Chiral Selectivity of Unusual Helimagnetic Transition in Iron Nanotubes: Chirality Makes Quantum Helimagnets

The remarkable interplay of chirality and magnetism in helical single-wall nanotubes of iron (FeSWNTs) is investigated using fully unconstrained spin-density-functional calculations. Spin-spiral waves exist and noncollinear helimagnetism appears only for the specific chirality of (6,3) and (5,3) FeSWNTs, whereas collinear ferromagnetism persists in other chiral FeSWNTs as unfolded monolayers, that is, chirality selectively involves the unusual helimagnetic phase transition (chiral selectivity). The emergence of quantum helimagnetism plays a variety of significant roles in (i) the stabilization of the chiral FeSWNTs as a long-lived “magic” structure in both freestanding and tip-suspended conditions, (ii) interference with quantum ballistic conductance by interband repulsion, and (iii) the involvement of chiral conductivity in which electric currents pass helically through the FeSWNTs. These chiral characteristics are a novel addition to the intriguing rich diversity of chirality-driven physics and phenomena

Ferroelectricity in Ruddlesden–Popper Chalcogenide Perovskites for Photovoltaic Application: The Role of Tolerance Factor

Chalcogenide perovskites with optimal band gap and desirable light absorption are promising for photovoltaic devices, whereas the absence of ferroelectricity limits their potential in applications. On the basis of first-principles calculations, we reveal the underlying mechanism of the paraelectric nature of Ba₃Zr₂S₇ observed in experiments and demonstrate a general rule for the appearance of ferroelectricity in chalcogenide perovskites with Ruddlesden–Popper (RP) A₃B₂X₇ structures. Group theoretical analysis shows that the tolerance factor is the primary factor that dominates the ferroelectricity. Both Ba₃Zr₂S₇ and Ba₃Hf₂S₇ with large tolerance factor are paraelectric because of the suppression of in-phase rotation that is indispensable to hybrid improper ferroelectricity. In contrast, Ca₃Zr₂S₇, Ca₃Hf₂S₇, Ca₃Zr₂Se₇, and Ca₃Hf₂S₇ with small tolerance factor exhibit in-phase rotation and can be stable in the ferroelectric <i>Cmc</i>2₁ ground state with nontrivial polarization. These findings not only provide useful guidance to engineering ferroelectricity in RP chalcogenide perovskites but also suggest potential ferroelectric semiconductors for photovoltaic applications

Multiferroic Domain Walls in Ferroelectric PbTiO₃ with Oxygen Deficiency

Atomically thin multiferroics with the coexistence and cross-coupling of ferroelectric and (anti)­ferromagnetic order parameters are promising for novel magnetoelectric nanodevices. However, such ferroic order disappears at a critical thickness in nanoscale. Here, we show a potential path toward ultrathin multiferroics by engineering an unusual domain wall (DW)-oxygen vacancy interaction in nonmagnetic ferroelectric PbTiO₃. We demonstrate from first-principles that oxygen vacancies formed at the DW unexpectedly bring about magnetism with a localized spin moment around the vacancy. This magnetism originates from the orbital symmetry breaking of the defect electronic state due to local crystal symmetry breaking at the DW. Moreover, the energetics of defects shows the self-organization feature of oxygen vacancies at the DW, resulting in a planar-arrayed concentration of magnetic oxygen vacancies, which consequently changes the deficient DWs into multiferroic atomic layers. This DW-vacancy engineering opens up a new possibility for novel ultrathin multiferroic

Unusual Multiferroic Phase Transitions in PbTiO₃ Nanowires

Unconventional phases and their transitions in nanoscale systems are recognized as an intriguing avenue for both unique physical properties and novel technological paradigms. Although the multiferroic phase has attracted considerable attention due to the coexistence and cross-coupling of electric and magnetic order parameters, mutually exclusive mechanism between ferroelectricity and ferromagnetism leaves conventional ferroelectrics such as PbTiO₃ simply nonmagnetic. Here, we demonstrate from first-principles that ultrathin PbTiO₃ nanowires exhibit unconventional multiferroic phases with emerging ferromagnetism and coexisting ferroelectric/ferrotoroidic ordering. Nanometer-scale and nonstoichiometric effects intrinsic to the nanowires bring about nonzero and nontrivial magnetic moments that coexist with the host ferroelectricity. The multiferroic order is susceptible to surface termination and nanowire morphology. Furthermore, calculations suggest that the nanowires undergo size-dependent ferroelectric-multiferroic-ferromagnetic phase transitions. This work therefore provides a route to multiferroic transitions in conventional nonmagnetic ferroelectric oxides

Modulation of Gas Adsorption and Magnetic Properties of Monolayer-MoS₂ by Antisite Defect and Strain

The flexible nature and high surface-to-volume ratio make monolayer-MoS₂ a novel paradigm for tunable nanoelectronic devices. However, for further improvement in the performance of these devices, a new design strategy is essential to modulate the properties of an inert MoS₂ basal plane. Here, we demonstrate from first-principles that the gas adsorption and magnetic properties of MoS₂ can be modulated through Mo_S antisite doping and strain. The Mo_S defect with localized d-orbital electron density significantly promotes the catalytic activity which leads to highly enhanced adsorption of NO, NO₂, NH₃, CO, and CO₂ gas molecules. On application of a biaxial tensile strain, the adsorption of NH₃ is further enhanced for the antisite-doped MoS₂. In addition, strain-induced switching of magnetic states is also realized in antisite-doped MoS₂ with and without adsorbed gas species. The superior strain modulation of antisite-doped MoS₂ is explained by quantum confinement effect and strain-induced accumulation/depletion of charge density at the defect site. These results suggest that antisite-doped MoS₂ can be a promising avenue to design nanoscale spintronic devices and gas sensors

Multiferroic Dislocations in Ferroelectric PbTiO₃

Ultrathin multiferroics with coupled ferroelectric and ferromagnetic order parameters hold promise for novel technological paradigms, such as extremely thin magnetoelectric memories. However, these ferroic orders and their functions inevitably disappear below a fundamental size limit of several nanometers. Herein, we propose a novel design strategy for nanoscale multiferroics smaller than the critical size limit by engineering the dislocations in nonmagnetic ferroelectrics, even though these lattice defects are generally believed to be detrimental. First-principles calculations demonstrate that Ti-rich PbTiO₃ dislocations exhibit magnetism due to the local nonstoichiometry intrinsic to the core structures. Highly localized spin moments in conjunction with the host ferroelectricity enable these dislocations to function as atomic-scale multiferroic channels with a pronounced magnetoelectric effect that are associated with the antiferromagnetic–ferromagnetic–nonmagnetic phase transitions in response to polarization switching. The present results thus suggest a new field of dislocation (or defect) engineering for the fabrication of ultrathin magnetoelectric multiferroics and ultrahigh density electronic devices

Thermal damage and ablation behavior of graphene induced by ultrafast laser irradiation

<p>Ultrafast laser-induced damage and ablation of graphene is the one of the most critical parts of precise nanopatterning of graphene by using laser ablation. In this article, we have studied the local damage and ablation behavior of monolayer graphene irradiated by femtosecond single pulse laser using molecular dynamics simulation. A theoretical model of phonon-dominated absorption of laser energy is proposed to describe the interaction between graphene and femtosecond single pulse laser. The simulation results based on this model are quantitatively consistent with experimental and theoretical ones. Furthermore, the effects of laser fluences on the atomic ablation behavior and nanogroove generation are investigated. The results show that the relationship between depth of the induced ablation and laser fluence follows a logarithmic function instead of a simple linear relationship. These results will be useful in providing guidance in femtosecond laser processing of graphene.</p

Griffith Criterion for Nanoscale Stress Singularity in Brittle Silicon

Brittle materials such as silicon fail <i>via</i> the crack nucleation and propagation, which is characterized by the singular stress field formed near the crack tip according to Griffith or fracture mechanics theory. The applicability of these continuum-based theories is, however, uncertain and questionable in a nanoscale system due to its extremely small singular stress field of only a few nanometers. Here, we directly characterize the mechanical behavior of a nanocrack <i>via</i> the development of <i>in situ</i> nanomechanical testing using a transmission electron microscope and demonstrate that Griffith or fracture mechanics theory can be applied to even 4 nm stress singularity despite their continuum-based concept. We show that the fracture toughness in silicon nanocomponents is 0.95 ± 0.07 MPa√m and is independent of the dimension of materials and therefore inherent. Quantum mechanics/atomistic modeling explains and provides insight into these experimental results. This work therefore provides a fundamental understanding of fracture criterion and fracture properties in brittle nanomaterials