arXiv subjects

V. J. Emery

Publications and source records attributed to V. J. Emery.

At least 19 recordsLinked to original sources

arXiv2002

Low temperature electronic properties of Sr_2RuO_4 I: Microscopic model and normal state properties

Starting from the quasi one-dimensional kinetic energy of the d_{yz} and d_{zx} bands we derive a bosonized description of the correlated electron system in Sr_2RuO_4. At intermediate coupling the magnetic correlations have a quasi one-dimensional component along the diagonals of the basal plane of the tetragonal unit cell that accounts for the observed neutron scattering results. Together with two-dimensional correlations the model consistently accounts for the normal phase specific heat, cyclotron mass enhancement, static susceptibility, and Wilson ratio and implies an anomalous high temperature resistivity.

cond-mat.str-el↗

arXiv2002

Concepts in High Temperature Superconductivity

It is the purpose of this paper to explore the theory of high temperature superconductivity. Much of the motivation for this comes from the study of the cuprate high temperature superconductors. However, our primary focus is on the core theoretical issues associated with the mechanism of high temperature superconductivity more generally. We concentrate on physics at intermediate temperature scales of order $T_c$ (as well as the somewhat larger "pseudogap" temperature) and energies of order the gap maximum, $Δ_0$. Prominent themes throughout the article are the need for a kinetic energy driven mechanism, and the role of mesoscale structure in enhancing pairing from repulsive interactions.

cond-mat.supr-con↗

arXiv2001

Nodal quasi-particles and coexisting orders in striped superconductors

We study the properties of a quasi-one dimensional superconductor which consists of an alternating array of two inequivalent chains. This model is a simple charicature of a locally striped high temperature superconductor, and is more generally a theoretically controllable system in which the superconducting state emerges from a non-Fermi liquid normal state. Even in this limit, ``d-wave like'' order parameter symmetry is natural, but the superconducting state can either have a complete gap in the quasi-particle spectrum, or gapless ``nodal'' quasiparticles. We also find circumstances in which antiferromagnetic order (typically incommensurate) coexists with superconductivity.

cond-mat.supr-con↗

arXiv2001

Dimensional Crossover in Quasi One-Dimensional and High T_c Superconductors

The one-dimensional electron gas exhibits spin-charge separation and power-law spectral responses to many experimentally relevant probes. Ordering in a quasi one-dimensional system is necessarily associated with a dimensional crossover, at which sharp quasiparticle peaks, with small spectral weight, emerge from the incoherent background. Using methods of Abelian bosonization, we derive asymptotically correct expressions for the spectral changes induced by this crossover. Comparison is made with experiments on the high temperature superconductors, which are electronically quasi one-dimensional on a local scale.

cond-mat.supr-con↗

arXiv2001

Evidence of Electron Fractionalization from Photoemission Spectra in the High Temperature Superconductors

In the normal state of the high temperature superconductors Bi_2Sr_2CaCu_2O_{8+delta} and La_{2-x}Sr_{x}CuO_4, and in the related ``stripe ordered'' material La_1.25Nd_0.6Sr_0.15CuO_4, there is sharp structure in the measured single hole spectral function A(k,w) considered as a function of k at fixed small binding energy w. At the same time, as a function of w at fixed k on much of the putative Fermi surface, any structure in A(k,w), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one dimensional electron gas.

cond-mat.supr-con↗

arXiv1999

Stripe phases in high-temperature superconductors

Stripe phases are predicted and observed to occur in a class of strongly-correlated materials describable as doped antiferromagnets, of which the copper-oxide superconductors are the most prominent representative. The existence of stripe correlations necessitates the development of new principles for describing charge transport, and especially superconductivity, in these materials.

cond-mat.supr-con↗

arXiv1999

Classification and Stability of Phases of the Multicomponent One-Dimensional Electron Gas

The classification of the ground-state phases of complex one-dimensional electronic systems is considered in the context of a fixed-point strategy. Examples are multichain Hubbard models, the Kondo-Heisenberg model, and the one-dimensional electron gas in an active environment. It is shown that, in order to characterize the low-energy physics, it is necessary to analyze the perturbative stability of the possible fixed points, to identify all discrete broken symmetries, and to specify the quantum numbers and elementary wave vectors of the gapless excitations. Many previously-proposed exotic phases of multichain Hubbard models are shown to be unstable because of the ``spin-gap proximity effect.'' A useful tool in this analysis is a new generalization of Luttinger's theorem, which shows that there is a gapless even-charge mode in any incommensurate N-component system.

cond-mat.str-el↗

arXiv1999

Topological doping and the stability of stripe phases

We analyze the properties of a general Ginzburg-Landau free energy with competing order parameters, long-range interactions, and global constraints (e.g., a fixed value of a total ``charge'') to address the physics of stripe phases in underdoped high-Tc and related materials. For a local free energy limited to quadratic terms of the gradient expansion, only uniform or phase-separated configurations are thermodynamically stable. ``Stripe'' or other non-uniform phases can be stabilized by long-range forces, but can only have non-topological (in-phase) domain walls where the components of the antiferromagnetic order parameter never change sign, and the periods of charge and spin density waves coincide. The antiphase domain walls observed experimentally require physics on an intermediate lengthscale, and they are absent from a model that involves only long-distance physics. Dense stripe phases can be stable even in the absence of long-range forces, but domain walls always attract at large distances, i.e., there is a ubiquitous tendency to phase separation at small doping. The implications for the phase diagram of underdoped cuprates are discussed.

cond-mat.supr-con↗

arXiv1999

Local Electronic Structure and High Temperature Superconductivity

It is argued that a new mechanism and many-body theory of superconductivity are required for doped correlated insulators. Here we review the essential features of and the experimental support for such a theory, in which the physics is driven by the kinetic energy.

cond-mat.str-el↗

arXiv1999

Classical Phase Fluctuations in High Temperature Superconductors

Phase fluctuations of the superconducting order parameter play a larger role in the cuprates than in conventional BCS superconductors because of the low superfluid density of a doped insulator. In this paper, we analyze an XY model of classical phase fluctuations in the high temperature superconductors using a low-temperature expansion and Monte Carlo simulations. In agreement with experiment, the value of the superfluid density at temperature T=0 is a quite robust predictor of Tc, and the evolution of the superfluid density with T, including its T-linear behavior at low temperature, is insensitive to microscopic details.

cond-mat.supr-con↗

arXiv1999

Charge Transport in Synthetic Metals

The phenomenology of charge transport in synthetic metals is reviewed. It is argued that the conventional quasiparticle picture and Boltzmann transport theory do not apply to these materials. The central ideas of Fermi liquid theory are reviewed, and the significant corrections produced by quasiparticle scattering from ferromagnetic spin fluctuations in liquid $^3$He are described. It is shown that Sr$_2$RuO$_4$ does not display the symptoms of a nearly-ferromagnetic Fermi liquid, so the source of its odd angular momentum pairing remains to be understood. The solution of an assisted-tunneling model of charge transport in quasi-one dimensional materials is described. This model has a quantum critical point and gives a resistivity that is linear in temperature or frequency, whichever is greater.

cond-mat.str-el↗

arXiv1998

Microscopic Theory of High Temperature Superconductivity

It is argued that the BCS many-body theory, which is outstandingly successful for conventional superconductors, does not apply to the high temperature superconductors and that a realistic theory must take account of the local electronic structure (stripes). The spin gap proximity effect is a mechanism by which the charge carriers on the stripes and the spins in the intervening regions acquire a spin gap at a relatively high temperature, with only strong repulsive interactions. Superconducting phase order is achieved at a lower temperature determined by the (relatively low) superfluid density of the doped insulator. This picture is consistent with the phenomenology of the high temperature superconductors. It is shown that, in momentum space, the spin gap first arises in the neighborhood of the points $(0,\pm π)$ and $(\pm π, 0)$ and then spreads along arcs of the Fermi surface. Some of the experimental consequences of this picture are discussed.

cond-mat↗

arXiv1998

Stripe Liquid, Crystal, and Glass Phases of Doped Antiferromagnets

A largely descriptive survey is given of the ordered phases of doped antiferromagnets, and of the long wavelength properties that can be derived from an order-parameter theory. In particular, we show that the competition between the long-range Coulomb repulsion and the strong short-distance tendency of doped holes to coalesce into regions of supressed antiferromagnetism leads to a variety of self-organized charge structures on intermediate length scales, of which ``stripes'' are the most common, both theoretically and experimentally. These structures lead to a rich assortment of novel electronic phases and crossover phenomena, as indicated in the title. We use the high temperature superconductors as the experimentally best-studied examples of doped antiferromagnets.

cond-mat↗

arXiv1997

Crossovers and Phase Coherence in Cuprate Superconductors

High temperature superconductivity is a property of doped antiferromagnetic insulators. The electronic structure is inhomogeneous on short length and time scales, and, as the temperature decreases, it evolves via two crossovers, before long range superconducting order is achieved. Except for overdoped materials, pairing and phase coherence occur at different temperatures, and phase fluctuations determine both T$_c$ and the temperature dependence of the superfluid density for a wide range of doping. A mechanism for obtaining a high pairing scale in a short coherence length material with a strong poorly-screened Coulomb interaction is described.

cond-mat.supr-con↗

arXiv1997

Doped Antiferromagnets in High Dimension

The ground-state properties of the t-J model on a d-dimensional hypercubic lattice are examined in the limit of large d. It is found that the undoped system is an ordered antiferromagnet, and that the doped system phase separates into a hole-free antiferromagnetic phase and a hole-rich phase. The latter is electron free if J > 4t and is weakly metallic (and typically superconducting) if J < 4t. The resulting phase diagram is qualitatively similar to the one previously derived for d=2 by a combination of analytic and numerical methods. Domain wall (or stripe) phases form in the presence of weak Coulomb interactions, with periodicity determined by the hole concentration and the relative strength of the exchange and Coulomb interactions. These phases reflect the properties of the hole-rich phase in the absence of Coulomb interactions, and, depending on the value of J/t, may be either insulating or metallic (i.e. an ``electron smectic'').

cond-mat.str-el↗

arXiv1997

Electronic Liquid Crystal Phases of a Doped Mott Insulator

The character of the ground state of an antiferromagnetic insulator is fundamentally altered upon addition of even a small amount of charge. The added charges agglomerate along domain walls at which the spin correlations, which may or may not remain long-ranged, suffer a $π$ phase shift. In two dimensions, these domain walls are ``stripes'' which are either insulating, or conducting, i.e. metallic rivers with their own low energy degrees of freedom. However, quasi one-dimensional metals typically undergo a transition to an insulating ordered charge density wave (CDW) state at low temperatures. Here it is shown that such a transition is eliminated if the zero-point energy of transverse stripe fluctuations is sufficiently large in comparison to the CDW coupling between stripes. As a consequence, there exist novel, liquid-crystalline low-temperature phases -- an electron smectic, with crystalline order in one direction, but liquid-like correlations in the other, and an electron nematic with orientational order but no long-range positional order. These phases, which constitute new states of matter, can be either high temperature supeconductors or two-dimensional anisotropic ``metallic'' non-Fermi liquids. Evidence for the new phases may already have been obtained by neutron scattering experiments in the cuprate superconductor, La_{1.6-x}Nd_{0.4}Sr_xCuO_{4}.

cond-mat↗

arXiv1997

Pairing and Phase Coherence in High Temperature Superconductors

Mobile holes in an antiferromagnetic insulator form a slowly fluctuating array of quasi one-dimensional metallic stripes, which induce a spin gap or pseudogap in the intervening Mott-insulating regions. The mobile holes on an individual stripe acquire a spin gap via pair hopping between the stripe and its environment; i.e. via a magnetic analog of the usual superconducting proximity effect. This process is the analog of pairing in conventional superconductors. At non-vanishing stripe densities, Josephson coupling between stripes produces a dimensional crossover to a state with long-range superconducting phase coherence. In contrast to conventional superconductors, the superconducting state is characterised by a high density of (spin) pairs, but the phase stiffness, which is determined by the density and mobility of holes on the stripes, is very low.

cond-mat↗

arXiv1997

Landau Theory of Stripe Phases in Cuprates and Nickelates

We consider a Landau theory of coupled charge and spin-density wave order parameters as a simple model for the ordering that has been observed experimentally in the La_2NiO_4 and La_2CuO_4 families of doped antiferromagnets. The period of the charge-density wave is generically half that of the spin-density wave, or equivalently the charges form antiphase domain walls in the antiferromagnetic order. A sharp distinction exists between the case in which the ordering is primarily charge driven (which produces a sequence of transitions in qualitative agreement with experiment) or spin driven (which does not). We also find that stripes with non-collinear spin order (i.e. spiral phases) are possible in a region of the phase diagram where the transition is spin driven; the spiral is circular only when there is no charge order, and is otherwise elliptical with an eccentricity proportional to the magnitude of the charge order.

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