Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J40: Invited Session: Superconductivity and Magnetism of Iron-based Superconductors II |
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Sponsoring Units: DCMP Chair: Jeffrey W. Lynn, National Institute of Standards and Technology Room: Mile High Ballroom 2B-3B |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J40.00001: Magnetism and its interplay with Superconductivity in the Doped Iron Chalcogenide Fe$_{\mathrm{1+y}}$Te$_{\mathrm{1-x}}$Se$_{\mathrm{x}}$ Invited Speaker: Vivek Thampy I examine the relationship of iron superconductivity with impurities, and low energy magnetic excitations in the structurally simple iron superconductor, (Fe$_{\mathrm{1+y}}$Te$_{\mathrm{1-x}}$Se$_{\mathrm{x}})$. In the first part of the talk, the pivotal role played by interstitial iron impurities in the microscopic origin of the quasi-static magnetism at (1/2,0) is demonstrated in Fe$_{\mathrm{1+y}}$Te$_{0}$ 0.38 [1]. We used polarized and unpolarized neutron scattering together with simulations of the scattering function based on structural data and a semi-metallic 5-band model with super-exchange interactions with the interstitial iron, to show that the formation of magnetic polarons around the interstitial iron atoms seeds the observed (1/2,0) magnetism. Though the quasi-static magnetism occurs at (1/2,0), the low energy spin dynamics are dominated by fluctuations at (1/2,1/2), like other iron based superconductors. In the second part of the talk, I will discuss these fluctuations and in particular the so-called spin resonance -- the signature feature in the low energy inelastic neutron scattering spectrum. We show that this scattering is quasi two dimensional and largely isotropic. Further, the first moment sum-rule for the dynamic correlation function is applied to the inelastic data in the normal and superconducting states to quantitatively determine the magnetic component of the superconducting condensation energy [2]. This method is sensitive to changes in the inter-site magnetic correlation energy, $\Delta $Eij, associated with superconductivity. We find that the length scale over which $\Delta $Eij is appreciable is similar to the superconducting coherence length, as determined by Scanning Tunneling Microscopy. Comparison of the inter-site magnetic correlation energy to the superconducting condensation energy determined through specific heat measurements indicates a significant role of magnetic fluctuations in stabilizing superconductivity. \\[4pt] [1] V. Thampy et al, Phys. Rev. Lett. 108, 107002 (2012).\\[0pt] [2] J. Leiner et al, Manuscript under preparation. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:42PM |
J40.00002: ARPES Study on the Strongly Correlated Iron Chalcogenides Fe$_{1+y}$Se$_x$Te$_{1-x}$ Invited Speaker: Zhongkai Liu The level of electronic correlation has been one of the key questions in understanding the nature of iron-based superconductivity. Using Angle Resolved Photoemission Spectroscopy (ARPES), we systematically investigated the correlation level in the iron chalcogenide family Fe$_{1+y}$Se$_x$Te$_{1-x}$. For the parent compound Fe$_{1.02}$Te, we discovered ``peak-dip-hump'' spectra with heavily renormalized quasiparticles in the low temperature antiferromagnetic (AFM) state, characteristic of coherent polarons seen in other correlated materials with complex electronic and lattice interactions. As the temperature (or Se ratio x) increases and Fe$_{1.02}$Se$_x$Te$_{1-x}$ is in the paramagnetic (PM) phase, we observed dissociation behavior of polarons, suggestive of connection between the weakening electron-phonon coupling and AFM [1]. Further increase of x leads to an incoherent to coherent crossover in the electronic structure, indicating a reduction in the electronic correlation as the superconductivity emerges. Furthermore, the reduction of the electronic correlation in Fe$_{1+y}$Se$_x$Te$_{1-x}$ evolves in an orbital-dependent way, where the d$_{xy}$ orbital is influenced most significantly [2]. At the other end of the phase diagram (FeSe) where the single crystal is not stable, we have studied the MBE-grown thin film which also reveals orbital-dependent strong correlation in the electronic structure [3]. Our findings provide a quantitative comprehension on the correlation level and its evolution on the phase diagram of Fe$_{1+y}$Se$_x$Te$_{1-x}$. We discuss the physical scenarios leading to strong correlations and its connection to superconductivity.\\[4pt] [1] Z. K. Liu, et al., Physical Review Letters 110, 037003(2013);\\[0pt] [2] Z. K. Liu, et al., submitted;\\[0pt] [3] M. Yi, et al., submitted. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 4:18PM |
J40.00003: Electron delocalization, orbital order, magnetism, and emergent superconductivity in Fe$_{1+y}$Te and Fe$_{1+y}$(Te,S/Se) Invited Speaker: Igor Zaliznyak Neutron scattering [1] reveales an unusual enhancement, on warming, of dynamical magnetism in iron telluride, Fe$_{1+y}$Te, the non-superconducting parent material of the chalcogenide family of iron-based superconductors, and in nearly critical Fe$_{1+y}$Te$_{1-x}$(S,Se)$_{x}$, where bulk measurements show the presence of filamentary superconductivity [2]. While these findings are consistent with both Kondo-like screening of local spins by conduction electrons, or a delocalization, on cooling, of one of the electrons, our more recent results shed light on this issue, favoring the latter scenario. Investigation of the magneto-structural phase diagram of the Fe$_{1+y}$Te series revealed that the low-temperature phase, which in the nearly stoichiometric (y $\approx $ 0) material is attained via the first order phase transition at T$_{N} \quad \approx $ 70 K, is characterized not only by antiferromagnetic and structural order, but also by a peculiar type of orbital order. By combining results of bulk characterization of electronic behavior and the diffraction data on the microscopic structural changes for samples with y $\approx $ 0.05 to 0.13, we were able to disentangle different low-temperature orders and identify new, electronically driven ferro-orbital ordering transition. The newly discovered orbital ordering is characterized by the formation of zigzag Fe-Fe chains similar to those in manganites, and is associated with the delocalization of one of the electrons. This has profound effect on magnetic and electronic properties, including marked decrease of resistivity and magnetic susceptibility.\\[4pt] In collaboration with D. Fobes, Z. Zhu, R. Zhong, G. Gu, J. Tranquada, C. Petrovic, V. Solovyov, Condensed Matter Physics and Material Science Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA; A. Savici, M. Lumsden, M. Stone, and B. Winn, NSSD, Oak Ridge National Laboratory, Oak Ridge, TN. This work was supported by the US DOE under Contract DE-AC02-98CH10886. \\[4pt] [1] I. A. Zaliznyak, Z. J. Xu, J. M. Tranquada, G. D. Gu, A. M. Tsvelik, M. B. Stone, Phys. Rev. Lett. 107, 216403 (2011). \\[0pt] [2] Rongwei Hu, E. S. Bozin, J. B. Warren, C. Petrovic, Phys. Rev. B 80, 214514 (2009).\\[0pt] [3] I. A. Zaliznyak, Z. J. Xu, J. S. Wen, J. M. Tranquada, G. D. Gu, V. Solovyov, V. N. Glazkov, A. I. Zheludev, V. O. Garlea, M. B. Stone, Phys. Rev. B 85, 085105 (2012); also unpublished (2014). [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:54PM |
J40.00004: High temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$ Invited Speaker: Lili Wang High transition temperature ($T_{C})$ superconductivity was discovered in single unit-cell thick FeSe films grown on a SrTiO$_{3}$(001) substrate by molecular beam epitaxy. \textit{In situ} scanning tunneling microscopy revealed a superconducting gap as large as 20 meV in single unit-cell thick FeSe films [1]. By \textit{ex situ} transport measurements on single unit-cell thick FeSe films protected with FeTe layer, we demonstrated an onset $T_{C}$ above 40 K and a critical current density $J_{C}$ $\sim$ 1.7 $\times$ 10$^{6}$ A/cm$^{2}$ at 2 K, which are much higher than $T_{C}$ $\sim$ 8 K and $J_{C}$ $\sim$ 10$^{4}$ A/cm$^{2}$ for bulk FeSe [2,3], and that the characteristics of the transition are consistent with a two-dimensional superconductor undergoing a Berezinskii-Kosterlitz-Thouless transition. The superconductivity is further confirmed by measuring Meissner effect. The simple structure of the current system provides an ideal platform for understanding the underlying physics of high-$T_{C}$ superconductivity.\\[4pt] [1] Wang, Q. Y. \textit{et al.}, Interface-induced high-temperature superconductivity in single unit-cell FeSe films on SrTiO$_{3}$. \textit{Chinese Physics Letters}, \textbf{29}, 037402 (2012).\\[0pt] [2] Hsu, F. C. \textit{et al.}, Superconductivity in the PbO-type structure $\alpha $-FeSe. \textit{Proc. Natl. Acad. Sci. USA}\textbf{ 105}, 14262 (2008).\\[0pt] [3] Lei, H. C.\textit{ et al.}, Critical fields, thermally activated transport, and critical current density of $\beta $-FeSe single crystals. \textit{Phys}.\textit{Rev. B} \textbf{84}, 014520 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:30PM |
J40.00005: Effects of electron-phonon coupling on the superconductivity of FeSe/SrTiO3 interface Invited Speaker: Fa Wang The maximal Tc in iron-based high temperature superconductors has remained around 55K since 2008. In 2012 a Chinese group reported STS evidences of enhanced superconductivity of one-unit-cell FeSe film on SrTiO3 substrate, with an estimate of Tc over 77K [1]. Similarly large gaps were later observed in ARPES experiments [2,3] and a recent transport measurement directly confirmed the superconductivity at this FeSe/STO interface [4]. These exciting progresses call for a better understanding of the mechanism of high Tc in this and other iron-based materials. In this talk I will discuss our work on the possible role of electron-phonon coupling in the FeSe/STO system [5]. We propose that electron-phonon coupling, which is largely overlooked in the studies of bulk Fe-based superconductors, can play a significant role here due to the soft ferroelectric phonon modes in SrTiO3. We generalize the phenomenological Eliashberg theory to this multiple-band case, and obtain generalized McMillan formula of Tc for conventional and unconventional s-wave pairing states. We can therefore demonstrate that moderate electron-phonon coupling will be able to produce the observed large enhancement of pairing gap. This result is further confirmed by a microscopic functional renormalization group calculation. We will also discuss the experimental signatures of electron-phonon coupling, and propose other substrate materials to utilize this mechanism. This work could foster further experimental and theoretical studies of Fe-based superconductivity, and may eventually lead to the discovery of even higher Tc systems.\\[4pt] [1] Q.-Y. Wang et al. Chin. Phys. Lett. 29, 037402 (2012). \\[0pt] [2] D. Liu et al. Nat. Commun. 3, 931 (2012). \\[0pt] [3] S.Y. Tan et al. Nat. Mater. 12, 634 (2012). \\[0pt] [4] Jian Wang et al. unpublished. \\[0pt] [5] Y.-Y. Xiang, F. Wang, D. Wang, Q.-H. Wang, D.-H. Lee, Phys. Rev. B 86, 134508 (2012). [Preview Abstract] |
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