Rashba Metamaterials and Metasurfaces with Zero Reflectivity and Effect of Surface States in Ultrathin Metal Films.

Metals, renowned for their high reflectivity, find extensive use in various technological applications, from mirrors to optical coatings in radars, telescopes, and mobile communications. However, their potential in antireflective coatings has remained largely untapped. In this study, we demonstrate that by applying an ultrathin metallic film onto an oxide layer, we can achieve a flawless optical surface with zero reflectivity. This phenomenon has been successfully observed across various metals, including Sn, Ag, Au, Pt, Bi, and Nb, showcasing its broad applicability. The underlying principle lies in the emergence of surface states, where the Rashba effect is strong, which give rise to the formation of Rashba metamaterial and metasurface (RMM) structures. Remarkably, these RMMs can be fine-tuned to act as high-resolution Veselago lenses. To illustrate, we achieved zero reflectivity with an RMM consisting of a 1 nm thick Sn metal film on a 1 nm Ge buffer, situated on a 60 nm Al2O3/Si substrate. Similar results were observed for other metals (Pt, Au, Ag, and Nb) and semimetals (Bi) by adjusting the film thickness to 2, 3, 5, 10, and 6 nm, respectively. The revelation of RMMs with zero reflectivity (R = 0) has tremendous potential to revolutionize optical device technologies, covering renewable energy, optoelectronics, and the telecommunications industry.

Photon Drag Currents and Terahertz Generation in α-Sn/Ge Quantum Wells.

We have fabricated α-Sn/Ge quantum well heterostructures by sandwiching nano-films of α-Sn between Ge nanolayers. The samples were grown via e-beam deposition and characterized by Raman spectroscopy, atomic force microscopy, temperature dependence of electrical resistivity and THz time-resolved spectroscopy. We have established the presence of α-Sn phase in the polycrystalline layers together with a high electron mobility µ = 2500 ± 100 cm2 V-1 s-1. Here, the temperature behavior of the resistivity in a magnetic field is distinct from the semiconducting films and three-dimensional Dirac semimetals, which is consistent with the presence of linear two-dimensional electronic dispersion arising from the mutually inverted band structure at the α-Sn/Ge interface. As a result, the α-Sn/Ge interfaces of the quantum wells have topologically non-trivial electronic states. From THz time-resolved spectroscopy, we have discovered unusual photocurrent and THz radiation generation. The mechanisms for this process are significantly different from ambipolar diffusion currents that are responsible for THz generation in semiconducting thin films, e.g., Ge. Moreover, the THz generation in α-Sn/Ge quantum wells is almost an order of magnitude greater than that found in Ge. The substantial strength of the THz radiation emission and its polarization dependence may be explained by the photon drag current. The large amplitude of this current is a clear signature of the formation of conducting channels with high electron mobility, which are topologically protected.

Perfect Impedance Matching with Meta-Surfaces Made of Ultra-Thin Metal Films: A Phenomenological Approach to the Ideal THz Sensors.

The terahertz (THz) frequency range is incredibly important as it covers electromagnetic emissions typical for biological and molecular processes. All molecules emit THz waves in a unique fingerprint pattern, although the intensity of such signals is usually too weak to be detected. To address the efficiency gap in existing THz devices it is extremely important to create surfaces with perfect anti-reflection properties. Although metals are absolutely reflective, here we show both theoretically and experimentally that by constructing meta-surfaces made of a superposition of ultra-thin metallic nano-films (a couple of nanometres thick) and oxide layers a unique property of perfect transmission and impedance matching may be realised. The perfect transmission rates can be as high as 100% and it may be achieved in both optical and THz regimes. The predicted effect has been observed for numerous meta-surfaces of different compositions. The effect found here is expected to impact the renewable energies sectors, optoelectronic and telecommunication industries, accelerating the arrival of the sensors for the new 6G-technology. The phenomenon is highly relevant to all scientific fields where minimising electromagnetic losses through reflection is important.

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating.

Manipulating the superconducting states of high transition temperature (high-Tc) cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr2CuO4±Î´ (PCO) films, based on two distinct mechanisms. Firstly, with positive electric fields, the film can be reversibly switched between superconducting and non-superconducting states, attributed to the carrier doping effect. Secondly, the film becomes more resistive by applying negative bias voltage up to - 4 V, but strikingly, a non-volatile superconductivity is achieved once the gate voltage is removed. Such phenomenon represents a distinctive route of manipulating superconductivity in PCO, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments. The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-Tc superconductors.

Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure.

The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni⋅⋅⋅Ni distance decreases from 3.19 Šat ambient pressure to 2.82 Šat 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Šfor the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.

Pressure suppression of charge order without metallisation in Cs2Au2I6.

Resistivity and powder X-ray diffraction measurements on Cs(2)Au(2)I(6) up to 320 kbar pressure show that the suppression of Au(+)/Au(3+) charge order above 55 kbar results in a non-metallic phase containing localised Au(2+) states that is irreversibly amorphised above 120 kbar, and a gradual metallisation observed above 175 kbar may result from decomposition within the amorphous material.

Electronic tuning of two metals and colossal magnetoresistances in EuWO(1+x)N(2-x) perovskites.

A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO(1+x)N(2-x). Ammonolysis of Eu(2)W(2)O(9) yields scheelite-type intermediates EuWO(4-y)N(y) with a very small degree of nitride substitution (y = 0.04) and then EuWO(1+x)N(2-x) perovskites that show a wide range of compositions -0.16 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO(1+x)N(2-x) phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.