Compact computer controlled biaxial tensile device for low-temperature transport measurements of layered materials.

A biaxial tensile device for the transport study of layered materials is described. The device is mounted on the standard 24 pin zero force connector and can be moved between various setups. The compact design of the device makes it suitable for a wide range of studies. In our case, it is placed inside a 50 mm diameter chamber in the cryocooler and is used in the temperature range 9-310 K. A sample is glued in the center of a polyimide cruciform substrate, the ends of which are connected to a tension system driven by four computer-controlled stepper motors providing tensile force up to 30 N. Computer simulation results and their experimental verification show that tensile strain along one axis depends on the tensile load along the perpendicular direction, and this dependence turns out to be relatively strong and exceeds 40%. The operation of the device is demonstrated by studying the effect of deformation on the electrical conductivity of the layered compound 2H-NbS2.

Numerical analysis of surface and edge states in slabs, stripes, rods and surface steps of topological insulators.

By numerically solving the effective continuous model of a topological insulator with parameters corresponding to the band structure of the topological insulator Bi2Se3, we analyze possible appearance of one-dimensional states in various geometries. Massless Dirac fermions are found at the edges of thin ribbons with surface oriented not only along the van der Waals gap but also in the perpendicular direction. Thick rods and slabs with surface steps host massive modes localized on surface faces. We argue that the the origin of the massive modes is due to the difference in the Dirac point energy of adjacent faces. The absence of one-dimensional states near edges of a large rectangular rod and surface steps is demonstrated.

Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of the Si(111)-7×7 surface in illuminated slightly doped crystals.

Physical properties of the Si(111)-7×7 surface of low-doped n- and p-type Si samples are studied in the liquid helium temperature region by scanning-tunnelling microscopy and spectroscopy. Conduction required for the study is provided by illumination of the surface. Application of illumination completely removes the band bending near the surface and restores the initial population of the surface states. Our results indicate the existence of the energy gap 2Δ = 40 ± 10 meV in the intrinsically populated Si(111)-7×7 surface.

'Quantized' states of the charge-density wave in microcrystals of K(0.3)MoO3.

Quantization of electrons in solids can typically be observed in microscopic samples if the mean free path of the electrons exceeds the dimensions of the sample. A special case is a quasi one-dimensional metal, in which electrons condense into a collective state. This state, a charge-density wave (CDW), is a periodic modulation of both the lattice and electron density. Here, we demonstrate that samples of K(0.3)MoO(3), a typical CDW conductor, show jumps in conduction, regular in temperature. The jumps correspond to transitions between discrete states of the CDW and reveal the quantization of the wave vector of electrons near the Fermi vector. The effect involves both quantum and classical features of the CDW: the quantum condensate demonstrates modes, resembling those of a classical wave in a resonator. The analysis of the steps allows extremely precise studies of the CDW wave-vector variations and reveals new prospects for structural studies of electronic crystals and fine effects in their electronic states and lattice motions.

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.

Evidence of collective charge transport in the ohmic regime of o-TaS(3) in the charge-density-wave state by a photoconduction study.

Using photoconduction techniques, we demonstrate that the low-temperature Ohmic conduction of o-TaS3 is not provided by band motion or hopping of single-particle excitations-electrons and holes excited over the Peierls gap. Instead, the low-temperature Ohmic conduction is mostly provided by collective excitations having an activation energy much less than the Peierls gap value and shunting the contribution of electrons and holes responsible for photoconduction.

One-dimensional conduction in charge-density-wave nanowires.

We report a systematic study of the transport properties of coupled one-dimensional metallic chains as a function of the number of parallel chains. When the number of parallel chains is less than 2000, the transport properties show power-law behavior on temperature and voltage, characteristic for one-dimensional systems.

Negative resistance and local charge-density-wave dynamics.

Charge-density-wave (CDW) dynamics is studied on a submicron length scale in NbSe(3) and o-TaS(3). Regions of negative absolute resistance are observed in the CDW sliding regime at sufficiently low temperatures. The origin of the negative resistance is attributed to the different forces that the deformed CDW and quasiparticles feel: the force on the CDW is merely caused by a difference of the electric potentials, while the quasiparticle current is governed by a difference of the electrochemical potentials.