229 research outputs found
Control of electron-hole pair generation by biharmonic voltage drive of a quantum point contact
A time-dependent electromagnetic field creates electron-hole excitations in a
Fermi sea at low temperature. We show that the electron-hole pairs can be
generated in a controlled way using harmonic and biharmonic time-dependent
voltages applied to a quantum contact and obtain the probabilities of the pair
creations. For a biharmonic voltage drive, we find that the probability of a
pair creation decreases in the presence of an in-phase second harmonic. This
accounts for the suppression of the excess noise observed experimentally
[Gabelli and Reulet, arXiv:1205.3638] proving that dynamic control and
detection of elementary excitations in quantum conductors are within the reach
of the present technology.Comment: 4+ pages, 4 figures; to appear in PRB Rapid Communication
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.Comment: 5 pages, 7 figure
Full counting statistics and shot noise of cotunneling in quantum dots and single-molecule transistors
We develop a conceptually simple scheme based on a master-equation approach
to evaluate the full-counting statistics (FCS) of elastic and inelastic
off-resonant tunneling (cotunneling) in quantum dots (QDs) and molecules. We
demonstrate the method by showing that it reproduces known results for the FCS
and shot noise in the cotunneling regime. For a QD with an excited state, we
obtain an analytic expression for the cumulant generating function (CGF) taking
into account elastic and inelastic cotunneling. From the CGF we find that the
shot noise above the inelastic threshold in the cotunneling regime is
inherently super-Poissonian when external relaxation is weak. Furthermore, a
complete picture of the shot noise across the different transport regimes is
given. In the case where the excited state is a blocking state, strongly
enhanced shot noise is predicted both in the resonant and cotunneling regimes.Comment: 14 pages, 7 figures, published versio
Elementary Andreev Processes in a Driven Superconductor-Normal Metal Contact
We investigate the full counting statistics of a voltage-driven normal
metal(N)-superconductor(S) contact. In the low-bias regime below the
superconducting gap, the NS contact can be mapped onto a purely normal contact,
albeit with doubled voltage and counting fields. Hence in this regime the
transport characteristics can be obtained by the corresponding substitution of
the normal metal results. The elementary processes are single Andreev transfers
and electron- and hole-like Andreev transfers. Considering Lorentzian voltage
pulses we find an optimal quantization for half-integer Levitons.Comment: 8 pages, 6 figures, contribution to the special issue in memory of
Markus B\"uttiker, accepted at Physica
Shot noise of charge and spin transport in a junction with a precessing molecular spin
Magnetic molecules and nanomagnets can be used to influence the electronic
transport in mesoscopic junction. In a magnetic field the precessional motion
leads to resonances in the dc- and ac-transport properties of a nanocontact, in
which the electrons are coupled to the precession. Quantities like the
dc-conductance or the ac-response provide valuable information like the level
structure and the coupling parameters. Here, we address the current noise
properties of such contacts. This encompasses the charge current and
spin-torque shot noise, which both show a step-like behavior as functions of
bias voltage and magnetic field. The charge current noise shows pronounced dips
around the steps, which we trace back to interference effects of electron in
quasienergy levels coupled by the molecular spin precession. We show that some
components of the noise of the spin-torque currents are directly related to the
Gilbert damping and, hence, are experimentally accessible. Our results show
that the noise characteristics allow to investigate in more detail the
coherence of spin transport in contacts containing magnetic molecules.Comment: 10 pages, 8 figure, published versio
Effective Quantum Theories for Transport in Inhomogeneous Systems with Non-trivial Band Structure
Starting from a general -band Hamiltonian with weak spatial and temporal
variations, we derive a low energy effective theory for transport within one or
several overlapping bands. To this end, we use the Wigner representation that
allows us to systematically construct the unitary transformation that brings
the Hamiltonian into band-diagonal form. We address the issue of gauge
invariance and discuss the necessity of using kinetic variables in order to
obtain a low energy effective description that is consistent with the original
theory. Essentially, our analysis is a semiclassical one and quantum
corrections appear as Berry curvatures in addition to quantities that are
related to the appearance of persistent currents. We develop a transport
framework which is manifestly gauge invariant and it is based on a quantum
Boltzman formulation along with suitable definitions of current density
operators such that Liouville's theorem is satisfied. Finally, we incorporate
the effects of an external electromagnetic field into our theory.Comment: 22 pages, 2 figure
Magnon-mediated spin current noise in ferromagnetnon-magnetic conductor hybrids
The quantum excitations of the collective magnetization dynamics in a
ferromagnet (F) - magnons - enable spin transport without an associated charge
current. This pure spin current can be transferred to electrons in an adjacent
non-magnetic conductor (N). We evaluate the finite temperature noise of the
magnon-mediated spin current injected into N by an adjacent F driven by a
coherent microwave field. We find that the dipolar interaction leads to
squeezing of the magnon modes giving them wavevector dependent non-integral
spin, which directly manifests itself in the shot noise. For temperatures
higher than the magnon gap, the thermal noise is dominated by large wavevector
magnons which exhibit negligible squeezing. The noise spectrum is white up to
the frequency corresponding to the maximum of the temperature or the magnon
gap. At larger frequencies, the noise is dominated by vacuum fluctuations. The
shot noise is found to be much larger than its thermal counterpart over a broad
temperature range, making the former easier to be measured experimentally
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.Comment: 9 pages, 4 figure
Fourth moments reveal the negativity of the Wigner function
The presence of unique quantum correlations is the core of quantum
information processing and general quantum theory. We address the fundamental
question of how quantum correlations of a generic quantum system can be probed
using correlation functions defined for quasiprobability distributions. In
particular we discuss the possibility of probing the negativity of a
quasiprobability by comparing moments of the Wigner function. We show that one
must take at least the fourth moments to find the negativity in general and the
eighth moments for states with a rotationally invariant Wigner function.Comment: 5 pages, published versio
Super-Poissonian shot noise of squeezed-magnon mediated spin transport
The magnetization of a ferromagnet (F) driven out of equilibrium injects pure
spin current into an adjacent conductor (N). Such FN bilayers have become
basic building blocks in a wide variety of spin based devices. We evaluate the
shot noise of the spin current traversing the FN interface when F is
subjected to a coherent microwave drive. We find that the noise spectrum is
frequency independent up to the drive frequency, and increases linearly with
frequency thereafter. The low frequency noise indicates super-Poissonian spin
transfer, which results from quasi-particles with effective spin . For typical ferromagnetic thin films, is
related to the dipolar interaction-mediated squeezing of F eigenmodes.Comment: 4 pages, 2 figure
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