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Department of Physics

The Cavendish Laboratory
 
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This list is intended to include all research talks taking place in the Department of Physics.
Updated: 1 hour 17 min ago

Thu 03 Jun 14:00: Non-conservative forces in nanoscale conductors

Fri, 07/05/2021 - 19:06
Non-conservative forces in nanoscale conductors

Over ten years ago our group showed that interatomic forces under current flow in nanostructures are non-conservative even under ideal steady-state conditions. In other words, the forces on atoms under current are not given by the gradient of a potential, opening up the questions of what the motion of atoms under these conditions looks like and what it can and cannot do. Other researchers world-wide have joined this area, with a number of new ideas and advances. This talk will summarise our research on this problem, both in the past and at present. In the past we have investigated the resultant atomic dynamics in a variety of systems, revealing a generic effect: the continuous supply of kinetic energy to the atomic motion by the work done by the non-conservative forces, which we have called the waterwheel effect. This effect can be seen both as a constructive aspect of the current-driven dynamics opening up the possibility of atomic-scale manipulation, and as a destructive agent limiting the stability of nanoscale conductors over and above Joule heating. Most recently we have started to investigate the influence of electron-electron interactions on the waterwheel effect, with some interesting preliminary findings.

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Thu 13 May 14:00: Thwarting long-lived dark states to speed up dynamic polarization

Thu, 06/05/2021 - 16:42
Thwarting long-lived dark states to speed up dynamic polarization

Long-lived dark states, in which an experimentally accessible qubit is not in thermal equilibrium with a surrounding spin bath, are pervasive in solid-state systems. In the first half of the talk, I will explain the ubiquity of dark states in a large class of inhomogenous central spin models using the proximity to integrable lines with exact dark eigenstates. I will develop a picture of the eigenstates away from the integrable lines in terms of many-body resonances, and argue that relaxation times are exponentially large in system size at accessible sizes.

Long-lived dark states stymie hyperpolarization protocols that aim to transfer spin polarization from the central qubit to the surrounding spin bath. In the second half of the talk, I will describe fast and efficient hyperpolarization protocols that exploit certain integrability-breaking terms to reduce the statistical weight on dark states. These protocols use approximate counter-diabatic driving and are experimentally accessible through Floquet engineering.

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Tue 11 May 16:00: Recent advances in the modelling of ttW at the LHC

Wed, 05/05/2021 - 11:17
Recent advances in the modelling of ttW at the LHC

In the light of recently reported discrepancies between data and theory for the ttW normalization in either ttH measurements or tttt searches in the multi-lepton channels the accurate modelling of these complex signatures is revisited. I will discuss the current status of the modelling of the complex ttW signatures with special emphasis on our recent calculation of top-quark pair production in association with a W boson in the POWHEG -BOX framework. I will show a comparison with other Monte Carlo generators for the two same-sign lepton signature commonly used by the experiments. Special focus will be put on the assessment of the importance of subleading electroweak contributions in the modelling of the production process. In addition, I will discuss theoretical uncertainties of our predictions, which are estimated by variations of the renormalization and factorization scales, also matching uncertainties are estimated.

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Thu 25 Nov 14:00: Title to be confirmed

Tue, 04/05/2021 - 18:25
Title to be confirmed

Abstract not available

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Thu 25 Nov 14:00: Title to be confirmed

Tue, 04/05/2021 - 18:25
Title to be confirmed

Abstract not available

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Wed 05 May 16:00: Physical models to predict the evolution of viruses and bacteria This lecture is a Livestream Event

Fri, 30/04/2021 - 13:35
Physical models to predict the evolution of viruses and bacteria

Abstract not available

This lecture is a Livestream Event

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Wed 26 May 16:00: Machine learning as a solution to the electronic structure problem

Fri, 30/04/2021 - 11:55
Machine learning as a solution to the electronic structure problem

An essential component of materials research is the use of simulations based on density functional theory (DFT), which imposes severe limitations on the size of the system under study. A promising development in recent years is the use of machine learning (ML) methodologies to train surrogate models with DFT data to predict quantum-accurate results for larger systems. Many successful ML models have been created to predict higher-level DFT results such as the total potential energy and atomic forces, and initial steps have been taken to create machine-learning based ML methodologies that can predict fundamental DFT outputs such as the charge density, wave functions and corresponding energy levels. In this talk, I will present our latest results using deep learning neural networks to learn and predict the electronic structure of a large variety of carbon allotropes, and its extension to hydrocarbons.

B. G. del Rio, C. Kuenneth, H. D. Tran, and R. Ramprasad, J. Phys. Chem. A 124 , 9496-9502 (2020).

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Wed 16 Jun 11:30: Title to be confirmed

Fri, 30/04/2021 - 11:47
Title to be confirmed

Abstract not available

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Wed 09 Jun 16:00: Title to be confirmed

Fri, 30/04/2021 - 11:46
Title to be confirmed

Abstract not available

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Wed 02 Jun 11:30: Title to be confirmed

Fri, 30/04/2021 - 11:45
Title to be confirmed

Abstract not available

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Wed 12 May 11:30: Electrically tunable stacking domains and ferroelectricity in Moiré superlattices

Fri, 30/04/2021 - 11:44
Electrically tunable stacking domains and ferroelectricity in Moiré superlattices

Twistronics is a new and exciting field in physics in which the components of layered systems are twisted or strained by small amounts with respect to one another, radically changing the properties of the system. In 2018 it was discovered that by twisting the layers in bilayer graphene by the ‘magic angle’ of 1.05 degrees, the system could exhibit superconducting or insulating behavior, neither of which are possible when untwisted. Very recently it was discovered twisted systems comprised of nonpolar layers could exhibit ferroelectricity [Nature 588, 71–76 (2020)]. However, the mechanism remains unclear.

Twisted systems form complex domain structures due to the interplay between the in-plane strains and the out-of-plane stacking energy. These domain structures are highly sensitive to the twist angle and lattice mismatch. I will discuss theoretical work and first-principles calculations which show that the domain structure can also be tuned using an applied electric field, and in a single sample. I will discuss how the domain structures may be responsible for ferroelectricity due to the polar response to their strain gradients via flexoelectricity.

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Wed 19 May 11:30: Hyperbonding in chalcogenides: origin of the electronic property contrast in phase-change memory materials

Fri, 30/04/2021 - 11:43
Hyperbonding in chalcogenides: origin of the electronic property contrast in phase-change memory materials

A new type of non-volatile (storage-class) electronic memory (Optane) has been developed by Intel, based on telluride materials. In this, binary information is stored as one of two metastable atomic-structural states of the memory material having differing electronic properties, rather than as trapped electronic charge. The glassy state is electrically resistive, whereas the crystalline state is electrically conductive. Voltage-pulse-induced reversible changes between these two phases are extremely rapid, ~ ns. However, the chemical-bonding nature of these two phases has been very controversial. In this talk, I will briefly describe the results of DFT -based simulations of these materials, and show how they support the notion of hyperbonding (3 centre-4 electron interactions) in these materials as an explanation for the property-contrast and ultra-rapid crystallization characteristics that they exhibit.

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Thu 29 Apr 11:30: Hyperbonding in chalcogenides: origin of the electronic property contrast in phase-change memory materials

Thu, 29/04/2021 - 10:28
Hyperbonding in chalcogenides: origin of the electronic property contrast in phase-change memory materials

A new type of non-volatile (storage-class) electronic memory (Optane) has been developed by Intel, based on telluride materials. In this, binary information is stored as one of two metastable atomic-structural states of the memory material having differing electronic properties, rather than as trapped electronic charge. The glassy state is electrically resistive, whereas the crystalline state is electrically conductive. Voltage-pulse-induced reversible changes between these two phases are extremely rapid, ~ ns. However, the chemical-bonding nature of these two phases has been very controversial. In this talk, I will briefly describe the results of DFT -based simulations of these materials, and show how they support the notion of hyperbonding (3 centre-4 electron interactions) in these materials as an explanation for the property-contrast and ultra-rapid crystallization characteristics that they exhibit.

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Wed 05 May 11:30: Electronic properties and structural relaxation of twisted bilayer graphene: insights from large-scale first principles calculations

Wed, 28/04/2021 - 17:11
Electronic properties and structural relaxation of twisted bilayer graphene: insights from large-scale first principles calculations

In this talk the electronic and structural properties of twisted bilayer graphene down to the first magic angle are presented. By combining state-of-the-art first principles calculations with a low-energy continuum model, we have studied the intriguing properties of twisted bilayer graphene at several twist angles. The crucial role played by the interlayer van der Waals interaction has been investigated, in particular as far as the properties of the flat bands showing up at the Fermi energy at special ‘magic’ angles are concerned. Our calculations revealed a strong interplay between the twist angle and the atomic relaxation arising after twisting.

References P. Lucignano, et al., Phys. Rev. B 99 , 195419 (2019) G.Cantele, et al., Phys. Rev. Research 2, 043127 (2020)

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Thu 06 May 14:00: Thermodynamics in the presence of coherences and strong coupling corrections

Wed, 28/04/2021 - 12:31
Thermodynamics in the presence of coherences and strong coupling corrections

I will give an introductory talk on quantum thermodynamics [1] covering a selection of my group’s results in recent years.

We will first ask what is “quantum” in quantum thermodynamics. To answer this question we set up a quantum thermodynamic process that removes quantum information in analogy to Landauer’s erasure of classical information. The thermodynamic analysis of such a process uncovers that work can be extracted from quantum coherences in addition to the work that can be extracted from classical non-equilibrium states [2].

In the later part of the talk, I will report on a new thermodynamic uncertainty relation that limits the accuracy of measuring the temperature and energy of a thermal quantum system [3]. Corrections to the standard uncertainty relation arise here because, unlike in standard thermodynamics, a small system’s interaction with its environment is not negligible. The emerging relation unites thermodynamic and quantum uncertainties for the first time.

[1] Quantum thermodynamics, S. Vinjanampathy, J. Anders, Cont. Phys. 57, 545 (2016).

[2] Coherence and measurement in quantum thermodynamics, P. Kammerlander, J. Anders, Sci. Rep. 6, 22174 (2016).

[3] Energy-temperature uncertainty relation in quantum thermodynamics, H. Miller, J. Anders, Nat. Comm. 9, 2203 (2018).

Most recent:

[4] Weak and ultrastrong coupling limits of the quantum mean force Gibbs state, J. Cresser, J. Anders, arxiv 2104.12606 (2021).

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Wed 12 May 11:30: Electrically tunable stacking domains and ferroelectricity in Moiré superlattices

Wed, 28/04/2021 - 11:13
Electrically tunable stacking domains and ferroelectricity in Moiré superlattices

Twistronics is a new and exciting field in physics in which the components of layered systems are twisted or strained by small amounts with respect to one another, radically changing the properties of the system. In 2018 it was discovered that by twisting the layers in bilayer graphene by the ‘magic angle’ of 1.05 degrees, the system could exhibit superconducting or insulating behavior, neither of which are possible when untwisted. Very recently it was discovered twisted systems comprised of nonpolar layers could exhibit ferroelectricity [Nature 588, 71–76 (2020)]. However, the mechanism remains unclear.

Twisted systems form complex domain structures due to the interplay between the in-plane strains and the out-of-plane stacking energy. These domain structures are highly sensitive to the twist angle and lattice mismatch. I will discuss theoretical work and first-principles calculations which show that the domain structure can also be tuned using an applied electric field, and in a single sample. I will discuss how the domain structures may be responsible for ferroelectricity due to the polar response to their strain gradients via flexoelectricity.

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Fri 14 May 16:00: Mixed QCD-electroweak corrections to W and Z production and their impact on W mass measurements at the LHC

Wed, 28/04/2021 - 09:12
Mixed QCD-electroweak corrections to W and Z production and their impact on W mass measurements at the LHC

The seminar will take place via Zoom here.

Abstract: The experimental collaborations at the LHC aim to eventually measure the mass of the W boson with an uncertainty of about 10 MeV, which would rival the precision available from global electroweak fits. At this level of precision, effects that were previously considered to be small can become relevant. One such effect are mixed QCD -electroweak corrections. I will discuss our recent calculation of mixed QCD -EW corrections to on-shell W and Z production at the LHC and how we use these results to estimate their impact on the W mass measurements at the LHC .

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Thu 20 May 14:00: TBC

Wed, 28/04/2021 - 08:51
TBC

TBC

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

Tue, 27/04/2021 - 14:32
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 .

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