Phosphorus-doped $n$-type diamond is currently one of the most promising wide-band-gap materials for next-generation high-power electronics and optoelectronics. Artificial diamond growth methods such as chemical vapor deposition involve hydrogen-containing precursors; therefore, the hydrogen atoms can be simultaneously introduced into the diamond lattice as a contamination and form complexes with other defects. In this work, we used the spin-polarized, hybrid density functional theory method to investigate the electronic structure, stability, and magnetic and optical properties of phosphorus, vacancy, and hydrogen clusters in diamond. Our results indicate a thermodynamic driving force for the formation of previously unidentified phosphorus-vacancy-hydrogen complexes that can be electrically, magnetically, or optically active centers. We found an unusual extremely large hyperfine coupling with the ${}^{31}\mathrm{P}$ nuclei ($A>2$ GHz) for some of the investigated defects that requires further experimental verification. Finally, we demonstrate that the ${\text{PV}}_2{\text{H}}^0$ complex has two metastable triplets between the ground- and excited-state singlets, and it may exhibit a highly selective spin decay channel to a ground state, which makes the defect a promising candidate for realizing long-living solid-state quantum memory. These results provide deep insight into the donor compensation effect associated with vacancy-related clusters, and they may be useful in future identification of P-related defects suitable for quantum information processing applications.

Electrostatic gating and optical pumping schemes enable efficient time modulation of graphene's free-carrier density, or Drude weight. We develop a theory for plasmon propagation in graphene under temporal modulation. When the modulation is on the timescale of the plasmonic period, we show that it is possible to create a backwards propagating or standing plasmon wave and to amplify plasmons. The theoretical models show very good agreement with direct Maxwell simulations.

We present a renormalization group analysis of the disorder effects on the low-energy behaviors of two-dimensional tilted Dirac-fermion systems, in which the fermions have two distinct orbitals unrelated by any symmetry. Four types of disordered potential, two interorbital and two intraorbital, are considered. If there is only one type of interorbital disorder, the fermion-disorder scattering induces logarithmic or power-law corrections to the fermion density of states and specific heat. In contrast, the intraorbital disorder can turn the system into a strongly disordered phase. In this disordered phase, calculations based on self-consistent Born approximation reveal that the Dirac point is destroyed and replaced by a bulk Fermi arc. We also study the interplay of four types of disorder, and find that the Dirac point can either remain intact or give place to a Fermi arc. We obtain the condition for the emergence of a Fermi arc in this case. Our results indicate that disorders can result in rich low-energy properties of tilted Dirac fermions.

Projected entangled pair states (PEPS) provide exact representations for many nonchiral topologically ordered states whereas their range of applicability to interacting chiral topological phases remains largely unsettled. In this context, the symmetries of the local PEPS tensors are crucial for determining the characteristic topological features of the state. In this paper we examine the constraints that arise when different symmetries are imposed simultaneously on the local tensor such as internal $\mathrm{SU}\left(2\right)$, point group, and translation symmetry. We show how the interplay of these symmetries manifests in the entanglement spectrum which is the main diagnostic tool for chiral topological order. We apply our results to a spin liquid PEPS introduced previously as a chiral generalization of the resonating valence bond state. Our findings explain the discrepancies observed between the entanglement spectrum of this state and the expected edge spectrum described by a chiral conformal field theory. Finally, in a certain parameter region where this PEPS possesses an additional $U\left(1\right)$ symmetry we are able to resolve these discrepancies and obtain an entanglement spectrum with the expected state countings and conformal weight.

The nitrogen-vacancy (NV) center in diamond is of high importance in quantum information processing applications. The operation of the NV center relies on the efficient optical polarization of its electron spin. However, the full optical spin-polarization process, which involves the intersystem crossing between the shelving singlet state and the ground-state triplet, is not understood. Here we develop a detailed theory of this process which involves a combination of pseudo- and dynamic Jahn-Teller interactions together with spin-orbit interaction. Our theory provides an explanation for the asymmetry between the observed emission and absorption spectra of the singlet states. We apply density functional theory to calculate the intersystem crossing rates and the optical spectra of the singlets, and we obtain a good agreement with the experimental data. Since the NV center serves as a template for other solid-state-defect quantum bit systems, our theory provides a toolkit to study them that might help optimize their quantum bit operation.

The antiferromagnetic semi-Heusler compound CuMnSb has been investigated under high pressure by electrical resistivity and angle dispersive synchrotron x-ray diffraction measurements to 53 and 36 GPa, respectively. The Néel temperature at $\sim 50$ K is found to initially increase rapidly with pressure, reaching 83 K at 7 GPa. However, near 8 GPa at ambient temperature a sluggish first-order structural transition begins from a semimetallic cubic phase to a likely semimetallic tetragonal phase; thermal cycling to $355{}^{\circ }\mathrm{C}$ at 9.6 GPa serves to complete the transition. In the tetragonal phase no sign of magnetic ordering is visible in the resistivity $R\left(T\right)$ over the measured temperature range 4–295 K. This suggests that magnetic ordering may have shifted to temperatures well above ambient. Indeed, density functional calculations find the magnetic ground state in the tetragonal phase to be antiferromagnetic. Following decompression to 1 bar at ambient temperature, the high-pressure tetragonal phase is retained.

Localization-protected quantum order extends the idea of symmetry breaking and order in ground states to individual eigenstates at arbitrary energy. Examples include many-body localized static and $\pi$ spin glasses in Floquet systems. Such order is inherently dynamical and difficult to detect as the order parameter typically varies randomly between different eigenstates, requiring specific superpositions of eigenstates to be targeted by the initial state. We show that two-time correlators overcome this, reflecting the presence or absence of eigenstate order even in fully mixed, infinite temperature states. We show how spatiotemporal correlators are generated by the recently introduced dynamical potentials, demonstrating this explicitly using an Ising spin glass and a Floquet $\pi$ spin glass and focusing on features mirroring those of equilibrium statistical mechanics such as bimodal potentials in the symmetry-broken phase.

We describe the phases of a solvable $t\text{−}J$ model of electrons with infinite-range, and random, hopping, and exchange interactions, similar to those in the Sachdev-Ye-Kitaev models. The electron fractionalizes, as in an “orthogonal metal,” into a fermion $f$, which carries both the electron spin and charge, and a boson $\varphi$. Both $f$ and $\varphi$ carry emergent ${\mathbb{Z}}_2$ gauge charges. The model has a phase in which the $\varphi$ bosons are gapped, and the $f$ fermions are gapless and critical, and so the electron spectral function is gapped. This phase can be considered as a toy model for the underdoped cuprates, without spatial structure. The model also has an extended, critical, “quasi-Higgs” phase where both $\varphi$ and $f$ are gapless, and the electron operator $\sim f\varphi$ has a Fermi liquid-like $1/\tau$ propagator in imaginary time, $\tau$. So while the electron spectral function has a Fermi liquid form, other properties are controlled by ${\mathbb{Z}}_2$ fractionalization and the anomalous exponents of the $f$ and $\varphi$ excitations. This quasi-Higgs phase is proposed as a toy model of the overdoped cuprates. We also describe the critical state separating these two phases.

At a normal-metal/superconductor interface, an incident electron from the normal-metal (N) side can be normally reflected as an electron or Andreev reflected as a hole. We show that pronounced lateral shifts along the interface between the incident and the reflected quasiparticles can happen in both reflection processes, which are analogous to the Goos-Hänchen effect in optics. Two concrete model systems are considered. For the simplest model in which the N side is of the two-dimensional electron gas, we find that while the shift in Andreev reflection stays positive, the shift in normal reflection can be made either positive or negative, depending on the excitation energy. For the second model with the N side taken by graphene, the shift in Andreev reflection can also be made negative, and the shifts have rich behavior due to the additional sublattice pseudospin degree of freedom. We show that the shift strongly modifies the dispersion for the confined waveguide modes in an SNS structure. We also suggest a possible experimental setup for detecting the shift.

${\mathrm{VO}}_2$ exhibits several insulating phases (monoclinic $M1,M2$, and triclinic $T)$, and the study of these phases is important for understanding the true nature of the insulator-to-metal transition (IMT) in ${\mathrm{VO}}_2$. These insulating phases have small but discernible crystallographic differences in the vanadium chains forming the dimers. Peculiarities of the electron localizations in the dimerized chains for many of the probes such as NMR make it difficult to characterize the true character of these phases [T. J. Huffman et al., Phys. Rev. B 95, 075125 (2017); J. Pouget et al., Phys. Rev. B 10, 1801 (1974)]. Here we present structural, electrical, ultrafast-reflectivity, and electronic structure studies of the $T$ phase Cr-doped ${\mathrm{VO}}_2$ and the $M1$ phase pure ${\mathrm{VO}}_2$ thin films, both grown by pulsed laser deposition under identical conditions. An intermediate $M2$ structure is observed in the Cr-doped ${\mathrm{VO}}_2$, while the pure ${\mathrm{VO}}_2$ directly goes from the insulating monoclinic $M1$ structure to a metallic rutile $R$ structure, manifested by temperature-dependent Raman spectroscopy. Temperature-dependent electronic structure studies utilizing x-ray near-edge absorption spectroscopy reveal that all these insulating phases (monoclinic $M1$ and $M2$ and triclinic $T)$ have similar electronic structures which place these insulating phases into the list of Mott-Hubbard insulators. This is a first combined experimental report on the electronic structure of all the three insulating phases, monoclinic $M1,M2$, and triclinic $T$.

The physics of the renormalization of the effective electron g factor by the confining potential in semiconductor nanostructures is theoretically investigated. The effective g factor for electrons in structures with interacting nanolayers, or coupled quantum wells (QWs), is obtained with an analytical and yet accurate multiband envelope-function solution, based on the linear $8\times 8\mathrm{k}·\mathrm{p}$ Kane model for the bulk band structure. Both longitudinal and transverse applied magnetic fields are considered and the g-factor anisotropy (i.e., the difference between the two field configurations) is analyzed over the entire space spanned by the two structure parameters: the thickness of the active layers and the thickness of the tunneling barrier separating them. Two-dimensional anisotropy maps are constructed for symmetric and asymmetric InGaAs coupled QWs, with InP tunneling barriers, that reproduce exactly known single-layer or QW results, in different limits. The effects of the structure inversion asymmetry on the mesoscopic g-factor renormalization are also discussed, in particular the negative anisotropies for thin-layer structures. Such multilayer structures form an excellent testing ground for the theory, and the analytical solution presented, which is perfectly consistent over the whole space of parameters, leads to helpful expressions and can guide further research on the mechanisms of this mesoscopic renormalization.

We study the optical properties of bianisotropic optical waveguides with nontrivial topological structure in wave-vector space, placed in an ordinary dielectric matrix. We derive an exact analytical description of the eigenmodes of the systems with arbitrary parameters that allows us to investigate topologically protected surface states (TPSS) in details. In particular, we find that the TPSS in the waveguides would disappear (1) if their radius is smaller than a critical value due to the dimensional quantization of azimuthal wave number, and also (2) if the permittivity of the host medium exceeds a critical value. Interestingly, we discover that the TPSS in the waveguides have negative refraction for some structures. We construct a TPSS phase diagram that will guide the development of topological waveguides for optical interconnects and devices.

Dicalcium nitride $\left({\mathrm{Ca}}_2\mathrm{N}\right)$ is a layered material that has been recently identified as a two-dimensional electride, based on first-principles calculations, where the electronic property and the optical complex dielectric function are studied using density functional theory. We find that the optical permittivity tensor is highly anisotropic, and ${\mathrm{Ca}}_2\mathrm{N}$ possesses metallic characters when the in-plane dielectric function is negative in midinfrared frequency. In addition, the enhanced midinfrared transmission property and the field distribution of one-dimensional photonic crystals composed of alternating ${\mathrm{Ca}}_2\mathrm{N}$ layers and the dielectric material are theoretically studied using the transfer-matrix method. It is found that the ${\mathrm{Ca}}_2\mathrm{N}$ photonic crystals support a series of passbands and stopbands the numbers of which increase with the thickness and the permittivity of the dielectrics. The field distributions show that the transmission resonances in the passband are attributed to the coupled Fabry-Pérot resonances of the individual reactively loaded dielectric slabs. It is also noticed that these resonances lie within certain characteristic frequency bands which are independent of the period of the photonic crystal. The low-frequency edge of the passband is highly tunable by the thickness of the electride material and the dielectric material, the permittivity of the dielectric layer, while the high-frequency edge is insensitive to the electride material thickness. Moreover, the first band gap in lower frequency is almost omnidirectional and polarization insensitive. When defect layers are introduced, the twin defect modes are found, and the frequency and the frequency interval of the two defect modes can be tuned just by changing the permittivity, the thickness and the position of the defect layer, respectively. Polarization and angular insensitive absorption bands can also be obtained for the electride-dielectric photonic crystal with a reflective substrate. These properties of electride material photonic crystals have potential applications in tunable multiband filters and absorbers in the midinfrared region.

By applying density functional theory calculations, we predict that the ground state of bilayer silicene at certain interlayer distances can be antiferromagnetic. At small electron or hole doping, it becomes half metallic under applied out-of-plane electric field, which can be used to produce a fully spin-polarized field-effect-driven current even in the absence of an external magnetic field, ferromagnetic substrates, doped magnetic ions, or spin-orbital coupling. Our finding points out a route to overcome the major challenge of spintronics.

The recently demonstrated unconventional superconductivity [Cao et al., Nature (London) 556, 43 (2018)] in twisted bilayer graphene (tBLG) opens the possibility for interesting applications of two-dimensional layers that involve correlated electron states. Here we explore the possibility of modifying electronic correlations by the application of uniaxial pressure on the weakly interacting layers, which results in increased interlayer coupling and a modification of the magic angle value and associated density of states. Our findings are based on first-principles calculations that accurately describe the height-dependent interlayer coupling through the combined use of density functional theory and maximally localized Wannier functions. We obtain the relationship between twist angle and external pressure for the magic angle flat bands of tBLG. This may provide a convenient method to tune electron correlations by controlling the length scale of the superlattice.

We study the low-energy electronic structure of three-dimensional Dirac semimetal, ${\mathrm{Cd}}_3{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ [$x=0$ and 0.34(3)], by employing angle-resolved photoemission spectroscopy (ARPES). We observe that the bulk Dirac states in ${\mathrm{Cd}}_3{\left({\mathrm{As}}_{0.66}{\mathrm{P}}_{0.34}\right)}_2$ are gapped out with an energy of 0.23 eV, contrary to the parent ${\mathrm{Cd}}_3{\mathrm{As}}_2$ in which the gapless Dirac states have been observed. Thus, our results confirm the earlier predicted topological phase transition in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ with perturbation. We further notice that the critical P substitution concentration, at which the two Dirac points that are spread along the $c$-axis in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ form a single Dirac point at $\mathrm{\Gamma }$, is much lower [$x_c\left(\mathrm{P}\right)<0.34\left(3\right)$] than the predicted value of $x_c\left(\mathrm{P}\right)=0.9.$ Therefore, our results suggest that the nontrivial band topology of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ is remarkably sensitive to the P substitution and can only survive over a narrow substitution range, i.e., $0\le x\left(\text{P}\right)<0.34\left(3\right)$.

Passive resonant metamaterials are limited by the narrow-band nature of the resonances they support. Here we show that by incorporating an active component into the structure of the commonly used split-ring resonator it is possible to tune the resonance frequency of this type of metamaterial atom. We make use of this tunability to examine the interaction between two resonators, one passive and one active, as the resonance frequency of the active resonator is swept through that of the passive resonator. The resultant modes of this coupled system exhibit an anticrossing and, by changing the separation between, and relative orientation of, the split-ring resonators, we investigate how the magnetic and electric coupling terms change. We find that the relative orientation of the resonators significantly effects the strength of the coupling. Through both structural and active tuning we are able to alter the relative sizes and signs of the coupling terms. We hope that the nature of these changes will be of use to those designing large actively tunable metamaterial systems.