J. Lorenzana

Coupling of a high-energy excitation to superconducting quasiparticles in a cuprate from coherent charge fluctuation spectroscopy

Dynamical information on spin degrees of freedom of proteins or solids can be obtained by NMR and electron spin resonance. A technique with similar versatility for charge degrees of freedom and their ultrafast correlations could move the understanding of systems like unconventional superconductors forward. By perturbing the superconducting state in a high-Tc cuprate, using a femtosecond laser pulse, we generate coherent oscillations of the Cooper pair condensate that can be described by an NMR/electron spin resonance formalism. The oscillations are detected by transient broad-band reflectivity and are found to resonate at the typical scale of Mott physics (2.6 eV), suggesting the existence of a nonretarded contribution to the pairing interaction, as in unconventional (non-Migdal–Eliashberg) theories.

Dynamics of Metallic Stripes in Cuprates

We study the dynamics of metallic vertical stripes in cuprates within the three-band Hubbard model based on a recently developed time-dependent Gutzwiller approximation. As doping increases, the optical conductivity shows transfer of spectral weight from the charge-transfer band towards (i) an incoherent band centered at 1.3 eV, (ii) a Drude peak, due mainly to motion along the stripe, and (iii) a low-energy collective mode which softens with doping and merges with (ii) at optimum doping in good agreement with experiment. The softening is related to the quasidegeneracy between Cu-centered and O-centered mean-field stripe solutions close to optimal doping.

Phase separation frustrated by the long-range Coulomb interaction. I. Theory

first-order density-driven phase transitions between two phases in the presence of a compensating rigid background. In the coexistence region we study mixed states formed by regions of one phase surrounded by the other in the case in which the scale of the inhomogeneities is much larger than the interparticle distance. Two geometries are studied in detail: spherical drops of one phase into the other and a layered structure of one phase alternating with the other. In the latter case we find the optimum density profile in an approximation in which the free energy is a function of the local density @local density approximation ~LDA!#. It is shown that an approximation in which the density is assumed to be uniform @uniform density approximation ~UDA!# within each phase region gives results very similar to those of the more involved LDA approach. Within the UDA we derive the general equations for the chemical potential and the pressures of each phase which generalize the Maxwell construction to this situation. The equations are valid for a rather arbitrary geometry. We find that the transition to the mixed state is quite abrupt; i.e., inhomogeneities of the first phase appear with a finite value of the radius and of the phase volume fraction. The maximum size of the inhomogeneities is found to be on the scale of a few electric field screening lengths. Contrary to the ordinary Maxwell construction, the inverse specific volume of each phase depends here on the global density in the coexistence region and can decrease as the global density increases. The range of densities in which coexistence is observed shrinks as the LRC interaction increases until it reduces to a singular point. We argue that close to this singular point the system undergoes a lattice instability as long as the inverse lattice compressibility is finite.

Doping dependence of spin excitations in the stripe phase of high- T c superconductors

Based on the time-dependent Gutzwiller approximation for the extended Hubbard model, we calculate the energy and momentum dependence of spin excitations for striped ground states. Our starting point correctly reproduces the observed doping dependence of the incommensurability in La-based cuprates and the dispersion of magnetic modes in the insulating parent compound. This allows us to make quantitative predictions for the doping evolution of the dispersion of magnetic modes in the stripe phase including the energy

Sum rules and missing spectral weight in magnetic neutron scattering in the cuprates

{\ensuremath{\omega}}_{0}

Time-Dependent Gutzwiller Approximation for the Hubbard Model

and intensity of the resonance peak as well as the velocity

Proximity of iron pnictide superconductors to a quantum tricritical point

c

Competing orders in FeAs layers.

of the spin-wave-like Goldstone mode. In the underdoped regime

Coulomb-Frustrated Phase Separation Phase Diagram in Systems with Short-Range Negative Compressibility

{n}_{h}l1∕8

Viscoelasticity and metastability limit in supercooled liquids

, we find a weak linear dependence of