172 research outputs found

### The One-Dimensional ANNNI model in a Transverse Field: Analytic and numerical study of Effective Hamiltonians

We consider a spin-$\frac{1}{2}$ chain with competing nearest and
next-nearest neighbor interactions within a transverse magnetic field, which is
known to be an equiavelent to the ANNNI model. When studing thermodynamics of
the 2D ANNNI model Villain and Bak arrived to a free fermion approximation that
neglects heavy excitations from the ferromagnetic ground state, which is an
appropriate description close to the paramagnetic-ferromagnetic transition. In
the vicinity of the floating-phase/anti-phase transition another sort of
quasiparticles, but free fermions too, appears to be convenient. Although free
fermions are a suitable tool for investigation of the phase diagram and the
critical properties, they are defined on the fictitious lattice which makes the
analysis non-rigorous. Here we deal with a proper fermion scheme which is
especially effective %devised to describe the floating-phase/anti-phase
transition. for performing exact diagonalization calculations for cyclic
chains. Systems up to size $L=32$ has been analysed and the predictions of the
effective fermion Hamiltonian has been confirmed. Various predictions for the
infinite system and the critical properties are derived.Comment: 30 RevTeX pages, 10 postscript figure

### Spin nematics in the bilinear-biquadratic S=1 spin chain

We report the existence of an extended critical, nondimerized region in the
phase diagram of the bilinear-biquadratic spin-one chain. The dominant power
law correlations are ferroquadrupolar, i.e. spin nematic in character. Another
known critical region is also characterized by dominant quadrupolar
correlations, although with a different wave vector. Our results show that spin
nematic correlations play an important role in quantum magnets with spin S >= 1
in regions between antiferromagnetic and ferromagnetic phases.Comment: 4 pages, 7 figure

### Three-sublattice ordering of the SU(3) Heisenberg model of three-flavor fermions on the square and cubic lattices

Combining a semi-classical analysis with exact diagonalizations, we show that
the ground state of the SU(3) Heisenberg model on the square lattice develops
three-sublattice long-range order. This surprising pattern for a bipartite
lattice with only nearest-neighbor interactions is shown to be the consequence
of a subtle quantum order-by-disorder mechanism. By contrast, thermal
fluctuations favor two-sublattice configurations via entropic selection. These
results are shown to extend to the cubic lattice, and experimental implications
for the Mott-insulating states of three-flavor fermionic atoms in optical
lattices are discussed.Comment: 4 pages, 3 figures, minor changes, references adde

### Magnetic phases of spin-3/2 fermions on a spatially anisotropic square lattice

We study the magnetic phase diagram of spin-3/2 fermions in a spatially
anisotropic square optical lattice at quarter filling (corresponding to one
particle per lattice site). In the limit of the large on-site repulsion the
system can be mapped to the so-called Sp(N) Heisenberg spin model with N=4. We
analyze the Sp(N) spin model with the help of the large-N field-theoretical
approach and show that the effective theory corresponds to the Sp(N) extension
of the CP^{N-1} model, with the Lorentz invariance generically broken. We
obtain the renormalization flow of the model couplings and show that although
the Sp(N) terms are seemingly irrelevant, their presence leads to a
renormalization of the CP^{N-1} part of the action, driving a phase transition.
We further consider the influence of the external magnetic field (the quadratic
Zeeman effect), and present the qualitative analysis of the ground state phase
diagram.Comment: 10 Revtex pages, 5 figures; (v2) corrected the last paragraph in
Appendix B and some typos; (v3) added references, extended discussion of the
phase diagra

### Magnetic Field Dependence of the Paramagnetic to the High Temperature Magnetically Ordered Phase Transition in CeB6

We have measured the magnetic field dependence of the paramagnetic to high
temperature magnetically ordered phase transition TQ(H) in CeB6 from 2 to 30 T
using cantilever magnetometry. It is found that the phase separation
temperature continuously increases in field with an increasingly positive
slope. In addition, we find that measurements in strong magnetic field
gradients have no effect on the phase transition.Comment: 6 total page including 3 figures, submitted to Physical Review B
(also available at
http://publish.aps.org/eprint/gateway/eplist/aps1999dec08_006) v2: Corrected
typos, added 1 reference, minor content changes (deleted 1 sentence in
introduction, added 2 sentences in discussion to explain added reference),
fixed the "et al"s in the bibliograph

### Exact trimer ground states on a spin-1 chain

We construct a new spin-1 model on a chain. Its ground state is determined
exactly which is three-fold degenerate by breaking translational invariance.
Thus we have trimerization. Excited states cannot be obtained exactly, but we
determine a few low-lying ones by using trial states, among them solitons

### Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides

We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur

### Concerning Order and Disorder in the Ensemble of Cu-O Chain Fragments in Oxygen Deficient Planes of Y-Ba-Cu-O

In connection with numerous X-ray and neutron investigations of some high
temperature superconductors (YBa$_2$Cu$_3$O$_{6+x}$ and related compounds) a
non-trivial part of the structure factor, coming from partly disordered
Cu-O-$\dots$-O-Cu chain fragments, situated within basal planes, CuO$_x$, can
be a subject of theoretical interest. Closely connected to such a diffusive
part of the structure factor are the correlation lengths, which are also
available in neutron and X-ray diffraction studies and depend on a degree of
oxygen disorder in a basal plane. The quantitative measure of such a disorder
can be associated with temperature of a sample anneal, $T_q$, at which oxygen
in a basal plane remains frozen-in high temperature equilibrium after a fast
quench of a sample to room or lower temperature. The structure factor evolution
with $x$ is vizualized in figures after the numerical calculations. The
theoretical approach employed in the paper has been developed for the
orthorhombic state of YBCO.Comment: Revtex, 27 pages, 14 PostScript figures upon request, ITP/GU/94/0

### Centipede ladder at quarter filling

We study the ground state and excitation spectrum of a quasi one-dimensional
nanostructure consisting of a pole and rungs oriented in the opposite
directions ("centipede ladder", CL) at quarter filling. The spin and charge
excitation spectra are found in the limits of small and large longitudinal
hopping $t_\|$ compared to the on-rung hopping rate $t_\perp$ and exchange
coupling $I_\perp$. At small $t_\|$ the system with ferromagnetic on-rung
exchange demonstrates instability against dimerization. Coherent propagation of
charge transfer excitons is possible in this limit. At large $t_\|$ CL behaves
like two-orbital Hubbard chain, but the gap opens in the charge excitation
spectrum thus reducing the symmetry from SU(4) to SU(2). The spin excitations
are always gapless and their dispersion changes from quadratic magnon-like for
ferromagnetic on-rung exchange to linear spinon-like for antiferromagnetic
on-rung exchange in weak longitudinal hopping limit.Comment: 10 pages, 7 eps figure

### Fluctuation Effects on the Quadrupolar Ordering in Magnetic Field

Effects of magnetic field on the quadrupolar ordering are investigated with
inclusion of fluctuation of order parameters. For the simplest model with the
nearest-neighbor quadrupolar interaction, the transition temperature and the
specific heat are derived by the use of the recently proposed effective medium
theory. It is shown that magnetic field H has two competing effects on the
quadrupolar ordering; one is to encourage the ordering by suppressing the
fluctuation among different components of order parameters, and the other is to
block the ordering as in antiferromagnets. The former is found to be of order
H^2 and the latter of order H^4. Hence the fluctuation is suppressed for weak
fields, and the transition temperature increases with magnetic field. The
fluctuation effect is so strong that the entropy released at the quadrupolar
ordering is only about half of the full value ln 4 even without the Kondo
effect.Comment: 10 pages including 3 Postscript figure

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