We study features of mobile carriers' skew scattering in nonmagnetic semiconductors emerging due to a combination of spin-orbit coupling in a crystal band structure and a nontrivial inner structure of impurities. In particular, we show that a nonzero magnetic moment of the impurity generally leads to the anomalous Hall effect (AHE) in the absence of the spin polarization of the mobile carriers, the effect arising from spin-independent scattering asymmetry due to exchange interaction. We analyze the skew scattering in bulk zinc-blende semiconductors for both electron and hole states and emphasize the crucial role of the impurity spin polarization for the emergent AHE for the valence band holes. We also revisit the skew scattering in quantum wells showing that the cancellation of the extrinsic contribution to the AHE common for two-dimensional systems can be lifted off depending on both the electron wave function and the impurity structure.

Resonance Raman scattering, which probes electrons, phonons, and their interplay in crystals, is extensively used in two-dimensional materials. Here we investigate Raman modes in $\mathrm{MoSe}_2$ at different laser excitation energies from 2.33 eV down to the near infrared 1.16 eV. The Raman spectrum at 1.16 eV excitation energy shows that the intensity of high-order modes is strongly enhanced if compared to the first-order phonon modes' intensity due to resonance effects with the $\mathrm{MoSe}_2$ indirect band gap. By comparing the experimental results with the two-phonon density of states calculated with density functional theory, we show that the high-order modes originate mostly from two-phonon modes with opposite momenta. In particular, we identify the momenta of the phonon modes that couple strongly with the electrons to produce the resonance process at 1.16 eV, while we verify that at 2.33 eV the two-phonon modes' line shape compares well with the two-phonon density of states calculated over the entire Brillouin zone. We also show that by lowering the crystal temperature, we actively suppress the intensity of the resonant two-phonon modes and we interpret this as the result of the increase of the indirect band gap at low temperature that moves our excitation energy out of the resonance condition.

The electron-electron scattering does not affect the electrical current in Galilean-invariant systems. We show that, nevertheless, electron-electron collisions may contribute to the electric resistivity of systems with parabolic spectrum provided that they have multiply connected Fermi surfaces, and there is an additional mechanism of scattering. To this end, we calculate the resistivity of a two-dimensional electron gas with two filled transverse subbands in a presence of electron-electron and impurity scattering. Although the collisions between the electrons do not directly affect the current in such systems, they cause a redistribution of the electrons between the Fermi contours, which results in a noticeable change in resistivity for realistic mechanisms of impurity scattering.

Recent progress in fabricating high-quality conductors with small densities of defects has initiated studies of a viscous electron fluid and has motivated the search for evidence of a hydrodynamic regime of electron transport. In this Letter we come up with the spin imaging technique, which can allow to attest to the emergence of electron hydrodynamic flows. Using numerical calculations, we examine specific inhomogeneous distributions of the electron spin density injected in a sample with a Poiseuille flow of a viscous electron fluid. We also demonstrate that the Hanle curves (the dependencies of the electron spin on magnetic field) at different positions across the channel acquire relative phase shifts resulting from the space variation of the hydrodynamic velocity. The studied effects can be employed to detect and map a viscous electron fluid noninvasively.

To address the effects of lattice anharmonicity across the perovskite to postperovskite transition in $\mathrm{MgSi}{\mathrm{O}}_3$, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on $abinitio$ molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit $\left(N\to \infty \right)$ are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation and the generalized gradient approximation calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Grüneisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope $\left(dP/dT\right)$ that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

Recently, the presence of an out-of-plane ferroelectric polarization was experimentally confirmed in bilayer two-dimensional (2D) hexagonal boron nitride (h-BN). In this work, we employ first-principles computations to reveal that (i) such slidetronic ferroelectricity is electronic in nature and arises from in-plane interlayer translation, which is distinctive among 2D slidetronic ferroelectric materials; and (ii) there is a coexistence of a negative longitudinal piezoelectric effect and an out-of-plane negative Poisson's ratio in the ferroelectric state of bilayer h-BN, which we refer to as anomalous double-negative effects.

MXenes are an emerging family of two-dimensional (2D) materials with highly tunable properties. Their fundamental mechanical properties have received comparatively less research attention than other functional properties due to the difficulty of performing such experiments on 2D materials. Therefore, we have developed a bond-order potential for surface-terminated titanium carbide MXene monolayers $({\mathrm{Ti}}_{n+1}{\mathrm{C}}_n{T}_x, n=1,2,\text{or}3,T=\text{−}\mathrm{O}\text{or}\text{−}\mathrm{F})$, enabling large-scale atomistic simulations which can probe both mechanical properties and deformation mechanisms. The bond-order potential does an excellent job of capturing relevant structural, elastic, and defect properties of the studied MXenes with and without surface terminations and is computationally scalable to allow for molecular dynamics simulations of monolayers hundreds of nanometers in size, only an order of magnitude below typical monolayer experiments. Crucially, these large-scale simulations open the possibility to study more realistic MXenes containing distributions of both defects and nonuniform surface terminations. We demonstrate in this work that these variables can have significant effects on the mechanical response of MXenes and therefore offer additional property-tuning capabilities which can be utilized to inform synthesis and postprocessing techniques.

$n$-alkane/urea inclusion compounds are prototypical examples of aperiodic crystals with uniformly aligned alkane guest molecules contained within linear channels formed by the supramolecular urea host. Here, we investigate single-crystal diffuse scattering, which is present in the form of sharp layers, from short-chain guest molecules (from octane to dodecane). The strong modulation of diffuse scattering within these layers shows that the positions of alkane molecules in neighboring channels are correlated. Using the three-dimensional difference pair distribution function method, we have extracted the effective interaction potentials and have shown that the interaction is mediated by the relaxation of urea molecules.

The dynamical quantum phase transitions (DQPTs) and the associated winding numbers have been extensively studied in the context Hermitian system. We consider the non-Hermitian analog of $p$-wave superconductor, supporting Hermitian gapless phase with complex hopping, in presence of on-site or superconducting loss term. This allows us to investigate the effect of non-Hermitian gapless phases on the DQPTs in addition to the Hermitian gapless phases. Our findings indicate that contour analysis of the underlying Hamiltonian, enclosing the origin and/or exceptional points, can predict the occurrences of DQPTs except the quench within the gapless phases. For the Hermitian case with initial and final Hamiltonians both being Hermitian, we find nonmonotonic integer jump for the winding number as the hallmark signature of the gapless phase there. For the hybrid case with initial and final Hamiltonians being Hermitian and non-Hermitian respectively, winding number exhibits integer spike in addition to the nonmonotonic integer jumps. For the non-Hermitian case with initial and final Hamiltonians both being non-Hermitian, the winding number show half-integer jumps for lossy superconductivity that does not have any Hermitian analog. On the other hand, the integer jumps in winding number is observed for lossy chemical potential. We understand our findings by connecting them with the profile of Fisher zeros and number of exceptional points and/or origin.

The possibility of experimental observation of the ultrasonic attenuation in the ferromagnet $\mathrm{LuCo}{}_3$ near the low-spin-high-spin crossover is discussed. We show that ultrasound propagation gives rise to transitions between states of the magnon band due to absorption of phonons, and this process is highly sensitive to the value of magnetization. The high magnetic field, which governs the crossover, alters the ultrasound propagation regime from off-resonant to resonant and we formulate a criterion of the change. Calculated temperature and field dependences of the ultrasonic wave number and attenuation clearly demonstrate anomalies in these characteristics in the vicinity of the crossover at intermediate temperatures far below the Curie temperature.

In the context of non-Hermitian Anderson localization, we study the physics of wave propagation in one-dimensional waveguide lattices, with randomly distributed gain or loss. Despite the Anderson localization of all eigenstates, the system exhibits counterintuitive propagation by quantized jumps between states located around distant sites. Such a novel effect was recently experimentally demonstrated in optical fiber loop networks. We provide a systematic way of understanding the underlying physical mechanism of such an effect. The role of eigenstates' nonorthogonality and biorthogonal projection is of central importance to our work, and is systematically examined in the symmetric non-Hermitian model, as well as the nonsymmetric Hatano-Nelson Hamiltonian. Our methodology can be applied to any non-Hermitian disordered system that contains complex elements with loss and/or gain, and thus exploits the meaning of wave transport in complex open systems.

Solid-state cooling using barocaloric materials is a promising avenue for ecofriendly, inexpensive, and highly efficient cooling. To design barocaloric compounds ready for deployment, it is essential to understand their thermodynamic behavior under working conditions. To this end, we have studied the rotational dynamics in the molecular-ionic crystal ammonium sulfate under pressure, providing detailed insight into the origin of its large barocaloric effect. Using quasielastic neutron scattering experiments, we show that rotation of the ammonium cations is facilitated by pressure in the low-entropy phase, with the rotational “hopping” motion increasing in frequency as the pressure-induced phase transition is approached. We explain this unusual behavior in terms of the competing hydrogen-bond networks represented by the two phases. This work includes the first results of a recently developed low-background, high-pressure gas cell for neutron scattering, showcasing its power in obtaining high-precision measurements of molecular dynamics under pressure.

We present a first-principles investigation of the magnetic and ferroelectric properties of ${\mathrm{BaCuF}}_4$. Our calculations indicate that the magnetic topology is one-dimensional, with a Néel temperature smaller than 1 K, if existing. We also show that applying high-pressure values up to 40 GPa on ${\mathrm{BaCuF}}_4$ does not induce three-dimensional magnetic order. In addition, our calculations predict that the polar phase is destabilized under pressure. The first consequence is to reduce the energy barrier between $Cmc{2}_1$ polar and $Cmcm$ nonpolar phases, enhancing the ability to reverse the polarization. The second consequence is the disappearance of ferroelectricity above a critical pressure of 12.6 GPa.

Isotropic helimagnets are known to host a diverse range of chiral magnetic states. In 2016, Rybakov et al., theorized the presence of a surface-pinned stacked spin spiral phase [F. N. Rybakov et al., New J. Phys. 18, 045002 (2016)], which has yet to be observed experimentally. The phase is characterized by surface spiral periods exceeding the host material's fundamental winding period $L_{\mathrm{D}}$. Here, we present experimental evidence for the observation of this state in lamellae of FeGe using resonant x-ray holographic imaging data and micromagnetic simulations. We find images of FeGe lamellae, exceeding a critical thickness of 300 nm ($4.3L_{\mathrm{D}}$), exhibit contrast modulations with a field-dependent periodicity of $\lambda \ge 1.4L_{\mathrm{D}}$, consistent with theoretical predictions of the stacked spiral state. The identification of this spiral state has significant implications for the stability of other coexisting spin textures, and will help complete our understanding of helimagnetic systems.

The collapse of Landau levels under an electric field perpendicular to the magnetic field is one of the distinctive features of Dirac materials. So is the coupling of lattice deformations to the electronic degrees of freedom in the form of gauge fields, which allows the formation of pseudo-Landau levels from strain. We analyze the collapse of Landau levels induced by strain on realistic Weyl semimetals hosting anisotropic, tilted Weyl cones in momentum space. We perform first-principles calculations to establish the conditions on the external strain for the collapse of Landau levels in TaAs which can be experimentally accessed.

Many physical quantities in solid-state physics are calculated from $k$-space summation. For spectral functions, the frequency-dependent factor can be decomposed into the energy-conserving $\delta$-function part and the nondissipative principal value part. A very useful scheme for this $k$-space summation is the tetrahedron method. Tetrahedron methods have been widely used to calculate the summation of the energy-conserving $\delta$-function part, such as the imaginary part of the dielectric function. On the other hand, the corresponding tetrahedron method for the nondissipative part, such as the real part of the dielectric function has not been used much. In this paper, we address the technical difficulties in the tetrahedron method for the nondissipative part and present an easy-to-implement stable method to overcome those difficulties. We demonstrate our method by calculating the static and dynamical spin Hall conductivity of platinum. Our method can be widely applied to calculate linear static or dynamical conductivity, self-energy of an electron, and electric polarizability, to name a few.

Layered material $\alpha \text{−}{\mathrm{RuCl}}_3$ has caught wide attention due to its possible realization of Kitaev's spin liquid and its electronic structure that involves the interplay of electron-electron correlations and spin-orbit effects. Several $\mathrm{DFT}+U$ studies have suggested that both electron-electron correlations and spin-orbit effects are crucial for accurately describing the band gap. This work studies the importance of these two effects using fixed-node and fixed-phase diffusion Monte Carlo calculations both in spin-averaged and explicit spin-orbit formalisms. In the latter, the Slater-Jastrow trial function is constructed from two-component spin orbitals using our recent quantum Monte Carlo (QMC) developments and thoroughly tested effective core potentials. Our results show that the gap in the ideal crystal is already accurately described by the spin-averaged case, with the dominant role being played by the magnetic ground state with significant exchange and electron correlation effects. We find qualitative agreement between hybrid DFT, $\text{DFT}+U$, and QMC. In addition, QMC results agree very well with available experiments, and we identify the values of exact Fock exchange mixing that provide comparable gaps. Explicit spin-orbit QMC calculations reveal that the effect of spin-orbit coupling on the gap is minor, of the order of 0.2 eV, which corresponds to the strength of the spin orbit of the Ru atom.