arXiv subjects

Alexander P. Fellows

Publications and source records attributed to Alexander P. Fellows.

8 recordsLinked to original sources

arXiv2026

Visualizing the Hidden Architecture of Molecular Films with Phase-Resolved Rotational SFG Microscopy

The highly variable physico-chemical properties of thin molecular films play an essential role in numerous research fields ranging from biophysics to the fabrication of functional devices such as molecular sensors. The properties of molecular films are largely governed by their three-dimensional molecular structure which often exhibits important spatial heterogeneity, either naturally, or introduced deliberately. In order to understand and control these properties microscopic insight into structural parameters such as composition, molecular orientation and conformation, as well as molecular order is required, which, so far, represents a mostly unachieved experimental target. In this contribution we present a powerful experimental approach that can overcome this limitation. Using phase-resolved rotational sum-frequency generation (SFG) microscopy all of these structural parameters can be obtained with sub-monolayer sensitivity and at sub-micron resolution. In measurements of monolayer assemblies of mixed phospholipids, we uncover the molecular packing structure in previously-unattained detail and demonstrate the large potential of the technique for the elucidation of the complex architecture inside molecular films. The structural insight provided by this nonlinear microscopy approach spans all the way from the molecular to the macroscopic scale opening the door to a completely new type of interfacial studies.

physics.chem-ph↗

arXiv2026

Beyond the Electric Dipole Approximation: Electric and Magnetic Multipole Contributions Reveal Biaxial Water Structure from SFG Spectra at the Air-Water Interface

The interpretation of sum-frequency-generation (SFG) spectra has been severely limited by the absence of quantitative theoretical predictions of higher-order multipole contributions. Magnetic dipole and electric quadrupole contributions are determined by bulk properties but appear in all experimental SFG spectra, obscuring the connection between measured spectra and interfacial structure. We present the simulation-based framework to predict the full set of multipole spectral contributions. This framework also yields depth-resolved spectra, enabling the precise spatial localization of spectroscopic features. Applied to the air-water interface, our approach achieves quantitative agreement with experimental spectra for different polarization combinations in both the bending and stretching regions. Higher-order multipole contributions are crucial for correctly interpreting SFG spectra: in the bending band, the electric dipole and the magnetic dipole contributions have similar intensities, while the electric quadrupole contribution is significantly larger. In the OH-stretch region, the electric quadrupole contribution is found to be in large part responsible for the characteristic shoulder at 3600/cm. Crucially, subtracting the quadrupole and magnetic contributions isolates the second-order electric dipole susceptibility, which is a quantitative probe for interfacial molecular orientational anisotropy. This electric-dipole susceptibility reveals a pronounced biaxial ordering of water at the air-water interface. By resolving a fundamental limitation of the interpretation of SFG spectroscopy, our framework allows for the detailed extraction of interfacial water ordering from SFG spectra.

cond-mat.stat-mech↗

arXiv2025

Terahertz switching of antiferromagnetic order by Néel spin-orbit torques

Ultrafast electric manipulation of magnetic order in solids is critical for the development of future terahertz data processing. A fascinating concept for such high-speed operation is offered in metallic antiferromagnets by Néel spin-orbit torque. It should allow one to coherently rotate the ordered spins by simply applying an electric current of suitable amplitude and polarity. However, such switching has been severely hampered by competing heat-induced effects, and it has not yet been achieved on the intrinsically ultrafast time scales of antiferromagnets. Here, we report robust, direction-controlled and non-thermal rotation of the Néel vector $\mathbf{L}$ by $\pm$90° at room temperature in the antiferromagnet Mn$_2$Au driven by phase-locked terahertz current pulses. All observed features are consistent with ultrafast Néel spin-orbit torque: First, nonlinear optical imaging reveals that the terahertz current direction sets the final orientation of $\mathbf{L}$ in the absence of any bias field for at least two months. Second, transient optical birefringence shows that the switching proceeds ultrafast in less than 15 picoseconds. Finally, atomistic spin-dynamics simulations reproduce the observed dynamics and confirm the minor role of thermal effects. While the switching is already one order of magnitude faster than in ferromagnets at comparable dissipated energy, our simulations predict routes toward switching times and energies which are another order of magnitude lower. Our approach can be transferred to electric-field-driven switching in many more antiferromagnets, including magnetoelectric insulators. The engineering of spin torques, resonance frequencies and read-out mechanisms provides an exciting pathway toward on-chip applications of terahertz antiferromagnetic spin-orbitronics.

cond-mat.mes-hall↗

arXiv2025

Anisotropic Water Structure at Charged Interfaces Studied by Depth Resolved Vibrational SFG/DFG Spectroscopy

The molecular water structure at charged aqueous interfaces is shaped by interfacial electric fields, which can induce significant anisotropy in the molecular orientations extending over nanometer-scale distances. Despite great relevance, very little is known about the details of this depth-dependent anisotropic water structure, mainly due to the lack of appropriate experimental techniques. Here, we present a depth-resolved study of the water anisotropy at the interface to insoluble charged surfactants using a newly developed technique which allows for directly correlating nonlinear vibrational spectra with depth information on the nanometer scale. We demonstrate that the obtained data allows for a full reconstruction of the nonlinear vibrational responses as function of depth. The results for the case of low salinity solutions show the presence of two pronounced regions within the interfacial anisotropy with largely deviating degrees of preferential molecular orientations. A spectral analysis of the depth-dependent vibrational responses furthermore reveals that the natural local hydrogen-bond structure of bulk water remains largely unperturbed throughout the interfacial region, including water in direct proximity of the surface charges. These findings significantly refine our understanding of the anisotropic water structure at the interface to hydrophilic charged surfactants and showcase the large potential of our depth-resolved spectroscopic technique.

physics.chem-ph↗

arXiv2025

The Importance of Layer-Dependent Molecular Twisting for the Structural Anisotropy of Interfacial Water

The unique structural properties of interfacial water are at the heart of a vast range of important processes in electrochemistry, climate science, and biophysics. At interfaces, water molecules exhibit preferential orientations and an altered intermolecular H-bond connectivity. Characterising this layer-dependent anisotropic structure for such a thin molecular boundary, however, is a veritable challenge, with many important details remaining unknown. Here, we combine a novel depth-resolved second-order spectroscopy with molecular dynamics simulations to study the anisotropic structure at the air-water interface through the H-O-H bending vibration. We firstly show that the experimental nonlinear spectrum contains a large bulk like (quadrupolar) contribution that has hampered the assessment of the interfacial structure in previous investigations. By subtracting this contribution, we uncover the elusive anisotropic interfacial response that quantitatively matches the simulated prediction. Thereafter, by analysing both the vibrational line-shape of the interfacial spectrum and its depth-dependence, we demonstrate that both the molecular tilt and twist angles of water must be highly restricted at the interface, which is confirmed by the simulated orientational distribution. Finally, by analysing the depth and orientation dependence of the bending frequency, we show substantial deviations from the expected behaviour, revealing an anomalous character to the interfacial H-bond network.

physics.chem-ph↗

arXiv2025

Full Crystallographic Imaging of Hexagonal Boron Nitride Monolayers with Phonon-Enhanced Sum-Frequency Microscopy

Hexagonal boron nitride (hBN) is an important 2D material for van der Waals heterostructures, single photon emitters, and infrared nanophotonics. The optical characterization of mono- and few-layer samples of hBN however remains a challenge as the material is almost invisible optically. Here we introduce phase-resolved sum-frequency microscopy as a technique for imaging monolayers of hBN grown by chemical vapor deposition (CVD) and visualize their crystal orientation. A combination of femtosecond mid-infrared (IR) and visible laser pulses is used for sum-frequency generation (SFG), which is imaged in a wide-field optical microscope. The IR laser resonantly excites a phonon of hBN that leads to an ~800-fold enhancement of the SFG intensity, making it possible to image large 100x100 μm2 sample areas in less than 1 s. Implementing heterodyne detection in combination with azimuthal rotation of the sample further provides full crystallographic information. Through combined knowledge of topography and crystal orientation, we find that triangular domains of CVD-grown monolayer hBN have nitrogen-terminated zigzag edges. Overall, SFG microscopy can be used as an ultra-sensitive tool to image crystal structure, strain, stacking sequences, and twist angles, and is applicable to the wide range of van der Waals structures, where location and identification of monolayer regions and interfaces with broken inversion symmetry is of paramount importance.

cond-mat.mtrl-sci↗

arXiv2024

How Thick is the Air-Water Interface? -- A Direct Experimental Measurement of the Decay Length of the Interfacial Structural Anisotropy

The air-water interface is a highly prevalent phase boundary with a far-reaching impact on natural and industrial processes. Water molecules behave differently at the interface compared to the bulk, exhibiting anisotropic orientational distributions, reduced intermolecular connectivity in the hydrogen bond network, and significantly slower dynamics. Despite many decades of research, the thickness of the structural anisotropy in the interfacial layer remains controversial, with a direct experimental measurement being absent. In this study, we utilise an advancement in non-linear vibrational spectroscopy to gain access to this important parameter. Combining phase-resolved sum- and difference-frequency generation (SFG and DFG) responses, we directly measure the decay in structural anisotropy of the air-water interface. We find a decay length of ~6-8Å, in excellent agreement with depth-resolved SFG spectra calculated from ab initio parameterised molecular dynamics (MD) simulations. The result reveals surprisingly short anisotropic orientational correlations from the interfacial layer that are even shorter than in the bulk. Furthermore, the recorded SFG and DFG responses are decomposed into a vibrationally resonant and non-resonant contribution through isotopic exchange measurements. Through their separate analysis, we show that the resonant response is a sensitive probe of the structural anisotropy at the interface whereas the non-resonant contribution contains a significant isotropic contribution from the bulk and therefore only partially reports on the interfacial structure. This finding places stringent restrictions on the insight available through both purely non-resonant and second-order intensity studies.

physics.chem-ph↗

arXiv2023

Spiralling molecular structures and chiral selectivity in model membranes

Since the lipid raft model was developed at the end of the last century, it became clear that the specific molecular arrangements of phospholipid assemblies within a membrane have profound implications in a vast range of physiological functions. Studies of such condensed lipid islands in model systems using fluorescence and Brewster angle microscopies have shown a wide range of sizes and morphologies, with suggestions of substantial in-plane molecular anisotropy and mesoscopic structural chirality. Whilst these variations can significantly alter many membrane properties including its fluidity, permeability, and molecular recognition, the details of the in-plane molecular orientations underlying these traits remain largely unknown. Here, we use phase-resolved sum-frequency generation microscopy on model membranes of phospholipid monolayers with mixed molecular chirality, which form micron-scale circular domains of condensed lipids, to fully determine their three-dimensional molecular structure. We find that the domains possess curved molecular directionality with spiralling mesoscopic packing. By comparing different enantiomeric mixtures, both the molecular and spiral turning directions are shown to depend on the lipid chirality, but with a clear deviation from mirror symmetry in the formed structures. This demonstrates strong enantioselectivity in the domain growth process, which has potential connections to the evolution of homochirality in all living organisms as well as implications for enantioselective drug design.

cond-mat.soft↗