arXiv subjects

M. Avignon

Publications and source records attributed to M. Avignon.

18 recordsLinked to original sources

arXiv2014

Magnetic polaron structures in the one-dimensional double and super-exchange model

An analytical and numerical study of the one-dimensional double and super-exchange model is presented. A phase separation between ferromagnetic and anti-ferromagnetic phases occurs at low super-exchange interaction energy. When the super-exchange interaction energy gets larger, the conduction electrons are self-trapped within separate small magnetic polarons. These magnetic polarons contain a single electron inside two or three sites depending on the conduction electron density and form a Wigner crystallization. A new phase separation is found between these small polarons and the anti-ferromagnetic phase. Our results could explain the spin-glass-like behavior observed in the nickelate one-dimensional compound Y(2-x)CaxBaNiO5.

cond-mat.str-el↗

arXiv2014

Spin and charge ordering in three-leg ladders in oxyborates

We study the spin ordering within the 3-leg ladders present in the oxyborate Fe(3)O(2)BO(3) consisting of localized classical spins interacting with conduction electrons (one electron per rung). We also consider the competition with antiferromagnetic superexchange interactions to determine the magnetic phase diagram. Beside a ferromagnetic phase we find (i) a phase with ferromagnetic rungs ordered antiferromagnetically (ii) a zig-zag canted spin ordering along the legs. We also determine the induced charge ordering within the different phases and the interplay with lattice instability. Our model is discussed in connection with the lattice dimerization transition observed in this system, emphasizing on the role of the magnetic structure.

cond-mat.str-el↗

arXiv2014

Spin-polarons in an exchange model

Spin-polarons are obtained using an Ising-like exchange model consisting of double and super-exchange interactions in low dimensional systems. At zero temperature, a new phase separation between small magnetic polarons, one conduction electron self-trapped in a magnetic domain of two or three sites, and the anti-ferromagnetic phase was previously reported. On the other hand the important effect of temperature was missed. Temperature diminishes Boltzmann probability allowing excited states in the system. Static magnetic susceptibility and short-range spin-spin correlations at zero magnetic field were calculated to explore the spin-polaron formation. At high temperature Curie-Weiss behavior is obtained and compared with the Curie-like behavior observed in a nickelate one-dimensional compound.

cond-mat.str-el↗

arXiv2013

On Ultrafast Spin Dynamics: Spin Dependent Fast Response of Hot Electrons, of Band--Structure

Different energy shifts for majority and minority electrons occur. Thus, for example in case of (laser) excited ferromagnetic metals majority and minority electrons may respond differently in time during closing the exchange splitting. Spin flip transitions of the hot electrons due to electron interactions cause quasi hybridization of the spin split states. This is also the case in itinerant ferromagnetic metals due to hopping between sites having magnetic moments pointing in direction of the magnetization (+) and opposite direction (-) and with energy levels $\varepsilon^+_{iσ}$ and $\varepsilon^-_{iσ}$. For energetic reasons the molecular field acts asymmetrically on the spins of the electrons and on spin flip transitions and thus causes different lifetimes of minority and majority electrons and spin dependent electron energy shifts. Quite general minority hot electrons in spin split states may respond faster than majority electrons at non--equilibrium. The molecular field acting on the spins delays spin flip transitions $\uparrow \rightarrow \downarrow$ and thus a response of the hot majority electrons and their energy levels. The closing of the exchange splitting in the electron spectrum of ferromagnetic transition and rare--earth metals, ferromagnetic semiconductors, spin split quantum well states in thin ferromagnetic films, etc. will reflect this. The time and spin dependent energy shifts of electrons at non--equilibrium may cause interesting behavior, in particular of magnetic tunnel junctions, spin currents etc.. In ferromagnets the moment reversal lifetime of (local) magnetic moments parallel to the global magnetization is larger than of moments pointing in opposite direction.

cond-mat.mes-hall↗

arXiv2012

Spin-orbit induced chirality of Andreev states in Josephson junctions

We study Josephson junctions (JJs) in which the region between the two superconductors is a multichannel system with Rashba spin-orbit coupling (SOC) where a barrier or a quantum point contact (QPC) is present. These systems might present unconventional Josephson effects such as Josephson currents for zero phase difference or critical currents that \textit{depend on} the current direction. Here, we discuss how the spin polarizing properties of the system in the normal state affect the spin characteristic of the Andreev bound states inside the junction. This results in a strong correlation between the spin of the Andreev states and the direction in which they transport Cooper pairs. While the current-phase relation for the JJ at zero magnetic field is qualitatively unchanged by SOC, in the presence of a weak magnetic field a strongly anisotropic behavior and the mentioned anomalous Josephson effects follow. We show that the situation is not restricted to barriers based on constrictions such as QPCs and should generically arise if in the normal system the direction of the carrier's spin is linked to its direction of motion.

cond-mat.mes-hall↗

arXiv2008

The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions

We study the effect of Coulomb interactions in transition metal oxides junctions. In this paper we analyze charge transfer at the interface of a three layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose a charge model considering a few atomic planes within each layer and obtain results for the magnetic coupling between the ferromagnetic layers. For large number of planes in the paramagnetic spacer we find that the coupling oscillates with the same period as in RKKY but the amplitude is sensitive to the Coulomb energy. At small spacer thickness however, large differences may appear as function of : the number of electrons per atom in the ferromagnetics and paramagnetics materials, the dielectric constant at each component, and the charge defects at the interface plane emphasizing the effects of charge transfer.

cond-mat.str-el↗

arXiv2008

Anomalous Josephson Current in Junctions with Spin-Polarizing Quantum Point Contacts

We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects.

cond-mat.mes-hall↗

arXiv2006

Surface criticality and multifractality at localization transitions

We develop the concept of surface multifractality for localization-delocalization (LD) transitions in disordered electronic systems. We point out that the critical behavior of various observables related to wave functions near a boundary at a LD transition is different from that in the bulk. We illustrate this point with a calculation of boundary critical and multifractal behavior at the 2D spin quantum Hall transition and in a 2D metal at scales below the localization length.

cond-mat.mes-hall↗

arXiv2004

Magnetic Moment Formation in Quantum Point Contacts

We study the formation of local magnetic moments in quantum point contacts. Using a Hubbard-like model to describe point contacts formed in a two dimensional system, we calculate the magnetic moment using the unrestricted Hartree approximation. We analyze different type of potentials to define the point contact, for a simple square potential we calculate a phase diagram in the parameter space (Coulomb repulsion - gate voltage). We also present an analytical calculation of the susceptibility to give explicit conditions for the occurrence of a local moment, we present a simple scaling argument to analyze how the stability of the magnetic moment depends on the point contact dimensions.

cond-mat.mes-hall↗

arXiv2004

Spin order in the one-dimensional Kondo and Hund lattices

We study numerically the one-dimensional Kondo and Hund lattices consisting of localized spins interacting antiferro or ferromagnetically with the itinerant electrons, respectively. Using the Density Matrix Renormalization Group we find, for both models and in the small coupling regime, the existence of new magnetic phases where the local spins order forming ferromagnetic islands coupled antiferromagnetically. Furthermore, by increasing the interaction parameter $|J|$ we find that this order evolves toward the ferromagnetic regime through a spiral-like phase with longer characteristic wave lengths. These results shed new light on the zero temperature magnetic phase diagram for these models.

cond-mat.str-el↗

arXiv2003

Charge and spin inhomogeneous phases in the Ferromagnetic Kondo Lattice Model

We study numerically the one-dimensional ferromagnetic Kondo lattice. This model is widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic polarons induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion. There is an insulating gap associated to the charge structure formation. We also study the insulator-metal transition induced by a magnetic field which removes simultaneously both charge and spin ordering.

cond-mat.str-el↗

arXiv2001

Phase diagram and optical conductivity of the one-dimensional spinless Holstein model

The effects of quantum lattice fluctuations on the Peierls transition and the optical conductivity in the one-dimensional Holstein model of spinless fermions have been studied by developing an analytical approach, based on the unitary transformation method. We show that when the electron-phonon coupling constant decreases to a finite critical value the Peierls dimerization is destroyed by the quantum lattice fluctuations. The dimerization gap is much more reduced by the quantum lattice fluctuations than the phonon order parameter. The calculated optical conductivity does not have the inverse-square-root singularity but have a peak above the gap edge and there exists a significant tail below the peak. The peak of optical-conductivity spectrum is not directly corresponding to the dimerized gap. Our results of the phase diagram and the spectral-weight function agree with those of the density matrix renormalization group and the exact diagonalization methods.

cond-mat.str-el↗

arXiv2000

Island phases and charge order in two-dimensional manganites

The ferromagnetic Kondo lattice model with an antiferromagnetic interaction between localized spins is a minimal description of the competing kinetic t and magnetic K energy terms which generate the rich physics of manganite systems. Motivated by the discovery in one dimension of homogeneous ``island phases'', we consider the possibility of analogous phases in higher dimensions. We characterize the phases present at commensurate fillings, and consider in detail the effects of phase separation in all filling and parameter regimes. We deduce that island and flux phases are stable for intermediate values of K/t at the commensurate fillings n = 1/4, 1/3, 3/8, and 1/2. We discuss the connection of these results to the charge and magnetic ordering observed in a wide variety of manganite compounds.

cond-mat.str-el↗

arXiv2000

Ferromagnetic Polarons in Manganites

Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h \simeq t).

cond-mat.str-el↗

arXiv1999

New Type of Charge and Magnetic order in the Ferromagnetic Kondo Lattice

We study numerically the one dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. By including a nearest neighbor Coulomb interaction (V) and a superexchange interaction between the localized moments (K), we obtain the phase diagram in parameter space for several dopings at T=0. Due to the competition between double and superexchange, we find a region where the formation of magnetic polarons induces a charge ordered (CO) state which survives also for V=0. This mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.

cond-mat.str-el↗

arXiv1998

Electron-Doped Manganese Perovskites: The Polaronic State

Using the Lanczos method in linear chains we study the ground state of the double exchange model including an antiferromagnetic super-exchange in the low concentration limit. We find that this ground state is always inhomogeneous, containig ferromagnetic polarons. The extention of the polaron spin distortion, the dispersion relation and their trapping by impurities, are studied for diferent values of the super exchange interaction and magnetic field. We also find repulsive polaron polaron interaction.

cond-mat↗

arXiv1996

Dynamical Mean Field Theory for Perovskites

Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states with perovskite geometry, we present a dynamical mean field theory which becomes exactin the limit of large coordination numbers or equivalently large spatial dimensions $D$. The theory is based on a new description of these systems for large $D$ using a selective treatment of different hopping processes which can not be generated by a unique scaling of the hopping element. The model is solved using a perturbational approach and an extended non-crossing approximation. We discuss the breakdown of the perturbation theory near half filling, the origin of the various 3d and 2p bands, the doping dependence of its spectral weight, and the evolution of quasi particles at the Fermi-level upon doping, leading to interesting insight into the dynamical character of the charge carriers near the metal insulator instability of transition metal oxide systems, three dimensional perovskites and other strongly correlated transition metal oxides.

cond-mat↗

arXiv1995

A new approach for perovskites in large dimensions

Using the Hubbard Hamiltonian for transition metal-3d and oxygen-2p states with perovskite geometry, we propose a new scaling procedure for a nontrivial extension of these systems to large spatial dimensions $D$. The scaling procedure is based on a selective treatment of different hopping processes for large $D$ and can not be generated by a unique scaling of the hopping element. The model is solved in the limit $D \rightarrow \infty$ by the iterated perturbation theory and using an extended non-crossing approximation. We discuss the evolution of quasi particles at the Fermi-level upon doping, leading to interesting insight into the dynamical character of the charge carriers near the metal insulator instability of transition metal oxide systems, three dimensional perovskites and other strongly correlated transition metal oxides.

cond-mat↗