arXiv subjects

Xavier Waintal

Publications and source records attributed to Xavier Waintal.

108 records · Page 6Linked to original sources

arXiv2003

Current induced distortion of a magnetic domain wall

We consider the spin torque induced by a current flowing ballistically through a magnetic domain wall. In addition to a global pressure in the direction of the electronic flow, the torque has an internal structure of comparable magnitude due to the precession of the electrons' spins at the "Larmor" frequency. As a result, the profile of the domain wall is expected to get distorted by the current and acquires a periodic sur-structure.

cond-mat.mes-hall↗

arXiv2002

Enhanced mesoscopic fluctuations in the crossover between random matrix ensembles

In random-matrix ensembles that interpolate between the three basic ensembles (orthogonal, unitary, and symplectic), there exist correlations between elements of the same eigenvector and between different eigenvectors. We study such correlations, using a remarkable correspondence between the interpolating ensembles late in the crossover and a basic ensemble of finite size. In small metal grains or semiconductor quantum dots, the correlations between different eigenvectors lead to enhanced fluctuations of the electron-electron interaction matrix elements which become parametrically larger than the non-universal fluctuations.

cond-mat.mes-hall↗

arXiv2001

From the Fermi Liquid Towards the Wigner Solid in Two Dimensions

The quantum-classical crossover from the Fermi liquid towards the Wigner solid is numerically revisited, considering small square lattice models where electrons interact via a Coulomb U/r potential. We review a series of exact numerical results obtained in the presence of weak site disorder for fully polarized electrons (spinless fermions) and when the spin degrees of freedom are included. A novel intermediate regime between the Fermi system of weakly interacting localized particles and the correlated Wigner solid is obtained. A detailed analysis of the non disordered case shows that the intermediate ground state is a solid entangled with an excited liquid. For electrons in two dimensions, this raises the question of the existence of an unnoticed intermediate liquid-solid phase. Using the Coulomb energy to kinetic energy ratio r_s ~ U ~ n_s^{-1/2}, we discuss certain analogies between the numerical results obtained as a function of U for a few particles and the low temperature behaviors obtained as a function of the carrier density n_s in two dimensional electron gases. Notably, the new ``exotic state of matter'' numerically observed at low energies in small clusters occurs at the same intermediate ratios r_s than the unexpected low temperature metallic behavior characterizing dilute electron gases. The finite size effects in the limit of strong disorder are eventually studied in the last section, providing two numerical evidences that the weak coupling Fermi limit is delimited by a second order quantum phase transition when one increases U.

cond-mat.mes-hall↗

arXiv2001

Spin torque in a magnetic trilayer coupled to a superconductor

A ferromagnet-normal-metal-ferromagnet trilayer does not conserve spin current and as a result, the conduction electrons can create a torque on the magnetic moments. When the trilayer is connected to a superconducting electrode, the spin torque created by a current flowing perpendicularly to the trilayer can drive the system to a configuration where the two magnetic moments are perpendicular to each other. Here we argue that in contrast to the non-equilibrium torque, the equilibrium torque (or magnetic exchange interaction) does not single out the perpendicular configuration. We calculate the equilibrium torque for a one-dimensional lattice model and find that it is sensitive to the presence of the superconductivity.

cond-mat.supr-con↗

arXiv2001

Magnetic exchange interaction induced by a Josephson current

We show that a Josephson current flowing through a ferromagnet-normal-metal-ferromagnet trilayer connected to two superconducting electrodes induces an equilibrium exchange interaction between the magnetic moments of the ferromagnetic layers. The sign and magnitude of the interaction can be controlled by the phase difference between the order parameters of the two superconductors. We present a general framework to calculate the Josephson current induced magnetic exchange interaction in terms of the scattering matrices of the different layers. The effect should be observable as the periodic switching of the relative orientation of the magnetic moments of the ferromagnetic layers in the ac Josephson effect.

cond-mat.mes-hall↗

arXiv2001

Current induced switching of magnetic domains to a perpendicular configuration

In a ferromagnet--normal-metal--ferromagnet trilayer, a current flowing perpendicularly to the layers creates a torque on the magnetic moments of the ferromagnets. When one of the contacts is superconducting, the torque not only favors parallel or antiparallel alignment of the magnetic moments, as is the case for two normal contacts, but can also favor a configuration where the two moments are perpendicular. In addition, whereas the conductance for parallel and antiparallel magnetic moments is the same, signalling the absence of giant magnetoresistance in the usual sense, the conductance is greater in the perpendicular configuration. Thus, a negative magnetoconductance is predicted, in contrast with the usual giant magnetoresistance.

cond-mat.mes-hall↗

arXiv2000

The Role of Spin-Dependent Interface Scattering in Generating Current-Induced Torques in Magnetic Multilayers

We present a calculation of current-induced torques in metallic magnetic multilayers derived from the spin-dependent transmission and reflection properties of the magnetic layers. A scattering formalism is employed to calculate the torques in a magnetic-nonmagnetic-magnetic trilayer, for currents perpendicular to the layers, in both the ballistic and diffusive regimes.

cond-mat.mes-hall↗

arXiv2000

Signatures of an intermediate 2d Coulomb phase at low temperatures

The study of the ground state of spinless fermions in 2d disordered clusters (Phys. Rev. Lett. {\bf 83}, 1826 (1999)) has suggested the existence of a new quantum phase for intermediate Coulomb energy to kinetic energy ratios $r_s$. Exact diagonalization of the same small clusters show that its low energy excitations (quantum ergodicity above a few ``hexatic'' excitations characterized by oriented currents) significantly differ from those occuring in the Fermi glass (weak $r_s$) and in the pinned Wigner crystal (large $r_s$). The ``hexatic'' excitations vanish for temperatures of order of the Fermi temperature.

cond-mat↗

arXiv2000

Compressibility crossover and quantum opening of a gap for two dimensional disordered clusters with Coulomb repulsion

Using Hartree-Fock orbitals with residual Coulomb repulsion, we study spinless fermions in a two dimensional random potential. When we increase the system size $L$ at fixed particle density, the size dependence of the average inverse compressibility exhibits a smooth crossover from a $1/L^2$ towards a 1/L decay when the Coulomb energy to Fermi energy ratio $r_s$ increases from 0 to 3. In contrast, the distribution of the first energy excitation displays a sharp Poisson-Wigner-like transition at $r_s \approx 1$.

cond-mat↗

arXiv1999

Delocalized Coulomb phase in two dimensions

Extending finite size scaling theory to the many body ground state, one finds that Coulomb repulsion can drive a system of spinless fermions in a random potential from the Anderson insulator (Fermi glass of localized states) towards a new extended phase in dimension $d=2$. The transition occurs at a Coulomb energy to Fermi energy ratio $r_s \approx 4$, where a change in the nature of the persistent currents has been previously observed. Relevance to the recently observed $2d$ metallic phase is suggested.

cond-mat↗

arXiv1999

Two interacting particles in a disordered chain III: Dynamical aspects of the interplay disorder-interaction

The interplay between the quantum interferences responsible for one particle localization over a length L_1, and the partial dephasing induced by a local interaction of strength U with another particle leading to partial delocalization over a length L_2 > L_1, is illustrated by a study of the motion of two particles put close to each other at the initial time. Localization is reached in two steps. First, before the time t_1 necessary to propagate over L_1, the interaction slows down the ballistic motion. On the contrary, after t_1 the interaction favors a very slow delocalization, characterized by a $\log(t)$ spreading of the center of mass, until L_2 is reached. This slow motion is related to the absence of quantum chaos in this one dimensional model, the interaction being only able to induce weaker chaos with critical spectral statistics. Under appropriate initial conditions, the motion remains invariant under the duality transformation mapping the behavior at small U onto the behavior at large U.

cond-mat.str-el↗

arXiv1999

Metallic phase between the Fermi glass and the Wigner crystal in two dimensions

For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless fermions in a two-dimensional random potential form a new quantum phase, different from the Fermi glass (weakly interacting Anderson localized states) and the Wigner crystal (regular array of charges pinned by the disorder). The intermediate phase is characterized by an ordered flow of persistent currents with a typical value decreasing with excitation energy. Extending finite size scaling analysis to the many body ground state, we find that electron-electron interactions can drive the Fermi glass towards an intermediate metallic phase (Coulomb metal).

cond-mat↗

arXiv1999

Quantum opening of the Coulomb gap in two dimensions

For a constant density of spinless fermions in strongly disordered two dimensional clusters, the energy level spacing between the ground state and the first excitation is studied for increasing system sizes. The average indicates a smooth opening of the gap when the Coulomb energy to Fermi energy ratio $r_s$ increases from 0 to 3, while the distribution exhibits a sharp Poisson-Wigner-like transition at $r_s \approx 1$. The results are related to the transition from Mott to Efros-Shklovskii hopping conductivity recently observed at a similar ratio $r_s$.

cond-mat↗

arXiv1999

A new quantum phase in two dimensions

For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless fermions in a random potential form a new quantum phase which is different from the Fermi glass and the Wigner crystal. From a numerical study of small clusters, we show that this phase is characterized by an ordered flow of enhanced persistent currents and occurs for $r_s$ values where a metallic phase has been observed in two dimensions.

cond-mat↗

arXiv1999

A new quantum phase between the Fermi glass and the Wigner crystal in two dimensions

For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless fermions in a random potential form a new quantum phase which is nor a Fermi glass, neither a Wigner crystal. Studying small clusters, we show that this phase gives rise to an ordered flow of enhanced persistent currents, for disorder strength and ratios $r_s$ where a metallic phase has been recently observed in two dimensions.

cond-mat↗

arXiv1998

New state of the matter between the Fermi glass and the Wigner crystal in two dimensions

Spinless fermions with Coulomb interaction in a square disordered lattice form a new state of the matter which is nor a Fermi glass, neither a Wigner crystal for intermediate Coulomb interaction. From a numerical study of small clusters, we find that this new state occurs between the two critical carrier densities where insulator-metal-insulator transitions of a hole gas have been observed in GaAs.

cond-mat↗

arXiv1998

Two interacting particles in a disordered chain I: Multifractality of the interaction matrix elements

For $N$ interacting particles in a one dimensional random potential, we study the structure of the corresponding network in Hilbert space. The states without interaction play the role of the ``sites''. The hopping terms are induced by the interaction. When the one body states are localized, we numerically find that the set of directly connected ``sites'' is multifractal. For the case of two interacting particles, the fractal dimension associated to the second moment of the hopping term is shown to characterize the Golden rule decay of the non interacting states and the enhancement factor of the localization length.

cond-mat.str-el↗

arXiv1998

Two interacting particles in a disordered chain II: Critical statistics and maximum mixing of the one body states

For two particles in a disordered chain of length $L$ with on-site interaction $U$, a duality transformation maps the behavior at weak interaction onto the behavior at strong interaction. Around the fixed point of this transformation, the interaction yields a maximum mixing of the one body states. When $L \approx L_1$ (the one particle localization length), this mixing results in weak chaos accompanied by multifractal wave functions and critical spectral statistics, as in the one particle problem at the mobility edge or in certain pseudo-integrable billiards. In one dimension, a local interaction can only yield this weak chaos but can never drive the two particle system to full chaos with Wigner-Dyson statistics.

cond-mat.str-el↗