A new Raman technique of superior spectral resolution

Raman-active vibrational modes are coherently excited by the transient stimulated Raman process. A subsequent delayed probe of relatively long duration interacts with the freely relaxing vibrations. Raman spectra are generated with higher resolution and more accurate peak positions than in conventional Raman spectroscopy. In liquid cyclohexane four new Raman lines were readily detected in the frequency range 2870–2920 cm−1

Narrowing of spectral lines beyond the natural or dephasing line width

Transient excitation and gated or delayed observation provides a narrowing of the inherent line width of the transition. Possible experimental one- and two-photon systems are discussed and the loss of signal with line narrowing is calculated. The general case of lines broadened by dephasing processes is treated. Contact is made with the line narrowing in recent Raman type experiments

Quasi-particle Lifetimes in a d_{x^2-y^2} Superconductor

We consider the lifetime of quasi-particles in a d-wave superconductor due to scattering from antiferromagnetic spin-fluctuations, and explicitly separate the contribution from Umklapp processes which determines the electrical conductivity. Results for the temperature dependence of the total scattering rate and the Umklapp scattering rate are compared with relaxation rates obtained from thermal and microwave conductivity measurements, respectively.Comment: 14 pages, 4 figure

The influence of phase-modulation on femtosecond time-resolved coherent Raman spectroscopy

The influence of phase-modulation on femtosecond time-resolved coherent Raman scattering is investigated theoretically and experimentally. The coherent Raman signal taken as a function of the spectral position shows unexpected temporal oscillations close to time zero. A theoretical analysis of the coherent Raman scattering process indicates that the femtosecond light pulses are amplitude and phase modulated. The pulses are asymmetric in time with more slowly decaying trailing wings. The phase of the pulse amplitude contains quadratic and higher-order contributions

Microwave conductivity of YBa2_2Cu3_3O6.99_{6.99} including inelastic scattering

The fluctuation spectrum responsible for the inelastic scattering in YBa$_2$Cu$_3$O$_{6.99}$ which was recently determined from consideration of the in-plane optical conductivity in the infrared, is used to calculate the temperature dependence of the microwave conductivity at several measured frequencies. Reasonable overall agreement can only be achieved if, in addition, some impurity scattering is included within a model potential intermediate between weak (Born) and strong (unitary) limit.Comment: 15 pages, 5 figures accepted for publication in Phys. Rev.

Dynamic structure selection and instabilities of driven Josephson lattice in high-temperature superconductors

We investigate the dynamics of the Josephson vortex lattice in layered high-T$_{c}$ superconductors at high magnetic fields. Starting from coupled equations for superconducting phases and magnetic field we derive equations for the relative displacements [phase shifts] between the planar Josephson arrays in the layers. These equations reveal two families of steady-state solutions: lattices with constant phase shifts between neighboring layers, starting from zero for a rectangular configuration to $\pi$ for a triangular configuration, and double-periodic lattices. We find that the excess Josephson current is resonantly enhanced when the Josephson frequency matches the frequency of the plasma mode at the wave vector selected by the lattice structure. The regular lattices exhibit several kinds of instabilities. We find stability regions of the moving lattice in the plane lattice structure - Josephson frequency. A specific lattice structure at given velocity is selected uniquely by boundary conditions, which are determined by the reflection properties of electromagnetic waves generated by the moving lattice. With increase of velocity the moving configuration experiences several qualitative transformations. At small velocities the regular lattice is stable and the phase shift between neighboring layers smoothly decreases with increase of velocity, starting from $\pi$ for a static lattice. At the critical velocity the lattice becomes unstable. At even higher velocity a regular lattice is restored again with the phase shift smaller than $\pi/2$. With increase of velocity, the structure evolves towards a rectangular configuration.Comment: 28 pages, 12 figures, submitted to Phys. Rev.

Optical properties of the pseudogap state in underdoped cuprates

Recent optical measurements of deeply underdoped cuprates have revealed that a coherent Drude response persists well below the end of the superconducting dome. In addition, no large increase in optical effective mass has been observed, even at dopings as low as 1%. We show that this behavior is consistent with the resonating valence bond spin-liquid model proposed by Yang, Rice, and Zhang. In this model, the overall reduction in optical conductivity in the approach to the Mott insulating state is caused not by an increase in effective mass, but by a Gutzwiller factor, which describes decreased coherence due to correlations, and by a shrinking of the Fermi surface, which decreases the number of available charge carriers. We also show that in this model, the pseudogap does not modify the low-temperature, low-frequency behavior, though the magnitude of the conductivity is greatly reduced by the Gutzwiller factor. Similarly, the profile of the temperature dependence of the microwave conductivity is largely unchanged in shape, but the Gutzwiller factor is essential in understanding the observed difference in magnitude between ortho-I and -II YBa$_2$Cu$_3$O$_y$.Comment: 9 pages, 6 figures, submitted to Eur. Phys. J.

Optical Sum Rule anomalies in the High-Tc Cuprates

We provide a brief summary of the observed sum rule anomalies in the high-T$_c$ cuprate materials. A recent issue has been the impact of a non-infinite frequency cutoff in the experiment. In the normal state, the observed anomalously high temperature dependence can be explained as a `cutoff effect'. The anomalous rise in the optical spectral weight below the superconducting transition, however, remains as a solid experimental observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal

Optical Sum Rule in Finite Bands

In a single finite electronic band the total optical spectral weight or optical sum carries information on the interactions involved between the charge carriers as well as on their band structure. It varies with temperature as well as with impurity scattering. The single band optical sum also bears some relationship to the charge carrier kinetic energy and, thus, can potentially provide useful information, particularly on its change as the charge carriers go from normal to superconducting state. Here we review the considerable advances that have recently been made in the context of high $T_c$ oxides, both theoretical and experimental.Comment: Review article accepted for publication in J. Low Temp. Phys. 29 pages, 33 figure