133 research outputs found
Sudden interaction quench in the quantum sine-Gordon model
We study a sudden interaction quench in the weak-coupling regime of the
quantum sine-Gordon model. The real time dynamics of the bosonic mode
occupation numbers is calculated using the flow equation method. While we
cannot prove results for the asymptotic long time limit, we can establish the
existence of an extended regime in time where the mode occupation numbers relax
to twice their equilibrium values. This factor two indicates a non-equilibrium
distribution and is a universal feature of weak interaction quenches. The
weak-coupling quantum sine-Gordon model therefore turns out to be on the
borderline between thermalization and non-thermalization.Comment: 18 pages, 4 figures, published in New Journal of Physic
Scaling and Decoherence in the Out-of-Equilibrium Kondo Model
We study the Kondo effect in quantum dots in an out-of-equilibrium state due
to an applied dc-voltage bias. Using the method of infinitesimal unitary
transformations (flow equations), we develop a perturbative scaling picture
that naturally contains both equilibrium coherent and non-equilibrium
decoherence effects. This framework allows one to study the competition between
Kondo effect and current-induced decoherence, and it establishes a large regime
dominated by single-channel Kondo physics for asymmetrically coupled quantum
dots.Comment: 4 pages, 3 figures; v2: minor changes (typos corrected, esp. in Eqs.
(3), (4), references updated, improved layout for figures
Non-linear feedback effects in coupled Boson-Fermion systems
We address ourselves to a class of systems composed of two coupled subsystems
without any intra-subsystem interaction: itinerant Fermions and localized
Bosons on a lattice. Switching on an interaction between the two subsystems
leads to feedback effects which result in a rich dynamical structure in both of
them. Such feedback features are studied on the basis of the flow equation
technique - an infinite series of infinitesimal unitary transformations - which
leads to a gradual elimination of the inter-subsystem interaction. As a result
the two subsystems get decoupled but their renormalized kinetic energies become
mutually dependent on each other. Choosing for the inter - subsystem
interaction a charge exchange term (the Boson-Fermion model) the initially
localized Bosons acquire itinerancy through their dependence on the
renormalized Fermion dispersion. This latter evolves from a free particle
dispersion into one showing a pseudogap structure near the chemical potential.
Upon lowering the temperature both subsystems simultaneously enter a
macroscopic coherent quantum state. The Bosons become superfluid, exhibiting a
soundwave like dispersion while the Fermions develop a true gap in their
dispersion. The essential physical features described by this technique are
already contained in the renormalization of the kinetic terms in the respective
Hamiltonians of the two subsystems. The extra interaction terms resulting in
the process of iteration only strengthen this physics. We compare the results
with previous calculations based on selfconsistent perturbative approaches.Comment: 14 pages, 16 figures, accepted for publication in Phys. Rev.
Evolving Nuclear Many-Body Forces with the Similarity Renormalization Group
In recent years, the Similarity Renormalization Group has provided a powerful
and versatile means to soften interactions for ab initio nuclear calculations.
The substantial contribution of both induced and initial three-body forces to
the nuclear interaction has required the consistent evolution of free-space
Hamiltonians in the three-particle space. We present the most recent progress
on this work, extending the calculational capability to the p-shell nuclei and
showing that the hierarchy of induced many-body forces is consistent with
previous estimates. Calculations over a range of the flow parameter for 6Li,
including fully evolved NN+3N interactions, show moderate contributions due to
induced four-body forces and display the same improved convergence properties
as in lighter nuclei. A systematic analysis provides further evidence that the
hierarchy of many-body forces is preserved.Comment: 26 pages, 15 figures, and 5 table
Evolution of Nuclear Many-Body Forces with the Similarity Renormalization Group
The first practical method to evolve many-body nuclear forces to softened
form using the Similarity Renormalization Group (SRG) in a harmonic oscillator
basis is demonstrated. When applied to He4 calculations, the two- and
three-body oscillator matrix elements yield rapid convergence of the
ground-state energy with a small net contribution of the induced four-body
force.Comment: 4 pages, 5 figures, PRL published versio
First Order Superfluid to Bose Metal Transition in Systems with Resonant Pairing
Systems showing resonant superfluidity, driven by an exchange coupling of
strength between uncorrelated pairs of itinerant fermions and tightly bound
ones, undergo a first order phase transition as increases beyond some
critical value . The superfluid phase for is characterized by
a gap in the fermionic single particle spectrum and an acoustic sound-wave like
collective mode of the bosonic resonating fermion pairs inside this gap. For
this state gives way to a phase uncorrelated bosonic liquid with a
spectrum.Comment: 5 pages, 3 figure
Flow equation analysis of the anisotropic Kondo model
We use the new method of infinitesimal unitary transformations to calculate
zero temperature correlation functions in the strong-coupling phase of the
anisotropic Kondo model. We find the dynamics on all energy scales including
the crossover behaviour from weak to strong coupling. The integrable structure
of the Hamiltonian is not used in our approach. Our method should also be
useful in other strong-coupling models since few other analytical methods allow
the evaluation of their correlation functions on all energy scales.Comment: 4 pages RevTeX, 2 eps figures include
Flow equation solution for the weak to strong-coupling crossover in the sine-Gordon model
A continuous sequence of infinitesimal unitary transformations, combined with
an operator product expansion for vertex operators, is used to diagonalize the
quantum sine-Gordon model for 2 pi < beta^2 < infinity. The leading order of
this approximation already gives very accurate results for the single-particle
gap in the strong-coupling phase. This approach can be understood as an
extension of perturbative scaling theory since it links weak to strong-coupling
behavior in a systematic expansion. The approach should also be useful for
other strong-coupling problems that can be formulated in terms of vertex
operators.Comment: 4 pages, 1 figure, minor changes (typo in Eq. (3) corrected,
references added), published versio
The X-ray edge singularity in Quantum Dots
In this work we investigate the X-ray edge singularity problem realized in
noninteracting quantum dots. We analytically calculate the exponent of the
singularity in the absorption spectrum near the threshold and extend known
analytical results to the whole parameter regime of local level detunings.
Additionally, we highlight the connections to work distributions and to the
Loschmidt echo.Comment: 7 pages, 2 figures; version as publishe
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