Condensed Matter

We have carried out a study of the momentum distribution and of the spectrum of elementary excitations of liquid 4 He across the normal-superfluid transition temperature, using the path integral Monte Carlo method. Our results for the momentum distribution in the superfluid regime show that a kink is present in the range of momenta corresponding to the roton excitation. This effect disappears when crossing the transition temperature to the normal fluid, in a behavior currently unexplained by theory.

The paper investigates dynamics of nonsingular vortices in a ferromagnetic spin-1 BEC, where spin and mass superfluidity coexist in the presence of uniaxial anisotropy (linear and quadratic Zeeman effect). The analysis is based on hydrodynamics following from the Gross-Pitaevskii theory. Cores of nonsingular vortices are skyrmions with charge, which is tuned by uniaxial anisotropy and can have any fractal value between 0 and 1. There are circulations of mass and spin currents around these vortices. The results are compared with the equation of vortex motion derived earlier in the Landau-Lifshitz-Gilbert theory for magnetic vortices in easy-plane ferromagnetic insulators. In the both cases the transverse gyrotropic force (analog of the Magnus force in superfluid and classical hydrodynamics) is proportional to the charge of skyrmions in vortex cores.

Flexural mode vibrations of miniature piezoelectric tuning forks (TF) are known to be highly sensitive to superfluid excitations and quantum turbulence in 3 He and 4 He quantum fluids, as well as to the elastic properties of solid 4 He , complementing studies by large scale torsional resonators. Here we explore the sensitivity of a TF, capable of simultaneously operating in both the flexural and torsional modes, to excitations in the normal and superfluid 4 He . The torsional mode is predominantly sensitive to shear forces at the sensor - fluid interface and much less sensitive to changes in the density of the surrounding fluid when compared to the flexural mode. Although we did not reach the critical velocity for quantum turbulence onset in the torsional mode, due to its order of magnitude higher frequency and increased acoustic damping, the torsional mode was directly sensitive to fluid excitations, linked to quantum turbulence created by the flexural mode. The combination of two dissimilar modes in a single TF sensor can provide a means to study the details of elementary excitations in quantum liquids, and at interfaces between solids and quantum fluid.

We measure the response of a rotating sample of superfluid 3 He-B to spin-down to rest in the zero-temperature limit. Deviations from perfect cylindrical symmetry in the flow environment cause the initial response to become turbulent. The remaining high polarization of vortices along the rotation axis suppresses the turbulent behavior and leads to laminar late-time response. We determine the dissipation during laminar decay at (0.13−0.22) T c from the precession frequency of the remnant vortex cluster. We extract the mutual friction parameter α and confirm that its dependence on temperature and pressure agrees with theoretical predictions. We find that the zero-temperature extrapolation of α has pressure-independent value α(T=0)∼5⋅ 10 −4 , which we attribute to a process where Kelvin waves, excited at surfaces of the container, propagate into the bulk and enhance energy dissipation via overheating vortex core-bound fermions.

Magnetic materials generate demagnetizing field that depends on geometry of the sample and results in a shift of magnetic resonance frequency. This phenomenon should occur in porous nanostructures as well, e.g., in globally anisotropic aerogels. Here we report results of nuclear magnetic resonance (NMR) experiments with liquid 3 He confined in anisotropic aerogels with different types of anisotropy (nematic and planar aerogels). Strands of aerogels in pure 3 He are covered by a few atomic layers of paramagnetic solid 3 He which magnetization follows the Curie-Weiss law. We have found that in our samples the NMR shift in solid 3 He is clearly seen at ultralow temperatures and depends on value and orientation of the magnetic field. The obtained results are well described by a model of a system of non-interacting paramagnetic cylinders. The shift is proportional to the magnetization of solid 3 He and may complicate NMR experiments with superfluid 3 He in aerogel.

We present experimental observation of a new phenomenon, that we interpret as NMR-like effect on anisotropic magnetic moment of the surface Andreev bound states in topological superfluid 3 He-B at zero temperature limit. We show that an anisotropic magnetic moment formed near the horizontal surface of a mechanical resonator due to symmetry violation of the superfluid 3 He-B order parameter by the resonator's surface may lead to anomalous damping of the resonator motion in magnetic field. In difference to classical NMR technique, here NMR was excited using own harmonic motion of the mechanical resonator, and nuclear magnetic resonance was detected as a maximum in damping when resonator's angular frequency satisfied the Larmor resonance condition.

The NTMpy code package allows for simulating the one-dimensional thermal response of multilayer samples after optical excitation, as in a typical pump-probe experiment. Several Python routines are combined and optimized to solve coupled heat diffusion equations in one dimension, on arbitrary piecewise homogeneous material stacks, in the framework of the so-called three-temperature model. The energy source deposited in the material is modelled as a light pulse of arbitrary cross-section and temporal profile. A transfer matrix method enables the calculation of realistic light absorption in presence of scattering interfaces as in multilayer samples. The open source code is fully object-oriented to enable a user-friendly and intuitive interface for adjusting the physically relevant input parameters. Here, we describe the mathematical background of the code, we lay out the workflow, and we validate the functionality of our package by comparing it to commercial software, as well as to experimental transient reflectivity data recorded in a pump-probe experiment with femtosecond light pulses.

We study the spectral properties of the thermal force giving rise to the Brownian motion of a continuous mechanical system -- namely, a nanomechanical beam resonator -- in a viscous liquid. To this end, we perform two separate sets of experiments. First, we measure the power spectral density (PSD) of the position fluctuations of the resonator around its fundamental mode at its center. Then, we measure the frequency-dependent linear response of the resonator, again at its center, by driving it with a harmonic force that couples well to the fundamental mode. These two measurements allow us to determine the PSD of the Brownian force noise acting on the structure in its fundamental mode. The PSD of the force noise extracted from multiple resonators spanning a broad frequency range displays a "colored spectrum". Using a single-mode theory, we show that, around the fundamental resonances of the resonators, the PSD of the force noise follows the dissipation of a blade oscillating in a viscous liquid -- by virtue of the fluctuation-dissipation theorem.

The physical properties of the spinel LiGaCr4S8 have been studied with neutron diffraction, X-ray diffraction, magnetic susceptibility and heat capacity measurements. The neutron diffraction and synchrotron X-ray diffraction data reveal negative thermal expansion (NTE) below 111(4) K. The magnetic susceptibility deviates from Curie-Weiss behavior with the onset of NTE. At low temperature a broad peak in the magnetic susceptibility at 10.3(3) K is accompanied by the return of normal thermal expansion. First principles calculations find a strong coupling between the lattice and the simulated magnetic ground state. These results indicate strong magnetoelastic coupling in LiGaCr4S8.

Neutron diffraction and muon spin relaxation ( μ SR) studies are presented for the newly characterized polymorph of NiNb 2 O 6 ( β -NiNb 2 O 6 ) with space group P4 2 /n and μ SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4 2 /n form was determined from neutron diffraction using both powder and single crystal data. Powder neutron diffraction determined an ordering wave vector k ?? = ( 1 2 , 1 2 , 1 2 ). Single crystal data confirmed the same k ?? -vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni 2+ layers perpendicular to the c-axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related tri-rutile phases, NiSb 2 O 6 and NiTa 2 O 6 . μ SR data finds a transition temperature of T N ??15 K for this system, while the columbite polymorph exhibits a lower T N = 5.7(3) K. Our μ SR measurements also allowed us to estimate the critical exponent of the order parameter β for each polymorph. We found β= 0.25(3) and 0.16(2) for the β and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent β= ~0.28(3) for β -NiNb 2 O 6 , in agreement with the μ SR value. While both systems have β values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to well-described by S=1 spin chains, the interchain interactions in the β -polymorph are likely much larger.