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Issue 11, 2024
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Extracts from the Internet


Electron-phonon interactions in HTSCs

According to angle-resolved photoemission spectroscopy data obtained by a group of scientists from the USA and Japan, the electron-phonon interaction (EPI) plays a central role in the conduction mechanism in high-temperature superconductors (HTSCs). The phenomenon of superconductivity relates to the Bose condensation of Cooper pairs of electrons. Electrons in metallic (low-temperature) superconductors pair up by exchanging phonons, quasi-particles corresponding to crystal lattice vibrations. According to certain theoretical and experimental arguments (see Phys. Usp. 43 965 (2000)), a relatively strong EPI should also exist in HTSC systems, although the electron-phonon mechanism is insufficient to fully account for the superconducting properties of these systems. Therefore a more complex, yet-unidentified electron pairing mechanism was believed to be at work in HTSCs. Indeed, it was widely held that there is no noticeable EPI at all in these systems. Still, the photoemission experiment by A Lanzara and his colleagues now shows that EPI may be important after all. This experiment measured the energy spectrum of the holes which synchrotron radiation produced by knocking out electrons in samples of three types of cuprate HTSCs. The spectrum showed a kink which could be interpreted as a change in the hole effective mass due to the interaction with a certain boson field. The authors present strong evidence that this occurs due to the interaction of electrons with phonons. Earlier, this kink has been repeatedly observed in metallic superconductors, where it is known for certain that it is due to the electron- phonon interaction. To elucidate the role of this interaction in HTSCs, new theoretical ideas and experimental studies are needed. Source: Nature 412 510 (2001) ; http://xxx.lanl.gov/abs/cond-mat/0108381

Electron waves in nanotubes

Carbon nanotubes display distinct quantum mechanical properties owing to their microscopic size. Calculations show, in particular, that an electron wave function in a nanotube should be a superposition of two vibrations with close wavelengths, and that the addition of the modes should lead to spatial beats in the electron density distribution. If the mode wavelengths are approximately equal to the nearest-neighbour carbon-carbon distance, the beats occur on the time scale of several atoms. This effect was first discovered by C Dekker and his colleagues at Delft technological University in Netherlands using an improved version of a scanning tunneling microscope. The team not only measured the variation of the tunneling current with the position of the microscope probe but also determined the dependence of the current on the electric voltage for each position of the probe. The high accuracy of the new technique allowed the researchers to determine the density distribution of electrons as a function of their energy and also to confirm the prediction of beats. Source: Nature 412 617 (2001)

Semiconducting detector of IR radiation

A superconducting detector capable of capturing single photons of IR radiation has been developed by a team led by R Sobolevski at the University of Rochester. The possibility of creating superconducting detectors has been discussed for long in the scientific literature. The sensor developed by the team is a one- atom-thick, 0.2-mkm-wide ribbon of niobium nitride laid on a sapphire substrate. When cooled to 4.2 K, the ribbon becomes superconducting. The absorption of an IR photon leads to the breakup of Cooper pairs and destroys superconductivity locally in a region of about a ribbon width in size - thus affecting the current flowing through the ribbon. Shortly after, superconductivity reappears, though. The superconducting IR detector is thousands of times more sensitive and faster (several GHz) than their semiconductor-based counterparts. Source: Appl. Phys. Lett. 79 705 (2001)

Possible variation in the fine structure constant

Absorption lines in quasar spectra form when radiation from the quasar travels through the intergalactic gas clouds in the quasar's line of sight. The relative position of the lines of various chemical elements depends on the magnitude of the fine structure constant alpha= e2/hc . Using the high-precision spectrometer HIRES on the 10-meter Keck II telescope in Hawaii, J Webb and his colleagues in Australia analyzed spectral lines for a number of metals, spanning redshifts 0.5 < z < 3.5 and hence emitted at various epochs. The researchers conclude that over the last 6 billion years alpha grew by about 0.001% for each of four independent line sets, it being claimed that the result has a good statistical significance. Because this finding would be of immense importance if confirmed, its careful verification and further investigations along these lines are needed. The possibility that physical constants may vary with time has been discussed since the 1930s and is not inconsistent with current unified theories of fundamental interactions. Source: Phys. Rev. Lett. 87 091301 (2001)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

It is compiled from a multitude of Internet sources.
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