J. F. Zasadzinski

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.

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.

Tunneling spectroscopy in superconducting Ba1−xKxBiO3: Direct evidence for phonon-mediated coupling

Abstract Tunneling measurements have been performed on bulk samples of Ba1−xKxBiO3 (x=0.375) using the natural, surface oxide as the tunnel barrier and an In thin film counterelectrode. High bias conductance and second derivative measurements show reproducible features in the energy range 10 to 65 meV which are characteristic of phonon effect as seen in conventional superconductors. The structures appear at energies which correspond to peaks in the phonon density of states as measured by inelastic neutron scattering and Raman spectroscopy. Estimates of the electron-phonon coupling strength, λ, and the average phonon frequency, , from the tunneling data indicate that phonon mediated pairing is responsible for superconductivity in this compound. Strong lifetime broadening effects are observed in the gap-region conductance of these junctions indicating gapless superconductivity for samples with concentration, x=0.375.

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.

Magnetic properties of Er2Fe14B and Nd2Fe14B thin films

Thin films of Nd2Fe14B and Er2Fe14B have been synthesized by dc sputtering onto single‐crystal sapphire substrates. X‐ray diffraction revealed that the films were single phase and that the texture was sensitive to the substrate temperature. Films deposited onto substrates at 600 °C showed directed crystalline growth with the c axis of the tetragonal structure predominantly perpendicular to the film plane. Magnetization measurements of the Er2Fe14B films and Kerr rotation of the Nd2Fe14B films indicate an intrinsic anisotropy consistent with the preferred orientation. Electrical transport measurements of the Nd2Fe14B film display a change of slope near T=150 K which is interpreted as a consequence of the spin reorientation.

Thermal Management in Large Bi2212 Mesas Used for Terahertz Sources

We present a thermal analysis of a patterned mesa on a Bi2Sr2CaCu2O8 (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ~800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40 times 300 times 1.2 mum3) and power dissipation that result in self-heating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, Rqp(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular Rqp(T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ~60 K.

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.

Ideal tunneling characteristics in Ba1-xKxBiO3 point-contact junctions with Au and Nb tips

One important potential application of high‐temperature superconductors is for tunnel junctions, especially hysteretic Josephson junctions. Unfortunately, tunnel junctions of oxide superconductors with a useful Tc≳20 K have exhibited nonideal I(V) characteristics with a normalized zero‐bias conductance, σs/σn(0), of 15–50%. We present data on point‐contact junctions of Ba1−xKxBiO3 which clearly demonstrate that these problems are not intrinsic. Using a normal‐metal counterelectrode, σs/σn(0) was <1% and σs/σn(V) agreed with the BCS theory by including only thermal smearing. Using a superconducting Nb counterelectrode, the I(V) showed very sharp features with evidence of a proximity effect, and σs/σn(0) was <0.2%. These results prove that there is no fundamental limitation for tunnel junctions using this HTS material, and one can anticipate operation of devices with two Ba1−xKxBiO3 electrodes at 15–20 K.

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 .