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EuCsFe4As4 pnictide (1144 family) single crystals with a critical temperature of Tc ≈ 36.8 K and optimal superconducting properties in the stoichiometric composition are grown. Multiple-gap superconductivity determined by bulk order parameters $$\Delta _{{\text{L}}}^{{{\text{out}}}}$$, $$\Delta _{{\text{L}}}^{{{\text{in}}}}$$, and $${{\Delta }_{{\text{S}}}}$$ with characteristic ratios $$2\Delta _{{\text{L}}}^{{{\text{out}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 5.3$$, $$2\Delta _{{\text{L}}}^{{{\text{in}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 3.2$$, and $$2{{\Delta }_{{\text{S}}}}(0){\text{/}}{{k}_{{\text{B}}}}{{T}_{{\text{c}}}} \approx 1.3$$, respectively, is found for the first time below Tc using incoherent multiple Andreev reflection effect spectroscopy. It is shown that the gap structure of EuCsFe4As4 is similar to that of BaFe2 – xNixAs2 superconducting pnictides of the sister 122 family.

Cobalt pyroborate Co2B2O5 single crystals have been obtained by spontaneous crystallization from solution–melt. Powder X-ray diffraction measurements have revealed the triclinic symmetry $$P\overline 1 $$ with the lattice parameters $$a = 3.1666(7)$$ A, $$b = 6.1543(6)$$ A, $$c = 9.2785(12)$$ A, $$\alpha = 104.240(5)^\circ $$ , $$\beta = 90.841(14)^\circ $$ , $$\gamma = 92.064(16)^\circ $$ , and $$V = 175.10(5)$$ A3. Magnetic properties have been studied in the temperature range of 4.2–300 K and in magnetic fields up to 90 kOe by measuring the static magnetization and molar heat capacity. A transition to an antiferromagnetic state has been detected at TN = 45 K. A spin-flop transition occurs in the sample in strong magnetic fields.

We have studied the electrical-transport, thermophysical, and magnetic properties of the YRh4B4 and HoRh3.8Ru0.2B4 compounds. YRh4B4 has been shown to be a classic type II superconductor with Tc ~ 10 K, whereas the superconducting transition of HoRh3.8Ru0.2B4 (at Tc ~ 6 K) occurs along with its ferrimagnetic transition (Tm ~ 22–23 K). We have measured magnetic irreversibility loops for the YRh4B4 and HoRh3.8Ru0.2B4 compounds and calculated Jc(B) in the Bean model. The reduced pinning force Fp/Fp max as a function of hp = B/Bc2 for YRh4B4 has been shown to obey a “scaling” law, whereas the magnetic superconductor HoRh3.8Ru0.2B4 exhibits a significant deviation from the scaling law for hp > 0.2.

―A new anion-deficient Sr2.4Dy0.6Co2O7‒δ (δ = 0.33–1.1) perovskite phase with a structure of the A3B2O7 Raddlesden‒Popper homologous series has been obtained by the solid-state synthesis in the reducing/oxidizing atmosphere and its structural characterization has been performed by the Rietveld refinement of the X-ray powder diffraction data. It has been stablished that the Sr2.4Dy0.6Co2O7‒δ compound (sp. gr. I4/mmm) has parameters of a = b = 3.8526(1) and c = 19.9431(7) A in the reduced form (δ = 1.1) and a = b = 3.8086(1) and c = 19.9190(6) A in the oxidized form (δ ≈ 0.33) and oxygen vacancies occupy mainly the sites linking CoO5 polyhedra inside two perovskite layers. It has been established using differential scanning calorimetry and thermogravimetry that, at T

By the example of the Dy0.2Sr0.8CoO3-δ compound undergoing an order–disorder phase transition with increasing temperature, we demonstrate a significant dependence of the kinetic properties on the morphology of the internal spatially inhomogeneous structure, which forms in the sample depending on the rate of its transition from the high-temperature disordered cubic phase to the ground tetragonal ordered phase upon cooling. The results of transmission electron microscopy visualization of the spatially inhomogeneous state in the Dy0.2Sr0.8CoO3-δ ceramic samples are presented and compared with the X-ray diffraction data.

The thermoelectric properties of the NdCoO3 and SmCoO3 rare-earth cobalt oxides with a perovskite structure have been investigated in a wide temperature range. It is shown that, in the low-temperature region, the thermal conductivity of the compounds has a sharp maximum and the electrical conductivity of the samples increases with temperature, whereas the Seebeck coefficient behaves nonmonotonically with increasing temperature. The SmCoO3 oxide is characterized by the positive thermopower over the entire investigated range with a sharp growth in the low-temperature region, attaining the maximum value (S ≈ 1000 μV/K) near room temperature, and a further decrease. It has been established that, in the NdCoO3 oxide, the Seebeck coefficient changes its sign, which was rarely observed in the La-based compounds and is atypical of the undoped rare-earth cobalt oxides. The thermopower maximum obtained at a temperature of 450 K is 400 μV/K. The regions of the fastest growth of the thermoelectric power factor correspond to the anomalies caused by the spin transition of Со3+ ions and the dielectric–metal transition.

A structural reason for superconductivity in a Cu-, Sr-, or Nb-atom-doped Bi2Se3 topological insulator is still unclear. To understand this reason, a codoping approach has been developed and BaySrxBi2Se3 single crystals with different x and y values have been grown. The composition and structural and transport properties of the grown crystals have been studied. With X-ray diffraction data, it has been shown that barium and strontium intercalate the system, although barium is present in the structure in a very small amount. The addition of barium surprisingly destroys superconductivity, slightly changing the lattice constants, the strontium doping level of the crystal matrix, and the electron density. Thus, a key role of a certain coordination arrangement of positions of strontium atoms between Bi2Se3 quintuples for achieving superconductivity in this material has been demonstrated.

A method for calculating the critical state of inhomogeneous superconducting films using the Ginzburg–Landau (GL) theory is proposed. From the condition of minimizing the functional of the GL free energy, a system of equations describing the superconducting state of an inhomogeneous plate is derived. The dependences of the critical current of the plate on the external magnetic field are numerically calculated. It is shown that the critical current in a zero external magnetic field decreases with increasing degree of film inhomogeneity, while the critical magnetic field (at a zero transport current) increases. An analysis of the calculated dependences of the product of the critical current and the external magnetic field (an analogue of the volume pinning force) of superconducting films on the external magnetic field shows that the maximum of this value shifts to stronger fields with increasing degree of inhomogeneity, i.e., an increase in inhomogeneity leads to an increase in the pinning force.

The samples of (Mn$_{1-x}$Co$_x$)Si with $x=0.15$ and $x=0.17$ were grown and their physical properties: magnetization and magnetic susceptibility, resistivity and heat capacity were studied. The data analysis included also the previous results at $x=0.057, 0.063, 0.09$. The indicated doping MnSi with Co completely destroys the helical phase transition whereas basically saves the helical fluctuation area normally situated slightly above the phase transition temperature. This area spreading from $\sim$5 to 0 K is not changed much with doping and forms some sort of helical fluctuation cloud revealing the quantum critical properties: $C_p/T\rightarrow\infty$ at $T\rightarrow0$., Comment: 5 pages, 13 figures

A detailed study of the structural phase transition in ${\mathrm{Eu}}_{1\text{--}x}{\mathrm{La}}_{x}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ mixed crystals as a function of composition is reported. By analyzing a frequency shift of an electronic f-f transition in high-resolution optical spectra of ${\mathrm{Eu}}^{3+}$ ions, we detected a decrease in the phase transition temperature ${T}_{\mathrm{s}}$ from 87.05 to 12.2 K (upon cooling) and a simultaneous increase in thermal hysteresis $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}$ from 0.29 to 4.7 K with increasing $x$ from $x=0$ to $x=0.12$. A rectangular hysteresis loop was observed. The experimental ${T}_{\mathrm{s}}(x)$ and $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}(x)$ dependences are described within the developed analytical model utilizing linear decrease in ${T}_{\mathrm{s}}$ with $x$ and treating the increase in $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{s}}$ in terms of the impurity-related decrease in the interaction between some local order parameters. We argue that ${R}_{1\ensuremath{-}x}{R}_{x}^{\ensuremath{'}}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ solid solutions, where $R$ and $R$\ensuremath{'} are different rare-earth elements, can be used to implement optical storage devices and switches operating at any chosen temperature between 0 and 450 K. It is found that the changes in the composition and, correspondingly, structural phase transition parameters do not affect the magnetic phase transformation. ${\mathrm{Eu}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_{3}{(\mathrm{B}{\mathrm{O}}_{3})}_{4}$ demonstrates the structural phase transition at about 12 K, well below the N\'eel temperature ${T}_{\mathrm{N}}=32\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.

The vapor phase growth of Cd3As2—Zn3As2 (in the following (Cd1−x Znx)3As2 solid solutions process is described. The (Cd0,993 Zn0,007)3As2 solid solution single crystals were synthesized. Scanning electron microscopy and electron diffraction data suggest high crystalline quality of studied sample. Its structure and surface morphology, indicating the presence of growth nuclei and cleavage planes, were investigated. Giant anisotropic magnetoresistance and Shubnikov — de Haas oscillations were observed at low temperatures. Obtained results suggests that peculiarities of Dirac semimetal phase persist in (Cd1−x Znx)3As2 solid solution at low zinc content. At the same time, there are indications of some differences with initial Cd3As2 properties.

The effect of cationic and anionic orderings on the crystal structure and magnetic properties of substituted rare-earth cobaltites Gd0.1Sr0.9CoO3 – δ was studied using X-ray diffraction, measurement of XANES spectra, and magnetic and thermodynamic characteristics. The effects of ordering cause a decrease in symmetry to tetragonal and a distortion of the coordination octahedra of CoO6. Anomalous magnetic and thermodynamic quantities are observed at 260 and 360 K, respectively, for disordered and ordered samples. The XANES spectra measured at the CoK edge did not reveal a noticeable shift in the absorption edge compared with the spectrum of original GdCoO3. This suggests that the charge compensation process is associated not only with a change in the electronic state of cobalt ions, but also with the emergence of holes in the 2p states of oxygen.

The evolution of the Schottky anomalies in the temperature dependence of the specific heat (in the temperature range T = 2–10 K and the magnetic field range H = 0–9 T) of polycrystalline YBa2Cu3Oy HTSCs is studied when the oxygen content in coarse-crystalline samples changes and when the degree of structural disordering in optimally doped fine-crystalline samples of the same compound changes. The number of paramagnetic centers with uncompensated spin moments, which generate the Schottky anomalies, and the splitting of the lower energy levels are estimated. A correlation between these parameters and both the number of oxygen vacancies in chain planes and the method of oxygen ordering in chains is found. The experimental data are compared with the results of studying the magnetic susceptibility at T > Tc in the same samples. Based on this comparison, we can conclude that the nature of the Schottky centers is related to the formation of paramagnetic centers at breaks in the Cu1–O4 copper–oxygen chains in the basal planes of the crystal structure.

We propose a new method for calculating the dependence of the critical current on an external magnetic field, and the distribution of this current over the layers in superconducting multilayered structures. The method is based on a numerical solution of a system of nonlinear Ginzburg–Landau equations that describe the behavior of a superconducting plate carrying a transport current in a magnetic field, provided that it does not contain Abrikosov vortices. The way in which boundary conditions in the Ginzburg–Landau theory influence the critical state of superconducting layered structures is also considered. From a mathematical point of view, application of a general boundary condition to the system of Ginzburg–Landau equations leads to a change in the order parameter along the thickness of the thin superconducting plates. The physical nature of this phenomenon is explained by the proximity effect at the superconductor–normal metal (SN) interface, which leads to the suppression of the order parameter near the SN interface. The resulting calculated dependences of the plates’ critical current on the magnetic field strength applied parallel to the layers are used to determine the critical current of multilayer structures. It is assumed that the mutual influence of superconducting layers occurs only via the magnetic field that they create.We propose a new method for calculating the dependence of the critical current on an external magnetic field, and the distribution of this current over the layers in superconducting multilayered structures. The method is based on a numerical solution of a system of nonlinear Ginzburg–Landau equations that describe the behavior of a superconducting plate carrying a transport current in a magnetic field, provided that it does not contain Abrikosov vortices. The way in which boundary conditions in the Ginzburg–Landau theory influence the critical state of superconducting layered structures is also considered. From a mathematical point of view, application of a general boundary condition to the system of Ginzburg–Landau equations leads to a change in the order parameter along the thickness of the thin superconducting plates. The physical nature of this phenomenon is explained by the proximity effect at the superconductor–normal metal (SN) interface, which leads to the suppression of the order parameter near t...

We report a highly anisotropic magnetoresistance of (Cd0.993Zn0.007)3As2 single crystals, which have been synthesized by the vapor-phase growth. Scanning electron microscopy and electron diffraction data confirm the high crystalline quality of obtained samples. Studied samples exhibit specific features such as octahedral nuclei growth and step-like morphology of the surface formed by {112} planes. The giant anisotropic magnetoresistance and Shubnikov-de Haas oscillations have been observed at low temperatures. The results suggest the existence of the Dirac semimetal phase in (Cd1 −xZnx)3As2 solid solution with low zinc content. Thus, the observed magnetoresistance anisotropy is partially attributed to the chiral anomaly.

A method for calculating the critical current of an inhomogeneous superconducting film (plate) in different external magnetic fields has been proposed. The superconducting properties are changed by varying the coherence length and London penetration depth over the thickness. The coherence length is maximum at the center of the plate and decreases upon approaching its boundaries and the London depth, on the contrary, is minimum at the center of the plate and increases toward its boundaries. Using the proposed approach, the magnetic field dependences of the critical current on the external magnetic field for the cases of nonuniform and uniform distributions of the superconducting properties over the film thickness have been calculated and compared. It has been established that at the nonuniform distribution of the superconducting properties the critical current of the plate is higher than at the uniform distribution at the same external magnetic fields. This is due to the fact that the order parameter is redistributed over the inhomogeneous plate thickness in such a way that the superconducting state becomes more stable against the current and magnetic field. It has been shown that the vortex-free Meissner state in inhomogeneous films is maintained at higher fields than in homogeneous films. The greater the film nonuniformity, the higher the stability of the Meissner state against an external magnetic field.

We have grown and characterized three samples of Co-doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity, and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results it follows that impurities (Co and Fe) eliminate the first-order phase transition peaks and spread the fluctuation maxima in such a way that the low-temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low-temperature parts of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The $d\ensuremath{\rho}/dT$ data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio ${C}_{p}/T$ and hence to the diverging effective electron mass. We found that at a large concentration of the dopant there are no distinct phase transition points. What we observe is probably a region of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0 K.

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100–250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296–703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state., We are grateful to the Russian Foundation for Basic Research (Project No. 20-02-00559) and President Council on Grants (Project No. МК-2339.2020.2) for supporting this paper. This paper was carried out within the state assignment of the Russian Ministry of Science and High Education via program “Quantum”’ (No. AAAA-A18-118020190095-4). We also acknowledge support by Russian Ministry of Education and Science via Contract No. 02.A03.21.0006. We acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2017-83468-R) and from the regional Government of Aragon (E12-20R RASMIA project).

The magnetic phase diagram of Tm1–xYbxB12 antiferromagnets is determined from the specific heat measurements performed at low and ultralow temperatures (0.07–10 K). Precise experimental studies of the features of the crystal structure at room temperature suggest that a metal−insulator transition occurring in Tm1–xYbxB12 with the growth of x is accompanied by a significant (by a factor of 2–6) increase in the amplitude of atomic displacements in the boron and rare-earth sublattices. There is also a signature of an instability arising in the electron configuration of ytterbium ions at this transition. At room temperature, near the quantum critical point at xc ≈ 0.25, anomalies in the structural characteristics of dodecaborides under study are observed.

The specific heat of Ho0.01Lu0.99B12 single crystals with dynamic charge stripes is studied at low and ultralow temperatures (0.07–20 K) in applied magnetic fields of 0–9 Т. It is shown that additional contributions to the specific heat appear at magnetic fields exceeding 1 T. These contributions depend on the orientation of the magnetic field and can be as large as 15% of the specific heat related to the subsystem of holmium ions in the cubic crystal field.

The effect of substitution of Sr2+ ions for Gd3+ ions on the phase composition, electrical resistivity, thermoelectricity, and thermal conductivity of rare-earth cobalt oxides Gd1-xSrxCoO3-δ (x = 0.8 and 0.9) has been investigated. It has been determined that at the investigated strontium concentrations, the single-phase disordered nonstoichiometric cubic perovskites and superstructures with ordered Sr2+/Gd3+ ions and anion vacancies can be formed. The influence of ordering/disordering of Gd and Sr cations over crystal-lattice A- sites on the thermoelectric figure of merit and sample stability at high temperatures has been studied. The thermoelectric figure of merit of the disordered samples was found to exceed by far the analogous parameter of the ordered samples, which allows us to consider the disordering as a way of improving the thermoelectric parameters. Two contributions to the conductivities are discussed: high-temperature thermoactivation and low-temperatures variable range hopping. The parameters of the Mott electronic structure, including DOS N(eF), hopping energy e (the energy of hopping conductivity activation), and hopping length Rh, have been estimated

We present detailed study of specific heat of FeSe single crystals with critical temperature T c = 8.45 K at 0.4 − 200 K in magnetic fields 0 − 9 T. Analysis of the electronic specific heat at low temperatures shows the coexistence of isotropic s-wave gap and strongly anisotropic extended s-wave gap without nodes. It was found two possibilities of superconducting gap parameters which give equally description of experimental data: (i) two gaps with approximately equal amplitudes and weight contribution to specific heat: isotropic Δ 1 = 1.7 meV ( 2 Δ 1 / k B T c =4.7) and anisotropic gap with the amplitude Δ 2 max = 1.8 meV ( 2 Δ 2 max / k B T c =4.9 and anisotropy parameter m = 0.85 ); (ii) two gaps with substantially different values: isotropic large gap Δ 1 = 1.65 meV ( 2 Δ 1 / k B T c = 4.52 ) and anisotropic small gap Δ 2 max = 0.75 meV ( 2 Δ 2 max / k B T c = 2 ) with anisotropy parameter m = 0.71 . These results are confirmed by the field behavior of the residual electronic specific heat γ r .

Fe substitution effect on the magnetic behavior of Mn2−xFexBO4 (x = 0.3, 0.5, 0.7) warwickites has been investigated combining Mossbauer spectroscopy, dc magnetization, ac magnetic susceptibility, and heat capacity measurements. The Fe3+ ions distribution over two crystallographic nonequivalent sites is studied. The Fe introduction breaks a long-range antiferromagnetic order and leads to onset of spin-glass ground state. The antiferromagnetic short-range-order spin correlations persist up to temperatures well above TSG reflecting in increasing deviations from the Curie-Weiss law, the reduced effective magnetic moment and “missing” entropy. The results are interpreted in the terms of the progressive increase of the frustration effect and the formation of spin-correlated regions.

The XANES spectra (X-ray absorption near-edge spectra) at the K edge of Co and the L3 edge of Gd in polycrystalline Gd0.4Sr0.6CoO3 – δ rare-earth oxides with an ordered and disordered distribution of Gd3+ and Sr2+ cations over the A sites in the crystal lattice are measured. The results of XANES measurements do not reveal a noticeable shift in the absorption edge with increasing Sr concentration as compared to the GdCoO3 parent composition. The measured temperature dependences of the heat capacity of polycrystalline ordered and disordered samples and a single-crystal ordered Gd0.4Sr0.6CoO2.85 sample exhibit two Schottky anomalies. These anomalies are thought to be related to the high-spin state of the Co3+ ions in the pyramidal environment caused by oxygen deficiency and to the magnetic state of oxygen p electrons induced by the doping-assisted generation of a hole in the 2p state. The absence of a noticeable shift in the absorption edge and the presence of two Schottky anomalies support the fact that the charge state of cobalt remains unchanged in the compounds under study.

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS3 on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T0 ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T0. The negative magnetoresistance (at B ∥ c) below T0 is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.

The vortex pinning and liquid-glass transition have been studied in BaFe2–xNixAs2 single crystals with different doping levels (x = 0.065, 0.093, 0.1, 0.14, 0.18). We found that Ni-doped Ba-122 has rather narrow vortex-liquid state region. Our results show that the temperature dependence of the resistivity as well as I−V characteristics of Ni-doped Ba-122 is consistent with 3D vortex-glass model. It was found that -pinning gives the main contribution to overall pinning in 122 Ni-doped system. The vortex phase diagrams for different doping levels were built based on the obtained data of temperature of the vortex-glass transition Tg and the upper critical magnetic field Hc2.

Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected., This work has been financed by the Russian Foundation for Basic Research (project no. 20-02-00559). We acknowledge financial support from the Spanish MINECO DWARFS project MAT2017-83468-R and Gobierno de Aragón (Group, E12-20R).

We report the results of synthesizing the MgB2-based material in the layerwise combustion and thermal explosion modes. For the initial temperature of 500 °C, the combustion temperatures in the layerwise combustion and thermal explosion modes are identical. The sample surface after the synthesis is coated with a friable white coating, up to 10 µm thick, consisting of whisker-like MgO crystals 1.5 µm long and 200 nm in diameter. It is possible to dope MgB2 with aluminum and carbon atoms. Time-resolved X-ray diffraction studies demonstrate that the (Mg,Al)B2 phase emerges without formation of any intermediate compounds. The absence of Al demonstrates that it is contained in MgB2. Aluminum and carbon doping of MgB2 alters the lattice parameters, while its structural type remains unchanged. Doping of MgB2 with carbon black is found to be a more effective method than graphite doping. Superconducting properties of the synthesized samples were studied.

In this paper, we present a descriptive “electrotechnical” model for calculating the distribution of induction currents in a superconducting film near which a measuring coil inducing a local alternating low-frequency magnetic field is placed. The change in the coil inductance, caused by a superconductor with a set London penetration depth of a magnetic field is calculated within the model. The possibility of determining the penetration depth from experimental data is shown; the sensitivity of the proposed approach for films of various thicknesses and various problem configurations is considered.

The low-temperature (2 K ≤ T ≤ 10 K) specific heat of the series of fine-crystalline samples of YBa2Cu3O y high-Tc superconductor optimally doped with oxygen and having different degrees of nanoscale structural inhomogeneity has been studied at the applied magnetic field H = 8 T. The result are compared to those obtained for the equilibrium polycrystalline samples with different oxygen contents y. Information on the quasiparticle excitations in the magnetic field near d-wave nodes of the gap function is obtained. The changes introduced by the structural inhomogeneity to the nodal gap slope (νΔ) in the k-space, which is a key parameter of high-Tc superconductors, have also been studied. It is found that νΔ increases with the degree of structural disorder, but the superconducting transition temperature remains nearly the same (Tc = (91.5 ± 0.5) K). It is shown that this is possible if superconductivity is suppressed not only at the nodal point but also near it (owing to the structural disorder). In this case, the density of states in such parts of the Fermi surface is nonzero even at zero temperature, promoting the existence of the linear in temperature contribution to the specific heat (~γ(0)T), which is characteristic of metals.

The results of the study of magnetization and dynamic magnetic susceptibility are correlated with changes in the microstructure of superconducting intermetallic Nb3Sn plates irradiated at the Kurchatov Institute cyclotron with fast protons with an energy of 12.8 MeV to a fluence of 1 · 1018 cm−2. The dependence of the superconducting transition temperature on the total irradiation dose is determined. For one of the samples, the dynamic magnetic susceptibility exhibits several steps corresponding to superconducting transitions at different temperatures. It is assumed that a Nb-enriched phase is separated in the region of maximum radiation damages. A microstructural analysis shows the appearance of randomly oriented Nb-enriched regions from 0.1 to 0.5 μm in size.

A comparative study of the low-temperature specific heat for two types of YBa2Cu3O y high-T c superconductor samples is performed within the temperature range of 2−10 K. The samples of the first type are fine-crystalline optimally doped ones with different degrees of nanoscale structural inhomogeneity. The second type includes coarse-crystalline equilibrium samples with different hole doping levels. A similarity in the behavior of different contributions to the specific heat for structurally inhomogeneous and underdoped samples is revealed. The samples of both types exhibit a metal-like contribution linear in temperature to the specific heat ~γT, which is not characteristic of the superconducting phase. It is found that this contribution moderately grows with the decrease in the oxygen content, whereas with the increase in the structural inhomogeneity, such growth of the linear contribution (γT) becomes anomalously large. This leads to the conclusion about the coexistence of metallic and superconducting states in the bulk of the samples under study. Such common feature of electron systems could be related to the formation of the pseudogap regime. It is demonstrated that this regime suppresses just the superconducting states, leaving intact the metallic ones.

We report results of studying the magnetization, specific heat, and thermal expansion of a single crystal with nominal composition $({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Si}$ with $x=0.15$. We found no thermodynamic evidence in favor of a second-order phase transition in this material. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e., approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.

The model strongly correlated electron system ${\mathrm{Ho}}_{0.8}{\mathrm{Lu}}_{0.2}{\mathrm{B}}_{12}$, which demonstrates a cooperative Jahn-Teller instability of the boron sublattice in combination with rattling modes of Ho(Lu) ions, dynamic charge stripes, and an unusual antiferromagnetic (AF) ground state, has been studied in detail at low temperatures by magnetoresistance (\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}), magnetization, and heat capacity measurements. Based on received results it turns out that the angular H-\ensuremath{\varphi}-T magnetic phase diagrams of this nonequilibrium AF metal can be reconstructed in the form of a ``Maltese cross''. The dramatic AF ground state symmetry lowering of this dodecaboride with fcc crystal structure can be attributed to the redistribution of conduction electrons. These leave the Ruderman-Kittel-Kasuya-Yosida oscillations of the electron spin density to participate in the dynamic charge stripes providing extraordinary changes in the indirect exchange interaction between magnetic moments of $\mathrm{H}{\mathrm{o}}^{3+}$ ions and resulting in the emergence of a number of various magnetic phases. It is also shown that the two main contributions to magnetoresistance in the complex AF phase, the (i) positive linear on magnetic field and the (ii) negative quadratic $\ensuremath{-}\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}/\ensuremath{\rho}\ensuremath{\sim}{H}^{2}$ component can be separated and analyzed quantitatively, correspondingly, in terms of charge carrier scattering on the spin density wave ($5d$) component of the magnetic structure and on local $4f\ensuremath{-}5d$ spin fluctuations of holmium sites.

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio $2\Delta/T_c = 3.48 - 3.73$ is very close to the BCS value in the weak coupling limit., Comment: 12 pages, 15 figures, 1 table

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 $2\Delta_L/k_BT_C > 4.1$ evidences for a strong coupling in the $\Delta_L$-bands., Comment: 6 pages, 4 figures. To be published in Proceedings of Superstripes 2016 Conference

Magnetic and thermodynamic properties of perovskite Gd0.2Sr0.8CoO3−δ, with ordered and disordered states of Gd and Sr in the A-sites of the crystal lattice have been studied revealing remarkable differences in the physical properties of the ordered and disordered states. The ordered samples have larger oxygen nonstoichiometry, heat capacity, and thermal expansion anomalies, and abnormal temperature dependence of the magnetization around 350 K.

High-precision measurements of the Hall effect and Raman scattering have been performed for single crystals of ZrB12 superconductor in the wide temperature range of 5–300 K. For ZrB12, the boson peak with ωmax ~ 100 cm–1 has been observed for the first time within the low-frequency range of the Raman spectrum I(ω). The sizes of vibrational clusters with the correlation length ranging from 25 to 35 A are estimated. The relation between the renormalization of the low-frequency density of vibrational states accompanying the transition to the cage-glass phase (T* ~ 90 K) and the enhancement of superconductivity in ZrB12 is discussed.

It is proposed to use the HTSC quantum levitation phenomenon in magnetic fields of various configurations to develop the systems of contact-free positioning and transport of cryogenic fuel targets (CFTs) to the focus of a high-power laser installation or the IFE reactor. The results are presented of a large cycle of experimental studies using YBa2Cu3O7−x superconducting ceramics and permanent magnet guideways based on various combinations of permanentmagnets to develop “CFT-MAGLEV” delivery systems.

Static magnetic susceptibility χ(T) in the normal state (Tc ≤ T ≤ 400 K) and specific heat C(T) near temperature Tc of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa2Cu3Oy superconductor, having y and Tc close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.

The influence of 200 keV He+ ion irradiation on superconducting and magnetotransport properties of Ba(Fe0.94Co0.06As)2 films has been studied. It was shown that in the film under the corresponding irradiation conditions, mainly nonmagnetic defects are generated. It was found that suppression of superconductivity at increasing concentration of nonmagnetic defects is noticeably slower than expected from a simple theory assuming s ± symmetry of the superconducting order parameter. The influence of defects on the magnetotransport properties has been analyzed. It was shown that the results of Hall measurements can be explained on the assumption that carriers are localized in the vicinity of radiation defect. The conclusion was made that the complete suppression of superconductivity occurs at a critical disorder in the system, which implies s ++ symmetry of the order parameter. The results are explained basing on the assumption about electron pairing in real space on definite centers of pairing.

Tensometric study of n-type Bi2Se3 single crystals in dc magnetic fields to 6 T in a temperature range of 7–23 K detected a weak negative thermal expansion (NTE) in the basal plane. The NTE increases with the field strength and depends on its orientation with respect to the trigonal c axis. In a magnetic field of 6 T, parallel to the c axis, the linear NTE coefficient reaches −7 · 10−7 K−1, and a minimum sample length is reached at a temperature of 13 K, where a Hall carrier concentration maximum is also detected. The found magnetoelastic anomaly can be associated with the topological insulator state.

We report experimental study of the electronic band structure of SnAs superconductor with the NaCl type lattice structure by angular resolved photoelectron spectroscopy (ARPES). The determined band structure, in general, is in a good agreement with the calculated one. However, at odd with the calculated band structure, the experimental data reveals splitting of one of the upper valence bands into three branches along the $$\bar K - \bar \Gamma - \bar K$$ and $$\bar M - \bar \Gamma - \bar M'$$ symmetry directions. We assume this splitting can be caused by the spin orbit coupling of electrons or a mixed valence of Sn atoms in the compound.

Vibrating-sample magnetometry techniques has been used for investigation of the electrophysical characteristics of several types of modern commercial multistrand composite wires based on Nb3Sn superconducting compound at temperatures that were measured 4.2, 7, and 12 K in magnetic fields up to 8 T before and after irradiation of the samples by fast (32-MeV) protons to fluences values 3 × 1016, 1 × 1017, 3 × 1017, and 1 × 1018 cm–2. All samples irradiated to fluences values up to 1 × 1017 cm–2 exhibited growth of the critical current density, which was most sharply pronounced in strong magnetic fields at lower temperatures. Further irradiation to a fluence value of 1 × 1018 cm–2 led to decrease of the critical current density below the initial level. The critical temperature of samples monotonically decreased with increasing irradiation dose.

Single-phase samples of the layered perovskite-like cobalt oxides Ln0.2Sr0.8CoO3-δ (Ln = Sm, Gd, Dy) with the same oxygen nonstoichiometry index δ = 0.37 ± 0.01 were synthesized. All samples are characterized by a tetragonal unit cell with the space group I4/mmm. The structural, magnetic, electric transport and dilatation properties of the obtained samples are investigated. The studied samples are characterized by two anomalies in magnetic properties, a high-temperature maximum near Тm = 350 К with magnetic field hysteresis below Tm, and a diffuse peak in the intermediate temperature range, which shifts with ionic radius decrease of the rare-earth element to higher temperatures. The high-temperature maxima of the magnetic susceptibility correlate with anomalies in thermal expansion, heat capacity and the features in the temperature dependences of the electrical resistivity, pointing to a strong relationship between the structural, magnetic and electronic degrees of freedom. The given comparative analysis of the experimental data of various substituting rare-earth elements with the same oxygen nonstoichiometry has been carried out for the first time.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers 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.