James Hamp

Dr James Hamp

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

Research

I am currently interested in spin ice and related frustrated magnets. Spin ice is a magnetic compound that does not order ferro- or antiferromagnetically down to the lowest temperatures, in contrast to what is expected for conventional magnets. Instead, the spins remain in a liquid-like state, with strong correlations but no long-range order. This lack of conventional order is due to the geometry of the pyrochlore lattice, which means the interactions between spins are frustrated. However, the system is not trivially disordered either. Indeed, spin ice in its ground state obeys the `ice rules' whereby two spins point in, and two out, of every tetrahedron which makes up the lattice. This constraint can be coarse-grained and recast in terms of an emergent divergenceless field, whose excitations are monopolar in nature and interact via a magnetic Coulomb interaction. These emergent magnetic monopoles are a classical instance of fractionalisation, and spin ice is best understood as a classical topologically-ordered state.

Spin ice compounds such as Dy₂Ti₂O₇ and Ho₂Ti₂O₇ display an array of fascinating phenomena at different temperatures and under different magnetic fields, many of which can be understood in terms of a fluid of magnetic monopoles. I am interested in studying these phenomena, both in- and out-of-equilibrium.

In Plain English

Matter can exist in different states, or phases, familiar examples being the solid (ice), liquid (water), and gas (steam) phases of water. Recently, physicists have been confronted with many systems, often with peculiar and unique properties, which do not neatly fit into the conventional classification of states of matter.

I work on a novel state of matter, called `spin ice', which is realised in certain magnetic materials. In a similar way to how a metal hosts mobile electric charges—electrons—spin ice at low temperatures hosts mobile magnetic charges that look very similar to the long-sought elementary magnetic monopoles. The motion of these emergent monopoles leads to the magnetic analogue of electricity—magnetricity.

I am interested in the varied and often surprising phenomena displayed by spin ice materials, many of which can be understood in terms of magnetic monopoles.

Featured Publications

Automated regime detection in multidimensional time series data using sliced Wasserstein k-means clustering Qinmeng Luan and James Hamp [arXiv2310.01285]
Supercooling and fragile glassiness in a dipolar kagome Ising magnet James Hamp, Roderich Moessner, and Claudio Castelnovo Phys. Rev. B 98 144439 (2018) [arXiv1806.02471]
Emergent order in the kagome Ising magnet Dy₃Mg₂Sb₃O₁₄ J. A. M. Paddison, H. S. Ong, J. O. Hamp, P. Mukherjee, X. Bai, M. G. Tucker, N. P. Butch, C. Castelnovo, M. Mourigal, and S. E. Dutton Nature Communications 7 13842 (2016) [arXiv1605.01423]
Emergent Coulombic criticality and Kibble-Zurek scaling in a topological magnet James Hamp, Anushya Chandran, Roderich Moessner, and Claudio Castelnovo Phys. Rev. B 92 075142 (2015) [arXiv1506.03472]
Spontaneous rotating vortex rings in a parametrically driven polariton fluid J. O. Hamp, A. K. Balin, F. M. Marchetti, D. Sanvitto, and M. H. Szymanska EPL 110 57006 (2015) [arXiv1403.3234] Featured on the cover of EPL