Tunable charge-density wave transport in a current-effect transistor

Charge Density Wave Phase Transitions in Large-Scale Few-Layer 1T-VTe2 Grown by Molecular Beam Epitaxy

Charge density wave (CDW) as a novel effect in two-dimensional transition metal dichalcogenides (TMDs) has obtained a rapid rise of interest for its physical nature and potential applications in oscillators and memory devices. Here, we report var der Waals epitaxial growth of centimeter-scale 1T-VTe₂ thin films on mica by molecular beam epitaxy. The VTe₂ thin films showed sudden resistance change at temperatures of 240 and 135 K, corresponding to two CDW phase transitions driven by temperature. Moreover, the phase transitions can be driven by an electric field due to local Joule heating, and the corresponding resistance states are nonvolatile and controllable, which could be applied to the memory device where the logic states can be switched by an electric field. The multistage CDW phase transitions in the VTe₂ thin films could be contributed to electron-phonon coupling in the two-dimensional VTe₂, which is supported by twice pronounced Raman blue shifts of the vibration modes associated with in-plane phonons at CDW phase transition temperature. The results open up a new platform for understanding the microscopic physical essence and electrical control of CDW phases of TMDs, expanding the functionalities of these materials for memory applications.

Electrically driven reversible insulator-metal phase transition in 1T-TaS2

In this work, we demonstrate abrupt, reversible switching of resistance in 1T-TaS2 using dc and pulsed sources, corresponding to an insulator-metal transition between the insulating Mott and equilibrium metallic states. This transition occurs at a constant critical resistivity of 7 mohm-cm regardless of temperature or bias conditions and the transition time is significantly smaller than abrupt transitions by avalanche breakdown in other small gap Mott insulating materials. Furthermore, this critical resistivity corresponds to a carrier density of 4.5 × 10(19) cm(-3), which compares well with the critical carrier density for the commensurate to nearly commensurate charge density wave transition. These results suggest that the transition is facilitated by a carrier driven collapse of the Mott gap in 1T-TaS2, which results in fast (3 ns) switching.

Smectic-A to -C phase transition in isotropic disordered environments

We study theoretically the smectic-A to -C phase transition in isotropic disordered environments. Surprisingly, we find that, as in the clean smectic-A to -C phase transition, smectic layer fluctuations do not affect the nature of the transition, in spite of the fact that they are much stronger in the presence of the disorder. As a result, we find that the universality class of the transition is that of the "random field XY model" (RFXY).

Smectic-A-smectic-C phase transition in biaxial disordered environments

We study the smectic-A-smectic-C phase transition in biaxial disordered environments, e.g., fully anisotropic aerogel. We find that both the A and C phases belong to the universality class of the "XY Bragg glass," and therefore have quasi-long-ranged translational smectic order. The phase transition itself belongs to a new universality class, which we study using an ε=7/2-d expansion. We find a stable fixed point, which implies a continuous transition, the critical exponents of which we calculate.

Nonlinear driven response of a phase-field crystal in a periodic pinning potential

We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems introduced recently for both one- and two-dimensional systems. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition.

Convective terms and transversely driven charge-density waves

We derive the convective terms in the damping which determine the structure of the moving charge-density wave (CDW), and study the effect of a current flowing transverse to conducting chains on the CDW dynamics along the chains. In contrast to a recent prediction we find that the effect is orders of magnitude smaller, and that contributions from transverse currents of electron- and holelike quasiparticles to the force exerted on the CDW along the chains act in the opposite directions. We discuss recent experimental verification of the effect and demonstrate experimentally that geometry effects might mimic the transverse current effect.