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71.
Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high-Tc superconductor 总被引:1,自引:0,他引:1
Kaminski A Rosenkranz S Fretwell HM Campuzano JC Li Z Raffy H Cullen WG You H Olson CG Varma CM Höchst H 《Nature》2002,416(6881):610-613
A change in 'symmetry' is often observed when matter undergoes a phase transition-the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field transition as seen in other superconductors. Rather, there is a region in the phase diagram above the superconducting transition temperature Tc (where phase coherence and superconductivity begin) but below a characteristic temperature T* where a 'pseudogap' appears in the spectrum of electronic excitations. It is therefore important to establish if T* is just a cross-over temperature arising from fluctuations in the order parameter that will establish superconductivity at Tc (refs 3, 4), or if it marks a phase transition where symmetry is spontaneously broken. Here we report that, for a material in the pseudogap state, left-circularly polarized photons give a different photocurrent from right-circularly polarized photons. This shows that time-reversal symmetry is spontaneously broken below T*, which therefore corresponds to a phase transition. 相似文献
72.
A 5-bp deletion in ELOVL4 is associated with two related forms of autosomal dominant macular dystrophy 总被引:20,自引:0,他引:20
Zhang K Kniazeva M Han M Li W Yu Z Yang Z Li Y Metzker ML Allikmets R Zack DJ Kakuk LE Lagali PS Wong PW MacDonald IM Sieving PA Figueroa DJ Austin CP Gould RJ Ayyagari R Petrukhin K 《Nature genetics》2001,27(1):89-93
Stargardt-like macular dystrophy (STGD3, MIM 600110) and autosomal dominant macular dystrophy (adMD) are inherited forms of macular degeneration characterized by decreased visual acuity, macular atrophy and extensive fundus flecks. Genetic mapping data suggest that mutations in a single gene may be responsible for both conditions, already known to bear clinical resemblance. Here we limit the minimum genetic region for STGD3 and adMD to a 0.6-cM interval by recombination breakpoint mapping and identify a single 5-bp deletion within the protein-coding region of a new retinal photoreceptor-specific gene, ELOVL4, in all affected members of STGD3 and adMD families. Bioinformatic analysis of ELOVL4 revealed that it has homology to a group of yeast proteins that function in the biosynthesis of very long chain fatty acids. Our results are therefore the first to implicate the biosynthesis of fatty acids in the pathogenesis of inherited macular degeneration. 相似文献
73.
The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal-insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05-0.1 cm2 V(-1) s(-1)) at room temperature. At temperatures below approximately 2.35 K with sheet carrier densities exceeding 2.5 x 10(14) cm(-2), the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible-from insulating to superconducting. 相似文献
74.
Integration of telomere sequences with the draft human genome sequence 总被引:15,自引:0,他引:15
Riethman HC Xiang Z Paul S Morse E Hu XL Flint J Chi HC Grady DL Moyzis RK 《Nature》2001,409(6822):948-951
Telomeres are the ends of linear eukaryotic chromosomes. To ensure that no large stretches of uncharacterized DNA remain between the ends of the human working draft sequence and the ends of each chromosome, we would need to connect the sequences of the telomeres to the working draft sequence. But telomeres have an unusual DNA sequence composition and organization that makes them particularly difficult to isolate and analyse. Here we use specialized linear yeast artificial chromosome clones, each carrying a large telomere-terminal fragment of human DNA, to integrate most human telomeres with the working draft sequence. Subtelomeric sequence structure appears to vary widely, mainly as a result of large differences in subtelomeric repeat sequence abundance and organization at individual telomeres. Many subtelomeric regions appear to be gene-rich, matching both known and unknown expressed genes. This indicates that human subtelomeric regions are not simply buffers of nonfunctional 'junk DNA' next to the molecular telomere, but are instead functional parts of the expressed genome. 相似文献
75.
Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors 总被引:3,自引:0,他引:3
Lanzara A Bogdanov PV Zhou XJ Kellar SA Feng DL Lu ED Yoshida T Eisaki H Fujimori A Kishio K Shimoyama JI Noda T Uchida S Hussain Z Shen ZX 《Nature》2001,412(6846):510-514
Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. 相似文献
76.
Khalifah P Nelson KD Jin R Mao ZQ Liu Y Huang Q Gao XP Ramirez AP Cava RJ 《Nature》2001,411(6838):669-671
Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization. 相似文献
77.
Melanocortin receptors: their functions and regulation by physiological agonists and antagonists 总被引:14,自引:0,他引:14
Abdel-Malek ZA 《Cellular and molecular life sciences : CMLS》2001,58(3):434-441
The melanocortins are a family of bioactive peptides derived from proopiomelanocortin, and share significant structural similarity.
Those peptides are best known for their stimulatory effects on pigmentation and steroidogenesis. Melanocortins are synthesized
in various sites in the central nervous system and in peripheral tissues, and participate in regulating multiple physiological
functions. Research during the past decade has provided evidence that melanocortins elicit their diverse biological effects
by binding to a distinct family of G protein-coupled receptors with seven transmembrane domains. To date, five melanocortin
receptor genes have been cloned and characterized. Those receptors differ in their tissue distribution and in their ability
to recognize the various melanocortins and the physiological antagonists, agouti signaling protein and agouti-related protein.
These advances have opened new horizons for exploring the significance of melanocortins, their antagonists, and their receptors
in a variety of important physiological functions.
Received 5 October 2000; accepted 10 November 2000 相似文献
78.
Analysis of nuclear apoptotic process in a cell-free system 总被引:2,自引:0,他引:2
We report an analysis of the apoptotic process of mouse liver nuclei induced in a cell-free carrot cytosol system by cytochrome
c. Typical characteristics of apoptosis were observed, such as chromatin condensation, margination, apoptotic bodies and DNA
ladders. Furthermore, transmission and scanning electron microscope analysis of the apoptotic nuclei detected chromatin-free
nuclear vesicles before apoptotic bodies appeared at a comparatively late phase. When AC-YVAD-CHO, an inhibitor of caspase
6, was introduced into the system, these vesicles and apoptotic bodies disappeared completely within our study sections. We
confirmed the results using whole-mount electron microscopy, and found that although the nuclear lamina was destroyed early,
the nuclear matrix largely remained intact during the course of apoptosis. The nuclear matrix played an important role in
maintaining the integrity of apoptotic cells and connecting the apoptotic bodies and apoptotic nucleus.
Received 29 September 2000; revised 10 December 2000; accepted 13 December 2000 相似文献
79.
Opposing effects of Ets and Id proteins on p16INK4a expression during cellular senescence 总被引:46,自引:0,他引:46
Ohtani N Zebedee Z Huot TJ Stinson JA Sugimoto M Ohashi Y Sharrocks AD Peters G Hara E 《Nature》2001,409(6823):1067-1070
80.
Observation of coherent optical information storage in an atomic medium using halted light pulses 总被引:36,自引:0,他引:36
Electromagnetically induced transparency is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a 'coupling' laser is used to create the interference necessary to allow the transmission of resonant pulses from a 'probe' laser. This technique has been used to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud. Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations. We anticipate applications of this phenomenon for quantum information processing. 相似文献