789 research outputs found
Critical spin-flip scattering at the helimagnetic transition of MnSi
We report spherical neutron polarimetry (SNP) and discuss the spin-flip
scattering cross sections as well as the chiral fraction close to the
helimagnetic transition in MnSi. For our study, we have developed a
miniaturised SNP device that allows fast data collection when used in small
angle scattering geometry with an area detector. Critical spin-flip scattering
is found to be governed by chiral paramagnons that soften on a sphere in
momentum space. Carefully accounting for the incoherent spin-flip background,
we find that the resulting chiral fraction decreases gradually above the
helimagnetic transition reflecting a strongly renormalised chiral correlation
length with a temperature dependence in excellent quantitative agreement with
the Brazovskii theory for a fluctuation-induced first order transition.Comment: 5 pages, 3 figure
Configurable multiplier modules for an adaptive computing system
The importance of reconfigurable hardware is increasing steadily. For example, the primary approach of using adaptive systems based on programmable gate arrays and configurable routing resources has gone mainstream and high-performance programmable logic devices are rivaling traditional application-specific hardwired integrated circuits. Also, the idea of moving from the 2-D domain into a 3-D design which stacks several active layers above each other is gaining momentum in research and industry, to cope with the demand for smaller devices with a higher scale of integration. However, optimized arithmetic blocks in course-grain reconfigurable arrays as well as field-programmable architectures still play an important role. In countless digital systems and signal processing applications, the multiplication is one of the critical challenges, where in many cases a trade-off between area usage and data throughput has to be made. But the a priori choice of word-length and number representation can also be replaced by a dynamic choice at run-time, in order to improve flexibility, area efficiency and the level of parallelism in computation. In this contribution, we look at an adaptive computing system called 3-D-SoftChip to point out what parameters are crucial to implement flexible multiplier blocks into optimized elements for accelerated processing. The 3-D-SoftChip architecture uses a novel approach to 3-dimensional integration based on flip-chip bonding with indium bumps. The modular construction, the introduction of interfaces to realize the exchange of intermediate data, and the reconfigurable sign handling approach will be explained, as well as a beneficial way to handle and distribute the numerous required control signals
Glassy dynamics in monodisperse hard ellipsoids
We present evidence from computer simulations for glassy dynamics in
suspensions of monodisperse hard ellipsoids. In equilibrium, almost spherical
ellipsoids show a first order transition from an isotropic phase to a rotator
phase. When overcompressing the isotropic phase into the rotator regime, we
observe super-Arrhenius slowing down of diffusion and relaxation, accompanied
by two-step relaxation in positional and orientational correlators. The effects
are strong enough for asymptotic laws of mode-coupling theory to apply. Glassy
dynamics are unusual in monodisperse systems. Typically, polydispersity in size
or a mixture of particle species is prerequisite to prevent crystallization.
Here, we show that a slight particle anisometry acts as a sufficient source of
disorder. This sheds new light on the question of which ingredients are
required for glass formation.Comment: included data of prolate system in all figures and text, extended
discussion of indicators of glassy dynamics, fixed symbol ambiguities,
preprint forma
Spin Transfer Torques in MnSi at Ultra-low Current Densities
Spin manipulation using electric currents is one of the most promising
directions in the field of spintronics. We used neutron scattering to observe
the influence of an electric current on the magnetic structure in a bulk
material. In the skyrmion lattice of MnSi, where the spins form a lattice of
magnetic vortices similar to the vortex lattice in type II superconductors, we
observe the rotation of the diffraction pattern in response to currents which
are over five orders of magnitude smaller than those typically applied in
experimental studies on current-driven magnetization dynamics in
nanostructures. We attribute our observations to an extremely efficient
coupling of inhomogeneous spin currents to topologically stable knots in spin
structures
Ferromagnetic phases in spin-Fermion systems
Spin-Fermion systems which obtain their magnetic properties from a system of
localized magnetic moments being coupled to conducting electrons are
considered. The dynamical degrees of freedom are spin- operators of
localized spins and spin-1/2 Fermi operators of itinerant electrons.
Renormalized spin-wave theory, which accounts for the magnon-magnon
interaction, and its extension are developed to describe the two ferrimagnetic
phases in the system: low temperature phase , where all electrons
contribute the ordered ferromagnetic moment, and high temperature phase
, where only localized spins form magnetic moment. The
magnetization as a function of temperature is calculated. The theoretical
predictions are utilize to interpret the experimentally measured
magnetization-temperature curves of ..Comment: 9 pages, 5 figure
Quantum Tricritical Points in NbFe
Quantum critical points (QCPs) emerge when a 2nd order phase transition is
suppressed to zero temperature. In metals the quantum fluctuations at such a
QCP can give rise to new phases including unconventional superconductivity.
Whereas antiferromagnetic QCPs have been studied in considerable detail
ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs
are avoided through either a change to 1st order transitions or through an
intervening spin-density-wave (SDW) phase. Here, we study the prototype of the
second case, NbFe. We demonstrate that the phase diagram can be modelled
using a two-order-parameter theory in which the putative FM QCP is buried
within a SDW phase. We establish the presence of quantum tricritical points
(QTCPs) at which both the uniform and finite susceptibility diverge. The
universal nature of our model suggests that such QTCPs arise naturally from the
interplay between SDW and FM order and exist generally near a buried FM QCP of
this type. Our results promote NbFe as the first example of a QTCP, which
has been proposed as a key concept in a range of narrow-band metals, including
the prominent heavy-fermion compound YbRhSi.Comment: 21 pages including S
Hidden Quantum Critical Point in a Ferromagnetic Superconductor
We consider a coexistence phase of both Ferromagnetism and superconductivity
and solve the self-consistent mean-field equations at zero temperature. The
superconducting gap is shown to vanish at the Stoner point whereas the
magnetization doesn't. This indicates that the para-Ferro quantum critical
point becomes a hidden critical point. The effective mass in such a phase gets
enhanced whereas the spin wave stiffness is reduced as compared to the pure FM
phase. The spin wave stiffness remains finite even at the para-Ferro quantum
critical point.Comment: 4 pages, Phys. Rev. B (Rapid) accepte
Substitution of Cytosine with Guanylurea Decreases the Stability of i-Motif DNA
Both 5-aza-2′-deoxycytidine (decitabine) and its primary breakdown product, 2′-deoxyriboguanylurea (GuaUre-dR), have been shown to act as mutagens and epimutagens that cause replication stress and alter both DNA methylation and gene expression patterns. As cytosine analogues, both are expected to be preferentially incorporated into regions of GC skew where runs of cytosine residues are sequestered on one strand and guanine residues on the other. Given that such regions have been identified as sites with the potential for effects on gene expression and replication stress linked to formation of alternative DNA secondary structures, it is of interest to determine the influence that these base analogues might have on the stability of structures of this kind. Here we report that incorporation of GuaUre-dR into an i-motif-forming sequence decreases both the thermal and pH stability of an i-motif despite the apparent ability of GuaUre-dR to base pair with cytosine
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