Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4

Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2 Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron scattering measurements on this material. The magnetic susceptibility, $\chi(T)$, shows a rounded maximum at T = 8 K indicative of a low dimensional antiferromagnet with no zero field magnetic phase transition. We compare the $\chi(T)$ data to exact diagonalization results for various one dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intra-rung coupling $J_1 = 1.143(3)$ meV and two mutually frustrating inter-rung interactions: $J_2 = 0.21(3)$ meV and $J_3 = 0.09(5)$ meV. The specific heat in zero field is exponentially activated with an activation energy $\Delta = 0.89(1)$ meV. A spin gap is also found through inelastic neutron scattering on powder samples which identify a band of magnetic excitations for $0.8 < \hbar\omega < 1.5$ meV. Using sum-rules we derive an expression for the dynamic spin correlation function associated with non-interacting propagating triplets in a spin ladder. The van-Hove singularities of such a model are not observed in our scattering data indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} are more complicated. For magnetic fields above $H_{c1} \simeq 7.2$ T specific heat data versus temperature show anomalies indicating a phase transition to an ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail Comments to [email protected]

Study of the magnetic susceptibility in the spin-Peierls system CuGeO3_3

We study numerically, using a one-dimensional Heisenberg model, the spin-Peierls transition in the linear Cu$^{2+}$ spin-1/2 chains in the inorganic compound CuGeO$_3$ which has been recently observed experimentally. We suggest that the magnetic susceptibility, the temperature dependence of the spin gap and the spin-Peierls transition temperature of this material can be reasonably described by including nearest and next nearest neighbor antiferromagnetic interactions along the chain. We estimate that the nearest neighbor exchange parameter J is approximately $160\:\rm K$, and that the next nearest neighbor exchange parameter is approximately $0.36\:\rm J$.Comment: 14 pages, Revtex v2.0, 4 figures available upon reques

A Combined Spitzer and Chandra Survey of Young Stellar Objects in the Serpens Cloud Core

We present Spitzer and Chandra observations of the nearby (~260 pc) embedded stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC and MIPS instruments, in six wavelength bands from 3 to 70 ${\mu}m$, to detect thermal emission from circumstellar disks and protostellar envelopes, and to classify stars using color-color diagrams and spectral energy distributions (SEDs). These data are combined with Chandra observations to examine the effects of circumstellar disks on stellar X-ray properties. Young diskless stars were also identified from their increased X-ray emission. We have identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17 transition disk, and 21 class III stars; 60 of which exhibit X-ray emission. Our primary results are the following: 1.) ten protostars detected previously in the sub-millimeter are detected at lambda < 24 microns, seven at lambda < 8 microns, 2.) the protostars are more closely grouped than more evolved YSOs (median separation : ~0.024 pc, and 3.) the luminosity and temperature of the X-ray emitting plasma around these YSOs does not show any significant dependence on evolutionary class. We combine the infrared derived values of AK and X-ray values of NH for 8 class III objects and find that the column density of hydrogen gas per mag of extinctions is less than half the standard interstellar value, for AK > 1. This may be the result of grain growth through coagulation and/or the accretion of volatiles in the Serpens cloud core.Comment: 69 pages, 16 figures, accepted to ApJ. Higher Resolution Figures at: http://www.cfa.harvard.edu/~ewinston

Magnetic excitations and effects of magnetic fields on the spin-Peierls transition in CuGeO3_3

We analyze the magnetic excitations of a spin-1/2 antiferromagnetic Heisenberg model with alternating nearest neighbor interactions and uniform second neighbor interactions recently proposed to describe the spin-Peierls transition in CuGeO$_3$. We show that there is good agreement between the calculated excitation dispersion relation and the experimental one. We have also shown that this model reproduces satisfactorily the experimental results for the magnetization vs. magnetic field curve and its saturation value. The model proposed also reproduces qualitatively some features of the magnetic phase diagram of this compound and the overall behavior of the magnetic specific heat in the presence of applied magnetic fields.Comment: 12 pages Revtex v2.0 + 4 figures postscripts include

Addressing Behavioral Barriers to COVID-19 Testing With Health Literacy-Sensitive eHealth Interventions: Results From 2 National Surveys and 2 Randomized Experiments.

BACKGROUND: Polymerase chain reaction (PCR) testing for COVID-19 was crucial in Australia's prevention strategy in the first 2 years of the pandemic, including required testing for symptoms, contact with cases, travel, and certain professions. However, several months into the pandemic, half of Australians were still not getting tested for respiratory symptoms, and little was known about the drivers of and barriers to COVID-19 PCR testing as a novel behavior at that time. OBJECTIVE: We aimed to identify and address COVID-19 testing barriers, and test the effectiveness of multiple eHealth interventions on knowledge for people with varying health literacy levels. METHODS: The intervention was developed in 4 phases. Phase 1 was a national survey conducted in June 2020 (n=1369), in which testing barriers were coded using the capability-opportunity-motivation-behavior framework. Phase 2 was a national survey conducted in November 2020 (n=2034) to estimate the prevalence of testing barriers and health literacy disparities. Phase 3 was a randomized experiment testing health literacy-sensitive written information for a wide range of barriers between February and March 2021 (n=1314), in which participants chose their top 3 barriers to testing to view a tailored intervention. Phase 4 was a randomized experiment testing 2 audio-visual interventions addressing common testing barriers for people with lower health literacy in November 2021, targeting young adults as a key group endorsing misinformation (n=1527). RESULTS: In phase 1, barriers were identified in all 3 categories: capability (eg, understanding which symptoms to test for), opportunity (eg, not being able to access a PCR test), and motivation (eg, not believing the symptoms are those of COVID-19). Phase 2 identified knowledge gaps for people with lower versus higher health literacy. Phase 3 found no differences between the intervention (health literacy-sensitive text for top 3 barriers) and control groups. Phase 4 showed that a fact-based animation or a TikTok-style video presenting the same facts in a humorous style increased knowledge about COVID-19 testing compared with government information. However, no differences were found for COVID-19 testing intentions. CONCLUSIONS: This study identified a wide range of barriers to a novel testing behavior, PCR testing for COVID-19. These barriers were prevalent even in a health system where COVID-19 testing was free and widely available. We showed that key capability barriers, such as knowledge gaps, can be improved with simple videos targeting people with lower health literacy. Additional behavior change strategies are required to address motivational issues to support testing uptake. Future research will explore health literacy strategies in the current context of self-administered rapid antigen tests. The findings may inform planning for future COVID-19 variant outbreaks and new public health emergencies where novel testing behaviors are required. TRIAL REGISTRATION: Australian New Zealand Clinical Trials Registry ACTRN12621000876897, https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=382318 ; Australian New Zealand Clinical Trials Registry ACTRN12620001355965, https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=380916&isReview=true

Quasi-Elastic Scattering in the Inclusive (3^3He, t) Reaction

The triton energy spectra of the charge-exchange $^{12}$C($^3$He,t) reaction at 2 GeV beam energy are analyzed in the quasi-elastic nucleon knock-out region. Considering that this region is mainly populated by the charge-exchange of a proton in $^3$He with a neutron in the target nucleus and the final proton going in the continuum, the cross-sections are written in the distorted-wave impulse approximation. The t-matrix for the elementary exchange process is constructed in the DWBA, using one pion- plus rho-exchange potential for the spin-isospin nucleon- nucleon potential. This t-matrix reproduces the experimental data on the elementary pn $\rightarrow$np process. The calculated cross-sections for the $^{12}$C($^3$He,t) reaction at $2^o$ to $7^o$ triton emission angle are compared with the corresponding experimental data, and are found in reasonable overall accord.Comment: 19 pages, latex, 11 postscript figures available at [email protected], submitted to Phy.Rev.

Comment on piNN Coupling from High Precision np Charge Exchange at 162 MeV

In this updated and expanded version of our delayed Comment we show that the np backward cross section, as presented by the Uppsala group, is seriously flawed (more than 25 sd.). The main reason is the incorrect normalization of the data. We show also that their extrapolation method, used to determine the charged piNN coupling constant, is a factor of about 10 less accurate than claimed by Ericson et al. The large extrapolation error makes the determination of the coupling constant by the Uppsala group totally uninteresting.Comment: 5 pages, latex2e with a4wide.sty. This is an updated and extended version of the Comment published in Phys. Rev. Letters 81, 5253 (1998

Natural Thermal and Magnetic Entanglement in 1D Heisenberg Model

We investigate the entanglement between any two spins in a one dimensional Heisenberg chain as a function of temperature and the external magnetic field. We find that the entanglement in an antiferromagnetic chain can be increased by increasing the temperature or the external field. Increasing the field can also create entanglement between otherwise disentangled spins. This entanglement can be confirmed by testing Bell's inequalities involving any two spins in the solid.Comment: 4 pages, 5 figure

Magnetization Curves of Antiferromagnetic Heisenberg Spin-1/2 Ladders

Magnetization processes of spin-1/2 Heisenberg ladders are studied using strong-coupling expansions, numerical diagonalization of finite systems and a bosonization approach. We find that the magnetization exhibits plateaux as a function of the applied field at certain rational fractions of the saturation value. Our main focus are ladders with 3 legs where plateaux with magnetization one third of the saturation value are shown to exist.Comment: 5 pages REVTeX, 4 PostScript figures included using psfig.sty; this is the final version to appear in Phys. Rev. Let

Predicting total reaction cross sections for nucleon-nucleus scattering

Nucleon total reaction and neutron total cross sections to 300 MeV for 12C and 208Pb, and for 65 MeV spanning the mass range, are predicted using coordinate space optical potentials formed by full folding of effective nucleon-nucleon interactions with realistic nuclear ground state densities. Good to excellent agreement is found with existing data.Comment: 10 pages, 4 figure