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Quantum 1/f Biochemical Detection Limits in THz Signatures Revealed by Scanning Tunneling Microscopy Currents

It may be possible to amplify the characteristic THz frequency oscillations of chemical and biological agents that contaminate surfaces or samples examined with the scanning tunneling microscope (STM). This THz spectral signature of the biochemical molecules competes with the frequency fluctuations introduced as phase noise close to the examined THz frequency by the 1/f fluctuations of the tunneling resistance, of the resistance of the STM tip, and of the spreading resistance in the sample. Indeed, as we show below, the achievable spectral sensing resolution is Deltaomega/omega = [2 C ln 2rceil1/2 = [2 ln 2(fSR/R2)/4Qt 4rceil1/2 = [1.66 ldr 10-12rceil1/2 = 1.3 ldr 10-6. Therefore, we have calculated the quantum 1/f noise of STMs for the first time, also determining the STM image resolution limits, including both the conventional and coherent quantum 1/f (Q1/f) effects. The main contributions are found to be quantum 1/f tunneling current noise, and quantum 1/f effect in the piezoelectric transducers in the feedback loop that controls the position of the needle above the sample. The results are compared with experimental data on STM resolution.