Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

Electron transport,penetration depth,and the upper critical magnetic field in ZrB<Subscript>12</Subscript> and MgB<Subscript>2</Subscript>

We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field H_c2, for polycrystalline samples of dodecaboride ZrB₁₂ and diboride MgB₂. We conclude that ZrB₁₂ behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature T_R = 280 K (to be compared to T_R = 900 K for MgB₂). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB₁₂ at T_c = 6.0 K. Although a clear exponential λ(T) dependence in MgB₂ thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB₁₂ below T_c/2. These features indicate an s-wave pairing state in MgB₂, whereas a d-wave pairing state is possible in ZrB₁₂. In disagreement with conventional theories, we found a linear temperature dependence, of H_c2(T) for ZrB₁₂ (H_c2(0) = 0.15 T).     

Specific features of dielectric anomalies in Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te(Ga) near a ferroelectric phase transition

The dielectric properties of Pb_{1 ? x} Ge _x Te(Ga) (x = 0.02, 0.03, 0.05) were studied in the temperature range 77–150 K at frequencies of 10⁴–10⁶ Hz. It is revealed that the ferroelectric phase transition temperature T _c and the permittivity ? of Pb_{1 ? x} Ge _x Te(Ga) increase substantially with the Ge content. The temperature dependence of the permittivity of Pb_{1 ? x} Ge _x Te shows two peaks; the main peak is at the ferroelectric phase transition temperature T _c , and an additional peak is at T ₁ > T _c .     

Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB<Subscript>2</Subscript>

The critical magnetic fields H _c‖ and H _c2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The H _c‖/H _c2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence H _c‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below T _c . As the electron mean free path decreases under irradiation of the film with a low dose of He⁺ ions, the critical field H _c‖ remains unchanged near T _c but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB₂ films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field H _c‖(T).     

Electron localization during an electronic-topological transition in Mo-Re alloys

Oscillations in the superconducting transition temperature ΔT _c (P), in the critical magnetic field ΔH _c (P), in the thermopower α / T (T ²), and in electrical resistivity ρ(T) (P is pressure) of Mo_1?x -Re _x alloys are observed at low temperatures against the background of specific features related to an electronic-topological transition (ETT) in these alloys. The oscillations are sensitive to the impurity concentration: they increase when the Re impurity concentration is close to the critical concentration C _c at which the ETT occurs. Oscillations are also detected in the concentration dependences of the temperature coefficient of resistivity (?ρ / ?T (C)) and the thermopower derivative (?(α/T) / ?T ² (C)) of Mo_1?x -Re _x alloys at low temperatures. The former and latter oscillations are shown to correlate with each other. These specific features are assumed to result from the ETT and to be related to the localization of the part of the electrons that fill a new cavity in the Fermi surface during this transition.     

New superconductivity dome in LaFeAsO1−xFx far away from magnetism and accompanied by structural transition

We report on the discovery and novel physics of a new superconductivity dome in LaFeAsO_1?xF_x with high-doping rate (0.25 ≤x≤0.75) synthesized by using the high-pressure technique. The maximal critical temperature T_c = 30 K peaked at x_opt = 0.5 ～0.55, which is even higher than that at x≤ 0.2. By nuclear magnetic resonance (NMR), we find that the new superconducting dome is far away from a magnetically ordered phase without low-energy magnetic fluctuations. Instead, NMR and transmission electron microscopy measurements indicate that a C4 rotation symmetry-breaking structural transition takes place for x> 0.5 above T_c. The electrical resistivity shows a temperature-linear behavior around the doping level where the crystal transition temperature extrapolate to zero and T_c is the maximal, suggesting the importance of quantum fluctuations associated with the structural transition. Our results point to a new paradigm of high temperature superconductivity.     

Surface and volume phase states of Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>x</Subscript>BO<Subscript>3</Subscript> crystals in the vicinity of the Néel point

The behavior of the magnetic system of a surface layer of macroscopic Fe_1?xGa_xBO₃ crystal (x=0, 0.3) in the vicinity of the Néel temperature T_N was studied. The studies were made by a method involving simultaneous gamma, x-ray, and electron Mössbauer spectroscopy that made it possible to obtain information simultaneously from surface layers and from the bulk of a macroscopic crystal. It was found that the temperature T_N(L) at which a thin layer at a depth L from the surface switches to a disordered state is lower than T_N for the bulk and is lower the closer this layer is to the surface. In the vicinity of T_N, a nonuniform state is observed in which the bulk of the crystal is magnetically ordered and the surface layer is disordered. The transition temperature T_N(L) decreases from T_N to its surface value within a surface layer of a “critical” thickness.     

Crystallographic and dynamic characteristics of K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiOPO<Subscript>4</Subscript> solid solutions in the low-temperature ranges

The unit cell parameters a and c of K_{1 ? x} Rb _x TiOPO₄ (x= 0, 0.3, and 0.5) crystals are investigated using x-ray diffraction in the temperature range 90–320 K. The thermal expansion coefficients along the principal crystallographic axes of the crystals are determined from the obtained temperature dependences of the unit cell parameters. It is found that the parameter a increases with increasing temperature, while the parameter c decreases. For the crystals under investigation, the elastic moduli c ₁₁ and c ₃₃ along the [100] and [001] directions are determined by the ultrasonic pulse-echo technique in the temperature range 100–350 K. It is shown that c ₃₃ > c ₁₁. The anomalies revealed in the temperature dependences of the crystallographic and dynamic characteristics of the samples at a temperature T ≈ 280 K indicate the occurrence of a phase transition. The temperature of the phase transition is found to increase with increasing rubidium content x.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Observation of bosonic resonances in GdO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeAs by intrinsic multiple Andreev reflection effect spectroscopy

The current–voltage characteristics and dynamic conductance spectra of Andreev arrays in nearly optimal GdO_{1 ? x} F _x FeAs superconductors with the critical temperature T _c = 46–50 K have been studied. A reproducible fine structure that accompanies large gap features and caused by boson emission during the process of multiple Andreev reflections has been observed. At T = 4.2 K, the bosonic mode energy ε₀ = (12 ± 2) meV has been obtained.     

Heat capacity anomaly in the vicinity of the tricritical and upper critical points

The anomalous behavior of the isochoric heat capacity of a mixture of methane, pentane and heptane is studied experimentally in the vicinity of the liquid-vapor critical point in the cases when (a) the critical temperature T _c approaches the tricritical point T _TCP and (b) the critical temperature approaches the upper critical end point T _U . It is shown that in all cases, the singular part of the heat capacity of the mixture has the form C_sing=A¦τ¦^?α, where τ=(T ? T _c )/T _c and α≈0.11. When T _c →T _U , amplitude A of the heat capacity anomaly is found to be approximately constant. At the same time, the amplitude of the anomaly tends to zero in the vicinity of the tricritical point: A∝¦τ_c¦^ε, where τ_c=(T _c ? T _TCP )/T _TCP and ε=1.6?1.7. The inevitable vanishing of this mode of the heat capacity anomaly leads to a negative value of the critical index \(\tilde \alpha\) characterizing the heat capacity anomaly at the tricritical point, while the tricritical point theory and the isomorphism hypothesis predict \(\tilde \alpha = 0.5\).     

Switching of spontaneous electric polarization in the DyMnO<Subscript>3</Subscript> multiferroic

For the DyMnO₃ multiferroic with a modulated magnetic structure, switching of its spontaneous electric polarization (P ‖ c axis) near the ferroelectric transition (T < T _FE ～ 20 K) is revealed by measuring the dielectric hysteresis loops. It is found that the coercive field strongly increases as the temperature decreases (up to 2.6 kV/mm at 17.6 K). The values obtained for the spontaneous polarization are found to agree well with the data obtained from pyroelectric measurements. In addition, anomalies are observed in the temperature dependences of the spontaneous polarization P _c , dielectric constant ? _c , and magnetic susceptibility x _b at T ～ 6 K; these anomalies are attributed to the antiferromagnetic ordering of the Dy³⁺ ions.     

Anisotropy of microwave conductivity in the superconducting and normal states of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−<Emphasis Type="Italic">x</Emphasis>: 3D-2D crossover

The imaginary parts of microwave conductivity σ″(T<T_c) and resistivity ρ (_T)=1/σ(T>T_c) along (σ _ab ^″ and ρ_ab) and across and (σ _c ^″ and ρ_c the cuprate ab planes of a YBa₂Cu₃O_7?x crystal with the oxygen doping level x varying from 0.07 to 0.47 were measured in the temperature range 5≤ T≤200 K. In the superconducting state, the σ _ab ^″ (T)/σ _ab ^″ (0) and σ _c ^″ (T/σ _c ^″ (0) curves coincide for an optimally doped (x=0.07) crystal, but, with an increase in x, the slopes of the σ _c ^″ (T)/σ _c ^″ (0) curves decrease noticeably at T<T_c/3, on the background of small changes happening to the σ _ab ^″ (T/σ _ab ^″ (0) curves. The two-dimensional (2D) transport along the ab planes in the normal state of YBa₂Cu₃O_7?x is always metallic, but there is a crossover (at x=0.07) from the Drude to hopping (at x>0.07) conductivity along the c axis. This is confirmed both by the estimates of the lowest metallic and the highest tunneling conductivities along the c axis and by quantitative comparison of the measured ρ_c(T) curves with the curves calculated in the polaron model of quasiparticle transport along the c axis.     

Pseudogap and its temperature dependence in YBCO from the data of resistance measurements

The temperature dependence of the excess conductivity Δσ for Δσ = A(1 ? T/T^*)exp(Δ^*/T) (YBCO) epitaxial films is analyzed. The excess conductivity is determined from the difference between the normal resistance extrapolated to the low-temperature range and the measured resistance. It is demonstrated that the temperature dependence of the excess conductivity is adequately described by the relationship Δσ = A(1 ? T/T^*)exp(Δ^*/T). The pseudogap width and its temperature dependence are calculated under the assumption that the temperature behavior of the excess conductivity is associated with the formation of the pseudogap at temperatures well above the critical temperature T _c of superconductivity. The results obtained are compared with the available experimental and theoretical data. The crossover to fluctuation conductivity near the critical temperature T _c is discussed.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Effect of phase transitions on the exciton absorption spectrum of Rb<Subscript>x</Subscript>K<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript>4</Subscript>I<Subscript>5</Subscript> superionic conductors

The absorption spectrum of thin films of solid solutions of Rb_xK_1?xAg₄I₅ superionic conductors is investigated in the energy range 3–6 eV at temperatures from 90 to 290 K upon heating and cooling. It is established that the temperature dependences of the spectral position E _m and the half-width Γ of the long-wavelength exciton band are determined by the exciton-phonon interaction and the generation of Frenkel defects at phase transitions to the superionic state. Fluctuations of the composition of solid solutions do not affect the behavior of the dependences E _m (x) and Γ(x) at 90 K, which is indicative of the localization of excitons in the AgI sublattice of these compounds. A difference in the dependences T _c 1(x) and T _c 2(x) (the γ → β and β → α phase transitions, respectively) is revealed: the curve T _c 1(x) has a minimum at x ≈ 0.5, whereas the curve T _c 2(x) shows a weak maximum.     

Existence of two types of perfect Bi<Subscript>2</Subscript>Sr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>6+δ</Subscript> single crystals

It is found that perfect Bi₂Sr_2?x La _x CuO_6+δ single crystals with the same concentrations of lanthanum x = 0.64 and excess oxygen δ = 0.237 exist in two types. Single crystals of the first type are obtained by slow cooling (the synthesis time is 90–105 h). They have a monoclinic superlattice and exhibit no superconducting transition down to 2 K. Crystals of the second type are obtained by rapid cooling (the synthesis time is 30–40 h) and are characterized by a orthorhombic superlattice and T _c = 18 K. Thus, the superconducting transition temperature is determined not only by the concentration of carriers but also by the configuration of defects. A rhombic superlattice prevails in single crystals obtained by slow cooling in the lanthanum concentration range x = 0.3–0.5, while a monoclinic superlattice dominates in the range x = 0.75–0.85. This fact explains the high values of T _c at optimal doping (x = 0.4) and the absence of high-T _c superconductivity at p < 0.10.     

Symmetry and lattice parameters of SrTiO<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

The lattice parameters of ceramic samples of (1 ? x)SrTiO₃-xPbTiO₃ solid solutions are measured at room temperature. It is found that the samples have cubic symmetry in the concentration range x = 0?0.3 and tetragonal symmetry for x > 0.3. The lattice parameter a is virtually independent of temperature for x < 0.8 and slightly decreases in the range x = 0.8?1.0, while the lattice parameter c increases with increasing x. The reduced cubic parameter varies nonlinearly and deviates from Vegard’s linear law as the concentration x increases.     

Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

The recently discovered (Li_1-xFe_x)OHFeSe superconductor with T_c about 40 K provides a good platform for investigating the magnetization and electrical transport properties of FeSe-based superconductors. By using a hydrothermal ion-exchange method, we have successfully grown crystals of (Li_1-xFe_x)OHFeSe. X-ray diffraction on the sample shows the single crystalline PbO-type structure with the c-axis preferential orientation. Magnetic susceptibility and resistive measurements show an onset superconducting transition at around T_c=38.3 K. Using the magnetization hysteresis loops and Bean critical state model, a large critical current J_s is observed in low temperature region. The critical current density is suppressed exponentially with increasing magnetic field. Temperature dependencies of resistivity under various currents and fields are measured, revealing a robust superconducting current density and bulk superconductivity.