2-D electron liquid solidifies in a magnetic field

ScienceDaily (Nov. 5, 2011) — Physicists from the Georgia Institute of Technology have developed a theory that describes, in a unified manner, the coexistence of liquid and pinned solid phases of electrons in two dimensions under the influence of a magnetic field. The theory also describes the transition between these phases as the field is varied. The theoretical predictions by Constantine Yannouleas and Uzi Landman, from Georgia Tech's School of Physics, aim to explain and provide insights into the origins of experimental findings published last year by a team of researchers from Princeton, Florida State and Purdue universities.

The research appears in the Oct. 27 edition of the journal Physical Review B.

The experimental discovery in 1982 of a new Hall conductance step at a fraction ?=1/m with m=3, that is at (1/3)e2/h (with more conductance steps, at other m, found later) -- where h is the Planck constant and e is the electron charge -- was made for two-dimensional electrons at low temperatures and strong magnetic fields and was greeted with great surprise. The theoretical explanation of this finding a year later by Robert Laughlin in terms of a new form of a quantum fluid, earned him and the experimentalists Horst Störmer and Daniel Tsui the 1998 Nobel Prize with the citation "for the discovery of a new form of quantum fluid with fractionally charged excitations." These discoveries represent conceptual breakthroughs in the understanding of matter, and the fractional quantum Hall effect (FQHE) liquid states, originating from the highly correlated nature of the electrons in these systems, have been termed as new states of matter.

"The quantum fluid state at the 1/3 primary fraction is the hallmark of the FQHE, whose theoretical understanding has been formulated around the antithesis between a new form of quantum fluid and the pinned Wigner crystal," said Landman, Regents' and Institute Professor in the School of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech. "Therefore, the discovery of pinned crystalline signatures in the neighborhood of the 1/3 FQHE fraction, measured as resonances in the microwave spectrum of the two-dimensional electron gas and reported in the Physical Review Letters in September 2010 by a group of researchers headed by Daniel Tsui, was rather surprising," he added.

Indeed, formation of a hexagonally ordered two-dimensional electron solid phase, a so called Wigner crystal (WC) named after the Nobel laureate physicist Eugene Wigner who predicted its existence in 1934, has been anticipated for smaller quantum Hall fractional fillings, ?, of the lowest Landau level populated by the electrons at high magnetic fields, for example ? = 1/9, 1/7 and even 1/5. However, the electrons in the ?=1/3 fraction were believed to resist crystallization and remain liquid.

The Georgia Tech physicists developed a theoretical formalism that, in conjunction with exact numerical solutions, provides a unified microscopic approach to the interplay between FQHE liquid and Wigner solid states in the neighborhood of the 1/3 fractional filling. A major advantage of their approach is the use of a single class of variational wave functions for description of both the quantum liquid and solid phases.

"Liquid characteristics of the fractional quantum Hall effect states are associated with symmetry-conserving vibrations and rotations of the strongly interacting electrons and they coexist with intrinsic correlations that are crystalline in nature," Senior Research Scientist Yannouleas and Landman wrote in the opening section of their paper. "While the electron densities of the fractional quantum Hall effect liquid state do not exhibit crystalline patterns, the intrinsic crystalline correlations which they possess are reflected in the emergence of a sequence of liquid states of enhanced stability, called cusp states, that correspond in the thermodynamic limit to the fractional quantum Hall effect filling fractions observed in Hall conductance measurements," they added.

The key to their explanation of the recent experimental observations pertaining to the appearance of solid characteristics for magnetic fields in the neighborhood of the 1/3 filling fraction is their finding that "away from the exact fractional fillings, for example near ?=1/3, weak pinning perturbations, due to weak disorder, may overcome the energy gaps between adjacent good angular momentum symmetry-conserving states. The coupling between these states generates broken-symmetry ground states whose densities exhibit spatial crystalline patterns. At the same time, however, the energy gap between the ground state at ?=1/3 and adjacent states is found to be sufficiently large to prevent disorder-induced mixing, thus preserving its quantum fluid nature."

Furthermore, the work shows that the emergence of the crystalline features, via the pinning perturbations, is a consequence of the aforementioned presence of crystalline correlations in the symmetry-conserving states. Consequently, mixing rules that govern the nature of the disorder-pinned crystalline states have been formulated and tested. Extrapolation of the calculated results to the thermodynamic limit shows development of a hexagonal Wigner crystal with enhanced stability due to quantum correlations.

"In closing, the nature of electrons in the fractional quantum Hall regime continues now for close to three decades to be a subject of great fascination, a research field that raises questions whose investigations can lead to deeper conceptual understanding of matter and many-body phenomena, and a rich source of surprise and discovery," said Landman.

This work was supported by the Office of Basic Energy Sciences of the US Department of Energy.

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Journal Reference:

Constantine Yannouleas, Uzi Landman. Unified microscopic approach to the interplay of pinned-Wigner-solid and liquid behavior of the lowest Landau-level states in the neighborhood of ?=1/3. Physical Review B, 2011; 84 (16) DOI: 10.1103/PhysRevB.84.165327

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Ionic liquid catalyst helps turn emissions into fuel

ScienceDaily (Oct. 7, 2011) — An Illinois research team has succeeded in overcoming one major obstacle to a promising technology that simultaneously reduces atmospheric carbon dioxide and produces fuel.

University of Illinois chemical and biological engineering professor Paul Kenis and his research group joined forces with researchers at Dioxide Materials, a startup company, to produce a catalyst that improves artificial photosynthesis. The company, in the university Research Park, was founded by retired chemical engineering professor Richard Masel. The team reported their results in the journal Science.

Artificial photosynthesis is the process of converting carbon dioxide gas into useful carbon-based chemicals, most notably fuel or other compounds usually derived from petroleum, as an alternative to extracting them from biomass.

In plants, photosynthesis uses solar energy to convert carbon dioxide (CO2) and water to sugars and other hydrocarbons. Biofuels are refined from sugars extracted from crops such as corn. However, in artificial photosynthesis, an electrochemical cell uses energy from a solar collector or a wind turbine to convert CO2 to simple carbon fuels such as formic acid or methanol, which are further refined to make ethanol and other fuels.

"The key advantage is that there is no competition with the food supply," said Masel, a co-principal investigator of the paper and CEO of Dioxide Materials, "and it is a lot cheaper to transmit electricity than it is to ship biomass to a refinery."

However, one big hurdle has kept artificial photosynthesis from vaulting into the mainstream: The first step to making fuel, turning carbon dioxide into carbon monoxide, is too energy intensive. It requires so much electricity to drive this first reaction that more energy is used to produce the fuel than can be stored in the fuel.

The Illinois group used a novel approach involving an ionic liquid to catalyze the reaction, greatly reducing the energy required to drive the process. The ionic liquids stabilize the intermediates in the reaction so that less electricity is needed to complete the conversion.

The researchers used an electrochemical cell as a flow reactor, separating the gaseous CO2 input and oxygen output from the liquid electrolyte catalyst with gas-diffusion electrodes. The cell design allowed the researchers to fine-tune the composition of the electrolyte stream to improve reaction kinetics, including adding ionic liquids as a co-catalyst.

"It lowers the overpotential for CO2 reduction tremendously," said Kenis, who is also a professor of mechanical science and engineering and affiliated with the Beckman Institute for Advanced Science and Technology. "Therefore, a much lower potential has to be applied. Applying a much lower potential corresponds to consuming less energy to drive the process."

Next, the researchers hope to tackle the problem of throughput. To make their technology useful for commercial applications, they need to speed up the reaction and maximize conversion.

"More work is needed, but this research brings us a significant step closer to reducing our dependence on fossil fuels while simultaneously reducing CO2 emissions that are linked to unwanted climate change," Kenis said.

Graduate students Brian Rosen, Michael Thorson, Wei Zhu and Devin Whipple and postdoctoral researcher Amin Salehi-Khojin were co-authors of the paper. The U.S. Department of Energy supported this work.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Illinois at Urbana-Champaign.

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B. A. Rosen, A. Salehi-Khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis, R. I. Masel. Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials. Science, 2011; DOI: 10.1126/science.1209786

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Liquid can turn into solid under high electric field, physicists show in simulations

ScienceDaily (Oct. 12, 2011) — Physicists have demonstrated in simulations that under the influence of sufficiently high electric fields, liquid droplets of certain materials will undergo solidification, forming crystallites at temperature and pressure conditions that correspond to liquid droplets at field-free conditions. This electric-field-induced phase transformation is termed electrocrystallization.

The study, performed by scientists at the Georgia Institute of Technology, appears online and is scheduled as a feature and cover article in the 42nd issue of Volume 115 of the Journal of Physical Chemistry C.

"We show that with a strong electric field, you can induce a phase transition without altering the thermodynamic parameters," said Uzi Landman, Regents' and Institute Professor in the School of Physics, F.E. Callaway Chair and director of the Center for Computational Materials Science (CCMS) at Georgia Tech.

In these simulations, Landman and Senior Research Scientists David Luedtke and Jianping Gao at the CCMS set out first to explore a phenomenon described by Sir Geoffrey Ingram Taylor in 1964 in the course of his study of the effect of lightning on raindrops, expressed as changes in the shape of liquid drops when passing through an electric field. While liquid drops under field-free conditions are spherical, they alter their shape in response to an applied electric field to become needle-like liquid drops. Instead of the water droplets used in the almost decade-old laboratory experiments of Taylor, the Georgia Tech researchers focused their theoretical study on a 10 nanometer (nm) diameter liquid droplet of formamide, which is a material made of small polar molecules each characterized by a dipole moment that is more than twice as large as that of a water molecule.

With the use of molecular dynamics simulations developed at the CCMS, which allow scientists to track the evolution of materials systems with ultra-high resolution in space and time, the physicists explored the response of the formamide nano-droplet to an applied electric field of variable strength. Influenced by a field of less than 0.5V/nm, the spherical droplet elongated only slightly. However, when the strength of the field was raised to a critical value close to 0.5 V/nm, the simulated droplet was found to undergo a shape transition resulting in a needle-like liquid droplet with its long axis -- oriented along the direction of the applied field -- measuring about 12 times larger than the perpendicular (cross-sectional) small axis of the needle-like droplet. The value of the critical field found in the simulations agrees well with the prediction obtained almost half a decade ago by Taylor from general macroscopic considerations.

Past the shape transition further increase of the applied electric field yielded a slow, gradual increase of the aspect ratio between the long and short axes of the needle-like droplet, with the formamide molecules exhibiting liquid diffusional motions.

"Here came the Eureka moment," said Landman. "When the field strength in the simulations was ramped up even further, reaching a value close to 1.5V/nm, the liquid needle underwent a solidification phase transition, exhibited by freezing of the diffusional motion, and culminating in the formation of a formamide single crystal characterized by a structure that differs from that of the x-ray crystallographic one determined years ago under zero-field conditions. Now, who ordered that?" he added.

Further analysis has shown that the crystallization transition involved arrangement of the molecules into a particular spatial ordered lattice, which optimizes the interactions between the positive and negative ends of the dipoles of neighboring molecules, resulting in minimization of the free energy of the resulting rigid crystalline needle. When the electric field applied to the droplet was subsequently decreased, the crystalline needle remelted and at zero-field the liquid droplet reverted to a spherical shape. The field reversal process was found to exhibit a hysteresis.

Analysis of the microscopic structural changes that underlie the response of the droplet to the applied field revealed that accompanying the shape transition at 0.5 V/nm is a sharp increase in the degree of reorientation of the molecular electric dipoles, which after the transition lie preferentially along the direction of the applied electric field and coincide with the long axis of the needle-­­like liquid droplet. The directional dipole reorientation, which is essentially complete subsequent to the higher field electrocrystallization transition, breaks the symmetry and transforms the droplet into a field-induced ferroelectric state where it possesses a large net electric dipole, in contrast to its unpolarized state at zero-field conditions.

Along with the large-scale atomistic computer simulations, researchers formulated and evaluated an analytical free-energy model, which describes the balance between the polarization, interfacial tension and dielectric saturation contributions. This model was shown to yield results in agreement with the computer simulation experiments, thus providing a theoretical framework for understanding the response of dielectric droplets to applied fields.

"This investigation unveiled fascinating properties of a large group of materials under the influence of applied fields," Landman said. "Here the field-induced shape and crystallization transitions occurred because formamide, like water and many other materials, is characterized by a relatively large electric dipole moment. The study demonstrated the ability to employ external fields to direct and control the shape, the aggregation phase (that is, solid or liquid) and the properties of certain materials."

Along with the fundamental interest in understanding the microscopic origins of materials behavior, this may lead to development of applications of field-induced materials control in diverse areas, ranging from targeted drug delivery, nanoencapsulation, printing of nanostructures and surface patterning, to aerosol science, electrospray propulsion and environmental science.

This research was supported by a grant from the U.S. Air Force Office of Scientific Research.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Georgia Institute of Technology.

Journal Reference:

William D. Luedtke, Jianping Gao, Uzi Landman. Dielectric Nanodroplets: Structure, Stability, Thermodynamics, Shape Transitions and Electrocrystallization in Applied Electric Fields. The Journal of Physical Chemistry C, 2011; 110901150249036 DOI: 10.1021/jp206673j

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Ionic liquid catalyst helps turn emissions into fuel

ScienceDaily (Oct. 7, 2011) — An Illinois research team has succeeded in overcoming one major obstacle to a promising technology that simultaneously reduces atmospheric carbon dioxide and produces fuel.

University of Illinois chemical and biological engineering professor Paul Kenis and his research group joined forces with researchers at Dioxide Materials, a startup company, to produce a catalyst that improves artificial photosynthesis. The company, in the university Research Park, was founded by retired chemical engineering professor Richard Masel. The team reported their results in the journal Science.

Artificial photosynthesis is the process of converting carbon dioxide gas into useful carbon-based chemicals, most notably fuel or other compounds usually derived from petroleum, as an alternative to extracting them from biomass.

In plants, photosynthesis uses solar energy to convert carbon dioxide (CO2) and water to sugars and other hydrocarbons. Biofuels are refined from sugars extracted from crops such as corn. However, in artificial photosynthesis, an electrochemical cell uses energy from a solar collector or a wind turbine to convert CO2 to simple carbon fuels such as formic acid or methanol, which are further refined to make ethanol and other fuels.

"The key advantage is that there is no competition with the food supply," said Masel, a co-principal investigator of the paper and CEO of Dioxide Materials, "and it is a lot cheaper to transmit electricity than it is to ship biomass to a refinery."

However, one big hurdle has kept artificial photosynthesis from vaulting into the mainstream: The first step to making fuel, turning carbon dioxide into carbon monoxide, is too energy intensive. It requires so much electricity to drive this first reaction that more energy is used to produce the fuel than can be stored in the fuel.

The Illinois group used a novel approach involving an ionic liquid to catalyze the reaction, greatly reducing the energy required to drive the process. The ionic liquids stabilize the intermediates in the reaction so that less electricity is needed to complete the conversion.

The researchers used an electrochemical cell as a flow reactor, separating the gaseous CO2 input and oxygen output from the liquid electrolyte catalyst with gas-diffusion electrodes. The cell design allowed the researchers to fine-tune the composition of the electrolyte stream to improve reaction kinetics, including adding ionic liquids as a co-catalyst.

"It lowers the overpotential for CO2 reduction tremendously," said Kenis, who is also a professor of mechanical science and engineering and affiliated with the Beckman Institute for Advanced Science and Technology. "Therefore, a much lower potential has to be applied. Applying a much lower potential corresponds to consuming less energy to drive the process."

Next, the researchers hope to tackle the problem of throughput. To make their technology useful for commercial applications, they need to speed up the reaction and maximize conversion.

"More work is needed, but this research brings us a significant step closer to reducing our dependence on fossil fuels while simultaneously reducing CO2 emissions that are linked to unwanted climate change," Kenis said.

Graduate students Brian Rosen, Michael Thorson, Wei Zhu and Devin Whipple and postdoctoral researcher Amin Salehi-Khojin were co-authors of the paper. The U.S. Department of Energy supported this work.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Illinois at Urbana-Champaign.

Journal Reference:

B. A. Rosen, A. Salehi-Khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis, R. I. Masel. Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials. Science, 2011; DOI: 10.1126/science.1209786

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Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Ultrathin copper-oxide layers behave like quantum spin liquid

ScienceDaily (June 11, 2011) — Magnetic studies of ultrathin slabs of copper-oxide materials reveal that at very low temperatures, the thinnest, isolated layers lose their long-range magnetic order and instead behave like a "quantum spin liquid" -- a state of matter where the orientations of electron spins fluctuate wildly. This unexpected discovery by scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and collaborators at the Paul Scherrer Institute in Switzerland may offer support for the idea that this novel condensed state of matter is a precursor to the emergence of high-temperature superconductivity -- the ability to carry current with no resistance.

The hope is that this research, just published online in Physical Review Letters, will lead to a deeper understanding of the physics of high-temperature superconductivity and advance the quest for new and better superconductors for meeting the nation's and world's energy needs.

The idea of quantum spin liquids is credited to Nobel laureate Philip W. Anderson, who also proposed the possible link to the emergence of high-temperature superconductivity when copper-oxide, or "cuprate," materials are doped with mobile charge carriers -- that is, when atoms supplying additional electrons or electron vacancies are added. However, some past experimental findings haven't supported this proposal: Without doping, lanthanum-copper-oxide, one of the most studied cuprates, shows a form of long-range magnetic order known as anti-ferromagnetism -- where spin orientations on adjacent electrons alternately point in exactly opposite directions -- even at room temperature. But the new Brookhaven Lab/Scherrer Institute experiments suggest a different picture when one looks at thin enough layers.

"The crystal structure of lanthanum-copper-oxide is layered; it consists of parallel copper-oxide and lanthanum-oxide sheets," explained Brookhaven physicist Ivan Bozovic, one of the lead authors on the paper. "The interaction among the spins within one copper-oxide plane is strong, while their interaction with the spins in the nearest copper-oxide plane (about 0.66 nanometers away) is ten thousand times weaker. Still, this weak interaction between layers may be sufficient to suppress fluctuations and stabilize the anti-ferromagnetic order."

The key to finding out if there was fluctuation-suppressing interaction among layers was to look for magnetic order in thinner films, with fewer layers and better insulation.

Bozovic used a specialized atomic-layer-by-layer molecular beam epitaxy method he's developed to assemble lanthanum-copper-oxide samples with varying numbers of layers. The layers were well separated and insulated to prevent any "crosstalk." The thickness was controlled with atomic precision and varied digitally, down to a single copper-oxide plane. This precision was critically important for the success of the experiment.

These unique samples were studied at the Paul Scherrer Institute by Elvezio Morenzoni and his team, who had developed an exquisite diagnostic technique called low-energy muon spin spectroscopy to detect and investigate magnetism in such ultrathin layers.

The magnetic measurements revealed that when the slabs contained four or more copper-oxide layers, they showed anti-ferromagnetic ordering -- just like thick, bulk crystals of the same materials, and even up to the same temperature. However, thinner slabs that contained just one or two copper-oxide layers showed an unexpected result: "While the magnetic moments, or spins, were still present and had about the same magnitude, there was no long-range static anti-ferromagnetic order, not even on the scale of a few nanometers. Rather, the spins were fluctuating wildly, changing their direction very fast," Bozovic said.

Even more telling, this effect was stronger the lower the temperature of the sample. "That means these fluctuations could not be of thermal origin and must be of quantum origin -- quantum objects fluctuate even at zero temperature," Bozovic explained.

"Altogether, this experiment indicates that once a copper-oxide plane is well isolated and not interacting with other such layers, it in fact seems to behave, at low temperature, like some sort of quantum spin liquid." Bozovic said. So perhaps the idea that high-temperature superconductivity emerges from this quantum spin liquid state could, after all, be true.

"We certainly need to do more experiments to test the implications of our discovery and how it relates to this theoretical prediction," Bozovic said.

This work was supported by the DOE Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Brookhaven National Laboratory.

Journal Reference:

A. Suter, E. Morenzoni, T. Prokscha, B. Wojek, H. Luetkens, G. Nieuwenhuys, A. Gozar, G. Logvenov, I. Božovic. Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La_{2-x}Sr_{x}CuO_{4} Superlattices Measured by Muon-Spin Rotation. Physical Review Letters, 2011; 106 (23) DOI: 10.1103/PhysRevLett.106.237003

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Ultrathin copper-oxide layers behave like quantum spin liquid

ScienceDaily (June 11, 2011) — Magnetic studies of ultrathin slabs of copper-oxide materials reveal that at very low temperatures, the thinnest, isolated layers lose their long-range magnetic order and instead behave like a "quantum spin liquid" -- a state of matter where the orientations of electron spins fluctuate wildly. This unexpected discovery by scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and collaborators at the Paul Scherrer Institute in Switzerland may offer support for the idea that this novel condensed state of matter is a precursor to the emergence of high-temperature superconductivity -- the ability to carry current with no resistance.

The hope is that this research, just published online in Physical Review Letters, will lead to a deeper understanding of the physics of high-temperature superconductivity and advance the quest for new and better superconductors for meeting the nation's and world's energy needs.

The idea of quantum spin liquids is credited to Nobel laureate Philip W. Anderson, who also proposed the possible link to the emergence of high-temperature superconductivity when copper-oxide, or "cuprate," materials are doped with mobile charge carriers -- that is, when atoms supplying additional electrons or electron vacancies are added. However, some past experimental findings haven't supported this proposal: Without doping, lanthanum-copper-oxide, one of the most studied cuprates, shows a form of long-range magnetic order known as anti-ferromagnetism -- where spin orientations on adjacent electrons alternately point in exactly opposite directions -- even at room temperature. But the new Brookhaven Lab/Scherrer Institute experiments suggest a different picture when one looks at thin enough layers.

"The crystal structure of lanthanum-copper-oxide is layered; it consists of parallel copper-oxide and lanthanum-oxide sheets," explained Brookhaven physicist Ivan Bozovic, one of the lead authors on the paper. "The interaction among the spins within one copper-oxide plane is strong, while their interaction with the spins in the nearest copper-oxide plane (about 0.66 nanometers away) is ten thousand times weaker. Still, this weak interaction between layers may be sufficient to suppress fluctuations and stabilize the anti-ferromagnetic order."

The key to finding out if there was fluctuation-suppressing interaction among layers was to look for magnetic order in thinner films, with fewer layers and better insulation.

Bozovic used a specialized atomic-layer-by-layer molecular beam epitaxy method he's developed to assemble lanthanum-copper-oxide samples with varying numbers of layers. The layers were well separated and insulated to prevent any "crosstalk." The thickness was controlled with atomic precision and varied digitally, down to a single copper-oxide plane. This precision was critically important for the success of the experiment.

These unique samples were studied at the Paul Scherrer Institute by Elvezio Morenzoni and his team, who had developed an exquisite diagnostic technique called low-energy muon spin spectroscopy to detect and investigate magnetism in such ultrathin layers.

The magnetic measurements revealed that when the slabs contained four or more copper-oxide layers, they showed anti-ferromagnetic ordering -- just like thick, bulk crystals of the same materials, and even up to the same temperature. However, thinner slabs that contained just one or two copper-oxide layers showed an unexpected result: "While the magnetic moments, or spins, were still present and had about the same magnitude, there was no long-range static anti-ferromagnetic order, not even on the scale of a few nanometers. Rather, the spins were fluctuating wildly, changing their direction very fast," Bozovic said.

Even more telling, this effect was stronger the lower the temperature of the sample. "That means these fluctuations could not be of thermal origin and must be of quantum origin -- quantum objects fluctuate even at zero temperature," Bozovic explained.

"Altogether, this experiment indicates that once a copper-oxide plane is well isolated and not interacting with other such layers, it in fact seems to behave, at low temperature, like some sort of quantum spin liquid." Bozovic said. So perhaps the idea that high-temperature superconductivity emerges from this quantum spin liquid state could, after all, be true.

"We certainly need to do more experiments to test the implications of our discovery and how it relates to this theoretical prediction," Bozovic said.

This work was supported by the DOE Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Brookhaven National Laboratory.

Journal Reference:

A. Suter, E. Morenzoni, T. Prokscha, B. Wojek, H. Luetkens, G. Nieuwenhuys, A. Gozar, G. Logvenov, I. Božovic. Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La_{2-x}Sr_{x}CuO_{4} Superlattices Measured by Muon-Spin Rotation. Physical Review Letters, 2011; 106 (23) DOI: 10.1103/PhysRevLett.106.237003

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Water's surface not all wet: Some water molecules split the difference between gas and liquid

ScienceDaily (June 9, 2011) — Air and water meet over most of Earth's surface, but exactly where one ends and the other begins turns out to be a surprisingly subtle question.

A new study in Nature narrows the boundary to just one quarter of water molecules in the uppermost layer -- those that happen to have one hydrogen atom in water and the other vibrating freely above.

Such molecules straddle gas and liquid phases, according to senior author Alexander Benderskii of the University of Southern California: The free hydrogen behaves like an atom in gas phase, while its twin below acts much like the other atoms that make up "bulk" water.

The finding matters for theoretical reasons and for practical studies of reactions at the water's surface, including the processes that maintain a vital supply of nitrogen, oxygen and carbon dioxide in the atmosphere.

"The air-water interface is about 70 percent of the Earth's surface," Benderskii said. "A lot of chemical reactions that are responsible for our atmospheric balance, as well as many processes important in environmental chemistry, happen at the air-water interface."

He added that the study provided a new way for chemists and biologists to study other interfaces, such as the boundary between water and biomembranes that marks the edge of every living cell.

"Water interfaces in general are important," Benderskii said, calling the study "an open door that now we can walk through and broaden the range of our investigations to other, perhaps more complex, acqueous interfaces."

In their study, Benderskii and his colleagues used techniques they invented to test the strength of hydrogen bonds linking water molecules (from the hydrogen of one molecule to the oxygen of another). These are the bonds that keep water a liquid at room temperature.

Specifically, the researchers inferred the bond strength by measuring the hydrogen-oxygen vibration frequency. The bond gets stronger as the frequency decreases, similar to the pull one feels when slowing down a child on a swing.

In the case of straddling molecules with one hydrogen in water, when compared to bonds below the surface, "the hydrogen bond is surprisingly only slightly weaker," according to Benderskii.

Likewise, the bond for the hydrogen atom sticking out of the water is similar in strength to bonds in the gas phase.

The researchers concluded that the change between air and water happens in the space of a single water molecule.

"You recover the bulk phase of water extremely quickly," Benderskii said.

While the transition happens in the uppermost layer of water molecules, the molecules involved change constantly. Even when they rise to the top layer, molecules for the most part are wholly submerged, spending only a quarter of their time straddling air and water.

The study raises the question of how exactly to define the air-water boundary.

If the straddling molecules constitute the boundary, it would be analogous to a wood fence where three of every four boards are missing -- except that since water molecules always are moving between submerged and straddling positions, the location of the fourth board would change millions of times per second.

If the boundary were the entire top layer of water molecules, the analogy would be a fence where one in four boards is sticking out at any one time.

A physical chemist, Benderskii began the study at Wayne State University in Detroit before joining the USC Dornsife College of Letters, Arts and Sciences in 2009 as an associate professor.

Benderskii's collaborators were lead author Igor Stiopkin, formerly at Wayne State and now at the University of Wisconsin-Madison; Champika Weeraman, also previously at Wayne State and now at Canada's National Research Council in Ottawa; Piotr Pieniazek and James Skinner of the University of Wisconsin-Madison; and Fadel Shalhout, formerly at Wayne State and now at USC Dornsife College.

The National Science Foundation and the U.S. Department of Energy funded the study.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Southern California. The original article was written by Carl Marziali.

Journal Reference:

Igor V. Stiopkin, Champika Weeraman, Piotr A. Pieniazek, Fadel Y. Shalhout, James L. Skinner, Alexander V. Benderskii. Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy. Nature, 2011; 474 (7350): 192 DOI: 10.1038/nature10173

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View the original article here