Accidental Discovery Dramatically Improves Electrical Conductivity

Accidental Discovery Dramatically Improves Electrical Conductivity

Quite by accident, Washington State University researchers have achieved a 400-fold increase in the electrical conductivity of a crystal simply by exposing it to light. The effect, which lasted for days after the light was turned off, could dramatically improve the performance of devices like computer chips.

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WSU doctoral student Marianne Tarun chanced upon the discovery when she noticed that the conductivity of some strontium titanate shot up after it was left out one day. At first, she and her fellow researchers thought the sample was contaminated, but a series of experiments showed the effect was from light.

The phenomenon they witnessed -- "persistent photoconductivity" -- is a far cry from superconductivity, the complete lack of electrical resistance pursued by other physicists, usually using temperatures near absolute zero. But the fact that they've achieved this at room temperature makes the phenomenon more immediately practical.

And while other researchers have created persistent photoconductivity in other materials, this is the most dramatic display of the phenomenon. The research, which was funded by the National Science Foundation, appears this month in the journal Physical Review Letters.

"The discovery of this effect at room temperature opens up new possibilities for practical devices," said Matthew McCluskey, co-author of the paper and chair of WSU's physics department. "In standard computer memory, information is stored on the surface of a computer chip or hard drive. A device using persistent photoconductivity, however, could store information throughout the entire volume of a crystal."

This approach, called holographic memory, "could lead to huge increases in information capacity," McCluskey said. Strontium titanate and other oxides, which contain oxygen and two or more other elements, often display a dizzying variety of electronic phenomena, from the high resistance used for insulation to superconductivity's lack of resistance.

"These diverse properties provide a fascinating playground for scientists but applications so far have been limited," said McCluskey, Tarun and physicist Farida Selim, now at Bowling Green State University, exposed a sample of strontium titanate to light for 10 minutes. Its improved conductivity lasted for days. They theorize that the light frees electrons in the material, letting it carry more current.

Friendly App Attacks Detect Vulnerabilities

Friendly App Attacks Detect Vulnerabilities

Hacking programs disguised as games are helping Apple to improve the security of devices operating on its iOS platform.

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Software companies work hard to protect their customers personal data from malicious applications, or 'apps', but even the most secure devices can be vulnerable. Skilled and independent computer scientists, such as Jin Han and co-workers at the A*STAR Institute for Infocomm Research and the Singapore Management University, can greatly assist companies by spotting security weaknesses before they are exploited.

Han and co-workers recently published a detailed comparison of the two very different security models used by the big players in mobile software, Apple's iOS platform and Google's Android1. Now, the researchers have developed subtle attack apps that test the secretive model of mobile security used in iOS2.

Apple's preferred security model is 'closed source'. This means that the company does not publish details of how apps are vetted before becoming available in their iTunes Store. Apple also refrains from publishing the internal code that decides whether apps can control phone functions such as contacts, calendars or cameras.

Despite this secrecy, the researchers were able to develop generic attack codes that enabled third-party control of iOS devices. They demonstrated seven different attack apps, disguised as games, that performed malicious actions including cracking the device's PIN, taking photographs and sending text messages without the user's awareness.

"We utilized private function calls to gain privileges that are not intended for third-party developers," explains Han. "Furthermore, we found a way to bypass Apple's vetting process so that our apps, embedded with proof-of-concept attacks, could be published on iTunes."

The attack apps worked on both iOS 5 and 6, although the team was careful to include secret triggers to protect any public users. The researchers have shared all of their findings with Apple and published recommendations on how the company should fix these vulnerabilities.

"Apple responded very quickly after we informed them about our findings, and before the release of the new iOS 7 platform," says Han. He expects that the company adopted countermeasures similar to those described in his team's paper, but cannot confirm this since iOS is closed source.

The ongoing debate over open- versus closed-source development will continue to rage among information technology specialists. Nevertheless, Han notes that their attack-app codes could, with some modifications, probably also bypass the permissions-based security model used in Android. "My personal opinion is that closed-source development is not good for security. A cryptosystem should be secure even if everything about the system, except the key, is public knowledge. I think the same principle applies to operating systems."

Key Processes of Photosynthesis Simulated On Quantum Level

Key Processes of Photosynthesis Simulated On Quantum Level

An artificial quantum system, physicists at Heidelberg University have simulated key processes of photosynthesis on a quantum level with high spatial and temporal resolution. In their experiment with Rydberg atoms the team of Prof. Dr. Matthias Weidemüller and Dr. Shannon Whitlock discovered new properties of energy transport. This work is an important step towards answering the question of how quantum physics can contribute to the efficiency of energy conversion in synthetic systems, for example in photovoltaics.

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The new discoveries, which were made at the Center for Quantum Dynamics and the Institute for Physics of Heidelberg University, have now been published in the journal Science.

In their research, Prof. Weidemüller and his team begin with the question of how the energy of light can be efficiently collected and converted elsewhere into a different form, e.g. into chemical or electric energy. Nature has found an especially efficient way to accomplish this in photosynthesis. Light energy is initially absorbed in light-harvesting complexes -- an array of membrane proteins -- and then transported to a molecular reaction centre by means of structures called nanoantennae; in the reaction centre the light is subsequently transformed into chemical energy. "This process is nearly 100 percent efficient. Despite intensive research we're still at a loss to understand which mechanisms are responsible for this surprisingly high efficiency," says Prof. Weidemüller. Based on the latest research, scientists assume that quantum effects like entanglement, where spatially separated objects influence one another, play an important role.

In their experiments the researchers used a gas of atoms that was cooled down to a temperature near absolute zero. Some of the atoms were excited with laser light to high electric states. The excited electron of these "atomic giants," which are called Rydberg atoms, is separated by macroscopic distances of almost a hair's breadth from the atomic nucleus. Therefore these atoms present an ideal system to study phenomena at the transition between the macroscopic, classical world and the microscopic quantum realm. Similar to the light-harvesting complexes of photosynthesis, energy is transported from Rydberg atom to Rydberg atom, with each atom transmitting its energy packages to surrounding atoms, similar to a radio transmitter.

"To be able to observe the energy transport we first had to find a way to image the Rydberg atoms. At the time it was impossible to detect these atoms using a microscope," explains Georg Günter, a doctoral student in Prof. Weidemüller's team. A trick from quantum optics ensured that up to 50 atoms within a characteristic radius around a Rydberg atom were able to absorb laser light. In this way each Rydberg atom creates a tiny shadow in the microscope image, allowing the scientists to measure the positions of the Rydberg atoms.

The fact that this technique would also facilitate the observation of energy transport came as a surprise, as PhD student Hanna Schempp emphasises. However, the investigations with the "atomic giants" showed how the Rydberg excitations, which are immersed in a sea of atoms, diffused from their original positions to their atomic neighbours, similar to the spreading of ink in water. Aided by a mathematical model the team of Prof. Weidemüller showed that the atomic sea crucially influences the energy transport from Rydberg atom to Rydberg atom.

"Now we are in a good position to control the quantum system and to study the transition from diffusive transport to coherent quantum transport. In this special form of energy transport the energy is not localised to one atom but is distributed over many atoms at the same time," explains Prof. Weidemüller. As with the light-harvesting complexes of photosynthesis, one central question will be how the environment of the nanoantennae influences the efficiency of energy transport and whether this efficiency can be enhanced by exploiting quantum effects. "In this way we hope to gain new insights into how the transformation of energy can be optimised in other synthetic systems as well, like those used in photovoltaics," the Heidelberg physicist points out.