arXiv subjects

Johann Kroha

Publications and source records attributed to Johann Kroha.

At least 55 records · Page 3Linked to original sources

arXiv2015

Theory of Curie temperature enhancement in electron-doped EuO

We present a comparative, theoretical study of the doping dependence of the critical temperature $T_C$ of the ferromagnetic insulator-metal transition in Gd-doped and O-deficient EuO, respectively. The strong $T_C$ enhancement in Eu$_{1-x}$Gd$_x$O is due to Kondo-like spin fluctuations on the Gd sites, which are absent in EuO$_{1-x}$. Moreover, we find that the $T_C$ saturation in Eu$_{1-x}$Gd$_x$O for large $x$ is due to a reduced activation of dopant electrons into the conduction band, in agreement with experiments, rather than antiferromagnetic long-range contributions of the RKKY interaction. The results shed light on possibilities for further increasing $T_C$.

cond-mat.mtrl-sci↗

arXiv2015

Diagrammatic self-consistent theory of Anderson localization for the tight-binding model

A self-consistent theory of the frequency dependent diffusion coefficient for the Anderson localization problem is presented within the tight-binding model of non-interacting electrons on a lattice with randomly distributed on-site energy levels. The theory uses a diagrammatic expansion in terms of (extended) Bloch states and is found to be equivalent to the expansion in terms of (localized) Wannier states which was derived earlier by Kroha, Kopp and Wölfle. No adjustable parameters enter the theory. The localization length is calculated in 1, 2 and 3 dimensions as well as the frequency dependent conductivity and the phase diagram of localization in 3 dimensions for various types of disorder distributions. The validity of a universal scaling function of the length dependent conductance derived from this theory is discussed in the strong coupling region. Quantitative agreement with results from numerical diagonalization of finite systems demonstrates that the self-consistent treatment of cooperon contributions is sufficient to explain the phase diagram of localization and suggests that the system may be well described by a one-parameter scaling theory in certain regions of the phase diagram, if one is not too close to the transition point.

cond-mat.dis-nn↗

arXiv2014

Temporal non-equilibrium dynamics of a Bose Josephson junction in presence of incoherent excitations

The time-dependent non-equilibrium dynamics of a Bose-Einstein condensate (BEC) typically generates incoherent excitations out of the condensate due to the finite frequencies present in the time evolution. We present a detailed derivation of a general non-equilibrium Green's function technique which describes the coupled time evolution of an interacting BEC and its single-particle excitations in a trap, based on an expansion in terms of the exact eigenstates of the trap potential. We analyze the dynamics of a Bose system in a small double-well potential with initially all particles in the condensate. When the trap frequency is larger than the Josephson frequency, $Δ> ω_J$, the dynamics changes at a characteristic time $τ_c$ abruptly from slow Josephson oscillations of the BEC to fast Rabi oscillations driven by quasiparticle excitations in the trap. For times $t<τ_c$ the Josephson oscillations are undamped, in agreement with experiments. We analyze the physical origin of the finite scale $τ_c$ as well as its dependence on the trap parameter $Δ$.

cond-mat.quant-gas↗

arXiv2013

Tuning the Ultrafast Spin Dynamics in Carrier-Density-Controlled Ferromagnets

Ultrafast strengthening or quenching of the ferromagnetic order of semiconducting Eu1-xGdxO was achieved by resonant photoexcitation. The modification of the magnetic order is established within 3 ps as revealed by optical second harmonic generation. A theoretical analysis shows that the response is determined by the interplay of chemically and optically generated carriers in a nonequilibrium scenario beyond the three-temperature model. General criteria for the design of spintronics materials with tunable ultrafast spin dynamics are given.

cond-mat.str-el↗

arXiv2012

Coherent feedback from dissipation: the lasing mode volume of random lasers

In any quantum or wave system dissipation leads to decoherence. Therefore, it was surprising in first instance when experiments on strongly lossy random lasers showed unambiguously by measurements of the photon statistics and of the lasing mode volume that coherent feedback is possible in such systems3-5,8. In coherent-feedback lasers the photons form a far-from-equilibrium condensate in the sense that a single quantum state is occupied by a macroscopic number of photons15. We demonstrate that the lossy dynamics of random lasers alone imply a three dimensional finite lasing mode extent, thus resolving the puzzle about coherent feedback without resonator. Our theory of random lasing including nonlinear gain and gain saturation predicts a characteristic dependence of this lasing mode volume on the pump intensity, which can be tested experimentally.

cond-mat.dis-nn↗

arXiv2012

Rotational Quantum Impurities in a Metal: Stability of the 2-Channel Kondo Fixed Point in a Magnetic Field

A three-level system with partially broken SU(3) symmetry immersed in a metal, comprised of a unique non-interacting ground state and two-fold degenerate excited states, exhibits a stable two-channel Kondo fixed point within a wide range of parameters, as has been shown in previous work. Such systems can, for instance, be realized by protons dissolved in a metal and bound in the interstitial space of the host lattice, where the degeneracy of excited rotational states is guaranteed by the space inversion symmetry of the lattice. We analyze the robustness of the 2CK fixed point with respect to a level splitting of the excited states and discuss how this may explain the behavior of the well-known dI/dV spectra measured by Ralph and Buhrman on ultrasmall quantum point contacts in a magnetic field.

cond-mat.str-el↗

arXiv2011

A Tunable Two-impurity Kondo system in an atomic point contact

Two magnetic atoms, one attached to the tip of a Scanning Tunneling Microscope (STM) and one adsorbed on a metal surface, each constituting a Kondo system, have been proposed as one of the simplest conceivable systems potentially exhibiting quantum critical behaviour. We have succeeded in implementing this concept experimentally for cobalt dimers clamped between an STM tip and a gold surface. Control of the tip-sample distance with sub-picometer resolution allows us to tune the interaction between the two cobalt atoms with unprecedented precision. Electronic transport measurements on this two-impurity Kondo system reveal a rich physical scenario which is governed by a crossover from local Kondo screening to non-local singlet formation due to antiferromagnetic coupling as a function of separation of the cobalt atoms.

cond-mat.str-el↗

arXiv2011

Efficient construction of maximally localized photonic Wannier functions: locality criterion and initial conditions

Wannier function expansions are well suited for the description of photonic- crystal-based defect structures, but constructing maximally localized Wannier functions by optimizing the phase degree of freedom of the Bloch modes is crucial for the efficiency of the approach. We systematically analyze different locality criteria for maximally localized Wannier functions in two- dimensional square and triangular lattice photonic crystals, employing (local) conjugate-gradient as well as (global) genetic-algorithm-based, stochastic methods. Besides the commonly used second moment (SM) locality measure, we introduce a new locality measure, namely the integrated modulus (IM) of the Wannier function. We show numerically that, in contrast to the SM criterion, the IM criterion leads to an optimization problem with a single extremum, thus allowing for fast and efficient construction of maximally localized Wannier functions using local optimization techniques. We also present an analytical formula for the initial choice of Bloch phases, which under certain conditions represents the global maximum of the IM criterion and, thus, further increases the optimization efficiency in the general case.

physics.optics↗

arXiv2009

Nonequilibrium Josephson oscillations in Bose-Einstein condensates without dissipation

We perform a detailed field theoretical study of nonequilibrium Josephson oscillations between interacting Bose-Einstein condensates confined in a finite-size double-well trap. We find that the Josephson junction can sustain multiple undamped Josephson oscillations up to a characteristic time scale $τ_c$ without quasipartcles being excited in the system. This may explain recent related experiments. At larger times the dynamics of the junction is governed by fast Rabi oscillations between the descrete quasiparticle levels. We predict that a selftrapped BEC state will be destroyed by these Rabi oscillations.

cond-mat.quant-gas↗

arXiv2009

Bistability and mode interaction in microlasers

We investigate the possibility of bistable lasing in microcavity lasers as opposed to bulk lasers. To that end, the dynamic behavior of a microlaser featuring two coupled, interacting modes is analytically investigated within the framework of a semiclassical laser model, suitable for a wide range of cavity shapes and mode geometries. Closed-form coupled mode equations are obtained for all classes of laser dynamics. We show that bistable operation is possible in all of these classes. In the simplest case (class-A lasers) bistability is shown to result from an interplay between coherent (population-pulsation) and incoherent (hole-burning) processes of mode interaction. We find that microcavities offer better conditions to achieve bistable lasing than bulk cavities, especially if the modes are not degenerate in frequency. This results from better matching of the spatial intensity distribution of microcavity modes. In more complicated laser models (class-B and class-C) bistability is shown to persist for modes even further apart in frequency than in the class-A case. The predictions of the coupled mode theory have been confirmed using numerical finite-difference time-domain calculations.

physics.optics↗

arXiv2009

Theory of Cherenkov radiation in periodic dielectric media: Emission spectrum

The Cherenkov radiation is substantially modified in the presence of a medium with a nontrivial dispersion relation. We consider Cherenkov emission spectra of a point charge moving in general three- (3D) and two-dimensional (2D) photonic crystals. Exact analytical expressions for the spectral distribution of the radiated power are obtained in terms of the Bloch mode expansion. The resulting expression reduces to a simple contour integral (3D case) or a one-dimensional sum (2D case) over a small fraction of the reciprocal space, which is defined by the generalized Cherenkov condition. We apply our method to a specific case of an electron moving with different velocities in a 2D square-lattice photonic crystal. Our method demonstrates an excellent agreement with numerically rigorous finite-difference time-domain calculations while being less demanding on computational resources.

physics.optics↗

arXiv2008

Self-consistent study of Anderson localization in the Anderson-Hubbard model in two and three dimensions

We consider the change in electron localization due to the presence of electron-electron repulsion in the \HA model. Taking into account local Mott-Hubbard physics and static screening of the disorder potential, the system is mapped onto an effective single-particle Anderson model, which is studied within the self-consistent theory of electron localization. We find rich nonmonotonic behavior of the localization length $ξ$ in two-dimensional systems, including an interaction-induced exponential enhancement of $ξ$ for small and intermediate disorders although $ξ$ remains finite. In three dimensions we identify for half filling a Mott-Hubbard-assisted Anderson localized phase existing between the metallic and the Mott-Hubbard-gapped phases. For small $U$ there is re-entrant behavior from the Anderson localized phase to the metallic phase.

cond-mat.dis-nn↗

arXiv2007

Simultaneous ferromagnetic metal-semiconductor transition in electron-doped EuO

We present a general framework to describe the simultaneous para-to-ferromagnetic and semiconductor-to-metal transition in electron-doped EuO. The theory correctly describes detailed experimental features of the conductivity and of the magnetization, in particular the doping dependence of the Curie temperature. The existence of correlation-induced local moments on the impurity sites is essential for this description.

cond-mat.str-el↗

arXiv2007

Coupled nanopillar waveguides: optical properties and applications

In this paper we review basic properties of coupled periodic and aperiodic nanopillar waveguides. A coupled nanopillar waveguide consists of several rows of periodically or aperiodically placed dielectric rods (pillars). In such a waveguide, light confinement is due to the total internal reflection, while guided modes dispersion is strongly affected by the waveguide structure. We present a systematic analysis of the optical properties of coupled nanopillar waveguides and discuss their possible applications for integrated optics.

physics.optics↗

arXiv2007

Stable two-channel Kondo fixed point of an SU(3) quantum defect in a metal: renormalization group analysis and conductance spikes

We propose a physical realization of the two-channel Kondo (2CK) effect, where a dynamical defect in a metal has a unique ground state and twofold degenerate excited states. In a wide range of parameters the interactions with the electrons renormalize the excited double downward below the bare defect ground state, thus stabilizing the 2CK fixed point. In addition to the Kondo temperature T_K the three-state defect exhibits another low-energy scale, associated with ground-to-excited-state transitions, which can be exponentially smaller than T_K. Using the perturbative nonequilibrium renormalization group we demonstrate explicitly that this can provide the long-sought explanation of the sharp conductance spikes observed by Ralph and Buhrman in ultrasmall metallic point contacts.

cond-mat.mes-hall↗

arXiv2007

Switchable lasing in coupled multimode microcavities

We propose the new concept of a switchable multimode microlaser. As a generic, realistic model of a multimode microresonator a system of two coupled defects in a two-dimensional photonic crystal is considered. We demonstrate theoretically that lasing of the cavity into one selected resonator mode can be caused by injecting an appropriate optical pulse at the onset of laser action (injection seeding). Temporal mode-to-mode switching by re-seeding the cavity after a short cool-down period is demonstrated by direct numerical solution. A qualitative analytical explanation of the mode switching in terms of the laser bistability is presented.

cond-mat.mes-hall↗

arXiv2006

Strong vs. Weak Coupling Duality and Coupling Dependence of the Kondo Temperature in the Two-Channel Kondo Model

We perform numerical renormalization group (NRG) as well as analytical calculations for the two-channel Kondo model to obtain the dependence of the Kondo temperature $T_K$ on the dimensionless (bare) spin exchange coupling $g$ over the complete parameter range from $g\ll 1$ to $g\gg 1$. We show that there exists a duality between the regimes of small and large coupling. It is unique for the two-channel model and enables a mapping between the strong and the weak coupling cases via the identification $g\leftrightarrow 3/(2g)$, implying an exponential dependence of $T_K$ on $1/g$ and $g$, respectively, in the two regimes. This agrees quantitatively with our NRG calculations where we extract $T_K(g)$ over the complete parameter range and obtain a non-monotonous $T_K(g)$ dependence, strongly peaked at the 2CK fixed point coupling $g^*$. These results may be relevant for resolving the long-standing puzzle within the 2CK interpretation of certain random defect systems, why no broad distribution of $T_K$ is observed in those systems.

cond-mat.str-el↗

arXiv2006

Universality in Voltage-driven Nonequilibrium Phase Transitions

We consider the non-equilibrium ferromagnetic transition of a mesoscopic sample of a resistive Stoner ferromagnet coupled to two paramagnetic leads. The transition is controlled by either the lead temperature T or the transport voltage V applied between the leads. We calculate the T and V dependence of the magnetization. For systems with a flat density of states we find within mean-field theory that even at finite bias the magnetization does not depend on the position along the sample axis, although the charge density and other quantities do vary. This may be relevant for possible spintronics applications. In addition, we establish a generalized control parameter in terms of T and V which allows for a universal description of the temperature- and voltage-driven transition.

cond-mat.mes-hall↗