We develop an irreversible thermodynamics framework for the description of creep deformation in crystalline solids by mechanisms that involve vacancy diffusion and lattice site generation and annihilation. The material undergoing the creep deformation is treated as a nonhydrostatically stressed multicomponent solid medium with nonconserved lattice sites and inhomogeneities handled by employing gradient thermodynamics. Phase fields describe microstructure evolution, which gives rise to redistribution of vacancy sinks and sources in the material during the creep process. We derive a general expression for the entropy production rate and use it to identify of the relevant fluxes and driving forces and to formulate phenomenological relations among them taking into account symmetry properties of the material. As a simple application, we analyze a one-dimensional model of a bicrystal in which the grain boundary acts as a sink and source of vacancies. The kinetic equations of the model describe a creep deformation process accompanied by grain boundary migration and relative rigid translations of the grains. They also demonstrate the effect of grain boundary migration induced by a vacancy concentration gradient across the boundary.

We have used polarized and unpolarized neutron diffraction to determine the spatial distribution of the magnetization density induced by a magnetic field of 9 T in the tetragonal phase of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$. The maximum entropy reconstruction shows clearly that most of the magnetization is confined to the region around the iron atoms whereas there is no significant magnetization associated with either Se or K atoms. The distribution of magnetization around the Fe atom is slightly nonspherical with a shape which is extended along the $〈001〉$ direction in the projection. Multipolar refinement results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms and their distribution suggests that they occupy 3$d$ $t_{2g}$-type orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the magnetic form factor indicates the presence of an orbital moment to the total paramagnetic moment of Fe${}^{2+}$.

It is shown that in antiferromagnetic open or closed corrals of magnetic adatoms grown on surfaces, the attachment of a single extra adatom anywhere in the corral impacts on the geometrical topology of the nanosystem and generates complex magnetic structures when a magnetic field is applied or a magnetic coupling to a ferromagnetic substrate exists. The spin configuration of the corral can be tuned to a nonplanar state or a planar noncollinear or ferrimagnetic state by adjusting its number of sites, the location of the extra adatom, or the strength of the coupling to the ferromagnetic substrate. This shows the possibility to generate nontrivial magnetic textures with atom-by-atom engineering anywhere in the corral and not only at the edges.

Building on spin-modified Coulomb blockade theory, we derive an approximate analytical solution which matches closely to the existing numerical models in the limit of a large difference in the resistance of the two tunnel junctions. Using this, we are able to explore the properties of an $I\left(V\right)$ measurement for a double tunnel junction incorporating a nanoparticle in various magnetic configurations. It is shown that this information allows, under certain conditions, seven of the device's physical parameters to be determined in a single $I\left(V\right)$ sweep, including individual junction capacitances and spin-dependent resistances. The tunnel magnetoresistance and spin polarizations are also examined in the light of this solution and are shown to be similar in nature and dominated by the magnetic properties of the most resistive barrier. It is shown that spin injection and detection are theoretically possible in a single device containing a nonmagnetic nanoparticle, suggesting a novel means of measuring the tunnel spin polarization of ferromagnetic materials.

Magnetic exchange force microscopy (MExFM) is one of the most important methods to investigate the spin structure of magnetic surfaces. However, a deep understanding of the measured spin contrast and the spin-dependent forces is missing. For the prototypical antiferromagnetic MnO(001) and NiO(001) surfaces probed by ferromagnetic Fe tips we demonstrate by means of spin-polarized density functional theory that the differences in the tip-surface interaction are due to the distance-dependent occupation of the transition-metal $3d$ states. While agreement is achieved with recent MExFM measurements of NiO(001), we predict opposite spin-contrast for MnO(001).

The magnetism of a metallic two-dimensional triangular antiferromagnetic (AF) compound, Ag${}_2$CrO${}_2$, has been investigated by muon-spin rotation and relaxation (${\mu }^{+}$SR) using a powder sample in the temperature range between 1.8 and 40 K. Below $T_{\mathrm{N}}=24$ K, a muon-spin precession signal was clearly observed in the zero-field spectrum, indicating the formation of static AF order. It was also found that the internal field is temperature independent except for in the vicinity of $T_{\mathrm{N}}$, as in the case for the susceptibility versus temperature curve. This suggests that the AF transition is induced by a first-order structural phase transition at $T_{\mathrm{N}}$. Combining the ${\mu }^{+}$SR result with the prediction for muon sites in the lattice by first-principles calculations, a partially disordered AF state was found to be the most reasonable spin structure for Ag${}_2$CrO${}_2$.

Recent experiments on Zn-doped 122-type iron pnictides, Ba(Fe${}_{1-x-y}$Co${}_y$Zn${}_x$)${}_2$As${}_2$, are challenging our understanding of electron doping the 122s and the interplay between doping and impurity scattering. To resolve this enigma, we investigate the disorder effects of nonmagnetic Zn impurities in the strong (unitary) scattering limit on various properties of the system in the $s_{\pm }$-wave superconducting pairing state. The lattice Bogoliubov–de Gennes equation (BdG) is solved self-consistently based on a minimal two-orbital model with an extended range of impurity concentrations. We find that Zn impurity is best modeled as a defect, where charge is mainly localized, but scattering is extended over a few lattice sites. With increasing Zn concentration, the density of states shows a gradual filling of the gap, revealing the impurity-induced pair-breaking effect. Moreover, both the disorder configuration-averaged superconducting order parameter and the superfluid density are dramatically suppressed toward the dirty limit, indicating the violation of the Anderson theorem for conventional $s$-wave superconductors and the breakdown of the Abrikosov-Gorkov theory for impurity-averaged Green's functions. Furthermore, we find that the superconducting phase is fully suppressed close to the critical impurity concentration of roughly $n_{\mathrm{imp}}\approx 10\%$, in agreement with recent experiments.

We investigate the topological phase transition on interacting square lattices via the non-Abelian potential by employing the real-space cellular dynamical mean-field theory combining with the continuous-time Monte Carlo method. For a weak on-site Hubbard interaction, a topological band insulating state with a pair of gapless edge states is induced by a next-nearest-neighbor hopping. A phase transition from the metallic phase to the Mott insulating phase is observed when the interaction is increased. These two phases can be distinguished by detecting whether a bulk gap in the $K$-dependent spectral function exists. The whole phase diagrams as functions of the interaction, next-nearest-neighbor hopping energy, and temperature are presented. The experimental setup to observe these new interesting phase transitions is also discussed.

We consider a double quantum dot (DQD) qubit which contains six electrons instead of the usual one or two. In this spin qubit, quantum information is encoded in a low-lying singlet-triplet space much as in the case of a two-electron DQD qubit. We find that initialization, manipulation, and readout can be performed similarly to the two-electron case, and that energy gaps remain large enough that these operations can be performed robustly. We consider DQD potentials with parameters chosen to be representative of current experimental capabilities. Results are obtained using two complementary full configuration interaction methods.

Several proposed applications and exotic effects in topological insulators rely on the presence of helical Dirac states at the interface between a topological insulator and a normal insulator. In the present work, we have used low-energy angle-resolved photoelectron spectroscopy to uncover and characterize the interface states of Bi${}_2$Se${}_3$ thin films and Bi${}_2$Te${}_3$/Bi${}_2$Se${}_3$ heterostructures grown on Si(111). The results establish that Dirac fermions are indeed present at the topological-normal-insulator boundary and absent at the topological-topological-insulator interface. Moreover, it is demonstrated that band bending present within the topological-insulator films leads to a substantial separation of the interface and surface states in energy. These results pave the way for further studies and the realization of interface-related phenomena in topological-insulator thin-film heterostructures.

We study the Hall conductivity in single-layer gapped Dirac fermion materials including coupling to a phonon field, which not only modifies the quasiparticle dynamics through the usual self-energy term but also renormalizes directly the gap. Consequently the Berry curvature is modified. As the temperature is increased the sign of the renormalized gap can change and the material can cross over from a band insulator to a topological insulator at higher temperature ($T$). The effective Chern numbers defined for valley and spin Hall conductivity show a rich phase diagram with increasing temperature. While the spin and valley dc Hall conductivity is no longer quantized at elevated temperature a change in sign with increasing $T$ is a clear indication of a topological crossover. The chirality of the circularly polarized light which is dominantly absorbed by a particular valley can change with temperature as a result of a topological crossover.

We present a procedure for the optical coherent control of quantum bits within a quantum dot spin-exciton system, as a preliminary step to implementing a proposal by Yao, Liu, and Sham [Phys. Rev. Lett. 95, 030504 (2005)] for deterministic spin-photon entanglement. The experiment proposed here utilizes a series of picosecond optical pulses from a single laser to coherently control a single self-assembled quantum dot in a magnetic field, creating the precursor state in 25 ps with a predicted fidelity of 0.991. If allowed to decay in an appropriate cavity, the ideal precursor superposition state would create maximum spin-photon entanglement. Numerical simulations using values typical of InAs quantum dots give a predicted entropy of entanglement of 0.929, largely limited by radiative decay and electron spin flips.

We investigate resonant sideband processes in the transmission through a time-modulated-potential region in graphene. Valley-dependent features in the time-dependent transmission due to trigonal-warping effects in the electronic structures are explored within a tight-binding model. Three main results obtained are dip structures in the transmission, valley dependence of the dip structures, and nontypical-Fabry-P$\acute{\mathrm{e}}$rot behavior in the dip-structure amplitudes. Dip structures in the transmission are obtained when a relevant band edge is available for the sideband processes. The relevant band edges are shown to become valley dependent, when the incident flow is formed from states of the same group-velocity direction. This is a consequence of the trigonal-warping effects, and it leads to the valley dependence in the dip structures. The dip-structure amplitudes, on the other hand, are found to exhibit a nontypical Fabry-P$\acute{\mathrm{e}}$rot oscillatory behavior, in their dependence on the width of the time-modulated region. This is shown, in our multiple sideband scattering analysis, to result from resonant sideband processes to a relevant band edge. As such, the nontypical Fabry-P$\acute{\mathrm{e}}$rot oscillatory behavior serves as another evidence for the key role the relevant band edges play in the transmission dip structures.

The accurate description of the O 1$s$ excitation in BaTiO${}_3$ has been elusive so far. In this Rapid Communication, the electronic structure and the high-resolution electron energy-loss near-edge structures of the O $K$ edge in tetragonal BaTiO${}_3$ are investigated using first-principles calculations. The results demonstrate a clear correlation between the broadening of the lower energy fine structure and the anisotropic effects induced by the core-hole potential, which are directly related to the structural distortion occurring in tetragonal BaTiO${}_3$. Moreover, we show that a significant improvement of the description of higher-lying structures can be obtained when correcting the energy position of the Ba 4$f$ states. This demonstrates that the O 1$s$ spectrum can be a very effective probe of the distortion and changes in the local electronic structure, and be used as a sensitive tool for studying new materials and ferroelectric transitions.

We discuss the low-frequency response of charge and heat transport to oscillatory voltage and temperature shifts in mesoscopic capacitors. We obtain, within scattering theory, generic expressions for the quantum admittances up to second order in the ac frequencies in terms of electric, thermoelectric, and heat capacitances and relaxation resistances. Remarkably, we find that the thermocurrent can lead or lag the applied temperature depending on the gate voltage applied to a quantum $RC$ circuit. Furthermore, the relaxation resistance for cross terms becomes nonuniversal as opposed to the purely electric or thermal cases.

We apply the concept of quasicrystals to metamaterials and experimentally demonstrate metasurfaces with isotropic properties and high resonance strength. By comparing quasicrystalline, periodic, and amorphous metasurfaces we quantify the impact of symmetry on their properties. This is achieved by studying the eigenpolarizations’ ellipticity and circular dichroism induced by mutual coupling of the meta-atoms. The advantage of the quasicrystalline in comparison to a periodic arrangement originates from the ability to reach a higher rotational symmetry in $k$ space, therefore opening a route towards isotropic metasurfaces.

We show that coupling to vibrational degrees of freedom can drive a semimetal excitonic-insulator quantum phase transition in a one-dimensional two-band $f$-$c$-electron system at zero temperature. The insulating state typifies an excitonic condensate accompanied by a finite lattice distortion. Using the projector-based renormalization method we analyze the ground-state and spectral properties of the interacting electron-phonon model at half filling. In particular we calculate the momentum dependence of the excitonic order-parameter function and determine the finite critical interaction strength for the metal-insulator transition to appear. The electron spectral function reveals the strong hybridization of $f$- and $c$-electron states and the opening of a single-particle excitation gap. The phonon spectral function indicates that the phonon mode involved in the transition softens (hardens) in the adiabatic (nonadiabatic and extreme antiadiabatic) phonon frequency regime.

Symmetry-protected topological (SPT) phases with gapless edge excitations have been shown to exist in strongly interacting bosonic/fermionic systems and it is highly desirable to identify practical systems realizing such phases through numerical simulation. A central question to be addressed is how to determine the SPT order in the system given the simulation result, while no local order parameter can be measured to distinguish the phases from a trivial one. In the tensor network approach to simulate strongly interacting systems, the quantum state renormalization algorithm has been shown to be effective in identifying intrinsic topological orders. Here, we show that a modified algorithm can identify SPT orders by extracting the symmetry-protected fixed point entanglement pattern in the ground-state wave function, which is essential for the existence of SPT order. The key to this approach is to add proper symmetry protection to the renormalization process. We demonstrate the effectiveness of this algorithm with examples of nontrivial SPT phases with internal symmetry in 1D and internal and translation symmetry in 2D.

We study the superlattice minibands produced by the interplay between a moiré pattern induced by a hexagonal BN substrate on a graphene layer and the interlayer coupling in bilayer graphene with Bernal stacking (BLG). We compare moiré miniband features and zero-energy gaps in BLG, where they are affected by the interlayer asymmetry of BLG/$h$-BN heterostructure and trigonal warping characteristic for electrons in Bernal-stacked bilayers, with those found in monolayer graphene.