arXiv subjects

Ryan Wagner

Publications and source records attributed to Ryan Wagner.

4 recordsLinked to original sources

arXiv2026

Spin Vector Control for Heisenberg-Inspired Probabilistic Computing

Probabilistic bits (p-bits) have emerged as a cornerstone of probabilistic computing, enabling energy-efficient hardware implementation for probabilistic inference and combinatorial optimization. A critical challenge in advancing this field beyond binary p-bits lies in realizing and manipulating vector spin information, essential for mapping complex energy-based models such as the Heisenberg Hamiltonian.Here, we demonstrate a spintronic platform capable of real-space vector summation by using dual ferromagnetic spin injections into a monolayer graphene channel. By electrically tuning the spin polarization through independently controlled injection currents, we achieve continuous control over the magnitude and direction of the resulting spin accumulation vector. Experimental observations, supported by theoretical vector summation models and spin-circuit simulations, reveal coherent vector interactions and angular tunability of the spin state. This approach enables direct implementation of vector-based spin logic and lays the groundwork for mapping classical Heisenberg models using stochastic low-barrier magnets. Our results establish a scalable pathway for realizing probabilistic spin circuits based on two-dimensional materials, offering new opportunities for low-power, non-Boolean computing architectures.

cond-mat.mes-hall↗

arXiv2024

3D Nanoscale Electromechanical Imaging with Interferometric Atomic Force Microscopy

Forces acting between an Atomic Force Microscope (AFM) tip and sample are three dimensional. Despite this, most AFM force measurements are confined to one or two dimensions. Extending AFM force measurements into three dimensions has previously required complex, difficult and time-consuming workflows. Here, we demonstrate an accurate, interferometric method for quantifying the full, three-dimensional response of an AFM tip to localized forces. We demonstrate this approach on a series of piezoelectric materials and show that this approach yields quantitative 3D measurement independent of the sample orientation beneath the tip. This approach simplifies existing, angle-resolved piezoresponse force microscopy (PFM) techniques. Our measurements benefit from the greatly reduced noise floor (5 fm per root Hz) and intrinsic accuracy of our interferometric measurements. One important result is that the vertical piezo sensitivity was systematically 2 to 3 times larger than the in-plane piezo sensitivities. A simple analysis of vertical and lateral contact stiffnesses, due to the difference in the Young (vertical) and Shear (lateral) sample yields a factor of 2.5, in good agreement with the measurements. While this work was confined to ferroelectric materials, it provides a general workflow and framework for other AFM based mechanical measurements.

cond-mat.mes-hall↗

arXiv2023

Nanoscale rheology: Dynamic Mechanical Analysis over a broad and continuous frequency range using Photothermal Actuation Atomic Force Microscopy

Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e. their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on a styrene-butadiene rubber (SBR), demonstrating the method's ability to quantify nanoscale viscoelasticity over a continuous frequency range up to five orders of magnitude (0.2 Hz to 20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude-modulation frequency-modulation (AM-FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range, and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials, and soft matter in general at the nanoscale, for any application.

cond-mat.mes-hall↗

arXiv2023

Accurate Vertical Nanoelectromechanical Measurements

Accurate measurements of the nanoscale electromechanical coupling in materials, including piezo and ferroelectrics, twisted 2D layers, and biological systems is of both fundamental scientific and applied importance. Piezoresponse Force Microscopy (PFM) is capable of detecting strains in these materials, down to the picometer range. Following the emergence of weaker materials, the smaller signals associated with them have revealed various crosstalk challenges that have limited the accuracy of measurements. Previous work demonstrated that the use of an interferometric displacement sensor (IDS) positioned appropriately above the tip of the cantilever (x/L~1), where x is the spot position and L is the cantilever length, has enabled sensitive and artifact-free electromechanical measurements. A similar approach has been employed in removing unwanted electrostatic and in-plane response contributions for the optical beam deflection (OBD) measurement technique commonly used in most atomic force microscopes. In the present study, extensive automated sub-resonance spot position dependent PFM measurements were conducted on periodically poled lithium niobate (PPLN). In this work, both IDS and OBD responses were measured simultaneously, allowing direct comparisons of the two approaches. The IDS showed a blind spot at x/L~1, as expected. However, for OBD measurements, the blind spot's location exhibited wider variation, ranging from 0.15<x/L<0.61. Furthermore, the magnitudes of the amplitudes measured with IDS and OBD were typically different, sometimes approaching disagreement by a factor of two. These measurements have important implications, not only for the PPLN measured here, but for more complex and unknown samples that have a heterogeneous polarization and electrical characteristic.

cond-mat.mes-hall↗