We report a new family of skyrmion materials originated from the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Based on the symmetric tensor technique, the molybdenum nitrides with the β-manganese structure, $A_2\mathrm{M}{\mathrm{o}}_3\mathrm{N}$ with $A=\mathrm{Fe}$, Co, Rh, are predicted to support the skyrmion phase. This predication is directly proved in doped $\mathrm{F}{\mathrm{e}}_x\mathrm{C}{\mathrm{o}}_{1-x}\mathrm{R}{\mathrm{h}}_{0.5}\mathrm{M}{\mathrm{o}}_3\mathrm{N}$ components by high-resolution Lorentz transmission electron imaging. Interestingly, the parent compounds $\mathrm{F}{\mathrm{e}}_2\mathrm{M}{\mathrm{o}}_3\mathrm{N}, \mathrm{C}{\mathrm{o}}_2\mathrm{M}{\mathrm{o}}_3\mathrm{N}$, and $\mathrm{R}{\mathrm{h}}_2\mathrm{M}{\mathrm{o}}_3\mathrm{N}$ exhibit ferromagnetic, antiferromagnetic, and possible superconducting orderings, respectively. Magnetism in these parent phases is theoretically clarified by the first-principle calculations, where the corrected nature of the magnetism is revealed.

The rare-earth nitrides have recently regained attention due to findings that most members of the series are intrinsic ferromagnetic semiconductors, a class of materials that is crucial for the development of spintronics devices. Here we present a study of NdN thin films, with films grown via molecular beam epitaxy. Optical transmission measurements revealed a band gap of about 0.9 eV, while resistivity measurements confirmed semiconducting behavior with a negative temperature coefficient of resistance, though semimetallic behavior could not be ruled out. The room temperature resistivity of 0.$6\mathrm{m}\mathrm{\Omega }\mathrm{cm}$ indicates strong doping by nitrogen vacancies. Magnetization measurements show a ferromagnetic moment of $1.0\pm 0.2{\mu }_B$ below the Curie temperature $T_C$ of $43\pm 1$ K, strongly suppressed from the Hund's rules value of $3.27{\mu }_B$ per ion. The ferromagnetic moment is strongly quenched and the $T_C$ is enhanced compared to previously studied bulk NdN, and crystal field calculations reveal that the quenched moment is likely due to lattice strain. X-ray magnetic circular dichroism measurements show that the magnetic moment is orbital dominant, placing NdN in the same category as SmN, an intrinsic ferromagnetic semiconductor with an orbital-dominant ferromagnetic moment.

We perform atomistic modeling of Co/CoO nanoparticles with a diameter of a few nanometers and realistic values of the exchange and anisotropy parameters in order to study the field-dependent energy barriers under forward and backward reversal of the magnetization. The barriers are extracted from the constrained energy minimization using the integration of the Landau-Lifshitz-Gilbert equations and the Lagrange multiplier method. We show that the applied field and the interface exchange strength have opposite effects on the values of the energy barriers. In particular, while the backward (forward) energy barrier increases (decreases) linearly with the strength of the interface exchange coupling, it decreases (increases) almost quadratically with the applied magnetic field. Our results are in good agreement with the well-known Meiklejohn-Bean model of exchange bias, and allow us to analyze the limits of applicability of the macrospin approach to the study of energy barriers in core-shell Co/CoO nanoparticles.

We study the effect of adding disorder to the Kitaev model on the hyperhoneycomb lattice, which hosts both gapped and gapless spin liquid phases with an emergent ${\mathbb{Z}}_2$ gauge field. The latter has an unusual gapless spectrum of Majorana fermion excitations, with a co-dimension-two Fermi ring. We thus address the question of the interplay of topological physics and disorder by considering the properties of isolated single and pairs of vacancies. We show that near the vacancies, the local magnetic response to a field $h_z$ is parametrically enhanced in comparison to the pristine bulk. Unlike the previously studied case of the 2D honeycomb Kitaev model, the vacancies do not bind a flux of the ${\mathbb{Z}}_2$ gauge field. In the gapped phase, an isolated vacancy gives rise to effectively free spin-half moments with a nonuniversal coupling to an external field. In the gapless phase, the low-field magnetization is suppressed parametrically to ${(-ln{h}_z)}^{-1/2}$ because of interactions with the surrounding spin liquid. We also show that a pair of vacancies is subject to a sublattice-dependent interaction on account of coupling through the bulk spin liquid, which is spatially anisotropic even when all Kitaev couplings have equal strength. This coupling is thus exponentially suppressed with distance in the gapped phase. In the gapless phase, two vacancies on the same (opposite) sublattice exhibit an enhanced (suppressed) low-field response, amounting to an effectively (anti-)ferromagnetic interaction.

A subharmonic structure in Josephson junctions appears due to Andreev reflections within the junction. Here we report on experimental observation of a subharmonic half-gap singularity in interlayer tunneling characteristics of a layered high temperature superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The singularity is most pronounced in optimally doped crystals and vanishes with decreasing doping. It indicates the existence of nonvanishing electronic density of states and certain metallic properties in the intermediate BiO layers, which grows stronger with increasing doping. This provides an additional coherent interlayer transport channel and can explain a gradual transition from an incoherent quasi-two-dimensional $c$-axis transport in underdoped to a coherent metallic transport in overdoped cuprates. Furthermore, due to a very small subgap current, the singularity allows unambiguous extraction of the superconducting gap, without distortion by self-heating.

A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy experiments on the interstitial Fe impurity (IFI) and its nearest-neighboring sites in an ${\mathrm{FeTe}}_{0.5}{\mathrm{Se}}_{0.5}$ superconducting (SC) compound. By introducing an impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubov–de Gennes equations, we investigate the low-lying energy states of the IFI and the underlying Fe plane. The calculations are performed in the presence as well as in the absence of a magnetic field. We find the IFI-induced ZBS does not shift or split in a magnetic field as long as the tunneling parameter between the IFI and the Fe plane is sufficiently small and the Fe plane is deep in the SC state. Our results are in good agreement with experiments. We also show that in the underdoped cases, modulation of the spin density wave or charge density wave will suppress the intensity of the ZBS on the Fe plane in a vortex state.

Spin-driven nematic order that has been proposed for iron-based superconductors is generated by pronounced fluctuations of a striped density wave state. On the other hand it is a well known fact that the nematic order parameter couples bilinearly to the strain, which suppresses the fluctuations of the nematic order parameter itself and lowers the upper critical dimension, yielding mean-field behavior of the nematic degrees of freedom for $d>2$. This is consistent with the measured Curie-Weiss behavior of the nematic susceptibility. Here we reconcile this apparent contradiction between pronounced magnetic fluctuations and mean-field behavior of the nematic degrees of freedom. We show, by developing a ${\phi }^4$ theory for the nematic degrees of freedom, that the coupling to elastic strain does not suppress the fluctuations that cause the nematic order in the first place (magnetic fluctuations), yet it does transform the Ising-nematic transition into a mean-field transition. In addition, we demonstrate that the mean-field behavior occurs in the entire temperature regime where a softening of the shear modulus is observed.

We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a ${\mathrm{Cu}}_{2}0$ crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.

Most recently, black phosphorus has come into focus as a promising material for future applications in nanoelectronic devices due to its unique electronic and transport properties. Here, we use angle-resolved photoemission spectroscopy in conjunction with ab initio calculations within the framework of density-functional theory to disentangle surface from the bulk contributions in the electronic structure of black phosphorus. We find good agreement between our theoretical predictions for the intra- and interlayer energy-momentum dispersions and the experimentally obtained three-dimensional band structure of this material. Our results provide compelling evidence for the existence of surface-resonant states near the top of the valence band, which can play an important role in the performance of electronic devices based on black phosphorus.

We address the topological nature of electronic states of step edges of Bi(111) films by first-principles band structure calculations. We confirm that the dispersion of step-edge states reflects the topological nature of underlying films, which become topologically trivial at a thickness larger than eight bilayers. This result clearly conflicts with recent claims that the step-edge state at the surface of a bulk Bi(111) crystal or a sufficiently thick Bi(111) film represents nontrivial edge states of the two-dimensional topological insulator phase expected for a very thin Bi(111) film. The trivial step-edge states have a gigantic spin splitting of one-dimensional Rashba bands and substantial intermixing with electronic states of the bulk, which might be exploited further.

Resonant exchange qubits are a promising addition to the family of experimentally implemented encodings of single qubits using semiconductor quantum dots. We have shown previously that it ought to be straightforward to perform a CPHASE gate between two resonant exchange qubits with a single exchange pulse. This approach uses energy gaps to suppress leakage rather than conventional pulse sequences. In this paper we present analysis and simulations of our proposed two-qubit gate subject to charge and Overhauser field noise at levels observed in current experiments. Our main result is that we expect implementations of our two-qubit gate to achieve high fidelities, with errors at the percent level and gate times comparable to single-qubit operations. As such, exchange-coupled resonant exchange qubits remain an attractive approach for quantum computing.

Rhombohedral graphite behaves like a topological semimetal, possessing flat surface subbands while being semimetallic in the bulk. The bulk-surface correspondence arises from the ABC-stacking configuration of graphene layers. The bulk subbands in rhombohedral graphite can be interpreted as a three-dimensional Dirac cone structure, whose Dirac points form continuous lines spiraling in momentum space. In this paper, we study the evolution of gapped bulk subbands in ABC-stacked $N$-layer graphene with an increase of $N$, and their dimensional crossover to the three-dimensional Dirac cone structure in the bulk limit, where the bulk gap closes up at the Dirac-point spirals. To clarify the effect of coupling to the surface subbands, we use a nonperturbative effective Hamiltonian closed in the bulk subspace. As a consequence, the wavelength of the standing-wave function across the stack of layers depends on the in-plane Bloch momentum. In the bulk limit, the coupling vanishes and hence the wavelength is irrelevant to the surface.

We explore the electronic structure and transport properties of a metal on top of a (weakly coupled) two-dimensional topological insulator. Unlike the widely studied junctions between topological nontrivial materials, the systems studied here allow for a unique band structure and transport steering. First, states on the topological insulator layer may coexist with the gapless bulk and, second, the edge states on one edge can be selectively switched off, thereby leading to nearly perfect directional transport of charge and spin even in the zero bias limit. We illustrate these phenomena for Bernal stacked bilayer graphene with Haldane or intrinsic spin-orbit terms and a perpendicular bias voltage. This opens a path for realizing directed transport in materials such as van der Waals heterostructures, monolayer, and ultrathin topological insulators.

Defects in silicon carbide are of intense and increasing interest for quantum-based applications due to this material's properties and technological maturity. We calculate the multiparticle symmetry-adapted wave functions of the negatively charged silicon vacancy defect in hexagonal silicon carbide via use of group theory and density functional theory and find the effects of spin-orbit and spin-spin interactions on these states. Although we focused on ${\text{V}}_{\mathrm{Si}}^{-}$ in $4H$-SiC because of its unique fine structure due to the odd number of active electrons, our methods can be easily applied to other defect centers of different polytypes, especially to the $6H$-SiC. Based on these results, we identify the mechanism that polarizes the spin under optical drive, obtain the ordering of its dark doublet states, point out a path for electric field or strain sensing, and find the theoretical value of its ground-state zero-field splitting to be 68 MHz, in good agreement with experiment. Moreover, we present two distinct protocols of a spin-photon interface based on this defect. Our results pave the way toward quantum information and quantum metrology applications with silicon carbide.

A nanophononic metamaterial is a new type of nanostructured material that features an array, or a forest, of intrinsically distributed resonating substructures. Each substructure exhibits numerous local resonances, each of which may hybridize with the phonon dispersion of the underlying host material, causing significant reductions in the group velocities and consequently a reduction in the lattice thermal conductivity. In this Rapid Communication, molecular dynamics simulations are utilized to investigate both the dynamics and the thermal transport properties of a nanophononic metamaterial configuration consisting of a freely suspended silicon membrane with an array of silicon nanopillars standing on the surface. The simulations yield results consistent with earlier lattice-dynamics-based predictions which showed a reduction in the thermal conductivity due to the presence of the local resonators. Using a spectral energy density approach, in which only simulation data are utilized and no a priori information on the nanostructure resonant phonon modes is provided, we show direct evidence of the existence of resonance hybridizations as an inherent mechanism contributing to the slowing down of thermal transport in the host medium.

While the possibility to create a single-atom-thick two-dimensional layer from any material remains, only a few such structures have been obtained other than graphene and a monolayer of boron nitride. Here, based upon ab initio theoretical simulations, we propose a new stable graphenelike single-atomic-layer ${\mathrm{Si}}_2\mathrm{BN}$ structure that has all of its atoms with $s{p}^2$ bonding with no out-of-plane buckling. The structure is found to be metallic with a finite density of states at the Fermi level. This structure can be rolled into nanotubes in a manner similar to graphene. Combining first- and second-row elements in the Periodic Table to form a one-atom-thick material that is also flat opens up the possibility for studying new physics beyond graphene. The presence of Si will make the surface more reactive and therefore a promising candidate for hydrogen storage.

Recently, the nodal line semimetals have attracted considerable interest in condensed matter physics. We show that their distinct band structure can be detected by measuring the collective modes. In particular, we find that the dependence of the plasmon frequency ${\omega }_p$ on the electron density $n$ follows a ${\omega }_p\sim n^{1/4}$ law in the long wavelength limit. Our results will be useful in the ongoing search for new candidates of nodal line semimetals.

Manganites are famous mostly for the colossal magnetoresistive effect, which involves the phase separation between ferromagnetic phase and charge-ordered $CE$-type antiferromagnetic phases. Furthermore, manganites contain some typical magnetic ferroelectrics, e.g., $E$-type antiferromagnetic $o-{\mathrm{HoMnO}}_3$. Here we re-examined these zigzag-winding antiferromagnetic phases ($CE$-type and $E$-type antiferromagnets) from the topological perspective. Our theoretical analysis proved that the $E$-type phase is a weak topological insulator belonging to the $\mathbb{Z}$ class. In momentum space, we classify the symmetries of this phase, and find the three symmetry operators for the chiral, particle-hole, and time-reversal symmetry. The $CE$-type phase can be described by the Duffin-Kemmer-Petiau algebra, implying that it is a different class of topological insulator and hence extends the existing classification. The corresponding topological end states are demonstrated via numerical calculations, which may implicate the experimental observed ferromagnetic edge states in manganite strips [Nat. Commun. 6, 6179 (2015)] and may play a crucial role in the colossal magnetoresistive effect.

The conductance across an atomically narrow metallic contact can be measured by using scanning tunneling microscopy. In certain situations, a jump in the conductance is observed right at the point of contact between the tip and the surface, which is known as “jump to contact” (JC). Such behavior provides a way to explore, at a fundamental level, how bonding between metallic atoms occurs dynamically. This phenomenon depends not only on the type of metal but also on the geometry of the two electrodes. For example, while some authors always find JC when approaching two atomically sharp tips of Cu, others find that a smooth transition occurs when approaching a Cu tip to an adatom on a flat surface of Cu. In an attempt to show that all these results are consistent, we make use of atomistic simulations; in particular, classical molecular dynamics together with density functional theory transport calculations to explore a number of possible scenarios. Simulations are performed for two different materials: Cu and Au in a [100] crystal orientation and at a temperature of 4.2 K. These simulations allow us to study the contribution of short- and long-range interactions to the process of bonding between metallic atoms, as well as to compare directly with experimental measurements of conductance, giving a plausible explanation for the different experimental observations. Moreover, we show a correlation between the cohesive energy of the metal, its Young's modulus, and the frequency of occurrence of a jump to contact.

We study Bogoliubov excitations of a spinor Bose-Einstein condensate in a honeycomb periodic potential, in the presence of a Zeeman field and of a spin-orbit coupling specific for photonic systems, which is due to the energy splitting between TE and TM polarized eigenstates. We also consider spin-anisotropic interactions typical for cavity polaritons. We show that the nontrivial topology of the single-particle case is also present for the interacting system. At low condensate density, the topology of the single-particle bands is transferred to the bogolon dispersion. At a critical value, the self-induced Zeeman field at the Dirac points of the dispersion becomes equal to the real Zeeman field and then exceeds it. The gap is thus closed and then reopened with inverted Chern numbers. This change of topology is accompanied by a change of the propagation directions of the one-way edge modes. This result demonstrates that the chirality of a topological insulator can be reversed by collective effects in a Bose-Einstein condensate.