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This brief review recalls some chapters in theory of sliding incommensurate density waves which may have appeared after inspirations from studies of I.E Dzyaloshinskii and collaborations with him. First we address the spin density waves which rich order parameter allows for an unusual object of a complex topological nature: a half-integer dislocation combined with a semi-vortex of the staggered magnetization. It becomes energetically preferable with respect to an ordinary dislocation due to the high Coulomb energy at low concentration of carriers. Generation of these objects should form a sequence of pi-phase slips in accordance with experimental doubling of phase-slips rate. Next, we revise the commonly employed TDGL approach which is shown to suffer from a violation of the charge conservation law resulting in nonphysical generation of particles which is particularly pronounced for electronic vortices in the course of their nucleation or motion. The suggested consistent theory exploits the chiral transformations taking into account the principle contribution of the fermionic chiral anomaly to the effective action. The derived equations clarify partitions of charges,currents and rigidity among subsystems of the condensate and normal carriers and the gluing electric field. Being non-analytical with respect to the order parameter, contrarily the conventional TDGL type, the resulting equations still allow for a numerical modeling of transient processes related to space- and spatio-temporal vorticity in DWs., arXiv admin note: text overlap with arXiv:1807.05512 more...

We present a general scheme to approach the space - time evolution of deformations, currents, and the electric field in charge density waves related to appearance of intrinsic topological defects: dislocations, their loops or pairs, and solitons. We derive general equations for the multi-fluid hydrodynamics taking into account the collective mode, electric field, normal electrons, and the intrinsic defects. These equations may allow to study the transformation of injected carriers from normal electrons to new periods of the charge density wave, the collective motion in constrained geometry, and the plastic states and flows. As an application, we present analytical and numerical solutions for distributions of fields around an isolated dislocation line in the regime of nonlinear screening by the gas of phase solitons. more...

The fundamental idea that the constituents of interacting many body systems in complex quantum materials may self-organise into long range order under highly non-equilibrium conditions leads to the notion that entirely new and unexpected functionalities might be artificially created. However, demonstrating new emergent order in highly non-equilibrium transitions has proven surprisingly difficult. In spite of huge recent advances in experimental ultrafast time-resolved techniques, methods that average over successive transition outcomes have so far proved incapable of elucidating the emerging spatial structure. Here, using scanning tunneling microscopy, we report for the first time the charge order emerging after a single transition outcome in a prototypical two-dimensional dichalcogenide 1T-TaS$_2$ initiated by a single optical pulse. By mapping the vector field of charge displacements of the emergent state, we find surprisingly intricate, long-range, topologically non-trivial charge order in which chiral domain tiling is intertwined with unique unpaired dislocations which play a crucial role in enhancing the emergent states remarkable stability. The discovery of the principles that lead to metastability in charge-ordered systems open the way to designing novel emergent functionalities, particularly ultrafast all-electronic non-volatile cryo-memories., preprint version of the paper published in npj Quantum Materials more...

The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T–TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states., The control of a material's state via external stimuli is the basis of modern information storage technology. Here, the authors use pulsed currents to induce fast switching between Mott insulator and metallic states in the charge density wave system 1T-TaS2, presenting an all-electronic storage mechanism. more...

International audience; Many recent experiments addressed manifestations of electronic crystals, particularly the charge density waves, in nano-junctions, under electric field effect, at high magnetic fields, together with real space visualizations by STM and micro X-ray diffraction. This activity returns the interest to stationary or transient states with static and dynamic topologically nontrivial configurations: electronic vortices as dislocations, instantons as phase slip centers, and ensembles of microscopic solitons. Describing and modeling these states and processes calls for an efficient phenomenological theory which should take into account the degenerate order parameter, various kinds of normal carriers and the electric field. Here we notice that the commonly employed time-depend Ginzburg–Landau approach suffers with violation of the charge conservation law resulting in unphysical generation of particles which is particularly strong for nucleating or moving electronic vortices. We present a consistent theory which exploits the chiral transformations taking into account the principle contribution of the fermionic chiral anomaly to the effective action. The resulting equations clarify partitions of charges, currents and rigidity among subsystems of the condensate and normal carriers. On this basis we perform the numerical modeling of a spontaneously generated coherent sequence of phase slips – the spacetime vortices – serving for the conversion among the injected normal current and the collective one. more...

Most common types of symmetry breaking in quasi-one-dimensional electronic systems possess a combined manifold of states degenerate with respect to both the phase $\ensuremath{\theta}$ and the amplitude $A$ sign of the order parameter $Aexp(i\ensuremath{\theta})$. These degrees of freedom can be controlled or accessed independently via either the spin polarization or the charge densities. To understand statistical properties and the phase diagram in the course of cooling under the controlled parameters, we present here an analytical treatment supported by Monte Carlo simulations for a generic coarse-grained two-field model of $XY$-Ising type. The degeneracies give rise to two coexisting types of topologically nontrivial configurations: phase vortices and amplitude kinks, i.e., the solitons. In two- and three-dimensional states with long-range (or Berezinskii---Kosterlitz---Thouless--type) orders, the topological confinement sets in at a temperature $T={T}_{1}$ which binds together the kinks and unusual half-integer vortices. At a lower $T={T}_{2}$, the solitons start to aggregate into walls formed as rods of amplitude kinks which are ultimately terminated by half-integer vortices. With lowering $T$, the walls multiply, passing sequentially across the sample. The presented results indicate a possible physical realization of a peculiar system of half-integer vortices with rods of amplitude kinks connecting their cores. Its experimental realization becomes feasible in view of recent successes in real-space observations and even manipulations of domain walls in correlated electronic systems. more...

Recent demonstrations of controlled switching between different ordered macroscopic states by impulsive electromagnetic perturbations in complex materials have opened some fundamental questions on the mechanisms responsible for such remarkable behavior. Here we experimentally address the question of whether two-dimensional (2D) Mott physics can be responsible for unusual switching between states of different electronic order in the layered dichalcogenide 1T-TaS2, or it is a result of subtle inter-layer “orbitronic” re-ordering of its stacking structure. We report on in-plane (IP) and out-of-plane (OP) resistance switching by current-pulse injection at low temperatures. Elucidating the controversial theoretical predictions, we also report on measurements of the anisotropy of the electrical resistivity "Equation missing""Equation missing"below room temperature. From the T-dependence of ρ⊥ and ρ||, we surmise that the resistivity is more consistent with collective motion than single particle diffusive or band-like transport. The relaxation dynamics of the metastable state for both IP and OP electron transport are seemingly governed by the same mesoscopic quantum re-ordering process. We conclude that 1T-TaS2 shows resistance switching arising from an interplay of both IP and OP correlations. more...

Experiments on optical and STM injection of carriers in layered $\mathrm{MX_2}$ materials revealed the formation of nanoscale patterns with networks and globules of domain walls. This is thought to be responsible for the metallization transition of the Mott insulator and for stabilization of a "hidden" state. In response, here we present studies of the classical charged lattice gas model emulating the superlattice of polarons ubiquitous to the material of choice $1T-\mathrm{TaS_2}$. The injection pulse was simulated by introducing a small random concentration of voids which subsequent evolution was followed by means of Monte Carlo cooling. Below the detected phase transition, the voids gradually coalesce into domain walls forming locally connected globules and then the global network leading to a mosaic fragmentation into domains with different degenerate ground states. The obtained patterns closely resemble the experimental STM visualizations. The surprising aggregation of charged voids is understood by fractionalization of their charges across the walls' lines., Comment: supplementary added more...

We suggest a phenomenological theory of dynamical phase transitions and the subsequent spaciotemporal evolution induced by a short optical pulse in a system which is already prone to a thermodynamic instability. We address the case of pumping to excitons whose density contributes additively to the thermodynamic order parameter like for charge-transfer excitons in electronic charge-ordering transitions. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the ``excitonic insulator'' (EI) and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number-preserved and phase-locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons, and the ground state of the EI. Modeling for an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domain merges. more...

We collect evidences on existence of microscopic solitons, and their determining role in electronic processes of quasi-1D conductors. The ferroelectric charge ordering gives access to several types of solitons in conductivity and permittivity, and to solitons' bound pairs in optics - both in insulating and conducting cases of TMTTF and TMTSF subfamilies. The excursion to physics of conjugated polymers allows to suggest further experiments. Internal tunnelling in Charge Density Waves goes through the channel of "amplitude solitons", which correspond to the long sought quasi-particle - the spinon. The same experiment gives an access to the reversible reconstruction of the junction via spontaneous creation of a lattice of 2Pi solitons - a grid of dislocations. The individual 2Pi solitons have been visually captured in recent STM experiments. Junctions of organic and oxide conductors are anticipated to show similar effects of reconstruction., Proceedings of ISCOM 2007, to be published in Solid State Sciences (2008) more...

We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor–acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh. more...

Victor Emery made seminal contributions to the theory of one-dimensional electronic systems and to its applications to organic metals. His inventions became illuminated recently when the joint effect of the ferroelectricity and the charge disproportionation has been discovered in (TMTTF)2X compounds and beyond. Several of his contributions came to agenda at once: separate gaps in spin/charge channels and the route to solitons, 4kF anomaly, dimerization gap, role of ionic transitions. New phenomena unify an unusual variety of concepts: ferroelectricity of good conductors, structural instability towards Mott-Hubbard state, Wigner crystallization in a dense electronic system, ordered 4kF density wave, richness of physics of solitons, interplay of structural and electronic symmetries. The ferroelectric state gives rise to several types of solitons carrying the electron charge, a noninteger charge, spin or both the spin and the charge in special cases. They are clearly observed via conductivity, electric and magnetic susceptibilities. Solitons are challenging for optics where they already seem to determine the pseudogap in absorption. Various features also appear, or are expected, from collective electronic and coupled electron-phonon modes. The last topic, as well as some aspects of physics of solitons, recalls also the contributions of M.J. Rice. Moreover, the observation of Mott-Hubbard states refers to classical results of A.A. Ovchinnikov., To be published in Synthetic Metals: proceedings of ICSM-04, the session in memory of V.J. Emery, M.J. Rice and A.A. Ovchinnikov. More information is available via presentations deposited at http://ipnweb.in2p3.fr/~lptms/membres/brazov/seminars.html more...

Recently the ferroelectric anomaly (Nad, Monceau, et al) followed by the charge disproportionation (Brown, et al) have been discovered in (TMTTF)2X compounds. The corresponding theory of the combined Mott-Hubbard state describes both effects by interference of the build-in nonequivalence of bonds and the spontaneous one of sites. The state gives rise to three types of solitons: \pi solitons (holons) are observed via the activation energy \Delta in the conductivity $G$; noninteger \alpha solitons (the FE domain walls) provide the frequency dispersion of the ferroelectric response; combined spin-charge solitons determine G(T) below subsequent structural transitions of the tetramerisation. The photoconductivity gap 2\Delta is determined by creations of soliton - antisoliton pairs. The optical edge lies well below, given by the collective ferroelectric mode which coexists with the combined electron-phonon resonance and the phonon antiresonance. The charge disproportionation and the ferroelectricity can exist hiddenly even in the Se subfamily giving rise to the unexplained yet low frequency optical peak, the enhanced pseudogap and traces of phonons activation. more...

We review the solid state physics approach to electronic and optical properties of conducting polymers also with attempts to bring together languages of the solid state theory for polymers and the quantum chemistry of oligomers. We consider polymers as generic one-dimensional semiconductors with features of strongly correlated electronic systems. Our model combines the long range electron–hole Coulomb attraction with a specific effect of strong intra-monomer electronic correlations, which results in effective intra-monomer electron–hole repulsion. We explain, exploit and organize various experimental and numerical findings. For example we connect such different questions as shallow singlet and deep triplet excitons in phenylenes, A g –B u exciton levels crossing in polyenes, common 1/ N energy dependencies in oligomers. more...

The review is devoted to the theory of collective and it local pinning effects in various disordered non-linear driven systems. Although the emphasis is put on charge and spin density waves and magnetic domain walls, the theory has also applications to flux lines and lattices thereof, dislocation lines, adsorbed mono-layers and related systems. In the first part we focus on the theory of the collective pinning which includes the equilibrium properties of elastic systems with frozen-in disorder as well as the features close to the dynamic depinning transition enforced by an external driving force and at finite temperatures. Thermal fluctuations smear out this transition and allow for a creep motion of the elastic objects even at small forces. An ac-driving force also destroys the sharp transition which is replaced by a velocity hysteresis. The second part is devoted to the local pinning picture and its applications. Inclusion of plastic deformations results in a rich cross-over behavior of the force-velocity relation as well as of the frequency dependence of the dynamic response. The local pinning recovers and exploits new elements of the energy landscape such as termination points of metastable branches or irreversibility of other ones related to generation of topological defects in the course of sliding. It also gives access to the quantum creep described as a tunneling between retarded and advanced configurations., 73 pages, 29 figures; Advances in Physics 2004 (in press) more...

The quasi one-dimensional organic conductor (TMTTF)2AsF6 shows the charge ordering transition at Tc101K to a state of the ferroelectric Mott insulator which is still well conducting. We present and interpret the experimental data on the gigantic dielectric response in the vicinity of TCO, concentrating on the frequency dependence of the inverse $1/\epsilon$ of the complex permittivity $\epsilon=\epsilon^\prime+i\epsilon^{\prime\prime}$. Surprisingly for a ferroelectric, we could closely approach the 2nd order phase transition and to deeply reach the critical dynamics of the polarization. We could analyse the critical slowing-down when approaching Tc from both sides and to extract the anomalous power law for the frequency dependence of the order parameter viscosity. Moreover, below Tc we could extract a sharp absorption feature coming from a motion of domain walls which shows up at a frequency well below the relaxation rate., Comment: Proceedings of the international school-workshop on Electronic Crystals: ECRYS-2014, Physica B (2014) more...

We derive and study equations for dissipative transient processes in a constraint incommensurate charge density wave (CDW) with remnant pockets or a thermal population of normal carriers. The attention was paid to give the correct conservation of condensed and normal electrons, which was problematic at presence of moving dislocation cores if working within an intuitive Ginzburg-Landau like model. We performed a numeric modelling for stationary and transient states in a rectangular geometry when the voltage V or the normal current are applied across the conducting chains. We observe creation of an array of electronic vortices, the dislocations, at or close to the junction surface; their number increases stepwise with increasing V. The dislocation core strongly concentrates the normal carriers but the CDW phase distortions almost neutralize the total charge. At other regimes, the lines of the zero CDW amplitude flash across the sample working as phase slips. The studies were inspired by, and can be applied to experiments on mesa-junctions in NbSe3 and TaS3 (Yu.I. Latyshev et al in proceedings of ECRYS 2008 and 2011)., Comment: "Special Issue on the International Workshop on Electronic Crystals (ECRYS-2014)", Physica B (2015) more...

We review existing manifestations and prospects for ferroelectricity in electronically and optically active carbon-based materials. The focus point is the proposal for the electronic ferroelectricity in conjugated polymers from the family of substituted polyacetylenes. The attractive feature of synthetic organic ferroelectrics is a very high polarizability coming from redistribution of the electronic density, rather than from conventional displacements of ions. Next fortunate peculiarity is the symmetry determined predictable design of perspective materials. The macroscopic electric polarization follows ultimately from combination of two types of a microscopic symmetry breaking which are ubiquitous to qusi-1D electronic systems. The state supports anomalous quasi-particles - microscopic solitons, carrying non-integer electric charges, which here play the role of nano-scale nucleus of ferroelectric domain walls. Their spectroscopic features in optics can interfere with low-frequency ferroelectric repolarization providing new accesses and applications. In addition to already existing electronic ferroelectricity in organic crystals and donor-acceptor chains, we point to a class of conducting polymers and may be also to nano-ribbons of the graphene where such a state can be found. These proposals may lead to potential applications in modern intensive searches of carbon ferroelectrics., Comment: To be published in the Int. J. of Synthetic Metals, March 2016; the special issue in honor of Yung Woo Park more...

International audience; We have performed scanning tunneling microscopy experiments in the quasi-one dimensional charge density wave (CDW) system NbSe3, where we could image and study in detail individual solitons corresponding to the self-trapping of a single electron. Our analysis shows that the type of soliton we observed is an ``Amplitude Soliton'', characterized by a vanishing CDW amplitude at the soliton center and by a pi-shift of its phase along the chain direction. Pairs of solitons or multiple solitons were also observed. Such observations could be made only in the high-temperature COW phase. Additionally, one-dimensional Friedel oscillations around charged defects could also be imaged and are found to be superimposed to the COW. The distinction between amplitude solitons and Friedel oscillations can be made without any ambiguity. (C) 2014 Elsevier B.V. All rights reserved. more...

Phase transitions induced by short optical pulses is a new mainstream in studies of cooperative electronic states. Its special realization in systems with neutral-ionic transformations stands out in a way that the optical pumping goes to excitons rather than to electronic bands. We present a semi-phenomenological modeling of spacio-temporal effects applicable to any system where the optical excitons are coupled to a symmetry breaking order parameter. In our scenario, after a short initial pulse of photons, a quasi-condensate of excitons appears as a macroscopic quantum state which then evolves interacting with other degrees of freedom prone to instability. This coupling leads to self-trapping of excitons; that locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains which evolve through dynamical phase transitions and may persist even after the initiating excitons have recombined. We recover dynamic interplays of fields such as the excitons wave function, electronic charge transfer and polarization, lattice deformations., Comment: Proceedings of international school-conference on electronic crystals: ECRYS-2014 more...

International audience; Using high-resolution X-ray scattering techniques, we measure the variation, q(x), of the position in reciprocal space of the CDW satellite, in the sliding state, along the length of NbSe_3 whiskers. We show that structural defects and intentionally X-ray radiation-damaged regions increase locally the CDW pinning force, and induce CDW phase distortions which are consistent with those observed near contacts. Using the semi-microscopic model from Brazovskii describing the normal-condensed carrier conversion, with spatially varying parameters, we account for the experimental spatial dependence of the CDW phase gradient near both types of defects. more...

We present experimental evidence and a corresponding theory for the ferroelectric transition in the family of quasi-one-dimensional conductors $(\mathrm{TMTTF}{)}_{2}X$. We interpret this new transition in the frame of the combined Mott-Hubbard state taking into account the double action of the spontaneous charge disproportionation on the TMTTF molecular stacks and of the $X$ anionic potentials. more...

We present an x-ray diffraction study of the vanadium-doped blue bronze ${\mathrm{K}}_{0.3}({\mathrm{Mo}}_{0.972}{\mathrm{V}}_{0.028}){\mathrm{O}}_{3}.$ At low temperature, we have observed both an intensity asymmetry of the $\ifmmode\pm\else\textpm\fi{}{2k}_{F}$ satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that the profile asymmetry is due to Friedel oscillation around the V substituant and that the intensity asymmetry is related to the charge-density-wave (CDW) pinning. These two effects, intensity, and profile asymmetries, give access to the local properties of CDW in disordered systems, including the pinning and even the phase shift of Friedel oscillations. more...

International audience; We compare the ferroelectricity for two families of synthetic conductors: the experimentally found ferroelectricity in organic crystal like (TMTTF)2X and a plausilble ferroelectricity in specially designed polyene chains. The difference of the polymer with respect to the organic conductor is the reverse of the build-in (now the sites) and spontaneous (now the bonds) effects of dimerizations. The theory predicts the existence of solitons (dimerization kinks) with non-integer variable charges, both with and without the spin. With today's understanding, we see them as walls separating domains with opposite electric polarization. Their physics will serve to relate transient ferroelectric processes and the visible range optics. more...

International audience; Transformations of cooperative electronic states by impacts of optical pumping or/and electrostatic doping is a new mainstream in physics of correlated systems. Here we present a semi-phenomenological modelling of spacio-temporal effects in a system where the light absorption goes through a channel creating the excitons – intra-molecular ones or bound electron-hole pairs -and finally the condensate of optical excitons feeds and stimulates phase transformations. Interacting with a near-critical order parameter and deformations, the excitons are subject to self-trapping. That locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The model can be used e.g. as a simplified version of optically induced neutral-ionic transitions in organic chain compounds. more...

International audience; Polymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the non-linear local excitations such as solitons, polarons and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here we report our observations of a qualitatively different MC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite) MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge). more...

We have studied by scanning tunneling microscopy (STM) the two charge-density wave (CDW) transitions in NbSe3 on in situ cleaved ( b , c ) plane. We could identify the three types of chains existing inside a single unit cell as well as characterize how both CDWs are distributed on these elementary chains. We also followed between 5 and 140 K the temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, to extract the surface critical temperatures ( T s ) . Whereas the high-temperature CDW appears to have comparable critical temperature to the bulk one, the low-T CDW transition occurs at T 2 s = 70 – 75 K , more than 15 K above the bulk T 2 b = 59 K while at exactly the same wave number. A reasonable mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation. more...

International audience; We report on scanning-tunneling microscopy experiments in a charge-density wave (CDW) system allowing visually capturing and studying in detail the individual solitons corresponding to the self-trapping of just one electron. This ``Amplitude Soliton'' is marked by vanishing of the CDW amplitude and by the pi shift of its phase. It might be the realization of the spinon-the long-sought particle (along with the holon) in the study of science of strongly correlated electronic systems. As a distinct feature we also observe one-dimensional Friedel oscillations superimposed on the CDW which develop independently of solitons. more...

Nonlinear local excitations like solitons, polarons, and bipolarons are known to be responsible for physical properties of conducting polymers. Recent experiments on nano-fibers in high electric and magnetic fields provide a further insight by demonstrating an effect of vanishing magnetoconductance (MC) in the polyacetylene (PA)—in contrast to other polymers. Here we present new experimental data and describe the theoretical model based on notion of solitons—dimerization kinks which can carry either the spin or the charge; they are allowed only in the PA with its degenerate ground state. The solitons experience a confinement force due to the interchange coupling which is erased by the electric field and disappears above critical field strength. The unbinding by tunneling allows for the transport of individual solitons, which sweeps off the spins residing at electronic intragap states associated with polarons, hence the vanishing MC. more...

The two charge-density wave (CDW) transitions in ${\mathrm{NbSe}}_{3}$ were investigated by scanning tunneling microscopy (STM) on an in situ cleaved ($\mathbit{b}$, $\mathbit{c}$) plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5 and 140 K to extract the surface critical temperatures (${T}_{s}$). The low-$T$ CDW transition occurs at ${T}_{2s}=70--75\text{ }\text{ }\mathrm{K}$, more than 15 K above the bulk ${T}_{2b}=59\text{ }\text{ }\mathrm{K}$ while at exactly the same wave number. A plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions. more...

We report a comprehensive investigation of the organic superconductor ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ in the vicinity of the endpoint of the spin-density-wave metal phase transition where phase coexistence occurs. At low temperature, the transition of metallic domains toward superconductivity is used to reveal the various textures. In particular, we demonstrate experimentally the existence of one-dimensional and two-dimensional (2D) metallic domains with a crossover from a filamentary superconductivity mostly along the ${c}^{\ensuremath{\star}}$ axis to a 2D superconductivity in the ${b}^{\ensuremath{'}}c$-plane perpendicular to the most conducting direction. The formation of these domain walls may be related to the proposal of a soliton phase in the vicinity of the critical pressure of the ${(\text{TMTSF})}_{2}{\text{PF}}_{6}$ phase diagram. more...

The temporal evolution of systems undergoing symmetry breaking phase transitions (SBTs) is of great fundamental interest not only in condensed matter physics, but extends from cosmology to brain function and finance \cite{topology,Kibble,Eltsov,Finance}. However, the study of such transitions is often hindered by the fact that they are difficult to repeat, or they occur very rapidly. Here we report for the first time on a high-time-resolution study of the evolution of both bosonic and fermionic excitations through a second order electronic charge-ordering SBT in a condensed matter system. Using a new three-pulse femtosecond spectroscopy technique, we periodically quench our model system into the high-symmetry state, detecting hitherto unrecorded coherent aperiodic undulations of the order parameter (OP), critical slowing down of the collective mode, and evolution of the particle-hole gap appearing through the Peierls-BCS mechanism as the system evolves through the transition. Numerical modeling based on Ginzburg-Landau theory is used to reproduce the observations without free parameters. The close analogy with other Higgs potentials in particle physics\cite{Higgs} gives new insight into hitherto unexplored dynamics of both single particle and collective excitations through a SBT. Of particular interest is the observation of spectro-temporal distortions caused by disturbances of the field arising from spontaneous annihilation of topological defects, similar to those discussed by the Kibble-Zurek cosmological model., Comment: 4 figures more...

We study theoretically the effects of impurities and of gap distortions in binding and self-trapping of particles added or excited over the insulating state of an antiferromagnetic Mott insulator. Wave functions of single particles or of bound excitons are affected by interactions with collective degrees of freedom, as shown by ARPES and optics experiments. The resulting self-localized state lowers the total particle energy, enhances its effective mass and splits off the electronic level below the nominal insulating gap. Most of the features measured experimentally in lightly n-doped cuprates are related to electronic states originated by anti-nodal points ( π , 0 ) of the Brillouin zone. In these points the van Hove singularity plays a major role by enhancing bound states of the electron with neutral and charged impurities and of inter-gap excitons. more...

International audience

International audience; The charge density wave (CDW) can be strained by applied voltage or by charge transfer at junctions, or by an electric field which can also put the CDW into sliding. Such strains can give rise to local or long-range modulational instabilities. We consider and perform a numerical modeling for two cases of strained CDW, the case of voltage is applied along and across conducting chains. We consider the effect of random and periodic dislocation array on the X-ray profile and present the plausible scenario of a very long-wave coherent periodic unharmonic superstructure. For the voltage allied across the chains, we confirm the reconstruction of the junction by creating an array of dislocations, whose number increases by steps with increasing voltage. The dislocation cores concentrate most of the voltage drop across the junction, giving rise to an effective tunneling. We compare the results with the experiments on coherent micro-diffraction in blue bronzes and with tunneling experiments in NbSe3 and TaS3. more...

This short review aims to summarize on "What the Charge Density Waves can tell to other inhomogeneous states in strongly correlated systems, particularly to spin-polarized superconductors". We shall update on expanding observations of solitons in quasi 1D CDW conductors and link them to the growing information and demands related to inhomogeneous spin-polarized states in superconductors. The related theory, existent or awaited for, stretches from solitons in 1D models to vortex-like elementary excitations in 2D,3D ordered incommensurate CDWs and superconductors., Comment: After presentations given at the conferences STRIPES 2008 and ECRYS 2008 more...

We suggest a theory of internal coherent tunneling in the pseudogap region when the applied voltage U is below the free electron gap 2{delta}{sub 0}. We address quasi-one-dimensional (quasi 1D) systems, where the gap is originated by spontaneous lattice distortions of the incommensurate charge density wave type. The results can be adjusted also to quasi 1D superconductors. The instanton approach allows one to calculate the interchain tunneling current both in single electron (amplitude solitons, i.e., spinons) and bielectron (phase slips) channels. Transition rates are governed by a dissipative dynamics originated by the emission of gapless phase excitations in the course of the instanton process. We find that the single-electron tunneling is allowed down to the true pair-breaking threshold at U{sub c1}=2W{sub as} more...

We present an x-ray diffuse scattering study of the charge-density wave (CDW) structure in the doped blue bronzes K{sub 0.3}(Mo{sub 1-x}V{sub x})O{sub 3} (x=2.8%), Rb{sub 0.3}(Mo{sub 1-x}V{sub x})O{sub 3} (x=1.44% and 0.28%), and K{sub 0.3}(Mo{sub 1-x}W{sub x})O{sub 3} (x=2%). At low temperature, the {+-}2k{sub F} satellite reflections are broadened in all four compounds, and shifted in V-doped compounds. Moreover, we have observed an intensity asymmetry of the {+-}2k{sub F} satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that both effects, intensity and profile asymmetry, give access to the local properties of CDW in disordered systems, including the pinning and the phase shifts around impurities. This leads us to propose a complete scenario of the pinning in doped blue bronzes. In the V-doped compounds, we show that the profile asymmetry is due to Friedel oscillations around the charged V substituent and that the intensity asymmetry is related to the strong pinning of the CDW. In the W-doped crystal, the CDW is weakly pinned in regions containing a few tens of neutral W substituents and the CDW phase is slightly distorted around the W impurities. Exact calculations are presented to sustain these ideas. more...

Tunneling spectra of chain materials NbSe{sub 3} and TaS{sub 3} were studied in nanoscale mesa devices. Current-voltage I-V characteristics related to all charge density waves (CDWs) reveal universal spectra within the normally forbidden region of low V, below the electronic CDW gap 2{delta}. The tunneling always demonstrates a threshold V{sub t}{approx_equal}0.2{delta}, followed, for both CDWs in NbSe{sub 3}, by a staircase fine structure. T dependencies of V{sub t}(T) and {delta}(T) scale together for each CDW, while the low T values V{sub t}(0) correlate with the CDWs' transition temperatures T{sub p}. Fine structures of CDWs perfectly coincide when scaled along V/{delta}. The results evidence the sequential entering of CDW vortices (dislocations) in the junction area with the tunneling current concentrated in their cores. The subgap tunneling proceeds via the phase channel: coherent phase slips at neighboring chains. more...

We report on direct observation of microscopic solitons in single electronic processes of the coherent interlayer tunneling in charge density waves. Special nanoscale devices were fabricated from the chain compound using focused ion beams. The spectra were drastically refined by working at high (up to 27 T) magnetic fields. Internal quantum tunneling of electrons can go through solitons that are energetically more favorable quantum particles than electrons. In addition to the interband tunneling across the gap 2Delta, we observe a clear peak at the intermediate voltage approximately 2Delta/3, which we associate with the creation of microscopic solitons, the energy of which must be 2Delta/pi. These solitons might correspond to the long sought special quasiparticle--the spinon. more...

International audience; Recent experiments on coherent interlayer tunneling in Charge Density Wave materials can be interpreted as direct observation of microscopic solitons in single electronic processes. Tunneling of electrons can go through the channel of solitons which are more favorable quantum particles than electrons. These solitons might correspond to the well searched special quasiparticle - the spinon. We shall summarize the necessary elements of theory in conjunction with both new experiments and earlier available data. more...

The creep motion of the charge-density wave (CDW) and its modulation through optical excitation in the low-temperature state of K 0.3 MoO 3 are analyzed in terms of a local pinning theory, in which local metastable states created by pinning induced phase dislocations are considered. The nonlinear I-V curve below the sliding threshold field (V s ) can be well reproduced with a numerical calculation, and it is shown that photoexcitation increases the transition probability of phase slips, which promotes the creep motion, and also leads to a relaxation of the phase strain. more...

We suggest a theory of internal coherent tunneling in the pseudogap region where the applied voltage is below the free electron gap. We consider quasi 1D systems where the gap is originated by a lattice dimerization like in polyacethylene, as well as low symmetry 1D semiconductors. Results may be applied to several types of conjugated polymers, to semiconducting nanotubes and to quantum wires of semiconductors. The approach may be generalized to tunneling in strongly correlated systems showing the pseudogap effect, like the family of High Tc materials in the undoped limit. We demonstrate the evolution of tunneling current-voltage characteristics from smearing the free electron gap down to threshold for tunneling of polarons and further down to the region of bi-electronic tunneling via bipolarons or kink pairs., 14 pages, 8 postscript figures more...

Charge density waves in constraint geometry experience stresses which can easily exceed a plastic threshold. Relevant situations include macroscopic crystal defects, surface strain, kinks at the surface, junctions and their edges, field effect. The stress is resolved by creation of dislocations which can be observed by modern scattering techniques of space resolved diffraction. We present theoretical considerations on the appearance and the role of enforced dislocations in such situations. We perform precise calculations for a special model of the surface scattering at presence of an array of dislocation lines which are assumed to be well aligned, submerged at the same depth but spaced randomly. The results recover such generally important features as nonsymmetry of intensity profiles I(q) and competition between their single and double peak shapes. more...

Using high-resolution x-ray scattering in the presence of an applied current, we report evidence for a dynamical decoupling between the two NbSe3 charge-density waves (CDWs), Q1 (T(C1)=145 K) and Q2 (T(C2)=59 K), coexisting below T(C2). Simultaneous and oppositely directed shifts of the relevant CDW superlattice spots develop above a threshold current which we identify as the depinning threshold I(C1) for the more strongly pinned upper CDW Q1 (I(C1) approximately 10I(C2)). In contrast with shifts induced by current conversion processes, the present effect is not current polarized and is not limited to the current-contact regions. We propose a model which explains this instability through a sliding-induced charge transfer between the two electronic reservoirs corresponding to the Q1 and Q2 CDWs. more...

International audience; Using small staked junctions of high quality we have measured interlayer tunnelling spectra of the layered quasi-one-dimensional material NbSe3. We identified a number of new features in the spectra: zero bias conductance peak (ZBCP), charge density wave (CDW) gap structure for lower and upper CDW, the sub-gap structure inside the CDW gap. The ZBCP dominates in low-temperature spectra. We discuss its origin as being related to the interlayer coherent tunnelling of the carriers uncondensed into CDW. We found that ZBCP is sensitive to magnetic field and its orientation. The field perpendicular to the layers broadens ZBCP while the parallel field narrows it. We consider the sub-gap structure as associated with self-localized states inside the gap. more...

For a one-dimensional electron-phonon system we consider the photon absorption involving electronic excitations within the pseudogap energy range. Within the adiabatic approximation for the electron - phonon interactions these processes are described by nonlinear configurations of an instanton type. We calculate intensities of the photoelectron spectroscopy PES including the momentum resolved one ARPES and supplement to known results for the optical subgap absorption. We start with the generic case of a one dimensional semiconductor with pronounced polaronic effect. In details we consider the Peierls model for a half-filled band of electrons coupled to the lattice which describes the polyacethylene and some commensurate Charge Density Waves. Particular attention was required for studies of momentum dependencies for the ARPES where we face an intriguing interference between the time evolution and the translational motion of the instantons., RevTex, 17 pages, 3 Postscript figures, added a theory of ARPES more...

The 2-fold degeneracy of the ground state of a quasi-one-dimensional system allows it to support topological excitations such as solitons. We study the combined effects of Coulomb interactions and confinement due to interchain coupling on the statistics of such defects. We concentrate on a 2D case which may correspond to monolayers of polyacetylene or other charge density waves. The theory is developped by a mapping to the 2D Ising model with long-range 4-spin interactions. The phase diagram exhibits deconfined phases for liquids and Wigner crystals of kinks and confined ones for bikinks. Also we find aggregated phases with either infinite domain walls of kinks or finite rods of bikinks. Roughening effects due to both temperature and Coulomb repulsion are observed. Applications may concern the melting of stripes in doped correlated materials., Comment: 16 pages, 7 figures more...

Topologically nontrivial states are common in symmetry broken phases at macroscopic scales. Low dimensional systems bring them to a microscopic level where solitons emerge as single particles. The earliest and latest applications are conducting polymers and spin-Peierls chains. After a history introduction, we shall discuss the topological aspects of elementary excitations, especially the confinement and the dimensional D crossover. At the 1D level we exploit results of different exact and approximate techniques in theory of solitons, for both quantum and semiclassical models, and the related knowledge on anomalous charges and currents. At higher D the topological requirements for the combined symmetry originate the spin- or charge- roton like excitations with charge- or spin- kinks localized in the core. In quasi 1D world they can be viewed as resulting from a spin-charge recombination due to the 3D confinement., Comment: 9 pages, 3 figures more...

We present an x-ray study of doped charge density waves systems. When a 2k_f-charge density wave is strongly pinned to impurities, an interference effect gives rise to an asymmetry between the intensities of the +2k_f and -2k_f satellite reflections. Moreover, profile asymmetry of the satellite reflections indicates the existence of Friedel oscillations (FOs) around the defects. We have studied these effects in V- and W-doped blue bronzes. A syncrotron radiation study of the V-doped blue bronze clearly reveals the presence of FO around the V atoms., Comment: To be published in Proceedings of ECRYS-99, J. de Physique, Coll., December 1999. http://www.lps.u-psud.fr/physol/Organisation-et-dynamique.html http://ipnweb.in2p3.fr/~lptms/membres/brazov/ more...