arXiv subjects

Raymond Fresard

Publications and source records attributed to Raymond Fresard.

At least 37 records · Page 2Linked to original sources

arXiv2007

Spin-rotationally symmetric domain flux phases in underdoped cuprates

We propose a new form of inhomogeneous phases consisting of out-of-phase staggered flux domains separated by diagonal charged domain walls centered on bonds or on sites. Remarkably, such domain flux phases are spin-rotationally symmetric and exhibit cone-like quasiparticle dispersion as well as incommensurate order of orbital currents. Such features are consistent with the pseudogap behavior and the diagonal stripes observed experimentally in lightly doped cuprates. A renormalized mean field theory shows that such solutions are competitive candidates within the $t$--$J$ model.

cond-mat.str-el↗

arXiv2006

Effect of the next-nearest neighbor hopping on the stability and band structure of the incommensurate phases in the cuprates

Using a spin-rotation invariant version of the slave-boson approach we investigate the relative stability and band structure of various incommensurate phases in the cuprates. Our findings obtained in the Hubbard model with next-nearest neighbor hopping $-t'/t\simeq 0.15$, as appropriate for the La$_{2-x}$Sr$_x$CuO$_4$ family, support the formation of diagonal (vertical) stripe phases in the doping regime $x=1/16$ ($x=1/8$), respectively. In contrast, based on the fact that a larger value $-t'/t=0.3$ expected for YBa$_2$Cu$_3$O$_{6+δ}$ triggers a crossover to the diagonal (1,1) spiral phase at increasing doping, we argue that it might explain why the static charge order has been detected in YBa$_2$Cu$_3$O$_{6+δ}$ only in the highly underdoped regime.

cond-mat.str-el↗

arXiv2006

Mechanism of the reorientation of stripes in the cuprates

Using the mean field theory in the slave-boson approach we analyzed the electron correlation effects in the stripe phases. One finds that a finite next-nearest neighbor hopping $t'$ plays an important role in the low doping regime, where it controls the crossover from the filled diagonal to half-filled vertical/horizontal stripes at doping $ x\simeq 1/16$.

cond-mat.str-el↗

arXiv2006

Magnetic and orbital correlations in a two-site molecule

We analyze the role of orbital degeneracy in possible magnetic and orbital instabilities by solving exactly a two-site molecule with two orbitals of either $e_g$ or $t_{2g}$ symmetry at quarter-filling. As a generic feature of both models one finds that the spin and orbital correlations have opposite signs in the low temperature regime when the orbitals are degenerate, in agreement with the Goodenough-Kanamori rules. While Hund's exchange coupling $J_H$ induces ferromagnetic spin correlations in both models, it is more efficient for $t_{2g}$ orbitals where the orbital quantum number is conserved along the hopping processes. We show that the ground state and finite temperature properties may change even qualitatively with increasing Coulomb interaction when the crystal field splitting of the two orbitals is finite, and the Goodenough-Kanamori rules may not be followed.

cond-mat.str-el↗

arXiv2006

Interplay between incommensurate phases in the cuprates

We establish the qualitative behavior of the incommensurability $ε$, optimal domain wall filling $ν$ and chemical potential $μ$ for increasing doping by a systematic slave-boson study of an array of vertical stripes separated by up to $d=11$ lattice constants. Our findings obtained in the Hubbard model with the next-nearest neighbor hopping $t'=-0.15t$ agree qualitatively with the experimental data for the cuprates in the doping regime $x\lesssim 1/8$. It is found that $t'$ modifies the optimal filling $ν$ and triggers the crossover to the diagonal (1,1) spiral phase at increasing doping, stabilized already at $x\simeq 0.09$ for $t'=-0.3t$.

cond-mat.str-el↗

arXiv2006

Slave-boson approach to the metallic stripe phases with large unit cells

Using a rotationally invariant version of the slave-boson approach in spin space we analyze the stability of stripe phases with large unit cells in the two-dimensional Hubbard model. This approach allows one to treat strong electron correlations in the stripe phases relevant in the low doping regime, and gives results representative of the thermodynamic limit. Thereby we resolve the longstanding controversy concerning the role played by the kinetic energy in stripe phases. While the transverse hopping across the domain walls yields the largest kinetic energy gain in the case of the insulating stripes with one hole per site, the holes propagating along the domain walls stabilize the metallic vertical stripes with one hole per two sites, as observed in the cuprates. We also show that a finite next-nearest neighbor hopping $t'$ can tip the energy balance between the filled diagonal and half-filled vertical stripes, which might explain a change in the spatial orientation of stripes observed in the high $T_c$ cuprates at the doping $x\simeq 1/16$.

cond-mat.str-el↗

arXiv2006

Microscopic origin of diagonal stripe phases in doped nickelates

We investigate the electron density distribution and the stability of stripe phases in the realistic two-band model with hopping elements between e_g orbitals at Ni sites on the square lattice, and compare these results with those obtained for the doubly degenerate Hubbard model with two equivalent orbitals and diagonal hopping. For both models we determine the stability regions of filled and half-filled stripe phases for increasing hole doping x=2-n in the range of x<0.4, using Hartree-Fock approximation for large clusters. In the parameter range relevant to the nickelates, we obtain the most stable diagonal stripe structures with filling of nearly one hole per atom, as observed experimentally. In contrast, for the doubly degenerate Hubbard model the most stable stripes are somewhat reminiscent of the cuprates, with half-filled atoms at the domain wall sites. This difference elucidates the crucial role of the off-diagonal e_g hopping terms for the stripe formation in La_2-xSr_xNiO_4. The influence of crystal field is discussed as well.

cond-mat.str-el↗

arXiv2005

Stripe phases - possible ground state of the high-Tc superconductors

Based on the mean-field method applied either to the extended single-band Hubbard model or to the single-band Peierls-Hubbard Hamiltonian we study the stability of both site-centered and bond-centered charge domain walls. The difference in energy between these phases is found to be small. Therefore, moderate perturbations to the pure Hubbard model, such as next nearest hopping, lattice anisotropy, or coupling to the lattice, induce phase transitions, shown in the corresponding phase diagrams. In addition, we determine for stable phases charge and magnetization densities, double occupancy, kinetic and magnetic energies, and investigate the role of a finite electron-lattice coupling. We also review experimental signatures of stripes in the superconducting copper oxides.

cond-mat.str-el↗

arXiv2005

Non-linear effects and Joule heating in I-V curves in manganites

We study the influence of the Joule effect on the non-linear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit cell parameters as an internal thermometer in X-ray and neutron diffraction. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath are at the origin of the non-linearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.

cond-mat.str-el↗

arXiv2001

Induced Ferromagnetism and Colossal Magnetoresistance by Ir-Doping in Pr1-xCaxMnO3

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_{1-x}Ca_xMnO_3 (0.4 < x < 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180K and CMR ratio in 7T as large as 10^4. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr, ...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state.

cond-mat.str-el↗

arXiv2001

Large Thermopower in Metallic Oxides: Misfit Cobaltites and Mangano-Ruthenates

Two different kinds of metal transition oxides have been studied for their large thermopower values. The first one corresponds to the Tl-based misfit cobaltite which is a hole-doped metal. We demonstrate that the partial Bi-substitution for Tl in this phase induces an increase of the room temperature (RT) thermopower (TEP) value. Same result is obtained with the new Pb_{1/3}SrCoO_{3+delta} misfit corresponding to the Tl complete replacement by lead. Simultaneously, the T dependence of their resistivity exhibits a re-entrance below 70-90K where a large negative magnetoresistance is observed. Magnetic measurements reveal a strong interplay between spins and charges for this class of materials. Electron-doped (n-type) perovskite manganites are a second class of potential candidates for applications. In particular, the Ru^{4+/5+} substitution for Mn in the CaMnO_3 semi-conductor induces a drastic drop of the resistivity values. Metals with large RT TEP values and not too large thermal conductivities are generated. A comparison with best known materials, Bi_2Te_3 and NaCo_2O_4 is made.

cond-mat.str-el↗

arXiv2001

Ferromagnetism in a Realistic Two-Band Model: A Slave-Boson Study

Using a slave boson representation of multi-band Hubbard models, we investigate a two-band model relevant to layered perovskites in the vicinity of half-filling. Beside the strong influence of the Hund's rule coupling, we obtain that the phase diagram separates into two regions: a weak to moderate coupling region where the effective mass is weakly renormalized, and a strong coupling regime where it is strongly renormalized. The transition between these two regimes is very sharp. It takes place in a (vanishingly) small domain. A ferromagnetic instability in only found in the strongly correlated regime, and is triggered by the Hund's rule coupling. The results are compared to La-doped layered ruthenates.

cond-mat.str-el↗

arXiv2001

Rhodium Doped Manganites : Ferromagnetism and Metallicity

The possibility to induce ferromagnetism and insulator to metal transitions in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown for the first time. Colossal magnetoresistance (CMR) properties are evidenced for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior.

cond-mat.mtrl-sci↗

arXiv2001

Slave Bosons in Radial Gauge: the Correct Functional Integral Representation and Inclusion of Non-Local Interactions

We introduce a new path integral representation for slave bosons in the radial gauge which is valid beyond the conventional fluctuation corrections to a mean-field solution. For electronic lattice models, defined on the constrained Fock space with no double occupancy, all phase fluctuations of the slave particles can be gauged away if the Lagrange multipliers which enforce the constraint on each lattice site are promoted to time-dependent fields. Consequently, only the amplitude (radial part) of the slave boson fields survives. It has the special property that it is equal to its square in the physical subspace. This renders the functional integral for the radial field Gaussian, even when non-local Coulomb-type interactions are included. We propose i) a continuum integral representation for the set-up of further approximation schemes, and ii) a discrete representation with an Ising-like radial variable, valid for long-ranged interactions as well. The latter scheme can be taken as a starting point for numerical evaluations.

cond-mat.str-el↗

arXiv1997

Spin and Charge Structure Factor of the 2-d Hubbard Model

The spin and charge structure factors are calculated for the Hubbard model on the square lattice near half-filling using a spin-rotation invariant six-slave boson representation. The charge structure factor shows a broad maximum at the zone corner and is found to decrease monotonically with increasing interaction strength and electron density and increasing temperature. The spin structure factor develops with increasing interaction two incommensurate peaks at the zone boundary and along the zone diagonal. Comparison with results of Quantum Monte Carlo and variational calculations is carried out and the agreement is found to be good. The limitations of an RPA-type approach are pointed out.

cond-mat.str-el↗

arXiv1996

Interplay of Mott Transition and Ferromagnetism in the Orbitally Degenerate Hubbard Model

A slave boson representation for the degenerate Hubbard model is introduced. The location of the metal to insulator transition that occurs at commensurate densities is shown to depend weakly on the band degeneracy M. The relative weights of the Hubbard sub-bands depend strongly on M, as well as the magnetic properties. It is also shown that a sizable Hund's rule coupling is required in order to have a ferromagnetic instability appearing. The metal to insulator transition driven by an increase in temperature is a strong function of it.

cond-mat.str-el↗

arXiv1996

Temperature Dependence of Transport Coefficients in Liquid and Amorphous Metals

We apply the muffin-tin effective medium approximation to calculate the temperature dependence of the resistivity and of the thermopower of amorphous and liquid metals. The results show unambiguously that a large resistivity is accompanied by a negative temperature coefficient, in agreement with the experimental situation. This behavior is shown to result from a pseudo-gap which opens in the 1-particle spectrum due to strong scattering at the quasi zone boundary. In turn the thermopower is found to have non-trivial density and temperature dependences.

cond-mat↗

arXiv1996

Metal to Insulator Transition in the 2-D Hubbard Model: A Slave Boson Approach

We use the Kotliar-Ruckenstein slave boson technique to treat the strong correlation of the Hubbard model. In a first part we discuss the metal to insulator transition that is occurring on the honeycomb lattice at half filling. We find the critical value for the interaction strength to be U/t=3.1. Above it a gap opens in the spectrum which behaves like (U-U_c) in the vicinity of the transition, and like U for strong coupling. In a second part we calculate the phase diagram of the Hubbard model on the square lattice at finite temperature. The Mermin-Wagner theorem is fulfilled in the largest part of the phase diagram. In the strong coupling regime, the uniform magnetic susceptibility shows a maximum in its doping dependence in agreement with numerical simulations.

cond-mat↗