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The discovery of superconductivity in pressurized La3Ni2O7 with a transition temperature of approximately 80 K above the boiling point of liquid nitrogen has sparked significant attention. It is essential to search for high-temperature superconductivity in bulk samples and at ambient pressure in nickelates. In this study, we report influential factors that affect the appearance of superconductivity in La3Ni2O7 at ambient pressure. From direct-current magnetic measurements, we observe a clear diamagnetic response at 80 K in post-annealed single crystals of La3Ni2O7 in oxygen. The superconducting volume fraction is estimated to be within 0.2%, resulting in a decrease in resistivity. This work presents a practical approach for further investigating high-temperature superconductivity in nickelates at ambient pressure., Comment: 4 figures, 11 pages

The recent discovery of superconductivity in $\rm La_3Ni_2O_7$ has attracted significant attention due to its high critical temperature and analogy to cuprate oxides. The oxidation and spin states of Ni ions are among the most important local properties in this compound, extensively discussed in the context of its superconductivity. Despite their direct link to the electron filling configurations of the relevant $\rm 3d_{x^2-y^2}$ and $\rm 3d_{z^2}$ orbitals, these local electronic properties of $\rm La_3Ni_2O_7$ yet to be systematically investigated. In this work, we address this issue using x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) measurements under pressure. Comparison of Ni \textit{K}-edge XAS and $\rm K\beta$ XES with the reference spectra of $\rm NiO$ and $\rm LaNiO_3$ shows that Ni ions, with an average valence of $\sim 2.53+$, are in a low-spin ($\rm S = 1/2$) ground state under ambient conditions. High pressure XAS and XES data clearly show that the oxidation ($\sim 2.5+$) and spin ($\rm S = 1/2$) states of Ni ions remain stable across the investigated pressure (up to 30 GPa) and temperature (down to 10 K) ranges, ruling out previously proposed spin transition scenarios.

The discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$ under pressure has drawn extensive attention. Herein, we report systematic investigations on the evolutions of structure, magnetism, and electrical resistance of Pr$_4$Ni$_3$O$_{10-\delta}$ polycrystalline samples under various pressures. Pr$_4$Ni$_3$O$_{10-\delta}$ exhibits density wave transitions on Ni and Pr sublattices at about 158 K and 4.3 K, respectively, and the density wave can be progressively suppressed by pressure. A structural transformation from the monoclinic $P2_1/a$ space group to the tetragonal $I4/mmm$ occurs at around 20 GPa. An apparent drop in resistance with evident magnetic field dependence is observed as pressure above 20 GPa, indicating the emergence of superconductivity in Pr$_4$Ni$_3$O$_{10-\delta}$ polycrystalline samples. The discovery of the signature of superconductivity in Pr$_4$Ni$_3$O$_{10-\delta}$ broadens the family of nickelate superconductors and provides a new platform for investigating the mechanisms of superconductivity in the Ruddlesden-Popper phases of nickelates., Comment: 12 pages with 3 figures in the main article and supplementary files

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, resistance, and Meissner effect in single crystals of La$_3$Ni$_2$O$_7$ with hydrostatic pressures up to 104 GPa. X-ray diffraction measurements reveal a structural transition from the orthorhombic to a tetragonal phase above 40 GPa. Zero resistance of the superconductivity was achieved with a maximum onset $T_c^{onset}$ of 83 K at 18.0 GPa. Superconductivity is gradually suppressed until it disappears above 80 GPa, resulting in a right-triangle-like superconducting region. The direct-current magnetic susceptibility technique successfully detected the Meissner effect in La$_3$Ni$_2$O$_7$ under pressure; the maximum superconducting volume fraction is estimated to be 62.7% at 22.0 GPa. Thus, we demonstrate the bulk nature of superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ single crystals under high pressure. The results reveal intimate connections among the superconductivity, oxygen content, and structure in La$_3$Ni$_2$O$_7$., Comment: This is a renewed version with Meissner effect measurements

Kagome system usually attracts great interest in condensed matter physics due to its unique structure hosting various exotic states such as superconductivity (SC), charge density wave (CDW), and nontrivial topological states. Topological semimetal RbTi3Bi5 consisting of the kagome layer of Ti shares a similar crystal structure to topological correlated materials AV3Sb5 (A = K, Rb, Cs) but with the absence of CDW and SC. Systematic de Haas-van Alphen (dHvA) oscillation measurements are performed on the single crystals of RbTi3Bi5 to pursue nontrivial topological physics and exotic states. Combining with theoretical calculations, detailed Fermi surface topology and band structure are investigated. A two-dimensional (2D) Fermi pocket \b{eta} is revealed with a light-effective mass in consistent with the semimetal predictions. Landau Fan of RbTi3Bi5 reveals a zero Berry phase for the \b{eta} oscillation in contrast to that of CsTi3Bi5. These results suggest the kagome RbTi3Bi5 is a good candidate to explore nontrivial topological exotic states and topological correlated physics.

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Ruddlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6-13.4 GPa is identified, accompanying with a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni 3dz2 orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates., Comment: 19 pages, 4 figures

The two-dimensional triangular-lattice antiferromagnet (TLAF) is a textbook example of frustrated magnetic systems. Despite its simplicity, the TLAF model exhibits a highly rich and complex magnetic phase diagram, featuring numerous distinct ground states that can be stabilized through frustrated next-nearest-neighbor couplings or anisotropy. In this paper, we report low-temperature magnetic properties of the TLAF material CsCeSe$_2$. The inelastic neutron scattering (INS) together with specific heat measurements and density functional theory calculations of crystalline electric field suggest that the ground state of Ce ions is a Kramers doublet with strong easy-plane anisotropy. Elastic neutron scattering measurements demonstrate the presence of stripe-$yz$ magnetic order that develops below $T_{\rm N} = 0.35$ K, with the zero-field ordered moment of $m_{\rm Ce} \approx 0.65~\mu_{\rm B}$. Application of magnetic field first increases the ordering temperature by about 20% at the intermediate field region and eventually suppresses the stripe order in favor of the field-polarized ferromagnetic state via a continuous quantum phase transition (QPT). The field-induced response demonstrates sizable anisotropy for different in-plane directions, $\mathbf{B}\parallel{}\mathbf{a}$ and $\mathbf{B}\perp{}\mathbf{a}$, which indicates the presence of bond-dependent coupling in the spin Hamiltonian. We further show theoretically that the presence of anisotropic bond-dependent interactions can change the universality class of QPT for $\mathbf{B}\parallel{}\mathbf{a}$ and $\mathbf{B}\perp{}\mathbf{a}$., Comment: 15 pages, 8 figures