Publications (163)416.39 Total impact
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ABSTRACT: We propose a theory of longitudinal resistivity in the normal phase of quasionedimensional organic superconductors near the quantum critical point where antiferromagnetism borders with superconductivity under pressure. The linearized semiclassical Boltzmann equation is solved numerically, fed in by the halffilling electronic umklapp scattering vertex as derived from oneloop renormalization group calculations for the quasionedimensional electron gas model. The momentum and temperature dependence of umklapp scattering has an important impact on the behaviour of longitudinal resistivity in the the normal phase. Resistivity is found to be linear in temperature around the quantum critical point at which spindensitywave order joins superconductivity along the antinesting axis, to gradually evolve towards the Fermi liquid behaviour in the limit of weak superconductivity. A comparison is made between theory and experiments performed on the (TMTSF)$_2$PF$_6$ member of the Bechgaard salt series under pressure.  [Show abstract] [Hide abstract]
ABSTRACT: The renormalization group technique is applied to onedimensional electronphonon Hubbard models at halffilling and zero temperature. For the HolsteinHubbard model, the results of oneloop calculations are congruent with the phase diagram obtained by quantum Monte Carlo simulations in the $(U,g_{\rm ph})$ plane for the phononmediated interaction $g_{\rm ph}$ and the Coulomb interaction $U$. The incursion of an intermediate phase between a fully gapped chargedensitywave state and a Mott antiferromagnet is supported along with the growth of its size with the molecular phonon frequency $\omega_0$. We find additional phases enfolding the base boundary of the intermediate phase. A Luttinger liquid line is found below some critical $ U^*\approx g^*_{\rm ph}$, followed at larger $U\sim g_{\rm ph}$ by a narrow region of bondorderwave ordering which is either charge or spin gapped depending on $U$. For the PeierlsHubbard model, the region of the $(U,g_{\rm ph})$ plane with a fully gapped Peierlsbondorderwave state shows a growing domination over the Mott gapped antiferromagnet as the Debye frequency $\omega_D$ decreases. A power law dependence $g_{\rm ph} \sim U^{2\eta}$ is found to map out the boundary between the two phases, whose exponent is in good agreement with the existing quantum Monte Carlo simulations performed when a finite nearestneighbor repulsion term $V$ is added to the Hubbard interaction. 
Article: Role of electronphonon interaction in a magnetically driven mechanism for superconductivity
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ABSTRACT: We use the renormalization group method to examine the effect of phonon mediated interaction on dwave superconductivity, as driven by spin fluctuations in a quasionedimensional electron system. The influence of a tightbinding electronphonon interaction on the spindensitywave and dwave superconducting instability lines is calculated for arbitrary temperature, phonon frequency and antinesting of the Fermi surface.The domain of electronphonon coupling strength where spindensitywave order becomes unstable against the formation of a bondorderwave or Peierls state is determined at weak antinesting. We show the existence of a positive isotope effect for spindensitywave and dwave superconducting critical temperatures which scales with the antinesting distance from quantum critical point where the two instabilities merge. We single out a low phonon frequency zone where the bondoderwave ordering gives rise to triplet fwave superconductivity under nesting alteration, with both orderings displaying a negative isotope effect. We also study the electronphonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasionedimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spindensitywave state under pressure is emphasized.  [Show abstract] [Hide abstract]
ABSTRACT: We report an investigation of charge, spin, and lattice effects in the spinPeierls state of the organic compound MEM(TCNQ)2. The 16.5GHz dielectric function along the chain axis shows an enhancement below the spin Peierls transition temperature near 18 K consistent with the charge coupling to the elastic strain involved in the transition. The velocity of two elastic modes perpendicular to the chain axis presents anomalies at the transition, which can be explained with a Landau freeenergy model including a linearquadratic coupling energy term between the appropriate elastic strain e and the spinPeierls magnetic gap �q . The analysis of the dielectric and elastic features aims toward an order parameter with an associated critical exponent β ∼ 0.36, which is similar to the threedimensional behavior seen in other spinPeierls materials. All these effects studied in a magnetic field up to 18 Teslas appear also compatible with a meanfield model of a quasionedimensional spinPeierls system.  [Show abstract] [Hide abstract]
ABSTRACT: We report an investigation of charge, spin and lattice effects in the spinPeierls state of the organic compound MEM(TCNQ)$_2$. The 16.5 GHz dielectric function along the chain axis shows an enhancement below the spinPeierls transition temperature near 18 K consistent with the charge coupling to the elastic strain involved in the transition. The velocity of two elastic modes perpendicular to the chain axis presents anomalies at the transition which can be explained with a Landau free energy model including a linearquadratic coupling energy term between the appropriate elastic strain $e$ and the spinPeierls magnetic gap $\Delta_q$. The analysis of the dielectric and elastic features aims toward an order parameter with an associated critical exponent $\beta \sim$ 0.36, which is similar to the threedimensional behavior seen in other spinPeierls materials. All these effects studied in a magnetic field up to 18 Teslas appear also compatible with a meanfield model of a quasionedimensional spinPeierls system.  [Show abstract] [Hide abstract]
ABSTRACT: An inductive method is used to follow the magneticfield–dependent susceptibility of the coupled charge density wave (CDW) and spinPeierls (SP) ordered state behavior in the dualchain organic conductor perylene2[Pt(mnt)2]. In addition to the coexisting SPCDW state phase below 8 K and 20 T, the measurements show that a second spingapped phase appears above 20 T that coincides with a fieldinduced insulating phase. The results support a strong coupling of the CDW and SP order parameters even in high magnetic fields, and provide new insight into the nature of the magnetic susceptibility of dualchain spin and charge systems.  [Show abstract] [Hide abstract]
ABSTRACT: The Peierls instability in onedimensional electronphonon systems is known to be qualitatively well described by the meanfield theory, however the related selfconsistent problem so far has only been able to predict a partial suppression of the transition even with proper account of classical lattice fluctuations. Here the HartreeFock approximation scheme is extended to the full quantum regime, by mapping the momentumfrequency spectrum of orderparameter fluctuations onto a continuous twoparameter space. For the onedimensional halffilled SuSchriefferHeeger model the ratio d=Ω/2π Tc0, where Ω is the characteristic phonon frequency and 2π Tc0 the lowest finite phonon Matsubara frequency at the meanfield critical point Tc0, provides a natural measure of the adiabaticity of lattice fluctuations. By integrating out finitefrequency phonons, it is found that a variation of d from the classical regime d=0 continuously connects Tc0 to a zerotemperature chargedensitywave transition setting up at a finite crossover d=dc. This finite crossover decreases within the range 0≤ d≈ 1 as the electronphonon coupling strength increases but remaining small enough for weakcoupling considerations to still hold. Implications of Tc suppression on the Ginzburg criterion is discussed, and evidence is given of a possible coherent description of the chargedensitywave problem within the framework of a renormalized meanfield theory encompassing several aspects of the transition including its thermodynamics close to the quantum critical point. 
Article: On the Origin of the Anomalous Upper Critical Field in QuasiOneDimensional Superconductors
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ABSTRACT: Upper critical field, H_c2, in quasi1D superconductors is investigated by the weak coupling renormalization group technique. It is shown that H_c2 greatly exceeds not only the Pauli limit, but also the conventional paramagnetic limit of the FludeFerrellLarkinOvchinnikov (FFLO) state. This increase is mainly due to quasi1D fluctuations effect as triggered by interference between unconventional superconductivity and densitywave instabilities. Our results give a novel viewpoint on the large H_c2 observed in TMTSFsalts in terms of a dwave FFLO state that is predicted to be verified by the H_c2 measurements under pressure.  [Show abstract] [Hide abstract]
ABSTRACT: We report an ultrasonic study of the magnetoelastic coupling of the hydrogenated and deuterated (TMTTF)2PF6 organic salts. For both salts the temperature dependence of the longitudinal velocity along the c∗ axis displays a monotonic stiffening of the C33 compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhanced, in concomitance with the reduction of spin degrees of freedom previously seen in magnetic measurements as lowdimensional precursors of the spinPeierls transition. The magnetoelastic coupling appears to be much weaker in the hydrogenated salt due to the highly inhomogeneous elastic behavior induced by the proximity of the charge ordering transition to the spinPeierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spinPeierls transition temperature TSP in agreement with scaling of the elastic deformation with the spinPeierls order parameter. In spite of the weakly inhomogeneous character of the spinPeierls phase transition, the magnetic field dependence of TSP is well captured with the meanfield prediction for the lattice distorted Heisenberg spin chain.  [Show abstract] [Hide abstract]
ABSTRACT: We report an ultrasonic study of the magnetoelastic coupling of the hydrogenated and deuterated (TMTTF)2PF6 organic salts. For both salts the temperature dependence of the longitudinal velocity along the c* axis displays a monotonic stiffening of the C33 compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhanced, in concomitance with the reduction of spin degrees of freedom previously seen in magnetic measurements as lowdimensional precursors of the spinPeierls transition. The magnetoelastic coupling appears to be much weaker in the hydrogenated salt due to the highly inhomogeneous elastic behavior induced by the proximity of the charge ordering transition to the spinPeierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spinPeierls transition temperature TSP in agreement with scaling of the elastic deformation with the spinPeierls order parameter. In spite of the weakly inhomogeneous character of the spinPeierls phase transition, the magnetic field dependence of TSP is well captured with the meanfield prediction for the lattice distorted Heisenberg spin chain.  [Show abstract] [Hide abstract]
ABSTRACT: Comparing resistivity data of the quasionedimensional superconductors (TMTSF)(2)PF(6) and (TMTSF)(2)ClO(4) along the least conducting c(⋆)axis and along the high conductivity aaxis as a function of temperature and pressure, a low temperature regime is observed in which a unique scattering time governs the transport along both directions of these anisotropic conductors. However, the pressure dependence of the anisotropy implies a large pressure dependence of the interlayer coupling. This is in agreement with the results of firstprinciples density functional theory calculations implying methyl group hyperconjugation in the TMTSF molecule. In this low temperature regime, both materials exhibit for ρ(c) a temperature dependence aT + bT(2). Taking into account the strong pressure dependence of the anisotropy, the Tlinear ρ(c) is found to correlate with the suppression of the superconducting T(c), in close analogy with ρ(a) data. This work reveals the domain of existence of the threedimensional coherent regime in the generic (TMTSF)(2)X phase diagram and provides further support for the correlation between Tlinear resistivity and superconductivity in nonconventional superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: Superconductivity in the Bechgaard salts series of quasionedimensional organic conductors occurs on the verge of spindensitywave ordering when hydrostatic pressure is applied. The sequence of instabilities is intimately connected to normal state anomalies in various quantities like the temperature dependence of electrical transport and nuclear spinlattice relaxation rate. We discuss how such a connection takes its origin in the interference between the different pairing mechanisms responsible for antiferromagnetism and superconductivity, a duo that can be comprehended in terms of a weak coupling renormalization group theory. The recent developments along this line of though are presented in relation to experiments.  [Show abstract] [Hide abstract]
ABSTRACT: The stabilization of unconventional superconductivity (SCd) close to a spindensitywave state (SDW) under pressure in organic conductors like the Bechgaard salts points out the primary importance of the repulsive Coulomb term in the origin of these phases. However, the electron(acoustic) phonon interaction is known to be finite in practice, as borne out for example by diffuse Xray scattering experiments. The question then arises about the role of this coupling, if any, in the mechanism of interaction between SDW and SCd orders in such materials. In this work, we address this issue using the renormalization group method. This is done in the framework of the quasi1D electron gas model with repulsive direct Coulomb terms and weak retarded electronphonon interaction, which are treated on equal footing. The impact of electronphonon interaction on the SDW and SCd instability lines of the phase diagram and on the strength of spin correlations in the normal phase are analyzed at arbitrary phonon frequency, and discussed in connection with experiments in organic superconductors like the Bechgaard salts.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we discuss the limitations of classical field treatments of onedimensional systems in the static approximation. Two exactly solvable Hamiltonians, the ferromagnetic Ising model, and its extension to a zerowidth halffilled band, are studied after their transformation to a classical field form via the Hubbard–Stratonovich identity. The more usual twofield transformation consists of using one field to describe the divergent order parameter and another to represent the nondivergent modes. The fluctuations in this latter one are usually neglected and this is shown to lead to incorrect thermodynamic behavior throughout the critical region, which is unusually large in onedimensional systems, and even beyond to the high temperature limit. Any limited expansion of the free energy is further seen to lead to incorrect treatment of the amplitude fluctuations. A rigorous treatment of both fields is required. Alternately, a onefield transformation can assure a simpler approach although all terms in the free energy expansion must be retained. The findings are extrapolated to other known Hamiltonians: Hubbard, Peierls and spinPeierls, and Bardeen–Cooper–Schrieffer (BCS) superconductivity. The Peierls case is examined in some detail because the usual onefield free energy functional is not obtained by a straightforward use of the Hubbard–Stratonovich transformations. As for the BCS Hamiltonian, it is seen to be in a special class because both symmetry fields are equally divergent and are automatically treated on an equal footing.  [Show abstract] [Hide abstract]
ABSTRACT: An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)2ClO4 and (TMTSF)2PF6 in the pressuretemperature phase diagram between T = 0 and 20 K and a theoretical description based on the weak coupling renormalization group method are reported. The analysis of the data reveals a high temperature domain (T ≈ 20 K) in which a regular T 2 electronelectron Umklapp scattering obeys a KadowakiWoods law and a low temperature regime (T < 8 K) where the resistivity is dominated by a linearin temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spinlattice relaxation due to antiferromagnetic fluctuations. In addition, a tight connection is clearly established between linear transport and T c . We propose a theoretical description of the anomalous resistivity based on a weak coupling renormalization group determination of electronelectron scattering rate. A linear resistivity is found and its origin lies in antiferromagnetic correlations sustained by Cooper pairing via constructive interference. The decay of the linear resistivity term under pressure is correlated with the strength of antiferromagnetic spin correlations and T c , along with an unusual buildup of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts.  [Show abstract] [Hide abstract]
ABSTRACT: The phase diagram of the onedimensional extended Hubbard model at halffilling is investigated by a weak coupling renormalization group method applicable beyond the usual continuum limit for the electron spectrum and coupling constants. We analyze the influence of irrelevant momentum dependent interactions on asymptotic properties of the correlation functions and the nature of dominant phases for the lattice model under study.  [Show abstract] [Hide abstract]
ABSTRACT: The problem of electronic correlations in organic conductors like the Bechgaard salt series [(TMTSF)2X] is well known to be at the heart of the remarkable richness of phase transitions observed in these materials [1], The well established quasionedimensionality of these materials makes both the phenomenological and the microscopic descriptions of correlations rather delicate. Correlations are actually characterized by various energy scales. This is mainly imposed for example by strongly anisotropic hopping integrals for the electrons and the sizeable onchain Coulomb repulsion. The scaling hypothesis [3] and its extended version [3b] for the inclusion of crossover effects between various energy scales then appears as a natural description of correlations in these materials [2]. The microscopic calculations must in turn confirm that this ansatz actually holds. Here, we give a brief survey of the applicability of the scaling hypothesis for the description of itinerant antiferromagnetism in the Bechgaard salts. We will focuss our attention on the recent NMR relaxation rate and EPR line width data. These two observable quantities are directly coupled to magnetic fluctuations and scaling allows a remarkable clarification of several aspects of correlations. Dimensionality and the respective weight associated to uniform and antiferromagnetic correlations are analyzed as function temperature and hydrostatic pressure. 
Article: Interfering antiferromagnetism and superconductivity in quasionedimensional organic conductors
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ABSTRACT: In this short note we summarize recent results obtained by the renormalization group approach to quasionedimensional electron gas model The approach is applied to the Bechgaard salts series (TMTSF)(2)X and the results are shown to give a satisfactory account of the Interdependence between antiferromagnetism and superconductivity featured by their phase diagram the anomalous enhancement of the nuclear relaxation and the electronelectron scattering rate under pressure (C) 2009 Elsevier B V All rights reserved 
Article: Microwave dielectric study of spinPeierls and chargeordering transitions in (TMTTF) 2 PF 6 salts
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ABSTRACT: We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts �TMTTF�2PF6. The temperature behavior of the dielectric function is consistent with shortrange polar order whose relaxation time decreases rapidly below the chargeordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. We give the first account of anomalies in the dielectric function related to the spinPeierls ground state revealing some interaction between both phases in their domain of coexistence in temperature. The variation of the spinPeierls ordering temperature obtained under magnetic field completes the structure of the phase diagram at low field and are analyzed in the framework of the meanfield prediction.  [Show abstract] [Hide abstract]
ABSTRACT: We use the weak coupling renormalization group method to examine the interplay between chargedensitywave and swave superconducting orders in a quasionedimensional model of electrons interacting with acoustic phonons. The relative stability of both types of order is mapped out at arbitrary nesting deviations and Debye phonon frequency $\omega_D$. We singled out a power law increase of the superconducting $T_c\sim \omega_D^{0.7}$ from a quantum critical point of chargedensitywave order triggered by nesting alterations. The results capture the key features shown by the proximity between the two types of ordering in the phase diagram of the recently discovered Perylene based organic superconductor under pressure. The impact of Coulomb interaction on the relative stability of the competing phases is examined and discussed in connection with the occurrence of swave superconductivity in low dimensional chargedensitywave materials. Comment: 6 pages, 5 figures
Publication Stats
3k  Citations  
416.39  Total Impact Points  
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Institutions

19842015

Université de Sherbrooke
 Department of Physics
Шербрук, Quebec, Canada


19832007

Université ParisSud 11
 Laboratoire de Physique des Solides
Paris, IledeFrance, France


2004

Universidad Autónoma de Madrid
Madrid, Madrid, Spain


1993

University of Bayreuth
Bayreuth, Bavaria, Germany
