arXiv subjects

Samuel W. LaGasse

Publications and source records attributed to Samuel W. LaGasse.

8 recordsLinked to original sources

arXiv2026

Demonstrating magnetic memory in iron-rhodium structures using a quantum diamond microscope

Iron-rhodium (FeRh) has a first-order phase transition near room temperature between antiferromagnetic (AFM) and ferromagnetic (FM) phases, making it a promising material for magnetic memory technologies like heat-assisted magnetic recording (HAMR). It has a comparatively sharper phase transition and lower writing temperature than alternative materials, implying less thermal engineering constraints and an increase in write/read head lifetime. Despite great effort, however, AFM-based magnetic memory using FeRh has not yet been realized. Here, we employ both wide-field and scanning nanoscale quantum diamond microscopes (QDMs) to image directly the magnetic field of a patterned FeRh thin film structure under ambient conditions, demonstrating a magnetic recording technique that is reliable and robust. We experimentally identify coupling between the Néel and magnetization vector directions; and also, that the magnetic orientation of the FM phase uniquely determines the Néel vector in the AFM phase, due to pinned uncompensated magnetic moments (UMMs) in the FeRh structure. Thus, the magnetic orientation is maintained when the system is cycled between AFM and FM phases, providing the foundation for a practical, AFM-based magnetic memory.

cond-mat.mtrl-sci↗

arXiv2026

Transport Enhancement and In Situ Control of Electronic Correlation via Photoinduced Modulation Doping of van der Waals Heterostructures

Modulation doping, a well-established technique for traditional semiconductor heterostructures, is a promising approach for tailoring carrier concentration in 2D materials devices. In this letter we report on photoinduced modulation doping in hBN-graphene-hBN-SiO2 heterostructures utilizing standard white light sources and no additional fabrication complexity. We establish the use of this technique to both dope the channel material and to photoanneal devices, providing control over electronic doping and disorder in the graphene channel. We analyze the transport properties by employing Drude and Landauer transport models, highlighting the ability to reversibly tune the mobility and mean scattering length of the graphene with a high degree of accuracy. This tunability allows us to switch our device between the diffusive and quasi-ballistic transport regimes in situ. We utilize the exceptional control our technique provides over local disorder to realize quantum Hall isospin ferromagnetic states in a device whose initial quality would otherwise leave such states unobservable. These results demonstrate precise manipulation of carrier density and charge disorder in van der Waals heterostructures, providing a highly accessible approach to creating high-quality devices capable of realizing correlated electronic states.

cond-mat.mes-hall↗

arXiv2026

Spatially-resolved voltage-reversal due to Bernoulli potentials in dissipative Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We measure magneto-transport and critical currents in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ Hall bar devices. Above critical current in an applied magnetic field, we observe longitudinal differential voltage along one edge comparable in magnitude but opposite in sign to the other edge. This phenomenon is unaffected by reversal of the applied field, and seems unique to devices with invasive voltage contacts. We attribute the source of this behavior to particle-hole symmetry breaking in moving vortices and the formation of opposite Bernoulli potentials due to opposing vortex velocities at the edges where the invasive contacts create hotspots for rapid vortex nucleation and flux flow. These results are fundamental to the composition and flow of dissipative currents in layered superconductors.

cond-mat.supr-con↗

arXiv2025

Nitrogen-Vacancy Magnetometry of Edge Magnetism in WS2 Flakes

Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature wide-field quantum diamond magnetometry to image pristine and Fe-implanted WS2 flakes of varying thicknesses (45-160 nm), exfoliated from bulk crystals and transferred to NV-doped diamond substrates. We observe direct evidence of edge-localized stray magnetic fields, which scale linearly with applied external magnetic field (4.4-220 mT), reaching up to 4.7 uT. The edge signal shows a limited dependence on the flake thickness, consistent with dipolar field decay and sensing geometry. Magnetic simulations using five alternative models favor the presence of edge magnetization aligned along an axis slightly tilted from the normal to the WS2 flake plane, consistent with spin canting in antiferromagnetically coupled edge states. Our findings establish WS2 as a promising platform for edge-controlled 2D spintronics.

cond-mat.mes-hall↗

arXiv2025

Direct-Write Printed Contacts to Layered and 2D Materials

Advancements in fabrication methods have shaped new computing device technologies. Among these methods, depositing electrical contacts to the channel material is fundamental to device characterization. Novel layered and two-dimensional (2D) materials are promising for next-generation computing electronic channel materials. Direct-write printing of conductive inks is introduced as a surprisingly effective, significantly faster, and cleaner method to contact different classes of layered materials, including graphene (semi-metal), MoS2 (semiconductor), Bi-2212 (superconductor), and Fe5GeTe2 (metallic ferromagnet). Based on the electrical response, the quality of the printed contacts is comparable to what is achievable with resist-based lithography techniques. These devices are tested by sweeping gate voltage, temperature, and magnetic field to show that the materials remain pristine post-processing. This work demonstrates that direct-write printing is an agile method for prototyping and characterizing the electrical properties of novel layered materials.

cs.ET↗

arXiv2025

Hydrated Cable Bacteria Exhibit Protonic Conductivity Over Long Distances

This study presents the direct measurement of proton transport along filamentous Desulfobulbaceae, or cable bacteria. Cable bacteria are filamentous multicellular microorganisms that have garnered much interest due to their ability to serve as electrical conduits, transferring electrons over several millimeters. Our results indicate that cable bacteria can also function as protonic conduits because they contain proton wires that transport protons at distances greater than 100 um. We find that protonic conductivity (σP) along cable bacteria varies between samples and is measured as high as 114 +/- 28 uS cm^-1 at 25-degrees C and 70-percent relative humidity (RH). For cable bacteria, the protonic conductance (GP) and σP are dependent upon the RH, increasing by as much as 26-fold between 60-percent and 80-percent RH. This observation implies that proton transport occurs via the Grotthuss mechanism along water associated with cable bacteria, forming proton wires. In order to determine σP and GP along cable bacteria, we implemented a protocol using a modified transfer-printing technique to deposit either palladium interdigitated protodes (IDP), palladium transfer length method (TLM) protodes, or gold interdigitated electrodes(IDE) on top of cable bacteria. Due to the relatively mild nature of the transfer-printing technique, this method should be applicable to a broad array of biological samples and curved materials. The observation of protonic conductivity in cable bacteria presents possibilities for investigating the importance of long-distance proton transport in microbial ecosystems and to potentially build biotic or biomimetic scaffolds to interface with materials via proton-mediated gateways or channels.

physics.bio-ph↗

arXiv2022

High-speed metamagnetic resistive switching of FeRh through Joule heating

Due to its proximity to room temperature and demonstrated high degree of temperature tunability, the metamagnetic ordering transition in FeRh is attractive for novel high-performance computing devices seeking to use magnetism as the state variable. We demonstrate electrical control of the transition via Joule heating in FeRh wires. Finite element simulations based on abrupt state transition within each domain result in a globally smooth transition that agrees with the experimental findings and provides insight into the thermodynamics involved. We measure a 150 K decrease in transition temperature with currents up to 60 mA, limited only by the dimensions of the device. The sizeable shift in transition temperature scales with current density and wire length, suggesting the absolute resistance and heat dissipation of the substrate are also important. The FeRh phase change is evaluated by pulsed I-V using a variety of bias conditions. We demonstrate high speed (~ ns) memristor-like behavior and report device performance parameters such as switching speed and power consumption that compare favorably with state-of-the-art phase change memristive technologies.

cond-mat.mes-hall↗

arXiv2018

Super-ideal diodes at the Schottky-Mott limit in gated graphene-WSe$_2$ heterojunctions

Metal-semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. We report measurements on a boron nitride encapsulated graphene-tungsten diselenide (WSe$_2$) Schottky junction which exhibits ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe$_2$, enabling experimental verification of the Schottky-Mott limit in a single device. Utilizing this exceptional gate control, we realize a super-ideal gated-Schottky diode which surpasses the ideal diode limit. Our results provide a pathway for defect-free electrical contact to two-dimensional semiconductors and open up possibilities for circuits with efficient switching characteristics and higher efficiency optoelectronic devices.

cond-mat.mes-hall↗