Fermi Surfaces without Quasiparticles

Tom Lancaster, Stephen J. Blundell

J. W. Clark, VICTOR A. KHODEL, MIKHAIL V. ZVEREV

Opportunities for topological phase transitions in strongly correlated Fermi systems near a quantum critical point are explored as an alternative to collective scenarios for experimentally observed departures from standard Fermi-liquid behavior. Attention is focused on a quantum critical point at which the effective mass is divergent due to vanishing of the quasiparticle group velocity at the Fermi surface. Working within the original Landau quasiparticle theory, it is demonstrated that the quasiparticle picture can remain meaningful beyond the quantum critical point through rearrangements of the unstable normal Fermi surface and quasiparticle spectrum. Two possibilities emerge at zero temperature, depending on whether the quasiparticle interaction is regular or singular at zero momentum transfer. In the regular case, one type of topological phase transformation leads to a state with a multiconnected Fermi surface. In the singular case, another type of topological phase transition leads to an exceptional state containing a fermion condensate – the Fermi surface swells into a volume in momentum space, within which partial occupation prevails and quasiparticle energies are pinned to the chemical potential. As the temperature increases from zero to a characteristic value Tm, a crossover can occur from the state with multiple Fermi surfaces to that containing a fermion condensate.

G. Benfatto, Giovanni Gallavotti

We describe an approach to the theory of the Fermi surface based on the renormalization group. A notion of the quasiparticle appears naturally in the theory. The \ensuremath{\beta} function is finite to all orders of the perturbation theory and we compute and discuss it to second order. The normality of the Fermi surface is linked to the repulsivity of the quasiparticle potential; the remarkable cancellations that occur in the \ensuremath{\beta} function lead one to think that a normal Fermi surface can perhaps occur, but only when the interaction between the quasiparticles is repulsive. The approach was first proposed in an earlier paper and we give an improved version of it here referring to the earlier work only to make use of technical estimates derived there.

R. N. Bhatt, Matteo Ippoliti

Delong Fang

The spatial distribution of vortex bound states is often anisotropic, which is correlated with the underlying property of materials. In this work, we examine the effects of Fermi surface anisotropy on vortex bound states. The large-scale calculation of vortex bound states is introduced in the presence of fourfold or twofold Fermi surface by solving the Bogoliubov–de Gennes (BdG) equations. Two kinds of quasiparticles' behaviors can be extracted from the local density of states (LDOS) around a vortex. The angle-dependent quasiparticles will move from high energy to low energy when the angle varies from curvature maxima to minima of the Fermi surface, while the angle-independent quasiparticles tend to stay at a relatively higher energy. In addition, the weight of angle-dependent quasiparticles can be enhanced by the increasing anisotropy degree of Fermi surface.