Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q5: Focus Session: Search for New Fe-based Superconductors II |
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Sponsoring Units: DMP DCOMP Chair: Clarina R. Dela Cruz, Oak Ridge National Laboratory Room: Juan Gorman Room 005 |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q5.00001: Itinerant magnetism in metallic CuFe$_2$Ge$_2$ K. V. Shanavas, D. J. Singh Discovery of superconductivity in iron pnictides and chalcogenides has generated interest in the coexistence and interplay of superconductivity and magnetism. Antiferromagnetic spin fluctuations are believed to be mediating superconductivity in these systems. The large spin-fluctuations may arise as a consequence of nearness to a quantum critical point (QCP), which can also lead to non-Fermi liquid behavior, unusual transport and novel ground states. Thus, it is of interest to look for other materials that share similar characteristics. Using density functional theory based calculations we have studied the electronic structure and magnetic properties of CuFe$_2$Ge$_2$ based on it's structural similarities with recently discovered YFe$_2$Ge$_2$. We find large density of states at the Fermi level [$N(E_F)$], consistent with itinerant character. Fermi surfaces in this system have a sheet like structure amenable to nesting and consequently to magnetic instabilities. Our results suggest that CuFe$_2$Ge$_2$ is an antiferromagnetic metal, with similarities to the Fe-based superconductors; such as magnetism with substantial itinerant character and coupling between magnetic order and electrons at the Fermi energy. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q5.00002: Universal V-shaped phase diagram in the iron-based superconductors XFe$_2$As$_2$ (X = K, Rb, Cs) Alexandre Ouellet, Fazel Fallah Tafti, Alexandre Juneau-Fecteau, Samuel Faucher, Maxime Lapointe-Major, Nicolas Doiron-Leyraud, Louis Taillefer, Aifeng Wang, Xigang Luo, Xianhui Chen Following the discovery of a sharp reversal in the dependence of T$_{c}$ on pressure in KFe$_{2}$As$_{2}$ [1] and CsFe$_{2}$As$_{2}$ [2], we report a similar behavior in RbFe$_{2}$As$_{2}$. The application of hydrostatic pressure initially decreases T$_{c}$ until a critical pressure P$_{c}$ = 11 kbar where T$_{c}$ suddenly starts to increase. For the three materials, we find a universal V-shaped temperature-pressure phase diagram, with identical slopes $\frac{dT_{c}}{dP}$ below and above P$_{c}$. Upon crossing P$_{c}$, the upper critical field H$_{c2}$ is observed to jump, by a similar factor for KFe$_{2}$As$_{2}$ and RbFe$_{2}$As$_{2}$. We interpret these universal features in terms of a pressure-induced change in the pairing state of these superconductors. [1] F. F. Tafti et al., Nature Physics 9, 349 (2013). [2] F. F. Tafti et al., Physical Review B 89, 134502 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q5.00003: Field dependence of thermal conductivity in \textit{X}Fe$_2$As$_2$ (\textit{X} = K, Rb, Cs) Patrick Bourgeois-Hope, F.F. Tafti, B. Vincent, N. Doiron-Leyraud, L. Taillefer, A.F. Wang, X.-G. Luo, X.H. Chen There is ongoing debate over the pairing symmetry in the hole-overdoped iron-based superconductor KFe$_2$As$_2$. While thermal conductivity [1] and penetration depth [2] data have been taken as evidence of a d-wave pairing state, heat capacity [3] and ARPES [4] have been interpreted within an s-wave state with accidental nodes on some parts of the Fermi surface. Here we report a complete study of the magnetic field dependence of thermal conductivity in the T = 0 limit for the isostructural materials XFe$_2$As$_2$ with X = K, Rb and Cs. Extending our previous study of KFe$_2$As$_2$ to RbFe$_2$As$_2$ and CsFe$_2$As$_2$ reveals a universal behaviour, implying that all three materials must have a very similar nodal structure. All data are found to be in excellent agreement with calculations for a d-wave superconductor. A similar nodal quasiparticle behaviour across different materials is natural within a d-wave state, a common nodal structure being automatically imposed by symmetry. By contrast, such similarity would be highly coincidental if nodes are accidental, as in an s-wave state. \\[4pt] [1] J.-Ph. Reid et al., PRL 109, 087001 (2012).\\[0pt] [2] H. Kim et al., PRB 89, 174519 (2014).\\[0pt] [3] F. Hardy et al., JPSJ 83, 014711 (2014).\\[0pt] [4] K. Okazaki et al., Science 337, 1314 (2012). [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:42PM |
Q5.00004: Novel Materials {\&} Multi-scale Analysis of the Superconducting State in Iron Based Superconductors Invited Speaker: Athena S. Sefat The understanding of the fundamental nature of a material's superconducting state is of crucial importance, if superconductors are to fulfill their promise for widespread use in energy-related needs. Our research applies multi-scale characterization techniques to study and probe the nuclear, electronic, and magnetic details of single crystals. The importance of such broad investigative work is demonstrated in our recent publication on praseodymium-doped BaFe$_{2}$As$_{2}$ for which non-uniform local distortions through isolated Pr atoms do not provide percolation path superconductivity [1]. For CaFe$_{2}$As$_{2}$, it is found that large Fermi-surface reconstruction in the non-magnetic phase causes a non-superconducting ground state [2], while different crystalline domains with varying lattice parameters are identified [3]. For Cu-doped BaFe$_{2}$As$_{2}$ it is found that orthorhombic distortion below T$_{\mathrm{s}}$ leads to magnetically ordered state of FeAs planes, hence no superconductivity [4]. Studies of this nature can yield groundbreaking results by demonstrating that many parameters can compete in a bulk material and even be spatially and electronically non-homogenous on nanometers. \\[4pt] [1] Phys. Rev. Lett. 112 (2014), 047005;\\[0pt] [2] Phys. Rev. Lett. 112 (2014), 186401;\\[0pt] [3] Sci. Reports 4 (2014), 4120;\\[0pt] [4] Phys. Rev. Lett. 113 (2014), 117001. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q5.00005: Enhancement of transition temperature in iron based superconductor KFe$_2$As$_2$ under pressure Yasuyuki Nakajima, Renxiong Wang, Tristin Metz, Xiangfeng Wang, Jason Jeffries, Johnpierre Paglione Superconducting pairing symmetry in the iron pnictides is one of the key issues to clarify the origin of high T$_c$ superconductivity. The versatile pairing symmetry, including sign-reversed full gap, symmetry-imposed, or accidental nodal states, has been proposed theoretically and experimentally [1,2], and it can undergo a transition from one to another by chemical substitution or pressure. For instance, recent pressure study on heavily hole-doped KFe$_2$As$_2$ may imply a possible symmetry change accompanied by sudden reversal in pressure dependence of T$_c$ [3]. To explore the phase diagram of KFe$_2$As$_2$ in a wider pressure range, we here report low-temperature transport study under high pressure up to 33 GPa by utilizing a designer diamond anvil cell. We will discuss the evolution of the superconducting and structural properties of this material, highlighting novel changes in the system at high pressure. [1] J.P. Reid et al., PRL 109, 087001 (2012). [2] T. Shibauchi et al., Annu. Rev. Condens. Matter Phys. 5, 113 (2014). [3] F. Tafti et al., Nat. Phys. 6, 349 (2013). [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q5.00006: Fermi-Surface Reconstruction in the collapsed-tetragonal phase of KFe2As2 Limin Wang, Yasuyuki Nakajima, Xiangfeng Wang, Jason Jeffries, Johnpierre Paglione Recent experiments reveal that applying pressure to KFe2As2 produces a rich phase diagram that includes the reversal in pressure dependence of Tc and without electronic structure change [1] and evolution of structural properties. Inspired by these, we investigate the electronic structure of KFe2As2 as a function of pressure using first-principles theory. Comparing with the ambient pressure tetragonal phase, we find that the Fermi surface topology undergoes significant changes at high pressures, with possible Lifshitz transition in the Fermi surface at high pressure. Together, our results suggest an interesting scenario of superconductivity in this material. \newline \newline [1] F. Tafti et al., Nat. Phys. 6, 349 (2013). [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q5.00007: Microscopic, Transport and Thermodynamic properties of Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$TM$_{x}$)$_{2}$As$_{2}$)$_{5}$ (TM=Co, Ni) single crystals Shan Jiang, Ni NI Here we report detailed microscopic, transport and thermodynamic measurements on two series of high quality single crystals Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$TM$_{x}$)$_{2}$As$_{2}$)$_{5}$ (TM=Co, Ni). With electron doping on Fe sites, the structural/magnetic phase transitions in the parent compound were suppressed at a rate of roughly -7K per 0.01Co doping and -9K per 0.01Ni doping. Superconductivity emerges in the region of $0.048<\mathrm{x}<0.20$ for Co doping with optimal Tc of 13.5K ($\mathrm{x}=0.11$) while it occurs in the region of $0.035<\mathrm{x}<0.11$ for Ni doping with optimal Tc of 9.6K ($\mathrm{x}=0.064$). No coexistence of AFM and SC is observed, which is different from the well-studied 122 Fe-pnictides but reminiscent the one of La1111. The comparison of the effect between Co- and Ni- doping on 10-3-8 shows that rigid band approximation is likely working for these two dopants in this superconducting family. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q5.00008: Vortex Lattice Anisotropy and Pauli Limiting in KFe$_2$As$_2$ S.J. Kuhn, M.R. Eskildsen, H. Kawano-Furukawa, M. Ono, E.M. Forgan, E. Jellyman, R. Riyat, C.H. Lee, K. Kihou, J. Gavilano In superconducting KFe$_2$As$_2$ (KFA), $H_{c2}$ (in Tesla) parallel to the basal plane is much larger than $T_c$ (in Kelvin) suggesting Pauli limiting. We have used small-angle neutron scattering (SANS) with $H$ applied close to the basal plane to study the vortex lattice (VL) anisotropy and scattered intensity in KFA. The VL anisotropy reflects the intrinsic anisotropy of the superconducting state ($\Gamma_{ac}$), and may differ significantly from the $H_{c2}$ anisotropy ($\Gamma_{H_{c2}}$) as recently seen in Sr$_2$RuO$_4$ [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 087003 (2013)]. Our data indicates a field dependent VL anisotropy in KFA, increasing with $H$, possibly caused by multi-band effects. At high fields $\Gamma_{ac} > \Gamma_{H_{c2}}$, supporting Pauli limiting. Due to the moderately high anisotropy, both the longitudinal and transverse VL field modulation may contribute to the SANS signal, occurring as non-spin flip and spin flip scattering respectively. This allow an independent determination of possible Pauli paramagnetic effects (spin polarization of un-paired quasiparticles in the vortex cores) along the two different directions. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q5.00009: Influence of doping on the physical properties of Ca$_{10-x}$RE$_x$Pt$_3$As$_8$(Fe$_{2-y}$Pt$_y$As$_2$)$_5$ Jiayun Pan, Amar Karki, Rongying Jin Ca$_{10-x}$RE$_x$Pt$_3$As$_8$(Fe$_{2-y}$Pt$_y$As$_2$)$_5$ is a new FeAs-based superconductor. We report the change of its superconducting transition temperature $T_c$ and physical properties upon chemical doping in either Ca (using La or Gd) or Fe (using Pt) site. While partial replacement of Fe by Pt results in $T_c$ up to $21 K$, we find that the substitution of Ca by La is most effective pushing $T_c$ to 30 K. The doping in both sites reduces the in-plane resistivity and anisotropy. The doping dependance of electrical transport properties will be presented and discussed. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q5.00010: Field dependence of thermal conductivity in the iron-based superconductor KFe$_{2}$As$_{2}$: Evidence of a $d$-wave state Fazel Fallah Tafti, Alexandre Juneau-Fecteau, Patrick Bourgeois-Hope, Samuel Rene De Cotret, Jean-Philippe Reid, Nicolas Doiron-Leyraud, Louis Taillefer, Aifeng Wang, Xigang Luo, Xianhui Chen Pairing symmetry in the iron-arsenide superconductor KFe$_{2}$As$_{2}$ is the subject of active debate. Thermal conductivity at milliKelvin temperatures is a sensitive and directional probe of the superconducting gap structure. Thermal conductivity measured along both [100] and [001] directions reveal that all Fermi surfaces in KFe$_{2}$As$_{2}$ must have vertical line nodes. This stringent constraint is automatically satisfied by $d$-wave symmetry, but not likely to be satisfied by an $s$-wave state, where nodes are accidental. Here, we report a detailed study of the magnetic field dependence of thermal conductivity in KFe$_{2}$As$_{2}$, measured in the $T =$ 0 limit. The data are found to be in excellent agreement with $d$-wave calculations. Our data are also compatible with low-temperature specific heat data as a function of field, within a multi-band $d$-wave scenario. Using Fermi surface parameters from quantum oscillations, we estimate the thermal conductivity expected of the gap structure extracted from ARPES measurements on KFe$_{2}$As$_{2}$ for the different Fermi surface sheets. We find a result that is incompatible with our thermal conductivity data, and conclude that the superconducting state at the surface, accessed by ARPES, must be different from the state in the bulk, accessed by transport and thermodynamic measurements. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q5.00011: Huge enhancement of superconductivity in the collapsed tetragonal KFe$_{2}$As$_{2}$ Jianjun Ying, Ling-yun Tang, Ho-Kwang Mao, Viktor Struzhkin, Ai-Feng Wang, Xian-Hui Chen, Xiao-Jia Chen Recent work (F. F. Tafti, \textit{et al. Nature Phys.}\textbf{9}, 349 (2013)) on hole-overdoped iron pnictide KFe$_{2}$As$_{2}$ indicated a paring symmetry change at pressure of around 1.7 GPa. The investigation for the low-pressure region (below 7 GPa) revealed oscillation of T$_{\mathrm{c}}$ with pressure. Here we report results of high-pressure transport and XRD measurements on KFe$_{2}$As$_{2}$ single crystals at high pressures up to 30 GPa. We map out the phase diagram of KFe$_{2}$As$_{2}$ and find a huge enhancement of $T_{\mathrm{c}}$ in the collapsed tetragonal phase. The correlation between $T_{\mathrm{c}}$, electronic and crystal structures is discussed. The strong electronic correlations are proposed to account for such an unexpected $T_{\mathrm{c}}$ enhancement in KFe$_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q5.00012: Superconducting Properties of KFe2Se2 Yoshihiko Takano Layer structured iron selenide, FeSe has the simplest crystal structures among iron-based superconductors. It shows superconductivity with transition temperature (Tc) of ~13 K under ambient pressure. The Tc increases up to 37 K by applying high pressure [1-3]. These facts indicate that the FeSe-layers are favorable structures to show superconductivity. When potassium is doped to the interlayer of FeSe, the resulting compound KFe2Se2 shows superconductivity at around 31 K under ambient pressure. However, the superconducting properties have no consensus even in the ambient pressure condition, because of its reproducibility, inhomogeneity, and instability, and so on. It is necessary to obtain the high-quality single crystals to clarify the intrinsic properties. In this study, we cultivate the preparation method for the single crystalline KFe2Se2 [4], and investigate its superconducting properties by electrical properties using ARPES, magnetic susceptibility and transport properties, and single crystal structural analysis. [1] Y. Mizuguchi et al., Appl. Phys. Lett. 93, 152505 (2008). [2] S. Margadonna et al., Phys. Rev. B 80, 064506 (2009). [3] S. Masaki et al., J. Phys. Soc. Jpn. 78, 063704 (2009). [4] T. Ozaki et al., Euro. Phys. Lett., 98, 27002 (2012). [Preview Abstract] |
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