arXiv subjects

A. N. Vasiliev

Publications and source records attributed to A. N. Vasiliev.

At least 37 records · Page 2Linked to original sources

arXiv2020

Trigonal layered rosiaite-related antiferromagnet MnSnTeO6: ion-exchange preparation, structure and magnetic properties

Ion-exchange treatment of Na2SnTeO6 in molten salt mixtures resulted in rosiaite (PbSb2O6)-related MnSnTeO6. Its crystal structure was refined by the Rietveld method. Of the three possible models of Sn/Te distribution, the disordered model ((P3 Im, a = 5.23093(11) A, c = 4.62430(16) A was preferred based on bond distances. However, it is supposed that each individual (SnTeO6)2- layer retains complete ordering of the precursor and the apparent disorder is only due to stacking faults. The magnetic studies have shown that MnSnTeO6 orders antiferromagnetically at Neel temperature TN = 9.8 K. The effective magnetic moment reasonably agrees with theoretical estimations assuming high-spin configuration of Mn2+ (S = 5/2). Electron spin resonance reveals spin dynamics in accordance with aniferromagnetic ordering with critical broadening of ESR linewidth indicating the low-dimentional type of exchange interactions. Based on the temperature and field-dependent magnetization studies, the magnetic phase diagram of the new compound was constructed.

cond-mat.mtrl-sci↗

arXiv2020

Coexistence of Superconductivity and Charge Density Wave in Tantalum Disulfide: Experiment and Theory

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS$_2$) at ambient pressure, is boosted by applying hydrostatic pressures up to 30GPa, thereby inducing a typical dome-shaped superconducting phase. The ambient pressure CDW ground state which begins at TCDW = 76 K, with critically small Fermi surfaces, was found to be fully suppressed at Pc = 8.7GPa. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc = 9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding lower critical field H$_{c1}$ was deduced. While a d wave and single-gap BCS prediction cannot describe our H$_{c1}$ experiments, the temperature dependence of the H$_{c1}$ can be well described by a single-gap anisotropic s-wave order parameter.

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

Multiband effect in elastoresistance of Fe(Se,Te)

We have investigated the elastoresistance of two FeSe${}_{1-x}$Te${}_{x}$ (x about 0.4 - 0.5) compounds that have a close chemical composition but differ significantly in electronic properties. The first compound has a negative temperature coefficient of resistance and does not show any phase transitions other than superconducting. The elastoresistance of this compound approximately follows $1/T$ low as it usually occurs in Fe(Se,S) with metallic conductivity. The second compound has a metallic type of conductivity and in addition to the superconducting transition, there is also a phase transition at a temperature of about 30 K. The elastoresistance of the second compound is sign-reversing and can be approximated with the sum of two Curie-Weiss type terms with opposite signs and different critical temperatures which suggest a competition of contributions to the elastoresistance from different band valleys.

cond-mat.supr-con↗

arXiv2020

The polarized proton and antiproton beam project at U-70 accelerator

The design and parameters of the polarized-beam facility at U-70 proton synchrotron of NRC Kurchatov Institute - IHEP are presented. The new beamline 24A will provide the polarized proton and antiproton beams for carrying out the rich physics program of the SPASCHARM experiment for comprehensive studies of spin phenomena in a wide spectrum of hadronic reactions in the energy range of 10-45 GeV.

physics.acc-ph↗

arXiv2019

Observation of orbital ordering and origin of the nematic order in FeSe

To elucidate the origin of nematic order in FeSe, we performed field-dependent 77Se-NMR measurements on single crystals of FeSe. We observed orbital ordering from the splitting of the NMR spectra and Knight shift and a suppression of it with magnetic field B0 up to 16 T applied parallel to the Fe-planes. There is a significant change in the distribution and magnitude of the internal magnetic field across the orbital ordering temperature Torb while stripe-type antiferromagnetism is absent. Giant antiferromagnetic (AFM) spin fluctuations measured by the NMR spin-lattice relaxation are gradually developed starting at ~ 40 K, which is far below the nematic ordering temperature Tnem. These results demonstrate that orbital ordering is the origin of the nematic order, and the AFM spin fluctuation is the driving mechanism of superconductivity in FeSe under the presence of the nematic order.

cond-mat.supr-con↗

arXiv2019

Observation of Feynman scaling violations and evidence for a new resonance at RHIC

We report measurements of forward jets and dijets produced in Cu+Au collisions at $\sqrt{s_{NN}}=200$ GeV at the Relativistic Heavy Ion Collider. We also report dijet production cross sections in p+p collisions at $\sqrt{s}=510$ GeV. We use the invariant dijet mass to search for indications of new particles. The p+p dijet results are compatible with string fragmentation models tuned to fit LHC data. The Cu+Au jet results far exceed Feynman scaling limits, and are compatible with models that incorporate string fusion to increase parton energy, acting as a QCD accelerator. The Cu+Au dijet results can be mostly explained by double parton scattering due to a parton flux from multiple p+p interactions with $\sqrt{s}>>\sqrt{s_{NN}}$. Further indication of the increased parton energy is obtained from evidence of single- and double-$Υ$(1S) production in the forward direction in Cu+Au collisions. Finally, we report evidence for the production of a new resonance, reconstructed from its dijet decay.

nucl-ex↗

arXiv2019

Nematic properties of FeSe$_{1-x}$Te$_{x}$ crystals with a low Te content

We report on the synthesis and physical properties of FeSe$_{1-x}$Te$_x$ single crystals with a low Te content (x = 0.17, 0.21, 0.25), where the replacement of Se with Te partially suppresses superconductivity. Resistivity and Hall effect measurements indicate weak anomalies at elevated temperatures ascribed to nematic transitions. A quasi-classical analysis of transport data, including in a pulsed magnetic field of up to 25 T, confirms the inversion of majority carriers type from holes in FeSe to electrons in FeSe$_{1-x}$Te$_x$ at x $>$ 0.17. The temperature-dependent term in the elastoresistance of the studied compositions has a negative sign, which means that for substituted FeSe compositions, the elastoresistance is positive for hole-doped materials and negative for electron-doped materials just like in semiconductors such as silicon and germanium.

cond-mat.supr-con↗

arXiv2019

3D visualizations of nano-scale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)0.95Fe1.75Se2

Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (AxFe2-ySe2) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of AxFe2-ySe2 superconductors. Here we report that the 3D visualizations of nano-scale phase separation in (Na0.32K0.68)0.95Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in (Na0.32K0.68)0.95Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change (ΔR/R) identify two characteristic temperatures T*~100 K (the onset temperature of coupling between nano-metallic and AFM phases) and TH~230 K (the onset temperature of metallic-interface-phase). An energy-transfer channel between the nano-metallic and AFM phases is inferred. This proposed channel provides a new insight into the pairing mechanism of alkaline iron selenide superconductors.

cond-mat.supr-con↗

arXiv2019

The decisive role of magnetic anisotropy in honeycomb layered Li$_3$Ni$_2$SbO$_6$ and Na$_3$Ni$_2$SbO$_6$

The decisive role of magnetic anisotropy even in systems with small anisotropy is illustrated for the honeycomb-layered antiferromagnets The decisive role of magnetic anisotropy in honeycomb layered Li$_3$Ni$_2$SbO$_6$ and Na$_3$Ni$_2$SbO$_6$ with $A$ = Li and Na. Both systems evolve long range magnetic order below $T_{\rm N}$ = 14 and 16.5~K, respectively. The magnetic phase diagrams obtained from static magnetisation studies up to 15~T imply competing antiferromagnetic phases and a tricritical point at $T_{\rm N}$. The phase boundaries are visible in the dynamic response of the antiferromagnetic resonance modes, too, which investigation by means of high frequency/high field electron spin resonance enables precise determination of magnetic anisotropy. The anisotropy gap amounts to $Δ= 360 \pm 2$~GHz in Na$_3$Ni$_2$SbO$_6$ while in Li$_3$Ni$_2$SbO$_6$ orthorhombicity is associated with $Δ= 198 \pm 4$ and $218 \pm 4$~GHz. Above $T_{\rm N}$, the data imply short-range antiferromagnetic order up to at least 80~K. The data suggest a crucial role of anisotropy for selecting the actual spin structure at $B=0$~T.

cond-mat.str-el↗

arXiv2019

Anomalous Knight shift and low-energy spin dynamics in the nematic state of FeSe$_{\rm 1-x}$S$_{\rm x}$

The interplay between the nematic order and magnetism in FeSe is not yet well understood. There is a controversy concerning the relationship between orbital and spin degrees of freedom in FeSe and their relevance for superconductivity. Here we investigate the effect of S substitution on the nematic transition temperature ($T_{\rm n}$) and the low-energy spin fluctuations (SF) in FeSe single crystals. We show that the low-energy SF emerge below the nematic transition. The difference between the onset temperature for the critical SF ($T_{\rm SF}$) and $T_{\rm n}$ is small for FeSe but significantly increases with S substitution. Below $T_{\rm SF}$ the Korringa relation is violated and the effective muon hyperfine coupling constant changes a sign. Our results exclude a direct coupling of the low-energy SF to the electronic nematic order indicating a presence of multiple spin degrees of freedom in FeSe$_{\rm 1-x}$S$_{\rm x}$.

cond-mat.str-el↗

arXiv2019

Flat-band spin dynamics and phonon anomalies of the saw-tooth spin-chain system Fe$_2$O(SeO$_3$)$_2$

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T = 150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

cond-mat.str-el↗

arXiv2018

Revealing Color Forces with Transverse Polarized Electron Scattering

The Spin Asymmetries of the Nucleon Experiment (SANE) measured two double spin asymmetries using a polarized proton target and polarized electron beam at two beam energies, 4.7 GeV and 5.9 GeV. A large-acceptance open-configuration detector package identified scattered electrons at 40$^{\circ}$ and covered a wide range in Bjorken $x$ ($0.3 < x < 0.8$). Proportional to an average color Lorentz force, the twist-3 matrix element, $\tilde{d}_2^p$, was extracted from the measured asymmetries at $Q^2$ values ranging from 2.0 to 6.0 GeV$^2$. The data display the opposite sign compared to most quark models, including the lattice QCD result, and an apparently unexpected scale dependence. Furthermore when combined with the neutron data in the same $Q^2$ range the results suggest a flavor independent average color Lorentz force.

nucl-ex↗

arXiv2018

Majority carrier type inversion in FeSe family and "doped semimetal" scheme in iron-based superconductors

The field and temperature dependencies of the longitudinal and Hall resistivity have been studied for high-quality FeSe${}_{1-x}$S${}_{x}$ (x up to 0.14) single crystals. Quasiclassical analysis of the obtained data indicates a strong variation of the electron and hole concentrations under the studied isovalent substitution and proximity of FeSe to the point of the majority carrier-type inversion. On this basis, we propose a `doped semimetal' scheme for the superconducting phase diagram of the FeSe family, which can be applied to other iron-based superconductors. In this scheme, the two local maxima of the superconducting temperature can be associated with the Van Hove singularities of a simplified semi-metallic electronic structure. The multicarrier analysis of the experimental data also reveals the presence of a tiny and highly mobile electron band for all the samples studied. Sulfur substitution in the studied range leads to a decrease in the number of mobile electrons by more than ten times, from about 3\% to about 0.2\%. This behavior may indicate a successive change of the Fermi level position relative to singular points of the electronic structure which is consistent with the `doped semimetal' scheme. The scattering time for mobile carriers does not depend on impurities, which allows us to consider this group as a possible source of unusual acoustic properties of FeSe.

cond-mat.supr-con↗

arXiv2018

Proton Form Factor Ratio, $μ_p G_E^p/G_M^p$ from Double Spin Asymmetry

The ratio of the electric and magnetic form factor of the proton, $μ_p G_E^p/G_M^p$, has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared, $Q^2=5.66$ (GeV/c)$^2$ using the double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction. This measurement of $μ_p G_E^p/G_M^p$ agrees with the $Q^2$ dependence of previous recoil polarization data and reconfirms the discrepancy at high $Q^2$ between the Rosenbluth and the polarization-transfer method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The form factor ratio at $Q^2$=2.06 (GeV/c)$^2$ has been measured as $μ_p G_E^p/G_M^p = 0.720 \pm 0.176_{stat} \pm 0.039_{sys}$, which is in agreement with an earlier measurement with the polarized target technique at similar kinematics. The form factor ratio at $Q^2$=5.66 (GeV/c)$^2$ has been determined as $μ_p G_E^p/G_M^p=0.244\pm0.353_{stat}\pm0.013_{sys}$, which represents the highest $Q^2$ reach with the double spin asymmetry with polarized target to date.

nucl-ex↗

arXiv2018

High-Pressure Phase Diagram of NdFeAsO$_{0.9}$F$_{0.1}$: Disappearance of superconductivity on the verge of ferromagnetism from nd moments

We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M$($H$) loops and the strong increase in the field cooled data, $M$($T$), toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi-liquid to non-Fermi-liquid to Fermi-liquid. These results give access to the high-pressure side of the superconducting phase diagram in 1111 type of materials.

cond-mat.supr-con↗

arXiv2018

Magnetism and the phase diagram of MnSb$_2$O$_6$

Static and dynamic magnetic properties of P\={3}1$m$-phase MnSb$_2$O$_6$ have been studied by means of muon spin relaxation ($μ$SR), high-frequency electron spin resonance (HF-ESR), specific heat, and magnetisation studies in magnetic fields up to 25\,T. The data imply onset of long-range antiferromagnetic order at $T_N$ =~8~K and a spin-flop-like transition at $B_{SF}\approx 0.7 - 1$~T. Below $T_N$, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below $T_2$ =~5.3~K is evidenced by a step in the magnetisation and a slight kink of the relaxation rate. Above $T_N$ , both $μ$SR and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least $12\cdot T_{\rm N}\approx 100$\,K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of $Δ\approx 18$\,GHz which implies small but finite magnetic anisotropy.

cond-mat.str-el↗

arXiv2018

Superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductor

We report on the Andreev spectroscopy and specific heat of high-quality single crystals BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps $Δ_L \approx 3.2 \textendash 4.5$\,meV, $Δ_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_xk_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps $Δ_L \approx 3.2$\,meV and $Δ_S \approx 1.6$\,meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers a further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

cond-mat.supr-con↗