Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.     

Study of the structure of a superconducting state of Co-doped BaFe2As2 multiband compounds

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe_{1 ? x} Co _x )₂As₂ compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ₁ ≈ 15 cm^?1, Δ₂ ≈ 21 cm^?1, and Δ₃ ≈ 30–35 cm^?1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe₂As₂ multiband compounds.     

Superconductivity in W5SiB2 with the T2-phase structure studied by the specific heat measurement and band structure calculation

The superconducting state of W₅SiB₂ with the T₂-phase structure has been investigated by a specific heat measurement and a density of state calculation. The estimated ΔC_p/γT_c and 2Δ(0)/k_BT_c values are 1.49 and 3.32, which are close to 1.43 and 3.53 within the weak coupling regime. The electronic specific heat data clearly indicates the absence of gap nodes in the superconducting order parameter, suggesting an isotropic s-wave superconductor. From the density of state calculations, we found that W d-orbital plays an important role for the superconductivity in W₅SiB₂.     

<Superscript>10</Superscript>B–<Superscript>11</Superscript>B isotope substitution and superconductivity in ZrB<Subscript>12</Subscript>

The specific heat of the ZrB₁₂ compound in the normal and superconducting states (T _C ≈ 6 K) has been studied in the 1.9–7 K temperature range for high-quality single crystals with different relative contents of boron isotopes. For Zr¹⁰B₁₂, Zr^natB₁₂, and Zr¹¹B₁₂ dodecaborides, the electron density of states and the electronphonon coupling constant, λ_e-ph ∼ 0.4, are found. The dependence of the thermodynamic and upper critical fields, as well as of the Ginzburg-Landau parameter (κ = 0.8–1.14) on temperature and isotope composition is determined. The results suggest the existence of the magnetic field induced phase transition at T* = 4–5 K, which is not related to the transition from type-I to type-II superconductivity. The possibilities of the existence of two-gap superconductivity and a structural phase transition at T* in zirconium dodecaboride are discussed.     

The superconducting specific heat jump of UBe13 and a narrow peak in the electronic density-of-states

The superconducting specific heat jump for the s-f; hybridization model of UBe₁₃ of Overhauser and Appel [2] is calculated. The narrow peak in the density-of-states is found to enhance the size of the jump relative to the BCS value of 1.43, but in weak coupling the increase is insufficient to agree with the measured value for UBe₁₃. The Kondo lattice model of Razafimandimby, Fulde and Keller [9] is considered as a possible modification, but in weak coupling a simple estimate shows that this cannot increase the specific heat jump significantly. It is speculated, however, that a consistent picture may still be possible with conventional strong coupling corrections.     

Temperature dependence of the electronic specific heat of superconductors with quantum defects

The electronic specific heat of metals with quantum defects in the superconducting state is examined. The role of the electron-polaron effect, as well as that of the level population factor of two-level states, is analyzed in an adiabatic approach. The cases of intermediate and strong coupling are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 413–418 (March 1998)     

Effects of strong coupling on pairing fluctuations in layered superconductors

The influence of short inelastic lifetimes due to strong coupling of fermionic quasiparticles to bosons on superconducting fluctuation effects aboveT _c is calculated. Considering a strong-coupling model for a layered superconducting metal, it is shown that pairing fluctuation corrections to the spin-lattice relaxation rate in weak coupling and very strong coupling are qualitatively different if the pairing fluctuation spectrum has s-wave symmetry. For weak coupling the corrections are positive, whereas for very strong coupling γ = 2? d ω α² F(ω)/ω > 2 the corrections are negative. In contrast, the Pauli spin susceptibility is insensitive to strong-coupling corrections.     

Influence of lattice instability on the superconducting properties of “La3S4”

The normal state properties (the electronic specific heat constant, Debye temperature and electrical resistivity) and superconducting state properties [the superconducting transition temperature, T_c, and the upper critical field at 0 K, H_c²(0)] have been studied in the La₃S₄-La₂S₃ system. The superconducting properties and the electronic specific heat constant exhibit the maximum values in the alloy with the lowest sulfur content that does not undergo a low temperature crystallographic transformation. At lower sulfur contents the alloys exhibit a cubic to tetragonal transformation at ～80 K with a serious degradation in their superconducting properties, especially H_c² (0). These alloys clearly illustrate that materials which are almost but not quite unstable are good superconductors, relative to the more stable compositions.     

Superconducting properties of LaOs2

For superconducting LaOs₂ of the pure C-15 phase we measured the low temperature specific heat, the upper critical field and the susceptibility in the normal state. From the specific heat results we confirm the transition temperature of the C-15 phase and derive several thermodynamic parameters of the normal and superconducting state like γ, θ_D, ΔC, Δ(0), and H_c(0). The results suggest that LaOs₂ is an extremely strong coupling superconductor like La₃In and La₃Tl.     

Spin-orbit coupling in the superconducting phase and DDW states of high-T<Subscript>c</Subscript> cuprates

The effects of the spin-orbit coupling are considered for the high T _c cuprates with asymmetric superconducting gap (SC) and the d-density wave (DDW) phase due to its vital role in the experimental determination of the DDW state. Experiments predict an anisotropy in the DSC gap where |Δ(0,π)|>|Δ(π,0)| and the gap node deviates from the diagonal direction towards the k _x axis. Measurements also demonstrate DDW to be a possible candidate for the pseudogap in the underdoped phase. Due to the spin-orbit (SO) coupling in the low temperature orthorhombic (LTO) phase, the phase diagram of the cuprates suffers a change due to the modification of the T* value, the temperature characteristic of pseudogap, although T _c remains unaltered. Moreover, for a more generalized SO coupling, the DDW gap decreases with the angle but has no effect on the SC gap. We calculate the density of states in the various regimes of doping for the mixed SC+DDW states in the underdoped (UD) phase, SC state in the overdoped phase and also the DDW state in the UD phase and compare them with various theoretical and experimental works. The temperature dependence of the specific heat does not exhibit any qualitative change due to the SO coupling.     

Competition mechanism between singlet and triplet superconductivity in the tight-binding model with anisotropic attractive potential

Based upon the tight-binding formalism a model of a high-T_c superconductor with isotropic and anisotropic attractive interactions is considered analytically. Symmetry facets of the group C_4v are included within a method of successive transformations of the reciprocal space. Complete sets of basis functions of C_4v irreducible representations are given. Plausible spin-singlet and spin-triplet superconducting states are classified with regard to the chosen basis functions. It is displayed that pairing interaction coefficients and the dispersion relation, which can be characterized by the parameter η= 2t₁/t₀, have a diverse and mutually competing influence on the value of the transition temperature. It is also shown that in the case of a nearly half-filled conduction band and an anisotropic pairing interaction the spin-singlet d-wave symmetry superconducting state is realized for small values of the parameter η, whereas in the opposite limit, for sufficiently large values, the spin-triplet p-wave symmetry superconducting state has to be formed. This result cannot be obtained within the Van Hove scenario or BCS-type approaches, where the p-wave symmetry superconducting state absolutely dominates. The specific heat jump and the isotope shift as functions of the parameter η are assessed and discussed for the d-wave symmetry singlet and the p-wave symmetry triplet states.     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Theoretical analysis of the thermal conductivity of high-temperature superconductors

Summary The thermal conductivity of YBa₂Cu₃O_7−δ high-T _c superconductor is analysed self-consistently on both normal and superconducting states on the base of the Bardeen-Rickayzen-Tewordt extended theory to take into account the effects of magnetic field and superconducting fluctuations. It is shown that experimental data are in a quantitative agreement with theory even if the number and variation intervals of adjustable parameters are substantially reduced in comparison with previous works. Phonon relaxation rates due to different mechanisms of phonon scattering as well as the parameters of electron-phonon interaction are estimated. It is shown that thermal conductivity in YBa₂Cu₃O_7−δ is consistent with the BCS model with intermediate electron-phonon coupling λ=1–3 the phonon-electron and electron-phonon relaxation times near critical temperature are evaluated to be 10⁻¹⁰s and 10⁻¹²s, respectively.     

Condensation energy of the superconducting bilayer cuprates

In the present work, we report the interplay of single particle and Cooper pair tunnelings on the superconducting state of layered high-T _c cuprate superconductors. For this we have considered a model Hamiltonian incorporating the intra-planar interactions and the contributions arising due to the coupling between the planes. The interplanar interactions include the single particle tunneling as well as the Josephson tunneling of Cooper pairs between the two layers. The expression of the out-of-plane correlation parameter which describes the hopping of a particle from one layer to another layer in the superconducting state is obtained within a Bardeen-Cooper-Schriefer (BCS) formalism using the Green’s function technique. This correlation is found to be sensitive to the various parameter of the model Hamiltonian. We have calculated the out-of-plane contribution to the superconducting condensation energy. The calculated values of condensation energy are in agreement with those obtained from the specific heat and the c-axis penetration depth measurements on bilayer cuprates.     

Muon spin relaxation and knight shift in the heavy-fermion superconductor UPt3

Positive muon spin relaxation experiments have been conducted on the heavy-fermion superconductor UPt₃ in both the normal and superconducting states for zero, transverse, and longitudinally applied magnetic fields. Below 6 K in zero applied field, the μ⁺ relaxation rate is approximately twice that expected from¹⁹⁵Pt nuclear dipolar relaxation alone. Transverse- and longitudinal-field measurements show that the observe relaxation rate depends on magnetic field and is quasistatic in origin. It is suggested that the onset of very weak (≈10⁻³ μ_B/U atom) magnetic ordering below approximately 6 K is responsible for the observed increase in the relaxation rate. μ⁺ Knight shift measurements in the normal state of UPt₃ show a temperature dependent shift K_μ which tracks the bulk susceptibility X. From the K_μ vs. X plot, a μ⁺ hyperfine field of approximately 100 Oe/μ_B is extracted.     

Possible Fulde-Ferrell-Larkin-Ovchinnikov inhomogeneous superconducting state in CeCoIn5

We present specific heat and thermal conductivity of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical fieldH _c2, measured with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase diagram and the first order nature of the superconducting phase transition at high fields close to a critical fieldH _c2 indicate the importance of the Pauli limiting effect in CeCoIn₅. In the same range of magnetic field we observe a second specific heat anomaly within the superconducting state, and interpret it as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a fieldH _k below the superconducting critical field, which closely coincides with the low temperature anomaly in specific heat tentatively identified with the appearance of the FFLO superconducting state. The enhancement of thermal conductivity within the FFLO state calls for further theoretical investigations of the real space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.     

Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La_1.87Sr_0.13CuO₄ are directly measured with the electric field polarized perpendicular to the CuO planes (E∥c) covering the frequency range 10-40 cm⁻¹ and temperatures 5-300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper-oxygen planes of La_1.87Sr_0.13CuO₄. A strongly enhanced low-frequency (below 15 cm⁻¹) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

Superconducting state parameters of CuCZr100-C metallic glasses

The theoretical investigation of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _c, isotope effect exponent α and effective interaction strength N ₀ V of ten Cu _C Zr_100−C metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential. Three local field correction functions proposed by Hartree (H), Taylor (T) and Ichimaru-Utsumi (IU) are used in the current investigation to study the screening influence on the aforesaid properties. It is observed that the electron-phonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show weak dependences on local field correction functions. The T _c obtained from IU-local field correction function are found an excellent agreement with available theoretical or experimental data. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in metallic glasses.      

Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ⁺ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U_1−xTh_xBe₁₃, x=0 and 0.033. The Knight shift in UBe₁₃ shows a strong decrease as the temperature is reduced in the superconducting state, unlike U_0.967Th_0.033Be₁₃ in which the shift remains at about the normal state value. If the superconducting state in UBe₁₃ has odd-parity, the decrease of K_μ below T_c suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be₁₃ system would explain the differences observed in the Th-doped and pure UBe₁₃ materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K.