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Primeur weekly 2014-12-22

Focus

SimScale engineering simulation platform is perfectly running in a web browser now ...

EuroFlash

RSC Group has entered in the Top10 of the world leading supercomputer vendors by TOP500 list ...

ISC High Performance introduces workshops in 2015 ...

ADVA Optical Networking transforms fiber access networks with new Access Link Monitoring solution ...

Telindus Telecom deploys ADVA Optical Networking's encryption technology to secure operator network and Cloud services ...

Institute of Aircraft Design at the University of Stuttgart selects Bright Cluster Manager for its HPC data centre ...

Imec partners with Huawei on high-bandwidth optical data link technology ...

Six new projects enabling scientific discovery to start in 2015 ...

ENTRA: Better software cuts computer energy use ...

USFlash

Cray awarded $30 million contract from the Department of Defense High Performance Computing Modernization Programme ...

High Performance Computing Modernization Programme (HPCMP) doubles capabilities ...

Ancient wisdom boosts sustainability of biotech cotton ...

Compute Canada attracts High Performance Computing talent to lead upgrades to the national advanced research platform ...

Pacific Northwest National Laboratory a major contributor to Washington state economy ...

U.S. Department of Veterans Affairs taps IBM Watson to help accelerate and enhance care delivery ...

More than 100 teams expected for ASC15, road-showed at Melbourne and Nanjing ...

Stanford team combines logic, memory to build a 'high-rise' chip ...

Tulane has one of the world's fastest computers ...

Funding ended for University of California High-Performance AstroComputing Center ...

Simulations at NERSC help illuminate attosecond laser experiment findings ...

Riding a 100G InfiniBand backbone, InfiniCortex, the Intercontinental Supercomputing Architecture, debuts in New Orleans ...

Quantum physics just got less complicated ...

Switching to spintronics ...

Lasers, multiple languages support and more with wysiwyg R34 ...

Simulations at NERSC help illuminate attosecond laser experiment findings


19 Dec 2014 Berkeley - The entire semiconductor industry, not to mention Silicon Valley, is built on the propensity of electrons in silicon to get kicked out of their atomic shells and start to move through the material. These mobile electrons are routed and switched though transistors, carrying the digital information that characterizes our age. An international team of physicists and chemists at the University of California, Berkeley, has for the first time taken snapshots of this ephemeral event using attosecond pulses of soft x-ray light lasting only a few billionths of a billionth of a second. The researchers then used supercomputing resources at Lawrence Berkeley National Laboratory's National Energy Research Scientific Computing Center (NERSC) to help them better understand their findings.

While earlier femtosecond lasers were unable to resolve the jump from the valence shell of the silicon atom across the band-gap into the conduction electron region, the new experiments now show that this transition takes less than 450 attoseconds.

"Though this excitation step is too fast for traditional experiments, our novel technique allowed us to record individual snapshots that can be composed into a 'movie' revealing the timing sequence of the process", stated Stephen Leone, UC Berkeley professor of chemistry and physics.

Stephen Leone, his UC Berkeley colleagues and collaborators from the Ludwig-Maximilians Universität, the University of Tsukuba and Berkeley Lab reported their achievement in the December 12 issue ofScience.

More than a century has elapsed since the discovery that light can make certain materials conductive, according to Stephen Leone. The first movie of this transition follows the excitation of electronics across the band-gap in silicon with the help of attosecond extreme ultraviolet (XUV) spectroscopy, developed in the Leone and Neumark Attosecond Physics Laboratory.

In semiconducting materials, electrons are initially localized around the individual atoms forming the crystal and thus cannot move or contribute to electrical currents. When light hits these materials or a voltage is applied, some of the electrons absorb energy and get excited into mobile states in which the electrons can move through the material. The localized electrons take a "quantum jump" into the conduction band, tunneling through the barrier that normally keeps them bound to atoms.

These mobile electrons make the semiconductor material conductive so that an applied voltage results in a current flowing through. This behaviour allows engineers to make silicon switches, known as transistors, which have become the basis of all digital electronics.

For this study, the researchers used attosecond XUV spectroscopy like an attosecond stop watch to follow the electron's transition. They exposed a silicon crystal to ultrashort flashes of visible light emitted by a laser source. The subsequent illumination with x-ray-pulses of only a few tens of attoseconds (10-18 seconds) in duration allowed the researchers to take snapshots of the evolution of the excitation process triggered by the laser pulses.

They then ran simulations on NERSC's Hopper Cray XE6 supercomputer to interpret the experimental findings, using BerkeleyGW code and an in-house code named Shirley.

"The experiments were very novel, but the role of theory was primarily to explain the various physical mechanisms that led to specific features of interest observed in the experimental spectra", explained co-author Das Pemmaraju, a project scientist in Berkeley Lab's Chemical Sciences Division who ran the simulations. "The theoretical simulations help make sense of the experimental data and improve our understanding."

The excitation of a semiconductor with light is traditionally conceived as a process involving two distinct events. First, the electrons absorb light and get excited. Afterwards, the lattice, composed of the individual atoms in the crystal, rearranges in response to this redistribution of electrons, turning part of the absorbed energy into heat carried by vibrational waves called phonons.

In analyzing their data, the team found clear indications that this hypothesis is true. They showed that initially, only the electrons react to the impinging light while the atomic lattice remains unaffected. Long after the excitation laser pulse has left the sample - some 60 femtoseconds later - they observed the onset of a collective movement of the atoms, that is, phonons. This is near the 64 femtosecond period of the fastest lattice vibrations.

Based on current theory, the researchers calculated that the lattice spacing rebounded about 6 picometers (10-12 meters) as a result of the electron jump, consistent with other estimates.

"These results represent a clean example of attosecond science applied to a complex and fundamentally important system", stated Daniel Neumark, UC Berkeley professor of chemistry.

The unprecedented temporal resolution of this attosecond technology will allow scientists to resolve extremely brief electronic processes in solids that so far seemed too fast to be approached experimentally, said Martin Schultze, who was a guest researcher in Stephen Leone's lab last year, visiting from the Ludwig-Maximilians Universität München. That imposes new challenges to the theory of light-matter interaction theory, including the excitation step and its timescale and the interpretation of experimental x-ray spectra.

"But here is also an advantage", Martin Schultze added. "With our ultrashort excitation and probing pulses, the atoms in the crystal can be considered frozen during the interaction. That eases the theoretical treatment a lot."
Source: National Energy Research Scientific Computing Center - NERSC

Back to Table of contents

Primeur weekly 2014-12-22

Focus

SimScale engineering simulation platform is perfectly running in a web browser now ...

EuroFlash

RSC Group has entered in the Top10 of the world leading supercomputer vendors by TOP500 list ...

ISC High Performance introduces workshops in 2015 ...

ADVA Optical Networking transforms fiber access networks with new Access Link Monitoring solution ...

Telindus Telecom deploys ADVA Optical Networking's encryption technology to secure operator network and Cloud services ...

Institute of Aircraft Design at the University of Stuttgart selects Bright Cluster Manager for its HPC data centre ...

Imec partners with Huawei on high-bandwidth optical data link technology ...

Six new projects enabling scientific discovery to start in 2015 ...

ENTRA: Better software cuts computer energy use ...

USFlash

Cray awarded $30 million contract from the Department of Defense High Performance Computing Modernization Programme ...

High Performance Computing Modernization Programme (HPCMP) doubles capabilities ...

Ancient wisdom boosts sustainability of biotech cotton ...

Compute Canada attracts High Performance Computing talent to lead upgrades to the national advanced research platform ...

Pacific Northwest National Laboratory a major contributor to Washington state economy ...

U.S. Department of Veterans Affairs taps IBM Watson to help accelerate and enhance care delivery ...

More than 100 teams expected for ASC15, road-showed at Melbourne and Nanjing ...

Stanford team combines logic, memory to build a 'high-rise' chip ...

Tulane has one of the world's fastest computers ...

Funding ended for University of California High-Performance AstroComputing Center ...

Simulations at NERSC help illuminate attosecond laser experiment findings ...

Riding a 100G InfiniBand backbone, InfiniCortex, the Intercontinental Supercomputing Architecture, debuts in New Orleans ...

Quantum physics just got less complicated ...

Switching to spintronics ...

Lasers, multiple languages support and more with wysiwyg R34 ...