spin-Peierls Ground State Research Articles

Basic aspects of the metal–insulator transition in vanadium dioxide VO2: a critical review

Vanadium dioxide exhibits a first order metal to insulator transition (MIT) at 340 K (T MI ) from a rutile (R) structure to a monoclinic (M 1 ) 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 VO 2 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 (M 1 , T, M 2 ) phases of VO 2 , 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 VO 2 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 V 4+ zig-zag chains and V–V pairs, stabilizes the M 2 and M 1 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 M 1 is analyzed via a mechanism of dimerization, in the T phase, of the spin 1/2 V 4+ zig-zag Heisenberg chains formed in the M 2 phase. This review summarizes in a critical manner the main results of the large literature on fundamental aspects of the MIT of VO 2 . It is completed by unpublished old results.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.

Complex changes in structural parameters hidden in the universal phase diagram of the quasi-one-dimensional organic conductors (TMTTF)2X ( X=NbF6, AsF6, PF6 , and Br)

Structural parameters in quasi-one-dimensional organic conductors ${(\mathrm{TMTTF})}_{2}X$ ($X=\mathrm{Nb}{\mathrm{F}}_{6}, \mathrm{As}{\mathrm{F}}_{6}, \mathrm{P}{\mathrm{F}}_{6}$, and Br) are investigated by synchrotron x-ray diffraction. The temperature dependences of the crystal structures differ significantly between octahedral and monatomic anion systems, corresponding to the presence or absence of a charge-ordering transition. Changes in temperature and chemical pressure control the dimensionality of the lattice and the degree of dimerization. Furthermore, the antiferromagnetic and spin-Peierls ground states in this system depend on the spin frustration due to transfer integral paths between the one-dimensional chain molecules.

The Optical Conductivity for a Spin-Peierls Ground State of $$\hbox {(TMTTF)}_{2}\hbox {PF}_{6}$$ (TMTTF) 2 PF 6 with Tetramer Formation

We theoretically investigate the optical conductivity of (TMTTF)$_{2}$PF$_{6}$ in the spin-Peierls ground state within the framework of the exact diagonalization method at absolute zero temperature ($T=0$). As an effective model, a 1/4-filled 1D (one-dimensional) extended Hubbard model with tetramerization is employed. Using appropriate parameters of the model which have already been reported, we clarify the electronic photoexcitation energies from the spin-Peierls ground state. Since some experiments indicate the formation of a tetramer in the spin-Peierls ground state of (TMTTF)$_{2}$PF$_{6}$, our results are useful to understand the effects of tetramerization on the optical properties of (TMTTF)$_{2}$PF$_{6}$.

Inelastic neutron scattering investigation of magnetostructural excitations in the spin-Peierls organic system (TMTTF)2PF6

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 ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$, 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 ${T}_{\mathrm{SP}}=13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In this paper, we present the first inelastic neutron scattering investigation of SP magnetic excitations in organics. Our paper reveals the presence above ${T}_{\mathrm{SP}}$ 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 ${T}_{\mathrm{SP}}^{\mathrm{MF}}\ensuremath{\approx}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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 ${T}_{\mathrm{SP}}$, 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 ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ in the classical (adiabatic) limit close to the classical/quantum crossover line. Then we provide arguments suggesting that pressurized ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ 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.

Correlation-assisted phonon softening and the orbital-selective Peierls transition in VO2

To explore the driving mechanisms of the metal-insulator transition (MIT) and the structural transition in VO${}_{2}$, we have investigated phonon dispersions of rutile VO${}_{2}$ ($R$-VO${}_{2}$) in the density functional theory (DFT) and the DFT$+U$ ($U$: Coulomb correlation) band calculations. We have found that the phonon softening instabilities occur in both cases, but the softened phonon mode only in the DFT$+U$ describes properly both the MIT and the structural transition from $R$-VO${}_{2}$ to monoclinic VO${}_{2}$ (M${}_{1}$-VO${}_{2}$). The present ab initio phonon dispersion calculations clearly demonstrate that the Coulomb correlation effect plays an essential role of assisting the Peierls transition in $R$-VO${}_{2}$ and producing the spin-Peierls ground state in M${}_{1}$-VO${}_{2}$.

1H and 195Pt NMR Study of the Parallel Two-Chain Compound Per2[Pt(mnt)2

1H and 195Pt NMR are used to probe the spin ½ anion chain in the quasi-one-dimensional conductor Per2[Pt(mnt)2], which exhibits nearly simultaneous charge density wave (CDW) and spin-Peierls (SP) transitions at low temperatures (Tc ~ 8 K). Below Tc the [Pt(mnt)2] chain forms a spin-singlet state that is evident in 1H NMR spectra and spin relaxation (1/T1) rates; however minority unpaired Pt spins may remain in the SP ground state. With increasing magnetic field, the SP and CDW order parameters decrease in unison, indicating they are coupled up to a critical field Bc ~ 20 T. Above Bc, the spin singlet evolves into a spin-polarized configuration. The 195Pt NMR signals vanish as either Tc or Bc are approached from within the SP ground state, suggesting the hyperfine field of the Pt nucleus is significantly stronger than at the proton sites. Simulations yield a consistent picture of the angular, temperature, and magnetic field-dependent spectral features.

Phase diagram of the correlated quarter-filled-band organic salt series (o-DMTTF)2X(X = Cl, Br, I)

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 techniques such as resistivity, thermopower, electron spin resonance, static magnetic measurements, and x-ray diffraction. In particular, we show that at ambient pressure (o-DMTTF)2X with X = Br and Cl undergoes two successive phase transitions setting successively a 4kF charge density and bond order wave order, then a spin-Peierls (SP) ground state. We discuss the symmetry of these phases and its relationship with the transport and magnetic properties. These phases are also followed under pressure by transport experiments, allowing the establishment of a generic phase diagram for this series of salts, where, with the onset of a one-dimensional to three-dimensional deconfinement transition, the 4kF order vanishes and the SP ground state transforms into a Peierls one. Interestingly, this phase diagram differs significantly from the one previously reported in other three-quarter-filled systems such as (TMTTF)2X and δ-(EDT-TTF-CONMe2)2X.

Interaction of magnetic field-dependent Peierls and spin-Peierls ground states in(Per)2[Pt(mnt)2

The interaction of parallel conducting perylene and insulating spin-1/2 $[\text{Pt}{(mnt)}_{2}]$ molecular chains in ${(Per)}_{2}{[\text{Pt}(mnt)}_{2}$] is investigated through the magnetic field dependence of their coupled Peierls (charge density wave - CDW) and spin-Peierls (SP) ground states. Proton NMR is used to monitor the spin-chain behavior, and signatures in the NMR spectrum and spin-relaxation rate are compared with previous electrical transport measurements that describe the metal-insulator transition in the perylene chains associated with the CDW phase boundary. We find that the SP and CDW chains remain coupled to high fields, at variance with a simple mean-field description, and that symmetry breaking in the perylene CDW transition may drive the nearly simultaneous SP transition.

Absence of static magnetic order in lightly-doped Ti1−xScxOCl down to 1.7 K

Impurity-induced magnetic order has been observed in many quasi-1D systems including doped variants of the spin-Peierls system CuGeO3. TiOCl is another quasi-1D quantum magnet with a spin-Peierls ground state, and the magnetic Ti sites of this system can be doped with non-magnetic Sc. To investigate the role of non-magnetic impurities in this system, we have performed both zero field and longitudinal field muSR experiments on polycrystalline Ti1-xScxOCl samples with x = 0, 0.01, and 0.03. We verified that TiOCl has a non-magnetic ground state, and we found no evidence for spin freezing or magnetic ordering in the lightly-doped Sc samples down to 1.7 K. Our results instead suggest that these systems remain non-magnetic up to the x = 0.03 Sc doping level.

Microwave dielectric study of spin-Peierls and charge-ordering transitions in(TMTTF)2PF6salts

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.

Inelastic neutron scattering investigation of magnetic excitations in the spin-Peierls ground state of

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)2PF6. The study, performed with a 98% deuterated powder, reveals the double gap structure of magnetic excitation soft the SP chain. These responses, as well as the observation of a high erenergy mode, occur at energies consistent with microscopic coupling deduced from magnetic measurements performed on the same sample.

Plaquette ground state of the Shastry-Sutherland model: Density-matrix renormalization-group calculations

I use the two-step density-matrix renormalization-group (DMRG) method based on two-leg ladder expansion to show numerical evidence of a plaquette ground state for ${J}_{2}=1.3{J}_{1}$ in the Shastry-Sutherland model. I argue that the DMRG method is very efficient in the strong frustration regime of two-dimensional spin models where a spin-Peierls ground state is expected to occur. It is thus complementary to quantum Monte Carlo algorithms, which are known to work well in the small frustration regime but which are plagued by the sign problem in the strong frustration regime.

Spin-Peierls and incommensurate states in layered S=1/2 system TiOCl

We demonstrate that TiOCl is a good model inorganic system to investigate spin-Peierls state. Our 35Cl and 47,49Ti NMR data show that a pseudo spin-gap behavior below T*=135 K precedes successive phase transitions at T c =94 K and T c ′ = 66 K into a singlet spin-Peierls ground state with a large energy gap E g / k B =430 K.

Incommensurate structure of the spin-Peierls compound TiOCl in zero and finite magnetic fields

We report on a detailed single crystal x-ray diffraction study of the unconventional spin-Peierls compound TiOCl. The intermediate phase of TiOCl is characterized by an incommensurate modulation which is virtually identical to that recently found in the homologue compound TiOBr. The first order phase transition between the spin-Peierls ground state and the incommensurate phase reveals the same kind of thermal hysteresis in both its crystal structure and magnetic susceptibility. A weak, but significant magnetic field effect is found for this phase transition with a field induced shift of the transition temperature of $\ensuremath{\Delta}{T}_{c1}=\ensuremath{-}0.13\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for an applied field of $B=10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ along the chain direction. The field induced changes of the incommensurate crystal structure are compatible with a scenario of competing intrachain and interchain interactions.

Neutron-scattering evidence for a spin-peierls ground state in (TMTTF)2PF6

Neutron-scattering evidence for a spin-peierls ground state in (TMTTF)2PF6

Magnetic and optical studies of spin and charge ordering in (TMTTF)2AsF6

We studied the magnetic and electronic properties of the insulating charge-transfer salt (TMTTF) 2 AsF 6 by electron spin resonance (ESR), SQUID magnetometer and optical conductivity measurements. For the first time, the lattice dynamics was investigated in the vicinity of the charge order transition. We found a splitting of the electron-molecular-vibration coupled totally symmetric intramolecular a g (v 3 ) mode below the charge-order transition at 100 K. From this result a charge disproportionation on the TMTTF molecular stacks of about 2:1 can be estimated. The spin degrees of freedom were investigated by ESR and SQUID. At high temperatures (TMTTF)2AsF6 behaves like a antiferromagnetic S=1/2 Heisenberg chain. Below 13 K, we observed an exponential decrease of the spin susceptibility indicating the phase transition into the spin-Peierls ground state.

Spin-Peierls lattice fluctuations and disorders in CuGeO$_\mathsf{3}$ and its solid solutions

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 which have been detected until 36 K. Furthermore we show that these fluctuations are 1D above 24 K, a feature expected for a structural instability triggered by the Cu2 + chains of spin 1/2. By extrapolating the thermal dependence of the correlation length in the chain direction, we show that formation of singlet dimers begins at about 50 K, a temperature that we identify as the mean field temperature of the SP chain. The critical nature of the pretransitional fluctuations does not change when low amounts (<1%) of non-magnetic dopants substitute either the Cu site (case of Zn and Mg) or the Ge site (case of Si and Al) of CuGeO3. However, the spatial extension of the fluctuations is considerably reduced when the magnetic dopant Ni substitutes the Cu site. In the SP ground state of doped materials we have been able to detect, in addition to the superlattice SP reflections previously observed, a very weak anisotropic diffuse scattering. We give evidences that this scattering originates from dopant-induced quasi-1D domains in which the dimerisation is perturbed. If we assume that each domain is limited by a soliton-antisoliton pair, pinned either on the substituent of the Cu site or by the deformation field induced by the substituent of the Ge site, we deduce that the soliton and antisoliton are separated by a distance of about $L_{0}\sim 28{-}45$ A, and that the soliton half width amounts to about $\xi_{SP}\sim 16{-}20$ A. With these numbers we are able to account for the rate of decrease of T SP as a function of the dopant concentration, and to deduce the critical concentration above which the long-range SP order vanishes. The overall size of the perturbed domains thus obtained, $L_{0} + 2\xi_{SP}\sim 70$ A, is comparable with the size of the magnetic inhomogeneities determined by muon spin spectroscopy in the AF phase of doped CuGeO3.

Spin-Peierls transition of the quasi-one-dimensional electronic system(DMe−DCNQI)2M(M=Li,Ag)probed by heat capacity

Through ac and thermal relaxation calorimetry techniques, we have performed a thermodynamic investigation of the spin-Peierls systems, $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Ag}$ and $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Li},$ where DMe-DCNQI is 2,5-dimethyl-${N,N}^{\ensuremath{'}}$-dicyanoquinonediimine. The fully gapped nature characteristic of the spin-Peierls (sP) ground state was confirmed by the absence of \ensuremath{\gamma} (T-linear) term in the low-temperature heat capacity for a single crystal. At higher temperatures, two distinct peaks were observed at 71 K and 86 K in heat capacity of $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Ag},$ while only a single peak was observed at 52 K for $(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}{)}_{2}\mathrm{Li}.$ The two-step structure in the Ag salt suggests that the sP transition of this material involves an intermediate state which is probably attributable to the degrees of freedom inside the dimers. For compacted pellets of numerous pieces of tiny crystals, however, the thermal anomaly around the sP transition disappears, but instead a \ensuremath{\gamma} term probably associated with the one-dimensional antiferromagnetic spin excitations appears. This implies that the present sP state is quite fragile against disorder or stress. These findings can be regarded as novel features of the sP transition in the quarter-filled band systems.

Evidence for spin solitons in spin-Peierls system (DMe-DCNQI)2Li

(DMe-DCNQI) 2 Li has a 1/4-filled π electron system and a spin-Peierls ground state. At low temperatures below 30K, the quasi-one dimensional spin diffusive motion of Curie electron spins are observed with variable frequency ESR measurement. It is a strong evidence for a presence of the spin solitons in a spin gap state of the tetramerized DCNQI chains. The intrachain diffusion rate obeys ∼T 2 below 30 K. The interchain hopping would be originated by the exchange interaction. Under pressure up to 1.5GPa, (DMe-DCNQI) 2 Li maintains highly one dimensional, and the spin soliton diffusion is affected only a little by the pressures.

Structural properties of strongly correlated quasi-1D organic systems

Correlated quasi-1D electron gas exhibit 2kF and 4kF CDW/BOW instabilities widely observed among the organic conductors. The Bechgaard and Fabre salts exhibit mixed SDW-CDW, Anti-ferromagnetism (AF) and spin peierls (SP) ground states achieving a subtle coupling between the spin and lattice degrees of freedom. The SP and AF nature of the Fabre salts’ ground state is analyzed in relation with the presence of a high temperature 4kF CDW-ferroelectric phase transition.