Euler's Disk

Guitar Oscillations

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http://physical-review.blogspot.de

The Nobel Prize in Physics 2011

Saul Perlmutter

Brian P. Schmidt

Adam G. Riess

The Nobel Prize in Physics 2011 was awarded "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae" with one half to Saul Perlmutter and the other half jointly to Brian P. Schmidt and Adam G. Riess.

http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/

Growth of platinum ultrathin films on Al2O3(0001)

http://www.sciencedirect.com/science/article/pii/S0039602811002895

The early stages of Pt growth on Al₂O₃(0001) are investigated by means of electron microscopy and X-ray diffraction. We deposit Pt ultrathin films of thicknesses ranging from 0.5 nm to 10 nm using DC sputtering deposition at 650 °C and 750 °C. We demonstrate that the growth is of Volmer-Weber type and show that epitaxy of islands could be reached at elevated temperature. The island morphology is governed by surface energy minimization, leading to well defined islands whose size depends on the nominal thickness. At 750 °C, the appearance of two island populations reveals that although the two studied growth temperatures are quite close, the growth kinetic is static at 650 °C and dynamic at 750 °C. Due to the lattice misfit between Pt and Al₂O₃, X-ray and electron diffractions reveal that the islands undergo an in-plane compression accompanied with an out-of-plane tension and such strains relax with thickness to reach the bulk lattice parameter above 10 nm when the Pt film becomes continuous. Electron microscopy in high resolution mode allows measuring the strain in isolated islands and corroborates X-ray diffraction measurements.

One-loop omega-potential of quantum fields with ellipsoid constant-energy surface dispersion law

http://www.sciencedirect.com/science/article/pii/S0003491611001138
Rapidly convergent expansions of a one-loop contribution to the partition function of quantum fields with ellipsoid constant-energy surface dispersion law are derived. The omega-potential is naturally decomposed into three parts: the quasiclassical contribution, the contribution from the branch cut of the dispersion law, and the oscillating part. The low- and high-temperature expansions of the quasiclassical part are obtained. An explicit expression and a relation of the contribution from the cut with the Casimir term and vacuum energy are established. The oscillating part is represented in the form of the Chowla–Selberg expansion of the Epstein zeta function. Various resummations of this expansion are considered. The general procedure developed is then applied to two models: massless particles in a box both at zero and nonzero chemical potential, and electrons in a thin metal film. Rapidly convergent expansions of the partition function and average particle number are obtained for these models. In particular, the oscillations of the chemical potential of conduction electrons in graphene and a thin metal film due to a variation of size of the crystal are described.

Fig. 3. On the top panel: I. The chemical potential of electrons in the thin metal film at the effective electron mass m_*=m, the temperature 

, and the average particle number N=1.6×10¹⁶ what corresponds to the undeformed metal film with the area 

, the width L_x=1 nm, and the chemical potential 

. The small plots depict the total chemical potential at 

(II) and 

(III). The small plot (IV) depicts the quasiclassical part 

of the chemical potential at 

. On the bottom panel: (I) The quasiclassical contribution to the average number of conduction electrons at 

and 

. The total number of conduction electrons in the thin metal film with the area 

at the fixed chemical potential 

, and the temperatures 

(II), 

(III), 

(IV), and  

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Auger effect boosts doped devices

New experiments by researchers at the University of Washington, Seattle, have looked at the roles of dopants in the photoluminescence of electrically active quantum dot films for the first time and have revealed remarkably large effects never previously observed. Daniel Gamelin and colleagues have found that an "Auger process" involving Mn²⁺ dopants in the quantum dots is much more effective than that in undoped dots – a result that bodes wells for various device technologies, such as field-effect transistors or solar cells.

Auger processes generally begin with the removal of an inner shell atomic electron to form a vacancy. There are many ways to produce this vacancy, with the most common being bombardment with an electron beam. The inner shell vacancy is then filled by a second atomic electron from a higher shell and energy is released at the same time in this step. Finally, a third electron, known as an Auger electron, escapes, so carrying off the excess energy in a "radiationless" process.

The Auger processes studied in Gamelin and colleagues' work are similar, radiationless de-excitation processes in which an excited state is quenched by transferring its energy to an electron. In previous studies on quantum dots, Auger processes have involved excitons (electron-hole pairs) coupling with other excitons, or excitons coupling with electrons.

"In our study, the process involves a Mn²⁺ excited state coupling with an electron," explained Gamelin. "The energy of the Mn²⁺ excited state is rapidly transferred to an electron at the bottom of the quantum dot conduction band, promoting that electron to a much higher 'hot electron' level, which can then cool back to the bottom of the conduction band by giving off heat."

A million times longer
The researchers found that the Auger process involving Mn²⁺dopants in quantum dots are much more effective than those of their undoped counterparts. This difference arises because the excited state lifetime of Mn²⁺ is about a million times longer than that of the undoped quantum dot. This gives electrons more time to diffuse around in the doped nanocrystal film and find the excited nanocrystal before the Mn²⁺ decays

This Auger process is in fact the microscopic reverse of the so-called impact excitation that forms the basis of many electroluminescent devices. These devices generally run at the highest possible current densities to be as bright as possible but their performance is ultimately believed to be limited by Auger processes like the one that Gamelin's team has observed. "By studying these Auger processes, we hope to learn more about how they may impact electroluminescence device performance, and about how to exploit them for other device applications where they may actually be beneficial."

For their measurements, the researchers began by depositing a film of Mn²⁺-doped CdS quantum dots on top of a transparent conductive oxide. They then used this as the working electrode in an electrochemical cell. When they subsequently applied a potential to the cell, electrons were transferred into the quantum dots.

"We measured the absorption and photoluminescence of the quantum dots as we performed the electrochemical experiments – something that allowed us to determine how the added electrons changed the absorption and photoluminescence properties of the quantum dots," Gamelin toldnanotechweb.org.

Fundamentally important
The work could be fundamentally important for interpreting various photoluminescence and electroluminescence results obtained for doped semiconductor nanocrystals. "Generally, scientists collect data and then try to interpret the data based on models they build to account for all known processes," said Gamelin. "By demonstrating that this Auger process can be extremely effective in doped nanocrystals, we believe that we are alerting the research community to the fact that they must now include the possibility of this process when analysing their data."

For example, a review of the literature has already helped the team identify several examples where this Auger process may provide a more plausible explanation for the results observed over the conclusions published.

For the practical side of things, analysing this Auger process could help researchers understand fundamental performance limits of doped quantum dot electroluminescence devices.

The team is now performing similar measurements to probe electron mobilities in quantum dot films at low carrier densities (where traditional measurements are limited). For example, quantum-dot Schottky junction solar cells are an important class of device where such information would be very welcome. "We are also currently working on exploiting the Auger effect to quench Mn²⁺ photoluminescence at specific times after a laser excitation pulse, which would allow the Mn²⁺ luminescence to be modulated on timescales faster than its intrinsic lifetime of around 2 ms," revealed Gamelin. "We would also like to capture the hot electrons produced by the Auger process."

These future experiments will teach the researchers something new about Auger processes in general, and about Auger processes in quantum dots in particular. "They will certainly advance our ability to harness the physical properties of doped semiconductor nanocrystals in future device technologies."

The work was detailed in ACS Nano.
http://nanotechweb.org/

Ultimate Energy Efficiency: Magnetic Microprocessors Could Use Million Times Less Energy Than Today's Silicon Chips

 Future computers may rely on magnetic microprocessors that consume the least amount of energy allowed by the laws of physics, according to an analysis by University of California, Berkeley, electrical engineers.

Today's silicon-based microprocessor chips rely on electric currents, or moving electrons, that generate a lot of waste heat. But microprocessors employing nanometer-sized bar magnets -- like tiny refrigerator magnets -- for memory, logic and switching operations theoretically would require no moving electrons.

Such chips would dissipate only 18 millielectron volts of energy per operation at room temperature, the minimum allowed by the second law of thermodynamics and called the Landauer limit. That's 1 million times less energy per operation than consumed by today's computers.

"Today, computers run on electricity; by moving electrons around a circuit, you can process information," said Brian Lambson, a UC Berkeley graduate student in the Department of Electrical Engineering and Computer Sciences. "A magnetic computer, on the other hand, doesn't involve any moving electrons. You store and process information using magnets, and if you make these magnets really small, you can basically pack them very close together so that they interact with one another. This is how we are able to do computations, have memory and conduct all the functions of a computer."

Lambson is working with Jeffrey Bokor, UC Berkeley professor of electrical engineering and computer sciences, to develop magnetic computers.

"In principle, one could, I think, build real circuits that would operate right at the Landauer limit," said Bokor, who is a codirector of the Center for Energy Efficient Electronics Science (E3S), a Science and Technology Center founded last year with a $25 million grant from the National Science Foundation. "Even if we could get within one order of magnitude, a factor of 10, of the Landauer limit, it would represent a huge reduction in energy consumption for electronics. It would be absolutely revolutionary."

One of the center's goals is to build computers that operate at the Landauer limit.

Lambson, Bokor and UC Berkeley graduate student David Carlton published a paper about their analysis online in the journal Physical Review Letters.

Fifty years ago, Rolf Landauer used newly developed information theory to calculate the minimum energy a logical operation, such as an AND or OR operation, would dissipate given the limitation imposed by the second law of thermodynamics. (In a standard logic gate with two inputs and one output, an AND operation produces an output when it has two positive inputs, while an OR operation produces an output when one or both inputs are positive.) That law states that an irreversible process -- a logical operation or the erasure of a bit of information -- dissipates energy that cannot be recovered. In other words, the entropy of any closed system cannot decrease.

In today's transistors and microprocessors, this limit is far below other energy losses that generate heat, primarily through the electrical resistance of moving electrons. However, researchers such as Bokor are trying to develop computers that don't rely on moving electrons, and thus could approach the Landauer limit. Lambson decided to theoretically and experimentally test the limiting energy efficiency of a simple magnetic logic circuit and magnetic memory.

The nanomagnets that Bokor, Lambson and his lab use to build magnetic memory and logic devices are about 100 nanometers wide and about 200 nanometers long. Because they have the same north-south polarity as a bar magnet, the up-or-down orientation of the pole can be used to represent the 0 and 1 of binary computer memory. In addition, when multiple nanomagnets are brought together, their north and south poles interact via dipole-dipole forces to exhibit transistor behavior, allowing simple logic operations.

"The magnets themselves are the built-in memory," Lambson said. "The real challenge is getting the wires and transistors working."

Lambson showed through calculations and computer simulations that a simple memory operation -- erasing a magnetic bit, an operation often called "restore to one" -- can be conducted with an energy dissipation very close, if not identical to, the Landauer limit.

He subsequently analyzed a simple magnetic logical operation. The first successful demonstration of a logical operation using magnetic nanoparticles was achieved by researchers at the University of Notre Dame in 2006. In that case, they built a three-input majority logic gate using 16 coupled nanomagnets. Lambson calculated that a computation with such a circuit would also dissipate energy at the Landauer limit.

Because the Landauer limit is proportional to temperature, circuits cooled to low temperatures would be even more efficient.

At the moment, electrical currents are used to generate a magnetic field to erase or flip the polarity of nanomagnets, which dissipates a lot of energy. Ideally, new materials will make electrical currents unnecessary, except perhaps for relaying information from one chip to another.

"Then you can start thinking about operating these circuits at the upper efficiency limits," Lambson said.

"We are working now with collaborators to figure out a way to put that energy in without using a magnetic field, which is very hard to do efficiently," Bokor said. "A multiferroic material, for example, may be able to control magnetism directly with a voltage rather than an external magnetic field."

Other obstacles remain as well. For example, as researchers push the power consumption down, devices become more susceptible to random fluctuations from thermal effects, stray electromagnetic fields and other kinds of noise.

"The magnetic technology we are working on looks very interesting for ultra low power uses," Bokor said. "We are trying to figure out how to make it more competitive in speed, performance and reliability. We need to guarantee that it gets the right answer every single time with a very, very, very high degree of reliability."

The work was supported by NSF and the Defense Advanced

http://www.sciencedaily.com

Chemists Discover Freezing Point of Supercooled Water

Scientists have long known that water can stay liquid at temperatures well below zero. Now they've discovered exactly how low they can go

K

It's easy to imagine that water must be one of best understood materials in science. After all, this liquid is possibly the best studied substance on Earth. But the truth is that many of its properties still mystify scientists.

One unsolved puzzle is its freezing point. Scientists have known for many years that you can cool liquid water well below zero degrees centigrade without it freezing. That's because water needs some nucleation event to trigger the process of ice formation. Without ice nucleation, it remains liquid.

But how low can you go?

Today, we have an answer of sorts thanks to the work of Emily Moore and Valeria Molinero at the University of Utah in Salt Lake City.

Part of the problem is that experiments to measure the freezing temperature are so difficult to perform that nobody has managed them. But the evidence points to the likelihood that ice crystals begin to form anyway at temperatures of about -41 C.

Supercoooled water should freeze at around this temperature but nobody has succeeded in measuring it because it always begins to freeze earlier.

Moore and Molinero get around this problem by simulating the freezing behaviour of over 250,000 water molecules on a computer. What they find is that once the natural process of ice formation begins to occur, then water cannot stay liquid at much lower temperatures.

In fact, their simulation indicates that the natural freezing point of supercooled water is about -43 C, just below the temperature at which ice crystals form naturally. That's as expected but the simulation also gives new insights into the way in which this freezing occurs.

In this state, water is a mixture of low density ice and water molecules that are on the verge of becoming ice, what chemists call "four co-ordinated" meaning that each molecule is linked to four others. The structure of "four co-ordinated" water seems to have important impact on the rate at which ice can form and this is what determines the freezing point.

There is an important caveat, however. The simulations require a major correction before they produce a physically realistic result. For some reason, they suggest that the natural ice formation begins to occur at about -71 C and that supercooled water freezes at about -73 C.

That's 30 degrees lower than in the real world. To get around this, Moore and Molinero simply add 30 degrees to all their results. Just why the simulation is out by so much isn't clear.

If the work is valid, however, it could have a major impact in other areas of science.

The temperature at which supercooled water freezes is an important factor in cloud formation. And small changes in this process, when entered into in climate change models, can have a big impact on the predictions about the future of the Earth.

Exactly how the new numbers will change climate predictions isn't yet clear. And of course, climatologists will want better evidence than a slightly wonky computer simulation. But it's a decent step forward and worth keeping on eye on for its influence elsewhere in science.

Ref: arxiv.org/abs/1107.1622: Structural Transformation In Supercooled Water Controls The Crystallization Rate Of Ice

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