1,065 research outputs found
Bond order wave instabilities in doped frustrated antiferromagnets: "Valence bond solids" at fractional filling
We explore both analytically and numerically the properties of doped t-J
models on a class of highly frustrated lattices, such as the kagome and the
pyrochlore lattice. Focussing on a particular sign of the hopping integral and
antiferromagnetic exchange, we find a generic symmetry breaking instability
towards a twofold degenerate ground state at a fractional filling below half
filling. These states show modulated bond strengths and only break lattice
symmetries. They can be seen as a generalization of the well-known valence bond
solid states to fractional filling.Comment: slightly shortened and reorganized versio
Phase diagram of interacting spinless fermions on the honeycomb lattice: A comprehensive exact diagonalization study
International audienceWe investigate the phase diagram of spinless fermions with nearest and next-nearest neighbour density-density interactions on the honeycomb lattice at half-filling. Using Exact Diagonalization techniques of the full Hamiltonian and constrained subspaces, combined with a careful choice of finite-size clusters, we determine the different charge orderings that occur for large interactions. In this regime we find a two-sublattice N\'eel-like state, a charge modulated state with a tripling of the unit cell, a zig-zag phase and a novel charge ordered states with a 12 site unit cells we call N\'eel domain wall crystal, as well as a region of phase separation for attractive interactions. A sizeable region of the phase diagram is classically degenerate, but it remains unclear whether an order-by-disorder mechanism will lift the degeneracy. For intermediate repulsion we find evidence for a Kekul\'e or plaquette bond-order wave phase. We also investigate the possibility of a spontaneous Chern insulator phase (dubbed topological Mott insulator), as previously put forward by several mean-field studies. Although we are unable to detect convincing evidence for this phase based on energy spectra and order parameters, we find an enhancement of current-current correlations with the expected spatial structure compared to the non-interacting situation. While for the studied tâV1âV2 model the phase transition to the putative topological Mott insulator is preempted by the phase transitions to the various ordered states, our findings might hint at the possibility for a topological Mott insulator in an enlarged Hamiltonian parameter space, where the competing phases are suppressed
Effective Spin Couplings in the Mott Insulator of the Honeycomb Lattice Hubbard Model
Motivated by the recent discovery of a spin liquid phase for the Hubbard
model on the honeycomb lattice at half-filling, we apply both perturbative and
non-perturbative techniques to derive effective spin Hamiltonians describing
the low-energy physics of the Mott-insulating phase of the system. Exact
diagonalizations of the so-derived models on small clusters are performed, in
order to assess the quality of the effective low-energy theory in the
spin-liquid regime. We show that six-spin interactions on the elementary loop
of the honeycomb lattice are the dominant sub-leading effective couplings. A
minimal spin model is shown to reproduce most of the energetic properties of
the Hubbard model on the honeycomb lattice in its spin-liquid phase.
Surprisingly, a more elaborate effective low-energy spin model obtained by a
systematic graph expansion rather disagrees beyond a certain point with the
numerical results for the Hubbard model at intermediate couplings.Comment: 20 pages, 10 figure
Quantum Critical Scaling of Fidelity Susceptibility
The behavior of the ground-state fidelity susceptibility in the vicinity of a
quantum critical point is investigated. We derive scaling relations describing
its singular behavior in the quantum critical regime. Unlike it has been found
in previous studies, these relations are solely expressed in terms of
conventional critical exponents. We also describe in detail a quantum Monte
Carlo scheme that allows for the evaluation of the fidelity susceptibility for
a large class of many-body systems and apply it in the study of the quantum
phase transition for the transverse-field Ising model on the square lattice.
Finite size analysis applied to the so obtained numerical results confirm the
validity of our scaling relations. Furthermore, we analyze the properties of a
closely related quantity, the ground-state energy's second derivative, that can
be numerically evaluated in a particularly efficient way. The usefulness of
both quantities as alternative indicators of quantum criticality is examined.Comment: 13 pages, 7 figures. Published versio
Establishing the boundaries: the hippocampal contribution to imagining scenes
When we visualize scenes, either from our own past or invented, we impose a viewpoint for our âmind's eyeâ and we experience the resulting image as spatially coherent from that viewpoint. The hippocampus has been implicated in this process, but its precise contribution is unknown. We tested a specific hypothesis based on the spatial firing properties of neurons in the hippocampal formation of rats, that this region supports the construction of spatially coherent mental images by representing the locations of the environmental boundaries surrounding our viewpoint. Using functional magnetic resonance imaging, we show that hippocampal activation increases parametrically with the number of enclosing boundaries in the imagined scene. In contrast, hippocampal activity is not modulated by a nonspatial manipulation of scene complexity nor to increasing difficulty of imagining the scenes in general. Our findings identify a specific computational role for the hippocampus in mental imagery and episodic recollection
Asymmetric spin-1/2 two-leg ladders
We consider asymmetric spin-1/2 two-leg ladders with non-equal
antiferromagnetic (AF) couplings J_|| and \kappa J_|| along legs (\kappa <= 1)
and ferromagnetic rung coupling, J_\perp. This model is characterized by a gap
\Delta in the spectrum of spin excitations. We show that in the large J_\perp
limit this gap is equivalent to the Haldane gap for the AF spin-1 chain,
irrespective of the asymmetry of the ladder. The behavior of the gap at small
rung coupling falls in two different universality classes. The first class,
which is best understood from the case of the conventional symmetric ladder at
\kappa=1, admits a linear scaling for the spin gap \Delta ~ J_\perp. The second
class appears for a strong asymmetry of the coupling along legs, \kappa J_|| <<
J_\perp << J_|| and is characterized by two energy scales: the exponentially
small spin gap \Delta ~ J_\perp \exp(-J_|| / J_\perp), and the bandwidth of the
low-lying excitations induced by a Suhl-Nakamura indirect exchange ~ J_\perp^2
/J_|| . We report numerical results obtained by exact diagonalization, density
matrix renormalization group and quantum Monte Carlo simulations for the spin
gap and various spin correlation functions. Our data indicate that the behavior
of the string order parameter, characterizing the hidden AF order in Haldane
phase, is different in the limiting cases of weak and strong asymmetry. On the
basis of the numerical data, we propose a low-energy theory of effective spin-1
variables, pertaining to large blocks on a decimated lattice.Comment: 18 pages, 11 figure
Resource optimisation in a wireless sensor network with guaranteed estimator performance
New control paradigms are needed for large networks of wireless sensors and actuators in order to
efficiently utilise system resources. In this study, the authors consider the problem of discrete-time state
estimation over a wireless sensor network. Given a tree that represents the sensor communications with the
fusion centre, the authors derive the optimal estimation algorithm at the fusion centre, and provide a closedform
expression for the steady-state error covariance matrix. They then present a tree reconfiguration
algorithm that produces a sensor tree that has low overall energy consumption and guarantees a desired
level of estimation quality at the fusion centre. The authors further propose a sensor tree construction and
scheduling algorithm that leads to a longer network lifetime than the tree reconfiguration algorithm.
Examples are provided throughout the paper to demonstrate the algorithms and theory developed
Spin gap and string order parameter in the ferromagnetic Spiral Staircase Heisenberg Ladder: a quantum Monte Carlo study
We consider a spin-1/2 ladder with a ferromagnetic rung coupling J_\perp and
inequivalent chains. This model is obtained by a twist (\theta) deformation of
the ladder and interpolates between the isotropic ladder (\theta=0) and the
SU(2) ferromagnetic Kondo necklace model (\theta=\pi). We show that the ground
state in the (\theta,J_\perp) plane has a finite string order parameter
characterising the Haldane phase. Twisting the chain introduces a new energy
scale, which we interpret in terms of a Suhl-Nakamura interaction. As a
consequence we observe a crossover in the scaling of the spin gap at weak
coupling from \Delta/J_\| \propto J_\perp/J_\| for \theta < \theta_c \simeq
8\pi/9 to \Delta/J_\| \propto (J_\perp/J_\|)^2 for \theta > \theta_c. Those
results are obtained on the basis of large scale Quantum Monte Carlo
calculations.Comment: 4 page
Antiholons in one-dimensional t-J models
Using a newly developed hybrid Monte Carlo algorithm for the nearest-neighbor
(n.n.) t-J model, we show that antiholons identified in the supersymmetric
inverse squared (IS) t-J model are clearly visible in the electron addition
spectrum of the n.n. t-J model at J=2t and also for J=0.5t, a value of
experimental relevance.Comment: 4 pages, 4 figure
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