SCANNING TUNNELING MICROSCOPE TIP CURRENT EXCITED BY MODULATED X-RAYS

We measured the current passing between a sample and a scanning tunnel ing microscope (STM) tip under conditions of X-ray irradiation. As sho wn in our previous reports, this STM tip current originated from elect ron emission on the sample surface. For high precision STM tip current measurement, we applied an X-ray modulation technique using an X-ray chopper and a lock-in amplifier. X-rays modulated by the X-ray chopper irradiated the sample surface of the STM, and the STM tip current was detected using the lock-in amplifier. The largest and most stable out put from the lock-in amplifier was obtained under the experimental con ditions of low modulation frequency (similar to 10 Hz), strong X-ray i ntensity, and high STM bias voltage. Compared with measuring the STM t ip current directly without the modulation technique, the precision or this measurement is threefold better.