Jian-Huang She

Inelastic electron tunneling spectroscopy for topological insulators.

Inelastic electron tunneling spectroscopy is a powerful spectroscopy that allows one to investigate the nature of local excitations and energy transfer in the system of interest. We study inelastic electron tunneling spectroscopy for topological insulators and investigate the role of inelastic scattering on the Dirac node states on the surface of topological insulators. Local inelastic scattering is shown to significantly modify the Dirac node spectrum. In the weak coupling limit, peaks and steps are induced in second derivative d2I/dV2. In the strong coupling limit, the local negative-U centers are formed at impurity sites, and the Dirac cone structure is fully destroyed locally. At intermediate coupling, resonance peaks emerge. We map out the evolution of the resonance peaks from weak to strong coupling, which interpolate nicely between the two limits. There is a sudden qualitative change of behavior at intermediate coupling, indicating the possible existence of a local quantum phase transition. We also find that, even for a simple local phonon mode, the inherent coupling of spin and orbital degrees in topological insulators leads to the spin-polarized texture in inelastic Friedel oscillations induced by the local mode.

Noncontact friction and relaxational dynamics of surface defects.

The motion of a cantilever near sample surfaces exhibits additional friction even before two bodies come into mechanical contact. Called noncontact friction (NCF), this friction is of great practical importance to the ultrasensitive force detection measurements. The observed large NCF of a micron-scale cantilever found an anomalously large damping that exceeds theoretical predictions by 8-11 orders of magnitude. This finding points to a contribution beyond fluctuating electromagnetic fields within the van der Waals approach. Recent experiments reported by Saitoh et al. [Phys. Rev. Lett. 105, 236103 (2010)] also found a nontrivial distance dependence of NCF. Motivated by these observations, we propose a mechanism based on the coupling of a cantilever to the relaxation dynamics of surface defects. We assume that the surface defects couple to the cantilever tip via spin-spin coupling and their spin relaxation dynamics gives rise to the backaction terms and modifies both the friction coefficient and the spring constant. We explain the magnitude, as well as the distance dependence of the friction due to these backaction terms. Reasonable agreement is found with the experiments.

Proximity of the Superconducting Dome and the Quantum Critical Point in the Two-Dimensional Hubbard Model

We use the dynamical cluster approximation to understand the proximity of the superconducting dome to the quantum critical point in the two-dimensional Hubbard model. In a BCS formalism, T(c) may be enhanced through an increase in the d-wave pairing interaction (V(d)) or the bare pairing susceptibility (χ(0d)). At optimal doping, where V(d) is revealed to be featureless, we find a power-law behavior of χ(0d)(ω=0), replacing the BCS log, and strongly enhanced T(c). We suggest experiments to verify our predictions.

Berezinskii-Kosterlitz-Thouless Transition to the Superconducting State of Heavy-Fermion Superlattices

We propose an explanation of the superconducting transitions discovered in the heavy-fermion superlattices by Mizukami et al. [Nature Phys. 7, 849 (2011)] in terms of Berezinskii-Kosterlitz-Thouless (BKT) transition. We observe that the effective mass mismatch between the heavy-fermion superconductor and the normal metal regions provides an effective barrier that enables quasi-2D superconductivity in such systems. We show that the resistivity data, both with and without magnetic field, are consistent with BKT transition. Furthermore, we study the influence of a nearby magnetic quantum critical point on the vortex system and find that the vortex core energy can be significantly reduced due to magnetic fluctuations. Further reduction of the gap with decreasing number of layers is understood as a result of pair breaking effect of Yb ions at the interface.

Negative- U superconductivity on the surface of topological insulators

We study the effects of a finite density of negative-U centers (NUCs) on the surface of a three-dimensional topological insulator. The surface Dirac fermions mediate a power-law interaction among the local Cooper pairs at the NUCs, and the interaction remains long-ranged for weak disorder. Superconductivity can be generated in the presence of a random distribution of NUCs. The NUCs play dual roles as both pair creators and pair breakers, and the competition of the two effects results in a nonmonotonic dependence of the mean-field superconducting transition temperature on the density of NUCs. Global phase coherence is established through coupling the locally superconducting puddles via Josephson coupling. Rare fluctuations play important roles, and a globally superconducting phase can only be achieved at large enough concentration of NUCs. The p-wave component of the superconducting order parameter gives rise to frustration among the superconducting grains, which is captured by a Potts-XY type model. New phases with chiral order, glass order, and, possibly, topological order can then emerge in the system of superconducting grains.

Statistics, condensation, and the Anderson-Higgs mechanism : Worldline path integral view

We explain, in the first quantized path integral formalism, the mechanism behind the Anderson-Higgs effect for a gas of charged bosons in a background magnetic field, and then use the method to prove the absence of the effect for a gas of fermions. The exchange statistics are encoded via the inclusion of additional Grassmann coordinates in a manner that leads to a manifest worldline supersymmetry. This extra symmetry is key in demonstrating the absence of the effect for charged fermions.

Inelastic electron tunneling spectroscopy at local defects in graphene

We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling ...

Stability of Quantum Critical Points in the Presence of Competing Orders

We investigate the stability of Quantum Critical Points (QCPs) in the presence of two competing phases. These phases near QCPs are assumed to be either classical or quantum and assumed to repulsively interact via square-square interactions. We find that for any dynamical exponents and for any dimensionality strong enough interaction renders QCPs unstable, and drives transitions to become first order. We propose that this instability and the onset of first-order transitions lead to spatially inhomogeneous states in practical materials near putative QCPs. Our analysis also leads us to suggest that there is a breakdown of Conformal Field Theory (CFT) scaling in the Anti de Sitter models, and in fact these models contain first-order transitions in the strong coupling limit. PACS numbers:

Strange Metals from Quantum Geometric Fluctuations of Interfaces

Our current understanding of strongly correlated electron systems is based on a homogeneous framework. Here we take a step going beyond this paradigm by incorporating inhomogeneity from the beginning. Specifying to systems near the Mott metal-insulator transition, we propose a real-space picture of itinerant electrons functioning in the fluctuating geometries bounded by interfaces between metallic and insulating regions. In 2+1 dimensions, the interfaces are closed bosonic strings, and we have a system of strings coupled to itinerant electrons. When the interface tension vanishes, the geometric fluctuations become critical, which gives rise to non-Fermi-liquid behavior for the itinerant electrons. In particular, the poles of the fermion Greens function can be converted to zeros, indicating the absence of propagating quasiparticles. Furthermore, the quantum geometric fluctuations mediate Cooper pairing among the itinerant electrons, indicating the intrinsic instability of electronic systems near the Mott transition.