Fall 2024
Wednesday, September 18, 11:00 am at QNC 1101
Speaker: Taylor Hughes (University of Illinois)
Title: Topological Crystalline Responses in Gapped and Gapless Systems
Abstract: Topological phases protected by crystalline symmetry are ubiquitous in nature. In the past decade, new classification schemes have identified thousands of candidate crystalline materials. The set of relevant systems includes topological insulators, topological semimetals, and even symmetry-enriched topological order. Even with such amazing progress we are left with the problems of trying to find probes that detect the topology, and then eventually trying to use the topological phenomena in robust applications. In this talk I will discuss a family of interconnected topological crystalline responses, and discuss the conditions for their appearance in materials. Along the way we will discuss new types of chirality, anomalies, and response coefficients. The phenomena we discuss are relevant for realizations in both metamaterials and quantum, solid-state materials.
Wednesday, October 23, 11:00 am at QNC 1101
Speaker: Leo Radzihovsky (University of Colorado)
Title: Coulomb Universality
Abstract: Motivated by a number of realizations of long-range interacting systems, I will discuss a neutral plasma with power-law interactions longer-ranged than Coulombic. I will show that beyond a crossover length, such interactions are universally screened down to a standard Coulomb form in all spatial dimensions. This implies, counterintuitively, that in two dimensions and below, such a “super-Coulombic” gas is asymptotically confining at low temperatures and undergoes a deconfining transition that in two dimensions is the same Kosterlitz-Thouless transition as a conventional Coulomb gas. I will also explain that the super-Coulomb to Coulomb crossover is followed at longer length scales by an unconventional ”Debye-Huckel” screening, which leads to faster-than-Coulombic, power-law decay of the screened potential, in contrast to the usual exponentially decaying Yukawa potential. I will conclude with showing absence of such universal screening in a related XY model with power-law exchange.
Wednesday, October 30, 11:00 am at QNC 1101
Speaker: Guo-Xing Miao (University of Waterloo)
Title: Programmable Iontronic Memristor for Neuromorphic Computing Applications
Abstract: Iontronics benefits from the controllable motion and detection of ions in electronics devices. We demonstrate an ion-coupled memristive system with solid-state electrolyte lithium phosphorus oxynitride as the ion source and the embedding and releasing of Li ions inside the cathodic like TiOx for volatile conductance responses. The system exhibits synapse-like short-term plasticity behaviour without requiring a forming process beforehand or a compliance current during switching, rendering a natural platform for hardware simulating neuron functionalities. Different short-term pulse-based phenomena, including paired pulse facilitation, post-tetanic potentiation, and spike rate-dependent plasticity were observed with unique self-relaxation characteristics. Based on the voltage excitation period, the timescale of the volatile memory can be tuned. In addition, the volatile analog devices can be configured into non-volatile memory units with multibit storage capabilities after an electroforming process. Therefore, on the same platform, we can configure volatile units as nonlinear dynamic reservoirs for performing neuromorphic training and the non-volatile units as the weight storage layer. These phenomena can be generalized to other ion active systems and can effectively process and update temporal information for reservoir and neuromorphic computing paradigms. We proceed to simulate voice recognition as an example with the variable time scale and a minimal training dataset. We also show their actual back-end-of-line (BOEL) integration on 180 nm CMOS chips as a demonstration of principle.
Wednesday, November 20, 11:00 am at QNC 1101
Speaker: Joe Checkelsky (MIT)
Title: Flat Band Effects in Model Lattice Crystals
Abstract: The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Here we present recent developments in realizing flat bands in transition element-based crystalline materials. We will present recent experiments in which a partial filling of a flat band is associated with unusual transport and thermodynamic that recall those of strongly correlated systems. We will also comment on the potential role of band topology and prospects for using similar lattice and orbital engineering to realize new correlated systems.
Wednesday, December 4, 11:00 am at QNC 1101
Speaker: Pengcheng Dai (Rice University)
Title: Spin and Lattice coupling in kagome metal FeGe
Abstract: Two-dimensional (2D) kagome lattice metals are interesting because they display flat electronic bands, Dirac points, Van Hove singularity, and can have interplay amongst charge density wave (CDW), magnetic order, and superconductivity. In kagome lattice antiferromagnet FeGe, a short-range CDW order was found deep within an antiferromagnetically ordered state interacting with magnetic order [1]. Surprisingly, the post-growth annealing process of FeGe at 560◦C can suppress the CDW order while annealing at 320◦C induces a long-range CDW order, with the ability to cycle between the states repeatedly by annealing [2]. Here we use transport, neutron scattering, scanning transmission electron microscopy (STEM), and muon spin rotation (μSR) experiments to unveil the microscopic origin of the annealing process and its impact on magneto-transport, CDW, and magnetic properties of FeGe. We find that 560◦C annealing creates germanium vacancies uniformly distributed throughout the FeGe kagome lattice that prevent the formation of Ge-Ge dimers necessary for the CDW order. Upon annealing at 320◦C, the system segregates into stoichiometric FeGe regions with long-range CDW order and regions with stacking faults that act as nucleation sites for the CDW. The presence or absence of CDW order greatly affects the anomalous Hall effect, incommensurate magnetic order, and spin-lattice coupling in FeGe, thus placing FeGe as the only known kagome lattice material with a tunable CDW and magnetic order potentially useful for sensing and information transmission.
[1] Nature 609, 490 (2022); Nature Physics 19, 814 (2023); Nature Communications 14, 6183 (2023); Nature Communications 15, 1918 (2024); Phys. Rev. Lett. 133, 046502 (2024).
[2] Phys. Rev. Lett. 132, 256501 (2024); Nature Communications 15, 6262 (2024).
Winter 2024
Wednesday, January 17, 11:00 am at QNC 1101
Speaker: David Hawthorn (University of Waterloo)
Title: Nematic and Charge Density Wave Orders in Cuprate Superconductors
Wednesday, January 31, 10:30 am at QNC 1101
Speaker: Eslam Khalaf (Harvard University)
Title: Spin polarons and skyrmion superconductivity in topological bands: application to graphene moire heterostructures
Abstract: Understanding the phase diagram of twisted bilayer graphene and related moir ́e systems is a central theoretical challenge. While the ground states at inte- ger fillings have been shown in many cases to be simple flavor ferromagnets, the charge excitations above such states can be non-trivial due to band topology. Conventional approaches to understand such excitations as real space topologi- cal textures fail to account for the distinct momentum space features of Chern bands and obscures their comparison to single particle excitations. Here, we present a general fully momentum space formulation for the problem of charge excitations in Chern bands. In the limit of (normal-ordered) contact interac- tions in an ideal flat bands, we construct exact analytical wavefunctions for the lowest energy excitation with charge ±e and spin n+1/2, a spin polaron. Away from this ideal limit, we show that these analytical wavefunctions are excellent variational states describing a bound state of an electron/hole with n spin flips. We apply our formalism to study charge excitations in twisted bilayer graphene and find that (i) in the chiral limit, multispin flip polarons are the lowest en- ergy charge excitations at charge neutrality and at non-zero integer fillings when doping towards neutrality. In the realistic limit, we find that the multispin flip states are the lowest charged excitations at ν = ±(1 − ε) for any strong cou- pling state and at ν = ±(2 − ε) for the time-reversal intervalley coherent state (TIVC) but not the Krammers intervalley coherent state (KIVC). We discuss the experimental implications of these results for low strain devices.
Wednesday, February 7, 10:30 am at QNC 1101
Speaker: Arun Paramekanti (University of Toronto)
Title: Sleuthing hidden orders and hidden spin liquids in quantum magnets
Abstract: The combination of spin-orbit interaction and crystal field physics can stabilize novel multipolar magnetism beyond conventional dipolar order. Such "hidden orders" are hard to probe. I will discuss aspects of this physics in certain spin-1/2 magnets where various puzzling experiments can be reconciled in terms of octupolar magnetism with orbital loop currents and how doped impurities can lead to a distinct type of transverse field Ising magnetism - some of our computed results are in agreement with recent impurity NMR experiments. I will also discuss possible "hidden spin liquid" physics lurking behind certain results on honeycomb cobaltate compounds where our numerical results suggest a "Dirac"-type spin liquid might be a proximate state of the XXZ spin model which appears to explain a number of experiments including THz spectroscopy and thermal transport.
Wednesday, February 28, 10:30 am at QNC 1101
Speaker: Sung-Sik Lee (McMaster University & Perimeter Institute)
Title: Emergence of curved momentum-spacetime in quantum critical metals
Abstract: In metals near quantum phase transitions, critical modes can significantly slow down the motion of electrons through a red shift that dilates frequency. If such quantum corrections are anisotropic, the momentum-dependent red shift can create a curved momentum-spacetime geometry. In this talk, we will discuss an example where a strongly curved momentum-spacetime geometry emerges dynamically - in a certain limit, the emergent geometry becomes analogous to that of the black hole horizon. Its experimental implication for the cyclotron motion of electron will be discussed.
Fall 2023
Wednesday, October 4, 10:30 am at QNC 1101
Speaker: Dominic Else (Perimeter Institute)
Title: On Fermi surfaces
Abstract: Metals are a very general concept that can apply beyond conventional theories such as Fermi liquid theory. In this talk, I address the question: what are the most general properties of metals, and how should we define them? In particular, I argue that the most fundamental property of a metal is the existence of a "Fermi surface" in momentum space. I moreover argue that the most general definition of Fermi surface, applicable even in strongly interacting systems, is via the associated emergent symmetry, and the anomaly of that emergent symmetry. I discuss several applications of these ideas: firstly, I use them to develop improved "holographic" models of non-Fermi liquid metals that explicitly incorporate the Fermi surface; secondly, I show that applying hydrodynamics to the conservation laws resulting from the emergent symmetry associated with the Fermi surface strongly constrains the dynamical modes of the system, essentially recovering the same equations of motion as in Fermi liquid theory without needing to assume anything about the existence of quasiparticles.
Wednesday, October 11, 10:30 am at QNC 1101
Speaker: Alex Frano (UCSD)
Title: Lessons from dilute antiferromagnetic domains towards a new wave of coherent x-ray scattering experiments
Abstract: In the effort to explore quantum matter using x-rays, spatial coherence in x-ray beams is the new frontier, promising fresh insights across various spatiotemporal scales. Yet, fully capitalizing on beam coherence remains a challenge. This seminar introduces a novel approach: by restricting sampling to simple spatial structures, we can more easily track well-defined Fourier transforms. This technique is especially useful during the onset of first-order phase transitions, when antiferromagnetic domains begin to form.
Using resonant coherent X-ray scattering, we examine these domains in PrNiO3, revealing that sparse domains lead to simpler, invertible diffraction patterns. Building on this, we suggest re-engineering the beam's wavefront using zone plate optics. The outcome is a well-defined spatial footprint that can be Fourier-reconstructed to extract new details about spatiotemporal sample dynamics.
Our findings not only validate resonant Bragg coherent diffractive imaging but also pave the way for real-time studies using spatially structured x-rays, offering an exciting outlook for future probes of quantum materials.
Wednesday, October 18, 10:30 am at QNC 1101
Speaker: Hae-Young Kee (University of Toronto)
Title: Bridging spin models and magnetic materials
Abstract: Magnetism has a long history, dating back to the use of compasses in ancient voyages. However, our modern understanding of magnetism began to take shape with the development of quantum mechanics. To elucidate and manipulate the magnetic properties of quantum materials, a deep dive into the microscopic Hamiltonians specific to each materials is essential. While the Heisenberg model is widely recognized in the magnetism community, recent advancements have revealed that various forms of spin interactions can emerge through the interplay of spin-orbit coupling and electron interactions. In this talk, I will illustrate a selection of such models in different magnetic materials and discuss their significance in the context of quantum spin liquids featuring fractionalized excitations.
Wednesday, November 15, 10:30 am at QNC 1101
Speaker: Ziliang Ye (UBC)
Title: Tuning the optical properties of layered semiconductors through sliding ferroelectricity
Abstract: Controlling the stacking order in van der Waals materials is a powerful way to engineer their emergent properties. In parallel-stacked transition metal dichalcogenides, also known as the rhombohedral stacking order, the asymmetric atomic registry breaks the layer symmetry and generates an interfacial electrical polarization associated with an interlayer potential. Recently, we quantified the polarization strength and its spatial distribution in rhombohedral MoS2. Interestingly, we observed that the domain size distribution follows a power-law distribution, suggesting that the shear strain occurring during the mechanical exfoliation can induce an avalanche of domain wall motion. These pre-existing domain walls are found to be critical for the polarization switching behavior and can be utilized to control the optical response of these layered semiconductors in a non-volatile way.
Wednesday, December 6, 10:30 am at QNC 1101
Speaker: Sergey Frolov (University of Pittsburgh)
Title: No, you have not discovered a Majorana Fermion
Abstract: Is what I tell myself. There was a time when I thought I may have discovered it, others did too. Around 2012 several groups including ours found evidence of these quantum excitations in electrical circuits containing nanowires of semiconductor covered by a superconductor. The dramatic signatures were peaks in conductance that appeared under conditions expected from theory for Majorana modes, which are their own anti-modes and may possess non-Abelian properties. But a few years later, similar features in the data were identified due to an interesting, but a more mundane effect - which we call trivial states such as Andreev bound states. Over time more and more data pointed at the trivial and not at the exotic explanation. But because Majorana claims kept coming, this led to some digging and even retractions. What we learned after 10 years is that we have a much better handle on what effects show up in these nanowires, which positions us well for the ultimate Majorana discovery which we should be able to tell apart from all the non-Majorana things we saw. The second lesson we learned is that materials quality of device constituents, superconductors and semiconductors, as well as how samples are fabricated - are the make-or-break factors for making this happen. So while I cannot report an exciting physics discovery, I can walk you through the scientific process that took place, a 10-year event of independent value which taught me how to do science better.
Wednesday, December 13, 10:30 am at QNC 1101
Speaker: Kin Fai Mak (Cornell University)
Title: Semiconductor moiré materials
Abstract: The discovery of moiré materials has enabled condensed matter experimentation in new regimes. In this talk, I will discuss the general features of moiré materials built on 2D semiconductors, with a particular focus on the interplay between strong electronic correlations and non-trivial band topology. Specifically, I will discuss how we can explore Hubbard physics and quantum Hall physics under zero magnetic field in these materials. The results may shed light on some of the deepest problems in condensed matter physics.
Winter 2023
Wednesday, January 18, 11 am at QNC 1201
Speaker: Adam Wei Tsen (University of Waterloo)
Title: Tunneling Probe of 2D Kitaev and Moiré Magnetism
Abstract: I will discuss our recent results using tunneling magnetoresistance and spectroscopy to probe the underlying magnetic order and/or excitations in two “complex” atomically thin spin systems: the Kitaev material alpha-RuCl3 in monolayer form and twisted bilayers of the layer antiferromagnet CrI3. We find a reversal of the magnetic anisotropy together with an enhancement of the Kitaev interaction in the former and multiple skyrmion states in the latter that can be tuned by magnetic field.
Wednesday, February 1, 11 am at QNC 1201
Speaker: Young-June Kim (University of Toronto)
Title: alpha-RuCl3: a progress report
Abstract: A bond-dependent anisotropic magnetic interaction called the Kitaev interaction can be found in honeycomb lattice materials with strong spin-orbit coupling, which has made a profound impact on quantum magnetism research. In particular, alpha-RuCl3 has been heralded as a realization of the Kitaev quantum spin liquid state, an elusive new state of matter that harbours Majorana fermions. In this talk, I will give a brief overview of the current status of research on alpha-RuCl3 and discuss recent experimental developments and a few surprising findings using ultra-high-quality samples grown in our laboratory. Our samples have minimal stacking faults even at low temperatures, allowing us to determine the low-temperature crystal structure unambiguously. We also found that the magnetic properties are surprisingly sensitive to the inter-layer configuration, giving rise to various magnetic transition temperatures. We also compare low-energy spin-orbit excitations in various Kitaev materials using resonant inelastic x-ray scattering (RIXS). We found that non-local physics is important for describing the spin-orbit excitations in these materials, in contrast to the conventional belief that local Jeff=1/2 physics is sufficient in these compounds.
Wednesday, March 1, 11 am at QNC 1201
Speaker: Yong-Baek Kim (University of Toronto)
Title: Quantum Spin Liquids and Criticality in Multipolar Materials
Abstract: Multipolar quantum materials possess local moments carrying higher-rank quadrupolar or octupolar moments. These higher-rank multipolar moments arise due to strong spin-orbit coupling and local symmetry of the crystal-electric-field environment. In magnetic insulators, the interaction between multipolar local moments on frustrated lattices may promote novel quantum spin liquids. In heavy fermion systems, the interaction between multipolar local moments and conduction electrons may lead to unusual non-Fermi liquids and quantum criticality. In this talk, we first discuss a novel quantum spin ice state, a three-dimensional quantum spin liquid with emergent gauge field, that may have been realized in Ce2Zr2O7 and Ce2Sn2O7, where Ce3+ ions carry dipolar-octupolar moments. We present a theoretical analysis of possible quantum spin ice states in this system and compare the theoretical results of dynamical spin structure factors with recent neutron scattering experiments. Next, we present a theoretical model to describe the unusual Kondo effect and quantum criticality in Ce3Pd20Si6, where Ce3+ moments carry a plethora of dipolar, quadrupolar, and octupolar moments. We show that two consecutive Kondo-destruction-type phase transitions can occur with the corresponding Fermi surface reconstructions. We compare these results with existing experiments and suggest future ultrasound experiments for the detection of emergent quantum critical behaviors.
Wednesday, March 15, 11 am at QNC 1201
Speaker: Roger Melko (University of Waterloo & Perimeter Institute)
Title: Language models for quantum simulation
Abstract: As the frontiers of artificial intelligence advance more rapidly than ever before, generative language models like ChatGPT are poised to unleash vast economic and social transformation. In addition to their remarkable performance on typical language tasks (such as writing undergraduate research papers), language models are being rapidly adopted as powerful ansatze states for quantum many-body systems. In this talk, I will discuss the use of language models for learning quantum states realized in experimental Rydberg atom arrays. By combining variational optimization with data-driven learning using qubit projective measurements, I will show how language models are poised to become one of the most powerful computational tools in our arsenal for the design and characterization of quantum simulators and computers.
Wednesday, April 5, 11 am at QNC 1201
Speaker: Brad Ramshaw (Cornell University)
Title: Strange metals from not-so-strange quasiparticles
Abstract: Strange metals have linear-in-temperature (T-linear) down to low temperature. Strange metals are found in many families of correlated electron materials, leading to the conjecture that a universal bound - the "Planckian" bound - limits the scattering rate of electrons to a value set by fundamental constants. If the Planckian bound exists, it would provide a natural explanation for why a host of seemingly disparate systems, including high-temperature superconductors and twisted bilayer graphene, all have T-linear resistivity. Perhaps more dramatically, T-linear resistivity suggests that electron-electron interactions are so strong that conventional concepts such as quasiparticles and Boltzmann transport do not apply in strange metals. I will present our work on the cuprate Nd-LSCO and the 5-layer superconducting nickelate that shows that conventional quasiparticle transport is alive and well, even in the strange metal regime where the Planckian bound is saturated. This suggests that we may not need to abandon the quasiparticle picture entirely, but that we need to better understand the source of scattering in these materials.
Wednesday, April 12, 11 am at QNC 1201
Speaker: Debanjan Chowdhury (Cornell University)
Title: The good, the bad and the strange: unconventional metallic behavior in the vicinity of Mott insulators
Abstract: In recent years, we have witnessed remarkable experimental breakthroughs in uncovering the intriguing properties of correlated metals in the vicinity of Mott transitions. Describing these phenomena theoretically remains an open challenge. This talk will focus on three recent examples of puzzling electronic behavior near Mott insulating phases and address the various conundrums. In the first part of the talk, I will discuss the microscopic origin of an unconventional T-linear resistivity with Planckian scattering in a quasi-two-dimensional “good” metal with long mean-free path, consisting of highly conducting metallic and Mott insulating layers, respectively. In the second part, I will address the origin of a low-temperature “bad” metallic behavior in the vicinity of a continuous bandwidth-tuned metal-insulator transition in a moiré semiconductor. I will end by presenting some new theoretical insights into the experimental observation of an anomalous particle-hole continuum and overdamped plasmon in the density response of cuprate “strange” metals.