arXiv subjects

Felix von Oppen

Publications and source records attributed to Felix von Oppen.

At least 163 records · Page 9Linked to original sources

arXiv2007

Anomalous Hall effect in a two dimensional electron gas with magnetic impurities

Magnetic impurities play an important role in many spintronics-related materials. Motivated by this fact, we study the anomalous Hall effect in the presence of magnetic impurities, focusing on two-dimensional electron systems with Rashba spin-orbit coupling. We find a highly nonlinear dependence on the impurity polarization, including possible sign changes. At small impurity magnetizations, this is a consequence of the remarkable result that the linear term is independent of the spin-orbit coupling strength. Near saturation of the impurity spins, the anomalous Hall conductivity can be resonantly enhanced, due to interference between potential and magnetic scattering.

cond-mat.mtrl-sci↗

arXiv2007

Vibrational absorption sidebands in the Coulomb blockade regime of single-molecule transistors

Current-driven vibrational non-equilibrium induces vibrational sidebands in single-molecule transistors which arise from tunneling processes accompanied by absorption of vibrational quanta. Unlike conventional sidebands, these absorption sidebands occur in a regime where the current is nominally Coulomb blockaded. Here, we develop a detailed and analytical theory of absorption sidebands, including current-voltage characteristics as well as shot noise. We discuss the relation of our predictions to recent experiments.

cond-mat.mes-hall↗

arXiv2007

Flexural phonons in free-standing graphene

Rotation and reflection symmetries impose that out-of-plane (flexural) phonons of free-standing graphene membranes have a quadratic dispersion at long wavelength and can be excited by charge carriers in pairs only. As a result, we find that flexural phonons dominate the phonon contribution to the resistivity $ρ$ below a crossover temperature T_x where we obtain an anomalous temperature dependence $ρ\propto T^{5/2}_{}\ln T$. The logarithmic factor arises from renormalizations of the flexural phonon dispersion due to coupling between bending and stretching degrees of freedom of the membrane.

cond-mat.mes-hall↗

arXiv2007

Electron Pair Resonance in the Coulomb Blockade

We study many-body corrections to the cotunneling current via a localized state with energy $ε_d$ at large bias voltages $V$. We show that the transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the localized level, results in ionization resonance peaks in the third derivative of the current with respect to $V$, centered at $eV=\pm 2ε_d/3$. Our results predict the existence of previously unnoticed structure within Coulomb-blockade diamonds.

cond-mat.mes-hall↗

arXiv2007

Zero-bias anomaly in the tunneling density of states of graphene

In the vicinity of the Fermi energy, the band structure of graphene is well described by a Dirac equation. Impurities will generally induce both a scalar potential as well as a (fictitious) gauge field acting on the Dirac fermions. We show that the angular dependence of the zero-bias anomaly in the spatially resolved tunneling density of states (TDOS) around a particular impurity allows one to distinguish between these two contributions. Our predictions can be tested in scanning-tunneling-microscopy measurements on graphene.

cond-mat.mes-hall↗

arXiv2006

Theory of the Franck-Condon blockade regime

Strong coupling of electronic and vibrational degrees of freedom entails a low-bias suppression of the current through single-molecule devices, termed Franck-Condon blockade. In the limit of slow vibrational relaxation, transport in the Franck-Condon-blockade regime proceeds via avalanches of large numbers of electrons, which are interrupted by long waiting times without electron transfer. The avalanches consist of smaller avalanches, leading to a self-similar hierarchy which terminates once the number of transferred electrons per avalanche becomes of the order of unity. Experimental signatures of self-similar avalanche transport are strongly enhanced current (shot) noise, as expressed by giant Fano factors, and a power-law noise spectrum. We develop a theory of the Franck-Condon-blockade regime with particular emphasis on effects of electron cotunneling through highly excited vibrational states. As opposed to the exponential suppression of sequential tunneling rates for low-lying vibrational states, cotunneling rates suffer only a power-law suppression. This leads to a regime where cotunneling dominates the current for any gate voltage. Including cotunneling within a rate-equation approach to transport, we find that both the Franck-Condon blockade and self-similar avalanche transport remain intact in this regime. We predict that cotunneling leads to absorption-induced vibrational sidebands in the Coulomb-blockaded regime as well as intrinsic telegraph noise near the charge degeneracy point.

cond-mat.mes-hall↗

arXiv2006

Nonequilibrium charge-Kondo transport through negative-U molecules

Low-temperature transport through molecules with effectively negative charging energy U exhibits a charge-Kondo effect. We explore this regime analytically by establishing an exact mapping between the negative-U and the positive-U Anderson models, which is suitable for the description of nonequilibrium transport. We employ this mapping to demonstrate the intimate relation between nonequilibrium tranport in the spin-Kondo and charge-Kondo regimes, and derive analytical expressions for the nonlinear current-voltage chracteristics as well as the shot noise in the latter regime. Applying the mapping in the opposite direction, we elucidate the finding of super-Poissonian noise in the positive-U Anderson model at high temperatures, by relating the correlations between spin flips to pair-tunneling processes at negative U.

cond-mat.mes-hall↗

arXiv2006

Current-induced nonequilibrium vibrations in single-molecule devices

Finite-bias electron transport through single molecules generally induces nonequilibrium molecular vibrations (phonons). By a mapping to a Fokker-Planck equation, we obtain analytical scaling forms for the nonequilibrium phonon distribution in the limit of weak electron-phonon coupling $λ$ within a minimal model. Remarkably, the width of the phonon distribution diverges as $\simλ^{-α}$ when the coupling decreases, with voltage-dependent, non-integer exponents $α$. This implies a breakdown of perturbation theory in the electron-phonon coupling for fully developed nonequilibrium. We also discuss possible experimental implications of this result such as current-induced dissociation of molecules.

cond-mat.mes-hall↗

arXiv2006

Fractional Shot Noise in the Kondo Regime

Low temperature transport through a quantum dot in the Kondo regime proceeds by a universal combination of elastic and inelastic processes, as dictated by the low-energy Fermi-liquid fixed point. We show that as a result of inelastic processes, the charge detected by a shot-noise experiment is enhanced relative to the noninteracting situation to a universal fractional value, $e^*=5/3 e$. Thus, shot noise reveals that the Kondo effect involves many-body features even at low energies, despite its Fermi-liquid nature. We discuss the influence of symmetry breaking perturbations.

cond-mat.str-el↗

arXiv2006

Pair tunneling through single molecules

By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here, we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Due to the restricted phase space for pair tunneling, the current-voltage characteristics exhibits striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching.

cond-mat.mes-hall↗

arXiv2005

Full counting statistics of strongly non-Ohmic transport through single molecules

We study analytically the full counting statistics of charge transport through single molecules, strongly coupled to a weakly damped vibrational mode. The specifics of transport in this regime - a hierarchical sequence of avalanches of transferred charges, interrupted by "quiet" periods - make the counting statistics strongly non-Gaussian. We support our findings for the counting statistics as well as for the frequency-dependent noise power by numerical simulations, finding excellent agreement.

cond-mat.mes-hall↗

arXiv2005

Effects of charge-dependent vibrational frequencies and anharmonicities in transport through molecules

As a step towards a more realistic modeling of vibrations in single-molecule devices, we investigate the effects of charge-dependent vibrational frequencies and anharmonic potentials on electronic transport. For weak phonon relaxation, we find that in both cases vibrational steps split into a multitude of substeps. This effectively leads to a bias-dependent broadening of vibrational features in current-voltage and conductance characteristics, which provides a fingerprint of nonequilibrium vibrations whenever other broadening mechanisms are secondary. In the case of an asymmetric molecule-lead coupling, we observe that frequency differences can also cause negative differential conductance.

cond-mat.mes-hall↗

arXiv2005

Franck-Condon blockade and giant Fano factors in transport through single molecules

We show that Franck-Condon physics leads to a significant current suppression at low bias voltages (termed Franck-Condon blockade) in transport through single molecules with strong coupling between electronic and vibrational degrees of freedom. We find that transport in this regime is characterized by remarkably large Fano factors ($10^2$--$10^3$), which arise due to avalanche-like transport of electrons. Avalanches occur in a self-similar manner over a wide range of time scales, as reflected in power-law dependences of the current noise on frequency and vibrational relaxation rate.

cond-mat.mes-hall↗

arXiv2005

Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions

We show that the resistivity rho(T) of disordered ferromagnets near, and above, the Curie temperature T_c generically exhibits a stronger anomaly than the scaling-based Fisher-Langer prediction. Treating transport beyond the Boltzmann description, we find that within mean-field theory, dρ/dT exhibits a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or diluted magnetic semiconductors, whose mobility near T_c is limited by disorder.

cond-mat.mes-hall↗

arXiv2005

Microwave photoconductivity of a two-dimensional electron gas due to intra-Landau-level transitions

Motivated by a recent experiment, we study the microwave-induced photoconductivity of a two-dimensional electron gas arising from intra-Landau-level transitions within a model where the electrons are subject to a unidirectional periodic potential in addition to a weaker impurity potential. With appropriate identifications, our results can be compared to experiment and allow us to explain the sign of the photocurrent, its dependence on magnetic field and microwave frequency as well as the microwave-induced suppression of the Shubnikov-deHaas oscillations.

cond-mat.mes-hall↗

arXiv2004

Theory of charge sensing in quantum-dot structures

Charge sensing in quantum-dot structures is studied by an exactly solvable reduced model and numerical density-matrix renormalization group methods. Charge sensing is characterized by the repeated cycling of the occupation of current-carrying states due to the capacitive coupling to trap states which are weakly coupled to the leads. In agreement with recent experiments, it results in a variety of characteristic behaviors ranging from asymmetric Coulomb-blockade peaks to sawtooth- and dome-like structures. Temperature introduces distinct asymmetric smearing of these features and correlations in the conductance provide a fingerprint of charge-sensing behavior.

cond-mat.mes-hall↗

arXiv2004

Thermopower of Single-Molecule Devices

We investigate the thermopower of single molecules weakly coupled to metallic leads. We model the molecule in terms of the relevant electronic orbitals coupled to phonons corresponding to both internal vibrations and to oscillations of the molecule as a whole. The thermopower is computed by means of rate equations including both sequential-tunneling and cotunneling processes. Under certain conditions, the thermopower allows one to access the electronic and phononic excitation spectrum of the molecule in a linear-response measurement. In particular, we find that the phonon features are more pronounced for weak lead-molecule coupling. This way of measuring the excitation spectrum is less invasive than the more conventional current-voltage characteristic, which, by contrast, probes the system far from equilibrium.

cond-mat.mes-hall↗

arXiv2004

Discrete charging of a quantum dot strongly coupled to external leads

We examine a quantum dot with $N_{\rm dot}$ levels which is strongly coupled to leads for varying number of channels $N$ in the leads. It is shown both analytically and numerically that for strong couplings between the dot and the leads, at least $N_{\rm dot}-N$ bound states (akin to subradiant states in optics) remain on the dot. These bound states exhibit discrete charging and, for a significant range of charging energies, strong Coulomb blockade behavior as function of the chemical potential. The physics changes for large charging energy where the same (superradiant) state is repeatedly charged.

cond-mat.mes-hall↗