Session J33: Superconductivity: Novel Materials

Superconductivity at T$_{C }$=38 K in Pristine and Sulfur Doped Amorphous Carbon

Detailed dc magnetic studies perormed on pristine amorphous carbon and on sulfur doped amorphous carbon, clearly indicate the existence of inhomogeneous superconductivity with T$_{C }$ranging from 32 to 38 K. The superconducting phase fraction is about 0.5{\%}. Superconductivity is manifested by: (i) the diamagnetic shielding state, (ii) the Meissner effects and (iii) the typical M(H) hysteresis loops. The results indicate that amorphous carbon is a system with non-perculative superconducting phase. Chemical analysis shows that the amorphous carbon contains a small amount of sulfur. It is propossed that superconductivity stems from an unknown C-S phase immersed in amorphous carbon, thus it is concluded that s\'{u}lfur is a effective dopant to induce superconductivity in carbon (graphite) based materials. The various options for the C-S superconducting phase will be discussed.   

Crushing Elemental Calcium into a High Temperature Superconductor.

The high temperature superconductivity (up to 25 K) observed in elemental Ca at high pressure extends across several phase boundaries, making understanding the crystal structures of Ca under high pressure of great importance. Above 100 GPa, both experiment and theory indicate three possible structures, having space groups P4$_3$2$_1$2, Cmca and Pnma. The reported room temperature structure, primitive simple cubic in the 32-109 GPa range, is dynamically unstable at T=0 throughout this range, according to linnear response calculations of the phonon spectrum. Structure optimization and constant pressure enthalpy calculations using density functional theory reveal that several structures (the structures mentioned above, an I-43m structure, and simple cubic)are quasi-degenerate, and hence are competing, in the pressure ranges 40-80 GPa and 100-130 GPa. Volume collapse transitions of the Cmca and Pnma structure will also be described. We discuss the implications of these findings for the observed room temperature ``simple cubic'' phase. Linear response calculations give T$_c\sim 20$ K for several of the phases at high pressure. Predictions of electron-phonon coupling in the 120-220 GPa regime will also be discussed.   

Competing s and d-wave superconducting order in V$_{3}$Si

Competing phases have generated a lot of interest in the study of cuprate and pnictide superconductors. Here we examine the competition between s and d-wave superconducting order in V$_{3}$Si. We provide microscopic arguments as to why both of these phases have a comparable transition temperature in this material. We further argue that many experimental probes provide evidence for this competition.   

Theoretical study of FFLO phases and related phases in non-centrosymmetric superconductors

Superconducting order can break translational invariance, leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure, such as in a FFLO phase. Some such superconductors break inversion symmetry, leading to helical and multiple-q (FFLO-like) phases. We study these related phases with and without vortices. We show that for a FFLO phase, a crisscrossing lattice solution arises from the decay of conventional Abrikosov vortices into pairs of fractional vortices. We further show that the fractional vortex solution can also exist in the multiple-q phase of non-centrosymmetric superconductors.   

Giant Magnetoresistance and Unusual Magnetic Behavior in Single Crystals of the Layered Arsenide EuRh$_2$As$_2$

Magnetic susceptibility $\chi$, isothermal magnetization $M$, resistivity $\rho$, Hall effect, and heat capacity $C$ measurements on EuRh$_2$As$_2$ reveal complex and unusual magnetic behavior. The $\chi(T)$ data gave a small Weiss temperature $\theta \approx$~12~K indicating predominantly ferromagnetic interactions between the Eu$^{2+}$ moments. Below $T$~=~47~K, however $\chi(T)$ indicates that an antiferromagnetic transition occurs instead. The unusually high $T_{\rm N}$ compared to $\theta$ ($\theta$/$T_{\rm N}\approx$0.26) suggests novel physics. A metamagnetic transition is observed in the $M$ versus $H$ data at $T < T_{\rm N}$ when $H$ is applied in the $ab$ plane. The metamagnetic field shows an unusual $T$ dependence, decreasing slightly between $T$~=~2~K and 30~K and increasing again on approaching $T_{\rm N}$ before vanishing abruptly at $T_{\rm N}$. In zero field the $\rho(T)$ data indicate metallic behavior between 2~K and 300~K. However, at low temperatures $T \leq 30$~K, $\rho(T)$ increases dramatically in an applied field $H$ and we observe a giant positive magnetoresistance of $\approx$90\% at $T$~=~2~K and $H$~=~8~T. For $T < 30$~K $\rho(T)$ increases for $H \geq$~1~T. A monotonic reduction of the electronic specific heat coefficient $\gamma$ with $H$ and a change in sign of the Hall coefficient from negative above $T$~=~15~K to positive for lower $T$ are also observed. \\[0pt] $^*$Supported by DOE-BES under Contract No.\ DE-AC02-07CH11358.   

Non-centrosymmetric superconductor La$_3$Bi$_4$Pt$_3$

Recently, we have discovered that the metallic La$_3$Bi$_4$Pt$_3$ (Y$_3$Au$_3$Sb$_4$ structure) becomes superconducting below a transition temperature $T_c$ of about 1.4K. Our interest in the superconducting phase of La$_3$Bi$_4$Pt$_3$ stems from the fact that it lacks a center of inversion, which may lead to unconventional superconductivity, including nodes in the superconducting gap function, even if the pair wave function exhibits the full spatial symmetry of the crystal. Compared to other non-centrosymmetric \emph{magnetic} compounds in which superconductivity has recently been discovered, like CePt$_3$Si, UIr, CeRhSi$_3$ (under pressure), the nature of the superconducting state in La$_3$Bi$_4$Pt$_3$ is not complicated by strong electron correlations nor the coexistence of magnetism. This makes it a good model system to study superconductivity without inversion symmetry. In our presentation we will focus on the first basic characterization of our La$_3$Bi$_4$Pt$_3$ single crystals (X-ray, specific heat, resistivity, penetration depth, etc.).   

On d-Wave Superconductors with a Zeeman or Exchange Splitting of the Spin-Up and --Down Fermi Surfaces

For a given Zeeman (or exchange) energy $h$, we used the Fermi-surface splitting, $\delta $\textit{$\mu $}, as a variational parameter, and showed: (1) For an s-wave superconductor, the Sarma state is actually an \underline {unstable} equilibrium state, which is known to exist for 0.5$ [Preview Abstract]

Superconductivity from two dimensional interfaces: CuCl/Si, GaP/Si, ZnS/Si

Two-dimensional (2D) interfaces of hetero-bonded semiconductor superlattices are studied using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. The 2D system, of metal-insulator-metal, is one of the candidate geometries to realize the excitonic mechanism of superconductvity, \footnote{V.L. Ginzburg, Sov. Phys. JETP {\bf 20},1549 (1965)} where $T_C$ can be greatly enhanced over phonon mediation. Epitaxially grown CuCl on Si (111) was reported to exhibit an anomalous diamagnetic susceptibility at 60$\sim$150 K. \footnote{Mattes and Foiles,Physica 135B, 139 (1985)} For all superlattices, 2D metallicity was found at the interfaces due to charge transfer from the polarity mismatch, \footnote{Rhim, Saniz, Yu, Ye, and Freeman, Phys. Rev. B {\bf 76}, 184505 (2007)} as evidenced by their bands, Fermi surfaces, and charge densities. The $T_C$, calculated within the crude RMTA and the McMillan-Hopfield formula, is 0.04$\sim$4.4K for the CuCl/Si case, but vanishes for the other cases. To pursue the excitonic mechanism, we are determining the Kernel function $K(\omega)$, i.e. the average of the effective Coulomb interfaction, with {\bf q} dependent dynamic screening. First results for CuCl/Si show $K(\omega)$ to be attractive for a certain energy range   

Standard Model for Superconductivity in Graphite Intercalation Compounds: Prediction of Optimum $T_c$

Based on the model that was successfully applied to the explanation of superconductivity with the transition temperature $T_c$ of about 0.1K or less in the alkali- intercalated graphite compounds such as KC$_8$, RbC$_8$, and CsC$_8$ in 1982 [Y. Takada, {\it J. Phys. Soc. Jpn.} {\bf 51}, 63 (1982) ], we have calculated $T_c$ for the alkaline-earth- intercalated graphite compounds including CaC$_6$, YbC$_6$, and SrC$_6$ with $T_c$ of about 10K or less to find that the same model reproduces the observed $T_c$ in those compounds as well, indicating that it is a standard model for superconductivity in the graphite intercalation compounds with $T_c$ ranging over three orders of magnitude. The difference in $T_c$ by two orders between KC$_8$ and CaC$_6$ can be accounted for by (i) doubling $Z$ the valency of the metal ions, which enhances $T_c$ by one order, and (ii) tripling $m^*$ the effective mass of the superconducting three-dimensional electrons in the interlayer band, which also enhances $T_c$ by one order. Enhancement of $T_c$ well beyond 10 K is also predicted in this model, if intercalant metals are judiciously chosen so that both $Z$ and $m^*$ are increased further.   

Crystal structure and physical properties of new layered oxysulfides (Cu$_{2}$S$_{2})$(Sr$_{4}$Sc$_{2}$O$_{6})$, (Cu$_{2}$S$_{2})$(Ba$_{3}$Sc$_{2}$O$_{5})$ and (Cu$_{2}$S$_{2})$(Ba$_{3}$In$_{2}$O$_{5})$

Recently high-$T_{c}$ superconductors were discovered in layered oxypnictide systems with stacking of fluorite-based oxide layers and anti fluorite-based pnictide layers. Materials having similar stacking structure with perovskite-based oxide layers and anti-fluorite chalcogenide or pnictide layers have been discovered in some oxypnictides and oxychalcogenides, but such systems are relatively less explored. Some materials belongs to these systems show interesting properties such as p-type transparent conductivity in (Cu$_{2}$S$_{2})$(Sr$_{3}$Sc$_{2}$O$_{5})$. This motivated us to explore perovskite-based layered oxysulfide system. New layered oxysulfides (Cu$_{2}$S$_{2})$(Sr$_{4}$Sc$_{2}$O$_{6})$, (Cu$_{2}$S$_{2})$(Ba$_{3}$Sc$_{2}$O$_{5})$ and (Cu$_{2}$S$_{2})$(Ba$_{3}$In$_{2}$O$_{5})$ have been synthesized by conventional solid state reaction. X-ray diffraction patterns show that the structure of these materials consists of stacking of semi-conducting Cu$_{2}$S$_{2}$ layers and perovskite-based oxide layers. Structural features and physical properties of these new materials will be presented.   

High-T$_{c}$ superconductivity in nanostructured Na$_{x}$WO$_{3-y}$: Sol-gel route

Tungsten trioxide, WO$_{3-y}$ infiltrated into various nanoporous matrix structures such as carbon inverse opal, carbon nanotubes paper, or platinum sponge and then intercalated with alkaline ions (Li$^{+}$, Na$^{+})$ exhibits a pronounced diamagnetic onset in ZFC magnetization in a wide range of temperatures, 125-132 K. Resistivity measurements show non zero jump and intensive fluctuations of electrical resistance below observed transition points. The observed magnetic and electrical anomalies in nanostructured tungsten bronzes (Li$_{x}$WO$_{3-y}$, Na$_{x}$WO$_{3-y})$ suggest the possibility of localized non-percolated superconductivity. The direct evidence of polaron formation from temperature dependence of EPR and photoemission spectra and formation of bipolarons in weakly reduced to WO$_{3-y}$, with 3-y typically in the order of 2.95 suggest bipolarons mechanism of a Bose-Einstein condensation of trapped electron pairs in doped WO$_{3-y}$. On the other hand the strong lattice instabilities in 2D systems like layered cuprates and tungsten bronzes place the upper limit on T$_{c}$. Than, the percolative self-organized mechanism on the metal/insulator interface like Na/WO$_{3}$ and NaWO$_{3}$/nanostructured matrix can facilitate the high T$_{c}$ obtained in sodium bronzes infiltrated into inverted carbon opal or carbon nanotube matricies.   

Search for superconductivity in surface-doped WO$_{3}$ films

We report the search for superconductivity in surface-doped WO$_{3}$ films. Possible evidence for high temperature superconductivity in this system has been reported in the literature. In our work, WO$_{3}$ films were grown by MBE and characterized by \textit{in-situ} XPS, UPS, and \textit{ex-situ} XRD and resistivity measurements. For some films, Na was deposited on the surface at room temperature, and the resultant Na+WO$_{3-x}$ films annealed in vacuum ($\sim $ 10$^{-8}$ Torr) for 1h at various temperatures (300 -- 800C). With increasing thickness of Na, the intensity of the Na1$s$ peak in XPS spectra and area of the W5$d$ state in UPS spectra increased, suggesting some charge transfer to the WO$_{3}$ film. After annealing below 500C, XPS and UPS spectra did not change, while after annealing above 500C, the area of the W5$d$ state drastically increased. The same behavior was observed for pure WO$_{3-x}$ films, however, suggesting that oxygen vacancies are created during the annealing. The films started to decompose above 700C. The resistivity of our films decreased with increasing annealing temperature. However, so far we have not observed any sign of superconductivity down to 2 K in any of our films. This work is supported by Air Force Office of Scientific Research.   

Combinatorial Study of the Hall-Effect Sign Change in Overdoped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$ Films

We have made a high-resolution study of the sign change of the Hall effect in combinatorially grown samples of over-doped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$, using our dedicated system for creating and measuring samples with ultrafine stoichiometry resolution. The data are from MBE films grown with a linear stoichiometry gradient and were taken with a characterization system that can measure both the Hall effect and resistivity simultaneously at 31 different locations on the film. Recently improved growth techniques, as well as sample processing, lithography and subsequent handling give both Hall and resistance measurements confirming a linear stoichiometry gradient to better than 1 \%. We report on the variation of the Hall coefficient, T$_{\rm c}$, and the resistivity in the region of the Fermi surface topology change.