Anomalous optical phonons in FeTe chalcogenides: Spin state, magnetic order, and lattice anharmonicity (original) (raw)
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We report interesting anomalies in the temperature dependent Raman spectra of FeSe 0.82 measured from 3K to 300K in the spectral range from 60 to 1800 cm -1 and determine their origin using complementary first-principles density functional calculations. A phonon mode near 100 cm -1 exhibits a sharp increase by ~ 5% in frequency below a temperature T s (~ 100K) attributed to strong spin-phonon coupling and onset of shortrange antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm -1 and 1600 cm -1 , attributed to electronic Raman scattering from (x 2 -y 2 )to xz / yz d-orbitals of Fe.
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We report interesting anomalies in the temperature dependent Raman spectra of FeSe 0.82 measured from 3K to 300K in the spectral range from 60 to 1800 cm -1 and determine their origin using complementary first-principles density functional calculations. A phonon mode near 100 cm -1 exhibits a sharp increase by ~ 5% in frequency below a temperature T s (~ 100K) attributed to strong spin-phonon coupling and onset of shortrange antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm -1 and 1600 cm -1 , attributed to electronic Raman scattering from (x 2 -y 2 )to xz / yz d-orbitals of Fe.
Lattice dynamics of iron chalcogenides: Raman scattering study
The discovery of superconductivity in FeSe led to a new subclass of high-temperature superconductors – iron chalcogenides. Materials from this group exhibit variety of specific features, from superconductivity with relatively high critical temperatures to low-dimensional magnetic properties. This review presents the most important results regarding the iron chalcogenides, with special emphasis on their vibrational properties investigated by means of Raman spectroscopy. Temperature-and/or doping-dependent Raman scattering spectra of iron chalcogenides provide a valuable insight into the complex relationships between the vibrational, electronic and magnetic properties of these materials. The results presented in this review demonstrated that Raman spec-troscopy provides new insights which may significantly improve our understanding of the fundamental properties of iron chalcogenides.
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The nature of metallicity and the level of electronic correlations in the antiferromagnetically ordered parent compounds are two important open issues for the iron-based superconductivity. We perform a temperature-dependent angle-resolved photoemission spectroscopy study of Fe1.02Te, the parent compound for iron chalcogenide superconductors. Deep in the antiferromagnetic state, the spectra exhibit a "peak-dip-hump" line shape associated with two clearly separate branches of dispersion, characteristics of polarons seen in manganites and lightly-doped cuprates. As temperature increases towards the Neel temperature (TN ), we observe a decreasing renormalization of the peak dispersion and a counterintuitive sharpening of the hump linewidth, suggestive of an intimate connection between the weakening electron-phonon (e-ph) coupling and antiferromagnetism. Our finding points to the highly-correlated nature of Fe1.02Te ground state featured by strong interactions among the charge, spin and lattice and a good metallicity plausibly contributed by the coherent polaron motion. PACS numbers: 74.25.Jb, 74.70.Xa, 79.60.-i, 71.38.-k
Journal of physics. Condensed matter : an Institute of Physics journal, 2014
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm( - 1) from 5 to 300 K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160 K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving the displacement of Fe atoms, show a sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400 and 1200 cm( - 1) are attributed to electronic Raman scattering involving the crystal field levels of d-orbitals of Fe(2+). The splitting between xz and yz d-orbital levels is shown to be ~25 meV, which increases as temperature decreases below Tsm. A broad Raman band observed at ~3200 cm( - 1) is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.
Journal of Physics: Condensed Matter, 2011
Inelastic light scattering studies on single crystal of electron-doped Ca(Fe 0.95 Co 0.05 ) 2 As 2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at T SM ~ 140 K and superconducting transition temperature T c ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm -1 shows hardening below T c , signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and T c i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at T SM . Using first-principles density functional theory-based calculations, we show that the low temperature phase (T c < T < T SM ) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spinphonon couplings that are responsible for these phonon anomalies.
Journal of Physics: Condensed Matter, 2010
We present first-principles density-functional-theory-based calculations to determine the effects of the strength of on-site electron correlation, magnetic ordering, pressure and Se vacancies on phonon frequencies and electronic structure of FeSe 1−x. The theoretical equilibrium structure (lattice parameters) of FeSe depends sensitively on the value of the Hubbard parameter U of on-site correlation and magnetic ordering. Our results suggest that there is a competition between different antiferromagnetic states due to comparable magnetic exchange couplings between first-and second-neighbor Fe sites. As a result, a short range order of stripe antiferromagnetic type is shown to be relevant to the normal state of FeSe at low temperature. We show that there is a strong spin-phonon coupling in FeSe (comparable to its superconducting transition temperature) as reflected in large changes in the frequencies of certain phonons with different magnetic ordering, which is used to explain the observed hardening of a Raman-active phonon at temperatures (∼100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phase permits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as a secondary order parameter at the temperature of magnetic ordering. The presence of Se vacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy, which could enhance the superconducting transition temperature within the BCS-like picture.