Scanning Probe Microscopy

Interfacial exchange coupling plays a critical role in enabling novel phenomena in magnetic heterostructures, such as spin triplet superconductivity, quantum anomalous Hall effect (QAHE), and advanced spintronic functionalities. While microscopic characterization of this coupling is essential for elucidating the underlying mechanism, it remains technically challenging. Here, using spin-polarized scanning tunneling microscopy (SP-STM) and quasiparticle interference, we directly observed interfacial exchange coupling in a magnetic tunnel junction formed by an Fe coated tip and a Cr(001) surface. We found the ferromagnetic tip induces significant energy shift (up to 10 meV) in the spin-polarized surface state of Cr(001). This shift is highly sensitive to the tip-surface distance and the spin-alignment between Fe tip and Cr surface, which can be switched by external magnetic field. Our results demonstrate that extended 2D surface states can mediate strong exchange coupling across a heterojunction, enabling local control of interfacial exchange interaction induced phenomena.

The quantum geometric phase intrinsically dictates the geometry, topology, and many-body correlations of electronic wave functions. While quantum geometric phases are conventionally inferred through momentum-space probes or macroscopic transport measurements, their direct visualization and quantification in real space have historically been restricted by the spatial averaging of bulk techniques. Scanning tunneling microscopy and spectroscopy (STM/STS) circumvent this limitation, leveraging atomic-scale spatial resolution and high energy sensitivity to resolve local electronic phase profiles directly. This review highlights recent progress across four representative methodologies: probing the Aharonov-Bohm (AB) geometric phase via nanoscale real space interferometry; extracting the Berry phase from defect-induced quasiparticle interference and wavefront dislocations; reconstructing the complex phase structure in symmetric systems, such as magic-angle graphene, using order parameter decomposition; and mapping the phase textures and topological defects of pair density wave (PDW) and charge density wave (CDW) in unconventional superconductors utilizing the numerical 2D lock-in technique. Together, these developments show how quantum phases can be translated onto real space and locally resolvable observables. Phase-resolved STM imaging provides stringent constraints on topological states of matter, symmetry-breaking patterns, and strong electronic correlations, outlining a robust framework for in situ phase engineering in quantum materials.

Altermagnetism is an emerging class of magnetic order characterized by momentum-dependent spin-split electronic structures despite vanishing net magnetization. Although momentum-space signatures consistent with altermagnetism have been reported in a growing number of materials, their relationship to the underlying real-space magnetic configurations remains incompletely understood, because similar spin-split electronic structures can arise from distinct magnetic orders. In the candidate d-wave altermagnet KV2Se2O, the magnetic origin of the observed momentum-dependent spin splitting has remained controversial. Here, we employ spin-polarized scanning tunnelling microscopy combined with magnetic-field-dependent quasiparticle interference imaging to determine the magnetic configuration of KV2Se2O at the atomic scale. Spin-resolved quasiparticle interference reveals a checkerboard-like antiparallel spin texture within the V2O layer and determines its interlayer spin arrangement across unit-cell step edges. Remarkably, we identify both C-type and G-type magnetic configurations, both of which generate similar spin-split electronic structures at the single-layer level but correspond to d-wave altermagnetic and conventional antiferromagnetic orders, respectively. These observations reveal a complex magnetic landscape arising from nearly degenerate magnetic states. Our results establish a direct connection between momentum-space spin splitting and real-space magnetic order, providing a framework for identifying the microscopic origin of spin-split electronic structures in altermagnetic materials.

Pair density wave (PDW) exhibits periodic amplitude and sign modulations of the superconducting order parameter. Such a pairing state has long been proposed to be highly sensitive to nonmagnetic scattering, but its experimental realization remains elusive. Here we discover a nonmagnetic PDW-breaking effect in a kagome superconductor, using designer atomic nonmagnetic impurities and high-precision scanning tunneling microscopy (STM) at a base temperature of 30mK. We detect 2x2 pair density modulations by Josephson STM with a superconducting tip and 2x2 pairing gap modulations by normal STM. We find that the pairing modulations in both cases are substantially suppressed upon doping the kagome lattice with dilute isovalent nonmagnetic impurities, whereas the charge order and uniform superconductivity remain robust. We further identify the correlation between atomic dopants and the local suppression of PDW. We attribute these findings to a nonmagnetic pair-breaking effect, arising from the phase modulation of PDW in the kagome d-orbital. Taken together with its signatures in other state-of-the-art spectroscopy and transport measurements linked by theory, our findings support the ground state of the kagome superconductor as a correlated topological phase with superconducting loop currents.

Characterizing electronic band structures requires precise knowledge of wave functions and their quantum geometry. Here, we introduce a tomography method to reconstruct the density matrix of electron states from quasiparticle interference maps around single impurities. We consider two-orbital models on a honeycomb lattice, relevant to graphene heterostructures and direct-gap semiconductors. For on-site impurities, backscattering between time-reversed states directly maps the density matrix populations and coherences into distinct orbital contributions in the interference map. While local probes usually lack orbital resolution, these orbital contributions transform under distinct symmetry group representations and can thus be disentangled to reveal the density matrix and quantum geometric tensor of the scattering states. This establishes impurities as tomographic probes for band structures in scanning tunneling microscopy using conventional, unpolarized tips.

Using mean-field theory, we determine the electronic phase diagram of undoped AB-stacked bilayer graphene in the presence of a transverse electric field. In addition to multiple competing electronic instabilities characterized by excitonic order parameters, our framework incorporates the long-range Coulomb energy associated with interlayer polarization. This long-range interaction plays a crucial role, as it significantly influences both the structure and the relative energies of the competing ordered states. We derive a set of self-consistency equations and solve them both numerically and analytically. Our findings reveal that, as the bias field is varied, the bilayer undergoes a cascade of first-order transitions between several ordered insulating phases for which order-parameter structures are explicitly identified. Some of these phases are characterized by two inequivalent single-particle gaps, whose magnitudes depend on the valley and spin quantum numbers. Field-driven transitions are accompanied by discontinuous and non-monotonic variations of the single-electron gap. We relate our results to Hartree-Fock numerical calculations and to experimental research, including observations of fractional metallic phases that emerge upon doping the bilayer system.

The nature of superconductivity in 2H-NbSe2 has generated sustained debate in the recent past. While angle resolved photoemission spectroscopy data have been interpreted as evidence for multiband superconductivity, the data from scanning tunneling microscope experiments relate to strongly anisotropic single-band superconductivity. In the later case, the charge density wave (CDW) order mimics the multigap character. Because the CDW reconstructs the Fermi surface and modifies the superconducting gap distribution, disentangling intrinsic multiband pairing from CDW-related effects is challenging. To address this issue, we investigate single-crystalline 2H-NbSeS, a mixed-chalcogen analogue of 2H-NbSe2 in which random Se/S substitution suppresses long-range CDW order while preserving the layered crystal structure P63/mmc. The material becomes superconducting below 6.0 K with moderate magnetic anisotropy. The upper critical field exhibits a pronounced upward curvature that cannot be described within a single-band framework but is well captured by a dirty-limit two-band model with a large diffusivity ratio. This indicates strong band-dependent scattering. The in-plane upper critical field exceeds the weak-coupling Pauli limit. Measurements of the lower critical field, superfluid density, and electronic specific heat are consistent with an interpretation of a fully gapped superconducting state with two nodeless gaps of different magnitudes.

Two-dimensional (2D) materials often require specific substrate terminations for epitaxial stabilization, yet the search for suitable templates has largely focused on low-index metal surfaces, which may not provide the optimal conditions for the growth of new phases. Here, we show that crystal-facet engineering on curved Cu surfaces enables the stabilization, within a single preparation step, of two distinct 2D phosphorus phases with different electronic properties. Hexagonal blue phosphorene forms on Cu(111) terraces, whereas a previously unreported skewed-square phosphorus phase is stabilized on Cu(513) facets. By combining complementary microscopy and spectroscopy techniques with theoretical calculations, we determine the structural and electronic properties of this new phase, which displays semiconducting character, in contrast to the metallic behavior of blue phosphorene. The coexistence of these two competing phases gives rise to a metal-to-semiconducting transition of the 2D phosphorus layer over the substrate. Locally, the competition between the two phases gives rise to self-aligned nanoarrays of alternating metallic and semiconducting phosphorus terraces. These results establish crystal-facet engineering as a practical route for discovering and stabilizing emergent 2D material phases on high-index substrates, while also enabling the engineering of nanostructures with tailored electronic properties through a simple and scalable growth process.

The parent compound FeTe hosts a complex magnetic landscape that is highly susceptible to lattice distortions. Although theoretical models have predicted a bicollinear to dimer antiferromagnetic (AFM) phase transition under tensile strain, its experimental realization and deterministic control has remained elusive owing to severe magnetic frustration. Here, combining high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we demonstrate the selective stabilization of bicollinear and dimer AFM orders in few-layer FeTe films via local uniaxial strain engineering. By mapping the strain fields near dislocation areas in FeTe films and FeTe/FeSe heterostructures, we establish a direct correspondence between specific strain components and the resulting magnetic ground states. We find that uniaxial compression along the Fe-Fe next-nearest-neighbor direction stabilizes the bicollinear AFM order, with the stripe orientation aligning parallel to the compression axis. Crucially, we report the experimental realization of the long-range dimer AFM order, which emerges under anisotropic strain along the Fe-Fe nearest-neighbor direction. This phase manifests as a distinct $\sqrt{2} \times \sqrt{2}$ electronic reconstruction and shares a common Neel temperature with the bicollinear phase. Our findings reveal that anisotropic strain effectively lifts the magnetic degeneracy among competing states. This work provides a robust strategy for the manipulation of elusive magnetic orders and offers insights into the interplay between lattice, spin, and electronic degrees of freedom in iron-based superconductors.

Strongly correlated two-dimensional systems provide compelling platforms for investigating exotic quantum phenomena. Niobium chloride (Nb$_3$Cl$_8$), a single-band Mott insulator, exhibits a remarkable out-of-plane polarization in its topmost layer that oscillates with layer parity, manifesting as an odd-even effect. Using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), this layer-parity-dependent polarization can be effectively characterized through surface morphology and potential mapping, enabling the unambiguous identification of different surface phases. We then fabricated dual-gate Hall devices by coupling different surface phases of Nb$_3$Cl$_8$ with monolayer graphene to investigate how the topmost-layer out-of-plane polarization influences interfacial coupling and the resulting transport behavior. Our results reveal significant phase-dependent variations in charge transfer, carrier densities, and hybridization gaps (25.2 meV for Phase 1 and 30.0 meV for Phase 2). Density functional theory calculations corroborate these experimental findings, showing that distinct out-of-plane polarizations in the topmost layer lead to different orbital overlaps and interfacial coupling strengths. These findings highlight the critical importance of surface polarization and orbital orientation in engineering the properties of strongly correlated van der Waals heterostructures.

Tip-induced switching in ferroelectrics is commonly framed as a field-driven process controlled by domain-wall kinetics. Here we argue for a more universal viewpoint: under a scanning probe, polarization reversal is fundamentally constrained by how fast screening charge can be supplied and redistributed. By combining local switching experiments with finite-element simulations, we identify a screening-current-limited mechanism in which charge injection and its space-charge-limited relaxation set the pace of switching and the scaling of domain growth. This framework naturally explains regime changes and the apparent breakdown of intrinsic domain-wall laws often inferred in the nanoscale piezoelectric hysteresis measurements. Beyond a reinterpretation of tip switching, the results position screening currents as a hidden control parameter of ferroelectric reversal across materials, turning current regulation into a practical handle for deterministic nanoscale domain engineering.

We present a compact millikelvin scanning tunneling microscope (STM) operating at 270mK with topographic navigation to micron-scale targets. Two piezoelectric low-temperature nanopositioners extend the accessible sample area, while a multi-stage copper powder and capillary filter scheme preserves millikelvin energy resolution, verified by BCS spectroscopy on aluminium thin films. A lithographically fabricated binary-encoded gold pattern encodes unique 16-bit coordinates in 4x4 pixels of 200nm$\times$200nm each. We demonstrate absolute positioning across a 350$\times$350$\mathrm{\mu}$m$^2$ area from a single STM scan. Requiring only a single lithography step and no hardware modifications to existing STM setups, the navigation system provides a versatile platform for scanning tunneling spectroscopy of nanoflakes and nanoscale devices.

Atomic-resolution imaging on molten metal/solid interfaces at temperatures above 200 {\deg}C was achieved using a high-temperature, high-speed atomic force microscope (AFM) equipped with a qPlus sensor. A tip-scanning high-speed Quadpod scanner for a large mass load of qPlus sensor (2.3 g) was developed to enhance thermal drift tolerance by high-speed scanning and thermal insulation from the heated specimen. This scanner has dominant resonant frequencies of 7.05 kHz (lateral) / 29.7 kHz (vertical) without a load. In addition, the Hybrid-loop frequency demodulation technique for low-resonant-frequency ($f_0$) sensors with a wider bandwidth than conventional phase-locked loop was also established, providing a demodulation bandwidth of $B_{\Delta f_\mathrm{inst}}\sim 0.26 f_0$ without exceeding the theoretical noise of the input deflection signal. Combining these techniques enabled atomic-resolution imaging on the molten $\mathrm{Ga/PtGa_x}$ interface at $\sim$210 {\deg}C. The topographic images obtained at $\sim$210 {\deg}C showed a relatively low-symmetry surface with an oblique lattice with a superstructure, which differed from the primitive rectangular lattice observed in the non-heated sample left at room temperature for 96 h. This demonstrates that the developed high-temperature, high-speed AFM techniques for qPlus sensors enable visualization of non-aqueous liquid/solid interfaces above 200 {\deg}C at atomic resolution, which has various potential applications, such as injection modeling, soldering, and the fabrication of liquid-metal-based catalysts.

Hybrid systems composed of graphene (Gr) and carbon nanotubes (CNTs), such as films and aerogels, have attracted broad attention for applications in electronics, mechanics, energy, and environmental science. Since the microstructures of Gr-CNT hybrids strongly affect their properties, it is essential to establish mechanical principles that govern these structures. In this study, we investigated the structural stability and mechanical behavior of Gr-CNT hybrid systems by combining molecular dynamics (MD) simulations and nanoindentation experiments. MD simulations of stacked Gr-CNT structures, in which two Gr layers confine CNTs between them, identified the energetically stable configurations and their governing parameters, i.e., intertube spacing, CNT diameter, and wall number. Specifically, under certain conditions, the structures exhibit mechanical bistability with two stable configurations: adhesion and separation of the Gr layers, arising from the competition between interlayer van der Waals attraction and elastic deformation of Gr and CNTs. Simulated loading--unloading curves display hysteresis and energy dissipation related to the stable configurations. In addition, reduced graphene oxide (rGO)-CNT hybrid films were experimentally fabricated as macroscopic assemblies of the unit structures modeled in the simulations. Atomic force microscopy-based nanoindentation measurements on the rGO-CNT films exhibit clear hysteresis and higher dissipation energy compared with pure rGO, in good agreement with the simulation results. These results provide valuable insights into Gr-CNT hybrid systems and offer guidance for designing microstructures with enhanced properties for advanced applications.

Strongly correlated quantum states, such as charge density waves (CDWs), are exquisitely sensitive to Fermi surface topology and lattice symmetry, and are typically quenched by heavy carrier doping. In two-dimensional (2D) systems, however, macroscopic geometric curvature emerges as a novel structural degree of freedom to modulate microscopic quantum coherence. This raises a compelling physical question: can non-Euclidean geometric deformations compete with extreme electronic perturbations to reshape, or even revive, a quenched macroscopic quantum order? Here, by constructing monolayer TiSe$_2$-NbSe$_2$ heterostructure on a BLG/SiC substrate for the first time, we report the curvature-driven revival of a frustrated charge order in a non-Euclidean space. Low-temperature angle-resolved photoemission spectroscopy (ARPES) reveals a massive interfacial charge transfer, which destroys the global Fermi surface nesting and completely suppresses the long-range CDW order in Euclidean flat regions. Strikingly, high-resolution scanning tunneling microscopy (STM) reveals that a novel, non-linear CDW state miraculously survives, remaining strictly localized within morphologically distorted, non-Euclidean nanoscale curved regions. Atomistic simulations unravel the structural origin of this phenomenon, demonstrating that interfacial twist and lattice mismatch spontaneously generate a corrugated superlattice.

In conventional superconductors, Cooper pairs form in an even-parity spin-singlet state. Noncentrosymmetric superconductors, which lack inversion symmetry, exhibit antisymmetric spin-orbit coupling (ASOC) that can combine even-parity spin-singlet and odd-parity spin-triplet pairs into a mixed-parity order parameter. Spin-triplet components are highly beneficial for superspintronic devices. Whether ASOC alone $-$ without strong electronic correlations $-$ is sufficient to generate a measurable triplet component remains a central open question. Here, we resolve this question in Nb$_{18}$Re$_{82}$ (Nb-Re), a weakly-correlated noncentrosymmetric metal whose superconducting pairing symmetry has been actively debated. Using low-temperature scanning tunneling spectroscopy on single crystals with four distinct crystallographic orientations, find a pronounced orientation-dependent anisotropy in the local density of states. Supported by a symmetry-constrained model, we show that the complete set of tunneling spectra requires a mixed-parity order parameter with the triplet amplitude reaching up to half of the singlet component. These results reconcile the conflicting reports in the literature on Nb-Re and demonstrate that ASOC is sufficient to foster a sizable spin-triplet component even without strong electronic correlations, suggesting that mixed-parity superconducting states may be more widespread than previously assumed. Since Nb-Re can be readily fabricated in thin-film form, these findings position it as an accessible platform for superspintronic devices and establish orientation-resolved tunneling spectroscopy as a general protocol for the detection of mixed-parity order parameters.

The realization of the axion insulator phase in magnetic topological insulators is often hindered by crystalline symmetries that protect gapless surface states, even when time-reversal symmetry is broken. Here, we use variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), complemented with density functional theory (DFT), to investigate the local electronic structure of the antiferromagnetic (AFM) topological insulator EuSn$_2$As$_2$ across its N\'eel transition at $T_N = 24$ K. On the (001) surface, we observe a substantial density of intrinsic Sn vacancies that introduce nanoscale electronic inhomogeneity and p-type doping. Upon cooling below $T_N$, we resolve the emergence of two distinct magnetically driven gaps: a $\sim$100 meV gap near the Fermi level and a $\sim$50 meV gap at the ARPES-resolved Dirac point. We attribute the former gap to AFM Brillouin-zone folding and hybridization. The characteristics of the 50 meV gap point toward the lifting of mirror-symmetry protection by Sn vacancies and the consequent mass gapping of the Dirac point, although contributions from AFM-induced folding hybridization cannot be entirely ruled out. Our findings provide real-space evidence for strong coupling between localized moments and itinerant topological states, highlighting exfoliable EuSn$_2$As$_2$ as a potential candidate for realizing axion-insulator-based devices.

We investigate electronic instabilities in 2H-TaS\textsubscript{2} and a self-intercalated variant, 2H$^\dagger$-Ta\textsubscript{1+$\delta$}S\textsubscript{2}. In conventional samples, which we determine to be slightly hole-doped, spectral gaps and backfolded features are found as fingerprints of the $3\times3$ charge density wave (CDW). Notably, the backfolded features emerge only at a temperatures below $T\approx$~65~K, substantially lower than the established CDW temperature of 78~K, suggesting an incommensurate-commensurate lock-in transition analogous to the phenomenology of the 2H-TaSe\textsubscript{2}. In contrast, the self-intercalated 2H$^\dagger$ sample exhibits substantial electron doping and signatures of a novel \tworootthree CDW. Using \textit{ab initio} calculations of the phonon spectrum, we demonstrate that the \threebythree instability ($\mathbf{q}=\sfrac{2}{3}\mathbf{\Gamma M}$) is highly sensitive to band filling. Furthermore, with increased interlayer spacing, a competing soft phonon mode emerges near $\mathbf{q}=\sfrac{1}{2}\mathbf{\Gamma K}$, corresponding to the superstructure observed in the 2H$^\dagger$ phase, although in our calculations this instability arises under hole doping rather than the electron doping inferred experimentally. These results establish band filling and interlayer spacing as key control parameters for CDW ordering vectors in 2H-TaS\textsubscript{2}, and highlight a route to engineering electronic instabilities in a prototypical layered material.

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.