Copyright (c) 2013 Purdue University All rights reserved. http://docs.lib.purdue.edu/physics_articles Recent documents in Physics Research Publications en-us Thu, 24 Jan 2013 18:03:51 PST 3600 http://docs.lib.purdue.edu/physics_articles/1360 http://docs.lib.purdue.edu/physics_articles/1360 Fri, 24 Jun 2011 02:25:27 PDT We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of similar to +/- 0.015 from filling factor v = 1/3. The resonance is remarkably similar to resonances previously observed near integer v, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of e/3-charged carriers.

]]> H. Zhu et al. http://docs.lib.purdue.edu/physics_articles/1359 http://docs.lib.purdue.edu/physics_articles/1359 Fri, 24 Jun 2011 02:25:02 PDT We use inelastic neutron scattering to show that for the optimally electron-doped BaFe1.9Ni0.1As2 (T-c = 20 K) iron arsenide superconductor, application of a magnetic field that partially suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results demonstrate that the energy of the resonance is intimately connected to the electron pairing energy, and thus indicate that the mode is a direct probe for measuring electron pairing and superconductivity in iron arsenides.

]]> J. Zhao et al. http://docs.lib.purdue.edu/physics_articles/1358 http://docs.lib.purdue.edu/physics_articles/1358 Fri, 24 Jun 2011 02:24:39 PDT We report the transport properties of graphene in the presence of topological and (non-topological) long-range impurities, in the framework of a tight-binding model. We first investigate the electronic transport properties of a zigzag edge graphene nanoribbon with several kinds topological defects: a pentagon, a heptagon, and a pentagon-heptagon pair at the center. On the aspect of non-topological impurities, we perform systematic calculations to investigate the effect of the impurity potential range and its density on the conductance of graphene nanoribbon. Moreover, we demonstrate that the nonlinear dependence in the recent experiment can also be explained when scattering due to the low density and long-range impurities are accounted. We also calculate the scaling properties of disordered graphene with long-range impurities in the framework of finite-size scaling. On the marginal metallic side, we find that the conductance is independent of the system size, which is a characteristic of the Kosterlitz-Thouless type transition in conventional 2D systems with random magnetic field. We explicitly probe the K-T transition nature of the MIT by identifying the bounding and unbounding vortex-anti-vortex local currents in the system. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

]]> Y. Y. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1357 http://docs.lib.purdue.edu/physics_articles/1357 Fri, 24 Jun 2011 02:24:15 PDT Pairing symmetry is a fundamental property that characterizes a superconductor. For the iron-based high-temperature superconductors, an s(+/-)-wave pairing symmetry has received increasing experimental and theoretical support. More specifically, the superconducting order parameter is an isotropic s-wave type around a particular Fermi surface, but it has opposite signs between the hole Fermi surfaces at the zone centre and the electron Fermi surfaces at the zone corners. Here we report the low-energy electronic structure of the newly discovered superconductors, A(x)Fe(2)Se(2) (A = K, Cs) with a superconducting transition temperature (T-c) of about 30 K. We found A(x)Fe(2)Se(2) (A = K, Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces are observed around the zone corners, with an almost isotropic superconducting gap of similar to 10.3 meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s(+/-) pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.

]]> Y. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1356 http://docs.lib.purdue.edu/physics_articles/1356 Fri, 24 Jun 2011 02:23:53 PDT The three-dimensional band structure and superconducting gap of Ba0.6K0.4Fe2As2 are studied with angle-resolved photoemission spectroscopy. In contrast with previous results, we have identified three holelike Fermi surface sheets near the zone center with sizable out-of-plane or k(z) dispersion. The superconducting gap on certain Fermi surface sheets shows significant k(z) dependence. Moreover, the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries (one odd, one even). Our results further reveal the three-dimensional and orbital-dependent structure of the superconducting gap in iron pnictides, which facilitates the understanding of momentum-integrated measurements and provides a distinct test for theories.

]]> Y. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1355 http://docs.lib.purdue.edu/physics_articles/1355 Fri, 24 Jun 2011 02:23:25 PDT The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multiband/multiorbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe1.85Co0.15As2 by studying the polarization and photon-energy dependence of angle-resolved photoemission data. While the distributions of the d(xz), d(yz), and d(3z2-r2) orbitals agree with the prediction of density functional theory, those of the d(xy) and d(x2-y2) orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations and, more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.

]]> Y. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1354 http://docs.lib.purdue.edu/physics_articles/1354 Fri, 24 Jun 2011 02:23:01 PDT The electronic structure of alpha-Fe1.06Te is studied with angle-resolved photoemission spectroscopy. We show that there is substantial spectral weight around Gamma and X, and line shapes are intrinsically broad in the paramagnetic state. The magnetic transition is characterized by a massive spectral weight transfer over an energy range as large as the bandwidth, which even exhibits a hysteresis loop that marks the strong first-order transition. Coherent quasiparticles emerge in the magnetically ordered state due to decreased spin fluctuations, which accounts for the change in transport properties from insulating behavior to metallic behavior. Our observation demonstrates that Fe1.06Te distinguishes itself from other iron-based systems with more local characters and much stronger interactions, and how a spin-density wave is formed in the presence of strong correlation.

]]> Y. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1353 http://docs.lib.purdue.edu/physics_articles/1353 Fri, 24 Jun 2011 02:22:40 PDT We performed As-75 NMR studies on two overdoped high-quality Ba1-xKxFe2As2 (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation (1/T-75(1)) from the x =0.7 to the x=1.0 samples. In KFe2As2, the ratio of 1/(T1TKn2)-T-75, where K-75(n) is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in KFe2As2 which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of 1/T-75(1) below T-c in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior 1/(T1T)-T-75 similar to T-0.5 is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.

]]> S. W. Zhang et al. http://docs.lib.purdue.edu/physics_articles/1352 http://docs.lib.purdue.edu/physics_articles/1352 Fri, 24 Jun 2011 02:22:15 PDT We use quantum Monte Carlo simulations to study effects of disorder on the quantum phase transition occurring versus the ratio g = J/J' in square-lattice dimerized S = 1/2 Heisenberg antiferromagnets with intradimer and interdimer couplings J and J'. The dimers are either randomly distributed (as in the classical dimer model), or come in parallel pairs with horizontal or vertical orientation. In both cases the transition violates the Harris criterion, according to which the correlation-length exponent should satisfy v >= 1. We do not detect any deviations from the three-dimensional O(3) universality class obtaining in the absence of disorder (where v approximate to 0.71). We discuss special circumstances which allow v < 1 for the type of disorder considered here.

]]> D. X. Yao et al. http://docs.lib.purdue.edu/physics_articles/1351 http://docs.lib.purdue.edu/physics_articles/1351 Fri, 24 Jun 2011 02:21:55 PDT We study the magnetic excitations of undoped iron oxypnictides using a three-dimensional Heisenberg model with single-ion anisotropy. Analytic forms of the spin-wave dispersion, velocities, and structure factor are given. Aside from quantitative comparisons that can be made to inelastic neutron scattering experiments, we also give qualitative criteria that can distinguish various regimens of coupling strength. The magnetization reduction due to quantum zero point fluctuations shows clear dependence on the c-axis coupling.

]]> D. X. Yao et al. http://docs.lib.purdue.edu/physics_articles/1350 http://docs.lib.purdue.edu/physics_articles/1350 Fri, 24 Jun 2011 02:21:26 PDT The electronic structure of LaFeAsO, a parent compound of iron-arsenic superconductors, is studied by angle-resolved photoemission spectroscopy. By examining its dependence on photon energy, polarization, and sodium dosing, both the bulk and the surface contributions are identified. We find that a bulk band moves toward high binding energies below the structural transition temperature, and shifts smoothly across the spin-density-wave transition by about 25 meV. Our data suggest that the band reconstruction may play a crucial role in the spin-density-wave and the structural transitions. For the surface states, both the LaO-terminated and FeAs-terminated components are revealed. Certain small band shifts are observed for the FeAs-terminated surface states in the spin-density-wave state, which might be a reflection of the bulk electronic-structure reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low temperatures, indicating drastic reduction in the scattering rate. A kink structure in one of the surface band is shown to be possibly related to the enhanced electron-phonon interactions on the polar surface.

]]> L. X. Yang et al. http://docs.lib.purdue.edu/physics_articles/1349 http://docs.lib.purdue.edu/physics_articles/1349 Fri, 24 Jun 2011 02:20:58 PDT The iron-pnictide superconductors have a layered structure formed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional(1) (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k(z) capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba0.6K0.4Fe2As2 samples. We found a marked kz dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer Delta(1) and interlayer Delta(2) pairing. The anisotropy ratio Delta(1)/Delta(2), determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J(c)/J(ab) in the parent compound(2). The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrains the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.

]]> Y. M. Xu et al. http://docs.lib.purdue.edu/physics_articles/1348 http://docs.lib.purdue.edu/physics_articles/1348 Fri, 24 Jun 2011 02:20:29 PDT Graphene with a large area was synthesized on Cu foils by chemical vapor deposition under ambient pressure A 4 '' x 4 '' graphene film was transferred onto a 6 '' Si wafer with a thermally grown oxide film Raman mapping indicates monolayer graphene dominates the transferred graphene film Gas sensors were fabricated on a 4 mm x 3 mm size graphene film with a 1 nm palladium film deposited for hydrogen detection Hydrogen in air with concentrations in 00025-1% (25-10,000 ppm) was used to test graphene-based gas sensors The gas sensors based on palladium-decorated graphene films show high sensitivity, fast response and recovery, and can be used with multiple cycles The mechanism of hydrogen detection is also discussed (C) 2010 Elsevier B V All rights reserved

]]> W. Wu et al. http://docs.lib.purdue.edu/physics_articles/1347 http://docs.lib.purdue.edu/physics_articles/1347 Fri, 24 Jun 2011 02:20:04 PDT We study the s-wave quintet Cooper pairing phase (S-pair = 2) in spin-3/2 cold atomic systems and identify various novel features which do not appear in spin-1/2 pairing systems. A single quantum vortex is shown to be energetically less stable than a pair of half-quantum vortices. The half-quantum vortex exhibits the global analogue of the non-Abelian Alice string and SO(4) Cheshire charge in gauge theories. The non-Abelian half-quantum vortex loop enables topological generation of quantum entanglement.

]]> C. J. Wu et al. http://docs.lib.purdue.edu/physics_articles/1346 http://docs.lib.purdue.edu/physics_articles/1346 Fri, 24 Jun 2011 02:19:37 PDT Carbon nanotubes (CNTs) are promising candidates to create new thermionic- and photoemission materials. Intercalation of CNTs with alkali metals, such as potassium, greatly reduces their work functions, and the low electron scattering rates of small-diameter CNTs offer the possibility of efficient photoemission. This work uses a Nd:YAG (YAG denotes yttrium aluminum garnet) laser to irradiate single- and multiwalled CNTs intercalated with potassium, and the resultant energy distributions of photo- and thermionic emitted electrons are measured using a hemispherical electron energy analyzer over a wide range of temperatures. For both single- and multiwalled CNTs intercalated with potassium, the authors observe a temperature dependent work function that has a minimum of approximately 2.0 eV at approximately 600 K. At temperatures above 600 K, the measured work function values increase with temperature presumably due to deintercalation of potassium atoms. Laser illumination causes the magnitudes of collected electron energy distributions to increase substantially but in many cases has little effect on their shape. Simple theoretical models are also developed that relate the photo- and thermionic emission processes and indicate that large numbers of photoexcited electrons partially thermalize (i.e., undergo one or more scattering events) before escaping from the emitter surface.

]]> T. L. Westover et al. http://docs.lib.purdue.edu/physics_articles/1345 http://docs.lib.purdue.edu/physics_articles/1345 Fri, 24 Jun 2011 02:19:14 PDT K. C. Welten et al. http://docs.lib.purdue.edu/physics_articles/1344 http://docs.lib.purdue.edu/physics_articles/1344 Fri, 24 Jun 2011 02:18:46 PDT On October 7, 2008, a small F-class asteroid, 2008 TC3, exploded in Earth's atmosphere, and produced a strewn field of meteorites in the Nubian Desert of Sudan. Subsequent searches yielded several hundred meteorite fragments, known as Almahata Sitta. This fall was classified as a polymict ureilite. We measured cosmogenic radionuclides in six fragments and noble gases in four fragments of the Almahata Sitta ureilite. The concentrations of 10Be, 26Al, and 36Cl in the meteorite fragments of asteroid 2008 TC3 indicate a preatmospheric radius of 300 +/- 30 g cm-2. Combined with an absolute radius of 1.8 +/- 0.2 m, which was derived from its brightness in space, the cosmogenic radionuclides constrain the bulk density of asteroid 2008 TC3 to 1.66 +/- 0.25 g cm-3 and the bulk porosity to 50 +/- 7%. The bulk density of asteroid TC3 is on the low end of the range of 1.6-3.3 g cm-3 determined for 40 of the recovered ureilite fragments. Since the denser materials have a higher chance of surviving atmospheric fragmentation, the low-density ureilites are probably more representative of the bulk asteroid. We thus conclude that the high porosity of asteroid 2008 TC3 is mainly due to microporosity, which implies that not all low-density asteroids are necessarily rubble pile structures. Finally, from the cosmogenic 21Ne concentrations, we determined a cosmic-ray exposure (CRE) age of 19.5 +/- 2.5 Myr. This age represents the time since asteroid 2008 TC3 was ejected from a large F-class parent body in the asteroid belt, until its collision with Earth.

]]> K. C. Welten et al. http://docs.lib.purdue.edu/physics_articles/1343 http://docs.lib.purdue.edu/physics_articles/1343 Fri, 24 Jun 2011 02:18:19 PDT The collection of approximately 3300 meteorites from the Queen Alexandra Range (QUE) area, Antarctica, is dominated by more than 2000 chondrites classified as either L5 or LL5. Based on concentrations of the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca in the metal and stone fraction of 16 QUE L5 or LL5 chondrites, we conclude that 13 meteorites belong to a single meteorite shower, QUE 90201, with a large preatmospheric size and a terrestrial age of 125 kyr. Members of this shower have properties typical of L (e.g., pyroxene composition) and LL chondrites (e.g., metal abundance and composition), as well as properties intermediate between the L and LL groups (e.g., olivine composition), and is thus best described as an L/LL5 chondrite. Based on comparison with model calculations, the measured radionuclide concentrations in the metal and stone fractions of QUE 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150 cm, representing one of the largest chondrites known so far. Based on the abundance of small L5 and LL5 chondrites at QUE and their distinct mass distribution, we conclude that the QUE 90201 shower includes up to 2000 fragments with a total recovered mass of 60-70 kg, < 1% of the preatmospheric mass of approximately 50,000 kg. The mass distribution of the QUE 90201 shower suggests that the meteoroid experienced catastrophic atmospheric fragmentation(s), either because it was a fragile object or it had a high entry velocity.

]]> K. C. Welten et al. http://docs.lib.purdue.edu/physics_articles/1342 http://docs.lib.purdue.edu/physics_articles/1342 Fri, 24 Jun 2011 02:17:54 PDT We measured the concentrations of the cosmogenic radionuclides Be-10, Al-26, Cl-36 and Ca-41 in the metal and stone fractions of three large chondrite showers to determine their pre-atmospheric size. Large chondrites are characterized by substantial contributions of neutron-capture Ca-41 in the stone fraction (up to similar to 2 dpm/gCa), low radionuclide concentrations in the metal fraction and high Be-10(stone)/Be-10(metal) ratios. Based on the measured concentrations in comparison with calculated cosmogenic nuclide depth profiles, using a semi-empirical and a purely physical model, we conclude that these objects had pre-atmospheric radii ranging from similar to 80 cm to >3 m. We conclude that the semi-empirical model is more reliable for spallogenic production rates in large objects, while the purely physical model is more reliable for neutron-capture products. (C) 2009 Elsevier B.V. All rights reserved.

]]> K. C. Welten et al. http://docs.lib.purdue.edu/physics_articles/1341 http://docs.lib.purdue.edu/physics_articles/1341 Fri, 24 Jun 2011 02:17:30 PDT We propose a new entanglement switch of qubits consisting of electric dipoles oriented along or against an external electric field and coupled by the electric dipole-dipole interaction. The pairwise entanglement can be tuned and controlled by the ratio of the Rabi frequency and the dipole-dipole coupling strength. Tuning the entanglement can be achieved for one, two, and three-dimensional arrangements of the qubits. The feasibility of building such an entanglement switch is also discussed.

]]> Q. Wei et al.