# Extracts from the Internet

## Superconductivity in lithium

K Shimizu and his colleagues at the Universities of Osaka and Tokyo have discovered that under pressure above 30 GPa lithium becomes a superconductor and that at 48 GPa the superconducting transition temperature increases to Tc=20K, a record for simple substances. This temperature, however, is about four times less than the theoretical Tc. Except for metallic hydrogen, for which no definitive evidence of superconductivity has yet been found, Li is the lightest of superconductors. As long ago as in the mid- 1980s, T H Lin and K J Dunn (University of California) have observed some evidence of superconductivity in lithium under pressure, and the new experiments by the Japan scientists have now confirmed this result. A sample of lithium was compressed in a diamond anvil cell. The main difficulty with the experiment was that lithium is chemically very active and forms a compound with diamond. The team has overcome this problem by giving the diamond anvil a special form, with an indentation for the sample under study. The appearance of superconductivity was signalled by the disappearance of electrical resistance, but the Meissner effect was not observed because of technical difficulties. It is found only that Tc decreases with increasing magnetic field and that the field of 3 T destroys superconductivity. Source: Nature 419 597 (2002)

## The wavelength of bi-photons

Quantum theory predicts that de Broglie wavelength of an ensemble of N photons in an entangled quantum state is L/N, where L is the wavelength of each individual photon. The quantum states of such photons are correlated, so that the ensemble of photons resembles the Bose condensate in some respect. The verification of this theoretical prediction for photon pairs (bi-photons) has been made at the University of Osaka in Japan. Bi-photons were obtained by splitting single photons in a nonlinear crystal. Measurements using a Mach-Zender interferometer revealed that a bi-photon shows the properties of a single particle with wavelength half that of each individual photon. That is, the wavelength of the pair equals the wavelength of the parent photon's before its splitting in the crystal. Bi-photons will be especially attractive for applications when it proves possible to make a bi-photon of two ordinary photons without changing the photon wavelengths. One promising approach to this problem is to use the process of hyper-parametric scattering'. Source: Phys. Rev. Lett. 89 213601 (2002)

## Superfast measurements

The generation of increasingly short laser pulses has now approached its natural limit, when pulse durations equal to several electromagnetic wave periods (i. e., a few femtoseconds for the optical range) have been obtained. While this duration is sufficiently short for investigating the dynamics of molecular structures, it is not short enough for the study of much faster electronic processes. One way to overcome this fundamental limit is by using short-wavelength radiations, for example X-rays. Recently, F Krausz, M Drescher and their colleagues in Germany and Austria have developed a technique for producing isolated pulses of soft X-ray radiation with a duration of less than 1 fs and found a way to synchronize these pulses with short pulses of visible light. In their new experiment, the same authors have used this method to study the rearrangement of the electron cloud around a krypton atom with a time resolution of about 100 attoseconds (1as=10-18s). First, a short X-ray pulse threw out an electron from an orbit close to the nucleus, and then the resulting electron vacancy was filled by one of the outer electrons. Almost simultaneously with the X-ray pulse, a laser pulse with a duration of several femtoseconds was applied to the atom. The energy spectrum of the emitted and scattered photons and electrons was measured by a spectrometer, and from this spectrum the nature of the processes occurring in the atom was reconstructed. Source: Nature 419 803 (2002)

## Conductivity of the hydrogen molecule

Jan van Ruitenbeek and his colleagues at Leiden University (the Netherlands) have performed an experiment which measured the electrical resistance of a single hydrogen molecule placed between two platinum electrodes. The electrodes and the gap between them formed the so-called break-junction', a device which resulted from the platinum wire being slowly stretched to the point where a microscopic neck formed and then broke up. The team used a piezoelectric element to stretch the wire and to measure how the separation between the contacts changed. From the electric resistance of the contact, the researchers were able to catch the moment when a few single metal atoms remained in the neck and the break-up took place. To the presence of only single platinum atoms in the contact there corresponded electrical conductivity equal to 1.4-1.9 quantum units of conductivity, G0=2e2/h. The whole of the setup was placed in a high vacuum and cooled to a temperature of 4.2 K. If the experiment was repeated in molecular hydrogen, a conductivity of 1G0 was found in many cases. According to theoretical calculations, this quantity corresponds to single hydrogen molecules getting into the contact. Another interesting point was that the contact with a molecule in it remained stable for a long period of time. Source: Nature 419 906 (2002)

## A black hole at the centre of the Galaxy

A team at the Max Planck Institute for Extraterrestrial Physics (Germany) led by R Schoedel has performed new high-precision observations of stars near the object Sgr A* at the centre of our Galaxy. This object emits radio waves and X-rays and is presumed to be a supermassive black hole. Observations of one of the stars in infrared wavelengths have been performed since 1992. The use of the adaptive optics system CONICA/NAOS in the European observatory's VLT telescope in Chile has greatly improved the accuracy in position determining in recent years. Over ten years of observation, the star passed 2/3 of its orbit. In particular, the passage of the star through the apocenter and perihelion was observed. The star's orbit was found to be elliptic, with an orbital period of 15.2 years. The relative position of the star and Sgr A* was accurately determined by superimposing infrared and radio images, a procedure which greatly benefitted from the presence in the vicinity of Sgr A* of several maser sources whose positions are well known from radio observations. The results indicate with considerable confidence that Sgr A* is indeed a black hole with mass M=(3.7 + -1.5)x106 solar masses. The distance of closest approach between Sgr A* and the star is only 2000 times greater than the Schwarzschild radius corresponding to M. This rules out the idea that Sgr A* is a cluster of neutron stars or black holes, or a ball of a degenerate fermion gas of elementary particles of some kind. The only alternative is a ball of bosons, but the formation of such a ball is problematic from the theoretical point of view. Source: Nature 419 694 (2002)

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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