Subband structure of II-VI modulation-doped magnetic quantum wells

Here we investigate the spin-dependent subband structure of newly-developed Mn-based modulation-doped quantum wells. In the presence of an external magnetic field, the s-d exchange coupling between carriers and localized d electrons of the Mn impurities gives rise to large spin splittings resulting in a magnetic-field dependent subband structure. Within the framework of the effective-mass approximation, we self-consistently calculate the subband structure at zero temperature using Density Functional Theory (DFT) with a Local Spin Density Approximation (LSDA). We present results for the magnetic-field dependence of the subband structure of shallow ZnSe/ZnCdMnSe modulation doped quantum wells. Our results show a significant contribution to the self-consistent potential due to the exchange-correlation term. These calculations are the first step in the study of a variety of interesting spin-dependent phenomena, e.g., spin-resolved transport and many-body effects in polarized two-dimensional electron gases.Comment: 3 pages, 3 postscript figures, submitted to the proceedings of the 10th Brazilian Workshop on Semiconductor Physics (BWSP10

Many-body effects on the ρxx\rho_{xx} ringlike structures in two-subband wells

The longitudinal resistivity $\rho_{xx}$ of two-dimensional electron gases formed in wells with two subbands displays ringlike structures when plotted in a density--magnetic-field diagram, due to the crossings of spin-split Landau levels (LLs) from distinct subbands. Using spin density functional theory and linear response, we investigate the shape and spin polarization of these structures as a function of temperature and magnetic-field tilt angle. We find that (i) some of the rings "break" at sufficiently low temperatures due to a quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of spin polarization ($\sim 50$%) within, consistent with the NMR data of Zhang \textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for increasing tilting angles the interplay between the anticrossings due to inter-LL couplings and the exchange-correlation (XC) effects leads to a collapse of the rings at some critical angle $\theta_c$, in agreement with the data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].Comment: 4 pages, 3 figure

Explicit Soliton for the Laplacian Co-Flow on a Solvmanifold

We apply the general Ansatz in geometric flows on homogeneous spaces proposed by Jorge Lauret for the Laplacian co-flow of invariant $G_2$-structures on a Lie group, finding an explicit soliton on a particular almost Abelian $7$-manifold.Comment: Minor corrections, proof's Lemma 4.1 modified. To appear in the S\~ao Paulo Journal of mathematical scienc

Hysteretic resistance spikes in quantum Hall ferromagnets without domains

We use spin-density-functional theory to study recently reported hysteretic magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases [Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips in the Hall resistance \rho_{xy}. Our theory, without domain walls, satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same figures as in v1

Nonequilibrium scaling explorations on a 2D Z(5)-symmetric model

We have investigated the dynamic critical behavior of the two-dimensional Z(5)-symmetric spin model by using short-time Monte Carlo (MC) simulations. We have obtained estimates of some critical points in its rich phase diagram and included, among the usual critical lines the study of first-order (weak) transition by looking into the order-disorder phase transition. Besides, we also investigated the soft-disorder phase transition by considering empiric methods. A study of the behavior of $\beta /\nu z$ along the self-dual critical line has been performed and special attention has been devoted to the critical bifurcation point, or FZ (Fateev-Zamolodchikov) point. Firstly, by using a refinement method and taking into account simulations out-of-equilibrium, we were able to localize parameters of this point. In a second part of our study, we turned our attention to the behavior of the model at the early stage of its time evolution in order to find the dynamic critical exponent z as well as the static critical exponents $\beta$ and $$ of the FZ-point on square lattices. The values of the static critical exponents and parameters are in good agreement with the exact results, and the dynamic critical exponent $z\approx 2.28$ very close of the 4-state Potts model ($z\approx 2.29$).Comment: 11 pages, 7 figure