arXiv subjects

R. Fresard

Publications and source records attributed to R. Fresard.

16 recordsLinked to original sources

arXiv2022

Charge instabilities of the extended attractive Hubbard Model on the cubic lattice

The paramagnetic phase of the extended attractive Hubbard model on the cubic lattice is studied within the spin rotation invariant Kotliar-Ruckenstein slave-boson representation at zero temperature. It is obtained that the quasiparticle residue of the Fermi liquid phase vanishes for all densities at an interaction strength slightly smaller than $U_c$ that signals the Brinkman-Rice transition, and that it weakly depends on density. While for vanishing non-local interaction parameters homogeneous static charge instabilities are found in a rather narrow window centered around quarter filling and $U \simeq 0.8\ U_c$, increasing them to $V = -0.2\ U$ results into a severe narrowing of this window. On the contrary, when all interaction parameters are attractive, for example for $V = 0.2\ U$, a large parameter range in which homogeneous static charge instabilities is found. Yet, this systematically happens inside the Fermi liquid phase.

cond-mat.str-el↗

arXiv2013

ω/T scaling of the optical conductivity in strongly correlated layered cobalt oxide

We report infrared spectroscopic properties of the strongly correlated layered cobalt oxide [BiBa$_{0.66}$K$_{0.36}$O$_2$]CoO$_2$. These measurements performed on single crystals allow us to determine the optical conductivity as a function of temperature. In addition to a large temperature dependent transfer of spectral weight, an unconventional low energy mode is found. We show that both its frequency and damping scale as the temperature itself. In fact, a basic analysis demonstrates that this mode fully scales onto a function of $ω$/T up to room temperature. This behavior suggests low energy excitations of non-Fermi liquid type originating from quantum criticality.

cond-mat.str-el↗

arXiv2009

Room temperature magnetism in LaVO3/SrVO3 superlattices by geometrically confined doping

Based on the Hubbard model of strongly correlated systems, a reduction in the bandwidth of the electrons can yield a substantial change in the properties of the material. One method to modify the bandwidth is geometrically confined doping, i.e. the introduction of a (thin) dopant layer in a material. In this paper, the magnetic properties of LaVO$_3$/SrVO$_3$ superlattices, in which the geometrically confined doping is produced by a one monolayer thick SrVO$_3$ film, are presented. In contrast to the solid solution La$_{1-x}$Sr$_x$VO$_3$, such superlattices have a finite magnetization up to room temperature. Furthermore, the total magnetization of the superlattice depends on the thickness of the LaVO$_3$ layer, indicating an indirect coupling of the magnetization that emerges at adjacent dopant layers. Our results show that geometrically confined doping, like it can be achieved in superlattices, reveals a way to induce otherwise unaccessible phases, possibly even with a large temperature scale.

cond-mat.str-el↗

arXiv2007

Magnetic ordering in the striped nickelate La5/3Sr1/3NiO4: A band structure point of view

We report on a comprehensive study of the electronic and magnetic structure of the striped nickelate La5/3Sr1/3NiO4. The investigation is carried out using band structure calculations based on density functional theory. A magnetic structure compatible with experiment is obtained from spin-polarized calculations within the generalized gradient approximation (GGA), whereas inclusion of a local Coulomb interaction in the LDA+U framework results in a different ground state. The influence of the various interaction parameters is discussed in detail.

cond-mat.str-el↗

arXiv2007

Thickness-dependence of the electronic properties in V2O3 thin films

High quality vanadium sesquioxide V2O3 films (170-1100 Å) were grown using the pulsed laser deposition technique on (0001)-oriented sapphire substrates, and the effects of film thickness on the lattice strain and electronic properties were examined. X-ray diffraction indicates that there is an in-plane compressive lattice parameter (a), close to -3.5% with respect to the substrate and an out-of-plane tensile lattice parameter (c) . The thin film samples display metallic character between 2-300 K, and no metal-to-insulator transition is observed. At low temperature, the V2O3 films behave as a strongly correlated metal, and the resistivity (ρ) follows the equation ρ=ρ_0 + A T^2, where A is the transport coefficient in a Fermi liquid. Typical values of A have been calculated to be 0.14 μΩcm K^{-2}, which is in agreement with the coefficient reported for V2O3 single crystals under high pressure. Moreover, a strong temperature-dependence of the Hall resistance confirms the electronic correlations of these V2O3 thin films samples.

cond-mat.str-el↗

arXiv2006

The antiferromagnetic insulator Ca3FeRhO6: characterization and electronic structure calculations

We investigate the antiferromagnetic insulating nature of Ca3FeRhO6 both experimentally and theoretically. Susceptibility measurements reveal a Neel temperature T_N = 20 K, and a magnetic moment of 5.3 muB/f. u., while Moessbauer spectroscopy strongly suggests that the Fe ions, located in trigonal prismatic sites, are in a 3+ high spin state. Transport measurements display a simple Arrhenius law, with an activation energy of 0.2 eV. The experimental results are interpreted with LSDA band structure calculations, which confirm the Fe 3+ state, the high-spin/low-spin scenario, the antiferromagnetic ordering, and the value for the activation energy.

cond-mat.str-el↗

arXiv2006

Scaling behavior in thermoelectric misfit cobalt oxides

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$. A large negative magnetothermopower is found to scale with both magnetic field and temperature revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S$_0\approx$ 60 $μ$V K$^{-1}$ equal to the high temperature asymptotic value of the spin 1/2 entropy. Low temperature specific heat measurements allow us to determine an enhanced electronic part with $γ\approx$ 50 mJ (mol K$^{2}$)$^{-1}$ attesting of strong correlations. Thereby, a critical comparison between [Bi$_{1.7}$Co$_{0.3}$Ca$_{2}$O$_{4}$]$^{RS}_{0.6}$CoO$_{2}$, other cobaltites as well as other materials reveals a universal behavior of the thermopower low temperature slope as a function of $γ$ testifying thus a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one.

cond-mat.str-el↗

arXiv2005

Melting of the stripe phases in the t-t'-U Hubbard model

We investigate melting of stripe phases in the overdoped regime x>0.3 of the two-dimensional t-t'-U Hubbard model, using a spin rotation invariant form of the slave boson representation. We show that the spin and charge order disappear simultaneously, and discuss a mechanism stabilizing bond-centered and site-centered stripe structures.

cond-mat.str-el↗

arXiv2005

Unconventional Hall effect in oriented Ca$_3$Co$_4$O$_9$ thin films

Transport properties of the good thermoelectric misfit oxide Ca$_3$Co$_4$O$_9$ are examined. In-plane resistivity and Hall resistance measurements were made on epitaxial thin films which were grown on {\it c}-cut sapphire substrates using the pulsed laser deposition technique. Interpretation of the in-plane transport experiments relates the substrate-induced strain in the resulting film to single crystals under very high pressure ($\sim$ 5.5 GPa) consistent with a key role of strong electronic correlation. They are confirmed by the measured high temperature maxima in both resistivity and Hall resistance. While hole-like charge carriers are inferred from the Hall effect measurements over the whole investigated temperature range, the Hall resistance reveals a non monotonic behavior at low temperatures that could be interpreted with an anomalous contribution. The resulting unconventional temperature dependence of the Hall resistance seems thus to combine high temperature strongly correlated features above 340 K and anomalous Hall effect at low temperature, below 100 K.

cond-mat.str-el↗

arXiv2005

Half-filled stripes in the t-t'-U Hubbard model

Using a self-consistent Hartree-Fock approximation we investigate the relative stability of various stripe phases in the extended $t$-$t'$-$U$ Hubbard model. One finds that a negative ratio of next- to nearest-neighbor hopping $t'/t<0$ expells holes from antiferromagnetic domains and reinforces the stripe order. Therefore the half-filled stripes not only accommodate holes but also redistribute them so that the kinetic energy is gained, and these stripes take over in the regime of $t'/t\simeq -0.3$ appropriate for YBa$_2$Cu$_3$O$_{6+δ}$.

cond-mat.str-el↗

arXiv2005

Magnetic ordering in trigonal chain compounds

We present electronic structure calculations for the one-dimensional magnetic chain compounds Ca_3CoRhO_6 and Ca_3FeRhO_6. The calculations are based on density functional theory and the local density approximation. We use the augmented spherical wave (ASW) method. The observed alternation of low- and high-spin states along the Co-Rh and Fe-Rh chains is related to differences in the oxygen coordination of the transition metal sites. Due to strong hybridization the O 2p states are polarized, giving rise to extended localized magnetic moments centered at the high-spin sites. Strong metal-metal overlap along the chains leads to a substantial contribution of the low-spin Rh 4d_{3z^2-r^2} orbitals to the exchange coupling of the extended moments. Interestingly, this mechanism holds for both compounds, even though the coupling is ferromagnetic for the cobalt and antiferromagnetic for the iron compound. However, our results allow to understand the different types of coupling from the filling dependence of the electronic properties.

cond-mat.str-el↗

arXiv2005

Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9

We have performed both in-plane resistivity, Hall effect and specific heat measurements on the thermoelectric cobalt oxide Ca$_{3}$Co$_{4}$O$_{9}$. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to $T_{min}\approx $63 K, a strongly correlated Fermi liquid up to $T^*\approx $140 K, with $ρ=ρ_0+AT^2$ and $A\approx 3.63$ $10^{-2} μΩcm/K^{2}$, followed by an incoherent metal with $k_Fl\leq 1$ and a high temperature insulator above T$^{**}\approx $510 K . Specific heat Sommerfeld coefficient $γ= 93$ mJ/(mol.K$^{2}$) confirms a rather large value of the electronic effective mass and fulfils the Kadowaki-Woods ratio $A/γ^2 \approx 0.45$ 10$^{-5}$ $μΩcm.K^2/(mJ^2mol^{-2})$. Resistivity measurements under pressure reveal a decrease of the Fermi liquid transport coefficient A with an increase of $T^*$ as a function of pressure while the product $A(T^*)^2/a$ remains constant and of order $h/e^2$. Both thermodynamic and transport properties suggest a strong renormalization of the quasiparticles coherence scale of order $T^*$ that seems to govern also thermopower.

cond-mat.str-el↗

arXiv2005

Mean-field phase diagram of interacting e_g electrons

We investigate the magnetic phase diagram of the two-dimensional model for e_g electrons which describes layered nickelates. One finds a generic tendency towards magnetic order accompanied by orbital polarization. For two equivalent orbitals with diagonal hopping such orbitally polarized phases are induced by finite crystal field.

cond-mat.str-el↗

arXiv2005

Interplay of orbitally polarized and magnetically ordered phases in doped transition metal oxides

We investigate the magnetic instabilities of the two-dimensional model of interacting e_g electrons for hole doping away from two electrons per site in the mean-field approximation. In particular, we address the occurrence of orbitally polarized states due to the inequivalent orbitals, and their interplay with ferromagnetic and antiferromagnetic spin order. The role played by the Hund's exchange coupling J_H and by the crystal field orbital splitting E_z in stabilizing one of the competing phases is discussed in detail.

cond-mat.str-el↗

arXiv2004

Extended moment formation and magnetic ordering in the trigonal chain compound Ca3Co2O6

The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca3Co2O6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low- and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal-metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d_{3z^2-r^2} orbitals of the low-spin cobalt atoms.

cond-mat.str-el↗

arXiv1999

One-electron spectral functions of the attractive Hubbard model at intermediate coupling

We calculate the one-electron spectral function of the attractive-U Hubbard model in two dimensions. We work in the intermediate coupling and low density regime and evaluate analytically the self-energy. The results are obtained in a framework based on the self-consistent T-matrix approximation. We also calculate the chemical potential of the bound pairs as a function of temperature. On the basis of this calculation we analyze the low-temperature resistivity and specific heat in the normal state of this system. We compare our results with recent beautiful tunneling experiments in the underdoped regime of HTSC-materials.

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