Session Q21: Novel Superconductivity in New and Low Dimensional Materials

Superconductivity in ThCoC$_{2}$

We report bulk superconductivity in the metallic carbide compound ThCoC$_{2}$. This compound crystallizes in the orthorhombic CeNiC$_{2}$ prototype structure, a non-centrosymmetric system. Despite the presence of Cobalt and the lack of inversion symmetry, we find bulk superconductivity with a critical temperature of T$_{c}$-2.6K. Details of the superconducting state with specific heat, magnetization, and resistivity measurements will be presented. This study was made possible by the generous support of AFOSR MURI ``Search for New Superconductors for Energy and Power Applications.''   

Superconductivity in ScGa$_3$ and LuGa$_3$

We are reporting low-temperature superconductivity in single crystals of ScGa$_3$ and LuGa$_3$. While the latter compound had been listed as a superconductor before, superconductivity in the former compound had never been observed, and characterization of the low-temperature state was lacking in both compounds. By measurements of magnetization, specific heat and resistivity, we show that RGa$_3$ (R = Sc and Lu) are conventional BCS superconductors with T$_c$ around 2.3 K and the upper critical field less than 240 Oe. The experimental results agree with band structure calculation estimate of the critical temperature.   

Physical properties of single crystalline SrSn$_{4}$ and BaSn$_{5}$ superconductors

We present the growths and detailed thermodynamic and transport measurements on single crystals of the recently discovered binary intermetallic superconductors, SrSn$_{4}$ and BaSn$_{5}$. Their superconducting transition temperatures T$_{c}$ are found to be 4.8 K and 4.4 K respectively. Both materials are strongly-coupled, possibly multi-band superconductors. Hydrostatic pressure causes a decrease in the superconducting transition temperature at the rate of $\approx \quad -$0.068 K/kbar for SrSn$_{4}$, and $\approx \quad -$0.053 K/kbar for BaSn$_{5}$. Band structure and upper superconducting critical field anisotropy of SrSn$_{4}$ suggest complex, multi-sheet Fermi surface formed by four bands. De Hass-van Alphen oscillations are observed in BaSn$_{5}$, which indicates a more complex topology of Fermi surface.   

Synthesis, Structure, Physical Properties of several Zirconium Chalcogenides

Compounds of KxZr2Se6, RbxZr2Se6 and ZrTe1.3As0.7 have been fabricated by high temperature solid state synthesis technique. All these compounds have the same space group Immm. They can be generally considered as the compounds derived from ZrSe3 and ZrTe3, which accommodate the quasi 2D type structure composed by (Zr2Se2)(Se4) and (Zr2Te2)(Te4) Layers. KxZr2Se6 and RbxZr2Se6 could be considered as the anionic layers [(Zr2Se2)(Se4)]x- intercalated with alkali cations. One the other hand, ZrTe1.3As0.7 isn't a layered compound. The compound has the same structure with NbPS, with disordered As and Te occupying the P sites. This structure could be considered as a derivative structure of ZrTe3 with the retained (Zr2Te2) layers interspersed with linear (Te0.3As0.7) chains. We also measured the magnetic and transport properties of these samples. We shall present and discuss their interesting structural and physical properties.   

Superconductivity in Pd, Ir, and Pt chalcogenide

Large spin-orbit coupling in materials with heavy chalcogens can result in unique quantum states or functional properties such as topological insulator, giant thermoelectric power, and superconductivity. When materials contain heavy cations in addition to heavy chalcogens, spin-orbit coupling can be further enhanced. For these reasons, we have studied the superconductivity of Pd, Ir, and Pt tellurides and selenides. In the exploration of these chalcogenides, we have focused on the competition between superconductivity and charge density wave that is associated with superlattice reflections.   

Elementary excitations and elusive superconductivity in palladium hydride -- \textit{ab initio} perspective. I. Paramagnons

\newcommand{\el}{\textit{et~al}.} Motivated by a experimental reports on possible high temperature superconductivity in palladium hydride [Tripodi \el, \textit{Physica C} \textbf{388-389}, 571 (2003)], we present a first principle study of spin fluctuations, electron-phonon coupling and critical temperature in PdH$_{x}$, $0 \leq x \leq 1$. A prerequisite for any qualitative study of exchange-enhanced materials is the knowledge of spin flip fluctuation spectrum. It is generally believed [Berk \& Schrieffer, \textit{Phys. Rev. Lett.}, \textbf{17}, 433 (1966)] that the ferromagnetic-like paramagnons of Pd are destructive for the conventional, i.e.\ $s$-wave, superconductivity. We describe them using linear response time dependent density functional theory, recently implemented to study complex metals [Buczek \el, \textit{Phys. Rev. Lett.} \textbf{105}, 097205 (2010)] . We find that hydrogenation suppresses the intense spin fluctuations of pure Pd, driving it away from a magnetic critical point. Under the assumption of $s$-wave pairing, this could lead to the formation of the superconducting state. The \textit{ab-initio} estimated electron-phonon coupling is strong enough to support superconductivity. Please look for the complementary contribution of Christophe Bersier.   

Elementary excitations and elusive superconductivity in palladium hydride -- \textit{ab initio} perspective. II. Phonons

Motivated by a experimental reports on possible high temperature superconductivity in palladium hydride [Tripodi et al., \textit{Physica C} \textbf{388-389}, 571 (2003)], we present a first principle study of spin fluctuations, electron-phonon coupling and critical temperature ($T_{c}$) in PdH$_{x}$ , $0 < x < 1$. Our results described in terms of (i) electronic structure, (ii) phonon density of states and (iii) Eliashberg function show that the hydrogenation of Pd clearly enhance the electron-phonon coupling in this material. Assuming phonons to be the driving force for superconductivity, fcc Pd features a vanishingly small $T_{c}$, while for the stochiometric $x=1$ PdH the resulting $T_{c}$ is around 10K in agreement with experiment. It is generally believed [Berk \& Schrieffer, \textit{Phys. Rev. Lett.}, \textbf{17}, 433 (1966)] that intense spin-\&flip fluctuations of Pd are destructive for the conventional, i.e. $s$-wave, superconductivity. However, the H doping leads to a drastic reduction of spin-flip scattering. Please look for complementary presentation of Pawe\l{} Buczek.   

Structural and Electronic properties of the 2D Superconductor CuS covellite with 113-valent Copper

Hexagonal CuS has recently been reported to superconduct at 40 K. At the same time, earlier experiments had found superconductivity at 1.5 K. Some authors reported local magnetic moments in this compound, while others observe pure Pauli paramagnetism. It exhibits a rather unusual for a good metal symmetry-lowering transition at 55 K of unknown origin. Even the Cu valency in CuS has been debated, with suggested ionic models such as $(Cu^{+1})_3(S_2^{-2})(S^{-1})$ and $(Cu^{+1})_3(S_2^{-1})(S^{-2})$, as well as noninteger valencies. To gain more insight, we've performed DFT calculations and found that the actual valency of Cu is 4/3 (2), so that the Cu d band is not sufficiently ionized in the ideal compound to provide enough proximity to Mott dielectric for either local moments or unconventional superconductivity. On the other hand, the reason that Cu in not divalent here is that some sulfurs form covalent S$_2$ molecules with the effective S$_2^{-2}$ valency. If some of these covalent S-S bonds are broken, local moments $can$ form, and in that case superexchange in the hexagonal lattice can induce $f-$wave pairing. Finally, regarding the structural transition, it is well reproduced in DFT calculations. Possible microscopic origin of this transition will be discussed in the talk.   

Moving Beyond Quantum Mechanics in Search for a Generalized Theory of Superconductivity

Though there are infinite number of theories currently in the literature in the search for a generalized theory of superconductivity (SC), there may be three domineering mechanisms for the Cooper pair formation (CPF) and their emergent theories of SC. Two of these mechanisms, electron-phonon interactions and electron-electron correlations which are based on the quantum theory axiom of action-at-a distance, may be only an approximation of the third mechanism which is contact interaction of the wavepackets of the two electrons forming the Cooper pair as envisaged in hadronic mechanics to be responsible for natural bonding of elements. The application of this hydronic --type interaction to the superconducting cuprates, iron based compounds and heavy fermions leads to interesting results. It is therefore suggested that the future of the search for the theory of SC may be considered from this natural possible bonding that at short distances, the CPF is by a nonlinear, nonlocal and nonhamiltonian strong hadronic-type interactions due to deep wave-overlapping of spinning particles leading to Hulthen potential that is attractive between two electrons in singlet couplings while at large distances the CPF is by superexchange interaction which is purely a quantum mechanical affairs.   

Material Specific Design for Room Temperature Superconductivity

The transition temperature, Tc, of superconductors has been increased sevenfold from 23K in Nb$_{3}$Ge to 164K in Hg-1223. A further two-fold increase would get us to above room temperature superconductivity. Studying high temperature superconductors (HTSCs), we have developed a formula that expresses Tc in terms of electronegativity, valence electrons, Ne, atomic number, Z, formula mass and a coupling constant, Ko. We observe an increasing linear relationship between Tc and Ko. Ko also correlates with formula mass and atomic number and the number of atoms in the compound. By our formula, Hg-1223 has Ko = 70. We propose, using our design algorithm, that room temperature superconductivity may be realized in a system with ko = 160; electronegativity = 2.5, Ne/Sqrt Z = 0.8. We proceed to show combinations of oxides and elements that will yield the required parameters for synthesizing reproducible room temperature superconductivity.   

Thermopower and Resistivity in the Spin Density Wave- Metal co-existence regime of (TMTSF)2PF6

The organic conductor (TMTSF)2PF6 is studied in the pressure regime where it is believed to exhibit co-existence between Metallic and Spin Density Wave domains. In this pressure regime, an anisotropy in the superconductivity transition - seen previously in resistivity data on separate samples in different experimental runs - Is here reproduced using both resistivity and thermopower as probes, on three slices of the same crystal measured simultaneously. The simultaneous use of these two measurements along the a ,b and c axes , enables us to extract complementary information about the remarkable anisotropy in the superconducting transition and may shed light on the possibly inhomogeneous ground states in the co-existence regime of (TMTSF)2PF6.   

Non-analytical Angular Dependence of the Upper Critical Magnetic Field in a Quasi-One-Dimensional Superconductor

We have derived the so-called gap equation, which determines the upper critical magnetic field, perpendicular to conducting chains of a quasi-one-dimensional superconductor. By analyzing this equation at zero temperature, we have found that the calculated angular dependence of the upper critical magnetic field is qualitatively different than that in the so-called effective mass model. In particular, our theory predicts a non-analitical angular dependence of the upper critical magnetic field, $H_{c2}(0) - H_{c2}(\alpha) \sim \alpha^{3/2}$, when magnetic field is close to one of the crystallographic axes and makes an angle $\alpha$ with the axis. We discuss possible experiments on the superconductor (DMET)$_2$I$_3$ to discover this non-analytical dependence.   

Hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell Superconducting Phases in a Magnetic Field in (TMTSF)$_{2}$ClO$_{4}$

We solve a long-standing problem about a theoretical description of the upper critical magnetic field, parallel to conducting layers and perpendicular to conducting chains, in (TMTSF)$_{2}$ClO$_{4}$ superconductor. In particular, we explain why the experimental upper critical field, H$^{b}_{c2}$ = 6T, is higher than both the quasi-classical upper critical field and Clogston paramagnetic limit. We show that this property is due to the coexistence of the hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic field in a form of three plane waves with non-zero momentums of the Cooper pairs. Our results are in good qualitative and quantitative agreement with the recent experimental measurements of H$^{b}_{c2}$ and support a singlet d-wave-like scenario of superconductivity in (TMTSF)$_{2}$ClO$_{4}$.   

$^{1}$H NMR Spin-Lattice Relaxation Study of $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br

The discovery of an anomalous Nernst signal in the organic superconductor $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br suggests the presence of magnetic flux vortices above T$_{c}^{[1]}$. Previous studies below the transition temperature have shown that the additional fluctuating field created by vortices dramatically increases the $^{1}$H NMR spin-lattice relaxation rate$^{[2]}$. We revisit $^{1}$H spin-lattice relaxation in $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br and provide new evidence of inhomogeneous behavior both above and below $T_{c}$. $^{[1]}$M. S. Nam et al, Nature 449, 584-587 (2007) $^{[2]}$H. Mayaffre et al, Phys. Rev. Lett. 76, 4951-4954 (1996) Work at UIUC supported by NSF DMR 10-05708 and the Center for Emergent Superconductivity, USDOE Award No. DE-AC0298CH1088. Work at Argonne supported by UChicago Argonne, LLC, Operator of Argonne National Laboratory, DOE Contract No. DE-AC02-06CH11357.   

$^{13}$C NMR Study of Slow Motions in $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br

Like the high-$T_{C}$ cuprates, the 2D organic superconductor $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$ ($T_{C}=11.9$~K) exhibits a pseudo-gapped phase above the superconducting transition, as indicated by the $^{13}\mathrm{C}$ spin-lattice relaxation rate ($1/T_{1}T$) peak at about 50~K. While $^{13}\mathrm{C}$ NMR has been used extensively to probe the pseudo-gapped regime, $T_{1}$ is only sensitive to fast motions in the MHz scale (Larmor frequency), and $T_{2}$ remains relatively constant in the pseudo-gapped regime. Neither $T_{1}$ nor $T_{2}$ give us any clue about any possible slow motions. We report measurements using the stimulated echo pulse sequence\footnote{L. R. Becerra, C. A. Klug, C. P. Slichter, and J. H. Sinfelt, J. Phys. Chem. \textbf{97}, 12014 (1993).} which is capable of providing more detailed information on possible slow motions in the pseudo-gapped regime.