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First-principles calculations of the magnetic anisotropic constants of Co–Pd multilayers: Effect of stacking faults¹
by G. Wu, K. H. Khoo, M. H. Jhon, H. Meng, S. Y. H. Lua, R. Sbiaa and C. K. Gan

Using first-principles density functional theory calculations with spin-orbit coupling, we systematically investigate the magnetic anisotropic energy (MAE) of Co_nPd_m (n+m=5 ) magnetic multilayers. We consider the influences of the relative atomic weight of Co, w_Co , stacking fault, and external stress on the MAE. We find that out-of-plane lattice constant, saturation magnetization, and magnetic moments are almost linearly correlated with w_Co . The magnetic anisotropic constant (MAC) curve of Co_nPd_m without stacking fault shows a near-linear dependence on w_Co that agrees with our derived effective MAC K_u^eff which includes shape, magneto-crystalline, and magneto-elastic contributions. We also show that the contributions from Pd layers to both the total magnetic moments and magnetic anisotropy are significant. The stress anisotropy due to the substrate has a weak effect on the MAC. However the stacking fault has a strong effect on the MAC. When the Co layer is thin, a Co–Pd interface without stacking fault is necessary for higher K_u^eff . However, when the Co layer is thick, creating stacking faults inside the Co region may produce a larger K_u^eff . Our study suggests the ways to increase the perpendicular magnetic anisotropy in Co–Pd multilayer systems and subsequently leads to the development of novel magnetic recording devices.

Covariant form of the ideal magnetohydrodynamic “connection theorem” in a relativistic plasma²
by F. Pegoraro

The magnetic connection theorem of ideal magnetohydrodynamics by Newcomb (Newcomb W. A., Ann. Phys. (N.Y.), 3 (1958) 347) and its covariant formulation are rederived and reinterpreted in terms of a “time resetting” projection that accounts for the loss of simultaneity in different reference frames between spatially separated events.

Superluminal tachyon-like excitations of Dirac fermions in a topological insulator junction³
by Vadim M. Apalkov and Tapash Chakraborty

We have considered a system of two topological insulators and have determined the properties of the surface states at the junction. Here we report that these states, under certain conditions exhibit superluminous (tachyonic) dispersion of the Dirac fermions. Although superluminal excitations are known to exist in optical systems, this is the first demonstration of possible tachyonic excitations in a purely electronic system. The first ever signature of tachyons could therefore be found experimentally in a topological insulator junction.

Measurement with a rotating magnetic field of the surface viscosity of a nematic liquid crystal⁴
by P. Oswald

We show how to measure with a rotating magnetic field the surface viscosity of a nematic liquid crystal in contact with a thin polymercaptan layer allowing for a planar sliding anchoring of the molecules.

1. G. Wu et al. 2012. First-principles calculations of the magnetic anisotropic constants of Co–Pd multilayers: Effect of stacking faults. EPL, 99, 17001. DOI: 10.1209/0295-5075/99/17001 [↩]
2. F. Pegoraro. 2012. Covariant form of the ideal magnetohydrodynamic “connection theorem” in a relativistic plasma. EPL, 99, 35001. DOI: 10.1209/0295-5075/99/35001 [↩]
3. Vadim M. Apalkov and Tapash Chakraborty. 2012. Superluminal tachyon-like excitations of Dirac fermions in a topological insulator junction. EPL, 100, 17002. DOI: 10.1209/0295-5075/100/17002 [↩]
4. P. Oswald. 2012. Measurement with a rotating magnetic field of the surface viscosity of a nematic liquid crystal. EPL, 100, 26001. DOI: 10.1209/0295-5075/100/26001 [↩]

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