Dr. Eslam Khalaf

University of Texas-Austin

Title: Strong Coupling Theory of Magic-Angle Graphene

Abstract: In this talk, I will review a recently developed strong coupling theory of magic-angle twisted bilayer graphene. An advantage of this approach is that a single formulation can capture the insulating and superconducting states, and with a few simplifying assumptions, can be treated analytically. I begin by reviewing the electronic structure of magic angle graphene’s flat bands, in a limit that exposes their peculiar band topology and geometry. I will show how similarities between the flat bands and the lowest Landau level can provide valuable insights into the effect of interactions and form the basis for an analytic treatment of the problem. At certain fractional fillings, the similarity to Landau level physics suggests a promising route for realizing fractional Chern insulators. At integer fillings, this approach points to flavor ordered insulators, which can be captured by a sigma-model in its ordered phase. Remarkably, topological textures of the sigma model carry electric charge which enables the same theory to describe the doped phases away from integer filling. I will show how this approach can lead to superconductivity on disordering the sigma model, and estimate the Tc for the superconductor. I will highlight the important role played by an effective super-exchange coupling both in pairing and in setting the effective mass of Cooper pairs. At the end, I will show how this theory provides criteria to predict which multilayer graphene stacks are expected to superconduct including the recently discovered alternating twist trilayer platform.

Dr. Eslam Khalaf

University of Texas-Austin

Title: Strong Coupling Theory of Magic-Angle Graphene

Abstract: In this talk, I will review a recently developed strong coupling theory of magic-angle twisted bilayer graphene. An advantage of this approach is that a single formulation can capture the insulating and superconducting states, and with a few simplifying assumptions, can be treated analytically. I begin by reviewing the electronic structure of magic angle graphene’s flat bands, in a limit that exposes their peculiar band topology and geometry. I will show how similarities between the flat bands and the lowest Landau level can provide valuable insights into the effect of interactions and form the basis for an analytic treatment of the problem. At certain fractional fillings, the similarity to Landau level physics suggests a promising route for realizing fractional Chern insulators. At integer fillings, this approach points to flavor ordered insulators, which can be captured by a sigma-model in its ordered phase. Remarkably, topological textures of the sigma model carry electric charge which enables the same theory to describe the doped phases away from integer filling. I will show how this approach can lead to superconductivity on disordering the sigma model, and estimate the Tc for the superconductor. I will highlight the important role played by an effective super-exchange coupling both in pairing and in setting the effective mass of Cooper pairs. At the end, I will show how this theory provides criteria to predict which multilayer graphene stacks are expected to superconduct including the recently discovered alternating twist trilayer platform.

Oksana Ostroverkhova

Professor of Physics

Oregon State University

Title: Photophysics of organic materials: from ancient pigments to high-performance organic semiconductors

Abstract: Organic (opto)electronic materials have been explored in a variety of applications in electronics and photonics. They offer several advantages over traditional silicon technology, including low-cost processing, fabrication of large-area flexible devices, and widely tunable properties through functionalization of the molecules. Over the past decade, remarkable progress in the material design has been made, which led to a considerable boost in performance of organic thin-film transistors, solar cells, and other applications that rely on photophysics and/or (photo)conductive properties of the material. Nevertheless, a number of fundamental questions pertaining to light-matter interactions and charge carrier photogeneration and transport in these materials remain. In this presentation, I will give examples of our efforts aiming to understand and tune exciton, polariton, and charge carrier dynamics in high-performance organic materials and to develop novel, sustainable organic materials.

Oksana Ostroverkhova

Professor of Physics

Oregon State University

Title: Photophysics of organic materials: from ancient pigments to high-performance organic semiconductors

Abstract: Organic (opto)electronic materials have been explored in a variety of applications in electronics and photonics. They offer several advantages over traditional silicon technology, including low-cost processing, fabrication of large-area flexible devices, and widely tunable properties through functionalization of the molecules. Over the past decade, remarkable progress in the material design has been made, which led to a considerable boost in performance of organic thin-film transistors, solar cells, and other applications that rely on photophysics and/or (photo)conductive properties of the material. Nevertheless, a number of fundamental questions pertaining to light-matter interactions and charge carrier photogeneration and transport in these materials remain. In this presentation, I will give examples of our efforts aiming to understand and tune exciton, polariton, and charge carrier dynamics in high-performance organic materials and to develop novel, sustainable organic materials.

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.

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.

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 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).