Zahlenwerte und Funktionen aus Naturwissenschaften und Technik Von Landolt-Bornstein. Neue Serie. Gesamtherausgabe: K. H. Hellwege. Gruppe II: Atom- und Molekularphysik. Band 1: Magnetische Eigenschaften freier Radikale. Von H. Fischer. Herausgeber: K. H. Hellwege und A. M. Hellwege. Pp. x + 154. (Berlin: Springer-Verlag, 1965.) 68 D.M.

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Electron Spin Resonance

We report synthesis and superconductivity at 3.7 K in PrO0.5F0.5BiS2. The newly discovered material belongs to the layered sulfide based REO0.5F0.5BiS2 compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ∘C in a single step. Detailed structural analysis has shown that the as synthesized PrO0.5F0.5BiS2 is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) A, c=13.362(4) A. Bulk superconductivity is observed in PrO0.5F0.5BiS2 below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (Tc) onset at 3.7 K and Tc(ρ=0) at 3.1 K. The hump at Tc related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ∼6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (Hc1) of around 8 Oe.

Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We study the pairing symmetry of a two-orbital J1-J2 model for FeAs layers in oxypnictides. We show that the mixture of an intraorbital unconventional s_{x;{2}y;{2}} approximately cos(k_{x})cos(k_{y}) pairing symmetry, which changes sign between the electron and hole Fermi surfaces, and a very small d_{x;{2}-y;{2}} approximately cos(k_{x})-cos(k_{y}) component is favored in a large part of the J1-J2 phase diagram. A pure s_{x;{2}y;{2}} pairing state is favored for J2>J1. The signs of the d_{x;{2}-y;{2}} order parameters in the two different orbitals are opposite. While a small d_{xy} approximately sin(k_{x})sin(k_{y}) interorbital pairing coexists in the above phases, the intraorbital d_{xy} pairing is not favored even for large J2.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-001927.pdf

Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides

A very recent report on the observation of superconductivity in Bi(4)O(4)S(3) [Mizuguchi, Y.; http://arxiv.org/abs/1207.3145] could potentially reignite the search for superconductivity in a broad range of layered sulfides. We report here the synthesis of Bi(4)O(4)S(3) at 500 °C by a vacuum encapsulation technique and its basic characterizations. The as-synthesized Bi(4)O(4)S(3) was contaminated with small amounts of Bi(2)S(3) and Bi impurities. The majority phase was found to be tetragonal (space group I4/mmm) with lattice parameters a = 3.9697(2) A and c = 41.3520(1) A. Both AC and DC magnetization measurements confirmed that Bi(4)O(4)S(3) is a bulk superconductor with a superconducting transition temperature (T(c)) of 4.4 K. Isothermal magnetization (M-H) measurements indicated closed loops with clear signatures of flux pinning and irreversible behavior. The lower critical field (H(c1)) at 2 K for the new superconductor was found to be ~15 Oe. Magnetotransport measurements showed a broadening of the resistivity (ρ) and a decrease in T(c) (ρ = 0) with increasing magnetic field. The extrapolated upper critical field H(c2)(0) was ~31 kOe with a corresponding Ginzburg-Landau coherence length of ~100 A . In the normal state, the ρ ~ T(2) dependence was not indicated. Hall resistivity data showed a nonlinear magnetic field dependence. Our magnetization and electrical transport measurements substantiate the appearance of bulk superconductivity in as-synthesized Bi(4)O(4)S(3). On the other hand, Bi heat-treated at the same temperature is not superconducting, thus excluding the possibility of impurity-driven superconductivity in the newly discovered superconductor Bi(4)O(4)S(3).

Bulk superconductivity in bismuth oxysulfide Bi4O4S3.

We report a strategy to induce superconductivity in the BiS$_2$-based compound LaOBiS$_2$ Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$ for trivalent La$^{+3}$ It is found that both the LaOBiS$_2$ and ThOBiS$_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with \emph{T$_c$} values of up to 285 K The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr$^{+2}$ for La$^{+3}$ does not induce superconductivity

Superconductivity induced by electron doping in the system La1-xMxOBiS2 (M = Ti, Zr, Hf, Th)

We report the electrical, magneto transport and specific heat of the layered polycrystalline RECoPO (RE = La, Nd and Sm) samples. These compounds are iso-structural to recently discovered superconductor LaFeAs(O/F). Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Cobalt in these compounds is in itinerant state with its paramagnetic moment above 1.4\muB and the same orders ferromagnetically (FM) with saturation moment of around 0.20\muB below say 80K. Though, LaCoPO shows single paramagnetic (PM) to ferromagnetic (FM) transition near 35K, the NdCoPO and SmCoPO exhibit successive PM-FM-AFM transitions. Both FM and AFM transition temperatures vary with applied field. Although the itinerant ferromagnetism occurs with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 69K and 14K for Nd and 57K and 45K for Sm. This FM-AFM transition of Co spins in NdCoPO and SmCoPO is both field and temperature dependent. The Magneto-transport of NdCoPO and SmCoPO distinctly follows their successive PM-FM-AFM transitions. It is clear that Sm/Nd (4f) interacts with the Co (3d) in first time synthesized Sm/NdCoPO.

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Synthesis, magnetization and magneto transport study of RECoPO (RE = La, Nd & Sm)

We report an easy-route single-step synthesis of BiOCuS with and without Cu deficiency. The title compound is synthesized via solid-state reaction route by encapsulation in an evacuated (10−3 Torr) quartz tube. Mixed components of the ingredients in stoichiometric ratio (
 $\frac{1}{3}$
 Bi2O3 + 0.34Bi + Cu1−x
 + S) are pelletized, sealed in evacuated quartz tube and heat-treated for 30 hours at 500 °C. Finally, the sample is allowed to cool down to room temperature. The resultant compound is black in color and could not hold in pellet form, but is powdered. X-ray diffraction Reitveld analysis is carried out on all three samples of a series BiOCu1−x
 S with x=0.0, 0.10 and 0.15. These samples crystallize in single phase with space group P4/nmm and with cell parameters a=3.868 A and c=8.557 A for stoichiometric BiOCuS. The volume of the cell slightly increases with an increase in Cu deficiency. The coordinate positions are determined by fitting the observed XRD patterns of the studied samples.

Synthesis and Structural Details of BiOCu1−xS: Possible New Entrant in a Series of Exotic Superconductors?

We report high field magneto transport of Sm/PrFeAsO. Below spin density wave transition (TSDW), the magneto-resistance (MR) of Sm/PrFeAsO is positive and increasing with decreasing temperature. The MR of SmFeAsO, is found 16%, whereas the same is 21.5% in case of PrFeAsO, at 2.5 K under applied magnetic field of 14 Tesla (T). In case of SmFeAsO, the variation of isothermal MR with field below 20 K is nonlinear at lower magnetic fields (< 2 Tesla) and the same is linear at moderately higher magnetic fields (H \geq 3.5 T). On the other hand PrFeAsO shows almost linear MR at all temperatures below 20 K. The anomalous behavior of MR being exhibited in PrFeAsO is originated from Dirac cone states. The stronger interplay of Fe and Pr ordered moments is responsible for this distinct behavior. PrFeAsO also shows a hump in resistivity (R-T) with possible conduction band (FeAs) mediated ordering of Pr moments at around 12 K. However the same is absent in SmFeAsO even down to 2 K. Our results of high field magneto-transport of up to 14 Tesla brings about clear distinction between ground states of SmFeAsO and PrFeAsO.

High Field (14Tesla) Magneto Transport of Sm/PrFeAsO

We report the electrical, magnetotransport, and specific heat of the layered polycrystalline RECoPO (RE = La, Nd, and Sm) samples. These compounds are iso-structural to recently discovered superconductor LaFeAs(O/F). Bulk polycrystalline samples are synthesized by a solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. Cobalt in these compounds is in an itinerant state, with its paramagnetic moment above 1.4 μB and the same orders ferromagnetically (FM) with saturation moment of around 0.20 μB below, say, 80 K. Though, LaCoPO shows single paramagnetic (PM) to ferromagnetic (FM) transition near 35 K, the NdCoPO and SmCoPO exhibit successive PM-FM-anti-ferromagnetic (AFM) transitions. Both FM and AFM transition temperatures vary with applied fields. Although the itinerant ferromagnetism occurs with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 60 K and 14 K for Nd and 70 K and 40 K for Sm. This FM-AFM transition of Co spins in NdCoPO and SmCoPO is both field and temperature dependent. The magneto-transport of NdCoPO and SmCoPO distinctly follows their successive PM-FM-AFM transitions. It is clear that Sm/Nd (4f) interacts with the Co (3d) in first-time-synthesized Sm/NdCoPO.

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Complex magnetism and magneto-transport of RECoPO (RE = La, Nd, and Sm)

The bulk polycrystalline sample FeSe1/2Te1/2 is synthesized via the solid state reaction route in an evacuated, sealed quartz tube at 750°C. The presence of superconductivity is confirmed through magnetization/thermoelectric/resistivity studies. It is found that the superconducting transition temperature (Tc) is around 12 K. The heat capacity (Cp) of superconducting FeSe1−xTex exhibits a hump near Tc, instead of a well-defined lambda transition. X-ray photoelectron spectroscopy studies reveal well-defined positions for divalent Fe, Se, and Te, but with sufficient hybridization of the Fe (2p) and Se/Te (3d) core levels. In particular, divalent Fe is shifted to a higher binding energy, and Se and Te to a lower one. The situation is similar to that observed previously for the famous Cu-based high Tc superconductors, where the Cu (3d) orbital hybridizes with O (2p). We also found the satellite peak of Fe at 712.00 eV, which is attributed to the charge-carrier localization induced by Fe at the 2c site.

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Anand Pal

Papers

Synthesis, magnetization and magneto transport study of RECoPO (RE = La, Nd & Sm)

Synthesis and Structural Details of BiOCu1−xS: Possible New Entrant in a Series of Exotic Superconductors?

High Field (14Tesla) Magneto Transport of Sm/PrFeAsO

Complex magnetism and magneto-transport of RECoPO (RE = La, Nd, and Sm)

Anomalous heat capacity and x-ray photoelectron spectroscopy of superconducting FeSe1/2Te1/2