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 

Abstract: Charge density wave (CDW) order has been established as a generic feature of the cuprate phase diagram, competing and co-existing with superconductivity. While most commonly characterized by a translational symmetry breaking order parameter, CDW order can also involve a nematic order parameter. How these orders manifest and couple to each other can depend on the structure of the material and hole doping, and may play an important role in understanding differences in the cuprate phase diagrams of various materials. In this talk, we will show how we can use equilibrium and pump-probe resonant x-ray scattering measurements to measure both CDW order and nematic order in the (La,M)₂CuO₄ family of cuprate superconductors. These measurements demonstrate orbital/nematic order is coupled to but distinct from CDW order, existing above the CDW onset temperature in some samples. By measuring the nematic order as a function of doping, we show nematic order to be associated with the pseudogap phase of the cuprates, with signatures of nematic order present within the pseudogap phase, but vanishing above the pseudogap critical doping, p*, and temperature, T* in overdoped cuprates. We will also examine the role that structural symmetry plays in stabilizing CDW order. By applying uniaxial stress to modify the structure, we show that the CDW order is suppressed and superconductivity is enhanced.

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.