Luwei Ge

Ba 8 CoNb 6 O 24 : A spin- 1 2 triangular-lattice Heisenberg antiferromagnet in the two-dimensional limit

The perovskite Ba8CoNb6O24 comprises equilateral effective spin-1/2 Co2+ triangular layers separated by six non-magnetic layers. Susceptibility, specific heat and neutron scattering measurements combined with high-temperature series expansions and spin-wave calculations confirm that Ba8CoNb6O24 is basically a twodimensional (2D) magnet with no detectable spin anisotropy and no long-range magnetic ordering down to 0.06 K. In other words, Ba8CoNb6O24 is very close to be a realization of the paradigmatic spin-1/2 triangular Heisenberg model, which is not expected to exhibit symmetry breaking at finite temperature according to the Mermin and Wagner theorem.

Spin order and dynamics in the diamond-lattice Heisenberg antiferromagnets CuRh 2 O 4 and CoRh 2 O 4

Antiferromagnetic insulators on the diamond lattice are candidate materials to host exotic magnetic phenomena ranging from spin-orbital entanglement to degenerate spiral ground-states and topological paramagnetism. Compared to other three-dimensional networks of magnetic ions, such as the geometrically frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism in real materials is less mature. In this work, we characterize the magnetic properties of model A-site spinels CoRh2O4 (cobalt rhodite) and CuRh2O4 (copper rhodite) by means of thermo-magnetic and neutron scattering measurements and perform group theory analysis, Rietveld refinement, mean-field theory, and spin wave theory calculations to analyze the experimental results. Our investigation reveals that cubic CoRh2O4 is a canonical S=3/2 diamond-lattice Heisenberg antiferromagnet with a nearest neighbor exchange J = 0.63 meV and a Neel ordered ground-state below a temperature of 25 K. In tetragonally distorted CuRh2O4, competiting exchange interactions between up to third nearest-neighbor spins lead to the development of an incommensurate spin helix at 24 K with a magnetic propagation vector k = (0,0,0.79). Strong reduction of the ordered moment is observed for the S=1/2 spins in CuRh2O4 and captured by our 1/S corrections to the staggered magnetization. Our work identifies CoRh2O4 and CuRh2O4 as reference materials to guide future work searching for exotic quantum behavior in diamond-lattice antiferromagnets.

Operation of SiGe HBTs Down to 70 mK

We present the first measurement results of a highly scaled, 90-nm silicon-germanium heterojunction bipolar transistor (SiGe HBT) operating at cryogenic temperatures as low as 70 mK. The SiGe HBT exhibits a transistor-like behavior down to 70 mK, but below 40 K, the transconductance suggests the presence of nonequilibrium transport mechanisms. Despite the non-ideal base current at cryogenic temperatures, a dc current gain (β) > 1 is achieved for IC > 1 nA, suggesting that ultralow-power low-noise amplifiers should be viable. Exposure of the SiGe HBT to strong magnetic fields (±14 T) is also presented to help understand the nature of the non-ideal