Operating characteristics and their anisotropy in a high-power laser (1.5 W, 300 K) with a quantum-dot active region

Continuous-wave lasing has been demonstrated in a vertically coupled quantum-dot laser with a high output power (1.5 W) at room temperature. It was shown that anisotropy of the quantum dot profile leads to anisotropy of the laser operating characteristics. Pis’ma Zh. Tekh. Fiz. 24, 50–55 (May 12, 1998)     

Evidence for a surface state with broken symmetry in3He-B from ROTA experiments at Helsinki

An unusual, newly observed, rotation-direction-dependent parity effect in nuclear magnetic resonance (NMR) experiments on rotating³He-B in a state void of vortices is here first explained in terms of the broken time-inversion symmetry (T) for the surface layer of the³He-B. Two possible types of such surface structures, both with brokenT symmetry, for superfluid³He-B near the wall of the container are consistent with properties of the parity effect: one has superfluid A phase on the³He-B surface, while the other one is characterized by a nonzero counterflow of the spin componentsS=0 andS=+-1 of the superfluid along the normal to the wall, but with the total mass supercurrent and the total spin supercurrent into the wall vanishing identically. The first surface structure is in better agreement with details of the experiment and our numerical results, which demonstrate, for the first time, absolute stability of an A-phase layer on the³He-B boundary in a certain region of the phenomenological parameters for the surface tension. We find it also possible for a first-order surface-wetting transition to occur by the A-phase layer between two different A-phase states: one with little A phase, the other with well developed A phase. We estimate the magnitude of the new surface-orientating effects by comparing the NMR data with computed spin-wave spectra. New types of point vortices on the surface, boojums, are possible for the³He-B with superfluid A phase on the boundary: unlike the usual boojums on the surface of³He-A, only these novel boojums on the³He-B surface display isolated half-integer quanta of superfluid circulation.     

Half-solitons in superfluid3He-A: Novel π/2-quanta of phase slippage

Core structures of nontopological solitons between inequivalent vacua in superfluid³He-A are considered. We analyze the symmetries of these A-A interfaces, and compute their hard-core structures in the Ginzburg-Landau regime. We discuss both domain walls where the orbital anisotropy l-vector is flipped (l–l), and those with the same l(x=–) and l(x=+) asymptotics. In particular, we find new classes of A-A boundaries: these novel /2-solitons, which can occur in the absence of a change in the asymptotic l-vector field, constitute the elementary quanta of phase slippage in superfluid³He-A. We ascribe these half-solitons to a new topological scenario for the flaring-out of vorticity in the extended (k, r)-space. Edges of such walls serve to provide vortices with 1/4 quantum of circulation in³He-A. In analogy with the B-B domain walls in superfluid³He-B, solitons of pure phase slippage by —with a normal core—prove unstable in³He-A; they either fission into a pair of ordinary l-solitons—domain walls with superfluid cores both flipping the orbital anisotropy axis (i.e., l–ll), or form a bound pair of walls, each of which constitutes an l-soliton with a phase shift of /2. Our investigation of the superfluid A-A vacuum interfaces may prove useful in a broader context since the A-A boundaries exemplify the possible domain walls relevant for the Higgs-field solitons (cosmic domain walls) within the Weinberg-Salam model.     

Boojums on the fermi surface in3He-A and Hamiltonian dynamics of the orbital momentum

Boojums (nodes) on the Fermi surface of³He-A lead to peculiar behavior of the intrinsic orbital momentum L in this superfluid. AtT=0, L has the form L=(/2m ₃)l(–C ₀), whereC ₀ is the prefactor of the anomalous term in the supercurrent l(l rot l). From the algebra of the hydrodynamical Poisson brackets and from microscopic theory it follows thatC ₀ is a dynamical invariant, (/t)C ₀=0. The closed system of nonlinear hydrodynamic equations of³He-A atT=0 is constructed in the framework of the Poisson brackets scheme.     

NMR experiments on rotating superfluid3He-A and3He-B and their theoretical interpretation

We have constructed a rotating nuclear demagnetization cryostat and used it for continuous-wave NMR experiments on superfluid³He-A and³He-B. The measurements were performed in a long cylindrical geometry of 5 mm diameter, with the cylinder axis parallel to the axis of rotation and with the external magnetic field H₀=284 or 142 Oe in the same direction. The angular velocity of rotation was varied between 0.2 and 1.5 rad/sec, and the experiments were done under 29.3 bar pressure at temperatures between T_c=2.72 and about 1.4 mK. As a guide to the new and esoteric field of superfluid³He in rotation, we first review the general theory at some length in relatively simple terms. Pictorial explanations are often given.In³He-A, a rotation-dependent NMR satellite was found; its intensity a rotation-dependent NMR satellite peak was discovered; its relative intensity increases linearly with . The position of the satellite is independent of and H, and does not depend on whether the sample was cooled from the Fermi-liquid region to the A phase while rotating or at rest. At temperatures 0.1<1–T/T_c<0.3, the frequency shift of the satellite can be described by the parameter R_t=0.86–1.1(1–T/T_c). Cooldown under rotation produced systematically larger satellite intensities than cooldown at rest. A second, metastable satellite, best seen at rest and disappearing in less than 30 min, was also discovered. Furthermore, the main NMR peak broadens during rotation, while the total NMR absorption remains the same. The behavior of the rotation-dependent satellite strongly supports the existence of vortices in³He-A, their number being proportional to ; the satellite is caused by localized spin wave modes trapped by vortex cores. Theoretical calculations agree quite well with our experimental data if continuous vortices, without a singularity in the order parameter, are assumed. Their presence is also responsible for the additional broadening of the main peak, due either to increased spin diffusion or to scattering of spin waves. The metastable satellite is caused by textural boundaries, probably by twist solitons in the superfluid, created by the rapid cooldown of the sample.In³He-B, a series of nearly equally spaced NMR satellites was found on the high-frequency side of the main peak with the cryostat at rest. Under rotation the separation between the satellites increases linearly with . The spacing displays a jump, proportional to , at 1–T/T_c=0.40. The discontinuity occurred only during start/stop experiments, not if the cryostat was continuously rotated while warming over the transition region. Immediately after rotation had been started the whole NMR spectrum shifted toward higher frequencies for about 30 sec; these transients were seen only at >0.25 rad/sec. In³He-B, the order parameter is strongly influenced by the wall of the container, producing the so-called flareout texture, with the angle between the vector andH equal to 63° at the walls. The satellites can be explained as spin wave modes arising from an almost harmonic potential well formed by the texture. The creation of vortices changes the texture and increases the steepness of the potential and therefore increases the satellite spacing during rotation. The vortices themselves perturb the texture due to the long-range orientating effect of their cores on the order parameter. The discontinuity in the satellite splitting at 1–T/T_c=0.40 is explained as being due to a first-order phase change in the vortex core at this temperature. The transient shift in the NMR spectrum, immediately after the start of rotation when vortices are not yet present, is caused by the large superfluid vs. normal liquid counterflow; this phenomenon thus gives an estimate for the time needed to create vortices in³He-B.     

Horizons and Ergoregions in Superfluids

Ripplons—gravity-capillary waves on the free surface of a liquid or at the interfaces between two superfluids—are the most favorable excitations for simulation of the general-relativistic effects related to horizons and ergoregions. The white-hole horizon for the “relativistic” ripplons at the surface of the shallow liquid is easily simulated using the kitchen-bath hydraulic jump. The same white-hole horizon is observed in quantum liquid—superfluid ⁴He. The ergoregion for the “non-relativistic” ripplons is generated in the experiments with two sliding ³He superfluids. The common property experienced by all these ripplons is the Miles instability inside the ergoregion or horizon. Because of the universality of the Miles instability, one may expect that it could take place inside the horizon of the astrophysical black holes, if there is a preferred reference frame which comes from the trans-Planckian physics. If this is the case, the black hole would evapotate much faster than due to the Hawking radiation. Hawking radiation from the artificial black hole in terms of the quantum tunneling of phonons and ripplons is also discussed.      

Gravity from Symmetry Breaking Phase Transition

Journal of Low Temperature Physics - The paper is devoted to the memory of Dmitry Diakonov. We discuss gravity emerging in the fermionic vacuum as suggested by Diakonov 10 years ago in his paper...     

Vortices in 3He-A in a weak magnetic field

The structure and the energy of vortices in rotating ³He-A are considered in the presence of a weak magnetic field. It is shown, using the logarithmic approximation for the free energy, that there will be a sequence of textural transitions when the magnetic field is increased. The transition from a nonsingular texture to a singular texture is studied in detail. It is shown that the l vector is almost uniform outside the cores of vortices. To verify these results, measurements by NMR and by ultrasound are suggested.This work has been financially supported by the Academy of Science of the USSR, by the Academy of Finland, and by the Committee of Scientific and Technical Cooperation between Finland and the USSR.