We applied two-phase simulations to directly calculate the high-pressure melting line of helium from 425 to 10 000 K and from 15 GPa to 35 TPa by using molecular dynamics based on density-functional theory. The implementation of the two-phase simulation method and the relaxation of the simulation to an equilibrium state was studied in detail, as well as its convergence with respect to particle number. We performed extensive two-phase simulations with the Perdew, Burke and Ernzerhof and the van der Waals density functional exchange-correlation functional and found almost identical results.

The incorporation of Fe impurities in hexagonal barium titanate is studied in the framework of density functional theory. Formation energies are calculated to find the most probable defect structure. The substitution of Fe at the two inequivalent Ti sites accompanied by oxygen vacancies in different positions is studied. Additionally, different charge states of defects are taken into account. The structural aspects of the different defects are also studied in detail. In summary, the theoretical results are in agreement with recent experimental results found by means of electron paramagnetic resonance.

Rare earth element polyhydrides have been predicted to exhibit high-$T_c$ superconductivity at extreme compressions. Through a series of in situ high-pressure high-temperature x-ray powder diffraction experiments combined with density functional theory calculations, we report the emergence of polyhydride species in the praseodymium-hydrogen system. We initially observe the formation of ${\mathrm{PrH}}_3$, which continuously increases in hydrogen content on compression towards ${\mathrm{PrH}}_4$. Laser heating ${\mathrm{PrH}}_4$ in a hydrogen medium at pressures of 85 GPa leads to the synthesis of both ${\mathrm{PrH}}_9$ and ${\mathrm{PrH}}_7$. Both structures are characterized by hexagonal arrays of praseodymium atoms surrounded by hydrogen clathrate cages.

We measured current-voltage characteristics on both sides of the magnetic-field-driven superconductor-insulator transition. On both sides, these show strong nonlinearities leading to a conduction branch that is independent of phonon temperature. We show that a picture of electron overheating can quantitatively explain our data over the entire magnetic field range. We find that electron-phonon coupling strength remains roughly constant throughout the insulating state and across the superconductor-insulator transition, dropping dramatically as the magnetic field approaches zero. Our findings shed light on the origin of the highly debated saturation of resistance at low temperature, which has been interpreted by some as evidence for a new anomalous metallic phase and by others as a result of electrons failing to cool down. At the heart of this treatment lies the assumption that resistance is a function of electron temperature and not the phononic one. The applicability of this framework implies that the conduction mechanism, present in the superconductor and throughout the insulating phase, does not rely on a phonon bath.

High-field electrical transport and point-contact tunneling spectroscopy are used to investigate superconducting properties of spinel oxide ${\mathrm{LiTi}}_2{\mathrm{O}}_{4-\delta }$ films with various oxygen contents. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an isotropic upper critical field $B_{\mathrm{c}2}$ up to 26.0 T is observed in the oxygen-rich sample, which is more than twice the $B_{\mathrm{c}2}$ of 11.3 T in the anoxic one. The change of the dominating pair-breaking mechanism from the orbital effect to the spin flip at $B_{\mathrm{c}2}$ is achieved by tuning oxygen contents, which can be explained by the appearance of small Fermi pockets due to extra oxygen. Our paper provides deep understanding of the intrinsic relation between $B_{\mathrm{c}2}$ and the complex Fermi surface, and contributes a promising way to enhance $B_{\mathrm{c}2}$ for practical superconductors.

We report the growth of films of the intrinsic ferromagnetic semiconductor NdN and investigate their optical and transport properties. There is clear evidence of a strong anomalous Hall effect as expected from a $4f$ conduction channel, supported by an optical absorption into a $4f$ or $4f/5d$ hybridized tail at the base of the conduction band. The results reveal a heavy-fermion $4f/5d$ band lying where it can be occupied at controllable levels with nitrogen-vacancy donors.

We report on a thermal-expansion study of the ferroelectric phase transition in insulating ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{TiO}}_3$ ($x=0.009$) and its evolution upon increasing charge-carrier concentration up to $n\simeq 60\times {10}^{19}{\mathrm{cm}}^{-3}$. Although electric polarization is screened by mobile charge carriers, we find clear signatures of the ferroelectric phase transition in the thermal-expansion coefficient $\alpha$ of the weakly doped metallic samples. Upon increasing $n$, the transition temperature $T_{\mathrm{C}}\left(n\right)$ and the magnitude of the anomalies in $\alpha$ rapidly decrease up to a threshold carrier density $n^{★}$ above which broadened anomalies remain present. There is no indication for a sign change of $\alpha$ as is expected for a pressure-dependent quantum phase transition with $n$ as the control parameter. Thus, the ferroelectriclike transition is either continuously fading away or it transforms to another low-temperature phase above $n^{★}$, but this change hardly affects the temperature-dependent $\alpha \left(T\right)$ data.

Experimental studies of materials at extreme conditions are challenging, and as a consequence, P-T phase diagrams are still unknown for many elements and materials. In this work, we present the P-T phase diagram of uranium calculated up to extreme conditions. First, we searched for possible crystal structures using the evolutionary algorithm USPEX. Their free energies were then calculated using thermodynamic integration (TI) and temperature-dependent effective potential techniques. TI was performed using molecular dynamics, employing a machine learning (ML) force field trained on energies and forces from density-functional calculations at the generalized gradient approximation level. The prediction error of the ML force field for the energy was less than 10 meV/atom. Using thermodynamic perturbation theory (including first and second order corrections), from the free energies of the ML force field, we obtained free energies and phase diagram at the level of quality of the underlying density-functional calculations at pressures up to 800 GPa and temperatures up to 16 000 K.

Measurements of proton nuclear magnetic resonance (${}^1\mathrm{H}\mathrm{NMR}$) spectra and relaxation and of muon spin relaxation (${\mu }^{+}\mathrm{SR}$) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) [where ${\mathrm{H}}_3\mathrm{trensal}=2,2^{\prime },2^{\prime \prime }-\mathrm{tris}-\left(\mathrm{salicylideneimino}\right)\mathrm{triethylamine}$], featuring crystallographically imposed trigonal symmetry. Both the nuclear $1/T_1$ and muon λ longitudinal relaxation rates (LRRs) exhibit a peak for temperatures $T$ < 30 K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at $T$ ∼ 50 K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all molecular nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time ${\tau }_{\mathrm{c}}$ determined by the spin slowing down at low temperature. Further, for $T<50\mathrm{K}$ the spectral width does not follow the temperature behavior of the magnetic susceptibility χ. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times ${\tau }_{\mathrm{LT}}$ and ${\tau }_{\mathrm{HT}}$, dominating for $T$ < 30 K and $T>50\mathrm{K}$, respectively. Finally, the observed flattening of LRR for $T$ < 5 K is suggested to have a quantum origin.

Epitaxial strain has been extensively used to tailor the functionality of perovskite oxides, in which strain control of magnetism is highly desirable, especially for perovskite manganites. Here on the basis of the first-principles calculations we demonstrated the control of magnetic phase and orbital order in ${\mathrm{LuMnO}}_3$ film by epitaxial strain imposed by a square substrate and revealed the surface and interface effects by combining the strain-bulk and heterostructure models. The spiral and $E$ -type multiferroic phases present in perovskite manganites bulks can exist in the tensile strained films, while a ferromagnetic half-metal phase with high Curie temperature and an antiferromagnetic polar-metal phase arise in the range of compressive strain. Increasing compressive strain changes the sign of the $Q_3$ mode of Jahn-Teller distortion, resulting in the transition of the in-plane stagger orbital order to a uniform orbital order. The reconstruction of Jahn-Teller distortion and orbital order occur in the first two layers near the surface. The symmetry breaking of the crystal field at the surface leads to a uniform $d_{3z^2-r^2}$ type orbital order and surface metallization. The electron accumulation at the interface with ${\mathrm{SrTiO}}_3$ substrate has been demonstrated and decreases dramatically with the increase in the number of ${\mathrm{LuMnO}}_3$ layers.

We report the theoretical prediction of a class of spintronic materials, namely, spin semimetals, which is indirectly supported by experimental data measured for one such system. The band structure of this class of materials is such that one of the spin bands resembles that of a semimetal, while the other is similar to that of an insulator/semiconductor. We show that quaternary Heusler alloy FeRhCrGe possesses band features characterizing it as a spin semimetal. It is found to possess a magnetic moment of $3{\mu }_B$ and a Curie temperature of 550 K. Measurements below 300 K show weakly temperature dependent electrical conductivity and a moderate Hall effect. Band structure calculations also reveal that the spin-up (semimetallic) band shows a strong signature of type-II Weyl semimetallic behavior. As such, this study opens up the possibility of a class of materials with combined spintronic and topological properties, which is important from both fundamental and applied perspectives.

In this paper the influence of $2s$ Bloch wave states on high angle annular dark field image contrast is studied quantitatively by Bloch wave and multislice simulations. We show that $2s$ states are excited beyond a critical $Z$ value and cause a long period oscillation of the electron probe wave function that significantly influences the $Z$ contrast. As a result, we find that the $Z$ contrast for high $Z$ alloys remains thickness dependent up to high sample thicknesses and this effect has to be considered for compositional analysis. In the special case of ordered alloys, the impact of $2s$ excitations on intensity provides a way to quantify long-range order phenomena in alloys with known composition.

We demonstrate that small but finite ferroelectric polarization ($\sim 0.01\mu \mathrm{C}/{\mathrm{cm}}^2$) emerges in orthorhombic ${\mathrm{LuFeO}}_3$ ($Pnma$) at $T_N$ ($\sim 600$ K) because of commensurate ($k=0$) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from the lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna{2}_1$ phase below $T_N$. The $Pna{2}_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic ${\mathrm{LuFeO}}_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric ${\mathrm{LaFeO}}_3$ or ${\mathrm{NdFeO}}_3$. Application of electric field induces finite piezostriction in ${\mathrm{LuFeO}}_3$ via electrostriction resulting in clear domain contrast in piezoresponse force microscopy images.

The $\mathit{MX}$-enes are favorable two-dimensional materials for a wide variety of applications such as electronic and spintronic devices. The quantum confinement effects arising from reduced dimensionality and the presence of narrow $d$ bands suggest the importance of electron correlation effects in these materials. We calculate the strength of the effective screened Coulomb interaction $U$ for pristine $M_2\mathrm{C}$ and functionalized $M_2{\mathrm{CO}}_2 (M=\mathrm{Sc},\mathrm{Ti},\mathrm{V},\mathrm{Cr},\mathrm{Fe},\mathrm{Zr},\mathrm{Nb},\mathrm{Mo},\mathrm{Hf},\mathrm{Ta}) \mathit{MX}$-enes by employing ab initio calculations in conjunction with the random-phase approximation. The calculated $U$ values lie between 1.9 and 5.2 eV and depend on the ground-state electronic structure, $d$-electron number, and chemical functionalization. The $U$ for the $3d$-based systems shows oscillation when $M$ is varied, which can be explained by the $3d$ partial density of states around the Fermi energy. For nonmetallic functionalized $\mathit{MX}$-enes $M_2{\mathrm{CO}}_2$, the reduced dimensionality and the existence of a band gap result in a small screening of the Coulomb interaction. The long-range Coulomb interactions for both $M_2\mathrm{C}$ and $M_2{\mathrm{CO}}_2$ are also reported. For metallic systems, the interactions turn out to be local, and the nonlocal part is strongly screened due to $d$ states, making metallic $\mathit{MX}$-enes correlated systems. Furthermore, on the basis of the calculated effective Coulomb parameters $U$ and $J$ we discuss the appearance of ferromagnetism at pristine and functionalized $\mathit{MX}$-enes within the Stoner model.

We propose an effective lattice model of graphene moiré superlattices on hexagonal boron nitride (MLG/BN) based on ab initio calculations to study the evolution of electronic properties of relaxed MLG/BN superlattices with relative twist angle ($\theta$). The parameters for obtaining hopping terms and on-site energies in the lattice Hamiltonian are directly extracted from the ab initio electronic structure of shifted commensurate bilayers. The effect of the in-plane strain induced by the structural relaxation can be taken into account straightforwardly for the lattice model. We consider completely free MLG/BN heterostructures and those with constant interlayer distances. We find that the structural distortion due to the relaxation exhibits similar magnitude for $\theta$ smaller than about $0.5^{\circ }$. The gap at the primary Dirac points (${\mathrm{\Delta }}_P$) is maintained for $\theta <0.5^{\circ }$, while the gap at the secondary Dirac points (${\mathrm{\Delta }}_S$) decreases rapidly with small $\theta$, and such distinct behavior of ${\mathrm{\Delta }}_P$ and ${\mathrm{\Delta }}_S$ is roughly consistent with recent experimental observations. In addition, decreasing interlayer distance can significantly enhance ${\mathrm{\Delta }}_P$. This lattice model is sufficiently transparent and flexible to be employed to investigate various configurations of MLG/BN moiré superlattices.

The breaking of the doubly degenerate in-plane lattice vibrations in monolayer, bilayer, and bulk molybdenum disulfide (${\mathrm{MoS}}_2$) are oftentimes ignored in the analysis of the material's lattice behaviors due to the lack of variation of polarization for the excitation light. In this work, we have observed variations in the relative intensity of the two most dominant Raman peaks of ${\mathrm{MoS}}_2$ via polarized micro-Raman spectroscopy using elliptically polarized incident light. The use of elliptically polarized light has demonstrated distinct properties in its incident Raman polarization vector. This gives an additional degree of freedom affecting the relationship between the $x$ ($E_{2gx}^1$) and $y$ ($E_{2gy}^1$) components of the material's in-plane lattice vibrations. The different ratio of the magnitudes for the $E_{2gx}^1$ and $E_{2gy}^1$ in the lattice vibrations can be induced by changing the polarization state of the incident light. This work investigates the material's unexplored fundamental phonon property that may enlighten past and future studies involving phonon behaviors.

While the topological insulator and topological crystalline insulator phases rely on time-reversal symmetry and crystalline mirror symmetry, respectively, here, we predict that two-dimensional ${\mathrm{Na}}_2\mathrm{MgPb}$ and ${\mathrm{Na}}_2\mathrm{CdSn}$ can possess the properties of both of them, realizing dual topological insulators with nonzero ${\mathbb{Z}}_2$ invariant ${\mathbb{Z}}_2=1$ and mirror Chern number ${\mathcal{C}}_{\mathcal{M}}=-1$. Remarkably, the large band gap, as much as 0.58 eV for ${\mathrm{Na}}_2\mathrm{MgPb}$, and stable crystal structure indicate a high possibility for an experimental observation at room temperature. Moreover, we find that dual topological insulators can be turned into topological nodal-line semimetals by mimicking a magnetic field that keeps the crystalline mirror symmetry. In addition, we test and explore the emergence of such topological nodal-line semimetals on a model Hamiltonian and a realistic two-dimensional ferromagnet.