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Laser mastery narrows down sources of superconductivity
Feb. 24, 2013 ? Identifying the mysterious mechanism underlying high-temperature superconductivity (HTS) remains one of the most important and tantalizing puzzles in physics. This remarkable phenomenon allows electric current to pass with perfect efficiency through materials chilled to subzero temperatures, and it may play an essential role in revolutionizing the entire electricity chain, from generation to transmission and grid-scale storage. Pinning down one of the possible explanations for HTS -- fleeting fluctuations called charge-density waves (CDWs) -- could help solve the mystery and pave the way for rapid technological advances.
Now, researchers at the Massachusetts Institute of Technology and the U.S. Department of Energy's Brookhaven National Laboratory have combined two state-of-the-art experimental techniques to study those electron waves with unprecedented precision in two-dimensional, custom-grown materials. The surprising results, published online February 24, 2013, in the journal Nature Materials, reveal that CDWs cannot be the root cause of the unparalleled power conveyance in HTS materials. In fact, CDW formation is an independent and likely competing instability.
"It has been difficult to determine whether or not dynamic or fluctuating CDWs even exist in HTS materials, much less identify their role," said Brookhaven Lab physicist and study coauthor Ivan Bozovic. "Do they compete with the HTS state, or are they perhaps the very essence of the phenomenon? That question has now been answered by targeted experimentation."
Custom-grown Superconductors
Electricity travels imperfectly through traditional metallic conductors, losing energy as heat due to a kind of atomic-scale friction. Impurities in these materials also cause electrons to scatter and stumble, but superconductors can overcome this hurdle -- assuming the synthesis process is precise.
For this experiment, Bozovic used a custom-built molecular beam epitaxy system at Brookhaven Lab to grow thin films of LaSrCuO, an HTS cuprate (copper-oxide) compound. The metallic cuprates, assembled one atomic layer at a time, are separated by insulating planes of lanthanum and strontium oxides, resulting in what's called a quasi-two-dimensional conductor. When cooled down to a low enough temperature -- less than 100 degrees Kelvin -- strange electron waves began to ripple through that 2D matrix. At even lower temperatures, these films became superconducting.
Electron Sea
"In quasi-two-dimensional metals, low temperatures frequently bring about interesting collective states called charge-density waves," Bozovic said. "They resemble waves rolling across the surface of a lake under a breeze, except that instead of water, here we actually have a sea of mobile electrons."
Once a CDW forms, the electron density loses uniformity as the ripples rise and fall. These waves can be described by familiar parameters: amplitude (height of the waves), wavelength (distance between waves), and phase (the wave's position on the material). Detecting CDWs typically requires high-intensity x-rays, such as those provided by synchrotron light sources like Brookhaven's NSLS and, soon, NSLS-II. And even then, the technique only works if the waves are essentially frozen upon formation. However, if CDWs actually fluctuate rapidly, they may escape detection by x-ray diffraction, which typically requires a long exposure time that blurs fast motion.
Measuring Rolling Waves
To catch CDWs in action, a research group at MIT led by physicist Nuh Gedik used an advanced ultrafast spectroscopy technique. Intense laser pulses called "pumps" cause excitations in the superconducting films, which are then probed by measuring the film reflectance with a second light pulse -- this is called a pump-probe process. The second pulse is delayed by precise time intervals, and the series of measurements allow the lifetime of the excitation to be determined.
In a more sophisticated variant of the technique, largely pioneered by Gedik, the standard single pump beam is replaced by two beams hitting the surface from different sides simultaneously. This generates a standing wave of controlled wavelength in the film, but it disappears rapidly as the electrons relax back into their original state.
This technique was applied to the atomically perfect LaSrCuO films synthesized at Brookhaven Lab. In films with a critical temperature of 26 degrees Kelvin (the threshold beyond which the superconductivity breaks down), the researchers discovered two new short-lived excitations -- both caused by fluctuating CDWs.
Gedik's technique even allowed the researchers to record the lifetime of CDW fluctuations -- just 2 picoseconds (a millionth of a millionth of a second) under the coldest conditions and becoming briefer as the temperatures rose. These waves then vanished entirely at about 100 Kelvin, actually surviving at much higher temperatures than superconductivity.
Ruling out a Suspect
The researchers then hunted for those same signatures in cuprate films with slightly different chemical compositions and a greater density of mobile electrons. The results were both unexpected and significant for the future of HTS research.
"Interestingly, the superconducting sample with the highest critical temperature, about 39 Kelvin, showed no CDW signatures at all," Gedik said.
The consistent emergence of CDWs would have bolstered the conjecture that they play an essential role in high-temperature superconductivity. Instead, the new technique's successful detection of such electron waves in one sample but not in another (with even higher critical temperature) indicates that another mechanism must be driving the emergence of HTS.
"Results like this bring us closer to understanding the mystery of HTS, considered by many to be one of the greatest problems in physics today," Bozovic said. "The source of this extraordinary phenomenon is slowly but surely running out of places to hide."
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The above story is reprinted from materials provided by DOE/Brookhaven National Laboratory.
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শনিবার, ২৩ ফেব্রুয়ারি, ২০১৩
Falcon Northwest Mach V (Triple Titan)
The launch of Nvidia's new GeForce GTX Titan graphics card has lit a fire underneath multiple boutique computer manufacturers. Falcon Northwest has built a new version of its Mach V desktop tower?the Mach V (Triple Titan)?to take advantage of the Titan's capabilities, with support for up to three of the new graphics cards and a six-core Intel Core i7 3970X CPU overclocked to 4.7GHz. The system's price tag as configured was $7,394. Yes, you read that right. The three graphics cards account for about $3000 of the eye-popping figure; this much power doesn't come cheap. The system's performance blew away the competition for the most part, setting a new standing for PC gaming. Thus, the Mach V (Triple Titan) takes the Editors' Choice for high-end gaming rigs.
Design and Features
The Mach V uses the same vertical chassis design as the Maingear Shift Super Stock (Core i7-3930K) ($5289 4.5); both cases orient video cards and components upwards to take advantage of the fact that hot air naturally rises. From the outside, the chassis design is simple, even understated, but the hardware inside the box is some of the most powerful that money can buy. There's 32GB of DDR3-1866 RAM, the aforementioned Intel Core i7-3970X, and a 256GB Crucial M4 SSD, backed up by a 2TB Western Digital Caviar Green hard drive. The entire system is powered by a 1200W PSU; optical storage comes courtesy of a 16x Asus DVD-RW drive.
The disadvantage to this triple-GPU configuration is that the GPUs consume every available slot; there's literally no room for any additional hardware. Falcon Northwest makes up for the lack of free PCI-Express ports with a whopping nine USB 2.0 ports and six USB 3.0 ports. Two of the latter are mounted at the top of the case for convenient access while working.
The vertical design presents two other challenges. First, a desk that's designed to exhaust hot air at the back may end up trapping that air if the system exhausts at the top. Second, there's the cabling issue. If you don't have many external drives or peripherals, the Mach V presents a very clean, crisp profile. Add three monitors' worth of cables, a few external drives, and a keyboard + mouse, and the top of the system looks like a rat's nest.
The trade-off is that cooling performance is typically better in a vertical case?and with three GPUs and a 4.7GHz overclocked CPU, the Mach V needs to balance cooling performance and noise. It does so quite well. The next-generation GPU Boost technology built into the Titan family keeps each GPU's temperature pegged at 80C. Customers can override this manually if they wish, but the system automatically adjusts frequencies to ensure that even a three-GPU system runs no risk of overheating.
Performance
The three Titan graphics cards are the showcase of the system and its strongest point. The Mach V (Triple Titan) sweeps even the Maingear F131 Super Stock (GTX Titan) in our 3D benchmark tests. Its 3DMark 11 Extreme score of 12,505 is 18% faster than the previous record-holder ? the Falcon Northwest Mach V (Intel Core i7-3770K), which was equipped with a pair of dual-GPU GTX 690s (four GPUs in total). In Aliens vs. Predator at 1,920-by-1,080 with 4x anti-aliasing (AA) and 4x anisotropic filtering (AF) turned on, the Mach V (Triple Titan) turned in 222 frames per second (fps) compared with the dual-Titan-equipped Maingear F131's 182fps. Performance in version 3.0 of Unigine's Heaven benchmark test at 1,920-by-1080 with 4x AA / AF was 198fps, compared to 155fps for the Maingear F131.
In other areas, the Maingear F131 and Mach V (Triple Titan) came in neck and neck. Both systems completed a Handbrake decode in 23 seconds. The Maingear F131 led in PCMark 7 (6,523 vs. 6,241 for the Mach V) and in Photoshop CS6 (2 minutes, 35 seconds to apply a dozen filters, vs. the Mach V's time of 2:41). The Mach V (Triple Titan) outperformed Maingear F131 in Cinebench, where the hexa-core 3970 got to stretch its legs. The multithreaded Cinebench R11.5 score for the Mach V (Triple Titan) was 13.62. Only the Origin Genesis (Core i7-3930K), with its 4.9GHz CPU, was faster.
Those performance figures provide an accurate model of how the Falcon-Northwest compares to other gaming systems. What they don't explain is how Falcon and its boutique competitors have leveraged the new cards to create better gaming experiences. We had the opportunity to test the Mach V in a 5,760-by-1,080 configuration across three 27-inch displays. A standard 1,920-by-1,080 display contains two megapixels? 5,760-by-1080 more than triples that, to 6.2MP. For comparison, 2,560-by-1,600 is 4 MP.
Yes, three GTX Titan GPUs pushing nearly 200fps in Heaven is impressive. What's more impressive yet is seeing the Mach V (Triple Titan) maintain 90fps in the same benchmark test when running across three displays at once. It's not a capability that's limited to synthetic benchmark tests?while multi-monitor gaming is still not standard, many recent titles like Shogun 2, Serious Sam BFE, World of Warcraft, and Batman: Arkham City all support it.
With three GTX Titan GPUs, the Falcon Northwest Mach V (Triple Titan) is capable of fielding game experiences that other configurations don't offer. Serious Sam BFE at 5,760-by-1,080 with 4x super-sampled AA, 16x AF, and all GPU and CPU options set to maximum, for example, still maintains a consistent 75fps.
The three-monitor experience isn't perfect?the view on the side panels is often distorted and, in some games, magnifies the landscape and enemies to appear closer than they are. Despite these flaws, multi-monitor gaming support is growing, and the immersion in these games is superior to single-panel play. Game for a few hours across three monitors, and you'll find that dropping back to one is jarring.
That said, what's also jarring is the Mach V (Triple Titan)'s price tag. At $999 each, there's no such thing as a cheap Titan GPU, but gamers who are interested without having quite that much scratch to spare should consider a single GPU configuration built around one of Falcon's lower-cost alternatives like the Tiki or FragBox.
If you've got the funds, this is an excellent system. It's a configuration that can handle multi-monitor gaming, 3D gaming, or both without breaking a sweat. Given the strength of the underlying hardware, this Mach V could be the center of a gaming platform for years to come. Buyers who prefer to buy a high-end system and use it for a number of years rather than upgrading on a regular basis should give the Mach V (Triple Titan) serious consideration. Of course, if you want to use this to replace the high-end gaming rig you bought last year because that system was the "fastest," that's also your prerogative. Regardless, with its record-breaking performance, triple-monitor capability, and smart power-saving functionality, the Falcon Northwest Mach V (Triple Titan) earns our Editors' Choice for high-end gaming desktops.
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University closes Muslim prayer room
21 February 2013
Last updated at 19:48 ET
City University in London has been accused of discrimination by a group of Muslim students after it closed a dedicated room used for Friday prayers.
A group of students have formed Muslim Voices on Campus, calling on the university to reverse its decision.
The group said it was being "unjustly targeted".
City said it acted because students had refused to submit the proposed content of sermons to the university before prayers to check its "appropriateness".
Wasif Sheikh, who leads the group, said: "We feel we are being unjustly targeted. All of our sermons are open, we welcome all students and all staff.
"But when you start submitting your sermons to be monitored and scrutinized then there's a chance for it to be dictated what's allowed and what's not allowed. We, as students, don't accept that."
Confrontational atmosphereThree years ago, the counter extremism think tank the Quilliam Foundation released a report on the sermons at Friday prayers at City University, in central London.
It claimed hard line views and a confrontational atmosphere were being encouraged.
In one sermon, which was recorded, the speaker said: "The Islamic state teaches to cut the hand of the thief. Yes it does. And it also teaches us to stone the adulterer.
"When they tell us that, the Islamic state tells us and teaches us to kill the apostate, yes it does."
BBC London has seen no evidence those views are still being spread now, but some argue the episode shows the need for greater scrutiny.
'Vicious history'Dr Usama Hasan from the Quilliam Foundation said: "If there was no past history at the university, I think what the students are saying would sound extremely reasonable. However, there has been quite a vicious and nasty history there.
"They have to deal with that and give a bit more to the university.
Continue reading the main story?Start Quote
End Quote Spokeswoman City University LondonThe university could not continue to condone an activity taking place on its premises where it cannot exercise reasonable supervision?
"Both sides need to compromise here and negotiate with some difficult conversations."
The challenge faced by many universities is how to deal with a potential threat - without being seen to restrict freedom of speech.
A statement from the university said it needed to be sure of the "appropriateness" of what was being discussed in sermons as authorized university events.
It said it also needed to be assured that all "students eligible to deliver" prayers and sermons "are considered equally and given the opportunity to do so".
The statement went on: "The university could not continue to condone an activity taking place on its premises where it cannot exercise reasonable supervision."
In addition, the university said it had suggested a number of alternative places for students to pray nearby.
Source: http://www.bbc.co.uk/news/uk-england-london-21542041
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