Ultrahigh-resolution photoemission spectroscopy of Ni borocarbides: direct observation of the superconducting gap and a change in gap anisotropy by impurity

Superconducting and Superhard Ice

Superconducting and superhard materials are essential for a myriad of scientific, biomedical and industrial applications. The contradiction between covalent bonds in superhard materials and metallic bonds in superconductors makes superconductivity and superhardness in the same material a very interesting and precious effect. Their abilities of zero-resistance and anti-pressure stem from the relationship between the crystal structure, chemical composition, and microstructure. The complexity of this interdependence limits researchers to conduct comprehensive experimental investigations, but can be supported by the theoretical calculations. Here, we report a general ab initio computational method to study three ice structures (Pmc2₁ , P2₁ and C2/m) and predict their phase transitions quantitatively at terapascal pressure. We predict that the ice structure will become a superhard material above 1.3 TPa (corresponding to P2₁ and C2/m), and turn into a superconductor above 5.0 TPa (corresponding to C2/m) with a critical temperature of 1.782 K. The proposed work benefits the applications of low temperature superconductors in high energy physics and fusion research and provides opportunities to advance the development of superconducting and superhard materials through computation.

Bulk electronic structure of the antiferromagnetic superconducting phase in ErNi2B2C

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

Angular-dependent I-V characteristics in borocarbide superconductor YNi(2)B(2)C

We present angular-dependent current-voltage (I-V) measurements in borocarbide YNi(2)B(2)C single crystals near the vortex-glass irreversible line. External magnetic fields are applied along the angle θ with respect to the c-axis. The nonlinear I-V curves reveal scaling behaviour near the transition. Using the scaling analysis, the relevant critical exponents and vortex transition temperatures are determined for all orientations. The data agrees well with the vortex-glass (VG) model. No evidence was found that supports the existence of a Bose-glass (BG) type of transition.

Anisotropic diamagnetic response in type-II superconductors with gap and Fermi-surface anisotropies

The effects of anisotropic gap structures on a diamagnetic response are investigated in order to demonstrate that the field-angle-resolved magnetization [ML(chi)] measurement can be used as a spectroscopic method to detect gap structures. Our microscopic calculation based on the quasiclassical Eilenberger formalism reveals that ML(chi) in a superconductor with a fourfold gap displays a fourfold oscillation reflecting the gap and Fermi-surface anisotropies, and the sign of this oscillation changes at a field between Hc1 and Hc2. As a prototype of unconventional superconductors, magnetization data for borocarbides are also discussed.

Photoemission spectroscopic evidence of gap anisotropy in an f-electron superconductor

We used low-temperature ultrahigh-resolution (360 microeV) photoemission spectroscopy with a laser as a photon source (Laser-PES) to study the superconducting (SC) gap of an f-electron superconductor CeRu2. The unique combination of the large escape depth expected from the known universal behavior and extremely high-energy resolution has enabled us to directly measure the bulk SC gap of an f-electron superconductor for the first time. The present study provides direct evidence for an anisotropic SC gap in CeRu2, and also demonstrates the potential of Laser-PES in investigating unconventional superconductivity realized in correlated d- and f-electron superconductors.

Evidence of gap anisotropy in superconducting YNi2B2C using directional point-contact spectroscopy

We present a study of the anisotropy in the superconducting energy gap in a single crystal of YNi2B2C (T(c) approximately 14.6 K) using directional point-contact spectroscopy. The superconducting energy gap at 2.7 K, when measured for I||c, is 4.5 times larger than that for I||a. The energy gaps in the two directions also have different temperature dependences. Our results support a scenario with s + g like symmetry.

Anisotropy of the superconducting gap of the borocarbide superconductor YNi2B2C with ultrasonic attenuation

The ultrasonic attenuation alpha of the highly anisotropic s-wave superconductor YNi2B2C has been measured for all the symmetrically independent elastic modes to explore the location of the zero superconducting gap region on the Fermi surface. The attenuation of the longitudinal mode shows a pronounced anisotropy in the superconducting state: While alpha shows a thermally activated behavior along [110] and [001] directions, it shows T-linear dependence along [100]. These results together with those for the transverse modes demonstrate the presence of point nodes or zero-gap regions along [100] and [010] directions. This is a clear demonstration of ultrasonic attenuation as a powerful probe for the structure of the anisotropic superconducting gap.

Reentrant vortex lattice transformation in fourfold symmetric superconductors

The physics behind the rhombic-->square-->rhombic flux line lattice transformation in increasing fields is clarified on the basis of Eilenberger theory. We demonstrate that this reentrance observed in LuNi2B2C is due to intrinsic competition between the superconducting gap and Fermi surface anisotropies. The calculations not only reproduce it but also predict the not yet found lock-in transition to a square lattice with different orientation in a higher field. In view of the physical origin given, this sequence of transitions is rather generic to occur in fourfold symmetric superconductors.

Gap function with point nodes in borocarbide superconductor YNi2B2C

To determine the superconducting gap function of YNi2B2C, the c-axis thermal conductivity kappa(zz) was measured in H rotated in various directions. The angular variation of kappa(zz) in H rotated within the ab plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c axis with a tilt angle of 45 degrees. These results provide the first compelling evidence that the gap function has point nodes located along the a and b axes. This unprecedented gap structure challenges the current view on the pairing mechanism.

Fermi surface sheet-dependent superconductivity in 2H-NbSe2

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectral function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

Highly anisotropic gap function in borocarbide superconductor LuNi(2)B(2)C

The thermal conductivity of borocarbide superconductor LuNi(2)B(2)C was measured down to 70 mK (T(c)/200) in a magnetic field perpendicular to the heat current from H = 0 to above H(c2) = 7 T. As soon as vortices enter the sample, the conduction at T-->0 grows rapidly, showing unambiguously that delocalized quasiparticles are present at the lowest energies. The field dependence is very similar to that of UPt(3), a heavy-fermion superconductor with a line of nodes in the gap, and very different from the exponential dependence characteristic of s-wave superconductors. This is strong evidence for a highly anisotropic gap function in LuNi(2)B(2)C, possibly with nodes.