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A schematic illustration of the monoclinic β-Ga2O3 crystal structure and its application in enhancing the photonic spin Hall effect.

PurposeThe Premier League (PL) and the Bundesliga (BL) were chosen for this study due to their fundamentally different approaches to ownership regulation and the distribution of media revenues. Regulation in the PL is very liberal if compared to the BL's 50+1-rule. In the BL, the distribution of media revenues is mainly based on past performance, whereas equal distribution is dominant in the PL. The specific aim of this paper was a longitudinal analysis with a focus on the final outcome of the seasons.Design/methodology/approachThis study looks at competitive intensity (CI) in the men's BL and the English PL because it is a crucial indicator for the long-term success of a sports league and the participants. To calculate the CI of both leagues and of all relevant sub-competitions (championship, Champions League (CL), Europa League (EL), Conference League (CoL) and fight against relegation), a CI index (CII) model was generalised and applied for an examination period spanning from 1998/99 to 2020/21.FindingsUntil 2008/09, seasonal CI in the BL was somewhat higher than in the PL. But afterwards, the BL's championship race's CI dropped considerably, while the PL's CI for qualification for the CL rose profoundly. Results also showed that the introduction of the CoL raised the leagues' CI indices.Originality/valueBesides a methodological contribution with the generalisation of the applied CI index model, the findings are discussed in the context of the above-mentioned regulatory and distribution mechanisms also taking into account the very current discussion regarding general regulatory changes within European football.

The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase1–4. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales5. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response6,7. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic1,3,4 and hexagonal8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals10, many common oxides11 and organic crystals12, greatly expanding the material base and extending design opportunities for compact photonic devices.

Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today's magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton-Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light-matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17)., Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future., Journal of Physics D: Applied Physics, 55 (46), ISSN:0022-3727, ISSN:1361-6463

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-freestanding-bilayer epitaxial graphene on 4H-SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasifree-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene. (C) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license. Funding Agencies|Swedish Research Council VRSwedish Research Council [2016-00889, 2016-05362]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [FL120181, RIF14-055, GMT14-0077, RMA15024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Link_oping University, Faculty Grant SFO Mat LiU [2009-00971]; National Science FoundationNational Science Foundation (NSF) [DMR 1808715]; Air Force Office of Scientific ResearchUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360]; Nebraska Materials Research Science and Engineering Center [DMR 1420645]; European Unions Horizon 2020 research and innovation program [696656, 785219]; University of Nebraska Foundation; J. A. Woollam Foundation

Natural particles possess varying degrees of surface heterogeneities. Although the impact of collector surface heterogeneity on particle deposition is more or less established, the impact of particle surface heterogeneity on the deposition of colloids in porous media is not well studied. In this work, Janus SiO2 microparticles (Janus SiO2 MPs) were engineered by partially covering SiO2 microspheres (500 nm in diameter) with positively charged poly(L-lysine) (PLL) as model particles to study the impact of particle surface heterogeneity on the deposition. The surface heterogeneity of Janus SiO2 MP was confirmed using Kelvin probe force microscopy. Up to 4–5 original SiO2 microspheres tended to assemble in the solution to form relatively stable clusters, with an isoelectric point slightly higher than pH 7. Compared with unmodified SiO2 microspheres, Janus SiO2 MPs had a slower attachment rate on silicon dioxide surfaces. While the deposition of Janus SiO2 MPs onto silicon dioxide surfaces was observed at pH values of 3 and 7, no deposition was observed at pH 9. No deposition was observed for Janus SiO2 MPs onto aluminum oxide surfaces for pH values ranging from 3 to 9, even under an overall favorable attractive condition. This work found that the deposition of colloids is more sensitive to particle surface heterogeneity properties than to the overall surface potential of the colloids.

We find that a five-phase (substrate, mixed native oxide and roughness interface layer, metal oxide thin film layer, surface ligand layer, ambient) model with two-dynamic (metal oxide thin film layer thickness and surface ligand layer void fraction) parameters (dynamic dual box model) is sufficient to explain in-situ spectroscopic ellipsometry data measured within and across multiple cycles during plasma-enhanced atomic layer deposition of metal oxide thin films. We demonstrate our dynamic dual box model for analysis of in-situ spectroscopic ellipsometry data in the photon energy range of 0.7-3.4eV measured with time resolution of few seconds over large numbers of cycles during the growth of titanium oxide (TiO2) and tungsten oxide (WO3) thin films, as examples. We observe cyclic surface roughening with fast kinetics and subsequent roughness reduction with slow kinetics, upon cyclic exposure to precursor materials, leading to oscillations of the metal thin film thickness with small but positive growth per cycle. We explain the cyclic surface roughening by precursor-surface interactions leading to defect creation, and subsequent surface restructuring. Atomic force microscopic images before and after growth, x-ray photoelectron spectroscopy, and x-ray diffraction investigations confirm structural and chemical properties of our thin films. Our proposed dynamic dual box model may be generally applicable to monitor and control metal oxide growth in atomic layer deposition, and we include data for SiO2 and Al2O3 as further examples. Funding Agencies|National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC)National Science Foundation (NSF) [DMR-1420645, CMMI 1337856, DMR 1808715]; Air Force Research Office [FA9550-18-1-0360]; American Chemical SocietyAmerican Chemical Society [ACS PRF 59374-ND5]; University of Nebraska-Lincoln; J. A. Woollam Co., Inc.; J. A. Woollam Foundation

We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/AlxGa1-xN high electron mobility transistor structure with a high aluminum content (x = 0.78). The 2DEG sheet density N s = ( 7.3 +/- 0.7 ) x 10 12 cm(-2), sheet mobility mu s = ( 270 +/- 40 ) cm(2)/(Vs), sheet resistance R- s = ( 3200 +/- 500 ) omega/ ?, and effective mass m( eff) = ( 0.63 +/- 0.04 ) m( 0) at low temperatures ( T = 5 K ) are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson-Schrodinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al0.78Ga22N ( m( eff) = 0.334 m( 0)). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson-Schrodinger calculations .Published under an exclusive license by AIP Publishing. Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program [2016-05190]; Chalmers University of technology; Ericsson; United Monolithic Semiconductors (UMS); Volvo Cars; Swedish Research Council Vetenskapsradet (VR) [2016-00889]; Swedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoeping University, Faculty Grant Stark forskningsmiljoe (SFO) Mat LiU [2009-00971]; National Science Foundation [DMR 1808715, OIA-2044049]; Air Force Office of Scientific Research [FA9550-18-1-0360, FA9550-19-S-0003, FA9550-21-1-0259]

We demonstrate calibration and operation of a Mueller matrix imaging microscope using dual continuously rotating anisotropic mirrors for polarization state generation and analysis. The mirrors contain highly spatially coherent nanostructure slanted columnar titanium thin films deposited onto optically thick titanium layers on quartz substrates. The first mirror acts as polarization state image generator and the second mirror acts as polarization state image detector. The instrument is calibrated using samples consisting of laterally homogeneous properties such as straight-through-air, a clear aperture linear polarizer, and a clear aperture linear retarder waveplate. Mueller matrix images are determined for spatially varying anisotropic samples consisting of a commercially available (Thorlabs) birefringent resolution target and a spatially patterned titanium slanted columnar thin film deposited onto a glass substrate. Calibration and operation are demonstrated at a single wavelength (530 nm) only, while, in principle, the instrument can operate regardless of wavelength. We refer to this imaging ellipsometry configuration as rotating-anisotropic-mirror-sample-rotating-anisotropic-mirror ellipsometry (RAM-S-RAM-E). (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Funding Agencies|National Science FoundationNational Science Foundation (NSF) [DMR 1808715, OIA-2044049]; Air Force Office of Scientific ResearchUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360, FA9550-19-S-0003, FA9550-21-1-0259]; Knut och Alice Wallenbergs StiftelseKnut & Alice Wallenberg Foundation

We experimentally demonstrate and theoretically verify spectrally tunable, extremely large, and ultrabroadband circular dichroism by designing new hybrid nanohelical metamaterial configurations operating at the technologically important visible to ultraviolet spectrum. The demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the broad fields of classical and quantum optics leading to significantly enhanced chiral light-matter interactions at the nanoscale.

In recent years, sport governing bodies (SGB) have been the subject of serious questions regarding their governance structures and decision-making processes. SGB that fail to implement regulatory mechanisms and to improve their governance structures and processes risk being confronted with severe ethically sensitive issues outside and inside the fields, which may eventually result in negative publicity and reduced demand (e.g., fans, sponsors) or financial support (e.g., from governments). This study examines selected regulations and practices of North American professional sports leagues in light of good governance principles. By adopting a qualitative research design, we investigate if there is a need for reforms to be employed by the leagues to comply with core dimensions of governance and thus reduce the risk of not being prepared to deal with ethically sensitive issues that may come up. Our critical analysis suggests that essential reforms need to be employed by the leagues to comply with core principles of good governance. In terms of democracy, professional leagues need to recognise stakeholder interests, implement innovative participation mechanisms, and apply diversity and inclusion policies for board composition. On transparency, it is required to publish internal regulations and financial information despite lax regulations on disclosure policies in the United States. Concerning accountability, professional leagues should separate their disciplinary and executive branches to avoid the concentration of power and potential conflict of interest in the relationship between the commissioner and team owners.

We report the effect of remote surface optical (RSO) phonon scattering on carrier mobility in monolayer graphene gated by ferroelectric oxide. We fabricate monolayer graphene transistors back-gated by epitaxial (001) Ba0.6Sr0.4TiO3 films, with field effect mobility up to 23 000 cm2 V−1 s−1 achieved. Switching ferroelectric polarization induces nonvolatile modulation of resistance and quantum Hall effect in graphene at low temperatures. Ellipsometry spectroscopy studies reveal four pairs of optical phonon modes in Ba0.6Sr0.4TiO3, from which we extract RSO phonon frequencies. The temperature dependence of resistivity in graphene can be well accounted for by considering the scattering from the intrinsic longitudinal acoustic phonon and the RSO phonon, with the latter dominated by the mode at 35.8 meV. Our study reveals the room temperature mobility limit of ferroelectric-gated graphene transistors imposed by RSO phonon scattering.

Along with the rapid rise of esports across the globe, the esports industry is increasingly confronted with issues concerning governance, regulations and manipulations. Software-cheating, match-fixing as well as doping seem to be widespread phenomena within professional esports competitions. Addressing these issues is challenging due to complex and fragmented governance structures and the lack of any central steering authority. This study looks into doping and performance enhancement in particular. Based on semi-structured interviews with (n = 9) selected professional FIFA esports players, we explore their perceptions of the use of drugs and other (allegedly) performance-enhancing substances such as energy drinks and gaming boosters. The core findings that emerged out of the data are a high performance pressure related to FIFA competitions as perceived by players, an ambivalent perception towards the legitimacy of different forms of performance enhancement and a lack of anti-doping measures on the part of organisations involved. The study is among the first to explore the view of players as one of the key stakeholders in the esports ecosystem. ispartof: Performance Enhancement and Health vol:10 issue:4 status: published

The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light-matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid- to far-infrared due to shear dissipation in the dielectric response. This feature emerges in materials where the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals where multiple oscillators with non-orthogonal relative orientations contribute to the optical response. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic and hexagonal crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear dissipation in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals, many common oxides and organic crystals, significantly expanding the material base and extending design opportunities for compact photonic devices.

Understanding the influence of surface roughness on the deposition of nanoparticles is important to a variety of environmental and industrial processes. In this work, slanted columnar thin films (SCTFs) were engineered to serve as an analogue for rough surfaces with controlled height and surface properties. The deposition of titanium dioxide nanoparticles (TiO2NPs) onto alumina- or silica-coated SCTFs (Al2O3-Si-SCTF, SiO2-Si-SCTF) with varying heights (50 nm, 100 nm, and 200 nm) was measured using a combined quartz crystal microbalance with dissipation monitoring (QCM-D) and generalized ellipsometry (GE) technique. No TiO2NP deposition was observed on flat, silica-coated QCM-D sensors or rough, 100 nm thick SiO2-Si-SCTF. TiO2NP deposition onto Al2O3-Si-SCTFs in ultra-pure water was significantly higher than on the flat alumina-coated QCM-D sensor, and deposition increased as the roughness height increased. The nanoparticle attachment was sensitive to the local flow field and the interaction energy between nanoparticles and the QCM-D sensor. At a higher ionic strength condition (100 mM NaCl), TiO2NP aggregates with varying sizes formed a rigid layer on top of SCTFs. For the first time, deposition of nanoparticles was measured as a function of roughness height, and the impact of roughness on the properties of the attached nanoparticle layers was revealed. This finding indicates that key parameters describing surface roughness should be explicitly included into models to accurately predict the transport of nanoparticles in the subsurface.

Electrically conducting polymers (ECPs) are becoming increasingly important in areas such as optoelectronics, biomedical devices, and energy systems. Still, their detailed charge transport properties produce an anomalous optical conductivity dispersion that is not yet fully understood in terms of physical model equations for the broad range optical response. Several modifications to the classical Drude model have been proposed to account for a strong non-Drude behavior from terahertz (THz) to infrared (IR) ranges, typically by implementing negative amplitude oscillator functions to the model dielectric function that effectively reduce the conductivity in those ranges. Here we present an alternative description that modifies the Drude model via addition of positive-amplitude Lorentz oscillator functions. We evaluate this so-called Drude-Lorentz (DL) model based on the first ultra-wide spectral range ellipsometry study of ECPs, spanning over four orders of magnitude: from 0.41 meV in the THz range to 5.90 eV in the ultraviolet range, using thin films of poly(3,4-ethylenedioxythiophene): tosylate (PEDOT: Tos) as a model system. The model could accurately fit the experimental data in the whole ultrawide spectral range and provide the complex anisotropic optical conductivity of the material. Examining the resonance frequencies and widths of the Lorentz oscillators reveals that both spectrally narrow vibrational resonances and broader resonances due to localization processes contribute significantly to the deviation from the Drude optical conductivity dispersion. As verified by independent electrical measurements, the DL model accurately determines the electrical properties of the thin film, including DC conductivity, charge density, and (anisotropic) mobility. The ellipsometric method combined with the DL model may thereby become an effective and reliable tool in determining both optical and electrical properties of ECPs, indicating its future potential as a contact-free alternative to traditional electrical characterization. Funding Agencies|Swedish Research Council; Swedish Foundation for Strategic Research; Wenner-Gren Foundations; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]

Understanding the transport of engineered nanoparticles (ENPs) and their interaction with environmental surfaces is critical to predicting their fate and environmental implications. Here, we report ...

We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous Si-Au slanted columnar heterostructures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of Si-Au slanted columnar heterostructures. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of Si-Au slanted columnar heterostructures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water. Funding Agencies|National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC) [DMR-1420645, CMMI 1337856, EAR 1521428, DMR 1808715]; German Research Foundation [FE 1532/1-1]; Air Force Research Office [FA9550-18-1-0360]; American Chemical Society [ACS PRF 59374-ND5]; University of Nebraska-Lincoln; J.A. Woollam Co., Inc.; J.A. Woollam Foundation

Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al xGa1− x)2O3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga2O3 and α-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga2O3 whereas for α-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M1 type in α-Ga2O3 to M0 type van Hove singularity in α-Al2O3.

In this study, by using glancing angle deposition technique, subsequent and repeated depositions of silicon(Si) and silver(Ag) lead to nanometer-dimension chiral subsegments, and thereby, we successfully fabricated spatially coherent, highly porous, super lattice type helical heterostructure thin films. We theoretically and experimentally investigate the chiro-optical properties of this new type plasmonic metamaterial via finite element modeling calculations and Mueller matrix spectroscopic ellipsometry method, respectively. The systematic changes in the morphology of helical structures by incorporating the plasmonic subsegments reveal an extra-ordinary chiro-optical response with fine spectral tunability over the entire visible spectral range into the ultra-violet.

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an $\alpha$-Ga$_2$O$_3$ thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on $m$-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with $M_0$-type van Hove singularities for polarization perpendicular to the $c$ axis, $E_{0,\perp}=5.46(6)$ eV and $E_{0,\perp}=6.04(1)$ eV, and one direct band-to-band transition with $M_1$-type van Hove singularity for polarization parallel with $E_{0,||}=5.44(2)$ eV. We further identify excitonic contributions with small binding energy of 7 meV associated with the lowest ordinary transition, and a hyperbolic exciton at the $M_1$-type critical point with large binding energy of 178 meV., Comment: 7 pages, 4 figures, in submission to Applied Physics Letters

Mechanical-strain-gated switches are cornerstone components of material-embedded circuits that perform logic operations without using conventional electronics. This technology requires a single material system to exhibit three distinct functionalities: strain-invariant conductivity and an increase or decrease of conductivity upon mechanical deformation. Herein, mechanical-strain-gated electric switches based on a thin-film architecture that features an insulator-to-conductor transition when mechanically stretched are demonstrated. The conductivity changes by nine orders of magnitude over a wide range of tunable working strains (as high as 130%). The approach relies on a nanometer-scale sandwiched bilayer Au thin film with an ultrathin poly(dimethylsiloxane) elastomeric barrier layer; applied strain alters the electron tunneling currents through the barrier. Mechanical-force-controlled electric logic circuits are achieved by realizing strain-controlled basic (AND and OR) and universal (NAND and NOR) logic gates in a single system. The proposed material system can be used to fabricate material-embedded logics of arbitrary complexity for a wide range of applications including soft robotics, wearable/implantable electronics, human-machine interfaces, and Internet of Things. Funding Agencies|Alexander von Humboldt postdoc fellowshipAlexander von Humboldt Foundation; German Science FoundationGerman Research Foundation (DFG) [404941515, SPP 2100]; German Research Foundation (DFG)German Research Foundation (DFG) [MA 5144/9-1, MA 5144/13-1, MA 5144/28-1]; Helmholtz Association of German Research CentresHelmholtz Association [HIL-A04]; Projekt DEAL

A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principles calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (also known as point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution(. )A four-parameter semi-quantum model is applied to determine all 16 pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, epsilon(infinity) = 4.05 +/- 0.05. The Lyddane-Sachs-Teller relation then gives access to the static dielectric constant, epsilon(DC) = 10.55 +/- 0.07. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped n-type sample a free electron density of n = (2.81 +/- 0.01) x 10(17) cm(-3), a mobility of mu = (112 +/- 3) cm(2)/(Vs), and an effective mass parameter of (0.208 +/- 0.006)m(e). Density and mobility parameters compare very well with the results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the Gamma point in In2O3 in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In2O3, and our model validates the static dielectric constant obtained from the Lyddane-Sachs-Teller relation. Published under an exclusive license by AIP Publishing. Funding Agencies|National Science Foundation (NSF)National Science Foundation (NSF) [NSF DMR 1755479, NSF DMR 1808715]; Nebraska Materials Research Science and Engineering Center, GraFOx, a Leibniz-Science Campus - Leibniz Association-Germany [DMR 1420645]; Air Force Office of Scientific Research (AFOSR)United States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360, FA9550-19-S-0003]; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Governmental Agency for Innovation Systems VINNOVA under the Competence Center Program [2016-05190]; Knut and Alice Wallenbergs Foundation supported grant "Wide-bandgap semiconductors for next generation quantum components"; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Faculty Grant SFO Mat LiU [2009-00971]; University of Nebraska Foundation; J. A. Woollam Foundation

We report the elevated temperature (22 °C [Formula: see text] T [Formula: see text] 600 °C) dielectric function properties of melt grown single crystal ZnGa2O4 using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) × 10−5 K−1. A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M0-type critical point associated with the direct bandgap transition in ZnGa2O4 is shown to red-shift linearly as the temperature is increased with a subsequent slope of −0.72(4) meV K−1. Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions.

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(Al xGa1− x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

This Review highlights basic and transition metal conducting and semiconducting oxides. We discuss their material and electronic properties with an emphasis on the crystal, electronic, and band structures. The goal of this Review is to present a current compilation of material properties and to summarize possible uses and advantages in device applications. We discuss Ga2O3, Al2O3, In2O3, SnO2, ZnO, CdO, NiO, CuO, and Sc2O3. We outline the crystal structure of the oxides, and we present lattice parameters of the stable phases and a discussion of the metastable polymorphs. We highlight electrical properties such as bandgap energy, carrier mobility, effective carrier masses, dielectric constants, and electrical breakdown field. Based on literature availability, we review the temperature dependence of properties such as bandgap energy and carrier mobility among the oxides. Infrared and Raman modes are presented and discussed for each oxide providing insight into the phonon properties. The phonon properties also provide an explanation as to why some of the oxide parameters experience limitations due to phonon scattering such as carrier mobility. Thermal properties of interest include the coefficient of thermal expansion, Debye temperature, thermal diffusivity, specific heat, and thermal conductivity. Anisotropy is evident in the non-cubic oxides, and its impact on bandgap energy, carrier mobility, thermal conductivity, coefficient of thermal expansion, phonon modes, and carrier effective mass is discussed. Alloys, such as AlGaO, InGaO, (AlxInyGa1− x− y)2O3, ZnGa2O4, ITO, and ScGaO, were included where relevant as they have the potential to allow for the improvement and alteration of certain properties. This Review provides a fundamental material perspective on the application space of semiconducting oxide-based devices in a variety of electronic and optoelectronic applications.

Strain-stress relationships for physical properties are of interest for heteroepitaxial material systems, where strain and stress are inherent due to thermal expansion and lattice mismatch. We report linear perturbation theory strain and stress relationships for optical phonon modes in monoclinic crystals for strain and stress situations which maintain the monoclinic symmetry of the crystal. By using symmetry group analysis and phonon frequencies obtained under various deformation scenarios from density-functional perturbation theory calculations on $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, we obtain four strain and four stress potential parameters for each phonon mode. We demonstrate that these parameters are sufficient to describe the frequency shift of the modes regardless of the stress or strain pattern which maintain the monoclinic symmetry of the crystal. The deformation potentials can be used together with experimentally determined phonon frequency parameters from Raman or infrared spectroscopy to evaluate the state of strain or stress of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, for example, in epitaxial heterostructures.

Piezoresistive soft composite materials are widely used in strain sensing and typically exhibit a decrease in conductivity upon elongation—the so-called positive gauge effect. We demonstrate a thin-film architecture that features the inverse behavior: a strain-induced transition from insulating to metallic conductivity, spanning nine orders of magnitude in conductivity. Our approach is based on a nanometer-scale sandwiched bilayer Au thin film with a polydimethylsiloxane elastomeric barrier layer. Upon application of strain, the thickness of the thin soft barrier decreases because of the strain governed by the Poisson effect, followed by electron-tunneling currents through the barrier, forming an interconnected bilayer metal electrode. An extremely high on–off electrical conductivity ratio (~ 109) is observed over a wide range of working strains (as high as 130%), which mimics the ideal features of a mechanical-force-controlled electric transistor. This conceptual design strategy is expected to benefit a wide range of applications in which operation under minimal standby power could be an essential feature, such as in implantable soft strain sensors and in prosthetic long-term monitoring systems for detecting sudden a swelling/volume expansion of human body organs or blood vessels, thereby helping to avoid acute and severe syndromes.

Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry–Perot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.

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