Polaronic features in the optical properties of the Holstein-t-J model

We derive the exact solution for the optical conductivity $\sigma(\omega)$ of one hole in the Holstein-t-J model in the framework of dynamical mean-field theory (DMFT). We investigate the magnetic and phonon features associated with polaron formation as a function of the exchange coupling $J$, of the electron-phonon interaction $\lambda$ and of the temperature. Our solution directly relates the features of the optical conductivity to the excitations in the single-particle spectral function, revealing two distinct mechanisms of closing and filling of the optical pseudogap that take place upon varying the microscopic parameters. We show that the optical absorption at the polaron crossover is characterized by a coexistence of a magnon peak at low frequency and a broad polaronic band at higher frequency. An analytical expression for $\sigma(\omega)$ valid in the polaronic regime is presented.Comment: improved version, as submitted to Phys. Rev.

Magnetization vector in the reversible region of a highly anisotropic cuprate superconductor: anisotropy factor and the role of 2D vortex fluctuations

By using a high quality Tl2Ba2Ca2Cu3O10 (Tl-2223) single crystal as an example, the magnetization vector was probed in the reversible region of highly anisotropic cuprate superconductors. For that, we have measured its components along and transverse to the applied magnetic field for different crystal orientations. The analysis shows that the angular dependence of the perpendicular component of the magnetization vector follows the one predicted by a London-like approach which includes a contribution associated with the thermal fluctuations of the 2D vortex positions. For the Tl-2223 crystal studied here, a lower bound for the anisotropy factor was estimated to be about 190.Comment: 6 pages, 3 figure

Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

There is an ongoing debate about the nature of the bosonic excitations responsible for the quasiparticle self energy in high temperature superconductors -- are they phonons or spin fluctuations? We present a careful analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV in angle resolved photoemission data using Eliashberg theory for a d-wave superconductor. Starting from the assumption that nodal quasiparticles are not coupled to the $(\pi,\pi)$ magnetic resonance, the sharp structure at $70$meV can be assigned to phonons. We find that not only can we account for the shifts of the kink energy seen on oxygen isotope substitution but also get a quantitative estimate of the fraction of the area under the electron-boson spectral density which is due to phonons. We conclude that for optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ phonons contribute $\sim 10$% and non-phononic excitations $\sim 90$%.Comment: 6 pages, 3 figure

Spin Dynamics in Cuprates: Optical Conductivity of HgBa2CuO4

The electron-boson spectral density function I^2ChiOmega responsible for carrier scattering of the high temperature superconductor HgBa2CuO4 (Tc = 90 K) is calculated from new data on the optical scattering rate. A maximum entropy technique is used. Published data on HgBa2Ca2Cu3O8 (Tc = 130 K) are also inverted and these new results are put in the context of other known cases. All spectra (with two notable exceptions) show a peak at an energy (Omega_r) proportional to the superconducting transition temperature Omega_r ~= 6.3 kB.Tc. This charge channel relationship follows closely the magnetic resonance seen by polarized neutron scattering, Omega_r^{neutron} ~= 5.4 kB.Tc. The amplitudes of both peaks decrease strongly with increasing temperature. In some cases, the peak at Omega_r is weak and the spectrum can have additional maxima and a background extending up to several hundred meV

Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates

We propose a new phase of matter, an electronic phase separation transition that starts near the upper pseudogap and segregates the holes into high and low density domains. The Cahn-Hilliard approach is used to follow quantitatively this second order transition. The resulting grain boundary potential confines the charge in domains and favors the development of intragrain superconducting amplitudes. The zero resistivity transition arises only when the intergrain Josephson coupling $E_J$ is of the order of the thermal energy and phase locking among the superconducting grains takes place. We show that this approach explains the pseudogap and superconducting phases in a natural way and reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig

Spectroscopy of H3_3S: evidence of a new energy scale for superconductivity

The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K has provided a new impetus to the search for even higher $T_c$. Theory predicted and experiment confirmed that the phase involved is H$_3$S with Im-3m crystal structure. The observation of a sharp drop in resistance to zero at $T_c$, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Here, we provide a first optical spectroscopy study of this new superconductor. Experimental results for the optical reflectivity of H$_3$S, under high pressure of 150 GPa, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density $\alpha^2$F($\Omega$). Two significant features stand out: some remarkably strong infrared active phonons at $\approx$ 160 meV and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. In this energy range, as predicted by theory, H$_3$S is found to become a better reflector with increasing temperature. This temperature evolution is traced to superconductivity originating from the electron-phonon interaction. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom.Comment: 10 pages, 8 figures. Main manuscript and supplementary informatio

Theory of the Fermi Arcs, the Pseudogap, TcT_c and the Anisotropy in k-space of Cuprate Superconductors

The appearance of the Fermi arcs or gapless regions at the nodes of the Fermi surface just above the critical temperature is described through self-consistent calculations in an electronic disordered medium. We develop a model for cuprate superconductors based on an array of Josephson junctions formed by grains of inhomogeneous electronic density derived from a phase separation transition. This approach provides physical insights to the most important properties of these materials like the pseudogap phase as forming by the onset of local (intragrain) superconducting amplitudes and the zero resistivity critical temperature $T_c$ due to phase coherence activated by Josephson coupling. The formation of the Fermi arcs and the dichotomy in k-space follows from the direction dependence of the junctions tunneling current on the d-wave symmetry on the $CuO_2$ planes. We show that this semi-phenomenological approach reproduces also the main future of the cuprates phase diagram.Comment: 5 pages 7 fig

High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc

The advent of high transverse-field muon spin rotation (TF-muSR) has led to recent muSR investigations of the magnetic-field response of cuprates above the superconducting transition temperature T_c. Here the results of such experiments on hole-doped cuprates are reviewed. Although these investigations are currently ongoing, it is clear that the effects of high field on the internal magnetic field distribution of these materials is dependent upon a competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the response to the external field above Tc is dominated by heterogeneous spin magnetism. However, the magnetism that dominates the observed inhomogeneous line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely different kind of magnetism in the heavily overdoped regime. The origin of the magnetism above x ~ 0.19 is currently unknown, but its presence hints at a competition between superconductivity and magnetism that is reminiscent of the underdoped regime. In contrast, the width of the internal field distribution of underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of superconducting carriers. This observation suggests that the magnetic response above Tc is not dominated by electronic moments, but rather inhomogeneous fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference