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Owing to the unique electronic structure, kagome materials AV3Sb5 (A=K, Rb, Cs) provide a fertile platform of quantum phenomena such as the strongly correlated state and topological Dirac band. It is well known that RbV3Sb5 exhibits a 2×2 unconventional charge density wave (CDW) state at low temperature, and the mechanism is controversial. Here, by using scanning tunneling microscopy/spectroscopy (STM/STS), we successfully manipulated the CDW state in the Sb plane of RbV3Sb5, and realized a new √ 3×√ 3 modulation together with the ubiquitous 2×2 period in the CDW state of RbV3Sb5. The work provides a new understanding of the collective quantum ground states in the kagome materials.

The van der Waals (vdW) MnSb4Te7 is a newly synthesized antiferromagnetic (AFM) topological insulator hosting robust axion insulator state irrelative of the specific spin structure. However, the intrinsic hole doped character of MnSb4Te7makes the Fermi level far away from the Dirac point of about 180 meV, which is unfavorable for the exploration of exotic topological properties such as the quantum anomalous Hall effect (QAHE). To shift up the Fermi level close to the Dirac point, the strategy by partially replacing Sb by Bi as Mn(Sb1-x Bi x )4Te7 was tried and the magnetotransport properties, in particularly, the anomalous Hall effect, were measured and analyzed. Through the electron doping, the anomalous Hall conductance σ AHchanges from negative to positive between x = 0.3 and 0.5, indicative of a possible topological transition. Besides, a charge neutrality point (CNP) also appears between x = 0.6 and 0.7. The results would be instructive for further understanding the interplay between nontrivial topological states and the magnetism, as well as for the exploration of exotic topological properties.

The study of magnetic order in few- and monolayer van der Waals materials poses a challenge to the most commonly employed magnetic characterization techniques as they normally lack magnetic sensitivity and/or lateral resolution enabling their thickness-dependent probing. Here we demonstrate the usefulness of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L 2 , 3 and Te M 5 edges, for the study of the ferromagnetic semiconductor CrSiTe3 (CST) in the form of single- and few-layer flakes. By scanning the sample under the incident x-ray beam, a map of the exfoliated system was obtained, which reproduced the optical micrographs showing the detailed distribution and thicknesses of the flakes. In this way, XAS/XMCD was performed at selected sample areas, revealing the thickness-resolved spectroscopic and magnetic properties of the flakes, such as the spin and orbital magnetic moments. The spin moment, in line with the saturation field, is decreasing with film thickness, revealing a single-domain and out-of-plane magnetization for the thinnest films. For CST, the electronic properties are governed by the strong covalent bond between the Cr 3d(e g ) and Te 5p states, giving rise to a superexchange scenario. We observed a gradually increasing ratio of orbital to spin moment for thinner flakes, which could be due to a further increase of the covalent mixing. Hysteresis loops were recorded at the Cr L 3 edge, showing an open loop for 10 down to ∼3 layers, while the bulk shows a wasp-waist shaped loop. With the transition temperature from the soft to the hard ferromagnetic state decreasing with thickness, the monolayer shows a narrowed, closed loop at 10 K, suggesting its transition temperature > 10 K. Our study demonstrates the unique capabilities of XAS/XMCD for the study of few-layer van der Waals magnets, highlighting the interplay between electron correlation and ferromagnetism in CST.

Room-temperature two-dimensional antiferromagnetic (AFM) materials are highly desirable for various device applications. In this letter, we report the low-energy electronic structure of KMnBi measured by angle-resolved photoemission spectroscopy, which confirms an AFM ground state with the valence band maximum located at −100 meV below the Fermi level and small hole effective masses associated with the sharp band dispersion. Using complementary Raman, atomic force microscope and electric transport measurement, we systematically study the evolution of electric transport characteristics of micro-mechanically exfoliated KMnBi with varied flake thicknesses, which all consistently reveal the existence of a probable AFM ground state down to the quintuple-layer regime. The AFM phase transition temperature ranges from 220 K to 275 K, depending on the thickness. Our results suggest that with proper device encapsulation, multilayer KMnBi is indeed a promising 2D AFM platform for testing various theoretical proposals for device applications.

Monoclinic α-MoP2, with the OsGe2-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 β = 119.263°, was synthesized under a pressure of 4~GPa at a temperature between 1100 °C and 1200 °C. The structure of α-MoP2 and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic β-MoP2 (space group Cmc21) is denser in structure than α-MoP2. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from β-MoP2 to α-MoP2, suggesting that α-MoP2 is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.

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 XCd2Sb2 (X = Eu and Yb), which have the same crystal structure. In antiferromagnetic (AFM) Weyl semimetal EuCd2Sb2, the Néel temperature (T 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 YbCd2Sb2, pressure-induced superconductivity appears at 1.94 GPa, then T 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 YbCd2Sb2, revealing the robustness of pressure-induced superconductivity against structural instability. Similar structural phase transition may also occur in EuCd2Sb2, causing the disappearance of magnetism. Our results show that XCd2Sb2 (X = Eu and Yb) is a novel platform for exploring the interplay among magnetism, topology, and superconductivity.

We report herein the nontrivial topological states in an intrinsic type-II superconductor BaSn5 (Tc ~ 4.4 K) probed via measuring the magnetizations, specific heat, de Haas-van Alphen (dHvA) effect and performing first principles calculations. The first principles calculations reveal a topological nodal ring structure centering at the H point in the kz = {\pi} plane of the Brillouin zone (BZ), which could be gapped by spin-orbit coupling (SOC), yielding rather small gaps below and above the Fermi level about 0.04 eV and 0.14 eV, respectively. The SOC also results in a pair of Dirac points along the {\Gamma}-A direction and located ~ 0.2 eV above the Fermi level. The analysis of the dHvA quantum oscillations supports the calculations by revealing nontrivial Berry phase originated from three hole and one electron pockets related to the bands forming the Dirac cones. Our study thus provides an excellent avenue for investigating the interplay between superconductivity and nontrivial topological states., Comment: 15 pages, 6 figures, 1 tables

As a van der Waals ferromagnet with high Curie temperature, Fe5–x GeTe2 has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe5–x GeTe2 crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight kz 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 Fe5–x GeTe2 and the understanding of magnetism in a two-dimensional ferromagnetic system.

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.

Topological superconductors (TSCs) have been widely investigated in recent years due to their novel physics and ability to host Majorana fermions (MFs) which are key to topological quantum computation. Despite the great interest, only a few compounds have been proposed as candidates of intrinsic TSCs, such as iron-based superconductor FeSe0.55Te0.45 and 2M-WS2. Among them, quasi-one-dimensional superconductor TaSe3 possesses fascinating properties such as its simple stoichiometry, layered nature and chemical stability. Here, using scanning tunneling microscope/spectroscopy (STM/STS), we systematically investigate the topography and electronic structure of TaSe3. Our STM/STS measurement reveals large atomically flat, defect-free surfaces suitable for the search of MF; electronic density of states consistent with our angle-resolved photoemission result and band-structure calculations, and a uniform superconducting gap with a typical size of ∼ 0.25 meV. Remarkably, additional edge states are observed in the vicinity of the terrace edge, suggesting they may have a topological origin. Our result proves the coexistence of superconductivity and topological electronic structure in TaSe3, making it an intriguing platform to investigate topological superconductivity.

We reveal the electronic structure in YbCd2Sb2, a thermoelectric material, by angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (trARPES). 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.

Characterization of Fermi surface of the BaSn3 superconductor (T c ∼ 4.4 K) by de Haas–van Alphen (dHvA) effect measurement reveals its non-trivial topological properties. Analysis of non-zero Berry phase is supported by the ab initio calculations, which reveals a type-II Dirac point setting and tilting along the high symmetric K–H line of the Brillouin zone, about 0.13 eV above the Fermi level, and other two type-I Dirac points on the high symmetric Γ–A direction, but slightly far below the Fermi level. The results demonstrate BaSn3 as an excellent example hosting multiple Dirac fermions and an outstanding platform for studying the interplay between nontrivial topological states and superconductivity.

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 300nm SiO 2 dielectric can be dramatically enhanced by about two orders of magnitude (from 11 nF/cm 2 to 880 nF/cm 2 ), reaching good efficiency of ion-liquid 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.

We report the fabrication of YBa2Cu3O7−δ/Nb-doped (0.8 wt%) SrTiO3 p–n junctions by pulsed electron deposition, photolithography and Ar-ion etching techniques. In the junctions, the abnormal rectifying behaviour and the Esaki tunnelling diode-like characteristic have been observed above and below the superconducting transition temperature TC of YBa2Cu3O7−δ, respectively. The tunnelling process is responsible for these behaviours and the depletion layer formed at the junction interface with very low resistance is the dominating factor. The intriguing properties in this work indicate that it is possible to realize the functions of traditional semiconductor diodes by using high-temperature superconductor-based heterojunctions.

Mn3InN antiperovskite is synthesized under high-pressure conditions, followed by measurements of magnetic susceptibility, isothermal magnetization, electrical resistivity, and specific heat. Multiple magnetic interactions are observed between 2 K and 400 K: the paramagnetic state turns to an antiferromagnetic state at 300 K on cooling, and a weak ferromagnetic contribution appears at 175 K by further cooling. Below 50 K, an enhanced ferromagnetic contribution is obvious. Although a long-range antiferromagnetic order is clearly established at room temperature, the material retains a good metallic conduction over a wide temperature range (the Sommerfeld coefficient is 43.3 mJ mol−1 K−2), suggesting a possible use of the material in spintronics applications.

We report a Zn-doping effect on BaFe1.92Pt0.08As2 (Tc =24.5 K) single crystal. Tc rapidly decreases with increasing the Zn content in the lattice, revealing a distinct feature which is highly contrasted with the intrinsic scattering feature induced by Pt. Doped Zn completely kills the superconductivity by no more than 8 atomic %, while intrinsic scattering induced by doped Pt shows a good coexistence with the superconductivity. The results are discussed in connection with the s±- and s++-wave models.

75As nuclear spin-lattice relaxation rate 1/T1 was measured in Zn-free and Zn-substituted La(Fe1−xZnx)AsO0.85 (x = 0, and 0.05), Tc of which is 25 K and 0 K, respectively. Although the temperature dependence of 1/T1T above Tc is the same between two samples, 1/T1T of x = 0 shows T3.0±0.1 dependence (1/T1 ∝ T4) in the superconducting (SC) state, but 1/T1T of non-SC x = 0.05 sample continuously decreases below 50 K. The normalized 1/T1T [(1/T1T)x=0/(1/T1T)x=0.05], which is related to the SC gap structure, is proportional to T2.4±0.1 below Tc. These results indicate that the temperature dependence of 1/T1T in the SC state can be affected by the temperature dependence of the normal-state 1/T1T.