Events List for the Academic Year
Event Time:
Wednesday, August 3, 2022 | 12:30 pm - 1:30 pm
Event Location:
HENN 318
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2022-08-03T12:30:00
2022-08-03T13:30:00
Equity & Inclusion Journal Club Meeting
Event Information:
Hello everyone!
The E&I in PHAS group is excited to host a Journal Club next Wednesday August 3rd 12:30 - 1:30 pm in Henn 318 (Zoom option available, see below).
Join us for FREE LUNCH AND SNACKS as we discuss gender and the pandemic, including caregiving, working from home, personal outcomes, career outcomes, and abortion!
Please RSVP here: https://forms.gle/tcfDu2arRLwM5VWdA
(if possible, by July 29th 11:00 am so we know how much food to order)
Our discussion will be based on the following two articles (see attached):
Stefanova (2021) Gender and the pandemic: Associations between caregiving, working from home, personal and career outcomes for women and men
Goldberg (2022) How Roe Shaped the World of Work for Women
We encourage everyone to prepare for the event by doing the readings and bringing one quote or statistic that stood out for them. For those of us who will inevitably forget, we will also begin the event with a brief summary of the readings.
Some things to keep in mind while reading:
How does this resonate with your own experiences? The experiences of those around you?
What stood out for you? Did anything surprise you?
Is there anything you feel was missed?
Submit anonymous discussion points here: https://forms.gle/e1NzG4V36PtTuSBp9
Join Zoom Meeting https://ubc.zoom.us/j/63169843132?pwd=WFpYanBSbDVCbDMrVDhFd2cxQ0JGdz09
Meeting ID: 631 6984 3132 Passcode: 828870
Hope to see you there!
The E&I Team
----
Equity and Inclusion in PHAS at UBC
Website: equity-inclusion.phas.ubc.ca
Subscribe to our mailing list here
Event Location:
HENN 318
Event Time:
Tuesday, July 26, 2022 | 1:00 pm - 3:00 pm
Event Location:
https://ubc.zoom.us/j/61385552566?pwd=WDg4UDhxZy8rQWphb3FxOFdyWFBGZz09 Passcode: 791344
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2022-07-26T13:00:00
2022-07-26T15:00:00
Peierls Coupling in Multi-Orbital Superconducting Oxides
Event Information:
We study the effects of the Peierls electron-phonon coupling in different multi-band systems. In contrast to the more commonly employed Holstein coupling, which is used in single-band models and is momentumindependent, the momentum-dependent Peierls coupling can explicitly treat coupling to multiple bands. Our results demonstrate the importance of using the Peierls coupling in modelling complex systems
First, we investigate single polaron physics on a perovskite lattice inspired by BaBiO 3 . We find that with Peierls coupling, the ground state momentum of the polaron jumps between high-symmetry points in Brillouin zone as the coupling strength is increased. Because such sharp transitions are not possible in the Holstein model, it follows that it is not always feasible to map the more complex Peierls model onto the simpler Holstein model.
Then we further investigate the nontrivial behaviour brought by the momentum-dependent Peierls coupling by studying the simplest multi-band system, namely the two-band model on a 1D chain. We show that the Peierls coupling leads to a self-energy that is neither local nor diagonal, opposite to the Holstein model. Peierls coupling can also change the orbital character of the lowest band. To explore whether experiments like ARPES can observe differences in the spectral weights corresponding to the two different couplings, we calculate the intensity measured by ARPES. We demonstrate that various components of the spectral weights are weighted by momentum dependent matrix elements, making it hard to identify the sources of momentum-dependence in the measured intensity.
Lastly, we add the Peierls coupling to Emery’s three-band model for cuprates and study its effect on the polaron effective mass. We show that although the hole-phonon coupling strength is moderate to strong, it only causes a negligble increase in the effective mass, indicating that the effective coupling to the magnon-dressed quasiparticle is much reduced by the dressing. We explain the reason for this and describe how to treat the difference between lattice coupling to bare holes versus to correlations-dressed quasiparticles.
Event Location:
https://ubc.zoom.us/j/61385552566?pwd=WDg4UDhxZy8rQWphb3FxOFdyWFBGZz09 Passcode: 791344
Event Time:
Friday, July 8, 2022 | 3:00 pm - 5:00 pm
Event Location:
https://ubc.zoom.us/j/67592194700?pwd=NHdUb1pqQ08wZUNwQ3dsQjB5blZ6UT09 Passcode: 695934
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2022-07-08T15:00:00
2022-07-08T17:00:00
Quantum Information in Electromagnetism and Gravity
Event Information:
The electromagnetic and gravitational fields transfer information between physical systems. This work is an attempt to better understand how matter systems communicate quantum information with one another using these fields, and also how quantum information about matter is broadcast into the fields themselves. We study the former process in Part I and the latter in Part II, by answering two distinct but related questions.
Part I of this work studies experimental proposals to observe gravity-induced quantum entanglement between matter systems. If these are successful, it can be argued that they would be the first experimental witness of a quantum superposition of space-time geometries, although this interpretation of the proposals has been the subject of vigorous debate. To address this, we first utilize the "quantum action principle" to quantize the electromagnetic and
(linearized) gravitational fields. We find that for the quantization to be self-consistent, physical quantum states in the theory must be gauge- and diffeomorphism-invariant. We then show that these constraints are the root cause of the confusion surrounding the proposed gravity-induced entanglement experiments. A deeper understanding, however, of how these constraints change when assigning quantum states to different hypersurfaces in space-time provides a satisfying resolution to the debate over the experimental proposals. We conclude that if gravity-induced entanglement is observed, it should be interpreted as experimental evidence in favor of the quantum nature of space-time.
Part II then addresses the quantum information content of low energy "soft" radiation. We first show that whether matter particles emit such radiation depends entirely upon the past and future boundaries of the worldlines they follow. This observation explains all tree-level "soft theorems" in both electromagnetism and gravity. We then quantize the electromagnetic and gravitational fields asymptotically, at null infinity. Again, the quantization compels us to ensure physical states are invariant under gauge transformations and diffeomorphisms which persist in the asymptotic limit. Invariance under these "large"
transformations fully constrains the state of the leading-order soft radiation, meaning that this radiation cannot carry quantum information.
Finally, we illustrate how our construction also avoids infrared divergences when predicting decoherence rates in a model interferometry experiment.
Lay Summary:
Most of the Universe is now known to be described by quantum mechanics.
Light, previously understood as waves in the electromagnetic field, is now also understood as consisting of particles called photons. Many expect that the gravitational field too consists of particles called gravitons, but so far these have not been observed. Thankfully there are proposals for experiments which might be able to do this, but not everyone believes they can.
In the first part of this thesis we show that these experiments should be able to observe the graviton, if it exists.
Photons and gravitons also carry information (photons are probably carrying these words to your eyes) and energy (the light from the Sun feels warm on our skin). In the second part of this thesis, we ask whether these particles can carry quantum information without carrying any energy at all, and show that they cannot.
Event Location:
https://ubc.zoom.us/j/67592194700?pwd=NHdUb1pqQ08wZUNwQ3dsQjB5blZ6UT09 Passcode: 695934
Event Time:
Friday, June 10, 2022 | 2:00 pm - 4:30 pm
Event Location:
BRIM 311
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2022-06-10T14:00:00
2022-06-10T16:30:00
PhD defense Graham Baker
Event Information:
Electrical conduction becomes non-local when an inhomogeneous electronic distribution is induced with spatial variation shorter than the mean free path (MFP) between momentum-relaxing electronic scattering processes. Two important methods of inducing such a distribution are via the size and skin effects. In the size effect, one or more dimensions of a medium are reduced below the MFP. The scattering of electrons from the medium's boundaries then induces an inhomogeneous electronic distribution under an applied direct current. In the skin effect, the exponential decay of alternating electromagnetic fields as they propagate into a medium gives rise to a so-called skin layer. The electronic distribution within the skin layer becomes inhomogeneous as the skin depth falls below the MFP.
Here we study the size and skin effects in PdCoO2, both experimentally and theoretically. While previous theoretical treatments of non-local electrical conductivity have assumed a free-electron dispersion, resulting in an isotropic Fermi surface (FS), we observe that the anisotropic FS in PdCoO2 results in behaviour that is incompatible with this assumption.
Measurements of the size effect in PdCoO2 revealed two novel phenomena, both of which are symmetry-forbidden for local conduction: anisotropy in the in-plane longitudinal resistivity, and a non-zero transverse resistivity at zero magnetic field. We developed a theory of the size effect for arbitrary FS geometry and used it to reproduce the key features of these measurements.
Motivated by recent interest in the possibility that electrons in solids may behave viscously as a result of frequent internal momentum-conserving scattering, we developed a generalized theory of the skin effect, taking into account separate rates of momentum-conserving and momentum-relaxing scattering for arbitrary FS geometry. For an isotropic FS, our theory encompasses several known limiting behaviours. For anisotropic FSs, we explored geometries which lead to changes in the scaling of the surface impedance.
By applying bolometric broadband microwave spectroscopy, we studied the skin effect in PdCoO2 for three different directions of electromagnetic propagation. Using symmetry-based arguments, we determined that our measurements were neither in the local nor purely viscous regime. We argued instead that the data demonstrate a novel, predominantly ballistic effect as a result of the faceted FS.
Event Location:
BRIM 311
Event Time:
Tuesday, June 7, 2022 | 12:30 pm - 3:00 pm
Event Location:
room 203 of the graduate student centre
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2022-06-07T12:30:00
2022-06-07T15:00:00
Red giant stars as standard candles
Event Information:
This thesis introduces two new extragalactic distance determination
methods; the first uses the median magnitude of carbon-rich asymptotic
giant branch stars (CS), while the second uses the combined luminosity
function of the red-giant branch (RGB) and oxygen-rich asymptotic giant
branch stars (AGB). The sample of CS and RGB+AGB stars are selected from
near-infrared JHK_s-bands colour-magnitude diagrams (CMD), as in these
filters RGs are bright and easy to identify. Both methods use the
Magellanic Clouds as the fundamental calibrators and are tested in four
Magellanic type galaxies: NGC 6822, IC 1613, WLM and NGC 3109, these
target galaxies are all members of the Local Group.
For the CS method, the CS J-band luminosity function is fitted using a
Lorentzian distribution modified to allow the distribution to be
asymmetric. The parameters of the best-fit distribution are then used to
determine if the CS luminosity function of a given galaxy resembles that
of the Large or Small Magellanic Clouds (LMC or SMC). Based on this
resemblance, either the LMC or SMC is used as the calibrator and the
distance to the given galaxy is estimated using the median J magnitude
of the CS samples.
The second method uses an un-binned maximum likelihood estimator to find
the distance modulus that minimizes the difference between the
luminosity function of the RGB+AGB stars in a target galaxy and the
model distribution given by the luminosity function of the RGB+AGB stars
in the LMC and SMC. The model luminosity function can be given by the
LMC and SMC individually or as a linear combination (LC) of both. The LC
includes a "shape" parameters that quantifies how "LMC-" or "SMC-like" a
target galaxy is. Except for the NGC 3109 K_s luminosity function, the
LC "shape" results agree with the CS "LMC/SMC-like" classification.
Estimations of the distances through the tip of the RGB method are also
included to test the performance and compare the results when the three
different methods are applied to the same data set. The distance
estimates for the target galaxies from the three different methods
presented in this thesis are in good agreement within the error bars.
Event Location:
room 203 of the graduate student centre
Event Time:
Tuesday, May 24, 2022 | 2:00 pm - 5:00 pm
Event Location:
Henn 318 & https://cern.zoom.us/j/63497174616?pwd=UDNRN1JLSUNVemd0SnJhalVuYlcvQT09
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2022-05-24T14:00:00
2022-05-24T17:00:00
PhD defense Robin Hayes
Event Information:
The Standard Model (SM) is the governing theory of particle physics. Although its predictions are in excellent agreement with experimental observations, it does not provide a full picture of the physical universe. The Higgs boson is the SM's most recently-discovered particle and a crucial ingredient of the theory. Measuring any deviation between its observed and expected properties could pave the way toward a more complete theory.
This thesis describes a study of the Higgs boson using proton-proton collisions at the Large Hadron Collider. The collision data was collected between 2015 and 2018 by the ATLAS detector. This research identifies the Higgs boson using its decay to two W bosons, H->WW*. It examines Higgs boson production via gluon fusion and vector boson fusion, making it sensitive to the Higgs boson's interactions with both heavy quarks and vector bosons. It results in the first observation of vector boson fusion production followed by H->WW* decay and measurements of Higgs boson cross-sections inclusively and across various kinematic regions. The measurements are made with unprecedented precision in this decay channel, and are compatible with the SM prediction.
Event Location:
Henn 318 & https://cern.zoom.us/j/63497174616?pwd=UDNRN1JLSUNVemd0SnJhalVuYlcvQT09
Event Time:
Thursday, May 12, 2022 | 2:00 pm - 5:00 pm
Event Location:
Zoom: https://ubc.zoom.us/j/61164285940?pwd=QWFiV0JZT3VwTXVacnc4SEh5UVErdz09. Password: 053582.
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2022-05-12T14:00:00
2022-05-12T17:00:00
Apocalyptic quantum gravity
Event Information:
Black holes are regions of spacetime from which nothing can escape. This is already strange, but more puzzling is the fact that, over time, quantum mechanics causes black holes to leak energy and disappear. What happens to the objects that fell inside? The unitarity of quantum mechanics suggests one answer, and computations in semiclassical gravity another. To determine which is correct, we need to understand how quantum and gravitational effects interact.
This thesis develops techniques to peer inside black holes and track the flow of information. We exploit and extend the AdS/CFT correspondence, an approach to quantum gravity in which data at the boundary of spacetime is related to geometric structure in the bulk. The basic strategy is to make highly symmetric incisions along the boundary, dual to surfaces in the bulk called end-of-the-world (ETW) branes, since they literally terminate spacetime. In some cases, these branes can reach inside the black hole interior and teach us about what happens there. We also carefully study the quantum mechanics of ETW branes and find that the scalpels needed to make the incisions are finely tuned, hidden like the needle in a proverbial haystack of consistent microscopic boundary conditions.
Event Location:
Zoom: https://ubc.zoom.us/j/61164285940?pwd=QWFiV0JZT3VwTXVacnc4SEh5UVErdz09. Password: 053582.
Event Time:
Friday, May 6, 2022 | 12:00 pm - 2:30 pm
Event Location:
University of Winnipeg or Watch live! (See webinar registration link below)
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2022-05-06T12:00:00
2022-05-06T14:30:00
2022 Western Regional Three-Minute Thesis Competition: PHAS Graduate student Emilie Carpentier represents UBC
Event Information:
2022 Western Regional Three-Minute Thesis Competition is being hosted by the University of Winnipeg on May 6, 2022 from 2:00-4:30 pm (CST)
Seventeen graduate schools across western Canada are sending their local Three-Minute Thesis Competition winners to compete in the 2022 Western Regional Competition!
Originally developed by the University of Queensland, the Three-Minute Thesis is an annual research communication competition challenging graduate students to communicate their scholarly research and its significance in three minutes or less.
Join us in supporting Emilie and other finalists in the Regionals! Register for the livestream webinar this Friday, May 6th from 2:00-4:30pm CST (12:00-2:30pm PDT).
See the full 2022 3MT competition schedule here: Schedule 2022 | Three Minute Thesis (3MT) (ubc.ca)
Event Location:
University of Winnipeg or Watch live! (See webinar registration link below)
Event Time:
Thursday, April 28, 2022 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201 (and via zoom)
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2022-04-28T16:00:00
2022-04-28T17:00:00
EVENT CANCELLED: Equity and Inclusion Survey Town Hall
Event Information:
-- This event is cancelled for Thursday, April 28th --
In 2020, the Physics and Astronomy (PHAS) Equity and Inclusion Committee at UBC conducted a Departmental Climate Survey to identify current issues in the department and to provide recommendations to create a more welcoming workplace. The responses to this survey strengthen our understanding of the demographics and diversity of experiences in PHAS, which are key to determining how we can make the Department more inclusive. In the following years, various new projects have been started to address the issues highlighted by the survey results.
This colloquium will feature a short presentation of the survey results from Dept. Head Colin Gay and Deputy Dept. Head Ingrid Stairs, followed by a town hall-style discussion focused on the future directions our department will be making in order to promote a more inclusive environment.
The survey report is now available on the PHAS internal site (link here), and we encourage everyone to review the findings and recommendations.
In the coming weeks and months, the Equity and Inclusion Committee will also be hosting a series of additional discussion forums with various groups (grads, staff, etc.) in the department. These will provide opportunities, in addition to the town hall event this Thursday, for others to share their thoughts and experiences in smaller settings.
Event Location:
Hennings 201 (and via zoom)
Event Time:
Thursday, April 28, 2022 | 10:00 am - 12:00 pm
Event Location:
zoom : https://ubc.zoom.us/j/63707269353?pwd=YnZKMzd4K0NKWGtESUJSS2JXNGU5Zz09 Passcode: 486279
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2022-04-28T10:00:00
2022-04-28T12:00:00
PhD Defense: Electron-phonon coupling in the time domain: TR-ARPES studies by a cavity-based XUV laser
Event Information:
Quantum materials manifest exciting macroscopic electronic properties that emerge from microscopic electron interactions -- such as those between the electron and the lattice. Extensive research effort has been dedicated to understanding the physics of these materials; among these, angle-resolved photoemission spectroscopy (ARPES) has the unique capability of taking ``photos" of the electronic band structure. This band structure is a fingerprint of the electronic properties of each material and encodes microscopic electron interactions. By combining ARPES with an ultrafast laser source, we can turn “photos" of the electronic band structure into “movies" and study how electrons respond to external perturbation, such as a short pulse of light. This technique is called time-resolved ARPES (TR-ARPES).
In particular, the interaction between electrons and the lattice is responsible for a host of quantum phenomena, including the well-known superconductivity. In this thesis, we study graphite using a new extreme ultraviolet laser. We observe that electrons in graphite preferentially interact with a specific mode of lattice vibration. From the electron dynamics, we quantitatively extract the interaction strength between the electrons and this lattice vibration with a simple, transparent model and find that it agrees well with first-principles calculations. With this demonstration, we show that the maturation of laser technology has significantly advanced the capabilities of TR-ARPES, which readily serves to further our understanding of quantum materials.
Event Location:
zoom : https://ubc.zoom.us/j/63707269353?pwd=YnZKMzd4K0NKWGtESUJSS2JXNGU5Zz09 Passcode: 486279
Event Time:
Wednesday, April 13, 2022 | 12:00 pm - 1:00 pm
Event Location:
Room 318 - Hennings Building
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2022-04-13T12:00:00
2022-04-13T13:00:00
“Causality constraints on corrections to Einstein gravity"
Event Information:
In this talk I will describe constraints from causality and unitarity on 2→2 graviton scattering in four-dimensional weakly-coupled effective field theories. Together, causality and unitarity imply dispersion relations that connect low-energy observables to high-energy data. Using such dispersion relations, I will explain how to derive two-sided bounds on gravitational Wilson coefficients in terms of the mass M of new higher-spin states. Such bounds have many implications. For instance, they show that gravitational interactions must shut off uniformly in the limit G→0, prove the scaling with M expected from dimensional analysis, and impose a species bound on theories with a large number of matter fields. In addition they demonstrate the gravity must be weakly coupled at all scales below Planck. Time permitting, I will also comment on the experimental implications of the bounds.
Event Location:
Room 318 - Hennings Building
Event Time:
Tuesday, April 12, 2022 | 11:00 am - 1:00 pm
Event Location:
Hennings 318 or https://ubc.zoom.us/j/64946668591?pwd=a2xCZEI5Uk5vTVQzR2g0cHlZMlU2QT09
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2022-04-12T11:00:00
2022-04-12T13:00:00
Searches for Higgs pair production in the 4 b-jet final state with the ATLAS Detector at the LHC
Event Information:
The Standard Model of Particle Physics is the prevailing theory for describing the interactions of all observed fundamental particles and three of the four known fundamental interactions. However, despite its profound success, the Standard Model fails to explain some observations, such as dark matter and matter-antimatter asymmetry. Additionally, incorporating Einstein’s theory of general relativity has proven difficult. Many proposed extensions to the Standard Model resolve these and other open questions through modifications of the Higgs potential or through new particles which interact with the Higgs boson.
This dissertation presents a search for Higgs boson pair production in the signal rich 4 b-jet final state using 136 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider. This search consists of two separate analysis strategies targeting resonant and non-resonant Higgs boson pair production. The resonant search provides sensitivity to the resonant decay of new massive scalar and spin-2 particles predicted by many extensions to the Standard Model. The non-resonant search is directly sensitive to the Higgs potential via the Higgs boson self-coupling and the four-point Higgs to vector boson couplings.
No significant excesses are observed in the resonant search. World-leading upper limits are placed on cross-sections for new massive scalar and spin-2 resonances. These limits exclude a wide range of Kaluza-Klein graviton models and provide leading sensitivity amongst the ATLAS Higgs pair searches for masses above 600 GeV.
In the non-resonant search, no evidence for Standard Model Higgs production is found in the non-resonant search. An observed (expected) upper limit of 5.49 (6.73) times the Standard Model cross-section is set. Limits are placed on the Higgs self-coupling and the four-point Higgs to vector boson couplings, with observed (expected) constraints of κλ ∈[-4.7, 12.2] ([-4.1, 10.4]) and κ2V ∈ [0.02, 2.05] ([0.01, 2.06]), respectively. These results are then reinterpreted in the more general Higgs Effective Field Theory and Standard Model Effective Field Theory frameworks. The resulting limits are found to provide leading sensitivity within the HH channel for several parameters and benchmark signals.
Event Location:
Hennings 318 or https://ubc.zoom.us/j/64946668591?pwd=a2xCZEI5Uk5vTVQzR2g0cHlZMlU2QT09
Event Time:
Tuesday, April 12, 2022 | 10:00 am - 1:00 pm
Event Location:
https://ubc.zoom.us/j/68698080843?pwd=cEJ3S283RENpSWROR0I3a0lzbkY2dz09
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2022-04-12T10:00:00
2022-04-12T13:00:00
PhD defense Xunyu Liang - "Dark Matter in Form of Axion Quark Nuggets: Formation, Detections, and Evidence"
Event Information:
Over two decades of development since its establishment, the axion quark nugget (AQN) is one of the best-studied macroscopic dark matter candidate with characteristic mass and size of order grams and 0.1 μm respectively. It naturally explains the observed similarity between the dark and visible density in the Universe, i.e. ΩDM ∼ Ωvis, with no fitting parameters, while many conventional dark matter candidates such as the weakly interacting massive particles (WIMPs) and axion do not. This dissertation presents numerous latest key progress of the AQN model, including its formation mechanism in early Universe, and new search strategies such as axion detection and global network. Lastly, we discuss potential evidence of AQNs from the anomalous events recently observed by the Telescope Array(TA) and ANtarctic Impulse Transient Antenna (ANITA) experiments.
Event Location:
https://ubc.zoom.us/j/68698080843?pwd=cEJ3S283RENpSWROR0I3a0lzbkY2dz09
Event Time:
Thursday, April 7, 2022 | 4:00 pm - 5:00 pm
Event Location:
Connect via zoom
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2022-04-07T16:00:00
2022-04-07T17:00:00
Verification of Quantum Computation
Event Information:
Abstract: Quantum computers promise to efficiently solve not only
problems believed to be intractable for classical computers, but also
problems for which verifying the solution is also considered
intractable. This raises the question of how one can check whether
quantum computers are indeed producing correct results. This task,
known as quantum verification, has been highlighted as a significant
challenge on the road to scalable quantum computing technology. We
review the existing approaches and compare them in terms of structure,
complexity and required resources. We also comment on the use of
cryptographic techniques which, for many of the presented protocols,
has proven extremely useful in performing verification. Finally, we
discuss issues related to fault tolerance, experimental
implementations and the outlook for this field of research.
Bio: Elham Kashefi is Professor of Quantum Computing at the School of
Informatics, University of Edinburgh, and Directeur de recherche au
CNRS at LIP6 Sorbonne Universite. She co-founded the fields of quantum
cloud computing and quantum computing verification, and has pioneered
a trans-disciplinary interaction of hybrid quantum-classical solutions
from theoretical investigation all the way to actual experimental and
industrial commercialisation (Co-Founder of VeriQloud Ltd). She has
been awarded several UK, EU and US grants and fellowships for her
works in developing applications for quantum computing and
communication and was awarded the French 2021 les Margaret
Intrapreneur award. She is the senior science team leader of the
quantum computing and simulation hub in the UK and member of the
executive team of the EU quantum internet alliance. She has been
recently elected as the executive director of the Quantum Algorithm
Institute in Canada.
Event Location:
Connect via zoom
Event Time:
Thursday, April 7, 2022 | 10:00 am - 11:00 am
Event Location:
Zoom link in description
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2022-04-07T10:00:00
2022-04-07T11:00:00
Dr. Valentino R. Cooper: Exploring the Chemical Landscape of High Entropy Oxides
Event Information:
https://ubc.zoom.us/j/68470173961?pwd=RTZEak9Pd01WajVOZHN5SW5YZHcyQT09
Meeting ID: 684 7017 3961
Passcode: 113399
Speaker: Dr. Valentino R. Cooper
Title: Exploring the Chemical Landscape of High Entropy Oxides
Abstract: High entropy, multi-component metal alloys (HEA), have superior mechanical properties and high radiation tolerances; which are, in part, driven by configurational entropy. Recently, an oxide analogue comprised of MgO, CoO, NiO, CuO and ZnO was synthesized; exhibiting a truly entropy-stabilized, reversible phase transition from a multiphase material to a single rock salt-ordered phase above 850-900°C. This entropy-driven stabilization may engender many unique properties, such as high melting temperatures, radiation resistance and other anomalous responses. Here, we discuss a design strategy for the prediction of synthesizable disordered oxides. Our effort employs first principles studies of 2-component oxides to develop design rules based on the relationship between pairwise enthalpies of formation, DH, and configurational entropy of the disordered material. A similar chemical identity-to-DH map was previously explored using the class of high entropy alloys, where the stability of multicomponent metal alloys was correlated to the enthalpy of mixing of binary and ternary compounds.
In this presentation, I will focus on our recent efforts to employ this local enthalpy map as an effective strategy for the discovery of new classes of entropy stabilized oxides. In particular, we are able to use our first principles calculations with Monte Carlo simulations in order to build chemical bonding maps to study the local environment preferences that determine whether a material will phase segregate, to form a single phase with clustered regions, or form a disordered solid solution. This enables us to identify compounds that may be synthesizable or could be stabilized by entropy – thus allowing for more reliable materials discovery and design.
This work was supported by the U.S. D.O.E., Office of Science, BES, MSED (first principles calculations) and the LDRD Program of ORNL (simulations), managed by UT-Battelle, LLC, for the U. S. DOE using resources at NERSC and OLCF.
Bio:
Dr. Valentino R. Cooper is the Section Head for the Materials Theory Modeling and Simulations section in the Materials Sciences and Technology Division at Oak Ridge National Laboratory. He received his Ph. D from the Chemistry Department at the University of Pennsylvania in 2005. Prior to joining ORNL in 2008, he was a post-doctoral associate in the Physics Department of Rutgers University. His research focuses on electronic structure methods for understanding dispersion interactions and in the prediction of functional materials including piezoelectrics and ferroelectrics. Dr. Cooper is a 2013 recipient of the Department of Energy Early Career award.
Event Location:
Zoom link in description
Event Time:
Thursday, April 7, 2022 | 9:00 am - 12:00 pm
Event Location:
https://ubc.zoom.us/j/67782233773?pwd=aU5JNkV0K2g3Y3EvcTV1c09JcmQvUT
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2022-04-07T09:00:00
2022-04-07T12:00:00
PhD defense Daniel Bruns
Event Information:
Atomistic modeling of phonon-mediated heat transport in single-walled carbon nanotubes (CNTs) dates to the year 2000, when Berber, Kwon and Tománek, by means of molecular dynamics (MD) simulations, predicted a thermal conductivity of up to 6600 W/mK, suggesting extremely efficient heat transfer in these one-dimensional carbon materials. Since then, many modeling efforts have been undertaken, but some aspects of CNT phonon transport, in particular its domain-size dependence, remain controversial.
In this thesis, we first revisit the two main-stream approaches of modeling phonon-mediated heat transport in CNTs: quantum mechanical calculations in the framework of the Peierls-Boltzmann transport theory
(PBTT) and classical MD simulations. Looking at domain size and temperature dependencies of CNT heat transport, we evaluate the strength and limitations of both modeling approaches. In regard to domain size effects, our numerical results offer new insights into the problem of weakly damped acoustic phonons with ever-increasing mean free path.
In the framework of the PBTT, we then focus specifically on the spectrum of three-phonon scattering channels in CNTs. From lowest-order anharmonic perturbation theory, we derive exact asymptotic scaling relations of phonon-phonon scattering rates in the limit of low phonon energies. By adopting a relaxation time approximation of phonon transport, we are then able to unambiguously clarify tube-length effects of CNT heat transport, which we further demonstrate to depend very sensitively on tensile lattice strain. With respect to earlier numerical PBTT calculations on CNTs, we show that a clear line can be drawn between physical and unphysical results.
Event Location:
https://ubc.zoom.us/j/67782233773?pwd=aU5JNkV0K2g3Y3EvcTV1c09JcmQvUT
Event Time:
Monday, April 4, 2022 | 3:00 pm - 4:00 pm
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2022-04-04T15:00:00
2022-04-04T16:00:00
Resonant Chains versus More "Typical" Exoplanetary Systems
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In the field of exoplanets, the most extreme systems often capture our attention, and they teach us interesting lessons. However, statistical modeling of survey data is important too, as it identifies what are the more common processes involved in planet formation.
For some systems, three or more planets are linked by mean-motion resonances, forming a "resonant chain." The observable transit timing variations allow masses and orbital parameters to be measured to excellent precision. Their current orbits, including orbital phase information, teaches us about the interactions of planets with disks. The spreading of resonant chains from exact resonance implicates tidal dissipation in the planets.
Resonant configurations are rare in the transit survey data though, and we report methods for characterizing the more "typical" close-in exoplanetary systems. After we had gotten used to extreme orbits among exoplanetary gas giants, we found surprisingly small mutual inclinations and eccentricities of the very common close-in systems of super-earths and sub-neptunes. Despite being on sub-AU scales, these architectural properties are very similar to the Solar System. Confronting planet formation theories with all this fossil evidence is an ongoing project.
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Event Time:
Thursday, March 31, 2022 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201 (or via zoom)
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2022-03-31T16:00:00
2022-03-31T17:00:00
Heavy Water: a Canadian (and BC) story
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In the mid-90s I found myself, as a member of the Sudbury Neutrino Observatory (SNO) collaboration, a recipient and custodian of 1000 tonnes of “spare” heavy water, book value $300M. How such a rare asset came to exist in Canada is a complex story of nuclear physics, geopolitics, world war, flight and exile. For a while the tale runs along the fringe of the Manhattan Project saga, but it largely concerns reactors rather than bombs. The story crosses continents (Norway-France-Canada) in circumstances anyone familiar with the news at this moment can readily imagine. Part of it is however set far away from conflict and from well-known centres of nuclear research, in the south-eastern corner of British Columbia.
Event Location:
Hennings 201 (or via zoom)
Event Time:
Thursday, March 31, 2022 | 10:00 am - 11:00 am
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2022-03-31T10:00:00
2022-03-31T11:00:00
Kwabena Bediako: New Twists on Chemistry and Physics in Moiré Superlattices
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https://ubc.zoom.us/j/68470173961?pwd=RTZEak9Pd01WajVOZHN5SW5YZHcyQT09
Meeting ID: 684 7017 3961
Passcode: 113399
Speaker: Kwabena Bediako, Assistant Professor at UC Berkeley, Dept. of Chemistry
Title: New twists on chemistry and physics in moiré superlattices
Abstract: Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. Likewise, these highly tunable materials are useful platforms for exploring fundamental questions of interfacial chemical/electrochemical reactivity. One exciting and relatively recent example is the formation of moiré superlattices from azimuthally misoriented (twisted) layers. These moiré superlattices result in flat bands that lead to an array of correlated electronic phases. However, in these systems, complex strain relaxation can also strongly influence the fragile electronic states of the material. Precise characterization of these materials and their properties is therefore critical to the understanding of the behavior of these novel moiré materials (and 2D heterostructures in general). In this talk, I will discuss how spontaneous mechanical deformations (atomic reconstruction) and resultant intralayer strain fields at moiré superlattices of twisted bilayer graphene have been quantitatively imaged using 4D-STEM Bragg interferometry. I will also discuss the impact of these mechanical deformations to the electronic band structure of these moiré superlattices and the correlated electronic phases they host. The talk will then explore how various degrees of freedom that are unique to 2D materials may be used to tailor interfacial chemistry at well-defined mesoscopic electrodes and the outlook for new paradigms of functional materials for energy conversion and low-power electronic devices.
Bio: Kwabena was born in Ghana, West Africa. He moved to the US in 2004 for his undergraduate studies in Chemistry at Calvin College, MI, graduating with honors in 2008. After a year working at UOP Honeywell in IL where he researched new catalysts for the petrochemical and gas processing industries, he traveled from the Midwest to the East Coast to begin his graduate studies in Inorganic Chemistry with Prof. Daniel Nocera at MIT (and later Harvard University). His graduate research focused on structural and mechanistic studies of water splitting electrocatalysis at cobalt and nickel compounds. After receiving his Ph.D. in 2015 from Harvard University, Kwabena began postdoctoral work in Prof. Philip Kim's group in the Department of Physics at Harvard, where he studied ion intercalation and quantum transport in 2D van der Waals heterostructures. In July 2018, Kwabena joined the faculty of the UC Berkeley Department of Chemistry. Awards received include: AFOSR Young Investigator award, ONR Young investigator award, DOE Early Career Award, Gordon and Betty Moore Materials Synthesis Fellow, and CIFAR–Azrieli Global Scholar.
Event Location:
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Event Time:
Monday, March 28, 2022 | 3:00 pm - 4:00 pm
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2022-03-28T15:00:00
2022-03-28T16:00:00
Localizations and Lenses: Looking towards Cosmology with CHIME/FRB
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The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has discovered thousands of fast radio bursts (FRBs). The extremely high all-sky rate of FRBs implies that they have the potential to become powerful cosmological probes. Unlocking this potential requires localizing a large sample of FRBs to their host galaxies. Until now, precise localization within the host galaxy has only been accomplished in follow-up observations of repeating sources. Here, we demonstrate the localization of FRB 20210603A using very long baseline interferometry (VLBI) at its time of first detection. This is an important milestone towards realizing CHIME/FRB Outriggers: a widefield, blind VLBI survey dedicated to localizing a large sample of FRBs. Finally, I will discuss a novel time-domain search for gravitationally-lensed FRBs, as a first application of FRBs to cosmological measurements. Our results imply that the cosmological dark matter at redshift z~0.1 cannot be composed exclusively of ~10^-3 solar mass compact objects.
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