Lattice Anomalies in the Oxygen Deficient NdFeAsO Superconductor

We compare the results of high-resolution synchrotron X-ray powder diffraction with low-temperature spectroscopic studies on the oxygen deficient NdFeAsO_0.85 superconductor. IR results indicate that a certain mode related to vibrations of the As ions softens at low temperatures. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around ∼180 K, as the anomaly of the IR mode, and disappear at the transition temperature. The data show smaller structural changes in the Nd–O charge reservoir layer, while in the superconducting layer the FeAs₄ tetrahedron shows gradual modifications below ∼180 K, which peak around 140 K and then suddenly disappear at T _c . The observed lattice anomalies are not connected with a structural phase transition or an AF ordering; they are more evident in the superconducting Fe–As planes, and disappear across T _c , pointing to a connection to superconductivity.     

Two Domains of the Parameter Space of the 2D Hubbard Model for High-Tc Superconductors

High-T _c superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ∼ −0.1 and t″ ∼ 0 versus t′ ∼ −0.3 and t″ ∼ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ∼ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.     

Single Crystal Growth and Effect of Doping on Structural, Transport and Magnetic Properties of A1?xKxFe2As2 (A = Ba, Sr)

We demonstrate the preparation of large, free-standing iron pnictide single crystals with a size up to 20×10×1 mm³ using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties have been investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at T _s ∼138 K in BaFe₂As₂ single crystals from self-flux shifts to T _s ∼85 K due to Sn solvent growth. We show evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature T _c ∼38.5 K for a single crystal of Ba_0.68K_0.32Fe₂As₂. A nearly 100% shielding fraction and the bulk nature of the superconductivity for the single crystal are confirmed by magnetic susceptibility data. A sharp transition T _c ∼25 K occurs for the single crystal of Sr_0.85K_0.15Fe₂As₂. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr_1−x K _x Fe₂As₂ single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.     

Competition between Superconductivity and Charge-density Wave Order in Na0.3CoO2⋅1.3H2O

Three samples of Na_0.3CoO₂⋅1.3H₂O that differed in sample age showed different ordering: after three and five days, superconductivity with T _c∼4.5 K but different superconducting condensates; after 40 days, no superconductivity but charge-density wave order at T _CDW∼7 K. A pair-breaking action that progresses with sample age and acts preferentially in one of two electron bands produces the changes in the superconducting condensate, and ultimately destroys the superconductivity. Theoretical calculations predicted the charge-density wave transition.     

Growth and Accurate Characterization of Bi2Sr2?xLaxCuO6+δ Single Crystals

High-quality and various doped Bi₂Sr_2−x La _x CuO_6+δ (x=0–0.90) single crystals were obtained by floating-zone method. Analysis of the thermal behavior indicated an incongruent melt for all the doped compounds. The segregation coefficient of La related to Sr was estimated to be ∼1.02. Chemical compositions including the La doping in the crystals were accurately determined to study the effect of doping on the structural, chemical and superconducting property of the compounds. Raman spectra were performed to show the high-frequency modes 627 cm⁻¹ softened with increasing the doping level of La. Implications of the doping effect on crystals for understanding the superconductivity are discussed.     

High-Temperature Superconductivity in Sulfur-Doped Amorphous Carbon Systems

Magnetization measurements performed on amorphous carbon (aC) powder that contained a small amount of sulfur revealed traces of inhomogeneous superconductivity (SC) at T _c=65 K. Thin films of granular aC-W composite obtained by Electron Beam Induced Deposition showed no sign of SC. However, SC at T _c≈40 K was induced upon treating this film with sulfur at 250 ^∘C for 24 hours. Although the superconducting volume fraction in both cases is very low, our results prove the necessity of sulfur for inducing SC in aC, and open new pathways to achieve high-temperature SC in the unique system of aC-based materials.     

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

High-T c superconductivity in thed-p electron system

The relaxation time with spin flipτ _s and the parametersξ, δ, χ of superconducting phase have been calculated on the basis of the kinematical mechanism of superconductivity in strongly correlated oxide models. An inter-relation between the superconducting gap Δ₀ and the specific heat jump Δ _c allowing the experimental verification was obtained and the Ginsburg-Landau equation derived.     

Pressure Effect on Superconductivity and Magnetism in α-FeSe x

In this paper, the pressure effect on superconductivity and magnetism has been investigated in FeSe _x (x=0.80,0.88). The magnetization curves display anomaly at T _s1∼106 K and T _s2∼78 K except for the superconducting diamagnetic transition around T _c ∼8 K. The magnetic anomaly at T _s1 and T _s2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises T _c , but also increases both T _s1 and T _s2.      

Occurrence of Superconductivity and Magnetism in Nominally Undoped LaOFeAs

Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO_1−x F _x FeAs) even under ambient conditions and oxygen deficiency (LaO_1−x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools–resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T _con∼17 K and a tail-like behavior when R goes to zero at T _c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼−5.5 and −1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At T<T _c0 the change in MMA with B shows a low field (B∼1 mT) peak, which vanishes at T>T _c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T _min∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T _con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature.     

Synthesis and Optical Studies of Superconducting MgB2 Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealed ex situ and rendered superconducting with T _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼10 min at 550°C. The optical absorption coefficient, α and photon energy, E followed a Tauc-type behavior, = _T (E - E_g )(\alpha E)^{1/2} = \beta _T (E - E_{\rm g} ). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameter β _T behaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Photoluminescence and thermal stability of yellow-emitting Sr-α-SiAlON:Eu<Superscript>2+</Superscript> phosphor

Novel α-SiAlON:Eu²⁺-based yellow oxynitride phosphors with the formula Sr_0.375−x Eu _x ²⁺Si_12−m−n Al _m+n O _n N_16−n (m = 0.75, n = x = 0.004–0.04) have been prepared by firing the powder mixture of SrSi₂, α-Si₃N₄, AlN, and Eu₂O₃ at 2,000 °C for 2 h under 1 MPa nitrogen atmosphere. The luminescence properties, the dependence of the activator concentration of Eu²⁺ and the thermal stability of Sr-α-SiAlON:Eu²⁺ phosphor have been investigated in comparison with Ca-α-SiAlON:Eu²⁺ phosphor. Similar to Ca-α-SiAlON:Eu²⁺ phosphor, Sr-α-SiAlON:Eu²⁺ phosphor has the excitation wavelength ranging from the ultraviolet region to 500 nm, and exhibit intense yellow light. The strongest luminescence was achieved at about x = 0.02 with the emission peak at 578 nm, slightly shorter than that of Ca-α-SiAlON:Eu²⁺ phosphor at 581 nm. Temperature-dependent emission intensity of Sr-α-SiAlON:Eu²⁺ phosphor is comparable to that of Ca-α-SiAlON:Eu²⁺ phosphor. The results suggest that the different position of the emission peak for Sr- and Ca-α-SiAlON:Eu²⁺ depends on the composition and the Stokes shift, and the thermal stability is nearly independent of Sr and Ca or fixed by the network of (Si, Al)–(O, N) in α-SiAlON at the same Eu²⁺ concentration.     

Electronic Energy Levels in High-Temperature Superconductors

Parent materials of high-temperature superconductors (HTSC) need to be doped to become superconducting. The optimum doping for maximum critical transition temperature T _c has been analyzed for more than 20 materials. Assuming a uniform doping distribution the distance x between doped unit cells—projected into the CuO₂ plane for cuprates—shows a strong linear correlation to the inverse of T _c in the form (2x)²=m ₁1/T _c with a slope of m ₁=(2.786±0.029)×10⁻¹⁵ m² K. The mercury cuprate homologous series HgBa₂Ca _n−1Cu _n O_2n+2+δ with n=1,2,3 has been used to demonstrate the procedure deriving the doping distance x from the optimum doping value δ.     

Quench-condensed indium-hydrogen films. II. Changes in electric and superconducting characteristics near the metal-insulator transition

Electrical and superconducting properties of indium films condensed in a H₂ atmosphere (pressurep _{H 2}=6×10⁻⁶ to 1.4×10⁻⁴ Torr) onto a substrate cooled with liquid helium are investigated. As hydrogen content is increased, a continuous increase in residual resistivity ρ* is observed, permitting systematic study of the resistance vs. temperature dependenceR(T) and the superconducting transition temperatureT _c on approaching the metal-insulator transition (MIT). With regard to ρ*, four regimes of conductivity can be observed: (1) conductivity with a positive temperature resistance coefficient (TRC), (2) conductivity with a small, constant, negative TRC, (3) conductivity under weak localization with ΔR (T) ∼ln T or type corrections, (4) hopping conductivity.T _c rises continuously with ρ* and reaches its peak (∼5.2K) in the second regime. A further increase of ρ* leads to a decrease ofT _c and complete suppression of superconductivity. The experimental dependenceR(T) is compared with theory. TheT _c variation on approaching the MIT and the relation between Mooij's rule and the superconducting properties are discussed.     

Thermodynamics of the Superconducting State in Calcium at 200 GPa

The thermodynamic parameters of the superconducting state in Calcium under the pressure at 200 GPa were calculated. The Coulomb pseudopotential values (μ ^⋆) from 0.1 to 0.3 were taken into consideration. It has been shown that the specific heat’s jump at the critical temperature and the thermodynamic critical field near zero Kelvin strongly decrease with μ ^⋆. The dimensionless ratios r ₁≡ΔC(T _C)/C ^N (T _C) and r₂ o T_CC^N(T_C)/H²_C(0)r_{2}\equiv T_{\mathrm{C}}C^{N}(T_{\mathrm{C}})/H^{2}_{\mathrm{C}}(0) significantly differ from the predictions based on the BCS model. In particular, r ₁ decreases from 2.64 to 1.97 with the Coulomb pseudopotential; whereas r ₂ increases from 0.140 to 0.157. The numerical results have been supplemented by the analytical approach.     

Probing the Superconducting Gap from Tunneling Conductance on NdFeAsO<Subscript>0.7</Subscript> with <Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript>=51 K

We report the tunneling spectroscopy of an iron-based oxypnictide NdFeAsO_0.7 with T _C=51 K, measured by a mechanical point contact technique. Mainly two kinds of tunneling spectra have been observed reproducibly. One is tunneling conductance displaying sharp superconducting gap peaks at 6.0±1.0 mV, in which hump (or kink) structures are also observed at 20–30 mV. Another is that showing dominantly the larger superconducting gap Δ _L with sharp conductance peaks at 14±1.0 meV, in which the trace of a smaller gap (Δ _S=5–7 meV) is simultaneously observed. Our results give direct evidence for the existence of multiple gaps in the quasiparticle excitation spectrum of this multiband system, although the origin of the hump at 20–30 mV is still unclear.     

Superfluidity of 4He Adsorbed on Nanoporous Activated Carbon Fibers

⁴He confined in nanoporous media is one of the most interesting nano-sized systems in the context of an interacting Bose system. In the present work, we study superfluidity of ⁴He adsorbed in nanoporous activated carbon fibers (ACFs). Up to a ⁴He coverage of n=22.6 μmol m⁻², no superfluidity is observed. Over 23.7 μmol m⁻², superfluid transition is observed at T _c ∼550 mK. With an increase in n the superfluid density enhances, but T _c is independent of n. These observations indicate that the thickness of the superfluid ⁴He films on the pore wall is restricted by a slit type pore shape of ACFs.     

On the generalized large set of Kasami sequences

In this paper, we present a new family of binary Kasami sequences of length 2 ⁿ − 1 for even integer n = 2m. It generalizes the recently proposed large set F^(l){\mathcal{F}^{(l)}} of Kasami sequences by relaxing the restriction on l to a more general case. The presented sequence family takes five nontrival correlation values −1, −1 ± 2 ^m , −1 ± 2 ^{m + gcd(m,l)}, and has family size 2^3m  + 2 ^m or 2^3m  + 2 ^m −1. We also completely determine the distribution of these correlation values.     

Neodymium-Doping Induced Superconductivity in 1111-SrFeAsF Iron-Pnictide System

Polycrystalline Sr_1−x Nd _x FeAsF samples at various Nd-doping levels have been prepared by conventional solid-state synthesis close to ambient pressure. Susceptibility and resistivity were studied down to 4 K showing superconductivity competing with a low-temperature magnetic ordering and probably the coexistence of both, well into the superconducting region. The temperature dependence of the resistivity of all the Nd-doped samples clearly show the presence of this transition below 15 K, most likely originating from the magnetic ordering of Nd moments. This transition is gradually attenuated but not completely suppressed even at optimum-doping region at x≈0.54. The spin-density wave anomaly at 175 K survives up to 0.35 Nd-doping, while superconductivity occurs only above x=0.40. The maximum onset transition temperature, T _c , reaches as high as 54.3 K for x=0.70, but superconductivity disappears beyond that. The estimated upper critical fields, H _c2, are high, with values exceeding 98 Tesla obtained from two samples of different doping level.     

Effect of temperature on W-shaped excess molar heat capacities and volumetric properties: Oxaalkane-nonane systems

Excess molar volumesV _m ^f of the mixtures diglyme (2,5,8-trioxanonane: TON), triglyme (2.5,8,11-tetraoxadodecane: TODD), or tetraglyme (2,5,8,11,14-pen-taoxapentadecane: POPD; E181)+n-nonane have been obtained from density measurements at 278.15, 288.15,298.15, and 308. 15 K. In addition, a micro DSC II differential scanning calorimeter was used to obtain excess molar heat capacitiesC _p ^E at constant pressure for the same mixtures except for TON+n-nonane and at the same temperatures except for 278.15 K. These results allowed us to calculate the following mixing quantities in the complete range of concentration: α(∂V _m ^υ /∂T) _p , and (∂H ^υ/∂p)nT at 298.15K. The excess molar volumes are positive with large maximum values located in the central concentration range with the exception of POPD +n-nonane at 278.15 K, which has a central miscibility gap. For these mixtures,C _p ^F has a W-shaped concentration dependence: two minima separated by a maximum.