Group news

In a recent paper [Nanoscale 13, 15156 (2021)], Jorge Quereda et al. identify a rapidly-responding photogating effect in monolayer MoS2 phototransistors that becomes  the dominant contribution to photoresponse under high-frequency light modulation, in contrast with previous understanding. Using a Hornbeck–Haynes model for the photocarrier dynamics, they fit the illumination power dependence of this photogating effect and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS2 caused by the presence of sulfur vacancies.

In a recent paper [New Journal of Physics 23, 083003 (2021)], Álvaro Díaz Fernández et al present a theoretical study of the effect of a single magnetic impurity in Weyl semimetals as a first step towards considering a larger number of point-like impurities. We find that resonances appear in the local density of states (LDOS) with a Friedel-like behaviour, oscillating as a function of distance. By studying the spin-resolved LDOS, they observe non-trivial and anisotropic spin textures where the spin components perpendicular to the spin of the impurity wind around the latter, until the spin becomes completely parallel to the impurity right at the impurity’s location. Friedel oscillations also play a relevant role in the form of the spin textures, forming an oscillatory pattern. They claim that the results can pave the way to further studies which consider the presence of a large number of random magnetic impurities.

In a recent paper [Journal of Colloid and Interface Science 596, 64 (2021)], Elena Díaz et al present a luminescence platform that can be used as point of care system for determining the presence and concentration of specific oligonucleotide sequences. This sensor exhibited a limit of detection as low as 50 fM by means of: (i) the use of single-stranded DNA (ssDNA) functionalized magnetic microparticles that captured and concentrated ssDNA-upconverting nanoparticles (ssDNA-UCNPs) on a solid support, when the target sequence (miR-21-5p DNA-analogue) was in the sample, and (ii) a photoligation reaction that covalently linked the ssDNA-UCNPs and the ssDNA magnetic microparticles, allowing stringent washes. The presented sensor showed a similar limit of detection when the assays were conducted in samples containing total miRNA extracted from human serum, demonstrating its suitability for detecting small specific oligonucleotide sequences under real-like conditions. The strategy of combining UCNPs, magnetic microparticles, and a photoligation reaction provides new insight into low-cost, rapid, and ultra-sensitive detection of oligonucleotide sequences.

In a recent paper [Nanoscale 13, 6117 (2021)], Leonor Chico et al present a theoretical study of the symmetry based electronic and optical properties of two-dimensional pentagonal structures based on the Cairo tiling. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, the electronic band structures and the local spin textures in momentum space are analyzed for four examples of these materials, namely, PdSeTe, PdSeS, InP5 and GeBi2, all of which are dynamically stable. Their results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, they computed the linear and nonlinear optical features of the proposed pentagonal materials and discuss some particular features such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

In a recent paper [Scientific Reports 11, 5810 (2021)], José Luis Hernando et al theoretically address the impact of a random distribution of non-magnetic impurities on the electron states formed at the surface of a topological insulator. The interaction of electrons with the impurities is accounted for by a separable pseudo-potential method that allows us to obtain closed expressions for the density of states. Spectral properties of surface states are assessed by means of the Green’s function averaged over disorder realisations. For comparison purposes, the configurationally averaged Green’s function is calculated by means of two different self-consistent methods, namely the self-consistent Born approximation (SCBA) and the coherent potential approximation (CPA). The latter is often regarded as the best single-site theory for the study of the spectral properties of disordered systems. However, although a large number of works employ the SCBA for the analysis of many-impurity scattering on the surface of a topological insulator, CPA studies of the same problem are scarce in the literature. In this work, they find that the SCBA overestimates the impact of the random distribution of impurities on the spectral properties of surface states compared to the CPA predictions. The difference is more pronounced when increasing the magnitude of the disorder.

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