177 research outputs found
Coherent motion in the interaction model of cold glasses
We have studied the collective phenomena of multicomponent glasses at ultra
low temperatures [Strehlow, et. al, Phys. Rev. Lett 80, 5361 (1998)] by taking
into account the proper interaction between tunneling centers. We have
considered both double and triple well potentials with different types of
interactions. We show that a phase with coherent motion appears for a range of
parameters when the path of tunneling is coursed by an interaction of the XY
type, while the usual Ising like interaction does not lead to the expected
collective phenomena. In the phase of coherent motion, the dipole moment and
the low-energy levels oscillate with a frequency proportional to the number of
tunneling centers in the system. Simultaneous level crossing occurs between the
ground and first excited states. The effects of long-range interactions and
also of random couplings have been also studied for a one- and two-dimensional
array of tunneling centers. We find that long-range interactions do not affect
the coherent motion, while a wide distribution of random couplings destroys the
collective effects.Comment: 11 pages, 11 figures, shorter version appears in Phys. rev.
Phase diagram of J1-J2 transverse field Ising model on the checkerboard lattice: a plaquette-operator approach
We study the effect of quantum fluctuations by means of a transverse magnetic
field () on the antiferromagnetic Ising model on the
checkerboard lattice, the two dimensional version of the pyrochlore lattice.
The zero-temperature phase diagram of the model has been obtained by employing
a plaquette operator approach (POA). The plaquette operator formalism bosonizes
the model, in which a single boson is associated to each eigenstate of a
plaquette and the inter-plaquette interactions define an effective Hamiltonian.
The excitations of a plaquette would represent an-harmonic fluctuations of the
model, which lead not only to lower the excitation energy compared with a
single-spin flip but also to lift the extensive degeneracy in favor of a
plaquette ordered solid (RPS) state, which breaks lattice translational
symmetry, in addition to a unique collinear phase for . The bosonic
excitation gap vanishes at the critical points to the N\'{e}el ()
and collinear () ordered phases, which defines the critical phase
boundaries. At the homogeneous coupling () and its close neighborhood,
the (canted) RPS state, established from an-harmonic fluctuations, lasts for
low fields, , which is followed by a transition to the
quantum paramagnet (polarized) phase at high fields. The transition from RPS
state to the N\'{e}el phase is either a deconfined quantum phase transition or
a first order one, however a continuous transition occurs between RPS and
collinear phases.Comment: To appear in EPJB, 12 pages, 15 figures, 1 tabl
Thermodynamic Properties of XXZ model in a Transverse Field
We have numerically studied the thermodynamic properties of the spin 1/2 XXZ
chain in the presence of a transverse (non commuting) magnetic field. The
thermal, field dependence of specific heat and correlation functions for chains
up to 20 sites have been calculated. The area where the specific heat decays
exponentially is considered as a measure of the energy gap. We have also
obtained the exchange interaction between chains in a bulk material using the
random phase approximation and derived the phase diagram of the three
dimensional material with this approximation. The behavior of the structure
factor at different momenta verifies the antiferromagnetic long range order in
y-direction for the three dimensional case. Moreover, we have concluded that
the Low Temperature Lanczos results [M. Aichhorn et al., Phys. Rev. B 67,
161103(R) (2003)] are more accurate for low temperatures and closer to the full
diagonalization ones than the results of Finite Temperature Lanczos Method [J.
Jaklic and P. Prelovsek, Phys. Rev. B 49, 5065 (1994)].Comment: 7 pages, 10 eps figure
Ground-state fidelity of the spin-1 Heisenberg chain with single ion anisotropy: quantum renormalization group and exact diagonalization approaches
We study the phase diagram of the anisotropic spin-1 Heisenberg chain with
single ion anisotropy (D) using a ground-state fidelity approach. The
ground-state fidelity and its corresponding susceptibility are calculated
within the quantum renormalization group scheme where we obtained the
renormalization of fidelity preventing to calculate the ground state. Using
this approach, the phase boundaries between the antiferromagnetic N\'{e}el,
Haldane and large-D phases are obtained for the whole phase diagram, which
justifies the application of quantum renormalization group to trace the
symmetery protected topological phases. In addition, we present numerical exact
diagonalization (Lanczos) results in, which we employ a recently introduced
non-local order parameter to locate the transition from Haldane to large-D
phase accurately.Comment: 9 pages, 11 figures, 1 table, to appear in JPC
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