99 research outputs found
Simulation of fermionic lattice models in two dimensions with Projected Entangled-Pair States: Next-nearest neighbor Hamiltonians
In a recent contribution [Phys. Rev. B 81, 165104 (2010)] fermionic Projected
Entangled-Pair States (PEPS) were used to approximate the ground state of free
and interacting spinless fermion models, as well as the - model. This
paper revisits these three models in the presence of an additional next-nearest
hopping amplitude in the Hamiltonian. First we explain how to account for
next-nearest neighbor Hamiltonian terms in the context of fermionic PEPS
algorithms based on simulating time evolution. Then we present benchmark
calculations for the three models of fermions, and compare our results against
analytical, mean-field, and variational Monte Carlo results, respectively.
Consistent with previous computations restricted to nearest-neighbor
Hamiltonians, we systematically obtain more accurate (or better converged)
results for gapped phases than for gapless ones.Comment: 10 pages, 11 figures, minor change
Comment on "Topological quantum phase transitions of attractive spinless fermions in a honeycomb lattice" by Poletti D. et al
In a recent letter [D. Poletti et al., EPL 93, 37008 (2011)] a model of
attractive spinless fermions on the honeycomb lattice at half filling has been
studied by mean-field theory, where distinct homogenous phases at rather large
attraction strength , separated by (topological) phase transitions,
have been predicted. In this comment we argue that without additional
interactions the ground states in these phases are not stable against phase
separation. We determine the onset of phase separation at half filling
by means of infinite projected entangled-pair states
(iPEPS) and exact diagonalization.Comment: 2 pages, 1 figur
Entanglement Entropy of Random Fractional Quantum Hall Systems
The entanglement entropy of the and quantum Hall
states in the presence of short range random disorder has been calculated by
direct diagonalization. A microscopic model of electron-electron interaction is
used, electrons are confined to a single Landau level and interact with long
range Coulomb interaction. For very weak disorder, the values of the
topological entanglement entropy are roughly consistent with expected
theoretical results. By considering a broader range of disorder strengths, the
fluctuation in the entanglement entropy was studied in an effort to detect
quantum phase transitions. In particular, there is a clear signature of a
transition as a function of the disorder strength for the state.
Prospects for using the density matrix renormalization group to compute the
entanglement entropy for larger system sizes are discussed.Comment: 29 pages, 16 figures; fixed figures and figure captions; revised
fluctuation calculation
Entanglement renormalization and boundary critical phenomena
The multiscale entanglement renormalization ansatz is applied to the study of
boundary critical phenomena. We compute averages of local operators as a
function of the distance from the boundary and the surface contribution to the
ground state energy. Furthermore, assuming a uniform tensor structure, we show
that the multiscale entanglement renormalization ansatz implies an exact
relation between bulk and boundary critical exponents known to exist for
boundary critical systems.Comment: 6 pages, 4 figures; for a related work see arXiv:0912.164
Tensor network states and geometry
Tensor network states are used to approximate ground states of local
Hamiltonians on a lattice in D spatial dimensions. Different types of tensor
network states can be seen to generate different geometries. Matrix product
states (MPS) in D=1 dimensions, as well as projected entangled pair states
(PEPS) in D>1 dimensions, reproduce the D-dimensional physical geometry of the
lattice model; in contrast, the multi-scale entanglement renormalization ansatz
(MERA) generates a (D+1)-dimensional holographic geometry. Here we focus on
homogeneous tensor networks, where all the tensors in the network are copies of
the same tensor, and argue that certain structural properties of the resulting
many-body states are preconditioned by the geometry of the tensor network and
are therefore largely independent of the choice of variational parameters.
Indeed, the asymptotic decay of correlations in homogeneous MPS and MERA for
D=1 systems is seen to be determined by the structure of geodesics in the
physical and holographic geometries, respectively; whereas the asymptotic
scaling of entanglement entropy is seen to always obey a simple boundary law --
that is, again in the relevant geometry. This geometrical interpretation offers
a simple and unifying framework to understand the structural properties of, and
helps clarify the relation between, different tensor network states. In
addition, it has recently motivated the branching MERA, a generalization of the
MERA capable of reproducing violations of the entropic boundary law in D>1
dimensions.Comment: 18 pages, 18 figure
Many body physics from a quantum information perspective
The quantum information approach to many body physics has been very
successful in giving new insight and novel numerical methods. In these lecture
notes we take a vertical view of the subject, starting from general concepts
and at each step delving into applications or consequences of a particular
topic. We first review some general quantum information concepts like
entanglement and entanglement measures, which leads us to entanglement area
laws. We then continue with one of the most famous examples of area-law abiding
states: matrix product states, and tensor product states in general. Of these,
we choose one example (classical superposition states) to introduce recent
developments on a novel quantum many body approach: quantum kinetic Ising
models. We conclude with a brief outlook of the field.Comment: Lectures from the Les Houches School on "Modern theories of
correlated electron systems". Improved version new references adde
A quantum magnetic analogue to the critical point of water
At the familiar liquid-gas phase transition in water, the density jumps
discontinuously at atmospheric pressure, but the line of these first-order
transitions defined by increasing pressures terminates at the critical point, a
concept ubiquitous in statistical thermodynamics. In correlated quantum
materials, a critical point was predicted and measured terminating the line of
Mott metal-insulator transitions, which are also first-order with a
discontinuous charge density. In quantum spin systems, continuous quantum phase
transitions (QPTs) have been investigated extensively, but discontinuous QPTs
have received less attention. The frustrated quantum antiferromagnet
SrCu(BO) constitutes a near-exact realization of the paradigmatic
Shastry-Sutherland model and displays exotic phenomena including magnetization
plateaux, anomalous thermodynamics and discontinuous QPTs. We demonstrate by
high-precision specific-heat measurements under pressure and applied magnetic
field that, like water, the pressure-temperature phase diagram of
SrCu(BO) has an Ising critical point terminating a first-order
transition line, which separates phases with different densities of magnetic
particles (triplets). We achieve a quantitative explanation of our data by
detailed numerical calculations using newly-developed finite-temperature
tensor-network methods. These results open a new dimension in understanding the
thermodynamics of quantum magnetic materials, where the anisotropic spin
interactions producing topological properties for spintronic applications drive
an increasing focus on first-order QPTs.Comment: 8+4 pages, 4+3 figure
Non-Fermi-liquid d-wave metal phase of strongly interacting electrons
Developing a theoretical framework for conducting electronic fluids
qualitatively distinct from those described by Landau's Fermi-liquid theory is
of central importance to many outstanding problems in condensed matter physics.
One such problem is that, above the transition temperature and near optimal
doping, high-transition-temperature copper-oxide superconductors exhibit
`strange metal' behaviour that is inconsistent with being a traditional Landau
Fermi liquid. Indeed, a microscopic theory of a strange-metal quantum phase
could shed new light on the interesting low-temperature behaviour in the
pseudogap regime and on the d-wave superconductor itself. Here we present a
theory for a specific example of a strange metal---the 'd-wave metal'. Using
variational wavefunctions, gauge theoretic arguments, and ultimately
large-scale density matrix renormalization group calculations, we show that
this remarkable quantum phase is the ground state of a reasonable microscopic
Hamiltonian---the usual t-J model with electron kinetic energy and two-spin
exchange supplemented with a frustrated electron `ring-exchange' term,
which we here examine extensively on the square lattice two-leg ladder. These
findings constitute an explicit theoretical example of a genuine
non-Fermi-liquid metal existing as the ground state of a realistic model.Comment: 22 pages, 12 figures: 6 pages, 7 figures of main text + 16 pages, 5
figures of Supplementary Information; this is approximately the version
published in Nature, minus various subedits in the main tex
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