arXiv subjects

N. N. Kolesnikov

Publications and source records attributed to N. N. Kolesnikov.

At least 55 records · Page 3Linked to original sources

arXiv2018

Spin wave effects in transport between a ferromagnet and a Weyl semimetal surface

We experimentally investigate spin-polarized transport between a ferromagnetic Ni electrode and a surface of Weyl semimetal, realized in a thick WTe$_2$ single crystal. For highly-transparent Ni-WTe$_2$ planar junctions, we observe non-Ohmic $dV/dI(I)$ behavior with an overall increase of differential resistance $dV/dI$ with current bias, which is accomplished by current-induced switchings. This behavior is inconsistent with trivial interface scattering, but it is well known for spin-polarized transport with magnon emission. Thus, we interpret the experimental results in terms of spin wave excitation in spin textures in the WTe$_2$ topological surface states, which is supported by the obtained magnetic field and temperature $dV/dI(I)$ dependencies.

cond-mat.mes-hall↗

arXiv2018

Signature of Fermi arc surface states in Andreev reflection at the WTe$_2$ Weyl semimetal surface

We experimentally investigate charge transport through the interface between a niobium superconductor and a three-dimensional WTe$_2$ Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependencies, we can interpret them as an analog of Tomasch oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe$_2$ interface. Observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is a hallmark of the Fermi arc surface states.

cond-mat.mes-hall↗

arXiv2018

Realization of a double-slit SQUID geometry by Fermi arc surface states in a WTe$_2$ Weyl semimetal

We experimentally study electron transport between two superconducting indium leads, coupled to the WTe$_2$ crystal surface. WTe$_2$ is characterized by presence of Fermi arc surface states, as a predicted type-II Weyl semimetal candidate. We demonstrate Josephson current in unprecedentedly long 5~$μ$m In-WTe$_2$-In junctions, which is confirmed by $I-V$ curves evolution with temperature and magnetic field. The Josephson current is mostly carried by the topological surface states, which we demonstrate in a double-slit SQUID geometry, realized by coupling the opposite WTe$_2$ crystal surfaces.

cond-mat.mes-hall↗

arXiv2015

Photoluminescence and Raman investigation of stability of InSe and GaSe thin films

Layered III-chalcogenide compounds belong to a variety of layered crystals that can be implemented in van der Waals heterostructures. Here we report an optical study of the stability of two of these compounds: indium selenide (InSe) and gallium selenide (GaSe). Micro-photoluminescence (PL) and Raman spectroscopy are used to determine how the properties of thin films of these materials change when they are exposed to air at room temperature. We find that in GaSe films, PL signal decreases on average below 50% over 24 (72) hours of exposure for films with thicknesses 10-25 (48-75) nm. In contrast, weak PL decrease of less than 20% is observed for InSe nm films after exposure of 100 hours. Similar trends are observed in Raman spectroscopy: within a week, the Raman signal decreases by a factor of 10 for a 24 nm thick GaSe, whereas no decrease was found for a 16 nm InSe film. We estimate that when exposed to air, the layers adjacent to the GaSe film surface degrade and become non-luminescent with a rate of 0.14$\pm$0.05 nm/hour. We show that the life-time of the GaSe films can be increased by up to two orders of magnitude (to several months) by encapsulation in dielectric materials such as SiO$_2$ or Si$_x$N$_y$.

cond-mat.mes-hall↗

arXiv2015

Optical properties of two-dimensional gallium chalcogenide films

Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thickness ranging from from 200 nm to a single unit cell. In both materials, PL shows dramatic decrease by 10$^4$-10$^5$ when film thickness is reduced from 200 to 10 nm. Based on evidence from cw and time-resolved PL, we propose a model explaining the PL decrease as a result of non-radiative carrier escape via surface states.

cond-mat.mes-hall↗

arXiv2014

Two-dimensional metal-chalcogenide films in tunable optical microcavities

Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic properties. However, photonic applications of thin films may be limited owing to weak light absorption and surface effects leading to reduced quantum yield. Integration of 2D films in optical microcavities will permit these limitations to be overcome owing to modified light coupling with the films. Here we present tunable microcavities with embedded monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; PL life-time shortening by a factor of 10 is achieved, a consequence of Purcell enhancement of the spontaneous emission rate. This work has potential to pave the way to microcavity-enhanced light-emitting devices based on layered 2D materials and their heterostructures, and also opens possibilities for cavity QED in a new material system of van der Waals crystals.

cond-mat.mtrl-sci↗

arXiv2012

Binding energy and stability of heavy and superheavy nuclei

Three different ways for description of binding energy of superheavy nuclei are discussed. First, one can consider superheavy nuclei as a part of a whole system of nuclei for which a global mass formula is found. Another way is the detailed local description of energy of superheavy nuclei taking into account the effects of shells and subshells. The third way of description, applied for nuclei in the region limited by principal magic numbers, is attached to the beta-stability line.

nucl-th↗

arXiv2008

Origins of large critical temperature variations in single layer cuprates

We study the electronic structures of two single layer superconducting cuprates, Tl$_2$Ba$_2$CuO$_{6+δ}$ (Tl2201) and (Bi$_{1.35}$Pb$_{0.85}$)(Sr$_{1.47}$La$_{0.38}$)CuO$_{6+δ}$ (Bi2201) which have very different maximum critical temperatures (90K and 35K respectively) using Angular Resolved Photoemission Spectroscopy (ARPES). We are able to identify two main differences in their electronic properties. First, the shadow band that is present in double layer and low T$_{c,max}$ single layer cuprates is absent in Tl2201. Recent studies have linked the shadow band to structural distortions in the lattice and the absence of these in Tl2201 may be a contributing factor in its T$_{c,max}$.Second, Tl2201's Fermi surface (FS) contains long straight parallel regions near the anti-node, while in Bi2201 the anti-nodal region is much more rounded. Since the size of the superconducting gap is largest in the anti-nodal region, differences in the band dispersion at the anti-node may play a significant role in the pairing and therefore affect the maximum transition temperature.

cond-mat.supr-con↗

arXiv2007

Doping dependence of the superconducting gap in Tl_2Ba_2CuO_{6+delta} from heat transport

We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime.

cond-mat.str-el↗

arXiv2002

Exponential Basis in Two-Sided Variational Estimates of Energy for Three-Body Systems

By the use of the variational method with exponential trial functions the upper and lower bounds of energy are calculated for a number of non-relativistic three-body Coulomb and nuclear systems. The formulas for calculation of upper and lower bounds for exponential basis are given, the lower bounds for great part of systems were calculated for the first time. By comparison of calculations for different bases the efficiency of exponential trial functions and their universality in respect to masses of particles and interaction are demonstrated. The advantage of exponential basis manifests mostly evident for the systems with comparable masses, though its use in one-center and two-center problems is justified too. For effective solution of two-center problem a carcass modification of the trial function is proposed. The stability of various three-particle Coulomb systems is analyzed.

physics.atom-ph↗

arXiv2002

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB$_2$ samples

The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $μ$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $λ_L(T)=X_s(T)/ωμ_0$ and $R_s(T)$ results in values of the London penetration depth $λ_L(0)\approx 600$ Åand residual surface resistance $R_{res}\approx 0.8$ m$Ω$. In the entire temperature range the dependences $R_s(T)$ and $X_s(T)$ are well described by the modified two-fluid model.

cond-mat.supr-con↗

arXiv2002

Magnetic Resonant Mode in the Single-Layer High Temperature Superconductor Tl$_2$Ba$_2$Cu$_{6+δ}$

An unusual spin excitation mode observed by neutron scattering has inspired numerous theoretical studies of the interplay between charged quasiparticles and collective spin excitations in the copper oxide high temperature superconductors. The mode has thus far only been observed in materials with crystal structures consisting of copper oxide bilayers, and it is notably absent in the single-layer compound La$_{2-x}$Sr$_{x}$CuO$_{4+δ}$. Neutron scattering data now show that the mode is present in Tl$_2$Ba$_2$Cu$_{6+δ}$, a single-layer compound with T$_c$ $\sim$ 90 K, thus demonstrating that it is a generic feature of the copper oxide superconductors, independent of the layer sequence. This restricts the theoretical models for the origin of the resonant mode and its role in the mechanism of high temperature superconductivity.

cond-mat.supr-con↗

arXiv2001

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of the MgB2 ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas beyond this region the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the slope as well as a "hysteresis" effect in the frequency vs. pressure behavior. These singularities in the E2g mode behavior under pressure support the suggestion that MgB2 may undergo a pressure-induced topological electronic transition.

cond-mat.mtrl-sci↗

arXiv2001

Synthesis of MgB2 from elements

Superconducting at 40 K MgB2 -samples were obtained by direct reaction from elements in molybdenum crucibles under argon pressure. Pressure allows to provide annealing at temperature up to 1400oC, that resulting in rise of Tc and compactness of the ceramics, suggesting that there is a homogeneity range of composition for the compound.

cond-mat.supr-con↗

arXiv2001

Crystal Growth and Characterization of Doped CZT Crystals

Cd1-xZnxTe crystals with x in the range of 0.1-0.2 were grown by the high-pressure vertical Bridgman method from pre-synthesized CZT. Resistive graphite heaters were used to control the temperature profiles within the furnaces, and an argon overpressure was used to reduce the cadmium loss. The crystals were doped with either Al, Ni, In, Ga, Ge or Sn. The doping was carried out by three different ways: 1) by adding of the pure metals during growth runs; 2) by adding of the tellurides of the metals during growth runs; or 3) by inserting of the metal tellurides during synthesis of the starting CZT material. Some of the growth process parameters were also varied. The as-grown CZT ingots had diameters of either 15 or 38 mm. The influence of the doping on CZT properties, particularly the conductivity type and specific electrical resistivity, will be discussed. Energy spectra from alpha particles (U-233, Ra-226, and U-233+Pu-239+Pu-238) and from different gamma sources (Cs-137, Co-60, Co-57, Am-241) will be reported.

cond-mat.mtrl-sci↗

arXiv2001

Effect of pressure on the superconducting Tc of MgB2

Measurements of the superconducting transition temperature Tc have been performed by the inductive method on MgB2 at pressures up to 28 GPa. Tc decreases with applied pressure, exhibiting a cusp at about 9 GPa. We interpret the appearance of this anomaly in the Tc(P) dependence as a result of the pressure-induced electronic transition. Recent band-structure calculations for MgB2 give some support for such an explanation.

cond-mat.supr-con↗

arXiv1999

Systematics of c-axis Phonons in the Thallium and Bismuth Based Cuprate Superconductors

We present grazing incidence reflectivity measurements in the far infrared region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6, Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly obtained. The reflectivity curves are well fitted by a series of Lorentz oscillators. In this way the transverse (TO) phonon frequencies were accurately determined. On the basis of the comparative study of the Bi and Tl based cuprates with different number of CuO2 layers per unit cell, we suggest modifications of the assignment of the main oxygen modes. We compare the LO frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic Josephson junction characteristics with our measurements, and explain the discrepancy in LO frequencies obtained by the two different methods.

cond-mat.supr-con↗

arXiv1998

Electronic Raman scattering in Tl2Ba2CuO6+x: symmetry of the order parameter, oxygen doping effects, and normal state scattering

Single crystals of the optimally doped, moderately and strongly overdoped high temperature superconductor Tl2Ba2CuO6+x (Tl-2201) with Tc=80, 56 and 30K, respectively, have been investigated by polarized Raman scattering. By taking the peak position of the B_1g component of electronic Raman scattering as 2Delta_0 we found that the reduced gap value (2Delta_0/k_BT_c) strongly decreases with increasing doping. The behavior of the low frequency scattering for the B_1g and B_2g scattering components is similar for optimally doped and overdoped crystals and can be described by a w^3 - and w -law, respectively, which is consistent with a d-wave symmetry of the order parameter. In contrast to the optimally doped Tl-2201 in both, moderately and strongly overdoped Tl-2201, the relative (compared to the B_1g) intensity of the A_1g scattering component is suppressed. We suggest that the van Hove singularity is responsible for the observed changes of Raman intensity and reduced gap value with doping. Electronic Raman scattering in the normal state is discussed in the context of the scattering from impurities and compared to the existing infrared data. The scattering rate evaluated from the Raman measurements is smaller for the overdoped samples, compared to the moderately overdoped samples.

cond-mat.supr-con↗