Fossil fuel usage causing rising CO2 levels and leading to climate change is, perhaps, the most pressing issue of our time. However, our economic dependence on energy necessitates its usage such that reducing energy usage is not possible leaving transitioning to renewable energy technologies as the only sustainable option. Currently, the largest barrier to large scale incorporation of renewable energy sources (e.g., solar, wind) is the high cost of energy storage technologies. Electrochemical energy storage technologies (e.g., lithium-ion batteries and supercapacitors) have been identified as a key approach for enabling the transition to renewable energy technologies.
Graphene is a material with exceptional properties that is receiving much attention for application in various energy storage technologies and could help reduce the cost of energy storage technologies. This thesis describes a novel fabrication procedure for low-cost and efficient synthesis of high-quality graphene nanostripes (GNSPs) and their application in lithium-ion battery and supercapacitor electrodes.
This thesis is structured as follows. Chapter 1 outlines the motivation and technical background of this research. Chapter 2 describes the instrumentation and procedures for fabricating GNSPs. Chapter 3 describes in situ exfoliation of GNSPs as electrodes in supercapacitors to increase the capacitance. Chapter 4 describes synthesis and application of pyridinic-type nitrogen-doped GNSPs as a lithium-ion battery anode. Chapter 5 describes the synthesis and application of silicon-, germanium-, and tin-doped GNSPs and their application in lithium-ion battery anodes. Chapter 6 concludes and synthesizes the findings of the thesis holistically. Additionally, future outlook and potential research objectives are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/bgw9-d234, author = {Chen, Chien-Chang}, title = {Investigation of the Physical Properties of Dirac Materials}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/bgw9-d234}, url = {https://resolver.caltech.edu/CaltechTHESIS:07092020-003626793}, abstract = {This thesis focuses on the investigation of two types of Dirac materials: topological insulators (TI) and graphene. Both materials have received much attention and stimulated intense research activities over the last decade. Although massless Dirac electron are wonderful, there will be more industrial applications if we can open the gap and make Dirac electrons massive. For topological insulators, we focus on studies of the TI/Magnetic TI (MTI) bilayer structures to induce a gap on the surface state. For graphene, the author focuses on the Moiré pattern and interlayer interaction.
For bilayer TI/MTI samples, they were investigated with scanning tunneling microscopy and spectroscopy (STM/STS), and with electrical transport measurements by means of a Physical Property Measurement System (PPMS). Details of the experimental setups for this research and their upgrades were described. For the current STM system, both the tube scanner and sample stage in the STM head had been redesigned and rebuilt, which led to better XYZ fine approach control, improved wire protection, and enhanced noise shielding. A new back gate capability was added to the sample stage. A customized commercial STM system has been commissioned, which is expected to provide a better sample holder with improved vacuum seals and easier temperature control, as well as more convenient approaches to loading samples and switching STM or AFM (atomic force microscope) tips. For PPMS, an optical probe had been designed and constructed, which enabled light-induced effects on the electrical transport properties of TIs. A new custom-made glove box has been installed, which provides a computer-controlled and self-circling gas environment to minimize the concentration of air while reduces the waste of argon. The glove box is also easy to use. This upgrade helps expand our abilities to conduct research more efficiently.
STM/STS studies of both the binary and ternary types of magnetic topological insulators (MTIs) are presented. For both binary and ternary bilayer TI/MTI systems, the majority of the density of states (DOS) spectra evolved with the temperature. At room temperature, all samples showed massless Dirac spectra. However, for temperatures below 200 K, all bilayer samples with the top pure TI layer thinner than 5QL revealed opening of a surface gap. Generally, binary TI/MTI samples exhibited smaller gapped domains, which was consistent with the finding of nearly negligible hysteretic behavior for Hall resistance vs, magnetic field sweeps at low temperatures. In contrast, ternary TI/MTI samples exhibited larger gapped domains, which implied longer range ferromagnetic order and was indeed corroborated by the apparent hysteretic behavior in the electrical transport measurements at low temperatures. Additionally, the application of c-axis magnetic fields led to slighter larger surface gaps and more uniform gap distributions, which further confirmed the physical origin of the surface gap as magnetic in nature. Besides the U or V-shaped DOS spectra, double-peak or single peak impurity resonances were also observed. These spatially localized minority spectra were found to mostly appear along the boundaries of gapped and gapless domains. Moreover, the number of impurities was founded to reach a maximum around 240 K, which corresponded to the onset temperature of localized surface gaps.
Detailed studies of the electrical transport properties of both the binary and ternary MTIs by the PPMS provided a comparison between the macroscopic information thus obtained with the microscopic information derived from STS studies. Binary TI/MTI showed an anonymous Hall effect (AHE) at 25 K while ternary TI/MTI showed AHE around 20 K. Binary TI/MTI systems exhibited weak localization (WL) behavior in the longitudinal resistance vs. magnetic field data at 2 K. The binary TI/MTI samples with a thinner top pure TI layer revealed sharper and stronger WL behavior. In contrast, for the 3QL-TI/6QL-MTI ternary sample, weak antilocalization (WAL) behavior was present for all temperatures, while WL also showed up below 13 K. The Hall resistance vs. magnetic field data for all samples of ternary TI/MTI bilayers and ternary MTI monolayer samples revealed strong hysteresis at low temperatures, in contrast to the negligible hysteretic behavior in all binary TI/MTI samples. Finally, circularly polarized light was found to enhance the AHE of the bilayer ternary TI/MTI sample while weakening that of the monolayer ternary MTI. These experimental phenomena may be mostly attributed to the different band structures and Fermi levels among the binary and ternary TI/MTI samples. In particular, we note that the observation of quantum anomalous Hall effect (QAHE) only in ternary MTI monolayers at extremely low temperatures (at T ≤ 30 mK < < Tcbulk ~ 30 K) may be attributed to the finite contributions of bulk carriers to excess conduction unless T → 0.
Simulations have been carried out to account for the Moiré patterns of graphene on Cu (111), graphene on Cu (100), twisted bilayer graphene, and Cr-doped topological insulators. The physical origin for empirically observed structural superlubricity between graphene layers has also been modeled by simulations based on the density functional theory (DFT).
Finally, the key findings of this thesis work and the suggested future research directions are summarized.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/y5a2-zx57, author = {Kishore Kumar, Deepan}, title = {Novel Light-Matter Interaction in Quasi-One-Dimensional Graphene Nanomaterials for Photonics}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/y5a2-zx57}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282021-182147719}, abstract = {
Nonlinear light-matter interaction in two-dimensional (2D) materials like graphene with unique nanostructured quasi-one-dimensionality (quasi-1D) holds the potential to address major technology opportunities in photonics from on-chip photo detection, modulation of light, and even possibly coherent light sources. In this work, we propose to use graphene, a gapless two-dimensional nanomaterial, for both nano-photonic applications and potentially energy harvesting by nano-structuring the material into nearly quasi-one-dimensional effective optical cavities with defects that act like color centers. These defects are naturally formed during its synthesis or can be engineered in the material by selective plasma radiation, is found to support a broad spectral distribution of color centers that exhibit excitation dependent photoluminescence. Through detailed investigation on the temperature and power dependence of photoluminescence from such defects, excitation dependent photoluminescence emission, we have established that these graphene nanomaterials with metastable energy states can support material excitations (e.g., excitons) that are strongly coupled to the optical modes confined within the nanostructured cavities to produce polaritonic quasiparticles, leading to many interesting nonlinear behaviors. In particular, the manifestation of blue-shifted photoluminescence, polariton lasing-like emission, multimode lasing-like emission, and distinct interference fringes, all points to the presence of novel light-matter interaction in quasi-one-dimensional graphene. Such novel light matter interactions can be exploited, among other applications, within photonic integrated circuits (PIC) by directly synthesizing graphene on silicon from a low temperature, single-step, plasma-enhanced chemical vapor deposition (PECVD) with feedstock gases of methane and hydrogen.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/vh7k-4w84, author = {Lin, Wei-Hsiang}, title = {Synthesis of 2D Quantum Materials for Nanoelectronic and Nanophotonic Applications}, school = {California Institute of Technology}, year = {2021}, doi = {10.7907/vh7k-4w84}, url = {https://resolver.caltech.edu/CaltechTHESIS:04262021-085536699}, abstract = {2D materials have attracted tremendous attention for a variety of properties such as ultra-low body thickness, ultra-high mobility, and tunable bandgap. These unique merits of the 2D materials bring in the significant improvements and new perspectives in the digital CMOS scaling, analog performance, as well as the 3D integration of wafer stacking.
In this thesis, we explore van der Waals materials for future CMOS technologies. Chapter 2 introduces a compatible and a single-step method for synthesizing high-mobility monolayer graphene (MLG) in merely a few minutes by means of plasma-enhanced chemical vapor deposition (PECVD) techniques without the need of active heating. This environment enables graphene growth on different surfaces at relatively low temperatures, which paves ways to a CMOS-compatible approach to graphene synthesis. Chapter 3 describes the development of a synthesis method that controls the growth of large-area h-BN films from monolayer to 30 atomic layers, and summarizes the characterizations of the properties of these h-BN films that demonstrate the high-quality of these materials.
New degrees of freedom possess the immense potential and attract huge attentions as the imminent end of “Moore’s Law”. Compared with the traditional charge degree of freedom, spin and valley are the other two additional internal degree of freedom in solid-state electronics which enable the spintronic and valleytronic devices with high integration density, fast processing speed, low power dissipation, and non-volatility. Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are semiconductors promising for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. In chapter 4, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS₂ single crystals. By illuminating WS₂ with off-resonance circularly-polarized light and measuring the resulting spatially resolved circularly-polarized emission (Pcirc), we find large circular polarization increases significantly to nearly 90% at 80 K. In Chapter 5, it is reported that valley polarized PL of monolayer WS₂ can be efficiently tailored at room temperature (RT) through the surface plasmon-exciton interaction with plasmonic Archimedes spiral (PAS) nanostructures. The DVP of WS₂ using 2 turns (2T) and 4 turns (4T) of PAS can reach up to 40% and 50% at RT, respectively. Further enhancement and continuous control of excitonic valley polarization in electrostatically doped monolayer WS₂ are demonstrated. Under the circularly polarized light on WS₂-2TPAS heterostructure, 40% valley polarization of exciton without electrostatic doping is icreased to 70% by modulating the carrier doping via a backgate. This enhancement of valley polarization may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The demonstration of electrical tunability in the valley-polarized emission from WS₂-PAS heterostructures provides new strategies to harness valley excitons for application in ultrathin valleytronic devices.
In contrast to future optical switch applications, in Chpater 6, it is reported that Ternary tellurides based on alloying different 2D transition metal dichalcogenides can result in interesting new 2D materials with tunable optical and electrical properties. Additionally, such alloys can provide opportunities for significantly improving the electrical contact properties at the metal-semiconductor interface. In particular, realization of practical devices based on the 2D materials will require overcoming the typical Fermi-level pinning limitations of the electrical contacts at the metal-semiconductor interface and ultimately approaching the ideal Schottky-Mott limit. In this work, we develop a simple method of stacking 3D/2D electrical metal contacts onto dangling-bond-free 2D semiconductors in order to surmount the typical issue of Fermi-level pinning. Specifically, contacts of Au, graphene/Au, and WTe₂/Au are transferred onto WS1.94Te0.06 alloy-based devices via a new transfer method. The WS1.94Te0.06 field-effect transistors (FETs) with WTe₂/Au contacts reveal a field-effect mobility of 25 cm²V⁻¹s⁻¹, an on/off current ratio of 10⁶, and extremely low contact resistance of 8 kΩ μm. These electrical properties are far more superior to similar devices with either Au or graphene/Au contacts, which may be attributed to the fact that the work function of WTe₂ is close to the band edge of the WS1.94Te0.06 alloy so that the resulting metal-semiconductor interface of the FETs are free from Fermi-level pinning. The Schottky barrier heights of the WS1.94Te0.06-FETs with WTe₂/Au contacts also follow the general trend of the Schottky-Mott limit, implying high-quality electrical contacts. Finally, in Chapter 7, several promising opportunities were proposed for future CMOS integrated circuits based on monolayer semiconductors.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert}, } @phdthesis{10.7907/6T02-4X35, author = {Hsu, Chen-Chih}, title = {Physics and Applications of Graphene-Based Nanostructures and Nano-Meta Materials}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/6T02-4X35}, url = {https://resolver.caltech.edu/CaltechTHESIS:03172020-153749505}, abstract = {Graphene, a single layer of carbon atoms forming a honeycomb lattice structure, has been considered a wonder material for both scientific research and technological applications. Structural distortions in nano-materials can induce dramatic changes in their electronic properties. In particular, strained graphene can result in both charging effects and pseudo-magnetic fields, so that controlled strain on a perfect graphene lattice can be tailored to yield desirable electronic properties.
In the first part of this thesis (Chapter 2 to 5), we explore a new approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields, global valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene. We have also observed pseudo-magnetic field-induced quantum oscillations and valley Hall signals, including quantum valley Hall effect, by transport measurements at 1.8K.
The second part of this thesis focuses on the development and applications of other graphene-based nanostructures. We report PECVD techniques for the synthesis of various graphene and graphene-based nanostructures, including horizontal growth of graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, and direct and selective deposition of multi-layer graphene on nanostructured substrates. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting, and insulating materials.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/M8FW-S641, author = {Teague, Marcus Lawrence}, title = {Scanning Tunneling Spectroscopic Studies on High-Temperature Superconductors and Dirac Materials}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/M8FW-S641}, url = {https://resolver.caltech.edu/CaltechTHESIS:05142013-151159910}, abstract = {This thesis details the investigations of the unconventional low-energy quasiparticle excitations in electron-type cuprate superconductors and electron-type ferrous superconductors as well as the electronic properties of Dirac fermions in graphene and three-dimensional strong topological insulators through experimental studies using spatially resolved scanning tunneling spectroscopy (STS) experiments.
Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type cuprate La0.1Sr0.9CuO2 (La-112) TC = 43 K, are investigated experimentally. For temperature (T) less than the superconducting transition temperature (TC), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T < TC, first ever observation of vortices in La-112 are reported. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of VPG = 8.5 ± 0.6 meV, while the inter-vortex quasiparticle spectra shows larger peak-to-peak gap values characterized by Δpk-pk(H) >VPG, and Δpk-pk (0)=12.2 ± 0.8 meV > Δpk-pk (H > 0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the VPG energy scale.
Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type “122” iron-based Ba(Fe1-xCox)2As2 are investigated for multiple doping levels (x = 0.06, 0.08, 0.12 with TC= 14 K, 24 K, and 20 K). For all doping levels and the T < TC, two-gap superconductivity is observed. Both superconducting gaps decrease monotonically in size with increasing temperature and disappear for temperatures above the superconducting transition temperature, TC. Magnetic resonant modes that follow the temperature dependence of the superconducting gaps have been identified in the tunneling quasiparticle spectra. Together with quasiparticle interference (QPI) analysis and magnetic field studies, this provides strong evidence for two-gap sign-changing s-wave superconductivity.
Additionally spatial scanning tunneling spectroscopic studies are performed on mechanically exfoliated graphene and chemical vapor deposition grown graphene. In all cases lattice strain exerts a strong influence on the electronic properties of the sample. In particular topological defects give rise to pseudomagnetic fields (B ~ 50 Tesla) and charging effects resulting in quantized conductance peaks associated with the integer and fractional Quantum Hall States.
Finally, spectroscopic studies on the 3D-STI, Bi2Se3 found evidence of impurity resonance in the surface state. The impurities are in the unitary limit and the spectral resonances are localized spatially to within ~ 0.2 nm of the impurity. The spectral weight of the impurity resonance diverges as the Fermi energy approaches the Dirac point and the rapid recovery of the surface state suggests robust topological protection against perturbations that preserve time reversal symmetry.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/TX09-6W39, author = {Hughes, Cameron Richard}, title = {Investigations of Nanoscale Variations in Spin and Charge Transport in Manganites and Organic Semiconductors Using Spin Polarized Scanning Tunneling Spectroscopy}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/TX09-6W39}, url = {https://resolver.caltech.edu/CaltechTHESIS:02082010-153048901}, abstract = {
Spintronics is a new class of spin-dependent electronics with great potential for nonvolatile memory and logic technology. Additionally, spintronics may be combined with optoelectronic applications to achieve higher efficiency and novel capabilities. All of these developments require growth and characterization of new materials to polarize and transport electron spin currents. In this context, spin-polarized and non spin-polarized spatially resolved conductance measurements performed by scanning tunneling microscopy (STM) are effective means to investigate the spin and charge quantum transport in magnetic and organic systems, particularly for systems that are prone to phase separations and complex magnetic properties, such as the colossal magnetoresistive (CMR) manganites La1-xCaxMnO3 (LCMO) that are known to exhibit intrinsic electronic heterogeneity due to strong electronic correlation and competing orders in the ground state. Additionally, STM measurements can provide direct information about the band structure and mobility of the organic semiconductor 8-hydroxyquinoline aluminum (Alq3) in the Alq3/LCMO heterostructures to further understand their performance in spintronic devices.
The manganite compound La1-xCaxMnO3 (LCMO) with a bulk doping level x = 0.3 is a ferromagnetic metal with a relatively high Curie temperature Tc = 270K. This system is promising for spintronic device applications, and may be used as a spin current injector because of the gapped band structure for minority spins, a property known as half-metallicity. On the other hand, even in this bulk ferromagnetic metallic phase, inherent electronic inhomogeneity at microscopic scales is expected. To further study this effect, we have investigated x = 0.3 LCMO thin films using scanning tunneling microscopy in spectroscopic mode under varied temperature, magnetic field and spin polarization of the tunneling current. Spatially resolved maps of tunneling conductance taken with non polarized Pt/Ir tip show variations on the scale of a few hundred nanometers in size in the bulk ferromagnetic state, which are believed to be the result of intrinsic inhomogeneity of the manganites due to their tendency toward phase separation. Maps of tunneling conductance taken with spin-polarized Cr coated tips are consistent with the convolution of the LCMO and Cr density of states, and below the Tc of LCMO the spin-polarized tunnel junction can be described as a spin valve configuration. The electronic homogeneity in the material increases above the magnetic ordering temperature, or with application of magnetic field in the bulk ferromagnetic state. We identified gaps in the conductance at two separate characteristic energies. The first gap of energy approximately 0.6 eV is believed to arise from a ferromagnetic insulator (FI) surface phase due to its disappearance above the Curie temperature (Tc) and the dependence of gap energy on relative tip and sample magnetic orientation. The surface phase may be stabilized by Ca deficiency at the LCMO surface, corroborated by x-ray photoemission spectroscopy (XPS). Second, we observe a nearly temperature independent and spatially varying gap of approximately 0.4 eV for all zero-field tunneling spectra, which is believed to be associated with the psuedogap (PG) phenomena in the manganites. Application of a magnetic field converts the regions of PG phenomena to FI, in conjunction with an increase in the homogeneity of the lm conductance. These findings suggest that the PG phenomena arise from electronic inhomogeneity in the manganite film, in agreement with theoretical investigations, and that the vertical and lateral electronic inhomogeneity, along with its dependence on temperature and applied magnetic field, has important implications for use of these materials in high-density nanoscale spintronic devices.
We have also successfully deposited and investigated Alq3/LCMO heterostructures of varying thicknesses to investigate charge transport in Alq3. Bulk Alq3 structural properties are preserved down to 10 nm in thickness with a -0.3 eV offset in band energies. The lack of band bending between LCMO and Alq3 is suggestive of a shift in the preferred isomer from meridinial to facial at the interface. The absence of polaron states from our STM studies implies the relative unimportance of polarons in Alq3 for this heterostructure. In addition, the measured mobilities on the order of 10-5cm2(Vs)-1 for electrons and holes in Alq3 lms deposited on heated LCMO substrates more closely resemble values of the intrinsic mobility estimated from the muon spin relaxation measurements than those from studies of the bulk LED structures, suggesting that superior film conductivity close to the fundamental limit is possible with a heated substrate during sublimation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/MM6C-AS16, author = {Beyer, Andrew David}, title = {Studies of the Low-Energy Quasiparticle Excitations in High-Temperature Superconducting Cuprates with Scanning Tunneling Spectroscopy and Magnetization Measurements}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/MM6C-AS16}, url = {https://resolver.caltech.edu/CaltechETD:etd-06082009-200539}, abstract = {
This thesis details the investigation of the unconventional low-energy quasiparticle excitations in both hole-and electron-type cuprate superconductors through experimental studies and theoretical modeling. The experimental studies include spatially resolved scanning tunneling spectroscopy (STS) experiments and bulk magnetization measurements, and the theoretical modeling involves developing a phenomenology that incorporates coexisting competing orders and superconductivity in the ground state of the cuprates.
Magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the electron-type cuprate La0.1Sr0.9CuO2 (La-112), the simplest structured cuprate superconductor with TC = 43 K, are investigated experimentally for the first time. For temperature (T) less than the superconducting transition temperature (TC), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T << TC, vortices are observed in La-112, which is the first direct observation of vortices among electron-type cuprate superconductors. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of VPG=(8.5±0.6)meV, while the inter-vortex quasiparticle spectra show larger peak-to-peak gap values characterized by Δpk-pk(H) ≥ VPG, and Δpk-pk(0)=(12.2±0.8)meV ≥Δpk-pk(H>0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the VPG energy scale. This finding is in stark contrast to the vortex-state quasiparticle spectra in conventional superconductors, where the intra-vortex spectra near vortex cores exhibit a sharp zero-bias conductance peak due to the complete suppression of superconductivity and the presence of continuous bound quasiparticle states. The lack of a zero-bias peak and the observation of pseudogap-like spectra in the intra-vortex quasiparticle spectra of La-112 suggest that superconductivity alone cannot describe the STS results.
Similar studies of the magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the hole-type cuprate YBa2Cu3O7-δ (Y-123) have also been carried out. The quasiparticle spectra for T << TC(~93 K) show satellite features at an energy higher than the superconducting gap, and the superconducting gap is found to be associated with a set of coherence peaks for H = 0. The coherence peaks are homogeneous, with a energy gap given by ΔSC=(20±1)meV, and may be attributed to superconductivity. The satellite features are less homogeneous, with a effective gap energy Δeff=(37.8±2.0)meV. The application of magnetic fields reveal vortices in Y-123, and the intra-vortex quasiparticle spectra show two energy gaps, with one gap at the pseudogap energy scale VPG~32meV and the other gap at the subgap energy scale Δ’ ~ 7-12meV < ΔSC. In contrast, the inter-vortex quasiparticle spectra reveal only one energy gap at ΔSC~20meV. A dramatic shift in the peak-to-peak gaps, Δpk-pk(H), from ΔSC to both VPG and Δ’ with increasing magnetic field is observed. In addition, higher spatial resolution STS measurements were performed in Y-123 to investigate the spatial dependence of the quasiparticle spectra in more detail. The experimental resolution allowed Fourier-transformed local density of states analysis to be performed. Energy-dependent dispersive diffraction modes attributable to quasiparticle scattering interferences (QPI) were seen, as well as three energy-independent modes not due to QPI. The energy-independent modes corresponded to periodic real-space conductance modulations along the Cu-O bonding and the nodal directions attributable to a pair-density wave, a charge-density wave, and a spin-density wave. The totality of data in Y-123 suggests that the ground state of Y-123 contains competing orders coexisting with superconductivity and not superconductivity alone.
In addition to the STS experiments, the effects of unconventional quasiparticle excitations on macroscopic superconductivity and vortex phase diagrams are investigated from bulk magnetization measurements on several different families of superconducting cuprate samples. Evidence for strong field-induced quantum phase fluctuations and quantum criticality are observed in the vortex phase diagrams of all samples considered. The origin of the apparent quantum criticality and strong field-induced quantum phase fluctuations due to the nearby presence of competing orders is discussed.
Finally, a “two-gap” phenomenological model, describing the excitations from a ground state of coexisting superconductivity and a competing order, is used to quantitatively model the unconventional quasiparticle excitations observed in the measurements of the local tunneling density of states and the angle-resolved photoemission spectroscopy (ARPES) experiments. The phenomenological model is found to provide consistent accounts for the quasiparticle tunneling data from our measurements in La-112 and Y-123, as well as experimental data by others on different cuprates.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/YGPG-W942, author = {Corcovilos, Theodore Allen}, title = {Fluid Phase Thermodynamics: I) Nucleate Pool Boiling of Oxygen under Magnetically Enhanced Gravity and II) Superconducting Cavity Resonators for High-Stability Frequency References and Precision Density Measurements of Helium-4 Gas}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/YGPG-W942}, url = {https://resolver.caltech.edu/CaltechETD:etd-07172007-132955}, abstract = {
Although fluids are typically the first systems studied in undergraduate thermodynamics classes, we still have only a rudimentary phenomenological understanding of these systems outside of the classical and equilibrium regimes. Two experiments will be presented. First, we present progress on precise measurements of helium-4 gas at low temperatures (1 K-5 K). We study helium because at low densities it is an approximately ideal gas but at high densities the thermodynamic properties can be predicted by numerical solutions of Schroedinger’s equation. By utilizing the high resolution and stability in frequency of a superconducting microwave cavity resonator we can measure the dielectric constant of helium-4 to parts in 109, corresponding to an equivalent resolution in density. These data will be used to calculate the virial coefficients of the helium gas so that we may compare with numerical predictions from the literature. Additionally, our data may allow us to measure Boltzmann’s constant to parts in 108, a factor of 100 improvement over previous measurements. This work contains a description of the nearly-completed apparatus and the methods of operation and data analysis for this experiment. Data will be taken by future researchers.
The second experiment discussed is a study of nucleate pool boiling. To date, no adequate quantitative model exists of this everyday phenomenon. In our experiment, we vary one parameter inaccessible to most researchers, gravity, by applying a magnetic force to our test fluid, oxygen. Using this technique, we may apply effective gravities of 0-80 times Earth’s gravitational acceleration (g). In this work we present heat transfer data for the boiling of oxygen at one atmosphere ambient pressure for effective gravity values between 1g and 16g . Our data describe two relationships between applied heat flux and temperature differential: at low heat flux the system obeys a power law and at high heat flux the behavior is linear. We find that the transition heat flux between these two regimes scales as the 4th root of the gravitational acceleration, which may indicate a relationship to the critical heat flux. Additionally, we find that the low heat flux power law exponent is independent of gravity and the power law scale coefficient increases linearly with gravity.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/11FP-3M88, author = {Chen, Ching-Tzu}, title = {Scanning Tunneling Spectroscopy Studies of High-Temperature Cuprate Superconductors}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/11FP-3M88}, url = {https://resolver.caltech.edu/CaltechETD:etd-05222006-124257}, abstract = {
This thesis presents the scanning tunneling spectroscopic studies of the non-universal electronic properties among electron- and hole-doped cuprates. Tunneling spectra of the electron-doped Sr0.9La0.1CuO2 and the hole-doped YBa2Cu3O6+delta reveal distinctly different behavior in the pairing symmetries, pseudogap phenomena, satellite features, and low-energy excitations. While underdoped and optimally doped YBa2Cu3O6+delta exhibits d-wave and overdoped Ca-doped YBa2Cu3O6+delta (d+s)-wave pairing symmetry, the electron-doped Sr0.9La0.1CuO2 shows fully gapped s-wave pairing symmetry. The absence of the satellite features and pseudogap in tunneling spectra of electron-doped cuprates sharply contrasts with their general presence in hole-doped cuprates. Furthermore, the subgap low-energy spectral characteristics of Sr0.9La0.1CuO2 deviate substantially from the mean-field Bardeen-Cooper-Schrieffer theory, while those of YBa2Cu3O6+delta can be fully accounted for by the mean-field generalized Blonder-Tinkham-Klapwijk formalism.
Despite the aforementioned disparities, several experimental results reveal important connections between the two types of cuprates. For instance, the coexistence of the pseudogap and superconducting spectra in hole-doped cuprates and the observations of the current- and field-induced pseudogap in electron-doped cuprates suggest that competing orders, manifested as the pseudogap, coexist with superconductivity in both types of cuprates. In addition, by comparing the tunneling spectra with the high-field vortex dynamics measurements, we find that the quasiparticle spectral characteristics of Sr0.9La0.1CuO2 and YBa2Cu3O6+delta correlate with the degree of field-induced quantum phase fluctuations of the two compounds.
Based on these findings, we propose a simple model of coexisting density waves with superconductivity to unify the apparent non-universal phenomena among cuprate superconductors. By incorporating quantum phase fluctuations and adopting realistic band structures, numerical simulations of the quasiparticle tunneling spectra demonstrate excess subgap low-energy excitations, which is consistent with the empirical observations in Sr0.9La0.1CuO2. Furthermore, by tuning the ratio of the density waves to superconductivity, the theoretical calculations reproduce the absence of pseudogap phenomena in electron-doped cuprates and the general presence of he pseudogap in hole-doped cuprates. Thereby, we conclude that the competing orders that coexist with superconductivity in cuprate superconductors contribute to the rich cuprate phenomenology.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/RYWM-VX48, author = {Fu, Chu-Chen}, title = {Spin-Polarized Quasiparticle Transport in Cuprate Superconductors}, school = {California Institute of Technology}, year = {2003}, doi = {10.7907/RYWM-VX48}, url = {https://resolver.caltech.edu/CaltechETD:etd-10242002-021518}, abstract = {The effects of spin-polarized quasiparticle transport in superconducting YBa2 Cu3O7-δ (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. Transport and magnetic properties of these CMR perovskites are first investigated by inducing lattice distortions using lattice mismatching substrates. The half-metallic nature of these perovskites provides an epitaxially grown heterostructure, ideal for injection of spin-polarized current. These effects are compared with the injection of simple quasiparticles into control samples of perovskite non-magnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density (Jc) as a function of the injection current density, temperature, and the thickness of the superconductor demonstrate the “self-injection effect” and reveal dramatic differences between the F-I-S and N-I-S heterostructures, with strong suppression of Jc and a rapidly increasing characteristic transport length near the superconducting transition temperature Tc only in the F-I-S samples. The temperature dependence of the efficiency in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveals that a substantial chemical potential shift in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO2 planes.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/bjp0-2k03, author = {Jiang, Wen}, title = {Effects of Controlled Disorder on the Vortex Phases of High-Temperature Superconductors}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/bjp0-2k03}, url = {https://resolver.caltech.edu/CaltechETD:etd-06212007-152240}, abstract = {The high transition temperature, short coherence length and long penetration depth of high-temperature superconductors result in novel vortex properties associated with the large thermal and disorder fluctuations. This thesis presents systematic experimental investigations on the vortex phases of YBa2Cu3O7 single crystals with different types of controlled disorder. Measurements of dc current-voltage characteristics and ac impedance as a function of frequency are carried out on dilutely twinned YBa2Cu3O7 single crystals irradiated with 3.0 MeV protons. It is found that the moderately increased vortex pinning caused by the increasing density of controlled point defects does not change the nature of the second-order vortex-glass transitions in YBa2Cu3O7 single crystals with dilute twin boundaries, as manifested by the universal critical exponents and scaling functions for samples with different densities of point defects. In YBa2Cu3O7 single crystals with c-axis correlated columnar defects created by 0.9 GeV Pb-ion irradiation, a Bose-glass to “superfluid” transition is demonstrated by the universal critical scaling behavior of the ac impedance versus frequency isotherms. The static and dynamic critical exponents and the universal scaling functions are determined from our self-consistent critical scaling analyses. The Bose-glass transition temperature is found to decrease with the increasing angle between the applied magnetic field and the column orientation, in contrast to the angular dependence of the vortex-glass transition temperature which increases monotonically with the increasing angle due to the intrinsic sample anisotropy.
The interplay of vortex pinning and thermodynamic vortex phase transitions is further studied in the weak pinning limit by investigating the vortex transport properties of a nearly defect-free, untwinned YBa2Cu3O7 single crystal. Two novel phenomena are observed and studied quantitatively. The resistive hysteresis near the vortex-solid melting transition is ascribed to a current-induced non-equilibrium effect; the resistive “peak effect” below the vortex-solid transition is found to be a general phenomenon existing in extreme type-II superconductors, and is associated with the softening of the vortex-solid before the thermodynamic melting transition. It is therefore concluded that the current-induced effects are of particular importance in determining the vortex properties of extreme type-II superconductors with weak pinning.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, } @phdthesis{10.7907/31s8-mm15, author = {Reed, Daniel Seymour}, title = {Frequency Dependent Investigation of the Vortex Dynamics in High Temperature Superconductors}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/31s8-mm15}, url = {https://resolver.caltech.edu/CaltechETD:etd-06212007-155251}, abstract = {The high transition temperature and short coherence length of the high temperature superconducting cuprates lead to new vortex dynamics not found in conventional low temperature superconductors. The existence of a vortex-liquid state and a second-order phase transition between this vortex-liquid and the vortex-solid states has raised important questions about the vortex dynamics near this phase transition and about the interaction between the vortices and sample defects. In this thesis, new frequency dependent measurements systems are developed to measure the ac impedance and ac magnetic susceptibility of YBa2Cu3O7 single crystals over a broad frequency range from 10(2)Hz to 10(7)Hz. In addition, a miniature Hall probe magnetometer is used to measure the third harmonic susceptibility of a YBa2Cu3O7 single crystal. New critical scaling relations and analysis techniques are developed for ac magnetic susceptibility and the third harmonic transmissivity which for the first time enable critical scaling analysis to be applied to the measurements of these physical quantities. In twinned YBa2Cu3O7 single crystals with only point defects, we provide the most conclusive evidence yet for the existence of a second-order vortex-solid to vortex-liquid phase transition by demonstrating consistent critical scaling for three different experimental techniques: dc voltage versus current, ac impedance versus frequency, and ac susceptibility versus frequency. We also use the new frequency dependent techniques to investigate the effects of symmetry breaking in the vortex system by introducing columnar defects with heavy ion irradiation. In the case of parallel columnar defects, the symmetry is broken parallel and perpendicular to the columns, and the vortex behavior is consistent with a Bose-glass to vortex-liquid phase transition. If the symmetry is broken further by introducing columnar defects at two different orientations, a new splayed-glass phase is found. The presence of canted columnar defects leads to stronger disorder and possible entanglement of the vortices resulting in slower critical dynamics near the vortex phase transition. The experimental techniques developed in this thesis provide valuable new tools for probing the vortex dynamics of these systems and future work on other vortex and spin systems.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yeh, Nai-Chang}, }