Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L43: Metals I |
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Sponsoring Units: DCMP Chair: David Singh, Oak Ridge National Laboratory Room: Mile High Ballroom 4B |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L43.00001: Metal-based photocathode materials able to sustain high currents Zhaozhu Li, Kaida Yang, Jose Riso, Rosa Lukaszew Existing photocathode technology may not meet the various requirements for long photocathode lifetime, high current and repetition rate, high polarization and/or low emittance that are required for next generation light sources and nuclear physics accelerator capabilities, particularly for electron ion colliders (EIC). Specifically, next-generation light sources will need MHz repetition rates with high charge, high energy, low emittance, and a very high repetition rate while new EIC proposals stipulate hundreds of mA of current. Metallic photocathodes offer several advantages over present semiconductor photocathodes for these stringent requirements but also exhibit low QE. Coupling to the surface Plasmon polariton (SPP) modes on the metal surface offers an ideal solution to decrease the optical penetration depth and reduce the metal reflectivity thus leading to higher QE. We will present our results exploring metallic photocathode performance by enabling Surface Plasmon Polariton excitation as well as the use of adequate over-layers since it has been shown that this can also reduce the work function thus enhancing QE. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L43.00002: Majorana Fermion Induced Non-local Current Correlations in Spin-orbit Coupled Superconducting Wires Jie Liu, Fu-Chun Zhang, K.T. Law The observation of zero bias conductance peaks in semiconductor wire-superconductor heterostructures has generated great interest, and there is a hot debate on whether the observation is associated with Majorana Fermions (MFs) or other effects which enhance local Andreev reflections. In this work, we study the transport of a normal lead/semiconductor wire-superconductor heterostructure /normal lead junction. We show that when MF end states from the two ends of the wire are strongly coupled, the MF end states can suppress local Andreev reflections and strongly enhance crossed Andreev reflections (CARs), in which an electron from one lead is reflected as a hole in a different lead. In the CAR dominated regime, the current-current correlations between the two leads are strongly enhanced. Moreover, the Fano factor of a normal lead, which is the ratio of the shot noise to the average current, is reduced from 2e to e. Since the CAR associated effects are non-local effects and they cannot be induced by processes which enhance local Andreev reflections, therefore, the measurement of Fano factors and current-current correlations of the normal leads can be used to identify MFs. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L43.00003: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L43.00004: Launching low energy surface plasmons in purple gold (AuAl$_{2}$) Panupon Samaimongkol, Hans D. Robinson The intermetallic compound AuAl$_{2}$ is sometimes known as ``purple gold'' due to its intense purple color. It was long assumed this color originated from an interband absorption transition, but, as has recently be pointed out [1], it is related to a surface plasmon resonance which exists in this material at a lower energy than in any of the pure metals (including Au, Ag, Cu, etc.). Thin films of purple gold can readily be prepared by layered evaporation of Au and Al in the proper~ratios, followed by annealing at moderately elevated temperatures. Fabricating AuAl$_{2}$ films on the hypotenuse of high index glass prisms, we have been able to launch surface plasmons in this material in the Kretschmann configuration, from which we directly extract the dispersion relation of surface plasmons, in good agreement with predictions from previous measurements of its dielectric function. Surface plasmon sensing results using AuAl$_{2}$ will also be presented. Finally, we will discuss the possible applications for purple gold in nanoparticle form, where it has the property of being an excellent light absorber across the entire visible spectrum.\\[4pt] [1] Keast, V.J., et al., Appl. Phys. Lett.~99, 111908~(2011). [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L43.00005: Infrared optical conductivity for Ni$_{1-x}$Pt$_{x}$ alloys and Ni$_{1-x}$Pt$_{x}$Si monosilicides Lina Abdallah, S. Zollner, C. Lavoie, A. Ozcan, M. Raymond We used infrared spectroscopic ellipsometry to measure the optical constants of Ni$_{1-x}$Pt$_{x}$ alloys and Ni$_{1-x}$Pt$_{x}$Si monosilicides in the infrared region (200-6000 wave numbers). Nickel platinum alloys (up to 30\% Pt) were deposited on top of a thick layer of thermal oxide. Similar alloys were deposited on top of silicon and were annealed at 500 $^{\circ}$ for 30 seconds to create monosilicides. The Pt composition dependence of the optical conductivity of the unreacted metal and monosilicide was investigated. We also studied the thickness dependence of Ni1-xPtx alloys. Four different fitting techniques were applied to our data using a point-by-point fit as well as an oscillator fit where we found a two carrier effect in the dielectric function of the unreacted metal. The data obtained from the IR region was combined with previous data in the visible spectrum to get more comprehensive optical constants for both unreacted metal and reacted silicide. Results indicate that optical conductivity is higher for thicker samples. Furthermore SiO$_{2}$ has a strong absorption peak at 1040 wave numbers caused by bond stretching vibration. This absorption peak appears in the ellipsometric data of the thinner films and is absent in thicker ones. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L43.00006: Anomalous Kondo transport in a single-electron transistor driven by microwave field Zhan Cao, Cheng Chen, Fu-Zhou Chen, Hong-Gang Luo The Kondo transport in a single-electron transistor continues to provide unexpected physics due to the interplay between magnetic field and microwave applied, as shown in a recent experiment(B. Hemingway et al., arXiv:1304.0037). For a given microwave frequency, the Kondo differential conductance shows an anomalous magnetic field dependence, and a very sharp peak is observed for certain field applied. Additionally, the microwave frequency is found to be larger of about one order than the corresponding Zeeman energy. These two features are not understood in the current theory. Here we propose a phenomenological mechanism to explain these observations. When both magnetic field and microwave are applied in the SET, if the frequency matches the (renormalized) Zeeman energy, it is assumed that the microwave is able to induce spin-ip in the single-electron transistor, which leads to two consequences. One is the dot level shifts down and the other is the renormalization of the Zeeman energy. This picture can not only explain qualitatively the main findings in the experiment but also further stimulate the related experimental study of the Kondo transport. Additional microwave modulation may provide a novel way to explore the functional of the SET in nanotechnology and quantum information processing. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L43.00007: Kondo effect of an adatom in graphene and metallic surface Hong-Gang Luo I will present the Kondo effect of a single magnetic adatom on the surface of graphene[1] and metal[2]. The unique linear dispersion relation near the Dirac points in graphene favors magnetic moment[3], which simply means that the Kondo resonance can be observed in a more wider parameter region than in the metallic host. Our work indicated that the Kondo resonance, whenever the chemical potential is tuned away from the Dirac points, indeed can form ranged from the Kondo regime, to the mixed valence, even to the empty orbital regime defined in the conventional metal host. Correspondingly, the Kondo resonance can exhibit as a sharp peak, a peak-dip or an anti-resonance in different regimes. These lineshapes result from the Fano resonance [4] due to the significant background leaded by dramatically broadening of the impurity level in graphene. The scanning tunneling microscopy (STM) spectra of the adatom are also showed and have obvious particle-hole asymmetry when the chemical potential is tuned by the gate voltages applied to the graphene. References: [1] L. Li et al., New J. Phys. 15, 053018 (2013). [2] H.-G. Luo et al., Phys. Rev. Lett. 92, 256602(2004); \textit{ibid}., 96, 019702 (2006). [3] B. Uchoa et al., Phys. Rev. Lett. 101, 026805(2008). [4] U. Fano, Phys. Rev. 124, 1866 (1961). [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L43.00008: Low temperature thermal conductivity of alloys used in cryogenic coaxial cables Akihiro Kushino, Soichi Kasai We have developed thin seamless coaxial cables applied for readout in low temperature experiments below liquid helium temperature. Stainless steel employed as the center and outer electrical conductors of the coaxial cable has adequately low thermal conductivity compared to pure metals and can be used when heat penetration into low temperature stages through cables should be lowered however it has large electrical resistivity which can disturb sensitive measurements. Superconducting NbTi alloy has good performance with rather low thermal conductivity and high electrical conductivity. Meanwhile coaxial cables using normal conducting copper alloys such as cupro-nickel, brass, beryllium-copper, phosphor-bronze are advantageous with their good electrical, thermal and cost performances. We investigated thermal conductivity of such alloys after the drawing process into coaxial cables, and compared to expected values without drawing. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L43.00009: Doping dependence of structural transitions in LaCu$_{6-x}$Ag$_x$ L. Poudel, M. Koehler, M. McGuire, V. Keppens, C. Dela Cruz, D. Mandrus, A. Christianson CeCu$_{6-x}$Au$_x$ is a well known heavy fermion system that exhibits a quantum critical point (QCP) at x $\simeq$ 0.1. One interesting feature of the CeCu$_{6-x}$Au$_x$ system is the doping dependence of the structural phase transition. The end member CeCu$_6$ undergoes a structural transition from an orthorhombic to a monoclinic phase at 230 K. The transition temperatures drop linearly with Au concentration until the transition is suppressed at x $\simeq$ 0.1. This is the same composition where the antiferromagnetic quantum critical point occurs. We study the related system, LaCu$_{6-x}$Ag$_x$, in order to determine the behavior of structural transition as is it suppressed without the complicating influence of magnetism. In analogy with the CeCu$_{6-x}$Au$_x$ system, LaCu$_6$ displays a structural transition from an orthorhombic to a monoclinic phase at 460 K, which is suppressed by Ag doping. The suppression of the transition temperature occurs in conjunction with an expansion on of the lattice. Both the transition temperature and the monoclinic order parameter $(ac Cos\beta)^2$ scale linearly with Ag doping. Extrapolation of either the transition temperature or the monoclinic order parameter indicates that the structural transition is suppressed completely for x $\simeq$0.225 [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L43.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L43.00011: The Role of Lattice Dynamics on The Thermal Properties of Cu-Ni Alloys Berk Onat, Sondan Durukanoglu We have investigated Cu-Ni alloys with both disorder and order phases in fcc structures to analyze the effect of temperature dependent vibrational thermodynamical properties. The interactions between the atoms in the model systems are defined using an EAM type potential, specifically developed for Cu-Ni alloys. Vibrational thermodynamic functions are determined within the harmonic approximation of lattice dynamics and the vibrational densities of states are calculated using real space Green's function technique. In addition, through ab-initio calculations we have estimated the electronic contributions to set the ground for a comparative discussion. Our results show that the overall characteristics of thermodynamic functions of Cu-Ni alloys of varying concentrations are governed by the lattice vibrations. We will present our results for free energy, heat capacity and entropy of ordered/disordered Cu-Ni alloys with the experimental findings and discuss the electronic, anharmonic and lattice dynamic contributions. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L43.00012: Defect Interaction in Iron and Iron-based Alloys Haixuan Xu, G. Malcolm Stocks, Roger Stoller Magnetism has a profound influence on the defect properties in iron and iron-based alloys. For instance, it has been shown from first principles calculations that the helium interstitial occupies the tetrahedral site instead of octahedral site in contrast to all previous work that neglected the magnetic effects. In this study, we explore the effects of magnetism on the defect interaction, primarily interstitial-type defects, in bcc iron and Fe-Cr systems. The magnetic moment change during the interaction of two 1/2 \textless 111\textgreater interstitial loops in bcc iron was calculated using the \textit{ab initio} locally self-consistent multiple-scattering (LSMS) method and a significant fluctuation was observed. Adding Cr significantly modifies the magnetic structure of the defects and defect interactions. In addition, the effects of magnetism on the defect energetics are evaluated. This study provides useful insights on whether magnetism can be used as a effective means to manipulate the defect evolution in iron-based structural alloys. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L43.00013: Prediction of A2 to B2 Phase Transition in the High Entropy Alloy Mo-Nb-Ta-W William Huhn, Michael Widom In this talk we show that an effective Hamiltonian fit with first principles calculations predicts an order/disorder transition occurs in the high entropy alloy Mo-Nb-Ta-W. Using the Alloy Theoretic Automated Toolset, we find T=0K enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in particular we find the stable structures for binaries at equiatomic concentrations are close in energy to the associated B2 structure, suggesting that at intermediate temperatures a B2 phase is stabilized in Mo-Nb-Ta-W. Our ``hybrid Monte Carlo/molecular dynamics'' results for the Mo-Nb-Ta-W system are analyzed to identify certain preferred chemical bonding types. A mean field free energy model incorporating nearest neighbor bonds will be presented, allowing us to predict the mechanism of the order/disorder transition. We find the temperature evolution of the system is driven by strong Mo-Ta bonding. Comparison of the free energy model and our MC/MD results suggest the existence of additional low-temperature phase transitions in the system likely ending with phase segregation into binary phases. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L43.00014: Predictors of the stability of high entropy alloys: is the entropy of mixing sufficient? M. Claudia Troparevsky, James R. Morris, Paul Kent, G. Malcolm Stocks High entropy alloys (HEAs) have attracted extensive attention due to their remarkable combination of strength, ductility, thermal stability, corrosion and wear resistance. However, little is known about why these alloys are stable in a single-phase solid solution or how to predict which combinations of elements will form a single phase HEA. Here, we present density functional theory calculations of the heat of formation of several HEAs in an effort to assess the role of the entropy of mixing in the stability of these alloys. The systems studied here include both single-phase and multi-phase alloys. The heats of formation show no significant differences, regardless of their single or multi phase formation, and no trends that could explain the stability of the single phase materials. Moreover, all of the calculated heats of formation are positive. These findings indicate that the entropy of mixing is insufficient to explain the unique stability of these alloys, and highlights the need for new criteria to explain the formation of single-phase solid solutions. We also discuss the minimum energy structures of several FCC and BCC alloys as well as their relative phase stability. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L43.00015: The interaction of Cr and Ni solute atoms with core of screw and edge dislocation in bcc Fe Yuri Osetsky, Odbadrakh Khorgolkhuu, German Samolyuk, Don Nicholson, Roger Stoller, Malcolm Stocks Mobility of dislocations controls the plasticity in metals. Density functional theory (DFT) is an effective tool in providing {\it ab initio} information on the energetic and magnetic properties of defects including dislocations and its interaction with other defects. We present DFT calculations on atomic properties of $1/2<111>$ screw and $1/2<111>(110)$ and $1/2<111>(112)$ edge dislocations in Fe-Cr/Ni system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr/Ni solute atoms. The size of supercell changes from 130 atoms for screw to 1800 atoms for edge dislocations. We investigated sensitivity of the binding energy of impurity atoms with a dislocation to lattice relaxation and size of modeling supercell. It was demonstrated that magnetic moment of solute atoms is ordered in the same direction as that of Fe matrix atoms for the case of Ni and in the opposite direction for Cr. Binding energy was found to be very sensitive to magnetic ordering. [Preview Abstract] |
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