Customised Solution for 2.5 - 350 K STM Operation in 3D Magnetic Fields

High-performance STM operation at low temperatures and high magnetic fields puts stringent requirements on microscope design. With the Cryogenic STM, Omicron has developed an ultrastable microscope for delicate experimental conditions.


Customised Cryogenic STM UHV system with Oxford ‘bottom-loader’ cryostat.
STM at cryogenic temperatures is a well-established technique and known to be key to acquiring enhanced spectroscopic details due to reduced thermal broadening of the local density of states. The combination with high magnetic ﬁelds gives access to a large variety of physical phenomena but experimental requirements are even more stringent.The recently installed system at the University of the City of London (Christoph Renner´s group) represents a fully UHV-compatible turnkey solution for continuous STM operation from 2.5 K to 350 K in 3D vector magnetic ﬁelds. The ‘bottom-loader’ concept is based on a pumped ⁴He magnet cryostat from Oxford Instruments, which employs a variable temperature insert (VTI) and a 3D vector magnet. The maximal magnetic ﬁeld is B_Z = 6 T if operated in vertical ﬁelds only, \|B_XYZ\|= 1 T over full 4π, and B_Z = 2 T and \|B_XY\| = 1 T in combined operation.

Using a vertical manipulator, the VTI insert with attached microscope is transferred to the STM chamber for in-situ tip/sample exchange (situ-ated below the cryostat). A second chamber with the fast entry load lock serves sample prepara-tion purposes. Three mechanisms ensure optimal STM/STS performance: (1) extremely rigid bench frame with air-damping legs, (2) compact STM design for high intrinsic stability to minimise the response to external mechanical or acoustic excitations, and (3) spring-suspended microscope head for mechanical decoupling from the cryostat cold plate. Within only 3 weeks, the system was installed and successfully passed the acceptance test with atomic resolution on Au(111) and meV energy resolution of the superconducting gap of NbSe₂.