Single-electron electrostatic force microscopy (e-EFM) has emerged as a powerful technique to probe single-electron tunneling events in nanoscale systems with discrete charge states. In this AFM-based method, an oscillating conductive cantilever acts as both a local gate and charge sensor, enabling the detection of tunneling events in quantum dots, nanoparticles, and molecules. These events appear as concentric “charging rings” in frequency shift and dissipation images, from which quantum properties such as Coulomb blockade energies, tunneling rates, and reorganization energies can be extracted via bias spectroscopy. However, unambiguously linking these charging rings to specific particles, particularly gold nanoparticles (AuNPs) as small as 1.6 nm, remains challenging, since the particles may not be clearly visible in the topography or may appear indistinguishable from other surface features. In this work, we present a methodology to extract charging ring radii directly from e-EFM dissipation images and establish a quantitative correlation with AuNP diameters. By comparing the distribution of ring radii to the nanoparticle size distribution obtained from transmission electron microscopy (TEM), which confirms a narrow range centered around 1.6 nm, we validate this approach for identifying particles of the appropriate size. In addition, this approach also provides insight into the surface density of 1.6 nm AuNPs, which will inform future estimates of enzyme coverage following nanoparticle functionalization. This is particularly important for upcoming experiments where 1.6 nm AuNPs will be attached to the metal center of metalloenzymes, requiring a precise size match with the enzyme’s active site. By validating this ring-to-radius relationship, we build a foundation for targeted single-particle measurements in our future metalloenzyme-AuNP systems.
Nanoscale defects such as Nitrogen Vacancy (NV) centers can serve as sensitive and non-invasive probes of electromagnetic fields and fluctuations from materials, which in turn can be used to characterize these systems. Here, we specifically discuss how NV centers can probe time-reversal symmetry breaking (TRSB) phenomena in low-dimensional electronic systems. We argue that the difference in relaxation rates $\Gamma_{\pm \hat{z}}$ of NV centers starting from $m = \pm 1$ spin states to the ground state with $m = 0$ directly probes TRSB. The effect arises from the difference in the fluctuation spectrum of left and right-polarized electromagnetic fields emanating from such materials. In the quantum Hall setting, the NV center experiences (nearly zero) large additional contribution to its relaxation due to the presence of the material when its magnetic dipole (anti-) aligns with the external field. More generally, the difference in the relaxation rates is sensitive to the imaginary part of the wave-vector-dependent Hall conductivity. We argue that this can be used to determine the Hall viscosity, which can potentially distinguish candidate fractional quantum Hall states and be used to infer the pairing angular momentum in TRSB superconductors. We compute the average relaxation rate $\left[\Gamma_{+\hat{z}} + \Gamma_{-\hat{z}}\right]$ near thin film superconductors and find that it exhibits a Hebel-Slichter like enhancement below $T_c$. The difference $\Gamma_{+\hat{z}} - \Gamma_{-\hat{z}}$ also inherits this peak but is only non-zero for $T < T_c$ and only if the superconductivity is chiral. We provide concrete estimates for observing this effect in stacked twisted Bismuth strontium calcium copper oxide.
Observation of Localized Edge States in a Mechanical Topological Insulator
Extreme alkali doping in graphene is a promising path toward correlated electron behaviour such as superconductivity, with largely unexplored flat-band physics near the van Hove singularity. That said, understanding alkali intercalation, diffusion, and desorption in graphene remains an open area of study. We present the development of an operando Raman and hyperspectral imaging platform to probe alkali-doped graphene. Alkali (K+, Rb+, Cs+) doping will be achieved with getter deposition sources. The system guides an external laser into an ultra-high vacuum chamber, with a secondary imaging path which enables precise laser alignment onto the sample. Mechanical stabilization reduced sample vibrations from approximately 30 µm to 1 µm. The system will measure Raman spectra at 355 nm and 532 nm excitation, and 400-1000nm hyperspectral reflectance to assess doping uniformity and diffusion dynamics. The system will also be fitted with a cryostat and a heating stage to enable temperature-dependent studies, alongside 4-contact resistance measurements to correlate optical signatures with transport. Future work includes implementing low-energy electron diffraction and angle-resolved photoemission spectroscopy to probe Kekulé order and band structure. More broadly, the system will be useful in the study of a wide variety of materials at extreme charge density, including fullerenes, fullertubes, and 2D materials in general.
Precipitates that nucleate at crystalline defects govern the strength of additively manufactured (AM) metals, yet AM's rapid, cyclic thermal histories produce defect-precipitate structures that traditional coarsening theories cannot capture. We adapt a Structural Phase-Field-Crystal (XPFC) alloy model, implemented in the high-performance OpenPFC framework, to resolve at atomic scale the interplay between dislocations and second-phase (beta) precipitate nucleation in a two-component eutectic alloy. To our knowledge, this is the first atomistic-scale study to model precipitation under an experimentally determined thermal history. We find that precipitates nucleate preferentially at grain boundaries decorated by dislocations.
Thiourea ($SC(NH_2)_2$) stands as a classic example of molecular ferroelectricity,
exhibiting a complex phase diagram driven by temperature-dependent lattice distortions.
Upon cooling, the material leaves the paralectric phase and passes through a sequence of
three distinct incommensurate phases characterized by molecular canting, before settling into
a ferroelectric ground state [1]. A signature of this structural change is an infrared-active
phonon mode that exhibits softening during cooling, followed by anomalous hardening
subsequent to the ferroelectric phase transition. While linear spectroscopy can track these
frequency shifts, it lacks the capacity to resolve the anharmonic interactions and microscopic
couplings that fundamentally drive these transitions.
In this work, we employ two-dimensional terahertz (2D THz) coherent spectroscopy to
directly probe the nonlinear lattice dynamics of thiourea across the temperature range where
the structural phase transitions occur. While 2D THz methods have been widely applied to
magnon and free-carrier dynamics [2,3], their application to structural phase transitions and
phonon nonlinearities have been limited [4,5]. By transmitting a pair of intense time-delayed
THz pulses through the sample, we generate 2D frequency-frequency correlation spectra that
isolate the nonlinear vibrational response from the linear background. This multi-dimensional
approach allows us to map the evolution of the soft mode’s anharmonicity and its
homogeneous dephasing. We present temperature-dependent spectra that elucidate how the
vibrational nonlinearities and inter-mode couplings influence the lattice instability. Our
findings provide a microscopic view of the energetic landscape governing the
incommensurate-to-ferroelectric transformation, demonstrating the utility of 2D THz
spectroscopy in characterizing the order parameters of soft molecular quantum materials.
Existing nanofluidic systems can perform parallel manipulation of single biopolymers via geometric confinement in nanoscale structures such as nanoslits, nanochannels and nanocavities. Such technology is powerful but the need to introduce molecules intact from bulk solution into nanoconfined environments introduces certain challenges and limitations (such as fragmentation of large molecules). Here we present a device that can perform molecular confinement electrically and allows direct loading of dsDNA from bulk solution. In our approach, we locally sculpt an electric field applied between two parallel electrodes via coating the lower electrode with a dielectric layer that contains arrays of etched holes. The field lines are concentrated at the position of the etched holes, leading to a locally enhanced field at the holes that acts electrokinetically to capture dsDNA; the holes thus act as attractive potential wells for the DNA. We find periodic driving of the device using signals with frequencies in the 1kHz to 1MHz range leads to long-range capture and reversible confinement of the molecules in the field wells while avoiding electrochemical degradation of the device and analytes. We find that the degree of confinement is frequency-dependent, allowing fine sub nanometer control of the molecules; different tunable confinement regimes can be described by the dynamics of resetting. Trapping of larger dsDNA leads to multi-well states where a molecule spans multiple wells with a resulting frequency dependent well occupancy.
In this study, we fabricate twisted bilayer graphene with different carbon isotopes (¹²C and ¹³C) using chemical vapor deposition (CVD). The use of isotopically distinct layers enables clear identification via Raman. Upon annealing the stacked graphene from 21°C to 250°C, we observe an irreversible change in interlayer orientation. An increase in the relative intensity of the middle 2D′ peak (3190cm⁻¹) in the same sampled region suggests enhanced interlayer coupling and a reorientation toward Bernal (AB) stacking. These results demonstrate how thermal treatment can be used to tune the stacking configuration in bilayer graphene