Anmin Zhang

Superconductivity at 44 K in K intercalated FeSe system with excess Fe

We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 Å. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.

Resonance Raman scattering in bulk 2H-MX2 (M = Mo, W; X = S, Se) and monolayer MoS2

We have performed a comparative study of resonance Raman scattering in transition-metal dichalcogenides 2H-MX2 semiconductors (M = Mo, W; X = S, Se) and single-layer MoS2. Raman spectra were collected using excitation wavelengths 633 nm (1.96 eV), 594 nm (2.09 eV), 532 nm (2.33 eV), 514 nm (2.41 eV), and 488 nm (2.54 eV). In bulk-MoS2 and WS2, the resonant energies appear to coincide with their exciton excitations. The resonance can be fine tuned by varying sample temperatures, which confirms its excitonic origin in both MoS2 and WS2. Temperature dependence of Raman intensities is analyzed in the context of resonance Raman theory, which agrees well with the existing absorption data. While in WSe2, the resonance has been observed in a wider range of excitations from 633 to 514 nm, which cannot be explained with its excitonic energies of 1.6 and 2.0 eV. It is considered that additional excitonic bands induced by band splitting are involved in the inter-band transitions and substantially extend the resonance energy range. The Raman resonance energy range remains unchanged in single-layer MoS2 compared with that in the bulk sample. However, most phonon modes in single-layer MoS2 are significantly broadened or strongly suppressed under resonance conditions. This change could be related to the modification of acoustic modes by the substrate.

Ultralow-Frequency Collective Compression Mode and Strong Interlayer Coupling in Multilayer Black Phosphorus

The recent renaissance of black phosphorus (BP) as a two-dimensional (2D) layered material has generated tremendous interest, but its unique structural characters underlying many of its outstanding properties still need elucidation. Here we report Raman measurements that reveal an ultralow-frequency collective compression mode (CCM) in BP, which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling, and this result is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than previously achieved by using high-frequency Raman modes. These findings offer fundamental insights and practical tools for further exploration of BP as a highly promising new 2D semiconductor.

Raman spectra in iron-based quaternary CeO1-xFxFeAs and LaO1-xFxFeAs

Raman spectra have been measured on iron-based quaternary CeO1?xFxFeAs and LaO1?xFxFeAs with different amounts of fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on first-principle calculations, the observed phonon modes can be assigned accordingly. In LaO1?xFxFeAs, the Eg and A1g modes related to the vibrations of La are suppressed with increasing F doping. However, F doping only has a small effect on the Eg and A1g modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO1?xFxFeAs, and the Eg mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450?cm?1 and shows a very sharp peak. The electronic scattering background is low, and electron?phonon coupling is not evident for the observed phonon modes. Three features are found above 500?cm?1, which may be associated with multi-phonon processes. Nevertheless, it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals, considering their energies.

Raman phonons of α -FeTe and Fe 1.03 Se 0.3 Te 0.7 single crystals

The polarized Raman scattering spectra of nonsuperconducting

Effects on superconductivity of transition-metal doping in FeSe0.5Te0.5

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Two-magnon Raman scattering in A(0.8)Fe(1.6)Se(2) systems (A = K, Rb, Cs, and Tl): Competition between superconductivity and antiferromagnetic order

-FeTe and of the newly discovered, As-free superconductor Fe

Interplay of Dirac electrons and magnetism in CaMnBi2 and SrMnBi2

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Vacancy ordering and phonon spectrum of the iron-based superconductor K0.8Fe1.6Se2

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Giant magneto-optical Raman effect in a layered transition metal compound

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