Hydrogen-Induced Ultralow Optical Absorption and Mechanical Loss in Amorphous Silicon for Gravitational-Wave Detectors.

The sensitivity of gravitational-wave detectors is limited by the mechanical loss associated with the amorphous coatings of the detectors' mirrors. Amorphous silicon has higher refraction index and lower mechanical loss than current high-index coatings, but its optical absorption at the wavelength used for the detectors is at present large. The addition of hydrogen to the amorphous silicon network reduces both optical absorption and mechanical loss for films prepared under a range of conditions at all measured wavelengths and temperatures, with a particularly large effect on films grown at room temperature. The uptake of hydrogen is greatest in the films grown at room temperature, but still below 1.5 at.% H, which show an ultralow optical absorption (below 10 ppm) measured at 2000 nm for 500-nm-thick films. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may enable fabrication of mirror coatings for gravitational-wave detectors with improved sensitivity.

Excess specific heat in evaporated amorphous silicon.

The specific heat C of e-beam evaporated amorphous silicon (a-Si) thin films prepared at various growth temperatures T(S) and thicknesses t was measured from 2 to 300 K, along with sound velocity v, shear modulus G, density n(Si), and Raman spectra. Increasing T(S) results in a more ordered amorphous network with increases in n(Si), v, G, and a decrease in bond angle disorder. Below 20 K, an excess C is seen in films with less than full density where it is typical of an amorphous solid, with both a linear term characteristic of two-level systems (TLS) and an additional (non-Debye) T3 contribution. The excess C is found to be independent of the elastic properties but to depend strongly on density. The density dependence suggests that low energy glassy excitations can form in a-Si but only in microvoids or low density regions and are not intrinsic to the amorphous silicon network. A correlation is found between the density of TLS n0 and the excess T3 specific heat c(ex) suggesting that they have a common origin.

Electron-mediated ferromagnetic behavior in CoO/ZnO multilayers.

CoO/Al-doped ZnO (AZO) multilayers exhibit ferromagnetism up to ~300 K. The magnetic behavior oscillates with odd vs even number of Co layers in the insulating antiferromagnetic CoO and (separately) with the thickness of the AZO layers and vanishes if AZO is replaced by intrinsic ZnO. Magnetization is due to uncompensated (111) ferromagnetic planes of insulating CoO for odd numbers of atomic planes per layer that are coupled together via RKKY exchange mediated by electron carriers in the nonmagnetic AZO layers. The period of the oscillation with AZO thickness qualitatively matches the Fermi wave vector calculated from the carrier concentration measured by ordinary Hall effect. Magnetic polarization of the AZO carriers is confirmed via an anomalous Hall effect that is proportional to the magnetization.

Thermodynamic measurements of Fe-Rh alloys.

FeRh undergoes an unusual antiferromagnetic-to-ferromagnetic (AFM-FM) transition just above room temperature (T(AFM>FM)) that can be tuned or even completely suppressed with small changes in composition. The underlying temperature-dependent entropy difference between the competing AFM and FM states that drives this transition is measured by specific heat as a function of temperature from 2 to 380 K on two nearly equiatomic epitaxial Fe-Rh films, one with a ferromagnetic ground state (Fe-rich) and the other with an antiferromagnetic ground state (Rh-rich). The FM state shows an excess heat capacity near 100 K associated with magnetic excitations that are not present in the AFM state. The integrated entropy and enthalpy differences between the two alloys up to T(AFM>FM) agree with the previously measured entropy of the transition (ΔS = 17 ± 3 J/kg/K) and yield a T=0 energy difference of 3.4 J/g, consistent with literature calculations and experimental data; this agreement supports the use of the Fe-rich FM sample as a proxy for the (unstable) FM state of the AFM Rh-rich sample. From the low-temperature specific heat, along with sound velocity and photoemission measurements, the lattice contribution to the difference (ΔS(latt) = -33 ± 9 J/kg/K) and electronic contribution (ΔS(el) = 8 ± 1 J/kg/K) to the difference in entropy are calculated, from which the excess heat capacity in the FM phase and the resulting entropy difference are shown to be dominated by magnetic fluctuations (ΔS(mag) = 43 ± 9 J/kg/K). The excess magnetic heat capacity is dominated by the magnetic heat capacity of the FM phase, which can be fit to a Schottky-like anomaly with an energy splitting of 16 ± 1 meV and a multiplicity of 1 per unit cell.

Fe spin reorientation across the metamagnetic transition in strained FeRh thin films.

A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. (57)Fe conversion electron Mössbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.

Electronic structure changes across the metamagnetic transition in FeRh via hard X-ray photoemission.

Stoichiometric FeRh undergoes a temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition at ~350 K. In this Letter, changes in the electronic structure accompanying this transition are investigated in epitaxial FeRh thin films via bulk-sensitive valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 5.95 keV. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density-functional theory. Changes in the 2p core levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results indicate that significant electronic structure changes over the entire valence-band region are involved in this AFM-FM transition.

Band gap and electronic structure of an epitaxial, semiconducting Cr0.80Al0.20 thin film.

Cr(1-x)Al(x) exhibits semiconducting behavior for x = 0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr(0.80)Al(0.20) thin film show several features in the valence band region, including a gap at the Fermi energy (E(F)) for which the valence band edge is 95 ± 14 meV below E(F). Theory agrees well with the valence band measurements, and shows an incomplete gap at E(F) due to the hole band at M shifting almost below E(F).

Thermodynamic measurements of submilligram bulk samples using a membrane-based "calorimeter on a chip".

Calorimetry offers a direct measurement of thermodynamic properties of materials, including information on the energetics of phase transitions. Many materials can only be prepared in thin film or small crystal (submilligram) form, negating the use of traditional bulk techniques. The use of micromachined, membrane-based calorimeters for submilligram bulk samples is detailed here. Numerical simulations of the heat flow for this use have been performed. These simulations describe the limits to which this calorimetric technique can be applied to the realm of small crystals (1-1000 microg). Experimental results confirm the feasibility of this application over a temperature range from 2 to 300 K. Limits on sample thermal conductivity as it relates to the application of the lumped and distributed tau 2 models are explored. For a typical sample size, the simulations yield 2.5% absolute accuracy for the heat capacity of a sample with thermal conductivity as low as 2 x 10(-5) W/cm K at 20 K, assuming a strong thermal link to the device. Silver paint is used to attach (both thermally and physically) the small samples; its heat capacity and reproducibility are discussed. Measurements taken of a submilligram single crystal of cobalt oxide (CoO) compare favorably to the results of a bulk calorimetric technique on a larger sample.

Surface-induced phenomena in uncompensated collinear antiferromagnets.

The net spontaneous magnetization of antiferromagnets with modified surfaces was computed using mean-field theory. For ordinary phase transitions the net magnetization of uncompensated AFM is smaller than the surface magnetization and the Néel vector, whereas for extraordinary phase transitions the net magnetization is larger than the surface magnetization and the Néel vector at finite temperature. Moreover, the temperature dependence of these three observable internal parameters changes drastically with the surface properties, i.e. the surface exchange coupling JS. Based on these findings, contour plots showing different regions of magnetization and Néel vector behavior as functions of temperature and surface exchange strength are proposed.

Temperature-driven growth of antiferromagnetic domains in thin-film FeRh.

The evolution of the antiferromagnetic phase across the temperature-driven ferromagnetic (FM) to antiferromagnetic (AF) phase transition in epitaxial FeRh thin films was studied by x-ray magnetic linear and circular dichroism (XMLD and XMCD) and photoemission electron microscopy. By comparing XMLD and XMCD images recorded at the same temperature, the AF phase was identified, its structure directly imaged, and its evolution studied across the transition. A quantitative analysis of the correlation length of the images shows differences between the characteristic length scale of the two phases with the AF phase having a finer feature size. The asymmetry of the transition from FM to AF upon cooling and AF-FM upon heating is evidenced: upon cooling the formation of AF phase is dominated by nucleation at defects, with little subsequent growth, resulting in a small and non-random final AF domain structure, while upon heating, heterogeneous nucleation at different sites followed by significant domain size growth of the FM phase is observed, resulting in a non-reproducible final FM large domain structure.

Barriers to the treatment of social anxiety.

OBJECTIVE: This article evaluates barriers to treatment reported by adults with social anxiety who participated in the 1996 National Anxiety Disorders Screening Day. METHOD: The background characteristics of screening day participants with symptoms of social anxiety (N=6,130) were compared with those of participants without social anxiety (N=4,507). Barriers to previous mental health treatment reported by participants with and without symptoms of social anxiety were compared. RESULTS: Social anxiety was strongly associated with functional impairment, feelings of social isolation, and suicidal ideation. Compared to participants without social anxiety, those with social anxiety were significantly more likely to report that financial barriers, uncertainty over where to go for help, and fear of what others might think or say prevented them from seeking treatment. However, they were significantly less likely to report they avoided treatment because they did not believe they had an anxiety disorder. Roughly one-third (N=1,400 of 3,682, 38.0%) of the participants with symptoms of social anxiety who were referred for further evaluation were specifically referred for an evaluation for social phobia. CONCLUSIONS: Social anxiety is associated with a distinct pattern of treatment barriers. Treatment access may be improved by building public awareness of locally available services, easing the psychological and financial burden of entering treatment, and increasing health care professionals' awareness of its clinical significance.

Comorbidity, impairment, and suicidality in subthreshold PTSD.

OBJECTIVE: Reliance on the categorical model of psychiatric disorders has led to neglected study of posttraumatic sequelae that fall short of full criteria for posttraumatic stress disorder (PTSD). Substantial disability and suicidal risk is associated with subthreshold PTSD, but this association has not been well studied. In addition, no studies have examined the role of comorbidity in explaining disability and impairment in subthreshold PTSD. METHOD: On National Anxiety Disorders Screening Day 1997, 2,608 out of 9,358 individuals screened for affective and anxiety disorders at 1,521 sites across the United States reported at least one PTSD symptom of at least 1 month's duration. Impairment, comorbid anxiety disorders, major depressive disorder, and rates of suicidality were determined and compared for individuals with no, one, two, three, or four (full PTSD) symptoms on a screening questionnaire. Regression analyses examined the relative contribution of subthreshold PTSD and comorbid disorders to impairment and suicidal ideation. RESULTS: Impairment, number of comorbid disorders, rates of comorbid major depressive disorder, and current suicidal ideation increased linearly and significantly with each increasing number of subthreshold PTSD symptoms. Individuals with subthreshold PTSD were at greater risk for suicidal ideation even after the authors controlled for the presence of comorbid major depressive disorder. CONCLUSIONS: Higher numbers of subthreshold PTSD symptoms were associated with greater impairment, comorbidity, and suicidal ideation. Disability and impairment found in previous studies of subthreshold PTSD symptoms may be related in part to the presence of comorbid disorders. However, the presence of subthreshold PTSD symptoms significantly raised the risk for suicidal ideation even after the authors controlled for major depressive disorder. Given the broad public health implications of these findings, more efforts are needed to identify subthreshold PTSD symptoms in clinical populations, epidemiologic surveys, and treatment studies.

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).

Spin-glass freezing and RKKY interactions near the metal-insulator transition in amorphous Gd-Si alloys

Magnetization measurements of giant negative magnetoresistive amorphous GdxSi1-x ( 0.04

Passive inhalation of free-base cocaine ('crack') smoke by infants.

Cocaine is one of the most widely abused substances in the United States, in part due to the availability of its inexpensive alkaloidal free-base form, "crack". A variety of medical complications, including sudden death, are known to occur in the adult-user population, regardless of the route of cocaine administration. We report 16 cases of infant death registered by the Philadelphia (Pa) Medical Examiner's Office over a 2-year period (1987 through 1989), where toxicologic analyses revealed the presence of cocaine and/or its metabolite, benzoylecgonine. Scene investigation documented that these infants, shortly before death, had been exposed to environments that contained the smoke from crack. We conclude that the route of cocaine administration in this infant population was the passive inhalation of crack smoke. It is possible that the cocaine may have contributed to the death of these infants. Thus, in addition to the adult users, infants and children exposed to environments where crack is smoked may inhale cocaine and potentially suffer from its adverse effects.

Stigma as a barrier to recovery: The extent to which caregivers believe most people devalue consumers and their families.

OBJECTIVE: The extent to which 461 caregivers of persons with serious mental disorders believed that most people devalue consumers and their families was assessed, and the magnitude of the relationships between these beliefs and the diagnostic status of consumers was estimated. METHODS: Caregivers of 180 consumers with schizophrenia, major depression, or bipolar disorder and caregivers of 281 consumers with bipolar disorder or schizoaffective disorder, manic type, completed a 15-item instrument comprising two scales: eight of the 15 items operationally defined the devaluation of individual consumers, and seven items operationally defined the devaluation of consumers' families. RESULTS: No significant differences were observed between the two samples on the two devaluation scales or on 14 of the 15 items that constituted the scales. About 70 percent of all caregivers indicated a belief that most people devalue consumers, and 43 percent expressed a belief that most people also devalue the families of consumers. CONCLUSIONS: Strong evidence from previous research indicates that the caregiving role is very demanding, is frequently distressing, and may be harmful to health and injurious to one's quality of life. The addition of a community that is perceived to be rejecting makes life even more difficult for the caregivers and families of people with serious mental disorders. The development and implementation of effective interventions to create more supportive and understanding communities would be a challenging and worthwhile endeavor.

Rubber and plastic ammunition lethal injuries: the Israeli experience.

Rubber and plastic ammunition was used by the Israeli Defence Forces between the years 1987 and 1993 (Intifada) almost exclusively as a deterrent to aggression by the Palestinian civilian population in the occupied territories of Gaza, Judea and Samaria. While designed to avoid the serious wounds and deaths caused by the use of conventional military ammunition, the firing of rubber and plastic bullets has resulted in extensive injuries and more than 20 deaths. The data from the autopsies of 17 of these victims, mostly teenagers, were analysed. Ten of the victims died from injuries inflicted by Improved Rubber Bullets, while the other seven succumbed to injuries caused by Plastic Bullets. In most instances, the cause of death was related to injuries to the brain. Ballistic features of non-conventional bullets are discussed, and suggestions to curtail the serious injuries and fatalities arising from their use are given, with emphasis on firing from a safe range. British and South African experience with this type of ammunition is compared with our observations.

Calorimetry of epitaxial thin films.

Thin film growth allows for the manipulation of material on the nanoscale, making possible the creation of metastable phases not seen in the bulk. Heat capacity provides a direct way of measuring thermodynamic properties of these new materials, but traditional bulk calorimetric techniques are inappropriate for such a small amount of material. Microcalorimetry and nanocalorimetry techniques exist for the measurements of thin films but rely on an amorphous membrane platform, limiting the types of films which can be measured. In the current work, ion-beam-assisted deposition is used to provide a biaxially oriented MgO template on a suspended membrane microcalorimeter in order to measure the specific heat of epitaxial thin films. Synchrotron x-ray diffraction showed the biaxial order of the MgO template. X-ray diffraction was also used to prove the high quality of epitaxy of a film grown onto this MgO template. The contribution of the MgO layer to the total heat capacity was measured to be just 6.5% of the total addenda contribution. The heat capacity of a Fe(.49)Rh(.51) film grown epitaxially onto the device was measured, comparing favorably to literature data on bulk crystals. This shows the viability of the MgO∕SiN(x)-membrane-based microcalorimeter as a way of measuring the thermodynamic properties of epitaxial thin films.

Heat transfer simulation and thermal measurements of microfabricated x-ray transparent heater stages.

A microfabricated amorphous silicon nitride membrane-based nanocalorimeter is proposed to be suitable for an x-ray transparent sample platform with low power heating and built-in temperature sensing. In this work, thermal characterization in both air and vacuum are analyzed experimentally and via simulation. Infrared microscopy and thermoreflectance microscopy are used for thermal imaging of the sample area in air. While a reasonably large isothermal area is found on the sample area, the temperature homogeneity of the entire sample area is low, limiting use of the device as a heater stage in air or other gases. A simulation model that includes conduction, as well as radiation and convection heat loss, is presented with radiation and convection parameters determined experimentally. Simulated temperature distributions show that the homogeneity can be improved by using a thicker thermal conduction layer or reducing the pressure of the gas in the environment but neither are good solutions for the proposed use. A new simple design that has improved temperature homogeneity and a larger isothermal area while maintaining a thin thermal conduction layer is proposed and fabricated. This new design enables applications in transmission x-ray microscopes and spectroscopy setups at atmospheric pressure.

Evidence for nanoscale two-dimensional Co clusters in CoPt3 films with perpendicular magnetic anisotropy.

The length scale of the local chemical anisotropy responsible for the growth-temperature-induced perpendicular magnetic anisotropy of face-centered cubic CoPt(3) alloy films was investigated using polarized extended x-ray absorption fine structure (EXAFS). These x-ray measurements were performed on a series of four (111) CoPt(3) films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preference for Co-Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, and random chemical order otherwise. Furthermore, atomic pair correlation anisotropy was evidenced only in the EXAFS signal from the next neighbors to the absorbing Co atoms and from multiple scattering paths focused through the next neighbors. This suggests that the Co clusters are no more than a few atoms in extent in the plane and one monolayer in extent out of the plane. Our EXAFS results confirm the correlation between perpendicular magnetic anisotropy and two-dimensional Co segregation in CoPt(3) alloy films, and establish a length scale on the order of 10 Å for the Co clusters.