58,243 research outputs found
Nonlinear transport through a dynamic impurity in a strongly interacting one-dimensional electron gas
We analyze the transport properties of a Luttinger liquid with an imbedded
impurity of explicitly time-dependent strength. We employ a radiative boundary
condition formalism to describe the coupling to the voltage sources. Assuming
the impurity time dependence to be oscillatory we present a full analytic
perturbative result in impurity strength for arbitrary interaction parameter
calculated with help of Coulomb gas expansion (CGE). Moreover, a full analytic
solution beyond the above restriction is possible for a special non-trivial
interaction strength which has been achieved independently by full resummation
of CGE series as well as via refermionization technique. The resulting
nonlinear current-voltage characteristic turns out to be very rich due to the
presence of the additional energy scale associated with the impurity
oscillation frequency. In accordance with the previous studies we also find an
enhancement of the linear conductance of the wire to values above the unitary
limit G0 = 2e2/h.Comment: 8 pages, 3 figures, submitted to PR
Transient dynamics of a molecular quantum dot with a vibrational degree of freedom
We investigate the transient effects occurring in a molecular quantum dot
described by an Anderson-Holstein Hamiltonian which is instantly coupled to two
fermionic leads biased by a finite voltage. In the limit of weak
electron-phonon interaction, we use perturbation theory to determine the
time-dependence of the dot population and the average current. The limit of
strong coupling is accessed by means of a self-consistent time-dependent
mean-field approximation. These comple- mentary approaches allow us to
investigate the dynamics of the inelastic effects occurring when the applied
bias voltage exceeds the phonon frequency and the emergence of bistability.Comment: 7 pages, 4 figure
Charge transfer statistics of a molecular quantum dot with strong electron-phonon interaction
We analyze the nonequilibrium transport properties of a quantum dot with a
harmonic degree of freedom (Holstein phonon) coupled to metallic leads, and
derive its full counting statistics (FCS). Using the Lang-Firsov (polaron)
transformation, we construct a diagrammatic scheme to calculate the cumulant
generating function. The electron-phonon interaction is taken into account
exactly, and the employed approximation represents a summation of a diagram
subset with respect to the tunneling amplitude. By comparison to Monte Carlo
data the formalism is shown to capture the basic properties of the strong
coupling regime
Full counting statistics of spin transfer through ultrasmall quantum dots
We analyze the spin-resolved full counting statistics of electron transfer
through an ultrasmall quantum dot coupled to metallic electrodes. Modelling the
setup by the Anderson Hamiltonian, we explicitly take into account the onsite
Coulomb repulsion . We calculate the cumulant generating function for the
probability to transfer a certain number of electrons with a preselected spin
orientation during a fixed time interval. With the cumulant generating function
at hand we are then able to calculate the spin current correlations which are
of outmost importance in the emerging field of spintronics. We confirm the
existing results for the charge statistics and report the discovery of the new
type of correlation between the spin-up and -down polarized electrons flows,
which has a potential to become a powerful new instrument for the investigation
of the Kondo effect in nanostructures.Comment: 5 pages, 1 figur
Understanding co-operative R&D activity: evidence from four European countries
This paper investigates co-operative research activity by firms using data from
the 3rd Community Innovation Survey for four countries, France, Germany, Spain and the
UK. We build on the Cassiman and Veugelers (CV) (2002) study of Belgian manufacturing
firms, by incorporating information on the service sector, and considering the role of public
support in affecting firms’ decisions to co-operate. Our results support those in CV, in that
we find a positive relationship between the likelihood of undertaking co-operative R&D
and both incoming knowledge spillovers and the extent to which firms find strategic
methods important in appropriating the returns to innovative activity. We find that public
support is positively related to the probability of undertaking co-operative agreements
particularly with regard to the likelihood of co-operation with the research base. We find
some evidence, in particular for Spain, that firms carry out co-operative R&D to overcome
excessive perceived risks and financial constraints
Spin Effects in the Local Density of States of GaAs
We present spin-resolved measurements of the local density of states in Si
doped GaAs. Both spin components exhibit strong mesoscopic fluctuations. In the
magnetic quantum limit, the main features of the spin-up and spin-down
components of the local density of states are found to be identical apart from
Zeeman splitting. Based on this observation, we introduce a mesoscopic method
to measure the -factor in a material where macroscopic methods are severely
restricted by disorder. Differences between the spin-up and spin-down
components are discussed in terms of spin relaxation due to spin-orbit
coupling.Comment: 4 pages and 5 figure
Renormalization Group Evolution in the type I + II seesaw model
We carefully analyze the renormalization group equations in the type I + II
seesaw scenario in the extended standard model (SM) and minimal supersymmetric
standard model (MSSM). Furthermore, we present analytic formulae of the mixing
angles and phases and discuss the RG effect on the different mixing parameters
in the type II seesaw scenario. The renormalization group equations of the
angles have a contribution which is proportional to the mass squared difference
for a hierarchical spectrum. This is in contrast to the inverse proportionality
to the mass squared difference in the effective field theory case.Comment: 13 pages, 4 figures; corrected error due to wrong superfield
normalization in RG equations (24-28,C1-4) as well as error in RG equations
of mixing parameters (38,43); RG equations of mixing angles depend on
Majorana phase
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