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R. Mélin

Publications and source records attributed to R. Mélin.

At least 55 records · Page 3Linked to original sources
arXiv2002

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) ``interrupted'' ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically $T_g \simeq 100 - 200$ mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal and C. Monthus, Phys. Rev. Lett. {\bf 80}, 3539 (1998)]. The typical relaxation time scales like $τ^{\rm typ} \sim τ_0 \exp{(T_g/T)}$. The glass cross-over temperature $T_g$ related to correlations among solitons is equal to the average energy barrier and scales like $T_g \sim 2 x ξ_0 Δ$. $x$ is the concentration of defects, $ξ_0$ the correlation length of the CDW or SDW and $Δ$ the charge or spin gap.

cond-mat.stat-mech↗
arXiv2001

Strongly disordered spin ladders

The effect of quenched disorder on the low-energy properties of various antiferromagnetic spin ladder models is studied by a numerical strong disorder renormalization group method and by density matrix renormalization. For strong enough disorder the originally gapped phases with finite topological or dimer order become gapless. In these quantum Griffiths phases the scaling of the energy, as well as the singularities in the dynamical quantities are characterized by a finite dynamical exponent, z, which varies with the strength of disorder. At the phase boundaries, separating topologically distinct Griffiths phases the singular behavior of the disordered ladders is generally controlled by an infinite randomness fixed point.

cond-mat.dis-nn↗
arXiv2001

Unconventional antiferromagnetic correlations of the doped Haldane gap system Y$_2$BaNi$_{1-x}$Zn$_x$O$_5$

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y$_2$BaNi$_{1-x}$Zn$_x$O$_5$. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility $χ(T) \sim C / (Θ+T)$ as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the ``impurity'' susceptibility $χ_{imp}(T)$ by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by $T χ_{imp}(T) = C_{imp} (1 + T_{imp}/T )^{-γ}$. In the temperature range [100 mK, 1 K] the experimental data are well fitted by $T χ_{imp}(T) = A \ln{(T/T_c)}$, where $T_c$ increases with $x$. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility $χ'(T)$ which suggests the existence of antiferromagnetic correlations at very low temperature.

cond-mat.str-el↗
arXiv2001

Microscopic theory of non local pair correlations in metallic F/S/F trilayers

We consider a microscopic theory of F/S/F trilayers with metallic or insulating ferromagnets. The trilayer with metallic ferromagnets is controlled by the formation of non local pair correlations among the two ferromagnets which do not exist with insulating ferromagnets. The difference between the insulating and ferromagnetic models can be understood from lowest order diagrams. Metallic ferromagnets are controlled by non local pair correlations and the superconducting gap is larger if the ferromagnetic electrodes have a parallel spin orientation. Insulating ferromagnets are controlled by pair breaking and the superconducting gap is smaller if the ferromagnetic electrodes have a parallel spin orientation. The same behavior is found in the presence of disorder in the microscopic phase variables and also in the presence of a partial spin polarization of the ferromagnets. The different behaviors of the metallic and insulating trilayers may be probed in experiments.

cond-mat.mes-hall↗
arXiv2001

Transport theory of multiterminal hybrid structures

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 $(e^2/h) (a_0/D)^2 \exp{[-D/ξ(ω^*)]}$, where $D$ is the distance between the contacts, $a_0$ the lattice spacing, $ξ(ω)$ is the superconducting coherence length, and $ω^*$ is the cross-over frequency between a perturbative regime ($ω< ω^*$) and a non perturbative regime ($ω^* < ω< Δ$). 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.

cond-mat.mes-hall↗
arXiv2001

Superconducting crossed correlations in ferromagnets: implications for thermodynamics and quantum transport

It is demonstrated that non local Cooper pairs can propagate in ferromagnetic electrodes having an opposite spin orientation. In the presence of such crossed correlations, the superconducting gap is found to depend explicitly on the relative orientation of the ferromagnetic electrodes. Non local Cooper pairs can in principle be probed with dc-transport. With two ferromagnetic electrodes, we propose a ``quantum switch'' that can be used to detect correlated pairs of electrons. With three or more ferromagnetic electrodes, the Cooper pair-like state is a linear superposition of Cooper pairs which could be detected in dc-transport. The effect also induces an enhancement of the ferromagnetic proximity effect on the basis of crossed superconducting correlations propagating along domain walls.

cond-mat.mes-hall↗
arXiv2001

Electronic EPR-like experiments with superconductors

We discuss a few situations related to non separable correlations in multiterminal hybrid structures. We show that the existence of such correlations can modify the strength of the gap of the superconductor. We discuss linear combinations of non local Cooper pairs. We discuss a possible transport experiment intended to probe non separable correlations. The models are worked out at the heuristic level of effective single site Green's functions.

cond-mat.supr-con↗
arXiv2001

Detection of non separable correlations with dc transport: a new type of Aharonov-Bohm effect

I construct a minimal formalism to describe transport of non separable correlations arising when a superconductor is in contact with several electrodes (being ferromagnetic or normal metal). Transport theory is expressed in terms of effective single site Green's functions. Part of the circuit is decimated exactly by renormalizing only one physical parameter (the density of states). I show that the physical current is obtained by acting on the infinite series of Feynman diagrams with an operator counting the charge of a given diagram. I use this method to propose a new Aharonov-Bohm experiment intended to detect non separable correlations. In this experiment, spin-$σ$ electrons are forced to travel around an Aharonov-Bohm loop in the presence of an applied magnetic flux, while spin-($- σ$) electrons are not directly coupled to the magnetic flux. It is shown that the spin-($-σ$) current oscillates as a function of the flux to which spin-$σ$ electrons are coupled. It is predicted that the effect exists both with high and low transparency contacts, and that the electrodes can be normal metal in which phase coherence can propagate over large distances. Three other aspects of the problem are investigated: (i) The dependence of the superconducting gap on spin polarization of ferromagnetic electrodes connected to the superconductor; (ii) The detection of linear superposition of correlated pairs of electrons; (iii) The proximity effect version of cryptomagnetism.

cond-mat.mes-hall↗
arXiv2000

Néel ordered versus quantum disordered behavior in doped spin-Peierls and Haldane gap systems

I consider a theoretical description of recent experiments on doping the spin-Peierls compound CuGeO$_3$ and the Haldane gap compounds PbNi$_2$V$_2$O$_8$ and Y$_2$BaNiO$_5$. The effective theory is the one of randomly distributed spin-$1/2$ moments interacting with an exchange decaying exponentially with distance. The model has two phases in the (doping, interchain coupling) plane: (i) A Néel ordered phase at small doping; (ii) A quantum disordered phase at larger doping and small interchain interactions. The spin-Peierls compound CuGeO$_3$ and the Haldane gap Nickel oxides PbNi$_2$V$_2$O$_8$ and Y$_2$BaNiO$_5$ fit well into this phase diagram. At small temperature, the Néel phase is found to be reentrant into the quantum disordered region. The Néel transition relevant for CuGeO$_3$ and PbNi$_2$V$_2$O$_8$ can be described in terms of a classical disordered model. A simplified version of this model is introduced, and is solved on a hierarchical lattice structure, which allows to discuss the renormalization group flow of the model. It is found that the system looks non disordered at large scale, which is not against available susceptibility experiments. In the quantum disordered regime relevant for Y$_2$BaNiO$_5$, the two spin model and the cluster RG in the 1D regime show a power law susceptibility, in agreement with recent experiments on Y$_2$BaNiO$_5$. It is found that there is a succession of two distinct quantum disordered phases as the temperature is decreased. The classical disordered model of the doped spin-$1$ chain contains already a physics relevant to the quantum disordered phase.

cond-mat.str-el↗
arXiv2000

Spin accumulation in the semi classical and quantum regimes

We consider spin accumulation at a ferromagnet--normal metal interface in the presence of magnetic scattering in the normal metal. In the classical regime, we discuss the inverse Drude scaling of the conductance as a function of the interface transparencies. We present a treatment based on an exact solution of the Boltzmann equation. In the quantum regime, we solve a single impurity ``spin-flip Fabry Perot interferometer'' for quantum coherent multiple scatterings, in which we find a resonance in the spin flip channels. This resonance appears to be the quantum analog of the semi classical inverse Drude scaling of the conductance.

cond-mat.mes-hall↗
arXiv1999

Antiferromagnetism in a doped spin-Peierls model: classical and quantum behaviors

We address the problem of antiferromagnetism in a two dimensional model of doped spin-Peierls system, at the classical and quantum levels. A Bethe-Peierls solution is derived for the classical model, with an ordering temperature proportional to the doping concentration. The quantum model is treated in a cluster renormalization group showing a finite randomness behavior and an antiferromagnetic susceptibility at low temperature.

cond-mat.str-el↗
arXiv1999

Spin glass behavior upon diluting frustrated magnets and spin liquids: a Bethe-Peierls treatment

A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids is given. A spin glass phase is present at low temperatures and close to the percolation point as soon as frustration takes a finite value in the dilute magnet model; the spin glass phase is reentrant inside the ferromagnetic phase. An extension of the model is given, in which the spin glass / ferromagnet phase boundary is shown not to reenter inside the ferromagnetic phase asymptotically close to the tricritical point whereas it has a turning point at lower temperatures. We conjecture similar phase diagrams to exist in finite dimensional models not constraint by a Nishimori's line. We increase frustration to study the effect of dilution in a spin liquid state. This provides a ``minimal'' ordering by disorder from an Ising paramagnet to an Ising spin glass.

cond-mat.stat-mech↗
arXiv1999

Spin resolved Andreev reflection in ferromagnet-superconductor junctions with Zeeman splitting

Andreev reflection in ferromagnet-superconductor junctions is derived in a regime in which Zeeman splitting dominates the response of the superconductor to an applied magnetic field. Spin-up and spin-down Andreev reflections are shown to be resolved as voltage is increased. In the metallic limit, the transition from Andreev to tunnel conductivity in the spin-up channels has a non trivial behavior when spin polarization is increased. The conductance is asymmetric in a voltage reversal.

cond-mat.supr-con↗
arXiv1999

Spin accumulation in FS junctions

We consider Andreev reflection in ferromagnet-superconductor junctions and extend the treatment by de Jong and Beenakker to incorporate the effect of a repulsive potential at the interface. We show the existence of a spin accumulation at a finite voltage in this model.

cond-mat.supr-con↗
arXiv1999

Phase diagram of doped spin-Peierls systems

The phase diagram of a model describing doped CuGeO$_3$ is derived. The model emphasizes the role of local moments released by the impurities and randomly distributed inside the gaped singlet background. The phase diagram is investigated by two methods: (i) in a mean field treatment of the interchain coupling and (ii) in a real space decimation procedure in a two dimensional model of randomly distributed moments. Both methods lead to similar results, in a qualitative agreement with experiments. In particular, a transition to an inhomogeneous Néel phase is obtained for arbitrary small doping. From the decimation procedure, we interpret this phase at very low doping as a {\sl Griffith antiferromagnet}. Namely, it does not have a true long range order down to zero temperature. Nonetheless, large magnetically ordered clusters appear already at relatively high temperatures. This demonstrates the role of disorder in the theoretical description of doping in CuGeO$_3$. A detailed comparison with other approaches is also given.

cond-mat.str-el↗
arXiv1999

Enhancement of Andreev reflection by spin polarization in out-of-equilibrium ferromagnet-superconductor junctions

We report on transport measurements on Nb/Al/Gd/Al/Nb junctions.Bulk Gadolinium is a weakly polarized ferromagnet (5-7%), and is present in the junction in granular form (superparamagnet). We show that Andreev reflection is strongly enhanced by a weak polarization, obtained by applying an external magnetic field. A new model is proposed that accounts for this effect. The transport is described in terms of ``hot'' carriers that experience Zeeman splitting due to the spin polarized background. The Landauer formula is $G=(4e^2)/h A(h_b)$, $h_b$ being the exchange field at the grain boundary, and A(E) the Andreev current coefficient, that increases with E for resistive junctions.

cond-mat.supr-con↗
arXiv1998

Coarsening on percolation clusters: out-of-equilibrium dynamics versus non linear response

We analyze the violations of linear fluctuation-dissipation theorem (FDT) in the coarsening dynamics of the antiferromagnetic Ising model on percolation clusters in two dimensions. The equilibrium magnetic response is shown to be non linear for magnetic fields of the order of the inverse square root of the number of sites. Two extreme regimes can be identified in the thermoremanent magnetization: (i) linear response and out-of-equilibrium relaxation for small waiting times (ii) non linear response and equilibrium relaxation for large waiting times. The function $X(C)$ characterizing the deviations from linear FDT cross-overs from unity at short times to a finite positive value for longer times, with the same qualitative behavior whatever the waiting time. We show that the coarsening dynamics on percolation clusters exhibits stronger long-term memory than usual euclidian coarsening.

cond-mat.dis-nn↗