The design and the performance of an ultrahigh vacuum 250 mK scanning tunneling microscope with a 7 Tesla magnetic field and a double deck sample stage for in-situ tip treatment

Scanning tunneling microscope (STM) is a powerful tool for studying the structural and electronic properties at the atomic scale. The combination of low temperature and high magnetic field for STM and related spectroscopy techniques allows us to investigate the physical properties of novel materials at these extreme conditions with high energy resolution. Here, we present the construction and the performance of a 250 mK STM system with the 7 Tesla magnetic field. Both sample and tip can be treated and exchanged in ultrahigh vacuum. Furthermore, a double deck sample stage is designed for the STM head so we can clean the tip by field emission or prepare a spin-polarized tip in situ without removing the sample. The energy resolution of scanning tunneling spectroscopy (STS) at T = 300 mK is determined by measuring the superconducting gap with a niobium tip on a gold surface. We demonstrate the performance of this STM system by measuring BiTeI surface and imaging the bicollinear magnetic order of $Fe_{1+x}Te$ at liquid helium temperature. We further show the superconducting vortex imaging on PbTaSe2 at T = 260 mK.