Spin-dependent THz oscillator based on hybrid graphene superlattices

We theoretically study the occurrence of Bloch oscillations in biased hybrid graphene systems with spin-dependent superlattices. The spin-dependent potential is realized by a set of ferromagnetic insulator strips deposited on top of a gapped graphene nanoribbon, which induce a proximity exchange splitting of the electronic states in the graphene monolayer. We numerically solve the Dirac equation and study Bloch oscillations in the lowest conduction band of the spin-dependent superlattice. While the Bloch frequency is the same for both spins, we find the Bloch amplitude to be spin dependent. This difference results in a spin-polarized ac electric current in the THz range.Comment: 4 pages, 6 figure

Dynamics and stability of Bose-Einstein solitons in tilted optical lattices

Bloch oscillations of Bose-Einstein condensates realize sensitive matter-wave interferometers. We investigate the dynamics and stability of bright-soliton wave packets in one-dimensional tilted optical lattices with a modulated mean-field interaction $g(t)$. By means of a time-reversal argument, we prove the stability of Bloch oscillations of breathing solitons that would be quasistatically unstable. Floquet theory shows that these breathing solitons can be more stable against certain experimental perturbations than rigid solitons or even non-interacting wave packets.Comment: final, published versio

Anomalous optical absorption in a random system with scale-free disorder

We report on an anomalous behavior of the absorption spectrum in a one-dimensional lattice with long-range-correlated diagonal disorder with a power-like spectrum in the form S(k) ~ 1/k^A. These type of correlations give rise to a phase of extended states at the band center, provided A is larger than a critical value A_c. We show that for A < A_c the absorption spectrum is single-peaked, while an additional peak arises when A > A_c, signalling the occurrence of the Anderson transition. The peak is located slightly below the low-energy mobility edge, providing a unique spectroscopic tool to monitor the latter. We present qualitative arguments explaining this anomaly.Comment: 4 pages, 4 postscript figures, uses revtex

Environment effects on the electric conductivity of the DNA

We present a theoretical analysis of the environment effects on charge transport in double-stranded synthetic poly(G)-poly(C) DNA molecules attached to two ideal leads. Coupling of the DNA to the environment results in two effects: (i) localization of carrier functions due to the static disorder and (ii) phonon-induced scattering of the carrier between these localized states, resulting in hopping conductivity. A nonlinear Pauli master equation for populations of localized states is used to describe the hopping transport and calculate the electric current as a function of the applied bias. We demonstrate that, although the electronic gap in the density of states shrinks as the disorder increases, the voltage gap in the $I-V$ characteristics becomes wider. Simple physical explanation of this effect is provided.Comment: 8 pages, 2 figures, to appear in J. Phys.: Condens. Matte

Intraband exciton relaxation in a biased lattice with long-range correlated disorder

We study numerically the intraband exciton relaxation in a one-dimensional lattice with scale-free disorder, in the presence of a linear bias. Exciton transport is considered as incoherent hoppings over the eigenstates of the static lattice. The site potential of the unbiased lattice is long-range-correlated with a power-law spectral density $S(k) \sim 1/k^{\alpha}$, $\alpha > 0$. The lattice supports a phase of extended states at the center of the band, provided $\alpha$ is larger than a critical value $\alpha_c$ [F. A. B. F. de Moura and M. L. Lyra, Phys. Rev. Lett. \textbf{81}, 3735 (1998)]. When the bias is applied, the absorption spectrum displays clear signatures of the Wannier-Stark ladder [E. D\'{\i}az \emph{et al.}, Phys. Rev. B\textbf{73}, 172410 (2006)]. We demonstrate that in unbiased lattices and in weakly correlated potentials the decay law is non-exponential. However, the decay is purely exponential when the bias increases and $\alpha$ is large. We relate this exponential decay to the occurrence of the Wannier-Stark ladder in the exciton band

Electronic properties of topological rough nanowires for thermoelectrical performance

We study the electronic states in topological nanowires of narrow-gap semiconductors, such as PbTe or SnTe, with rough surfaces, using a continuous two-band model. We calculate the subband structure and identify topological conducting states located at the surface of the nanowire. In addition, a novel approach to study a nanowire with rough surface demonstrates that the topological surface states are mostly confined in the widest areas of the nanowire. This effect leads to a flattening of the subbands, thus raising the effective mass of carriers. Finally, we analyze the thermoelectric properties of the topological nanowires. The reduction of the radius causes a noticeable enhancement of the thermoelectric efficiency due surface phonon scattering, as expected. However, we also observe that the appearance of topological surface states can play a detrimental role, reducing the thermoelectric efficiency. We conclude that, in addition to nanostructuring, the modulation of the radius of the nanowires, which partially suppress the conduction of the surface states, may be a potential strategy to improve the thermoelectric response of narrow-gap semiconductor nanowires

Spin dynamics in helical molecules with nonlinear interactions

It is widely admitted that the helical conformation of certain chiral molecules may induce a sizable spin selectivity observed in experiments. Spin selectivity arises as a result of the interplay between a helicity-induced spin-orbit coupling (SOC) and electric dipole fields in the molecule. From the theoretical point of view, different phenomena might affect the spin dynamics in helical molecules, such as quantum dephasing, dissipation and the role of metallic contacts. With a few exceptions, previous studies usually neglect the local deformation of the molecule about the carrier, but this assumption seems unrealistic to describe charge transport in molecular systems. We introduce an effective model describing the electron spin dynamics in a deformable helical molecule with weak SOC. We find that the electron-lattice interaction allows the formation of stable solitons such as bright solitons with well defined spin projection onto the molecule axis. We present a thorough study of these bright solitons and analyze their possible impact on the spin dynamics in deformable helical molecules

Lump solitons in a higher-order nonlinear equation in 2+1 dimensions

We propose and examine an integrable system of nonlinear equations that generalizes the nonlinear Schrodinger equation to 2 + 1 dimensions. This integrable system of equations is a promising starting point to elaborate more accurate models in nonlinear optics and molecular systems within the continuum limit. The Lax pair for the system is derived after applying the singular manifold method. We also present an iterative procedure to construct the solutions from a seed solution. Solutions with one-, two-, and three-lump solitons are thoroughly discussed