arXiv subjects

E. Schachinger

Publications and source records attributed to E. Schachinger.

33 records · Page 2Linked to original sources

arXiv2004

On the Relation between Optical Conductivity and Quasiparticle Dynamics: Boson Structures

An extended Drude form is often used to analyze optical data in terms of an optical scattering rate and renormalized mass corresponding, respectively, to the real and imaginary part of the memory function. We study the relationship between memory function and quasiparticle self energy for an isotropic system. We emphasize particularly boson signatures. We find it useful to introduce a new auxiliary model scattering rate and its Kramers-Kronig transform determined solely from optics which are much closer to the self energy than is the memory function itself in the normal state. In the superconducting state the simplification fails because the quasiparticle density of states acquires an essential energy dependence.

cond-mat.supr-con↗

arXiv2004

Optical conductivity in non-equilibrium d-wave superconductors

We consider the optical conductivity of a d-wave BCS superconductor in the presence of a non-equilibrium distribution of excess quasiparticles. Two different simplified models used in the past for the s-wave case are considered and results compared. In the $T^\ast$-model of Parker the excess quasiparticles are assumed to be in a thermal distribution at some temperature $T^\ast$ larger than the equilibrium sample temperature. In the $μ^\ast$- model of Owen and Scalapino a chemical potential is introduced to accommodate the excess quasiparticles. Some of the results obtained are specific to the model, most are qualitatively similar in both.

cond-mat.supr-con↗

arXiv2004

Optical spectral weight distribution in d-wave superconductors

The distribution in frequency of optical spectral weight remaining under the real part of the optical conductivity in the superconducting state of a d-wave superconductor depends on impurity concentration, on the strength of the impurity potential as well as on temperature and there is some residual absorption even at T = 0. In BCS theory the important weight is confined to the microwave region if the scattering is sufficiently weak. In an Eliashberg formulation substantial additional weight is to be found in the incoherent, boson assisted background which falls in the infrared and is not significantly depleted by the formation of the condensate, although it is shifted as a result of the opening of a superconducting gap.

cond-mat.supr-con↗

arXiv2003

ARPES and optical renormalizations: phonons or spin fluctuations

Improved resolution in both, energy and momentum in ARPES-data has lead to the establishment of a definite energy scale in the dressed quasiparticle dispersion relations. The observed structure around 80 meV has been taken as evidence for coupling to phonons and has re-focused the debate about the mechanism of superconductivity in the cuprates. Here we address the relative merits of phonon as opposed to spin fluctuation mechanisms. Both possibilities are consistent with ARPES. On the other hand, when considerations are extended to infrared optical data, a spin fluctuation mechanism provides a more natural interpretation of the combined sets of data in Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212).

cond-mat.supr-con↗

arXiv2003

Residual absorption at zero temperature in d-wave superconductors

In a d-wave superconductor with elastic impurity scattering, not all the available optical spectral weight goes into the condensate at zero temperature, and this leads to residual absorption. We find that for a range of impurity parameters in the intermediate coupling regime between Born (weak) and unitary (strong) limit, significant oscillator strength remains which exhibits a cusp like behavior of the real part of the optical conductivity with upward curvature as a function of frequency, as well as a quasilinear temperature dependence of the superfluid density. The calculations offer an explanation of recent data on ortho-II YBa$_2$Cu$_3$O$_{6.5}$ which has been considered anomalous.

cond-mat.supr-con↗

arXiv2002

Application of an Extended Eliashberg Theory to High-T_c Cuprates

In recent years a unified phenomenological picture for the hole doped high-T_c cuprates has emerged for a spin and charge spectroscopy. Spectral anomalies have been interpreted as evidence of charge carrier coupling to a collective spin excitation present in the optical conductivity, in ARPES (angular resolved photoemission), and in tunneling data. These anomalies can be used to derive an approximate picture of a charge carrier-exchange boson interaction spectral density I^2 chi(omega) which is then be used within an extended Eliashberg formalism to analyze normal and superconducting properties of optimally doped and overdoped cuprates. This paper reviews recent developments and demonstrates the sometimes astonishing agreement between experiment and theoretical prediction.

cond-mat.supr-con↗

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

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

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↗

arXiv2001

Superconducting properties of Y_xLu_(1-x)Ni2B2C and La3Ni2B2N3: A comparison between experiment and Eliashberg theory

Specific heat and resistivity measurements were performed on polycrystalline samples of the solid-solution Y_xLu_(1-x)Ni2B2C in order to determine thermodynamic properties such as the specific-heat difference, the thermodynamic critical field H_c(T), as well as the upper critical field H_{c2}(T). These properties were analyzed within the Eliashberg theory including anisotropy effects, yielding electron-phonon coupling anisotropy parameters ranging beween 0.02 and 0.03 for the whole series, and Fermi velocity anisotropy parameters of = 0.245-0.3. Excellent agreement between theory and experiment was achieved for these parameters, the Sommerfeld constant and model phonon spectra determined from specific heat measurements. An analysis of the previously investigated boronitride La3Ni2B2N3 for comparison revealed the electron-phonon anisotropy to be of great significance in describing its thermodynamic properties and the calculations yielded \~ 0.08 and ~ 0.245. The T_c-behavior within the series Y_xLu_(1-x)Ni2B2C is discussed in terms of coupling and impurity effects, and the density of states at the Fermi level N(0).

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

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

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

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.

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