GAITHERSBURGH, Md. A new type of magnetism that might have applications to quantum computing has been observed in an exotic zincochromite (ZnCrO4) crystal lattice. The Commerce Department's National Institute of Standards and Technology made the discovery here using neutron beams to investigate the structure's spin behavior.
Zinc-chromium-oxide structures were known to exhibit exotic magnetic properties, which result from the inability of the lattice's magnetic spins to adopt a stable configuration because of the peculiar geometry of the crystal lattice. The new observations have detected emergent structures that were not predicted by theory, showing that the spin configurations do have a type of order.
"These systems, called 'frustrated magnets,' are interesting because the spin configurations can never settle down into a stable long-range configuration; they have an infinite number of ground states," said Seung-Hun Lee, a physicist at NIST's Center for Neutron Research. The neutron observation of zincochromite's magnetic spin configurations was performed by Lee and his colleagues at the NIST center along with physicists at Johns Hopkins University (Baltimore) and Rutgers University (Piscataway, N.J.).
The geometry of zincochromite produces tetrahedral corners, which make it impossible for the spin vectors of the lattice's atoms to line up in stable pairs, as they do in other crystal structures such as the common cubic lattice. "In any material, at high temperatures, the spins are free to fluctuate it's basically a liquid state and as you cool the material, they normally line up, making a phase change to a stable solid," Lee explained. "In this particular lattice they don't know where to go, and they remain in the liquid state no matter how cool the system becomes."
That makes for a physical system ripe for manifesting "emergent" behavior, which is essentially what Lee's group found.
Emergent behavior is a key concept in neural-network theory for example, where it is able to explain how high-level "intelligent" components arise from the unintelligent behavior of electrons. Other unusual electronic properties, such as the various forms of superconductivity, depend on large-scale coordinated movements that suddenly emerge from the random movements of electrons.
"Condensed-matter physicists are beginning to discover similar higher-order organization principles in materials. There are now two approaches to understanding the behavior of materials," Lee said. "The reductionist approach starts with individual particles, solves Shroedinger's equation for the system and then generalizes from there. This has a long tradition in science. The problem is computational complexity. If the number of particles exceeds 10, the computational complexity increases exponentially, making simulations impossible.
"The newer approach looks for emergent, higher-order patterns that maintain their stability over time. These structures, called a protectorate, have their own intrinsic properties and form an essentially new entity."
The neutron observations uncovered a protectorate in the zincochromite spin liquid consisting of a hexagonal arrangement of six anti-parallel spins. Two spin vectors form a stable unit when they can line up in parallel but opposite directions (called an anti-parallel configuration). Thus three spins cannot line up in a magnetically neutral configuration at the corner of the lattice, but six spin vectors can cyclically line up so that any vector is anti-parallel to its neighboring spin vectors. The larger hexagonal arrays can therefore orient themselves so that a single stable spin direction appears.
This configuration is stable and repeats throughout the lattice, forming a crystalline structure with long-range order, although that structure would not be detected by looking at the orientation of spins at individual corners in the lattice.
The discovery solves some mysteries about the magnetic behavior of zincochromite and may provide some important clues to such other areas as protein folding and quantum computing, said Lee. "Of course, it is too soon to talk about applications, but the emergence of stable patterns of many different quantum levels might be useful in the design of quantum computers," he said.
The quantized variable physically represented by magnetic spin has been used in some rudimentary logic circuits based on the theory of quantum information processing. A fundamental problem facing such information encoding schemes is how to preserve the quantum coherence of the system for a long enough time to perform an algorithm. An emergent protectorate of quantum spins could open a route to solving that problem.
Neutron beams have become a highly useful tool for investigating electron-spin behavior. "Neutron scattering gives us the most detailed information on spin correlation in electrons and can be used as a local probe to detect spin behavior," Lee said. Neutrons have no electric charge but do have a magnetic moment, and refined beams of the particles operate like a magnetic version of an electron microscope.
In the NIST experiment, the researchers applied Fourier transforms to the scattered neutron beam to isolate the unusual magnetic configuration.