A high bandwidth and high transmission STM bias line for ESR and quantum stochastic resonance measurements at 0.4 K

We present a scanning tunneling microscope operating in a 3He cryostat at a base temperature of 0.4 K, suitable for electron spin resonance (ESR) [S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, Science 350, 417 (2015)], quantum stochastic resonance (QSR) [M. Hänze, G. McMurtrie, S. Baumann, L. Malavolti, S. N. Coppersmith, and S. Loth, Sci. Adv. 7, eabg2616 (2021)], and electric pump-probe experiments. We achieve excellent signal transmission of -2 to -3 dB between 100 kHz and 100 MHz, -23 dB at 10 GHz, -35 to -40 dB between 25 and 35 GHz, and -45 dB at 40 GHz, using a combination of normal conducting and superconducting semi-rigid, as well as flexible RF cables. SMK type 2.92 mm and mini-SMP connectors are used to minimize reflections and losses over the entire frequency range. The ESR capability of our instrument is demonstrated on possibly hydrogenated titanium atoms adsorbed on two and three monolayers of MgO on Ag(100) at 0.4 and at 4.2 K. We further show QSR measurements for Fe/MgO/Ag(100) and compare the driven relaxation time with the intrinsic energy relaxation time T1 reported for that system [W. Paul, K. Yang, S. Baumann, N. Romming, T. Choi, C. P. Lutz, and A. J. Heinrich, Nat. Phys. 13, 403 (2017)] as a function of magnetic field and tunnel current.