arXiv subjects

J. P. Carbotte

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

At least 163 records · Page 9Linked to original sources

arXiv1999

Magnetic field as a probe of gap energy scales in YBCO

Among high T_c materials, the YBCO (YBa_2Cu_3O_{7-x}) compounds are special since they have superconducting chains as well as planes. We show that a discontinuity in the density of states as a function of magnetic field may appear at a new energy scale, characteristic of the chain and distinct from that set by the d wave gap. This is observable in experimental studies of the thermodynamical properties of these systems, such as the specific heat.

cond-mat.supr-con↗

arXiv1999

Coupling strength of charge carriers to spin fluctuations in high-temperature superconductors

In conventional superconductors, the most direct evidence of the mechanism responsible for superconductivity comes from tunneling experiments in which a clear image of the electron-phonon interaction is revealed. The observed structure in the current voltage characteristics at the phonon energies can be used to measure, through inversion of the Eliashberg equations, the electron phonon spectral density α^2F(ω). The coherence length in conventional materials is long and the tunneling process probes several atomic layers into the bulk of the superconductor. On the contrary, in the high-T_c oxides, particularly for c-axis tunneling, the coherence length can be quite short and in an optical experiment or in neutron scattering experiments the bulk of the sample is probed. Therefore, these spectroscopies become the methods of choice for the investigation of mechanisms of high-T_c superconductivity. Accurate reflectance measurements in the infrared range and precise polarized neutron scattering data are available for a variety of oxides. In this paper we show that conducting carriers studied by means of infrared spectroscopy reveal strong coupling to a resonance structure in the spectrum of spin fluctuations examined with neutron scattering. The coupling strength inferred from experiment is sufficient to account for high values of T_c which signals the prominent role of spin excitations in the superconductivity of oxides.

cond-mat.supr-con↗

arXiv1998

Anisotropic thermodynamics and $T/\sqrt H$ scaling of d-wave superconductors in the vortex state

The density of states of a 2D d-wave superconductor in the vortex state with applied magnetic field $\bf H$ in the plane is shown to exhibit fourfold oscillations as a function of the angle of the field with respect to the crystal axes. We find further that the frequency dependence of the density of states and the temperature dependence of transport coefficients obey different power laws, thus leading to different magnetic scaling functions, for field in the nodal and anti-nodal direction. We discuss the consequences of this anisotropy for measurements of the specific heat.

cond-mat.supr-con↗

arXiv1998

Effect of anisotropy on universal transport in unconventional superconductors

We investigate the universal electronic transport for a mixed $d_{x^2-y^2}$+s-wave superconductor in the presence of an anisotropic elliptical Fermi surface. Similar to the universal low-temperature transport predicted in a $d_{x^2-y^2}$-wave superconductor with a circular Fermi surface, anisotropic universal features are found in the low-temperature microwave conductivity, and thermal conductivity in the anisotropic system. The effects of anisotropy on the penetration depth, impurity induced $T_c$ suppression, and the zero-frequency density of states are also considered. While a small amount of anisotropy can lead to a strong suppression of the effective scattering rate and hence the density of states at zero frequency, experimental data suggests that large effects are restored by a negative $s$-component gap admixture.

cond-mat.supr-con↗

arXiv1998

Absence of anisotropic universal transport in YBCO

There exists significant in-plane anisotropy between $a$ and $b$ axis for various properties in YBCO. However recent thermal conductivity measurement by Chiao et al. which confirms previous microwave conductivity measurement by Zhang et al., shows no obvious anisotropy in the context of universal transport. We give a possible explanation of why the anisotropy is seen in most properties but not seen in the universal transport.

cond-mat.supr-con↗

arXiv1998

Quasiparticle transport in the vortex state of d-wave superconductors

We calculate the magnetic field and temperature dependence of the Raman response, superfluid density and the NMR relaxation rate in the vortex state of a d-wave superconductor arising from the Doppler energy shift of extended quasiparticle states. Our results are valid both at low temperatures, where we observe scaling with variable $TH^{-1/2}$ and obtain explicit form of the scaling functions, and beyond this region. We derive a universal frequency dependent scaling relation for the Raman response, and discuss the breakdown of the single relaxation rate approach to NMR response.

cond-mat.supr-con↗

arXiv1998

Inversion of K$_3$C$_{60}$ Reflectance Data

We outline a procedure for obtaining the electron-phonon spectral density by inversion of optical conductivity data, a process very similar in spirit to the McMillan-Rowell inversion of tunelling data. We assume both electron-impurity (elastic) and electron-phonon (inelastic) scattering processes. This procedure has the advantage that it can be utilized in the normal state. Furthermore, a very good qualitative result can be obtained explicitly, without iteration. We illustrate this technique on recently acquired far-infrared data in K$_3$C$_{60}$. We show that the electron-phonon interaction is most likely responsible for superconductivity in these materials.

cond-mat↗

arXiv1997

The role of intermediate layers in the c-axis conductivity of layered superconductors

A simplified model of c axis transport in the high T_c superconductors is presented. Expressions are found for the c axis optical conductivity, the d.c. resistivity, and the c axis penetration depth. Within the framework of this model, the pseudogap in the optical conductivity arises naturally as a result of the layered band structure of the high T_c materials. We discuss the occurence of the pseudogap in terms of three parameters: a band gap Delta_{ps}, a temperature dependent scattering rate Gamma(T), and the strength of the interlayer coupling t_{perp}. We are also able to find analytic expressions for the d.c. conductivity and the low temperature penetration depth in terms of these three parameters. This work is an attempt to present a simple, unified picture of c axis properties in the high T_c cuprates.

cond-mat.supr-con↗

arXiv1997

Quasiparticle Lifetimes and the Conductivity Scattering Rate

We compute the single-particle inverse lifetime, along with the conductivity-derived scattering rate, for a metallic system in an s-wave superconducting state. When both electron-phonon and electron-impurity scattering are included, we find that while these scattering rates are in qualitative agreement, in general quantitative agreement is lacking. We also derive results for the quasiparticle lifetime within the BCS framework with impurity scattering, which makes it clear that impurity scattering is suppressed for electrons near the Fermi surface in the superconducting state.

cond-mat↗

arXiv1997

Universal zero-frequency Raman slope in a d-wave superconductor

It is known that for an unconventional superconductor with nodes in the gap, the in-plane microwave or dc conductivity saturates at low temperatures to a universal value independent of the impurity concentration. We demonstrate that a similar feature can be accessed using channel-dependent Raman scattering. It is found that, for a $d_{x^2-y^2}$-wave superconductor, the slope of low-temperature Raman intensity at zero frequency is universal in the $A_{1g}$ and $B_{2g}$ channels, but not in the $B_{1g}$ channel. Moreover, as opposed to the microwave conductivity, universal Raman slopes are sensitive not only to the existence of a node, but also to different pairing states and should allow one to distinguish between such pairing states.

cond-mat.supr-con↗

arXiv1997

Normal-state magnetic susceptibility in a bilayer cuprate

The magnetic susceptibility of high-T_c superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in c-axis optical conductivity on underdoped samples, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence suggest that the mechanism governing the c-axis optical pseudogap is different from that for the $a-b$ plane magnetic pseudogap.

cond-mat↗

arXiv1997

Effects of Interlayer Interaction on the Superconducting State in YBCO

For a two layer system in a weak coupling BCS formalism any interlayer interaction, regardless of its sign, enhances the critical temperature. The sign has an effect upon the relative phase of the order parameter in each of the two planes but not upon its magnitude. When one of the planes has a dispersion consistent with CuO chains and no intrinsic pairing interaction there is both an enhancement of the critical temperature and an s+d mixing in both layers as the interlayer interaction is increased. The magnetic penetration depth, c-axis Josephson tunneling, density of states and Knight shift are calculated for several sets of model parameters.

cond-mat.supr-con↗

arXiv1997

C-axis Transport in Bilayer Cuprates and Relation to Pseudogap

We consider the effect of interband transitions on the c-axis conductivity, DC resistivity, and thermal conductivity in a plane-chain bilayer model of a cuprate. The relation between the c-axis resistivity and thermal conductivity is governed by the Wiedemann-Franz law. When the perpendicular hopping matrix element between chain and plane ($t_\perp$) is small, the c-axis DC resistivity shows a characteristic upturn as the temperature is lowered and the infrared conductivity develops a pseudogap. As $t_\perp$ is increased, intraband transitions start to dominate and a more conventional response is obtained. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity.

cond-mat↗

arXiv1997

The c axis optical conductivity of layered systems in the superconducting state

In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)] in the high T_c superconductors, in the superconducting state. The basic premise of this work is that electrons travelling along the c axis between adjacent CuO_2 layers must pass through several intervening layers. In earlier work we found that, for weak inter-layer coupling, it is preferable for electrons to travel along the c axis by making a series of interband transitions rather than to stay within a single (and very narrow) band. Moreover, we found that many of the properties of the normal state optical conductivity, including the pseudogap could be explained by interband transitions. In this work we examine the effect of superconductivity on the interband conductivity. We find that, while the onset of superconductivity is clearly evident in the spectrum, there is no clear signature of the symmetry of the superconducting order parameter.

cond-mat.supr-con↗

arXiv1996

C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap

We investigate the Raman spectra in the geometry where both incident and scattered photon polarizations are parallel to the $\hat{z}$-direction, for a plane-chain bilayer coupled via a single-particle tunneling $t_\perp$. The Raman vertex is derived in the tight-binding limit and in the absence of Coulomb screening, the Raman intensity can be separated into intraband ($\propto t_\perp^4$) and interband ($\propto t_\perp^2$) transitions. In the small-$t_\perp$ limit, the interband part dominates and a pseudogap will appear as it does in the conductivity. Coulomb interactions bring in a two-particle coupling and result in the breakdown of intra- and interband separation. Nevertheless, when $t_\perp$ is small, the Coulomb screening ($\propto t_\perp^4$) has little effect on the intensity to which the unscreened interband transitions contribute most. In general, the total Raman spectra are strongly dependent on the magnitude of $t_\perp$.

cond-mat↗

arXiv1996

Triplanar Model for the Gap and Penetration Depth in YBCO

YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2 bilayer with a CuO plane of chains in between. Starting with a model of isolated planes coupled through a transverse matrix element, we consider the possibility of intra as well as interplane pairing within a nearly antiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS equations for the gap exhibit orthorhombic symmetry with s- as well as d-wave contributions. The temperature dependence and a-b in plane anisotropy of the resulting penetration depth is discussed and compared with experiment.

cond-mat.supr-con↗

arXiv1996

Effect of Interband Transitions on the c axis Penetration Depth of Layered Superconductors

The electromagnetic response of a system with two planes per unit cell involves, in addition to the usual intraband contribution, an added interband term. These transitions affect the temperature dependence and the magnitude of the zero temperature c-axis penetration depth. When the interplane hopping is sufficiently small, the interband transitions dominate the low temperature behaviour of the penetration depth which then does not reflect the linear temperature dependence of the intraband term and in comparison becomes quite flat even for a d-wave gap. It is in this regime that the pseudogap was found in our previous normal state calculations of the c-axis conductivity, and the effects are connected.

cond-mat.supr-con↗

arXiv1996

Interband effects in the c-axis optical conductivity in YBaCuO

The normal state optical conductivity is calculated for a layered metal with two layers per unit cell coupled through a transverse hopping matrix element $t_\perp$. The optical response involves an interband term in addition to the more familiar intraband term which leads to the usual Drude form. The interband term is only weakly temperature dependent, even for an inelastic scattering rate which is linear in T. It gives a $c$-axis response which extends in frequency over the entire band width although there can be structure on this energy scale which reflects details of the electronic structure. In particular, at low energy, the $c$-axis response can develop a gap or pseudogap as the temperature is lowered. At high temperature, a Drude response will be seen only if the intraband transitions, which are of order $t_\perp^4$, become important compared with the interband transitions which are of order $t_\perp^2$.

cond-mat↗