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			Paglione Research Group
			Center for Nanophysics and Advanced Materials Department of Physics University of Maryland
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Latest News         Graduate Student Brings Liquid Science to 2nd Grade As part of an NSF outreach activity, grad student Kevin Kirshenbaum performed demonstrations to a 2nd grade class at Dodge Park Elementary School in Landover, MD, using liquid nitrogen. As a result, Kevin earned the prestigious title of "real physicist" - nice work! See link here.     Neutron Work Highlighted in NIST 2011 Annual Report Aliovalent substitution of trivalent rare earth R3+ ions for divalent Ca2+ into CaFe2As2 acts to suppress AFM order and induce superconductivity at temperatures as high as 47 K. With a close match between ionic radii of La, Ce, Pr, and Nd with that of Ca, we are able to selectively tune the structural parameters of this compound by using the slightly decreasing variation in rare earth size with f-electron count, known as the lanthanide contraction, to tune the unit cell volume in a controllable manner. Single-crystal diffraction studies were carried out to study the Ca1-xRxFe2As2 structure, revealing dramatic changes in the structure that include not only an unprecedented thermal expansion, but also a structural collapse of the tetragonal unit cell akin to that previously found under pressure. Read more in the NIST Annual Report. Also see NIST Tech Beat and UMD News Desk.     Superconductivity in Topological Semimetal YPtBi Superconductivity was discovered in the non-centrosymmetric half-Heusler compound YPtBi below a critical temperature Tc = 0.77 K. While having a low Tc value, our magnetoresistance and Hall measurements support theoretical predictions that this material is a topologically nontrivial semimetal, making the appearance of superconductivity even more interesting beyond the fact that this material has a non-centrosymmetric crystal structure. Furthermore, the surprisingly low positive charge carrier density of ~1018 cm-3 is unprecedented, making this the lowest carrier density superconductor known to date. The combination of non-centrosymmetry and strong spin-orbit coupling in YPtBi presents a promising platform for the investigation of topological superconductivity. Read more here.   High-temperature Superconductivity Reveals its Secret High-temperature superconductivity, the ability of certain materials to conduct electricity with zero electrical resistance at temperatures above the boiling point of liquid nitrogen, was unexpectedly discovered in copper oxide (cuprate) materials in 1987. High-temperature superconductivity could revolutionize technologies ranging from magnetically-levitated trains to electrical power transmission. However, the mechanism by which these cuprate materials become superconducting has remained a mystery for almost 25 years. Now, scientists at the University of Maryland, College Park, have found the strongest evidence yet that the cause of high-temperature superconductivity involves the pairing of electrons by magnetic excitations (spin fluctuations). The new experimental results suggest ways to improve the superconductivity in these novel materials, with the ultimate goal of finding superconductors that operate at room temperature.  Read more here.     A hundred years of superconductivity: special issue The stunning discovery of superconductivity in iron-based materials has uncovered a new family of high-temperature superconductors with properties that are both similar to and different than those of the copper-oxide family of superconductors. With transition temperatures approaching the boiling point of liquid nitrogen, these materials promise to provide a rich playground in which to study the fundamentals of superconductivity, while advancing the prospects for widespread technological applications. See feature article in this special issue of Physics in Canada.       STM study of SrFe2As2 reveals maze-like adatom structures Michael Dreyer of the NSA Laboratory for Physical Sciences has used a scanning tunneling microscope to image the surface of SrFe2As2 single crystals at 4.2 K, finding maze-like regions of atoms on the surface sitting on As bridge positions of the underlying Fe2As2 layer. Atomically resolved images of the corner positions within the maze-like reconstruction reveal the presence of adatoms rather than As dimers. A link to the published article in Journal of Physics is available here.       Quest for better superconductors News feature on superconductivity research at the University of Maryland and elsewhere has been published in Australia’s #1 science print and online media brand, Cosmos Magazine.     Structural collapse and 45 K superconductivity in electron-doped CaFe2As2 The interplay between structural, magnetic and superconducting properties in the newly discovered iron-based superconducting compounds has been a central theme in attempts to elucidate the nature of Cooper pairing in this new family of high-temperature superconductors. This work reports the stunning observation of 45 K superconductivity in electron-doped CaFe2As2, presenting the highest Tc in the intermetallic class of iron-based superconductors. The use of aliovalent rare earth substitution into the alkaline earth site allows us to tune both the lattice density and charge doping in this system, resulting in a controllably induced structural collapse of the tetragonal unit cell by choice of substituent ion size. Remarkably, the superconductivity appears to persist independent of the presence of a structural collapse, despite an abrupt change in electronic structure at the onset of interlayer bonding. [preprint available here]     Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3 The group of Hui Zhao at the University of Kansas has used ultrafast pump-probe techniques to study the dynamic response of topological insulator material Bi2Se3. A link to the published article in Phys. Rev. B is available here.   IOP paper highlighted in superconductivity centenary Our publication reporting superconductivity at 23 K in Pt-doped BaFe2As2 has been chosed by IoP as one of 100 articles made freely available to celebrate the centenary of the discovery of superconductivity     Science News feature on iron-based superconductivity Science writer Gwyneth Dickey has written a feature called "Iron in the mix" in the bi-weekly periodical Science News discussing current efforts to understand high-temperature superconductivity in the iron-pnictides, highlighting some of the work going on in CNAM and our group.   Progress Article published in Nature Physics High-temperature superconductivity in iron-based materials (Nature Physics) The surprising discovery of superconductivity in layered iron-based materials, with transition temperatures climbing as high as 55 K, has lead to thousands of publications on this subject over the past two years. While there is general consensus on the unconventional nature of the Cooper pairing state of these systems, several central questions remain - including the role of magnetism, the nature of chemical and structural tuning, and the resultant pairing symmetry - and the search for universal properties and principles continues. In this Progress Article we review recent advances of research on iron-based superconducting materials, highlighting the major experimental benchmarks that have been so far reached and the important questions that remain to be conclusively answered.     Probing the surface states of a topological insulator Strong surface scattering in ultra-high mobility Bi2Se3 crystals (Phys Rev. B - Rapid Communications)  While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi2Se3, a significant experimental concern is that no signatures have been observed in bulk transport. We have studied nominally undoped single crystals of Bi2Se3 with carrier densities approaching 1016 cm−3 and very high mobilities exceeding 2 m2 V−1  s−1 in order to characterize these "protected" surface states. However, Shubnikov-de Haas oscillations, Hall effect, and optical reflectivity measurements indicate that the measured electrical transport can be attributed solely to bulk states, even at low Landau-level filling factor and in the quantum limit. This article has been highlighted as an Editor's Suggestion.                               ...older posts