3,862 research outputs found
A numerical canonical transformation approach to quantum many body problems
We present a new approach for numerical solutions of ab initio quantum
chemistry systems. The main idea of the approach, which we call canonical
diagonalization, is to diagonalize directly the second quantized Hamiltonian by
a sequence of numerical canonical transformations.Comment: 10 pages, 3 encapsulated figures. Parts of the paper are
substantially revised to refer to previous similar method
Neel order in square and triangular lattice Heisenberg models
Using examples of the square- and triangular-lattice Heisenberg models we
demonstrate that the density matrix renormalization group method (DMRG) can be
effectively used to study magnetic ordering in two-dimensional lattice spin
models. We show that local quantities in DMRG calculations, such as the on-site
magnetization M, should be extrapolated with the truncation error, not with its
square root, as previously assumed. We also introduce convenient sequences of
clusters, using cylindrical boundary conditions and pinning magnetic fields,
which provide for rapidly converging finite-size scaling. This scaling behavior
on our clusters is clarified using finite-size analysis of the effective
sigma-model and finite-size spin-wave theory. The resulting greatly improved
extrapolations allow us to determine the thermodynamic limit of M for the
square lattice with an error comparable to quantum Monte Carlo. For the
triangular lattice, we verify the existence of three-sublattice magnetic order,
and estimate the order parameter to be M = 0.205(15).Comment: 4 pages, 5 figures, typo fixed, reference adde
Enhanced Pairing in the "Checkerboard" Hubbard Ladder
We study signatures of superconductivity in a 2--leg "checkerboard" Hubbard
ladder model, defined as a one--dimensional (period 2) array of square
plaquettes with an intra-plaquette hopping and inter-plaquette hopping
, using the density matrix renormalization group method. The highest
pairing scale (characterized by the spin gap or the pair binding energy,
extrapolated to the thermodynamic limit) is found for doping levels close to
half filling, and . Other forms of modulated
hopping parameters, with periods of either 1 or 3 lattice constants, are also
found to enhance pairing relative to the uniform two--leg ladder, although to a
lesser degree. A calculation of the phase stiffness of the ladder reveals that
in the regime with the strongest pairing, the energy scale associated with
phase ordering is comparable to the pairing scale.Comment: 9 pages, 9 figures; Journal reference adde
Weak plaquette valence bond order in the honeycomb Heisenberg model
Using the density matrix renormalization group, we investigate the
Heisenberg model on the honeycomb lattice with first- () and
second-neighbor () interactions. We are able to study long open cylinders
with widths up to 12 lattice spacings. For near 0.3, we find an
apparently paramagnetic phase, bordered by an antiferromagnetic phase for
and by a valence bond crystal for . The
longest correlation length that we find in this intermediate phase is for
plaquette valence bond (PVB) order. This correlation length grows strongly with
cylinder circumference, indicating either quantum criticality or weak PVB
order.Comment: 9 pages, 15 figures, minor changes are made for publication in Phys.
Rev. Let
Unexpected z-Direction Ising Antiferromagnetic Order in a frustrated Spin-1/2 XY Model on the Honeycomb Lattice
Using the density matrix renormalization group (DMRG) on wide cylinders, we
study the phase diagram of the spin-1/2 XY model on the honeycomb lattice, with
first-neighbor () and frustrating second-neighbor ()
interactions. For the intermediate frustration regime , we find a surprising antiferromagnetic Ising phase, with
ordered moments pointing along the z axis, despite the absence of any S_z_z
interactions in the Hamiltonian. Surrounding this phase as a function of
are antiferromagnetic phases with the moments pointing in the plane for
small and a close competition between an plane magnetic collinear
phase and a dimer phase for large values of . We do not find any spin
liquid phases in this model.Comment: 5 pages, 5 figures, minor changes made for publication on PR
Dynamical Correlation Functions using the Density Matrix Renormalization Group
The density matrix renormalization group (DMRG) method allows for very
precise calculations of ground state properties in low-dimensional strongly
correlated systems. We investigate two methods to expand the DMRG to
calculations of dynamical properties. In the Lanczos vector method the DMRG
basis is optimized to represent Lanczos vectors, which are then used to
calculate the spectra. This method is fast and relatively easy to implement,
but the accuracy at higher frequencies is limited. Alternatively, one can
optimize the basis to represent a correction vector for a particular frequency.
The correction vectors can be used to calculate the dynamical correlation
functions at these frequencies with high accuracy. By separately calculating
correction vectors at different frequencies, the dynamical correlation
functions can be interpolated and pieced together from these results. For
systems with open boundaries we discuss how to construct operators for specific
wavevectors using filter functions.Comment: minor revision, 10 pages, 15 figure
Real time evolution using the density matrix renormalization group
We describe an extension to the density matrix renormalization group method
incorporating real time evolution into the algorithm. Its application to
transport problems in systems out of equilibrium and frequency dependent
correlation functions is discussed and illustrated in several examples. We
simulate a scattering process in a spin chain which generates a spatially
non-local entangled wavefunction.Comment: 4 pages, 4 eps figures, some minor corrections in text and Eq.(3
Topography of Spin Liquids on a Triangular Lattice
Spin systems with frustrated anisotropic interactions are of significant
interest due to possible exotic ground states. We have explored their phase
diagram on a nearest-neighbor triangular lattice using the density-matrix
renormalization group and mapped out the topography of the region that can
harbor a spin liquid. We find that this spin-liquid phase is continuously
connected to a previously discovered spin-liquid phase of the isotropic
model. The two limits show nearly identical spin correlations,
making the case that their respective spin liquids are isomorphic to each
other.Comment: Accepted to PRL; 5 p., 11+ p. supplemental; main text is longer than
the accepted versio
Disorder-Induced Mimicry of a Spin Liquid in YbMgGaO
We suggest that a randomization of the pseudo-dipolar interaction in the
spin-orbit-generated low-energy Hamiltonian of YbMgGaO due to an
inhomogeneous charge environment from a natural mixing of Mg and
Ga can give rise to orientational spin disorder and mimic a
spin-liquid-like state. In the absence of such quenched disorder, and
density matrix renormalization group calculations both show robust ordered
states for the physically relevant phases of the model. Our scenario is
consistent with the available experimental data and further experiments are
proposed to support it.Comment: 5+ main text, 7+ supplemental, text asymptotically close to PR
Binding of holons and spinons in the one-dimensional anisotropic t-J model
We study the binding of a holon and a spinon in the one-dimensional
anisotropic t-J model using a Bethe-Salpeter equation approach, exact
diagonalization, and density matrix renormalization group methods on chains of
up to 128 sites. We find that holon-spinon binding changes dramatically as a
function of anisotropy parameter \alpha=J_\perp/J_z: it evolves from an exactly
deducible impurity-like result in the Ising limit to an exponentially shallow
bound state near the isotropic case. A remarkable agreement between the theory
and numerical results suggests that such a change is controlled by the
corresponding evolution of the spinon energy spectrum.Comment: 4 pages, 5 figures, published versio
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