Solid-state quantum emitters are playing a crucial role in the development of photonic quantum technologies. However, several decoherence mechanisms are known to impact the device performances and scalability. In various quantum emitter materials the fluctuation of the two-level system has been investigated by different methods to capture the evolution of coherence reduction. However, resolving the dynamics of spectral diffusion in the fundamental framework of on-demand single-photon generation remained unsolved. Hereby, the challenge is to observe with high time and energy resolution the impact of fluctuations directly in the properties of the emitted photons. In this Rapid Communication, we demonstrate the use of dispersion in a slow-light medium to map the photons frequency domain into time domain, observed as frequency-dependent time-of-flight. This allows for the measurement of the emission spectrum and the quantification of the spectral diffusion dynamics in one intensity correlation measurement. On exemplary semiconductor quantum dots, the impact of charge and spin noise on the spectral diffusion are revealed to follow an Ornstein-Uhlenbeck process. By a single measurement, broadening from the excitation repetition up to the stationary limit is resolved. This enables one to extract time-dependent two-photon interference visibilities for various timescales, which is a key performance measure for quantum emitters.

Weyl semimetals exhibit exotic Fermi-arc surface states, which strongly affect their electromagnetic properties. We derive analytical expressions for all components of the composite density-spin response tensor for the surfaces states of a Weyl-semimetal model obtained by closing the band gap in a topological insulating state and introducing a time-reversal-symmetry-breaking term. Based on the results, we discuss the electromagnetic susceptibilities, the current response, and other physical effects arising from the density-spin response. We find a magnetoelectric effect caused solely by the Fermi arcs. We also discuss the effect of electron-electron interactions within the random phase approximation and investigate the dispersion of surface plasmons formed by Fermi-arc states. Our work is useful for understanding the electromagnetic and optical properties of the Fermi arcs.

The polaron concept captures physical situations involving an itinerant quantum particle (excitation) that interacts strongly with bosonic degrees of freedom and becomes heavily boson-dressed. While the Gerlach-Löwen theorem rules out the occurrence of nonanalyticities of ground-state-related quantities for a broad class of polaron models, examples were found in recent years of sharp transitions pertaining to strongly momentum-dependent interactions of an excitation with dispersionless (zero-dimensional) phonons. On the example of a lattice model with Peierls-type excitation-phonon interaction, such level-crossing-type small-polaron transitions are analyzed here through the prism of the entanglement spectrum of the excitation-phonon system. By evaluating this spectrum in a numerically exact fashion it is demonstrated that the behavior of the entanglement entropy in the vicinity of the critical excitation-phonon coupling strength chiefly originates from one specific entanglement-spectrum eigenvalue, namely the smallest one. While the discrete translational symmetry of the system implies that those eigenvalues can be labeled by the bare-excitation quasimomentum quantum numbers, here it is shown numerically that they are predominantly associated with the quasimomenta 0 and $\pi$, including cases where a transition between the two takes place deeply in the strong-coupling regime.

Electrons in a lattice exhibit time-periodic motion, known as Bloch oscillation, when subject to an additional static electric field. Here, we show that a corresponding dynamics can occur upon replacing the spatially periodic potential by a time-periodic driving: Floquet oscillations of charge carriers in a spatially homogeneous system. The time lattice of the driving gives rise to Floquet bands that take on the role of the usual Bloch bands. For two different drivings (harmonic driving and periodic kicking through pulses) of systems with linear dispersion we demonstrate the existence of such oscillations, both by directly propagating wave packets and based on a complementary Floquet analysis. The Floquet oscillations feature richer oscillation patterns than their Bloch counterpart and enable the imaging of Floquet bands. Moreover, their period can be directly tuned through the driving frequency. Such oscillations should be experimentally observable in effective Dirac systems, such as graphene, when illuminated with circularly polarized light.

The magneto-optic Voigt effect is observed in a synthetic diamond membrane with a substitutional nitrogen defect concentration in the order of 200 ppm and a nitrogen-vacancy defect subensemble generated through neutron irradiation and annealing. The measured polarization rotation in the reflected light is observed to be quadratically proportional to the applied magnetic field and to the incident reflection angle. Additionally, it is observed to be modifiable by illuminating the diamond with a 532 nm laser. Spectral analysis of the reflected light under 532 nm illumination shows a slow narrowing of the spectral distribution, indicating a small increase in the overall magnetization, as opposed to magnetization degradation caused by heating. Further analysis of the optical power dependence suggest this may be related to a shift in the spin ensembles charge state equilibrium and, by extension, the resulting ensemble magnetization.

In the present study, the martensitic transformation (MT) and magnetic properties exhibited by the Ni-Mn-Sn Heusle-type magnetic shape memory alloys (MSMAs) doped with Zn have been investigated experimentally and theoretically. The inverse magnetocaloric effect (MCE) in ${\text{Ni}}_{47}{\text{Mn}}_{40}{\text{Sn}}_{13-x}{\text{Zn}}_x$ (x = 0, 1) was studied by direct measurements of the adiabatic temperature change, $\mathrm{\Delta }{T}_{\text{ad}}$, in pulsed magnetic fields of 5, 10, and 20 T. The Zn doping of the Ni-Mn-Sn alloy led to a striking enhancement of the value of $\mathrm{\Delta }{T}_{\text{ad}}$, e.g., from –2.5 for undoped to –11 K for Zn-doped alloys under a magnetic field amplitude of 20 T. The first-principles calculations were used to understand the origin of Zn-doping influence on MT, magnetic, and magnetocaloric properties. Particularly, the crystal structure and magnetic ordering influenced by the site occupancy in the undoped and Zn-doped alloys were analyzed. The results show that, whereas the usual transition metal elements with more valence electrons tend to enter the Ni sites, Zn atom prefers to occupy the Sn sublattice. The underlying physics of the drastic enhancement of MCE by Zn doping is discussed in terms of a partial disorder in the occupation sites of Zn atoms.

We scrutinize the magnetic properties of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$ through its first-order metal-insulator transition at $T_{\mathrm{CO}}=30\mathrm{K}$ by means of ${}^1\mathrm{H}$ nuclear magnetic resonance (NMR). While in the metallic state we find Fermi-liquid behavior with temperature-independent ${\left(T_{1}T\right)}^{-1}$, the relaxation rate exhibits a pronounced enhancement when charge order sets in. The NMR spectra remain unchanged through the transition and we find no evidence for magnetic order down to 25 mK. Similar to the isostructural spin-liquid candidates $\kappa {\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ and $\kappa {\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$, $T_1^{-1}$ acquires a dominant maximum (here around 5 K). An examination of the field dependence identifies the low-temperature feature as a dynamic inhomogeneity contribution that is typically dominant over the intrinsic relaxation but is suppressed with increasing magnetic field.

We propose a way to include magnetic dipole-dipole interactions in density functional theory calculations. To this end, we derive an approximation to the exchange-correlation energy functional associated with the spin-spin correction to the Coulomb force in the Breit-Pauli Hamiltonian. The local spin-density approximation is shown to be identically zero. First order nonlocal corrections are evaluated analytically within linear response to a noncollinear external magnetic field. The functional obtained is based on the exact-exchange energy of the magnetic electron gas with dipolar interactions and is estimated to be relevant at interatomic distances, or in the low electron density limit, where it amounts to one quarter of the magnetostatic energy. We expect our functional to improve over the current description of ground-state properties of inhomogeneous magnetic structures at the nanoscale and dipolar spin systems.

We present a data set of calculated Curie temperatures for the main-phase compounds of rare-earth permanent magnets. We employ ab initio electronic structure calculations for itinerant ferromagnetism and an effective spin model for finite-temperature magnetism. Curie temperatures are derived on the basis of a classical Heisenberg model mapped via Liechtenstein's formula for atomic pairwise exchange couplings. Relative trends with respect to the species of rare-earth elements in calculated Curie temperatures for $R_2{\mathrm{Fe}}_{14}\mathrm{B}$ are in agreement with experimental trends. A quantitative comparison between calculation and experimental data found in the literature points to an effective range of exchange couplings imposing a limit on the validity range of the effective spin model.

Artificial spin ices are arrays of correlated nanoscale magnetic islands that prove an excellent playground in which to study the role of topology in critical phenomena. Here, we investigate a continuum of spin ice geometries, parametrized by rotation of the islands. In doing so, we morph from the classic square ice to the recently studied pinwheel geometry, with the rotation angle acting as a proxy for controlling interisland interactions. We experimentally observe a transition from antiferromagnetic ordering in square ice to a slight preference for ferromagnetic vertices in the weakly coupled pinwheel ice using Lorentz transmission electron microscopy on thermally annealed cobalt arrays. The rotation angle also affects the relaxation timescales for individual arrays, leading to varying degrees of thermalization, and an apparent change in the nature of the defects supported: from one-dimensional strings in square ice to two-dimensional vortexlike structures for geometries similar to pinwheel. The numerical scaling of these quantities is consistent with that predicted by the Kibble-Zurek mechanism. Our results show how magnetic order in artificial spin ices can be tuned by changes in geometry and suggest the possibility of realizing a truly frustrated ice-rule phase in two-dimensional systems. Furthermore, we demonstrate this system as a test bed to investigate out-of-equilibrium dynamics across phases.

The outstanding transport properties expected at the edge of two-dimensional time-reversal invariant topological insulators have proven to be challenging to realize experimentally, and have so far only been demonstrated in very short devices. In search for an explanation to this puzzling observation, here we report a full first-principles calculation of topologically protected transport at the edge of novel quantum spin Hall insulators—specifically, bismuth and antimony halides—based on the nonequilibrium Green's functions formalism. Our calculations unravel two different scattering mechanisms that may affect two-dimensional topological insulators, namely, time-reversal symmetry breaking at vacancy defects and inter-edge scattering mediated by multiple cooperating impurities, possibly nonmagnetic. We discuss their drastic consequences for typical nonlocal transport measurements as well as strategies to mitigate their negative impact. Finally, we provide an instructive comparison of the transport properties of topologically protected edge states to those of the trivial edge states in ${\mathrm{MoS}}_2$ ribbons. Although we focus on a few specific cases (in terms of materials and defect types), our results should be representative for the general case and thus have significance beyond the systems studied here.

Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work, we report first-principles calculations and an analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence- and conduction-band edges employing density functional theory. Further, our calculated phonon renormalization of the $GW$-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperature. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.

The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to realize unconventional magnetic effects, which results in a demand for experimental techniques to determine the magnetization profile with nanoscale resolution. Magnetic small-angle neutron scattering (SANS) probes both the chemical and magnetic nanostructure and is thus a powerful technique, e.g., for the characterization of magnetic nanoparticles. Here, we show that the conventionally used particle-matrix approach to describe SANS of magnetic particle assemblies, however, leads to a flawed interpretation. As a remedy, we provide general expressions for the field-dependent two-dimensional magnetic SANS cross section of correlated moments. It is shown that for structurally disordered ensembles the magnetic structure factor is in general, and contrary to common assumptions, (i) anisotropic also in zero field and (ii) that even in saturation the magnetic structure factor deviates from the nuclear one. These theoretical predictions explain qualitatively the intriguing experimental, polarized SANS data of an ensemble of dipolar-coupled iron oxide nanoparticles.

We examine the role of doped impurity atoms on the conductivity anisotropy of the spin-density wave state in iron pnictides. The conductivity is calculated in a self-consistent spin-density wave state with random impurities in a two-orbital model. We find that the increase in impurity concentration leads to an increased suppression of conductivity in both the antiferromagnetic and ferromagnetic directions. However, the conductivity anisotropy is larger in comparison to the Drude-weight anisotropy in the hole-doped regions. The difference between the two is explained in terms of the anisotropic scattering by the impurities. We demonstrate the effect of the anisotropic impurity scattering by calculating the modulation in the density of states introduced by a single impurity. It is also shown that an increase in the Drude-weight anisotropy with changing carrier concentration, which results mainly from the reconstructed band characteristics, may be directly linked to a similar anisotropy in the density-of-states modulation caused by the impurity atom.

A calculation of the photonic Green's tensor of a structure is at the heart of many photonic problems, but for nontrivial nanostructures, it is typically a prohibitively time-consuming task. Recently, a general normal-mode expansion (GENOME) was implemented to construct the Green's tensor from eigenpermittivity modes. Here, we employ GENOME to study the response of a cluster of nanoparticles. To this end, we use the rigorous mode-hybridization theory derived earlier by D. J. Bergman [Phys. Rev. B 19, 2359 (1979)], which constructs the Green's tensor of a cluster of nanoparticles from the sole knowledge of the modes of the isolated constituent. The method is applied to a scatterer with a nontrivial shape (namely, a pair of elliptical wires) within a fully electrodynamic setting and for the computation of the Purcell enhancement and Förster resonant energy transfer rate enhancement, showing good agreement with direct simulations. The procedure is general, is trivial to implement using standard electromagnetic software, and holds for arbitrary shapes and number of scatterers forming the cluster. Moreover, it is orders of magnitude faster than conventional direct simulations for applications requiring the spatial variation of the Green's tensor, promising wide use in quantum technologies, free-electron light sources, and heat transfer, among others.

We demonstrate tunable dissipative interactions between optically trapped exciton-polariton condensates. We apply annular shaped nonresonant optical beams to both generate and confine each condensate to their respective traps, pinning their natural frequencies. Coupling between condensates is realized through the finite escape rate of coherent polaritons from the traps leading to robust phase locking with neighboring condensates. The coupling is controlled by adjusting the polariton propagation distance between neighbors. This permits us to map out regimes of both strong and weak dissipative coupling, with the former characterized by clear in-phase and antiphase synchronization of the condensates. With robust single-energy occupation governed by dissipative coupling of optically trapped polariton condensates, we present a system that offers a potential optical platform for the optimization of randomly connected $XY$ Hamiltonians.

In this paper we employ finite difference time-domain (FDTD) calculations to demonstrate the enhancement of chiroptical activity in high refractive index dielectric slotted nanodisks. The chiral enhancement mechanism consists of pairing the electric and magnetic modes of the slit cavity with the resonances of the rest of the disk, i.e., the weak chiroptical activity ($\sim {10}^{-1}$) inside the chiral gap region (which is chiral itself) is enhanced through the interaction with the electric and magnetic resonances in the nanodisk. These results are extended to a two-dimensional periodic arrangement of slotted nanosdisks forming a metasurface. The chiral electromagnetic near field at the slotted region can be harnessed for increasing the asymmetry in energy absorption of circularly polarized light (CPL). This chiral enhancement makes this novel metasurface useful for sensing or to control the optical forces that can be exerted on chiral molecules placed in those slotted regions. The latter can be relevant to the pharmaceutical industry since distinct enantiomers of racemic drugs frequently exhibit different biological activities.

In the Falicov-Kimball model, a model for (annealed) disorder, we expect weak localization corrections to the optical conductivity. However, we get such weak localization effects only when employing a $pp$-ladder approximation in the dual fermion approach. In the full parquet approach, these $pp$ contributions are suppressed by $ph$-reducible diagrams. For the optical conductivity, we find that the $\overline{ph}$ channel yields the main contribution, even in the region where weak localization in the $pp$ ladder was indicated.