Non Equilibrium Structural Dynamics Of Incommensurate Charge Density Waves

In recent years, charge-density wave (CDW) systems have been studied extensively, as they provide a diverse testing field for basic concepts in electron-phonon coupling, electron correlation, and structural phase transitions. In particular, time-resolved techniques have participated in that process, disentangling the dynamics of the various degrees of freedoms in such complex materials. As a recently developed pump-probe technique, ultrafast low-energy electron diffraction provides complementary insight into the CDW-coupled structural dynamics at the surface. This cumulative thesis covers t...

Below I present two related studies on the dynamics of spin-/charge-density-waves (SDW/CDW) in chromium. In the first study we measured equilibrium fluctuations of antiferromagnetic SDW/CDW domains in a bulk sample by correlating speckle patterns obtained with partially coherent synchrotron x-rays. The autocorrelation functions follow a double compressing exponential decay similar to that observed in other `jammed' condensed matter systems. This hyper-diffusive behavior indicates the existence of collective relaxation modes which we interpret to arise from the coherent rotation of weakly coupled blocks of spins. Time scales for this collective process range from tens of seconds at 150 Kelvin to thousands of seconds at low temperatures. Even near absolute zero, we observed fluctuations of the magnetic structure which can most directly be interpreted as arising from quantum mechanical zero-point motion. Because microscopic domains of energetically similar spin/charge ordered states are quantum mechanically linked, there exists a dynamic competition between alternate ground states that provides insight into the free energy landscape of domain wall configurations. We obtain estimates of the effective soliton mass of a domain wall and the Arrhenius activation energy.

The latest addition to this series covers a field which is commonly referred to as charge density wave dynamics. The most thoroughly investigated materials are inorganic linear chain compounds with highly anisotropic electronic properties. The volume opens with an examination of their structural properties and the essential features which allow charge density waves to develop. The behaviour of the charge density waves, where interesting phenomena are observed, is treated both from a theoretical and an experimental standpoint. The role of impurities in statics and dynamics is considered and an examination of the possible role of solitons in incommensurate charge density wave systems is given. A number of ways to describe charge density waves theoretically, using computer simulations as well as microscopical models, are presented by a truely international board of authors.

Charge density wave (CDW) is a periodic charge modulation in a metal, induced by electron-phonon or electron-electron interaction, which breaks the translational symmetry of the underlying electron gas. The charge density wave order is ubiquitous among condensed matter systems, and its equilibrium properties have been well characterized by static probes, such as X-ray scattering. However, little is known about their nonequilibrium properties following photoexcitation. In this thesis, we use time resolved optical measurements to characterize the nonequilibrium dynamics of charge density wave systems. In the time resolved optical experiments in this work, an ultrashort (

Incommensurate structural phase transformations involve the appearance of modulated atomic displacements with spatial periodicity unrelated to the fundamental periodicity of the basic lattice. In the case of some quasi one- or two-dimensional metals such transformations are the result of Fermi-surface instabilities that also produce electronic charge density waves (CDW's) and soft phonon modes due to metallic electron screening singularities. Incommensurate soft mode instabilities have been found in insulators as well. Recent neutron scattering studies of both the statics and dynamics of incommensurate structural instabilities will be reviewed.

This thesis presents analytical theoretical studies on the interplay between charge density waves (CDW) and superconductivity (SC) in the actively studied transition-metal dichalcogenide 1T-TiSe2. It begins by reapproaching a years-long debate over the nature of the phase transition to the commensurate CDW (CCDW) state and the role played by the intrinsic tendency towards excitonic condensation in this system. A Ginzburg-Landau phenomenological theory was subsequently developed to understand the experimentally observed transition from commensurate to incommensurate CDW (ICDW) order with doping or pressure, and the emergence of a superconducting dome that coexists with ICDW. Finally, to characterize microscopically the effects of the interplay between CDW and SC, the spectrum of CDW fluctuations beyond mean-field was studied in detail. In the aggregate, the work reported here provides an encompassing understanding of what are possibly key microscopic underpinnings of the CDW and SC physics in TiSe2.

This book advances understanding of light-induced phase transitions and nonequilibrium orders that occur in a broken-symmetry system. Upon excitation with an intense laser pulse, materials can undergo a nonthermal transition through pathways different from those in equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. The work in this book reveals new insights into this phenomena via investigation of photoinduced melting and recovery of charge density waves (CDWs). Using several time-resolved diffraction and spectroscopic techniques, the author shows that the light-induced melting of a CDW is characterized by dynamical slowing-down, while the restoration of the symmetry-breaking order features two distinct timescales: A fast recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence, the latter of which is dictated by the presence of topological defects in the CDW. Furthermore, after the suppression of the original CDW by photoexcitation, a different, competing CDW transiently emerges, illustrating how a hidden order in equilibrium can be unleashed by a laser pulse. These insights into CDW systems may be carried over to other broken-symmetry states, such as superconductivity and magnetic ordering, bringing us one step closer towards manipulating phases of matter using a laser pulse.