The origin of the band gap bowing in dilute nitrogen doped gallium based III-V semiconductors is largely debated. In this paper we show the dilute GaAs${}_{1-x}$N${}_x$ and GaP${}_{1-x}$N${}_x$ as representative examples that the nitrogen-induced states close to the conduction band minimum propagate along the zigzag chains on the {110} planes. Thereby states originating from different N atoms interact with each other resulting in broadening of the nitrogen-induced states which narrows the band gap. Our modeling based on ab initio theoretical calculations explains the experimentally observed N concentration dependent band gap narrowing both qualitatively and quantitatively.

We derive a transparent and easy-to-use analytic expression for the selection rules and the optical dipole matrix elements for carbon nanotubes of arbitrary chirality in the presence of axial magnetic fields using a single-orbital π-electron tight-binding model. From this, we calculate the linear absorption spectrum for arbitrary polarization directions of the incident light, providing insight into all optically allowed transition. We show that the transverse absorption peaks can be selectively excited with circularly polarized light and spectrally resolved in an axial magnetic field.

We present a combined analytical and numerical study of the early stages (sub-100-fs) of the nonequilibrium dynamics of photoexcited electrons in graphene. We employ the semiclassical Boltzmann equation with a collision integral that includes contributions from electron-electron (e-e) and electron–optical phonon interactions. Taking advantage of circular symmetry and employing the massless Dirac fermion (MDF) Hamiltonian, we are able to perform an essentially analytical study of the e-e contribution to the collision integral. This allows us to take particular care of subtle collinear scattering processes—processes in which incoming and outgoing momenta of the scattering particles lie on the same line—including carrier multiplication (CM) and Auger recombination (AR). These processes have a vanishing phase space for two-dimensional MDF bare bands. However, we argue that electron-lifetime effects, seen in experiments based on angle-resolved photoemission spectroscopy, provide a natural pathway to regularize this pathology, yielding a finite contribution due to CM and AR to the Coulomb collision integral. Finally, we discuss in detail the role of physics beyond the Fermi golden rule by including screening in the matrix element of the Coulomb interaction at the level of the random phase approximation (RPA), focusing in particular on the consequences of various approximations including static RPA screening, which maximizes the impact of CM and AR processes, and dynamical RPA screening, which completely suppresses them.

We have determined the equilibrium shape of graphene domains grown on Ni(111) via carbon segregation at 925${}^{\circ }$C. In situ, spatially resolved electron diffraction measurements were used to determine the crystallographic orientation of the edges of the graphene domains. In contrast to recent theoretical predictions of a nearly circular shape, we show that graphene domains, which nucleate with random shapes, all evolve toward a triangular equilibrium shape with “zigzag” edges. Only one of the two possible zigzag edge orientations is observed.

We show that condensate-induced transitions between two-dimensional topological phases provide a general framework to relate one-dimensional spin models at their critical points. We demonstrate this using two examples. First, we show that two well-known spin chains, namely, the $\mathit{XY}$ chain and the transverse field Ising chain with only next-nearest-neighbor interactions, differ at their critical points only by a nonlocal boundary term and can be related via an exact mapping. The boundary term constrains the set of possible boundary conditions of the transverse field Ising chain, reducing the number of primary fields in the conformal field theory that describes its critical behavior. We argue that the reduction of the field content is equivalent to the confinement of a set of primary fields, in precise analogy to the confinement of quasiparticles resulting from a condensation of a boson in a topological phase. As the second example we show that when a similar confining boundary term is applied to the $\mathit{XY}$ chain with only next-nearest-neighbor interactions, the resulting system can be mapped to a local spin chain with the $u{\left(1\right)}_2\times u{\left(1\right)}_2$ critical behavior predicted by the condensation framework.

We apply a circularly and linearly polarized terahertz field on a monolayer of graphene, taking into account spin-orbit interactions of the intrinsic and Rashba types. It turns out that the field can be used not only to induce a gap in the energy spectrum, but also to close an existing gap due to the different reaction of the spin components with circularly polarized light. Signatures of spin-orbit coupling in the density of states of the driven system can be observed even for energies where the static density of states is independent of spin-orbit interactions. Furthermore it is shown that the time evolution of the spin polarization and the orbital dynamics of an initial wave packet can be modulated by varying the ratio of the spin-orbit-coupling parameters. Assuming that the system acquires a quasistationary state, the optical conductivity of the irradiated sample is calculated. Our results confirm the multistep nature of the conductivity obtained recently, where the number of intermediate steps can be changed by adjusting the spin-orbit-coupling parameters and the orientation of the field.

We report a combined high-resolution photoemission (XPS) and photoelectron diffraction (XPD) investigation of the three layer system MgO/Fe/GaAs(001). Each layer is investigated with regard to its structure. The two dimers model of the GaAs $(4\times 2)$ reconstruction was confirmed by XPD patterns. We find the intermediate Fe layer in a crystalline structure. Further, the study clearly shows a well-ordered epitaxial MgO film on Fe. A careful analysis of the interface signals indicates an interdiffusion at the Fe/GaAs interface and partially shifted magnesium layers at the MgO/Fe interface.

The nature of the electronic ground states in strained undoped graphene at weak interaction between electrons is discussed. After providing a lattice realization of the strain-induced axial magnetic field we numerically find the self-consistent solution for the time reversal symmetry breaking quantum anomalous Hall order parameter, at weak second-nearest-neighbor repulsion between spinless fermions. The anomalous Hall state is obtained in both uniform and nonuniform axial magnetic fields, with the spatial profile of the order parameter resembling that of the axial field itself. When the electron spin is included, the time reversal symmetric anomalous spin Hall state becomes slightly preferred energetically at half filling, but the additional anomalous Hall component should develop at a finite doping.

We study different aspects of quantum von Neumann and Rényi entanglement entropy of one-dimensional long-range harmonic oscillators that can be described by well-defined nonlocal field theories. We show that the entanglement entropy of one interval with respect to the rest changes logarithmically with the number of oscillators inside the subsystem. This is true also in the presence of different boundary conditions. We show that the coefficients of the logarithms coming from different boundary conditions can be reduced to just two different universal coefficients. We also study the effect of the mass and temperature on the entanglement entropy of the system in different situations. The universality of our results is also confirmed by changing different parameters in the coupled harmonic oscillators. We also show that more general interactions coming from general singular Toeplitz matrices can be decomposed to our long-range harmonic oscillators. Despite the long-range nature of the couplings, we show that the area law is valid in two dimensions and the universal logarithmic terms appear if we consider subregions with sharp corners. Finally, we study analytically different aspects of the mutual information such as its logarithmic dependence to the subsystem, effect of mass, and influence of the boundary. We also generalize our results in this case to general singular Toeplitz matrices and higher dimensions.

The elastic interaction, induced by the lattice distortion due to the difference of the molecular size, causes an effective long-range interaction. In spin-crossover (SC) compounds, local bistable states, i.e., high-spin and low-spin states, have different molecular sizes, and the elastic interaction is important. In bipartite lattices, e.g., the square lattice, the ground state can be two types of phases: ferromagneticlike and antiferromagneticlike phases. In systems like SC compounds, the former phase consists of all small or large molecules, and the latter phase has the configuration of alternating small and large molecules. In fact, both cases are observed in SC systems. In this paper we have studied the effect of the short-range interaction in the elastic system on the properties of those order-disorder phase transitions. We have obtained a phase diagram in the coordinates of the temperature and the strength of the short-range interaction, including the metastable structures. We show that effects of the short-range interaction are essentially different for ferromagneticlike and antiferromagneticlike phase transitions. In the ferromagneticlike transition, the long-range interaction of elasticity is relevant, and the system exhibits a phase transition in the mean-filed universality class. In this case, the long-range interaction strongly enhances the ferromagneticlike order, and it works cooperatively with the short-range interaction. In contrast, in the antiferromagneticlike transition, the elastic interaction slightly enhances the antiferromagneticlike order, but essentially it does not contribute to the ordering, and the system shows a transition in the Ising universality class. We have found that in the border region between ferromagneticlike and antiferromagneticlike phases, the antiferromagneticlike phase has an advantage at finite temperatures. We discuss the critical properties of two-step SC transitions with comparison between the elastic interaction model and conventional SC models (Ising-like models).

We introduce a simple lattice model with Ising spins as a zeroth-order approximation of the hollandite-type magnetic compounds. We argue that geometrical frustration of the lattice in combination with nearest-neighbor antiferromagnetic (AFM) interactions are responsible for the appearance of a spin-glass phase in presence of disorder. We investigate this system numerically using parallel tempering. The model reproduces magnetic transitions present in some oxides with hollandite structure and displays a rich phenomenology: in the absence of disorder we have identified five different ground states, depending on the relative strength and sign of the interactions: one ferromagnetically ordered, three antiferromagnetically ordered, and one disordered, macroscopically degenerate ground state. Remarkably, for the sets of AFM couplings having an AFM ground state in the clean system, there exists a critical value of the disorder above which the ground state becomes a spin glass while maintaining all the couplings antiferromagnetically. This model presents this kind of transition with nearest-neighbor frustrated AFM interactions. We argue that this model is useful for understanding the relation between AFM coupling, disorder, and appearance of spin-glass phases.

We report the ab initio density functional theory calculation of phonons in the cubic phase of ZrW${}_2$O${}_8$ in the entire Brillouin zone and identify specific anharmonic phonons that are responsible for large negative thermal expansion (NTE) in terms of the translation, rotation, and distortion of WO${}_4$ and ZrO${}_6$. We have used density functional calculations to interpret the experimental phonon spectra as a function of pressure and temperature as reported in literature. We discover that the phonons showing anharmonicity with temperature are not necessarily the same as those showing anharmonicity with pressure although both are of similar frequencies. Only the latter phonons are associated with NTE. Therefore, the cubic and/or quadratic anharmonicity of phonons is not relevant to NTE but just the volume dependence of frequencies. The calculations are able to reproduce the observed anomalous trends, namely, the softening of the low-frequency peak at about 4 meV in the phonon spectra with pressure and its hardening with temperature, whereas, both changes involve a compression of the lattice.

We present a neutron powder-diffraction study of the monoclinic double perovskite systems Nd${}_2$Na$B^{\prime }$O${}_6$ ($B^{\prime }=\text{Ru}$, Os), with magnetic atoms occupying both the $A$ and $B^{\prime }$ sites of the $A_{2}B{B}^{\prime }{\text{O}}_6$ structure. Our measurements reveal coupled spin ordering between the Nd and $B^{\prime }$ atoms with magnetic transition temperatures of 14.3(1) K for Nd${}_2$NaRuO${}_6$ and 16.2(2) K for Nd${}_2$NaOsO${}_6$. There is a type-I antiferromagnetic structure associated with the Ru and Os sublattices, with the ferromagnetic planes stacked along the $c$ axis and [110] direction respectively, while the Nd sublattices exhibit complex, canted antiferromagnetism with different spin arrangements in each system.

Recent studies of the global phase diagram of quantum-critical heavy-fermion metals prompt consideration of the interplay between the Kondo interactions and quantum fluctuations of the local moments alone. Toward this goal, we study a Bose-Fermi Kondo model (BFKM) with Ising anisotropy in the presence of a local transverse field that generates quantum fluctuations in the local-moment sector. We apply the numerical renormalization-group method to the case of a sub-Ohmic bosonic bath exponent and a constant conduction-electron density of states. Starting in the Kondo phase at zero transverse field, there is a smooth crossover with increasing transverse field from a fully screened to a fully polarized impurity spin. By contrast, if the system starts in its localized phase, then increasing the transverse field causes a continuous, Kondo-destruction transition into the partially polarized Kondo phase. The critical exponents at this quantum phase transition exhibit hyperscaling and take essentially the same values as those of the BFKM in zero transverse field. The many-body spectrum at criticality varies continuously with the bare transverse field, indicating a line of critical points. We discuss implications of these results for the global phase diagram of the Kondo lattice model.

We report on an experimental observation of high-energy magnon excitations in ultrathin ferromagnetic films with a perpendicular easy axis. We demonstrate that a transversally spin-polarized beam can be used to excite and probe the high-energy magnons within spin-polarized electron energy-loss spectroscopy experiments. The magnon dispersion relation and lifetime are probed over the entire surface Brillouin zone for a set of body-centered tetragonal FeCo films with a large perpendicular magnetic anisotropy. First-principles calculations reveal that in addition to the tetragonal distortion, which is the origin of the large perpendicular magnetic anisotropy, the interfacial electronic hybridization also has a considerable impact on the properties of magnons.

Electronic-Structure and high-pressure phase transitions of stoichiometric uranium dioxide (UO${}_2$) and hyperstoichiometric UO${}_{2.03}$ were investigated using first-principles calculations. Density functional theory calculations using the generalized gradient approximation and the projector-augmented wave method with the on-site Coulomb repulsive interactions were applied in order to reasonably calculate the equilibrium volume, total and partial density of states, band gap of UO${}_{2+x}$, and energetics of the high-pressure phase transitions. Structure optimizations were completed with and without the Hubbard $U$-ramping method separately. The $U$-ramping method was intended to remove the metastable states of the 5$f$ electrons of both stoichiometric and hyperstoichiometric UO${}_{2+x}$. Using the Hubbard $U$ parameters ($U$ $=$ 3.8, $J$ $=$ 0.4), whose values are based on the experimental band gap width of 2.1 eV as a reference, the calculated cell parameter for the stoichiometric UO${}_2$ with cubic fluorite structure is about 1$\%$ greater than the experimental unit cell parameter; in contrast, it is about 1$\%$ smaller than the experimental value without the on-site Coulomb repulsive interactions. For hyperstoichiometric UO${}_{2.03}$ with the cubic fluorite structure, the interstitial oxygen at the octahedral interstitial site induces new bands at the top of the band gap of the stoichiometric UO${}_2$, similar to those of the high-pressure phase with orthorhombic cotunnite structure. The orbitals associated with the charge transfers to the interstitial oxygen in hyperstoichiometric UO${}_{2.03}$ are partially delocalized and partially localized in both the cubic fluorite structure and orthorhombic cotunnite structure. The energy required for the incorporation of an interstitial O atom is 0.3 eV higher for the orthorhombic phase than for the cubic phase at ambient pressure and increases to 0.5 eV at 10 GPa. The calculated transition pressures from the cubic to the orthorhombic structure are 18 and 27 GPa for UO${}_2$ and UO${}_{2.03}$, respectively. The dramatic increase in the calculated transition pressure for the hyperstoichiometric UO${}_2$ is related to structural incompatibility of the interstitial oxygen in the cotunnite structure (high-pressure phase), which is less in the case of the fluorite structure. These results suggest that experimentally determined pressure values for the phase transition can be significantly affected by small compositional deviations off the ideal stoichiometric UO${}_2$. Comparisons of the results using the $U$-ramping method and without using the $U$-ramping method suggest that the electronic metastability of UO${}_2$ affects the calculated total energy and electronic structure and could lead to a different local defect configuration for the hyperstoichiometric orthorhombic phase. However, the metastability has a negligible effect on the calculated phase transition pressure.

We investigate the finite-temperature phase diagram of the classical Kitaev-Heisenberg model on the hexagonal lattice. Due to the anisotropy introduced by the Kitaev interaction, the model is magnetically ordered at low temperatures. The ordered phase is stabilized entropically by an order-by-disorder mechanism where thermal fluctuations of classical spins select collinear magnetic states in which magnetic moments point along one of the cubic directions. We find that there is an intermediate phase between the low-temperature ordered phase and the high-temperature disordered phase. We show that the intermediate phase is a critical Kosterlitz-Thouless phase exhibiting correlations of the order parameter that decay algebraically in space. Using finite-size-scaling analysis, we determine the boundaries of the critical phase with reasonable accuracy. We show that the Kitaev interaction plays a crucial role in understanding the finite-temperature properties of $A$${}_2$IrO${}_3$ systems.

${\mathrm{SrHo}}_2{\mathrm{O}}_4$ is a geometrically frustrated magnet in which the magnetic Ho${}^{3+}$ ions form honeycomb layers connected through a network of zigzag chains. At low-temperature two distinct types of short-range magnetic order can be inferred from single-crystal diffraction data, collected using both polarized and unpolarized neutrons. In the ($hk0$) plane the diffuse scattering is most noticeable around the $\mathbf{k}=0$ positions and its intensity rapidly increases at temperatures below 0.7 K. In addition, planes of diffuse scattering at $Q=(hk\pm \frac{l}{2})$ are visible at temperatures as high as 4.5 K. These planes coexist with the broad peaks of diffuse scattering in the ($hk0$) plane at low temperatures. Correlation lengths associated with the broad peaks are $L\approx 150$ Å in the a–b plane and $L\approx 190$ Å along the $c$ axis, while the correlation length associated with the diffuse scattering planes is $L\approx 230$ Å along the $c$ axis at the lowest temperature. Both types of diffuse scattering are elastic in nature. The highly unusual coexistence of the two types of diffuse scattering in ${\mathrm{SrHo}}_2{\mathrm{O}}_4$ is likely to be the result of the presence of two crystallographically inequivalent sites for Ho${}^{3+}$ in the unit cell.

We present a study of exchange bias generated at the interface between a polycrystalline Co film sputtered on a cleavage plane of single-crystal Ca${}_2$Ru${}_{0.98}$Fe${}_{0.02}$FeO${}_4$. The exchange bias is accompanied by an extremely large vertical magnetization shift that is characterized by 60$\%$ of the saturation magnetization in a 70-kOe cooling field. This phenomenon is seldom observed in other heterostructures. The effects of cooling-field amplitude and temperature on the exchange bias indicate that the magnetization shift results from a ferromagnetic contribution of canted moments in Ca${}_2$Ru${}_{0.98}$Fe${}_{0.02}$O${}_4$. A Type-I training effect is also observed, in which the hysteresis loop shrinks from both sides with cycling of the applied field.