Response In Cuprate Superconductors Research Articles

Electromagnetic response of cuprate superconductors with coexisting electronic nematicity

ABSTRACT The electronically nematic order has emerged as a key feature of cuprate superconductors, however, its correlation with the fundamental properties such as the electromagnetic response remains unclear. Here the nematic-order state strength dependence of the electromagnetic response in cuprate superconductors is investigated within the framework of the kinetic-energy-driven superconductivity. It is shown that a significant anisotropy of the electromagnetic response is caused by the electronic nematicity. In particular, in addition to the pure d-wave component of the superconducting gap, the pure s-wave component of the superconducting gap is generated by the electronic nematicity, therefore there is a coexistence and competition of the d-wave component and the s-wave component. This coexistence and competition leads to that the maximal condensation energy appears at around the optimal strength of the electronic nematicity, and then decreases in both the weak and strong strength regions, which in turn induces the enhancement of superconductivity, and gives rise to the dome-like shape of the nematic-order state strength dependence of the superfluid density.

Doping Dependence of Electromagnetic Response in Cuprate Superconductors

The study of the electromagnetic response in cuprate superconductors plays a crucial role in the understanding of the essential physics of these materials. Here the doping dependence of the electromagnetic response in cuprate superconductors is studied within the kinetic energy–driven superconducting mechanism. The kernel of the response function is evaluated based on the linear response approximation for a purely transverse vector potential and can be broken up into its diamagnetic and paramagnetic parts. In particular, this paramagnetic part exactly cancels the corresponding diamagnetic part in the normal-state, and then the Meissner effect is obtained within the entire superconducting phase. Following this kernel of the response function, the electromagnetic response calculation in terms of the specular reflection model qualitatively reproduces many of the striking features observed in the experiments. In particular, the local magnetic field profile follows an exponential law, while the superfluid density exhibits the nonlinear temperature behavior at the lowest temperatures, followed by the linear temperature dependence extending over the most of the superconducting temperature range. Moreover, the maximal value of the superfluid density occurs at around the critical doping δcritical ∼ 0.16 and then decreases in both lower doped and higher doped regimes. The theory also shows that the nonlinear temperature dependence of the superfluid density at the lowest temperatures can be attributed to the nonlocal effects induced by the d-wave gap nodes on the electron Fermi surface.

Pseudogap-induced anisotropic suppression of electronic Raman response in cuprate superconductors

It has become clear that the anomalous properties of cuprate superconductors are intimately related to the formation of a pseudogap. Within the framework of the kinetic-energy-driven superconducting mechanism, the effect of the pseudogap on the electronic Raman response (ERR) of cuprate superconductors in the superconducting-state is studied by taking into account the interplay between the superconducting gap and pseudogap. It is shown that the low-energy spectra almost rise as the cube of energy in the channel and linearly with energy in the channel. It is also shown that the pseudogap is strongly anisotropic in momentum space, where the magnitude of the pseudogap around the nodes is smaller than that around the antinodes, which leads to that the low-energy spectral weight of the spectrum is suppressed heavily by the pseudogap, while the pseudogap has a more modest effect on the ERR in the orientation.

The Temperature Evolution of the Inhomogeneous Magnetic Response of Cuprate Superconductors Above T c

Many different experiments and probes have displayed some form of anomalous behavior that may be related to charge inhomogeneity in different families of cuprate superconductors. In some materials, it appears to be associated with charge density waves and in others as local static domains of varying densities. The doping and temperature evolution of such charge instability is a matter of current intense research. We present here a model based on a phase separation transition to the temperature evolution of transverse field muon spin relaxation (TF- μSR) magnetic inhomogeneous response of cuprates above T c recently measured.

Dynamical spin response in cuprate superconductors from low-energy to high-energy

Within the framework of the kinetic energy driven superconducting mechanism, the dynamical spin response of cuprate superconductors is studied from low-energy to high-energy. The spin self-energy is evaluated explicitly in terms of the collective charge carrier modes in the particle–hole and particle–particle channels, and employed to calculate the dynamical spin structure factor. Our results show the existence of damped but well-defined dispersive spin excitations in the whole doping phase diagram . In particular, the low-energy spin excitations in the superconducting-state have an hour-glass-shaped dispersion, with commensurate resonance that appears in the superconducting-state only , while the low-energy incommensurate spin fluctuations can persist into the normal-state. The high-energy spin excitations in the superconducting-state on the other hand retain roughly constant energy as a function of doping, with spectral weights and dispersion relations comparable to those in the corresponding normal-state. The theory also shows that the unusual magnetic correlations in cuprate superconductors can be ascribed purely to the spin self-energy effects which arise directly from the charge carrier–spin interaction in the kinetic energy of the system. • We provide an explanation to dynamical spin response in cuprate superconductors. • Low-energy spin excitations have an hour-glass-shaped dispersion. • Commensurate resonance appears in the superconducting-state only. • High-energy spin excitations are similar to those found in the parent compound.

Effect of the pseudogap on the infrared response in cuprate superconductors

One of the most essential aspects of cuprate superconductors is a large pseudogap coexisting with a superconducting gap, then some anomalous properties can be understood in terms of the formation of the pseudogap. Within the kinetic energy-driven superconducting mechanism, the effect of the pseudogap on the infrared response of cuprate superconductors in the superconducting state is studied. By considering the interplay between the superconducting gap and pseudogap, the electron current–current correlation function is evaluated based on the linear response approach and it then is employed to calculate finite-frequency conductivity. It is shown that in the underdoped and optimally doped regimes, the transfer of the part of the low-energy spectral weight of the conductivity spectrum to the higher energy region to form a midinfrared band is intrinsically associated with the presence of the pseudogap.

ELECTRONIC RAMAN SCATTERING IN CUPRATE SUPERCONDUCTORS

Since the discovery of cuprate superconductors, the mechanism of high temperature superconductivity is still a mystery. Among the investigation tools, the electronic Raman scattering is a powerful one to probe electronic excitations in different regions of the Fermi surface of cuprate superconductors by a simple choice of the incident and scattered polarization vectors. Thus the symmetry of the superconducting Cooper pairs can be indicated. In this article we review our investigations of electronic Raman scattering in cuprate superconductors based on the t–J model within the kinetic energy driven superconductivity. The theory of electronic Raman response in cuprate superconductors is presented together with an overview of the charge-spin separation fermion-spin theory to handle the t–J model. Some theoretical results of electronic Raman response are presented in comparison with the experimental results. Special emphasize is given to the doping and temperature dependent of electronic Raman spectra.

Doping and temperature dependence of electronic Raman response in cuprate superconductors

The doping and temperature dependence of the electronic Raman response in cuprate superconductors is studied within the kinetic energy driven superconducting mechanism. It is shown that the temperature dependent depletion at low-energy shifts is faster in the B 1 g symmetry than in the B 2 g symmetry. In analogy to the domelike shape of the doping dependent superconducting transition temperature, the maximal peak energy in the B 2 g channel occurs around the optimal doping, and then decreases in both underdoped and overdoped regimes. Moreover, the overall density of Cooper pairs increases with increasing doping in the underdoped regime.

Electromagnetic response in kinetic energy driven cuprate superconductors: Linear response approach

Within the framework of the kinetic energy driven superconductivity, the electromagnetic response in cuprate superconductors is studied in the linear response approach. The kernel of the response function is evaluated and employed to calculate the local magnetic field profile, the magnetic field penetration depth, and the superfluid density, based on the specular reflection model for a purely transverse vector potential. It is shown that the low temperature magnetic field profile follows an exponential decay at the surface, while the magnetic field penetration depth depends linearly on temperature, except for the strong deviation from the linear characteristics at extremely low temperatures. The superfluid density is found to decrease linearly with decreasing doping concentration in the underdoped regime. The problem of gauge invariance is addressed and an approximation for the dressed current vertex, which does not violate local charge conservation is proposed and discussed.

Lattice Instability in High Temperature Superconducting Cuprates and FeAs Systems: Polarons Probed by EXAFS

Carrier-induced lattice distortion (signature of polaron) in oxypnictide superconductors is found by an instantaneous local probe, extended X-ray absorption fine structure (EXAFS). Polaron formation is detected as two distinct nearest neighbor distances (Fe-As),implying an incoherent local mode that develops coherence at the critical temperature. Comparing the results with the unusual lattice response in cuprate superconductors, intimate correlation between evolution of local lattice mode and superconductivity is revealed. The results suggest that strong electron-lattice interaction is present as a common ingredient in the microscopic mechanism of superconducting transition. The effect of magnetic impurity atoms in cuprates further indicates that magnetic scattering becomes diluted as long as polaron formation is conserved. We argue that polaron coherence dominates electrical conduction and magnetic interaction in oxypnictide and cuprate superconductors.

Local lattice response in LaFeAsO0.93F0.07 probed by x-ray absorption spectroscopy: Evidence for carrier-induced lattice distortion

Carrier-induced lattice distortion (signature of strong electron-lattice interaction) in oxypnictide superconductors is found by a local structural study using extended x-ray absorption fine structure (EXAFS). Local lattice instability (polaron formation) is detected as an anomalous upturn of the mean-square relative displacement for the nearest neighbor distances (Fe-As) in LaFeAsO1-xFx (x = 0.07), implying a local lattice distortion that develops below 70 K and disappears at the superconducting critical temperature. Comparing the results with the lattice response in cuprate superconductors, intimate correlation between local lattice mode and superconductivity is revealed. The results suggest that strong electron-lattice interaction is present as a common ingredient in the microscopic mechanism of s uperconducting transition.

Optical conductivity and electronic Raman response of cuprate superconductors

We present the results of detailed analytical calculations for the in-plane optical conductivity and the electronic Raman susceptibility in quasi two-dimensional systems possessing a ground state with two competing order parameters: a d-wave density wave (dDW) and d-wave superconductor (dSC). In the coexisting dDW+dSC phase we determine the frequency dependence of these correlation functions in the presence of randomly distributed non-magnetic impurities in the unitary limit.

Electromagnetic response of cuprate superconductors: Meissner effect in the kinetic energy driven superconductivity

The Meissner effect in cuprate superconductors is studied within the model of kinetic energy driven d-wave superconductivity. The kernel of the response function is derived and employed to calculate doping evolution of the superfluid density within the specular reflection model for a purely transverse vector potential.

Singularities in the optical response of cuprates

We argue that the detailed analysis of the optical response in cuprate superconductors allows one to verify the magnetic scenario of superconductivity in cuprates, as for strong coupling charge carriers to antiferromagnetic spin fluctuations, the second derivative of optical conductivity should contain detectable singularities at $2\Delta +\Delta_{\rm spin}$, $4\Delta$, and $2\Delta+2\Delta_{\rm spin}$, where $\Delta$ is the amplitude of the superconducting gap, and $\Delta_{s}$ is the resonance energy of spin fluctuations measured in neutron scattering. We argue that there is a good chance that these singularities have already been detected in the experiments on optimally doped $YBCO$.

Electrodynamic response in cuprate superconductors

We present results from a microscopic theory of the electrodynamic response (E∥ĉ) in high-Tc superconductors for an array of coupled superconducting layers. In this model each layer is described by a two-dimensional Fermi liquid, and interlayer coupling is mediated by thermally activated incoherent scattering processes. We discuss the surface resistance in the c-direction and show that a d-state symmetry of the order parameter leads to an anomalous T-dependence at frequencies below Δo, and imposes rather stringent conditions on the interlayer coupling.