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Journal Articles

Lattice dynamics in FeSe via inelastic X-ray scattering and first-principles calculations

Murai, Naoki; Fukuda, Tatsuo; Nakajima, Masamichi*; Kawamura, Mitsuaki*; Ishikawa, Daisuke*; Tajima, Setsuko*; Baron, A. Q. R.*

Physical Review B, 101(3), p.035126_1 - 035126_6, 2020/01

https://doi.org/10.1103/PhysRevB.101.035126
 Times Cited Count：5 Percentile：33.01(Materials Science, Multidisciplinary)

We report an inelastic X-ray scattering investigation of phonons in FeSe superconductor. Comparing the experimental phonon dispersion with density functional theory (DFT) calculations in the non-magnetic state, we found a significant disagreement between them. A better overall agreement was obtained by allowing for spin-polarization in DFT calculations, despite the absence of magnetic order in FeSe. This calculation gives a realistic approximation, at DFT level, of the disordered paramagnetic state of FeSe, in which strong spin fluctuations are present.

Journal Articles

Experimental investigation of the suppressed superconducting gap and double-resonance mode in Ba$$_{1-x}$$K$$_{x}$$Fe$$_{2}$$As$$_{2}$$

Ideta, Shinichiro*; Murai, Naoki; Nakajima, Masamichi*; Kajimoto, Ryoichi; Tanaka, Kiyohisa*

Physical Review B, 100(23), p.235135_1 - 235135_7, 2019/12

https://doi.org/10.1103/PhysRevB.100.235135
 Times Cited Count：3 Percentile：16.31(Materials Science, Multidisciplinary)

Journal Articles

Effect of electron correlations on spin excitation bandwidth in Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ as seen via time-of-flight inelastic neutron scattering

Murai, Naoki; Suzuki, Katsuhiro*; Ideta, Shinichiro*; Nakajima, Masamichi*; Tanaka, Kiyohisa*; Ikeda, Hiroaki*; Kajimoto, Ryoichi

Physical Review B, 97(24), p.241112_1 - 241112_6, 2018/06

https://doi.org/10.1103/PhysRevB.97.241112
 Times Cited Count：5 Percentile：25.82(Materials Science, Multidisciplinary)

We use inelastic neutron scattering (INS) to investigate the effect of electron correlations on spin dynamics in iron-based superconductor Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$. Our INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A first principles analysis of dynamical spin susceptibility, incorporating the mass renormalization factor of 3, as determined by angle-resolved photoemission spectroscopy, provides a reasonable description of the observed spin excitations. This analysis shows that electron correlations in the Fe-3d bands yield enhanced effective electron masses, and consequently, induce substantial narrowing of the spin excitation bandwidth. Our results highlight the importance of electron correlations in an itinerant description of the spin excitations in iron-based superconductors.

Journal Articles

Effect of magnetism on lattice dynamics in SrFe$$_2$$As$$_2$$ using high-resolution inelastic X-ray scattering

Murai, Naoki*; Fukuda, Tatsuo; Kobayashi, Tatsuya*; Nakajima, Masamichi*; Uchiyama, Hiroshi*; Ishikawa, Daisuke*; Tsutsui, Satoshi*; Nakamura, Hiroki; Machida, Masahiko; Miyasaka, Shigeki*; et al.

Physical Review B, 93(2), p.020301_1 - 020301_5, 2016/01

https://doi.org/10.1103/PhysRevB.93.020301
 Times Cited Count：7 Percentile：34.12(Materials Science, Multidisciplinary)

Journal Articles

Strong carrier-scattering in iron-pnictide superconductors ${it Ln}$FeAsO$$_{1-y}$$ (${it Ln}$ = La and Nd) obtained from charge transport experiments

Ishida, Shigeyuki*; Nakajima, Masamichi*; Tomioka, Yasuhide*; Ito, Toshimitsu*; Miyazawa, Kiichi*; Kito, Hijiri*; Lee, C.-H.*; Ishikado, Motoyuki; Shamoto, Shinichi; Iyo, Akira*; et al.

Physical Review B, 81(9), p.094515_1 - 094515_6, 2010/03

https://doi.org/10.1103/PhysRevB.81.094515
 Times Cited Count：21 Percentile：64.67(Materials Science, Multidisciplinary)

Characteristic normal-state charge transport is found in the oxygen-deficient iron-arsenides ${it Ln}$FeAsO$$_{1-y}$$ (${it Ln}$: La and Nd) with the highest ${it T}$ $$_{c}$$'s among known Fe-based superconductors. The effect of "doping" in this system is mainly on the carrier scattering, quite distinct from that in high-${it T}$ $$_{c}$$ cuprates. In the superconducting regime of the La system with maximum ${it T}$ $$_{c}$$ = 28 K, the low-temperature resistivity is dominated by a ${it T}$ $$^{2}$$ term. On the other hand, in the Nd system with ${it T}$ $$_{c}$$ higher than 40 K, the carriers are subject to stronger scattering showing ${it T}$ -linear resistivity and small magnetoresistance. Such strong scattering appears crucial for high-${it T}$ $$_{c}$$ superconductivity in the iron-based system.

Oral presentation

Effect of magnetism on phonon dispersion of SrFe$$_2$$As$$_2$$ via inelastic X-ray scattering

Murai, Naoki*; Fukuda, Tatsuo; Uchiyama, Hiroshi*; Tsutsui, Satoshi*; Ishikawa, Daisuke*; Kobayashi, Tatsuya*; Nakamura, Hiroki; Machida, Masahiko; Nakajima, Masamichi*; Miyasaka, Shigeki*; et al.

no journal, , 

no abstracts in English

Oral presentation

Magneto-elastic coupling in detwinned Sr(Fe$$_{1-x}$$Co$$_x$$)$$_2$$As$$_2$$ by inelastic X-ray scattering

Murai, Naoki*; Fukuda, Tatsuo; Nakajima, Masamichi*; Kobayashi, Tatsuya*; Uchiyama, Hiroshi*; Tsutsui, Satoshi*; Ishikawa, Daisuke*; Nakamura, Hiroki; Machida, Masahiko; Miyasaka, Shigeki*; et al.

no journal, , 

Oral presentation

Effect of magnetism on lattice dynamics of SrFe$$_{2}$$As$$_{2}$$ via high-resolution inelastic X-ray scattering

Murai, Naoki; Fukuda, Tatsuo; Kobayashi, Tatsuya*; Nakajima, Masamichi*; Uchiyama, Hiroshi*; Ishikawa, Daisuke*; Tsutsui, Satoshi*; Nakamura, Hiroki; Machida, Masahiko; Miyasaka, Shigeki*; et al.

no journal, , 

We report the first IXS study of single crystals of SrFe$$_{2}$$As$$_{2}$$ that have been detwinned via application of in-plane uniaxial stress. Our results clearly show anisotropy in phonon structure below $$it T_{s}$$ = $$it T_{N}$$ characterized by mode splitting at tetragonally-equivalent momentum transfers. To the best of our knowledge, this is the first observation of phonon anisotropy in iron-pnictides at finite momentum transfers. We obtain good agreement with our results using calculations that are modified to account for a reduced magnetic moment and fluctuating magnetism. Previous efforts using either non-magnetic calculations or static magnetism tend to either get the dispersion wrong from non-magnetic calculations or strongly over-estimate the phonon mode splitting when static magnetism is included. Our model for the response below $$it T_{N}$$ incorporates a phenomenological reduction in force constant anisotropy, by about 65%, that can be attributed to residual fast magnetic fluctuations. Above $$it T_{N}$$, all fluctuations are assumed to be fast and the response becomes the average of the results from the static magnetism. This serves as a starting point for a general model of phonons in iron-pnictides applicable to both non-magnetic and magnetic phases.

Oral presentation

Two distinct resonance-like excitations in underdoped Ba_${1-x}$K_${x}$Fe_${2}$As_${2}$ (x = 0.25) as seen via inelastic neutron scattering

Murai, Naoki; Kajimoto, Ryoichi; Ikeuchi, Kazuhiko*; Tanaka, Kiyohisa*; Nakajima, Masamichi*; Ideta, Shinichiro*; Suzuki, Katsuhiro*

no journal, , 

We report an inelastic neutron scattering (INS) study of underdoped Ba_${1-x}$K_${x}$Fe_${2}$As_${2}$ that shows the presence of double-resonance at the same commensurate wave vector in the superconducting (SC) state. By comparing our INS data with the SC gap structure of the same crystal, as measured by angle-resolved photoemission spectroscopy (ARPES), we show that the observed double-resonance feature originates neither from SC gap anisotropy nor from multiple SC gaps on different Fermi-surface sheets. This suggests that the double-resonance feature in iron-pnictides cannot be interpreted within a spin-exciton scenario.

Oral presentation

Large downward renormalization of spin excitation energies in Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ due to electron correlation effects

Murai, Naoki; Kajimoto, Ryoichi; Suzuki, Katsuhiro*; Ideta, Shinichiro*; Nakajima, Masamichi*; Ikeda, Hiroaki*; Tanaka, Kiyohisa*

no journal, , 

Using inelastic neutron scattering (INS) and angle-resolved photoemission spectroscopy (ARPES), we investigate the spin dynamics of iron-based superconductor Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$. The INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A random phase approximation (RPA) treatment of the dynamical spin susceptibility, incorporating the band renormalization factor of 3 derived from the ARPES measurements on the same crystals, provides a reasonable description of the observed spin excitations. This analysis shows that the experimental spin excitation peak lies at a much lower energy than the bare value, reflecting a strong renormalization of the quasiparticle band dispersion near the Fermi level due to electron correlation effect. The present results point to a unified framework that connects the magnetic response to the underlying electronic structure of the materials.

Oral presentation

Large downward renormalization of spin excitation energies in Ba$$_{1-x}$$K$$_{x}$$Fe$$_{2}$$As$$_{2}$$ due to electron correlation effects

Murai, Naoki; Kajimoto, Ryoichi; Suzuki, Katsuhiro*; Ikeda, Hiroaki*; Nakajima, Masamichi*; Ideta, Shinichiro*; Tanaka, Kiyohisa*

no journal, , 

no abstracts in English

Oral presentation

Large downward renormalization of spin excitation energies in Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ due to electron correlation effects

Murai, Naoki; Kajimoto, Ryoichi; Suzuki, Katsuhiro*; Nakajima, Masamichi*; Ikeda, Hiroaki*; Ideta, Shinichiro*; Tanaka, Kiyohisa*

no journal, , 

We report inelastic neutron scattering (INS) study of hole-doped iron-based superconductor Ba$$_{1-x}$$K$$_{x}$$Fe$$_{2}$$As$$_{2}$$ (x = 0.25). The INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A random phase approximation (RPA) treatment of the dynamical spin susceptibility, incorporating the band renormalization factor of 3 derived from angle-resolved photoemission spectroscopy (ARPES) measurements on the same sample, provides a reasonable description of the observed spin excitations. This analysis shows that the Fe-3d bandwidth narrowing due to electron correlation is directly reflected in the spin excitation energy scale. We will discuss the current state of our analysis relating the magnetic response to the underlying electronic structure of the materials.

Oral presentation

Effect of electron correlations on spin excitation bandwidth in Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ as seen via time-of-flight inelastic neutron scattering

Murai, Naoki; Kajimoto, Ryoichi; Suzuki, Katsuhiro*; Ikeda, Hiroaki*; Nakajima, Masamichi*; Ideta, Shinichiro*; Tanaka, Kiyohisa*

no journal, , 

We use inelastic neutron scattering (INS) to investigate the effect of electron correlations on spin dynamics in iron-based superconductor Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$. Our INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A first principles analysis of dynamical spin susceptibility, incorporating the mass renormalization factor of 3, as determined by angle-resolved photoemission spectroscopy, provides a reasonable description of the observed spin excitations. This analysis shows that electron correlations in the Fe-3d bands yield enhanced effective electron masses, and consequently, induce substantial narrowing of the spin excitation bandwidth. Our results highlight the importance of electron correlations in an itinerant description of the spin excitations in iron-based superconductors.

Oral presentation

Three-dimensional electronic structure and multiple superconducting gaps of Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ as seen via time-of-flight neutron spectroscopy

Kajimoto, Ryoichi; Kofu, Maiko; Kawamura, Seiko; Suzuki, Katsuhiro*; Nakajima, Masamichi*; Murai, Naoki

no journal, , 

The spin dynamics of iron-based superconductors is believed to play an essential role in the pairing mechanism, but realistic modeling of magnetic neutron scattering spectra in these materials remains challenging owing to their complex multi-band structure. Here we report the results of inelastic neutron scattering measurements of Ba$$_{0.75}$$K$$_{0.25}$$Fe$$_{2}$$As$$_{2}$$ that provide a benchmark of an ab-initio approach of the dynamical spin susceptibility. Our main results are two-fold. First, the magnetic neutron scattering response exhibits periodic intensity modulation along the out-of-plane direction. The observed periodicity corresponds to the out-of-plane warping of Fermi surfaces, revealing the importance of nesting in three-dimensional momentum space. Second, our careful measurements reveal the existence of the theoretically predicted multiple resonant modes that result from the multiple superconducting gaps. The agreement between theory and experiment in the present work highlights the potential of neutron spectroscopy to investigate the complex multi-band structure of iron-based superconductors.

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