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 Physics at Virginia

"Nonlinear Optical Spectroscopies for Resolution of Electronic Structure and Dynamics"


Veronica Policht , U.S. Naval Research Laboratory
[Host: Despina Louca]
ABSTRACT:

Rapid and efficient charge transfer following absorption of light is a process of intense interest from the
perspectives of both fundamental physics and optoelectronic applications. Among the exciting systems which
host charge transfer are photosynthetic reaction centers (RC), proteins packed with light-absorbing molecules
which yield a charge separated state with near unity quantum efficiency, and Transition Metal Dichalcogenide
Heterostructures (TMD HS), which host interlayer charge transfer to form spatially separated interlayer
excitons. Despite intense interest in understanding charge transfer in these systems, their complex electronic
structure and the ultrafast timescales of their dynamics have presented a significant challenge in clearly
resolving the underlying fundamental physics. Two-Dimensional Electronic Spectroscopy (2DES) is a nonlinear
optical spectroscopic technique with simultaneously high frequency and temporal resolution and is an ideal
tool for studying systems with complex electronic structure and femtosecond-timescale dynamics. In this talk I
will present on my work applying 2DES to resolving the excitonic structure of photosynthetic RCs as well as
resolving ultrafast interlayer exciton dynamics in TMD HS.

Condensed Matter Seminar
Thursday, January 18, 2024
3:30 PM
Gibson Hall, Room 211
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A recording of this talk is available at this link (enter the passcode UEd*U6Wr).


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"Room-temperature and many-body quantum states in topological materials "


Md. Shafayat Hossain , Princeton University
[Host: Seunghun Lee]
ABSTRACT:

Topological states of matter combine quantum physics with topology—a branch of mathematics that explores geometric properties preserved under deformation. Quantum topology can lead to incredible properties. For instance, in a topological insulator, conducting edge states exist within an insulating bulk. Despite continuing progress, the search for such new quantum phases remains a central theme of condensed matter physics. In this talk, I will introduce two of the most sought-after quantum states—room-temperature topology and topological exciton insulator. I will first discuss our spectroscopic observation of topological edge states in Bi4Br4. I will show that the topological states, which typically can only be observed at temperatures around absolute zero, survive here at room temperature. I will also show how we probe the quantum transport response of this edge state using quantum interference. These observations mark the first steps in demonstrating the potential of topological materials for energy-saving applications. In the second part of my talk, I will discuss our discovery of a unique topological state in Ta2Pd3Te5. Here, the Coulomb interactions pair fermions (electrons and holes) into bosons (excitons), leading to a superfluid condensate state in the bulk while hosting topological edge states on the boundary. Finally, I will touch upon how these discoveries suggest exciting possibilities. This includes new devices and experimental techniques to discover the fundamental physics of topological quantum matter, opening doors for more efficient room-temperature devices and quantum information technology. 

Condensed Matter Seminar
Monday, January 22, 2024
2:00 PM
Physics, Room 323
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"Next-generation artificial van der Waals quantum materials "


Dr. Bumho Kim , University of Pennsylvania
[Host: Seunghun Lee]
ABSTRACT:

If one can control the atomic symmetries of a material at will, the intrinsic properties of the material will be significantly modified. However, the atomic structures of conventional materials are often constrained by the equilibrium phase of matter. Here, we overcome this fundamental limitation using recent advances in twistronics, enabling precise control over all the individual point group symmetry elements – inversion, mirror, and rotational symmetries – in twisted van der Waals (vdW) material in a new 3D configuration [1]. The resulting 3D twisted materials exhibit emerging optical responses that are fundamentally different from those of natural vdW materials. This novel approach to control symmetries can enable nearly infinite vdW quasicrystalline phases, promising a practical platform to study less-explored structure-property relationships of quasicrystals. In addition, we will discuss an ultraclean vdW crystal synthesis method [2]. A self-flux synthesis method we developed has yielded vdW materials with ~ 2 orders of magnitude lower point defect density compared to commercial vdW materials grown by a chemical vapor transport method. These ultraclean vdW materials reveal intrinsic excitonic properties that were previously obscured by low-quality materials. The combination of these ultraclean materials with the symmetry design approach holds great promise for the development of high-performance artificial material systems for next-generation technologies.

 

References:

 

  1. Bumho Kim, Jicheng Jin, Zhi Wang, Li He, Thomas Christensen, Eugene J. Mele, and Bo Zhen, Nature Photonics 18, 91-98 (2024).
  2. Bumho Kim, Yue Luo, Daniel Rhodes, Yusong Bai, Jue Wang, Song Liu, Abraham Jordan, Baili Huang, Zhaochen Li, Takashi Taniguchi, Kenji Watanabe, Jonathan Owen, Stefan Strauf, Katayun Barmak, Xiaoyang Zhu, and James Hone, ACS Nano 16, 140-147 (2022).

 

Condensed Matter Seminar
Thursday, January 25, 2024
3:30 PM
Physics Building, Room 323
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A recording of this talk is available at this link (enter the passcode @mJua4k1).


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"Novel Fabrication of Quantum Wires: Towards Fractionalized Excitations"


Tomoya Asaba , Kyoto University
[Host: Seunghun Lee]
ABSTRACT:

The quest for novel quantum states in condensed matter physics often hinges on the reduction of system dimensionality. In particular, one-dimensional systems are theoretically predicted to host a range of fractionalized excitations. These include the Tomonaga-Luttinger liquid, which exhibits spin and charge separation, and the Majorana particle, a cornerstone for fault-tolerant quantum computing. However, fabricating near-perfect one-dimensional quantum wires has been a significant challenge, especially those involving strongly correlated electrons.

In our research, we have developed a novel method to fabricate quantum wires of a Mott insulator on graphite substrates using pulsed-laser deposition, achieving structures such as stripes, junctions, and nanorings. These single-crystalline wires are one unit cell in thickness and precisely two to four unit cells in width, and can extend to several micrometers in length. The spectroscopy measurements along with theoretical calculations reveal the existence of strong electron correlations in this system. Moreover, our findings emphasize the importance of nonequilibrium reaction-diffusion processes in atomic-scale self-organization, opening up exciting avenues for the exploration of exotic fractionalized states in purely one-dimensional quantum wires.

Condensed Matter Seminar
Monday, January 29, 2024
2:00 PM
Physics, Room 323
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A recording of this talk is available at this link (enter passcode ^Sa3J2OZ).


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"Nanoscale quantum sensing of programmable quantum matter"


Shaowen Chen , Harvard University
[Host: Seunghun Lee]
ABSTRACT:

Characterization and quantum control of complex quantum matter is one of the shared goals for condensed matter and quantum information science research. Toward this end, my research uses van der Waals materials to synthesize topological and correlated states, and quantum sensors based on spin defects to uncover their microscopic picture. Focusing on superconductivity as the theme of this talk, I will first present pathways to program the electron correlation by exploiting the lattice degree of freedom, both in the planar and vertical directions of moiré materials. The challenges to fully characterize the moiré superconductivity will be discussed. In the second part, I will show new experimental observables unlocked by the nanoscale quantum sensing platform can uncover hidden physics. As an example, quantitative visualization of the super current flow in a Josephson junction is used to reveal electrically configurable ground states in the zero-resistance regime. A surprising role of the kinetic inductance and the implications for the Josephson diode effect will be discussed. Finally, I will share my vision to explore intertwined topology and correlation by integrating the programmable quantum materials with nanoscale quantum sensors.

Condensed Matter Seminar
Thursday, February 1, 2024
3:30 PM
Physics, Room 323
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A recording of this talk is available at this link (enter passcode *0m4DSym).


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"Ultranodal state in multiband spin-1/2 superconductors"


Peter Hirschfeld , University of Florida
[Host: Bellave Shivaram]
ABSTRACT:

Recent measurements on the tetragonal phase of the iron-based superconductor FeSe,S support the existence of a remarkable phase where the superconducting state supports a finite residual  density of states arising from patchlike nodal surfaces[1,2].  This ``ultranodal"> state can arise in situations where conventional intraband spin singlet pairing is highly anisotropic and coexists with time-reversal symmetry breaking  interband spin triplet interactions [3].  Here I present a  microscopic scenario including ferromagnetic interactions that can account for nonunitary pairing and C4 symmetry breaking in the superconducting state that is also observed in recent experiments.

 

1) Sato, Y. et al. Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1-xSx. Proc. Natl Acad. Sci. 115, 1227??1231 (2018).

2) Hanaguri, T. et al. Two distinct superconducting pairing states divided by the nematic end point in FeSe1-xSx. Sci. Adv. 4, eaar6419 (2018).

3) ``Topologically protected ultranodal state in iron-based superonductors", S. Setty, S.

Bhattacharyya, Y. Cao, A. Kreisel and P.J. Hirschfeld,  Nat. Comm. 11, 523 (2020).

 

Condensed Matter Seminar
Thursday, February 8, 2024
3:30 PM
Physics, Room 323
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"Exploring emergent quantum phases in two-dimensional flat band systems"


Jiang-Xiazi Lin , Brown University
[Host: Seunghun Lee]
ABSTRACT:

Quantum phases such as superconductivity and ferromagnetism are among the most important topics in condensed matter physics research. Recently, a family of two-dimensional flat band systems, including magic-angle twisted graphene, uncovered an abundance of symmetry breaking and novel quantum phases.

In this talk, I will introduce the recent advances in these materials and give two examples of how we engineered and revealed new quantum phases of matter in twisted graphene. These include an orbital ferromagnetic state induced by spin-orbit coupling and a zero-field superconducting diode effect. Towards the end of the talk, I will mention our on-going effort of studying a new type of Coulomb-driven rotational symmetry breaking state in the moiré-less bilayer graphene. These examples establish the two-dimensional flat band systems as a versatile platform with multiple tuning knobs, where new physics emerges from the interplay between various quantum phases.

Condensed Matter Seminar
Monday, February 12, 2024
2:00 PM
Physics, Room 323
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A recording of this talk is available at this link (use passcode #D8FWkr?).


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ABSTRACT:

Despite identical R3 crystal structures, honeycomb layered MTiO3 ilmenites exhibit diverse magnetic orders and transition temperatures (TN): G-type antiferromagnetic for MnTiO3 (TN=68 K) and A-type antiferromagnetic for CoTiO3(TN=38 K) and NiTiO3 (TN=22 K). This work focuses on this intriguing interplay between local structure, electronic properties, and magnetic configurations. CoTiO3 has two magnon peaks around 5-14 meV with a distinct gapless Dirac node nestled between them are observed and the magnon modes are renormalized to lower energies. For CoTiO3, magnetic excitations attributed to spin-orbit exciton multiplet transitions show the same temperature dependance as magnon with the intensity dissipating quickly above TN.  The energy levels arising from crystal field and spin-orbit coupling are gradually thermally populated through T and reaching maximum at 100 K. However, the NiTiO3 system shows a single low energy magnon peak around 2-4 meV which is renormalized into lower energies, but it does not show Dirac magnon properties. The calculated exchange interactions using SpinW confirm the weaker inter-plane interaction in CoTiO3 than NiTiO3. Across three system, both transition metal M+2 ion and Ti+4 ions are in distorted octahedra environment, and the first four nearest neighbors are Ti-O < M-O < Ti-O < M-O with the given bond length order. Across three systems Ti-O and short M-O bond length variation is minimum. However, M-O bond length (MnTiO3=2.28 Å, CoTiO3=2.17 Å and NiTiO3=2.12 Å) variation is significant which follows the same variation as reported dielectric constants (MnTiO3=20.4, CoTiO3=19.5 and NiTiO3=17.8 ) and TN and confirms the interplay between these parameters. Within the measured 100 K to 500 K, temperature dependance of local structure is insignificant and for the reported relative dielectric values, the variation is almost constant. This suggests that the interplay between local geometry and magnetic interactions governs the diverse behaviors observed in these honeycomb materials.

Condensed Matter Seminar
Thursday, February 22, 2024
11:00 AM
Physics, Room 323
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ABSTRACT:

In this talk, I will discuss our recent work on transport phenomena stemming from the topological properties of magnetic textures. As a specific illustrative case, we study the transport of vorticity on curved dynamical two-dimensional magnetic membranes. We find that topological transport can be controlled by geometrically reducing symmetries, which enables processes that are not present in flat magnetic systems. To this end, we construct a vorticity 3-current obeying a continuity equation, which is immune to arbitrary local disturbances of the magnetic texture as well as spatiotemporal fluctuations of the membrane. We show how electric current can manipulate vortex transport in geometrically nontrivial magnetic systems. As an example, we propose a minimal setup that realizes an experimentally feasible energy storage device and discuss its thermodynamic efficiency in terms of a vorticity-transport counterpart of the thermoelectric “ZT” figure of merit.

Condensed Matter Seminar
Thursday, March 21, 2024
3:30 PM
Gibson Hall, Room 211
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"Linear-in-temperature conductance in electron hydrodynamics"


Leonid Levitov , MIT
[Host: Dima Pesin]
ABSTRACT:

Linear temperature dependence of transport coefficients in metals is habitually ascribed to non-Fermi-liquid physics. In this talk we establish this behavior for 2D electron fluids, systems in which carrier collisions assist conduction, leading to resistance decreasing with temperature. As we will see, electron fluids with simple Fermi surfaces obey nonclassical hydrodynamics described by a loop representing the Fermi surface shape evolving in space and time. Replacing the fluid velocity dynamics with an amoeba-like loop dynamics leads to a large family of long-lived excitations manifest as multiple viscous modes. A cascade of these modes results in a linear T dependence that extends down to lowest temperatures, as well as a Kolmogorov-like fractional power -5/3 scaling of conductivity vs. wavenumber. These dependences provide a smoking gun for nonclassical hydrodynamics and are expected to be generic for strongly-correlated 2D systems with near-circular Fermi surfaces.

Condensed Matter Seminar
Thursday, March 28, 2024
2:00 PM
Monroe Hall, Room 118
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ABSTRACT:

Pairing PbS quantum dots (QDs) with photochromic molecules (PCMs) allows for the synthesis of efficient and reversible near infrared photoluminescence (PL) photo-switches. In our work, we explore the utility space of this hybrid system by systematically comparing and contrasting different types of PCMs and different sizes of QDs. We demonstrate that the amount of photo-switching observed can be affected by (1) varying the size of the QDs, (2) varying the length of the PCMs, (3) fluorinating the PCMs, (4) varying the end group of the PCMs. We further investigate this system to parse out the mechanisms which may be responsible for this behavior. We present strong evidence to suggest that the mechanism driving this switching effect is an inter-QD tunneling process. We demonstrate a possible link between the energy levels of the PCMs and the magnitude of the switching effect and outline a rough empirical model which can guide the future design of QD/PCM photo switches to produce customized switching properties.

Condensed Matter Seminar
Thursday, March 28, 2024
3:30 PM
Gibson Hall, Room 211
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"Computational Approach to Compositionally Complex Materials "


Diego Ibarra , University of Virginia
[Host: Joe Poon]
ABSTRACT:

High Entropy Alloys (HEAs), also known as Compositionally Complex Alloys (CCAs), embody a transformative class of materials consisting of at least four elemental components. These new types of material open new horizons in alloy design for exploring new structural and functional material properties unknown in traditional alloys. However, the combinatorial nature of HEAs can result in compositional possibilities reaching into the billions or even trillions, making traditional studies challenging. This talk presents an in-depth exploration of HEAs, starting with a foundational understanding of their unique characteristics and the importance of their complex phase behavior. It highlights the inherent challenges posed by the expansive compositional space and limitations of conventional materials discovery and design methodologies. The talk emphasizes the pivotal role of computational techniques that provides a strategic blueprint for high-throughput alloy design that accelerates the exploration and optimization of HEAs but also provides a deeper insight into their fundamental behaviors.

Condensed Matter Seminar
Thursday, April 4, 2024
11:00 AM
Physics Building, Room 323
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"Voxelated Bioprinting: Digital Assembly of Viscoelastic Bio-ink Particles"


Liheng Cai , University of Virginia
[Host: Bellave Shivaram]
ABSTRACT:

Analogues of pixels to two-dimensional (2D) pictures, voxels –– in the form of small cubes or spheres –– are the basic units of three-dimensional (3D) objects. Digital assembly of bio-ink voxels may provide an approach to engineering heterogeneous yet tightly organized 3D tissue mimics. However, this approach requires precisely manipulating highly viscoelastic bio-ink voxels in 3D space, which represents a grand challenge in both soft matter science and biomanufacturing. In this talk, I will introduce a voxelated bioprinting technology that enables the Digital Assembly of Spherical bio-ink Particles (DASP). First, I will discuss the criteria for the on-demand generation, disposition, and assembly of viscoelastic bio-ink droplets in an aqueous environment without the help of large interfacial tension. Second, I will describe how to use DASP to create 3D structures consisting of interconnected yet distinguishable bio-ink particles. Finally, I will share our recent progress in applying DASP to encapsulate islets into multiscale porous scaffolds to treat type 1 diabetes. I will also discuss immediate applications and emerging challenges associated with voxelated bioprinting.

Condensed Matter Seminar
Thursday, April 11, 2024
3:30 PM
Gibson Hall, Room 211
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