Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires

We present electronic transport measurements of GaAs nanowires grown by catalyst-free metal-organic chemical vapor deposition. Despite the nanowires being doped with a relatively high concentration of substitutional impurities, we find them inordinately resistive. By measuring sufficiently high aspect-ratio nanowires individually in situ, we decouple the role of the contacts and show that this semi-insulating electrical behavior is the result of trap-mediated carrier transport. We observe Poole-Frenkel transport that crosses over to phonon-assisted tunneling at higher fields, with a tunneling time found to depend predominantly on fundamental physical constants as predicted by theory. By using in situ electron beam irradiation of individual nanowires we probe the nanowire electronic transport when free carriers are made available, thus revealing the nature of the contacts

Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.ope

Suppression of mid-infrared plasma resonance due to quantum confinement in delta-doped silicon

The classical Drude model provides an accurate description of the plasma resonance of three-dimensional materials, but only partially explains two-dimensional systems where quantum mechanical effects dominate such as P:$\delta$-layers - atomically thin sheets of phosphorus dopants in silicon that induce novel electronic properties beyond traditional doping. Previously it was shown that P:$\delta$-layers produce a distinct Drude tail feature in ellipsometry measurements. However, the ellipsometric spectra could not be properly fit by modeling the $\delta$-layer as discrete layer of classical Drude metal. In particular, even for large broadening corresponding to extremely short relaxation times, a plasma resonance feature was anticipated but not evident in the experimental data. In this work, we develop a physically accurate description of this system, which reveals a general approach to designing thin films with intentionally suppressed plasma resonances. Our model takes into account the strong charge density confinement and resulting quantum mechanical description of a P:$\delta$-layer. We show that the absence of a plasma resonance feature results from a combination of two factors: i), the sharply varying charge density profile due to strong confinement in the direction of growth; and ii), the effective mass and relaxation time anisotropy due to valley degeneracy. The plasma resonance reappears when the atoms composing the $\delta$-layer are allowed to diffuse out from the plane of the layer, destroying its well-confined two-dimensional character that is critical to its novel electronic properties

Observation of Space-Charge-Limited Transport in InAs Nanowires

Recent theory and experiment have suggested that space-charge-limited transport should be prevalent in high aspect-ratio semiconducting nanowires. We report on InAs nanowires exhibiting this mode of transport and utilize the underlying theory to determine the mobility and effective carrier concentration of individual nanowires, both of which are found to be diameter-dependent. Intentionally induced failure by Joule heating supports the notion of space-charge-limited transport and proposes reduced thermal conductivity due to the nanowires polymorphism

Professional learning for distributed leadership:Primary headteachers’ perspectives

This article draws from a small-scale study of headteachers motivated to positively impact on the quality of pupil experience by involving all staff in a distributed perspective on leadership. Each headteacher perceived leadership as involving learned processes requiring support and experience, expending considerable effort in providing a fertile environment for learning about its practice. This perspective developed from their personal experience of challenging established leadership orthodoxies prior to and since appointment to headship. The article explores the impact of formal work-based postgraduate leadership preparation and experiential professional learning on each headteacher’s understandings of distributed leadership and its practice. It then explores the ways in which they supported the professional learning of staff. The article concludes by suggesting that headteachers and staff encounter a range of challenges in developing school practices inherent in distributed leadership and can benefit from ongoing support with informed reflection on practice beyond initial preparation for headship

Redefining what It means to be a teacher through professional standards:Implications for continuing teacher education

This article connects with an international debate around the place of professional standards in educational policy targeted at enhancing teacher quality, with associated implications for continuing teacher education. Scotland provides a fertile context for discussion, having developed sets of professional standards in response to a recent national review of career-long teacher education. That review called for a reprofessionalisation of the teaching profession and the revision of the standards was an element of this process. Scotland is utilised as a lens through which one country’s response to international trends is viewed, with a focus on ‘teacher leadership’ and ‘practitioner enquiry’ as policy endorsed sets of practices. The analysis demonstrates the complex and contested nature of these terms and the tensions posed between the need to meet professional standards as part of teacher education and aspirational dimensions of the current policy project of reprofessionalisation. The article concludes by considering the implications for continuing teacher education

The shifting discourses of educational leadership:International trends and Scotland’s response

Increasing emphasis has been placed on leadership within educational theory, policy and practice. Drawing on a wide range of academic literature and policy documents, this paper explores how the discourse of leadership has shifted and for what purposes. The authors are critical of the lack of conceptual underpinning for that discourse, evident both nationally and internationally, and they identify key issues that the teaching profession has been left to try to understand and make sense of. They caution that, despite attempts to align contemporary policy developments to position leadership as inherent in the role of every teacher, flaws in the conceptualisation of leadership, and particularly in favoured forms such as ‘distributed leadership’ and ‘teacher leadership’, pose significant challenges to a serious attempt to ‘reprofessionalise’ teachers. Contemporary developments in Scottish education are referred to, exemplifying key tensions inherent in translating international trends into practice

Adolescent bereavement and social support perception of need according to gender

Includes bibliographical references