Quantum Shot Noise Characteristics in Atomic Scale Junctions at Liquid Nitrogen and Room Temperatures Using Novel Measurement Technique
Doctor of Philosophy
Shot noise encodes information not directly inferable from simple electronic transport measurements. Previous measurements in atomic-scale junctions have shown suppression of the shot noise at particular conductance values. This suppression shows that transport in these structures proceeds via discrete quantum channels. A novel measuring technique was used to probe these quantum shot noise characteristics at liquid nitrogen and room tem- peratures. This technique utilized high–frequency, broad band RF signal measurements of square wave biased junctions and simultaneous extraction of shot noise power and conduc- tance measurement data at high sampling rates. Junctions were created and measured at room temperature utilizing both MCBJ and STM–BJ experimental design hardware. Noise suppression was observed at up to three conductance quanta in junction configuration en- sembles, with possible indications of current-induced local heating and 1/f noise in the contact region at high biases. Lithographically created junctions were also measured using the same electronics at liquid nitrogen temperature enabling the examination of individual junction configurations. Non–linearity and asymmetry were found in a significant number of the point contacts. Discrete changes were found in the bias dependence at threshold val- ues of the bias, consistent with electronic excitation of local vibrational modes. Moreover, with some regularity, significant mesoscopic variation in the magnitude of the noise was found in particular junctions even with small changes in the accompanying conductance. Pronounced asymmetries in the inferred noise magnitude were also observed as a func- tion of bias polarity, suggesting current-driven ionic motion in the electrodes even at biases well below those used for deliberate electromigration. These measurements demonstrate the quantum character of transport at room temperature at the atomic scale. This high– frequency broadband technique provides an additional tool for studying correlations in nanodevices.