45 research outputs found
Spin 3/2 dimer model
We present a parent Hamiltonian for weakly dimerized valence bond solid
states for arbitrary half-integral S. While the model reduces for S=1/2 to the
Majumdar-Ghosh Hamiltonian we discuss this model and its properties for S=3/2.
Its degenerate ground state is the most popular toy model state for discussing
dimerization in spin 3/2 chains. In particular, it describes the impurity
induced dimer phase in Cr8Ni as proposed recently. We point out that the
explicit construction of the Hamiltonian and its main features apply to
arbitrary half-integral spin S.Comment: 5+ pages, 6 figures; to appear in Europhysics Letter
Vortex Dynamics and Hall Conductivity of Hard Core Bosons
Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum
spin model representation, appropriately gauged on the torus to allow for an
external magnetic field. We find strong lattice effects near half filling. An
effective quantum mechanical description of the vortex degrees of freedom is
derived. Using semiclassical and numerical analysis we compute the vortex
hopping energy, which at half filling is close to magnitude of the boson
hopping energy. The critical quantum melting density of the vortex lattice is
estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed
from the Chern numbers of the low energy eigenstates. At zero temperature, it
reverses sign abruptly at half filling. At precisely half filling, all
eigenstates are doubly degenerate for any odd number of flux quanta. We prove
the exact degeneracies on the torus by constructing an SU(2) algebra of
point-group symmetries, associated with the center of vorticity. This result is
interpreted as if each vortex carries an internal spin-half degree of freedom
('vspin'), which can manifest itself as a charge density modulation in its
core. Our findings suggest interesting experimental implications for vortex
motion of cold atoms in optical lattices, and magnet-transport of short
coherence length superconductors.Comment: 15 pages, 15 figure
A magnetic analog of the isotope effect in cuprates
We present extensive magnetic measurements of the
(Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} (CLBLCO) system with its four
different families (x) having a Tc^max(x) variation of 28% and minimal
structural changes. For each family we measured the Neel temperature, the
anisotropies of the magnetic interactions, and the spin glass temperature. Our
results exhibit a universal relation Tc=c*J*n_s for all families, where c~1, J
is the in plane Heisenberg exchange, and n_s is the carrier density. This
relates cuprate superconductivity to magnetism in the same sense that phonon
mediated superconductivity is related to atomic mass.Comment: With an additional inset in Fig.
Formation of energy gap in higher dimensional spin-orbital liquids
A Schwinger boson mean field theory is developed for spin liquids in a
symmetric spin-orbital model in higher dimensions. Spin, orbital and coupled
spin-orbital operators are treated equally. We evaluate the dynamic correlation
functions and collective excitations spectra. As the collective excitations
have a finite energy gap, we conclude that the ground state is a spin-orbital
liquid with a two-fold degeneracy, which breaks the discrete spin-orbital
symmetry. Possible relevence of this spin liquid state to several realistic
systems, such as CaVV and NaSbTiO, are discussed.Comment: 4 pages with 1 figur
The Expansion and Spin Correlations in Constrained Wavefunctions
We develop a large-N expansion for Gutzwiller projected spin states. We
consider valence bonds singlets, constructed by Schwinger bosons or fermions,
which are variational ground states for quantum antiferromagnets. This
expansion is simpler than the familiar expansions of the quantum Heisenberg
model, and thus more instructive. The diagrammatic rules of this expansion
allow us to prove certain identities to all orders in 1/N. We derive the
on-site spin fluctuations sum rule for arbitrary N. We calculate the
correlations of the one dimensional Valence Bonds Solid states and the
Gutzwiller Projected Fermi Gas upto order 1/N. For the bosons case, we are
surprised to find that the mean field, the order 1/N and the exact correlations
are simply proportional. For the fermions case, the 1/N correction enhances the
zone edge singularity. The comparison of our leading order terms to known
results for N=2, enhances our understanding of large-N approximations in
general.Comment: 36 pages, LaTe
Atomic Model of Susy Hubbard Operators
We apply the recently proposed susy Hubbard operators to an atomic model. In
the limiting case of free spins, we derive exact results for the entropy which
are compared with a mean field + gaussian corrections description. We show how
these results can be extended to the case of charge fluctuations and calculate
exact results for the partition function, free energy and heat capacity of an
atomic model for some simple examples. Wavefunctions of possible states are
listed. We compare the accuracy of large N expansions of the susy spin
operators with those obtained using `Schwinger bosons' and `Abrikosov
pseudo-fermions'. For the atomic model, we compare results of slave boson,
slave fermion, and susy Hubbard operator approximations in the physically
interesting but uncontrolled limiting case of N->2. For a mixed representation
of spins we estimate the accuracy of large N expansions of the atomic model. In
the single box limit, we find that the lowest energy saddle-point solution
reduces to simply either slave bosons or slave fermions, while for higher boxes
this is not the case. The highest energy saddle-point solution has the
interesting feature that it admits a small region of a mixed representation,
which bears a superficial resemblance to that seen experimentally close to an
antiferromagnetic quantum critical point.Comment: 17 pages + 7 pages Appendices, 14 figures. Substantial revision
Spin Dynamics of the Triangular Heisenberg Antiferromagnet: A Schwinger Boson Approach
We have analyzed the two-dimensional antiferromagnetic Heisenberg model on
the triangular lattice using a Schwinger boson mean-field theory. By expanding
around a state with local order, we obtain, in the limit of
infinite spin, results for the excitation spectrum in complete agreement with
linear spin wave theory (LSWT). In contrast to LSWT, however, the modes at the
ordering wave vectors acquire a mass for finite spin. We discuss the origin of
this effect.Comment: 15 pages REVTEX 3.0 preprint, 6 postscript figures ( uuencoded and
compressed using the script uufiles ) are submitted separately
Superfluids and Supersolids on Frustrated 2D Lattices
We study the ground state of hard-core bosons with nearest-neighbor hopping
and nearest-neighbor interactions on the triangular and Kagom\'e lattices by
mapping to a system of spins (), which we analyze using spin-wave
theory. We find that the both lattices display superfluid and supersolid (a
coexistence of superfluid and solid) order as the parameters and filling are
varied. Quantum fluctuations seem large enough in the Kagom\'e system to raise
the interesting possibility of a disordered ground state.Comment: Latex format, 24 figures available by email upon request. Submitted
to Physical Review
Kondo Insulator: p-wave Bose Condensate of Excitons
In the Anderson lattice model for a mixed-valent system, the
hybridization can possess a -wave symmetry. The strongly-correlated
insulating phase in the mean-field approximation is shown to be a -wave Bose
condensate of excitons with a spontaneous lattice deformation. We study the
equilibrium and linear response properties across the insulator-metal
transition. Our theory supports the empirical correlation between the lattice
deformation and the magnetic susceptibility and predicts measurable ultrasonic
and high-frequency phonon behavior in mixed-valent semiconductors.Comment: 5 pages, 3 encapsulated PostScript figure
Rotational invariance and order-parameter stiffness in frustrated quantum spin systems
We compute, within the Schwinger-boson scheme, the Gaussian-fluctuation
corrections to the order-parameter stiffness of two frustrated quantum spin
systems: the triangular-lattice Heisenberg antiferromagnet and the J1-J2 model
on the square lattice. For the triangular-lattice Heisenberg antiferromagnet we
found that the corrections weaken the stiffness, but the ground state of the
system remains ordered in the classical 120 spiral pattern. In the case of the
J1-J2 model, with increasing frustration the stiffness is reduced until it
vanishes, leaving a small window 0.53 < J2/J1 < 0.64 where the system has no
long-range magnetic order. In addition, we discuss several methodological
questions related to the Schwinger-boson approach. In particular, we show that
the consideration of finite clusters which require twisted boundary conditions
to fit the infinite-lattice magnetic order avoids the use of ad hoc factors to
correct the Schwinger-boson predictions.Comment: 9 pages, Latex, 6 figures as ps files, fig.1 changed and minor text
corrections, to appear in Phys.Rev.