The negatively charged nitrogen-vacancy $\left({\mathrm{NV}}^{-}\right)$ center in diamond is a promising candidate for many quantum applications. Here, we examine the splitting and broadening of the center's infrared (IR) zero-phonon line (ZPL). We develop a model for these effects that accounts for the strain induced by photodependent microscopic distributions of defects. We apply this model to interpret observed variations of the IR ZPL shape with temperature and photoexcitation conditions. We identify an anomalous temperature-dependent broadening mechanism and that defects other than the substitutional nitrogen center significantly contribute to strain broadening. The former conclusion suggests the presence of a strong Jahn-Teller effect in the center's singlet levels and the latter indicates that major sources of broadening are yet to be identified. These conclusions have important implications for the understanding of the center and the engineering of diamond quantum devices. Finally, we propose that, once the major sources of broadening are identified, the IR ZPL has the potential to be a sensitive spectroscopic tool for probing microscopic strain fields and performing defect tomography.

We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20-nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps pulses. We demonstrate that the transient profile of the photon echo from the localized trion (${\mathrm{X}}^{-}$) and the donor-bound exciton (${\mathrm{D}}^0\mathrm{X}$) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than $\pi$ due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (${\mathrm{D}}^0\mathrm{X}$). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are incorporated in our model, which is able to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for ${\mathrm{X}}^{-}$ and ${\mathrm{D}}^0\mathrm{X}$ complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as $\mu ({\mathrm{X}}^{-}$)=73 D and $\mu ({\mathrm{D}}^0\mathrm{X}$)=58 D.

We show that the parallel magnetic field-induced increase in the critical electron density for the Anderson transition in a strongly interacting two-dimensional electron system is caused by the effects of exchange and correlations. If the transition occurs when electron spins are only partially polarized, additional increase in the magnetic field is necessary to achieve the full spin polarization in the insulating state due to the exchange effects.

Commensurability oscillations in the magnetotransport of periodically patterned systems, emerging from the interplay of cyclotron orbit and pattern periodicity, are a benchmark of mesoscopic physics in electron gas systems. Exploiting similar effects in two-dimensional materials would allow exceptional control of electron behavior, but it is hindered by the requirement to maintain ballistic transport over large length scales. Recent experiments have overcome this obstacle and observed distinct magnetoresistance commensurability peaks for perforated graphene sheets (antidot lattices). Interpreting the exact mechanisms behind these peaks is of key importance, particularly in graphene, where a range of regimes are accessible by varying the electron density. In this work, a fully atomistic, device-based simulation of magnetoresistance experiments allows us to analyze both the resistance peaks and the current flow at commensurability conditions. Magnetoresistance spectra are found in excellent agreement with experiment, but we show that a semiclassical analysis, in terms of simple skipping or pinned orbits, is insufficient to fully describe the corresponding electron trajectories. Instead, a generalized mechanism in terms of states bound to individual antidots, or to groups of antidots, is required. Commensurability features are shown to arise when scattering between such states is enhanced. The emergence and suppression of commensurability peaks is explored for different antidot sizes, magnetic field strengths, and electron densities. The insights gained from our study will guide the design and optimization of future experiments with nanostructured graphene.

The broadband and ultrafast photoresponse of graphene has been extensively studied in recent years, although the photoexcited carrier dynamics is still far from being completely understood. Different experimental approaches imply either one of two fundamentally different scattering mechanisms for hot electrons. One is high-energy optical phonons, while the other is disorder-driven supercollisions with acoustic phonons. However, the concurrent relaxation via both optical and acoustic phonons has not been considered so far, hindering the interpretation of different experiments within a unified framework. Here we expand the optical phonon-mediated cooling model, to include electron scattering with the acoustic phonons. By assuming the enhancement of electron-acoustic phonon supercollisions from the localized defect at the photothermoelectric current-generating interface, we provide a broader perspective to the ultrafast photoresponse of graphene, highlighting the previously overlooked effect of the interface for cooling dynamics. We show that the transient photothermoelectric response, which has been attributed exclusively to supercollisions, can be successfully explained without rejecting the established optical phonon relaxation pathway, demonstrating that the two cooling mechanisms are not mutually exclusive but complement each other.

We present ultralow-temperature thermal conductivity measurements on single crystals of the prototypical charge-density-wave material $1T\text{−}{\mathrm{TaS}}_2$, which was recently argued to be a candidate for a quantum spin liquid. Our experiments show that the residual linear term of thermal conductivity at zero field is essentially zero, within experimental accuracy. Furthermore, the thermal conductivity is found to be insensitive to the magnetic field up to 9 T. These results clearly demonstrate the absence of itinerant magnetic excitations with fermionic statistics in bulk $1T\text{−}{\mathrm{TaS}}_2$, and thus put a strong constraint on the theories of the ground state of this material.

We investigate the influence of nuclear spins on the resistance of helical edge states of two-dimensional topological insulators (2DTIs). Via the hyperfine interaction, nuclear spins allow electron backscattering, otherwise forbidden by time-reversal symmetry. We identify two backscattering mechanisms, depending on whether the nuclear spins are ordered or not. Their temperature dependence is distinct but both give resistance, which increases with the edge length, decreasing temperature, and increasing strength of the electron-electron interaction. Overall, we find that the nuclear spins will typically shut down the conductance of the 2DTI edges at zero temperature.

Based solely on the analytical properties of the single-particle Green's function of fermions at finite temperatures, we show that the generalized Luttinger theorem inherently possesses topological aspects. The topological interpretation of the generalized Luttinger theorem can be introduced because (i) the Luttinger volume is represented as the winding number of the single-particle Green's function and, thus, (ii) the deviation of the theorem, expressed with a ratio between the interacting and noninteracting single-particle Green's functions, is also represented as the winding number of this ratio. The formulation based on the winding number naturally leads to two types of the generalized Luttinger theorem. Exploring two examples of single-band translationally invariant interacting electrons, i.e., simple metal and Mott insulator, we show that the first type falls into the original statement for Fermi liquids given by Luttinger, where poles of the single-particle Green's function appear at the chemical potential, while the second type corresponds to the extended one for nonmetallic cases with no Fermi surface such as insulators and superconductors generalized by Dzyaloshinskii, where zeros of the single-particle Green's function appear at the chemical potential. This formulation also allows us to derive a sufficient condition for the validity of the Luttinger theorem of the first type by applying the Rouche's theorem in complex analysis as an inequality. Moreover, we can rigorously prove in a nonperturbative manner, without assuming any detail of a microscopic Hamiltonian, that the generalized Luttinger theorem of both types is valid for generic interacting fermions as long as the particle-hole symmetry is preserved. Finally, we show that the winding number of the single-particle Green's function can also be associated with the distribution function of quasiparticles, and therefore the number of quasiparticles is equal to the Luttinger volume. This implies that the fundamental hypothesis of the Landau's Fermi-liquid theory, the number of fermions being equal to that of quasiparticles, is guaranteed if the Luttinger theorem is valid since the theorem states that the number of fermions is equal to the Luttinger volume. All these general statements are made possible because of the finding that the Luttinger volume is expressed as the winding number of the single-particle Green's function at finite temperatures, for which the complex analysis can be readily exploited.

In this paper, we propose a generalization of the $S$-duality of four-dimensional quantum electrodynamics (${\text{QED}}_4$) to ${\text{QED}}_4$ with fractionally charged excitations, the fractional $S$-duality. Such ${\text{QED}}_4$ can be obtained by gauging the $\text{U(1)}$ symmetry of a topologically ordered state with fractional charges. When time-reversal symmetry is imposed, the axion angle ($\theta$) can take a nontrivial but still time-reversal-invariant value $\pi /t^2$ ($t\in \mathbb{Z}$). Here, $1/t$ specifies the minimal electric charge carried by bulk excitations. Such states with time-reversal and $\text{U(1)}$ global symmetry (fermion number conservation) are fractional topological insulators (FTIs). We propose a topological quantum field theory description, which microscopically justifies the fractional $S$-duality. Then, we consider stacking operations (i.e., a direct sum of Hamiltonians) among FTIs. We find that there are two topologically distinct classes of FTIs: type I and type II. Type I ($t\in {\mathbb{Z}}_{\mathrm{odd}}$) can be obtained by directly stacking a noninteracting topological insulator and a fractionalized gapped fermionic state with minimal charge $1/t$ and vanishing $\theta$. But type II ($t\in {\mathbb{Z}}_{\mathrm{even}}$) cannot be realized through any stacking. Finally, we study the surface topological order of fractional topological insulators.

Evaluating the time-dependent dynamics of driven open quantum systems is relevant for a theoretical description of many systems, including molecular junctions, quantum dots, cavity-QED experiments, cold atoms experiments, and more. Here, we formulate a rigorous microscopic theory of an out-of-equilibrium open quantum system of noninteracting particles on a lattice weakly coupled bilinearly to multiple baths and driven by periodically varying thermodynamic parameters like temperature and chemical potential of the bath. The particles can be either bosonic or fermionic and the lattice can be of any dimension and geometry. Based on the Redfield quantum master equation under Born-Markov approximation, we derive a linear differential equation for an equal time two point correlation matrix, sometimes also called a single-particle density matrix, from which various physical observables, for example, current, can be calculated. Various interesting physical effects, such as resonance, can be directly read off from the equations. Thus, our theory is quite general and gives quite transparent and easy-to-calculate results. We validate our theory by comparing with exact numerical simulations. We apply our method to a generic open quantum system, namely, a double quantum dot coupled to leads with modulating chemical potentials. The two most important experimentally relevant insights from this are as follows: (i) Time-dependent measurements of current for symmetric oscillating voltages (with zero instantaneous voltage bias) can point to the degree of asymmetry in the system-bath coupling and (ii) under certain conditions time-dependent currents can exceed time-averaged currents by several orders of magnitude, and can therefore be detected even when the average current is below the measurement threshold.

We use density functional theory to study intrinsic defects and oxygen related defects in indium selenide. We find that InSe is prone to oxidation, but however not reacting with oxygen as strongly as phosphorene. The dominant intrinsic defects in In-rich material are the In interstitial, a shallow donor, and the Se vacancy, which introduces deep traps. The latter can be passivated by oxygen, which is isoelectronic with Se. The dominant intrinsic defects in Se-rich material have comparatively higher formation energies.

Analysis of statistical distributions and auto- and cross correlations of polarization and electric field during the field-driven polarization reversal in a bulk polycrystalline ferroelectric is performed. A mesoscopic switching model is used which accounts self-consistently for the development of depolarization fields. Correlations mediated by electrostatic fields are shown to be mostly isotropic and short range at the typical scale of the grain size which is explained by an effective screening via adapting bound charges. The short-range screening clarifies the paradoxical ability of common statistical concepts neglecting the feedback effect of depolarization fields to adequately describe the polarization switching kinetics. The statistical distribution of the local electric field magnitudes is continuously spreading in the course of the global polarization reversal due to mismatching of both dielectric tensor and spontaneous polarization at grain boundaries. The increasing field dispersion substantially contributes to the well-known deceleration of the polarization reversal at long times.

The Andreev conductance across realistic two-dimensional (2D) normal-metal (N)/superconductor (SC) junctions with a relativistic Dirac spectrum is theoretically investigated within the Blonder-Tinkham-Klapwijk formalism with tunable tunneling transparency. It is known that due to the effect of Klein tunneling, impurity potentials at the interface of 2D relativistic materials will enhance (not suppress) the tunneling and therefore are not suitable to model a realistic tunnel junction of these materials. Here, we propose a way to construct a more realistic tunnel junction by adding a narrow, homogeneous local strain, which effectively generates a $\delta$-gauge potential and variations of electron hopping at the interface, to adjust the transparency of the N/SC junction. Remarkable suppression of the Andreev conductance is indeed observed in the graphene N/SC junction as the strength of the local strain increases. We also explore the Andreev conductance in a topological N/SC junction at the two inequivalent Dirac points and predict the distinctive behaviors for the conductance across the chiral-to-helical topological phase transition. The relevance of our results for the adatom-doped graphene is discussed.

Spatiotemporal modulations of the elastic properties of materials can be used to break time and parity symmetry of elastic waves. We show that the form of the elastic band structure depends not only on the spatial and temporal periodicity of a spatiotemporal modulation but also on its shape through its Fourier components. We demonstrate that hybridization gaps open from interactions between the Bloch modes of the periodic medium in absence of the temporal variation of the modulation and the combined sinusoidal components of the Fourier decomposition of the periodic modulation.

The nonplanar sample surface is a crucial feature that must be taken into account for a good interpretation of magneto-optical observations of magnetic cylindrical microwires. This is due to the fact that a curved topography gives rise to a spatial distribution of local planes of incidence as a function of position on the circumference of a cylinder. Analytical calculations of the magnetic contrast of magnetic cylinders spontaneously magnetized in the axial direction reveal that an axial magnetization reversal produces a characteristic black-and-white magneto-optical contrast and that it does not correspond to two oppositely magnetized domains, as would be concluded in the case of planar samples. Optical and magneto-optical observations of thin cylindrical FeSiB wires support this model. Calculations are also performed for the cases of circular and polar magnetizations.

The magnetic properties of ${\mathrm{Fe}}_2\mathrm{Ta}$ and ${\mathrm{Fe}}_2\mathrm{W}$ in the hexagonal Laves phase are computed using density functional theory in the generalized gradient approximation, with the full potential linearized augmented plane-wave method. The alloy ${\mathrm{Fe}}_2{\mathrm{Ta}}_{1-x}{\mathrm{W}}_x$ is studied using the virtual crystal approximation to treat disorder, with some comparisons to supercell calculations. ${\mathrm{Fe}}_2\mathrm{Ta}$ is found to be ferromagnetic with a saturation magnetization of ${\mu }_0{M}_{\text{s}}=0.66\mathrm{T}$ while, in contrast to earlier computational work, ${\mathrm{Fe}}_2\mathrm{W}$ is found to be ferrimagnetic with ${\mu }_0{M}_{\text{s}}=0.35\mathrm{T}$. The transition from the ferri- to the ferromagnetic state occurs for $x\le 0.1$. The magnetocrystalline anisotropy energy (MAE) is calculated to $1.25\mathrm{MJ}/{\mathrm{m}}^3$ for ${\mathrm{Fe}}_2\mathrm{Ta}$ and $0.87\mathrm{MJ}/{\mathrm{m}}^3$ for ${\mathrm{Fe}}_2\mathrm{W}$. The MAE is found to be smaller for all values $x$ in ${\mathrm{Fe}}_2{\mathrm{Ta}}_{1-x}{\mathrm{W}}_x$ than for the end compounds and it is negative (in-plane anisotropy) for $0.1\le x\le 0.9$. The MAE is carefully analyzed in terms of the electronic structure. Even though there are weak $5d$ contributions to the density of states at the Fermi energy in both end compounds, a reciprocal space analysis, using the magnetic force theorem, reveals that the MAE originates mainly from regions of the Brillouin zone with strong $3d\text{−}5d$ hybridization near the Fermi energy. Perturbation theory and its applicability in relation to the MAE is discussed.

We study the superconducting properties of the thin film BCS superconductor proximity coupled to a magnetically doped time-reversal invariant topological insulator (TI). Using mean-field theory, we show that Fulde-Ferrell (FF) pairing can be induced in the conventional superconductor through the inverse proximity effect (IPE). This occurs when the IPE of the TI to the superconductor is large enough that the normal bands of the superconductor possess a proximity induced spin-orbit coupling and magnetization. We find that the energetics of the different pairings are dependent on the coupling strength between the TI and the BCS superconductor and the thickness of the superconductor film. As the thickness of the superconductor film is increased, we find a crossover from the FF pairing to the BCS pairing phase. This is a consequence of the increased number of the superconducting bands, which favor the BCS pairing, implying that the FF phase can only be observed in the thin film limit. In addition, we also propose transport experiments that show distinct signatures of the FF phase.

We present a detailed theoretical study of the electronic, magnetic, and structural properties of magnesiowüstite ${\mathrm{Fe}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ with $x$ in the range between 0 and 0.875 using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method. In particular, we compute the electronic structure and phase stability of the rocksalt $B1$-structured (Fe,Mg)O at high pressures relevant for the Earth's lower mantle. We find that upon compression paramagnetic (Fe,Mg)O exhibits a spin-state transition of ${\mathrm{Fe}}^{2+}$ ions from a high-spin to low-spin (HS-LS) state which is accompanied by a collapse of local magnetic moments. The HS-LS transition results in a substantial drop in the lattice volume by about 4%–8%, implying a complex interplay between electronic and lattice degrees of freedom. Our results reveal a strong sensitivity of the calculated transition pressure $P_{\mathrm{tr}.}$ upon addition of Mg. While, for Fe-rich magnesiowüstite with Mg $x<0.5, P_{\mathrm{tr}.}$ is about 80 GPa, for Mg $x=0.75$ it drops to 52 GPa, i.e., by 35%. This behavior is accompanied by a substantial change in the spin transition range from 50 to 140 GPa in FeO to 30 to 90 GPa for $x=0.75$. In addition, the calculated bulk modulus (in the HS state) is found to increase by $\sim 12\%$ from 142 GPa in FeO to 159 GPa in (Fe,Mg)O with Mg $x=0.875$. We find that the pressure-induced HS-LS transition has different consequences for the electronic properties of the Fe-rich and -poor (Fe,Mg)O. For the Fe-rich (Fe,Mg)O, the transition is found to be accompanied by a Mott insulator to a (semi)metal phase transition. In contrast to that, for $x>0.25$, (Fe,Mg)O remains insulating up to the highest studied pressures, implying a Mott-insulator to band-insulator phase transition at the HS-LS transformation.