T. Dong

Structural phase transition in IrTe2: A combined study of optical spectroscopy and band structure calculations

Ir1−xPtxTe2 is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe2 in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (px, py) and Te pz bands.

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.

Optical spectroscopy study of Nd(O,F)BiS2 single crystals

We present an optical spectroscopy study on F-substituted NdOBiS2 superconducting single crystals grown using the KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 cm(-1). The plasma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of x = 0.5, but very close to the value based on a doping level of 7% of itinerant electrons per Bi site as determined by the angle-resolved photoemission spectroscopy (ARPES) experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a charge-density wave (CDW) instability as being widely discussed for a doping level of x = 0.5.

Revealing multiple charge-density-wave orders in TbTe3 by optical conductivity and ultrafast pump-probe experiments

Rare-earth tri-tellurium RTe3 is a typical quasi-two-dimensional system which exhibits obvious charge-density-wave (CDW) orders. So far, RTe3 with heavier R ions (Dy, Ho, Er, and Tm) are believed to experience two CDW phase transitions, while the lighter ones only hold one. TbTe3 is claimed to belong to the latter. However, in this work we present evidences that TbTe3 also possesses more than one CDW order. Aside from the one at 336 K, which was extensively studied and reported to be driven by imperfect Fermi surface nesting with a wave vector q = (2/7c*), a new CDW energy gap (260 meV) develops at around 165 K, revealed by both infrared reflectivity spectroscopy and ultrafast pump-probe spectroscopy. More intriguingly, the origin of this energy gap is different from the second CDW order in the heavier R ions-based compounds RTe3 (R = Dy, Ho, Er, and Tm).

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.

Coexistence of superconductivity and density wave in Ba2Ti2Fe2As4O: An optical spectroscopy study

We performed an optical spectroscopy measurement on single crystals of Ba2Ti2Fe2As4O, which is a newly discovered superconductor showing a coexistence of superconductivity and density wave orders. The study reveals spectral changes associated with both density wave and superconductivity phase transitions. The density wave phase transition at T-DW approximate to 125 K leads to the reconstruction of Fermi surfaces which removes about half of Drude spectral weight. The ratio of 2 Delta(DW)/k(B)T(DW) approximate to 11.9 is considerably larger than the mean-field value based on the weak-coupling BCS theory. At the lowest temperature in the superconducting state, further spectral change associated with the superconducting condensate is observed. The low frequency optical conductivity could be well modeled within the Mattis-Bardeen approach with two isotropic gaps of Delta(1)(0) = 3.4 meV and Delta(2)(0) = 7.9 meV. The superconducting properties of the Ba2Ti2Fe2As4O compound are similar to those of BaFe1.85Co0.15As2.