Quantum and frustrated magnetism

Quantum Criticality in an Ising Chain: Experimental Evidence for Emergent E8 Symmetry

A transverse magnetic field induces quantum disorder in the ferromagnetic Ising chain material CoNb₂O₆. This system was used to explore the remarkable symmetry described by the E8 Lie group where a spectrum of eight particles is predicted. Just below the quantum critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean predicted for the first two meson particles of the E8 spectrum.

R Coldea, DA Tennant, EM Wheeler E Wawrzynski, D Prabhakaran, M Telling, K Habicht, P Smeibidl, K Kiefer, Science 327, 177 (2010).

Press release: Golden ratio discovered in a quantum world

Confinement of fractional quantum number particles in a condensed-matter system

Using time of flight neutron scattering a completely new type of quantum critical behaviour was discovered in CaCu₂O₃. This result is important because it has analogies with the most famous example of confinement found in particle physics where baryons and mesons are produced by the confinement of quarks.

B Lake, AM Tsvelik, S. Notbohm , DA Tennant, TG Perring, M Reehuis, C Sekar, G Krabbes, B Büchner, Nature Physics 6, 50 (2010).

Press release: Spinons - confined like Quarks

Dirac Strings and Magnetic Monopoles in the Spin Ice Dy₂Ti₂O₇

Topological materials require new experimental techniques. Such a state in spin ice described by interweaving networks of aligned dipoles known as Dirac strings is argued to have defects resembling magnetic monopoles when individual strings end. By applying a symmetry-breaking magnetic field the density and orientation of the strings can be manipulated and compared to theories of monopoles. To address this we combined 3D diffuse scattering using the E2 spectrometer with millikelvin temperatures and applied field and showed agreement with magnetic monopoles in the rare earth pyroclore Dy₂Ti₂O₇.

DJP Morris, DA Tennant, SA Grigera, B Klemke, C Castelnovo, R Moessner, C Czternasty, M Meissner, KC Rule, JU Hoffmann, K Kiefer, S Gerischer, D Slobinsky, RS Perry, Science 326, 411 (2009).

Press release: Magnetic monopoles detected in a real magnet

Azurite - The Blue Magnet

In addition to being useful as a painting pigment, the natural mineral, azurite is also a curious low-dimensional quantum magnet. Azurite comprises a diamond chain arrangement of spin-½ Cu2+ atoms and is considered a candidate for the first system to exhibit spin-1/3 excitations. This material exhibits a plateau in magnetization at one-third of its saturation value in which one third of the copper spins are polarised.

Recently we have determined the magnetically ordered structure of azurite by neutron diffraction [1] and investigated its spin dynamics by inelastic neutron scattering [2].

[1] KC Rule, M Reehuis, MCR Gibson, B Ouladdiaf, MJ Gutmann, J-U Hoffmann, S Gerischer, DA Tennant, S Süllow, and M Lang, Physical Review B 83 (2011) 104401

[2] KC Rule, DA Tennant, J-S Caux, MCR Gibson, MTF Telling, S Gerischer, S Süllow, and M Lang, Physical Review B 84 (2011), 184419