95 research outputs found
Locally tunable disorder and entanglement in the one-dimensional plaquette orbital model
We introduce a one-dimensional plaquette orbital model with a topology of a
ladder and alternating interactions between and pseudospin components
along both the ladder legs and on the rungs. We show that it is equivalent to
an effective spin model in a magnetic field, with spin dimers that replace
plaquettes and are coupled along the chain by three-spin interactions. Using
perturbative treatment and mean field approaches with dimer correlations we
study the ground state spin configuration and its defects in the lowest excited
states. By the exact diagonalization approach we find that the quantum effects
in the model are purely short-range and we get estimated values of the ground
state energy and the gap in the thermodynamic limit from the system sizes up to
dimers. Finally, we study a class of excited states with classical-like
defects accumulated in the central region of the chain to find that in this
region the quantum entanglement measured by the mutual information of
neighboring dimers is locally increased and coincides with disorder and
frustration. Such islands of entanglement in otherwise rather classical system
may be of interest in the context of quantum computing devices.Comment: 12 pages, 12 figure
Symmetry properties and spectra of the two-dimensional quantum compass model
We use exact symmetry properties of the two-dimensional quantum compass model
to derive nonequivalent invariant subspaces in the energy spectra of clusters up to L=6. The symmetry allows one to reduce the original compass cluster to the one with modified interactions.
This step is crucial and enables: (i) exact diagonalization of the
quantum compass cluster, and (ii) finding the specific heat for clusters up to
L=6, with two characteristic energy scales. We investigate the properties of
the ground state and the first excited states and present extrapolation of the
excitation energy with increasing system size. Our analysis provides physical
insights into the nature of nematic order realized in the quantum compass model
at finite temperature. We suggest that the quantum phase transition at the
isotropic interaction point is second order with some admixture of the
discontinuous transition, as indicated by the entropy, the overlap between two
types of nematic order (on horizontal and vertical bonds) and the existence of
the critical exponent. Extrapolation of the specific heat to the
limit suggests the classical nature of the quantum compass model and high
degeneracy of the ground state with nematic order.Comment: 15 pages, 12 figures; accepted for publication in Physical Review
One-dimensional frustrated plaquette compass model: Nematic phase and spontaneous multimerization
We introduce a one-dimensional (1D) pseudospin model on a ladder where the
Ising interactions along the legs and along the rungs alternate between
and for even/odd bond (rung). We include also the
next nearest neighbor Ising interactions on plaquettes' diagonals that
alternate in such a way that a model where only leg interactions are switched
on is equivalent to the one when only the diagonal ones are present. Thus in
the absence of rung interactions the model can interpolate between two 1D
compass models. The model posses local symmetries which are the parities within
each cell (plaquette) of the ladder. We find that for different
values of the interaction it can realize ground states that differ by the
patterns formed by these local parities. By exact diagonalization we derive
detailed phase diagrams for small systems of , 6 and 8 plaquettes, and use
next to identify generic phases that appear in larger systems as well.
Among them we find a nematic phase with macroscopic degeneracy when the leg and
diagonal interactions are equal and the rung interactions are larger than a
critical value. The nematic phase is similar to the one found in the
two-dimensional compass model. For particular parameters the low-energy sector
of the present plaquette model reduces to a 1D compass model with spins
which suggests that it realizes peculiar crossovers within the class of compass
models. Finally, we show that the model can realize phases with broken
translation invariance which can be either dimerized, trimerized,
\textit{etcetera}, or completely disordered and highly entangled in a~well
identified window of the phase diagram.Comment: 18 pages, 14 figures, accepted by Physical Review
Entangled Spin-Orbital Phases in the d Model
We investigate the phase diagrams of the spin-orbital Kugel-Khomskii
model for a bilayer and a monolayer square lattice using Bethe-Peierls-Weiss
method. For a bilayer we obtain valence bond phases with interlayer singlets,
with alternating planar singlets, and two entangled spin-orbital (ESO) phases,
in addition to the antiferromagnetic and ferromagnetic order. Possibility of
such entangled phases in a monolayer is under investigation at present.Comment: 3 pages, 2 figures, presented at Euroconference Physics of Magnetism
201
Noncollinear Magnetic Order Stabilized by Entangled Spin-Orbital Fluctuations
Quantum phase transitions in the two-dimensional Kugel-Khomski model on a
square lattice are studied using the plaquette mean field theory and the
entanglement renormalization ansatz. When orbitals are favored by
the crystal field and Hund's exchange is finite, both methods give a
noncollinear exotic magnetic order which consists of four sublattices with
mutually orthogonal nearest neighbor and antiferromagnetic second neighbor
spins. We derive effective frustrated spin model with second and third neighbor
spin interactions which stabilize this phase and follow from spin-orbital
quantum fluctuations involving spin singlets entangled with orbital
excitations.Comment: 5 pages, 5 figures and supplemental material (4 pages, 3 figures);
accepted for publication in Phys. Rev. Let
Driving Topological Phases by Spatially Inhomogeneous Pairing Centers
We investigate the effect of periodic and disordered distributions of pairing
centers in a one-dimensional itinerant system to obtain the microscopic
conditions required to achieve an end Majorana mode and the topological phase
diagram. Remarkably, the topological invariant can be generally expressed in
terms of the physical parameters for any pairing center configuration. Such a
fundamental relation allows us to unveil hidden local symmetries and to
identify trajectories in the parameter space that preserve the non-trivial
topological character of the ground state. We identify the phase diagram with
topologically non-trivial domains where Majorana modes are completely
unaffected by the spatial distribution of the pairing centers. These results
are general and apply to several systems where inhomogeneous perturbations
generate stable Majorana modes.Comment: 9 pages, 5 figure
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