Single crystal synthesis, structure, and magnetism of Pb10−x_{10-x}Cux_x(PO4_4)6_6O

The recent claim of superconductivity above room temperature in Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O with 0.9 < $x$ < 1 (referred to as LK-99) has sparked considerable interest. To minimize the influence of structural defects and impurity phases on the physical properties, we have synthesized phase-pure single crystals with $x \sim 1$. We find that the crystals are highly insulating and optically transparent. X-ray analysis reveals an uneven distribution of the substituted Cu throughout the sample. Temperature ($T$) dependent magnetization measurements for $2 \leq T \leq 800$ K reveal the diamagnetic response characteristic of a non-magnetic insulator, as well as a small ferromagnetic component, possibly originating from frustrated exchange interactions in Cu-rich clusters in the Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O structure. No anomalies indicative of phase transitions are observed. We therefore rule out the presence of superconductivity in Pb$_{9}$Cu(PO$_4$)$_6$O crystals, and provide some considerations on the origin of anomalies previously reported in experiments on polycrystalline specimen

Phase formation in hole- and electron-doped rare-earth nickelate single crystals

The recent discovery of superconductivity in hole-doped infinite-layer nickelates has triggered a great interest in the synthesis of novel nickelate phases, which have primarily been examined in thin film samples. Here, we report the high-pressure optical floating zone (OFZ) growth of various perovskite and perovskite-derived rare-earth nickelate single-crystals, and investigate the effects of hole-, electron-, and self-doping. For hole-doping with Ca and Sr, we observe phase separations during the growth process when a substitution level of 8% is exceeded. A similar trend emerges for electron-doping with Ce and Zr. Employing lower doping levels allows us to grow sizeable crystals in the perovskite phase, which exhibit significantly different electronic and magnetic properties than the undoped parent compounds, such as a decreased resistivity and a suppressed magnetic response. Our insights into the doping-dependent phase formation and the resulting properties of the synthesized crystals reveal limitations and opportunities for the exploration and manipulation of electronic states in rare-earth nickelates

Controlling frustrated magnetism on the kagome lattice by uniaxial-strain tuning

It is predicted that strongly interacting spins on a frustrated lattice may lead to a quantum disordered ground state or even form a quantum spin liquid with exotic low-energy excitations. However, a thorough tuning of the frustration strength, separating its effects from those of disorder and other factors, is pending. Here we break the symmetry of a kagome-lattice compound in a controlled manner by applying $in$ $situ$ uniaxial stress. The transition temperature of $\rm Y_3Cu_9(OH)_{18}OCl_8$ is linearly enhanced with strain, $\Delta T_{\rm N}/T_{\rm N} \approx 10\%$ upon in-plane compression of order $1\%$, providing clear evidence for a release of frustration and its pivotal role for magnetic order. Our comprehensive $^1$H NMR results suggest a $\overrightarrow{Q}=(1/3\times 1/3)$ state under unstrained conditions and further reveal an incomplete antiferromagnetic transition with fluctuating moments in this strongly frustrated system.Comment: 7 pages total, 4 pages main text, 5 figure

Crystal Growth with Oxygen Partial Pressure of the BaCuSi2O6 and Ba1-xSrxCuSi2O6 Spin Dimer Compounds

BaCuSi2O6 is a quasi-two-dimensional spin dimer system and a model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. The new Bai(1-x)Sr(x)CuSi(2)O(6) mixed system, which can be grown with x <= 0.3, and BaCuSi2O6, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure (I4(1)/acd) at room temperature. The mixed system shows no structural phase transition so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as a control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure

Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study

We report a comprehensive study of magnetic correlations in LaNiO$_{2}$, a parent compound of the recently discovered family of infinite-layer (IL) nickelate superconductors, using multiple experimental and theoretical methods. Our specific heat, muon-spin rotation ($\mu$SR), and magnetic susceptibility measurements on polycrystalline LaNiO$_{2}$ show that long-range magnetic order remains absent down to 2 K. Nevertheless, we detect residual entropy in the low-temperature specific heat, which is compatible with a model fit that includes paramagnon excitations. The $\mu$SR and low-field static and dynamic magnetic susceptibility measurements indicate the presence of short-range magnetic correlations and glassy spin dynamics, which we attribute to local oxygen non-stoichiometry in the average infinite-layer crystal structure. This glassy behavior can be suppressed in strong external fields, allowing us to extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures, in analogy to doped cuprates that possess robust non-local spin fluctuations. The distinct temperature dependence of the intrinsic susceptibility of LaNiO$_{2}$ can be theoretically understood by a multi-method study of the single-band Hubbard model in which we apply complementary cutting-edge quantum many-body techniques (dynamical mean-field theory, cellular dynamical mean-field theory and the dynamical vertex approximation) to investigate the influence of both short- and long-ranged correlations. Our results suggest a profound analogy between the magnetic correlations in parent (undoped) IL nickelates and doped cuprates.Comment: 18 pages, 14 figure

Magnetism and anomalous transport in the Weyl semimetal PrAlGe: Possible route to axial gauge fields

In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall and Nernst effects. With sizes of both the anomalous Hall conductivity and Nernst effect in good quantitative agreement with first principles calculations, we identify PrAlGe as a system where magnetic fields can connect directly to Weyl nodes via the Pr magnetization. Furthermore, we find the predominantly easy-axis ferromagnetic ground state co-exists with a low density of nanoscale textured magnetic domain walls. We describe how such nanoscale magnetic textures could serve as a local platform for tunable axial gauge fields of Weyl fermions.Comment: 42 pages, 5 figure