Overview

The optical and electronic properties of semiconductors underpin modern technology and will define its future. Our programme encompasses high electron mobility semiconductors for sensitive ballistic sensors, mid-infrared light sources for molecular fingerprinting and cancer diagnosis, as well as quantum dots for quantum technologies.

Nanoscale Magnetism

Academic Staff: Dr Will Branford and Professor Lesley Cohen

We have ongoing work engineering arrays of ferromagnetic nanobars to induce a type of magnetic frustration known as the Ice rules and characterizing their magnetic and transport properties. Frustrated lattices such as these "artificial spin ice" nanoarrays have no ordered state and the excitations act as mobile magnetic charges. Last year we explored how to inject magnetic charges into specific locations to control the magnetic state. To do this we developed new methods of exploring the dynamics of domain walls in magnetic nanostructures and to control the direction of flow with the angle of the applied field using focussed MOKE spectroscopy. We explored the challenges associated with controlling the direction of propagation solely with the chirality of the domain wall. We discovered that injecting domain walls directly into nanowires is possible using the tip of a magnetic force microscope.

We have recently begun a programme of work on Mn based antiperovskite nitride thin films (Mn3AN where A = Ni, Sn and Ga) in collaboration with the Materials Department at Imperial College. These materials are antiferromagnetic in the bulk but the frustrated magnetic structure that the material takes below the Neel temperature is highly sensitive to strain. In particular we have shown that biaxial strain produces a magnetic moment through a process known as piezomagnetism. The work maybe important for new types of spintronics where active strain control (coming from a peizo substrate for example), could be used to control magnetism.

Superconductivity

Academic Staff: Dr Will Branford, Professor David Caplin and Professor Lesley Cohen

The effort in superconductivity research is an established activity in the group and we have an internationally recognised reputation in the development of characterisation tools such as length-scale analysis for examining the connectivity issue, flux creep (and the E-J-B surface), Hall probe imaging and a fast and ultra-sensitive calorimeter for studies of heat capacity of very small samples in high magnetic fields. More recently we have developed point contact spectroscopy (PCS) as a means to chart the evolution of the superconducting energy gap (D) in magnetic field.

Over recent times we have explored the interplay between structure and magnetotransport properties in the BaFe2As2 family of materials, using crystals grown at Oak Ridge. These compounds can exhibit either superconductivity or Dirac cone transport properties with correct doping.

We have used point contact Andreev spectroscopy to interrogate ferromagnetic metals, originally studying the degree of spin polarisation of the transport current and more recently searching for evidence of Andreev bound states within the forbidden energy gap. These bound states are the precursor to the formation of exotic long range superconducting triplet superconductivity which survives within the bulk of a ferromagnet when it is in close proximity with a superconductor.

Ballistic and high mobility nano-sensors

Academic Staff: Profesor Lesley Cohen, Dr Will Branford, Dr Rupert Oulton and Professor Stefan Maier 

In collaboration with the National Physical Laboratory we examined the performance of InSb quantum well Hall structures as multifunctional sensors of electric, magnetic and optical fields, utilising our newly developed scanning photoconductivity rig. Sensitivity to electron charge of the order of 0.05e/√Hz from this simple semiconductor micro-Hall cross is comparable to single electron transistors.

We have extended our work to examine the multifunctional sensor characteristics of CVD graphene sensors in comparison to InSb. We also study the dynamic carrier response in hybrid graphene – nanostructured metal structures, finding plasmon- induced optical anisotropy and electron temperatures in the graphene of the order of 500K at short timescales. This has led us to think further about potential enhancement of the graphene photothermoelectric effect using plasmonic nanostructures. 

Quantum Ratchet Solar Cells

Academic Staff: Professor Chris Phillips and Dr Ned Ekins-Daukes

We have assembled a team to put our “Quantum Ratchet” Solar cell idea to the test. It uses extra optical transitions, designed into a nanostructured PV material that enable conversion efficiencies up to ~ 60%, i.e. the sort of numbers that are critical if PV is to become a significant contribution to carbon mitigation. We are exploring practical implementations, with collaborators in Japan, the US, Diamond, and Sheffield with GaAs/AlGaAs and antimonide nanostructures. The race is on to be the first to convincingly demonstrate the sequential photon absorption that lies at the heart of the idea. 

Mid Infrared Technologies

Academic Staff: Professor Chris Phillips

Our “Digistain” IR imaging method for earlier cancer detection is undergoing clinical trials in Charing Cross Hospital, and work with Southampton and Bath promises a new form of coherent radiation emitter, an asymmetric nanostructure, to generate tunable THz beams[3] in ultra-strongly coupled semiconductor microcavities.