arXiv subjects

N. I. Fedotov

Publications and source records attributed to N. I. Fedotov.

8 recordsLinked to original sources

arXiv2025

Strain-induced splitting of the CCDW-NCCDW phase transition in 1T-TaS$_2$

The effects of uniaxial and biaxial tensile strain on the $ρ_{xx}$ and $ρ_{yy}$ components of the resistivity tensor, and the commensurable-nearly commensurate CDW (CCDW-NCCDW) transition temperature in 1T-TaS$_2$ are studied. At room temperature, uniaxial tensile strain increases the resistivity tensor components by a comparable magnitude both parallel and perpendicular to the strain axis. In the case of biaxial strain, up to 20~K decrease in the CCDW-NCCDW phase transition temperature is observed. In the case of uniaxial strain, a new phase with two different CCDW-NCCDW phase transition temperatures is observed, the splitting exceeds 10 K. The occurrence of such a phase is associated with the transition of the CDW into the commensurate state along the tensile strain direction while maintaining nearly commensurability along the perpendicular one. The results allow to justify various models widely used in analysis of transport properties of 1T-TaS$_2$ in commensurate and nearly commensurate states.

cond-mat.mtrl-sci↗

arXiv2022

Scanning tunneling microscopy of Bi$_2$Te$_3$ films prepared by pulsed laser deposition: from nanocrystalline structures to van der Waals epitaxy

Bi$_2$Te$_3$ is a material with high efficiency of thermoelectric energy conversion. Recently, it was also recognized as a topological insulator, and is often used as the basis for creation of other types of topological matter. Pulsed laser deposition (PLD) is widely considered as a simple method for growing of multicomponent films, but not as a tool for van der Waals epitaxy. We demonstrate here that the van der Waals epitaxy of Bi$_2$Te$_3$ is indeed impossible in vacuum PLD, but is possible in the presence of a background gas, which is confirmed by the results of scanning tunneling microscopy and spectroscopy studies. Results of {\it ab initio} calculations reproduce tunneling spectra of the first three terraces of epitaxial films of Bi$_2$Te$_3$. In addition, an unusual hexagonal superstructure resembling a charge-density wave is observed in overheated films.

cond-mat.mtrl-sci↗

arXiv2019

Experimental Observation of Bound States of 2D Dirac Electrons at Surface Steps of the Topological Insulator Bi$_2$Se$_3$

Topologically protected surface states of three-dimensional topological insulators provide a model framework for studying massless Dirac electrons in two dimensions. Usually a step on the surface of a topological insulator is treated as a scatterer for the Dirac electrons, and the study of its effect is focused on the interference of the incident and scattered electrons. Then a major role plays the warping of the Dirac cone far from the Dirac point. The existence of another significant effect near the Dirac point brought about by the presence of steps is experimentally demonstrated here. Namely the band bending in the vicinity of steps leads to formation of 1D bound states in the corresponding potential wells. The observation of bound states in such potential wells in our scanning tunneling microscopy and spectroscopy investigation of the surface of the topological insulator Bi$_2$Se$_3$ is reported. Numerical simulations support our conclusion and provide a recipe for the identification of such states.

cond-mat.mes-hall↗

arXiv2018

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 modes are massive and their origin 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.

cond-mat.mes-hall↗

arXiv2017

Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi$_2$Se$_3$: Distinguishing between effects and artifacts

The results of a detailed study of the topological insulator Bi$_2$Se$_3$ surface state energy structure in the vicinity of surface steps using scanning tunneling microscopy and spectroscopy methods are presented. An increase in the chemical potential level $μ$ near the step edge is observed. The value of the increase $δμ\sim 0.1$~eV is found to correlate with the step height. The effect is caused by redistribution of electron wave functions between the outer and inner edges of surface steps, as known for normal metals. The smaller value of the chemical potential shift and its larger characteristic length of $\sim 10$~nm reflect specifics of the helical surface states. This increase is accompanied by enlargement of the normalized differential tunneling conductance in the helical surface states energy region and thereby produces the illusion of the appearance of edge states. We show that the enlargement is reproduced in the framework of the tunneling model taking into account the tunneling gap transparency change when the chemical potential moves away from the Dirac point.

cond-mat.mtrl-sci↗

arXiv2016

Electronic correlation effects and Coulomb gap in the Si(111)-$(\sqrt{3}\times\sqrt{3})$-Sn surface

Electronic transport properties of the Si(111)-$(\sqrt{3}\times\sqrt{3})$-Sn surface formed on low doped Si substrates are studied using two-probe conductivity measurements and tunnelling spectroscopy. We demonstrate that the ground state corresponds to Mott-Hubbard insulator with a band gap $2Δ= 70$meV, which vanishes quickly upon temperature increase. The temperature dependence of the surface conductivity above $T > 50$K corresponds to the Efros-Shklovskii hopping conduction law. The energy gap at the Fermi level observed in tunnelling spectroscopy measurements at higher temperatures could be described in terms of dynamic Coulomb blockade approximation. The obtained localization length of electron is $ξ= 7$Å.

cond-mat.str-el↗

arXiv2016

Energy gap in tunneling spectroscopy: effect of the chemical potential shift

We study the effect of a shift of the chemical potential level on the tunneling conductance spectra. In the systems with gapped energy spectra, significant chemical-potential dependent distortions of the differential tunneling conductance curves, $dI/dV$, arise in the gap region. An expression is derived for the correction of the $dI/dV$, which in a number of cases was found to be large. The sign of the correction depends on the chemical potential level position with respect to the gap. The correction of the $dI/dV$ associated with the chemical potential shift has a nearly linear dependence on the tip-sample separation $z$ and vanishes at $z\to 0$.

cond-mat.mes-hall↗

arXiv2014

Effect of surface defects and few-atomic steps on the local density of states of the atomically-clean surface of topological insulator Bi$_2$Se$_3$

The results of ultra-high vacuum low-temperature scanning-tunneling microscopy (STM) and spectroscopy (STS) of atomically clean (111) surface of the topological insulator Bi$_2$Se$_3$ are presented. We observed several types of new subsurface defects whose location and charge correspond to p-type conduction of grown crystals. The sign of the thermoelectric effect also indicates p-type conduction. STM and STS measurements demonstrate that the chemical potential is always located inside the bulk band gap. We also observed changes in the local density of states in the vicinity of the quintuple layer steps at the studied surface. This changes correspond either to the shift of the Dirac cone position or to the shift of the chemical potential near the step edge.

cond-mat.mes-hall↗