Physics and Astronomy

While ferromagnets have been known and exploited for millennia, antiferromagnets were only discovered in the 1930s. The elusive nature indicates antiferromagnets’ unique properties: At large scale, due to the absence of global magnetization, antiferromagnets may appear to behave like any non-magnetic material; At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. We study this exciting possibility in an antiferromagnetic Axion insulator, even-layered MnBi2Te4 flakes. We report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions.

Reference:

A. Gao, et al.  “Layer Hall effect in a 2D topological axion antiferromagnet.” Nature 595, 521 (2021).

In one spatial dimension, enhanced thermal and quantum fluctuations should preclude the existence of any long range ordered superfluid phase of matter.  Instead, the quantum liquid should be described at low energies by an emergent hydrodynamic framework known as Tomonaga-Luttinger liquid theory.  In this talk I will present details on some complimentary experimental and theoretical searches for this behavior in helium-4 including: (1) pressure driven superflow through nanopores, and (2) the excitation spectrum of a confined superfluid inside nano-engineered porous silica-based structures. For flow experiments, we have devised a framework that is able to quantitatively describe dissipation at the nanoscale leading to predictions for the critical velocity borne out by recent superflow measurements in nanopores.  In confined porous media, with radii reduced via pre-plating with rare gases, I will discuss ab initio simulations of phase and density correlations inside the pore that are in agreement with recent neutron scattering measurements.   Taken together, these results indicate significant progress towards the experimental observation of a truly one-dimensional quantum liquid.

This work was supported by the NSF through grants DMR-1809027 and DMR-1808440.  

Materials manipulation via ion or laser beams can achieve precisely tuned atomic geometries that are necessary, e.g. to engineer interactions between defects in quantum materials and for fabricating novel electronic devices with nanoscale dimensions. In addition, such beams are also used to characterize and probe materials properties by means of electronic and optical excitations. I will discuss recent quantum- mechanical first-principles predictions for electron dynamics and the subsequent ionic motion that follows after an excitation of the electronic system. Using real-time time-dependent density functional theory we simulated the underlying ultrafast time scales of electron dynamics in semiconductors and metals. Examples include long-lived electronic excitations in proton, electron, and laser irradiated bulk semiconductors that facilitate diffusion of point defects, such as oxygen vacancies in MgO. We compare such bulk simulations to aluminum surfaces under irradiation, for which we quantify electron emission, charge capture, and pre-equilibrium effects that are unique to thin films or two-dimensional materials. Limitations and possible extensions of the theoretical description will be included in the discussion.

Spintronics-based technology, which uses spins to represent and propagate information, holds promise to realize devices that surpass the current CMOS transistor technology in power, density and speed. For example, magnetic random-access memory (MRAM) based on magnetic tunnel junctions were identified as promising non-volatile memory but its use has been limited. A second generation MRAM-based on spin transfer torque has reduced currents. However, next generation MRAM based on pure spin currents may provide even more energy efficiency. My research is focused on developing power-efficient ways to generate, propagate and manipulate spins via pure spin currents. In order to develop such pure spin current technologies, the development of new materials such as topological insulators must come hand in hand with the development of new devices. In this talk, I will discuss (i) low damping ferromagnetic insulating thin films for achieving efficient spin current generation in spintronic devices, (ii) spin current generation in these films and large spin-charge interconversion in neighboring layers, (iii) spin interactions in ferromagnetic insulator/topological insulator heterostructures. Together these results lay the foundation for new energy-efficient pure spin current-based electronics.

Reference:

1. Li, P. et al. Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Lett. 21, 1, 84 (2021).

2. Li, P. et al. Switching magnetization utilizing topological surface state. Science Advances 5, eaaw3415 (2019).

In this talk, I will discuss rich topological behavior in two related models – the Majorana wire and a Su-Schrieffer-Heeger ladder- in the presence of potential energy landscapes. An introduction of the two models and of techniques that directly provide information on edge-state properties will form the starting point for obtaining topological phase diagrams in these models. In the case of these systems subject to a quasiperiodic potential, a beautiful topological phase diagram emerges mimicking Hofstadter’s butterfly patterns. In the case of disordered potential landscapes, Anderson localization physics informs the behavior of the disordered topological phase diagram.  Finally, I will discuss the possible implementation of this physics in a variety of experimental systems, including solid state, cold atomic and electro-mechanical settings. 

Amorphous oxide semiconductors (AOS)—ternary or quaternary oxides of post-transition metals—have attracted a lot of attention due to high carrier mobility which is an order of magnitude larger than that of amorphous silicon (a-Si:H). Unlike Si-based semiconductors, AOS exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. However, the properties of AOS are extremely sensitive to deposition conditions, oxygen stoichiometry, and metal composition, rendering the available research data inconsistent or hard to reproduce, thus, hampering further progress. Moreover, owing to the weak metal-oxygen bonding as well as many degrees of freedom in disordered materials, defects in AOS have the structural, thermal, and electronic characteristics that differ fundamentally from those in the crystalline transparent conducting oxides.

To navigate the large parameter space for AOS materials, computationally-intensive ab-initio Molecular Dynamics simulations followed by comprehensive structural analysis and accurate Density-Functional calculations, are performed for several AOS families. Integrated with systematic experimental measurements, the results provide microscopic understanding of complex relationships between the morphology, carrier generation, and electron transport across the crystalline-amorphous transition and help derive versatile design principles for next-generation transparent amorphous semiconductors with a combination of properties not achievable in Si-based architectures.

Magnetic tweezers allow the user to apply force and torque to magnetic beads attached to single DNA molecules, and to observe the resulting changes in DNA extension. This technique, however, is limited to measuring a single degree of freedom: the distance between the magnetic bead and the microscope slide surface. Total internal reflection fluorescence microscopy enables visualization of single molecules that have been tagged with fluorescent dyes. By combining TIRF microscopy and magnetic tweezers, we can simultaneously manipulate DNA molecules and use fluorescence to detect additional parameters, such as the presence of a protein or orthogonal changes in the DNA structure. We have recently installed a custom MT-TIRF instrument. In this talk I will discuss the instrument design, the physics underlying the two techniques, and how we plan to utilize them together to extract more information from our systems of interest.

I will describe how the geometry of the band structure of metals manifests itself in their optical and transport properties. I particular, I will show that the natural optical activity of metals, equivalent to the so-called dynamic chiral magnetic effect, stems from the intrinsic magnetic moments of quasiparticles, and demonstrate that these magnetic moments can be of both intrinsic and extrinsic origin. I will then discuss optical Hall response of chiral crystals in the presence of a DC transport current – the gyrotropic Hall effect – and show that it is related to the Berry curvature dipole. The latter fact makes the gyrotropic Hall effect a diagnostic tool for topological properties of three-dimensional chiral metals. If time permits, I will discuss how to observe the chiral magnetic effect in Weyl semimetals using the heating effect of a transport electric field.

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Abstract: We will discuss the formation and instabilities of quantum droplet and bubble phases arising in two-component attractively interacting bosonic mixtures. Insights regarding the interplay of atom and interaction imbalance as well as the impact of dimensionality giving rise, for instance, to a transition from mixed droplet-gas to pure self-bound states will be elucidated. These phases are addressed within the suitable extended Gross-Pitaevskii framework incorporating the first-order Lee-Huang-Yang quantum correction. An effective model based on the established Lee-Huang-Yang theory is constructed and found to provide qualitative analytical predictions. The stability properties of these configurations will be analyzed resorting to the Bogoliubov-de-Gennes excitation spectrum. Additionally, the two-body correlation patterns of the above-mentioned structural phase transitions will be explicated. Finally, a plethora of applications will be provided ranging from the snake instability of dark soliton stripes, to the generic stability of kink configurations and the destabilization of oblique dispersive shock-waves.

We will then extend the discussion to emergent many-body phenomena in ultracold chemistry, where coherent reactions provide an additional mechanism for engineering quantum phases and dynamics. Specifically, we will consider quantum degenerate atom-molecule mixtures in which two-body Fano-Feshbach conversion competes with coherent three-body recombination. When the latter becomes prominent, a structural deformation from a second-order to a first-order quantum phase transition of the atom-molecule mixture takes place. This process originates from a double-well structure of the ensuing energy landscape and it is accompanied by bistability, molecular metastability, and enhanced atom-molecule entanglement. To probe the dynamical signatures of the metastable molecular condensate, we will utilize quenches across the relevant phase boundaries, revealing how coherent few-body reaction processes can be exploited to steer chemical reactions at ultralow temperatures.

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Abstract: Conventional synthesis of solid-state materials typically requires high temperatures above 600 °C and long heating times, from days to weeks, to enable atomic diffusion and formation of the thermodynamically stable phase. Although many important materials have been discovered under these conditions, some combinations of atomic structure and elemental composition that could produce desirable electronic and magnetic properties remain inaccessible by conventional solid-state synthesis. I present progress in two alternative synthetic pathways aimed at accessing these structures and properties: (1) the use of single-source precursors to pre-form metal-metal bonds and trap metastable intermetallics with potential quantum properties and (2) the formation of Au-based hybrid inorganic-organic materials.

Speaker bio: Kelly Powderly is an Assistant Professor of Chemistry at Washington University in St. Louis, where she joined the faculty in 2024. Her research seeks to develop and utilize new synthetic pathways to discover extended solids with magnetic, electronic, and non-trivial topological properties of interest in quantum information science, and to explore new fundamental bonding in materials. Kelly earned her bachelor degree at Northwestern University. She completed her PhD in Chemistry and Materials with Professor Robert J Cava at Princeton University as a National Science Foundation Graduate Research Fellow, where she discovered magnetic inorganic solids with properties ranging from ferrimagnetism to quasi-1D antiferromagnetism. She completed her postdoctoral work with Professor Catherine Murphy at University of Illinois Urbana-Champaign where she was awarded a Beckman-Brown Postdoctoral Research Fellowship.

Abstract: Scalable quantum computation has the potential to solve certain problems far more efficiently than classical computation. Trapped ions offer a promising platform for scalable quantum computation. However, as the system size scales up, both computational speed and gate fidelity can deteriorate.

       In this talk, I will present two Hamiltonian engineering approaches to enhance the speed and fidelity of quantum gates in trapped-ion systems, thereby addressing key challenges associated with scaling. The first approach employs parametric amplification of the ions’ motion to enhance coherent interactions [1, 2]. The second approach uses amplitude modulation of laser pulses that couple the ions’ internal qubit states and their collective motional modes, enabling optimal control of the system [3].

       To demonstrate the effectiveness of these methods, I will present examples showing significant improvements in the speed and the fidelity of two-qubit gates. These results open new avenues towards scalable trapped-ion quantum computing and have direct relevance to a wide range of other physical platforms.

References:

1. Wenchao Ge, B. Sawyer, J. Britton, K. Jacobs, J. Bollinger, M. Foss-Feig, “Trapped Ion Quantum Information Processing with Squeezed Phonons”, Phys. Rev. Lett. 122, 030501 (2019)
2. SC Burd, Raghavendra Srinivas, HM Knaack, Wenchao Ge, AC Wilson, DJ Wineland, Dietrich Leibfried, JJ Bollinger, DTC Allcock, DH Slichter, “Quantum amplification of boson-mediated interactions”, Nature Physics 17, 898 (2021)
3. Luke Ellert-Beck, and Wenchao Ge, “Power-optimized amplitude modulation for robust trapped-ion entangling gates: a study of gate-timing errors”, Phys. Rev. A 111, 062422 (2025)

Abstract: Two-dimensional materials offer a vast range of band structures, yet experiments have explored only a narrow portion of them, in part because conventional electrostatic gating limits the accessible carrier density. We use two approaches to extreme doping of 2D materials while preserving high mobility: modulation doping by proximity to α-RuCl3 and electrostatic gating by using a high-k dielectric SrTiO3. In modulation-doped bilayer graphene we populate the second hole sub-band and analyze quantum oscillations, while monolayer graphene allows us to identify a magnetic transition in α-RuCl3. Finally, I will discuss our efforts to use thin films of the high-k dielectric to reversibly dope 2D materials.