174 research outputs found
Evidence for the Coexistence of Anisotropic Superconducting Gap and Nonlocal Effects in the Non-magnetic Superconductor LuNi2B2C
A study of the dependence of the heat capacity Cp(alpha) on field angle in
LuNi2B2C reveals an anomalous disorder effect. For pure samples, Cp(alpha)
exhibits a fourfold variation as the field H < Hc2 is rotated in the [001]
plane, with minima along (alpha = 0). A slightly disordered sample,
however, develops anomalous secondary minima along for H > 1 T, leading
to an 8-fold pattern. The anomalous pattern is discussed in terms of coexisting
superconducting gap anisotropy and non-local effects.Comment: 5 pages, 4 figure
Renormalisation and fixed points in Hilbert Space
The energies of low-lying bound states of a microscopic quantum many-body
system of particles can be worked out in a reduced Hilbert space. We present
here and test a specific non-perturbative truncation procedure. We also show
that real exceptional points which may be present in the spectrum can be
identified as fixed points of coupling constants in the truncation procedure.Comment: 4 pages, 1 tabl
A Quantum Monte Carlo Method and Its Applications to Multi-Orbital Hubbard Models
We present a framework of an auxiliary field quantum Monte Carlo (QMC) method
for multi-orbital Hubbard models. Our formulation can be applied to a
Hamiltonian which includes terms for on-site Coulomb interaction for both
intra- and inter-orbitals, intra-site exchange interaction and energy
differences between orbitals. Based on our framework, we point out possible
ways to investigate various phase transitions such as metal-insulator, magnetic
and orbital order-disorder transitions without the minus sign problem. As an
application, a two-band model is investigated by the projection QMC method and
the ground state properties of this model are presented.Comment: 10 pages LaTeX including 2 PS figures, to appear in J.Phys.Soc.Jp
Origin of G-type Antiferromagnetism and Orbital-Spin Structures in
The possibility of the distortion of octahedra is
examined theoretically in order to understand the origin of the G-type
antiferromagnetism (AFM(G)) and experimentally observed puzzling properties of
. By utilizing an effective spin and pseudospin Hamiltonian with
the strong Coulomb repulsion, it is shown that AFM(G) state is stabilized
through the lift of the -orbital degeneracy accompanied by a tiny
-distortion . The estimated spin-exchange interaction is in agreement
with that obtained by the neutron scattering. Moreover, the level-splitting
energy due to the distortion can be considerably larger than the spin-orbit
interaction even when the distortion becomes smaller than the detectable limit
under the available experimental resolution. This suggests that the orbital
momentum is fully quenched and the relativistic spin-orbit interaction is not
effective in this system, in agreement with recent neutron-scattering
experiment.Comment: 9 pages, 6 figure
How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study
In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the
transition-metal d electron becomes increasingly localized and undergoes a Mott
transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard
Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving
it in the single-site DMFT approximation, it was recently shown[1] that
simultaneously with the Mott transition there occurs a strong suppression of
orbital fluctuations due to splitting of the t2g levels. The present paper
reviews and expands this work, in particular in the direction of exposing the
underlying chemical mechanisms by means of ab initio LDA Wannier functions
generated with the NMTO method. The Wannier functions for the t2g band exhibit
covalency between the transition-metal t2g, the large cation-d, and the
oxygen-p states; this covalency, which increases along the series, turns out to
be responsible not only for the splittings of the t2g levels, but also for
non-cubic perturbations of the hopping integrals, both of which are decisive
for the Mott transition. We find good agreement with the optical and
photoemission spectra, with the crystal-field splittings and orbital
polarizations recently measured for the titanates, and with the metallization
volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using
super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and
YTiO3, but the results are sensitive to detail, in particular for YTiO3 which,
without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM.
Finally, we show that it possible to unfold the orthorhombic t2g LDA
bandstructure to a pseudocubic zone. In this zone, the lowest band is separated
from the two others by a direct gap and has a width, W_I, which is
significantly smaller than that, W, of the entire t2g band. The progressive
GdFeO3-type distortion favours electron localization by decreasing W, by
increasing the splitting of the t2g levels and by decreasing W_I. Our
conclusions concerning the roles of GdFeO3-type and JT distortions agree with
those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see
http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd
Many-body perturbation calculation of spherical nuclei with a separable monopole interaction: I. Finite nuclei
We present calculations of ground state properties of spherical, doubly
closed-shell nuclei from O to Pb employing the techniques of
many-body perturbation theory using a separable density dependent monopole
interaction. The model gives results in Hartree-Fock order which are of similar
quality to other effective density-dependent interactions. In addition, second
and third order perturbation corrections to the binding energy are calculated
and are found to contribute small, but non-negligible corrections beyond the
mean-field result. The perturbation series converges quickly, suggesting that
this method may be used to calculate fully correlated wavefunctions with only
second or third order perturbation theory. We discuss the quality of the
results and suggest possible methods of improvement.Comment: 20 Pages, 11 figure
Comparison of techniques for computing shell-model effective operators
Different techniques for calculating effective operators within the framework
of the shell model using the same effective interaction and the same excitation
spaces are presented. Starting with the large-basis no-core approach, we
compare the time-honored perturbation-expansion approach and a model-space
truncation approach. Results for the electric quadrupole and magnetic dipole
operators are presented for Li. The convergence trends and dependence of
the effective operators on differing excitation spaces and Pauli Q-operators is
studied. In addition, the dependence of the electric-quadrupole effective
charge on the harmonic-oscillator frequency and the mass number, for A=5,6, is
investigated in the model-space truncation approach.Comment: 18 pages. REVTEX. 4 PostScript figure
Magnetic Phase Transition of the Perovskite-type Ti Oxides
Properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state is hardly changed so that the spin-exchange coupling along the
c-axis changes nearly continuously from positive to negative and takes
approximately zero at the phase boundary. The resultant strong
two-dimensionality in the spin coupling causes a rapid suppression of the
critical temperature as is observed experimentally.Comment: 9 pages, 5 figure
The Upper Critical Field in Disordered Two-Dimensional Superconductors
We present calculations of the upper critical field in superconducting films
as a function of increasing disorder (as measured by the normal state
resistance per square). In contradiction to previous work, we find that there
is no anomalous low-temperature positive curvature in the upper critical field
as disorder is increased. We show that the previous prediction of this effect
is due to an unjustified analytical approximation of sums occuring in the
perturbative calculation. Our treatment includes both a careful analysis of
first-order perturbation theory, and a non-perturbative resummation technique.
No anomalous curvature is found in either case. We present our results in
graphical form.Comment: 11 pages, 8 figure
Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach
The properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state changes only little whereas the spin-exchange coupling along the
c-axis is expected to change from positive to negative nearly continuously and
approaches zero at the phase boundary. The resultant strong two-dimensionality
in the spin coupling causes rapid suppression of the critical temperature, as
observed experimentally. It may induce large quantum fluctuations in this
region.Comment: 13 pages, 15 figure
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