643 research outputs found
Nuclear surface diffuseness revealed in nucleon-nucleus diffraction
Nuclear surface provides useful information on nuclear radius, nuclear
structure as well as properties of nuclear matter. We discuss the relationship
between the nuclear surface diffuseness and elastic scattering differential
cross section at the first diffraction peak of high-energy nucleon-nucleus
scattering as an efficient tool in order to extract the nuclear surface
information from limited experimental data involving short-lived unstable
nuclei. The high-energy reaction is described by a reliable microscopic
reaction theory, the Glauber model. Extending the idea of the black sphere
model, we find one-to-one correspondence between the nuclear bulk structure
information and proton elastic scattering diffraction peak. This implies that
we can extract both the nuclear radius and diffuseness simultaneously, using
the position of the first diffraction peak and its magnitude of the elastic
scattering differential cross section. We confirm the reliability of this
approach by using realistic density distributions obtained by a mean-field
model.Comment: 12 pages, 12 figures, to appear in Phys. Rev.
Nuclear transparencies for nucleons, knocked-out under various semi-inclusive conditions
Using hadron dynamics we calculate nuclear transparencies for protons,
knocked-out in high-, semi-inclusive reactions. Predicted transparencies
are, roughly half a standard deviation above the NE18 data. The latter contain
the effects of binned proton missing momenta and mass, and of finite detector
acceptances. In order to test sensitivity we compare computed transparencies
without restrictions and the same with maximal cuts for missing momenta and the
electron energy loss. We find hardly any variation, enabling a meaningful
comparison with data and predictions based on hadron dynamics. Should
discrepancies persist in high-statistics data, the above may with greater
confidence be attributed to exotic components in the description of the
outgoing proton.Comment: 13 pages + 3 figsin appended PS file, report # WIS-94/43/Oct-P
Absolute accuracy in membrane-based ac nanocalorimetry
To achieve accurate results in nanocalorimetry a detailed analysis and
understanding of the behavior of the calorimetric system is required. There are
especially two system-related aspects that should be taken in consideration:
the properties of the empty cell and the effect of the thermal link between
sample and cell. Here we study these two aspects for a membrane-based system
where heater and thermometer are both in good contact with each other and the
center of the membrane. Practical, analytical expressions for describing the
frequency dependence of heat capacity, thermal conductance, and temperature
oscillation of the system are formulated and compared with measurements and
numerical simulations. We finally discuss the experimental conditions for an
optimal working frequency, where high resolution and good absolute accuracy are
combined
Radio Frequency Electrical Resistance Measurement under Destructive Pulsed Magnetic Fields
We developed a resistance measurement using radio frequency reflection to
investigate the electrical transport characteristics under destructive pulsed
magnetic fields above 100 T. A homemade flexible printed circuit for a sample
stage reduced the noise caused by the induced voltage from the pulsed magnetic
fields, improving the accuracy of the measurements of the reflected waves. From
the obtained reflectance data, the absolute value of the magnetoresistance was
successfully determined by using a phase analysis with admittance charts. These
developments enable more accurate and comprehensive measurements of electrical
resistance in pulsed magnetic fields.Comment: 7 pages, 5 figure
Pion interferometry with pion-source-medium interactions
An extended pion source, which can be temporarily created by a high energy
nuclear collision, will also absorb and distort the outgoing pions. We discuss
how this effect alters the interferometric pattern of the two-pion momentum
correlation function. In particular, we show that the two-pion correlation
function decreases rapidly when the opening angle between the pions increases.
The opening-angle dependence should serve as a new means of obtaining
information about the pion source in the analysis of experimental data.Comment: 14 pages (revtex) and 9 figures (uuencoded), Caltech preprint
MAP-175, Indiana Univ. preprint IU/NTC 914-1
Quantum Criticality of an Ising-like Spin-1/2 Antiferromagnetic Chain in Transverse Magnetic Field
We report on magnetization, sound velocity, and magnetocaloric-effect
measurements of the Ising-like spin-1/2 antiferromagnetic chain system
BaCoVO as a function of temperature down to 1.3 K and applied
transverse magnetic field up to 60 T. While across the N\'{e}el temperature of
K anomalies in magnetization and sound velocity confirm the
antiferromagnetic ordering transition, at the lowest temperature the
field-dependent measurements reveal a sharp softening of sound velocity
and a clear minimum of temperature at T,
indicating the suppression of the antiferromagnetic order. At higher fields,
the curve shows a broad minimum at T, accompanied by a
broad minimum in the sound velocity and a saturation-like magnetization. These
features signal a quantum phase transition which is further characterized by
the divergent behavior of the Gr\"{u}neisen parameter . By contrast, around the critical field, the
Gr\"{u}neisen parameter converges as temperature decreases, pointing to a
quantum critical point of the one-dimensional transverse-field Ising model.Comment: Phys. Rev. Lett., to appea
Field-induced Bose-Einstein Condensation of triplons up to 8 K in Sr3Cr2O8
Single crystals of the spin dimer system Sr3Cr2O8 have been grown for the
first time. Magnetization, heat capacity, and magnetocaloric effect data up to
65 T reveal magnetic order between applied fields of Hc1 ~ 30.4 T and Hc2 ~ 62
T. This field-induced order persists up to ~ 8 K at H ~ 44 T, the highest
observed in any quantum magnet where Hc2 is experimentally-accessible. We fit
the temperature-field phase diagram boundary close to Hc1 using the expression
Tc = A(H-Hc1)^v. The exponent v = 0.65(2), obtained at temperatures much
smaller than 8 K, is that of the 3D Bose-Einstein condensate (BEC) universality
class. This finding strongly suggests that Sr3Cr2O8 is a new realization of a
triplon BEC where the universal regimes corresponding to both Hc1 and Hc2 are
accessible at He-4 temperatures.Comment: 4 pages, 3 figures, accepted by PR
Hall coefficient and Hc2 in underdoped LaFeAsO0.95F0.05
The electrical resistivity and Hall coefficient of LaFeAsO0.95F0.05
polycrystalline samples were measured in pulsed magnetic fields up to m0H = 60
T from room temperature to 1.5 K. The resistance of the normal state shows a
negative temperature coefficient (dr/dT < 0) below 70 K for this composition,
indicating insulating ground state in underdoped LaFeAsO system in contrast to
heavily doped compound. The charge carrier density obtained from Hall effect
can be described as constant plus a thermally activated term with an energy gap
DE = 630 K. Upper critical field, Hc2, estimated from resistivity measurements,
exceeds 75 T with zero-field Tc = 26.3 K, suggesting an unconventional nature
for superconductivity.Comment: 12 pages and 4 figure
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