Redistribution of excess space charge in structures based on single-crystal strontium titanate

A band mechanism for transport in single-crystal strontium titanate (STO) is used as a basis for estimating the parameters characterizing the localization and redistribution of space charge in STO for T<100 K. A comparison of the experimental and calculated temperature dependences of the charge carrier concentration yield an estimate for the location of the Fermi level in STO at T=4.2 K with E _F-E _v∼10 meV. The phenomena at metal/STO and YBCO/STO contacts are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 349–352 (February 1997)     

Nature of conduction via impurities in uncompensated silicon

The static conductivity σ(E) and photoconductivity (PC) at radiation frequencies ħω=10 and 15 meV in Si doped with shallow impurities (density N=10¹⁶−6×10¹⁶ cm⁻³, ionization energy ε₁≃45 meV) with compensation K=10⁻⁴−10⁻⁵ in electric fields E=10–250 V/cm are measured at liquid-helium temperatures T. Special measures are taken to prevent the high-frequency part of the background radiation (ħω>16 meV) from striking the sample. It is found that the conductivity σ(E) is due to carrier motion along the D ⁻ band, which is filled with carriers under the influence of the field E. In fields E<E _q (E _q ≃100–200 V/cm) the carrier motion consists of hops along localized D ⁻ states in a 10–15 meV energy band below the bottom of the free band (energy ε=ε₁); for E>E _q carriers drift along localized D ⁻ states with energy ε∞ε₁−10 meV. An explanation is proposed for the threshold behavior of the field dependence of the photo-and static conductivities. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 232–236 (25 August 1997)     

Stimulated polariton-polariton scattering and dynamic Bose-Einstein condensation of polaritons in GaAs microcavities under excitation near the exciton resonance

The dynamics of formation of the macroscopically occupied polariton mode at the bottom of the polariton band E _LP(k = 0) and its spin polarization under the quasiresonant pulse excitation of excitons (E = E _X ) with large values of quasi-momentum have been studied in planar GaAs microcavities. It has been found that the growth in the depth E _X − E _LP(k = 0) of the polariton band leads to the change in the formation mechanism for the k = 0 condensate state from the direct parametric decay of the photoexcited mode (due to the polariton-polariton interaction) to the dynamic condensation of polaritons, which results from the multiple scattering of polaritons by both phonons and polaritons. At the same time, in microcavities with E _X − E _LP(k = 0) > 3.5 meV, the direct decay of the photoexcited mode does not disappear, becoming an efficient mechanism for the filling of the states located at the k-space ring, corresponding to the energies E _LP(k) ≈ E _X − 2.6 meV.     

Magnetooptical microwave spectroscopy of the coherent magnetic state in the mixed valence compound SmB6 in the frequency range 40–120 GHz

In undoped pure single crystals of the mixed valence compound SmB₆ anomalous ESR absorption is observed in the frequency range v=40–120 GHz at temperatures of 1.8–4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g ₁=1.907±0.003 and g ₂=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=g ₁ increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB₆ crystalline lattice, with a densit ∼10¹⁵−10¹⁶ cm⁻³, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 707–712 (25 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB₆, Sm_0.8B₆, and Sm_1–x La_xB₆ (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB₆. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated. Zh. éksp. Teor. Fiz. 115, 1024–1038 (March 1999)     

EPR observations of anomalous triplet states in Ba1−x KxBiO3 and BaPbyBi1− y O3

Electron paramagnetic resonance (EPR) spectra of samples of the systems Ba_1−x K_xBiO₃ and BaPb_yBi_1−y O₃ are investigated over wide ranges of composition and temperature. Two main lines in the EPR spectrum with factors g ₁≈2.1 and g ₂≈4.2 are found for all compositions. It is shown that the observed EPR line with g ₂≈4.2 is due to oxygen ions. This probably indicates the presence of oxygen ions with different effective charges, i.e., the existence of charge density waves in the oxygen-ion sublattice in addition to charge density waves in the bismuth sublattice. Zh. éksp. Teor. Fiz. 115, 1326–1336 (April 1999)     

Cooper pairing and superfluidity in the Emery model

The energy E of the system as a function of the gauge phase Φ is calculated by exact diagonalization in a two-dimensional Cu₄O₈ cluster and by the slave-boson method for large systems. It is shown that motion of carriers with charge 2e, i.e., Cooper pairs, is observed for certain values of the parameters in the Hamiltonian. This motion is identified from the onset of a characteristic maximum of E(Φ) at Φ≈Φ₀/2, where Φ₀ is the flux quantum. The phase diagram is constructed and the range of values of the model parameters where the effect is observed is determined. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 78–82 (25 January 1996)     

Electron density redistribution in Sn-doped Bi2Te3

X-ray photoelectron spectroscopy is used to investigate the redistribution of the density of electronic states in the valence band, and of the binding energies and chemical shifts of core levels in bismuth telluride caused by introduction of impurity tin atoms. A substantial increase in the density of electronic states below the valence-band top at energies μ≈15–30 meV has been revealed. This feature in the energy spectrum accounts for the unusual behavior of the kinetic coefficients in p-Bi₂Te₃:Sn crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1969–1972 (November 1999)     

Condition of strong magnetic field in the mode of quantum reluctance oscillations

The volume (3D), quasi-two-dimentional (Q2D), and two-dimentional (2D) oscillations of degenerated electrons of the Shubnikov-de Haase reluctance reveal common properties. The region of quantum oscillations is bounded (in the magnetic field) from below by the weak field condition (ω_cτ_t≤1) and from above by the quantum limit condition (ξ_F≥ħω_c/2). The monotonic oscillation component is saturated for the occupied main conduction E_m-subband and excitation E_p-subband of dimensional quantization for the Q2D and 2D electron systems in strong magnetic fields. The reluctance of the Q2D system in the quantum limit changes according to a law ∼B ^α with α ≈ 2.6–2.8. The oscillation amplitude is described by an exponential dependence on the magnetic field strength and temperature, similarly to the 3D case. This is caused by the identity of physical conditions and the topology of resonant escape of the Landau levels out of the Fermi surface. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 33–39, March, 2006.     

Effect of pressure and mechanical stress on the electronic properties of AlN and GaN

The fundamental properties of the AlN and GaN compounds with a wurtzite structure under external hydrostatic pressure, uniaxial mechanical stress σ_⊥ along the hexagonal axis, and biaxial mechanical stress σ_⊥ in the basal plane of the unit cell have been considered in terms of first-principles calculations in the frame-work of the density functional theory. The pressures of the phase transitions from the structures of wurtzite and zinc blende to the structure of rock salt have been obtained. The behavior of the structural parameters, interband transitions, and positions of the charge neutrality level has been investigated. The calculated pressure coefficients of the band gap are as follows: ∂E _g /∂p = 40.9 meV/GPa, −∂E _g /∂σ _| = −4.2 meV/GPa, and −∂E _g /∂σ _⊥ = 45.2 meV/GPa for AlN and ∂E _g /∂p = 33.0 meV/GPa, −∂E _g /∂σ _| = 23.6 meV/GPa, and −∂E _g /∂σ _⊥ = 9.6 meV/GPa for GaN. The pressure coefficients of the charge neutrality level in almost all cases are substantially smaller than the corresponding values obtained for the band gap E _g .     

Application of abrupt cut-off models in the analysis of the capacitance spectra of conjugated polymer devices

In this paper, a detailed study of the capacitance spectra obtained from Au/doped-polyaniline/Al structures in the frequency domain (0.05 Hz–10 MHz), and at different temperatures (150–340 K) is carried out. The capacitance spectra behavior in semiconductors can be appropriately described by using abrupt cut-off models, since they assume that the electronic gap states that can follow the ac modulation have response times varying rapidly with a certain abscissa, which is dependent on both temperature and frequency. Two models based on the abrupt cut-off concept, formerly developed to describe inorganic semiconductor devices, have been used to analyze the capacitance spectra of devices based on doped polyaniline (PANI), which is a well-known polymeric semiconductor with innumerous potential technological applications. The application of these models allowed the determination of significant parameters, such as Debye length (≈20 nm), position of bulk Fermi level (≈320 meV) and associated density of states (≈2×10¹⁸ eV⁻¹ cm⁻³), width of the space charge region (≈70 nm), built-in potential (≈780 meV), and the gap states’ distribution.     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Temporal structure of the muon disk at large distances from the axis of extensive air showers with E 0⩾6 ×1016eV

Preliminary results are reported from an investigation of the temporal structure of the muon disk in extensive air showers (EASs) with primary energy E ₀≥6×10¹⁶eV at distances 100–1500 m from the axis. The investigation is performed at the Yakutsk array using the large muon detector, which commenced operation in November 1995, with a planned area 184 m² and a detection threshold E _μ≈0.5· sec θ GeV. For E ₀>10¹⁸eV it is found that the thickness of the muon disk tends to decrease. This requires substantial changes in our notions of the development of EASs. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 361–366 (25 March 1998)     

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La₂CuO₄, La_2−x M_xCuO₄ (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV. Fiz. Tverd. Tela (St. Petersburg) 39, 1000–1004 (June 1997)     

Nature of low-energy excitations in La<Subscript>1.87</Subscript>Sr<Subscript>0.13</Subscript>CuO<Subscript>4</Subscript> superconducting cuprate

The spectra of the conductivity and dielectric constant of La_1.87Sr_0.13CuO₄ cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm⁻¹) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm⁻¹ at liquid helium temperatures.     

Evolution of the optical properties of single-crystal La1−x SrxMnO3

An ellipsometric method is used to study the dispersion of the real ɛ ₁(ω) and imaginary ɛ ₂(ω) parts of the complex dielectric permittivity of single-crystal La_1−x Sr_xMnO₃ (x=0.1, 0.2, and 0.3) for energies from 100 meV to 5 eV at room temperature. It is found that, when lanthanum is replaced by strontium, the optical spectrum changes fundamentally. A shift in the main features of the spectrum of initial LaMnO₃ at 1.9 and 4.7 eV to lower energies takes place, as well as a partial redistribution of the optical-conductivity spectral weight in the band gap region E<1.7 eV. For compositions with x=0.2 and 0.3, a fine structure of the interband absorption is observed against a background of non-Drude optical conductivity at low energies. Fiz. Tverd. Tela (St. Petersburg) 41, 1445–1449 (August 1999)     

Low-temperature (T<180 K) relaxation processes and possible “electronic phase separation” in RBa2Cu3O6+x (R=Y,Tm, Lu) single crystals

Low-temperature (120–180 K) relaxational effects are observed in oxygen-deficient RBa₂Cu₃O_6+x (R=Y, Tm, Lu) single crystals. Isothermal holding of the crystals after rapid cooling increases their resistance. The characteristic times and the activation energy of the relaxation process, E _a ≈0.46 eV, are determined. The possible relation between the observed effects and the appearance of “electronic phase separation” is discussed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 10, 788–791 (25 May 1996)     

Andreev spectroscopy of FeSe: Evidence for two-gap superconductivity

Current-voltage characteristics (CVCs) of Andreev superconductor-constriction-superconductor (ScS) contacts in polycrystalline samples of FeSe with the critical temperature T _C = (12 ± 1) K have been measured using the break-junction technique. In Sharvin-type nanocontacts, two sets of subharmonic gap structures were detected due to multiple Andreev reflections, indicating the existence of two nodeless superconducting gaps Δ _L = (2.75 ± 0.3) meV and Δ _S = (0.8 ± 0.2) meV. Well-shaped CVCs for stacks of Andreev contacts with up to five contacts were observed due to the layered structure of FeSe (the intrinsic multiple Andreev reflections effect). An additional fine structure in the CVCs of Andreev ScS nanocontacts is attributed to the existence of a Leggett mode. A linear relation between the superconducting gap Δ _L and the magnetic resonance energy E _magres ≈ 2Δ _L is found to be valid for layered iron pnictides.     

Effect of electric field on neutron scattering in lead magnoniobate

The temperature dependence of the intensity of the Bragg and the transverse component of quasi-elastic neutron scattering from the single-crystal model relaxor PbMg_1/3Nb_2/3O₃ (PMN) has been studied for various applied electric fields. It is shown that application of a field E>E _th ≈1.6 kV/cm increases the elastic scattering intensity and reduces the intensity of the transverse diffuse-scattering component and that, below 230 K and for E>6 kV/cm, the elastic-scattering intensity saturates while the temperature-dependent part of the transverse diffuse-scattering component becomes practically suppressed. The measured temperature and field dependences of the intensity of type (h00) and (hh0) Bragg reflections provide supportive evidence both for the presence of considerable lead-ion displacements relative to the ideal perovskite sites and for the existence in strong electric fields of an induced transition to the ferroelectric phase below 250 K. Fiz. Tverd. Tela (St. Petersburg) 40, 1905–1910 (October 1998)