Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A39: Focus Session: Magnesium Diboride and Related Compounds: Multi Gap Superconductivity |
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Sponsoring Units: DMP Chair: Lance Cooley, Brookhaven National Laboratory Room: Baltimore Convention Center 342 |
Monday, March 13, 2006 8:00AM - 8:12AM |
A39.00001: Current-induced phase textures and pairbreaking in multilayered structures and two-gap superconductors. Alex Gurevich, Valerii Vinokur We predict an inhomogeneous state in current-carrying multilayers or superconductors with multicomponent order parameters. Using the GL theory, we show that the current I flowing along a weakly coupled bilayer (with two different superconducting layers) can result in a two-stage pairbreaking process: 1. Current-induced interlayer decoupling due to a transition from the phase-locked state at $I < I_b$ to a periodic chain of interlayer $2\pi$ phase slips for $I > I_b$. This phase texture re-distributes currents between the layers, the period of the texture L(I) decreases as I increases similar to the period of the vortex lattice L(H) in a long Josephson junction. 2. The global pairbreaking at the depairing current $I_d >> I_b$. The same current-induced band decoupling and interband phase textures occur in two-gap superconductors with two weakly coupled s-wave order parameters, such as $MgB_2$.Using the Usadel equations we derive an equation for the interband phase difference valid for all temperatures. This equation shows that at small currents $I < I_b$, the $\pi$ and $\sigma$ bands are phase-locked, while for $I > I_b$, the band decoupling transition occurs as an equilibrium interband phase slip structure forms along the direction of current flow. These interband phase textures can manifest themselves in dc transport, vortex properties and nonlinear rf impedance. [Preview Abstract] |
Monday, March 13, 2006 8:12AM - 8:24AM |
A39.00002: Theory of superconducting fluctuations in magnesium diboride Andrei Varlamov, Alexei Koshelev, Valerii Vinokur A theory of fluctuations in two-band superconductor MgB$_{2}$ is developed. Since the standard Ginzburg-Landau (GL) approach fails in description of its properties, we generalize it basing on the microscopic theory of a two-band superconductor. Calculating the microscopic fluctuation propagator, we build up the nonlocal two-band GL functional and the corresponding time-dependent GL equations. This allows us to calculate the main fluctuation observables. Temperature dependencies of the fluctuation specific heat, magnetic susceptibility, and in-plane conductivity are determined by the same function which interpolates between two regimes: the standard GL regime very close to $T_c$, where superconductivity is described by the unique order parameter for both bands, and the regime of dominating $\sigma$-band which is settled at temperatures slightly further away from $T_c$. This work was supported by the U.S. DOE, Office of Science, under contract \# W-31-109-ENG-38. A.A.V. acknowledges the support of the FIRB project of the Italian Ministry of Science and Education. [Preview Abstract] |
Monday, March 13, 2006 8:24AM - 8:36AM |
A39.00003: Multi-Gap Superconductivity in MgB$_{2}$: Magneto-Raman Spectroscopy A. Mialitsin, B.S. Dennis, G. Blumberg, N. D. Zhigadlo, J. Karpinski We report polarization-resolved Raman scattering measurements on MgB$_{2}$ single crystals as a function of excitation, temperature and magnetic field with special attention paid to the superconductivity induced electronic collective modes and phonon renormalization. In addition to the two previously reported SC features: a fundamental gap below 32 and a SC coherence peak at 109 cm$^{-1}$ - we observe a continuum onset shoulder at 48 and a collective mode at 78 cm$^{-1}$ in the $A_{1g} $ channel. Because MgB$_{2}$ optical conductivity exhibits inter-band excitation in the visible range resonant Raman coupling allows us to turn on and off SC features induced by different coupling mechanisms as we scan the Raman excitation energy. Since the absorption at 2.6 eV originates from distinctive inter-band transitions we are able to selectively determine inter-band coupling constant. Finally we demonstrate how the frequency and the line width of the Raman active $E_{2g}$ phonon evolve with temperature and magnetic field. Tracking this evolution across the SC phase transition illustrates the self energy effect as the electronic spectra renormalize along with the blue-shift of the $E_{2g}$ phonon. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A39.00004: Feshbach shape resonance in a superlattice of superconducting layers: the case of aluminum doped magnesium diboride A. Bianconi, M. Filippi, M. Fratini, V. Palmisano, L. Simonelli, N. L. Saini, E. Liarokapis We have synthesized highly pure cristalline magnesium diboride samples where Mg is substituted by Al. We show that by electron doping magnesium diboride it is possible to tune the chemical potential to the Feshbach shape resonance in a superlattice of metallic layers. In this multiband superconductor in the clean limit, showing two gap superconductivity, the interband exchange like pairing term shows a Feshbach shape resonance in the proximity of the 2D to 3D Lifshitz electronic topological transition (2D/3D ETT). This is shown that the Feshbach resonance is the key term controlling the increase of the critical temperature in the low temperature range to 40K. The variation of the electron phonon interaction is probed by micro-Raman. The 2D/3D ETT is shown by micro Raman and a lattice anomaly. The control of the nanoscale phase separation tuning the chemical potential in the proximity of the ETT is discussed. The Feshbach resonance inthe interband pairing is indicated by the suppression of the isotope coefficient in the critical temperature as a function of aluminum substitution. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A39.00005: Ground electronic state of MgB$_{2}$: Intrinsic nonadiabatic state at broken translation symmetry. Pavol Banacky Study of the band structure of MgB$_{2}^{ }$has shown that electron coupling to E$_{2g}$ phonon mode induces not only $\sigma $-bands splitting at $\Gamma $ point but also fluctuation of the top of one of $\sigma $ band at the Fermi level, resulting in dramatic decrease of the Fermi energy. As a consequence, the original adiabatic state ($\omega $/E$_{F} \quad <$ 1) corresponding to the equilibrium nuclear geometry has been changed to the intrinsic nonadiabatic state ($\omega $/E$_{F} \quad >>$ 1) at distorted geometry, already at the displacement which is smaller than the rms. displacement of B-B atoms corresponding to zero-point energy of the E$_{2g}$ phonon mode. At these circumstances, not only Migdal theorem but also Born-Oppenheimer approximation has been broken, and standard treatment of EP interactions, including calculation of nonadiabatic corrections to adiabatic ground state by means of perturbation theory, can not be applied. Study of the electron-nuclear Hamiltonian by means of the quasiparticle unitary transformation (Q,P--dependent) which treats electrons and nuclei on the same footing, has revealed that EP interactions in the intrinsic nonadiabatic state stabilize the fermionic ground state of MgB$_{2}$ at broken translation symmetry and corresponding wave function is dependent not only on nuclear coordinates, but it is strongly modulated mainly by nuclear momenta. From the results follow that in this case, instead of Cooper pairs formation, condensation process is represented rather by real-space, T-dependent formation of mobile bipolarons. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A39.00006: Elucidating strong coupling between $E_{2g} $ phonon mode and electrons in MgB$_{2}$ Richard Perez , Yong-Jihn Kim, Oswald Uwakeh, Eric Hellstrom Although the isotope effect and the Testardi correlation provide definite proof of the phonon mechanism in MgB$_{2}$, the detailed electron-phonon coupling is not clear yet. For instance, point contact and tunneling spectroscopy experiments show only qualitative information on the (strong) coupling between the $E_{2g} $ phonon mode and the electrons that was predicted by theory. Raman scattering and inelastic x-ray scattering demonstrate the anomalously damped and broad $E_{2g} $ phonon mode due to strong electron-phonon coupling. We follow the change of the anomalously broadened linewidth of the$E_{2g} $mode as the system becomes disordered by impurity substitution and/or ball milling. We expect that the broadening due to the electron-phonon coupling will decrease significantly when T$_{c }$ is reduced to zero due to weak localization. We combine Raman scattering and resistance ratio data to clarify the importance of the coupling between the $E_{2g} $ phonon mode and the electrons in MgB$_{2}$. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A39.00007: Electron phonon coupling and phonon renormalization in covalent metals Peihong Zhang, Steven G. Louie, Marvin L. Cohen Electron-phonon (el-ph) coupling contributes to the finite lifetime of both electrons and phonons, renormalizes their energy, and is responsible for conventional superconductivity. Therefore, phonon renormalization relates directly to the el-ph coupling in metals and is a convenient indication of the coupling strength. In this talk, we will discuss phonon renormalization effects in materials with strong el-ph coupling using a recently developed formalism [1]. We will focus on covalent metals such as MgB$_{2}$ and related systems and heavily doped covalent semiconductors. \newline \newline [1] P. Zhang, S. G. Louie, and M. L. Cohen, PRL \textbf{94}, 225502 (2005). [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A39.00008: Hole-doped diamond: a 3D version of MgB2? Jens Kortus, Lilia Boeri, Ole Krogh Andersen The discovery of superconductivity in heavily-boron doped diamond in 2004 has caused great excitement both in the fields of superconductors and of semiconductors. In this contribution we show, via first-principles and analytical calculations, that the observed superconductivity can be explained with an electron-phonon mechanism very similar to the one which is causing superconductivity in MgB2: holes at the top of the zone-centered, degenerate $\sigma$ bands couple strongly to the optical bond-stretching modes. We discuss similarities and differences between the two materials, the doping dependence of the critical temperature and the possibility to observe superconductivity in the other group-IV semiconductors. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A39.00009: Studying the electronic structure in pure and electron doped MgB$_{2}$ Y. Zhu, R.F. Klie, L. Wu, J.C. Zheng, L.D. Cooley We use high-energy electrons to reveal electronic structure information to understand the effects of electron doping in MgB$_{2}$ superconductors. Angle-resolved electron energy-loss spectroscopy was used to investigate the difference in the excited states, while image-coupled quantitative electron diffraction was used to map the valence electron distribution, in pure and Al-doped MgB$_{2}$. The results were compared with density functional theory calculations. We found significant changes in the B K-edge fine structure as a function of electron doping concentration, suggesting the corresponding $\sigma $ and $\pi $ bands are being filled simultaneously. The filling of the $\sigma $-band states near the Fermi level reduces the critical temperature T$_{c}$ of highly doped MgB$_{2}$ to a level comparable to that of other $\pi $-band superconductors such as intercalated graphite. Valence electron maps reveal that electron doping causes considerable charge transfer and accumulation in charge density between both Al-B and B-B bonds. This results in a shortened c-axis of the unit cell and higher phonon frequency, which eventually quenches superconductivity altogether. The relationship between charge transfer and inter-band scattering are also examined. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A39.00010: MgB$_{2}$; Al and C doping, $\sigma $-band filling and anisotropy reduction Sabina Ruiz-Chavarria, Pablo de la Mora, Gustavo Tavizon Al and C-MgB$_{2}$ doping adds an electron to the system for each atom. This extra electron fills up the $\sigma $-bands thus diminishing the number of $\sigma $-carriers; this has been the usual explanation for the $T_{c}$ reduction. Nevertheless in this work we show that there is also a large reduction of anisotropy in the electrical conductivity due to the $\sigma $-carriers which should also have an effect on the $T_{c}$ reduction. Al and C doping produce a different $T_{c}$ pattern; this difference can be largely explained by the relative shift between the $\sigma $-bands and $\pi $-bands. After adjusting to this shift there is a small but visible difference, at low doping $T_{c}$ in the Al compounds drops faster than in the C compounds, this can be directly related to the faster loss of conductivity anisotropy in the Al compounds. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A39.00011: Electrons and phonons in the hexagonal layered superconducting alloy CaAl$_{2-x}$Si$_x$ Giovanni B. Bachelet, Lilia Boeri, Matteo Giantomassi We report a first-principles study of structural, electronic and vibrational properties of the superconducting C$_{32}$ phase of the ternary alloy CaAl$_{2-x}$Si$_x$, both in the experimental range of stability, $0.6 \leq x \leq 1.2$, and outside, in the limits of high Al and high Si concentration. We find that the dependence of the electronic bands on composition is well described by a rigid-band model, which breaks down outside the experimental range of stability. This breakdown, in the limit of high Al concentration, is connected to vibrational instabilities, and results in important differences between CaAl$_2$ and MgB$_2$. Unlike MgB$_2$, the interlayer band and the out-of-plane phonons play a major role on the stability and superconductivity of CaAlSi and related C$_{32}$ intermetallics. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A39.00012: Direct observation of superconducting gap anisotropy in YNi$_{2}$B$_{2}$C : Angle-resolved photoemission spectroscopy Teruhisa Baba, Takayoshi Yokoya, Shunshuke Tsuda, Shik Shin, Tadataka Watanabe, Minoru Nohara, Hideaki Takagi, Tamio Oguchi In a borocarbide superconductor YNi$_{2}$B$_{2}$C(T$_{c}$=15K), various experimental results have shown the existence of a gap anisotropy with node. More recently even the direction and type of node have been reported. However, the type of the nodal structure (point or line) seems to be controversial. Also, the position of the node on Fermi surfaces, which is essential for determining the origin of the large anisotropy, has not been clarified, yet. Therefore, we have performed low-temperature ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) to clarify the origin of the large anisotropy. We successfully observed experimental valence band dispersions, Fermi surfaces, and momentum-dependent superconducting gap of YNi$_{2}$B$_{2}$C (001). Superconducting gaps have shown large anisotropy. From these results, we discuss possible orgins of the large anisotropy in YNi$_{2}$B$_{2}$C. [Preview Abstract] |
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