Quantum dots in GaAs/InGaAs structures have been proposed as a candidate system for realizing quantum computing. The short coherence time of the electronic quantum state that arises from coupling to the nuclei of the substrate is dramatically increased if the system is subjected to a magnetic field and to repeated optical pulsing. This enhancement is due to mode locking: oscillation frequencies resonant with the pulsing frequencies are enhanced, while off-resonant oscillations eventually die out. Because the resonant frequencies are determined by the pulsing frequency only, the system becomes immune to frequency shifts caused by the nuclear coupling and by slight variations between individual quantum dots. The effects remain even after the optical pulsing is terminated. In this work, we explore the phenomenon of mode locking from a quantum mechanical perspective. We treat the dynamics using the central-spin model, which includes coupling to 10–20 nuclei and incoherent decay of the excited electronic state, in a perturbative framework. Using scaling arguments, we extrapolate our results to realistic system parameters. We estimate that the synchronization to the pulsing frequency needs time scales in the order of $1\mathrm{s}$.

The application of the Pancharatnam-Berry (PB) phase approach to the design of nonlinear metasurfaces has recently enabled subdiffractive phase control over the generated nonlinear fields, embedding phased array features in ultrathin structures. Here, we rigorously model, analyze, and design highly efficient nonlinear metasurfaces with advanced functionalities, including the generation of pencil beams steered in arbitrary directions in space, as well as vortex beams with polarization-dependent angular momentum, and we extend the PB approach to various nonlinear processes. To this purpose, we develop an accurate and efficient theoretical framework—inspired by the linear phase array theory—based on the effective nonlinear susceptibility method, thus avoiding the use of time-consuming numerical simulations. Our findings allow exploiting the flat nonlinear optics paradigm, enabling exciting applications based on subwavelength field control over flat and large-scale structures with giant nonlinear responses.

Single-crystalline ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ has been grown by a floating-zone melting method, and its magnetic, thermal, and transport properties have been investigated through measurements of magnetization, specific heat, and electrical resistivity to reveal its peculiar magnetism. It is shown that ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ undergoes a second-order phase transition at $T_{\text{m}}$ = 19 K, which had been believed to be ferromagnetic ordering in the literature, from a paramagnetic phase to an uncompensated antiferromagnetic phase with spontaneous magnetization along the tetragonal $c$ axis (the easy magnetization direction). The magnetic entropy analysis points to the itinerant character of $5f$ electrons in the magnetic ordered state of ${\mathrm{UAu}}_2{\mathrm{Si}}_2$ with large enhancement of the electronic specific heat coefficient of $\gamma \sim 150$ mJ/${\mathrm{K}}^2\mathrm{mol}$ at 2 K. It also reveals the relatively isotropic crystalline electric field effect of this compound, with contrast to the other relative isostructural compounds. The observed magnetization curves strongly suggest that there is a parasitic ferromagnetic component developing below $\sim 50$ K in high coercivity with the easy axis along the tetragonal $c$ axis. The results are discussed in the context of evolution of magnetism within the entire family of isostructural $\mathrm{U}{T}_2{\mathrm{Si}}_2$ compounds.

Using the duality between seemingly different (2+1)-dimensional $\left[\right(2+1\left)d\right]$ conformal field theories (CFT) proposed recently [D. T. Son, Phys. Rev. X 5, 031027 (2015); M. A. Metlitski and A. Vishwanath, Phys. Rev. B 93, 245151 (2016); C. Wang and T. Senthil, Phys. Rev. X 6, 011034 (2015); C. Wang and T. Senthil, Phys. Rev. X 5, 041031 (2015); C. Wang and T. Senthil, Phys. Rev. B 93, 085110 (2016); C. Xu and Y.-Z. You, Phys. Rev. B 92, 220416 (2015); D. F. Mross et al., Phys. Rev. Lett. 117, 016802 (2016); A. Karch and D. Tong, arXiv:1606.01893; N. Seiberg et al., arXiv:1606.01989; P.-S. Hsin and N. Seiberg, arXiv:1607.07457], we study a series of $(2+1)d$ stable self-dual interacting CFTs. These CFTs can be realized (for instance) on the boundary of the $3d$ bosonic topological insulator protected by U(1) and time-reversal symmetry $\left(\mathcal{T}\right)$, and they remain stable as long as these symmetries are preserved. When realized as a boundary system, these CFTs can be driven into anomalous fractional quantum Hall states once $\mathcal{T}$ is broken. We demonstrate that the newly proposed dualities allow us to study these CFTs quantitatively through a controlled calculation, without relying on a large flavor number of matter fields. We also propose a numerical test for our results, which would provide strong evidence for the originally proposed duality between Dirac fermion and QED.

We have studied the magnetic properties of ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, which features a honeycomb lattice of magnetic ${\mathrm{Co}}^{2+}$ ions, through macroscopic characterization and neutron diffraction on a powder sample. We have shown that this material orders in a zigzag antiferromagnetic structure. In addition to allowing a linear magnetoelectric coupling, this magnetic arrangement displays very peculiar spatial magnetic correlations, larger in the honeycomb planes than between the planes, which do not evolve with the temperature. We have investigated this behavior by classical Monte Carlo calculations using the $J_1\text{−}{J}_2\text{−}{J}_3$ model on a honeycomb lattice with a small interplane interaction. Our model reproduces the experimental neutron structure factor, although its absence of temperature evolution must be due to additional ingredients, such as chemical disorder or quantum fluctuations enhanced by the proximity to a phase boundary.

In $s$-wave systems, it has been theoretically shown that a ferromagnetic film hosting a skyrmion can induce a bound state embedded in the opposite-spin continuum. In this work, we consider a case of skyrmion-induced state in a $p$-wave superconductor. We find that the skyrmion induces a bound state that generally resides within the spectral gap and is isolated from all other states, in contrast to the case of conventional superconductors. To this end, we derive an approximate expression for the $T$ matrix, through which we calculate the spin-polarized local density of states which is observable in scanning tunneling microscopy measurements. We find the unique spectroscopic features of the skyrmion-induced bound state and discuss how our predictions could be employed as experimental probes for $p$-wave superconducting states.

Cubic boron arsenide (BAs) was predicted to have an exceptionally high thermal conductivity $\left(k\right)\sim 2000\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperature, comparable to that of diamond, based on first-principles calculations. Subsequent experimental measurements, however, only obtained a $k$ of $\sim 200\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. To gain insight into this discrepancy, we measured phonon dispersion of single-crystal BAs along high symmetry directions using inelastic x-ray scattering and compared these with first-principles calculations. Based on the measured phonon dispersion, we have validated the theoretical prediction of a large frequency gap between acoustic and optical modes and bunching of acoustic branches, which were considered the main reasons for the predicted ultrahigh $k$. This supports its potential to be a super thermal conductor if very-high-quality single-crystal samples can be synthesized.

The scattering of elastic waves by three-dimensional obstacles in isotropic elastic zero-index-metamaterials (ZIM) is theoretically investigated. We show that the zero values of each single effective parameter and their various combinations of the elastic ZIM can produce different types of wave propagation. Particularly, there is no mode conversion when either longitudinal ($P$) wave or transverse ($S$) wave is scattered by the obstacles in a specific type of double-ZIM (DZIM), possessing near zero reciprocal of shear modulus and near zero mass density. When the obstacle is off resonance, elastic waves are scarcely scattered; nevertheless, the scattering cross section of the obstacle can be drastically enhanced by orders of magnitude when it is on resonance. While in another type of DZIM possessing near zero reciprocal of bulk modulus and near zero mass density, mode conversion occurs during the scattering process and many other transmission characteristics are also different to the former. Moreover, enhanced transmission can be realized for various types of single-ZIM (SZIM) by introducing obstacles, and numerical analysis shows that the enhanced transmission is due to resonant modes arisen in the embedded obstacles. We expect that our findings could have potential practical application, such as seismic protection and on-chip phononic devices.

Dopants of transition metal ions in II-VI semiconductors exhibit native 2+ valency. Despite this, 3+ or mixed 3+/2+ valency of iron ions in ZnO was reported previously. Several contradictory mechanisms have been put forward for explanation of this fact so far. Here we analyze Fe valency in ZnO by complementary theoretical and experimental studies. Our calculations within the generalized gradient approximation (GGA+$U)$ indicate that the Fe ion is a relatively shallow donor. Its stable charge state is ${\mathrm{Fe}}^{2+}$ in ideal ZnO, however, the high energy of the (+/0) transition level enhances the compensation of ${\mathrm{Fe}}^{2+}$ to ${\mathrm{Fe}}^{3+}$ by nonintentional acceptors in real samples. Using several experimental methods like electron paramagnetic resonance, magnetometry, conductivity, excitonic magnetic circular dichroism, and magnetophotoluminescence we confirm the 3+ valency of the iron ions in polycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%. We find a predicted increase of $n$-type conductivity upon the Fe doping with the Fe donor ionization energy of $0.25\pm 0.02$ eV consistent with the results of theoretical considerations. Moreover, our magneto-optical measurements confirm the calculated nonvanishing s,p-d exchange interaction between band carriers and localized magnetic moments of the ${\mathrm{Fe}}^{3+}$ ions in the ZnO, being so far an unsettled issue.

We analyze the spin anisotropy of the magnetic susceptibility of ${\mathrm{Sr}}_2\mathrm{RuO}4$ in the presence of spin-orbit coupling and anisotropic strain using quasi-two-dimensional tight-binding parametrization fitted to the angle-resolved photoemission spectroscopy results. Similar to the previous observations we find the in-plane polarization of the low-$\mathbf{q}$ magnetic fluctuations and the out-of-plane polarization of the incommensurate magnetic fluctuation at the nesting wave-vector ${\mathbf{Q}}_1=(2/3\pi ,2/3\pi )$ but also nearly isotropic fluctuations near ${\mathbf{Q}}_2=(\pi /6,\pi /6)$. Furthermore, one finds that, apart from the high-symmetry direction of the tetragonal Brillouin zone, the magnetic anisotropy is maximal, i.e., ${\chi }^{xx}\ne {\chi }^{yy}\ne {\chi }^{zz}$ reflected in the $x$ polarization of the intraband nesting wave-vector ${\mathbf{Q}}_3=(\pi /2,\pi )$. This is a consequence of the orbital anisotropy of the $t_{2g}$ orbitals in momentum space. We also study how the magnetic anisotropy evolves in the presence of the strain and find strong Ising-like ferromagnetic fluctuations near the Lifshitz transition for the $xy$ band.

Prior to the superconducting transition at $T_{\mathrm{c}}\approx 2.3\mathrm{K}, \mathrm{M}{\mathrm{o}}_3\mathrm{S}{\mathrm{b}}_7$ undergoes a symmetry-lowering, cubic-to-tetragonal structural transition at $T_{\mathrm{s}}=53\mathrm{K}$. We have monitored the pressure dependence of these two transitions by measuring the resistivity of $\mathrm{M}{\mathrm{o}}_3\mathrm{S}{\mathrm{b}}_7$ single crystals under various hydrostatic pressures up to 15 GPa. The application of external pressure enhances $T_{\mathrm{c}}$ but suppresses $T_{\mathrm{s}}$ until $P_{\mathrm{c}}\approx 10\mathrm{GPa}$, above which a pressure-induced first-order structural transition takes place and is manifested by the phase coexistence in the pressure range $8\le P\le 12\mathrm{GPa}$. The cubic phase above 12 GPa is also found to be superconducting with a higher $T_{\mathrm{c}}\approx 6\mathrm{K}$ that decreases slightly with further increasing pressure. The variations with pressure of $T_{\mathrm{c}}$ and $T_{\mathrm{s}}$ satisfy the Bilbro-McMillan equation, i.e. ${T_{\mathrm{c}}}^n{T_{\mathrm{s}}}^{1-n}=$ constant, thus suggesting the competition of superconductivity with the structural transition that has been proposed to be accompanied with a spin-gap formation at $T_{\mathrm{s}}$. Our first-principles calculations suggest the importance of magnetism that competes with the superconductivity in $\mathrm{M}{\mathrm{o}}_3\mathrm{S}{\mathrm{b}}_7$.

The distributional exact diagonalization (DED) scheme is applied to the description of Kondo physics in the Anderson impurity model. DED maps Anderson's problem of an interacting impurity level coupled to an infinite bath onto an ensemble of finite Anderson models, each of which can be solved by exact diagonalization. An approximation to the self-energy of the original infinite model is then obtained from the ensemble-averaged self-energy. Using Friedel's sum rule, we show that the particle number constraint, a central ingredient of the DED scheme, ultimately imposes Fermi liquid behavior on the ensemble-averaged self-energy, and thus is essential for the description of Kondo physics within DED. Using the numerical renormalization group (NRG) method as a benchmark, we show that DED yields excellent spectra, both inside and outside the Kondo regime for a moderate number of bath sites. Only for very strong correlations ($U/\mathrm{\Gamma }\gg 10$) does the number of bath sites needed to achieve good quantitative agreement become too large to be computationally feasible.

We consider the subgap physics of a hybrid double-quantum dot Cooper-pair splitter with large single-level spacings, in the presence of tunneling between the dots and finite Coulomb intra- and interdot Coulomb repulsion. In the limit of a large superconducting gap, we treat the coupling of the dots to the superconductor exactly. We employ a generalized master-equation method, which easily yields currents, noise, and cross-correlators. In particular, for finite inter- and intradot Coulomb interaction, we investigate how the transport properties are determined by the interplay between local and nonlocal tunneling processes between the superconductor and the dots. We examine the effect of interdot tunneling on the particle-hole symmetry of the currents with and without spin-orbit interaction. We show that spin-orbit interaction in combination with finite Coulomb energy opens the possibility to control the nonlocal entanglement and its symmetry (singlet/triplet). We demonstrate that the generation of nonlocal entanglement can be achieved even without any direct nonlocal coupling to the superconducting lead.

We develop in detail a model of the charge order in ${\mathrm{NbSe}}_2$ deriving from a strong electron-phonon coupling dependent on the ingoing and outgoing electron momenta as well as the electronic orbitals scattered between. Including both dependencies allows us to reproduce the full range of available experimental observations on this material. The stability of both experimentally-observed charge-ordered geometries (1Q and 3Q) is studied within this model as a function of temperature and uniaxial strain. It is found that a small amount of bulk strain suffices to stabilize the unidirectional order, and that in both ordering geometries, lattice fluctuations arising from the strong electron-phonon coupling act to suppress the onset temperature of charge order, giving a pseudogap regime characterized by local order and strong phase fluctuations.

We investigate the energy spectrum of single-layer black phosphorene nanoribbons (BPNs) by means of a low-energy expansion of a recently proposed tight-binding model that describes electron and hole bands close to the Fermi energy level. Using the continuum approach, we propose boundary conditions based on sublattice symmetries for BPNs with zigzag and armchair edges and show that our results for the energy spectra exhibit good agreement with those obtained by using the five-parameter tight-binding model. We also explore the behavior of the energy gap versus the nanoribbon width $W$. Our findings demonstrate that band gaps of armchair BPNs scale as $1/W^2$, while zigzag BPNs exhibit a $1/W$ tendency. We analyze the different possible combinations of the zigzag edges that result in twofold degenerate and nondegenerate edge states. Furthermore, we obtain expressions for the wave functions and discuss the limit of validity of such an analytical model.

A theoretical model which relates the binding energy of a positively charged exciton in a quantum dot with the confinement energy is presented. It is shown that the binding energy, defined as the energy difference between the corresponding charged and neutral complexes confined on the same excitonic shell, strongly depends on the shell index. Moreover, we found that the ratio of the binding energy for positively charged excitons from the $p$ and $s$ shells of a dot depends mainly on the nearly perfect confinement inside the dot, which is due to the “hidden symmetry” of the multielectron-hole system. We applied the theory to the excitons confined to a single GaAlAs/AlAs quantum dot. The relevant binding energy was determined using microphotoluminescence and microphotoluminescence excitation magnetospectroscopy. We show that within our theory, the confinement energy determined using the ratio of the binding energy corresponds well to the actual confinement energy of the investigated dot.

Hanle spin precession measurements are a common method to extract the spin transport properties of graphene. In epitaxial graphene on silicon carbide, these measurements show unexpected behavior, due to presumed localized states in the carbon buffer layer that is present between the channel and the substrate. As a consequence, the Hanle curve narrows in its magnetic field dependence and can show an unconventional shape, which has been experimentally observed and modeled in previous studies. Here, we extend the previously developed model by assuming that the localized states are charge traps, that have a power-law distribution of trapping times. Our simulations show that the energy dependence of these trapping times can be extracted from the temperature evolution of the Hanle curve, which was previously observed in experiments. Our extended model gives better insight into what processes play a role when a spin channel is coupled to localized states and their relation to the experimental observations.