Ilya Vekhter: "Probing anisotropic superconductivity with magnetic field"

Ilya presented his work done with Anton Vorontsov on extracting the nodal structure of an order parameter by applying a magnetic field in varying directions and predicting what the zero-bias density of states averaged over the vortex unit cell looks like as a function of field direction. Starting with an experimental overview, in particular specific heat and heat conductivity measurements were singled out for testing nodal quasiparticles. Based on work by Volovik '93, the local density was calculated in the Doppler-shift approximation and averaged over the vortex unit cell. Ilya pointed out that although many experiments contain information about the presence or absence of nodes, not so many tell where in the Brillouin zone the nodes are situated. In order to tackle this problem, Ilya presented studies where a magnetic field is rotated and the averaged zero-bias DOS is monitored.


Question by Dirk Morr: is the field in plane? Answer by Ilya: we do not assume any Josephson vortex structures to be present, all vortices are usual Abrikosov vortices.


Ilya then draw a picture with an oscillating behavior of the averaged N(w=0) as a function of field angle, with minima where the field is parallel to the node. Ilya then pointed out contradicting experimental findings for CeCoIn_5, with nodes either consistent with a d_x^2-y^2 or a d_xy order parameter. Ilya suggested that this can be explained by a more careful study in the high-field region. Solving Eilenberger-Larkin-Ovchinnikov equations employing the Brandt-Pesch-Teward approximation again the averaged zero-bias DOS was calculated, this time also covering the high field region. Ilya summarized the results of these calculations in a H-T phase diagram, with a low-T low-field region showing minima in N(w=0) as tested by C/T when the field points in nodal direction, another not so interesting region near T_c, and an 'inverted' region in the rest of the phase diagram where a maximum occurs in C/T as function of field direction when the field points in nodal direction.


Q: Hartmut Monien asked if subdominant order parameters in the vortex core regions would change the results. Ilya answered that he does not think that this happenes in the materials that were studied.


Q: Ben Powell was interested in the role of pancake vortices, however this was beyond the model Ilya was considering.