Layered van der Waals Topological Metals of TaTMTe4 (TM = Ir, Rh, Ru) Family.

Layered van der Waals materials of the family TaTMTe4 (TM = Ir, Rh, Ru) are showing interesting electronic properties. We report the growth and characterization of TaIrTe4, TaRhTe4, TaIr1-xRhxTe4 (x = 0.06, 0.14, 0.78, 0.92), Ta1+xRu1-xTe4 single crystals. X-ray powder diffraction confirms that TaRhTe4 is isostructural to TaIrTe4. All these compounds are metallic with diamagnetic behavior. Below T ≈ 4 K we observed signatures of the superconductivity in the TaIr1-xRhxTe4 compounds for x = 0.92. All samples show weak quadratic-in-field magnetoresistance (MR). However, for TaIr1-xRhxTe4 with x ≈ 0.78, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe4 series the MR is almost isotropic. Electronic structure calculations for TaIrTe4 and TaRhTe4 reveal appearance of the Rh band close to the Fermi level.

Strongly correlated two-dimensional plasma explored from entropy measurements.

Charged plasma and Fermi liquid are two distinct states of electronic matter intrinsic to dilute two-dimensional electron systems at elevated and low temperatures, respectively. Probing their thermodynamics represents challenge because of lack of an adequate technique. Here, we report a thermodynamic method to measure the entropy per electron in gated structures. Our technique appears to be three orders of magnitude superior in sensitivity to a.c. calorimetry, allowing entropy measurements with only 10(8) electrons. This enables us to investigate the correlated plasma regime, previously inaccessible experimentally in two-dimensional electron systems in semiconductors. In experiments with clean two-dimensional electron system in silicon-based structures, we traced entropy evolution from the plasma to Fermi liquid regime by varying electron density. We reveal that the correlated plasma regime can be mapped onto the ordinary non-degenerate Fermi gas with an interaction-enhanced temperature-dependent effective mass. Our method opens up new horizons in studies of low-dimensional electron systems.

Comment on "Connecting the reentrant insulating phase and the zero-field metal-insulator transition in a 2D hole system".

A Comment on the Letter by R. L. J. Qiu et al., Phys. Rev. Lett. 108, 106404 (2012). The authors of the Letter offer a Reply.

Spin-droplet state of an interacting 2D electron system.

We report thermodynamic magnetization measurements of two-dimensional electrons in several high-mobility Si metal-oxide-semiconductor field-effect transistors. We provide evidence for an easily polarizable electron state in a wide density range from insulating to deep into the metallic phase. The temperature and magnetic field dependence of the magnetization is consistent with the formation of large-spin droplets in the insulating phase. These droplets melt in the metallic phase with increasing density and temperature, though they survive up to large densities.

Metal-insulator transition in two dimensions: experimental test of the two-parameter scaling.

We report a detailed scaling analysis of resistivity rho(T,n) measured for several high-mobility 2D electron systems in the vicinity of the 2D metal-insulator transition. We analyzed the data using the two-parameter scaling approach and general scaling ideas. This enables us to determine the critical electron density, two critical indices, and temperature dependence for the separatrix in the self-consistent manner. In addition, we reconstruct the empirical scaling function describing a two-parameter surface which fits well the rho(T,n) data.

Interaction effects in conductivity of Si inversion layers at intermediate temperatures.

We compare the temperature dependence of resistivity rho(T) of Si-metal-oxide-semiconductor field-effect transistors with the recent theory by Zala et al. In this comparison, the effective mass m* and g* factor for mobile electrons have been determined from independent measurements. An anomalous increase of rho with temperature, which has been considered as a signature of the "metallic" state, can be described quantitatively by the interaction effects in the ballistic regime. The in-plane magnetoresistance rho(B(axially)) is only qualitatively consistent with the theory; the lack of quantitative agreement indicates that the magnetoresistance is more sensitive to sample-specific effects than rho(T).

Low-density spin susceptibility and effective mass of mobile electrons in Si inversion layers.

We studied the Shubnikov-de Haas (SdH) oscillations in high-mobility Si-MOS samples over a wide range of carrier densities n approximately (1-50)x10(11) cm(-2), which includes the vicinity of the apparent metal-insulator transition in two dimensions (2D MIT). Using a novel technique of measuring the SdH oscillations in superimposed and independently controlled parallel and perpendicular magnetic fields, we determined the spin susceptibility chi(*), the effective mass m(*), and the g(*) factor for mobile electrons. These quantities increase gradually with decreasing density; near the 2D MIT, we observed enhancement of chi(*) by a factor of approximately 4.7.

Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high-mobility (100) Si-inversion layers.

We report studies of the magnetoresistance (MR) in a two-dimensional electron system in (100) Si-inversion layers, for perpendicular and parallel orientations of the current with respect to the magnetic field in the 2D plane. The magnetoresistance is almost isotropic; this result does not support the suggestion of its orbital origin. In the hopping regime, however, the MR contains a weak anisotropic component that is nonmonotonic in the magnetic field. We found that the field, at which the MR saturates, varies for different samples by a factor of 2 at a given carrier density. Therefore, the saturation of the MR cannot be identified with the complete spin polarization of free carriers.

Exclusion of quantum coherence as the origin of the 2D metallic state in high-mobility silicon inversion layers.

The temperature and density dependence of the phase coherence time tau(phi) in high-mobility silicon inversion layers was determined from the magnetoresistivity due to weak localization. The upper temperature limit for single-electron quantum interference effects was delineated by comparing tau(phi) with the momentum relaxation time tau. A comparison between the density dependence of the borders for quantum interference effects and the strong resistivity drop reveals that these effects are not related to each other. As the strong resistivity drop occurs in the Drude regime, the apparent metallic behavior cannot be caused by quantum coherent effects.