Criticality in a disordered quantum antiferromagnet by neutron diffraction

Abstract

Field-induced magnetic ordering in the structurally disordered quantum magnets Ni(Cl${}_{1-x}$Br${}_x$)${}_2$$\cdot$4SC(NH${}_2$)${}_2$, $x=8$% and 13%, is studied by means of neutron diffraction. The order parameter critical exponent is found to be very close to its value $\beta =0.5$ expected for magnetic Bose-Einstein condensation in the absence of disorder. This result applies to temperatures down to 40 mK, and a 1 T range in magnetic field. The crossover exponent is found to be $\varphi \sim 0.4$, for temperature ranges as small as 300 mK. Possible reasons for a discrepancy with recent numerical simulations and bulk measurements are discussed.

submitted to PRB

References in corpus (3)

• Bose-Einstein Condensation in Magnetic Insulators
• Particle-hole symmetry and the dirty boson problem
• Quantum Phase Transitions of Hard-Core Bosons in Background Potentials

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