Colloidal Mn${}^{2+}$-doped semiconductor nanocrystals are solution processable analogs of classic phosphor and diluted magnetic semiconductor materials with promising applications ranging from fluorescence microscopy to spintronic information processing. At doping levels of only a few cation mole percent, Mn${}^{2+}$ dimers form in appreciable concentration and cause shortened photoluminescence decay times and reduced luminescence circular polarization under applied magnetic fields. Here, we show that these differences allow the use of time-resolved magnetophotoluminescence measurements to investigate the magnetic properties of the luminescent dimer excited state in Zn${}_{1-x}$Mn${}_x$Se nanocrystals. These measurements reveal that Mn${}^{2+}$-Mn${}^{2+}$ dimers are coupled ferromagnetically in their luminescent excited state, in contrast with the antiferromagnetic coupling of their ground state. We find that Mn${}^{2+}$-Mn${}^{2+}$ dimers also luminesce with much purer circular polarization than Mn${}^{2+}$ monomers under applied magnetic fields. These results are explained well by perturbation theory and density functional theory analyses of the microscopic orbital exchange interactions within the photoexcited Mn${}^{2+}$-Mn${}^{2+}$ dimers. This discovery of photoswitchable dimer magnetism (from $S$ = 0 to $S$ = 4) with strong associated circularly polarized luminescence raises intriguing possibilities for optical spin manipulation in doped semiconductors.

We present a comprehensive computational study of the electronic, thermal, and thermoelectric (TE) properties of gallium nitride nanowires (NWs) over a wide range of thicknesses (3–9 nm), doping densities (${10}^{18}$–${10}^{20}$ cm${}^{-3}$), and temperatures (300–1000 K). We calculate the low-field electron mobility based on ensemble Monte Carlo transport simulation coupled with a self-consistent solution of the Poisson and Schrödinger equations. We use the relaxation-time approximation and a Poisson-Schrödinger solver to calculate the electron Seebeck coefficient and thermal conductivity. Lattice thermal conductivity is calculated using a phonon ensemble Monte Carlo simulation, with a real-space rough surface described by a Gaussian autocorrelation function. Throughout the temperature range, the Seebeck coefficient increases while the lattice thermal conductivity decreases with decreasing wire cross section, both boding well for TE applications of thin GaN NWs. However, at room temperature these benefits are eventually overcome by the detrimental effect of surface roughness scattering on the electron mobility in very thin NWs. The highest room-temperature $ZT$ of 0.2 is achieved for 4-nm-thick NWs, while further downscaling degrades it. In contrast, at 1000 K, the electron mobility varies weakly with the NW thickness owing to the dominance of polar optical phonon scattering and multiple subbands contributing to transport, so $ZT$ increases with increasing confinement, and reaches 0.8 for optimally doped 3-nm-thick NWs. The $ZT$ of GaN NWs increases with increasing temperature beyond 1000 K, which further emphasizes their suitability for high-temperature TE applications.

Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy-band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low-temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibit a gate-voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.

We investigate the spin relaxation and decoherence in a single-electron graphene quantum dot with Rashba and intrinsic spin-orbit interactions. We derive an effective spin-phonon Hamiltonian via the Schrieffer-Wolff transformation in order to calculate the spin relaxation time $T_1$ and decoherence time $T_2$ within the framework of the Bloch-Redfield theory. In this model, the emergence of a nonmonotonic dependence of $T_1$ on the external magnetic field is attributed to the Rashba spin-orbit coupling-induced anticrossing of opposite spin states. A rapid decrease of $T_1$ occurs when the spin and orbital relaxation rates become comparable in the vicinity of the spin-mixing energy-level anticrossing. By contrast, the intrinsic spin-orbit interaction leads to a monotonic magnetic field dependence of the spin relaxation rate which is caused solely by the direct spin-phonon coupling mechanism. Within our model, we demonstrate that the decoherence time $T_2\simeq 2T_1$ is dominated by relaxation processes for the electron-phonon coupling mechanisms in graphene up to leading order in the spin-orbit interaction. Moreover, we show that the energy anticrossing also leads to a vanishing pure spin dephasing rate for these states for a super-Ohmic bath.

We show how the entanglement contained in states of spins arranged on a lattice may be lower bounded with observables arising in scattering experiments. We focus on the partial differential cross section obtained in neutron scattering from magnetic materials but our results are sufficiently general such that they may also be applied to, e.g., optical Bragg scattering from ultracold atoms in optical lattices or from ion chains. We discuss resonating valence bond states and ground and thermal states of experimentally relevant models—such as the Heisenberg, Majumdar-Ghosh, and XY models—in different geometries and with different spin numbers. As a by-product, we find that for the one-dimensional XY model in a transverse field such measurements reveal factorization and the quantum phase transition at zero temperature.

Hole localization accompanying the formation of a cation vacancy in bulk MgO and CaO and at the (100) MgO/CaO interface is described using the Heyd-Scuseria-Ernzerhof hybrid functionals and DFT + $U$ method. The ground state is found to be the ${\text{O}}^1\text{−}{\text{O}}^1$ bipolaronic configuration both in bulk oxides and at their interfaces. The ground-state magnetic configuration is a triplet, which has an energy only about 1–2 meV lower than that of the singlet state. The one-centered ${\text{O}}^2\text{−}{\text{O}}^0$ bipolaron was found to be metastable with its stability being enhanced at the interfaces compared to that in bulk oxides. Possible transition between different biplaronic configurations at the interface are analyzed for chosen configurations.

Gapped phases with long-range entanglement may admit gapped boundaries. If the boundary is gapped, the ground-state degeneracy is well defined and can be computed using methods of topological quantum field theory. We derive a general formula for the ground-state degeneracy for Abelian fractional quantum Hall phases, including the cases when connected components of the boundary are subdivided into an arbitrary number of segments, with a different boundary condition on each segment, and in the presence of an arbitrary number of boundary domain walls.

Here, we report on the ac conductivity $[{\sigma }^{\prime }\left(\omega \right);10\mathrm{mHz}<\omega <0.1\mathrm{MHz}]$ measurements performed on atomically thin, two-dimensional layers of MoS${}_2$ grown by chemical vapor deposition (CVD). ${\sigma }^{\prime }\left(\omega \right)$ is observed to display a “universal” power law, i.e., ${\sigma }^{\prime }\left(\omega \right)\sim {\omega }^s$ measured within a broad range of temperatures, 10 K $<T<$ 340 K. The temperature dependence of ‘$`s$” indicates that the dominant ac transport conduction mechanism in CVD-grown MoS${}_2$ is due to electron hopping through a quantum mechanical tunneling process. The ac conductivity also displays scaling behavior, which leads to the collapse of the ac conductivity curves obtained at various temperatures into a single master curve. These findings establish a basis for our understanding of the transport mechanism in atomically thin, CVD-grown MoS${}_2$ layers.

Two-dimensional (2D) materials often adopt a hexagonal lattice. We report on a class of 2D materials, Cu${}_2$$M{X}_4$ ($M$ = Mo, W; $X$ = S, Se), that has a square lattice. Up to three monolayers, the systems are kinetically stable. All of them are semiconductors with band gaps from 2.03 to 2.48 eV. Specifically, the states giving rise to the valence band maximum are confined to the Cu and $X$ atoms, while those giving rise to the conduction band minimum are confined to the $M$ atoms, suggesting that spontaneous charge separation occurs. The semiconductive nature makes the materials promising for transistors, optoelectronics, and solar energy conversion. Moreover, the ferromagnetism on the edges of square Cu${}_2$$M{X}_4$ nanoribbons opens applications in spintronics.

We illustrate the possibility of realizing band gaps in graphene-like systems that fall outside the existing classification of gapped Dirac Hamiltonians in terms of masses. As our primary example we consider a band gap arising due to time-dependent distortions of the honeycomb lattice. By means of an exact, invertible, and transport-preserving mapping to a time-independent Hamiltonian, we show that the system exhibits Chern-insulating phases with quantized Hall conductivities $\pm e^2/h$. The chirality of the corresponding gapless edge modes is controllable by both the frequency of the driving and the manner in which sublattice symmetry is broken by the dynamical lattice modulations. Finally, we discuss a promising possible realization of this physics in photonic lattices.

We point out that, contrary to an assertion by Chen et al. [Phys. Rev. B 88, 125110 (2013)], the nonquantized part of the intrinsic anomalous Hall conductivity can indeed be expressed as a Fermi-surface property even when Weyl points are present in the band structure.

Magnetopolariton in bilayer graphene (BLG) is theoretically investigated with the consideration of the influence of asymmetry between on-site energies in the two layers of BLG. The results show that an ultrastrong light-matter coupling regime can be achieved in a high filling factor and asymmetry has a strong effect on it. Although BLG in the low-energy regime and semiconductor have a similar quadratic dispersion of quasiparticles, a remarkably different cavity quantum electrodynamics (QED) effect occurs in BLG. In particular, a quantum phase transition, as predicted by the Dicke model, occurs in BLG in spite of the Schrödinger-like term $p^2$/2$m$ in the system Hamiltonian, while such quantum phase transition does not exist in semiconductors. Most noticeably, the ultrastrong light-matter coupling can be easily controlled by modulating the asymmetry in BLG, which provides an excellent platform to observe interesting QED effects and can lead to tunable polariton-based devices and cavity-controlled magnetotransport in BLG.

Finite-temperature properties such as thermodynamic state functions can be obtained for a range of crystalline materials by combining density functional theory (DFT) with lattice-dynamics approaches. Despite the usefulness of such first-principles predictions, their results must be carefully checked for accuracy, especially in cases where the DFT description of the material itself is nontrivial. Here, we investigate a prototypical layered semiconductor, namely, germanium selenide (GeSe) by dispersion-corrected DFT, lattice-dynamics computations, and a thermodynamic framework that relies on the quasiharmonic approximation (QHA). We study phonon band structures, their evolution under pressure, and finite-temperature thermodynamic state functions. Besides the layered orthorhombic structure, this analysis includes the high-temperature cubic (rocksalt-type) polymorph of GeSe, which is shown to exhibit imaginary vibrational modes but emerging dynamic stability under increasing external pressure. The effect of these imaginary vibrational modes on the QHA and hence on computed thermochemical properties is critically evaluated. First-principles thermodynamics correctly predict a high-temperature transition from the orthorhombic to the cubic structure, albeit the transition temperature is severely underestimated. This simple compound allows us to address important methodological questions regarding the QHA treatment of crystalline solids.

Non-180° ferroelectric domains are also ferroelastic domains; their existence in polycrystalline materials is to relieve internal stresses generated during solid-solid phase transitions and minimize the elastic distortion energy. Therefore, grains with random orientations in polycrystalline ceramics are always occupied by many domains, especially in the regions close to grain boundaries. In this Rapid Communication, we report the observation of a single-domain state in a BaTiO${}_3$-based polycrystalline ceramic at intermediate poling electric fields with in situ transmission electron microscopy. The grains in the virgin ceramic and under high poling fields are found multidomained. The unique single-domain state is believed to be responsible for the ultrahigh piezoelectric property observed in this lead-free composition and is suggested to be of orthorhombic symmetry for its exceptionally low elastic modulus.

We propose a mechanism for negative isotropic magnetoresistance in the hopping regime. It results from a memory effect encrypted into spin correlations that are not taken into account by the conventional theory of hopping conductivity. The spin correlations are generated by the nonequilibrium electric currents and lead to the decrease of the conductivity. The application of the magnetic field destroys the correlations thus enhancing the conductance. This effect can occur even at magnetic fields as small as a few gauss.

Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of $\beta$-TeVO${}_4$. Two different approaches, based either on a perturbative treatment of the multiorbital Hubbard model in the strongly correlated limit or on the calculation of supercell total energy differences, have been employed to evaluate magnetic couplings in this compound. The picture provided by these two approaches is that of weakly coupled frustrated chains with ferromagnetic nearest-neighbor and antiferromagnetic second-nearest-neighbor couplings. These results, differing substantially from previous reports, should motivate further experimental investigations of the magnetic properties of this compound.

The electronic behavior of various solid metals (Al, Ni, Cu, Au, Ti, and W) under ultrashort laser irradiation is investigated by means of density functional theory. Successive stages of extreme nonequilibrium on picosecond time scale impact the excited material properties in terms of optical coupling and transport characteristics. As these are generally modelled based on the free-electron classical theory, the free-electron number is a key parameter. However, this parameter remains unclearly defined and dependencies on the electronic temperature are not considered. Here, from first-principles calculations, density of states are obtained with respect to electronic temperatures varying from ${10}^{-2}$ to ${10}^5$ K within a cold lattice. Based on the concept of localized or delocalized electronic states, temperature dependent free-electron numbers are evaluated for a series of metals covering a large range of electronic configurations. With the increase of the electronic temperature we observe strong adjustments of the electronic structures of transition metals. These are related to variations of electronic occupation in localized $d$ bands, via change in electronic screening and electron-ion effective potential. The electronic temperature dependence of nonequilibrium density of states has consequences on electronic chemical potentials, free-electron numbers, electronic heat capacities, and electronic pressures. Thus electronic thermodynamic properties are computed and discussed, serving as a base to derive energetic and transport properties allowing the description of excitation and relaxation phenomena caused by rapid laser action.

Weak near-infrared and strong terahertz excitation are applied to study excitonic Rabi splitting in (GaIn)As/GaAs quantum wells. Pronounced anticrossing behavior of the split peaks is observed for different terahertz intensities and detunings relative to the intra-excitonic heavy-hole $1s$–$2p$ transition. At intermediate to high electric fields the splitting becomes highly asymmetric and exhibits significant broadening. A fully microscopic theory is needed to explain the experimental results. Comparisons with a two-level model reveal the increasing importance of higher excitonic states at elevated excitation levels.

We construct the nonrelativistic parity-violating hydrodynamic description of a two-dimensional dissipative, normal fluid in the presence of small $U\left(1\right)$ background fields and vorticity. This is achieved by taking the nonrelativistic limit of the recently developed relativistic hydrodynamics in $2+1$ dimensions. We identify and interpret the resulting parity-violating contributions to the nonrelativistic constitutive relations, which include the Hall current flowing perpendicular to the temperature gradient, the Hall viscosity, and the Leduc-Righi energy current. Also a comparison of our findings is made with the nonrelativistic parity-violating hydrodynamics obtained from a light-cone dimensional reduction.