arXiv subjects

A. Chan

Publications and source records attributed to A. Chan.

9 recordsLinked to original sources

arXiv2019

Backfolded acoustic phonons in metal-oxide superlattices

We report the observation of low-frequency modes in the Raman spectra of thin-film superlattices of the high-temperature superconductor YBa$ _{2} $Cu$ _{3} $O$ _{7-\delta} $ and various manganite perovskites. Our study shows that these modes are caused by the backfolding of acoustic phonons due to the additional periodicity introduced by the superlattice. Such modes were previously only observed for ultra-pure semiconductor superlattices. They can be used to determine the bilayer thickness of the superlattice and its speed of sound. Moreover, we use the spatial resolution of Raman microscopy to map the film thickness inhomogeneity across a sample, making these modes a useful tool to characterize thin-film superlattices.

cond-mat.supr-con↗

arXiv2017

The INSuRE Project: CAE-Rs Collaborate to Engage Students in Cybersecurity Research

Since fall 2012, several National Centers of Academic Excellence in Cyber Defense Research (CAE-Rs) fielded a collaborative course to engage students in solving applied cybersecurity research problems. We describe our experiences with this Information Security Research and Education (INSuRE) research collaborative. We explain how we conducted our project-based research course, give examples of student projects, and discuss the outcomes and lessons learned.

cs.CY↗

arXiv2007

VoIP over Multiple IEEE 802.11 Wireless LANs

Prior work indicates that 802.11 is extremely inefficient for VoIP transport. Only 12 and 60 VoIP sessions can be supported in an 802.11b and an 802.11g WLAN, respectively. This paper shows that the bad news does not stop there. When there are multiple WLANs in the vicinity of each other, the already-low VoIP capacity can be further eroded in a significant manner. For example, in a 5-by-5, 25-cell multi-WLAN network, the VoIP capacities for 802.11b and 802.11g are only 1.63 and 10.34 sessions per AP, respectively. This paper investigates several solutions to improve the VoIP capacity. Based on a conflict graph model, we propose a clique-analytical call-admission scheme, which increases the VoIP capacity by 52% and 37% in 802.11b and 802.11g respectively. If all the three orthogonal frequency channels available in 11b and 11g are used, the capacity can be nearly tripled by the call-admission scheme. This paper also proposes for the first time the use of coarse-grained time-division multiple access (CoTDMA) in conjunction with the basic 802.11 CSMA to eliminate the performance-degrading exposed-node and hidden-node problems. We find that CoTDMA can further increase the VoIP capacity in the multi-WLAN scenario by an additional 35%.

cs.NI↗

arXiv2005

delta S = 2 nonleptonic hyperon decays

A sensitive search for the rare decays Ω^- \to Λπ^- and Ξ^0 \to p π^- has been performed using data from the 1997 run of the HyperCP (Fermilab E871) experiment. Limits on other such processes do not exclude the possibility of observable rates for |ΔS| = 2 nonleptonic hyperon decays, provided the decays occur through parity-odd operators. We obtain the branching-fraction limits B(Ω^- \to Λπ^-)< 2.9 x 10^{-6} and B(Ξ^0 \to p π^-)< 8.2 x 10^{-6}, both at 90% confidence level.

hep-ex↗

arXiv2005

Measurement of the Alpha Asymmetry Parameter for the Omega- to Lambda K- Decay

We have measured the alpha parameter of the Omega- to Lambda K- decay using data collected with the HyperCP spectrometer during the 1997 fixed-target run at Fermilab. Analyzing a sample of 0.96 million Omega- to Lambda K^-, Lambda to p pi- decays, we obtain alpha_Omega*alpha_Lambda = [1.33+/-0.33(stat)+/-0.52(syst)] x 10^{-2}. With the accepted value of alpha_Lambda, alpha_Omega is found to be [2.07+/-0.51(stat)+/-0.81(syst)] x 10^{-2}.

hep-ex↗

arXiv2004

Search for CP Violation in Charged-Xi and Lambda Hyperon Decays

We have compared the proton and antiproton angular distributions in 117 million Xi-minus -> Lambda + pi-minus -> proton + pi-minus + pi-minus and 41 million anti-Xi-minus -> anti-Lamba + pi-plus -> antiproton + pi-plus + pi-plus decays using a subset of the data from the HyperCP experiment (E871) at Fermilab. We find no evidence of CP violation, with the direct-CP-violating parameter $A_{ΞΛ} = \frac{α_Ξα_Λ - \barα_Ξ\barα_Λ}{α_Ξα_Λ + \barα_Ξ\barα_Λ} = [0.0+/-5.1(stat)+/-4.4(syst)]{\times}10^{-4}$.

hep-ex↗

arXiv2004

HyperCP: A high-rate spectrometer for the study of charged hyperon and kaon decays

The HyperCP experiment (Fermilab E871) was designed to search for rare phenomena in the decays of charged strange particles, in particular CP violation in $Ξ$ and $Λ$ hyperon decays with a sensitivity of $10^{-4}$. Intense charged secondary beams were produced by 800 GeV/c protons and momentum-selected by a magnetic channel. Decay products were detected in a large-acceptance, high-rate magnetic spectrometer using multiwire proportional chambers, trigger hodoscopes, a hadronic calorimeter, and a muon-detection system. Nearly identical acceptances and efficiencies for hyperons and antihyperons decaying within an evacuated volume were achieved by reversing the polarities of the channel and spectrometer magnets. A high-rate data-acquisition system enabled 231 billion events to be recorded in twelve months of data-taking.

hep-ex↗

arXiv2003

Software Scalability Issues in Large Clusters

The rapid development of large clusters built with commodity hardware has highlighted scalability issues with deploying and effectively running system software in large clusters. We describe here our experiences with monitoring, image distribution, batch and other system administration software tools within the 1000+ node Linux cluster currently in production at the RHIC Computing Facility.

physics.comp-ph↗

arXiv2003

The GRID and the Linux Farm at the RCF

The emergence of the GRID architecture and related tools will have a large impact in the operation and design of present and future large clusters. We present here the ongoing efforts to equip the Linux Farm at the RHIC Computing Facility with Grid-like capabilities.

physics.comp-ph↗