Shi studied fractionalization in frustrated quantum matter, where the interacting spins are unable to order. Instead, they create long-range patterns of entanglement leading to states of matter such as quantum spin liquids, heralding the topological quantum matter with novel fractionalized particles and emergent gauge fields. These states are characterized by topological order: ground state degeneracy on a manifold of non-zero genus, and fractionalized excitations with abelian and non-abelian quantum statistics. In these states, the original localized spin degrees undergo further fractionalization to give new degrees of freedom, such as Majorana fermions and spinons. In these states, both charges and spins are localized. However, the emergent fractionalized degrees of freedom can be remarkably delocalized and able to transport energy. Identifying and studying the phenomena of fractionalization presents a dual challenge: discerning fractionalized particles and finding material candidates that realize fractionalization. His dissertation presents a comprehensive theoretical study of fractionalization in both one and two dimensions, focusing on these challenges. [Link to his dissertation]
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Joseph Szabo has successfully defended his thesis. Congratulations!
Joseph studied the Fermi-Hubbard model, a theoretical model that captures the insulating and conducting phases of high temperature superconducting materials, where he characterized novel quantum phases and dynamics realized on cutting-edge quantum simulation platforms. By correlating maps of the local density of states, the local magnetization, and the local bond conductivity, he found a collapse of the Mott gap toward a V-shape pseudo-gapped density of states that occurs concomitantly with the decrease of magnetism around the highly disordered sites, while the electronic bond conductivity increases. His results provide one of the first microscopic investigations of dynamical response and how these two phases (correlated metal and Mott insulator) coexist microscopically and lead to an overall macroscopic phase transition. Expanding beyond the ground state properties of interacting matter, he also venture exploring the field of nonequilibrium quantum dynamics that bridges foundational atomic, molecular, and optical and condensed matter models. In relevant works he presented a new framework that connects physical spin-fluctuations, quantum Fisher information, and bipartite entanglement entropy between collective quantum systems.
Franz Utermohlen has successfully defended his thesis. Congratulations!
In Franz’s work he presented an analysis of the magnetic anisotropy that enables 2D ferromagnetism in CrI3 , one of the first 2D ferromagnets discovered. He determined the most general microscopic spin interactions allowed by this material’s crystal symmetries and estimate the strength and sign of these interactions by fitting angle-dependent ferromagnetic resonance data using this spin model. He also presented a symmetry analysis of the most general forms of tensors describing physical responses in 2D materials composed of a honeycomb lattice of edge-sharing octahedra relevant for the aforesaid materials. [Link to his thesis]
Wenjuan Zhang has successfully defended her thesis. Congratulations!
Wenjuan’s research focuses how the interplay between strong correlations and spin-orbit coupling gives rise to unconventional magnetic structures with unusual properties, where both the orbital and spin orderings dictate the nature of magnetism. Since orbitals are directional and couple to the lattice, magnetism in these materials can be manipulated by external strain, providing a new knob to tune their magnetic properties. [Link to her thesis]
Magnetic field-induced intermediate quantum spin liquid with a spinon Fermi surface
In a quantum spin liquid, the spins remain disordered down to zero temperature, and yet, it displays topological order that is stable against local perturbations. The Kitaev model with anisotropic interactions on the bonds of a honeycomb lattice is a paradigmatic model for a quantum spin liquid. We explore the effects of a magnetic field and discover an intermediate gapless spin liquid sandwiched between the known gapped Kitaev spin liquid and a polarized phase. We show that the gapless spin liquid harbors fractionalized neutral fermionic excitations, dubbed spinons, that remarkably form a Fermi surface in a charge insulator.
Signatures of magnetic-field-driven quantum phase transitions in the entanglement entropy and spin dynamics of the Kitaev honeycomb model
The main question we address is how to probe the fractionalized excitations of a quantum spin liquid (QSL), for example, in the Kitaev honeycomb model. By analyzing the energy spectrum and entanglement entropy, for antiferromagnetic couplings and a field along either [111] or [001], we find a gapless QSL phase sandwiched between the non-Abelian Kitaev QSL and polarized phases. Increasing the field strength towards the polarized limit destroys this intermediate QSL phase, resulting in a considerable reduction in the number of frequency modes and the emergence of a beating pattern in the local dynamical correlations, possibly observable in pump-probe experiments.
Paper on topological Weyl semimetals published in Scientific Reports
We consider the electromagnetic response of a topological Weyl semimetal (TWS) with a pair of Weyl nodes in the bulk and corresponding Fermi arcs in the surface Brillouin zone. We compute the frequency-dependent complex conductivities σαβ(ω) and also take into account the modification of Maxwell equations by the topological θ-term to obtain the Kerr and Faraday rotations in a variety of geometries. For TWS films thinner than the wavelength, the Kerr and Faraday rotations, determined by the separation between Weyl nodes, are significantly larger than in topological insulators. In thicker films, the Kerr and Faraday angles can be enhanced by choice of film thickness and substrate refractive index. We show that, for radiation incident on a surface with Fermi arcs, there is no Kerr or Faraday rotation but the electric field develops a longitudinal component inside the TWS, and there is linear dichroism signal. Our results have implications for probing the TWS phase in various experimental systems.
Paper on SrRu2O6 published in PRB Rapid Communications
A quasi-two-dimensional honeycomb ruthenate has been synthesized by members of IRG-1. Neutron diffraction shows antiferromagnetic ordering in each layer and between layers up to a temperature of 565 K. At this critical temperature, the layers magnetically decouple due to the weak inter-layer coupling which we can understand through a combinations of density functional theory calculations and Monte-Carlo simulations.
Paper on Higgs mode in disordered superconductors published in Nature Physics!
“The Higgs mode in disordered superconductors close to a quantum phase transition,” was just published in Nature Physics! The recent paper is a theory-experiment collaboration where our theoretical predictions of a Higgs mode going soft at a quantum critical point in a disordered superconductor are put to the test in dynamical conductivity experiments. This is the first unequivocal observation of the Higgs mode in a superconductor. In contrast to previous attempts where there was considerable mixing of the Higgs mode with broken pairs, in the experiments reported here the energy scale for the Higgs mode could be reduced well below the pair breaking scale. Importantly, the Higgs mass was shown to vanish at the quantum critical point between a superconductor and an insulator leaving no doubt that its origin lay in the amplitude fluctuations of the superconducting order parameter. Our theory was first published in Swanson, Loh, Randeria, Trivedi Phys. Rev. X 4, 021007 (2014). Phil Anderson has written a historical and insightful News and Views on our Nature Physics paper.