Nonlinear optical responses are commonly used as a probe for studying the electronic properties of materials. For topological materials, studies thus far focused on photogalvanic electric currents, which are forbidden in centrosymmetric materials because they require broken inversion symmetry. In this Letter, we propose a class of symmetry-allowed responses for inversion-symmetric topological insulators with two doubly degenerate bands. We consider a specific example of such a response, the orbital current, and show that the sign of the response reflects the ${\mathbb{Z}}_2$ topological index, i.e., the orbital current changes sign at the transition between trivial and topological insulator phases. This is illustrated in two models of topological insulators: the Bernevig-Hughes-Zhang model and the $1T^{\prime }$ phase of transition metal dichalcogenides.

Solid systems with strong correlations and interactions under light illumination have the potential for exhibiting interesting bulk photovoltaic behavior in the nonperturbative regime, which has remained largely unexplored in past theoretical studies. We investigate the bulk photovoltaic response of a perovskite manganite with strongly coupled electron-spin-lattice dynamics using real-time simulations performed with a tight-binding model. The transient changes in the band structure and the photoinduced phase transitions, emerging from spin and phonon dynamics, result in a nonlinear current versus intensity behavior beyond the perturbative limit. The current rises sharply across a photoinduced magnetic phase transition, which later saturates at higher light intensities due to excited phonon and spin modes. The predicted peak photoresponsivity is an order of magnitude higher than another known transition metal ferroelectric oxide ${\mathrm{BiFeO}}_3$. We disentangle phonon-and spin-assisted components to the ballistic photocurrent, showing that they are comparable in magnitude. Our results illustrate a promising alternative way for controlling and optimizing the bulk photovoltaic response through the photoinduced phase transitions in strongly correlated systems.

Generation of entanglement between spatially separated macroscopic systems enables quantum networks and fundamental tests of quantum theory. Here, we propose a scheme to generate a long-range entangled state between two macroscopic spheres. The hybrid system under consideration consists of two massive ferromagnetic spheres and a superconducting transmon qubit, which are both coupled to a microwave cavity array. When the qubit or magnon frequency lies outside of the frequency band of the cavity array, the qubit or magnon mode can seed a tunable, localized cavity mode with an exponentially decaying envelope around its position. This effective cavity mediates the long-range qubit-magnon and magnon-magnon interactions in a coherent way, enabling the long-distance magnon-magnon entanglement via the superconducting qubit.

We report the results of a combined experimental and theoretical study on nonstoichiometric $\mathrm{Cr}{\mathrm{N}}_{1+\delta }$ thin films grown by reactive magnetron sputtering on $c$-plane sapphire and MgO (100) substrates in an $\mathrm{Ar}/{\mathrm{N}}_2$ gas mixture using different percentages of ${\mathrm{N}}_2$. There is a transition from $n$-type to $p$-type behavior in the layers as a function of nitrogen concentration varying from 48 to 52 at. % in CrN films. The compositional change follows a similar trend for all substrates, with a N/Cr ratio increasing from approximately 0.7 to 1.06–1.11 by increasing the percentage of ${\mathrm{N}}_2$ in the gas flow ratio. As a result of the change in stoichiometry, the lattice parameter and the Seebeck coefficient increase together with the increase of N in $\mathrm{Cr}{\mathrm{N}}_{1+\delta }$; in particular, the Seebeck value coefficient transitions from $\text{–}50\mu \mathrm{V}{\mathrm{K}}^{\text{–}1}$ for ${\mathrm{CrN}}_{0.97}$ to $+75\mu \mathrm{V}{\mathrm{K}}^{\text{–}1}$ for ${\mathrm{CrN}}_{1.1}$. Density functional theory calculations show that Cr vacancies can account for the change in the Seebeck coefficient, since they push the Fermi level down in the valence band, whereas N interstitial defects in the form of ${\mathrm{N}}_2$ dumbbells are needed to explain the increasing lattice parameter. Calculations including both types of defects, which have a strong tendency to bind together, reveal a slight increase in the lattice parameter and a simultaneous formation of holes in the valence band. To explain the experimental trends, we argue that both Cr vacancies and ${\mathrm{N}}_2$ dumbbells, possibly in combined configurations, are present in the films. We demonstrate the possibility of controlling the semiconducting behavior of CrN with intrinsic defects from $n$ to $p$ type, opening possibilities to integrate this compound in energy-harvesting thermoelectric devices.

Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to atomic displacement and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e., how the magnetic interactions depend on atomic displacement, of the prototypical two-dimensional ferromagnet ${\mathrm{CrI}}_3$. We extract an effective measure of the spin-lattice coupling in ${\mathrm{CrI}}_3$, which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to antiferromagnetic (AFM) for atomic displacements larger than 0.16 (0.18) Å for Cr (I) atoms. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split $\mathrm{Cr}\text{−}3d$ orbitals. The competition between the AFM $t_{2g}\text{−}{t}_{2g}$ and FM $t_{2g}\text{−}{e}_g$ contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign changes when the I-Cr-I bond angle approaches ${90}^{\circ }$. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis, can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

We study the dopant-induced inhomogeneity effect on the electronic properties of heavy fermion ${\mathrm{CeCoIn}}_5$ using a combined approach of density functional theory (DFT) and dynamical mean-field theory (DMFT). The inhomogeneity of the hybridization between Ce-$4f$ and conduction electrons is introduced to impose the inequivalent Ce atoms with respect to the dopant. From the DFT to the $\mathrm{DFT}+\mathrm{DMFT}$ results, we demonstrate a variation of the hybridization strength depending on the hole or electron doping. A drastic asymmetric mass renormalization could be reproduced in the $\mathrm{DFT}+\mathrm{DMFT}$ calculation. Finally, the calculated Kondo temperature reflects the different development of the heavy quasiparticle states, depending on the dopant.

Electrons in graphene are theoretically expected to retain spin states much longer than most materials, making graphene a promising platform for spintronics and quantum information technologies. Here, we use first-principles density-matrix (FPDM) dynamics simulations to show that interaction with electric fields and substrates strongly enhances spin relaxation through scattering with phonons. Consequently, the relaxation time at room temperature reduces from microseconds in free-standing graphene to nanoseconds in graphene on the hexagonal boron nitride (hBN) substrate, which is the order of magnitude typically measured in experiments. Further, inversion symmetry breaking by hBN introduces a stronger asymmetry in electron and hole spin lifetimes than predicted by the conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from the conventional DP model are stronger for in-plane spin relaxation, resulting in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a maximum close to the Dirac point. These FPDM results, independent of symmetry-specific assumptions or material-dependent parameters, also validate recent modifications of the DP model to explain such deviations. Overall, our results indicate that spin-phonon relaxation in the presence of substrates may be more important in graphene than typically assumed, requiring consideration for graphene-based spin technologies at room temperature.

The carrier dynamics in various types of epitaxial graphene layers on SiC substrates was investigated by means of time- and angle-resolved photoemission spectroscopy (TARPES). Layer-dependent electron doping was observed in the Dirac bands of quasicrystalline bilayer graphene after optical pumping, leading to generation of transient voltage between the upper and lower layers. The amount of photoinduced carrier transport depends on the distance from the substrate. A comparison of the TARPES results of single-layer graphene between flat and stepped SiC substrates experimentally indicates that a source of the doping carriers likely originates from interface step states. The dynamic model is described based on the electronic structure, calculated using density functional theory.

Biased bilayer graphene (BBG) is a semiconductor with a strongly field-dependent band gap of up to $300\text{meV}$, making it of particular interest for graphene-based nanoelectronic and nanophotonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory$+$Bethe-Salpeter-equation modeling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30–300\text{mV/Å}$. The exciton properties prove to have a strong field dependence, with both energy ordering and dipole alignment varying significantly between the low and high field regimes. Namely, at low fields we find a mostly dark ground state exciton, as opposed to high fields, where the lowest exciton is bright. Also, excitons preferentially align with a dipole moment opposite the field, due to the field-induced charge transfer in the ground state of BBG. However, in stronger fields, this alignment becomes energetically less favorable. Additionally, the bright excitons show particle- and lightlike bands similar to monolayer transition metal dichalchogenides. Finally, we model the radiative lifetimes and emission properties of BBG, which prove to be strongly dependent on temperature in addition to field strength.

Optical communications, imaging, and biomedicine require efficient detection of infrared radiation. Growing demand pushes for the integration of such detectors on chips. It is a challenge for conventional semiconductor devices to meet these specs due to spectral limitations arising from their finite band gap, as well as material incompatibilities. Single layer graphene (SLG) is compatible with complementary metal-oxide-semiconductor (CMOS) Si technology, while its broadband (UV to THz) absorption makes the SLG/Si junction a promising platform for photodetection. Here we model the thermionic operation of SLG/Si Schottky photodetectors, considering SLG's absorption, heat capacity, and carrier cooling dependence on temperature and carrier density. We self-consistently solve coupled rate equations involving electronic and lattice temperatures, and nonequilibrium carrier density under light illumination. We use as an example the infrared photon energy of 0.4 eV, below the threshold for direct photoemission over the Schottky barrier, to study the photothermionic response as a function of voltage bias, input power, pulse width, electronic injection, and relaxation rates. We find that device and operation parameters can be optimized to reach responsivities competitive with the state of the art for any light frequency, unlike conventional semiconductor-based devices. Our results prove that the SLG/Si junction is a broadband photodetection platform.

After extensive investigation of the Floquet second-order topological insulator (FSOTI) in two dimensions, here we propose two driving schemes to systematically engineer the hierarchy of the Floquet first-order topological insulator, the FSOTI, and the Floquet third-order topological insulator in three dimensions. Our driving protocols allow these Floquet phases to showcase regular 0, anomalous $\pi$, and hybrid 0-$\pi$-modes in a unified phase diagram, obtained for both two- and three-dimensional (2D and 3D) systems, while starting from the lower-order topological or nontopological phases. Both the step drive and the mass kick protocols exhibit the analogous structure of the evolution operator around the high symmetry points. These eventually enable us to understand the Floquet phase diagrams analytically and the Floquet higher-order modes numerically based on finite-size systems. The number of 0 and $\pi$ modes can be tuned irrespective of the frequency in the step drive scheme, while we observe frequency-driven topological phase transitions for the mass kick protocol. We topologically characterize some of these higher-order Floquet phases (harboring either 0 or anomalous $\pi$ mode) by a suitable topological invariant in 2D and 3D cases.

Topological semimetals exhibit novel quantum states and electronic structures with interesting Fermi surfaces. In contrast to Weyl and Dirac nodal points, nodal lines can have multiple topological configurations in momentum space that lead to new properties, such as long-range Coulomb interaction and flat Landau levels. Herein, we report the discovery of a Dirac nodal-line semimetal polynitride ${\mathrm{ZnN}}_4$, synthesized from the elements under high pressure via laser-heated diamond anvil cell technique. The crystal structure of ${\mathrm{ZnN}}_4$ has an $Ibam$ space group, and features infinite one-dimensional nitrogen chains in the pressure range of 14–130 GPa. The Dirac cone is observed midway between $S$ and $X$ points in the Brillouin zone and the Dirac nodal-line is found near the $T$ point. The measured electrical resistance of the ${\mathrm{ZnN}}_4$ sample shows an anomalous increase with pressure, which is attributed to the localization of valence electrons. The case of ${\mathrm{ZnN}}_4$ provides a unique example of Dirac nodal-line semimetal in nitrides.

Electron correlation in finite and extended systems is often described in an effective single-particle framework within the $GW$ approximation. Here, we use the statically screened second-order exchange (SOX) contribution to the self-energy ($G3W2$) to calculate a perturbative correction to the $GW$ self-energy. We use this correction to calculate total correlation energies of atoms, relative energies, as well as charged excitations of a wide range of molecular systems. We show that the second-order correction improves correlation energies with respect to the random-phase approximation and also improves relative energies for many, but not all, considered systems. The dynamically screened SOX term has previously been shown to consistently lower the highest occupied molecular orbital (HOMO) quasiparticle (QP) energies and to increase the lowest unoccupied molecular orbitals (LUMO) QP energies. We show here that the statically screened $G3W2$ correction consistently increases the LUMO QP energies, while no consistent trend can be observed for the HOMO levels. Also, confirming previous results, the magnitude of the correction is much smaller with the statically screened interaction than with the dynamically screened one. Quasiparticle self-consistent $\mathit{GW}$ by itself is shown to be an excellent method for the calculation of charged excitation of finite systems, and it cannot consistently be improved upon by the $G3W2$ correction. For range-separated hybrid starting points, the description of fundamental gaps and HOMO QP energies is slightly worsened. However, tremendous improvements upon the $GW$ LUMO energies, leading to almost perfect agreement with high-level coupled cluster reference methods, are observed. The evaluation of the statically screened $G3W2$ correction only comes with small additional computational cost compared to $G_0{W}_0$ for systems with up to 100 atoms and should therefore be suitable for practical applications.

The confinement of elementary excitations induces distinctive features in the non-equilibrium quench dynamics. One of the most remarkable is the suppression of entanglement entropy, which in several instances turns out to oscillate rather than grow indefinitely. While the qualitative physical origin of this behavior is clear, till now no quantitative understanding away from the field theory limit was available. Here we investigate this problem in the weak quench limit, when mesons are excited at rest, hindering entropy growth and exhibiting persistent oscillations. We provide analytical predictions of the entire entanglement dynamics based on a Gaussian approximation of the many-body state, which captures numerical data with great accuracy and is further simplified to a semiclassical quasiparticle picture in the regime of weak confinement. Our methods are valid in general and we apply explicitly to two prototypical models: the Ising chain in a tilted field and the experimentally relevant long-range Ising model.

Employing the recently developed momentum-space quantum Monte Carlo scheme, we study the dynamic response of single-particle and collective excitations in realistic continuum models of twisted bilayer graphene. At charge neutrality with small flat-band dispersion, this unbiased numerical method reveals single-particle spectra and collective excitations at finite temperature. Single-particle spectra indicate that repulsive interactions push the fermion spectral weight away from the Fermi energy and open up an insulating gap. The spectra of collective excitations suggest an approximate valley $\text{SU}\left(2\right)$ symmetry. At low energy, long-lived valley waves are observed, which resemble spin waves of Heisenberg ferromagnetism. At high energy, these sharp modes quickly become overdamped, when their energy reaches the fermion particle-hole continuum.

The development of future spintronic applications requires a thorough and fundamental understanding of the magnetization dynamics. Of particular interest are magnetic nanodisks, in which the vortex state emerges as a stable spin configuration. Here, we focus on how the vortex-core polarization can be reversed periodically by an oscillating magnetic field, applied perpendicularly to the disk's surface. By means of micromagnetic simulations, we demonstrate the presence of several subharmonic switching modes, i.e., the commensurate ratio between the switching frequency of the core and the driving frequency. The underlying mechanism of this periodic behavior depends on the disk thickness. For thin disks, the core switches periodically due to resonant excitation of radial spin wave modes, while it is due to the breathing mode in the case of thick disks. However, overlap of both modes impedes periodic vortex-core switching. For thin disks, the threshold field amplitude required for periodic switching can be substantially lowered by increasing the disk diameter. For thick disks, in contrast, the diameter is less decisive for the minimal field amplitude, as only the average energy density in a central region around the vortex core is relevant to excite the breathing mode. Our results contribute to the understanding of the switching mechanisms in magnetic nanodisks, which are of technological interest due to their potential in nonvolatile memory devices.

We investigate reorientation effects under external periodic drive in the nanomagnet dynamics coupled to a Josephson junction. The Kapitza pendulum is introduced as a mechanical analog to this system and we demonstrate the reorientation of the easy axis of the nanomagnet. The magnetic field generated by the Josephson junction and external drive plays the role of the oscillating force of the suspension point in the Kapitza pendulum. The high-frequency oscillations change the orientation of the magnetic moment. The magnetic field of the quasiparticle current determines the frequency dependence of the magnetic moment's orientation. We obtain simple analytical formulas for the stable position of the magnetic moment, both under the external periodic drive and without it. The influence of external periodic drive on the voltage of complete reorientation have been demonstrated.