Shu Kurokawa

Electron transport through Ni/1,4-benzenedithiol/Ni single-molecule junctions under magnetic field

We have studied electron transport through Ni/1,4-benzenedithiol (BDT)/Ni single molecule junctions at cryogenic temperatures under magnetic field up to 250 mT. Instead of examining magnetoresistance (MR) of individual junctions, we measured the conductance of many junctions under a constant magnetic field and investigated how a single-molecule peak in a conductance histogram shifts with the field strength. We found that the single-molecule resistance at 77 K, deduced from the conductance peak position, shows a hysteresis against the field strength and takes a maximum around 50 mT when the magnetic field increases from 0 T to 150 mT. The observed resistance change yields a MR of ∼(80−90)% for Ni/BDT/Ni single molecule junctions. This MR is higher than experimental MR reported for non-conjugating molecules but consistent with high theoretical MR predicted for π-conjugated molecules such as BDT. We have also investigated the nonlinearity of the current-voltage (I−V) characteristics of Ni/BDT/Ni junctions un...

Conductance and I-V characteristics of Au/BPY/Au single molecule junctions

We have investigated the low bias conductance and I-V characteristics of single 4,4′-bipyridine (BPY) molecule connected to gold electrodes at room temperature. Exploiting the mechanical controllable break junction technique we statistically determined the most preferred conductance value of Au/BPY/Au single molecule junctions. The measured conductance, 0.01G0 (G0 is the conductance quantum unit), is in good agreement with two of the previous experiments using scanning tunnel microscopy break junctions. The I-V characteristics of the Au/BPY/Au junction have been directly measured by varying the bias up to ±0.5 V while holding the junction at its single molecule state. By comparing the obtained I-V curves with those of the Au/1,4-benzenedithiol (BDT)/Au junction, we observed differences in the energy level alignment of BPY and BDT molecules with respect to the Au electrode.

Admittance of Au/1,4-benzenedithiol/Au single-molecule junctions

Employing the admittance formula for double-barrier junctions [Fu and Dudley, Phys. Rev. Lett. 70, 65 (1993)], we have estimated an ac susceptance (imaginary part of admittance) of Au/1,4-benzenedithiol/Au single-molecule junctions from their current-voltage characteristics. In the MHz regime, we find that the junction susceptance shows a very small (∼0.1 aF) capacitive component that can be entirely masked by a larger electrode capacitance. Direct ac signal transmission measurements up to 1 GHz reveal no molecular signals and confirm the smallness of the molecular capacitance in the MHz regime.

Transmission of Voltage Pulses through the Cu Single-Atom Contact

We have conducted a pulse transmission experiment on the single-atom contact of Cu in air at room temperature. Employing a thin coaxial cable as a sample of the mechanically controllable break junction (MCBJ) method, we can produce stable Cu single-atom contacts and transmit through them pulses of 50–500 ns width. Analysis of the detected signal waveform indicates that a Cu single-atom contact behaves as a pure resistance, which is consistent with the theoretical admittance of quantum point contacts.