Coexistence and competition of multiple charge-density-wave orders in rare-earth tri-telluride RTe3

The occurrences of collective quantum states, such as superconductivity (SC) and charge- or spin-densitywaves (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 tri-telluride 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.

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

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

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

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.

Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K$_{0.83}$Fe$_{1.53}$Se$_2$

We report an infrared spectroscopy study on K$_{0.83}$Fe$_{1.53}$Se$_2$, a semiconducting parent compound of the new iron-selenide system. 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 4000-6000 cm$^{-1}$ and abundant phonon modes much more than those 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.

Optical spectroscopy study on CeTe$_3$: evidence for multiple charge-density-wave orders

We performed optical spectroscopy measurement on single crystal of CeTe$_3$, a rare-earth element tri-telluride charge density wave (CDW) compound. The optical spectra are found to display very strong temperature dependence. Besides a large and pronounced CDW energy gap being present already at room temperature as observed in earlier studies, the present measurement revealed the formation of another energy gap at smaller energy scale at low temperature. The second CDW gap removes the electrons near E$_F$ which undergo stronger scattering. The study yields evidence for the presence of multiple CDW orders or strong fluctuations in the light rare-earth element tri-telluride.

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 towards high frequency upon rare-earth element substitutions for La, suggesting a strong enhancement of the bonding strength. Furthermore, an asymmetric line-shape of the in-plane phonon mode is observed, implying the presence of an electron-phonon coupling effect in Fe-pnictides.