Step bunching of vicinal 6H-SiC{0001} surfaces

We use kinetic Monte Carlo simulations to understand growth- and etching-induced step bunching of 6H-SiC{0001} vicinal surfaces oriented towards [1-100] and [11-20]. By taking account of the different rates of surface diffusion on three inequivalent terraces, we reproduce the experimentally observed tendency for single bilayer height steps to bunch into half unit cell height steps. By taking account of the different mobilities of steps with different structures, we reproduce the experimentally observed tendency for adjacent pairs of half unit cell height steps to bunch into full unit cell height steps. A prediction of our simulations is that growth-induced and etching-induced step bunching lead to different surface terminations for the exposed terraces when full unit cell height steps are present.Comment: 10 pages, 12 figure

Absence of the Transition into Abrikosov Vortex State of Two-Dimensional Type-II Superconductor with Weak Pinning

The resistive properties of thin amorphous NbO_{x} films with weak pinning were investigated experimentally above and below the second critical field H_{c2}. As opposed to bulk type II superconductors with weak pinning where a sharp change of resistive properties at the transition into the Abrikosov state is observed at H_{c4}, some percent below H_{c2} (V.A.Marchenko and A.V.Nikulov, 1981), no qualitative change of resistive properties is observed down to a very low magnetic field, H_{c4} < 0.006 H_{c2}, in thin films with weak pinning. The smooth dependencies of the resistivity observed in these films can be described by paraconductivity theory both above and below H_{c2}. This means that the fluctuation superconducting state without phase coherence remains appreciably below H_{c2} in the two-dimensional superconductor with weak pinning. The difference the H_{c4}/H_{c2} values, i.e. position of the transition into the Abrikosov state, in three- and two-dimensional superconductors conforms to the Maki-Takayama result 1971 year according to which the Abrikosov solution 1957 year is valid only for a superconductor with finite dimensions. Because of the fluctuation this solution obtained in the mean field approximation is not valid in a relatively narrow region below H_{c2} for bulk superconductors with real dimensions and much below H_{c2} for thin films with real dimensions. The superconducting state without phase coherence should not be identified with the mythical vortex liquid because the vortex, as a singularity in superconducting state with phase coherence, can not exist without phase coherence.Comment: 4 pages, 4 figure

Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity

The mass-loss rates of hot, massive, luminous stars are considered a decisive parameter in shaping the evolutionary tracks of such stars and influencing the interstellar medium on galactic scales. The small-scale structures (clumps) omnipresent in such winds may reduce empirical estimates of mass-loss rates by an evolutionarily significant factor of >=3. So far, there has been no direct observational evidence that wind-clumping may persist at the same level in environments with a low ambient metallicity, where the wind-driving opacity is reduced. Here we report the results of time-resolved spectroscopy of three presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud, where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale emission peaks moving outwards in the accelerating parts of the stellar winds.The general properties of the moving features, such as their velocity dispersions,emissivities and average accelerations, closely match the corresponding characteristics of small-scale inhomogeneities in the winds of Galactic Wolf-Rayet stars.Comment: 9 pages, 3 figures; accepted by ApJ Letter

Structural compliance, misfit strain and stripe nanostructures in cuprate superconductors

Structural compliance is the ability of a crystal structure to accommodate variations in local atomic bond-lengths without incurring large strain energies. We show that the structural compliance of cuprates is relatively small, so that short, highly doped, Cu-O-Cu bonds in stripes are subject to a tensile misfit strain. We develop a model to describe the effect of misfit strain on charge ordering in the copper oxygen planes of oxide materials and illustrate some of the low energy stripe nanostructures that can result.Comment: 4 pages 5 figure

On a systematic approach to defects in classical integrable field theories

We present an inverse scattering approach to defects in classical integrable field theories. Integrability is proved systematically by constructing the generating function of the infinite set of modified integrals of motion. The contribution of the defect to all orders is explicitely identified in terms of a defect matrix. The underlying geometric picture is that those defects correspond to Backlund transformations localized at a given point. A classification of defect matrices as well as the corresponding defect conditions is performed. The method is applied to a collection of well-known integrable models and previous results are recovered (and extended) directly as special cases. Finally, a brief discussion of the classical $r$-matrix approach in this context shows the relation to inhomogeneous lattice models and the need to resort to lattice regularizations of integrable field theories with defects.Comment: 27 pages, no figures. Final version accepted for publication. References added and section 5 amende

Qualitative features of periodic solutions of KdV

In this paper we prove new qualitative features of solutions of KdV on the circle. The first result says that the Fourier coefficients of a solution of KdV in Sobolev space $H^N,\, N\geq 0$, admit a WKB type expansion up to first order with strongly oscillating phase factors defined in terms of the KdV frequencies. The second result provides estimates for the approximation of such a solution by trigonometric polynomials of sufficiently large degree

Effective masses for zigzag nanotubes in magnetic fields

We consider the Schr\"odinger operator with a periodic potential on quasi-1D models of zigzag single-wall carbon nanotubes in magnetic field. The spectrum of this operator consists of an absolutely continuous part (intervals separated by gaps) plus an infinite number of eigenvalues with infinite multiplicity. We obtain identities and a priori estimates in terms of effective masses and gap lengths

Octupolar order in the multiple spin exchange model on a triangular lattice

We show how a gapless spin liquid with hidden octupolar order arises in an applied magnetic field, in a model applicable to thin films of 3He with competing ferromagnetic and antiferromagnetic (cyclic) exchange interactions. Evidence is also presented for nematic -- i.e., quadrupolar -- correlations bordering on ferromagnetism in the absence of a magnetic field.Comment: 4 pages, 5 figure