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Details of Grant 

EPSRC Reference: EP/K040359/1
Title: MilliKelvin Experiments Utilising Vector Magnetic Field
Principal Investigator: Pepper, Professor Sir M
Other Investigators:
Kumar, Dr S Thomas, Dr K
Researcher Co-Investigators:
Project Partners:
Department: London Centre for Nanotechnology
Organisation: UCL
Scheme: Standard Research - NR1
Starts: 01 July 2013 Ends: 30 June 2015 Value (£): 8,379
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Panel History:
Panel DatePanel NameOutcome
12 Feb 2013 EPSRC Equipment Business Case - February 2013 Announced
Summary on Grant Application Form
Many striking physical effects have been found using a two dimensional electron gas at low temperatures in the presence of a strong magnetic field, for example the Integer and Fractional Quantum Hall effects. In this work we propose to investigate electronic properties of semiconductor nanostructures at milliKelvin temperatures in the presence of a high magnetic field when the angle between the plane of the electron gas and the field can be altered. This will open up a new range of physical investigations as the direction of the field affects different properties of the electron system. For example, the spin splitting is determined by the total field whereas the wavefunction is affected by the field component transverse to the electron gas. If coupled layers of electron gas are used then the interlayer coupling is affected by the component of field parallel to the plane.

This will allow a much greater exploration of a number of effects. In most situations electrons in semiconductors can be regarded as free with their energy determined by their total number and their effective mass with the mutual repulsion only slightly modifying this free electron picture. However at low values of carrier concentration the repulsion can dominate the manner in which the electrons diffuse in the solid, theory has shown that at sufficiently low temperatures the electrons can arrange themselves into a regular array. This is termed a Wigner Crystal, or Wigner Lattice, after Wigner who first predicted such a phenomenon.

In one dimension the electrons form a single line and the Wigner Crystal is the trivial case of the electrons seeking a regular periodicity. However, as the confinement weakens, or the electron repulsion increases, so it is possible for the line of electrons to distort as electrons attempt to maximise their separation. In the limit the row splits into two or more separate rows.

By following the values of conductance as the confinement is changed so the movement of energy levels can be obtained as a function of confinement potential. This has been observed and we call the two rows formed as a result of the electron-electron repulsion the Incipient Wigner Lattice, IWL. Analysis of the results on the movement of energy levels has shown that prior to the formation of the two separate rows a hybridised state is formed in which two electrons are shared between the two rows such that they form a distorted single row. Quantum Mechanics dictates that two electrons shared in this way must have opposite spins and they can be entangled as a consequence of which they each "know" the quantum state the other is in..

It is now proposed to study the IWL and magnetically modify the hybrid state in which the electrons are entangled. In similar studies a variety of properties of electrons will be investigated such as the spin incoherent regime which occurs when temperature causes the spins to rotate rapidly and randomly and so it is no longer a defined quantum parameter. The localisation of electrons by disorder is very much affected by a magnetic field which can drive a system insulating or conversely at low fields remove the interference characteristic of electron waves, This latter effect results in an insulating sample increasing in conductivity.

The flexibility of this facility will be applied to the study of new materials where the surface and bulk contributions to the overall properties can be determined by varying the field direction. Energy level splittings in a surface conduction layer will vary as the transverse component whereas the bulk properties are determined by the total field.

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