Condens. Matter, Volume 7, Issue 2 (June 2022) – 14 articles

The results of direct synthesis of composite powder based on boron nitride (BN) are considered. Concentrated light heating of the initial boron powder was carried out in a xenon high-flux optical furnace in a nitrogen flow. Formation of particles of the desired sizes and architecture highly dependent of the synthesis conditions. The flow of nitrogen separates the particles depending on their architecture and size. An increase in the distance from the reaction zone leads to the formation of powder with a wider bandgap, increases the amount of amorphous phase, and decreases the amount of oxide in the collected composite powder. However, the close distance to the reaction zone and high temperatures provide a denser packing of the structure on the particle surface and the disappearance of the BN transition phases. Incorporation of the nickel sulfate hexahydrate to initial boron contributes to the formation of graphene-like structures. Full article

A phonon of appropriate momentum $k_F$ will open a band gap at the Fermi energy $E_F$. The gap within the electronic density-of-states (DOS), $N\left(E_F\right)$, leads to a gain in electronic energy and a loss of elastic energy because of the gap-generating phonon. A BCS-like simulation shows that the energy gain is larger than the loss for temperatures below a certain transition temperature, $T_C$. Here, it is shown that the energy count can be almost as favorable for gaps a little below or above $E_F$. Such gaps can be generated by auxiliary phonons (or even spin- and charge-density waves) with k-vectors slightly different from $k_F$. Gaps not too far from $E_F$ will add to the energy gain at the superconducting transition. In addition, a DOS-peak can appear at $E_F$ and thereby increase $N\left(E_F\right)$ and $T_C$. A dip in the DOS below $E_F$ will result for temperatures below $T_C$, which is similar to what often is observed in cuprate superconductors. The roles of spin waves and thermal disorders are discussed. Full article

The Chern gap is a unique topological feature that can host non-abelian particles. The Kagome lattice hosts Chern fermions. Upon the inclusion of magnetism, the Kagome system hosts a Chern gap at the K points in the lattice. In this work, the effect of Ge doping on HoMn${}_6$Sn${}_6$ is investigated. It is seen that with increased doping, a multi-stack Chern gap in formed in HoMn${}_6$Sn${}_{6-x}$Ge${}_x$. In addition, the Chern gaps are much more pronounced and disperse more in energy in HoMn${}_6$Ge${}_6$ then in HoMn${}_6$Sn${}_6$. Full article

The present study evaluates the effect of mesoporous multiphase titanium dioxide (TiO₂) nanoparticles (NPs) as an electron transporting layer and investigates the influence of phase composition on the perovskite solar cell (PSC) performances. This study also aims to evaluate PSC performance using conductive silver ink as an alternative counter electrode. The heterogeneous PSC thin-film solar cells were successfully fabricated and assembled by using a simple a doctor blade and two-step spin coating methods under ambient conditions. Scanning electron microscopy (SEM) micrograph images investigate methyl ammonium lead iodide (MAPbI₃) crystal formation on the mesoporous TiO₂ surface structure. Energy-dispersive x-ray spectroscopy (EDX) spectra reveal excellent qualitative and quantitative analysis corresponding to the SEM images in the TiO₂/MAPbI₃ heterogeneous thin films. Thermogravimetric analysis (TGA) characterization reveals that the TiO₂/MAPbI₃ thin films are thermally stable recording a maximum of 15.7% mass loss at 800 °C elevated temperatures. Photoluminescence spectroscopy (PL) characterized the effect of multiphase TiO₂ phase transformation on the TiO₂/MAPbI₃ recombination efficiencies. A maximum of 6% power conversion efficiency (PCE) with the open-circuit voltage (Voc) of 0.58 ± 0.02 V and short circuit current (Jsc) of 3.89 ± 0.17 mAcm⁻² was achieved for devices with an active area of 3 × 10⁻⁴ m² demonstrating that the synthesized multiphase TiO₂ nanoparticles are promising for large surface area manufacturing. Therefore, it is apparent that multiphase TiO₂ NPs play a significant role in the performance of the final device. Full article

Artificial neural networks have been widely adopted as ansatzes to study classical and quantum systems. However, for some notably hard systems, such as those exhibiting glassiness and frustration, they have mainly achieved unsatisfactory results, despite their representational power and entanglement content, thus suggesting a potential conservation of computational complexity in the learning process. We explore this possibility by implementing the neural annealing method with autoregressive neural networks on a model that exhibits glassy and fractal dynamics: the two-dimensional Newman–Moore model on a triangular lattice. We find that the annealing dynamics is globally unstable because of highly chaotic loss landscapes. Furthermore, even when the correct ground-state energy is found, the neural network generally cannot find degenerate ground-state configurations due to mode collapse. These findings indicate that the glassy dynamics exhibited by the Newman–Moore model caused by the presence of fracton excitations in the configurational space likely manifests itself through trainability issues and mode collapse in the optimization landscape. Full article

Pure and Li-doped CuO nanofilms were synthesized on heated glass substrates using the spray-pyrolysis technique. The deposited pure CuO nanofilms were achieved at a precursor molarity of 0.2 M using a solution prepared from copper nitrate trihydrate (Cu(NO₃)₂·3H₂O). Doped Li–CuO nanofilms were obtained using several doping concentrations (3, 6, 9, 12 and 15%) by adding a solution prepared from lithium nitrate (LiNO₃). The pure and Li–CuO samples were investigated by different techniques. XRD revealed three dominant peaks (-111), (111) and (211), which are the properties of monoclinic CuO. The increase in Li-doping concentration showed the appearance of other peaks of low intensities detected at 2θ ranging from 49 to 68°. AFM images showed a textured and inhomogeneous surface composed of spherical grains whose size decreased with increasing Li doping. UV–visible spectroscopy showed that the CuO samples were of low transparency; the transmittance was less than 50%. The band-gap energy determined from Tauc’s equation plot increased from 2.157 to 3.728 eV with the increase in Li doping. These values correspond well to the band gap of semiconducting CuO. The photocatalytic properties were accelerated by Li doping, as revealed by the discoloration of aqueous methylene-blue (MB) solution under ultraviolet irradiation. Full article

We report a comprehensive theoretical investigation on phosphorus–boron mixed neutral, anionic, and cationic clusters P₂B_n/P₂B_n⁻/P₂B_n⁺ (n = 3–7) with two phosphorus atoms and three to seven boron atoms. We reveal the common character of all the structures (i.e., the phosphorus atoms choose to occupy the peripheral position), whereas the boron atoms tend to be in the central and inside position of the ground state phosphorus—boron mixed clusters at each stoichiometry. Any three atoms preferentially form a stable triangle and grow with zigzag shape in a planar network. Interestingly, a series of planar motifs (including tetra-, penta-, and hexa-coordination) have been discovered in the phosphorus–boron clusters. The large binding energies (3.6 to 4.6 eV/atom) and quite large HOMO–LUMO gaps (5 to 10 eV) indicate the high stability of the clusters. The energy differences Δ₁E, Δ₂E, and energy gaps display oscillating behavior with increasing numbers of boron atoms. The electron affinity (EA) and ionization potential (IP) generally have small variations, with the EA values ranging from 2 to 3 eV, and the IP values ranging from 7 to 9 eV. Chemical bond analysis shows that the existence of multi-center delocalized bonds stabilize the clusters. Full article

We use first-principles calculations within the density functional theory (DFT) to explore the electronic properties of stage-1 Li- and Li⁺-graphite-intercalation compounds (GIC) for different concentrations of LiC_x/Li⁺C_x, with x = 6, 12, 18, 24, 32 and 36. The essential properties, e.g., geometric structures, band structures and spatial charge distributions are determined by the hybridization of the orbitals, the main focus of our work. The band structures/density of states/spatial charge distributions display that Li-GIC shows a blue shift of Fermi energy just like metals, but Li⁺-GIC still remains as in the original graphite or exhibits so-called semi-metallic properties, possessing the same densities of free electrons and holes. According to these properties, we find that there exist weak but significant van der Waals interactions between interlayers of graphite, and 2s-2p_z hybridization between Li and C. There scarcely exist strong interactions between Li⁺-C. The dominant interaction between the Li and C is 2s-2p_z orbital-orbital coupling; the orbital-orbital coupling is not significant in the Li⁺ and C cases, but dipole-diploe coupling is. Full article

In functional materials such as thermoelectric materials and superconductors, the interplay between functionality, electronic structure, and phonon characteristics is one of the key factors to improve functionality and to understand the underlying mechanisms. In the first part of this article, we briefly review investigations on lattice anharmonicity in functional materials on the basis of the Grüneisen parameter (γ_G). We show that γ_G can be a good index for large lattice anharmonicity and for detecting a change in anharmonicity amplitude in functional materials. Then, we show original results on the estimation of γ_G for recently developed high-entropy alloy-type (HEA-type) functional materials with a layered structure and a NaCl-type structure. As a common trend for those two systems with two- and three-dimensional structures, we found that γ_G increased with a slight increase in the configurational entropy of mixing (ΔS_mix) and then decreased with increasing ΔS_mix in the high-entropy region. Full article

Layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) have two polymorphs, including α- and β-forms, which have the FeOCl and SmSI structures, respectively. These compounds are band insulators and become metals and show superconductivity after electron doping by intercalating alkali metals between the layers. The superconductivity of β-form had been extensively characterized from decades ago, but it is not easy to consistently interpret all experimental results using conventional phonon-mediated Bardeen–Cooper–Schriefer mechanisms. The titanium compound TiNCl crystallizes only in the α-form structure. TiNCl also exhibits superconductivity as high as ~16 K after electron doping by intercalating metals and/or organic basis. It is important to compare the superconductivity of different M–N networks. However, α-form compounds are vulnerable to moisture, unlike β-form ones. The intercalation compounds are even more sensitive to humid air. Thus, there are few experimental studies on the superconducting mechanism of α-form, although it has been discussed for exotic Cooper-pairing mechanisms. This short review gathers the recent progress in experimental studies of TiNCl. Full article

We studied the miscibility of two dipolar quantum gases in the limit of zero temperature. The system under study is composed of a mixture of two Bose gases with dominant dipolar interaction in a two-dimensional harmonic confinement. The dipolar moments are all considered to be perpendicular to the plane, turning the dipolar potential in a purely repulsive and isotropic model. Our analysis is carried out by using the diffusion Monte Carlo method, which allows for an exact solution to the many-body problem within some statistical noise. Our results show that the miscibility between the two species is rather constrained as a function of the relative dipolar moments and masses of the two components. A narrow regime is predicted where both species mix and we introduce an adimensional parameter whose value quite accurately predicts the miscibility of the two dipolar gases. Full article

The origin of the pseudogap and superconducting behaviors in high-$T_c$ superconductors is proposed, based on the picture of Euclidean Q-balls formation that carry Cooper/local-pair condensates inside their volumes. Euclidean Q-balls that describe bubbles of collective spin-/charge density fluctuations (SDW/CDW) oscillating in Matsubara time are found as a new self-consistent solution of the Eliashberg equations in the ‘nested’ repulsive Hubbard model of high-$T_c$ superconductors. The Q-balls arise due to global invariance of the effective theory under the phase rotation of the Fourier amplitudes of SDW/CDW fluctuations, leading to conservation of the ‘Noether charge’ Q in Matsubara time. Due to self-consistently arising local minimum of their potential energy at finite amplitude of the density fluctuations, the Q-balls provide greater binding energy of fermions into local/Cooper pairs relative to the usual Frohlich mechanism of exchange with infinitesimal lattice/charge/spin quasiparticles. We show that around some temperature $T^{*}$ the Q-balls arise with a finite density of superconducting condensate inside them. The Q-balls expand their sizes to infinity at superconducting transition temperature $T_c$. The fermionic spectral gap inside the Q-balls arises in the vicinity of the ‘nested’ regions of the bare Fermi surface. Solutions are found analytically from the Eliashberg equations with the ‘nesting’ wave vectors connecting ‘hot spots’ in the Brillouin zone. The experimental ‘Uemura plot’ of the linear dependence of $T_c$ on superconducting density $n_s$ in high-$T_c$ superconducting compounds follows naturally from the proposed theory. Full article

We provide a detailed description of the path-integral Monte Carlo worm algorithm used to exactly calculate the thermodynamics of Bose systems in the canonical ensemble. The algorithm is fully consistent with periodic boundary conditions, which are applied to simulate homogeneous phases of bulk systems, and it does not require any limitation in the length of the Monte Carlo moves realizing the sampling of the probability distribution function in the space of path configurations. The result is achieved by adopting a representation of the path coordinates where only the initial point of each path is inside the simulation box, the remaining ones being free to span the entire space. Detailed balance can thereby be ensured for any update of the path configurations without the ambiguity of the selection of the periodic image of the particles involved. We benchmark the algorithm using the non-interacting Bose gas model for which exact results for the partition function at finite number of particles can be derived. Convergence issues and the approach to the thermodynamic limit are also addressed for interacting systems of hard spheres in the regime of high density. Full article