Evaluating Mandarin language students’ online experience during Covid-19: A case study from London

Given the impact of coronavirus, all schools across all sectors public and private, in the United Kingdom closed at the end of March 2020. Closures affected every type of establishment across the UK as well as private language schools. Our case study takes place in London and looks into the student experience in a language Mandarin Chinese school. These language students, as very many across the globe, who attended face-to-face sessions up until that point, have since been studying remotely and joining virtual classrooms via Zoom, one of the multiple video conferencing platforms available. To better evaluate students’ online learning experience of remote learning, this study examines the teacherstudent interactions that take place via online means and the students’ sense of ‘presence’ (i.e., cognitive, social, and teaching presence) in order to evaluate the lessons we can learn from the online learning experience, going forward in terms of teaching and learning. This study also presents how meaningful and worthwhile the experience has been and how the sense of ‘presence’ plays a significant role in the process of online teaching and learning

What Fraction of the Young Clusters in the Antennae Galaxies are "Missing"?

A reexamination of the correspondence between 6 cm radio continuum sources and young star clusters in the Antennae galaxies indicates that 85 % of the strong thermal sources have optical counterparts, once the optical image is shifted 1.2 arcsec to the southwest. A sample of 37 radio-optical matches are studied in detail showing correlations between radio properties and a variety of optical characteristics. There is a strong correlation between the radio flux and the intrinsic optical brightness. In particular, the brightest radio source is also the intrinsically brightest optical cluster (WS80). It is also the most extincted cluster in the sample, the strongest CO source and the strongest 15 micron source . Furthermore, the brightest ten radio sources are all amongst the youngest clusters with ages in the range 0 - 4 Myr and extinctions from A_V = 0.5 to 7.6 mag (with a median value of 2.6 mag). Only a few of the very red clusters originally discovered by Whitmore & Schweizer are radio sources, contrary to earlier suggestions. Finally, a new hybrid method of determining cluster ages has been developed using both UBVI colors and H_alpha equivalent widths to break the age-reddening degeneracy.Comment: 51 pages, 13 postscript figures, LaTex. To appear in the Astronomical Journal, 124, 2002, Septembe

Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

Spin orientation of photoexcited carriers and their energy relaxation is investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield high polarization degree, namely larger than 40%, as well as a fine-tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multi-valley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime

Stabilizing confined quasiparticle dynamics in one-dimensional polar lattice gases

The disorder-free localization that occurred in the study of relaxation dynamics in far-from-equilibrium quantum systems has been widely explored. Here we investigate the interplay between the dipole-dipole interaction (DDI) and disorder in the hard-core polar bosons in a one-dimensional lattice. We find that the localized dynamics will eventually thermalize in the clean gas, but can be stabilized with the existence of a small disorder proportional to the inverse of DDI strength. From the effective dimer Hamiltonian, we show that the effective second-order hopping of quasiparticles between nearest-neighbor sites is suppressed by the disorder with strength similar to the effective hopping amplitude. The significant gap between the largest two eigenvalues of the entanglement spectrum indicates the dynamical confinement. We also find that the disorder related sample-to-sample fluctuation is suppressed by the DDI. Finally, we extend our research from the uncorrelated random disorder to the correlated quasiperiodic disorder and from the two-dimer model to the half-filling system, obtaining similar results.Comment: 10 pages, 10 figure

Higgs Boson Search Sensitivity in the H→WWH \to WW Dilepton Decay Mode at s=7\sqrt s = 7 and 10 TeV

Prospects for discovery of the standard model Higgs boson are examined at center of mass energies of $7$ and $10$ TeV at the CERN Large Hadron Collider. We perform a simulation of the signal and principal backgrounds for Higgs boson production and decay in the $W^+ W^-$ dilepton mode, finding good agreement with the ATLAS and CMS collaboration estimates of signal significance at 14 TeV for Higgs boson masses near $m_H = 160$~GeV. At the lower energy of $7$~TeV, using the same analysis cuts as these collaborations, we compute expected signal sensitivities of about $2$ standard deviations ($\sigma$'s) at $m_H = 160$~GeV in the ATLAS case, and about 3.6~$\sigma$ in the CMS case for $1$~fb$^{-1}$ of integrated luminosity. Integrated luminosities of 8~$\rm{fb}^{-1}$ and 3~$\rm{fb}^{-1}$ are needed in the ATLAS case at $7$ and $10$~TeV, respectively, for $5~\sigma$ level discovery. In the CMS case, the numbers are 2~$\rm{fb}^{-1}$ and 1~$\rm{fb}^{-1}$ at $7$ and $10$~TeV. Our different stated expectations for the two experiments arise from the more restrictive analysis cuts in the CMS case. Recast as exclusion limits, our results show that with $1~{\rm fb}^{-1}$ of integrated luminosity at 7~TeV, the LHC may be able to exclude $m_H$ values in the range 160 to 180~GeV provided no signal is seen.Comment: 29 pages, 8 figures. New results on estimated discovery reach for both CMS and ATLAS, as well as exclusion limits, along with comparisons with Tevatron possibilities. References added

Evidence for a full energy gap for nickel-pnictide LaNiAsO_{1-x}F_x superconductors by ^{75}As nuclear quadrupole resonance

We report systematic ^{75}As-NQR and ^{139}La-NMR studies on nickel-pnictide superconductors LaNiAsO_{1-x}F_x (x=0, 0.06, 0.10 and 0.12). The spin lattice relaxation rate 1/T_1 decreases below T_c with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened, and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T_1T is constant in the temperature range T_c \sim 4 K < T <10 K for all compounds and up to T=250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron-pnictides.Comment: 4 pages, 5 figure

Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance

We report a systematic study by 75As nuclear-quadrupole resonance in LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime 0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of the superconducting transition temperature Tc is found, with the highest Tc = 27 K at x = 0.06 which is realized under significant AFSF. With increasing x further, the AFSF decreases, and so does Tc. These features resemble closely the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1) below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates that the energy gaps are fully-opened. The temperature variation of 1/T1 below Tc is rendered nonexponential for other x by impurity scattering.Comment: 5 pages, 5 figures, more references adde

2*2 random matrix ensembles with reduced symmetry: From Hermitian to PT-symmetric matrices

A possibly fruitful extension of conventional random matrix ensembles is proposed by imposing symmetry constraints on conventional Hermitian matrices or parity-time- (PT-) symmetric matrices. To illustrate the main idea, we first study 2*2 complex Hermitian matrix ensembles with O(2) invariant constraints, yielding novel level-spacing statistics such as singular distributions, half-Gaussian distribution, distributions interpolating between GOE (Gaussian Orthogonal Ensemble) distribution and half Gaussian distributions, as well as gapped-GOE distribution. Such a symmetry-reduction strategy is then used to explore 2*2 PT-symmetric matrix ensembles with real eigenvalues. In particular, PT-symmetric random matrix ensembles with U(2) invariance can be constructed, with the conventional complex Hermitian random matrix ensemble being a special case. In two examples of PT-symmetric random matrix ensembles, the level-spacing distributions are found to be the standard GUE (Gaussian Unitary Ensemble) statistics or "truncated-GUE" statistics

Rotational Symmetry of Classical Orbits, Arbitrary Quantization of Angular Momentum and the Role of Gauge Field in Two-Dimensional Space

We study the quantum-classical correspondence in terms of coherent wave functions of a charged particle in two-dimensional central-scalar-potentials as well as the gauge field of a magnetic flux in the sense that the probability clouds of wave functions are well localized on classical orbits. For both closed and open classical orbits, the non-integer angular-momentum quantization with the level-space of angular momentum being greater or less than $\hbar$ is determined uniquely by the same rotational symmetry of classical orbits and probability clouds of coherent wave functions, which is not necessarily $2\pi$-periodic. The gauge potential of a magnetic flux impenetrable to the particle cannot change the quantization rule but is able to shift the spectrum of canonical angular momentum by a flux-dependent value, which results in a common topological phase for all wave functions in the given model. The quantum mechanical model of anyon proposed by Wilczek (Phys. Rev. Lette. 48, 1144) becomes a special case of the arbitrary-quantization.Comment: 6 pages, 5 figure