Motivated by recent high resolution results on the inversion domain boundaries (IDB) in gallium nitride, we refine by ab initio DFT calculations the well established atomic model ${\mathrm{IDB}}^{*}$ derived by Northrup et al. This allows us to recover these experimental results obtained by coherent x-ray diffraction and showing small additional shifts of the polarity domains, in particular 8 pm shift along the hexagonal direction. The influence of boundary conditions and electrostatic fields (IDB-IDB and IDB-surface interactions) on the results and the existence of metastable solutions is carefully discussed to stress the accuracy of the method. These results demonstrate a cross-talk between advanced characterization tools and state-of-the-art ab initio calculations that opens perspectives for the structural analysis of defects in the picometer range.

We study the transport at the domain wall (DW) in a magnetic topological insulator. The low-energy behaviors of the magnetic topological insulator are dominated by the chiral edge states (CESs). Here, we find that the spectrum and transport of the CESs at the DW are strongly dependent on the DW configuration. For a Bloch wall, two copropagating CESs at the DW are doubly degenerate, and the incoming electron is totally reflected. However, for a Néel wall, the two CESs are split, and the transmission is determined by the interference between the CESs. Moreover, the effective Hamiltonian for the CESs indicates that the component of magnetization perpendicular to the wall leads to the distinct transport behaviors. These findings may pave the way to realizing low-power-dissipation spintronics devices based on magnetic DWs.

Based on a kinematic approach in defining a geometric phase for a density matrix, we define the generalized Loschmidt overlap amplitude (GLOA) for an open system for arbitrary quantum evolution. The GLOA reduces to the Loschmidt overlap amplitude (LOA) with a modified dynamic phase for unitary evolution of a pure state, with the argument of the GLOA well defined by the geometric phase, thus possessing a similar physical interpretation to that of the LOA. The rate function for the GLOA exhibits nonanalyticity at a critical time, which corresponds to the dynamical quantum phase transition. We observe that the dynamical quantum phase transition related to GLOA is not destroyed under a finite temperature and weak enough dissipation. In particular, we find that an alternate type of dynamical quantum phase transition emerges in a dissipation system. The proposed GLOA provides a powerful tool in the investigation of a dynamical quantum phase transition in an arbitrary quantum system, which not only can characterize the robustness of the dynamical quantum phase transition but also can be used to search for new transitions.

Calcium ruthenate ($\mathrm{CaRu}{\mathrm{O}}_3$) is widely believed to be located close to a quantum critical point due to the strong non-Fermi-liquid behavior expressed in the temperature dependence of electronic transport, specific heat, optical conductivity, etc. However, the corresponding anomalous behavior, marking crossover temperature regimes in the magnetic response of $\mathrm{CaRu}{\mathrm{O}}_3$, is still lacking. Here we report detailed AC and DC magnetic susceptibility measurements of $\mathrm{CaRu}{\mathrm{O}}_3$ and $\mathrm{CaR}{\mathrm{u}}_{0.97}\mathrm{T}{\mathrm{i}}_{0.03}{\mathrm{O}}_3$. The AC magnetic susceptibility measurements of $\mathrm{CaRu}{\mathrm{O}}_3$ show a slight dependence on the frequency of AC magnetic field below $\sim 40\mathrm{K}$, and an additional subtle change of curvature around 12 K. We interpret these results as a critical slowing down of spin fluctuations towards $T=0\mathrm{K}$. We confirm these observations by magnetic measurements of $\mathrm{CaR}{\mathrm{u}}_{0.97}\mathrm{T}{\mathrm{i}}_{0.03}{\mathrm{O}}_3$, which show a pronounced magnetic response corresponding to the above temperatures.

The Kitaev-Heisenberg model on the honeycomb lattice has been studied for the purpose of finding exotic states such as quantum spin liquid and topological orders. On the kagome lattice, in spite of a spin-liquid ground state in the Heisenberg model, the stability of the spin-liquid state has hardly been studied in the presence of the Kitaev interaction. Therefore, we investigate the ground state of the classical and quantum spin systems of the kagome Kitaev-Heisenberg model. In the classical system, we obtain an exact phase diagram that has an eightfold degenerated canted ferromagnetic phase and a subextensive degenerated Kitaev antiferromagnetic phase. In the quantum system, using Lanczos-type exact diagonalization and cluster mean-field methods, we obtain two quantum spin-liquid phases, an eightfold degenerated canted ferromagnetic phase similar to the classical spin system, and an eightfold degenerated, $\mathbf{q}=\mathbf{0},{120}^{\circ }$ ordered phase induced by quantum fluctuation. These results may provide a crucial clue to recently observed magnetic structures of the rare-earth-based kagome lattice compounds $A_2{\mathrm{RE}}_3{\mathrm{Sb}}_3{\mathrm{O}}_{14}(A=\text{Mg}$, Zn; $\text{RE}=\text{Pr}$, Nd, Gd, Tb, Dy, Ho, Er, Yb).

We use the maximum entropy method (MaxEnt) to estimate the dynamic structure factor of superfluid ${}^4\mathrm{He}$ at $T=1$ K, by inverting imaginary-time density correlation functions computed by quantum Monte Carlo (QMC) simulation. Our procedure consists of a Metropolis random walk in the space of all possible spectral images, sampled from a probability density which includes the entropic prior, in the context of the so-called “classic” MaxEnt. Comparison with recent work by other authors shows that contrary to what is often stated, sharp features in the reconstructed image are not “washed out” by the entropic prior if the underlying QMC data have sufficient precision. Only spurious features that tend to appear in a straightforward ${\chi }^2$ minimization are suppressed.

We theoretically study self-consistent proximity effects in finite-sized systems consisting of ferromagnet (F) layers coupled to an $s$-wave superconductor (S). We consider both ${\mathrm{SF}}_1{\mathrm{F}}_2$ and $\mathrm{SH}$ nanostructures, where the ${\mathrm{F}}_1{\mathrm{F}}_2$ bilayers are uniformly magnetized and the ferromagnetic $\mathrm{H}$ layer possesses a helical magnetization profile. We find that when the ${\mathrm{F}}_1{\mathrm{F}}_2$ layers are weakly ferromagnetic, a hard gap can emerge when the relative magnetization directions are rotated from parallel to antiparallel. Moreover, the gap is most prominent when the thicknesses of ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$ satisfy $d_{\text{F}1}\le d_{\text{F}2}$, respectively. For the $\mathrm{SH}$ configuration, increasing the spatial rotation period of the exchange field can enhance the induced hard gap. Our investigations reveal that the origin of these findings can be correlated with the propagation of quasiparticles with wave vectors directed along the interface. To further clarify the source of the induced energy gap, we also examine the spatial and energy-resolved density of states, as well as the spin-singlet and spin-triplet superconducting correlations, using experimentally accessible parameter values. Our findings could be beneficial for designing magnetic hybrid structures where a tunable superconducting hard gap is needed.

We report a continuous phase transition between quantum-anomalous-Hall and trivial-insulator phases in a magnetic topological insulator upon magnetization rotation. The Hall conductivity transits from one plateau of quantized Hall conductivity $e^2/h$ to the other plateau of zero Hall conductivity. The transition curves taken at various temperatures cross almost at a single point, exemplifying the critical behavior of the transition. The slope of the transition curves follows a power-law temperature dependence with a critical exponent of $0.61$. This suggests a common underlying origin in the plateau transitions between the quantum anomalous Hall and quantum Hall systems, which is a percolation of one-dimensional chiral edge channels.

Transport, magnetic, and optical investigations on ${\mathrm{EuRbFe}}_4{\mathrm{As}}_4$ single crystals evidence that the ferromagnetic ordering of the ${\mathrm{Eu}}^{2+}$ magnetic moments at $T_m=15$ K, below the superconducting transition ($T_c=36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_m$, the zero resistance state is preserved and the diamagnetic response is only slightly affected by the emerging ferromagnetism; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_c$, associated with the opening of a complete superconducting gap at $2\mathrm{\Delta }=10$ meV. A gap smaller than the weak-coupling limit suggests strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.

It is well known that spontaneous symmetry breaking in one spatial dimension is thermodynamically forbidden at finite energy density. Here we show that mirror-symmetric disorder in an interacting quantum system can invert this paradigm, yielding spontaneous breaking of mirror symmetry only at finite energy density and giving rise to “mirror-glass” order. The mirror-glass transition, which occurs via the energetic activation of a finite density of emergent Ising degrees of freedom, is enabled by many-body localization and appears to occur simultaneously with the localization transition. This counterintuitive manifestation of localization-protected order can be viewed as a quantum analog of inverse freezing, a phenomenon that occurs, e.g., in certain models of classical spin glasses.

The propagation of magnetostatic forward volume waves excited by a constricted coplanar waveguide is studied via inductive spectroscopy techniques. A series of devices consisting of pairs of submicrometer size antennae is used to perform a discrete mapping of the spin wave amplitude in the plane of a 30-nm thin YIG film. We found that the spin wave propagation remains well focused in a beam shape of width comparable to the constriction length and that the amplitude within the constriction displays oscillations, two features which are explained in terms of near-field Fresnel diffraction theory.

In some of the ferropnictide materials, spin-density wave order coexists with superconductivity over a range of doping and temperature. In this paper, we show that odd-frequency pairing emerges on the edges of pnictides in such a coexistence phase. In particular, the breaking of spin-rotation symmetry by spin-density wave and translation symmetry by the edge can lead to the development of odd-frequency spin-triplet Cooper pairing. In this case, the odd-frequency pairing has even-parity components, which are immune to disorder. Our results show that pnictides are a natural platform to realize odd-frequency superconductivity, which has been mainly searched for in heterostructures of magnetic and superconducting materials. The emergence of odd-frequency pairing on the edges and in the defects can be potentially detected in magnetic response measurements.

We study the electronic and optical properties of twisted trilayer and tetralayer graphene structures using a combination of tight-binding and density-functional theory methods. Band structures, densities of states, and optical absorption spectra are calculated for a variety of layer stackings and twisting angles. Systematic trends of all properties are obtained and compared to the more well known case of twisted bilayer graphene. For trilayer and tetralayer structures, we find, respectively, two and three well-defined absorption peaks in the infrared/visible range that shift with twisting angle, in contrast to the single peak observed in bilayer graphene. In addition, systems containing Bernal-stacked layers present an extra peak in the infrared which is related to transitions between parabolic bands and does not shift with twisting angle. The observed trends may be used to identify the twisting angle and the number of layers in multilayer graphene samples. In particular, magic angles are predicted for the trilayer and tetralayer structures.

In the majority of optomechanical experiments, the interaction between light and mechanical motion is mediated by radiation pressure, which arises from momentum transfer of reflecting photons. This is an inherently weak interaction, and optically generated carriers in semiconductors have been predicted to be the mediator of different and potentially much stronger forces. Here we demonstrate optomechanical forces induced by electron-hole pairs in coupled quantum wells embedded in a free-free nanomembrane. We identify contributions from the deformation potential and piezoelectric coupling and observe optically driven motion about three orders of magnitude larger than expected from radiation pressure. The amplitude and phase of the driven oscillations are controlled by an applied electric field, which tunes the carrier lifetime to match the mechanical period. Our work opens perspectives not only for enhancing the optomechanical interaction in a range of experiments but also for interfacing mechanical objects with complex macroscopic quantum objects, such as excitonic condensates.

Recent work has extended topological band theory to open, non-Hermitian Hamiltonians, yet little is understood about how non-Hermiticity alters the topological quantization of associated observables. We address this problem by studying the quantum anomalous Hall effect (QAHE) generated in the Dirac surface states of a three-dimensional time-reversal-invariant topological insulator (TI) that is proximity coupled to a metallic ferromagnet. By constructing a contact self-energy for the ferromagnet, we show that in addition to generating a mass gap in the surface spectrum, the ferromagnet can introduce a non-Hermitian broadening term, which can obscure the mass gap in the spectral function. We calculate the Hall conductivity for the effective non-Hermitian Hamiltonian describing the heterostructure and show that it is no longer quantized despite being classified as a Chern insulator based on non-Hermitian topological band theory. Our results indicate that the QAHE will be challenging to experimentally observe in ferromagnet-TI heterostructures due to the finite lifetime of quasiparticles at the interface.

The dynamics of differently spin-polarized carriers photoexcited in a system of Landau levels is investigated in an InGaAs/InP quantum well. Shake-up emission from Landau levels above the Fermi level is observed, and it is shown to significantly affect the recombination dynamics of Landau levels. The shake-up effect is found to occur due to the inter-Landau-level scattering, which manifests itself in a rapid decay of the emission from Landau levels. In addition, the inter-Landau-level scattering is shown to determine the time delay of photoluminescence response due to the relaxation of photoexcited electrons among Landau levels. The sharp minimum of the recombination time is observed between the quantum Hall phases with the filling factors $\nu =1$ and 2, and it is attributed to the formation of the metallic intermediate phase, which causes increasing overlap between the electron and hole wave functions.

Graphene grown by large-scale synthesis methods usually contains grain boundaries. They can strongly affect the electronic and mechanical properties of graphene and it is promising to exploit them for the design of electronic components and sensors. Here, we consider semiconducting graphene bicrystals and study how grain boundary structure variations influence electron transport using density functional theory in conjunction with the nonequilibrium Green function method. We find that the size of the transport gap in these bicrystals is not changed by structure variations. Interestingly however, electron transport outside the transport gap is very sensitive to modifications of the grain boundary. We show that these results can be understood within the ballistic transport approximation and by inspecting the electronic density of states resolved in energy-momentum space. Our findings suggest that the electronic response of graphene bicrystals can be controlled not only by grain misorientation but also by manipulation of the grain boundary structure.