Student Talks

(Decay) Dark-Matter admixed Stars

Mr. Tin Lok Chan
10:40-10:55 in Astrophysics & Cosmology Session

Dark-matter admixed white dwarfs, where fermionic single species dark matter (DM) is assumed, are under investigation using different parameters including the central densities, the particle mass of dark matter and different equations of state (EOS) in hope of attaining super-Chandrasekhar white dwarfs. The central density in this study ranges from white dwarfs’ density to neutron stars’ density, so nuclear EOS are involved in the calculation for high density domain. Hydrostatic equilibrium formula with TOV correction is accountable for the error arising from the general relativistic effects at high density. Discovered by LIGO and Virgo, a compact object of 2.6 solar mass is suggested to be either the lightest black hole or the heaviest neutron star observed. This study shows that DM dominated white dwarfs is also a possible explanation of the 2.6 solar mass compact object. On the other hand, exploration on decay dark-mark admixed stars is initiated. The focus on the second study is the radius, instead of the mass, of the stars. The gravity becomes weaker as DM in a star decays, so the stars will then be loosened, thus its radii increases. One hypothesis is that a neutron star may become a white dwarf after DM decays. This research is still in progress, not much conclusion could be drawn for the time being.

Implementation of Loop-TNR algorithm on 2D classical Ising Model

Mr. Kin Yip Fung
10:40-10:55 in Computational & Data Session

Evaluation of the physical quantities for a many-body system has always been a tough job in computational physics. The Loop-optimization Tensor Network Renormalization (Loop-TNR) is a more stable algorithm to compute them using the tools of tensor network and renormalization group. I will discuss the basic theory of tensor network and renormalization, and the result of the applying Loop-TNR to the 2D classical Ising model.

Self-organization of swimmers drives long-range fluid transport in bacterial colonies

Mr. Haoran Xu
10:40-10:55 in Quantum Physics & Biophysics Session

Motile subpopulations in microbial communities are believed to be important for dispersal, quest for food, and material transport. Here, we show that motile cells in sessile colonies of peritrichously flagellated bacteria can self-organize into two adjacent, centimeter-scale motile rings surrounding the entire colony. The motile rings arise from spontaneous segregation of a homogeneous swimmer suspension that mimics a phase separation; the process is mediated by intercellular interactions and shear-induced depletion. As a result of this self-organization, cells drive fluid flows that circulate around the colony at a constant peak speed of ~30 µm s−1, providing a stable and high-speed avenue for directed material transport at the macroscopic scale. Our findings present a unique form of bacterial self-organization that influences population structure and material distribution in colonies.

Probing Halo Substructure with Gravitational-wave Lensing

Mr. Mark Cheung
10:55-11:10 in Astrophysics & Cosmology Session

As gravitational waves couple weakly with matter, they can be used to probe the centers of dark matter halos and galaxies. In particular, when they are strongly lensed by halos, we can potentially recover the structures of their centers, which can help resolve the core-cusp problem. We use Fisher Information to show that the core of a halo can be detected or constrained by the space based gravitational-wave detector LISA if strong lensing occurs: for halos with masses greater than 10^8 solar masses, a core radius of 1% the Einstein radius of the halo can be measured with a relative error less than 10%.

Prediction of crystal systems from radial distribution function by machine learning

Mr. Man Kit Chan
10:55-11:10 in Computational & Data Session

In 3-dimensional space, there are a total of 230 space groups that refer to all unique crystal symmetry configurations. The 230 space groups can be regrouped and classified into 7 crystal systems, such that crystals within the same crystal system are likely to exhibit similar symmetry. In this project, radial distribution function (RDF) are computed for theoretical crystal structures generated by computers, then the neural network predicts its crystal system based on the RDF dataset. The main objective of this project is to understand the relationship between symmetry and RDF, i.e. radial spatial density of atoms in primitive cells of crystal.

Structure and mechanism of the plant enzyme EPS1 examined by MD simulations

Mr. Ziqi WANG
10:55-11:10 in Quantum Physics & Biophysics Session

The plant enzyme EPS1 is a peculiar BAHD-family protein with the highly conserved catalytic histidine mutated, suggesting a distinctive catalytic activity different from the canonical acyltransferase activity. Together with PBS3, EPS1 form a two-step metabolic pathway to produce Salicylic acid (SA) from isochorismate in the plant Arabidopsis, in which EPS1 is in charge of catalyzing the conversion of isochorismoyl-glutamate A (IGA) to SA and N-pyruvoyl-L-glutamate (NpG). In order to understand the structure-mechanism relation in this catalytic process, we performed molecular dynamics (MD) simulations to identify the binding pose and analyze the geometry of EPS1’s substrate and product. We delineate a possible mechanism that emphasizes the enzyme's role in breaking the C-O bond of IGA, based on a previously proposed pericyclic reaction mechanism. By inspecting simulated conformations, key residues in contact with the substrate are identified, and subsequent experimental verification is carried out.

Search for Strongly lensed Gravitational Wave Images

Mr. Wing Lok Chan, Mr. Wai Yin Wong
11:10-11:25 in Astrophysics & Cosmology Session

Gravitational lensing is the spacetime curvature due to massive objects, which could give rise to multiple gravitational wave images. As the images can be different in magnitude, arrival time, signal-to-noise ratio, and the global phase, there are many possible lensed pair candidates. It is nearly impossible to analyze all the possible candidates. As a reference, now there are 56 single event candidates detected, which could give at least 1540 possible pairs of lensed event candidates. With each lensed event candidates, a full parameter estimation could take days to weeks. In our research, we expect that the lensed pair should come from the same position from the sky. The parameter estimation of BAYESTAR and BILBY is used for locating the g-wave source. The overlaps of 200 lensed event pairs generated are then computed to investigate the efficiency of using skymap overlap to filter unrelated pairs.

Optimization of Targeted Sub-threshold Search for Strongly Lensed Gravitational Waves Signals

Mr. Man Chun Yeung
11:10-11:25 in Computational & Data Session

Strong gravitational lensing can produce multiple gravitational waves with separated arrival times and different amplitudes. Assuming confirmed events as lensed images, we attempt to search for weaker sub-threshold lensed counterparts for these events through a targeted-search. With the main diffference between a targeted search and a usual search for gravitational waves being the large size reduction in template bank, the default settings for bank splitting may not be nominal for the sensitivity of the former search, if not worsen. In this research, we explore how template binning affects the sensitivty of the targeted-search for lensed gravitational waves. In addition, plot of chi against chirp mass and frequencies of the templates will be investigated as new tools to study the characteristics of reduced template bank.

The dynamic nature of lower processivity observed in SRPK2 than SRPK1

Mr. Yitao GOU
11:10-11:25 in Quantum Physics & Biophysics Session

The SR proteins are phosphorylated by SR protein kinase (SRPK1 and SRPK2). Previous investigation has demonstrated that both SRPK2 and SRPK1 can phosphorylate SRSF1, a typical SR protein, in a processive manner, but the processivity of SRPK2 is lower than that of SRPK1. To understand the dynamic mechanism behind this difference, we performed molecular dynamics (MD) simulations and found that when HIS601, a residue at the center of the docking groove in SRPK2, is protonated (+1 charged), the SR substrate becomes much more flexible. Indeed, HIS601 is surrounded by proton donors (i.e., acidic side chains in the docking groove) and its protonation is likely facilitated by such an environment. From an ensemble perspective, we demonstrate that the overall protonation state of HIS601, which is a weighted average over its neutral and protonated states, will result in an overall weaker docking interaction, thereby, reducing the processivity of SRPK2 relative to that of SRPK1.

Relative Astrometry of Fast-Moving Pulsars

Ms. So Yee Cheung
13:10-13:25 in Astrophysics & Cosmology Session

X-ray observations have led to the discovery of many interesting pulsars. To better understand the nature and emission features of the pulsars, their properties should be investigated. By determining the proper motion and transverse velocity of pulsars, we can better understand neutron-star natal kicks. We performed relative astrometry on Chandra images of 4 pulsars: PSR B2224+65, PSR J0357+3205, PSR J1101-6101 and CXOU J061705.3+222127, and determined their proper motion, transverse velocity and the position angle (P.A.) of the proper motion. The time baseline ranges from 1.85 yr to 11.79 yr. PSR B2224+65, PSR J0357+3205 and PSR J1101-6101 displayed a proper motion of 192.6+/-4.9 mas yr-1 at a P.A. of 50.8 deg, 172.7+/-5.9 mas yr-1 at P.A. 319.7 deg and 179.3+/-5.1 mas yr-1 at P.A. 10.7 deg respectively. Due to the diffuse structure of IC443, the position of the pulsar CXOU J061705.3+222127 was diffcult to determine, and thus its proper motion is uncertain, although we attempted to calculate its proper motion. All the other three pulsars exhibited a significant angular displacement.

Exploring the relationship between lightning density and weather radar echoes

Mr. Chen Qian
13:10-13:25 in Computational & Data Session

Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of energy. This discharge may produce a wide range of electromagnetic radiation, from very hot plasma created by the rapid movement of electrons to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms. We quantify the relationship between lightning density (i.e. number per unit area) and radar echoes (i.e. the intensity of rain area) using Big Data analysis. We apply the results to the prediction of thunderstorms by advanced Numerical Weather Prediction (NWP) models.

What can we learn from quantum sensing under extreme conditions?

Mr. Kin On Ho
13:10-13:25 in Quantum Physics & Biophysics Session

In last year's student conference, I talked about quantum sensing of local magnetic field texture under extreme conditions. In more detail, we utilize nitrogen vacancy (NV) centers in diamond as a powerful, spatially-resolved vector field sensor for material research under pressure at cryogenic temperatures. We extract the superconducting transition temperature (Tc), the local magnetic field profile in the Meissner state, and the critical fields (Hc1 and Hc2). This protocol will become a unique tool for tuning, probing, and understanding quantum many-body systems. This year, I will extend the discussion on quantum sensing under extreme conditions. Recently, there are lots of breakthroughs using ultra-high pressure as a tuning parameter to reach near-room temperature or even room-temperature superconductor. Therefore, it is of utter importance that we should know thoroughly about the pressure effect. In our latest work, we utilize NV centers in diamonds as robust spatially-resolved pressure sensors. We present the study of pressure distributions inside the pressure medium under different experimental conditions. These studies not only show a good spatial resolution, wide temperature, and pressure working ranges, compatibility of the existing pressure cell design with the new method, but also demonstrate the usefulness to measure with these sensors as the pressure distribution is sensitive to various factors. The method and the results will benefit many disciplines such as material research and phase transitions in fluid dynamics.

Properties of Cooled Intracluster Gas in Elliptical Galaxies

Mr. James M.G. Nianias
13:25-13:40 in Astrophysics & Cosmology Session

Elliptical galaxies lying at the center of cool-core groups and clusters display a puzzling set of characteristics, including sprawling filamentary nebulae showing strong H-alpha and H2 emission lines. The physics of these nebulae (temperature, mass, origin, excitation and potential for star formation) are poorly understood. I present the first detailed comparison of the molecular and atomic hydrogen emission (H2 1-0 S(1) ro-vibrational and H-alpha lines, respectively) from a cool-core central elliptical galaxy (NGC 5044). I find that the spectral profiles are consistent with molecular and atomic emission from common gas clouds. In addition, I show that the ratio of H2 to H-alpha surface brightness is ~0.05, close to the value found in NGC 1275 (the central elliptical galaxy in the Perseus cluster) by Lim et al. (2011). I compare these results with the excitation model presented by Ferland et al. (2009) and Fabian et al. (2011) in which electrons from the surrounding X-ray emitting intracluster medium penetrate the nebula and excite the gas.

Active Noise Reduction of Vibration Isolation Systems in Gravitational Wave Experiment

Mr. Yee Ching Lam
13:25-13:40 in Computational & Data Session

At KAGRA, high seismic activity increases mirror residual displacements inside the interferometer, causing difficulty for stable cavity lock and thus introduce noise to the sensors. Active control has been employed to reduce optic's motion at low frequencies. The current configuration of the feedback system at the inverted pendulum stage utilizes only relative sensors (LVDTs) with sensor correction, which is sub-optimal at attenuating seismic noise between 100&150 mHz (microseism). To improve the active control system, customized filters and Normalized least mean square algorithm (NLMS) are employed to calibrate the sensors. The platform displacement is measured by inertial sensors, which act as an indicator of the sensor correction performance involving the relative sensors and seismometer. The signal is then filtered and regarded as the error signal (since ideal displacement is zero) feed to the NLMS while the ground motion signal is regarded as the input signal of the algorithm. The result of the above method is successful with error less than 2% in simulations of the system.

In situ Studies of Stress Environment in Amorphous Solids Using Negatively Charged Nitrogen Vacancy (NV-) Centers in Nanodiamond

Ms. Man Yin Leung
13:25-13:40 in Quantum Physics & Biophysics Session

Amorphous solids, which show characteristic differences from crystals, are common in daily usage. Glasses, gels, and polymers are familiar examples, and polymers are particularly important in terms of their role in construction and crafting. Previous researches have mainly focused on the bulk properties of polymeric products, but the local properties have rarely been discussed. Here, we designed a distinctive protocol using the negatively charged nitrogen vacancy (NV-) center in nanodiamond to study properties inside polymeric products in situ. Choosing the curing of polydimethylsiloxane (PDMS) and the polymerization of cyanoacrylate as subjects of investigation, we measured the time dependence of local pressure and strain in the materials during the chemical processes. From the measurements, we were able to probe the local shear stress inside the two polymeric substances in situ. By regarding the surprisingly large shear stress as the internal tension, we attempted to provide a microscopic explanation for the ultimate tensile strength (UTS) of a bulk solid. Our methodology is applicable to any kind of transparent amorphous solids and to the study of in situ properties in nanoscale.

Some Small-Scale Challenges to the λCDM model

Ms. Ying Chan, Mr. Hoi Tim Cheung
13:40-13:55 in Astrophysics & Cosmology Session

The Missing Satellite Problem and the Too-Big-to-Fail Problem are the two small-scale challenges to the λCDM model studied in this work. We employed Decaying Dark Matter (DDM) to account for the two problems using the SemiCore code. We simulate a wide mass range of dwarf galaxy in a DDM halo. We compare the evolutions of these haloes to study the effect of DDM to their rotation velocities. We find out DDM gives an encouraging result that mimics the Too-Big-To-Fail Problem well, but the Missing Satellite Problem partially.

An Investigation on the Effects of Non-Gaussian Noise Transients and Their Mitigations to Gravitational-wave Tests of General Relativity

Mr. Jack Kwok
13:40-13:55 in Computational & Data Session

The detection of gravitational waves from compact binary coalescence by Advanced LIGO and Advanced Virgo provides an opportunity to study the strong-field, highly-relativistic regime of gravity. Gravitational-wave tests of General Relativity (GR) typically assume Gaussian and stationary noise, thus do not account for non-Gaussian, transient noise features (glitches). We present the false deviations from GR obtained by performing parameterized gravitational-wave tests on simulated signals from binary-black-hole coalescence overlapped with instrumental glitches. We then separately apply three common glitch mitigation methods and evaluate their effect on reducing false deviations from GR.

Linear magnetoresistance with a universal energy scale in a strong-coupling superconductor

Mr. Wei Zhang
13:40-13:55 in Quantum Physics & Biophysics Session

Magnetoresistance (MR) which is defined as the change of the electrical resistance when a magnetic field is applied, usually has a quadratic field dependence at low field range and then saturates as the field increases. The recent discovery of a non-saturating linear magnetoresistance in several correlated electron systems near a quantum critical point has revealed an interesting interplay between the linear magnetoresistance and the zero-field linear-in-temperature resistivity. These studies suggest a possible role of quantum criticality on the observed linear magnetoresistance. Here, I am going to show our discovery of a non-saturating, linear magnetoresistance in Mo8Ga41, a nearly isotropic strong electron-phonon coupling superconductor with a linear-in-temperature resistivity from the transition temperature to ∼55 K. The growth of the resistivity in field is comparable to that in temperature, provided that both quantities are measured in the energy unit. A new empirical scaling formula is developed, which is able to capture the key features of the low-temperature magnetoresistance data of Mo8Ga41.

Effects of Sterile Neutrinos on CDM Halo

Mr. Hoi Lun Ng, Mr. Cheuk Yeung Wang
14:50-15:05 in Astrophysics & Cosmology Session

Sterile neutrino is a proposed theoretical model for dark matter (DM) due to its non-interaction with other fundamental forces except gravity. On the other hand, DM Halos are gigantic structures that surround galaxies. Since the Halos are rich in DM, they are excellent hunting grounds for the sterile neutrinos. The study adopts the widely-accepted ∧CDM model, which assume the DM to be slow-moving and non-decaying, as the basis of the study and uses a simplified semi-analytic model to compute the effects and the resultant CDM distribution after the addition of sterile neutrinos into the CDM. After the addition of sterile neutrinos, it is found that the CDM part of the halo shrinks with an increased density near the centre, and thus gives hints into the possibility of sterile neutrinos as DM.

Membrane permeability of rifampicin and tetracycline from free energy and diffusivity calculations

Mr. Hui Shek Fung
14:50-15:05 in Computational & Data Session

Drugs that violate the rule of five (Ro5) are commonly considered to be poorly absorbed with a low permeability across cellular membranes. However, there are exceptions where compounds that are deemed beyond the rule of five (bRo5) can still well penetrate a lipid membrane. In this work, we examine two such bRo5 drugs, namely, rifampicin and tetracycline, and determine their permeability through a POPC bilayer from first principles. Specifically, we have performed microsecond-long extended Adaptive Biasing Force (eABF) calculations to determine the potential of mean force associated with their membrane permeation. Employing the Bayesian inference method, we then determined their positional dependent diffusivity along the membrane normal. Our results compare favorably with experimental findings and reveal detailed mechanisms of how these two bRo5 drugs manage to cross the lipid membrane barrier.

Single-exposure absorption imaging of ultracold atoms using deep learning

Mr. Rongzi Zhou
14:50-15:05 in Quantum Physics & Biophysics Session

The traditional method to extract line-of-sight integrated optical density (OD) image the special atomic distribution from absorption images is formed by subtracting the logarithms of the pixel counts in the two frames, with and without the atoms. However, due to acoustic noises and other dynamical processes, the noise patterns in the two images are typically not identical, which results in some residual structured noise patterns. Here we solve this problem by performing absorption imaging with only a single exposure, where instead of a second exposure the reference frame is generated by an unsupervised image-completion autoencoder neural network, which can be shown to simplify the experimental systems and reduces the hardware requirements maintaining a good accuracy.

Dark-matter admixed Neutron Stars

Mr. Sheung Chit Ethan Lee
15:05-15:20 in Astrophysics & Cosmology Session

The project investigates the effect of adding dark matter into Neutron Stars. There was a recent paper discovered a 2.6 Solar mass compact object in a binary system together with a black hole, its identity is unknown because it is too heavy to be a Neutron Star, but too light to be a black hole. Thus, this project aims to provide a possible candidate for the 2.6 Solar mass compact object –– Dark-matter admixed Neutron Stars (DANS). By implementing different conditions to the star and solve the TOV equation, the resulting DANS will have different profiles. Multiple 2.6 Solar mass DANS was constructed, with some of them being normal matter dominated, which is possible to be observed.

Molecular dynamics simulation of Te vacancy-driven phase transition of MoTe2 ribbon

Mr. Chi Ho Lam
15:05-15:20 in Computational & Data Session

Transition metal dichalcogenides are known to exhibit different physical properties due to structural difference and fine control on phase transition is required to utilize the properties. Previous paper has shown that photoinduced 2H-to-1T’ phase transition in MoTe2 monolayer is energetically favorable due to vacancy ordering. In the present work, the phase transition of Te-deficient MoTe2 ribbon with vacancies located near the edges was investigated with molecular dynamics simulation. Periodic boundary condition in NVT ensemble were used. Traces of Te-vacancy induced phase transition was observed in monovacancy and divacancy setups, however most were found to be temporary. In addition, straight edge configuration was found in multiple setups at initial zigzag-Mo edge, which might suggest possible edge reconstruction.

Quantum droplet in a mixture of Rb and Na Bose-Einstein condensates

Mr. Zhichao Guo
15:05-15:20 in Quantum Physics & Biophysics Session

According to the mean-field theory, an atomic Bose-Einstein condensate (BEC) will collapse when the interaction between atoms is attractive. However, the mixture of two BECs with attractive inter-species interaction can be stabilized by the beyond mean-field Lee-Huang-Yang(LHY) correction in the format of self-bound quantum droplets. [1,2,3] In this talk, We will present our progress in studying the hetero-nuclear quantum droplet with the double BEC of Rb and Na atoms. With the help of an interspecies Feshbach resonance(FR), we have created double BECs with nearly arbitrary interaction strengths and signs. When setting the inter-species scattering length to larger enough negative values, we observe the self-bound behavior as the signature of the Na-Rb droplet during the time of flight expansion upon releasing the mixture from the optical trap. With careful compensation of magnet field gradient and calibration of atomic number, we studied the liquid-gas phase diagram vs. the atomic number. Due to the high sensitivity of the phase diagram to the magnetic field, a precision measurement of FR has been implemented. Moreover, we will study a mixture sample with only LHY correction by tuning the magnetic field to its mean-field energy vanishing. In this case, the sample can be held in a trap and have a longer lifetime thanks to less three-body loss. This work was supported by the Hong Kong RGC General Research Fund (grant No. 14301815) and the Collaborative Research Fund ((grant No. C6026-16W). References: [1] D. S. Petrov, Phys. Rev. Lett. 115, 155302 [2] C. R. Cabrera et al, Science Vol. 359, Issue 6373, pp. 301-304 [3] G. Semeghini et al, Phys. Rev. Lett. 120, 235301

Parameter Estimation of Gravitational Wave Ringdown

Ms. Sonja Choi
15:20-15:35 in Astrophysics & Cosmology Session

General Relativity predicts that accelerating masses, such as black hole mergers, produce spacetime ripples known as gravitational waves. The perturbed single black hole observed during a gravitational wave ringdown radiates GW that only depends on its final mass and spin. It serves as an arena to test whether compact binary coalescence produces a Kerr black hole. Parameter recovery from a ringdown signal is a crucial part of the test, which motivates this study. In our work, we adopt a Bayesian approach and show that the credible intervals of parameters converge for louder signal. We also show that numerical noise and non-linearities do not affect the recovery.

X-ray Reverberation on super-Eddington accretion disks

Mr. Lars Lund Thomsen
15:35-15:50 in Astrophysics & Cosmology Session

X-ray reverberation is a proven technique capable of probing the innermost region of accretion disks around compact objects. Current theoretical effort assumes that the disk is geometrically thin, optically thick and rotating at Keplerian speed. Thus, these theoretical models cannot be applied to super-Eddington accretion systems because the thin disk approximation fails in this accretion regime. State-of-the-art numerical simulations of super-Eddington accretion show optically thick winds being launched from the geometrically and optically thick disks. Therefore, the reflection geometry of the super-Eddington case is morphologically different from the thin disk picture, and, thus, we need new theoretical methods to handle X-ray reverberation on super-Eddington accretors. The main focus of the talk is a morphological comparison between the X-ray reverberation signal from the Fe K-alpha alpha line produced in super-Eddington and classical thin disks geometries. I show the spectral Fe line profiles produced are morphological different - with the super-Eddington reflection geometry producing substantially more blueshifted and symmetric line profiles. The same tendencies are translated into the temporal aspect of the line profiles. These theoretical results of X-ray reverberation on super-Eddington accretors are consistent with and fitted to the observed Fe K-alpha line from the jetted tidal disruption event, Swift J1644. A transient super-Eddington accretion disk is believed to have formed out of stellar debris after the tidal disruption event, Swift J1644.