Core Electron Spectroscopic Studies for 11-Fe-based Superconductors
ScienceDirect [International Conference on Advances in Nanomaterials and Devices for Energy andEnvironment, (ICAN-2019)]
Core Electron Spectroscopic Studies for 11-Fe-based Superconductors
Soumyadeep Ghosh a,b * , Haranath Ghosh a,b a Human Resources Development Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India b Homi Bhabha National Institute, 2nd floor, Training School Complex, Anushakti Nagar, Mumbai 400094, India
Abstract
Electron energy loss near edge spectra (ELNES) has been calculated for two different Fe based superconducting material FeSeand FeTe using density functional theory (DFT). Fe K-edge absorption spectra consist of several features, origins of which arethoroughly described in light of partial density of states of constituent atoms. Here we have included “core-hole effect” and founddrastic change in absorption spectra. Our results including core hole effect matches very well with experimental X-rayAbsorption Near Edge Structure (XANES) results available in literature.
Keywords: Core-electron spectroscopy; 11 iron based superconducting materials; core-hole effect Introduction
High temperature superconductivity in iron based superconducting materials had been one of the frontier researcharea since last 10 years. There exist a large number of families of Fe-based superconducting materials, FeSe, FeTeand their variants belong to the simplest structured 11 family, in which Fe atoms forms a square lattice and Se/Te areabove and below the Fe-plane. Even though 11 family members have the simplest structures among all otherfamilies [1], it holds the record highest Tc ~ 100 K [2] when grown on single layered SrTiO . Both Electron energyloss near edge structure (ELNES) and x-ray absorption near edge structure (XANES) spectroscopy measure theelectric dipole transitions from a selected core-orbital to unoccupied states. Due to the high energy resolution ofEELS spectrometer, it can provide information on bonding, local composition, structure, site occupancy andelectronic structure of materials. The probing electron or X-ray (depending on ELNES or XANES studies) interactswith the system (like atom/molecule/solid) which thus gets perturbed by the probe itself. The system being studiedwill have an inner shell electron removed, leaving a core hole, in general known as “core-hole” effect. The core holeeffect, essentially have a net positive charge of the target atom, provides a pull to the unoccupied density of statestowards the conduction band edge, causes redistribution of the unoccupied electronic density of state. Such studiesin Fe-based superconducting materials provided a special role [3,4]. In this work we provide Fe-K edge absorptionfeatures for the two Fe-based superconducting materials FeSe and FeTe within first principles density functionaltheory (DFT) framework. Our results show excellent agreement with experimental observations. Computational Methodology
DFT based simulations of ELNES and calculation of partial density of states (PDOS) are performed usingCASTEP [5], which is based on the plane-wave pseudo-potential method [6]. Exchange correlation is taken asgeneralized gradient approximation (GGA) using using Perdew-Burke-Enzerhof (PBE) functional [7]. Here we use * Corresponding author. Tel.: 0731244-2580
E-mail address: [email protected]
FGS geometry optimization scheme [8] for fully relaxed structure. We used experimentally determined crystalstructure for FeSe having tetragonal Bravais lattices with space group P4/nmm (No.-129) [9]. Results and Discussions
In Fig. (1) theoretically calculated Fe K-edge absorption spectra for two iron based superconductors FeSe (Fig.-1:i) and FeTe (Fig.-1:ii) with and without including core hole effect is presented. In case of FeSe there are fivedistinct features in ELNES spectra (A, B, C, D and E). The feature A is known as ‘pre-edge’ feature which signifiesamount of distortion of FeSe / FeTe tetrahedrons. Feature A is due to transition from to , which is known tobe very weak due to quadrupolar nature of transition. However due to Fe(Se/Te) distortion there are mixingbetween orbitals of the legends and transition metals. Due to such mixing of different angular momentumelectronic states, to optical transition gains moment and shows in the core-level excited spectra. This indicatesthat FeSe has larger tetrahedral distortion compared to FeTe. Such mixing between electronic states have beenelaborately shown in the third row figures of Figs. (2,3:iii). The features B, C, D are due to to transitionswhere the state can come either from Se/Te or Fe as distinctly presented in Figs. (2, 3). However, modificationsin the core-level excited spectra are worth noticing in presence of core-hole effect. Feature B becomes most intenseamong all others differing so much from no core hole spectra in which feature C is most intense. Feature C issuppressed with inclusion of core hole and shifted toward higher energy region. This is because the feature D isenhanced largely and spectral weight is transferred. Feature D and E both are also significantly enhanced, becomemore intense and wider by the inclusion of core hole without any horizontal shift in loss energy. In case of FeTe weget four distinct features (A, B, C and D). Like FeSe feature A, B becomes pronounced with inclusion of core hole.Feature C shifts towards low energy region as well as suppressed and feature D becomes pronounced due to corehole effect, also width of both feature increases. Fig. 1. Normalized Fe K-edge absorption spectra with (solid lines) and without (dashed lines) core hole effects for FeSe (left), FeTe (right)respectively. Special features are indicated by vertical lines. Conclusions
From the above results we can conclude that core hole effect plays significant role in determination of Fe K-edgeabsorption spectra for FeSe and FeTe. For FeSe pre-edge feature is prominent and pronounced in presence of corehole effect. In case of FeTe pre-edge feature is less prominent than FeSe even after considering core hole effect.From electronic structure point of view transition from state to any empty p state is most intense since dipolarransitions are dominant. All the main peaks and separation between different peaks including core hole effect are inclose agreement with experimental observations [3,10]. Fig. 2. Theoretical ELNES spectra of FeSe: Fe K-edge absorptionspectra (i) and site projected PDOS (ii-iii) Fig. 3. Theoretical ELNES spectra of FeTe: Fe K-edge absorptionspectra (i) and site projected PDOS (ii-iii)
Acknowledgements
We thank Dr. P. A. Naik for his encouragement and A. Ghosh for his involvement during this work. HG and SGacknowledges RRCAT computer center for support. SG acknowledges the HBNI, RRCAT for financial assistance.
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