The lattice parameter of Ge films doped with phosphorus, arsenic, and antimony was measured as a function of the dopant concentration. When the observed trends are compared with similar measurements in doped Si, a clear pattern emerges in support of the Cargill-Keyes theory that computes the doping dependence of the lattice parameter as the sum of a purely electronic contribution, proportional to the absolute deformation potential for the states occupied by the dopant carriers, plus a size mismatch contribution that depends on the universal topological rigidity parameter for Si-Ge systems. It is shown that when considered from the same global perspective, ab initio calculations of the structural effects of doping are in remarkable agreement with the Cargill-Keyes theory.

We model the remarkable thin-film Ba-Ti-O structures formed by heat treatment of an initial perovskite ${\mathrm{BaTiO}}_3$ thin film on a Pt(111) surface. All structures contain a rumpled Ti-O network with all Ti threefold coordinated with O and with Ba occupying the larger. mainly ${\mathrm{Ti}}_7{\mathrm{O}}_7$, pores. The quasicrystal structure is a simple decoration of three types of tiles: square, triangle, and ${30}^{\circ }$ rhombus, with edge lengths 6.85 Å, joined edge-to-edge in a quasicrystalline pattern; observed periodic crystals in ultrathin film Ba-Ti-O are built from these and other tiles. Simulated STM images reproduce the patterns seen experimentally and identify the bright protrusions as Ba atoms. The models are consistent with all experimental observations.

We observe trion emission from suspended carbon nanotubes where carriers are introduced electrostatically using field-effect transistor structures. The trion peak emerges below the $E_{11}$ emission energy at gate voltages that coincide with the onset of bright exciton quenching. By investigating nanotubes with various chiralities, we verify that the energy separation between the bright exciton peak and the trion peak becomes smaller for larger diameter tubes. Trion binding energies that are significantly larger compared to surfactant-wrapped carbon nanotubes are obtained, and the difference is attributed to the reduced dielectric screening in suspended tubes.

We propose a protocol to preserve the spin state of rare-earth ions when they are optically cycled. This technique uses large magnetic fields to increase the probability of an optically excited ion returning to its initial spin state. This Zeeman enhanced cyclicity is shown to be applicable to non-Kramers ions in various crystals irrespective of the site symmetry. The specific example of ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ is investigated to demonstrate that the protocol can create closed optical transitions even where the point group symmetry of the site is $C_1$. In this example, the predicted cyclicity exceeds ${10}^4$. This high level of cyclicity extends the usefulness of rare-earth ion crystals for applications in quantum and classical information processing. We explore the use of this technique to enable single-ion, spin-state optical readout and the creation of ensemble-based spectral features that are robust against optical cycling.

Recently, several new materials exhibiting massless Dirac fermions have been proposed. However, many of these do not have the typical graphene honeycomb lattice, which is often associated with Dirac cones. Here, we present a classification of these different two-dimensional Dirac systems based on the space groups and discuss our findings within the context of a minimal two-band model. In particular, we show that the emergence of massless Dirac fermions can be attributed to the mirror symmetries of the materials. Moreover, we uncover several novel Dirac systems that have up to 12 inequivalent Dirac cones and show that these can be realized in (twisted) bilayers. Hereby, we obtain systems with an emergent $\text{SU}\left(2N\right)$ valley symmetry with $N=1,2,4,6,8,12$. Our results pave the way to engineer different Dirac systems, in addition to providing a simple and unified description of materials ranging from square and $\beta \text{-graphynes}$ to $Pmmn$ boron, ${\mathrm{TiB}}_2$, phosphorene, and anisotropic graphene.

While controlling particle diffusion in a confined geometry is a popular approach taken by both natural and artificial systems, it has not been widely adopted for controlling light transport in random media, where wave interference effects play a critical role. The transmission eigenchannels determine not only light propagation through the disordered system but also the energy concentrated inside. Here, we propose and demonstrate an effective approach to modify these channels, whose structures are considered to be universal in conventional diffusive waveguides. By adjusting the waveguide geometry, we are able to alter the spatial profiles of the transmission eigenchannels significantly and deterministically from the universal ones. In addition, evanescent channels may be converted to propagating channels by gradually increasing the waveguide cross-section. Our approach allows to control not only the transmitted and reflected light, but also the depth profile of energy density inside the scattering system. In particular geometries, perfect reflection channels are created, and their large penetration depth into the turbid medium as well as the complete return of probe light to the input end would greatly benefit sensing and imaging applications. Absorption along with geometry can be further employed for tuning the decay length of energy flux inside the random system, which cannot be achieved in a common waveguide with uniform cross-section. Our approach relies solely on confined geometry and does not require any modification of intrinsic disorder, thus it is applicable to a variety of systems and also to other types of waves.

We present a numerical study on the minimum reflection channel of a disordered waveguide and its modification by coherent amplification of light. The minimum reflection channel is formed by destructive interference of quasinormal modes at the front surface of a random medium. While the lowest reflection eigenvalue increases with optical gain in most random realizations, the minimum reflection channel can adjust its modal composition to enhance destructive interference and slow down the growth of reflectance with gain. Some of the random realizations display a further reduction of the minimum reflectance by adding optical gain. The differential amplification of the modes can make their destructive interference so effective that it dominates the amplitude growth of the modes, causing the reflectance to drop with gain in these cases. Therefore, the interplay between interference and amplification can further minimize light reflection from a strong scattering medium; indicating optical gain may provide an additional degree of freedom for coherent control of mesoscopic transport.

We report the observation of two pressure-induced quantum phase transitions in a $\mathrm{Yb}{\mathrm{B}}_6$ single crystal, i.e., from a topologically trivial semiconductor to an intermediate semimetal and then from the semimetal to a possible topologically nontrivial high-pressure gapped phase through in situ high-pressure transport and synchrotron x-ray diffraction measurements. Our high-pressure absorption results reveal that the mixed-valence state in pressurized $\mathrm{Yb}{\mathrm{B}}_6$ above 15 GPa plays an important role in the formation of the high-pressure state with a reopened gap. These high-pressure results may be helpful for shedding light on the intriguing relation between the topology and the $4f$ electrons in the rare-earth hexaborides.

We study the electrical behavior of multiferroic ${\mathrm{BiFeO}}_3$ by means of first-principles calculations. We do so by constraining a specific component of the electric displacement field along a variety of structural paths, and by monitoring the evolution of the relevant physical properties of the crystal along the way. We find a complex interplay of ferroelectric, antiferroelectric, and antiferrodistortive degrees of freedom that leads to an unusually rich electrical phase diagram, which strongly departs from the paradigmatic double-well model of simpler ferroelectric materials. In particular, we show that many of the structural phases that were recently reported in the literature, e.g., those characterized by a giant aspect ratio, can be in principle accessed via application of an external electric field starting from the $R3c$ ground state. Our results also reveal ways in which nonpolar distortions (e.g., the antiferrodistortive ones associated with rotations of the oxygen octahedra in the perovskite lattice) can be controlled by means of applied electric fields, as well as the basic features characterizing the switching between the ferroelectric and antiferroelectric phases of ${\mathrm{BiFeO}}_3$. We discuss the multimode couplings behind this wealth of effects, while highlighting the implications of our work as regards both theoretical and experimental literature on ${\mathrm{BiFeO}}_3$.

Commonly, excitons in semiconductors are regarded as a low-temperature, low carrier density phenomenon, becoming unstable as the temperature and carrier density increase. Contrary to the common expectation, our ultrafast conductivity and luminescence measurements in GaInN/GaN quantum wells reveal a highly efficient formation of radiative excitons from a high-density electron-hole plasma at room temperature, and provide a quantitative measure of the exciton fraction to reach more than 40% at a total carrier population as high as $\sim {10}^{13}{\mathrm{cm}}^{-2}$. Driven by the mass action of electrons and holes, this effect is believed to contribute to the extraordinarily high quantum efficiency of group-III nitride light emitters.

We introduce exactly solvable models of interacting (Majorana) fermions in $d\ge 3$ spatial dimensions that realize a new kind of fermion topological quantum order, building on a model presented by S. Vijay, T. H. Hsieh, and L. Fu [Phys. Rev. X 5, 041038 (2015)]. These models have extensive topological ground-state degeneracy and a hierarchy of pointlike, topological excitations that are only free to move within submanifolds of the lattice. In particular, one of our models has fundamental excitations that are completely stationary. To demonstrate these results, we introduce a powerful polynomial representation of commuting Majorana Hamiltonians. Remarkably, the physical properties of the topologically ordered state are encoded in an algebraic variety, defined by the common zeros of a set of polynomials over a finite field. This provides a “geometric” framework for the emergence of topological order.

We demonstrate a compact, spectrally selective, and tunable delay line for single photons emitted by quantum dots. This is achieved by fine-tuning the wavelength of the optical transitions of such “artificial atoms” into a spectral window in which a cloud of natural atoms behaves as a slow-light medium. By employing the ground-state fine-structure-split exciton confined in an InGaAs/GaAs quantum dot as a source of single photons at different frequencies and the hyperfine-structure-split $D_1$ transition of Cs-vapors as a tunable delay medium, we achieve a differential delay of up 2.4 ns on a 7.5-cm-long path for photons that are only $60\mu \mathrm{eV}$ (14.5 GHz) apart. To quantitatively explain the experimental data, we develop a theoretical model that accounts for both the inhomogeneous broadening of the quantum-dot emission lines and the Doppler broadening of the atomic lines. The concept we proposed here may be used to implement time-reordering operations aimed at erasing the “which-path” information that deteriorates entangled-photon emission from excitons with finite fine-structure splitting.

We present how macroscopically coherent ordering within a chiral state can be manifested in the physical properties within the context of an archetypical system—the chiral spin soliton lattice in a monoaxial chiral magnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$. Using magnetotransport measurements and state-of-the-art Lorentz electron microscopy, we demonstrate spin soliton confinement in $1-\mu \mathrm{m}$-wide grains with different crystalline chirality and discretized magnetoresistance in $10-\mu \mathrm{m}$-wide crystals. Discretization effects are found to be prominent when the system size is reduced to the order of $10\mu \mathrm{m}$ along the chiral axis. A consequence that we identify is a robust coherence of the chiral soliton lattice against deformation. The spin configuration at the grain boundaries, which leads to soliton confinement, is experimentally clarified.

The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semiconductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions, a systematic investigation of the role of screening on two-dimensional (2D) excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model, which assumes a dielectric function of the form $\epsilon \left(q\right)=1+2\pi \alpha q$, and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons and holes are described as in-plane point charges with a finite extension in the perpendicular direction, and their interaction is screened by a dielectric function with a nonlinear $q$ dependence which is computed ab initio. The screened interaction is used in a generalized Mott-Wannier model to calculate exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model, and both are in good agreement with ab initio many-body calculations. On the other hand, for more complex structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function, and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless connection between the strict 2D limit of isolated monolayer materials and the more bulk-like screening characteristics of supported 2D materials or van der Waals heterostructures.

Building on recent solutions of the fermion sign problem for specific models we present two continuous-time quantum Monte Carlo methods for efficient simulation of mass-imbalanced Hubbard models on bipartite lattices at half filling. For both methods we present the solutions to the fermion sign problem and the algorithms to achieve efficient simulations. As applications, we calculate the dependence of the spin correlation on the mass imbalance in a one-dimensional lattice and study the thermal and quantum phase transitions to an antiferromagnetic Ising long-range ordered state in two dimensions. These results offer unbiased predictions for experiments on ultracold atoms and bridge known exact solutions of the Falicov-Kimball model and previous studies of the $\text{SU}\left(2\right)$-symmetric Hubbard model.

We consider the phenomenon of natural optical activity, and related chiral magnetic effect in metals with low carrier concentration. To reveal the correspondence between the two phenomena, we compute the optical conductivity of a noncentrosymmetric metal to linear order in the wave vector of the light wave, specializing to the low-frequency regime. We show that it is the orbital magnetic moment of quasiparticles that is responsible for the natural optical activity, and thus the chiral magnetic effect. While for purely static magnetic fields the chiral magnetic effect is known to have a topological origin and to be related to the presence of Berry curvature monopoles (Weyl points) in the band structure, we show that the existence of Berry monopoles is not required for the dynamic chiral magnetic effect to appear; the latter is thus not unique to Weyl metals. The magnitude of the dynamic chiral magnetic effect in a material is related to the trace of its gyrotropic tensor. We discuss the conditions under which this trace is nonzero; in noncentrosymmetric Weyl metals it is found to be proportional to the energy-space dipole moment of Berry curvature monopoles. The calculations are done within both the semiclassical kinetic equation, and Kubo linear-response formalisms, with coincident results.

We investigate experimentally and theoretically the interference at avoided crossings which are repeatedly traversed as a consequence of an applied ac field. Our model system is a charge qubit in a serial double quantum dot connected to two leads. Our focus lies on effects caused by simultaneous driving with two different frequencies. We work out how the commensurability of the driving frequencies affects the symmetry of the interference patterns both in real space and in Fourier space. For commensurable frequencies, the symmetry depends sensitively on the relative phase between the two modes, whereas for incommensurable frequencies the symmetry of monochromatic driving is always recovered.

Inhomogeneous dephasing from uncontrolled environmental noise can limit the coherence of a quantum sensor or qubit. For solid-state spin qubits such as the nitrogen-vacancy (NV) center in diamond, a dominant source of environmental noise is magnetic field fluctuations due to nearby paramagnetic impurities and instabilities in a magnetic bias field. In this work, we use ac stress generated by a diamond mechanical resonator to engineer a dressed spin basis in which a single NV center qubit is less sensitive to its magnetic environment. For a qubit in the thermally isolated subspace of this protected basis, we prolong the dephasing time $T_2^{*}$ from $2.7\pm 0.1$ to $15\pm 1\mu \mathrm{s}$ by dressing with a $\mathrm{\Omega }/2\pi =581\pm 2$ kHz mechanical Rabi field. Furthermore, we develop a model that quantitatively predicts the relationship between $\mathrm{\Omega }$ and $T_2^{*}$ in the dressed basis. Our model suggests that a combination of magnetic field fluctuations and hyperfine coupling to nearby nuclear spins limits the protected coherence time over the range of $\mathrm{\Omega }$ accessed here. We show that amplitude noise in $\mathrm{\Omega }$ will dominate the dephasing for larger driving fields.

Multipolar electromagnetic phenomena in subwavelength resonators are at the heart of metamaterial science and technology. In this Rapid Communication, we demonstrate selective and enhanced coupling to specific multipole resonances via beam engineering. We first derive an analytical method for determining the scattering and absorption of spherical nanoparticles (NPs) that depends only on the local electromagnetic field quantities within an inhomogeneous beam. Using this analytical technique, we demonstrate the ability to drastically manipulate the scattering properties of a spherical NP by varying illumination properties and demonstrate the excitation of a longitudinal quadrupole mode that cannot be accessed with conventional illumination. This work enhances the understanding of fundamental light-matter interactions in metamaterials and lays the foundation for researchers to identify, quantify, and manipulate multipolar light-matter interactions through optical beam engineering.

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index ($\sim 1.52$). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials having isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as microlenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.