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International audience; A detailed investigation of the charge-ordered charge-transfer salt δ−(EDT−TTF−CONMe2)2Br by thermal-expansion measurements and dielectric spectroscopy reveals three dynamic processes of relaxational character. The slowest one exhibits the characteristics of glassy freezing and is ascribed to the conformational dynamics of terminal ethylene groups of the organic molecules. Such a process was previously found for related charge-transfer salts where, however, the anions form polymerlike chains, in contrast to the spherical anions of the present material. Dielectric spectroscopy reveals two additional relaxational processes. The characteristics of the faster one are consistent with excitations of a one-dimensional Wigner lattice as recently observed in this material by infrared spectroscopy, which are also accompanied by conformational changes of the molecules. However, at low temperature the ethylene-group relaxation exhibits the cooperativity-induced dramatic slowing down that is typical for glassy freezing, while the defect-related Wigner-lattice excitation follows thermally activated behavior as expected for single-dipole relaxations.

The quantum critical state of organic quantum spin liquids (QSL) exhibits large sensitivity even to weak perturbations. For example, the antisymmetric exchange, the Dzyaloshinskii-Moriya (DM) interaction, which is present in all spin systems without inversion symmetry, could result in a phase transition from the quantum critical phase to an antiferromagnetic phase already at moderate magnetic fields. Using the combination of multi-frequency Electron Spin Resonance spectroscopy (ESR) in the 1-500 GHz frequency range and muon spin rotation (mSR), we studied the influence of the DM interaction in two-dimensional and quasi-one-dimensional organic QSL candidates.In the triangular lattice QSL, k-(ET)2Ag2(CN)3 (J’/J=0.94, J=175 K), our ESR measurements found a static staggered moment of 6×10-3 mB at T=1.5 K and at B=15 T [1]. The magnetic field dependence of the ESR linewidth, which measures the spectral density of the antiferromagnetic fluctuations, proves that this staggered moment stems from the DM interaction (DM0=4 K) in a perfectly crystalline two-dimensional structure. In a new quasi-one-dimensional QSL candidate, (EDT-TTF-CONH2)2+BABCO-, which is a weak Mott insulator with a distorted triangular lattice (J’/J=3, J=360 K), our combined ESR and mSR study confirmed the absence of magnetic ordering down to 20 mK [2]. This remarkable observation is partially attributed to a unique structural motif of the (EDT-TTF-CONH2)2+BABCO- salt. Here, the (EDT-TTF-CONH2)2+ conducting layers are separated by the highly disordered BABCO- molecular rotors. Importantly, despite the presence of a sizable DM interaction (DM0=0.6 K), the staggered moment is smaller than 4×10-4 mB at T=1.5 K and B=15 T. The magnetic field dependence of the ESR linewidth does not show the effect of the DM interaction. Instead, the linear dependence is indicative of the presence of fast spin fluctuations, which is supported by longitudinal-field mSR measurements that reveal the spin excitations to possess one-dimensional diffusive character. The quenching of the effect of the DM interaction is explained by the strong disorder introduced by the anion layer.Despite the fact that the magnitude of the DM interaction is 2 to 3 orders of magnitude weaker than the symmetric exchange, it can substantially alter the phase diagram of QSLs. Our work gives a novel explanation to the field-induced phase transitions, and it demonstrates that high-frequency ESR is a powerful technique to study the spin dynamics of QSLs.

International audience; The 1/4-filled organic compound, δ-(EDT-TTF-CONMe2)2AsF6 is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40 K, but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4 K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.

International audience

International audience; Optical investigations of quarter-filled charge ordered insulators delta-(EDT-TTF-CONMe2)(2)X, X = AsF6, Br are reported. The spectra evidence the one-dimensional nature of the systems at all temperatures. The charge order exists already at room temperature, the optical gap is estimated to be of the order of 550 cm(-1) for the X = Br compound and about 700 cm(-1) for the X = AsF6 material. The optical response in the direction of the stacks is discussed in terms of domain-wall excitations of one-dimensional quarter-filled Wigner lattice at moderate values of nearest-neighbor electronic repulsion. A similarity between the effects of temperature contraction and chemical pressure is found.

International audience

Date du colloque : 07/2013

Date du colloque : 12/02/2013; National audience

International audience; Transport measurements were performed on the organic layered compound alpha-(BEDT-TTF)(2)I-3 under hydrostatic pressure. The carrier types, densities, and mobilities are determined from the magnetoconductance of alpha-(BEDT-TTF)(2)I-3. While evidence of high-mobility massless Dirac carriers has already been given, we report here their coexistence with low-mobility massive holes. This coexistence seems robust as it has been found up to the highest studied pressure. Our results are in agreement with recent DFT calculations of the band structure of this system under hydrostatic pressure. A comparison with graphene Dirac carriers has also been done.

International audience; We report on a series of amino acid derivatives of tetrathiafulvalene as well as on the structure-directing abilities of their peptide residues in the crystalline solid state to stabilize patterns of interactions such as beta strands and sheet motifs. Characteristic hydrogen-bonding motifs are indeed identified within ethylenedithiotetrathiafulvalene (EDT-TTF) and dimethyltetrathiafulvalene (Me-2-TTF) based compounds 1-5. Esters 1-3 contain hydrogen-bond acceptors, namely carbonyl groups, as well as one strong (NH) and one weak (C-sp(2)-H) hydrogen-bond donor. In addition to the hydrogen-bonded sets of ester derivatives, acids 4 and 5 present the carboxylic acid moiety, which acts as both a hydrogen-bond donor and acceptor. EDT-TTF-CO-GlyOH has been previously used to afford a new type of hydrogen-bonded acid/zwitterion (1:1) hybrid admixture of redox peptidics.

Date du colloque : 06/2012

International audience

International audience

Date du colloque: 01/2011

National audience; 1,4-Bis(carboxylatoethynyl)bicyclo[2.2.2]octane: un rotor ultra-rapide dans un sel dilué est freiné au sein d’un métal multifonctionnel1L’objectif de ce travail est la conception de systèmes dans lesquels les propriétés dynamiques d’une machine moléculaire seraient corrélées aux propriétés électroniques que l’on rencontre habituellement dans les matériaux conçus au laboratoire par la technique d’électrocristallisation. Pour cela un sel dilué de formulation [nBu4N+]2[BABCO][BABCO–]2 a été préparé à partir du 1,4-di(ethynyl)bicyclo[2.2.2]octane synthétisé en neuf étapes2. Des expériences de temps de relaxation spin-réseau du 1H sur cristal statique montrent que deux environnements différents des rotors sont associés à des énergies d’activation et des fréquences de rotation différentes.Ce sel a été engagé dans un processus d’électrocristallisation avec le donneur d’électrons de type π EDT-TTF-CONH2 pour donner un nouveau métal organique cristallin : (EDT-TTF-CONH2)2[BABCO–], pour la première fois avec une fonctionnalité dynamique. Une seule position d’équilibre est observée pour le rotor dont on observe que l’énergie d’activation est grandement augmentée: ceci pourrait être attribué à des contraintes à la rotation accrues, expression d’un réseau dense d’interactions non covalentes (liaisons hydrogènes et interactions dihydrogène attractives3).Ce matériau organique métallique ouvre la voie à d’autres systèmes multifonctionnels amphidynamiques.1C. Lemouchi, C. Mézière, L. Zorina, E. Canadell, P. Wzietek, P. Auban-Senzier, C. Pasquier, P. Batail, soumis2C. Lemouchi, C. Vogelsberg, S. Simonov, L. Zorina, P. Batail, S. Brown, M. A. Garcia-Garibay, J. Am. Chem. Soc., 133, 6371-6379 (2011).3J. Echeverrıa, G. Aullon, D. Danovich, S. Shaik, S. Alvarez, Nature Chem., 2011, DOI: 10.1038/nchem.1004

International audience; A methodology is proposed to provide direct access in good yields to peptide residues-appended perylenediimides PDI-(Cl-4)-[Gly-Ala(OEt)](2), 2a, PDI-(Cl-4)-[Gly-Val(OEt)](2), 2b and PDI-(Cl-4)-[Gly-Gly(OEt)](2), 2c from a generic perylenediimide (PDI) platform symmetrically functionalized with carboxylic acids at the imide sites, PDI-(Cl-4)-[Gly(OH)](2), 1. The latter is obtained in good purity by a non classical two-steps route avoiding the many, notoriously cumbersome successive chromatography steps typical of PDI chemistry, and including a single final purification allowing to crystallize the water soluble pure diacid 1, of great interest in its own right for further developments in a variety of fields. Then, the synthesis, crystallization and analysis of the crystal structures of 2a and 2b reveal a common pattern of self-assembly of the outer peptide residues based on collections of parallel N-H center dot center dot center dot O peptidic hydrogen bonds running alongside stacks where the constraints imposed upon on the inner PDI skeletons by long range interaction of these parallel electric dipoles reduce the dihedral angles around the bay regions by as much as 11% down to 32 degrees.

Date du colloque: 10/2011

Date du colloque : 09/2011

Date du colloque: 09/2011; International audience

Date du colloque: 08/2011; International audience

International audience

We present a multifrequency electron spin resonance study in the range of 4-420 GHz of the quasi-one-dimensional, nondimerized, quarter-filled Mott insulators, delta-(EDT-TTF-CONMe2)(2)X (X=AsF6, Br). In the high-temperature orthorhombic phase above T similar to 190 K, the magnitude and the temperature dependence of the high-temperature spin susceptibility are described by a S=1/2 Heisenberg antiferromagnetic chain with J(AsF6)=298 K and J(Br)=474 K coupling constants for X=AsF6 and Br, respectively. We estimate from the temperature dependence of the linewidth (Delta H) an exchange anisotropy, J'/J of similar to 2 X 10(-3). The frequency dependence of Delta H and the g shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii-Moriya) interaction.

Date du colloque: 06/2010

Date du colloque : 10/2010; National audience; L'électrocristallisation est devenue à ce jour une technique incontournable pour la préparation de cristaux moléculaires d'espèces radicalaires, obtenus sur l'électrode par oxydation ou réduction électrochimique de précurseurs moléculaires. C’est un outil à fort potentiel car il n'est pas limité à la seule préparation de cristaux conducteurs mais peut être mis en œuvre facilement pour différents types de cristaux isolants de molécules électro-actives [1].Une collaboration active entre les laboratoires de Rennes et Angers s’est développée autour de cette technique d’élaboration de monocristaux qui ont l’avantage d’être très purs. Les études physiques sont actuellement menées au niveau international entre différents partenaires, notamment dans le cadre de l’ANR blanche intitulée « ¾-Filled » coordonnée par le Laboratoire de Physique des Solides d’Orsay.Cette activité commune a été soutenue par le réseau CRISTECH avec un financement de 4000 € qui a permis l’achat de deux générateurs de micro-courant pour renouveler et étoffer le parc à Angers ainsi qu’une enceinte thermostatée à Rennes pour équiper le service installé plus récemment. L’affiche présente le principe sur lequel repose cette technique particulière de cristallogénèse, les molécules type qui peuvent être engagées, le matériel nécessaire ainsi que les paramètres qui influent sur la croissance.Deux exemples d’études récentes qui ont pu voir le jour grâce à cette technique sont également mis en exergue. L’une sur le système δ-(EDT-TTF-CONMe2)2X avec X = AsF6– ou Br–, un isolant de Mott à bande ¾ remplie [2] et [3]. L’autre concerne des sels d'un autre tétrathiafulvalène dissymétrique, à savoir o-DMTTF qui cristallise dans une structure quadratique très originale avec des anions halogénures comme Cl–, Br– et I– [4]. En particulier, nous avons pu accéder à deux séries de solutions solides [(o-DMTTF)2(X)x(Y)1–x] avec X, Y = Cl–/Br– ou Br–/I–, dans lesquelles des phénomènes d'insertion préférentielle de Br– ont été mis en évidence[1] P. Batail et al., Chem. Mater., 10, 3005 (1998).[2] L. Zorina et al., J. of Mater. Chem., 19, 6980 (2009)[3] P. Auban-Senzier et al., Phys. Rev. Lett., 102, 257001 (2009)[4] M. Fourmigué et al., Dalton Trans, 4652 (2008)

An investigation of the P/T phase diagram of the quarter-filled organic conductors, [EDT-TTF-CONMe2]2X, is reported on the basis of transport and NMR studies of two members, X=AsF6 and Br of the family. The strongly insulating character of these materials in the low pressure regime has been attributed to a remarkably stable charge ordered state confirmed by 13C NMR and the only existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of the 1D Mott localization seen in (TM)2X which are quarter-filled compounds with dimerization. A non magnetic insulating phase instead of the spin density wave state is stabilized in the deconfined regime of the phase diagram. This sequence of phases observed under pressure may be considered as a generic behavior for 1/4-filled conductors with correlations.

Date du colloque: 10/2009

International audience; An investigation of the P-T phase diagram of the quarter-filled organic conductors, [EDT-TTF-CONMe2]2X is reported on the basis of transport and NMR studies of two members, X=AsF6 and Br of the family. The strongly insulating character of these materials in the low-pressure regime has been attributed to a remarkably stable charge-ordered state confirmed by 13C NMR and the only existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of the 1D Mott localization seen in (TMTTF)2X and (TMTSF)2X which are 1/4-filled compounds with dimerization. A nonmagnetic insulating phase instead of the spin density wave state is stabilized in the deconfined regime of the phase diagram. This sequence of phases observed under pressure may be considered as a generic behavior for 1/4-filled conductors with correlations.

International audience; In the two-dimensional organic charge-transfer salts κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x a systematic variation in the Br content from x=0 to 0.9 allows us to tune the Mott transition by increasing the bandwidth. At temperatures below 50 K, an energy gap develops in the Cl-rich samples and grows to approximately 1000 cm−1 for T→0. With increasing Br concentration spectral weight shifts into the gap region and eventually fills it up completely. As the samples with x=0.73, 0.85, and 0.9 become metallic at low temperatures, a Drude-type response develops due to the coherent quasiparticles. Here, the quasiparticle scattering rate shows a ω2 dependence and the effective mass of the carriers is enhanced in agreement with the predictions for a Fermi liquid. These typical signatures of strong electron-electron interactions are more pronounced for compositions close to the critical value xc≈0.7, where the metal-to-insulator transition occurs.

Date du colloque: 06/2008

Date du colloque: 10/2008

Champ "Titre" tronqué, voir ci-dessous pour le titre complet:Modulating the Framework Negative Charge Density in the System [BDT−TTP•+]/[Re6S5Cl91-]/[Re6(S/Se)6Cl82-]/[Re6S7Cl73-]: Templating by Isosteric Cluster Anions of Identical Symmetry and Shape, Variations of Incommensurate Band Filling, and Electronic Structure in 2D Metals; International audience; A series of 2D metals, β-(BDT-TTP)6[Re6Se6Cl8]·(CHCl2−CHCl2)2, 2; β-(ST-TTP)6[Re6S6Cl8]·(CH2Cl−CHCl2)2, 3; β-(BDT-TTP)7[Re6S6Cl8]0.5[Re6S7Cl7]0.5·(CH2Cl2), 4; β-(BDT-TTP)7[Re6Se6Cl8]0.5[Re6S7Cl7]0.5·(CH2Cl2), 5; β-(BDT-TTP)8[Re6S7Cl7]·(CH2Cl2)4, 6 (BDT-TTP and ST-TTP are 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene and 2-(1,3-diselenol-2-ylidene)-5(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, respectively) is reported to have one single β-slab layered topology despite successive increases of the cluster anion negative charge. The charge density within the templating composite inorganic−neutral molecule slab is shown to remain above a threshold of ca. one negative charge per square nanometer, that is, for cluster anions with two negative charges and higher. Conversely, discrete stacks are shown to be stabilized instead in the semiconducting salts (BDT-TTP)2[Re6S5Cl9], 1 where the cluster anion bears one negative charge only. The electronic structure of salts 2−6 is shown to be very stable and kept almost intact across the series. The templating strategy is shown to fulfill its anticipated potential for deliberate installment of incommensurate band fillings in molecular metals. The deliberate admixture of the 6:1 and 8:1 structures yields novel phases with a 7:1 stoichiometry with the anticipated crystal and electronic structures. The action at the organic−inorganic interface triggered by changing the anion charge yet keeping its shape and volume identical, which ultimately governs the shape of the unit cell, is of paramount importance in defining the Fermi surface of these metallic salts. The present BDT-TTP salts thus provide a series of materials with strongly related but subtly different Fermi surfaces worthy of many physical studies. Shubnikov−de Haas measurements are expected to be particularly interesting since they are especially sensitive to the details of the Fermi surface.

Date du colloque: 12/2008

International audience; Infrared reflection measurements of the half-filled two-dimensional organic conductorskappa-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x (BEDT-TTF denotes bis-(ethylenedithio)tetrathiafulvalene] were performedas a function of temperature (5 K300 K) and Br substitution (x=0%, 40%, 73%, 85%, and 90%)in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant andlocalized charge carriers. The broad midinfrared absorption has two contributions: transitions between the twoHubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)2 dimer. Since thelatter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational featuresprovides a tool to disentangle these contributions and to follow their temperature and electronic-correlationdependence. Calculations based on the cluster model support our interpretation.

A report of the solid state synthesis and X-ray crystal structure analysis of [NBun4]4[(Re6S5OCl7)2O], the first oxo-bridged, tetravalent hexanuclear oxochalcohalide rhenium cluster.

International audience; La synthèse en phase solide à haute temperature des sels de chalcohalogénures de rhenium a clusters octaédriques de cations alcalino-terreux, CaRe$_6$S$_6$Cl$_8$ , CaRe$_6$Se$_6$Cl$_8$, MgRe$_6$S$_6$Cl$_8$ et MgRe$_6$Se$_6$Cl$_8$ est décrite. Ces composés sont solubles dans une variété de solvants organiques ce qui permet l'accès avec des rendements élevés aux sels (n-$Bu_4N$)$_2$Re$_6$Q$_6$Cl$_8$ (Q = S, Se). Nous démontrons par ailleurs que la cristallisation des sels de calcium et de magnésium à partir de solvants organiques permet d'obtenir des monocristaux de grande qualité des sels de complexes heptaaqua-, and pentaaqua-calcium(II) et hexaaqua-magnesium(II), Ca(H$_2$ O)$_7$Re$_6$Q$_6$Cl $_8$.3H$_2$O (Q = S, Se), Ca(H$_2$O)$_8$Re$_6$S$_6$Cl$_8$ and Mg(H$_2$O)$_6$Re$_6S_6$Cl$_8$.2H$_2$O. Ces derniers ont été caracterisés par LSIMS et détermination de leurs structures cristallines. Celles-ci révèlent des topologies inusitées en chimie de coordination, résultant d'associations mono- et tri-dimensionnelles des sous-unités moléculaires par liaisons hydrogène.