Mark Christopher Hersam

Magnetic tunnel junctions based on ferromagnets are canonical devices in spintronics, with wide-ranging applications in data storage, computing and sensing. They simultaneously exhibit mechanisms for electrical detection and control of magnetic order through the tunnelling magnetoresistance (TMR)^1-5 and spin-transfer torque^6-9 effects, respectively. It was long assumed that neither of these effects could be sizeable in all-antiferromagnetic tunnel junctions (AATJs), considering that they have no net magnetization. However, it was recently shown that AATJs based on chiral antiferromagnets do exhibit TMR due to their non-relativistic momentum-dependent spin polarization and cluster magnetic octupole moment, which are manifestations of their spin-split band structure^10-13. However, the reciprocal effect, that is, the antiferromagnetic counterpart of spin-transfer torque, has been assumed non-existent due to the total electric current being spin-neutral. Here we report nanoscale AATJs that display this reciprocal effect, which we term octupole-driven spin-transfer torque (OTT). We demonstrate current-induced OTT switching of PtMn₃|MgO|PtMn₃ AATJs, with a TMR value of 363% at room temperature and switching current densities of the order of 10 MA cm^-2. Our theoretical modelling explains the origin of OTT in terms of the imbalance between intra- and inter-sublattice spin currents across the AATJ, and, equivalently, in terms of the non-zero net cluster octupole polarization of each PtMn₃ layer. This work establishes a new materials platform for antiferromagnetic spintronics and provides a pathway towards deeply scaled magnetic memory and room-temperature terahertz technologies.

While the Josephson junction is central to qubit operation, overall quantum coherence requires thorough understanding of the entire metallic environment---leads, capacitive pads, and readout resonators. The established contributors to decoherence are surface oxides and hydroxides, as well as absorbed species, such as hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with nonoxidizing metals is a successful strategy to mitigate these issues. To achieve guided optimization, it is significant to understand the microscopic behavior of capped films. Although most attention was paid to the surface and interfaces, bulk properties were not systematically studied. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium gold and compare them to bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature and the lowest upper critical field, London penetration depth, and critical current. These observations agree well with the microscopic theory of anisotropic normal and superconducting states of niobium with varying amounts of scattering. We conclude that surface encapsulation significantly affects bulk properties, and gold-coated films are the cleanest. Since these changes reside in the bulk of the niobium rather than in the overlayer, they may in turn influence the processes that limit quantum coherence. Considering such a strong variation of properties, we believe that the question of whether the surface or bulk is the dominant contributor to quantum decoherence is wide open. However, they likely both contribute via different mechanisms, which makes it very hard to pinpoint where the bottleneck is.

Quantum spin Hall insulators, also known as 2D topological insulators, are crucial 2D systems hosting topologically protected edge states. The working temperature of this topological quantum phase is dictated by the inverted bandgap. However, the previously identified large-gap 2D topological insulators are either extremely chemically unstable or cannot be made with atomistic precision over macroscopic scales. Here, we establish two-quintuple-layer Bi2Te3 and MnBi2Te4/Bi2Te3 heterostructures as atomically controlled, millimeter-scale 2D topological insulators, enabled by precision layer-by-layer growth that yields a carpet-like morphology extending coherently over macroscopic distances. This carpet-like growth mode renders the films amenable to mechanical exfoliation and subsequent wet or dry transfer. Multimodal spectroscopies and microscopies reveal the integer-layer-tuned electronic structure of (Bi2Te3)n in excellent agreement with theory. Photon-energy-dependent photoemission and time-resolved photoemission identify band inversion and band dynamics, respectively, while scanning tunneling spectroscopy resolves topological edge states that are characteristic of the 2D topological insulator phase. Thickness- and photon-energy-dependent photoemission further validate MnBi2Te4/Bi2Te3 as a robust 2D topological insulator. The large inverted gaps of ∼100 meV in (Bi2Te3)2 and ∼150 meV in MnBi2Te4/Bi2Te3 suggest operation near ambient temperature. These results define a scalable materials platform for next-generation, low-loss quantum and energy-efficient devices.

Organic electrochemical transistors (OECTs) efficiently convert ionic signals into measurable electronic currents, enabling applications in bioelectronic sensors, neuromorphic devices, and low‐voltage integrated circuits. High device performance relies on conjugated polymer structures capable of balanced mixed ionic‐electronic conduction, motivating side‐chain engineering design strategies. Here we report a systematic study comparing the properties of DPP‐EDOT polymers in which branched alkyl side chains are progressively replaced by linear glycolated EG ₅ substituents (0%–100%) and evaluate their performance metrics in both conventional (cOECT) and vertical (vOECT) device architectures. Increasing EG ₅ content monotonically increases both steady‐state ( g _{m, max, norm} = 1.27 → 70.73 S·cm ⁻¹ ) and transient performance ( τ _ON/ τ _OFF = 903.91/473.9 → 2.4/0.4 ms) in cOECTs. In contrast, vOECTs exhibit a more pronounced composition‐dependent transition: the vertical architecture delivers outstanding performance only when EG ₅ content exceeds ∼50%, revealing a critical hydrophilicity requirement imposed by the reduced ion‐injection area and extended lateral ion‐transport pathways intrinsic to the vertical geometry. These findings establish architecture‐dependent molecular design principles for effectively pairing OMIECs with planar and vertical OECT configurations.

Superconducting qubits show great promise for the realization of fault-tolerant quantum computing, but lossy, amorphous dielectrics limit current technology. Identifying highly crystalline and stoichiometric dielectrics with intrinsically low microwave loss is therefore a central materials challenge, yet experimentally validated platforms remain scarce. This work integrates a crystalline dielectric into a heteroepitaxial TiN/γ-Al2O3/TiN trilayer grown via pulsed laser deposition. Correlative high-resolution imaging, diffraction, and spectroscopy measurements confirm the single-crystal quality and chemical integrity of all layers, with minimal defects and limited anion interdiffusion across the oxide–nitride interfaces. Using microwave lumped-element resonators with parallel-plate capacitors, this work establishes an experimentally measured upper bound for the dielectric loss of epitaxial γ-Al2O3, which has a low intrinsic two-level system loss of δTLS0 = (2.8 ± 0.1) × 10–5. These results establish heteroepitaxial oxides on transition metal nitrides as an attractive materials platform for superconducting quantum circuits, particularly for integration into compact device architectures such as merged-element transmons and microwave kinetic inductance detectors.

Borophene research has predominantly focused on its monolayer forms due to the challenges associated with synthesizing multilayer boron sheets, despite the desirable properties predicted theoretically beyond the single-atomic-layer limit. Here, we present the successful synthesis of bilayer borophene on Ir(111) substrates, which possesses a distinct atomic structure compared to previously reported bilayer borophene phases. Using scanning tunneling microscopy and spectroscopy, a significantly reduced local work function is observed for bilayer borophene compared to monolayer borophene on iridium. Specifically, bilayer islands show strong peak position shifts toward lower energy in field emission resonance spectroscopy, which is further corroborated by density functional theory calculations. This ultralow-work-function bilayer phase presents expanded application opportunities for borophene including field emission sources, photoelectric devices, and quantum technologies.

Base baths, which typically employ strong bases dissolved in alcohols, are frequently used to clean laboratory glassware but present significant fire, chemical, and handling hazards when improperly prepared or maintained. This work evaluates prior awareness, documentation practices, and daily use of base baths within an academic research environment. It seeks to improve safety education on users’ knowledge and behaviors. Initial surveys revealed knowledge gaps regarding base bath fire risks, the correct use of personal protective equipment (PPE), proper secondary containment measures, and improper labeling and documentation practices. In response, standardized training materials, safety briefings, and a formal standard operating procedure were developed and distributed through departmental seminars and interactive training sessions. Follow-up surveys demonstrate improved recognition of proper base bath preparation, increased awareness of appropriate spill/splash response, more precise identification of associated fire hazards, and greater confidence in working with base baths. The reported use of labeling, lids, secondary containment, and appropriate PPE increased following these safety interventions. Collectively, these results indicate that structured, low-barrier safety programming combined with accessible written guidance measurably improved hazard recognition and risk mitigation during handling base baths. This framework provides a practical model for strengthening the chemical safety culture through student-led initiatives focused on high-risk cleaning practices, which are standard in research laboratories and may be adapted to other commonly overlooked laboratory hazards.

The charge storage capacitance of δ-MnO ₂ -based pseudocapacitors stems from a combination of bulk cation intercalation/deintercalation and surface proton chemisorption/desorption. Here, we investigate the mechanistic origins of the enhanced capacitance in δ-MnO ₂ with pre-intercalated Cu ²⁺ . To this end, we synthesize Au-core/δ-MnO ₂ -shell nanostructures with and without Cu ²⁺ pre-intercalation, enabling real-time in situ spectroscopic monitoring of structure-function relationships during electrochemical cycling. Transition metal pre-intercalation preserves interlayer-confined water, which in turn supports proton-coupled charge storage via the reversible reaction of MnO ₂ + H ₂ O + e ^- ⇌ MnOOH + OH ^- . This confined water forms a hydrogen-bonded network that lowers the energy barrier for proton transport within the interlayer space. Similar mechanistic transition is also evident in δ-MnO ₂ systems pre-intercalated with other transition metal ions, such as Co ²⁺ and Mg ²⁺ . By tuning the MnO ₂ shell thickness, we decouple the relative contributions of proton- and cation-driven processes, revealing that proton intercalation delivers a markedly higher specific capacitance than cation intercalation. Electrolyte-dependent studies further reveal that Cu ²⁺ pre-intercalation promotes OH ^- transport within the interlayer space while preserving proton accessibility at active sites. These findings suggest that proton-coupled transport may offer further increases in charge storage performance in pseudocapacitors.

Scalable manufacturing of graphene-based solid-contact ion-selective electrodes (SC-ISEs) is constrained by reliance on mined graphite and on patterning methods that are costly, energy-intensive, or poorly suited to high-throughput production. Here, we present a sustainable, manufacturing-ready approach in which graphene derived from hardwood biochar is formulated into printable inks and patterned by high-throughput screen printing to produce bioderived graphene solid-contact ion-selective electrodes (BioG-SC-ISEs). Unlike prior biomass-derived electrodes based on amorphous or activated carbons, this work demonstrates the conversion of biochar into graphene-like nanosheets compatible with scalable printing. By functionalizing the screen-printed electrodes with poly(vinyl chloride) (PVC)-based ion-selective membranes doped with distinct ionophores, we produced sensors capable of selectively monitoring six plant nutrients (K⁺, Na⁺, NH₄⁺, Ca²⁺, Mg²⁺, and NO₃^-). The resulting sensors exhibited near-Nernstian sensitivities with detection limits between 0.064 and 0.526 mg·L^-1 with wide sensing ranges (10^-6 to 10^-1 M for monovalent ions, 10^-5 to 10^-1 M for Mg²⁺, and 10^-5 to 10^-2 M for Ca²⁺). As an application-relevant demonstration, K⁺ BioG-SC-ISEs tracked progressive potassium depletion across modified Hoagland formulations (100%, 50%, and 25% K⁺), achieving a sensitivity of 57.4 ± 0.7 mV·dec^-1 and a detection limit nearly three orders of magnitude below typical hydroponic potassium levels, with measurements in close agreement with inductively coupled plasma optical emission spectrometry (ICP-OES). Overall, this work establishes a sustainable strategy for scalable SC-ISE manufacturing and provides a versatile platform for high-performance ion sensing with direct implications for hydroponic agriculture, environmental monitoring, and other electrochemical sensing applications requiring selective ion detection.