Surface-induced near-field scaling in the Knudsen layer of a rarefied gas

We report on experiments performed within the Knudsen boundary layer of a low-pressure gas. The non-invasive probe we use is a suspended nano-electro-mechanical string (NEMS), which interacts with $^4$He gas at cryogenic temperatures. When the pressure $P$ is decreased, a reduction of the damping force below molecular friction $\propto P$ had been first reported in Phys. Rev. Lett. Vol 113, 136101 (2014) and never reproduced since. We demonstrate that this effect is independent of geometry, but dependent on temperature. Within the framework of kinetic theory, this reduction is interpreted as a rarefaction phenomenon, carried through the boundary layer by a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced by surface scattering. Adsorbed atoms are shown to play a key role in the process, which explains why room temperature data fail to reproduce it.Comment: Article plus supplementary materia

Non-linear Frequency Transduction of Nano-mechanical Brownian Motion

We report on experiments addressing the non-linear interaction between a nano-mechanical mode and position fluctuations. The Duffing non-linearity transduces the Brownian motion of the mode, and of other non-linearly coupled ones, into frequency noise. This mechanism, ubiquitous to all weakly-nonlinear resonators thermalized to a bath, results in a phase diffusion process altering the motion: two limit behaviors appear, analogous to motional narrowing and inhomogeneous broadening in NMR. Their crossover is found to depend non-trivially on the ratio of the frequency noise correlation time to its magnitude. Our measurements obtained over an unprecedented range covering the two limits match the theory of Y. Zhang and M. I. Dykman, Phys. Rev. B 92, 165419 (2015), with no free parameters. We finally discuss the fundamental bound on frequency resolution set by this mechanism, which is not marginal for bottom-up nanostructures.Comment: Article plus Supplementary Materia

On the nonlinear NMR and magnon BEC in antiferromagnetic materials with coupled electron-nuclear spin precession

We present a new study of nonlinear NMR and Bose-Einstein Condensation (BEC) of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and nuclear spins. In particular, we show that the observed behaviour of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a new theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with BEC of nuclear spin waves with k=0.Comment: 12 pages, 8 figure

Observation of majorana quasiparticles’ edge states in superfluid3he

© Springer-Verlag Wien 2014. We suggest in this article the nuclear magnetic resonance (NMR) method of observation and investigations of Majorana fermions at the edge of Topological Insulator, superfluid 3He-B. The Majorana fermions form the remarkable quantum state of condensed matter where particle-like and antiparticle (holelike) excitations are indistinguishable. They have been observed recently by deviation of the temperature dependence of the superfluid3He-B heat capacity from the well-known exponential law for Bogoliubov quasiparticles at the world limit of ultra-low temperatures. The experimental data are well described by adding the heat capacity of Majorana quasiparticles’ edge states with zero energy gap. We report here the results of the similar experiments with extended temperature range down to 125 lK. The possible way to detect these states by means of NMR is also discussed

Measuring frequency fluctuations in nonlinear nanomechanical resonators

Advances in nanomechanics within recent years have demonstrated an always expanding range of devices, from top-down structures to appealing bottom-up MoS$_2$ and graphene membranes, used for both sensing and component-oriented applications. One of the main concerns in all of these devices is frequency noise, which ultimately limits their applicability. This issue has attracted a lot of attention recently, and the origin of this noise remains elusive up to date. In this Letter we present a very simple technique to measure frequency noise in nonlinear mechanical devices, based on the presence of bistability. It is illustrated on silicon-nitride high-stress doubly-clamped beams, in a cryogenic environment. We report on the same $T/f$ dependence of the frequency noise power spectra as reported in the literature. But we also find unexpected {\it damping fluctuations}, amplified in the vicinity of the bifurcation points; this effect is clearly distinct from already reported nonlinear dephasing, and poses a fundamental limit on the measurement of bifurcation frequencies. The technique is further applied to the measurement of frequency noise as a function of mode number, within the same device. The relative frequency noise for the fundamental flexure $\delta f/f_0$ lies in the range $0.5 - 0.01~$ppm (consistent with literature for cryogenic MHz devices), and decreases with mode number in the range studied. The technique can be applied to {\it any types} of nano-mechanical structures, enabling progresses towards the understanding of intrinsic sources of noise in these devices.Comment: Published 7 may 201

informal adjustment of association?

In our paper, we look at the conditions for successful transfer of European Union (EU) rules in the areas of transport, environment and energy to the associated Eastern Partnership countries. We assume that in these areas there are fewer indirect external benefits of implementing EU rules than in the areas of trade and visa free regime and therefore the adoption of these rules should depend more on their direct relevance to the governments of associated countries. Our review of law harmonization in all three countries is complemented by three in-depth case studies in all three areas. These offer an analysis of how EU standards and templates travel to this neighbourhood by delving into their adoption and implementation and assessing the degree to which they fit with governmental priorities. The first case study considers transport and focuses on the implementation of the road safety directive (2009/40/EC) in just one country, Georgia, where implementation proved challenging. The second case study concerns Ukraine and Moldova, focusing on the role of environmental impact assessment regulations in discussions between the two countries on the possible construction of hydropower plants on the Dniester River. In the area of energy, the third case study focuses on unbundling in the electricity sector in all three associated countries. Our main finding is that transposition and implementation in these areas is patchy, but better than expected. This is due to the on-going informal adjustment of the Association Agreements, which has reduced the scope of the commitments taken. While this informal adjustment helps to lighten the burden of law harmonization and facilitate transfer of the EU acquis, it does not seem to follow any blueprint, and thus creates uncertainty among the different stakeholders over future regulation