Incommensurate Charge-density Wave Systems Research Articles

Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of 2H−TaS2

We report an in-depth Angle Resolved Photoemission Spectroscopy (ARPES) study on $2H$-TaS$_2$, a canonical incommensurate Charge Density Wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, $2H$-TaSe$_2$ and $2H$-NbSe$_2$, the energy gap ($\Delta_{\text{cdw}}\,$) of $2H$-TaS$_2$ is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of $\Delta_{\text{cdw}}\,$ even at temperatures higher than the CDW transition temperature $\it{T}_{\text{cdw}}\,$ in $2H$-TaS$_2$, reflects the similar pseudogap (PG) behavior observed previously in $2H$-TaSe$_2$ and $2H$-NbSe$_2$. However, in sharp contrast to $2H$-NbSe$_2$, where $\Delta_{\text{cdw}}\,$ is non-zero only in the vicinity of a few "hot spots" on the inner K-centered Fermi surface barrels, $\Delta_{\text{cdw}}\,$ in $2H$-TaS$_2$ is non-zero along the entirety of both K-centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of $\Delta_{\text{cdw}}\,$ between otherwise similar CDW compounds to the different orbital orientations of their electronic states that are involved in CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium.

We report on time-resolved x-ray diffraction measurements following femtosecond laser excitation in pure bulk chromium. Comparing the evolution of incommensurate charge-density-wave (CDW) and atomic lattice reflections, we show that, a few nanoseconds after laser excitation, the CDW undergoes different structural changes than the atomic lattice. We give evidence for a transient CDW shear strain that breaks the lattice point symmetry. This strain is characteristic of sliding CDWs, as observed in other incommensurate CDW systems, suggesting the laser-induced CDW sliding capability in 3D systems. This first evidence opens perspectives for unconventional laser-assisted transport of correlated charges.

Electric Field Gradient Tensor Distributions in 2-Dimensional Incommensurate Charge Density Wave Systems

The incommensurate charge density modulation in 2-dimensional conducting systems leads to characteristic profiles of the charge density and to characteristic distributions of electric field gradient (EFQ) tensor components which can be probed by Knight-shift and nuclear quadrupole interaction measurements, respectively. The Fourier expansion of the charge density reveals how the density distribution translates into EFG tensor component distributions. Lineshape simulations for I = 1 illustrate that the local model is inadequate.

Fermi surface and photoemission lineshape of incommensurate CDW systems

We study the single-particle spectra of a model which shows an incommensurate charge-density wave (CDW) instability arising from the competition between phase separation and long-range Coulomb interactions. Starting from an open Fermi surface we find that the resulting CDW is oriented along the (1,0)- and (or) (0,1)-direction which (besides a purely one-dimensional structure) allows for a two-dimensional "eggbox type" charge modulation. This leads to a substantial enhancement of the van Hove singularities and the spectral weight of Fermi surface states near the M-points tends to be suppressed. Remarkably, a leading edge gap arises near these points, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless. Extending this approach to the inclusion of dynamic incommensurate CDW fluctuations in the superconducting state we calculate the photoemission lineshape which agrees well with experimental data of underdoped Bi2212 compounds.

Fermi Surface and Electronic Structure of Incommensurate Charge-Density Wave Systems

We study the single-particle spectra of a model that shows an incommensurate charge-density wave (CDW) instability arising from the competition between phase separation and long-range Coulomb interactions. Starting from an open Fermi surface, we find that the resulting CDW is oriented along the (1, 0) and (or) (0, 1) direction, which allows for a purely one-dimensional (1D) or a two-dimensional (2D) eggbox type charge modulation. In both cases, the van Hove singularities are substantially enhanced, and the spectral weight of Fermi surface states near the M points tends to be suppressed. Remarkably, a leading edge gap arises near these point, which, in the eggbox case, leaves finite arcs of the Fermi surface gapless.

Spin-lattice relaxation via sliding of the modulation wave in incommensurate charge density wave systems

It is shown that nuclear spin-lattice relaxation time measurements can be used for a precise determination of the sliding velocity of the electrically depinned modulation wave in incommensurate charge density wave systems. NMR relaxation time measurements are much more suited than are NMR lineshape measurements for a determination of the distribution of sliding velocities of multi-domain samples.

Microwave Conductivity of Incommensurate Charge Density Wave Systems

We calculate the microwave conductivity due to phase mode taking into account impurity effects and long-range Coulomb interaction between the phason and quasi-particles (and/or free carriers). It is shown by using experimental results that the damping of the phason, which is expressed by the dc conductivity of quasi-particles (and/or of free carriers), is fairly large. We obtain the microwave conductivity (σ(ω)), taking into account this damping in the calculation of the self-energy of the phason Green's function and explain frequency and delicate temperature dependences of σ(ω) of TaS 3 observed experimentally.

Coherent polaron superlattices and long-range Coulomb forces in quasi-one-dimensional conductors.

We discuss the formation of a spatially coherent layered polaron superlattice in a dilute one-dimensional electron gas with strong Fr\"ohlich-type electron-phonon interaction and elastic interchain coupling. We consider the effect of both long- and short-range Coulomb repulsions and present results for the long-wavelength phason excitations of such a superstructure. Our results apply to doping-induced superstructures in quasi-one-dimensional semiconductors and to three-dimensional-discommensuration domain walls in incommensurate charge-density wave systems.

Amplitude solitons in incommensurate Peierls systems: Implications for TTF-TCNQ

Amplitude soliton excitations are described for both dimerized [i.e. commensurate with 2k f = ( π a ) ] and incommensurate charge-density- wave (CDW) ground states. For the dimerized case the soliton excitations can have charge + e, 0, − e with spin 0, ± 1 2 , 0 depending on the occupancy of the localized gap state. For the incommensurate case only the neutral spin- 1 2 soliton exists. Utilizing the results of Takayama, Lin-Liu and Maki, we show that E s = ( 2 π )Δ where 2Δ is gap for electron-hole excitations ( Δ i = W exp ( −1 λ ) for the incommensurate case and Δ c = W exp (− 1 2 λ) for the dimerized case where λ is the electron-phonon coupling constant and W is the bandwidth). We suggest that neutral soliton excitations may make significant contributions to the magnetic susceptibility of incommensurate CDW systems, and that this may be the origin of observed differences between the low-temperature magnetic and transport activation energies in TTF-TCNQ.