Condensed Matter

We present an experimental and theoretical study of the 2D dynamics of electrically charged nanoparticles trapped under a free surface of superfluid helium in a static vertical electric field. We focus on the dynamics of particles driven by the interaction with quantized vortices terminating at the free surface. We identify two types of particle trajectories and the associated vortex structures: vertical linear vortices pinned at the bottom of the container and half-ring vortices travelling along the free surface of the liquid.

Quantum turbulence in superfluid He-4 in narrow channels often takes the form of moving localized vortex tangles. Such tangles, called turbulent plugs, also serve as building blocks of quantum turbulence in wider channels. We report on a numerical study of various aspects of the dynamics and structure of turbulent plugs in a wide range of governing parameters. The unrestricted growth of the tangle in a long channel provides a unique view on a natural tangle structure including superfluid motion at many scales. We argue that the edges of the plugs propagate as turbulent fronts, following the advection-diffusion-reaction dynamics. This analysis shows that the dynamics of the two edges of the tangle have distinctly different nature. We provide an analytic solution of the equation of motion for the fronts that define their shape, velocities and effective diffusivity, and analyze these parameters for various flow conditions.

We report on the first ESR study of atomic hydrogen and tritium stabilized in a solid T 2 and T 2 :H 2 matrices down to 70 mK. The concentrations of T atoms in pure T 2 approached 2× 10 20 cm −3 and record-high concentrations of H atoms ∼1× 10 20 cm −3 were reached in T 2 :H 2 solid mixtures where a fraction of T atoms became converted into H due to the isotopic exchange reaction T+H 2 → TH+H. The maximum concentrations of unpaired T and H atoms was limited by their recombination which becomes enforced by efficient atomic diffusion due to a presence of a large number of vacancies and phonons generated in the matrices by β -particles. Recombination also appeared in an explosive manner both being stimulated and spontaneously in thick films where sample cooling was insufficient. We suggest that the main mechanism for H and T migration is physical diffusion related to tunneling or hopping to vacant sites in contrast to isotopic chemical reactions which govern diffusion of H and D atoms created in H 2 and D 2 matrices by other methods.

We carried out quartz crystal microbalance experiments of a 5 MHz AT-cut crystal for superfluid 4He films on Grafoil (exfoliated graphite) with a small amount of 3He up to 0.40 atoms/nm2. We found that the mass decoupling from oscillating substrate is considerable sensitive even in a small amount of 3He doping. In a 4He film of 29.3 atoms/nm2, we observed a small drop in resonance frequency at T3 of ~0.4 K for a small amplitude, which is attributed to sticking of 3He atoms on the 4He solid atomic layer. For a large amplitude, the 4He solid layer shows a reentrant mass decoupling at TR close to T3. This decoupling can be explained by the suppression of the superfluid counterflow due to the adsorption of 3He atoms on edge dislocations. As the 4He areal density increases, TR shifts to the lower temperature, and vanishes around a 4He film of 39.0 atoms/nm2.

We report measurements of elastic moduli of hcp solid 4 He down to 15 mK when the samples are rotated unidirectionally. Recent investigations have revealed that the elastic behavior of solid 4 He is dominated by gliding of dislocations and pinning of them by 3 He impurities, which move in the solid like Bloch waves (impuritons). Motivated by the recent controversy of torsional oscillator studies, we have preformed direct measurements of shear and Young's moduli of annular solid 4 He using pairs of quarter-circle shape piezoelectric transducers (PZTs) while the whole apparatus is rotated with angular velocity Ω up to 4 rad/s. We have found that shear modulus μ is suppressed by rotation below 80 mK, when shear strain applied by PZT exceeds a critical value, above which μ decreases because the shear strain unbinds dislocations from 3 He impurities. The rotation - induced decrement of μ at Ω=4 rad/s is about 14.7 (12.3) % of the total change of temperature dependent μ for solid samples of pressure 3.6 (5.4) MPa. The decrements indicate that the probability of pinning of 3 He on dislocation segment, G , decreases by several orders of magnitude. We propose that the motion of 3 He impuritons under rotation becomes strongly anisotropic by the Coriolis force, resulting a decrease in G for dislocation lines aligning parallel to the rotation axis.

Understanding the temperature dependence of thermal boundary resistance, or Kapitza resistance, between liquid helium and sintered metal has posed a problem in low temperature physics for decades. In the ballistic regime of superfluid 3 He-B, we find the Kapitza resistance can be described via scattering of thermal excitations (quasiparticles) with a macroscopic geometric area, rather than the sintered metal's microscopic area. We estimate that a quasiparticle needs on the order of 1000 collisions to successfully thermalise with the sinter. Finally, we find that the Kapitza resistance is approximately doubled with the addition of two mono-layers of solid 4 He on the sinter surface, which we attribute to an extra magnetic channel of heat transfer being closed as the non-magnetic solid 4 He replaces the magnetic solid 3 He.

We discuss the effective metric experienced by the Nambu-Goldstone mode propagating in the broken symmetry spin-superfluid state of coherent precession of magnetization. This collective mode represents the phonon in the RF driven or pulsed out-of-equilibrium Bose-Einstein condensate (BEC) of optical magnons. We derive the effective BEC free energy and consider the phonon spectrum when the spin superfluid BEC is formed in the anisotropic polar phase of superfluid 3He, experimentally observed in uniaxial aerogel 3He-samples. The coherent precession of magnetization experiences an instability at a critical value of the tilting angle of external magnetic field with respect to the anisotropy axis. From the action of quadratic deviations around equilibrium, this instability is interpreted as a Minkowski-to-Euclidean signature change of the effective phonon metric. We also note the similarity between the magnon BEC in the unstable region and an effective vacuum scalar "ghost" condensate.

In pure superfluid 3 He-B at ultra-low temperatures, quartz tuning fork oscillator response is expected to saturate when the dissipation caused by the superfluid medium becomes substantially smaller than the internal dissipation of the oscillator. However, even with small amount of 4 He covering the surfaces, we have observed saturation already at significantly higher temperatures than anticipated, where we have other indicators to prove that the 3 He liquid is still cooling. We found that this anomalous behavior has a rather strong pressure dependence, and it practically disappears above the crystallization pressure of 4 He. We also observed a maximum in the fork resonance frequency at temperatures where the transition in quasiparticle flow from the hydrodynamic to the ballistic regime is expected. We suggest that such anomalous features derive from the superfluid 4 He film on the oscillator surface.

Nonlinear phononics play important role in strong laser-solid interactions. We discuss nonlinear dynamical protocols which allow for efficient excitation and control of nonlinear phonons. We consider recent inspiring proposals: inducing ferroelectricity in paraelectric material such as KTaO 3 and inducing structural deformations in cuprates like La 2 CuO 4 [A. Subedi this http URL, Phys. Rev. B 89,220301 (2014), A.Subedi, Phys. Rev. B 95, 134113 (2017)]. High-frequency phonon modes are driven by mid-infrared pulses, and coupled to lower-frequency modes those indirect excitation causes structural deformations. Such proposals are in line with a series of recent experiments on light-induced phase transitions. We study in a more detail the case of KTaO 3 without strain, where (at first glance) it was not possible to excite the needed low frequency phonon mode by resonant driving of the higher frequency one. Behaviour of the phonon system is explained using a reduced model of coupled driven nonlinear oscillators. We find a dynamical mechanism which prevents effective excitation at resonance driving, and show that for certain detunings of driving frequency from the exact resonance the system response is, counterintuitively, greatly amplified. In order to induce ferroelectricity in KTaO 3 without a strain we employ driving with sweeping frequency, realizing so called capture into resonance. The method works for realistic femtosecond pulses. Our approach can be applied to other related systems, e.g. laser driven orthorombic perovskites like ErFeO 3 .

The eigenvalues of a parameter-dependent N?N Hamiltonian matrix form a band structure in parameter space. Quantum geometric properties (Berry curvature, quantum metric, etc.) of such N -band systems are usually computed from parameter-dependent eigenstates. This approach faces several difficulties, including gauge ambiguities and singularities in the multicomponent eigenfunctions. In order to circumvent this problem, this work exposes an alternative approach based on eigenprojectors and (generalized) Bloch vectors. First, an expansion of each eigenprojector as a matrix polynomial in the Hamiltonian is deduced, and using SU( N ) Gell-Mann matrices an equivalent expansion of each Bloch vector is found. In a second step, expressions for the N -band Berry curvature and quantum metric in terms of Bloch vectors are obtained. This leads to new explicit Berry curvature formulas in terms of the Hamiltonian vector, generalizing the well-known two-band formula to arbitrary N . Moreover, a detailed treatment is given for the case of a particle-hole symmetric energy spectrum, which occurs in systems with a chiral or charge conjugation symmetry. For illustrating the formalism, several model Hamiltonians featuring a multifold linear band crossing are discussed; they have identical energy spectra but completely different geometric and topological properties. The methodology used in this work is more broadly applicable to compute any physical quantity, or to study the quantum dynamics of any observable without the explicit construction of energy eigenstates.