Materials Science (cond-mat.mtrl-sci)

Abstract: Four-nitrogen-containing 5,6,13,14-Tetraazapentacene (BTANC) has attracted attention as a new n-type organic semiconductor with a rigid crystalline phase due to intermolecular CH-N hydrogen bonding. However, in the thin film transistor of BTANC, poor carrier transport properties and low stability in the ambient condition have been reported so far; thus further refining and understanding of the thin film of BTANC will be required. Here, by means of carefully-controlled vacuum deposition of BTANC in the narrow window of temperature avoiding impurity sublimation and thermal degradation of molecules, we produced a well-defined monolayer on Cu(111) for molecular-level investigations. Synchrotron photoemission of the monolayer revealed a noticeable alteration of the chemical state of N atoms, which is unexpected for the pure BTANC molecule. In addition, molecular imaging of the monolayer by scanning tunneling microscope (STM) revealed that the molecular packing structure in the monolayer significantly differed from that in the single crystal of BTANC. These observations can be interpreted as a result of the partial hydrogenation of N atoms in BTANC and the emergence of the NH-N type intermolecular hydrogen bonding in the monolayer. These findings will provide a general remark and strategy to control the molecular packing structure and electronic property in the molecular films of the nitrogen-containing acenes, by means of controlled hydrogenation.

Abstract: The intermetallic Mn-phase, which precipitates in steels and superalloys, can noticeably soften the mechanical properties of their matrix. Despite the importance of developing superalloys and steels, the thermodynamic properties and directions of thermal expansion of the Mu-phase are still poorly studied. In this work, the thermal expansion paths, elastic, thermal and thermodynamic properties of the Fe23Mo16 and Fe7Mo6 Mu-phases have been studied using first-principles based quasi-harmonic Debye-Gruneisen approach. A method allowing avoids differentiation in many variables is used. The free energies consisting of the electronic, vibrational and magnetic energy contributions, calculated along different paths of thermal expansions were compared between themselves. A path with the least free energy was chosen as the trajectory of thermal expansion. Negative thermal expansion of the Fe7Mo6 compound was predicted, while the Fe23Mo16 has a conventional thermal expansion and negative TEC in the parameter c. The thermal expansions of both these compounds are not isotropic. The elastic constants, modulus, heat capacities, Curie and Debye temperatures were predicted. The obtained results satisfactorily agree with the available experimental data. Physical factors affecting the stability of Fe23Mo16 and Fe7Mo6 have been studied. The paper presents an essential feature of thermal expansions of the Mu-phase of the Fe-Mo system, which can provide an insight into future developments.

Abstract: Magnetic skyrmions have raised high hopes for future spintronic devices. For many applications it would be of great advantage to have more than one metastable particle-like texture available. The coexistence of skyrmions and antiskyrmions has been proposed in inversion symmetric magnets with exchange frustration. However, so far only model systems have been studied and the lifetime of coexisting metastable topological spin structures has not been obtained. Here, we predict that skyrmions and antiskyrmions with diameters below 10 nm can coexist at zero magnetic field in a Rh/Co bilayer on the Ir(111) surface -- an experimentally feasible system. We show that the lifetimes of metastable skyrmions and antiskyrmions in the ferromagnetic ground state are above one hour for temperatures up to 75 K and 48 K, respectively. The entropic contribution to the nucleation and annihilation rates differs for skyrmions and antiskyrmions. This opens the route to thermally activated creation of coexisting skyrmions and antiskyrmions in frustrated magnets with Dzyaloshinskii-Moriya interaction.

Abstract: The accurate description of electronic properties and optical absorption spectra is a long-standing challenge for density functional theory. Recently, the introduction of screened range-separated hybrid (SRSH) functionals for solid-state materials has allowed for the calculation of fundamental band gaps and optical absorption spectra that are in very good agreement with many-body perturbation theory. However, since solid-state SRSH functionals are typically tuned to reproduce the properties of bulk phases, their transferability to low-dimensional structures, which experience substantially different screening than in the bulk, remains an open question. In this work, we explore the transferability of SRSH functionals to several prototypical van der Waals materials, including transition-metal sulfides and selenides, indium selenide, black phosphorus, and hexagonal boron nitride. Considering the bulk and a monolayer of these materials as limiting cases, we show that the parameters of the SRSH functional can be determined systematically, using only the band-edge quasiparticle energies of these extremal structural phases as fitting targets. The resulting SRSH functionals can describe both electronic bandstructures and optical absorption spectra with accuracy comparable to more demanding ab initio many-body perturbation theory (GW and Bethe-Salpeter equation) approaches. Selected examples also demonstrate that the SRSH parameters, obtained from the bulk and monolayer reference structures, display good accuracy for bandstructures and optical spectra of bilayers, indicating a degree of transferability that is independent of the fitting procedure.

Abstract: The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in-operando x-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a five-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.