1,222 research outputs found
Influence of the Dzyaloshinskii-Moriya exchange interaction on quantum phase interference of spins
Magnetization measurements of a Mn12mda wheel single-molecule magnet with a
spin ground state of S = 7 show resonant tunneling and quantum phase
interference, which are established by studying the tunnel rates as a function
of a transverse field applied along the hard magnetization axis.
Dzyaloshinskii-Moriya (DM) exchange interaction allows the tunneling between
different spin multiplets. It is shown that the quantum phase interference of
these transitions is strongly dependent on the direction of the DM vector.Comment: 5 pages, 5 figure
Fabrication of Nano-Gapped Single-Electron Transistors for Transport Studies of Individual Single-Molecule Magnets
Three terminal single-electron transistor devices utilizing Al/Al2O3 gate
electrodes were developed for the study of electron transport through
individual single-molecule magnets. The devices were patterned via multiple
layers of optical and electron beam lithography. Electromigration induced
breaking of the nanowires reliably produces 1-3 nm gaps between which the SMM
can be situated. Conductance through a single Mn12(3-thiophenecarboxylate)
displays the coulomb blockade effect with several excitations within +/- 40
meV.Comment: 10 pages, 5 figure
Shaping the global communications milieu : the EU's influence on internet and telecommunications governance
This article evaluates the European Union's (EU) influence in shaping the global governance for telecommunications and the Internet. Through analysing EU behaviour within an actorness framework, we demonstrate how the external opportunity structure and the EU's internal environment has impacted on its ability to exert and maximize its presence in order to meet its goals and aims in these two very different sub-sectors of global communications in terms of evolution and development. Such a comparison of EU actorness, we argue, is revealing in terms of uncovering the underlying factors and conditions that allow the EU to influence two important and dynamic communications sub-sectors
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
Keck Adaptive Optics Imaging of Nearby Young Stars: Detection of Close Multiple Systems
Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have
surveyed 24 of the nearest young stars known in search of close companions. Our
sample includes members of the MBM 12 and TW Hydrae young associations and the
classical T Tauri binary UY Aurigae in the Taurus star-forming region. We
present relative photometry and accurate astrometry for 10 close multiple
systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high
in comparison to other young regions, though the significance of this result is
low because of the small number statistics. We resolve S 18 into a triple
system including a tight 63 mas (projected separation of 17 AU at a distance of
275 pc) binary for the first time, with a hierarchical configuration
reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample
-- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass
measurement using astrometric observations within a few years, and thus could
be important for testing pre-main sequence evolutionary models. Our
observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is
bound to TWA 5A. We find that the flux ratio of UY Aur has changed
dramatically, by more than a magnitude in the H-band, possibly as a result of
variable extinction. With a smaller flux ratio, the system may once again
become detectable as an optical binary, as it was at the time of its discovery
in 1944. Taken together, our results demonstrate that adaptive optics on large
telescopes is a powerful tool for detecting tight companions, and thus
exploring the frequency and configurations of close multiple systems.Comment: accepted for publication in The Astronomical Journa
Quantum phase interference and spin parity in Mn12 single-molecule magnets
Magnetization measurements of Mn12 molecular nanomagnets with spin ground
states of S = 10 and S = 19/2 showresonance tunneling at avoided energy level
crossings. The observed oscillations of the tunnel probability as a function of
the magnetic field applied along the hard anisotropy axis are due to
topological quantum phase interference of two tunnel paths of opposite
windings. Spin-parity dependent tunneling is established by comparing the
quantum phase interference of integer and half-integer spin systems.Comment: 5 pages, 5 figure
Quantum Phase Interference and Neel-Vector Tunneling in Antiferromagnetic Molecular Wheels
The antiferromagnetic molecular wheel Fe18 of eighteen exchange-coupled
Fe(III) ions has been studied by measurements of the magnetic torque, the
magnetization, and the inelastic neutron scattering spectra. The combined data
show that the low-temperature magnetism of Fe18 is very accurately described by
the Neel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory.
In addition, the magnetic torque as a function of applied field exhibits
oscillations that reflect the oscillations in the NVT tunnel splitting with
field due to quantum phase interference.Comment: 5 pages, 4 figures, REVTEX4, to appear in PR
Huge Transverse Magnetization in the Field-Induced Phase of the Antiferromagnetic Molecular Wheel CsFe8
The 1H-NMR spectrum and nuclear relaxation rate 1/T_1 in the
antiferromagnetic wheel CsFe8 were measured to characterize the previously
observed magnetic field-induced low-temperature phase around the level crossing
at 8 T. The data show that the phase is characterized by a huge staggered
transverse polarization of the electronic Fe spins, and the opening of a gap,
providing microscopic evidence for the interpretation of the phase as a
field-induced magneto-elastic instability.Comment: 5 pages, 4 figures, REVTEX4, to appear in PR
Realization of random-field dipolar Ising ferromagnetism in a molecular magnet
The longitudinal magnetic susceptibility of single crystals of the molecular
magnet Mn-acetate obeys a Curie-Weiss law, indicating a transition to a
ferromagnetic phase due to dipolar interactions. With increasing magnetic field
applied transverse to the easy axis, the transition temperature decreases
considerably more rapidly than predicted by mean field theory to a T=0 quantum
critical point. Our results are consistent with an effective Hamiltonian for a
random-field Ising ferromagnet in a transverse field, where the randomness is
induced by an external field applied to Mn-acetate crystals that are
known to have an intrinsic distribution of locally tilted magnetic easy axes.Comment: 4 pages, 4 figure
Tuning Magnetic Avalanches in Mn12-ac
Using micron-sized Hall sensor arrays to obtain time-resolved measurements of
the local magnetization, we report a systematic study in the molecular magnet
Mn-acetate of magnetic avalanches controllably triggered in different
fixed external magnetic fields and for different values of the initial
magnetization. The speeds of propagation of the spin-reversal fronts are in
good overall agreement with the theory of magnetic deflagration of Garanin and
Chudnovsky \cite{Garanin}.Comment: 8 pages, 7 figures; discussion expanded and revise
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