192 research outputs found
U(1) symmetry breaking in one-dimensional Mott insulator studied by the Density Matrix Renormalization Group method
A new type of external fields violating the particle number preservation is
studied in one-dimensional strongly correlated systems by the Density Matrix
Renormalization Group method. Due to the U(1) symmetry breaking, the ground
state has fluctuation of the total particle number, which implies injection of
electrons and holes from out of the chain. This charge fluctuation can be
relevant even at half-filling because the particle-hole symmetry is preserved
with the finite effective field. In addition, we discuss a quantum phase
transition obtained by considering the symmetry-breaking fields as a mean field
of interchain-hopping.Comment: 7 pages, 4 figure
Nontrivial quantized Berry phases for itinerant spin liquids
Quantized Berry phases as local order parameters in t-J models are studied. A
texture pattern of the local order parameters is topologically stable due to
the quantization of non-Abelian Berry phases defined by low-energy states below
a spin gap, which exists in the large J/t case with a few electrons. We have
confirmed that itinerant singlets in the wide class of t-J models carry the
nontrivial Berry phase pi. In the large J/t case for the one-dimensional t-J
model, Berry phases are uniformly pi when the number of electrons is N =4n +2,
().Comment: 8 pages, 4 figure
Edge states and topological orders in the spin liquid phases of star lattice
A group of novel materials can be mapped to the star lattice, which exhibits
some novel physical properties. We give the bulk-edge correspondence theory of
the star lattice and study the edge states and their topological orders in
different spin liquid phases. The bulk and edge-state energy structures and
Chern number depend on the spin liquid phases and hopping parameters because
the local spontaneous magnetic flux in the spin liquid phase breaks the time
reversal and space inversion symmetries. We give the characteristics of bulk
and edge energy structures and their corresponding Chern numbers in the
uniform, nematic and chiral spin liquids. In particular, we obtain analytically
the phase diagram of the topological orders for the chiral spin liquid states
SL[\phi,\phi,-2\phi], where \phi is the magnetic flux in two triangles and a
dodecagon in the unit cell. Moreover, we find the topological invariance for
the spin liquid phases, SL[\phi_{1},\phi_{2},-(\phi_{1}+\phi_{2})] and
SL[\phi_{2},\phi_{1},-(\phi_{1}+\phi_{2})]. The results reveal the relationship
between the energy-band and edge-state structures and their topological orders
of the star lattice.Comment: 7 pages, 8 figures, 1 tabl
Simple Exactly Solvable Models of non-Fermi Liquids
We generalize the model of Hatsugai and Kohmoto [J. Phys. Soc. Jpn, 61, 2056
(1992)] and find ground states which do not show the properties of Fermi
liquids. We work in two space dimensions, but it is straightforward to
generalize to higher dimensions. The ground state is highly degenerate and
there is no discontinuity in the momentum distribution; i.e., there is no Fermi
surface. The Green's function generically has a branch cut.Comment: Revte
Valley Spin Sum Rule for Dirac Fermions: Topological Argument
We consider a two-dimensional bipartite lattice system. In such a system, the
Bloch band spectrum can have some valley points, around which Dirac fermions
appear as the low-energy excitations. Each valley point has a valley spin +1 or
-1. In such a system, there are two topological numbers counting vortices and
merons in the Brillouin zone, respectively. These numbers are equivalent, and
this fact leads to a sum rule which states that the total sum of the valley
spins is absent even in a system without time-reversal and parity symmetries.
We can see some similarity between the valley spin and chirality in the
Nielsen-Ninomiya no-go theorem in odd-spatial dimensions.Comment: 5 pages, 1 figure, some comments are added/revised, accepted for
publication in J. Phys. Soc. Jp
Transitions from the Quantum Hall State to the Anderson Insulator: Fa te of Delocalized States
Transitions between the quantum Hall state and the Anderson insulator are
studied in a two dimensional tight binding model with a uniform magnetic field
and a random potential. By the string (anyon) gauge, the weak magnetic field
regime is explored numerically. The regime is closely related to the continuum
model. The change of the Hall conductance and the trajectoy of the delocalized
states are investigated by the topological arguments and the Thouless number
study.Comment: 10 pages RevTeX, 14 postscript figure
Entanglement Entropy of One-dimensional Gapped Spin Chains
We investigate the entanglement entropy (EE) of gapped S=1 and spin
chains with dimerization. We find that the effective boundary degrees of
freedom as edge states contribute significantly to the EE. For the
dimerized Heisenberg chain, the EE of the sufficiently long chain is
essentially explained by the localized effective spins on the
boundaries. As for S=1, the effective spins are also causing a Kennedy
triplet that yields a lower bound for the EE. In this case, the residual
entanglement reduces substantially by a continuous deformation of the
Heisenberg model to that of the AKLT Hamiltonian.Comment: 5 pages, 6 figure
Scaling near random criticality in two-dimensional Dirac fermions
Recently the existence of a random critical line in two dimensional Dirac
fermions is confirmed. In this paper, we focus on its scaling properties,
especially in the critical region. We treat Dirac fermions in two dimensions
with two types of randomness, a random site (RS) model and a random hopping
(RH) model. The RS model belongs to the usual orthogonal class and all states
are localized. For the RH model, there is an additional symmetry expressed by
. Therefore, although all non-zero energy states
localize, the localization length diverges at the zero energy. In the weak
localization region, the generalized Ohm's law in fractional dimensions,
, has been observed for the RH model.Comment: RevTeX with 4 postscript figures, To appear in Physical Review
Topological Classification of Gapped Spin Chains :Quantized Berry Phase as a Local Order Parameter
We characterize several phases of gapped spin systems by local order
parameters defined by quantized Berry phases. This characterization is
topologically stable against any small perturbation as long as the energy gap
remains finite. The models we pick up are dimerized Heisenberg chains
and S=2 Heisenberg chains with uniaxial single-ion-type anisotropy.
Analytically we also evaluate the topological local order parameters for the
generalized Affleck-Kennedy-Lieb-Tasaki (AKLT) model. The relation between the
present Berry phases and the fractionalization in the integer spin chains are
discussed as well.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Combinatorial interpretation of Haldane-Wu fractional exclusion statistics
Assuming that the maximal allowed number of identical particles in state is
an integer parameter, q, we derive the statistical weight and analyze the
associated equation which defines the statistical distribution. The derived
distribution covers Fermi-Dirac and Bose-Einstein ones in the particular cases
q = 1 and q -> infinity (n_i/q -> 1), respectively. We show that the derived
statistical weight provides a natural combinatorial interpretation of
Haldane-Wu fractional exclusion statistics, and present exact solutions of the
distribution equation.Comment: 8 pages, 2 eps-figure
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