title: Emergence of long-range order in sheets of magnetic dimers.
abstract: Quantum spins placed on the corners of a square lattice can dimerize and form singlets, which then can be transformed into a magnetic state as the interactions between dimers increase beyond threshold. This is a strictly 2D transition in theory, but real-world materials often need the third dimension to stabilize long-range order. We use high pressures to convert sheets of Cu(2+) spin 1/2 dimers from local singlets to global antiferromagnet in the model system SrCu2(BO3)2. Single-crystal neutron diffraction measurements at pressures above 5 GPa provide a direct signature of the antiferromagnetic ordered state, whereas high-resolution neutron powder and X-ray diffraction at commensurate pressures reveal a tilting of the Cu spins out of the plane with a critical exponent characteristic of 3D transitions. The addition of anisotropic, interplane, spin-orbit terms in the venerable Shastry-Sutherland Hamiltonian accounts for the influence of the third dimension.
authors.affiliation: The James Franck Institute and Department of Physics, The University of Chicago, Chicago, IL 60637; Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439;
authors.forename: S
authors.initials: S
authors.lastname: Haravifard
authors.affiliation: The James Franck Institute and Department of Physics, The University of Chicago, Chicago, IL 60637; Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831;
authors.forename: A
authors.initials: A
authors.lastname: Banerjee
authors.affiliation: School of Physics, The University of Bristol, Bristol BS8 1TL, United Kingdom; and.
authors.forename: J
authors.initials: J
authors.lastname: van Wezel
authors.affiliation: The James Franck Institute and Department of Physics, The University of Chicago, Chicago, IL 60637;
authors.forename: D M
authors.initials: DM
authors.lastname: Silevitch
authors.affiliation: Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831;
authors.forename: A M
authors.initials: AM
authors.lastname: dos Santos
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439;
authors.forename: J C
authors.initials: JC
authors.lastname: Lang
authors.affiliation: Department of Physics and Astronomy and Brockhouse Institute for Material Research, McMaster University, Hamilton, ON, Canada L8S 4M1.
authors.forename: E
authors.initials: E
authors.lastname: Kermarrec
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439;
authors.forename: G
authors.initials: G
authors.lastname: Srajer
authors.affiliation: Department of Physics and Astronomy and Brockhouse Institute for Material Research, McMaster University, Hamilton, ON, Canada L8S 4M1.
authors.forename: B D
authors.initials: BD
authors.lastname: Gaulin
authors.affiliation: Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN 37831;
authors.forename: J J
authors.initials: JJ
authors.lastname: Molaison
authors.affiliation: Department of Physics and Astronomy and Brockhouse Institute for Material Research, McMaster University, Hamilton, ON, Canada L8S 4M1.
authors.forename: H A
authors.initials: HA
authors.lastname: Dabkowska
authors.affiliation: The James Franck Institute and Department of Physics, The University of Chicago, Chicago, IL 60637; tfr@caltech.edu.
authors.forename: T F
authors.initials: TF
authors.lastname: Rosenbaum
keywords: condensed matter physics; dimensional cross-over; neutron and X-ray scattering; phase transition; quantum magnetism