7,314 research outputs found
Exact ground states of spin-2 chains
We use the matrix product approach to construct all optimum ground states of
general anisotropic spin-2 chains with nearest neighbour interactions and
common symmetries. These states are exact ground states of the model and their
properties depend on up to three parameters. We find three different
antiferromagnetic Haldane phases, one weak antiferromagnetic and one weak
ferromagnetic phase. The antiferromagnetic phases can be described as spin
liquids with exponentially decaying correlation functions. The variety of
phases found with the matrix product ansatz also gives insight into the
behaviour of spin chains with arbitrary higher spins.Comment: 7 pages, 2 figures, to be published in europhysics letters, uses
epl.cl
On the nature of pressureâinduced coordination changes in silicate melts and glasses
Progressive decreases in the SiâOâSi angles between cornerâshared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edgeâ or faceâshared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressureâinduced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals
A sensitive optical pyrometer for shock-temperature measurements
A new optical system was used to determine temperatures above 2400 K in shocked materials by measuring the spectral radiance of sub-microsecond pulses of light emitted from initially transparent solid samples in the visible and near infrared (450 to 900 nm). The high sensitivity of this optical pyrometer is attributed to the small number of channels, large aperture (0.03 steradian), the large bandwidth per channel (40 nm), and large photodiode detection area (0.2 sq cm). Improved calibration techniques reduce systematic errors encountered in previous shock-temperature experiments
Shock-induced CO2 loss from CaCO3: Implications for early planetary atmospheres
Recovered samples from shock recovery experiments on single crystal calcite were subjected to thermogravimetric analysis to determine the amount of post-shock CO2, the decarbonization interval and the activation energy, for the removal of remaining CO2 in shock-loaded calcite. Comparison of post-shock CO2 with that initially present determines shock-induced CO2 loss as a function of shock pressure. Incipient to complete CO2 loss occurs over a pressure range of approximately 10 to approximately 70 GPa. Optical and scanning electron microscopy reveal structural changes, which are related to the shock-loading. The occurrence of dark, diffuse areas, which can be resolved as highly vesicular areas as observed with a scanning electron microscope are interpreted as representing quenched partial melts, into which shock-released CO2 was injected. The experimental results are used to constrain models of shock-produced, primary CO2 atmospheres on the accreting terrestrial planets
Satisfaction with college outcomes and career services: a study of students enrolled at public 2-year and 4-year institutions in the Midwest
The purpose of this study was to examine student perception of the importance of attaining skills and to measure college satisfaction as students were about to graduate or leave their current public college. The study utilized American College Testing\u27s (ACT) World-of-Work Map and College Outcomes Survey instrument;Participants in this study, 4,421 college students at 36 public 2- and 4-year institutions in 13 Midwestern states, rated the perceived importance of attaining 26 skills, expressed their level of satisfaction with 39 aspects of college life and identified their occupational choice. In addition, descriptive information was gathered including lifetime educational aspirations, education attainment of parents and the perceived amount the college educational experiences contributed to their growth and preparation;A factor analysis of the items related with aspects of perceived college satisfaction identified academic, individual student services, student support services, student life services, career services and student activities satisfaction. Additionally, a factor analysis of the importance to attain certain skills was performed and identified five skill areas: social, analytical, technical, basic and career related outcomes skills;GLM were performed by job clusters and gender and by grade point average and lifetime educational aspiration. T-tests of independent means identified differences in perceived importance to attain skills by gender and job clusters across factors. Similar tests were performed to identify mean differences with grade point average and lifetime educational aspirations. Lifetime educational aspiration was a significant independent variable in students rating the importance to attain career related outcomes skills and career services satisfaction. Significant differences were noted in all six factor areas with significant interactions occurring in technical skills and career services satisfaction by job cluster and gender;The perceived college contribution to student growth and preparation was examined by institution type. T-tests of independent means were conducted for the analysis. Areas included intellectual growth, personal growth, social growth, preparation for further study and preparation for a career. Significant differences between public 2- and 4-year institutions occurred in personal growth, preparation for further study and preparation for a career
From AMANDA to IceCube
The first string of the neoteric high energy neutrino telescope IceCube
successfully began operating in January 2005. It is anticipated that upon
completion the new detector will vastly increase the sensitivity and extend the
reach of AMANDA to higher energies. A discussion of the IceCube's discovery
potential for extra-terrestrial neutrinos, together with the prospects of new
physics derived from the ongoing AMANDA research will be the focus of this
paper. Preliminary results of the first antarctic high energy neutrino
telescope AMANDA searching in the muon neutrino channel for localized and
diffuse excess of extra-terrestrial neutrinos will be reviewed using data
collected between 2000 and 2003. Neutrino flux limits obtained with the
all-flavor dedicated UHE and cascade analyses will be described. A first
neutrino spectrum above one TeV in agreement with atmospheric neutrino flux
expectations and no extra-terrestrial contribution will be presented, followed
by a discussion of a limit for neutralino CDM candidates annihilating in the
center of the Sun.Comment: 15 pages, 8 figures Invited talk contribution at 5th International
Conference on Non-accelerator New Physics (NANP 05), Dubna, Russia, 20-25 Jun
200
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