arXiv subjects

V. A. Vlasenko

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

17 recordsLinked to original sources

arXiv2025

Ferromagnetic resonance in an antiferromagnetic crystal EuSn$_2$As$_2$

We report results of electron spin resonance (ESR) measurements in single crystals of EuSn$_2$As$_2$. In the temperature range of antiferromagnetic (AFM) ordering of Eu atoms, $T \leq T_N\approx 24$\,K, the ESR signal splits into two resonance lines, one of which, at high-field (or low-frequency), is the conventional acoustic AFM resonance mode that occurs at temperatures below $T_N$. The lower-field (high-frequency) line, as we have proven here, is the ferromagnetic resonance associated with the presence in the layered AFM crystal of a small amount ($\sim 3\%$) of planar nanodefects with a non-zero ferromagnetic (FM) moment. The existence of ferromagnetic nano-inclusions in the bulk of the antiferromagnetic compound makes EuSn$_2$As$_2$ a peculiar example of a natural magnetic metamaterial. We believe that the planar FM nanodefects are also inherent in other layered AFM compounds, which explains often observed increase in their magnetic susceptibility upon cooling at $T< T_N\rightarrow 0$.

cond-mat.mtrl-sci↗

arXiv2025

Emergence of Ferromagnetism from Planar Defects in EuSn2As2 Antiferromagnet

We report a study of nano-scale structural peculiarities of the antiferromagnetic layered semimetal EuSn2As2, and show that they are responsible for its puzzling magnetic properties. The high resolution transmission electron microscopy revealed the presence of planar defects in the lattice of the studied single crystals. Using a combination of microstructural and DFT analysis we demonstrated that a single planar nano-defects forms a layer of a distinct phase EuSnAs2, that is different from the EuSn2As2 phase of the bulk lattice. The smaller distance between Eu layers in the planar nano-defect promotes formation of local ferromagnetic (FM) ordering of the Eu atoms. On average, the planar defects form a weak ferromagnetic phase in the antiferromagnetic (AFM) host lattice. The obtained results explain several puzzling features in magnetic properties of A-type AFM materials: the nonlinear magnetization in low in-plane fields, ferromagnetic-type hysteresis in low field, and the upturn of the magnetic susceptibility in the AFM state at temperatures approaching zero.

cond-mat.str-el↗

arXiv2024

Intrinsic Negative Magnetoresistance in Layered AFM Semimetals: the Case of EuSn$_2$As$_2$

Here, by applying a comprehensive approach including magnetic, transport measurements, ARPES band structure measurements, DFT calculations, and analytical theory consideration, we unveil the puzzling origin of the negative isotropic magnetoresistance in the highly anisotropic semimetals, particularly, Eu$_2$Sn$_2$As$_2$ with AFM ordering of Eu atoms. The isotropic magnetoresistance developing along with the magnetization changes up to the complete spin polarization field was reported previously in several experimental studies, though its theoretical explanation was missing up to date. Recently, we proposed a novel theoretical mechanism to describe the observed magnetoresistance in layered AFM compounds by exchange splitting of the electron energy levels and by confining the electron wave functions with different spin projection in the vicinity of the respective magnetic layer. In this paper, we present more detailed experimental studies of the negative magnetoresistance with several samples of EuSn$_2$As$_2$ in order to identify its sample-independent features including temperature dependence. We also substantiate the proposed theory by comparing it with magnetotransport data, with ARPES measurements of the energy band structure, and DFT energy spectrum calculations.

cond-mat.supr-con↗

arXiv2023

Vortex phase dynamics in yttrium superhydride YH$_6$ at megabar pressures

A comprehensive study of the vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor YH$_6$ with T$_C$ (onset) of 215 K under pressure of 200 GPa.Thermal activation energy (U$_0$) is derived in the framework of thermally activated flux flow (TAFF) theory. The activation energy yields a power law dependence U$_0$ $\propto$ H$^α$ on magnetic field with a possible crossover at a field around 8-10 Tesla. Furthermore, we have depicted the vortex phase transition from vortex-glass to vortex-liquid state according to the vortex-glass theory. Finally, vortex phase diagram is constructed for the first time for superhydrides. Very high estimated values of flux flow barriers U$_0$(H) = 1.5-7*10$^4$ K together with high crossover fields makes YH$_6$ a rather outstanding superconductor as compared to most cuprates and iron-based systems. The Ginzburg number for YH$_6$ Gi = 3-7*10$^{-3}$ indicates that thermal fluctuations are not so strong and cannot broaden superconducting transitions in weak magnetic fields.

cond-mat.supr-con↗

arXiv2022

Magnetic resonances in EuSn$_2$As$_2$ single crystal

In this work, we report the broad-band ferromagnetic resonance spectroscopy of EuSn$_2$As$_2$ single crystals at different temperatures in combination with magnetization measurements and structural characterization. We observe conventional collective acoustic resonance mode of the A-type antiferromagnetic spin-flop phase in the Eu sub-lattice, and its transition to the paramagnetic resonance above the ordering temperature. Furthermore, we observe reproducibly additional well-defined spectral line. The origin of the additional line remains unclear. However, its temperature dependence attributes it to magnetism in the Eu sub-lattice.

cond-mat.supr-con↗

arXiv2022

Antiferromagnetic resonances in twinned EuFe2As2 single crystal

In this work, we report ferromagnetic resonance spectroscopy of EuFe2As2 single crystals. We observe ferromagnetic resonance responses, which are attributed to antiferromagnetic resonances of Eu sub-lattice with orthorhombic crystal structure and with different orientations of twin domains relative to the external field. We confirm validity of the recently-proposed spin Hamiltonian with anisotropic Eu-Eu exchange interaction and biquadratic Eu-Fe exchange interaction.

cond-mat.supr-con↗

arXiv2021

Superconducting Order Parameters in overdoped BaFe$_{1.86}$Ni$_{0.14}$As$_2$ Revealed by Multiple Andreev Reflection Spectroscopy of Planar Break-Junctions

Using multiple Andreev reflection (MAR) spectroscopy, we studied superconductor - normal metal - superconductor (SnS) semiballistic contacts with incoherent transport in overdoped pnictide superconductors BaFe$_{1.86}$Ni$_{0.14}$As$_2$ with critical temperature $T_c \approx 12$ K. We directly determined an anisotropic large gap with the characteristic ratios $2Δ_L^{\rm out}(0)/k_BT_c \approx 6$ and $2Δ_L^{\rm in}(0)/k_BT_c \approx 4$, almost isotropic small gap with $2Δ_S(0)/k_BT_c \approx 2$, and the temperature dependence of the resolved superconducting order parameters. Above the large gap position $2Δ_L$, a fine dI(V)/dV structure fully vanishing at $T_c$ and possibly caused by a resonant coupling with a bosonic mode was observed.

cond-mat.supr-con↗

arXiv2021

Novel magnetic stoichiometric superconductor EuRbFe4As4

In the novel stoichiometric iron-based material RbEuFe4As4 superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states; their coexistance is puzzling and represents a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy, resonant photoemission spectroscopy, Andreev reflection spectroscopy and scanning tunneling spectroscopy we have addressed this puzzle and unambigously shown that Fe- and Eu-derived states are largely decoupled and that superconducting and a long range magnetic orders exist almost independently from each other.

cond-mat.supr-con↗

arXiv2021

Helical spin order in EuFe2As2 and EuRbFe4As4 single crystals

In this work, fabrication and characterization of magnetic properties of EuFe2As2 and EuRbFe4As4 single crystals is reported. Magnetization measurements of samples with well defined thin film geometry and crystal orientation demonstrate a striking similarity in ferromagnetic properties of Eu subsystems in these two compounds. Measurements with magnetic field applied along ab crystal planes reveal meta-magnetic transition in both compounds. Numerical studies employing the Jz1-Jz2 Heisenberg model suggest that the ground state of the magnetic order in Eu subsystem for both compounds is the helical spin order with the helical angle about 2pi/5, while the meta-magnetic transition is the helix-to-fan first order phase transition.

cond-mat.supr-con↗

arXiv2020

When superconductivity does not fear magnetism: Insight into electronic structure of RbEuFe$_{4}$As$_{4}$

In the novel stoichiometric iron-based material RbEuFe$_{4}$As$_{4}$ superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long range magnetic orders exist independently from each other. The established electronic structure of RbEuFe$_{4}$As$_{4}$ opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

cond-mat.supr-con↗

arXiv2020

Evolution of vortex pinning in the FeSe$_{1-x}$S$_x$ system

We present a comprehensive study of vortex matter and pinning evolution in the FeSe$_{1-x}$S$_x$ system with various doping degree. The influence of sulphur substitution on vortex pinning and peak effect occurrence is studied. We show that there is a complex interplay among various pinning contributions in the FeSe$_{1-x}$S$_x$ system. Additionally, we study a possible vortex liquid-vortex glass/lattice transition and find an evidence that the vortex liquid-vortex glass phase transition in FeSe has a quasi two-dimensional nature. We investigate the upper critical field behaviour in FeSe$_{1-x}$S$_x$ system, and found that the upper critical field is higher than that predicted by the Werthamer-Helfand-Hohenberg (WHH) model, whereas its temperature dependence could be fitted within a two-band framework. Finally, a detailed H-T phase diagram is presented.

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

Vortex dynamic, pinning and irreversibility field investigation in EuRbFe4As4 superconductor

We performed systematic AC susceptibility and magnetic moment measurements to investigate the vortex dynamics and pinning in the $EuRbFe_4As_4$ single crystal as a function of temperature, frequency, and DC magnetic field. The vortex solid-liquid line was determined and it fits well with $H(T_p)=H_0(1-t_p)^β$ using $β$=1.74-1.91, for $H\parallel ab$. It indicates a rather high pinning strength of the vortex system. The activation energy $U_0$ was determined from thermally activated flux creep theory and reached 6700 K at low fields, suggesting strong vortex pinning. A field dependence of $U_0(H\parallel ab)\sim H^a$ with $a=0.47$ suggests thermally activated plastic pinning or caused by planar defects. Magnetic moment measurements also confirmed strong pinning in a $EuRbFe_4As_4$ superconductor and the superconducting response gives the main contribution to the $M(H)$ hysteresis. Additionally, we found evidence of long-range magnetic interactions in $Eu^{2+}$ sublattice and the FM-like nature of $Eu^{2+}$ atoms ordering.

cond-mat.supr-con↗

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↗

arXiv2016

Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]$_2$As$_2$

We studied nearly optimally Ni-substituted BaFe$_{2-x}$Ni$_x$As$_2$ (BFNA) single crystals with $T_C \approx 18.5$\,K. In irreversible magnetization measurements, we determined the field dependence of the critical-current density and discuss the nature of observed strong bulk pinning. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we directly determine two distinct superconducting gaps and resolve their moderate anisotropy in the momentum space. The BCS-ratio for the large gap $2Δ_L/k_BT_C > 4.1$ evidences for a strong coupling in the $Δ_L$-bands.

cond-mat.supr-con↗

arXiv2015

Superconducting properties of sulfur-doped iron selenide

The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_{c}$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth $λ_{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $λ_{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$-wave and single-gap BCS theory under the weak-coupling approach can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.

cond-mat.supr-con↗

arXiv2012

Bulk magnetization and strong intrinsic pinning in Ni-doped BaFe2As2 single crystals

We report on measurements of isothermal irreversible magnetization loops (M(H)) of Ni-doped BaFe2-xNixAs2 single crystals with x=0.1 and x=0.14 in magnetic fields up to 14T. For both samples in field H//c-axis and H//ab-plane, critical current density calculated from M(H) curves exceeds 106 A/cm2 at low temperatures suggesting strong intrinsic pinning in our crystals. In a broad temperature range 2-17K in both field orientations nearly optimally doped single crystal displays second magnetization peak (fish-tail effect). For H//c-axis, curves of the normalized pinning force, as a function of reduced field, measured at different temperatures obey scaling relation with peak position 0.33 for BaFe1.86Ni0.14As2 crystal and 0.4 for BaFe1.9Ni0.1As2 sample indicating single normal point pinning mechanism. In striking contrast, in H//ab-plane orientation, the scaling of curves is absent.

cond-mat.supr-con↗