Ba2YCu3O7- delta : Electrodynamics of crystals with high reflectivity

Analysis of optical data using extended Drude model and generalized Allen's formulas

Extended Drude model formalism has been successfully utilized for analyzing optical spectra of strongly correlated electron systems including heavy-fermion systems and high-T _c superconducting iron pnictides and cuprates. Furthermore, generalized Allen's formulas have been developed and applied to extract the electron-boson spectral density function from measured optical data of high temperature superconductors including cuprates in various material phases. Here we used a reverse process to obtain various optical quantities starting from two typical electron-boson spectral density model functions for three intriguing (normal, pseudogap, and d-wave superconducting) material phases in cuprates. We also assigned the calculated optical results to designated regions in the phase diagram of hole-doped cuprates and compared them with the corresponding measured optical spectra of Bi₂Sr₂CaCu₂ [Formula: see text] (Bi-2212). This comparison suggested that this way of optical data analysis can be a convincing method to study correlated electrons in the copper oxide superconductors and other superconducting systems as well.

Anomalous density fluctuations in a strange metal

A central mystery in high-temperature superconductivity is the origin of the so-called strange metal (i.e., the anomalous conductor from which superconductivity emerges at low temperature). Measuring the dynamic charge response of the copper oxides, [Formula: see text], would directly reveal the collective properties of the strange metal, but it has never been possible to measure this quantity with millielectronvolt resolution. Here, we present a measurement of [Formula: see text] for a cuprate, optimally doped Bi_2.1Sr_1.9CaCu₂O_8+x (T_c = 91 K), using momentum-resolved inelastic electron scattering. In the medium energy range 0.1-2 eV relevant to the strange metal, the spectra are dominated by a featureless, temperature- and momentum-independent continuum persisting to the electronvolt energy scale. This continuum displays a simple power-law form, exhibiting q² behavior at low energy and q²/ω² behavior at high energy. Measurements of an overdoped crystal (T_c = 50 K) showed the emergence of a gap-like feature at low temperature, indicating deviation from power law form outside the strange-metal regime. Our study suggests the strange metal exhibits a new type of charge dynamics in which excitations are local to such a degree that space and time axes are decoupled.

Physics of the resonating valence bond (pseudogap) state of the doped mott insulator: spin-charge locking

The properties of the pseudogap phase above T(C) of the high-T(C) cuprate superconductors are described by showing that the Anderson-Nambu SU(2) spinors of a resonating valence bond spin gap "lock" to those of the electron charge system because of the resulting improvement of kinetic energy. This enormously extends the range of the vortex liquid state in these materials. A heuristic description of the nonlocal electrodynamics of this pseudogap-vortex liquid state is proposed.

High-transition-temperature superconductivity in the absence of the magnetic-resonance mode

The fundamental mechanism that gives rise to high-transition-temperature (high-T(c)) superconductivity in the copper oxide materials has been debated since the discovery of the phenomenon. Recent work has focused on a sharp 'kink' in the kinetic energy spectra of the electrons as a possible signature of the force that creates the superconducting state. The kink has been related to a magnetic resonance and also to phonons. Here we report that infrared spectra of Bi2Sr2CaCu2O8+delta (Bi-2212), shows that this sharp feature can be separated from a broad background and, interestingly, weakens with doping before disappearing completely at a critical doping level of 0.23 holes per copper atom. Superconductivity is still strong in terms of the transition temperature at this doping (T(c) approximately 55 K), so our results rule out both the magnetic resonance peak and phonons as the principal cause of high-T(c) superconductivity. The broad background, on the other hand, is a universal property of the copper-oxygen plane and provides a good candidate signature of the 'glue' that binds the electrons.

Collective density-wave excitations in two-leg Sr14-xCaxCu24O41 ladders

Raman measurements in the 1.5-20 cm(-1) energy range were performed on single crystals of Sr14-xCaxCu24O41. A quasielastic scattering peak (QEP) which softens with cooling is observed only in the polarization parallel to the ladder direction for samples with x=0, 8, and 12. The QEP is a Raman fingerprint of pinned collective density wave excitations screened by uncondensed carriers in the ladder structures. Our results suggest that transport in metallic samples, which is similar to transport in underdoped high-T(c) cuprates, is driven by a collective electronic response.