Magnetic properties of the three-dimensional Hubbard model at half filling

We study the magnetic properties of the 3d Hubbard model at half-filling in the TPSC formalism, previously developed for the 2d model. We focus on the N\'eel transition approached from the disordered side and on the paramagnetic phase. We find a very good quantitative agreement with Dynamical Mean-Field results for the isotropic 3d model. Calculations on finite size lattices also provide satisfactory comparisons with Monte Carlo results up to the intermediate coupling regime. We point out a qualitative difference between the isotropic 3d case, and the 2d or anisotropic 3d cases for the double occupation factor. Even for this local correlation function, 2d or anisotropic 3d cases are out of reach of DMF: this comes from the inability of DMF to account for antiferromagnetic fluctuations, which are crucial.Comment: RevTex, 9 pages +10 figure

Crossover from two- to three-dimensional critical behavior for nearly antiferromagnetic itinerant electrons

The crossover from two- to three-dimensional critical behavior of nearly antiferromagnetic itinerant electrons is studied in a regime where the inter-plane single-particle motion of electrons is quantum-mechanically incoherent because of thermal fluctuations. This is a relevant regime for very anisotropic materials like the cuprates. The problem is studied within the Two-Particle Self-Consistent approach (TPSC), that has been previously shown to give a quantitative description of Monte Carlo data for the Hubbard model. It is shown that TPSC belongs to the $n\rightarrow \infty$ limit of the $O\left( n\right)$ universality class. However, contrary to the usual approaches, cutoffs appear naturally in the microscopic TPSC theory so that parameter-free calculations can be done for Hubbard models with arbitrary band structure. A general discussion of universality in the renormalized-classical crossover from $d=2$ to $d=3$ is also given.Comment: Revtex, 23 pages + 6 postcript figures (with epsfile

Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping

A combination of analytical approaches and quantum Monte Carlo simulations is used to study both magnetic and pairing correlations for a version of the Hubbard model that includes second-neighbor hopping $t^{\prime }=-0.35t$ as a model for high-temperature superconductors. Magnetic properties are analyzed using the Two-Particle Self-Consistent approach. The maximum in magnetic susceptibility as a function of doping appears both at finite $$ and at $t^{\prime }=0$ but for two totally different physical reasons. When $t^{\prime }=0$, it is induced by antiferromagnetic correlations while at $t^{\prime }=-0.35t$ it is a band structure effect amplified by interactions. Finally, pairing fluctuations are compared with $$-matrix results to disentangle the effects of van Hove singularity and of nesting on superconducting correlations. The addition of antiferromagnetic fluctuations increases slightly the $d$-wave superconducting correlations despite the presence of a van Hove singularity which tends to decrease them in the repulsive model. Some aspects of the phase diagram and some subtleties of finite-size scaling in Monte Carlo simulations, such as inverted finite-size dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure

Pairing fluctuations and pseudogaps in the attractive Hubbard model

The two-dimensional attractive Hubbard model is studied in the weak to intermediate coupling regime by employing a non-perturbative approach. It is first shown that this approach is in quantitative agreement with Monte Carlo calculations for both single-particle and two-particle quantities. Both the density of states and the single-particle spectral weight show a pseudogap at the Fermi energy below some characteristic temperature T*, also in good agreement with quantum Monte Carlo calculations. The pseudogap is caused by critical pairing fluctuations in the low-temperature renormalized classical regime $\omega < T$ of the two-dimensional system. With increasing temperature the spectral weight fills in the pseudogap instead of closing it and the pseudogap appears earlier in the density of states than in the spectral function. Small temperature changes around T* can modify the spectral weight over frequency scales much larger than temperature. Several qualitative results for the s-wave case should remain true for d-wave superconductors.Comment: 20 pages, 12 figure

Le modèle de Hubbard à faible densité et à proximité du demi-remplissage : quelques aspects

Ce travail, qui concerne quelques aspects du modèle de Hubbard, se divise en deux volets. Dans une première partie, nous étudions une approximation simple basée sur l'idée de renormalisation de l'interaction par les effets à courte portée. Elle se justifie à basse densité et possède la propriété de satisfaire les relations de croisement fermioniques. Des comparaisons détaillées aux résultats de simulations Monte Carlo pour les fonctions de corrélation de spin, de charge et de paire de diverses symétries, nous permettent d'identifier les effets essentiels dans ce régime d'interaction modérée, et de préciser les régions où les effets non triviaux se manifestent. Cette approximation est également reliée à la théorie des liquides de Fermi, et nous permet d'interpréter les résultats à faible remplissage de bande, et sur petits réseaux, d'un point de vue liquide de Fermi faiblement corrélé. Dans une deuxième partie, nous nous placerons à proximité du demi-remplissage de bande, pour lequel une approche récemment développée est particulièrement adéquate: elle prédit conformément au théorème de Mermin-Wagner, l'absence de transition magnétique à température finie en 2D. Cette théorie satisfait quelques exigences des relations de croisement. Nous nous intéresserons à l'apparition et à la caractérisation d'une transition de phase antiferromagnétique par l'inclusion d'un faible effet tridimensionnel. Quelques comparaisons aux résultats expérimentaux sur le composé de La₂CuO₄ sont discutées

Phénoménologie du transfert et de la délocalisation électronique : traitement exact d'un modèle simple

Un modèle simple de la délocalisation et du transfert d'électron induits par un potentiel dépendant du temps est résolu numériquement. L'équation de Schrodinger étudiée est celle d'une particule plongée dans un potentiel à une dimension, constitué de plusieurs potentiels à courte portée (fonctions delta de Dirac). Les résultats, obtenus sans traitement perturbatif, concernent la probabilité d'ionisation dépendante du temps, le spectre d'énergie et le temps de transfert

Phénoménologie du transfert et de la délocalisation électronique : traitement exact d'un modèle simple

Un modèle simple de la délocalisation et du transfert d'électron induits par un potentiel dépendant du temps est résolu numériquement. L'équation de Schrodinger étudiée est celle d'une particule plongée dans un potentiel à une dimension, constitué de plusieurs potentiels à courte portée (fonctions delta de Dirac). Les résultats, obtenus sans traitement perturbatif, concernent la probabilité d'ionisation dépendante du temps, le spectre d'énergie et le temps de transfert

Theoretical demonstration of a hot-carrier effect in ultra-thin solar-cell

International audienceBased on a quantum modeling of the electronic transport, this work shows that ultra-thin solar cells can exhibit an improved open-circuit voltage VOC, without current reduction. This improvement is obtained when an energy-selective contact is considered between the absorber and the reservoir, and is attributed to a hot-carrier effect. While extraction with a nonselective contact does not generate hot carriers, the use of energy-selective contact induces an increase of carrier temperature up to 130 K and a corresponding VOC enhancement of 41 meV, considering an (In,Ga)As absorber. This enhancement agrees with a simple and general expression formulated in the quantum thermal machine field. Concerning the current, we show that current through an energy-selective contact is of the same order of magnitude as the one obtained without selectivity. This remarkable behavior, which is explained by the hybridization of states in the absorber with the state of the contact, requires a quantum confinement and thus an ultrathin absorber. Finally, we propose a simple rate model enabling an intuitive interpretation of the numerical results