Photoluminescence spectra show that monolayer transition-metal dichalcogenides (ML-TMDs) possess charged exciton binding energies, conspicuously similar to the energy of optical phonons. This enigmatic coincidence has offered opportunities to investigate many-body interactions between trion $X_{-}$, exciton $X$, and phonon and led to efficient excitonic anti-Stokes processes with the potential for laser refrigeration and energy harvesting. In this study, we show that in ${\mathrm{WSe}}_2$ materials, the trion binding energy matches two phonon modes, the out-of-plane $A_1^{\prime }$ and the in-plane $E^{\prime }$ modes. In this respect, using the Fermi golden rule together with the effective mass approximation, we investigate the rate of the population transfers between $X$ and $X_{-}$, mediated by a single phonon. We demonstrate that, while the absolute importance of the two phonon modes on the up-conversion process strongly depends on the experimental conditions such as the temperature and the dielectric environment (substrate), both modes lead to an up-conversion process on time scales in the range of few picoseconds to subnanoseconds, consistent with recent experimental findings. The conjugate process is also investigated in our study, as a function of temperature $T$ and electron density $N_e$. We prove that the exciton to trion down-conversion process is very unlikely at low electron density $N_e<{10}^{10}{\text{cm}}^{-2}$ and high temperature $T>50\text{K}$ while it increases dramatically to reach few picoseconds time scale at low temperature and for electron density $N_e>{10}^{10}{\text{cm}}^{-2}$. Finally, our results show that the conversion processes occur more rapidly in exemplary monolayer molybdenum-based dichalcogenides (${\mathrm{MoSe}}_2$ and ${\mathrm{MoTe}}_2$) than tungsten dichalcogenides.

The optical properties of infinite planar array of scattering particles, metasurfaces and metagratings, are attracting special attention lately for their rich phenomenology, including both plasmonic and high-refractive-index dielectric meta-atoms with a variety of electric and magnetic resonant responses. Herein we derive a coupled electric and magnetic dipole analytical formulation to describe the reflection and transmission of such periodic arrays, including specular and diffractive orders, valid in the spectral regimes where only dipolar multipoles are needed. The two-dimensional lattice Green function is rewritten in terms of a one-dimensional (chain) version that fully converges in the complex frequency plane and can be easily calculated. Modes emerging as poles of such lattice Green function can be extracted, as evidenced by calculating resonances and bound states in the continuum for an array of Si spheres. This formulation can be applied to investigate a wealth of plasmonic, all-dielectric, and hybrid metasurfaces and metagratings of interest throughout the electromagnetic spectrum.

Detailed theoretical study of the magneto-optical properties of weakly confining GaAs/AlGaAs quantum dots is provided. We focus on the diamagnetic coefficient and the $g$ factor of the neutral and the charged excitonic states, respectively, and their evolution with various dot sizes for the magnetic fields applied along the [001] direction. For the calculations we utilize the combination of $\mathbf{k}·\mathbf{p}$ and the configuration interaction methods. We decompose the theory into four levels of precision, i.e., (i) single-particle electron and hole states, (ii) noninteracting electron-hole pair, (iii) electron-hole pair constructed from the ground state of both quasiparticles and interacting via the Coulomb interaction (i.e., with minimal amount of correlation), and (iv) that including the effect of correlation. The aforementioned approach allows us to pinpoint the dominant influence of various single-particle and multiparticle effects on the studied magneto-optical properties, allowing the characterization of experiments using models which are as simple as possible, yet retaining the detailed physical picture.

We explore the moiré magnon bands in twisted bilayer magnets with next-nearest-neighbor Dzyaloshinskii-Moriya interactions, assuming that the out-of-plane collinear magnetic order is preserved under weak interlayer coupling. By calculating the magnonic band structures and the topological Chern numbers for four representative cases, we find that (i) the valley moiré bands are extremely flat over a wide range of continuous twist angles, (ii) the topological Chern numbers of the lowest few flat bands vary significantly with the twist angle, and (iii) the lowest few topological flat bands in bilayer antiferromagnets entail nontrivial thermal spin transport in the transverse direction. These properties make twisted bilayer magnets an ideal platform to study the magnonic counterparts of moiré electrons, where the statistical distinction between magnons and electrons leads to fundamentally new physical behavior.

By means of spin current, the flow of spin angular momentum, we find a regime of “spin treacle” in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conversion yield between spin current and charge current, is independent of temperature. The spin Hall angle starts to decrease at a certain temperature $T^{*}$ and completely vanishes at a lower temperature. We argue that the latter corresponds to the spin freezing temperature $T_{\mathrm{f}}$ of the nanometer-scale spin glass, where the direction of conduction electron spin is randomized by the exchange coupling with the localized moments. The present experiment quantitatively verifies the existence of a distinct spin treacle between $T_{\mathrm{f}}$ and $T^{*}$. We have also quantified a timescale of fluctuation of local magnetic moments in the spin treacle from the spin relaxation time of conduction electrons.

Nitrogen doping is a promising approach to induce the basal-plane localized magnetic moments in $\mathit{sp}$-electron-only graphene sheets. Enhancing the magnetic couplings among the moments can contribute to the high Curie temperature $T_C$ and large ratio of ferromagnetism (FM), which is vital for real applications in spintronic devices; however, it is still challenging. Here, we experimentally demonstrate that graphitic nitrogen (N-Q) could enhance the ferromagnetic couplings of localized magnetic moments provided by the defective N in nitrogen-doped graphene (NG). Nearly pure FM at 2 K is obtained in ${\mathrm{NG}}_{16.04}$ (N-Q/N = 16.04 at. %) with a $T_C$ up to 397.5 K, while the room-temperature saturated magnetization is high as $\sim 0.4$ emu/g. The rich $\mathrm{C}\to \mathrm{N}$ electrons contributed by the high ratio of the N-Q complex dominantly devote itinerant magnetism and enhance the interactions among the pyrrolic N (N-5) and/or pyridinic N complexes. Our density functional theory simulation results provide the configurations of (trimerized N-5)-triazinic N-Q along certain directions with enhanced sublattice-independent FM, which are proposed to be the magnetic structures in ${\mathrm{NG}}_{16.04}$. Our investigation reveals that introducing the N-Q can serve as an ideal route for the long-range spin correlations and thus full ferromagnetic couplings in graphene.

We control the transition frequency of a superconducting flux qubit coupled to a frequency-tunable resonator comprising a direct current superconducting quantum interference device (dc SQUID) by microwave driving. The dc SQUID mediates the coupling between microwave photons in the resonator and a flux qubit. The polarity of the frequency shift depends on the sign of the flux bias for the qubit and can be both positive and negative. The absolute value of the frequency shift becomes larger by increasing the photon number in the resonator. These behaviors are reproduced by a model considering the magnetic interaction between the flux qubit and dc SQUID. The tuning range of the transition frequency of the flux qubit reaches $\approx 1.9\mathrm{GHz}$. The effect of photon number fluctuation in the resonator to the dephasing rate of the flux qubit is also discussed.

We introduce higher-order topological Dirac superconductor (HOTDSC) as a gapless topological phase of matter in three dimensions, which extends the notion of Dirac phase to a higher-order topological version. Topologically distinct from the traditional topological superconductors and known Dirac superconductors, a HOTDSC features Majorana hinge modes between adjacent surfaces, which are direct consequences of the symmetry-protected higher-order band topology manifesting in the system. Specifically, we show that rotational, spatial inversion, and time-reversal symmetries together protect the coexistence of bulk Dirac nodes and hinge Majorana modes in a seamless way. We define a set of topological indices that fully characterizes the HOTDSC. We further show that a practical way to realize the HOTDSC phase is to introduce unconventional odd-parity pairing to a three-dimensional Dirac semimetal while preserving the necessary symmetries. As a concrete demonstration of our idea, we construct a corresponding minimal lattice model for HOTDSC obeying the symmetry constraints. Our model exhibits the expected topological invariants in the bulk and the defining spectroscopic features on an open geometry, as we explicitly verify both analytically and numerically. Remarkably, the HOTDSC phase offers an example of a “higher-order topological quantum critical point, which enables realizations of various higher-order topological phases under different symmetry-breaking patterns. In particular, by breaking the inversion symmetry of a HOTDSC, we arrive at a higher-order Weyl superconductor, which is yet another gapless topological state that exhibits hybrid higher-order topology.

The magnetic response of $\mathrm{Ca}\mathrm{K}{\left({\mathrm{Fe}}_{0.949}{\mathrm{Ni}}_{0.051}\right)}_4{\mathrm{As}}_4$ was investigated by means of the muon spin rotation/relaxation. The long-range commensurate magnetic order sets in below the Néel temperature $T_{\mathrm{N}}=50.0\left(5\right)$ K. The density-functional theory calculations have identified three possible muon stopping sites. The experimental data were found to be consistent with only one type of magnetic structure, namely, the long-range magnetic spin-vortex-crystal order with the hedgehog motif within the $ab$ plane and the antiferromagnetic stacking along the $c$ direction. The value of the ordered magnetic moment at $T\approx 3$ K was estimated to be $m_{\mathrm{Fe}}=0.38\left(11\right){\mu }_{\mathrm{B}}$ (${\mu }_{\mathrm{B}}$ is the Bohr magneton). A microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature $T_{\mathrm{c}}\simeq 9$ K is observed. Comparison with 11, 122, and 1144 families of Fe-based pnictides points to existence of correlation between the reduction of the magnetic order parameter at $T\to 0$ and the ratio of the transition temperatures $T_{\mathrm{c}}/T_{\mathrm{N}}$. Such correlations were found to be described by Machida's model for coexistence of itinerant spin-density-wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195 (1981); S. L. Bud'ko et al., Phys. Rev. B 98, 144520 (2018)].

It has recently been shown that the ab initio anharmonic free energy of fcc crystals can be approximated to meV/atom accuracy by a lattice of anharmonic nearest-neighbor bonds, where the bonding potential can be efficiently parametrized from the target system. We develop a mean-field approach for the free energy of a general bond lattice, analytically accounting for strong bond-bond correlations while enforcing material compatibility and thermodynamic self-consistency. Applying our fundamentally anharmonic model to fcc crystals yields free energies within meV/atom of brute force thermodynamic integration for core seconds of computational effort. Potential applications of this approach in computational materials science are discussed.

It is widely known that spin-locking noise spectroscopy is a powerful technique for the characterization of low-frequency noise mechanisms in superconducting qubits. Here we show that the relaxation rate of the driven spin-locking state of a qubit can be significantly affected by the presence of an off-resonant high-frequency two-level-system defect. Thus, both low- and high-frequency defects should be taken into account in the interpretation of spin-locking measurements and other types of driven-state noise spectroscopy.

Ferromagnetic resonance (FMR) measurements using a broadband microstrip excitation technique were performed at room temperature for an antiferromagnetic/ferromagnetic (AF/FM) bilayer of ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}\left(35.0\mathrm{nm}\right)/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}\left(7.5\mathrm{nm}\right)$. The angular dependence of the resonance field was measured for several excitation frequencies ranging from $1.5$ to $10.0$ GHz. From the numerical fitting of the experimental data, it was observed that the exchange bias field $\left(H_E\right)$ and the rotatable anisotropy field $\left(H_{\mathrm{RA}}\right)$ originating from the exchange interaction at the AF/FM interface are frequency dependent. The results were explained considering the coexistence of ferromagnetic and antiferromagnetic coupling at the AF/FM interface. We also investigated the spin pumping voltage generated in the structure of ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}\left(35.0\mathrm{nm}\right)/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}\left(7.5\mathrm{nm}\right)$/Pt(4 nm) and showed that from the voltage peaks occurring at the ferromagnetic resonance condition it is possible to reproduce the FMR dispersion relation.

In many interacting-electron systems, the microscopic spin angular momentum of electrons is conserved along with the macroscopic angular momentum in electrical current flows, i.e., vorticity. Such spin-vorticity coupling expands the choice of materials for spintronics devices. In this paper, we evaluate the magnitude of an alternating spin current generated by the spin-vorticity coupling with a gigahertz-order surface acoustic wave in a Cu thin film. We measure the gigahertz alternating spin current by an electrical method based on the inverse spin-Hall effect. From the amplitude of the spin current, we can determine the conversion efficiency of the angular momentum between local lattice rotation and electron spin in the Cu film. The conversion efficiency is four orders of magnitude larger than the case of spin current generation via kilohertz-order vorticity in turbulent flow of liquid mercury [M. Matsuo et al., Phys. Rev. B 96, 020401(R) (2017)]. Such a huge conversion efficiency is attributable to a smaller inconsistency of energy scale between lattice rotation and electron spin than the case of liquid vorticity.

Nanometer-scale magnetization configurations known as magnetic skyrmions have mostly been studied in cubic chiral helimagnets, in which they are Bloch-type and their axes align along the applied magnetic field. In contrast, the orientation of Néel-type skyrmions is locked to the polar axis of the host material's underlying crystal structure. In the lacunar spinels $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{S}}_8$ and $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{Se}}_8$, the Néel-type skyrmion lattice phase exists for externally applied magnetic fields parallel to this axis and withstands oblique magnetic fields up to some critical angle. Here, we map out the stability of the skyrmion lattice phase in both crystals as a function of field angle and magnitude using dynamic cantilever magnetometry. The measured phase diagrams reproduce the major features predicted by a recent theoretical model, including a reentrant cycloidal phase in $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{Se}}_8$. Nonetheless, we observe a greater robustness of the skyrmion phase to oblique fields, suggesting possible refinements to the model. Besides identifying transitions between the cycloidal, skyrmion lattice, and ferromagnetic states in the bulk, we measure additional anomalies in $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{Se}}_8$ and assign them to magnetic states confined to polar structural domain walls.

The “dipolar-octupolar” pyrochlore oxides ${\mathrm{R}}_2{\mathrm{M}}_2{\mathrm{O}}_7$ (R=Ce, Sm, Nd) represent an important opportunity in the search for three-dimensional quantum spin liquid (QSL) ground states. Their low-energy physics is governed by an alluringly simple ‘$‘XYZ$” Hamiltonian, enabling a theoretical description with only a small number of free parameters. Meanwhile, recent experiments on Ce pyrochlores strongly suggest QSL physics. Motivated by this, we present here a complete analysis of the ground-state phase diagram of dipolar-octupolar pyrochlores. Combining cluster mean-field theory, variational arguments, and exact diagonalization we find multiple $U\left(1\right)$ QSL phases, which together occupy a large fraction of the parameter space. These results give a comprehensive picture of the ground-state physics of an important class of QSL candidates and support the possibility of a $U\left(1\right)$ QSL ground state in ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$.

We demonstrate that a dynamical modification of the Hubbard $U$ in the model charge-transfer insulator NiO can be observed with state-of-the-art time-resolved absorption spectroscopy. Using a self-consistent time-dependent density functional theory plus $U$ computational framework, we show that the dynamical modulation of screening and Hubbard $U$ significantly changes the transient optical spectroscopy. Whereas we find the well-known dynamical Franz-Keldysh effect when the $U$ is frozen, we observe a dynamical band-gap renormalization for dynamical $U$. The renormalization of the optical gap is found to be smaller than the renormalization of $U$. This work opens up the possibility of driving a light-induced transition from a charge transfer into a Mott insulator phase.

We numerically study the physical properties of quasiperiodic superconductors with the aim of understanding superconductivity in quasicrystals. Considering the attractive Hubbard model on the Penrose tiling as a simple theoretical model, we calculate various basic superconducting properties and find deviations from the universal values of the Bardeen-Cooper-Schrieffer theory with a constant density-of-states approximation. In particular, we find that the jump of the specific heat at the superconducting transition is about 10%–20% smaller than that universal value, consistent with the experimental results obtained for the superconducting Al-Mg-Zn quasicrystalline alloy. Furthermore, we calculate current-voltage characteristics and find that the current gradually increases with the voltage on the Penrose tiling in contrast to a rapid increase in the periodic system. These distinctions originate from the nontrivial Cooper pairing characteristic of the quasiperiodic system.

We develop a Markovian master equation in the Lindblad form that enables the efficient study of a wide range of open quantum many-body systems that would be inaccessible with existing methods. The validity of the master equation is based entirely on properties of the bath and the system-bath coupling, without any requirements on the level structure within the system itself. The master equation is derived using a Markov approximation that is distinct from that used in earlier approaches. We provide a rigorous bound for the error induced by this Markov approximation; the error is controlled by a dimensionless combination of intrinsic correlation and relaxation timescales of the bath. Our master equation is accurate on the same level of approximation as the Bloch-Redfield equation. In contrast to the Bloch-Redfield approach, our approach ensures preservation of the positivity of the density matrix. As a result, our method is robust, and can be solved efficiently using stochastic evolution of pure states (rather than density matrices). We discuss how our method can be applied to static or driven quantum many-body systems, and illustrate its power through numerical simulation of a spin chain that would be challenging to treat by existing methods.

Mie-resonant high-index dielectric particles are at the core of modern all-dielectric photonics. In many situations, their response to the external fields is well captured by the dipole model which neglects the excitation of higher-order multipoles. In that case, it is commonly assumed that the dipole moments induced by the external fields are given by the product of particle polarizability tensor and the field in the particle center. Here, we demonstrate that the dipole response of nonspherical subwavelength dielectric particles is significantly more complex since the dipole moments are defined not only by the field in the particle center but also by the second-order spatial derivatives of the field. As we prove, such nonlocal response is especially pronounced in the vicinity of anapole minimum in the scattering cross section. We examine the excitation of high-index dielectric disk in microwave domain and silicon nanodisk in near infrared applying group-theoretical analysis and retrieving the nonlocal corrections to the dipole moments. Extending the discrete dipole model to include nonlocality of the dipole response, we demonstrate an improved agreement with full-wave numerical simulations. These results provide important insights into metaoptics of Mie-resonant nonspherical particles as well as metamaterials and metadevices based on them.

The interplay between magnetism and nontrivial topology in magnetic topological insulators (MTIs) is expected to give rise to exotic topological quantum phenomena like the quantum anomalous Hall effect and the topological axion states. A key to assessing these novel properties is to realize gapped topological surface sates. $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ possesses nontrivial band topology with an intrinsic antiferromagnetic state. However, the highly electron-doped nature of the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ crystals obstructs the exhibition of the surface band gap. Here, we tailor the material through Sb substitution to reveal the gapped surface states in $\mathrm{Mn}{\mathrm{Bi}}_{2\text{−}x}{\mathrm{Sb}}_x{\mathrm{Te}}_4$. By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.