Unless otherwise specified, all talks are held starting at 1.15pm in the Theory Library on the 4th floor of Physics East.

Everyone is encouraged to ask questions during the talk. You are welcome to leave when you need to, otherwise the talk will usually wrap up by 2.30pm, at which point there will be biscuits.

Thursday 12th December 2024: Ilya Gruzberg (Ohio State)

Multifractals and conformal invariance at Anderson transitions

Multifractal measures arise in such diverse subjects as dynamical chaos, weather and climate, turbulence, fractal growth, critical clusters in statistical mechanics, disordered magnets and other random critical points, Anderson transitions, mathematical finance, random energy landscapes, Gaussian multiplicative chaos, and rigorous approaches to conformal field theory. A multifractal measure is characterized by a continuous spectrum of multifractal exponents Δ_q that describe the scaling of the moments of the measure with the system size. In the context of Anderson transitions, the multifractality of critical wave functions is described by operators with scaling dimensions Δ_q in a field-theory description of the transitions. The operators O_q satisfy the so-called Abelian fusion expressed as a simple operator product expansion. Assuming conformal invariance and Abelian fusion, we use the conformal bootstrap framework to derive a constraint that implies that the multifractal spectrum Δ_q must be quadratic in its arguments in any dimension d ≥ 2 (parabolicity of the spectrum). We confront this finding with available numerical and analytical data for various Anderson transitions that unambiguously show clear deviations of the multifractal spectra Δ_q from parabolicity and discuss possible reasons for the discrepancy.

Thursday 25th April 2024: Martin Speight (University of Leeds)

Soliton crystals

Topological solitons are smooth spatially localized lump-like solutions of nonlinear field theories that can move around and interact in a particle-like manner. Examples are superconducting vortices, magnetic monopoles, domain walls and skyrmions. In condensed matter contexts they are usually observed in the laboratory in the form of spatially periodic arrays, analogous to crystals. The Abrikosov vortex lattice in type II superconductors is a prominent and influential example. Theoretical studies of soliton crystals almost always impose some plausible period lattice geometry (cubic, square or triangular) a priori, and optimize only over the size of the unit cell. I will argue that this assumption is, in general, ill-founded and that one should really optimize the system over the space of fields _and_ period lattices. The latter variation has an elegant reformulation in terms of the stress tensor of the field theory. When implemented numerically, one finds that much more exotic period lattices are possible than those conventionally imposed. The ideas will be illustrated by concentrating on skyrmion crystals in 2 and 3 spatial dimensions.

Joint work with Derek Harland and Paul Leask (also at Leeds).

Thursday 2nd May 2024: Alexander Mietke (University of Oxford)

How constraints and chirality guide self-organisation in living systems

Living matter has the fascinating ability to autonomously organise itself in space and time. Self-organisation refers typically to the fact that local interactions among microscopic agents enable an emerging dynamic that is seemingly coordinated on macroscopic length scales much larger than the agents. Classical examples are the interactions of actin and actin-binding molecules within the eukaryotic cell cortex, of cells within tissues or of entire organisms interacting within populations. While details of each self-organisation phenomenon will depend on the concrete biological system, mechanistic insights can still be developed by identifying generic features that are shared among systems – an abstraction that physics approaches provide many useful tools for. Two such generic features this talk focuses on are mechanical constraints and broken chiral symmetry, which are both ubiquitous in biological systems. Specifically, we will discuss the impact of mechanical constraints in guiding tissue morphogenesis in the flour beetle, the role of chirality in guiding the emergent properties of living crystals made of starfish embryos, as well as a mechanism to robustly establish a left-right body axis during nematode development when both, mechanical constraints and chirality, act in concert.

Thursday 9th May 2024: Orazio Scarlatella (University of Cambridge)

Strongly-coupled atomic arrays: a dynamical mean-field theory study

Subwavelength arrays of quantum emitters have emerged as an interesting platform displaying prominent collective effects. In this talk I will discuss the steady-states of such arrays under coherent driving, realizing an open quantum many-body problem with long range interactions and dissipation. I will discuss a Dynamical Mean Field Theory approach to the problem, for which few theoretical methods are available. I will show that the combination of dipolar interactions and regular geometry have a dramatic effect on the spectrum of emitted light in the strong-drive regime: the famous Mollow triplet characterizing the emission of a single atom develops a structured broadening with flat sidebands, distinguishing the emission of atomic arrays from that of disordered atomic clouds or of non-interacting emitters. For moderate drive strengths instead, I will show that the steady-state is influenced by the existence of guided modes in the single-particle regime that are completely decoupled from dissipation. 

Thursday 25th July 2024: Bryce Gadway (Penn State)

NB unusual time of 11am!

Synthetic dimensions in Rydberg atom arrays

Arrays of dipolar-interacting spins – magnetic atoms, polar molecules, and Rydberg atoms – represent powerful and versatile platforms for analog quantum simulation experiments. The internal state dynamics in such dipolar arrays provide a natural setting to explore problems of equilibrium and non-equilibrium quantum magnetism. The presence of many different internal states of the atoms and molecules in such experiments enables studies of large-spin magnetism, but also holds promise for more general quantum simulation studies. Here we describe how the simple addition of multi-frequency microwave fields to Rydberg arrays enables highly controllable studies of few- and many-body dynamics along an internal-state “synthetic” dimension. I’ll discuss several early studies in the Rydberg synthetic dimension platform, touching on interaction-driven phenomena relevant to topology, artificial gauge fields, and disorder-induced localization. Looking forward, such microwave manipulation opens up several new directions for exploring complex, driven quantum matter in dipolar arrays.

Wednesday 21st August 2024: Richard Brierley (Nature Physics) — Physics East Seminar Room 217

NB unusual time of 11am and unusual venue!

Inside Nature Physics

For nearly twenty years Nature Physics has aimed to publish high-quality research, reviews and commentary that are accessible and of interest to the whole physics community. I will give an introduction to the journal, its editors and how it fits into the wider Nature portfolio. I’ll also try to give insight into the editorial process and how we make our decisions.

Thursday 26th September 2024: Laura Messio (Sorbonne University)

Overview of the possibilities of high temperature series expansions

In the search for exotic properties of spin lattices, many numerical methods focus on the ground state and on the low-energy excitations of a model. Here, we use the opposite approach, with high temperature series expansions. The coefficients of the free energy, the specific heat or the magnetic susceptibility series are obtained up to beta^20 (depending on the model complexity). But the convergence radius limits the range of temperature where information can been obtained by simple summation. Thus, alternative methods are required. One of them is the entropy method, that uses some hypothesis on the nature of the ground state to reconstruct thermodynamic quantities over the whole temperature range. This method has been used on several compounds to propose exchange parameter values. When a finite temperature phase transition occurs, a singularity forbids to use the entropy method, but then, informations on the critical exponents and on the critical temperature can be extracted. A review of these methods and their limitations, together with some applications, will be presented.

Thursday 3rd October 2024: Nick Manton (Cambridge)

NB unusual time of 1.30pm!

Skyrme’s Original Skyrmions

Tony Skyrme (1922-87) spent the the majority of his career at Birmingham. His main interest was nuclear physics, which at that time was close to elementary particle physics. He proposed a model for the interaction of nucleons (protons and neutrons) inside larger nuclei; this is the “Skyrme force”, still regularly used by nuclear theorists. But his most bold and original idea was the Skyrmion, a model for an individual nucleon created out of a topologically twisted configuration of a nonlinear pion field. When a Skyrmion’s rotational motion is quantised, the lowest-energy states are naturally identified with spin-half states of protons or neutrons. There has been much progress since Skyrme’s time on multi-Skyrmion solutions, modelling medium-sized nuclei like Carbon-12. These solutions have fascinating symmetries that constrain their quantised rotational and vibrational excitations, matching known nuclear spectra quite well. A current challenge is to model nuclear reactions in terms of Skyrmion collisions.

Thursday 10th October 2024: Martin Völkl (Birmingham)

Learning about QCD from heavy-ion collisions

Understanding the properties of the strong interaction from first principles is a difficult endeavour. The large coupling strength at energies usually found in nature, as well as the fact that quarks and gluons are confined in hadrons mean that usual perturbative methods can often not be applied. Heavy-ion collisions at high energies, such as those measured by the ALICE detector at the LHC provide a laboratory to understand quantum chromodynamics in a vastly different regime. The quark-gluon plasma produced there is a state of equilibrated, deconfined, strongly interacting matter. During this seminar we will discuss what how we can learn about QCD from these collisions, what we have found out so far, and which questions remain to be explored in the future.

Thursday 17th October 2024: Eli Hawkins (York)

Quantization of Angular Momentum

I shall outline the mathematical perspective of Strict Deformation Quantization. In this context, I can prove that for a well-behaved quantization of the 2-sphere, the values of the quantization parameter (formally, Planck’s constant) are restricted. This is a robust version of the quantization of angular momentum that applies when rotational symmetry is only approximate.

Thursday 24th October 2024: Douglas Abraham (Oxford)

Phase Separation in Uni-axial Ferromagnets: Recent exact results

The phenomenological view from the 19th century of phase separation in simple fluids and the associated structure of the resulting interface will be described, along with its shortcomings. The statistical-mechanical discussion of these matters begins with the work of Peierls (formerly of this department) and continues with that of Griffiths and Dobrushin. This is combined with the exact solution approach of Kaufman and of Lieb, Mattis and Schultz to obtain new results. For the first time, an approach to the Gibbs dividing surface is given, as well as a way to incorporate fluctuations in that picture. A short discussion will be given of how the calculations work, beginning with Kaufman.

Thursday 7th November 2024: Enrico Amico (Birmingham)

Higher-order connectomics of human brain function

Traditional models of human brain activity often represent it as a network of pairwise interactions between brain regions. Going beyond this limitation, recent approaches have been proposed to infer higher-order interactions from temporal brain signals involving three or more regions. However, to this day it remains unclear whether methods based on inferred higher-order interactions outperform traditional pairwise ones for the analysis of fMRI data. In this talk I will introduce a novel approach to the study of interacting dynamics in brain connectomics, based on higher-order interaction models. Our method builds on recent advances in simplicial complexes and topological data analysis, with the overarching goal of exploring macro-scale and time-dependent higher-order processes in human brain networks. I will present our preliminary findings along these lines, and discuss limitations and potential future directions for the exciting field of higher-order brain connectomics.

Thursday 14th November 2024: Daniele Toniolo (UCL)

Stability of slow Hamiltonian dynamics and Dynamical generation of α-Rényi entropies

In this talk I will present two results. The first regards the stability of the slow dynamics for spin systems from the point of view of Lieb-Robinson (L-R) bounds. L-R bounds quantify the speed at which a generic local operator spreads across the system due to Heisenberg evolution. A slow dynamics happens when to spread across a region the operator takes a time proportional to the exponential of such region. This is relevant in particular for so called many-body localized systems. In this case when the system is perturbed, for example with the insertion of perturbations that would lead to regions of fast dynamics, I will show that the dynamics remains locally slow. With this theory I can also consider the non perturbative scenario of the junction among systems with fast and slow dynamics. This is based upon [1].

The second result that I will discuss is the dynamical generation of entanglement as quantified by α-Rényi entropies, with 0<α<1, including the case α=1 of the von Neuman entropy, starting from a generic product state. I will show that this follows from the Lieb-Robinson bound of the dynamics. For a generic local spin system in one dimension then the entanglement grows at most linearly in time. On the other hand when a slow dynamics is considered, like in the first part of the talk, I will obtain the log t-law of entanglement that has been shown before for MBL systems only by numerics or by assuming the existence of local integrals of motions. The second part of the talk is based upon [2], and [3].

[1] D. Toniolo, S. Bose, arXiv:2405.05958

[2] D. Toniolo, S. Bose, arXiv:2408.00743

[3] D. Toniolo, S. Bose, arXiv:2408.02016

Thursday 21st November 2024: Michele Simoncelli (Cambridge)

Unified theories of transport in solids: from crystals to glasses, and from diffusion to viscous hydrodynamics

Crystals and glasses have dramatically different properties which intrigued scientists long before the development of atomistic theories, and nowadays play a pivotal role in a variety of technologies. I will explore the quantum mechanisms that determine the macroscopic conduction properties of solids, extending established formulations[1] and developing the computational framework[2] to solve them. Starting from a density matrix formalism, I will show how the semiclassical Boltzmann equation for heat transport is missing a tunneling term that becomes pivotal in disordered or defective materials[3]. Second, I will discuss how this formulation can be extended to describe coupled transport phenomena — involving, e.g., heat and light, charge, or spin — which are critical for applications ranging from zero-carbon jet engines[4] to neuromorphic computing. Finally, I will elucidate how the microscopic transport equations can be coarse-grained into mesoscopic, viscous equations; these transcend ordinary diffusion, rationalizing the recent observation of hydrodynamic behaviour[5] and paving the way for its control and technological exploitation.

[1] M. Simoncelli, N. Marzari, and F. Mauri, Phys. Rev. X 12, 041011 (2022).
[2] B. Póta, P. Ahlawat, G. Csányi, and M. Simoncelli, arXiv:2408.00755.
[3] A. F. Harper, K. Iwanowski, W. C. Witt, M. C. Payne, and M. Simoncelli, Phys. Rev. Mater. 8, 043601 (2024).
[4] A. Pazhedath, L. Bastonero, N. Marzari, and M. Simoncelli, Phys. Rev. Appl. 22, 024064 (2024).
[5] J. Dragašević and M. Simoncelli, arXiv:2303.12777.

Thursday 28th November 2024: Benedikt Placke (Oxford)

Topological Quantum Spin Glasses: the low temperature phase of qLDPC codes with expansion

Understanding the nature of glasses and the glass transition remains one of the most important unsolved problems in statistical mechanics, with wide ranging implications, from materials to machine learning and optimisation problems. Here, we establish a rigorous connection between (spin-)glass order and expansion, a property of classical and quantum error-correcting codes defined on non-Euclidean graphs that guarantees efficient decodability. This allows us to rigorously prove the existence of finite-temperature spin glass order in classical error correcting codes, as well as the existence of a novel form of topological quantum spin glass order in quantum error correcting codes with expansion. Topological Quantum Spin Glasses are characterized by the same complex (free) energy landscape as their classical counterparts, but with every local and global minimum corresponding to a topologically ordered quantum state. We complement our rigorous results by extensive numerical studies and argue that (topological) glassiness may be observed in a large class of (quantum) error correcting codes.

Thursday 5th December 2024: Nick Jones (Oxford)

Symmetry, topology and entanglement in the chiral clock family

Global symmetries greatly enrich the phase diagram of quantum many-body systems. As well as symmetry-breaking phases, symmetry-protected topological (SPT) phases have symmetric ground states that cannot be connected to a trivial state without a phase transition. There can also be symmetry-enriched critical points between these phases of matter, and ‘unnecessary’ critical points. I will demonstrate these phenomena in phase diagrams constructed using an integrable family of N-state chiral clock chains, where an underlying Onsager algebra allows for many exact results.

The talk is based on joint work with Abhishodh Prakash and Paul Fendley arXiv:2406.01680 and ongoing work.