Condensed Matter Physics Seminars

Spatial-Spectral Single-Molecule Microscopy

Multicolour and spectrally resolved single-molecule microscopy can reveal molecular interactions, nanoscale environments and dynamics, but usually depends on experimentally complex detection architectures based on beam splitting,[1] spectral dispersion[2] or engineered point spread functions.[3] Here we show that spatially patterned detectors offer a conceptually simpler route to spectral single-molecule imaging.[4] By replacing a conventional monochrome camera with a commercially available colour CMOS detector and fitting the raw detector response directly, we recover both molecular position and spectral fingerprint from a single image without optical splitting, channel registration or demosaicing.

We term this approach spatial-spectral single-molecule microscopy, or S3M. We show that S3M retains single-molecule sensitivity across the visible spectrum, enables robust spectral multiplexing, and supports applications spanning multicolour single-molecule tracking, single-molecule Förster resonance energy transfer, multicolour localisation microscopy and spectral PAINT. Although spatial patterning necessarily trades photon efficiency for spectral information, current low-noise detectors already provide sufficient performance for a broad range of experiments. Spatially patterned detection therefore establishes a widely accessible strategy for simplified spectral microscopy and single-molecule spectroscopy, and points towards a new class of detector-informed photonic measurement schemes for nanoscale imaging.

References

<sub>1. Kumar, A. et al. Multispectral live-cell imaging with uncompromised spatiotemporal resolution. Nat. Photon. 1–11 (2025) doi:10.1038/s41566-025-01745-7.
2. Zhang, Z., Kenny, S. J., Hauser, M., Li, W. & Xu, K. Ultrahigh-throughput single-molecule spectroscopy and spectrally resolved super-resolution microscopy. Nat. Methods 12, 935–938 (2015).
3. Shechtman, Y., Weiss, L. E., Backer, A. S., Lee, M. Y. & Moerner, W. E. Multicolour localization microscopy by point-spread-function engineering. Nat. Photon. 10, 590–594 (2016).
4. Beckwith, J. S. et al. Spatially patterned, spectral single-molecule microscopy. 2026.04.08.715690 Preprint at https://doi.org/10.64898/2026.04.08.715690 (2026).</sub>

Profiling biomolecular heterogeneity with label-free detection of individual solution-phase molecules

Understanding heterogeneous molecular systems requires measurements that go beyond ensemble averages to resolve the properties and dynamics of individual molecules. Yet achieving single-molecule sensitivity without fluorescent labels, surface immobilisation or other molecular modification remains a considerable experimental challenge. In this talk, I will briefly describe our development of cavity-enhanced single-molecule profiling¹, a label-free approach based on high-finesse fibre Fabry-Pérot optical microcavities. By enhancing the interaction between light and freely diffusing molecules, we are able to detect molecules as small as 1.2 kDa with signal-to-noise ratios exceeding 100, as well as distinguishing differences in molecular size and conformation in molecular mixtures without the need for external labels.

In the next phase of this work at Warwick, my group will develop this from a single-molecule detection technology into a platform for interrogating heterogeneous biomolecular systems, combining improved molecular profiling, microfluidic sample handling and complementary single-molecule fluorescence measurements. A major focus will be protein self-assembly and aggregation, where structurally and functionally distinct molecular species coexist within the same sample but are obscured by ensemble measurements.

Ultimately, the aim is to use label-free single-molecule measurements not simply to detect individual molecules, but to uncover the molecular heterogeneity and dynamic processes that govern complex biological systems.

References
^{1. Needham, L. M. et al. Label-free detection and profiling of individual solution-phase molecules.
Nature 629, 1062–1068 (2024).}

The cell plasma membrane and its underlying actin cytoskeleton are the central processing platform for any signalling going into or out of the cell. Growing evidence highlights that mechanical forces are driving signalling and that local membrane organisation is driven by actin cytoskeleton dynamics. Our focus lies to understand how the force generating machinery of the cell cortex, actin filaments and motor proteins, govern and regulate the mechanical properties of the cell membrane and dynamics of cell membrane components, and vice versa, how membrane organisation and signalling events feed-back to the regulation of the cortex machinery. Using reconstituted minimal systems of the membrane – cortex composite, we shed light on the physical mechanisms that govern the active cell membrane organisation. We use quantitative imaging approaches, controlled mechanical and biochemical manipulations and work with theoretical physicists to understand how cells might use the active cell cortex to locally and temporally alter membrane-protein organisation in the cell plasma membrane. In a second part, we discuss how simplified cell surface mimicking substrates offer insights into the cell cortex dynamics during E-cadherin mediated cell-cell adhesion.

Point-contact spectroscopy provides a unique window into the microscopic processes governing electron transport in quantum materials [1]. By injecting electrons through nanometre-sized ballistic contacts, it becomes possible to probe scattering mechanisms, electron-phonon interactions, and quantum interference effects on their natural length scales. They have also been used for magnetotransport characterisation [2-3]

In this colloquium, I will discuss our efforts to realise point-contact spectroscopy within a scanning tunnelling microscope and its application to graphite, a model system combining an unusual electronic structure with long electronic coherence lengths. I will show how transport through atomic-scale contacts provides information complementary to conventional tunnelling spectroscopy, revealing the influence of both the electronic structure and the scattering processes that govern charge transport. Measurements of conductance, current fluctuations, and magnetotransport demonstrate how local point contacts can be used to investigate electron-phonon interactions, quantum interference phenomena, and electron coherence in low-dimensional systems.

Finally, I will present our ongoing development of a radio-frequency scanning tunnelling microscope for point-contact experiments.

References

<sup>1. Yu. G. Naidyuk and I. K. Yanson, Point-Contact Spectroscopy (Springer, 2005/2019)
2. Saxegaard, Magne; Zheng, Yang de; Wahlström, Erik; Brucas, Rimantas; Hanson, Maj.(2010) Field and current-induced magnetization reversal studied through spatially resolved point-contacts.Link opens in a new windowJournal of Applied Physics
3. Monsen, Åsmund Fløystad; Boschker, Jos Emiel; Nordblad, Per; Mathieu, R.; Tybell, Per Thomas Martin; Wahlström, Erik. (2015) Point contact investigations of film and interface magnetoresistance of La_0.7Sr_0.3MnO₃ heterostructures on Nb:SrTiO₃ .Link opens in a new windowJournal of Magnetism and Magnetic Materials</sup>

Ultrasonic devices feature in all aspects of our lives, from healthcare to industry. Common examples include instruments for minimally invasive and precision surgeries, targeted drug delivery, and imaging of various parts of the body to detect illnesses or monitor health conditions. Industrial applications include underwater sensing and communications (sonar), flow measurement in the water and energy industries, and the nondestructive testing and evaluation of large infrastructure like railways and bridges, and inspection in the automotive and aerospace industries. Current ultrasonic devices for these applications are often limited by the reliance on conventional materials and manufacturing approaches. For example, the stack Langevin transducers used as the foundation of modern ultrasonic surgical devices are tuned to a single mode of vibration, meaning they are tailored to operating on specific tissue types. Innovative strategies for realising ultrasonic devices for operation and measurement at multiple frequencies, tuneable across wide bandwidths, and able to undertake different processes within a single device, are emerging. One approach is through incorporating tailored or engineered materials in these devices. For example, the adjustable elastic properties of shape memory alloys can be used to control resonance frequency, and acoustic metamaterials can be designed to focus ultrasonic wave propagation in specific directions to improve performance. This presentation will provide an overview of emerging strategies to optimise acoustic and ultrasonic devices to meet current industrial and medical demands. There will also be time given for Q&A and further discussion, to explore potential collaborations between Warwick and Glasgow.

I will begin by introducing the field of altermagnetism, summarising defining concepts and materials currently under study. I will then present our recent resonant X-ray diffraction and density functional theory study on CoF₂, which motivate a real-space multipolar interpretation of altermagnetism. In this light, I will summarise our theoretical results derived from the quantum theory of angular momentum that serve to link phenomenological multipolar models to localised electronic states.

Nearly half of the world’s CO₂ emissions can be attributed to heating and cooling [1]. This is primarily due to heating with natural gas and cooling with compression of greenhouse gases, which are neither environmentally friendly nor energy efficient. Therefore there is great interest in developing alternative technologies that can replace these gas-based environmentally damaging systems [2,3]. Barocaloric materials are at the core of novel solid-state heat-pump technologies [4]. During this talk I will describe our work on mechanically responsive barocaloric materials, and present our recent advances on barocaloric systems for heating and cooling applications.

_{[1] Climate change: science and solutions low-carbon heating and cooling, Royal Society, 2021.
[2] X. Moya, S. Kar-Narayan and N. D. Mathur, Nature Materials 13, 439 (2014).
[3] X. Moya and N. D. Mathur, Science 370, 797 (2020).
[4] www.barocal.com.}

Atomic and molecular spins on surfaces offer a uniquely powerful platform to investigate magnetism and quantum behaviour at the ultimate length scale, where adsorption geometry, symmetry, and coupling to the environment can be engineered one building block at a time. This controllability makes surface spins an ideal testbed to explore coherent spin phenomena and identify practical strategies to preserve coherence while maintaining fast, local addressability and scalability.

In this talk, I will present how we drive and coherently control molecular and atomic spins on surfaces using electron spin resonance (ESR). First, I will introduce our surface-sensitive ensemble ESR spectrometer, which offers low-temperature X-band operation under ultra-high-vacuum conditions on single-crystal surfaces, with sensitivity reaching below a single molecular layer [1]. We employ this setup to explore how the spins of metal–organic complexes are altered when interfaced with solid-state surfaces. Using ESR and surface characterisation, we identify decoupling strategies to preserve the YPc₂ radical spin from quenching in close proximity to a metallic substrate, highlighting the central role of interfacial hybridisation in spin protection [2]. In contrast, for the molecular spin-qubit candidate FluTi(CoT) [3], where the spin is localised on the metal centre, we observe a robust spin response even in contact with metal surfaces. Moreover, the anisotropy of its g-tensor provides a direct spectroscopic fingerprint to distinguish molecular orientations within a single layer.

In the second part, I will move to the single-spin regime and discuss all-electrical ESR scanning tunnelling microscopy (ESR-STM) of individual Er atoms on MgO/Ag(100) [4], where the lanthanide ground state can be engineered to couple efficiently to microwaves [5]. We demonstrate coherent manipulation with sub-GHz Rabi frequencies, an unusually fast regime for a 4f spin system, establishing that atoms on surfaces can enable highly efficient coherent drive beyond traditional approaches. By exploiting atomic-precision assembly and symmetry control at the adsorption complex, this platform provides a clean route to study and optimise the microscopic mechanisms that set driving efficiency and coherence in surface-based atomic qubits.

References
_{[1] F. H. Cho et al., Rev. Sci. Instrum. 95, 063904 (2024). [2] S. Oh et al., Nanoscale 17, 22163 (2025). [3] S. Wisbeck et al., Chem. Sci. 15, 14390 (2024). [4] S. Reale et al., Nat. Commun. 15, 5289 (2024). [5] S. Reale et al., Phys. Rev. B 107, 045427 (2023).}

With Moore’s law approaching its physical limit, beyond-CMOS concepts are needed to address the rising power demands of Internet of Things (IoT) and artificial intelligence (AI) systems [1]. Among these concepts, magnetoelectric spin-orbit (MESO) logic offers a promising route by harnessing electric-field control of magnetism via multiferroics [2]. The room-temperature multiferroic BiFeO₃ is central to this approach as a magnetoelectric medium due to the coupling between its antiferromagnetic spin cycloid and ferroelectric polarization. However, the coexistence of multiple ferroelectric variants in bulk crystals [3] and thin films [4,5] leads to complex switching pathways. Achieving deterministic, energy-efficient operation therefore requires stabilization of ferroelectric-domain-wall-free, single-variant BiFeO₃ that enables reproducible magnetoelectric control.

In this talk, I will present two parts. First, I will show how we simplified the multiferroic landscape of epitaxial BiFeO₃ thin films by engineering them into single-domain variants through anisotropic-strain strategies. These single-domain systems allowed us to identify the fundamental limit of the magnetoelectric interaction and enabled narrow-band THz emission from this configuration [6,7]. The second part revolves around the deterministic electric-field control of ferroelectric polarization in substrate-vicinality-driven single ferroelectric variants (P) as well as single spin-cycloid propagation directions (k). In these heterostructures, we achieved nonvolatile, and deterministic toggle switching between two distinct antiferromagnetic states [8]. This heterostructure provides a promising platform for magnetoelectric-based devices, advancing the development of MESO technology toward practical applications.

References:

_{[1] G. E. Moore: Electronics 38, 8 (1965); [2] S. Manipatruni, et. al., Nat. Phys., 14, 338 (2018); [3] JT. Heron, et. al., Nat., 516, 370 (2014); [4] A. Haykal, et. al., Nat. Commun. 11, 1704 (2020); [5] P. Dufour*, A. Abdelsamie*, et. al., Nano Lett. 23, 9073 (2023).; [6] A. Abdelsamie, et. al., Appl. Phys. Lett. 24, 242902 (2024); [7] S. Massabeau, A. Abdelsamie, et. al., submitted (2025); [8] A. Abdelsamie, et. al., under review (2026).}

Operando electrochemical characterisation aims to reveal catalytic mechanisms under working conditions in reactions such as electrochemical CO2 reduction and water splitting. However, many operando measurements are still performed under conditions that differ substantially from those used for catalytic performance evaluation. First, operando measurements are often conducted at relatively low current densities, whereas product selectivity is evaluated at much higher, practically relevant current densities. Second, operando measurements are commonly performed in H-cells or modified flow cells, while catalytic selectivity is often assessed in fundamentally different reaction environments, such as practical flow-cell or membrane-electrode-assembly systems. As a result, the interface probed during operando measurements may not be the same interface that governs catalytic performance under realistic operating conditions. This presentation discusses strategies to make operando electrochemical characterisation more truly operando by bridging the gap between mechanistic measurements and practical reaction environments. I will highlight our first demonstration of operando proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) for electrochemical CO2 reduction and its subsequent development for quantitative, time-resolved detection of complex intermediates and products under operating conditions. I will then present recent advances in operando Raman spectroscopy under practically relevant electrochemical conditions, enabling direct observation of catalyst surface restructuring and evolution of the local interfacial chemical environment. Finally, I will discuss data analysis strategies developed for these measurements to more clearly resolve mechanistic evolution and improve the reliability of structure–performance interpretation under real operating conditions.

Electrocaloric cooling is an emerging solid-state refrigeration technology that offers low energy consumption, high efficiency, and operation without greenhouse-gas refrigerants. It relies on ferroelectric materials, in which an applied electric field induces a reversible phase transition, resulting in heat absorption and release. In recent years, multilayer ceramic capacitors, or MLCCs, have attracted significant attention as a promising platform for electrocaloric cooling because they can generate high electric fields within a compact geometry. However, high-performance EC devices still mainly rely on lead scandium tantalate, which contains toxic lead as well as relatively expensive elements such as scandium and tantalum. At the same time, commercial lead-free barium titanate-based MLCCs are not optimized for electrocaloric cooling, particularly in terms of their thermal mass, heat transfer, and electrocaloric performance. I will first present recent advances in electrocaloric cooling, including various materials and laboratory-scale prototypes. Then, I will show our recent progress in developing lead-free, manganese-doped barium strontium titanate MLCCs with a protective-layer-free architecture, using an industrially scalable fabrication process. The resulting device achieves both a higher regeneration factor and a higher specific cooling power compared with state-of-the-art lead scandium tantalate-based prototypes. These results demonstrate the potential of scalable, lead-free MLCC technology for high-performance electrocaloric cooling. Finally, I will discuss possible approaches to further enhance the cooling performance of electrocaloric systems and improve their compatibility with practical applications.

Low-dimensional van der Waals (vdW) ferroelectric crystals provide a versatile platform for exploring unconventional ferroelectric phenomena beyond those commonly encountered in conventional oxide ferroelectrics. Their highly anisotropic bonding, structurally discontinuous lattice, and mobile ionic species introduce additional degrees of freedom that can couple strongly to ferroelectric polarization, giving rise to unusual electromechanical responses, ionic–polarization dynamics, and stacking-driven ferroelectricity. In this talk, I will present our recent studies of several such phenomena in layered vdW ferroelectrics. First, in CuInP₂S₆ (CIPS), we reveal its negative longitudinal piezoelectricity through experimental verification of both the direct and inverse piezoelectric effects, and uncover its microscopic origin from the distinctive anharmonicity of the lattice. Second, I will discuss the interplay between ionic conduction and ferroelectric switching in CIPS, where the migration and redistribution of mobile Cu ions are intimately coupled to polarization reversal, resulting in anomalous switching kinetics and polarization fatigue behavior. Finally, I will present our recent work on NbOCl₂, where nearly degenerate ferroelectric and antiferroelectric states emerge from competing polar instabilities. The delicate energetic balance between these states enables reversible control of the polar order by small mechanical strain, providing a route to manipulate competing ferroic states through lattice engineering. Together, these studies demonstrate how the structural softness, ionic mobility, and competing interactions inherent to low-dimensional vdW crystals give rise to a rich landscape of unconventional ferroelectric behaviors.