3,147 research outputs found
Spintronics via non-axisymmetric chiral skyrmions
Micromagnetic calculations demonstrate a peculiar evolution of
non-axisymmetric skyrmions driven by an applied magnetic field in confined
helimagnets with longitudinal modulations. We argue that these specific
solitonic states can be employed in nanoelectronic devices as an effective
alternative to the common axisymmetric skyrmions which occur in magnetically
saturated states
The properties of isolated chiral skyrmions in thin magnetic films
Axisymmetric solitonic states (chiral skyrmions) have been predicted
theoretically more than two decades ago. However, until recently they have been
observed in a form of skyrmionic condensates (hexagonal lattices and other
mesophases). In this paper we report experimental and theoretical
investigations of isolated chiral skyrmions discovered in PdFe/Ir(111) bilayers
two years ago (Science 341 , 636 (2013)). The results of spin-polarized
scanning tunneling microscopy analyzed within the continuum and discrete models
provide a consistent description of isolated skyrmions in thin layers. The
existence region of chiral skyrmions is restricted by strip-out instabilities
at low fields and a collapse at high fields. We demonstrate that the same
equations describe axisymmetric localized states in all condensed matter
systems with broken mirror symmetry, and thus our findings establish basic
properties of isolated skyrmions common for chiral liquid crystals, different
classes of noncentrosymmetric magnets, ferroelectrics, and multiferroics.Comment: 12 pages, 12 figure
On the nature of the solar-wind-Mars interaction
Plasma measurements near Mars on the U.S.S.R. Mars-2, -3, and -5 spacecraft are considered. The data are compared with simultaneous magnetic measurements. Strong evidence is obtained in favor of a direct interaction and mass exchange between the solar wind plasma and the gaseous envelope of Mars
Topological defects in antiferromagnetically coupled multilayers with perpendicular anisotropy
A rich variety of specific multidomain textures recently observed in
antiferromagnetically coupled multilayers with perpendicular anisotropy include
regular (equilibrium) multidomain states as well as different types of
topological magnetic defects. Within a phenomenological theory we have
classified and analyzed the possible magnetic defects in the antiferromagnetic
ground state and determine their structures. We have derived the optimal sizes
of the defects as functions of the antiferromagnetic exchange, the applied
magnetic field, and geometrical parameters of the multilayer. The calculated
magnetic phase diagrams show the existence regions for all types of magnetic
defects. Experimental investigations of the remanent states (observed after
different magnetic pre-history) in [Co/Pt]/Ru multilayers with wedged Co layers
reveal a corresponding succession of different magnetic defect domain types.Comment: 3 pages, 4 figure
A Protocol for Generating Random Elements with their Probabilities
We give an AM protocol that allows the verifier to sample elements x from a
probability distribution P, which is held by the prover. If the prover is
honest, the verifier outputs (x, P(x)) with probability close to P(x). In case
the prover is dishonest, one may hope for the following guarantee: if the
verifier outputs (x, p), then the probability that the verifier outputs x is
close to p. Simple examples show that this cannot be achieved. Instead, we show
that the following weaker condition holds (in a well defined sense) on average:
If (x, p) is output, then p is an upper bound on the probability that x is
output. Our protocol yields a new transformation to turn interactive proofs
where the verifier uses private random coins into proofs with public coins. The
verifier has better running time compared to the well-known Goldwasser-Sipser
transformation (STOC, 1986). For constant-round protocols, we only lose an
arbitrarily small constant in soundness and completeness, while our public-coin
verifier calls the private-coin verifier only once
On Basing Search SIVP on NP-Hardness
The possibility of basing cryptography on the minimal assumption NPBPP is at the very heart of complexity-theoretic cryptography. The closest we have gotten so far is lattice-based cryptography whose average-case security is based on the worst-case hardness of approximate shortest vector problems on integer lattices. The state-of-the-art is the construction of a one-way function (and collision-resistant hash function) based on the hardness of the -approximate shortest independent vector problem .
Although SIVP is NP-hard in its exact version, Guruswami et al (CCC 2004) showed that is in NPcoAM and thus unlikely to be NP-hard. Indeed, any language that can be reduced to (under general probabilistic polynomial-time adaptive reductions) is in AMcoAM by the results of Peikert and Vaikuntanathan (CRYPTO 2008) and Mahmoody and Xiao (CCC 2010). However, none of these results apply to reductions to search problems, still leaving open a ray of hope: can NP be reduced to solving search SIVP with approximation factor ?
We eliminate such possibility, by showing that any language that can be reduced to solving search with any approximation factor lies in AM intersect coAM. As a side product, we show that any language that can be reduced to discrete Gaussian sampling with parameter lies in AM intersect coAM
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