Topological confinement in graphene bilayer quantum rings

We demonstrate the existence of localized electron and hole states in a ring-shaped potential kink in biased bilayer graphene. Within the continuum description, we show that for sharp potential steps the Dirac equation describing carrier states close to the K (or K') point of the first Brillouin zone can be solved analytically for a circular kink/anti-kink dot. The solutions exhibit interfacial states which exhibit Aharonov-Bohm oscillations as functions of the height of the potential step and/or the radius of the ring

Simplified model for the energy levels of quantum rings in single layer and bilayer graphene

Within a minimal model, we present analytical expressions for the eigenstates and eigenvalues of carriers confined in quantum rings in monolayer and bilayer graphene. The calculations were performed in the context of the continuum model, by solving the Dirac equation for a zero width ring geometry, i.e. by freezing out the carrier radial motion. We include the effect of an external magnetic field and show the appearance of Aharonov-Bohm oscillations and of a non-zero gap in the spectrum. Our minimal model gives insight in the energy spectrum of graphene-based quantum rings and models different aspects of finite width rings.Comment: To appear in Phys. Rev.

Teor e rendimento de extrato das folhas de três morfotipos de Arrabidaea chica (Bonpl.) B. Verl. em função de adubação orgânica em Manaus, AM.

A espécie Arrabidaea chica (Bonpl.) B. Verl., também conhecida como crajiru, pariri, chica, cipó-cruz, dentre outros nomes, pertence à família Bignoniaceae. Este trabalho teve o objetivo de avaliar o teor e rendimento de extrato de crajiru em função de diferentes fontes de adubo orgânico em Manaus, AM. As mudas foram obtidas por estaquia e plantadas em bandejas de poliestireno expandido (72 células) contendo substrato comercial, as quais permaneceram em viveiro durante 60 dias até serem plantadas em campo, no espaçamento de 1,0 m x 1,0 m. O experimento foi conduzido em esquema fatorial 5x3 com delineamento em blocos casualizados onde os níveis dos fatores foram as diferentes fontes de adubo orgânico: composto ? 5,0 kg m-2, esterco de aves ? 3,0 kg m-2, casca de guaraná ? 4,0 kg m-2, esterco de gado ? 4,0 kg m-2 e controle (ausência), e os três morfotipos de crajiru. Após 240 dias foram avaliadas a produção de folhas, teor e rendimento de extratos. Os dados foram submetidos à análise de variância e as médias ao Teste de Tukey, a 5% de probabilidade. Observou-se que as fontes de adubo orgânico promoveram melhor produção de folhas para as plantas de A. chica, exceto para o morfotipo 1 que não apresentou diferenças significativas em relação às fontes utilizadas, sendo que o esterco de aves foi o adubo que proporcionou maior biomassa foliar, teor e rendimento de extratos

Conditions for non-monotonic vortex interaction in two-band superconductors

We describe a semi-analytic approach to the two-band Ginzburg-Landau theory, which predicts the behavior of vortices in two-band superconductors. We show that the character of the short-range vortex-vortex interaction is determined by the sign of the normal domain - superconductor interface energy, in analogy with the conventional differentiation between type-I and type-II superconductors. However, we also show that the long-range interaction is determined by a modified Ginzburg-Landau parameter $\kappa^*$, different from the standard $\kappa$ of a bulk superconductor. This opens the possibility for non-monotonic vortex-vortex interaction, which is temperature-dependent, and can be further tuned by alterations of the material on the microscopic scale

Fast optimum-predictive control and capacitor voltage balancing strategy for bipolar back-to-back NPC converters in high-voltage direct current transmission systems

Multilevel power converters have been introduced as the solution for high-power high-voltage switching applications where they have well-known advantages. Recently, full back-to-back connected multilevel neutral point diode clamped converters (NPC converter) have been used inhigh-voltage direct current (HVDC) transmission systems. Bipolar-connected back-to-back NPC converters have advantages in long-distance HVDCtransmission systems over the full back-to-back connection, but greater difficulty to balance the dc capacitor voltage divider on both sending and receiving end NPC converters. This study shows that power flow control and dc capacitor voltage balancing are feasible using fast optimum-predictive-based controllers in HVDC systems using bipolar back-to-back-connected five-level NPC multilevel converters. For both converter sides, the control strategytakes in account active and reactive power, which establishes ac grid currents in both ends, and guarantees the balancing of dc bus capacitor voltages inboth NPC converters. Additionally, the semiconductor switching frequency is minimised to reduce switching losses. The performance and robustness of the new fast predictive control strategy, and its capability to solve the DC capacitor voltage balancing problem of bipolar-connected back-to-back NPCconverters are evaluated

Enhanced Optical Dichroism of Graphene Nanoribbons

The optical conductivity of graphene nanoribbons is analytical and exactly derived. It is shown that the absence of translation invariance along the transverse direction allows considerable intra-band absorption in a narrow frequency window that varies with the ribbon width, and lies in the THz range domain for ribbons 10-100nm wide. In this spectral region the absorption anisotropy can be as high as two orders of magnitude, which renders the medium strongly dichroic, and allows for a very high degree of polarization (up to ~85) with just a single layer of graphene. The effect is resilient to level broadening of the ribbon spectrum potentially induced by disorder. Using a cavity for impedance enhancement, or a stack of few layer nanoribbons, these values can reach almost 100%. This opens a potential prospect of employing graphene ribbon structures as efficient polarizers in the far IR and THz frequencies.Comment: Revised version. 10 pages, 7 figure

Static Recrystallization of Austenite in a Medium-Carbon Vanadium Microalloyed Steel and Inhibition by Strain-Induced Precipitates

The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V microalloyed steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation.Peer reviewe