Tunable positions of Weyl nodes via magnetism and pressure in the ferromagnetic Weyl semimetal CeAlSi.

The noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi with simultaneous space-inversion and time-reversal symmetry breaking provides a unique platform for exploring novel topological states. Here, by employing multiple experimental techniques, we demonstrate that ferromagnetism and pressure can serve as efficient parameters to tune the positions of Weyl nodes in CeAlSi. At ambient pressure, a magnetism-facilitated anomalous Hall/Nernst effect (AHE/ANE) is uncovered. Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrated that the Weyl nodes with opposite chirality are moving away from each other upon entering the ferromagnetic phase. Under pressure, by tracing the pressure evolution of AHE and band structure, we demonstrate that pressure could also serve as a pivotal knob to tune the positions of Weyl nodes. Moreover, multiple pressure-induced phase transitions are also revealed. These findings indicate that CeAlSi provides a unique and tunable platform for exploring exotic topological physics and electron correlations, as well as catering to potential applications, such as spintronics.

Fluctuating magnetic droplets immersed in a sea of quantum spin liquid.

The search of quantum spin liquid (QSL), an exotic magnetic state with strongly fluctuating and highly entangled spins down to zero temperature, is a main theme in current condensed matter physics. However, there is no smoking gun evidence for deconfined spinons in any QSL candidate so far. The disorders and competing exchange interactions may prevent the formation of an ideal QSL state on frustrated spin lattices. Here we report comprehensive and systematic measurements of the magnetic susceptibility, ultralow-temperature specific heat, muon spin relaxation (µSR), nuclear magnetic resonance (NMR), and thermal conductivity for NaYbSe2 single crystals, in which Yb3+ ions with effective spin-1/2 form a perfect triangular lattice. All these complementary techniques find no evidence of long-range magnetic order down to their respective base temperatures. Instead, specific heat, µSR, and NMR measurements suggest the coexistence of quasi-static and dynamic spins in NaYbSe2. The scattering from these quasi-static spins may cause the absence of magnetic thermal conductivity. Thus, we propose a scenario of fluctuating ferrimagnetic droplets immersed in a sea of QSL. This may be quite common on the way pursuing an ideal QSL, and provides a brand new platform to study how a QSL state survives impurities and coexists with other magnetically ordered states.

Modulating Charge-Density Wave Order and Superconductivity from Two Alternative Stacked Monolayers in a Bulk 4Hb-TaSe2 Heterostructure via Pressure.

Two-dimensional (2D) van der Waals heterostructures (VDWHs) containing a charge-density wave (CDW) and superconductivity (SC) have revealed rich tunability in their properties, which provide a new route for optimizing their novel exotic states. The interaction between SC and CDW is critical to its properties; however, understanding this interaction within VDWHs is very limited. A comprehensive in situ study and theoretical calculation on bulk 4Hb-TaSe2 VDWHs consisting of alternately stacking 1T-TaSe2 and 1H-TaSe2 monolayers are investigated under high pressure. Surprisingly, the superconductivity competes with the intralayer and adjacent-layer CDW order in 4Hb-TaSe2, which results in substantially and continually boosted superconductivity under compression. Upon total suppression of the CDW, the superconductivity in the individual layers responds differently to the charge transfer. Our results provide an excellent method to efficiently tune the interplay between SC and CDW in VDWHs and a new avenue for designing materials with tailored properties.

Magnetism-induced topological transition in EuAs3.

The nature of the interaction between magnetism and topology in magnetic topological semimetals remains mysterious, but may be expected to lead to a variety of novel physics. We systematically studied the magnetic semimetal EuAs3, demonstrating a magnetism-induced topological transition from a topological nodal-line semimetal in the paramagnetic or the spin-polarized state to a topological massive Dirac metal in the antiferromagnetic ground state at low temperature. The topological nature in the antiferromagnetic state and the spin-polarized state has been verified by electrical transport measurements. An unsaturated and extremely large magnetoresistance of ~2 × 105% at 1.8 K and 28.3 T is observed. In the paramagnetic states, the topological nodal-line structure at the Y point is proven by angle-resolved photoemission spectroscopy. Moreover, a temperature-induced Lifshitz transition accompanied by the emergence of a new band below 3 K is revealed. These results indicate that magnetic EuAs3 provides a rich platform to explore exotic physics arising from the interaction of magnetism with topology.

Anomalous Dome-like Superconductivity in RE2(Cu1-xNix)5As3O2 (RE = La, Pr, Nd).

A significant manifestation of interplay of superconductivity and charge density wave, spin density wave, or magnetism is a dome-like superconducting critical temperature (Tc) in cuprate, iron-based, and heavy Fermion superconductors. Pesudogap, quantum critical point, and strange metals emerge in different doping ranges. Exploring dome-like Tc in new superconductors is of interest to detect emergent effects. Here we report the superconductivity in a new layered Cu-based compound RE2Cu5As3O2 (RE = La, Pr, Nd), in which the Tc exhibits dome-like variation with a maximum Tc of 2.5, 1.2, and 1.0 K with substitution of Cu by large amount of Ni ions. Simultaneously, the structural parameters like As-As bond length and c/a ratio exhibit unusual variations as the Ni-doping level goes through the optimal value. The robustness of superconductivity, up to 60% of Ni doping, reveals the unexpected impurity effect on inducing and enhancing superconductivity in these novel layered materials.

Structure and Transport Properties in Itinerant Antiferromagnet RE2(Ni1- xCu x)5As3O2 (RE = Ce, Sm).

We report the crystal structure and physical properties of two Ni5As3-based compounds RE2Ni5As3O2 (RE = Ce, Sm). The former exhibits structural phase transition from tetragonal (space group I4/ mmm, 139) to orthorhombic (space group Immm, 71) symmetry at 230 K, while the latter undergoes a charge-density-wave-like structural distortion with abrupt change of Ni-As bond length. Both compounds show antiferromagnetic transitions due to RE3+ ions ordering at 4.4 and 3.4 K, accompanying with the large enhancement of Sommerfeld coefficients comparing to the nonmagnetic La analogue. Although the Cu substitution for Ni induces structural anomalies and suppression of structural transition like the behaviors in La/Pr/Nd analogues, the superconductivity is not observed in both Cu-doped RE2Ni5As3O2 (RE = Ce, Sm) above 0.25 K. Our structural refinements reveal that the lacking of superconductivity in RE2(Ni1- xCu x)5As3O2 might relate to the anomalous increase of As height, h1.

V2Te2O: A Two-Dimensional van der Waals Correlated Metal.

A metastable vanadium oxytelluride V2Te2O is prepared via a topochemical deintercalation of interlayer Rb+ cations in Rb1-δV2Te2O. The new ternary mixed-anion compound crystallizes in a body-centered tetragonal lattice with a = 3.9282(1) Å and c = 13.277(5) Å, containing V2O square nets that are sandwiched by Te-atomic sheets. The charge-neutral [V2OTe2] block layers stack along the c axis with van der Waals forces, which shows a metallic behavior with a dominant T2 dependence for resistivity at low temperatures. The electronic specific-heat coefficient reaches 33.9 mJ K-2 mol-1, ∼4 times that of the electronic structure calculations, suggesting a significant electron-mass renormalization. The electron correlation effect is concurrently demonstrated by the Wilson and Kadowaki-Woods ratios. Neither charge/spin-density wave nor superconductivity was observed down to 0.03 K.

Cs0.9Ni3.1Se3: A Ni-Based Quasi-One-Dimensional Conductor with Spin-Glass Behavior.

In this work, we report the discovery of a new Ni-based quasi-one-dimensional selenide: Cs0.9Ni3.1Se3. This compound adopts the TlFe3Te3-type structure with space group P63/ m, which consists of infinite [Ni3Se3] chains with face-sharing Ni6 octahedra along the c direction. The lattice parameters are calculated as a = 9.26301(4) Å and c = 4.34272(2) Å, with the Ni-Ni distance in the ab plane as 2.582(3) Å, suggesting the formation of a Ni-Ni metallic bond in this compound. Interestingly, it has been found that Cs0.9Ni3.1Se3 is nonstoichiometric, which is different from the other TlFe3Te3-type phases reported so far. Structure refinement shows that the extra Ni atom in the structure may occupy the 2c site, together with Cs atoms. Cs0.9Ni3.1Se3 shows metallic behavior with monotonously decreased resistivity with temperatures from 300 to 0.5 K. Measurements on the magnetic susceptibility display a spin-glass state below 7 K. The specific heat curve gives a Sommerfeld coefficient of 14.6 mJ·K-2·mol-1 and a Debye temperature of 143.6 K. The discovery of this new compound enriches the diversity of low-dimensional materials in a transition-metal-based family and also sheds light on the structure-property relationship of this system.