[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields. Bulk Bi(1-x)Sb(x) single crystals are predicted to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher-dimensional analogues of the edge states that characterize a quantum spin Hall insulator. In addition to its interesting boundary states, the bulk of Bi(1-x)Sb(x) is predicted to exhibit three-dimensional Dirac particles, another topic of heightened current interest following the new findings in two-dimensional graphene and charge quantum Hall fractionalization observed in pure bismuth. However, despite numerous transport and magnetic measurements on the Bi(1-x)Sb(x) family since the 1960s, no direct evidence of either topological Hall states or bulk Dirac particles has been found. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy (IPEM-ARPES), we report the direct observation of massive Dirac particles in the bulk of Bi(0.9)Sb(0.1), locate the Kramers points at the sample's boundary and provide a comprehensive mapping of the Dirac insulator's gapless surface electron bands. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the 'topological metal'. They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate 'light-like' bulk carriers and spin-textured surface currents.

/pdf/a-topological-dirac-insulator-in-a-quantum-spin-hall-phase-dyc1pws69i.pdf

A topological Dirac insulator in a quantum spin Hall phase

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

This paper shows that the nanostructures deposited at room temperature in scanning tunneling microscopy experiments are produced by mechanical contact between tip and sample. Gold mounds are deposited in gold substrates and it is observed that the current flowing between tip and sample is quantized and the resistance can be as low as 100 \ensuremath{\Omega}.

Quantum contact in gold nanostructures by scanning tunneling microscopy.

The polarization-dependent absorption spectra and wavelength-modulated absorption spectra of Ti${\mathrm{O}}_{2}$ have been measured under very-high-resolution conditions between 1.6 and 300\ifmmode^\circ\else\textdegree\fi{}K. For the first time the fine structure of the fundan ental absorption edge is resolved and a detailed investigation is presented. We find an indirect allowed transition ${\ensuremath{\Sigma}}_{4v}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1c}$, characterized by a small doublet (3.061-3.065 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K) which constitutes the fundamental transition in polarization $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$ parallel to $\stackrel{\ensuremath{\rightarrow}}{\mathrm{C}}$. In polarization perpendicular to $\stackrel{\ensuremath{\rightarrow}}{\mathrm{C}}$, we find the same indirect transition plus a new forbidden direct transition ${\ensuremath{\Gamma}}_{3v}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1c}$ which appears at slightly lower energy (3.031 eV at 1.6\ifmmode^\circ\else\textdegree\fi{}K). The fine structure of the forbidden exciton is resolved and the ionization energy is 4 meV.

Fine structure in the intrinsic absorption edge of Ti O 2

Magnetic and superconducting interactions couple electrons together to form complex states of matter. We show that, at the atomic scale, both types of interactions can coexist and compete to influence the ground state of a localized magnetic moment. Local spectroscopy at 4.5 kelvin shows that the spin-1 system formed by manganese-phthalocyanine (MnPc) adsorbed on Pb(111) can lie in two different magnetic ground states. These are determined by the balance between Kondo screening and superconducting pair-breaking interactions. Both ground states alternate at nanometer length scales to form a Moire-like superstructure. The quantum phase transition connecting the two (singlet and doublet) ground states is thus tuned by small changes in the molecule-lead interaction.

https://science.sciencemag.org/content/sci/332/6032/940.full.pdf

Competition of Superconducting Phenomena and Kondo Screening at the Nanoscale

The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.

Selectivity in vibrationally mediated single-molecule chemistry.

Material structures of reduced dimensions exhibit electrical and mechanical properties different from those in the bulk. Measurements of room-temperature electronic transport in pulled metallic nanowires are presented, demonstrating that the conductance characteristics depend on the length, lateral dimensions, state and degree of disorder, and elongation mechanism of the wire. Conductance during the elongation of short wires (length l ∼ 50 angstroms) exhibits periodic quantization steps with characteristic dips, correlating with the order-disorder states of layers of atoms in the wire predicted by molecular dynamics simulations. The resistance R of wires as long as l ∼ 400 angstroms exhibits localization characteristics with In R ( l ) ∼ l 2 .

https://science.sciencemag.org/content/sci/267/5205/1793.full.pdf

Jose Ignacio Pascual

Papers

Quantum contact in gold nanostructures by scanning tunneling microscopy.

Fine structure in the intrinsic absorption edge of Ti O 2

Competition of Superconducting Phenomena and Kondo Screening at the Nanoscale

Selectivity in vibrationally mediated single-molecule chemistry.

Properties of Metallic Nanowires: From Conductance Quantization to Localization