Charge Density Wave Superstructure Research Articles

Ultrafast spot-profile LEED of a charge-density wave phase transition

We investigate the optically driven phase transition between two charge-density wave (CDW) states at the surface of tantalum disulfide (1T-TaS2). Specifically, we employ a recently improved ultrafast low-energy electron diffraction setup to study the transition from the nearly commensurate to the incommensurate (IC) CDW state. The experimental setup allows us to follow transient changes in the diffraction pattern with high momentum resolution and 1-ps electron pulse duration. In particular, we trace the diffraction intensities and spot profiles of the crystal lattice, including main and CDW superstructure peaks, as well as the diffuse background. Harnessing the enhanced data quality of the instrumental upgrade, we follow the laser-induced transient disorder in the system and perform a spot-profile analysis that yields a substantial IC-peak broadening for very short time scales followed by a prolonged spot narrowing.

Origin of the multiple charge density wave order in 1T−VSe2

Transition-metal dichalcogenide $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ experimentally exhibits multiple charge density wave (CDW) orders, but its origin is still under debate. Using first-principles calculations, we investigate the origin of CDW orders in $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ and clarify the ground state of CDW in the freestanding monolayer. Our results show that both Fermi-surface nesting and electron-phonon coupling account for the $4\ifmmode\times\else\texttimes\fi{}4\ifmmode\times\else\texttimes\fi{}3$ CDW superstructure in bulk $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$, while the momentum-dependent electron-phonon coupling-induced $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ CDW superstructure is most stable in the freestanding monolayer $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$. For monolayer $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$, the substrate-induced compressive strain can turn the ground state into the $4\ifmmode\times\else\texttimes\fi{}4$ CDW superstructure, while tensile strain preserves the $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superstructure. Our results demonstrate the origin of the CDW orders in $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ and shed light on the experimental observation of multiple CDW orders in monolayer $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$.

Coexistence of superconductivity and charge-density-wave domain in 1T-FexTa1−xSSe

A series of 1T-FexTa1−xSSe (0 ≤ x ≤ 0.1) single crystals were fabricated via the chemical-vapor-transport method and investigated by structure, transport, and magnetic measurements along with the density-functional-theory (DFT) calculations. The superconductivity (SC) in parent 1T-TaSSe can be gradually suppressed by Fe-substitution (x ≤ 0.03), accompanied by the disappearance of charge-density-wave (CDW). DFT calculations show that the Fe-substitution effectively inhibits the CDW superstructure and thereby the CDW domains are destroyed. With further increasing x (x > 0.03), the disorder-induced scattering increases, and the system enters into the possible Anderson localization state. Our results prove the SC develops in the CDW phase and coexists with the CDW domain in 1T-TaSSe system.

Ultrasonic properties near 50 K of the quasi-one-dimensional conductorsA0.30MoO3 (A = K, Rb) andRb0.30(Mo1−xVx)O3

The charge density wave (CDW) nonlinear conductivity of the blue bronzesA0.30MoO3 (A = K, Rb) shows two different regimes depending on the temperature: a strongly damped CDW motion above∼50 K and a CDW motion with almost no damping below∼50 K.In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurementson A0.30MoO3 single crystals in the temperature range 4–300 K. InRb0.30MoO3,at T∼50 K, upon cooling, a large increase of the sound velocity for the longitudinal mode measuredalong the , [102] and b directions is observed. The ultrasonic attenuation coefficient shows anincrease down to 50 K followed by a plateau. Similar results are found inK0.30MoO3. In V-dopedsamples, Rb0.30(Mo1−xVx)O3 (x = 0.4%) the anomaly broadens and is shifted towards higher temperatures. The results are discussedin relation to the changes in the CDW rigidity, disorder and dielectric response. Ascenario based on a glass transition for the CDW superstructure is proposed.

Glass transition and secondary relaxation in the charge-density-wave systemK0.3MoO3

Low-frequency dielectric spectroscopy and thermally stimulated discharge measurements of charge-density wave (CDW) system ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}$ are presented. Below 80 K two distinct relaxational processes are observed, which freeze at finite temperatures bearing close resemblance to the phenomenology of the dielectric response of glasses. We compare our results to the case of $\mathrm{o}\ensuremath{-}{\mathrm{TaS}}_{3}$ in which the glass transition on the level of CDW superstructure has been recently reported and discuss the possibility that it is a universal feature of CDW systems.

Determination of the Fermi velocity by angle-dependent periodic orbit resonance measurements in the organic conductorα−(BEDT−TTF)2KHg(SCN)4

We report on detailed angle-dependent studies of the microwave $(\ensuremath{\nu}=50--90\mathrm{GHz})$ interlayer magnetoelectrodynamics of a single crystal sample of the organic charge-density-wave (CDW) conductor $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4}.$ Recently developed instrumentation enables both magnetic-field $(\mathbf{B})$ sweeps for a fixed sample orientation and angle sweeps at fixed $\ensuremath{\nu}/\mathbf{B}.$ We observe series' of resonant absorptions, which we attribute to periodic orbit resonances (POR)---a phenomenon closely related to cyclotron resonance. The angle dependence of the POR indicates that they are associated with the low-temperature quasi-one-dimensional (Q1D) Fermi surface (FS) of the title compound; indeed, all of the resonance peaks collapse onto a single set of $\ensuremath{\nu}/\mathbf{B}$ versus angle curves, generated using a semiclassical magnetotransport theory for a single Q1D FS. We show that Q1D POR measurements provide one of the most direct methods for determining the Fermi velocity, without any detailed assumptions concerning the band structure; our analysis yields an average value of ${v}_{F}=6.5\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{m}/\mathrm{s}.$ Quantitative analysis of the POR harmonic content indicates that the Q1D FS is strongly corrugated. This is consistent with the assumption that the low-temperature FS derives from a reconstruction of the high-temperature quasi-two-dimensional FS, caused by the CDW instability. Detailed analysis of the angle dependence of the POR yields parameters associated with the CDW superstructure, which are consistent with published results. Finally, we address the issue as to whether or not the interlayer electrodynamics are coherent in the title compound. We obtain a relaxation time from the POR linewidths, which is considerably longer than the interlayer hopping time, indicating that the transport in this direction is coherent.

Metal–insulator transition in 1 T-TaS 2− xSe x

The transition metal dichalcogenide, 1 T-TaS 2, shows metal–insulator transition (MIT) accompanying the nearly commensurate (NC)–commensurate (C) charge density wave (CDW) phase transition at 180 K. It has been proposed that the low-temperature C-CDW phase is a Mott insulator. However, our specific heat and magnetic susceptibility measurements in the 1 T-TaS 2− x Se x system revealed no indications of effective mass enhancement or intrinsic localized spins in the insulating phase. So we propose a new model for the MIT in which the interlayer coupling of the CDW superstructure plays an important role.

Electronic interference, elasticity, and scanning tunneling microscopy studies of the charge density wave conductor K 0.3MoO 3

Charge density wave (CDW) dynamics and statics are explored in the blue bronze K 0.3MoO 3. For very thin (optically transparent, d≈0.2 μm) platelets highly coherent response obtains, and in the presence of dc electric fields the ac conductivity shows high frequency interference structure. This interference is discussed in terms of a resonant circuit analog. At low temperatures, the zero differential resistance state is associated with dramatic changes in the elastic constants of the crystal. However, the effects of sample core heating may be important. Both above and below the CDW transition temperature, the surface of K 0.3MoO 3 has been examined by atomic resolution scanning tunneling microscopy. Although the underlying lattice is clearly resolved, no CDW superstructure has been imaged on the crystal surface.

Surface effects of charge density waves by molecular beam diffraction

Abstract The diffraction pattern of He atoms scattered by the 2H-Ta Se2 plane at 20 K shows the presence of a charge density-wave superstructure at the topmost layer of the crystal. The intensity of the superstructure peaks shows vertical shifts of Se atom tops of about 0.02a%.

Surface charge density waves observed by atomic beam diffraction

Abstract The diffraction pattern of He atoms scattered by the 1T-TaS2 basal plane at 80 K demonstrates the presence of a charge-density wave superstructure at the topmost layer of the crystal. The intensity of the superstructure peaks is as large as that of the main peaks, suggesting a strong deformation of the surface. A model calculation, which assumes sinusoidal corrugations for both the sulphur and the superstructure mesh, shows vertical shifts of the sulphur atom tops ranging from -0.12 to +0.16 A.