Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A11: Focus Session: MgB2-like: Exotic Behavior in MgB2-like Materials |
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Sponsoring Units: DMP Chair: Xiaoxing Xi, Pennsylvania State University Room: Morial Convention Center RO9 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A11.00001: High quality MgB$_{2}$ thin films in clean and dirty limit realized via HPCVD Chenggang Zhuang, Sheng Meng, Qingrong Feng, Zizhao Gan, Huan Yang, Ying Jia, Haihu Wen, Xiaoxing Xi Pure and carbon-doped MgB$_{2}$ thin films are fabricated using hybrid physical-chemical vapor deposition. Excellent properties are obtained in the pure samples, including T$_{c}\sim $41.4K, \textit{$\rho $}${\rm g}$(42K)$\sim $0.3$\mu \Omega$ cm, which indicates that films are in the clean limit. Ultrahigh critical current density, up to 1x10$^{8}$A/cm$^{2}$ at 5K and zero fields, approaching the theoretical deparing current of MgB$_{2}$, was observed in a 150nm-wide microbridge, in agreement with the results derived using a Bean Model from the magnetization measurements. Large normal state magnetoresistance before the transition, about 40{\%} at 7T, and absence of flux jump at low temperature and low field regions in the magnetization curves strongly support that the film is very clean. To enhance upper critical field and flux pinning, carbon doping is employed and realized in a dual-heater HPCVD setup from the thermal decomposition of CH$_{4}$. $\mu _{0}$H$_{c2}$(0)$\sim $54T, J$_{c}$(5K,6T)$\sim $3x10$^{5}$A/cm$^{2}$, as well as T$_{c}$(0)$\sim $34K and \textit{$\rho $}${\rm g}$(42K)$\sim $47$\mu \Omega $cm are achieved in the carbon-doped films. By controlling the carbon concentration, MgB$_{2}$ films in dirty limit can be prepared. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A11.00002: Superconducting properties of MoN microfibers and thin films Amar Karki, Edem Okudzeto, Phil Adams, David Young We present the superconducting transport properties measurements on polycrystalline MoN synthesized in the form of thin films and microfibers. The samples were prepared by heating Mo films and Mo-coated carbon fibers to temperatures between 850 $^{o}$C and 1000 $^{o}$C for different time periods in a flowing stream of ammonia (NH$_{3})$ gas under atmospheric pressure. The microfibers and thin films had a transition temperature Tc $\sim $ 12.2 K, which is substantially higher than that reported for MoN films grown by other methods. We also present critical current measurements on microfibers which consisted of a 50-nm thick layer of polycrystalline MoN synthesized directly onto 5 micron-diameter carbon fibers. The microfibers supported current densities in excess of 10$^{7}$ A/cm$^{2}$ well below Tc. Near Tc, Jc was well described by the power law [1-(T/Tc)$^{2}$]$^{3/2}$. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A11.00003: Theory of Raman Scattering from Leggett's Collective Mode in a Multiple Band Superconductor: Application to MgB$_{2}$ Miles Klein Using an extension of BCS theory to a two-band superconductor, Leggett showed that if the relevant parameters obeyed certain conditions a collective mode would exist corresponding to the counter flow of the two condensates.$^{1}$ I have extended earlier work on electronic Raman in superconductors$^{2}$ to the multiple band case in order to incorporate Leggett's theory. The following effects have been included: (a) Vertex correction in the particle/hole channel where the Raman vertex acts. (b) Realistic parameters that apply to MgB$_{2}$ yielding a counter flow mode that decays into the pair-breaking continuum associated with the lower gap $\pi $ band. (c) Large finite wave-vector effects due to the relatively large Fermi velocity of the $\pi $ band. (d) Integration over the wave-vector in part (c) necessitated by the exponential decay of the photon fields traveling into and out of the metallic sample. A comparison to the results of Blumberg$^{3}$ will be given. $^{1}$A.J. Leggett, Progr. Theor. Phys. \textbf{36}, 901 (1966). $^{2}$M.V. Klein and S.B. Dierker, Phys. Rev. B\textbf{29}, 4976 (1984). $^{3}$G. Blumberg et al., Phys. Rev. Lett. \textbf{99}, (2007); arXiv:0710.2803. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A11.00004: MgB$_{2}$: Novel properties due to multibands Invited Speaker: About 40 years ago A.J. Leggett proposed a new collective mode arising from cross-tunneling of Cooper pairs residing on different Fermi surfaces of a multiband superconductor: Leggett's collective mode is caused by a counter flow of the interacting superfluids leading to small fluctuations of the relative phase of the condensates while the total electron density is locally conserved.\footnote{A.J. Leggett, Progr. Theor. Phys. \textbf{36}, 901 (1966).} Here we present direct spectroscopic observation of the Leggett's excitation in the MgB$_{2}$ superconductor containig two pairs of Fermi surfaces resulting from $\pi$- and $\sigma$-bands. Electronic Raman scattering studies have revealed three distinct superconducting (SC) features: (i) a clean threshold of Raman intensity at 4.6 meV consistent with the $\pi$-band SC gap; (ii) the SC pair breaking coherence peak at 13.5 meV consistent with excitations above the $\sigma$-band gap; and (iii) the SC collective mode at 9.4 meV which we assign to an excitation first discussed by Leggett.\footnote{G. Blumberg et al., Phys. Rev. Lett. \textbf{99} (2007); \urllink{arXiv:0710.2803}{http://arxiv.org/pdf/0710.2803}.} Our calculation of the Raman response function for MgB$_{2}$ superconductor based on multiband interaction matrices by first principle computations show good agreement with spectroscopic observations. The temperature and field dependencies for all three features (i) -- (iii) have been established;\footnote{G. Blumberg et al., Physica (Amsterdam) \textbf{456C}, 75 (2007).} the effects of magnetic field on the pair cross-tunneling in multiband system will be discussed. In addition, anharmonicity and superconductivity-induced self-energy effects for the E$_{2g}$ boron stretching phonon have been studied.\footnote{A. Mialitsin et al., Phys. Rev. B \textbf{75}, 020509(R) (2007).} We show that anharmonic two-phonon decay is mainly responsible for the unusually large linewidth of the E$_{2g}$ mode. We observe 2.5\% hardening of the E$_{2g}$ phonon frequency upon cooling into the SC state and estimate the electron-phonon coupling strength associated with this renormalization. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A11.00005: Microwave properties of epitaxial MgB2 films and micro-bridges M.V. Costache, G.X. Miao, J.S. Moodera The high transition temperature and simple AlB$_{2}$ structure make the MgB$_{2}$ superconductor a promising new material for application in superconducting electronic devices. In order to reach this goal, in addition to the development of MgB2 Josephson Junctions (JJ), we have explored the superconducting properties of lithographically patterned micron sized bridges in high quality epitaxial MgB2 films. These thin film micron size wire bridges show JJ characteristics. The current-voltage (I-V) characteristics show the expected JJ behavior as a function of temperature, magnetic field and microwave radiation including hysteresis. Large change in the hysteresis behavior is observed when the microwave power is varied. The large hysteretic I-V can have potential as a memory element. The details will be discussed. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A11.00006: Force-Detected NMR Study of Single-Crystal MgB$_2$ using Ultrasensitive Oscillators* Han-Jong Chia, Mark Monti, Samaresh Guchhait, John Markert, Jae-Hyuk Choi, Sung-Ik Lee MgB$_2$ is a unique superconductor with a relatively high $T_c$ and two nearly independent electronic bands. An NMR study of $^{11}$B in MgB$_2$ using polycrystalline samples [1] did not observe any two-band effects, nor a Hebel-Slichter coherence peak, possibly due to large $H_{c2}$ anisotropy (and thus a distribution of $T_c$'s). Anisotropic NMR studies of MgB$_2$ have proven difficult due to the small size ($\sim 10\ \mu$m) of high-quality crystals. A large-single-crystal conventional NMR study [2] could not probe the superconducting state due to line broadening. We have set out to use the exquisite sensitivity of Nuclear Magnetic Resonance Force Microscopy (NMRFM) to probe the behavior of $^{11}$B in single crystal MgB$_2$. We have fabricated ultrasensitive mechanical oscillators using e-beam lithography to facilitate detection of the weak $^{11}$B resonance; these have resonance frequencies of 1--10 kHz, spring constants of $\sim 10^{-4}$ N/m, and quality factors $>$3000 at 77 K. We report our initial detection of the $^{11}$B nuclear resonance and our plans to study relaxation rates in single crystal MgB$_2$. [1] H. Kotegawa {\it et al.}, {\it Phys.~Rev.~Lett.} {\bf 87}, 127001 (2001). [2] S.~Str\"assle {\it et al.}, {\it Physica C} {\bf 466}, 168 (2007). *Supported by NSF DMR-0605828 and DGE-0549417. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A11.00007: High pressure synthesis of single crystalline MgB$_{2}$ M. Tillman, G. Lapertot, R. Prozorov, C. Martin, S.L. Bud'ko, P.C. Canfield We report the results of single crystal growth of MgB$_{2}$.~ A high pressure furnace, using a 19 mm truncation edge length, cubic anvil capable of reaching 3.3 GPa and 2000 C was used to grow crystals of MgB$_{2}$ out of the Mg-B-N ternary.~ Design, setup, and calibration will be discussed as well as correlations between pressure and temperature profiles and crystal size.~ Results of measurements of penetration depth and H$_{c2}$(T) on single crystals will be shown as well as the results of initial doping studies. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A11.00008: Electric-field-effect studies of atomically thin flakes of superconducting NbSe$_2$ prepared by mechanical exfoliation Neal Staley, Jain Wu, Peter Eklund, Ying Liu, Linjun Li, Zhuan Xu Recent years have yielded many studies on electric field modulated superconductivity. In order to achieve the high carrier density changes needed to modulate superconductivity, two main approaches have been tried, ultra thin films grown by in situ quench deposition or few-layer single crystalline films of superconductors grown by pulsed laser deposition or molecular beam epitaxy. However in both cases, difficulties have been encountered, ultra thin films are subject to large amounts of disorder, and it is difficult if not impossible to grow single layered superconductors. Using a simple micromechanical exfoliation technique, we are able to create single crystal single layered graphite, featuring a linear density of states, allowing its physical properties to be tuned by gate voltage. Using this mechanical exfoliation procedure we fabricated ultra thin single crystalline NbSe$_2$ flakes ranging from double layered to many layered as estimated using an optical technique correlated to AFM and Raman spectroscopy measurements. Using a lithography-free, ``all dry'' process we fabricated devices showing modulated T$_c$ with applied gate voltage as well as a superconductor insulator transition tuned by the number of layers. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A11.00009: Low Energy Electronic Structure of an Excitonic CDW Melted Novel Superconductor Dong Qian, David Hsieh, Lewis Wray, Yuqi Xia, E. Morosan, R.J. Cava, M.Z. Hasan A superconducting (SC) state has very recently been observed upon successful doping of the charge-density wave (CDW) ordered triangular lattice TiSe$_2$ with copper. Using angle-resolved photoemission spectroscopy (ARPES) we studied the doping evolution of the electronic structure of Cu$_x$TiSe$_2$. The momentum space locations of the doped electrons that form the Fermi sea of the parent superconductor is identified. With increasing electron doping, we observe a significant rise of chemical potential which is found to destabilize the long range CDW order. At the same time the emergence of a large density of states in the form of a narrow electron pocket near the L- points of the Brillouin zone favors the onset of superconductivity within the BCS-Eliashberg scenario. With doping, we find that the kinematic nesting volume increases whereas the coherence of the CDW order sharply drops. The k- space electron distributions highlight the unconventional interplay of CDW to SC cross-over achieved through non-magnetic copper doping. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A11.00010: Sandwich-type MgB$_{2}$ tunnel junctions Ke Chen, Yi Cui, Qi Li, Xiaoxing Xi Properties of superconductors, such as band gap, density of states (DOS) of quasiparticles, and effective phonon spectra, can be studied by electrical tunneling with a high energy resolution of several $k_{B}T$. Sandwich-type MgB$_{2}$ tunnel junctions with Pb or Ag as the counter-electrodes were made on Hybrid Physical-Chemical Vapor Deposition MgB$_{2}$ films with thermal oxide tunnel barrier. The tunnel junctions exhibit very small subgap current at 4.2 K. Fit to the BCS DOS relation, the $\pi $-band and $\sigma $-band gaps of MgB$_{2}$ are found to be 2.3 and 7.4 meV at 4.2 K respectively. Josephson tunneling was observed with $I_{c}R_{n}$ product following the BCS temperature dependence relation, and the critical current showing a Fraunhofer pattern modulated by the applied magnetic field. Effective phonon modes are revealed at 42, 61, and 74 meV. These superb tunnel junctions give us deep insights into the properties of MgB$_{2}$. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A11.00011: Hybridized Abrikosov Flux-lines and Pancake Vortices in Two-band Superconductors with Mixed Dimensionality K. Tanaka, M. Eschrig We study electronic structure and thermodynamic properties of a two-band superconductor, in which one band is ballistic and quasi-two dimensional (2D), and the other is diffusive and three dimensional (3D). We assume that superconductivity in the 3D diffusive band is ``weak'', i.e., mostly induced, as is the case in MgB$_2$. Hybridization with the ``weak'' 3D diffusive band has significant and intriguing influence on the electronic properties of the ``strong'' 2D ballistic band. In particular, the effects of Coulomb interactions in the diffusive band and unusual Kramer-Pesch effect are examined. Furthermore, based on a circular-cell approximation within the quasiclassical theory of superconductivity, we explore the effects of magnetic field on vortex structure in such a two-band superconductor. We discuss hybridization of Abrikosov flux lines in the 3D diffusive band with pancake vortices in the 2D ballistic band. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A11.00012: Low-temperature transport in highly disordered films of superconducting magnesium diboride Nicholas Breznay, Aharon Kapitulnik Many features make magnesium diboride an interesting model system for understanding the behavior of two dimensional BCS superconductors, including its sensitivity to disorder and low spin-orbit coupling. We study the transport behavior of highly disordered magnesium diboride films, and will review preliminary low-temperature magnetoresistance measurements in the regime of high magnetic fields. We connect these results to recent work on multilayered films prepared using a similar process and also to similar work on other model systems, and review prospects for future study. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A11.00013: T$_{c}$ vs Isotopic Mass and vs Residual Resistivity Investigation in MgB$_{2}$ Marina Putti, Matteo Tropeano, Paolo Brotto, Carlo Ferdeghini, Enrico Galleani , Pietro Manfrinetti, Andrea Palenzona Almost five years after the discovery of superconductivity in MgB$_{2}$ the isotope effect on T$_{c}$ is not yet understood (M. Calandra et al, Physica C456, 38 (2007) and references therein). The isotope effect is mainly due to the B atoms reflecting the important role of B vibrations in determining T$_{c}$. Detailed two bands calculation leads to $\alpha $(B) of the order of 0.4--0.45, in disagreement with experiments which evaluated $\alpha $(B) = 0.30. Anharmonicity was proposed as a possible explanation for the reduced B isotope coefficient, but recently it was emphasized that such an explanation needs to be reconsidered. On the other hand, recent investigations on the effect of disorder on T$_{c}$ pointed out that samples with residual resistivity ($\rho _{0})$ of few $\mu \Omega $cm present T$_{c}$ variations comparable with the intrinsic variations due to isotopic effect. This calls for new investigations of isotopic effect in samples with controlled amount of disorder. Ultra clean Mg$^{10}$B$_{2}$ and Mg$^{11}$B$_{2}$ samples ($\rho _{0}\sim $0.5 $\mu \Omega $cm) were damaged respectively with annealing and neutron irradiation and T$_{c}$ and resistivity were measured. T$_{c}$ vs $\rho _{0}$ plot shows in both cases a linear relationship allowing us to extrapolate T$_{c}$ ($\rho $=0)${\rm g}$ for both the sample series. $\alpha $(B) evaluated by these intrinsic T$_{c}$ values confirms results of previous report and the crucial role of disorder in determining T$_{c}$ has been proved. [Preview Abstract] |
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