Session B21: Focus Session: Search for New Superconductors: Methodologies and New Materials

An Enlightened Combinatorial Search for New Superconductors

I describe a methodology for the fast search for new superconducting materials. This method consists of a parallel synthesis of a highly inhomogeneous alloy covering large areas of the metallurgical phase diagram combined with a very sensitive, fast, microwave-based method, which allows large non-superconducting portions of the sample to be discarded. Once an inhomogeneous sample containing a minority phase superconductor is identified, we revert to well-known, thorough identification methods, which include standard physical and structural methods. We show how a systematic structural study helps in avoiding miss-identification of new superconducting materials when there are indications from other methods of new discoveries. The application of these ideas to the La-Si-C system, which exhibits promising normal state properties, sometimes correlated with superconductivity, will be discussed. Although this system shows indications of a new superconducting compound, the careful analysis described here shows that the superconductivity in this system can be attributed to intermediate binary and single phases. Searches in other Rare Earth-Si based systems will also be described. Work done in collaboration with J. de la Venta, Ali C. Basaran, J. G. Ramirez, T. Grant, A. J. S. Machado, M. R. Suchomel, R. T. Weber, Z Fisk, P. Guptasarma, O. Shpyrko and D. Basov.   

Search for New Superconductors in RE-Si and Al-B Systems: a High Pressure High Temperature Approach

We have searched for the presence of superconductivity in the RE-Si and Al-B systems using HP-HT synthesis. The RE-Si system has some of the common features that are present in high TC superconducting materials. We have synthesized Ce, Pr, Nd and Gd silicides undoped and doped with C and B. On the other hand, AlB$_{2}$ has some similarities with the superconducting MgB$_{2}$. We have tried to synthesize AlB$_{2}$ way off stoichiometry using HP-HT and thin films Phase Spread Alloy. The Al$_{0.67}$ B$_{2}$ would be the MgB$_{2}$ equivalent and good reason to expect superconductivity. We discuss the results for both systems after a careful analysis of several physical properties (SQUID, Modulated Microwave Absorption) and x-ray powder diffraction.   

Search for new materials: phase spread alloy thin film fabrication and characterization

We use the phase spread alloy (PSA) method of fabricating compositionally heterogeneous thin films as an efficient way to produce and screen new, interesting materials (e.g. superconductors, magnetoresistive compounds, etc.). This method uses co-sputtering to deposit material with smoothly varying element concentration across a substrate. Both local and non-local probes are used to verify the composition of the sample. Using the La-Si-C system as an example, we perform x-ray fluorescence from a synchrotron source, x-ray diffraction from a lab source, atomic force microscopy, and infrared spectroscopy on one sample to verify the presence of different phases and their properties.   

Search for new superconductors in rare-earth silicide systems

We have searched for the presence of superconductivity in the RE$_{5}$Si$_{3}$ system doped with C or B as a light element (RE: La, Ce, Pr, and Eu). High temperature superconductors and RE$_{5}$Si$_{3}$ systems have some common properties. Both systems have a layered tetragonal crystal structure. They are multi-element compounds and are also doped with a light element to introduce the superconductivity. In this study, multiphase bulk samples were made using arc-melting. Phase spread alloy thin films were also prepared in a sputtering system. We used magnetic field modulated microwave absorption spectroscopy (MFMMS), which is a very sensitive contactless technique to detect superconductivity, as the first screening for the existence of superconductivity in an inhomogeneous sample. We will present some of our results from MFMMS and SQUID measurements and compare them with structural refinement from X-Ray data.   

Superconductivity in the K-Mo-O system

The rutile-type structure belongs to space group P4$_{2}$\textit{/mnm}. Some transition metals form dioxides with variants rutile structure are known as pseudorutiles. These dioxides have interesting physical properties but they are still poorly understood. MoO$_{2}$ is one of them. Polycrystalline samples of MoO$_{2}$ can be easily prepared using stoichiometric amounts of Mo and MoO$_{3}$ through solid state reaction at temperatures near 700\r{ }C. This material is a highly conductive oxide and exhibits Mo-Mo metallic bounds along $c$-axis. On the other hand, previous results show that the physical properties of the MoO$_{2}$ are substantially changed with potassium doping [1]. This work unambiguously demonstrates that the K$_{x}$MoO$_{2}$ system exhibits superconductivity. Electrical resistivity and magnetization measurements were carried out from 2 to 300 K. The electrical and magnetic measurements show that the superconducting critical temperature ranges from 3 to 10 K. The phase composition responsible for the superconductivity is still under investigation. \\[4pt] [1] L. M. S. Alves et al., Phys. Rev. B \textbf{81, }174532 (2010).   

High spin-low spin transition in insulating CaMn$_2$Sb$_2$

Layered manganese pnictides are often interesting compounds to compare with the iron pnictide superconductors. To this end, we have synthesized high quality flux-grown single crystals of CaMn$_2$Sb$_2$, which forms in a trigonal CaAl$_2$Si$_2$-type structure characterized by corrugated triangular Mn layers. Previously reported as a bad metal, we observe instead that this compound exhibits a distinct insulating trend in temperature-dependent resistivity measurements, including an enhancement of up to two orders of magnitude between 200 K and T$_N$ = 85 K. Measurements of ac susceptibility exhibit an orientation- and highly field-dependent plateau across the same temperature range, while heat capacity measurements reveal a sharp feature at 85 K as well as a broad anomaly centered near 195 K. Curie-Weiss behavior above 300 K indicates the presence fluctuating moments with prevailing ferromagnetic interactions, corresponding to less than half the static moment reported for the antiferromagnetic ordered state. These results imply a temperature-induced high spin-low spin insulator-insulator transition.   

An infrared study of electron delocalization in Mn-based relatives of the pnictides

Current data suggest that the viability of parent compounds to become superconducting is intimately tied to electron correlations.\footnote{M. Quazilbash \textit{et. al.} Nature Physics 5, 647 (July 2009)}$^,$\footnote{P.A. Lee \textit{et. al.} Reviews of Modern Physics 78, 17 (January 2006)} Further comparisons between the ground states of the cuprate and pnictide parent compounds indicate that doping across an electron delocalization transition (EDT) may be key to obtaining a higher critical temperature.\footnote{J. Simonson \textit{et. al.} ArXiv:11105938} These insights lead us to study the Mn-based compounds that are isostructural with pnictides and are antiferromagnetic insulators like the cuprates. Specifically, we have explored the effects of doping on LaMnPO$_{1-x}$F$_x$ and Ca$_{1-x}$La$_x$Mn$_2$Sb$_2$ via optical spectroscopy, transport, and magnetic measurements in parallel to theoretical band structure calculations. Our studies show that LaMnPO is highly resistant to electron delocalization. Likewise, in CaMn$_2$Sb$_2$, full delocalization was not attained even though a shift in the band edge was observed.   

Strong Electronic Correlations in YMn$_{2}$Ge$_{2}$

Exotic phases, like superconductivity, often emerge near electron delocalization transitions in strongly interacting systems. Magnetization, heat capacity and resistivity measurements were performed on single crystals of the antiferromagnetic metal YMn$_{2}$Ge$_{2}$, which is isostructural to the ThCr$_{2}$Si$_{2}$-type iron pnictides. Above the antiferromagnetic ordering temperature T$_{N}$=425 K, the magnetic susceptibility displays Curie-Weiss like behaviour with a fluctuating moment $\mu$ = 3.3 $\mu_{B}$/Mn atom, larger than the ordered moment of 2.2 $\mu_{B}$/Mn atom. Heat capacity measurements yield a Sommerfeld coefficient $\gamma$ = $\frac{C}{T}$ = 8.5 mJ/mol Mn K$^{2}$, nearly three times larger than $\gamma_{Ru}$ = 3.3 mJ/mol Mn K$^{2}$ for its non-magnetic isostructual analog YRu$_{2}$Ge$_{2}$, indicating strong electronic correlations in YMn$_{2}$Ge$_{2}$. The quasiparticle mass enhancement $\frac{m*}{m_{Ru}}$ = $\frac{\gamma}{\gamma_{Ru}}$ = 2.6 is similar to the value observed in the 122-type iron pinctides. Fermi-liquid behaviour of the resistivity $\rho = \rho_{0} + A T^{2}$ is observed over a very broad range of temperatures between 0.5 K and 300 K, with the resistivity at low temperature $\rho$(0.5 K) = 8 $\mu\Omega$ cm indicating high sample quality   

Magnetic, Thermal and Transport Properties of LaNi$_2$(Ge$_{1- x}$P$_x$)$_2$

Polycrystalline samples of LaNi$_2$(Ge$_{1-x}$P$_x$)$_2$ ($x=$ 0, 0.25, 0.50, 0.75, 1) with the tetragonal ${\rm ThCr_2Si_2}$ structure were investigated by heat capacity $C_{\rm p}$, magnetic susceptibility $\chi$, and electrical resistivity $\rho$ measurements for temperatures $1.8~{\rm K}\leq T \leq 300$~K\@. The $\rho(T)$ data for each sample reveal metallic behavior that follows the Bloch-Gr\"uniesen theory. The low-$T$ $C_{\rm p}(T)$ data for the series yielded Sommerfeld coefficients $\gamma = 6$--12~mJ/mol\,K$^2$ and Debye temperatues $\Theta_{\rm D} = 300$--480~K\@. The $\chi(T)$ data showed nearly $T$-independent paramagnetism except for LaNi$_2$Ge$_2$, where data up to 1000~K exhibit a broad peak at $\approx 300$~K\@. A possible onset of superconductivity is seen for ${\rm LaNi_2P_2}$ at 2.1~K\@. Analytic functions accurately representing the Bloch-Gr\"uniesen and Debye functions are presented that are very useful for fitting $\rho(T)$ and lattice $C_{\rm p}(T)$ data, respectively.   

Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with Teflon

WO3-xFx (x $<$ 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (Teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was discovered in the samples with fluorine contents of 0.41 $<$ x $<$ 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.   

Type-I Superconductivity in Ytterbium Diantimonide

The layered antimonide compound YbSb$_2$ crystallizes with a ZrSi$_2$-type orthorhombic structure, different from other rare earth diantimonides. Unusual for a binary compound, Type-I superconductivity has been observed in YbSb$_2$. In this talk, we present the results from anisotropic magnetization, resistivity, heat capacity and magneto-optical Faraday effect measurements on YbSb$_2$ single crystals, showing a clear superconducting transition at $T_c$ = 1.25 K. The estimated electron-phonon coupling $\lambda$ = 0.51, together with the jump in electronic specific heat $\Delta C_{es}/\gamma T_{c}$ = 1.36, indicate the system to be a weak-coupling BCS superconductor. Magnetization, as well as heat capacity measured under field, clearly suggest a Type-I behavior, which is confirmed by the estimated Ginzburg-Landau parameter $\kappa$ = 0.13. According to the $H$-$T$ phase diagram, the critical field $H_c$ is around 60 Oe.   

Cu$_{1-x}$BiSO: the first Fe-pnictide-structured compound without Fe or Pnictogen

The electronic structure of 1111 transition metal pnictides offers a large variety of low-energy phenomena depending on the electronic filling explored. Based on first principles calculations I study the electronic filling $d^{10-x}$ represented by Cu$_{1-x}$BiSO: a band insulator that becomes metallic upon hole doping. I argue that the electron-phonon coupling is very strong in this material, and probably drives superconductivity. The critical temperature is however strongly depressed by the proximity to ferromagnetism. The competition between these two different order parameters brings about a high tunability of the system that can go from conventional to unconventional superconductivity by varying such parameters as doping or pressure.   

Possible Evidence for Novel Superconductivity in LaRu$_{3}$Si$_{2}$

Superconductivity in LaRu$_{3}$Si$_{2}$ with the honeycomb structure of Ru has been investigated. It is found that the normal state specific heat C/T exhibits a deviation from the Debye model down to the lowest temperature. A relation C/T = $\gamma _{n}+\beta $T$^{2}$-ATlnT which concerns the electron correlations can fit the data very well. The suppression to the superconductivity by the magnetic field is not the mean-field like, which is associated well with the observation of strong superconducting fluctuations. The field dependence of the induced quasiparticle density of states measured by the low temperature specific heat shows a non-linear feature, indicating the significant contributions given by the delocalized quasiparticles. The Wilson ratio estimated here is about 3.3, indicating also a strong correlation effect. Interestingly, the Fe-doping can suppress the superconductivity very easily, while Co-doping kills the superconductivity very slowly. The possible reasons are discussed. All these results suggest that the electronic correlation effect exists in LaRu$_{3}$Si$_{2}$ and superconductivity may be novel.