Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z13: Focus Session: Fe-based Superconductors-Nematicity |
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Sponsoring Units: DMP Chair: Chris Homes, Brookhaven National Laboratory Room: 207 |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z13.00001: Nematic charge fluctuations and electron-phonon coupling in EuFe$_{2}$As$_{2}$ and SrFe$_{2}$As$_{2}$ Wei-Lu Zhang, Verner Thorsmolle, Hsiang-Hsi Kung, Philip Lubik, Alexander Lee, Girsh Blumberg, Pierre Richard, Hong Ding, Athena S. Sefat, Jack Gillett, Suchitra Sebastian We study phononic and electronic Raman scattering in the 122 iron pnictide parent compounds EuFe$_{2}$As$_{2}$ and SrFe$_{2}$As$_{2}$. Using polarized Raman spectra we identify all four Raman active phonon modes through the temperature range of 5-300 K from \textit{ab} plane and \textit{ac} plane. Strong interference of the phonon and the electronic continuum is observed in the A$_{g}$ channel below the tetragonal (D$_{4h}$) to orthorhombic (D$_{2h}$) (T-O) phase transition. Quasi-elastic Raman scattering induced by nematic fluctuations observed in the B$_{2g}$ channel in a wide temperature range above the T-O transition. The temperature dependence of the static Raman susceptibility derived from the Raman response $\chi'(0)= \frac{1}{\pi}P\int^{\infty}_{-\infty}\frac{\chi''(\omega)}{\omega}d\omega$ follows a Curie-Weiss like law A(T-T$_{0}$)$^{-1}$, where T$_{0}$ is about 50K below the T-O transition temperature. WLZ acknowledges ICAM support (NSF-IMI grant DMR-0844115), GB, VT and HHK acknowledge support by NSF DMR-1104884 and by U.S. DOE, BES, Award DE-SC0005463, AS acknowledges the support by US DOE, BES, Materials Sciences and Engineering Division. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z13.00002: Spin excitations in the A-type antiferromagnet CaCo$_{2}$As$_{2}$ R.J. McQueeney, B.G. Ueland, A. Sapkota, Abhishek Pandey, G.S. Tucker, D.L. Abernathy, J.L. Niedziela, A. Kreyssig, D.C. Johnston, A.I. Goldman CaCo$_{2}$As$_{2}$ is an A-type antiferromagnetic (AFM) metal with net ferromagnetic interactions between Co ions within the square-lattice layer and AFM interactions between layers. The material is isostructural to AFe$_{2}$As$_{2}$ (A=Ca, Sr, Ba) based high-temperature superconductors, although the stripe AFM magnetism is manifestly different from the nearly ferromagnetic CaCo$_{2}$As$_{2}$ compound. Surprisingly, AFM stripe spin correlations similar to those found in the iron arsenides are observed in the closely related paramagnetic SrCo$_{2}$As$_{2}$ system. We have studied spin excitations in CaCo$_{2}$As$_{2}$ using inelastic neutron scattering. While we find that low-energy Goldstone modes appear at the A-type AFM wavevector, broad and quasi-one-dimensional spin excitations spanning both A-type and stripe AFM wavevectors persist up to very high energies ($>$ 150 meV) and share many similarities to SrCo$_{2}$As$_{2}$. Combined with the small ordered moment ($<$ 0.5 $\mu_{B}$/Co) of CaCo$_{2}$As$_{2}$, the results suggest that the cobalt arsenides are itinerant magnetic systems where the net magnetic exchange interactions place them close to a quantum critical point separating stripe AFM and ferromagnetic ground states. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z13.00003: Anomalous behaviour of critical fields near a superconducting quantum critical point in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ C. Putzke, A. Carrington, P. Walmsley, L. Malone, J.D. Fletcher, P. See, D. Vignolles, C. Proust, S. Badoux, S. Kasahara, Y. Mazukami, T. Shibauchi, Y. Matsuda BaFe$_2$(As$_{1-x}$P$_x$)$_2$ presents one of the cleanest and clearest systems in which to study the influence of quantum critical fluctuations on high temperature superconductivity. In this material a sharp maximum in the magnetic penetration depth has been found at the quantum critical point (QCP $x=0.3$) where $T_c$ is maximal$^1$. Specific heat and de Haas-van Alphen effect measurements$^2$ show that this peak is driven by a corresponding increase in the quasiparticle effective mass. Based on these previous results a simple one-band theory would suggest that at the QCP we should expect a large increase in $H_{c2}$ and a corresponding dip in $H_{c1}$ . Actual measurements of these critical fields, which we present here, shows quite different behavior which we suggest is caused by an anomalous enhancement in the vortex core energy close to the QCP. \\ $^1$ K.Hashimoto \emph{et.al.}, Science \textbf{336}, 1554 (2012)\\ $^2$ P.Walmsley, C.Putzke \emph{et.al.}, Phys. Rev. Lett. \textbf{110}, 257002 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z13.00004: Electronic nematicity and its relation to magnetism in EuFe$_{2}$As$_{2}$ based superconductors Invited Speaker: Philipp Gegenwart The interplay of electronic, magnetic and structural degrees of freedom leads to the competition of various phases in iron pnictude superconductors. We have recently established the thermoelectric power under uniaxial pressure as sensitive novel probe of electronic nematicity in these materials, which is able to distinguish the influence of anisotropic magnetic fluctuations and orbital polarization [1]. We focus our attention on EuFe$_{2}$As$_{2}$ based systems, where the presence of local 4f moments leads to intriguing behavior: their magnetic ordering below 20 K, mediated by RKKY interaction, sensitively depends on the electronic structure, which can be tuned by chemical or hydrostatic pressure [2,3]. Such tuning results in different magnetic phases which coexist or compete with superconductivity [4]. Application of magnetic field below 20 K, acts differently along the orthorhombic a and b axis before the polarization of 4f moments is reached at 1.5 T. A giant magnetostriction of order 10$^{-3}$ is found, related to field induced de-twinning, which (partially) remains up to 200 K. Application of this effect to the study of the in-plane anisotropy in various properties is demonstrated. Work in collaboration with H.S. Jeevan, Y. Tokiwa, J. Maiwald, N. Bach, C. Stingl, S. Jiang, S. Zapf and M. Dressel.\\[4pt] [1] S. Jiang, H.S. Jeevan, J. Dong, P. Gegenwart, Phys. Rev. Lett. 110, 067001 (2013).\\[0pt] [2] H.S. Jeevan, D. Kasinathan, H. Rosner, P. Gegenwart. Phys. Rev. B 83, 054511 (2011).\\[0pt] [3] Y. Tokiwa, H.-S. H\"{u}bner, O. Beck, H.S. Jeevan, P. Gegenwart, Phys. Rev. B 86, 220505(R) (2012).\\[0pt] [4] S. Zapf, H.S. Jeevan, T. Ivek, F. Pfister, F. Klingert, S. Jiang, D. Wu, P. Gegenwart, R.K. Kremer, M. Dressel, Phys. Rev. Lett. 110, 237002 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z13.00005: The gap structure of BaFe$_2$(As$_{x}$P$_{1-x}$)$_2$ determined from specific heat measurements L. Malone, Y. Mizukami, P. Walmsley, C. Putzke, S. Kasahara, T. Terashima, T. Shibauchi, Y. Matsuda, A. Carrington The structure of the superconducting gap of the pncitide superconductors is an unresolved but crucial issue to understanding their mechanism of superconductivity. While some experiments and theories support a fully gapped s+/s- state, several experiments have revealed evidence for nodes in some families of pnictides. Detailed knowledge of the superconducting gap structure and how it varies as a function of material properties can be useful in helping to decide between microscopic theories. BaFe$_2$(As$_x$P$_{1-x}$)$_2$ is a pnictide family with a nodal gap structure and evidence for quantum critical behavior [1]. We have measured the specific heat of several samples of BaFe$_2$(As$_{x}$P$_{1-x}$)$_2$ in a range of $x$ values. We examine the temperature, field and field angle dependence of the specific heat to deduce the changes in the superconducting gap structure as the material is tuned to the quantum critical point. [1] K. Hashimoto et al, Science, 336, [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z13.00006: Doping Dependence of Resonant Spin Excitations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Raymond Osborn, John-Paul Castellan, Stephan Rosenkranz, Keith Taddei, Jared Allred, Omar Chmaissem, Sevda Avci, Duck-Young Chung, Helmut Claus, Mercouri Kanatzidis, Doug Abernathy, Matthew Stone The first spectroscopic evidence of unconventional s$_\pm$-symmetry in the iron-based superconductors was provided by inelastic neutron scattering on Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ with the observation of a resonant spin excitation at the wavevector, Q, that connected the hole and electron Fermi surfaces, centered at the zone center and zone boundary, respectively. Subsequent measurements as a function of hole doping showed that the resonant excitations split into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes and the resonant enhancement below T$_c$ falls to zero as the magnetic interactions weaken, in good agreement with RPA theory. We have now extended these measurements to investigate the doping dependence of BaFe$_2$(As$_{1-x}$P$_x$)$_2$, whose nominal isovalence has been the subject of debate, from $x=0.2$ to 0.6. We find no evidence of a splitting of the resonance with increasing $x$, but see a monotonic decrease in the resonant enhancement across the superconducting dome falling to zero close to $x=0.6$. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z13.00007: Magnetoresistance Near the Quantum Critical Point of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Ian Hayes, Arkady Shekhter, Ross McDonald, Nicholas Breznay, James Analytis We report on the magnetoresistance of the Iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ up to 65 Tesla. In addition to showing unconventional superconductivity, this compound has a quantum critical point associated with the suppression of the anti-ferromagnetic transition to zero temperature at a x = 0.33. We propose a simple model for the magnetoresistance of a quantum critical system; at the quantum critical point of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ this model captures the data quite well, while significant deviations are observed for samples far from the critical point. I will discuss this analysis in detail, as well as extensions to other systems. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z13.00008: Effect of Electron Irradiation on isovalenly substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$ C.P. Strehlow, Jason Murphy, Makariy Tanatar, Ruslan Prozorov, M. Konczykowski, N. Salovich, R.W. Giannetta, T. Kobayashi, S. Miyasaka, S. Tajima The effects of electron irradiation on the termperature-dependent London penetration depth, $\lambda$(T), have been investigated in annealed optimally doped single crystals of isovalently substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$, using the tunnel diode resonator technique. The low temperature behavior of $\lambda$(T) changes under electron irradiation from almost $T-$linear to practically exponentially saturated behavior, similar to the observations in another isovalently substituted 122 pnictide, BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Furthermore, aluminum - coating technique was used to measure the absolute values of the London penetration depth, $\lambda$(0), which allowed calculations of the superfluid density, $\rho_s$. We conclude that, similar to BaFe$_2$(As$_{1-x}$P$_x$)$_2$, the superconducting properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ are compatible with $s_{\pm}$ pairing with accidental line nodes that are lifted by pair-breaking disorder. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z13.00009: Quantum Phase Transitions Inside the Superconducting Dome of the iron-based Superconductors Saurabh Maiti, Rafael Fernandes, Peter Woelfle, Andrey Chubukov In several iron-based superconductors, a nematic transition from the tetragonal to the orthorhombic phase precedes the onset of long-range magnetic order. As doping increases, both the nematic and magnetic ordered states are suppressed, and the two transitions lines cross separately the superconducting dome. In this talk, we discuss the fate of these two instabilities inside the superconducting dome. Using a microscopic electronic model in which nematicity arises from magnetic fluctuations, we show that both ordered states are able to coexist with superconductivity for a wide range of parameters. As the temperature is lowered below Tc, the two transitions merge, giving rise to a single simultaneous first-order nematic-magnetic transition at T=0. The changes in the magnetic spectrum caused by the coexistence with superconductivity makes this quantum phase transition weakly first-order, allowing strong fluctuations to existence in its vicinity. Our results are consistent with experimental findings in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, which indicate the existence of a single quantum phase transition inside the superconducting dome. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z13.00010: Linear magnetoconductivity in multiband spin-density-wave metals with nonideal nesting Alexei Koshelev In several parent iron-pnictide compounds the resistivity has an extended range of linear magnetic field dependence. We argue that there is a simple and natural explanation of this behavior. Spin density wave transition leads to Fermi-surface reconstruction corresponding to strong modification of the electronic spectrum near the nesting points. It is difficult for quasiparticles to pass through these points during their orbital motion in magnetic field, because they must turn sharply. As the area of the Fermi surface affected by the nesting points increases proportionally to magnetic field, this mechanism leads to the linear magnetoresistance. The crossover between the quadratic and linear regimes takes place at the field scale set by the SDW gap and scattering rate. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z13.00011: $T$-linear scattering rate in optimally doped K- and P-Ba122 iron-pnictides Y.M. Dai, C.C. Homes, R.P.S.M. Lobo, B. Xu, B. Shen, H. Xiao, X.G. Qiu, H.H. Wen The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reveal two groups of carriers with different scattering rates ($1/\tau$), described by two Drude components in the optical conductivity. A ``broad'' Drude component results in an incoherent background with a $T$-independent $1/\tau_b$, while a ``narrow'' Drude component reveals a $T$-linear scattering rate $1/\tau_n$ resulting in a resistivity $\rho_n \equiv 1/\sigma_{1n}(\omega \rightarrow 0)$ also linear in temperature. This fact explains the $T$-linear $\rho$ at low temperatures and the tendency to saturation at room temperature observed by transport measurements in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. The low frequency spectral weight increases with decreasing $T$, following an arctan($T$) dependence, which is also strong evidence for a $T$-linear scattering rate. A comparison to other materials with similar behavior suggests that the $T$-linear $1/\tau_n$ and $\rho_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ may arise out of scattering from spin fluctuations due to the proximity to a quantum critical point (QCP). Similar behaviors are found in the optimally doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_{2}$. [Preview Abstract] |
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