Featured Research

Probing Maxwell's Demon with a Nanoscale Thermometer

Describing the measurement of temperature across extremely tiny distances such as individual molecules, UA physicists have glimpsed a phenomenon mimicking the actions of Maxwell's Demon, a hypothetical figure in a thought experiment that seemingly defies the laws of physics.

"Maxwell's Demon can't exist because it would violate the laws of thermodynamics," said Charles Stafford, a professor at the department of physics in the UA College of Science. "So you can imagine we were quite surprised to see it appear in our computer-based experiments."

"We showed that if you try to measure the temperature of a system of particles, in this case, electrons, not molecules in a box, with a spatial precision down to the size of individual atoms, then the laws of quantum mechanics result in an effect that is almost identical to what Maxwell's demon would do," Stafford explained.

In their theoretical work, the group simulated a system consisting of a small molecule of carbon and hydrogen atoms with three electrodes attached to it. One electrode transfers heat into the molecule, the second electrode drains heat out of the molecule, and the third measures the temperature at different places within the molecule. The whole setup is called scanning thermal microscopy: A scanning electron microscope uses an ultrafine tip whose apex consists of a single atom to measure temperatures on an atomic scale.

"In our simulations, we found that it is possible to separate the hot from the cold electrons within that single molecule without expending any energy to make this happen, which is exactly what Maxwell's Demon does," Stafford said.

However, it turns out this sorting process does not violate the laws of thermodynamics because of the peculiarities of quantum physics, he explained.

"In the quantum state of the molecule, the hot and cold electrons never mix despite the fact that they exist in the same place at the same time. But that's because they 'remember' where they came from due to quantum wave effects - not because there is a demon at play," he said.

The research project and its unexpected results were several years in the making, Stafford said. The investigation began when undergraduate researcher Shauna Story, who graduated with a Bachelor of Science in physics in 2010, discovered the strange effect while studying simple molecules. This led the group to tests with more complex structures, resulting in a publication co-authored by former graduate student Justin Bergfield and organic chemist Robert Stafford.

For the full story, see http://uanews.org/story/chasing-demons-with-a-microscope

Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Testing Einstein’s E=mc2 in outer space

The most important problem in physics is known to be a creation of the so-called Theory of Everything – a unification of all forces in nature, including the electro-weak and strong interactions and gravitation. One of the most difficult steps in this direction is a developing of the quantum gravitation theory. To make some efforts in an achieving of the above mentioned goal, Prof. Andrei Lebed has recently started to consider a problem about a weight of the simplest quantum object – a hydrogen atom - in the Earth’s gravitational field in the framework of the Theory of General Relativity. The results of his research show how unexpected novel physics might be. Indeed, the most famous equation, E = mc², where m is a gravitational mass, has been demonstrated to survive only in average. Prof. Lebed has also suggested how to observe those rare events, where weight of a hydrogen atom is not equivalent to its energy (i.e., where the equation E = mc² is broken), using experiments inside satellite (spacecraft) on the Earth’s orbit (see the picture). More information about these theoretical results and the suggested experiment can be found at: http://uanews.org/story/testing-einsteins-emc2-outer-space

A talk “Breakdown of the Equivalence between Passive Gravitational Mass and Energy for a Quantum Body” was delivered by Andrei Lebed at the Marcel Grossmann Meeting-13 on Recent Developments in Theoretical and Experimental General Relativity (July 1-7, Stockholm, Sweden) and will be published in the proceedings of the Meeting. For details of the calculations, see Prof. Lebed’s preprints in Cornell University Library:

http://xxx.lanl.gov/abs/1111.5365 http://xxx.lanl.gov/abs/1205.3134

http://xxx.lanl.gov/abs/1208.5756

This research was supported by the National Science Foundation.

Anderson localization in random lasers

Wave coherence effects influence a broad range of physical phenomena, ranging from the propagation of ocean waves and the transmission of light through interstellar clouds, down to the motion of individual electrons in nanoscale electrical circuits and the transmission of photons through optical waveguides. These interference effects notoriously modify the propagation of waves in so-called random media - those that contain so many defects and impurities that the waves propagating through them are almost constantly scattered into multitudes of partial waves. In certain circumstances, the resulting interference effects can even stop the propagation of waves. This is the phenomenon of Anderson localization, named after Nobel laureate Philip Warren Anderson who predicted it in 1958. Anderson localization is by now well understood for noninteracting waves, however its fate in the presence of interactions or other nonlinearities is the subject of much debate.

In a recent article in Nature Photonics, researchers at the University of Arizona and at the University of Basel, Switzerland have found that in certain strongly nonlinear optical devices called random lasers, Anderson localization is immune to nonlinear effects. Random microlasers pose a persistent theoretical challenge. Unlike conventional lasers, they have no resonator to trap light, they are highly multimode with potentially strong modal interactions, and they are based on disordered gain media, where photons undergo random multiple scattering. Using a combination of analytical calculations and numerical simulations, Peter Stano and Philippe Jacquod surmounted all these obstacles and calculated the properties characterizing the lasing modes of microcavity lasers where impurities and defect in the gain medium result in Anderson localization. Their conclusion sounds almost trivial, yet it is totally unexpected: despite spatial and spectral mode overlap, and even in regimes of strong pumping, lasing modes are no different from cold-cavity modes - those that exist in the cavity in the absence of pumping and nonlinearity. The spatial extension of lasing modes and their frequency does not change as more modes start to lase and even overlap.

A number of interesting applications of random microlasers have been proposed, such as medical applications where e.g. human tissue is pumped and the more strongly scattering cancerous cells are identified as those that lase first; applications in encryption, where information is encoded in the sample-specific lasing spectrum; and applications in flat-panel or vehicle control displays and so forth. The results reported by Stano and Jacquod are important because the presence of long-lived, Anderson localized lasing modes guarantees low lasing thresholds, and thus laser operation at low pump power.

"Suppression of Interactions in Multimode Random Lasers in the Anderson Localized Regime" by Peter Stano and Philippe Jacquod appears in the advance online publication of Nature Photonics at http://dx.doi.org/10.1038/nphoton.2012.298

The research at the University of Arizona was sponsored by the National Science Foundation.

Superexcited oxygen breakup captured in real-time

Neutral superexcited molecules, such as those formed through solar irradiation of the upper atmosphere, are energetically unstable and fragment rapidly through multiple competing pathways. Using attosecond spectroscopy, Prof. Sandhu's group captured the ultrafast dynamics of superexcited oxygen and directly measured its dissociation and autoionization lifetimes. The breakup of superexcited oxygen molecules is an interesting process, as it yields excited atoms that participate in the formation and decay of ozone. Over the past three decades, many theoretical and experimental efforts have focused on understanding the complex non-adiabatic interactions governing the dynamics of superexcited oxygen. In contrast to these energy-resolved studies, the Sandhu group used a direct time-domain approach. An ultrashort extreme-ultraviolet pulse was used to create a temporally and spatially localized wave packet, whose evolution was then resolved using a second time-delayed laser pulse. This work resolves an important longstanding question, namely, how fast does the c-state of an oxygen molecule break up? The Sandhu group found that this spontaneous dissociation timescale is 1100 femtoseconds. They also measured the previously unknown autoionization lifetimes of superexcited oxygen states to be 90 femtoseconds. The knowledge these competing relaxation mechanisms and associated lifetimes is vital for understanding how energy redistribution occurs in elementary molecular processes. The attosecond techniques can also be utilized for real-time control of ultrafast electronic motion and chemical reactions in the high-energy regime. More information about these results and attosecond spectroscopy can be found at: http://uanews.org/story/freezing-electrons-flight.

"Ultrafast Dynamics of Neutral Superexcited Oxygen: A Direct Measurement of the Competition between Autoionization and Predissociation" by Henry Timmers, Niranjan Shivaram and Arvinder Sandhu, was published in Physical Review Letters. The link to the paper is: http://link.aps.org/doi/10.1103/PhysRevLett.109.173001
This research was supported by the National Science Foundation.

Intnl. Workshop in Honor of Prof. Lizhi Fang

The Department of Physics and Department of Astronomy at The University of Arizona
are pleased to announce the international workshop

"Exploring the Dark Universe: Frontier of Cosmology and Astrophysics in
the 21st Century",

to be held at the Westward Look Resort in Tucson, Arizona, USA, October
7-8, 2012.

We will celebrate the extraordinary scientific career of Prof. Lizhi Fang,
and his many contributions to astrophysics and cosmology. We will discuss the current status and future prospects of a number of frontier topics in astrophysics and cosmology that were the focus of Prof. Fang's scientific career, including:

- Relativistic Astrophysics
- Early Universe and particle astrophysics
- The Nature of Dark Matter and Dark Energy
- Cosmic Dark Ages and Cosmic Reionization
- Growth of Large Scale Structures in the Universe
- Other Topics in Astrophysics and Cosmology

The conference website is http://www.physics.arizona.edu/EDU2012 , which
includes details of registration and accommodations.

Other activities:

There will be a plaque dedication ceremony for a tree planted in honor of
Prof. Fang in the lawn in front of PAS building at 4:45 pm on October 8,
Monday.

UA ATLAS Group Celebrates Higgs Discovery

UA Physicists and ATLAS experiment collaborators Elliott Cheu, Ken Johns, John Rutherfoord, Mike Shupe,and Erich Varnes had a particularly rousing Fourth of July. On that day, spokespersons for the ATLAS and CMS experiments at the CERN LHC announced the discovery of a new particle that is presumed to be the long sought-after Higgs boson. The quantum field of the Higgs hides a beautiful symmetry that unites the quantum version of Maxwell's electromagnetism with the weak force, leading to the origin of mass in all elementary particles (including the zero mass of the photon). Profs. Rutherfoord and Shupe were founding members of the ATLAS experiment and played a major role in its construction. Prof. Varnes had been engaged in the search for the Higgs boson in his research at the Fermilab Tevatron. Profs. Cheu and Johns were excited that now more attention than ever will be focused on discovering Beyond-the-Standard-Model particles which is their current domain on ATLAS. More information can be found at http://www.arizona.edu/features/hunting-higgs and the ATLAS paper can be read at http://arxiv.org/pdf/1207.7214v1.pdf

New Dirac Point Created in Graphene

Graphene, a two-dimensional lattice of hexagonally bonded carbon atoms, continues to be a hot topic in condensed matter physics. Researchers from the Physics department along with collaborators from M.I.T. and Japan have studied the effects of graphene under the influence of a periodic potential. A weak periodic potential is achieved experimentally using a substrate of hexagonal boron nitride, a wide band gap insulator with the same hexagonal bond structure as graphene.

At low energies, charge carriers in graphene behave like photons; they follow a linear energy versus momentum dispersion relation and have zero effective mass. The crossing of the linear valence and conduction bands, known as the Dirac point, occurs with exactly zero density of electronic states. The density of electron or hole carriers can easily be tuned by applying an electrical potential, but the direction of their motion cannot be easily controlled. When graphene is placed on hexagonal boron nitride, the bonding mismatch and relative rotation of the two lattices forms a long range hexagonal superlattice which acts as a weak periodic potential for the graphene charge carriers. Researchers found this creates new linearly vanishing points in the band structure known as superlattice Dirac points.

The wavelength of the hexagonal superlattice varies randomly from sample to sample. With scanning tunneling microscopy, researchers were able to accurately measure this wavelength for many different samples. Using the corresponding spectroscopy, researchers were able to verify theoretical predictions for the energy separation of the original and superlattice Dirac points. Furthermore, researchers found new electron and hole carriers exist at these superlattice Dirac points, characterized by a drastically reduced group velocity with a preferential direction of motion. Hexagonal boron nitride is a very popular substrate for graphene, and understanding these novel electronic effects is crucial for future advancement of the field.

"Emergence of superlattice Dirac points in graphene on hexagonal boron nitride," by Matthew Yankowitz, Jiamin Xue, Daniel Cormode, Javier D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, Pablo Jarillo-Herrero, Philippe Jacquod and Brian J. LeRoy, appears in the advance online publication of Nature Physics at http://dx.doi.org/10.1038/nphys2272.

The research at the University of Arizona was sponsored by the U.S. Army Research Office and the National Science Foundation.

Higgs Boson Update

The search for the Higgs boson, which has spanned decades and continents, may be coming to an end. Results from the Large Hadron Collider at CERN and the Tevatron at Fermilab indicate that if the Higgs exists, it must have a mass near 125 GeV. Among the contributors to the Tevatron results were postdoc Xiaowen Lei and professor Erich Varnes. Continued analysis and further data from the LHC should settle the issue of the Higgs' existence by the end of the year. Also, see the associated story at UANews .

Art, Science, or Both?

The cover of Europhysics Letters, features a specially adapted figure out of one of Prof. Philippe Jacquod's recent publications on superconductivity [Europhys. Lett. 91, 67009 (2010)]. It was Grenoble's Dr. Robert Whitney - Jacquod's co-author on that manuscript - who made the figures for their article but artist Frederique Swist who turned it into a piece of art.

"My knowledge of the subject of physics is limited, which I use as an asset in my work. I approach my reference material by creating new functions to the forms and colours that I manipulate, in parallel to retaining the scientific information," Swist says.

You can get more information on Swist's work on her web site http://www.nsqi.bris.ac.uk/artscience.php .

Deuteron vs. LHC

Now also on: http://uanews.org/node/42347

Graduate student Emanuele Mereghetti, Prof. Bira van Kolck and their collaborators have shown that experiments with the tiniest nucleus, the deuteron, could be sensitive to the origin of time-reversal asymmetry in the universe at a scale comparable to that probed by the largest particle accelerator in the world, the LHC in Geneva.

That the laws of physics do not change when past and future are interchanged (time reversal) might be considered a particularly compelling symmetry. Nevertheless, in the realm of particle physics it is broken, mysteriously, because there are three families of quarks. This known breaking, however, is insufficient to explain the absence of antimatter in the universe, a connection first pointed out by Andrei Sakharov. Therefore, new sources of time-reversal symmetry breaking, originating at ultrahigh energy, are anticipated.

UA graduate student Emanuele Mereghetti, Prof. Bira van Kolck and their collaborators at the University of Groningen in the Netherlands have now shown that the tiny deuteron, the nucleus of the hydrogen-isotope deuterium, holds a crucial key needed to unravel such high-energy signals. A simple bound state of one proton and one neutron, the deuteron has a time-honored role to test models of nuclear forces and the interaction of photons with atomic nuclei. The deuteron possesses exotic electromagnetic properties that break time-reversal symmetry, and, moreover, reveal unique information about the expected sources of symmetry breaking.

These results, available at the ArXiv (http://arxiv.org/abs/arXiv:1102.4068)
and about to be published in Physical Review Letters, are relevant for existing plans to search for electric dipole moments of neutrons and light ions, in particular the proton and the deuteron, in experiments with such an exquisite sensitivity that they probe the same energy scales as the LHC.

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