Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W11: Cuprates: Josephson Junctions |
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Sponsoring Units: DMP Chair: Mike Osofsky, Naval Research Laboratory Room: 007B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W11.00001: Direct Observation of Superconducting Pair States in Atomic Resolution on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Sang Hyun Joo, Inhee Lee, Chung Koo Kim, Jung Hoon Yoo, Min Seok Park, Gyoung Seok Lee, Jae-joon Kim, Genda Gu, Kyle McElroy, Shin-ichi Uchida, J.C. Davis, Jinho Lee Conventional Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) utilizes metal tips which limits ones only to access the information of quasiparticles, not superconducting pairs. We report the first atomically resolved Scanning Josephson Tunneling Microscopy (SJTM) on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) using Bi-2212 tips which is formed \textit{in situ}. Conductance maps measured by Bi-2212 tips show clear Josephson $I$-$V$ characteristics which can be explained by considering Josephson phase diffusion due to thermal fluctuations. Zero Bias Conductance Peaks (ZBCPs) following Ambegaokar-Baratoff (AB) theory were directly measured by lock-in technique by which we can probe SC pairs directly. We will also present studies of Zn impurities on Bi-2212 using SJTM. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W11.00002: High-resolution Josephson spectroscopy with a scanning tunneling microscope Mallika T. Randeria, Benjamin E. Feldman, Ilya K. Drozdov, Ali Yazdani Conventional scanning tunneling microscopy (STM) measurements use a normal metal tip to probe local quasi-particle density of states with atomic resolution. Using a superconducting tip to conduct spectroscopy significantly boosts the energy resolution of the measurements, thus expanding the STM capabilities. Moreover, superconducting tips make it possible to probe superconductivity via the Josephson effect, which provides a direct measure of the local superconducting order parameter. Therefore, scanning Josephson spectroscopy measurements have the potential to characterize of a wide variety of superconducting materials on the atomic scale. I will present superconducting Pb tip measurements performed at temperatures below 250mK in a dilution refrigerator STM. By controlling the junction resistance, we are able to explore a wide range of tunneling regimes. Josephson measurements on Pb samples exhibit features including multiple Andreev reflections, and I will discuss the extension of these techniques to study atomic scale variations in Josephson current. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W11.00003: Studies of superconductivity (SC) and competing-order (CO) interplay in cuprates and Fe-base compounds using scanning tunneling spectroscopy (STS) M.L. Teague, C.-C. Chen, N.-C. Yeh, Z.J. Feng STS studies of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (Y-123) and Ca-doped Y-123 from under- to over-doped regimes demonstrate that the origin of the pseudogap (PG) is due to competing orders (COs), and that the presence (absence) of PG above the SC transition $T_{c}$ is associated with a CO energy $\Delta_{\mathrm{CO}}$ larger (smaller) than the SC gap $\Delta_{\mathrm{SC}}$. We find that for hole doping level $p$ $\le $ 0.16, $\Delta_{\mathrm{CO}}$ \textit{\textgreater } $\Delta_{\mathrm{SC}}$, whereas both $\Delta_{\mathrm{SC}}$ and $\Delta_{\mathrm{CO}}$ decrease with $p$ for $p$ \textgreater 0.16, and $\Delta_{\mathrm{CO}}$ ($\sim$ 10 meV) \textit{\textless } $\Delta_{\mathrm{SC}}$ ($\sim$ 13 meV) at $p$ $\sim$ 0.23. The CO wave-vectors Q$_{\mathrm{CDW}}$ and Q$_{\mathrm{PDW}}$ along the Cu-O bond are determined from Fourier transformation of the STS as a function of $p$, and are found to occur at 1/3 and 2/3 of the reciprocal lattice constant (2$\pi $/$a)$ for $p \quad =$ 0.16. The pairing symmetry also evolves from pure $d_{x^2-y^2}$ to ($d_{x^2-y^2}+s)$ for $p$ \textgreater 0.16, where the $s$-wave component increases with $p$. Moreover, under a finite magnetic field the ratio of the vortex ``halo'' radius ($\xi _{\mathrm{halo}})$ relative to the SC coherence length ($\xi _{\mathrm{SC}})$ decreases with $p$, from $\sim$ 8 for $p \quad =$ 0.16 to $\sim$ 3 for $p \quad =$ 0.216, suggesting PG contributions to the vortex halo. Magnetic resonance mode at $\Omega_{\mathrm{r}}$ $\sim$ 2$\Delta_{\mathrm{SC}}$ is also observed as a function of $p$. Finally, we present comparative STS studies of Fe-based superconductors, including Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ and Rb$_{0.8}$Fe$_{1.6}$Se$_{2}$. This work was supported by NSF. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W11.00004: Interpretation of scanning tunneling quasiparticle interference and impurity states Andreas Kreisel, P. Choubey, T. Berlijn, B.M. Andersen, P.J. Hirschfeld We use a simple method of calculating inhomogeneous, atomic-scale phenomena in superconductors to obtain real-space conductance maps as measured in scanning tunneling spectroscopy (STM). Our approach makes use of first principles Wannier functions in conjunction with self-consistent solutions of the Bogoliubov-de Gennes equations on a lattice to image superconducting phenomena[1]. This method is a powerful tool since it captures correctly local symmetries on the surface that can be lower than the global lattice symmetry; it improves the spatial resolution from one pixel per lattice point to the sub-atomic scale; and simplifies the interpretation of STM data. As an example, we show how the pattern observed around a Zn impurity in BSCCO-2212, can be understood by accounting for the tails of the Cu Wannier functions[2], and thus compare perfectly to experimental findings. Further applications of this method include the investigation of impurity states in multiorbital systems as well as the study of quasi particle interference phenomena to enable a better understanding of novel phenomena in high temperature superconductors. [1] Choubey, \textit{et al.}, Phys. Rev. B \textbf{90}, 134520 (2014) [2] Kreisel, \textit{et al.}, arxiv.org:1407.1846 (2014) [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W11.00005: Relaxation Oscillations in Josephson STM Junctions at mK Temperatures Michael Dreyer, Anita Roychowdhurry, Rami Dana, WanTing Li, Shu-Chu Liao Small Josephson junctions can exhibit relaxation oscillations [1] between the superconducting and normal state. The switching time depends on the charging time of the junction capacitance and the R-L time constant of the electrical connections, usually in the $\mu$s range. We observed similar oscillations in the tunnel current between a Nb sample and a Nb tip in our STM operated at 30 mK. The oscillations occur in two forms, either of which is triggered by lowering the gap resistance. The first type occurs in voltage ranges where the I(V) curves show negative differential conductance, which in turn is caused by coupling to the electrical environment [4]. The oscillations span only a fraction of the superconducting gap and run at maximum frequencies below 10 kHz. The possible existence of ``minor'' loops was already mentioned in the original article [1], though thought to be a result of an applied in plane magnetic field. The second type appears at lower gap resistances and affects the whole bias range. The frequency was too high to be determined by our current setup, and thus could be due to a conventional relaxation oscillation.Our results will be discussed in detail. \\[4pt] [1] F. L. Vernon Jr. and R. J. Pedersen, J. Appl. Phys., \textbf{39} (6), 2661 (1968). [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W11.00006: Improved heat exhaust and the characteristics of the high $T_{\mathrm{c}}$ superconducting terahertz emitter T. Kashiwagi, T. Yamamoto, T. Kitamura, K. Asanuma, T. Yasui, Y. Shibano, C. Watanabe, K. Nakade, Y. Saiwai, H. Kubo, K. Sakamoto, T. Katsuragawa, M. Tsujimoto, R. Yoshizaki, H. Minami, R.A. Klemm, K. Kadowaki In our previous study it is known that THz emitting efficiency improves greatly when the stand-alone type of mesa structure is used for the THz emitting device.\footnote{K. Kadowaki \textit{et al.,} Physica C \textbf{491}, (2013) 2} The principle reason for that lies in the heat removal from the mesa, in which a gigantic amount of heat is generated while the mesa is in the resistive state. Recently, we developed a new device structure based on the stand-alone type of mesa structure of Bi2212 single crystal in order to make high exhaust of Joule heating. The results show that although the power is comparable and is not significantly increased, very wide the radiation frequencies ranging from 0.3 to 1.6 THz were obtained. We will discuss the details of the radiation characteristics of this one. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W11.00007: Comparison of luminescent and scanning laser thermal micro-imaging of self-heating in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesa THz sources Timothy Benseman, Alexei Koshelev, Vitalii Vlasko-Vlasov, Yang Hao, Wai-Kwong Kwok, Ulrich Welp, Courtney Keiser, Boris Gross, Matthias Lange, Dieter Koelle, Reinhold Kleiner, Kazuo Kadowaki Scanning laser thermal microscopy of stacked Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ Josephson junction terahertz sources has revealed both electromagnetic cavity resonance modes and strongly non-uniform self-heating in these devices. However, this technique -- in which a modulated laser beam is rastered across the surface of a device -- excites a number of physical phenomena, and thus the resulting images can be difficult to interpret. Here we compare scanning laser images taken on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesa THz sources with micro-images collected via a thermoluminescent technique under identical conditions. The latter technique directly measures the device surface temperature, and we find excellent agreement with the scanning laser results, confirming that scanning laser thermal microscopy is indeed primarily probing device temperature. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W11.00008: The stability of current filaments in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ observed via luminescent thermal microscopy Yang Hao, Timothy Benseman, Alexei Koshelev, Vitalii Vlasko-Vlasov, Wai-Kwong Kwok, Ulrich Welp, Courtney Keiser, Boris Gross, Matthias Lange, Dieter Koelle, Reinhold Kleiner, Kazuo Kadowaki Stacks of Intrinsic Josephson Junctions (IJJs) in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ (Bi-2212) designed as emitters of THz-radiation are prone to strong self-heating and thermal instability due to the poor thermal conductivity and semiconducting resistivity along the c-axis. Recent theory and experimental evidence indicate a possible correlation between strong self-heating and THz power emission. Here we study the temperature distribution in stacks of IJJs using current-voltage (I-V) characteristics and direct thermal imaging. At low bias currents and at low temperature, we observe the nucleation of small hot-spots near the corners or edges of the sample. These hot-spots carry 20-30{\%} of the entire bias current thus forming current filaments. With increasing current and at elevated temperatures the size of the hot-spot increases and it moves to the center of the sample. These observations are in excellent agreement with theoretical analysis regarding the stability of current filaments. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W11.00009: A role of temperature inhomogeneity and hot-spot formation on the THz emission from high-$T_{c}$ superconducting intrinsic Josephson junction mesa devices Chiharu Watanabe, Hidetoshi Minami, Takeo Kitamura, Kentaro Asanuma, Kurama Nakade, Takaki Yasui, Yoshihiko Saiwai, Yuki Shibano, Takashi Yamamoto, Takanari Kashiwagi, Richard Klemm, Kazuo Kadowaki It is well known that the mesa device fabricated from single crystalline Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi2212) gets heated enormously and forms very inhomogeneous temperature distribution and often ``hot-spot,'' where temperature is well above $T_{c}$ when it is current biased and turns to the resistive state. One very surprising phenomena, there is that the mesa can still emit THz radiation even in such a hectic thermal condition in the mesa if two conditions are satisfied: ac-Josephson effect and the cavity resonance condition. In order to understand such a curious behavior we have carried out photoluminescent measurement to investigate temperature inhomogeneous distribution and emission spectra simultaneously while the mesa is generating strong THz emission. Furthermore, we were able to control the hot-spot position by shining the LASER beam focused on the mesa to see the interplay between hot-spot formation, inhomogeneous temperature distribution, and THz emission intensity and the frequency. The results will be shown in detail at the meeting. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W11.00010: Nano-(Bi$_{0.7}$Pb$_{0.3}$)Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10}$ crystals synthesis by sol-gel improved with acrylamide and microwaves E. Chavira, F. Flores, A. Conde, H. Montiel, D. Cabrero, C. Flores, O. Novelo, A. Tejeda, G. Zavala, L. Almeida, G.E. Torres We obtain (Bi$_{0.7}$Pb$_{0.3}$)Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10}$ nano-crystals by sol-gel improved with acrylamide and microwaves, not reported in the literature. TGA gives an idea of the reaction temperatures (200-550 $^\circ$ C) for the formation of binary, ternary and unknown materials. SEM and TEM shows morphology and crystal size 30-33 nm. We studied the thermodynamic and kinetic stability of the gel quenching, by varying the temperature and time according to a previous thermal analysis. Starting compounds (bismuth oxide, strontium carbonate, copper acetate, lead nitrate and calcium sulfate) were analyzed by XRD. By AFM we observed the dehydrated gel surface absorbed water from the environment. From the micrographs we measured the size of the fibers, grains and nano-crystals. We found at 560 $^\circ$ C Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ compound with tetragonal crystal structure, corresponding to the 2:2:2:3 compound, with T$_{c}$ 110 K. At 860 $^\circ$ C seen a shift of some reflections corresponding to two phases. Xerogel magnetic measurement shows antiferromagnetic behavior at 63 K. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W11.00011: A Study on Nucleation, Crystallization Kinetics, Microstructure and Mechanical Properties of Ru-Bi Partial Substituted BSCCO Glass Ceramics Ahmet Tolga Tasci, Ozgur Ozturk, Tugba Gokcen, Sukru Cavdar, Haluk Koralay, Abdulkadir Senol This study deals with, the effects of Ru-Bi partial substitutions on the thermal, structural and mechanical properties of Bi$_{\mathrm{1.8-x}}$Ru$_{\mathrm{x}}$Pb$_{0.2}$Sr$_{2}$CaCu$_{2}$O$_{10+\delta}$ (x$=$0.0, 0.025, 0.050, 0.075), produced with glass-ceramics method have been investigated. The effects of Ru-Bi Partial substitutions on glass transition, nucleation and crystallization temperature are analyzed by differential thermal analyzer (DTA). Furthermore, micro-structure and micro-mechanical properties of Ru-Bi partial substituted BSCCO glass ceramics have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Vickers microhardness measurements. From the DTA results, nucleation kinetics have been obtained by using Ozawa, Augis-Bennett, Takher and Kissinger equations. Also activation energies and Avrami parameters have been found. Oxidation amount is seen to be increased with increasing Ru concentration in consequence of thermogravimetric analyses results. Moreover, Lattice parameters, volume fractions and surface morphologies of the samples are obtained from XRD and SEM measurements, respectively. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W11.00012: Oxygen Annealing in the Synthesis of the Electron-Doped Cuprates J. S. Higgins, P. L. Bach, W. Yu, B. D. Weaver, R. L. Greene Post-synthesis oxygen reduction (annealing) in the electron-doped, high-temperature superconducting cuprates is necessary for the establishment of superconductivity. It is not established what effect this reduction has microscopically on the lattice structure. Several mechanisms have been put forth as explanations; they range from disorder minimization$^{1}$, antiferromagnetic suppression$^{2}$, and copper migration$^{3}$. Here we present an electronic transport study on electron-doped cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4\pm \delta }$ (PCCO) thin films in an attempt to better understand the need for this post-synthesis process. Several different cerium doping concentrations of PCCO were grown. Within each doping, a series of films were grown with varying levels of oxygen concentration. As a measure of disorder on the properties of PCCO, several films were irradiated with various doses of 2 MeV protons. Analysis within each series, and among the different dopings, favors disorder minimization through the removal of apical oxygen as the explanation for the necessary post-synthesis annealing process. $^{1}$P. K. Mang, \textit{et al}., Physical Review Letters, \textbf{93 }(2):027002, 2004. $^{2}$P. Richard, \textit{et al}., Physical Review \textbf{B}, 70 (6), 064513, 2004. $^{3}$Hye Jung Kang, \textit{et al}., Nature Materials, 2007. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W11.00013: Fabrication of Small Edge Josephson Junctions Between Sr$_2$RuO$_4$ and Al Brian Zakrzewski, Xinxin Cai, Yiqun Ying, David Fobes, Tijiang Liu, Zhiqiang Mao, Ying Liu Sr$_2$RuO$_4$ is predicted to have a chiral p-wave orbital pairing. However, attempts to measure the chiral edge currents have yielded results inconsistent with theoretical predictions. Josephson junctions between Sr$_2$RuO$_4$ and an s-wave superconductor such as Al may provide an avenue for directly measuring the edge currents. We report progress on fabricating these junctions, using Al electrodes with no oxide barrier. The Josephson junctions are placed on the naturally formed edges of cleaved Sr$_2$RuO$_4$ thin crystal, which is expected to feature a surface less disordered than ramped junctions prepared by focused ion beam and ion mills. Transmission electron microscope studies provide a powerful tool to characterize the interface. We have systematically investigated the effects of nanofabrication processes on the quality of the junction interface. In particular, several post-lithography processes appear to cause irreversible damage to the surface layer of Sr2RuO4, which highlights potential issues for general small scale device fabrication. We also report preliminary measurements of Josephson tunneling from these devices. [Preview Abstract] |
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