Searcharxiv⌕ Search

arXiv subjects

S. W. Koch

Publications and source records attributed to S. W. Koch.

46 records · Page 3Linked to original sources
arXiv2003

Terahertz response of optically excited semiconductors

The induced terahertz response of semiconductor systems is investigated with a microscopic theory. In agreement with recent terahertz experiments, the developed theory fully explains the ultrafast build up of the plasmon resonance and the slow formation of incoherent excitonic populations. For incoherent conditions, it is shown that a terahertz field exclusively probes the correlated electron-hole pairs via a symmetry breaking between many-body correlations with even and odd functional form.

cond-mat.str-el↗
arXiv2002

Influence of Coulomb and Phonon Interaction on the Exciton Formation Dynamics in Semiconductor Heterostructures

A microscopic theory is developed to analyze the dynamics of exciton formation out of incoherent carriers in semiconductor heterostructures. The carrier Coulomb and phonon interaction is included consistently. A cluster expansion method is used to systematically truncate the hierarchy problem. By including all correlations up to the four-point (i.e. two-particle) level, the fundamental fermionic substructure of excitons is fully included. The analysis shows that the exciton formation is an intricate process where Coulomb correlations rapidly build up on a picosecond time scale while phonon dynamics leads to true exciton formation on a slow nanosecond time scale.

cond-mat.str-el↗
arXiv2002

Interacting electrons in a one-dimensional random array of scatterers - A Quantum Dynamics and Monte-Carlo study

The quantum dynamics of an ensemble of interacting electrons in an array of random scatterers is treated using a new numerical approach for the calculation of average values of quantum operators and time correlation functions in the Wigner representation. The Fourier transform of the product of matrix elements of the dynamic propagators obeys an integral Wigner-Liouville-type equation. Initial conditions for this equation are given by the Fourier transform of the Wiener path integral representation of the matrix elements of the propagators at the chosen initial times. This approach combines both molecular dynamics and Monte Carlo methods and computes numerical traces and spectra of the relevant dynamical quantities such as momentum-momentum correlation functions and spatial dispersions. Considering as an application a system with fixed scatterers, the results clearly demonstrate that the many-particle interaction between the electrons leads to an enhancement of the conductivity and spatial dispersion compared to the noninteracting case.

cond-mat.dis-nn↗
arXiv2001

Dynamics of Coulomb-correlated electron-hole pairs in disordered semiconductor nanowires

The dynamics of optically generated electron-hole pairs is investigated in a disordered semiconductor nanowire. The particle pairs are generated by short laser pulses and their dynamics is followed using the Heisenberg equation of motion. Is is shown that Coulomb-correlation acts against localization in the case of the two-interacting particles (TIP) problem. Furthermore, currents are generated using a coherent combination of full-gap and half-gap pulses. The subsequent application of a full-gap pulse after time $τ$ produces an intraband echo phenomenon $2τ$ time later. The echo current is shown to depend on the mass ratio between the electrons and the holes.

cond-mat.dis-nn↗
arXiv2001

Optically induced coherent intra-band dynamics in disordered semiconductors

On the basis of a tight-binding model for a strongly disordered semiconductor with correlated conduction- and valence band disorder a new coherent dynamical intra-band effect is analyzed. For systems that are excited by two, specially designed ultrashort light-pulse sequences delayed by tau relatively to each other echo-like phenomena are predicted to occur. In addition to the inter-band photon echo which shows up at exactly t=2*tau relative to the first pulse, the system responds with two spontaneous intra-band current pulses preceding and following the appearance of the photon echo. The temporal splitting depends on the electron-hole mass ratio. Calculating the population relaxation rate due to Coulomb scattering, it is concluded that the predicted new dynamical effect should be experimentally observable in an interacting and strongly disordered system, such as the Quantum-Coulomb-Glass.

cond-mat.dis-nn↗
arXiv2000

Propagation of Coulomb-correlated electron-hole pairs in semiconductors with correlated and anticorrelated disorder

Local ultrafast optical excitation of electron-hole pairs in disordered semiconductors provides the possibility to observe experimentally interaction-assisted propagation of correlated quantum particles in a disordered environment. In addition to the interaction driven delocalization known for the conventional single-band TIP-(two-interacting-particles)-problem the semiconductor model has a richer variety of physical parameters that give rise to new features in the temporal dynamics. These include different masses, correlated vs. anticorrelated disorder for the two particles, and dependence on spectral position of excitation pulse.

cond-mat.dis-nn↗
arXiv1996

Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling

In this first of three articles on the optical absorption of electrons in half-filled Peierls-distorted chains we present analytical results for non-interacting tight-binding electrons. We carefully derive explicit expressions for the current operator, the dipole transition matrix elements, and the optical absorption for electrons with a cosine dispersion relation of band width $W$ and dimerization parameter $δ$. New correction (``$η$''-)terms to the current operator are identified. A broad band-to-band transition is found in the frequency range $Wδ< ω< W$ whose shape is determined by the joint density of states for the upper and lower Peierls subbands and the strong momentum dependence of the transition matrix elements.

cond-mat↗
arXiv1996

Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain

In this second of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain we present exact results for strongly correlated tight-binding electrons. In the limit of a strong on-site interaction $U$ we map the Hubbard model onto the Harris-Lange model which can be solved exactly in one dimension in terms of spinless fermions for the charge excitations. The exact solution allows for an interpretation of the charge dynamics in terms of parallel Hubbard bands with a free-electron dispersion of band-width $W$, separated by the Hubbard interaction $U$. The spin degrees of freedom enter the expressions for the optical absorption only via a momentum dependent but static ground state expectation value. The remaining spin problem can be traced out exactly since the eigenstates of the Harris-Lange model are spin-degenerate. This corresponds to the Hubbard model at temperatures large compared to the spin exchange energy. Explicit results are given for the optical absorption in the presence of a lattice distortion $δ$ and a nearest-neighbor interaction $V$. We find that the optical absorption for $V=0$ is dominated by a peak at $ω=U$ and broad but weak absorption bands for $| ω-U | \leq W$. For an appreciable nearest-neighbor interaction, $V>W/2$, almost all spectral weight is transferred to Simpson's exciton band which is eventually Peierls-split.

cond-mat↗
arXiv1996

Optical absorption of strongly correlated half-filled Mott-Hubbard chains

In this last of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain we address the dimerized extended Hubbard model in the limit of a large on-site interaction $U$. When the Hubbard interaction is large both compared to the band width $W$ and the nearest neighbor interaction $V$ the charge dynamics is properly described by the Harris-Lange model. This model can be exactly mapped onto a model of free spinless Fermions in parallel (Hubbard-)bands of width $W$ which are eventually Peierls-split. To determine the coherent absorption features at low temperatures we design and employ the ``no-recoil approximation'' in which we assume that the momentum transfer to the spin degrees of freedom can only be $Δq_S=0$ or $Δq_S=π/a$ during an optical excitation. We present explicit analytical results for the optical absorption in the presence of a lattice dimerization $δ$ and a nearest-neighbor interaction $V$ for the Néel and dimer state. We find that the coherent part of the optical absorption for $V=0$ is given by a single peak at $ω=U$ and broad but weak absorption bands for $Wδ\leq |ω-U| \leq W$. The central peak at $ω=U$ only vanishes for $δ=0$ in the Néel state. For an appreciable nearest neighbor interaction $V>W/2$ almost all spectral weight is transferred to the $Δq_C=0$-exciton and the $Δq_C=π/a$-exciton whose relative spectral weights very sensitively depend on both the lattice and the spin dimerization of the ground state.

cond-mat↗