Department of Physics

This list is intended to include all research talks taking place in the Department of Physics.
Updated: 45 min 51 sec ago

### Thu 24 Jun 14:00: TBC

Tue, 27/04/2021 - 08:13
TBC

TBC

### Thu 03 Jun 14:00: TBC

Tue, 27/04/2021 - 08:11
TBC

TBC

### Fri 07 May 16:00: Instanton/Sphaleron processes in hadronic, heavy ion collisions and cosmology

Mon, 26/04/2021 - 08:59
Instanton/Sphaleron processes in hadronic, heavy ion collisions and cosmology

The seminar will take place via Zoom here.

Abstract: I start describing the (pure gauge) topological landscape as a function of Chern-Simons number and size. It was originally derived by reduction from 4d Instanton-Antiinstanton valley, as well as direct energy minimization with constraints. The instanton-sphaleron processes combines an “inclined tunneling” and Minkowskian solution, describing a sphaleron explosion. It has been proposed to observe the QCD sphaleron production in pp collisions at LHC and RHIC , via double diffractive events . At small masses $M\sim 3\, GeV$ the key observable is proposed to be $\bar u u \bar d d \bar s s$ flavor content, at $M> 20\, Gev$ multi-jet clusters. We then briefly discuss sounds in heavy ion collisions and the Big Bang, and ongoing search for sphaleron production via chiral imbalance and chiral magnetic effect. At the end we consider electroweak and QCD cosmological phase transitions, focusing on possible role of sphalerons in production of sounds, gravity waves and magnetic fields.

### Tue 27 Apr 16:00: Quantum computing for simulating high energy collisions

Sun, 25/04/2021 - 22:20
Quantum computing for simulating high energy collisions

The simulation of high energy collisions at experiments like the Large Hadron Collider (LHC) relies on the performance of full event generators and their accuracy and speed in modeling the complexity of multi-particle final states. The rapid improvement of quantum devices presents an exciting opportunity to construct dedicated algorithms to exploit the potential quantum computers can provide.

I will present general and extendable quantum computing algorithms to calculate two key stages of an LHC collision; the hard interaction via helicity amplitudes and the subsequent parton shower process. These algorithms fully utilise the quantum nature of the calculations and the machine’s ability to remain in a quantum state throughout the computation. It is a first step towards a quantum computing algorithm to describe the full collision event at the LHC .

### Tue 04 May 13:30: Anharmonic Lattice Dynamics in Organic Semiconductors

Fri, 23/04/2021 - 12:15
Anharmonic Lattice Dynamics in Organic Semiconductors

What is the relation between the structural dynamics of crystals and their functional properties at finite temperatures? Recent studies of halide perovskite semiconductors showed that despite being mechanically soft and having many structural defects, they have excellent optoelectronic properties. We and others have shown that a key fundamental property for their surprising optoelectronic properties is their strongly anharmonic (i.e., nonlinear) lattice dynamics. Inspired by these findings, we turned to investigate anharmonic effects in the lattice dynamics of organic semiconducting crystals that are even softer, mechanically, than the halide perovskites. I will present Raman scattering studies that show a plethora of expressions of anharmonicity in the structural dynamics of organic crystals. Then, I will discuss how the character of the structural dynamics can be tuned by introduction of molecular sidechains. Finally, I will discuss implications of the anharmonic lattice dynamics on the electronic properties of these crystals.

### Thu 29 Apr 16:00: Towards an all-orders calculation of the electroweak bubble wall velocity

Fri, 23/04/2021 - 10:21
Towards an all-orders calculation of the electroweak bubble wall velocity

The seminar will take place via Zoom here.

Abstract: In this talk I discuss recent work where we calculate the velocity of the Higgs condensate bubble wall during a first-order electroweak phase transition in the early Universe. The interaction of particles with the bubble wall can be accompanied by the emission of multiple soft gauge bosons. When computed at fixed order in perturbation theory, this process exhibits large logarithmic enhancements which must be resummed to all orders when the wall velocity is large. We perform this resummation both analytically and numerically at leading logarithmic accuracy. The numerical simulation is achieved by means of a particle shower in the broken phase of the electroweak theory. The two approaches agree to the 10% level. For fast-moving walls, we find the scaling of the thermal pressure exerted against the wall to be P~γ^2T4, independent of the particle masses, implying a significantly slower terminal velocity than previously suggested. This is impactful for baryogenesis, gravitational wave radiation and generation of other cosmic relics.

### Fri 18 Jun 16:00: TBA

Fri, 23/04/2021 - 08:01
TBA

The seminar will take place via Zoom here.

Abstract:

### Fri 11 Jun 16:00: TBA

Fri, 23/04/2021 - 08:00
TBA

The seminar will take place via Zoom here.

Abstract:

### Fri 28 May 16:00: TBA

Fri, 23/04/2021 - 07:59
TBA

The seminar will take place via Zoom here.

Abstract:

### Fri 21 May 16:00: TBA

Fri, 23/04/2021 - 07:58
TBA

The seminar will take place via Zoom here.

Abstract:

### Fri 04 Jun 16:00: TBA

Fri, 23/04/2021 - 07:58
TBA

Abstract not available

### Fri 14 May 16:00: TBA

Fri, 23/04/2021 - 07:56
TBA

The seminar will take place via Zoom here.

Abstract:

### Fri 07 May 16:00: TBA

Fri, 23/04/2021 - 07:49
TBA

The seminar will take place via Zoom here.

Abstract:

### Thu 29 Apr 16:00: TBA

Fri, 23/04/2021 - 07:47
TBA

The seminar will take place via Zoom here.

Abstract:

### Tue 27 Apr 15:30: Controlling charge, spin and light in Lead-Halide Inspired Hybrid Semiconductors

Thu, 22/04/2021 - 22:52
Controlling charge, spin and light in Lead-Halide Inspired Hybrid Semiconductors

Hybrid organic/metal-halide inorganic semiconductors offer tremendous opportunities to control fundamental properties that underpin energy technologies. While currently there are enormous worldwide efforts exploring, exploiting and improving a narrow class of these hybrid semiconductors (metal-halide perovskite semiconductors (MHP), such as methylammonium lead iodide (MAPbI3)), primarily for photovoltaic (PV) applications, an opportunity exists to transcend this initial focus and seek deeper understanding and control of their fundamental properties. In this presentation I will discuss our studies of controlling the charge carrier dynamics, light/matter interactions, and spin populations in these novel hybrid systems. In one effort we are exploring the use of novel organic hybrid systems at and near interfaces to control the carrier dynamics and reduce surface recombination but also to protect grain boundary surfaces from degradation. With respect to controlling spins we have recently studied and developed a novel class of chiral hybrid semiconductors based upon layered metal-halide perovskite 2D Ruddlesden-Popper type structures. These systems exhibit chiral induced spin selectivity whereby only one spin sense can transport across the film and the other spin sense is blocked. From these systems we can achieve a high degree of spin current polarization and injection when used as a contact layer. We have developed novel spin-based LEDs using mixed NCs as the light emitting layer that promotes light emission at a highly spin-polarized interface. The LED spin-polarization is limited by spin-depolarization within the MHP N Cs. In a separate effort we have explored the use of chiral copper-halide hybrid systems for circular light polarized detection. Chiral based copper-halide systems combined with highly conductive carbon nanotube networks can be employed to detect circular polarized light with the use of polarizers. Our chiral heterostructure shows high photoresponsivity of 452 A/W, a competitive anisotropy factor of up to 21%, a current response in microamperes, and low working voltage down to 0.01 V. These results demonstrate that the emergent properties of organic−inorganic hybrid systems offer unique opportunities in controlling light, charge and spin.

### Thu 29 Apr 14:00: Topology vs. interaction in fragile bands

Tue, 20/04/2021 - 09:57
Topology vs. interaction in fragile bands

Topological systems of non-interacting electrons may be forced by their topological nature to have a gapless spectrum. As two examples, Integer quantum Hall states and topological insulators must carry gapless edge states, which allow them to carry current. Whether interaction between the electrons may turn a gapless conductor into a fully gapped insulator depends on the case at hand, with a negative answer for the first example and a positive for the second.

In this talk I will review these two examples, and then focus on bands of fragile topology. I will explain what fragile topology is as well as how and when it forbids a gapped spectrum at the non-interacting level. Then, I will show that interactions may be powerful enough to overcome the decree issued by topology, and explain how they do that. Finally, I will discuss relevance to twisted bi-layers of graphene.

### Thu 10 Jun 14:00: Title to be confirmed

Mon, 19/04/2021 - 15:04
Title to be confirmed

Abstract not available

### Tue 22 Jun 16:00: Observables in 3-jet and Z+2 jet final states

Tue, 13/04/2021 - 08:38
Observables in 3-jet and Z+2 jet final states

Abstract not available

### Tue 18 May 16:00: Probabilistic definition of the perturbative theoretical uncertainty from missing higher orders

Tue, 13/04/2021 - 08:38

### Tue 11 May 16:00: ttW theory overview

Tue, 13/04/2021 - 08:37
ttW theory overview

Abstract not available