Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T26: Focus Session: Iron Based Superconductors -- Magnetic Properties & Phase Diagrams |
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Sponsoring Units: DMP DCOMP Chair: Stephen Hayden, Bristol University Room: D162/164 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T26.00001: Magnetic and Structural Phase Diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Sevda Avci, Duck-Young Chung, Stephan Rosenkranz, John-Paul Castellan, Ray Osborn, Omar Chmaissem, Mercouri Kanatzidis, Eugene Goremychkin, Aziz Daoud-Aladine It is well known that the partial substitution of Ba by K in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ causes a steep suppression of both the antiferromagnetic and tetragonal-orthorhombic transitions, leading to the onset of superconductivity over a large substitution range peaking at 38 K for x = 0.4. We report high resolution neutron powder diffraction results, which show that the magnetic and structural transitions are coincident over the entire phase diagram, in contrast to Ba(Fe$_{1-x}$,Co$_{x})_{2}$As$_{2}$. Volume discontinuities show that the combined transitions are first-order. The superconducting phase diagram has been refined with greater precision and a narrow region of phase coexistence have been delineated. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T26.00002: Coexistence of Superconductivity and Magnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$ )$_{2}$ A.A. Aczel, T.J. Williams, T. Goko, F.L. Ning, Y.J. Uemura, C. Arguello, W. Yu, G.F. Chen, G.M. Luke We have performed resistivity, magnetization, and $\mu $SR studies on single crystalline EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$. These measurements provide clear evidence for bulk superconductivity in this system, with the sample resistance dropping to zero around 12 K. This work has also revealed ferromagnetic ordering of the S = 7/2 Eu$^{2+}$ moments along the c-axis (T$_{curie} \quad \sim $ 19 K). Finally, our $\mu $SR results indicate that the Eu magnetism is very homogeneous and occupies the full-volume fraction, pointing to real-space coexistence of magnetism and superconductivity in this material. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T26.00003: Physical and magnetic properties of Ba(Fe$_{1-x}$Mn$_x$)$_2$As$_2$ single crystals Alexander Thaler, Sheng Ran, Alfred Kracher, Warren Straszheim, Jiaqiang Yan, Sergey Bud'ko, Paul Canfield Single crystals of Ba(Fe$_{1-x}$Mn$_x$)$_2$As$_2$, $0 |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T26.00004: The magnetic form factor of SrFe2As2 William Ratcliff, P.A. Kienzle, J.W. Lynn, S. Li, P. Dai, G.F. Chen, N.L. Wang The Fe-pnictide based superconductors have recently been the subject of great interest. In this talk, we discuss recent neutron diffraction measurements of the magnetic form factor of SrFe2As2. These measurements reveal that while the form factor is primarily isotropic, a maximum entropy reconstruction reveals that there is evidence of hybridization between the Fe and As orbitals. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T26.00005: Temperature-pressure-composition phase diagram for Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ (x$\le $0.285 and P$\le $84kbar) S.K. Kim, M.S. Torikachvili, A. Thaler, E.C. Colombier, S.L. Bud'ko, P.C. Canfield BaFe$_{2}$As$_{2}$ shows a structural/magnetic (SM) phase transition near T$_{SM}$=134K, which is suppressed upon partial substitutions at the Ba (K), Fe (Cr, Mn, Co, Ni, Cu, Ru, Rh, Pd, Ir) or As (P) sites, and also by adding pressure (P), most times leading to the emergence of superconductivity (SC) at low T. Here, we report on the P-dependence of the electrical resistivity in under- to near-optimally doped Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ (0$\le $x$\le $0.285) for P$\le $84kbar. Pressure suppresses the SM transition at a rate that increases with Ru content, e.g. for x=0.092, 0.161, and 0.210, dT$_{SM}$/dP $\sim $-0.2, -0.4, and -0.6K/kbar, respectively. Although the x=0.092 and x=0.161 compositions are not SC at P=0, SC is seen when T$_{SM}$ is sufficiently reduced by P. For x=0.161, T$_{c}$ at first increases to a maximum near 25K at $\sim $20kbar, after which it decreases at a rate of $\sim $-0.4K/kbar. Likewise, the T$_{c}$ values for the higher dopings peak $\sim $20kbar and decrease at higher P at similar rates. We find that the application of pressure on Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ enhances T$_{c}$ beyond that which was achieved with only doping, seen before in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. -- Supported by U.S. DOE (DE-AC02-07CH11358), AFOSR-MURI (FA9550-09-1-0603), and NSF (DMR-0805335). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T26.00006: Phase diagram of superconductivity and antiferromagnetism in single crystals of Sr(Fe1-xCox)2As2 and Sr1-yEuy (Fe0.88Co0.12)2As2 Rongwei Hu, Sergey Bud'ko, Paul Canfield We report magnetic susceptibility, resistivity and heat capacity measurements on single crystals of Sr(Fe$_{1-x}$Co$_{x}$)$_{2} $As$_{2}$ and Sr$_{1-x}$Eu$_{x}$(Fe$_{0.88}$Co$_{0.12}$)$_{2} $As$_{2}$ series. The optimal Co concentration for superconductivity in Sr(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2} $ is determined to be $x=0.117$. Based on this we grew members of the Sr$_{1-y}$Eu$_{y}$ series to examine the effects of well defined local moment scattering on the superconducting state. We show the evolution of superconductivity and development of antiferromagnetism across the whole doping range. The suppression of superconductivity within Abrikosov-Gor'kov's theory and de Gennes scaling as well as the antiferromagnetic transition temperature will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T26.00007: Lattice distortion and magnetic quantum phase transition in CeFeAs$_{1-x}$P$_{x}$O Invited Speaker: With the advent of Fe-based superconductivity initially discovered in the prototypical electron doped Fe-pnictide LaFeAsO$x$F1$-x$, came a surge of renewed interest in high temperature superconductivity. The discovery of ubiquitous antiferromagnetic (AFM) order in the parent compounds of iron arsenide superconductors has brought attention to the understanding of the interplay between magnetism and high-transition temperature (high-$Tc)$ superconductivity in these materials. Although superconductivity in iron arsenides arises from charge carrier doping of their semimetal parent compounds, the resulting electronic phase diagrams are dramatically materials dependent, ranging from first-order-like AFM to superconductivity phase transition for LaFeAsO1-$x$F$x$, to the gradual suppression of the AFM order before superconductivity for CeFeAsO1-$x$F$x$, and finally to the co-existing AFM order with superconductivity in SmFeAsO1-$x$F$x$. A feature of the parent compounds is the structural distortion that occurs in the vicinity of the onset of long range magnetic order of the Fe-spins. In the RFeAsO(R=rare earth) family, the magneto-structural transition is suppressed in favor of superconductivity upon doping charge carriers into the system, which alters the system electronically and crystallographically as well. To understand the lattice effect on the suppression of the AFM ground state itself by quantum fluctuations, it is important to isoelectronically tune the crystal lattice structure without the influence of charge carrier doping and superconductivity. Here we use neutron scattering to show that replacing the larger arsenic with smaller phosphorus in CeFeAs1-$x$P$x$O simultaneously suppresses the AF order and orthorhombic distortion near $x $= 0.4, providing evidence for a magnetic quantum critical point. Furthermore, we find that the pnictogen height in iron arsenide is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity. Preliminary work on systematic phosphorous doping in LaFeAs1-$x$P$x$O was also done to possibly identify characteristic changes in the lattice that may be correlated with the phosphorous doping induced superconductivity in the La system and in turn give insights as to the absence of superconductivity in the Ce system. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T26.00008: Rare earth substitution in AFe2As2 single crystals Shanta Saha, Nicholas Butch, Tyler Drye, Jeff Mcgill, Johnpierre Paglione, Peter Zavalij, Jeffrey Lynn Synthesis and characterization of aliovalent light rare earth substitutions for alkaline earth atoms are studied in single crystals of FeAs-based compounds with the ThCr2Si2 structure. Electrical resistivity, magnetic susceptibility and structural parameters determined via x-ray and neutron scattering techniques are investigated as a function of chemical pressure and charge doping induced by substitution. Measured physical properties are compared to the effects of external applied pressure on CaFe2As2, known to induce a collapse of the tetragonal unit cell. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T26.00009: Magneto-elastic Coupling in Single-crystal CeFeAsO H.-F. Li, J.-Q. Yan, J.W. Kim, R.W. McCallum, T.A. Lograsso, D. Vaknin Single-crystal synchrotron X-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave-vector at low temperatures. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. We find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the domains in the AFM phase, indicating a distribution of the in-plane lattice constants. This strongly suggests that the phase above the structural transition is virtually T with strong O-T fluctuations that are induced by magnetic fluctuations. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T26.00010: Competing magnetic ground states in Ba(Fe$_{1-x}$Cr$_x$)$_2$As$_2$ K. Marty, M.D. Lumsden, A.D. Christianson, C. Wang, M. Matsuda, H. Cao, L. VanBebber, J.L. Zarestky, D.J. Singh, A.S. Sefat Understanding the origin of unconventional superconductivity is a great challenge of condensed matter physics. In the so called 122 family, doping in the conductive layer (i.e. on the Fe site) of the BaFe$_2$As$_2$ iron pnictide parent compound leads to superconductivity for almost any transition metal, except for Cr and Mn. The absence of superconductivity in these cases remains an unresolved issue. We report here neutron diffraction measurements of Ba(Fe$_{1-x}$Cr$_x$)$_2$As$_2$ for concentrations up to x=0.47. The results show that Cr doping stabilizes magnetism across the phase diagram with a competing magnetic order favoured at high Cr-doping, in contrast to the other superconducting Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T26.00011: Superconductivity and magnetism in Eu$_{1-x}$K$_{x}$Fe$_{2}$(As$_{1-y}$P$_y$)$_2$ H.S. Jeevan, J. Maiwald, Philip Gegenwart, Deepa Kasinathan, Helge Rosner We report detail investigation of superconductivity and magnetism in EuFe$_{2}$As$_{2}$ by doping of K in Eu site and P in As site. In this new class of FeAs-based superconductors, it is found that superconductivity appears close to a magnetic instability, suggesting a possible unconventional pairing mechanism. We have synthesized single crystals of both doped and undoped samples and investigated their physical properties by means of heat capacity, resistivity, magnetization and thermal conductivity measurements. The parent compound shows an antiferromagnetic spin-density-wave (T$_{SDW}$) at $\approx190$K related to the Fe$_{2}$As$_{2}$ layers and magnetic ordering of Eu$^{2+}$ (T$_N$) moments at $\approx20$K. Upon doping Eu with K $>$30$\%$, T$_{SDW}$ gets suppressed and superconductivity (SC) appears at $\approx32$K and also Eu$^{2+}$ ordering suppressed to the low temperature. On the other hand, P doping to the As site suppresses the SDW transition and results in SC, but Eu ordering remains undisturbed. Further increasing the P doping, Eu order transitions from AFM to FM phase which leads to disappearance of SC. We will compare our experimental findings with density functional theory based calculations. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T26.00012: Effects of annealing time and temperature on the transition temperature and low temperature state of AFe2As2 materials Sheng Ran, Sergey Bud'ko, Alex Thaler, Dominic Ryan, Yuji Furukawa, Beas Roy, Andreas Kreyssig, Robert McQueeney, Daniel Pratt, Alan Goldman, Paul Canfield Over the past couple of years the AFe2As2 (A = Ca, Sr, Ba) family of compounds has become a model system for the study of FeAs-based superconductivity. Superconductivity can be stabilized by hole and electron doping (on A and Fe sites) as well as hydrostatic pressure and isoelectronic substitutions on both the Fe and As sites. In all cases the adequate suppression of the structural / antiferromagnetic phase transition appears to be a necessary condition for the appearance of superconductivity. In this talk we will review the effects of annealing time and temperature on the structural / antiferromagnetic phases of AFe2As2. Transition temperature / time and transition temperature / annealing temperature plots will be presented and discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T26.00013: Miscibility gap between BaMn$_2$As$_2$ and BaFe$_2$As$_2$ Abhishek Pandey, David C. Johnston ${\rm BaMn_2As_2}$ and ${\rm BaFe_2As_2}$ both crystallize at room temperature in the same tetragonal ThCr$_2$Si$_2$-type structure but with divergent unit cell volumes of 234.12 and 204.38~\AA$^3$, respectively, suggesting that the Mn$^{+2}$ is in a high-spin state while Fe$^{+2}$ is in a low-spin state. The physical properties of the two compounds are therefore also highly divergent; \emph{e.g.}, ${\rm BaMn_2As_2}$ is an insulating local moment antiferromagnet with a high N\'eel temperature $T_{\rm N} = 625$~K whereas BaFe$_2$As$_2$ is a metallic itinerant antiferromagnet with a much lower $T_{\rm N} = 137$~K\@.\footnote{D. C. Johnston, Adv. Phys. {\bf 59}, 803--1061 (2010).} We have discovered a miscibility gap in the pseudobinary phase diagram between these two isostructural compounds, probably arising from their divergent chemistry. Our investigations of the miscibility gap and of the structural, magnetic, electronic transport and thermal properties of various compositions in this system will be discussed. [Preview Abstract] |
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