Physicians' database searches as a tool for early detection of epidemics.

We analyzed retrospectively the use of Physician Desk Reference Database searches to identify epidemics of tularemia, nephropathy, Pogosta disease, and Lyme disease and compared the searches with mandatory laboratory reports to the National Infectious Diseases Register in Finland during 1995. Continuous recording of such searches may be a tool for early detection of epidemics

``Cosmological'' scenario for A-B phase transition in superfluid 3He

At a very rapid superfluid transition in $^3$He, follows after a reaction with single neutron, the creation of topological defects (vortices) has recently been demonstrated in accordance with the Kibble-Zurek scenario for the cosmological analogue. We discuss here the extension of the Kibble-Zurek scenario to the case when alternative symmetries may be broken and different states nucleated independently. We have calculated the nucleation probability of the various states of superfluid $^3$He during a superfluid transition. Our results can explain the transition from supercooled $A$ phase to the $B$ phase, triggered by nuclear reaction. The new scenario is an alternative to the well-known ``baked Alaska'' scenario.Comment: RevTex file, 4 pages, 3 figures, submitted to Phys. Rev. Let

Composite defect extends cosmology - 3He analogy

Spin-mass vortices have been observed to form in rotating superfluid 3He-B following the absorption of a thermal neutron and a rapid transition from the normal to superfluid state. The spin-mass vortex is a composite defect which consists of a planar soliton (wall) which terminates on a linear core (string). This observation fits well within the framework of a cosmological scenario for defect formation, known as the Kibble-Zurek mechanism. It suggests that in the early Universe analogous cosmological defects might have formed.Comment: RevTeX file, 5 pages, 2 figures, submitted to Phys. Rev. Lett., modified according to referee repor

Wetting of Superfluid 4He by Liquid 3He

We have investigated optically the spreading of He3 on top of the He4-rich solution in phase-separated helium-mixture films, 20-50 μm thick. In equilibrium, the He3 layer wets the He4-rich phase completely, but nearly circular or stripelike pools of He3-rich phase are stabilized instead when He4 atoms are condensed to the liquid sample at the rate 10 exp 15-5 × 10 exp 15 atoms/cm2s. For the contact angle we obtain about 10 mrad, which suggests a fractional change of the He3 surface tension by a factor of 10 exp −5 from the equilibrium value.Peer reviewe

Defect formation and local gauge invariance

We propose a new mechanism for formation of topological defects in a U(1) model with a local gauge symmetry. This mechanism leads to definite predictions, which are qualitatively different from those of the Kibble-Zurek mechanism of global theories. We confirm these predictions in numerical simulations, and they can also be tested in superconductor experiments. We believe that the mechanism generalizes to more complicated theories.Comment: REVTeX, 4 pages, 2 figures. The explicit form of the Hamiltonian and the equations of motion added. To appear in PRL (http://prl.aps.org/

Critical Velocity of Vortex Nucleation in Rotating Superfluid 3He-A

We have measured the critical velocity v_c at which 3He-A in a rotating cylinder becomes unstable against the formation of quantized vortex lines with continuous (singularity-free) core structure. We find that v_c is distributed between a maximum and minimum limit, which we ascribe to a dependence on the texture of the orbital angular momentum l(r) in the cylinder. Slow cool down through T_c in rotation yields l(r) textures for which the measured v_c's are in good agreement with the calculated instability of the expected l texture.Comment: 4 pages, 3 figure

Defect Formation in Quench-Cooled Superfluid Phase Transition

We use neutron absorption in rotating 3He-B to heat locally a 10 micrometer-size volume into normal phase. When the heated region cools back in microseconds, vortex lines are formed. We record with NMR the number of lines as a function of superflow velocity and compare to the Kibble-Zurek theory of vortex-loop freeze-out from a random network of defects. The measurements confirm the calculated loop-size distribution and show that also the superfluid state itself forms as a patchwork of competing A and B phase blobs. This explains the A to B transition in supercooled neutron-irradiated 3He-A.Comment: RevTex file, 4 pages, 3 figures, resubmitted to Phys. Rev. Let

Vortex Multiplication in Applied Flow: the Precursor to Superfluid Turbulence

The dynamics of quantized vortices in rotating $^3$He-B is investigated in the low density (single-vortex) regime as a function of temperature. An abrupt transition is observed at $0.5 T_{\rm c}$. Above this temperature the number of vortex lines remains constant, as they evolve to their equilibrium positions. Below this temperature the number of vortices increases linearly in time until the vortex density has grown sufficiently for turbulence to switch on. On the basis of numerical calculations we suggest a mechanism responsible for vortex formation at low temperatures and identify the mutual friction parameter which governs its abrupt temperature dependence.Comment: 5 pages, 4 figures; version submitted to Phys. Rev. Let

Transition to superfluid turbulence governed by an intrinsic parameter

Hydrodynamic flow in both classical and quantum fluids can be either laminar or turbulent. To describe the latter, vortices in turbulent flow are modelled with stable vortex filaments. While this is an idealization in classical fluids, vortices are real topologically stable quantized objects in superfluids. Thus superfluid turbulence is thought to hold the key to new understanding on turbulence in general. The fermion superfluid 3He offers further possibilities owing to a large variation in its hydrodynamic characteristics over the experimentally accessible temperatures. While studying the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp transition at 0.60Tc between two regimes, with regular behaviour at high-temperatures and turbulence at low-temperatures. Unlike in classical fluids, this transition is insensitive to velocity and occurs at a temperature where the dissipative vortex damping drops below a critical limit. This discovery resolves the conflict between existing high- and low-temperature measurements in 3He-B: At high temperatures in rotating flow a vortex loop injected into superflow has been observed to expand monotonically to a single rectilinear vortex line, while at very low temperatures a tangled network of quantized vortex lines can be generated in a quiescent bath with a vibrating wire. The solution of this conflict reveals a new intrinsic criterion for the existence of superfluid turbulence.Comment: Revtex file; 5 pages, 2 figure

Are there static texture?

We consider harmonic maps from Minkowski space into the three sphere. We are especially interested in solutions which are asymptotically constant, i.e. converge to the same value in all directions of spatial infinity. Physical 3-space can then be compactified and can be identified topologically (but not metrically!) with a three sphere. Therefore, at fixed time, the winding of the map is defined. We investigate whether static solutions with non-trivial winding number exist. The answer which we can proof here is only partial: We show that within a certain family of maps no static solutions with non-zero winding number exist. We discuss the existing static solutions in our family of maps. An extension to other maps or a proof that our family of maps is sufficiently general remains an open problem.Comment: 12 page Latex file, 1 postscript figure, submitted to PR