Session T66: Frontiers in Fundamental Physics I

Numerical Studies of One-Dimensional Gratings of Heliconical Liquid Crystals

Numerical Studies of One-Dimensional Gratings of Heliconical Liquid Crystals

 

Sha Liu, Hao Yu, Miao Jiang, Qi-Huo Wei

Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China

Corresponding author: weiqh@sustech.edu.cn

 

In this work, we numerically study one-dimensional gratings made of heliconical liquid crystals by using finite-difference time-domain approach. In the gratings, the molecular orientations at the surface are varied periodically in one direction. In interestingly, we found that in contrast to what observed in typical cholesteric liquid crystals, the heliconical gratings exhibit two reflection bands, one corresponds to the band wavelengths of the heliconical liquid crystals, the other is at exactly twice the band wavelengths. We will present the physical mechanisms behind this phenomenon.

 

Key words: Heliconical liquid crystal, finite-difference time-domain   

Length scale formation and the localization in the Landau levels of quasicrystals

The quasicrystals have motivated extensive studies of quantum phenomena such as critical states. Nevertheless, the Landau levels of quasicrystals have been barely understood because of the complication of tilings without periodic length scales. Thus, it has been thought that the system cannot possess any universal features of the Landau levels. Contrary to these assertions, we discuss new quantum phenomena in quasicrystals where electrons localization behavior is distinct in short and large length scales under magnetic fields. In short length scales, electrons are completely localized forming islands with a certain radius, however, in large length scales, such localized islands are extensively distributed and interfere with each other under field control. In the two-dimensional rhombic quasicrystals with rotational symmetries, we derive exact flux conditions and the localization length for such electron localization. Furthermore, macroscopically degenerate zero-energy Landau levels are present due to the chiral symmetry of the rhombic tilings. This gives an important message that the critical state can be completely removable under a magnetic field and the emergence of such quantum states can be applied to control the fractality of electron wave functions in quasicrystals.   

Emptiness Formation in Polytropic Quantum Liquids

We study large deviations in interacting quantum liquids with the polytropic equation of state P(ρ) ∼ ρ^γ , where ρ is density and P is pressure. By solving hydrodynamic equations in imaginary time we evaluate the instanton action and calculate the emptiness formation probability (EFP), the probability that no particle resides in a macroscopic interval of a given size. Analytic solutions are found for a certain infinite sequence of rational polytropic indexes γ and the result can be analytically continued to any value of γ ≥ 1. Our findings agree with (and significantly expand on) previously known analytical and numerical results for EFP in quantum liquids. We also discuss interesting universal spacetime features of the instanton solution.   

Theoretical Investigation of Resonance Energy Transfer Using Discrete and Continuous Donor and Acceptor Models

Resonance energy transfer (RET) is the transport of electronic energy from an excited atom or molecule to another. Besides being an essential process in photosynthesis, RET presents useful applications such as spectroscopic nanoruler and photosensitization of semiconductors. In this study, we investigate the ways to harness high RET rates using various models. First, we consider systems that consist of a single donor-acceptor pair and a metal nanoparticle (NP) to identify the geometrical factors that maximize RET rates when the donor-acceptor pair is coupled to the plasmon-enhanced near-field. It is demonstrated that conversely, rate information can be used to obtain geometrical information of nanophotonic systems. Then, it is shown that RET involving multiple donors may invoke collective mechanisms that give rise to higher RET efficiency, exemplified by activation of surface lattice resonance (SLR) modes and energy transfer among the donors. Finally, energy transfer from a medium consisting of donors as a four-level system to a medium of acceptors as a two-level system is described by solving integral equations involving polarization and electric field.   

Bootstrap bounds on conformal field theories with Lieb-Schultz-Mattis anomalies

We put bounds on 1+1d conformal field theories (CFTs) that saturate certain mixed anomalies arising in spin chains subject to the Lieb-Schultz-Mattis (LSM) theorem using conformal bootstrap techniques. To do this, we will present a new hybrid bootstrap approach, which augments modular bootstrap by additionally imposing crossing symmetry on correlation functions of certain symmetry defect operators. The hybrid bootstrap accounts for a more complete picture of 't Hooft anomalies of discrete symmetries in 1+1d CFTs than modular bootstrap alone and still gives universal bounds on local operators. In particular, we will present universal bounds on charged operators, some of which are only possible with our hybrid approach. Our bounds exhibit distinctive kinks, some of which are approximately saturated by known theories such as the su(3) level 1 WZW model, and others which are unexplained.   

Static and Dynamics Evolution of Charge-carriers in Shape Engineered Metal Halide Perovskite Nanocrystals

Metal halide perovskites (MHPs) nanocrystals (NCs) with their unique structure and exotic properties, emerged as promising and potential material for highly efficient and low costing photovoltaics and optoelectronic applications. For their wide and diverse applications, it is desirable to engineer and tune their properties, which is achieved previously via cation-anion exchange, doping and variation in size. Herein we study the photo-physics of CsPbBr₃ NCs by engineering their shape from usual cubic to rhombic-dodecahedrons and rhombicuboctahedron by steady state and time resolved spectroscopy. In our analysis we observe a remarkably contrasting behaviour of dynamic Burstein-Moss effect (BME) and amplified spontaneous emission (ASE). The BME is observed in the cubic and rhombic-dodecahedrons NCs with shift 24 and 20 meV respectively, on the other hand ASE appeared only in rhombicuboctahedron NCs.  Further it is observed that second-order decay is the major channel for carrier recombination with the rate of 3.68×10^-10, 3.65×10^-10 and 2.03×10^-10 cm³ s^-1 in cubic, rhombic-dodecahedrons and rhombicuboctahedron. These results offer insights into intrinsic photophysics in MHPs with direct implications for photovoltaic and optoelectronic applications by engineering shape of NCs.   

Universal empirical and theoretical for an updated compilation of K x-ray production cross sections by protons Gregory Lapicki, Department of Physics, East Carolina University, Greenville NC 27858, USA

     The relevance of x-ray production cross sections (XRPCS) and the related ionization cross sections (ISC) in many research areas has been described at length and analyzed in detail [1].  X-ray emission cross sections by ion impact are a relevant input in many areas such as studies of track structure in biological matter. Particle Induced X-ray Emission (PIXE) strongly requires trustworthy databases for XRPCS and/or reliable predictions of inner-shell ionization theories as periodically evaluated in Monte Carlo Geant4 simulations [2]. To check if theories are accurate across the periodic table of elements and a large range of projectile energies, equally comprehensive databases are essential and a universal fit for them is desired.    

     For each target element, the compiled XRPCS [1] follow a single curve when plotted versus the ratio of the proton velocity v₁ to the orbital velocity of v_2L=Z_2L//n of the inner-shell electron. Furthermore, for all elements XRPCS peak at σ_LX^max(Z_2L) when v₁ = v_2L. With v≡ v₁/v_2L, a universal fit to all compiled data σ_LX= σ_LX^max(Z_2L)·exp[-(1+a₁Z_2L)v²+a₂v⁷] is made with just two adjustable parameters a₁=0.00484 and a₂=0.005. The predictions of the ECUSAR theory [3] can be also fitted in a similar fashion as done recently based on recently updated data and the revised universal empirical fit [4].  With a standard deviation of 29%,  97% of these ratios are within a factor of 2 from the ideal ratio of 1.00.

      For this meeting, 1) universal empirical and theoretical fits to the updated database from [5,6] will be reviewed as it was done the L shell [4], and 2) similar fits will be presented for the K shell.

[1] J. Miranda and G. Lapicki 2014 ADNDT 100 651.

[2] S. Incerti et al. 2015 NIM.B 358 210.

[3] G. Lapicki 2001 NIM.B 189 6.

[4] G. Lapicki 2020 NIM B 467 123.

[5] G. Lapicki 1989 JPCRD 18 111.

[6] H Paul and J. Sacher 1989 ADNDT 42 105.   

Identification of Higgs boson events from the background by machine learning.

Almost a decade since its detection in the Large Hadron Collider, the Higgs boson remains a key component in the confirmation of the standard model (SM) of particle physics. Current experimental efforts to determine the Higgs boson decay channels, predicted by the SM, rely on the accurate identification of Higgs boson events from a noisy background. Machine learning methods have emerged as new ways to accurately classify rare events present in large datasets. This study compares three supervised classification models in their ability to accurately and efficiently identify signal events: (i) boosted decision trees, (ii) support vector machine, and (iii) neural networks. With an accuracy of 83.88%, F1-score of 81.72% and a training time of 8.4 seconds using the simulated dataset from the 2014 ATLAS Higgs boson Machine Learning Challenge, a histogram gradient boosted decision tree was determined to be the most effective classification model for identifying Higgs boson events. The worst performing algorithm was the support vector machine with the lowest accuracy at 80.35%, F1-score of 77.12%, and the second lowest training time of 50 minutes.   

Three-Dimensional Real Space Invariants: Theory and Applications

In this work, we have developed the three-dimensional real space invariants (3D RSIs) for all the 1,651 Shubnikov space groups (SSGs) with spin-orbit coupling (SOC), including the 230 paramagnetic space groups (SGs) and the 1,421 magnetic space groups (MSGs), and provide the expressions of all the RSIs in the Bilbao Crystallographic Server. The 3D RSIs have allowed for the identification of a special set of 3D topologically trivial insulators which we call Obstructed Atomic Insulators (OAIs), and electrides which we call obstructed metals. Using the theoretical formulation of RSIs, we have performed the high throughput scanning of the Topological Quantum Chemistry database materials and found more than 3300  OAIs. and  5000 obstructed metals. Furthermore, we analyze the potential applications of OAIs and obstructed metals in catalysis and superconductivities.   

Lorentz transformations and Clifford algebra as real and imaginary components respectively of δ(δz+δzδz)=0

Dirac had to postulate his Clifford algebra γ s inside the relativistic energy E² equation to create his linear equation. In contrast here we get both special relativity and the Clifford algebra from the single equation δ(δz+δzδz)=0 since it  splits into the real component Minkowski metric (Lorentz transformations)  and imaginary component Clifford algebra. But it also gives 4D and the operator formalism and so first derivatives. We thereby get the Dirac equations for the electron e and neutrino v. In addition in this formulation the e,v composite is the Standard electroweak Model and the 3e composite gives the rest of particle physics.

Also postulate of 1 implies min(z-zz) which implies z=zz+C  (and δC=0 and δ(δz+δzδz)=0 using z=1+δz). Then plug the left side z into the right side zz and thereby start a sequence of Lemniscates whose limit is the Fiegenbaum point and thereby get GR. So from postulate of 1 (and resulting δ(δz+δzδz)=0) we have found a far more rigorous derivation of the Dirac equation and also a simple origin to physics.   

Discovery of Millisecond Pulsars in Globular Clusters

Globular clusters are known to be rich pulsar laboratories. Their extreme stellar densities imply high stellar interaction rates, which in turn lead to the production of many X-ray binaries and, hence, to spun-up millisecond pulsars (MSPs). The high interactions rates in GCs can also alter the typical evolution of binary systems, resulting in the formation of exotic binaries with unconventional properties. These are valuable, as they can help us put limits on neutron star (NS) masses and constrain NS equation of state. With 206 known pulsars in 35 different clusters, GCs currently account for about 40% of the total known MSP population in our Galaxy. In this talk, I will present the results of two pulsar search surveys carried out in different globular clusters using the MeerKAT and upgraded GMRT radio telescopes. I will discuss some of the interesting binary pulsar discoveries formed as a result of a "secondary" exchange encounter, where the original companion that spun the pulsar up was ejected and replaced by the current companion. The pulsars discovered by MeerKAT represent a notable test case of the capability of the MeerKAT telescope as a precursor for SKA science.   

Multi-component multiscatter capture of dark matter

In recent years, the usefulness of astrophysical objects as dark matter probes has become increasingly evident. The potentially observable signatures of DM gravitationally trapped inside a compact astrophysical object have been used to forecast stringent constraints on the nucleon-dark matter interaction cross-section. Currently, the probes of interest are: at high red-shifts, Population III stars that form in isolation, or in small numbers, in very dense DM minihalos at z=15-40, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. None of those objects are truly single-component, and, as such, capture rates calculated with the common assumption made in the literature of single-component capture, i.e. capture of DM by multiple scatterings with one single type of nucleus inside the object, are not accurate. In this work, we present an extension of this formalism to multi-component objects and apply it to Pop III stars, investigating the role of He on the capture rate. As expected, we find that the inclusion of helium leads to an enhancement of the overall capture rate, improving the potential of Pop III stars as Dark Matter probes.    

Dark Sound

The Axion is a candidate particle for dark matter in our universe, and it is currently a pressing topic to develop new detection methods for this particle. Owing to its high occupancy number, the Axion can display properties of a Bose-Einstein condensate. In particular, self-interactions and gravity produce a sound mode (``Dark Sound") in the system, whose dynamics allows us to probe with new frequency regimes at detectors. We present our analysis and numerical estimates of detection parameters.   

Search for the Expected Radiation due to Photoejection of an Inner Shell Electron

Pratt et al have called attention to the unsettled observational status for the radiation (intraatomic bremstahllung, IAB) expected upon the photoejection of inner shell electrons.  We tested for such radiation in the 3 to 7 keV band produced by photoelectrons from the K shell of a copper target upon absorption of an incident 46 keV photon. Exploting a prediction for the major background process: ordinary bremsstrahlung due to  the encounter of the photoelectron with atoms other than that of its point of origin, we conclude that for our thinnest (40 nm) targets within statistical limits we observe no IAB radiation. In  this manner,  contemporary theory is ruled out by over 5 sigmas.  Assuming no particular secondary process the intensity of observed IAB radiation is 4 sigmas below expectations. We conclude that in contrast to the sister nuclear process of internal bremsstrahlung observed upon beta decay, our understanding of  intratomic bremsstrahlung remains lacking.