2,065 research outputs found
Resummation for Nonequilibrium Perturbation Theory and Application to Open Quantum Lattices
Lattice models of fermions, bosons, and spins have long served to elucidate
the essential physics of quantum phase transitions in a variety of systems.
Generalizing such models to incorporate driving and dissipation has opened new
vistas to investigate nonequilibrium phenomena and dissipative phase
transitions in interacting many-body systems. We present a framework for the
treatment of such open quantum lattices based on a resummation scheme for the
Lindblad perturbation series. Employing a convenient diagrammatic
representation, we utilize this method to obtain relevant observables for the
open Jaynes-Cummings lattice, a model of special interest for open-system
quantum simulation. We demonstrate that the resummation framework allows us to
reliably predict observables for both finite and infinite Jaynes-Cummings
lattices with different lattice geometries. The resummation of the Lindblad
perturbation series can thus serve as a valuable tool in validating open
quantum simulators, such as circuit-QED lattices, currently being investigated
experimentally.Comment: 15 pages, 9 figure
Tuning thermal transport in graphene via combinations of molecular antiresonances
We propose a method to engineer the phonon thermal transport properties of
low dimensional systems. The method relies on introducing a predetermined
combination of molecular adsorbates, which give rise to antiresonances at
frequencies specific to the molecular species. Despite their dissimilar
transmission spectra, thermal resistances due to individual molecules remain
almost the same for all species. On the other hand, thermal resistance due to
combinations of different species are not additive and show large differences
depending on the species. Using a toy model, the physics underlying the
violation of resistance summation rule is investigated. It is demonstrated that
equivalent resistance of two scatterers having the same resistances can be
close to the sum of the constituents or 70\% of it depending on the
relative positions of the antiresonances. The relative positions of the
antiresonances determine the net change in transmission, therefore the
equivalent resistance. Since the entire spectrum is involved in phonon spectrum
changes in different parts of the spectrum become important. Performing
extensive first-principles based computations, we show that these distinctive
attributes of phonon transport can be useful to tailor the thermal transport
through low dimensional materials, especially for thermoelectric and thermal
management applications.Comment: 4 figures (1 figure and 2 videos as supplemental material
Charge gaps and quasiparticle bands of the ionic Hubbard model
The ionic Hubbard model on a cubic lattice is investigated using analytical
approximations and Wilson's renormalization group for the charge excitation
spectrum. Near the Mott insulating regime, where the Hubbard repulsion starts
to dominate all energies, the formation of correlated bands is described. The
corresponding partial spectral weights and local densities of states show
characteristic features, which compare well with a hybridized-band picture
appropriate for the regime at small , which at half-filling is known as a
band insulator. In particular, a narrow charge gap is obtained at half-filling,
and the distribution of spectral quasi-particle weight reflects the fundamental
hybridization mechanism of the model
Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation
By coupling a first-principles, spin-density functional calculation with an
exact diagonalization study of the Hubbard model, we have searched over various
functional groups for the best case for the flat-band ferromagnetism proposed
by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic
polymers of five-membered rings. The original proposal (poly-aminotriazole) has
turned out to be the best case among the materials examined, where the reason
why this is so is identified here. We have also found that the ferromagnetism,
originally proposed for the half-filled flat band, is stable even when the band
filling is varied away from the half-filling. All these make the ferromagnetism
proposed here more experimentally inviting.Comment: 11 pages, 13figure
Theory of Unconventional Superconductivity in Strongly Correlated Systems: Real Space Pairing and Statistically Consistent Mean-Field Theory - in Perspective
In this brief overview we discuss the principal features of real space
pairing as expressed via corresponding low-energy (t-J or periodic
Anderson-Kondo) effective Hamiltonian, as well as consider concrete properties
of those unconventional superconductors. We also rise the basic question of
statistical consistency within the so-called renormalized mean-field theory. In
particular, we provide the phase diagrams encompassing the stable magnetic and
superconducting states. We interpret real space pairing as correlated motion of
fermion pair coupled by short-range exchange interaction of magnitude J
comparable to the particle renormalized band energy , where is the
carrier number per site. We also discuss briefly the difference between the
real-space and the paramagnon - mediated sources of superconductivity. The
paper concentrates both on recent novel results obtained in our research group,
as well as puts the theoretical concepts in a conceptual as well as historical
perspective. No slave-bosons are required to formulate the present approach
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