Charge Density Wave Systems Research Articles

Holographic charge density waves

We discuss a gravity dual of a charge density wave consisting of a U(1) gauge field and two scalar fields in the background of a Schwarzschild–AdS4 black hole together with an antisymmetric field (probe limit). Interactions drive the system to a phase transition below a critical temperature. We numerically compute the ground states characterized by modulated solutions for the gauge potential corresponding to a dynamically generated unidirectional charge density wave in the conformal field theory. Signatures of the holographic density waves are retrieved by studying the dynamical response to an external electric field. We find that this novel holographic state shares many common features with the standard condensed matter version of charge density wave systems.

Detection of charge density wave ground state in granular thin films of blue bronze K0.3MoO3 by femtosecond spectroscopy

During the last years, femtosecond time-resolved spectroscopy (fsTRS) has become an important new tool to investigate low energy excitations in strongly correlated systems. By studying energy relaxation pathways linking various degrees of freedom (e.g., electrons, spin, or lattice), the interaction strengths between different subsystems can be deduced. Here we report on yet another application of fsTRS, where the technique is used to unambiguously determine the nature of the ground state in granular thin films of a prototype charge density wave system blue bronze, K0.3MoO3. These, potassium blue bronze, films, obtained for the first time ever, have been prepared by pulsed laser deposition and investigated by various standard characterization methods. While the results of all used methods indicate that the thin films consist of nanometer grains of K0.3MoO3, it is only the non-destructive fsTRS that demonstrates the charge density wave nature of the ground state. Furthermore, the comparison of the fsTRS data obtained in thin films and in single crystals shows the reduction of the charge density wave transition temperature and of the photoinduced signal strength in granular thin films in respect to single crystals, which is attributed to the granularity and crystal growth morphology.

Anharmonic order-parameter oscillations and lattice coupling in strongly driven 1 T−TaS2andTbTe3charge-density-wave compounds: A multiple-pulse femtosecond laser spectroscopy study

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in the systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent to the broken symmetry state of the CDW systems.

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

Chiral Charge-Density Waves

We discovered the chirality of charge-density waves (CDW) in 1T-TiSe₂ by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia₁∶Ia₂∶Ia₃ = 1∶0.7 ± 0.1∶0.5 ± 0.1, where Ia(i) (i=1,2,3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H(CDW) ≡ q₁·(q₂×q₃), where q(i) are the CDW q vectors, which is independent of the symmetry of components. The nonzero H(CDW)-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system.

Charge redistribution in the formation of one-dimensional lithium wires on Cu(001)

We describe the formation of one-dimensional lithium wires on a Cu(001) substrate, providing an atomic-scale description of the onset of metallization in this prototypical adsorption system. A combination of helium atom scattering and density-functional theory reveals pronounced changes in the electronic charge distribution on the formation of the $c(5\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R45\ifmmode^\circ\else\textdegree\fi{}$ Li/Cu(001) structure, as in-plane bonds are created. Charge donation from Li-substrate bonds is found to facilitate the formation of stable, bonded, and depolarized chains of Li adatoms that coexist with an interleaved phase of independent adatoms. The resultant overlayer has a commensurate charge distribution and lattice modulations but differs fundamentally from structurally similar charge-density wave systems.

ChemInform Abstract: Synthesis and Characterization of the Pseudo-1-D Mixed-Valence (Et4- nMenN)Cu2X4 Salts: Pinned Charge Density Wave Systems Exhibiting Intervalence Charge Transfer.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Disentanglement of the Electronic and Lattice Parts of the Order Parameter in a 1D Charge Density Wave System Probed by Femtosecond Spectroscopy

We report on the high resolution studies of the temperature (T) dependence of the q=0 phonon spectrum in the quasi-one-dimensional charge density wave (CDW) compound K(0.3)MoO(3) utilizing time-resolved optical spectroscopy. Numerous modes that appear below T(c) show pronounced T dependences of their amplitudes, frequencies, and dampings. Utilizing the time-dependent Ginzburg-Landau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2k(F) phonons, while the (electronic) CDW amplitude mode is overdamped.

The influence of temperature and electric field history on the conductivity of the charge density wavesystem o-TaS3

We have studied the dependence of the low electric field conductivity of the charge density wave (CDW)system o-TaS3 on the temperature and applied electric field history. Without the application of highelectric fields, the conductivity is higher on cooling than on heating in a wide temperaturerange below the CDW transition. With the application of increasing field at a giventemperature the conductivity evolves towards a stable, history-independent value situatedbetween the cooling and heating values. The evolution is gradual but most pronouncedaround the threshold field for nonlinear conductivity. The relative change of conductivityafter the application of high field is asymmetric in respect of the temperaturehistory. We discuss the results within the model of quantized changes of the CDWwavelength induced by the variation of the temperature and the electric field.

Femtosecond nonequilibrium dynamics in quasi-1D CDW systems and

Abstract Here we report on detailed studies of the photoexcited carrier and collective mode dynamics in prototype quasi-1D charge density wave (CDW) systems K 0.3 MoO 3 and Rb 0.3 MoO 3 focusing on their low temperature and low excitation range. We show that the order parameter relaxation dynamics are inconsistent with the Rothwarf–Taylor model describing the relaxation of the coupled quasi-particle and high frequency phonon system in a system with a narrow gap in the density of states. Therefore we propose an alternative model that accounts for the photoexcited carrier relaxation phenomena commonly observed in CDW systems.

Transport properties of , a highly disordered charge–density wave system

Abstract Differential resistivity and broadband noise measurements of La 0.5 Ca 0.5 MnO 3 reveal behaviour typical of a highly disordered charge–density wave system. In addition, the differential resistivity measurements reveal a large hysteresis, with the upper part of the hysteresis curve only appearing when the sample has been annealed by heating to room temperature and then cooling. The variation of the area of the hysteresis loop with temperature is found to be governed by a power law.

Dynamics of Photoinduced Charge-Density-Wave to Metal Phase Transition inK0.3MoO3

We present the first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast nonthermal CDW melting is achieved at the absorbed energy density that corresponds to the electronic energy difference between the metallic and CDW states. The results imply that on the subpicosecond time scale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

Observation of correlations up to the micrometer scale in sliding charge-density waves

High-resolution coherent X-ray diffraction experiment has been performed on the Charge-Density Wave (CDW) system K0.3MoO3K0.3MoO3. The 2kF2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the 2kF2kF satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long range order is 1500 times larger than the CDW period itself.

Raman spectroscopy of collective modes in charge‐density‐wave systems: the mean‐field microscopic theory

AbstractThe electron‐mediated coupling of external electromagnetic fields and Raman‐active oscillations is derived for a general electronic model with multiple bands using the adiabatic approach and the explicit diagrammatic approach. The theory is illustrated on the quasi‐one‐dimensional (Q1D) quarter‐filled charge‐density‐wave (CDW) model. It is shown how the long‐range Coulomb forces and the single‐electron relaxation processes affect the Raman spectroscopy of the amplitude‐oscillation mode in clean CDW systems. It is also argued that the adiabatic treatment of the photon‐phonon coupling functions can be safely used in this case. Copyright © 2008 John Wiley & Sons, Ltd.

Observation of Correlations Up To the Micrometer Scale in Sliding Charge-Density Waves

A high resolution coherent x-ray diffraction experiment has been performed on the charge-density wave (CDW) system K0.3MoO3. The 2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the 2kF satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long-range order is 1500 times larger than the CDW period itself. This new type of electronic correlation seems inherent to the collective dynamics of electrons in charge-density wave systems. Several scenarios are discussed.

Freezing of low energy excitations in charge density wave glasses

Thermally stimulated discharge current measurements were performed to study slow relaxation processes in two canonical charge density wave systems K(0.3)MoO(3) and o-TaS(3). Two relaxation processes were observed and characterized in each system, corroborating the results of dielectric spectroscopy. Our results are consistent with the scenario of the glass transition on the charge density wave superstructure level. In particular, the results directly prove the previously proposed criterion of charge density wave freezing based on the interplay of charge density wave pinning by impurities and screening by free carriers. In addition, we obtained new information on distribution of relaxation parameters, as well as on nonlinear dielectric response both below and above the threshold field for charge density wave sliding.

Sliding charge-density wave in manganites

Stripe and chequerboard phases appear in many metal oxide compounds, and are thought to be linked to exotic behaviour such as high-temperature superconductivity and colossal magnetoresistance. It is therefore extremely important to understand the fundamental nature of such phases. The so-called stripe phase of the manganites has long been interpreted as the localization of charge at atomic sites. Here, we present resistance measurements on La(0.50)Ca(0.50)MnO(3) that strongly suggest that this state is in fact a prototypical charge-density wave (CDW) that undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. The CDW-type behaviour of the manganite superstructure suggests that unusual transport and structural properties do not require exotic physics, but could emerge when a well-understood phase (the CDW) coexists with disorder.

Nonadiabatic Effects and the Quantum Geometric Tensor in Charge Density Wave Systems

We studied the energy corrections to the mean-field theory on a charge density wave (CDW) system in terms of the random phase approximation (RPA). An energy correction with a logarithmic singular behavior has been found in the ground-state energy. There exists a region where this correction gives a considerable contribution to the mean-field gap equation. By estimating the validity of the adiabatic approximation, the global phase diagram of the ground state in the \((\Delta/\varepsilon_{\text{F}},\hbar\omega/\varepsilon_{\text{F}})\)-plane (Δ: CDW mean-field gap, e F : Fermi energy, \(\hbar\omega\): phonon energy quantum) has been revealed. In particular, we have found a crossover between adiabatic states and nonadiabatic states in the quantum liquid region in the phase diagram. In this paper, we also explain that the logarithmic singular term corresponds to the fourth-order energy correction, which is represented by the so-called quantum geometric tensor, in the adiabatic approximation.

Thermoelectric studies of the non-thermal equilibrium dynamics in chiral metals

The conventional pyroelectric effect is intimately connected to the symmetry, or rather lack of center of symmetry, of the material. Although the experiments we discuss involve studies of low symmetry materials, the pyroelectric currents observed are of an entirely new origin. Systems with broken-translational-symmetry phases that incorporate orbital quantization can exhibit significant departures from thermodynamic equilibrium due to a change in magnetic induction. For example, orbitally quantized field-induced spin- or charge density wave systems, in which the competition between the elastic forces of the density wave and pinning leads to a critical state analogous to the vortex phase of type II superconductors. This metastable state consists of a balance between the density-wave pinning force and the Lorentz force on the extended currents due to the drift of cyclotron orbits. This results in the establishment of a three-dimensional chiral metal that can extend deep into the bulk of the crystal. In this way the density wave pinning potential plays a similar role to the edge potential in a two-dimensional electron gas, leading to a large Hall angle and quantization of the Hall resistance. A thermal perturbation that reduces the pinning potential returns the system toward thermal equilibrium, which can only be achieved by current flow orthogonal to the surface. The observation of this new form of pyroelectric effect in the high magnetic field phase ( B > 30 T ) of the organic charge transfer salt α - ( BEDT - TTF ) 2 KHg ( SCN ) 4 is conclusive proof of the existence of a three-dimensional chiral metal.

Fermi surface of layered compounds and bulk charge density wave systems

A review is given of recent angle-resolved photoemission (ARPES) experimentsand analyses on a series of layered charge density wave materials. Importantaspects of ARPES are recalled in view of its capability for bulk band, Fermisurface and spectral function mapping despite its surface sensitivity. Discussed areTaS2,TaSe2,NbTe2,TiSe2 andTiTe2 with structuresrelated to the so-called 1T polytype. Many of them undergo charge density wave transitions or exist with a distortedlattice structure. Attempts to explain the mechanism behind the structural reconstructionare given. Depending on the filling of the lowest occupied band a drastically differentbehaviour is observed. Whereas density functional calculations of the electronic energy andmomentum distribution reproduce well the experimental spectral weight distribution at theFermi energy, the ARPES energy distribution curves reveal that for some of the compoundsthe Fermi surface is pseudo-gapped. Two different explanations are given, the first based ondensity functional calculations accounting for the charge-density-wave-induced latticedistortion and the second relying on many-body physics and polaron formation.Qualitatively, both describe the observations well. However, in the future, in order to beselective, quantitative modelling will be necessary, including the photoemission matrixelements.