Low energy inelastic neutron scattering on single crystals of the kagome spin-liquid compound ${\mathrm{ZnCu}}_3{\left(\mathrm{OD}\right)}_6{\mathrm{Cl}}_2$ (herbertsmithite) reveals antiferromagnetic correlations between impurity spins for energy transfers $\hslash \omega <0.8\mathrm{meV} \left(\sim J/20\right)$. The momentum dependence differs significantly from higher energy scattering which arises from the intrinsic kagome spins. The low energy fluctuations are characterized by diffuse scattering near wave vectors (100) and $\left(00\frac{3}{2}\right)$, which is consistent with antiferromagnetic correlations between pairs of nearest-neighbor Cu impurities on adjacent triangular (Zn) interlayers. The corresponding impurity lattice resembles a simple cubic lattice in the dilute limit below the percolation threshold. Such an impurity model can describe prior neutron, NMR, and specific heat data. The low energy neutron data are consistent with the presence of a small spin gap $\left(\mathrm{\Delta }\sim 0.7\mathrm{meV}\right)$ in the kagome layers, similar to that recently observed by NMR. The ability to distinguish the scattering due to Cu impurities from that of the planar kagome Cu spins provides an important avenue for probing intrinsic spin-liquid physics.

Superconducting FeSe single crystals of (001) orientation are synthesized via a hydrothermal ion-release route. An Ising spin-nematic order is identified by our systematic measurements of in-plane angular-dependent magnetoresistance (AMR) and static magnetization. The turn-on temperature of anisotropic AMR signifies the Ising spin-nematic ordering temperature $T_{\mathrm{sn}}$, below which a twofold rotational symmetry is observed in the iron plane. A downward curvature appears below $T_{\mathrm{sn}}$ in the temperature dependence of static magnetization for the weak in-plane magnetic field as reported previously. Remarkably, we find a universal linear relationship between $T_{\mathrm{c}}$ and $T_{\mathrm{sn}}$ among various superconducting samples, indicating that the spin nematicity and the superconductivity in FeSe have a common microscopic origin.

The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. Here we report the formation of ordered Cu(100) $p(2\times 2)$ oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitride $(h$-BN) on Cu after annealing. By using scanning tunneling microscopy and x-ray photoelectron spectroscopy, we identify the superstructure and reveal its roles in passivating the exposed Cu surfaces, decoupling $h$-BN and Cu, and disintegrating $h$-BN monolayers. The oxygen superstructure appears as a 2D pattern on the exposed Cu surface or quasi-1D stripes of paired oxygen intercalated in the interface of $h$-BN and Cu predominantly oriented along the moiré modulations. The oxygen superstructure is shown to etch the overlaying $h$-BN monolayer in a thermal annealing process. After extended annealing, the $h$-BN monolayer disintegrates into nanoislands with zigzag edges. We discuss the implications of these findings on the stability and oxidation resistance of $h$-BN and relate them to challenges in process integration and 2D heterostructures.

The possibilities of turning intrinsically $n$-type oxide semiconductors like ZnO and ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ into $p$-type materials are investigated. Motivated by recent experiments on ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ doped with nitrogen, we analyze the electronic defect levels of point defects ${\mathrm{N}}_{\mathrm{O}},{\mathrm{v}}_{\mathrm{Zn}}$, and ${\mathrm{N}}_{\mathrm{O}}\text{−}{\mathrm{v}}_{\mathrm{Zn}}$ pairs in ZnO and ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$ by means of self-interaction-corrected density functional theory calculations. We show how the interplay of defects can lead to shallow acceptor defect levels, although the levels of individual point defects ${\mathrm{N}}_{\mathrm{O}}$ are too deep in the band gap to be responsible for $p$-type conduction. We relate our results to $p$-type conduction paths at grain boundaries seen in polycrystalline ZnO and develop an understanding of a $p$-type mechanism which is common to ZnO, ${\mathrm{Zn}}_{1-x}{\mathrm{Mg}}_x\mathrm{O}$, and related materials.

The electronic structure of the chiral helimagnet $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ has been studied with core level and angle-resolved photoemission spectroscopy (ARPES). Intercalated Cr atoms are found to be effective in donating electrons to the $\mathrm{Nb}{\mathrm{S}}_2$ layers but also cause significant modifications of the electronic structure of the host $\mathrm{Nb}{\mathrm{S}}_2$ material. In particular, the data provide evidence that a description of the electronic structure of $\mathrm{C}{\mathrm{r}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ on the basis of a simple rigid band picture is untenable. The data also reveal substantial inconsistencies with the predictions of standard density functional theory. The relevance of these results to the attainment of a correct description of the electronic structure of chiral helimagnets, magnetic thin films/multilayers, and transition metal dichalcogenides intercalated with $3d$ magnetic elements is discussed.

The exciton dynamics in monolayer black phosphorus is investigated over a very wide range of photoexcited exciton densities using time resolved photoluminescence. At low excitation densities, the exciton dynamics is successfully described in terms of a double exponential decay. With increasing exciton population, a fast, nonexponential component develops as exciton-exciton annihilation takes over as the dominant recombination mechanism under high excitation conditions. Our results identify an upper limit for the injection density, after which exciton-exciton annihilation reduces the quantum yield, which will significantly impact the performance of light emitting devices based on single-layer black phosphorus.

We demonstrate that the dynamics of the $ab$-spiral-spin order in a magnetoelectric multiferroic ${\mathrm{Eu}}_{0.55}{\mathrm{Y}}_{0.45}{\mathrm{MnO}}_3$ can be unambiguously probed through optical second harmonic signals, generated via spin-induced ferroelectric polarization. In the case of weak excitation, the ferroelectric and the spiral-spin order remains interlocked, both relaxing through spin-lattice relaxation in the nonequilibrium state. When the additional optical pulse illuminating the sample is intense enough to induce a local phase transition thermally, the system creates a metastable state of the $bc$-spiral-spin order (with the electric polarization $P\parallel c)$ via supercooling across the first-order phase transition between the $ab$ and $bc$ spiral. The supercooled state of the $bc$-spiral spin is formed in the thermodynamical ground state of the $ab$ spiral $\left(P\parallel a\right)$, displaying a prolonged lifetime with strong dependence on the magnetic field along the $a$ axis. The observed phenomena provide a different paradigm for photoswitching between the two distinct multiferroic states, motivating further research into a direct observation of the photocreated supercooled $bc$-spiral spin in multiferroic manganites.

We use our high resolution He-lamp-based, tunable laser-based angle-resolved photoemission spectroscopy measurements and density functional theory calculations to study the electronic properties of LaBi, a binary system that was proposed to be a member of a new family of topological semimetals. Both bulk and surface bands are present in the spectra. The dispersion of the surface state is highly unusual. It resembles a Dirac cone, but upon closer inspection we can clearly detect an energy gap. The bottom band follows roughly a parabolic dispersion. The dispersion of the top band remains very linear, “V” -shape like, with the tip approaching very closely to the extrapolated location of Dirac point. Such asymmetric mass acquisition is highly unusual and opens a possibility of a new topological phenomenon that has yet to be understood.

We have carried out ${}^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) in a wide temperature range of 1.5–300 K to investigate the electronic properties of ${\mathrm{Ge}}_{50}{\mathrm{Te}}_{50},{\mathrm{Ag}}_2{\mathrm{Ge}}_{48}{\mathrm{Te}}_{50}$, and ${\mathrm{Sb}}_2{\mathrm{Ge}}_{48}{\mathrm{Te}}_{50}$ from a microscopic point of view. From the temperature dependence of the NMR shift $\left(K\right)$ and nuclear spin lattice relaxation rate $\left(1/T_1\right)$, we found that two bands contribute to the physical properties of the materials. One band overlaps the Fermi level providing the metallic state where no strong electron correlations are revealed by Korringa analysis. The other band is separated from the Fermi level by an energy gap of $E_{\mathrm{g}}/k_{\mathrm{B}}\sim 67$ K, which gives rise to semiconductorlike properties. First-principles calculation reveals that the metallic band originates from the Ge vacancy while the semiconductorlike band is related to the fine structure of the density of states near the Fermi level. Low-temperature ${}^{125}\mathrm{Te}$ NMR data for the materials studied here clearly show that Ag substitution increases hole concentration while Sb substitution decreases it.

Using the tight-binding (TB) approximation with inclusion of the spin-orbit interaction, we predict a topological phase transition in the electronic band structure of phosphorene in the presence of axial strains. We derive a low-energy TB Hamiltonian that includes the spin-orbit interaction for bulk phosphorene. Applying a compressive biaxial in-plane strain and perpendicular tensile strain in ranges where the structure is still stable leads to a topological phase transition. We also examine the influence of strain on zigzag phosphorene nanoribbons (zPNRs) and the formation of the corresponding protected edge states when the system is in the topological phase. For zPNRs up to a width of 100 nm the energy gap is at least three orders of magnitude larger than the thermal energy at room temperature.

We investigate spin-dependent thermoelectric transport through a system of two coupled quantum dots attached to reservoirs of spin-polarized electrons. Generally, we focus on the strongly correlated regime of transport. To this end, a slave-boson method for finite $U$ is employed. Our main goal is to show that, apart from complex low-temperature physics, such a basic multilevel system provides a possibility to examine various quantum interference effects, with particular emphasis put on the influence of such phenomena on thermoelectric transport. Apart from the influence of interference effects on spin-degenerate charge transport, we show how spin-dependent transport, induced by ferromagnetic leads, can be modified as well. Finally, we also consider the case where the spin-relaxation time in the ferromagnetic leads is relatively long, which leads to the so-called spin thermoelectric effects.

The lattice thermal conductivity of silicon clathrate framework ${\mathrm{Si}}_{23}$ and two Zintl clathrates, $\left[{\mathrm{Si}}_{19}{\mathrm{P}}_4\right]{\mathrm{Cl}}_4$ and ${\mathrm{Na}}_4\left[{\mathrm{Al}}_4{\mathrm{Si}}_{19}\right]$, is investigated by using an iterative solution of the linearized Boltzmann transport equation in conjunction with ab initio lattice dynamical techniques. At 300 K, the lattice thermal conductivities for ${\mathrm{Si}}_{23}, \left[{\mathrm{Si}}_{19}{\mathrm{P}}_4\right]{\mathrm{Cl}}_4$, and ${\mathrm{Na}}_4\left[{\mathrm{Al}}_4{\mathrm{Si}}_{19}\right]$ were found to be 43 W/(m K), 25 W/(m K), and 2 W/(m K), respectively. In the case of ${\mathrm{Na}}_4\left[{\mathrm{Al}}_4{\mathrm{Si}}_{19}\right]$, the order-of-magnitude reduction in the lattice thermal conductivity was found to be mostly due to relaxation times and group velocities differing from ${\mathrm{Si}}_{23}$ and $\left[{\mathrm{Si}}_{19}{\mathrm{P}}_4\right]{\mathrm{Cl}}_4$. The difference in the relaxation times and group velocities arises primarily due to the phonon spectrum at low frequencies, resulting eventually from the differences in the second-order interatomic force constants (IFCs). The obtained third-order IFCs were rather similar for all materials considered here. The present findings are similar to those obtained earlier for some skutterudites. The predicted lattice thermal conductivity of ${\mathrm{Na}}_4\left[{\mathrm{Al}}_4{\mathrm{Si}}_{19}\right]$ is in line with the experimentally measured thermal conductivity of recently synthesized type-I Zintl clathrate ${\mathrm{Na}}_8\left[{\mathrm{Al}}_8{\mathrm{Si}}_{38}\right]$ (polycrystalline samples).

Superparamagnetic tunnel junctions are nanostructures that auto-oscillate stochastically under the effect of thermal noise. Recent works showed that despite their stochasticity, such junctions possess a capability to synchronize to subthreshold voltage drives, in a way that can be enhanced or controlled by adding noise. In this work, we investigate a system composed of two electrically coupled junctions, connected in series to a periodic voltage source. We make use of numerical simulations and of an analytical model to demonstrate that both junctions can be phase locked to the drive, in phase or in antiphase. This synchronization phenomenon can be controlled by both thermal and electrical noises, although the two types of noises induce qualitatively different behaviors. Namely, thermal noise can stabilize a regime where one junction is phase locked to the drive voltage while the other is blocked in one state; on the contrary, electrical noise causes the junctions to have highly correlated behaviors and thus cannot induce the latter. These results open the way for the design of superparamagnetic tunnel junctions that can perform computation through synchronization, and which harvest the largest part of their energy consumption from thermal noise.

Topologically protected swirl of the magnetic texture known as the skyrmion has become ubiquitous in both metallic and insulating chiral magnets. Meanwhile the existence of its three-dimensional analog, known as the magnetic monopole, has been suggested by various indirect experimental signatures in MnGe compound. Although Ginzburg-Landau arguments in favor of the formation of a three-dimensional crystal of monopoles and antimonopoles have been put forward, no microscopic model Hamiltonian was shown to support such a phase. Here we present strong numerical evidence from Monte Carlo simulations for the formation of a rocksalt crystal structure of monopoles and antimonopoles in short-period chiral magnets. Real-time simulation of the spin dynamics suggests there is only one internal excitation mode in the monopole crystal state in the frequency range of several gigahertz for the material parameters of MnGe.

Several powerful techniques exist to detect topologically protected surface states of weakly interacting electronic systems. In contrast, surface modes of strongly interacting systems which do not carry electric charge are much harder to detect. We propose resonant light scattering as a means of probing the chargeless surface modes of interacting quantum spin systems, and illustrate its efficacy by a concrete calculation for the three-dimensional hyperhoneycomb Kitaev quantum spin liquid phase. We show that resonant scattering is required to efficiently couple to this model's sublattice polarized surface modes, comprised of emergent Majorana fermions that result from spin fractionalization. We demonstrate that the low-energy response is dominated by the surface contribution for thin films, allowing identification and characterization of emergent topological band structures.

Imaging the magnetic structure of a material is essential to understanding the influence of the physical and chemical microstructure on its magnetic properties. Magnetic imaging techniques, however, have been unable to probe three-dimensional micrometer-size systems with nanoscale resolution. Here we present the imaging of the magnetic domain configuration of a micrometer-thick FeGd multilayer with hard x-ray dichroic ptychography at energies spanning both the Gd $L_3$ edge and the Fe $K$ edge, providing a high spatial resolution spectroscopic analysis of the complex x-ray magnetic circular dichroism. With a spatial resolution reaching $45\mathrm{nm}$, this advance in hard x-ray magnetic imaging is a first step towards the investigation of buried magnetic structures and extended three-dimensional magnetic systems at the nanoscale.

We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a skyrmion texture. Using the $T$-matrix calculations and numerical modeling we calculate the spin-polarized local density of states in the superconductor in the vicinity of the skyrmion. We predict the skyrmion bound states that are induced in the superconductor, similar to the well-known Yu-Shiba-Rusinov states. The bound-state wave functions have spatial power-law decay. It is suggested that superconductivity could facilitate an effective long-range interaction between skyrmions when bound-state wave functions overlap.

We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr- doped $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependent terms in the scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La-doped and Pr-doped $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. For temperatures below the structure collapse transition in Pr-doped $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ at $\sim 70\mathrm{K}$, the scattering rate decreases due to weakening of electronic correlations, and the Drude spectral weight decreases due to modification of the low-energy electronic structure.

We study the vortex lattice in a two-dimensional $s$-wave topological superconductor with Rashba spin-orbit coupling and Zeeman field by solving the Bogoliubov-de Gennes equations self-consistently for the superconducting order parameter. We find that when spin-orbit coupling is relatively weak, one of the two underlying chiralities in the topological superconducting state can be strongly manifest in the vortex core structure and govern the response of the system to vorticity and a nonmagnetic impurity where the vortex is pinned. The Majorana zero mode in the vortex core is found to be robust against the nonmagnetic impurity in that it remains effectively a zero-energy bound state regardless of the impurity potential strength and the major chirality. The spin polarization of the Majorana bound state depends on the major chirality for weak spin-orbit coupling, while it is determined simply by the vorticity when spin-orbit coupling is relatively strong.

Motivated by the numerous examples of 1/3 magnetization plateaux in the triangular-lattice Heisenberg antiferromagnet with spins ranging from 1/2 to 5/2, we revisit the semiclassical calculation of the magnetization curve of that model, with the aim of coming up with a simple method that allows one to calculate the full magnetization curve and not just the critical fields of the 1/3 plateau. We show that it is actually possible to calculate the magnetization curve including the first quantum corrections and the appearance of the 1/3 plateau entirely within linear spin-wave theory, with predictions for the critical fields that agree to order $1/S$ with those derived a long time ago on the basis of arguments that required going beyond linear spin-wave theory. This calculation relies on the central observation that there is a kink in the semiclassical energy at the field where the classical ground state is the collinear up-up-down structure and that this kink gives rise to a locally linear behavior of the energy with the field when all semiclassical ground states are compared to each other for all fields. The magnetization curves calculated in this way for spin 1/2, 1, and 5/2 are shown to be in good agreement with available experimental data.