For long wavelengths, three-dimensional connected metallic wire meshes are impenetrable by light and have an electromagnetic response similar to that of an electron gas below the plasma frequency. Surprisingly, here it is shown that when two opaque metallic meshes are spatially interlaced, the combined structure enables an anomalous light tunneling in the long wavelength regime. The effect is due to the destructive interference of the waves scattered by the two wire meshes, which leads to a Fano-type resonance. The Fano resonance occurs when the longitudinal (plasmon) mode satisfies the Fabry-Pérot condition.

Strongly interacting topological states in multiparticle quantum systems pose great challenges to both theory and experiment. Recently, bound states of elementary spin waves (magnons) in quantum magnets have been experimentally observed in quantum Heisenberg chains comprising ultracold Bose atoms in optical lattices. Here, we explore a strongly interacting topological state called topological magnon bound state in the quantum Heisenberg chain under cotranslational symmetry. We find that the cotranslational symmetry is the key to the definition of a topological invariant for multiparticle quantum states, which enables us to characterize the topological features of multimagnon excitations. We calculate energy spectra, density distributions, correlations, and Chern numbers of the two-magnon bound states and show the existence of topological protected edge bound states. Our study not only opens a prospect to pursue topological magnon bound states, but also gives insights into the characterization and understanding of strongly interacting topological states.

In one spatial dimension, anyons in the original description of Leinaas and Myrheim are formally equivalent to locally interacting bosons described by the Lieb-Liniger model. This allows an interesting reinterpretation of interacting bosons in the context of anyons. We elaborate on this parallel, particularly including the many-body bound states from the attractive Lieb-Liniger model. In the anyonic context these bound states are created solely by quantum-statistical attraction and coined the quantum-statistical condensate, which is shown to be more robust than the Bose-Einstein condensate. We introduce the second quantization formalism for the present anyons and construct the generalized Jordan-Wigner transformation that connects them to the bosons of the Lieb-Liniger model.

We show evidence of the backscattering of quantum Hall edge channels in a narrow graphene Hall bar, induced by the gating effect of the conducting tip of a scanning gate microscope, which we can position with nanometer precision. We show full control over the edge channels and are able, because of the spatial variation of the tip potential, to separate copropagating edge channels in the Hall bar, creating junctions between regions of different charge carrier density, that have not been observed in devices based on top or split gates. The solution of the corresponding quantum scattering problem is presented to substantiate these results, and possible follow-up experiments are discussed.

The kagome lattice is a paragon of geometrical frustration, long-studied for its association with novel ground states including spin liquids. Many recently synthesized kagome materials feature rare-earth ions, which may be expected to exhibit highly anisotropic exchange interactions. The consequences of this combination of strong exchange anisotropy and extreme geometrical frustration are yet to be fully understood. Here, we establish a general picture of the interactions and resulting ground states arising from nearest-neighbor exchange anisotropy on the kagome lattice. We determine a generic anisotropic exchange Hamiltonian from symmetry arguments. In the high-symmetry case where reflection in the kagome plane is a symmetry of the system, the generic nearest-neighbor Hamiltonian can be locally defined as an XYZ model with out-of-plane Dzyaloshinskii-Moriya interactions. We proceed to study its phase diagram in the classical limit, making use of an exact reformulation of the Hamiltonian in terms of irreducible representations (irreps) of the lattice symmetry group. This reformulation in terms of irreps naturally explains the threefold mapping between three families of models supporting spin liquids, as recently studied by the present authors [Nat. Commun. 7, 10297 (2016)]. In addition, a number of unusual states are stabilized in the regions where different forms of ground-state order compete, including a stripy phase with a local ${\mathbb{Z}}_8$ symmetry and a classical analog of a chiral spin liquid. As a peculiar property of the kagome lattice, the generic model turns out to be a fruitful hunting ground for the coexistence, in the same ground-state configuration, of multiple forms of long-range magnetic orders. In exotic instances, partial long-range order may also coexist in the ground state with a finite fraction of disordered spin degrees of freedom. These results are compared and contrasted with those obtained on the pyrochlore lattice, and connection is made with recent progress in understanding quantum models with $S=1/2$.

We report the development of an ab initio electronic structure method applicable to generic substitutionally disordered real materials. A charge self-consistent orbital-based extension of the coherent potential approximation due to Blackman, Esterling, and Berk (BEB-CPA) is combined with the mixed-basis pseudopotential density functional theory approach. The general formalism in terms of a nonorthogonal basis set including subtleties of the pseudopotential framework is outlined. The BEB-CPA is validated on a binary tight-binding toy model against exact diagonalization of a randomly occupied cluster. Finally, the developed ab initio approach is benchmarked for a binary CuZn alloy, which confirms the robustness of the charge self-consistent procedure with respect to initial conditions.

We demonstrate that the metallic topological surface states wrap on all sides the three-dimensional topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magnetotransport and magnetization at tilted magnetic fields, which enables us to observe simultaneous contributions from neighboring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir, we successfully describe theoretically the de Haas–van Alphen oscillations of magnetization. The determined $\pi$–Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magnetotransport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has a universal character.

Two-dimensional density-matrix renormalization group method is employed to examine the ground-state phase diagram of the Hubbard model on the triangular lattice at half-filling. The calculation reveals two discontinuities in the double occupancy with increasing the repulsive Hubbard interaction $U$ at $U_{\mathrm{c}1}\sim 7.8t$ and $U_{\mathrm{c}2}\sim 9.9t$ ($t$ being the hopping integral), indicating that there are three phases separated by first-order transitions. The absence of any singularity in physical quantities for $0\le U<U_{\mathrm{c}1}$ implies a metallic phase in this regime. For $U>U_{\mathrm{c}2}$, the local spin density induced by an applied pinning magnetic field exhibits a three sublattice feature, which is compatible with the ${120}^{\circ }$ Néel-ordered state realized in the limit of $U\to \infty$. For $U_{\mathrm{c}1}<U<U_{\mathrm{c}2}$, a response to the applied pinning magnetic field is comparable to that in the metallic phase with a relatively large spin correlation length, but showing neither valence bond nor chiral magnetic order, which therefore resembles gapless spin liquid. However, the spin structure factor for the intermediate phase exhibits the maximum at the $\mathrm{K}$ and ${\mathrm{K}}^{\prime }$ points in the momentum space, which is not compatible to spin liquid with a large spinon Fermi surface. The calculation also finds that the pairing correlation function monotonically decreases with increasing $U$ and thus the superconductivity is unlikely in the intermediate phase.

The phonon dispersion, density of states, Grüneisen parameters, and the lattice thermal conductivity of single-layer and multilayer boron nitride were calculated using first-principles methods. For the bulk $h$-BN we also report the two-phonon density of states. We also present simple analytical solutions to the acoustic vibrational mode-dependent lattice thermal conductivity. Moreover, computations based on the elaborate Callaway-Klemens and the real-space supercell methods are presented to calculate the sample length and temperature-dependent lattice thermal conductivity of single-layer and multilayer hexagonal boron nitride which shows good agreement with experimental data.

Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. To date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance ($R_{\mathrm{c}}$), particularly in the monolayer limit. In this paper, we use a combination of theory and experiments in Au and Ni contacted monolayer $\mathrm{Mo}{\mathrm{S}}_2$ device to elucidate the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers—the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multidimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such a composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.

Edge/surface states often appear in a topologically nontrivial phase when the system has a boundary. The edge state of a one-dimensional topological insulator is one of the simplest examples. Electron spin resonance (ESR) is an ideal probe to detect and analyze the edge state for its high sensitivity and precision. We consider ESR of the edge state of a generalized Su-Schrieffer-Heeger model with a next-nearest-neighbor (NNN) hopping and a staggered spin-orbit coupling. The spin-orbit coupling is generally expected to bring about nontrivial changes on the ESR spectrum. Nevertheless, in the absence of the NNN hoppings, we find that the ESR spectrum is unaffected by the spin-orbit coupling thanks to the chiral symmetry. In the presence of both the NNN hopping and the spin-orbit coupling, on the other hand, the edge ESR spectrum exhibits a nontrivial frequency shift. We derive an explicit analytical formula for the ESR shift in the second-order perturbation theory, which agrees very well with a nonperturbative numerical calculation.

We combine the theoretical method of calculating spin wave excitation with the finite-temperature modeling of lattice vibration and calculate the magnon-phonon relaxation time in the technologically important material yttrium iron garnet (YIG) from first principles. The finite lifetime of magnon excitation is found to arise from the fluctuation of the exchange interaction of magnetic atoms in YIG. At room temperature, the magnon spectra have significant broadening that is used to extract the magnon-phonon relaxation time quantitatively. The latter is a phenomenological parameter of great importance in YIG-based spintronics research. We find that the magnon-phonon relaxation time for the optical magnon is a constant, while that for the acoustic magnon is proportional to $1/k^2$ in the long-wavelength regime.

We demonstrate emergence of both the chiral soliton lattice and skyrmion lattice and investigate their magnetic-field variation in the strained ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ thin plate by means of small-angle resonant soft x-ray scattering. The tensile strain stabilizes a helical spin structure with the modulation vector along the strain direction. Consequently, when increasing the field perpendicular to the modulation vector, it undergoes large shrinkage and higher-order diffraction peaks are also observed, which is in accord with a typical feature of the chiral soliton lattice. The skyrmion lattice also appears in the presence of tensile strain but is elongated along the strain direction in reciprocal space. The magnetic-field dependence of the modulation vector along the strain direction agrees with the theoretical model quantitatively, while that along other directions does not. The agreement, however, becomes worse at lower temperatures, which may be attributed to the large potential barrier for skyrmion annihilation.

We studied structural and magnetic ordering in ${\mathrm{Yb}}_2{\mathrm{TiO}}_5$ using synchrotron x-ray diffraction, neutron diffraction and total scattering, ac and dc susceptibility, and inelastic neutron scattering. Diffraction measurements reveal an average disordered fluorite structure with additional diffuse scattering features, which are caused by structural short-range orthorhombic order, as evidenced by the neutron pair distribution function measurements. The ac susceptibility measurements show a broad peak at $T_f\approx 0.35$ K that displays Arrhenius behavior with an activation energy of 2.51(5) meV. Zero-field neutron scattering measurements show broad magnetic diffuse scattering in the elastic channel with an antiferromagnetic-type gapless excitation extending to 1.5 meV. A polarized state with partial spin order is induced with an applied magnetic field which opens a gapped excitation that increases monotonically with field strength.

We investigate the electronic and magnetic structures of the $122\left({AM}_2{B}_2\right)$ hexagonal transition metal pnictides with $A=(\text{Sr},\text{Ca}),M=(\text{Cr},\text{Mn},\text{Fe},\text{Co},\text{Ni})$, and $B=(\text{As},\text{P},\text{Sb})$. It is found that the family of materials shares critical similarities with those of tetragonal structures that include the famous iron-based high-temperature superconductors. In both families, the next-nearest-neighbor (NNN) effective antiferromagnetic (AFM) exchange couplings reach the maximum value in the iron-based materials. While the NNN couplings in the latter are known to be responsible for the $C$-type AFM state and to result in the extended $s$-wave superconducting state upon doping, they cause the former to be extremely frustrated magnetic systems and can lead to a time-reversal symmetry-broken $d+id$ superconducting state upon doping. Thus, if synthesized, iron-based compounds with hexagonal structure can help us to determine the origin of high-temperature superconductivity.

Despite numerous studies the exact nature of the order parameter in superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ remains unresolved. We have extended previous small-angle neutron scattering studies of the vortex lattice in this material to a wider field range, higher temperatures, and with the field applied close to both the $\langle 100\rangle$ and $\langle 110\rangle$ basal plane directions. Measurements at high field were made possible by the use of both spin polarization and analysis to improve the signal-to-noise ratio. Rotating the field towards the basal plane causes a distortion of the square vortex lattice observed for $\mathit{H}\parallel \langle 001\rangle$ and also a symmetry change to a distorted triangular symmetry for fields close to $\langle 100\rangle$.The vortex lattice distortion allows us to determine the intrinsic superconducting anisotropy between the $c$ axis and the Ru-O basal plane, yielding a value of $\sim 60$ at low temperature and low to intermediate fields. This greatly exceeds the upper critical field anisotropy of $\sim 20$ at low temperature, reminiscent of Pauli limiting. Indirect evidence for Pauli paramagnetic effects on the unpaired quasiparticles in the vortex cores are observed, but a direct detection lies below the measurement sensitivity. The superconducting anisotropy is found to be independent of temperature but increases for fields $\gtrsim 1$ T, indicating multiband superconductvity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Finally, the temperature dependence of the scattered intensity provides further support for gap nodes or deep minima in the superconducting gap.

The piezomagnetoelectric effect is considered to be the appearance of a piezoelectric response caused by a magnetic field in magnetically ordered media. The possibility of its existence in antiferromagnets was predicted in the 1960's; however, attempts at its experimental detection were unsuccessful. In this Rapid Communication, our experiments confirm the appearance of the piezomagnetoelectric effect in a ${\mathrm{LiCoPO}}_4$ single crystal.

We discuss a general framework to address spin decoherence resulting from fluctuations in a spin Hamiltonian. We performed a systematic study on spin decoherence in the compound ${\mathrm{K}}_6\left[{\mathrm{V}}_{15}{\mathrm{As}}_6{\mathrm{O}}_{42}\left({\mathrm{D}}_2\mathrm{O}\right)\right]·8{\mathrm{D}}_2\mathrm{O}$, using high-field electron spin resonance. By analyzing the anisotropy of resonance linewidths as a function of orientation, temperature, and field, we find that the spin-orbit term is a major decoherence source. The demonstrated mechanism can alter the lifetime of any spin qubit and we discuss how to mitigate it by sample design and field orientation.