89 research outputs found
Tunable ohmic environment using Josephson junction chains
We propose a scheme to implement a tunable, wide frequency-band dissipative
environment using a double chain of Josephson junctions. The two parallel
chains consist of identical SQUIDs, with magnetic-flux tunable inductance,
coupled to each other at each node via a capacitance much larger than the
junction capacitance. Thanks to this capacitive coupling, the system sustains
electromagnetic modes with a wide frequency dispersion. The internal quality
factor of the modes is maintained as high as possible, and the damping is
introduced by a uniform coupling of the modes to a transmission line, itself
connected to an amplification and readout circuit. For sufficiently long
chains, containing several thousands of junctions, the resulting admittance is
a smooth function versus frequency in the microwave domain, and its effective
dissipation can be continuously monitored by recording the emitted radiation in
the transmission line. We show that by varying in-situ the SQUIDs' inductance,
the double chain can operate as tunable ohmic resistor in a frequency band
spanning up to one GHz, with a resistance that can be swept through values
comparable to the resistance quantum R_q = (h/4e^2) ~ 6.5 k{\Omega}. We argue
that the circuit complexity is within reach using current Josephson junction
technology.Comment: 11 pages, 9 figure
Ground-state cooling of a carbon nanomechanical resonator by spin-polarized current
We study the nonequilibrium steady state of a mechanical resonator in the
quantum regime realized by a suspended carbon nanotube quantum dot contacted by
two ferromagnets. Because of the spin-orbit interaction and/or an external
magnetic field gradient, the spin on the dot couples directly to the flexural
eigenmodes. Accordingly, the nanomechanical motion induces inelastic spin flips
of the tunneling electrons. A spin-polarized current at finite bias voltage
causes either heating or active cooling of the mechanical modes. We show that
maximal cooling is achieved at resonant transport when the energy splitting
between two dot levels of opposite spin equals the vibrational frequency. Even
for weak electron-resonator coupling and moderate polarizations we can achieve
ground-state cooling with a temperature of the leads, for instance, of
Charge-vibration interaction effects in normal-superconductor quantum dots
We study the quantum transport and the nonequilibrium vibrational states of a
quantum dot embedded between a normal and a superconducting lead with the
charge on the quantum dot linearly coupled to a harmonic oscillator of
frequency . To the leading order in the charge-vibration interaction,
we calculate the current and the nonequilibrium phonon occupation by the
Keldsyh Green's function technique. We analyze the inelastic,
vibration-assisted tunneling processes in the regime , with the
superconducting energy gap , and for sharp resonant transmission
through the dot. When the energy of the dot's level is close to
the Fermi energy , i.e. , inelastic Andreev
reflections dominate up to voltage . The inelastic
quasiparticle tunneling becomes the leading process when the dot's level is
close to the gap . In both cases,
the inelastic tunneling processes appear as sharp and prominent peaks - not
broadened by temperature - in the - characteristic and pave the way for
inelastic spectroscopy of vibrational modes even at temperatures . We also found that inelastic Andreev reflections as well as
quasiparticle tunneling induce a strong nonequilibrium state of the oscillator.
In different ranges on the dot's level, we found that the current produces: (i)
ground-state cooling of the oscillator with phonon occupation , (ii)
accumulation of energy in the oscillator with and (iii) a mechanical
instability which is a precursor of self-sustained oscillations. We show that
ground-state cooling is achieved simultaneously for several modes of different
frequencies. Finally, we discuss how the nonequilibrium vibrational state can
be detected by the asymmetric behavior of the inelastic current peaks respect
to the gate voltage
Control of vibrational states by spin-polarized transport in a carbon nanotube resonator
We study spin-dependent transport in a suspended carbon nanotube quantum dot
in contact with two ferromagnetic leads and with the dot's spin coupled to the
flexural mechanical modes. The spin-vibration interaction induces spin-flip
processes between the two energy levels of the dot. This interaction arises
from the spin-orbit coupling or a magnetic field gradient. The inelastic
vibration-assisted spin flips give rise to a mechanical damping and, for an
applied bias voltage, to a steady nonequilibrium occupation of the harmonic
oscillator. We analyze these effects as function of the energy-level separation
of the dot and the magnetic polarization of the leads. Depending on the
magnetic configuration and the bias-voltage polarity, we can strongly cool a
single mode or pump energy into it. In the latter case, we find that within our
approximation, the system approaches eventually a regime of mechanical
instability. Furthermore, owing to the sensitivity of the electron transport to
the spin orientation, we find signatures of the nanomechanical motion in the
current-voltage characteristic. Hence, the vibrational state can be read out in
transport measurements
Finite frequency current noise in the Holstein model
We investigate the effects of local vibrational excitations in the
nonsymmetrized current noise of a nanojunction. For this purpose,
we analyze a simple model - the Holstein model - in which the junction is
described by a single electronic level that is coupled to two metallic leads
and to a single vibrational mode. Using the Keldysh Green's function technique,
we calculate the nonsymmetrized current noise to the leading order in the
charge-vibration interaction. For the noise associated to the latter, we
identify distinct terms corresponding to the mean-field noise and the vertex
correction. The mean-field result can be further divided into an elastic
correction to the noise and in an inelastic correction, the second one being
related to energy exchange with the vibration. To illustrate the general
behavior of the noise induced by the charge-vibration interaction, we consider
two limit cases. In the first case, we assume a strong coupling of the dot to
the leads with an energy-independent transmission whereas in the second case we
assume a weak tunneling coupling between the dot and the leads such that the
transport occurs through a sharp resonant level. We find that the noise
associated to the vibration-charge interaction shows a complex pattern as a
function of the frequency and of the transmission function or of the
dot's energy level. Several transitions from enhancement to suppression of the
noise occurs in different regions, which are determined, in particular, by the
vibrational frequency. Remarkably, in the regime of an energy-independent
transmission, the zero order elastic noise vanishes at perfect transmission and
at positive frequency whereas the noise related to the charge-vibration
interaction remains finite enabling the analysis of the pure
vibrational-induced current noise
Decoherence and relaxation of topological states in extended quantum Ising models
We study the decoherence and the relaxation dynamics of topological states in
an extended class of quantum Ising chains which can present a manyfold ground
state subspace. The leading interaction of the spins with the environment is
assumed to be the local fluctuations of the transverse magnetic field. By
deriving the Lindblad equation using the many-body states, we investigate the
relation between decoherence, energy relaxation and topology. In particular, in
the topological phase and at low temperature, we analyze the dephasing rates
between the different degenerate ground states
Nonequilibrium Andreev bound states population in short superconducting junctions coupled to a resonator
Inspired by recent experiments, we study a short superconducting junction of
length (coherence length) inserted in a dc-SQUID containing an
ancillary Josephson tunnel junction. We evaluate the nonequilibrium occupation
of the Andreev bound states (ABS) for the case of a conventional junction and a
topological junction, with the latter case of ABS corresponding to a Majorana
mode. We take into account small phase fluctuations of the Josephson tunnel
junction, acting as a damped LC resonator, and analyze the role of the
distribution of the quasiparticles of the continuum assuming that these
quasiparticles are in thermal distribution with an effective temperature
different from the environmental temperature. We also discuss the effect of
strong photon irradiation in the junction leading to a nonequilibrium
occupation of the ABS. We systematically compare the occupations of the bound
states and the supercurrents carried by these states for conventional and
topological junctions.Comment: 20 pages, 9 figures; added references, corrected typos, edited the
tex
Quantum phase transition with dissipative frustration
We study the quantum phase transition of the one-dimensional phase model in
the presence of dissipative frustration, provided by an interaction of the
system with the environment through two non-commuting operators. Such a model
can be realized in Josephson junction chains with shunt resistances and
resistances between the chain and the ground. Using a self-consistent harmonic
approximation, we determine the phase diagram at zero temperature which
exhibits a quantum phase transition between an ordered phase, corresponding to
the superconducting state, and a disordered phase, corresponding to the
insulating state with localized superconducting charge. Interestingly, we find
that the critical line separating the two phases has a non monotonic behavior
as a function of the dissipative coupling strength. This result is a
consequence of the frustration between (i) one dissipative coupling that
quenches the quantum phase fluctuations favoring the ordered phase and (ii) one
that quenches the quantum momentum (charge) fluctuations leading to a vanishing
phase coherence. Moreover, within the self-consistent harmonic approximation,
we analyze the dissipation induced crossover between a first and second order
phase transition, showing that quantum frustration increases the range in which
the phase transition is second order. The non monotonic behavior is reflected
also in the purity of the system that quantifies the degree of correlation
between the system and the environment, and in the logarithmic negativity as
entanglement measure that encodes the internal quantum correlations in the
chain
Interplay of magneto-elastic and polaronic effects in electronic transport through suspended carbon-nanotube quantum dots
We investigate the electronic transport through a suspended carbon-nanotube
quantum dot. In the presence of a magnetic field perpendicular to the nanotube
and a nearby metallic gate, two forces act on the electrons: the Laplace and
the electrostatic force. They both induce coupling between the electrons and
the mechanical transverse oscillation modes. We find that the difference
between the two mechanisms appears in the cotunneling current
Quantum Phase-Slip Junction Under Microwave Irradiation
We consider the dynamics of a quantum phase-slip junction (QPSJ) -- a dual
Josephson junction -- connected to a microwave source with frequency
. With respect to an ordinary Josephson junction, a QPSJ
can sustain dual Shapiro steps, consisting of well-defined current plateaus at
multiple integers of in the current-voltage (I-V)
characteristic. The experimental observation of these plateaus has been elusive
up to now. We argue that thermal as well as quantum fluctuations can smear the
I-V characteristic considerably. In order to understand these effects, we study
a current-biased QPSJ under microwave irradiation and connected to an inductive
and resistive environment. We find that the effect of these fluctuations are
governed by the resistance of the environment and by the ratio of the
phase-slip energy and the inductive energy. Our results are of interest for
experiments aimed at the observation of dual Shapiro steps in QPSJ devices for
the definition of a new quantum current standard.Comment: 12 pages, 9 figures, comments and suggestions would be greatly
appreciate
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