Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H _c2(T)|,T ∼ 0 andT →T _c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|H_c2(0)|/[-Tδ|H _c2 T|δT]_T{s0} for the case of a FS open ink _z-direction with thek _z-bandwidth, 4t ₃, very small compared to the Fermi energy,E _F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT _c and the temperature dependence of H _c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa₂Cu₃O₇ used elsewhere, we find reasonable values for the upper critical fieldH _c2(0), the slope (dH _c2/dT)_{T c0}, anisotropic coherence lengths ζ_i(T=0),i=x, y, z, and (dT _c/d|H|)_{|H| → 0}.     

Magnetic and crystalline properties of GdxLa1−x FeSi compounds

The magnetic and crystalline properties of the polycrystalline compounds Gd_xLa_1−x FeSi (x=0, 0.1, 0.3, 0.45, 0.55, 0.65, 0.8, 1) and single crystal GdFeSi are studied. All the compounds have the CeFeSi tetragonal structure. The temperature dependences of the magnetization and initial susceptibility show that ferromagnetic behavior is characteristic of these compounds; the exception is the Pauli paramagnetic LaFeSi. Substitution of gadolinium by the nonmagnetic lanthanum leads to a sharp drop in the Curie temperature T _c, and to a reduction in the saturation magnetic moment μ ₀, effective magnetic moment μ _eff, and paramagnetic Curie temperature Θ_p. Measurements on single crystal GdFeSi imply that the easy magnetization axis of this compound is the c axis and the anisotropy field is H _a∼4 kOe. No magnetic moment was observed on the iron ion in any of these compounds. Fiz. Tverd. Tela (St. Petersburg) 39, 325–329 (February 1997)     

Segregation of impurities and vacancies on phase and antiphase boundaries in alloys

The equilibrium distribution of low-concentration impurities or vacancies is investigated in the region of a coherent phase boundary or antiphase boundary in a binary alloy. A general expression for the free energy of an inhomogeneous multicomponent alloy, which generalizes the expression previously derived for a binary alloy, is presented. Explicit formulas for the impurity concentration profile c _im(x) in terms of the distribution of the principal components of the alloy near a boundary are obtained from this expression in the mean-field and pair-cluster approximations. The shape of this profile is determined by a “preference potential” P, which characterizes the attraction of an impurity to one of the alloy components, as well as by the temperature T and the phase transition temperature T c. At small values of P/T impurities segregate on a phase boundary, and the degree of this segregation, i.e, the height of the maximum of c _im(x), in the region of the boundary increases exponentially as the ratio T _c/T increases. For P ≠ 0 the c _im(x) profile near a phase boundary is asymmetric, and as P/T increases, it takes on the form of a “worn step.” The maximum on the c _im(x) curve then decreases, and at a certain |P|≳T _c it vanishes. Segregation on an antiphase boundary is investigated in the case of CuZn ordering in a bcc alloy. The form of c _im(x) near an antiphase boundary depends significantly both on the form of the potential P and on the stoichiometry of the alloy. At small P impurities segregate on an antiphase boundary, and at fairly large P “antisegregation,” i.e., a decrease in the impurity concentration on the antiphase boundary in comparison with the value within the antiphase domains, is also possible. Zh. éksp. Teor. Fiz. 112, 714–728 (August 1997)     

The Ginzburg-Landau expansion and the slope of the upper critical field in superconductors with anisotropic normal-impurity scattering

We carry out the Ginzburg-Landau expansion for superconductors with anisotropic s and d pairing in the presence of anisotropic normal-impurity scattering, which enhances the stability of d pairing with respect to disordering. We find that the slope of the curve of the upper critical field, |dH _c2/dT|_{T c}, in superconductors with d pairing behaves nonlinearly as disorder grows: at low scattering anisotropy the slope rapidly decreases with increasing impurity concentration, then gradually but nonlinearly increases with concentration, reaches its maximum, and then rapidly decreases, vanishing at the critical impurity concentration. In superconductors with anisotropic s pairing, |dH _c2/dT|_{T c} always increases with impurity concentration, finally reaching the familiar asymptotic value characteristic of the isotropic case, irrespective of whether there is anisotropic impurity scattering. Zh. éksp. Teor. Fiz. 112, 2124–2133 (December 1997)     

Effect of uniaxial stress on the dielectric properties of BaTiO3+0.1wt.%Eu2O3 ceramics

ABSTRACT

BaTiO₃+0.1wt.%Eu₂O₃ ceramics were prepared by a solid-state reaction method. The dielectric behavior of these ceramics as a function of uniaxial pressure has been systematically studied. The external stress showed obvious effects on these properties. An increase of the Curie point (T_c) and decrease of the Curie–Weiss temperature (T₀) was observed with increasing pressure, resulting in an increase in the first-order nature of the phase transformation (T_c–T₀ increases). Broadening and flattening of the permittivity versus temperature curves near their maximum was found. The pressure behavior of thermal hysteresis and the ??/?T vs. T plot suggests that the phase transition changes to second-order type with increasing pressure. Furthermore, the Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type.     

Paramagnetic-ferromagnetic and insulator-metal phase transitions in La<Subscript>0.88</Subscript>MnO<Subscript>2.95</Subscript>

We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La_0.88MnO_2.95. This compound exhibits an insulator-metal (IM) phase transition at T _IM ≈ 253 K (above the Curie temperature, T _C ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T _C , isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La_0.88MnO_2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.     

Magnetic,electrical, magnetoelectrical,and magnetoelastic properties of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> manganites

This paper reports on a study of the influence of oxygen deficiency on the magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance, and volume magnetostriction of the La_0.9Sr_0.1MnO_{3 − y} manganite with y = 0.03, 0.10, and 0.15. The magnetization M(T) behaves in a complex way with temperature; for T < 80 K, it only weakly depends on T, and at 80 ≤ T ≤ 300 K, the M(T) curve shows a falloff. Within the interval 240 K ≤ T ≤ 300 K, the long-range magnetic order breaks up into superparamagnetic clusters. For T < 80 K, the magnetic moment per formula unit is about one-fourth that which should be expected for complete ferromagnetic alignment of Mn ion moments. Although the composition with y = 0.03, in which part of acceptor centers is compensated by donors (oxygen vacancies), the negative magnetoresistance Δρ/ρ and volume magnetostriction ω are observed to pass through maxima near the Curie point, their values are one to two orders of magnitude smaller than those for the y = 0 composition. In compositions with y = 0.10 and 0.15 with electronic doping, the values of Δρ/ρ and ω are smaller by one to two orders of magnitude than those observed for the y = 0.03 composition. They do not display giant magnetoresistance and volume magnetostriction effects, which evidences the absence of ferrons near unionized oxygen vacancies. This allows the conclusion that the part played by both compensated and uncompensated doubly charged donors consists in forming dangling Mn-O-Mn bonds, which lead to a decrease in the Curie temperature with increasing y and to the formation above it of superparamagnetic clusters of the nonferron type.     

Generalized Ginzburg-Landau equation and the properties of superconductors with Ginzburg-Landau parameter κ close to 1

An expression is derived for the free energy of a superconductor near the critical temperature, taking account of the terms of next highest order in the parameter 1−T/T _c. These terms become important for Ginzburg-Landau parameter values |κ−1|≪1, and in this case, in an external magnetic field H ₀ close to H _c2, the structure of the order parameter is determined by the relative values of the three small parameters |κ−1|, 1−T/T _c, and (H _c2−H ₀)/H _c2. Three types of lattices are investigated: triangular with one and two flux quanta per cell and square with one flux quantum per cell. Zh. éksp. Teor. Fiz. 115, 726–739 (February 1999)     

Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.     

Magnetic states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe₁₂O₁₉) are performed in the phase transition region around the Curie temperature (T _c). The method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe₁₂O₁₉ the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below T _c. It is established that the transition temperature T _c(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value T _c at the lower boundary of the “critical” surface layer. Therefore, near T _c a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Néel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.     

Inelastic scattering of phonons by quadrupole defects with internal degrees of freedom

We solve the quantum mechanical problem of the inelastic scattering of phonons by a quadrupole defect in a crystal lattice for the case of solid parahydrogen whose matrix contains pair complexes of H₂ orthomolecules. By employing the pseudospin approximation for the operator of the energy of quadrupole-quadrupole interaction of the molecules in an orthopair we derive an effective Hamiltonian that describes the interaction of phonons with a pair quadrupole orthodefect in the lattice. We set up the scattering matrix and calculate the effective phonon relaxation time τ(ω, T) as a function of the frequency ω and the crystal temperature T. We also find that a pair quadrupole defect, which has a complicated system of levels, can be replaced by an effective two-level system with temperature-dependent parameters. The fact that a pair quadrupole orthocluster has internal degrees of freedom results in a resonant scattering peak near a certain critical temperature T ₀. Our estimates for H₂ yield T ₀≃ 6–7 K. Finally, we discuss the contribution of this mechanism to the low-temperature thermal conductivity of solid hydrogen. Zh. éksp. Teor. Fiz. 114, 555–569 (August 1998)     

Specific heat of Nickel-Iron and Nickel-Cobalt alloys between 600° K and 1500° K

The -specific heat anomalies of ferromagnetic fcc Ni–Co and Ni–Fe alloys were investigated near the Curie temperatures. The magnetic contribution to the specific heat was found to be logarithmically divergent [c _M=log|T/T _c–1| ^A ] above the Curie temperature. The exponentA was composition dependent. Entropy and energy values associated with magnetic transions were determined experimentally and compared with theoretical predictions of the Heisenberg and Ising localized electron models obtained from series expansion calculations.     

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

A self-consistent microscopic theory of the relaxation of the crystal-field levels of an impurity ion in a state with an integer valence implanted in a normal metal is devised. A microscopic approach based on the Coqblin-Schrieffer-Cooper approach, rather than the formal model of the sf exchange interaction, makes it possible to take into account the specific details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width Γ_MM′(T) of transitions between the crystal-field states |M〉 of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width γ _M (T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width γ _G (T) of the crystal-field ground state | G〉 of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*=Γ _G(T=0)/2 from the quasielastic scattering width at zero temperature Γ _G (T=0)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr³⁺ ions implanted in CeAl₃ and LaAl₃. The results of the calculations are in quantitative agreement with the experimental data. Zh. éksp. Teor. Fiz. 113, 1843–1865 (May 1998)     

Magnetically two-phase state in Eu1−x AxMnO3 (A=Ca, Sr)

It is observed that low-temperature magnetic properties (dependence of the magnetization on the cooling conditions and the presence of a maximum in the initial magnetic susceptibility) of Eu_{1− x} A_xMnO₃ (A=Ca, Sr; x=0,0.3) samples are similar to those of spin glasses. However, there are also substantial differences: The magnetization depends on the cooling conditions right up to the maximum measurement fields H=45 kOe, and the temperature of T _N of the maximum of the initial magnetic susceptibility is independent of the frequency of the ac magnetic field in which the susceptibility is measured. The magnetization isotherms for T<T _N are a superposition of a linear part, characteristic for an antiferromagnet, and a small spontaneous part. For compositions containing Sr a maximum of the resistivity ρ (ρ_max∼10⁸ Ω ·cm) is observed near T _N; in a 120 kOe magnetic field this maximum is lowered by four orders of magnitude and the temperature of the maximum is two times higher. In compositions with x=0.3 the paramagnetic Curie point is much higher than for the composition with x=0: θ=110 K (A=Ca), 175 K (A=Sr), and −100 K (x=0). These characteristic features of the magnetic and electric properties are explained by the existence of a magnetically two-phase state in this system, consisting of ferromagnetic clusters, in which the charge carriers are concentrated, embedded in an insulating antiferromagnetic matrix. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 375–380 (10 March 1999)     

Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton dispersion relationsk _⊥ (ω), andk _‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points.     

Insulator-metal transition in the single-crystal high-T c superconductor Bi-2201

Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T _c superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T ^−1/3, changes to ln ρ ∝ T ^−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator state. The behavior of the width of the superconducting transition and of the temperature of its onset, T _con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity of the IM transition the superconducting transition becomes broader, and the onset of the transition T _con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in the crystal of superconducting droplets with different values of T _c , which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions, superconductivity has a percolation character. Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998)     

Phase transitions and the magnetocaloric effect in Mn rich Ni–Mn–Ga Heusler alloys

In Mn rich polycrystalline Heusler alloys, Ni₅₀Mn_25+−xGa_25−x, prepared by Arc melting, it is found that the structural/first-order magnetic transition temperature T_m increases as the Mn content increases. The Curie temperature T_c is higher than that of Ni rich alloys (Ni_50+xMn_25−xGa₂₅ ) of the same series, and is less affected by composition x. Magnetic entropy change of |ΔS_M| also increases as Mn content increases, while behaviour of the field dependence of ΔS_M is similar to that of single crystal Ni_52.6Mn_23.1Ga_24.3.     

Microwave absorption in a Y1Ba2Cu3O7−x single crystal near the superconducting transition

Y₁Ba₂Cu₃O_7−x single crystals are investigated near T _c ≈92 K in fields 0<H⩽9 kOe using a modified ESR spectrometer. The temperature modulation method is used for the first time, together with the traditional magnetic modulation method, to detect microwave responses in single crystals. Superconducting-transition peaks shifted relative to one another in temperature and differing in shape are observed in the temperature dependence of the corresponding signals ∂R/∂H and ∂R/∂T (R — microwave absorption). The evolution of these peaks as a function of the field and the angle π between H and the c axis of the single crystal is traced. It is shown that the difference in the temperature dependences of the derivatives ∂R/∂H and ∂R/∂T is due to the broadening of the superconducting transition characteristic of HTSCs. Fiz. Tverd. Tela (St. Petersburg) 41, 14–17 (January 1999)     

Phase transition from spin glass to long-range magnetic order in semiconductor spinels CuCr1.5+0.5x Sb0.5−0.5x S4 (x=0.34 and 0.4)

Reentrant behavior to a spin glass state is discovered in the solid solution system CuCr_{1.5+0.5x Sb x0.5−0.5 x S 4} with x=0.34 and 0.4. The spin-glass transition temperature _{T f} determined from the kink in the temperature dependence of the initial susceptibility in an alternating magnetic field depends on the measurement frequency ω. It is shown that the frequency dependence _{T f} (ω) is a power-law function 1/ω=(1/ω₀)[T _f /( _{T f} − _T *)]^zv with _zv =7.7 for both compositions. For the composition with _x =0.34 a maximum is observed near _T * in the temperature dependence of the resistivity. These facts indicate that the transition from the spin glass to long-range magnetic order is a phase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 265–269 (25 August 1996)