K. E. Gray

Predominantly Superconducting Origin of Large Energy Gaps in Underdoped Bi 2 Sr 2 CaCu 2 O 8 + δ from Tunneling Spectroscopy

New tunneling data are reported in underdoped Bi2Sr2CaCu2O8-d using superconductor-insulator-superconductor break junctions. Energy gaps, Delta, of 51+2, 54+2 and 57+3 meV are observed for three crystals with Tc=77, 74, and 70 K respectively. These energy gaps are nearly three times larger than for overdoped crystals with similar Tc. Detailed examination of tunneling spectra over a wide doping range from underdoped to overdoped, including the Josephson IcRn product, indicate that these energy gaps are predominantly of superconducting origin.

Evidence for two-band superconductivity from break-junction tunneling on MgB2.

Superconductor-insulator-superconductor tunnel junctions have been fabricated on MgB2 that display Josephson and quasiparticle currents. These junctions exhibit a gap magnitude, Delta approximately 2.5 meV, that is considerably smaller than the BCS value, but which clearly and reproducibly closes near the bulk T(c). In conjunction with fits of the conductance spectra, these results are interpreted as direct evidence of two-band superconductivity.

Coated conductor development : an assessment.

This paper summarizes the current status of basic research related to the understanding and development of coated conductor tapes for use in large scale superconducting applications. It reports highlights of a workshop and discussion meeting which dealt with many of the central issues associated with both thick film deposition methods and the defects that control the critical current densities. Topics included texture control in the metal substrate, smoothing the substrate surface before the grain aligned buffer layers are deposited, deposition methods for the buffer layers, deposition methods for high performance in the superconducting layer. The ionized beam-assisted deposition (IBAD) method, and the rolling-assisted biaxially textured substrate (RABiTS) method were the primary approaches to texture control. Emphasis was placed on the use of microscopic probes to study defects and grain boundaries in order to develop a science base for the factors that determine the superconducting critical current of practical conductors. In all of the discussion, an effort was made to identify basic research that would lead to a fundamental understanding of the mechanisms at work so that we can improve processing methods and enhance performance of the superconducting tape.

Correlation of Tunneling Spectra inBi2Sr2CaCu2O8+δwith the Resonance Spin Excitation

New break-junction tunneling data are reported in Bi(2)Sr(2)CaCu(2)O(8+delta) over a wide range of hole concentration from underdoped (T(c) = 74 K) to optimal doped (T(c) = 95 K) to overdoped (T(c) = 48 K). The conductances exhibit sharp dips at a voltage, Omega/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Omega scales as 4.9kT(c) over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.

Energy gap from tunneling and metallic contacts onto MgB{sub 2}: Possible evidence for a weakened surface layer

Point-contact tunnel junctions using a Au tip on sintered MgB{sub 2} pellets reveal a sharp superconducting energy gap that is confirmed by subsequent metallic contacts made on the same sample. The peak in the tunneling conductance and the metallic contact conductance follow the BCS form, but the gap values of 4.3{minus}4.6 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk T{sub c} of 39 K. The low value of {Delta} compared to the BCS value for the bulk T{sub c} is possibly due to chemical reactions at the surface.

Application of NbN films to the development of very high field superconducting magnets

We report the preliminary results of a program recently begun at Argonne National Laboratory to demonstrate the feasibility of using niobium nitride (NbN) films as practical high field superconductors. Films of varying thicknesses (2-9 μm) have been deposited on sapphire and Hastelloy substrates, using d.c. magnetron sputtering. The superconducting transition temperatures T c of these films range from 12-15 K. Using the WHH extrapolation, upper critical fields H c2 (O) up to 36 T in the parallel direction and 43T in the perpendicular direction were obtained. Critical current densities J c (H,4.2 K) were \approx 1\times10^{4} A/cm2at 20 T in both the parallel and perpendicular directions. The preliminary results from novel sample preparation technique for TEM studies of film cross-sections is briefly described. This technique shows the variation in film structure as a function of distance from the substrate.

Retention of two-band superconductivity in highly carbon-doped MgB{sub 2}.

Tunneling data on MgB{sub 1.8}C{sub 0.2} show a reduction in the energy gap of the {pi} bands by nearly a factor of 2 from undoped MgB{sub 2} that is consistent with the T{sub c} reduction, but inconsistent with the expectations of the dirty limit. Dirty-limit theory for undoped MgB{sub 2} predicts a single gap about three times larger than measured and a reduced T{sub c} comparable to that measured. Our heavily doped samples exhibit a uniform dispersion of C suggestive of significantly enhanced scattering, and we conclude that the retention of two-band superconductivity in these samples is caused by a selective suppression of interband scattering.

Break-junction tunneling on MgB2

Abstract Tunneling data on magnesium diboride, MgB 2 , are reviewed with a particular focus on superconductor–insulator–superconductor (SIS) junctions formed by a break-junction method. The collective tunneling literature reveals two distinct energy scales, a large gap, Δ L ∼7.2 meV, close to the expected BCS value, and a small gap, Δ S ∼2.4 meV. The SIS break junctions show clearly that the small gap closes near the bulk critical temperature, T c =39 K. The SIS spectra allow proximity effects to be ruled out as the cause for the small gap and therefore make a strong case that MgB 2 is a coupled, two-band superconductor. While the break junctions sometimes reveal parallel contributions to the conductance from both bands, it is more often found that Δ S dominates the spectra. In these cases, a subtle feature is observed near Δ S + Δ L that is reminiscent of strong-coupling effects. This feature is consistent with quasiparticle scattering contributions to the interband coupling which provides an important insight into the nature of two-band superconductivity in MgB 2 .

Point contact tunnelling apparatus with temperature and magnetic field control

The design and testing of a new device for point contact tunnelling measurements in superconductors are described. The insert is designed for use with a continuous flow cryostat which allows for a large range of sample temperatures from 1.5 K to room temperature. The use of nonmagnetic parts allows tunnelling measurements to be performed in high magnetic fields. Testing was carried out on a conventional superconductor, Nb, in fields up to 6 T using Nb and Au tips to obtain superconductor‐ insulator‐superconductor and superconductor‐insulator‐normal metal junctions. In addition, a moderate-Tc oxide superconductor, Ba1 - xKxBiO3, was used to obtain temperature and magnetic field dependencies of the energy gap.

Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.