Experience for Undergraduates Program |
UC Davis Physics Department
June 16 to August 24, 2019
Dr. Chiang's laboratory uses high-resolution microscopy to study the surfaces of metals deposited on both semiconductor and metal substrates. This work improves our understanding of two-dimensional materials growth and transformation, which are of technological importance in semiconductor devices, magnetic disks, and heterogeneous catalysis. Understanding how the surface orders during the processes of surface reconstruction, adsorption of atoms and molecules, and phase transformations allows better control of the surface structure and properties. Dr. Chiang's group has recently measured the unusual collective motion of Ag and Au islands that form on Ge(110). An REU student will work with a graduate student in growing thin films of metals on semiconductors and imaging their surface changes at the nanometer to micron scale. The student will learn to operate sophisticated ultrahigh vacuum surface equipment: either a low energy electron microscope for real-time measurements at 10 nm lateral resolution of surface dynamical processes as a function of both temperature and adsorbate coverage, or a variable temperature scanning tunneling microscope for atomic-scale imaging of nucleation and growth at steps and defects. The student will also use image processing software to analyze the measurements.
Dr. Curro's group studies the behavior of strongly correlated electron materials at low temperature using Nuclear Magnetic Resonance (NMR). There are numerous cases in the natural world where the collective behavior of a group differs dramatically from that of the individual particles. A prime example is the flocking of birds, which cannot be understood by investigating a single bird. These beautiful and unexpected phenomena are known as emergent behavior, and play a role in all realms of science, from the behavior of animals and people, to the quantum mechanical realm of the electron. In some cases electrons in condensed matter exhibit behavior typical of individual particles, but they can also do unexpected and surprising things. Superconductivity, for example, arises because of interactions between electrons that enables them to enter into a new quantum phase of matter. The group uses NMR to investigate these new types of quantum behavior of electrons, in materials such as heavy fermions, iron pnictides, and transition metal oxides. An REU student can participate in hands-on or computational aspects of the work.
Dr. Taufour's group designs, grows, and studies new materials with bizarre ground states. These are often strongly correlated electron materials in which the interactions of many electrons give rise to unusual phenomena, sometimes with the potential for practical applications. These physical properties can be explored and controlled with low temperatures, high magnetic fields, and high pressures. An REU student will take part in the Taufour group's ongoing research efforts and learn how to grow single crystals of novel intermetallic compounds. The student will collaborate with other research groups at UC Davis to further characterize and understand the physical properties of the new crystals.
Dr. Yu takes advantage of the monocrystallinity and the large surface-to-volume ratio in low dimensional nanocrystals to explore both fundamental science and possible applications. One possible project for an REU student is to study vanadium dioxide (VO2) nanowires. VO2 undergoes an insulator to metal phase transition at about 68 degrees Celsius where the conductivity changes by orders of magnitude, with the phase transition mechanism still under debate. Since the transition can also be induced by electric field, it may lead to a new type of low-power, high-speed transistor. Another possible project is on hybrid methyl ammonium lead halide (CH3NH2PbX3, X=I, Br, Cl), a candidate material for solar cells. The Yu group aims to synthesize nanowires and nanoplates composed of this material and examine the photo-conversion and carrier transport properties. For either project the REU student will synthesize samples by vapor deposition, fabricate nano-devices through lithography, measure them optoelectronically, and analyze the data.
Granular materials such as sand, composed of large numbers of solid particles, do not behave exactly like any of the standard phases of matter. For example there can be liquid-like flow in avalanches, but structures like sand castles are instead solid-like. Dr. Zieve's lab studies the effects of grain shape on the stability of an artificial sandpile. The "grains" are ball bearings welded together to construct more complex shapes. These grains are placed in a pile inside a rotating drum. As the drum tilts, the pile gets steeper and eventually undergoes an avalanche. The angle reached before the avalanche depends on the shapes of the individual particles that make up the pile. The current goal is to understand how the exact configuration of grains within a pile causes the onset of the avalanche to change. To increase the variation among configurations, each pile is a mixture of two grain shapes. A student will improve the existing Python code for recognzing the location of grain shapes and will use the code to analyze thousands of avalanches and find the connections between configuration and stability.
Dr. Scalettar's group uses Quantum Monte Carlo simulations to study magnetic, metal-insulator, superfluid and superconducting transitions in condensed matter and in atomic condensates. Projects generally involve studying a simple model to see if it can capture the qualitative physics of a particular experimental system. Typically an REU student begins by learning some of the fundamentals of the Monte Carlo method and statistical mechanics before learning about an open question in the field and writing a research code to address it. (An introductory college programming class provides sufficient background.) Dr. Scalettar's group includes postdoctoral researchers, graduate and undergraduate students with whom the REU student can work. Work of past REU students can be found here:
Dr. Singh's project involves series expansion methods, such as high-temperature series expansions or expansions in coupling constants, which provide controlled ways to study thermodynamic properties of macroscopic many-body systems. They are straightforward to calculate and analyze, and useful for understanding a variety of systems and experimental probes, including magnetic susceptibility and specific heat of spin models, spin-wave spectra of magnetically ordered phases, critical phenomena near quantum phase transitions, and quantum entanglement in many-body systems. A summer project will focus on one of these problems. The student will learn and apply certain expansion methods. To complete the project within the REU timeframe, the student should have some prior knowledge of quantum mechanics and computer programming.
The Cox group, in collaboration with two labs in the chemistry department, is working to develop self-assembling proteins as scaffolding for materials applications from energy storage to tissue engineering. The relevant proteins have a "solenoid" structure in which the protein backbone forms a coil, stabilized by the packing of hydrophobic amino acid side chains in the interior. The outward-facing side chains can be modified for multiple purposes from templating the growth of nanoparticles to binding living cells or grabbing graphene layers. The diversity of the protein structures and their applications enable a range of student projects including: (i) realistically modeling the growth of protein assemblies on the computer; (ii) searching for and building simulation models of genetically homologous proteins whose structures have not been characterized; (iii) designing interface structures that can grab nanoparticles or bind to materials; (iv) theoretically modeling characteristics of self-assembled devices for energy storage or energy generation. Projects with experimental components are also possible.
With the nuclear group's experiments running at Brookhaven's Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) at CERN, an REU student will mainly do calculations and data analysis. As one example, the production of hadrons in heavy ion collisions at RHIC has been parameterized as a function of the number of participating nucleons and the number of binary collisions. These numbers are useful when comparing measured quantities as a function of the centrality of the collision to calculations done for the same centralities. Unfortunately, neither of the numbers can be measured directly in the experiment. Instead their values are obtained by comparing the measured distribution of charge multiplicity to the corresponding distributions obtained from phenomenological Glauber calculations. Applied to nucleus-nucleus collisions, Glauber theory calculates cross-sections from quantitative considerations of the geometrical configuration of the nuclei. An REU student will write, compile and validate code for a Glauber calculation. Ultimately, from a probability distribution for nucleons within the nucleus (based on the measurements of nuclear matter density) and a fundamental cross-section for nucleon-nucleon collisions, the code will calculate the numbers of participating nucleons and binary collisions as a function of impact parameter. Time permitting, the student can apply phenomenological models to obtain the charge multiplicity and compare the results to measured distributions.
Dr. Tripathi works on the Large Underground Xenon (LUX) experiment, which is the world's most sensitive device for detecting interactions of dark matter with normal matter, and its follow-up larger version called LZ. LUX is a large two-phase xenon time-projection chamber, located nearly one mile underground in the Homestake mine in Lead, South Dakota. It has recorded 300 days of data, which offers several opportunities to search for rare event phenomena. Meanwhile the group is also involved in developing analog front-end electronics for LZ. The group's work at UC Davis allows various possible computer-based or hands-on REU projects: analyzing data from LUX; designing, prototyping and testing electronics for LZ; developing silicon photomultipliers, which are the future for single photon detection; and radiation hardness testing of materials and electronics. The last of these is aimed at the Compact Muon Solenoid experiment at the Large Hadron Collider, in which Dr. Tripathi is working on data analyses involving searches for dark matter and dark photon production.
Accelerating charges radiate energy and momentum, and therefore experience a drag force. This force is not included in the Lorentz force. The problem of correctly accounting for this "radiation reaction" force is over 100 years old, and has been called a "perpetual problem" in physics. A student will begin by carrying out calculations of radiation reaction using conservation of total energy and momentum as a guiding principle. The goal is to understand how to compute the radiation reaction effect as a small perturbation to the motion given by the Lorentz force. This should lead to an improved treatment of the topic compared to the half-truths contained in textbooks. Time permitting, the student will then investigate how quantum effects cut off the runaway solutions that plague traditional approaches to the problem.
Dr. Knox is a theorist who works directly with high-resolution data on the cosmic microwave background radiation as a member of the Planck and South Pole Telescope observational teams. An REU student will create visualizations of the propagation of gravitationally-driven sound waves in the plasma that filled the Universe for its first 380,000 years. By adding in new components such as extra species of neutrinos or "early dark energy," the visualizations will show how the evolution of the plasma depends on the matter content of the universe. The student will learn basics of cosmology, Fourier analysis, and relativistic perturbation theory. He or she will use existing Einstein-Boltzmann code solvers (in C++ or in Fortran) and write additional software for graphical analysis of the output through still plots and animations.
Hidden photon dark matter experiment: Many types of astronomical observations show decisively that most of the mass in the Universe is of an unknown form, unlike ordinary matter. This "dark matter" fills the universe and clumps over cosmic time under its own gravitational self attraction. Our current understanding of physics cannot explain dark matter; its existence is evidence for new physics! Its physical nature is a central unanswered question in science. Sensitive searches for weakly interacting massive particles have found nothing. Other possibilities for dark matter, such as the ultra-low mass regime, remain unexplored. The REU student would work with Prof. Tyson and grad student Ben Godfrey on an extremely sensitive laboratory experiment: "dark E-field radio." The result will be at least a 10,000-fold improvement over current astrophysical limits in dark matter detection searches in the vast unexplored ultra-low mass regime. Experience in electronics and radio highly desired.