Doctor of Philosophy, The Ohio State University, 2021, Physics

Two-dimensional crystals have become a prominent part of condensed-matter research for many years due to the unique adaptability of their interlayer interactions. This ability to isolate and stack individual crystal layers has led to the exploration of low dimensional physics and has spurred the creation of countless van der Waals heterostructure devices. Graphene, a monolayer of graphite, has spearheaded this research and continued to remain at the forefront since the tremendous activity directed towards it following the experiment of 2004 [1]. More recently, graphene has regained the spotlight with the discovery of twisted bilayer graphene, a heterostructure consisting of two overlapping graphene layers with an interlayer twist. As a result of the twist, a geometric interference pattern whose periodicity can be controlled by the relative angular alignment between them is formed. This is known as a moire pattern and it creates a superlattice potential that modifies the electronic structure of the system. For twist angles of about 1.1°, the “magic angle”, flat bands form near zero Fermi energy, resulting in a number of correlated phases including Mott-like insulators, superconductivity, and magnetism. [2-12] Another atomically thin two-dimensional material of interest is black phosphorus (BP), a semiconductor with high electron mobility and a tunable direct band gap. BP is an allotrope of phosphorus that can be obtained by heating up white phosphorus under high pressure. Known as phosphorene when it is mono- or few-layered, BP possesses an anisotropic crystal structure, which translates to anisotropic electronic, optical, and thermal properties [13, 14]. As a result of these particularly interesting features, phosphorene has spurred its own set of studies [15-26] The focus of this dissertation can be divided into two parts. In the first part (Chapters 1-6), it will explore Hall transport in TBG near the magic angle [27]. In the second part (Chapters (open full item for complete abstract)

Committee: Marc Bockrath (Advisor); Chun Ning Lau (Committee Member); Ilya Gruzberg (Committee Member); Richard Furnstahl (Committee Member) Subjects: Materials Science; Physics

MS, University of Cincinnati, 2017, Engineering and Applied Science: Computer Engineering

A series of simulations which utilize a Non-equilibrium Green's function (NEGF) formalism is suggested which can provide indirect evidence of the fine and non-local electrostatic tuning of the onset of spin polarization in two closely spaced quantum point contacts (QPCs) that experience a phenomenon known as lateral spin-orbit coupling (LSOC). Each of the QPCs that create the device also has its own pair of side gates (SGs) which are in-plane with the device channel. Numerical simulations of the conductance of the two closely spaced QPCs or four-gate QPC are carried out for different biasing conditions applied to two leftmost and rightmost SGs. Conductance plots are then calculated as a function of the variable, Vsweep, which is the common sweep voltage applied to the QPC. When Vsweep is only applied to two of the four side gates, the plots show several conductance anomalies, i.e., below G0 = 2e2/h, characterized by intrinsic bistability, i.e., hysteresis loops due to a difference in the conductance curves for forward and reverse common voltage sweep simulations. The appearance of hysteresis loops is attributed to the co-existence of multistable spin textures in the narrow channel of the four-gate QPC. The shape, location, and number of hysteresis loops are very sensitive to the biasing conditions on the four SGs. The shape and size of the conductance anomalies and hysteresis loops are shown to change when the biasing conditions on the leftmost and rightmost SGs are swapped, a rectifying behavior providing an additional indirect evidence for the onset of spontaneous spin polarization in nanoscale devices made of QPCs. The results of the simulations reveal that the occurrence and fine tuning of conductance anomalies in QPC structures are highly sensitive to the non-local action of closely spaced SGs. It is therefore imperative to take into account this proximity effect in the design of all electrical spin valves making use of middle gates to fine tune the spin preces (open full item for complete abstract)

Committee: Marc Cahay Ph.D. (Committee Chair); Rashmi Jha Ph.D. (Committee Member); Punit Boolchand Ph.D. (Committee Member) Subjects: Nanotechnology

MS, University of Cincinnati, 2014, Engineering and Applied Science: Electrical Engineering

Hysteresis has been observed between the forward and reverse sweeps of a common mode bias applied to the two in-plane side gates of asymmetrically biased GaAs quantum point contacts (QPCs) in the presence of lateral spin orbit coupling. The size of the hysteresis loop was found to be increasing with the amount of bias asymmetry ΔVg between the two side gates and dependent on the polarity of ΔVg. Some of the experimental results can be explained using a non-equilibrium Green's function approach to model the conductance through the devices. A detailed numerical study of the onset of intrinsic bistability and accompanying hysteresis in the QPCs was performed. Plots of the conductance versus common gate voltage applied to the two side gates show single or multiple hysteresis loops depending on the QPC dimensions and biasing conditions. The hysteresis only appears for sufficiently long QPCs if the electron-electron interaction is strong enough. The shape of the hysteresis loops depends on the polarity and magnitude of ΔVg and the strength of the electron-electron interaction. The hysteresis loops are affected by the presence of dangling bonds on the sidewalls. The rich plethora of hysteresis loops is intimately related to a wide variety of metastable spin textures and linked to the onset of a net spin polarization in the narrow portion of the QPC for biasing conditions leading to conductance anomalies, i.e., less than 2e2/h.

Committee: Marc Cahay Ph.D. (Committee Chair); Punit Boolchand Ph.D. (Committee Member); Altan Ferendeci Ph.D. (Committee Member) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2014, Engineering and Applied Science: Electrical Engineering

To realize the full potential of spin-based devices, ways must be found to inject, manipulate, and detect the spin of the electron by purely electrical means. Previously, our group has shown that a quantum point contact (QPC) with lateral spin orbit coupling (LSOC) can be used to create a strongly spin-polarized current by purely electrical means. The LSOC results from the lateral in-plane electric field created by the confining potential in QPCs with in-plane side gates (SGs). Strongly spin-polarized currents can be generated by tuning the asymmetric bias voltages on the side gates. A conductance anomaly in the form of a plateau at conductance G=0.5G0 (where G0 =2e2/h) was observed in the ballistic conductance of a QPC based in the absence of magnetic field – which was established to be a signature of complete spin polarization. A Non-Equilibrium Green's Function (NEGF) analysis was used to model a small QPC and three ingredients were found to be essential to generate a strong spin polarization: (1) LSOC, (2) an asymmetric lateral confinement, and (3) a strong electron-electron (e-e) interaction. We have also shown that all-electric control of spin polarization can be achieved for different materials, electron mobility, heterostructure design, QPC dimensions and strength of LSOC.

Committee: Marc Cahay Ph.D. (Committee Chair); Steven T. Herbert Ph.D. (Committee Member); Punit Boolchand Ph.D. (Committee Member); Altan Ferendeci Ph.D. (Committee Member); Peter Kosel Ph.D. (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy (PhD), Ohio University, 2013, Physics and Astronomy (Arts and Sciences)

In this document we present a study of the thermodynamic and transport properties of two kinds of quantum impurity systems in the Kondo regime. The first system consists of a spin-1 molecule in which mechanical stretching along the transport axis produces a magnetic anisotropy. We find that a generic coupling between a vibrational mode along this axis and the molecular spin induces a correction to the magnetic anisotropy, driving the ground state of the system into a non-Fermi-liquid phase. A transition into a Fermi-liquid ground state can then be induced by means of stretching, going through an underscreened spin-1 Kondo ground state at zero effective anisotropy. In the second system we study the effects of a charge detector, implemented by a quantum point-contact (QPC), on the Kondo state of a nearby spin-1/2 quantum dot (QD). While making the charge detection possible, the Coulomb interaction between the electrons traversing the QPC and those within the QD contribute to decoherence of the Kondo state. By modeling the QPC as two metallic terminals connected to an intermediate localized level, we can explore three transport regimes of the detector: a zero-conductance regime, a finite-conductance regime in mixed valence, and unitary conductance in a Kondo ground state that has been suggested as an explanation to the "0.7 anomaly" in QPCs. Transitions between these different ground states can be achieved by tuning the strength of a capacitive coupling that parameterizes the electrostatic interaction.

Committee: Sergio Ulloa Prof. (Advisor); Wojciech Jadwisienczak Ph.D (Committee Member); Saw Hla Prof. (Committee Member); Horacio Castillo Ph.D. (Committee Member); Nancy Sandler Ph.D. (Committee Member) Subjects: Condensed Matter Physics; Low Temperature Physics; Nanoscience; Nanotechnology; Physics; Quantum Physics; Solid State Physics

PhD, University of Cincinnati, 2012, Engineering and Applied Science: Electrical Engineering

This dissertation explores the use of side-gated semiconductor quantum point contacts (QPCs) to generate strongly spin polarized current by purely electrical means. An anomalous conductance plateau (at conductance value, G = 0.5x(2e2/h)) was observed [P. Debray et al., Nature Nanotechnology 4, 759 (2009)] in an asymmetrically-biased, InAs quantum point contact with in-plane side gates, at 4.2 Kelvin in the ballistic transport regime. This was a clear experimental signature of spontaneous spin polarization in the narrow channel of the QPC. A non-equilibrium Green's function (NEGF) simulation revealed that three ingredients are necessary to create the spin polarization: an asymmetric lateral confinement, a lateral spin-orbit coupling (LSOC) induced by the lateral confining potential of the QPC, and a strong electron-electron interaction. In order to use such QPC devices to make all-electric spin valves, it is necessary to probe the sensitivity of these three ingredients to the bias difference between the two side gates that confine the electrons in the narrow channel. Towards that goal, this thesis shows experimentally that the anomalous conductance plateaus are quite robust and remain over a wide range of asymmetric bias voltages. In addition, a very systematic study is conducted to understand the appearance of conductance anomalies which range from 0.4 to 0.7x(2e2/h) depending on the biasing conditions. These results are interpreted as evidence for the sensitivity of the QPC spin polarization to the defects (surface roughness and impurity (dangling bonds) scattering) generated during the etching process that defines the QPC side walls or gates. This assertion is supported by NEGF simulations of the conductance of a QPC in presence of dangling bonds on its walls. We also show that a spin polarization over 90% can be achieved despite the presence of these defects. NEGF simulations show that the maximum spin polarization is not necessarily reached where the conductanc (open full item for complete abstract)

Committee: Marc Cahay PhD (Committee Chair) Subjects: Physics

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

This dissertation reports the first experimental observation of spontaneous spin polarization due to lateral spin orbit coupling (LSOC) in side- gated (SG) quantum point contacts (QPCs). The QPC devices are fabricated on InAs/InGaAs quantum well structures using e-beam lithography. The low band gap InAs semiconductor was chosen because of its large intrinsic spin-orbit coupling. The side gates are realized by wet etching technique which is optimized to pattern the QPC devices. The width of the QPC is varied from 200 nm to 500 nm, while the length of the QPC is kept in the range 150-200 nm. The gradient in the lateral potential confinement in a side gated (SG) quantum point contact (QPC) causes a spin-orbit coupling (SOC). This LSOC induces a spontaneous spin polarization of opposite nature at the two edges of the QPC in the absence of any applied magnetic field. We have observed an anomalous conductance plateau at G @ 0.5 (2e2/h) (0.5 structure) in the SG QPCs fabricated on InAs/InGaAs QW structures. The 0.5 structure moves up in perpendicular magnetic field and approaches the normal conduction quantization at G = (2e2/h) in high magnetic field, whereas in-plane magnetic field has no effect on it. The evolution in magnetic field clearly indicates LSOC is responsible for the 0.5 structure. We believe it is the asymmetry in the confining potential of the QPC that leads to a net spin polarization giving the 0.5 structure. By electrically modulating the asymmetry of the QPC confinement, we have succeeded in making this structure appear and disappear. Such a QPC can conceivably be used as a spin polarizer or detector on demand by tuning the gate voltages. We also have proposed a dual-QPC device to experimentally validate the spin polarization by electrical means.

Committee: Philippe Debray (Advisor) Subjects: Physics, Condensed Matter