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.

Thermochemical Mechanism of Optimized Lanthanum Chromite Heaters for High-Pressure and High-Temperature Experiments.

High-pressure heaters in large volume presses must reconcile potentially contradictory properties, and the whole high-pressure and high-temperature (HPHT) community has been engaged for years to seek a better heater. LaCrO3 (LCO)-based ceramic heaters have been widely applied in multianvil apparatus; however, their performance is far from satisfactory, motivating further research on the chemical optimization strategy and corresponding thermochemical mechanism. Here, we adopted a chemical-screening strategy and manufactured tubular heaters using the electrically, chemically, and mechanically optimized Sr-Cu codoped La0.9Sr0.1Cr0.8Cu0.2O3-δ (LSCCuO-9182). HPHT examinations of cylindrical LSCCuO-9182 heaters on Walker-type multianvil apparatuses demonstrated a small temperature gradient, robust thermochemical stability, and excellent compatibility with high-pressure assemblies below 2273 K and 10 GPa. Thermochemical mechanism analysis revealed that the temperature limitation of the LSCCuO-9182 heater was related to the autoredox process of the Cu dopant and Cr and the exchanging ionic migration of Cu and Mg between the LSCCuO-9182 heater and the MgO sleeve. Our combinatorial strategy coupled with thermochemical mechanism analysis makes the prioritization of contradictory objectives more rational, yields reliable LCO heaters, and sheds light on further improvement of the temperature limitation and thermochemical stability.

First-principles calculations of the magnetic and electronic structures of MnP under pressure.

Manganese monophosphide (MnP) shows complicated magnetic states varying with both temperature and pressure. We calculate the magnetic and electronic structures of MnP at different pressures using first-principles methods and obtain spiral ground states whose propagation vector changes from the c-axis at low pressure to the b-axis at high pressure. In between, we find a ferromagnetic state, as observed in the experimental phase diagram. The propagation vector of the spiral states is found to vary nonmonotonically with pressure, consistent with neutron measurements. Our results indicate that the complicated magnetic phase diagram originates from a delicate competition between neighboring exchange interactions between the Mn-ions. At all pressures, the electronic structures indicate the existence of quasi-one-dimensional charge carriers, which appear in the ferromagnetic state and become gapped in the spiral state, and anisotropic three-dimensional charge carriers. We argue that this two-fluid behavior originates from the special crystal structure of MnP and may be relevant for understanding the pairing mechanism of the superconductivity at the border of the high pressure spiral phase.

A new pnictide superconductor without iron.

LiCu(2)P(2) and LiFeP have been synthesized by conventional solid-state reaction. LiCu(2)P(2) has a crystal structure similar to that of BaFe(2)As(2); LiFeP has the same crystal structure as that of LiFeAs. Resistivity and magnetization measurements reveal that they become superconductive at 3.5 K for LiCu(2)P(2) and 4.1 K for LiFeP.

A new perovskite polytype in the high-pressure sequence of BaIrO(3).

The high-pressure sequence of the perovskite polytypes of BaIrO(3) has been investigated in the pressure range up to 10 GPa. At ambient pressure the so-called "9R" polytype has been prepared by solid-state reaction and slow cooling in air to yield an almost fully oxygen-stoichiometric BaIrO(2.96(1)) composition. The crystal structure has been refined from XRD data in the monoclinic C2/m space group with a = 10.0046(3) A, b = 5.75362(14) A, c = 15.1839(4) A, beta = 103.27(1) degrees ; it contains trimers of face-sharing octahedra (or Ir(3)O(12) trioctahedra) that are linked by their vertices to form columns parallel to the c-axis with a stacking of layers of corner sharing (c) and face sharing (h) IrO(6) octahedra along the sequence hhchhc. This structure is stable up to 3 GPa; at 4 GPa a new 5H polytype has been stabilized as a pure phase. The crystal structure has been solved by ab initio procedures from powder XRD data. It is monoclinic with a = 9.9511(2) A, b = 5.7503(1) A, c = 13.71003(3) A, beta = 118.404(2) degrees , and it was refined in the C2/m space group from NPD data collected at room temperature. This polytype can be described as a stacking of IrO(6) octahedra along the sequence hchcc. The structure contains chains of double dimer units of face-sharing octahedra; the twin dimers are connected to single layers of vertex-sharing octahedra, forming infinite chains along c. This is a unique stacking that, with this repetition length, has never been described before among the hexagonal polytypes of ABO(3) perovskites. The 5H polytype is stable in a narrow pressure range; at 5 GPa the 6H structure is formed, stable up to 10 GPa. The 6H-BaIrO(3) polytype is monoclinic, space group C2/c, with a = 5.7483(2) A, b = 9.9390(3) A, c = 14.3582(5) A, beta = 91.319(2) degrees . The structure consists of dimers of face-sharing octahedra separated by single corner-sharing octahedra, showing the sequence hcchcc along the c-axis. At 10 GPa the cubic 3C perovskite structure could be identified as a minority phase, with a = 4.0611(7) A, defined in the Pm3m space group. The precarious stability of the 5H polytype, as well as the novel pressure sequence displayed by BaIrO(3) that is distinct from the classical sequence 9R-4H-6H-3C exhibited by many transition metal oxides, for instance BaRuO(3), is a result of the particular stability of the "9R" ambient-pressure structure, which is reinforced by a strong Ir-Ir bond across the octahedral faces, and the Ir-Ir Coulombic repulsion across shared faces that destabilizes the 4H polytype relative to the 6H phase to allow stabilization of the hybrid 5H polytype in a narrow presure range.