We study the drag force on objects moving in a Fermi superfluid at velocities on the order of the Landau velocity vL. The expectation has been that vL is the critical velocity beyond which the drag force starts to increase towards its normal-state value. This expectation is challenged by a recent experiment measuring the heat generated by a uniformly moving wire immersed in superfluid 3 He. We introduce the basis for the calculation of the drag force on a macroscopic object using the Fermi-liquid theory of superfluidity. As a technical tool in the calculations we propose a boundary condition that describes diffuse reflection of quasiparticles from a surface on a scale that is larger than the superfluid coherence length. We calculate the drag force on steadily moving objects of different sizes. For an object that is small compared to the coherence length, we find a drag force that is in accordance with the expectation. For a macroscopic object we need to take into account the spatially varying flow field around the object. At low velocities this arises from ideal flow of the superfluid. At higher velocities the flow field is modified by excitations that are created when the flow velocity locally exceeds vL. The flow field causes Andreev reflection of quasiparticles and thus leads to change in the drag force. We calculate multiple limiting cases for a cylinder-shaped object. In the absence of quasiparticle-quasiparticle collisions we find that the critical velocity is larger than vL and the drag force (per cross-sectional area) at 2vL is reduced by an order of magnitude compared to the case of a small object. In a collision-dominated limit the flow shows signs of instability at a velocity below vL. arXiv:1806.02554v2 [cond-mat.supr-con]

We study the quantum ground-state phases of the one-dimensional disordered Bose-Hubbard model with attractive interactions, realized by a chain of superconducting transmon qubits or cold atoms. We map the phase diagram using perturbation theory and exact diagonalization. Compared to the repulsive Bose-Hubbard model, the quantum ground-state behavior is dramatically different. At strong disorder of the on-site energies, all the bosons localize into the vicinity of a single site, contrary to the Bose glass behavior of the repulsive model. At weak disorder, depending on hopping, the ground state is either superfluid or a W state, which is a multisite and multiparticle entangled superposition of states where all the bosons occupy a single site. We show that the robustness of the W phase against disorder diminishes as the total number of bosons increases.

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We consider a vortex line in the B phase of superfluid 3 He under uniformly precessing magnetization. The magnetization exerts torque on the vortex, causing its order parameter to oscillate. These oscillations generate spin waves, which is analogous to an oscillating charge generating electromagnetic radiation. The spin waves carry energy, causing dissipation in the system. Solving the equations of spin dynamics, we calculate the energy dissipation caused by spin wave radiation for arbitrary tipping angles of the magnetization and directions of the magnetic field, and for both vortex types of 3 He-B. For the double-core vortex we also consider the anisotropy of the radiation and the dependence of the dissipation on twisting of the half cores. The radiated energy is compared with experiments in the mid-temperature range T ∼ 0.5Tc. The dependence of the calculated dissipation on several parameters is in good agreement with the experiments. Combined with numerically calculated vortex structure, the radiation theory produces the order of magnitude of the experimental dissipation. The agreement with the experiments indicates that spin wave radiation is the dominant dissipation mechanism for vortices in superfluid 3 He-B in the mid-temperature range. arXiv:1809.10484v1 [cond-mat.supr-con]