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(Quasi-)one-dimensional systems exhibit various fascinating properties such as Luttinger liquid behavior, Peierls transition, novel topological phases, and the accommodation of unique quasiparticles (e.g., spinon, holon, and soliton, etc.). Here we study molybdenum blue bronze A0.3MoO3 (A = K, Rb), a canonical quasi-one-dimensional charge-density-wave material, using laser-based angle-resolved photoemission spectroscopy. Our experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, we observe strong renormalizations of band dispersions, which is recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. We argue that the strong electron-phonon coupling plays a dominant role in electronic properties and the charge-density-wave transition in blue bronzes. Our results not only reconcile the long-standing heavy debates on the electronic properties of blue bronzes but also provide a rare platform to study novel composite quasiparticles in Luttinger liquid materials., 19 pages, 4 figures, accepted by Nature Communications

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge_{2}Sb_{2}Te_{5} has been widely used in optical data storage and nonvolatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation, and time-dependent density-functional theory molecular dynamic simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5}. The randomly oriented Peierls distortion among unit cells in the averaged rocksalt phase of Ge_{2}Sb_{2}Te_{5} is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in the individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ∼0.3 ps. In addition, the impact of the carrier relaxation and the large number of vacancies to the ultrafast structural response is quantified and discussed. Our Letter provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature of one such compound, \cis . We show that this hitherto overlooked fluctuating symmetry lowering is electronic in origin and will significantly modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties., 10 pages 3 figures

Recently the structural transition in Na2Ti3Cl8 at 200 K with the formation of triangular Ti3 clusters was studied by ab-initio calculations in PRL 124, 167203 (2020). It was concluded on the basis of the DFT+U results that the usual band effects are not sufficient to explain this transition. The authors then invoked magnetic mechanisms, including complicated factors such as biquadratic and ring exchanges. We point out that this overcomplicated description is unnecessary, and that the formation of trimers, or breathing kagome lattice in Na2Ti3Cl8, and in many other systems, has very simple, almost trivial explanation basing on the orbital-driven Peierls transition.

The spinel-structure CuIr$_{2}$S$_{4}$ compound displays a rather unusual orbitally-driven three-dimensional Peierls-like insulator-metal transition. The low-T symmetry-broken insulating state is especially interesting due to the existence of a metastable irradiation-induced disordered weakly conducting state. Here we study intense femtosecond optical pulse irradiation effects by means of the all-optical ultrafast multi-pulse time-resolved spectroscopy. We show that the structural coherence of the low-T broken symmetry state is strongly suppressed on a sub-picosecond timescale above a threshold excitation fluence resulting in a structurally inhomogeneous transient state which persists for several-tens of picoseconds before reverting to the low-T disordered weakly conducting state. The electronic order shows a transient gap filling at a significantly lower fluence threshold. The data suggest that the photoinduced-transition dynamics to the high-T metallic phase is governed by first-order-transition nucleation kinetics that prevents the complete ultrafast structural transition even when the absorbed energy significantly exceeds the equilibrium enthalpy difference to the high-T metallic phase. In contrast, the dynamically-decoupled electronic order is transiently suppressed on a sub-picosecond timescale rather independently due to a photoinduced Mott transition., arXiv admin note: substantial text overlap with arXiv:2005.02207

A correlation is established between the theories of superconductivity based on the concept of charge density waves (CDW) and the translation invariant (TI) bipolaron theory. It is shown that CDW are originated from TI-bipolaron states in the pseudogap phase due to Kohn anomaly and form a pair density wave (PDW) for wave vectors corresponding to nesting. Emerging in the pseudogap phase, CDW coexist with superconductivity at temperatures below that of superconducting transition while their wave amplitudes decrease as a Bose condensate is formed from TI-bipolarons, vanishing at zero temperature., Slightly edited and enhanced version. Published in Front. Phys. 7 pages, 1 figure

We have studied the magnetic, magnetotransport, and galvanomagnetic properties of ${\mathrm{TmNiC}}_{2}$. We find that the antiferromagnetic and field induced metamagnetic and ferromagnetic orderings do not suppress the charge density wave. The persistence of Fermi surface pockets, open as a result of imperfect nesting accompanying the Peierls transition, results in an electronic carriers mobility of the order of $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{ }{\mathrm{cm}}^{2}\text{ }{\mathrm{V}}^{\ensuremath{-}1}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ in ferromagnetic state, without any signatures for a significant deterioration of nesting properties. This is independently evidenced by high, nonsaturating linear magnetoresistance reaching 440% at $T=2\text{ }\text{ }\mathrm{K}$ and an analysis of the Hall conductivity. We thus demonstrate that, the coexistence of charge density wave and magnetism provides an alternative route to maintain high electronic mobility in the magnetically ordered state.

Soft-phonon modes of an undistorted phase encode a material’s preference for symmetry lowering. However, the evidence is sparse for the relationship between an unstable phonon wavevector’s reciprocal and the number of formula units in the stable distorted phase. This “1/q*-criterion” holds great potential for the first-principles design of materials, especially in low-dimension. We validate the approach on the Q1D organometallic materials space containing 1199 ring–metal units and identify candidates that are stable in undistorted (1 unit), Peierls (2 units), charge density wave (3–5 units), or long wave (>5 units) phases. We highlight materials exhibiting gap-opening as well as an uncommon gap-closing Peierls transition and discuss an example case stabilized as a charge density wave insulator. We present the data generated for this study through an interactive publicly accessible Big Data analytics platform (https://moldis.tifrh.res.in/data/rmq1d) facilitating limitless and seamless data-mining explorations.

The Peierls instability toward a charge density wave is a canonical example of phonon-driven strongly correlated physics and is intimately related to topological quantum matter and exotic superconductivity. We propose a method to realize an analogous photon-mediated Peierls transition, using a system of one-dimensional tubes of interacting Bose or Fermi atoms trapped inside a multimode confocal cavity. Pumping the cavity transversely engineers a cavity-mediated metal--to--insulator transition in the atomic system. For strongly interacting bosons in the Tonks-Girardeau limit, this transition can be understood (through fermionization) as being the Peierls instability. We extend the calculation to finite values of the interaction strength and derive analytic expressions for both the cavity field and mass gap. They display nontrivial power law dependence on the dimensionless matter-light coupling., Comment: 6 pages, 2 figures; supplement of 4 pages; published version

In the present Letter, a numerical study of the dynamic properties of one-dimensional (1D) incommensurate charge density wave (CDW) compound in the presence of free carriers (not affected by the Peierls transition) is carried out. The results obtained showed that the CDW sliding state is very sensitive to this local effect. The CDW depinning threshold field, excess current normalized and amplitude of Narrow-Band-Noise (NBN) are affected by these single particle’s excitations. This latter can be viewed as a screening effect of the applied electrical field by free carriers which relax in order to reduce the CDW polarization and generate a homogeneous system response.

Two-dimensional (2D) transition metal carbides (MXenes) exhibit excellent thermodynamic stability and remarkable mechanical strength and flexibility, as well as rich functionality, which attract considerable interest due to their potential application for high-performance flexible and stretchable devices. However, premature phonon instability of some non-hybrid MXenes was recently found to intrinsically limit their strength and flexibility, evoking passionate curiosity in pursuing an effective solution for more impressive mechanical properties. In this work, on the basis of an alloying strengthening mechanism, a combinational strategy is proposed to build ordered hybrid M2''M'C2O2 (M'' = Mo, W; M' = Ti, Zr, Hf) with remarkable dynamic stability and superior mechanical properties by hindering the premature phonon instability originating from the outer transition metals. By means of comprehensive screening, symmetrical-Mo2TiC2O2 is interestingly found to possess excellent stability at equilibrium and outstanding tolerance to phonon instability during straining compared to its Ti counterpart, being attributed to the character of the robust Mo-dz2 and O-pz hybridization. Although similar optical phonon soft modes appear in Ti3C2O2 and Mo2TiC2O2 under multiple loadings, the latter is much stiffer during straining. An in-depth analysis of deformed electronic structures reveals that a strain-induced increasing density of states in the vicinity of the Fermi level mainly composed of Mo-dz2 states facilitates the fatal phonon softening in Mo2TiC2O2 under biaxial tension, while differing from the mechanical instability in Ti3C2O2 triggered by a Peierls transition. Our findings provide a novel stabilization and strengthening strategy for 2D materials, and pave a new way for searching for 2D material candidates in designing flexible devices.

Using time- and angle-resolved photoemission spectroscopy, we investigate the photoexcited electron dynamics in a prototypical charge density wave (CDW) material, Rb0.3MoO3. We observe a pronounced slow relaxation component within the CDW-gap energies. This anomalously slow dynamics is independent of the wave number, and cannot be explained by the presence of impurities. We propose that phase-coherent domains are temporarily established after a transient closure of the CDW gap, and that the intragap excitations are attributed to relaxation of the domain boundaries.

Twin grain boundaries in MoSe2 are metallic and undergo a metal to insulator Peierls transition at low temperature. Growth of MoSe2 by molecular beam epitaxy results in the spontaneous formation of a high density of these twin grain boundaries, likely as a mechanism to incorporate Se deficiency in the film. Using scanning tunneling microscopy, we study the grain boundary network that is formed in homoepitaxially grown MoSe2 and for MoSe2 grown heteroepitaxially on MoS2 and HOPG substrates. No statistically relevant variation of the grain boundary concentration has been found for the different substrates, indicating that the grain boundary formation is substrate independent and depends mainly on the growth conditions. Twin grain boundaries exhibit three crystallographically identical orientations, and thus they form an intersecting network. Different intersection geometries are identified that imply varying defect configurations. These intersection points act as preferential nucleation sites for vapor-depo...

The uniaxial strain of quasi-one-dimensional conductor whiskers of orthorhombic TaS3 at a strain higher than e c ~ 0.8% leads to a sharp increase in the coherence of the properties of a charge density wave (CDW), which manifests itself in its motion in fields higher than threshold field E t . During uniaxial elongation, TaS3 is shown to exhibit the following unusual properties even in weak fields: Peierls transition temperature T P depends nonmonotonically on e, one-dimensional fluctuations weaken near T P , and the coherence length of a charge density increases at T < T P . Investigations in fields higher than E t show that the ultracoherent properties of CDW exist in a wide temperature range and are retained when temperature increases up to T P . These properties of CDW make it possible to observe a sharp increase in E t near T P and an almost jumplike increase in E t at T < 90 K. The increase in E t at T P is explained by a decrease in the coherence volume of CDW because of a fluctuational suppression of the Peierls gap.

The local structure of NaTiSi$_{2}$O$_{6}$ is examined across its Ti-dimerization orbital-assisted Peierls transition at 210 K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that on warming the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490 K. The ODL state is correlated over the length scale spanning $\sim$6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems., Comment: 7 pages, 5 figures

We study a one-dimensional system of strongly correlated bosons on a dynamical lattice. To this end, we extend the standard Bose-Hubbard Hamiltonian to include extra degrees of freedom on the bonds of the lattice. We show that this minimal model exhibits phenomena reminiscent of fermion-phonon models. In particular, we discover a bosonic analog of the Peierls transition, where the translational symmetry of the underlying lattice is spontaneously broken. This provides a dynamical mechanism to obtain a topological insulator in the presence of interactions, analogous to the Su-Schrieffer-Heeger model for electrons. We characterize the phase diagram numerically, showing different types of bond order waves and topological solitons. Finally, we study the possibility of implementing the model using atomic systems.

We use the density matrix renormalization group method to investigate the role of longitudinal quantized phonons on the Peierls transition in the spin-Peierls model. For both the $XY$ and Heisenberg spin-Peierls model we show that the staggered phonon order parameter scales as $\sqrt{\ensuremath{\lambda}}$ (and the dimerized bond order scales as $\ensuremath{\lambda}$) as $\ensuremath{\lambda}\ensuremath{\rightarrow}0$ (where $\ensuremath{\lambda}$ is the electron-phonon interaction). This result is true for both linear and cyclic chains. Thus, we conclude that the Peierls transition occurs at $\ensuremath{\lambda}=0$ in these models. Moreover, for the $XY$ spin-Peierls model we show that the quantum predictions for the bond order follow the classical prediction as a function of inverse chain size for small $\ensuremath{\lambda}$. We therefore conclude that the zero $\ensuremath{\lambda}$ phase transition is of the mean-field type.

The effect of an elastic spontaneous distortion of the crystal lattice of a doped semiconductor Ge:As near the insulator–metal (IM) phase transition has been discovered. The effect is manifested in the electron spin resonance (ESR) of neutral As atoms as a splitting of the single resonance absorption line. It observed at electron concentrations in the range 0.8 < n/nC < 1 at low temperatures T < 100 K (nC = 3.7 × 1017 cm-3 is the critical electron concentration for the IM phase transition). The splitting is the strongest along each of the six [110] directions, which indicates that the local lattice distortion occurs just in these directions. As a result, a sample is possibly divided into separate domains differing in the directions of compressive or tensile deformations. A study of concentration, temperature, and angular dependences of the effect has shown that the phenomenon discovered can be understood in terms of the Peierls spin transition model.

Electron spin resonance and electron–proton double resonance (Overhauser shift method) are used for the comparison of proton radiation damaged and as-grown (fluoranthene) 2 PF 6 single crystals. Chemical modification and various consequences of the nonuniform distribution of radiation induced defects in this quasi-one-dimensional organic conductor with defect dependent Peierls transition are worked out.

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic- metal transition at T-C = 180 K and transforms into a ferromagnetic insulator below T-MI = 95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above T-C to T-MI, which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4+:Cr3+ states in a 3:1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d-electrons/Cr) and the half-metallic ferromagnetism in the t(2g) up-spin band favor a low-energy Peierls metal-insulator transition.

One-dimensional (1D) solids exhibit a number of striking electronic structures including charge-density wave (CDW) and spin-density wave (SDW). Also, the Peierls theorem states that at zero temperature, a 1D system predicted by simple band theory to be a metal will spontaneously dimerize and open a finite fundamental bandgap, while at higher temperatures, it will assume the equidistant geometry with zero bandgap (a Peierls transition). We computationally study these unique electronic structures and transition in polyyne and all-trans polyacetylene using finite-temperature generalizations of ab initio spin-unrestricted Hartree–Fock (UHF) and spin-restricted coupled-cluster doubles (CCD) theories, extending upon previous work [He et al., J. Chem. Phys. 140, 024702 (2014)] that is based on spin-restricted Hartree–Fock (RHF) and second-order many-body perturbation (MP2) theories. Unlike RHF, UHF can predict SDW as well as CDW and metallic states, and unlike MP2, CCD does not diverge even if the underlying RHF reference wave function is metallic. UHF predicts a gapped SDW state with no dimerization at low temperatures, which gradually becomes metallic as the temperature is raised. CCD, meanwhile, confirms that electron correlation lowers the Peierls transition temperature. Furthermore, we show that the results from all theories for both polymers are subject to a unified interpretation in terms of the UHF solutions to the Hubbard–Peierls model using different values of the electron-electron interaction strength, U/t, in its Hamiltonian. The CCD wave function is shown to encompass the form of the exact solution of the Tomonaga–Luttinger model and is thus expected to describe accurately the electronic structure of Luttinger liquids.

Low dimensional (LD) metallic oxides have been a subject of continuous interest in the last two decades, mainly due to the electronic instabilities that they present at low temperatures. In particular, charge density waves (CDW) instabilities associated with a strong electron-phonon interaction have been found in Molybdenum metallic oxides such as KMo 6 O 17 purple bronze. We report an angle resolved photoemission (ARPES) study from room temperature (RT) to T ∼440 K well below the Peierls transition temperature for this material, with CDW transition temperature T CDW ∼120 K. We have focused on photoemission spectra along ΓM high symmetry direction as well as photoemission measurements were taken as a function of temperature at one representative k F point in the Brillouin zone in order to look for the characteristic gap opening after the phase transition. We found out a pseudogap opening and a decrease in the density of states near the Fermi energy, E F , consistent with the partial removal of the nested portions of the Fermi surface (FS) at temperature below the CDW transition. In order to elucidate possible Fermi liquid (FL) or non-Fermi liquid (NFL) behaviour we have compared the ARPES data with that one reported on quasi-lD K 0.3 MoO 3 blue bronze.

Influence of the crystal topology on the Peierls transition was investigated by means of ultrafast optical pump-probe measurements. The comparison of the exponential single particle decay between the whisker and ring NbSe 3 compounds reveals several pronounced differences around a phase transition temperature (T P ). A discrepancy from BCS-like temperature dependence of the gap was observed in the ring, suggesting a loss of phase coherence between adjacent chains in the closed-loop topology.

A simple consideration demonstrates that if the temperature of a second-order phase transition is suppressed by fluctuations, the dominating effect at the transition is a maximum, but not a jump of specific heat, c p . One can attribute a certain value of latent heat to the transition; an estimate of the latent heat is proposed and compared with the behavior of c p and of thermal expansion at the Peierls and superconducting transitions.

We report in this Letter a numerical study of the response of an incommensurate charge density wave (CDW) to an external electrical field in the presence of free carriers. The results show that the single particles excitations affect the local dynamical properties of the CDW, the CDW drift velocity and the time scale of the CDW relaxation phenomena.

Here, we report on the direct observation of lattice distortion in the new quasi-one-dimensional spin system TiOBr by X-ray diffraction experiment using synchrotoron radiation. By comparing the crystal structure between TiOBr and TiOCl, the difference in b -axis length was considered to play a key role in determining the physical properties of both systems. Using a single crystal sample of TiOBr, superlattice reflections were observed at (0,1.5,0) and (0,2.5,0). The temperature dependence of the superlattice reflections implied the first-order transition at T c1 =27 K. We discussed the phase transition and analyzed the data using a simple dimerization model.

We examine the transition temperature of spin-Peierls state, T SP , in the one-dimensional quarter-filled Peierls–Hubbard model with lattice dimerization and intersite repulsive interaction, V , by applying the bosonization and the renormalization-group method. It is shown that for large V the transition temperature T SP is suppressed due to the appearance of the charge ordering. In the spin-Peierls state, the location of the spin-singlet state is shifted continuously from the bond of the lattice to the on-site by the increase of the charge ordering. The result is compatible with our previous results of the ground state. We discuss the pressure effect on T SP , which has been observed in the NMR measurement.

The phonon dispersion relations in the two-dimensional Peierls phase (Peierls insulator) are studied numerically, focusing on a two-dimensional SSH (Su–Shrieffer–Heeger) model having a half-filled ...

Recent studies of finite-size effects in charge-density wave conductors are reviewed. Various manifestations of finite-size effects, including the transverse-size dependence of the nonlinear-conduction threshold field, the Peierls transition temperature, high-frequency conduction, and the relaxation rates of metastable states, are discussed. Resistivity jumps in thin samples, the smeared threshold field for nonlinear conduction, and threshold conduction above the Peierls transition temperature are considered, as are mesoscopic oscillations of the threshold field, one-dimensional conduction in thin crystals, absolute negative conductivity of quasi-one-dimensional conductors, the length dependence of the phase-slip voltage, and the Aharonov–Bohm oscillations in sliding CDWs. Problems yet to be solved are discussed.

The dipole P(F) of systems with periodic boundary conditions (PBC) in a static electric field F is applied to one-dimensional Peierls-Hubbard models for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for finite systems of N = 14 and 16 sites that are almost converged to infinite chains in deformable lattices subject to a Peierls transition. The electronic polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid stacks with alternating transfer integrals t(1 +/- \delta) diverges at the neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in dimerized chains. The Peierls or dimerization mode couples to charge fluctuations along the stack and results in large vibrational contributions, \alpha_{vib}, that are related to \partial P/\partial \delta and that peak sharply at the Peierls transition. The extension of P(F) to correlated electronic states yields the dielectric response \kappa of models with neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with parameters used previously for variable ionicity \rho and vibrational spectra of CT salts. The calculated \kappa accounts for the dielectric response of CT salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs (chloranil). The role of lattice stiffness appears clearly in models: soft systems have a Peierls instability at small \rho and continuous crossover to large \rho, while stiff stacks such as TTF-CA have a first-order transition with discontinuous \rho that is both a neutral-ionic and Peierls transition. The transitions are associated with tuning the electronic ground state of insulators via temperature or pressure in experiments, or via model parameters in calculations., Comment: 10 pages, 9 figures; J.Chem.Phys., in press

Pulsed ESR is used for microscopic imaging of the conduction electron spin dynamics in the organic crystal (FA) 2 PF 6 . Previous conductivity and X-ray measurements of this crystal show a Peierls transition at 180 K. The `local' diffusion measurements reveal that the spins diffuse freely far from the crystal boundaries. This enables the extraction of the free diffusion coefficient by a one-parametric fit. The temperature dependence of the diffusion coefficient was found to correlate with a previously reported structural transition occurring well above the Peierls transition.

Vanadium dioxide (VO2) is well known for its metal-insulator transition (MIT) at 341 K.Normally,the VO2 presents a metallic rutile (R) phase above the Tc,but an insulator (monoclinic,M) phase below the Tc.Besides the thermally driven mode,the phase transition can also be triggered electrically,which is common in electron devices like field effect transistors and actuators.Due to the electron correlation,the Mott transition associated with electronelectron interaction as well as the Peierls transition involving electron-lattice interaction are both believed to drive the transition of VO2,although the actual MIT mechanism is still under debate in condensed matter physics.The Coulomb screening of the electron hopping can be broken by injecting enough carriers.However,the issue is more complicated in the electrically-triggered MIT of VO2 due to the Joule heat of current and the carrier injection of field effect.In this work, we study the electrically induced MIT in VO2 nanowires by in-situ transmission electron microscopy (TEM).We build a closed circuit under the TEM by using in-situ electric TEM holder to capture the changes of VO2 in electron structure and phase structure simultaneously.An alternating bias voltage is applied to the VO2 nanowire while the selected area electron diffraction (SAED) patterns of VO2 nanowire are recorded using Gatan Oneview fast camera.The current rises or drops suddenly in the current-voltage curve (I-V curve),indicating a phase transition,through which the SAED pattern of nanowire is recoded every 5 ms.By correspondence analysis between the SAED patterns and the I-V data at every moment,a transition state of insulating R phase is observed,which is obviously different from the normal state of the metallic R phase or the insulating M phase.The existence of the insulating R phase indicates that electron structure transforms prior to the phase transition.The decoupling phenomenon reveals a predominant role of electron-electron interaction.Moreover,by feedback strategy of the circuit,the current through the metallic nanowire of VO2 remains unchanged,and thus keeping the Joule heating in the nanowire constant,the phase transition from metal to insulator does not happen until the voltage decreases to about 1 V.When phase transition to insulator happens in voltage stepdown,even stronger Joule heating is generated because of the increased resistance of VO2 nanowire.Therefore,the VO2 phase transition is triggered electrically by the carrier injection instead of the Joule heating.The injecting of enough carriers can break the screening effect to activate the electron hopping and initiate the phase transition.The deduction is confirmed by the decoupling phenomenon in the insulating R phase.Additionally,the polarized shift rather than the phase transition of the VO2 nanowire is observed in the non-contact electric field mode,which also supports the cause of the carrier injection for the electric induced MIT.The results prove the electron-correlation-driven MIT mechanism, or so called Mott mechanism,and open the new way for electron microscopy used to study the electron correlated MIT.

We report measurements of the dynamics of the charge-density-wave systems K0.3MoO3 and NbSe3 applying the quite recent technique of meV-resolution inelastic X-ray scattering (IXS). In K0.3MoO3, serving as a test-case with existing reference results from neutron scattering, the very steep slope of the phason dispersion has been reproduced by IXS together with a very good agreement for the observed “phason velocity": 26$km/sec$ (IXS) $vs.$ 23$km/sec$ (neutrons). In NbSe3, where IXS allows for dispersion studies on thin whiskers, the longitudinal acoustic and one transverse acoustic (TA1) branch propagating along the NbSe3-chain direction have been determined at room temperature. Low temperature TA1-data have been taken around the upper-CDW satellite position (0, 0.241, 0) at temperatures down to below the upper Peierls transition (Tp=145K). The low temperature results indicate the absence of a Kohn anomaly, in contrast to the CDW-related dynamics observed in K0.3MoO3 and KCP, but similar as in (TaSe4)2I.

The effect of correlation on Peierls transition, which is accompanied by a dimerization, t_d, of a bond alternation for transfer energy, has been examined for a half-filled one-dimensional electron system with on-site repulsive interaction (U). By applying the renormalization group method to the interaction of the bosonized Hamiltonian, the dimerization has been calculated variationally and self-consistently with a fixed electron-phonon coupling constant (\lambda) and it is shown that t_d takes a maximum as a function of U. The result is examined in terms of charge gap and spin gap and is compared with that of the numerical simulation by Hirsch [Phys. Rev. Lett 51 (1983) 296]. Relevance to the spin Peierls transition in organic conductors is discussed., Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jpn. 71 No.3 (2002)

We report results from a systematic strong-coupling expansion of a spin-1/2 Heisenberg chain coupled to Einstein phonons. In the non-adiabatic regime this model is used to describe zero temperature properties of CuGeO3. The linked cluster expansion allows the determination of observables in the thermodynamic limit preserving the full lattice dynamics without a truncation of the phononic Hilbert space. In particular, the spin gap and the dispersion of the elementary triplet excitation are calculated up to 10th order in a dimer expansion. The magnetic structure factor of the ground state is evaluated up to 6th order. We show that the spin-phonon coupling leads to a renormalization of the elementary triplet dispersion. Surprisingly in the non-adiabatic regime a substantial renormalization of the spin gap only sets in at much larger couplings than those proposed for CuGeO3. The ground state magnetic correlations are found to be hardly effected by the spin-phonon coupling, but dominated by the frustrating magnetic interaction in the parameter regime relevant for CuGeO3., revtex, 4 pages, 3 eps-figures, see also http://brahms.physik.uni-bonn.de/ClusterExpansion/DOC/Intro.html

Nuclear magnetic resonance of 19F nuclei in the radical cation salt (pyrene)12(SbF6)7 is analysed in dependence of temperature and frequency. Linewidth and spin-lattice relaxation reflect the rotational motion of the SbF6 octahedra that is frozen-in on approaching the Peierls transition. The relevance of this mode of molecular motions for the conduction electrons is demonstrated via Overhauser-shift detected pyrene-1H spin-lattice relaxation.

Temperature dependence of resistivity in quasi-two-dimensional purple bronzes AMo6O17 (A=Na, K, and Tl) has been comparatively studied between 2 and 300 K at different magnetic fields. Huge positive magnetoresistance (MR) behavior below the Peierls transition temperature TP∼110 K was found in both K/Tl purple bronzes for H∥c, but only a weak one in the Na purple bronze. At lower temperature TM∼16 K, a second anomaly of the resistivity, which moved to higher temperatures under a magnetic field, was also observed in both K/Tl compounds, with no corresponding anomaly in the Na purple bronze. For H⊥c, the MR effect becomes much weaker in all three compounds. These results suggest that the second anomaly is possibly related to the second Peierls instability, and the huge MR effect results from the large modification of the third FS along the [100] direction induced by the magnetic field, in which the third FS is imperfectly nested in K/Tl purple bronzes and absent in the Na purple bronze.

We show that the Peierls instability can result in softening of acoustic phonons with small wave vectors and suggest that this unusual transition takes place in carbon nanotubes, resulting in a static twist deformation of the nanotube lattice. The topological excitations in the ordered phase are immobile and propagate only in pairs.

The magnetic-field-induced 3D ordered phase of the two-leg spin ladder Cu2(C5H12N2)2Cl4 has been probed through measurements of 1H NMR spectra and 1/T1 in the temperature range 70 mK-1.2 K. The second order transition line T(c)(H) has been determined between H(c1) = 7.52 T and H(c2) = 13.5 T and varies as (H-H(c1))(2/3) close to H(c1). From the observation of anomalous shifts and a crossover in 1/T1 above T(c), the mechanism of the 3D transition is argued to be magnetoelastic as in spin-Peierls chains, here involving a displacement of the protons along the longitudinal exchange ( J( parallel)) path.

Results of local density approximation (LDA) and Hartree–Fock (HF) calculations for even numbered monocyclic rings are reported. Small Cn rings satisfying n=4N+2 show aromatic stability with equal bond-length structures, whereas rings of size n=4N show antiaromatic destabilization with bond-length alternation. For large rings a transition, the Peierls transition, from aromatic and antiaromatic to nonaromatic behavior, takes place. Above the Peierls transition, both n=4N and n=4N+2 rings show bond-length alternation and no differences in stability. The critical size for the transition to nonaromatic behavior depends on the electron-phonon coupling strength and therefore depends on the choice of ab initio method. HF predicts nonaromatic behavior for ring sizes n=14 and above. Fully optimized LDA results are presented up to n=42, which still has a cumulenic structure. Calculations based on periodic infinite ring systems show that within LDA the onset of nonaromatic behavior does not occur until n=82. Experim...

The magnetic susceptibility and specific heat of the one-dimensional S=1 bilinear-biquadratic Heisenberg model are calculated using the transfer matrix renormalization group. By comparing the results with the experimental data of ${\rm LiVGe_2O_6}$ measured by Millet et al. (Phys. Rev. Lett. {\bf 83}, 4176 (1999)), we find that the susceptibility data of this material, after subtracting the impurity contribution, can be quantitatively explained with this model. The biquadratic exchange interaction in this material is found to be ferromagnetic, i.e. with a positive coupling constant., 4 pages, 4 postscript figures

We conclude from 23Na and 51V NMR measurements in alpha(')-NaxV2O5 (x = 0.996) (i) a charge-ordering transition starting at T

The Peierls conductor o-TaS 3 enriched with isotope ( 34 5 (99%)) has been synthesized. The temperature and electric field dependencies of the conduction of these samples were studied. The results were compared with ones for samples of the same quality with natural isotope composition. The following parameters of the linear conduction were extracted: the Peierls transition temperature (Tp), the width (&T p ) and height of the peak of temperature derivative of conduction, activation energy (A). Small increase of the Peierls transition temperature by 1 ± 1 K was observed in Ta 34 S 3 samples.

Resistivity and magnetoresistance have been measured on single crystals of the monophosphate tungsten bronzes (PO 1 ) 4 (WO 3 ) 2m for m=5. The m = 5 compound exists in two structural varieties which lead to different resistivity and magnetoresistance properties. Shubnikov de Haas oscillations with large amplitude are observed in one of the variety. The origin of the observed anomalies in the resistivity vs temperature curves is discussed.

Very thin samples of TaS 3 (cross-section area ∼ 10 -3 μm 2 ) are found to demonstrate smearing of the I-V curves near the threshold field in the Peierls state. With approaching the Peierls transition temperature, Tp, the smearing evolves into smooth growth of conductance from zero voltage, interpreted by us as the contribution of fluctuations to the non-linear conductance. The fluctuation contribution to the linear conductance near Tp is identified independently. Both linear and non-linear contributions depend on temperature with close activation energies ∼ (2 - 4) x 10 3 K and apparently reveal the same process. We reject creep of the continuous charge-density waves (CDW) as an origin of the effect and show that it is spontaneous phase slippage that results to creep of the CDW. A model is proposed accounting for both the linear and non-linear parts of the fluctuation conduction up to Tp.

The magnetic susceptibility of a new one-dimensional S=1 system, the vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is consistent with a gapless spectrum above this temperature. We propose that this behavior is due to the presence of a significant biquadratic exchange interaction, a suggestion supported by quantum chemistry calculations that take into account the quasidegeneracy of the t2g levels.

We present the polarized reflection spectra of non-metallic DMTTA-BF 4 and the isostructural metallic DMTSA-BF 4 . From these spectra, both compounds were regarded as quasi-one-dimensional conductors. From the analysis of these spectra we show that DMTTA-BF 4 is a Mott insulator with U /4 t ≈0.8–1.2 and DMTSA-BF 4 is a weakly correlated metal with structural fluctuation near the room temperature. Resistivity, thermopower, and ESR are consistent with the above picture, and demonstrate the phase transitions at c a . 100 K for DMTTA-BF 4 and at c a . 150 K for DMTSA-BF 4 . The low-temperature reflection spectra of both compounds strongly suggest the breaking of screw-axis symmetry along the conducting axis. These phase transitions are regarded as the spin Peierls transition (DMTTA-BF 4 ) and Peierls (DMTSA-BF 4 ) transition. Assuming a dimerized stack, we estimate the transfer integrals t 1 and t 2 as 0.25 and 0.21 eV from the 10 K spectrum of DMTSA-BF 4 . The coupling constants of molecular vibration of DMT...

We describe quantitatively the structural change in finite Peierls chains. Full quantum mechanical calculations show: (i) how the smooth phase transition occurs by the continuous change of the ground state wave function which preserves the symmetry of the Hamiltonian; (ii) the structural change is accompanied by an anharmonicity which becomes very pronounced in the broad transition region; (iii) the dimerization is suppressed for sizes shorter than a certain critical size. Quantum phonon fluctuations can make the latter sufficiently large (∼ 0.1 - 1 µm) to be detected in mesoscopic systems ( e.g. perylene radical cation salts) with weaker electron-phonon coupling. As possible techniques for experimental observation, we suggest the nonlinear optical absorption and the ultrasonic attenuation. Our calculations are based on numerical diagonalization done exactly in small chains and within the adiabatic ansatz for longer chains. We indicate how to test selfconsistently the latter results and show that they are...

Measurements of the magnetic susceptibility and the powder X-ray diffraction pattern of diluted p-CyDOV radical crystals, (p-CyDOV) 1− x (p-CyDOV-H) x ( x =0∼0.10), were performed to study the effects of non-magnetic impurities, p-CyDOV-H (amine precursor of p-CyDOV), on the spin–Peierls transition of p-CyDOV. The transition temperature reduced rapidly upon p-CyDOV-H substitution and the spin–Peierls state disappeared around x =0.07. The effect of p-CyDOV-H substitution was not observed in the X-ray diffraction pattern of p-CyDOV.