Figure 6 - uploaded by Jean-Paul POUGET
Content may be subject to copyright.
(Colour online) Thermal dependence of the integrated intensity of the two sets of {(210), (-1-21), (1612)} and {(200), (004)} reflections of (TMTSF) 2 PF 6 -D12 (black squares and red dots, respectively). The continuous
Source publication
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...
Contexts in source publication
Context 1
... the rate of increase of the intensity is enhanced upon cooling below 30 K. A somewhat similar thermal behaviour is observed in the set of {(0-11), (110), (10-1), (101), (10-1)} superimposed Bragg reflections, labelled (0-11) eq in figure 4 and located at 2θ ≈ 46°, and whose intensity increases by about 25% below ~ 30 K. A different thermal dependence is shown by Bragg reflections located at larger reciprocal wave vectors. For example, figure 6 gives the thermal dependence of the integrated intensity of two different sets of { (210), (-1-21), (1-12)} and {(200), (004)} superimposed Bragg reflections located at 2θ ≈ 84° and ≈ 90°, respectively. They exhibit a quite large increase of intensity of 100% and 35%, respectively, upon cooling. ...
Context 2
... in contrast to the (001) reflection, the enhanced rate of increase of the intensity starts at a higher temperature 50-60 K, and the intensity saturates at low temperatures. Several other Bragg reflections also located at large 2θ values exhibit a low-temperature increase of intensity resembling the one shown in figure 6. For example, the intensity of the set of {(-1-21), (1-12)} Bragg reflections increases by about 75% between 60 K and 1.5 K. ...
Context 3
... their Miller indices are different from those reported above for the anomalous thermal variation in (TMTSF) 2 PF 6 -D12. Their thermal variation resembles the ones shown in figure 6 for PF 6 -D12, however the intensity anomalies in (TMTTF) 2 PF 6 -D12 occur at a higher temperature T CO ≈ 84 K which corresponds to lattice changes accompanying the charge-ordering transition [38]. No additional Bragg intensity anomalies can be detected at low temperature. ...
Context 4
... the Bragg reflections located at low 2θ, the intensity increases below ~ 30 K without apparent saturation ( figure 5). For the Bragg reflections located at large 2θ the intensity increases below 50-60 K (noted ~55K below) and saturates at low temperature ( figure 6). These distinct effects should have different structural origins for the following reason. ...
Context 5
... same modes should induce a drastic thermal reduction of the intensity of the Bragg reflections located at larger 2θ angles because the Debye-Waller factor varies with the square of the reciprocal lattice wave vector G². This is in contradiction with the thermal dependence observed in figure 6. Consequently, another structural effect must be invoked to explain the thermal behaviour of the large-2θ Bragg reflections. ...
Context 6
... addition to the lattice dynamics of the methyl groups and the anions the effects of which have never been analysed before, another noticeable structural feature shown by our data is the anomalous change around ~55 K of the intensity of several Bragg reflections located at large 2θ ( figure 6). In agreement with earlier 1 H and 19 F NMR studies [24][25][26][27]29] we attribute this change to the ordering or freezing of the methyl group classical motion and to the linkage of these methyl groups to the F atoms of the anion revealed by the formation of H- bonds in the low-temperature structural refinements. ...
Context 7
... loss of inversion centres should lead to a P1 structure similar to the one probably stabilized in the Fabre (TMTTF) 2 X salts below the CO/ferroelectric transition. In this respect the Bragg intensity variation shown in figure 6 resembles the one observed at the 84 K CO transition in (TMTTF) 2 PF 6 -D12 [38]. However, our structural refinement of PF 6 -D12 at 4 K, as well as earlier low- temperature refinement performed in PF 6 -H12 [23] does not provide evidence of a P1 non-centrosymmetric structure. ...
Context 8
... a possibility is sustained by the thermal behaviour of the low-frequency dielectric constant of (TMTSF) 2 PF 6 , measured along the interlayer c* direction, which exhibits a significant increase below 60 K with a rate of increase depending strongly on the measurement frequency [50]. Interestingly the onset of the enhanced charge response coincides with the ~ 55 K structural change depicted in figure 6. In our scenario this means that the H-bond linkage of the anion with the methyl groups probably induces a more polarizable medium revealed by dielectric measurements. ...
Citations
... The partially filled bands were those taken into account in the calculations. The DFT calculations reported here have been performed using three different crystal structures: (i) protonated (TMTSF) 2 PF 6 -H 12 at 4 K [46] and ambient pressure, (ii) protonated (TMTSF) 2 PF 6 -H 12 at 1.7 K and 7 kbar [47], and (iii) 98 % deuterated (TMTSF) 2 PF 6 -D 12 at 4 K and ambient pressure [48]. Note that the thermal dependences of the Lindhard functions reported below are due to the Fermi function in equation (1). ...
... T CO ) and vanish below 50 K [20,55]. This vanishing can be understood as resulting from the increase of the lattice rigidity caused by the strengthening of the linkage of the PF 6 anions with the methyl terminal groups of the TMTSF through hydrogen bonding [48]. ...
... Another feature which can be discussed in the context of the present calculation concerns the (T,P) SDW phase diagram. In this respect note that (i) (TMTSF) 2 PF 6 -D 12 is equivalent to the application of a negative pressure of 5 kbar in (TMTSF) 2 PF 6 -H 12 at P atm [48] and, (ii) there is a strong reduction of T SDW to about 4 K and coexistence between the SDW modulation and superconductivity in (TMTSF) 2 PF 6 -H 12 at 7 kbar [74]. The finding in our calculations that the q 2 maximum of the Lindhard response at 5 K decreases by 3% from (TMTSF) 2 PF 6 -D 12 to (TMTSF) 2 PF 6 -H 12 at P atm and by the same amount in (TMTSF) 2 PF 6 -H 12 from P atm to 7 kbar shows that within the RPA approximation, the decrease of T SDW under pressure can be simply rationalized by the decrease of the Lindhard response. ...
We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2kF spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2kF broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron-hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2kF Bond Order Wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron-hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2kF SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.
... In this picture, anion displacement should also control the dynamics of SP pairing. As anions are certainly linked by H bonds to the outer methyl groups of TMTTF, delimiting its cavity, at T MF SP and below, as found in (TMTSF) 2 PF 6 [48], such dynamics could be particularly slow. Thus anions could impose a relaxation or order-disorder dynamics exhibiting the critical slowing down observed at the SP transition. ...
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 decisive role to stabilize a spin-Peierls (SPs) ground state at TSP=13K. In this paper, we present the first inelastic neutron scattering investigation of SP magnetic excitations in organics. Our paper reveals the presence above TSP of sizable critical fluctuations leading to the formation of a pseudogap in the 1D antiferromagnetic (AF) S=1/2 magnetic excitation spectrum of the donor stack, concomitant with the local formation of singlet of paired spins into dimers below TSPMF≈40K. In addition, the inelastic neutron scattering investigation allows us also to probe the SP critical lattice dynamics and to show that at ambient pressure these dynamics are of relaxation or order-disorder type. Below TSP, our paper reveals the emergence of a two gap SP magnetic excitation spectrum towards a well-defined S=1 magnon mode and a continuum of two excitations, as theoretically predicted. Our measurements allow us to locate the ambient pressure SP phase of (TMTTF)2PF6 in the classical (adiabatic) limit close to the classical/quantum crossover line. Then we provide arguments suggesting that pressurized (TMTTF)2PF6 shifts to the quantum (antiadiabatic) SP gapped phase, which ends in a quantum critical point allowing the stabilization of an AF phase that competes with superconductivity at higher pressure. Finally, we propose that the magnetostructural coupling mechanism in the Fabre salts is caused by dimer charge/spin fluctuations driven by the coupling of donors with anions.
... Indeed, employing such an experimental technique, phase transitions involving distinct types of excitations, i.e. lattice, charge, magnetic and orbital degrees of freedom, can be precisely detected. For instance, ultra-high resolution expansivity measurements have been recently employed to detect and explore subtle lattice effects accompanying the charge-ordering transition in molecular solids [7,8], spin-liquidlike lattice instability [9], the Mott metal-to-insulator transition [2,4,10,11], magnetic [12], multiferroic [13] and superconducting transitions as well [14]. Analogously, the bulk properties of a solid can be obtained by means of high-resolution specific heat measurements [15]. ...
... Employing equations (7), (8) and the definition of c V , equation (4) can be easily deduced. It is worth mentioning that the GR (equation (7)) holds true for solids, liquids and gases. ...
The Grüneisen ratio (Γ), i.e. the ratio of the thermal expansivity to the specific heat at constant pressure, quantifies the degree of anharmonicity of the potential governing the physical properties of a system. While Γ has been intensively explored in solid state physics, very little is known about its behavior for gases. This is most likely due to the difficulties posed in carrying out both thermal expansion and specific heat measurements in gases with high accuracy as a function of pressure and temperature. Furthermore, to the best of our knowledge a comprehensive discussion about the peculiarities of the Grüneisen ratio is still lacking in the literature. Here we report on a detailed and comprehensive overview of the Grüneisen ratio. Particular emphasis is placed on the analysis of Γ for gases. The main findings of this work are: (i) for the van der Waals gas Γ depends only on the co-volume b due to interaction effects, it is smaller than that for the ideal gas (Γ = 2/3) and diverges upon approaching the critical volume; (ii) for the Bose-Einstein condensation of an ideal boson gas, assuming the transition as first-order, Γ diverges upon approaching a critical volume, similarly to the van der Waals gas; (iii) for ⁴He at the superfluid transition Γ shows a singular behavior. Our results reveal that Γ can be used as an appropriate experimental tool to explore pressure-induced critical points.
... Whereas this model does not exhibit ''perfect'' quantitative performance [2], it is the very first realistic solid state model to consider the effect of the crystal lattice vibrations on the thermodynamic properties, see e.g. Refs. [3,4]. As a matter of fact, there are only a few realistic systems, whose multiplicities can be calculated using elementary methods, see e.g. ...
... Ref. [5] and references cited therein. Essentially, the model proposed by Einstein in 1907 [1] to describe the thermal properties of a simple crystalline solid, treating the solid as an array of atoms consisting of independent threedimensional harmonic oscillators, is still of great interest [3,4]. The aim of this work is to extend the model proposed by Einstein for the case of finite number of harmonic oscillators. ...
The theoretical model proposed by Einstein to describe the phononic specific
heat of solids as a function of temperature consists the very first application
of the concept of energy quantization to describe the physical properties of a
real system. Its central assumption lies in the consideration of a total energy
distribution among N (in the thermodynamic limit $N \rightarrow \infty$)
non-interacting oscillators vibrating at the same frequency ($\omega$).
Nowadays, it is well-known that most materials behave differently at the
nanoscale, having thus some cases physical properties with potential
technological applications. Here, a version of the Einstein's model composed of
a finite number of particles/oscillators is proposed. The main findings
obtained in the frame of the present work are: (i) a qualitative description of
the specific heat in the limit of low-temperatures for systems with nano-metric
dimensions; (ii) the observation that the corresponding chemical potential
function for finite solids becomes null at finite temperatures as observed in
the Bose-Einstein condensation and; (iii) emergence of a first-order like phase
transition driven by varying $N$.
... In (TMTSF) 2 PF 6 the classical motion of the PF 6 and of the methyl groups probed by NMR stops below about 70 K [43] and 55 K [41] respectively, but quantum tunneling of the methyl groups remains below these temperatures. The 4 K structural refinements of (TMTSF) 2 PF 6 [45,46] and (TMTTF) 2 PF 6 [37,47] show that each anion locks its orientation by establishing two short F-Se contact distances and four F· · ·H-CH 2 bonds with four neighboring TMTCF molecules located nearly in a plane perpendicular to a. In (TMTSF) 2 PF 6 , a cooperative locking between anions and methyl groups occurs at T ≈ 55 K [41,46] via the formation of H-bonds between these two entities. ...
... The 4 K structural refinements of (TMTSF) 2 PF 6 [45,46] and (TMTTF) 2 PF 6 [37,47] show that each anion locks its orientation by establishing two short F-Se contact distances and four F· · ·H-CH 2 bonds with four neighboring TMTCF molecules located nearly in a plane perpendicular to a. In (TMTSF) 2 PF 6 , a cooperative locking between anions and methyl groups occurs at T ≈ 55 K [41,46] via the formation of H-bonds between these two entities. Non-centro-symmetric anions X, located in similar methyl group cavities, are also subject to an orientational disorder at RT. ...
... These findings reveal the strong sensitivity of the superconductivity to lattice parameters in the family of κ-(BEDT-TTF) 2 X, as discussed in more detail in [89]. Very recently it has been reported [46] that in the Bechgaard salt (TMTSF) 2 PF 6 the phononic Grüneisen parameter , defined by expression (22), is dominated by the anion rotation ( (PF 6 ) ≈ 28). As this finding remains true for the Fabre salts with octahedral anions, the lattice expansion data of section 6 should reveal the electronic instabilities on the TMTTF stack via the modification of the anion vibrational spectra (generally caused by a modification of the volume and shape of the methyl group cavity where the anion is located or by a modification of the linkage of the anion with the neighboring organic molecules). ...
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
tetramethyltetrathiafulvalene and X for a monovalent anion (X D PF6, AsF6 and SbF6).
However, the relationship between the electron localization phenomena and the role of the
lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the
literature. Here we present a brief overview of selected literature results, with emphasis placed
on recent thermal expansion experiments probing the charge-ordering transition of these salts.
An outstanding challenge in synthesis and theory is to develop molecular materials at ambient conditions that exhibit highly efficient energy transfer. Here we demonstrate the potential of a recently synthesized, highly conductive amorphous material—a nickel tetrathiafulvalene-tetrathiolate (NiTTFtt) polymer—to become an exciton condensate—a Bose-Einstein condensate of particle-hole pairs, known as excitons, that supports dissipationless flow of excitation energy. While exciton condensates have recently been realized in ordered materials, we show by advanced electronic structure calculations that this highly correlated phenomenon can potentially be realized in molecularly tailored, amorphous materials. In contrast to the Bechgaard salts that support superconductivity at compressed geometries requiring high pressures, we show that the recently synthesized, amorphous NiTTFtt polymer exhibits the computational signature of exciton condensation at experimentally realizable geometries, occurring at ambient pressures. Results suggest that superfluidity in this system and related systems—including van der Waals structures, molecular metals with extended-TTF dithiolate ligands, and Bechgaard salts—may occur via a nontraditional excitonic mechanism tuneable according to system composition, geometry, size, and charge. This study prompts further experimental investigation of the rational design of molecularly scaled exciton condensates with potential applications to efficient transport in technologically relevant materials.
The ionic-liquid-gating technique can be applied to the search for novel physical phenomena at low temperatures because of its wide controllability of the charge carrier density. Ionic-liquid-gated field-effect transistors are often fragile upon cooling, however, because of the large difference between the thermal expansion coefficients of frozen ionic liquids and solid target materials. In this paper, we provide a practical technique for setting up ionic-liquid-gated field-effect transistors for low-temperature measurements. It allows stable measurements and reduces the electronic inhomogeneity by reducing the shear strain generated in frozen ionic liquid.
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 transport properties of many of these systems. After reviewing the basic interactions and different ground states occurring in molecular conductors we consider in detail how anions influence the structure of the donor stacks and often guide them toward different types of transitions. Consideration of the Bechgaard and Fabre salts illustrates how anions play a crucial role in directing these salts through complex phase diagrams where different conducting and localized states are in competition. We also emphasize the important role of hydrogen bonding and conformational flexibility of donors related to BEDT-TTF and we discuss how anions have frequently a strong control of the electronic landscape of these materials. Charge ordering, metal to metal and metal to insulator transitions occurring in these salts are considered.
By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intra-dimer charge degrees of freedom are of particular importance in correlated $\kappa$-(BEDT-TTF)$_2$X salts and can create novel states, such as electronically-driven multiferroicity or charge-order-induced quasi-1D spin liquids.
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 30 K at normal incidence. Spectra taken around RT were also studied as a function of the angle () between the electric field of the X-ray beam and the 1D conducting direction. In contrast with a previous study of the S L2/3-edges spectra in (TMTTF)2AsF6, the Se M2/3-edges of (TMTSF)2PF6 do not exhibit a well resolved spectrum. Surprisingly, the C K-edge spectra contain three well defined peaks exhibiting strong and non-trivial and temperature dependence. The nature of these peaks as well as those of the F K-edge spectra could be rationalized on the basis of first-principles DFT calculations. Despite the structural similarity, the NEXAFS spectra of (TMTSF)2PF6 and (TMTTF)2AsF6 exhibit important differences. In contrast with the case of (TMTTF)2AsF6, the F K-edge spectra of (TMTSF)2PF6 do not change with temperature despite stronger donor-anion interactions. All these features reveal subtle differences in the electronic structure of the TMTSF and TMTTF families of salts.
