Category Archives: LASSP Seminar

The Theory of the Nodal Nematic Quantum Chirality

Posted on by

Eun-Ah Kim, Stanford University

2/7/08 – 4:30 p.m., 700 Clark Hall

In the last several years, experimental evidence has accumulated in a variety of highly correlated electronic systems of new quantum phases which (for purely electronic reasons) spontaneously break the rotational (point group) symmetry of the underlying crystal.  Such “nematic” phases have been seen in quantum Hall systems, in the metamagnetic SrRuO, and more recently in magnetic neutron scattering studies of the high temperature superconductor, YBCO.

The quantum dynamics of the nematic order parameter, especially near a putative quantum critical point, naturally couples strongly to low energy fermionic excitations (quasiparticles).  Such coupling can affect the nature and even the fate of quasiparticles making a study of nematic quantum criticality a   highly nontrivial problem. In this talk, I will discuss our recent results on one case in which this problem can be attacked in a tractable manner: the nodal nematic quantum criticality. Deep inside a d-wave superconducting state in a two-dimensional tetragonal crystal, the phase space for the low energy fermions reduce from the entire Fermi surface to four nodal points. The limited phase space for scattering allows for a controlled analysis of the nematic quantum criticality for nodal fermions.

We study the character of this quantum phase transition.  We investigated the problem by solving a model system with $N$ flavors of quasiparticles in the large N limit.   We find the transition to be continuous in this limit and the critical nematic fluctuations to have drastic effects on the nature of nodal quasiparticles.  To leading order in 1/N, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strongly angle dependent scattering leading to a nontrivial structure in the single particle spectral function. I will discuss possible implications of our results for cuprate physics from the spectral function of the nematic mode and the single fermion spectral function.

3D Topological Insulators and 2D Anderson Delocalization

Posted on by

Speaker: Shinsei Ryu, University of California, Santa Barbara

2/5/08

4:30 p.m., 700 Clark Hall

Recently, a new type of insulators, called Z2 topological insulator, has been discovered. They can be thought of as a close cousin of the integer quantum Hall insulators, but different from the IQHE in many essential ways: Z2 topological insulators exist in systems that respect time reversal symmetry, can be either in 2D or 3D, and are characterized by a Z2 topological number, unlike the integral Hall conductance in the IQHE. Several candidate materials possessing the non-trivial Z2 topological features, such as HgTe quantum wells, and Bismuth-Antimony alloys, have been proposed and tested experimentally. In this talk, we will discuss quantum transport of 2D surface massless Dirac fermion states supported by 3D Z2 topological insulators terminated by a 2D boundary. Although these Dirac fermion states, from the symmetry point of view, belong to the 2D spin-orbit (symplectic) symmetry class of Anderson localization, they inherit the Z2 topological character of the bulk, and exhibit different response to impurities from conventional disorderedconductors with spin-orbit interactions. We also briefly discuss that this surface physics of 3D Z2 topological insulators can be simulated by graphene.