Transport properties of armchair graphene nanoribbon junctions between graphene electrodes

The transmission properties of armchair graphene nanoribbon junctions between graphene electrodes are investigated by means of first-principles quantum transport calculations. First the dependence of the transmission function on the size of the nanoribbon has been studied. Two regimes are highlighted: for small applied bias transport takes place via tunneling and the length of the ribbon is the key parameter that determines the junction conductance; at higher applied bias resonant transport through HOMO and LUMO starts to play a more determinant role, and the transport properties depend on the details of the geometry (width and length) of the carbon nanoribbon. In the case of the thinnest ribbon it has been verified that a tilted geometry of the central phenyl ring is the most stable configuration. As a consequence of this rotation the conductance decreases due to the misalignment of the $pi$ orbitals between the phenyl ring and the remaining part of the junction. All the computed transmission functions have shown a negligible dependence on different saturations and reconstructions of the edges of the graphene leads, suggesting a general validity of the reported results

First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and found that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon-metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurties present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1 Bohr magnetons and are increasingly delocalized. The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while is non-magnetic at the level of GGA calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th, 200

Magnetic field induced transition in a wide parabolic well superimposed with superlattice

We study a $Al_{x}Ga_{x-1}As$ parabolic quantum wells (PQW) with $GaAs/Al_{x}Ga_{x-1}As$ square superlattice. The magnetotransport in PQW with intentionally disordered short-period superlattice reveals a surprising transition from electrons distribution over whole parabolic well to independent-layer states with unequal density. The transition occurs in the perpendicular magnetic field at Landau filling factor $\nu\approx3$ and is signaled by the appearance of the strong and developing fractional quantum Hall (FQH) states and by the enhanced slope of the Hall resistance. We attribute the transition to the possible electron localization in the x-y plane inside the lateral wells, and formation of the FQH states in the central well of the superlattice, driven by electron-electron interaction.Comment: 5 pages, 4 figure

V, R, I and Halpha photometry of circumnuclear star forming regions in four galaxies with different levels of nuclear activity

V,R,I continuum bands and Halpha + [NII] photometry for circumnuclear star forming regions (CNSFR) is presented, for galaxies with different kinds of nuclear activity: NGC 7469 (Sy 1), NGC 1068 (Sy 2), NGC 7177 (LINER) and NGC 3310 (Starburst). Their Halpha luminosities (0.02 - 7x10**40 erg/s; uncorrected for internal extinction), are comparable to those observed in other galaxies. No systematic differences in the broad band colours are found for the CNSFR in the different galaxies, except for those in NGC 3310 which are considerably bluer, partially due to a younger stellar population. Using theoretical evolutionary synthesis models some colours can be reproduced by single populations (between 7 and 300 Myr) and extinction 0.5-1.5 mag). However, in many cases, the observed equivalent width of Halpha, require a younger population. NGC 1068, 7177 and 7469, are fitted by a two-burst population model at solar metallicity; the age difference between both populations is around 5-7 Myr and the younger burst involves 3 - 61% of the total mass of the cluster,consistent with the younger burst being originated by supernova activity from the previous one. NGC 3310, requires younger ionizing population ages and lower metallicity. An excess (R-I) observed/predicted, not consistent with a normal reddening law can be atributted to red supergiants present in the older population, and not properly accounted for by the models. The two-population scenario seems to show a trend of the evolutionary state of the CNSFR with the nuclear type of the parent galaxy that remains to be explored.Comment: Accepted for publication in MNRAS, 22pages, 11 figure

High order fractional microwave induced resistance oscillations in 2D systems

We report on the observation of microwave-induced resistance oscillations associated with the fractional ratio n/m of the microwave irradiation frequency to the cyclotron frequency for m up to 8 in a two-dimensional electron system with high electron density. The features are quenched at high microwave frequencies independent of the fractional order m. We analyze temperature, power, and frequency dependencies of the magnetoresistance oscillations and discuss them in connection with existing theories.Comment: 5 pages, 5 figure

Non-linear transport phenomena in a two-subband system

We study non-linear transport phenomena in a high-mobility bilayer system with two closely spaced populated electronic subbands in a perpendicular magnetic field. For a moderate direct current excitation, we observe zero-differential-resistance states with a characteristic 1/B periodicity. We investigate, both experimentally and theoretically, the Hall field-induced resistance oscillations which modulate the high-frequency magneto-intersubband oscillations in our system if we increase the current. We also observe and describe the influence of direct current on the magnetoresistance in the presence of microwave irradiation.Comment: 8 pages, 6 figure

Microwave zero-resistance states in a bilayer electron system

Magnetotransport measurements on a high-mobility electron bilayer system formed in a wide GaAs quantum well reveal vanishing dissipative resistance under continuous microwave irradiation. Profound zero-resistance states (ZRS) appear even in the presence of additional intersubband scattering of electrons. We study the dependence of photoresistance on frequency, microwave power. and temperature. Experimental results are compared with a theory demonstrating that the conditions for absolute negative resistivity correlate with the appearance of ZRS.Comment: 5 pages, 4 figure

Envelhecimento acelerado de aguardentes vínicas na presença de fragmentos de madeira. Influência nos compostos odorantes

A tecnologia de envelhecimento tradicional, que consiste na colocação da aguardente vínica em vasilhas de madeira durante vários anos, é uma técnica morosa e onerosa. Assim, têm vindo a ser introduzidas e desenvolvidas novas técnicas, com o intuito de optimizar o processo e reduzir os custos. Uma dessas novas técnicas consiste em introduzir pedaços de madeira (aparas, toros ou outras formas) na bebida a envelhecer, condicionada em depósitos de inox. Têm sido realizados muitos estudos em vinhos, mas em aguardentes a experimentação é escassa [1, 2]. Assim, este trabalho teve como objectivo avaliar a influência da utilização de alternativas ao envelhecimento de aguardentes em vasilhas de madeira, na composição química das aguardentes obtidas, dando particular atenção aos compostos odorantes provenientes da madeira. Para tal, uma mesma aguardente vínica da Lourinhã foi submetida a um processo de envelhecimento, com três formas de madeira: aguardente colocada em vasilha de madeira (V), aguardente colocada em vasilha de inox com introdução de madeira sob a forma de dominós (D) e aguardente colocada em vasilha de inox com introdução de madeira sob a forma de tábuas (T), tendo sido colhidas amostras de aguardente, ao fim de 180 dias de envelhecimento, para análise e quantificação dos compostos odorantes. Os resultados obtidos mostram que a forma da madeira teve um efeito altamente significativo na maioria dos compostos analisados. No caso dos compostos odorantes derivados da lenhina da madeira (fenois voláteis e vanilina), os teores mais elevados foram encontrados nas aguardentes envelhecidas na presença de fragmentos (dominós ou tábuas), enquanto para os compostos odorantes derivados das hemiceluloses da madeira (ácido acético e aldeídos furânicos) os teores mais elevados foram determinados nas aguardentes envelhecidas em vasilha de madeira. Os resultados deste trabalho sugerem a possibilidade de diferenciação química das aguardentes, em função da tecnologia de envelhecimento