One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials.

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

By means of an ab initio investigation, exploiting many-body perturbation theory approaches on top of ground-state density functional theory calculations, we disclose the impact of the electronic correlation on the band structure and optical properties of a class of recently synthesized Pb-free 2D-layered halide perovskites, namely, (BA)4AgBiBr8 and (BA)2CsAgBiBr7. We observe strong out-of-plane excitonic dipoles characterizing the optical spectra, a result that suggests the potential applicability of these layered hybrid materials in photonics and plasmonics, where the perpendicular field direction plays a relevant role.

We report a cathodoluminescence (CL) study of layered germanium sulfide (GeS) where we observe a sharp emission peak from flakes covered with a thin hexagonal boron nitride film. GeS is a material that has recently attracted considerable interest due to its emission in the visible region and its strong anisotropy. The measured CL peak is at ~1.69 eV for samples ranging in thickness from 97 nm to 45 nm, where quantum-confinement effects can be excluded. By performing ab initio ground- and excited-state simulations for the bulk compound, we show that the measured optical peak can be unambiguously explained by radiative recombination of the first free bright bound exciton, which is due to a mixing of direct transitions near the Γ-point of the Brillouin Zone and it is associated to a very large optical anisotropy. The analysis of the corresponding excitonic wave function shows a Wannier–Mott interlayer character, being spread not only in-plane but also out-of-plane.

Citation: D Fornalski et al. "Simultaneous thermodynamic and dynamical characterisation using in situ calorimetry with neutron spectroscopy." Low Both Differential Scanning Calorimetry (DSC) and Quasi-elastic Neutron Scattering (QENS) are powerful analytical tools actively used in studies of phase transitions in complex solid and liquid systems. DSC is typically used to map phase transition temperatures and identify sample states, and QENS provides information on the molecular scale dynamical motions, such as molecular self-diffusion or glassy dynamics, associated with such transitions. Both techniques provide highly valuable complementary information about the sample and in many cases it would be advantageous to measure in parallel with a view to linking the two observables. The biggest challenge is that the cell design which differs greatly between the two methods. Here we present a first attempt at designing a cryogenic system which will allow the simultaneous measurement of calorimetric transitions and QENS measurements, as tested on the neutron spectrometer IRIS at ISIS neutron scattering facility. The system temperature range is from 10 K to 300 K. We present and discuss the initial design of the system, preliminary test results, current challenges and limitations, and future directions.