1,041 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
Analytische Modellierung des Zeitverhaltens und der Verlustleistung von CMOS-Gattern
In modernen CMOS-Technologien werden die Verzögerungszeit, die Ausgangsflankensteilheit und der Querstrom eines Gatters sowohl durch die LastkapazitĂ€t als auch durch die Steilheit des Eingangssignals beeinflusst. Die heute verwendeten Technologiebibliotheken beinhalten Tabellenmodelle mit 25 oder mehr StĂŒtzpunkten dieser AbhĂ€ngigkeiten, woraus durch Interpolation die benötigten Zwischenwerte berechnet werden. Bisherige Versuche, analytische Modelle abzuleiten beruhten darauf, den Querstrom zu vernachlĂ€ssigen oder Transistorströme als stĂŒckweise linear anzunĂ€hern. Der hier gezeigte Ansatz beruht auf einer nĂ€herungsweisen Lösung der Differentialgleichung, die aus den beiden Transistorströmen und einer LastkapazitĂ€t besteht und damit das Schaltverhalten eines Inverters beschreibt. Mit wenigen Technologieparametern können daraus fĂŒr einen beliebig dimensionierten Inverter die fĂŒr eine Timing- und Verlustleistungsanalyse notwendigen GröĂen berechnet werden. Das Modell erreicht bei einem Vergleich zu Referenzwerten aus SPICE Simulationen eine Genauigkeit von typischerweise 5%.</p><p style="line-height: 20px;">In modern CMOS-technologies the gate delay, output transition time and the short-circuit current depend on the capacitive load as well as on the input transition time. Todayâs technology libraries use table models with 25 or more samples for these dependencies. Intermediate values have to be calculated through interpolation. Attempts to derive analytical models are based on neglecting the short-circuit current or approximating it by piecewise linear functions. The approach shown in this paper provides an approximate solution for the differential equation describing the dynamic behavor of an inverter circuit. It includes the influence of both transistor currents and a single load capacitance. The required values for timing and power analysis can be calculated with a small set of technology parameters for an arbitrary designed inverter. Compared to reference values extracted from SPICE simulations, the model achieves a typical precision of 5%
Hardwarearchitektur fĂŒr einen universellen LDPC Decoder
Im vorliegenden Beitrag wird eine universelle Decoderarchitektur fĂŒr einen Low-Density Parity-Check (LDPC) Code Decoder vorgestellt. Anders als bei den in der Literatur hĂ€ufig beschriebenen Architekturen fĂŒr strukturierte Codes ist die hier vorgestellte Architektur frei programmierbar, so dass jeder beliebige LDPC Code durch eine Ănderung der Initialisierung des Speichers fĂŒr die PrĂŒfmatrix mit derselben Hardware decodiert werden kann. Die gröĂte Herausforderung beim Entwurf von teilparallelen LDPC Decoder Architekturen liegt im konfliktfreien Datenaustausch zwischen mehreren parallelen Speichern und Berechnungseinheiten, wozu ein Mapping und Scheduling Algorithmus benötigt wird. Der hier vorgestellte Algorithmus stĂŒtzt sich auf Graphentheorie und findet fĂŒr jeden beliebigen LDPC Code eine fĂŒr die Architektur optimale Lösung. Damit sind keine Wartezyklen notwendig und die ParallelitĂ€t der Architektur wird zu jedem Zeitpunkt voll ausgenutzt
A Phenomenological Description of the Non-Fermi-Liquid Phase of MnSi
In order to understand the non-Fermi-liquid behavior of MnSi under pressure
we propose a scenario on the basis of the multispiral state of the magnetic
moment.
This state can describe the recent critical experiment of the Bragg sphere in
the neutron scattering which is the key ingredient of the non-Fermi-liquid
behavior.Comment: 3 page
Peculiar behavior of the electrical resistivity of MnSi at the ferromagnetic phase transition
The electrical resistivity of a single crystal of MnSi was measured across
its ferromagnetic phase transition line at ambient and high pressures. Sharp
peaks of the temperature coefficient of resistivity characterize the transition
line. Analysis of these data shows that at pressures to ~0.35 GPa these peaks
have fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is
symptomatic that this structure disappears at pressures higher than ~0.35 GPa,
which was identified earlier as a tricritical poin
Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction
We report simultaneous measurements of the distribution of lattice constants
and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and
conventional neutron diffraction, as a function of temperature and pressure up
to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is
purely parasitic and quantitatively originates from the distribution of lattice
constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high
pressure are separated by a line of first-order phase transitions, which ends
in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must
have different symmetry, as expected of the HO being, e.g., incommensurate
orbital currents, helicity order, or multipolar order.Comment: 4 pages, 4 figure
Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator CuOSeO
Linear dichroism -- the polarization dependent absorption of electromagnetic
waves -- is routinely exploited in applications as diverse as structure
determination of DNA or polarization filters in optical technologies. Here
filamentary absorbers with a large length-to-width ratio are a prerequisite.
For magnetization dynamics in the few GHz frequency regime strictly linear
dichroism was not observed for more than eight decades. Here, we show that the
bulk chiral magnet CuOSeO exhibits linearly polarized magnetization
dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical
filters that are assembled from filamentary absorbers, the magnet provides
linear polarization as a bulk material for an extremely wide range of
length-to-width ratios. In addition, the polarization plane of a given mode can
be switched by 90 via a tiny variation in width. Our findings shed a
new light on magnetization dynamics in that ferrimagnetic ordering combined
with anisotropic exchange interaction offers strictly linear polarization and
cross-polarized modes for a broad spectrum of sample shapes. The discovery
allows for novel design rules and optimization of microwave-to-magnon
transduction in emerging microwave technologies.Comment: 20 pages, 4 figure
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
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