Spectral Analysis of Markov Chains

The paper deals with the problem of a statistical analysis of Markov chains connected with the spectral density. We present the expressions for the function of spectral density. These expressions may be used to estimate the parameter of the Markov chain

Switching ferroelectricity in SnSe across diffusionless martensitic phase transition

We experimentally investigate transport properties of a hybrid structure, which consists of a thin single crystal SnSe flake on a top of 5~$\mu$m spaced Au leads. The structure initially is in highly-conductive state, while it can be switched to low-conductive one at high currents due to the Joule heating of the sample, which should be identified as $\alpha$-$Pnma$ -- $\beta$-$Cmcm$ diffusionless martensitic phase transition in SnSe. For highly-conductive state, there is significant hysteresis in $dI/dV(V)$ curves at low biases, so the sample conductance depends on the sign of the applied bias change. This hysteretic behavior reflects slow relaxation due to additional polarization current in the ferroelectric SnSe phase, which we confirm by direct measurement of time-dependent relaxation curves. In contrast, we observe no noticeable relaxation or low-bias hysteresis for the quenched $\beta$-$Cmcm$ low-conductive phase. Thus, ferroelectric behavior can be switched on or off in transport through hybrid SnSe structure by controllable $\alpha$-$Pnma$ -- $\beta$-$Cmcm$ phase transition. This result can also be important for nonvolatile memory development, e.g. phase change memory for neuromorphic computations or other applications in artificial intelligence and modern electronics

Gate-dependent non-linear Hall effect at room temperature in topological semimetal GeTe

We experimentally investigate non-linear Hall effect as zero-frequency and second-harmonic transverse voltage responses to ac electric current for topological semimetal GeTe. A thick single-crystal GeTe flake is placed on the Si/SiO$_2$ substrate, where the p-doped Si layer serves as a gate electrode. We confirm, that electron concentration is not gate-sensitive in thick GeTe flakes due to the gate field screening by bulk carriers. In contrast, by transverse voltage measurements, we demonstrate that the non-linear Hall effect shows pronounced dependence on the gate electric field at room temperature. Since the non-linear Hall effect is a direct consequence of a Berry curvature dipole in topological media, our observations indicate that Berry curvature can be controlled by the gate electric field. This experimental observation can be understood as a result of the known dependence of giant Rashba splitting on the external electric field in GeTe. For possible applications, the zero-frequency gate-controlled non-linear Hall effect can be used for the efficient broad-band rectification

Current-induced control of the polarization state in a polar metal based heterostructure SnSe/WTe2_2

The concept of a polar metal proposes new approach of current-induced polarization control for ferroelectrics. We fabricate SnSe/WTe$_2$ heterostructure to experimentally investigate charge transport between two ferroelectric van der Waals materials with different polarization directions. WTe$_2$ is a polar metal with out-of-plane ferroelectric polarization, while SnSe ferroelectric semiconductor is polarized in-plane, so one should expect complicated polarization structure at the SnSe/WTe$_2$ interface. We study $dI/dV(V)$ curves, which demonstrate sharp symmetric drop to zero $dI/dV$ differential conductance at some threshold bias voltages $\pm V_{th}$, which are nearly symmetric in respect to the bias sign. While the gate electric field is too small to noticeably affect the carrier concentration, the positive and negative threshold positions are sensitive to the gate voltage. Also, SnSe/WTe$_2$ heterostructure shows re-entrant transition to the low-conductive $dI/dV=0$ state for abrupt change of the bias voltage even below the threshold values. This behavior can not be observed for single SnSe or WTe$_2$ flakes, so we interpret it as a result of the SnSe/WTe$_2$ interface coupling. In this case, some threshold value of the electric field at the SnSe/WTe$_2$ interface is enough to drive 90$^\circ$ change of the initial SnSe in-plane polarization in the overlap region. The polarization mismatch leads to the significant interface resistance contribution, analogously to the scattering of the charge carriers on the domain walls. Thus, we demonstrate polarization state control by electron transport through the SnSe/WTe$_2$ interface

Genetic determinants of complicated pregnancy

The connections of polymorphic variants of the gene of vasoactive hormones with the level of arterial pressure in pregnant women, depending on the development of preeclampsia (PE), have been studie

Candidate genes and clinical-laboratory parameters in pregnant women with preeclampsia

They studied the associations of genetic polymorphisms with clinico-laboratory indicators among pregnant women with preeclampsia depending on hereditary complicatio

Surface ferromagnetism in a chiral topological semimetal CoSi

Despite the chiral topological semimetal CoSi is known as bulk diamagnetic, it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic ordering has been attributed to the distorted bonds, the superlattice of ordered vacancies, or even to topological surface textures due to the spin polarization in the neighboring Fermi arcs. We experimentally compare magnetization reversal curves for initially oxidized CoSi single crystals and cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi samples do not show sizable ferromagnetism, the fresh CoSi surface gives a strong ferromagnetic response, which is accompanied by the pronounced modulation of the angle dependence of magnetization, as it can be expected for easy and hard axes in a ferromagnet. In addition to the first order reversal curves analysis, this observation allows us to distinguish between different mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude that the surface states-induced RKKY interaction between distorted bonds near the sample surface is responsible for the strong ferromagnetic multi-domain behavior for freshly cleaved samples

Surface spin polarization in the magnetic response of GeTe Rashba ferroelectric

We experimentally investigate magnetization reversal curves for a GeTe topological semimetal. In addition to the known lattice diamagnetic response, we observe narrow magnetization loop in low fields, which should not be expected for non-magnetic GeTe. The hysteresis is unusual, so the saturation level is negative in positive fields, and the loop is passed clockwise, in contrast to standard ferromagnetic behavior. The experimental hysteresis curves can not be obtained from usual ferromagnetic ones by adding/subtracting of any linear dependence, or even by considering several interacting magnetic phases. The possibility of several phases is also eliminated by the remanence plots technique (Henkel or {\delta}M plots). We explain our results as a direct consequence of the correlation between ferroelectricity and spin-polarized surface states in GeTe, similarly to magnetoelectric structures

«New Normal» of Students’ Educational Practices in the Coronavirus Pandemic Reality

The article presents the results of qualitative sociologic survey aimed at analyzing students’ subjective evaluations related to “new” educational practices from the perspective of the experience gained in the pandemic period, as well as the dynamics of changes in the content of these evaluations during the year due to the transition to new learning formats. The purpose of the study is to assess the impact of forced online learning on the transformation of traditional educational practices. Following the results of work with qualitative data, the main trends observed in the informants’ answers are shown – increase of academic workload and, as a consequence, the time spent for its performance; absence of former attitude and concentration of attention on the learning material caused by the impossibility to draw boundaries between private and public, that in its turn had an impact on the quality of education and gave rise to new practices of academic dishonesty and cheating; and as a whole, downturn of psychological wellbeing. Main positive moments named by the informants were increase of the capabilities of the educational field and a certain level of comfort in the sphere of everyday life, logistics and work. It is noted that even with the outright victory over the COVID-19 it will be impossible to return completely to pre-covid variant of organization of educational process; whether we like it or not, electronic educational environments have already become an integral part of our life and higher education. The main questions that must be asked now – which of pre-covid and gained practices should be taken into the future, how to find the balance between intramural and online learning formats. Search for solutions depends to a large extent on the success of interaction and cooperation between the direct participants of educational process. Main method of data collection is semi-structured interviews; field works have been carried out in spring of 2020 and 2021. The interview method made it possible to study the situation in detail as it has developed. Based on the generalization of qualitative data, the substantial characteristics of the informants’ value judgments are formed, strong and weak sides of the learning process related to introduction of new formats of interaction “university – teacher – student”. The ambivalent attitude of students to full-time and distance learning was revealed. The scientific novelty of the study lies in the data acquisition, systematization and analytical generalization of the original material on the transformation of students’ educational practices as a result of a forced mass transition to distance education. The obtained materials provide grounds for clarifying the process and results of these changes and can be used to justify management decisions