S. Brazovskii

Ferroelectric Mott-Hubbard Phase of Organic (TMTTF)2X Conductors

We present experimental evidence and a corresponding theory for the ferroelectric transition in the family of quasi-one-dimensional conductors (TMTTF)2X. 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.

Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal

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-TaS2 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 samples conducting properties, the phenomenon might also lead to practical applications. A 35-femtosecond laser pulse causes the dichalcogenide 1T-TaS2 to enter a stable phase not present in the equilibrium phase diagram. Hidden states of matter may be created if a system out of equilibrium follows a trajectory to a state that is inaccessible or does not exist under normal equilibrium conditions. We found such a hidden (H) electronic state in a layered dichalcogenide crystal of 1T-TaS2 (the trigonal phase of tantalum disulfide) reached as a result of a quench caused by a single 35-femtosecond laser pulse. In comparison to other states of the system, the H state exhibits a large drop of electrical resistance, strongly modified single-particle and collective-mode spectra, and a marked change of optical reflectivity. The H state is stable until a laser pulse, electrical current, or thermal erase procedure is applied, causing it to revert to the thermodynamic ground state.

Coherent dynamics of macroscopic electronic order through a symmetry breaking transition

The speed with which symmetry breaking transitions occur in the solid state makes them difficult to study in the time domain. State-of-the-art pump–probe measurements of the dynamics of charge-density waves in terbium telluride enable the evolution of the symmetry breaking charge-order transition of this system to be studied with unprecedented temporal resolution.

PINNING AND SLIDING OF DRIVEN ELASTIC SYSTEMS: FROM DOMAIN WALLS TO CHARGE DENSITY WAVES

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. †Permanent address: Institute for Theoretical Physics, University of Cologne, Zülpicher Str. 77, D-50937 Cologne, Germany 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.

A systematic theory for optical properties of phenylene-based polymers

A complete picture of phenylene-based polymers is developed which unifies features of band and molecular exciton models. It incorporates major experimental findings in direct and photoinduced optical absorption, in stimulated photoemission and photoconductivity. Our theoretical picture is based upon a band description for electronic states, while invoking corrections from Coulomb interactions. We demonstrate the existence of different types of Coulomb excitons and their roles in optical absorption. We show that a low binding energy-emitting exciton also dominates in the fundamental absorption. Contradictions in the current modeling state-of-the-art are displayed and discussed. For poly(phenylene vinylene) (PPV)-type polymers, we give new assignments for the most disputed features; for oligomers we identify new ones as edge states. In applying our model to the poly(phenylene) (PPP) family, more progress is available due to analytical results covering not only spectra but also the oscillator strength. We conclude similar assignments as for PPV and emphasize the delocalized nature of basic features. An important and intriguing peculiarity of PPP- compared with PPV-based polymers is the possible level inversion between excitons. Comparison is made to available optical and electron energy loss spectroscopy (EELS) data. Our conclusions are suggestive for future light-polarized experiments.

Excitations and optical properties of phenylene-based conjugated polymers and oligomers

Abstract We present a combined experimental and theoretical study of the ground and photoexcited optical properties of a model oligomer of PPV, MEH-DSB. Our theoretical picture is based upon a band description of electronic states of PPV oligomers, while invoking corrections from Coulomb interactions. The necessary discrete energy levels at low and intermediate energies appear naturally, in addition to the lower energy delocalized states. On this basis we identify the most important features in direct optical absorption for both high (4–6 eV) and low (2–4 eV) photon energies as well as in photoinduced absorption (PA) and stimulated photoemissions (SE) in MEH-DSB solutions, which represent the limit of noninteracting oligomers. While in agreement with previous interpretations for three absorption peaks (2.74, 4.46 and 6.2 eV), we give a new assignment for the most disputed 3.62 eV one as well as for the two PA peaks.

Fast electronic resistance switching involving hidden charge density wave states

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.

Insulator-metal transition in Rb4C60 under pressure from 13C-NMR

Abstract We present an NMR study of Rb 4 C 60 performed under pressure up to 12 kbar. The temperature dependence of the 13 C spin-lattice relaxation rate is activated at ambient pressure and becomes Korringa-like under pressure. The behaviour of the relaxation is interpreted in terms of one channel due to excited intramolecular triplet states above the Jahn-Teller ground state plus another one related to electron-hole excitations through an indirect band gap. In spite of a finite value of the density of states at the Fermi level indicated by the NMR data under pressure, no superconductivity could be detected at 15 kbar above 0.4 K.

Controlling the metal-to-insulator relaxation of the metastable hidden quantum state in 1T-TaS2

Revealing the relaxation mechanisms of hidden states in transition metal dichalcogenides leads to control of metastability. Controllable switching between metastable macroscopic quantum states under nonequilibrium conditions induced either by light or with an external electric field is rapidly becoming of great fundamental interest. We investigate the relaxation properties of a “hidden” (H) charge density wave (CDW) state in thin single crystals of the layered dichalcogenide 1T-TaS2, which can be reached by either a single 35-fs optical laser pulse or an ~30-ps electrical pulse. From measurements of the temperature dependence of the resistivity under different excitation conditions, we find that the metallic H state relaxes to the insulating Mott ground state through a sequence of intermediate metastable states via discrete jumps over a “Devil’s staircase.” In between the discrete steps, an underlying glassy relaxation process is observed, which arises because of reciprocal-space commensurability frustration between the CDW and the underlying lattice. We show that the metastable state relaxation rate may be externally stabilized by substrate strain, thus opening the way to the design of nonvolatile ultrafast high-temperature memory devices based on switching between CDW states with large intrinsic differences in electrical resistance.

Theory of electronic states and excitations in PPV

Abstract We present a consistent theoretical picture for optical properties of phenyl based polymers, especially for the PPV family. The model is based upon an analytical solution for the band structure of PPV oligomers, while invoking the dominant Coulomb corrections for electron-hole interactions. The adjustable parameters are only the common shift for the bands centers of gravity and a dielectric susceptibility at small distances. Our picture gives a clear understanding for the origin of all possible transitions in linear and nonlinear optics. We describe both tightly bound localized excitons and excitons of intermediate range (i.e. of both the Frenkel and Wannier-Mott types). The quantitative description of excitons is obtained from the long range Coulomb interactions, We emphasize where the ring torsion plays a role in the overall energy minimization of the excited state. This article provides theory details for the joint article [S. Brazovskii, N. Kirova, A.R. Bishop, V. Klimov, D. McBranch, N.N. Barashkov, J.P. Ferraris, Opt. Mater. 9 (1998) 472], where a complete picture was outlined.