283 research outputs found
Effective Hamiltonians for fastly driven many-body lattice systems
We consider 1D lattices described by Hubbard or Bose-Hubbard models, in the
presence of periodic high-frequency perturbations, such as uniform ac force or
modulation of hopping coefficients. Effective Hamiltonians for interacting
particles are derived using an averaging method resembling classical canonical
perturbation theory. As is known, a high-frequency force may renormalize
hopping coefficients, causing interesting phenomena such as coherent
destruction of tunnelling and creation of artificial gauge fields. We find
explicitly additional corrections to the effective Hamiltonians due to
interactions, corresponding to non-trivial processes such as single-particle
density-dependent tunnelling, correlated pair hoppings, nearest neighbour
interactions, etc. Some of these processes arise also in multiband lattice
models, and are capable to give rise to a rich variety of quantum phases. The
apparent contradiction with other methods, e.g. Floquet-Magnus expansion, is
explained. The results may be useful for designing effective Hamiltonian models
in experiments with ultracold atoms, as well as in the field of ultrafast
nonequilibrium magnetism. An example of manipulating exchange interaction in a
Mott-Hubbard insulator is considered, where our corrections play an essential
role
Therapieresistente systemische Sarkoidose mit kutaner Manifestation : erfolgreiche Behandlung mit einer Kombination von Infliximab, Azathioprin und neiderdosierten Steroiden
Fermi acceleration in time-dependent rectangular billiards due to multiple passages through resonances
We consider a slowly rotating rectangular billiard with moving boundaries and
use the canonical perturbation theory to describe the dynamics of a billiard
particle. In the process of slow evolution certain resonance conditions can be
satisfied. Correspondingly, phenomena of scattering on a resonance and capture
into a resonance happen in the system. These phenomena lead to destruction of
adiabatic invariance and to unlimited acceleration of the particle.Comment: 20 pages. Presented on School-Conference "Mathematics and Physics of
Billiard-Like Systems" (Ubatuba, 2011). Accepted to Chao
Semiclassical Spectrum of Small Bose-Hubbard Chains: A Normal Form Approach
We analyze the spectrum of the 3-site Bose-Hubbard model with periodic
boundary conditions using a semiclassical method. The Bohr-Sommerfeld
quantization is applied to an effective classical Hamiltonian which we derive
using resonance normal form theory. The derivation takes into account the 1:1
resonance between frequencies of a linearized classical system, and brings
nonlinear terms into a corresponding normal form. The obtained expressions
reproduce the exact low-energy spectrum of the system remarkably well even for
a small number of particles N corresponding to fillings of just two particles
per site. Such small fillings are often used in current experiments, and it is
inspiring to get insight into this quantum regime using essentially classical
calculations.Comment: Minor corrections to the coefficients of the effective Hamiltonian in
Eqs 14,15,18,19. Figs 1,2 are slightly modified, correspondingl
Universality in nonadiabatic behaviour of classical actions in nonlinear models with separatrix crossings
We discuss dynamics of approximate adiabatic invariants in several nonlinear
models being related to physics of Bose-Einstein condensates (BEC). We show
that nonadiabatic dynamics in Feshbach resonance passage, nonlinear
Landau-Zener (NLZ) tunnelling, and BEC tunnelling oscillations in a double-well
can be considered within a unifying approach based on the theory of separatrix
crossings. The separatrix crossing theory was applied previously to some
problems of classical mechanics, plasma physics and hydrodynamics, but has not
been used in the rapidly growing BEC-related field yet. We derive explicit
formulas for the change in the action in several models. Extensive numerical
calculations support the theory and demonstrate its universal character. We
also discovered a qualitatively new nonlinear phenomenon in a NLZ model which
we propose to call {\em separated adiabatic tunnelling}Comment: Accepted for publication in Physical Review E; Several misprints are
corrected; main results are emphasized in the end of Introduction (including
finite conversion efficiency in Feshbach resonance passage due to geometric
jump in the action); bibliography is extende
Partial reconstitution of cutaneous microvessels in long-term survivors after allogeneic bone marrow transplantation
BACKGROUND: Graft-versus-host disease (GVHD) is a major complication after allogeneic hematopoietic stem cell transplantation (HSCT) and skin is involved in acute and chronic disease. Immune-mediated vessel attack and subsequent microvessel loss have been observed in skin of patients with chronic GVHD. OBJECTIVES: To test whether long-term survivors (LTS) after allogeneic HSCT without cutaneous GVHD show signs of persistent vascular remodeling. METHODS: Microvessels in skin biopsies were investigated in a cohort of 32 LTS with a median follow-up of 17 years (range 11-26). Five were currently classified as having chronic GVHD other than skin involvement. RESULTS: LTS showed no significant difference in median microvessel density and relative vessel size distribution pattern compared to healthy controls. Past experience of GVHD and current status of chronic GVHD other than skin involvement had no impact on vessel density. In contrast, recipients with chronic cutaneous GVHD of sclerotic type and patients with lichen sclerosus have significant microvessel loss in the upper dermis. CONCLUSION: The complex therapy of allogeneic HSCT had no sustained effect on the microvascular architecture of LTS when clinicopathological evidence of cutaneous GVHD is absent. Microvascular remodeling as observed during chronic GVHD recovers completely after resolution of chronic cutaneous GVHD
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