arXiv subjects

Raymond Fresard

Publications and source records attributed to Raymond Fresard.

At least 19 recordsLinked to original sources

arXiv2022

Bad metal and negative compressibility transitions in a two-band Hubbard model

We analyze the paramagnetic state of a two-band Hubbard model with finite Hund's coupling close to integer filling at $n=2$ in two spacial dimensions. Previously, a Mott metal-insulator transition was established at $n=2$ with a coexistence region of a metallic and a bad metal state in the vicinity of that integer filling. The coexistence region ends at a critical point beyond which a charge instability persists. Here we investigate the transition into negative electronic compressibility states for an extended filling range close to $n=2$ within a slave boson setup. We analyze the separate contributions from the (fermionic) quasiparticles and the (bosonic) multiparticle incoherent background and find that the total compressibility depends on a subtle interplay between the quasiparticle excitations and collective fields. Implementing a Blume-Emery-Griffiths model approach for the slave bosons, which mimics the bosonic fields by Ising-like pseudospins, we suggest a feedback mechanism between these fields and the fermionic degrees of freedom. We argue that the negative compressibility can be sustained for heterostructures of such strongly correlated planes and results in a large capacitance of these structures. The strong density dependence of these capacitances allows to tune them through small electronic density variations. Moreover, by resistive switching from a Mott insulating state to a metallic state through short electric pulses, transitions between fairly different capacitances are within reach.

cond-mat.str-el↗

arXiv2015

Charge instabilities of the two-dimensional Hubbard model with attractive nearest neighbour interaction

Attractive non-local interactions jointly with repulsive local interaction in a microscopic modelling of electronic Fermi liquids generate a competition between an enhancement of the static charge susceptibility---ultimately signalling charge instability and phase separation---and its correlation induced suppression. We analyse this scenario through the investigation of the extended Hubbard model on a two-dimensional square lattice, using the spin rotation invariant slave-boson representation of Kotliar and Ruckenstein. The quasiparticle density of states, the renormalised effective mass and the Landau parameter $F_0^s$ are presented, whereby the positivity of $F_0^s-1$ constitutes a criterion for stability. Van Hove singularities in the density of states support possible charge instabilities. A (negative) next-nearest neighbour hopping parameter $t'$ shifts their positions and produces a tendency towards charge instability even for low filling whereas the $t'$-controlled particle-hole asymmetry of the correlation driven effective mass is small. A region of instability on account of the attractive interaction $V$ is identified, either at half filling in the absence of strong electronic correlations or, in the case of large on-site interaction $U$, at densities far from half filling.

cond-mat.str-el↗

arXiv2012

On the ferromagnetic character of (LaVO$_3$)$_m$/SrVO$_3$ superlattices

The experimental observation that vanadate superlattices (LaVO$_3$)$_m$/SrVO$_3$ show ferromagnetism up to room temperature [U.\ Lüders {\it et al.}, Phys.\ Rev.\ B {\bf 80}, 241102R (2009)] is investigated by means of density functional theory. First, the influence of the density functional on the electronic and magnetic structure of bulk ${\rm LaVO_3}$ is discussed. Second, the band structure of a (LaVO$_3$)$_m$/SrVO$_3$ slab for $m=5$ and 6 is calculated. Very different behaviors for odd and even values of $m$ are found: In the odd case lattice relaxation results into a buckling of the interface VO$_2$ layers that leads to spin-polarized interfaces. In the even case a decoupling of the interface VO$_2$ layers from the LaO layers is obtained, confining the interface electrons into a two-dimensional electron gas. The orbital reconstruction at the interface due to the lattice relaxation is discussed.

cond-mat.str-el↗

arXiv2012

Magnetic transitions in strong coupling expansions for nearly degenerate states

We consider a strong coupling expansion for a two-band Hubbard model on two sites with nearly degenerate states. A comparative analysis is performed for different schemes of perturbation theory which are applicable to systems with nearly degenerate states. A fourth order approach which builds on a four-dimensional low-energy subspace with nearly degenerate states captures accurately the transition from an antiferromagnetic to a ferromagnetic ground state at large on-site Coulomb interaction.

cond-mat.str-el↗

arXiv2012

Exact results with the Kotliar-Ruckenstein slave-boson representation

Radial slave boson representations have the particular advantage that the expectation values of their respective fields are finite even without the formal introduction of spurious Bose condensates for each of the bosonic fields. The expectation values of the radial (real) fields are in fact to be interpreted as the density of empty or singly occupied sites. Whereas the radial representation of the Barnes slave bosons has been investigated before, a setup for the functional integral of radial bosonic fields in the more physical Kotliar-Ruckenstein representation has not been accomplished to date. We implement a path integral procedure with suitable renormalization factors for a strongly correlated two-site model which allows to control the formal steps in the intricate evaluation, as the results for the partition function and the expectation values are known from exact diagonalization for such a minimal single impurity Anderson model. The partition function is shown to be a trace over a product of matrices local in time and therefore can be calculated analytically. Eventually, we establish the scheme for the evaluation of correlation functions and thermodynamic properties.

cond-mat.str-el↗

arXiv2009

Influence of the vacuum interface on the charge distribution in V2O3 thin films

The electronic structure of V2O3 thin films is studied by means of the augmented spherical wave method as based on density functional theory and the local density approximation. We establish that effects of charge redistribution, induced by the vacuum interface, in such films are restricted to a very narrow surface layer of approx. 15 Angstroem thickness. As a consequence, charge redistribution can be ruled out as a source of the extraordinary thickness-dependence of the metal-insulator transition observed in V2O3 thin films of 100-1000 Angstroem thickness.

cond-mat.str-el↗

arXiv2009

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.

cond-mat.str-el↗

arXiv2009

Electronic structure and thermoelectric properties of CuRh(1-x)MgxO2

Electronic structure calculations using the augmented spherical wave method have been performed for CuRhO2. For this semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the 4d t2g orbitals of Rh^{3+}. The structural characterizations of CuRh(1-x)MgxO2 show a broad range of Mg^{2+} substitution for Rh^{3+} in this series, up to about 12%. Measurements of the resistivity and thermopower of the doped systems show a Fermi liquid-like behavior for temperatures up to about 1000K, resulting in a large weakly temperature dependent power factor. The thermopower is discussed both within the Boltzmann equation approach as based on the electronic structure calculations and the temperature independent correlation functions ratio approximation as based on the Kubo formalism.

cond-mat.str-el↗

arXiv2009

Stripe segregation and magnetic coupling in the nickelate La_5/3 Sr_1/3 NiO_4

We investigate the consequences of the stripe formation in the nickelate La5/3Sr1/3NiO4 for the details of its crystal structure and electronic states. Our data are based on numerical simulations within density functional theory (DFT) and the generalized gradient approximation (GGA). The on-site Coulomb interaction is included in terms of the LDA+U scheme. Structure optimization of preliminary experimental data indicates a strong interaction between the structural and electronic degrees of freedom. In particular, we find a segregation of the diagonal filled stripes induced by a delicate interplay with the magnetic coupling. Beyond the cooperative effect of stripe segregation and spin order, distinct octahedral distortions are essential for the formation of an insulating state.

cond-mat.str-el↗

arXiv2008

Substrate effects on V2O3 thin films

We apply density functional theory and the augmented spherical wave method to analyze the electronic structure of V2O3 in the vicinity of an interface to Al2O3. The interface is modeled by a heterostructure setup of alternating vanadate and aluminate slabs. We focus on the possible modifications of the V2O3 electronic states in this geometry, induced by the presence of the aluminate layers. In particular, we find that the tendency of the V 3d states to localize is enhanced and may even cause a metal-insulator transition.

cond-mat.str-el↗

arXiv2008

Dual electronic states in thermoelectric cobalt oxide

We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hund's coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties.

cond-mat.str-el↗

arXiv2008

Exact results in a slave boson saddle point approach for a strongly correlated electron model

We revisit the Kotliar-Ruckenstein (KR) slave boson saddle point evaluation for a two-site correlated electron model. As the model can be solved analytically, it is possible to compare the KR saddle point results to the exact many particle levels. The considered two site cluster mimics an infinite-$U$ single-impurity Anderson model with a nearest neighbor Coulomb interaction: one site is strongly correlated with an infinite local Coulomb repulsion which hybridizes with the second site, on which the local Coulomb repulsion vanishes. Making use of the flexibility of the representation we introduce appropriate weight factors in the KR saddle point scheme. Ground state and all excitation levels agree with the exact diagonalization results. Thermodynamics and correlation functions may be recovered in a suitably renormalized saddle point evaluation.

cond-mat.str-el↗

arXiv2008

Barnes slave boson approach to the two-site single impurity Anderson model with non-local interaction

The Barnes slave boson approach to the $U=\infty$ single impurity Anderson model extended by a non-local Coulomb interaction is revisited. We demonstrate first that the radial gauge representation facilitates the treatment of such a non-local interaction by performing the \emph{exact} evaluation of the path integrals representing the partition function, the impurity hole density and the impurity hole density autocorrelation function for a two-site cluster. The free energy is also obtained on the same footing. Next, the exact results are compared to their approximations at saddle-point level, and it is shown that the saddle point evaluation recovers the exact answer in the limit of strong non-local Coulomb interaction, while the agreement between both schemes remains satisfactory in a large parameter range.

cond-mat.str-el↗

arXiv2008

Long-range magnetic order and spin-lattice coupling in the delafossite CuFeO2

The electronic and magnetic properties of the delafossite CuFeO2 are investigated by means of electronic structure calculations. They are performed using density functional theory in the generalized gradient approximation as well as the new full-potential augmented spherical wave method. The calculations reveal three different spin states at the iron sites. Taking into account the correct crystal structure, we find long-range antiferromagnetic ordering in agreement with experiment. Contrasting previous work, our calculations show that non-local exchange interactions lead to a semiconducting ground state.

cond-mat.str-el↗

arXiv2008

On the metallic conductivity of the delafossites PdCoO2 and PtCoO2

The origin of the quasi two-dimensional behavior of PdCoO2 and PtCoO2 is investigated by means of electronic structure calculations. They are performed using density functional theory in the generalized gradient approximation as well as the new full-potential augmented spherical wave method. We show that the electric conductivity is carried almost exclusively by the in-plane Pd (Pt) d orbitals. In contrast, the insulating CoO2 sandwich layers of octahedrally coordinated Co atoms may be regarded as charge carrier reservoirs. This leads to a weak electronic coupling of the Pd (Pt) layers. The obtained nearly cylindrical Fermi surface causes the strong anisotropy of the electric conductivity.

cond-mat.mtrl-sci↗

arXiv2007

Unidirectional d-wave superconducting domains in the two-dimensional t-J model

Motivated by the recently observed pattern of unidirectional domains in high-T_c superconductors [Y. Kohsaka et al., Science 315, 1380 (2007)], we investigate the emergence of spontaneous modulations in the d-wave superconducting resonating valence bond phase using the t-J model at x=1/8 doping. Half-filled charge domains separated by four lattice spacings are found to form along one of the crystal axis leading to modulated superconductivity with out-of-phase d-wave order parameters in neighboring domains. Both renormalized mean-field theory and variational Monte Carlo calculations yield that the energies of modulated and uniform phases are very close to each other.

cond-mat.str-el↗

arXiv2007

Slave bosons in radial gauge: a bridge between path integral and hamiltonian language

We establish a correspondence between the resummation of world lines and the diagonalization of the Hamiltonian for a strongly correlated electronic system. For this purpose, we analyze the functional integrals for the partition function and the correlation functions invoking a slave boson representation in the radial gauge. We show in the spinless case that the Green's function of the physical electron and the projected Green's function of the pseudofermion coincide. Correlation and Green's functions in the spinful case involve a complex entanglement of the world lines which, however, can be obtained through a strikingly simple extension of the spinless scheme. As a toy model we investigate the two-site cluster of the single impurity Anderson model which yields analytical results. All expectation values and dynamical correlation functions are obtained from the exact calculation of the relevant functional integrals. The hole density, the hole auto-correlation function and the Green's function are computed, and a comparison between spinless and spin 1/2 systems provides insight into the role of the radial slave boson field. In particular, the exact expectation value of the radial slave boson field is finite in both cases, and it is not related to a Bose condensate.

cond-mat.str-el↗

arXiv2007

Anisotropic susceptibility of the geometrically frustrated spin-chain compound Ca3Co2O6

Ca3Co2O6 is a system exhibiting a series of fascinating properties, including magnetization plateaus and remarkably slow dynamics at low-T. These properties are intimately related to the geometrical frustration, which results from a particular combination of features: (i) the chains are arranged on a triangular lattice; (ii) there is a large uniaxial anisotropy; (iii) the intrachain and interchain couplings are ferromagnetic and antiferromagnetic, respectively. The uniaxial anisotropy is thus an issue of crucial importance for the analysis of the physical properties of Ca3Co2O6. However, it turns out that no precise investigation of this magnetic anisotropy has been performed so far. On the basis of susceptibility data directly recorded on single crystals, the present study reports on quantitative information about the anisotropy of Ca3Co2O6.

cond-mat.str-el↗