arXiv subjects

Raymond Frésard

Publications and source records attributed to Raymond Frésard.

22 records · Page 2Linked to original sources

arXiv2011

Mg substitution in CuCrO2 delafossite compounds

A detailed investigation of the series CuCr(1-x)MgxO2 (x = 0.0 - 0.05) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg2+ for Cr3+ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x ~ 0.02 - 0.03, indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr2O4 spinels as soon as x = 0.005. The thermopower is discussed in the temperature-independent correlation functions ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.

cond-mat.mtrl-sci↗

arXiv2004

The Origin of Magnetic Interactions in Ca3Co2O6

We investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt compound Ca3Co2O6. In particular, we establish a local model which stabilizes a ferromagnetic alignment of the S=2 spins on the cobalt sites with trigonal prismatic symmetry, for a sufficiently strong Hund's rule coupling on the cobalt ions. The exchange is mediated through a S=0 cobalt ion at the octahedral sites of the chain structure. We present a strong coupling evaluation of the Heisenberg coupling between the S=2 Co spins on a separate chain. The chains are coupled antiferromagnetically through super-superexchange via short O-O bonds.

cond-mat.str-el↗

arXiv1998

Magnetic and Charge Correlations of the 2-dimensional $t-t'-U$ Hubbard model

Using a spin-rotation invariant six-slave boson representation, we study the square lattice Hubbard model with nearest-neighbor hopping $t$ and next-nearest neighbor hopping $t'$. We discuss the influence of $t'$ on the charge and magnetic properties. In the hole-doped domain, we find that a negative $t'$ strongly favors itinerant ferromagnetism over any incommensurate phase, especially in the strong coupling regime. For positive $t'$ magnetic fluctuations are suppressed. A tight connection between frustrated charge dynamics and large magnetic fluctuations is pointed out. A clear tendency towards striped charge ordering is found in the regime of large positive $t'$.

cond-mat.str-el↗

arXiv1997

One-Electron Spectral Functions of the Attractive Hubbard Model for Intermediate Coupling

We calculate the one-electron spectral function of the attractive (negative-$U$) Hubbard model. We work in the intermediate coupling and low density regime and obtain the self-energy in an approximate analytical form. The excitation spectrum is found to consist of three branches. The results are obtained in a framework, based on the self-consistent T-matrix approximation, which is compatible with the Mermin-Wagner theorem.

cond-mat↗