On the phase transitions for the electronic excitations in semiconductors

A model Hamiltonian for a system of interacting electrons, holes and Wannier excitons is derived. This system of electronic excitations is assumed to be in a quasi-equilibrium state. With the aid of Bogolubov's variational principal the thermodynamic potential is calculated. Using the most general mean-field Hamiltonian as a trial Hamiltonian, a set of coupled integral equations is obtained for the self-energies. These equations are solved numerically for equal effective masses of the electrons and holes. Below a critical temperature ofk _B T _c0.65E _ex ^b whereE _ex ^b is the exciton binding energy, we find a first order phase transition from an exciton rich phase into a degenerate electron-hole phase. The mechanical and thermal stability of both phases is proven. Below a critical temperaturek _B T _c0.11E _ex ^b the exciton system becomes degenerate (Bose-Einstein condensation). A complete phase diagram of these three phases is given.This is a project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft     

Magnetoexciton absorption in coupled quantum wells

A Mott exciton in coupled quantum wells in a transverse magnetic field H is considered. An expression for the exciton spectrum in an arbitrary magnetic field for large separations D between quantum wells containing an electron (e) and a hole (h) is given. The exciton spectrum in a strong magnetic field for different Landau levels at arbitrary D has been calculated. Changes in the parameter D/l, where is the magnetic length, cause rearrangement of the magnetoexciton dispersion curves ℰ(P), where P is the conserved “magnetic” momentum, which is a function of the separation between the electron and hole in the plane of the quantum wells. Off-center (“roton”) extrema occur only for D/l,<(D/l)_cr, where (D/l)_cr is a function of the exciton quantum numbers n and m. The magnetoexciton effective mass in states with magnetic quantum number m=0 monotonically increases with H and D, while in states with m≠0 it is a nonmonotonic function of D/l. The probability of generating an exciton in coupled quantum wells increases with H. Absorption of electromagnetic radiation due to transitions between excitonic levels in coupled quantum wells is discussed. For an exciton containing a heavy hole the oscillator strengths increase with H and the oscillator strengths decrease. Zh. éksp. Teor. Fiz. 112, 1791–1808 (November 1997)     

Magnetoexciton light absorption in inhomogeneous quasi-two-dimensional systems

The problem of exciton light absorption in quasi-two-dimensional inhomogeneous systems in a strong transverse magnetic field H is analyzed. We assume that a random Gaussian field (“white noise”) acting separately on an electron and a hole is due to (1) fluctuations in the quantum well thickness or (2) fluctuations in the concentrations of the solid solution components. The problem of a magnetoexciton in a random Gaussian white noise field has been reduced to the problem of the motion in an H-dependent effective field of a single particle with the effective magnetic mass of the exciton, which is a function of the magnetic field and parameters of the quantum wells, in a field characterized by “colored noise,” whose correlation function is different from that of the white noise field. In this approximation, the problem of a magnetoexciton in isolated and coupled quantum dots is considered. In the coherent-potential approximation, the exciton absorption in random fields of the first and second type in single and coupled quantum wells has been calculated. The absorption decreases as H increases in the range of strong magnetic fields, which is in agreement with experimental data. Zh. éksp. Teor. Fiz. 114, 1451–1465 (October 1998)     

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromangetic exchange interaction is found for the ground state. In the magnetic field range μH∼0.5T _c (T _c is the Kondo temperature) there is an inflection point, and in the strong magnetic field range μH≫T _c , the correction to the average spin is proportional to (T _c /μ H)². In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states. Zh. éksp. Teor. Fiz. 115, 1263–1284 (April 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Theory and experimental evidence for superconductors with short coherence length

We sketch the theory of London superconductors with a complex order parameter undergoing a phase transition dominated by thermal fluctuations. This theory is essentially that of superfluid helium, appropriately modified. We derive relations between the transition temperatureT _c , the London penetration depth, the phase correlation length, the fall ofT _c with reduced thickness in thin slabs and the associated appearance of Kosterlitz-Thouless behavior. AtH _c2 thermal fluctuations drive the transition first order. Accordingly, magnetic properties will exhibit hysteresis phenomena and glassy behavior. Our analysis of recent specific heat and SR measurements and of experiments on ultrathin slabs as well as the experimental evidence for hysteresis phenomena and glassy behavior in magnetic properties reveal that this theory describes a large class of superconductors that encompasses the cuprates, bismuthates and fullerenes.     

Metal-insulator transition in a two-layer electron-hole system

This paper discusses a quantum-mechanical metal-insulator transition that occurs in an anisotropic electron-hole system with the electrons and holes separated and confined to a double quantum well. The critical concentration n _c of carriers in the system above which the excitonic (insulating) phase becomes an electron-hole (metallic) phase is investigated, along with its dependence on the distance between wells D. Fiz. Tverd. Tela (St. Petersburg) 39, 1654–1656 (September 1997)     

Spin-wave theory for finite classical magnets and superparamagnetic relation

Analytical calculations based on finite-size spin-wave theory and Monte Carlo (MC) simulations are performed to investigate the validity of the well-known relation m(H, T) = M(H, T)B _D[M(H, T) H/T] between the induced magnetization m of the magnetic particle and its intrinsic magnetization M for the Ising and isotropic classical models (B _D(x) is the Langevin function, D is the number of spin components, is the number of atoms in the particle). It follows from general arguments and from our analytical results for the Heisenberg model at T≪T _c that this relation is not exact for any finite D and nonzero temperature. Nevertheless, corrections to this formula remain very small practically in the whole range T < T _c if ≫ 1, as confirmed by our Monte Carlo calculations. At T T _c/4 there is a good agreement between the MC and finite-size spin-wave calculations for the field dependence of m and M for the Heisenberg model with free boundary conditions. Received 1st December 2000     

Phase transitions in a system of spatially separated electrons and holes

The formation of a superfluid exciton liquid in a system of spatially separated electrons and holes in a system of two coupled quantum wells is predicted and its properties are investigated. The ground-state energy and the equilibrium density of the exciton liquid are calculated as functions of distance D between the quantum wells. The properties of a rarefied exciton gas with dipole-dipole repulsions are considered, where this gas is the metastable phase for D<1.9a* and the stable phase for D<1.9a* (a* is the radius of the two-dimensional exciton). The gas-liquid quantum transition is examined for increasing D. The Berezinskii-Kosterlitz-Thouless transition temperatures, at which superfluidity arises in the system, are found for different values of D. Possible experimental manifestations of the predicted effects are discussed. Zh. éksp. Teor. Fiz. 111, 1879–1895 (May 1997)     

Magnetic-field-induced valence transition in rare-earth systems

The magnetic-field-induced valence transition in rare-earth systems has been investigated using the periodic Anderson model supplemented by the Falicov-Kimball term. This model has been solved by first decoupling the Falicov-Kimball term as proposed by Khomskii and Koharjan and then taking the limit of infinite intra-site Coulomb repulsion. The valence transition both in the absence and in the presence of magnetic field as a function of temperature is studied. It has been found that the system makes transition from non-magnetic to magnetic state when the magnetic field increases beyond a critical value H _c. The phase boundary defined in terms of reduced field H _c(T)/H _c(0) and reduced temperature T/T _v (T _v being the valence transition temperature in the absence of field) is almost independent of the position of the localized level. The results are in qualitative agreement with experimental observations in Yb- and Eu-compounds.     

有机-无机杂化钙钛矿自组装量子阱结构的能带调控和光电性能的研究

有机-无机杂化钙钛矿材料具有分子尺度上调节能带结构的特点,在光、电、磁等领域均表现出了优异的性能.通过简单的旋涂方法,成功的制备了具有不同无机层层数的杂化钙钛矿材料(C₆H₁₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (简写为C₆Pb关键词： 杂化钙钛矿 量子阱 带隙 光电性能     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

Observation of the “Inverse” spin valve effect in a Ni/V/Ni trilayer system

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T _c for samples comprising Ni layers with similar values of the coercive field H _c reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H _c amounts up to ΔH _c ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the T _c for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.     

New phase transition in an easy-axis “triangular” antiferromagnet

The NMR of ⁵⁵Mn in the quasi-one-dimensional noncollinear anti-ferromagnet CsMnI₃ is investigated at T=1.3 K in magnetic fields up to ∼80 kOe and angles between the field and C ₆ axis ϕ≈ 0.5° and ϕ=7°. A new reorientational magnetic phase transition is observed in a field H _c1≈39.0 kOe. The magnetic structure for H>H _c1 is determined. The average Mn²⁺ spins of the magnetic sublattices in the new phase are determined from an analysis of the NMR spectrum to be 〈 S _C 〉=1.63 and 〈S _D 〉=1.72. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 988–993 (25 June 1998)     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO₃ in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature T _sp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾T _sp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at T<T _sp, λ is positive and λ _b >λ _c . At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 156–159 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

On the isotope effect of Tc in squaric acid (H2C4O4)

The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on T_c is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H₂SQ) is stretched to 2R(D₂SQ) the phase transition temperature 7^H _c of H₂SQ coincides with T^D _c of D₂SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.     

Triplet superfluidity in neutron matter with Skyrme forces at subnuclear and supranuclear densities in a strong magnetic field

Within a generalized non-relativistic Fermi-liquid approach we have found general analytical formulae for phase-transition temperatures T _c,1(n, H) and T _c,2(n, H) (which are nonlinear functions of density, n, and linear of magnetic field, H) for phase transitions in spatially uniform, dense, pure neutron matter from normal to superfluid states with spin-triplet p-wave pairing (similar to anisotropic superfluid phases ³He - A₁ and ³He - A₂) in steady and homogeneous sufficiently strong magnetic field (but |μ _n |H ≪ E _c < ɛ _F (n), where μ _n is the magnetic dipole moment of a neutron, E _c is the cutoff energy and ɛ _F (n)is the Fermi energy in neutron matter). General formulae for T _c,1,2(n,H) are valid for arbitrary parameterization of the effective Skyrme forces in neutron matter. We have used for definiteness the so-called SLy2, Gs and RATP parameterizations of the Skyrme forces with different exponents in their power dependence on density n (at sub- and supranuclear densities) from the interval 0.7 n ₀ ≲ n < n _c (Skyrme)< 2 n ₀, where n ₀ =0.17 fm⁻³ is the nuclear density and n _c (Skyrme)is the the critical density of the ferromagnetic instability in superfluid neutron matter. These phase transitions might exist in the liquid outer core of magnetized neutron stars.     

Superconductivity

The superconducting and normal state properties are reviewed for exemplary matrix-impurity systems in the three distinct regimes of magnetic character of the impurity which have been identified. It is shown that these three regimes can be distinguished by thedetailed behavior of the depressions of 1) the superconducting transition temperatureT _c as a function of impurity concentrationn and 2) the specific heat jump ΔC atT _c as a function ofT _c . These systematics of superconductivity in the presence of local moments appear to be sufficiently well established that it is possible to 1) ascertain whether the solute spin is long-lived (magnetic) or short-lived (nonmagnetic) compared to thermal fluctuation lifetimes at superconducting temperatures, 2) determine the sign and magnitude of the conduction electron-impurity spin exchange interaction parameter ϑ and the temperature dependence of the exchange scattering of conduction electrons bylong-lived solute spins, 3) derive, in favorable cases, information pertaining to the energy level structure of rare earth ions in the crystalline electric field of their superconducting metallic host, and 4) observe magnetic-nonmagnetic transitions of an impurity induced by the application of an external pressure or variation of the composition of a binary alloy matrix. This article is based upon an invited presentation at the 1973 Summer meeting of the American Physical Society in East Lansing, Michigan, 18–20 June 1973 [Bull. Amer. Phys. Soc.18, 779 (1973)]. Supported by the U.S. Energy Research and Development Administration under Contract No. ERDA E(04-3)-34 PA 227.     

Metal-insulator transition induced by a magnetic field in the compounds Eu0.7 A 0.3MnO3 (A=Ca, Sr)

The temperature R(T) and field R(H) dependences of the electrical resistance in the compound Eu_0.7 A _0.3MnO₃ (A=Ca, Sr) are investigated in the temperature range 4.2–200 K in magnetic fields up to 14 T. Above the antiferromagnetic transition temperature T _a, the function R(T) is semiconducting in character. Application of a magnetic field H that exceeds a certain critical value H _c changes the character of the function R(T) for Eu_0.7Sr_0.3MnO₃ to metallic (dR/dT>0). For T<T _a and H<H _c a jump in the resistance is observed indicating instability of the electronic state caused by competition between charge and spin ordering of the Mn ions of different valences. Fiz. Tverd. Tela (St. Petersburg) 40, 708–712 (April 1998)     

Quantum crystallization of two-dimensional dipole systems

An analysis is made of the region of existence of crystalline order in a system of spatially separated electrons (e) and holes (h) in two coupled quantum wells for various concentrations n, temperatures T, and distances D between the layers. A study is also made of crystallization in a system of electrons in semiconductor structures near a metal electrode for various distances d between the semiconductor and the metal. Calculations of the crystalline phase were made using variational calculations of the ground-state energy of the system allowing for pairing of quasiparticles with nonzero momentum. For a system of two coupled quantum wells, regions in (T,n,D) space are determined in which electron (or hole) charge-density waves exist in each layer and regions where these charge-density waves are in phase, in other words, indirect excitons (or pairs with spatially separated electrons and holes) interacting as electric dipoles, become crystallized. In the electron system in semiconductor structures near a metal electrode, regions of existence of an electron crystal are also obtained in (T,n,D) space, where over large distances the electrons interact as electric dipoles because of image forces. Fiz. Tverd. Tela (St. Petersburg) 40, 1350–1355 (July 1998)