Development of high-field STM and its application to the study on magnet ically-tuned criticality in Sr3RU2O7

Scanning tunneling microscopy/spectroscopy (STM/STS) is a powerful method to investigate the spatial variation of electronic states with an atomic resolution. The energy resolution of STM/STS is also very high, better than -meV, which enable us to study the details of low-lying quasi-particle excitation spectrum. In addition, STM/STS works even in extreme environments e.g. at low temperatures and under high magnetic fields. Taking these advantages into account, we have built a versatile STM/STS system working under combined extreme conditions of ultra-high vacuum, very-low temperature and high magnetic field. We apply this system to the study of strongly correlated electron systems in which subtle interplay among charge, spin and orbital degrees of freedom brings about various electronic phenomena. As one of the examples, we describe the evolution of the tunneling spectrum across the metamagnetic transition in Sr3Ru2O7.