arXiv subjects

Stefan Kettemann

Publications and source records attributed to Stefan Kettemann.

52 records · Page 3Linked to original sources

arXiv2007

Spin-polarized tunneling microscopy and the Kondo effect

We present a theory for spin-polarized scanning tunneling microscopy (SP-STM) of a Kondo impurity on an unpolarized metallic substrate. The spin polarization of the SP-STM breaks the spin symmetry of the Kondo system, similar to an applied magnetic field, leading to a splitting of the Abrikosov-Suhl-Kondo resonance. The amount of splitting is controlled by the strength of the coupling between the impurity and the SP-STM tip and also the overall spin polarization of the SP-STM.

cond-mat.str-el↗

arXiv2007

Disorder-quenched Kondo effect in mesosocopic electronic systems

Nonmagnetic disorder is shown to quench the screening of magnetic moments in metals, the Kondo effect. The probability that a magnetic moment remains free down to zero temperature is found to increase with disorder strength. Experimental consequences for disordered metals are studied. In particular, it is shown that the presence of magnetic impurities with a small Kondo temperature enhances the electron's dephasing rate at low temperatures in comparison to the clean metal case. It is furthermore proven that the width of the distribution of Kondo temperatures remains finite in the thermodynamic (infinite volume) limit due to wave function correlations within an energy interval of order $1/τ$, where $τ$ is the elastic scattering time. When time-reversal symmetry is broken either by applying a magnetic field or by increasing the concentration of magnetic impurities, the distribution of Kondo temperatures becomes narrower.

cond-mat.mes-hall↗

arXiv2006

Free Magnetic Moments in Disordered Metals

The screening of magnetic moments in metals, the Kondo effect, is found to be quenched with a finite probability in the presence of nonmagnetic disorder. Numerical results for a disordered electron system show that the distribution of Kondo temperatures deviates strongly from the result expected from random matrix theory. A pronounced second peak emerges for small Kondo temperatures, showing that the probability that magnetic moments remain unscreened at low temperatures increases with disorder. Analytical calculations, taking into account correlations between eigenfunction intensities yield a finite width for the distribution in the thermodynamic limit. Experimental consequences for disordered mesoscopic metals are discussed.

cond-mat.mes-hall↗

arXiv2005

Nonchiral Edge States at the Chiral Metal Insulator Transition in Disordered Quantum Hall Wires

The quantum phase diagram of disordered wires in a strong magnetic field is studied as a function of wire width and energy. The two-terminal conductance shows zero-temperature discontinuous transitions between exactly integer plateau values and zero. In the vicinity of this transition, the chiral metal-insulator transition (CMIT), states are identified that are superpositions of edge states with opposite chirality. The bulk contribution of such states is found to decrease with increasing wire width. Based on exact diagonalization results for the eigenstates and their participation ratios, we conclude that these states are characteristic for the CMIT, have the appearance of nonchiral edges states, and are thereby distinguishable from other states in the quantum Hall wire, namely, extended edge states, two-dimensionally (2D) localized, quasi-1D localized, and 2D critical states.

cond-mat.mes-hall↗

arXiv2004

Tails of the Density of States in a Random Magnetic Field

We study the tails of the density of states of fermions subject to a random magnetic field with non-zero mean with the Optimum Fluctuation Method (OFM). Closer to the centres of the Landau levels, the density of states is found to be Gaussian, whereas the energy dependence is non-analytic near the lower bound of the spectrum.

cond-mat.mes-hall↗

arXiv2003

Localization in the quantum Hall regime

The localization properties of electron states in the quantum Hall regime are reviewed. The random Landau model, the random matrix model, the tight-binding Peierls model, and the network model of Chalker and Coddington are introduced. Descriptions in terms of equivalent tight-binding Hamiltonians, and the 2D Dirac model, are outlined. Evidences for the universal critical behavior of the localization length are summarized. A short review of the supersymmetric critical field theory is provided. The interplay between edge states and bulk localization properties is investigated. For a system with finite width and with short-range randomness, a sudden breakdown of the two-point conductance from $ne^{2}/h$ to 0 ($n$ integer) is predicted if the localization length exceeds the distance between the edges.

cond-mat.mes-hall↗

arXiv2002

Thermoelectrical Field Effects in Low Dimensional Structure Solar Cells

Taking into account the temperature gradients in solar cells, it is shown that their efficiency can be increased beyond the Shockley-Queisser limit (J. Appl. Phys. 32 (1961) 510). The driving force for this gain is the temperature gradient between this region and its surroundings. A quantitative theory is given. Though the effect is found to be weak in conventional solar cells, it is argued that it can be substantially increased by proper choice of materials and design of the device. In particular, it is shown that the insertion of a quantum well can enhance the efficiency beyond one of the single gap cell, due to the presence of temperature jumps at the heterojunctions.

cond-mat.mtrl-sci↗

arXiv2001

Magnetolocalization in disordered quantum wires

The magnetic field dependent localization in a disordered quantum wire is considered nonperturbatively. An increase of an averaged localization length with the magnetic field is found, saturating at twice its value without magnetic field. The crossover behavior is shown to be governed both in the weak and strong localization regime by the magnetic diffusion length L_B. This function is derived analytically in closed form as a function of the ratio of the mean free path l, the wire thickness W, and the magnetic length l_B for a two-dimensional wire with specular boundary conditions, as well as for a parabolic wire. The applicability of the analytical formulas to resistance measurements in the strong localization regime is discussed. A comparison with recent experimental results on magnetolocalization is included.

cond-mat.mes-hall↗

arXiv2000

Symmetry Dependence of Localization in Quasi- 1- dimensional Disordered Wires

The crossover in energy level statistics of a quasi-1-dimensional disordered wire as a function of its length L is used, in order to derive its averaged localization length, without magnetic field, in a magnetic field and for moderate spin orbit scattering strength. An analytical function of the magnetic field for the local level spacing is obtained, and found to be in excellent agreement with the magnetic field dependent activation energy, recently measured in low-mobility quasi-one-dimensional wires\cite{khavin}. This formula can be used to extract directly and accurately the localization length from magnetoresistance experiments. In general, the local level spacing is shown to be proportional to the excitation gap of a virtual particle, moving on a compact symmetric space.

cond-mat.dis-nn↗

arXiv2000

Correlated Persistent Tunneling Currents in Glasses

Low temperature properties of glasses are derived within a generalized tunneling model, considering the motion of charged particles on a closed path in a double-well potential. The presence of a magnetic induction field B violates the time reversal invariance due to the Aharonov-Bohm phase, and leads to flux periodic energy levels. At low temperature, this effect is shown to be strongly enhanced by dipole-dipole and elastic interactions between tunneling systems and becomes measurable. Thus, the recently observed strong sensitivity of the electric permittivity to weak magnetic fields can be explained. In addition, superimposed oscillations as a function of the magnetic field are predicted.

cond-mat.dis-nn↗

arXiv1998

Information about the Integer Quantum Hall Transition Extracted from the Autocorrelation Function of Spectral Determinants

The Autocorrelation function of spectral determinants (ASD) is used to probe the sensitivity of a two-dimensional disordered electron gas to the system's size L. For weak magnetic fields ASD is shown to depend only trivially on L, which is a strong indication that all states are localized. From nontrivial dependence of ASD on L for infinite L at a Hall conductance of 1/2 e^2/h we deduce the existence of critical wave functions at this point, as long as the disorder strength does not exceed a critical value.

cond-mat.mes-hall↗

arXiv1998

Spin Flip Torsion Balance

The spin flip of the conduction electrons at the interface of a ferromagnetic and a nonmagnetic part of a metallic wire, suspended between two electrodes, is shown to tort the wire when a current is driven through it. In order to enhance the effect it is suggested to use an alternating current in resonance with the torsional oscillations. Thereby the magnetic polarization of the conduction electrons in the ferromagnet can be measured directly, and compared to the total magnetization. This may yield new information on the transport properties of the narrow band electrons in itinerant ferromagnets.

cond-mat.str-el↗

arXiv1998

Exploring Level Statistics from Quantum Chaos to Localization with the Autocorrelation Function of Spectral Determinants

The autocorrelation function of spectral determinants (ASD) is used to characterize the discrete spectrum of a phase coherent quasi- 1- dimensional, disordered wire as a function of its length L in a finite, weak magnetic field. An analytical function is obtained depending only on the dimensionless conductance g= xi/L where xi is the localization length, the scaled frequency x= omega/Delta, where Delta is the average level spacing of the wire, and the global symmetry of the system. A metal- insulator crossover is observed, showing that information on localization is contained in the disorder averaged ASD.

cond-mat.mes-hall↗

arXiv1997

Transport through a Constriction in a FQH Annulus

The composite fermion perspective is used, to study the flux dependence of thermodynamic properties of an annulus in the fractional quantum hall state at odd inverse filling factor. It is shown that $ϕ_0$- periodicity is restored, if there is tunneling of composite fermions between the edges of the annulus. In addition, the result for the finite magnitude of the persistent current across a very strong constriction in the annulus is presented, as obtained from an extension of Wen's edge state theory.

cond-mat.mes-hall↗

arXiv1997

Persistent Hall Voltage and Current in the Fractional Quantum Hall Regime

The persistent Hall voltage and current in an isolated annulus in a strong perpendicular magnetic field, at odd inverse filling factor, and in the presence of a weak constriction is obtained as a function of temperature, and flux piercing the annulus. A thermodynamic Hall conductance is found which has a universal value even with back scattering at the constriction. Included are detailed appendices on the bosonization of chiral soliton operators and the derivation of the chiral action, which has an additional topological term originating from the dynamics of the zero modes.

cond-mat.mes-hall↗

arXiv1994

Localization and dynamic persistent currents in long cylinders

A dynamic response to a magnetic field in a long disordered cylinder is considered. We show that, although at high frequencies conduction is classical in all directions, the low frequency behavior corresponds to localization in the longitudinal direction and to a diamagnetic dynamic persistent current in the transversal one. The current density does not vanish even in the limit of the infinitely long cylinder.

cond-mat↗