Exposing a Hidden State: Shining intense laser light on a material can temporarily alter its properties. The effect usually subsides after a few picoseconds, unless the system is trapped in a metastable state, in which case the transient period may last as long as microseconds. Stojchevska et al. (p. 177 ) observed that, following exposure to a 35-femtosecond laser pulse, the layered dichalcogenide 1T -TaS ₂ entered a stable “hidden” state not present in the equilibrium phase diagram and stayed there indefinitely. The switch to the hidden state could be reversed by heat or a train of laser pulses. Because the switch alters the sample's conducting properties, the phenomenon might also lead to practical applications.

The study of the temporal evolution of systems undergoing symmetry breaking phase transitions—whether it is in condensed-matter physics, cosmology or finance^1,2,3,4,5,6—is difficult because they are hard to repeat, or they occur very rapidly. Here we report a high-time-resolution study of the evolution of both bosonic and fermionic excitations through an electronic charge-ordering symmetry breaking phase transition. Periodically quenching our system with femtosecond optical pulses, we subsequently detect hitherto-unrecorded coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode and evolution of the particle–hole gap as the system evolves through the transition. Modelling on the basis of Ginzburg–Landau theory is used to reproduce the observations without free parameters. Of particular interest is the observation of spectrotemporal distortions arising from spontaneous annihilation of topological defects, analogous to those discussed by the Kibble–Zurek cosmological model^2,3.

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.

This review is devoted to the theory of collective and local pinning effects in various disordered nonlinear driven systems. A common feature of both approaches is the emergence of metastability. Although the emphasis is put on charge and spin density waves and magnetic domain walls, the theory also has applications to flux lines and lattices thereof, dislocation lines, adsorbed monolayers and related systems. In the first part of the article we focus on the theory of 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. At zero temperature and for adiabatic changes of the force, the dynamic depinning transition is continuous, the correlation length is large, the behaviour is dominated by scaling laws with non-trivial static and dynamical critical indices. The application of functional renormalization group methods allows for the detailed description of both equilibrium as well as non-equilibrium properties. The depinning transition is also characterized by the appearance of new scaling laws. Thermal fluctuations smear out this transition and allow for a creep motion of the elastic objects even at small forces. The application of an ac-driving force also destroys the sharp transition which is replaced by a velocity hysteresis. The second part of the review is devoted to the picture of local pinning and its applications. Local theories apply in the region where correlation effects are less important, i.e. not too close to the depinning transition, at low temperatures, at high enough frequencies or velocities. The inclusion of plastic deformations results in a rich cross-over behaviour of the force–velocity relation as well as of the frequency dependence of the dynamic response. Being easily affected at higher frequencies or velocities, the local pinning becomes an easily accessed source of dispersion, relaxation and dissipation. The picture of the local pinning can be effectively used to explain experimental data: qualitatively and even quantitatively. The advantages come from the explicit treatment of metastable states, their creation and relaxation, and their relation to plasticity and topological defects. The local pinning recovers and exploits new elements of the energy landscape such as termination points of some branches or irreversibility of other ones related to generation of topological defects in the course of sliding. It also provides a clue to quantum effects describing quantum creep as tunnelling between retarded and advanced configurations.

In this tutorial review, we cover the solid state physics approach to electronic and optical properties of conducting polymers. We attempt to bring together languages and advantages of the solid state theory for polymers and of the quantum chemistry for monomers. 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. Our approach allows to go beyond the single-chain picture and to compare excitons for polymers in solutions and in films. The approach helps connecting such different questions as shallow singlet and deep triplet excitons, stronger binding of interchain excitons in films, crossings of excitons’ branches, 1/N energies shifts in oligomers. We describe a strong suppression of the luminescence from free charge carriers by long-range Coulomb interactions. Main attention is devoted to the most requested in applications phenyl based polymers. The specifics of the benzene ring monomer give rise to existence of three possible types of excitons: Wannier-Mott, Frenkel and intermediate ones. We discuss experimental manifestations of various excitons and of their transformations. We touch effects of the time-resolved self-trapping by libron modes leading to formation of torsion polarons.

The exciton binding energy (E_b) and the band gap energy (E_g) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band π-π* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with E_b ≈ 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.