This week in cond-mat

One paper today in the arxiv:
arxiv:0811.2914 - Zwanenburg et al., Spin states of the first four holes in a silicon nanowire quantum dot
This is another typically exquisite paper by the Kouwenhoven group at Delft, in collaboration with Charlie Lieber at Harvard. The Harvard folks have grown a Si wire segment in the middle of a long NiSi wire. The NiSi ends act as source and drain electrodes for conduction measurements, and the Si segment acts as a quantum dot, with the underlying substrate acting as a gate electrode. As usual, the small size of the Si segment leads to a discrete level spectrum, and the weak electronic coupling of the Si segment to the NiSi combined with the small size of the Si segment results in strong charging effects (Coulomb blockade, which I'll explain at length for nonexperts sometime soon). By measuring at low temperatures very carefully, the Delft team can see, in the conductance data as a function of source-drain voltage and gate voltage, the energy level spectrum of the dot. By looking at the spectrum as a function of magnetic field, they can deduce the spin states of the ground and excited levels of the dot for each value of dot charge. That's cute, but the part that I found most interesting was the careful measurement of excited states of the empty dot. The inelastic excitations that they see are not electronic in nature - they're phonons. They have been able to see evidence for the launching (via inelastic tunneling) of quantized acoustic vibrations. Figure 5 is particularly nice.