Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophysics Seminars are in Room NPB 2205 Contact: Yasu Takano or Dmitrii Maslov |
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August 26 |
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September 2 (No seminar – Labor Day) |
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September 9 |
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Speaker |
Kathryn McGill (UF) |
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Title |
Two-dimensional materials: from Berry curvature to wrapping a microsphere |
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Abstract |
The study of atomically-thin, truly two-dimensional (2D) materials has morphed into its own field since the experimental isolation of graphene and similar 2D materials in 2005. Graphene, as a single layer of carbon atoms with a unique band structure, and monolayer molybdenum disulfide (MoS2), a three-atom-thick semiconductor, have been of particular interest both for the physics accessible in 2D crystals and the applications achievable with highly flexible materials. Here I present a variety of experiments exploring the optoelectronic and mechanical properties of both monolayer MoS2 and graphene. In particular, I will discuss three studies: (1) the experimental realization of the valley Hall effect, an effect based on the Berry curvature of a material's energy bands, in monolayer MoS2; (2) methods for directly measuring the bending stiffness of graphene (and related 2D materials); and (3) an investigation of the wrapping of micro-spherical droplets by monolayer MoS2. I will conclude by discussing the future outlook of both "valleytronics" and microencapsulation by atomically thin materials. |
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Host |
Amlan Biswas |
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September 16 |
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Speaker |
Laura Fanfarillo (SISSA and UF) |
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Title |
Unconventional superconductivity and Hund's induced electron correlations: a cooperative mechanism |
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Abstract |
Thirty years of research has established the emergence of a new paradigm in which strong electron-electron correlations and superconductivity are strongly intertwined. A number of successful approaches have improved our understanding of this link in high-temperature copper-based superconductors and in other families, including the iron-based materials. However, a general framework to understand how superconductivity emerges in systems dominated by electron-electron repulsion is still lacking. In this talk I will focus on the analysis of this problem in the context of iron-based superconductors. First I will provide a brief overview of the experimental evidences of the strong orbital-dependent electronic correlations characterizing the paramagnetic phase of iron-based superconductors and I will discuss this phenomenology in terms of Hund's metal physics. Then, I will present our results on the role of those electronic correlations on the superconductivity driven by a generic weak-coupling mechanism (e.g. the coupling to a boson). The key novelty of the study is the inclusion of the dynamical properties that make a Hund's metal substantially different with respect to both a weakly interacting metal and to an ordinary correlated metal with a large effective mass renormalization. This allows us to unveil the crucial role of the redistribution of spectral weight of the Hund's metal to promote superconductivity and to enhance the orbital-selective character of the gap functions. |
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Host |
Peter Hirschfeld |
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September 23 |
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Speaker |
BingKan Xue (UF) |
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Associative memory, attractor dynamics, and neural networks |
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Abstract |
I will describe some basic ideas in the theoretical study of neural networks, originating from the classic work of Hopfield. The Hopfield model provides an abstract characterization of associative memory, the ability of a neural network to retrieve stored patterns. In this model, memories are encoded as attractors of the network dynamics. The analogy to statistical mechanics allows us to understand the behavior of the network, whereas the biological context brings new types of questions, such as the network's capacity for storing different memories. The idea has since then been developed and generalized in many ways. I will describe some recent developments that incorporate temporal structures, such as transition between attractors and recognition of dynamic patterns, or spatial structures, such as spatial maps of environments and assembly of macro-molecules. |
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Host |
Steve Hagen |
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September 30 |
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Speaker |
Matthew Matheny (Caltech) |
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Applications of nanomechanical lattices: from dynamics to computation |
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Nanomechanical systems have been studied for over 20 years with most efforts only exploring a single resonant device at a time. Due to the current limitations in nanoscale fabrication, it proves nearly impossible to achieve the uniformity required to study even basic lattice models using nanomechanics. Here I will describe an experiment based on piezoelectric nanomechanical systems which achieves strong uniformity of lattice parameters. Amazingly, this experimental system is able to spontaneously generate exotic symmetry breaking states akin to those found in spin systems in condensed matter. I will also describe how the experimental architecture could be used for novel computational schemes. Finally, I will describe a proposed effort to use piezoelectric nanomechanics in quantum computational architectures. |
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Host |
Yoonseok Lee |
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October 7 |
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Speaker |
Manh-Huong Phan (USF) |
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Two-dimensional magnetism: new discoveries, opportunities and challenges |
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Two-dimensional (2D) magnetic van der Waals materials and their heterostructures are emerging candidates for ultralow-power and ultra-compact spintronic device applications. Although the Mermin-Wagner theorem predicts suppression of long-range magnetic order at finite temperatures in such 2D materials, recent experiments have demonstrated the existence of long-range ferromagnetic ordering in bulk van der Waals materials at the single layer limit [1,2]. In particular, our recent discovery of the strong room temperature ferromagnetism in epitaxially grown transition metal dichalcogenide (TMD) monolayers of VSe2 grown on various van der Waals substrates (graphene, graphite, MoS2, WS2) has the potential to transform the fields of spintronics and quantum computing [3]. In this talk, I will present research progress in 2D magnetism, including our new findings of tunable exchange bias effect and room temperature light-controlled magnetism in monolayer VSe2/MoS2 or VSe2/WS2 heterostructures, as well as the development of a new class of highly sensitive magnetic sensor using this single layer magnet. Opportunities and challenges in 2D magnetic materials research will be discussed . [1] C. Gong et al., Nature 546, 265 (2017). [2] B. Huang et al., Nature 546, 270 (2017). [3] M. Bonilla et al., Nat. Nanotech. 13, 289 (2018). |
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Host |
Mark Meisel |
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October 14 |
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Speaker |
Xiaoyu Wang (NHMFL, Tallahassee) |
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Title |
Scattering mechanisms and electrical transport near and Ising-nematic quantum critical point |
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Electrical transport properties near an Ising-nematic quantum critical point are of both theoretical and experimental interest. The difficulty of the problem is in part due to the fact that the electronic scattering mediated by critical fluctuations are momentum-conserving. As a result, despite destroying coherent Landau quasi-particles, critical fluctuations do not naturally lead to a finite electrical resistivity. One is led to carefully address the interplay with various current relaxation mechanisms that are absent from the low-energy theory, namely impurity scattering, umklapp scattering, and so on. In this talk, I will first derive a generalized kinetic equation valid in a broad temperature regime near the quantum critical point. Next, I discuss several current-relaxation mechanisms and how they lead to very rich features in the temperature scaling of dc electrical resistivity beyond what has been studied in the literature. Finally I will discuss Raman scattering in the B1g channel, in particular the properties of a quasi-elastic peak due to critical fluctuations. |
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Host |
Dmitrii Maslov, Yuxuan Wang |
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October 21 |
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Speaker |
Dixit Purushottam (UF) |
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Metastability transition in genome evolution and population structure of bacterial species |
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Multiple evolutionary forces such as point mutation, horizontal gene transfer (HGT), adaptation, and ecological niche separation simultaneously shape genomic diversity within bacterial species. Of these, the interaction between point mutations and HGT is of particular interest as it pertains to a fundamental question: what are bacterial species? On the one hand, binary cell division and vertical inheritance (mother to daughter) of point mutations imposes a clonal population structure. On the other hand, transfer of small genetic fragments between related bacteria corrupts the phylogenetic tree. Whether bacteria can retain clonal phylogeny in the presence of HGT currently remains unknown.? Using a theoretical model, we identify two qualitatively distinct modes of bacterial evolution. In the divergent mode the cohesion due to recombination is not sufficient to overcome vertical inheritance of mutations. As a consequence divergence between genomes increases linearly with time. At the population level, transient clusters of sexually isolated sub-populations are continuously formed and dissolved. The species as a whole retains a clonal population structure. In contrast, in the metastable mode, recombination has the upper hand. Here, genomes remain closely related to each other for very long periods of time before eventually escaping the pull of recombination (hence the name metastable). The population remains genetically cohesive and stable over time. Analysis of real bacteria shows that bacterial species belong to both these modes. Generalizations and future directions are discussed. |
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Host |
Dmitrii Maslov |
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October 28 |
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Speaker |
Philip Feng (UF Electrical & Computer Engineering) |
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Emerging semiconductor nanoscale devices and systems for classical and quantum information processing |
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Emerging semiconductors, ranging from atomic layer semiconducting crystals (such as transition metal dichalcogenides (TMDCs) and black phosphorus) to wide and ultrawide bandgap materials (such as SiC and Ga2O3), along with their heterostructures, offer compelling new platforms for electronic, photonic devices and transducers, where the unconventional and unique properties of these crystals can be harnessed for engineering both classical and quantum signal processing and sensing schemes. In this presentation, I will describe some of my research group’s latest endeavors and results on advancing solid-state device physics and engineering, by employing some of these emerging semiconductors. In classical domain, we build atomically thin transistors, optoelectronic devices, and a new class of nanoscale transducers, 2D nanoelectromechanical systems (NEMS), all enabled by 2D semiconductors and their van der Waals heterostructures. We demonstrate how the unconventional properties of these structures and their internal strong coupling effects have led to novel transistors and logic circuits, optoelectronic devices, and resonant NEMS transducers with remarkably broad dynamic range and electrical tunability, as well as new phenomena and device functions. Toward quantum engineering, atomistic defects in SiC and emerging 2D crystals support single-photon quantum emitters promising for enabling quantum bits (qubits) at room temperature. Built on our recent attainments in SiC photonics and 2D devices, we explore such platforms and heterogeneous integration, toward realizing quantum transduction and information processing in chip-scale integrated systems. |
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Host |
Yoonseok Lee |
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November 4 |
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Speaker |
Mathias Scheurer (Harvard Univ.) |
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Gauge theories for the cuprates: thermal Hall effect and optimal doping |
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Recent experiments [1] have revealed an enhanced thermal Hall effect in the pseudogap phase of several different cuprate compounds. The large signal even persists in the undoped system and, thus, challenges our understanding of the square-lattice antiferromagnet fundamentally. In the first part of the talk, I will analyze possible mechanisms that can give rise to a thermal Hall effect in the antiferromagnet [2,3]. In particular, I will discuss the possibility [3] that the magnetic field can drive the Néel state close to a transition to a phase where Néel order coexists with a chiral spin liquid. A spinon lattice model for this transition is shown to give rise to a large thermal Hall conductivity that also features a magnetic-field and temperature dependence similar to experiment. We will derive the low-energy continuum field theory for the transition, which is characterized by an emergent global SO(3) symmetry and has four different formulations in terms of relativistic gauge theories that are all related by dualities. In the second part of the talk, I will present a non-Abelian gauge theory that we propose [4] as an effective field theory for the cuprates near optimal doping. In this theory, spin-density-wave order is fractionalized into Higgs fields while all low-energy fermionic excitations are electron-like and gauge neutral. The conventional Fermi-liquid state corresponds to the confining phase of the theory at large doping and there is a quantum phase transition to a Higgs phase, describing the pseudogap, at low doping. It will be shown that the topological order of the Higgs phase is very naturally intertwined with charge-density-wave, Ising-nematic, and scalar spin-chirality order. We will also discuss the quantum critical point of the model. [1] Nature 571, 376 (2019). [2] Phys. Rev. B 99, 165126 (2019). [3] arXiv:1903.01992 [Nature Physics (2019)]. [4] Phys. Rev. B 99, 054516 (2019). |
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Host |
Yuxuan Wang |
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November 11 (No seminar – Veterans Day) |
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November 18 |
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Adrian H. C. Po (MIT) |
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Title |
Modeling twisted bilayer graphene |
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Superconductivity and correlated insulators have been observed in “magic-angle” twisted bilayer graphene when the nearly flat bands close to charge neutrality are partially filled. The observed phenomenology resembles that of high-temperature superconductors like cuprates. Yet, the building blocks of the two systems are vastly different, since the states in TBG descend from graphene's Dirac dispersion. We argue that the Dirac character of the relevant states endows the nearly flat bands with a nontrivial band topology, which forbids any fully symmetric tight-binding description for the nearly flat bands alone. We will argue that such band topology is “fragile” in nature, in that it can be neutralized by supplementing additional trivial degrees of freedom. Correspondingly, we construct faithful tight-binding models for twisted bilayer graphene by incorporating higher-energy atomic bands. Not only do these models circumvent the problem of band topology, but they also lead to more localized effective orbitals and therefore shorter-range bonds and interactions. |
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Host |
Yuxuan Wang |
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November 25 (Monday of Thanksgiving Week) |
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Speaker |
Alex Levchenko (Uni. Wisconsin--Madison) |
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Mesoscopic, topological, anomalous Josephson effects |
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I will present three interconnected stories of the Josephson effect. First, is an old problem of the proximity effect and Josephson current in chaotic quantum dots where an interesting feature of the problem -- formation of secondary gaps -- was overlooked for many decades. Second, is relatively new topic of multi-terminal Josephson junctions as a practical platform for creating and manipulating band topologies of Andreev bound states when materials forming the junction are not topological. Third, is the topic of anomalous Josephson effect via Rashba 2DEG and its recent measurements in epitaxial Al/InAs junctions. |
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Host |
Dmitrii Maslov |
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December 2 (Monday after Thanksgiving Holiday) |
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Speaker |
Rongying Jin (Louisiana State Univ.) |
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TBA |
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Host |
Xiaoguang Zhang |
Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophyics Seminars are in Room NPB 2205 Contact: Yasu Takano or Dmitrii Maslov |
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January 6 |
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Speaker |
Thomas Searles (Howard Univ.) |
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TBA |
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Host |
Chris Stanton |
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January 13 |
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Speaker |
Juan Guan (UF) |
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Host |
Dmitrii Maslov |
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January 20 (No seminar - Martin Luther King Jr. Day) |
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January 27 |
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Constantin Schrade (MIT) |
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Host |
Dominique Laroche |
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February 3 |
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(Tentatively reserved) |
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February 10 |
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(Tentatively reserved) |
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February 17 |
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(Tentatively reserved) |
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February 24 |
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(Tentatively reserved) |
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March 2 (No Seminar - UF Spring Break Week, APS March Meeting in Denver) |
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March 9 |
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March 16 |
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Speaker |
Valentin Stanev (Univ. Maryland) |
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TBA |
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Host |
Peter Hirschfeld |
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March 23 |
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Speaker |
Se Kwon Kim (Univ. Missouri) |
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TBA |
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Host |
Yoonseok Lee |
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March 30 |
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Speaker |
Andrey Chubukov (Univ. Minnesota) |
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TBA |
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Host |
Dmitrii Maslov |
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April 6 |
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Speaker |
Alimamy Bangura (NHMFL, Tallahassee) |
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TBA |
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Host |
Mark Meisel |
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April 13 |
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Speaker |
Kater Murch (Washington Univ.) |
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Host |
James Hamlin |
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April 20 |
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Speaker |
Yi Li (Johns Hopkins Univ.) |
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TBA |
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Host |
Yuxuan Wang |