Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J37: Focus Session: Fe-based Superconductors: Pressure effects |
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Sponsoring Units: DMP DCOMP Chair: Xiao-Jia Chen, Carnegie Institution of Washington Room: 345/346 |
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J37.00001: Evolution of superconductivity in Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ under pressure Invited Speaker: Shanta Saha The evolution of superconductivity in single crystals of the aliovalent La-doped CaFe$_2$As$_2$ is studied with both quasi-hydrostatic and hydrostatic applied pressures measuring transport, magnetic, and neutron scattering properties. The application of pressure to under doped samples of Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Fe$_2$As$_2$ suppresses the antiferromagnetic (AFM) transition and causes an abrupt appearance of superconductivity with $T_{\mathrm{c}}$ values similar to those (about 45 K) recently been reported at ambient pressure. This superconducting phase appears under both quasi-hydrostatic and hydrostatic pressures, indicating an intrinsic property of the observed superconducting state. Unlike transition metal-doped 122 iron-superconductors where superconductivity happily coexists with AFM, the little coexistence of SC and AFM appears to mimic that found in 1111 iron-superconductors, suggesting a similar phase diagram. The unusual dichotomy between lower-$T_{\mathrm{c}}$ systems that happily coexist with AFM and tendency for the highest-$ T_{\mathrm{c}}$ systems to show phase separation provides an important clue to the pairing mechanism in iron-based superconductors. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J37.00002: c-axis transport in rare-earth doped CaFe$_2$As$_2$ Johnpierre Paglione, T. Drye, R. Hu, S.R. Saha The discovery of a low volume fraction phase of superconductivity in rare-earth doped Ca$_{1-x}$R$_x$Fe$_2$As$_2$ with $T_c$= 47 K has sparked controversy over the nature of the observed superconductivity. However, an important aspect to understanding the behavior in these systems lies in understanding the role of the structural collapse wherein interlayer As atoms abruptly form a bond at sufficiently low temperatures, resulting in a $\sim$10\% reduction of the $c$-axis through a first-order transition. We will present measurements of electrical transport with currents directed along the crystallographic $c$-axis, discussing the implications for the superconducting phase and the nature of the band structure change through the structural collapse transition. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J37.00003: Inter-plane resistivity in single crystals Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with doping level variation Makariy Tanatar, S. Ran, S.L. Bud'ko, P.C. Canfield, Ruslan Prozorov CaFe$_2$As$_2$ undergoes sharp first order tetragonal-to-orthorhombic phase transition on cooling below $T_{SM}$=175 K, accompanied by stripe type antiferromagnetic ordering. The transition temperature can be suppressed to zero by application of pressure, revealing collapsed tetragonal high pressure phase, and partial superconductivity. It can also be suppressed by Co substitution of Fe. This doping suppresses structural and magnetic instabilities and induces bulk superconductivity with $T_c$ up to 17~K Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=2.8\% [1]. Here we report systematics of the temperature-dependent inter-plane resistivity in this Co-doped series of compounds over complete doping phase diagram.\\[4pt] [1] S. Ren S.L. Bud'ko, W.E.Straszheim, J. Soh, M.G.Kim, A.Kreyssig, A.I.Goldman and P.C.Canfield, Phys. Rev. B {\bf 85}, 224528 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J37.00004: Thermal expansion of CaFe$_2$As$_2$: effect of annealing and cobalt doping Sergey L. Bud'ko, Sheng Ran, Paul C. Canfield Careful choice of Co concentration and annealing/quenching temperature in the Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ series allows for tuning the ground state of the from orthorhombic-antiferromagnetic to superconducting to collapsed tetragonal [1].In this talk temperature-dependent, $c$-axis, thermal expansion measurements on several sets of Co-doped CaFe$_2$As$_2$ single crystals that were subjected to a variety of annealing conditions will be presented. These samples were chosen to cover all salient regions of the 3D $x - T_{anneal} - T$ phase diagram. The thermal expansion signatures of different types of phase transitions observed in these series will be discussed and comparison with the other measurements will be made.\\[4pt] [1] S. Ran, {\it et al.}, PRB {\bf 85}, 224528 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J37.00005: London penetration depth under pressure in Fe-based superconductors Kyuil Cho, Makariy A. Tanatar, Charles P. Strehlow, Ruslan Prozorov Precisely measured low - temperature London penetration depth can be used as a sensitive tool to study the superconducting gap structure. Tunnel diode resonator (TDR) technique provides the ultimate sensitivity and resolution and it has been employed to study conventional and unconventional superconductors [1]. In case of Fe-based superconductors the dome - like evolution of the superconducting properties, including the gap anisotropy, has been found as a function of doping. While easier to work with, the doping also changes the scattering, especially important in Fe-based superconductors [1]. Pressure provides potentially cleaner alternative way for systematic study of the superconducting gap evolution. However, thus far no successful measurements on London penetration depth in the pressure cell were possible due to various background contributions. We have recently developed the pressure dependent London penetration depth measurement technique by combining a TDR technique with BeCu piston cell. Technical characteristics as well as the first results will be discussed. \\ $\lbrack$1$\rbrack$ R. Prozorov and V. G. Kogan, Rep. Prog. Phys. 74, 124505 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J37.00006: Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering Steven Hahn, Gregory Tucker, Jiaqiang Yan, Ayman Said, Bogdan Leu, R.W. McCallum, Ercan Alp, Thomas Lograsso, Robert McQueeney, Bruce Harmon The phonon dispersion was measured at room temperature (above T$_{\mathrm{N}}$ ) along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well documented in the AFe$_{2}$As$_{2}$-based (A$=$Ca,Sr,Ba,Eu) systems. Only recently have single crystals of RFeAsO (R$=$La,Ce,Pr,Nd,Sm,Gd)-based compounds become available. The experimentally observed splitting between two A$_{\mathrm{1g}}$ phonon modes at 22 and 26 meV is only produced in spin-polarized first-principles calculations imposing various types of antiferromagnetic order. Magnetostructural effects similar to those observed in the AFe$_{2}$As$_{2}$ materials are confirmed present in LaFeAsO. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist above T$_{\mathrm{N}}$ and into the tetragonal phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J37.00007: Ru $L_2$ edge X-ray resonant magnetic scattering from Ba(Fe$_{0.795}$Ru$_{0.205})_2$As$_2$ compound Min Gyu Kim, J. Soh, J.C. Lang, Y. Choi, A. Thaler, E. Bourret-Courchesne, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. Birgeneau We have investigated the magnetic polarization of the Ru 4$d$ dopant states in Ba(Fe$_{0.795}$Ru$_{0.205})_2$As$_2$ using Ru $L_2$ edge x-ray resonant magnetic scattering (XRMS). We observed a XRMS signal at Q $=$ (1/2, -1/2, 3), which is consistent with the magnetic propagation vector for the stripe AFM ordering found in the parent BaFe$_2$As$_2$ compound. We find that the temperature dependence of the XRMS signal follows closely the temperature dependence of the Fe order as determined by our previous neutron diffraction measurement. Our observations show evidence that the Ru 4$d$ states may be spin-polarized. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J37.00008: High Pressure Low Temperature Studies of the Iron-Based Superconductor SrFe$_{2}$As$_{2}$ Gary Chesnut, Walter Uhoya, Jeffrey Montgomery, Antonio dos Santos, Jamie Molaison Iron-based superconductors are a critical clue in understanding the mechanism behind high temperature superconductivity. It is well-known that superconductivity is highly influenced by magnetic fields. Recent neutron scattering experiments were performed on SrFe$_{2}$As$_{2}$ to examine the nuclear and magnetic structure to a temperature of 89 K and a pressure of 4.3 GPa. The structural phase transition from tetragonal to orthorhombic was observed at T$_{\mathrm{o}} =$ 196 K with an increase in orthorhombic distortion with decreasing temperature. The neutron diffraction experiments revealed subtle, but interesting results at elevated pressures. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J37.00009: Pressure induced structural modifications in NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor Elissaios Stavrou, Xiao-Jia Chen, Alexander Goncharov, A. Wang, Y. Yan, X. Luo, X. Chen NaFe$_{1.99}$Co$_{0.01}$As$_2$ superconductor with the tetragonal ThCr$_2$Si$_2$-type structure ($I4/mmm$) has been studied using x-ray diffraction and Raman spectroscopy up to 25 GPa (at RT). Recently, it was found that, for this compound, $T_c$ increases with pressure to a maximum of 32 K at 2.5 GPa. With further compression $T_c$ decreases up to 6 GPa, the highest pressure superconductivity has been detected. We report that, although NaFe$_{1.99}$Co$_{0.01}$As$_2$ remains in the ambient pressure phase, the lattice parameters evolution with pressure shows distinct behavior below and above a critical pressure Pc=2.5 GPa. This is accompanied by a subtle change of Raman spectra at Pc. Below Pc, a-axis increases while both the c-axis and the c/a axial ratio decrease. In contrast above Pc, both axes show a normal decrease and c/a remains almost constant. The different behavior of c-axis, below and above Pc, can be viewed as a modification of the initial tetragonal phase (T) to a collapsed tetragonal (CT) one. This is in line with previous studies on 122 iron-based superconductors. We conclude that the high compressibility of c-axis, in the T phase, enhances superconductivity since layers are brought together. Above Pc, compression of CT phase seems to have the opposite effect. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J37.00010: Magneto-transport properties of single crystal LaFeAsO at ambient and high pressure Colin McElroy, James Hamlin, Benjamin White, M. Brian Maple Electrical resistivity and magneto-transport measurements were performed on single crystals of LaFeAsO, which were grown using a NaAs flux. The magneto-transport measurements were made at ambient pressure in magnetic fields up to 9 T using the van der Pauw technique, which yielded the magnetoresistance and the Hall coefficient, from which the carrier density and mobility were inferred. The dominant charge carriers were identified as electrons, and a second anomaly was observed below the spin-density wave (SDW) transition. In order to study the evolution of these two anomalies with pressure, electrical resistivity measurements were performed under applied pressures up to 36.7 GPa. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J37.00011: Fe moments in the pressure-induced collapsed tetragonal phase of (Ca$_{0.67}$Sr$_{0.33}$)Fe$_{2}$As$_{2}$ Jason Jeffries, Nicholas Butch, Joseph Bradley, Yuming Xiao, Paul Chow, Shanta Saha, Kevin Kirshenbaum, Johnpierre Paglione The tetragonal AEFe$_{2}$As$_{2}$ (AE=alkaline earth element) family of iron-based superconductors exhibits magnetic order at ambient pressure and low temperature. Under pressure, the magnetic order is suppressed, and an isostructural volume collapse is induced due to increased As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase has been shown to support superconductivity under some conditions, and theoretical calculations suggest an unconventional origin. Theoretical calculations also reveal that enhanced As-As bonding and the magnitude of the Fe moments are correlated, suggesting that the Fe moments can be quenched in the collapsed tetragonal phase. Whether the Fe moments persist in the collapsed tetragonal phase has implications for the pairing mechanism of the observed, pressure-induced superconductivity in these compounds. We will present pressure-dependent x-ray emission spectroscopy (XES) measurements that probe the Fe moments through the volume collapse transition of (Ca$_{0.67}$Sr$_{0.33}$)Fe$_{2}$As$_{2}$, and compare these measurements with the occurrence of superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J37.00012: Interplay of superconductivity with the SDW order and Eu$^{2+}$ AFM order in the Ca$_{\mathrm{1-x}}$Eu$_{\mathrm{x}}$Fe$_2$As$_2$ system at ambient and under pressures Keshav Shrestha, Kui Zhao, Ben Jawdat, Liangzi Deng, Xiyu Zhu, Yuyi Xue, Bing Lv, Paul Chu Single crystals of Eu doped Ca$_{\mathrm{1-x}}$Eu$_{\mathrm{x}}$Fe$_2$As$_2$ (0$\le $ x $\le $1) with size up to 5 x 5 mm size were grown from FeAs self-flux technique. Detailed magnetic and resistivity data a systematical evolution of a spin-density-wave (SDW) transition from $\sim$ 170K at x$=$0 to $\sim$ 190K at x $=$1. Moreover, the Eu$^{2+}$ antiferromagnetic (AFM) emerged at 3.7K at a threshold doping x $\sim$ 0.2, and systematically increased up to $\sim$ 20K with the increase of the Eu content. High pressure was applied to some of these compounds to explore the competition among SDW, collapsed phase and superconductivity. Superconductivity up to 18K was observed in samples without the structural ``collapsed tetragonal'' phase before the emergence of the superconductivity signal. The data suggest that the superconductivity in the doped Ca122 under pressure is not associated with the structural collapsed-tetragonal phase. The complex phase diagram of the SDW, Eu$^{2+}$ AFM order and superconductivity at ambient and under pressure will be presented and its implication will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J37.00013: Concentration dependence of magnetic characteristics in EuFe$_2$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_2$ single crystals Takanari Kashiwagi, Takuya Ishikawa, Tomoki Gota, Youhei Jono, Akihiko Nozawa, Kasumi Tashima, Kazuo Kadowaki In order to understand the structural and magnetic characteristics of EuFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$, we have performed electron spin resonance (ESR) and magnetization measurements. The experimental results of the doping dependence of both measurements for the single crystal of EuFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$ samples suggest the change of the exchange interaction and/or the anisotropy of Eu-site. The angular dependence of the ESR signal in the \textit{ab}-plane clearly shows the change of the in-plane anisotropy by the doping of P. The details will be discussed in the meeting. [Preview Abstract] |
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