Table of contents

Research on topological and topological crystalline insulators (TCIs) is one of the most intense and exciting topics due to its fascinating fundamental science and potential technological applications. Pressure (strain) is one potential pathway to induce the non-trivial topological phases in some topologically trivial (normal) insulating or semiconducting materials. In the last ten years, there have been substantial theoretical and experimental efforts from condensed-matter scientists to characterize and understand pressure-induced topological quantum phase transitions (TQPTs). In particular, a promising enhancement of the thermoelectric performance through pressure-induced TQPT has been recently realized; thus evidencing the importance of this subject in society. Since the pressure effect can be mimicked by chemical doping or substitution in many cases, these results have opened a new route to develop more efficient materials for harvesting green energy at ambient conditions. Therefore, a detailed understanding of the mechanism of pressure-induced TQPTs in various classes of materials with spin–orbit interaction is crucial to improve their properties for technological implementations. Hence, this review focuses on the emerging area of pressure-induced TQPTs to provide a comprehensive understanding of this subject from both theoretical and experimental points of view. In particular, it covers the Raman signatures of detecting the topological transitions (under pressure), some of the important pressure-induced topological and TCIs of the various classes of spin–orbit coupling materials, and provide future research directions in this interesting field.

Here, we study B20-type RhGe, a representative of a class of non-centrosymmetric monosilicides and monogermanides, which possess unique topological and magnetic properties important for many possible applications. The stability and phase transitions of the non-equilibrium B20-RhGe phase that can only be obtained under high pressure, are investigated theoretically using ab initio calculations and experimentally by means of differential scanning calorimetry. For RhGe and, for comparison, for its analogue RhSi, we conducted an evolutionary search for low-energy polymorphic modifications at zero temperature and then performed simulations of their behavior at finite temperatures. The ($P,T$) conditions of stability for the found polymorphs are determined. Our calorimetric studies on high-pressure-synthesized RhGe samples allowed us to reveal peculiarities in thermal stability and heating-induced phase transformations. X-ray diffraction analysis and microstructure analysis of the samples were carried out before and after the heating. We also determined the specific heat from calorimetric measurements and compared the results with our calculations in the quasi-harmonic approximation.

Hyperfine parameters and the pressure dependence of the magnetic transition temperatures of FeRhGe₂ have been investigated. Sample has been prepared using high pressure—high temperature synthesis technique. FeRhGe₂ consists of two B20 structure phases with close lattice constants. The phase separation stays constant in the temperature range 4–300 K. The magnetic transition temperatures $T_{\mathrm{c1}}$ = 213 K and $T_{\mathrm{c2}}$ = 135 K of FeRhGe₂ slightly increases with pressure in the range 0–4.5 GPa. We have compared this pressure dependence with some others compounds in the family Fe$_{1-x}$Rh_xGe. The two phases in FeRhGe₂ have slightly different values of the hyperfine magnetic fields.

Light–matter interaction is one of the key means to manipulate the structural and electronic properties of materials, especially in two-dimensional (2D) layered materials, which are optically accessible due to their atomic thickness. We propose that an ultrashort laser pulse could drastically enhance the ferroelectric polarization of bilayer WTe₂ by our real-time time-dependent density functional theory simulations. It is noted that bilayer WTe₂ is a 2D sliding ferroelectric material recently discovered whose vertical polarization can be controlled by a slight horizontal displacement. We demonstrate that interlayer sliding and compression are simultaneously achieved upon illumination of linearly polarized near-infrared laser pulse, leading to an ultrafast electric polarization enhancement by ∼230% within hundreds of femtosecond. Two major contributions have been identified: (a) the piezoelectric effect due to laser-induced interlayer compression, caused by interlayer charge transfer and dipole-dipole interaction; (b) the interlayer sliding along the opposite direction of ferroelectric switching, induced by inhomogeneous excited carrier distribution and specific electron-phonon couplings. This work provides new insights on controlling ferroelectricity of layered materials, which may extend to other van der Waals bilayers and even bulk materials.

In this paper, we analyze the influence of the electron-vibration interaction on the charge transfer process at the donor-acceptor interface in an organic solar cell. We present an essentially exact numerical analysis for a minimal model with only one vibration mode which is coupled to the charge transfer state. We show that the charge transfer state can be hot or cold depending on the parameters and in particular on the value of the energy of the lowest unoccupied molecular orbital on the donor side. We analyze also different regimes where electron–hole attraction or hybridization effects at the interface can modify the quantum yield of the transfer. We discuss also briefly the possible effects of the other vibration modes that are coupled to the charge either on the donor side or on the acceptor side.

We present results for the steady-state nonlinear response of a $d_{x^2-y^2}$ superconducting film connected to normal-metal reservoirs under voltage bias, allowing for a subdominant s-wave component appearing near the interfaces. Our investigation is based on a current-conserving theory that self-consistently includes the non-equilibrium distribution functions, charge imbalance, and the voltage-dependencies of order parameters and scalar impurity self-energies. For a pure d-wave superconductor with [110] orientation of the interfaces to the contacts, the conductance contains a zero-bias peak reflecting the large density of zero-energy interface Andreev bound states. Including a subdominant s-wave pairing channel, it is in equilibrium energetically favorable for an s-wave order parameter component $\Delta_\mathrm{s}$ to appear near the interfaces in the time-reversal symmetry breaking combination d + is. The Andreev states then shift to finite energies in the density of states. Under voltage bias, we find that the non-equilibrium distribution in the contact area causes a rapid suppression of the s-wave component to zero as the voltage $eV\rightarrow\Delta_\mathrm{s}$. The resulting spectral rearrangements and voltage-dependent scattering amplitudes lead to a pronounced non-thermally broadened split of the zero-bias conductance peak that is not seen in a non-selfconsistent Landauer–Büttiker scattering approach.

The topological study of the complicated one-dimensional (1D) systems with multi-band-gap structures, including quasi-crystals (QCs), is very hard since the lack of effective topological invariants to describe the non-triviality of gaps. A generalized method, based on the contracted wave-function, is proposed in this work to calculate the real-space winding number for the complicated 1D systems with multi-band-gap structures. First, the effectiveness of the generalized method is demonstrated to obtain the quantized real-space winding number for the gaps and correctly predict the topological phase transition and the existing fractional charge on the edges for the periodic 4-atoms SSH model (4A-SSH model). Then, we apply the generalized method to more complicated 1D Thue–Morse (TM) systems, which is one kind of QCs. The quantized real-space winding number is obtained for two traditional gaps and two fractal gaps for the TM systems and can also correctly predict the existence of topological edge-states and fractional charge on the ends. Several new phenomena are observed, e.g. the topological phase transition and the edge-states for the gaps in multi-band-gap structures, the $1/4$ fractional charge for the 4A-SSH model, the fluctuation of local charge and the asymmetric (but still with a quantized difference) fractional charge at the ends of TM system. The generalized method could be a powerful tool to study the topology of gaps in the complicated periodic systems or QCs.

The melting thermodynamic characteristics of 2- to 20-layered onion-like fullerenes (OLF_n) (C₆₀@C₂₄₀ to C₆₀@···@C₆₀₀₀···@C₂₄₀₀₀) are comprehensively explored using first-principles-based ReaxFF atomistic simulations and random forest machine learning (RF ML). It is revealed that OLF_n shows lower thermal stability than the counterparts of single-walled fullerenes (SWF_n). The melting point of SWF_n increases monotonically with increasing size, whereas for OLF_n, an unusual size-dependent melting point is observed; OLF_n with intermediate size shows the highest melting point. For small OLF_n, the melting occurs from the inner to the outer, whereas for large OLF_n, it nucleates from the inner to the outer and to intermediate fullerenes. The melting and erosion behaviors of both SWF_n and OLF_n are mainly characterized by the nucleation of non-hexagons, nanovoids, carbon chains and emission of C₂. RF ML model is developed to predict the melting points of both SWF_n and OLF_n. Moreover, the analysis of the feature importance reveals that the Stone-Wales transformation is a critical pathway in the melting of SWF_n and OLF_n. This study provides new insights and perspectives into the thermodynamics and pyrolysis chemistry of fullerenic carbons, and also may shed some lights onto the understanding of thermally-induced erosion of carbon-based resources and spacecraft materials.

High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity, $2 \times 2 \times 2$ supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT.

For realistic crystals, the free energy strictly formulated in ensemble theory can hardly be obtained because of the difficulty in solving the high-dimension integral of the partition function, the dilemma of which makes it even a doubt if the rigorous ensemble theory is applicable to phase transitions of condensed matters. In the present work, the partition function of crystal vanadium under compression up to 320 GPa at room temperature is solved by an approach developed very recently, and the derived equation of state is in a good agreement with all the experimental measurements, especially the latest one covering the widest pressure range up to 300 GPa. Furthermore, the derived Gibbs free energy proves the very argument to understand most of the experiments reported in the past decade on the pressure-induced phase transition, and, especially, a novel phase transition sequence concerning three different phases observed very recently and the measured angles of two phases agree with our theoretical results excellently.

We use two-site quantum nonlocality to identify the topological quantum phase transitions (TQPTs) of the extended Ising model driven by varying system parameters. We investigate how the system parameters, including the anisotropies of the nearest-neighbor and the next-nearest-neighbor spin pairs, the transverse magnetic field, and the three-spin interaction, affect the quantum nonlocality. We show that the nonlocality cannot mark any TQPTs while its first derivative can perfectly characterize the TQPTs. By making the influences of the thermal fluctuations and the site distance of spin pairs on the critical behavior of the TQPTs analysis, we show that the sufficiently low temperature has a slight impact on the features of nonlocality and its first derivative while the site distance of spin pairs can significantly alter the properties of nonlocality and its first derivative. We further present the energy spectra and the trajectories of the winding vectors of the model to demonstrate that the quantum nonlocality can be employed to successfully signalize the TQPTs.

Using momentum microscopy with sub-µm spatial resolution, allowing momentum resolved photoemission on individual antiferromagnetic domains, we observe an asymmetry in the electronic band structure, $E(k)\,\neq\,E(-k)$, in Mn₂Au. This broken band structure parity originates from the combined time and parity symmetry, ${\cal PT}$, of the antiferromagnetic order of the Mn moments, in connection with spin–orbit coupling. The spin–orbit interaction couples the broken parity to the Néel order parameter direction. We demonstrate a novel tool to image the Néel vector direction, N, by combining spatially resolved momentum microscopy with ab-initio calculations that correlate the broken parity with the vector N.

PtGa is a topological semimetal with giant spin-split Fermi arcs. Here, we report on angular-dependent de Haas–van Alphen (dHvA) measurements combined with band-structure calculations to elucidate the details of the bulk Fermi surface of PtGa. The strong spin–orbit coupling leads to eight bands crossing the Fermi energy that form a multitude of Fermi surfaces with closed extremal orbits and results in very rich dHvA spectra. The large number of experimentally observed dHvA frequencies make the assignment to the equally large number of calculated dHvA orbits challenging. Nevertheless, we find consistency between experiment and calculations verifying the topological character with maximal Chern number of the spin-split Fermi surface.

In this work, far from equilibrium Hall response of semi-Dirac materials is studied. This required preparing the system in non-equilibrium states through a quantum quench protocol. We show that in the non-equilibrium setting, there is non-zero Hall response even when instantaneous time reversal symmetry (TRS) is present and the Hall current persists for long times. This is in contrast to the equilibrium case where the system is required to break TRS for a Hall response. This highlights unique features of far from equilibrium response in semi-Dirac materials that are not present in the corresponding equilibrium state.

Anisotropic transport, Shubnikov-de Haas (SdH), and de Haas-van Alphen (dHvA) quantum oscillations studies are reported on a high-quality CoSi single crystal grown by the Czochralski method. Temperature-dependent resistivities indicate the dominating electron-electron scattering. Magnetoresistance (MR) at 2 K reaches 610% for $I~\parallel~[111]$ and $B~\parallel~$[01$\bar{1}$], whereas it is 500% for $I~\parallel~$[01$\bar{1}$] and $B~\parallel~[111]$. A negative slope in field-dependent Hall resistivity suggests electrons are the majority carriers. The carrier concentration extracted from Hall conductivity indicates no electron–hole compensation. In 3D CoSi, the electron transport lifetime is found to be approximately in the same order as the quantum lifetime, whereas in 2D electron gas the long-range scattering drives the transport life much larger than the quantum lifetime. From MR and Hall SdH oscillations, the effective masses and Dingle temperatures have been calculated. The dHvA oscillation reveals three frequencies at 18 T (γ), 558 T (α) and 663 T (β), whereas, SdH oscillation results in only two frequencies α and β. The γ frequency observed in dHvA oscillation is a tiny hole pocket at the Γ point.

The value and the nature of the bandgap of In₄Se₃ are still not well defined, with a large spread of the experimental data between 0.42 and 1.68 eV and an uncertain nature, predicted to be indirect by ab initio band structure calculations. Here we report on the optical transmission and photoluminescence (PL) performed in In₄Se₃ thin films grown by coevaporation on (0001)-oriented sapphire wafers. The quality of the polycrystalline layers allows the first detection of the excitonic-like transition in the optical absorption of this compound at low temperature. The PL detected under weak laser excitation shows a bound exciton emission at 0.75 eV. Strong laser irradiation reveals a quadratic dependence of the PL intensity on the optical excitation, which demonstrates a stimulated emission at 0.79 eV in relation with an exciton–exciton scattering process. On the basis of a reasonable estimate of the exciton energy, equal to $10-15$ meV, we evaluate the direct bandgap of In₄Se₃ to $0.82\pm0.01$ eV at low temperature.

Our first-principles evidence shows that the two-dimensional (2D) multiferroic VSe₂/In₂Se₃ experiences continuous change of electronic structures, i.e. with the change of the ferroelectric (FE) polarization of In₂Se₃, the heterostructure can possess type-I, -II, and -III band alignments. When the FE polarization points from In₂Se₃ to VSe₂, the heterostructure has a type-III band alignment, and the charge transfer from In₂Se₃ into VSe₂ induces half-metallicity. With reversal of the FE polarization, the heterostructure enters the type-I band alignment, and the spin-polarized current is turned off. When the In₂Se₃ is depolarized, the heterostructure has a type-II band alignment. In addition, influence of the FE polarization on magnetism and magnetic anisotropy energy of VSe₂ was also analyzed, through which we reveal the interfacial magnetoelectric coupling effects. Our investigation about VSe₂/In₂Se₃ predicts its wide applications in the fields of both 2D spintronics and multiferroics.