Changes in the spin excitation spectrum in the Ising chain magnet CoNb2O6 as the applied transverse field quantum melts the spontaneous magnetic order, as observed by neutron scattering on OSIRIS . a) Schematic phase diagram showing a caricature of the spin quasiparticles in the two phases. b,c) In the ordered phase neutrons scatter by creating two independently-propagating domain-wall quasiparticles leading to a broad continuum with dispersive boundaries. e) In contrast, in the high-field paramagnetic phase excitations are spin-flip quasiparticles with a sharp single-particle dispersion.
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Quantum systems have zero-point fluctuations even at zero temperature. When such fluctuations become strong, they can drive transitions between distinct phases of matter.
One of the most theoretically studied paradigms for such a quantum phase transition is the one-dimensional (1D) chain of Ising spins in a transverse magnetic field. The field stimulates quantum tunneling between the ‘up’ and ‘down’ spin orientations and, above a critical field, these quantum fluctuations become strong enough to ‘melt’ the spontaneous magnetic order and stabilise a paramagnetic state.
We have realised this system experimentally for the first time by applying strong magnetic fields to the quasi-1D Ising ferromagnet CoNb2O6. Using high-resolution single-crystal neutron scattering on Osiris and Iris we have observed a dramatic change in the fundamental quantum character of spin quasiparticles as the magnetic field quantum melts the spontaneous magnetic order. Our results emphasise that quantum criticality opens up new avenues to experimentally realise and explore otherwise inaccessible and novel correlated quantum states of matter.
R Coldea (Oxford University), DA Tennant (Helmholtz Zentrum Berlin, Technical University Berlin), EM Wheeler (Oxford University), E Wawrzynska (Bristol University), D Prabhakaran (Oxford University), M Telling (ISIS), K Habicht, P Smeibidl, K Kiefer (Berlin)
Research date: January 2010
Contact: R Coldea, firstname.lastname@example.org
Further reading: R Coldea et al., Science 327 (2010) 177
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