Condensed Matter

In superfluid 3 He-B confined in a slab geometry, domain walls between regions of different order parameter orientation are predicted to be energetically stable. Formation of the spatially-modulated superfluid stripe phase has been proposed. We confined 3 He in a 1.1 μ m high microfluidic cavity and cooled it into the B phase at low pressure, where the stripe phase is predicted. We measured the surface-induced order parameter distortion with NMR, sensitive to the formation of domains. The results rule out the stripe phase, but are consistent with 2D modulated superfluid order.

Superfluid helium II contains excitations known as rotons. Their properties have been studied experimentally for more than 70 years but their structure is not fully understood. Feynman's 1954 description, involving rotating flow patterns, does not fully explain later experimental data. Here we identify volumetric, thermodynamic, colloidal, excitation, x-ray and neutron scattering evidence that rotons are composed of interstitial helium atoms. We show in particular that they have the same mass, effective mass and activation energy within experimental accuracy. They readily move through the substrate, and couple through lattice vibrations to produce quantized, loss-free flow which corresponds to the observed superflow. Our observations revive London's 1936 conclusion that helium II has a relatively open crystal-like lattice with enough free volume for atoms to move relative to one another, and reconcile it with London's 1938 description of a quantum fluid.

For two prototype systems, we calculate the exact exchange-correlation kernels f xc (x, x ′ ,ω) of time-dependent density functional theory. f xc , the key quantity for optical absorption spectra of electronic systems, is normally subject to uncontrolled approximation. We find that, up to the first excitation energy, the exact f xc has weak frequency-dependence and a simple, though non-local, spatial form. For higher excitations, the spatial behavior and frequency-dependence become more complex. The accuracy of the underlying exchange-correlation potential is of crucial importance.

A common property of topological systems is the appearance of topologically protected zero-energy excitations. In a superconductor or superfluid such states set the critical velocity of dissipationless flow v cL , proposed by Landau, to zero. We check experimentally whether stable superflow is nevertheless possible in the polar phase of p-wave superfluid 3 He, which features a Dirac node line in the energy spectrum of Bogoliubov quasiparticles. The fluid is driven by rotation of the whole cryostat, and superflow breakdown is seen as the appearance of single- or half-quantum vortices. Vortices are detected using the relaxation rate of a Bose-Einstein condensate of magnons, created within the fluid. The superflow in the polar phase is found to be stable up to a finite critical velocity v c ≈0.2 cm/s, despite the zero value of the Landau critical velocity. We suggest that the stability of the superflow above v cL but below v c is provided by the accumulation of the flow-induced quasiparticles into pockets in the momentum space, bounded by Bogoliubov Fermi surfaces. In the polar phase this surface has non-trivial topology which includes two pseudo-Weyl points. Vortices forming above the critical velocity are strongly pinned in the confining matrix, used to stabilize the polar phase, and hence stable macroscopic superflow can be maintained even when the external drive is brought to zero.

The possibility of using an optocoupler "pulsed fiber laser - photo thyristor" as a switch in excitation circuits of copper vapor lasers (CVL) is investigated. It is shown that such switch has a nanosecond speed, is able to pass monopolar or alternating current pulses through CVL with a power of up to 10 MW and a repetition rate of up to tens of kilohertz with an electric efficiency of excitation circuit of more than 95%. A simple but very accurate model of photo thyristor is proposed, which can be used in full-scale CVL modeling programs.

We consider exciting surface plasmon polaritons in the Kretschmann configuration. Contrary to common belief, we show that a plane wave incident at an angle greater than the angle of total internal reflection does not excite surface plasmon polaritons. These excitations do arise, however, if the incident light forms a narrow beam composed of an infinite number of plane waves. The surface plasmon polariton is formed at the geometrical edge of the beam as a result of interference of reflected plane waves.

Topological Lifshitz transitions involve many types of topological structures in momentum and frequency-momentum spaces: Fermi surfaces, Dirac lines, Dirac and Weyl points, etc. Each of these structures has their own topological invariant ( N 1 , N 2 , N 3 , N ~ 3 , etc.), which supports the stability of a given topological structure. The topology of the shape of Fermi surfaces and Dirac lines, as well as the interconnection of the objects of different dimensions, lead to numerous classes of Lifshitz transitions. The consequences of Lifshitz transitions are important in different areas of physics. The singularities emerging at the transition may enhance the transition temperature to superconductivity; the Lifshitz transition can be in the origin of the small masses of elementary particles in our Universe; the black hole horizon serves as the surface of Lifshitz transition between the vacua with type-I and type-II Weyl points; etc.

Physics of supercritical state is understood to a much lesser degree compared to subcritical liquids. Carbon dioxide in particular has been intensely studied, yet little is known about the supercritical part of its phase diagram. Here, we combine neutron scattering experiments and molecular dynamics simulations and demonstrate the structural crossover at the Frenkel line. The crossover is seen at pressures as high as 14 times the critical pressure and is evidenced by changes of the main features of the structure factor and pair distribution functions.

The experimentally demonstration of Casimir force transition from attraction to repulsion is still challenging. Herein, the Casimir forces for a sphere above a plate immersed in different liquids were precisely measured using Atomic force microscope, and the long-range repulsive Casimir force in the gold-cyclohexane-PTFE system is observed for the first time. The experimental data are consistent with the calculation by Lifshitz theory, which offers the direct evidence for the system of {\epsilon}1<{\epsilon}3<{\epsilon}2. It further verifies the reasonability of van Zwol et al. dielectric model to describe the intervening fluids. This study is promising for potential applications on quantum levitation and frictionless devices in MEMS and NEMS by Casimir repulsion.

Three dimensional (3D) topology optimization problems always involve huge numbers of Degrees of Freedom (DOFs) in finite element analysis (FEA) and design variables in numerical optimization, respectively. This will inevitably lead to large computational efforts in the solution process. In the present paper, an efficient and explicit topology optimization approach which can reduce not only the number of design variables but also the number of degrees of freedom in FEA is proposed based on the Moving Morphable Voids (MMVs) solution framework. This is achieved by introducing a set of geometry parameters (e.g., control points of B-spline surfaces) to describe the boundary of a structure explicitly and removing the unnecessary DOFs from the FE model at every step of numerical optimization. Numerical examples demonstrate that the proposed approach does can overcome the bottleneck problems associated with a 3D topology optimization problem in a straightforward way and enhance the solution efficiency significantly.