6,755 research outputs found
Charge transfer in heterostructures of strongly correlated materials
In this manuscript, recent theoretical investigations by the authors in the
area of oxide multilayers are briefly reviewed. The calculations were carried
out using model Hamiltonians and a variety of non-perturbative techniques.
Moreover, new results are also included here. They correspond to the generation
of a metallic state by mixing insulators in a multilayer geometry, using the
Hubbard and Double Exchange models. For the latter, the resulting metallic
state is also ferromagnetic. This illustrates how electron or hole doping via
transfer of charge in multilayers can lead to the study of phase diagrams of
transition metal oxides in the clean limit. Currently, these phase diagrams are
much affected by the disordering standard chemical doping procedure, which
introduces quenched disorder in the material.Comment: 14 pages, 9 figures. Invited article for a special issue of JPCM on
Metal Oxide Thin Films; minor changes in the tex
Evolution of the electronic structure across the filling-control and bandwidth-control metal-insulator transitions in pyrochlore-type Ru oxides
We have performed photoemission and soft x-ray absorption studies of
pyrochlore-type Ru oxides, namely, the filling-control system
SmCaRuO and the bandwidth-control system
SmBiRuO, which show insulator-to-metal transition with
increasing Ca and Bi concentration, respectively. Core levels and the O 2
valence band in SmCaRuO show almost the same amount of
monotonous upward energy shifts with Ca concentration, which indicates that the
chemical potential is shifted downward due to hole doping. The Ru 4 band in
SmCaRuO is also shifted toward the Fermi level () with
hole doping and the density of states (DOS) at increases. The core levels
in SmBiRuO, on the other hand, do not show clear energy
shifts except for the Ru 3 core level, whose line shape change also reflects
the increase of metallic screening with Bi concentration. We observe pronounced
spectral weight transfer from the incoherent to the coherent parts of the Ru 4d
band with Bi concentration, which is expected for a bandwidth-control
Mott-Hubbard system. The increase of the DOS at is more abrupt in the
bandwidth-control SmBiRuO than in the filling-control
SmCaRuO, in accordance with a recent theoretical
prediction. Effects of charge transfer between the Bi 6 band and the Ru
4 band are also discussed.Comment: 11 pages, 6 figure
On the Axisymmetric Force-Free Pulsar Magnetosphere
We investigate the axisymmetric magnetosphere of an aligned rotating magnetic
dipole surrounded by an ideal force-free plasma. We concentrate on the magnetic
field structure around the point of intersection of the separatrix between the
open and closed field-line regions and the equatorial plane. We first study the
case where this intersection point is located at the Light Cylinder. We find
that in this case the separatrix equilibrium condition implies that all the
poloidal current must return to the pulsar in the open-field region, i.e., that
there should be no finite current carried by the separatrix/equator current
sheet. We then perform an asymptotic analysis of the pulsar equation near the
intersection point and find a unique self-similar solution; however, a Light
Surface inevitably emerges right outside the Light Cylinder. We then perform a
similar analysis for the situation where the intersection point lies somewhere
inside the Light Cylinder, in which case a finite current flowing along the
separatrix and the equator is allowed. We find a very simple behavior in this
case, characterized by a 90-degree angle between the separatrix and the equator
and by finite vertical field in the closed-field region. Finally, we discuss
the implications of our results for global numerical studies of pulsar
magnetospheres.Comment: 31 pages, including 5 figure
Magnetic properties of the spin Heisenberg chain with hexamer modulation of exchange
We consider the spin-1/2 Heisenberg chain with alternating spin exchange %on
even and odd sites in the presence of additional modulation of exchange on odd
bonds with period three. We study the ground state magnetic phase diagram of
this hexamer spin chain in the limit of very strong antiferromagnetic (AF)
exchange on odd bonds using the numerical Lanczos method and bosonization
approach. In the limit of strong magnetic field commensurate with the
dominating AF exchange, the model is mapped onto an effective Heisenberg
chain in the presence of uniform and spatially modulated fields, which is
studied using the standard continuum-limit bosonization approach. In absence of
additional hexamer modulation, the model undergoes a quantum phase transition
from a gapped string order into the only one gapless L\"uttinger liquid (LL)
phase by increasing the magnetic field. In the presence of hexamer modulation,
two new gapped phases are identified in the ground state at magnetization equal
to 1/3 and 2/3 of the saturation value. These phases reveal themselves also in
magnetization curve as plateaus at corresponding values of magnetization. As
the result, the magnetic phase diagram of the hexamer chain shows seven
different quantum phases, four gapped and three gapless and the system is
characterized by six critical fields which mark quantum phase transitions
between the ordered gapped and the LL gapless phases.Comment: 21 pages, 5 figures, Journal of Physics: Condensed Matter, 24,
116002, (2012
{\bf -Function Evaluation of Gap Probabilities in Orthogonal and Symplectic Matrix Ensembles}
It has recently been emphasized that all known exact evaluations of gap
probabilities for classical unitary matrix ensembles are in fact
-functions for certain Painlev\'e systems. We show that all exact
evaluations of gap probabilities for classical orthogonal matrix ensembles,
either known or derivable from the existing literature, are likewise
-functions for certain Painlev\'e systems. In the case of symplectic
matrix ensembles all exact evaluations, either known or derivable from the
existing literature, are identified as the mean of two -functions, both
of which correspond to Hamiltonians satisfying the same differential equation,
differing only in the boundary condition. Furthermore the product of these two
-functions gives the gap probability in the corresponding unitary
symmetry case, while one of those -functions is the gap probability in
the corresponding orthogonal symmetry case.Comment: AMS-Late
Magnetic properties of the distorted diamond chain at T=0
We explore, at T=0, the magnetic properties of the antiferromagnetic
distorted diamond chain described by the Hamiltonian {\cal H}
= \sum_{j=1}^{N/3}{J_1 ({\bi S}_{3j-1} \cdot {\bi S}_{3j}
+ {\bi S}_{3j} \cdot {\bi S}_{3j+1})
+ J_2 {\bi S}_{3j+1} \cdot {\bi S}_{3j+2}
+ J_3 ({\bi S}_{3j-2} \cdot {\bi S}_{3j}
+ {\bi S}_{3j} \cdot {\bi S}_{3j+2})}
\allowbreak - H \sum_{l=1}^{N} S_l^z with , which well
models with , and azurite . We employ the physical
consideration, the degenerate perturbation theory, the level spectroscopy
analysis of the numerical diagonalization data obtained by the Lanczos method
and also the density matrix renormalization group (DMRG) method. We investigate
the mechanisms of the magnetization plateaux at and , and
also show the precise phase diagrams on the plane
concerning with these magnetization plateaux, where
and is the saturation magnetization. We also calculate the magnetization
curves and the magnetization phase diagrams by means of the DMRG method.Comment: 21 pages, 29 figure
Thermodynamics of a black hole in a cavity
We present a unified thermodynamical description of the configurations
consisting on self-gravitating radiation with or without a black hole. We
compute the thermal fluctuations and evaluate where will they induce a
transition from metastable configurations towards stable ones. We show that the
probability of finding such a transition is exponentially small. This indicates
that, in a sequence of quasi equilibrium configurations, the system will remain
in the metastable states till it approaches very closely the critical point
beyond which no metastable configuration exists. Near that point, we relate the
divergence of the local temperature fluctuations to the approach of the
instability of the whole system, thereby generalizing the usual fluctuations
analysis in the cases where long range forces are present. When angular
momentum is added to the cavity, the above picture is slightly modified.
Nevertheless, at high angular momentum, the black hole loses most of its mass
before it reaches the critical point at which it evaporates completely.Comment: 27 pages, latex file, contains 3 figures available on request at
[email protected]
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.
Theory of Raman Scattering from Orbital Excitations in Manganese Oxides
We present a theory of the Raman scattering from the orbital wave excitations
in manganese oxides. Two excitation processes of the Raman scattering are
proposed. The Raman scattering cross section is formulated by using the
pseudospin operator for orbital degree of freedom in a Mn ion. The Raman
spectra from the orbital wave excitations are calculated and their implications
in the recent experimental results reported in LaMnO are discussed.Comment: 10 pages, 7 figure
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