Topological nodal line in superfluid 3^3He and the Anderson theorem

We have found an experimental evidence for the existence of the Dirac nodal line in the quasiparticle spectrum of the polar phase of superfluid $^3$He. The polar phase is stabilized by confinement of $^3$He between nm-sized cylinders. The temperature dependence of the gap, measured via frequency shift in the NMR spectrum, follows expected $\propto T^3$ dependence. The results support the Fomin extension of the Anderson theorem to the polar phase with columnar defects: perfect columnar non-magnetic defects do no perturb the magnitude of the gap in the polar phase. The existence of the node line opens possibilities to study Bogoliubov Fermi surfaces and flat-band fermions in the polar phase.Comment: 6 pages, 4 figures, references adde

Conceptual design of the orbit correctors for D2 and Q4

In the luminosity upgrade of the Large Hadron Collider, many dipole, quadrupole, and corrector magnets around the ATLAS and CMS detectors are replaced with larger aperture magnets. The purpose is to reduce the beam size at the interaction point by a factor of two and thus to increase the number of particle collisions. This article presents the results of a preliminary design study of the replacements for double-aperture orbit corrector magnets positioned next to the first matching section quadrupole Q4 and the new correctors to be placed next to the recombination dipole D2. The apertures of the correctors are increased from the current 70 mm diameter to 105 mm. The larger apertures and the fixed 188/194 mm distance between the beams pose design challenges due to magnetic coupling between the apertures. The design proposal described in this report consists of a two-in-one Nb-Ti magnet with one aperture providing horizontal and the other vertical correction. The magnetic forces are taken primarily by stainless steel collars with possibly further support from the rigid iron yoke. The design of the yoke is essential in shielding the two apertures from each other

Quasiparticle damping of surface waves in superfluid 3He and 4He

Oscillations on free surfaces of superfluids at the inviscid limit are damped by quasiparticle scattering. We study this effect in both superfluid 3He and superfluid 4He, deep below the respective critical temperatures. Surface oscillators offer several benefits over immersed mechanical oscillators traditionally used for similar purposes. Damping is modeled as specular scattering of ballistic quasiparticles from the moving free surface. The model is in reasonable agreement with our measurements for superfluid 4He but significant deviation is found for 3He.Peer reviewe

Spin, Orbital, Weyl and Other Glasses in Topological Superfluids

| openaire: EC/H2020/694248/EU//TOPVACOne of the most spectacular discoveries made in superfluid 3He confined in a nanostructured material like aerogel or nafen was the observation of the destruction of the long-range orientational order by a weak random anisotropy. The quenched random anisotropy provided by the confining material strands produces several different glass states resolved in NMR experiments in the chiral superfluid 3He-A and in the time-reversal-invariant polar phase. The smooth textures of spin and orbital order parameters in these glasses can be characterized in terms of the randomly distributed topological charges, which describe skyrmions, spin vortices and hopfions. In addition, in these skyrmion glasses the momentum-space topological invariants are randomly distributed in space. The Chern mosaic, Weyl glass, torsion glass and other exotic topological states are examples of close connections between the real-space and momentum-space topologies in superfluid 3He phases in aerogel.Peer reviewe

Polar Phase of 3 He in Nematic Aerogel and Quartz Tuning Fork as Sensitive Detectors of Surface Boundary Conditions

| openaire: EC/H2020/694248/EU//TOPVAC Funding Information: The experiments on Setup 1 are carried out at the Kapitza Institute (Moscow) and was supported by the Russian Science Foundation (project no. 18-12-00384). The experimental work on Setup 2 is carried out in the Low Temperature Laboratory, which is a part of the OtaNano research infrastructure of Aalto University, and was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 694248) and by Academy of Finland (project 334470). Publisher Copyright: © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.A new superfluid phase of 3He, a polar phase, which is known to stabilize in nematic aerogel, has already led to the observation of many new phenomena. One important condition for the existence of the polar phase is a 4He coverage, which removes solid paramagnetic 3He from the surface of the aerogel strands. We report here the results of NMR experiments with superfluid 3He in nematic aerogel demonstrating the influence of 4He coverage. Simultaneous measurements of resonance properties of a quartz tuning fork immersed in liquid 3He well correlate with the NMR data.Peer reviewe

Suppressing the Kibble-Zurek Mechanism by a Symmetry-Violating Bias

| openaire: EC/H2020/694248/EU//TOPVAC Funding Information: We thank V. V. Dmitriev for providing the NAFEN sample. This Letter has been supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 694248) and by Academy of Finland Project No. 332964. S. A. acknowledges support from the Jenny and Antti Wihuri Foundation via the Council of Finnish Foundations, and T. K. acknowledges support from the Finnish Cultural Foundation. This research made use of the OtaNano Low Temperature Laboratory infrastructure of Aalto University, which is part of the European Microkelvin Platform (European Union’s Horizon 2020 Grant No. 824109). Publisher Copyright: © 2021 American Physical Society.The formation of topological defects in continuous phase transitions is driven by the Kibble-Zurek mechanism. Here we study the formation of single- and half-quantum vortices during transition to the polar phase of He3 in the presence of a symmetry-breaking bias provided by the applied magnetic field. We find that vortex formation is suppressed exponentially when the length scale associated with the bias field becomes smaller than the Kibble-Zurek length. We thus demonstrate an experimentally feasible shortcut to adiabaticity - an important aspect for further understanding of phase transitions as well as for engineering applications such as quantum computers or simulators.Peer reviewe

Thermal conductivity measurements of impregnated Nb3_{3}Sn coil samples in the temperature range of 3.5 K to 100 K

In the framework of the luminosity upgrade of the LHC, high-field magnets are under development. Magnetic flux densities of up to 13 T require the use of Nb$_{3}$Sn superconducting coils. Quench protection becomes challenging due to the high stored energy density and the low stabilizer fraction. The thermal conductivity and diffusivity of the combination of insulating layers and Nb$_{3}$Sn based cables are an important thermodynamic input parameter for quench protection systems and superfluid helium cooling studies. A two-stage cryocooler based test stand is used to measure the thermal conductance of the coil sample in two different heat flow directions with respect to the coil package geometry. Variable base temperatures of the experimental platform at the cryocooler allow for a steady-state heat flux method up to 100 K. The heat is applied at wedges style copper interfaces of the Rutherford cables. The respective temperature difference represents the absolute value of thermal conductance of the sample arrangement. We report about the measurement methodology applied to this kind of non-uniform sample composition and the evaluation of the used resin composite materials