4,571 research outputs found
Quantum Hole Digging in Magnetic Molecular Clusters
Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum
relaxation regime. We showed recently that the predicted ``square-root time''
relaxation is obeyed, allowing us to develop a new method for watching the
evolution of the distribution of molecular spin states in the sample. We
measured the distribution P(H) of molecules which are in resonance at the
applied field H. Tunnelling initially causes rapid transitions of molecules,
thereby ``digging a hole'' in P(H). For small initial magnetisation values, the
hole width shows an intrinsic broadening which may be due to nuclear spins. We
present here hole digging measurements in the thermal activated regime which
may allow to study the effect of spin-phonon coupling.Comment: 3 pages, 2 figures, conference proceedings of LT22 (Helsinki,
Finland, August 4-11, 1999
Impact of end effector technology on telemanipulation performance
Generic requirements for end effector design are briefly summarized as derived from generic functional and operational requirements. Included is a brief summary of terms and definitions related to end effector technology. The second part contains a brief overview of end effector technology work as JPL during the past ten years, with emphasis on the evolution of new mechanical, sensing and control capabilities of end effectors. The third and major part is devoted to the description of current end effector technology. The ongoing work addresses mechanical, sensing and control details with emphasis on mechanical ruggedness, increased resolution in sensing, and close electronic and control integration with overall telemanipulator control system
Answer to the comment of Chudnovsky: On the square-root time relaxation in molecular nanomagnets
Answer to the comment of E. Chudnovsky concerning the following papers:
(1) N.V. Prokof'ev, P.C.E. Stamp, Phys. Rev. Lett.80, 5794 (1998).
(2) W. Wernsdorfer, T. Ohm, C. Sangregorio, R. Sessoli, D. Mailly, C.
Paulsen, Phys. Rev. Lett. 82, 3903 (1999).Comment: 1 page
Relaxation in the 3D ordered CoTAC spin chain by quantum nucleation of 0D domain walls
We have shown that resonant quantum tunnelling of the magnetisation (QTM),
until now observed only in 0D cluster systems (SMMs), occurs in the molecular
Ising spin chain, CoTAC ([(CH_3)_3NH]CoCl_3 - 2H_2O) which orders as a canted
3D-antiferromagnet at T_C=4.15 K. This effect was observed around a resonant
like field value of 1025 Oe. We present here measurements of the relaxation of
the magnetisation as a function of time, from the zero field cooled (ZFC)
antiferromagnet state and from the saturated ferromagnet state. We show that,
at the resonant field, the relaxation from the saturated state occurs in a
complicated process, whereas, surprisingly, in the case of the ZFC state, the
relaxation is exponential.Comment: 4 pages, 5 figures, LT25 proceeding
Robust preconditioners for a new stabilized discretization of the poroelastic equations
In this paper, we present block preconditioners for a stabilized
discretization of the poroelastic equations developed in [45]. The
discretization is proved to be well-posed with respect to the physical and
discretization parameters, and thus provides a framework to develop
preconditioners that are robust with respect to such parameters as well. We
construct both norm-equivalent (diagonal) and field-of-value-equivalent
(triangular) preconditioners for both the stabilized discretization and a
perturbation of the stabilized discretization that leads to a smaller overall
problem after static condensation. Numerical tests for both two- and
three-dimensional problems confirm the robustness of the block preconditioners
with respect to the physical and discretization parameters
The ATLAS beam pick-up based timing system
The ATLAS BPTX stations are comprised of electrostatic button pick-up
detectors, located 175 m away along the beam pipe on both sides of ATLAS. The
pick-ups are installed as a part of the LHC beam instrumentation and used by
ATLAS for timing purposes.
The usage of the BPTX signals in ATLAS is twofold: they are used both in the
trigger system and for LHC beam monitoring. The BPTX signals are discriminated
with a constant-fraction discriminator to provide a Level-1 trigger when a
bunch passes through ATLAS. Furthermore, the BPTX detectors are used by a
stand-alone monitoring system for the LHC bunches and timing signals. The BPTX
monitoring system measures the phase between collisions and clock with a
precision better than 100 ps in order to guarantee a stable phase relationship
for optimal signal sampling in the subdetector front-end electronics. In
addition to monitoring this phase, the properties of the individual bunches are
measured and the structure of the beams is determined.
On September 10, 2008, the first LHC beams reached the ATLAS experiment.
During this period with beam, the ATLAS BPTX system was used extensively to
time in the read-out of the sub-detectors. In this paper, we present the
performance of the BPTX system and its measurements of the first LHC beams.Comment: 3 pages. Submitted to NIM A for the proceedings of TIPP09 (Tsukuba,
Japan
In Search of a Meaningful Story: Oral History and Triathlon Memory in Australia
High Efficiency Video Coding (HEVC) is currently being prepared as the newest video coding standard of the ITU-T Video Coding Experts Group and the ISO/IEC Moving Picture Experts Group. The main goal of the HEVC standardization effort is to enable significantly improved compression performance relative to existing standards-in the range of 50% bit-rate reduction for equal perceptual video quality. This paper provides an overview of the technical features and characteristics of the HEVC standard
Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets
We investigated the effect of intermolecular dipolar interactions on a cubic
3D ensemble of 5X5X4=100 nanomagnets, each with spin . We employed the
Landau-Lifshitz-Gilbert equation to solve for the magnetization curves
for several values of the damping constant , the induction sweep rate,
the lattice constant , the temperature , and the magnetic anisotropy
field . We find that the smaller the , the stronger the maximum
induction required to produce hysteresis. The shape of the hysteresis loops
also depends on the damping constant. We find further that the system
magnetizes and demagnetizes at decreasing magnetic field strengths with
decreasing sweep rates, resulting in smaller hysteresis loops. Variations of
within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction
plays an important role in the magnetic hysteresis by controlling the
relaxation process. The dependencies of and of are presented
and discussed with regard to recent experimental data on nanomagnets.
enhances the size of the hysteresis loops for external fields parallel to the
anisotropy axis, but decreases it for perpendicular external fields. Finally,
we reproduce and test an curve for a 2D-system [M. Kayali and W. Saslow,
Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is
only weakly dependent on the shape anisotropy field and the sweep rate, but
depends sensitively upon the dipolar interactions. Although in 3D systems,
dipole-dipole interactions generally diminish the hysteresis, in 2D systems,
they strongly enhance it. For both square 2D and rectangular 3D lattices with
, dipole-dipole interactions can cause
large jumps in the magnetization.Comment: 15 pages 14 figures, submitted to Phys. Rev.
Suppression of Quantum Phase Interference in Molecular Magnets Fe₈ with Dipolar-Dipolar Interaction
Renormalized tunnel splitting with a finite distribution in the biaxial spin
model for molecular magnets is obtained by taking into account the dipolar
interaction of enviromental spins. Oscillation of the resonant tunnel splitting
with a transverse magnetic field along the hard axis is smeared by the finite
distribution which subsequently affects the quantum steps of hysteresis curve
evaluated in terms of the modified Landau-Zener model of spin flipping induced
by the sweeping field. We conclude that the dipolar-dipolar interaction drives
decoherence of quantum tunnelling in molcular magnets Fe₈, which explains
why the quenching points of tunnel spliting between odd and even resonant
tunnelling predcited theoretically were not observed experimentally.Comment: 5 pages including 3 figure and 1 table. To appear in Physical Review
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