Abstract: Dibismuth dioxychalcogenides, Bi2O2Ch (Ch = S, Se, Te), are a promising class of materials for next-generation electronics and thermoelectrics due to their ultrahigh carrier mobility and excellent air stability. An interesting member of this family is Bi2O2S, which has a stereochemically active 6s2 lone pair of Bi3+ cations, heterogeneous bonding, and a high mass contrast between its constituent elements. In the present study, we have used first-principles calculations in combination with Boltzmann transport theory to systematically investigate the effect of hydrostatic pressure on lattice dynamics and phonon transport properties of Bi2O2S. We found that the ambient Pnmn phase has a low average lattice thermal conductivity (kappa l) of 1.71 W/(m K) at 300 K. We also predicted that Bi2O2S undergoes a structural phase transition from a low-symmetry (Pnmn) to a high-symmetry (I4/mmm) structure at around 4 GPa due to centering of Bi3+ cations with pressure. Upon compression, the lone pair activity of Bi3+ cations is suppressed, which increases kappa l by almost 3 times to 4.92 W/ (m K) at 5 GPa for the I4/mmm phase. The computed phonon lifetimes and Gru''neisen parameters show that anharmonicity decreases with increasing pressure due to further suppression of the lone pair activity and strengthening of intra-and intermolecular interactions, leading to an average room-temperature kappa l of 12.82 W/(m K) at 20 GPa. Overall, this study provides a comprehensive understanding of the effect of hydrostatic pressure on the stereochemical activity of the lone pair of Bi3+ cations and its implications on the phonon transport properties of Bi2O2S.

Abstract: A Raman scattering study covering the entire substitution range of the FeSe1-xSx solid solution is presented. Data were taken as a function of sulfur concentration x for 0 <= x <= 1, of temperature and of scattering symmetry. All types of excitations including phonons, spins, and charges are analyzed in detail. It is observed that the energy and width of the iron-related B-1g phonon mode vary continuously across the entire range of sulfur substitution. The A(1g) chalcogenide mode disappears above x = 0.23 and reappears at a much higher energy for x = 0.69. In a similar way the spectral features appearing at finite doping in A(1g) symmetry vary discontinuously. The magnetic excitation centered at approximately 500 cm(-1) disappears above x = 0.23 where the A(1g) lattice excitations exhibit a discontinuous change in energy. The low-energy mode associated with fluctuations displays maximal intensity at the nematostructural transition and thus tracks the phase boundary.

Abstract: Nanoporous membranes based on two dimensional materials are predicted to provide highly selective gas transport in combination with extreme permeance. Here we investigate membranes made from multilayer graphdiyne, a graphene-like crystal with a larger unit cell. Despite being nearly a hundred of nanometers thick, the membranes allow fast, Knudsen-type permeation of light gases such as helium and hydrogen whereas heavy noble gases like xenon exhibit strongly suppressed flows. Using isotope and cryogenic temperature measurements, the seemingly conflicting characteristics are explained by a high density of straight-through holes (direct porosity of similar to 0.1%), in which heavy atoms are adsorbed on the walls, partially blocking Knudsen flows. Our work offers important insights into intricate transport mechanisms playing a role at nanoscale.

Abstract: We compare the image-force barrier lowering (IFBL) and calculate the resulting contact resistance for four different metal-dielectric-two-dimensional (2D) material configurations. We analyze edge contacts in three different geometries (a homogeneous dielectric throughout, including the 2D layer; a homogeneous dielectric surrounding the 2D layer, both ungated and back gated) and also a top-contact assuming a homogeneous dielectric. The image potential energy of each configuration is determined and added to the Schottky energy barrier which is calculated assuming a textbook Schottky potential. For each configuration, the contact resistivity is calculated using the WKB approximation and the effective mass approximation using either SiO2 or HfO2 as the surrounding dielectric. We obtain the lowest contact resistance of 1 k Omega mu m by n-type doping an edge contacted transition metal-dichalcogenide (TMD) monolayer, sandwiched between SiO2 dielectric, with similar to 1012 cm-2 donor atoms. When this optimal configuration is used, the contact resistance is lowered by a factor of 50 compared to the situation when the IFBL is not considered.

Abstract: We investigate a Josephson junction in an electron-hole superfluid in a double-layer transition metal dichalco-genide heterostructure. The observation of a critical tunneling current is a clear signature of superfluidity. In addition, we find the BCS-BEC crossover physics in the narrow barrier region controls the critical current across the entire system. The corresponding critical velocity, which is measurable in this system, has a maximum when the excitations pass from bosonic to fermionic. Remarkably, this occurs for the density at the boundary of the BEC to BCS-BEC crossover regime determined from the condensate fraction. This provides, in a semiconductor system, an experimental way to determine the position of this boundary.

Abstract: We fabricated an asymmetric nanoscale SQUID consisting of one nanobridge weak link and one Dayem bridge weak link. The current phase relation of these particular weak links is characterized by multivaluedness and linearity. While the latter is responsible for a particular magnetic field dependence of the critical current (so-called vorticity diamonds), the former enables the possibility of different vorticity states (phase winding numbers) existing at one magnetic field value. In experiments the observed critical current value is stochastic in nature, does not necessarily coincide with the current associated with the lowest energy state and critically depends on the measurement conditions. In this paper, we unravel the origin of the observed metastability as a result of the phase dynamics happening during the freezing process and while sweeping the current. Moreover, we employ special measurement protocols to prepare the desired vorticity state and identify the (hidden) phase slip dynamics ruling the detected state of these nanodevices. In order to gain insights into the dynamics of the condensate and, more specifically the hidden phase slips, we performed time-dependent Ginzburg-Landau simulations.

Abstract: All-inorganic lead-free halide double perovskites offer a promising avenue toward non-toxic, stable optoelec-tronic materials, properties that are missing in their prominent lead-containing counterparts. Their large ther-mopowers and high carrier mobilities also make them promising for thermoelectric applications. Here, we present a first-principles study of the lattice vibrations and thermal transport behaviors of Cs2SnI6 and gamma-CsSnI3, two prototypical compounds in this materials class. We show that conventional static zero temperature density functional theory (DFT) calculations severely underestimate the lattice thermal conductivities (kappa l) of these compounds, indicating the importance of dynamical effects. By calculating anharmonic renormalized phonon dispersions, we show that some optic phonons significantly harden with increasing temperature (T), which reduces the scattering of heat carrying phonons and enhances calculated kappa l values when compared with standard zero temperature DFT. Furthermore, we demonstrate that coherence contributions to kappa l, arising from wave like phonon tunneling, are important in both compounds. Overall, calculated kappa l with temperature-dependent inter-atomic force constants, built from particle and coherence contributions, are in good agreement with available measured data, for both magnitude and temperature dependence. Large anharmonicity combined with low phonon group velocities yield ultralow kappa l values, with room temperature values of 0.26 W/m-K and 0.72 W/m-K predicted for Cs2SnI6 and gamma-CsSnI3, respectively. We further show that the lattice dynamics of these compounds are highly anharmonic, largely mediated by rotation of the SnI6 octahedra and localized modes originating from Cs rattling motion. These thermal characteristics combined with their previously computed excellent electronic properties make these perovskites promising candidates for optoelectronic and room temperature thermoelectric applications.

Abstract: Motivated by the potential of atomically thin magnets towards achieving tunable high-frequency magnonics, we detail the spin-wave dispersion of bilayer CrI3. We demonstrate that the magnonic behavior of the bilayer strongly depends on its stacking configuration and the interlayer magnetic ordering, where a topological band gap opens in the dispersion caused by the Dzyaloshinskii-Moriya and Kitaev interactions, classifying bilayer CrI3 as a topological magnon insulator. We further reveal that both the size and the topology of the band gap in a CrI3 bilayer with an antiferromagnetic interlayer ordering are tunable by an external magnetic field.

Abstract: When using hexagonal boron-nitride (hBN) as a substrate for graphene, the resulting moire pattern creates secondary Dirac points. By encapsulating a multilayer graphene within aligned hBN sheets the controlled moire stacking may offer even richer benefits. Using advanced tight-binding simulations on atomistically-relaxed heterostructures, here we show that the gap at the secondary Dirac point can be opened in selected moire-stacking configurations, and is independent of any additional vertical gating of the heterostructure. On the other hand, gating can broadly tune the gap at the principal Dirac point, and may thereby strongly compress the first moire mini-band in width against the moire-induced gap at the secondary Dirac point. We reveal that in hBN-encapsulated bilayer graphene this novel mechanism can lead to isolated bands flatter than 10 meV under moderate gating, hence presenting a convenient pathway towards electronically-controlled strongly-correlated states on demand.

Abstract: Experimental realizations of two-dimensional materials are hardly free of structural defects such as e.g. vacancies, which, in turn, modify drastically its pristine physical defect-free properties. In this work, we explore effects due to point defect clustering on the electronic and transport properties of bilayer graphene nanoribbons, for AA and AB stacking and zigzag and armchair boundaries, by means of the tight-binding approach and scattering matrix formalism. Evident vacancy concentration signatures exhibiting a maximum amplitude and an universality regardless of the system size, stacking and boundary types, in the density of states around the zero-energy level are observed. Our results are explained via the coalescence analysis of the strong sizeable vacancy clustering effect in the system and the breaking of the inversion symmetry at high vacancy densities, demonstrating a similar density of states for two equivalent degrees of concentration disorder, below and above the maximum value.

Abstract: The investigation of thermal transport properties of novel two-dimensional materials is crucially important in order to assess their potential to be used in future technological applications, such as thermoelectric power generation. In this respect, the lattice thermal transport properties of the monolayer structures of group VA elements (P, As, Sb, Bi, PAs, PSb, PBi, AsSb, AsBi, SbBi, P3As1, P3Sb1, P1As3, and As3Sb1) with a black phosphorus like puckered structure were systematically investigated by first-principles calculations and an iterative solution of the phonon Boltzmann transport equation. Phosphorene was found to have the highest lattice thermal conductivity, , due to its low average atomic mass and strong interatomic bonding character. As a matter of course, anisotropic was obtained for all the considered materials, owing to anisotropy in frequency values and phonon group velocities calculated for these structures. However, the determined linear correlation between the anisotropy in the values of P, As, and Sb is significant. The results corresponding to the studied compound structures clearly point out that thermal (electronic) conductivity of pristine monolayers might be suppressed (improved) by alloying them with the same group elements. For instance, the room temperature of PBi along the armchair direction was predicted to be as low as 1.5 W m(-1) K-1, whereas that of P was predicted to be 21 W m(-1) K-1. In spite of the apparent differences in structural and vibrational properties, we peculiarly revealed an intriguing correlation between the values of all the considered materials as = c(1) + c(2)/m(2), in particular along the zigzag direction. Furthermore, our calculations on compound structures clearly showed that the thermoelectric potential of these materials can be improved by suppressing their thermal properties. The presence of ultra-low values and high electrical conductivity (especially along the armchair direction) makes this class of monolayers promising candidates for thermoelectric applications.

Abstract: We present a computational model to model the magnetic structure of two-dimensional (2D) dilute-magnetic-semiconductors (DMS) both the monolayers and multilayers using first-principles density functional theory (DFT), as well as their magnetic phase transition as a function of temperature using Monte-Carlo simulations. Using our method, we model the magnetic structure of bulk, bilayer, and monolayer MoS2 substitutionally doped with Fe atoms. We find that the out-of-plane interaction in bilayer MoS2 is weakly ferromagnetic, whereas in bulk MoS2 it is strongly anti-ferromagnetic. Finally, we show that the magnetic order is more robust in bilayer Fe-doped MoS2 compared to the monolayer and results in a room-temperature FM at an atomic substitution of 14-16%.

Abstract: Using first-principles calculations, we demonstrated that high lithium storage capacity and fast kinetics are achieved for Ti3C2O2 by preintercalating organic molecules. As a proof-of-concept, two different quinone molecules, namely 1,4-benzoquinone (C6H4O2) and tetrafluoro-1,4-benzoquinone (C6F4O2) were selected as the molecular linkers to demonstrate the feasibility of this interlayer engineering strategy for energy storage. As compared to Ti3C2O2 bilayer without linker molecules, our pillared structures facilitate a much faster ion transport, promising a higher charge/discharge rate for Li. For example, while the diffusion barrier of a single Li ion within pristine Ti3C2O2 bilayer is at least 1.0 eV, it becomes 0.3 eV in pillared structures, which is comparable and even lower than that of commercial materials. At high Li concentrations, the calculated diffusion barriers are as low as 0.4 eV. Out-of-plane migration of Li ions is hindered due to large barrier energy with a value of around 1-1.35 eV. Concerning storage capacity, we can only intercalate one monolayer of Li within pristine Ti3C2O2 bilayer. In contrast, pillared structures offer significantly higher storage capacity. Our calculations showed that at least two layers of Li can be intercalated between Ti3C2O2 layers without forming bulk Li and losing the pillared structure upon Li loading/unloading. A small change in the in-plane lattice parameters (<0.5%) and volume (<1.0%) and ab initio molecular dynamics simulations prove the stability of the pillared structures against Li intercalation and thermal effects. Intercalated molecules avoid the large contraction/expansion of the whole structure, which is one of the key problems in electrochemical energy storage. Pillared structures allow us to realize electrodes with high capacity and fast kinetics. Our results open new research paths for improving the performance of not only MXenes but also other layered materials for supercapacitor and battery applications.

Abstract: Stone Matrix Asphalt (SMA) is a gap-graded mixture which requires high contents of asphalt binder. To prevent draindown, natural or synthetic fibres and polymer-modified asphalt binders are conventionally used in SMA. Banana agribusiness is one of the major sources of post-harvest residue in Brazil. Amongst those residues, fibres extracted from the pseudostem of the banana plant are resistant and used in diverse purposes. The present study assesses the incorporation of fibres from the pseudostem of the banana plant in an SMA mixture. The fibre contents and lengths capable to prevent binder draindown were evaluated from draindown tests. Mechanical properties of an SMA mixture stabilised with different banana fibre lengths were analysed through the tests of Marshall stability, modified Lottman, Indirect Tensile Strength and Cantabro. The results indicated that the fibres studied are a viable alternative for SMA, stabilising draindown and improving its mechanical performance, especially at the length of 20 mm.

Abstract: Combining first-principles calculations with Landauer-Mittiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature p-type thermoelectric figure of merit (ZT) of 1.57 is obtained for 2H-HfSe2, which can be as high as 3.30 at T = 800 K. Additionally, 2H-ZrSe2, 2H-ZrTe2, and 2H-HfS2 have considerable ZT values (both nand p-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their ZT values to below 1 at room temperature.