arXiv subjects

Claude Bourbonnais

Publications and source records attributed to Claude Bourbonnais.

18 recordsLinked to original sources

arXiv2025

Superconducting and spin-density wave phases probed by scanning tunneling spectroscopy in the organic conductor $\mathrm{(TMTSF)_{2}ClO_{4}}$

By scanning tunneling microscopy (STM) we have probed the local quasi-particle density of states (DOS) of the Bechgaard salt organic superconductor $\mathrm{(TMTSF)_{2}ClO_{4}}$ in slowly cooled single crystals cleaved under ultrahigh vacuum conditions. In well STM imaged crystallographic surface planes, the local DOS has been probed for different surface areas at temperatures above and below the critical temperature of superconducting or insulating spin-density wave states. While a rather homogeneous superconducting state is expected in the bulk from previous studies, depending on the degree of disorder introduced by cleavage in the anion lattice, an inhomogeneous granular state is predominantly observed at the surface. A pronounced linear V-shape profile of the local DOS is observed from intermediate to the lowest energy scale in the less disordered superconducting surface areas. This supports the existence of an unconventional d-wave like order parameter with nodes at low energy, which is preceded by more energetic fluctuations attributed to quantum criticality of the material. At higher energy disorder combined to correlations deplete further the DOS. By contrast a non-linear U-shape characterizes the local low energy DOS profile for the more disordered and insulating surface areas of the spin-density wave state. The experimental results are compared quantitatively with those predicted by the renormalization group theory of the quasi-one dimensional electron gas model and its description of the superconducting and spin-density wave states that are interlinked by quantum criticality in the Bechgaard salts.

cond-mat.supr-con↗

arXiv2024

Functional Renormalization Group for fermions on a one dimensional lattice at arbitrary filling

A formalism based on the fermionic functional-renormalization-group approach to interacting electron models defined on a lattice is presented. One-loop flow equations for the coupling constants and susceptibilities in the particle-particle and particle-hole channels are derived in weak-coupling conditions. It is shown that lattice effects manifest themselves through the curvature of the spectrum and the dependence of the coupling constants on momenta. This method is then applied to the one-dimensional extended Hubbard model; we thoroughly discuss the evolution of the phase diagram, and in particular the fate of the bond-centered charge-density-wave phase, as the system is doped away from half-filling. Our findings are compared to the predictions of the field-theory continuum limit and available numerical results.

cond-mat.str-el↗

arXiv2023

Quasi one-dimensional organic conductors: from Fröhlich conductivity and Peierls insulating state to magnetically-mediated superconductivity, a retrospective

It is indisputable that the search for high-temperature superconductivity has stimulated the work on low-dimensional organic conductors at its beginning. Since the discovery of true metal-like conduction in molecular compounds more than 50 years ago, it appeared that the chemical composition and the quasi one-dimensional crystalline structure of these conductors were determining factors for their physical properties; materials with incommensurate conduction band filling favoring the low-dimensional electron-phonon diverging channel and the establishment of the Peierls superstructure and more rarely superconductivity at low temperature, while those with commensurate band filling favor either magnetic insulating or superconducting states depending on the intensity of the coupling between conductive chains. In addition, the simple structures of these materials have allowed the development of theoretical models in close cooperation with almost all experimental findings. Even though these materials have not yet given rise to true high-temperature superconductivity, the wealth of their physical properties makes them systems of choice in the field of condensed matter physics due to their original properties and their educational qualities. Research efforts continue in this field. The present retrospective, which does not attempt to be an exhaustive review of the field, provides a set of experimental findings alluding to the theoretical development while a forthcoming article will address in more details the theoretical aspect of low dimensional conductors and superconductors.

cond-mat.supr-con↗

arXiv2022

Theory of phonon instabilities in Weyl semimetals at high magnetic fields

The behavior of three-dimensional (3D) semimetals under strong magnetic fields is a topic of recurring interest in condensed matter physics. Recently, the advent of Weyl and Dirac semimetals has brought about an interesting platform for potentially uncovering phases of matter that combine nontrivial band topology and interactions. While electronic instabilities of such semimetals at strong magnetic fields have been explored theoretically and experimentally, the role of electron-phonon interactions therein has been largely neglected. In this paper, we study the interplay of electron-electron and electron-phonon interactions in a minimal two-node model of Weyl semimetal. Using a Kadanoff-Wilson renormalization group approach, we analyze lattice (Peierls) instabilities emerging from chiral and nonchiral Landau levels as a function of the magnetic field. We consider both the adiabatic and the nonadiabatic phonon regimes, in the presence or in the absence of improper symmetries that relate Weyl nodes of opposite chirality. We find that (i) the Cooper channel, often neglected in recent studies, can prevent purely electronic instabilities while enabling lattice instabilities that are not Bardeen-Cooper-Schrieffer-like; (ii) breaking the improper symmetry that relates the two Weyl nodes suppresses the Cooper channel, thereby increasing the critical temperature for the lattice instability; (iii) in the adiabatic phonon regime, lattice instabilities can preempt purely electronic instabilities; (iv) pseudoscalar phonons are more prone to undergo a Peierls instability than scalar phonons. In short, our study emphasizes the importance of taking electron-phonon interactions into account for a complete understanding of interacting phases of matter in Dirac and Weyl semimetals at high magnetic fields.

cond-mat.str-el↗

arXiv2015

Nature of ground states in one-dimensional electron-phonon Hubbard models at half-filling

The renormalization group technique is applied to one-dimensional electron-phonon Hubbard models at half-filling and zero temperature. For the Holstein-Hubbard model, the results of one-loop calculations are congruent with the phase diagram obtained by quantum Monte Carlo simulations in the $(U,g_{\rm ph})$ plane for the phonon-mediated interaction $g_{\rm ph}$ and the Coulomb interaction $U$. The incursion of an intermediate phase between a fully gapped charge-density-wave state and a Mott antiferromagnet is supported along with the growth of its size with the molecular phonon frequency $ω_0$. 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 bond-order-wave ordering which is either charge or spin gapped depending on $U$. For the Peierls-Hubbard model, the region of the $(U,g_{\rm ph})$ plane with a fully gapped Peierls-bond-order-wave state shows a growing domination over the Mott gapped antiferromagnet as the Debye frequency $ω_D$ decreases. A power law dependence $g_{\rm ph} \sim U^{2η}$ 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 nearest-neighbor repulsion term $V$ is added to the Hubbard interaction.

cond-mat.str-el↗

arXiv2014

Role of electron-phonon interaction in a magnetically driven mechanism for superconductivity

We use the renormalization group method to examine the effect of phonon mediated interaction on d-wave superconductivity, as driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and d-wave superconducting instability lines is calculated for arbitrary temperature, phonon frequency and antinesting of the Fermi surface.The domain of electron-phonon coupling strength where spin-density-wave order becomes unstable against the formation of a bond-order-wave or Peierls state is determined at weak antinesting. We show the existence of a positive isotope effect for spin-density-wave and d-wave 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 bond-oder-wave ordering gives rise to triplet f-wave superconductivity under nesting alteration, with both orderings displaying a negative isotope effect. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized.

cond-mat.str-el↗

arXiv2012

On the Origin of the Anomalous Upper Critical Field in Quasi-One-Dimensional Superconductors

Upper critical field, H_c2, in quasi-1D 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 Flude-Ferrell-Larkin-Ovchinnikov (FFLO) state. This increase is mainly due to quasi-1D fluctuations effect as triggered by interference between unconventional superconductivity and density-wave instabilities. Our results give a novel viewpoint on the large H_c2 observed in TMTSF-salts in terms of a d-wave FFLO state that is predicted to be verified by the H_c2 measurements under pressure.

cond-mat.supr-con↗

arXiv2012

Magneto-elastic coupling in the spin-Peierls ground state of hydrogenated and deuterated (TMTTF)$_2$PF$_6$ salts

We report an ultrasonic study of the magneto-elastic coupling of the hydrogenated and deuterated (TMTTF)$_2$PF$_6$ organic salts. For both salts the temperature dependence of the longitudinal velocity along the c* axis displays a monotonic stiffening of the $C_{33}$ 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 low dimensional precursors of the spin-Peierls transition. The magneto-elastic 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 spin-Peierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spin-Peierls transition temperature $T_{\rm SP}$ in agreement with scaling of the elastic deformation with the spin-Peierls order parameter. In spite of the weakly inhomogeneous character of the spin-Peierls phase transition, the magnetic field dependence of $T_{\rm SP}$ is well captured with the mean-field prediction for the lattice distorted Heisenberg spin chain.

cond-mat.str-el↗

arXiv2012

Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors

An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 in the Pressure-Temperature 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\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the resistivity is dominated by a linear-in temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spin-lattice 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 electron-electron 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 build-up of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts.

cond-mat.str-el↗

arXiv2011

The metallic transport of (TMTSF)_2X organic conductors close to the superconducting phase

Comparing resistivity data of quasi-one dimensional superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 along the least conducting c*-axis and along the high conductivity a -axis as a function of temperature and pressure, a low temperature regime is observed in which a unique scattering time governs 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 first-principles DFT calculations implying methyl group hyperconjugation in the TMTSF molecule. In this low temperature regime, both materials exhibit for rc a temperature dependence aT + bT^2. Taking into account the strong pressure dependence of the anisotropy, the T-linear rc is found to correlate with the suppression of the superconducting Tc, in close analogy with ra data. This work is revealing the domain of existence of the 3D coherent regime in the generic (TMTSF)_2X phase diagram and provides further support for the correlation between T-linear resistivity and superconductivity in non-conventional superconductors.

cond-mat.str-el↗

arXiv2010

Mechanism for the Singlet to Triplet Superconductivity Crossover in Quasi-One-Dimensional Organic Conductors

Superconductivity of quasi-one-dimensional organic conductors with a quarter-filled band is investigated using the two-loop renormalization group approach to the extended Hubbard model for which both the single electron hopping t_{\perp} and the repulsive interaction V_{\perp} perpendicular to the chains are included. For a four-patches Fermi surface with deviations to perfect nesting, we calculate the response functions for the dominant fluctuations and possible superconducting states. By increasing V_{\perp}, it is shown that a d-wave (singlet) to f-wave (triplet) superconducting state crossover occurs, and is followed by a vanishing spin gap. Furthermore, we study the influence of a magnetic field through the Zeeman coupling, from which a triplet superconducting state is found to emerge.

cond-mat.str-el↗

arXiv2009

Correlation between linear resistivity and $T_c$ in the Bechgaard salts and the pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

The quasi-1D organic Bechgaard salt (TMTSF)$_2$PF$_6$ displays spin-density-wave (SDW) order and superconductivity in close proximity in the temperature-pressure phase diagram. We have measured its normal-state electrical resistivity $ρ_a(T)$ as a function of temperature and pressure, in the $T \to 0$ limit. At the critical pressure where SDW order disappears, $ρ_a(T) \propto T$ down to the lowest measured temperature (0.1 K). With increasing pressure, $ρ_a(T)$ acquires a curvature that is well described by $ρ_a(T) = ρ_0 + AT + BT^2$, where the strength of the linear term, measured by the $A$ coefficient, is found to scale with the superconducting transition temperature $T_c$. This correlation between $A$ and $T_c$ strongly suggests that scattering and pairing in (TMTSF)$_2$PF$_6$ have a common origin, most likely rooted in the antiferromagnetic spin fluctuations associated with SDW order. Analysis of published resistivity data on the iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ reveals a detailed similarity with (TMTSF)$_2$PF$_6$, suggesting that antiferromagnetic fluctuations play a similar role in the pnictides.

cond-mat.supr-con↗

arXiv2009

Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)$_2$PF$_6$ salts

We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering 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. Anomalies in the dielectric function are also observed in the spin-Peierls ground state revealing some intricate lattice effects in a temperature range where both phases coexist. The variation of the spin-Peierls ordering temperature under magnetic field appears to follow a mean-field prediction despite the presence of spin-Peierls fluctuations over a very wide temperature range in the charge ordered state of these salts.

cond-mat.str-el↗

arXiv2007

Role of Interchain Hopping in the Magnetic Susceptibility of Quasi-One-Dimensional Electron Systems

The role of interchain hopping in quasi-one-dimensional (Q-1D) electron systems is investigated by extending the Kadanoff-Wilson renormalization group of one-dimensional (1D) systems to Q-1D systems. This scheme is applied to the extended Hubbard model to calculate the temperature ($T$) dependence of the magnetic susceptibility, $χ(T)$. The calculation is performed by taking into account not only the logarithmic Cooper and Peierls channels, but also the non-logarithmic Landau and finite momentum Cooper channels, which give relevant contributions to the uniform response at finite temperatures. It is shown that the interchain hopping, $t_\perp$, reduces $χ(T)$ at low temperatures, while it enhances $χ(T)$ at high temperatures. This notable $t_\perp$ dependence is ascribed to the fact that $t_\perp$ enhances the antiferromagnetic spin fluctuation at low temperatures, while it suppresses the 1D fluctuation at high temperatures. The result is at variance with the random-phase-approximation approach, which predicts an enhancement of $χ(T)$ by $t_\perp$ over the whole temperature range. The influence of both the long-range repulsion and the nesting deviations on $χ(T)$ is further investigated. We discuss the present results in connection with the data of $χ(T)$ in the (TMTTF)$_2X$ and (TMTSF)$_2X$ series of Q-1D organic conductors, and propose a theoretical prediction for the effect of pressure on magnetic susceptibility.

cond-mat.str-el↗

arXiv2005

Effect of Inter-Site Repulsions on Magnetic Susceptibility of One-Dimensional Electron Systems at Quarter-Filling

The temperature dependence of the magnetic susceptibility, χ(T), is investigated for one-dimensional interacting electron systems at quarter-filling within the Kadanoff-Wilson renormalization-group method. The forward scattering on the same branch (the g_4-process) is examined together with the backward (g_1) and forward (g_2) scattering amplitudes on opposite branches. In connection with lattice models, we show that χ(T) is strongly enhanced by the nearest-neighbor interaction, an enhancement that surpasses one of the next-nearest-neighbor interaction. A connection between our predictions for χ(T) and experimental results for χ(T) in quasi-one-dimensional organic conductors is presented.

cond-mat.str-el↗

arXiv1996

Organic Superconductors: Reduced Dimensionality and Correlation Effects

In this tutorial we will tackle the problem of electronic correlations in quasi-one-dimensional organic superconductors. We will go through different pieces of experimental evidence showing the range of applicability of the Fermi and Luttinger liquid descriptions of the normal phase of the Bechgaard salts series and their sulfur analogs.

cond-mat.str-el↗

arXiv1996

Theory of Lattice and Electronic Fluctuations in Weakly Localized Spin-Peierls Systems

A theoretical approach to the influence of one-dimensional lattice fluctuations on electronic properties in weakly localized spin-Peierls systems is proposed using the renormalization group and the functional integral techniques. The interplay between the renormalization group flow of correlated electrons and one-dimensional lattice fluctuations is taken into account by the one-dimensional functional integral method in the adiabatic limit. Calculations of spin-Peierls precursor effects on response functions are carried out explicitely and the prediction for the temperature dependent magnetic susceptibility and nuclear relaxation is compared with available experimental data for (TMTTF)$_{2}$PF$_{6}$.

cond-mat↗

arXiv1996

Luttinger liquids coupled by hopping

The stability of the Luttinger liquid to small transverse hopping has been studied from several points of view. The renormalization group approach in particular has been criticized because it does not take explicitly into account the difference between spin and charge velocities and because the interaction should be turned-on before the transverse hopping if the stability of the Luttinger liquid is a non-perturbative effect. An approach that answers both of these objections is explained here. It shows that the Luttinger liquid is unstable to arbitrarily small transverse hopping. The crossover temperatures below which either transverse coherent band motion or long-range order start to develop can be finite even when spin and charge velocities differ. Explicit scaling relations for these one-particle and two-particle crossover temperatures are derived in terms of transverse hopping, spin and charge velocities and anomalous exponents. The renormalization group results are recovered as special cases when spin and charge velocities are identical. The results compare well with recent experiments presented at this conference. Magnetic field effects are alluded to.

cond-mat↗