Upcoming Colloquia

Department of Physics & Astronomy

Department of Physics & Astronomy 

Colloquia Archive

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Higher-Order Topological Matter

Recently, a novel class of topological insulators and superconductors (TI and TS) coined "higher-order (HO) TI and TS" has become a major topic in condensed matter physics. A HO TI or TS hosts protected gapless states on boundaries of more than one dimensions lower, such as hinge and corner states. I will introduce three prototypes that we proposed in 2012, 2018, and 2020, respectively, featuring unique physics of TI and TS, in two and three dimensions, with and without time-reversal symmetry. I will also discuss the experimental efforts in realizing these rare phases of matter in solid-state systems and particularly in the unique Bi4X4 (X=Br,I) family, which not only represents the quasi-one-dimensional generation of TI but also brings the excitements to the room temperature and the correlated regime.

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Machine Learning in the Era of Time Domain Astronomy

Astronomy surveys like the Zwicky Transient Facility have been leading to discoveries that are orders of magnitude more than just a decade ago. The discoveries range from the Solar System (comets, NEAs etc.) to Galactic science (CVs, YSOs, binaries etc.), to extragalactic (distant supernovae, AGN, TDEs) and possible EM counterparts to multi-messenger transients. We will provide an overview of the plethora of discoveries, and the ongoing work. We will describe our plans for ramping up discoveries even further with the aid of machine
learning, especially by combining the archives with fresh alerts.  With newer surveys, as we go fainter, cover more wavelengths, and go multi-messenger, more novel objects will be found. The rarity of these objects will both push the boundaries of our understanding, and also, due to the lack of statistically significant samples sometimes lead to premature claims, especially when using cutting-edge machine learning. We explore some aspects of time domain astronomy and its inherent biases, and comment on the requirement to explain the results through various techniques often involving post-hoc interpretability.

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Superconductivity at interfaces of KTaO₃

In this talk I will discuss the recently discovered superconducting electron gas at interfaces between KTaO₃ (KTO) and insulating overlayers. The superconducting T_c can be as high as 2.2 K, about an order of magnitude higher than in the LaAlO₃/SrTiO₃ system. The superconductivity at KTaO₃ interfaces has a number of striking features. Firstly, it is strongly orientation selective. While the interfacial electron gas at the KTO (111) interface has a T_c as high as 2.2 K, for the (110) interface the T_c  saturates at about 1 K, and the (100) interface remains normal down to the lowest temperatures measured. Secondly, the superconductivity is strongly tunable – the Tc can be varied by both chemical doping and via electric field-effect gating. Notably, for KTO (111) interfaces,  T_c varies linearly with the carrier density as determined by the Hall effect, remaining superconducting down to very low carrier densities. These properties point to a highly tunable two-dimensional interfacial superconductor that may have relevance in superconducting electronics, while also suggesting an unconventional mechanism for superconductivity. I will focus on a theoretical proposal where we suggest that an inter-orbital pairing mechanism can address key aspects of superconductivity at KTO interfaces.

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How to Form a Habitable Planet

More than 20% of nearby main sequence stars are surrounded by debris disks, where planetesimals, larger bodies similar to asteroids and comets in our own Solar System, are ground down through collisions.  The resulting dusty material is directly linked to any planets in the system, providing an important probe of the processes of planet formation and subsequent dynamical evolution.  The Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized our ability to study planet formation, allowing us to see planets forming in disks and sculpting the surrounding material in high resolution.  I will present highlights from ongoing work using ALMA and other facilities that explores how planetary systems form and evolve by (1) connecting debris disk structure to sculpting planets and (2) understanding the impact of stellar flares on planetary habitability.  Together these results provide an exciting foundation to investigate the evolution of planetary systems through multi-wavelength observations.

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Imaging ultrafast and ultrasmall: Unraveling magnetic and electronic behavior using time-resolved coherent x-ray scattering

Ultrafast laser control of magnetic and correlated materials has emerged as a fascinating avenue of manipulating magnetic and electronic behavior at femtosecond to picosecond timescales. Ultrafast manipulation of these materials has also been envisioned as a new paradigm for next generation memory and data storage devices. Numerous studies have been performed for both magnetic metallic systems as well as complex oxides to understand the mechanism underlying laser excitation. However, it has been recently recognized that spatial domain structure and nanoscale heterogeneities can play a critical role in dictating ultrafast behavior.  In this talk, I will focus on utilizing time-resolved x-ray scattering and nanodiffraction studies to study spatial texture dependent dynamics in magnetic multilayers and correlated systems. I will describe our recent experimental studies using emerging synchrotron techniques and free electron laser such as European XFEL and FERMI. In magnetic multilayers, we uncover a symmetry-dependent behavior of the ultrafast response. Labyrinth domain structure with no translation symmetry exhibit an ultrafast shift in their isotropic diffraction peak position that indicates their spatial rearrangement. On the other hand, anisotropic domains with translation symmetry do not exhibit any modification of their anisotropic diffraction peak position. I will also show that spatially dependent ultrafast response is observed in complex oxides such as rare-earth nickelates. Theis intriguing observation suggests preferential, texture-dependent paths not only for the transport of angular momentum, but also for structural rearrangements. These measurements provide us with a unique way to study and manipulate spin, charge and lattice degrees of freedom.

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Spintronic Terahertz Emission from Two-Dimensional Hybrid Metal Halides

Terahertz (THz) technologies hold great promise for the development of future computing and communication systems. The ideal, energy-efficient, and miniaturized future THz devices should consist of lightweight, low-cost, and robust components with synergistic capabilities. However, a paucity of materials systems possessing both these desirable attributes and functionalities has made device realization difficult. Two-Dimensional Hybrid Metal Halides (2D-HMHs) have been shown to allow for facile and economical, solution-based synthesis while still maintaining high energy conversion efficiency, chemical flexibility, and defect tolerance. These attributes make them ideal candidates for high-performance THz communication applications. Spintronic toolkits, on the other hand, can serve as an effective ‘control knob’ for future THz devices when interfacing HMHs with ferromagnetic (FM) materials, taking advantage of the fast relaxation of spin (akin to a ‘switch’) and the couplings between magnons, photons, and spins. The broken inversion symmetry and resulting giant Rashba state formed at the 2D-HMH surface allow for the spintronic control of intensive THz generation. In this talk, we will present the observation of spintronic-THz radiation in 2D-HMHs interfaced with a ferromagnetic metal, produced by the ultrafast spin current under femtosecond laser excitation. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters.

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Nonreciprocal spin and charge transport in magnetic and topological materials systems

Experimental characterizations of transport phenomena in topological and magnetic materials have mainly been based on linear responses of spin and charge carriers to the applied electric field or current. Far less well understood are emerging nonreciprocal response effects that, in principle, can also be hosted in these materials systems, due to spin-orbit interaction, broken inversion symmetry, and/or band topology.

In this talk, I will present our theoretical works on three representative nonreciprocal transport phenomena that have recently been of particular interests: 1) a unidirectional magnetoresistance effect in conducting ferromagnet|nonmagnet bilayer systems, which is analogous to the current-in-plane giant magnetoresistance effect, but with a current-induced spin density – as opposed to a ferromagnetic metal layer – playing the role of a spin polarizer, 2) a bilinear magneto-electric resistance effect in 3D topological insulators, which may be deemed as a nonlinear version of spin-to-charge conversion, arising from spin-momentum locking of surface Dirac fermions and time-reversal symmetry breaking, and 3) a nonlinear Hall effect in Weyl semimetals, a new transport signature of chiral anomaly – which, in the presence of non-perpendicular electric and magnetic fields, creates an effective chemical potential difference between a pair of Weyl nodes with opposite chiralities and thereby becomes detectable in the nonlinear response regime.

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Public Talk 12/9 @ 7pm CR 118

Hollyweird Science: The Relationship between Science and Story in TV and Film

When there is a science mistake in a film or a favorite television show, do you notice? Does it pull you out of the drama? Does it make you angry? Does it leave you asking, “Why did they have to do that? It was easy enough to get that right if they’d just gone through the effort!”? It turns out that Hollywood has been going through the effort. Now more than ever, Hollywood productions are incorporating science advisors to help them get the science as right as possible, while still telling the stories they want to tell. The National Academy of Sciences places such an emphasis on science accuracy in TV and film that they have even opened an office in Los Angeles. Known as the Science and Entertainment Exchange, they provide free science consulting to creatives in the entertainment industry to raise the bar for science discourse in TV and film. Dr. Kevin Grazier has been a science advisor for many television series (Battlestar Galactica) and movies (Gravity). He is also the co-author of the Hollyweird Science series of books that explore the depiction of science, scientists, and the culture of science in TV and film. He will discuss the relationship between science and story in TV and film, and why sometimes it is important to get it right, while there are other instances where productions intentionally get the science wrong.

Astronomy Colloquia 12/10 @ 4pm in PS 234

Jupiter’s Role as a Cometary Shield Revisited

It has been widely reported, even on popular astronomy television series, that Jupiter has a profound role in shielding the terrestrial planets from comets raining in from the depths of space. The “Jupiter as a Shield” concept evolved such that some claimed that a jovian planet is a near-requirement for the evolution of life on Earth and in the formation of exo-Earths. The origin of this concept is often credited to a 1994 paper by George Wetherill in which he reported on the result of numerical simulations, but although Wetherill used the term “Jupiter barrier,” he never claimed that Jupiter acts as an impenetrable comet shield. His numerical methods have also since been shown to have failure modes that occur often. To evaluate whether Jupiter shields Earth, we re-created the work of Wetherill, and discovered that not only is Jupiter not a credible shield, it is at least as likely to divert non-threatening comets and asteroids into the inner Solar System.
Following on from that work, using techniques inspired by the type of Big Data analytics used by Hollywood and large retail firms, we mined data sets generated from that research to glean further insight into the many paths planetesimals may take as they wend their way throughout the Solar System—which includes a description of the mechanism for how Centaur objects are converted into Jupiter Family Comets. These results have wide-ranging implications, including a plausible mechanism for how the K-Pg impactor may have made its way to Earth.

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Ferroelectric Photovoltaics

All conventional solar cells use electric field at the interface between two doped semiconductors to drive electrons and holes, generated by absorption of photons of sunlight, in opposite directions which results in solar current. I will present a new concept of conversion of the solar energy into electric current which involves tunneling of photoelectrons through a thin ferroelectric layer deposited on top of pure semiconductor where photoelectrons are generated. Our initial results in Si/(Si-doped hafnia) system showed that electron-hole pairs generated by absorption of light in indirect bandgap semiconductor (Si) were separated by electric field within the ferroelectric layer and formed a photocurrent. Advantages of the next generation solar cells are that doping of semiconductors is not necessary and they can potentially provide larger voltage than the conventional cells based on silicone.

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Present and future astrophysics simulations using the Einstein Toolkit

General relativistic numerical magneto-hydrodynamics simulations are used to simulate the merger of neutron star and black hole binaries, disks around black holes, as well as supernovae. Driven by multi-messenger detections of gravitational waves and electromagnetic signals new groups that did not in past engage in these simulations are joining the simulation effort. These simulations however combine demanding hydrodynamics simulations using high-resolution shock capturing schemes with a fully general relativistic treatment of gravity cannot be developed by single researchers anymore. Instead, the Einstein Toolkit is a community driven framework for numerical these astrophysics simulations that is used by many group throughout the world.

I will first present an overview of current work using the Einstein Toolkit to simulate neutron star mergers, accretion disks, and binary black hole mergers, focusing on the aspects of the toolkit that facilitate these simulations. I will highlight the scientific results obtained using the toolkit by different researchers around the world.

In a second part I will introduce the next generation mesh refinement driver for the Einstein Toolkit: CarpetX. CarpetX leverages the AMReX mesh refinement library that is being developed as part the Exascale project. It improves on the current mesh refinement driver, Carpet, by supporting fine grained mesh refinement to track features in the simulations and native support for GPU accelerated computing. I will present an outline of CarpetX's design and current capabilities.

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Two-Dimensional Layered Materials: Effects of Substrates/Layer Thickness

Research in two-dimensional (2D) crystals started with the isolation of graphene in 2004, which was soon followed by the discovery of many other 2D-materials, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). Earlier theoretical works that studied 2D crystals, ignored the effect of substrates and the ambient gases on the properties of 2D materials. However, as the 2D-crystals are surface-only structures, it is natural to expect that their properties are perturbed by their immediate environment.  This talk will focus on our different density functional theory works that highlight how the effects of the immediate environment determine the properties of 2D crystals.

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Spin-based Quantum Sensing with Charge Carrier Spin States in Organic Semiconductors

Quantum sensors probe physical observables by utilizing the inherent volatility of quantum states due to interaction with their environment. Quantum sensors can overcome the sensitivity limits of classical sensors, and they can also probe inherently quantum mechanical observables such as entanglement and permutation symmetry; This presentation is about quantum sensing concepts based on electron spin states in organic condensed matter environments. Due to weak spin-orbit coupling, paramagnetic charge carrier states in π-conjugated polymer thin films are subject to pronounced spin selection rules and, also, they exhibit spin coherence times in the order of a s, limited only by unresolved proton hyperfine fields caused by the all-abundant hydrogen in these materials. The spin-selection rules enable straightforward electrical detection of coherent propagation of these spin states [1]. We have utilized these as spin-based quantum sensors [2] for the study of the non-perturbative drive regime of magnetic dipole excitations. These experiments have revealed previously not accessible magnetic resonant strong-drive phenomena, including spin-collectivity effects [3], multiple-quantum transitions occurring at integer and fractional g-factors, as well as a magnetic resonant Bloch-Siegert Shift [4,5].

[1] D.R. McCamey et al., Phys. Rev. Lett. 104 (1), 017601; [2] W. J. Baker et al., Nature Commun. 3, 898, (2012); [3] D. P. Waters et al., Nat. Phys., 11, 910 (2015); [4] S. Jamali et al., Nano Lett. 17, 4648 (2017); [5] S. Jamali et al., Nature Commun. 12 (1), 1 (2021).

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Boron in the 2D Limit: Borophene, Borophane, and Beyond

The recent experimental realization of 2D boron (i.e., ‘borophene’) has spurred broad interest in its unique material attributes such as in-plane anisotropy, seamless phase intermixing, high mechanical strength and flexibility, massless Dirac fermions, and phonon-mediated superconductivity [1]. The polymorphic nature of borophene, which is rooted in the rich bonding configurations among boron atoms, further distinguishes it from other 2D materials and offers an additional means for tailoring its material properties [2]. This presentation will explore the ultrahigh vacuum synthesis and atomic-scale characterization of borophene on noble metal substrates including Ag(111) [3] and Au(111) [4]. In addition to distinct borophene polymorphs, conditions for forming self-assembled intermixed phases [5], superlattices [6], and bilayers [7] will be delineated. Using field emission resonance spectroscopy, the work function and degree of electron transfer doping can be quantified, revealing subtle differences between the different borophene phases [8]. By exploiting spatially inhomogeneous surface chemistry, seamless 2D heterointerfaces can also be realized between borophene and other materials including organic semiconductors [9], graphene [10], and graphene nanoribbons [11], each of which show atomically sharp electronic interfaces as confirmed by scanning tunneling microscopy and spectroscopy. In an effort to further tune the chemical and electronic properties of 2D boron, covalent hydrogenation of borophene has been achieved, resulting in a series of ‘borophane’ polymorphs that possess significantly higher stability in ambient conditions compared to pristine borophene [12]. Overall, this work establishes a series of design rules for manipulating and integrating 2D boron into a range of next-generation electronic [13] and quantum technologies [14].

[1] A. J. Mannix, et al., Nature Nanotechnology, 13, 444 (2018). [2] H. Bergeron, et al., Chemical Reviews, 121, 2713 (2021). [3] X. Liu, et al., Nature Communications, 10, 1642 (2019). [4] B. Kiraly, et al., ACS Nano, 13, 3816 (2019). [5] X. Liu, et al., Nature Materials, 17, 783 (2018). [6] L. Liu, et al., Nano Letters, 20, 1315 (2020). [7] X. Liu, et al., Nature Materials, DOI: 10.1038/s41563-021-01084-2 (2021). [8] X. Liu, et al., Nano Letters, 21, 1169 (2021). [9] X. Liu, et al., Science Advances, 3, e1602356 (2017). [10] X. Liu, et al., Science Advances, 5, eaax6444 (2019). [11] Q. Li, et al., Nano Letters, 21, 4029 (2021). [12] Q. Li, et al., Science, 371, 1143 (2021). [13] V. K. Sangwan and M. C. Hersam, Nature Nanotechnology, 15, 517 (2020). [14] X. Liu and M. C. Hersam, Nature Reviews Materials, 4, 669 (2019).

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Chiral Effect in Twisted Van der Waals Heterostructures

Recent years have seen a surge of interest in twisted van der Waals heterostructures consisting of atomically thin crystal layers. From a structural point of view, twisted layers are interesting because not only are they chiral, but their chirality can be readily controlled by varying the twisting angle. For example, bilayers with opposite twisting angles are mirror images of each other; therefore, they possess opposite chirality. This structural flexibility makes twisted van der Waals heterostructures a versatile platform for investigating chirality-dependent phenomena.  In this talk, I will discuss two phenomena related to the chirality of twisted bilayers, namely, chiral charge pumping and layer circular photogalvanic effect.  The former relates mechanical motion to electricity, while the latter could be useful for frequency-sensitive, circularly polarized light detection, particularly in the infrared range.

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Electronic Properties of Novel Materials – Photovoltaic, 2D Magnetic, and Topological Materials

Electronic properties, such as the electronic band structures and the density of states, of a material are at the center of understanding the physical properties. The electronic properties are directly related to the optical, magnetic, and transport properties of the materials. Thus, the understanding of the electronic properties provides the fundamental basis of understanding the materials of interests. In this talk, I would like to share the results of our recent and on-going works focusing on three material categories: (1) photovoltaic materials (organic photovoltaic, and organometallic halide perovskite materials); (2) magnetic materials (CrBr₃, Eu-Si nanowires, and EuO); and (3) topological materials (2M-WS₂). Scanning tunneling microscopy and spectroscopy (STM/S) is the main tool used to provide the nm-scale understanding of the electronic properties in these materials. If time permits, I will also discuss the future directions.

 

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Measuring Magnetic Fields in the Solar Corona - Towards Understanding the Sources of Space Weather

The Sun influences the Earth due to its variable output of radiation, energetic particles, and magnetized plasma. Eruptive events on the Sun can drive radical disturbances in the near-Earth environment known as space weather. Many critical technologies upon which society has become increasingly dependent are vulnerable to the effects of space weather including satellites, communication networks, navigation systems and power grids. In addition, space weather events pose a hazard to humans in space and airline crews and passengers on polar routes.

The physical mechanisms at the Sun giving rise to space weather are poorly understood in detail. What is certain is that magnetism in the solar corona plays the central role in driving space weather events. While magnetism is well observed in the solar photosphere, it is not adequately measured in the solar corona due to the faintness of the corona compared to the solar disk (5 orders of magnitude)  and the weak strength of magnetic fields in the corona (~10 Gauss). Progress towards understanding the mechanisms that lead to space weather, and eventually predicting the occurrence of solar storms requires that we develop the ability to measure magnetic fields routinely in the corona.

In this talk, I will introduce the topic of space weather, and motivate the need to measure magnetic fields in the corona. I will describe the techniques we have at our disposal to remotely sense coronal magnetism. And I will describe the Coronal Solar Magnetism Observatory (COSMO), a project we are developing that has as its central element a 1.5-m aperture coronagraph. The COSMO project has been funded by NSF to perform a site survey to identify a location for the COSMO observatory and to work with a design firm to design the COSMO coronagraph.

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Spin Dynamics and Interplay with Carrier Dynamics in Lead Halide Perovskites

Lead halide perovskites are an emerging class of semiconductors, and remarkable progress has been made in photovoltaic and other optoelectronic applications in the last decade. While substantial research efforts are on solving practical issues impeding commercialization, many fundamental properties of the materials still need to be investigated in-depth to overcome current challenges and exploit their full potential for applications beyond optoelectronics. These materials hold great promise for spintronics, because of their large and tunable spin-orbit coupling, spin-dependent optical transitions, and potential easy spin manipulation. Previously we have demonstrated optical orientation of spin-polarized excitons and spin coherence in polycrystalline MAPbI₃ films [1]. Two mysteries had arisen in our study -- long spin lifetime (> 1 ns) with the presence of large spin-orbit coupling, and extremely small exchange couplings of the excitons (< 1micro eV). In order to unravel the mysteries, we studied spin dynamics on high quality MAPbI₃ single crystals with low defect density. Details of intrinsic spin dynamics are revealed in MAPbI₃, which offers insights into the interplay between spin dynamics and carrier dynamics following photo excitation. In this talk, we will present our current understanding about the spin-polarized exciton states in magnetic field and spin relaxation mechanisms in prototype bulk lead halide perovskite materials. All the recent studies on spin-dependent properties suggest that the halide perovskites are promising platform materials for spintronics, and many more interesting phenomena are yet to be explored and incorporated into spin-based device applications.

[1] P. Odenthal et al., Nature Physics 13, 894 (2017)

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Ferroelectric Metals and Perspective

Almost all ferroelectrics are insulators. Ferroelectricity arises due to the long-range dipole-dipole interaction. Metallicity and ferroelectricity are seemingly incompatible since free electrons screen the dipole-dipole interaction. The transition to a polar phase is possible in a metal as long as free electrons do not interact strongly with the transverse optical phonons as postulated by Anderson and Blount (AB) in 1965. The ferroelectric metal defined by AB is distinguishable from the metals with breaking inversion symmetry. LiOsO₃ discovered in recent years fulfills the requirement of a ferroelectric metal, but the origin for the ion displacement is different from the description given by AB. In this talk, I will give an overview of achievements on the ferroelectric metal in the literature and demonstrate the recent development, especially, the new results from my group. 

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Topological Spin Textures in Two-dimensions and Beyond

Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One important example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability, and gives rise to promising applications in future spintronic devices.

A key transport signature of the skyrmion is the topological Hall effect, that is, the electron moves in sideway under the effect of real space Berry phase induced by spin chirality. In the first part of the talk, I will discuss how transport signatures can be used as footprint of skyrmions. On the other hand, I will also set an alarm that topological Hall effect should not be abused, since it could originated from momentum space Berry phase or atomic scale chirality from thermal fluctuations.

In the second part of the talk, I will generalize the skyrmion texture from two dimensions (2D) to three dimensions (3D), and discuss two relevant 3D spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. A target skyrmion consists of concentric helical rings and can be stabilized in the absence of external magnetic field. The other texture to be discussed is the magnetic hopfion. We propose the presence of zero-field hopfion in synthetic chiral magnetic multilayers. The transition from hopfion to the ground state, a monopole-antimonopole pair, can be fully understood as the topological transition between torus and sphere. These works could stimulate the development of 3D spintronics.

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9/9 7pm Thursday Public Talk:  The Accelerating Expanding Universe: Dark Matter, Dark Energy, and Einstein's Cosmological Constant

Dark energy is the leading candidate for the mechanism that is responsible for causing the cosmological expansion to accelerate. Bharat Ratra will describe the astronomical data which persuade cosmologists that (as yet undetected) dark energy and dark matter are by far the main components of the energy budget of the universe at the present time. He will review how these observations have led to the development of a quantitative "standard" model of cosmology that describes the evolution of the universe from an early epoch of inflation to the complex hierarchy of structure seen today. He will also discuss the basic physics, and the history of ideas, on which this model is based.

9/10 4pm Friday Astronomy Colloquium: The "Standard" Model of Cosmology ... and Open Questions

Experiments and observations over the last two decades have provided strong support for a "standard" model of cosmology that describes the evolution of the universe from an early epoch of inflation to the complex hierarchy of structure seen today. I review the basic physics, astronomy, and history of ideas, on which this model is based. I describe the data which persuade cosmologists that (as yet not directly detected) dark energy and dark matter are by far the main components of the energy budget of the universe. I conclude with a list of open cosmological questions.

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High-Resolution Infrared Spectroscopy: An Opening Window for Galactic Chemical Evolution Research

Describing the birth and evolution of the Milky Way Galaxy is a fundamental goal of modern astrophysics.  A prime observational tool in this endeavor is high-resolution stellar spectroscopy, from which detailed chemical compositions can lead to the nucleosynthetic history of our Galaxy.  For half a century high-resolution spectroscopy was limited to the optical window, (0.3-1 micron), but advances of the last couple of decades in telescope/instrument/detector design has opened up the infrared to detailed  chemical composition research.  In this talk I will concentrate on two innovative instruments developed at McDonald Observatory, which have led to complete spectral coverage in atmospheric-transparent 0.8 to 2.4 micron wavelength areas.  This IR spectral domain is crucial for investigations in dust-obscured regions of our galaxy.  I will concentrate on three important observational questions:  (1) Do IR and optical chemical composition results agree?  (2) What unique spectral features bring value-added information in the IR?  (3) What are the most important Galactic large-sample studies to undertake now, and in the future with 30m-class telescopes?

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Low-Dimensional Materials for Advanced Logic, Memory and Photonics

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures and making outstanding progress on fundamental studies, practical applications of 2D materials will require a broader integration strategy. I will present our ongoing and recent work on integration of 2D materials with 3D electronic materials to realize logic switches and memory devices with novel functionality that can potentially augment the performance and functionality of Silicon technology. First, I will present our recent work on gate-tunable diode¹ and tunnel junction devices based on integration of 2D chalcogenides with Si and GaN. Following this I will present our recent work on non-volatile memories based on Ferroelectric Field Effect Transistors (FE-FETs) made using a heterostructure of MoS₂/AlScN² and I also will present our work on Ferroelectric Tunnel Junction (FTJ) devices³ also based on thin AlScN. 

   Next, I will present our work on light-trapping in 2D chalcogenides and halide perovskites. I will present the effect of nano-structuring on hybridization between excitons, plasmons and cavity photons.⁴ I will extend this concept to 2D halide perovskites⁵ and demonstration of hybrid exciton-polariton emission at room temperatures. If time permits I will also give a brief overview on other ongoing efforts in the lab including tunable 1D optical materials,⁶ in-situ electron-microscopy^{7, 8} and scanning probe microscopy efforts.⁹ I will end by giving a broad perspective on future opportunities of 2D and other low-dimensional materials in basic science and applied microelectronics technology.

1. Miao, J.; Liu, X.; Jo, K.; He, K.; Saxena, R.; Song, B.; Zhang, H.; He, J.; Han, M.-G.; Hu, W.; Jariwala, D. Nano Letters 2020, 20, (4), 2907-2915.
   2. Liu, X.; Wang, D.; Zheng, J.; Musavigharavi, P.; Miao, J.; Stach, E. A.; Olsson III, R. H.; Jariwala, D. arXiv preprint arXiv:2010.12062 2020.
   3. Liu, X.; Zheng, J.; Wang, D.; Musavigharavi, P.; Stach, E. A.; Olsson III, R.; Jariwala, D. arXiv preprint arXiv:2012.10019 2020.
   4. Zhang, H.; Abhiraman, B.; Zhang, Q.; Miao, J.; Jo, K.; Roccasecca, S.; Knight, M. W.; Davoyan, A. R.; Jariwala, D. Nature Communications 2020, 11, (1), 3552.
   5. Song, B.; Hou, J.; Wang, H.; Sidhik, S.; Miao, J.; Gu, H.; Zhang, H.; Liu, S.; Fakhraai, Z.; Even, J.; Blancon, J.-C.; Mohite, A. D.; Jariwala, D. ACS Materials Letters 2021, 3, (1), 148-159.
   6. Song, B.; Liu, F.; Wang, H.; Miao, J.; Chen, Y.; Kumar, P.; Zhang, H.; Liu, X.; Gu, H.; Stach, E. A.; Liang, X.; Liu, S.; Fakhraai, Z.; Jariwala, D. ACS Photonics 2020, 7, (10), 2896-2905.
   7. Han, M.-G.; Garlow, J. A.; Liu, Y.; Zhang, H.; Li, J.; DiMarzio, D.; Knight, M. W.; Petrovic, C.; Jariwala, D.; Zhu, Y. Nano Letters 2019, 19, 7859-7865.
   8. Kumar, P.; Horwath, J. P.; Foucher, A. C.; Price, C. C.; Acero, N.; Shenoy, V. B.; Stach, E. A.; Jariwala, D. npj 2D Materials and Applications 2020, 4, (1), 1-10.                                                                            9. Moore, D.; Jo, K.; Nguyen, C.; Lou, J.; Muratore, C.; Jariwala, D.; Glavin, N. R. npj 2D Materials and Applications 2020, 4, (1), 44.

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Life after Astronomy PhD; From Silicon Valley to Wall Street

I will talk about my PhD research on deep learning for gravitational wave astronomy at UIUC, and my experiences afterwards working on moonshots at Google X and on machine learning for finance at JPMorgan Chase. I will discuss opportunities and suggestions for students interested in data science/machine learning roles and also highlight differences between academia, tech industry, and finance. Following the talk, there will be an extended Q&A session where questions on any topic are welcome.

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Black hole simulations with AI: Do androids dream of electric black holes?

Black holes accrete gas from their surroundings in a chaotic, turbulent manner. This colloquium will describe the pilot application of deep learning for black hole weather forecasting. Early results indicate that black hole simulations can benefit tremendously from AI. Along the way, I will explain the difference between artificial intelligence (AI), machine learning, and deep learning and why the scientific community is so excited about the latter.

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Novel Layered Magnetic Topological Materials

The combination of magnetism and non-trivial band topology can generate various topological quantum states, such as quantum anomalous insulators, axion insulators and Weyl semimetals. Searching for magnetic topological materials and understanding their underlying physics have been a focused research direction in recent years. In this talk, I will first give a brief introduction to this research area and then talk about our recent studies on two classes of magnetic topological materials: 1) intrinsic magnetic topological insulators (MnBi₂Te₄)(Bi₂Te₃)_m (m=0, 1 & 2); 2) antiferromagnetic (AFM) Dirac semimetal (Sr/Ba)MnSb₂. For the (MnBi₂Te₄)(Bi₂Te₃)_m system, we have synthesized not only AFM but also ferromagnetic (FM) phases, which provides rich opportunities for observing novel magnetic topological phases. Through chemical potential tuning by Sb substitution for Bi, we have observed transport evidence of a long-sought topological state - an ideal type-II FM Weyl state with one pair of Weyl nodes [1, 2]. In (Sr/Ba)MnSb₂, we not only observed the interaction between Dirac fermions and magnetism [3], but also discovered a novel Dirac state featuring spin-valley locking and bulk quantum Hall effect [4].

References:

[1] Lee et al., Phys. Rev. Research 1, 012011(R) (2019).

[2] Lee et al., arXiv:2002.10683.

[3] Liu et al., Nature Materials 16, 905 (2017).

[4] Liu et al., arXiv:1907.06318 (2019).

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Emergent Topological Phenomena in Oriented Thin Films of Pyrochlore Iridates

The recent progress in synthesizing and discovering states and phenomena beyond the Landau symmetry-breaking paradigm in quantum materials has been quite extraordinary. These new modalities confront our views of fermions and bosons’ possible behavior in solids, yet in bulk remain frequently concealed from the modern experimental probes. To exacerbate, though by now topological phases are well-known for non-correlated compounds, they are scarcely found in correlated electron systems. In my talk, I will discuss a way of addressing these two challenges by creating new synthetic templates with rich many-body behavior derived from the rare-earth pyrochlore iridates and (ii) discovering quantum states and phenomena entwined with spin correlations and non-trivial band topology.

Specifically, I will focus on quantum states of (111)-oriented rare-earth iridium pyrochlore thin-films La2Ir2O7 (La = Pr ... Lu) which is (1) a model system for an exotic nodal non-Fermi liquid metal known as the Luttinger-Abrikosov-Beneslavskii state, and (2) time-reversal broken Weyl semimetal. During the talk, I will discuss the challenges of growing complex materials of platinum group metal oxides and finding direct signatures for the presence of Weyl fermions.

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Laser Shock Nano-Straining in 2D Materials

Straining nanomaterials to break their lattice symmetry is perhaps the most efficient approach toward realizing bandgap tunability.  Graphene has a great potential to replace silicon in prospective semiconductor industries due to its outstanding electronic and transport properties; nonetheless, its lack of energy bandgap is a substantial limitation for practical applications. In this talk, a large-scale strain engineering technique to control the local strains in 2D materials and their heterostructures will be discussed. First, laser shock nanostraining will be introduced to generate three-dimensional (3D) nanostructures and thus induce local strains in the graphene sheet. The size dependent straining limit of the graphene and the critical breaking pressure will be discussed.  Moreover, laser shock nanostraining induces modulated inhomogeneous local asymmetric elastic–plastic strains as results of 1-100 GPa‐level shock loading at a high strain rate of 10⁶–10⁸ s⁻¹. Currently, due to the weak lattice deformation induced by uniaxial or in‐plane shear strain, most strained graphene studies have yielded bandgaps <0.5 eV, laser shock nanostraining can induce tunable bandgaps in graphene of up to 2.1 eV.  Finally, laser shock nanostraining will be discussed to controllably tune the interlayer distance between VdW heterostructures and generate strain coupling between layers.  The strains in the 2D heterolayers, providing a simple and effective way to modify their optic and electronic properties.

 

1. Link to References

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Betelgeuse: Active Now, Doomed to Explode Later

Betelgeuse is in many ways a typical massive red giant star doomed to explode when its core burns out and collapses to form a neutron star. It is also special because it is close enough at 500 light years to be spatially resolved and hence studied in unprecedented detail. The Great Dimming of early 2019 has only added to its allure. Some years ago, I began work with a group of undergraduates, the initial goal of which was to try to better estimate when Betelgeuse might explode. Our first discovery was that a single-star picture failed drastically to accounting for  the  observed  rotational  velocity.  The  rotation  would  be  much  too slow. We proposed that Betelgeuse was originally a binary system that underwent a merger to form a single more rapidly rotating star. I will summarize recent work that has elucidated the nature of Betelgeuse and its Great Dimming.

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Wrangling the Beast – Precision Supermassive Black Hole Mass Measurement With ALMA

While supermassive black holes (BHs) gravitationally dominate only the innermost regions of galaxies, their masses correlate with large-scale galaxy properties, hinting that BHs co-evolve with their host galaxies over the age of the universe. These correlations suggest a distinct evolutionary pathway for the most luminous galaxies; however, an incomplete census of >10^9 solar mass BHs (and large measurement uncertainties) preclude any definitive conclusions. Emission-line observations with the Atacama Large Millimeter/submillimeter Array (ALMA) are opening a new avenue for studying BH demographics in nearby galaxies. I will present ongoing ALMA CO imaging that has resolved circularly-rotating molecular gas disks in the nuclei of a growing number of very luminous galaxies, providing ideal probes of their inner gravitational potentials. I will highlight results from recent gas-dynamical modeling efforts, which have enabled some of the most precise direct BH mass determinations to date and important cross-checks with other measurement techniques. I will also discuss the prospect of future telescopes like the next-generation Very Large Array (ngVLA) to expand on ALMA's revolutionary capabilities.

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Hubbard Model Simulation in Moiré Materials

The Hubbard model is a simple theoretical model of interacting quantum particles in a lattice. It is thought to capture the essential physics of high-temperature superconductors and other complex quantum many-body phenomena, but has proved difficult to solve accurately. Physical realizations of the Hubbard model therefore have a vital role to play in solving the strong-correlation puzzle. Moiré materials, metamaterials built on artificial “moiré atoms”, have recently emerged as a promising Hubbard model simulator. The ability to continuously control the Hubbard Hamiltonian in these materials has provided a unique opportunity to address some of the long-standing questions in condensed matter physics. For instance, can unconventional superconductivity and quantum spin liquids emerge from the Hubbard model? In this talk, I will discuss recent efforts to simulate Hubbard model physics in moiré materials. Specific examples on realizing the Mott insulating state, the Mott-Hubbard transition and electron crystals will be discussed. I will end with a brief discussion on opportunities and challenges for future studies.

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Advanced Materials for Condensed Matter Physics

The advance of condensed matter physics has been promoted by the discoveries of novel quantum materials. The recent examples include novel functional 2D materials and topological semimetals. In this talk, I will introduce a few advanced functional materials, including functional 1D semiconductors, 2D materials, and the emerging topological semimetals. The studies of these materials lead to the discovery of new physics concepts and technological useful properties.

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10x Rule that Drives Innovations in Academic and Industrial Research

Peter Theil, Paypal’s co-founder, wrote in his book Zero to One: “As a good rule of thumb, proprietary technology must be at least 10 times better than its closest substitute in some important dimension to lead to a real monopolistic advantage.” He suggests that only when a startup has something exponentially better than its competition, its success can be foreseen. While reflecting on my own experiences in conducting research, I find the “10x rule” also applies to academic activities. In this talk, I will share several examples that validate the rule, and discuss how to use it as a framework to think through the value of a research.

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Electron Tunneling Driven by a Time-Dependent Electric Field

This talk is about of electron tunneling driven by a strong time-dependent field. First, we discuss tunneling quantum transfer with dissipation introduced by Anthony Leggett. Quantum dissipation will be presented by a harmonic bath interacting with a particle. We also discuss the concept of coherent and incoherent tunneling. We introduce a two-level system interaction with an oscillator environment as the simplest model of a tunneling system. Then we discuss the electron tunneling in a time-dependent monochromatic electric field. In particular, we consider coherent and incoherent tunneling in a time-dependent electric field. Then we discuss the electron tunneling in a pulsed cw field where induced coherent oscillation can take place despite the strong dissipation. There are many interesting features of electron tunneling in a bichromatic electromagnetic field, which are discussed in the presentation. We apply the concepts of electron tunneling driven by a time-dependent electric filed to electron transfer in quantum well using the Bardeen’s Hamiltonian. We find the system can exhibit the effect of absolute negative resistance, i.e., the electron moves to the opposite direction. Finally, we discuss light absorption accompanied the electron transfer in a time-dependent field. The problem under consideration is solved by the methods that does not consider the perturbation theory. In this problem the absorption peak positions depend on the light intensity.

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