arXiv subjects

A. V. Muratov

Publications and source records attributed to A. V. Muratov.

16 recordsLinked to original sources

arXiv2026

Amorphous insulating MoS$_x$ films deposited by magnetron sputtering as a low-index optical coating

While much effort are now concentrated on exploration of crystalline form of MoS$_2$ as a two-dimensional material, we show that room-temperature DC magnetron sputtering from pressurized MoS$_2$ powder target produces amorphous MoS$_x$ films with optical properties fundamentally different from those of crystalline MoS$_2$. The films were found to have a sulfur content of approximately $x=1.8$, indicating sulfur deficiency. They are optically isotropic and exhibit rather low optical losses, with a refractive index of about 2 over the visible and near-infrared spectral range. Their compatibility with lift-off lithography and dry etching makes amorphous MoS$_x$ a promising material for integrated photonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Electronic localization on the structural inhomogeneities formed due to Bi and Te deficiency in the MBE grown films of AFM topological insulator MnBi2Te4: Evidence from spectroscopic ellipsometry and infrared studies

The intrinsic substitutional and antisite defects cause unintentional doping and shift of the E_F position above the conduction band minimum in the AFM topological insulator (TI) MnBi2Te4. This prevents measurements of the quantum anomalous Hall effect (QAH) and investigation of the topological Dirac states. In the present study, the Mn-Bi-Te films grown by the MBE technique onto Si(111) substrates with decreasing Bi and Te contents and increasing Mn content were investigated by 0.5-6.5 eV spectroscopic ellipsometry. In addition, the 0.004-0.9 eV infrared (IR) transmittance spectra were examined. An effective medium model was used to reproduce the measured ellipsometric angles, Psi(omega) and Delta(omega), of the Mn-Bi-Te films in terms of the constructed model, including film thickness, surface roughness, and volume fractions of two (MnTe and Bi2Te3) or three constituents, the latter being associated with the structural inhomogeneities contribution. The results obtained for the inhomogeneous Mn-Bi-Te films using the three-phase EMA model indicate that the defect-associated optical response systematically shifts to higher photon energies from ~1.95 to ~2.43 eV with decreasing Te and Bi contents and increasing Mn content, pointing out that the electrons become more deeply localized in the formed structural inhomogeneities. The obtained results indicate that the structure of the non-stoichiometric Mn-Bi-Te films is not continuous but represented by regions of nearly stoichiometric MnBi2Te4 phase, which includes hollows or quantum anti-dots (QADs). The measured FIR transmittance spectra for the non-stoichiometric Mn-Bi-Te films show substantially reduced (or absent) contribution(s) from free charge carriers, which supports the relevance of localization effects.

cond-mat.str-el↗

arXiv2023

Amorphous VO$_x$ films with high temperature coefficient of the resistivity grown by reactive e-beam evaporation of V metal

Amorphous VO$_x$ films without a hysteretic phase transition are stable with respect to thermal cycling and highly demanded as sensitive elements of the resistive thermometers and microbolometers. In this paper we present simple and low-temperature growth of amorphous vanadium oxide films by reactive electron beam evaporation of vanadium metal in $\sim 10^{-4}$ mBar oxygen atmosphere. The temperature coefficient of the resistivity (TCR) of the films is weakly sensitive to substrate material and temperature and could be tuned by oxygen pressure in the growth chamber up to -2.2\% /K. The resistivity value is stable for months. It depends on the substrate material and substrate temperature during the evaporation. Simplicity and controllability of the method should lead to various laboratory and industrial applications.

cond-mat.mtrl-sci↗

arXiv2023

Bismuth layer properties in the ultrathin Bi-FeNi multilayer films probed by spectroscopic ellipsometry

Using wide-band (0.5-6.5 eV) spectroscopic ellipsometry we study ultrathin [Bi(0.6-2.5 nm)-FeNi(0.8,1.2 nm)]N multilayer films grown by rf sputtering deposition, where the FeNi layer has a nanoisland structure and its morphology and magnetic properties change with decreasing the nominal layer thickness. From the multilayer model simulations of the ellipsometric angles, Psi(omega) and Delta(omega), the complex (pseudo)dielectric function spectra of the Bi layer were extracted. The obtained results demonstrate that the Bi layer can possess the surface metallic conductivity, which is strongly affected by the morphology and magnetic properties of the nanoisland FeNi layer in the GMR-type Bi-FeNi multilayer structures.

cond-mat.mtrl-sci↗

arXiv2020

Optical and Hidden Transport Properties of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ Film

Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ thin superconducting film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow ($D_1$) and another broad one ($D_2$). In the superconducting state, two gaps with $2Δ_{0}^{(2)}/k_BT_c=1.9$--2.0 and $2Δ_{0}^{(1)}/k_BT_c=4.0$--4.3 are formed from the narrow component $D_1$ while the broad component $D_2$ remains ungapped. The calculated total DC resistivity of the film and the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. The change of total electron-boson coupling ($λ_{tot}$) and representative energy ($Ω_{0}$) in the normal state with respect to the superconducting state is typical of other iron-based materials as well as high-temperature superconducting (HTSC) cuprates.

cond-mat.supr-con↗

arXiv2020

Electronic structure and superconductivity of the non-centrosymmetric Sn$_4$As$_3$

In a superconductor that lacks inversion symmetry, the spatial part of the Cooper pair wave function has a reduced symmetry, allowing for the mixing of spin-singlet and spin-triplet Cooper pairing channels and thus providing a pathway to a non-trivial superconducting state. Materials with a non-centrosymmetric crystal structure and with strong spin-orbit coupling are a platform to realize these possibilities. Here, we report the synthesis and characterisation of high quality crystals of Sn$_4$As$_3$, with non-centrosymmetric unit cell ($R3m$). We have characterised the normal and superconducting state using a range of methods. Angle-resolved photoemission spectroscopy shows a multiband Fermi surface and the presence of two surface states, confirmed by Density-functional theory calculations. Specific heat measurements reveal a superconducting critical temperature of $T_c\sim 1.14$ K and an upper critical magnetic field of $H_c\gtrsim 7$ mT, which are both confirmed by ultra-low temperature scanning tunneling microscopy and spectroscopy. Scanning tunneling spectroscopy shows a fully formed superconducting gap, consistent with conventional $s$-wave superconductivity.

cond-mat.supr-con↗

arXiv2020

THz electrodynamics of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film analyzed in the framework of multiband Eliashberg theory

The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from terahertz spectra of conductivity and dielectric permittivity of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film with critical temperature $T_c=19.6$~K. Part of experimental data were analyzed within a simple three-band Eliashberg model where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy $Ω_{0}$ scales with $T_{c}$ according to the empirical law $Ω_{0} = 4.65k_{B}T_{c}$, and with a total electron-boson coupling strength $λ_{tot} = 2.17$.

cond-mat.supr-con↗

arXiv2019

Electronic Band Structure and Superconducting Properties of SnAs

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio $2Δ/T_c = 3.48 - 3.73$ is very close to the BCS value in the weak coupling limit.

cond-mat.supr-con↗

arXiv2019

Low-temperature infrared spectroscopy of Tm0.19Yb0.81B12 dodecaboride with metal-insulator transition and dynamic charge stripes

Tm1-xYbxB12 dodecaborides represent model objects for the studies of quantum critical behavior, metal-insulator transition and complex charge-spin-orbital-phonon coupling phenomena. In spite of intensive investigations, the mechanism of the ground state formation in this strongly correlated electron system remains a subject of active debates. We have performed first systematic measurements of temperature-dependent spectra of infrared conductivity of Tm0.19Yb0.81B12 at frequencies 40-35000 cm-1 and in the temperature interval from 300 K down to 10 K. Analysis of the temperature evolution of the observed absorption resonances is performed. Their origin is associated with the Jahn-Teller instability in the cubic lattice which results in the rattling modes of the rare earth ions and leads to emergence of both the intra-gap mixed-type collective excitations and the dynamic charge stripes. Temperature dependent effective mass of charge carriers is determined and a picture is presented of the multiple relaxation channels and the transformation of the manybody states at different temperatures. We argue in favor of electronic phase separation scenario which is valid both for the metal-insulator transition and for the formation of the nanoscale filamentary structure in Tm1-xYbxB12 compounds.

cond-mat.str-el↗

arXiv2018

Collective infrared excitation in rare-earth Gd$_x$La$_{1-x}$B$_6$ hexaborides

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides Gd$_x$La$_{1-x}$B$_6$, $x$(Gd)$=0$, 0.01, 0.1, 0.78, 1 are determined in the frequency range 30$-$35000$~cm^{-1}$. In all compounds, in addition to the Drude free-carrier spectral component, a broad excitation is discovered with the unusually large dielectric contribution $Δ$$\varepsilon$=5000 -- 15000 and non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with the dynamic cooperative Jahn-Teller effect of B$_6$ clusters. Analysis of the spectra together with the results of DC and Hall resistivity measurements shows that only 30$-$50$\%$ of the conduction band electrons are contributing to the free carrier conductivity with the rest being involved in the formation of an overdamped excitation, thus providing possible explanation of remarkably low work function of thermoemission of Gd$_x$La$_{1-x}$B$_6$ and non-Fermi-liquid behavior in GdB$_6$ crystals.

cond-mat.mtrl-sci↗

arXiv2018

Unique interplay between superconducting and ferromagnetic orders in EuRbFe$_4$As$_4$

Transport, magnetic and optical investigations on EuRbFe$_4$As$_4$ single crystals evidence that the ferromagnetic ordering of the Eu$^{2+}$ magnetic moments at $T_N=15$ K, below the superconducting transition ($T_c=36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_N$, the zero resistance state is preserved and the superconductivity is affected by the in-plane ferromagnetism mainly at domain boundaries; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_c$, associated with the opening of a complete superconducting gap at $2Δ= 10$ meV. A gap smaller than the weak coupling limit suggests the strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.

cond-mat.supr-con↗

arXiv2018

Observation of dynamic charge stripes in Tm0.19Yb0.81B12 at the metal-insulator transition

Higher accuracy low temperature charge transport measurements in combination with precise X-ray diffraction experiment have allowed detecting the symmetry lowering in the single domain Tm0.19Yb0.81B12 crystals of the family of dodecaborides with metal-insulator transition. Basing on the fine structure analysis we discover formation of dynamic charge stripes within the semiconducting matrix of Tm0.19Yb0.81B12. The charge dynamics in these metallic nano-size conducting channels is characterized by broad-band optical spectroscopy that allowed estimating the frequency (~2.4 10^11 Hz) of quantum motion of the charge carriers. It is suggested that caused by cooperative Jahn-Teller effect in the boron sub-lattice, the large amplitude rattling modes of the Tm and Yb ions are responsible for modulation of the conduction band along [110] direction through the variation of 5d-2p hybridization of electron states.

cond-mat.str-el↗

arXiv2017

On the Structure of Superconducting Order Parameter in High-Temperature Fe-Based Superconductors

This paper discusses the synthesis, characterization, and comprehensive study of Ba-122 single crystals with various substitutions and various $T_c$. The paper uses five complementary techniques to obtain a self-consistent set of data on the superconducting properties of Ba-122. A major conclusion of the authors work is the coexistence of two superconducting condensates differing in the electron-boson coupling strength. The two gaps that develop in distinct Fermi surface sheets are nodeless in the $k_xk_y$-plane and exhibit s-wave symmetry, the two-band model represents a sufficient data description tool. A moderate interband coupling and a considerable Coulomb repulsion in the description of the two-gap superconducting state of barium pnictides favor the $s^{++}$-model.

cond-mat.supr-con↗

arXiv2017

Direct Evidence of Two Superconducting Gaps in FeSe$_{0.5}$Te$_{0.5}$: SnS-Andreev Spectroscopy and Lower Critical Field

We present direct measurements of the superconducting order parameter in nearly optimal FeSe$_{0.5}$Te$_{0.5}$ single crystals with critical temperature $T_C \approx 14$ K. Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of lower critical field, we directly determined two superconducting gaps, $Δ_L \approx 3.3 - 3.4$ meV and $Δ_S \approx 1$ meV, and their temperature dependences. We show that a two-band model fits well the experimental data. The estimated electron-boson coupling constants indicate a strong intraband and a moderate interband interaction.

cond-mat.supr-con↗

arXiv2017

Probing Bulk Superconducting Order Parameter in Ba(K)Fe$_2$As$_2$ by Four Complementary Techniques

Using four different experimental techniques, we performed comprehensive studies of the bulk superconductive properties of single crystals of the nearly optimally doped $Ba_{1-x}K_xFe_2As_2$ ($T_{c} \approx 36\,K$), a typical representative of the 122 family. We investigated temperature dependencies of the (i) specific heat $C_{el}(T)$, (ii) first critical magnetic field $H_{c1}(T)$, (iii) intrinsic multiple Andreev reflection effect (IMARE), and (iv) infrared reflectivity spectra. All data clearly show the presence of (at least) two superconducting nodeless gaps. The quantitative data on the superconducting spectrum obtained by four different techniques are consistent with each other: (a) the small energy gap $Δ_S(0) \approx 1.8 - 2.5\,meV$, and the large gap energy $Δ_L(0) \approx 9.5 - 11.3\,meV$ that demonstrates the signature of an extended s-wave symmetry ($\sim~33 \%$ in-plane anisotropy), (b) the characteristic ratio $2Δ_L/k_BT_C$ noticeably exceeds the BCS value.

cond-mat.supr-con↗

arXiv2012

Observation of Multiple Superconducting Gaps in the Infrared Reflectivity Spectra of Ba(Fe0.9Co0.1)2As2

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The reflectivity is found to be close to unity at frequencies w lower than 2Delta/h (2Delta is the superconducting gap and h is Planck's constant). This is evidence for the s+/- or s+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at temperatures lower than critical Tc.

cond-mat.supr-con↗