. It is shown that it impossible to adequately reproduce the observed deviation from the Cauchi relation δ(p) for compressed atomic cryocrystals, without accounting for the deformation of electron shells of atoms in a quadrupole approximation. The inputs from a threeparticle and quadrupole interaction for Ne, Kr, and Xe crystals are mutually compensated, which provides a weak dependence on pressure for δ(p). We found a good agreement between the calculated phonon frequencies, Birch and Fuchs elastic moduli, the deviation from the Cauchi relation for the total number of NeXe crystals in a wide range of pressures, and existing experiments.

Encapsulating “armchair” carbon nanotubes with “zigzag” chains of Fe atoms
View Description Hide DescriptionAb initio calculations of structural, electron, and magnetic properties of “armchair” carbon nanotubes (NT) encapsulated by a “zigzag” chain of Fe atoms Fe2@(n,n)m (m = 1, 2; n = 4, 5, 6, 7, 8, 9), are performed within the framework of the density functional theory. It is shown that optimizing the structure along the NT axis can significantly impact the binding energy of the NT and the Fe atom chain. It follows from the calculations that Fe2@(5,5) is the most stable of all the investigated encapsulated nanotubes. A twofold decrease in the concentration of Fe in an encapsulated NT converts the system from exothermic to endothermic (Fe2@(5,5)m) and vice versa (Fe2@(6,6)m)). For large radii of an encapsulated NT (>4.13 Å) the binding energy of the NT and the Fe atom chain goes to zero, and the magnetic moments of the Fe atoms and the deviation of the Fe atoms from the NT axis go toward analogous values of the free “zigzag” Fe atom chain.

Tunneling through localized barrier states in superconducting heterostructures
View Description Hide DescriptionThin film heterostructures composed of superconducting electrodes (molybdenum rhenium alloy) and a nanoscale silicon layer doped with tungsten, have been designed and experimentally studied. The currentvoltage characteristics of junctions exhibiting local maxima of the current against the background of abrupt current increases for the first time, were measured in the voltage range of −800 to 800 mV, at temperatures of 4.2–8 K. The positions of these singularities, which are symmetrical with respect to zero voltage, varied from sample to sample within the range of 40–300 mV. With increasing temperature, they became blurred and completely vanished with the disappearance of superconductivity in the electrodes. The nature of the observed singularities is associated with the properties of electron tunneling through the impurity states localized in the semiconducting barrier. The use of a superconducting electrode enhances the interaction of the localized electron with the conduction electrons thanks to the root divergence in the density of electron states of a superconductor.