R. H. Yuan

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

We report an in-plane optical spectroscopy study on the iron-selenide superconductor K0.75Fe1.75Se2. The measurement revealed the development of a sharp reflectance edge below Tc 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.

Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K0.83Fe1.53Se2

We report an infrared spectroscopy study on K0.83Fe1.53Se2, a semiconducting parent compound of the 245 iron selenide superconductors. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double-peak structure between 4000 and 6000 cm(-1) and abundant phonon modes-much more than expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron-diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites.

Formation of partial energy gap below the structural phase transition and the rare-earth element-substitution effect on infrared phonons in ReFeAsO (Re=La, Nd, and Sm)

Single crystals of LaFeAsO, NdFeAsO, and SmFeAsO have been prepared by means of a NaAs flux growth technique and studied by optical spectroscopy measurements. We show that the spectral features corresponding to the partial energy gaps in the spin-density-wave (SDW) state are present below the structural phase transition. This indicates that the electronic state below the structural phase transition is already very close to that in the SDW state. We also show that in-plane infrared phonon modes display systematic shifts toward high frequency upon rare-earth element Nd and Sm substitutions for La, suggesting a strong enhancement of the bonding strength. Furthermore, an asymmetric line shape of the in-plane phonon mode is observed, yielding evidence for the presence of electron-phonon coupling in Fe pnictides.

Optical spectroscopy of single-crystalline LaFeAsO

Millimeter-sized single crystals of LaFeAsO were grown from NaAs flux and the in-plane optical properties were studied over a wide frequency range. A sizable electronic correlation effect was indicated from the analysis of the free-carrier spectral weight. With decreasing temperature from 300 K, we observed a continuous suppression of the spectral weight near 0.6 eV. But a spin-density-wave gap formation at lower energy scale was seen only in the broken-symmetry state. We elaborate that both the itinerancy and local-spin interactions of Fe 3d electrons are present for the FeAs-based systems; however, the establishment of the long-range magnetic order at low temperature has a dominantly itinerant origin.

Field-induced spin-flop transitions in single-crystalline CaCo2As2

CaCo2As2, a ThCr2Si2-structure compound, undergoes an antiferromagnetic transition at T-N = 76 K with the magnetic moments being aligned parallel to the c axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo2As2 is much weaker compared to the parent compounds of iron pnictide. Applying magnetic field along the c axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with H parallel to c and H parallel to ab at 10 K indicate that the antiferromagnetic coupling along the c direction is very weak. The interlayer antiferromagntic coupling constant J(c) is estimated to be about 2 meV.

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

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 fairly 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 mainly caused by the strong Hunds rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. The coupling strength changes with the environment of the Fe atom. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture of the metallic magnetism in the materials.

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

The occurrences of collective quantum states, such as superconductivity (SC) and charge-or spin-density waves (CDWs or SDWs), are among the most fascinating phenomena in solids. To date, much effort has been made to explore the interplay between different orders, yet little is known about the relationship of multiple orders of the same type. Here we report optical spectroscopy study on CDWs in the rare-earth tritelluride compounds RTe3 (R = rare-earth elements). Besides the prior reported two CDW orders, the study reveals unexpectedly the presence of a third CDW order in the series, which evolves systematically with the size of R element. With increased chemical pressure, the first and third CDW orders are both substantially suppressed and compete with the second one by depleting the low-energy spectral weight. A complete phase diagram for the multiple CDW orders in this series is established.

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements, and the first-principles 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, a small amount of itinerant carriers with short life time tau is observed, which is gapped at 20 K with a direct gap of 0.24 eV. Our first-principles calculations indicate that the antiferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t(2g) bands and push them away from Fermi level (E-F). On the other hand, a noncollinear magnetic interaction is needed to push the Os(4c) bands away from E-F, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and noncollinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

In-plane optical spectroscopy study on FeSe epitaxial thin film grown on SrTiO3 substrate

We perform in-plane optical spectroscopy measurement on (001) FeSe thin film grown on SrTiO3 substrate by the pulsed laser deposition method. The study indicates that the low-frequency conductivity consists of two Drude components: a broad one which takes up most of the spectral weight and a very narrow one roughly below 100-150 cm(-1). The narrow Drude component locates at so low frequencies that no such behavior was observed in iron pnictides. The overall plasma frequency is smaller than that of the FeAs-based compounds, suggesting a stronger correlation effect. Similar to iron pnictides, a temperature-induced spectral weight transfer is observed for FeSe. However, the relevant energy scale is smaller. Additionally, different from a recent angle-resolved photoemission spectroscopy measurement which revealed a spin density wave (SDW) order at low temperature for FeSe thin films grown on SrTiO3 substrate, no signature of the band structure reconstruction arising from the formation of the SDW order is seen by optical measurement in the thick FeSe films.

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

Vibrational properties of iron-chalcogenide superconductor K0.75Fe1.75Se2 with T-c similar to 30 K have been measured by Raman and optical spectroscopies over the temperature range 3-300 K. The sample undergoes I4/m -> 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 similar to 165 cm(-1). Small vibration mode anomalies are also observed at T-2 similar to 160 K. From first-principles vibrational analysis of antiferromagnetic K0.8Fe1.6Se2 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.