Ontogenetic changes in cutaneous and branchial ionocytes and morphology in yellowfin tuna (Thunnus albacares) larvae.

The development of osmoregulatory and gas exchange organs was studied in larval yellowfin tuna (Thunnus albacares) from 2 to 25 days post-hatching (2.9-24.5 mm standard length, SL). Cutaneous and branchial ionocytes were identified using Na+/K+-ATPase immunostaining and scanning electron microscopy. Cutaneous ionocyte abundance significantly increased with SL, but a reduction in ionocyte size and density resulted in a significant decrease in relative ionocyte area. Cutaneous ionocytes in preflexion larvae had a wide apical opening with extended microvilli; however, microvilli retracted into an apical pit from flexion onward. Lamellae in the gill and pseudobranch were first detected ~ 3.3 mm SL. Ionocytes were always present on the gill arch, first appeared in the filaments and lamellae of the pseudobranch at 3.4 mm SL, and later in gill filaments at 4.2 mm SL, but were never observed in the gill lamellae. Unlike the cutaneous ionocytes, gill and pseudobranch ionocytes had a wide apical opening with extended microvilli throughout larval development. The interlamellar fusion, a specialized gill structure binding the lamellae of ram-ventilating fish, began forming by ~ 24.5 mm SL and contained ionocytes, a localization never before reported. Ionocytes were retained on the lamellar fusions and also found on the filament fusions of larger sub-adult yellowfin tuna; however, sub-adult gill ionocytes had apical pits. These results indicate a shift in gas exchange and NaCl secretion from the skin to branchial organs around the flexion stage, and reveal novel aspects of ionocyte localization and morphology in ram-ventilating fishes

Monte Carlo simulation method for Laughlin-like states in a disk geometry

We discuss an alternative accurate Monte Carlo method to calculate the ground-state energy and related quantities for Laughlin states of the fractional quantum Hall effect in a disk geometry. This alternative approach allows us to obtain accurate bulk regime (thermodynamic limit) values for various quantities from Monte Carlo simulations with a small number of particles (much smaller than that needed with standard Monte Carlo approaches).Comment: 13 pages, 6 figures, 2 table

Transverse force on a quantized vortex in a superconductor

The total transverse force acting on a quantized vortex in a type-II superconductor determines the Hall response in the mixed state, yet a consensus as to its correct form is still lacking. In this paper we present an essentially exact expression for this force, valid in the superclean limit, which was obtained by generalizing the recent work by Thouless, Ao, and Niu [D. J. Thouless, P. Ao, and Q. Niu, Phys. Rev. Lett. 76, 3758 (1996)] on the Magnus force in a neutral superfluid. We find the transverse force per unit length to be $f = \rho K \times V$, where $\rho = \rho_{n} + \rho_{s}$ is the sum of the mass densities of the normal and superconducting components, $K$ is a vector parallel to the line vortex with a magnitude equal to the quantized circulation, and $V$ is the vortex velocity.Comment: 4 pages, Revtex, 1 figur

Electrical current-driven pinhole formation and insulator-metal transition in tunnel junctions

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. The CIS effect is here studied in TJs with two thin (20 \AA) non-magnetic (NM) Ta electrodes inserted above and below the insulating barrier. We observe resistance (R) switching for positive applied electrical current (flowing from the bottom to the top lead), characterized by a continuous resistance decrease and associated with current-driven displacement of metallic ions from the bottom electrode into the barrier (thin barrier state). For negative currents, displaced ions return into their initial positions in the electrode and the electrical resistance gradually increases (thick barrier state). We measured the temperature (T) dependence of the electrical resistance of both thin- and thick-barrier states ($R_b$ and R$_B$ respectively). Experiments showed a weaker R(T) variation when the tunnel junction is in the $R_b$ state, associated with a smaller tunnel contribution. By applying large enough electrical currents we induced large irreversible R-decreases in the studied TJs, associated with barrier degradation. We then monitored the evolution of the R(T) dependence for different stages of barrier degradation. In particular, we observed a smooth transition from tunnel- to metallic-dominated transport. The initial degradation-stages are related to irreversible barrier thickness decreases (without the formation of pinholes). Only for later barrier degradation stages do we have the appearance of metallic paths between the two electrodes that, however, do not lead to metallic dominated transport for small enough pinhole radius.Comment: 10 pages, 3 figure

Hypernetted-chain study of broken rotational symmetry states for the ν\bm{\nu} = 1/3 fractional quantum Hall effect and other fractionally filled Landau levels

We investigate broken rotational symmetry (BRS) states for the fractional quantum Hall effect (FQHE) at 1/3-filling of the valence Landau level (LL). Recent Monte Carlo calculations by Musaelian and Joynt [J. Phys.: Condens.\ Matter {\bf 8}, L105 (1996)] suggest that Laughlin's state becomes unstable to a BRS state for some critical finite thickness value. We study in detail the properties of such state by performing a hypernetted-chain calculation that gives results in the thermodynamic limit, complementing other methods which are limited to a finite number of particles. Our results indicate that while Laughlin's state is stable in the lowest LL, in higher LLs a BRS instability occurs, perhaps indicating the absence of FQHE at partial fillings of higher LLs. Possible connections to the newly discovered liquid crystalline phases in higher LLs are also discussed.Comment: 7 pages including 3 eps figure

Scattering of Phonons by a Vortex in a Superfluid

Recent work gives a transverse force on an isolated moving vortex which is independent of the normal fluid velocity, but it is widely believed that the asymmetry of phonon scattering by a vortex leads to a transverse force dependent on the relative motion of the normal component and the vortex. We show that a widely accepted derivation of the transverse force is in error, and that a careful evaluation leads to a much smaller transverse force. We argue that a different approach is needed to get the correct expression. \pacs{67.40.Vs,67.57.Fg,47.37.+q,47.32.Cc}Comment: 4 page

Longitudinal Force on a Moving Potential

We show a formal result of the longitudinal force acting on a moving potential. The potential can be velocity-dependent, which appears in various interesting physical systems, such as electrons in the presence of a magnetic flux-line, or phonons scattering off a moving vortex. By using the phase-shift analysis, we are able to show the equivalence between the adiabatic perturbation theory and the kinetic theory for the longitudinal force in the dilute gas limit.Comment: RevTeX, 4 pages, revised tex

Acoustic Energy and Momentum in a Moving Medium

By exploiting the mathematical analogy between the propagation of sound in a non-homogeneous potential flow and the propagation of a scalar field in a background gravitational field, various wave ``energy'' and wave ``momentum'' conservation laws are established in a systematic manner. In particular the acoustic energy conservation law due to Blokhintsev appears as the result of the conservation of a mixed co- and contravariant energy-momentum tensor, while the exchange of relative energy between the wave and the mean flow mediated by the radiation stress tensor, first noted by Longuet-Higgins and Stewart in the context of ocean waves, appears as the covariant conservation of the doubly contravariant form of the same energy-momentum tensor.Comment: 25 Pages, Late