742 research outputs found
Magnetic and thermal properties of the S = 1/2 zig-zag spin-chain compound In2VO5
Static magnetic susceptibility \chi, ac susceptibility \chi_{ac} and specific
heat C versus temperature T measurements on polycrystalline samples of In2VO5
and \chi and C versus T measurements on the isostructural, nonmagnetic compound
In2TiO5 are reported. A Curie-Wiess fit to the \chi(T) data above 175 K for
In2VO5 indicates ferromagnetic exchange between V^{4+} (S = 1/2) moments. Below
150 K the \chi(T) data deviate from the Curie-Weiss behavior but there is no
signature of any long range magnetic order down to 1.8 K. There is a cusp at
2.8 K in the zero field cooled (ZFC) \chi(T) data measured in a magnetic field
of 100 Oe and the ZFC and field cooled (FC) data show a bifurcation below this
temperature. The frequency dependence of the \chi_{ac}(T) data indicate that
below 3 K the system is in a spin-glass state. The difference \Delta C between
the heat capacity of In2VO5 and In2TiO5 shows a broad anomaly peaked at 130 K.
The entropy upto 300 K is more than what is expected for S = 1/2 moments. The
anomaly in \Delta C and the extra entropy suggests that there may be a
structural change below 130 K in In2VO5.Comment: 6 pages, 7 figures, 1 tabl
Numerical Linked-Cluster Algorithms. II. t-J models on the square lattice
We discuss the application of a recently introduced numerical linked-cluster
(NLC) algorithm to strongly correlated itinerant models. In particular, we
present a study of thermodynamic observables: chemical potential, entropy,
specific heat, and uniform susceptibility for the t-J model on the square
lattice, with J/t=0.5 and 0.3. Our NLC results are compared with those obtained
from high-temperature expansions (HTE) and the finite-temperature Lanczos
method (FTLM). We show that there is a sizeable window in temperature where NLC
results converge without extrapolations whereas HTE diverges. Upon
extrapolations, the overall agreement between NLC, HTE, and FTLM is excellent
in some cases down to 0.25t. At intermediate temperatures NLC results are
better controlled than other methods, making it easier to judge the convergence
and numerical accuracy of the method.Comment: 7 pages, 12 figures, as publishe
Improved Mean-Field Scheme for the Hubbard Model
Ground state energies and on-site density-density correlations are calculated
for the 1-D Hubbard model using a linear combination of the Hubbard projection
operators. The mean-field coefficients in the resulting linearized Equations of
Motion (EOM) depend on both one-particle static expectation values as well as
static two-particle correlations. To test the model, the one particle
expectation values are determined self-consistently while using Lanczos
determined values for the two particle correlation terms. Ground state energies
and on-site density-density correlations are then compared as a function of
to the corresponding Lanczos values on a 12 site Hubbard chain for 1/2 and 5/12
fillings. To further demonstrate the validity of the technique, the static
correlation functions are also calculated using a similar EOM approach, which
ignores the effective vertex corrections for this problem, and compares those
results as well for a 1/2 filled chain. These results show marked improvement
over standard mean-field techniques.Comment: 10 pages, 3 figures, text and figures as one postscript file -- does
not need to be "TeX-ed". LA-UR-94-294
Antiferromagnetism of the 2D Hubbard Model at Half Filling: Analytic Ground State at Weak Coupling
We introduce a local formalism to deal with the Hubbard model on a N times N
square lattice (for even N) in terms of eigenstates of number operators, having
well defined point symmetry. For U -> 0, the low lying shells of the kinetic
energy are filled in the ground state. At half filling, using the 2N-2 one-body
states of the partially occupied shell S_{hf}, we build a set of (2N-2 N-1)^{2}
degenerate unperturbed ground states with S_{z}=0 which are then resolved by
the Hubbard interaction \hat{W}=U\sum_{r}\hat{n}_{r\ua}\hat{n}_{r\da}. In
S_{hf} we study the many-body eigenstates of the kinetic energy with vanishing
eigenvalue of the Hubbard repulsion (W=0 states). In the S_{z}=0 sector, this
is a N times degenerate multiplet. From the singlet component one obtains the
ground state of the Hubbard model for U=0^{+}, which is unique in agreement
with a theorem by Lieb. The wave function demonstrates an antiferromagnetic
order, a lattice step translation being equivalent to a spin flip. We show that
the total momentum vanishes, while the point symmetry is s or d for even or odd
N/2, respectively.Comment: 13 pages, no figure
Proposal for a cumulant-based Bell test for mesoscopic junctions
The creation and detection of entanglement in solid state electronics is of
fundamental importance for quantum information processing. We prove that
second-order quantum correlations can be always interpreted classically and
propose a general test of entanglement based on the violation of a classically
derived inequality for continuous variables by fourth-order quantum correlation
functions. Our scheme provides a way to prove the existence of entanglement in
a mesoscopic transport setup by measuring higher order cumulants without
requiring the additional assumption of a single charge detectionComment: 6 pages, 1 figure, detailed proof of weak positivity and Bell-type
inequalit
Flux quantization and superfluid weight in doped antiferromagnets
Doped antiferromagnets, described by a t-t'-J model and a suitable 1/N
expansion, exhibit a metallic phase-modulated antiferromagnetic ground state
close to half-filling. Here we demonstrate that the energy of latter state is
an even periodic function of the external magnetic flux threading the square
lattice in an Aharonov-Bohm geometry. The period is equal to the flux quantum
entering the Peierls phase factor of the hopping
matrix elements. Thus flux quantization and a concomitant finite value of
superfluid weight D_s occur along with metallic antiferromagnetism. We argue
that in the context of the present effective model, whereby carriers are
treated as hard-core bosons, the charge q in the associated flux quantum might
be set equal to 2e. Finally, the superconducting transition temperature T_c is
related to D_s linearly, in accordance to the generic Kosterlitz-Thouless type
of transition in a two-dimensional system, signaling the coherence of the phase
fluctuations of the condensate. The calculated dependence of T_c on hole
concentration is qualitatively similar to that observed in the high-temperature
superconducting cuprates.Comment: 5 pages, 2 figures, to be published in J. Phys. Condens. Matte
Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}
Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to
J. Mag. Mag. Ma
Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions
The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown
through X-ray diffraction and thermogravimetric studies to be one of a series
of hydrated phases of Na0.3CoO2. Further, it is shown that the material is
exceptionally sensitive to both temperature and humidity near ambient
conditions, easily dehydrating to a non-superconducting lower hydrate. The
observation of this stable lower hydrate with c=13.8 angstroms implies that the
superconductivity turns on in this system between CoO2 layer spacings of 6.9
and 9.9 angstroms at nominally constant chemical doping.Comment: 10 pages and 4 figure
Nonclassical time correlation functions in continuous quantum measurement
A continuous projective measurement of a quantum system often leads to a
suppression of the dynamics, known as the Zeno effect. Alternatively,
generalized nonprojective, so-called "weak" measurements can be carried out.
Such a measurement is parameterized by its strength parameter that can
interpolate continuously between the ideal strong measurement with no
dynamics-the strict Zeno effect, and a weak measurement characterized by almost
free dynamics but blurry observations. Here we analyze the stochastic
properties of this uncertainty component in the resulting observation
trajectory. The observation uncertainty results from intrinsic quantum
uncertainty, the effect of measurement on the system (backaction) and detector
noise. It is convenient to separate the latter, system-independent contribution
from the system-dependent uncertainty, and this paper shows how to accomplish
this separation. The system-dependent uncertainty is found in terms of a
quasi-probability, which, despite its weaker properties, is shown to satisfy a
weak positivity condition. We discuss the basic properties of this
quasi-probability with special emphasis on its time correlation functions as
well as their relationship to the full correlation functions along the
observation trajectory, and illustrate our general results with simple
examples.We demonstrate a violation of classical macrorealism using the
fourth-order time correlation functions with respect to the quasi-probability
in the twolevel system.Comment: 20 pages, 1 figure, published version (open access
Fermionic description of spin-gap states of antiferromagnetic Heisenberg ladders in a magnetic field
Employing the Jordan-Wigner transformation on a unique path and then making a
mean-field treatment of the fermionic Hamiltonian, we semiquantitatively
describe the spin-gap states of Heisenberg ladders in a field. The appearance
of magnetization plateaux is clarified as a function of the number of legs.Comment: 2 pages, 3 figures embedded, J. Phys. Soc. Jpn. Vol. 71, No. 6, 1607
(2002
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