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Schematic representation of the structure of CuGeO3 showing the bonding network between the CuO2 squares and the GeO4 tetrahedra. The O(2) atoms are shared between the squares and the tetrahedra and the O(1) atoms are only shared between the tetrahedra.
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CuGeO3 exhibits a Spin-Peierls (SP) transition, at T
SP
= 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which
can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering
shows that in this 1D direction the cor...
Context in source publication
Context 1
... Section 3 the pretransitional X-ray fluctuations will be quantitatively analyzed and their dy- namics will be discussed in relationship with the neutron scattering data in Section 4. Finally, in Section 5, these findings will be compared to those found in the organic SP and Peierls systems. Additional concluding remarks will be presented in Section 6. Figure 1 shows the anisotropic bonding network of the orthorhombic (Pbmm) high temperature structure of CuGeO 3 . The magnetic S = 1/2 Cu 2+ surrounded by a square of oxygen atoms, denoted O(2) below, forms ribbons running along the c-direction. ...
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Citations
... IR-active phonons are available over a wide range of energies in the high-temperature structure [29], and the rather large unit cell of the spin-Peierls phase makes their number significant, although for laser-driving purposes we note that all of the A u modes are silent. Inelastic neutron scattering (INS) has been used to characterize all the phonon modes of the high-temperature phase [32,34], finding the strongest response at frequencies of 3.2 and 6.8 THz, and suggesting that the spin-Peierls transition is of a type occurring without an accompanying soft mode [59,60]. Here we comment that ultrafast methods appear to offer a qualitatively different approach to the investigation of low-lying phonons around and below the spin-Peierls temperature [61]. ...
Magnetophononics, the modulation of magnetic interactions by driving infrared-active lattice excitations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnet with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit an intrinsic self-blocking effect, whereby only a fraction of the available laser power is absorbed by the phonon. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking arises from the self-consistent alteration of the resonance frequencies. We link this to the appearance of mutually repelling collective spin-phonon states, which in the regime of strong hybridization become composites of a phonon and two triplons. We then identify the mechanism and optimal phonon frequencies by which to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum and demonstrate that this effect should be observable in ultrafast THz experiments on a number of known quantum magnetic materials.
... IR-active phonons are available over a wide range of energies in the high-temperature structure [29], and the rather large unit cell of the spin-Peierls phase makes their number significant, although for laser-driving purposes we note that all of the A u modes are silent. Inelastic neutron scattering (INS) has been used to characterize all the phonon modes of the high-temperature phase [32,34], finding the strongest response at frequencies of 3.2 and 6.8 THz, and suggesting that the spin-Peierls transition is of a type occurring without an accompanying soft mode [52,53]. Here we comment that ultrafast methods appear to offer a qualitatively different approach to the investigation of lowlying phonons around and below the spin-Peierls temperature [54]. ...
Magnetophononics, the modulation of magnetic interactions by driving infrared-active lattice exci-tations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnet with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit an intrinsic self-blocking effect, whereby only a fraction of the available laser power is absorbed by the phonon. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking arises from the self-consistent alteration of the resonance frequencies. We link this to the appearance of mutually repelling collective spin-phonon states, which in the regime of strong hybridization become composites of a phonon and two triplons. We then identify the mechanism and optimal phonon frequencies by which to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum and demonstrate that this effect should be observable in ultrafast THz experiments on a number of known quantum magnetic materials.
... The observation of a pseudo-gap in χ spin is the signature of a classical (adiabatic) regime of instability. It is found in many SP materials [2,4,18,27], some of which are considered below. However, there are other compounds such as TTF-CuBDT, MEM-(TCNQ) 2 [4,18] and Per 2 Pt(mnt) 2 [25] where SP pre-transitional lattice effects do not affect the spin susceptibility. ...
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 pair into S = 0 singlet dimers. This is well illustrated by the observation of a spin-Peierls (SP) instability in the (TMTTF)2X Fabre salts and related salts with the o-DMTTF donor. These instabilities are revealed by the formation of a pseudo-gap in the spin degrees of freedom triggered by the development of SP structural correlations. The divergence of these 1D fluctuations, together with the interchain coupling, drive a 3D-SP ground state. More surprisingly, we show that the Per2-M(mnt)2 system, undergoing a Kondo coupling between the metallic Per stack and the dithiolate stack of localized AF coupled spin ½ (for M = Pd, Ni, Pt), enhances the SP instability. Then, we consider the zig-zag spin ladder DTTTF2-M(mnt)2 system, where unusual singlet ground state properties are due to a combination of a 4kF charge localization effect in stack direction and a 2kF SP instability along the zig-zag ladder. Finally, we consider some specific features of correlated 1D systems concerning the coexistence of symmetrically different 4kF BOW and 4kF CDW orders in quarter-filled organics, and the nucleation of solitons in perturbed SP systems.
... eV per Å for organic SP systems [137]. As the reduced spin-phonon coupling λ SP entering in the theory of the SP transition is proportional to g 2 [135,136], λ SP of VO 2 should be one order of magnitude larger than λ SP of "conventional" SP systems [135,138]. The M 2 to T SP transition should thus be treated in the strong coupling limit. ...
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 of chemical and physical properties of VO2 and of its alloys, benefits of a renewed interest due to possible applications coming from the realization of devices made of thin films.
We describe in this review the structural, electronic and magnetic properties of the different metallic (R) and insulating (M1, T, M2) phases of VO2, of its solid solutions and under constraint. We present in a synthetic manner the various phase diagrams and their symmetry analysis. This work allows us to revisit older interpretation and to emphasize in particular the combined role of electron–electron interactions in the various phase of VO2 and of structural fluctuations in the MIT mechanism. In this framework we show that the phase transition is surprisingly announced by anisotropic one-dimensional (1D) structural fluctuations revealing chain like correlations between the V due to an incipient instability of the rutile structure. This leads to an unexpected critical dynamics of the order–disorder (or relaxation) type. We describe how the two-dimensional (2D) coupling between these 1D fluctuations, locally forming uniform V4+ zig-zag chains
and V–V pairs, stabilizes the M2 and M1 insulating phases. These phases exhibit a 1D electronic anisotropy where substantial electron–electron correlations conduct to a spin–charge decoupling. The spin-Peierls ground state of M1 is analyzed via a mechanism of dimerization, in the T phase, of the spin 1/2 V4+ zig�zag Heisenberg chains formed in the M2 phase.
This review summarizes in a critical manner the main results of the large literature on fundamental aspects of the MIT of VO2. It is completed by unpublished old results. Interpretations are also placed in a large conceptual frame which is also relevant to interpret physical
properties of other classes of materials.
... On the other hand, in many systems where the spin-Peierls instability occurs, the transition takes place in the nonadiabatic limit [9,61,62]. In such systems, an order-disorder critical spin-Peierls dynamics is predicted [63][64][65] and observed in inorganic CuGeO 3 [66] and organic (TMTTF) 2 PF 6 [67] spin-Peierls compounds. ...
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 instability with a quasielastic critical scattering which has been fully characterized. These observations are discussed in the scope of a Peierls instability dominated by strong electron-phonon coupling and/or nonadiabatic effects.
... Interestingly, g ep is comparable to the spinphonon coupling energy, g sp , driving the SP transition of the Fabre salts, for instance g sp ~ 8.5meV in (TMTTF) 2 PF 6 . [68]. Note that the electron-phonon coupling interaction can be directly probed by Resonant Inelastic X-ray Scattering (RIXS). ...
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 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.
... Estimation of microscopic parameters shows that organic SP compounds are located either in the classical region, as in (BCP − TTF) 2 X, or in the quantum region, as in MEM-(TCNQ) 2 , while the Fabre salts should be located in the vicinity of the classical-quantum crossover [19] (see also Fig. 10 in Ref. [20]); BCP-TTF is benzo-cyclopentyltetrathiafulvalene, MEM is methyl-ethyl-morpholinium, and TCNQ is tetracyano-quinodimethane. The inorganic SP compound CuGeO 3 is also located in the quantum region but in the vicinity of the QCP line at which the SP ground state vanishes. ...
... In this limit, the damping is due to the slowing down of the SP cluster life time τ , while the critical phonon frequency does not appreciably vary. Furthermore the observation of the critical growth of a central peak in energy in the neutron scattering spectrum [ Fig. 5(a)], associated to a critical slowing down of the SP dynamics together with a slight frequency hardening of the TA mode (see Appendix A) means that the SP transition of the (TMMTF) 2 PF 6 actually exhibits an order-disorder character [19,40]. Finally, the occurrence of a slow SP critical lattice dynamics is assessed by the detection in (TMTTF) 2 AsF 6 of a divergent 75 As NMR T −1 1 relaxation rate [36]. ...
... A previous investigation of CuGeO 3 , revealing the critical growth of a central peak in energy [41] and the frequency hardening of the critical phonon mode [42], shows also that the SP critical dynamics of CuGeO 3 is of the order-disorder type [19]. Note, however, that a different dynamics of the displacive type, which consists in a critical phonon frequency softening, as predicted in Ref. [35], is observed in the SP parented compound TiOCl [43]. ...
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.
... In this framework, the mechanism driving the SP transition depends on the relative value of the mean-field gap ∆ MF with respect to the q SP critical phonon energy:hΩ C ≈ 50-100 K for TA-LA phonon frequencies in organics [44]. For the Heisenberg chain, the SP transition occurs in the classical (adiabatic) limit whenhΩ C ≤ ∆ MF /2 [45]. ...
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 fluorenthene, perylene derivatives and anions X such as PF6 or AsF6 exhibit a high temperature (TP ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)2[M(mnt)2] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts exhibit the physics expected for a two chain Kondo lattice.
... With the above quoted TSP MF values, one gets Δ MF ≈ 75 K for the Pt salts and Δ MF ≈ 250 K for the Pd and Ni salts. In this framework, the mechanism driving the SP transition depends on the relative value of the mean‐field gap Δ MF with respect to the critical phonon energy: ħΩC ≈ 50–100 K for TA‐LA phonon frequencies in organics [44]. For the Heisenberg chain, the SP transition occurs in the classical (adiabatic) limit when ħΩC ≤ Δ MF /2 [45]. ...
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 fluorenthene, perylene derivatives and anions X such as PF6 or AsF6 exhibit a high temperature (TP ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)2[M(mnt)2] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts exhibit the physics expected for a two chain Kondo lattice.
... Because of the importance of zero-point quantum fluctuations, the singlet-triplet gap measured in the SP ground state is significantly smaller than Δ MF SP . Also the obtaining of Δ MF SP values comparable to the energy of the SP critical phonon Ω SP [42] offers the possibility to study the classical-quantum crossover in the SP phase diagrams established for XY [43] or Heisenberg [44] AF chains. Fig. 7 locates different SP compounds [45] in the phase diagram established for the SP Heisenberg chain [44]. ...
... The lifetime of the fluctuating electron-hole pairs can be expressed by: (24) In expression (24), ξ 0 is the electron-hole coherence length and v eh is the velocity of the electron-hole pair excitations. ξ o given in Table 1 is obtained either from the curvature of the Kohn anomaly if it exists (42) or from the hightemperature (∼T MF P ) value of the CDW correlation length ξ (42,32). v eh , obtained from expression (11), is expressed in Table 1 in fraction of the Fermi velocity v F , this latter quantity being obtained either from ab-initio band calculations or from ARPES measurements. ...
... The lifetime of the fluctuating electron-hole pairs can be expressed by: (24) In expression (24), ξ 0 is the electron-hole coherence length and v eh is the velocity of the electron-hole pair excitations. ξ o given in Table 1 is obtained either from the curvature of the Kohn anomaly if it exists (42) or from the hightemperature (∼T MF P ) value of the CDW correlation length ξ (42,32). v eh , obtained from expression (11), is expressed in Table 1 in fraction of the Fermi velocity v F , this latter quantity being obtained either from ab-initio band calculations or from ARPES measurements. ...
