Crystallographic, electronic transport, thermal and magnetic properties are reported for SrMn2As2 and CaMn2As2 single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl2Si2-type structure (space group P3m1), in agreement with the literature. Electrical resistivity ρ versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn2As2 and 61 meV for CaMn2As2. In a local-moment picture, the Mn +2 3d 5 ions are expected to have high-spin S = 5/2 with spectroscopic splitting factor g ≈ 2. Magnetic susceptibility χ and heat capacity measurements versus T reveal antiferromagnetic (AFM) transitions at T N = 120(2) K and 62(3) K for SrMn2As2 and CaMn2As2, respectively. The anisotropic χ(T ≤ T N) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. The χ(T ) data for both compounds and the Cp(T ) for SrMn2As2 show strong dynamic short-range AFM correlations from TN up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.

at deep sub-wavelengths, [2] leading to a variety of applications including nanolasers, [3] sensors, [4] bioimaging, [5] surfaceenhanced Raman spectroscopy, [6,7] color displays, [8] and others. [9][10][11][12] To improve the performance of today's photonic systems, researchers have extensively investigated the fundamental relation between the wave vector and energy of an SPP wave-named dispersion relation. This quantity describes the propagation of light within a material (i.e., medium), extremely relevant for the abovementioned optical devices. Therefore, understanding the dispersion relation can allow the design of optical materials with superior response, ranging from 2D van der Waals to oxides and metals. Concerning 2D materials, it can uncover the origin of tunable polaritons in hyperbolic metamaterials based on graphene and hexagonal boron nitride (h-BN), which is due to the hybridization of SPP and surface phonon polaritons. [13] As another example, by utilizing the band-edge mode of the dispersion relation in metallic nanocavities, [3] lasing with a 200 times enhancement of the spontaneous emission rate of the dye has been reached. [14] As a class of emerging photonic materials, noble metal alloys with permittivity and localized surface plasmon resonances not achievable by pure metals [9,15,16] have been proposed as alternative candidates for plasmonics [12,[17][18][19][20] because of their tunable dielectric functions, which make it possible to engineer the alloy composition to attain optical properties that will meet desired resonances. In turn, this tunability could be used to Surface plasmon polaritons (SPPs) enable the deep subwavelength confinement of an electromagnetic field, which can be used in optical devices ranging from sensors to nanoscale lasers. However, the limited number of metals that satisfy the required boundary conditions for SPP propagation in a metal/dielectric interface severely limits its occurrence in the visible range of the electromagnetic spectrum. We introduce the strategy of engineering the band structure of metallic materials by alloying. We experimentally and theoretically establish the control of the dispersion relation in Ag-Au alloys by varying the film chemical composition. Through X-ray photoelectron spectroscopy (XPS) measurements and partial density-of-states calculations we deconvolute the d band contribution of the density-of-states from the valence band spectrum, showing that the shift in energy of the d band follows the surface plasmon resonance change of the alloy. Our density functional theory calculations of the alloys band structure predict the same variation of the threshold of the interband transition, which is in very good agreement with our optical and XPS experiments. By elucidating the correlation between the optical behavior and band structure of alloys, we anticipate the fine control of the optical properties of metallic materials beyond pure metals. Band Structure EngineeringThe ORCID identification number(s) for the author(s) of this article can b...

single, multiple, or broadband frequency of the electromagnetic spectrum. The ultrahigh light absorption is obtained due to an impedance match between the material and the medium. [9] Here, the electric and magnetic resonances are designed so that the bulk effective impedance is equal to the one of the free space (air or vacuum). As a result, most of the incident light is absorbed and the reflection is negligible. Salisbury [10] and Dallenbach [11] first idealized classical absorbers to operate in the microwave range of the electromagnetic spectrum. The former included a resistive layer located at a quarter wavelength from a metallic substrate while the latter consisted of a dielectric layer on top of a metallic substrate.In the past two decades, advances in nanofabrication have given rise to nanostructures with controlled geometry, recently inspiring the design of metamaterials and metasurfaces for superabsorbers. [12] With the flexibility of tuning nanostructures' geometry and periodicity, such absorbers have been demonstrated in the visible, [13,14] near-infrared (NIR), [15] mid-infrared (MIR), [16] and far-infrared (FIR) [17,18] frequency ranges, proving to be a powerful approach for producing optical responses that are not feasible by any conventional material. However, the cost of the current fabrication methods limits their commercial applications. For instance, metasurfaces consisting of arrays of nanostructures on a dielectric surface are difficult to manufacture through physical deposition approaches in large scale even by using state-of-the-art bottomup nanolithography methods.To overcome the scalability constraints of the fabrication methods currently implemented for metasurfaces, the use of thin films in superabsorbers has been explored for a broad range of the spectrum, extending from visible to the FIR. [19][20][21][22][23][24][25][26][27][28] The Dallenbach configuration provides significant benefits regarding the fabrication of ultrathin, planar, omnidirectional, and polarization independent structures with very high absorption. Recently, it was reported that more than 98% of the normally incident light could be absorbed in an ultrathin layer of Ge on top of Ag at a wavelength (λ) of 625 nm, decreasing to 80% for incident angles up to 66° for both polarizations. [21] In addition, highly doped Si has been used as a metallic-like substrate under a thin Ge layer to absorb light in the MIR, where the doping concentration in Si was the knob to engineer its Superabsorbers based on metasurfaces have recently enabled the control of light at the nanoscale in unprecedented ways. Nevertheless, the sub-wavelength features needed to modify the absorption band usually require complex fabrication methods, such as electron-beam lithography. To overcome the scalability limitations associated with the fabrication of metallic nanostructures, engineering the optical response of superabsorbers by metal alloying is proposed, instead of tuning the geometry/size of the nanoscale building blocks. The superior performance...

Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMnAs that crystallizes in a trigonal structure ([Formula: see text]) and undergoes an antiferromagnetic (AFM) transition at [Formula: see text] K. The magnetic susceptibility remains nearly constant at temperatures [Formula: see text] with [Formula: see text] whereas it decreases significantly with [Formula: see text]. This shows that the ordered Mn moments lie in the [Formula: see text] plane instead of aligning along the [Formula: see text]-axis as in tetragonal BaMnAs. Single-crystal neutron diffraction measurements on SrMnAs demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with [Formula: see text] AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMnAs as evident from the temperature dependence of the order parameter.