Synchrotron-radiation scanning tunneling microscopy (SR-STM, also called SX-STM) detects an element-specific modulation of the STM tip current when the incident photon energy is tuned across a core-level absorption edge, enabling nanoscale elemental mapping. In the 2008 hard-X-ray demonstration of nanoscale elemental identification — Cu nanoislands on Ge(111) — the lock-in-extracted tip current showed a picoampere-scale element-specific reduction on the Cu island above the Cu K edge, with no contrast below the edge, and the modulation reportedly reversed sign (to an increase) when the tip was retracted out of tunneling contact. The microscopic origin of this modulation was never established. The open problem is to quantitatively decompose the measured beam-synchronous tip-current modulation into its physical channels (local tunneling-current modification, nonlocal detection of emitted Auger and secondary electrons, and carrier production and redistribution) and to build predictive models that link the picoampere-scale signals to specific core-excitation and relaxation processes. Later work instrumented channel separation and demonstrated an atomically localized X-ray-excited tunneling current, but no quantitative microscopic theory explaining the sign, magnitude, and parameter dependence of the modulation exists, so the problem remains open.

Background

Scanning tunneling microscopy (STM) maps surfaces by measuring the tunneling current that flows between a sharp metal tip and a conductive sample across a sub-nanometer gap; the current depends exponentially on the tip-sample distance and on the local density of states (LDOS), the energy distribution of electronic states at the surface. Ordinary STM therefore images structure and electronic states, but it cannot by itself identify which chemical element is under the tip. Synchrotron radiation (SR) — the highly brilliant, energy-tunable X-ray beam produced by electron storage rings — offers element specificity through X-ray absorption: when the photon energy crosses an element's absorption edge (for a K edge, the threshold for exciting a 1s core electron), the photon absorption probability of that element rises sharply. Exciting a core electron leaves a core hole that decays within femtoseconds, mainly by emitting Auger electrons (an electron filling the hole while another is emitted) and a cascade of lower-energy secondary electrons.

SR-STM/SX-STM combines the two: an STM junction is illuminated by a chopped (mechanically modulated) X-ray microbeam, and a lock-in amplifier — an instrument that extracts only the signal component synchronized with the modulation frequency — isolates the beam-induced change of the tip current. Because the bare tip also collects electrons emitted by the illuminated sample area far from the junction (a nonlocal background of up to hundreds of picoamperes), the Osaka group developed insulator-coated tips that expose only the apex, together with a 2.5 kHz chopper, lock-in detection, and real-time beam monitoring. With this system they first imaged Ge nanoislands on Si(111) through the Ge K edge, and then obtained element-specific contrast for Cu nanoislands on a Ge(111) 2×8 surface: above the Cu K edge the lock-in tip-current image showed the Cu island darker than the Ge substrate by a picoampere-scale amount (Saito et al., 2008; tip-current changes of about 10 pA are cited for the same system in Yan et al., 2019), while below the edge no contrast appeared. A parallel line at ISSP Tokyo and Niigata University demonstrated similar element-specific imaging and interpreted the signal as a photon-induced tip current carried by secondary electrons generated by core excitation, and the Advanced Photon Source group developed SX-STM with nanofabricated coaxial 'smart tips', dual tip/sample current detection, and phase-sensitive chopping into an active user technique (the XTIP beamline), culminating in X-ray absorption detection from a single atom.

The physical picture that the community now uses is that the measured tip current is a superposition of channels: the conventional tunneling current ItunnelingI_{tunneling}, an X-ray-excited local tunneling current Itunneling,X−rayI_{tunneling,X-ray} produced at the junction, and a nonlocal photoelectron current Iphoto−ejected,X−rayI_{photo-ejected,X-ray} from Auger and secondary electrons leaving the illuminated surface and reaching exposed tip surfaces. What is missing is the reverse direction: a quantitative microscopic account of why and how resonant core excitation changes the current — via the transient core-hole potential and its decay cascade, local modification of the density of states, or modulation of the tunneling barrier — that predicts the observed signs, magnitudes, and their dependence on bias, tip height, and photon energy.

Problem Statement

Establish a quantitative, predictive physical model of the element-specific tunneling-current modulation in synchrotron-radiation scanning tunneling microscopy. Concretely: (1) decompose the lock-in-extracted, beam-synchronous tip-current modulation into its constituent physical channels — the local modification of the tunneling current at the junction, the nonlocal detection of Auger and secondary electrons emitted by the illuminated surface, and photo-induced carrier production and redistribution — with quantified relative contributions at the declared conditions of the canonical Cu/Ge(111) measurement: Cu nanoislands on a Ge(111) 2×8 surface under positive sample bias Vs=+2V_s=+2 V, constant-current operation with a setpoint of It=1.0I_t=1.0 nA, and chopped (2.5 kHz) X-ray illumination at 8.990 keV above and 8.970 keV below the Cu K edge; (2) identify the dominant microscopic mechanisms by which resonant core excitation changes the tunneling current; transient core-hole dynamics, Auger and secondary-electron processes, local density-of-states changes, and tunnel-barrier modulation are candidate examples (a suggested list, not a mandatory one), and mechanisms outside this list are not excluded; and (3) validate the model against the measured picoampere-scale signals at those declared conditions, treating the following as independently standing targets: (3a) the sign and order of magnitude of the element-specific reduction of the tip current on the Cu nanoislands above the Cu K edge — an averaged reduction of order 10–30 pA on the island relative to the Ge substrate, with no contrast below the edge; (3b) the reported reversal of the modulation to an increase when the tip is retracted out of tunneling contact (approximately 400 nm); and (3c) the photon-energy dependence, with the bias and tip-height dependence required only where the canonical observations constrain it. Channel weights may differ between the tunneling and retracted configurations, but they must follow from a single model whose microscopic parameters are not re-fitted per configuration; fitting each configuration's weights independently does not by itself satisfy deliverable (1). These three deliverables serve one objective — turning SR-STM elemental contrast from a phenomenological fingerprint into quantitatively understood physics — and that understanding is the prerequisite for making SR-STM elemental mapping quantitative and reproducible across elements, absorption edges, samples, and beamline conditions.

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