Electronic susceptibilities are a very popular tool to study electronic and magnetic properties of materials, both in experiment and theory. Unfortunately, the numerical evaluation of even the bare susceptibility, which depends on the computation of matrix elements and sums over energy bands, is very work intensive and therefore various approximations have been introduced to speed up such calculations. We present a reliable and efficient implementation of the tetrahedron method which allows us to accurately calculate both static and dynamic bare susceptibilities, based on full-potential density functional theory (DFT) calculations. In the light of the exact results we assess the effects of replacing the matrix elements by a constant and the impact of truncating the sum over the energy bands. Results will be given for representative and topical materials such as Cr, a classical transition metal, as well as the iron-based superconductor FeSe.

We determine the effective field theory in $(2+1)$-dimensional space and time so that it captures the long-wavelength and low-energy limit of fermions hopping on a honeycomb lattice at half-filling when both dominant intrinsic and subdominant Rashba spin-orbit couplings are present. This effective field theory for a ${\mathbb{Z}}_2^{}$ topological insulator (the Kane-Mele model at vanishing uniform and staggered chemical potentials) is a perturbation around a double Chern-Simons theory, with the $\text{U}\left(1\right)$ gauge invariance associated with spin conservation explicitly broken due to the Rashba spin-orbit coupling. Nonetheless, we find that the effective field theory has a Becchi-Rouet-Stora-Tyutin symmetry that allows us to construct the bulk-edge correspondence.

We explore the dynamics and directionality of spontaneous emission from self-assembled In(Ga)As quantum dots into transverse-electric–polarized guided modes in GaAs two-dimensional photonic-crystal waveguides. The local group velocity of the guided waveguide mode is probed, with values as low as $\sim 1.5\%c$ measured close to the slow-light band edge. By performing complementary continuous-wave and time-resolved measurements with detection along and perpendicular to the waveguide axis, we probe the fraction of emission into the waveguide mode ($\beta$ factor). For dots randomly positioned within the unit cell of the photonic-crystal waveguide, our results show that the emission rate varies from $\ge 1.55\mathrm{n}{\mathrm{s}}^{-1}$ close to the slow-light band edge to $\le 0.25\mathrm{n}{\mathrm{s}}^{-1}$ within the two-dimensional photonic band gap. We measure an average Purcell factor of $\sim 2\times$ for dots randomly distributed within the waveguide and maximum values of $\beta \sim 90\%$ close to the slow-light band edge. Spatially resolved measurements performed by exciting dots at a well-controlled distance of 0–45 $\mu \text{m}$ from the waveguide facet highlight the impact of disorder on the slow-light dispersion. Although disorder broadens the spectral width of the slow-light region of the waveguide dispersion from $\delta E_d\le 0.5$ to $>6\mathrm{m}\mathrm{e}\mathrm{V}$, we find that emission is nevertheless primarily directed into propagating waveguide modes. The ability to control the rate and directionality of emission from isolated quantum emitters by placing them in a tailored photonic environment provides much promise for the use of slow-light phenomena to realize efficient single-photon sources for quantum optics in a highly integrated setting.

The effect of nonconservative current-induced forces on the ions in a defect-free metallic nanowire is investigated using both steady-state calculations and dynamical simulations. Nonconservative forces were found to have a major influence on the ion dynamics in these systems, but their role in increasing the kinetic energy of the ions decreases with increasing system length. The results illustrate the importance of nonconservative effects in short nanowires and the scaling of these effects with system size. The dependence on bias and ion mass can be understood with the help of a simple pen and paper model. This material highlights the benefit of simple preliminary steady-state calculations in anticipating aspects of brute-force dynamical simulations, and provides rule of thumb criteria for the design of stable quantum wires.

Our main comment is that the data at low temperatures of Kube et al. [Phys. Rev. B 88, 085206 (2013)] and those of Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] may not have been due to intrinsic vacancies but rather to extrinsic vacancies generated at carbon and/or vacancy clusters, depending on the specimens used.

Suezawa et al. [Phys. Rev. B 90, 117201 (2014)] claim in their Comment that the data reported by Shimizu et al. [ Phys. Rev. Lett. 98, 095901 (2007)] and Kube et al. [Phys. Rev. B 88, 085206 (2013)] on silicon self-diffusion for temperatures between 900 and 735 °C are affected by carbon and vacancy clusters and, accordingly, do not reflect self-diffusion under thermal equilibrium conditions. We demonstrate in our Reply that an impact of carbon on self-diffusion can definitely be excluded. In addition it is rather unlikely that the self-diffusion data reported by Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] and Kube et al. [Phys. Rev. B 88, 085206 (2013)] are affected by the dissolution of vacancy clusters since strong differences exist not only in the preparation of the samples used for the experiments, but also in the time of diffusion. Finally, the vacancy formation enthalpy deduced by Suezawa et al. [J. Appl. Phys. 110, 083531 (2011)] from quenching experiments is consistent with the value obtained from the temperature dependence of the vacancy formation enthalpy reported by Kube et al. [Phys. Rev. B 88, 085206 (2013)]. Overall we conclude that the quenching experiments of Suezawa et al. [J. Appl. Phys. 110, 083531 (2011)] cannot disprove the interpretation of the low-temperature self-diffusion data reported by Shimizu et al. [ Phys. Rev. Lett. 98, 095901 (2007)] and Kube et al. [Phys. Rev. B 88, 085206 (2013)].

We study the different phases in the Quantum Electrodynamics of three-dimensional Dirac semimetals depending on the number $N$ of Dirac fermions, using renormalization group methods and the self-consistent resolution of the Schwinger-Dyson equation. We find that, for $N<4$, a phase with dynamical generation of mass prevails at sufficiently strong coupling, sharing the same physics of the excitonic instability in two-dimensional Dirac semimetals. For $N\ge 4$, we show that the phase diagram has instead a line of critical points characterized by the suppression of the quasiparticle weight at low energies, making the system fall into the class of marginal Fermi liquids. Such a boundary marks the transition to a kind of strange metal which can still be defined in terms of electron quasiparticles, but with parameters that have large imaginary parts implying an increasing deviation from the conventional Fermi liquid picture.

In periodically driven lattice systems, the effective (Floquet) Hamiltonian can be engineered to be topological; then, the principle of bulk-boundary correspondence guarantees the existence of robust edge states. However, such setups can also host edge states not predicted by the Floquet Hamiltonian. The exploration of such edge states and the corresponding unique bulk topological invariants has only recently begun. In this work we calculate these invariants for chiral symmetric periodically driven one-dimensional systems. We find simple closed expressions for these invariants, as winding numbers of blocks of the unitary operator corresponding to a part of the time evolution. This gives a robust way to tune these invariants using sublattice shifts. We illustrate our ideas on the periodically driven Su-Schrieffer-Heeger model, which, as we show, can realize a discrete-time quantum walk; this opens a useful connection between periodically driven lattice systems and discrete-time quantum walks. Our work helps interpret the results of recent simulations where a large number of Floquet Majorana fermions in periodically driven superconductors have been found.

We consider a one-dimensional system of interacting bosons in a random potential. At zero temperature, it can be either in the superfluid or in the insulating phase. We study the transition at weak disorder and moderate interaction. Using a systematic approach, we derive the renormalization group equations at two-loop order and discuss the phase diagram. We find the universal form of the correlation functions at the transitions and compute the logarithmic corrections to the main universal power-law behavior. In order to mimic large density fluctuations on a single site, we study a simplified model of disordered two-leg bosonic ladders with correlated disorder across the rung. Contrarily to the single-chain case, the latter system exhibits a transition between a superfluid and a localized phase where the exponents of the correlation functions at the transition do not take universal values.

The optical analysis of multilayer structures formed from the topmost layer of InGaAs/GaAs quantum rings (QRs) grown on a vertically stacked and laterally aligned InGaAs/GaAs quantum dot (QD) superlattice has been performed to elucidate the nature of the contribution from each layer. These hybrid structures representing a coupled QR chain layer and the layers of self-assembled QD chains display strong optical anisotropy. Unusually strong oscillations are observed in the circularly polarized photoluminescence (PL) intensities under magnetic field for emissions in the spectral range of the QD structure and these oscillations occur simultaneously with weaker oscillations related to the Aharonov-Bohm interference that modulates the emissions from the QR top layer of the structure. The behavior seen in the magneto-PL spectrum is interpreted in terms of joint effects associated to strain, spatial, and magnetic field confinements on the valence band states forming the magnetoexciton ground state of this multilayered structure. The result can be ascribed to a magnetically induced dark exciton contribution where the heavy-hole (type II) state becomes localized outside, whereas light-hole (type I) as well as electron states remain inside the spatial confinement area of the QD.

We consider the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities on the surface of a three-dimensional topological insulator with proximity induced superconductivity. A superconductor placed on the top of the topological insulator induces a gap in the surface electron states and gives rise to a long-range in-plane antiferromagnetic RKKY interaction. This interaction is frustrated due to strong spin-orbit coupling, inversely proportional to the distance between magnetic impurities if the distance between magnetic impurities is smaller than the superconducting coherence length, and dominates over the ferromagnetic and Dzialoshinskii-Moriya type interactions if the distance between magnetic impurities is larger than the superconducting coherence length. The condition for the subgap states that are bound to the magnetic impurities is found.

Electronic correlations control the normal state of bulk high-$T_c$ cuprates. Strong correlations also suppress the charge transport through cuprate grain boundaries (GBs). The question then arises if these correlations can produce magnetic states at cuprate GBs. We analyze the formation of local magnetic moments at the GB of a correlated two-dimensional electron system, which is represented by an inhomogeneous Hubbard model. The model Hamiltonian is diagonalized after the implementation of a mean-field decoupling. The formation of local magnetic moments is supported by a sufficiently strong variance in the bond kinetic energies at the GB. Local scattering potentials can assist or suppress the formation of a magnetic GB state, depending on the details of their spacial distribution. Grain-boundary-induced stripes are formed in the vicinity the GB and decay into the bulk. Moreover, we observe the buildup of conducting channels which are confined by magnetic clusters. The grain boundary resistance increases at decreasing temperatures. This low-temperature behavior is caused by the suppression of current correlations in the state with local magnetic GB moments. The resistance upturn at low temperatures is in qualitative agreement with experiments.

We compute the ac susceptibility of a weakly dipolar-interacting monodisperse assembly of magnetic nanoclusters with oriented anisotropy. For this purpose, we first compute the relaxation rate in a longitudinal magnetic field of a single nanomagnet taking account of both dipolar interactions in the case of dilute assemblies and surface anisotropy. We then study the behavior of the real and imaginary components of the ac susceptibility as functions of temperature, frequency, surface anisotropy, and interparticle interactions. We find that the surface anisotropy induces an upward shift of the temperature at the maximum of the ac susceptibility components and that its effects may be tuned so as to screen out the effects of interactions. The phenomenological Vogel-Fulcher law for the effect of dipolar interaction on the relaxation rate is revisited within our formalism and a semianalytical expression is given for the effective temperature in terms of inter alia the applied field, surface anisotropy, and dipolar interaction.

The quantum dynamics of pairs of two-level systems immersed in dissipative reservoirs with sub-Ohmic spectral distributions is studied by means of numerically exact path integral Monte Carlo methods. It is shown that this class of reservoirs, relevant for generic properties of strongly non-Markovian environments with possible realizations in solid state structures at cryogenic temperatures, supports bipartite entanglement in broad ranges of parameter space and even for broken symmetries and in the presence of thermal noise. The sensitivity of the creation of entanglement in nonequilibrium on the initial preparations of the reservoir is investigated in detail.

The upper critical field $H_{c2}$ and its anisotropy are calculated for order parameters with line nodes at the equators, $k_z=0$, of the Fermi surface of uniaxial superconductors. It is shown that characteristic features found in Fe-based materials (a nearly linear $H_{c2}\left(T\right)$ in a broad $T$ domain, a low and increasing on warming anisotropy ${\gamma }_H=H_{c2,ab}/H_{c2,c}$) can be caused by competing effects of the equatorial nodes and of the Fermi surface anisotropy. For certain material parameters, ${\gamma }_H\left(T\right)-1$ may change sign upon warming, in agreement with the recorded behavior of FeTeS systems. It is also shown that the anisotropy of the penetration depth ${\gamma }_{\lambda }={\lambda }_c/{\lambda }_{ab}$ decreases upon warming to reach ${\gamma }_H$ at $T_c$, in agreement with data available. For some materials ${\gamma }_{\lambda }\left(T\right)$ may change upon warming, from ${\gamma }_{\lambda }>1$ at low $T\mathrm{s}$ to ${\gamma }_{\lambda }<1$ at high $T\mathrm{s}$.

${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ is a quasi-one-dimensional antiferromagnetic spin-1/2 chain system with pronounced Ising anisotropy of the magnetic exchange. Due to finite interchain interactions, long-range antiferromagnetic order develops below $T_{\mathrm{N}}\simeq 5.5\mathrm{K}$, which is accompanied by a structural distortion in order to lift magnetic frustration effects. The corresponding temperature versus magnetic-field phase diagram is highly anisotropic with respect to the magnetic-field direction and various details are still under vivid discussion. Here, we report the influence of several substitutions on the magnetic properties and the phase diagrams of ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. We investigate the substitution series ${\mathrm{Ba}}_{1-\mathrm{x}}{\mathrm{Sr}}_{\mathrm{x}}{\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ over the full range $0\le x\le 1$ as well as the influence of a partial substitution of the magnetic ${\mathrm{Co}}^{2+}$ by small amounts of other magnetic transition metals or by nonmagnetic magnesium. In all cases, the phase diagrams were obtained on single crystals from magnetization data and/or high-resolution studies of the thermal expansion and magnetostriction.

We theoretically investigate the design of cavities in a three-dimensional (3D) inverse woodpile photonic crystal. This class of cubic diamondlike crystals has a very broad photonic band gap and consists of two perpendicular arrays of pores with a rectangular structure. The point defect that acts as a cavity is centered on the intersection of two intersecting perpendicular pores with a radius that differs from the ones in the bulk of the crystal. We have performed supercell band structure calculations with up to $5\times 5\times 5$ unit cells. We find that up to five isolated and dispersionless bands appear within the 3D photonic band gap. For each isolated band, the electric-field energy is localized in a volume centered on the point defect, hence the point defect acts as a 3D photonic band gap cavity. The mode volume of the cavities resonances is as small as 0.8 ${\lambda }^3$ (resonance wavelength cubed), indicating a strong confinement of the light. By varying the radius of the defect pores we found that only donorlike resonances appear for smaller defect radius, whereas no acceptorlike resonances appear for greater defect radius. From a 3D plot of the distribution of the electric-field energy density we conclude that peaks of energy are found in sharp edges situated at the point defect, similar to how electrons collect at such features. This is different from what is observed for cavities in noninverted woodpile structures. Since inverse woodpile crystals can be fabricated from silicon by CMOS-compatible means, we project that single cavities and even cavity arrays can be realized, for wavelength ranges compatible with telecommunication windows in the near infrared.

Symmetry-protected topological (SPT) phases are gapped short-range-entangled quantum phases with a symmetry $G$, which can all be smoothly connected to the trivial product states if we break the symmetry. It has been shown that a large class of interacting bosonic SPT phases can be systematically described by group cohomology theory. In this paper, we introduce a (special) group supercohomology theory which is a generalization of the standard group cohomology theory. We show that a large class of short-range interacting fermionic SPT phases can be described by the group supercohomology theory. Using the data of supercocycles, we can obtain the ideal ground state wave function for the corresponding fermionic SPT phase. We can also obtain the bulk Hamiltonian that realizes the SPT phase, as well as the anomalous (i.e., non-onsite) symmetry for the boundary effective Hamiltonian. The anomalous symmetry on the boundary implies that the symmetric boundary must be gapless for (1+1)-dimensional [(1+1)D] boundary, and must be gapless or topologically ordered beyond (1+1)D. As an application of this general result, we construct a new SPT phase in three dimensions, for interacting fermionic superconductors with coplanar spin order (which have $T^2=1$ time-reversal $Z_2^T$ and fermion-number-parity $Z_2^f$ symmetries described by a full symmetry group $Z_2^T\times Z_2^f$). Such a fermionic SPT state can neither be realized by free fermions nor by interacting bosons (formed by fermion pairs), and thus are not included in the $K$-theory classification for free fermions or group cohomology description for interacting bosons. We also construct three interacting fermionic SPT phases in two dimensions (2D) with a full symmetry group $Z_2\times Z_2^f$. Those 2D fermionic SPT phases all have central-charge $c=1$ gapless edge excitations, if the symmetry is not broken.

We report on a first-principles investigation of transient heat current through molecular devices under steplike pulse of external and gate voltages. Using the nonequilibrium Green's function (NEGF) approach, an exact solution of transient heat current is obtained that goes beyond the wide-band limit. Combining with density-functional theory (DFT), we propose a time-dependent NEGF-DFT formalism to study the transient heat current under a steplike pulse for molecular devices from first principles. Anticipating the huge computational cost in the transient regime, we develop an algorithm to speed up the calculation using the complex absorbing potential (CAP). By adding the CAP to replace the Hamiltonian of leads, the effective self-energy of the Green's function becomes independent of energy, allowing analytic calculation of the triple integrations in the exact solution of transient heat current using the theorem of residue. With this linear scaling algorithm, the computational complexity is greatly reduced, and a first-principles calculation of transient heat current of molecular devices becomes possible. As an example, we apply our NEGF-DFT-CAP formalism for a molecular device, the Di-thiol benzene molecule connected by two semi-infinite aluminum leads, and we calculate the transient heat current under an upward gate voltage pulse. The enhancement of heat current is observed.