arXiv subjects

M. Eschrig

Publications and source records attributed to M. Eschrig.

50 records · Page 3Linked to original sources

arXiv2002

The neutron resonance in the cuprates and its effect on fermionic excitations

We argue that the exciton scenario for the magnetic resonance in the cuprate superconductors yields a small spectral weight of the resonance, in agreement with experiment. We show that the small weight is related to its concentration in a small region of momentum and energy. Despite this, we find that a large fermionic self-energy can indeed be generated by a resonance with such properties, i.e., the scattering from the resonance substantially affects the electronic properties of the cuprates below $T_c$.

cond-mat.supr-con↗

arXiv2002

What the resonance peak does

High-resolution angle-resolved photoemission with variable excitation energies is used to disentangle bilayer splitting effects and intrinsic (self-energy) effects in the electronic spectral function near the ($π$,0)-point of differently doped (Pb,Bi)$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. In contrast to overdoped samples, where intrinsic effects at the ($π$,0)point are virtually absent, we find in underdoped samples \textit{intrinsic} effects in the superconducting-state ($π$,0) spectra of the antibonding band. This intrinsic effect is present only below the critical temperature and weakens considerably with doping. Our results give strong support for models which involve a strong coupling of electronic excitations with the resonance mode seen in inelastic neutron scattering experiments.

cond-mat.supr-con↗

arXiv2002

Dispersion Anomalies in Bilayer Cuprates and the Odd Symmetry of the Magnetic Resonance

We present a theoretical model which accounts for recent angle resolved photoemission data in bilayer cuprate superconductors. Lineshapes and dispersions of the various bonding and antibonding features in the spectra are quantitatively reproduced. The observed dispersion anomalies are consistent with the interaction of electrons with a bosonic mode which is odd with respect to the layer indices, a unique property of the magnetic resonance observed by inelastic neutron scattering.

cond-mat.supr-con↗

arXiv2001

Spatially resolved electronic structure inside and outside the vortex core of a high temperature superconductor

One of the puzzling aspects of high temperature superconductors is the prevalence of magnetism in the normal state and the persistence of superconductivity in very high magnetic fields. Generally, superconductivity and magnetism are not compatible. But recent neutron scattering results indicate that antiferromagnetism can appear deep in the superconducting state in an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines each one representing a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. To address this question it is important to perform electronic structural studies with spatial resolution. Here we report on implementation of a high field NMR imaging experiment that allows spatial resolution of the electronic behavior both inside and outside the vortex cores. Outside we find strong antiferromagnetic fluctuations, and localized inside there are electronic states rather different from those found in conventional superconductors.

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arXiv2001

Vortex Core Structure and Dynamics in Layered Superconductors

We investigate the equilibrium and nonequilibrium properties of the core region of vortices in layered superconductors. We discuss the electronic structure of singly and doubly quantized vortices for both s-wave and d-wave pairing symmetry. We consider the intermediate clean regime, where the vortex-core bound states are broadened into resonances with a width comparable to or larger than the quantized energy level spacing, and calculate the response of a vortex core to an {\em a.c.} electromagnetic field for vortices that are pinned to a metallic defect. We concentrate on the case where the vortex motion is nonstationary and can be treated by linear response theory. The response of the order parameter, impurity self energy, induced fields and currents are obtained by a self-consistent calculation of the distribution functions and the excitation spectrum. We then obtain the dynamical conductivity, spatially resolved in the region of the core, for external frequencies in the range, $0.1Δ< \hbarω\lsim 3Δ$. We also calculate the dynamically induced charge distribution in the vicinity of the core. This charge density is related to the nonequilibrium response of the bound states and collective mode, and dominates the electromagnetic response of the vortex core.

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arXiv2001

Momentum Distribution Curves in the Superconducting State

We demonstrate that the E vs. k dispersion in the superconducting state extracted from momentum distribution curves differs qualitatively from the traditional dispersion extracted from energy distribution curves. This occurs because of a combination of many-body effects and the presence of an energy gap, along with the associated coherence factors. Analysis of such MDC dispersions can give important information on the microscopics of high temperature superconductors.

cond-mat.supr-con↗

arXiv2001

Influence of vortices on the magnetic resonance in cuprate superconductors

We investigate several theoretical possibilities for the suppression in a c-axis magnetic field of the magnetic resonance recently observed in inelastic neutron scattering experiments on YBCO_{6.6}. We find that neither the Doppler shift of the quasiparticle states caused by supercurrents outside the vortex core, nor an assumed spatially uniform suppression of the coherence factors or spectral gap due to the applied field, can account for the observed effect. In contrast, suppressing the gap or the coherence factors in the vortex core to zero is consistent with the data. We demonstrate that an even simpler description of the data can be achieved by assuming that the resonance is not supported within an effective radius xi_{eff} around each vortex, where xi_{eff} is the sum of the superconducting and spin-spin correlation lengths. We use this simple idea to predict the doping dependence of the field suppression.

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arXiv2001

Mixed-Parity Superconductivity in Sr2RuO4

We show that in Sr2RuO4 the Fermi surface geometry as inferred from angle resolved photoemission experiments has important implications for a pairing interaction dominated by incommensurate, strongly anisotropic, spin fluctuations. For a spin fluctuation spectrum consistent with inelastic neutron scattering experiments the system is close to an accidental degeneracy between even parity spin singlet and odd parity spin triplet channels. This opens the possibility of a mixed parity order parameter state in Sr2RuO4. We determine the stable and metastable order parameter phases at low temperatures and discuss especially phases with order parameter nodes.

cond-mat.supr-con↗

arXiv2000

The neutron resonance: modeling photoemission and tunneling data in the superconducting state of Bi_2Sr_2CaCu_2O_{8+d}

Motivated by neutron scattering data, we develop a model of electrons interacting with a magnetic resonance and use it to analyze angle resolved photoemission (ARPES) and tunneling data in the superconducting state of Bi_2Sr_2CaCu_2O_{8+d}. We not only can explain the peak-dip-hump structure observed near the (pi,0) point, and its particle-hole asymmetry as seen in SIN tunneling spectra, but also its evolution throughout the Brillouin zone, including a velocity `kink' near the d-wave node.

cond-mat.supr-con↗

arXiv1999

Distribution functions in non-equilibrium theory of superconductivity and Andreev spectroscopy in unconventional superconductors

We present a new theoretical formulation of non-equilibrium superconducting phenomena, including singlet and triplet pairing. We start from the general Keldysh-Nambu-Gor'kov Green's functions in the quasiclassical approximation and represent them in terms of 2x2 spin-matrix coherence functions and distribution functions for particle-type and hole-type excitations. The resulting transport equations for the distribution functions may be interpreted as a generalization to the superconducting state of Landau's transport equation for the normal Fermi liquid of conduction electrons. The equations are well suited for numerical simulations of dynamical phenomena. Using our formulation we solve an open problem in quasiclassical theory of superconductivity, the derivation of an explicit representation of Zaitsev's nonlinear boundary conditions [A.V. Zaitsev, JETP 59, 1015 (1984)] at surfaces and interfaces. These boundary conditions include non-equilibrium phenomena and spin singlet and triplet unconventional pairing. We eliminate spurious solutions as well as numerical stability problems present in the original formulation. Finally, we formulate the Andreev scattering problem at interfaces in terms of the introduced distribution functions and present a theoretical analysis for the study of time reversal symmetry breaking states in unconventional superconductors via Andreev spectroscopy experiments at N-S interfaces with finite transmission. We include impurity scattering self consistently.

cond-mat.supr-con↗

arXiv1999

Superconducting Fluctuation Effects on the Electron Spin Susceptibility in YBa_2Cu_3O_{6.95}

The electronic spin susceptibility of YBa_2Cu_3O_{6.95} has been measured with high precision up to 24 Tesla with 17O nuclear magnetic resonance. Its temperature dependence can be accounted for by superconducting fluctuations that result in a smooth crossover from the normal to the vortex liquid state. A magnetic field-temperature phase diagram for this crossover has been established having strong upward curvature.

cond-mat.supr-con↗

arXiv1999

Comment on "Magnetic Field Independence of the Spin Gap in YBa_2Cu_3O_{7-d}" by K. Gorny et al., Phys. Rev. Lett. 82, 177 (1999)

In a recent Letter of Gorny et al. it was reported that the spin-lattice relaxation rate in YBCO_{7-d} is magnetic field independent. These authors point out that their results are inconsistent with prior reports of Borsa et al., Carretta et al., and Mitrovic et al., which concluded that there is a small but significant magnetic field dependence. In this comment we propose an explanation for this discrepancy.

cond-mat.supr-con↗

arXiv1999

Effects of Strong Magnetic Fields on Pairing Fluctuations in High Temperature Superconductors

We present the theory for the effects of superconducting pairing fluctuations on the nuclear spin-lattice relaxation rate, 1/T_1, and the NMR Knight shift for layered superconductors in high magnetic fields. These results can be used to clarify the origin of the pseudogap in high-T_c cuprates, which has been attributed to spin fluctuations as well as pairing fluctuations. We present theoretical results for s-wave and d-wave pairing fluctuations and show that recent experiments in optimally doped YBa_2Cu_3O_{7-d} are described by d-wave pairing fluctuations. In addition, we show that the orthorhombic distortion in YBa_2Cu_3O_{7-d} accounts for an experimentally observed discrepancy between 1/T_1 obtained by NQR and NMR at low field. We propose an NMR experiment to distinguish a fluctuating s-wave order parameter from a fluctuating strongly anisotropic order parameter, which may be applied to the system Nd_{2-x}Ce_xCuO_{4-d} and possibly other layered superconductors.

cond-mat.supr-con↗

arXiv1999

Superconducting Fluctuation Effects on the Spin-Lattice Relaxation Rate in YBa_2Cu_3O_{6.95}

We report 63Cu(2) spin-lattice relaxation rate measurements of YBa_2Cu_3O_{6.95} in magnetic fields from 2.1 T to 27.3 T obtained from 17O(2,3) nuclear magnetic resonance spin-spin relaxation. For T < 120 K, the spin-lattice rate increases with increasing magnetic field. We identify this magnetic field dependence with the change in the low-energy spectral weight originating from d-wave pairing fluctuation corrections to the density of states.

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