Anomalous Normal State Properties Research Articles

Conservation of the particle–hole symmetry in the pseudogap state in optimally-doped Bi2Sr2CuO6+δ superconductor

The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi2Sr1.6La0.4CuO6+δ superconductor, we report the observations of the particle–hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These results provide key insights in understanding the nature of the pseudogap and its relation with high temperature superconductivity.

Quantum Criticality Stabilizes High T c Superconductivity Against Competing Symmetry-Breaking Instabilities

The occurrence of high-T c superconductivity in systems including the cuprates and the iron-based superconductors, is known to coincide with the existence of anomalous normal-state properties which have been associated with quantum criticality. We argue here that this observation results from the fact that quantum criticality can allow the occurrence of very-strong-coupling superconductivity by preventing its suppression due to competing symmetry-breaking instabilities. Treating the electrons through a large-U ansatz yields their separation into boson quasiparticles which are directly involved in the formation of these instabilities, represented as their Bose condensates, and charge-carrying fermion quasiparticles which are affected by them indirectly. Within the critical regime, condensates corresponding to the different broken-symmetry states are combined; consequently their negative effect on the pairing of the fermions is strongly diminished, enabling high-T c superconductivity to occur. The observed phase diagram of the hole-doped cuprates then derives from a hidden T=0 quantum phase transition between a Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state. The pseudogap range within this diagram is found to include two distinct regimes, with partial pairing occurring in one of them. Conclusions are drawn concerning a strategy for searching for new high-T c systems with higher transition temperatures.

Link between spin fluctuations and electron pairing in copper oxide superconductors

Although it is generally accepted that superconductivity is unconventional in the high-transition-temperature copper oxides, the relative importance of phenomena such as spin and charge (stripe) order, superconductivity fluctuations, proximity to a Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped copper oxides, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic spin fluctuations are the dominant feature. Here we report a study of magnetotransport in thin films of the electron-doped copper oxide La(2 - x)Ce(x)CuO(4). We show that a scattering rate that is linearly dependent on temperature--a key feature of the anomalous normal state properties of the copper oxides--is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.

Evidence of a Precursor Superconducting Phase at Temperatures as High as 180 K inRBa2Cu3O7−δ (R=Y,Gd,Eu)Superconducting Crystals from Infrared Spectroscopy

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O(7-δ) (R=Y, Gd, Eu) provides important new information about the anomalous normal-state properties of underdoped cuprate high temperature superconductors. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*≫T(c), it enables us to identify the onset of a precursor superconducting state below T(ons)>T(c). We map out the doping phase diagram of T(ons) which reaches a maximum of 180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

A Theory for the High-T c Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing

A theory of highly correlated layered superconducting materials is applied for the cuprates. Differently from an independent-electron approximation, their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, where the introduction of a Lagrange Bose field enables treating them as bosons or fermions. The energy spectrum of this field accounts for the tendency of hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it induces a different analytical behavior for auxiliary particles corresponding to “antinodal” and “nodal” electrons, enabling the existence of different pairing temperatures at T ∗ and T c . This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T ∗ and of a “marginal-FL” critical behavior above it. The qualitative anomalous behavior of numerous physical quantities is accounted for, including kink- and waterfall-like spectral features, the drop in the scattering rates below T ∗ and more radically below T c , and an effective increase in the density of carriers with T and ω, reflected in transport, optical and other properties. Also is explained the correspondence between T c , the resonance-mode energy, and the “nodal gap”.

Theory of the coupling of quantum-critical fluctuations to fermions andd-wave superconductivity in cuprates

The quantum critical fluctuations of the time-reversal breaking order parameter which is observed in the pseudogap regime of the Cuprates are shown to couple to the lattice equivalent of the local angular momentum of the fermions. Such a coupling favors scattering of fermions through angles close to $\pm \pi/2$ which is unambiguously shown to promote d-wave pairing. The right order of magnitude of both $T_c$ and the normalized zero temperature gap $\Delta/T_c$ are calculated using the same fluctuations which give the temperature, frequency and momentum dependence of the the anomalous normal state properties for dopings near the quantum-critical value and with two parameters extracted from fit to such experiments.

What lies beneath the dome?

Numerous experiments on cuprate materials suggest that a zero-temperature phase transition is hidden beneath the superconducting dome. Is it the key to understanding high-temperature superconductivity, and can it explain the anomalous normal state properties?

Evolution of Hall Coefficient in Two-Dimensional Heavy Fermion CeCoIn5

We report on the pressure dependence of the Hall coefficient $R_H$ in quasi-2D heavy fermion CeCoIn$_5$. At ambient pressure, below a temperature associated with the emergence of non-Fermi liquid properties, $R_H$ is anomalously enhanced. We found that the restoration of the Fermi liquid state with applied pressure leads to a gradual suppression of this dramatic enhancement. Moreover, the enhancement in $R_H$ was found to be confined to an intermediate temperature window, where inelastic electron-electron scattering is dominant. Our results strongly support the presence of cold and hot spots on the Fermi surface probably due to anisotropic scattering by antiferromagnetic fluctuations, which may also prove relevant for the debate on the anomalous normal-state properties of high-$T_c$ cuprates.

Quantum-critical theory of the spin-fermion model and its application to cuprates: Normal state analysis

We present the full analysis of the normal state of the spin-fermion model near the antiferromagnetic instability in two dimensions. This model describes low-energy fermions interacting with their own collective spin fluctuations, which soften at the antiferromagnetic transition. We argue that in 2D, the system has two typical energies - an effective spin-fermion interaction $\bar g$ and an energy $\omega_{sf}$ below which the system behaves as a Fermi liquid. The ratio of the two determines the dimensionless coupling constant for spin-fermion interaction $\lambda^2 \propto {\bar g}/\omega_{sf}$. We show that $\lambda$ scales with the spin correlation length and diverges at criticality. This divergence implies that the conventional perturbative expansion breaks down. We developed a novel approach to the problem - the expansion in either the inverse number of hot spots in the Brillouin zone, or the inverse number of fermionic flavors - which allowed us to explicitly account for all terms which diverge as powers of $\lambda$, and treat the remaining, $O(\lambda)$ terms in the RG formalism. We applied this technique to study the properties of the spin-fermion model in various frequency and temperature regimes. We present the results for the fermionic spectral function, spin susceptibility, optical conductivity and other observables. We compare our results in detail with the normal state data for cuprates, and argue that the spin-fermion model is capable to explain the anomalous normal state properties of high $T_c$ materials. We also discuss the non -applicability of the conventional $\phi^4$ theory of the quantum-critical behavior in 2D.

Model for superconductivity in optimally doped electron cuprates

The very anomalous normal state properties and superconductivity of electron-doped superconducting cuprates with the T* structure are discussed. The high values of the electron and hole mobilities in the literature are explained by the electrons and holes existing in different layers. The annealing procedures necessary for superconductivity produce oxygen-vacancy negative-U pairing centers. The relative insensitivity of Tc to the annealing condition is attributed to stabilization of the chemical potential near the middle of the narrow oxygen band formed by oxygen vacancies, where the electronic entropy is large.

ChemInform Abstract: Finite-Temperature Properties of Doped Antiferromagnets

We review recent results for the properties of doped antiferromagnets, obtained by the numerical analysis of the planar t-J model using the novel finite-temperature Lanczos method for small correlated systems. First we briefly summarize our present understanding of anomalous normal-state properties of cuprates, and present the electronic phase diagram, phenomenological scenarios and models proposed in this connection. The numerical method is then described in more detail. The following sections are devoted to various static and dynamical properties of the t-J model. Among the thermodynamic properties the chemical potential, entropy and the specific heat are evaluated. Discussing electrical properties the emphasis is on the planar optical conductivity and the d.c. resistivity. Magnetic properties involve the static and dynamical spin structure factor, as measured via the susceptibility measurements, the NMR relaxation and the neutron scattering, as well as the orbital current contribution. The analysis of ...

High Tc: Top Down or Bottom Up?

We present and discuss recent tunneling results, which shed some light on the two contradicting approaches to the high-Tc problem. The “top-down” approach starts from the ansatz that the anomalous properties of the cuprates in their normal state above Tc need to be understood first, before the problem of superconductivity can be tackled successfully. On the other hand, in the “bottom-up” approach, both high Tc and the anomalous normal-state properties come about because of the vicinity of a quantum critical point. Our new tunneling results indicate that in the superconducting state, at some critical doping, the order parameter goes from a pure d-wave on the underdoped side to a complex one on the overdoped one. This symmetry breaking lends support to the hypothesis that there does exist a quantum critical point and, therefore, favors the “bottom-up” approach.

Finite-temperature properties of doped antiferromagnets

We review recent results for the properties of doped antiferromagnets, obtained by the numerical analysis of the planar t-J model using the novel finite-temperature Lanczos method for small correlated systems. First we briefly summarize our present understanding of anomalous normal-state properties of cuprates, and present the electronic phase diagram, phenomenological scenarios and models proposed in this connection. The numerical method is then described in more detail. The following sections are devoted to various static and dynamical properties of the t-J model. Among the thermodynamic properties the chemical potential, entropy and the specific heat are evaluated. Discussing electrical properties the emphasis is on the planar optical conductivity and the d.c. resistivity. Magnetic properties involve the static and dynamical spin structure factor, as measured via the susceptibility measurements, the NMR relaxation and the neutron scattering, as well as the orbital current contribution. The analysis of electron spectral functions, studied by photoemission experiments, and their relation to the c-axis conductivity, follows. Finally we discuss density fluctuations, the electronic Raman scattering and the thermoelectric power. Whenever feasible a comparison with experimental data is performed. A general conclusion is that the t-J model captures well the main features of anomalous normal-state properties of cuprates, for a number of quantities the agreement is even a quantitative one. It is shown that several dynamical quantities exhibit at intermediate doping a novel universal behaviour consistent with a marginal Fermi-liquid concept, which seems to emerge from a large degeneracy of states and a frustration induced by doping the antiferro magnet.

Charge confinement effect in cuprate superconductors: an explanation for the normal-state resistivity and pseudogap

The fact that the stripe phase and pseudogap in the cuprate superconductors occur in the same doping regime is emphasized. A model based on charge confinement in self-organized nanometer-scale stripe fragments is proposed to understand various generic features of the normal-state energy gap including the magnitude of the gap, its anti-correlation with the superconducting gap, and the d-wave symmetry in its κ-dependence. This model also provides a basis for understanding other anomalous normal-state properties such as the linear temperature dependence of electrical resistivity.

Thermodynamics of the 2D Hubbard model

A theoretical analysis of the thermodynamic response functions of the 2D single-band Hubbard model is realized by means of the composite operator method. It is shown that all the features of these quantities can be explained by looking at the dependence of the thermodynamic variables on their conjugate ones. It is found that the numerical data from quantum Monte Carlo techniques for the internal energy and electronic specific heat are well reproduced. The anomalous normal state properties in hole-doped cuprate high Tc superconductors are also well described. Actually, the results for the linear coefficient of the electronic specific heat are in agreement with those obtained by using a pure fermionic theoretical scheme. Finally, we obtain several characteristic crossing points for the response functions when reported as functions of some thermodynamic variables. These peculiar features indicate the existence of more than one energy scale competing with thermal excitations in the 2D Hubbard model.

Anisotropic renormalization of quasiparticles in cuprate-oxide superconductors

The Hubbard model on the simple square lattice in the vicinity of the Mott transition is considered by the 1/ d expansion method, with d being the spatial dimensionality. The superexchange interaction is enhanced by antiferromagnetic spin fluctuations, and its enhanced portion is similar to an interaction due to the so-called spin-fluctuation mechanism. Gutzwiller's heavy electrons are renormalized to be light by the enhanced superexchange interaction, which is responsible for high-temperature superconductivity; the formation of light quasiparticles occurs, in particular, for k s in the vicinity of saddle points in two dimensions. This anisotropic renormalization of quasi-particles can explain not only the anisotropic pseudogap in normal states but also other anomalous normal-state properties such as the large anisotropy of resistivity between the parallel and the perpendicular directions to CuO 2 planes.

Pseudogap aboveTcin a model withdx2−y2pairing

We study the anomalous normal state properties of a simple two-dimensional model whose ground state is a d-wave superconductor. Using a self-consistent, conserving formulation, we show that pairing correlations above T_c lead to the appearance of a highly anisotropic pseudogap in the electronic spectral function and the destruction of the Fermi surface. We discuss the similarities and differences between our results and ARPES experiments on underdoped cuprates.

Incommensurate charge-density-wave instability in the extended three-band Hubbard model

The infinite-U three-band Hubbard model is considered in order to describe the CuO_2 planes of the high temperature superconducting cuprates. The charge instabilities are investigated when the model is extended with a nearest-neighbor repulsion between holes on copper d and oxygen p orbitals and in the presence of a long-range Coulombic repulsion. It is found that a first-order valence instability line ending with a critical point is present like in the previously investigated model without long-range forces. However, the dominant critical instability is the formation of incommensurate charge-density-waves, which always occur before the valence-instability critical point is reached. An effective singular attraction arises in the proximity of the charge-density wave instability, accounting for both a strong pairing mechanism and for the anomalous normal state properties.

Deviations from Fermi-liquid behavior in (2+1)-dimensional quantum electrodynamics and the normal phase of high-Tc superconductors.

We argue that the gauge-fermion interaction in multiflavor quantum electrodynamics in (2+1) dimensions is responsible for non-Fermi-liquid behavior in the infrared, in the sense of leading to the existence of a nontrivial (quasi)fixed point that lies between the trivial fixed point (at infinite momenta) and the region where dynamical symmetry breaking and mass generation occurs. This quasifixed-point structure implies slowly varying, rather than fixed, couplings in the intermediate regime of momenta, a situation which resembles that of (four-dimensional) "walking technicolor" models of particle physics. The inclusion of wave-function renormalization yields marginal $O(\frac{1}{N})$ corrections to the "bulk" non-Fermi-liquid behavior caused by the gauge inter-action in the limit of infinite flavor number. Such corrections lead to the appearance of modified critical exponents. In particular, at low temperatures there appear to be logarithmic scaling violations of the linear resistivity of the system of order $O(\frac{1}{N})$. The connection with the anomalous normal-state properties of certain condensed-matter systems relevant for high-temperature superconductivity is briefly discussed. The relevance of the large (flavor) $N$ expansion to the Fermi-liquid problem is emphasized. As a partial result of our analysis, we point out the absence of charge-density-wave instabilities from the effective low-energy theory, as a consequence of gauge invariance.

Normal state properties of the attractive Hubbard model: moment approach

We introduce a two-δ-peak ansatz for the spectral density of the attractive Hubbard model in the normal state and in d-dimensions. The resulting two excitation branches and their weights are fixed in terms of the exact first four frequency moments. This ansatz correctly reproduces the atomic limit, the free case and the strong coupling limit to first order in the hopping amplitude. For this reason it is expected to provide a reasonable interpolation scheme covering the intermediate and strong coupling regimes for properties that do not depend sensitively on the damping of the two excitation branches. In the strong coupling regime, the upper excitation branch describes the creation of a fermion in an empty state, whereas the lower band arises from pair formation due to the creation of a fermion in a state already occupied by a fermion of opposite spin. Consequently, anomalous normal state properties can be expected whenever the chemical potential is within the correlation gap. This turns out to be the case when half-filling is approached for intermediate coupling as well as for any finite band filling in the strong coupling regime. In these filling and coupling regimes, there is no Fermi surface, and the formation and dissociation of pairs occur in the normal state as revealed by the temperature dependence of the double occupancy. For this reason the normal state properties exhibit an anomalous temperature dependence. Comparison with t-matrix calculations in the dilute and intermediate coupling regimes and quantum Monte Carlo estimates covering intermediate band fillings as well as intermediate and strong coupling suggest that the predictions of the moment approach are essentially correct whenever the correlation gap is not smeared out by damping effects or overlapping bands. Exploring the three-dimensional case as well, we conclude that this scenario applies to the d-dimensional attractive Hubbard model.