Normal State Of Cuprates Research Articles

Magnetic field reveals vanishing Hall response in the normal state of stripe-ordered cuprates

The origin of the weak insulating behavior of the resistivity, i.e. {rho }_{xx}propto {mathrm{ln}},(1/T), revealed when magnetic fields (H) suppress superconductivity in underdoped cuprates has been a longtime mystery. Surprisingly, the high-field behavior of the resistivity observed recently in charge- and spin-stripe-ordered La-214 cuprates suggests a metallic, as opposed to insulating, high-field normal state. Here we report the vanishing of the Hall coefficient in this field-revealed normal state for all T < (2-6){T}_{{rm{c}}}^{0}, where {T}_{{rm{c}}}^{0} is the zero-field superconducting transition temperature. Our measurements demonstrate that this is a robust fundamental property of the normal state of cuprates with intertwined orders, exhibited in the previously unexplored regime of T and H. The behavior of the high-field Hall coefficient is fundamentally different from that in other cuprates such as YBa2Cu3O6+x and YBa2Cu4O8, and may imply an approximate particle-hole symmetry that is unique to stripe-ordered cuprates. Our results highlight the important role of the competing orders in determining the normal state of cuprates.

Vortex phase diagram and the normal state of cuprates with charge and spin orders.

The phase diagram of underdoped cuprates in a magnetic field (H) is key to understanding the anomalous normal state of these high-temperature superconductors. However, the upper critical field (H c2), the extent of superconducting (SC) phase with vortices, and the role of charge orders at high H remain controversial. Here we study stripe-ordered La-214, i.e., cuprates in which charge orders are most pronounced and zero-field SC transition temperatures are lowest. This enables us to explore the vortex phases in a previously inaccessible energy scale window. By combining linear and nonlinear transport techniques sensitive to vortex matter, we determine the T - H phase diagram, directly detect H c2, and reveal novel properties of the high-field ground state. Our results demonstrate that quantum fluctuations and disorder play a key role as T → 0, while the high-field ground state is likely a metal, not an insulator, due to the presence of stripes.

Non-Fermi-liquid scattering against an emergent Bose liquid: Manifestations in the kink and other exotic quasiparticle behavior in the normal-state cuprate superconductors

The normal state of cuprate superconductors exhibits many exotic behaviors qualitatively different from the Fermi liquid, the foundation of condensed matter physics. Here we demonstrate that non-Fermi liquid behaviors emerge naturally from scattering against an emergent Bose liquid. Particularly, we find a finite zero-energy scattering rate at low-temperature limit that grows linearly with respect to temperature, against clean fermions' generic non-dissipative characteristics. Surprisingly, three other seemingly unrelated experimental observations are also produced, including the well-studied "kink" in the quasi-particle dispersion, as well as the puzzling correspondences between the normal and superconducting state. Our findings provide a general route for fermionic systems to generate non-Fermi liquid behavior, and suggest strongly that by room temperature the doped holes in the cuprates have already formed an emergent Bose liquid of tightly bound pairs, whose low-temperature condensation gives unconventional superconductivity.

Explanation of Non-linear In-Plane Resistivity and Hall Coefficient in the Normal State of Cuprates: Polaronic Approach

We present a theoretical study of the in-plane resistivity ρ a b (T) and Hall coefficient R H (T) within the polaronic model and precursor pairing scenario by considering a two-component charge carrier picture in the normal state of high-temperature superconducting cuprates (HTSC). Here, we use a Boltzmann-equation approach and extended BCS-like model to compute ρ a b (T) and R H (T) in the τ-approximation. The opening of the pseudogap (PG) in the normal state of the cuprates should affect their transport properties. We have found that the transition to the PG regime and the effective conductivity of charge carriers in the normal state are responsible for the pronounced non-linear temperature dependence of ρ a b and R H . With the two-component model analysis, we conclude that the opening of the BCS-like PG, while the non-linear temperature dependence of ρ a b and R H could be understood as a consequence of pairing fluctuations in the PG state of cuprate superconductors. The calculated results for ρ a b (T) and R H (T) were compared with the experimental data obtained for various hole-doped cuprates. For all the considered cases, a good quantitative agreement was found between theory and experimental data. We also show that the energy scales of the binding energies of charge carriers are identified by PG crossover temperature on the cuprate phase diagram.

Electron dynamics in the normal state of cuprates: Spectral function, Fermi surface and ARPES data

An influence of the electron-phonon interaction on excitation spectrum and damping in a narrow band electron subsystem of cuprates has been investigated. Within the framework of the t-J model an approach to solving a problem of account of both strong electron correlations and local electron–phonon binding with characteristic Einstein mode ω0 in the normal state has been presented. In approximation Hubbard-I it was found an exact solution for the polaron bands. We established that in the low-dimensional system with a pure kinematic part of Hamiltonian a complicated excitation spectrum is realized. It is determined mainly by peculiarities of the lattice Green's function. In the definite area of the electron concentration and hopping integrals a correlation gap may be possible on the Fermi level. Also, in specific cases it is observed a doping evolution of the Fermi surface. We found that the strong electron–phonon binding enforces a degree of coherence of electron-polaron excitations near the Fermi level and spectrum along the nodal direction depends on wave vector module weakly. It corresponds to ARPES data. A possible origin of the experimentally observed kink in the nodal direction of cuprates is explained by fine structure of the polaron band to be formed near the mode −ω0.

Fermi Arcs and Pseudogap in Cuprate Superconductors

Earlier, we have proposed [ arXiv:1110.0227 ] the model of HTSC electronic structure modification under doping. In this model, doping by localized charges plays the key role, being responsible for local closing of the gap Δct between excitonic-like 3d10L− state of cation and electronic 3d9L state of anion and formation (at a certain dopant concentration) of the percolation cluster with the Fermi surface located in the cation-anion band of peculiar nature. This electronic structure is favorable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron–electron interaction. Here, the nature of normal state of cuprates as well as the mechanism of pseudogap and Fermi arcs formation are considered in the framework of this model by example YBCO.

EFFECT OF EXTERNAL MAGNETIC FIELD ON AFM ORDER PARAMETER AND NEEL TEMPERATURE IN ELECTRON DOPED CUPRATE SUPERCONDUCTORS

A theoretical model is presented to study the effect of an external magnetic field on antiferromagnetic (AFM) order parameter and Neel temperature in electron doped high TC cuprates in normal state. The expression for the antiferromagnetic order parameter is derived by using double time electron Green function of Zubarev type and studied numerically. It is found that the magnetic field up to a certain parametric value considerably enhances the Neel temperature. Above this value, the Neel temperature decreases on increasing the field. The trend of field dependence of the AFM order parameter at 30 K is quite similar to that of Neel temperature. This parametric study of external magnetic field is in good agreement with experimental findings.

Nonmonotonic pseudogap in high-Tccuprates

The mechanism of high-temperature superconductivity has not been resolved for so long because the normal state of cuprates, which exhibits enigmatic pseudogap phenomena, is not yet understood. We performed careful temperature- and momentum-resolved photoemission experiments to show that the depletion of the spectral weight in slightly underdoped cuprate superconductor, usually called the “pseudogap,” exhibits an unexpected nonmonotonic temperature dependence: decreases linearly approaching T at which it reveals a sharp transition but does not vanish and starts to increase gradually again at higher temperature. The low-temperature behavior of the pseudogap is remarkably similar to one of the incommensurate charge ordering gap in the transitionmetal dichalcogenides, while the reopening of the gap at room temperature fits the scenario of temperaturedriven metal-insulator transition. This observation suggests that two phenomena, the electronic instability to density-wave formation and the entropy-driven metal-to-insulator crossover, may coexist in the normal state of cuprates.

Gauge approach to the specific heat in the normal state of cuprates

Many experimental features of the electronic specific heat and entropy of high-${T}_{c}$ cuprates in the normal state, including the nontrivial temperature dependence of the specific-heat coefficient $\ensuremath{\gamma}$ and the negative intercept of the extrapolated entropy to $T=0$ for underdoped cuprates, are reproduced using the spin-charge gauge approach to the $t\text{\ensuremath{-}}J$ model. The entropy turns out to be basically due to fermionic excitations but with a temperature dependence of the specific-heat coefficient controlled by fluctuations of a gauge field coupling them to gapful bosonic excitations. In particular the negative intercept of the extrapolated entropy at $T=0$ in the pseudogap ``phase'' is attributed to the scalar component of the gauge field, which implements the local no-double occupancy constraint.

NMR Spin-Lattice Relaxation Rates in Cuprates

An analysis of nuclear spin-lattice relaxation data in the normal state of cuprates that appropriately accounts for the highly anisotropic structures shows no contrasting temperature dependence of the Cu, O, and Y relaxations, which suggests that all nuclei relax by the same mechanism of the spin liquid. To investigate the temperature dependence of this mechanism, the model of fluctuating fields is used in which the rates are expressed in terms of hyperfine interaction energies and an effective correlation time τ eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added more coherently at low temperature but incoherently at high temperature. At low temperatures, τ eff grows linearly with temperature as in ordinary metals. At high temperatures, however, the nuclear spin-lattice relaxation rates in various cuprates unequivocally reflect local moment features. This behavior is similar to that observed for the magnetic transition metals Fe, Co, and Ni, where also some properties show a cross-over from an itinerant behavior of delocalized electrons at low to that of localized moments at high temperatures.

Identification of Raman peaks of high- [formula omitted] cuprates in normal state through density of states

We present a microscopic theory to explain and identify the Raman spectral peaks of high- T c cuprates R 2 - x M x CuO 4 in the normal state. We used electronic Hamiltonian prescribed by Fulde in presence of anti-ferromagnetism. Phonon interaction to the hybridization between the conduction electrons of the system and the f-electrons has been incorporated in the calculation. The phonon spectral density is calculated by the Green's function technique of Zubarev at zero wave vector and finite (room) temperature limit. The four Raman active peaks ( P 1 – P 4 ) representing the electronic states of the atomic sub-systems of the cuprate system are identified by the calculated quasi-particle energy bands and electron density of states (DOS). The effect of interactions on these peaks are also explained.

Incommensurate spin dynamics in underdoped cuprate perovskites

The incommensurate magnetic response observed in normal-state cuprate perovskites is interpreted based on Mori's projection operator formalism and the t– J model of Cu–O planes. In agreement with experiment, the calculated dispersion of maxima in the susceptibility has the shape of two parabolas with upward- and downward-directed branches which converge at the antiferromagnetic wave vector. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to the weakness of the interaction between the spin excitations and holes near the hot spots. Also in conformity with experiment, the low-frequency incommensurability parameter δ grows with the hole concentration x for x ≲ 0.12 and then saturates.

Incommensurate Magnetic Response in Cuprate Perovskites

AbstractFor Abstract see ChemInform Abstract in Full Text.

INCOMMENSURATE SPIN DYNAMICS IN UNDERDOPED CUPRATE PEROVSKITES

The incommensurate magnetic response observed in normal-state cuprate perovskites is interpreted based on the projection operator formalism and the t–J model of Cu-O planes. In agreement with experiment the calculated dispersion of maxima in the susceptibility has the shape of two parabolas with upward and downward branches which converge at the antiferromagnetic wave vector. The maxima are located at the momenta (½, ½ ± δ), (½ ± δ, ½) and at (½ ± δ, ½ ± δ), (½ ± δ, ½ ∓ δ) in the lower and upper parabolas, respectively. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the weakness of the interaction between the spin excitations and holes near the hot spots. The frequency dependence of the susceptibility is shown to depend strongly on the hole bandwidth and damping and varies from the shape observed in YBa 2 Cu 3 O 7-y to that inherent in La 2-x Sr x CuO 4.

Incommensurate magnetic response in cuprate perovskites

The incommensurate magnetic response of the normal-state cuprate perovskites is interpreted based on Mori's memory function approach and the t– J model of Cu–O planes. In agreement with experiment the calculated dispersion of the susceptibility maxima has the shape of two parabolas with upward and downward branches which converge at the antiferromagnetic wave vector. The maxima are located at ( 1 2 , 1 2 ± δ ) , ( 1 2 ± δ , 1 2 ) and at ( 1 2 ± δ , 1 2 ± δ ) , ( 1 2 ± δ , 1 2 ∓ δ ) in the lower and upper parabolas, respectively. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the weakness of the interaction between the spin excitations and holes near the hot spots.

Temperature dependence of electrical conductivity in systems with large polarons and bipolarons

Conductivity increase with temperature at constant carrier concentration is accustomed in systems of small polarons. We demonstrate it can occur in systems with large (bi)polarons at their transition into delocalized states. Calculated temperature dependence of conductivity demonstrates good consent with experiments on BaTiO 3, TiO 2, and superconducting cuprates in normal state.

Variational tests of current order parameters in the Hubbard model

Variational tests are performed for current order parameters as probable sources of the pseudogap normal state of cuprates. The calculations are carried out based on the states with correlations of the valence bond type whose formation can induce in principle both the superconducting order of the d symmetry and current phases. It is shown for the t-t′-U Hubbard models with a large value of U(∼8t) and the Hubbard splitting of the conduction band that (1) phases of alternating charge and longitudinal spin currents cannot be realized and (2) transverse spin currents are not compatible with the superconducting order and they could exist against the normal-state background only within a very narrow doping region near the optimal one. This region does not correspond to the region of existence of a pseudogap in cuprates, which refutes the above-mentioned hypothesis of the pseudogap origin. The requirements to the parameters of models for which the consideration of correlations of the valence bond type yields a reasonable phase curve. The existence of current phases in the t-t′-U-V Hubbard models with a strong interaction (V>0.25t) of particles in neighboring sites is predicted when the d-superconductivity is completely suppressed.

Pseudogap in the normal state of cuprates

We show that underdoped cuprates are disordered materials with the diffusion coefficients of carriers as low as 10 −5 m 2.s −1. In these conditions Coulomb interaction between electrons must be taken into account. The main effect is to open a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called “pseudogap” in the normal state and in particular its value, its anisotropy and its variation with doping.

Pseudogap in the Normal State of Cuprates

We consider underdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10 m 2 s -1 , In these conditions, Coulomb interaction between electrons must be taken into account. The main effect is to open a dip and even a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called pseudogap in the normal state and in particular its value, anisotropy, and variation with doping.

Electron-energy loss spectra and plasmon resonance in cuprates

The consequences of the non-Drude charge response in the normal state of cuprates and the effect of the layered structure on electron-energy loss spectra are investigated, both for experiments in the transmission and the reflection mode. It is shown that in the intermediate doping regime the plasmon resonance has to be nearly critically damped as a result of the anomalous frequency dependence of the relaxation rate. This also implies an unusual low-energy dependence of the loss function. Both facts are consistent with experiments in cuprates. Our study based on the t-J model shows good agreement with measured plasmon frequencies.