The structure and superconducting properties of the Fe 1-x (A) x (Se 0.2 Te 0.8 ) 0.82 (x = 0.1, 0.2) compound, where A is Cu, Zn, Bi or Group IV elements (Pb, Sn, Ge), were studied. It was shown that the effect of substitution of some iron atoms on the structure and superconducting properties of the Fe(Se,Te) compound depends on whether a solid solution with a substituting element was formed. If complete dissolution of the replacement element occured (Cu, Zn), then a significant degradation of the superconducting properties was observed. If a separate chalcogenide phase was formed (substitution by Pb, Sn, Ge), then a slight change in the superconducting properties was observed. Such a slight change and even improvement in the superconducting properties upon precipitation of the chalcogenide phase can be associated with both the positive effect of an increase in the relative content of iron and a decrease in the number of iron atoms in the interstitial sites of the β-phase. The preservation of the superconducting properties of the Fe(Se,Te) compound with a small substitution of iron for group IV elements can be used to create wires based on Fe(Se,Te), since impurity chalcogenide particles can act as effective pinning centers. Keywords: Iron-based superconductor, FeSeTe, critical parameters, substitution, structure, electrical resistivity, magnetoresistance, pressure effect.

In this work, we study the magnetization of two types of particles of a composite consisting of polystyrene and multilayer graphene chemically bonded to it, obtained by magnetic separation of the initial composite. For needleshaped particles, a ferromagnetic type of magnetization was observed for the first time, with values of saturation magnetization that are large for graphene. For flat, macroscopic particles of the composite, a superposition of several contributions to the magnetization was observed.

Bismuth ferrites have attracted much attention in recent years because of the possibility of their promising practical applications in sensing systems as well as digital memory. In this study, nanocrystalline material based on bismuth ferrite with mullite-like structure was synthesized by solution combustion technique. The results of XRD confirmed the formation of a Bi2Fe4O9 orthorhombic phase with an average crystallite size 60 +/- 3 nm. The composition, morphology and porosity were characterized by SEM/EDX and helium pycnometry. The peculiarities of the material formation from the initial mixture with a glycine deficiency by subsequent calcination of the combustion product in air, were characterized by XRD, EDX and simultaneous thermal analysis. Magnetic properties of the substance were studied by Mo center dot ssbauer spectroscopy and magnetometry which showed that its magnetization is enhanced in comparing to similar materials by several times. Specific features in the Bi2Fe4O9 magnetization with changing temperature were analyzed for the first time. The photocatalytic activity of Bi2Fe4O9 during the decay of rhodamine B has been studied. It is demonstrated that the degradation of rhodamine B increased by about 70% when irradiated with visible light.

Terbium orthoferrite (TbFeO3) perovskite demonstrates specific magnetic properties which are object of interest. Aggregated granular nanopowder of TbFeO3 particles of 85.6 nm were obtained by direct solution combustion method. The orthorhombic perovskite structure (space group Pbnm) of the terbium orthoferrite was confirmed by powder X-ray diffraction, Raman spectroscopy and 57Fe Mössbauer spectroscopy. The unusual granular morphology and mesoporous structure of the nanopowder were investigated by scanning electron microscopy and adsorption-structural analysis via low-temperature (77 K) adsorption-desorption of nitrogen with a total porosity of 0.0145 cm³/g and average pore width of 12 nm. The magnetic properties of TbFeO3 nanoparticles were thoughtfully studied, M = 34.2 emu/g at T = 10 K, Hc = 300 Oe. The onset of reorientation of the Fe3+ spin system at T 18 K and the antiferromagnetic ordering of Tb3+ ions at T 4 K were observed by vibrational magnetometry. Obtained results confirm the flexible paramagnetic to antiferromagnetic behavior of granular TbFeO3 nanopowder.

Thermomagnetic instabilities in the lead-porous glass (Pb-PG) nanocomposite in superconducting state were studied, and a model was proposed that relates magnetization jumps and heat release spikes with average critical current density. The samples were created by filling porous glass (characteristic pore size d = 7 nm) with lead from a melt under pressure, which created a 3D multiply connected system of lead filaments. The critical temperature of superconducting transition in Pb-PG T c = 7.2 K is the same as for bulk lead and the critical magnetic field H c (0) ≈ 40 kOe is much higher than H c bulk (0) ≈ 800 Oe for bulk lead. Hysteresis and magnetic flux jumps were observed in the magnetization curves m(H) of the Pb-PG nanocomposite, which are connected with the formation of magnetic flux gradients in a system of interconnected contours. Magnetic flux jumps are thermomagnetic processes accompanied by heat release which was directly observed by measuring adiabatic temperature change in the sample during magnetic field sweep. The flux jumps in the magnetization and heat release events were almost periodic in the magnetic field. The period dependences on the magnetic field are similar to the calculated average critical current density dependence j c (H) .

The transition from 'the initial graphite material, consisting of multilayer graphene of macroscopic size, to its oxidized form occurring as films, followed by their thermal reduction in an atmosphere of hydrogen, allows for obtaining submicron galleries of reduced graphene oxide with oxygen-containing groups on the surface. Such hydroxyl and carboxyl groups were used to functionalize the surface of graphene nanosheets with methacrylate groups to twist graphene layers relative to each other during in-situ copolymerization with styrene. In such a composite, mechanical stresses and defects are potential in the graphene nanosheets, which may be the reason for local superconductivity at room temperature. A similar effect was also recorded for photoreduced graphene oxide with a perforated surface as a component of a pol-ystyrene-based composite.

A simple and reliable method for determining the critical current of a HTSC tape is a non-contact method based on capturing the magnetic flux by a closed superconducting ring made of this tape and measuring the magnetic field in the center of the ring with a field sensor. This method also makes it possible to obtain the dependence of the critical current on the mag-netic field by applying a magnetic field locally on a small section of the tape. A local magnetic field can be created using strong permanent magnets, which also makes it possible to apply the field at different angles relative to the plane of the tape and to determine the anisotropy of critical currents. Using this method, experiments were carried out on rings made of a SuperOx HTSC tape at T = 77 K and in a magnetic field up to 4 kOe. The values obtained for the anisotropy of critical currents are in good agreement with the data given by the manufacturer.

Reflection spectra of quasi-two-dimensional organic conductors (EDT–TTF)4[Hg3I8]1–x, are considered, which, depending on the composition, demonstrate different phase transitions: x = 0.027—metal–superconductor with Tc = 8.1 K, x = 0—metal–insulator at a temperature of T < 35 K. The polarized reflection spectra were analyzed using the “phase phonons” model, which takes into account the electronic-vibrational interaction. Using this model, the optical functions of both compounds were adequately described and the energy parameters of the π-electron system were obtained. It is shown that for the compound with x = 0.027 these parameters change monotonically with temperature, while for x = 0 they show a jump at the phase transition temperature. A comparison of the model parameter Δ (the value of the periodic potential) for crystals with different stoichiometric coefficients x = 0 and 0.027 was carried out and an assumption was made about the dependence of the periodic potential on the average charge of one EDT molecule.

The magnetization M(H) of the superconducting niobium plate was measured for a series of angles α between the direction of the magnetic field and the plane of the sample. It was found that: 1) M(H) at α = 0° and α = 90° differ significantly; 2) the jumps in the magnetic flux, which are observed on M(H) at low temperatures, also differ significantly for α = 0° and α = 90°; 3) in a wide range of angles, the form of the dependences M(H) and the flux jumps, characteristic of α = 90°, are retained when the magnetic field deviates from the normal of the sample. The same is observed at a deviation from α = 0°, but in a smaller range of angles

In anisotropic layered films of a multicomponent n-Bi1.92In0.02Te2.85Se0.15 solid solution in strong magnetic fields from 2 to 14 T at low temperatures, quantum oscillations of the magnetoresistance associated with the surface states of electrons in three-dimensional topological insulators are studied. From analysis of the spectral distribution of the amplitudes of quantum magnetoresistance oscillations, the main parameters of the Dirac-fermion surface states are determined. The results are compared with data obtained by the method of scanning tunneling spectroscopy. It is shown that a high surface concentration determines the contribution of the Dirac-fermion surface states to the thermoelectric properties of n-Bi1.92In0.02Te2.85Se0.15 films.

In this work, we study the magnetization of two types of particles of a composite consisting of polystyrene and multilayer graphene chemically bonded to it, obtained by magnetic separation of the initial composite. For needle-shaped particles, a ferromagnetic type of magnetization was observed for the first time, with values of saturation magnetization that are large for graphene. For flat, macroscopic particles of the composite, a superposition of several contributions to the magnetization was observed.

The dependence of magnetization M on magnetic field H was measured for a thin superconducting niobium plate with strong magnetic flux pinning with the field oriented normally and parallel to the sample plane. At T < T c , dependences M ( H ) show a hysteretic behavior with a peak effect in large fields; the shape of the hysteresis loop depends on the field orientation. At temperatures below ~ T c /2 magnetic field jumps appear on dependences M ( H ), and their behavior also depends on the field orientation. When a good conductor is in the in a good thermal contact with the planar superconducting sample, the flux jumps are noticeably reduced in the perpendicular field orientation, while the flux jumps in the parallel orientation remain unchanged. The decrease in the magnitude of the flux jumps with a transverse orientation of the magnetic field occurs due to the electrodynamic braking of magnetic instability, which could lead to a magnetic flux jump.

Рассмотрены спектры отражения квазидвумерных органических проводников (EDT-TTF)4[Hg3I8]1-x, которые в зависимости от состава демонстрируют различные фазовые переходы: х=0.027 --- металл-сверхпроводник с Tc= 8.1 K, x=0 --- металл-диэлектрик при температуре T<35 K. Поляризованные спектры отражения были проанализированы с помощью модели "фазовых фононов", учитывающей электронно-колебательное взаимодействие. При использовании этой модели были адекватно описаны оптические функции обоих соединений и получены энергетические параметры π-электронной системы. Показано, что для соединения с x=0.027 эти параметры монотонно меняются с температурой, а для x=0 наблюдается их скачок при температуре фазового перехода. Проведено сравнение параметра модели Delta (периодический потенциал) для кристаллов с разным стехиометрическим коэффициентом x=0 и x=0.027, выдвинуто предположение о зависимости периодического потенциала от усредненного заряда одной молекулы EDT. Ключевые слова: органические проводники, EDT?TTF, слоистая структура, спектры отражения, модель "фазовых фононов", электронно-колебательное взаимодействие, оптическая проводимость.

On SmSb single crystals synthesized from elements, the electrical resistance R and magnetization M were measured as functions of temperature and magnetic field. A large magnetoresistance is observed over the entire temperature range of 2–300 K, increasing significantly with decreasing temperature. The temperature dependence of the magnetization exhibits a singularity at the transition temperature to the antiferromagnetic state TN=2.3 K. At temperatures below 8 K, de Haas-van Alphen oscillations are observed in the M(H) dependences, the frequencies of which do not change upon the transition through TN. Linear extrapolation of the dependence of the Landau indices N on the reciprocal of the magnetic field induction 1/B to zero gives the value N=0.75 at T=2 K, which indicates the presence of the Berry phase and indicates a nontrivial band topology in the SmSb compound.

On SmSb single crystals synthesized from elements, the electrical resistance R and magnetization M were measured as functions of temperature and magnetic field. A large magnetoresistance is observed over the entire temperature range of 2-300 K, increasing significantly with decreasing temperature. The temperature dependence of the magnetization exhibits a singularity at the transition temperature to the antiferromagnetic state TN=2.3 K. At temperatures below 8 K, de Haas--van Alphen oscillations are observed in the M(H) dependences, the frequencies of which do not change upon the transition through TN. Linear extrapolation of the dependence of the Landau indices N on the reciprocal of the magnetic field 1/B to zero gives the value NT=2 K=0.75, which indicates the presence of the Berry phase and a nontrivial band topology in the SmSb compound. Keywords: samarium monoantimonide, magnetization, magnetoresistance, antiferromagnetic phase transition, de Haas-van Alphen magnetic oscillations, Berry phase.

The magnetic characteristics of a nanocrystalline material based on bismuth ferrite Bi2Fe4O9 with 60±3 nm crystallites were studied. The obtained material demonstrates unusual magnetic properties, which are manifested in a significant increase in the value of magnetization in comparison with the data known from other works.

The magnetic characteristics of a nanocrystalline material based on bismuth ferrite Bi2Fe4O9 with 60±3 nm crystallites were studied. The obtained material demonstrates unusual magnetic properties, which are manifested in a significant increase in the value of magnetization in comparison with the data known from other works. Keywords: nanocrystals, bismuth ferrite, mullite, magnetic properties.

Interlayer surface defects and thermoelectric properties in layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators L. N. Lukyanova*, O. A. Usov, M. P. Volkov, V. A. Rusakov Ioffe Institute Russian academy of science, 194021 St.Petersburg, Russia *E-mail: lidia.lukyanova@mail.ioffe.ru Abstract In layered films of n-Bi2Te2.7Se0.15S0.15 topological insulators optimized for temperatures below room temperature, the morphology of the (0001) interlayer surface and thermoelectric properties were studied. On the profiles of the (0001) surface, we identified neutral impurity defects arising from the substitution of Se and S atoms for Te atoms and donor antisite defects of tellurium at bismuth sites, which affect the thermoelectric properties. The average value of thermoelectric figure of merit in n-Bi2Te2.7Se0.15S0.15 films increases to ≈ 3.0•10-3 K-1 in the range 80 – 215 K, while in bulk solid solution ≈ 2.0•10-3 K-1. An increase in the thermoelectric figure of merit in films is associated with an increase in the energy dependence of the relaxation time due to an increase in the effective scattering parameter reff. It is shown that in films the Seebeck coefficient, the density of states effective mass m/m0, and the material parameter proportional to the power factor increase, while the lattice κL and electronic thermal conductivity κe decrease, which determines the increase in thermoelectric figure of merit.

Измерены зависимости намагниченности M от магнитного поля H для сверхпроводящей тонкой ниобиевой пластины с сильным пиннингом магнитного потока при ориентации внешнего поля нормально и параллельно плоскости образца. При T<T c зависимости M(H) имеют гистерезисный характер с пик-эффектом в больших полях, форма петли гистерезиса зависит от ориентации магнитного поля. При температурах ниже ~ T c /2 на зависимостях M(H) появляются скачки магнитного потока, которые по-разному проявляются при различной ориентации магнитного поля. Размещение хорошо проводящего материала в тепловом контакте с плоским сверхпроводящим образцом заметно уменьшает скачки потока в поперечном магнитном поле, в то время как в продольном поле влияния на скачки потока не наблюдается. Уменьшение величины скачков потока при поперечной ориентации магнитного поля происходит из-за электродинамического торможения развития магнитной неустойчивости, приводящей к скачку магнитного потока. Ключевые слова: сверхпроводимость, намагниченность, скачки потока, ниобий, поверхностный барьер, электродинамическое торможение, ориентация магнитного поля.