We use first-principles calculations to find that in infinite-layer nickelates $R{\mathrm{NiO}}_2$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R=\mathrm{La}\text{−}\mathrm{Sm}$. For late lanthanide elements $R=\mathrm{Eu}\text{−}\mathrm{Lu}$, an imaginary phonon frequency appears at $A=(\pi ,\pi ,\pi )$ point. For those infinite-layer nickelates, condensation of this phonon mode into the $P4/mmm$ structure leads to a more energetically favorable $I4/mcm$ structure that is characterized by an out-of-phase rotation of “${\mathrm{NiO}}_4$ square.” Special attention is given to two borderline cases: ${\mathrm{PmNiO}}_2$ and ${\mathrm{SmNiO}}_2$, in which both the $P4/mmm$ structure and the $I4/mcm$ structure are local minima, and the energy difference between the two structures can be fine tuned by epitaxial strain. Compared to the $P4/mmm$ structure, $R{\mathrm{NiO}}_2$ in the $I4/mcm$ structure has a substantially reduced Ni $d_{x^2-y^2}$ bandwidth, a smaller Ni $d$ occupancy, a “cleaner” Fermi surface with a lanthanide $d$-derived electron pocket suppressed at the $\mathrm{\Gamma }$ point, and a decreased critical $U_{\text{Ni}}$ to stabilize long-range antiferromagnetic ordering. All these features imply enhanced correlation effects and favor Mott physics. Our work reveals the importance of structure-property relation in infinite-layer nickelates, in particular, the spontaneous “${\mathrm{NiO}}_4$ square” rotation provides a tuning knob to render $R{\mathrm{NiO}}_2$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.

Many material properties such as the electronic transport characteristics depend on the details of the electronic band structure in the vicinity of the Fermi level. For an accurate ab initio description of the material properties, the electronic band structure must be known and theoretically reproduced with high fidelity. Here, we ask a question which of the ab initio methods compare the best to the experimental photoemission intensities from bcc Fe. We confront the photoemission data from Fe(001) thin film grown on Au(001) to the photoemission simulations based on different ab initio initial band structures: density functional theory (DFT) in the local density approximation (LDA) and the generalized gradient approximation (GGA) and GGA corrected with many-body perturbation theory in the $GW$ approximation. We find the best comparison for the $GW$ results. As a second step, we discuss how the calculated intrinsic anomalous Hall conductivity (AHC) in bcc Fe depends on the choice of the method that describes the electronic band structure and Fermi level position. We find very large differences in AHC between the three theoretical approaches and show that the AHC found for the experimental Fermi level location within the $GW$ band structure is the closest to the literature results of transport experiments. This finding improves our understanding of not only the anomalous Hall effect itself, but also other related phenomena, such as the anomalous Nernst effect.

Ce compounds feature a remarkable diversity of electronic properties, which motivated extensive investigations over the last decades. Inelastic neutron scattering represents an important tool for understanding their underlying electronic structures but in certain cases a straightforward interpretation of the measured spectra is hampered by the presence of strong vibronic couplings. The latter may give rise to extra spectral features, which complicates the mapping of experimental data onto standard multiplet diagrams. To benchmark the performance of embedded-cluster quantum chemical computational schemes for the case of $4f$ systems, we here address the Ce $4f^1$ multiplet structure of $\mathrm{Na}\mathrm{Ce}{\mathrm{O}}_2$, an antiferromagnet with $D_{2d}$ magnetic-site symmetry for which neutron scattering measurements indicate only weak vibronic effects. Very good agreement with the experimental results is found in the computations, which validates our computational approach and confirms $\mathrm{Na}\mathrm{Ce}{\mathrm{O}}_2$ as a $4f$ magnet in the intermediate coupling regime with equally strong $4f$-shell spin-orbit and crystal-field interactions.

Theory of optical absorption of linearly and circularly polarized light in surface and edge states in topological insulators is developed for a nonlinear light intensity regime. The absorbance for surface states and the absorption width for the edge states both decrease as $1/\sqrt{I}$ at high light intensity $I$. Elastic scattering of the photoexcited electrons and holes is taken into account. The absorption bleaching at high intensity is shown to be strongly suppressed by the elastic scattering. The linear-circular dichroism of one- and two-photon absorption in surface states, emerging in the nonlinear intensity regime, decreases due to the elastic scattering of photocarriers. Absorption in the edge states of two-dimensional topological insulators also depends on the elastic scattering rate being suppressed when it is stronger than the energy relaxation rate. Linear dichroism—the dependence of the absorption length on the polarization plane orientation—is studied at arbitrary light intensities. We show that a degree of linear dichroism is governed by the ratio of the elastic and inelastic relaxation times. The photocurrents generated in the edge states by both linearly and circularly polarized light change their intensity dependencies from the linear one at low $I$ to the $\propto \sqrt{I}$ at high intensities. The variation of the photocurrents with both polarization and intensity is strongly dependent on the elastic scattering rate. The considered effects can be observed in the terahertz (THz) frequency range at laser intensities used in modern experiments.

Using the continuum model, we investigate the electronic properties of two types of bilayer graphene (BLG) quantum ring (QR) geometries: (i) An isolated BLG QR and (ii) a monolayer graphene (MLG) with a QR put on top of an infinite graphene sheet (hybrid BLG QR). Solving the Dirac-Weyl equation in the presence of a perpendicular magnetic field and applying the infinite mass boundary condition at the ring boundaries, we obtain analytical results for the energy levels and corresponding wave spinors for both structures. In the case of isolated BLG QR, we observe a sizable and magnetically tunable band gap which agrees with the tight-binding transport simulations. Our analytical results also show the intervalley symmetry $E_e^K\left(m\right)=-E_h^{K^{\prime }}\left(m\right)$ between the electron (e) and the hole (h) states ($m$ is the angular momentum quantum number) for the energy spectrum of the isolated BLG QR. The presence of interface boundary in a hybrid BLG QR modifies drastically the energy levels as compared with that of an isolated BLG QR. Its energy levels are tunable from MLG dot to isolated BLG QR and to MLG Landau energy levels as the magnetic field is varied. Our predictions can be verified experimentally using different techniques such as by magnetotransport measurements.

Due to its linear dispersion, monolayer graphene is expected to generate a third harmonic response at terahertz frequencies. There have been a variety of different models of this effect and recently it has been experimentally observed. However, there is still considerable uncertainty as to the role of scattering on harmonic generation in graphene. In this paper, we model third harmonic generation in doped monolayer graphene at THz frequencies by employing a nearest-neighbour tight-binding model in the length gauge. We include optical phonon and neutral impurity scattering at the microscopic level, and examine the effects of scattering on the third harmonic response. We also compare the results of a phenomenological semiclassical theory, using a field-dependent scattering time extracted from the simulation, and find a significantly lower third-harmonic field than that found from the microscopic model. This demonstrates that third harmonic generation is much more sensitive to the nature of the scattering than is the linear response. We also compare the results of our full simulation to recent experimental results and find qualitative agreement.

Simulations of interacting electrons and bosons out of equilibrium, starting from first principles and aiming at realistic multiscale scenarios, is a grand theoretical challenge. Here, using the formalism of nonequilibrium Green's functions and relying in a crucial way on the recently discovered time-linear formulation of the Kadanoff-Baym equations, we present a versatile toolbox for the simulation of correlated electron-boson dynamics. A large class of methods are available, from the Ehrenfest to the dressed $GD$ for the treatment of electron-boson interactions in combination with perturbative, i.e., Hartree-Fock and second-Born, or nonperturbative, i.e., $GW$ and $T$ matrices either without or with exchange effects, for the treatment of the Coulomb interaction. In all cases the numerical scaling is linear in time and the equations of motion satisfy all fundamental conservation laws.

Nonequilibrium dynamics of the open chain Holstein-Hubbard model is studied using the linear time-scaling $\mathrm{GKBA}+\mathrm{ODE}$ scheme developed in Pavlyukh et al. [Phys. Rev. B 105, 125134 (2022)]. We focus on the set of parameters relevant for photovoltaic materials, i.e., a pair of electrons interacting with phonons at the crossover between the adiabatic and antiadiabatic regimes and at moderately large electron-electron interaction. By comparing with exact solutions for two corner cases, we demonstrate the accuracy of the $T$ matrix (in the $pp$ channel) and the second-order Fan ($GD$) approximations for the treatment of electronic ($e\text{−}e$) and electron-phonon ($e$-ph) correlations, respectively. The feedback of electrons on phonons is consistently included and is shown to be mandatory for the total energy conservation. When two interactions are simultaneously present, our simulations offer a glimpse into the dynamics of doublons and polarons unveiling the formation, propagation and decay of these quasiparticles, energy redistribution between them and self-trapping of electrons.

We develop a formulation of the coherent potential approximation (CPA) on the basis of the Wannier representation to advance a computationally efficient method for the treatment of homogeneous random alloys that is independent of the applied first-principles electronic structure code. To verify the performance of this CPA implementation within the Wannier representation, we examine the Bloch spectral function, the density of states, and the magnetic moment in Fe-based transition metal alloys $\mathrm{Fe}-X$ ($X=\mathrm{V}$, Co, Ni, and Cu) and compare the results with those of the well-established CPA implementation based on the Korringa-Kohn-Rostoker (KKR) Green's function method. The Wannier-CPA and the KKR-CPA methods lead to very similar results. The presented Wannier-CPA method has a wide potential applicability to other physical quantities and large compound systems because of the low computational effort required.

We study how stable the Majorana-mediated spin transport in a quantum spin Kitaev model is against thermal fluctuations. Using the time-dependent thermal pure quantum state method, we examine finite-temperature spin dynamics in the Kitaev model. The model exhibits two characteristic temperatures, $T_L$ and $T_H$, which correspond to energy scales of the local flux and the itinerant Majorana fermion, respectively. At low temperatures $(T\ll T_L)$, an almost flux-free state is realized, and the spin excitation propagates in a similar way to that for the ground state. Namely, after the magnetic pulse is introduced at one of the edges, the itinerant Majorana fermions propagate the spin excitations even through the quantum spin liquid state region, and oscillations in the spin moment appear in the other edge with a tiny magnetic field. When $T\sim T_L$, larger oscillations in the spin moments are induced in the other edge, compared with the results at the ground state. At higher temperatures, excited $Z_2$ fluxes disturb the coherent motion of the itinerant Majorana fermions, which suppresses the spin propagation. Our results demonstrate a crucial role of thermal fluctuations in the Majorana-mediated spin transport.

Investigations on $\mathrm{II}\text{−}\mathrm{Sn}\text{−}{\mathrm{N}}_2$ ($\mathrm{II}=\mathrm{Mg}$, Ca) have been started very recently compared to the intense research of $\mathrm{Zn}\text{−}\mathrm{IV}\text{−}{\mathrm{N}}_2$ ($\mathrm{IV}=\mathrm{Si}$, Ge, Sn). In this work, we study the phase stability of ${\mathrm{MgSnN}}_2$ and ${\mathrm{ZnSnN}}_2$ in wurtzite and rocksalt phases by first principles calculations. The calculated phase diagram agrees with the experimental observation; i.e., ${\mathrm{MgSnN}}_2$ can form in the wurtzite and rocksalt phases while ${\mathrm{ZnSnN}}_2$ only crystallizes in the wurtzite phase. Due to the higher ionicity of Mg-N bonds compared to Sn-N bonds and Zn-N bonds, wurtzite-type ${\mathrm{MgSnN}}_2$ appears under Mg-rich conditions. The defect properties and doping behavior of ${\mathrm{MgSnN}}_2$ in the wurtzite phase are further investigated. We find that ${\mathrm{MgSnN}}_2$ exhibits self-doped $n$-type conductivity, and donor-type antisite defect ${\mathrm{Sn}}_{\mathrm{Mg}}$ is the primary source of free electrons. The high possibility of forming the stoichiometry-preserving ${\mathrm{Mg}}_{\mathrm{Sn}}+{\mathrm{Sn}}_{\mathrm{Mg}}$ defect complex leads to our study of cation disorder in ${\mathrm{MgSnN}}_2$ by using the cluster expansion method with first principles calculations. It is found that cation disorder in ${\mathrm{MgSnN}}_2$ induces a band-gap reduction because of a violation of the octet rule. The local disorder, namely, forming (4,0) or (0,4) tetrahedra, leads to an appreciable band-gap reduction and hinders the enhancement of the optical absorption.

Using an elegant model involving only ${\mathrm{\Gamma }}_{6c}$ and ${\mathrm{\Gamma }}_{8v}$ bands, massless Kane fermions were defined as the particles associated with the peculiar band structure of gapless HgCdTe crystals. Although their dispersion relation resembles that of a pseudo-spin-1 Dirac semimetal, these particles were originally considered to be hybrids of pseudospin-1 and -1/2 fermions. Here we unequivocally find that, by considering an additional ${\mathrm{\Gamma }}_{7c}$ conduction band inherent in HgCdTe crystals, the Kane fermions are ultimately two nested Dirac particles. This observation allows for the direct application of Lorentz transformations to describe the relativistic behavior of these particles in crossed electric and magnetic fields. By studying the relativistic collapse of their Landau levels at different orientations between the crossed fields and the main crystallographic axes, we demonstrate that the Kane fermions strikingly decay into two independent Dirac particles with increase of the electric field. Our results provide insight into semirelativistic effects in narrow-gap semiconductors in crossed electric and magnetic fields.

Topological electronic phases exist in a variety of naturally occurring materials but can also be created artificially. We used a cryogenic scanning tunneling microscope to create dimerized chains of identical quantum dots on a semiconductor surface and to demonstrate that these chains give rise to one-dimensional topological phases. The dots were assembled from charged adatoms, creating a confining potential with single-atom precision acting on electrons in surface states of the semiconductor. Coupling between the dots leads to electronic states localized at the ends of the chains, as well as at deliberately created internal domain walls, in agreement with the predictions of the Su-Schrieffer-Heeger model. Scanning tunneling spectroscopy also reveals deviations from this well-established model manifested in an asymmetric level spectrum and energy shifts of the boundary states. The deviations arise because the dots are charged and hence lead to an onsite potential that varies along the chain. We show that this variation can be mitigated by electrostatic gating using auxiliary charged adatoms, enabling fine-tuning of the boundary states and control of their superposition. The experimental data, which are complemented by theoretical modeling of the potential and the resulting eigenstates, reveal the important role of electrostatics in these engineered quantum structures.

An atomistic first-principles-based effective Hamiltonian is used to study the response of a prototype of antiferroelectric materials, namely, the perovskite sodium niobate system, to the application of terahertz electric pulses. So-called hidden phases are discovered due to such ultrafast field excitation. These phases are homogeneous in nature, and correspond to local minima of energy. In addition to improved piezoelectric responses, they also possess controllable magnitude of the electric polarization, leading to possible applications of antiferroelectrics for ultrafast neuromorphic computing.

Exceptional points (EPs) are critical points in the parameter space of non-Hermitian systems, where two or more eigenvalues and eigenvectors simultaneously coalesce. The remarkable physics and behavior of waves at these EPs have raised considerable attention. Previous research has accessed EPs in parity-time (PT) symmetric systems through spatially modulated parameters. Using acoustics, this Letter demonstrates a different family of EPs in classical wave systems that emerge from coordinated modulation of mass density and loss/gain in time. This condition can create nonreciprocal coupling between arbitrary modes at the EPs, leading to exotic behaviors such as unilateral frequency conversion and linear amplification of waves that are unattainable at conventional time-invariant systems. Moreover, these phenomena can be attained with only loss, and acoustic gain via modal energy transfer is demonstrated in a loss-only system at such EPs. Our work marries time-varying systems with EPs, which could open new avenues for wave manipulation.

We compare thermal-gradient-driven transverse voltages in ferrimagnetic-insulator/heavy-metal bilayers (${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{W}$ and ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Pt}$) to corresponding electrically driven transverse resistances at and above room temperature. We find for ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{W}$ that the thermal and electrical effects can be explained by a common spin-current detection mechanism, the physics underlying spin Hall magnetoresistance (SMR). However, for ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Pt}$ the ratio of the electrically driven transverse voltages (planar Hall signal/anomalous Hall signal) is much larger than the ratio of corresponding thermal-gradient signals, a result which is very different from expectations for a SMR-based mechanism alone. We ascribe this difference to a proximity-induced magnetic layer at the ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}/\mathrm{Pt}$ interface.

We study the nonlinear optical response in a superconducting NbN thin film with a strong terahertz (THz) wave. In. addition to the expected third-harmonic generation, we observe a transient oscillation which softens in frequency with temperature increasing towards the superconducting transition temperature $T_c$. We identify this feature as transient Higgs oscillation. To verify this proposal, we introduce a time-frequency resolved technique, a spectrogram for visualizing the THz spectrum. The dynamic decaying behavior of the oscillation is observed, which is consistent with the theoretical expectation of intrinsic Higgs oscillation. Moreover, we observe a higher-order nonlinear optics effect, i.e., fifth-harmonic generation, which we assign to the higher-order coupling between the Higgs mode and electromagnetic field.

We report that a perpendicular magnetic field introduces an anomalous interaction correction, $\delta \sigma$, to the static conductivity of doped graphene in the ballistic regime. The correction implies that the magnetoresistance, $\delta {\rho }_{xx}$ scales inversely with temperature $\delta {\rho }_{xx}\left(T\right)\propto 1/T$ in a parametrically large interval. When the disorder is scalarlike, the $\propto 1/T$ behavior is the leading contribution in the crossover between diffusive regime exhibiting weak localization and quantum magnetooscillations. The behavior originates from the field-induced breaking of the chiral symmetry of Dirac electrons around a single valley. The result is specific for generic two-dimensional Dirac materials which deviate from the half-filling. We conclude by proposing magnetotransport experiments, which have the capacity to detect the nature of impurities and defects in high-mobility Dirac monolayers such as recently fabricated ballistic graphene samples.

Strong Coulomb interaction in monolayer transition metal dichalcogenides can facilitate nontrivial many-body effects among excitonic complexes. Many-body effects such as exciton-exciton annihilation have been widely explored in this material system. However, a similar effect for charged excitons (or trions), that is, trion-trion annihilation (TTA), is expected to be relatively suppressed due to repulsive like-charges, and it has not been hitherto observed in such layered semiconductors. By a gate-dependent tuning of the spectral overlap between the trion and the charged biexciton through an “anticrossing”-like behavior in monolayer ${\mathrm{WS}}_2$, here we present an experimental observation of an anomalous suppression of the trion emission intensity with an increase in gate voltage. The results strongly correlate with time-resolved measurements, and they are inferred as direct evidence of a nontrivial TTA resulting from nonradiative Auger recombination of a bright trion, and the corresponding energy resonantly promoting a dark trion to a charged biexciton state. The extracted Auger coefficient for the process is found to be tunable ten-fold through a gate-dependent tuning of the spectral overlap.

When the continuous translation symmetry breaks, a continuous mode splits into two distinct discrete modes with different energies. Based on the fully discrete Peierls model, it is found that stability of such discrete modes for dislocations can be tuned by the stacking fault energy of a slip system. As the stacking fault energy changes continuously, the stable dislocation structure transforms periodically and the Peierls stress varies oscillatorily. Furthermore, at the transformation point, the lattice resistance nearly vanishes and thus the dislocation can almost move freely.