Canonical Transformation Approach to the Ultrafast Non-linear Optical Dynamics of Semiconductors

We develop a theory describing the effects of many-particle Coulomb correlations on the coherent ultrafast nonlinear optical response of semiconductors and metals. Our approach is based on a mapping of the nonlinear optical response of the ``bare'' system onto the linear response of a ``dressed'' system. The latter is characterized by effective time-dependent optical transition matrix elements, electron/hole dispersions, and interaction potentials, which in undoped semiconductors are determined by the single-exciton and two-exciton Green functions in the absence of optical fields. This mapping is achieved by eliminating the optically-induced charge fluctuations from the Hamiltonian using a Van Vleck canonical transformation. It takes into account all many-body contributions up to a given order in the optical fields as well as important Coulomb-induced quantum dynamics to all orders in the optical field. Our approach allows us to distinguish between optical nonlinearities of different origins and provides a physically-intuitive interpretation of their manifestations in ultrafast coherent nonlinear optical spectroscopy.Comment: 24 page

Electron momentum distribution of a single mobile hole in the t-J model

We investigate the electron momentum distribution function (EMDF) for the two-dimensional t-J model. The results are based on the self-consistent Born approximation (SCBA) for the self-energy and the wave function. In the Ising limit of the model we give the results in a closed form, in the Heisenberg limit the results are obtained numerically. An anomalous momentum dependence of EMDF is found and the anomaly is in the lowest order in number of magnons expressed analitycally. We interpret the anomaly as a fingerprint of an emerging large Fermi surface coexisting with hole pockets.Comment: M2S - submitted to Physica

Spectral Properties of the Attractive Hubbard Model

Deviations from Fermi liquid behavior are well documented in the normal state of the cuprate superconductors, and some of these differences are possibly related to pre-formed pairs appearing at temperatures above T_c. In order to test these ideas we have investigated the attractive Hubbard model within a self-consistent, conserving ladder approximation. In this version of the theory, no feature is present which can be related to the pseudo gap found in the high-T_c materials. Further, the interactions between two-particle bound states change the physics of the superconducting instability in a profound fashion, and lead to a completely different phenomenology that one predicts based on the non-self-consistent version of the same theory.Comment: 4 pages including 2 figures, to appear in the proceedings of the SNS'9

Evolution of Hole and Spin Dynamics in High Temperature Superconductors within the Small Hole Density Limit of the t-J Model

The evolution of hole and spin dynamics in high temperature superconductors is studied within the self-consistent noncrossing approximation of the t-J model in the small hole density limit. As the doping concentration is increased, long-range electron correlations disappear rapidly and the quasiparticle energy band becomes considerably narrow. At a small hole density long-range antiferromagnetic order is destroyed leading to the inadequacy of spin wave basis approximation near small wave vectors. Spin excitations near the antiferromagnetic zone boundary are strongly renormalized and damped but they are still well described within spin wave basis approximation.Comment: 4 pages, 4 figures, revetex, accepted for publication in Solid State Communicatio

On the interpretation of wave function overlaps in quantum dots

The spontaneous emission rate of excitons strongly confined in quantum dots is proportional to the overlap integral of electron and hole envelope wave functions. A common and intuitive interpretation of this result is that the spontaneous emission rate is proportional to the probability that the electron and the hole are located at the same point or region in space, i.e. they must coincide spatially to recombine. Here we show that this interpretation is not correct even loosely speaking. By general mathematical considerations we compare the envelope wave function overlap, the exchange overlap integral, and the probability of electrons and holes coinciding and find that the frequency dependence of the envelope wave function overlap integral is very different from that expected from the common interpretation. We show that these theoretical considerations lead to predictions for measurements. We compare our qualitative predictions with recent measurements of the wave function overlap and find good agreement.Comment: 4 pages, 3 figure

Prerequisite for superconductivity: appropriate spin-charge correlations

This work investigates the relation between superconductivity and correlations. A simple calculation shows that the appropriate spin-charge correlation is the key role to any superconductivity, and this calculation is consistent with the analyses of unusual properties of superconductors. (Note: the Tc of this model is not given clearly in this work, but we have advanced this mechanism to a t-x model which includes various superconductivities and magnetisms (please see arXiv:0707.3660 and following works).)Comment: 7page

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We present an absorption study of the neutral and positively charged exciton (trion) under the influence of a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the hole gas is created by the formation of trions. The evolution of these populations is studied, including the spin flip and trion formation processes. The contributions of several mechanisms to intensity changes are evaluated, including phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in p-doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding heavy holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blockedComment: 4 pages, 4 figure