Anthony J Wright

Strong nonlinear optical response of graphene in the terahertz regime

We demonstrate that within the model of massless Dirac fermions, graphene has a strong nonlinear optical response in the terahertz regime. It is found that the nonlinear contribution significantly alters both the single frequency and frequency tripled optical response at experimentally relevant field strengths. The optical activity of single layer graphene is significantly enhanced by nonlinear effects, and the frequency tripled response opens the gateway to photonic and optoelectronic device applications.

Orbital magnetization of graphene and graphene nanoribbons

We present a quantitative analysis on orbital magnetization of graphene under a magnetic field. The energy spectra were obtained by solving Harper equation for a honeycomb lattice. The effect of the next-nearest-neighbor hopping (NNNH) is to increase the period of the magnetic subbands in ϕ/ϕ0 from 1 to 6 (where ϕ is the magnetic flux through a unit cell and ϕ0 is the flux quanta). The shifts of the energy levels due to the NNNH vary with the magnetic field. The Holmholtz free energy at the points ϕ/ϕ0=6n+1 is lowered as compared to that with the nearest neighbor hopping only. For graphene nanoribbons, the magnetic susceptibility is very sensitive to the ribbon width and chirality. The zero-field susceptibility increase with the ribbon width.

Cambrian and Ordovician faunas and stratigraphy, Mt Patriarch, New Zealand

Abstract Forty‐one species of trilobites and two of graptolites are described from the Patriarch Formation and the overlying Summit Limestone of the northwest Nelson area, New Zealand. Most of the trilobites are recorded for the first time from Australasia. The stratigraphy of both formations is revised and stratigraphic columns presented. Five successive faunules, shown to span the Cambrian‐Ordovician boundary, are recognised in the sequence, and are correlated internationally. Stratigraphically important conodonts from the area are figured and briefly discussed. Faunule 1, in the lower 220 m of the Patriarch Formation, contains the Lotagnostus‐Hedinaspis‐Hysterolenus trilobite fauna, which is found around the world in Late Cambrian strata. Twenty‐three trilobite species occur, including three new species: the aulacopleurid Proteuloma ahu, the shumardiid Shumardia (Conophrys) tauwhena, and the nepeid Amzasskiella kupenga. Faunule 2, from the upper 250 m of the Patriarch Formation, contains the graptolite...

Silicified Early Devonian trilobites from Mudgee, New South Wales

A silicified trilobite fauna is described from an un-named Early Devonian (late Zlichovian-early Dalejan) limestone at Mudgee, N.S.W. The fauna includes eleven taxa as follows: Proetus sp., Cornuproetus ? sp., Astycoryphe sp., Cyphaspis sp., Harpidae gen. et sp. indet., Phacops (Phacops) sp., Crotalocephalus struszi sp.nov., Acanthopyge (Lobopyge) campbelli sp.nov., Terranovia? sp.nov., Dudleyaspis sp. cf. D. campbelli (Chatterton 1971) and Radiaspis bispinosa? (Chatterton 1971). Terranovia has not been previously described from Australia.

Spin-orbit interaction enhanced polaron effect in two-dimensional semiconductors

It is shown that in two-dimensional semiconductors, the electron-phonon interaction and polaron mass correction are both significantly enhanced by the Rashba spin-orbit coupling. The mass correction is positive for the upper Rashba branch and negative for the lower Rashba branch. Both Rashba branches have the same polaron binding energy, which is higher than that for systems in the absence of spin-orbit interaction. To the leading order, the correction to the binding energy is proportional to the square of the spin-orbit coupling strength.

Silurian and Early Devonian geochronology — a reappraisal, with new evidence from the Bungonia Limestone

The lower part of the Bungonia Limestone in the Bungonia-Marulan South area, N.S.W., has previously been dated as Ludlovian. New fossil localities from the upper part of the formation yield faunas including Scabriscutellum sp. cf. S. scabrum, Schizophoria, Eospirifer eastoni and Spathognathodus sp. cf. S. remscheidensis which indicate a Lochkovian age. The Bungonia Limestone is intruded by parts of the Marulan Batholith (398 ± 7m.y.) thus giving a minimum age for the Silurian-Devonian boundary. An appraisal of published data shows that there are 20 radiometric dates with good biostratigraphic control from Silurian and Early Devonian rocks. The relationship between the palaeontologically-based relative duration of Silurian and Early Devonian series and stages and the radiometric age of units confined to individual series or stages, provides an estimate of the age of the base of the following: Llandoverian 433 m.y.; Wenlockian 419 m.y.; Ludlovian 412 m.y.; Pridolian 405 m.y.; Gedinnian and Lochkovian 401 m....

A new Arenig (Ordovician) graptolite fauna from the Kerman District, east-central Iran

A single slab of silicified silty and cross-bedded mudstone from the Katkoyeh Formation, east-central Iran yields Yutagraptus cf. mantuanus Riva which indicates a late Arenig age. The slab also bears Dictyonema ghodsiae sp. nov., regarded as the only unequivocally planktonic dendroid from post-Tremadoc strata. The sicula is surrounded by at least twelve vesicular bodies which constitute a raft of floats. These are the only certain floats yet identified in the Order Dendroidea, and one of very few cases known in the Class Graptolithina.

Asteropygine trilobites from the Late Devonian of the Kerman region, Iran

The discovery of two new Frasnian species of the subfamily Asteropyginae, Bradocryphaeus kermanensis and Radiopyge hamedii, in the Haruz section north of Kerman, east-central Iran, demonstrates for the first time the presence of North Gondwanan trilobites in this region. The two new species provide a link between faunas from Afghanistan to the east and the Alborz Mountains in north Iran. The recognition of R. hamedii confirms the validity of Radiopyge Farsan, hitherto known only from Afghanistan, as well as the presence in some Frasnian Asteropyginae of six pairs of lateral pygidial spines.

The Pristiograptus dubius (Suess, 1851) species group and iterative evolution in the Mid- and Late Silurian

Synopsis Pristiograptus dubius (Suess, 1851), one of the longest-known graptolites, is also one of the simplest and longest ranging (about 20 Ma). As well as giving rise to new genera, such as Colonograptus Přibyl (1943c) and Saetograptus Přibyl (1943c) amongst others, it also repeatedly gave rise to both broad and narrow short-lived pristiograptids, cases of iterative development. These closely resemble P. d. dubius and are often difficult to distinguish from it, especially in cases of indifferent preservation or tectonic deformation: a stratigraphy known on independent criteria is a great facilitator to identification! There is no difference in the overall range of broad versus narrow species, but the latter are more numerous. The parent species P. d. dubius survives them all and extends almost to the top of the Přídolí. The origin of the P. dubius lineages is probably through P. initialis Kirste (1919) and P. pergratus in the latest Llandovery, and P. praedubius (Bouček, 1931) in the earliest Wenlock. Pristiograptus d. dubius is a stem lineage in the sense of Urbanek (1998).

Thermodynamic properties of graphene nanoribbons under zero and quantizing magnetic fields

The thermodynamic properties of graphene nanoribbons under zero and finite magnetic fields have been investigated. We have obtained qualitative and quantitative results of the free energy and the magnetic susceptibility in graphene nanoribbons. The electronic states in the tight-binding approximation were used to calculate the thermal and magnetic properties. In the case of a finite magnetic field, the Harper equation was solved for the electronic state for various wavevectors. The magnetic field and temperature dependence of the magnetic susceptibility over a wide field and temperature range are presented.