Two-dimensional normal-state quantum oscillations in a superconducting heterostructure

Kozuka, Y.,Kim, M.,Bell, C.,Kim, B. G.,Hikita, Y.,Hwang, H. Y. Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.462 No.7272

Semiconductor heterostructures provide an ideal platform for studying high-mobility, low-density electrons in reduced dimensions. The realization of superconductivity in heavily doped diamond, silicon, silicon carbide and germanium suggests that Cooper pairs eventually may be directly incorporated in semiconductor heterostructures, but these newly discovered superconductors are currently limited by their extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering, in single-crystal or polycrystalline samples, or atomic-scale disorder, in amorphous materials, confining these examples to the extreme ‘dirty limit’. Here we report the fabrication of a high-quality superconducting layer within a thin-film heterostructure based on SrTiO<SUB>3</SUB> (the first known superconducting semiconductor). By selectively doping a narrow region of SrTiO<SUB>3</SUB> with the electron-donor niobium, we form a superconductor that is two-dimensional, as probed by the anisotropy of the upper critical magnetic field. Unlike in previous examples, however, the electron mobility is high enough that the normal-state resistance exhibits Shubnikov–de Haas oscillations that scale with the perpendicular field, indicating two-dimensional states. These results suggest that delta-doped SrTiO<SUB>3</SUB> provides a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons. They also demonstrate that high-quality complex oxide heterostructures can maintain electron coherence on the macroscopic scales probed by transport, as well as on the microscopic scales demonstrated previously.

Near-field focusing and magnification through self-assembled nanoscale spherical lenses

Lee, Ju Young,Hong, Byung Hee,Kim, Woo Youn,Min, Seung Kyu,Kim, Yukyung,Jouravlev, Mikhail V.,Bose, Ranojoy,Kim, Keun Soo,Hwang, In-Chul,Kaufman, Laura J.,Wong, Chee Wei,Kim, Philip,Kim, Kwang S. Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.460 No.7254

It is well known that a lens-based far-field optical microscope cannot resolve two objects beyond Abbe’s diffraction limit. Recently, it has been demonstrated that this limit can be overcome by lensing effects driven by surface-plasmon excitation, and by fluorescence microscopy driven by molecular excitation. However, the resolution obtained using geometrical lens-based optics without such excitation schemes remains limited by Abbe’s law even when using the immersion technique, which enhances the resolution by increasing the refractive indices of immersion liquids. As for submicrometre-scale or nanoscale objects, standard geometrical optics fails for visible light because the interactions of such objects with light waves are described inevitably by near-field optics. Here we report near-field high resolution by nanoscale spherical lenses that are self-assembled by bottom-up integration of organic molecules. These nanolenses, in contrast to geometrical optics lenses, exhibit curvilinear trajectories of light, resulting in remarkably short near-field focal lengths. This in turn results in near-field magnification that is able to resolve features beyond the diffraction limit. Such spherical nanolenses provide new pathways for lens-based near-field focusing and high-resolution optical imaging at very low intensities, which are useful for bio-imaging, near-field lithography, optical memory storage, light harvesting, spectral signal enhancing, and optical nano-sensing.

Peierls distortion as a route to high thermoelectric performance in In₄Se_3-δ crystals

Rhyee, Jong-Soo,Lee, Kyu Hyoung,Lee, Sang Mock,Cho, Eunseog,Kim, Sang Il,Lee, Eunsung,Kwon, Yong Seung,Shim, Ji Hoon,Kotliar, Gabriel Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.459 No.7249

Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In<SUB>4</SUB>Se<SUB>3-δ</SUB>, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

High-Q surface-plasmon-polariton whispering-gallery microcavity

Min, Bumki,Ostby, Eric,Sorger, Volker,Ulin-Avila, Erick,Yang, Lan,Zhang, Xiang,Vahala, Kerry Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.457 No.7228

Surface plasmon polaritons (SPPs) are electron density waves excited at the interfaces between metals and dielectric materials. Owing to their highly localized electromagnetic fields, they may be used for the transport and manipulation of photons on subwavelength scales. In particular, plasmonic resonant cavities represent an application that could exploit this field compression to create ultrasmall-mode-volume devices. A key figure of merit in this regard is the ratio of cavity quality factor, Q (related to the dissipation rate of photons confined to the cavity), to cavity mode volume, V (refs 10, 11). However, plasmonic cavity Q factors have so far been limited to values less than 100 both for visible and near-infrared wavelengths. Significantly, such values are far below the theoretically achievable Q factors for plasmonic resonant structures. Here we demonstrate a high-Q SPP whispering-gallery microcavity that is made by coating the surface of a high-Q silica microresonator with a thin layer of a noble metal. Using this structure, Q factors of 1,376 ± 65 can be achieved in the near infrared for surface-plasmonic whispering-gallery modes at room temperature. This nearly ideal value, which is close to the theoretical metal-loss-limited Q factor, is attributed to the suppression and minimization of radiation and scattering losses that are made possible by the geometrical structure and the fabrication method. The SPP eigenmodes, as well as the dielectric eigenmodes, are confined within the whispering-gallery microcavity and accessed evanescently using a single strand of low-loss, tapered optical waveguide. This coupling scheme provides a convenient way of selectively exciting and probing confined SPP eigenmodes. Up to 49.7 per cent of input power is coupled by phase-matching control between the microcavity SPP and the tapered fibre eigenmodes.

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Kim, Keun Soo,Zhao, Yue,Jang, Houk,Lee, Sang Yoon,Kim, Jong Min,Kim, Kwang S.,Ahn, Jong-Hyun,Kim, Philip,Choi, Jae-Young,Hong, Byung Hee Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.457 No.7230

Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of ∼280 Ω per square, with ∼80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP> and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Interdimensional universality of dynamic interfaces

Kim, Kab-Jin,Lee, Jae-Chul,Ahn, Sung-Min,Lee, Kang-Soo,Lee, Chang-Won,Cho, Young Jin,Seo, Sunae,Shin, Kyung-Ho,Choe, Sug-Bong,Lee, Hyun-Woo Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.458 No.7239

Despite the complexity and diversity of nature, there exists universality in the form of critical scaling laws among various dissimilar systems and processes such as stock markets, earthquakes, crackling noise, lung inflation and vortices in superconductors. This universality is mainly independent of the microscopic details, depending only on the symmetry and dimension of the system. Exploring how universality is affected by the system dimensions is an important unresolved problem. Here we demonstrate experimentally that universality persists even at a dimensionality crossover in ferromagnetic nanowires. As the wire width decreases, the magnetic domain wall dynamics changes from elastic creep in two dimensions to a particle-like stochastic behaviour in one dimension. Applying finite-size scaling, we find that all our experimental data in one and two dimensions (including the crossover regime) collapse onto a single curve, signalling universality at the criticality transition. The crossover to the one-dimensional regime occurs at a few hundred nanometres, corresponding to the integration scale for modern nanodevices.

Unusual magnetic order in the pseudogap region of the superconductor HgBa₂CuO_4+δ

Li, Y.,Balé,dent, V.,Bariš,ić,, N.,Cho, Y.,Fauqué,, B.,Sidis, Y.,Yu, G.,Zhao, X.,Bourges, P.,Greven, M. Macmillan Publishers Limited. All rights reserved 2008 Nature Vol.455 No.7211

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T<SUB>c</SUB>) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>6+δ</SUB> (YBCO) resulted in contradictory claims of no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO and of circularly polarized photoemission experiments on Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8+δ</SUB>(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa<SUB>2</SUB>CuO<SUB>4+δ</SUB> (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.

Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts

Choi, Minkee,Na, Kyungsu,Kim, Jeongnam,Sakamoto, Yasuhiro,Terasaki, Osamu,Ryoo, Ryong Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.461 No.7261

Zeolites—microporous crystalline aluminosilicates—are widely used in petrochemistry and fine-chemical synthesis because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion. This has been realized by synthesizing zeolite nanocrystals, by exfoliating layered zeolites, and by introducing mesopores in the microporous material through templating strategies or demetallation processes. But except for the exfoliation, none of these strategies has produced ‘ultrathin’ zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.

A highly annotated whole-genome sequence of a Korean individual

Kim, Jong-Il,Ju, Young Seok,Park, Hansoo,Kim, Sheehyun,Lee, Seonwook,Yi, Jae-Hyuk,Mudge, Joann,Miller, Neil A.,Hong, Dongwan,Bell, Callum J.,Kim, Hye-Sun,Chung, In-Soon,Lee, Woo-Chung,Lee, Ji-Sun,Seo, Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.460 No.7258

Recent advances in sequencing technologies have initiated an era of personal genome sequences. To date, human genome sequences have been reported for individuals with ancestry in three distinct geographical regions: a Yoruba African, two individuals of northwest European origin, and a person from China. Here we provide a highly annotated, whole-genome sequence for a Korean individual, known as AK1. The genome of AK1 was determined by an exacting, combined approach that included whole-genome shotgun sequencing (27.8× coverage), targeted bacterial artificial chromosome sequencing, and high-resolution comparative genomic hybridization using custom microarrays featuring more than 24 million probes. Alignment to the NCBI reference, a composite of several ethnic clades, disclosed nearly 3.45 million single nucleotide polymorphisms (SNPs), including 10,162 non-synonymous SNPs, and 170,202 deletion or insertion polymorphisms (indels). SNP and indel densities were strongly correlated genome-wide. Applying very conservative criteria yielded highly reliable copy number variants for clinical considerations. Potential medical phenotypes were annotated for non-synonymous SNPs, coding domain indels, and structural variants. The integration of several human whole-genome sequences derived from several ethnic groups will assist in understanding genetic ancestry, migration patterns and population bottlenecks.

Observation of molecular orbital gating

Song, Hyunwook,Kim, Youngsang,Jang, Yun Hee,Jeong, Heejun,Reed, Mark A.,Lee, Takhee Macmillan Publishers Limited. All rights reserved 2009 Nature Vol.462 No.7276

The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.