Chern insulators arguably provide the simplest examples of topological phases. They are characterized by a topological invariant and can be identified by the presence of protected edge states. In this paper, we show that a local impurity in a Chern insulator induces a twofold response: bound states that carry a chiral current and a net current circulating around the impurity. This is a manifestation of broken time reversal symmetry and persists even for an infinitesimal impurity potential. To illustrate this, we consider a Coulomb impurity in the Haldane model. Working in the low-energy, long-wavelength limit, we show that an infinitesimal impurity strength suffices to create bound states. We find analytic wave functions for the bound states and show that they carry a circulating current. In contrast, in the case of a trivial analog—graphene with a gap induced by a sublattice potential—bound states occur but carry no current. In the many-body problem of the Haldane model at half-filling, we use a linear response approach to demonstrate a circulating current around the impurity. Impurity textures in insulators are generally expected to decay exponentially; in contrast, this current decays polynomially with distance from the impurity. Going beyond the Haldane model, we consider the case of coexisting trivial and nontrivial masses. We find that the impurity induces a local chiral current as long as time-reversal symmetry is broken. However, the decay of this local current bears a signature of the overall topology—the current decays polynomially in a nontrivial system and exponentially in a trivial system. In all cases, our analytic results agree well with numerical tight-binding simulations.

We discuss possible topologies of physically different metasurfaces with equivalent macroscopic electromagnetic responses. We specifically demonstrate that the presence of normal components (with respect to the metasurface plane) of the polarization currents does not add any functionality in engineering the reflection and transmission characteristics which cannot occur in metasurfaces maintaining only tangential currents. Therefore, if a certain property is achievable using general volumetric inclusions, polarizable along all three directions, the same response can be achieved using some equivalent perfectly planar elements, polarizable only in the metasurface plane. Our results clarify what physical properties are necessary for realizing desired metasurface responses and promote flat microwave and optical designs, providing possibilities for dramatic complexity reduction of metasurfaces for rather general field transformations, including asymmetric reflection and transmission.

We present a detailed theoretical study of the electronic and transport properties of monolayer black phosphorus (BP). This study is motivated by recent experimental activities in investigating $n$-type few-layer BP systems. The electron density of states, the screening length, and the low-temperature electron mobility are calculated for monolayer BP (MLBP). In particular, the electron transport mobilities along the armchair and zigzag directions are examined on the basis of the momentum-balance equation derived from a semiclassical Boltzmann equation. The anisotropic electron mobilities in MLBP along different directions are demonstrated where the electron-impurity scattering is considered. Furthermore, we compare the results obtained from two electronic band structures of MLBP and find that the simplified model can describe quite rightly the electronic and transport properties of MLBP. This study is relevant to the application of few-layer BP based electronic systems as advanced electronic devices.

We identify a topological $\mathbb{Z}$ index for three-dimensional chiral insulators with $P*T$ symmetry where two Hamiltonian terms define a nodal loop. Such systems may belong in the AIII or DIII symmetry class. The $\mathbb{Z}$ invariant is a winding number assigned to the nodal loop and has a correspondence to the geometric relation between the nodal loop and the zeros of the gap terms. Dirac cone edge states under open boundary conditions are in correspondence with the winding numbers assigned to the nodal loops. We verify our method with the low-energy effective Hamiltonian of a three-dimensional material of topological insulators in the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ family.

The relative energies of the low-pressure rutile, anatase, and brookite polymorphs and the high-pressure columbite polymorph of ${\mathrm{TiO}}_2$ have been calculated as a function of temperature using the diffusion quantum Monte Carlo (DMC) method and density functional theory (DFT). The vibrational energies are found to be important on the scale of interest and significant quartic anharmonicity is found in the rutile phase. Static-lattice DFT calculations predict that anatase is lower in energy than rutile, in disagreement with experiment. The accurate description of electronic correlations afforded by DMC calculations and the inclusion of anharmonic vibrational effects contribute to stabilizing rutile with respect to anatase. Our calculations predict a phase transition from anatase to rutile ${\mathrm{TiO}}_2$ at $630\pm 210$ K.

The class of topological semimetals comprises a large pool of compounds. Together they provide a wide platform to realize exotic quasiparticles, for example, Dirac, nodal-line Dirac, and Weyl fermions. In this Rapid Communication, we report the Berry phase, Fermi-surface topology, and anisotropic magnetoresistance of HfSiS which has recently been predicted to be a nodal-line semimetal. This compound contains a large carrier density, higher than most of the known semimetals. Massive amplitudes of de Haas–van Alphen and Shubnikov–de Haas oscillations up to 20 K in 7 T assist us in witnessing a nontrivial π-Berry phase, which is a consequence of topological Dirac-type dispersion of bands originating from the hybridization of $p_x+p_y$ and $d_{x^2-y^2}$ orbitals of square-net plane of Si and Hf atoms, respectively. Furthermore, we establish the three-dimensional Fermi surface which consists of very asymmetric water caltroplike electrons and barley seedlike hole pockets which account for the anisotropic magnetoresistance in HfSiS.

Phonon-assisted electronic tunneling through a vibrating quantum dot embedded between normal and superconducting leads is studied in the Kondo regime. In such a hybrid device, with the bias applied to the normal lead, we find a series of Kondo sidebands separated by half a phonon energy in the differential conductance, which are distinct from the phonon-assisted sidebands previously observed in conventional Andreev tunneling and in systems with only normal leads. These Kondo sidebands originate from the Kondo-Andreev cooperative cotunneling mediated by phonons, which exhibit an interesting Kondo transport behavior due to the interplay of the Kondo effect, the Andreev tunneling, and the mechanical vibrations. Our result could be observed in a recent experiment setup [J. Gramich et al., Phys. Rev. Lett. 115, 216801 (2015)], provided that their carbon nanotube device reaches the Kondo regime at low temperatures.

In the quasi-one-dimensional heavy-fermion system ${\mathrm{YbNi}}_4{\left({\mathrm{P}}_{1-x}{\mathrm{As}}_x\right)}_2$ the presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_{\mathrm{c}} \approx 0.1$ with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation ($\mu \mathrm{SR}$) experiments on polycrystals of this series to study the magnetic order and the low-energy $4f$-electronic spin dynamics across the FM QCP. The zero-field $\mu \mathrm{SR}$ measurements on pure ${\mathrm{YbNi}}_4{\mathrm{P}}_2$ proved static long-range magnetic order and suggested a strongly reduced ordered Yb moment of about $0.04{\mu }_{\mathrm{B}}$. With increasing As substitution, the ordered moment is reduced by half at $x=0.04$ and to less than $0.005{\mu }_{\mathrm{B}}$ at $x=0.08$. The dynamic behavior in the $\mu \mathrm{SR}$ response shows that magnetism remains homogeneous upon As substitution, without evidence of a disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows $1/T_{1}T\propto T^{-n}$ with $1.01\pm 0.04\le n\le 1.13\pm 0.06$. The critical fluctuations are very slow and become even slower when approaching the QCP.

The $\nu =\frac{5}{2}$ fractional quantum Hall effect is of experimental and theoretical interest due to the possible non-Abelian statistics of the excitations in the electron liquid. A small voltage difference across a sample applied in experiments to probe the system is often ignored in theoretical studies due to the Galilean invariance in the thermodynamic limit. No experimental sample, however, is Galilean invariant. In this Rapid Communication, we explore the effects of the probe electric fields in a disk geometry with finite thickness. We find that weak probe fields enhance the Moore-Read Pfaffian state, but sufficiently strong electric fields destroy the incompressible state. In a disk geometry, the behavior of the system depends on the polarity of the applied radial field, which can potentially be observed in experiments using a Corbino disk configuration. Our simulation also shows that the application of such a field enhances the coherence length of quasiholes propagating through the edge channels.

The electrical conductivity of metallic carbon nanotubes (CNTs) quickly saturates with respect to bias voltage due to scattering from a large population of optical phonons. The decay of these dominant scatterers in pristine CNTs is too slow to offset an increased generation rate at high voltage bias. We demonstrate from first principles that encapsulation of one-dimensional atomic chains within a single-walled CNT can enhance the decay of “hot” phonons by providing additional channels for thermalization. Pacification of the phonon population growth reduces the electrical resistivity of metallic CNTs by 51% for an example system with encapsulated beryllium.

We present the results of temperature- and polarization-dependent high-resolution optical spectroscopy studies of $\mathrm{DyF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ performed in spectral ranges $40-300\mathrm{c}{\mathrm{m}}^{-1}$ and $3000-23000\mathrm{c}{\mathrm{m}}^{-1}$. The crystal-field (CF) parameters for the $\mathrm{D}{\mathrm{y}}^{3+}$ ions in the $P{3}_{1}21$ ($P{3}_{2}21$) phase of $\mathrm{DyF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ are obtained from calculations based on the analysis of the measured f-f transitions. Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on $\mathrm{DyF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ using circularly polarized x rays [T. Usui, Y. Tanaka, H. Nakajima, M. Taguchi, A. Chainani, M. Oura, S. Shin, N. Katayama, H. Sawa, Y. Wakabayashi, and T. Kimura, Nat. Mater. 13, 611 (2014)]. Using the obtained set of the CF parameters, we calculate temperature dependencies of the electronic quadrupole moments of the $\mathrm{D}{\mathrm{y}}^{3+}$ ions induced by the low-symmetry ($C_2$) CF component and show that the quadrupole helix chirality can be explained quantitatively. We also consider the temperature dependencies of the bulk magnetic dc-susceptibility and the helix chirality of the single-site magnetic susceptibility tensors of the $\mathrm{D}{\mathrm{y}}^{3+}$ ions in the paramagnetic $P{3}_{1}21$ ($P{3}_{2}21$) phase and suggest the neutron and resonant x-ray diffraction experiments in a magnetic field to reveal the helix chirality of field-induced magnetic moments.

Rare-earth nickelates exhibit several temperature-driven phase transitions that are tunable by the size of the rare-earth ions, pressure, epitaxial strain in ultrathin films, etc. We investigate the metal-insulator and Neel transitions in a series of ${\mathrm{NdNiO}}_3$ thin films with varying degrees of lattice mismatch using ultralow frequency electrical noise measurements. The noise magnitude follows a 1/f  behavior and is Gaussian in the high temperature paramagnetic metallic phase of the films, however deviations are seen in the low temperature paramagnetic and antiferromagnetic insulating phases. The noise magnitude increases by orders of magnitude at temperatures below the metal-insulator transition and is indicative of inhomogeneous electrical conduction arising from phase separation. The inhomogeneous nature of conduction is corroborated by the presence of a large non-Gaussian noise signature in the low temperature phases. Well below the Neel temperature, the noise behavior evolves between Gaussian and non-Gaussian over several hours pointing to dynamically competing ground states with subtle variations within the antiferromagnetic insulating phase.

The effect of correlated hopping on the charge and heat transport of strongly correlated particles is studied for the Falicov-Kimball model on the Bethe lattice. Exact solutions for the one-particle density of states (DOS) and two-particle transport function (the “quasiparticle” scattering time) are derived using dynamical mean-field theory. For a wide range of the correlated hopping, the transport function exhibits singularities due to the resonant two-particle contribution, whereas the one-particle DOS does not show any anomalous features. By tuning the number of itinerant electrons, so as to bring the Fermi level close to the resonant frequency, we get a large increase of the electrical and thermal conductivities, and the thermoelectric power. When the hopping amplitude between the occupied sites is reduced sufficiently, the itinerant electrons localize in the clusters of sites occupied by $f$ electrons. This gives rise to an additional narrow band in the DOS between the lower and upper Hubbard bands but has only a minor effect on the thermoelectric properties.

We study quantum Hall states for two-component particles (hardcore bosons and fermions) loading in topological lattice models. By tuning the interplay of interspecies and intraspecies interactions, we demonstrate that two-component fractional quantum Hall states emerge at certain fractional filling factors $\nu =1/2$ for fermions ($\nu =2/3$ for bosons) in the lowest Chern band, classified by features from ground states including the unique Chern number matrix (inverse of the $\mathbf{K}$ matrix), the fractional charge and spin pumpings, and two parallel propagating edge modes. Moreover, we also apply our strategy to two-component fermions at integer filling factor $\nu =2$, where a possible topological Neel antiferromagnetic phase is under intense debate very recently. For the typical $\pi$-flux checkerboard lattice, by tuning the onsite Hubbard repulsion, we establish a first-order phase transition directly from a two-component fermionic $\nu =2$ quantum Hall state at weak interaction to a topologically trivial antiferromagnetic insulator at strong interaction, and therefore exclude the possibility of an intermediate topological phase for our system.

At low temperature, information can be stored in the orientation of the localized magnetic moment of an adatom. However, scattering of electrons and phonons with the nanomagnet leads its state to have incoherent classical dynamics and might cause fast loss of the encoded information. Recently, it has been understood that such scattering obeys certain selection rules due to the symmetries of the system. By analyzing the point-group symmetry of the surface, the time-reversal symmetry and the magnitude of the adatom effective spin, we identify which nanomagnet configurations are to be avoided and which are promising to encode a stable bit. A new tool of investigation is introduced and exploited: the quasispin quantum number. By means of this tool, our results are easily generalized to a broad class of bipartite cluster configurations where adatoms are coupled through Heisenberg-like interactions. Finally, to make contact with the experiments, numerical simulations have been performed to show how such stable configurations respond to typical scanning tunneling microscopy measurements.

Due to its simplicity the diffuse mismatch model (DMM) remains a popular description of phonon transmission across solid-solid boundaries. However, it remains unclear in which situations the DMM should be expected to be a valid model of the underlying physics. Here its validity is investigated mode-by-mode using a three-dimensional extension of the frequency domain, perfectly matched layer (FD-PML) method, to study the interface between face-centered cubic solids with interdiffused atoms. While submonolayer levels of interdiffusion are found to increase the number of available modes for transmission, consistent qualitatively with the DMM, we do not find quantitative or qualitative convergence toward the DMM at higher levels of interdiffusion. In particular, contrary to the fundamental assumption of the DMM, modes are not found to lose memory of their initial polarization and wave vector. The transmission coefficients of randomly interdiffused and smoothly graded interfaces are also compared. While smoothly graded interfaces show strong antireflection properties, selection rules still prohibit transmission of many modes, whereas interdiffused interfaces are not subject to such rules and achieve similar thermal interface conductance by transmitting with lower probability but using a wider range of modes.

The topological nature of Weyl semimetals (WSMs) is corroborated by the presence of chiral surface states, which connect the projections of the bulk Weyl points by Fermi arcs (FAs). We study a bilayer structure realized by introducing a thin insulating spacer into a bulk WSM. Employing a self-consistent mean-field description of the interlayer Coulomb interaction, we propose that this system can develop an interlayer electron-hole pair condensate. The formation of this excitonic condensate leads to partial gapping of the FA dispersion. We obtain the dependence of the energy gap and the critical temperature on the model parameters, finding, in particular, a linear scaling of these quantities with the separation between the Weyl points in momentum space. A detrimental role is played by the curvature of the FAs, although the pairing persists for moderately small curvature. A signature of the condensate is the modification of the quantum oscillations involving the surface FAs.

Transition-metal dichalcogenide $\mathrm{IrT}{\mathrm{e}}_2$ has attracted attention because of its striped lattice, charge ordering, and superconductivity. We have investigated the surface structure of $\mathrm{IrT}{\mathrm{e}}_2$, using low-energy electron diffraction and scanning tunneling microscopy. A complex striped lattice modulation as a function of temperature is observed, which shows hysteresis between cooling and warming. While the bulk 5 × 1 and 8 × 1 phases appear at high temperatures, the surface ground state has the 6 × 1 phase, not seen in the bulk, and the surface transition temperatures are distinct from the bulk. The broken symmetry at the surface creates a quite different phase diagram, with the coexistence of several periodicities resembling devil's staircase behavior.

We report measurements of quantum turbulence generated by a vibrating grid in superfluid ${}^3\mathrm{He}$-B at zero pressure in the zero temperature limit. Superfluid flow around individual vortex lines Andreev reflects incoming thermal ballistic quasiparticle excitations, and allows noninvasive detection of quantum vortices in ${}^3\mathrm{He}$-B. We have compared two Andreev reflection-based techniques traditionally used to detect quantum turbulence in the ballistic regime: quasiparticle transmission through and reflection from ballistic vortex rings and a turbulent tangle. We have shown that the two methods are in very good agreement and thus complement each other. Our measurements reveal that vortex rings and a tangle generated by a vibrating grid have a much larger spatial extent than previously realized. Furthermore, we find that a vortex tangle can either pass through an obstacle made from a mesh or diffuse around it. The measured dependence of vortex signal as a function of the distance from the vibrating grid is consistent with a power-law behavior in contrast to turbulence generated by a vibrating wire which is described by an exponential function.

We use neutron scattering to study the electron-doped superconducting ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ ($T_c=14$ K), which has coexisting static antiferromagnetic (AF) order ($T_N=31$ K) and exhibits two neutron spin resonances ($E_{r1}\approx 3.5$ meV and $E_{r2}\approx 6$ meV) at the in-plane AF ordering wave vector ${\mathbf{Q}}_{\mathrm{AF}}={\mathbf{Q}}_1=(1,0)$ in reciprocal space. In the twinned state below the tetragonal-to-orthorhombic structural transition $T_s$, both resonance modes appear at ${\mathbf{Q}}_1$ but cannot be distinguished from ${\mathbf{Q}}_2=(0,1)$. By detwinning the single crystal with uniaxial pressure along the orthorhombic $b$ axis, we find that both resonances appear only at ${\mathbf{Q}}_1$ with vanishing intensity at ${\mathbf{Q}}_2$. Since electronic bands of the orbital $d_{xz}$ and $d_{yz}$ characters split below $T_s$ with the $d_{xz}$ band sinking $\sim 10$ meV below the Fermi surface, our results indicate that the neutron spin resonances in ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ arise mostly from quasiparticle excitations between the hole and electron Fermi surfaces with the $d_{yz}$ orbital character.