114 research outputs found
Time-dependent quantum transport: causal superfermions, exact fermion-parity protected decay mode, and Pauli exclusion principle for mixed quantum states
We extend the recently developed causal superfermion approach to the
real-time transport theory to time-dependent decay problems.Its usefulness is
illustrated for the Anderson model of a quantum dot with tunneling rates
depending on spin due to the ferromagnetic electrodes and/or spin polarization
of the tunnel junction. We set up a second quantization scheme for density
operators in the Liouville-Fock space constructing causal field superoperators
using the fundamental physical principles of causality/probability conservation
and the fermion-parity superselection (univalence). The time-dependent
perturbation series for the time-evolution is renormalized by explicitly
performing the wide-band limit on the superoperator level. The short and
long-time reservoir correlations are shown to be tightly linked to the
occurrence of causal field destruction and creation superoperators,
respectively. The effective theory takes as a reference a damped local system,
providing an interesting starting point for numerical calculations of memory
kernels in real-time. A remarkable feature of this approach is the natural
appearance of a measurable fermion-parity protected decay mode. It already can
be calculated exactly in the Markovian, infinite temperature limit by leading
order perturbation theory, yet persists unaltered for the finite temperature,
interaction and tunneling spin polarization. Furthermore, we show how a
Liouville-space analog of the Pauli principle directly leads to the exact
result in the noninteracting limit: surprisingly, it is obtained in finite
(second) order renormalized perturbation theory, both for the self-energy as
well as the time-evolution propagator. For this limit we calculate the
time-evolution of the full density operator starting from an arbitrary initial
state on the quantum dot, including spin and pairing coherences and
two-particle correlations.Comment: This version contains the more extensive introduction and the
conclusion, discussing an experimental relevance of the obtained exact result
for the new decay mode. A lot of new references have been added. The more
detailed comparison of the results obtained for the noninteracting case with
the known results has been done. Small typos have been fixe
Density-operator evolution: Complete positivity and the Keldysh real-time expansion
We study the reduced time-evolution of open quantum systems by combining
quantum-information and statistical field theory. Inspired by prior work [EPL
102, 60001 (2013) and Phys. Rev. Lett. 111, 050402 (2013)] we establish the
explicit structure guaranteeing the complete positivity (CP) and
trace-preservation (TP) of the real-time evolution expansion in terms of the
microscopic system-environment coupling.
This reveals a fundamental two-stage structure of the coupling expansion:
Whereas the first stage defines the dissipative timescales of the system
--before having integrated out the environment completely-- the second stage
sums up elementary physical processes described by CP superoperators. This
allows us to establish the nontrivial relation between the (Nakajima-Zwanzig)
memory-kernel superoperator for the density operator and novel memory-kernel
operators that generate the Kraus operators of an operator-sum. Importantly,
this operational approach can be implemented in the existing Keldysh real-time
technique and allows approximations for general time-nonlocal quantum master
equations to be systematically compared and developed while keeping the CP and
TP structure explicit.
Our considerations build on the result that a Kraus operator for a physical
measurement process on the environment can be obtained by 'cutting' a group of
Keldysh real-time diagrams 'in half'. This naturally leads to Kraus operators
lifted to the system plus environment which have a diagrammatic expansion in
terms of time-nonlocal memory-kernel operators. These lifted Kraus operators
obey coupled time-evolution equations which constitute an unraveling of the
original Schr\"odinger equation for system plus environment. Whereas both
equations lead to the same reduced dynamics, only the former explicitly encodes
the operator-sum structure of the coupling expansion.Comment: Submission to SciPost Physics, 49 pages including 6 appendices, 13
figures. Significant improvement of introduction and conclusion, added
discussions, fixed typos, no results change
Transport signature of pseudo-Jahn-Teller dynamics in a single-molecule transistor
We calculate the electronic transport through a molecular dimer, in which an
excess electron is delocalized over equivalent monomers, which can be locally
distorted. In this system the Born-Oppenheimer approximation breaks down
resulting in quantum entanglement of the mechanical and electronic motion. We
show that pseudo Jahn-Teller (pJT) dynamics of the molecule gives rise to
conductance peaks that indicate this violation. Their magnitude, sign and
position sharply depend on the electro-mechanical properties of the molecule,
which can be varied in recently developed three-terminal junctions with
mechanical control. The predicted effect depends crucially on the degree of
intramolecular delocalization of the excess electron, a parameter which is also
of fundamental importance in physical chemistry.Comment: 6 pages, 3 figure
Non-linear thermoelectrics of molecular junctions with vibrational coupling
We present a detailed study of the non-linear thermoelectric properties of a
molecular junction, represented by a dissipative Anderson-Holstein model. A
single orbital level with strong Coulomb interaction is coupled to a localized
vibrational mode and we account for both electron and phonon exchange with both
electrodes, investigating how these contribute to the heat and charge
transport. We calculate the efficiency and power output of the device operated
as a heat to electric power converter and identify the optimal operating
conditions, which are found to be qualitatively changed by the presence of the
vibrational mode. Based on this study of a generic model system, we discuss the
desirable properties of molecular junctions for thermoelectric applications.Comment: 8 pages, 5 figure
Pair-tunneling resonance in the single-electron transport regime
We predict a new electron pair-tunneling (PT) resonance in non-linear
transport through quantum dots with positive charging energies exceeding the
broadening due to thermal and quantum fluctuations. The PT resonance shows up
in the single-electron transport (SET) regime as a peak in the derivative of
the non-linear conductance when the electrochemical potential of one electrode
matches the average of two subsequent charge addition energies. For a single
level quantum dot (Anderson model) we find the analytic peak shape and the
dependence on temperature, magnetic field and junction asymmetry and compare
with the inelastic cotunneling peak which is of the same order of magnitude. In
experimental transport data the PT resonance may be mistaken for a weak SET
resonance judging only by the voltage dependence of its position. Our results
provide essential clues to avoid such erroneous interpretation of transport
spectroscopy data.Comment: 5 pages, 2 figures, published versio
Duality for open fermion systems: energy-dependent weak coupling and quantum master equations
Open fermion systems with energy-independent bilinear coupling to a fermionic
environment have been shown to obey a general duality relation [Phys. Rev. B
93, 81411 (2016)] which allows for a drastic simplification of time-evolution
calculations. In the weak-coupling limit, such a system can be associated with
a unique dual physical system in which all energies are inverted, in particular
the internal interaction. This paper generalizes this fermionic duality in two
ways: we allow for weak coupling with arbitrary energy dependence and describe
both occupations and coherences coupled by a quantum master equation for the
density operator. We also show that whenever generalized detailed balance holds
(Kolmogorov criterion), the stationary probabilities for the dual system can be
expressed explicitly in terms of the stationary recurrence times of the
original system, even at large bias.
We illustrate the generalized duality by a detailed analysis of the rate
equation for a quantum dot with strong onsite Coulomb repulsion, going beyond
the commonly assumed wideband limit. We present predictions for (i) the decay
rates for transient charge and heat currents after a gate-voltage quench and
(ii) the thermoelectric linear response coefficients in the stationary limit.
We show that even for pronouncedly energy-dependent coupling, all nontrivial
parameter dependence in these problems is entirely captured by just two
well-understood stationary variables, the average charge of the system and of
the dual system. Remarkably, it is the latter that often dictates the most
striking features of the measurable quantities (e.g., positions of resonances),
underscoring the importance of the dual system for understanding the actual
one.Comment: 25 pages + 2 pages appendix + 2 pages references, 7 figures. To be
submitted to Phys. Rev.
Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise
We demonstrate that transport spectroscopy of single molecular magnets shows
signatures of quantum tunneling at low temperatures. We find current and noise
oscillations as function of bias voltage due to a weak violation of spin
selection rules by quantum tunneling processes. The interplay with Boltzmann
suppression factors leads to fake resonances with temperature-dependent
position which do not correspond to any charge excitation energy. Furthermore,
we find that quantum tunneling can completely suppress transport if the
easy-plane anisotropy has a high symmetry.Comment: 4 pages, 3 figure
Qubit quantum-dot sensors: noise cancellation by coherent backaction, initial slips, and elliptical precession
We theoretically investigate the backaction of a sensor quantum dot with
strong local Coulomb repulsion on the transient dynamics of a qubit that is
probed capacitively. We show that the measurement backaction induced by the
noise of electron cotunneling through the sensor is surprisingly mitigated by
the recently identified coherent backaction [PRB 89, 195405] arising from
quantum fluctuations. This renormalization effect is missing in semiclassical
stochastic fluctuator models and typically also in Born-Markov approaches,
which try to avoid the calculation of the nonstationary, nonequilibrium state
of the qubit plus sensor. Technically, we integrate out the current-carrying
electrodes to obtain kinetic equations for the joint, nonequilibrium
detector-qubit dynamics. We show that the sensor-current response, level
renormalization, cotunneling, and leading non-Markovian corrections always
appear together and cannot be turned off individually in an experiment or
ignored theoretically. We analyze the backaction on the reduced qubit state -
capturing the full non-Markovian effects imposed by the sensor quantum dot on
the qubit - by applying a Liouville-space decomposition into quasistationary
and rapidly decaying modes. Importantly, the sensor cannot be eliminated
completely even in the simplest high-temperature, weak-measurement limit: The
qubit state experiences an initial slip that persists over many qubit cycles
and depends on the initial preparation of qubit plus sensor quantum dot. A
quantum-dot sensor can thus not be modeled as a 'black box' without accounting
for its dynamical variables. We furthermore find that the Bloch vector relaxes
(T1) along an axis that is not orthogonal to the plane in which the Bloch
vector dephases (T2), blurring the notions of T1 and T2 times. Finally, the
precessional motion of the Bloch vector is distorted into an ellipse in the
tilted dephasing plane.Comment: This is the version published in Phys. Rev.
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