1,439 research outputs found
Quantum phase transitions in multileg spin ladders with ring exchange
Four-spin exchange interaction has been raising intriguing questions
regarding the exotic phase transitions it induces in two-dimensional quantum
spin systems. In this context, we investigate the effects of a cyclic four-spin
exchange in the quasi-1D limit by considering a general N-leg spin ladder. We
show by means of a low-energy approach that, depending on its sign, this ring
exchange interaction can engender either a staggered or a uniform dimerization
from the conventional phases of spin ladders. The resulting quantum phase
transition is found to be described by the SU(2)_N conformal field theory. This
result, as well as the fractional value of the central charge at the
transition, is further confirmed by a large-scale numerical study performed by
means of Exact Diagonalization and Density Matrix Renormalization Group
approaches for N \le 4
Competing superconducting instabilities in the one-dimensional p-band degenerate cold fermionic system
The zero-temperature phase diagram of -orbital two-component fermionic
system loaded into a one-dimensional optical lattice is mapped out by means of
analytical and numerical techniques. It is shown that the -band model away
from half-filling hosts various competing superconducting phases for attractive
and repulsive interactions. At quarter filling, we analyze the possible
formation of incompressible Mott phases and in particular for repulsive
interactions, we find the occurrence of a Mott transition with the formation of
fully gapped bond-ordering waves.Comment: published versio
Quantum phase transitions in three-leg spin tubes
We investigate the properties of a three-leg quantum spin tube using several
techniques such as the density matrix renormalization group method, strong
coupling approaches and the non linear sigma model. For integer spins S, the
model proves to exhibit a particularly rich phase diagram consisting of an
ensemble of 2S phase transitions. They can be accurately identified by the
behavior of a non local string order parameter associated to the breaking of a
hidden symmetry in the Hamiltonian. The nature of these transitions are further
elucidated within the different approaches. We carry a detailed DMRG analysis
in the specific cases S = 1. The numerical data confirm the existence of two
Haldane phases with broken hidden symmetry separated by a trivial singlet
state. The study of the gap and of the von Neumann entropy suggest a first
order phase transition but at the close proximity of a tricritical point
separating a gapless and a first order transition line in the phase diagram of
the quantum spin tube.Comment: 20 pages, 18 figure
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
Numerical Contractor Renormalization Method for Quantum Spin Models
We demonstrate the utility of the numerical Contractor Renormalization (CORE)
method for quantum spin systems by studying one and two dimensional model
cases. Our approach consists of two steps: (i) building an effective
Hamiltonian with longer ranged interactions using the CORE algorithm and (ii)
solving this new model numerically on finite clusters by exact diagonalization.
This approach, giving complementary information to analytical treatments of the
CORE Hamiltonian, can be used as a semi-quantitative numerical method. For
ladder type geometries, we explicitely check the accuracy of the effective
models by increasing the range of the effective interactions. In two dimensions
we consider the plaquette lattice and the kagome lattice as non-trivial test
cases for the numerical CORE method. On the plaquette lattice we have an
excellent description of the system in both the disordered and the ordered
phases, thereby showing that the CORE method is able to resolve quantum phase
transitions. On the kagome lattice we find that the previously proposed twofold
degenerate S=1/2 basis can account for a large number of phenomena of the spin
1/2 kagome system. For spin 3/2 however this basis does not seem to be
sufficient anymore. In general we are able to simulate system sizes which
correspond to an 8x8 lattice for the plaquette lattice or a 48-site kagome
lattice, which are beyond the possibilities of a standard exact diagonalization
approach.Comment: 15 page
Symmetry-protected topological phases of alkaline-earth cold fermionic atoms in one dimension
We investigate the existence of symmetry-protected topological phases in
one-dimensional alkaline-earth cold fermionic atoms with general half-integer
nuclear spin I at half filling. In this respect, some orbital degrees of
freedom are required. They can be introduced by considering either the
metastable excited state of alkaline-earth atoms or the p-band of the optical
lattice. Using complementary techniques, we show that SU(2) Haldane topological
phases are stabilised from these orbital degrees of freedom. On top of these
phases, we find the emergence of topological phases with enlarged SU(2I+1)
symmetry which depend only on the nuclear spin degrees of freedom. The main
physical properties of the latter phases are further studied using a
matrix-product state approach. On the one hand, we find that these phases are
symmetry-protected topological phases, with respect to inversion symmetry, when
I=1/2,5/2,9/2,..., which is directly relevant to ytterbium and strontium cold
fermions. On the other hand, for the other values of I(=half-odd integer),
these topological phases are stabilised only in the presence of exact
SU(2I+1)-symmetry
Hidden symmetry and quantum phases in spin-3/2 cold atomic systems
Optical traps and lattices provide a new opportunity to study strongly
correlated high spin systems with cold atoms. In this article, we review the
recent progress on the hidden symmetry properties in the simplest high spin
fermionic systems with hyperfine spin , which may be realized with atoms
of Cs, Be, Ba, Ba, and Hg. A {\it generic}
SO(5) or isomorphically, ) symmetry is proved in such systems with the
s-wave scattering interactions in optical traps, or with the on-site Hubbard
interactions in optical lattices. Various important features from this high
symmetry are studied in the Fermi liquid theory, the mean field phase diagram,
and the sign problem in quantum Monte-Carlo simulations. In the s-wave quintet
Cooper pairing phase, the half-quantum vortex exhibits the global analogue of
the Alice string and non-Abelian Cheshire charge properties in gauge theories.
The existence of the quartetting phase, a four-fermion counterpart of the
Cooper pairing phase, and its competition with other orders are studied in one
dimensional spin-3/2 systems. We also show that counter-intuitively quantum
fluctuations in spin-3/2 magnetic systems are even stronger than those in
spin-1/2 systems
Numerical study of magnetization plateaux in the spin-1/2 kagome Heisenberg antiferromagnet
We clarify the existence of several magnetization plateaux for the kagome
antiferromagnetic Heisenberg model in a magnetic field. Using
approximate or exact localized magnon eigenstates, we are able to describe in a
similar manner the plateau states that occur for magnetization per site
, , and of the saturation value. These results are confirmed
using large-scale Exact Diagonalization on lattices up to 63 sites.Comment: 8 pages; minor changes; 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
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
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