We report Ru L3-edge resonant X-ray diffraction studies on single-crystal and (001)-oriented epitaxial films of RuO2. We investigate the distinct Q = (100) and (001) Bragg-forbidden reflections as a function of incident energy, azimuthal angle, and temperature. The results show that the observed resonant diffraction in RuO2 is fully consistent with a resonant charge anisotropy signal of structural origin permitted by the parent (nonmagnetic) rutile P42/mnm space group. These results significantly constrain the magnetic contribution to the resonant diffraction signal and indicate the unlikely existence of k = 0 antiferromagnetic order in RuO2.

Topotactic reduction is critical to a wealth of phase transitions of current interest, including synthesis of the superconducting nickelate Nd0.8Sr0.2NiO2, reduced from the initial Nd0.8Sr0.2NiO3/SrTiO3 heterostructure. Due to the highly sensitive and often damaging nature of the topotactic reduction, however, only a handful of research groups have been able to reproduce the superconductivity results. A series of in situ synchrotron-based investigations reveal that this is due to the necessary formation of an initial, ultrathin layer at the Nd0.8Sr0.2NiO3 surface that helps to mediate the introduction of hydrogen into the film such that apical oxygens are first removed from the Nd0.8Sr0.2NiO3 / SrTiO3 (001) interface and delivered into the reducing environment. This allows the square-planar / perovskite interface to stabilize and propagate from the bottom to the top of the film without the formation of interphase defects. Importantly, neither geometric rotations in the square planar structure nor significant incorporation of hydrogen within the films is detected, obviating its need for superconductivity. These findings unveil the structural basis underlying the transformation pathway and provide important guidance on achieving the superconducting phase in reduced nickelate systems.

Understanding the microscopic origin of the superior electromechanical response in relaxor ferroelectrics requires knowledge not only of the atomic-scale formation of polar nanodomains (PNDs) but also the rules governing the arrangements and stimulated response of PNDs over longer distances. Using x-ray coherent nanodiffraction, we show the staggered self-assembly of PNDs into unidirectional mesostructures that we refer to as polar laminates in the relaxor ferroelectric 0.68PbMg1/3Nb2/3O3-0.32PbTiO3(PMN-0.32PT). We reveal the highly heterogeneous electric-field–driven responses of intra- and interlaminate PNDs and establish their correlation with the local strain and the nature of the PND walls. Our observations highlight the critical role of hierarchical lattice organizations on macroscopic material properties and provide guiding principles for the understanding and design of relaxors and a wide range of quantum and functional materials.

The doping dependent phase diagram of the iron pnictide systems displays diverse electronic ground states including unconventional superconductivity and magnetic ordering. From previous bulk measurements, it was argued that the superconducting phase of Ba1-xKxFe2As2 might be described within a two band formalism where superconductivity emerges with significantly different magnitudes of the pairing amplitude in the different bands. We have performed point contact Andreev reflection spectroscopy on the optimally doped system (x x = 0.4) where we found features that misleadingly mimic the signature of multiple gap amplitudes with large energy difference, when the point contacts are away from the ballistic regime. Closer to the ballistic regime, we found two types of spectra. In one type, a single superconducting gap with unusual broadening was found. The broadening might be due to the presence of multiple gap amplitudes with small energy spacing. The other kind of spectra displayed spectral features at similar to 30 meV in the normal state that gradually diminished with increasing temperature and eventually disappeared at 140 K, the spin density wave transition temperature of parent BaFe2As2. We attribute the 30 meV spectral feature to a characteristic electron-magnon interaction energy scale in the system.

Recently, superconductivity at high temperatures has been observed in bulk La$_3$Ni$_2$O$_{7-{\delta}}$ under high pressure. However, the attainment of high-purity La$_3$Ni$_2$O$_{7-{\delta}}$ single crystals, exhibiting controlled and homogeneous stoichiometry through the post-annealing process in an oxygen-rich floating zone furnace, remains a formidable challenge. Here, we report the crystal structure and physical properties of single crystals of Sr-doped La$_3$Ni$_2$O$_7$ synthesized at high pressure (20 GPa) and high temperature (1400 {\deg}C). Through single crystal X-ray diffraction, we showed that high-pressure-synthesized paramagnetic Sr-doped La$_3$Ni$_2$O$_7$ crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2){\deg}out-of-plane and 175.0(2){\deg}and 176.7(2){\deg}in plane. The substitution of Sr alters in band filling and the ratio of Ni$^{2+}$/Ni$^{3+}$ in Sr-doped La$_3$Ni$_2$O$_7$, aligning them with those of "La$_3$Ni$_2$O$_{7.05}$", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La$_3$Ni$_2$O$_7$ exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr-doped La$_3$Ni$_2$O$_7$ transits back from a metal-like behavior to an insulator. The insulator-metal-insulator trend under high pressure dramatically differs from the behavior of the parent compound La$_3$Ni$_2$O$_{7-{\delta}}$, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

Material functionality can be strongly determined by structure extending only over nanoscale distances. The pair distribution function presents an opportunity for structural studies beyond idealized crystal models and to investigate structure over varying length scales. Applying this method with ultrafast time resolution has the potential to similarly disrupt the study of structural dynamics and phase transitions. Here we demonstrate such a measurement of CuIr2S4 optically pumped from its low-temperature Ir-dimerized phase. Dimers are optically suppressed without spatial correlation, generating a structure whose level of disorder strongly depends on the length scale. The redevelopment of structural ordering over tens of picoseconds is directly tracked over both space and time as a transient state is approached. This measurement demonstrates the crucial role of local structure and disorder in non-equilibrium processes as well as the feasibility of accessing this information with state-of-the-art XFEL facilities.

The R BaCo 4 O 7 system is a prototype geometrically frustrated magnet in which kagome planes and triangular layers of Co-O tetrahedra interleave. For R = Y, an antiferromagnetic ground state is realized due to a frustration-breaking trigonal-orthorhombic phase transition. For R = Lu, however, a long-range ordered state has rarely, if ever, been reported despite a similar symmetry-breaking transition, albeit at a significantly lower temperature. To explore this dichotomy, we present a comprehensive magnetic and structural phase diagram for Y1-xLuxBaCo4O7, 1- x Lu x BaCo 4 O 7 , established through complementary neutron diffraction and magnetization measurements. Our results outline the phase evolution of the nuclear structures in response to changes in composition and temperature. The temperature of the trigonal (P31c) P 31 c ) to orthorhombic (Pbn21) Pbn 2 1 ) transition, T s1 , decreases monotonically with increasing Lu content from 310 K for x = 0.0 . 0 to 110 K for x = 1.0. . 0. In Lu-rich compositions (0.7 . 7 x 1.0), . 0), first-order structural transitions are observed with coexisting and competing orthorhombic Pbn 2 1 and metastable monoclinic Cc phases. For the magnetically ordered Y-rich compositions, T- and x-dependent refinements of the magnetic structure reveal an antiferromagnetic "ribbonlike" arrangement of Co spin pairs in both the triangular and the kagome layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.

Multiple anomalous features in electronic spectra of metals with a kagome lattice structure-van Hove singularities, Dirac points, and flat bands-imply that materials containing this structural motif may lie at a nexus of topological and correlated electron physics. Due to the prospects of such exceptional electronic behavior, the recent discovery of superconductivity coexisting with charge-density wave (CDW) order in the layered kagome metals A V 3 Sb 5 (A A = K,Rb,Cs) has attracted considerable attention. Notably, these archetypal kagome metals express unconventional magnetotransport behavior, including an unexpected linear-in-H H diagonal resistivity at low fields, and an even more peculiar, nonmonotonic sign-changing behavior of the Hall resistivity, which has been speculated to arise from a chiral CDW. We argue here that this unusual magnetotransport derives not from such unconventional phenomena, but rather from the unique fermiology of the A V 3 Sb 5 materials. Specifically, it is caused by a large, concave hexagonal Fermi surface sheet formed in the close proximity to the van Hove singularities, which is backfolded into a small hexagonal sheet and two large triangular sheets in the CDW state. We introduce and analyze a model of the electronic structure of these Fermi surface sheets that allows for a full analytical treatment within Boltzmann kinetic theory and that enables semi-quantitative fits of our transport data. Specifically, we find that the anomalous magnetotransport behavior is caused by the confluence of strong reduction of the Fermi velocity near the van Hove singularities located near the vertices of the hexagonal sheet and sharp corners in Fermi surface generated by the CDW reconstruction. Our analytical approach not only explains the anomalous magnetotransport in the kagome superconductors but also can be extended to a variety of metallic systems hosting singular features in their Fermi surfaces.

We report on Raman scattering and 23 Na nuclear magnetic resonance (NMR) investigations of j eff = 1 / 2 hyperkagome antiferromagnet Na 4- x Ir 3 O 8 , which lies in the Mott insulating state. Our Raman scattering experiments unveil remarkable parallels between the magnetic excitations of the pristine Na 4 Ir 3 O 8 and the Kitaev honeycomb material Na 2 IrO 3 , which are characterized by dominant fractional spinon excitations. In the case of moderate Na-deficient Na 4- x Ir 3 O 8 , however, a substantial suppression of the magnetic excitations is observed, alluding to the notable influence of charge fluctuations on spin dynamics. In addition, our site -specific 23 Na NMR measurements offer further insights into the spin dynamics when a minor concentration of holes is introduced into a spin -orbit coupled Mott insulator. Specifically, the spin -lattice relaxation rate 1 / T 1 reveals the emergence of pseudogaplike correlations at the Na(2) site, alongside a critical slowing down behavior at the Na(1) and Na(3) sites. These findings showcase the intricate interplay between itinerant holes and magnetic correlations in a spin -liquid -like background.

We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide, which stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase $La_{3}Ni_{2}O_{7}$adopts a novel stacking sequence in which single and trilayer blocks of $NiO_{6}$ octahedra alternate in a 1313 sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imcm variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with onset at T = 134 K, which lies intermediate between that of the standard 2222 polymorph of $La_{3}Ni_{2}O_{7}$ (space group Amam) and the trilayer RP phase, $La_{4}Ni_{3}O_{10}$. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity with $T_{c}$ as high as 80 K above 14 GPa in bilayer $La_{3}Ni_{2}O_{7}$.

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of 6.0 T and 1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.

Polarons-fermionic charge carriers bearing a strong companion lattice deformation-exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La_{1.2}Sr_{1.8}Mn_{2}O_{7} across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase-a dynamic polaron liquid.

Magneto-transport measurements of Pr_4Ni_3O_10 single crystals performed under externally applied pressures up to 73 GPa in diamond anvil cells with either KBr or Nujol oil as pressure media yield signatures of superconductivity with a maximum onset temperature of approximately 31 K. True zero resistance was not observed, consistent with a non-percolating superconducting volume fraction. Magnetization measurements provided corroborating evidence of superconductivity, with a pressure-dependent diamagnetic signal occurring below the onset temperature, and an estimate from the absolute value of the susceptibility suggests a superconducting volume fraction on the order of 10 magnitude and pressure dependence of Tc as well as a dependence on the configuration of electrical contacts on a given sample. Possible causes of this behavior may be significant inhomogeneities in the pressure and/or damage to the samples induced by the pressure media as well as inhomogeneities in the crystals themselves. The results imply that our as-grown Pr_4Ni_3O_10 single crystals are not bulk superconductors but that there is a minority structure present within the crystals that is indeed superconducting.

The $R\mathrm{BaC}{\mathrm{o}}_{4}{\mathrm{O}}_{7}$ system is a prototype geometrically frustrated magnet in which kagome planes and triangular layers of Co-O tetrahedra interleave. For $R=\mathrm{Y}$, an antiferromagnetic ground state is realized due to a frustration-breaking trigonal-orthorhombic phase transition. For $R=\mathrm{Lu}$, however, a long-range ordered state has rarely, if ever, been reported despite a similar symmetry-breaking transition, albeit at a significantly lower temperature. To explore this dichotomy, we present a comprehensive magnetic and structural phase diagram for ${\mathrm{Y}}_{1--x}\mathrm{L}{\mathrm{u}}_{x}\mathrm{BaC}{\mathrm{o}}_{4}{\mathrm{O}}_{7}$, established through complementary neutron diffraction and magnetization measurements. Our results outline the phase evolution of the nuclear structures in response to changes in composition and temperature. The temperature of the trigonal ($P31c$) to orthorhombic ($Pbn{2}_{1}$) transition, ${T}_{\mathrm{s}1}$, decreases monotonically with increasing Lu content from 310 K for $x=0.0$ to 110 K for $x=1.0$. In Lu-rich compositions ($0.7\ensuremath{\le}x\ensuremath{\le}1.0$), first-order structural transitions are observed with coexisting and competing orthorhombic $Pbn{2}_{1}$ and metastable monoclinic $\mathit{Cc}$ phases. For the magnetically ordered Y-rich compositions, $T$- and $x$-dependent refinements of the magnetic structure reveal an antiferromagnetic ``ribbonlike'' arrangement of Co spin pairs in both the triangular and the kagome layers. A gradual suppression of long-range magnetic order is observed with increasing the Lu content, accompanied by the development of short-range magnetic correlations present in all the samples.

Pd_3Bi_2Se_2 is a rare realization of a superconducting metal with a non-zero Z_2 topological invariant. We report the growth of high-quality single crystals of layered Pd_3Bi_2Se_2 with a superconducting transition at T_c 0.80 K and upper critical fields of 10 mT and 5 mT for the in-plane and out-of-plane directions, respectively. Our density functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level, all displaying clear three-dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd_3Bi_2Se_2 is evident in magneto-transport measurements, yielding a sign-changing Hall resistivity at low temperatures. The magnetoresistance is non-saturating and follows Kohler's scaling rule. We interpret the magneto-transport data in terms of open orbits that are revealed in the DFT-calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F_α = (150 ± 26)T, F_β = (293 ± 10)T, F_γ = (375 ± 20)T, and F_η = (1017 ± 12)T, with the low frequency dominating the spectrum. Through the measurement of angular dependent dHvA oscillations and DFT calculations, we identify the F_α frequency with an approximately ellipsoidal electron pocket centered on the L_2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass: m^* = (0.11 ± 0.02) m_0 and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd_3Bi_2Se_2 as a potential candidate for exploring topological superconductivity.

Pd3Bi2Se2 is a rare realization of a superconducting metal with a nonzero Z(2) topological invariant. Here, we report the growth of high-quality single crystals of layered Pd3Bi2Se2 with a superconducting transition at T-c approximate to 0.80 K and upper critical fields of similar to 10 and similar to 5 mT for the in-plane and out-of-plane directions, respectively. Our density-functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level, all displaying clear three-dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd3Bi2Se2 is evident in magnetotransport measurements, yielding a sign-changing Hall resistivity at low temperatures. The magnetoresistance is nonsaturating and follows Kohler's scaling rule. We interpret the magnetotransport data in terms of open orbits that are revealed in the DFT-calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F-alpha = 150 +/- 26 T, F-beta = 293 +/- 10 T, F-gamma = 375 +/- 20 T, and F-eta = 1017 +/- 12 T, with the low frequency dominating the spectrum. Through the measurement of angular-dependent dHvA oscillations and DFT calculations we identify the F-alpha frequency with an approximately ellipsoidal electron pocket centered on the L-2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass m* = (0.11 +/- 0.02)m(0) and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd3Bi2Se2 as a potential candidate for exploring topological superconductivity.

The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, T a A s 2 , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ( 2 ‾ 01 ) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of T a A s 2 are trivial bulk bands. The absence of symmetry-protected surface state on the ( 2 ‾ 01 ) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of T a A s 2 .

Charge order is a central feature of the physics of cuprate superconductors and is known to arise from a modulation of holes with primarily oxygen character. Low-valence nickelate superconductors also host charge order, but the electronic character of this symmetry breaking is unsettled. Here, using resonant inelastic x-ray scattering at the Ni $L_2$-edge, we identify intertwined involvements of Ni $3d_{x^2-y^2}$, $3d_{3z^2-r^2}$, and O $2p_σ$ orbitals in the formation of diagonal charge order in an overdoped low-valence nickelate La$_{4}$Ni$_{3}$O$_{8}$. The Ni $3d_{x^2-y^2}$ orbitals, strongly hybridized with planar O $2p_σ$, largely shape the spatial charge distribution and lead to Ni site-centered charge order. The $3d_{3z^2-r^2}$ orbitals play a small, but non-negligible role in the charge order as they hybridize with the rare-earth $5d$ orbitals. Our results reveal that the low-energy physics and ground-state character of these nickelates are more complex than those in cuprates.

Quantum materials display rich and myriad types of magnetic, electronic, and structural ordering, often with these ordering modes either competing with one another or 'intertwining', that is, reinforcing one another. Low dimensional quantum materials, influenced strongly by competing interactions and/or geometric frustration, are particularly susceptible to such ordering phenomena and thus offer fertile ground for understanding the consequent emergent collective quantum phenomena. Such is the case of the quasi-2D materials R4Ni3O10(R=La, Pr), in which intertwined charge-and spin-density waves (CDW and SDW) on the Ni sublattice have been identified and characterized. Not unexpectedly, these density waves are largely 2D as a result of weak coupling between planes, compounded with magnetic frustration. In the case of R=Pr, however, we show here that exchange coupling between the transition metal and rare earth sublattices upon cooling overcomes both obstacles, leading to a dimensional crossover into a fully 3D ordered and coupled SDW state on both sublattices, as an induced moment on notionally nonmagnetic Pr3+ opens exchange pathways in the third dimension. In the process, the structure of the SDW on the Ni sublattice is irreversibly altered, an effect that survives reheating of the material until the underlying CDW melts. This 'bootstrapping' mechanism linking incommensurate SDWs on the two sublattices illustrates a new member of the multitude of quantum states that low-dimensional magnets can express, driven by coupled orders and modulated by frustrated exchange pathways.