Research Project Areas:

• Condensed Matter Experiment
• Condensed Matter Theory
• Complex Systems and Computational Physics
• Nuclear Physics Experiment
• Experimental Particle Physics
• Theoretical Particle Physics
• Precision Measurement
• Astrophysics

Project Descriptions

Condensed Matter Experiment

Shirley Chiang

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 unusually high rate of mass transport of Pb atoms during island formation at 220-283K on Ge(111). 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.

Nick Curro

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.

Valentin Taufour

Dr. Taufour's group designs, grows, and studies new materials with bizarre ground states. In these materials, the interactions of many electrons give rise to unusual phenomena, sometimes with the potential for practical applications. These are often strongly correlated electron materials, topological materials, unconventional superconductors, or compounds with weird magnetic orders. 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 learn how to prepare ampoules for the synthesis of single crystals using the solution growth, Czochralski, Bridgman, or vapor transport technique. After a successful synthesis, the student will learn basic characterization techniques such as x-ray powder and Laue diffraction, magnetization measurements, and electrical resistivity measurements. The student will collaborate with other research groups at UC Davis to further characterize and understand the physical properties of the new crystals. Some knowledge of solid-state physics or inorganic chemistry is preferred but not required. Good hands-on skills are helpful as well.

Inna Vishik

The Vishik group uses angle-resolved photoemission spectroscopy (ARPES) and other photoemission techniques, as well as ultrafast optics to learn about electronic structure and dynamics in quantum materials. Quantum materials are characterized by emergence, whereby the properties of a many-electron system cannot be derived in a reductionist manner from the properties of one electron. As a result, these materials often yield experimental surprises, which can be discovered with precision tools sensitive to electrons such as the ones in this lab. The emergent phenomena studied include unconventional superconductivity, strong electronic correlations, topologically protected electronic states, and exciton condensation. An REU student would study one of the materials systems currently under investigation (including but not limited to: copper oxide superconductors, transition metal dichalcogenides, 3D topological insulators, magnetic Weyl semimetals), and simultaneously learn about optical systems, ultrahigh vacuum systems, instrumentation programming, data analysis, cryogenic systems, and data analysis.

Dong Yu

Dr. Yu investigates charge transport in low-dimensional materials using spatially resolved optoelectronic techniques. Irradiating a spot on the sample with a laser creates charge carriers. They move depending on sample characteristics and applied fields, and are detected as currents reaching fixed electrodes on the sample. By successively focusing the laser on different spots, his group can determine the lifetime and distance traveled by the excited carriers, and much more. One possible project for an REU student is to study hybrid halide perovskites for solar energy harvesting and light emitting devices. The other one is on topological insulators for spintronics and quantum computing. The student will have the opportunity of synthesizing materials, fabricating nano-devices, as well as optoelectronic measurements.

Rena Zieve

Below 2 Kelvin, liquid helium becomes superfluid, with unusual properties from zero viscosity to high thermal conductivity to quantized vortices. Although helium is the only superfluid on Earth, neutrons deep inside neutron stars also form a superfluid. The behavior of vortices in the neutron superfluid may explain the observed glitches in neutron stars' rotation, where the angular momentum abruptly increases. Dr. Zieve's group has set up an experiment to monitor a vessel of rotating superfluid helium for similar glitch behavior. If observed, the conditions for glitches to occur can be tested far more easily in the lab than by observing distant stars. An REU student will help carry out low-temperature measurements, analyze data, and make improvements to the setup.

Condensed Matter Theory

Richard Scalettar

Work of past REU students can be found here:

• Nicole Hartman: Phys. Rev. B93, 235143 (2016).
• Thomas Blommel: Phys. Rev. Lett. 120, 187003 (2018).
• Amelia Broman and Jack Mucciaccio, Phys. Rev. B105, 195429 (2022).
• Claire Kvande, Phys. Rev. B108, 075119 (2023).

Rajiv Singh

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.

Complex Systems and Computational Physics

Jim Crutchfield

Nuclear Physics Experiment

Manuel Calderon and Daniel Cebra

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.

Experimental Particle Physics

Eric Prebys

The Crocker Nuclear Laboratory, supervised by Dr. Prebys, is adjacent to the Physics Building. It houses a cyclotron with proton beams tunable from 4 MeV to 67.5 MeV. This is a rare energy range for today's machines, and the machine has several specialized uses, from simulating radiation effects of outer space to medical treatment via proton therapy. This leads to numerous unique opportunities for hands-on experience for undergraduates, who can participate in all aspects of planning, simulation, data taking, and analysis. One possible project for a student working with Dr. Prebys is to measure the energy-dependent production cross section for protons on various nuclear targets, which surprisingly is unknown for many materials. The student will insert thin foils of the target material in a stack of aluminum plates, which lower the beam energy to the desired value. After irradiation the student will remove the foils, assay them with a sensitive photon detector, and identify the characteristic spectra of the daughter particles of interest.

Theoretical Particle Physics

Markus Luty

Hamiltonian truncation is a variational approximation method in quantum mechanics that involves truncating the space of states to a finite-dimensional vector space. In this approximation, the Hamiltonian is a finite-dimensional matrix that can be diagonalized numerically. In recent years, this approximation has been applied to study quantum field theories to perform calculations in strongly-coupled theories where no perturbative expansion is possible. In this project, the student will apply these methods to simple quantum field theories in 1+1 spacetime dimensions. Students should have a strong background in quantum mechanics at the advanced undergraduate level, some exposure to quantum field theory, and some experience in scientific computation, preferably using C++.

Precision Measurement

Nancy Aggarwal

Dr. Aggarwal's research group uses precision quantum measurements to unravel our mysterious universe via her hunt for gravitational waves (GW), dark matter, GWs produced by dark matter, and dark matter signatures in GW-detectors. One direct search for dark matter is the ARIADNE experiment, where Dr. Aggarwal and her collaborators look for spin-dependent forces mediated by the axion. The axion is a hypothetical particle which if discovered will not only solve the strong CP problem, but might also be a portion of dark matter. The experiment searches for a new beyond-the-standard model force mediated by the axion in the form of a fictitious magnetic field, of the order of 10^-21 T, many orders of magnitude smaller than the Earth's field. This requires an extremely precise magnetometer -- nuclear magnetic resonance on highly polarized He-3. Unfortunately the He-3 can easily be depolarized, which destroys the pristine axion signal. Since a main source of depolarization is magnetic field gradients, we need to characterize the magnetic fields emanating from the many components of the experiment.

An REU student will characterize the magnetism of the rotation stage being built for the ARIADNE experiment. This will require building the testing assembly, taking low-noise magnetometry measurements, analyzing the data and presenting it in a format useful to the collaboration. This result would be a part of a future publication and possibly a conference poster.

Astrophysics

Tucker Jones

Dr. Jones studies how galaxies form in the early universe and evolve over time, using the world's most sensitive telescopes. His group studies gravitationally lensed galaxies which appear larger and brighter on the sky thanks to strong magnification, allowing resolution of their spatial structure even at great distances. From spectroscopic observations they address several topics including the formation of the first galaxy disks, the cycle of gas into and out of galaxies, and the distribution of heavy elements. An REU student will work on one of these aspects using data collected from Keck Observatory or the James Webb Space Telescope. Familiarity with computer programming (especially Python) and some knowledge of statistics, including curve fitting, is desirable.

Lloyd Knox

The student will explore the implications of contemporary cosmological observations, including those from the third generation camera on the South Pole Telescope (SPT), for cosmological models. The standard model of cosmology has been highly successful at predicting statistical properties of the microwave background, light element abundances, and various observables sensitive to the distance-redshift relationship at low redshift. There are also some puzzling discrepancies with standard cosmological predctions such as with Cepheid-calibrated supernova luminosities at low redshifts (the 'Hubble tension'), with the amplitude of matter fluctuations at low redshift (the 'S8 tension') and with the density of non-relativistic matter at low redshifts (which is complicating interpretation of our constraints on neutrino masses). New SPT data will be available by Summer 2025 (see Prabhu et al. 2024 and Ge et al. 2024) and should significantly reduce the wiggle room for cosmological models to address these issues. The REU student will explore the best ideas we have for theories compatible with the SPT and other cosmological data.

Andrew Wetzel

Dr. Wetzel's group uses the nation's most powerful supercomputers to simulate the formation of galaxies, including the physics of dark matter, gas hydrodynamics, star formation, and stellar evolution. They use these simulations to model the formation of our own Milky Way galaxy. An REU student will work on a project to model the formation and dynamical evolution of stars in simulations of Milky Way-like galaxies and compare with observations of the Milky Way and similar galaxies today. The student ideally should have some familiarity with Python, including data visualization with Matplotlib and array manipulation with NumPy and SciPy.