# Talks

### 2016

Theoretically it has been long known that breaking spin-degeneracy to realize so-called “spinless fermions” is a promising path to topological superconductivity. However, topological superconductors are rare to date. We propose a new strategy for materializing “spinless fermions” by splitting the spin-degeneracy in momentum space. Specifically, we identify monolayer hole-doped (p-type) transition metal dichalcogenide(TMD)s as candidates that can materialize topological modulated superconductors out of such momentum space split “spinless fermions”. In fact, superconductivity in electron-doped (n-type) TMD’s is by now well established. However, light hole-doping puts these systems in an unusual state with spin-valley locking that is absent in the electron-doped side. Using a renormalization group analysis, we predicttopological intra-pocket pairing with finite center of mass momentum to emerge from electron-electron repulsion in hole-doped TMD’s. Realization of an atomically thin two-dimensional topological superconductor that hosts majorana zero modes at the vortex core will form an ideal platform for braiding the zero modes: a long sought after holy grail.

### 2015

### 2014

Video and audio available at http://online.kitp.ucsb.edu/online/ironic_c14/kim/

Sr2RuO4 is the leading material candidate for topological triplet superconductivity yet its low transition temperature (*T**c*) limits experimental investigation of the system. One of the leading proposals for the mechanism of the observed superconductivity is the one dimensional band driven superconductivity mediated by antiferromagnetic fluctuations. Within this proposal a perturbative RG approach on a microscopic model with purely repulsive interactions yielded dominant triplet pairing tendency. In this approach the fermiology plays a key role in tilting the balance among different pairing possibilities and the superconducting *T**c*. This implies one can manipulate superconductivity through Fermi surface engineering. Motivated by the recent experimental advance in the growth of Ba2RuO4 films where an isovalent substitution of Sr2+ by Ba2+ produces negative chemical pressure, we investigate how the resulting changes in Fermi surface affect superconducting instability using the perturbative RG approach. We take the band structure fitted to Fermi surface measured using angle resolved photoemission spectroscopy as our input. We then compare the results to known effects of hydrostatic pressure.

Abstract: Much interest in the superconducting proximity effect in 3D topological insulators (TI) has been driven by the potential to induce exotic pairing states at the interface surface. However most candidate materials for 3D TI's are in fact bulk metals, due to the presence of bulk conduction states at the Fermi level. Nevertheless, such systems can have well-defined surface states exhibiting robust spin-momentum locking when the doping level is low enough. For such topological metals (TM), superconducting proximity effect can be qualitatively different from that in TI's. By studying a model topological metal-superconductor (TM-SC) heterostructure within Bogoliubov-de Gennes formalism, we show that the pairing amplitude is not confined to the interface as it is in topological insulator-superconductor (TI-SC) heterostructure and rather it reaches the naked surface. Furthermore, we predict vortex bound state spectra to contain a Majorana zero mode localized at the naked surface, separated from the bulk vortex bound state spectra by a finite gap in such a TM-SC heterostructure. Such naked-surface-bound Majorana modes are amenable to experimental observation and manipulation and hence present experimental advantage of TM-SC structure over TI-SC structure.

Recently, systematic penetration depth (PD) measurements carried out over several families of unconventional superconductors suggest they are near quantum critical points (QCP). In particular, the temperature dependence of the PD shows anomalous power law scaling. We argue, because the momentum carried by critical fluctuations needs to connect nodal points, this anomalous behavior is not due to AFM ordering. So instead, we focus on instabilities of the d-wave superconducting state associated with developing additional Q=0 order that can alter the scaling behavior of the PD. This additional ordering can be in either the charge channel, the pairing channel or both. We find that fluctuations in the pairing channel leads to scaling exponents smaller than one, while fluctuations in the charge channel leads to scaling exponents larger than one. Based on these results, we argue that the temperature scaling of PD in CeCoIn5 is caused by close a proximity to a QCP associated predominantly with Fermi surface distortions such as a nematic QCP.

### 2009

Dresden, Germany

### 2008

Similar talks were given for Condensed Matter Seminars at Cornell, UCLA, and Stanford, February and January, 2008