7,827 research outputs found
A Millimeter-Wave Achromatic Half Wave Plate
We have constructed an achromatic half wave plate (AHWP) suitable for the
millimeter wavelength band. The AHWP was made from a stack of three sapphire
a-cut birefringent plates with the optical axes of the middle plate rotated by
50.5 degrees with respect to the aligned axes of the other plates. The measured
modulation efficiency of the AHWP at 110 GHz was %. In contrast,
the modulation efficiency of a single sapphire plate of the same thickness was
%. Both results are in close agreement with theoretical predictions.
The modulation efficiency of the AHWP was constant as a function of incidence
angles between 0 and 15 degrees. We discuss design parameters of an AHWP in the
context of astrophysical broad band polarimetry at the millimeter wavelength
band.Comment: In print - Applied Optics, 14 pages, 7 figure
A Cosmic Microwave Background Radiation Polarimeter Using Superconducting Bearings
Measurements of the polarization of the cosmic microwave background (CMB)
radiation are expected to significantly increase our understanding of the early
universe. We present a design for a CMB polarimeter in which a cryogenically
cooled half wave plate rotates by means of a high-temperature superconducting
(HTS) bearing. The design is optimized for implementation in MAXIPOL, a
balloon-borne CMB polarimeter. A prototype bearing, consisting of commercially
available ring-shaped permanent magnet and an array of YBCO bulk HTS material,
has been constructed. We measured the coefficient of friction as a function of
several parameters including temperature between 15 and 80 K, rotation
frequency between 0.3 and 3.5 Hz, levitation distance between 6 and 10 mm, and
ambient pressure between 10^{-7} and 1 torr. The low rotational drag of the HTS
bearing allows rotations for long periods of time with minimal input power and
negligible wear and tear thus making this technology suitable for a future
satellite mission.Comment: 6 pages, IEEE-Transactions of Applied Superconductivity, 2003, Vol.
13, in pres
Magnetic-Field-Induced 4f-Octupole in CeB6 Probed by Resonant X-ray Diffraction
CeB6, a typical Gamma_8-quartet system, exhibits a mysterious
antiferroquadrupolar ordered phase in magnetic fields, which is considered as
originating from the T_{xyz}-type magnetic octupole moment induced by the
field. By resonant x-ray diffraction in magnetic fields, we have verified that
the T_{xyz}-type octupole is indeed induced in the 4f-orbital of Ce with a
propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed
an asymmetric field dependence of the intensity for an electric quadrupole (E2)
resonance when the field was reversed, and extracted a field dependence of the
octupole by utilizing the interference with an electric dipole (E1) resonance.
The result is in good agreement with that of the NMR-line splitting, which
reflects the transferred hyperfine field at the Boron nucleus from the
anisotropic spin distribution of Ce with an O_{xy}-type quadrupole. The
field-reversal method used in the present study opens up the possibility of
being widely applied to other multipole ordering systems such as NpO2,
Ce_{x}La_{1-x}B_{6}, SmRu_{4}P_{12}, and so on.Comment: 5 pages, 4 figures, submitte
Screening in Supersymmetric Gauge Theories in Two Dimensions
We show that the string tension in N=1 two-dimensional super Yang-Mills
theory vanishes independently of the representation of the quark anti-quark
external source. We argue that this result persists in SQCD_2 and in
two-dimensional gauge theories with extended supersymmetry or in chiral
invariant models with at least one massless dynamical fermion. We also compute
the string tension for the massive Schwinger model, as a demonstration of the
method of the calculation.Comment: 8 pages, Latex. Minor changes. Accepted to Phys.Lett.
Hamiltonian study of Supersymmetric Yang-Mills Quantum Mechanics
New results obtained within the recently developed approach to supersymmetric
quantum mechanical systems are presented. The method does not suffer from the
sign problem in any dimensions and is capable to provide any quantum observable
with controllable systematic error. Discussed results include: the spectrum and
Witten index of the D=4 system, and the spectrum of zero volume glueballs in
higher 3 < D < 11 dimensions.Comment: 4 pages, 3 figures, contribution to Lattice200
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