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Solid solutions with the formula of Bi2–xA′xRu2O7–y (A′ = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) have been synthesized and characterized. The crystal structures f...

Geometrically frustrated systems populated with large spin-orbit coupled ions are an ideal setting for the exploration of novel exotic states of matter. Here we present an example of iridium on a mixed B-site spinel oxide structure: $\mathrm{Cu}[{\mathrm{Ir}}_{1.498(2)}{\mathrm{Cu}}_{0.502(2)}]{\mathrm{O}}_{4}$. Synchrotron XRD refinements reveal a face-centered-cubic structure with space group $Fd\overline{3}m$ and mixed Cu-Ir site disorder within the ${\mathrm{B}}_{2}{\mathrm{O}}_{4}$ rocksalt substructure. Electrical properties reveal a metallic state within the 50--600-K range with a Kondo effect at $Tl50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. X-ray absorption spectroscopy (XAS) measurements show a mixed ${\mathrm{Cu}}^{1+/2+}$ and ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, which suggests a metallic/band description with reduced on-site Coulomb interactions. Spin-glass-like freezing is seen at ${T}_{\mathrm{g}}=49\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the hysteresis behavior for $Tg{T}_{\mathrm{g}}$ resembles that of a strongly frustrated magnet. DFT calculations show sizable hybridization between the Cu $3d$ and Ir $5d$ states with an effective mixed ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, supporting the electronic and XAS results.

Osmium-containing oxides are rare due to the difficulty in stabilizing complex structures with a fixed stoichiometry and metastability of the phases. Bismuth-substituted thallium osmate pyrochlore samples, Tl2-xBixOs2O7-y, were synthesized using solid-state reactions where the solubility limit was found to be approximately x = 1.4. Members of this solid solution were characterized by their structural, electronic, magnetic, and thermal properties to understand the influence of Bi3+ substitution on the ground state. The Os-containing pyrochlores crystallize in the ideal cubic pyrochlore structure (Fd3m), and the lattice parameter a was found to slightly increase as a function of Bi content. A possible interplay between structure and cation valence states was explored using both neutron powder diffraction and X-ray absorption spectroscopy, suggesting that a combination of Os4+/Os5+ and Tl1+/Tl3+ mixed valency throughout the solid solution allows for the stabilization of the pyrochlore structure. The system is metallic for the entire solid solution and predominantly exhibits temperature-independent paramagnetism. Specific heat measurements show an enhanced Sommerfeld coefficient, a possible flat-band signature. This system gave insight into the bonding preferences of Os, indicating a dependence on high oxidation states and mixed valence for the stability of complex structures.

We investigate the spin-glass transition in the strongly frustrated well-known compound $Fe_2TiO_5$. A remarkable feature of this transition, widely discussed in the literature, is its anisotropic properties: the transition manifests itself in the magnetic susceptibly only along one axis, despite $Fe^{3+}$ $d^5$ spins having no orbital component. We demonstrate, using neutron scattering, that below the transition temperature $T_g = 55 K$, $Fe_2TiO_5$ develops nanoscale surfboard shaped antiferromagnetic regions in which the $Fe^{3+}$ spins are aligned perpendicular to the axis which exhibits freezing. We show that the glass transition may result from the freezing of transverse fluctuations of the magnetization of these regions and we develop a mean-field replica theory of such a transition, revealing a type of magnetic van der Waals effect., 14 pages, 4 figures, 6 page supplemental material, 2 figures

The plateau at $1/3$ of the saturation magnetization ${M}_{s}$ in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured $M$ and the specific heat $C$ in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with $\mathrm{\ensuremath{\Delta}}{S}_{z}=1$ as measured by $M$, coexist with a much larger population of $\mathrm{\ensuremath{\Delta}}{S}_{z}=0$ excitations measured by $C$ but invisible to $M$. The large density of $\mathrm{\ensuremath{\Delta}}{S}_{z}=0$ excitations, their energy gap, and their seeming lack of interaction with $\mathrm{\ensuremath{\Delta}}{S}_{z}=1$ excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in $M(H)$ are seen, the size of which, $0.15%{M}_{s}$, is consistent with fractional quantization of magnetization per site in the down-spin layers.

$\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ represents a borderline case in the range of solid solutions formed in the pseudobinary system $\mathrm{LaC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}\text{\ensuremath{-}}\mathrm{CeC}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$. The material undergoes ferromagnetic ordering at \ensuremath{\sim}225 K followed by a structural collapse at \ensuremath{\sim}190 K, which leads to a strong suppression of magnetization. The structural phase transition manifests itself in a gradual decrease in the parameter $c$ and a relatively smaller increase of the parameter $a$ of the tetragonal lattice. Interestingly, a combination of magnetic measurements and nonpolarized and polarized neutron scattering experiments suggests that the structural collapse does not lead to an antiferromagnetically ordered state, observed in samples with the higher Ce content. On the contrary, $\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{P}}_{2}$ appears to enter a disordered, spin glass state, with gradual dissipation of the ferromagnetic ordering taking place simultaneously with the structural collapse, as evidenced by temperature-dependent measurements of the depolarization factor for a polarized neutron beam passing through the sample. The observed behavior is analogous to that reported for so-called reentrant spin glasses. In the present case, however, the appearance of the reentrant spin glass regime is caused not by tuning the chemical composition but by the structural phase transition. Electronic structure calculations confirm that the loss of magnetic ordering is caused by the subtle change to the density of states at the Fermi level due to the variation of the crystal structure of the material.

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled $J=1/2$ local moments such as iridates and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. Here we propose the monoclinic ${\mathrm{YbCl}}_{3}$ with a ${\mathrm{Yb}}^{3+}$ honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on ${\mathrm{YbCl}}_{3}$ single crystal. We find that the ${\mathrm{Yb}}^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J}_{\text{eff}}=1/2$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range N\'eel-type antiferromagnetic order at 0.60 K, below which the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)${}^{\ensuremath{\circ}}$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the N\'eel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) ${\ensuremath{\mu}}_{B}$ at 0.25 K, all indicate that ${\mathrm{YbCl}}_{3}$ could be a two-dimensional spin system to proximate the Kitaev physics.

Recently, rare earth iridates, R2Ir2O7, with the pyrochlore structure have been intensively investigated due to their promise as either topological Mott insulators or Weyl semimetals. Single crystals of such pyrochlores with R = Nd, Sm, Eu, and Dy were prepared hydrothermally in sealed gold tubes at 975 K and show significantly higher electrical resistivities than previously reported for either crystals or polycrystalline samples. Furthermore, none of the present crystals exhibit the metal-insulator transition found for some samples of these phases. Lower resistivities are ascribed to lack of control of x and y in R2-xIr2O7-y in other more commonly used synthesis methods, yielding uncertainty in the Ir oxidation state. We also report resistivity of R2Ru2O7 crystals for R = Yb, Gd, Eu, and Nd, prepared in the same manner. These results suggest that the observed charge transport in hydrothermally grown iridate crystals is that of essentially stoichiometric phases and is consistent a with the existence of Weyl nodes.

The heavy fermion compound URu2Si2 continues to attract great interest due to the long- unidentified nature of the hidden order that develops below 17.5K. Here we discuss the implications of an angular survey of the linear and nonlinear susceptibility of URu2Si2 in the vicinity of the hidden order transition. While the anisotropic nature of spin fluctuations and low-temperature quasiparticles was previously established, our recent results suggest that the order parameter itself has intrinsic Ising anisotropy, and that moreover this anisotropy extends far above the hidden order transition. Consistency checks and subsequent questions for future experimental and theoretical studies of hidden order are discussed., Comment: Contributed article to SCES 2017, Prague

Large Fe3O4@SiO2 nanoparticles (∼200 nm) functionalized with gold and poly(vinylpyrrolidone) have been synthesized, characterized, and evaluated for bioseparation and sensing applications. The particles have been characterized using a combination of experimental techniques including ultraviolet visible spectroscopy, energy-dispersive spectroscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, superconducting quantum interference device magnetrometry, and surface-enhanced Raman sensing (SERS) spectroscopy. The particles have a unique surface morphology comprised of roughened gold nodules. The surface coatings prevent oxidation and render the particles easy to functionalize in order to target a wide range of moieties. The gold coverage is not only uniform across the entire particle surface but also ultrathin so as to maintain a high percentage of the cores magnetic saturation (∼68%) when compared to that of bare magnetite. The gold nodules facilitate the generation ...

An important challenge in magnetism is the unambiguous identification of a quantum spin liquid 1,2 , of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems 3,4 wherein classical order is suppressed by a frustrating lattice 5 , an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point—a region of highly fluctuating spins—by an applied magnetic field. Such systems suffer from a weak specific-heat peak at the quantum critical point, with little entropy available for quantum fluctuations 6 . Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K2PbCu(NO2)6. Across the temperature–magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point. Exploiting the magnetic field-induced shift of entropy in certain molecular salts when going from 1D short-range ordering to a 3D quantum critical point could provide a route for producing strongly fluctuating quantum materials.

Pyrochlore phases Bi2–xCaxIr2O6Oy′ with x from 0.0 to 1.0 have been evaluated based on Rietveld analysis of neutron diffraction data, electrical resistivity and thermopower data from 3 to 756 K, and magnetic susceptibility data from 3 to 298 K. The average Ir oxidation state is less than 4+ at low x, above 4+ for high x, and is very close to 4+ at x = 0.5. All samples show metallic properties with an electrical resistivity of ∼10–3 Ω·cm at room temperature. For low x, the sign of the Seebeck coefficient is negative at low temperature but becomes positive at high temperature. For high x, the sign of the Seebeck coefficient is positive from 3 to 756 K. Magnetic measurements indicate no magnetic ordering down to 3 K for all values of x. All Bi is in its ideal position for all values of x, but much of the Ca is strongly displaced from the ideal A cation site. This displacement of Ca apparently only occurs when there is an adjacent vacancy at the O′ site.

Two novel solid solution series, Ba2-xLaxInIrO6 (x=0–1.0) and BaLaIn1-yCayIrO6 (y=0–1.0), were prepared and several changes in structure, moment and charge transport were observed. The Ba2-xLaxInIrO6 series exhibits a transition from a 6M polytype to an orthorhombic perovskite structure with increased La content whereas the BaLaIn1-yCayIrO6 series transitioned from a disordered, orthorhombic perovskite to an ordered, cubic perovskite with increased Ca content. Seebeck measurements for both systems showed that Ir(IV)-rich compounds tended to have a n-type conduction mechanism while Ir(V)-rich compounds tended to be p-type. Both systems were found to be semiconducting and the magnitude of the resistivity is dependent on crystal structure and Ir environment. Magnetic measurements show that the μ eff values for both systems are significantly less than predicted for Ir(IV) (1.73 μ B ) and greater than predicted for Ir(V) (0 μ B ). These results are compared to other iridate compound families.

Osmium-containing oxides are rare due to the difficulty in stabilizing complex structures with a fixed stoichiometry and metastability of the phases. Bismuth-substituted thallium osmate pyrochlore samples, Tl

Samarium sulfide (SmS) is a nonmagnetic narrow-gap (0.06 eV) semiconductor that undergoes a transition to a metallic intermediate valence state at 6.5 kbar. Europium sulfide (EuS) is a ferromagnetic semiconductor with a Curie temperature of 16 K and a gap of 1.6 eV. Here we present a study of the lattice constant, magnetic susceptibility, and resistivity of the substitution series $\mathrm{S}{\mathrm{m}}_{1\ensuremath{-}x}\mathrm{E}{\mathrm{u}}_{x}\mathrm{S}$ for $0lxl1$. We observe a smooth interpolation of magnetic and transport behavior across the series, consistent with a virtual crystal scenario and Vegard's law. Surprisingly, however, the lattice constant deviates below Vegard's law in a manner that suggests parametric control of the Sm-Sm distance by the Eu moment in the manner of a magnetic polaron.

The solid solution series Li2Ir1-xRhxO3 is synthesized for several values of x between 0 and 1. The compounds possess a monoclinic layered structure (space group C2/m) throughout the solid solution range with the lattice constants following Vegard's relationship. Magnetization and resistivity data below room temperature are presented. The effective magnetic moment (μeff) is reduced below the value obtained by interpolating between the end-members, presumably due to nearest neighbor charge exchange leading to non-magnetic Ir5+/Rh3+ pairs. Surprisingly, the degree of reduction of μeff cannot be explained by a random mixture of Ir and Rh and, in particular, is strongly asymmetric around x = 0.5. This anomalous moment reduction possibly results from the difference in on-site Coulomb repulsion between Ir and Rh ions.

Dilution of iron in the AM12O19 has shown promising coupling of magnetic and electric properties in the hexaferrites. We have prepared samples of low iron content in the isostructural hibonite, CaAl12−xFexO19, to investigate the structural effects of larger iron substitution into the aluminum sites and the resultant properties. We found a solid solution exists for CaAl12−xFexO19 where x ≤ 5.5. Iron distributes into all sites with the tetrahedral M3 and octahedral M5 sites displaying nominally larger preference at higher Fe content. Paramagnetic behavior was observed for x ​= ​1–3, while x ​= ​4–5.5 displayed a broad ferromagnetic-like transition with increasing Curie temperature. Differences in field-cooled and zero-field-cooled magnetic susceptibility measurements and low magnetic moment indicate the presence of a frustrated antiferromagnetic structure as the source of the ferromagnetic-like behavior. Increasing Fe content also increases the dielectric constant up to 21 ​at room temperature with high dielectric loss.

Single-phase polycrystalline Li2PdO3 has been synthesized at 640 C in oxygen for the first time under ambient pressure. X-ray and neutron diffraction analyses show that the sample possesses a monoclinic layered structure belonging to the C2/m space group. Rietveld refinements of neutron powder diffraction data indicate ~10% Li–Pd site exchange and DIFFaX modelling manifest ~2% stacking faults present within LiPd2 layers. A band gap of ~2.23 eV was calculated for the golden Li2PdO3 using absorbance measurements. Thermogravimetric analysis of the sample shows that Li2PdO3 is stable up to 730 C under oxygen. A Curie tail is observed at low temperature magnetic measurements (T

Solid solutions of Bi12Mn1-xMxO20 (M ​= ​Ti, Si, Ge, x ​= ​0–1) and Bi12-xInxMn0.2Ti0.8O20 (x ​= ​0–4.0) having Bi12MO20 sillenite-type structure were synthesized through M-site and Bi-site substitutions and their properties were evaluated. Powder neutron and X-ray structure analyses confirm that all the compounds crystallize in space group I23 with Mn in the tetrahedral site. The color of Bi12Mn1-xMxO20 (M ​= ​Ti, Si, Ge) solid solutions varies from dark green (x ​= ​0) to intense green (x ​= ​0.8), and the x ​= ​1 phases Bi12MO20 (M ​= ​Ti, Si, Ge) are light yellow. Substituting indium for bismuth brings a yellowish hue to the Bi12Mn1-xTixO20 series as shown in Bi12-xInxMn0.2Ti0.8O20 (x ​= ​0–4.0). Optical measurements suggest that the d-d transition of Mn ions combined with the M4+–O2- charge transfer is responsible for the observed green colors. All compositions exhibit high near-infrared reflectance (NIR) which makes them potential candidates for cool pigment applications. The Bi12Mn1-xTixO20 (x ​= ​0.2, 0.6, 0.8) compounds show paramagnetic behavior in 5–300K temperature region, and the effective magnetic moment is slightly smaller than theoretical magnetic moment of tetravalent Mn. This is one of the few cases where tetrahedrally coordinated Mn4+ functions as a chromophore in inorganic metal oxides.

Polycrystalline samples of a new solid solution Li2Ru1-xRhxO3 have been synthesized at 1000 °C in oxygen. Miscibility is found to exist for 0.4 ≤ x ≤ 1. X-ray and neutron diffraction analyses show that the whole series possesses a monoclinic layered structure belonging to the C2/m space group with lattice constants decreasing with increasing Rh content. For 0 x ≠ 0,1 . Seebeck measurements show all of the compositions to be p-type above 300 K. The resistivity, ρ ( T ) , is well fit by a power law, ρ ( T ) ∝ T − ν , where 4.7 ≤ ν ≤ 9 depending on the composition, as opposed to Arrhenius-type behavior. This provides support for an electronic density of states that varies with energy as a power law, as opposed to that found for semiconductors or systems exhibiting variable range hopping, where charge transport is dictated by energy gaps.

The 1/8 fractional plateau phase (1/8 FPP) in Shastry-Sutherland lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB_{4} using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8 FPP is smoothly connected to the antiferromagnetic phase on ramping the field from H=0. Thus, the 1/8 FPP is not a distinct thermodynamic ground state of TmB_{4}. The implication of these results for Heisenberg spins on the SSL is discussed.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

Superconductivity in alkali-doped C 60 (A 3 C 60 , A = an alkali atom) is well described by an s-wave state produced by phonon mediated pairing. Moderate coupling of electrons to high-frequency shape-changing intra-molecular vibrational modes produces transition temperatures ( T c ) up to 33 K in single-phase material. The good understanding of pairing in A 3 C 60 offers a paradigm for the development of new superconducting materials.

The 1/8 fractional plateau phase (1/8 FPP) in Shastry-Sutherland lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB_{4} using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8 FPP is smoothly connected to the antiferromagnetic phase on ramping the field from H=0. Thus, the 1/8 FPP is not a distinct thermodynamic ground state of TmB_{4}. The implication of these results for Heisenberg spins on the SSL is discussed.

Quantum and frustrated magnets are a great platform to probe collective phenomena of fundamental and technological importance. Antiferromagnets on a diamond lattice have not been as extensively studied as other model magnetic materials due to the absence of obvious geometrical frustration. This is changing with the prediction --- and in some cases observation --- of several unique phenomena in magnetic A-site spinels like, e.g., degenerate spin spirals, spin-orbital entanglement and topological paramagnetism. In this work, the authors add two model materials to the growing family of diamond lattice antiferromagnets: CuRh${}_{2}$O${}_{4}$ and CoRh${}_{2}$O${}_{4}$. The authors deploy an arsenal of experimental and theoretical techniques to develop a detailed understanding of their results. Their work evidences that CoRh${}_{2}$O${}_{4}$ is a canonical diamond-lattice antiferromagnet while CuRh${}_{2}$O${}_{4}$ displays an unexpected spin-helix ground-state with a very strong influence from quantum fluctuations.

Pyrochlore phases Bi

We present a systematic study of the structural and magnetic properties of two branches of the rare-earth tripod-kagome-lattice (TKL) family ${A}_{2}{R}_{3}{\mathrm{Sb}}_{3}{\mathrm{O}}_{14}$ ($A=$ Mg, Zn; $R=$ Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb; here, we use abbreviation A-R, as in MgPr for ${\mathrm{Mg}}_{2}{\mathrm{Pr}}_{3}{\mathrm{Sb}}_{3}{\mathrm{O}}_{14}$), which complements our previously reported work on MgDy, MgGd, and MgEr [Z. L. Dun et al., Phys. Rev. Lett. 116, 157201 (2016)]. The present susceptibility (${\ensuremath{\chi}}_{\text{dc}}, {\ensuremath{\chi}}_{\text{ac}}$) and specific-heat measurements reveal various magnetic ground states, including the nonmagnetic singlet state for MgPr, ZnPr; long-range orderings (LROs) for MgGd, ZnGd, MgNd, ZnNd, and MgYb; a long-range magnetic charge ordered state for MgDy, ZnDy, and potentially for MgHo; possible spin-glass states for ZnEr, ZnHo; the absence of spin ordering down to 80 mK for MgEr, MgTb, ZnTb, and ZnYb compounds. The ground states observed here bear both similarities as well as striking differences from the states found in the parent pyrochlore systems. In particular, while the TKLs display a greater tendency towards LRO, the lack of LRO in MgHo, MgTb, and ZnTb can be viewed from the standpoint of a balance among spin-spin interactions, anisotropies, and non-Kramers nature of single-ion state. While substituting Zn for Mg changes the chemical pressure, and subtly modifies the interaction energies for compounds with larger $R$ ions, this substitution introduces structural disorder and modifies the ground states for compounds with smaller $R$ ions (Ho, Er, Yb).

Since its first application as a substrate for graphene field effect transistors (FETs), hexagonal boron nitride (hBN) has become a prominent component in two-dimensional (2D) material devices. In addition, hBN has been shown to host defects that can be manipulated to change the electronic properties of adjacent 2D materials. Despite the wide use of such defect manipulations, no focused efforts have been made to further the understanding of defect excitations and their influence in graphene/hBN FETs. In this study, we explore the effect of high electric fields ( ∼ 10 V / nm ) on graphene/hBN FETs and find that persistent and reversible shifts in graphene's charge neutrality point (CNP) occur. By increasing the applied electric field and temperature of our device, we find that this CNP shift is enhanced. With this insight, we propose a mechanism that explains these observations based on Poole–Frenkel emissions from defects in hBN. Finally, we show that such an effect may be suppressed by using graphite as a backgate, thus preventing unintended changes in the electrical properties of graphene/hBN FETs.Since its first application as a substrate for graphene field effect transistors (FETs), hexagonal boron nitride (hBN) has become a prominent component in two-dimensional (2D) material devices. In addition, hBN has been shown to host defects that can be manipulated to change the electronic properties of adjacent 2D materials. Despite the wide use of such defect manipulations, no focused efforts have been made to further the understanding of defect excitations and their influence in graphene/hBN FETs. In this study, we explore the effect of high electric fields ( ∼ 10 V / nm ) on graphene/hBN FETs and find that persistent and reversible shifts in graphene's charge neutrality point (CNP) occur. By increasing the applied electric field and temperature of our device, we find that this CNP shift is enhanced. With this insight, we propose a mechanism that explains these observations based on Poole–Frenkel emissions from defects in hBN. Finally, we show that such an effect may be suppressed by using graphit...

Advances in theoretical, computational and experimental materials science and engineering offer not only the promise to accelerate the pace at which new materials are discovered, but also to reduce the time required to bring new products to market. The so-called Materials Genome Initiative seeks to capitalize on that promise by identifying innovative research paradigms that integrate theory, computation, synthesis, and characterization in manners that, until recently, were not possible. A workshop was held at the National Science Foundation in December of 2013 to identify some of the central challenges and opportunities facing materials research within the context of that initiative. This article summarizes the findings of the workshop, and presents a series of concrete recommendations with the potential to facilitate its implementation. It also provides an overview of timely fundamental, technical and logistical challenges, organized according to distinct classes of materials, whose solution could have significant practical and societal benefits.

Weyl fermions scattering from a random Coulomb potential are predicted to exhibit resistivity versus temperature [Formula: see text] in a single particle model. Here we show that, in closed-environment-grown polycrystalline samples of Y2Ir2O7, [Formula: see text] over four orders of magnitude in [Formula: see text]. While the measured prefactor, [Formula: see text], is obtained from the model using reasonable materials parameters, the [Formula: see text] behavior extends far beyond the model's range of applicability. In particular, the behavior extends into the low-temperature, high-resistivity region where the Ioffe-Regel parameter, [Formula: see text]. Strong on-site Coulomb correlations, instrumental for predicting a Weyl semimetal state in Y2Ir2O7, are the possible origin of such 'bad' Weyl semimetal behavior.

The solid solution of In2Fe2−xGaxCuO7 (space group P63/mmc) was synthesized and investigated through X-ray diffraction and magnetic susceptibility studies. Limited changes to the lattice parameters were observed as a result of the similar ionic radii for Fe3+ and Ga3+ in the trigonal bipyramidal (TBP) crystallographic site. An increase in the Weiss temperature, along with spin glass behavior are observed from x=0 to x=2, but irregularities in the trend are apparent for x=0.75 and 1. With the highest concentration in magnetic ions, In2Fe2CuO7 appears to have competing nearest neighbor interactions that produce a suppression of the Curie tail and the experimental magnetic moment. In comparison to InFe1−xGaxCuO4, both solid solutions show an invariable progression of the lattice parameters, but the magnetic properties are greatly affected by the distinct TBP layering schemes.

We have successfully synthesized a rare example of an open framework nickel oxide with succinate capping the channels. A honeycomb-like layer of 14-membered rings centered in the (11) plane are connected by vertex-sharing NiO6 octahedra and water resides in the channels. The structure is only the second example of an extended inorganic–organic hybrid material containing 3-D Ni–O–Ni connectivity and was structurally characterized by single-crystal and powder X-ray diffraction. The material displays excellent chemical stability in aqueous solution from pH ∼ 1 to 13 and thermal stability to ∼375 °C as evidenced by thermogravimetric analysis coupled mass spectroscopy. The Ni2+ ions order ferromagnetically below Tc = 5.1 K and anisotropic exchange interactions lead to a field-induced metamagnetic transition and spin-glass-like dependence on cooling conditions in magnetic field.

The heavy fermion compound ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ continues to attract great interest due to the unidentified hidden order it develops below 17.5 K. The unique Ising character of the spin fluctuations and low-temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a ${\mathrm{cos}}^{4}\ensuremath{\theta}$ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.

The effects of partial substitution of Cu and Co for Ni in the intermetallic perovskite superconductor MgCNi3 are reported. Calculation of the expected electronic density of states suggests that electron (Cu) and hole (Co) doping should have different effects. For MgCNi3-xCux, solubility of Cu is limited to approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For MgCNi3-xCox, solubility of Co is much more extensive, but bulk superconductivity disappears for Co doping of 1% (x = 0.03). No signature of long range magnetic ordering is observed in the magnetic susceptibility of the Co doped material., submitted, Solid State Communications

The preparation and physical characterization of non-stoichiometric Ru 2 Ge 3+ x (0≤ x ≤1) are reported for the first time. The defect TiSi 2 -type chimney-ladder structure is maintained for the full stoichiometry range. The resistivity of Ru 2 Ge 3+ x increases systematically with x from 300 mΩ cm, x =0 –3 Ω cm, x =1 at 300 K. The temperature dependence is consistent with a variable range-hopping mechanism for x ≥0.6. The Seebeck coefficients of samples do not evolve simply with x . A low thermal conductivity ( κ 300 K =0.03 W/K cm) suggests that Ru 2 Ge 3 has some of the properties of a phonon-glass–electron-crystal. The low value of the thermoelectric figure of merit ZT =3.2×10 −3 ( T =300 K) calculated for Ru 2 Ge 3 is due primarily to a low conductivity.

In this work, the solid solution series Ba2In2–xIrxO5+δ (x = 0–1.4, 2) was synthesized, and its structural, magnetic, and charge-transport properties were measured. With increasing Ir content, three transitions in the room-temperature structure were observed: orthorhombic to tetragonal to cubic to a monoclinic distortion of a hexagonal BaTiO3 structure. Neutron diffraction shows Ba2In1.6Ir0.4O5.4 to be cubic and Ba2InIrO6 to be monoclinic, the latter contrary to previously published X-ray diffraction refinements. Magnetization measurements show Curie–Weiss behavior for x = 0.2–0.6, which arises from nearly 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid-state methods under ambient conditions. The electrical resistivity measurements show all the compounds studied are semiconducting and that resistivity decreases with increasing Ir content, suggesting the proximity to a metal–insulator transition. A sign reversal in the high-temperatur...

The heavy fermion compound URu_{2}Si_{2} continues to attract great interest due to the unidentified hidden order it develops below 17.5 K. The unique Ising character of the spin fluctuations and low-temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a cos^{4}θ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.

We present the structural and magnetic properties of a new compound family, Mg_{2}RE_{3}Sb_{3}O_{14} (RE=Gd,Dy,Er), with a hitherto unstudied frustrating lattice, the "tripod kagome" structure. Susceptibility (ac, dc) and specific heat exhibit features that are understood within a simple Luttinger-Tisza-type theory. For RE=Gd, we found long-ranged order (LRO) at 1.65 K, which is consistent with a 120° structure, demonstrating the importance of diople interactions for this 2D Heisenberg system. For RE=Dy, LRO at 0.37 K is related to the "kagome spin ice" physics for a 2D system. This result shows that the tripod kagome structure accelerates the transition to LRO predicted for the related pyrochlore systems. For RE=Er, two transitions, at 80 mK and 2.1 K are observed, suggesting the importance of quantum fluctuations for this putative XY system.

In this work, the solid solution series Ba2In(2-x)Ir(x)O5+δ (x = 0-1.4, 2) was synthesized, and its structural, magnetic, and charge-transport properties were measured. With increasing Ir content, three transitions in the room-temperature structure were observed: orthorhombic to tetragonal to cubic to a monoclinic distortion of a hexagonal BaTiO3 structure. Neutron diffraction shows Ba2In(1.6)Ir(0.4)O5.4 to be cubic and Ba2InIrO6 to be monoclinic, the latter contrary to previously published X-ray diffraction refinements. Magnetization measurements show Curie-Weiss behavior for x = 0.2-0.6, which arises from nearly 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid-state methods under ambient conditions. The electrical resistivity measurements show all the compounds studied are semiconducting and that resistivity decreases with increasing Ir content, suggesting the proximity to a metal-insulator transition. A sign reversal in the high-temperature Seebeck coefficient is observed indicating both electron and hole charge transport.

Research in functional materials is frequently driven by a desire to make informed choices in the quest for better, more effective materials. A great deal of recent attention has been focused on the modalities of how such informed choices can themselves be made in a better, more effective manner. The examples presented here examine some of these modalities, emphasizing the nexus between new synthesis, computational design and analysis, growth in high purity forms, and finally, end-use in terms of either application or of significant property measurement. The illustrations, many drawn from the recent literature, commence with the role that theory has played, both in property prediction and concomitant materials selection, in the areas of multiferroics and topological insulators. The importance of materials quality is emphasized, using examples from observation of the fractional Quantum Hall Effect, where new science has emerged as a result of improved materials. In the area of organic electronics, prospects for advancing the field are suggested, as are future directions in nanoscience. While the examples chosen here point to developments that require a highly collaborative “systems” approach to materials, the role that serendipity plays is not ignored.

The oxygen-enhanced photoconductivity observed in crystalline rubrene is investigated using electron paramagnetic resonance (EPR) spectroscopy and steady-state and time dependent photoconductivity (PC) measurements. The EPR data indicate the presence of rubrene radical cation and oxygen radical anion pairs formed within the crystalline structure when rubrene is irradiated in the presence of oxygen. Radical lifetimes determined using EPR spectroscopy correlate well with transient PC data and provide strong evidence that the rubrene radical cation is the charge carrier responsible for enhanced conduction. This process is reversible, although photodegradation is also observed. The oxygen-enhanced PC of rubrene is thus explained by an electron transfer mechanism that generates radical cation “hole” carriers within the crystal via the oxygen acceptor.

A cantilever-based uncooled IR imager was developed utilizing a novel optical readout scheme based on inter-pixel interference. A series of small arrays (approximately 100 × 100 pixels) were fabricated using 8-in. silicon MEMS processes. The array design and process development will be discussed and initial uniformity and imaging results presented. Future challenges in developing a direct-view IR imager will be addressed.