Mark Johnson

Member of Darwin College
PhD student in Prof Needs' group

Office: 540 Mott Bld
Phone: +44(0)1223 3 37354
Email: mj464 @ cam.ac.uk

TCM Group, Cavendish Laboratory
19 JJ Thomson Avenue,
Cambridge, CB3 0HE UK.

Research

I am part of a group developing an ab initio anharmonic vibrational model of crystal systems at finite temperature. Our model takes the Born-Oppenheimer approximation, and models finite temperature effects through mean-field phonon interactions.

My research focusses on harnessing crystal symmetries to accelerate computation, and developing adaptive methods to minimise calculation and verify convergence.

The model is being developed with intended application to systems with highly anharmonic character. Such systems include a variety of two-dimensional materials, and a number of systems exhibiting unusual thermal behaviour such as negative thermal expansion.

In Plain English

I develop computer software which studies crystals at non-zero temperatures.

Without heat, nothing moves. Studying an object which is completely still is useful, and comparatively easy, but can only tell you how that object behaves at a temperature of absolute zero (the coldest possible temperature). To learn how an object behaves at temperatures above absolute zero, you have to study that object when it is moving.

I am working on a model which treats movement in terms of vibrations around a fixed position. Under quantum mechanics, these vibrations can be thought of in terms of particle-like 'vibration portions' known as phonons. By studying how these phonons behave within a piece of crystal, I can learn about the crystal as a whole.

Using the computer model, I hope to investigate the properties of crystals which have not been investigated before, as well as discovering new behaviour in crystals which have already been studied. I hope to explore the properties of two-dimensional materials, superconductors, and materials which shrink as they are heated.