1,133 research outputs found
Gapless Excitation above a Domain Wall Ground State in a Flat Band Hubbard Model
We construct a set of exact ground states with a localized ferromagnetic
domain wall and with an extended spiral structure in a deformed flat-band
Hubbard model in arbitrary dimensions. We show the uniqueness of the ground
state for the half-filled lowest band in a fixed magnetization subspace. The
ground states with these structures are degenerate with all-spin-up or
all-spin-down states under the open boundary condition. We represent a spin
one-point function in terms of local electron number density, and find the
domain wall structure in our model. We show the existence of gapless
excitations above a domain wall ground state in dimensions higher than one. On
the other hand, under the periodic boundary condition, the ground state is the
all-spin-up or all-spin-down state. We show that the spin-wave excitation above
the all-spin-up or -down state has an energy gap because of the anisotropy.Comment: 26 pages, 1 figure. Typos are fixe
Microscopic analysis of the microscopic reversibility in quantum systems
We investigate the robustness of the microscopic reversibility in open
quantum systems which is discussed by Monnai [arXiv:1106.1982 (2011)]. We
derive an exact relation between the forward transition probability and the
reversed transition probability in the case of a general measurement basis. We
show that the microscopic reversibility acquires some corrections in general
and discuss the physical meaning of the corrections. Under certain processes,
some of the correction terms vanish and we numerically confirmed that the
remaining correction term becomes negligible; the microscopic reversibility
almost holds even when the local system cannot be regarded as macroscopic.Comment: 12 pages, 10 figure
Fluctuation theorem for currents in open quantum systems
A quantum-mechanical framework is set up to describe the full counting
statistics of particles flowing between reservoirs in an open system under
time-dependent driving. A symmetry relation is obtained which is the
consequence of microreversibility for the probability of the nonequilibrium
work and the transfer of particles and energy between the reservoirs. In some
appropriate long-time limit, the symmetry relation leads to a steady-state
quantum fluctuation theorem for the currents between the reservoirs. On this
basis, relationships are deduced which extend the Onsager-Casimir reciprocity
relations to the nonlinear response coefficients.Comment: 19 page
Entropy Production in a Persistent Random Walk
We consider a one-dimensional persisent random walk viewed as a deterministic
process with a form of time reversal symmetry. Particle reservoirs placed at
both ends of the system induce a density current which drives the system out of
equilibrium. The phase space distribution is singular in the stationary state
and has a cumulative form expressed in terms of generalized Takagi functions.
The entropy production rate is computed using the coarse-graining formalism of
Gaspard, Gilbert and Dorfman. In the continuum limit, we show that the value of
the entropy production rate is independent of the coarse-graining and agrees
with the phenomenological entropy production rate of irreversible
thermodynamics.Comment: 21 pages, 8 figures, to appear in Physica
Exact solutions of domain wall and spiral ground states in Hubbard models
We construct a set of exact ground states with a localized ferromagnetic
domain wall and an extended spiral structure in a deformed flat-band Hubbard
model. In the case of quarter filling, we show the uniqueness of the ground
state with a fixed magnetization. We discuss more realistic situation given by
a band-bending perturbation, which can stabilize these curious structures. We
study the scattering of a conduction electron by the domain wall and the spiral
spins.Comment: 4 pages, 2 figures. To be published in J. Phys. Soc. Jpn. 73 (2004
Slow decay of dynamical correlation functions for nonequilibrium quantum states
A property of dynamical correlation functions for nonequilibrium states is
discussed. We consider arbitrary dimensional quantum spin systems with local
interaction and translationally invariant states with nonvanishing current over
them. A correlation function between local charge and local Hamiltonian at
different spacetime points is shown to exhibit slow decay.Comment: typos correcte
Magnetic field effects on two-dimensional Kagome lattices
Magnetic field effects on single-particle energy bands (Hofstadter
butterfly), Hall conductance, flat-band ferromagnetism, and magnetoresistance
of two-dimensional Kagome lattices are studied. The flat-band ferromagnetism is
shown to be broken as the flat-band has finite dispersion in the magnetic
field. A metal-insulator transition induced by the magnetic field (giant
negative magnetoresistance) is predicted. In the half-filled flat band, the
ferromagnetic-paramagnetic transition and the metal-insulator one occur
simultaneously at a magnetic field for strongly interacting electrons. All of
the important magnetic fields effects should be observable in mesoscopic
systems such as quantum dot superlattices.Comment: 10 pages, 4 figures, and 1 tabl
Quantum Multibaker Maps: Extreme Quantum Regime
We introduce a family of models for quantum mechanical, one-dimensional
random walks, called quantum multibaker maps (QMB). These are Weyl
quantizations of the classical multibaker models previously considered by
Gaspard, Tasaki and others. Depending on the properties of the phases
parametrizing the quantization, we consider only two classes of the QMB maps:
uniform and random. Uniform QMB maps are characterized by phases which are the
same in every unit cell of the multibaker chain. Random QMB maps have phases
that vary randomly from unit cell to unit cell. The eigenstates in the former
case are extended while in the latter they are localized. In the uniform case
and for large , analytic solutions can be obtained for the time
dependent quantum states for periodic chains and for open chains with absorbing
boundary conditions. Steady state solutions and the properties of the
relaxation to a steady state for a uniform QMB chain in contact with
``particle'' reservoirs can also be described analytically. The analytical
results are consistent with, and confirmed by, results obtained from numerical
methods. We report here results for the deep quantum regime (large ) of
the uniform QMB, as well as some results for the random QMB. We leave the
moderate and small results as well as further consideration of the
other versions of the QMB for further publications.Comment: 17 pages, referee's and editor's comments addresse
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