The role of hydrogen in the photoluminescence (PL) of ZnO was investigated using four different types of bulk ZnO single crystal, with varying concentrations of unintentional hydrogen donor and Group I acceptor impurities. Photoluminescence spectra were measured at 3 K, with emission energies determined to ±50 μeV, before and after separate annealing in ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{H}}_2$ atmospheres. Using this approach, several new hydrogen-related neutral-donor-bound excitons, and their corresponding B exciton, ionized donor, and two electron satellite (TES) excited state transitions were identified and their properties further investigated using hydrogen and deuterium ion implantation. The commonly observed $I_4$ (3.36272 eV) emission due to excitons bound to multicoordinated hydrogen inside an oxygen vacancy (${\mathrm{H}}_{\mathrm{O}}$), that is present in most ZnO material, was noticeably absent in hydrothermally grown (HT) ZnO and instead was replaced by a doublet of two closely lying recombination lines $I_{4b,c}$ (3.36219, 3.36237 eV) due to a hydrogen-related donor with a binding energy ($E_D$) of 47.7 meV. A new and usually dominant recombination line $I_{6\text{−}H}$ (3.36085 eV) due to a different hydrogen-related defect complex with an $E_D$ of 49.5 meV was also identified in HT ZnO. Here, $I_{4b,c}$ and $I_{6\text{−}H}$ were stable up to approximately 400 and 600 °C, respectively, indicating that they are likely to contribute to the unintentional $n$-type conductivity of ZnO. Another doublet $I_5$ (3.36137, 3.36148 eV) was identified in hydrogenated HT ZnO single crystals with low Li concentrations, and this was associated with a defect complex with an $E_D$ of 49.1 meV. A broad near band edge (NBE) emission centered at 3.366 eV was associated with excitons bound to subsurface hydrogen. We further demonstrate that hydrogen incorporates on different lattice sites for different annealing conditions and show that the new features $I_{4b,c}$, $I_{6\text{−}H}$, and $I_5$ most likely originate from the lithium-hydrogen defect complexes $\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{\mathrm{O}}$, $\mathrm{A}{\mathrm{l}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{\mathrm{O}}\text{−}\mathrm{L}{\mathrm{i}}_{\mathrm{Zn}}$, and ${\mathrm{V}}_{\mathrm{Zn}}\text{−}{\mathrm{H}}_{3,4}$, respectively.

We study quantum quenches in the XXZ spin-$1/2$ Heisenberg chain from families of ferromagnetic and antiferromagnetic initial states. Using Bethe ansatz techniques, we compute short-range correlators in the complete generalized Gibbs ensemble (GGE), which takes into account all local and quasilocal conservation laws. We compare our results to exact diagonalization and numerical linked cluster expansion calculations for the diagonal ensemble, finding excellent agreement and thus providing a very accurate test for the validity of the complete GGE. Furthermore, we use exact diagonalization to compute the diagonal entropy in the postquench steady state. We show that the Yang-Yang entropy for the complete GGE is consistent with twice the value of the diagonal entropy in the largest chains or the extrapolated result in the thermodynamic limit. Finally, the complete GGE is quantitatively contrasted with the GGE built using only the local conserved charges (local GGE). The predictions of the two ensembles are found to differ significantly in the case of ferromagnetic initial states. Such initial states are better suited than others considered in the literature to experimentally test the validity of the complete GGE and contrast it to the failure of the local GGE.

We followed the evolution of the normal and superconducting properties of Al thin films after each session of various successive oxygen irradiations at ambient temperature. Such irradiated films, similar to the granular ones, exhibit enhanced superconductivity, Kondo behavior, and negative-curvature resistivity. Two distinct roles of oxygen are identified: as a damage-causing projectile and as an implanted oxidizing agent. The former gives rise to the processes involved in the conventional recovery stages. The latter, considered within the context of the Cabrera-Mott model, gives rise to a multistep process which involves charges transfer and creation of stabilized vacancies and charged defects. Based on the outcome of this multistep process, we consider (i) the negative-curvature resistivity as a manifestation of a thermally assisted liberation of trapped electric charges, (ii) the Kondo contribution as a spin-flip scattering from paramagnetic, color-center-type defects, and (iii) the enhancement of $T_{\text{c}}$ as being due to a lattice softening facilitated by the stabilized defects and vacancies. The similarity in the phase diagrams of granular and irradiated films as well as the aging effects are discussed along the same line of reasoning.

We investigate the scattering of an electron by phonons in a small structure between two one-dimensional tight-binding leads. This model mimics the quantum electron transport through atomic wires or molecular junctions coupled to metallic leads. The electron-phonon-coupled structure is represented by the Holstein model. We observe permanent energy transfer from the electron to the phonon system (dissipation), transient self-trapping of the electron in the electron-phonon-coupled structure (due to polaron formation and multiple reflections at the structure edges), and transmission resonances that depend strongly on the strength of the electron-phonon coupling and the adiabaticity ratio. A recently developed TEBD algorithm, optimized for bosonic degrees of freedom, is used to simulate the quantum dynamics of a wave packet launched against the electron-phonon-coupled structure. Exact results are calculated for a single electron-phonon site using scattering theory and analytical approximations are obtained for limiting cases.

Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\right)}_y{\mathrm{As}}_2$, $0\le x\le 1$, $1.86\le y\le 2$, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the $c$ axis, persists for $x\lesssim 0.12\left(1\right)$. The antiferromagnetic order is smoothly suppressed with increasing $x$, with both the ordered moment and Néel temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for $x\le 0.25$, nor does ferromagnetic order for $x$ up to at least $x=0.104$, and a smooth crossover from the collapsed-tetragonal (cT) phase of ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ to the tetragonal (T) phase of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ occurs. These results suggest that hole doping ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca.

We carry out a thorough study of the topological Hall and topological spin Hall effects in disordered skyrmionic systems: the dimensionless (spin) Hall angles are evaluated across the energy-band structure in the multiprobe Landauer-Büttiker formalism and their link to the effective magnetic field emerging from the real-space topology of the spin texture is highlighted. We discuss these results for an optimal skyrmion size and for various sizes of the sample and find that the adiabatic approximation still holds for large skyrmions as well as for nanoskyrmions. Finally, we test the robustness of the topological signals against disorder strength and show that the topological Hall effect is highly sensitive to momentum scattering.

Here we report the discovery of superconductivity in multiple phases of the compressed $\mathrm{GeS}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_4$ (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous $(a$-GST) and for the crystalline $(c$-GST) phases. The superconducting critical temperature $T_c$ continuously increases with applied pressure, reaching a maximum $T_c=6\mathrm{K}$ at $P=20\mathrm{GPa}$ for $a$-GST, whereas the critical temperature of the cubic phase reaches a maximum $T_c=8\mathrm{K}$ at 30 GPa. This material system, exhibiting a superconductor-insulator quantum phase transition, has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus.

Nonlocality is one of the most striking signatures of the topological nature of Weyl semimetals. We propose to probe the nonlocality in these materials via a measurement of a magnetic-field-dependent Coulomb drag between two sheets of graphene which are separated by a three-dimensional slab of Weyl semimetal. We predict a mechanism of Coulomb drag, based on cyclotron orbits that are split between opposite surfaces of the semimetal. In the absence of impurity scattering between different Weyl nodes, this mechanism does not decay with the thickness of the semimetal.

We study magnetic, transport, and thermodynamic properties of the half-filled two-dimensional $\left(2D\right)$ Hubbard model with layered distributed repulsive interactions using unbiased finite temperature quantum Monte Carlo simulations. Antiferromagnetic long-ranged correlations at $T=0$ are confirmed by means of the magnetic structure factor and the onset of short-ranged ones is at a minimum temperature, which can be obtained by peaks in susceptibility and specific heat following a random-phase-approximation (RPA) prediction. We also show that transport is affected in the large interaction limit and is enhanced in the nonrepulsive layers suggesting a change of dimensionality induced by increased interactions. Lastly, we show that by adiabatically switching the interactions in layered distributed patterns reduces the overall temperature of the system with a potential application in cooling protocols in cold atoms systems.

Iron oxide is a key compound to understand the state of the deep Earth. It has been believed that previously known oxides such as FeO and ${\mathrm{Fe}}_2{\mathrm{O}}_3$ will be dominant at the mantle conditions. However, the recent observation of ${\mathrm{FeO}}_2$ shed another light to the composition of the deep lower mantle (DLM), and thus understanding of the physical properties of ${\mathrm{FeO}}_2$ will be critical to model the DLM. Here, we report the electronic structure and structural properties of ${\mathrm{FeO}}_2$ by using density functional theory and dynamic mean-field theory. The crystal structure of ${\mathrm{FeO}}_2$ is composed of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{O}}_2{}^{2-}$ dimers, where the Fe ions are surrounded by the octahedral O atoms. We found that ${\mathrm{FeO}}_2$ shows a metal-insulator transition (MIT) under high pressure. The MIT is not a Mott type but a band insulator type which is driven by the ${\mathrm{O}}_2$ dimer bond length change. However, the correlation effect of Fe $3d$ orbitals should be considered to correctly describe ${\mathrm{O}}_2$ dimer bond length of ${\mathrm{FeO}}_2$ and the MIT.

In this work, we study the effects of spin-orbit and Coulomb anisotropy on the electronic and magnetic properties of the Mott insulator ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. We use the local-density approximation + dynamical mean-field approach and spin-wave theory. We show that, contrary to a recent proposal, the Mott metal-insulator transition is not induced by the spin-orbit interaction. We confirm that, instead, it is mainly driven by the change in structure from long to short $\mathbf{c}$-axis layered perovskite. We show that the magnetic ordering and the anisotropic Coulomb interactions play a small role in determining the the size of the gap. The spin-orbit interaction turns out to be essential for describing the magnetic properties. It not only results in a spin-wave gap, but it also enlarges significantly the magnon bandwidth.

The intense theoretical and experimental interest in topological insulators and semimetals has established band structure topology as a fundamental material property. Consequently, identifying band topologies has become an important, but often challenging, problem, with no exhaustive solution at the present time. In this work we compile a series of techniques, some previously known, that allow for a solution to this problem for a large set of the possible band topologies. The method is based on tracking hybrid Wannier charge centers computed for relevant Bloch states, and it works at all levels of materials modeling: continuous $\mathbf{k}·\mathbf{p}$ models, tight-binding models, and ab initio calculations. We apply the method to compute and identify Chern, ${\mathbb{Z}}_2$, and crystalline topological insulators, as well as topological semimetal phases, using real material examples. Moreover, we provide a numerical implementation of this technique (the Z2Pack software package) that is ideally suited for high-throughput screening of materials databases for compounds with nontrivial topologies. We expect that our work will allow researchers to (a) identify topological materials optimal for experimental probes, (b) classify existing compounds, and (c) reveal materials that host novel, not yet described, topological states.

Due to its wide band gap and availability as a single crystal, $\beta -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ has potential for applications in many areas of micro/optoelectronics and photovoltaics. Still, little is as yet known about its intrinsic defects, which may influence carrier concentrations and act as recombination centers. From a theoretical point of view, the problem is that standard (semi)local approximations of density functional theory usually cannot handle wide band-gap oxides, while results of tuned hybrid functional calculations so far have shown little quantitative coincidence with experimental data on $\beta -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. Here, we show a method for selecting the optimal hybrid, which reproduces not only the band gap, but also satisfies the generalized Koopmans’ theorem. Unless the screening is strongly orbital/direction dependent in the given material, such an optimal hybrid can reproduce the whole $GW$ band structure quite accurately. With the optimized functional, and introducing a modification into the charge correction process, we are able to give a consistent description of observed carrier trapping by intrinsic defects in $\beta -\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$. With the exception of gallium interstitials, which can act as shallow donors, all other intrinsic defects are deep. Gallium vacancies are the main compensating acceptors in $n$-type samples, while both oxygen interstitials and vacancies act as hole traps, in addition to small hole polarons. Considering the limitations imposed by a medium-sized (160-atom) supercell in an ionic solid, the calculated adiabatic and vertical transitions are in good agreement with available experimental data.

We create an interface of graphene with a metallic and magnetic support that leaves its electronic structure largely intact. This is achieved by exposing epitaxial graphene on ferromagnetic thin films of Co and Ni to vapor of the rare earth metal Eu at elevated temperatures, resulting in the intercalation of an Eu monolayer in between graphene and its substrate. The system is atomically well defined, with the Eu monolayer forming a $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ superstructure with respect to the graphene lattice. Thereby, we avoid the strong hybridization with the (Ni,Co) substrate $3d$ states that otherwise drastically modify the electronic structure of graphene. This picture is suggested by our x-ray absorption spectroscopy measurements which show that after Eu intercalation the empty $2p$ states of C atoms resemble more the ones measured for graphite in contrast to graphene directly bound to $3d$ ferromagnetic substrates. We use x-ray magnetic circular dichroism at the Co and Ni $L_{2,3}$ and Eu $M_{4,5}$ as an element-specific probe to investigate magnetism in these systems. An antiferromagnetic coupling between Eu and Co/Ni moments is found, which is so strong that a magnetic moment of the Eu layer can be detected at room temperature. Density functional theory calculations confirm the antiferromagnetic coupling and provide an atomic insight into the magnetic coupling mechanism.

We have studied the magnetoresistance of a quantum point contact fabricated on a high mobility two-dimensional electron gas (2DEG) exposed to microwave irradiation. The resistance reveals giant $B$-periodic oscillations with a relative amplitude $\mathrm{\Delta }R/R$ of up to $700\%$ resulting from the propagation and interference of the edge magnetoplasmons (EMPs) in the sample. This giant photoconductance is attributed to the considerably large local electron density modulation in the vicinity of the point contact. We have also analyzed the oscillation periods $\mathrm{\Delta }B$ of the resistance oscillations and, comparing the data with the EMP theory, extracted the EMP interference length $L$. We have found that the length $L$ substantially exceeds the distance between the contact leads, and rather corresponds to the distance between metallic contact pads measured along the edge of the 2DEG. This resolves existing controversy in the literature and should help to properly design highly sensitive microwave and terahertz spectrometers based on the discussed effect.

We study the interplay of particle-hole symmetry and fermion-vortex duality in multicomponent half-filled Landau levels, such as quantum Hall gallium arsenide bilayers and graphene. For the $\nu =1/2+1/2$ bilayer, we show that particle-hole-symmetric interlayer Cooper pairing of composite fermions leads to precisely the same phase as the electron exciton condensate realized in experiments. This equivalence is easily understood by applying the recent Dirac fermion formulation of $\nu =1/2$ to two components. It can also be described by Halperin-Lee-Read composite fermions undergoing interlayer $p_x+i{p}_y$ pairing. A renormalization group analysis showing strong instability to interlayer pairing at large separation $d\to \infty$ demonstrates that two initially decoupled composite Fermi liquids can be smoothly tuned into the conventional bilayer exciton condensate without encountering a phase transition. We also discuss multicomponent systems relevant to graphene, derive related phases including a $Z_2$ gauge theory with spin-half visons, and argue for symmetry-enforced gaplessness under full $\mathrm{SU}\left(N_f\right)$ flavor symmetry when the number of components $N_f$ is even.