243 research outputs found

### Theory for Phase Transitions in Insulating Vanadium Oxide

We show that the recently proposed S=2 bond model with orbital degrees of
freedom for insulating V$_{2}$O$_{3}$ not only explains the anomalous magnetic
ordering, but also other mysteries of the magnetic phase transition. The model
contains an additional orbital degree of freedom that exhibits a zero
temperature quantum phase transtion in the Ising universality class.Comment: 5 pages, 2 figure

### Pressure-tuning of the c-f hybridization in Yb metal detected by infrared spectroscopy up to 18 GPa

It has been known that the elemental Yb, a divalent metal at mbient pressure,
becomes a mixed-valent metal under external pressure, with its valence reaching
~2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the
evolution of microscopic electronic states associated with the valence
crossover in Yb at external pressures up to 18 GPa. The measured infrared
reflectivity spectrum R(w) of Yb has shown large variations with pressure. In
particular, R(w) develops a deep minimum in the mid-infrared, which shifts to
lower energy with increasing pressure. The dip is attributed to optical
absorption due to a conduction c-f electron hybridization state, similarly to
those previously observed for heavy fermion compounds. The red shift of the dip
indicates that the $c$-$f$ hybridization decreases with pressure, which is
consistent with the increase of valence.Comment: 2 pages, to appear in J. Phys. Soc. Jpn. Supp

### Plaquette Ordering in SU(4) Antiferromagnets

We use fermion mean field theory to study possible plaquette ordering in the
antiferromagnetic SU(4) Heisenberg model. We find the ground state for both the
square and triangular lattices to be the disconnected plaquette state. Our mean
field theory gives a first order transition for plaquette ordering for the
triangular lattice. Our results suggest a large number of low lying states.Comment: 16 pages, 5 figure

### Electronic structure of GdN, and the influence of exact exchange

GdN bulk is studied with the local density approximation, on the Hartree-Fock
level, and on the level of the hybrid functional B3LYP. A local basis set
formalism is used, as implemented in the present CRYSTAL06 release. It is
demonstrated that the code is technically capable of treating this system with
its 4f electrons explicitly, i.e. out of the core. The band structure at the
level of the local density approximation is in good agreement with earlier
calculations and is found to be half-metallic. The Hartree-Fock band structure
is insulating with a large gap. Interestingly, three solutions were found at
the B3LYP level. The lowest of them is insulating for majority spin, and the
Fermi surface for minority spin consists only of points, resulting in a very
low density of states around the Fermi level.Comment: to appear in J. Phys.: Condensed Matte

### Orbitally Degenerate Spin-1 Model for Insulating V2O3

Motivated by recent neutron, X-ray absorption and resonant scattering
experiments, we revisit the electronic structure of V2O3. We propose a model in
which S=1 V3+ ions are coupled in the vertical V-V pairs forming two-fold
orbitally degenerate configurations with S=2. Ferro-orbital ordering of the V-V
pairs gives a description which is consistent with all experiments in the
antiferromagnetic insulating phase.Comment: 4 pages, including three figure

### Magneto-x-ray effects in transition-metal alloys

We present a theory that combines the relativistic spin-polarized version of the Koringa-Kohn-Rostoker coherent-potential approximation theory and the macroscopic theory of magneto-optical effects enabling us to calculate magneto-x-ray effects from first principles. The theory is illustrated by calculation of Faraday and Kerr rotations and ellipticities for transition-metal alloys

### Electronic Structure and Phase Transition in V2O3: Importance of 3d Spin-Orbit Interaction and Lattice Distortion

The 3d electronic structure and phase transition in pure and Cr doped V2O3
are theoretically investigated in relation to the 3d spin-orbit interaction and
lattice distortion. A model consisting of the nearest-neighbor V ion pair with
full degeneracy of the 3d orbitals is studied within the many-body point of
view. It is shown that each V ion with S=1 spin state has a large orbital
magnetic moment $\sim 0.7 \mu_{\rm B}$ and no orbital ordering occurs in the
antiferromagnetic insulating (AFI) phase. The anomalous resonant Bragg
reflection found in the AFI phase is attributed to the magnetic ordering. In
the AFI and paramagnetic insulating (PI) phases, Jahn-Teller like lattice
instability leads to tilting of the V ion pairs from the corundum c-axis and
this causes large difference in the orbital occupation between the paramagnetic
metal and the insulating phases, which is consistent with linear dichroic V 2p
XAS measurements.
To understand the AFI to PI transition, a model spin Hamiltonian is also
proposed. The transition is found to be simultaneous order-disorder transition
of the magnetic moments and tilting directions of the V ion pairs. Softening of
elastic constant C44 and abrupt change in short range spin correlations
observed at the transition are also explained.Comment: 18 pages, 16 figure

### Zero temperature metal-insulator transition in the infinite-dimensional Hubbard model

The zero temperature transition from a paramagnetic metal to a paramagnetic
insulator is investigated in the Dynamical Mean Field Theory for the Hubbard
model. The self-energy of the effective impurity Anderson model (on which the
Hubbard model is mapped) is calculated using Wilson's Numerical Renormalization
Group method. Results for quasiparticle weight, spectral function and
self-energy are discussed for Bethe and hypercubic lattice. In both cases, the
metal-insulator transition is found to occur via the vanishing of a
quasiparticle resonance which appears to be isolated from the Hubbard bands.Comment: 4 pages, 3 eps-figures include

### Ground State and Excitations of Spin Chain with Orbital Degeneracy

The one dimensional Heisenberg model in the presence of orbital degeneracy is
studied at the SU(4) symmetric viewpoint by means of Bethe ansatz. Following
Sutherland's previous work on an equivalent model, we discuss the ground state
and the low-lying excitations more extensively in connection to the spin
systems with orbital degeneracy. We show explicitly that the ground state is a
SU(4) singlet. We study the degeneracies of the elementary excitations and the
spectra of the generalized magnons consisting of these excitations. We also
discuss the complex 2-strings in the context of the Bethe ansatz solutions.Comment: Revtex, 9 pages, 3 figures; typos correcte

### Incommensurate magnetic structure of CeRhIn5

The magnetic structure of the heavy fermion antiferromagnet CeRhIn5 is
determined using neutron diffraction. We find a magnetic wave vector
q_M=(1/2,1/2,0.297), which is temperature independent up to T_N=3.8K. A
staggered moment of 0.374(5) Bohr magneton at 1.4K, residing on the Ce ion,
spirals transversely along the c axis. The nearest neighbor moments on the
tetragonal basal plane are aligned antiferromagnetically.Comment: 4 pages, 4 figures There was an extra factor of 2 in Eq (2). This
affects the value of staggered moment. The correct staggered moment is
0.374(5) Bohr magneton at 1.4

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