Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q23: Focus Session: Iron Based Superconductors -- Fermi Topology |
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Sponsoring Units: DMP DCOMP Chair: Mark Lumsden, Oak Ridge National Laboratory Room: D165 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q23.00001: Ab initio study of de Haas van Alphen effect in BaRh$_{2}$P$_{2}$ and BaIr$_{2}$P$_{2}$ Simon Blackburn, Michel Cote, Bobby Prevost, Andrea Bianchi, Marek Bartkowiak, Beate Bergk, Oleg Ignatchik, Jochen Wosnitza, Gabriel Seyfarth, Cigdem Capan, Zachary Fisk The de Haas-van Alphen (dHvA) effect is a powerful probe of the Fermi surface (FS) of a metal. Since it measures the area of a cross-section of the Fermi surface, a theoretical description of this surface complements well these experiments. However, a very accurate description of the FS is required from the ab initio calculations in order to calculate the dHvA frequencies. This is achieved using maximally localized Wannier functions (MLWF) (Marzari {\&} Vanderbilt, \textit{Phys. Rev. B}, 56, 12847)to interpolate the Hamiltonian on a dense k-point grid. In this work, we present a dHvA study of BaRh$_{2}$P$_{2}$ and of its isovalent material BaIr$_{2}$P$_{2}$, both structurally analog to the iron pnictide BaFe$_{2}$As$_{2}$. We also present results concerning LaFe$_{2}$P$_{2}$ and CeFe$_{2}$P$_{2}$ which are also related to BaFe$_{2}$As$_{2}$ by a rigid electronic band shift to account for the difference in the number of electrons. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q23.00002: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q23.00003: Transport properties of 3D extended s-wave states appropriate for iron-based superconductors Vivek Mishra, Siegfried Graser, Peter Hirschfeld The Fermi surfaces of Fe-pnictide superconductors are fairly two-dimensional (2D), and it has thus come as a surprise that recent penetration depth and thermal conductivity measurements on some systems have reported $c$-axis transport at low temperatures in the superconducting state comparable to or even larger than that in the $ab$ plane. These results should provide important information on both the Fermi surface and the superconducting state. Here we consider the theory of the superfluid density and thermal conductivity in models of extended-$s$ wave superconducting states expected to be appropriate for Fe-pnictide systems. We include both intra- and interband disorder and consider a range of different Fermi surfaces where gap nodes might exist. We show that qualitative fits can be obtained to match recent experiments on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, and discuss their implications. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q23.00004: Fermi Surface of the Pnictide Superconductor LaRu$_2$P$_2$ studied by quantum oscillations Philip Moll, Fedor Balakirev, Ross McDonald, Janusz Karpinski, Zbigniew Bukowski, Peter Blaha, Karlheinz Schwarz, Bertram Batlogg LaRu$_2$P$_2$ is a stochiometric pnictide superconductor (T$_c$ $\sim$ 4.1K) and crystallizes in the ThCr$_2$Si$_2$ structure (the ``122'' pnictide family). We have mapped out its Fermi surface via the deHaas-vanAlphen effect in pulsed magnetic fields up to 60T (LANL/NHMFL). Pronounced oscillations were observed in the magnetic torque measured with a microcantilever setup. Two features are particularly noteworthy: The oscillations can be followed to surprisingly high temperatures beyond 20K, and the main frequency component at $\theta$ = 20$^{\circ}$ ($\theta$ = 0$^{\circ}$ at HIIc) is at 349T ($\alpha$-peak), significantly lower than in the related compounds LaFe$_2$P$_2$. A second frequency originating from a larger Fermi surface cross-section at 1921 T ($\beta$-peak) is identified. The temperature dependence of the amplitudes is well described by the Lifshitz- Kosevich formalism and gives low effective masses m*/m = 0.80 ($\alpha$ sheet) and 1.09 ($\beta$ sheet). Therefore, most ``122'' metals appear to have similarly low effective masses. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q23.00005: Campbell penetration depth in stoichiometric LiFeAs - evidence for static fishtail effect Plengchart Prommapan, Hyunsoo Kim, Makariy A. Tanatar, Ruslan Prozorov, Bumsung Lee, Seunghyun Khim, Kee Hoon Kim The ``fishtail'' or second magnetization peak is one of the most intriguing properties of high$-T_c$ cuprate superconductors. Now it has also been observed in iron-based materials and has been associated with weak collective pinning. To understand whether the fishtail effect has dynamic (due to field-dependent magnetic relaxation) or static behavior (due to actual non-monotonic field dependence of the true critical current) one needs to measure the clean system, which are rare in pnictide superconductors. A stoichiometric LiFeAs is one of the cleanest of the pnictides with RRR=65. We measured the Campbell penetration depth using a 10 MHz tunnel-diode resonator in DC magnetic fields of up to 9 T. As opposed to the ``apparent'' current density, estimated from the magnetization relaxed over tens of seconds, the Campbell penetration depth depends on the curvature of the pinning potential sampled at time intervals of 0.1 $\mu$sec, thus allowing one to estimate the unrelaxed, ``true'' $j_c(T,B)$. The obtained $j_c(T,B)$ shows a non-monotonic trend with a second peak shifting toward lower fields at higher temperatures implying a static origin of the fishtail effect in LiFeAs. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q23.00006: Measuring the Absolute Penetration Depth in Superconducting Materials. Nicholai Salovich, Russell Giannetta, Ruslan Prozorov, Paul Canfield, Sergey Bud'ko The absolute penetration depths of a variety of superconductors were measured using a method involving a high precision tunnel diode oscillator and an Aluminum film coating method [1]. Variations of the Al film geometry (thickness, coverage area, etc) and microstructure (grain size, RRR, etc) were used to test the reliability and versatility of the coating technique. A variety of supplemental techniques (dual beam SEM, EBS, AFM, XRD, etc) were used to independently characterize the films and control their quality. Special emphasis was placed on measurements of cobalt doped iron pnictide samples given the well established quality of such samples now available. Work at UIUC supported by NSF DMR 10-05708, and Center for Emergent Superconductivity USDOE Award No. DE-AC02-98CH10886. Work at the Ames Laboratory was supported by the division of Materials Science and Engineering, Basic Energy Sciences, Department of Energy (US DOE), under Contract No. DEAC02-07Ch11358. \\[4pt] [1] R. Prozorov, et al, Appl. Phys. Lett 77,1202 (2000) [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q23.00007: dHvA studies of the Fermi topology of Iron-based Superconductors and Metals Invited Speaker: Quantum oscillations studies on various non-magnetic iron pnictides reveal a Fermi surface in broad agreement with the details of the band structure calculations and moderate enhancement of the electronic correlations [1,2,3,4]. Whether or not the nesting of the electron and hole bands are essential for explaining the superconducting behaviour in these materials is still under debate but it is becoming clear that structural alteration have a significant effect in determining their electronic properties. In this talk I will present quantum oscillations studies in materials in which the Fermi surface suffers major topological changes. I will discuss the effect of isoelectronic substitution and doping on the Fermi surface and the quasiparticle masses and their relevance for understanding the complex physics of these materials. This work is in collaboration with groups at Bristol University, Stanford University and Kyoto University [1,2,3,4] and experiments were performed at high magnetic field facilities in Tallahassee, Nijmegen and Toulouse. \\[4pt] [1] A. I. Coldea et al., Phys. Rev. Lett. 101, 216402 (2008); \\[0pt] [2] A. I. Coldea, et al., Phys. Rev. Lett. 103, 026404 (2009). \\[0pt] [3] J. G. Analytis, et al., Phys. Rev. Lett.103 076401 (2009). \\[0pt] [4] H. Shishido, et al., Phys. Rev. Lett.104, 057008 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q23.00008: London penetration depth measurements in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ single crystals Ryan Gordon, Makariy Tanatar, Alexander Thaler, Ruslan Prozorov The variation of the in-plane London penetration depth with temperature, $\Delta\lambda_{ab}(T)$, has been measured in Ba (Fe$_{1-x}$Ru$_x$)$_2$As$_2$ single crystals by using a tunnel diode resonator (TDR). The crystals were grown out of self- flux, with superconductivity stabilized for $x>0.2$ and the maximum $T_c\approx16$ K corresponding to $x\approx0.29$. The substitution of Ru for Fe in this compound is particularly interesting because it is isovalent, so that no additional charge carriers are added by the Ru. The low-temperature penetration depth has been found to exhibit a power law dependence, $\Delta\lambda_{ab}(T)\propto T^n$, with an $x$- dependent exponent, $n$. Using the penetration depth data, the superfluid density has been constructed and compared to known theoretical models for different superconducting pairing symmetries and impurity scattering limits. These results will be compared to previous measurements on different iron-based superconductors. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q23.00009: Superfluid Density Measurements of Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ Films near Optimal Doping Jie Yong, Sanghan Lee, J. Jiang, C.W. Park, J.D. Weiss, E.E. Hellstorm, D.C. Larbalestier, C.B. Eom, T.R. Lemberger We report the first direct, low-frequency measurements of superfluid density, n$_{s}$(T) $\propto \quad \lambda ^{-2}$(T), in Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ thin films, near optimal doping. 100 nm thick films are fabricated by Pulsed Laser Deposition (PLD) in high vacuum and SrTiO$_{3}$ is used as template to match Ba layer in ba-122. Temperature dependence of superfluid density is measured by our two-coil mutual inductance apparatus down to 1.3K. The magnetic penetration depth, $\lambda $, at T $\approx $ 0 is 350 to 430 nm. The T-dependence of $\lambda ^{-2}$ is well characterized by a small s-wave gap, 2$\Delta $(0)/k$_{B}$T$_{c}$ = 2.2 $\pm $ 0.1. In detail, $\lambda $ has power-law behavior at low T: $\lambda $(T)/$\lambda $(0) -- 1 = 0.60*(T/T$_{c})^{2.5\pm 0.1}$. A tail of superfluid density near T$_{c}$ is the only possible indication of a bigger gap. The small gap, together with power-law behavior at low-T, suggests strong intraband scattering on the larger-gap Fermi surface plus significant interband scattering between large-gap and small-gap Fermi surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q23.00010: Linear magnetoresistance in the underdoped iron pnictide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Jiun-Haw Chu, Hsueh-Hui Kuo, Scott Riggs, James Analytis, Ian Fisher BaFe$_2$As$_2$ suffers an antiferromagnetic transition which has been described in terms of a nodal spin density wave. The material exhibits a striking linear magnetoresistance in the low temperature antiferromagnetic state, possibly related to the unique character of the reconstructed Fermi surface. Here we present data showing the evolution of the magnetoresistance as a function of both composition and temperature for the specific case of Ba(Fe$_{1- x}$Co$_x$)$_2$As$_2$, revealing a correlation with other transport properties, including the in-plane resistivity anisotropy. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q23.00011: Hall effect study of iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) T.J. Liu, J. Hu, B. Qian, Z.Q. Mao Our previous work reveals three composition regions with distinct physical properties in the phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) (Liu \textit{et al.}, Nature Materials \textbf{9}, 719 (2010)). Region I $(0 \leq x \leq 0.09)$ exhibits long range $(\pi, 0)$ antiferromagnetic (AFM) order, while Region II $(0.09 < x < 0.29)$ displays short range $(\pi, 0)$ magnetic correlations and is characterized by a weakly localized electronic state. Only in Region III $(x \geq 0.29)$ do we find evidence of bulk superconductivity. In this talk, we will report Hall effect studies of this system. In the AFM state of Region I, we find that the inverse Hall angle (IHA) exhibits a quadratic temperature dependence, consistent with the Fermi liquid behavior probed by resistivity and specific heat measurements. In the weakly localized state of Region II, however, the IHA shows a linear temperature dependence, implying that the quasiparticle scattering rate in this region changes significantly compared with the AFM phase. We will discuss how quasiparticle scattering is associated with $(\pi, 0)$ magnetic fluctuations. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q23.00012: Upper Critical Field and the Fulde-Ferrel-Larkin-Ovchinnikov Transition in Multiband Superconductors Alex Gurevich The effect of orbital and Zeeman pairbreaking on the upper critical field $H_{c2}$ and the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) transition in clean Fe-based superconductors is addressed using a multiband BCS theory. It is shown that the crystalline anisotropy and the $s^\pm$ pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase the orbitally-limited $H_{c2}(T)$ and facilitate the FFLO transition. Small pockets of the Fermi surface emerging upon doping can trigger the FFLO transition even for moderate values of the Maki parameter in the main bands. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q23.00013: Upper critical field study of a LiFeAs single crystal grown by Sn flux Seunghyun Khim, Bumsung Lee, Jae Wook Kim, Eun Sang Choi, G. R. Stewart, Kee Hoon Kim Temperaturedependence of the upper critical fields $H_{c2}(T)$ was investigated in a LiFeAs single crystal by measuring resistivity at a fixed magnetic field up to 36 T. $H_{c2}^{ab}$(0) and $H_{c2}^{ab}$ (0) values are obtained as 30 and 17.2 T respectively. $H_{c2}^{ab}$(0) is lower than expected one from the orbital limiting field $H_{c2}^{orb}$(0) because of the presence of a moderate Pauli limiting effect; upon fitting $H_{c2}^{ab}(T)$ with the WHH formula, the Maki parameter \textit{$\alpha $} = 0.65. For $H_{c2}^{c }$(T), rather a linearly increasing behavior of $H_{c2}(T)$ could be explained by a two-band model in a dirty limit. The anisotropy $H_{c2}^{ab}$ /$ H_{c2}^{c}$ ($T)$ is $\sim $2.3 near $T_{c }$and decreases with temperature being lowered to reaches $\sim $1.3 at $T$ = 0. We also compare $H_{c2}$ (T) of this 111 system with those of other Fe-based superconductors and conclude that the moderateness of the spin-paramagnetic effect is related with rather a weak slope of $H_{c2}(T)$ near $T_{c}$, which is inversely proportional to the Fermi velocity and mean free path. Thus, the comparison of the slope of $H_{c2}$ supports that LiFeAs is located in a rather clean limit among the Fe-based superconductors. [Preview Abstract] |
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