A quantum phase transition for a spin liquid

  Magnetism

Low-dimensional, geometrically frustrated quantum antiferromagnets can support a “spin liquid” state, which has no long-range order, even at very low temperatures, and with a finite energy gap. In some of these systems, a magnetic field can drive a quantum phase transition to an ordered phase that is mathematically equivalent to Bose-Einstein condensation or the Mott metal-insulator transition. This makes the study of criticality and dimensional crossover in spin liquids of wide significance.

In a Rapid Communication appearing in Physical Review B, Vasile Garlea and collaborators at the Oak Ridge National Laboratory, USA, the Hahn-Meitner Institut in Germany, and the Commissariat à l’Énergie Atomique in Grenoble, France, report an unusual magnetic-field-induced spin ordering in a geometrically frustrated quasi-one-dimensional compound, Sul-Cu₂Cl₄. At high magnetic fields, the spins order into a spiral that twists around the c axis. This spiral, or “helimagnetic,” phase has a definite chirality—the spins spiral in one direction but not the other.

With neutron scattering, Garlea et al. extract critical exponents for the phase transition that do not conform to well-understood theoretical models. These results pose interesting questions regarding the role of chirality in determining the critical behavior of a phase transition from a spin liquid to an ordered magnet. – Sarma Kancharla