Broadband CPW-based impedance-transformed Josephson parametric amplifier

Quantum-limited Josephson parametric amplifiers play a pivotal role in advancing the field of circuit quantum electrodynamics by enabling the fast and high-fidelity measurement of weak microwave signals. Therefore, it is necessary to develop robust parametric amplifiers with low noise, broad bandwidth, and reduced design complexity for microwave detection. However, current broadband parametric amplifiers either have degraded noise performance or rely on complex designs. Here, we present a device based on the broadband impedance-transformed Josephson parametric amplifier (IMPA) that integrates a horn-like coplanar waveguide (CPW) transmission line, which significantly decreases the design and fabrication complexity, while keeping comparable performance. The device shows an instantaneous bandwidth of 700(200) MHz for 15(20) dB gain with an average saturation power of -110 dBm and near quantum-limited added noise. The operating frequency can be tuned over 1.4 GHz using an external flux bias. We further demonstrate the negligible back-action from our device on a transmon qubit. The amplification performance and simplicity of our device promise its wide adaptation in quantum metrology, quantum communication, and quantum information processing.Comment: 11 pages, 8 figure

Realization of Quantum Spin Hall State in Monolayer 1T'-WTe2

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling. By investigating electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first principle calculations, we observe clear signatures of the topological band inversion and the band gap opening, which are the hallmarks of a QSH state. Scanning tunneling microscopy measurements further confirm the correct crystal structure and the existence of a bulk band gap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).Comment: 19 pages, 4 figures; includes Supplemental Material (11 pages, 7 figures

Experimentally Engineering the Edge Termination of Graphene Nanoribbons

The edges of graphene nanoribbons (GNRs) have attracted much interest due to their potentially strong influence on GNR electronic and magnetic properties. Here we report the ability to engineer the microscopic edge termination of high quality GNRs via hydrogen plasma etching. Using a combination of high-resolution scanning tunneling microscopy and first-principles calculations, we have determined the exact atomic structure of plasma-etched GNR edges and established the chemical nature of terminating functional groups for zigzag, armchair and chiral edge orientations. We find that the edges of hydrogen-plasma-etched GNRs are generally flat, free of structural reconstructions and are terminated by hydrogen atoms with no rehybridization of the outermost carbon edge atoms. Both zigzag and chiral edges show the presence of edge states.Comment: 16+9 pages, 3+4 figure

Manipulating Topological Domain Boundaries in the Single-layer Quantum Spin Hall Insulator 1T’–WSe_2

We report the creation and manipulation of structural phase boundaries in the single-layer quantum spin Hall insulator 1T′–WSe_2 by means of scanning tunneling microscope tip pulses. We observe the formation of one-dimensional interfaces between topologically nontrivial 1T′ domains having different rotational orientations, as well as induced interfaces between topologically nontrivial 1T′ and topologically trivial 1H phases. Scanning tunneling spectroscopy measurements show that 1T′/1T′ interface states are localized at domain boundaries, consistent with theoretically predicted unprotected interface modes that form dispersive bands in and around the energy gap of this quantum spin Hall insulator. We observe a qualitative difference in the experimental spectral line shape between topologically “unprotected” states at 1T′/1T′ domain boundaries and protected states at 1T′/1H and 1T′/vacuum boundaries in single-layer WSe_2

Manipulating 1D Conduction Channels; from Molecular Geometry to 2D Topology

This dissertation is divided into two segments, both of which focus on creating and manipulating one-dimensional (1D) conduction channels in novel 1D and two-dimensional (2D) systems, characterized by scanning tunneling microscopy (STM).The first half describes how the electronic properties of quasi-1D graphene nanoribbons (GNRs) are manipulated by controlling their width and edge geometry at the atomic scale. A bottom-up approach is used for fabricating three different armchair GNR (AGNR) systems, allowing the geometry and hence the electronic properties of resultant AGNRs to be controlled. Successful molecular bandgap engineering in 1D AGNR heterojunctions is described, as well as the electronic and topographic characterization of the concentration dependence of boron-doped AGNRs. The discovery of two new in-gap dopant states with different symmetry is described. The successful fabrication and characterization of S-AGNRs having sulfur atoms substitutionally doped at the AGNR edges is also described. Our results indicate that S-doping induces a rigid shift of the energies for both the valence and conduction bands.The second half of this thesis describes how the 1T’ phase of monolayer transition metal dichalcogenides (TMDs) can be used as a platform to create 2D topological insulators (TIs). These novel TI systems are characterized in great detail. The successful growth and characterization of single-layer 1T’–WTe2 is described. This material is shown to host a bulk bandgap and helical edge states at the 1T’–vacuum interface. The growth and characterization of mixed phase-WSe2 is described. New techniques for creation and manipulation of edge conduction channels at interfaces between materials of different topologies are described

Manipulating 1D Conduction Channels: From Molecular Geometry to 2D Topology

This dissertation is divided into two segments, both of which focus on creating and manipulating one-dimensional (1D) conduction channels in novel 1D and two-dimensional (2D) systems, characterized by scanning tunneling microscopy (STM). The first half describes how the electronic properties of quasi-1D graphene nanoribbons (GNRs) are manipulated by controlling their width and edge geometry at the atomic scale. A bottom-up approach is used for fabricating three different armchair GNR (AGNR) systems, allowing the geometry and hence the electronic properties of resultant AGNRs to be controlled. Successful molecular bandgap engineering in 1D AGNR heterojunctions is described, as well as the electronic and topographic characterization of the concentration dependence of boron-doped AGNRs. The discovery of two new in-gap dopant states with different symmetry is described. The successful fabrication and characterization of S-AGNRs having sulfur atoms substitutionally doped at the AGNR edges is also described. Our results indicate that S-doping induces a rigid shift of the energies for both the valence and conduction bands. The second half of this thesis describes how the 1T’ phase of monolayer transition metal dichalcogenides (TMDs) can be used as a platform to create 2D topological insulators (TIs). These novel TI systems are characterized in great detail. The successful growth and characterization of single-layer 1T’–WTe2 is described. This material is shown to host a bulk bandgap and helical edge states at the 1T’–vacuum interface. The growth and characterization of mixed phase-WSe2 is described. New techniques for creation and manipulation of edge conduction channels at interfaces between materials of different topologies are described

Bottom-up graphene nanoribbon field-effect transistors

Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nano-scale chemically synthesized GNR field-effect transistors, made possible by development of a reliable layer transfer process. We observe strong environmental sensitivity and unique transport behavior characteristic of sub-1 nm width GNRs. © 2013 AIP Publishing LLC.Research was supported by the Office of Naval Research BRC Program, by the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and by National Science Foundation award DMR-1206512. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Peer Reviewe

Tuning the band gap of graphene nanoribbons synthesized from molecular precursors

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps via covalent self-assembly of a new species of molecular precursors that yields n = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these n = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for n = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of n = 13 armchair GNRs that is associated with hydrogen-terminated sp2-hybridized carbon atoms at the zigzag termini.Research was supported by the Office of Naval Research BRC Program (molecular synthesis and characterization), by the Helios Solar Energy Research Center, which is supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 (STM instrumentation development, STM operation), and by National Science Foundation award DMR-1206512 (image analysis). D.G.O. acknowledges fellowship support by the European Union under FP7-PEOPLE-2010-IOF.Peer reviewe