Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D38: Focus Session: Superconductivity-Properties of Doped and Irradiated Magnesium Diboride and Related Compounds |
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Sponsoring Units: DMP Chair: David Larbalestier, University of Wisconsin Room: Baltimore Convention Center 341 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D38.00001: Scattering in MgB$_{2}$ produced by substitutions and damage Invited Speaker: The two-gap nature of superconductivity is a unique feature of MgB$_{2 }$that stimulates many theoretical and experimental investigations. From the beginning it was pointed out that the peculiar role of disorder in a two-gap superconductor In fact, inter-band scattering by non-magnetic impurities is expected to suppresses the critical temperature T$_{c}$ down to 20 K, where an equivalent one-gap BCS system stabilizes. The verification of these predictions has motivated several efforts to introduce defects in MgB$_{2}$ by substitutions (Al in sites of Mg and C in sites of B) and by irradiation. Substitutions modify electronic structure so that superconductivity can be influenced. To overcome this problem we studied two different kind of samples: co-doped Mg$_{1-x}$(AlLi)$_{x}$B$_{2}$ in which the disorder induced by the Mg substitution is accompanied by a rather complete charge compensation, and neutron irradiated Mg$^{11}$B$_{2}$. In both the sample series remarkable changes in the band structure are not expected. The superconducting properties of Mg$_{1-x}$(AlLi)$_{x}$B$_{2 }$are compared with those of Mg$_{1-x}$Al$_{x}$B$_{2}$. All the properties scale systematically as a function of the Al content rather than electron doping. This suggests that the lattice deformations induced by Al, namely point-like defects and lattice compression, are very effective in tuning the superconducting properties. Similar conclusion can be drawn by the study of neutron irradiated samples. By increasing the neutron fluence, T$_{c}$ monotonously decreases down to 9 K, the resistivity raises by two order of magnitude and the cell volume increases (1.7{\%}). Our results demonstrate that the critical temperature is suppressed by the disorder well down the value of 20 K. On the other hand, the two-gap feature evident in the temperature range above 21 K, disappears when T$_{c}$ is lowered down to 11 and 8.7 K a single-gap superconductivity is established. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D38.00002: Effects of helium ion damage on the two-band superconductivity in MgB$_{2}$ thin films L.H. Greene, W.K. Park, X. Lu, B. Moeckly, R. Singh, N. Newman, J.M. Rowell While the two-band superconductivity in MgB$_{2}$ has been well established, it remains controversial whether disorder in the Mg and B planes causes enhanced interband scattering, band filling or both. To address this, we have performed electronic transport and point-contact spectroscopy measurements on helium-ion irradiated MgB$_{2}$ thin films. Two sets of samples are prepared using: a) 1 MeV He$^{+}$ ions with uniform doses ranging from 1x10$^{15}$ to 1x10$^{17}$ ions/cm$^{2}$; b) 2 MeV alpha particles with gradient doses. The resistivity ($T_{c})$ is observed to increase (decrease) monotonically with increasing dose. The conductance spectra are taken from point-contact junctions between MgB$_{2}$ thin films and Au tips. The $T_{c}$ is determined by the onset of enhanced conductance at zero bias. Our preliminary results show that the two gaps tend to merge into one gap with increasing ion damage. Detailed results including $T_{c}$ vs. energy gap will be presented and discussed in terms of the electronic structure change caused by ion-induced point defects. WKP acknowledges Pavel Krasnochtchekov and Robert Averback for ion irradiation experiments at UIUC. This work is supported by the DoE DEFG02-91ER45439, through the FSMRL and the Center for Microanalysis of Materials at UIUC. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D38.00003: Effect of ion irradiation and annealing on scattering processes in MgB2 Raghuram Gandikota, Rakesh Singh, Jihoon Kim, Barry Wilkens, Nathan Newman, John Rowell, Alexej Pogrebnyakov, Xiaoxing Xi, Joan Redwing, Shengyong Xu, Qi Li, Brian Moeckly The effect of point defects introduced by ion irradiation on Tc, resistivity, and Hc2 of MgB2 films was studied. We will show that ion induced disorder in films with different as-made disorder, followed by annealing, offers advantages in the study of Hc2 of MgB2 films. Films, from three deposition processes, were damaged by 2 MeV alpha particles, followed by annealing after Tc was reduced to $<$10K. Damage increases Hc2 (0) in clean films, resulting in maxima ($\sim $34T) near Tcs of 33K, for fields parallel to the film. For Tcs from $\sim $25K to below 10K, Hc2(0) decreases almost linearly with Tc. Annealing the films, after Tc has been reduced below 10K, reproduces the resistivity value but not the Hc2(0), for a given Tc. These results suggest that Tc is reduced by smearing of the density of states, and that the measured resistivity and Hc2 are determined by scattering in pi and sigma bands respectively. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D38.00004: Magnetic and non-magnetic substitutions in MgB$_{2}$ single crystals: influence on superconducting properties and structure Janusz Karpinski, Nikolai D Zhigadlo, Krzysztof Rogacki, Bertram Batlogg, Getz Schuck, Roman Puzniak, Andrzej Wisniewski, Renato Gonnelli Pure and substituted single crystals of MgB$_{2 }$have been grown at high pressure (30 kbar) using the cubic anvil technique. The crystals have very low residual resistivity $\rho _{o}$(40 K)$\approx $0.5 and a sharp transition $\Delta $T$_{c}\approx $0.2 K. Magnetic (Mn, Fe) and non-magnetic (Al, C) ions have been substituted to study their effect on superconductivity and on the impurity scattering in and between the $\sigma $ and $\pi $ bands. Single-phase Mg$_{1-x}$Al$_{x}$B$_{2}$ and MgB$_{2-x}$C$_{x}$ crystals were grown for x=0-0.3. Al and C cause a similar moderate decrease of T$_{c}$. Magnetic ions, such as Fe$^{3+}$ and Mn$^{2+}$ suppress T$_{c}$ very effectively, due to magnetic pair breaking. Superconductivity is completely suppressed for by 2{\%} Mn. Fe substitution decreases T$_{c}$ less rapidly than Mn but much faster than Al and C. Carbon substitution increases the H$_{c2}$ twice, while Al, Fe and Mn substitutions decrease this field. H$_{c2}$ anisotropy decreases with all substitutions, but the temperature dependence of the anisotropy is different, due to different scattering rates in the $\pi $ and $\sigma $ bands. For Mn and Al, $\pi $ and $\sigma $ energy gaps exist up to the highest substitution level, while for C substitution, merging of these gaps is observed indicating interband scattering. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D38.00005: Evolution of two-gap superconductivity of MgB$_2$ by Al substitution Min-Seok Park, Heon-Jung Kim, Hye-Gyong Lee, Myung-Hwa Jung, Younghun Jo, Sung-Ik Lee The temperature dependence of the upper critical fields $(H_{c2} (T )$) of the two-gap superconductors Mg$_{1-x}$Al$_x$B$_2$ not only for low doping level ($x$ $<$ 0.1) but also for high doping level (0.1 $\leq$ $x$ $\leq$ 0.3) was obtained by resistivity measurements. The $H_{c2}(T)$ values were analyzed, within the dirty-limit two-gap model, using the calculated electron-phonon coupling constants and Coulomb pseudo potentials. As the Al contents increased, both $T_c$ and $H_{c2}(0)$ decreased, which was the direct manifestation of the increasing number of electrons. At the same time, the three-dimensional $\pi$ bands became much dirtier when Al was doped. In contrast, the intraband scattering of the two-dimensional $\sigma$ bands was relatively unaffected by the Al doping. These behaviors could be understood when the characters of both bands were considered. In all the samples that we investigated, the $\sigma$ bands were dirtier than the $\pi$ bands. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D38.00006: Thermal Expansion measurements on single crystals of the superconductor MgB$_2$ John J. Neumeier, R. Bollinger, C. A. M. dos Santos, N. D. Zhigadlo, Janusz Karpinski Thermal expansion measurements have been conducted along the a and c axes on single crystals of the superconductor MgB$_2$. The samples are platelets with typical thicknesses of 0.1 mm. The a axis lies in the plane of the platelets, this axis is typically 0.5 mm long. The measurements were conducted using a capacitive thermal expansion cell constructed entirely of fused silica; it is capable of detecting 0.1 $\AA$ changes in length. The results reveal that the linear thermal expansion $\Delta$$L/L$ = 21x10$^{-4}$ and 5.2x10$^{-4}$ along the c and a axes, respectively, in the temperature range 5 K $<$ T $<$ 300 K. At the superconducting transition temperature $T_c$, jumps in the thermal expansion coefficient $\alpha$ are observed to be positive along both axes. Regions where $\alpha$ $<$ 0 are observed along both axes. The results are in general agreement with recently published data [1] on polycrystalline MgB$_2$. [1] J. J. Neumeier, T. Tomita, M. Debassai, J. S. Schilling, P. W. Barnes, D. G. Hinks, and J. D. Jorgensen, Phys. Rev. B. Rapid Commun. December 2005. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D38.00007: Measurements of elastic constants for single crystals of magnesium diboride Kenneth Pestka, J.D. Maynard, Xiaxing Xi, Qi Li Important information concerning the nature of the high temperature BCS superconductor magnesium diboride, MgB$_2$, may be obtained from measurements of elastic constants with single crystal samples. As derivatives of the free energy with respect to atomic displacements, the elastic constants are a sensitive probe of the lattice environment in which all solid state phenomena occur. We have used the technique of resonant ultrasound spectroscopy (RUS) for very small samples in order to measure good quality single crystals. The samples were grown epitaxially on silicon carbide substrates using physical vapor deposition; samples were a few hundred microns wide and 600 to 750 nm thick. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D38.00008: Nanoscale spatial non-homogeneity of 3D $\Delta _{\pi }$ in Mg$_{1-x}$AlB$_{2}$ single crystals F. Bobba, F. Giubileo, A. Scarfato, A. M. Cucolo, D. Roditchev, N. Zhigadlo, S. Kazakov, J. Karpinski We have performed local I(V) and dI/dV(V) measurements on high quality Mg$_{1-x}$Al$_{x}$B$_{2}$ single crystals with different Al dopings by means of Scanning Tunneling Spectroscopy (STS). Both temperature and magnetic field dependences of the conductance spectra have been studied both in S-I-N and in S-I-S configurations. The measured spectra revealed only one gap at low temperatures as expected for pure c-axis tunneling. In the case of x=0.1, the measured superconducting gap $\Delta _{\pi {\rm g}}$ appeared highly non-homogeneous in its spatial distribution on nanometer scale, with an amplitude varying between 1.5 meV and 2.3 meV. In pure MgB$_ {2}$ the evolution of the zero bias conductance (ZBC) in magnetic field clearly showed two different regimes with a fast rise up to 1 T, followed by a slower refilling of states for higher fields. This effect results smoothed in doped single crystals. Consistently with the more recent theories, in comparison with the undoped material, we have found that for x=0.1 the $\Delta _{\pi }$ gap value and the H$_{c2}$ critical field increase of about 15{\%} and 30{\%}, respectively. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D38.00009: Superconductivity in Y$_2$C$_3$ Satoshi Akutagawa, Jun Akimitsu The discovery of superconductivity in MgB$_2$ revived the enthusiasm of non-oxides materials and initiated a search for novel superconductivity in intermetallic compounds including light elements, B and C. As a part of these researches, we have reported a relatively high-$T\rm_c$ superconductivity in Y$_2$C$_3$ at 18 K whose $T\rm_c$ could be changed from 10 K to 18 K by synthesis conditions, although this material with a maximum $T\rm_c$ of 11.5 K has already been investigated by Krupka {\itet al.}. The crystal structure of Y$_2$C$_3$ is a body-cente red cubic (Pu$_2$C$_3$-type) s tructure. In this structure, Y atoms are aligned along the $ <$111$>$ direction and C atoms form dimers. We synthesized a high-purity sample of the medium-$T\rm_c$ phase ($T\rm_c$ = 13.9 K) in Y$_2$C$_3$ and examined its physical properties in detail. From a specific heat measurement, the superconducting gap is estimated to be 4.50, indicating that the superconductivity in Y$_2$C$_3$ can be described by an $s$-wave strong coupling regime. From specific heat in various magnetic fields, the upper critical field $H\rm_{c2}$(0) is estimated to be 24.7 T. [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D38.00010: Li-Vacancy Ordering in LiBC Phase Ebru Gungor, Engin Ozdas The discovery of a phonon mediated superconductivity in MgB$_{2}$ with a T$_{c}$ of 40 K has accelarated the scientific interest in similar layered compounds and the electronic band structure calculations showed that the high-Tc superconductivity is possible for the hole-doped layered lithium borocarbide, Li$_{x}$BC [1,2]. However, the superconducting features for Li off-stoichiometric borocarbide compounds have not been observed in any experimental studies, because of the difficulties in the sample preparation. In this work, the effects of synthesis conditions on the structure of Li$_{x}$BC samples with the different Li-stoichiometries and the phase stability were investigated. The structural studies showed that the intercalation process has a staging behaviour as Li intercalated graphite and a novel Li vacancy ordered structure for off-stoichiometric Li$_{x}$BC phases. [1] Rosner H.et al., PRL 88, 12 (2002) [2] Dewhurst et al., PRB 68, 020504(R) (2003) [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D38.00011: Anomalous Magnetoresistance Effects in (CrO$_{2})_{1-x}$ : (MgB$_{2})_{x}$ powders Raghava Panguluri, B. Nadgorny It is known that the electrical conduction mechanism in compressed powders and polycrystalline CrO$_{2}$ films is due to the intergranular tunneling. Here, we focus on the percolation effects in the CrO$_{2}$ powders intermixed with a superconductor, MgB$_{2}$. A mixture of (CrO$_{2})_{1-x}$ : (MgB$_{2})_{x}$ powders with 0$\le $x$\le $1 were cold-pressed to form circular disks. The electrical resistance measured by a four probe technique, as expected showed a lower resistance for the mixture when compared to the pure pressed CrO$_{2}$. The observed hysteresis extrinsic magnetoresistance (MR) showed peaks corresponding to the coercive fields of CrO$_{2}$ which can be associated with the alignment of magnetization of adjacent CrO$_{2}$ particles. Interestingly, the MR changed signs at around the superconducting transition temperature of MgB$_{2}$ ($\sim $40K) and also the slope of MR at high magnetic fields changed from positive to negative. We will discuss a possible origin of the observed effects, which we believe are related to the ferromagnet/superconductor interaction. [Preview Abstract] |
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