Ubiquitous V-shape density of states in mixed state of clean limit type   II superconductors; STM experiment and theory
  It is demonstrated theoretically and experimentally that the low energy density of states $N(E)$ is described by a singular V-shape form $N(E)=N_0(H)+\alpha |E|+O(E^2)$ for all clean superconductors in a vortex state, irrespective of the underlying gap structure. The linear term $\alpha |E|$ which has been not recognized so far is obtained by exactly evaluating the vortex contribution. Based on microscopic Eilenberger theory $N(E)$ is evaluated for isotropic gap, line and point node gaps to yield a V-shape $N(E)$. STS-STM experiments on NbSe$_2$ and YNi$_2$B$_2$C give direct evidence for this. We provide arguments on significance of this finding and on relevance to other experiments. 
N. Nakai, P. Miranovic, M. Ichioka, H. F. Hess, K. Uchiyama, H.   Nishimori, S. Kaneko, N. Nishida, and K. Machida