arXiv subjects

A. Vasiliev

Publications and source records attributed to A. Vasiliev.

At least 19 recordsLinked to original sources

arXiv2025

Comparative study of terbium tellurides Tb2Te5 and TbTe3

Two terbium tellurides, TbTe3 and Tb2Te5, were studied by means of thermodynamics, ultrafast pump-probe spectroscopy and torque magnetometry. While crystal structure and some physical properties of TbTe3 were established previously, the crystal structure of Tb2Te5 was solved only in this work in the orthorhombic space group Cmcm with the parameters of unit cell a = 4.3120(5), b = 41.0305(76) and c = 4.2979(8) Å. In contrast to TbTe3, which experiences three successive magnetic phase transitions, Tb2Te5 orders antiferromagnetically in two steps at TN1 = 9.0 K and TN2 = 6.8 K, both readily suppressed by an external magnetic field. The third transition in TbTe3 is due to the interaction of the magnetic subsystem with the charge density waves. The interaction of magnetic and electronic subsystems in Tb2Te5 has been revealed by the pump probe. Torque measurements of TbTe3 show that the magnetic moments of Tb are oriented predominantly in the ac plane at high temperatures and switch to the b axis at low temperatures. In Tb2Te5, the magnetic moments of Tb are oriented predominantly in the ac plane at low temperatures.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic phase diagram and possible Kitaev-like behavior of honeycomb-lattice antimonate Na3Co2SbO6

Recent theoretical studies have suggested that Kitaev physics and such effects as formation of a mysterious spin-liquid state can be expected not only in RuCl3 and iridates, but also in conventional $3d$ transition metal compounds. Using DC and AC magnetometry, thermodynamic and $^{23}$Na nuclear magnetic resonance measurements (NMR) we studied such a candidate material Na3Co2SbO6. A full phase diagram of Na3Co2SbO6 in a wide range of magnetic fields and temperatures is presented. The results demonstrate transformation of the antiferromagnetic structure under the external magnetic field, gradual development of the saturation phase, as well as evidence of gapped behavior in certain parts of the phase diagram

cond-mat.str-el↗

arXiv2022

Linear magnetoelastic coupling and magnetic phase diagrams of the buckled-kagomé antiferromagnet Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl

Single crystals of Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl were investigated using high-resolution capacitance dilatometry in magnetic fields up to 15 T. Pronounced magnetoelastic coupling is found upon evolution of long-range antiferromagnetic order at T$_{\mathrm{N}} = 26.4(3)$ K. Grüneisen analysis reveals moderate effects of uniaxial pressure on T$_{\mathrm{N}}$, of $1.8(4)$ K/GPa, $-0.62(15)$ K/GPa and $0.33(10)$ K/GPa for $p \parallel a$, $b$, and $c$, respectively. Below 22 K, Grüneisen scaling fails which implies the presence of competing interactions. The structural phase transition at T$_{\mathrm{S}} = 120.7(5)$ K is much more sensitive to uniaxial pressure than T$_{\mathrm{N}}$, with strong effects of up to $27(3)$ K/GPa ($p \parallel c$). Magnetostriction and magnetization measurements reveal a linear magnetoelastic coupling for $B\parallel c$ below T$_{\mathrm{N}}$, as well as a mixed phase behavior above the tricritical point around 0.4 T. An analysis of the critical behavior in zero-field points to three-dimensional (3D) Ising-like magnetic ordering. In addition, the magnetic phase diagrams for fields along the main crystalline axes are reported.

cond-mat.str-el↗

arXiv2021

Electromagnetic conductivity of quark-gluon plasma at non-zero baryon density

In this preprint we present our results on the study of the electromagnetic conductivity in dense quark-gluon plasma obtained within lattice simulations with $N_f = 2 + 1$ dynamical quarks. We employ stout improved rooted staggered quarks at the physical point and the tree-level Symanzik improved gauge action. The simulations are performed at imaginary baryon chemical potential, and the Tikhonov regularisation method is used to extract the conductivity from current-current correlators. Our results indicate an increase of QGP electromagnetic conductivity with real baryon density, and this dependence is quite strong.

hep-lat↗

arXiv2020

Iterative Bayesian Monte Carlo for nuclear data evaluation

In this work, we explore the use of an iterative Bayesian Monte Carlo (IBM) procedure for nuclear data evaluation within a Talys Evaluated Nuclear data Library (TENDL) framework. In order to identify the model and parameter combinations that reproduce selected experimental data, different physical models implemented within the TALYS code, were sampled and varied simultaneously to produce random input files with unique model combinations. All the models considered were assumed to be equal a priori. Parameters to these models were then varied simultaneously using the TALYS code system to produce a set of random ENDF files which were processed into x-y tables for comparison with selected experimental data from the EXFOR database within a Bayesian framework. To improve our fit to experimental data, we iteratively update our 'best' file - the file that maximises the likelihood function - by re-sampling model parameters around this file. The method proposed has been applied for the evaluation of p+Cd-111 and Co-59 between 1 - 100 MeV incident energy region. Finally, the adjusted files were compared with experimental data from the EXFOR database as well as with evaluations from the TENDL-2017 and JENDL-4.0/HE nuclear data libraries.

physics.data-an↗

arXiv2019

In search of the best nuclear data file for proton induced reactions: varying both models and their parameters

A lot of research work has been carried out in fine tuning model parameters to reproduce experimental data for neutron induced reactions. This however is not the case for proton induced reactions where large deviations still exist between model calculations and experiments for some cross sections. In this work, we present a method for searching both the model and model parameter space in order to identify the 'best' nuclear reaction models with their parameter sets that reproduces carefully selected experimental data. Three sets of experimental data from EXFOR are used in this work: (1) cross sections of the target nucleus (2) cross sections of the residual nuclei and (3) angular distributions. Selected models and their parameters were varied simultaneously to produce a large set of random nuclear data files. The goodness of fit between our adjustments and experimental data was achieved by computing a global reduced chi square which took into consideration the above listed experimental data. The method has been applied for the adjustment of proton induced reactions on Co-59 between 1 to 100 MeV. The adjusted files obtained are compared with available experimental data and evaluations from other nuclear data libraries.

nucl-th↗

arXiv2019

MnSnTeO6: a Chiral Antiferromagnet Prepared by a Two-Step Topotactic Transformation

MnSnTeO6, a new chiral antiferromagnet, was prepared both by topotactic transformation of the metastable rosiaite-type polymorph and by direct synthesis from coprecipitated hydroxides. Its structure, static and dynamic magnetic properties were studied comprehensively both experimentally (through X-ray and neutron powder diffraction, magnetization, specific heat, dielectric permittivity and ESR techniques) and theoretically (by means of ab initio density functional theory (DFT) calculations within the spin-polarized generalized gradient approximation). MnSnTeO6 is isostructural with MnSb2O6 (space group P321) and does not show any structural transition between 3 and 300 K. The magnetic susceptibility and specific heat exhibit an antiferromagnetic ordering at TN=9.8 K, which is confirmed by low-temperature neutron data. At the same time, the thermodynamic parameters demonstrate an additional anomaly on the temperature dependences of magnetic susceptibility chi(T), specific heat Cp(T) and dielectric permittivity epsilon(T) at T*=4.9 K, which is characterized by significant temperature hysteresis. Clear enhancement of the dielectric permittivity at T* is most likely to reflect the coupling of dielectric and magnetic subsystems leading to development of electric polarization. It was established that the ground state of MnSnTeO6 is stabilized by seven exchange parameters, and neutron diffraction revealed incommensurate magnetic structure with propagation vector k= (0, 0, 0.183) analogous to that of MnSb2O6. Ab initio DFT calculations demonstrate that the strongest exchange coupling occurs between planes along diagonals. All exchange parameters are antiferromagnetic and reveal moderate frustration.

cond-mat.str-el↗

arXiv2019

High Precision Measurement of Compton Scattering in the 5 GeV region

The cross section of atomic electron Compton scattering $γ+ e \rightarrow γ^\prime + e^\prime $ was measured in the 4.40--5.475 GeV photon beam energy region by the {\em PrimEx} collaboration at Jefferson Lab with an accuracy of 2\% and less. The results are consistent with theoretical predictions that include next-to-leading order radiative corrections. The measurements provide the first high precision test of this elementary QED process at beam energies greater than 0.1 GeV.

nucl-ex↗

arXiv2019

Bayesian updating for data adjustments and multi-level uncertainty propagation within Total Monte Carlo

In this work, a method is proposed for combining differential and integral benchmark experimental data within a Bayesian framework for nuclear data adjustments and multi-level uncertainty propagation using the Total Monte Carlo method. First, input parameters to basic nuclear physics models implemented within the state of the art nuclear reactions code, TALYS, were sampled from uniform distributions and randomly varied to produce a large set of random nuclear data files. Next, a probabilistic data assimilation was carried out by computing the likelihood function for each random nuclear data file based first on only differential experimental data (1st update) and then on integral benchmark data (2nd update). The individual likelihood functions from the two updates were then combined into a global likelihood function which was used for the selection of the final 'best' file. The proposed method has been applied for the adjustment of $^{208}$Pb in the fast neutron energy region below 20 MeV. The 'best' file from the adjustments was compared with available experimental data from the EXFOR database as well as evaluations from the major nuclear data libraries and found to compare favourably.

nucl-th↗

arXiv2018

Technical Supplement to "Polarization Transfer Observables in Elastic Electron-Proton Scattering at Q$^2$ = 2.5, 5.2, 6.8, and 8.5 GeV$^2$"

The GEp-III and GEp-2$γ$ experiments, carried out in Jefferson Lab's Hall C from 2007-2008, consisted of measurements of polarization transfer in elastic electron-proton scattering at momentum transfers of $Q^2 = 2.5, 5.2, 6.8,$ and $8.54$ GeV$^2$. These measurements were carried out to improve knowledge of the proton electromagnetic form factor ratio $R = μ_p G_E^p/G_M^p$ at large values of $Q^2$ and to search for effects beyond the Born approximation in polarization transfer observables at $Q^2 = 2.5$ GeV$^2$. The final results of both experiments were reported in a recent archival publication. A full reanalysis of the data from both experiments was carried out in order to reduce the systematic and, for the GEp-2$γ$ experiment, statistical uncertainties. This technical note provides additional details of the final analysis omitted from the main publication, including the final evaluation of the systematic uncertainties.

nucl-ex↗

arXiv2018

Polarization Transfer Observables in Elastic Electron Proton Scattering at $Q^2 = $2.5, 5.2, 6.8, and 8.5 GeV$^2$

The GEp-III and GEp-2$γ$ experiments were carried out in Jefferson Lab's (JLab's) Hall C from 2007-2008, to extend the knowledge of $G_E^p/G_M^p$ to the highest practically achievable $Q^2$ and to search for effects beyond the Born approximation in polarization transfer observables of elastic $\vec{e}p$ scattering. This article reports an expanded description of the common experimental apparatus and data analysis procedure, and the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance data of the GEp-2$γ$ experiment. The Hall C High Momentum Spectrometer detected and measured the polarization of protons recoiling elastically from collisions of JLab's polarized electron beam with a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scattered electrons in coincidence to suppress inelastic backgrounds. The final GEp-III data are largely unchanged relative to the originally published results. The statistical uncertainties of the final GEp-2$γ$ data are significantly reduced at $ε= 0.632$ and $0.783$ relative to the original publication. The decrease with $Q^2$ of $G_E^p/G_M^p$ continues to $Q^2 = 8.5$ GeV$^2$, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At $Q^2 = 2.5$ GeV$^2$, the proton form factor ratio $G_E^p/G_M^p$ shows no statistically significant $ε$-dependence, as expected in the Born approximation. The ratio $P_\ell/P_\ell^{Born}$ of the longitudinal polarization transfer component to its Born value shows an enhancement of roughly 1.4\% at $ε= 0.783$ relative to $ε= 0.149$, with $\approx 1.9σ$ significance based on the total uncertainty, implying a similar effect in the transverse component $P_t$ that cancels in the ratio $R$.

nucl-ex↗

arXiv2018

Quantum-Assisted Blockchain

Bitcoin and blockchain in general is a hot topic nowadays. In the paper we propose a quantum empowering of this technology and show how to speed-up the mining procedure using the modified Grover's algorithm.

quant-ph↗

arXiv2017

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14\,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.

cond-mat.supr-con↗

arXiv2016

Longitudinal magnon, inversion breaking and magnetic instabilities in the pseudo-Kagome francisites Cu3Bi(SeO3)2O2X with X=Br, Cl

We performed Raman studies and a dielectric characterization of the pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour antiferromagnetic exchange as well as highly noncollinear magnetic ground state. However, at low temperature they differ with respect to the existence of inversion symmetry. For both compounds there exists a pronounced interplay of polar phonon modes with quantum magnetic fluctuations. A novel Raman mode appears for temperatures below the Neel temperature with a Fano lineshape and an enormous intensity that exceeds most of the phonon lines. We discuss a possible contribution of longitudinal magnons to this signal. In contrast, one magnon scattering based on linear transvers magnons is excluded based on a symmetry analysis of spin wave representations and Raman tensor calculations. There exists evidence that in these pseudo-kagome compounds magnetic quantum fluctuations carry an electric dipole moment. Our data as well as a comparison with previous far-infrared spectra allow us to conclude that Cu3Bi(SeO3)2O2Cl changes its symmetry most likely from Pmmn to P21mn with a second order structural phase transition at T*=120 K and becomes multiferroic. Cu3Bi(SeO3)2O2Br represents an interesting counter part as it does not show this instability and stays inversion symmetric down to lowest temperatures, investigated.

cond-mat.str-el↗

arXiv2015

Control of the magnetic phase coexistence in NdFe$_3$(BO$_3$)$_4$ by electric fields

We present a resonant x-ray diffraction study of the magnetic order in NdFe$_3$(BO$_3$)$_4$ and its coupling to applied electric fields. Our high-resolution measurements reveal A coexistence of two different magnetic phases, which can be triggered effectively by external electric fields. More in detail, the volume fraction of the collinear magnetic phase is found to strongly increase at the expense of helically ordered regions when an electric field is applied. These results confirm that the collinear magnetic phase is responsible for the ferroelectric polarization of NdFe$_3$(BO$_3$)$_4$ and, more importantly, demonstrate that magnetic phase coexistence provides an alternative route towards materials with a strong magnetoelectric response.

cond-mat.str-el↗

arXiv2015

Toward polarized antiprotons: Machine development for spin-filtering experiments

The paper describes the commissioning of the experimental equipment and the machine studies required for the first spin-filtering experiment with protons at a beam kinetic energy of $49.3\,$MeV in COSY. The implementation of a low-$β$ insertion made it possible to achieve beam lifetimes of $τ_{\rm{b}}=8000\,$s in the presence of a dense polarized hydrogen storage-cell target of areal density $d_{\rm t}=(5.5\pm 0.2)\times 10^{13}\,\mathrm{atoms/cm^{2}}$. The developed techniques can be directly applied to antiproton machines and allow for the determination of the spin-dependent $\bar{p}p$ cross sections via spin filtering.

physics.acc-ph↗

arXiv2012

Magnetic frustration, phase competition and the magneto-electric effect in NdFe3(BO3)4

We present an element selective resonant magnetic x-ray scattering study of NdFe3(BO3)4 as a function of temperature and applied magnetic field. Our measurements show that the magnetic order of the Nd sublattice is induced by the Fe spin order. When a magnetic field is applied parallel to the hexagonal basal plane, the helicoidal spin order is suppressed and a collinear ordering, where the moments are forced to align in a direction perpendicular to the applied magnetic field, is stabilized. This result excludes a non-collinear spin order as the origin of the magnetically induced electric polarization in this compound. Instead our data imply that magnetic frustration results in a phase competition, which is the origin of the magneto-electric response.

cond-mat.str-el↗

arXiv2012

Dynamical lattice instability versus spin liquid state in a frustrated spin chain system

The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T<T*=100K. We interpret these effects as evidence for a dynamical interplay of spin and lattice degrees of freedom that might lead to a further transition into a dimerized or structurally distorted phase at lower temperatures.

cond-mat.str-el↗