Large inverse and normal magnetocaloric effects in HoBi compound with nonhysteretic first-order phase transition

HoBi single crystal and polycrystalline compounds with NaCl-type structure are successfully obtained, and their magnetic and magnetocaloric properties are studied in detail. With temperature increasing, HoBi compound undergoes two magnetic transitions at 3.7 K and 6 K, respectively. The transition temperature at 6 K is recognized as an antiferromagnetic-to-paramagnetic (AFM-PM) transition, which belongs to the first-order magnetic phase transition (FOMT). It is interesting that the HoBi compound with FOMT exhibits good thermal and magnetic reversibility. Furthermore, a large inverse and normal magnetocaloric effect (MCE) is found in HoBi single crystal in the $H|| [100]$ direction, and the positive $\Delta S_{\rm M}$ peak reaches 13.1 J/kg$\cdot$K under a low field change of 2 T and the negative $\Delta S_{\rm M}$ peak arrives at $-18 $ J/kg$\cdot$K under a field change of 5 T. These excellent properties are expected to be applied to some magnetic refrigerators with special designs and functions.     

Mn、Ti掺杂对NaxCoO2体系超导电性和电荷有序化影响

本文研究了Mn、Ti掺杂对Na0.3Co1-xMxO2·1.3H2O超导电性和 Na0.5Co1-xMxO2材料电荷有序现象的影响.我们成功合成了Na0.7Co1-xMxO2(M= Mn和Ti,0≤x≤0.1)系列样品,并采用不同强度氧化剂氧化脱钠,分别获得Na0.5Co1-xMxO2和水合物Na0.3Co1-xMxO2·1.3H2O (0≤x≤0.1, M= Mn和Ti).X射线衍射分析表明所有样品都是比较好的单相,它们的晶格常数随着Ti和Mn的含量发生系列变化.物性检测显示Na0.3Co1-xMxO2·1.3H2O的超导电性随着Mn和Ti掺杂量的增加明显减弱,在Mn掺杂为x=0.02 或Ti掺杂为x=0.01时其超导电性消失;磁性检测发现随着掺杂量的增加,Na0.5Co1-xMxO2电荷有序态被逐渐破坏,其中掺Mn比掺Ti对电荷有序化行为的影响更为明显.     

Fe0.5Cu0.5BaSrYCu2Oy化合物的结构和超导电性研究

实验采用常压和高压分别制备出含铁的FexCu1-xBaSrYCu2Oy（x= 0～1）系列化合物，本文主要研究x=0.5时样品的结构和超导电性.测量结果显示，高压合成的样品均具有超导电性，对于x=0.5的样品超导转变温度Tc(onset)～57K， Tc(0)～40K，而常压合成的样品当Fe含量x>0.3时均不超导.为此，我们利用透射电子显微镜（TEM）研究了该体系的微观结构特性，并通过电子能量损失谱(EELS)揭示出高压合成导致样品中载流子浓度明显高于常压样品，证明样品制备过程中高压有利于超导电性的形成.同时对高压样品中的缺陷和局域晶体结构畸变进行了深入分析.     

Machine Learning to Instruct Single Crystal Growth by Flux Method

Growth of high-quality single crystals is of great significance for research of condensed matter physics. The exploration of suitable growing conditions for single crystals is expensive and time-consuming, especially for ternary compounds because of the lack of ternary phase diagram. Here we use machine learning(ML) trained on our experimental data to predict and instruct the growth. Four kinds of ML methods, including support vector machine(SVM), decision tree, random forest and gradient boosting decision tree, are adopted. The SVM method is relatively stable and works well, with an accuracy of 81% in predicting experimental results. By comparison,the accuracy of laboratory reaches 36%. The decision tree model is also used to reveal which features will take critical roles in growing processes.     

钠钴氧水合物新型超导体研究的新进展

文章介绍了Na0 .3 CoO2 ·1.3H2 O新型层状超导体的合成、超导电性及晶体结构等研究的新进展 .结构分析表明 ,这种超导体的晶格具有六方对称性 ,其空间对称群为P6 3 /mmc.电子显微镜观察清楚地显示 ,沿c轴方向超导材料呈现层状结构特征 ,并发现在Co-O面上存在一个超结构 .由电子能量损失谱分析得知 ,Co离子在Na0 .3CoO2 ·1.3H2 O超导体中的价态范围为 +3.6— +3.8.     

Resistance Anomaly and Linear Magnetoresistance in Thin Flakes of Itinerant Ferromagnet Fe3GeTe2

Research interests in recent years have expanded into quantum materials that display novel magnetism incorporating strong correlations, topological effects, and dimensional crossovers. Fe₃GeTe₂ represents such a twodimensional van der Waals platform exhibiting itinerant ferromagnetism with many intriguing properties. Up to date, most electronic transport studies on Fe₃GeTe₂ have been limited to its anomalous Hall responses while the longitudinal counte...     

Moiré superlattice modulations in single-unit-cell FeTe films grown on NbSe2 single crystals

Interface can be a fertile ground for exotic quantum states, including topological superconductivity, Majorana mode, fractal quantum Hall effect, unconventional superconductivity, Mott insulator, etc. Here we grow single-unit-cell (1UC) FeTe film on NbSe₂ single crystal by molecular beam epitaxy (MBE) and investigate the film in-situ with a home-made cryogenic scanning tunneling microscopy (STM) and non-contact atomic force microscopy (AFM) combined system. We find different stripe-like superlattice modulations on grown FeTe film with different misorientation angles with respect to NbSe₂ substrate. We show that these stripe-like superlattice modulations can be understood as moiré pattern forming between FeTe film and NbSe₂ substrate. Our results indicate that the interface between FeTe and NbSe₂ is atomically sharp. By STM-AFM combined measurement, we suggest that the moiré superlattice modulations have an electronic origin when the misorientation angle is relatively small (≤ 3°) and have structural relaxation when the misorientation angle is relatively large (≥ 10°).     

拓扑材料γ-PtBi2的变温晶体结构研究

利用X射线衍射（XRD）、角分辨光电子能谱（ARPES）和能带计算的方法研究了不同温度下γ-PtBi₂的晶体结构。利用单晶XRD确定了室温下晶体的结构为P31m。为了确定低温时样品的晶体结构,用ARPES测得了样品的电子结构并与计算结果进行了对比,结果显示样品的结构与P31m相吻合,这表明在低温时样品依然保持P31m结构。进一步的高温XRD研究表明,在高温时样品的晶体结构仍为P31m结构。     

Electronic structure,Dirac points and Fermi arc surface states in three-dimensional Dirac semimetal Na_3Bi from angle-resolved photoemission spectroscopy

The three-dimensional(3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A_3Bi(A = Na, K, Rb). Here we report comprehensive high-resolution angle-resolved photoemission(ARPES) measurements on the two cleaved surfaces,(001) and(100), of Na_3Bi. On the(001) surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the k_x–k_y plane and by varying the photon energy to get access to different out-of-plane k_zs. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the(100) surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the(100) plane. We directly observe two isolated 3D Dirac nodes on the(100) surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ~150 me V before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na_3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the3 D Dirac cones, on the possible formation of surface reconstruction of the(001) surface, and on the issue of basic Brillouin zone selection for the(100) surface.     

Crystal growth,structural and physical properties of the 5d noncentrosymmetric LaOsSi_3

LaOsSi3 single crystals are synthesized for the first time by an arc melting method. The crystal features a tetrag- onal BaNiSn3-type structure （space group 14mm） which lacks inversion symmetry along the crystallographic c axis and is isostructural with the intensively studied Rashba-type noncentrosymmetric superconductors LaRhSi3 and LaIrSi3. Un- like LaRhSi3 and LaIrSi3 displaying superconductivity, LaOsSi3 shows only metallic behavior over the measured temper- ature range of 2 K-300 K. The Sommerfeld coefficient ]/derived from specific heat is 5.76 mJ.mol-1 -K-2, indicating that LaOsSi3 has a weak electronic correlation effect. The absence of superconductivity in LaOsSi3 may lie in the Os 5d bands in the electronic structure. If it is true, it would be useful to provide complementary knowledge in understanding the relation between conduction and the d bands of M in LaMSi3 compounds （M = transition metals）.