1,820 research outputs found
Single-molecule probing of incommensurate biphenyl
Our data on the distribution of purely-electronic linewidths of terrylene single molecules in incommensurate biphenyl crystals are compared with the data of other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts — the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at 1.8 K cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts
Estoque de carbono em sistema de uso da terra em Rondônia.
Este trabalho teve como objetivo determinar sistemas de uso da terra praticados em Rondônia, que podem contribuir para o incremento do sequestro de carbono na fitomassa e no solo. Para medir o estoque de carbono, tomou-se como referência a floresta primária, que foi comparada com os seguintes sistemas de uso da da terra: capoeira natural; capoeira melhorada com espécies leguminosas (Inga edulis e Senna sieames); monocultivo de café; sistemas agroflorestais (café x bandarra; café x seringueira); pastagens tradicionais e pastagem manejada. A floresta primária estoca, em média, 188 t ha -1 de C, sendo que 148 t ha -1 está presente na fitomassa acima do solo.bitstream/item/77901/1/FOL-6097-0001.pd
Magnetic field induced lattice anomaly inside the superconducting state of CeCoIn: evidence of the proposed Fulde-Ferrell-Larkin-Ovchinnikov state
We report high magnetic field linear magnetostriction experiments on
CeCoIn single crystals. Two features are remarkable: (i) a sharp
discontinuity in all the crystallographic axes associated with the upper
superconducting critical field that becomes less pronounced as the
temperature increases; (ii) a distinctive second order-like feature observed
only along the c-axis in the high field (10 T ) low
temperature ( 0.35 K) region. This second order transition is
observed only when the magnetic field lies within 20 of the ab-planes and
there is no signature of it above , which raises questions regarding
its interpretation as a field induced magnetically ordered phase. Good
agreement with previous results suggests that this anomaly is related to the
transition to the Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.Comment: 3 figures, 5 page
A ferrofluid based neural network: design of an analogue associative memory
We analyse an associative memory based on a ferrofluid, consisting of a
system of magnetic nano-particles suspended in a carrier fluid of variable
viscosity subject to patterns of magnetic fields from an array of input and
output magnetic pads. The association relies on forming patterns in the
ferrofluid during a trainingdphase, in which the magnetic dipoles are free to
move and rotate to minimize the total energy of the system. Once equilibrated
in energy for a given input-output magnetic field pattern-pair the particles
are fully or partially immobilized by cooling the carrier liquid. Thus produced
particle distributions control the memory states, which are read out
magnetically using spin-valve sensors incorporated in the output pads. The
actual memory consists of spin distributions that is dynamic in nature,
realized only in response to the input patterns that the system has been
trained for. Two training algorithms for storing multiple patterns are
investigated. Using Monte Carlo simulations of the physical system we
demonstrate that the device is capable of storing and recalling two sets of
images, each with an accuracy approaching 100%.Comment: submitted to Neural Network
High-magnetic field lattice length changes in URu2Si2
We report high magnetic field (up to 45 T) c-axis thermal expansion and
magnetostriction experiments on URu2Si2 single crystals. The sample length
change associated with the transition to the hidden order phase becomes
increasingly discontinous as the magnetic field is raised above 25 T. The
re-entrant ordered phase III is clearly observed in both the thermal expansion
and magnetostriction above 36 T, in good agreement with previous results. The
sample length is also discontinuous at the boundaries of this phase, mainly at
the upper boundary. A change in the sign of the coefficient of
thermal-expansion is observed at the metamagnetic transition (B_M = 38 T) which
is likely related to the existence of a quantum critical end point.Comment: 5 pages, 4 figures, to be published in PR
Time-resolved dynamics of electron wave packets in chaotic and regular quantum billiards with leads
We perform numerical studies of the wave packet propagation through open
quantum billiards whose classical counterparts exhibit regular and chaotic
dynamics. We show that for t less or similar to tau (tau being the Heisenberg
time), the features in the transmitted and reflected currents are directly
related to specific classical trajectories connecting the billiard leads. In
contrast, the long-time asymptotics of the wave packet dynamics is
qualitatively different for classical and quantum billiards. In particularly,
the decay of the quantum system obeys a power law that depends on the number of
decay channels, and is not sensitive to the nature of classical dynamics
(chaotic or regular).Comment: 5 pages, 4 figure
Heavy holes: precursor to superconductivity in antiferromagnetic CeIn3
Numerous phenomenological parallels have been drawn between f- and d-
electron systems in an attempt to understand their display of unconventional
superconductivity. The microscopics of how electrons evolve from participation
in large moment antiferromagnetism to superconductivity in these systems,
however, remains a mystery. Knowing the origin of Cooper paired electrons in
momentum space is a crucial prerequisite for understanding the pairing
mechanism. Of especial interest are pressure-induced superconductors CeIn3 and
CeRhIn5 in which disparate magnetic and superconducting orders apparently
coexist - arising from within the same f-electron degrees of freedom. Here we
present ambient pressure quantum oscillation measurements on CeIn3 that
crucially identify the electronic structure - potentially similar to high
temperature superconductors. Heavy pockets of f-character are revealed in
CeIn3, undergoing an unexpected effective mass divergence well before the
antiferromagnetic critical field. We thus uncover the softening of a branch of
quasiparticle excitations located away from the traditional spin-fluctuation
dominated antiferromagnetic quantum critical point. The observed Fermi surface
of dispersive f-electrons in CeIn3 could potentially explain the emergence of
Cooper pairs from within a strong moment antiferromagnet.Comment: To appear in Proceedings of the National Academy of Science
Fermi Surface Measurements on the Low Carrier Density Ferromagnet Ca1-xLaxB6 and SrB6
Recently it has been discovered that weak ferromagnetism of a dilute 3D
electron gas develops on the energy scale of the Fermi temperature in some of
the hexaborides; that is, the Curie temperature approximately equals the Fermi
temperature. We report the results of de Haas-van Alphen experiments on two
concentrations of La-doped CaB6 as well as Ca-deficient Ca1-dB6 and
Sr-deficient Sr1-dB6. The results show that a Fermi surface exists in each case
and that there are significant electron-electron interactions in the low
density electron gas.Comment: 4 pages, 5 figures, submitted to PR
A versatile and compact capacitive dilatometer
We describe the design, construction, calibration, and operation of a
relatively simple differential capacitive dilatometer suitable for measurements
of thermal expansion and magnetostriction from 300 K to below 1 K with a
low-temperature resolution of about 0.05 angstroms. The design is characterized
by an open architecture permitting measurements on small samples with a variety
of shapes. Dilatometers of this design have operated successfully with a
commercial physical property measurement system, with several types of
cryogenic refrigeration systems, in vacuum, in helium exchange gas, and while
immersed in liquid helium (magnetostriction only) to temperatures of 30 mK and
in magnetic fields to 45 T.Comment: 8 pages, incorporating 6 figures, submitted to Rev. Sci. Instru
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