Summer Thesis 1-Fall 2016

and 1 collaborator

Interferometric Array Multi-Objective Visual Analytics

Transcranial direct current stimulation in chronic spinal cord injury: quantitative EEG study.

and 4 collaborators

Unearthing the contributions and ecology of uncultured soil bacteria to the metabolism of xylose and cellulose in soil.”

and 2 collaborators

Quantum Phase Transitions

# Introduction and previous work

The electronic properties of a material are primarelly dictated by three distinct effects\cite{2000}.

The ion potentials,

the electron-electron interaction, and

externally applied fields

A full solution accounting for all of these interaction would yield a many body problem that would typically be unsolvable. For this reason it is necessary to make approximations to obtain meaningful results. The most common of this approximations would be to ignore electron-electron interactions and assume a stationary periodic potential. This near-free electron problem leads to the Band Structure Theory that is often solved with assistance of Bloch Theorem \cite{kittel_1966}. This solution leads to a very simple classification of metals, and insulators based on their band structure and Fermi Energy.

This method, although often successful, it fails by omitting electron-electron interactions and lattice distortions. A clear example of this is NiO; by its half filed orbitals one would predict this material to be a metal- however it has insulating properties due to electron-electron interactions.

The presence of two opposite mechanism driving that drives to either metallic or insulating behavior can lead to some pretty interesting results. As an example of this consider a N hydrogen atoms distributed at a constant density. The system is described by two non-commuting operators; the Kinetic energy, and the ion potentials. The kinetic energy of the electrons will trend to spread the the electrons, and the ion potential (driven by the coulomb force) will trend to localize the electrons near the protons. The electrons therefore have to phases; bound, or unbound. At low densities, the attractive potential can act over long distances which localizes the electrons. This is an insulating state. As we increase the density we pass a critical density after which the attractive potential is screened by other particles in the systems for the electrons to be bound\cite{1982}. This leads to a metallic state at high densities.

This change in properties can be characterized as quantum phase transition. In general a quantum phase transition can be induce by varying the relative strength of two non-commuting operators. It is therefore possible to externally alter the relative strength of these mechanisms by varying an external parameter i.e. pressure \cite{2000} . This metal-insulator transition as quantum phase transition is shown in figure 1. It is worth noting that proper transition is only attained at zero temperature. This is because at higher temperature there is always some conduction, at which point it becomes a discussion of how good of an insulator it is.

Topologically Non-trivial RTD: Final Report

Topological Edge states

**Introduction:** Topological Insulator has drawn a lot of attention recently in condensed matter physics. It describes the phase of matter in a different way, and gives us a new perspective toward materials. What is Topological Insulator(TI)? It is a material with bulk bandgap. However, at the surface of TI, it has edge states that propagate like a metal. We can imagine this like plastic tube wrapped with a aluminum foil around the tube. And it is proposed that it might be a potential candidate as fault tolerant quantum computation because the edge states are protected by time reversal symmetry. Protected state means that it is robust against impurity or imperfections in the crystal. The first 3D TI was observed in semiconducting alloy *B**i*_{1 − x}*S**b*_{x} with angle resolved photoemission spectroscopy(ARPES) \cite{Hsieh_2008}. The reason to choose Bisumth is due to its strong spin orbit interaction which is essential to see TI edge states. But the *B**i*_{1 − x}*S**b*_{x} surface states are complicated, so it invokes other materials such as *B**i*_{2}*S**e*_{3} \cite{Xia_2009}.

**Integer Quantum Hall Effect:** To discuss TI surface edge states, we begin with the very similar cousin of TI edge states, which is quantum hall effect(QHE), or more accurately integer quantum hall effect. First observation of quantum hall effect was conducted in 1980 in MOSFET system in the low temperature and high magnetic field environment\cite{Klitzing_1980}. The astonishing experiment result is that the conductivity(or resistivity) of the system has plateau with increasing magnetic field as following.

**Charge Density Waves, their Transport in Layered Materials, and Potential Applications**