The Scaling of the Anomalous Hall Effect in the Insulating Regime

We develop a theoretical approach to study the scaling of anomalous Hall effect (AHE) in the insulating regime, which is observed to be $\sigma_{xy}^{AH}\propto\sigma_{xx}^{1.40\sim1.75}$ in experiments over a large range of materials. This scaling is qualitatively different from the ones observed in metals. Basing our theory on the phonon-assisted hopping mechanism and percolation theory, we derive a general formula for the anomalous Hall conductivity, and show that it scales with the longitudinal conductivity as $\sigma_{xy}^{AH}\sim\sigma_{xx}^{\gamma}$ with $\gamma$ predicted to be $1.38\leq\gamma\leq1.76$, quantitatively in agreement with the experimental observations. Our result provides a clearer understanding of the AHE in the insulating regime and completes the scaling phase diagram of the AHE.Comment: 4 pages, 4 figures, plus the supplementary information. Minor revisions made according to Referee report

Interaction between a fast rotating sunspot and ephemeral regions as the origin of the major solar event on 2006 December 13

The major solar event on 2006 December 13 is characterized by the approximately simultaneous occurrence of a heap of hot ejecta, a great two-ribbon flare and an extended Earth-directed coronal mass ejection. We examine the magnetic field and sunspot evolution in active region NOAA AR 10930, the source region of the event, while it transited the solar disk centre from Dec. 10 to Dec. 13. We find that the obvious changes in the active region associated with the event are the development of magnetic shear, the appearance of ephemeral regions and fast rotation of a smaller sunspot. Around the area of the magnetic neutral line of the active region, interaction between the fast rotating sunspot and the ephemeral regions triggers continual brightening and finally the major flare. It is indicative that only after the sunspot rotates up to 200$^{\circ}$ does the major event take place. The sunspot rotates at least 240$^{\circ}$ about its centre, the largest sunspot rotation angle which has been reported.Comment: 4 pages, 6 figures, ApJ Letters inpres

Diffusion-limited loop formation of semiflexible polymers: Kramers theory and the intertwined time scales of chain relaxation and closing

We show that Kramers rate theory gives a straightforward, accurate estimate of the closing time $\tau_c$ of a semiflexible polymer that is valid in cases of physical interest. The calculation also reveals how the time scales of chain relaxation and closing are intertwined, illuminating an apparent conflict between two ways of calculating $\tau_c$ in the flexible limit.Comment: Europhys. Lett., 2003 (in press). 8 pages, 3 figures. See also, physics/0101087 for physicist's approach to and the importance of semiflexible polymer looping, in DNA replicatio

Continuous Transition between Antiferromagnetic Insulator and Paramagnetic Metal in the Pyrochlore Iridate Eu2Ir2O7

Our single crystal study of the magneto-thermal and transport properties of the pyrochlore iridate Eu2Ir2O7 reveals a continuous phase transition from a paramagnetic metal to an antiferromagnetic insulator for a sample with stoichiometry within ~1% resolution. The insulating phase has strong proximity to an antiferromagnetic semimetal, which is stabilized by several % level of the off-stoichiometry. Our observations suggest that in addition to electronic correlation and spin-orbit coupling the magnetic order is essential for opening the charge gap.Comment: 6 pages, 6 figure

Quantized spin Hall effect in Helium three-A and other p-wave paired Fermi systems

In this paper we propose the quantized spin Hall effect (SHE) in the vortex state of a rotating p-wave paired Fermi system in an inhomogeneous magnetic field and in a weak periodic potential. It is the three dimensional extension of the spin Hall effect for a 3He-A superfluid film studied in Ref. [1]. It may also be considered as a generalization of the 3D quantized charge Hall effect of Bloch electrons in Ref. [2] to the spin transport. The A-phase of 3He or, more generally, the p-wave paired phase of a cold Fermi atomic gas, under suitable conditions should be a good candidate to observe the SHE, because the system has a conserved spin current (with no spin-orbit couplings).Comment: 6 pages, revised version

Comment on “Collisionless shock and supernova remnant simulations on VULCAN” [Phys. Plasmas 8, 2439 (2001)]

This recent paper reports some real advances in experimental technique, but is misleading or incorrect in several places. First, the design assumes without discussion that the magnetic field will completely penetrate the plasma, but this is not likely. Second, when the magnetic field is present the surfaces of the converging plasmas will be Rayleigh–Taylor unstable. Third, any shocks produced in experiments like those reported may be collisionless but have no relevance to shocks in supernova remnants. Fourth, the experiment is not a meaningful hydrodynamic simulation of a supernova remnant. Finally, the hydrodynamic simulation results reported are also in error, leading to incorrect values for some scaling parameters. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69649/2/PHPAEN-9-2-727-1.pd

Some results on homoclinic and heteroclinic connections in planar systems

Consider a family of planar systems depending on two parameters $(n,b)$ and having at most one limit cycle. Assume that the limit cycle disappears at some homoclinic (or heteroclinic) connection when $\Phi(n,b)=0.$ We present a method that allows to obtain a sequence of explicit algebraic lower and upper bounds for the bifurcation set ${\Phi(n,b)=0}.$ The method is applied to two quadratic families, one of them is the well-known Bogdanov-Takens system. One of the results that we obtain for this system is the bifurcation curve for small values of $n$, given by $b=\frac5 7 n^{1/2}+{72/2401}n- {30024/45294865}n^{3/2}- {2352961656/11108339166925} n^2+O(n^{5/2})$. We obtain the new three terms from purely algebraic calculations, without evaluating Melnikov functions

Mesoscopic Stern-Gerlach spin filter by nonuniform spin-orbit interaction

A novel spin filtering in two-dimensional electron system with nonuniform spin-orbit interactions (SOI) is theoretically studied. The strength of SOI is modulated perpendicular to the charge current. A spatial gradient of effective magnetic field due to the nonuniform SOI causes the Stern-Gerlach type spin separation. The direction of the polarization is perpendicular to the current and parallel to the spatial gradient. Almost 100 % spin polarization can be realized even without applying any external magnetic fields and without attaching ferromagnetic contacts. The spin polarization persists even in the presence of randomness.Comment: 6 pages, 5 figures (2 color figures), to appear in Phys. Rev. B, Rapid Commu