Tunneling spectroscopy of Novel Layered superconductors: MgB₂, Li_0.48(THF)_XHfNCl and related substances

The discovery of novel high-T_c superconductivity in MgB₂ (T_c = 39.5 K) andLi0.48(THF)yHfNCl (T_c = 25.5 K) initiated substantial progress in the field ofsuperconductivity physics and its applications despite the fact that competing high-Tccuprates remain the world leaders in almost all practically important superconductingparameters. This article describes electron tunneling and point-contact experimentalstudies of the indicated two materials and related substances, being crucial to elucidatethe character of the quasiparticle energy spectrum both in superconducting and normalstate. The account is based mostly on our own experiments, although works carried out inother laboratories are taken into account as well. We studied superconducting gap structures by means of break-junction tunnelingspectroscopy (BJTS), scanning tunneling spectroscopy (STS) and point-contactspectroscopy (PCS).In the case of MgB₂, tunnel conductance G(V) = dI/dV(V) reveals multiple-gapfeatures. Here I is the quasiparticle tunnel current and V is the bias voltage. The two-gapmodel including the proximity effect (the correlated two-gap model) was used to describethe observed multiple-gap features in BJTS and STS. Three specific gap values can beidentified as follows: Δ_S = 2 - 2.5, Δ_M = 4.5 - 7.5 and Δ_L = 10 - 12 meV. The observedvalues 2Δ_L(4.2 K)/k_BT_c > 5 - 6 constitute the largest known values of thesuperconducting gap to T_c ratio except for those appropriate to copper oxides and certainorganic materials. Here k_B is the Boltzmann constant. On the other hand, ratios of thesmall gaps Δ_S to corresponding T_c's, 2Δ_S(4.2 K)/k_BT_c, fall into the range 1.2 - 1.5, whichis considerably below the Bardeen-Cooper-Schrieffer (BCS) value 2Δ(4.2 K)/k_BT_c ≈ 3.5inherent to s-wave superconductors. The extrapolated highest gap-closing field B_c for thelargest gap agrees with the upper-critical field, thereby indicating that this gap ispredominant in MgB₂. Point-contact conductance G(V) also demonstrates multiple-gapstructures. Peak positions in the second derivative conductance of PCS are in areasonable agreement with the phonon spectrum frequencies revealed by the inelasticneutron scattering measurements. The high-energy boron vibration modes (̃ 75 meV) aresuggested to play the important role in the overall electron-phonon interaction in MgB₂. The BJTS data of other AlB₂ type superconductors show the strong-coupling-size valuesof the ratio 2Δ(0)/k_BT_c, specifically, 4.2 - 4.5 for NbB₂ and 4.2 - 4.6 for CaAlSi. Tunneling measurements have been carried out on layered nitride superconductors ofthe β(SmSI)-type Li_0.48(THF)_xHfNCl (THF;C₄H₈O) (T_c ≈ 25.5 K), HfNCl_0.7 (T_c ≈ 23-24K) and ZrNCl_0.7 (T_c ≈ 14 K). BJTS reveals Bardeen - Cooper - Schrieffer (BCS) - likegap structures with typical gap values of 2Δ(4.2 K) = 11-12 meV for Li_0.48(THF)_xHfNClwith the highest T_c ≈ 25.5 K. Our measurements revealed multiple gaps and dip-humpstructures, the largest gap 2Δ (4.2 K) = 17 - 20 meV closing at T_c. It comes about that thehighest obtained gap ratio 2Δ/k_BT_c ̃ 8 substantially exceeds the BCS weak-couplinglimiting values: ≈ 3.5 and ≈ 4.3 for s-wave and d-wave order parameter symmetry,respectively. For ZrNCl_0.7, two gap ratios 2Δ/k_BT_c = 6 - 8 and 2Δ/k_BT_c = 3 - 4 followfrom the BJTS data. Such huge values of 2Δ/k_BT_c are rather unusual for conventionalsuperconductors.