What If a Nonmagnetic Material Could Be Magnetic?

Electric fields in a crystal of Ni2Mo3O8 create spin excitons and elusive magnetic order.

Left: Nickel ions in Ni2Mo3O8 (spheres inside center of polygons) form a honeycomb lattice of tetrahedral (green) and octahedral (blue) polygons. Right: Excitations of the magnetism from the tetrahedral polygons form a neutron scattering pattern (orange). — Image courtesy of Bin Gao, University of Missouri

The Science

To work, quantum computers, quantum networks, and quantum information transfer (quantum teleportation) require that particles are synchronized in space and time. In nickel molybdate (Ni₂Mo₃O₈), nickel ions (Ni²⁺) form a triangular array of tetrahedrons and octahedrons. Each Ni²⁺ ion has a magnetic spin (akin to a compass needle). Spins in the tetrahedrons tend to point opposite to the spins in the octahedrons, so there is no net magnetic force and direction (or moment) in the ions. However, electric fields in the Ni₂Mo₃O₈ induce parallel alignment of the spins. This alignment changes with time. The fluctuating moments are called spin excitons. The result—Ni₂Mo₃O_8,should not be magnetic, but the excitons make it so.

The Impact

Spin alignment synchronizes particles. This provides the quantum entanglement that makes quantum information science possible. This experiment establishes a new means to achieve spin entanglement (synchronicity or coherence) by taking advantage of the competition between electric fields in the crystal, spin coupling between neighboring ions, and a preference for spins to point specific directions. The spin excitons that result from occasional alignment of spins form coherent waves. If controlled, these coherent waves of spin excitons can be used in future applications of quantum mechanics. The applications potentially include quantum computing at room temperature.

Summary

Physicists used a combination of materials synthesis, neutron scattering, magnetometry, calorimetry, and theory to identify the origins of coherent waves of spin excitons in nickel molybdate (Ni₂Mo₃O₈). Ni²⁺ ions in the material form a triangular lattice from a combination of two crystal environments bounded by octahedral or tetrahedral arrangements of oxygen atoms. The researchers used neutron scattering to obtain the momentum and energy dependence of spin excitations as functions of magnetic field and temperature. The crystal electric fields were weak for octahedral and strong for tetrahedral sites, inducing spin excitons in both environments. Weak excitations of spin singlets exhibited antiferromagnetic order. Higher energy excitations exhibited dependence of the excitation with momentum, thus, implying coherence of the spin exciton wave. These spin excitons are associated with the tetrahedrally coordinated site.

Calculations suggest the nickel molybdate is unique among materials in that the combination of crystal electric field strengths, strengths of spin coupling in and across the tetrahedra and octahedra, and directionality of the spins make possible a quantum phase of spin excitons. Remarkably, the quantum phase persists to temperatures well above the low temperature antiferromagnetic phase as observed with neutron scattering and magnetometry and calorimetry.

Contact

Pengcheng Dai
Rice University
pdai@rice.edu

Funding

This research was supported by the the Department of Energy (DOE) Office of Science, Basic Energy Sciences program, the Robert A. Welch Foundation, the National Science Foundation Condensed Matter and Materials Theory program, and the Ministry of Science and Technology in Taiwan. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science user facility operated by Oak Ridge National Laboratory, and the ISIS Neutron and Muon Source.