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Charge orders (COs) and entangled topology denote the fundamentals of correlated electron systems at low temperatures, and their profound revelation at atomic scale awaits experimental advances. Here, we report such a spatially resolved investigation using cryogenic atomic-scale electron microscopy and spectroscopy at 100 K, with the CO insulator of $\mathrm{Hg}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ below 240 K as an exemplification. By the cryogenic technique, we can resolve the charge and lattice characteristics of the insulating CO phase at atomic resolution and identify the order parameters (OPs) at play as charge-density-modulated (\ensuremath{\sim}31.4 \AA{}, modulation length) and antiphase (\ensuremath{\sim}10.5 \AA{}) OPs. Both OPs are largely unaddressed forms of COs in correlated oxides and display the emergent topology of intertwining mesoscopic stripes of 10--40 nm in width and \ensuremath{\ge}500 nm in length, distinctly different from the atomically narrow CO stripes conventional to correlated systems and depicting the unique electronic contour of the phase. This preliminary demonstration of the cryogenic unveiling of the OPs and topological landscape paves the way to disentangling the multifarious COs and hidden topology in condensed matters and, meanwhile, calls for further developments in the atomic-scale cryogenic spectral probing at an enhanced speed.

We have successfully grown single crystals of $\mathrm{Pb}{\mathrm{Mn}}_{2}{\mathrm{Ni}}_{6}{\mathrm{Te}}_{3}{\mathrm{O}}_{18}$ and present a comprehensive study of their magnetic, thermodynamic, and Raman spectroscopic properties. $\mathrm{Pb}{\mathrm{Mn}}_{2}{\mathrm{Ni}}_{6}{\mathrm{Te}}_{3}{\mathrm{O}}_{18}$ consists of a planar network of pairwise rotated $\mathrm{Ni}{\mathrm{O}}_{6}$ dimers coupled by corners. Similarities to ${\mathrm{Ni}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ exist, which forms honeycomb layers. The magnetic susceptibility \ensuremath{\chi} and heat capacity ${C}_{p}$ data reveal an antiferromagnetic phase transition around 84 K, which is evidently hysteretic on warming and cooling between 94 and 40 K with a loop width of about 1.83\ifmmode\pm\else\textpm\fi{} 0.41 K; thus the transition appears to be of first order. \ensuremath{\chi} is anisotropic, with larger values for in-plane fields over the entire measured temperature range. Raman spectroscopy has been employed to investigate the lattice and magnetic excitations of the $\mathrm{Pb}{\mathrm{Mn}}_{2}{\mathrm{Ni}}_{6}{\mathrm{Te}}_{3}{\mathrm{O}}_{18}$ from 5 to 300 K. Besides an anharmonic phonon behavior, i.e., a decay into acoustic phonons, we find a coupling to the spin system at ${T}_{N}=84\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, as well as weak anomalies at ${T}^{*}\ensuremath{\approx}200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This second characteristic temperature gives evidence for an instability of the coupled spin/lattice system, as here the phonon linewidths and intensities evidence similar behavior as at ${T}_{N}=84\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Furthermore, magnetic Raman scattering at $240\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ is used to estimate an exchange coupling of $J=--86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in general agreement with the Curie-Weiss temperature.

The $B$-site ordered double perovskites ${\mathrm{Sr}}_{2}\mathrm{Cu}({\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}){\mathrm{O}}_{6}$ provide an excellent arena for investigating exotic phases expected for the ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ square-lattice Heisenberg antiferromagnet. Here, combining magnetic susceptibility and specific-heat measurements with electron spin resonance (ESR) and muon spin rotation/relaxation $(\ensuremath{\mu}\mathrm{SR})$ techniques, we explore a spin-liquid-like state in the vicinity of the N\'eel critical end point $(x=0.05--0.1)$. The specific heat and the ESR and muon relaxation rates give evidence for an energy hierarchy of low-energy excitations, reminiscent of randomness-induced singlet states. In addition, the weak transverse $\ensuremath{\mu}\mathrm{SR}$ data show a fraction of frozen magnetic moments in the random-singlet background. The origin of a random-singlet-like state near the phase boundary is discussed in terms of concomitant exchange randomness and local strain generated by the ${\mathrm{W}}^{6+}\text{\ensuremath{-}}\mathrm{for}\text{\ensuremath{-}}{\mathrm{Te}}^{6+}$ substitution.

BaFexNi1–xO3 with end members of BaNiO3 (x = 0) and BaFeO3 (x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba4Fe3NiO12 (x = 0.75), that adopts the 12R perovskite structure with the space group R3m (a = 5.66564(7) A and c = 27.8416(3) A), was found to be stabilized. Mossbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. The magnetic properties of the compound are also discussed.

A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ -> 0.75Fe(3+) + 0.25Fe(5+)) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R (3) over barc crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3+ spins align antiferromagnetically and one-fourth of the Fe3+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins.

We have studied Fe1-xCoxSi (0 ≤ x ≤ 1) crystals using the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements, to examine the influence of Co substitution on the electrical and thermal transport properties of FeSi. Notably, a metal-insulator transition occurs at x = 0.3 for the Fe1-xCoxSi series, as the closure of band-gap of FeSi occurs upon Co substitution. Besides, the substituent (Co) induced a significant variation in the Seebeck coefficient, including two sign crossovers. These observations are ascribed to the asymmetric electronic band structure of the Fe1-xCoxSi compounds. The analysis of thermal conductivity demonstrated that Co substitution has a strong effect on the low-temperature phonon thermal transport of FeSi via boundary scattering and point-defect scattering. Most intriguingly, the substituted end compound CoSi has the largest room temperature thermoelectric figure of merit of about 0.06 which is four orders of magnitude larger than that of undoped FeSi.

dc and ac magnetic susceptibility, magnetization, specific heat, and Raman scattering measurements are combined to probe low-lying spin excitations in $\ensuremath{\alpha}$-${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{Cl}}_{3}$ ($x\ensuremath{\approx}0.2$), which realizes a disordered spin liquid. At intermediate energies ($\ensuremath{\hbar}\ensuremath{\omega}g3\text{ }\text{ }\mathrm{meV}$), Raman spectroscopy evidences linearly $\ensuremath{\omega}$-dependent Majorana-like excitations, obeying Fermi statistics. This points to robustness of a Kitaev paramagnetic state under spin vacancies. At low energies below 3 meV, we observe power-law dependences and quantum-critical-like scalings of the thermodynamic quantities, implying the presence of a weakly divergent low-energy density of states. This scaling phenomenology is interpreted in terms of the random hoppings of Majorana fermions. Our results demonstrate an emergent hierarchy of spin excitations in a diluted Kitaev honeycomb system subject to spin vacancies and bond randomness.

We report on magnetization M(H), dc and ac magnetic susceptibility χ(T), specific heat C_{m}(T) and muon spin relaxation (μSR) measurements of the Kitaev honeycomb iridate Cu_{2}IrO_{3} with quenched disorder. In spite of the chemical disorders, we find no indication of spin glass down to 260 mK from the C_{m}(T) and μSR data. Furthermore, a persistent spin dynamics observed by the zero-field muon spin relaxation evidences an absence of static magnetism. The remarkable observation is a scaling relation of χ[H,T] and M[H,T] in H/T with the scaling exponent α=0.26-0.28, expected from bond randomness. However, C_{m}[H,T]/T disobeys the predicted universal scaling law, pointing towards the presence of additional low-lying excitations on the background of bond-disordered spin liquid. Our results signify a many-faceted impact of quenched disorder in a Kitaev spin system due to its peculiar bond character.

We report the high pressure synthesis of a layered perovskite Ca2GeO4 which is found to have the Ruddlesden–Popper structure with I41/acd symmetry. Consonant with\ud our recent predictions [Ablitt et al., npj Computational Materials, 2017, 3, 44], the phase displays pronounced uniaxial negative thermal expansion over a large temperature range. Negative thermal expansion that persists over a large temperature range is very unusual in the perovskite structure, and its occurrence in this instance can be understood to arise due to both soft lattice vibrations associated with a phase competition and the unusually compliant nature of this structure, which effectively couples thermal expansion in the layer plane to lattice contractions perpendicular to the layering direction via a “corkscrew” mechanism.

We have investigated the nature of the short-range magnetic phase region of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{IrO}}_{4}$ ($0\ensuremath{\le}x\ensuremath{\le}0.14$). This so-called spin-orbit Mott insulator's predicted transition to superconductivity with electron doping has remained elusive. Instead, short-range magnetism prevails over an extensive doping range up to the La solubility limit. We find evidence for quantum critical-like fluctuations of the short-range magnetic phase, in both a scaling relation of the ac susceptibility $\ensuremath{\chi}{T}^{\ensuremath{\alpha}}$ and the magnetization $M{T}^{\ensuremath{\alpha}\ensuremath{-}1}$ with $\ensuremath{\alpha}=0.4--0.5$, as well as in the time-field scaling ($t/{H}^{\ensuremath{\gamma}}$) of the $\ensuremath{\mu}\mathrm{SR}$ asymmetry function. Our combined study of ac susceptibility and $\ensuremath{\mu}\mathrm{SR}$, on the one hand, uncovers a two-stage weak symmetry breaking to spin freezing or proximate spin glass and to a subsequent spin glass within the short-range magnetic phase. On the other hand, we demonstrate that magnetic correlations become more static above $x=0.06$ in spite of an apparent dilution of magnetic moments, indicative of enhanced inhomogeneities in a higher La-doping regime.

It is of great interest to design and make materials in which ferroelectric polarisation is coupled to other order parameters such as lattice, magnetic and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realised. Here we report detailed crystallographic studies of a novel perovskite Hg$^{\textbf{A}}$Mn$^{\textbf{A'}}_{3}$Mn$^{\textbf{B}}_{4}$O$_{12}$ that is found to exhibit a polar ground state on account of such couplings that arise from charge and orbital ordering on both the A' and B-sites, which are themselves driven by a highly unusual Mn$^{A'}$-Mn$^B$ inter-site charge transfer. The inherent coupling of polar, charge, orbital and hence magnetic degrees of freedom, make this a system of great fundamental interest, and demonstrating ferroelectric switching in this and a host of recently reported hybrid improper ferroelectrics remains a substantial challenge., 9 pages, 7 figures

We investigated moderate-temperature oxygen diffusion mechanisms in Sr2ScGaO5 with Brownmillerite structure type. From oxygen isotope 18O−16O exchange experiments we determined that oxygen mobility sets in above 550 °C. Temperature-dependent neutron and X-ray (synchrotron) diffraction experiments allowed us to correlate the oxygen mobility with a subtle phase transition of the orthorhombic room-temperature structure with I2mb space group toward Imma, going along with a disorder of the (GaO4)∞-tetrahedral chains. From lattice dynamical simulations we could clearly evidence that dynamic switching of the (GaO4)∞-tetrahedral chains from its R to L configuration sets in at 600 °C, thus correlating oxygen diffusion with the dynamic disorder. Oxygen ion diffusion pathways are thus constrained along the onedimensional oxygen vacancy channels, which is a different diffusion mechanism compared to that of the isostructural CaFeO2.5, where diffusion of the apical oxygen atoms into the vacant lattice sites are equally involved in the diffusion pathway. The proposed ordered room-temperature structure in I2mb is strongly supported by 17O, 45Sc, and 71Ga NMR measurements, which indicate the presence of crystallographically unique sites and the absence of local disordering effects below the phase transition. The electric field gradient tensor components measured at the nuclear sites are found to be in excellent agreement with calculated values using the WIEN2k program. The oxygen site assignment has been independently confirmed by 17O{45Sc} transfer of adiabatic populations double-resonance experiments.

The oxygen evolution reaction (OER) plays a key role in emerging energy conversion technologies such as rechargeable metal‐air batteries, and direct solar water splitting. Herein, a remarkably low overpotential of ≈150 mV at 10 mA cm−2 disk in alkaline solutions using one of the non‐Fermi liquids, Hg2Ru2O7, is reported. Hg2Ru2O7 displays a rapid increase in current density and excellent durability as an OER catalyst. This outstanding catalytic performance is realized through the coexistence of localized d‐bands with the metallic state that is unique to non‐Fermi liquids. The findings indicate that non‐Fermi liquids could greatly improve the design of highly active OER catalysts.

We report high-precision magnetization ([Formula: see text]), magnetic susceptibility ([Formula: see text]), specific heat (C p (T, H)) and 'zero-field' electrical resistivity, [Formula: see text], data taken on Gd2Te3 single crystal over wide ranges of temperature and magnetic field (H), with either [Formula: see text]-axis or [Formula: see text]-plane. [Formula: see text] and [Formula: see text] unambiguously establish that the b-axis is the easy direction of magnetization whereas any direction in the ac-plane is a hard direction. The [Formula: see text]-type anomaly in 'zero-field' specific heat, C p (T, H = 0), and an abrupt drop in [Formula: see text] (characteristic of the paramagnetic (PM) - antiferromagnetic (AFM) phase transition) are observed at the Neel temperature, [Formula: see text] K. [Formula: see text] and C p (T,H) clearly demonstrate that [Formula: see text] shifts to lower temperatures with increasing H irrespective of whether H points in the easy or hard direction. When [Formula: see text], the [Formula: see text] isotherms at temperatures in the range 2.5 K [Formula: see text] [Formula: see text] K reveal the existence of a field-induced spin-flop (SF) transition at fields 4.0 T [Formula: see text] [Formula: see text] [Formula: see text] 4.5 T. The first principles electronic band structure and density of states calculations, based on the density functional theory, correctly predict an AFM ground state (stabilized primarily by the 4f Gd3+ - 5p Te2-- 4f Gd3+ superexchange interactions) and the observed semi-metallic behavior for the Gd2Te3 compound. Moreover, these calculations yield the values [Formula: see text] [Formula: see text] for the ordered magnetic moment per Gd atom at T = 0, [Formula: see text] mJ mol-1 K-2 for the Sommerfeld coefficient for the electronic specific heat contribution and [Formula: see text] K for the Curie-Weiss temperature, respectively. These theoretical estimates conform well with the corresponding experimental values [Formula: see text] [Formula: see text], [Formula: see text] mJ mol-1 K-2 and [Formula: see text] K.

Using magnetization, dielectric constant, and neutron diffraction measurements on a high quality single crystal of YBaCuFeO5 (YBCFO), we demonstrate that the crystal shows two antiferromagnetic transitions at [Formula: see text] K and [Formula: see text] K, and displays a giant dielectric constant with a characteristic of the dielectric relaxation at T N2. It does not show the evidence of the electric polarization for the crystal used for this study. The transition at T N1 corresponds with a paramagnetic to antiferromagnetic transition with a magnetic propagation vector doubling the unit cell along three crystallographic axes. Upon cooling, at T N2, the commensurate spin ordering transforms to a spiral magnetic structure with a propagation vector of ([Formula: see text] [Formula: see text] [Formula: see text]), where [Formula: see text], [Formula: see text], and [Formula: see text] are odd, and the incommensurability δ is temperature dependent. Around the transition boundary at T N2, both commensurate and incommensurate spin ordering coexist.