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Single-phase metal dodecaboride solid solutions, Zr0.5Y0.5B12 and Zr0.5U0.5B12, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray di...

High-quality single crystals of ${(\mathrm{PbSe})}_{5}{({\mathrm{Bi}}_{2}{\mathrm{Se}}_{3})}_{3m}$ were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on $^{77}\mathrm{Se}$ and $^{207}\mathrm{Pb}$ NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology ${(\mathrm{PbSe})}_{5}{({\mathrm{Bi}}_{2}{\mathrm{Se}}_{3})}_{3m}$ with $m=1 ({\mathrm{Pb}}_{5}{\mathrm{Bi}}_{6}{\mathrm{Se}}_{14})$ and $m=2 ({\mathrm{Pb}}_{5}{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23})$. A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting ${\mathrm{Pb}}_{5}{\mathrm{Bi}}_{6}{\mathrm{Se}}_{14}$ to a semimetalliclike ${\mathrm{Pb}}_{5}{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23}$ system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer on the crystal and electronic structure of ${\mathrm{Pb}}_{5}{\mathrm{Bi}}_{12}{\mathrm{Se}}_{23}$. These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.

Tungsten borides, such as tungsten tetraboride (WB4) exhibit a wide range of appealing physical properties, including superhardness, chemical inertness and electronic conductivity. Among the various tungsten borides, the most puzzling remains WB4, with its crystal structure to linger in question for over half a century (Lech et al. in Proc Natl Acad Sci USA 112:3223–3228, 2015). In the present investigation, polycrystalline WB4 samples have been synthesized with two different methods and characterized at the atomic level by combining X-ray diffraction, scanning electron microscopy and nuclear magnetic resonance spectroscopy. The 11B multiple quantum MAS experiment revealed a range of boron sites that were not resolved within the experiment. This result is in contrast to the 11B MAS spectrum of WB2 with four resolved, discernible boron resonances. However, despite the structural complexity and boron-site variety in WB4, the detection of a single exponential of 11B spin–lattice relaxation recovery suggested that all of the boron sites relaxed with a single time constant. The Knight shift (K) was found to be independent of temperature while the $$ T_{1}^{ - 1} $$ was governed by the Korringa law with a Korringa product T1T = 350 sK across the entire temperature range (168–437 K) of this study. The measured Korringa product was small, indicating substantial spin–lattice relaxation resulting from coupling with the conduction carriers. The abovementioned experimental results not only clearly rule out structures, such as the “MoB4-type phase” of WB4, with the resulting Fermi level in the pseudo-gap as has previously been predicted theoretically; but they also provide a comprehensible and valuable insight into the structural and electronic properties of WB4 at the atomic level.

The presence of one-dimensional trigonal arrangements of chromium atoms (Cr-3) in the structure of TiCrIr2B2 leads to a magnetic transition near ambient temperature. Herein we report an investigation of the nature of electronic and magnetic properties of TiCrIr2B2 via ab initio calculations together with B-11 NMR Knight shift (K) and spin-lattice relaxation rate (1/T1) analysis. The presence of a characteristic rectangular powder pattern below 280 K, absence of a Korringa relation, strong enhancement of 1/TI.T at low temperatures and weak temperature dependence of K indicate competition between antiferromagnetic and ferromagnetic spin fluctuations in the itinerant d-band electrons, in agreement with ab initio calculations. One-dimensional trigonal arrangements of magnetically active elements in intermetallic compounds, as is found in TiCrIr2B2, are quite rare and could lead to exotic phenomena such as spin-chirality and quantum criticality in low-dimensional frustrated lattices.

The structure of tungsten diboride, WB2, differs from those of metal diborides with AlB2- or ReB2-type structures that have been previously studied by 11B NMR spectroscopy. As opposed to the single 11B resonance reported for the metal diborides having those AlB2- or ReB2-type structures, four groups of 11B resonances are found for WB2. There is also 11B NMR spectral evidence to support the proposal by Kiesling in 1947 of an additional boron atom at the center of some of the six-membered boron rings. The 11B quadrupolar frequencies of all four groups of resonances in WB2 are all quite small (

PURPOSE To investigate the fracture resistance and performance of zirconia when employed for the fabrication of implant abutments with different angulations, simulating anterior maxillary oral rehabilitation. MATERIALS AND METHODS Forty-five monolithic zirconia custom abutments of internal conical implant connection were CAD/CAM designed and fabricated. The specimens were divided into three groups (n = 15/group) according to implant-to-abutment angulation. The angulations used were; 0°, 15°, and 25°. The abutments were loaded until failure at 135° using the Universal Testing Machine (Instron, Canton, MA). Collected data were statistically analyzed using one-way ANOVA and post hoc Tukey test. RESULTS Mean (±standard deviation) load at fracture of the zirconia abutments for the three groups were 962.37 ± 93.81 N (Gr15) > 718.25 ± 93.71 N (Gr25) > 534.05 ±133.77 N (Gr0). Statistically significant difference (p < 0.0001) was found between all groups; Gr0 vs. Gr15, Gr0 vs. Gr25, Gr15 vs. Gr25. CONCLUSIONS Contrary to expectations, the non-angulated monolithic zirconia abutments presented the lowest fracture resistance values. Angulating the abutments 15 or 25 degrees, following the palatal resorption pattern of the premaxilla, significantly increased the in vitro fracture resistance.

Osseointegrated implants have been widely used for decades with high survival and success rates. However, mechanical complications continue to be reported in the literature, and their clinical management can be often very challenging for the clinician while there is no consensus on the ideal management. The aim of this manuscript was to review the risk factors of abutment screw complications, to identify the most recent incidence of screw failure in the clinical setting and report on the methodology used and the outcome of intervention. Clinical studies and reports were reviewed that reported on abutment screw looseness and/or fracture. A search of the electronic database PUBMED was conducted in November 2018, including manuscripts published in English from 2004 to 2018. Study selection: animal studies, narrative reviews, expert opinions and communications/letters were excluded. Further exclusion criteria included reports on occlusal prosthetic screws and fracture of abutments, and reports that did not provide adequate data. A total of 12 manuscripts were finally included that reported on single implant crowns or 2-unit implant fixed dental prosthesis. To conclude, the most current abutment screw loosening incidence ranges between 7% and 11%, while the abutment screw fracture incidence was found to be 0.6%. The majority reported on fracture of the screw body. Screw loosening or fracture was often located at the first molar restored area, while the maxillary central incisor area was also reported as an area that presented screw fracture. No single abutment screw failure management can be identified as the ideal treatment approach.

Xerostomia is a common adverse effect of radiation therapy at the head and neck area. Radiation-induced xerostomia can be severe and detrimental for the quality of life. Clinicians and radiologists have focused on the prevention of xerostomia as feasible, which has been significantly improved in the recent decades with the use of the contemporary radiation technology. However, radiation-induced xerostomia still remains one of the most devastating side effects of radiation therapy. Clinical risk factors have been identified, but the variation of its incidence and presentation has turned the focus on the investigation of parameters that would be able to predict the onset of acute or chronic xerostomia for each individual patient. Recently, potential imaging parameters and biomarkers are investigated in order for early prediction of the incidence and severity of xerostomia. Here, we compile the resulting imaging biomarkers as have been identified in the recent literature based on MRI and CT performed in correlation with radiation therapy. The identification of such biomarkers is very promising for the prevention and control of xerostomia in the head and neck radiation setting.

Non-oxide perovskites exhibit unusual properties such as negative thermal expansion, negative thermal coefficient of resistance, positive and negative giant magnetoresistance as well as superconductivity. These uncommon properties appear to originate from the basic structure only, in strong contrast to the oxides. Ordering in nonstoichiometric compounds may not only lead to different chemical compositions but also to the promotion of these physical properties. We present a combined NMR and first-principles study of the cubic Zr 2 Ir 6 B perovskite to investigate the boron ordering with boron deficiency leading to the formation of superstructure. Competing ionic and metallic interactions reflect the semimetallic character of this boride and result in the formation of a pseudogap, as predicted by our first principles calculations and verified experimentally by 11 B solid state NMR. Several avoided crossing scenarios were also found for the bands from the conducting states at +1 eV to the Fermi level along specific directions. This observation is of paramount importance for understanding the structure-property relationships in metal boride perovskites and the search for new cubic perovskites.

Crystal orbital Hamilton population (COHP) bonding analysis has predicted that ScPd3 B0.5 is the least stable compound of the entire series Sc2 Ir6-x Pdx B. Here, we report a systematic study of Sc2 Ir6-x Pdx B (x=3, 5 and 6) by means of 11 B nuclear magnetic resonance (NMR), Knight shift (K) and nuclear spin-lattice relaxation rate (1/T1 ). NMR results combined with theoretical band structure calculations provide a measure of s- and non-s-character Fermi-level density of states. We present direct evidence that the enhanced s-state character of the Fermi level density of states (DOS) in ScPd3 B0.5 reduces the strength of the B 2p and Pd 4d hybridized states across the entire Sc2 Ir6-x Pdx B series. This hybridization strength relates to the opening of a deep pseudogap in the density of states of Sc2 IrPd5 B and the chemical bonding instability of ScPd3 B0.5 . This study is an experimental realization of a chemical fine-tuning of the electronic properties in intermetallic perovskites.

Analyses of the one-dimensional 11B NMR spectra from static and MAS experiments of a polycrystalline ReB2 sample have extracted parameters characterizing both the electric field gradient and the shielding at the boron site. In contrast to that previously assumed for AlB2, the principal axis systems for these two interactions are not coincident in ReB2. A brief discussion of the literature regarding spin–lattice relaxation for half-integer quadrupolar nuclei and the mathematical functions used for the extraction of relaxation time constants is presented. With radiofrequency conditions chosen to excite the central and satellite transitions, the saturation recovery spin–lattice relaxation rate of ReB2 is better described by a single exponential rather than the Andrew–Tunstall multiexponential model for I = 3/2. In addition, relaxation measurements for 11B in ReB2 have been extended to temperatures above ambient and show a deviation from the Korringa relationship.

Magnetic resonance spectroscopy of biomolecules has recently gained attention for clinical diagnosis. Its combination with saliva collection and analysis can promote early disease detection and monitoring, by identifying biomarkers of specific underlying pathology or disease as detected in saliva. With this novel, non-invasive technique, certain salivary biomarkers have been linked to dental and periodontal tissues pathology, as well as to specific head and neck cancer malignancies. At present, diagnostic biomarkers are still in need for further identification (e.g., diagnosis and monitoring of Sjögren’s syndrome), and nuclear magnetic resonance spectroscopy has been found to be a promising technique to compliment the current analytic methodology. Moreover, this article reports on the various data collection and analysis parameters used in the literature. Protocol standardization is yet to be established not only for the laboratory procedures, but also for the clinical sample collection. Herein, we review the current status of utilizing nuclear magnetic resonance in order to further support data on health associated biomarkers, and we also propose a saliva sampling scheduling protocol with the potential to be used in the clinical and experimental setting for standardization of the testing methodology.

Understanding different bonding environments in various metal borides provides insight into their structures and physical properties. Polycrystalline aluminum diboride (AlB2) samples have been synthesized and compared both with a commercial sample and with the literature. One issue that arose is the relative ease with which boron-rich and aluminum deficient phases of aluminum borides can be presented in AlB2. Here, we report 27Al, 11B nuclear magnetic resonance (NMR) spectroscopy and first-principles calculations on AlB2 in order to shed light on these different bonding environments at the atomic level and compare the structural and electronic properties of the products of different preparations. Along with the aforementioned, the present study also takes an in-depth look at the nature of the 11B and 27Al nuclear spin–lattice relaxation recovery data for the AlB2 and other superhard materials. The nuclear spin–lattice relaxation has been measured for a static sample and with magic-angle spinning. The combination of NMR and band structure calculations highlights the synthetic challenges with superhard materials.

Numerous nuclear magnetic resonance (NMR) investigations of the KH2PO4 family of hydrogen-bonded compounds that display ferroelectric behavior have reported results related to structure, molecular motion, and phase transitions. The current study looks at how high magnetic field and high-resolution solid-state techniques affect the measured NMR parameters for ammonium dihydrogen phosphate (NH4H2PO4). Under magic-angle spinning (MAS), the ammonium and acid protons do not achieve a common spin temperature as previously stated in the literature [i.e., the spin–lattice relaxation times (T1) are different]. While the relaxation behavior in the static sample can be well-described with a single exponential at ambient temperature and below, the behavior becomes increasingly nonexponential as the sample is taken above ambient temperature. This behavior may arise from the motion of the ammonium moiety or from ionic conduction. The three principal values of the 1H shielding tensors for the different species of proton...

Cd

Cd3As2 is a Dirac semimetal that is a 3D analog of graphene. We investigated the local structure and nuclear-spin dynamics in Cd3As2 via Cd-113 NMR. The wideline spectrum of the static sample at 295K is asymmetric and its features are well described by a two-site model with the shielding parameters extracted via Herzfeld-Berger analysis of the magic-angle spinning spectrum. Surprisingly, the Cd-113 spin-lattice relaxation time (T-1) is extremely long (T-1 = 95 s at 295 K), in stark contrast to conductors and the effects of native defects upon semiconductors; but it is similar to that of C-13 in graphene (T-1 = 110 s). The temperature dependence of 1/T-1 revealed a complex bipartite mechanism that included a T-2 power-law behavior below 330 K and a thermally activated process above 330 K. In the high-temperature regime, the Arrhenius behavior is consistent with a field-dependent Cd atomic hopping relaxation process. At low temperatures, a T-2 behavior consistent with a spin-1/2 Raman-like process provides evidence of a time-dependent spin-rotation magnetic field caused by angular oscillations of internuclear vectors due to lattice vibrations. The observed mechanism does not conform to the conventional two-band model of semimetals, but is instead closer to a mechanism observed in high-Z element ionic solids with large magnetorotation constant [A.J. Vega et al., Phys. Rev. B 74, 214420 (2006)/. Published by AIP Publishing.

Crystal orbital Hamilton population (COHP) bonding analysis has predicted that ScPd

Considerable evidence suggests that variations in the properties of topological insulators (TIs) at the nanoscale and at interfaces can strongly affect the physics of topological materials. Therefore, a detailed understanding of surface states and interface coupling is crucial to the search for and applications of new topological phases of matter. Currently, no methods can provide depth profiling near surfaces or at interfaces of topologically inequivalent materials. Such a method could advance the study of interactions. Herein we present a non-invasive depth-profiling technique based on $\beta$-NMR spectroscopy of radioactive $^8$Li$^+$ ions that can provide "one-dimensional imaging" in films of fixed thickness and generates nanoscale views of the electronic wavefunctions and magnetic order at topological surfaces and interfaces. By mapping the $^8$Li nuclear resonance near the surface and 10 nm deep into the bulk of pure and Cr-doped bismuth antimony telluride films, we provide signatures related to the TI properties and their topological non-trivial characteristics that affect the electron-nuclear hyperfine field, the metallic shift and magnetic order. These nanoscale variations in $\beta$-NMR parameters reflect the unconventional properties of the topological materials under study, and understanding the role of heterogeneities is expected to lead to the discovery of novel phenomena involving quantum materials., Comment: 46 pages, 12 figures in Proc. Natl. Aca. Sci. USA (2015) Published online - early edition

While each atom species in PbSe corresponds to a single crystallographic site and transport measurements reveal a single carrier density, $^{207}$Pb NMR reveals a more complicated picture than previously thought comprising three discrete homogeneous carrier components, each associated with $n$- or $p$-type carrier fractions. The origins of these fractions are discussed in terms of electronic heterogeneity of the native semiconductor. The interaction mechanism between nuclear spins and lattice vibrations via fluctuating spin-rotation interaction, applicable to heavy spin-1/2 nuclei [Phys. Rev. B 74, 214420 (2006)], does not hold. Instead, a higher-order temperature dependence dominates the relaxation pathway. Shallow acceptor states and deep level defects in the midgap explain the complex temperature dependence of the direct band gap., 29 pages, 10 figures

In a topological crystalline insulator (TCI) the inversion of the bulk valence and conduction bands is a necessary condition to observe surface metallic states. Solid solutions of Pb$_{1-x}$Sn$_x$Te have been shown to be TCI, where band inversion occurs as a result of the band gap evolution upon alloying with Sn. The origins of this band inversion remain unclear. Herein the role of Sn insertion into the PbTe matrix is investigated for the $p$-type Pb$_{1-x}$Sn$_x$Te series with $x$ = 0, 0.35, 0.60, and 1.00 via nuclear magnetic resonance (NMR) and transport measurements. $^{207}$Pb, $^{119}$Sn, and $^{125}$Te line shapes, spin-lattice relaxation rates, and Knight shifts provide site-specific characterization of the electronic band structure. This probe of the electronic band structure shows that the band inversion is unaffected by lattice distortions but related to spatial electronic inhomogeneities formed by Sn incorporation into the PbTe matrix. Strong relativistic effects are found to be responsible for the band inversion, regardless of carrier type and concentration, suggesting a novel interpretation of the band gap evolution with composition. The temperature dependences of the NMR parameters reveal a negative temperature coefficient of the direct gap for SnTe and positive coefficient for PbTe., 34 pages, 9 figures, Published in Advanced Electronic Materials (2015)

In this study we present an alternative approach to separating contributions to the NMR shift originating from the Knight shift and chemical shielding by a combination of experimental solid-state NMR results and ab initio calculations. The chemical and Knight shifts are normally distinguished through detailed studies of the resonance frequency as function of temperature and carrier concentration, followed by extrapolation of the shift to zero carrier concentration. This approach is time-consuming and requires studies of multiple samples. Here, we analyzed $^{207}$Pb and $^{125}$Te NMR spin-lattice relaxation rates and NMR shifts for bulk and nanoscale PbTe. The shifts are compared with calculations of the $^{207}$Pb and $^{125}$Te chemical shift resonances to determine the chemical shift at zero charge carrier concentration. The results are in good agreement with literature values from carrier concentration-dependent studies. The measurements are also compared to literature reports of the $^{207}$Pb and $^{125}$Te Knight shifts of $n$- and $p$-type PbTe semiconductors. The literature data have been converted to the currently accepted shift scale. We also provide possible evidence for the "self-cleaning effect" property of PbTe nanocrystals whereby defects are removed from the core of the particles, while preserving the crystal structure., Comment: 34 pages, 9 figures

Non-invasive local probes are needed to characterize bulk defects in binary and ternary chalcogenides. These defects contribute to the non-ideal behavior of topological insulators. We have studied bulk electronic properties via $^{125}$Te NMR in Bi$_2$Te$_3$, Sb$_2$Te$_3$, Bi$_{0.5}$Sb$_{1.5}$Te$_3$, Bi$_2$Te$_2$Se and Bi$_2$Te$_2$S. A distribution of defects gives rise to asymmetry in the powder lineshapes. We show how the Knight shift, line shape and spin-lattice relaxation report on carrier density, spin-orbit coupling and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi$_2$Te$_2$Se is the best TI candidate material at the present time. Our results, which are in good agreement with transport and ARPES studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials., Comment: 24 pages, 6 figures

The requirement for large bulk resistivity in topological insulators has led to the design of complex ternary and quaternary phases with balanced donor and acceptor levels. A common feature of the optimized phases is that they lie close to the p- to n-transition. The tetradymite Bi2Te3−xSex system exhibits minimum bulk conductance at the ordered composition Bi2Te2Se. By combining local and integral measurements of the density of states, we find that the point of minimum electrical conductivity at x = 1.0 where carriers change from hole-like to electron-like is characterized by conductivity of the mixed type. Our experimental findings, which are interpreted within the framework of a two-band model for the different carrier types, indicate that the mixed state originates from different types of native defects that strongly compensate at the crossover point.

We report the NMR Knight shift and spin-lattice relaxation of $^{8}\mathrm{Li}{}^{+}$ implanted $\ensuremath{\sim}100$ nm into single crystals of semimetallic Sb, Bi, and topologically insulating ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$. We find small negative shifts (of order 100 ppm) in all three. In the insulator, the shift is nearly temperature independent, while in Bi and Sb it becomes more negative at low temperature without following the bulk susceptibility, suggesting two distinct temperature dependent contributions, possibly from the orbital and spin response. However, a simple model is unable to account for the observed shift. The spin-lattice relaxation differs in both scale and temperature dependence in all three. It is Korringa-like in Bi and remarkably is fastest in the insulating alloy and slowest in Sb with the highest bulk carrier density. These surprising results call for detailed calculations, but phenomenologically demonstrate that $\ensuremath{\beta}$-detected NMR of implanted $^{8}\mathrm{Li}{}^{+}$ is sensitive to the magnetic response of low-density carriers. The prospects for depth-resolved studies of conventional and topological surface states at lower implantation energies are good.

A 125Te NMR study of bismuth telluride nanoparticles as function of particle size revealed that the spin-lattice relaxation is enhanced below 33 nm, accompanied by a transition of NMR spectra from single to bimodal regime. The satellite peak features a negative Knight shift and higher relaxivity, consistent with core polarization from p-band carriers. Whereas nanocrystals follow a Korringa law in the range 140-420K, micrometer particles do so only below 200K. The results reveal increased metallicity of these nanoscale topological insulators in the limit of higher surface-to-volume ratios., 17 pages, 4 figures, 1 table

In the frame of the research domain of the production of metal matrix composite coatings, the aim of the present work was the production of copper electrodeposits containing micro- and nano-particles of SiC. The electrodeposition was carried out under dc conditions using a copper pyrophosphate plating bath into which micro- or nano-SiC particles were suspended. The composite coatings were tested and compared to pure copper coatings regarding their microstructure, mechanical and anticorrosive properties. The codeposition of SiC in the metal matrix changed the microstructure of the copper leading to improvement of both their mechanical and, in some cases, their protective properties. The Vickers microhardness presented an increase of about 35% and 61% in the case of SiC micro- and nano-particles incorporation while the increase of the abrasion resistance was 88% and 58%, respectively. The incorporation of the micro-particles lead to gaps formation among the SiC micro-particles and the metal matrix, thus lowering the resistance on both uniform and localized corrosion in comparison to the pure copper-coated specimens while the codeposition of SiC nano-particles produced deposits with higher resistance to both uniform and localized corrosion.