Quantum Dynamics Calculations on Massively Parallel Supercomputers: Implementation and Molecular Applications

Prof. Morten Ring Eskildsen, (University of Notre Dame, Notre Dame, USA)

Since the discovery of superconductivity in CeCoIn₅, a plethora of interesting phenomena has been observed in this material. Among these are one of the highest critical temperatures (T_c = 2.3 K) in any heavy fermion superconductor, d-wave pairing symmetry, a paramagnetically limited upper critical field which is first-order at low temperatures, and field- and pressure-induced quantum-critical points and non-Fermi liquid behavior. Finally, several bulk measurements indicate a phase transition to a non-uniform (Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state just below H_c2 at low temperatures.

Here we report on small-angle neutron scattering studies of the flux-line lattice (FLL) in CeCoIn₅ with fields up to H_c2 = 5 T applied parallel to the c-axis and temperatures from 60 mK to 1.3 K. The FLL undergoes a series of symmetry and reorientation transitions from hexagonal to rhombic to square and back, as the applied magnetic field is increased from zero to H_c2. While the low-field transitions can be understood as driven either by non-local effects coupled to an anisotropic Fermi surface or the d-wave pairing, the high-field behavior is presently unexplained.

The magnetic field distribution around the vortices was studied by measuring the FLL absolute scattered intensity which allow a determination of the vortex form factor. The form factor shows a striking departure from the usual exponential decrease with increasing field. Rather, the form factor remains constant in fields up to 2 T, above which it increases. At H_c2 the form factor drops abruptly to zero, probably reflecting the first order nature of the upper critical field in this material and/or the presence of a non-Fermi liquid state and a quantum critical point just above the upper critical field. While not understood in detail, these results indicates a strong field dependence of the penetration depth and/or coherence length, rendering the notion of characteristic length scales meaningless. Comparison will be made with measurements on TmNi₂B₂C in the paramagnetic state above T_N.

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