Jean-Paul POUGET

Université Paris-Sud 11 | Paris 11 · Laboratoire de Physique des Solides

The charge density wave (CDW) instability, usually occurring in low-dimensional materials, has been a topic of interest for longtime. However, some very fundamental aspects of the mechanism remain unclear. Recently, a plethora of new CDW materials, a substantial fraction of which is 2D (two-dimensional) or even 3D (three-dimensional), has been pre...

We review the magneto-structural properties of electron–electron correlated quasi-one- dimensional (1D) molecular organics. These weakly localized quarter-filled metallic-like systems with pronounced spin 1/2 antiferromagnetic (AF) interactions in stack direction exhibit a spin charge decoupling where magnetoelastic coupling picks up spin 1/2 to pa...

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals contain�ing three different types of chains and undergoing two different charge densitywave (CDW) Peierls transitions TP1and TP2 associated with type III and typeI chains, respectively. The nature of these transitions is discussed on the basisof first-principles DFT calculation of their Fermi S...

This article contains supplementary information at the main text: “Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3 and monoclinic-TaS3”

Vanadium dioxide exhibits a first order metal to insulator transition (MIT) at 340 K (TMI) from a rutile (R) structure to a monoclinic (M1) structure. The mechanism of this transition interpreted as due either to a Peierls instability or to a Mott–Hubbard instability is controversial since half a century. However, in the last twenty years the study...

Many charge density wave (CDW) systems exhibit q(T) electron-hole modulations continuously varying with T and saturating upon cooling at an incommensurate value even if the maximum occurring in the electron-hole Lindhard response does not exhibit such a thermal shift. Using a simple RPA argument we show that the experimental q(T) can be understood...

Lattice dynamics of low-dimensional BaVS3 is reported across the metal-insulator Peierls transition occurring at TP=69 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density-functional theory calculations. The nondetection of a Kohn anomaly points to a unique situation of an order-disorder Peierls...

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave (CDW) Peierls transitions at TP1 and TP2 . The nature of these transitions is discussed on the basis of first-principles DFT calculation of their electron-hole Lindhard response function. As a result of...

Here, the first accurate study is presented of the room-temperature and 100 K structures of one of the first organic spin liquids,-(BEDT-TTF) 2 Ag 2 (CN) 3. It is shown that the monoclinic structure determined previously is only the average one. It is shown that the exact structure presents triclinic symmetry with two non-equivalent dimers in the u...

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2X (X= ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures.The evolution of the electron-hole response with temper...

On the basis of first-principles DFT calculations the wave-vector and temperature dependencies of the Lindhard response function of the blue bronze K 0.3 MoO 3 have been calculated. The k I F + k II F interband component of the response, which is responsible for the Peierls instability, has been quantitatively analyzed. It is found that (i) the ele...

We report anomalies observed in photoelectron spectroscopy measurements performed on α-(BEDT-TTF)2I3 crystals. In particular, above its metal-insulator transition temperature (T=135 K), we observe the lack of a sharp Fermi edge in contradiction with the metallic transport properties exhibited by this quasi-bidimensional organic material. We interpr...

This contribution is a tribute to André Guinier and to the scientific school he created. We show how his thesis work allowed one to develop new X-ray scattering techniques of investigation of matter both at small and large scattering Bragg angles. In parallel to his pioneering experimental work, André Guinier introduced a well-adapted formalism all...

Anions have often been considered to act essentially as electron donors or acceptors in molecular conductors. However there is now growing evidence that they play an essential role in directing the structural and hence electronic properties of many of these systems. After reviewing the basic interactions and different ground states occurring in mol...

A mixed-valence conducting cation radical salt of the unsymmetricallysubstitutedo-Me2TTF donor molecule (TTF is tetrathiafulvalene) was obtainedupon electrocrystallization in the presence of the non-centrosymmetric NO3�anion. It crystallizes at room temperature in the monoclinicP21/cspace group,with the anion disordered on an inversion centre. The...

We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)2Cu2(CN)3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symm...

In strongly correlated organic materials it has been pointed out that charge ordering could also achieve electronic ferroelectricity at the same critical temperature T . A prototype of such phenomenon are the co quasi-one-dimensional (TMTTF) X Fabre salts. However, the stabilization of a long-range ferroelectric ground 2 requires the break of inver...

Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the κ-(BEDT-TTF) 2 Cu 2 (CN) 3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits us to excite different sets of phonon modes. The cyanide (CN) stretching mode is selected at the...

One-dimensional (1D) conductors such as Bechgaard and Fabre salts are a prototypal example of correlated systems where the phase diagram is controlled by sizable electron-electron repulsions. In deuterated (TMTTF)2PF6, where this interaction achieves charge localization at ambient pressure on donor stacks, magnetostructural coupling plays a decisiv...

All-optical femtosecond relaxation dynamics in a single crystal of monophosphate tungsten bronze (PO2)4(WO3)2m with alternate stacking m = 6 of WO3 layers was studied through the three consequent charge-density-wave (CDW) transitions. Several transient coherent collective modes associated with the different CDW transitions were observed and analyze...

All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions wer...

We consider structural instabilities exhibited by the one-dimensional (1D) (arene)2X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X, these salts exhibit a quarter-filled (hole) conduction band located on donor stacks. Compounds built with donors such as...

We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi one- dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 3...

We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi one- dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 3...

A first-principles DFT study of the electronic structure of the two-chain molecular conductor NMP-TCNQ is reported. It is shown that the charge transfer occurring in this salt is not 1 but 2/3, finally settling the debate concerning the real charge transfer in this molecular metal. These calculations also lead to a simple rationalization of the thr...

A first-principles density functional theory (DFT) study of theta -(BEDT-TTF)2X molecular conductors with X = I3, CsCo(SCN)4 (ambient pressure, 7.5 kbar and 10 kbar), CsZn(SCN)4,TlCo(SCN)4, RbCo(SCN)4 and RbZn(SCN)4 (220 K and 90 K) is reported. It is shown that these salts exhibit three different types of band structure each of them associated wit...

High-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the As M-edge, F K-edge and S L-edge of the Fabre salt (TMTTF)2AsF6 were performed from room temperature (RT) to 90 K, allowing to reach the charge localization regime below Tρ ≈ 230 K and to cross the charge ordering (CO) transition at TCO ≈ 102 K. The F K-edge and...

The paper presents a single-crystal ESR study of a series of pristine and X-ray irradiated (TMTTF)2X salts, where TMTTF is tetramethyltetrathiafulvalene and X is either a centro-symmetrical (SbF6 and Br) or a non-centro-symmetrical (ReO4, ClO4, BF4, SCN and NO3) monovalent anion. Besides standard line-width and spin-susceptibility measurements, the...

The electronic structure of TTF-TCNQ was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to detect a spectroscopic signature of the phase transitions, especially that occurring at 54 K, which is related to a Peierls gap opening on the TCNQ stacks. All unoccupied TCNQ orbitals predicted by first-principles calcula...

Core-level photoemission spectra of the Fabre salts with X = SbF6 and PF6 were taken using hard X-rays from PETRA III, Hamburg. In these salts TMTTF layers show a significant stack dimerization with a charge transfer of 1e per dimer to the anion SbF6 or PF6. At room temperature and slightly below the core-level spectra exhibit single lines, charact...

We have established a pressure-temperature phase diagram of TTF[Ni(dmit)2]2 based on spin susceptibility at ambient pressure as well as longitudinal and transverse resistivity measurements under pressure up to 30 kbar. The data were analyzed on the basis of first-principles density functional theory calculations. We were able to find several phase...

A first-principles density functional theory study of the Bechgaard salts (TMTSF) 2 ClO 4 and (TMTSF) 2 NO 3 , with special emphasis on the anion ordering transition, is reported. It is found that the main effect of the anion ordering on the band structure occurs through the deformations induced in the TMTSF layers and not through the Coulomb poten...

We report an investigation of charge, spin, and lattice effects in the spin-Peierls state of the organic compound MEM(TCNQ)2. The 16.5-GHz 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 ve...

Charge-ordering phenomena have been highly topical over the past few years. A phase transition towards a charge-ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one-dimensional organic charge-transfer salts .TMTTF/2X, where (TMTTF) stands for tetramethy...

We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)2PF6 and (TMTSF)2AsF6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF)2PF6-D12 salt allows locating precisely the methyl groups at...

BaVS3 presents a metal-to-insulator (MI) transition at ambient pressure due to the stabilization of a 2kF commensurate charge density wave (CDW) Peierls ground state built on the dz2 V orbitals. The MI transition vanishes under pressure at a quantum critical point (QCP) where the electronic properties exhibit a non-Fermi liquid behavior. In this pa...

We review structural aspects of the Bechgaard and Fabre salts in relationship with their electronic, magnetic and superconducting properties. We emphasize the role of bond and charge modulations of the quarter filled organic stack in the various instabilities and ground states exhibited by these salts. A special consideration is also devoted to the...

We report an ultrasonic study of the magneto-elastic 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 modu...

α-(BEDT-TTF)2I3 exhibits a metal to insulator transition around 135Kthat has been ascribed to charge ordering in the donor layers containing three different donors (A-A dimers, B and C). First-principles density-functional theory (DFT) calculations provide a description of the electronic structure of this system in agreement with the presently avai...

Combining density-functional theory and many-body approaches we investigate the origin of the ferroelectric phase generally observed in TMTTF2-X organic crystals. We solve by means of mean-field theory and exact diagonalization a two-dimensional tight-biding model built from density-functional theory calculations and we show that short-range intera...

The search for multiferroics has become an important research topic in the last few years. Almost all newly discovered multiferroics are transition metal compounds where spin, lattice and charge degrees of freedom are strongly entangled. The possibility of finding organic multiferroics can open up a new area of research where new mechanisms, differ...

The 2:1 family of organic salts (o-DMTTF)2X (X = Cl, Br, I) exhibits regular stacks and a high-symmetry structure, which provide a perfect three-quarter-filling-band system allowing a rich phase diagram in the presence of strong electronic correlations. In this paper, we present a detailed study of this series combining complementary experimental t...

Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non-superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origi...

The paper points out the importance of the coupling between anions and donors in order to achieve the 4k F charge localization observed in (TMTTF) 2 PF 6 , d-(EDT-TTF-CONMe 2) 2 Br and (o-DMTTF) 2 Cl/Br salts, the 2k F charge density wave (CDW) ground state of a-(BEDT-TTF) 2 KHg(SCN) 4 and the metal to insulator transition of a-(BEDT-TTF) 2 I 3. Th...

We present the first inelastic neutron scattering investigation of magnetic excitations in a one-dimensional (1D) organic conductor. This experiment is conducted in the spin-Peierls (SP) ground state of (TMTTF) 2 PF 6. The study, performed with a 98% deuterated powder, reveals the double gap structure of magnetic excitations of the SP chain. These...

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal,...

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal,...

Resonant magnetic x-ray scattering near the vanadium L2,3-absorption edges has been used to investigate the low temperature magnetic structure of high quality BaVS3 single crystals. Below TN=31K, the strong resonance revealed a triple-incommensurate magnetic ordering at the wave vector (0.2260.226xi) in hexagonal notation, with xi=0.033. The azimut...

Recent advances in processing of polyaniline and polyacetylene have resulted in a new generation of conducting polymers with higher dc conductivities. We present the temperature (T) dependent microwave frequency dielectric constant, dc conductivity, and Kramers-Kronig analysis of conducting polyaniline and polyacetylene. The low temperature dielect...

-�BEDT-TTF�2KHg�SCN�4 develops a density wave ground state below 8 K whose origin is still debated. Here we report a combined x-ray diffuse scattering and first-principles density functional theory study supporting the charge density wave �CDW� scenario. In particular, we observe a triply incommensurate anharmonic lattice modulation with intralayer...

The molecular charge order (CO)-ferroelectric transition of the strongly correlated organic conductors (TMTTF)(2)X is investigated We present here a direct evidence of a lattice deformation at the CO of deuterated (TMTTF)(2)PF6 using powder neutron diffraction Our data show that the intensity of several Bragg reflections is affected by the CO Their...

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 short-range polar order whose relaxation time decreases rapidly below the charge-ordering temperature. If this transition has more a relaxor character in the hydrogena...

Among the correlated electronic systems, BaVS3 which exhibits both itinerant and localized states as well as a subtle interplay betwen charge, orbital, spin and lattice degrees of freedom, is a model system. Its electronic structure consists in a broad quasi–one-dimensional (1D) dz2 band and two quasi-degenerate narrow e(t2g) bands. Pure BaVS3 exhi...

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal,...

We report on the synthesis and application of an internal chemical pressure to effectively control, and reduce, the Mott gap in the system d-(EDT-TTF-CONMe 2) 2 X, X ¼ Br, AsF 6 ; the detailed accounts of its Pmna, averaged room temperature structure and reversible phase transition at ca. 190 K towards a low temperature P2 1 /a structure; the synth...

On the basis of first-principles density functional theory calculations, it is shown that, in contrast with a recent suggestion, K0.25WO3 does not exhibit hidden one-dimensional Fermi surfaces. A charge-density wave instability should not be at the origin of the resistivity anomaly of this bronze,which most likely results from potassium vacancy ord...

High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)2X (X=PF(6) and AsF6), where charge ordering at T(CO) coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at T(CO) in the uniaxial expansivity along the interstack c direct...

We report results of high-resolution thermal expansion measurements on the charge-ordering ðT CO Þ and spin-Peierls ðT SP Þ transition in ðTMTTFÞ 2 X for X ¼ AsF 6 and PF 6. Based on distinct lattice effects observed at T CO in the c Ã-axis expansivity, a scheme is proposed which involves a charge modulation along the TMTTF stacks and its coupling...

We study the influence of Sr substitutions on the structural counterpart of the MI transition of BaVS3. When Sr content increases, the commensurate CDW modulation of pure BaVS3 is changed into an incommensurate short range modulation that we attribute to a charge ordering of the e(t2g) electrons. The same features are observed in S deficient sample...

Single crystal ESR data on �TMTTF�2X salts are presented. Taking advantage of the large dimensions of the crystals, skin effect is studied through the asymmetry of the ESR resonance lines. When the microwave magnetic field is perpendicular to the stacking axis, a very rich temperature dependence of this asymmetry is found although the three compone...

The high-temperature tetragonal to low-temperature orthorhombic phase transition of La//1//. //8//8Sr//0//. //1//2CuO//4 has been studied by single-crystal X-ray diffraction and diffuse scattering. The temperature dependence of superstructure reflections below T//T//-//O equals 215 K, precursor diffuse scattering above T//T//-//O and the orthorhomb...

X-Ray diffuse scattering from a series of examples where the reason for the existence of incommensurable features is fairly well understood, is described. They include in particular non stoichiometric compounds such as [DIPS Φ4(l3)0.76] and such as intercalated graphite RbC24. and quasi one dimensional conductors. A particular emphasis is given on...

We present an x-ray diffuse scattering study of the charge-density wave (CDW) structure in the doped blue bronzes K0.3(Mo1−xVx)O3 (x=2.8%), Rb0.3(Mo1−xVx)O3 (x=1.44% and 0.28%), and K0.3(Mo1−xWx)O3 (x=2%). At low temperature, the ±2kF satellite reflections are broadened in all four compounds, and shifted in V-doped compounds. Moreover, we have obse...

We present a study of the spin-Peierls (SP) transition in the ${(\mathrm{TMTTF})}_{2}{\mathrm{PF}}_{6}$ organic conductor by elastic neutron scattering. The $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ superstructure reflection associated with the SP transition is clearly detected up to $18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The intensity of the sat...

The quasi-one-dimensional conductor BaVS3 undergoes a metal-insulator transition at TMI=70 K accompanied by a formation of superstructure doubling the chain periodicity and a magnetic transition at TX=30 K.We present an anomalous x-ray scattering investigation of the charge-density wave �CDW� satellite reflections at the V K-edge. Our study shows t...

The one-dimensional (1D) 3d1 system BaVS3 undergoes a metal–insulator (MI) transition at TMI=70K, whose origin is still a matter of controversy. We show that the MI transition is accompanied by the formation of a superstructure which doubles the chain periodicity. The superstructure has been refined in the monoclinic Im space group from high resolu...

BaVS3 is an unusual 3d1 quasi-one dimensional (1D) correlated system presenting an interplay of charge, orbital and spin degrees of freedom. At TMI=70K, BaVS3 undergoes a metal–insulator transition whose origin is still a matter of controversy. We show that this transition is associated with a structural modulation preceded by an extended temperatu...

In this review we present three representative examples of molecular disorder: substitution disorder for carbon nanotubes in zeolite, coupling between substitution disorder and displacements through the pinning of charge density waves in one dimensional molecular conductors and combination of random packing with orientional disorder in the C60-C60...

(TMTTF) 2X, with X=PF 6, exhibits, at T SP, a spin-Peierls transition. Below TSP elastic neutron scattering measurements performed on large protonated and deuterated samples reveal the formation of very weak superstructure reflections at the (1/2,1/2, 1/2) reduced wave vector. Upon deuteration T SP decreases by 30% while the charge ordering (CO) tr...

A sequence of structural transitions occurring in the quasi-one-dimensional (1D) 3d1 system BaVS3 at low temperature was investigated by high resolution synchrotron X-ray diffraction. The orthorhombic Cmc21 structure of the intermediate-temperature (70K<T<240K) phase was confirmed. A model for the low-T (T<70K) k=(1 0 1/2)O superstructure (with Im...

Quasi-one dimensional (1D) conductors of the TTF-TCNQ family of charge transfer salts exhibit a Peierls transition which stabilizes a periodic lattice distortion (PLD), accompanied by a charge density wave (CDW) with an incommensurate 2kF (and sometimes 4kF) wave vector related to the band filling of the 1D electronic subsystem. The Peierls instabi...

Times Cited: 3

The inorganic quasi-one dimensional (1D) S = 1/2 antiferromagnetic (AF) system CuGeO3 undergoes a 2nd order spin-Peierls (SP) phase transition at T SP = 14.2 K. In this study we present an X-ray synchrotron radiation investigation which confirms that the SP instability is announced by an important regime of pretransitional structural fluctuations...