#### Invited Speakers

Speaker |
Title |
Abstract |

Andrew CaseyRoyal Holloway University of London |
SQUID detection of Nano-electro-mechanical systems | Nano-electro-mechanical systems at low temperatures can form elements of ultra-sensitive detection schemes. The integration of such devices into quantum fluids experiments provides a potential route for studying surface modes and properties of confined superfluids. In this work we investigate how the properties of a NEMs beam are modified by coupling to the input circuit of a remote SQUID, a configuration compatible with ultra-low temperatures. Here modification of the SQUID bias conditions and the magnetic field applied to the NEMs beam effect the strength and sign of the coupling to the SQUID. We demonstrate that the beam can be driven into a stable state of self-sustained oscillations. |

Moses ChanPenn State University, USA |
Superfluid mass flow through solid helium samples | Recent torsional oscillator measurements (PRB 90, 064503 (2014)) placed an upper limit of superfluid fraction in solid helium of 5 ppm. Interestingly, this is not the last word of supersolidity. By means of a clever design, Hallock et al., UMass., found evidence of superfluid-like mass flow through solid helium sandwiched between two superfluid reservoirs (PRL 100, 235301, 2008). The UMass results with a solid sample of 4 cm were recently replicated at Penn State, using samples 8, 50 and 1000 microns thick. Our measurements show interesting similarities and also differences from that found at UMass. Mass flow rate as a function of path length, temperature and pressure will be presented. Work carried out in collaboration with Jae-ho Shin, Ariel Haziot (currently at Institut Neel, France) and Dukyoung Kim (currently at Los Alamos, USA). |

Eddy CollinInstitut NEEL, Grenoble, France |
Nanomechanical beams for sub-coherence length studies in superfluid ^{3}He |
Mechanical probes such as vibrating wires and forks are neat devices for the studies of quantum fluids. The coherence length of superfluid ^{3}He is of the order of 100 nm, a lengthscale which is easily attained today using clean room fabrication: it is thus possible now to probe this scale using dedicated nano-electro-mechanical systems (NEMS). We present low temperature properties of these devices, both linear and non-linear, and introduce measuring techniques. In particular, the parametric amplification scheme is extremely useful for over-damped systems. We discuss preliminary fluid dynamics measurements using NEMS of cross dimensions about 100 nm, and lengths up to 300 microns. |

Vladimir DmitrievKapitza Institute, Moscow, Russia |
NMR Studies of Superfluid Polar Phase of ^{3}He |
We report results of NMR experiments in superfluid polar phase of ^{3}He. This phase is not realized in bulk ^{3}He, but can be stabilized in ^{3}He confined in a new type of aerogel (nafen) which strands are nearly parallel to one another. In our experiments the polar phase was observed in all used samples of nafen with porosities from 98% down to 77%. It was found that the region of existence of this phase essentially depends on nafen porosity, pressure and on ^{4}He coverage of the nafen strands. NMR properties of the polar phase and influence of spin supercurrents on the spin dynamics were investigated. Possible future experiments will be also discussed in the talk. |

Vladimir EltsovAalto University, Finland |
New faces of superfluid ^{3}He: Higgs bosons, Majorana fermions and Alice strings |
Topological superfluid 3He possesses collective modes of the order parameter, which are analogous to Higgs boson of the Standard Model of particle physics, and fermionic excitations of Majorana, Weyl or Dirac character, which can live in bulk or as bound states at interfaces and order-parameter defects. We discuss new possibilities for studies of those states opened by recent advances in the experimental techniques. The developments include ultra-sensitive probes based on Bose-Einstein condensates of magnon quasiparticles and new superfluid phases engineered with nanostructured confinement. One example is the polar phase, where a long-time elusive half-quantum vortex has been discovered. |

Hiroshi FukuyamaUniversity of Tokyo, Japan |
A possible quantum-liquid-crystal phase in helium monolayers adsorbed on graphite | Physics of 2D helium adsorbed on graphite is attracting renewed interests and developing rapidly in two directions. One is the possible supersolidity in 2D ^{4}He; the other the quantum spin liquid (QSL) magnetism in 2D ^{3}He. Both the astonishing phenomena possibly originate from the quantum liquid crystal (QLC) phase, a new state of matter, most probably a quantum hexatic phase, which is consistent with our latest heat capacity measurements on ^{4}He/^{4}He/gr, ^{3}He/^{3}He/gr and ^{3}He/HD/HD/gr systems. In the last system, we found two QSLs with very different magnetic properties. One is an incompressible commensurate phase found at a very low density. The other is a highly compressible phase sharing similar magnetic properties with the QSL already known in ^{3}He/^{3}He/gr. |

Richard HaleyLancaster University, UK |
Breaking the superfluid speed limit | Coherent condensates appear as emergent phenomena in many systems, sharing the characteristic feature of an energy gap separating the lowest excitations from the condensate ground state. An object moving with high enough velocity that the excitation spectrum becomes gapless can create excitations at no energy cost and initiate the breakdown of the condensate. This limit is the well-known Landau velocity. For the neutral Fermionic superfluid ^{3}He-B in the T=0 limit, flow around an oscillating body displays a very clear critical velocity for the onset of dissipation. However, to our considerable surprise, we have found that for uniform linear motion there is no discontinuity whatsoever in the dissipation as the Landau critical velocity is passed and exceeded. |

Victor L'vovWeizmann Institute, Rehovot, Israel |
Statistics of quantum turbulence | Based on the current understanding of statistics of quantum turbulence as well as on the results of its intensive ongoing analytical, numerical and experimental studies. We will discuss the following problems in the large-scale, space-homoge-neous, steady-state turbulence of superfluid ^{4}He and ^{3}He: (i) Energy spectra of the normal and superfluid velocity compo-nents, (ii) Cross-correlation function of the normal and superfluid velocities, (iii) Energy dissipation by mutual friction and viscosity, (iv) Energy exchange between the normal and superfluid components, (v) High-order statistics and intermittency effects. The statistical properties will be discussed for turbulence in different types of flows: co-flow of ^{4}He; co-flow of ^{3}He with the laminar normal-fluid; pure super-flow and counter-flow in ^{4}He. |

Takeshi MizushimaOsaka University, Japan |
Topology, emergent Ising order, and spontaneous symmetry breaking in superfluid ^{3}He-B |
Superfluid ^{3}He serves as a rich repository of topological quantum phenomena, and the marriage of the prototypical topological superfluid with nanofabrication techniques brings about a rich variety of spontaneous symmetry breaking, such as a variety of Nambu-Goldstone modes and Higgs modes. In this talk, I review the recent progress on understanding the nontrivial topological structure and symmetry breaking of superfluid ^{3}He. This includes the emergence of Majorana fermions, their quantum mass acquisition at the topological critical point, and new bosonic modes bound to the surface. A key ingredient to understand these novel phenomena is the emergence of the Ising order. I show that the emergent Ising order spontaneously forms the domain wall by increasing a magnetic field across the topological critical point. |

Petr MoroshkinRIKEN Center for Emergent Matter Science, Japan |
Electrohydrodynamic effects in superfluid ^{4}He |
We inject large quantities of electrically charged impurity particles into superfluid ^{4}He that can influence the properties and the dynamics of the superfluid. Applied large static electric field induces the motion of the charged superfluid and perturbations of the liquid surface that can be captured by a fast video camera. We observe several electrohydrodynamic phenomena that so far had not been addressed in relation to superfluid helium: electroconvection, formation of bubbles, surface waves, quasistatic dimples and humps (Taylor cones), jet emission (electrospraying), as well as a shuttle motion of charged microparticles and their trapping at the free surface of the liquid. |

Yury MukharskySPEC, CEA-CNRS, University Paris-Saclay, France |
Quantum turbulence in ^{4}He studied using the SHREK facility |
We present recent results from the SHREK collaboration. Different sensors have been developed, tested and are used to characterize the large scale cryogenic turbulent flows in this large Von Karman-type experiment, including hot wire anemometer, Pitot tubes, flexible cantilever anemometer and second-sound absorption. The Pitot tubes have new design which allows for potentially wider frequency range. We also present a novel noise-reduction method of spectral averaging, allowing to average down to zero almost all sources of sensor and preamplifier noise, including thermal noise. |

Jeevak ParpiaCornell University, USA |
The A-B transition for superfluid ^{3}He confined to a 1.08 micrometer tall geometry |
Motivated by recent experimental and theoretical work, we confine superfluid ^{3}He to a 1.08 μm thick nanofluidic cavity incorporated into the head of a torsion pendulum. For measurements between 0.1 and 5.6 bar we observe that the A phase is always interspersed between the B phase and the normal state and the superfluid fraction of the A phase is always greater than that of the B phase which is bulk-like. The dissipation is greater in the B phase than the A phase contrary to bulk behavior. Despite clean surfaces that do not pin the phase boundary, the non-monotonic supercooling of the A phase is seen to be much smaller than in the bulk. Our experiment did not observe the presence of the striped phase, likely due to bowing of the cell cavity at pressure. |

Silke Bühler-PaschenVienna University of Technology, Austria |
Quantum criticality and novel phases in heavy fermion metals | Heavy fermion materials are prototype systems to study quantum criticality: the application of non-thermal control parameters such as magnetic field or pressure frequently induces a continuous phase transition at absolute zero in temperature. Quantum fluctuations emerging from such a "quantum critical point" (QCP) lead to exotic behaviour that cannot be accounted for by Landau Fermi liquid theory and is thus called non-Fermi liquid behaviour. Frequently, new phases, including unconventional superconductivity, form in the vicinity of a QCP. After an overview of the field I will present recent efforts to extend the temperature scale of such studies to ultralow temperatures using cooling by nuclear demagnetization. |

Yutaka SasakiKyoto University, Japan |
Visualizing textural domain walls in superfluid ^{3}He by Magnetic Resonance Imaging |
A real space image of textural domain walls inside a single 100 micrometer thickness slab of superfluid ^{3}He-A was MRI-imaged. Straight lines, which appeared in between large domains of uniform textures, were textural domain walls (solitons) with particularly important feature. The observed NMR properties suggest that the domain wall has almost dipole-locked soliton structure inside the wall, which connects two regions of uniform d = l texture with different chirality, namely d,l and –d,-l. This soliton is accompanied with two surface chiral domain walls located back to back on both side of the slab surface. The surface chiral domain walls anchor the dipole-locked soliton in its place. |

Oleksandr SerhaUniversity of Kaiserslautern, Germany |
Supercurrent in a room temperature Bose-Einstein magnon condensate | A supercurrent of magnons is detected by Brillouin light scattering spectroscopy in a magnon Bose-Einstein condensate (BEC) prepared in a room temperature Y_{3}Fe_{5}O_{12} magnetic film. The local laser heating induces a frequency shift between different parts of the BEC and leads to an increasing phase gradient in the condensate. As a result, a phase-gradient-induced current – a magnon supercurrent – flowing out of the focal spot, is excited. This efflux reduces the BEC density in the probing point but it does not alter the dynamics of gaseous magnons. |

Oleksii ShevtsovNorthwestern University, USA |
Electron bubbles and Weyl fermions in chiral superfluid ^{3}He-A |
The A phase of ^{3}He is a chiral superfluid that spontaneously breaks parity and time-reversal symmetries. This was beautifully demonstrated by the observation of an anomalous Hall mobility of electron bubbles by the RIKEN group. We present the theory of the anomalous Hall effect for negative ions in superfluid ^{3}He-A, based on a full quantum mechanical treatment of multiple scattering of Bogoliubov quasiparticles by ions embedded in the chiral superfluid. Quantum interference leads to the transverse force on the ions. Our results are in quantitative agreement with the RIKEN experiments, providing a microscopic view of the underlying processes responsible for the anomalous Hall effect. |

Yong-il ShinSeoul National University, Korea |
Dynamics of half-quantum vortices in a spinor Bose-Einstein condensate | Quantum vortices with half the quantum circulation, known as half-quantum vortices (HQVs), have been observed in various spinor superfluid systems recently. In this presentation, we describe our recent experimental studies of HQV dynamics in a spin-1 antiferromagnetic Bose-Einstein condensate, where we observe the spontaneous dissociation of a singly charged vortex into a pair of HQVs and the collisional dynamics of HQV pairs, revealing the short range interactions arising from their ferromagnetic cores. Additionally, we investigate the relaxation dynamics of turbulent superflow containing many HQVs to find that spin waves are generated by the collisional motions of the HQVs. |

Martin ZwierleinMassachusetts Institute of Technology, USA |
Solitons and spin-charge correlations in strongly interacting Fermi gases | Ultracold atomic Fermi gases near Feshbach resonances or in optical lattices realize paradigmatic, strongly interacting forms of fermionic matter. Topological excitations and spin-charge correlations can be directly imaged in real time. In resonant fermionic superfluids, we observe the cascade of solitonic excitations following a pi phase imprint. A planar soliton decays, via the snake instability, into vortex rings and long-lived solitonic vortices. For fermions in optical lattices, realizing the Fermi-Hubbard model, we detect charge and antiferromagnetic spin correlations with single-site resolution. At low fillings, the Pauli and correlation hole is directly revealed. In the Mott insulating state, we observe strong doublon-hole correlations, which should play an important role for transport. |