Session B25: Superconducting Materials: Growth, Structure, and Properties

Superconductivity Study of LaO$_{0.5}$F$_{0.5}$BiS$_2$ using Nuclear Magnetic Resonance

LaO$_{0.5}$F$_{0.5}$BiS$_2$ is a member of the recently discovered class of BiS$_2$-layered superconductors. It has a superconducting temperature $T_c$ close to 3~K when prepared under normal conditions. Pressure makes $T_c$ as high as 10~K.\ $T_c$ is also close to 10~K at ambient pressure for samples synthesized under pressure (2~GPa). We are conducting a $^{19}$F-NMR study in a polycrystalline powder of LaO$_{0.5}$F$_{0.5}$BiS$_2$ ($T_c\sim$3~K). We report static and dynamic NMR parameters as functions of temperature, between 1.8 and 300 K. Our data show changes of sample behavior on cooling below both 10 and 3 K, and a modulation on the spin-echo-amplitude decay, which we discuss in some detail.   

Ferrodistortive lattice modes and polytypism in LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2}}$ superconductor

LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2}}$ is a disordered, non-magnetic superconductor with a transition temperature of 10.8 K at x $=$ 0.5. The crystal structure of LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2\thinspace }}$is investigated using synchrotron X-ray and neutron diffraction experiments. The Bragg pattern obtained in hk0 plane could not be reproduced by either the long-presumed nominal symmetry P4/nmm or other theoretically suggested symmetries and indicated the possibility that the symmetry is lower than expected. The Bragg structure can be reproduced by a model involving coordinated ferrodistortive in-plane displacements of sulfur. Several possibilities of sulfur displacement arrangements can reproduce the data equally well leading to the possibility of domains. When several domains are averaged together the fitting results improve. Therefore, the structure most likely consists of polytypes stacked along the c-axis. In the superconducting planes, the structure needs to be uniform, otherwise domain walls are created due to antiferrodistortive arrangements, giving rise to additional peaks not present in the data.   

Rattling induced superconductivity in RV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ (R $=$ Sc, Lu, Y) aluminides -- an experimental and theoretical study

Polycrystalline samples of four ternary intermetallics RV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ (R $=$ Sc, Y, La, and Lu) were synthesized. Structural studies carried out using powder x-ray diffraction and Rietveld analysis show that all compounds crystallize in CeCr$_{\mathrm{2}}$Al$_{\mathrm{20}}$-type structure composed of icosahedral Al-R cages. Results of physical properties measurements reveal that ScV$_{\mathrm{2}}$Al$_{\mathrm{20}}$, YV$_{\mathrm{2}}$Al$_{\mathrm{20}}$, and LuV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ are weakly-coupled BCS superconductors with critical temperatures T$_{\mathrm{c}}=$ 1.0, 0.57, and 0.60 K, respectively. Electronic and phonon structure calculations reveal the key role of low-frequency anharmonic vibrations of R atoms (rattling effect) for the appearance of superconductivity. A correlation between phonon and crystal structures was observed, allowing to search for new RV$_{\mathrm{2}}$Al$_{\mathrm{20}}$ superconductors.   

Influence of ZnO and Dy2O3 on MgB2 Bulks Fabricated by High Temperature and Pressure Reaction

ZnO and Dy$_{\mathrm{2}}$O$_{\mathrm{3}}$ have been considered as dopants for the improvement of superconducting properties in MgB$_{\mathrm{2}}$ bulks. However, the effect of these dopants is still unclear: some studies reported these metal oxides worked as new pinning centers and others was attributed the effects to Mg site substitution. In addition, low temperature reactions may explore limited solubility regimes for these dopants. In order to study the intrinsic effect of ZnO and Dy$_{\mathrm{2}}$O$_{\mathrm{3}}$ in MgB$_{\mathrm{2}}$, a high temperature solid state sintering method has been used to fabricate dense and homogeneous MgB$_{\mathrm{2}}$ bulks. Even higher temperature excursions above the peritectic allow us to explore the solubility limits. To do this we used an induction furnace built inside of a high pressure vessel which allowed us to reach 1700$^{\mathrm{o}}$C and 1500 Psi. A slow cooling rate (2$^{\mathrm{o}}$C/min) was used in an attempt to obtain a homogeneous nucleation and phase distribution. A series of MgB$_{\mathrm{2}}$ bulk samples with ZnO and Dy$_{\mathrm{2}}$O$_{\mathrm{3}}$ additives were synthesized through this high pressure and temperature procedures. The resulting microstructures of these bulk samples were revealed by SEM and TEM. Atomic substitution were evaluated by high resolution XRD. The upper critical field $B_{c2}$, irreversible field $B_{irr}$ and $T_{c}$ were obtained from both magnetic and resistivity measurements. The roles of substitution vs precipitate induced strain on $B_{c2} $enhancements with adding ZnO and Dy$_{\mathrm{2}}$O$_{\mathrm{3}}$ were discussed.   

Mo$_5$PB$_2$: a new superconductor in the Cr$_5$B$_3$ structure type with $T_c$ = 9.2 K

Superconductivity has been reported recently in several ternary silicide-borides adopting the tetragonal Cr$_5$B$_3$ structure type, including Nb$_5$Si$_{3-x}$B$_x$, Mo$_5$SiB$_2$, and W$_5$SiB$_2$, with critical temperatures ranging from 5.8-7.8 K. Here we report superconductivity with $T_c$ exceeding 9 K in the phosphorus-containing analogue Mo$_5$PB$_2$. We have synthesized polycrystalline samples of the compound, made measurements of electrical resistivity, magnetic susceptibility, and heat capacity, and performed first principles electronic structure calculations. The highest $T_c$ values occur in slightly phosphorus rich samples, with composition near Mo$_5$P$_{1.1}$B$_{1.9}$. Together with the measured properties, the calculations suggest the superconductivity in these materials may be multi-band.   

Superconductivity and ferromagnetism in Pd doped Y$_{\mathrm{9}}$Co$_{\mathrm{7}}$

The ferromagnetic superconductor Y$_{\mathrm{9}}$Co$_{\mathrm{7}}$ was chemically doped with Pd in an attempt to form Y$_{\mathrm{9}}$Co$_{\mathrm{7-x}}$Pd$_{\mathrm{x}}$ for 0\textless x\textless 0.4. The~lattice parameter $a$ does not depend on x; whereas, $c$ increases with increasing Pd content up to x $=$ 0.2, which turned out to be the palladium solubility limit. Superconductivity (T$_{\mathrm{sc}} \quad =$ 2.4 K) and ferromagnetism (T$_{\mathrm{C}} \quad =$ 4.5 K) were observed only for the parent Y$_{\mathrm{9}}$Co$_{\mathrm{7}}$ compound. For the lowest tested Pd doping level (x$=$0.05), strong enhancement of ferromagnetism is observed (T$_{\mathrm{C}} \quad =$ 9.35 K), but superconductivity is not seen above 1.8K. The Curie temperature rapidly increases from 4.5 K to about 10 K for a Pd concentration x$=$0.1 and remains almost unchanged for Y$_{\mathrm{9}}$Co$_{\mathrm{6.8}}$Pd$_{\mathrm{0.2}}$.   

Quench-condensing superconducting thin films using the Fab on a Chip approach

Micro-electromechanical systems (MEMS) being manufactured in a macroscopic fab inspires the idea of getting the process further down to fabricate even smaller structures, namely nano-structures, using MEMS. The Fab on a Chip concept was proposed based on such ideas. By implementing the final-step, additive fabrication approach, manufacturing, characterization and experiments of nano-structures are integrated \textit{in-situ}. Due to the miniature size of MEMS, the thickness precision is significantly improved while the power consumption is significantly depressed, making the quench-condensation of very thin films well controlled and easily achievable. Among various types of nano-structures, quench-condensed superconducting thin films are of great interest for physicists. Here we present such experiments done on superconducting thin films quench-condensed using the Fab on a Chip. We show that we are able to fabricate very thin films with its thickness precisely controlled, and the base temperature kept under \textasciitilde 3K during the process. The resistivity data demonstrates the high purity and uniformity of the film, as well as the annealing effect when cycling to higher temperatures. Based on the tremendous results obtained from the superconducting thin films, more complex nano-circuits can be fabricated and investigated using the Fab on a Chip, enabling a new approach for novel condensed matter physics experiments. This research is funded by the NSF through their CMMI division.   

Application of metamaterial nanoengineering to triple the superconducting critical temperature of bulk aluminum

Recent experiments have shown the viability of the metamaterial approach to dielectric response engineering for enhancing the transition temperature, Tc, of a superconductor [1]. In this report, we demonstrate the use of Al2O3-coated aluminium nanoparticles to form the recently proposed epsilon near zero (ENZ) core-shell metamaterial superconductor [2] with a Tc that is three times that of pure aluminium [3]. IR reflectivity measurements confirm the predicted metamaterial modification of the dielectric function thus demonstrating the efficacy of the ENZ metamaterial approach to Tc engineering. The developed approach advances potentials for efficient nanofabrication of bulk aluminium-based metamaterial superconductors. These results open up numerous new possibilities of considerable Tc increase in other simple superconductors. [1]. V. N. Smolyaninova et al., Scientific Reports 4, 7321 (2014) [2] I. I. Smolyaninov and V. N. Smolyaninova, Phys. Rev. B 91, 094501 (2015). [3] V. N. Smolyaninova et al., Scientific Reports 5, 15777 (2015)   

Interplay of superconductivity and magnetism in oxy-chalcogen cuprates YBaSrCu$_{3}$O$_{x}$Se$_{y}$

In YBa$_{2}$Cu$_{3}$O$_{7}$ superconductors we substituted one atom of Sr for Ba, and simultaneously doped the composition by Se. The resulting substance demonstrates two superconducting transitions: at 34K and 12K. Moreover, at 18K it reveals the Wohlleben effect, jumping from a diamagnetic to paramagnetic response, while keeping the resistance zero. At about 4K, a ferromagnetic state sets up yielding a re-entrance effect, noticeable as an upturn in magnetic susceptibility and, in some samples, in resistivity. This substance behaves very differently from the reported YBaSrCu$_{3}$O$_{7}$ or YBa$_{2}$Cu$_{3}$O$_{x}$Se$_{y}$: simultaneous presence of Se- and Sr-ions yields magnetic moments in the lattice. We present data on DC and AC magnetization, on heat capacity and resistivity in magnetic fields, on crystalline phases, as well as on composition. We also discuss possible mechanisms responsible for the observed effects.   

Novel Superoxygenated Phases in Superconducting Cuprate Thin Films

The superconducting critical temperature ($T_c$) of hole-doped cuprates tends to increase with their lattice complexity, which is generally correlated with higher states of oxidation. For $\mathrm{YBa_2Cu_3O_{7-\delta}}$ (YBCO-123), it is known that solid-state reaction in high-pressure oxygen can induce the formation of more complex and oxidized phases such as $\mathrm{Y_2Ba_4Cu_7O_{15-\delta}}$ (YBCO-247) and $\mathrm{Y_2Ba_4Cu_8O_{16}}$ (YBCO-248). In this work, we apply this superoxygenation concept of material synthesis to nanoscale thin films which, owing to their large surface-to-volume ratio, are more thermodynamically reactive than bulk samples. Epitaxial thin films of YBCO-123 were grown by pulsed laser deposition on $\mathrm{(La,Sr)(Al,Ta)O_3}$ substrates, and post-annealed in up to 500 atm of oxygen at 800C. Our post-annealed films show robust superconducting transitions with $T_c$ ranging from 80 to 93K. Transmission electron microscopy and X-ray absorption spectroscopy were used to probe the lattice structure and oxygen stoichiometry. Our measurements show clear evidence of conversion to YBCO-247 and YBCO-248 in the superoxygenated films, as well as YBCO-125, a novel YBCO phase that has three CuO chains per unit cell and potentially higher $T_c$.   

Quasi-One Dimensional Analogues of BiS$_{2}$-Based Superconductors

Many recently-reported superconductors have layered structures consisting of superconducting planes separated by insulating charge reservoir layers. Studies linking the width of the blocking layer to the critical temperature of the superconductivity onset draw a direct connection from the superconducting properties to the structure. We report three new compounds (Sr$_{2}$O$_{2}$Bi$_{2}$Se$_{3}$, Ba$_{2}$O$_{2}$Bi$_{2}$Se$_{3}$, and Sr$_{2}$O$_{2}$Sb$_{2}$Se$_{3})$ which are quasi-one dimensional analogues of the bismuth sulfide and bismuth selenide superconductors, providing a unique opportunity to study the role of dimensionality on superconductivity. The physical properties of the compounds were studied via magnetic susceptibility, thermal transport, resistivity, and heat capacity.   

Vertical Magnetic Levitation Force Measurement on Single Crystal YBaCuO Bulk at Different Temperatures

Magnetic levitation force measurements of HTS samples are performed with the use of liquid nitrogen. It is both convenient and cheap. However, the temperature of the sample cannot be changed (77 K) and there is problem of frost. So, it is necessary to build another type of system to measure the levitation force high Tc superconductor at different temperatures. In this study, we fabricated YBaCuO superconducting by top-seeding-melting-growth (TSMG) technique and measured vertical forces of them at FC (Field Cooling) and ZFC (Zero Field Cooling) regimes by using our new designed magnetic levitation force measurement system. It was used to investigate the three-dimensional levitation force and lateral force in the levitation system consisting of a cylindrical magnet and a permanent cylindrical superconductor at different temperatures (37, 47, 57, 67 and 77 K).   

Low Temperature Properties and Quantum Criticality of CrAs1-xPx single crystal.

We report a systematically study of resistivity and specific heat on phosphorus doped CrAs$_{1-x}$P$_{x}$ single crystals with x$=$0 to 0.2. With the increasing of phosphorus doping concentration x, the magnetic and structural transition temperature T$_{N}$ is suppressed. Non-fermi liquid behavior and quantum criticality phenomenon are observed from low temperature resistivity around critical doping with x$_{c}$ \textasciitilde 0.05 where the long-range antiferromagnetic ordering is completely suppressed. The low temperature specific heat of CrAs$_{1-x}$P$_{x}$ is contributed by the thermal excitation of phonons and electrons. The electronic specific heat coefficient $\gamma $, which reflects the effective mass of quasi-particles, shows maximum around x$_{c}$ \textasciitilde 0.05, also indicating the existence of quantum critical phenomenon around the critical doping. The value of Kadowaki-Woods ratio of CrAs$_{1-x}$P$_{x}$ shows no significant different from that of CrAs. Work is done in collaboration with Fukun Lin, Wei Wu, Ping Zheng, Guozhi Fan, Jinguang Cheng.