arXiv subjects

Claudine Lacroix

Publications and source records attributed to Claudine Lacroix.

18 recordsLinked to original sources

arXiv2025

From chessboard of bipolarons of size 4a in cubic La7/8Sr1/8MnO3 to stripes of the same bipolarons in layered high Tc cuprates

The compound La1-xSrxMnO3 exhibits a charge order (CO) state at $x\approx 1/8$ and $T T_{co}$ which become ordered on a chessboard organized in a 3D-order state of ferromagnetically paired polarons at $T<T_{co}$. Applied to the CuO$_2$ planes of the cuprates of the "214" family, this model produces stripes of bipolarons intertwined with stripes of antiferromagnetically arranged spins, hole-poor, both of size $4a$, leading to a spin density wave with a wave vector $δ=1/8$, a charge density wave with $q=1/4$, the Yamada laws $δ(x)=x$ and $T_c\propto δ$ and a decrease of $T_c$ at x=1/8. This work invokes relevance of a bipolaronic origin of high $T_c$ superconductivity, in which bipolarons of size $4a$ can play a major role.

cond-mat.supr-con↗

arXiv2024

Emergence and tunability of Fermi-pocket and electronic instabilities in layered Nickelates

Layered Nickelates have gained intensive attention as potential high-temperature superconductors, showing similarities and subtle differences to well-known Cuprates. This study introduces a modelling framework to analyze the tunability of electronic structures by focusing on effective orbitals and additional Fermi pockets, mimicking doping or external pressure qualitatively. It investigates the role of the $3d_{z^2}$ orbital in interlayer hybridization, which leads to the formation of a second pocket in the Fermi surface. The resulting effective model also predicts specific charge and spin susceptibility in the form of Lindhard susceptibility at wave vector $\mathbf{q_{0}} = (π, π)$, which can be tuned by doping or pressure. These results provide valuable insights into tunable orbital contributions and their influence on potential ordering and electronic instabilities in Layered Nickelates.

cond-mat.supr-con↗

arXiv2023

Metamagnetic transition in the two $f$ orbitals Kondo lattice model

In this work, we study the effects of a transverse magnetic field in a Kondo lattice model with two $f$ orbitals interacting with the conduction electrons. The $f$ electrons that are present on the same site interact through Hund's coupling, while on neighboring sites they interact through intersite exchange. We consider here that part of $f$ electrons are localized (orbital 1) while another part (orbital 2) are delocalized, as it is frequent in uranium systems. Then, only electrons in the localized orbital 1 interact through exchange interaction with the neighboring ones, while electrons in orbital 2 are coupled with conduction electrons through a Kondo interaction. We obtain a solution where ferromagnetism and Kondo effect coexist for small values of an applied transverse magnetic field for $T\rightarrow0$. Increasing the transverse field, two situations can be obtained when Kondo coupling vanishes: first, a metamagnetic transition occurs just before or at the same time of the fully polarized state, and second, a metamagnetic transition occurs when the spins are already pointing out along the magnetic field.

cond-mat.str-el↗

arXiv2021

Photo-emission signatures of coherence breakdown in Kondo alloys: dynamical mean-field theory approach

We study the Kondo alloy model on a square lattice using dynamical mean-field theory (DMFT) for Kondo substitution and disorder effects, together with static mean-field approximations. We computed and analyzed photoemission properties as a function of electronic filling $n_c$, Kondo impurity concentration $x$, and strength of Kondo temperature $T_K$. We provide a complete description of the Angle Resolved Photoemission Spectroscopy (ARPES) signals expected in the paramagnetic Kondo phases. By analyzing the Fermi surface, we observe the Lifshitz-like transition predicted previously for strong $T_K$ at $x=n_c$ and we discuss the evolution of the dispersion from the dense coherent to the dilute Kondo regimes. At smaller $T_K$, we find that this transition marking the breakdown of coherence at $x=n_c$ becomes a crossover. However, we identify another transition at a smaller concentration $x^\star$ where the effective mass continuously vanishes. $x^\star$ separates the one-branch and the two-branches ARPES dispersions characterizing respectively dilute and dense Kondo paramagnetic regimes. The $x-T_K$ phase diagrams are also described, suggesting that the transition at $x^\star$ might be experimentally observable since magnetically ordered phases are stabilized at much lower $T_K$. Fermi surface reconstructions in antiferromagnetic and ferromagnetic phases are also discussed.

cond-mat.str-el↗

arXiv2018

Breakdown of coherence in Kondo alloys: crucial role of concentration vs band filling

We study the low energy states of the Kondo alloy model (KAM) as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling JK, that may be separated by a transition at x=nc. Here, we analyze the KAM for finite JK on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination Z=infty, we show that the real part of the local potential scattering may be located outside the conduction electron band, revealing a possible breakdown of Luttinger 'theorem' for intermediate values of impurity concentration x. Unusual physical signatures are expected, e.g., in ARPES experiments. In order to take into account fluctuations associated with finite dimensionality,i.e., finite Z, we extend this analyze by also studying the KAM with an adaptation of the statistical-DMFT method that was developped elsewhere. We review the distributions of local potential scattering and their evolution with model parameters: concentration, strength of Kondo coupling, coordination number, local site neighborhood, connection with percolation issue. Relevence for Kondo alloys material with f-electrons is also discussed.

cond-mat.str-el↗

arXiv2016

s- and d-wave superconductivity in a two-band model

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials as the high Tc oxides, pnictides and heavy fermions. In this work we study a realistic model that includes the relevant physics of superconductivity in the presence of strong Coulomb correlations. We consider a two-band model, since most of these correlated systems have electrons from at least two different atomic orbitals coexisting at their Fermi surface. The Coulomb repulsion is taken into account through a local repulsive interaction. Pairing is considered among quasi- particles in neighbouring sites and we allow for different symmetries of the order parameter. In order to deal with the strong local correlations, we use the well known slave boson approach that has proved very successful for this problem. Here we are interested in obtaining the zero temperature properties of the model, specifically its phase diagram and the existence and nature of superconducting quantum critical points. We show that these can arise by increasing the mixing between the two bands. Since this can be controlled by external pressure or doping, our results have a direct relation with experiments. We show that the superconductor-to-normal transition can be either to a metal, a correlated metal or to an insulator. Also we compare the relative stability of s and d-wave paired states for different regions of parameter space and investigate the BCS- BEC crossover in the two-band lattice model as function of the strength of the pairing interaction.

cond-mat.supr-con↗

arXiv2014

Spin liquid versus long range magnetic order in the frustrated body-centered tetragonal lattice

The quantum Heisenberg model is studied in the geometrically frustrated body-centered tetragonal lattice(BCT lattice) with antiferromagnetic interlayer coupling J1 and intralayer first and second neighbor coupling J2 and J3. We introduce a variational method: each interaction term can be decoupled partially in the purely magnetic Weiss and in the spin-liquid (SL) mean-field channels. We find that the most stable variational solutions correspond to the three different possible long range magnetic orders that are respectively governed by J1, J2, and J3. We characterize three different purely SL non-magnetic solutions that are variationally the second most stable states after the purely magnetic ones. This suggests that quantum fluctuations induced by the frustration between J1-J2-J3 coupling should destroy magnetic orders and stabilize the formation of SL in large areas of parameters. The SL solution governed by J1 breaks the lattice translation symmetry. This Modulated SL is associated to a commensurate ordering wave vector (1,1,1). We discuss the relevance of our results for heavy fermion Uru2Si2 and cuprate superconductors that have a BCT lattice structure. Also, the general variational method introduced here can be applied to any other system where interaction terms can be decoupled in two different mean-field channels.

cond-mat.str-el↗

arXiv2014

Effect of anisotropy in the $S=1$ underscreened Kondo lattice

We study the effect of crystal field anisotropy in the underscreened $S=1$ Kondo lattice model. Starting from the two orbital Anderson lattice model and including a local anisotropy term, we show, through Schrieffer-Wolff transformation, that local anisotropy is equivalent to an anisotropic Kondo interaction ($J_{\parallel} \neq{J_{\perp}}$). The competition and coexistence between ferromagnetism and Kondo effect in this effective model is studied within a generalized mean-field approximation. Several regimes are obtained, depending on the parameters, exhibiting or not coexistence of magnetic order and Kondo effect. Particularly, we show that a re-entrant Kondo phase at low temperature can be obtained. We are also able to describe phases where the Kondo temperature is smaller than the Curie temperature ($T_K<T_C$). We propose that some aspects of uranium and neptunium compounds that present coexistence of Kondo effect and ferromagnetism, can be understood within this model.

cond-mat.str-el↗

arXiv2013

Lifshitz transition in Kondo alloys

We study the low energy states of Kondo alloys as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. Using two complementary approaches, the mean-field coherent potential approximation and the strong coupling limit, we identify and characterize two different Fermi liquid regimes. We propose that both regimes are separated by a Lifshitz transition at x = nc. Indeed, we predict a discontinuity of the number of quasiparticles which are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around x = nc.

cond-mat.str-el↗

arXiv2011

Direct observation of the influence of the FeAs4 tetrahedron on superconductivity and antiferromagnetic correlations in Sr2VO3FeAs

We measure the pressure dependence of the electrical resistivity and the crystal structure of iron superconductor Sr2VO3FeAs. Below ~10 GPa the structure compresses but remains undeformed, with regular FeAs4 tetrahedrons, and a constant Tc. Beyond 10GPa, the tetrahedron strongly distorts, while Tc goes gradually to zero. Band structure calculations of the undistorted structure show multiple nesting features that hinder the development of an antiferromagnetic ground state (AF), allowing the appearance of superconductivity. The deformation of the tetrahedra that breaks band degeneracy degrades multiple nesting, thus favouring one particular AF state at the expense of Tc.

cond-mat.supr-con↗

arXiv2008

Chiral two-dimensional electron gas in a periodic magnetic field

We study the energy spectrum and electronic properties of two-dimensional electron gas in a periodic magnetic field of zero average with a symmetry of triangular lattice. We demonstrate how the structure of electron energy bands can be changed with the variation of the field strength, so that we can start from nearly free electron gas and then transform it continuously to a system of essentially localized chiral electron states. We find that the electrons near some minima of the effective potential are responsible for occurrence of dissipationless persistent currents creating a lattice of current contours. The topological properties of the electron energy bands are also varied with the intensity of periodic field. We calculated the topological Chern numbers of several lower energy bands as a function of the field. The corresponding Hall conductivity is nonzero and, when the Fermi level lies in the gap, it is quantized.

cond-mat.str-el↗

arXiv2008

Ising Like Order by Disorder In The Pyrochlore Antiferromagnet with Dzyaloshinskii-Moriya Interactions

It is shown that the mechanism of order out of disorder is at work in the antisymmetric pyrochlore antiferromagnet. Quantum as well as thermal fluctuations break the continuous degeneracy of the classical ground state manifold and reduce its symmetry to $\mathbb{Z}_3 \times \mathbb{Z}_2$. The role of anisotropic symmetric exchange is also investigated and we conclude that this discrete like ordering is robust with respect to these second order like interactions. The antisymmetric pyrochlore antiferromagnet is therefore expected to order at low temperatures, whatever the symmetry type of its interactions, in both the classical and semi classical limits.

cond-mat.str-el↗

arXiv2006

Anomalous Hall Effect due to the spin chirality in the Kagomé lattice

We consider a model for a two dimensional electron gas moving on a kagomé lattice and locally coupled to a chiral magnetic texture. We show that the transverse conductivity $σ\_{xy}$ does not vanish even if spin-orbit coupling is not present and it may exhibit unusual behavior. Model parameters are the chirality, the number of conduction electrons and the amplitude of the local coupling. Upon varying these parameters, a topological transition characterized by change of the band Chern numbers occur. As a consequence, $σ\_{xy}$ can be quantized, proportional to the chirality or have a non monotonic behavior upon varying these parameters.

cond-mat.str-el↗

arXiv2003

Electronic states and magnetic excitations in LiV2O4: Exact diagonalization study

Motivated by recent inelastic neutron scattering experiment we examine magnetic properties of LiV2O4. We consider a model which describes the half-filled localized A1g spins interacting via frustrated antiferromagnetic Heisenberg exchange and coupled by local Hund's interaction with the 1/8-filled itinerant Eg band, and study it within an exact diagonalization scheme. In the present study we limited the analysis to the case of the cluster of two isolated tetrahedrons. We obtained that both the ground state structure and low-lying excitations depend strongly on the value of the Hund's coupling which favors the triplet states. With increasing temperature the triplet states become more and more populated which results in the formation of non-zero residual magnetic moment. We present the temperature dependence of calculated magnetic moment and of the spin-spin correlation functions at different values of Hund's coupling and compare them with the experimental results.

cond-mat.str-el↗

arXiv2001

Dzyaloshinski-Moriya interactions in the kagome lattice

The kagomé lattice exhibits peculiar magnetic properties due to its strongly frustated cristallographic structure, based on corner sharing triangles. For nearest neighbour antiferromagnetic Heisenberg interactions there is no Neel ordering at zero temperature both for quantum and classical s pins. We show that, due to the peculiar structure, antisymmetric Dzyaloshinsky-Moriya interactions (${\bf D} . ({\bf S}_i \times {\bf S}_j)$) are present in this latt ice. In order to derive microscopically this interaction we consider a set of localized d-electronic states. For classical spins systems, we then study the phase diagram (T, D/J) through mean field approximation and Monte-Carlo simulations and show that the antisymmetric interaction drives this system to ordered states as soon as this interaction is non zero. This mechanism could be involved to explain the magnetic structure of Fe-jarosites.

cond-mat.str-el↗

arXiv2001

Comparison of several tetrahedra-based lattices

A comparison of the quantum Heisenberg antiferromagnetic model (QHAM) on the pyrochlore lattice, the checkerboard lattice and the square lattice with crossing interactions is performed. The three lattices are constructed with the same tetrahedral unit cell and this property is used to describe the low energy spectrum by means of an effective hamiltonian restricted to the singlet sector. We analyze the structure of the effective hamiltonian and solve it within mean field approximation for the three lattices.

cond-mat.str-el↗

arXiv1998

Kondo Screening and Magnetic Ordering in Frustrated UNi4B

UNi4B exhibits unusual properties and, in particular, a unique antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the low temperature behavior of this compound, we propose that the remaining 1/3 U sites are nonmagnetic due to the Kondo effect. We derive a model in which the coexistence of magnetic and nonmagnetic U sites is the consequence of the competition between frustration of the crystallographic structure and instability of the 5f moments.

cond-mat.str-el↗

arXiv1998

Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid

The quantum pyrochlore antiferromagnet is studied by perturbative expansions and exact diagonalization of small clusters. We find that the ground state is a spin-liquid state: The spin-spin correlation functions decay exponentially with distance and the correlation length never exceeds the interatomic distance. The calculated magnetic neutron diffraction cross section is in very good agreement with experiments performed on Y(Sc)Mn2. The low energy excitations are singlet-singlet ones, with a finite spin gap.

cond-mat.str-el↗