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The potential of interface engineering to enhance electronic properties is exemplified in a single atomic layer of FeSe grown on SrTiO$_3$, which exhibits an order-of-magnitude increase in its superconducting transition temperature ($T_c$ up to 110 K) compared to bulk ($T_c$ = 8 K). Since this discovery in 2012, efforts to reproduce, understand, and extend this finding continue to draw both excitement and scrutiny. In this thesis, we report the use of a combined molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM) system to grow and image films of superconducting FeSe/SrTiO$_3$. In particular, we investigate and harness atomic defects in as-grown films to derive microscopic insights in two directions. First, we image quasiparticle interference (QPI) patterns generated around defects in order to reconstruct the electronic structure of the unperturbed system, and uncover pieces of the puzzle of high-$T_c$ superconductivity in a monolayer of FeSe. Second, we characterize the atomic structure of defects using density functional theory (DFT), with possible implications on film quality and nanostructuring., Physics

Charge carriers in graphene mimic two-dimensional massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range. Interband transitions in graphene give rise to pronounced optical absorption in the mid-infrared to visible spectral range, where the optical conductivity is close to a universal value $\sigma_0 = \pi e^2/2h$. Free-carrier intraband transitions, on the other hand, cause low-frequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density. These properties together suggest a rich variety of possible optoelectronic applications for graphene. In this thesis, we investigate the optoelectronic properties of graphene by measuring transient photoconductivity with optical pump-terahertz probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations are accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Our measurements also reveal the non-monotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions., Physics

In this dissertation, I study the physical behavior of nanoscale magnetic materials and build spin-based transistors that encode information in magnetic domain walls. It can be argued that energy dissipation is the most serious problem in modern electronics, and one that has been resistant to a breakthrough. Wasted heat during computing both wastes energy and hinders further technology scaling. This is an opportunity for physicists and engineers to come up with creative solutions for more energy-efficient computing. I present the device we have designed, called domain wall logic (DW-Logic). Information is stored in the position of a magnetic domain wall in a ferromagnetic wire and read out using a magnetic tunnel junction. This hybrid design uses electrical current as the input and output, keeping the device compatible with charge- based transistors. I build an iterative model to predict both the micromagnetic and circuit behavior of DW- Logic, showing a single device can operate as a universal gate. The model shows we can build complex circuits including an 18-gate Full Adder, and allows us to predict the device switching energy compared to complementary metal-oxide semiconductor (CMOS) transistors. Comparing 15 nm feature nodes, I find DW-Logic made with perpendicular magnetic anisotropy materials, and utilizing both spin torque transfer and the Spin Hall effect, could operate with 1000× reduced switching energy compared to CMOS. I fabricate DW-Logic device prototypes and show in experiment they can act as AND and NAND gates. I demonstrate that one device can drive two subsequent devices, showing gain, which is a necessary requirement for fanout. I also build a clocked ring oscillator circuit to demonstrate successful bit propagation in a DW-Logic circuit and show that properly scaled devices can have improved operation. Through building the devices, I develop a novel fabrication method for patterning sub-25 nm magnetic wires with very low (~ 2 nm) average edge roughness. I apply the fabrication method to measuring the Spin Hall angle in epitaxially grown thin films and to studying the repeatability of domain wall motion in narrow wires. I also present a number of modeling results, including the effect of edge roughness on both magnetic tunnel junctions and domain walls., Physics

The discovery of high-\(T_c\) superconductivity in 1986 generated tremendous excitement. However, despite over 25 years of intense research efforts, many properties of these complex materials are still poorly understood. For example, the cuprate phase diagram is dominated by a mysterious "pseudogap" state, a depletion in the Fermi level density of states which persists above the superconducting critical temperature \(T_c\). Furthermore, these materials are typically electronically inhomogeneous at the atomic scale, but to what extent the intrinsic chemical or structural disorder is responsible for electronic inhomogeneity, and whether the inhomogeneity is relevant to pseudogap or superconductivity, are unresolved questions. In this thesis, I will describe scanning tunneling microscopy experiments which probe the interplay of structural, chemical and electronic disorder in high-\(T_c\) superconductors. First, I will present the imaging of a picoscale orthorhombic structural distortion in Bi-based cuprates. Based on insensitivity of this structural distortion to temperature, magnetic field, and doping level we conclude that it is an omnipresent background not related to the pseudogap state. I will also present the discovery of three types of oxygen disorder in the high-\(T_c\) superconductor \(Bi_2Sr_2CaCu_2O_{8+x}\) two different interstitials as well as vacancies at the apical oxygen site. We find a strong correlation between the positions of these defects and the nanoscale inhomogeneity in the pseudogap phase, which highlights the importance of chemical disorder in these compounds. Furthermore, I will show the determination of the exact intra-unit-cell positions of these dopants and the effect of different types of intrinsic strain on their placement. I will also describe the identification of chemical disorder in another cuprate \(Y_{1−x}Ca_xBa_2Cu_3O_{7−x}\), and the first observation of electronic inhomogeneity of the spectral gap in this material. Finally, I will present definitive identification of the cleavage surfaces in \(Pr_xCa_{1−x}Fe_2As_2\), and imaging of Pr dopants which exhibit lack of clustering, thus ruling out Pr inhomogeneity as the likely source of the high-\(T_c\) volume fraction. To achieve the aforementioned results, we employ novel analytical and experimental tools such as an average supercell algorithm, high-bias dI/dV dopant mapping, and local barrier height mapping., Physics

Nanoscale imaging of materials through phase transitions can provide valuable insight into the local nature of the transition and the emergence of order. The scanning force microscope used in the studies presented here is an ideal instrument to investigate phase transitions with nanoscale spatial resolution. We study phase transitions in two different systems by operating in different modes: contact mode, in which we measure the local electronic properties of the sample; and non-contact mode, in which we probe the sample by monitoring the interaction between the sample and cantilever. We increased the versatility of this microscope by developing a method to control the quality factor Q of a conducting cantilever via capacitive coupling to the local environment. We show that Q may be reversibly tuned over a range of a factor of 260. We describe the underlying physics with a point-mass oscillator model. Tuning Q can enhance force-gradient sensitivity or scan speed, which we demonstrate with topographic scans of a \((VO_2)\) acquired in high vacuum. Scanning in contact mode with a conductive cantilever, we study local electronic properties of a vanadium dioxide \((VO_2)\) thin film through an insulator to metal transition. At each point in the scan, we sweep the voltage applied to the sample, obtaining current versus voltage sweeps with nanoscale resolution while inducing the insulator to metal transition. In some \((VO_2)\) grains, we see two electronic transitions, consistent with a locally stable intermediate insulating phase. We find large insulating state resistances and transition voltages at grain boundaries, underscoring the importance of Joule heating in triggering the transition in this type of measurement. Finally, we evaluate the conduction mechanism in the insulating regime, allowing the local determination of permittivity and temperature. We scan in non-contact mode with a magnetic tip to investigate the spin reorientation transition in single-crystal \(Nd_2Fe_{14}B\). This ferromagnetic system undergoes the spin reorientation transition near 135 K. We achieve nanoscale magnetic resolution at both room temperature and at a variety of temperatures around the phase transition. We demonstrate the ability to resolve the magnetic domain structure and monitor its evolution through the phase transition., Physics