Events List for the Academic Year
Event Time:
Thursday, September 5, 2019 | 2:00 pm - 3:00 pm
Event Location:
BRIMACOMBE 311
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2019-09-05T14:00:00
2019-09-05T15:00:00
CM Seminar: Can machine learning outperform a condensed-matter physicist?
Event Information:
Abstract: Machine learning models are usually trained by a large number of observations (big data) to make predictions through the evaluation of complex mathematical objects. However, in many applications in science, particularly in quantum condensed-matter physics, obtaining observables is expensive so information is limited. In the present work, we consider the limit of ‘small data’. Usually, ‘big data’ are for machines and ‘small data’ are for humans, i.e. humans can infer physical laws given a few isolated observations, while machines require a huge array of information for accurate predictions. Here, we explore the possibility of machine learning that could build physical models based on very restricted information. In this talk, I will show how to build such models using Bayesian machine learning and how to apply such models to two types of problems: (1) the inverse problems aiming to infer Hamiltonians from the dynamical observables; (2) the extrapolation problems aiming to make predictions in parts of the Hamiltonian parameter space, where neither experiment nor theory are feasible.
Event Location:
BRIMACOMBE 311
Event Time:
Wednesday, September 4, 2019 | 2:00 pm - 3:00 pm
Event Location:
BRIM 311
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2019-09-04T14:00:00
2019-09-04T15:00:00
CM Seminar: Imaging Viscous Flow of the Dirac Fluid in Graphene Using a Quantum Spin Magnetometer
Event Information:
Abstract: Hydrodynamic electron fluid has emerged as a paradigm of strongly-correlated electronic transport. In particular, the electron-hole plasma in charge-neutral graphene is predicted to realize a quantum critical fluid whose transport features a universal hydrodynamic description relevant to strongly-correlated electrons in high-Tc superconductors. This “Dirac fluid” is expected to have a shear viscosity close to a minimum bound, with an inter-particle scattering rate saturating at the Planckian time ħ/(kBT). While electrical transport measurements at finite carrier density are consistent with hydrodynamic electron flow in graphene, a clear demonstration of viscous behavior at the charge neutrality point (CNP) remains elusive. In this work, we directly image viscous Poiseuille flow of the Dirac fluid at room temperature via measurement of the associated stray magnetic field. Nanoscale magnetic imaging, performed using quantum spin magnetometers realized with nitrogen vacancy (NV) centers in diamond, reveals a parabolic Poiseuille profile for electron flow in a graphene channel near the CNP, establishing the viscous transport of the Dirac fluid. Via combined imaging-transport measurements, we obtain viscosity and scattering rates, and observe that these quantities are comparable to the universal values expected at quantum criticality. This finding establishes a nearly-ideal electron fluid in charge-neutral graphene at room temperature.
Biosketch: Mark Jen-Hao Ku is a postdoctoral associate working at Harvard University, with joint appointments at the Harvard-Smithsonian Center for Astrophysics and the University of Maryland Quantum Technology Center. He attended the University of British Columbia, where he received B.Sc in Mathematics and Physics and M.Sc in Physics. He obtained Ph.D in Physics at the Massachusetts Institute of Technology, where he used ultracold matter formed by atomic gases for quantum simulation of high-temperature superfluids. At Harvard, he uses nitrogen-vacancy centers in diamond as a quantum sensor to probe condensed matter phenomena in the nanoscale, including transport of correlated electrons in graphene and spin-waves in ferromagnets.
Event Location:
BRIM 311
Event Time:
Thursday, August 29, 2019 | 12:30 pm - 2:30 pm
Event Location:
Room 309, Hennings Bldg.
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2019-08-29T12:30:00
2019-08-29T14:30:00
Final PhD Oral Examination (Thesis Title: “Search for New High-Mass Phenomena in Events with Two Muons using the ATLAS Detector at the Large Hadron Collider”)
Event Information:
Abstract:
Although elementary particles and their interactions are extremely well modeled by the Standard Model of particle physics, some experimental measurements cannot be explained entirely by this theory. Many extensions of the Standard Model predict the existence of new phenomena at high energies. In particular, new resonance models and contact interaction models leading to dimuon final states are numerous.
This dissertation presents a search for new high-mass phenomena in events with two muons using the ATLAS detector at the Large Hadron Collider. The search results are found to be consistent with the Standard Model background prediction. Interpretations both in the context of resonant and non-resonant new physics models are carried out. In particular, lower limits on the mass of hypothetical Z' bosons are set between 4.0 TeV for the Z'SSM model and 3.3 TeV for the Z'ψ model, and lower limits on the contact interaction energy scale Λ are set between 18 TeV and 30 TeV, depending on the chiral structure of the contact interaction.
In addition to data analysis at the energy frontier, the performance of muon reconstruction and identification within the ATLAS experiment is detailed. More precisely, calculations of muon trigger scale factors for high-pΤ muons using events containing a leptonically decaying W boson and jets are presented. A new muon identification working point is also investigated.
Finally, as the ATLAS experiment enters its second long shutdown, the first layer of the endcap regions of the muon spectrometer will be replaced with the New Small Wheels (NSWs) in order to improve both the triggering and tracking capabilities of the ATLAS detector. One of the two main technologies used in the NSW is small-strip Thin Gap Chambers (sTGCs). Work carried out with the sTGC collaboration, which aims to characterize and integrate the NSW into the ATLAS detector in the coming years, is described. Particularly, results of various test beam campaigns carried out at Fermilab and at CERN are presented. Position resolution measurements of less than 50μm are obtained. Measurements using the latest electronics readout chain of the sTGC detectors under realistic conditions are also presented.
Event Location:
Room 309, Hennings Bldg.
Event Time:
Thursday, August 29, 2019 | 9:30 am - 11:30 am
Event Location:
AMPEL 488
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2019-08-29T09:30:00
2019-08-29T11:30:00
Departmental Oral Examination (Thesis Title: "Spin-Orbit Coupling in Iridates")
Event Information:
Abstract:
Transition-metal oxides (TMOs) are a widely studied class of materials with fascinating electronic properties and a great potential for applications. Sr2IrO4 is such a TMO, with a partially filled 5d t2g shell. Given the reduced Coulomb interactions in these extended 5d orbitals, the insulating state in Sr2IrO4 is quite unexpected. To explain this state, it has been proposed that spin-orbit coupling (SOC) entangles the t2g states into a filled jeff = 3/2 state and a half-filled jeff = 1/2 state, in which a smaller Coulomb interaction can open a gap. This new scheme extends filling and bandwidth, the canonical control parameters for metal-insulator transitions, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive such a transition.
In order to address this question, we have studied the behaviour of Sr2IrO4 when substituting Ir for Ru or Rh. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only SOC can satisfactorily explain the the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr2IrO4.
It has furthermore been proposed that the jeff = 1/2 model in Sr2IrO4 is an analogue to the superconducting cuprates, realizing a two-dimensional pseudo-spin 1/2 model. We test this directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr2IrO4 can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on direct comparisons to the cuprate superconductors.
We thus find that the insulating ground state in Sr2IrO4 is mediated by SOC, however, SOC is not strong enough to fully disentangle the jeff = 1/2 state, requiring that Sr2IrO4 is described as a multi-orbital relativistic Mott insulator.
Event Location:
AMPEL 488
Event Time:
Tuesday, August 27, 2019 | 12:30 pm - 2:30 pm
Event Location:
Room 203, Graduate Student Centre (6371 Crescent Road)
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2019-08-27T12:30:00
2019-08-27T14:30:00
Final PhD Oral Examination (Thesis Title: “Search for the Production of Higgs Bosons in Association with Top Quarks and Decaying into Bottom Quark Pairs with the ATLAS Detector”)
Event Information:
Abstract:
The Standard Model of particle physics (SM) describes mass generation of fundamental particles via the Brout-Englert-Higgs mechanism. It predicts Yukawa interactions between the Higgs boson and fermions, with interaction strengths proportional to the fermion masses. The largest Yukawa coupling is that of the top quark, and its value has implications in particle physics and cosmology. As the SM is not a complete theory of nature, detailed measurements of its predictions are a mandatory step towards improving the understanding of nature.
This dissertation presents a search for Higgs boson production in association with a top quark pair, a process directly sensitive to the top quark Yukawa coupling. The search uses 36.1 fb−1 of data at √s = 13 TeV, collected with the ATLAS detector at the Large Hadron Collider (LHC) in 2015 and 2016. It is designed for Higgs boson decays to bottom quarks, and decays of the top quark pair resulting in final states with one or two electrons or muons. The discrimination between the signal Higgs boson production process and background processes, dominated by the production of top quark pairs, is performed with multivariate analysis techniques. The matrix element method is used and optimized for this search. Possible machine learning extensions of the method are investigated to help overcome its large computational demand. The obtained ratio of the measured cross-section for the signal Higgs boson production process to the prediction of the SM is μ = 0.84 (+0.64, -0.61). The expected sensitivity of an extension of the search, using 139.0 fb−1 of data collected between 2015 and 2018, is 3.3σ. Data collected between 2016 and 2018 is also used in a measurement of the ATLAS muon trigger system efficiency.
A statistical combination of searches for Higgs bosons produced in association with top quark pairs is performed, including the search for Higgs boson decays to bottom quarks and additional final states. The combination results in the observation of this Higgs boson production process with an observed significance of 5.4σ, compared to an expected sensitivity of 5.5σ. It experimentally establishes top quark Yukawa interactions in the SM.
Event Location:
Room 203, Graduate Student Centre (6371 Crescent Road)
Event Time:
Monday, August 26, 2019 | 11:00 am - 12:00 pm
Event Location:
309
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2019-08-26T11:00:00
2019-08-26T12:00:00
Topological qauntum field theory in 3+1D and the emergence of quantum gravity.
Event Information:
Topological qauntum field theory(TQFT) is a very powerful theoretical tool
to study topological phases and phase transitions. In 2 + 1D, it is well
known that the Chern-Simons theory captures all the universal topological
data of topological phases, e.g., quasi-particle braiding statistics,
chiral central charge and even provides us a deep insight for the nature of
topological phase transitions. Recently, topological phases of quantum
matter are also intensively studied in 3 + 1D and it has been shown that
loop like excitation obeys the so-called three-loop-braiding statistics. In
this talk, I will discuss a TQFT framework to understand the quantum
statistics of loop like excitation in 3 + 1D. Most surprisingly, this new
class of 3+1D TQFT even provides us a new route towards a quantum theory of
gravity.
Event Location:
309
Event Time:
Thursday, August 15, 2019 | 11:00 am - 12:00 pm
Event Location:
Ampel 311
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2019-08-15T11:00:00
2019-08-15T12:00:00
STS Observations of the Spin-Polarized Edge State in Zigzag Graphene Nanoribbons
Event Information:
A peculiar electronic state in graphene nanoribbons (GNRs) which is localized only at the zigzag
type edge, the so called “edge state”, was proposed in 1996 by M. Fujita and his coworkers [1]. In
2005−2006, the existence of the edge state was verified by scanning tunneling microscopy and
spectroscopy (STM/S) measurements at zigzag step edges on graphite surfaces by two groups
including ourselves [2,3]. In very narrow GNRs of widths less than ∼20 nm with zigzag edges on
both sides (zGNRs), the edge atoms are predicted to possess high spin polarizations (10−20%) even
under weak electron interactions [1], and the edge magnetic moments on opposite sites will align
ferromagnetically (FM) or antiferromagnetically (AF) depending on the ribbon width [4]. Moreover,
the edge state in FM-zGNRs will be gapless (metallic), whereas that in the AF-zGNRs will have a
band gap of the order of 100−400 meV (semiconducting) again depending on the width [4]. Several
experimental groups have reported band gap features, which could be caused by the magnetic edge
state, in zGNRs prepared by various ways [5]. However, the results are not necessarily consistent
each other quantitatively, and the detailed atomic structures of the edges are not clear.
In this talk, I will show preliminary results of our new low temperature STM/S measurements on
the edge magnetism of zGNR samples of 6−20 nm widths which are synthesized in between two
adjacent hexagonal nanopits anisotropically etched by hydrogen plasma on graphite surfaces [6]. The
6 nm wide ribbon clearly shows two peaks separated by 50−60 meV in the local density of states on
the two opposite edges (gap-like structures), and the separation decreases with increasing width.
Details of the sample preparation by anisotropic H-plasma etching and the STM/S measurements will
be discussed.
This work was carried out as a collaboration with Tomohiro Matsui and André E. B. Amend.
* hiroshi@phys.s.u-tokyo.ac.jp
[1] M. Fujita et al., J. Phys. Soc. Jpn. 65, 1920 (1996); K. Nakada et al., Phys. Rev. B 54, 17954
(1996).
[2] Y. Niimi et al., Appl. Surf. Sci. 241, 43 (2005); Y. Niimi et al., Phys. Rev. B 73, 085421 (2006).
[3] Y. Kobayashi et al., Phys. Rev. B 71, 193406 (2005).
[4] Young-Woo Son et al., Phys. Rev. Lett. 97, 216803 (2006).
[5] C. Tao et al., Nature Physics 7, 616 (2011); M. Ziatdinov et al., Phys. Rev. B 87, 115427 (2013);
Y.Y. Li et al., Nature Comm. 5, 4311 (2014); G. Z. Magda et al., Nature 514, 608 (2014).
[6] T. Matsui et al., to appear; A. E. B. Amend et al., e-J. Surf. Sci. and Nanotech., 16, 72 (2018).
Event Location:
Ampel 311
Event Time:
Thursday, August 15, 2019 | 8:30 am - 10:30 am
Event Location:
Room 318, Hennings Bldg
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2019-08-15T08:30:00
2019-08-15T10:30:00
Departmental Oral Examination (Thesis Title: "Nuclear structure corrections in muonic atoms with statistical uncertainty quantification")
Event Information:
Abstract:
The discovery of the proton and deuteron radius puzzles from Lamb shift measurements of muonic atoms has initiated experimental efforts to probe heavier muonic systems and casts doubt on earlier analysis based on ordinary atoms. For muonic atoms, the large muon mass results in a Bohr radius about 200 times smaller with respect to their electronic counterparts, making them sensitive to nuclear structure effects. These effects dominate the uncertainty budget of the experimental analysis and diminish the attainable accuracy of charge radii determinations from Lamb shift spectroscopy. This dissertation investigates the precision of nuclear structure corrections relevant to the Lamb and hyperfine splitting in muonic deuterium to support ongoing experiments and shed light on the puzzles. Using state-of-the-art nuclear models, multivariate regression analysis and Bayesian techniques, we estimate the contribution of all relevant uncertainties for nuclear structure corrections in muonic deuterium and demonstrate that nuclear theory errors are well constrained and do not account for the deuteron radius puzzle. This uncertainty analysis was carried out using the “η-expansion” method that has also been applied to A ≥ 2 nuclei. This method relies on the expansion of a dimensionless parameter η, with η < 1, up to second order. To estimate the truncation uncertainty of this method and to improve future calculations of nuclear structure effects in other nuclei, we introduce an improved formalism based on a multipole expansion of the longitudinal and transverse response functions that contains higher order terms in η, and generalize the method to account for the cancellation of elastic terms such as the Friar moment (or third Zemach moment). This method is then adapted to address the nuclear structure corrections to the hyperfine splitting. iii The hyperfine splitting is dominated by magnetic dipole transitions that are sensitive to the effects of two-body currents. Therefore, we develop the formalism of the next-to-leading-order two-body magnetic moment contributions to the magnetic dipole. These operators are applied to A = 2,3 and A = 6 systems in anticipation of the upcoming experiments in µ 6,7Li2+ ions. We find that two-body contributions are important to reach agreement with experiment.
Event Location:
Room 318, Hennings Bldg
Event Time:
Thursday, August 8, 2019 | 10:00 am - 12:00 pm
Event Location:
UBC Life building (the old SUB), room 2201.
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2019-08-08T10:00:00
2019-08-08T12:00:00
ENPH 253 robot competition
Event Information:
Robots, plush dolls, and the smell of burning MOSFETS… it must be time for the annual ENPH 253 robot competition!
This year's contest will be held on Thursday August 8th at 10 am in the UBC Life building (the old SUB), room 2201.
Pairs of robots will face off in a head to head Avengers-themed competition to capture and return as many Infinity Stones and Avengers as possible. The rules are here.
We hope to see you there, and feel free to bring along any robot or Avengers enthusiasts in your life. Be sure to arrive 10-15 minutes early if you want a front row seat!
Event Location:
UBC Life building (the old SUB), room 2201.
Event Time:
Tuesday, August 6, 2019 | 12:30 pm - 2:30 pm
Event Location:
Room 203 of the Graduate Student Centre (6371 Crescent Road)
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2019-08-06T12:30:00
2019-08-06T14:30:00
Final PhD Oral Examination (Thesis Title: “Investigating the Dark Sector of the Universe Using Cosmological Observables”)
Event Information:
Abstract:
Although the Standard Model of particle physics has been a phenomenal success in modelling known particles and predicting new, theoretically founded particles, it is known to be incomplete. Although the Standard Model of cosmology has been a phenomenal success in modelling the evolution of the Universe, it too has open questions that remain unresolved. In this thesis, we aim to address properties of new physics models that are being developed that aim to answer these open questions. In particular, we wish to focus on and examine in detail the connection between the dark sector of the Universe and the visible sector. In examining this connection, we may use cosmological observables to place strict limits on new theories that go beyond the Standard Model.
In the first part of this thesis we will address the flow of energy from the visible sector to the hidden via a phenomenon known as freeze-in. Here, we explore the effects that early-time, ultraviolet energy transfer may have on the infrared, late-time evolution of a dark matter candidate. We use a simplified hidden-sector model to highlight the notion that operators that are typically considered early may have relevant late-time effects.
Following this, we consider the reverse energy flow, and consider how dark-sector energy injection via decays of electromagnetic radiation may affect the products of Big Bang Nucleosynthesis. In this section, we focus on arbitrary light particle (< 100 MeV) decays, and identify how direct and indirect alteration of the light element abundances can be constrained using the measured values today. Direct alteration is caused by photodissociation, while indirect effects are felt through changes in the radiation energy density.
Finally, we consider a full and rich dark sector, consisting of a non-Abelian SU(3) gauge force. This new gauge field presents itself as glueballs after a confining transition. We study the effects of this confining transition, as well as the subsequent dynamic evolution of the spectrum of glueballs produced. In the final chapter, we examine how decays to Standard Model particles via higher-dimensional, non-renormalizable operators can place stringent limits on the parameter space of this gauge force.
Event Location:
Room 203 of the Graduate Student Centre (6371 Crescent Road)
Event Time:
Monday, July 29, 2019 | 1:00 pm - 3:00 pm
Event Location:
Room 301, Hennings Bldg
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2019-07-29T13:00:00
2019-07-29T15:00:00
Departmental Oral Examination (Thesis Title: "Exploring the tumour microenvironment with non-invasive magnetic resonance imaging (MRI) techniques")
Event Information:
Abstract:
This thesis comprises development and application of several MRI techniques to improve our understanding of tumour growth, drug distribution, and drug effect using pre-clinical tumour models in mice. In the first part of the thesis, a novel high molecular weight contrast agent, HPG-GdF is introduced. This molecule is a hyperbranched polyglycerol labeled with an MRI contrast agent (Gd-DOTA) as well as a fluorescent tag. After injecting the agent into mice within an MRI scanner, contrast-agent kinetics were quantified using a two-parameter linear model and validated with quantitative immunohistochemistry via direct fluorescence imaging of HPG-GdF.
HPG-GdF was used to assess whether vascular function plays a role in how a chemotherapy (Herceptin) distributes within a tumour. Tumour vessel permeability and fractional plasma volume were quantified using the HPG-GdF and no relationship was found between vascular function and presence of drug. HPG-GdF was then applied to show that Avastin (an antiangiogenic agent) decreased vessel permeability in tumours. Using histological methods, a dramatic reduction in hypoxia (oxygen deficiency in tissues) was observed in treated tumours. Unfortunately, existing MRI methods to evaluate oxygenation were time-intensive and lacked sensitivity. In the second part of this thesis, we introduce, develop, validate, and apply a new method to assess tumour oxygenation using MRI.
Oxygen (O2) is a paramagnetic molecule that shortens the longitudinal relaxation time (T1) of protons in MRI. This subtle effect has been widely reported in the literature but its applications in cancer have been limited. Our technique - dynamic oxygen-enhanced MRI (dOE-MRI) - uses T1 weighted signal intensity images acquired during a cycling gas challenge (air or oxygen) and independent component analysis (ICA). Hypoxia staining with pimonidazole correlated strongly with dOE-MRI values in a murine tumour model (SCCVII) and only weakly in a colorectal xenograft model (HCT-116). Finally, we provide compelling evidence that treatment with Avastin improves tumour oxygenation in subcutaneous tumours. With dOE-MRI the sensitivity and speed of existing techniques was greatly improved. Since our technique requires no injectable contrast agent, special sequences or hardware, we anticipate that this technique can be quickly translated into the clinic.
Event Location:
Room 301, Hennings Bldg
Event Time:
Monday, July 29, 2019 | 8:00 am - 10:00 am
Event Location:
ISAC-II Room 223 (TRIUMF)
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2019-07-29T08:00:00
2019-07-29T10:00:00
Departmental Oral Examination (Thesis Title: "Dawning of Nuclear Magicity in N=32 Seen Through Precision Mass Spectrometry")
Event Information:
Abstract:
In the early days of nuclear science, physicists were astounded that specific "magic" combinations of neutrons or protons within nuclei seemed to bind together more tightly than other combinations. This phenomenon was related to the formation of shell structures in nuclei. More recently, nuclear shells were observed to emerge or vanish as we inspect nuclei further from stability. The structural evolution of these changing shells has been the object of intense experimental investigation, and their behavior has become a standard ruler to benchmark theoretical predictions.
In this work, we investigated the emergence of shell effects in systems with 32 neutrons (N=32) using mass spectrometry techniques. Evidence for "magicity" was observed in potassium (with 19 protons, or Z=19), calcium (Z=20) and scandium (Z=21), but not in vanadium (Z=23) and heavier elements. In between, the picture at titanium (Z=22) was unclear.
We produced neutron-rich isotopes of titanium and vanadium through nuclear reactions at the ISAC facility and measured their atomic masses at the TITAN facility, in the TRIUMF Laboratory in Vancouver. These measurements were performed with the newly commissioned Multiple-Reflection Time-of-Flight Mass Spectrometer at TITAN facility and were substantiated by independent measurements from the Penning trap mass spectrometer. The atomic masses of 52Ti to 55Ti and 52V to 55V isotopes were measured with high precision, right at the expected emergence of N=32 shell effects. Our results conclusively establish the existence of weak shell effects in titanium and confirm their absence in vanadium, narrowing down the precise onset of this shell closure.
Calculations of the N=32 nuclear shell are within reach of the so-called "ab initio" theories. In these, complex atomic nuclei are described theoretically from fundamental principles, by applying principles of Quantum Chromodynamics to many-body quantum methods. Our data were compared with a few state-of-the-art ab initio calculations which, despite very successfully describing the N=32 shell effects in Ca and Sc isotopes, overpredict its strength in Ti and erroneously assign V as its point of appearance. We hope the deficiencies revealed by our work will guide the development of the next generation of ab initio theories.
Event Location:
ISAC-II Room 223 (TRIUMF)
Event Time:
Thursday, July 25, 2019 | 1:00 pm - 3:00 pm
Event Location:
Room 203, Graduate Student Centre (6371 Crescent Road)
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2019-07-25T13:00:00
2019-07-25T15:00:00
Final PhD Oral Examination (Thesis Title: “Aspects of Quantum Information in Quantum Field Theory and Quantum Gravity”)
Event Information:
Abstract:
We discuss applications of quantum information theoretic concepts to quantum gravity and the low-energy regime of quantum field theories.
The first part of this thesis is concerned with how quantum information spreads in four-dimensional scattering experiments for theories coupled to quantum electro- dynamics or perturbative quantum gravity. In these cases, every scattering process is accompanied by the emission of an infinite number of soft photons or gravi-tons, which cause infrared divergences in the calculation of scattering probabilities.
There are two methods to deal with IR divergences: the inclusive and dressed formalisms. We demonstrate that in the late-time limit, independent of the method, the hard outgoing particles are entangled with soft particles in such a way that the reduced density matrix of the hard particles is essentially completely decohered.
Furthermore, we show that the inclusive formalism is ill-suited to describe scattering of wavepackets, requiring the use of the dressed formalism. We construct the Hilbert space for QED in the dressed formalism as a representation of the canonical commutation relations of the photon creation/annihilation algebra, and argue that it splits into superselection sectors which correspond to eigenspaces of the generators of large gauge transformations.
In the second part of this thesis, we turn to applications of quantum information theoretic concepts in the AdS/CFT correspondence. In pure AdS, we find an explicit formula for the Ryu-Takayanagi (RT) surface for special subregions in the dual conformal field theory, whose entangling surface lie on a light cone. The explicit form of the RT surface is used to give a holographic proof of Markovicity of the CFT vacuum on a light cone. Relative entropy of a state on such special subregions is dual to a novel measure of energy associated with a timelike vector iiiflow between the causal and entanglement wedge. Positivity and monotonicity of relative entropy imply positivity and monotonicity of this energy, which yields a consistency conditions for solutions to quantum gravity.
Event Location:
Room 203, Graduate Student Centre (6371 Crescent Road)
Event Time:
Wednesday, July 24, 2019 | 1:00 pm - 3:00 pm
Event Location:
Room 311, AMPEL/Brimacombe Bldg.
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2019-07-24T13:00:00
2019-07-24T15:00:00
Departmental Oral Examination (Thesis Title: "Designing quantum phases in monolayer graphene")
Event Information:
Abstract:
The physics of quantum materials is at the heart of current condensed matter research. The interactions in these materials between electrons themselves, with other excitations, or external fields can lead to a number of macroscopic quantum phases like superconductivity, the quantum Hall effect, or density wave orders. But the experimental study of these materials is often hindered by complicated structural and chemical properties as well as by the involvement of toxic elements. Graphene, on the other hand, is a purely two-dimensional material consisting of a simple honeycomb lattice of carbon atoms. Since it was discovered experimentally, graphene has become one of the most widely studied materials in a range of research fields and remains one of the most active areas of research today. However, even though graphene has proven to be a promising platform to study a plethora of phenomena, the material itself does not exhibit the effects of correlated electron physics.
In this thesis, we show two examples of how epitaxially grown large-scale graphene can be exploited as a platform to design quantum phases through interaction with a substrate and intercalation of atoms. Graphene under particular strain patterns exhibits pseudomagnetic fields. This means the Dirac electrons in the material behave as if they were under the influence of a magnetic field, even though no external field is applied. We are able to create large homogeneous pseudomagnetic fields using shallow nanoprisms in the substrate, which allows us to study the strain-induced quantum Hall effect in a momentum-resolved fashion using angle-resolved photoemission spectroscopy (ARPES). In the second part, we show how the intercalation of gadolinium can be used to couple flat bands in graphene to ordering phenomena in gadolinium. Flat bands near the Fermi level are theorised to enhance electronic correlations, and in combination with novel ordering phenomena, play a key role in many quantum material families. Our ARPES and resonant energy-integrated X-ray scattering (REXS) measurements reveal a complex interplay between different quantum phases in the material, including pseudogaps and evidence for a density wave order.
Event Location:
Room 311, AMPEL/Brimacombe Bldg.
Event Time:
Monday, July 22, 2019 | 3:00 pm - 5:00 pm
Event Location:
UBC Sage Restaurant (Leon and Thea Koerner University Centre, 6331 Crescent Road Vancouver BC, V6T 1Z2)
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2019-07-22T15:00:00
2019-07-22T17:00:00
Don Witt Celebration of Life
Event Information:
A celebration of Don Witt's life will be hosted at the UBC Sage Restaurant (Leon and Thea Koerner University Centre, 6331 Crescent Road Vancouver BC, V6T 1Z2) on Monday, July 22, 2019 at 3pm. Please RSVP via the link below.
RSVP for Don Witt's Celebration of Life
Leave a message on Don's Memorial Page
Event Location:
UBC Sage Restaurant (Leon and Thea Koerner University Centre, 6331 Crescent Road Vancouver BC, V6T 1Z2)
Event Time:
Friday, July 19, 2019 | 10:00 am - 12:00 pm
Event Location:
Room 309, Hennings Bldg
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2019-07-19T10:00:00
2019-07-19T12:00:00
Departmental Oral Examination (Thesis Title: "High Resolution Two-Photon Spectroscopy of 129Xe for Precision Optical Magnetometry")
Event Information:
Abstract:
This dissertation presents high precision two-photon xenon spectroscopy of the 5p5(2P3/2)6p 2[3/2]2 -> 5p6 (1S0) transition. Experimental results of the spectroscopic signal over pressures of 15 980 mTorr, combined with a theoretical discussion, led to the determination of the natural linewidth of the excited state. Specific attention is payed to the (F = 3/2) hyperfine level of 129Xe, motivated by the new experiment at TRIUMF to measure the electric dipole moment of the neutron (nEDM), where 129Xe will be used as an optical comagnetometer. A non-zero value of the nEDM would partially confirm the existence of the baryon asymmetry in the universe as predicted by the standard model. The experiment at TRIUMF aims increase the sensitivity of the current nEDM limit by increasing the number of ultra-cold neutrons and employing a cohabiting 199Hg/129Xe dual species comagnetometer.
The first experimental success was the technological development of the continuous wave 252 nm ultra-violet (UV) laser system, with the power and precision to selectively probe the hyperfine levels of 129Xe. Using this laser system the first high resolution spectrum of the two-photon transition was measured. Ten transition peaks are observed for the six most abundant isotopes in xenon gas, along with the hyperfine levels of the 129Xe and 131Xe isotopes. Analysis of this spectrum led to direct determinations of the hyperfine constants of the 5p5(2P3/2)6p2[3/2]2 excited state and constants relating to the isotope shift.
The final part of the dissertation focused on the parameterization of the 129Xe (F = 3/2) spectral line. In the nEDM experiment the pressure of 129Xe is limited to 3 mTorr, making it essential to characterize the signal at this level in order to maximize the sensitivity of the comagnetometer. This work presents a pressure dependence study of the spectral lineshape from 980-15 mTorr. These results illustrate the expected signal size and relative transition frequency to the (F = 3/2) level for precision laser tuning. In addition to the contributions of this work to optical magnetometry, the results present a complimentary experimental technique to determine the natural linewidth of the 5p5(2P3/2)6p2[3/2]2 excited state. The high precision of the results permit extrapolation of the lineshape to zero pressure yielding a natural linewidth of 4.3(1.6) MHz/Torr. Overall, the work presented is significantly important to optical magnetometry in nEDM experiments, as well as to precision spectroscopy and theories of atom-atom interactions.
Event Location:
Room 309, Hennings Bldg
Event Time:
Thursday, July 18, 2019 | 2:00 pm - 3:30 pm
Event Location:
Brimacombe 311
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2019-07-18T14:00:00
2019-07-18T15:30:00
CM Seminar: Three superconducting phases of 1111-type iron-based superconductor RFeAs1-xPnxO1-y(F,H)y (R=La and Nd, Pn=P and Sb)
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In one of 1111-type iron-based superconducting (SC) system LaFeAsO, the electron doping level and the local crystal structure can be controlled by the F/H substitution for O and P for As. By these chemical substitution effects, Fermi surface (FS) topology has been changed, and three different SC phases appears in LaFeAs1-xPxO1-y(F/H)y system. [1,2] The first and second SC phases (SC1 and SC2) are observed in the low F concentration region around LaFeAsO0.9F0.1 ((x, y) ~ (0, 0.1)) and LaFeAs0.6P0.4O0.9F0.1 ((x, y) ~ (0.4, 0.1)), respectively. On the other hand, the third SC phase (SC3) appears in the highly electron doping region of (x, y) ~ (0, 0.3). In the SC1 and SC2 states, the spin fluctuations due to the FS nesting in LaFeAsO-type FS with xy hole FS or LaFePO-type FS without xy hole FS play an important role in order to stabilize the superconductivity. [3,4] In the SC3 state, however, the heavily electron doping destabilizes the FS nesting. In this SC3 phase, the next nearest neighbor magnetic interaction in the xy direction in real space induces the superconductivity.
We also have investigated the transport properties and structural parameters of NdFeAs1-xPxO1-y(F/H)y (y=0~0.4) and LaFeAs1-xSbxO1-y(F/H)y (y=0~0.3) to clarify the correlation between the stability of superconductivity and the change of the FS accompanied by the P/Sb and F/H substitutions. In the Nd-1111 system with P substitution, the result of structural analysis revealed that the pnictogen height from the Fe plane hPn is larger than that in the La-1111 system. The Sb substitution in the La-1111 system also increases hPn. With increasing hPn, the FS nesting has been improved by the enlarging the xy FS near RFeAsO in the phase diagram, and the strength of the next nearest neighbor magnetic interaction is also enhanced in heavily H doping region. As a result, these effects has stabilized the SC1 and SC3 states, and merges the SC1 phase near RFeAsO and the SC3 one in the highly H doping region in the phase diagram. The phase diagram for the present systems can be explained by the scenario for FS nesting and next nearest neighbor magnetic interaction.
[1] S. Miyasaka et al., Phys. Rev. B 95, 214515 (2017).
[2] K. T. Lai, S. Miyasaka et al., Phys. Rev. B 90, 064504 (2014).
[3] S. Miyasaka et al., J. Phys. Soc. Jpn. 82, 124706 (2013).
[4] A. Takemori, S. Miyasaka et al., Phys. Rev. B 98, 100501(R) (2018).
Event Location:
Brimacombe 311
Event Time:
Wednesday, July 17, 2019 | 2:00 pm - 3:00 pm
Event Location:
BRIM 311
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2019-07-17T14:00:00
2019-07-17T15:00:00
Majorana-Hubbard Model on the Triangular Lattice
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Of late, there has been a growing body of experimental work that brings us closer to an undoubted realization of Majorana fermions in condensed matter systems. In two-dimensions these quasiparticles arise as zero energy vortex bound states on the surface of a topological superconductor. And in the presence of a lattice of these vortices, interactions between these Majorana zero modes (MZMs) fall off exponentially with the superconducting coherence length. This motivates the construction of a tight-binding model to describe the low energy physics. Given that the vortex lattice usually has a triangular geometry, we study the role of interactions in this setup using a combination of mean field theory and numerical simulation of ladder variants of the model using the density-matrix renormalization-group technique. Our analysis indicates an interaction driven phase transition into a critical phase.
Event Location:
BRIM 311
Event Time:
Monday, July 8, 2019 | 9:00 am - 4:00 pm
Event Location:
Hennings
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2019-07-08T09:00:00
2019-08-02T16:00:00
Phenomenal Physics Summer Camp
Event Information:
Event Location:
Hennings
Event Time:
Monday, July 8, 2019 | 9:00 am - 11:00 am
Event Location:
Room 200, Graduate Student Centre (6371 Crescent Road)
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2019-07-08T09:00:00
2019-07-08T11:00:00
Final PhD Oral Examination (Thesis Title: “Personalized Dosimetry Protocol for the Optimization of Lutetium-177 Dotatate Radionuclide Therapy”)
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Abstract:
Targeted radionuclide therapy has been shown to be one of the most effective treatment options for metastatic neuroendocrine tumours (NETs). In particular, peptide receptor radionuclide therapy (PRRT) with Lutetium-177 (177Lu) labeled DOTATATE results in significantly improved tumour control, while only low to moderate normal tissue toxicity. There is growing evidence that the efficacy of this treatment can be further improved by performing personalized administration of radiopharmaceutical. However, since dosimetry for PRRT is usually considered challenging, traditionally NET patients are treated with same or very similar amounts of 177Lu DOTATATE. The objective of this thesis was to propose a simple, yet accurate dosimetry protocol, which could be easily implemented in clinics for the optimization of 177Lu DOTATATE radionuclide therapy. To achieve this aim, the following physics questions, that are related to image-based dose calculation, were investigated:
The performance of camera calibration method using simple planar scans, was compared to that obtained from tomographic acquisitions. To assess the quantitative accuracy of commercial SPECT reconstruction software (Siemens Flash3D), a number of phantom experiments with different photon attenuation conditions were performed. The influence of camera dead-time correction on the estimated dose was investigated. The kidney doses obtained from four time-activity curve creation methods using three data points were compared. In order to simplify the dosimetry, the accuracy of dose estimated based on two data points, or even potentially one data point, was evaluated.
Our results show that gamma camera can be accurately calibrated with planar scan of a point-like source. The error of 177Lu activity quantification in large volume (>100mL) was about 5% when proper segmentation was applied to Siemens Flash3D reconstructions. Dead-time correction was found to have no impact on the estimated dose. Kidney dose estimated based on single data measured at 48-72 hours produced small error (<10%) for the majority of patients, thus could be recommended for clinical use. This single data point method can also be applied to other organ, as long as its bio-kinetics can be described by a monoexponential function and the statistical behavior of the population effective half-lives in that organ has been estimated.
Event Location:
Room 200, Graduate Student Centre (6371 Crescent Road)