arXiv subjects

Wolfgang Belzig

Publications and source records attributed to Wolfgang Belzig.

At least 163 records · Page 9Linked to original sources

arXiv2009

Phonon-assisted current noise in molecular junctions

We investigate the effects of phonon scattering on the electronic current noise through nanojunctions using the non-equilibrium Green's functions formalism extended to include the counting field. In the case of weak electron-phonon coupling and a single broad electronic level we derive an analytic expression for the current noise at arbitrary temperature and identify physically distinct contributions based on their voltage dependence. We apply our theory to the experimentally relevant case of a deuterium molecule placed in a break-junction and predict a significant inelastic contribution to the current noise.

cond-mat.mes-hall↗

arXiv2009

Electronic transport in ferromagnetic conductors with inhomogeneous magnetic order parameter -- domain-wall resistance

We microscopically derive transport equations for the conduction electrons in ferromagnetic materials with an inhomogeneous magnetization profile. Our quantum kinetic approach includes elastic scattering and anisotropic spin-flip scattering at magnetic impurities. In the diffusive limit, we calculate the resistance through a domain wall and find that the domain-wall resistance can be positive or negative. In the limit of long domain walls we derive analytical expressions and compare them with existing works, which used less general models or different theoretical frameworks.

cond-mat.mes-hall↗

arXiv2009

Two atomic quantum dots interacting via coupling to BECs

We consider a system of three weakly coupled Bose-Einstein condensates and two atomic quantum dots embedded in the barriers between the condensates. Each dot is coupled to two neighboring condensates by optical transitions and can be described as a two-state system, or a pseudospin 1/2. Although there is no direct coupling between the dots, an effective interaction between the pseudospins is induced due to their coupling to the condensate reservoirs. We investigate this effective interaction, depending on the strengths of the dot-condensate coupling T and the direct coupling J between the condensates. In particular, we show that an initially ferromagnetic arrangement of the two pseudospins stays intact even for large T/J. However, antiferromagnetically aligned spins undergo peculiar "breathing" modes for weak coupling T/J<1, while for strong coupling the behaviour of the spins becomes uncorrelated.

cond-mat.other↗

arXiv2009

Full counting statistics of crossed Andreev reflection

We calculate the full transport counting statistics in a three-terminal tunnel device with one superconducting source and two normal-metal or ferromagnet drains. We obtain the transport probability distribution from direct Andreev reflection, crossed Andreev reflection, and electron transfer which reveals how these processes' statistics are determined by the device conductances. The cross-correlation noise is a result of competing contributions from crossed Andreev reflection and electron transfer, as well as antibunching due to the Pauli exclusion principle. For spin-active tunnel barriers that spin polarize the electron flow, crossed Andreev reflection and electron transfer statistics exhibit different dependencies on the magnetization configuration, and can be controlled by relative magnetization directions and voltage bias.

cond-mat.mes-hall↗

arXiv2008

Width of the $0-π$ phase transition in diffusive magnetic Josephson junctions

We investigate the Josephson current between two superconductors (S) which are connected through a diffusive magnetic junction with a complex structure (F$_{c}$). Using the quantum circuit theory, we obtain the phase diagram of 0 and $π$ Josephson couplings for F$_{c}$ being a IFI (insulator-ferromagnet-insulator) double barrier junction or a IFNFI structure (where N indicates a normal metal layer). Compared to a simple SFS structure, we find that the width of the transition, defined by the interval of exchange fields in which a $0-π$ transition is possible, is increased by insulating barriers at the interfaces and also by the presence of the additional N layer. The widest transition is found for symmetric F$_{c}$ structures. The symmetric SIFNFIS presents the most favorable condition to detect the temperature induced $0-π$ transition with a relative width, which is five times larger than that of the corresponding simple SFS structure.

cond-mat.mes-hall↗

arXiv2008

Finite frequency noise of a superconductor/ferromagnet quantum point contact

We have calculated the finite-frequency current noise of a superconductor-ferromagnet quantum point contact (SF QPC). This signal is qualitatively affected by the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon reflection on the QPC. For a weakly transparent QPC, noise steps appear at frequencies or voltages determined directly by the SDIPS. These steps can occur at experimentally accessible temperatures and frequencies. Finite frequency noise is thus a promising tool to characterize the scattering properties of a SF QPC.

cond-mat.mes-hall↗

arXiv2008

Formulation of Time-Resolved Counting Statistics Based on a Positive-Operator-Valued Measure

We propose a derivation of the full counting statistics of electronic current based on a positive-operator-valued measure. Our approach justifies the Levitov-Lesovik formula in the long-time limit, but can be generalized to the detection of finite-frequency noise correlations. The combined action of the projection postulate and the quantum formula for current noise at high frequencies imply an additional white noise. Estimates for this additional noise are in accordance with known experiments. We propose an experimental test of our conjecture by a simultaneous measurement of high- and low-frequency noise.

cond-mat.mes-hall↗

arXiv2008

Elementary Charge Transfer Processes in Mesoscopic Conductors

We determine charge transfer statistics in a quantum conductor driven by a time-dependent voltage and identify the elementary transport processes. At zero temperature unidirectional and bidirectional single charge transfers occur. The unidirectional processes involve electrons injected from the source terminal due to excess dc bias voltage. The bidirectional processes involve electron-hole pairs created by time-dependent voltage bias. This interpretation is further supported by the charge transfer statistics in a multiterminal beam splitter geometry in which injected electrons and holes can be partitioned into different outgoing terminals. The probabilities of elementary processes can be probed by noise measurements: the unidirectional processes set the dc noise level while bidirectional ones give rise to the excess noise. For ac voltage drive, the noise oscillates with increasing the driving amplitude. The decomposition of the noise into the contributions of elementary processes identifies the origin of these oscillations: the number of electron-hole pairs generated per cycle increases with increasing the amplitude. The decomposition of the noise into elementary processes is studied for different time-dependent voltages. The method we use is also suitable for systematic calculation of higher-order current correlators at finite temperature. We obtain current noise power and the third cumulant in the presence of time-dependent voltage drive. The charge transfer statistics at finite temperature can be interpreted in terms of multiple charge transfers with probabilities which depend on energy and temperature.

cond-mat.mes-hall↗

arXiv2008

Conductance and current noise of a superconductor/ferromagnet quantum point contact

We study the conductance and current noise of a superconductor/ferromagnet (S/F) single channel Quantum Point Contact (QPC) as a function of the QPC bias voltage, using a scattering approach. We show that the Spin-Dependence of Interfacial Phase Shifts (SDIPS) acquired by electrons upon scattering by the QPC can strongly modify these signals. For a weakly transparent contact, the SDIPS induces sub-gap resonances in the conductance and differential Fano factor curves of the QPC. For high transparencies, these resonances are smoothed, but the shape of the signals remain extremely sensitive to the SDIPS. We show that noise measurements could help to gain more information on the device, e.g. in cases where the SDIPS modifies qualitatively the differential Fano factor of the QPC but not the conductance.

cond-mat.supr-con↗

arXiv2007

Proximity effect-assisted absorption of spin currents in superconductors

The injection of pure spin current into superconductors by the dynamics of a ferromagnetic contact is studied theoretically. Taking into account suppression of the order parameter at the interfaces (inverse proximity effect) and the energy-dependence of spin-flip scattering, we determine the temperature-dependent ferromagnetic resonance linewidth broadening. Our results agree with recent experiments in Nb|permalloy bilayers [C. Bell et al., arXiv:cond-mat/0702461].

cond-mat.mes-hall↗

arXiv2007

Circuit theory for crossed Andreev reflection and nonlocal conductance

Nonlocal currents, in devices where two normal metal terminals are contacted to a superconductor, are determined using the circuit theory of mesoscopic superconductivity. We calculate the conductance associated with crossed Andreev reflection and electron transfer between the two normal metal terminals, in addition to the conductance from direct Andreev reflection and quasiparticle tunneling. Dephasing and proximity effect are taken into account.

cond-mat.mes-hall↗

arXiv2007

Elementary Charge Transfer Processes in a Superconductor-Ferromagnet Entangler

We study the production of spatially separated entangled electrons in ferromagnetic leads from Cooper pairs in a superconducting lead. We give a complete description of the elementary charge transfer processes, i) transfer of Cooper pairs out of the superconductor by Andreev reflection and ii) distribution of the entangled quasiparticles among the ferromagnetic leads, in terms of their statistics. The probabilities that entangled electrons flow into spatially separated leads are completely determined by experimentally measurable conductances and polarizations. Finally, we investigate how currents, noise and cross correlations are affected by transport of entangled electrons.

cond-mat.supr-con↗

arXiv2007

Elementary events of electron transfer in a voltage-driven quantum point contact

We show that the statistics of electron transfer in a coherent quantum point contact driven by an arbitrary time-dependent voltage is composed of elementary events of two kinds: unidirectional one-electron transfers determining the average current and bidirectional two-electron processes contributing to the noise only. This result pertains at vanishing temperature while the extended Keldysh-Green's function formalism in use also enables the systematic calculation of the higher-order current correlators at finite temperatures.

cond-mat.mes-hall↗

arXiv2007

Nanospintronics with carbon nanotubes

One of the actual challenges of spintronics is the realization of a spin-transistor allowing to control spin transport through an electrostatic gate. In this review, we report on different experiments which demonstrate a gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyze spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

cond-mat.mes-hall↗

arXiv2006

Circuit theory of crossed Andreev reflection

We consider transport in a three terminal device attached to one superconducting and two normal metal terminals, using the circuit theory of mesoscopic superconductivity. We compute the nonlocal conductance of the current out of the first normal metal terminal in response to a bias voltage between the second normal metal terminal and the superconducting terminal. The nonlocal conductance is given by competing contributions from crossed Andreev reflection and electron cotunneling, and we determine the contribution from each process. The nonlocal conductance vanishes when there is no resistance between the superconducting terminal and the device, in agreement with previous theoretical work. Electron cotunneling dominates when there is a finite resistance between the device and the superconducting reservoir. Decoherence is taken into account, and the characteristic timescale is the particle dwell time. This gives rise to an effective Thouless energy. Both the conductance due to crossed Andreev reflection and electron cotunneling depend strongly on the Thouless energy. We suggest to experimentally determine independently the conductance due to crossed Andreev reflection and electron cotunneling in measurement of both energy and charge flow into one normal metal terminal in response to a bias voltage between the other normal metal terminal and the superconductor.

cond-mat.supr-con↗

arXiv2006

Quasiparticle transport in arrays of chaotic cavities

We find the distribution of transmission eigenvalues in a series of identical junctions between chaotic cavities using the circuit theory of mesoscopic transport. This distribution rapidly approaches the diffusive wire limit as the number of junctions increases, independent of the specific scattering properties of a single junction. The cumulant generating function and the first three cumulants of the charge transfer through the system are obtained both in the normal and in the superconducting state.

cond-mat.mes-hall↗

arXiv2005

Slow Vibrations in Transport through Molecules

We show how one can measure the signal from slow jumps of a single molecule between metastable positions using a setup where the molecule is fixed to one lead, and one of the coupling strengths is controlled externally. Such a measurement yields information about slow processes deforming the molecule in times much longer than the characteristic time scales for the electron transport process.

cond-mat.mes-hall↗

arXiv2005

Full counting statistics of Andreev scattering in an asymmetric chaotic cavity

We study the charge transport statistics in coherent two-terminal double junctions within the framework of the circuit theory of mesoscopic transport. We obtain the general solution of the circuit-theory matrix equations for the Green's function of a chaotic cavity between arbitrary contacts. As an example we discuss the full counting statistics and the first three cumulants for an open asymmetric cavity between a superconductor and a normal-metal lead at temperatures and voltages below the superconducting gap. The third cumulant shows a characteristic sign change as a function of the asymmetry of the two quantum point contacts, which is related to the properties of the Andreev reflection eigenvalue distribution.

cond-mat.supr-con↗