Scanning Probe Microscopy

Piezoresponse force microscopy (PFM) has established itself as a very successful and reliable imaging and spectroscopic tool for measuring a wide variety of nanoscale electromechanical functionalities. Quantitative imaging of nanoscale electromechanical phenomena requires high sensitivity while avoiding artifacts induced by large drive biases. Conventional PFM often relies on high voltages to overcome optical detection noise, leading to various non-ideal effects including electrostatic crosstalk, Joule heating, and tip-induced switching. To mitigate this situation, we introduce interferometrically detected, resonance-enhanced dual AC resonance tracking (iDART), which combines femtometer-scale displacement sensitivity of quadrature phase differential interferometry with contact resonance amplification. Through this combination, iDART achieves 10x or greater signal-to-noise improvement over current state of the art PFM approaches including both single frequency interferometric PFM or conventional, resonance enhanced PFM using optical beam detection. In this work, we demonstrate a >10x improvement of imaging sensitivity on PZT and Y-HfO. Switching spectroscopy shows similar improvements, where further demonstrates reliable hysteresis loops at small biases, mitigating nonlinearities and device failures that can occur at higher excitation amplitudes. These results position iDART as a powerful approach for probing conventional ferroelectrics with extremely high signal to noise down to weak piezoelectric systems, extending functional imaging capabilities to thin films, 2D ferroelectrics, beyond-CMOS technologies and bio-materials.

The engineering of superlattices in two-dimensional van der Waals materials has enabled the realization of rich phase diagrams hosting topological and strongly correlated phases. While incommensurability is widespread in three-dimensional systems, the role of moir\'e potentials in bulk materials remains largely unexplored. Here, using scanning tunneling microscopy, we demonstrate that a bulk transition-metal dichalcogenide polytype, 4Hb-TaS$_2$, hosts an emergent incommensurate potential between its alternating 1T and 1H layers. Interplay with a concomitant incommensurate charge-density wave suppresses the long-range order of this potential, leading to intricate coupling with electronic correlations in the doped 1T surface layer. Combining density functional theory with dynamical mean-field theory, we show that the lattice mismatch locally modulates the interlayer distance, thereby tuning both hybridization and charge transfer between the correlated 1T and metallic 1H layers. This redistribution of charge drives the system towards a doped Mott regime, in which the remaining local moments become self-screened, giving rise to a zero-bias resonance. We further find that bulk superconductivity competes with both the underlying landscape and the associated charge transfer. Our results establish incommensurate potentials as a previously overlooked ingredient in hybrid transition-metal dichalcogenides, highlighting their central role in the interplay between electronic correlations, charge-density-wave order, and unconventional superconductivity.

FeCl2 is a promising single-layer material with sizeable magnetic susceptibility and insulating character that can be easily grown by molecular beam epitaxy on various surfaces. In order to include it into the select palette of van der Waals materials used to engineer functional heterostructures, it is necessary to confirm its magnetic and electronic ground states, and understand the influence of the supporting substrate. In this work, we unambiguously demonstrate ferromagnetic ordering in a single-layer FeCl2 on Au(111) by means of spin-polarized scanning tunnelling microscopy. The material features a relatively wide insulating gap of 3.3 eV and a strongly spin-polarized conduction band that emerges at 1.5 eV above the Fermi level. Atomic scale defects with triangular shape play a primary role in the electronic gap and spin density distribution. Specifically, in a region of 1.6 nm around each defect, the conduction band is locally suppressed and the tunnelling magneto-conductance is reduced a factor of four. By tracking the spin-dependent tunnelling conductance as a function of the applied magnetic field, we record atomically resolved hysteresis loops, revealing a soft ferromagnetic ground state with pronounced out-of-plane anisotropy and coercive fields in the range of 15-50 mT.