Table of contents

Employing high resolution hard x-ray photoemission spectroscopy, we investigate the electronic structure of an exotic Fe-based superconductor, CaFe₂As₂, which exhibits rich temperature pressure phase diagram and dichotomy on achieving superconductivity on application of pressure. The experimental valence band spectra exhibit significant differences for experiments at different surface sensitivities. We discover that the change in angle between light polarization and surface normal leads to similar orbital selective spectral response suggesting requirement of different methodology to probe the surface-bulk differences. Thus, the final state effects of the core level spectroscopy has been exploited to reveal the depth-resolved information. Strong features related to plasmon excitations have been observed in various core level spectra. Ca 2p spectra exhibit evidence of significant hybridization with the conduction electrons, and distinct features corresponding to the surface and bulk electronic structures while As core levels remain unaffected. The depth-resolved Fe 2p spectra at different temperatures exhibit interesting features suggesting structural anomaly may be a bulk property. All these results reveal complexity in the hybridization physics between Fe, As and Ca states presumably leading to exoticity in this material.

Although the 1T^' phase is rare in the transition metal dichalcogenides (TMDCs) family, it has attracted rapid growing research interest due to the coexistence of superconductivity, unsaturated magneto-resistance, topological phases etc. Among them, the quantum spin Hall (QSH) state in monolayer 1T^'-TMDCs is especially interesting because of its unique van der Waals crystal structure, bringing advantages in the fundamental research and application. For example, the van der Waals two-dimensional (2D) layer is vital in building novel functional vertical heterostructure. The monolayer 1T^'-TMDCs has become one of the widely studied QSH insulator. In this review, we review the recent progress in fabrications of monolayer 1T^'-TMDCs and evidence that establishes it as QSH insulator.

Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals hetero-structures with a focus on hetero-bilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those hetero-bilayers are introduced. Most recent findings focusing on different transition metal dichalcogenides hetero-structures are presented and possible optical transitions between different valleys, appearance of moiré patterns and signatures of moiré excitons are discussed. The fascinating and fast growing research on van der Waals hetero-bilayers provide promising insights required for their application as emerging quantum-nano materials.

The manipulation of magnetism by electrical means is one of the most intensely pursued research topics of recent times aiming at the development of efficient and low-energy consumption devices in spintronics, microelectronics and bioelectronics. Herein, we successfully tuned the saturated magnetization of Fe₃O₄ by a supercapacitor. Through increasing the surface area of magnetic particles and activation of carbon cloth, fully reversible and robust saturation magnetization variation with low power consumption and remarkable switching speed can be realized on Fe₃O₄/ionic liquid interfaces at room temperature. The associated magnetism modulation can be attributed to ionic transition between Fe²⁺ and Fe³⁺ resulting from both electrostatic and electrochemical doping. This work paves the way for the development of high-performance spintronic devices.

We report investigation of phonons and oxygen diffusion in Bi₂O₃ and (Bi_0.7Y_0.3)₂O₃. The phonon spectra have been measured in Bi₂O₃ at high temperatures up to 1083 K using inelastic neutron scattering. Ab initio calculations have been used to compute the individual contributions of the constituent atoms in Bi₂O₃ and (Bi_0.7Y_0.3)₂O₃ to the total phonon density of states. Our computed results indicate that as temperature is increased, there is a complete loss of sharp peak structure in the vibrational density of states. Ab initio molecular dynamics simulations show that even at 1000 K in δ-phase Bi₂O₃, Bi–Bi correlations remain ordered in the crystalline lattice while the correlations between O–O show liquid like disordered behavior. In the case of (Bi_0.7Y_0.3)₂O₃, the O–O correlations broadened at around 500 K indicating that oxygen conductivity is possible at such low temperatures in (Bi_0.7Y_0.3)₂O₃ although the conductivity is much less than that observed in the undoped high temperature δ-phase of Bi₂O₃. This result is consistent with the calculated diffusion coefficients of oxygen and observation by quasielastic neutron scattering experiments. Our ab initio molecular dynamics calculations predict that macroscopic diffusion is attainable in (Bi_0.7Y_0.3)₂O₃ at much lower temperatures, which is more suited for technological applications. Our studies elucidate the easy directions of diffusion in δ-Bi₂O₃ and (Bi_0.7Y_0.3)₂O₃.

Sulfur vacancy in MoS₂ has been found to have an important influence on the performance of optoelectronic devices. Here, we study the effect of sulfur vacancy and O₂ adsorption on the electronic and optical properties in the two-dimensional ferroelectric CuInP₂S₆. It is revealed that a defect state appears at the top of valence band with the presence of sulfur vacancy. However, when O₂ is chemisorbed at sulfur vacancy, the defect state disappears. The variation of charge state and charge transfer are calculated and discussed. Although the ferroelectricity is greatly suppressed with the presence of sulfur vacancy, the ferroelectric state can be recovered when the O₂ is adsorbed. Within the framework of GW + BSE method, the optical absorption edge of CuInP₂S₆ monolayer exhibits a red-shift for the presence of sulfur vacancy and further O₂ adsorption gives rise to a blue-shift of the spectrum. Our findings have shown an effective way to improve the functionality of two-dimensional ferroelectrics via defect engineering.

Controlling sub-microsecond desorption of water and other impurities from electrode surfaces at high heating rates is crucial for pulsed power applications. Despite the short time scales involved, quasi-equilibrium ideas based on transition state theory (TST) and Arrhenius temperature dependence have been widely applied to fit desorption activation free energies. In this work, we apply molecular dynamics (MD) simulations in conjunction with equilibrium potential-of-mean-force (PMF) techniques to directly compute the activation free energies (ΔG*) associated with desorption of intact water molecules from Fe₂O₃ and Cr₂O₃ (0001) surfaces. The desorption free energy profiles are diffuse, without maxima, and have substantial dependences on temperature and surface water coverage. Incorporating the predicted ΔG* into an analytical form gives rate equations that are in reasonable agreement with non-equilibrium molecular dynamics desorption simulations. We also show that different ΔG* analytical functional forms which give similar predictions at a particular heating rate can yield desorption times that differ by up to a factor of four or more when the ramp rate is extrapolated by 8 orders of magnitude. This highlights the importance of constructing a physically-motivated ΔG* functional form to predict fast desorption kinetics.

We propose the model of a random polymer network, formed on the base on Erdös–Rényi random graph. In the language of mathematical graphs, the chemical bonds between monomers can be treated as vertices, and their chemical functionalities as degrees of these vertices. We consider graphs with fixed number of vertices N = 5 and variable parameter c (connectedness), defining the total number of links L = cN(N − 1)/2 between vertices. Each link in such graphs is treated as a Gaussian polymer chain. The universal rotationally invariant size and shape characteristics, such as averaged asphericity and size ratio of such structures are obtained both numerically by application of Wei's method and analytically within the continuous chain model. In particular, our results quantitatively indicate an increase of asymmetry of polymer network structure when its connectedness c decreases.

Two-dimensional (2D) materials have applications towards electronic devices, energy storages, and catalysis, et al. So far, most of elemental 2D materials are composed based on groups IIIA, IVA or VA. To expand the 2D material family, the orbital hybridization becomes a key factor to determine stability. Here we predict that sp²d³ hybridization of the outmost electrons in iodine and astatine can build up 2D triangle lattices, delta-iodiene and delta-astatiene, using first-principles calculations. Each atom is connected by σ bonds with nearest 6 atoms and the π bonds are thus introduced. The band gaps can approach zero because of interaction of unpaired single electron between each atom, if the identical bond length is reduced. By inducing compression strain, the Dirac points or topological nontrivial points can be created in the delta-iodiene and delta-astatiene. Our discovery paves a new way to construction of 2D materials.

The modern theory of orbital magnetization (OM) was developed by using Wannier function method, which has a formalism similar with the Berry phase. In this manuscript, we perform a numerical study on the fate of the OM under disorder, by using this method on the Haldane model in two dimensions, which can be tuned between a normal insulator or a Chern insulator at half filling. The effects of increasing disorder on OM for both cases are simulated. Energy renormalization shifts are observed in the weak disorder regime and topologically trivial case, which was predicted by a self-consistent T-matrix approximation. Besides this, two other phenomena can be seen. One is the localization trend of the band orbital magnetization. The other is the remarkable contribution from topological chiral states arising from nonzero Chern number or large value of integrated Berry curvature. If the fermi energy is fixed at the gap center of the clean system, there is an enhancement of |M| at the intermediate disorder, for both cases of normal and Chern insulators, which can be attributed to the disorder induced topological metal state before localization.

We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to ∼22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic oC4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the α-cF4–oC4–tI2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the oC4 phase, and 1–2 GPa lower than previously reported. We find the α-cF4 → tI2 phase boundary to have a positive gradient of 280 K (GPa)⁻¹, less steep than the 670 K (GPa)⁻¹ reported previously, and find the oC4 → tI2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2–3 GPa in the transition pressures at which the oC4 → tI2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the α-cF4 → tI2 is not second order at all temperatures up to 820 K.

Thermodynamic stability and vibrational anharmonicity of single layer black phosphorene (SLBP) are studied using a spectral energy density (SED) method. At finite temperatures, SLBP sheet undergoes structural deformation due to the formation of thermally excited ripples. Thermal stability of deformed SLBP sheet is analyzed by computing finite temperature phonon dispersion, which shows that SLBP sheet is thermodynamically stable and survives the crumpling transition. To analyze the vibrational anharmonicity, temperature evolution of all zone center optic phonon modes are extracted, including experimentally forbidden IR and Raman active modes. Mode resolved phonon spectra exhibits red-shift in mode frequencies with temperature. The strong anharmonic phonon–phonon coupling is the predominant reason for the observed red-shift of phonon modes, the contribution of thermal expansion is marginal. Further, temperature sensitivity of all optic modes are analyzed by computing their first order temperature co-efficient (χ), and it can be expressed as B_2g > ${A}_{\mathrm{g}}^{2}$ > ${B}_{3\mathrm{g}}^{1}$ > ${B}_{3\mathrm{g}}^{2}$ > B_1g > ${A}_{\mathrm{g}}^{1}$ & B_2u > B_1u for Raman and IR active modes, respectively. The quasi-harmonic χ values are much smaller than the SED and experimental values; which substantiate that quasi-harmonic methods are inadequate, and a full anharmonic analysis is essential to explain structure and dynamics of SLBP at finite temperatures.

We have studied correlational properties of quasi-one-dimensional electron gas at finite temperature T by incorporating the dynamics of electron correlations within the quantum version of the self-consistent mean-field approach of Singwi, Tosi, Land, and Sjölander. Static structure factor, pair-correlation function, static density susceptibility, excess kinetic energy, and free correlation energy are calculated covering a wide range of temperature and electron number density. As at absolute zero temperature, the inclusion of dynamics of correlations results in stronger spatial electron correlations, with a pronounced peak in the static structure factor at wave vector q ∼ 3.5k_F, which grows further with decreasing electron density. Below a critical density, the static density susceptibility seems to diverge at this value of q, signaling a transition from liquid to the Wigner crystal state-a prediction in qualitative agreement with recent simulations and experiment. However, thermal effects tend to impede crystallization with the consequence that the critical density decreases significantly with rising T. On the other hand, the pair-correlation function at short range exhibits a non-monotonic dependence on T, initially becoming somewhat stronger with rising T and then weakening continuously above a sufficiently high T. The calculated free correlation energy shows a noticeable dependence on T, with its magnitude increasing with increase in T. Further, we have looked into the effect of temperature on the frequency-dependence of dynamic local-field correction factor and the plasmon dispersion. It is found that with rising T the dynamics of correlations weakens, and the plasmon frequency exhibits a blue shift. Wherever interesting, we have compared our results with the lower-order approximate calculations and zero-T quantum Monte Carlo simulations.

A Heisenberg spin-s chain with alternating ferromagnetic (FM) ($-{J}_{1}^{\mathrm{F}}{< }0$) and antiferromagnetic (${J}_{1}^{\mathrm{A}}{ >}0$) nearest-neighbor (NN) interactions, exhibits the dimer and spin-2s Haldane phases in the limits ${J}_{1}^{\mathrm{F}}/{J}_{1}^{\mathrm{A}}\to 0$ and ${J}_{1}^{\mathrm{F}}/{J}_{1}^{\mathrm{A}}\to \infty$ respectively. These two phases are understood to be topologically equivalent. Induction of the frustration through the next NN FM interaction ($-{J}_{2}^{\mathrm{F}}{< }0$) produces a very rich quantum phase diagram. With frustration, the whole phase diagram is divided into a FM and a nonmagnetic (NM) phase. For s = 1/2, the full NM phase is seen to be of Haldane–dimer type, but for s > 1/2, a spiral phase comes between the FM and the Haldane–dimer phases. The study of a suitably defined string-order parameter and spin-gap at the phase boundary indicates that the Haldane–dimer and spiral phases have different topological characters. We also find that, along the ${J}_{2}^{\mathrm{F}}=\frac{1}{2}{J}_{1}^{\mathrm{F}}$ line in the NM phase, an NN dimer state is the exact groundstate, provided ${J}_{1}^{\mathrm{A}}{ >}{J}_{C}=\kappa {J}_{1}^{\mathrm{F}}$ where κ ⩽ s + h for applied magnetic field h. Without magnetic field, the position of J_C is on the FM–NM phase boundary when s = 1/2, but for s > 1/2, the location of J_C is on the phase separation line between the Haldane–dimer and spiral phases.

Analysis of the resistivity, thermoelectric power, and heat capacity of Fe_1−xNi_xSi is presented in this report. In-spite of Ni having two extra valence electrons as compared to Fe, the physical properties are observed to be dominated by holes. In this report, we have explained this unusual hole dominant scenario by a modified two narrow-band model. According to this model, the impurity electrons which are nearer to conduction band get shifted towards lower energy level thereby leaving holes around the Fermi level, and hence a hole dominated scenario at low temperatures. Due to this hole like density of states around the Fermi level, the nickel substitution could only produce a weak ferromagnetic behavior. Such a picture may assist in understanding the thermopower of similar systems i.e. Ni substituted on Fe site, such as Fe_2−xNi_xVAl. We have also found that the activation energy derived from resistivity and thermoelectric power decreases with increasing Ni concentration.

We report on the single crystal growth and transport properties of a topological semimetal CaAgBi which crystallizes in the hexagonal ABC-type structure with the non-centrosymmetric space group P6₃mc (No. 186). The transverse magnetoresistance measurements with current in the basal plane of the hexagonal crystal structure reveal a value of about 30% for I∥[10] direction and about 50% for I∥[10] direction at 10 K in an applied magnetic field of 14 T. The magnetoresistance shows a cusp-like behavior in the low magnetic field region, suggesting the presence of weak antilocalization effect for temperatures less than 100 K. The Hall measurements reveal that predominant charge carriers are p-type, exhibiting a linear behavior at high fields. The magnetoconductance of CaAgBi is analyzed based on the modified Hikami–Larkin–Nagaoka model. Our first-principle calculations within a density-functional theory framework reveal that the Fermi surface of CaAgBi consists of both the electron and hole pockets and the size of the hole pocket is much larger than electron pockets suggesting the dominant p-type carriers in accordance with our experimental results.

A generalized spin-1 Blume–Capel model with distance/volume dependent nearest-neighbor exchange interaction is introduced and investigated using the standard methods of statistical mechanics. Besides of the volume-dependent magnetic energy, the static electromagnetic energy and anharmonic Einstein phonon contribution are also taken into account. Taking the simple volume dependence of all energy contributions we have obtained the equation of state, magnetic moment, internal and Helmholtz free energy of the system. The ground-state and finite-temperature phase diagrams are obtained and discussed in detail. Is it shown that the generalized spin-1 Blume–Capel model exhibits a novel critical behavior appearing due to magnetostriction and thermal volume variations. The presented approach is very universal and easily applicable to many other theoretical models in different fields of solid state physics.

Magnetic materials are typically described in terms of the Heisenberg model, which provides an accurate account of thermodynamic properties when combined with first principles calculations. This approach is usually based on an energy mapping between density functional theory and a classical Heisenberg model. However, for two-dimensional systems the eigenenergies of the Heisenberg model may differ significantly from the classical approximation, which leads to modified expressions for exchange parameters. Here we demonstrate that density functional theory yields local magnetic moments that are in accordance with strongly correlated anti-ferromagnetic eigenstates of the Heisenberg Hamiltonian. This implies that density functional theory provides a description of these states that conforms with the quantum mechanical eigenstates of the model. We then provide expressions for exchange parameters based on a proper eigenstate mapping to the Heisenberg model and find that they may be reduced by up to 17% compared to a classical analysis. Finally, we calculate the corrections to critical temperature for magnetic ordering for a previously predicted set of two-dimensional ferromagnetic insulators and find that the inclusion of quantum effects may reduce the predictions of critical temperatures by up to 7%. The effect is, however, predicted to be much higher for spin-1/2 systems, which are not included in the predictions of critical temperatures.

Multiferroic materials endowed with both dielectric and magnetic orders, are ideal candidates for a wide range of applications. In this work, we reported two phase transitions of MnI₂ at 3.45 K and 4 K by systemically measuring the magnetic-field and temperature-dependent magnetization of the MnI₂ thin flakes. Furthermore, we observed similar temperature and field-dependent behaviours for the magnetic susceptibility of MnI₂ and electronic capacitance of the Ag/MnI₂/Ag devices below 3.5 K. Considering the related theory work, we discussed the relationship between the antiferromagnetic and ferroelectric orders in MnI₂. Our work reveals the in-plane magnetic and electric properties of MnI₂ materials, which might be helpful for the further investigation and application of MnI₂ multiferroics in the future.

Amorphous CoFeB films grown on GaAs(001) substrates demonstrating significant in-plane uniaxial magnetic anisotropy were investigated by vector network analyzer ferromagnetic resonance. Distinct in-plane anisotropy of magnetic damping, with a largest maximum–minimum damping ratio of about 109%, was observed via analyzing the frequency dependence of linewidth in a linear manner. As the CoFeB film thickness increases from 3.5 nm to 30 nm, the amorphous structure for all the CoFeB films is maintained while the magnetic damping anisotropy decreases significantly. In order to reveal the inherent mechanism responsible for the anisotropic magnetic damping, studies on time-resolved magneto-optical Kerr effect and high resolution transmission electron microscopy were performed. Those results indicate that the in-plane angular dependent anisotropic damping mainly originates from two-magnon scattering, while the Gilbert damping keeps almost unchanged.