Rocksalt CeO (001) epitaxial thin films were synthesized and their electronic properties were investigated. A simple $4f^1{5d}^1$ electronic configuration with $4f–5d$ hybridization in CeO was confirmed by x-ray photoemission and absorption spectroscopy. While $5d$ conduction holes governed the metallic conduction at high temperatures, a partially compensated $n$-type conduction appeared below ∼10 K with a rapid decrease in resistivity corresponding to a Fermi liquid state. The hole mobility was as high as $646\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{–1}{\mathrm{s}}^{–1}$ at 2 K in contrast to the two decades lower electron mobility, reflecting the large and small dispersion of $5d$ and $4f$ bands, respectively. At 0.8 K, a resistivity minimum was observed as a manifestation of the Kondo effect, indicating partial $4f$ localization. These results represented that the significant $4f–5d$ hybridization induced a Kondo coherent state in CeO with a short Ce-Ce interionic distance unlike the other antiferromagnetic Ce chalcogenides.

We propose the concept of valley coherence and superradiance in the reciprocal space and show that it leads to an $N$-fold decrease of the bright exciton radiative lifetime in quantum dots (QDs) of an $N$-valley semiconductor. Next we explain why, despite this, the exciton radiative lifetimes in $\mathrm{Pb}X$ ($X=$ S, Se, Te) QDs, measured from the photoluminescence decay are in the microsecond range. We also address peculiarities of the light-matter interaction in nanostructures made of narrow-gap materials with strong interband coupling.

Superconductors can be classified as topological or not based on whether time-reversal symmetry, chiral symmetry, and particle-hole symmetry are preserved or not. Further, topological superconductors can also be classified as chiral or helical. In this paper, using Hanbury Brown and Twiss (HBT) shot-noise correlations and the nonlocal conductance, we probe metal and two-dimensional unconventional superconductor and metal junctions to understand better the pairing topological vs nontopological or helical vs chiral or nodal vs gapful. We see that HBT correlations are asymmetric as a function of bias voltage for nontopological superconductors, whereas they are symmetric for topological superconductors irrespective of the barrier strength. Topological superconductors are associated with Majorana fermions which are important for topological quantum computation. By distinguishing topological superconductors from nontopological superconductors, our study will help search for Majorana fermions, which will aid in designing a topological quantum computer.

The concept of optical exciton—a photoexcited bound electron-hole pair within a crystal—is routinely used to interpret and model a wealth of excited-state phenomena in semiconductors. Beside originating subband gap signatures in optical spectra, optical excitons have also been predicted to condensate, diffuse, recombine, and relax. However, all these phenomena are rooted on a theoretical definition of the excitonic state based on the following simple picture: “excitons” are actual particles that both appear as peaks in the linear absorption spectrum and also behave as well-defined quasiparticles. In this paper, we show, instead, that the electron–phonon interaction decomposes the initial optical (i.e., “reducible”) excitons into elemental (i.e., “irreducible”) excitons, the latter being a different kind of bound electron-hole pairs lacking the effect caused by the induced, classical, electric field. This is demonstrated within a real-time, many-body perturbation theory approach starting from the interacting electronic Hamiltonian including both electron-phonon and electron-hole interactions. We then apply the results to two realistic and paradigmatic systems, monolayer ${\mathrm{MoS}}_2$ (where the lowest-bound optical exciton is optically inactive) and monolayer ${\mathrm{MoSe}}_2$ (where it is optically active), using first-principles methods to compute the exciton-phonon coupling matrix elements. Among the consequences of optical-elemental decomposition, we point to a homogeneous broadening of absorption peaks occurring even for the lowest-bound optical exciton , and we demonstrate this by computing exciton-phonon transition rates. More generally, our findings suggest that the optical excitons gradually lose their initial structure and evolve as elemental excitons. These states can be regarded as the real intrinsic excitations of the interacting system, the ones that survive when the external perturbation and the induced electric fields have vanished.

The layered iron chalcogenide materials have attracted considerable attention recently for their exotic superconductivity at relatively high temperatures. Topological phases are, however, seldom proposed in these materials. Based on density-functional theory calculations together with symmetry analysis, 100% spin-polarized Weyl semimetals, namely Weyl half semimetals (WHSMs), are predicted in two-dimensional (2D) TlFeSe and GaFeSe monolayers, built based on FeSe monolayers. The acquired Weyl fermions are protected by a nonsymmorphic symmetry. Dissimilarly, the InFeSe monolayer is found to be a quantum anomalous Hall (QAH) insulator with a large band gap (403 meV). By tuning the magnetization direction, the monolayers can vary from a WHSM to a QAH insulator or vice versa. The phase-transition mechanism is analyzed by using an effective $\mathit{k}·\mathit{p}$ model. Our work provides a pathway to carry out the fascinating 2D WHSMs and the QAH effect in one material which will have promising applications in not only spintronics but also topological microelectronics.

Exciton-polaritons in an optical microcavity can form a macroscopically coherent state despite being an inherently driven-dissipative system. In comparison with equilibrium bosonic fluids, polaritonic condensates possess multiple peculiarities that make them behave differently from well-known textbook examples. One such peculiarity is the presence of dark excitons which are created by the pump together with optically active particles. They can considerably affect the spectrum of elementary excitations of the condensate and hence change its superfluid properties. Here, we theoretically analyze the influence of the bright and dark “reservoir” populations on the sound velocity $c_s$ of incoherently driven polaritons. Both pulsed and continuous-wave pumping schemes characterized by essentially different condensate-to-reservoir ratios are considered. We show that the dark exciton contribution leads to considerable lowering of $c_s$ and to its deviation from the square-root-like behavior on the system's chemical potential (measurable condensate blueshift). Importantly, our model allows us to unambiguously define the density of dark excitons in the system by experimentally tracking $c_s$ against the condensate blueshift and fitting the dependence at a given temperature.

We show that a resonant pulsed field can induce a steady superconducting state even in the deep Mott insulating phase of the dimerized Hubbard model. The superconductivity found here in the nonequilibrium steady state is due to the $\eta$-pairing mechanism, characterized by the existence of the off-diagonal long-range order (ODLRO), and is absent in the ground-state phase diagram. The key of the scheme lies in the generation of the field-induced charge density wave (CDW) state that is from the valence bond solid. The dynamics of this state resides in the highly excited subspace of the dimerized Hubbard model and is dominated by a $\eta$-spin ferromagnetic model. The decay of such long-living excitation is suppressed because of energy conservation. We also develop a dynamical method to detect the ODLRO of the nonequilibrium steady state. Our finding demonstrates that the nonequilibrium many-body dynamics induced by the interplay between the resonant external field and electron-electron interaction is an alternative pathway to access a new exotic quantum state, and also provides an alternative mechanism for enhancing superconductivity.

Antiferromagnets (AFMs) are strong candidates for the future spintronic and memory applications largely because of their inherently fast dynamics and lack of stray fields, with ${\mathrm{Mn}}_2\mathrm{Au}$ being one of the most promising. For the numerical modeling of magnetic material properties, it is common to use ab initio methods, atomistic models, and micromagnetics. However, each method alone describes the physics within certain limits. Multiscale methods bridging the gap between these three approaches have been already proposed for ferromagnetic materials. Here we present a complete multiscale model of the AFM ${\mathrm{Mn}}_2\mathrm{Au}$ as an exemplar material, starting with results from ab initio methods going via atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model. First, bulk ${\mathrm{Mn}}_2\mathrm{Au}$ is modelled using a classical spin Hamiltonian constructed based on earlier first-principles calculations. Second, this spin model is used in the stochastic Landau-Lifshitz-Gilbert to calculate temperature-dependent equilibrium properties, such as magnetization and magnetic susceptibilities. Third, the temperature-dependent micromagnetic parameters are used in the AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for three paradigmatic cases: (i) damped magnetic oscillations, (ii) magnetization dynamics following a heat pulse resembling pump-probe experiments, and (iii) magnetic domain wall motion under thermal gradients.

Recently, great effort has been devoted to the search for two-dimensional (2D) ferromagnetic materials with inherent strong Dzyaloshinskii-Moriya interaction (DMI). Here, through a first-principles approach, we systematically investigate the effect of biaxial strain on the DMI, the Heisenberg exchange interaction, and the magnetic anisotropy energy (MAE) of Janus ${\mathrm{Cr}}_2{X}_3{Y}_3$ ($X,Y$ = Cl, Br, I, $X\ne Y$) monolayers. Both DMI and MAE can be significantly enhanced by tensile strain, whereas a reversal of the chirality of DMI in ${\mathrm{Cr}}_2{\mathrm{Cl}}_3{\mathrm{Br}}_3$ and a switch of MAE from off plane to in plane in ${\mathrm{Cr}}_2{\mathrm{I}}_3{\mathrm{Cl}}_3$ are induced by a compressive strain of $2\%$. Microscopically, DMI and MAE are associated mainly with the large spin-orbit coupling of the heavy nonmagnetic halogen atoms rather than that of the magnetic Cr atoms. In particular, the peculiar magnetic transition of ${\mathrm{Cr}}_2{\mathrm{I}}_3{\mathrm{Cl}}_3$ is explained by competition between direct exchange and superexchange interactions. Micromagnetic simulations show that a small external magnetic field of 65 mT stabilizes a skyrmion with a diameter of 9.8 nm in the ${\mathrm{Cr}}_2{\mathrm{I}}_3{\mathrm{Cl}}_3$ monolayer. Our results will provide guidance for further research on DMI and skyrmions in 2D Janus materials as well as a basis for the potential applications in spintronic devices.

Periodic driving has been established as a powerful technique for engineering novel phases of matter and intrinsically out-of-equilibrium phenomena such as time crystals. Recent paper by Bluvstein et al. [Science 371, 1355 (2021)] has demonstrated that periodic driving can also lead to a significant enhancement of quantum many-body scarring, whereby certain nonintegrable systems can display persistent quantum revivals from special initial states. Nevertheless, the mechanisms behind driving-induced scar enhancement remain poorly understood. Here we report a detailed study of the effect of periodic driving on the PXP model describing Rydberg atoms in the presence of a strong Rydberg blockade—the canonical static model of quantum many-body scarring. We show that periodic modulation of the chemical potential gives rise to a rich phase diagram, with at least two distinct types of scarring regimes that we distinguish by examining their Floquet spectra. We formulate a toy model, based on a sequence of square pulses, that accurately captures the details of the scarred dynamics and allows for analytical treatment in the large-amplitude and high-frequency driving regimes. Finally, we point out that driving with a spatially inhomogeneous chemical potential allows to stabilize quantum revivals from arbitrary initial states in the PXP model, via a mechanism similar to prethermalization.

Inhomogeneous broadening is well known to hinder individual characteristics of emitters, supplanting the single-particle properties by their broader probability distribution. Here, we present an analysis of the emission spectra of mesoscopic ensembles of inhomogeneously distributed emitters below the thermodynamic limit (${10}^1\text{−}{10}^4$ emitters). Based on a simple analytical model and an extensive numerical analysis, we show that the number and individual linewidths of the emitters can be directly estimated from the ensemble autocorrelation function in spite of an inhomogeneously broadened emission. As an application, we analyze the photoluminescence of colloidal nanocrystal aggregates embedded in a gold shell. Our general method can be applied to a wide range of mesoscopic many-body systems and could provide new insights into their first-order coherence properties.

We have studied experimentally spin-dependent photon echoes from excitons in an InGaAs/GaAs quantum well subject to a transverse magnetic field (Voigt geometry). Larmor precession of the spins of the electron and heavy hole in an exciton leads to a periodic transfer of coherence between bright and dark exciton states. The increase in dephasing time due to the transition to the dark states could be useful for coherent control development. A comprehensive analysis shows a good agreement between a four-wave mixing experiment and the predictions of a theoretical treatment based on a five-level exciton model comprising a ground state and two pairs of bright and dark states. The extracted optical dephasing time of bright and dark excitons is equal to 30 and 130 ps, respectively. Exploiting the photon echo reveals evidence of electron Larmor precession with in-plane $g$ factor $|g_{e,\perp }|=0.44\pm 0.05$ and heavy hole precession as well. The precession frequency of the latter depends nonlinearly on the applied magnetic field, and the corresponding $g$ factor reaches a value of $|g_{h,\perp }|\approx 0.3$ at $B=6$ T. Estimates for the heavy hole $g$-factor spreading as well as the isotropic exchange interaction constant are provided.

We have observed the motion of two-dimensional phases of Au on Ge(111) under an electric bias by in operando low energy electron microscopy. Electromigration of Au results in a complex dynamics that depends on the nature of the involved mobile phases: two-dimensional adatom gas or two-dimensional islands. We show that the Au adatoms move at the surface in the direction opposite to the electric current. The wind force induced by the electron flow is measured: the effective valence of Au ($Z^{*}=-82\pm 15$) is directly deduced from the coverage profile of a Au adatom gas spatially retained by a strong Erlich-Schwoebel barrier at a downhill step edge. The velocity of two-dimensional Au islands versus island size reveals a mass transport by terrace diffusion inside the islands. The energy barrier for diffusion above 820 K is $1.16\pm 0.08$ eV and it strongly increases up to $3.1\pm 0.6$ eV below. We attribute this change of regime to a modification of diffusing species from single Au atoms at high temperature to Au clusters at low temperature. The strong shape fluctuations of the 2D islands is consistent with a nearly vanishing line tension of $1.2\pm 0.4$ meV/nm at 800 K.

Magnetoelectric multiferroic materials are the potential candidates for the high-density nonvolatile data storage devices. However, up to now, multiferroic materials with strong magnetoelectric coupling are still rare. Here, based on the first-principles calculations and theoretical model, we predict a different class of single phase multiferroic materials, transition metal phosphorus chalcogenides $TM{\mathrm{P}}_2{X}_6\left(T=\mathrm{Cu},\mathrm{Ag};M=\mathrm{Cr},\mathrm{V};X=\mathrm{S},\mathrm{Se}\right)$ with multiple polarization phases and strong magnetoelectric coupling. The ferroelectric polarization originates from the movement of Cu/Ag atoms breaking the symmetry of spatial inversion and the magnetism arises from partially filled $d$ orbitals of the V/Cr atoms. It is predicted that the different ferroelectric phases of $TM{\mathrm{P}}_2{X}_6$ bulk have different band gaps, providing a way to control electronic and transport properties by the external electric field. Most prominently, for $\mathrm{CuV}{\mathrm{P}}_2{\mathrm{S}}_6$ bilayer and few layers, one of the ferroelectric phases has ferromagnetic ground state and the other has antiferromagnetic states, realizing the electric-field control of magnetism. We reveal that the physical mechanism of the strong magnetoelectric coupling is from the reduced dimension and symmetry by constructing a theoretical model including the crystal field splitting, electric polarization effect, and exchange interaction. This work not only predicts a different class of magnetoelectric multiferroic materials, but also proposes a strategy to design them by controlling the interlayer interaction in van der Waals layered materials.

Real Clifford algebras play a fundamental role in the eight real Altland-Zirnbauer symmetry classes and the classification tables of topological phases. Here, we present another elegant realization of real Clifford algebras in the $d$-dimensional spinless rectangular lattices with $\pi$ flux per plaquette. Due to the $T$-invariant flux configuration, real Clifford algebras are realized as projective symmetry algebras of lattice symmetries. Remarkably, $d$ mod 8 exactly corresponds to the eight Morita equivalence classes of real Clifford algebras with eightfold Bott periodicity, resembling the eight real Altland-Zirnbauer classes. The representation theory of Clifford algebras determines the degree of degeneracy of band structures, both at generic $k$ points and at high-symmetry points of the Brillouin zone. Particularly, we demonstrate that the large degeneracy at high-symmetry points offers a rich resource for forming topological states by various dimerization patterns, including a three-dimensional ($3\mathrm{D}$) higher-order semimetal state with double-charged bulk nodal loops and hinge modes, a $4\mathrm{D}$ nodal surface semimetal with $3\mathrm{D}$ surface solid-ball zero modes, and $4\mathrm{D}$ Möbius topological insulators with an eightfold surface nodal point or a fourfold surface nodal ring. Our theory can be experimentally realized in artificial crystals by their engineerable ${\mathbb{Z}}_2$ gauge fields and capability to simulate higher-dimensional systems.

We theoretically investigate topological spin transport of the magnon polarons in a bilayer magnet with two-dimensional square lattices. Our theory is motivated by recent reports on the van der Waals magnets which show the reversible electrical switching of the interlayer magnetic order between antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction opens band gaps and allows the interband transition between different excitation states. In the layered antiferromagnet, due to the interband transition between the magnon-polaron states, the spin Berry curvature which allows the topological spin transport occurs even if the time-reversal symmetry is preserved. We find that the spin Berry curvature in the layered antiferromagnet is very large due to the small energy spacing between two magnonlike states. As a result, the spin Nernst conductivity shows a sudden increase (or decrease) at the phase transition point between ferromagnetic and antiferromagnetic phases. Our results suggest that the ubiquity of tunable topological spin transports in two-dimensional magnetic systems.

We study the Hatano-Nelson model, i.e., a one-dimensional non-Hermitian chain of spinless fermions with nearest-neighbor nonreciprocal hopping, in the presence of repulsive nearest-neighbor interactions. At half filling, we find two $\mathcal{PT}$ transitions, as the interaction strength increases. The first transition is marked by an exceptional point between the first and the second excited state in a finite-size system and is a first-order symmetry-breaking transition into a charge-density wave regime. Persistent currents characteristic of the Hatano-Nelson model abruptly vanish at the transition. The second transition happens at a critical interaction strength that scales with the system size and can thus only be observed in finite-size systems. It is characterized by a collapse of all energy eigenvalues onto the real axis. We further show that in a strong interaction regime, but away from half filling, the many-body spectrum shows point gaps with nontrivial winding numbers, akin to the topological properties of the single-particle spectrum of the Hatano-Nelson chain, which indicates the skin effect of extensive many-body eigenstates under open boundary conditions. Our results can be applied to other models such as the non-Hermitian Su-Schrieffer-Heeger-type model and contribute to an understanding of fermionic many-body systems with non-Hermitian Hamiltonians.

We investigate the spin-dynamic properties of two carbon cross structures assembled from two carbon chains with two Ni atoms attached. Both the global spin transfer through the long-distance carbon cross and the local spin flip on the single Ni atom are achieved within the subpicosecond regime, demonstrating the feasibility of individual intrasite and intersite spin manipulation. We investigate the dependence of the two different types of spin-dynamics processes on an external magnetic field, and we compare them by analyzing the spin features of the populated intermediate states. The comparison explains well the addressability of the local spin flip and the conservation of the global spin transfer. In addition, by applying two identical laser pulses, we successfully enhance the sensitivity of the local spin flip with respect to the external magnetic field, which yields a higher spatial resolution of the individual spin addressing. The present investigation is a significant step towards integrated all-optical logic circuits based on nano-spintronics.