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Holographic storage sees light

Their technique works by using a process called two-colour holography. One wavelength of light records information on the crystal and another is used to read the data. Their method fires two light sources at the same time at the target.

The researchers doped a lithium niobate crystal with iron and manganese to create two different ‘deep electron traps’ between the valence and conduction bands in the crystal. An ultraviolet lamp was used to excite electrons from a low energy state (the manganese ion), to an intermediate state (on the iron ion) via the conductance band. Energy from a visible red laser can then excite the electrons in the intermediate state back up to the conduction band where it drops to the manganese low energy state. The ions left behind then change the refractive index of the material and record the diffraction pattern. Without the ultraviolet light, then the red light does not affect the refractive index of the crystal, so the material can be read without erasing the data. If more ultraviolet light is played on the crystal, the diffraction pattern is erased and new data can be written.

Light makes insulator conduct

The researchers cooled the manganese oxide compound (Pr_0.7Ca_0.3MnO₃) to below 200 K in a zero magnetic field. At this temperature the electrons drop into a low energy state, which minimizes their natural repulsion. The material is usually an insulator, but previous experiments indicated that it can be transformed to a conductor by using X-rays, visible light or electric fields.

Manfred Fiebig and colleagues at the University of Tokyo fabricated two gold electrodes on the surface of the material separated by 150 nm. When they fired rapid pulses of ultraviolet light between the electrodes, they noticed that the change to a conductor state remains localized between the electrodes. Once the laser pulses are switched off, the material remains a conductor until current stops flowing between the electrodes.

The effect appears to be similar to an effect called negative differential resistance, which is found in semiconductors. When a electron in a semiconductor is subjected to a high electric field, the field excites the electron enough for it to ionize impurities in the material. This causes a chain reaction in which other electrons released in the collisions ionize other molecules, changing the material to a weak conductor. However, in semiconductors this is thought to be a “single electron” effect – as the electric field increases, a given electron acquires more energy before it hits an impurity. A more complex many-body effect appears to be happening in the oxide which means that the conductor effect is more noticeable.

Nanotubes for the Mass Market

Carbon nanotubes – which can be thought of as a rolled-up graphite sheet – were discovered in 1991. One way to produce large quantities of nanotubes is to place two water-cooled carbon electrodes 1mm apart in a vacuum chamber filled with an inert gas such as argon. A high-current arc between the electrodes creates nanotubes on the negative electrode while removing carbon from the positive electode. A scraper then pushes the nanotubes to the floor of the chamber at regular intervals. As the positive electrode is ‘eaten’ away by the process, one of the electrodes has to be moved towards the other to retain the 1 mm gap. However, the need for these moving parts inside the vacuum chamber increases the complexity of the manufacturing process. Olk overcomes this problem by submerging the electrodes in a tank filled with liquid nitrogen or argon, avoiding the need for a vacuum chamber. Mechanisms for moving the electrodes can be positioned above the tank in the open air, simplifying the equipment and the production process.

Computer heal thyself

Researchers are investigating new ways to make computer chips in an effort to increase processing speed. It is likely that these new methods, such as chemical fabrication and assembly, will produce large numbers of defective chips. Rather than throw these chips away, James Heath of UCLA and colleagues at Hewlett-Packard are investigating how to build a computer that will work even if some of the chips inside are faulty. To test their ideas they built the Teramac using faulty silicon chips produced by existing fabrication techniques.

Instead of a traditional design with the whole computer on a single chip, Teramac has all the components for computation laid out in a lattice formation. Each component also has high speed connections with its nearest neighbours. The computer is programmed so that if one route or component is defective, another route or component is used. Calculations by the team indicate that 10 per cent of the 864 processor chips inside the Teramac – plus 3 per cent of its 220, 000 memory chips and 10 per cent of the signals – were defective. However, by building the computer first, and then detecting the faults and programming the system to bypass problems, the Teramac was significantly faster than a high-end workstation.

Crosswords and cross words on the Internet

The crossword puzzle was created using a software package developed by Lewis. The instructor simply types the clues and their answers into the program, which then creates the crossword grid. Although the students are given a hard copy of the puzzle, they have to enter their solutions over the Internet with a Java capable browser (see, for example, http://deborah.eastern.edu/crossword/crossword.html). The Java program then marks the puzzle and sends the grade to the lecturer. Bradstreet and Steelman hope to make the network programs available at little or no cost to other universities.

However, academics at the University of Washington have called the growth of interactive computer tools in education “naive” and potentially “disastrous”. Their letter was triggered by the actions of the 2020 Commission, a planning committee for higher education in the state. Reports from the committee down-played the role of universities as centres of learning and concentrated on the benefits of multimedia education. University staff were angry that the committee did not include any academics. “Distance learning should be a supplement to higher education, not a central feature of it, ” the letter says.

Doomsday Clock moves closer to midnight

The Doomsday Clock was set up in 1947 by atomic scientists concerned at the growing nuclear tensions between the US and Soviet Union. Initially the clock read seven minutes to midnight, and moved to two minutes to midnight after the first hydrogen bombs were tested. Until yesterday the time on the clock was 14 minutes to midnight.

The board claims that the tests by India and Pakistan are a symptom of the failure of the international community to fully commit itself to preventing the spread of nuclear weapons. It points in particular to the failure of the Russia Duma and the US Congress to ratify the Comprehensive Test Ban Treaty (CTBT). Of the five nuclear powers, only France and the UK have so far ratified the CTBT. The Duma has also refused to ratify the START II nuclear arms reduction treaty. Over 30, 000 nuclear weapons are currently kept in a state of readiness by Russia and the US.

Meanwhile the government in Pakistan has announced a unilateral moratorium on nuclear testing and says that it is ready to formalise this arrangement with India. However, Pakistan is still refusing to sign the CTBT.

Hackers attack Stanford accelerator

The hacker or hackers managed to gain password access to SLAC by intercepting the username and password of a registered user accessing data inside SLAC’s firewall. The random behaviour of the hackers inside the SLAC network suggests that they were trying to gain access to other government and university servers rather than access SLAC data.

“They covered their tracks very well, we have no idea who they were, ” said Patricia Anne Moore, a spokesperson for SLAC. Moore believes that the lab was “very fortunate” that no experimental data or programs appear to have been altered. “The potential damage we could have suffered was enormous” she said. During the shutdown an ISDN line was set-up so that some files could be down-loaded for users. The lab is now investigating a longer term solution for such problems.

Hydrogen exonerated in Hindenburg crash

William Van Vorst of the University of California at Los Angles and Addison Bain, an independent consultant, have gathered evidence that hydrogen could not have caused the explosion. Film footage and witnesses of the crash describe bright yellow flames burning downwards, but hydrogen would only burn in an upward direction with a colourless flame. Several helium-filled airships also crashed and appeared to burn in a similar way. A second piece of evidence comes from chemical analysis of fabric used to cover the airframe. Van Vorst and Bain discovered the that sealant used on the cotton – a mixture of iron oxide, cellulose acetate and aluminium powder – was extremely flammable. “The total mixture might well serve as a respectable rocket propellant, ” says Van Vorst. Finally, the design used to attach the cotton to the airframe did not have any mechanism for preventing the build up of electrostatic charge. They believe that when the skin finally discharged, it passed a current through the skin direct to the frame, igniting the sealant on the fabric.

After the crash Hugo Eckner, chairman of Zeppelin company, publicly blamed the US for the accident by cutting off his supplies of helium, forcing him to use hydrogen. However, Van Vorst and Bain point out that the Graf Zeppelin , which was being constructed at the time of the disaster, suddenly underwent a series of modifications to reduce the risk posed by the fabric. These included doping the fabric with a fireproofing agent and using copper rather than aluminium in the sealant to prevent the build up of static electricity. The researchers also discovered a letter from Otto Beyersdorff – an independent investigator hired by the Zeppelin Company – who blamed the fabric for causing the blaze. “Clearly, there must have been strong suspicion that the fabric was the real culprit, ” says Van Vorst.

Sloan Survey sees first light

The 2.5 m Sloan telescope is based at Apache Point Observatory in New Mexico. The telescope has an unusually wide field of view and one of the most advanced digital cameras ever built. The camera consists of 30 charge-coupled devices (CCDs), each containing four million picture elements. One night’s observing will produce up to 200 gigabytes of data on a dozen tapes. Astronomers record three-dimensional information by fitting an aluminium plate containing 640 small holes onto the focal plane of the telescope. These holes mark the location of objects observed by the telescope. Light passing through the holes is collected with fibre optics and piped to a spectrograph which analyses the redshift and hence the distance to the object. Astronomers hope that the survey will help refine theories of how the universe’s structure formed after the Big Bang.

Complexity holds thekey to predicting epileptic seizures

In a process called electroencephalogram (EEG), brain activity is measured by strapping electrodes to the scalp. The electrodes record weak electrical waves produced from different parts of the brain and the results plotted on a graph. The algorithm that is widely used to study different states of consciousness (e.g., being asleep or awake) can also be used to monitor brain activity during a seizure, no one has been able to identify a pre-seizure state in the brain until now.

Lehnertz and Elger recorded the EEG patterns from 128 electrodes implanted in 16 patients before, during and after a seizure. Using a time-resolved analysis of the algorithm they looked at the part of the brain most closely associated with epileptic fits. A computer automatically applied the algorithm to the data to produce a chart showing the complexity. The EEG scans picked up an increase in the frequency and duration of reduced ‘blips’ in brain activity as a seizure became more likely. The researchers say that when reductions in complexity reach a specified level, the patient could be said to be entering a seizure.

The technique could give people who suffer epileptic fits enough advance warning to take drugs that reduce the severity of the attack.

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