692 research outputs found
Microscopic mechanism for the 1/8 magnetization plateau in SrCu_2(BO_3)_2
The frustrated quantum magnet SrCu_2(BO_3)_2 shows a remarkably rich phase
diagram in an external magnetic field including a sequence of magnetization
plateaux. The by far experimentally most studied and most prominent
magnetization plateau is the 1/8 plateau. Theoretically, one expects that this
material is well described by the Shastry-Sutherland model. But recent
microscopic calculations indicate that the 1/8 plateau is energetically not
favored. Here we report on a very simple microscopic mechanism which naturally
leads to a 1/8 plateau for realistic values of the magnetic exchange constants.
We show that the 1/8 plateau with a diamond unit cell benefits most compared to
other plateau structures from quantum fluctuations which to a large part are
induced by Dzyaloshinskii-Moriya interactions. Physically, such couplings
result in kinetic terms in an effective hardcore boson description leading to a
renormalization of the energy of the different plateaux structures which we
treat in this work on the mean-field level. The stability of the resulting
plateaux are discussed. Furthermore, our results indicate a series of stripe
structures above 1/8 and a stable magnetization plateau at 1/6. Most
qualitative aspects of our microscopic theory agree well with a recently
formulated phenomenological theory for the experimental data of SrCu_2(BO_3)_2.
Interestingly, our calculations point to a rather large ratio of the magnetic
couplings in the Shastry-Sutherland model such that non-perturbative effects
become essential for the understanding of the frustrated quantum magnet
SrCu_2(BO_3)_2.Comment: 24 pages, 24 figure
Ising pyrochlore magnets: Low temperature properties, ice rules and beyond
Pyrochlore magnets are candidates for spin-ice behavior. We present
theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare
earth) supported by magnetothermal measurements on selected systems. By
considering long ranged dipole-dipole as well as short-ranged superexchange
interactions we get three distinct behaviours: (i) an ordered doubly degenerate
state, (ii) a highly disordered state with a broad transition to paramagnetism,
(iii) a partially ordered state with a sharp transition to paramagnetism. Thus
these competing interactions can induce behaviour very different from
conventional ``spin ice''. Closely corresponding behaviour is seen in the real
compounds---in particular Ho2Ti2O7 corresponds to case (iii) which has not been
discussed before, rather than (ii) as suggested earlier.Comment: 5 pages revtex, 4 figures; some revisions, additional data,
additional co-authors and a changed title. Basic ideas of paper remain the
same but those who downloaded the original version are requested to get this
more complete versio
The Kelvin Formula for Thermopower
Thermoelectrics are important in physics, engineering, and material science
due to their useful applications and inherent theoretical difficulty,
especially in strongly correlated materials. Here we reexamine the framework
for calculating the thermopower, inspired by ideas of Lord Kelvin from 1854. We
find an approximate but concise expression, which we term as the Kelvin formula
for the the Seebeck coefficient. According to this formula, the Seebeck
coefficient is given as the particle number derivative of the entropy
, at constant volume and temperature ,
. This formula is shown to be competitive compared to other
approximations in various contexts including strongly correlated systems. We
finally connect to a recent thermopower calculation for non-Abelian fractional
quantum Hall states, where we point out that the Kelvin formula is exact.Comment: 6 pages, 2 figure
Magnetic Raman scattering from 1D antiferromagnets
We study Raman scattering from 1D antiferromagnets within the Fleury-Loudon scheme by applying a finite temperature Lanczos method to a 1D spin-half Heisenberg model with nearest-neighbor ( J1) and second-neighbor ( J2) interactions. The low-temperature spectra are analyzed in terms of the known elementary excitations of the system for J2 = 0 and J2 = ½. We find that the low- T Raman spectra are very broad for |J2/J1|≤0.3. This broad peak gradually diminishes and shifts with temperature, so that at T > J1 the spectra are narrower and peaked at low frequencies. The experimental spectra for CuGeO3 are discussed in light of our calculations
Finite temperature properties of the triangular lattice t-J model, applications to NaCoO
We present a finite temperature () study of the t-J model on the
two-dimensional triangular lattice for the negative hopping , as relevant
for the electron-doped NaCoO (NCO). To understand several aspects of
this system, we study the -dependent chemical potential, specific heat,
magnetic susceptibility, and the dynamic Hall-coefficient across the entire
doping range. We show systematically, how this simplest model for strongly
correlated electrons describes a crossover as function of doping () from a
Pauli-like weakly spin-correlated metal close to the band-limit (density )
to the Curie-Weiss metallic phase () with pronounced
anti-ferromagnetic (AFM) correlations at low temperatures and Curie-Weiss type
behavior in the high-temperature regime. Upon further reduction of the doping,
a new energy scale, dominated by spin-interactions () emerges (apparent both
in specific heat and susceptibility) and we identify an effective interaction
, valid across the entire doping range. This is distinct from
Anderson's formula, as we choose here , hence the opposite sign of the
usual Nagaoka-ferromagnetic situation. This expression includes the subtle
effect of weak kinetic AFM - as encountered in the infinitely correlated
situation (). By explicit computation of the Kubo-formulae, we
address the question of practical relevance of the high-frequency expression
for the Hall coefficient . We hope to clarify some open questions
concerning the applicability of the t-J model to real experimental situations
through this study
Ground state of the spin-1/2 Heisenberg antiferromagnet on an Archimedean 4-6-12 lattice
An investigation of the N\'eel Long Range Order (NLRO) in the ground state of
antiferromagnetic Heisenberg spin system on the two-dimensional, uniform,
bipartite lattice consisting of squares, hexagons and dodecagons is presented.
Basing on the analysis of the order parameter and the long-distance correlation
function the NLRO is shown to occur in this system. Exact diagonalization and
variational (Resonating Valence Bond) methods are applied.Comment: 4 pages, 6 figure
Uncertainty Principle Enhanced Pairing Correlations in Projected Fermi Systems Near Half Filling
We point out the curious phenomenon of order by projection in a class of
lattice Fermi systems near half filling. Enhanced pairing correlations of
extended s-wave Cooper pairs result from the process of projecting out s-wave
Cooper pairs, with negligible effect on the ground state energy. The Hubbard
model is a particularly nice example of the above phenomenon, which is revealed
with the use of rigorous inequalities including the Uncertainty Principle
Inequality. In addition, we present numerical evidence that at half filling, a
related but simplified model shows ODLRO of extended s-wave Cooper pairs.Comment: RevTex 11 pages + 1 ps figure. Date 19 September 1996, Ver.
Thermoelectric effects in a strongly correlated model for NaCoO
Thermal response functions of strongly correlated electron systems are of
appreciable interest to the larger scientific community both theoretically and
technologically. Here we focus on the infinitely correlated t-J model on a
geometrically frustrated two-dimensional triangular lattice.
Using exact diagonalization on a finite sized system we calculate the
dynamical thermal response functions in order to determine the thermopower,
Lorenz number, and dimensionless figure of merit. The dynamical thermal
response functions is compared to the infinite frequency limit and shown to be
very weak functions of frequency, hence, establishing the validity of the high
frequency formalism recently proposed by Shastry for the thermopower, Lorenz
number, and the dimensionless figure of merit. Further, the thermopower is
demonstrated to have a low to mid temperature enhancement when the sign of the
hopping parameter is switched from positive to negative for the
geometrically frustrated lattice considered.Comment: 16 pages, 10 figures, color version available at
http://physics.ucsc.edu/~peterson/mrpeterson-condmat-NCO.pdf. V.2 has fixed
minor typos in Eq. 11, 19, 25, and 26. V.3 is a color versio
Quantum spin models with exact dimer ground states
Inspired by the exact solution of the Majumdar-Ghosh model, a family of
one-dimensional, translationally invariant spin hamiltonians is constructed.
The exchange coupling in these models is antiferromagnetic, and decreases
linearly with the separation between the spins. The coupling becomes
identically zero beyond a certain distance. It is rigorously proved that the
dimer configuration is an exact, superstable ground state configuration of all
the members of the family on a periodic chain. The ground state is two-fold
degenerate, and there exists an energy gap above the ground state. The
Majumdar-Ghosh hamiltonian with two-fold degenerate dimer ground state is just
the first member of the family.
The scheme of construction is generalized to two and three dimensions, and
illustrated with the help of some concrete examples. The first member in two
dimensions is the Shastry-Sutherland model. Many of these models have
exponentially degenerate, exact dimer ground states.Comment: 10 pages, 8 figures, revtex, to appear in Phys. Rev.
A lecture on the Calogero-Sutherland models
In these lectures, I review some recent results on the Calogero-Sutherland
model and the Haldane Shastry-chain. The list of topics I cover are the
following: 1) The Calogero-Sutherland Hamiltonian and fractional statistics.
The form factor of the density operator. 2) The Dunkl operators and their
relations with monodromy matrices, Yangians and affine-Hecke algebras. 3) The
Haldane-Shastry chain in connection with the Calogero-Sutherland Hamiltonian at
a specific coupling constant.Comment: (2 references added, small modifications
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