arXiv subjects

David Pines

Publications and source records attributed to David Pines.

33 records · Page 2Linked to original sources

arXiv2000

Quantum Protectorates in the Cuprate Superconductors

Following the identification of the pairing state, the major challenge in understanding the cuprate superconductors has been determining the evolution with doping and temperature of their anomalous normal state behavior. Key to this understanding is the experimentally determined magnetic phase diagram for the cuprates, which provides information on the protected magnetic properties of the normal state, generic behavior that is reliably the same one system to the next, regardless of details. I discuss the constraints this places on candidate quantum protectorates, and the status of microscopic model calculations for a protectorate consistent with these constraints, the nearly antiferromagnetic Fermi liquid.

cond-mat.supr-con↗

arXiv2000

Spin and charge inhomogeneities in high-T_c cuprates: Evidence from NMR and neutron scattering experiments

In this communication we consider the doping dependence of the strong antiferromagnetic spin fluctuations in the cuprate superconductors. We investigate the effect of an incommensurate magnetic response, as recently observed in inelastic neutron scattering (INS) experiments on several YBa_2Cu_3O_{6+x} compounds, on the spin-lattice and spin-echo relaxation rates measured in nuclear magnetic resonance (NMR) experiments. We conclude that a consistent theoretical description of INS and NMR can be reached if one assumes spatially inhomogeneous but locally commensurate spin correlations and that NMR and INS experiments can be described within a single theoretical scenario. We discuss a simple scenario of spin and charge inhomogeneities which includes the main physical ingredients required for consistency with experiments.

cond-mat.supr-con↗

arXiv1998

Effect of Superconductivity on the Incommensurate Magnetic Response of Cuprate Superconductors

We explain the effects of superconductivity on the incommensurate magnetic response $χ''(q,ω)$ observed in inelastic neutron scattering measurements on La$_{2-x}$Sr$_x$CuO$_4$. We show that a spin-fermion model correctly describes the frequency and momentum dependent changes of $χ''$ in the superconducting phase. We find these changes are generic features of an incommensurate spin structure and the d-wave symmetry of the superconducting gap and are thus expected for all cuprates with an incommensurate magnetic response. Our analysis of INS experiments in La$_{2-x}$Sr$_x$CuO$_4$ up to optimal doping suggests a Fermi surface which is closed around $(π,π)$.

cond-mat.supr-con↗

arXiv1998

The resonance peak in cuprate superconductors

We pursue the consequences of a theory in which the resonance peak observed in inelastic neutron scattering (INS) experiments on underdoped and optimally doped YBa$_2$Cu$_3$O$_{6+x}$ compounds arises from a spin-wave excitation. We find that it is heavily damped, and thus almost not observable, in the normal state, but becomes visible in the superconducting state due to the drastic decrease in spin damping. We show that a spin-fermion model correctly describes the temperature dependence of the peak position for YBa$_2$Cu$_3$O$_7$, as well as the doping dependence of the peak position and of the integrated intensity. We explain why no resonance peak has been observed in La$_{2-x}$Sr$_x$CuO$_4$, and make several predictions concerning resonance peaks in other cuprate superconductors.

cond-mat.str-el↗

arXiv1998

Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors I: General theory and quasiparticle properties

We derive in detail a novel solution of the spin fermion model which is valid in the quasi-static limit pi T<<omega_sf, found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, and use it to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of_hot_ (around k=(pi,0)) and_cold_ (around k=(pi/2,pi/2)) quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. The electron spin-fluctuation vertex function, i.e. the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors.

cond-mat↗

arXiv1997

Magnetotransport in Cuprates: a Test of the Spin Fluctuation Model

We report on a simple calculation of the magnetotransport in cuprate superconductors, based on the nearly antiferromagnetic Fermi liquid (spin fluctuation) model. We find that the model explains all important features seen experimentally: the violation of K"ohler's rule, the close relationship between the Hall angle and the magnetoresistance, the temperature dependence of the first high field correction to MR and the doping dependence of the low field MR data. In addition, the estimated values of omega_c tau, calculated using parameters obtained from the NMR measurements, yield values in close agreement with those found experimentally for overdoped and optimally doped cuprates.

cond-mat↗

arXiv1997

Weak Pseudogap Behavior in the Underdoped Cuprate Superconductors

We report on an exact solution of the nearly antiferromagnetic Fermi liquid spin fermion model in the limit πT << ω_{sf}, which demonstrates that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and precursor effects of an SDW state. We show that the onset temperature, T^{cr}, of weak pseudo-gap (pseudoscaling) behavior is determined by the strength, ξ, of the AF correlations, and obtain the generic changes in low frequency magnetic behavior seen in NMR experiments with ξ(T^{cr}) \approx 2, confirming the Barzykin and Pines crossover criterion.

cond-mat.supr-con↗

arXiv1997

Theory of the Optical Conductivity in the Cuprate Superconductors

We present a study of the normal state optical conductivity in the cuprate superconductors using the nearly antiferromagnetic Fermi liquid (NAFL) description of the magnetic interaction between their planar quasiparticles. We find that the highly anisotropic scattering rate in different regions of the Brillouin zone, both as a function of frequency and temperature, a benchmark of NAFL theory, leads to an average relaxation rate of the Marginal Fermi Liquid form for overdoped and optimally doped systems, as well as for underdoped systems at high temperatures. We carry out numerical calculations of the optical conductivity for several compounds for which the input spin fluctuation parameters are known. Our results, which are in agreement with experiment on both overdoped and optimally doped systems, show that NAFL theory explains the anomalous optical behavior found in these cuprate superconductors.

cond-mat.str-el↗

arXiv1997

Understanding High Temperature Superconductors: Progress and Prospects

I review progress in measurements of the dynamic spin susceptibility in the normal state which yield a new phase diagram and discuss microscopic calculations which yield qualitative, and in many cases, quantitative agreement with the measured changes in the quasiparticle, transport, magnetotransport, and optical properties of the cuprate superconductors as one varies doping and temperature provided one describes the systems as nearly anti-ferromagnetic Fermi liquids in which the effective magnetic interaction between planar quasiparticles mirrors the dynamic spin susceptibility measured in NMR and INS experiments. Together with the demonstration that the NAFL pairing potential leads inexorably to a d_x2-y2,pairing state, this work provides a "proof of concept" for the NAFL description of high Tc materials. I review Eliashberg calculations of the mean-field behavior found in overdoped systems and discuss the extent to which the crossovers to pseudoscaling and pseudogap behavior found in the effective magnetic interaction and quasiparticle behavior in the optimally doped and underdoped systems may be derived microscopically. I conclude with a tentative scenario for the dependence of Tc on doping level and imperfections in different systems.

cond-mat↗

arXiv1997

Nearly Antiferromagnetic Fermi Liquids: A Progress Report

I describe recent theoretical and experimental progress in understanding the physical properties of the two dimensional nearly antiferromagnetic Fermi liquids (NAFL's) found in the normal state of the cuprate superconductors. In such NAFL's, the magnetic interaction between planar quasiparticles is strong and peaked at or near the commensurate wave vector, $Q \equiv (π,π)$. For the optimally doped and underdoped systems, the resulting strong antiferromagnetic correlations produce three distinct magnetic phases in the normal state: mean field above $T_{cr}$, pseudoscaling between $T_{cr}$ and $T_*$, and pseudogap below $T_*$. I present arguments which suggest that the physical origin of the pseudogap found in the quasiparticle spectrum below $T_{cr}$ is the formation of a precursor to a spin-density-wave-state, describe the calculations based on this scenario of the dynamical spin susceptibility, Fermi surface evolution, transport, and Hall effect, and summarize the experimental evidence in its support.

cond-mat↗

arXiv1996

A Theory of the Longitudinal and Hall Conductivities of the Cuprate Superconductors

We establish the applicability to transport phenomena in the cuprate superconductors of a nearly antiferromagnetic Fermi liquid (NAFL) description of the magnetic interaction between planar quasiparticles by using it to obtain the temperature dependent resistivity and Hall conductivity seen experimentally in the normal state. Following a perturbative calculation of the anisotropic (as one goes around the Fermi surface) quasiparticle lifetimes which are the hallmark of a NAFL, we obtain simple approximate expressions for the longitudinal, $σ_{xx}$, and Hall, $σ_{xy}$, conductivities which reflect the magnetic crossovers seen experimentally as one varies the doping level and temperature. We present a simple phenomenological model for the variation in mean free path around the Fermi surface, and use this to extract from experiments on $σ_{xx}$ and $σ_{xy}$ quasiparticle lifetimes in the hot (strongly coupled quasiparticle) and cold (weakly coupled quasiparticle) regions of the Fermi surface which are consistent with the perturbation theory estimates. We improve upon the latter by carrying out direct numerical (non-variational) solutions of the Boltzmann equation for representative members of the YBa$_2$Cu$_3$O$_{6+x}$ and La$_{2-x}$Sr$_x$CuO$_4$ systems, with results for transport properties in quantitative agreement with experiment. Using the same numerical approach we study the influence of CuO chains on the a-b plane anisotropy and find results in agreement with experimental findings in YBa$_2$Cu$_4$O$_8$.

cond-mat↗

arXiv1996

Temperature crossovers in cuprates

We study the temperature crossovers seen in the magnetic and transport properties of cuprates using a nearly antiferromagnetic Fermi liquid model (NAFLM). For the overdoped cuprates, we find, in agreement with earlier work, mean-field $z=2$ behavior of the magnetic variables associated with the fact that the damping rate of their spin fluctuations is essentially independent of temperature, while the resistivity exhibits a crossover from Fermi liquid behavior at low temperature to linear-in-T above a certain temperature $T_0$, due to the proximity of the quasiparticle Fermi surface to the magnetic Brillouin zone boundary. For the underdoped cuprates we argue that the sequence of crossovers identified by Barzykin and Pines in the low frequency magnetic behavior (from mean field $z=2$ at high temperatures, $T>T_{cr}$, to non-universal $z=1$ scaling behavior at intermediate $T$, $T_*<T<T_{cr}$, to pseudogap behavior below $T_*$) reflects the development in the electronic structure of a precursor to a spin-density-wave state. This development begins at $T_{cr}$ with a thermal evolution of the quasiparticle spectral weight which brings about temperature dependent spin-damping and ends at $T_*$ where the Fermi surface has lost pieces near corners of the magnetic Brillouin zone. For $T_*<T<T_{cr}$ the resistivity is linear in $T$ because this change in spectral weight does not affect the resistivity significantly; below $T_*$ vertex corrections act to bring about the measured downturn in $(ρ(T)-ρ(0))/T$ and approximately quadratic in $T$ resistivity for $T\ll T_*$.

cond-mat↗

arXiv1995

The Anomalous Hall Effect in YBa$_2$Cu$_3$O$_7$

The temperature dependence of the normal state Hall effect and magnetoresistance in YBa$_2$Cu$_3$O$_7$ is investigated using the Nearly Antiferromagnetic Fermi Liquid description of planar quasiparticles. We find that highly anisotropic scattering at different regions of the Fermi surface gives rise to the measured anomalous temperature dependence of the resistivity and Hall coefficient while yielding the universal temperature dependence of the Hall angle observed for both clean and dirty samples. This universality is shown to arise from the limited momentum transfers available for the anomalous, spin fluctuation scattering and is preserved for any system with strong antiferromagnetic correlations.

cond-mat↗

arXiv1995

Model of C-Axis Resistivity of High-$\Tc$ Cuprates

We propose a simple model which accounts for the major features and systematics of experiments on the $c$-axis resistivity, $ρ_c$, for $\lsco$, $\ybco$ and $\bsco $. We argue that the $c$-axis resistivity can be separated into contributions from in-plane dephasing and the $c$-axis ``barrier'' scattering processes, with the low temperature semiconductor-like behavior of $ρ_c$ arising from the suppression of the in-plane density of states measured by in-plane magnetic Knight shift experiments. We report on predictions for $ρ_c$ in impurity-doped $\ybco$ materials.

cond-mat↗

arXiv1993

Prediction and Adaptation in an Evolving Chaotic Environment

We describe the results of analytic calculations and computer simulations of adaptive predictors (predictive agents) responding to an evolving chaotic environment and to one another. Our simulations are designed to quantify adaptation and to explore co-adaptation for a simple calculable model of a complex adaptive system. We first consider the ability of a single agent, exposed to a chaotic environment, to model, control, and predict the future states of that environment. We then introduce a second agent which, in attempting to model and control both the chaotic environment and the first agent, modifies the extent to which that agent can identify patterns and exercise control. We find that (i) optimal adaptive predictors have an optimal memory and an optimal complexity, which are small for a rapidly changing map dynamics and (ii) that the predictive power can be increased by imposing chaos or random noise onto the map dynamics. The competition between the two predictive agents can lead either to chaos, or to metastable emergent behavior, best described as a leader-follower relationship. Our results suggest a correlation between optimal adaptation, optimal complexity, and emergent behavior, and provide preliminary support for the concept of optimal co-adaptation near the edge of chaos.

adap-org↗