Quantum transport in hexagonal boron nitride-carbon nanotube heterostructures

Carbon nanotubes (CNTs) are a versatile fundamental building block for classical small scale electronics and quantum electronics, and for the investigation of novel quantum states. However, the ideal properties of CNTs are usually masked by electrical potential fluctuations induced by the substrate. In this context, realization of disorder-free and clean CNT devices with outstanding quality is crucial for the fundamental studies of various interesting phenomena, such as Fabry-Perot interference, Klein tunneling, tunable quantum dots (QDs), spin-orbit interactions, valley spin-blockade, and the formation of Luttinger liquids, Wigner crystals, and Wigner molecules in one-dimensional (1D) systems. In this PhD project, we introduce a new production scheme, where we combine the two-dimensional (2D) hexagonal boron nitride (hBN) with quasi-1D CNTs. This new approach aims to improve the device quality significantly, and eventually allow us to explore the electronic transport properties of CNTs in extended 1D geometries. In particular, we investigate hBN as clean substrates for CNT QDs, insulators for top finger gates, tunnel barriers to CNTs, and to fully encapsulate the CNTs. Our results are very promising first steps in the fabrication of substrate-bound very clean CNT devices. This allows us to explore many advantageous properties of CNTs in more versatile structures than possible in two-terminal devices with “ultra-clean” suspended CNTs. This thesis is structured as follows. In Chapter 2, we introduce the theoretical background of the studied material systems, namely, the CNTs and hBN, with a focus on the basics of the CNT QDs. We discuss the superconductivity phenomena that may occur when a CNT is brought into contact with superconductors. Chapter 3 describes the fabrication details of hBN-CNT heterostructures and demonstrates the low-temperature measurement set-up. The main results of this thesis are presented in Chapters 4-7. We investigate the scanning electron microscopy (SEM) imaging contrast for locating CNTs on hBN flakes in Chapter 4. We discuss the low-temperature characteristics of CNT QDs fabricated on hBN flakes and of the dual-gated CNT QD devices with hBN top-gate dielectrics. We demonstrate that very good electrical device quality and stability can be achieved simply by introducing hBN flakes into the system. In Chapter 5, we focus on the CNT devices with atomically thin hBN tunnel barriers. We first characterize a CNT parallel double-QD, where we study the avoided crossings observed in its finite bias spectroscopy. In the second part of Chapter 5, we turn to the discussion on challenges of integrating atomically thin hBN into a CNT device. In Chapter 6, hBN encapsulated CNTs with zero-dimensional (0D) normal metal side contacts are investigated, while devices with 0D superconducting side contacts are characterized in Chapter 7. We demonstrate that low contact resistance with high-yield can be realized with 0D side contacts. This system allows us to study induced superconductivity in hBN encapsulated CNTs, where different transport regimes are identified. In an intermediate coupling regime, we observe Coulomb blockade, quasiparticle transport, resonant Andreev tunneling, and Andreev bound states, while in a strong coupling regime, multiple Andreev reflections and the magnetic field dependence of the critical current are discussed. Chapter 8 summarizes the experimental results and provides an outlook

Carbon nanotube quantum dots on hexagonal boron nitride

We report the fabrication details and low-temperature characteristics of the first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride (hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard chemical vapor deposition and that standard scanning electron microscopy imaging and lithography can be employed to fabricate nanoelectronic structures when using optimized parameters. This proof of concept paves the way to more complex devices on hBN, with more predictable and reproducible characteristics and electronic stability.Comment: 4 pages, 4 figure

Identifizierung und funktionelle Analyse von 5' UTR-Spleissvarianten der mRNA des humanen Cathepsin L

Humanes Cathepsin L (hCATL) ist eine lysosomale Cystein-Protease und bei der Tumor-Invasion und -Metastasierung beteiligt. Die Expression des hCATL korreliert in vielen normalen Geweben und Tumorgeweben positiv mit der mRNA-Expression. Eine Ausnahme stellt das Nierenkarzinom dar, wo trotz erhöhter mRNA-Expression der hCATL-Proteingehalt erniedrigt ist. Um dieses Missverhältnis in malignen Nierentumoren gegenüber gesundem Nierengewebe zu klären, wurden in der vorliegender Arbeit die Spleissvarianten der hCATL-mRNA kloniert und charakterisiert

Titanate nanotubes combined with graphene-based materials as a novel anode material for lithium-ion batteries

Lithium-ion batteries are popular rechargeable energy storage devices due to their attractive properties such as good performance, high reliability, and a ordability. Due to the problametic dentride growth present in typical anode materials for conventional lithium-ion batteries, graphene-based materials have gotten a wide attention as alternative materials for graphite oxide. Graphene features a high electrical conductivity and good mechanical properties. Also being able to be functionalized makes them very attractive as support materials for other electrochemically active anode materials. Due to titanium dioxide (TiO2) being a non-toxic and cost-effective material with a capacity theoretically up to 335 mAh/g, it has become a hot research topic worldwide. Nevertheless, a poor electronic conductivity and a low rate capability are the main drawbacks which can be overcome by synergetic effects of composite materials. Titanate nanotubes (TiNTs) are promising materials because of their special morphology and high specific surface area. We introduce a novel one-step hydrothermal method to obtain TiNTs&graphene-based composites as our anode materials. The selforganized TiNTs (H2T3O7) are dispersed on the surface of various types of graphene based materials, namely graphene oxide (GO), reduced graphene oxide (rGO), nitrogen doped reduced graphene oxide (NrGO), popypyrrole functionalized graphene oxide (PPy-GO), graphene nanoplates (GNP), nitrogen noped graphene nanoplates (NGNP), and amino functionalized graphene oxide (GO-NH2). Material characterization such as X-ray powder di raction (XRD), Raman spectroscopy, Brunauer-Emmett- Teller (BET), scanning electron microscopy (SEM) are performed for all the as-prepared samples to examine the chemical compositions, elemental properties, and physical morphologies. Electrochemical characterizations such as charge and discharge, cyclic performance are conducted. The material characterization reveals well-aligned TiNTs which are homogeneously dispersed on the surface of the GO-based materials. A battery test is performed on four promising samples. Among all these samples, GO-NH2&TiNTs at pH=4 yields the best electrochemical performance. It exhibits a high capacity retention with only 11% capacity fading after the rst 4 cycles. Furthermore, its reversible capacity after 40 cycles is about 100 mAh/g with a high capacity stability. Charging and discharging cycle tests of our lithium-ion batteries reveal the anode materials have good stability in terms of capacity retention. Our findings suggest that the integrity of TiNTs are conserved well and the ion di usion rate is in good balance with the electron transfer

Research on Establishment of Abnormal Phlegmatic Syndrome with Premature Ovarian Failure Rat Model and Effects of Balgham Munziq Treatment

This study aimed to establish and explore the biological basis of abnormal phlegmatic syndrome with premature ovarian failure (POF) model in rats based on the Uighur medicine (UM) in the first place and investigate the effects of unique herbal medicine, Balgham Munziq (BMq). Mature female Wistar rats were fed with spinach and coriander in cold and humid condition for approximately 20 weeks until abnormal phlegmatic syndrome (APS) model was established. When APS model was confirmed by Uighur medical experts, APS with POF disease rats were subdivided into APS with POF disease model group and APS with POF disease treated with BMq group; the rest of them were subdivided into APS model group and APS treated with BMq group. The results show that biological characteristics of animals in the course of modeling period were in accordance with clinical features of abnormal phlegmatic syndrome (APS) in Uighur medicine. Herbal medicine BMq not only reverted reproductive hormone levels disorders but also improved the function of hypothalamic-pituitary-ovarian axis and regulated secretion of monoamine neurotransmitters. APS is most likely to cause pathological changes of hypothalamic-pituitary-ovarian axis and lead to the occurrence of POF and BMq is effective in the treatment of APS with POF disease

Geometric reduction of dynamical nonlocality in nanoscale quantum circuits

Nonlocality is a key feature discriminating quantum and classical physics. Quantum-interference phenomena, such as Young’s double slit experiment, are one of the clearest manifestations of nonlocality, recently addressed as dynamical to specify its origin in the quantum equations of motion. It is well known that loss of dynamical nonlocality can occur due to (partial) collapse of the wavefunction due to a measurement, such as which-path detection. However, alternative mechanisms affecting dynamical nonlocality have hardly been considered, although of crucial importance in many schemes for quantum information processing. Here, we present a fundamentally different pathway of losing dynamical nonlocality, demonstrating that the detailed geometry of the detection scheme is crucial to preserve nonlocality. By means of a solid-state quantum-interference experiment we quantify this effect in a diffusive system. We show that interference is not only affected by decoherence, but also by a loss of dynamical nonlocality based on a local reduction of the number of quantum conduction channels of the interferometer. With our measurements and theoretical model we demonstrate that this mechanism is an intrinsic property of quantum dynamics. Understanding the geometrical constraints protecting nonlocality is crucial when designing quantum networks for quantum information processing

Superconductivity in type-II Weyl-semimetal WTe2 induced by a normal metal contact

WTe$_2$ is a material with rich topological properties: it is a 2D topological insulator as a monolayer and a Weyl-semimetal and higher-order topological insulator (HOTI) in the bulk form. Inducing superconductivity in topological materials is a way to obtain topological superconductivity, which lays at the foundation for many proposals of fault tolerant quantum computing. Here, we demonstrate the emergence of superconductivity at the interface between WTe$_2$ and the normal metal palladium. The superconductivity has a critical temperature of about 1.2 K. By studying the superconductivity in perpendicular magnetic field, we obtain the coherence length and the London penetration depth. These parameters correspond to a low Fermi velocity and a high density of states at the Fermi level. This hints to a possible origin of superconductivity due to the formation of flat bands. Furthermore, the critical in-plane magnetic field exceeds the Pauli limit, suggesting a non-trivial nature of the superconducting state.Comment: As accepted to Journal of Applied Physics, 7 pages, 4 figure

Membrane-Bound sn-1,2-Diacylglycerols Explain the Dissociation of Hepatic Insulin Resistance from Hepatic Steatosis in MTTP Knockout Mice

Microsomal triglyceride transfer protein (MTTP) deficiency results in a syndrome of hypolipidemia and accelerated NAFLD. Animal models of decreased hepatic MTTP activity have revealed an unexplained dissociation between hepatic steatosis and hepatic insulin resistance. Here, we performed comprehensive metabolic phenotyping of liver-specific MTTP knockout (L-Mttp(-/-)) mice and age-weight matched wild-type control mice. Young (10-12-week-old) L-Mttp(-/-) mice exhibited hepatic steatosis and increased DAG content; however, the increase in hepatic DAG content was partitioned to the lipid droplet and was not increased in the plasma membrane. Young L-Mttp(-/-) mice also manifested normal hepatic insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamps, no PKC epsilon activation, and normal hepatic insulin signaling from the insulin receptor through AKT Ser/Thr kinase. In contrast, aged (10-month-old) L-Mttp(-/-) mice exhibited glucose intolerance and hepatic insulin resistance along with an increase in hepatic plasma membrane sn-1,2-DAG content and PKC epsilon activation. Treatment with a functionally liver-targeted mitochondrial uncoupler protected the aged L-Mttp(-/-) mice against the development of hepatic steatosis, increased plasma membrane sn-1,2-DAG content, PKC epsilon activation, and hepatic insulin resistance. Furthermore, increased hepatic insulin sensitivity in the aged controlled-release mitochondrial protonophore-treated L-Mttp(-/-) mice was not associated with any reductions in hepatic ceramide content. Taken together, these data demonstrate that differences in the intracellular compartmentation of sn-1,2-DAGs in the lipid droplet versus plasma membrane explains the dissociation of NAFLD/lipid-induced hepatic insulin resistance in young L-Mttp(-/-) mice as well as the development of lipid-induced hepatic insulin resistance in aged L-Mttp(-/-) miceThis work was supported by National Institutes of Health Grants R01 DK116774, R01 DK119968, R01 DK114793, R01 DK113984, K23 DK10287, P30 DK045735, DK121490, and HL137202 and the Veterans Health Administration Merit Review Awards I01 BX000901 and BX004113. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or the U.S. Department of Veterans Affair