Table of contents

Topological materials have become promising materials for next-generation devices by utilizing their exotic electronic states. Their exotic states caused by spin-orbital coupling usually locate on the surfaces or at the edges. Scanning tunneling spectroscopy (STS) is a powerful tool to reveal the local electronic structures of condensed matters. Therefore, STS provides us with an almost perfect method to access the exotic states of topological materials. In this topical review, we report the current investigations by several methods based on the STS technique for layered topological material from transition metal dichalcogenide Weyl semimetals (WTe₂ and MoTe₂) to two dimensional topological insulators (layered bismuth and silicene). The electronic characteristics of these layered topological materials are experimentally identified.

Complex metallic alloys belong to the vast family of intermetallic compounds and are hallmarked by extremely large unit cells and, in many cases, extensive crystallographic disorder. Early studies of complex intermetallics were focusing on the elucidation of their crystal structures and classification of the underlying building principles. More recently, ab initio computational analysis and detailed examination of the physical properties have become feasible and opened new perspectives for these materials. The present review paper provides a summary of the literature data on the reported compositions with exceptional structural complexity and their properties, and highlights the factors leading to the emergence of their crystal structures and the methods of characterization and systematization of these compounds.

We have investigated the intermediate range structure of amorphous Cu₂GeTe₃ based on ab initio molecular dynamics simulations. The highest population of ring size is three, which makes the triangle structure. This ring consists of mainly Cu₂Te. Rings may also consist of CuCuCu, Cu₂Ge, and CuGeTe, where approximately 88% of Cu atoms in the system are related with the three-membered ring. The second highest population of ring size is five. Three- and five-membered rings in the amorphous phase originate from six-membered ring in the crystalline phase. This situation can enhance the phase transition between crystalline and amorphous phases. In the phase change process, Cu atoms may diffuse in the amorphous state with changing bonds. The diffusion coefficient of Cu D_Cu is estimated to be approximately 0.12 × 10⁻⁹ m² s⁻¹. Such high diffusion coefficient of Cu atoms is contributed from only 10% of Cu atoms in the amorphous phase.

Amorphous metals display an extraordinary mechanical strength and elasticity and can at the same time be formed like thermoplastic polymers. These properties make them the ideal material for industrial applications where complex parts have to withstand high mechanical loads. In this work, the thermoplastic formability of amorphous metals is evaluated and discussed in connection to their thermophysical properties. Formability is experimentally assessed in thermoplastic deformation experiments with a constant heating rate, and in isothermal experiments. The results are compared to the theoretical formability values calculated from the thermophysical material properties and found to perfectly coincide. The formability of amorphous alloys can be reliably calculated based on a viscosity measurement in the supercooled liquid region. In isothermal experiments, the maximum formability is obtained at the highest temperatures where crystallization can still be avoided.

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the ⁵³Cr-NMR spectra of the Cr₈Cd ring to study the local spin densities on the Cr sites. Cr₈Cd can in fact be considered a model system of a finite AF open chain with an even number of spins. The NMR resonant frequencies are in good agreement with the theoretical local spin densities, by assuming a core polarization field A_C = −12.7 T μ_B⁻¹. Moreover, these NMR results confirm the theoretically predicted non-collinear spin arrangement along the Cr₈Cd ring, which is typical of an even-open AF spin chain.

The size, form and distribution function of catalyst particles define the quality of synthesized arrays of carbon nanotubes. In this work, we study the kinetics of catalyst particle formation from the thin nickel film (9 nm) deposited on the silicon substrate (SiO₂/Si) with a buffer layer of niobium nitride at the temperature of 880 °C. In the experiment, we have obtained the time dependences of the average radius, average height and concentration of nickel particles. The experimental data are satisfactorily described by simulations based on the wetting transition theory. Comparison of the simulation results and experimental data allows us to estimate the effective interaction potential between the nickel film and buffer layer of niobium nitride. Besides, we have estimated the viscosity of the nickel confirming an undercooled liquid state of the nanosized nickel film at the temperature of 880 °C.

The scattering of acoustic phonons by nonreciprocal spring defects inserted in an harmonic chain is investigated. The degree of nonreciprocity of the forces mediated by the defect springs is parameterized by a single quantity Δ that effectively takes into account the interaction of the coupled masses with hidden degrees of freedom of an underlying nonequilibrium system. We demonstrate a pronounced rectification effect with transmission having a preferential direction. Nonreciprocity also allows energy exchange between the system and the medium. Further, we show a cooperative action between defects mediated by resonant cavity modes. The influence of damping forces is also explored and shown to promote the rectification of the reflected vibrational wave component.

A new first-principles computation scheme to calculate 'branching ratio' has been applied to various 5d, 4d, and 3d transition metal elements and compounds. This recently suggested method is based on a theory which assumes the atomic core hole interacts barely with valence electrons. While it provides an efficient way to calculate the experimentally measurable quantity without generating spectrum itself, its reliability and applicability should be carefully examined especially for the light transition metal systems. Here we select 36 different materials and compare the calculation results with experimental data. It is found that our scheme well describes 5d and 4d transition metal systems whereas, for 3d materials, the difference between the calculation and experiment is quite significant. It is attributed to the neglect of core–valence interaction whose energy scale is comparable with the spin–orbit coupling of core p orbitals.

We report ¹⁰⁵Pd nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on a single crystal of Ce₃Pd₂₀Si₆, where antiferroquadrupolar and antiferromagnetic orders develop at low temperature. From the analysis of NQR and NMR spectra, we have determined the electric field gradient (EFG) tensors and the anisotropic Knight shift (K) components for both inequivalent Pd sites—Pd(32f) and Pd(48h). The observed EFG values are in excellent agreement with our state-of-the-art density functional theory calculations. The principal values of the quadrupolar coupling are MHz and MHz, for the Pd(32f) and Pd(48h) sites, respectively, which is large compared to the Larmor frequency defined by the gyromagnetic constant MHz/T for ¹⁰⁵Pd. Therefore, the complete knowledge of K and the EFG tensors is crucial to establish the correspondence between NMR spectra and crystallographic sites, which is needed for a complete analysis of the magnetic structure, static spin susceptibility, and the spin-lattice relaxation rate data and a better understanding of the groundstate of Ce₃Pd₂₀Si₆.

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti_1−xMn_xCo₂O₄ in the temperature interval 1.6 K T 300 K. Our neutron diffraction study reveals cooperative distortions of the TO₆ octahedra in the Ti_1−xMn_xCo₂O₄ system for all the Jahn–Teller active ions T  =  Mn³⁺ , Ti³⁺ and Co³⁺ , having the electronic configurations 3d¹, 3d⁴ and 3d⁶, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x  =  0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet–Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a  <  1, in which the apical bond length d_c(T_B-O) is longer than the equatorial bond length d_ab(T_B-O) due to the splitting of the e_g level of Mn³⁺ ions into and . For the composition x  =  0.4, the distortion in the T_BO₆ octahedra is stronger as compared to x  =  0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti_0.6Mn_0.4Co₂O₄ and Ti_0.8Mn_0.2Co₂O₄ sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti³⁺ , Mn³⁺ , and Co³⁺ occupy the octahedral, whereas, Co²⁺ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T_COMP  =  25.4 K in Ti_0.8Mn_0.2Co₂O₄ and a reentrant spin-glass behaviour in Ti_0.6Mn_0.4Co₂O₄ with a freezing temperature of  ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T_COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

The present work offers an insight into the magnetic properties of Mn-rich spinel zinc manganate. Rietveld refinement reveals the formation of Zn_0.67Mn_2.33O_4, where Zn²⁺ and Mn²⁺ ions are randomly distributed in the tetrahedral sublattice. DC and AC susceptibility measurement of Zn_0.67Mn_2.33O₄ infers the occurrence of two kinds of transition below 11 K. Paramagnetic to ferrimagnetic transition occur at 10.7 K and ferrimagnetic to spin glass-like transition occurs at 5.8 K. The long-range canted ferrimagnetic ordering is corroborated using modified Lotgering model and calculated the exchange interaction values (J_AA = 5.2 K,J_AB = 5.3 K, J_BB = 14.8 K). Further, the observed shift in freezing temperature with DC magnetic field obeys Almeida–Thouless behaviour and frequency dependence of AC susceptibility follows Vogel–Fulcher law. However, the complete establishment of the canonical spin glass state is denied since the manganese ions occupied at the tetrahedral site is equal to the percolation threshold (=33%). Subsequently, the observed spin glass-like behaviour (5.8 K) below Curie temperature (10.7 K) evidences the reentrant spin glass nature. Similarly, the strong frustration in Zn_0.67Mn_2.33O₄ lattice is observed through the substantial negative value of Curie–Weiss temperature (∼−599 K) and very high frustration factor (f = 56). Overall, the chosen Zn_0.67Mn_2.33O₄ is a highly frustrated magnetic system revealing re-entrant spin-glass behaviour.

Recently, it was reported that the VI₃ had a Mott insulator nature and also displayed the structural and magnetic phase transition at low temperature. Here, we explored the magnetic properties of the two-dimensional (2D) monolayer structure using the density functional theory. We found that the 2D VI₃ had an enhanced lattice constant compared with that in the bulk structure. Besides, the 2D monolayer had an indirect band gap of 0.98 eV, and this band gap was increased (decreased) with tensile (compressive) strain up to ±3%. The monolayer structure had a ferromagnetic ground state and this nature was preserved under both tensile and compressive strains. We obtained that the monolayer structure had a perpendicular magnetic anisotropy energy of 0.29 meV/cell. The perpendicular magnetic anisotropy still remained even after applying the tensile and compressive strains although the magnitude of magnetic anisotropy was slightly changed. Using the Metropolis Monte Carlo simulations, we found that the monolayer had a Curie temperature of 46 K. This Curie temperature was increased to 57 K with 3% tensile strain whereas it was decreased to 35 K with 3% compressive strain. Overall, we found that the magnetic property of 2D VI₃ monolayer was robust under the strain.

Single walled carbon nanotube (SWCNT) and alkaline metal oxide have been identified as potential materials for management of CO₂ emission. Yet the underlying operating mechanism is still not well understood, while an in-depth understanding would possibly lead to development of superior CO₂ monitoring, capture, and storage devices. Here we present ab initio density functional theory calculations to provide a comprehensive description of CO₂ gas interaction with SWCNT and CaO surface. In particular, our results revealed that CO₂ is chemisorbed on CaO surface with negligible effect on electronic properties of the absorbent, while CO₂ interaction with SWCNT can be categorized as physisorption interaction a process that can be easily reversed using thermal treating of the tube at 150 °C. Thus CaO is found to be ideal for long term storage of CO₂ while SWCNT reported superior performance in CO₂ sensing and capture. This work may guide the development of better devices based on CaO and SWCNT for CO₂ sensing, capture, and storage.

In our published paper entitled, 'All-optical spin switching under different spin configurations' by G P Zhang and M Murakami 2019 J. Phys.: Condens. Matter31 345802, figures 5(b)–(d) use zero exchange interactions. It came to our attention when we tried to simulate a ferrimagnet recently. The attached three figures, which replace those old figures, are now correct that all the data are computed with exchange interaction J = 0.1  eV/ℏ². The differences between the old and new figures are difficult to detect visually. When we check the data directly, we find a clear difference due to the exchange interaction. All the other figures are correct. This change does not affect the conclusion and finding.