The quasiparticle band structure and dielectric function for the so-called magic sequence SiGe${}_2$Si${}_2$Ge${}_2$SiGe${}_{12}$ (or ${\alpha }_{12}$) structure [Phys. Rev. Lett. 108, 027401 (2012)] are calculated by many-body perturbation theory (MBPT) within an ab initio framework. On top of density functional calculations within the local density approximation (LDA) leading to a fundamental band gap of 0.23 eV, we have computed the quasiparticle band structure within the $G_0{W}_0$ approach, opening the gap to 0.61 eV. Moreover, we have calculated the optical properties by solving the Bethe-Salpeter equation (BSE) for the electron-hole two-particle correlation function. When comparing the imaginary part of the dielectric function obtained at various levels of approximation–i.e., the independent particle approximation (or random phase approximation) based on (i) the LDA or (ii) $GW$ band structures, and (iii) the BSE including local field effects and electron-hole correlations—we observe that the important first transition is better explained by taking into account excitonic effects. Moreover, the onset transition originating from the direct transition of the magic sequence structure is also investigated.

It has been recently realized that strong interactions in topological Bloch bands give rise to the appearance of novel states of matter. Here we study connections between these systems—fractional Chern insulators and the fractional quantum Hall states—via generalization of a gauge-fixed Wannier-Qi construction in the cylinder geometry. Our setup offers a number of important advantages compared to the earlier exact diagonalization studies on a torus. Most notably, it gives access to edge states and to a single-cut orbital entanglement spectrum, hence to the physics of bulk-edge correspondence. It is also readily implemented in the state-of-the-art density matrix renormalization group method that allows for numerical simulations of significantly larger systems. We demonstrate our general approach on examples of flat-band models on ruby and kagome lattices at bosonic filling fractions $\nu =1/2$ and $\nu =1$, which show the signatures of (non)-Abelian phases, and establish the correspondence between the physics of edge states and the entanglement in the bulk. Notably, we find that the non-Abelian $\nu =1$ phase can be stabilized by purely on-site interactions in the presence of a confining potential.

The effect of a magnetic field on the charged vacuum is investigated. The field dependence of the energy levels causes jumps in the total vacuum charge that occur whenever an energy level crosses the Fermi level, and this leads to reentrant cycles of vacuum charging and discharging. In atomic systems these effects require astrophysical magnetic fields of around ${10}^8$ T, but in graphene with a mass gap they occur in laboratory fields of about 1 T or lower. It is suggested that an electrostatic graphene quantum dot defined by a gate electrode provides a solid state model of the as yet unobserved charged vacuum as well as a model of an atomic system in an extreme astrophysical environment. Phase diagrams are computed to show how the total vacuum charge depends on the confining potential strength and applied magnetic field. The vacuum charge density is also investigated and experimental consequences are discussed.

We have investigated the transport properties of CrO${}_2$ thin films deposited on TiO${}_2$ and sapphire substrates and find subtle differences with respect to earlier reports. The films are good metals down to low temperatures, with residual resistivities of the order of 6 $\mu \Omega$ cm for films deposited on TiO${}_2$ and two times higher for films on sapphire substrates. Magnetoresistance (MR) measurements in high fields show an as yet unobserved nonmonotonic behavior, which is particularly pronounced around the sign change that takes place from negative to positive at a temperature around 100 K. Moreover, both the ordinary and anomalous Hall coefficients show considerable changes around 100–150 K, suggesting a change in carrier density together with the onset of the influence of spin defects in this temperature window. At lower temperatures, the MR is a linear function of the applied field, which can be explained as intergrain tunneling MR. This interpretation is also suggested by the angular MR. Planar Hall effect measurements reveal that the CrO${}_2$ thin films are not in a single magnetic domain state even for films deposited on an isostructural TiO${}_2$ substrate.

By means of ${}^{27}$Al NMR studies, we have investigated magnetic excitations and the hybridization effect between Pr 4$f$ and conduction electrons in a pair of cubic compounds Pr$Tr$${}_2$Al${}_{20}$ ($Tr=\text{Ti}$, V). From Knight shift measurements we have evaluated comparative strengths for the $c$-$f$ hybridization effect in these compounds and confirm a definite increase of hybridization on replacing Ti with V. Analysis of the spin-lattice relaxation rate $1/T_1$ reveals that the spectral width of magnetic fluctuations in crystalline electric field excited states is strongly temperature dependent. This result indicates that magnetic fluctuations are dominated by strong $c$-$f$ exchange coupling at high temperatures and thus is nicely consistent with the Kondo picture that features $\ln T$ dependence of resistivity over the same temperature region.

We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors, the transit time of spin-polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance. However, in none of the experiments do we observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle-type effects indicates that the magnetoresistance is not based on the injection of spin-polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance. These measurements suggest that even for clear spin-valve signatures, control experiments should be established which can confirm or disprove the electrical detection of injected spin-polarized carriers, similar to the Hanle precession in inorganic spin valves.

A single-mode microcavity with an embedded Aharonov-Bohm quantum ring, which is pierced by a magnetic flux and subjected to a lateral electric field, is studied theoretically. It is shown that external electric and magnetic fields provide additional means of control of the emission spectrum of the system. In particular, when the magnetic flux through the quantum ring is equal to a half-integer number of the magnetic flux quantum, a small change in the lateral electric field allows tuning of the energy levels of the quantum ring into resonance with the microcavity mode providing an efficient way to control the quantum ring-microcavity coupling strength. Emission spectra of the system are calculated for several combinations of the applied magnetic and electric fields.

The behavior of electrons in strained graphene is usually described using effective pseudomagnetic fields in a Dirac equation. Here we consider the particular case of a spatially constant strain. Our results indicate that lattice corrections are easily understood using a strained reciprocal space, in which the whole energy dispersion is simply shifted and deformed. This leads to a directional-dependent Fermi velocity without producing pseudomagnetic fields. The corrections due to atomic wave function overlap changes tend to compensate such effects. Also, the analytical expressions for the shift of the Dirac points, which do not coincide with the $K$ points of the renormalized reciprocal lattice, as well as the corresponding Dirac equation are found. In view of the former results, we discuss the range of applicability of the usual approach of considering pseudomagnetic fields in a Dirac equation derived from the old Dirac points of the unstrained lattice or around the $K$ points of the renormalized reciprocal lattice. Such considerations are important if a comparison is desired with experiments or numerical simulations.

Based on the noncrossing approximation, we calculate both the linear and nonlinear conductance within the two-lead two-channel single-impurity Anderson model where the conduction electron density of states vanishes in a power-law fashion $\propto |\omega -{\mu }_F{|}^r$ with $r=1$ near the Fermi energy, appropriate for a hexagonal system. For given gate voltage, we address the universal crossover from a two-channel Kondo phase, argued to occur in doped graphene, to an unscreened local moment phase. We extract universal scaling functions in conductance governing charge transfer through the two-channel pseudogap Kondo impurity and discuss our results in the context of a recent scanning tunneling spectroscopy experiment on Co-doped graphene.

Hund's rule of maximum multiplicity controls the spin configuration of a system with degenerate midgap states, while Lieb's theorem determines the total spin of the ground state of a bipartite lattice. Most of graphene nanostructures follow both Hund's rule and Lieb's theorem, except that a number of nanoflakes with degenerate midgap states violates Hund's rule by exhibiting antiferromagnetic ground states as predicted by Lieb's theorem. To determine why Hund's rule fails and Lieb's theorem can predict correct spin configuration beyond the Hubbard model for which it was proved, we solve the strongly correlated electron states in such graphene nanoflakes where Hund's rule is violated by using a full configuration interaction approach. We find that long-range electron-electron interactions are quenched in both the ground and low-lying excited states of these systems. As there are no interactions between electrons with parallel spins in the system where long-range interactions are absent, we point out that a graphene nanoflake with degenerate midgap states and an antiferromagnetic ground state, in fact, falls very close to an ideal Hubbard system. Quenching of long-range interactions is therefore believed to be the natural explanation for the advantage of Lieb's theorem over Hund's rule in graphene nanostructures.

We present a three-band tight-binding (TB) model for describing the low-energy physics in monolayers of group-VIB transition metal dichalcogenides $M{X}_2$ ($M=\text{Mo}$, W; $X=\text{S}$, Se, Te). As the conduction- and valence-band edges are predominantly contributed by the $d_{z^2}$, $d_{xy}$, and $d_{x^2-y^2}$ orbitals of $M$ atoms, the TB model is constructed using these three orbitals based on the symmetries of the monolayers. Parameters of the TB model are fitted from the first-principles energy bands for all $M{X}_2$ monolayers. The TB model involving only the nearest-neighbor $M$-$M$ hoppings is sufficient to capture the band-edge properties in the $\pm K$ valleys, including the energy dispersions as well as the Berry curvatures. The TB model involving up to the third-nearest-neighbor $M$-$M$ hoppings can well reproduce the energy bands in the entire Brillouin zone. Spin-orbit coupling in valence bands is well accounted for by including the on-site spin-orbit interactions of $M$ atoms. The conduction band also exhibits a small valley-dependent spin splitting which has an overall sign difference between Mo$X_2$ and W$X_2$. We discuss the origins of these corrections to the three-band model. The three-band TB model developed here is efficient to account for low-energy physics in $M{X}_2$ monolayers, and its simplicity can be particularly useful in the study of many-body physics and physics of edge states.

The structural, electronic, and vibrational properties of glassy Ge${}_x$Se${}_{1-x}$ are studied using density-functional-based molecular dynamics. The focus is on four compositions ($x=10\%,20\%,25\%,33\%$) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments.

Spin-polarized electrical transport is investigated in ${\text{Al}}_2{\text{O}}_3/{\text{Ni}}_{80}{\text{Fe}}_{20}/{\text{Al}}_2{\text{O}}_3$ thin films for permalloy thickness between 6 and 20 nm. The degree of spin polarization of the current flowing in the plane of the film is measured through the current-induced spin-wave Doppler shift. We find that it decreases as the film thickness decreases from 0.63 at 20 nm to 0.42 at 6 nm. This decrease is attributed to a spin depolarization induced by the film surfaces. A model is proposed which takes into account the contributions of the different sources of electron scattering (alloy disorder, phonons, thermal magnons, grain boundaries, film surfaces) to the measured spin-dependent resistivities.

The role of the strain state in epitaxial (001)-oriented Cu/Ni(14 nm)/Cu rings is investigated using a combination of magnetic force microscopy and finite-element calculations. Rings with an external diameter of 3 and 2 $\mu$m and linewidth $W$ larger than 400 nm show two different structures: domains with magnetization oriented in the radial direction exist at the inner and outer radius, separated by an area in the interior of the ring consisting of stripe domains with perpendicular magnetization. The former is the sole magnetic structure observed for $W<400$ nm. Micromagnetic calculations on narrow-linewidth structures indicate that the radial domain-wall structure consists of elliptical Bloch lines with a shorter and longer length along the tangential and radial directions, respectively. Finite-element calculations show that the anisotropic relaxation of the in-plane strain is larger at the ring inner and outer edges than in the interior part of the ring and accounts for the reorientation of the magnetization direction.

Phase slips in a one-dimensional closed array of Josephson junctions hybridize the persistent current states and plasmon branches of excitations. The interference between phase slips passing through different junctions of the array makes the hybridization sensitive to the charges of the superconducting islands comprising the array. This in turn results in the Aharonov-Casher effect for plasmons, which in the absence of phase slips are insensitive to island charges.

The magnetic phase diagram of La${}_{2-x}$Sr${}_x$CuO${}_4$ thin films grown on single-crystal LaSrAlO${}_4$ substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La${}_{2-x}$Sr${}_x$CuO${}_4$, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Néel temperature $T_{\mathrm{N}}$ is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase ($x\gtrsim 0.02$) the transition temperature to the cluster spin-glass state $T_{\mathrm{g}}$ is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed.

We report the results of low-temperature heat capacity measurements of the pnictide oxide superconductor BaTi${}_2$Sb${}_2$O doped with sodium. The temperature- and field-dependent heat capacity data are well described by a single-gap BCS theory. The estimated values for the normal-state Sommerfeld constant, the heat capacity jump at $T_c$, and the electron-phonon coupling constant are in favor of a conventional weak coupling superconductivity mediated by electron-phonon interaction. The results are discussed with regard to and compared with recent first-principles calculations.

We address the question of the origin of the recently discovered chiral property of the charge-density-wave phase in 1$T$-TiSe${}_2$, which so far lacks a microscopic understanding. We argue that the lattice degrees of freedom seem to be crucial for this novel phenomenon. We motivate a theoretical model that takes into account one valence and three conduction bands, a strongly screened Coulomb interaction between the electrons, as well as the coupling of the electrons to a transverse optical phonon mode. The Falicov-Kimball model extended in this way possesses a charge-density-wave state at low temperatures, which is accompanied by a periodic lattice distortion. The charge ordering is driven by a lattice deformation and electron-hole pairing (excitonic) instability in combination. We show that both electron-phonon interaction and phonon-phonon interaction must be taken into account at least up to quartic order in the lattice displacement to achieve a stable chiral charge order. The chiral property is exhibited in the ionic displacements. Furthermore, we provide the ground-state phase diagram of the model and give an estimate of the electron-electron and electron-phonon interaction constants for 1$T$-TiSe${}_2$.

We study the temperature and frequency dependence of the dielectric constant and the ac conductivity in the dimer Mott insulator family ${\beta }^{\prime }$-type $\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2$ (where dmit denotes 1,3-dithiole–2-thione–4,5-dithiolate). We find that all compounds within this family have relaxorlike dielectric relaxation with a Curie-Weiss increase in the dielectric constant with decreasing temperature. An analysis of the dielectric data shows a clear relation between electronic correlation and the dipole-dipole interaction; also, geometrical frustration appears to weaken this interaction. Furthermore, the dc and ac conductivities are shown to be closely correlated and follow the same universal ac conductivity as disordered solids.

Spins of negatively charged nitrogen-vacancy (NV${}^{-}$) defects in diamond are among the most promising candidates for solid-state qubits. The fabrication of quantum devices containing these spin-carrying defects requires position-controlled introduction of NV${}^{-}$ defects having excellent properties such as spectral stability, a long spin coherence time, and a stable negative charge state. Nitrogen ion implantation and annealing enable the positioning of NV${}^{-}$ spin qubits with high precision, but to date, the coherence times of qubits produced this way are short, presumably because of the presence of residual radiation damage. In the present work, we demonstrate that a high-temperature annealing at 1000 ${}^{\circ }$C allows 2 ms coherence times to be achieved at room temperature. These results were obtained for implantation-produced NV${}^{-}$ defects in a high-purity, 99.99% ${}^{12}$C-enriched single crystal chemical vapor deposited diamond. We discuss these long coherence times in the context of the thermal behavior of residual defect spins.