Special Joint Semiar with Astronomy

Eugene Kolomeisky

Associate Professor of Physics

University of Virginia

Title: Coulomb Universe in a Jellium Droplet

Abstract: Analogy between the Coulomb law of interaction between charges and the Newton law of gravitational attraction between masses is familiar to every physics student.  In this talk I demonstrate that this analogy implies that a system of identical charges can evolve with time in a manner that parallels cosmological evolution of the physical Universe with hallmarks such as Hubble's law and Friedmann-type dynamics present.  The Coulomb and Newton laws are also dissimilar because the electrostatic force is many orders of magnitude larger than the gravitational force whose manifestations are only noticeable on astronomical scale.  On the other hand, analog cosmological evolutions driven by Coulomb interactions are predicted to be observable in laboratory experiments involving Coulomb explosions and electron density oscillations in conductors.

Yafis Barlas

Assistant Professor of Physics

University of Nevada, Reno

Title: Correlated and Topological Phases in flat bands of two-dimensional crystals

Abstract: Due to their vanishing density of states and gapless semi-metallic behavior at charge neutrality, honeycomb lattice two-dimensional (2D) crystals are ideal candidates to host topological states. Even more interesting are twisted or strained 2D crystals, as the electron dispersion in these systems can be weakly dispersing, (i.e exhibit a small bandwidth) or completely flat. In these situations, the interplay of topology and correlation driven phases in flat bands of 2D crystals can result in emergent topological order.  Similarly, at high magnetic fields, multi-layer graphene and twisted bilayer graphene exhibit topological bands and various correlated states. In this talk, I will discuss a new class of interacting and non-interacting symmetry protected topological phases stabilized by mirror symmetry in 2D Dirac semi-metals. This quantum parity Hall state, exhibits two one-dimensional counter-propagating metallic edge states, distinguished by even or odd parity under the system’s mirror reflection symmetry.​ I will also discuss some of our results in twisted 2D crystals at high magnetic fields.

Professor Mark Novotny

Mississippi State University

Department of Physics and Astronomy

Title: Generalization of Bloch’s Theorem for Tight-binding Models:Quantum Dragon Nanomaterials and Nanodevices

Abstract: Bloch’s Theorem arguably has the largest economic impact of any condensed matter theorem.  Bloch’s Theorem can only be used for translationally invariant systems, and together with the tight-binding model gives band structures and hence devices such as transistors and optoelectronics.  For tight-binding models, a generalization of Bloch’s Theorem for some strongly disordered (non-translationally invariant) systems is stated and proved.  The theorem generalization leads to predictions of novel electrical properties for some nanomaterials and nanodevices with strong disorder, whimsically called quantum dragons [1].  A number of different predictions of quantum dragon materials and devices [2] will be presented.  Furthermore, conjectures and testing of scaling for materials that are ‘close to quantum dragons’ are given [3].  For example, a perfect quantum wire is one in which impinging electrons from an appropriate incoming lead have probability unity of propagating through the device to the end of an outgoing lead, T(E)=1, for all electron energies E that propagate through the leads. Some quantum dragons are predicted to be perfect quantum wires (see the device in the figure below [3]), while others may make a unique FET (Field Effect Transistor) or quantum sensor or spintronics nanodevice. 

[1] M.A. Novotny, Energy-independent total quantum transmission of electrons through nanodevices with correlated disorder, Phys. Rev. B 90, 165103 (2014).

[2] G. Inkoom and M. Novotny, Quantum dragon solutions for electron transport through nanostructures based on rectangular graphs, J. Physics Commun. 11, 115019 (2018). 

[3] M.A. Novotny and T. Novotný, Order amidst Disorder in 2D+3D Quantum Dragon Composite Nanodevices with varying Breadth, J. Phys: Conf. Ser. 1740, 012002 (2021).

Professor Hiro Nakamura

Department of Physics

University of Arkansas

Host: Seo

Title: Complex geometrical phases in quantum materials probed by transport

Abstract: Recent advances in condensed matter physics brought the phase part of wavefunction in the forefront, as exemplified by the Berry phase in graphene. However, experimentally probing complex phases in a momentum space is not easy. In this talk, we present how advanced transport techniques such as quantum interference and planer Hall effect could shed light on higher-order and/or anisotropic quantum phases in solids. We show recent application of these techniques to three-dimensional (3D) topological antiperovskites, which points to a unique spin/pseudospin texture in this material [1,2]. Preliminary results from a 2D material with distinct spin texture (few-layer WSe₂) also showcase an impact of different anisotropy of such phase patterns.

1. H. Nakamura et al., Nature Comm. 11, 1161 (2020).
2. D. Huang, H. Nakamura, and H. Takagi, arXiv:2101.05512 (2021)

Prof. Yashar Komijani

Department of Physics

University of Cincinnati

Host: Gannon/Kaul

Title: Critical charge fluctuations and superconductivity in magnetic environments

Abstract:

Quantum electronic matter has long been understood in terms of two limiting behaviors of electrons: one of delocalized metallic states, and the other of localized magnetic states. Heavy fermions are miniature high-Tc superconductors whose small energy scales provide the possibility of tuning the ground state between these two limits and enable accessing the strange metallic behavior which develops at the brink of localization. I will discuss the attempts [1,2] to map out the phase diagram of heavy-fermions using dynamical large-N method and the Schwinger boson representation of the spins. I will then highlight the recent observation of quantum critical point and strange metal behavior in the stoichiometric ferromagnetic heavy-fermion CeRh6Ge4 [3]. This is surprising as the abrupt change in the patterns of entanglement and the Fermi surface that usually accompanies singular charge fluctuations are absent in a ferromagnet. I will argue that the innocuous easy-plane magnetic anisotropy that is present in this system, produces triplet resonating valence bond (tRVB) states, which lead to a highly entangled ordered phase, similar to a magnetically-frustrated anti-ferromagnet. Doping such a tRVB host provides a route towards realizing triplet superconductivity in a magnetic environment [4].

References: [1] Y. Komijani, P. Coleman, PRL 120, 157206 (2018); ibid 122, 217001 (2019).

[2] J. Wang, Y.-Y. Chang, C.-Y. Mou, S. Kirchner, C.-H. Chung, PRB 102, 115133 (2019).

[3] B. Shen, Y. Zhang, Y. Komijani, M. Nicklas, R. Borth, A. Wang, Y. Chen, Z. Nie, R. Li, X. Lu, H. Lee, M. Smidman, F. Steglich, P. Coleman, H. Yuan, Nature 579, 51 (2020).

[4] P. Coleman, Y. Komijani, E. König, PRL 125, 077001 (2020).

Prof. Kate Ross

Department of Physics

Colorado State University

Host: Kaul

Title: Microscopics of Quantum Annealing in the Disordered Dipolar Ising Ferromagnet LiHoxY1-xF4

Abstract:  The technique of “quantum annealing” (QA) involves using quantum fluctuations to find the global minimum of a rugged energy landscape. For some problems it has been shown to produce faster optimization than thermal annealing (TA), and it has been adopted as one technique used for quantum computing (adiabatic quantum computing).  Conceptually, QA is often framed in the context of the disordered transverse field Ising model, where a magnetic field applied perpendicular to the Ising axis tunes the quantum fluctuations and enables a “better” (lower energy) spin configuration to be obtained via quantum tunneling.  A celebrated material example of this model, LiHo0.45Y0.55F4, was shown decades ago to exhibit faster dynamics after a QA protocol, compared to a TA protocol.  However, little is known about the actual process of optimization involved and ultimately what the optimal spin configurations are like.

We have set out to understand the microscopics of QA in LiHo0.45Y0.55F4 using diffuse magnetic neutron scattering. We performed the same protocols as initially used to demonstrate QA in this material, and find that the QA protocol results in what appears to be the equilibrium state, whereas TA results in a state that continues to evolve over time.   This is as expected if QA indeed provides an optimization “speed up” compared to TA.  However, we also clearly observe evidence that the transverse field does more than just introduce quantum fluctuations; namely, it produces random longitudinal fields, which had been previously studied theoretically and experimentally.  Thus, while the material does respond to QA differently than TA, it is not a simple annealing problem; the energy landscape being optimized is changing as the optimization proceeds. Understanding this version of quantum annealing could be of interest in the context of adiabatic quantum computing, possibly for designing new algorithms, or for accounting for unwanted experimental effects.

Prof. Sara Haravifard

Department of Physics

Duke University

Host: Kaul

Title: Investigating real-world quantum spin liquid candidates

Abstract: The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit.  Recently, Yb-based triangular lattice antiferromagnets have garnered significant interest as possible QSL candidates, however, the presence of chemical disorder in real-world compounds has made directly measuring the Hamiltonian parameters challenging. To further elucidate role of chemical disorder and to explore phase diagram of these materials, we present neutron scattering and high-resolution magnetometry measurements of YbMgGaO4 and YbZnGaO4, covering a broad range of applied magnetic field at low temperature. We use key observations of the magnetic phase crossover to motivate an exploration of the field- and parameter-dependent phase diagram, providing an expanded view of the available magnetic states in applied field. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder. 

Prof. Adrian Del Maestro

Department of Physics and Astronomy

University of Tennessee

Host: Kaul

Title: Nanoscale confinement towards a one-dimensional superfluid

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

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

Dr. Paul McClarty

Max Planck Institute for the Physics of Complex Systems

Dresden, Germany

Host: Kaul

Title: Topological Magnons

Abstract: Coherent magnetic excitations can have nontrivial topological features. In this talk, I'll give examples of "gapped" and "gapless" band topology of magnons including Kitaev-Heisenberg honeycomb models as analogues of Chern insulators and Dirac nodal lines in CoTiO3. Then I'll talk about features of band topology that are special to magnon systems: (i) the robustness of topological edge states in the presence of interactions; (ii) the visualization of spin-momentum locking in neutron scattering; (iii) the non-Hermitian topology of spontaneous magnon decay.

Dr. Lucia Steinke

National High Magnetic Field Laboratory High B/T Facility

University of Florida

Host: Gannon

Title: Towards a universal measurement platform for calorimetric and thermal transport measurements at the combined extremes of high magnetic fields and sub-millikelvin temperatures

Abstract: The NHMFL High B/T facility at the University of Florida in Gainesville pursues the mission to enable user experiments at the combined extremes of high magnetic fields B and ultra-low temperatures (ULT) below one millikelvin. While almost all measurement techniques like electrical transport or magnetometry require adaptations to be successfully performed in this environment, calorimetry and thermal transport are particularly challenging due to a lack of suitable thermometry. Existing thermometers typically either lack ULT sensitivity, are incompatible with high magnetic fields, or have too high masses and long thermalization times to allow for measurements of small samples. Specific heat and thermal transport experiments at ULT and high B - especially on small crystals - would be essential to studies of quantum criticality, superconductors with very low transition temperatures, or to provide evidence for a quantum spin liquid state. To meet increased user demand for such experiments, our group is developing fast and compact ULT thermometers to form the basis of a universal measurement platform for calorimetry and thermal transport of a variety of samples. The measurement principle relies on the well-established temperature dependent viscosity of liquid ³He that is probed by tracking the resonance of quartz tuning forks immersed in the liquid. I will report on recent progress in the miniaturization of such tuning fork thermometers (TFT), and present results from field and temperature dependent performance tests. 

The National High Magnetic Field Laboratory is supported by the National Science Foundation through NSF DMR-1644779 and the State of Florida.