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Condensed Matter Seminar Thursday, January 19, 2023 4:00 PM , Room TBA Note special time. Note special room. |
RESERVED
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Condensed Matter Seminar Thursday, January 26, 2023 4:00 PM , Room TBA Note special time. Note special room. |
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Condensed Matter Seminar Thursday, February 2, 2023 4:00 PM , Room TBA Note special time. Note special room. |
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Condensed Matter Seminar Thursday, February 9, 2023 4:00 PM , Room TBA Note special time. Note special room. |
"Ultra-Low Energy Manipulation of Spin in Nanostructures"Weigang Wang , University of Arizona [Host: D. Louca]
ABSTRACT:
Information technology backed by sophisticated semiconductor devices have deeply changed our society in the past two decades, with AI-driven tools such as ChatGPT posited to bring even deeper impact. However, in the physics governing devices behind most of these applications, we have only utilized the charge carried by electrons, while ignoring the other inherent quantum property, the spin. My research focuses on the understanding of the spin degree of freedom of electrons at nanoscales. First I will give an introduction on a few key phenomena in the field of spintronics, such as the coherent tunneling of spins by controlling the symmetry of the wavefunctions, and the exchange scattering effect in materials with compensated magnetization where the spins can be manipulated in the picosecond time scale. Then I will present in detail one of our research directions in which we attempt to control the order parameter of magnetic systems by using electric fields, instead of magnetic fields or spin-polarized currents. Through the voltage controlled magnetic anisotropy effect where the energy of the system can be modified by the redistribution of wavefunctions induced by external electric potentials, a 100-fold reduction in switching current density has been realized. We have demonstrated that both the magnetic anisotropy and saturation magnetization of a metallic ferromagnet can be controlled by voltage, leading to a new method to modify the interlayer exchange coupling of the system, directly verified by in-situ X-ray magnetic circular dichroism experiment. In addition to the ferromagnetic order, the antiferromagnetic order can also be effectively manipulated by electric fields. Finally, I will describe our recent effort to reduce the switching energy of magnetic tunnel junctions. By controlling the spin-orbit interaction of the system using a remote doping technique, we have achieved a record-low switching energy of ~3 fJ using sub-ns voltage pulses. |
Condensed Matter Seminar Monday, February 13, 2023 3:30 PM Monroe Hall, Room 110 Note special date. Note special room. |
"Theoretical study on the spin dynamics induced by a magnetic field in a one-dimensional chiral magnet"Kotaro Shimizu , University of Tokyo [Host: Gia-Wei Chern]
ABSTRACT:
Swirling spin textures, such as helices and vortices, appear in chiral magnets, and these noncolinear and noncoplaner spin textures bring about characteristic resonance structures and quantum transports. Recently, a one-dimensional chiral magnet hosting a conical spin texture and a chiral soliton lattice (CSL) have attracted a lot of interest since resonance frequencies as well as a magnetic modulation period can be flexibly controlled by an external magnetic field [1,2]. However, the dynamical properties of these spin textures associated with the oscillating magnetic field and the resultant emergent electromagnetic phenomena have not been systematically clarified.
In this study, we theoretically study the resonance modes and the emergent electromagnetic phenomena in a one-dimensional chiral magnet by numerically solving the Landau-Lifshitz-Gilbert equation (LLG eq.) and by using the linear spin wave theory. We systematically clarified the magnon band structure by varying the external magnetic field and find that the band gaps increase with the magnetic field perpendicular to the chiral axis. We also find the edge modes appear within the band gap and clarify that the swirling spin textures penetrate into the system from the edges by activating the edge modes. In the talk, we will also discuss the enhancement of the emergent electric phenomena and AC magnetic field drive of swirling spin textures. |
Condensed Matter Seminar Thursday, February 16, 2023 4:00 PM Claude Moore Nursing, Room G120 Note special time. Note special room. [1] J. Kishine and A. S. Ovchinnikov, Phys. Rev. B 79, 220405(R) (2009). |
"X-ray and Neutron Scattering on Square Lattice Antiferromagnets: 214-nickelates and melilite Ba2CoGe2O7"Rajesh Dutta , Institut fur Kristallographie, RWTH Aachen University [Host: Despina Louca]
ABSTRACT:
Neutron scattering and its complementary X-ray scattering techniques help to probe many novel and [1] A. Maity, R. Dutta, and W. Paulus, Stripe discommensuration and spin dynamics of half-doped Pr3/2Sr1/2NiO4, Phys. Rev. |
Condensed Matter Seminar Monday, February 20, 2023 3:30 PM , Room Zoom Note special date. Note special room. Join Zoom Meeting https://virginia.zoom.us/j/97901615040?pwd=YTh4WEs0RmRCVHRKUjQwTmpDUzQ4QT09
Meeting ID: 979 0161 5040 Passcode: 920090 |
"Stripe discommensuration and spin dynamics in hole-doped 214-nickelates Pr2-xSrxNiO4+delta"Aveshek Maity , Research Neutron Source Heinz Maier-Leibnitz (FRM II), Technical University of Munich, Garching, Germany [Host: Despina Louca]
ABSTRACT:
Magnetic excitations in the spin-stripe phases of La-based 214-nickelates have been vigorously explored using inelastic neutron scattering (INS) study for almost last three decades and still have remained an exciting research field, especially to understand their differences yet of their structural similarities with high-Tc 214-cuprates. In view of so far reported two-dimensional antiferromagnetic nature, out-of-plane magnetic excitations are generally not expected in 214-nickelates. In this talk, I will present our recent results from INS measurements on the stripe discommensurated phases of Sr-doped Pr3/2Sr1/2NiO4 samples, showing a compelling evidence for the presence of a sizable out-of-plane interaction suggesting a three-dimensional nature of magnetic excitations near the half-doped region [1,2]. The measured magnetic excitations are in good agreement with our linear spin wave (LSW) theory based calculations in the discommensurated spin stripe models. Additionally, I will discuss the effect of short-range vs. long-range spin stripe correlations on the spin wave dispersion by comparing the results with our INS study on an O-doped sample Pr2NiO4+δ [3]. |
Condensed Matter Seminar Thursday, February 23, 2023 4:00 PM via Zoom, Room TBA Note special time. Note special room. Join Zoom Meeting https://virginia.zoom.us/j/91523887660?pwd=TE5NSEhzZy9RTVN4Nmg0V0VvVFM1Zz09
Meeting ID: 915 2388 7660 Passcode: 845220 |
"Non-Loudon-Fleury Raman scattering in spin-orbit coupled Mott insulators "Yang Yang , University of Minnesota [Host: Gia-Wei Chern]
ABSTRACT:
The Loudon-Fleury form of the Raman operator has been used to compute magnetic Raman responses inside magnetic insulators for decades. The formalism provided by Loudon and Fleury [1] only considers the light-induced direct hopping between two magnetic ions. However, this is an oversimplified scenario for spin-orbit coupled Mott insulators. For example, the microscopic origin of the superexchange interaction inside Kitaev materials shows multiple indirect superexchange paths involving ligand ions can also produce a significant anisotropic interaction [2]. Therefore, to correctly construct the Raman operator for spin-orbit coupled Mott insulators requires considering all the non-negligible direct and indirect superexchange paths.
In this talk, I will present our work on constructing the Raman operator for the spin-orbit coupled Mott insulators which involve multiple superexchange paths [3], and I will also show how our revised theory can be applied to the three-dimensional hyperhoneycomb Kitaev material β−Li2IrO3 [4], where we show a qualitative modification of the polarization dependence, including, e.g., the emergence of a sharp one-magnon peak at low energies, which is not expected in the traditional Loudon-Fleury theory.
[1] P. A. Fleury and R. Loudon, Phys. Rev. 166, 514 (1968). [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). [3] Yang Yang, Mengqun Li, Ioannis Rousochatzakis, and Natalia B. Perkins Phys. Rev. B 104, 144412 (2021). [4] Yang Yang, Yiping Wang, Ioannis Rousochatzakis, Alejandro Ruiz, James G. Analytis, Kenneth S. Burch, and Natalia B. Perkins Phys. Rev. B 105, L241101(2022). |
Condensed Matter Seminar Thursday, March 2, 2023 4:00 PM Chemistry, Room 402 Note special time. Note special room. |
Available
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Condensed Matter Seminar Monday, March 13, 2023 4:00 PM , Room TBA Note special date. Note special time. Note special room. |
Available
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Condensed Matter Seminar Thursday, March 16, 2023 4:00 PM Claude Moore Nursing, Room G120 Note special time. Note special room. |
ABSTRACT:
Simulation of quantum many-body physics, such as looking for ground state properties and real time dynamics, plays an important role in the study of condensed matter physics, high energy physics and quantum information science. The recent advancement of machine learning provides new opportunities for tackling challenges in simulating quantum many-body physics. In this talk, I will first discuss a class of wave functions via neural network transformation, neural network backflow, which can fulfill the anti-symmetry property and capture the correlation and the sign structure for strongly-interacting fermionic physics. Next, I will talk about recent progress of simulating continuum quantum field theories with neural quantum field state [2], and lattice gauge theories such as 2+1D quantum electrodynamics with finite density dynamical fermions using gauge symmetric neural networks [3,4]. Finally, I will present a neural network representation based on positive-value-operator measurements for quantum circuit and open quantum system dynamics simulation [5]. |
Condensed Matter Seminar Monday, March 20, 2023 4:00 PM Physics Building, Room 313 Note special date. Note special time. Note special room. |
"The Mpemba effect for phase transitions by Landau theory"Roi Holtzmann , Weizmann Institute of Science [Host: Marija Vucelja]
ABSTRACT:
The Mpemba effect describes the situation in which a hot system cools faster than an identical copy that is initiated at a colder temperature. In many of the experimental observations of the effect, e.g. in water and clathrate hydrates, it is defined by the phase transition timing. However, none of the theoretical investigations so far considered the timing of the phase transition, and most of the abstract models used to explore the Mpemba effect do not have a phase transition. In this talk, I will suggest a definition for the phase transition time in a non-equilibrium state using the Landau theory for phase transitions. Using this definition, I will show that a Mpemba effect with respect to phase transitions can exist in such models, namely that the hotter system undergoes the transition before the colder one when quenched to a cold temperature. |
Condensed Matter Seminar Thursday, March 23, 2023 4:00 PM Ridley Hall, Room 177 Note special time. |
"Altermagnetism: a third type of ordered collinear magnetism"Igor Mazin , George Mason University [Host: Dima Pesin]
ABSTRACT:
Since many years, the canonincal classification of ordered magnets included noncollinear (with many further subdivisions) and two collinear types: antiferromagnets (AF), which have net magnetization zero by symmetry, and ferro/ferrimagnets (FM), which do not have this property. The two have distinctly different micro- and macroscopic properties. It was supposed, for instance, that only FM can exhibit spin-splitting of the electronic bands in absence of spin-orbit coupling AND lack of inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven by variation of the Berry phase), magnetooptical effects, suppressed Andreev scattering in contact with a singlet superconductor etc. A surprisingly recent development (~2019) is that this classification is incomplete: there are collinear magnets that would belong to AF by this classification, but show all characteristics of FM, *except the net spin polarization*! They were recently dubbed by Mainz group "altermagnets", AM. Incidentally, what has also not been fully appreciated was that there are also materials that have strictly zero net magnetization, but enforced not by symmetry, but by the Luttinger's theorem, and therefore truly belonging to the FM class ("Luttinger-compensated ferrimagnets"). In this talk I will present the new classification and explain, in specific examples, what are the symmetry conditions for AM, why these are a truly new class deserving a new name, and how their unusual properties appear. |
Condensed Matter Seminar Monday, March 27, 2023 3:30 PM Physics Building, Room 313 Note special date. Note special room. |
"Scanning tunneling spectroscopy of unconventional superconductors"Pavlo Sukhachov , Yale [Host: Dmytro Pesin]
ABSTRACT:
Motivated by recent experimental observations of unconventional superconductivity in twisted bilayer and trilayer graphenes, we develop a theory describing the differential conductance between a normal STM tip and a 2D superconductor with an arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmission between the tip and the superconductor. Exploiting the dependence of Andreev reflection on the relative position of the STM tip with respect to the lattice symmetry points, we show that the structure of the superconducting gap can be extracted by combining weak- and strong-tunneling limits of differential conductance. Furthermore, the theory incorporates a tip/impurity-induced scattering potential within the 2D material, which allows us to describe subgap resonances. |
Condensed Matter Seminar Thursday, March 30, 2023 4:00 PM Ridley Hall, Room 177 Note special time. |
"Anomalous thermal relaxation in Langevin dynamics and reaction networks "Matthew Walker , UVA [Host: Marija Vucelja]
ABSTRACT:
In recent years anomalous cooling and heating effects in the far from equilibrium limit have gained attention. One anomaly is the so called Mpemba Effect, in which the time to relax towards thermal equilibrium does not grow monotonically as a function of distance to the target. Instead, it has been proposed that there exist shortcuts in the relaxation process that allow both faster, and even exponentially faster heating and cooling. In this talk I will discuss recent works [1,2] that have progressed our understanding of such shortcuts by studying the Mpemba effect using Overdamped Langevin dynamics. I will show when and where you can get the effect, and that our models are in good agreement with experimental findings. Lastly, I will touch upon current works where we study the effect using Markovian jump processes on linear reaction networks.
Matthew R Walker and Marija Vucelja J. Stat. Mech. (2021) 113105
Matthew R Walker and Marija Vucelja arXiv preprint arXiv:2212.07496 (2022)
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Condensed Matter Seminar Thursday, April 6, 2023 4:00 PM Ridley, Room 177 Note special time. |
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Condensed Matter Seminar Wednesday, April 12, 2023 3:00 PM Physics, Room 120 Note special date. Note special time. Note special room. |
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Condensed Matter Seminar Thursday, April 13, 2023 4:00 PM Ridley Hall, Room 177 Note special time. |
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Condensed Matter Seminar Thursday, April 20, 2023 4:00 PM Ridley Hall, Room 177 Note special time. |
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Condensed Matter Seminar Tuesday, April 25, 2023 12:00 PM Physics, Room 313 Note special date. Note special time. Note special room. |
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Condensed Matter Seminar Thursday, April 27, 2023 4:00 PM Ridley Hall, Room 177 Note special time. |
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