Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F8: Superconductivity: Novel Superconductors |
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Sponsoring Units: DCMP DMP Room: 304 |
Tuesday, March 15, 2016 11:15AM - 11:27AM |
F8.00001: Molecular orbital polarization in Na$_2$Ti$_2$Sb$_2$O: a scenario to metal-to-metal phase transition without spontaneous symmetry breaking Heung-Sik Kim, Hae-Young Kee We suggest a scenario of partial Fermi surface (FS) gapping related to metal-to-metal phase transition without a spontaneous symmetry breaking. This theory is applied to Na$_2$Ti$_2$Sb$_2$O, where the density polarization of spin-orbital entangled molecular orbitals occurs due to spin-orbit coupling. This is further enhanced by electronic correlations of Ti d-orbitals. Sharp increase of the polarization happens above a critical electronic interaction strength which then gaps out a part of FS made of d-orbitals, while the rest of FS associated with Sb p-orbitals remain almost intact. Experimental implications to test our proposal are also discussed. [Preview Abstract] |
Tuesday, March 15, 2016 11:27AM - 11:39AM |
F8.00002: Pressure dependence of the magnetic ground states in MnP Sachith Dissanayake, Masaaki Matsuda, J.-G. Cheng, F. Ye, S. Chi, J. Ma, H. D. Zhou, J.-Q. Yan, S. Kasamatsu, O. Sugino, T. Kato, K. Matsubayashi, T. Okada, Y. Uwatoko The newly discovered superconductor MnP shows a ferromagnetic order below T$_{\mathrm{C\thinspace }}\approx $ 290 K followed by a helical order below T$_{\mathrm{s}}\approx $ 50 K in ambient pressure. An antiferromagnetic order is suggested in the vicinity of the pressure induced superconducting phase. We have performed single crystal neutron diffraction experiments to determine the magnetic structure under pressure. Both T$_{\mathrm{C}}$ and T$_{\mathrm{s}}$ are gradually suppressed with increasing pressure and the helical order disappears at 1.2 GPa. At intermediate pressures of 1.8 and 2.0 GPa, the ferromagnetic order first develops and is gradually suppressed below a characteristic temperature. At 4 GPa no magnetic signal was observed down to 3.5 K. Our results suggested that the new magnetic phase in the vicinity of the superconducting phase is in a short-ranged magnetic state due to frustration or in an itinerant magnetic state, where the itinerant small Mn moments are weakly interacting. [Preview Abstract] |
Tuesday, March 15, 2016 11:39AM - 11:51AM |
F8.00003: Chemical pressure effect on dynamic spin properties in CrAs M. Matsuda, M. B. Stone, J.-G. Cheng, W. Wu, F. Lin, J. L. Luo, J.-Q. Yan, K. Matsubayashi, Y. Uwatoko CrAs is an antiferromagnetic metal, which shows a helical spin structure accompanied by an abrupt lattice expansion at $T\rm_{N}$$\sim$260 K in ambient pressure. With applying pressure, the magnetic transition is suppressed and superconductivity appears with a maximum transition temperature of $\sim$2 K. Since Cr has the spin degree of freedom, elucidating the magnetic contribution to the superconductivity is crucial to understand the pairing mechanism. However, inelastic neutron scattering (INS) measurement under high pressure is challenging due to sample space limitation. Therefore, we studied chemical pressure effect by substituting As by P, which is found to be almost the same as the external pressure. We performed INS experiments in undoped and P-doped CrAs using powder samples. The results in the P-doped CrAs clearly indicate that the antiferromagnetic fluctuations still remain above the critical P content, where the long range magnetic order is suppressed, suggesting a coupling between the magnetism and the superconductivity. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:03PM |
F8.00004: Reversal of the Upper Critical Field Anisotropy and Spin-Locked Superconductivity in K$_{2}$Cr$_{3}$As$_{3}$ Fedor F. Balakirev, Tai Kong, Marcelo Jaime, Ross D. McDonald, Charles H. Mielke, Alexander Gurevich, Paul C. Canfield, Sergey L. Bud'ko We report the first measurements of the anisotropic upper critical field $H_{c2}(T)$ for K$_{2}$Cr$_{3}$As$_{3}$ single crystals up to 60 T and $T$\textgreater 0.6K. $H_{c2}(T)$ was determined via resistivity and proximity detector oscillator techniques. Our results show that the upper critical field parallel to the Cr chains,$ H_{c2\parallel }$, exhibits a paramagnetically-limited behavior, whereas no evidence of paramagnetic pair breaking was observed with field perpendicular to the Cr chains. As a result, the curves $H_{c2\bot }$ ($T)$ and $H_{c2\parallel }$ ($T)$ cross at $T$\textasciitilde 4 K, so that the anisotropy parameter $\gamma (T)= H_{c2\bot }$ /$H_{c2\parallel }$ increases from $\gamma $\textasciitilde 0.35 near $T_{c}$ to $\gamma $\textasciitilde 1.7 at 0.6 K. This behavior of $H_{c2}(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains. [Preview Abstract] |
Tuesday, March 15, 2016 12:03PM - 12:15PM |
F8.00005: Material Specific Rational Design of A$_1$B$_2$C$_3$O$_7$ High-Tc Superconductors \textit{without} Copper [A, B, C = Cations] O'Paul Isikaku-Ironkwe, Michael J. Schaffer Soon after the discovery of YBa$_2$Cu$_3$O$_7$ with Tc $=$ 93K, a similar structured system with Ag replacing Cu was discovered with a Tc $=$ 50K. Also, the discovery of Ba$_0.6$K$_0.4$BiO$_3$ with Tc $=$ 30K indicated that Cu was not indispensable for high temperature superconductivity (HTSC). Latter, the discoveries of the Pnictide and Chalcogenide high-Tc superconductors confirmed those earlier experimental indications. Using our recently developed Material Specific Characterization Dataset (MSCD) model for analysis and design of superconductors, we have computed many designs that satisfy the MSCD characteristics of YBa$_2$Cu$_3$O$_7 $as a design model. Our design recognizes the valence state characteristics that make YBa$_2$Cu$_3$O$_6 $a semiconductor, while YBa$_2$Cu$_3$O$_7 $is a superconductor. Here we present ten material specific rational design examples of potential A$_1$B$_2$C$_3$O$_7$ HTSCs without Cu, using the YBa$_2$Cu$_3$O$_7$ design model. This MSCD design model opens the possibility for search and discovery of high-Tc oxide superconductor systems without copper. [Preview Abstract] |
Tuesday, March 15, 2016 12:15PM - 12:27PM |
F8.00006: Microscopic Origin of the Neutron Spin Resonance in Heavy Fermion Superconductor Ce1-xYbxCoIn5 Yu Song, John Van Dyke, I. K. Lum, B. D. White, L. Shu, Sooyoung Jang, A Schneidewind, Petr Cermak, Y. Qiu, M. B. Maple, Dirk K. Morr, Pengcheng Dai We have systematically studied the evolution of the neutron resonance mode in Ce1-xYbxCoIn5 (x $=$ 0, 0.05, 0.3) with neutron scattering. We uncover clear dispersive feature of the mode and show that it is quite robust to disorder due to doping. Our results suggest that the resonance in Ce1-xYbxCoIn5 is a paramagnon reminiscent of spin waves in CeRhIn5. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F8.00007: Consistency of measured phase boundaries of the FFLO superconducting phase for different materials and types of probes Charles Agosta, Nathanael Fortune, Scott Hannahs, Ju-Hyun Park, John Schleuter, Lucy Liang, Shuyao Gao, Logan Bishop-Van Horn, Max Newman, Shuyao Gu, Lucy Liang New magnetocaloric and specific heat measurements of the high field superconducting state in the organic superconductor $\kappa$- (BEDT-TTF)$_2$Cu(NCS)$_2$ are compared to rf penetration depth, magnetic torque, and NMR measurements. The position of the phase lines separating the uniform superconducting state with the FFLO state and the normal state are mostly in good agreement with each other. The order of the phase transitions can only be determined from the calorimetric measurements and will be compared to theory. Results from other organic superconductors show that there is universal behavior. As an example, the distance between the lower and upper magnetic field phase line containing the FFLO state is proportional to the upper critical field. The position of the lower phase line, the Clogston Chandrasakar paramagnetic limit, will be compared to semi empirical calculations based on the specific heat for five different superconductors. [Preview Abstract] |
Tuesday, March 15, 2016 12:39PM - 12:51PM |
F8.00008: Magnetocaloric Evidence for FFLO Superconductivity in $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ Nathanael Fortune, Charles Agosta, Scott Hannahs, Ju-Hyun Park, Shuyao Gu, Lucy Liang, John Schleuter We present new magnetocaloric and calorimetric measurements of the high field superconducting state in the layered structure superconductor $\kappa$ - (BEDT-TTF)$_2$Cu(NCS)$_2$. The strongly field-orientation dependent phase transition between the low field superconducting state and high field superconducting states is first order and is nearly temperature independent, occurring at the Clogston-Chandrasakar paramagnetic limit $H_p$. Magnetocaloric measurements $dT/dH$ as a function of magnetic field reveal that the system becomes strongly paramagnetic at the cross over from the low field to high field state. At lower temperatures, we are able to resolve small changes at the phase boundary due to the absorption/release of latent heat when increasing/decreasing field, indicating that the high field state is higher entropy than the low field state. These results provide strong new evidence for the formation of paramagnetic spin domains within an inhomogeneous FFLO superconducting state. They also allow us to rule out alternative explanations involving the formation of spin density waves within a homogenous superconducting state. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F8.00009: Charge fluctuations and superconductivity in organic conductors: the case of $\beta^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ Georgios Koutroulakis, H. Kuhne, H.-H. Wang, J. A. Schlueter, J. Wosnitza, S. E. Brown Superconductivity in most organic charge transfer salts is considered magnetically mediated, in part due to the proximity to antiferromagnetic ground states, as well as the preponderance of spin fluctuations in their normal state. An alternative proposal is based on mediation by charge fluctuations, close to collapsed charge-ordered insulating states. The all-organic salt $\beta^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, which undergoes a superconducting transition at $T_{\rm{C}}$=4.5K, has been suggested as a candidate material for the realization of charge-fluctuation pairing. Here, we report on a detailed $^{13}$C NMR study examining the normal and superconducting states of this material, and we discuss the results in the context of the proposal for charge-fluctuation driven superconductivity. [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F8.00010: Superconducting Properties of a Nanoparticle Assembly of the Organic Conductor (TMTSF)2ClO4 Laurel Winter, Eden Steven, James Brooks, Shermane Benjamin, Ju-Hyun Park, Dominique de Caro, Christopher Faulmann, Lydie Valade, Kane Jacob, Imane Chtioui, Belen Ballesteros, Jordi Fraxedas While the study of thin-film and nanoparticle geometries on semiconductor devices, type-I elemental superconductors, and even single-molecular magnet materials have been explored, progress on thin-film and nanoparticle organic superconductors -- in particular charge-transfer organic salts -- has remained elusive. Recent refinements of synthesis conditions have produced nanoparticles of the Bechgaard salt (TMTSF)$_2$ClO$_4$. High resolution TEM studies have determined these nanoparticles are approximately 3-5 nm in size, which form nanoparticle clusters that are on average 34 nm in size$^1$. In order to investigate the properties of these nanoparticles, randomly oriented assemblies were studied in magnetic fields up to 16 T, using a high sensitivity inductive method in a dilution refrigerator, the results of which show that the ground-state properties of the nanoparticle assembly compares favorably with the bulk-single-crystal material$^2$. $^1$ D. de Caro \textit{et al.}, \textit{Eur. J. Inorg. Chem.}, \textbf{2014}, 4010 (2014). $^2$ L. E. Winter \textit{et al.}, \textit{Phys. Rev. B.}, \textbf{91}, 035437 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F8.00011: Realization of insulating state and superconductivity in Rashba semiconductor BiTeCl Jianjun Ying, Viktor Struzhkin, Alexander Goncharov, Ho-Kwang Mao, Fei Chen, Xian-Hui Chen, Alexander Gavriliuk, Xiao-Jia Chen Measurements of the resistivity, Hall coefficient, and Raman spectroscopy are performed on a Rashba semiconductor BiTeCl single crystal at high pressures up to 50 GPa. We find that applying pressure first induces the theoretically predicted insulating state followed by a superconducting phase with the insulating normal state. Upon heavy compression, another different superconducting phase is entered with the metallic normal state. The dome-like evolution of the superconducting transition temperature with pressure is obtained with the crossover from the electron to hole carriers across the boundary of the two superconducting phases. These findings imply the possible realization of topological state of the insulating and superconducting phases in this material. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F8.00012: MgB$_{2\, }$Ultrathin$_{\, }$Films Fabricated by Hybrid Physical Chemical Vapor Deposition and Subsequent Ion Milling. Narendra Acharya, Matthaeus Wolak, Teng Tan, Daniel Cunnane, Boris Karasik, Xiaoxing Xi Hot electron bolometer (HEB) mixers are a great tool for measuring high-resolution spectroscopy at Terahertz frequencies. MgB$_{2\, }$offers a higher critical temperature (39 K) compared to commonly used Nb and NbN and boasts a shorter intrinsic electron-phonon relaxation time, giving rise to a broader intermediate frequency (IF) bandwidth. We have fabricated high quality ultrathin MgB$_{2\, }$films using hybrid physical-chemical vapor deposition (HPCVD) and employing ion milling to achieve thickness down to 2 nm. The thinnest achieved films show high $T_{c}_{\, }$of 28 K with residual resistivity below 28 \textmu $\Omega $cm and high critical current \textit{Jc}$_{\, }$of 1x10$^{6}$ A/cm$^{2\, }$at 20~K. As a result of the employed low angle ion milling process, the films remain well connected even after being thinned down since the initial thick films offer a better connectivity than as-grown thin films. The established process offers a way to realize MgB$_{2}$ based HEB mixers of extremely low thickness and therefore small local oscillator power requirements and increased IF bandwidth. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F8.00013: Epitaxial growth of MgB$_2$ films at ambient temperature Hiroaki Shishido, Takuya Yoshida, Takatoshi Nakagami, Takekazu Ishida We grew crystalline MgB$_2$ thin films using molecular beam epitaxy at a low substrate temperature of 110\,$^\circ$C under an ultrahigh vacuum of about 10$^{-6}$\,Pa. MgB$_2$ thin films were deposited on the (001) surface of a 4H-SiC substrate with an epitaxial Mg buffer layer. The epitaxial growth was confirmed by X-ray diffraction measurements. MgB$_2$ thin films show a sharp superconducting transition at 27.2\,K, with a relatively narrow superconducting transition width $\Delta T_{\rm c}$ = 0.9\,K. The growth temperature was lower than any in prior reports on superconducting MgB$_2$ thin films. The presence of the epitaxial Mg buffer layer is crucial for reducing the epitaxial temperature. [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F8.00014: Unconventional superconductivity in half-Heusler semimetals Philip Brydon, Limin Wang, Michael Weinert, Daniel Agterberg We consider the superconductivity of the topological half-Heusler semimetals YPtBi and LuPtBi, where pairing occurs between $j = 3/2$ quasiparticles. This permits Cooper pairs with quintet or septet total angular momentum, in addition to singlet and triplet states. Purely on-site interactions can generate unconventional (quintet) time-reversal symmetry-breaking states with topologically nontrivial point or line nodes. Furthermore, due to the broken inversion symmetry in these materials, the usual $s$-wave singlet state can mix with a $p$-wave septet state, also with topologically stable line-nodes. [Preview Abstract] |
Tuesday, March 15, 2016 2:03PM - 2:15PM |
F8.00015: Superfluid Density and Flux-Flow Resistivity Measurements of Multiple-Band Superconductor $\beta$-PdBi$_{2}$ Tatsunori Okada, Yoshinori Imai, Atsutaka Maeda $\beta$-PdBi$_2$ ($T_{\rm c}^{\rm max}=5.4$ K) is a newcomer of the multiple-band superconductors, revealed by the specific heat and the upper critical field measurements [1], and the angle-resolved photoemission spectroscopy [2]. In addition, authors of ref. [2] observed the spin-polarized band dispersion and proposed that $\beta$-PdBi$_2$ is a candidate of topological superconductor. However, there is less information on superconducting properties so far. In order to clarify the superconducting gap function, we measured the temperature ($T$) and magnetic field ($B$) dependence of microwave complex conductivity of $\beta$-PdBi$_2$ single crystals. We found that the superfluid density exhibits the thermally activated $T$ dependence, manifesting the absence of nodes in the superconducting gaps. We also found that the flux-flow resistivity increased with $B$ with downward-convex shape. Based on some theories, we considered that such a behavior originated from the backflow of supercurrents around vortices reflecting rather small Ginzburg-Landau parameter ($\kappa\simeq5$). [1] Y. Imai {\it et al}., JPSJ {\bf 81}, 113708 (2011). [2] M. Sakano {\it et al}., arXiv:1505.07231. [Preview Abstract] |
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