Shi Xun

Surface State Bands in Superconducting (PtxIr1−x)Te2*

Angle-resolved photoemission spectroscopy is performed to study the bulk and surface electronic structures of nonsuperconducting IrTe2 and superconducting Pt0.05Ir0.95Te2. In addition to the bulk electronic bands predicted by the local density approximation calculations, we observe two Dirac cone-like bands at the Brillouin zone center, which are non-dispersive along kz, suggesting that the extra bands are surface state bands. As the experimental results are well consistent with the ab initio calculations of surface states, the parity analysis proves that these surface state bands are topologically trivial and thus exclude (PtxIr1−x)Te2 as a possible topological superconductor candidate.

Filled skutterudites: from single to multiple filling

Research on traditional thermoelectric (TE) materials with narrow bandgap have made great progress in recent years. Here, we primarily reviewed the study on CoSb3 skuttuerdites with intrinsic crystal voids that could be partially filled with various filler atoms for TE performance optimization. By combining experimental measurement of electrical/thermal transport properties and theoretical study of electronic structure and structural stability, we analyzed the fundamentals in understanding the filling behavior of fillers atoms in the voids, and provided a few guidance rules in simultaneously optimizing electrical and thermal transport properties for thermoelectric performance optimization. By following those rules, nearly independent tunning of electron and phonon transports can be achieved in these caged compounds. As a result, these partially filled skutterudites basically satisfy the picture of the ‘phonon glass-electron crystal’ paradigm, and TE figure of merit ( ZT ) was pushed up to reach 1.7@850 K, highest among all reported bulk TE materials. The general discussions could also provide guidance in searching for novel thermoelectric materials.

An Alternating-Current Voltage Modulated Thermal Probe Technique for Local Seebeck Coefficient Characterization

An ac voltage-modulated thermal probe technique based on the atomic force microscope is developed to measure local Seebeck coefficients (S) of thermoelectric bulk and films. The characterization principle is based on the strictly quadratic relationship between the excited local dc Seebeck voltage and the applied ac voltage at high frequency. Excellent agreement is found between local S values and their corresponding macro-S values of thermoelectric bulk and thin films. This thermoelectric probe technique provides a very convenient, promising tool for local thermoelectric parameters with sub-micrometer scale resolution.

Experimental Investigation of the Electronic Structure of Ca 0.83 La 0.17 Fe 2 As 2

We performed a combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy study of the electronic structure of electron-doped Ca0.83La0.17Fe2As2. A surface reconstruction associated with the dimerization of As atoms is observed directly in the real space, as well as the consequent band folding in the momentum space. Besides this band folding effect, the Fermi surface topology of this material is similar to that reported previously for BaFe1.85Co0.15As2, with Gamma-centered hole pockets quasi-nested to M-centered electron pockets by the antiferromagnetic wave vector. Although no superconducting gap is observed by ARPES possibly due to low superconducting volume fraction, a gap-like density of states depression of 7.7 +/- 2.9 meV is determined by scanning tunneling microscopy.

高填充量Ba x Eu y Co 4 Sb 12 方钴矿热电性能研究

利用熔融法和等离子放电烧结(SPS)制备单相双原子填充Ba x Eu y Co 4 Sb 12 方钴矿材料并测试其高温热电性能。实验发现, 在高填充量下( x + y >40%), 材料在高温时具有高的功率因子(>60 W/(cm•K 2 ))。在方钴矿的晶格空洞中同时引入Ba和Eu两种填充原子, 能增强晶格声子散射, 从而大幅降低方钴矿的晶格热导。实验证实, Ba x Eu y Co 4 Sb 12 体系的晶格热导显著降低, 其室温晶格热导最低达1.7 W/(m•K)。与此对应的是双原子填充Ba x Eu y Co 4 Sb 12 方钴矿材料的热电优值( ZT 值)明显增大, 其中Ba 0.19 Eu 0.23 Co 4 Sb 12 的 ZT 值在850 K时达到了1.3。