Spin-resolved topology and partial axion angles in three-dimensional insulators.

Symmetry-protected topological crystalline insulators (TCIs) have primarily been characterized by their gapless boundary states. However, in time-reversal- ([Formula: see text]-) invariant (helical) 3D TCIs-termed higher-order TCIs (HOTIs)-the boundary signatures can manifest as a sample-dependent network of 1D hinge states. We here introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the intrinsic bulk topological properties of spinful 3D insulators. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetals, and [Formula: see text]-doubled axion insulator (T-DAXI) states with nontrivial partial axion angles indicative of a 3D spin-magnetoelectric bulk response and half-quantized 2D TI surface states originating from a partial parity anomaly. Using ab-initio calculations, we demonstrate that ß-MoTe2 realizes a spin-Weyl state and that α-BiBr hosts both 3D QSHI and T-DAXI regimes.

Design Rules, Accurate Enthalpy Prediction, and Synthesis of Stoichiometric Eu3+ Quantum Memory Candidates.

Stoichiometric Eu3+ compounds have recently shown promise for building dense, optically addressable quantum memory as the cations' long nuclear spin coherence times and shielded 4f electron optical transitions provide reliable memory platforms. Implementing such a system, though, requires ultranarrow, inhomogeneous linewidth compounds. Finding this rare linewidth behavior within a wide range of potential chemical spaces remains difficult, and while exploratory synthesis is often guided by density functional theory (DFT) calculations, lanthanides' 4f electrons pose unique challenges for stability predictions. Here, we report DFT procedures that reliably reproduce known phase diagrams and correctly predict two experimentally realized quantum memory candidates. We are the first to synthesize the double perovskite halide Cs2NaEuF6. It is an air-stable compound with a calculated band gap of 5.0 eV that surrounds Eu3+ with mononuclidic elements, which are desirable for avoiding inhomogeneous linewidth broadening. We also analyze computational database entries to identify phosphates and iodates as the next generation of chemical spaces for stoichiometric quantum memory system studies. This work identifies new candidate platforms for exploring chemical effects on quantum memory candidates' inhomogeneous linewidth while also providing a framework for screening Eu3+ compound stability with DFT.

Exfoliable Transition Metal Chalcogenide Semiconductor NbSe2I2.

As the field of exfoliated van der Waals electronics grows to include complex heterostructures, the variety of available in-plane symmetries and geometries becomes increasingly valuable. In this work, we present an efficient chemical vapor transport synthesis of NbSe2I2 with the triclinic space group P1̅. This material contains Nb-Nb dimers and an in-plane crystallographic angle γ = 61.3°. We show that NbSe2I2 can be exfoliated down to few-layer and monolayer structures and use Raman spectroscopy to test the preservation of the crystal structure of exfoliated thin films. The crystal structure was verified by single-crystal and powder X-ray diffraction methods. Density functional theory calculations show triclinic NbSe2I2 to be a semiconductor with a band gap of around 1 eV, with similar band structure features for bulk and monolayer crystals. The physical properties of NbSe2I2 have been characterized by transport, thermal, optical, and magnetic measurements, demonstrating triclinic NbSe2I2 to be a diamagnetic semiconductor that does not exhibit any phase transformation below room temperature.

Ultrafast X-Ray Scattering Reveals Composite Amplitude Collective Mode in the Weyl Charge Density Wave Material (TaSe_{4})_{2}I.

We report ultrafast x-ray scattering experiments of the quasi-1D charge density wave (CDW) material (TaSe_{4})_{2}I following ultrafast infrared photoexcitation. From the time-dependent diffraction signal at the CDW sidebands we identify a 0.11 THz amplitude mode derived primarily from a transverse acoustic mode of the high-symmetry structure. From our measurements we determine that this mode interacts with the valence charge indirectly through another collective mode, and that the CDW system in (TaSe_{4})_{2}I has a composite nature supporting multiple dynamically active structural degrees of freedom.

Observation of a massive phason in a charge-density-wave insulator.

The lowest-lying fundamental excitation of an incommensurate charge-density-wave material is believed to be a massless phason-a collective modulation of the phase of the charge-density-wave order parameter. However, long-range Coulomb interactions should push the phason energy up to the plasma energy of the charge-density-wave condensate, resulting in a massive phason and fully gapped spectrum1. Using time-domain terahertz emission spectroscopy, we investigate this issue in (TaSe4)2I, a quasi-one-dimensional charge-density-wave insulator. On transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrowband terahertz radiation. The frequency, polarization and temperature dependences of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interactions. Our observations underscore the role of long-range interactions in determining the nature of collective excitations in materials with modulated charge or spin order.

Angstrom-scale imaging of magnetization in antiferromagnetic Fe2As via 4D-STEM.

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe2As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods.

Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb4Se15I2)I2.

The discovery of new low-dimensional transition-metal chalcogenides is contributing to the already prosperous family of these materials. In this study, needle-shaped single crystals of a quasi-one-dimensional (1D) material, (Nb4Se15I2)I2, were grown by chemical vapor transport, and the structure was solved by single-crystal X-ray diffraction (XRD). The structure has 1D (Nb4Se15I2)n chains along the [101] direction, with two I- ions per formula unit directly bonded to Nb5+. The other two I- ions are loosely coordinated and intercalated between the chains. Individual chains are chiral and stack along the b axis in opposing directions, giving space group P21/c. The phase purity and crystal structure were verified by powder XRD. Density functional theory calculations show (Nb4Se15I2)I2 to be a semiconductor with a direct band gap of around 0.6 eV. Resistivity measurements of bulk crystals and micropatterned devices demonstrate that (Nb4Se15I2)I2 has an activation energy of around 0.1 eV, and no anomaly or transition was seen upon cooling. Low-temperature XRD shows that (Nb4Se15I2)I2 does not undergo a structural phase transformation from room temperature to 8.2 K, unlike related compounds (NbSe4)nI (n = 2, 3, or 3.33), which all exhibit charge-density waves. This compound represents a well-characterized and valence-precise member of a diverse family of anisotropic transition-metal chalcogenides.

Ultra-Wide Band Gap Ga2O3-on-SiC MOSFETs.

Ultra-wide band gap semiconductor devices based on ß-phase gallium oxide (Ga2O3) offer the potential to achieve higher switching performance and efficiency and lower manufacturing cost than that of today's wide band gap power electronics. However, the most critical challenge to the commercialization of Ga2O3 electronics is overheating, which impacts the device performance and reliability. We fabricated a Ga2O3/4H-SiC composite wafer using a fusion-bonding method. A low-temperature (≤600 °C) epitaxy and device processing scheme was developed to fabricate MOSFETs on the composite wafer. The low-temperature-grown epitaxial Ga2O3 devices deliver high thermal performance (56% reduction in channel temperature) and a power figure of merit of (∼300 MW/cm2), which is the highest among heterogeneously integrated Ga2O3 devices reported to date. Simulations calibrated based on thermal characterization results of the Ga2O3-on-SiC MOSFET reveal that a Ga2O3/diamond composite wafer with a reduced Ga2O3 thickness (∼1 µm) and a thinner bonding interlayer (<10 nm) can reduce the device thermal impedance to a level lower than that of today's GaN-on-SiC power switches.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes.

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO2 nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Pharmacologically Enhanced Regulatory Hematopoietic Stem Cells Revert Experimental Autoimmune Diabetes and Mitigate Other Autoimmune Disorders.

Type 1 diabetes (T1D) is characterized by the loss of immune self-tolerance, resulting in an aberrant immune responses against self-tissue. A few therapeutics have been partially successful in reverting or slowing down T1D progression in patients, and the infusion of autologous hematopoietic stem cells (HSCs) is emerging as an option to be explored. In this study, we proposed to pharmacologically enhance by ex vivo modulation with small molecules the immunoregulatory and trafficking properties of HSCs to provide a safer and more efficacious treatment option for patients with T1D and other autoimmune disorders. A high-throughput targeted RNA sequencing screening strategy was used to identify a combination of small molecules (16,16-dimethyl PGE2 and dexamethasone), which significantly upregulate key genes involved in trafficking (e.g., CXCR4) and immunoregulation (e.g., programmed death ligand 1). The pharmacologically enhanced, ex vivo-modulated HSCs (regulatory HSCs [HSC.Regs]) have strong trafficking properties to sites of inflammation in a mouse model of T1D, reverted autoimmune diabetes in NOD mice, and delayed experimental multiple sclerosis and rheumatoid arthritis in preclinical models. Mechanistically, HSC.Regs reduced lymphocytic infiltration of pancreatic ß cells and inhibited the activity of autoreactive T cells. Moreover, when tested in clinically relevant in vitro autoimmune assays, HSC.Regs abrogated the autoimmune response. Ex vivo pharmacological modulation enhances the immunoregulatory and trafficking properties of HSCs, thus generating HSC.Regs, which mitigated autoimmune diabetes and other autoimmune disorders.

Ga2O3-on-SiC Composite Wafer for Thermal Management of Ultrawide Bandgap Electronics.

ß-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor (EG ∼ 4.8 eV), which promises generational improvements in the performance and manufacturing cost over today's commercial wide bandgap power electronics based on GaN and SiC. However, overheating has been identified as a major bottleneck to the performance and commercialization of Ga2O3 device technologies. In this work, a novel Ga2O3/4H-SiC composite wafer with high heat transfer performance and an epi-ready surface finish has been developed using a fusion-bonding method. By taking advantage of low-temperature metalorganic vapor phase epitaxy, a Ga2O3 epitaxial layer was successfully grown on the composite wafer while maintaining the structural integrity of the composite wafer without causing interface damage. An atomically smooth homoepitaxial film with a room-temperature Hall mobility of ∼94 cm2/Vs and a volume charge of ∼3 × 1017 cm-3 was achieved at a growth temperature of 600 °C. Phonon transport across the Ga2O3/4H-SiC interface has been studied using frequency-domain thermoreflectance and a differential steady-state thermoreflectance approach. Scanning transmission electron microscopy analysis suggests that phonon transport across the Ga2O3/4H-SiC interface is dominated by the thickness of the SiNx bonding layer and an unintentionally formed SiOx interlayer. Extrinsic effects that impact the thermal conductivity of the 6.5 µm thick Ga2O3 layer were studied via time-domain thermoreflectance. Thermal simulation was performed to estimate the improvement of the thermal performance of a hypothetical single-finger Ga2O3 metal-semiconductor field-effect transistor fabricated on the composite substrate. This novel power transistor topology resulted in a ∼4.3× reduction in the junction-to-package device thermal resistance. Furthermore, an even more pronounced cooling effect is demonstrated when the composite wafer is implemented into the device design of practical multifinger devices. These innovations in device-level thermal management give promise to the full exploitation of the promising benefits of the UWBG material, which will lead to significant improvements in the power density and efficiency of power electronics over current state-of-the-art commercial devices.

Thermal Unequilibrium of PdSn Intermetallic Nanocatalysts: From In Situ Tailored Synthesis to Unexpected Hydrogenation Selectivity.

Effective control on chemoselectivity in the catalytic hydrogenation of C=O over C=C bonds is uncommon with Pd-based catalysts because of the favored adsorption of C=C bonds on Pd surface. Here we report a unique orthorhombic PdSn intermetallic phase with unprecedented chemoselectivity toward C=O hydrogenation. We observed the formation and metastability of this PdSn phase in situ. During a natural cooling process, the PdSn nanoparticles readily revert to the favored Pd3 Sn2 phase. Instead, using a thermal quenching method, we prepared a pure-phase PdSn nanocatalyst. PdSn shows an >96 % selectivity toward hydrogenating C=O bonds of various α,ß-unsaturated aldehydes, highest in reported Pd-based catalysts. Further study suggests that efficient quenching prevents the reversion from PdSn- to Pd3 Sn2 -structured surface, the key to the desired catalytic performance. Density functional theory calculations and analysis of reaction kinetics provide an explanation for the observed high selectivity.

Electrodeposition of atmosphere-sensitive ternary sodium transition metal oxide films for sodium-based electrochemical energy storage.

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion-based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (∼10-12 cm2⋅s-1), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm-2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks.

Inflexible stoichiometry in bulk pyrite FeS2 as viewed by in situ and high-resolution X-ray diffraction.

Non-stoichiometry is considered to be one of the main problems limiting iron pyrite, FeS2, as a photovoltaic absorber material. Although some historical diffraction experiments have implied a large solubility range of FeS2-δ with δ up to 0.25, the current consensus based on calculated formation energies of intrinsic defects has lent support to line-compound behavior. Here it is shown that pyrite stoichiometry is relatively inflexible in both reductive conditions and in autogenous sulfur partial pressure, which produces samples with precise stoichiometry of FeS2 even at different Fe/S ratios. By properly standardizing in situ gas-flow X-ray diffraction measurements, no significant changes in the lattice parameter of FeS2 can be resolved, which portrays iron pyrite as prone to forming sulfur-deficient compounds, but not intrinsic defects in the manner of NiS2-δ.

Oxygen-Induced Ordering in Bulk Polycrystalline Cu2ZnSnS4 by Sn Removal.

Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and Raman spectroscopy were used to show that Cu2ZnSnS4 (CZTS) bulk solids grown in the presence of oxygen had improved cation ordering compared to bulk solids grown without oxygen. Oxygen was shown to have negligible solubility in the CZTS phase. The addition of oxygen resulted in the formation of SnO2, leading to Sn-deficient CZTS. At the highest oxygen levels, other phases such as Cu9S5 and ZnS were observed. Beneficial ordering was only observed in samples produced with more than 2 at. % oxygen in the precursor materials but did not occur in samples designed with excess Sn and O. Thus, it is the removal of Sn and formation of Sn-deficient CZTS that improves ordering rather than the presence of SnO2 or O alone. These results indicate that using oxygen or air annealing to tailor the Sn content of CZTS followed by an etching step to remove SnO2 may significantly improve the properties of CZTS.

GSK3 Inhibition Drives Maturation of NK Cells and Enhances Their Antitumor Activity.

Maturation of human natural killer (NK) cells as defined by accumulation of cell-surface expression of CD57 is associated with increased cytotoxic character and TNF and IFNγ production upon target-cell recognition. Notably, multiple studies point to a unique role for CD57+ NK cells in cancer immunosurveillance, yet there is scant information about how they mature. In this study, we show that pharmacologic inhibition of GSK3 kinase in peripheral blood NK cells expanded ex vivo with IL15 greatly enhances CD57 upregulation and late-stage maturation. GSK3 inhibition elevated the expression of several transcription factors associated with late-stage NK-cell maturation including T-BET, ZEB2, and BLIMP-1 without affecting viability or proliferation. When exposed to human cancer cells, NK cell expanded ex vivo in the presence of a GSK3 inhibitor exhibited significantly higher production of TNF and IFNγ, elevated natural cytotoxicity, and increased antibody-dependent cellular cytotoxicity. In an established mouse xenograft model of ovarian cancer, adoptive transfer of NK cells conditioned in the same way also displayed more robust and durable tumor control. Our findings show how GSK3 kinase inhibition can greatly enhance the mature character of NK cells most desired for effective cancer immunotherapy. Cancer Res; 77(20); 5664-75. ©2017 AACR.

Structural, Electronic, and Optical Properties of K2Sn3O7 with an Offset Hollandite Structure.

We present the compound K2Sn3O7, a Sn4+-containing oxide with a unique structure type among oxides. The compound is orthorhombic and reminiscent of an offset hollandite, where open channels hold a row of four K+ per channel per cell. UV-visible spectroscopy indicates a wide band gap semiconductor, which is confirmed by first-principles electronic-structure calculations of band structures, densities of states, and optical properties. The continued discovery of new structure types in ternary tin oxides should remain a priority for the identification of prospective ion conductors and transparent conducting compounds.

Crystal growth and characterization of the narrow-band-gap semiconductors OsPn2 (Pn = P, As, Sb).

Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

In situ studies of a platform for metastable inorganic crystal growth and materials discovery.

Rapid shifts in the energy, technological, and environmental demands of materials science call for focused and efficient expansion of the library of functional inorganic compounds. To achieve the requisite efficiency, we need a materials discovery and optimization paradigm that can rapidly reveal all possible compounds for a given reaction and composition space. Here we provide such a paradigm via in situ X-ray diffraction measurements spanning solid, liquid flux, and recrystallization processes. We identify four new ternary sulfides from reactive salt fluxes in a matter of hours, simultaneously revealing routes for ex situ synthesis and crystal growth. Changing the flux chemistry, here accomplished by increasing sulfur content, permits comparison of the allowable crystalline building blocks in each reaction space. The speed and structural information inherent to this method of in situ synthesis provide an experimental complement to computational efforts to predict new compounds and uncover routes to targeted materials by design.

Prostaglandin-modulated umbilical cord blood hematopoietic stem cell transplantation.

Umbilical cord blood (UCB) is a valuable source of hematopoietic stem cells (HSCs) for use in allogeneic transplantation. Key advantages of UCB are rapid availability and less stringent requirements for HLA matching. However, UCB contains an inherently limited HSC count, which is associated with delayed time to engraftment, high graft failure rates, and early mortality. 16,16-Dimethyl prostaglandin E2 (dmPGE2) was previously identified to be a critical regulator of HSC homeostasis, and we hypothesized that brief ex vivo modulation with dmPGE2 could improve patient outcomes by increasing the "effective dose" of HSCs. Molecular profiling approaches were used to determine the optimal ex vivo modulation conditions (temperature, time, concentration, and media) for use in the clinical setting. A phase 1 trial was performed to evaluate the safety and therapeutic potential of ex vivo modulation of a single UCB unit using dmPGE2 before reduced-intensity, double UCB transplantation. Results from this study demonstrated clear safety with durable, multilineage engraftment of dmPGE2-treated UCB units. We observed encouraging trends in efficacy, with accelerated neutrophil recovery (17.5 vs 21 days, P = .045), coupled with preferential, long-term engraftment of the dmPGE2-treated UCB unit in 10 of 12 treated participants.