The angular momentum quantization of chiral gapless modes confined to a circularly shaped interface between two different topological phases is investigated. By examining several different setups, we show analytically that the angular momentum of the topological modes exhibits a highly chiral behavior, and can be coupled to spin and/or valley degrees of freedom, reflecting the nature of the interface states. A simple general one-dimensional model, valid for arbitrarily shaped loops, is shown to predict the corresponding energies and the magnetic moments. These loops can be viewed as building blocks for artificial magnets with tunable and highly diverse properties.

We experimentally demonstrate a three-dimensional x-ray ptychography in combination with precession measurements that can reconstruct multisection images of optically thick objects. We collect a tilt-series ptychographic diffraction data set of a four-layered object and then successfully reconstruct the phase images of each layer by using an improved phase retrieval algorithm with a multislice approach. The present method has great potential for nondestructively determining anisotropic structures in materials science and biology.

Using an optical interferometric method, the cavitation density of bulk superfluid helium at $T=0.96\mathrm{K}$ is measured and found to be ${\rho }_{\mathrm{cav}}=0.1338\pm 0.0002\mathrm{g}{\mathrm{cm}}^{-3}$. A well-established equation of state for liquid helium at negative pressures converts this to the cavitation pressure $P_{\mathrm{cav}}=-5.1\pm 0.1\mathrm{bars}$. This cavitation pressure is consistent with a model taking into account the presence of quantized vortices, but disagrees with previously published experimental values of $P_{\mathrm{cav}}$.

We apply Mie scattering theory to study the interaction of magnetic spheres with microwaves in cavities beyond the magnetostatic and rotating wave approximations. We demonstrate that both strong and ultrastrong coupling can be realized for stand alone magnetic spheres made from yttrium iron garnet (YIG), acting as an efficient microwave antenna. The eigenmodes of YIG spheres with radii of the order mm display distinct higher angular momentum character that has been observed in experiments.

We report depth sensitive investigations of the magnetic interaction between exchange-coupled stacked CoO and ferromagnetic Co bilayers (separated by thick Au layers) as we explore the degree of recovery of the untrained state after the first two field cycles. Such a recovery is expected by field cycling a reorientation field $\left(H_{\mathrm{RE}}\right)$ along a direction (${\mathrm{\Omega }}_{\mathrm{RE}}$) away from the initial field cooling direction. Measurements as a function of ${\mathrm{\Omega }}_{\mathrm{RE}}$ and the strength of $H_{\mathrm{RE}}$ (along each direction) map the influence of ${\mathrm{\Omega }}_{\mathrm{RE}}$ on the reversal mechanism in the layers and thereby the degree of recovery. Our results are consistent with the earlier observations in similar systems that was realized with ${\mathrm{\Omega }}_{\mathrm{RE}}={90}^{\circ }$. We ascribe these partial and/or significant recoveries to the unchanged sense of rotation after initial field cooling of the ferromagnetic magnetization upon each field cycling. Furthermore, in our system, we find that this recovery can be regulated by choosing various other $H_{\mathrm{RE}}$ and ${\mathrm{\Omega }}_{\mathrm{RE}}$ values without changing the rotational sense. The best recipe for recovery is identified for ${\mathrm{\Omega }}_{\mathrm{RE}}={45}^{\circ }$, that can be achieved partially with $H_{\mathrm{RE}}=3.0$ kOe and remain significant even with $H_{\mathrm{RE}}=10.0$ kOe. In this study we not only understand the fundamental mechanism in the recovery of training, but also instigate its technological prospects by lifting the directional restrictions of the reorientation field.

We examine the ionization of a P dopant in a Si(111) nanofilm using first-principles electronic structure calculations with 2D periodic boundary conditions. The electrostatic divergence of a charged periodic system is resolved by defining an electrostatic reference potential along the confined direction. After ionization, there is an overall electrostatic potential drop of the system. A nanofilm with larger periodicity can reduce the potential drop by screening the P ion, and leads to a smaller ionization energy. We compare the ionization energy calculated for the P-doped Si nanofilm with a P-doped Si nanocrystal and a P-doped Si(110) nanowire. As dimensionality decreases, quantum confinement tends to lower the ionization energy by raising the defect level. However, lower dimensionality also reduces screening after P ionization. This leads to a larger electrostatic potential drop and offsets the effect of quantum confinement on the ionization energy.

The pseudolayered character of 3D bulk crystals of antimony has led us to predict its 2D single-layer crystalline phase named antimonene in a buckled honeycomb structure like silicene. Sb atoms also form an asymmetric washboard structure like black phospherene. Based on an extensive analysis comprising ab initio phonon and finite-temperature molecular dynamics calculations, we show that these two single-layer phases are robust and can remain stable at high temperatures. They are nonmagnetic semiconductors with band gaps ranging from 0.3 eV to 1.5 eV, and are suitable for 2D electronic applications. The washboard antimonene displays strongly directional mechanical properties, which may give rise to a strong influence of strain on the electronic properties. Single-layer antimonene phases form bilayer and trilayer structures with wide interlayer spacings. In multilayers, this spacing is reduced and eventually the structure changes to 3D pseudolayered bulk crystals. The zigzag and armchair nanoribbons of the antimonene phases have fundamental band gaps derived from reconstructed edge states and display a diversity of magnetic and electronic properties depending on their width and edge geometry. Their band gaps are tunable with the widths of the nanoribbons. When grown on substrates, such as germanene or Ge(111), the buckled antimonene attains a significant influence of substrates.

In an oxide apertured quantum dot (QD) micropillar cavity-QED system, we observe strong QD hysteresis effects and line-shape modifications even at very low intensities corresponding to $<{10}^{-3}$ intracavity photons. We attribute this to the excitation of charges by the intracavity field; they get trapped at the oxide aperture, where they screen the internal electric field and blueshift the QD transition. This in turn strongly modulates light absorption by cavity-QED effects, eventually leading to the observed hysteresis and line-shape modifications. The cavity also enables us to observe the QD dynamics in real time, and all experimental data agree well with a power-law charging model. The observed charging effect can serve as a tuning mechanism for quantum dots.

Intrinsic disorder in photonic crystal waveguides occurs via rapid fluctuations of the air-dielectric interface and is typically characterized by a quadratic mean surface roughness and a surface correlation length. We theoretically study the impact of correlation length on extrinsic scattering losses and discuss the numerical implementation for several different waveguide designs. The role of correlation length is found to be strongly influenced by the underlying Bloch modes which are dependent on waveguide design and frequency, and can thus be partly controlled via spatial-dispersion engineering. For most frequencies and waveguide designs, we find an asymptotical increase in losses as the correlation length increases; however, we show that for some frequencies and designs, a maximum scattering loss is achieved for a finite correlation length. Our results also demonstrate the importance of choosing an appropriate correlation function for modeling quickly varying disorder.

Control of hyperfine interactions is a fundamental requirement for quantum computing architecture schemes based on shallow donors in silicon. However, at present, there is lacking an atomistic approach including critical effects of central-cell corrections and nonstatic screening of the donor potential capable of describing the hyperfine interaction in the presence of both strain and electric fields in realistically sized devices. We establish and apply a theoretical framework, based on atomistic tight-binding theory, to quantitatively determine the strain and electric-field-dependent hyperfine couplings of donors. Our method is scalable to millions of atoms, and yet captures the strain effects with an accuracy level of DFT method. Excellent agreement with the available experimental data sets allow reliable investigation of the design space of multiqubit architectures, based on both strain only as well as hybrid (strain + field) control of qubits. The benefits of strain are uncovered by demonstrating that a hybrid control of qubits based on (001) compressive strain and in-plane (100 or 010) fields results in higher gate fidelities and or faster gate operations, for all of the four donor species considered (P, As, Sb, and Bi). The comparison between different donor species in strained environments further highlights the trends of hyperfine shifts, providing predictions where no experimental data exists. While faster gate operations are realizable with in-plane fields for P, As, and Sb donors, only for the Bi donor, our calculations predict faster gate response in the presence of both in-plane and out-of-plane fields, truly benefiting from the proposed planar field control mechanism of the hyperfine interactions.

The family of one-dimensional localized solutions to dissipative nonlinear equations includes a variety of objects such as sources, sinks, shocks (kinks), and pulses. These states are in general accompanied by nontrivial density currents, which are not necessarily related to the movement of the object itself. We investigate the existence and physical properties of sink-type solutions in nonresonantly pumped exciton-polariton condensates modeled by an open-dissipative Gross-Pitaevskii equation. While sinks possess density profiles similar to bright solitons, they are qualitatively different objects as they exist in the case of repulsive interactions and represent a heteroclinic solution. We show that sinks can be created in realistic systems with appropriately designed pumping profiles. We also consider the possibility of creating sinks in a two-dimensional configuration with a ring-shaped pumping profile.

A recently discovered phase of orthorhombic iron carbide o-${\mathrm{Fe}}_7{\mathrm{C}}_3$ [Prescher et al., Nat. Geosci. 8, 220 (2015)] is assessed as a potentially important phase for interpretation of the properties of the Earth's core. In this paper, we carry out first-principles calculations on o-${\mathrm{Fe}}_7{\mathrm{C}}_3$, finding properties to be in broad agreement with recent experiments, including a high Poisson's ratio (0.38). Our enthalpy calculations suggest that o-${\mathrm{Fe}}_7{\mathrm{C}}_3$ is more stable than Eckstrom-Adcock hexagonal iron carbide (h-${\mathrm{Fe}}_7{\mathrm{C}}_3$) below approximately 100 GPa. However, at 150 GPa, the two phases are essentially degenerate in terms of Gibbs free energy, and further increasing the pressure towards Earth's core conditions stabilizes h-${\mathrm{Fe}}_7{\mathrm{C}}_3$ with respect to the orthorhombic phase. Increasing the temperature tends to stabilize the hexagonal phase at 360 GPa, but this trend may change beyond the limit of the quasiharmonic approximation.

We present a theoretical study of the electron transmission through the AB-BA stacking boundary in multilayer graphenes. Using the tight-binding model and the transfer matrix method, we calculate the electron transmission probability through the boundary as a function of electron Fermi energy in multilayers from bilayer to five-layer. We find that the transmission is strongly suppressed particularly near the band touching point, suggesting that the electronic conductivity in general multilayer graphenes is significantly interfered with by stacking fault. The conductivity suppression by stacking fault is the strongest in the bilayer graphene, while it is gradually relaxed as the number of layers increases. At a large carrier density, we observe an even-odd effect where the transmission is relatively lower in trilayer and five-layer than in bilayer and four-layer, and this is related to the existence of a monolayerlike linear band in odd layers. For bilayer graphene, we also study the effect of the perpendicular electric field opening an energy gap, and show that the band deformation enhances the electron transmission at a fixed carrier density.

We present scattering cross sections ${\sigma }_{\text{scatt}}$ of ultracold neutrons (UCNs) in liquid deuterium at $T=20.6\text{K}$, as recently measured by means of a transmission experiment. The indispensable thorough raw data treatment procedure is explained. A calculation model for coherent and incoherent scattering in liquid deuterium in the hydrodynamic limit based on appropriate physical concepts is provided and shown to fit the data well. The applicability of the incoherent approximation for UCN scattering in liquid deuterium was tested and found to deliver acceptable results.

We have systematically investigated the variation of the exchange parameters and the ground state in the $S=\frac{1}{2}$ kagome-lattice antiferromagnet (${\mathrm{Rb}}_{1-x}{\mathrm{Cs}}_x$)${}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$ via magnetic measurements using single crystals. One of the parent compounds, ${\mathrm{Rb}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$, which has a distorted kagome lattice accompanied by four sorts of nearest-neighbor exchange interaction, has a disordered ground state described by a pinwheel valence-bond-solid state. The other parent compound, ${\mathrm{Cs}}_2{\mathrm{Cu}}_3{\mathrm{SnF}}_{12}$, which has a uniform kagome lattice at room temperature, has an ordered ground state with the $q=0$ spin structure. The analysis of magnetic susceptibilities shows that with increasing cesium concentration $x$, the exchange parameters increase with the tendency to be uniform. It was found that the ground state is disordered for $x<0.53$ and ordered for $x>0.53$. The pseudogap observed for $x<0.53$ and the Néel temperature for $x>0.53$ approach zero at $x_{\mathrm{c}}\simeq 0.53$. This is indicative of the occurrence of a quantum phase transition at $x_{\mathrm{c}}$.

The magnetocaloric properties of Ni-Co-Mn-Cr-In Heusler alloys have been studied by means of ab initio calculations and Monte Carlo simulations. We discuss the resulting complex spin configurations, the temperature behavior of entropy, as well as the critical temperatures of the phase transitions. The substitution of 5% Co for Ni and 5% Cr for Mn results in a first-order magnetostructural transition from ferromagnetic austenite to antiferromagnetic martensite, which is accompanied by a spin-flip transition upon cooling. As a result, a large magnetization drop and giant inverse magnetocaloric effect can be achieved of $\mathrm{\Delta }{T}_{\text{ad}}\approx 10$ K in a 2 T field across the magnetostructural phase transition.

We utilize intense, single-cycle terahertz pulses to induce collective excitations in the charge-density-wave-ordered underdoped cuprate $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+x}$. These excitations manifest themselves as pronounced coherent oscillations of the optical reflectivity in the transient state, accompanied by minimal incoherent quasiparticle relaxation dynamics. The oscillations occur at frequencies consistent with soft phonon energies associated with the charge-density-wave, but vanish above the superconducting transition temperature rather than that at the charge-density-wave transition. These results indicate an intimate relationship of the terahertz excitation with the underlying charge-density-wave and the superconducting condensate itself.

We develop statistical mechanical methods to predict the thermodynamic properties of dilute vacancies in multicomponent solids from first principles. The approach relies on a coarse-graining procedure to predict dilute vacancy concentrations with Monte Carlo simulations in alloys exhibiting varying degrees of short- and long-range order. We apply this approach to a study of vacancies in hcp based Ti-Al binary alloys and find a strong dependence of the equilibrium vacancy concentration on the Al concentration and the degree of long-range order, especially at low temperature.

The electron-phonon interaction plays a crucial role in many fields of physics and chemistry. Nevertheless, its actual calculation by means of modern many-body perturbation theory is weakened by the use of model Hamiltonians that are based on parameters difficult to extract from the experiments. Such shortcoming can be bypassed by using density-functional theory to evaluate the electron-phonon scattering amplitudes, phonon frequencies, and electronic bare energies. In this work, we discuss how a consistent many-body diagrammatic expansion can be constructed on top of density-functional theory. In that context, the role played by screening and self-consistency when all the components of the electron-nucleus and nucleus-nucleus interactions are taken into account, is paramount. A way to avoid overscreening is notably presented. Finally, we derive cancellation rules as well as internal consistency constraints in order to draw a clear, sound, and practical scheme to merge the many-body perturbation and density-functional theory.