Five U-M Physics faculty have been promoted starting in the fall of 2019 - two to Associate Professor with Tenure and three to Professor. Typically, Assistant Professors are newly hired faculty in tenure-track positions. After approximately six years, faculty performance is reviewed for promotion to Associate Professor, often with tenure. To receive tenure, faculty members must demonstrate scholarly contributions such as journal publications, as well as service to their department through teaching, mentoring, or leadership roles. After continued demonstration of scholarly contributions, faculty can be promoted to Professor, sometimes called “Full Professor.”
This year, Professor Xiaoming Mao and Professor Tom Schwarz have been promoted to Associate Professor with Tenure. Professor Lu Li, Professor David Lubensky, and Professor Vanessa Sih have been promoted to Professor.
Professor Xiaoming Mao’s research is in theoretical soft matter physics. This branch of physics is relatively new and encompasses studies of gels, colloids, liquid crystals, and more. One of Professor Mao’s research interests is on the rigidity, or stiffness, of matter. Rigidity in physics is what separates a liquid from a solid, and it arises from interactions between the particles making up the material as well as from the geometry of how the particles pack together. In materials like gels, the division between liquid and solid is murky, so the classification of transitions between different phases becomes important. Professor Mao has studied phase transitions in colloidal gels, in which tiny solid or liquid particles are dispersed in the gel. The particles can form different structures depending on their properties and their concentration in the gel. Professor Mao’s research aims at understanding and controlling the emergence of rigidity of these gels as they are compressed and squeezed in various ways.
Another of Professor Mao’s research directions is in topological metamaterials. Topology has to do with properties of an object that do not change when the object is smoothly deformed, as if it were made of Play-Doh. A well-known example of a topological property is the number of holes an object has - one for a donut and zero for a ball. However, to shape a ball into a donut, it must undergo a “topological phase transition,” in this case tearing the Play-Doh to make the hole. In real materials, topology leads to unusual, “topologically protected,” physical properties that show up on the surfaces or edges. Professor Mao has extended her studies of rigidity to this type of material to understand how topology plays a role in the phase transition between solid and liquid. She predicted a metamaterial, or a material that does not occur in nature, where the rigidity of the surface can be changed from soft to hard based on the material’s topological properties. Professor Mao is collaborating with experimental groups to perform tests on these materials and design devices based on the exotic surface properties.
Outside of research, Professor Mao has taught upper-level undergraduate and graduate courses. An important aspect of her teaching method is project-based learning, which engages students more fully with the material than traditional lecture-based coursework. Projects help students explore topics that are interesting to them and allow them to apply course material, improving their understanding. Professor Mao employs project-based learning through final presentations, which also gives students speaking experience. In addition, Professor Mao plans to develop a new course on soft matter physics for graduate students and upper-level undergraduates.
Professor Tom Schwarz is a member of the ATLAS collaboration at the Large Hadron Collider (LHC). His research focuses on studies of the Higgs boson and searches for exotic particles. The Higgs boson is one of the particles of the Standard Model, the theory describing fundamental particles and their interactions. Standard Model particles all have different masses, from the photon, with zero mass, to the top quark, which is comparable in weight to a tungsten atom. In the 1960s, researchers predicted an additional particle which would influence how heavy the other fundamental particles are. This additional particle is the Higgs boson. The Higgs boson was first measured in 2012, and it was the final particle of the Standard Model to be detected. After the initial measurement, other properties and interactions involving the Higgs boson have been studied at the LHC.
Professor Schwarz studies interactions between the Higgs boson and the top quark, which is especially interesting due to the large mass of the top quark. In addition, he worked on the most sensitive probe of interactions that produce two Higgs bosons together. Not only does this experiment study properties of the Higgs boson, it can help search for new interactions beyond the Standard Model. Professor Schwarz also works on searches for vector-like quarks, a predicted heavy type of particle that is not part of the Standard Model. In addition to measurements, Professor Schwarz is leading the U.S. effort in a hardware upgrade for the muon spectrometer, or outer shell, of the ATLAS detector. At U-M, he and his collaborators work on design and testing of new pieces for the muon spectrometer.
In addition to research, Professor Schwarz has been involved in service and teaching. He organizes a semester abroad program for U-M physics students at CERN along with Professor Emeritus Jean Krisch. He also serves on the United States ATLAS committee for Diversity, Equity and Inclusion. At U-M, Professor Schwarz has taught a number of upper-level undergraduate courses, helping to implement computational work in some to provide programming experience to students.
Professor Lu Li is an experimental condensed matter physicist focusing on studies of novel electric, magnetic, and thermal properties of materials. To perform some of his measurements, Professor Li has developed a technique called quartz magnetometry, which employs a quartz crystal to measure electronic and magnetic properties of a material in a magnetic field. Quartz crystals are commonly used in devices like digital watches, clocks, and phones. They rely on a property called piezoelectricity, which describes the relationship between electric and mechanical responses of a material. This occurs when a voltage is applied to the quartz crystal, causing a distortion in the shape. When the voltage is removed, the crystal returns to its original shape. These electrical-mechanical effects are utilized to create a cyclic response occurring at a specific frequency, and this allows the crystal to be used, for example, to keep accurate time.
From a scientific perspective, specific frequencies of quartz crystals can be used to probe properties of materials in magnetic fields. One effect they can be used to measure is quantum oscillations, where quantum mechanical principles cause a “wavy” variation of a material property such as resistance. Measuring these waves can help researchers understand characteristics of a material, like how many electrons are present or how easily the electrons can move. Quantum oscillations can only be observed at very low temperatures and very high magnetic fields, and quartz is useful for this because its frequency response doesn’t change much under these extreme conditions. Professor Li has measured quantum oscillations in materials including bismuth, samarium hexaboride, and ytterbium dodecaboride to investigate some of their surprising properties.
Besides research, Professor Li enjoys teaching undergraduate courses. This semester, he is teaching a physics course geared toward future elementary school teachers. In addition, he coaches young students at Angell Elementary School for their competition in the Washtenaw Elementary Science Olympiad.
Professor David Lubensky is a theoretical biophysicist. He studies animal development and circadian rhythms using computational tools and physical principles. A central area in studies of animal development, or, the process by which an egg grows into an adult organism, is how newly born cells “know” to specialize into certain types of tissue. Each type of tissue is made up of cells of a different shape and arrangement. The cell shapes can be modeled as polygons, and physics principles in self-assembly, or how polygons arrange to make a certain structure, can be used to investigate their ordering. Professor Lubensky is especially interested in applying these techniques to epithelial cells, which make up tissues like skin and blood vessels. One specific system he has studied is the zebrafish retina, in which the cells have a very regular organizational pattern.
Another area of research for Professor Lubensky is in circadian clocks, like the internal clock that governs an animal’s sleep-wake cycle. Clocks respond to daily variations in light or temperature, but they can continue to function even when there is no environmental variation. From a physics perspective, circadian clocks can be modeled as oscillators that repeat every 24 hours. Professor Lubensky has investigated how these oscillators are created using proteins that undergo cyclic reactions. Specifically, he has studied the S. elongatus bacterium, which is relatively straightforward to study as only three proteins are needed to reproduce the bacterium’s circadian clock.
Outside of research, Professor Lubensky has taught a variety of courses, including introductory physics courses and statistical mechanics for undergraduates. He has worked to update the statistical mechanics course by introducing active learning methods as an alternative to traditional lecture-based learning. Also, he has served as a mentor for graduate student instructors who lead discussion sections in upper-level undergraduate courses. In addition to teaching, Professor Lubensky is an organizer of the Ann Arbor APS Local Links networking group, which aims to connect academic and industry workers to facilitate professional relationships.
Professor Vanessa Sih’s research is in experimental condensed matter physics, studying semiconductors using optical techniques. Semiconductors are materials that have electrical properties that are in between those of metals and insulators, and our understanding of how to control the motion of electrons in semiconductor devices has led to the development and miniaturization of computers. The electrons and nuclei in the semiconductor, like all particles, have a quantity called "spin" associated with them, which one can visualize like a spinning top. Like the spinning top, spin has a direction based on its axis and direction of rotation. The degree to which the spins of many particles in a material are aligned with each other is called “spin polarization.”
Professor Sih studies spin polarization in semiconductors by using a pulse of laser light to first create a spin polarization and a second pulse of laser light to measure the extent to which the spins remain aligned together. From these measurements, she can determine how long the alignment remains present in the semiconductor, as well as probe tiny magnetic fields. These measurements aid in understanding what affects electron spins, which is important for potential future advances in computing. Modern computing relies on controlling the motion of electrons through semiconductors, but processes could be sped up if spin polarizations could be used to carry data in what is called “spintronic” computing.
In addition to research, Professor Sih serves as the Associate Chair for the graduate program in physics. In this role, she works on graduate student admissions and recruitment, among other responsibilities. She has also organized a new Life in Graduate School seminar series, which aims to give advice to graduate students on topics including funding, attending conferences, and publishing journal papers.