Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2018"
TL;DR: In this article, the reproducibility of Tauc plot slopes for ZnO as a model direct-gap material and compare these experimental values with the theoretically derived slope was examined. But the experimental slope values varied by several orders of magnitude.
Abstract: One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression where the intercept gives the band gap and the fitting exponent identifies the electronic transition as direct or indirect. [See Tauc et al., Physica Status Solidi 15, 627 (1966); naturally, these are usually called “Tauc” plots.] In earlier work, we found that direct band gaps fitted using Tauc's method can be quite accurate, to ∼1% [see Viezbicke et al., Phys. Status Solidi B 252, 1700 (2015)]. Still, slopes of these Tauc plots are less frequently quantified, even though the slopes are directly rooted in key band-structure parameters. In this study, we examine the reproducibility of Tauc plot slopes for ZnO as a model direct-gap material and compare these experimental values with the theoretically derived slope. In contrast to the band gap accuracy, the experimental slope values varied by several orders of magnitude. The histogram of slope values was significantly more compact for Tauc plots exhibiting less Urbach tail contribution. In these cases, the Tauc slopes can provide an order-of-magnitude quantification of other key band characteristics such as carrier effective mass.
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TL;DR: In this article, the transmission spectra of h-BN flakes transferred onto quartz plates with the transfer-matrix method was determined in the wavelength range from 450 to 1200nm.
Abstract: Hexagonal boron nitride (h-BN) is being widely utilized as a platform for optical and electrical devices exploiting two-dimensional layered materials. By analyzing transmission spectra of h-BN flakes transferred onto quartz plates with the transfer-matrix method, the refractive index of h-BN is determined in the wavelength range from 450 to 1200nm. The transmission spectra are reproduced by applying the single oscillator model of n(λ)1⁄4 1þAλ/(λ λ0) as the wavelength-dependent refractive index. Averaging over the parameters from different samples, the parameters of λ01⁄4 164.4 nm and A1⁄4 3.263 are obtained.
72 citations
TL;DR: In this paper, a comprehensive study on three selected Heusler alloy systems with X=Al, In, Sn is presented with respect to the relevant magnetocaloric properties of their magnetostructural phase transition, namely martensitic transition temperature as well as its field dependence, magnetization change, and width of the thermal hysteresis.
Abstract: We present a comprehensive study on three selected Heusler alloy systems. Ni-Mn-X(-Co) systems with X=Al, In, Sn are compared with respect to the relevant magnetocaloric properties of their magnetostructural phase transition, namely martensitic transition temperature as well as its field dependence, magnetization change, and width of the thermal hysteresis. The latter one is strongly determining the reversibility of the magnetocaloric effect. Therefore the understanding of how to tailor it by extrinsic and intrinsic factors is of great importance. Our study of the magnetocaloric properties leads to the conclusion that the width of thermal hysteresis can be correlated to the magnetization change of the phase transition. Consequently, the adiabatic temperature change under cycling can largely vary despite similar values of isothermal entropy change for Ni-Mn-In-Co and Ni-Mn-Sn-Co. This result therefore shows the importance of tailoring sharpness, thermal hysteresis, and field dependence of the phase transition to achieve high values for the isothermal entropy change as well as a large magnetocaloric cooling effect in the different Heusler alloys.
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TL;DR: In this article, the fine structure and spin dynamics of spin-3/2 centers associated with silicon vacancies in silicon carbide are discussed and the experimental achievements in magnetometry and thermometry based on the spin state mixing at level anticrossings in an external magnetic field and the underlying microscopic mechanisms.
Abstract: We discuss the fine structure and spin dynamics of spin-3/2 centers associated with silicon vacancies in silicon carbide. The centers have optically addressable spin states which makes them highly promising for quantum technologies. The fine structure of the spin centers turns out to be highly sensitive to mechanical pressure, external magnetic and electric fields, temperature variation, etc., which can be utilized for efficient room-temperature sensing, particularly by purely optical means or through the optically detected magnetic resonance. We discuss the experimental achievements in magnetometry and thermometry based on the spin state mixing at level anticrossings in an external magnetic field and the underlying microscopic mechanisms. We also discuss spin fluctuations in an ensemble of vacancies caused by interaction with environment.
35 citations
TL;DR: In this article, phonon dispersion, thermodynamic and optical properties including charge density, Fermi surface, Mulliken population analysis, theoretical Vickers hardness of predicted Sc 2 InC have been performed for the first time.
Abstract: The calculations of phonon dispersion, thermodynamic and optical properties including charge density, Fermi surface, Mulliken population analysis, theoretical Vickers hardness of predicted Sc 2 InC have been performed for the first time. The Sc 2 InC is mechanically as well as dynamically stable. It is promising for optoelectronic devices in the visible and ultraviolet energy regions and as a coating material to avoid solar heating.
32 citations
TL;DR: Based on rigid cubic node rotation and the kinematic geometrical relations of a three-dimensional (3D) isotropic anti-tetrachiral structure, an analytical expression for the modulus of the 3D isotropically anti-tochiral structure is derived from strain energy analysis.
Abstract: Chiral metastructures consisting of ring (polygonal or cubic) nodes and elastic bending ligaments exhibit excellent design flexibility for compliant structures. In this paper, based on rigid cubic node rotation and the kinematic geometrical relations of a three-dimensional (3D) isotropic anti-tetrachiral structure, an analytical expression for the modulus of the 3D isotropic anti-tetrachiral structure is derived from strain energy analysis. The nylon powder Selected Laser Sintering (SLS) additive manufacturing technique is employed for fabricating two types of 3D isotropic anti-tetrachiral specimens with different geometrical parameters; then comparison between experiments, finite element analysis (FEA), and theoretical studies is performed for verifying the analytical formula. The mechanical properties of 3D isotropic anti-tetrachiral structures can be tuned with two independent dimensionless geometrical parameters. The proposed 3D anti-tetrachiral structures can be employed for designing advanced structures against impact damages, realizing flexibility of industrial components, and optimizing the vibration attenuation abilities of engineering structures.
TL;DR: In this article, the formation of active component of FeMo and Co-Mo catalysts for carbon nanotube growth with variable Mo content is studied with in situ XRD, XPS, and STEM&EDX.
Abstract: The formation of active component of Fe–Mo and Co–Mo catalysts for carbon nanotube growth with variable Mo content is studied with in situ XRD, XPS, and STEM&EDX. The activation of bimetallic Fe–Mo catalysts under the growth conditions leads to the formation of Fe–Mo alloy particles in contrast to Co-containing catalysts in which active alloy incorporates only a small portion of Mo (<2–3 at.%). The stable carbide formation for these systems is not detected. The effect of Mo content in Fe–Mo and Co–Mo catalysts on the properties of MWCNTs is additionally investigated using the Raman spectroscopy in combination with measurements of the temperature dependence of conductivity. It is shown that MWCNT defectiveness depends on the active component composition for both Fe–Mo and Co–Mo catalysts. The current carrier concentration for the MWCNTs grown on Fe–Mo catalysts slightly increases with the Mo content. The opposite effect is observed for the tubes grown on Co–Mo catalysts, showing a decrease in the current carrier concentration with the increase of Mo concentration.
TL;DR: In this article, the intrinsic energy cost associated with the transformation between metastable phases is investigated and a micro-calorimetric method that provides valuable insight into intrinsic behavior with the sensitivity to measure micro-crystallites is described.
Abstract: Hysteresis is detrimental to the refrigeration cooling cycle efficiency and yet many of the magnetocaloric materials under consideration as solid state refrigerants possess this property. This article discusses some aspects related to the factors leading to hysteresis in real materials. In the absence of high quality single crystals, determining the intrinsic energy cost associated with the transformation between metastable phases is an experimental challenge. We describe a micro-calorimetric method that provides valuable insight into intrinsic behavior with the sensitivity to measure micro-crystallites. We show that there is no correlation between the strength of first order character and magnitude of hysteresis between material families. We review some of the extrinsic factors which contribute to the hysteresis in real materials particularly those that can be accounted for using local imaging techniques such as scanning Hall probe microscopy. We discuss a number of mechanisms by which the extrinsic hysteretic properties of a material can be modified.
TL;DR: In this paper, the combined effects of electron-A(1g) phonon and Rashba spin-orbit couplings on the electronic spectrum of graphene in the presence of Haldane interaction induced by photo-irradiation were studied.
Abstract: The combined effects of electron-A(1g) phonon and Rashba spin-orbit couplings on the electronic spectrum of graphene in the presence of Haldane interaction induced by photo-irradiation have been studied. A Frohlich-type Hamiltonian has been proposed to model these interactions, and then a diagonalization procedure based on the Lee-Low-Pines (LLP) theory which includes two successive unitary transformations has been applied. The results show that combined effects arising from the Rashba spin-orbit interaction and electron-A(1g) phonon interaction in the presence of Haldane interaction remove the valley degeneracy of electronic bands, and thus allow one to tune the resulting band gap just by adjusting the associated strengths of the interactions.
TL;DR: In this paper, the problem of free carrier screening of strongly bound excitons in two-dimensional monolayer semiconductors is addressed and theoretical considerations concerning the applicability of the commonly used static approximation of the screening effect and the implications are discussed.
Abstract: The authors address the problem of free carrier screening of exciton states in two‐dimensional monolayer semiconductors Basic theoretical considerations are presented concerning the applicability of the commonly used static approximation of the screening effect and the implications are discussed The authors show that the low‐frequency models leads to a major overestimation of the free carrier response and are inadequate to describe the screening of strongly bound excitons in monolayer materials The presented arguments are consistent with existing high‐level many‐body theories and transparently illustrate the underlying physics
TL;DR: In this article, the effect of differently strained epitaxial layers on Fe-segregation was investigated, and it was shown that by growing the Fe-doped layers under unusual growth conditions (low temperature, or as AlGaN), or by adding an interlayer (AlN, 5× AlN/GaN, or C-Doped GaN), they succeeded in limiting the Fe segregation within 200nm of undoped GaNs layer instead of the typically required 800 nm.
Abstract: AlGaN/GaN High Electron Mobility Transistors (HEMTs) require a semi-insulating buffer to compensate a high background donor concentration and to prevent parasitic effects, such as parallel conduction. Iron and carbon are typical impurities used for such purpose, since they can behave as deep acceptors in GaN layers. The former (Fe) brings as drawback a well-known memory effect which consists in the segregation of Fe atoms through the GaN layers, requiring thick undoped layers to keep the two-dimensional electron gas (2DEG) away from such electron traps. The latter (C), although easier to incorporate in the GaN layers and free of any memory effect, could cause current collapse. In this study we investigate the effect of differently strained epitaxial layers on Fe-segregation. HEMT structures were grown on sapphire substrates by MOCVD. By growing the Fe-doped layers under unusual growth conditions (low temperature, or as AlGaN), or by adding an interlayer (AlN, 5× AlN/GaN, or C-doped GaN) between the doped and undoped epitaxial layers, we have succeeded in limiting the Fe segregation within 200 nm of undoped GaN layer instead of the typically required 800 nm, as proven by SIMS. While the morphology and the crystal quality of the HEMT structures have been affected to a very low extent, the electrical characteristics have benefitted from such interlayers. Higher carrier mobility and lower sheet resistance distinguish such epitaxial structures. Further benefits are expected by the device performance such as reduced soft-subthreshold behavior (soft-breakdown) and dispersion effects.
TL;DR: In this article, the dielectric, ferroelectric, and piezoelectric properties of the conventional polyvinylidene fluoride/trifluoroethylene, P(VDF-TrFE), copolymer of composition 70/30
Abstract: In this study we report on the dielectric, ferroelectric, and piezoelectric properties of the conventional polyvinylidene fluoride/trifluoroethylene, P(VDF-TrFE), copolymer of composition 70/30 mol.% with fillers on the basis of lead zirconate titanate (PZT). (Pb0.75Ba0.24Sr0.01)(Zr0.53Ti0.47)O3 (BPZT) fillers with concentrations from 10 to 50 vol.% are additional components in the P(VDF-TrFE) piezoelectric polymer. Dielectric spectroscopy and the characterization of ferroelectricity and piezoelectricity in the P(VDF-TrFE)/BPZT composites are performed over a wide temperature range from 150 to 420 K. The dependence of the effective dielectric permittivity for the composites under investigation on the filler concentration is analyzed by the Lichtenecker mixing rule. The approximation model of effective medium is also applied to explain the impact of BPZT fillers on the ferroelectric and piezoelectric properties of P(VDF-TrFE)/BPZT composites.
TL;DR: In this article, the reactivity of different types of gaseous NOx molecules were compared with the surface properties of the graphene-based sensors, and the results were used for the development of selective graphenebased gas sensors.
Abstract: Graphene derivatives are promising sensor materials due to their high surface area available for molecule adsorption and conductivity changes under the adsorbate impact. The selectivity of such materials can be tuned through the attaching of certain functional groups preferably interacting with the defined gases. In the present work, we compare the reactivity of graphene oxide, oxyfluorinated graphene, and fluorinated graphene toward gaseous NOx molecules. The interaction of the molecules with the graphene-based films was monitored by in situ X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy measurements. The spectra before and after exposure of the films to a gaseous NOx mixture detected equal concentrations of adsorbed NO2 and NO species on graphene oxide, the preferable interaction of oxyfluorinated graphene with NO2 and the absence of the adsorbed molecules on the fluorinated graphene surface. These results are useful for the development of selective graphene-based gas sensors.