Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J52: Superconductors Phonons and Electrons |
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Sponsoring Units: DCMP DCOMP Chair: Lilia Boeri, TU-Graz Room: Mile High Ballroom 1F |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J52.00001: Superconducting properties in Li$_x$ZrNCl from first principles Paolo Zoccante, Matteo Calandra, Francesco Mauri ZrNCl is a layered band insulator. Upon Li intercalation it undergoes a metal-superconductor transition at 5$\%$ Li content. The superconducting critical temperature displays a very peculiar behaviour as it is maximal (T$_C$ =15.2K) for $\approx 5\%$ intercalation, at the edge of the insulator-superconductor transition. When the Li concentration is increased the critical temperature decreases monotonically, stabilizing at T$_C$ =11.5 K for x=0.3. We present calculations beyond DFT on Li$_x$ZrNCl as a function of doping. We show that an ultradense sampling of the Brillouin zone (both for phonon and electron momentum) is necessary to describe the phonon spectra and the electron-phonon coupling in this material. We show that marked Kohn-anomalies, overlooked in all previous calculations, occur in the phonon spectrum at different energy scales. Finally, we discuss the role of correlation effects in determining the superconducting properties of Li$_x$ZrNCl. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J52.00002: Ambipolar Transport and Gate-Induced Superconductivity in Layered Transition Metal Dichalcogenides Wu Shi, Jianting Ye, Yijing Zhang, Ryuji Suzuki, Yu Saito, Yoshihiro Iwasa Transition metal dichalcogenides (TMDs) are well known van der Waals layered materials that are easy to be exfoliated into atomically flat nano scale flakes. Owing to high efficiency of electrical double layer (EDL) dielectrics, thin flakes of TMDs have achieved high performance ambipolar transistor operation and established metallic states with high mobility, which are ideal for inducing superconductivity. Here, we report a comprehensive study of ambipolar transport behaviors in the EDL transistors (EDLTs) of MoS$_{2}$, MoSe$_{2}$ and MoTe$_{2}$ thin flakes down to 2 K. In comparison, MoSe$_{2}$ EDLT displayed a well-balanced ambipolar transistor operation while the other two showed opposite predominance in electron and hole accumulation, respectively. By modulation of carrier densities, the metal insulator transition (MIT) was observed in both electron and hole transport measurements. Particularly, superconducting transitions were reached after the formation of metallic states in the electron side. The phase diagram of transition temperature-carrier density was established and a dome-shaped structure was confirmed, revealing a universal feature of gate-induce superconductivity in layered band insulators. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J52.00003: Intertwined Order Parameters in a Charge-Ordered Superconductor Aleksej Mialitsin, Ilia Solv'yov, Anna Toth, Igor Mazin, Andriy Nevidomskyy, Brian Dennis, Girsh Blumberg We present a spectroscopic study of the low temperature state in NbSe$_2$, exhibiting charge-density wave and superconductivity orders. Raman scattering reveals that the spectrum of quasi-particle excitations out of the condensate is characterized by an energy gap derived from both order parameters in a way that suggests intertwining between them. Supported by a calculation of NbSe$_2$ Raman vertices, and by earlier photoemission studies, we conclude that in NbSe$_2$ an isotropic superconductivity interplays with a strongly anisotropic charge-density wave order on selected parts of the Fermi surface, as characterized by admixture of particle-particle and particle-hole excitations. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J52.00004: Structural transition and doping induced superconductivity in IrTe$_2$ Huibo Cao, Bryan Chakoumakos, Xin Chen, Jiaqiang Yan, Michael McGuire, Hui Yang, Radu Custelcean, Haidong Zhou, Andrew Christianson, David Singh, David Mandrus Doped IrTe$_2$ compounds are of current interest as they offer the opportunity to investigate the relationship between a structural transition and the appearance of superconductivity. Here we present the results of an investigation of the structural transition of Ir$_{1-x}$Pt$_x$Te$_2$ (x=0, 0.03, and 0.05) by X-ray and neutron diffraction. In IrTe$_2$ a structural modulation was observed with a wave vector of $k$ = (1/5, 0, 1/5) below 285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. First principles calculations suggest the local bonding instability associated with the Te 5$p$ states is likely the origin of the structural phase transition. Pt-doping (x=0.03) suppresses the structural transition down to 70 K and the superconductivity appears at 3 K. No response to onset of superconductivity was observed in the structural parameters suggesting that strong electron-lattice coupling does not play a role in IrTe$_2$. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J52.00005: Raman scattering of IrTe$_2$ Alexander Lee, Verner Thorsmolle, Sergey Artyukhin, Jun Yang, Sang-Wook Cheong, Girsh Blumberg IrTe$_2$ presents a layered compound with a triangular lattice. It is known to exhibit a first order structural phase transition at approximately 260~K which is of a first order, corresponding to a formation of a superstructure with a period of five unit cells. Using polarized Raman spectroscopy we have studied the temperature dependence of 14 observed Raman allowed phononic modes. These phonons couple strongly to this transition and one additional first order transition at approximately 170~K. In the high-temperature phase only 3 modes are observed, while below approximately 280~K all 14 modes become visible. Below approximately 170~K only 11 modes are observed. Our results shed light on the possible mechanism driving the transitions. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J52.00006: Synthesis of the $Ba_{1-x}Na_{x}Ti_{2}Sb_{2}O_{z}$ system with x= 0.00, 0.15, 0.25 and 1.00, by solid-state reaction method Carolina Ju\'arez, E. Chavira, A. Tejeda, D. Cabrero, V. Garc\'Ia-V\'azquez, J.T. Elizalde Galindo The present study of this system is a superconductor without Cu-O planes using solid-state reaction synthesis with slow cooling. The samples used were considered with an initial weight of 1 g after the losses to $CO_2$ owing to the reaction. Used ThermoGravimetric Analysis, we determined the reaction temperatures. The samples were characterized using X-Ray powder Diffraction and Scanning Electron Microscopy, expose the formation to secondary and ternary compounds, also the changes in the oxidation numbers of some initial reagents in each reaction temperature. A new contribution of this work is obtain the conditions of the monophase, $NaSbO_3$ with PDF 42-0223 in x=1.00 at 550 $^\circ$C/62 h 50 min. not reported in the literature.We observed in XRD an amorphous phase at the same temperature, which not observed at 700 $^\circ$C. By SEM we distinguish that the amorphous phase continue exist, but using XRD is not perceptible because it is in less proportion and showing a grains grow in the samples. We determine the thermodynamic conditions to obtain a $NaSbO_3$ monophase at x= 1.00, different that was be reported. This system exhibit an amorphous phase between 550 - 700 $^\circ$C. To optimize the connection between polycrystals grain pellets were manufactured at 800 $^\circ$C. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J52.00007: Pressure-induced superconductivity in $n$-type bismuth telluride Mari Einaga, Tomoko Kagayama, Katsuya Shimizu, Ayako Ohmura, Yuh Yamada Stoichiometric bismuth telluride (Bi$_2$Te$_3$), which is a $p$-type semiconductor, has the rhombohedral structure with space group $R$-3$m$ at ambient condition. We have previously reported that pressure-induced superconductivity of stoichiometric $p$-type Bi$_2$Te$_3$ occurs in the high-pressure phases which appear above 8 GPa. However Bi$_2$Te$_3$ shows the variations of the carrier types, the carrier density, and the transport properties with the atomic composition. In this study, we performed the x-ray diffraction study and the electrical resistivity measurement of Bi$_{35}$Te$_{65}$ as the $n$-type Bi$_{2}$Te$_{3}$ sample under high pressure to investigate structural phase transition and pressure-induced superconductivity. At ambient condition, Bi$_{35}$Te$_{65}$ has also the $R$-3$m$ structure. It remains stable up to 8 GPa at room temperature. The superconducting transition is observed at 6 GPa below 2.9 K. The electrical resistivity at room temperature decreases rapidly at pressures from 7 to 8 GPa, indicating the occurrence of structural phase transition. It suggests that the superconducting transition at 6 GPa occurs at the ambient pressure phase with the $R$-3$m$ structure. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J52.00008: Photon energy and carrier density dependent dynamics of the coherent A1g phonon in bismuth Crystal Bray, Eamonn Murray, Stephen Fahy, David Reis We investigate the dynamics of the coherent A1g phonon as a function of photon energy and carrier density for photo-excited single-crystal thin-film bismuth. Previous experimental and theoretical studies on group V semimetals such as bismuth show strong softening of the mode with photo-excitation associated with electronic softening and a reduction in the Peierls distortion; however, theoretical models differ on the detailed dependence for how the carriers populate the conduction band states immediately following excitation [Murray et al., Phys. Rev. B \textbf{72, }060301 (2005); Zijlstra et al., Phys. Rev. B \textbf{74, }220301 (2006); Sheu et al., Phys. Rev. B \textbf{87, }075429 (2013)]. By carefully controlling the total energy deposition and the incident photon number as a function of different pump wavelengths, we are able to test two different models for the filling near the Fermi surface: a one-chemical potential model whereby the carrier density depends on electronic temperature and a two-chemical potential model whereby the carrier density depends on the number of photons absorbed. We find evidence that neither model suffices, likely due to different relaxation mechanisms depending on which bands are involved in the initial excitation. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J52.00009: First-principles calculation of the polarization-dependent force driving the E$_\textrm{g}$ mode in bismuth under optical excitation. Eamonn Murray, Stephen Fahy Using first principles electronic structure methods, we calculate the induced force on the E$_\textrm{g}$ (zone centre transverse optical) phonon mode in bismuth immediately after absorption of polarized light. When radiation with polarization perpendicular to the c-axis is absorbed in bismuth, the distribution of excited electrons and holes breaks the three-fold rotational symmetry and leads to a net force on the atoms in the direction perpendicular to the axis. We calculate the initial excited electronic distribution as a function of photon energy and polarization and find the resulting transverse and longitudinal forces experienced by the atoms. Using the measured, temperature-dependent rate of decay of the transverse force\footnote{J.J. Li et al, Phys. Rev. Lett. 110, 047401 (2013)}, we predict the approximate amplitude of induced atomic motion in the E$_\textrm{g}$ mode as a function of temperature and optical fluence. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J52.00010: Intrinsic anisotropy of critical current in c-tilted MgB$_{2}$ thin films Anatolii Polyanskii, Dmytro Abraimov, Fumitake Kametani, David Larbalestier, Alex Gurevich, Aki Yamamoto, I. Pallecchi, Marina Putti, C. Zhuang, T. Tan, X.X. Xi The intense investigations of MgB$_{2}$ in recent years have revealed many unusual effects of the two-band superconductivity on the properties of this material. However, given the size and geometry of available single crystals, the critical current along the c-direction, mostly affected by the anisotropy of the two bands, has not been thoroughly investigated. To address the issue of the c-axis current transport, we succeeded to measure J$_{\mathrm{c}}$ along the c axis by growing epitaxial films with tilted c-axis, which offers a unique possibility of probing both the in-plane and the out-of-plane critical currents. We show that anisotropic band parameters determine the anisotropy of vortex pinning with respect to the current direction. This yields a temperature and field dependent anisotropy of the critical current density J$_{\mathrm{c}}$ related to the anisotropic penetration depths at low temperature and low field and to the ratio of $\sigma $ band effective masses along the c and ab crystalline axes at temperatures close to T$_{\mathrm{c}}$ or at applied fields larger than the virtual upper critical field of the $\pi $ band H $\approx $ 0.2T. Our results suggest that the out-of-plane current transport could set the ultimate J$_{\mathrm{c}}$ limit in randomly oriented MgB$_{2}$ polycrystals. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J52.00011: THz pump-THz probe Cooper pair dynamics in MgB$_{2}$ Jingdi Zhang, Teng Tan, Mengkun Liu, Wenqing Dai, Kun Geng, Qi Li, Xiaoxing Xi, Richard Averitt THz pump-THz probe spectroscopy is used to study non-equilibrium superconducting carrier dynamics in MgB$_{2}$ thin films. An intense THz pump pulse resonantly breaks Cooper pairs, exciting quasiparticles across the lowest energy superconducting gap (2$\Delta _{\mathrm{0}}\approx $3.6meV corresponding to $\sim$ 0.8THz). The condensate and quasiparticle dynamics are temporally resolved by measuring the real and imaginary part of THz conductivity. Following THz excitation, the condensate density is strongly suppressed on a picosecond time scale coinciding with the emergence of a Drude peak. These results are compared with optical pump-THz probe experiments where, consistent with previous measurements, much slower Copper pair-breaking dynamics are observed. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J52.00012: Superconducting graphene sheets in CaC$_{6}$ enabled by phonon-mediated interband interactions Shuolong Yang, Jonathan Sobota, Chris Howard, Chris Pickard, Makoto Hashimoto, Donghui Lu, Sung-Kwan Mo, Patrick Kirchmann, Zhi-Xun Shen The superconducting mechanism of graphite intercalation compounds has been under intense debate. To reveal this mechanism, we studied a prototypical compound CaC$_{6}$ using angle-resolved photoelectron spectroscopy. Both the calcium-derived and graphene-derived bands were clearly resolved. We performed analysis on the superconducting gaps and electron-phonon coupling constants. We will also discuss the important implications in fabricating superconducting graphene devices. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J52.00013: Dynamical Jahn-Teller Effect and Antiferromagnetism in insulating Cs$_3$C$_{60}$ Naoya Iwahara, Liviu Chibotaru The dynamical Jahn--Teller effect on fullerene sites in insulating Cs$_3$C$_{60}$ is investigated fully {\it ab initio} [1]. The vibronic excitations of rotational type are at $\ge$ 65 cm$^{-1}$, while the net kinetic contribution to the Jahn--Teller stabilization energy constitutes ca 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides, therefore, free rotations of deformations take place independently on each C$_{60}$. The latter destroy the orbital ordering and establish a conventional exchange interaction between $S=1/2$ on fullerene sites. The corresponding exchange model is derived and predicts the N\'{e}el temperature for A15 Cs$_3$C$_{60}$ close to experiment. \\[4pt] [1] N. Iwahara and L. F. Chibotaru, Phys. Rev. Lett. {\bf 111}, 056401 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J52.00014: Theory of Jahn Teller signatures in the infrared absorption of C$_{60}^{3-}$ S. Shahab Naghavi, Michele Fabrizio, Erio Tosatti Among the molecular superconductors, trivalent fullerides such as Cs$_{3}$C$_{60}$, with three folded degenerate HOMO and a fully ordered pressure induced superconductor-insulator are still intriguing. The orbital degeneracy of the fulleride ion C$_{60}^{-3}$ implies that besides a Jahn-Teller distorted state with S=1/2 and high-lying spin (S=3/2) excitation known from NMR, another undetected orbital excitation with S=1/2 should exist. Building upon accurate density hybrid functional theory calculations where properties such as the infrared (IR) spectrum and its Jahn-Teller features are well described, we extracted the {\sc ab-initio} orbital and spin spectrum of a C$_{60}^{-3}$ ion in different spin and orbital states including a new low lying L=2 S=1/2 excitation. Despite a Jahn-Teller distortion so small to be observable in its IR spectrum, this state is found to gain a large zero-point energy, placing it just above the L=1, S=1/2 ion ground state, and way below the L=0, S=3/2 high lying excitation. We can now elegantly explain the surprising early thermal disappearance of the low-temperature Jahn-Teller IR spectral features and splitting without a concurrent rise of spin susceptibility that would instead be required by population of the high spin S=3/2 excitation. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J52.00015: Phonons in solid picene at high pressures Lilia Boeri, F. Capitani, M. Hoeppner, B. Joseph, G.A. Artioli, L. Baldassarre, A. Perucchi, M. Piccinini, S. Lupi, P. Dore, L. Malavasi, P. Postorino Intercalated hydrocarbons have attracted considerable interest as a new class of superconductors. Calculations of the $e-ph$ interaction in different approximations yield conflicting results on the role of inter and intra-molecular vibrations in the pairing. We present an experimental and theoretical study of the phonon spectrum of solid picene under high pressure. We introduce a new theoretical analysics, based on the projection of phonon eigenvectors, to quantify the increase of intermolecular character under pressure. F. Capitani et al., Phys. Rev. B 88, 144303 (2013). [Preview Abstract] |
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