arXiv subjects

Patrick J. Taylor

Publications and source records attributed to Patrick J. Taylor.

7 recordsLinked to original sources

arXiv2025

Direct Comparison of Static and Dynamic Measurements of Spin Generation in a Topological Insulator Thin Film

The competition between intrinsic spin-orbit physics, magnetic phenomena, and the quality of materials and interfaces governs the charge-to-spin conversion processes that are essential to the implementation of spintronic devices. Direct comparisons of spin parameters, which serve as metrics of device quality, obtained by different measurement techniques are scarce, leading to uncertainty regarding discrepancies and the reliability of the methods. Here, we directly compare the spin Hall coefficient ($θ_{SH}$) in molecular beam epitaxy grown films of $(Bi_{1-x}Sb_{x})_{2}Te_{3-y}Se_{y}$ (BSTS, x = 0.58, y = 1) at room temperature using two complementary techniques: a static method using non-local voltage (NLV) measurements in BSTS Hall bars with DC charge current, and a dynamic method using spin-torque ferromagnetic resonance (ST-FMR) measurement in $BSTS/Ni_{80}Fe_{20}$ heterostructures at GHz frequencies. We obtain comparable spin Hall coefficients in angular-dependent ST-FMR ($θ_{SH}$=$4.7\pm1.1$) and in NLV ($θ_{SH}$=$2.8\pm0.6$). The complex effects of ferromagnetic interfaces while determining spin Hall coefficients using static or dynamic techniques becomes evident by contrasting our results to literature.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic Spin Transport in a Topological Insulator Thin Film

Topological insulators (TIs) are intriguing materials for advanced computing applications based on spintronics because they can host robust spin effects. For instance, TIs have intrinsically large spin generation enabled by their large spin-orbit coupling. Furthermore, topological surface states (TSS) with spin-momentum locking and Dirac dispersion lead to long spin diffusion. Future spintronic device technology will require scalable film growth of high-quality material. We grow epitaxial films of Bi$_{1-x}$Sb$_x$Te$_{3-y}$Se$_y$ (BSTS, $x = 0.58, y = 1$) and confirm the gapless band structure with optimal doping using angle-resolved photoelectron spectra. The temperature dependence of longitudinal resistivity shows bulk transport is suppressed as temperature is decreased, and at low temperature surface transport dominates. We evaluate the spin transport properties in BSTS without using ferromagnetic tunnel contacts via a non-local resistance experiment as a function of temperature and applied charge current. As expected, these experiments reveal the necessity of decreasing the bulk conduction to best enhance the spin transport. In the TSS, we find high efficiency of charge-to-spin conversion (spin Hall angle, $θ_{SH} \approx 1$) and spin diffusion over several microns. Further development of high-quality TIs will make them viable candidates for efficient and lossless spintronics.

cond-mat.mes-hall↗

arXiv2024

Topological Antenna: A Non-Classical Beam-Steering Micro-Antenna Based on Spin Injection from a Topological Insulator

Antennas are the quintessential means to communicate information wirelessly over long distances via electromagnetic waves. Traditional antennas have two shortcomings that have prevented miniaturization: (1) their radiation efficiencies plummet and (2) they radiate isotropically when miniaturized to small fractions of the radiated wavelength. Here, we report a new genre of non-classical antennas that overcome these limitations by employing non-traditional principles and harnessing topological insulators. An alternating charge current of frequency 1-10 GHz injected into a thin film of a three-dimensional topological insulator (3D-TI) injects a spin current of alternating spin polarization into a periodic array of cobalt nanomagnets deposited on the surface of the 3D-TI. This generates spin waves in the nanomagnets, which radiate electromagnetic waves in space, thereby implementing an antenna. The frequency of the electromagnetic wave is the same as that of the current. The antenna dimension is only 0.6-1.8% of the free space wavelength and yet it radiates with an efficiency several orders of magnitude larger than the theoretical limit for conventional antennas. Furthermore, it radiates anisotropically (despite being a "point source") and one can change the anisotropic radiation pattern by changing the direction of the injected alternating charge current, which changes the spin wave patterns within the nanomagnets because of spin-momentum locking in the 3D-TI. This enables beam steering without the use of a phased array. We have overcome several limitations of classical antennas by harnessing the quantum mechanical attributes of a quantum material, namely a 3D-TI.

cond-mat.mes-hall↗

arXiv2023

Spintronic Quantum Phase Transition in a $Graphene/Pb_{0.24}Sn_{0.76}Te$ Heterostructure with Giant Rashba Spin-Orbit Coupling

Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a two-dimensional electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. We study the spintronic properties of this heterostructure with non-local spin valve devices. We observe spin-momentum locking at lower temperatures that transitions to regular spin channel transport only at ~40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin-momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition-like behavior in the Landé g-factor and spin relaxation time.

cond-mat.mes-hall↗

arXiv2023

Emergence of Three-fold Symmetric Helical Photocurrents in Epitaxial Low Twinned Bi$_2$Se$_3$

We observe enhanced three-fold symmetric helicity-dependent topological photocurrents using time-domain THz spectroscopy in epitaxially-grown Bi2Se3 with reduced crystallographic twinning. It is established how twinned crystal domains introduce competing responses that obscure inherent nonlinear optical responses of the intrinsic crystal structure. Minimizing this defect reveals strong nonlinear optical response currents whose magnitude and direction depend on the alignment of the excitation to the crystal axes and follow the three-fold rotational symmetry of the crystal structure. Notably, the azimuthal dependence of the photoresponse persists for helical excitations -- an unprecedented result we attribute to the photon drag effect, where the photon momentum acts as an applied in-plane field that is stationary in the laboratory frame. Additionally, the sign of the resultant THz signal inverts when the helicity of incident light is switched from right to left circularly polarized, indicating a reversal of the photocurrent. Our results demonstrate that even extended domain defects can obscure intrinsic physical processes, making the study of single domain thin films crucial to the observation of phenomena that couple topological order and crystal symmetries.

cond-mat.mes-hall↗

arXiv2022

Strain-tunable Berry curvature in quasi-two-dimensional chromium telluride

Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics.

cond-mat.mtrl-sci↗

arXiv2022

Magnetotransport in graphene/Pb0.24Sn0.76Te heterostructures: finding a way to avoid catastrophe

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb0.24Sn0.76Te (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20,000 cm2/Vs and a magnetoresistance approaching 100 percent, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/non-polar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of remarkable behaviors in these novel materials.

cond-mat.mtrl-sci↗