arXiv subjects

J. P. Carbotte

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

At least 145 records · Page 8Linked to original sources

arXiv2001

Microwave conductivity of YBa$_2$Cu$_3$O$_{6.99}$ including inelastic scattering

The fluctuation spectrum responsible for the inelastic scattering in YBa$_2$Cu$_3$O$_{6.99}$ which was recently determined from consideration of the in-plane optical conductivity in the infrared, is used to calculate the temperature dependence of the microwave conductivity at several measured frequencies. Reasonable overall agreement can only be achieved if, in addition, some impurity scattering is included within a model potential intermediate between weak (Born) and strong (unitary) limit.

cond-mat.supr-con↗

arXiv2001

An effective Hamiltonian for phase fluctuations on a lattice: an extended XY model

We derive an effective Hamiltonian for phase fluctuations in an s-wave superconductor starting from the attractive Hubbard model on a square lattice. In contrast to the common assumption, we find that the effective Hamiltonian is not the usual XY model but is of an extended XY type. This extended feature is robust and leads to essential corrections in understanding phase fluctuations on a lattice. The effective coupling in the Hamiltonian varies significantly with temperature.

cond-mat.supr-con↗

arXiv2001

Sum rule for the optical scattering rates

An important quantity in electronic systems is the quasiparticle scattering rate (QPSR). A related optical scattering rate (OSR) is routinely extracted from optical data, and, while it is not the same as the QPSR, it nevertheless displays many of the same features. We consider a sum rule which applies to the area under a closely related quantity, almost equal to the OSR in the low energy region. We focus on the readjustment caused by, for example, a quasiparticle density of states change due to the superconducting transition. Unfortunately, no general statement about mechanism can be made solely on the energy scale in which the spectral weight readjustment on the OSR occurs.

cond-mat.supr-con↗

arXiv2001

C-axis Response of a High-T_{c} Superconductor with D-Density Wave Order

The microscopic origin of the pseudogap state which exists in the underdoped cuprates remains unknown. The $c$-axis properties in the pseudogap regime are particularly anomalous. We use a recently proposed model of a $d$-density wave which leads to staggered currents and the doubling of the unit cell to investigate the $c$-axis kinetic energy and the optical sum rule. The density of states expected in the model is also considered.

cond-mat.supr-con↗

arXiv2001

Electron - Phonon Superconductivity

In this chapter we review the essentials of conventional electron-phonon superconductivity, as provided through the Migdal-Eliashberg formalism. Signatures of the electron-phonon interaction, as they appear in conventional superconductors, are described in detail. An appendix briefly describes more recent attempts to understand the electron-phonon interaction for all coupling strengths.

cond-mat.supr-con↗

arXiv2001

C-axis Optical Sum Rule in Josephson Coupled Vortex State

Observed violations of the $c$-axis optical sum rule can give important information on deviations from in-plane Fermi liquid behavior and on the nature of interlayer coupling between adjacent copper oxide planes. Application of a magnetic field perpendicular to these planes is another way to probe in-plane dynamics. We find that the optical sum rule is considerably modified in the presence of the $c$-axis magnetic field. Interlayer correlation of pancake vortices is involved in the sum rule modification; however, details of the vortex distribution in the plane are less important.

cond-mat.supr-con↗

arXiv2001

Quasiparticle States around a Nonmagnetic Impurity in D-Density-Wave State of High-$T_c$ Cuprates

Recently Chakravarty {\em et al.} proposed an ordered $d$-density wave (DDW) state as an explanation of the pseudogap phase in underdoped high-temperature cuprates. We study the competition between the DDW and superconducting ordering based on an effective mean-field Hamiltonian. We are mainly concerned with the effect of the DDW ordering on the electronic state around a single nonmagnetic impurity. We find that a single subgap resonance peak appears in the local density of state around the impurity. In the unitary limit, the position of this resonance peak is always located at $E_r=-μ$ with respect to the Fermi energy. This result is dramatically different from the case of the pure superconducting state for which the impurity resonant energy is approximately pinned at the Fermi level. This can be used to probe the existence of the DDW ordering in cuprates.

cond-mat.supr-con↗

arXiv2001

Comparison of in- and out-of-plane charge dynamics in YBa_2Cu_3O_{6.95}

The in-plane optical conductivity has been successfully employed to obtain information about the coupling of the charge carriers to the spin degrees of freedom in the high T_c oxides. We investigate how this inelastic scattering affects the out-of-plane charge dynamics. We consider both coherent (in-plane momentum is conserved) and incoherent (no momentum conservation) c-axis transfers and find that the two cases give quite a distinct c-axis conductivity as a function of energy $ω$. Comparison of our theoretical calculations with the available data does not allow a definitive conclusion, but a momentum dependent coherent matrix element characteristic of the CuO_2 chemistry is favored with the possibility of a subdominant incoherent contribution.

cond-mat.supr-con↗

arXiv2001

Magnetic resonance at 41 meV and charge dynamics in YBa_2Cu_3O_6.95

We report an Eliashberg analysis of the electron dynamics in YBa_2Cu_3O_6.95. The magnetic resonance at 41 meV couples to charge carriers and defines the characteristic shape in energy of the scattering rate τ^{-1}(T,ω) which allows us to construct the charge-spin spectral density I^2χ(ω,T) at temperature T. The T dependence of the weight under the resonance peak in I^2χ(ω,T) agrees with experiment as does that of the London penetration depth and of the microwave conductivity. Als, at T=0 condensation energy, the fractional oscillator strength in the condensate, and the ratio of gap to critical temperature agree well with the data.

cond-mat.supr-con↗

arXiv2000

Signatures of Spin Degrees of Freedom in Charge Dynamics of YBa_2Cu_3O_6.95

Charge carrier-spin excitation spectral densities constructed from optical data and varying with temperature are used to calculate various properties of the charge carrier dynamics. Temperature (T) variations of such properties are sensitive to the changes in the spectral density brought about by the growth in the 41 meV spin resonance as T is lowered. The microwave conductivity and the thermal conductivity are stressed and comparison with experiment is given. The imaginary part of the conductivity as a function of frequency ωis also featured.

cond-mat.supr-con↗

arXiv2000

Interlayer coupling and the c-axis quasiparticle transport in high-$T_{c}$ cuprates

The c-axis quasiparticle conductivity shows different behavior depending on the nature of the interlayer coupling. For coherent coupling with a constant hopping amplitude $t_{\perp}$, the conductivity at zero frequency and zero temperature $σ(0,0)$ depends on the direction of the magnetic field, but it does not for angle-dependent hopping $t(ϕ)$ which removes the contribution of the nodal quasiparticles. For incoherent coupling, the conductivity is also independent of field direction and changes only when paramagnetic effects are included. The conductivity sum rule can be used to determine the admixture of coherent to incoherent coupling. The value of $σ(0,0)$ can be dominated by $t_{\perp}$ while at the same time $t(ϕ)$ dominates the temperature dependence of the superfluid density.

cond-mat.supr-con↗

arXiv2000

Effects of an in-plane magnetic field on c-axis sum rule and superfluid density in high-$T_{c}$ cuprates

In layered cuprates, the application of an in-plane magnetic field $({\bf H})$ changes the c-axis optical sum rule and superfluid density $ρ_{s}$. For pure incoherent c-axis coupling, ${\bf H}$ has no effect on either quantities but it does if an additional coherent component is present. For the coherent contribution, different characteristic variations on ${\bf H}$ and on temperature result from the constant part $(t_{\perp})$ of the hopping matrix element and from the part $(t_ϕ)$ which has zero on the diagonal of the Brillouin zone. Only the constant part $(t_{\perp})$ leads to a dependence on the direction of ${\bf H}$ as well as on its magnitude.

cond-mat.supr-con↗

arXiv2000

Conductivity sum rule, implication for in-plane dynamics and c-axis response

Recently observed $c$-axis optical sum rule violations indicate non-Fermi liquid in-plane behavior. For coherent $c$-axis coupling, the observed flat, nearly frequency independent $c$-axis conductivity $σ_{1}(ω)$ implies a large in-plane scattering rate $Γ$ around $(0,π)$ and therefore any pseudogap that might form at low frequency in the normal state will be smeared. On the other hand incoherent $c$-axis coupling places no restriction on the value of $Γ$ and gives a more consistent picture of the observed sum rule violation which, we find in some cases, can be less than half.

cond-mat.supr-con↗

arXiv2000

Coupling to spin fluctuations from conductivity scattering rates

A recent analysis of optical conductivity data which has provided strong evidence for coupling of the charge carriers to the 41 meV spin resonance, seen in the superconducting state of optimally doped Y123, is extended to other systems. We find that the corresponding spin resonance is considerably broader in Tl2201 and Y124 than it is in Bi2212. On the other hand, in overdoped Tl2201 with T_c = 23 K no resonance forms. A more specific interpretation of the optical data in terms of a spin fluctuation exchange mechanism of superconductivity gives an underlying effective spectral density which is strongly temperature dependent and which contains important feedback effects that further stabilize superconductivity as the temperature is reduced.

cond-mat.supr-con↗

arXiv2000

Conductivity sum rule; comparison of coherent and incoherent c-axis coupling

We calculate the $c$-axis kinetic energy difference between normal and superconducting state for coherent and for incoherent interlayer coupling between CuO$_{2}$ planes. For coherent coupling the ratio of the missing conductivity spectral weight to the superfluid density is equal to one and there is no violation of the conventional sum rule, but for the incoherent case we find it is always greater than one whatever the nature of the impurity potential may be. To model more explicitly YBa$_{2}$Cu$_{3}$O$_{7-x}$ around optimum doping, which is found to obey the sum rule, we consider a plane-chain model and show that the sum rule still applies. A violation of the sum rule of either sign is found even for coherent coupling when the in-plane density of electronic states depends on energy on a scale of the order of the gap.

cond-mat.supr-con↗

arXiv2000

Doppler shift on local density of states and local impurity scattering in the vortex state

The vortex state thermal and transport properties of the high T_c copper oxides can be understood in a d-wave gap model and are dominated by the extended quasiparticle states that exist along the nodal directions in momentum space. The Doppler shift on these states due to the circulating supercurrents around the vortex core, introduces new van Hove ridges into the energy dependent local density of states (LDOS) as a function of distance in the region between cores. We emphasize the topology of these ridges and the effect on them of local impurity scattering in Born and unitary limit. We treat possible orthorhombicity. Effective local scttering rates are also obtaines.

cond-mat.supr-con↗

arXiv1999

Ultrasonic attenuation in clean d-wave superconductors

We consider the attenuation of longitudinal ultrasonic waves in a clean two-dimensional d-wave superconductor. We show that the attenuation coefficient is linear in temperature at low temperatures for all in-plane directions of the propagation of the ultrasound, and that the coefficient of the linear term can be used to determine the parameters crucial for the low temperature transport in these compounds.

cond-mat.supr-con↗

arXiv1999

Anisotropic thermodynamics of d-wave superconductors in the vortex state

We show that the density of states and the thermodynamic properties of a 2D d-wave superconductor in the vortex state with applied magnetic field $\bf H$ in the plane depend on the angle between $\bf H$ and the order parameter nodes. Within a semiclassical treatment of the extended quasiparticle states, we obtain fourfold oscillations of the specific heat, measurement of which provides a simple probe of gap symmetry. The frequency dependence of the density of states and the temperature dependence of thermodynamic properties obey different power laws for field in the nodal and anti-nodal direction. The fourfold pattern is changed to twofold when orthorhombicity is considered.

cond-mat.supr-con↗