Search

It is shown that the spin resolved current shot noise can probe attractive or repulsive interactions in mesoscopic systems. This is illustrated in two physical situations : i) a normal-superconducting junction where the spin current noise is found to be zero, and ii) a single electron transistor (SET), where the spin current noise is found to be Poissonian. Repulsive interactions may also lead to weak attractive correlations (bunching of opposite spins) in conditions far from equilibrium. Spin current shot noise can be used to measure the spin relaxation time $T_1$, and a set-up is proposed in a quantum dot geometry., Comment: 5 pages, 4 Figures, revised version, added references

Based on the model of F/S/F trilayer with atomic thickness [A. Buzdin and M. Daumens, cond-mat/0305320] we discuss the relative roles of pair-breaking and proximity effects, as a function of the exchange field, of disorder and of a finite thickness in the superconducting layer. The exchange field can be small (weak ferromagnets) or large (strong ferromagnets) compared to the superconducting gap. With weak ferromagnets we show the existence of a reentrant superconducting gap for the F/S/F trilayer with atomic thickness in the parallel alignment (equivalent to the F/S bilayer). Qualitatively small disorder is equivalent to reducing the value of the hopping parameters. In the presence of a finite thickness in the superconducting layer the superconducting gap in the antiparallel alignment is larger than in the parallel alignment, meaning that pair breaking dominates over the proximity effect., Comment: 7 pages, 3 figures

Approaching a two-component tip made of a superconductor (S) and a ferromagnet (F) from a magnetic sample allows for two distinct tunneling processes between the ferromagnets, through S: i) Charge and spin are conserved; ii) Charge and spin are reversed, e.g. a Cooper pair flows from S, one electron going into F, the other into the sample. At subgap voltages, this allows two currents to flow from the tip : one is insensitive to the spin polarizations and allows for surface topography, the other directly tracks the relative spin polarizations of F and the sample. The whole device acts as a STM sensitive to the spin polarization at the Fermi level (MSTM). Its sensitivity is studied and optimized with respect to the tip geometry., Comment: 12 pages, 2 figures

We consider tunneling in a hybrid system consisting of a superconductor with two or more probe electrodes which can be either normal metals or polarized ferromagnets. In particular we study transport at subgap voltages and temperatures. Besides Andreev pair tunneling at each contact, in multi-probe structures subgap transport involves additional channels, which are due to coherent propagation of two particles (electrons or holes), each originating from a different probe electrode. The relevant processes are electron cotunneling through the superconductor and conversion of two electrons stemming from different probes in a Cooper pair. These processes are non-local and decay when the distance between the pair of involved contacts is larger than the superconducting coherence length. The conductance matrix of a the three terminal hybrid structure is calculated. The multi-probe processes enhance the conductance of each contact. If the contacts are magnetically polarized the contribution of the various conduction channels may be separately detected., 7 pages, 1 figure, accepted in Europhysics Letters, minor changes, 3 references added

The crucial role of the cooperative Jahn–Teller deformations in determining the magnetic properties of stoechiometric LaMnO3 is studied by means of exact diagonalization. The layered antiferromagnetic order mainly results form the strong cooperative Jahn–Teller deformations, stabilizing a certain type of orbital ordering. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) can be understood only if the Jahn–Teller energy is much larger than the superexchange couplings, which is consistent with experiments. The crucial role of the deformation anisotropy Q2/Q3 is also emphasized: realistic values of Q2/Q3 are needed to stabilize the layered order. The experimental values of the superexchange couplings can be recovered using reasonable sets of parameters.

We derive a microscopic transport theory of multiterminal hybrid structures in which a superconductor is connected to several spin-polarized electrodes. We discuss the non-perturbative physics of extended contacts, and show that it can be well represented by averaging out the phase of the electronic wave function. The maximal conductance of a two-channel contact is proportional to $(e^2/h) (a_0/D)^2 \exp{[-D/\xi(\omega^*)]}$, where $D$ is the distance between the contacts, $a_0$ the lattice spacing, $\xi(\omega)$ is the superconducting coherence length, and $\omega^*$ is the cross-over frequency between a perturbative regime ($\omega< \omega^*$) and a non perturbative regime ($\omega^* < \omega < \Delta$). The intercontact Andreev reflection and elastic cotunneling conductances are not equal if the electronic phases take a fixed value. However, these two quantities do coincide if one can average out the electronic phase. The equality between the Andreev and cotunneling conductances is also valid in the presence of at least one extended contact in which the phases take deterministic values., Comment: 20 pages, 11 figures, revised version, biblio updated

We consider the phase diagram of a classical fluid in the presence of a random pinning potential of arbitrary strength. Introducing replicas for averaging over the quenched disorder, we use the hypernetted chain approximation to calculate the correlations in the replicated liquid. The freezing transition of the liquid into a nearly crystalline state is studied using a density functional approach, and the liquid-to-glass transition is studied using a phenomenological replica symmetry breaking approach introduced by Mezard and Parisi. The first-order liquid-to-crystal transition is found to change to a continuous liquid-to-glass transition as the strength of the disorder is increased above a threshold value., Comment: 7 pages, 4 figures, to appear in EuroPhysics Letters

An image method is developed in order to analyze the contribution of screening currents to the interaction of vortices with cylindrical defects. In the case of layered superconductors the interaction between elementary (pancake) vortices and a circular dielectric region is calculated. The method can be easily generalized to an arbitrary configuration of vortices, and the deformation of the vortex lattice near a single defect is considered as an example. It also shows that the global effect of a collection of defects is to weaken screening of the magnetic field and renormalize the London penetration length for long-range field components. More generally, the analogy between the London equations at short distances and electrostatics is discussed in detail.

The spectral function of a single electron coupled to Einstein phonons (Holstein model) is calculated exactly in the limit of large space dimensionality, for any coupling strength and phonon frequency. A self-consistent mapping onto an effective impurity model leads to the solution in terms of a continued-fraction expansion for the electron propagator at zero temperature. True self-trapping is obtained only for finite bandwidth, such as the case of a Bethe lattice in infinite dimensions. It is shown that at intermediate coupling multiphonon resonances develop, corresponding to polaron states. Only the lowest one has long lifetime, and the polaron mass is calculated.

Some specific properties of vortices and vortex lines in layered superconductors are reviewed. Emphasis is put on new features originating from very short coherence lengths, especially along the normal to the layers. As far as bulk properties are concerned, vortex lines close to the layer direction can lock-in to the layer direction and experience intrinsic pinning by the layered structure itself. On the other hand, surface properties are also modified with respect to conventional superconductors. This is illustrated by entrance fields at a perfect surface, for both “Josephson” vortices and “pancake” vortices.

The interaction of vortices with cylindrical defects is analyzed by an image method. In the case of layered superconductors, it allows to calculate the interaction between elementary (pancake) vortices and a circular dielectric region. The obtained results are used for analyzing the strong pinning of vortices by dielectric precipitates in the superconducting matrix.

An exact solution for the problem of a point vortex near the lateral surface of a layered superconductor is presented in the limiting case of no Josephson coupling between the layers. The surface barrier against penetration of vortices is discussed for a field normal to the layers. Contrary to the usual Bean-Livingston barrier in a 3D superconductor, an energy minimum is found at a distance of order λ from the surface, and thermal activation allows vortex penetration at fields only slightly higher than Hcl.

The Bean-Livingston entrance field Hs for vortex penetration at the surface is calculated for layered superconductors with Josephson interlayer coupling and field parallel to the layers. Two regimes must be distinguished: close to Tc, one can use the anisotropic London theory, so Hs is of the order of Hc, the thermodynamic critical field, and is the same as for a field normal to the layers. On the opposite, when the transverse coherence length ξc is smaller than the interlayer distance d, Hs becomes smaller than Hc and is of the order of ( ξ c d )H c . Contrary to the entrance field for pure Josephson vortices (fluxons) in junctions, this field is still much larger than the first critical field Hc1∥. This behaviour is a consequence of the specific structure of the vortex core in a layered superconductor and results in a crossover from a linear to a square root temperature dependence as the temperature is lowered.

A study of the dc Josephson current between two superconducting leads in the presence of a precessing classical spin is presented. The precession gives rise to a time-dependent tunnel potential which not only creates different tunneling probabilities for spin-up and spin- down quasiparticles, but also introduces a time-dependent spin-flip term. In particular, we study the effects of the spin-flip term alone on the Josephson current between two spin-singlet superconductors as a function of precession frequency and junction transparency. The system displays a steady-state solution although the magnitude and nature of the current is indeed affected by the precession frequency of the classical spin.

A single Abrikosov line is considered in a field perpendicular to the layers in superconductors with Josephson coupling of the layers. It is shown that the vortex line evaporates above some critical temperature ${\mathit{T}}^{\mathrm{*}}$ in compounds with perpendicular superconducting correlation length ${\ensuremath{\xi}}_{\mathit{z}}$(0)\ensuremath{\ll}d/\ensuremath{\kappa} where \ensuremath{\kappa} is the Ginzburg parameter and d is the interlayer distance. In a system with identical layers, ${\mathit{T}}^{\mathrm{*}}$ coincides with the critical temperature of the Kosterlitz-Thouless transition.

A multilayered superconducting system with a purely electromagnetic coupling between layers is considered. A general formula is derived for the energy of an arbitrary configuration of point vortices localized in the layers. It is shown that the interaction energy of a vortex and an antivortex located on the same layer depends logarithmically on their distance for large distances. The structure of a vortex line in perpendicular fields is also considered. Two different regimes appear, depending on the ratio of the interlayer distance d to the effective one-layer screening length λ eff = λ 2L/ d0 where λL is the London screening length in the layers and do the layer thickness. Due to the temperature dependence of λL, the behaviour of the first critical field H c1 is expected to show a crossover below Tc.

The effect of quantum lattice fluctuations is investigated in a half-filled one-dimensional Peierls-Hubbard model. The nonadiabatic effects due to finite phonon frequency \ensuremath{\omega} are treated by introducing the polaronic transformation and the squeezed-phonon state. The electronic correlations are treated by means of a modified Gutzwiller approach and a unified description for the long-range charge-density-wave (CDW) ordering state and the short-range antiferromagnetic correlated state has been developed. The main result is a rather good description of the continuous variation of the dimerization as functions of the Hubbard U and \ensuremath{\omega} is given, which is in good agreement with that of the numerical simulations. The results of the squeezed-polaron theory are shown to be better than those of the ordinary polaron theory. A continuous phase transition is found between the long-range CDW ordering state and the short-range antiferromagnetic correlated state with a transition point different from the prediction of the Hartree-Fock decoupling. The effect of quantum lattice fluctuations on the transition point is also discussed.

A mean-field approach for the dynamics of a vortex lattice is presented, which is suited for strong pinning. The self-consistent equation of the obtained effective single-particle dynamics is evaluated for simplicity in a reduced dimensionality and exhibits various regimes with temperature and drift velocity.

The spectral function of a single electron coupled to Einstein phonons (Holstein model) is calculated exactly in the limit of large space dimensionality, for any coupling strength and phonon frequency. A self-consistent mapping onto an effective impurity model leads to the solution in terms of a continued fraction expansion for the electron Green function at zero temperature, depending strongly on the shape of the lattice density of states (lorentzian, gaussian or semi-circular). It is also shown that the small polaron states are long-lived only at low energies.

The electronic and magnetic correlations in diatomic molecules are studied rigorously on the basis of a Hubbard Hamiltonian. The problem is treated in three phases separately, firstly for electrons with nondegenerate orbitals, then for electrons with degenerate orbitals, and finally for a mixture of both. In this last case special emphasis is put on the interaction between the electrons in the nondegenerate orbital levels with those in the degenerate orbital levels. In this context some qualitative features of how electronic correlations influence the cohesive forces are sketched. We highlight the extent to which various factors such as the total number of electrons in the system, the Pauli principle, Hund's rule, the level splitting, and the relative size of the transfer integrals are influential in determining the ground-state configurations. Some simple physical guidelines are provided for accurately guessing the various possible ground-state configurations.

An elastic theory of CDW deformations under electric fields is presented, on the basis of a simple phase Hamiltonian. The emphasis is put on the internal rigidity of the modulation which indeed resembles an «electronic solid», and on the anisotropy of its elastic constants. CDW's continous deformations are described by a strain tensor given by the relative variations of the wavevector of the modulation. Stresses occur due to the combined effect of external electric field and pinning points. Under low stresses the globally pinned condensate retains phase continuity: the structure has no defects. Under larger stresses dislocation loops are nucleated involving local amplitude variations of the CDW. We describe the nucleation process for elementary loops under an external field and compare as a consequence thresholds for impurity depinning and boundary depinning (surfaces or interfaces). A characteristic sample length is found under which contact pinning prevails and a coherent deformation (polarization) is built Theorie de la deformation sous champ electrique utilisant un simple hamiltonien de phase, basee sur la rigidite interne de la modulation et sur l'anisotropie des constantes elastiques de l'ODC. Sous faibles contraintes le condensat, globalement ancre, conserve la continuite de phase et la structure est sans defauts; sous de plus fortes contraintes on observe la nucleation de boucles de dislocation. Analyse du processus de nucleation des boucles elementaires et comparaison des seuils de desancrage des impuretes et des bords; longueur caracteristique au-dessous de laquelle l'ancrage aux contacts domine et une deformation coherente (polarisation) apparait

Certain narrow-band materials such as transition metal oxides can be thought of as composites of loosely coupled diamagnetic pairs of 3d electrons. Interactions between these pairs via electron hopping leads to configuration mixing of the two lowest-lying singlet and triplet states for the isolated pairs, which introduces antiferromagnetic correlation between them. A Green function formalism is developed which rigorously takes into account the electronic properties of the isolated pairs and treats their interaction in a self-consistent way. As a function of the interpair hopping rate, the authors calculate the density of states, spin correlations and internal energy for a corundum structure-a case of an idealised Ti2O3.

With use of the symmetry properties of the half-filled single-band Hubbard Hamiltonian, there is derived an effective Hamiltonian on a decimated lattice in which the spin and charge operators occur explicitly. Being a generalization of the Blume-Emery-Griffiths Hamiltonian for ${\mathrm{He}}^{3}$-${\mathrm{He}}^{4}$ mixtures, this new statistical mechanical model permits one to give a preliminary discussion of the phase diagram of the correlated electron gas by establishing analogies with their results.

Lattice modulations are usually coupled with charge density waves (CDW) in crystals which are conductors in the higher temperature range where the lattice modulations are unstable [1, 2, 3] (Figure 1).

Edge states are one-dimensional transport channels, emerging in quantum wells in the integer Quantum Hall regime, with remarkable properties of chirality and quantum coherence. In this thesis we present the idea of manipulating electronic currents mixed over two co-propagating edge channels, and discuss its potential impact for quantum interferometry and transport of spin-qubit states. We introduce the characteristics of edge states and evaluate the effect of local, non adiabatic potentials and their efficiency to transfer charge between two channels. We show that sharp potential variations whose energies are smaller than the Landau gap provide weak mixing, and we identify some experimental strategies that can achieve good mixing percentages. We develop numerical techniques of simulation to model existing experiments that employ mixed edge channels, and analytical methods in order to treat the effect of Coulomb interactions between edge states in a future spin-interferometry experiment.; Les états de bord sont des canaux de transport unidimensionnels qui se développent dans des puits quantiques en régime d'Effet Hall entier, avec de remarquables propriétés de chiralité et de cohérence quantique. Dans cette thèse nous présentons l'idée d'une manipulation de courants électroniques mettant en jeu le mélange de deux canaux de bord co-propageants, et nous discutons son impact potentiel pour l'interférométrie quantique et le transport de qubits de spin. Nous présentons les caractéristiques des états de bord et évaluons l'effet de potentiels locaux et non-adiabatiques, et de leur efficacité pour transférer la charge entre les deux canaux. Il est montré que des variations rapides du potentiel, d'amplitude plus petite que le gap de Landau, donnent lieu à un faible mélange, et nous identifions des stratégies expérimentales permettant d'atteindre un bon pourcentage de mélange. Nous développons des techniques de simulation numérique afin de modéliser de expériences qui mettent en jeu des canaux avec mélange, ainsi que des méthodes analytiques permettant de traiter les interactions coulombiennes entre états de bord, en vue de futures expériences d'interférométrie de spin.

Edge states are one-dimensional transport channels, emerging in quantum wells in the integer Quantum Hall regime, with remarkable properties of chirality and quantum coherence. In this thesis we present the idea of manipulating electronic currents mixed over two co-propagating edge channels, and discuss its potential impact for quantum interferometry and transport of spin-qubit states. We introduce the characteristics of edge states and evaluate the effect of local, non adiabatic potentials and their efficiency to transfer charge between two channels. We show that sharp potential variations whose energies are smaller than the Landau gap provide weak mixing, and we identify some experimental strategies that can achieve good mixing percentages. We develop numerical techniques of simulation to model existing experiments that employ mixed edge channels, and analytical methods in order to treat the effect of Coulomb interactions between edge states in a future spin-interferometry experiment., Les états de bord sont des canaux de transport unidimensionnels qui se développent dans des puits quantiques en régime d'Effet Hall entier, avec de remarquables propriétés de chiralité et de cohérence quantique. Dans cette thèse nous présentons l'idée d'une manipulation de courants électroniques mettant en jeu le mélange de deux canaux de bord co-propageants, et nous discutons son impact potentiel pour l'interférométrie quantique et le transport de qubits de spin. Nous présentons les caractéristiques des états de bord et évaluons l'effet de potentiels locaux et non-adiabatiques, et de leur efficacité pour transférer la charge entre les deux canaux. Il est montré que des variations rapides du potentiel, d'amplitude plus petite que le gap de Landau, donnent lieu à un faible mélange, et nous identifions des stratégies expérimentales permettant d'atteindre un bon pourcentage de mélange. Nous développons des techniques de simulation numérique afin de modéliser de expériences qui mettent en jeu des canaux avec mélange, ainsi que des méthodes analytiques permettant de traiter les interactions coulombiennes entre états de bord, en vue de futures expériences d'interférométrie de spin.

In this manuscript, we are interested in a quantum dotconnected to three reservoirs. The Kondo cloud essentially develops inthe third reservoir which is strongly coupled to the quantum dot. Theother two reservoirs are weakly coupled to the quantum dot and areused to probe the system by transport measurements.After modeling such a three-terminal geometry, we have calculated the conductance matrix at zero temperature by Fermi liquids theory.In the remaining of the manuscript, we focus on the case where the third reservoir has a finite size, which confers to its density ofstate a peaks structure.We have first studied the system by the perturbative renormalisation group and have evaluated the Kondo temperature, which is the main energy scale of the problem. Next, we have calculated the conductance matrix of the system atdifferent temperatures. For temperatures much stronger than the Kondotemperature, we have used a perturbative approach. For temperaturesmuch weaker than the Kondo temperature, we have used a Fermi liquidtype theory. In the intermediate regime, we have used a numericalmethod called slave bosons mean field theory. In the latter regime, we have also performed a spectroscopic analysis of the dot density of states., Dans cette thèse, nous nous intéressons à un point quantiqueconnecté à trois réservoirs. Le nuage Kondo se développeessentiellement dans le troisième réservoir, fortement couplé au pointquantique, alors que les deux autres réservoirs, faiblement couplés aupoint quantique, servent à sonder le système par des mesures detransport.Après avoir modélisé une telle géométrie triterminale, nousavons calculé la matrice de conductance à température nulle par lathéorie des liquides de Fermi.Dans le reste de la thèse, nous nous sommes intéressés au casoù le troisième réservoir est de taille finie, ce qui confère à sa densité d'état une structure en pics.Dans un premier temps, nous avons étudié le système par legroupe de renormalisation perturbatif et nous avons calculé latempérature Kondo, principale échelle d'énergie du problème.Ensuite, nous avons calculé la matrice de conductance dusystème dans différents régimes de température. Pour des températurestrès grandes devant la température Kondo, nous avons utilisé uneapproche perturbative. Pour des températures très petites devant latempérature Kondo, nous avons utilisé une théorie de type liquides deFermi. Et dans le régime intermédiaire, nous avons utilisé une méthodenumérique appelée théorie des bosons esclaves en champ moyen. Dans cedernier régime a également été menée à terme une analysespectroscopique de la densité d'état du point quantique.

In this manuscript, we are interested in a quantum dotconnected to three reservoirs. The Kondo cloud essentially develops inthe third reservoir which is strongly coupled to the quantum dot. Theother two reservoirs are weakly coupled to the quantum dot and areused to probe the system by transport measurements.After modeling such a three-terminal geometry, we have calculated the conductance matrix at zero temperature by Fermi liquids theory.In the remaining of the manuscript, we focus on the case where the third reservoir has a finite size, which confers to its density ofstate a peaks structure.We have first studied the system by the perturbative renormalisation group and have evaluated the Kondo temperature, which is the main energy scale of the problem. Next, we have calculated the conductance matrix of the system atdifferent temperatures. For temperatures much stronger than the Kondotemperature, we have used a perturbative approach. For temperaturesmuch weaker than the Kondo temperature, we have used a Fermi liquidtype theory. In the intermediate regime, we have used a numericalmethod called slave bosons mean field theory. In the latter regime, we have also performed a spectroscopic analysis of the dot density of states.; Dans cette thèse, nous nous intéressons à un point quantiqueconnecté à trois réservoirs. Le nuage Kondo se développeessentiellement dans le troisième réservoir, fortement couplé au pointquantique, alors que les deux autres réservoirs, faiblement couplés aupoint quantique, servent à sonder le système par des mesures detransport.Après avoir modélisé une telle géométrie triterminale, nousavons calculé la matrice de conductance à température nulle par lathéorie des liquides de Fermi.Dans le reste de la thèse, nous nous sommes intéressés au casoù le troisième réservoir est de taille finie, ce qui confère à sa densité d'état une structure en pics.Dans un premier temps, nous avons étudié le système par legroupe de renormalisation perturbatif et nous avons calculé latempérature Kondo, principale échelle d'énergie du problème.Ensuite, nous avons calculé la matrice de conductance dusystème dans différents régimes de température. Pour des températurestrès grandes devant la température Kondo, nous avons utilisé uneapproche perturbative. Pour des températures très petites devant latempérature Kondo, nous avons utilisé une théorie de type liquides deFermi. Et dans le régime intermédiaire, nous avons utilisé une méthodenumérique appelée théorie des bosons esclaves en champ moyen. Dans cedernier régime a également été menée à terme une analysespectroscopique de la densité d'état du point quantique.