February 7, 2011 — The Higgs boson is being sought at CERN’s Large Hadron Collider (LHC). According to physicists at Madrid’s Institute for Material Science, ripples in atom-thick graphene arise from a spontaneous symmetry-breaking process similar to that which separated the weak and electromagnetic forces in the early universe. This graphene research could shed light on the Higgs boson particle

At the high energies of the early universe, the weak nuclear force and the electromagnetic force are thought to converge into one electroweak force. As the temperature of the universe fell below a certain point, the two forces suddenly became separate. This "electroweak symmetry breaking" can be explained in terms of a field — the Higgs field — shifting from an effectively empty high-energy state to its ground state, filling space with a field that gives some particles their mass.

The yet-to-be-detected Higgs boson is the particle associated with vibrations of this field, and is currently being sought in the LHC. What drives the Higgs field between the high-energy and ground states? Although the parameters in the Standard Model of particle physics can be adjusted to force the symmetry-breaking, it is possible that the Higgs field may not need any coaxing. Pablo San-Jose and colleagues at Madrid’s Institute for Material Science now argue that studies of the emergence of ripples in graphene could shed some light on the process.

Solid State Technology editor Robert Haavind has speculated that Higgs boson research and discovery could land physicists a Nobel prize. The 2010 Nobel Prize-garnering (in Physics) material graphene may be a stepping stone to that discovery.

Graphene loses some symmetry in the transition from a flat shape to a rippled one, in the same way as the "activation" of the Higgs field is tied to the breaking of the electroweak symmetry. "Measuring the rippling of graphene under variable tension could give us information about the details of the intrinsic condensation of the Higgs," San-Jose says.

Working with colleagues Francisco Guinea and Jose Gonzalez, San-Jose claims to have shown that the energy landscape of graphene rippling in 2D, and that of the Higgs field in 3D, are described by similar "Mexican hat" potentials. Like a sombrero, the potential energy starts high in the center but quickly falls away to a minimum in any direction. The negative curvature at the top ensures that symmetry will break spontaneously – any push from the centre sends the system down towards a stable point in the brim, just where the edge of the hat begins to climb again.

In the case of graphene, the negative curvature is a result of how graphene responds to being stretched or compressed. In particle physics, negative curvature is a result of the relationship between the Higgs field and the "bare" mass of the Higgs boson. In order to be unstable, this bare mass must be imaginary – the Higgs boson acquires a real, effective mass when the field reaches its true stable ground state.

According to San-Jose, studying how graphene responds to compression by buckling into ripples could give hints about how the Higgs condenses. Small, spontaneous ripples in the absence of compression, for example, would suggest that the Higgs field may condense without requiring an imaginary bare mass for the Higgs boson. Experiments could also probe the structure of the potential in cases where the ripples are larger, providing information about mathematically difficult details of the Higgs quantum field theory.

Vitor Pereira, a condensed-matter physicist from the National University of Singapore, is interested in the Spanish team’s explanation for how these ripples arise in graphene, which it sees as being via interactions between electrons and deformities in the structure that soften the material. He suggests that graphene’s structure might be tunable through the control of electrons.

Pereira adds that graphene’s properties may mimic other experimentally difficult processes, such as how particle–antiparticle pairs arise in the vacuum. Although some researchers are sceptical about accuracy of such models, he thinks otherwise. Graphene, arguably the simplest of the condensed-matter systems, is a good testbed for such sophisticated ‘high-energy’ phenomena, he says.

The work is reported in Phys. Rev. Lett. 106 045502. Access the article at http://prl.aps.org/abstract/PRL/v106/i4/e045502
Abstract: We show that the interaction between flexural phonons, when corrected by the exchange of electron-hole excitations, may drive the graphene sheet into a quantum critical point characterized by the vanishing of the bending rigidity of the membrane. Ripples arise then due to spontaneous symmetry breaking, following a mechanism similar to that responsible for the condensation of the Higgs field in relativistic field theories, and leading to a zero-temperature buckling transition in which the order parameter is given by the square of the gradient of the flexural phonon field.

Courtesy of Kate McAlpine, science writer based in the UK, http://physicsworld.com

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February 7, 2011 — The Higgs boson is being sought at CERN’s Large Hadron Collider (LHC). According to physicists at Madrid’s Institute for Material Science, ripples in atom-thick graphene arise from a spontaneous symmetry-breaking process similar to that which separated the weak and electromagnetic forces in the early universe. This graphene research could shed light on the Higgs boson particle

At the high energies of the early universe, the weak nuclear force and the electromagnetic force are thought to converge into one electroweak force. As the temperature of the universe fell below a certain point, the two forces suddenly became separate. This "electroweak symmetry breaking" can be explained in terms of a field — the Higgs field — shifting from an effectively empty high-energy state to its ground state, filling space with a field that gives some particles their mass.

The yet-to-be-detected Higgs boson is the particle associated with vibrations of this field, and is currently being sought in the LHC. What drives the Higgs field between the high-energy and ground states? Although the parameters in the Standard Model of particle physics can be adjusted to force the symmetry-breaking, it is possible that the Higgs field may not need any coaxing. Pablo San-Jose and colleagues at Madrid’s Institute for Material Science now argue that studies of the emergence of ripples in graphene could shed some light on the process.

Solid State Technology editor Robert Haavind has speculated that Higgs boson research and discovery could land physicists a Nobel prize. The 2010 Nobel Prize-garnering (in Physics) material graphene may be a stepping stone to that discovery.

Graphene loses some symmetry in the transition from a flat shape to a rippled one, in the same way as the "activation" of the Higgs field is tied to the breaking of the electroweak symmetry. "Measuring the rippling of graphene under variable tension could give us information about the details of the intrinsic condensation of the Higgs," San-Jose says.

Working with colleagues Francisco Guinea and Jose Gonzalez, San-Jose claims to have shown that the energy landscape of graphene rippling in 2D, and that of the Higgs field in 3D, are described by similar "Mexican hat" potentials. Like a sombrero, the potential energy starts high in the center but quickly falls away to a minimum in any direction. The negative curvature at the top ensures that symmetry will break spontaneously – any push from the centre sends the system down towards a stable point in the brim, just where the edge of the hat begins to climb again.

In the case of graphene, the negative curvature is a result of how graphene responds to being stretched or compressed. In particle physics, negative curvature is a result of the relationship between the Higgs field and the "bare" mass of the Higgs boson. In order to be unstable, this bare mass must be imaginary – the Higgs boson acquires a real, effective mass when the field reaches its true stable ground state.

According to San-Jose, studying how graphene responds to compression by buckling into ripples could give hints about how the Higgs condenses. Small, spontaneous ripples in the absence of compression, for example, would suggest that the Higgs field may condense without requiring an imaginary bare mass for the Higgs boson. Experiments could also probe the structure of the potential in cases where the ripples are larger, providing information about mathematically difficult details of the Higgs quantum field theory.

Vitor Pereira, a condensed-matter physicist from the National University of Singapore, is interested in the Spanish team’s explanation for how these ripples arise in graphene, which it sees as being via interactions between electrons and deformities in the structure that soften the material. He suggests that graphene’s structure might be tunable through the control of electrons.

Pereira adds that graphene’s properties may mimic other experimentally difficult processes, such as how particle–antiparticle pairs arise in the vacuum. Although some researchers are sceptical about accuracy of such models, he thinks otherwise. Graphene, arguably the simplest of the condensed-matter systems, is a good testbed for such sophisticated ‘high-energy’ phenomena, he says.

The work is reported in Phys. Rev. Lett. 106 045502. Access the article at http://prl.aps.org/abstract/PRL/v106/i4/e045502
Abstract: We show that the interaction between flexural phonons, when corrected by the exchange of electron-hole excitations, may drive the graphene sheet into a quantum critical point characterized by the vanishing of the bending rigidity of the membrane. Ripples arise then due to spontaneous symmetry breaking, following a mechanism similar to that responsible for the condensation of the Higgs field in relativistic field theories, and leading to a zero-temperature buckling transition in which the order parameter is given by the square of the gradient of the flexural phonon field.

Courtesy of Kate McAlpine, science writer based in the UK, http://physicsworld.com

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February 3, 2011 — The same properties that make engineered nanoparticles attractive for numerous applications — small as a virus, biologically and environmentally stable, and water-soluble — also cause concern about their long-term impacts on environmental health and safety (EHS). One particular characteristic, the tendency for nanoparticles to clump together in solution, is of great interest because the size of these clusters may be key to whether or not they are toxic to human cells. Researchers at the National Institute of Standards and Technology (NIST) have demonstrated for the first time a method for producing nanoparticle clusters in a variety of controlled sizes that are stable over time so that their effects on cells can be studied properly.*
 

Figure. Transmission electron micrograph of gold nanoparticles clustering in solution. The distance between the two red arrows is approximately 280 nanometers, some 200 times smaller than the diameter of a human hair. The individual nanoparticles are approximately 15nm in diameter, about the distance across three side-by-side sodium atoms. Credit: A. Keene, U.S. Food and Drug Administration

In their tests, the NIST team made samples of gold, silver, cerium oxide and positively charged polystyrene nanoparticles and suspended them separately in cell culture medium, allowing clumping to occur in each. They stopped the clumping by adding a protein, bovine serum albumin (BSA), to the mixtures. The longer the nanoparticles were allowed to clump together, the larger the size of the resulting cluster. For example, a range of clustering times using 23nm silver nanoparticles produced a distribution of masses between 43 and 1,400nm in diameter. Similar size distributions for the other three nanoparticle types were produced using this method.

The researchers learned that using the same "freezing times" — the points at which BSA was added to halt the process — yielded consistent size distributions for all four nanoparticle types. Additionally, all of the BSA-controlled dispersions remained stable for 2-3 days, which is sufficient for many toxicity studies.

Having successfully shown that they could control the production of nanoparticle clumps of different sizes, the researchers wanted next to prove that their creations could be put to work. Different-sized silver nanoparticle clusters were mixed with horse blood in an attempt to study the impact of clumping size on red blood cell toxicity. The presence of hemoglobin, the iron-based molecule in red blood cells that carries oxygen, would tell researchers if the cells had been lysed (broken open) by silver ions released into the solution from the clusters. In turn, measuring the amount of hemoglobin in solution for each cluster size would define the level of toxicity — possibly related to the level of silver ion release — for that specific average size.

What the researchers found was that red blood cell destruction decreased as cluster size increased. They hypothesize that large nanoparticle clusters dissolve more slowly than small ones, and therefore, release fewer silver ions into solution.

In the future, the NIST team plans to further characterize the different cluster sizes achievable through their production method, and then use those clusters to study the impact on cytotoxicity of coatings (such as polymers) applied to the nanoparticles.

*J.M. Zook, R.I. MacCuspie, L.E. Locascio, M.D. Halter and J.T. Elliott. Stable nanoparticle aggregates/agglomerates of different sizes and the effect of their size on hemolytic cytotoxicity. Nanotoxicology, published online Dec. 13, 2010 (DOI: 10.3109/17435390.2010.536615).

The National Institute of Standards and Technology (NIST) is an agency of the U.S. Commerce Department. Learn more at http://www.nist.gov/index.html

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February 3, 2011 — The École Polytechnique Federale de Lausanne (EPFL) Laboratory of Nanoscale Electronics and Structures (LANES) recently demonstrated that single-layer MoS₂ can be used to fabricate transistors with extremely low leakage currents (25fA/um). Single-layer MoS₂ is 0.65nm thick and is similar to graphene, except that it is a direct gap semiconductor, with a band gap of 1.8eV.

In addition to low leakage for 0.5V supply voltage, LANES’ transistors also show mobility comparable to 2nm thin silicon films and on/off ratio higher than 108. All this is measured at room temperature.

LANES researchers believe that single layer MoS₂ could therefore complement graphene in applications that require a band gap, such as electronics.

LANES single-layer MoS₂ was produced by scotch-tape peeling of naturally occuring molybdenite crystals.

The paper describing this research, written by B. Radisavljevic,A. Radenovic, J. Brivio, V. Giacometti & A. Kis, titled Single-layer MoS₂ transistors, was published in Nature Nanotechnology at http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2010.279.html

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