Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the ultrastructural changes in bacteria induced by antimicrobial peptides (AMPs). Both the ␤-stranded gramicidin S and the ␣-helical peptidyl-glycylleucine-carboxyamide (PGLa) are cationic amphiphilic AMPs known to interact with bacterial membranes. One representative Gram-negative strain, Escherichia coli ATCC 25922, and one representative Gram-positive strain, Staphylococcus aureus ATCC 25923, were exposed to the AMPs at sub-MICs and supra-MICs in salt-free medium. SEM revealed a shortening and swelling of the E. coli cells, and multiple blisters and bubbles formed on their surface. The S. aureus cells seemed to burst upon AMP exposure, showing open holes and deep craters in their envelope. TEM revealed the formation of intracellular membranous structures in both strains, which is attributed to a lateral expansion of the lipid membrane upon peptide insertion. Also, some morphological alterations in the DNA region were detected for S. aureus. After E. coli was incubated with AMPs in medium with low ionic strength, the cells appeared highly turgid compared to untreated controls. This observation suggests that the AMPs enhance osmosis through the inner membrane, before they eventually cause excessive leakage of the cellular contents. The adverse effect on the osmoregulatory capacity of the bacteria is attributed to the membrane-permeabilizing action of the amphiphilic peptides, even at low (sub-MIC) AMP concentrations. Altogether, the results demonstrate that both TEM and SEM, as well as appropriate sample preparation protocols, are needed to obtain detailed mechanistic insights into peptide function.

We investigate nanocontact formation by thermally assisted electromigration of gold nanowires. An automatic cycling process allows us to follow a line of constant power dissipated at the nanocontact up to resistances corresponding to 10–20 conductance quanta. The contacts are thinned in a controlled way by voltage-induced heating. In the ballistic regime, oscillations of the conductance histograms show oscillations typical for atomic discreteness.

Spintronic structures are extensively investigated for their spin–orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular, the intrinsic inverse spin-Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The spin-charge conversion is probed by complementary techniques—ultrafast THz time-domain spectroscopy in the dynamic regime for THz pulse emission and ferromagnetic resonance spin-pumping measurements in the GHz regime in the steady state—to determine the role played by the material properties, resistivities, spin transmission at metallic interfaces, and spin-flip rates. These measurements show the correspondence between the THz time-domain spectroscopy and ferromagnetic spin-pumping for the different set of samples in term of the spin mixing conductance. The latter quantity is a critical parameter, determining the strength of the THz emission from spintronic interfaces. This is further supported by ab initio calculations, simulations, and analysis of the spin-diffusion and spin-relaxation of carriers within the multilayers in the time domain, permitting one to determine the main trends and the role of spin transmission at interfaces. This work illustrates that time-domain spectroscopy for spin-based THz emission is a powerful technique to probe spin-dynamics at active spintronic interfaces and to extract key material properties for spin-charge conversion.

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Terahertz (THz) spin‐to‐charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here, nanometric trilayers of Co/X/Pt (X = Ti, Au or an Au:W alloy) are investigated as a function of the “X” layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the “X” layer it is shown that the ultrafast spin current injected in the non‐magnetic layer defines a direct‐spin‐conductance, whereas the Gilbert damping leads to an effective spin‐mixing‐conductance of the trilayer. Importantly, it is shown that these two parameters are connected to each other and that spin‐memory‐losses can be modeled via an effective Hamiltonian with Rashba fields. This work highlights that magneto‐circuit concepts can be successfully extended to ultrafast spintronic devices, as well as enhancing the understanding of spin‐to‐charge conversion processes through the complementarity between ultrafast THz spectroscopy and steady state techniques.

Spin‐to‐charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin‐locked TSS. In this work, sizable THz emission from a nanometric thick topological insulator (TI)/ferromagnetic junction—SnBi2Te4/Co—specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi2Te3 is demonstrated. THz emission time domain spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by investigating the TI thickness and angular dependence of the THz emission. This work illustrates THz emission TDS as a powerful tool alongside angular resolved photoemission spectroscopy (ARPES) methods to investigate the interfacial spintronic properties of TI/ferromagnet bilayers.

Pyramidal GaAs structures on top of GaAs∕AlAs distributed Bragg reflectors are investigated as candidates for true three-dimensional cavities with potentially low mode volume and high quality-factor. Different types of single and coupled resonators with base lengths of a few microns are realized using a combination of molecular-beam epitaxy, electron-beam lithography, and wet chemical etching. Embedded InGaAs quantum dots are utilized as light sources to verify the resonator modes. Furthermore, a spatially localized emission through the pyramid facets indicates the future possibility of coupling cavity modes to optical fibers. This could be interesting within the context of single photon emitters.

Atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy were applied to characterize single walled carbon nanotubes (SWNTs) deposited on sapphire. The electronic properties and structure of luminescent semiconducting SWNTs can be probed by PL spectroscopy. The diameter, length, spatial position, and angular orientation of deposited SWNTs can be accurately determined by intermittent contact AFM. We describe an approach to combine and compare the spectroscopic (PL) and topographical (AFM) information for the same individual nanotubes as well as nanotube aggregates.