Browsing by Author "Emori, Satoru"
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- Decoherence of Transverse Electronic Spin Current in Magnetic MetalsLim, Youngmin (Virginia Tech, 2022-05-31)Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914.
- High Performance Broadband Photodetectors Based on Graphene/Semiconductor HeterostructuresWang, Yifei (Virginia Tech, 2022-04-15)Graphene, a monolayer of carbon atoms, has gained prominence to augment existing chip-scale photonic and optoelectronic applications, especially for sensing in optical radiation, owing to its distinctive electrical properties and bandgap as well as its atomically thin profile. As a building block of photodetection, graphene has been co-integrated with mature silicon technology to realize the on-chip, high-performance photo-detecting platforms with broad spectral response from the deep-ultraviolet (UV) to the mid-infrared (MIR) regime. The recent state-of-the-art graphene-based photodetectors utilizing the combination of colloidal quantum dots (QDs) and graphene have been intensively studied, where QDs function as the absorber and the role of graphene is as a fast carrier recirculating channel. With such a configuration, an ultrahigh sensitivity can be achieved on account of the photogating mechanism; however, the response time is slow and limited to the millisecond-to-second range. To achieve balance between high sensitivity and fast response time, we have demonstrated a new photodetector that is based on graphene/two-dimensional heterostructures. The homogeneous thickness and the large contact of the heterostructure give rise to fast carrier transporting between the thin absorber layer and the graphene, leading to a fast response time. This thesis carefully investigates the optimization of fabrication as well as optoelectronic characterization of photodetectors based on graphene/semiconductor heterostructures field-effect transistors (GFETs). GFETs with different architectures were demonstrated and systematically studied under optical illumination ranging from deep-UV to MIR at varying optical powers. Noise behaviors have been studied under different device parameters such as device structure, area and gate-bias. Results show that the flicker noise of graphene-based devices can be explained by the McWhorter model in which the fluctuation of carrier numbers is the dominant process of noise in low frequencies; thus, it can be scaled down by reducing the number of introduced charged carriers with optimized fabrication. Besides, the impact of absorber on top of graphene and the bottom substrate has been comprehensively explored through various experimental techniques including current-voltage (IV), photo-response dynamics, and noise characterization measurements. With our configuration, the high sensitivity and fast response time of photodetectors can be obtained at the same time. In addition to this, the study of the bottom substrate with different doping levels suggests a concept of dual-photogating effect which is induced by the top absorbent material and the photoionization of the doped silicon substrate. In summary, this thesis showcases novel device architecture and fabrication procedures of GFETs photodetectors, optimizes device structure, quantifies the performance and evaluates the effect of various absorbent materials and substrate. It provides insight into the improvement of possible routes to achieve a broadband photo-detecting system with higher sensitivity, faster response time and lower noise level.
- Hydrodynamic and ballistic transport in high-mobility GaAs/AlGaAs heterostructuresGupta, Adbhut (Virginia Tech, 2021-09-24)The understanding and study of electron transport in semiconductor systems has been the instigation behind the growth of semiconductor electronics industry which has enabled technological developments that are part of our everyday lives. However, most materials exhibit diffusive electron transport where electrons scatter off disorder (impurities, phonons, defects, etc.) inevitably present in the system, and lose their momentum. Advances in material science have led to the discovery of materials which are essentially disorder-free and exhibit exceptionally high mobilities, enabling transport physics beyond diffusive transport. In this work, we explore non-diffusive transport regimes, namely, the ballistic and hydrodynamic regimes in a high-mobility two-dimensional electron system in a GaAs quantum well in a GaAs/AlGaAs heterostructure. The hydrodynamic regime exhibits collective fluid-like behavior of electrons which leads to the formation of current vortices, attributable to the dominance of electron-electron interactions in this regime. The ballistic regime occurs at low temperatures, where electron-electron interactions are weak, constraining the electrons to scatter predominantly against the device boundaries. To study these non-diffusive regimes, we fabricate mesoscopic devices with multiple point contacts on the heterostructure, and perform variable-temperature (4.1 K to 40 K) zero-field nonlocal resistance measurements at various locations in the device to map the movement of electrons. The experiments, along with interpretation using kinetic simulations, demarcate hydrodynamic and ballistic regimes and establish the dominant role of electron-electron interactions in the hydrodynamic regime. To further understand the role of electron-electron interactions, we perform nonlocal resistance measurements in the presence of magnetic field in transverse magnetic focusing geometries under variable temperature (0.39 K to 36 K). Using our experimental results and insights from the kinetic simulations, we quantify electron-electron scattering length, while also highlighting the importance of electron-electron interactions even in ballistic transport. At a more fundamental level, we reveal the presence of current vortices in both hydrodynamic and surprisingly, ballistic regimes both in the presence and absence of magnetic field. We demonstrate that even the ballistic regime can manifest negative nonlocal resistances which should not be considered as the hallmark signature of hydrodynamic regime. The work sheds a new light on both hydrodynamic and ballistic transport in high-mobility solid-state systems, highlighting the similarities between these non-diffusive regimes and at the same time providing a way of effectively demarcating them using innovative device design, measurement schemes and one-to-one modeling. The similarities stem from total electron system momentum conservation in both the hydrodynamic and ballistic regimes. The work also presents a sensitive and precise experimental technique for measuring electron-electron scattering length, which is a fundamental quantity in solid-state physics.
- Nanostructures for Coherent Light Sources and PhotodetectorsHo, Vinh Xuan (Virginia Tech, 2020-05-14)Large-scale optoelectronic integration is limited by the lack of efficient light sources and broadband photodetectors, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission as well as broadband photodetection from silicon in extending the silicon technology into fully integrated optoelectronic circuits. Recent breakthroughs, including the demonstration of high-speed optical modulators, photodetectors, and waveguides in silicon, have brought the concept of transition from electrical to optical interconnects closer to realization. The on-chip light sources based on silicon are still a key challenge due to the indirect bandgap of silicon that impedes coherent light sources. To overcome this issue, we have studied, fabricated, and characterized nanostructures including single semiconductor epilayers, multiple quantum wells, and graphene-semiconductor heterostructures to develop coherent light sources and photodetectors in silicon. To develop coherent light sources, we reported the demonstration of room-temperature lasing at the technologically crucial 1.5 m wavelength range from Er-doped GaN epilayers and Er-doped GaN multiple-quantum wells grown on silicon and sapphire. The realization of room-temperature lasing at the minimum loss window of optical fiber and in the eye-safe wavelength region of 1.5 m is highly sought-after for use in many applications in various fields including defense, industrial processing, communication, medicine, spectroscopy and imaging. The results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics. Silicon photodiodes contribute a large portion in the photodetector market. However, silicon photodetectors are sensitive in the UV to near infrared region. Photodetection in the mid-infrared is based on thermal radiation detectors, narrow bandgap materials (InGaAs, HgCdTe) semiconductors, photo-ionization of shallow impurities in semiconductors (Si:As, Ge:Ga), and quantum well structures. Such technology requires complicated fabrication processes or cryogenic operation, resulting in manufacturing costs and severe integration issues. To develop broadband photodetectors, we focus on graphene photodetectors on silicon. Graphene generates photocarriers by absorbing photons in a broadband spectrum from the deep-ultraviolet to the terahertz region. Graphene can be realized as the next generation broadband photodetection material, especially in the infrared to terahertz region. Here, we have demonstrated high-performance hybrid photodetectors operating from the deep-ultraviolet to the mid-infrared region with high sensitivity and ultrafast response by coupling graphene with a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 thin film.
- Non-equilibrium dynamics in three-dimensional magnetic spin models and molecular motor-inspired one-dimensional exclusion processesNandi, Riya (Virginia Tech, 2021-03-10)We investigate the relaxation dynamics of two distinct non-equilibrium processes: relaxation of three-dimensional antiferromagnetic lattice spin models with Heisenberg interaction following a critical quench, and a one-dimensional exclusion process inspired by the gear-like motion of molecular motors. In a system of three-dimensional Heisenberg antiferromagnets the non-conserved staggered magnetization components couple non-trivially to the conserved magnetization densities inducing fully reversible terms that enter the Langevin dynamic equation. We simulate the exact microscopic dynamics of such a system of antiferromagnets by employing a hybrid simulation algorithm that combines the reversible spin precession implemented by the fourth-order Runge-Kutta integration method with the standard relaxational dynamics at finite temperatures using Monte Carlo updates. We characterize the dynamic universality class of this system by probing the early temporal window where the system exhibits aging scaling properties. We also verify an earlier renormalization group prediction that the temporal decay exponent in the two-time spin autocorrelation function exhibits non-universality, specifically it depends on the width of the initial spin orientation distribution. We employ a similar numerical technique to study the critical dynamics of an anisotropic Heisenberg antiferromagnet in the presence of an external field. The phase diagram of this system exhibits two critical lines that meet at a bicritical point. We study the aging scaling dynamics for the model C critical line, probe the model F critical line by investigating the system size dependence of the characteristic spin-wave frequencies near criticality, and measure the dynamic critical exponents for the order parameter including its aging scaling at the bicritical point. We introduce a one-dimensional non-equilibrium lattice gas model representing the processive motion of dynein molecular motors over the microtubule. We study both dynamical and stationary state properties for the model consisting of hardcore particles hopping on the lattice with variable step sizes. We find that the stationary state gap-distribution exhibits striking peaks around gap sizes that are multiples of the maximum step size, for both open and periodic boundary conditions, and verify this using a mean-field calculation. For open boundary conditions, we observe intriguing damped oscillator-like distribution of particles over the lattice with a periodicity equal to the maximum step size. To characterize transient dynamics, we measure the mean square displacement that shows weak superdiffusive growth with exponent γ≈ 1.34 for periodic boundary and ballistic growth ( γ≈ 2) for open boundary conditions at early times. We also study the effect of Langmuir dynamics on the density profile.
- Ordering processes and pattern formation in systems far from equilibriumStidham III, James Edward (Virginia Tech, 2022-05-12)In this work, we present our investigations into two different systems, both far from equilibrium. We first present the relaxation and ordering processes in magnetic skyrmion systems. This is followed by a study of the behavior of many species interacting on a spatially heterogeneous lattice. Magnetic skyrmions have been a subject of great interest in recent years. They have been proposed to be at the heart of next-generation computing and information storage devices. One interesting feature of magnetic skyrmions is the presence of the non-dissipative Magnus force. The Magnus force causes the skyrmions to be deflected from their direction of motion. In this work, we examine the effect the strength of this Magnus force has on the late-time ordering behavior of magnetic skyrmions. We show that the late-time ordering also shows enhanced relaxation with an increase in the Magnus force. We also studied the behavior of magnetic skyrmions when confined to a narrow channel. We show that, like before, the Magnus force helps the system order faster while experiencing a constant drive. Interestingly, when the drive was periodic, the Magnus force inhibited the relaxation in the system. Interacting populations have been a topic of scientific interest since the late eighteenth century. We studied the effect of spatial heterogeneity on a two-dimensional lattice. Using cyclic predator-prey interaction schemes, we numerically simulated the effect of asymmetric predation rates inside "habitats." We show that, due to the non-linearity of the system, the species that had a chance to escape predation did not see the largest benefit from this change. Instead, the predator of this prey saw the largest benefit from this change. The material on skyrmion systems is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Award Number DE-SC0002308. The population dynamics research was sponsored by the Army Research Office and was accomplished under Grant No. W911NF17-1-0156.
- Photogating Effect and Diffractive Optics in Low-Dimensional StructuresHowe, Leslie (Virginia Tech, 2024-09-10)The development of nanostructures is the driving force of many scientific and technological fields. Among these myriad applications, important technologies such as advanced detection and ranging capabilities for infrared wavelengths and enhanced sensing of chemical molecules have been obtained recently, advancing our understanding of the earth's climate and space imaging. This kind of advancement is made possible through the thorough understanding of the performance of these devices which are fabricated from low-dimensional materials such as graphene, as well as the interaction of light with the materials at the microscopic scales, as is included in this dissertation. As such, the techniques of fabrication and theoretical understanding of graphene-field effect transistors (GFETs) as well as multi-level Fresnel zone plates (MLFZPs) are provided in detail. The photogating effect, understood as the ability of charge carriers to generate photocurrent when excited by an incident photon within a material, is crucial to the device physics of GFETs. We can utilize this property, as well as the resultant band bending of the interfacial band structure created within these transistors, to measure and predict the power of light as it interacts with the optical sensing area of the graphene. This allows for graphene transistors to be effective photodetectors, and we can accurately model the behavior of these detectors at many testing conditions, such as differing ambient temperatures, varying wavelengths, and multiple sensing area sizes. This work elucidates the capabilities and efficacy of these devices as photodetectors within both experimental and simulated conditions. In addition to photodetectors, GFETs prove to be capable biosensors as well, as graphene modulation due to the interaction of a molecule on its surface has a similar effect on the current of the channel as the photogating effect. When coupling these mechanisms, we find that there is a measurable effect due to the deposition of a photoreactive biomolecule of interest on the surface of the device. In particular, utilizing photoactive yellow protein (PYP), which has an incredibly strong reaction to blue light, allows us record concentration levels of the protein in solution down to the femtomole on the graphene surface of the detector when illuminated with the appropriate wavelength. This ability to measure the amount of PYP present in solution has many exciting implications, both to the understanding of the protein itself, as well as to the capability of the devices in detecting other proteins or biological molecules. Finally, nanostructures are an important component to diffraction, which allows for the construction of very precise diffractive lenses. This work entails the fabrication and simulation of MLFZPs, which are useful in their ability to tune the wavelength and focal length of the lenses to strict parameters. In addition, it is shown that these devices may be fabricated on thin polyimide films, allowing for flexibility and usefulness in mechanical applications. We have been able to fabricate lenses with features in precise control down to the nanometer in depth, and this results in incredibly precise and powerful optics which align well with simulated values.
- Probing Coherent States and Nonlinear Properties in Multifunctional Material SystemsHerath Mudiyanselage, Rathsara Rasanjalee Herath (Virginia Tech, 2021-04-15)The rapid progress on developing new and improved multifunctional materials, for optoelectronic and spin based phenomena/devices, have increased the importance of the fundamental understanding of their coherent states and nonlinear optical properties. This study is aimed at characterizing, modeling, and controlling the fundamental electronic, phononic, and spin properties of several classes of materials through nonequilibrium and nonlinear light-matter interactions, coupled with a novel design of the material phases, interfaces, and heterostructures. This research directly addresses the Grand Challenges identified in the Basic Energy Sciences Advisory Committee report "Directing Matter and Energy: Five Challenges for Science and the Imagination" (Hemminger, 2007) [1], in particular, the area: "Matter far beyond equilibrium" and addresses the questions, "How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties?" and "How do we design and perfect atom- and energy-efficient synthesis of revolutionary new forms of matter with tailored properties?". The knowledge gained from these fundamental studies can provide new information for a broad community to provide concepts for the next generation of multifunctional materials and devices, and resulted in several publications and conference presentations. The materials studied in this dissertation included multiferroic BaTiO3-BiFeO3 [2], ferroelectric Pb0.52Zr0.48TiO3 (PZT), InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7]. Probing multiferroics, which are materials that can exhibit ferromagnetic, ferroelectric, and ferroelastic orders simultaneously in a single phase, was a main focus of this study. BiFeO3 (BFO) is the most widely investigated multiferroic due to its high Neel and Curie temperatures and has antiferromagnetic and ferroelectric properties [8]. An inherent drawback of BFO is its large leakage currents. In this project, (1 − x)BaTiO3-(x)BiFeO3, x = 0.725 (BTO-BFO) heterostructures were investigated [9], where the conductivity of the solid solution can be reduced by adding another perovskite material, BaTiO3 [2]. We aimed to study optically induced coherent states in our BTO-BFO structures. Time resolved pumpprobe spectroscopic measurements were performed at room temperature as well as at low temperature (100 K) up to 10 T. Coherent acoustic phonons were observed both in a film and nanorods, resulting in coherent phonon frequencies of 27 and 33 GHz, respectively [2]. Coherent phonon spectroscopy is a sensitive tool to characterize the interfaces and can be employed as an effective ultrasensitive quantum sensor [10]. Furthermore, in the nanorods arrays of BTO-BFO, an additional oscillation with frequency in the range of 8.1 GHz was observed. This frequency is close to a theoretically predicted magnon frequency which could indicate the coexistence of coherent phonons and magnons in the nanorods arrays [2]. In an analogy to photonics which relies on electromagnetic waves, magnonics utilizes spin waves to carry and process information, offering several advantages such as an operation frequency in the THz range. Recently, "a quantum tango" [11] was reported where coupled coherent magnon and phonons modes were formed on a surface patterned ferromagnet. Furthermore, BTO-BFO heterostructures were probed using transient birefringence and magneto-optical Kerr effect spectroscopy. The results demonstrated that the magnetic field dependence of coherent phonons, measured by these two techniques, exhibits more sensitivity to the external magnetic fields compared to the differential reflectivity technique [2]. Moreover, nonlinear optical properties of this structure were investigated via second harmonic generation spectroscopy, where wavelength and polarization dependence of this nonlinear observation will be discussed in this dissertation. As part of this study, another class of multiferroic materials, with strong ferroelectric and piezoelectric properties, Pb0.52Zr0.48TiO3 (PZT) was studied [12]. In this project, the nonlinear optical properties of PZT nanorod arrays were investigated. Clear signatures of second harmonic generations from 490-525 nm (2.38-2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties in the range of 690-1000 nm where multiphoton processes were responsible for the photoexcitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼ 3.1 eV) was close to the bandgap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications. Furthermore, the established techniques during my research provided new insights on optical properties of InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7], and the results will be briefly presented in this dissertation.
- Quantifying the orbital-to-spin moment ratio under dynamic excitationEmori, Satoru; Maizel, Rachel E.; Street, Galen T.; Jones, Julia L.; Arena, Dario A.; Shafer, Padraic; Klewe, Christoph (AIP Publishing, 2024-03-18)The orbital component of magnetization dynamics, e.g., excited by ferromagnetic resonance (FMR), may generate “orbitronic” effects in nanomagnetic devices. Yet, distinguishing orbital dynamics from spin dynamics remains a challenge. Here, we employ x-ray magnetic circular dichroism (XMCD) to quantify the ratio between the orbital and spin components of FMR-induced dynamics in a Ni80Fe20 film. By applying the XMCD sum rules at the Ni L 3 , 2 edges, we obtain an orbital-to-spin ratio of 0.108 ± 0.005 for the dynamic magnetization. This value is consistent with 0.102 ± 0.008 for the static magnetization, probed with the same x-ray beam configuration as the dynamic XMCD experiment. The demonstrated method presents a possible path to disentangle orbitronic effects from their spintronic counterparts in magnetic media.
- Room-Temperature Intrinsic and Extrinsic Damping in Polycrystalline Fe Thin FilmsWu, Shuang; Smith, David A.; Nakarmi, Prabandha; Rai, Anish; Clavel, Michael; Hudait, Mantu K.; Zhao, Jing; Michel, F. Marc; Mewes, Claudia; Mewes, Tim; Emori, Satoru (2021-09-08)We examine room-temperature magnetic relaxation in polycrystalline Fe films. Out-of-plane ferromagnetic resonance (FMR) measurements reveal Gilbert damping parameters of $\approx$ 0.0024 for Fe films with thicknesses of 4-25 nm, regardless of their microstructural properties. The remarkable invariance with film microstructure strongly suggests that intrinsic Gilbert damping in polycrystalline metals at room temperature is a local property of nanoscale crystal grains, with limited impact from grain boundaries and film roughness. By contrast, the in-plane FMR linewidths of the Fe films exhibit distinct nonlinear frequency dependences, indicating the presence of strong extrinsic damping. To fit our in-plane FMR data, we have used a grain-to-grain two-magnon scattering model with two types of correlation functions aimed at describing the spatial distribution of inhomogeneities in the film. However, neither of the two correlation functions is able to reproduce the experimental data quantitatively with physically reasonable parameters. Our findings advance the fundamental understanding of intrinsic Gilbert damping in structurally disordered films, while demonstrating the need for a deeper examination of how microstructural disorder governs extrinsic damping.
- Spin States in Bismuth and Its Surfaces: Hyperfine InteractionJiang, Zijian (Virginia Tech, 2021-01-07)The hyperfine interaction between carrier spins and nuclear spins is an important component in exploring spin-dependent properties in materials with strong spin orbit interaction.However hyperfine interaction has been less studied in bismuth (Bi), a heavy element exhibiting a strong Rashba-like spin-orbit interaction in its two-dimensional surface states due to the broken spatial inversion symmetry. In this dissertation we experimentally explore the carrier spin polarization due to transport under strong spin-orbit interaction and the nuclear polarization resulting from the relatively unexplored hyperfine interaction on Bi(111) films.The carrier and nuclear spin polarizations are expected to dynamically interact, a topic with ramifications to other materials where surface states with noteworthy properties play a role.To achieve this goal, an optimized van der Waals epitaxy growth technique for Bi(111) on mica substrates was developed and used, resulting in flat Bi surfaces with large grain sizes and a layered step height of 0.39±0.015 nm, corresponding to one Bi(111) bilayer height. A comparison between Bi(111) films grown on three different substrates (mica, InSb(111)B, and Si(111)) is discussed, for which scanning electron microscopy and atomic force microscopy are applied to obtain the structural and morphological characteristics on the film surface. Magnetotransport measurements are carried out to extract the transport properties of theBi(111) films. Using the high quality Bi(111) film deposited on mica, we develop quantum magnetotransport techniques as delicate tools to study hyperfine interaction. The approach is based on measuring quantum corrections to the conductivity due to weak antilocalization, which depend on the coherence of the spin state of the carriers. The carrier spin polarization is generated by a strong DC current in the Bi(111) surface states (here called the Edelstein effect), which then induces dynamic nuclear polarization by hyperfine interaction. Quantum transport antilocalization measurements in the Bi(111) thin-films grown on mica indicate a suppression of antilocalization by the in-plane Overhauser field from the nuclear polarization, and allow for the quantification of the Overhauser field, which is shown to depend on both polarization duration and the DC current magnitude. Various delay times between the polarization and the measurement result in an exponential decay of the Overhauser field, driven by relaxation time T1. We observe that in the Bi surface states, the appreciable electron density and strong spin-orbit interaction allow for dynamic nuclear polarization in the absence of an external magnetic field.
- Taking Steps Towards Superfluid-like Spin Transport in Metallic FerromagnetsSmith, David Acoya (Virginia Tech, 2022-05-12)Conventional electronics rely on the transport of electrons through a circuit to carry information. This comes with ever-present Joule heating as a result of the resistive scattering of electrons. Recent works in the field of spintronics have focused on using magnetic excitations (e.g., spin waves) instead of electrons as a means of information transport without Joule heating. However, realizing long distance information transport using conventional spin waves has proven difficult owing to their diffusive nature and the exponential decay of spin current. Theoretical studies have proposed a new form of magnetization dynamics, referred to as superfluid-like spin transport, as a way to overcome this shortfall. Instead of decaying exponentially with distance, the spin current associated with superfluid-like spin transport decays linearly with distance, potentially allowing for information transport beyond the micron-scale. In this dissertation, I discuss the work that I have done towards realizing this novel phenomenon in a metallic, ferromagnetic system. Results on a reduced damping and reduced magnetic moment Fe-based alloy, micromagnetic simulations that use established domain wall physics to explain superfluid-like spin transport, and an investigation of spin torques found in a current-in-plane spin valve structure with broken in-plane symmetry for excitation of superfluid-like spin transport dynamics are discussed. I conclude by discussing what steps remain before superfluid-like spin transport can be measured in an experimental system as well as the impacts this work could have on the wider spintronics field. This work was supported in part by National Science Foundation, Grant No. DMR-2003914.
- Time-Variant Components to Improve Bandwidth and Noise Performance of AntennasLoghmannia, Pedram (Virginia Tech, 2021-01-18)Without noise, a wireless system would be able to transmit and receive signals over an arbitrary long-distance. However, practical wireless systems are not noise-free, leading to a limited communication range. Thus, the design of low-noise devices (such as antennas, amplifiers, and filters) is essential to increase the communication range. Also, it is well known that the noise performance of a receiving radio is primarily determined by the frontend including the antenna, filter, and a low-noise amplifier. In our first design, we intend to reduce the noise level of the receiving system by integrating a parametric amplifier into the slot antenna. The parametric amplifier utilizes nonlinear and/or time-variant properties of reactive elements (capacitors and/or inductors) to amplify radio frequency signals. Also, the parametric amplifier offers superior noise performance due to its reactive nature. We utilize the parametric amplifier to design a low-noise active matching circuit for electrically small antennas in our second design. Using Chu's limit and the Bode-Fano bound, we show a trade-off between the noise and bandwidth of the electrically small antennas. In particular, to make the small antenna wideband, one needs to introduce a mismatch between the antenna and the amplifier. Due to the mismatch, the effect of the low-noise amplifier becomes even more critical and that is why we choose the parametric amplifier as a natural candidate. As a realized design, a loop antenna is configured as a receiver, and the up-converter parametric amplifier is connected to it leading to a low-noise and wideband active matching circuit. The structure is simulated using a hybrid simulation technique and its noise performance is compared to the transistor counterpart. Our simulation and measurement results show more than 20 times bandwidth improvement at the expense of a 2 dB increase in the noise figure compared to the passive antenna counterpart.
- Uncovering Structure-Property Relations in Biomimetic Lipid Membranes with Molecular AdditivesLihiniya Kumarage, Teshani Omanthika (Virginia Tech, 2024-08-15)The lipid bilayer, the fundamental structure of cell membranes, exemplifies a highly adaptable molecular assembly with characteristics that have been fine-tuned through evolution to meet the diverse functional needs of cells. These bilayers must strike a delicate balance: they need to be sufficiently rigid to act as protective barriers, yet fluid enough to facilitate the diffusion of proteins and molecular clusters crucial for various biological processes. Owing to their multifunctional nature, lipid membranes are not only vital in biological contexts but also in numerous practical applications, such as artificial cells, drug-delivery nanocarriers, and biosensors. Both biological and synthetic lipid membranes frequently incorporate molecular or nanoscale additives that modify their properties through a range of mechanisms. Gaining a comprehensive understanding of how lipid membranes interact with these additives is an area of active research, particularly with the advent of advanced high-resolution characterization techniques that reveal both the static and dynamic behaviors of these systems. This dissertation investigates the impact of small molecular additives – specifically natural and synthetic sterols – on the structure, elasticity, and organization of biomimetic lipid membranes. Utilizing advanced scattering techniques and other methods, the research elucidates the intricate interplay between the membrane composition, structure, and elasticity. Key findings demonstrate that, unlike previous observations, cholesterol significantly affects the bending rigidity of lipid membranes regardless of chain unsaturation, when measured on mesoscopic length and time scales. Interestingly, the replacement of cholesterol with engineered molecules, comprised of a sterol unit that is chemically conjugated to one or both of the lipid chains, results in further enhancement in the membrane bending rigidity and mechanical stability, making them a promising additive for advanced liposomal drug delivery systems. Further studies on phase-separating membranes illustrate the effective use of sterol-modified lipids in regulating the formation and size of distinct lipid domains implicated in protein recruitment and biological function. This work advances the current understanding of membrane biophysics and paves the way for novel therapeutic strategies and biomaterial designs.
- Vertically graded Fe-Ni alloys with low damping and a sizable spin-orbit torqueMaizel, Rachel E.; Wu, Shuang; Balakrishnan, Purnima P.; Grutter, Alexander J.; Kinane, Christy J.; Caruana, Andrew J.; Nakarmi, Prabandha; Nepal, Bhuwan; Smith, David A.; Lim, Youngmin; Jones, Julia L.; Thomas, Wyatt C.; Zhao, Jing; Michel, F. Marc; Mewes, Tim; Emori, Satoru (American Physical Society, 2024-10-21)Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to approximately 1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10-nm-thick single-layer ferromagnet to attain both low damping and a sizable SOT. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of αeff<5×10-3 in the Fe-Ni alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable antidamping SOT efficiency of |θAD|≈0.05, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.