Tuning magnetic frustration in a dipolar trident lattice

Frustrated interactions are key to a wide range of phenomena, from protein folding and magnetic memory to fundamental studies of emergent exotic states.

Geometrical frustration and “spin ice”

When bar magnets are brought together, opposite poles will attract and like poles will repel, and the magnets will arrange themselves accordingly, to minimize energy. However, if the magnets are constrained to a lattice structure where each one has just two possible orientations, some magnets could end up geometrically “frustrated,” with neither orientation being lower in energy than the other. The system becomes what’s known as a “spin ice,” analogous to water ice, which retains intrinsic randomness (residual entropy) even at absolute zero.

Systems incorporating geometrical frustration are fascinating because their hard-to-predict behavior is key to a wide range of phenomena, from protein folding and magnetic memory to the emergence of exotic states of matter. For example, the emergence of magnetic monopole–like excitations in spin ice raises the intriguing possibility of “magnetic-charge” circuitry based on currents of magnetic monopole excitations.

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Image: (extract, entire image here) Magnetic scattering patterns calculated from XMCD data for various lattice parameters. While relatively sharp peaks indicative of long-range order are seen in (a) and (c), the diffuse patterns in (b) indicate highly disordered configurations.


Observation and Control of Laser-Enabled Auger Decay

When isolated atoms are electronically excited, they have two possible ways of releasing electronic energy: by radiation or by Auger decay. The Auger process, in which the decaying electron transfers its energy to another electron causing it to detach (ionization), has played an important part in modern physics, particularly surface science, because it is by far the strongest decay channel for core holes of light elements such as carbon, nitrogen, and oxygen. In some cases, the Auger process is energetically forbidden, because the energy being exchanged is not sufficient for ionization. In this case, new electronic mechanisms for deexcitation may be discovered that “borrow” energy from the surroundings. One of these is interatomic Coulombic decay (ICD) where the energy is “borrowed” from surrounding atoms. Another mechanism is laser enabled Auger Decay (LEAD), where the energy is “borrowed” from an ancillary laser field; up to now LEAD has been observed with low-energy photons, meaning that more than one photon must be absorbed to make the process possible.

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