Superconductivity in Ca<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>FBiSe<Subscript>2</Subscript>

Through the measurement of resistivity, magnetic susceptibility, and Hall effect, we discovered a novel BiSe₂-based superconductor Ca_0.5La_0.5FBiSe₂ with T _c of 3.9 K. A strong diamagnetic signal below T _c in susceptibility χ(T) is observed indicating the bulk superconductivity. The negative Hall coefficient throughout the whole temperature regime implies the dominant electron-type carriers in the sample. Different to most of BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, its resistivity in the present compound exhibits a metallic behavior down to T _c . Together with the enhanced T _c , the metallic character of the normal state implies that the electronic structure of Ca_0.5La_0.5FBiSe₂ may be different to those in the other BiS₂-based compounds.     

The H-T and P-T Phase Diagram of the Superconducting Phase in Pd:Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

We study the magnetic field vs. temperature (H – T) and pressure vs. temperature (P – T) phase diagrams of the T _c ≈ 5.5 K superconducting phase in Pd _x Bi₂Te₃ (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The H – T phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H _c (0) ≈ 6–10 kOe. The T _c is suppressed approximately linearly with pressure at a rate d T _c /d P ≈ ?0.28 K/GPa.     

Inducement of Itinerant Electron Transport in Charge-Ordered Pr<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> by Ba Doping

The effects of Ba ²⁺ doping on the electrical and magnetic properties of charge-ordered Pr_0.6Ca_0.4MnO₃ were investigated through electrical resistivity and AC susceptibility measurements. X-ray diffraction data analysis showed an increase in unit cell volume with increasing Ba ²⁺ content indicating the possibility of substituting Ba ²⁺ for the Ca-site. Electrical resistivity measurements showed insulating behavior and a resistivity anomaly at around 220 K. This anomaly is attributed to the existence of charge ordering transition temperature, \(T^{\mathrm {R}}_{\text {CO}}\) for the x = 0 sample. The Ba-substituted samples exhibited metallic to insulator transition (MI) behavior, with transition temperature, T _MI, increasing from ～98 K (x = 0.1) to ～122 K (x = 0.3). AC susceptibility measurements showed ferromagnetic to paramagnetic (FM-PM) transition for Ba-substituted samples with FM-PM transition temperature, T _c, increasing from ～121 K (x = 0.1) to ～170 K (x = 0.3), while for x = 0, an antiferromagnetic to paramagnetic transition behavior with transition temperature, T _N, ～170 K was observed. In addition, inverse susceptibility versus T plot showed a deviation from the Curie–Weiss behavior above T _c, indicating the existence of the Griffiths phase with deviation temperature, T _G, increasing from 160 K (x = 0.1) to 206 K (x = 0.3). Magnetoresistance, MR, behavior indicates intrinsic MR mechanism for x = 0.1 which changed to extrinsic MR for x > 0.2 as a result of Ba substitution. The weakening of charge ordering and inducement of ferromagnetic metallic (FMM) state as well as increase in both T _c and T _MI are suggested to be related to the increase of tolerance factor, τ, and increase of e _g ?electron bandwidth as average ionic radius at A-site, <r _A> increased with Ba substitution. The substitution may have reduced MnO₆ octahedral distortion and changed the Mn–O–Mn angle which, in turn, promotes itinerancy of charge carrier and enhanced double exchange mechanism. On the other hand, increase in A-site disorder, which is indicated by the increase in σ ² is suggested to be responsible for the widening of the difference between T _c and T _MI.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

High Pressure Studies on the Transport Properties and Upper Critical Field (<Emphasis Type="Italic">H</Emphasis><Subscript>c2</Subscript>) of Hole-Doped Pr<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>FeAsO Iron Pnictides

We report the pressure dependence of superconductivity for hole-doped polycrystalline Pr _0.8Sr _0.2FeAsO by magnetization and the electrical resistivity using cubic anvil. The superconducting transition temperature is initially increased from 16 K (P = 0) to 32 K (P = 6 GPa) with positive pressure coefficient and then it decreases on increasing the pressure further which is similar to an electrondoped systems. The upper critical field of the sample with T _c = 16 K was estimated to be ～56(1) T. Our results suggest that the hole-doped samples may have higher upper critical fields comparing to the electrondoped ones, due to the higher quasi particle density of states at the Fermi level.     

Crystal Growth and Magneto-transport of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals

In this letter, we report on the growth and characterization of bulk Bi ₂Se ₃ single crystals. The studied Bi ₂Se ₃ crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 ℃/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) Å. Temperature versus resistivity (ρ?T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ _{300 K}) of around 0.53 m Ω-cm and residual resistivity (ρ _{0 K}) of 0.12 m Ω-cm. Resistivity under magnetic field [ ρ(T)H] measurements exhibited large + ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ ρ _oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi ₂Se ₃ single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.     

Comparative Experimental and Density Functional Theory (<Emphasis Type="Italic">DFT</Emphasis>) Study of the Physical Properties of MgB<Subscript>2</Subscript> and AlB<Subscript>2</Subscript>

In the present study, we report an intercomparison of various physical and electronic properties of MgB₂ and AlB₂. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C _P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB₂ shrinks in c-direction for AlB₂ with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB₂. Superconductivity is not observed for AlB₂. Interestingly, the sign of S(T) is +ve for MgB₂ the same is ?ve for AlB₂. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB₂ to Debye–Einstein model and estimated the value of Debye temperature (Θ _D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E _F) is calculated. From the ratio of experimental N(E _F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB₂ in the strong coupling BCS category. The electronic specific heat of MgB₂ is also fitted below T _c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB₂ is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB₂, and hence it is not superconducting. DFT calculations demonstrated that for MgB₂, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB₂ is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB₂ is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB₂ and AlB₂.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Synthesis and Superconductivity of Layered Compounds with Orthogonal [Zr<Subscript>2</Subscript>N<Subscript>2</Subscript>] Network

Layered α-form ZrNX (X: Cl and Br) compounds with high quality were prepared by chemical vapor transport. The intercalation of alkali metal A (A: Li, Na, K, Rb) was carried out to realize electron doping into the orthogonal [Zr₂N₂] layers. The Rietveld refinement analysis reveals that the [Zr₂N₂] crystalline layers in the intercalation compounds shift mutually in the ab plane when compared with the hosts. Magnetic measurements show that the intercalation compounds A _x ZrNX are changed into superconductors with transition temperature T _c of up to 12 K. Upon the cointercalation of solvent molecules such as THF, T _c decreases to as low as 6.1 K with increasing the interlayer spacing d up to 14 Å, which is similar to the d dependence of T _c recently found in electron-doped α-form TiNX series. We also succeeded in synthesizing another new polymorph of α-Zr₂N₂S by the topochemical reaction between α-form ZrNX and Na₂S. α-Zr₂N₂S (space group: Immm, a = 4.1375(1) Å, b = 3.5422(1) Å, and c = 11.5204(3) Å) has the same α-[Zr₂N₂] layers, whereas the interlayer spacing between two adjacent [Zr₂N₂] layers is effectively decreased by 1/3 when compared with the parent compounds of ZrNX.     

Peculiarities of phase transitions in polytypes of monoclinic TlInS<Subscript>2</Subscript>

The existence of two polytypes at room temperatures, C-TlInS₂ and 2C-TlInS₂, with different monoclinic cell parameters, c and 2c, has been revealed. Significant differences in crystal lattice dynamics of these polytypes have been found. In particular, two phase transitions (PTs) have been detected for the polytype C-TlInS₂ as the temperature varies: a second-order PT from paraphase to incommensurate phase at T _i = 215 K and a first-order ferroelectric PT accompanied by a quadrupling of the parameter c at T _c = 197 K. No PT accompanied by an increase in unit cell parameter c has been found in the polytype 2C-TlInS₂, but a global temperature hysteresis characteristic of crystals with an incommensurately modulated structure has been detected at T = 180–230 K.     

Appearance of Superconductivity at 15.9 K in Layered V<Subscript>2</Subscript>AlN

A bulk polycrystalline sample with the nominal compositions represented by V₂AlN_1?δ was synthesized by a two-step solid-state reaction. The structural characterization has been done via X-ray diffraction, followed by Rietveld refinements, which revealed that the layered V₂AlN is crystalized in cubic Fm3m space group with lattice parameters a = b = c = 6.127 Å. Both DC resistivity and magnetization measurements confirmed that V₂AlN is a bulk superconductor with superconducting transition temperature (T _c ) of 15.9 K.     

The enhanced interface effect induced by thermal pressure in Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> ceramics

Polycrystalline ceramics Nd_0.7Sr_0.3MnO _y prepared by solid-state reaction were treated under high pressure of 9 GPa and temperature of 1000 K. The electrical transport behaviour of samples were investigated by a variable temperature system and a peculiar transport character was found at low temperature of 120 K, the I–V showed an obvious step-shape behaviour with the increase in the measurement voltages; at much lower temperature of 12.3 K, the I–V curves exhibited a notable symmetric hysteresis at a critical voltage of 4.5 V, although a linear I–V behaviour at 293 K. On the other hand, the R–T measurement revealed that the resistivity peak (resistivity at T _{M I} ) disappeared gradually and is replaced by a resistivity platform with the increase in the load currents, surprisingly, the resistivity platform broadened with continuous increase in the load currents but weakened when an external magnetic field was applied. All of these phenomena were not observed for the un-treated sample. The particle attenuated and the enhanced interface effect which resulted from the violent thermal-pressure treatment was responsible for the unique electromagnetic transport.     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Superconducting Properties of NdO<Subscript>0.5</Subscript><Emphasis Type="Bold">F</Emphasis><Subscript>0.5</Subscript>BiS<Subscript>2</Subscript> Single Crystals

We report on superconducting properties of high-quality single crystals of F-substituted NdOBiS₂ using low-temperature magnetization and transport measurements. Using the mixture of CsCl and KCl as the flux, we have synthesized our single crystals. This compound exhibits bulk superconductivity with a transition temperature of about T _c～4.6 K. The critical current density J _c as a function of temperature has been derived and decreases with the increasing temperature. We construct the phase diagram H _c2(T). The zero-temperature value for \(H_{\mathrm {c2}}^{B\parallel c}\) for value for \(T_{c}^{90~\%}\) and \(T_{c}^{0~\%}\) is estimated to be approximately 2.17 and 1.72 T respectively by using Werthamer-Helfand-Hohenberg model.     

Ho<Subscript>2</Subscript>S<Subscript>3</Subscript>-Ga<Subscript>2</Subscript>S<Subscript>3</Subscript> phase diagram

The Ho₂S₃-Ga₂S₃ system has been studied using differential thermal analysis, X-ray diffraction, microstructural analysis, microhardness tests, and density measurements, and its phase diagram has been constructed. The system contains three ternary compounds: Ho₃GaS₆, HoGaS₃, and Ho₆Ga_10/3S₁₄. Their melting behavior has been studied for the first time. The compound Ho₆Ga_10/3S₁₄ melts congruently at 1435 K; Ho₃GaS₆ and HoGaS₃ melt incongruently at 1370 and 1250 K, respectively. The Ho₂S₃-Ga₂S₃ system is a pseudobinary join of the ternary system Ho-Ga-S. At room temperature, the β-Ga₂S₃-based solid solution extends to 1.5 mol % Ho₂S₃; the Ho₂S₃ solubility in γ-Ga₂S₃ is 10 mol %. The compounds HoGaS₃ and Ho₃GaS₆ crystallize in orthorhombic symmetry (Ho₃GaS₆: a = 10.40 Å, b = 13.20 Å, c = 6.44 Å, Z = 4; HoGaS₃: a = 6.8 Å, b = 9.92 Å, a = 3.08 Å, Z = 4). Ho₆Ga_10/3S₁₄ has a hexagonal structure (a = 9.62 Å, c = 6.04 Å).     

Effects of Iron Contents on the Vortex State in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript>

We investigate the effects of iron content on the upper critical field (H _c2) and the activation energy U(T) in thermally activated flux flow in Fe _x Se_0.5Te_0.5 near the superconducting transition temperature T _c . The variations in H _c2(T) with temperature are analyzed using Ginzburg-Landau (GL), Werthamer-Helfand-Hohenberg (WHH) models along with the empirical relation (ER). The obtained values of H _c2(0) depend strongly on the model and the criteria used to determine the transition temperature. However, the general trend is that that H _c2(0) increases with the increasing Fe content. The activation energy U(T) is maximum for x =? 1 and rapidly suppressed by excess or deficiency of iron. The low values of U(T) (～10 meV) reflect the low vortex-pinning nature (due to defects, vacancies, etc.) in the Fe _x Se_0.5Te_0.5 superconductor.     

Phase relations in the Ag<Subscript>8</Subscript>SnS<Subscript>6</Subscript>-Ag<Subscript>2</Subscript>SnS<Subscript>3</Subscript>-AgBr system and crystal structure of Ag<Subscript>6</Subscript>SnS<Subscript>4</Subscript>Br<Subscript>2</Subscript>

The T-x phase diagram of the Ag-Sn-S-Br system has been studied in the composition region Ag₈SnS₆-Ag₂SnS₃-AgBr, and a compound of composition Ag₆SnS₄Br₂ has been identified. Ag₆SnS₄Br₂ has a new structure, closely related to that of Ag₆GeS₄Br₂: sp. gr. Pnma, a = 6.67050(10), b = 7.82095(9), c = 23.1404(3) Å, Z = 4, R _B = 0.0519, R _wp = 0.0782, χ² = 1.36.     

X-ray Photoelectron Spectroscopy,Magnetotransport and Magnetisation Study of Nb<Subscript>2</Subscript>PdS<Subscript>5</Subscript> Superconductor

In the present report, we investigate various properties of the Nb₂PdS₅ superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb₂PdS₅ while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity (\(T_{c}^{\text {onset}} =?7\) K and T cρ=?0 = 6.2 K) of the studied Nb₂PdS₅ material is quite robust against magnetic field with the upper critical field (H _c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb₂PdS₅ is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T _c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H _c1) is extracted from DC magnetisation measurements at various T below T _c. It is found that H _c1(T) of Nb₂PdS₅ material seemingly follows the multiband nature of superconductivity.     

The Effect of Substrate Temperature on Magnetic and Electrical Properties of (CeO<Subscript>2</Subscript>)<Subscript>14</Subscript>Fe<Subscript>86</Subscript> Films

(CeO₂)₁₄Fe₈₆ films were fabricated by a radio frequency magnetron sputtering method at different substrate temperature. The results reveal that the films deposited at substrate temperature lower than 773 K exhibit a strong perpendicular anisotropy, and the correlated dynamic permeability spectrum measured over the frequency range of 0.5–7 GHz shows a high resonance frequency. The study on the relation R～T shows that the resistivity of the thin film has a minimum near room temperature and tends to saturation as the temperature approaches zero, exhibiting a behavior reminiscent of Kondo scattering. However, as the substrate temperature increases to 973 K, the films possess an in-plane anisotropy and lower H _c. The resistivity exhibits a transition from metal to insulator characterized by a maximum of resistivity at 220 K.     

Crystal and molecular structure of uranyl acetylacetonate dimer, [UO<Subscript>2</Subscript>(C<Subscript>5</Subscript>H<Subscript>7</Subscript>O<Subscript>2</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>

The crystal and molecular structure of uranyl acetylacetonate dimer was determined by single crystal X-ray diffraction. The compound crystallizes in the tetragonal system, a = 7.9420(2), c = 40.1240(13) Å (at 100 K), Z = 4, space group P4₁2₁2. Dimeric uranyl acetylacetonate molecules in the crystal are formed by bridging bonding of one of O atoms of the acetylacetonate ligands with U atoms, so that the coordination polyhedra of U atoms (distorted pentagonal bipyramids) share a common equatorial edge. The dimer has a nonplanar structure, being significantly bent along the conventional line connecting the bridging O atoms.