48 research outputs found
Unconventional superconducting phases in a correlated two-dimensional Fermi gas of nonstandard quasiparticles: a simple model
We discuss a detailed phase diagram and other microscopic characteristics on
the applied magnetic field - temperature (H_a-T) plane for a simple model of
correlated fluid represented by a two-dimensional (2D) gas of heavy
quasiparticles with masses dependent on the spin direction and the effective
field generated by the electron correlations. The consecutive transitions
between the Bardeen-Cooper-Schrieffer (BCS) and the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases are either continuous or
discontinuous, depending on the values of H_a and T. In the latter case, weak
metamagnetic transitions occur at the BCS-FFLO boundary. We single out two
different FFLO phases, as well as a reentrant behaviour of one of them at high
fields. The results are compared with those for ordinary Landau quasiparticles
in order to demonstrate the robustness of the FFLO states against the BCS state
for the case with spin-dependent masses (SDM). We believe that the mechanism of
FFLO stabilization by SDM is generic: other high-field low-temperature (HFLT)
superconducting phases benefit from SDM as well.Comment: 10 pages, 4 figure
Electronic states, Mott localization, electron-lattice coupling, and dimerization for correlated one-dimensional systems. II
We discuss physical properties of strongly correlated electron states for a
linear chain obtained with the help of the recently proposed new method
combining the exact diagonalization in the Fock space with an ab initio
readjustment of the single-particle orbitals in the correlated state. The
method extends the current discussion of the correlated states since the
properties are obtained with varying lattice spacing. The finite system of N
atoms evolves with the increasing interatomic distance from a Fermi-liquid-like
state into the Mott insulator. The criteria of the localization are discussed
in detail since the results are already convergent for N>=8. During this
process the Fermi-Dirac distribution gets smeared out, the effective band mass
increases by ~50%, and the spin-spin correlation functions reduce to those for
the Heisenberg antiferromagnet. Values of the microscopic parameters such as
the hopping and the kinetic-exchange integrals, as well as the magnitude of
both intra- and inter-atomic Coulomb and exchange interactions are calculated.
We also determine the values of various local electron-lattice couplings and
show that they are comparable to the kinetic exchange contribution in the
strong-correlation limit. The magnitudes of the dimerization and the zero-point
motion are also discussed. Our results provide a canonical example of a
tractable strongly correlated system with a precise, first-principle
description as a function of interatomic distance of a model system involving
all hopping integrals, all pair-site interactions, and the exact one-band
Wannier functions.Comment: 18 pages, REVTEX, submitted to Phys. Rev.
Four left renal arteries — a rare variant of kidney arterial supply
We describe the arterial supply of a human kidney harvested post-mortem from a 75-year-old female volunteer body donor. The kidney was analysed with con- trast-enhanced computed tomography (CT), and corrosion casting was used to reveal the kidney’s angio-architecture. In the left kidney, we observed four renal arteries, each originating directly from the abdominal aorta. Three renal arteries, including the main renal artery, coursed through the renal hilum, and the fourth renal artery reached the lower kidney pole. The supply areas of each of the four renal arteries were analysed with a three-dimensional reconstruction of CT images and with corrosion casting. There were no clear boundaries between the areas supplied by the four renal arteries because their branches overlapped in most kidney segments.
Verwey transition in FeO at high pressure: quantum critical behavior at the onset of metallization
We provide evidence for the existence of a {\em quantum critical point} at
the metallization of magnetite FeO at an applied pressure of GPa. We show that the present ac magnetic susceptibility data
support earlier resistivity data. The Verwey temperature scales with pressure
, with . The resistivity data shows a
temperature dependence , with above and
2.5 at the critical pressure, respectively. This difference in with
pressure is a sign of critical behavior at . The magnetic susceptibility
is smooth near the critical pressure, both at the Verwey transition and near
the ferroelectric anomaly. A comparison with the critical behavior observed in
the Mott-Hubbard and related systems is made.Comment: 5 pages, 5 figure
Spin-triplet superconducting pairing due to local (Hund's rule, Dirac) exchange
We discuss general implications of the local spin-triplet pairing among
fermions induced by local ferromagnetic exchange, example of which is the
Hund's rule coupling. The quasiparticle energy and their wave function are
determined for the three principal phases with the gap, which is momentum
independent. We utilize the Bogolyubov-Nambu-De Gennes approach, which in the
case of triplet pairing in the two-band case leads to the four-components wave
function. Both gapless modes and those with an isotropic gap appear in the
quasiparticle spectrum. A striking analogy with the Dirac equation is briefly
explored. This type of pairing is relevant to relativistic fermions as well,
since it reflects the fundamental discrete symmetry-particle interchange. A
comparison with the local interband spin-singlet pairing is also made.Comment: 16 pages, LaTex, submitted to Phys. Rev.
Electronic and phononic states of the Holstein-Hubbard dimer of variable length
We consider a model Hamiltonian for a dimer including all the electronic one-
and two-body terms consistent with a single orbital per site, a free Einstein
phonon term, and an electron-phonon coupling of the Holstein type. The bare
electronic interaction parameters were evaluated in terms of Wannier functions
built from Gaussian atomic orbitals. An effective polaronic Hamiltonian was
obtained by an unrestricted displaced-oscillator transformation, followed by
evaluation of the phononic terms over a squeezed-phonon variational wave
function. For the cases of quarter-filled and half-filled orbital, and over a
range of dimer length values, the ground state was identified by simultaneously
and independently optimizing the orbital shape, the phonon displacement and the
squeezing effect strength. As the dimer length varies, we generally find
discontinuous changes of both electronic and phononic states, accompanied by an
appreciable renormalization of the effective electronic interactions across the
transitions, due to the equilibrium shape of the wave functions strongly
depending on the phononic regime and on the type of ground state.Comment: 11 pages, RevTeX, 10 PostScript figures; to appear in Phys. Rev.
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V_2O_3
Magnetic correlations in all four phases of pure and doped vanadium
sesquioxide V_2O_3 have been examined by magnetic thermal neutron scattering.
While the antiferromagnetic insulator can be accounted for by a Heisenberg
localized spin model, the long range order in the antiferromagnetic metal is an
incommensurate spin-density-wave, resulting from a Fermi surface nesting
instability. Spin dynamics in the strongly correlated metal are dominated by
spin fluctuations in the Stoner electron-hole continuum. Furthermore, our
results in metallic V_2O_3 represent an unprecedentedly complete
characterization of the spin fluctuations near a metallic quantum critical
point, and provide quantitative support for the SCR theory for itinerant
antiferromagnets in the small moment limit. Dynamic magnetic correlations for
energy smaller than k_BT in the paramagnetic insulator carry substantial
magnetic spectral weight. However, the correlation length extends only to the
nearest neighbor distance. The phase transition to the antiferromagnetic
insulator introduces a sudden switching of magnetic correlations to a different
spatial periodicity which indicates a sudden change in the underlying spin
Hamiltonian. To describe this phase transition and also the unusual short range
order in the paramagnetic state, it seems necessary to take into account the
orbital degrees of freedom associated with the degenerate d-orbitals at the
Fermi level in V_2O_3.Comment: Postscript file, 24 pages, 26 figures, 2 tables, accepted by Phys.
Rev.
Magnetic Properties of Monomer and Dimer Tetrahedral VOx Entities Dispersed on Amorphous Silica-based Materials: Prediction of EPR Parameters from Relativistic DFT Calculations and Broken Symmetry Approach to Exchange Couplings
Molecular structures of the isolated tetrahedral oxovanadium(IV) and bridged μ-oxo-divanadium(IV) complexes hosted by the clusters mimicking surfaces of amorphous silica-based materials were investigated using density functional theory (DFT) calculations. Principal values of the g and A tensors for the monomer vanadyl species were obtained using the coupled-perturbed DFT level of theory and the spin–orbit mean-field approximation (SOMF). Magnetic exchange interaction for the μ-oxo bridged vanadium(IV) dimer was investigated within the broken symmetry approach. An antiferromagnetic coupling of the individual magnetic moments of the vanadium(IV) centers in the [VO–O–VO]2+ bridges was revealed and discussed in detail. The coupling explains pronounced decrease of the electron paramagnetic resonance signal (EPR) intensity, observed for the reduced VOx/SiO2 samples with the increasing coverage of vanadia, in terms of transformation of the paramagnetic monomer species into the dimers with S = 0 ground state