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Register a new userImpurity scattering effects on the low-temperature specific heat of d-wave superconductors
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Very recently impurity scattering effects on quasiparticles in d-wave superconductors have attracted much attention. Especially, the thermodynamic properties in magnetic fields H are of interest. We have measured the low-temperature specific heat C(T,H) of La_1.78Sr_0.22Cu_1-xNi_xO4. For the first time, the impurity scattering effects on C(T,H) of cuprate superconductors were clearly observed, and are compared with theory of d-wave superconductivity. It is found that impurity scattering leads to gamma(H)=gamma(0)(1+D((H/H_c2)(ln(H_c2/H)) in small magnetic fields. Most amazingly, the scaling of C(T,H) breaks down due to impurity scattering.
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The low-temperature specific heat of a superconductor Mo3Sb7 with T_c = 2.25 (0.05) K has been measured in magnetic fields up to 5 T. In the normal state, the electronic specific heat coefficient gamma_n, and the Debye temperature Theta_D are found to be 34.5(2) mJ/molK^2 and 283(5) K, respectively. The enhanced gamma_n value is interpreted due to a narrow Mo-4d band pinned at the Fermi level. The electronic specific heat in the superconducting state can be analyzed in terms a phenomenological two BCS-like gap model with the gap widths 2Delta_1/k_BT_c = 4.0 and 2Delta_2/k_BT_c = 2.5, and relative weights of the mole electronic heat coefficients gamma_1/gamma_n = 0.7 and gamma_2/gamma_n = 0.3. Some characteristic thermodynamic parameters for the studied superconductor, like the specific heat jump at T_c, DeltaC_p(T_c)/gamma_nT_c, the electron-phonon coupling constant,lambda_eph, the upper H_c2 and thermodynamic critical H_c0 fields, the penetration depth, lambda, coherence length xi, and the Ginzburg-Landau parameter kappa are evaluated. The estimated values of parameters like 2Delta/k_BT_c, DeltaC_p(T_c)/gamma_nT_c, N(E_F), and lambda_eph suggest that Mo3Sb7 belongs to intermediate-coupling regime. The electronic band structure calculations indicate that the density of states near the Fermi level is formed mainly by the Mo-4d orbitals and there is no overlapping between the Mo- 4d and Sb-sp orbitals.
We investigate the effect of thermal fluctuations on the two-particle spectral function for a disordered $s$-wave superconductor in two dimensions, focusing on the evolution of the collective amplitude and phase modes. We find three main effects of thermal fluctuations: (a) the phase mode is softened with increasing temperature reflecting the decrease of superfluid stiffness; (b) remarkably, the non-dispersive collective amplitude modes at finite energy near ${bf q}=[0,0]$ and ${bf q}=[pi,pi]$ survive even in presence of thermal fluctuations in the disordered superconductor; and (c) the scattering of the thermally excited fermionic quasiparticles leads to low energy incoherent spectral weight that forms a strongly momentum-dependent background halo around the phase and amplitude collective modes and broadens them. Due to momentum and energy conservation constraints, this halo has a boundary which disperses linearly at low momenta and shows a strong dip near the $[pi,pi]$ point in the Brillouin zone.
Motivated by recent proposals of correlation induced insensitivity of d-wave superconductors to impurities, we develop a simple pairing theory for these systems for up to a moderate strength of disorder. Our description implements the key ideas of Anderson, originally proposed for disordered s-wave superconductors, but in addition takes care of the inherent strong electronic repulsion in these compounds, as well as disorder induced inhomogeneities. We first obtain the self-consistent one-particle states, that capture the effects of disorder exactly, and strong correlations using Gutzwiller approximation. These `normal states, representing the interplay of strong correlations and disorder, when coupled through pairing attractions following the path of Bardeen-Cooper-Schrieffer (BCS), produce results nearly identical to those from a more sophisticated Gutzwiller augmented Bogoliubov-de Gennes analysis.
We study suppression of superconductivity by disorder in d-wave superconductors, and predict the existence of (at least) two sequential low temperature transitions as a function of increasing disorder: a d -wave to -wave, and then an s-wave to metal transition. This is a universal property of the system which is independent of the sign of the interaction constant in the s-channel
We analyze the complex interplay of the strong correlations and impurities in a high temperature superconductor and show that both the nature and degree of the inhomogeneities at zero temperature in the local order parameters change drastically from what are obtained in a simple Hartree-Fock-Bogoliubov theory. While both the strong electronic repulsions and disorder contribute to the nanoscale inhomogeneity in the population of charge-carriers, we find them to compete with each other leading to a relatively smooth variation of the local density. Our self-consistent calculations modify the spatial fluctuations in the pairing amplitude by suppressing all the double-occupancy within a Gutzwiller formalism and prohibit the formation of distinct superconducting-`islands. In contrast, presence of such `islands controls the outcome if strong correlations are neglected. The reorganization of the spatial structures in the Gutzwiller method makes these superconductors surprisingly insensitive to the impurities. This is illustrated by a very weak decay of superfluid stiffness, off-diagonal long range order and local density of states up to a large disorder strength. Exploring the origin of such a robustness we conclude that the underlying one-particle normal states reshape in a rich manner, such that the superconductor formed by pairing these states experiences a weaker but spatially correlated effective disorder. Such a route to superconductivity is evocative of Andersons theorem. Our results capture the key experimental trends in the cuprates.
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