Quantum hall ferromagnetism in a two-dimensional electron system

Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction nu = 1/3) and spin-unpolarized (nu = 2/5) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.     

Very large capacitance enhancement in a two-dimensional electron system

Increases in the gate capacitance of field-effect transistor structures allow the production of lower-power devices that are compatible with higher clock rates, driving the race for developing high-κ dielectrics. However, many-body effects in an electronic system can also enhance capacitance. Onto the electron system that forms at the LaAlO(3)/SrTiO(3) interface, we fabricated top-gate electrodes that can fully deplete the interface of all mobile electrons. Near depletion, we found a greater than 40% enhancement of the gate capacitance. Using an electric-field penetration measurement method, we show that this capacitance originates from a negative compressibility of the interface electron system. Capacitance enhancement exists at room temperature and arises at low electron densities, in which disorder is strong and the in-plane conductance is much smaller than the quantum conductance.     

Montmorillonite: electron diffraction from two-dimensional single crystals

Some Selected area electron diffraction patterns of isolated monocrystalline particles of both Wyoming and Camp Berteaux sodiumti-monfmorillonites fail to show the mirror platne symmetry required for the accepted structure. A triclinic model deserves consideraction.     

Pi electron systems at high pressure

In the paper "Pi electron systems at high pressure" [Science 156, 1183 (1967)] a mechanism is suggested for the high-pressure reactions of perylene and azulene complexes with tetracyanoethylene (Figs. lOb and lOc) which is clearly impossible as drawn. What I had meant to suggest is the reaction shown below (with Fig. lOb as an example), which is at least conceivable, if not very probable from the chemical viewpoint. A similar modification would apply to Fig. lOc. As indicated in the paper, any serious study of the reaction requires the synthesis of enough product for more complete analysis, as well as a fairly detailed knowledge of the crystal geometry of the unreacted complex. See pdf for figure.     

Protein carbon-13 spin systems by a single two-dimensional nuclear magnetic resonance experiment

By applying a two-dimensional double-quantum carbon-13 nuclear magnetic resonance experiment to a protein uniformly enriched to 26 percent carbon-13, networks of directly bonded carbon atoms were identified by virtue of their one-bond spin-spin couplings and were classified by amino acid type according to their particular single- and double-quantum chemical shift patterns. Spin systems of 75 of the 98 amino acid residues in a protein, oxidized Anabaena 7120 ferredoxin (molecular weight 11,000), were identified by this approach, which represents a key step in an improved methodology for assigning protein nuclear magnetic resonance spectra. Missing spin systems corresponded primarily to residues located adjacent to the paramagnetic iron-sulfur cluster.     

Coherent spin oscillations in a disordered magnet

Most materials freeze when cooled to sufficiently low temperature. We find that magnetic dipoles randomly distributed in a solid matrix condense into a spin liquid with spectral properties on cooling that are the diametric opposite of those for conventional glasses. Measurements of the nonlinear magnetic dynamics in the low-temperature liquid reveal the presence of coherent spin oscillations composed of hundreds of spins with lifetimes of up to 10 seconds. These excitations can be labeled by frequency and manipulated by the magnetic fields from a loop of wire and can permit the encoding of information at multiple frequencies simultaneously.     

Ultrastrong coupling of the cyclotron transition of a 2D electron gas to a THz metamaterial

Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems because of the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz (THz) metamaterial where the cyclotron transition of a high-mobility two-dimensional electron gas (2DEG) is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio, Ω/ω(c) = 0.58, between the vacuum Rabi frequency, ?, and the cyclotron frequency, ω(c). Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG.     

A mechanism of lithium storage in disordered carbons

High-resolution electron microscopy and lithium-7 nuclear magnetic resonance measurements were carried out for a disordered carbon material, prepared by heat treatment of polyphenylene, in which lithium was stored electrochemically. The nuclear magnetic resonance spectrum suggests the existence of Li(2) covalent molecules in the carbon material. This extra covalent site of lithium storage promises extraordinarily high energy density for secondary batteries.     

IPV4向IPV6网络体系的过渡与安全问题

伴随互联网用户数膨胀所出现的问题越来越严重,人们提出下一代互联网采用的核心协议IPv6.现行的IPv4向IPv6过渡势在必行,因此对IPv4向IPv6过渡过程中网络体系结构的更替进行研究具有重要的现实意义.另外,新的网络体系也面临诸多安全问题同样不可忽视.     

Long-range order in electronic transport through disordered metal films

Ultracold atom magnetic field microscopy enables the probing of current flow patterns in planar structures with unprecedented sensitivity. In polycrystalline metal (gold) films, we observed long-range correlations forming organized patterns oriented at +/-45 degrees relative to the mean current flow, even at room temperature and at length scales larger than the diffusion length or the grain size by several orders of magnitude. The preference to form patterns at these angles is a direct consequence of universal scattering properties at defects. The observed amplitude of the current direction fluctuations scales inversely to that expected from the relative thickness variations, the grain size, and the defect concentration, all determined independently by standard methods. Ultracold atom magnetometry thus enables new insight into the interplay between disorder and transport.     

Ultralow thermal conductivity in disordered, layered WSe2 crystals

The cross-plane thermal conductivity of thin films of WSe2 grown from alternating W and Se layers is as small as 0.05 watts per meter per degree kelvin at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe2 and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. We attribute the ultralow thermal conductivity of these disordered, layered crystals to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe2 sheets. Disordering of the layered structure by ion bombardment increases the thermal conductivity.     

二维图像编辑中透视效应的研究

“龟纹图”的变化规律,揭示了二维图像编辑中的透视效应。通过对平行透视的分析,研究视觉艺术的单点透视与散点透视,使之能在工程和艺术方面,得到更深层次的研究与应用。