Variable-range hopping conduction in LaMnO<Subscript>3 + δ</Subscript>

The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO_{3 + δ} (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO_{3 + δ}, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ₀(T)exp[(T ₀/T)^1/2], where ρ₀(T) = AT ^9/2 (A is a constant). The density of localized states g(?) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(T _v)(T/T _v)^1/2, where T _v is the temperature of the onset of variable-range hopping conduction and γ(T _v) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 Å are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO_{3 + δ} with δ = 0.154 is analyzed, and the dependences Δ(P) and γ_v(P) are obtained.     

On the relation between the normal fluid density and the one particle Green function for Bose fluids

The normal fluid density is defined through the moment of inertia of a cylinder rotating about its axis, in the limit of infinitesimal angular velocity. The angular momentum may be calculated from the one particle Green function, and this leads to the relation between ?_n and the Green function.We then consider an ensemble in which the fluid is constrained to irrotational flow. For statistical states which are “locally gauge invariant” we show that ρ_n, as defined previously, is identical to the full density. This demonstrates the connection between superfluidity and the breaking of local gauge symmetry.Finally, we apply the result to the gas of non-interacting quasi-particles and the fully interacting Bose fluid in the limit of very low temperatures. In both cases we reproduce the Landau expression for ρ_n in terms of the quasiparticle occupation numbers, and derive the well-known result for real superfluid helium that ρ_n ～ T⁴ as T→0.     

Nonmonotonic temperature dependence of the Hall resistance of a 2D electron system in silicon

For a 2D electron system in silicon, the temperature dependence of the Hall resistance ρ_xy(T) is measured in a weak magnetic field in the range of temperatures (1–35 K) and carrier concentrations n where the diagonal resistance component exhibits a metallic-type behavior. The temperature dependences ρ_xy(T) obtained for different n values are nonmonotonic and have a maximum at T_max ～ 0.16T_F. At lower temperatures T < T_max, the change δρ_xy(T) in the Hall resistance noticeably exceeds the interaction quantum correction and qualitatively agrees with the semiclassical model, where only the broadening of the Fermi distribution is taken into account. At higher temperatures T > T_max, the dependence ρ_xy(T) can be qualitatively explained by both the temperature dependence of the scattering time and the thermal activation of carriers from the band of localized states.     

Mean-field vs. Monte Carlo equation of state for the expansion of a Fermi superfluid in the BCS-BEC crossover

The equation of state (EOS) of a Fermi superfluid is investigated in the BCS-BEC crossover at zero temperature. We discuss the EOS based on Monte Carlo (MC) data and asymptotic expansions and the EOS derived from the extended BCS (EBCS) mean-field theory. Then we introduce a time-dependent density functional, based on the bulk EOS and Landau’s superfluid hydrodynamics with a von Weizsäcker-type correction, to study the free expansion of the Fermi superfluid. We calculate the aspect ratio and the released energy of the expanding Fermi cloud showing that MC EOS and EBCS EOS are both compatible with the available experimental data of ⁶Li atoms. We find that the released energy satisfies and approximate analytical formula that is quite accurate in the BEC regime. For an anisotropic droplet, our numerical simulations show an initially faster reversal of anisotropy in the BCS regime, later suppressed by the BEC fluid.     

The velocity autocorrelation function and self-diffusion coefficient of fluids with steeply repulsive potentials

We consider the velocity autocorrelation function, vacf, or C_v(t) and self-diffusion coefficients, D, of steeply repulsive inverse power fluids (SRP) in which the particles interact with a pair potential, ? (r) = ?(σ/r)ⁿ. The C_v(t) are calculated numerically by molecular dynamics simulations. Accurate expressions for the short time expansion of C_v(t) to order O(t⁴) for n large are derived for this fluid. We propose novel expressions for C_v (t) that, for n large, spans the transition from the short time regime (expandable in even powers of time) and the longer time exponential-like regime characteristic of hard spheres. Inter alia we introduce relaxation times that characterize the duration of a collision and the decay of the velocity correlation within the mean-collision or Enskog-like relaxation time, T_E.     

Effect of correlated disorder on the temperature of unconventional cooper pairing: 3He in aerogel

It has been shown by the example of ³He in aerogel that the correlation in the position of impurities may have a considerable effect on the transition temperature T _c of a Fermi fluid to an unconventional superfluid or superconducting state if the correlation radius of the system of impurities exceeds the correlation length ξ₀ of the emerging superfluid phase. A decrease in T _c of ³He in aerogel has been expressed in terms of the structure factor of aerogel. Taking into account the fractal structure of aerogel provides a simple formula that satisfactorily describes the observed decrease in T _c.     

Anomalous frequency dependence of giant quantum attenuation in Bismuth below 1 K

Giant quantum attenuation of longitudinal sound waves in Bi has been measured at temperatures down to 8 mK for the case that the electron (n = 0, s = + 1) and hole (n = 1, s = ? 1) Landau levels cross simultaneously Fermi level at a field of about 90 kOe. The temperature and frequency dependences of the peak attenuation due to these two levels are expressed by α_p ∝ T^-μω^ν with $\text{μ ? 1}$ and $\text{ν ? 1}$ at T > 1 K. When T ? 1 K the value of μ decreases by decreasing temperature and below 0.1 K, α_p takes almost constant value. At T ? 0.5 K, ν becomes larger than 1 and maximum value of ν observed is 2 at T ～ 0.05 K. These features of the attenuation peak at very low temperatures are consistent with what are expected in the fluctuation region of the gas-liquid type transition of the electron-hole system.     

The role of specific features of the electronic structure in electrical resistivity of band ferromagnets Co<Subscript>2</Subscript>Fe<Emphasis Type="Italic">Z</Emphasis> (<Emphasis Type="Italic">Z</Emphasis> = Al,Si, Ga,Ge, In,Sn, Sb)

The electrical resistivity ρ(T) of the band ferromagnets Co₂FeZ (where Z = Al, Si, Ga, Ge, In, Sn, and Sb are s- and p-elements of Mendeleev’s Periodic Table) has been investigated in the temperature range 4.2 K < T < 1100 K. It has been shown that the dependences ρ(T) of these alloys in a magnetically ordered state at temperatures T < T _C are predominantly determined by the specific features of the electronic spectrum in the vicinity of the Fermi level. The processes of charge carrier scattering affect the behavior of the electrical resistivity ρ(T) only in the vicinity of the Curie temperature T _C and above, as well as in the low-temperature range (at T ? T _C).     

High-temperature absorption coefficient of methane at 3.392μ

The spectral absorption coefficient of methane at 3.392μ has been measured in the temperature range 965 ?T, °K≤2710 behind incident and reflected shock waves. It is given by the relation P'=P'₀(T₀/T)ⁿ where P'₀=(1.34±0.58)x10²cm^-1atm^-1 at T₀=300°K and n=2.88±0.21. The empirically determined temperature exponent n may be approximately accounted for by a simplified theoretical analysis.     

Some Features in the behavior of the density of superconducting condensate in superconductors

The correlation between the density ρ_s(T→0) of superconducting condensate and the superconducting transition temperature T _c in underdoped HTSC systems is considered. It is shown that the linear relation between ρ_s(0) and T _c observed in some experiments can easily be interpreted in the framework of the conventional Bardeen-Cooper-Schrieffer (BCS) model without invoking any exotic superconductivity models.     

Study of insulating electrical conductivity in hydrogenated amorphous silicon-nickel alloys at very low temperature

On the insulating side of the metal-insulator transition (MIT), the study of the effect of low temperatures T on the electrical transport in amorphous silicon-nickel alloys a-Si_1−yNi_y:H exhibits that the electrical conductivity follows, at the beginning, the Efros-Shklovskii Variable Range Hopping regime (ES VRH) with T^−1/2. This behaviour showed that long range electron-electron interaction reduces the Density Of State of carriers (DOS) at the Fermi level and creates the Coulomb gap (CG). For T higher than a critical value of temperature T_C, we obtained the Mott Variable Range Hopping regime with T^−1/4, indicating that the DOS becomes almost constant in the vicinity of the Fermi level. The critical temperature T_C decreases with the content of nickel in the alloys.     

1D-3D crossover in hopping conduction of carbynes

D.C. and A.C. conductivities of carbyne samples were studied over the temperature range 1.8–300 K at frequencies 10 MHz–1 GHz. It was established that a variation in the fraction of sp ² bonds in carbynes induces transition from one-to two-and then to three-dimensional conduction. In the one-dimensional hopping conduction regime, the resistivity of carbynes depends on temperature as ρ=ρ₀exp[(T ₀/T)^1/2], whereas Coulomb correlations are insignificant. The Hunt model of one-dimensional A.C. conductivity is experimentally confirmed.     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

Galvanomagnetic properties of La0.7Sr0.3Mn0.9Cu0.1O3

La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ ceramic samples have been obtained by the conventional method of the solid-phase reaction, and their resistivity ρ has been investigated in a temperature range from 50 to 300 K in magnetic fields B = 0–20 T. Dependences are typical of perovskite manganites with a maximum at T _max = 140–150 K and an increase in ρ near T _max with increasing external magnetic field B. It has been established that the behavior of resistivity is caused by the variable range hopping conduction mechanism ρ(T) = ρ₀(T)exp[(T ₀/T)^1/4], where ρ₀(T) ～ T ^25/4. The Mott variable range hopping conduction has been observed below the Curie temperature for La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ samples (T _C ～ 300 K) in a temperature range from 300 to 200 K. The influence of Cu doping on the properties of La_0.7Sr_0.3MnO₃ samples is apparently caused by an additional distortion introduced into the crystal lattice of the material and by a weakening of the double-exchange mechanism.     

Scaling in the quantum Hall effect regime in n-InGaAs/GaAs nanostructures

The longitudinal ρ _xx (B) and Hall ρ _xy (B) magnetoresistances are investigated experimentally in the integer quantum Hall effect (QHE) regime in n-InGaAs/GaAs double quantum well nanostructures in the range of magnetic fields B = (0–16) T and temperatures T = (0.05–70) K before and after IR illumination. The results are evaluated within the scaling hypothesis with regard to electron-electron interaction.     

Effect of two-impurity tunnel resonance on the lower critical field of a long S-I-S tunneling contact with weak structural disorder in the I layer

The region where the effect of two-impurity quantum resonance-percolation trajectories (QRPTs) [1] on the lower critical field value of a disordered S-I-S (superconductor-insulator-superconductor) contact predominates is found on the (μ — ?₀, c) parameter plane, where μ is the contact Fermi level, ?0 is the energy of the single-impurity local electron level, and c is the density of impurities in the I layer. The crossover line between the regions where the effect of single-impurity and two-impurity QRPTs predominates is plotted. The contribution from two-impurity QRPTs to mesoscopic fluctuations of the lower critical field of a disordered S-I-S contact is studied.     

Efficient second harmonic generation in CDA

Efficient second harmonic generation at 0.532 μ has been achieved in a temperature tuned CDA crystal when a narrow beam divergence Nd:YAG laser was used. At a crystal temperature of 40.3°C, a peak second harmonic power of 7.5 MW was obtained with a peak power conversion efficiency of 33%. The temperature variation of the refractive indices in CDA has been determined to be d(n^e_2ω?n⁰_ω)/dT = 7.2 × 10^?5°C^?1.     

Studies of vibrational surface modes in ionic crystals: III. Vibrational contributions to the surface thermodynamic functions for the (001) face of LiF,MgO, NaF,NaCl, Nal,RbF, and RbCl

The temperature dependence of the vibrational contributions to surface specific heat, surface entropy, surface energy, and surface Helmholtz free energy have been calculated for the (001) face of seven crystals having the rocksalt structure. The calculations assume a perfect, unrelaxed surface and make use of shell models fitted to bulk phonon spectra determined from inelastic neutron scattering. In terms of the bulk zero-temperature Debye temperature θ₀, the surface specific heat C_v^s exhibits an effective power law behavior, T^α, from at least T = 0.02 θ₀ up to 0.05 θ₀ in most cases (and up to 0.07 θ₀ for NaF), with α ≈ 2.5 in most cases — in contrast with the result of α = 2 in a Debye-like model. (Below 0.02 θ₀, results derived for our 15-layer films depart significantly from intrinsic surface effects because of the finite thickness.) C_v^s attains a maximum at a temperature T(C_max^s) ranging from 0.14 θ₀ to 0.20 θ₀, in contrast with the result T(C_max^s) = 0.21 θ₀ for the Debye-like model. The peak value C_max^s ranges from 0.34 k_BA_SUC to 0.41 k_BA_SUC, where A_SUC is the area of the surface unit cell. The shap the peak in C_v^s differs characteristically between that class of crystals in which there is some overlap of the acoustical and optical bulk bands and that class in which there is an appreciable absolute gap between the acoustical and optical bulk bands; in the latter class the peak is flattened on the low side of the maximum, with the maximum pushed to somewhat higher temperature. On those points of comparison with the rather sparse existing data for surface-excess heat capacity in which the value of specific surface area is not required (e.g., the value of T(C^s_max)), the agreement ranges from encouraging to equivocal. On those comparisons which require the surface area of the experimental samples (e.g., the magnitude of C^s_max) the agreement ranges from only fair to bad. Further experimental work is needed, and great care in surface area determinations is necessary.     

The emission spectrum of 13C16O2 near 4.5 μm

The emission spectrum of ¹³C¹⁶O₂ near 4.5 μm, the ν₃ transition region, was measured in high resolution. The sequence 00v₃ → 00(v₃ ? 1) was measured to v₃ = 9, 01¹v₃ → 01¹(v₃ ? 1) to v₃ = 6, and 02²v₃ → 00²(v₃ ? 1) to v₃ = 3. Several bands involving Fermi resonant Σ state were also observed. The data were analyzed and spectroscopic constants were determined for all bands identified.     

Energy-scale phenomenology and pairing via resonant spin-charge motion in FeAs, CuO, heavy-fermion and other exotic superconductors

Muon spin relaxation (μSR) studies of the “1111” and “122” FeAs systems have detected static magnetism with variably sized ordered moments in their parent compounds. The phase diagrams of FeAs, CuO, organic BEDT, A₃C₆₀ and heavy-fermion systems indicate competition between static magnetism and superconductivity, associated with first-order phase transitions at quantum phase boundaries. In both FeAs and CuO systems, the superfluid density n_s/m^* at T→0 exhibits a nearly linear scaling with T_c. Analogous to the roton-minimum energy scaling with the lambda transition temperature in superfluid ⁴He, clear scaling with T_c was also found for the energy of the magnetic resonance mode in cuprates, (Ba,K)Fe₂As₂, CeCoIn₅ and CeCu₂Si₂, as well as the energy of the superconducting coherence peak observed by angle resolved photo emission (ARPES) in the cuprates near (π,0). Both the superfluid density and the energy of these pair-non-breaking soft-mode excitations determine the superconducting T_c via phase fluctuations of condensed bosons. Combining these observations and common dispersion relations of spin and charge collective excitations in the cuprates, we propose a resonant spin-charge motion/coupling, “traffic-light resonance,” expected when the charge energy scale ε_F becomes comparable to the spin fluctuation energy scale ?ω_SF~J, as the process which leads to pair formation in these correlated electron superconductors.