Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of doped cadmium-oxide, epsilon-near-zero nanofilms. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.Comment: 19 pages, 5 figure

High-entropy high-hardness metal carbides discovered by entropy descriptors

High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor - entropy forming ability - for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides - promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.Comment: 12 pages, 2 figure

Science of entropy-stabilized ultra-high temperature materials: predictive and multi-physics modelling

Our team is exploring a new concept in the development of ultra-high temperature materials - engineering the configurational entropy contribution to the free energy. We are doing this by using compositions with multiple refractory elements in near equi-molar concentrations. The work has focused primarily on unique refractory alloys that combine multi- and single-component sublattices; these are di-borides with layered sublattices, as well as nitrides, carbides, and carbo-nitrides with interpenetrating multi-and single-component face-centered-cubic sublattices. Please click Additional Files below to see the full abstract

Science of entropy-stabilized ultra-high temperature materials: synthesis, validation and properties

A multidisciplinary effort to identify new ultra-high temperature materials combines both computational and experimental efforts exploring the potential of multi-principle element transition metal carbides, nitrides, and borides for improved properties. The elements of focus for this project include the group IV, V, and VI transition metals (Ti, Zr, Hf, V, Bn, Ta, Cr, Mo, W) combined as carbides, nitrides, or borides. This presentation will describe synthesis, validation, and evaluation of these Entropy Stabilized Ultra-High Temperature Materials (ES-UHTM). Both thin film physical vapor deposition and field-assisted bulk sintering techniques are used with the goal to synthesize single phase ES-UHTM. A five-cathode deposition tool has been successfully used to prepare thin film carbides such as (Nb,W,Ti,Zr,Ta)C as a single phase with the rocksalt structure. Films are prepared with thicknesses up to several microns to facilitate fundamental property measurements. Multiple compositions of equimolar, five-component, metal diborides and as well as several metal carbides have also been successfully fabricated as bulk specimens via high-energy ball milling and spark plasma sintering. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and aberration corrected scanning transmission electron microscopy with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping have all been used to evaluate the success in achieving single phase states with random and homogeneous distributions of cations. Experimental successes in achieving single phase compositions are compared to theoretical predictions described in a companion presentation. Material properties such as thermal conductivity, diffusivity and oxidation resistance are evaluated to assess performance relative to theoretical prediction and conventional UHTCs. Thermal conductivity is determined using the time domain thermal reflectance technique on thin film specimens for comparison to theoretical predictions for these multiple principle element compositions. Thin film diffusion couples (two-layer films), in which four of the principle transition metals are found in both films and one element of each thin film differs, are prepared to study diffusivity in these ES-UHTMs. Oxidation resistance is been characterized using Joule heating of bulk specimens to ultra-high temperatures (T\u3e1500C) in controlled argon-oxygen atmospheres to determine oxide phases formed and their distribution, comparing carbides and borides of the same metal composition. Properties of these unique ES-UHTM are evaluated with the intent to enable tailoring of material performance via exploration of the large compositional space available

First principles computational descriptor for entropy forming ability

Entropy stabilized materials [1], where the mixing of the components is driven by configurational entropy rather than formation enthalpy, are potential candidates for ultra-high temperature applications. The prediction of which compositions will form entropy stabilized materials is difficult since calculating the entropic contribution to the free energy from first principles is computationally expensive. Therefore, we have formulated a descriptor for the synthesizability of disordered materials based on the energy distribution of the thermodynamic density of states (TDOS) for an ensemble of ordered configurations generated using the AFLOW (Automatic FLOW) partial occupation (AFLOW-POCC) methodology [2,3] and calculated with DFT. This descriptor has been used to successfully predict which refractory metal carbide compositions can be experimentally synthesized as single-phase entropy stabilized materials [4]. This work is supported by the U.S. Office of Naval Research MURI program (grant No. N00014-15- 1-2863). [1] C. M. Rost, E. Sachet, T. Borman, A. Moballegh, E. C. Dickey, D. Hou, J. L. Jones, S. Curtarolo, and J.-P. Maria, Entropy Stabilized Oxides, Nat. Commun. 6, 8485 (2015). [2] S. Curtarolo, W. Setyawan, G. L. W. Hart, M. Jahnatek, R. V. Chepulskii, R. H. Taylor, S. Wang, J. Xue, K. Yang, O. Levy, M. J. Mehl, H. T. Stokes, D. O. Demchenko, and D. Morgan, AFLOW: an automatic framework for high-throughput materials discovery, Comput. Mater. Sci. 58, 218-226 (2012). [3] K. Yang, C. Oses, and S. Curtarolo, Modeling off-stoichiometry materials with a high-throughput ab-initio approach, Chem. Mater. 28, 6484-6492 (2016). [4] P. Sarker, T. Harrington, C. Toher, K. Vecchio, and S. Curtarolo, First principles materials design using a spectral descriptor for entropy forming ability, in preparation (2017)

Single-peak and narrow-band mid-infrared thermal emitters driven by mirror-coupled plasmonic quasi-BIC metasurfaces

Wavelength-selective thermal emitters (WS-EMs) hold considerable appeal due to the scarcity of cost-effective, narrow-band sources in the mid-to-long-wave infrared spectrum. WS-EMs achieved via dielectric materials typically exhibit thermal emission peaks with high quality factors (Q factors), but their optical responses are prone to temperature fluctuations. Metallic EMs, on the other hand, show negligible drifts with temperature changes, but their Q factors usually hover around 10. In this study, we introduce and experimentally verify a novel EM grounded in plasmonic quasi-bound states in the continuum (BICs) within a mirror-coupled system. Our design numerically delivers an ultra-narrowband single peak with a Q factor of approximately 64, and near-unity absorptance that can be freely tuned within an expansive band of more than 10 {\mu}m. By introducing air slots symmetrically, the Q factor can be further augmented to around 100. Multipolar analysis and phase diagrams are presented to elucidate the operational principle. Importantly, our infrared spectral measurements affirm the remarkable resilience of our designs' resonance frequency in the face of temperature fluctuations over 300 degrees Celsius. Additionally, we develop an effective impedance model based on the optical nanoantenna theory to understand how further tuning of the emission properties is achieved through precise engineering of the slot. This research thus heralds the potential of applying plasmonic quasi-BICs in designing ultra-narrowband, temperature-stable thermal emitters in mid-infrared. Moreover, such a concept may be adaptable to other frequency ranges, such as near-infrared, Terahertz, and Gigahertz.Comment: 39 pages, 12 figure

Mechanisms of Resilience in Children of Mothers Who Self-Report with Depressive Symptoms in the First Postnatal Year.

BACKGROUND: Symptoms of maternal postnatal depression are associated with an increased risk of adverse effects on child development. However, some children exposed to postnatal depression have outcomes similar to unexposed children, and can be referred to as resilient. This study aimed to determine the mechanisms of resilience in children exposed to depressive symptoms postnatally. METHOD: Data are from a prospective cohort study, the Avon Longitudinal Study of Parents and Children. Self-report questionnaire data were collected during pregnancy and the child's first 2 years regarding maternal views of parenting and her perception of the child. The Edinburgh Postnatal Depression Scale (EPDS) was completed postnatally at 8 months and the Strengths and Difficulties Questionnaire (SDQ) at age 11 years. Exposed children who scored above the median score of non-exposed children were defined as resilient. Structural equation modeling was used to investigate the development of resilience. RESULTS: From the core ALSPAC cohort, 1,009 children (6.9%) were exposed to maternal depression at 8 months postnatally. The SDQ total difficulties scores at 11 years of age indicated that 325 (32.2%) were resilient, 684 were non-resilient. Maternal positive feelings about parenting and child non-verbal communication at 15 months increased the likelihood of later resilience. CONCLUSIONS: In this study, resilience was associated with two factors: the child's nonverbal communication at 15 months and by maternal positive feelings about parenting. Early intervention to support mother-child interaction and foster child development in women identified with postnatal depressive symptoms may benefit later child resilience