R. H. Yuan

Beijing Information Science & Technology University

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Tao Dong

University of Michigan–Ann Arbor

Zhiguo Chen

Institute of Physics

Buwen Cheng

Chinese Academy of Sciences

Nanlin Wang

Beijing Academy of Quantum Information Sciences

P. Zheng

Chinese Academy of Sciences

Bin Hu

Chinese Academy of Sciences

D. Casa

Advanced Photon Source

A. Lupascu

University of Toronto

Ignace Jarrige

Brookhaven National Laboratory

Young‐June Kim

University of Toronto

Cooperative Institutions

Chinese Academy of Sciences

Institute of Physics

University of Chinese Academy of Sciences

National Laboratory for Superconductivity

Czech Academy of Sciences, Institute of Physics

Brookhaven National Laboratory

Songshan Lake Materials Laboratory

Peking University

Argonne National Laboratory

University of Toronto

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High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy

Journal of Physics Condensed Matter (2012)

N L Wang W. Z. Hu Zhiguo Chen R. H. Yuan Gang Li

We report optical spectroscopic measurements on electron- and hole-doped BaFe2As2. We show that the compounds in the normal state are not simple metals. The optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at rather high energy scale near 0.6 eV. This suppression evolves with the As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that the feature is caused by the strong Hund's rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture about the metallic magnetism in the materials.

Pseudogap

Optical conductivity

Magnetism

10.1088/0953-8984/24/29/294202

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Citations (43)

Optical spectroscopy studies on FeTe_1−xSe_xandA_xFe_2−ySe₂(A= K, Rb, Cs): A brief overview

Chinese Physics B (2013)

R. H. Yuan Nanlin Wang

In this short overview, we summarize the optical spectroscopy studies on iron selenide superconducting systems FeTe1−xSex and A−xFe2−ySe2. We elaborate that optical spectroscopy measurements yield fruitful information about the band structure evolution across the AFM phase transition temperature, the electronic correlation effect, the superconducting pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.

Selenide

10.1088/1674-1056/22/8/087404

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Structural phase transition below 250 K in superconducting K0.75Fe1.75Se2

Physical Review B (2012)

Alexander Ignatov Anil Kumar Phil Lubik R. H. Yuan Weihua Guo

Vibrational properties of iron-chalcogenide superconductor K${}_{0.75}$Fe${}_{1.75}$Se${}_{2}$ with ${T}_{c}\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}$ 30 K have been measured by Raman and optical spectroscopies over the temperature range 3--300 K. The sample undergoes $I4/m\ensuremath{\rightarrow}I4$ structural phase transition accompanied by loss of inversion symmetry at ${T}_{1}$, below 250 K, observed as the appearance of new fully symmetric Raman mode at $\ensuremath{\sim}$165 cm${}^{\ensuremath{-}1}$. Small vibration mode anomalies are also observed at ${T}_{2}\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}$ 160 K. From first-principles vibrational analysis of antiferromagnetic K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ utilizing pseudopotentials all observed Raman and infrared modes have been assigned and the displacement patterns of the new Raman mode identified as involving predominantly the Se atoms.

Atmospheric temperature range

10.1103/physrevb.86.134107

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Nanoscale phase separation of antiferromagnetic order and superconductivity in K0.75Fe1.75Se2

Scientific Reports (2012)

R. H. Yuan Tao Dong Y. J. Song P. Zheng G. F. Chen

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Furthermore, the feature could be noticeably suppressed and shifted to lower frequency by a moderate magnetic field. Our analysis indicates that this edge structure arises from the development of a Josephson-coupling plasmon in the superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale phase separation between superconductivity and magnetism. The results also enable us to understand various seemingly controversial experimental data probed from different techniques.

Selenide

Magnetism

10.1038/srep00221

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Citations (104)

Resonant inelastic x-ray scattering study of electronic excitations in insulatingK0.83Fe1.53Se2

Physical Review B (2015)

H. Gretarsson T. Nomura Ignace Jarrige A. Lupascu M. H. Upton

We report an Fe $K$-edge resonant inelastic x-ray scattering study of ${\mathrm{K}}_{0.83}{\mathrm{Fe}}_{1.53}{\mathrm{Se}}_{2}$. This material is an insulator, unlike many parent compounds of iron-based superconductors. We found a sharp excitation around 1 eV, which is resonantly enhanced when the incident photon energy is tuned near the pre-edge region of the absorption spectrum. The spectral weight and lineshape of this excitation exhibit clear momentum dependence. In addition, we observe momentum-independent broad interband transitions at higher excitation energies of 3--7 eV. Calculations based on a 70-band $dp$ orbital model, using a moderate ${U}_{\mathrm{eff}}\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, indicate that the $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ feature originates from the correlated Fe $3d$ electrons, with a dominant ${d}_{xz}$ and ${d}_{yz}$ orbital character. We find that a moderate ${U}_{\mathrm{eff}}$ yields a satisfying agreement with the experimental spectra, suggesting that the electron correlations in the insulating and metallic iron-based superconductors are comparable.

Absorption edge

Photon energy

10.1103/physrevb.91.245118

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Nanoscale stripe-type phase separation of antiferromagnetic order and superconductivity in K$_{0.75}$Fe$_{1.75}$Se$_2$

arXiv (Cornell University) (2011)

R. H. Yuan Tao Dong Y. J. Song G. F. Chen Jiayuan Hu

We report an in-plane optical spectroscopy study on the newly discovered iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge at very low frequency below T$_c$ on the relatively incoherent electronic background. The phenomenon was explained as arising from the Josephson-coupling plasmon of superconducting condensate. Together with the transmission electron microscopy analysis, our study yields compelling evidence for the presence of nanoscale stripe-type phase separation between superconductivity and magnetism.

Magnetism

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Coexistence and competition of multiple charge-density-wave orders in rare-earth tritellurides

Physical Review B (2014)

Bin Hu Buwen Cheng R. H. Yuan Tao Dong Nanlin Wang

A family of quasi-two-dimensional earth telluride compounds exhibits a rich phase diagram. By measuring optical conductivity researchers now report the observation of a third, previously unknown charge-density wave order.

Charge Density Wave

10.1103/physrevb.90.085105

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Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using x-ray emission spectroscopy

Bulletin of the American Physical Society (2012)

H. Gretarsson A. Lupascu Young‐June Kim Wenlong Wu S. R. Julian

Magnetism

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Observation of a small superconducting energy gap in K$_{0.7}$Fe$_{1.8}$Se$_2$ by optical spectroscopy

arXiv (Cornell University) (2011)

R. H. Yuan Tao Dong G. F. Chen J. B. He D. M. Wang

Selenide

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Citations (1)

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

Physical Review B (2012)

P. Zheng Y. G. Shi Quansheng Wu Gang Xu Tao Dong

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A \lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time \tau are observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

Ferrimagnetism

10.1103/physrevb.86.195108

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Citations (6)