We investigate the bulk orbital angular momentum (AM) in a two-dimensional hole-doped topological superconductor (SC) which is composed of a hole-doped semiconductor thin film, a magnetic insulator, and an $s$-wave SC and is characterized by the Chern number $C=-3$. In the topological phase, $L_z/N$ is strongly reduced from the intrinsic value by the non-particle-hole-symmetric edge states as in the corresponding chiral $f$-wave SCs when the spin-orbit interactions (SOIs) are small, while this reduction of $L_z/N$ does not work for the large SOIs. Here $L_z$ and $N$ are the bulk orbital AM and the total number of particles at zero temperature, respectively. As a result, $L_z/N$ is discontinuous or continuous at the topological phase transition depending on the strengths of the SOIs. We also discuss the effects of the edge states by calculating the radial distributions of the orbital AM.

A variational Monte Carlo method for bosonic lattice models is introduced. The method is based on the Baeriswyl projected wave function. The Baeriswyl wave function consists of a kinetic energy based projection applied to the wave function at infinite interaction, and is related to the shadow wave function already used in the study of continuous models of bosons. The wave function at infinite interaction, and the projector, are represented in coordinate space, leading to an expression for expectation values which can be evaluated via Monte Carlo sampling. We calculate the phase diagram and other properties of the bosonic Hubbard model. The calculated phase diagram is in excellent agreement with known quantum Monte Carlo results. We also analyze correlation functions.

Superconducting ${\mathrm{MgB}}_2$ shows an $E_{2g}$ zone center phonon, as measured by Raman spectroscopy, that is very broad in energy and temperature dependent. The Raman shift and lifetime show large differences with the values elsewhere in the Brillouin zone measured by inelastic x-ray scattering (IXS), where its dispersion can be accounted for by standard harmonic phonon theory, adding only a moderate electron-phonon coupling. Here we show that the effects rapidly disappear when electron-phonon coupling is switched off by Al substitution on the Mg sites. Moreover, using IXS with very high wave-vector resolution in ${\mathrm{MgB}}_2$, we can follow the dispersion connecting the Raman and the IXS signal, in agreement with a theory using only electron-phonon coupling but without strong anharmonic terms. The observation is important in order to understand the effects of electron-phonon coupling on zone center phonon modes in ${\mathrm{MgB}}_2$, and also in all metals characterized by a small Fermi velocity in a particular direction, typical for layered compounds.

We report a detailed muon-spin rotation study of the pressure evolution of the magnetic order in the manganese-based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors $\mathbf{Q}$ aligned along the crystallographic $c$ and $b$ directions, respectively) which transform into each other through first-order phase transitions as a function of pressure and temperature. Our data suggest that the magnetic state from which superconductivity develops at higher pressures is an incommensurate helical phase.

We report the structural changes of three $\mathrm{YF}{\mathrm{e}}_2{\mathrm{O}}_{4-\delta }$ ($\delta <0.1$) specimens using high resolution synchrotron x-ray powder diffraction between 80 and 300 K. All samples adopt a rhombohedral cell at room temperature (space group $R\overline{3}m$). This cell becomes unstable for the three samples on cooling, and the oxygen-poor specimen ($\delta \sim 0.1$) shows a single transition at 240 K. The nearly stoichiometric ($\delta \le 0.03$) compounds exhibit two structural transitions with decreasing temperature at about 240 and 200 K. Each transition is revealed by an anomaly in the heat capacity measurements and a jump in the electric resistivity. Below 240 K, a strong splitting of some diffraction peaks is accompanied by the occurrence of superstructure peaks that follow the propagation vector $\mathbf{k}=(1/7,-2/7,9/7)$. The cell symmetry is then triclinic, and the structural transition is characterized by an expansion of the $c$ axis coupled to a contraction of the other two lattice parameters. There are 49 nonequivalent sites for Fe atoms with a maximum charge disproportionation of $\sim 0.5{\mathrm{e}}^{-}$. Upon cooling at 200 K, the previous superstructure peaks begin to vanish, and finally they are replaced by a new set of superstructure peaks following the propagation vector $\mathbf{k}=(1/4,1/2,1/4)$ with respect to the rhombohedral cell. The transition is also reflected in sudden changes in the lattice parameters that seem to smooth the changes observed in the previous transition. The new cell is also triclinic, and there are 48 nonequivalent Fe sites with a maximum charge disproportionation of $\sim 0.7{\mathrm{e}}^{-}$. Both phases coexist in a wide temperature range because this second transition is not completed at 80 K. A symmetry mode analysis indicates a complicated pattern for the charge distribution in the Fe sublattice of both distorted structures but clearly discard any bimodal distribution of only two types of Fe cations. Therefore, the sharp jumps in the electric resistivity at the phase transitions are clearly correlated with two different structural changes. Finally, the oxygen stoichiometry seems to be a key factor in the stabilization of the different distorted structures.

We present a dynamical approach to generate defect-free continuous-random-network (CRN) models of hydrogenated amorphous silicon ($a$-Si:H). Using the atomic coordination number of silicon as a collective variable and few configurational constraints, we have shown that classical metadynamics can be used to construct CRN models of $a$-Si with arbitrary concentrations of dangling-bond coordination defects. These defective networks have been subsequently hydrogenated to produce high-quality models of $a$-Si:H using ab initio total-energy calculations to generate hydrogen (H) microstructures for H concentrations from 7 to 22 at. %. The structural, electronic, optical, and vibrational properties of the models are examined, and the microstructure of the hydrogen distribution is analyzed and compared with experimental data from neutron scattering, spectroscopic ellipsometry, infrared spectroscopy, and nuclear magnetic resonance studies. The results obtained from the models are found to be in excellent agreement with the experimental data.

We demonstrate that short-wavelength exchange-coupled magnons can be identified electrically through the combination of their spin-pumping and damping properties in a metal/magnetic insulator heterostructure. We present clear wave-vector dependence of the spin pumping from parametrically excited exchange-coupled magnons in the heterostructure. The efficiency of spin pumping by dipole-exchange magnons was measured to be sensitive to their wave numbers and propagation angles, while the efficiency was found to be almost constant in the exchange limit. This dependence enabled us to uncover nontrivial dual bistability in the system originating from drastic change in dominant nonequilibrium magnon modes. These findings pave a path for direct electric access to short-wavelength spin dynamics that cannot be probed by optical techniques.

Observation of significant negative exchange bias ($H_E$) is reported in solid solutions of single-phase samples of $\mathrm{D}{\mathrm{y}}_{1-x}\mathrm{N}{\mathrm{d}}_x\mathrm{Cr}{\mathrm{O}}_3$ for $x=0.33,0.67,$ and 1 with corresponding Néel temperatures at $T_N^{\mathrm{Cr}}=175\mathrm{K},200\mathrm{K},$ and 225 K, respectively, and the spin-reorientation transitions $T_{SR}$ at 48 K, 58 K, and 38 K, respectively. In contrast, no $H_E$ was observed for the sample with $x=0$ (i.e., $\mathrm{DyCr}{\mathrm{O}}_3$) shows no $H_E$ below its $T_N^{\mathrm{Cr}}=145\mathrm{K}$, and no reorientation of spins at lower temperatures was observed. More importantly, the (negative) $H_E$ for the $x=0.33,0.67,$ and 1 samples is observed only in the temperature range between $T_N$ and $T_{SR}$ with the corresponding observation of magnetic coercivity. These results show that the canted antiferromagnetic ${\mathrm{\Gamma }}_{7,\mathrm{Cr}}$ structure of the $\mathrm{C}{\mathrm{r}}^{3+}$ moments present between $T_N^{\mathrm{Cr}}$ and $T_{SR}$ is essential for the observed $H_E$. By comparing this result to the literature, common features of single-phase materials with $H_E$ were identified. The present bulk powder samples were prepared by the citrate method and structurally characterized by x-ray diffraction and Raman spectroscopy techniques. Neutron diffraction measurements for the $x=0.33,0.67,$ and 1 samples at select temperatures were done to verify the presence of the ${\mathrm{\Gamma }}_{7,\mathrm{Cr}}$ structure between $T_N^{\mathrm{Cr}}$ and $T_{SR}$ and the ${\mathrm{\Gamma }}_{1,\mathrm{Cr}}$ structure (with no canting of $\mathrm{C}{\mathrm{r}}^{3+}$ moments) below $T_{SR}$.

In this paper, we illustrate the dilemma of inverse magnetocaloric materials using the example of Heusler alloys. For such materials, the magnetic and lattice contribution to the total entropy change are competing with each other. For the two paradigmatic Heusler systems of Ni-Mn-In and Ni-Mn-In-Co, we provide a systematic comparison of experimental data under different magnetic fields and hydrostatic pressures with magnetic and the magnetocaloric properties obtained from the Heisenberg model. This allows us to separate the lattice and the magnetic contribution to the total entropy of the martensitic transition. Our analysis reveals that a large magnetization change is parasitic, but at the same time it is necessary to drive the magnetocaloric effect. This contradicting role of the magnetic contribution—the dilemma—is a general characteristic of inverse magnetocaloric Heusler materials.

We study light propagation in a neutral Weyl semimetal with the Fermi level lying at the Weyl nodes in the weak self-interacting regime. The nontrivial topology induces a screening effect in one of the two transverse gauge fields, for which we find two branches of attenuated collective excitations. In addition to the known topologically gapped photon mode, a different massless and slightly damped excitation appears. Strikingly, at low energies, this excitation has a linear dispersion and it propagates with the same velocity as the electrons, while at energies well above the electron-hole continuum threshold it behaves as a massive attenuated photon with velocity similar to the speed of light in the material. There is a crossover at a certain momentum in the direction perpendicular to the separation of the Weyl nodes above which this gapless mode enters into an overdamped regime. Regarding the unscreened gauge field, we show that it is also attenuated, which is a nontopological property shared by Dirac semimetals as well.

Transition-metal-based Heusler semiconductors are promising materials for a variety of applications ranging from spintronics to thermoelectricity. Employing the $GW$ approximation within the framework of the FLAPW method, we study the quasiparticle band structure of a number of such compounds being almost gapless semiconductors. We find that in contrast to the $sp$-electron based semiconductors such as Si and GaAs, in these systems, the many-body corrections have a minimal effect on the electronic band structure and the energy band gap increases by less than 0.2 eV, which makes the starting point density functional theory (DFT) a good approximation for the description of electronic and optical properties of these materials. Furthermore, the band gap can be tuned either by the variation of the lattice parameter or by the substitution of the $sp$-chemical element.

An alternative approach of using a distributed transmission line analogy for solving transport equations for ballistic nanostructures is applied for solving the three-dimensional problem of electron transport in gated ballistic nanostructures with periodically changing width. The structures with varying width allow for modulation of the electron drift velocity while keeping the plasma velocity constant. We predict that in such structures biased by a constant current, a periodic modulation of the electron drift velocity due to the varying width results in the instability of the plasma waves if the electron drift velocity to plasma wave velocity ratio changes from below to above unity. The physics of such instability is similar to that of the sonic boom, but, in the periodically modulated structures, this analog of the sonic boom is repeated many times leading to a larger increment of the instability. The constant plasma velocity in the sections of different width leads to resonant excitation of the unstable plasma modes with varying bias current. This effect (that we refer to as the superplasmonic boom condition) results in a strong enhancement of the instability. The predicted instability involves the oscillating dipole charge carried by the plasma waves. The plasmons can be efficiently coupled to the terahertz electromagnetic radiation due to the periodic geometry of the gated structure. Our estimates show that the analyzed instability should enable powerful tunable terahertz electronic sources.

One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power-broadening regime and demonstrate an enhanced coherence at exciton saturation where the detected count rates are highest. We detect single-photon count rates of 460 000 counts per second under pulsed laser excitation while maintaining a single-photon purity greater than 99%. Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire band gap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence twofold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, $T_2$, of 1.2 ns, and two-photon interference visibility as high as 83% under strong temporal postselection ($\sim 5\%$ without temporal postselection).

We present the first-principles studies on the hydrostatic pressure effect of the electronic properties of black phosphorus. We show that the energy bands crossover around the critical pressure $P_c=1.23$ GPa; with increasing pressure, the band reversal occurs at the $Z$ point and evolves into 4 twofold-degenerate Dirac cones around the $Z$ point, suggesting that pressured black phosphorus is a 3D Dirac semimetal. With further increasing pressure the Dirac cones in the $\mathrm{\Gamma }-Z$ line move toward the $\mathrm{\Gamma }$ point and evolve into two hole-type Fermi pockets, and those in the $Z-M$ lines move toward the $M$ point and evolve into two tiny electron-type Fermi pockets, and a band above the $Z-M$ line sinks below ${\mathrm{E}}_F$ and contributes four electron-type pockets. A clear Lifshitz transition occurs at $P_c$ from semiconductor to 3D Dirac semimetal. Such a 3D Dirac semimetal is protected by the nonsymmorphic space symmetry of bulk black phosphorus. These suggest the bright perspective of black phosphorus for optoelectronic and electronic devices due to its easy modulation by pressure.

Relying on generalized-gradient and hybrid first-principles simulations, this work provides a complete characterization of the electronic properties of ZnO ultrathin films, cut along the body-centered-tetragonal(010), cubane(100), hexagonal boron nitride(0001), zinc-blende(110), and wurtzite ($10\overline{1}0$) and (0001) orientations. The characteristics of the local densities of states are analyzed in terms of the reduction of the Madelung potential on undercoordinated atoms and surface states/resonances appearing at the top of the valence band and bottom of the conduction band. The gap width in the films is found to be larger than in the corresponding bulks, which is assigned to quantum confinement effects. The components of the high-frequency dielectric constant are determined and the absorption spectra of the films are computed. They display specific features just above the absorption threshold due to transitions from or to the surface resonances. This study provides a first understanding of finite-size effects on the electronic properties of ZnO thin films and a benchmark which is expected to foster experimental characterization of ultrathin films via spectroscopic techniques.

The ferromagnetic superconductor URhGe has been known to exhibit an extremely large enhancement of the upper critical field when the field is confined and rotated in the $\left(ab\right)$ crystal plane. Our high-field ${}^{59}\mathrm{Co}$ NMR measurements on 10% Co-doped URhGe up to 30 T prove that this unconventional behavior of the superconductivity (SC) is associated with a strong anisotropy of field-dependent quantum fluctuations near a tricritical point. These fluctuations are rapidly suppressed by the field component along the $c$ axis, while being unaffected by any field component along the $a$ axis. The observed close interplay between the SC and quantum fluctuations strongly supports a pairing mechanism mediated by these fluctuations in URhGe.

The rich phenomenology of plasmonic excitations in the dichalcogenides is analyzed as a function of doping. The many-body polarization, the dielectric response function, and electron energy loss spectra are calculated using an ab initio based model involving material-realistic Coulomb interactions, band structure, and spin-orbit coupling. Focusing on the representative case of ${\mathrm{MoS}}_2$, a plethora of plasmon bands are observed, originating from scattering processes within and between the conduction or valence band valleys. We discuss the resulting square-root and linear collective modes, arising from long-range versus short-range screening of the Coulomb potential. We show that the multiorbital nature of the bands and spin-orbit coupling strongly affects intervalley scattering processes by gapping certain two-particle modes at large momentum transfer.