Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J34: Focus Session: Iron Pnictides and Other Novel Superconductors VII: Pressure Effects and Thermal Expansion |
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Sponsoring Units: DCMP Chair: Neil Sullivan, University of Florida Room: 404 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J34.00001: Effects of pressure on CaFe$_{2}$As$_{2}$ and related materials Invited Speaker: The discovery of CaFe$_{2}$As$_{2 }$[1] and its extreme pressure dependence [2] (even for pressures below 1 GPa) has lead to it being used as a model system for understanding the effects of pressure on the (AE)Fe$_{2}$As$_{2 }$(AE = Ba, Sr, Ca) compounds.[3-5] We have found that the combination of extreme pressure sensitivity with a first order structural phase transition that involves significant changes in the unit cell dimensions makes CaFe$_{2}$As$_{2}$ very sensitive to the pressure medium used. In liquid medium, self clamping cells the higher temperature transitions, while generally detectable and highly reproducible [2,6] are smeared and broadened, especially near the first order phase transition between the low temperature orthorhombic and collapsed tetragonal phases. In He-pressure cells these transitions remain extremely sharp.[3-5] Superconductivity is detected between $\sim $0.3 and $\sim $0.7 GPa in the liquid medium cells and is essentially absent in the He-pressure cell.[2,5,6] This superconducting region can be associated with a coexistence of low temperature phases brought on by non-hydrostatic components associated with the medium's inability to respond to the high-temperature structural phase transitions.[2-5] The origin of the superconductivity in this mixed region remains a topic of keen experimental and theoretical interest. \\[4pt] [1] N. Ni et al., Phys. Rev. B \textbf{78}, 014523 (2008), [2] M. Torikachvili et al., Phys. Rev. Lett. \textbf{101}, 057006 (2008), [3] A. Kreyssig et al., arXiv:0807.3032, [4] A. I. Goldman et al., arXiv:0811.2013, [5] W. Yu et al., arXiv:0811.2554, [6] H. Lee et al., arXiv:0809.3550. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J34.00002: Discovery of a pressure-induced ``collapsed'' phase in CaFe$_{2}$As$_{2}$ A. Kreyssig$^{1,2}$, M.A. Green$^{3,4}$, Y. Lee$^{1,2}$, G.D. Samolyuk$^{1,2}$, P. Zajdel$^{3,5}$, J.W. Lynn$^{3}$, S.L. Bud'ko$^{1,2}$, M.S. Torikachvili$^{6}$, N. Ni$^{1,2}$, S. Nandi$^{1,2}$, J.B. Le\~{a}o$^{3}$, S.J. Poulton$^{3,4}$, D.N. Argyriou$^{7}$, B.N. Harmon$^{1,2}$, R.J. McQueeney$^{1,2}$, P.C. Canfield$^{1,2}$, A.I. Goldman$^{1,2}$ Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe$_{2}$As$_{2}$ undergoes a pressure-induced transition to a non-magnetic, volume ``collapsed'' tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. -- The support by U.S. DOE (DE-AC02-07CH11358) and NSF (DMR-0306165 and DMR-0805335) is acknowledged. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J34.00003: Pressure-induced superconducting state of CaFe$_{2}$As$_{2}$ from an antiferromagnetic spin-density-wave state Hanoh Lee, Eunsung Park, Tuson Park, F. Ronning, E.D. Bauer, J.D. Thompson The spin-density-wave (SDW) antiferromagnet CaFe2As2 has been reported as superconducting under pressure. By measuring electrical resistivity and magnetic susceptibility under pressure in silicon fluid as a pressure medium, we show that bulk superconductivity is present in a narrow pressure range where orthogonal and collapsed tetragonal state coexist. At higher pressures, where the collapsed tetragonal structure is proposed, distinctive behavior appears in resistivity with strong thermal and pressure hysteresis. Magnetic fluctuations combined with structural instability appear to be important for superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J34.00004: Structural changes in pressure induced superconducting BaFe$_{2}$As$_{2}$: Similarities to chemical doping Simon Kimber, Andreas Kreyssig, Fabiano Yokaichiya, Dimitri Argyriou, Jiaqiang Yan, Thomas Hansen, Tapan Chatterji, Robert McQueeney, Paul Canfield, Alan Goldman We have determined the crystal structure of BaFe$_{2}$As$_{2}$ as a function of temperature (4$-$150 K) and pressure (0$-$6 GPa) using neutron powder diffraction. The structural features important to superconductivity, namely suppression of the T$-$O phase transition and reduction in the As$-$Fe$-$As bond angle and Fe$-$Fe distance, show exactly the same behaviour under pressure up to the optimal T$_{C}$ value, as found in chemically doped samples. This result suggests that chemical doping and pressure have similar effects on the electronic degrees of freedom in this family of iron pnictide superconductors. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J34.00005: Dffraction Studies of the P-T Phase Diagram with Single-Crystal Ca122 A.I. Goldman, A. Kreyssig, K. Prokes, D.K. Pratt, D.N. Argyriou, J.W. Lynn, S. Nandi, S.A.J. Kimber, Y. Chen, Y.B. Lee, G. Samolyuk, J.B. Leao, S.J. Poulton, S.L. Budko, N. Ni, P.C. Canfield, B.N. Harmon, R.J. McQueeney Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the nonmagnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J34.00006: Pressure-induced shift of $T_c $ in $K_x Sr_{1-x} Fe_2 As_2 $ (x = 0.2, 0.4, 0.7): Analogy to the high-T$_{c}$ cuprate superconductors Melissa Gooch, Bing Lv, Bernd Lorenz, Arnold Guloy, Ching-Wu Chu Through a systematic study of $K_x Sr_{1-x} Fe_2 As_2 $ (x = 0.2, 0.4, 0.7), by pressure shifts of the $T_c $, similarities between the FeAs and high T$_{c}$ superconductors can be observed. These similarities develop directly from the layered structure seen in both superconductors, which consists of an active superconducting layer and a charge reservoir block. The pressure coefficient of Tc depends on the doping level: dTc/dp$>$0 (underdoped, x=0.2), dTc/dp=0 (optimally doped, x=0.4), and dTc/dp$<$0 (overdoped, x=0.7). This is understood in terms of a pressure-induced charge transfer between the active and charge reservoir layers. In addition to the measured pressure shift in the T$_{c}$, the suppression of the spin density wave can clearly be demonstrated for the x = 0.2 case. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J34.00007: Pressure Effect on the Structural and Magnetic Transition in CaFe$_2$As$_2$ Shiliang Li, Ying Chen, Jeffrey Lynn, Xianhui Chen, Pengcheng Dai We use neutron scattering technique to study both the structural and magnetic phase transitions of CaFe$_2$As$_2$. We confirmed that the nuclear structure changes from orthorhombic to collapsed tetragonal phase with increasing pressure at low temperatures. Strong hysteresis is found in increasing and decreasing temperature processes. The c-axis lattice constant of the orthorhombic phase is found to increase with deceasing temperature under pressure while that of the collapsed tetragonal phase shows almost no change, which suggests a strong magnetic-lattice coupling. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J34.00008: Pressure dependence of the inelastic neutron scattering response of CaFe$_{2}$As$_{2}$. S. Rosenkranz, R. Osborn, E. Goremychkin, I.S. Todorov, D.Y. Chung, H. Claus, J.A. Schlueter, C.D. Malliakas, M.G. Kanatzidis, A.D. Christianson, R.I. Bewley, T. Guidi Application of $\sim $2.5 kbar pressure induces superconductivity in CaFe$_{2}$As$_{2}$ with a T$_{C} \quad \sim $12K that remains constant up to $\sim $ 7kbar, where superconductivity is again suppressed. This modest pressure enables the use of neutron scattering to study in detail changes of the spin and lattice correlations between normal to the superconducting state as a function of pressure. The elastic part of the scattering measured on the MERLIN spectrometer utilizing a He gas pressure cells shows that at 4kbar pressure and 2K only $\sim $ 50{\%} of the sample has transformed to the collapsed tetragonal phase. The inelastic spectra show a suppression of spectral weight at low energies and small momentum transfer on going from ambient pressure to the superconducting state at 4kbar and 2K. The spectral weight is transferred to higher energies and wavevectors, leading to a V-shaped excitation branch in the collapsed tetragonal phase. \newline \newline Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J34.00009: Thermal expansion and pressure derivatives of $T_c$ in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals S.L. Bud'ko, M.S. Torikachvili, N. Ni, J.-Q. Yan, P.C. Canfield, G.M. Schmiedeshoff We present heat capacity and anisotropic thermal expansion data for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Evolution of structural/magnetic phase transition with Co-concentration is clearly detected by both measurements. Anisotropic pressure derivatives of the superconducting transition temperature are evaluated via the Ehrenfest relations and compared with directly measured hydrostatic $dT_c/dP$. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J34.00010: High-Resolution Thermal Expansion Measurements of CaFe$_2$As$_2$ Ariana de Campos, M.S. da Luz, J.J. Neumeier, E.D. Bauer, F. Ronning, J.D. Thompson, Hanoh Lee, Tuson Park, Eunsung Park The discovery of superconductivity in doped LaFeAsO initiated a surge of interest in layered FeAs systems. The recent discoveries in CaFe$_2$As$_2$ [1,2] of: (i) suppression of the first-order structural phase transition under modest hydrostatic pressure, (ii) superconductivity under pressure and (iii) at higher pressures the suppression of superconductivity with stabilization of a potentially different high temperature phase, establish pressure as a valuable parameter for tuning the behavior of these fascinating compounds. In this work, thermal expansion measurements of a high-quality single crystal of CaFe$_2$As$_2$ are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T$_S$ $\approx$180 K. [1] M.S. Torikachvili, et al. PRL 101, 057006(2008). [2] Park T., et al., J. Phys.Cond.Matter, 20, 322204 (2008). This material is based upon work supported by the Brazilian Agency CNPq (Grant No. 201439/2007-7), the NSF (Grant No. DMR- 0504769) and U.S. DOE Office of Basic Energy Sciences (Grant No. DE-FG-06ER46269). Work at LANL was performed under the auspices of the U.S. DOE Office of Basic Energy Sciences and supported by the LDRD program. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J34.00011: Magnetic Ordering and Negative Thermal Expansion in PrFeAsO D.N. Argyriou, S.A.J. Kimber, F. Yokaichiya, K. Habicht, S. Gerischer, R Klingeler, C. Hess, G. Behr, A. Kondrat, B. B\"uchner, T. Hansen, T. Chatterji We report the structure and magnetism of PrOFeAs, one of the parent phases of the newly discovered Fe-As superconductors, as measured by neutron powder diffraction. In common with other REOFeAs materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with $k =$(1,0,1) is detected below $\sim$ 85 K. Our magnetic order parameter measurements show that the ordered Fe moment along the $a$ axis reaches a maximum at $\sim$ 40 K, below which an anomalous expansion of the $c$ axis sets in, which results in a negative thermal volume expansion of 0.015 \% at 2 K. We propose that this effect, which is suppressed in superconducting samples, is driven by a delicate interplay between Fe and Pr ordered moments. [Preview Abstract] |
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