Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S13: Focus Session: Fe Based Superconductors-DFT and Pressure Effect |
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Chair: Rong Yu, Renmin University of China Room: 207 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S13.00001: Lifshitz Transitions in 122-Pnictides Under Pressure Khandker Quader, Michael Widom The issue of Lifshitz transitions [1] in solids has been of considerable interest over several decades. These occur due to topological change(s) of the Fermi surface caused by external pressure or chemical substitution. Resulting anomalies in lattice parameters, density of states near the Fermi energy, elastic properties, and electron dynamics manifest in thermodynamic and transport properties that may be observed experimentally. At sufficiently high pressures, the 122 pnictides, AFe$_{2}$As$_{2}$ (A = Ca, Sr, Ba), display transition from a tetragonal phase (T) to a ``collapsed'' phase (cT). Based on our T = 0 first principles total energy density functional theory calculations as a function of pressure, we propose that the observed T-cT transitions result from T= 0 K Lifshitz transitions. Our results for energy band dispersions and spectra, c- and a-axis lattice parameters, and elastic constants over a wide range of hydrostatic pressure support our view. \\[4pt] [1] I. M. Lifshitz, Sov. Phys. JETP 11, 1130 (1960) [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S13.00002: First-principles investigation of the effect of pressure on CaFe$_2$As$_2$ and Pr-doped Ca$_{1-x}$Pr$_{x}$Fe$_ {2}$ As$_ {2}$ Tanju Gurel, A. V. Lukoyanov, Esra Erturk, Guven Akcay, Resul Eryigit, V. I. Anisimov In a recent study (arXiv:1310.3842), superconductivity has been observed at critical temperature ~51 K under pressure 1.9 GPa for rare-earth doped Ca$_{0.86}$Pr$_{0.14}$Fe$_2$As$_2$ which is the highest T$_c$ reported in the class of 1-2-2 iron-based superconductors. Motivated by this, we present density functional theory calculations on iron-based pnictide undoped CaFe$_2$As$_2$ and Pr-doped Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$=0.25 and 0.125). The calculations have been carried out using plane-waves and pseudopotential approach within generalized gradient approximation (GGA) and also within GGA+U in order to investigate the influence of correlation effects. The effect of pressure on crystal structure, magnetic order, and electronic structure are investigated for both undoped and Pr-doped structures for comparison and discussed with experimental findings. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S13.00003: Ab initio study on rare-earth iron-pnictides $R$FeAsO ($R=$Pr, Nd, Sm, Gd) in low-temperature \emph{Cmma} phase Resul Eryigit, Tanju Gurel, Esra Erturk, A.V. Lukoyanov, Guven Akcay, V.I. Anisimov We present density functional theory calculations on iron-based pnictides $R$FeAsO ($R=$Pr, Nd, Sm, Gd). The calculations have been carried out using plane-waves and projector augmented wave (PAW) pseudopotential approach. Structural, magnetic and electronic properties are studied within generalized gradient approximation (GGA) and also within GGA+U in order to investigate the influence of electron correlation effects. Low-temperature $Cmma$ structure is fully optimized by GGA considering both non-magnetic and magnetic cells. We have found that spin-polarized structure improves the agreement with experiments on equilibrium lattice parameters, particularly $c$ lattice parameter and Fe-As bond-lengths. Electronic band structure, total density of states, and spin-dependent orbital-resolved density of states are also analyzed in the frameworks of GGA and GGA+U and discussed. For all materials, by including on-site Coulomb correction, rare earth 4f states move away from the Fermi level and the Fermi level features of the systems are found to be mostly defined by the 3d electron-electron correlations in Fe. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S13.00004: Density functional theory for plasmon-assisted superconductivity Ryosuke Akashi, Ryotaro Arita The predictive calculation of superconducting transition temperatures (Tc) is a fascinating but extremely difficult problem in the field of superconductivity. For a conventional phonon-induced superconducting mechanism, an accurate predictive scheme to calculate Tc is established by the recent progress in the density functional theory for superconductors (SCDFT) [Lueders et al., PRB 72, 024545 (2005); Marques et al., PRB 72, 024546 (2005)]; the current SCDFT-based scheme systematically reproduces Tc observed by experiments in the conventional systems such as niobium and MgB2, with discrepancies no more than a few kelvin. However, further extensions including other mechanisms are essential to treat more general materials. Recently, we extended the SCDFT-based scheme to include a plasmon mechanism of superconductivity [Akashi and Arita, PRL 111, 057006 (2013)]. The plasmon mechanism, which has been considered solely in rather dilute electron systems, is also expected to be relevant in a wider range of materials because it can cooperate with the conventional phonon mechanism. Our extended scheme enables us to evaluate the effects on Tc of the plasmon and phonon mechanisms on equal footing. In the talk, we present recent applications to elemental metals. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S13.00005: Pressure-driven magnetic and structural transitions in the 122-pnictides Michael Widom, Khandker Quader Pnictides of the family AFe2As2, where A is an alkali earth element, exhibit several phase transitions in their structure and magnetic order as functions of applied pressure. We employ density functional theory total energy calculations at T=0K to model these transitions for the entire set of alkali earths (A=Ca, Sr, Ba, Ra) which form the 122 family. Three distinct types of transition occur: an enthalpic transition [1] in which the striped antiferromagnetic orthorhombic (OR-AFM) phase swaps thermodynamic stability with a competing tetragonal phase; a magnetic transition in which the OR-AFM phase loses its magnetism and orthorhombicity; a lattice parameter anomaly in which the tetragonal c-axis collapses. We identify this last transition as a ``Lifshitz transition'' caused by a change in Fermi surface topology. Depending on the element A, the tetragonal state exhibiting the Lifthitz transition might be metastable (A=Ca) or stable (A=Sr, Ba and Ra). \\[4pt] [1] M. Widom and K. Quader, Phys. Rev. B 88 (2013) 045117 [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S13.00006: Direct Detection of Interstitial Oxygen and its Electronic Structure in Superconducting Fe$_{\mathrm{1+y}}$TeO$_{\mathrm{x}}$ Thin-Films Jian-Min Zuo, Hefei Hu, Mao Zheng, Can Zhang, Laura Greene, James Eckstein, Ji-Hwan Kwon Fe$_{\mathrm{1+y}}$Te thin films become a superconductor when doped with oxygen, in which the interstitially incorporated oxygen plays crucial role for emergence of superconductivity. In this study, we investigated atomic and electronic structure of the oxygen-doped Fe$_{\mathrm{1+y}}$Te thin films using electron energy loss spectroscopy (EELS) and first principles calculation based on density functional theory (DFT). Atomic-resolution EELS reveals interstitial oxygen position next to the Fe layer, four fold hollow site, which location is consistent with DFT calculation result showing that the experimentally-found structure is the most energetically stable interstitial site. The relaxed geometry of oxygen-incorporated FeTe through DFT calculation shows drastic structural distortions in the planar structure of FeTe layer, which is associated with large amount of magnetic moment fluctuation in the Fe atoms with the bicollinear antiferromagnetic configuration. The detailed analysis including the charge density and electronic structures of the oxygen-doped FeTe will be presented. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S13.00007: Correlations and effects of pressure in Fe-pnictides Invited Speaker: Roser Valenti In this talk we will explore the effects of correlations and pressure in Fe-based superconductors by considering a combination of density functional theory calculations and dynamical mean field theory and compare our results with recent ARPES and de Haas van Alphen experiments. We will discuss the importance of orbital-selective correlations in the 111 (LiFeAs, LiFeP) and 122 families (BaFe$_2$As$_2$,CaFe$_2$As$_2$,KFe$_2$As$_2$) [1,2] and indicate how the topology of the Fermi surface, specially in KFe2As2, is influenced by these effects. In this context, we will show why MgFeGe, an isostructural and isoelectronic system to LiFeAs, doesn't superconduct [3]. In the case of the 122 systems, we will predict and analyze changes in the electronic and magnetic properties under hydrostatic, tensile and compressive pressure and will discuss our results in relation to (i) superconductivity, (ii) magnetism and (iii) the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal a [4].\\[4pt] [1] J. Ferber, H. O. Jeschke, R. Valenti, Phys. Rev. Lett 109, 236403 (2012).\\[0pt] [2] D. Guterding, S. Backes, H.O. Jeschke, R. Valenti, in preparation.\\[0pt] [3] H.O. Jeschke, I. I. Mazin, and R. Valenti, Phys. Rev. B (RC) 87, 241105 (2013).\\[0pt] [4] M. Tomic, H. O. Jeschke, R. M. Fernandes, R. Valenti, Phys. Rev B 87, 174503 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S13.00008: Superconductivity from repulsion in LiFeAs: novel $s$-wave symmetry and potential time-reversal symmetry breaking Ilya Eremin, Felix Ahn, Johannes Knolle, Vladimir Zabolotnyy, S. Borisenko, Roderich Moessner, Bernd B\"uchner, Andrey Chubukov Using the ten orbital tight-binding model, derived from the ab-initio LDA calculations and fitted to the ARPES experiments, we analyze the structure of the superconducting gap in LiFeAs. We treat superconductivity as quasi-2D and decompose the pairing interaction for various $k_z$ cuts into $s-$ and $d$-wave components. Analyzing the leading superconducting instabilities we find that in addition to the conventional $s^{+-}$-wave superconducting order parameter where the gap changes sign between electron and hole pockets LiFeAs possesses another instability where the superconducting gap also changes sign between two smaller inner hole pockets. This occurs due to relatively large repulsion between these two small pockets and also relatively weak interaction between outer and inner hole pockets. The sizes of the gaps on the inner hole pockets is larger than the average value of the superconducting gap on the outer hole pockets and electron pockets which agrees with experimental data. Depending on the input parameters this gap structure is either a leading instability in the s-wave channel or a subleading one to the usual $s^{+-}$. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S13.00009: Effect of uniaxial strain on structural and magnetic phase transitions in Ba (Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$(0$\le $x$\le $0.04) Chetan Dhital, Zahra Yamani, Zhensong Ren, Tom Hogan, Masa Matsuda, Stephen Wilson Most of the parent compounds of iron based superconductors have either simultaneous or nearly simultaneous structural and magnetic transitions from the high temperature paramagnetic tetragonal phase to the low temperature orthorhombic antiferromagnetic phase. Different measurement probes either directly/indirectly using uniaxial strain (dc transport, optical, thermodynamic and spectroscopic) indicate the presence of a high-temperature electronically anisotropic, nematic phase in both doped and undoped iron pnictide compounds which persists well beyond the nominal magnetic and structural transition temperatures. Here we will discuss the influence of uniaxial strain on the magnetic and structural transition temperatures across a series of Ba (Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$ As$_{2}$ (0$\le $x$\le $0.04) single crystals studied via elastic neutron diffraction. We will present in detail how the effect of strain field varies as function of concentration of cobalt and discuss the relevance of our result to the previously observed electronic anisotropies. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S13.00010: Do Se vacancies electron dope monolayer FeSe? Tom Berlijn, Hai-Ping Cheng, P.J. Hirschfeld, Wei Ku Following the discovery of the potentially very high temperature superconductivity in monolayer FeSe we investigate [1] the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore we find that the disorder induced broadening causes an effective hole doping. The surprising net result is that in terms of the Fe-$d$ bands Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measured by angle resolved photoemission spectroscopy. TB was supported by DOE CMCSN and as a Wigner Fellow at the Oak Ridge National Laboratory.\\[4pt] [1] T. Berlijn, H.-P. Cheng, P. J. Hirschfeld, and W. Ku, arXiv:1307.0140. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S13.00011: Phonon spectrum of SrFe$_{2}$As$_{2}$ determined using multizone phonon refinement D. Parshall, R. Heid, J.L. Niedziela, Th. Wolf, M.B. Stone, D.L. Abernathy, D. Reznik The ferropnictide superconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23~meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in $\approx$ 40~Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to better than 0.1~meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S13.00012: Space group symmetry, spin-orbit coupling, and the low-energy effective Hamiltonian for iron-based superconductors Vladimir Cvetkovic, Oskar Vafek Iron-based superconductors are multiband semimetals with competing instabilities. This motivates us to use the method of invariants when constructing a low-energy effective theory for these materials. In the construction we use the space group which, being non-symmorphic, leads to peculiar consequences at the Brillouin zone corner, precisely where the low-energy states reside. Our model displays good agreement with the multiband tight-binding models. The spin-orbit coupling, significant in iron, is easily incorporated in our model. We predict its consequences. The nodal spin-density wave (SDW) is proven to be unstable toward any finite spin-orbit coupling. Both colinear or coplanar SDW are shown to induce magnetic moments on pnictogen atoms. The quasiparticle dispersion in the presence of an s-wave spin singlet superconducting order is studied. In the absence of spin-orbit coupling, our minimal model yields isotropic gaps on bothhole Fermi surfaces. The gap structure on the electron Fermi surfaces is determined by the ratio of pairing parameters. The presence of spin-orbit interaction results in the gap anisotropy on the hole Fermi surfaces and a qualitative change of the gap structure on the electron Fermi surfaces. [1] V. Cvetkovic and O. Vafek, Phys. Rev. 88, 134510 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S13.00013: Quasi-particle spectrum around vortex core states in iron-selenide superconductors Qianen Wang, Fuchun Zhang We study electronic structure of vortex core states of iron-selenide superconductors based on a three-orbital model by solving the Bogoliubov-de Gennes equation self-consistently. The absence of hole pocket at Gamma point and multi-orbital band structure are two special features of FeSe superconductors. We calculate quasi-particle wavefunctions and local density of states of the vortex core states for isotropic s-wave, anisotropic s-wave, and $d_{x^2-y^2}$-wave pairing symmetries, respectively. It turns out that the orbital-resolved vortex core states in different paring symmetries manifest themselves as distinguishable structures due to different behavior of the qusi-particle wavefunctions. [Preview Abstract] |
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