10,983 research outputs found

### Localization of strongly correlated electrons as Jahn-Teller polarons in manganites

A realistic modeling of manganites should include the Coulomb repulsion
between $e_g$ electrons, the Hund's rule coupling to $t_{2g}$ spins, and
Jahn-Teller phonons. Solving such a model by dynamical mean field theory, we
report large magnetoresistances and spectra in good agreement with experiments.
The physics of the unusual, insulating-like paramagnetic phase is determined by
correlated electrons which are-due to strong correlations-easily trapped as
Jahn-Teller polarons.Comment: 4 pages, 3 figure

### Microscopic conditions favoring itinerant ferromagnetism: Hund's rule coupling and orbital degeneracy

The importance of Hund's rule coupling for the stabilization of itinerant
ferromagnetism is investigated within a two-band Hubbard model. The magnetic
phase diagram is calculated by finite-temperature quantum Monte Carlo
simulations within the dynamical mean-field theory. Ferromagnetism is found in
a broad range of electron fillings whereas antiferromagnetism exists only near
half filling. The possibility of orbital ordering at quarter filling is also
analyzed.Comment: 5 pages, 6 figures, RevTeX, final version contains an additional
phase diagram for smaller Hund's rule coupling. to appear in Eur. Phys. J. B
(1998

### Electronic structure of CeRu4Sn6: a density functional plus dynamical mean field theory study

The Kondo system CeRu$_4$Sn$_6$ shows a strong anisotropy in its electric,
optic and magnetic properties. We employ density functional theory plus
dynamical mean field theory and show that the predominant Ce-$f$ state has
total angular moment $J=5/2$ and $z$-component $m_J=\pm 1/2$ in agreement with
recent X-ray absorption experiments. Even though CeRu$_4$Sn$_6$ has the direct
gap of a Kondo insulator through most of the Brillouin zone it remains weakly
metallic. This is because of (i) a band crossing in the $z$-direction and (ii)
a negative indirect gap.Comment: 6 pages, 9 figure

### Physics behind the minimum of relative entropy measures for correlations

The relative entropy of a correlated state and an uncorrelated reference
state is a reasonable measure for the degree of correlations. A key question is
however which uncorrelated state to compare to. The relative entropy becomes
minimal for the uncorrelated reference state that has the same one-particle
density matrix as the correlated state. Hence, this particular measure, coined
nonfreeness, is unique and reasonable. We demonstrate that for relevant
physical situations, such as finite temperatures or a correlation enhanced
orbital splitting, other choices of the uncorrelated state, even educated
guesses, overestimate correlations.Comment: 4 pages, 1 figure, final version as to appear European Physical
Journal

### From d- to p-wave pairing in the t-t' Hubbard model at zero temperature

We develop a DCA(PQMC) algorithm which employs the projective quantum Monte
Carlo (PQMC) method for solving the equations of the dynamical cluster
approximation (DCA) at zero temperature, and apply it for studying pair
susceptibilities of the two-dimensional Hubbard-model with next-nearest
neighbor hopping. In particular, we identify which pairing symmetry is dominant
in the U-n parameter space (U: repulsive Coulomb interaction; n: electron
density). We find that p_{x+y}- (d_{x^2-y^2}-) wave is dominant among triplet
(singlet) pairings -at least for 0.3<n<0.8 and U<=4t. The crossover between
d_{x^2-y^2}-wave and p_{x+y}-wave occurs around n~0.4.Comment: 5 pages 5 figures; two additional panels in Fig. 2; as to appear in
Phys. Rev.

### Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static
and dynamical mean-field theories (LDA+U and LDA+DMFT) indicate that the
metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room
temperature is not a Mott-Hubbard transition, but is caused by orbital
splitting of the majority-spin eg bands. For LaMnO3 to be insulating at
pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller
distortion are needed.Comment: 4 pages, 3 figure

### Thermodynamic and spectral properties of compressed Ce calculated by the merger of the local density approximation and dynamical mean field theory

We have calculated thermodynamic and spectral properties of Ce metal over a
wide range of volume and temperature, including the effects of 4f electron
correlations, by the merger of the local density approximation and dynamical
mean field theory (DMFT). The DMFT equations are solved using the quantum Monte
Carlo technique supplemented by the more approximate Hubbard I and Hartree Fock
methods. At large volume we find Hubbard split spectra, the associated local
moment, and an entropy consistent with degeneracy in the moment direction. On
compression through the volume range of the observed gamma-alpha transition, an
Abrikosov-Suhl resonance begins to grow rapidly in the 4f spectra at the Fermi
level, a corresponding peak develops in the specific heat, and the entropy
drops rapidly in the presence of a persistent, although somewhat reduced local
moment. Our parameter-free spectra agree well with experiment at the alpha- and
gamma-Ce volumes, and a region of negative curvature in the correlation energy
leads to a shallowness in the low-temperature total energy over this volume
range which is consistent with the gamma-alpha transition. As measured by the
double occupancy, we find a noticeable decrease in correlation on compression
across the transition; however, even at the smallest volumes considered, Ce
remains strongly correlated with residual Hubbard bands to either side of a
dominant Fermi-level structure. These characteristics are discussed in light of
current theories for the volume collapse transition in Ce.Comment: 19 pages including 14 eps figure

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