Impact of Bismuth Incorporation into (Ga,Mn)As Dilute Ferromagnetic Semiconductor on Its Magnetic Properties and Magnetoresistance.

The impact of bismuth incorporation into the epitaxial layer of a (Ga,Mn)As dilute ferromagnetic semiconductor on its magnetic and electromagnetic properties is studied in very thin layers of quaternary (Ga,Mn)(Bi,As) compound grown on a GaAs substrate under a compressive misfit strain. An addition of a small atomic fraction of 1% Bi atoms, substituting As atoms in the layer, predominantly enhances the spin-orbit coupling strength in its valence band. The presence of bismuth results in a small decrease in the ferromagnetic Curie temperature and a distinct increase in the coercive fields. On the other hand, the Bi incorporation into the layer strongly enhances the magnitude of negative magnetoresistance without affecting the hole concentration in the layer. The negative magnetoresistance is interpreted in terms of the suppression of weak localization in a magnetic field. Application of the weak-localization theory for two-dimensional ferromagnets by Dugaev et al. to the experimental magnetoresistance results indicates that the decrease in spin-orbit scattering length accounts for the enhanced magnetoresistance in (Ga,Mn)(Bi,As).

Facile and Electrically Reliable Electroplated Gold Contacts to p-Type InAsSb Bulk-Like Epilayers.

Narrow band-gap semiconductors, namely ternary InAsSb alloys, find substantial technological importance for mid-infrared application as photodetectors in medical diagnostics or environmental monitoring. Thus, it is crucial to develop electrical contacts for these materials because they are the fundamental blocks of all semiconductor devices. This study demonstrates that electroplated gold contacts can be considered as a simple and reliable metallization technology for the electrical-response examination of a test structure. Unalloyed electroplated Au contacts to InAsSb exhibit specific contact resistivity even lower than vacuum-deposited standard Ti-Au. Moreover, temperature-dependent transport properties, such as Hall carrier concentration and mobility, show similar trends, with a minor shift in the transition temperature. It can be associated with a difference in metallization technology, mainly the presence of a Ti interlayer in vacuum-deposited contacts. Such a transition may give insight into not only the gentle balance changes between conductivity channels but also an impression of changing the dominance of carrier type from p- to n-type. The magnetotransport experiments assisted with mobility spectrum analysis clearly show that such an interpretation is incorrect. InAsSb layers are strongly p-type dominant, with a clear contribution from valence band carriers observed at the whole analyzed temperature range. Furthermore, the presence of thermally activated band electrons is detected at temperatures higher than 220 K.

Tunable Planar Hall Effect in (Ga,Mn)(Bi,As) Epitaxial Layers.

We have thoroughly investigated the planar Hall effect (PHE) in the epitaxial layers of the quaternary compound (Ga,Mn)(Bi,As). The addition of a small amount of heavy Bi atoms to the prototype dilute ferromagnetic semiconductor (Ga,Mn)As enhances significantly the spin-orbit coupling strength in its valence band, which essentially modifies certain magnetoelectric properties of the material. Our investigations demonstrate that an addition of just 1% Bi atomic fraction, substituting As atoms in the (Ga,Mn)As crystal lattice, causes an increase in the PHE magnitude by a factor of 2.5. Moreover, Bi incorporation into the layers strongly enhances their coercive fields and uniaxial magneto-crystalline anisotropy between the in-plane ⟨110⟩ crystallographic directions in the layers grown under a compressive misfit strain. The displayed two-state behaviour of the PHE resistivity at zero magnetic field, which may be tuned by the control of applied field orientation, could be useful for application in spintronic devices, such as nonvolatile memory elements.

Structural Quality and Magnetotransport Properties of Epitaxial Layers of the (Ga,Mn)(Bi,As) Dilute Magnetic Semiconductor.

Structural analysis of epitaxial layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor (DMS), together with investigations of their magnetotransport properties, has been thoroughly performed. The obtained results are compared with those for the reference (Ga,Mn)As layers, grown under similar conditions, with the aim to reveal an impact of Bi incorporation on the properties of this DMS material. Incorporation of Bi into GaAs strongly enhances the spin-orbit coupling strength in this semiconductor, and the same has been expected for the (Ga,Mn)(Bi,As) alloy. In turn, importantly for specific spintronic applications, strong spin-orbit coupling in ferromagnetic systems opens a possibility of directly controlling the direction of magnetization by the electric current. Our investigations, performed with high-resolution X-ray diffractometry and transmission electron microscopy, demonstrate that the (Ga,Mn)(Bi,As) layers of high structural quality and smooth interfaces can be grown by means of the low-temperature molecular-beam epitaxy method, despite a large difference between the sizes of Bi and As atoms. Depending on the applied buffer layer, the DMS layers can be grown under either compressive or tensile misfit strain, which influences their magnetic properties. It is shown that even small 1% Bi content in the layers strongly affects their magnetoelectric properties, such as the coercive field and anisotropic magnetoresistance.

Polarization of Magnetoplasmons in Grating Metamaterials Based on CdTe/CdMgTe Quantum Wells.

Grating metamaterials were fabricated with electron beam lithography on CdTe/CdMgTe modulation doped structures with two non-interacting quantum wells. Two types of samples were studied: with etched gratings and with gratings formed by deposition of Au stripes. The polarization properties at THz frequencies of the gratings were determined at room temperature. It was shown that Au gratings formed a linear polarizer, while etched gratings did not polarize THz radiation. Transmission of circularly polarized THz radiation at low temperatures through a sample with no grating showed a strongly circularly polarized cyclotron resonance transition. Transmission of this radiation through a sample with an etched grating showed a magnetoplasmon transition that was almost perfectly linearly polarized. We concluded that magnetoplasmons in metamaterials with etched gratings are linearly polarized excitations, possibly with a small contribution of a circular component. This work opens the possibility of the detailed study of the polarization of magnetoplasmons, which has not been explored in the past.

Ultrafast electrical switching of nanostructured metadevice with dual-frequency liquid crystal.

Shortening of switching times of various soft-matter-based tunable metamaterials is one of the key challenges to improve the functionality of modern active devices. Here we show an effective strategy in the evolution of soft-matter-based tunable metamaterials that makes possible acceleration of both on and off switching processes by using a dual-frequency liquid crystal mixture. The frequency-convertible dielectric anisotropy of the dual-frequency mixture enabled us to create a fast-response in-plane switching metasurface at the nanoscale, which could be tuned by an electrical signal with different frequencies. The results clearly show that the resonance of the metamaterial can be continuously and reversibly controlled within a wavelength range of 100 nm as the applied frequency is inverted between 1 kHz and 40 kHz, with a total response time (τ = τON + τOFF) of 1.89 ms. Furthermore, experimental characteristics of the hybrid metamaterial are in great agreement with numerical calculations, which allow us to anticipate active epsilon-near-zero behavior of the metadevice. This work indicates the future development direction of liquid-crystal-based active plasmonic systems.

Tryptophan-derived sulfur-containing phytoalexins--a general overview.

Phytoalexins are low molecular weight antimicrobial compounds that are synthesized and accumulated in plants after their exposure to pathogenic microorganisms (bacteria, fungi, viruses and protozoans). They are extensively studied now as promising antifungal, potentially anticancer and plant diseases controlling agents. The article pertains to a group of indole-derived phytoalexins--brassinins, containing at least one sulfur atom in the side chain or in the ring(s), isolated from the cruciferous plants. Up today more than 20 compounds, closely related biogenetically, but exhibiting diversified biological activity have been identified. The survey summerises most promising recent results pertaining practical application of brassinins and camalexins.

Novel bisindole derivatives of Catharanthus alkaloids with potential cytotoxic properties.

Pantropical plant Catharanthus roseus (L) G. Don is known as a source of valuable bisindole alkaloids: vinblastine (VBL) and vincristine (VCR), oncolytics widely used as sulfates in therapy of malignant diseases. They are biosynthesized in the plant from monoindolic vindoline and catharanthine, both derived from L-tryptophan and loganine units. In the course of phytochemical screening of this plant cultivated in Poland and considered as a home source of VBL and VCR we developed a new isolation method based on the solid phase extraction. Mild conditions used during the isolation procedure enabled the isolation of some labile compounds and so the monomeric alkaloids with high yields. Vindoline and so its congeners and subjected to various oxidative conditions gave 15,15'-bisindolic derivatives in quite good yield. Biological activities of the above mentioned bisindolic compounds are under study.