Determination of the surface states from the ultrafast electronic states in a thermoelectric material

We reveal the electronic structure in Yb Cd₂Sb₂,a thermoelectric material,by angle-resolved photoemission spectroscopy(ARPES)and time-resolved ARPES(tr ARPES).Specifically,three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center,consistent with the density functional theory(DFT)calculation.It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right-and left-handed circularly polarized probe.In addition,a hole band of surface states,which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation,is identified.We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states.Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.     

Nontrivial Topological States in BaSn5 Superconductor Probed by de Haas–van Alphen Quantum Oscillations

We report the nontrivial topological states in an intrinsic type-Ⅱ superconductor BaSn₅（T_c～4.4 K） probed by measuring the magnetization, specific heat, de Haas–van Alphen（d Hv A） effect, and by performing first-principles calculations. The first-principles calculations reveal a topological nodal ring structure centered at the H point in the k_z = π plane of the Brillouin zone, which could be gapped by spin-orbit coupling（SOC）, yielding relatively small gaps below ...     

A new transition metal diphosphide α-MoP2 synthesized by a high-temperature and high-pressure technique

Monoclinic $\alpha $-MoP$_{2}$, with the OsGe$_{2}$-type structure (space group $C2/m$, $Z = 4$) and lattice parameters $a = 8.7248(11) $ Å, $b = 3.2322(4) $ Å, $c = 7.4724(9) $ Å, and $\beta =119.263^\circ $, was synthesized under a pressure of 4 GPa at a temperature between 1100 ${^\circ}$C and 1200 ${^\circ}$C. The structure of $\alpha $-MoP$_{2}$ and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic $\beta $-MoP$_{2}$ (space group Cmc2$_{1}$) is denser in structure than $\alpha $-MoP$_{2}$. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from $\beta $-MoP$_{2}$ to $\alpha $-MoP$_{2}$, suggesting that $\alpha $-MoP$_{2}$ is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.     

Film Thickness Dependence of Rectifying Properties of La1.85Sr0.15CuO4/Nb-SrTiO3 Junctions

In this work, p-n junctions are made from directly depositing optimal doped La1.85Sr0.15CuO4 （LSCO） films on n-type Nb-doped SrTiO3 substrates. Film thickness controlled rectifying behaviors are strikingly displayed. The starting points of the diffusion voltage reduction Vd-on change clearly with varying film thickness. Vd-on and Tc coincide with each other when the film thickness is larger than 300 nm, indicating a close relation between the two parameters. However, when the film is very thin （〈 350 nm） a departure between the two parameters was also observed. A possible reason for this is discussed within the framework of an inhomogeneous Sehottky contact. Enhanced interface inhomogeneity due to the tensile strain appears to be the origin.     

Enhancing low-field magnetoresistance of La0.67Ca0.33MnO3 films deposited on anodized aluminium-oxide membranes

In this paper we report a new method to fabricate nanostructured films, La0.67Ca0.33MnO3 （LCMO） nanostructured films have been fabricated by using pulsed electron beam deposition （PED） on anodized aluminium oxide （AAO） membranes, The magnetic and electronic transport properties are investigated by using the Quantum Design physics properties measurement system （PPMS） and magnetic properties measurement system （MPMS）. The resistance peak temperature （Tp） is about 85 K and the Curie temperature （To） is about 250 K for the LCMO film on an AAO membrane with a pore diameter of 20nm. Large magnetoresistance ratio （MR） is observed near Tp. The MR is as high as 85% under 1 T magnetic field. The great enhancement of MR at low magnetic fields could be attributed to the lattice distortion and the grain boundary that are induced by the nanopores on the AAO membrane.     

Measurement of electronic structure in van der Waals ferromagnet Fe5-xGeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe_5-xGeTe₂ has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe_5-xGeTe₂ crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight k_z evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe_5-xGeTe₂ and the understanding of magnetism in a two-dimensional ferromagnetic system.     

Observation of large in-plane anisotropic transport in van der Waals semiconductor Nb2SiTe4

Two-dimensional (2D) van der Waals material is a focus of research for its widespread application in optoelectronics, memories, and spintronics. The ternary compound Nb₂SiTe₄ is a van der Waals semiconductor with excellent air stability and small cleavage energy, which is suitable for preparing a few layers counterpart to explore novel properties. Here, properties of bulk Nb₂SiTe₄ with large in-plane electrical anisotropy are demonstrated. It is found that hole carriers dominate at a temperature above 45 K with a carrier active energy of 31.3 meV. The carrier mobility measured at 100 K is about 213 cm²·V^-1·s^-1 in bulk Nb₂SiTe₄, higher than the reported results. In a thin flake Nb₂SiTe₄, the resistivity ratio between the crystalline axes of a and b is reaching about 47.3 at 2.5 K, indicating that there exists a large anisotropic transport behavior in their basal plane. These novel transport properties provide accurate information for modulating or utilizing Nb₂SiTe₄ for electronic device applications.     

A New Effect of Oxygen Plasma on Two-Dimensional Field-Effect Transistors:Plasma Induced Ion Gating and Synaptic Behavior

Plasma treatment is a powerful tool to tune the properties of two-dimensional materials. Previous studies have utilized various plasma treatments on two-dimensional materials. We find a new effect of plasma treatment. After controlled oxygen-plasma treatment on field-effect transistors based on two-dimensional SnSe_2, the capacitive coupling between the silicon back gate and the channel through the 300 nm SiO_2 dielectric can be dramatically enhanced by about two orders of magnitude(from 11 n F/cm~2 to 880 nF/cm~2), reaching good efficiency of ionliquid gating. At the same time, plasma treated devices show large hysteresis in the gate sweep demonstrating memory behavior. We reveal that this spontaneous ion gating and hysteresis are achieved with the assistance of a thin layer of water film automatically formed on the sample surface with water molecules from the ambient air, due to the change in hydrophilicity of the plasma treated samples. The water film acts as the ion liquid to couple the back gate and the channel. Thanks to the rich carrier dynamics in plasma-treated two-dimensional transistors, synaptic functions are realized to demonstrate short-and long-term memories in a single device. This work provides a new perspective on the effects of plasma treatment and a facile route for realizing neuromorphic devices.     

改性光刻胶制备纳米压印模版

制备纳米压印中的模版是压印技术的基本条件.目前很多压印模版是利用高硬度材料制作的，但是这些材料比较难以加工，从而限制了纳米压印技术的发展.提出一种利用光刻胶制备纳米压印模版的方法.利用聚焦离子束对光刻胶的改性作用，控制加工的条件，将柔性的光刻胶改性为硬度很高的材料，从而形成纳米压印模版.这种方法具有速度快、制备简单等特点，是一种新颖的加工方式，扩展了聚焦离子束的加工范围，可用于其他的纳米加工领域. 关键词： 纳米压印 光刻胶 聚焦离子束 纳米孔阵列     

High-pressure study of topological semimetals XCd2Sb2 (X = Eu and Yb)

Topological materials have aroused great interest in recent years, especially when magnetism is involved. Pressure can effectively tune the topological states and possibly induce superconductivity. Here we report the high-pressure study of topological semimetals $X$Cd$_{2}$Sb$_{2}$ ($X = {\rm Eu} $ and Yb), which have the same crystal structure. In antiferromagnetic (AFM) Weyl semimetal EuCd$_{2}$Sb$_{2}$, the Néel temperature (${T}_{\rm N}$) increases from 7.4 K at ambient pressure to 50.9 K at 14.9 GPa. When pressure is above 14.9 GPa, the AFM peak of resistance disappears, indicating a non-magnetic state. In paramagnetic Dirac semimetal candidate YbCd$_{2}$Sb$_{2}$, pressure-induced superconductivity appears at 1.94 GPa, then ${ T}_{\rm c}$ reaches to a maximum of 1.67 K at 5.22 GPa and drops to zero at about 30 GPa, displaying a dome-shaped temperature-pressure phase diagram. High-pressure x-ray diffraction measurement demonstrates that a crystalline-to-amorphous phase transition occurs at about 16 GPa in YbCd$_{2}$Sb$_{2}$, revealing the robustness of pressure-induced superconductivity against structural instability. Similar structural phase transition may also occur in EuCd$_{2}$Sb$_{2}$, causing the disappearance of magnetism. Our results show that $X$Cd$_{2}$Sb$_{2}$ ($X = {\rm Eu}$ and Yb) is a novel platform for exploring the interplay among magnetism, topology, and superconductivity.