Eric Sembrat's Test Bonanza

Abstract: We explore light-activated processes in materials and molecules using first principles, quantum mechanical calculations. Our aim is to address two outstanding challenges: designing sustainable materials to efficiently capture and store solar energy, and predicting systems, for example optically addressable spin qubits, to build novel sensors and computers.

Abstract: My lab studies how the movement and shape of living cells is controlled by living materials constructed by protein assemblies within the cell interior. In this talk, I will describe my lab’s recent efforts to understand the design principles of the active, soft materials that drive morphogenesis of epithelial tissue. In particular, we are interested in the design principles by which protein-based materials generate, relax, sense and adapt to mechanical force. Here I will describe our current experimental efforts to study the regulation of the shape and size of epithelial cells. If time allows, I will discuss how physical constraints govern cell size regulation in epithelial tissue.

Abstract: Academic collaboration with China was once encouraged by the US government and universities. As tension between the two countries rises rapidly, those who did, especially scientists of Chinese descent, are under heightened scrutiny by the federal government. Law enforcement officials consider collaborating with Chinese colleagues “by definition conveying sensitive information to the Chinese.” In 2015, I became a casualty of this campaign despite being innocent. “China Initiative” established by the Justice Department in 2018 has resulted in numerous prosecutions of university professors for alleged failure to disclose China ties.

In this talk, I argue that academic decoupling is not in America’s interest. It is a tall order to convince the public and policy makers of this fact, but the scientific community must try lest the American leadership in science and technology will be irreparably damaged.

I will review the growth of quantum physics from its inception in the beginning of the 20th century to its maturity 50 years later. My narrative includes dramatic stories of the quantum pioneers between two wars. Caught between two evil regimes –– those of the National Socialist German Workers’ Party (known as Nazis) and the SU Communist Party (known as Bolsheviks) –– they had to make difficult choices in pursuing their scientific goals and aspirations.

Abstract: The magnetic fields generated by spins and currents provide a unique window into the physics of correlated-electron materials and devices. Proposed only a decade ago, magnetometry based on the electron spin of nitrogen-vacancy (NV) defects in diamond is emerging as a platform that is exceptionally suited for probing condensed matter systems. It can be operated from cryogenic temperatures to above room temperature, has a dynamic range spanning from DC to GHz, and allows sensor-sample distances as small as a few nanometers. As such, NV magnetometry provides access to static and dynamic magnetic and electronic phenomena with nanoscale spatial resolution. While pioneering work focused on proof-of-principle demonstrations of its nanoscale imaging resolution and magnetic field sensitivity, now experiments are starting to probe the correlated-electron physics of magnets and superconductors and to explore the current distributions in low-dimensional materials.

In this talk, I will review some of our recent work that uses NV center magnetometry to image skyrmions in thin magnetic films, measure the spin chemical potential in magnetic insulators, and image hydrodynamic electron flow in layered materials. In addition, I will describe the use of NV centers in a new scattering platform that uses spin waves as the probing excitation.

Abstract: Recent developments [1] have shown that the collapse hypothesis is self-inconsistent and is no longer a viable theory. (The collapse hypothesis in quantum mechanics is that the state of a system is collapsed by a measurement, by some process of unknown character.) Based on earlier work by Ballentine [2] on the Ensemble Interpretation (in spite of the familiar-sounding name, this is almost entirely unknown), more recent work [3] has filled it out and demonstrated clearly that it provides a simple and natural resolution to the problem of measurement in quantum mechanics. We discuss the Ensemble Interpretation and recent developments, measurement theory in quantum mechanics (correcting many common misconceptions), and the resolutions of Schrödinger Cat, Wigner’s Friend, and Extended Wigner’s Friend experiments. Given that almost all current QM textbooks are based on the now defunct collapse hypothesis [4], we propose switching to textbooks based on the Ensemble Interpretation [5] as a means of restoring consistency to QM. [1] D. Frauchiger & R. Renner, “Quantum theory cannot consistently describe the use of itself”, Nature Comm [2018] [2] L. Ballentine, “The statistical interpretation of quantum mechanics”, Rev Mod Phys [1970] [3] A. Rizzi, “How the natural interpretation of QM avoids the recent no-go theorem” Foundations of Physics [2020] and “A simple approach to measurement in quantum mechanics” arXiv [2020] [4] Two common examples include Griffiths and Morrison. Even though both discuss the use of ensembles, the statistical understanding is undermined completely by use of the collapse hypothesis in discussing measurements. [5] See L. Ballentine, “Quantum Mechanics” and A. Rizzi, “Physics for Realists: Quantum Mechanics”.

Bio: Dr. Jasmine Nirody is an Independent Fellow at the Rockefeller Center for Theoretical Studies, where she is supported partially by the James S. McDonnell Foundation Fellowship for Complex Systems. She is also a Fellow of All Souls College, University of Oxford. Dr. Nirody is interested in the physics of how organisms interact with natural environments, and the interplay between structure / morphology and mechanical function. Dr. Nirody's work aims to provide insight into the evolutionary history of organismal design, as well as into design principles for the development of bioinspired synthetic systems. Abstract: Natural environments are heterogeneous and can fluctuate with time. As such, biomechanical systems from proteins to whole organisms have developed strategies to deal with considerable spatial and temporal variability. Understanding the physics behind these strategies is important both in an evolutionary context and for the development of bioinspired systems. I will discuss two (quite different!) broadly successful locomotive modes: flagellated motility in bacteria and interfacial locomotion in geckos. (1) A bacterium's life can be complicated: it must swim through fluids of varying viscosity as well as interact with surfaces and other bacteria. We characterized the mechanosensitive adaptation in bacterial flagella that facilitates these transitions by using magnetic tweezers to manipulate external torque on the bacterial flagellar motor. Our model for the dynamics of load-dependent assembly in the flagellar motor illustrates how this nanomachine allows bacteria to adapt to changes in their surroundings. (2) Animals that live in areas with periodic flooding must deal with seasonal fluctuations in their habitats. In the field, we showed that tropical geckos can run across the water’s surface as fast as they can on land. In the lab, we showed that these geckos use both surface slapping and surface tension, as well as take advantage of their superhydrophobic skin, to transition between terrestrial and semi-aquatic locomotion.

Abstract: With the detection of gravitational waves emitted during black hole and neutron star mergers, LIGO has recently opened the field of gravitational wave astrophysics. In this talk I will review the astrophysical processes that may be responsible for the formation of the observed events. The event rate distribution with mass, spins, eccentricity and redshift may be used to discriminate among different processes that lead to black hole mergers. I will show that the standard astrophysical merger pathways are already in tension with LIGO/VIRGO observations. New ideas may be needed to explain the origin of the detected sources. I will discuss the possibility that black hole mergers happen in active galactic nuclei where the interaction with a gaseous disk helps to form binaries, and a combination of dynamical and gas effects facilitate the merger of the binaries. Bio: Prof. Kocsis is a theoretical astrophysicist working on a wide range of topics including gravitational wave astrophysics using LIGO/VIRGO, LISA, and pulsar timing arrays, astrophysical dynamics of dense star clusters, astrophysical general relativity, black holes physics from stellar mass to supermassive scales, accretion disks, disk-satellite interactions, and statistical mechanics. Before coming to Oxford, Prof. Kocsis was a faculty member at Eotvos University, and even earlier, a postdoctoral fellow at the Institute for Advanced Study and at the Harvard Center for Astrophysics. Prof. Kocsis is the PI of the GALNUC project funded by an ERC Starting Grant which strives to develop a comprehensive model to describe the long term behavior of astrophysical multibody systems using multidisciplinary methods.

Abstract

Investigating the exotic quantum phenomena and the related ground state of the frustrated quantum magnets has caught a lot of attention over the past years .While the realization of quantum spin liquid (QSL) without conventional magnetic ordering even down to zero temperature is a challenging task, the Kitaev model is a prominent example with an exactly solvable spin model. The Kitaev materails so far have been focused on 4d/5d based systems with effective spin-1/2, such as H3LiIr2O6, α-Li2IrO3, α-Na2IrO3, and α-RuCl3, which exhibit zigzag antiferromagnetic (AFM) ordering at zero magnetic field owing to non-Kitaev interactions such as Heisenberg exchange or off-diagonal exchanges. Recently, the theoretical studies propose that the 3d-cobalt honeycomb magnets with effective spin-1/2 can be another playground to realize the Kitaev model. For example, non-Kitaev terms have been predicted to be almost vanishing with the small trigonal crystal fields acting on Co2+ ions, which makes the cobaltates as one good system for Kitaev physics. We studied the potential candidate Na2Co2TeO6 (NCTO) and performed magnetization, heat capacity, high-field electric spin resonance, and inelastic neutron scattering (INS) measurements. A complicated magnetic field-temperature phase diagram was demonstrated with a zigzag AFM state and spin gap was observed in the spin dynamics. A quantum spin disordered state is induced by an ab-plane magnetic field between ~ 7.5 and 10.5 T like the field-induced QSL in α-RuCl3. Through detailed theoretical simulations on the INS spin-wave spectrum, we could obtain different ratios of the Kitaev interaction and the Heisenberg exchange. Hence, we anticipate our work will generate significant interest: it will trigger new experimental efforts for other realizations of magnetic field inducing the exotic quantum states and rejuvenate theoretical and computational efforts to understand the elusive dynamics in the Kitaev model for frustrated quantum magnetism.