arXiv subjects

J. P. Carbotte

Publications and source records attributed to J. P. Carbotte.

169 records · Page 10Linked to original sources

arXiv1996

c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions

Within a microscopic two band model of planes and chains with a pairing potential in the planes and off diagonal pairing between planes and chains we find that the chains make the largest contribution to the Josephson tunnelling current and that through them the d-wave part of the gap contributes to the current. This is contrary to the usual assumption that for a d-wave tetragonal superconductor the c-axis Josephson current for incoherent tunnelling into an s-wave superconductor is zero while that of a d-wave orthorhombic superconductor with a small s-wave component to its gap it is small but non-zero. Nevertheless it has been argued that the effect of twins in YBCO would lead to cancellation between pairs of twins and so the observation of a current in c-axis YBCO-Pb experiments is evidence against a d-wave type order parameter. We argue that both theory and experiment give evidence that the two twin orientations are not necessarily equally abundant and that the ratio of tunnelling currents in twinned and untwinned materials should be related to the relative abundance of the two twin orientations.

cond-mat↗

arXiv1996

a-b Plane Anisotropy in YBCO

The zero temperature in plane penetration depth in an untwinned single crystal of optimally doped YBa_2Cu_3O_6.93 is highly anisotropic. This fact has been interpreted as evidence that a large amount of the condensate resides in the chains. On the other hand, the temperature dependence of lambda_a(T)/lambda_a(0) and lambda_b(T)/lambda_b(0) (where the b-direction is along the chains) are not very different. This constrains theories and is, in particular, difficult to understand within a proximity model with d-wave pairing only in the CuO_2 plane and none on the CuO chains but instead supports a more three dimensional models with interplane interactions.

cond-mat↗

arXiv1996

Quasiparticle Lifetimes in Superconductors: Relationship with the Conductivity Scattering Rate

We compute the single particle inverse lifetime, evaluated in the superconducting state. Within the BCS framework, the calculation can be done non-perturbatively, i.e. poles can be found well away from the real axis. We find that perturbative calculations are in good agreement with these results, even for non-zero impurity scattering. With electron-phonon scattering added to the problem, we use the Eliashberg equations, with impurity scattering, to calculate the quasiparticle inverse lifetime perturbatively. In all cases we find that the inverse lifetime is significantly suppressed in the superconducting state, particularly in the presence of impurity scattering. We also compute the far-infrared and microwave conductivity, and describe procedures for extracting scattering rates from conductivity measurements. While these procedures lead to scattering rates in qualitative agreement with the inverse lifetime, we find that quantitative agreement is lacking, in general.

cond-mat↗

arXiv1996

The Imaginary Part of the Optical Conductivity of $Ba_{1-x}K_xBiO_3$

The frequency dependence of the imaginary part of the infrared conductivity is calculated for a superconductor. Sharp structure, characteristic of superconductivity with an order parameter with s-wave symmetry, appears in the BCS limit as a minimum at a frequency equal to twice the gap value. This structure scales with temperature but gets progressively smeared and shifted as impurity scattering is increased. The relationship between low frequency results and the zero frequency limit is investigated. Experimental results on $\BKBO$ are also presented. The low frequency imaginary part of the conductivity displays a minimum at $2Δ\approx 12$ meV, and provides unequivocal evidence of an s-wave superconducting order parameter. Strong coupling (Eliashberg) results show similar trends. Using this formulation we find that the electron-phonon coupling in $\BKBO$ must necessarily be small, with coupling constant $λ\approx 0.2$, in agreement with conclusions drawn from measurements of the real part of the conductivity. Thus $\BKBO$ is an s-wave superconductor that is not driven by the electron-phonon interaction.

cond-mat↗

arXiv1995

Effect of Proximity Coupling of Chains and Planes on the Penetration Depth Anisotropy in Y_1Ba_2Cu_3O_7

We calculate the penetration depth $λ$ in the $a$, $b$ and $c$ directions for a simple model of YBa$_2$Cu$_3$O$_7$. In this model there are two layers---representing a CuO$_2$ plane and a CuO chain---per unit cell. There is a BCS--like pairing (both $s$ wave and $d$ wave are considered) interaction localised in the CuO$_2$ planes. The CuO chains become superconducting at temperatures lower than $T_c$ because of their proximity to the planes, and there is an induced gap in the chains. Since the temperature dependence of the penetration depth in the $b$ direction (along the chains) is sensitive to the size of the induced gap, the difference between the shapes of the penetration depth curves in the $a$ and $b$ directions reveals a great deal about the nature of the condensate in the chains. We find that in our proximity model there are always regions of the chain Fermi surface on which the induced gap is much smaller than $T_c$, so that the temperature dependence of $λ_b$ is always different than that of $λ_a$. Experimental observations of the of the $ab$ anisotropy show nearly identical temperature dependences. The main result of our paper, then, is that a simple proximity model in which the pairing interaction is localized to the planes, and the planes are coherently coupled to the chains cannot account for the superfluid on the chains.

cond-mat↗

arXiv1995

Critical Josephson Current in a Model Pb/YBa_2Cu_3O_7 Junction

In this article we consider a simple model for a c--axis Pb/YBa_2Cu_3O_{7-δ} Josephson junction. The observation of a nonzero current in such a junction by Sun et al. [A. G. Sun, D. A. Gajewski, M. B. Maple, R. C. Dynes, Phys. Rev. Lett. 72, 2267 (1994)] has been taken as evidence against d--wave superconductivity in YBa_2Cu_3O_{7-δ}. We suggest, however, that the pairing interaction in the CuO_2 planes may well be d--wave but that the CuO chains destroy the tetragonal symmetry of the system. We examine two ways in which this happens. In a simple model of an incoherent junction, the chains distort the superconducting condensate away from d_{x^2-y^2} symmetry. In a specular junction the chains destroy the tetragonal symmetry of the tunneling matrix element. In either case, the loss of tetragonal symmetry results in a finite Josephson current. Our calculated values of the critical current for specular junctions are in good agreement with the results of Sun and co-workers.

cond-mat↗

arXiv1994

Density of states of a layered S/N d-wave superconductor

We calculate the density of states of a layered superconductor in which there are two layers per unit cell. One of the layers contains a d-wave pairing interaction while the other is a normal metal. The goal of this article is to understand how the d-wave behaviour of the system is modified by the coupling between the layer-types. This coupling takes the form of coherent, single particle tunneling along the c-axis. We find that there are two physically different limits of behaviour, which depend on the relative locations of the Fermi surfaces of the two layer-types. We also discuss the interference between the interlayer coupling and pairing interaction and we find that this interference leads to features in the density of states.

supr-con↗