Field-angle-dependent specific heat measurements and gap determination of a heavy fermion superconductor URu2Si2.

To identify the superconducting gap structure in URu2Si2, we perform field-angle-dependent specific heat measurements for the two principal orientations in addition to field rotations, and a theoretical analysis based on microscopic calculations. The Sommerfeld coefficient gamma(H)'s in the mixed state exhibit a distinctly different field dependence. This comes from point nodes and the substantial Pauli paramagnetic effect of URu2Si2. These two features combined give rise to a consistent picture of superconducting properties, including a possible first order transition of Hc2 at low temperatures.

Ubiquitous V-shape density of states in a mixed state of clean limit type II superconductors.

It is demonstrated theoretically and experimentally that the low energy density of states N(E) is described by a singular V-shape form N(E)=N(0)(H)+alpha|E|+O(E2) for all clean superconductors in a vortex state, irrespective of the underlying gap structure. The linear term alpha|E| which has not been recognized so far is obtained by exactly evaluating the vortex contribution. Based on microscopic Eilenberger theory N(E) is evaluated for the isotropic gap, line, and point-node gaps to yield a V-shape N(E). Scanning tunneling spectroscopy-STM experiments on NbSe2 and YNi2B2C give direct evidence for this. We provide arguments on the significance of this finding and on the relevance to other experiments.

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.

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.

Elastic moduli of vortex lattices within nonlocal London model.

Vortex lattice (VL) elastic response is analyzed within the nonlocal London model which holds for high-kappa clean superconductors. The squash modulus vanishes at the field H( square) where VL undergoes a square-to-rhombus transition. For H>H( square), where the square VL is stable, the rotation modulus turns zero at H = H(r), indicating VL instability to rotations. The shear modulus depends on the shear direction; the dependence is strong in the vicinity of H( square) where the square VL is soft with respect to the shear along [110]. The H dependences of the moduli are evaluated for LuNi(2)B(2)C.