arXiv subjects

Johann Kroha

Publications and source records attributed to Johann Kroha.

69 records · Page 4Linked to original sources

arXiv2006

Theory of strong localization effects of light in disordered loss or gain media

We present a systematical theory for the interplay of strong localization effects and absorption or gain of classical waves in 3-dimensional, disordered dielectrics. The theory is based on the selfconsistent Cooperon resummation, implementing the effects of energy conservation and its absorptive or emissive corrections by an exact, generalized Ward identity. Substantial renormalizations are found, depending on whether the absorption/gain occurs in the scatterers or in the background medium. We find a finite, gain-induced correlation volume which may be significantly smaller than the scale set by the scattering mean free path, even if there are no truly localized modes. Possible consequences for coherent feedback in random lasers as well as the possibility of oscillatory in time behavior induced by sufficiently strong gain are discussed.

cond-mat.dis-nn↗

arXiv2006

Low-loss resonant modes in deterministically aperiodic nanopillar waveguides

Quasiperiodic Fibonacci-like and fractal Cantor-like single- and multiple-row nanopillar waveguides are investigated theoretically employing the finite difference time domain (FDTD) method. It is shown that resonant modes of the Fibonacci and Cantor waveguides can have a Q-factor comparable with that of a point-defect resonator embedded in a periodic nanopillar waveguide, while the radiation is preferably emitted into the waveguide direction, thus improving coupling to an unstructured dielectric waveguide located along the structure axis. This is especially so when the dielectric waveguide introduces a small perturbation in the aperiodic structure, breaking the structure symmetry while staying well apart from the main localization area of the resonant mode. The high Q-factor and increased coupling with external dielectric waveguide suggest using the proposed deterministically aperiodic nanopillar waveguides in photonic integrated circuits.

cond-mat.mtrl-sci↗

arXiv2005

pi-Junction behavior and Andreev bound states in Kondo quantum dots with superconducting leads

We investigate the temperature- and coupling-dependent transport through Kondo dot contacts with symmetric superconducting s-wave leads. For finite temperature T we use a superconducting extension of a selfconsistent auxiliary boson scheme, termed SNCA, while at T=0 a perturbative renormalization group treatment is applied. The finite-temperature phase diagram for the 0--pi transition of the Josephson current in the junction is established and related to the phase-dependent position of the subgap Kondo resonance with respect to the Fermi energy. The conductance of the contact is evaluated in the zero-bias limit. It approaches zero in the low-temperature regime, however, at finite T its characteristics are changed through the coupling- and temperature-dependent 0--pi transition.

cond-mat.str-el↗

arXiv2002

Self-consistent Conserving Theory for Quantum Impurity Systems: Renormalization Group Analysis

We review the diagrammatic, conserving theory for quantum impurities with strong on-site repulsion. The method is based on auxiliary particle technique, where Wick's theorem is valid, which opens up the possibility for generalizations to more complicated situations. An analysis in terms of the perturbative renormalization group (RG) shows that on the level of the Conserving T--matrix Approximation the theory correctly describes the RG flow, including the non-scaling of potential scattering terms and the correct Kondo temperature.

cond-mat.str-el↗

arXiv2001

Kondo effect in non-equilibrium - Theory of energy relaxation induced by dynamical defects in diffusive nanowires

In diffusive Cu and Au quantum wires at finite transport voltage $U$ the non-equilibrium distribution function $f(E,U)$ exhibits scaling behavior, $f(E,U)=f(E/eU)$, indicating anomalous energy relaxation processes in these wires. We show that in nonequilibrium the Kondo effect, generated either by magnetic impurities (single-channel Kondo effect) or possibly by non-magnetic, degenerate two-level systems (two-channel Kondo effect), produces such scaling behavior as a consequence of a Korringa-like (pseudo)spin relaxation rate $\propto U$ and of damped powerlaw behavior of the impurity spectral density as a remnant of the Kondo strong coupling regime at low temperature but high bias. The theoretical, scaled distribution functions coincide quantitatively with the experimental results, the impurity concentration being the only adjustable parameter. This provides strong evidence for the presence of Kondo defects, either single- or two-channel, in the experimental systems. The relevance of these results for the problem of dephasing in mesoscopic wires is discussed briefly.

cond-mat.mes-hall↗

arXiv2001

Energy and phase relaxation in non-equilibrium diffusive nano-wires with two-level systems

In recent experiments the non-equilibrium distribution function $f(E,U)$ in diffusive Cu and Au quantum wires at a transport voltage $U$ shows scaling behavior, $f(E,U)=f(E/eU)$, indicating a non-Fermi liquid interaction with {\it non-vanishing} T=0 scattering rate. The two-channel Kondo (2CK) effect, possibly produced by degenerate two-level systems, is known to exhibit such behavior. Generalizing the auxiliary boson method to non-equilibrium, we calculate $f(E,U)$ in the presence of 2CK impurities. We show that the 2CK equations reproduce the scaling form $f(E/eU)$. For all measured samples the theoretical, scaled distribution functions coincide quantitatively with the experimental results, the impurity concentration being the only adjustable parameter. This provides a microscopic explanation for the experiments and, considering that no other mechanism producing the scaling form is known to date, lends strong evidence for the presence of degenerate two-level defects in these systems. The relevance of these results for the problem of dephasing in mesoscopic wires is discussed.

cond-mat.mes-hall↗

arXiv2000

Auxiliary particle theory of threshold singularities in photoemission and X-ray absorption spectra: Test of a conserving T-matrix approximation

We calculate the exponents of the threshold singularities in the photoemission spectrum of a deep core hole and its X-ray absorption spectrum in the framework of a systematic many-body theory of slave bosons and pseudofermions (for the empty and occupied core level). In this representation, photoemission and X-ray absorption can be understood on the same footing; no distinction between orthogonality catastrophe and excitonic effects is necessary. We apply the conserving slave particle T-matrix approximation (CTMA), recently developed to describe both Fermi and non-Fermi liquid behavior systems with strong local correlations, to the X-ray problem as a test case. The numerical results for both photoemission and X-ray absorption are found to be in agreement with the exact infrared powerlaw behavior in the weak as well as in the strong coupling regions. We point out a close relation of the CTMA with the parquet equation approach of Nozi{è}res et al.

cond-mat.str-el↗

arXiv2000

Diagrammatic Theory of Anderson Impurity Models: Fermi and Non-Fermi Liquid Behavior

We review a recently developed method, based on a pseudoparticle representation of correlated electrons, to describe both Fermi liquid and non-Fermi liquid behavior in quantum impurity systems. The role of the projection onto the physical Hilbert space and the impossibility of slave boson condensation are discussed. By summing the leading coherent spin and charge fluctuation processes in a fully self-consistent and gauge invariant way one obtains the correct infrared behavior of the pseudoparticles. The temperature dependence of the spin susceptibility for the single channel and two-channel Anderson models is calculated and found to agree well with exact results.

cond-mat.str-el↗

arXiv1998

Fermi and non-Fermi liquid behavior in quantum impurity systems: Conserving slave boson theory

The question of Fermi liquid vs. non-Fermi liquid behavior induced by strong correlations is one of the prominent problems in metallic local moment systems. As standard models for such systems, the SU(N)xSU(M) Anderson impurity models exhibit both Fermi liquid and non-Fermi liquid behavior, depending on their symmetry. Taking the Anderson model as an example, these lectures first give an introduction to the auxiliary boson method to describe correlated systems governed by a strong, short-range electronic repulsion. It is then shown how to include the relevant low-lying excitations (coherent spin flip and charge fluctuation processes), while preserving the local gauge symmetry of the model. This amounts to a conserving T-matrix approximation (CTMA). We prove a cancellation theorem showing that the CTMA incorporates all leading and subleading infrared singularities at any given order in a self-consistent loop expansion of the free energy. As a result, the CTMA recovers the correct infrared behavior of the auxiliary particle propagators, indicating that it correctly describes both the Fermi and the non-Fermi regimes of the Anderson model.

cond-mat.str-el↗

arXiv1998

Correlation-enhanced Friedel oscillations in amorphous alloys and quasicrystals

We show that quantum correlations induced by electron-electron interactions in the presence of random impurity scattering can play an important role in the thermal stabilization of amorphous Hume-Rothery systems: When there is strong backscattering off local, concentrical ion clusters, the static electron density response $χ(0,q)$ acquires a powerlaw divergence at $q=2k_F$ even at elevated temperature. This leads to an enhancement as well as to a systematical phase shift of the Friedel oscillations, both consistent with experiments. The possible importance of this effect in icosahedral quasicrystals is discussed.

cond-mat.mtrl-sci↗

arXiv1998

The Kondo Box: A Magnetic Impurity in an Ultrasmall Metallic Grain

We study the Kondo effect generated by a single magnetic impurity embedded in an ultrasmall metallic grain, to be called a ``Kondo box''. We find that the Kondo resonance is strongly affected when the mean level spacing in the grain becomes larger than the Kondo temperature, in a way that depends on the parity of the number of electrons on the grain. We show that the single-electron tunneling conductance through such a grain features Kondo-induced Fano-type resonances of measurable size, with an anomalous dependence on temperature and level spacing.

cond-mat.str-el↗

arXiv1998

Fermi and Non-Fermi Liquid Behavior of Local Moment Systems within a Conserving Slave Boson Theory

The question of Fermi liquid vs. non-Fermi liquid behavior induced by strong correlations is one of the prominent problems in metallic local moment systems. As standard models for such systems, the SU(N) x SU(M) Anderson impurity models exhibit both Fermi liquid and non-Fermi liquid behavior, depending on their symmetry. Using an auxiliary boson method, we present a generally applicable scheme to select the relevant contributions in the low frequency regime, while preserving the local gauge symmetry of the model. It amounts to a conserving T-matrix approximation (CTMA) including coherent spin flip as well as charge fluctuation processes, which are found to dominate in the Kondo and in the mixed valence regime, respectively. The infrared threshold exponents of the auxiliary particle spectral functions are indicators for the presence of Fermi or non-Fermi liquid behavior in any given model with strong on-site repulsion. We show that, in contrast to earlier auxiliary boson theories, the CTMA recovers the correct exponents in both cases, indicating that it correctly describes both the Fermi and the non-Fermi regimes of the Anderson model.

cond-mat.str-el↗

arXiv1997

Correlation-enhanced Friedel oscillations in amorphous and quasicrystalline metals

The exponentially strong damping of the conventional Friedel oscillations at elevated temperature T as well as due to disorder poses a severe problem to the Hume-Rothery (HR) stabilization mechanism of amorphous and quasicrystalline alloys. We show that quantum correlations induced by electron-electron interactions in the presence of random impurity scattering can play an important role in stabilizing these systems: When there is strong backscattering off local ion clusters, the static electron density response chi(0,q) acquires a powerlaw divergence at q=2k_F even at T>0. This Fermi surface singularity leads to an enhancement as well as to a systematical phase shift of the Friedel oscillations, consistent with experiments. In addition, the spatial decay exponent is reduced, strongly supporting the validity of a HR-like mechanism at T>0. This effect may be accounted for in pseudopotential calculations through the local field factor.

cond-mat.str-el↗

arXiv1994

Quantum Interference of Coulomb Interaction and Disorder: Phase Shift of Friedel Oscillations and an Instability of the Fermi Sea

We investigate the influence of interference between Coulomb interaction and impurity scattering on the static electronic response $χ(0,q)$ in disordered metals to leading order in the effective Coulomb interaction. When the transport relaxation time $τ_{tr}$ is much shorter than the quasiparticle life time, we find a $\mbox{sgn}(2p_F-q)/\sqrt{|2p_F-q|}$ divergence of the polarization function at the Fermi surface ($q=2p_F$). It causes a phase shift of the Friedel oscillations as well as an enhancement of their amplitude. Our results are consistent with experiments and may be relevant for understanding the stability of the amorphous state of certain alloys against crystallization.

cond-mat↗

arXiv1994

Nonlinear Conductance for the Two Channel Anderson Model

Using the integral equations of the Noncrossing Approximation, the differential conductance is computed as a function of voltage for scattering from a two channel Kondo impurity in a point contact. The results compare well to experimental data on Cu point contacts by Ralph and Buhrman. They support a recently proposed scaling hypothesis, and also show finite temperature corrections to scaling in agreement with experiment. The conductance signal is predicted to be asymmetric in the bias when the impurity is not equally coupled to left and right moving electrons.

cond-mat↗