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(a) 3D rendering of the main body of the STM head. (b) Mesh used for finite element analysis. (c)–(f) Fundamental modes of the STM head (fixed at the top) in the same order as the frequencies in Table I. (c) and (d) correspond to bending modes (↔ and ⊙), (e) to a torsional mode (↺), and (f) to a longitudinal mode along the long axis of the body (↕), which causes the strongest stress around the edges of the central cavity, as indicated by the color scale.
(a) CAD rendering of the assembled STM head. (b) Photograph of the STM head.
Sapphire STM head modes measured in situ with a lock-in amplifier, coarse approach piezo stacks are used for excitation (1 V) and response detection. The highlighted area is zoomed for a more detailed view; calculated positions of the four primary modes of the bare body (as listed in Table I) are denoted.
Tunnel junction noise at , with the 1K-pot turned on and off (i.e., being pumped or not pumped), as measured via the feedback loop control voltage applied to the scan piezo during tunneling. For comparison, the environmental noise on top of the isolation table was measured with a commercial geophone18 and plotted alongside.
Topographic images (raw data) of NbSe2 at (a) 4.2 K and (b) 1.7 K (while operating the 1 K pot) acquired with V = 100 mV and I = 0.5 nA, taken with different tips on different samples. (c) and (d) show line cuts through the topographic images in (a) and (b) along the indicated paths, demonstrating the vertical stability of the system.
Tunneling spectroscopy characterization of NbSe2, revealing the superconducting gap. (a) Superconducting gap as a function of temperature (lock-in modulation: 0.5 mV, 405 Hz, 30 ms lock-in averaging); spectra are averaged from small maps. (b) Spectra calculated using Eq. (1) with the lock-in broadening used in the experiment. Spectra in (a) and (b) are normalized and spread out vertically for clarity.
Comparison of fundamental mode frequencies (given in Hz) for different choices of material. Listed are the four lowest frequency modes. Finite element calculations were performed with the upper end of the STM head rigidly fixed, as it is in the experimental setup. The first two modes correspond to horizontal (↔ and ⊙) bending of the body, the third to twisting (↺), and the fourth to asymmetric vertical (↕) distortion. The simulated body and the corresponding mesh used are shown in Fig. 1. In addition, the density ρ, shear modulus G, Young's modulus E, and the thermal conductivity at room temperature κ(300 K) are listed (values for MACOR from Ref. 38, Al2O3 from Ref. 11, and sapphire from Ref. 39; isotropic averages were used which do not account for material anisotropy).
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