5,070 research outputs found
Frequency Evolution of Neutron Peaks Below Tc: Commensurate and Incommensurate Structure in LaSrCuO and YBaCuO
We study the evolution of the neutron cross-section with variable frequency
and fixed below in two different cuprate families. Our
calculations, which predominantly probe the role of d-wave pairing, lead to
generic features, independent of Fermi surface shapes. Among our findings,
reasonably consistent with experiment, are (i) for near the gap
energy , both optimal {LaSrCuO} and slightly underdoped YBCO exhibit
(comparably) incommensurate peaks (ii) peak sharpening below is seen in
{LaSrCuO}, (iii) quite generically, a frequency evolution from incommensurate
to commensurate and then back to incommensurate structure is found with
increasing . Due to their narrow regime of stability,
commensurate peaks in {LaSrCuO} should be extremely difficult to observe.Comment: RevTex 5pages, 4figures; Manuscript rewritten, figures revised, and
direct comparisons with experiments adde
Commensurate and Incommensurate Structure of the Neutron Cross Section in LaSrCuO and YBaCuO
We study the evolution of the d-wave neutron cross-section with variable
frequency \omega and fixed T (below and above Tc) in two different cuprate
families. The evolution from incommensurate to commensurate to incommensurate
peaks is rather generic within an RPA-like scheme. This behavior seems to be in
reasonable accord with experiments, and may help distinguish between this and
the "stripe" scenario.Comment: 2 pages; submitted to Proceedings of M2S-HTSC-V
Hole dynamics in an antiferromagnet across a deconfined quantum critical point
We study the effects of a small density of holes, delta, on a square lattice
antiferromagnet undergoing a continuous transition from a Neel state to a
valence bond solid at a deconfined quantum critical point. We argue that at
non-zero delta, it is likely that the critical point broadens into a non-Fermi
liquid `holon metal' phase with fractionalized excitations. The holon metal
phase is flanked on both sides by Fermi liquid states with Fermi surfaces
enclosing the usual Luttinger area. However the electronic quasiparticles carry
distinct quantum numbers in the two Fermi liquid phases, and consequently the
limit of the ratio A_F/delta, as delta tends to zero (where A_F is the area of
a hole pocket) has a factor of 2 discontinuity across the quantum critical
point of the insulator. We demonstrate that the electronic spectrum at this
transition is described by the `boundary' critical theory of an impurity
coupled to a 2+1 dimensional conformal field theory. We compute the finite
temperature quantum-critical electronic spectra and show that they resemble
"Fermi arc" spectra seen in recent photoemission experiments on the pseudogap
phase of the cuprates.Comment: 33 pages, 8 figures, Longer version of cond-mat/0611536, with
additional results for electron spectrum at non-zero temperatur
Superconducting Order Parameter in Bi-Layer Cuprates: Occurrence of Phase Shifts in Corner Junctions
We study the order parameter symmetry in bi-layer cuprates such as YBaCuO,
where interesting phase shifts have been observed in Josephson junctions.
Taking models which represent the measured spin fluctuation spectra of this
cuprate, as well as more general models of Coulomb correlation effects, we
classify the allowed symmetries and determine their associated physical
properties. phase shifts are shown to be a general consequence of
repulsive interactions, independent of whether a magnetic mechanism is
operative. While it is known to occur in d-states, this behavior can also be
associated with (orthorhombic) s-symmetry when the two sub-band gaps have
opposite phase. Implications for the magnitude of are discussed.Comment: 5 pages, RevTeX 3.0, 9 figures (available upon request
Quantum Criticality and Global Phase Diagram of Magnetic Heavy Fermions
Quantum criticality describes the collective fluctuations of matter
undergoing a second-order phase transition at zero temperature. It is being
discussed in a number of strongly correlated electron systems. A prototype case
occurs in the heavy fermion metals, in which antiferromagnetic quantum critical
points have been explicitly observed. Here, I address two types of
antiferromagnetic quantum critical points. In addition to the standard
description based on the fluctuations of the antiferromagnetic order, a local
quantum critical point is also considered. It contains inherently quantum modes
that are associated with a critical breakdown of the Kondo effect. Across such
a quantum critical point, there is a sudden collapse of a large Fermi surface
to a small one. I also consider the proximate antiferromagnetic and
paramagnetic phases, and these considerations lead to a global phase diagram.
Finally, I discuss the pertinent experiments on the antiferromagnetic heavy
fermions, briefly address the case of ferromagnetic heavy fermions, and outline
some directions for future studies.Comment: (v2) reference added, and typos corrected; (v1) 10 pages, 2 figures,
based on a plenary talk given at the International Conference on Quantum
Criticality and Novel Phases (QCNP09, Dresden
Magnetic interactions in the metallic phase of the copper oxides: A Fermi-liquid description
One of the central issues in the field of high-temperature superconductivity is whether the normal state can be described by Fermi-liquid theory. Recent photoemission experiments along with a growing body of Fermi-liquid-based theoretical work have provided some support for this viewpoint. However, one major concern in ascertaining the validity of a Fermi-liquid approach is whether the magnetic interactions in the metallic cuprates are sufficiently strong so as to undermine the usual Fermi-liquid description. In this paper we address this question by examining the nature of the magnetic interactions in the metallic state. These interactions, which are dominantly between the Cu spins, arise via the intermediating oxygenlike states. Since the oxygen character changes as the hole doping is increased, it is expected that the Cu-Cu interactions are doping sensitive and evolve away from their value in the insulating limit. We deduce these interactions within a physical picture in which the Cu d electrons are nearly localized and the oxygen bandwidth assumes a finite value. While our qualitative results are general, we use a 1/N expansion as a convenient theoretical tool. In the extended Hubbard Hamiltonian the exchange terms are evaluated at order (1/N2). Both superexchange (JS) and Ruderman-Kittel-Kasuya-Yosida interactions (JR) emerge on a similar footing. With increasing carrier concentration, JS decreases rapidly, while JR abruptly increases from zero. We find that, because of the reduction in the strength of the superexchange, there is an enhanced stability of the Fermi-liquid phase. The dynamical susceptibility is also calculated within this scheme and the consequences for NMR and neutron experiments are discussed elsewhere
Phenomenological description of the copper oxides as almost localized Fermi liquids
We demonstrate that a variety of normal-state properties of the cuprates are similar to what is observed in heavy-fermion metals, provided that the temperature or frequency are rescaled. This leads to a characteristic energy scale Tcoh 150 K and suggests that the non-Fermi-liquid-like behavior of the cuprates is related to the high-temperature breakup of coherence of a Fermi-liquid ground state. We propose a concrete physical picture for future theories and give predictions which can be tested
Comparison of spin dynamics in YBa2Cu3O7-δ and La2-xSrxCuO4: Effects of Fermi-surface geometry
Neutron experiments have indicated that the structure factor S(q,ω) for the two cuprates YBa2Cu3O7-δ and La2-xSrxCuO4 has a different q dependence. Commensurate peaks at (π/a,π/a) are observed in the former case, whereas clearly incommensurate peaks are seen in the latter, for metallic hole concentrations. We attribute this contrasting q dependence to differences in the Fermi-surface geometry, obtained in band-structure approaches, and (for the YBaCuO system) also corroborated by photoemission experiments. Using a large Coulomb-U, Fermi-liquid-based scheme, we present results for the q,ω and temperature dependence of the neutron cross section as well as for the temperature dependence of the NMR relaxation, in both cuprate families at various hole concentrations. When antiferromagnetic quasiparticle interactions of moderate strength are included, these calculations compare favorably with experiment. It should be stressed that the Fermi-surface shape must be accurately represented in both systems in order to produce this good agreement with the neutron data. We conclude that the close correspondence found, thus far, between band-structure-derived spin dynamics and the detailed fermiology of both cuprates provides support for Fermi-liquid-based schemes. Furthermore, this correspondence suggests important constraints which should be included in theoretical schemes ranging from the marginal and nearly antiferromagnetic Fermi liquid to more exotic scenarios for the normal state. Within this context, it is extremely important to determine the characteristic energy scales of the Fermi liquid. Comparison of our calculations with the measured energy scales of the spin dynamics indicates that these are sufficiently low so that one can reconcile deviations from canonical behavior above Tc with a Fermi-liquid ground state. Explicit effects of these low-energy scales are discussed in the context of the quasiparticle lifetime as a function of frequency and temperature. Our detailed studies also yield predictions for future experiments which will help to test futher the validity of this approach
NMR relaxation and neutron scattering in a Fermi-liquid picture of the metallic copper oxides
Using a Fermi-liquid formalism in which the Cu d electrons are quasilocalized, we calculate the NMR relaxation for the Cu and O sites, the temperature-dependent magnetic susceptibility, and the neutron-scattering cross section. Our microscopic calculations of the dynamical susceptibility help to decouple narrow-band phenomena from spin-fluctuation effects. The former lead to a low (coherence) energy scale and considerable structure in the Lindhard function. The latter help to enhance these effects
Theory of Neutron Scattering in the Normal and Superconducting State of YBCO
We analyze neutron experiments on \ybco at various stoichiometries in the
superconducting state, within the context of a bi-layer theory which yields
good agreement with the normal state Cu-NMR and neutron data as a function of
\omega, q and T. A d-wave superconducting state exhibits peaks at q = ( \pi ,
\pi , \pi ) and sharp maxima as a function of \omega, at twice the gap
frequency. This behavior may have been observed experimentally. The counterpart
behavior for other choices of order parameter symmetry is discussed.Comment: uuencoded postscript file for the entire paper enclose
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