Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the $f$ and $d$ orbitals. In this study, aluminum $K$-edge x-ray absorption spectroscopy and density functional theory calculations are used to evaluate the electronic structure of the dialuminides, $M{\mathrm{Al}}_2$ ($M$ = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results show how the energy and occupancy of the $4f$ or $5f$ orbitals impacts mixing of Al $3p$ character into the $5d$ or $6d$ conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

We report an angle-resolved photoemission (ARPES) study of $\beta -{\mathrm{YbAlB}}_4$, which is known to harbor unconventional quantum criticality (QC) without any tuning. We directly observe a quasiparticle peak (QP) emerging from hybridization, characterized by a binding energy and an onset of coherence both at about 4 meV. This value conforms with a previously observed reduced Kondo scale at about 40 K. Consistency with an earlier study of carriers in $\beta -{\mathrm{YbAlB}}_4$ via the Hall effect strongly suggests that this QP is responsible for the QC in $\beta -{\mathrm{YbAlB}}_4$. A comparison with the sister polymorph $\alpha -{\mathrm{YbAlB}}_4$, which is not quantum critical at ambient pressure, further supports this result. Indeed, within the limitation of our instrumental resolution, our ARPES measurements do not show tangible sign of hybridization in this locally isomorphic system, while the conduction band we observe is essentially the same as in $\beta -{\mathrm{YbAlB}}_4$. We therefore claim that we identified by ARPES the carriers responsible for the QC in $\beta -{\mathrm{YbAlB}}_4$. The observed dispersion and the underlying hybridization of this QP are discussed in the context of existing theoretical models.

We report first-principles microscopic calculations of the properties of defects with dangling bonds in crystalline $3C$-SiC. Specifically, we focus on hydrogenated Si and C vacancies, divacancies, and multivacancies. The latter is a generic model for an isolated dangling bond within a bulk SiC matrix. Hydrogen serves to passivate electrically active defects to allow the isolation of a single dangling-bond defect. We used hybrid density-functional methods to determine energetics and electrical activity. The present results are compared to previous $3C$-SiC calculations and experiments. Finally, we identify homopolar carbon dangling-bond defects as the leakage causing defects in nanoporous SiC alloys.

The ready optical detection and manipulation of bright nitrogen vacancy center spins in diamond plays a key role in contemporary quantum information science and quantum metrology. Other optically dark defects such as substitutional nitrogen atoms (‘P1 centers') could also become potentially useful in this context if they could be as easily optically detected and manipulated. We develop a relatively straightforward continuous wave protocol that takes advantage of the dipolar coupling between nitrogen vacancy and P1 centers in type 1b diamond to detect and polarize the dark P1 spins. By combining mutual spin flip transitions with radio frequency driving, we demonstrate the simultaneous optical polarization and detection of the electron spin resonance of the P1 center. This technique should be applicable to detecting and manipulating a broad range of dark spin populations that couple to the nitrogen vacancy center via dipolar fields, allowing for quantum metrology using these spin populations.

Here we examine the role of anharmonicity in the elastic constants of bulk metallic glasses and develop an anharmonic model to consider the effects of both pressure and temperature across the glass transition. By comparing against reported experimental data and elastic constants obtained from molecular dynamics simulations, we show that the model is able to capture reported elastic constants from cryogenic temperatures through the glass transition and under hydrostatic pressures up to 18 GPa. Microstructural indicators based on short-range order analysis also display strong correlations with the bulk and shear moduli across the range of pressures and temperatures studied. These results not only greatly expand our understanding in the physical origins of elastic properties of bulk metallic glasses but are also of practical interest for application to processing routes for bulk metallic glass materials.

Cubic $B2$ FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM) to ferromagnet (FM)] around 353 K and remains structurally stable at higher temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit imaginary modes at $M$ points in the Brillouin zone, indicating a premartensitic instability, which is a precursor to a martensitic transformation at low temperatures. Combining electronic-structure calculations with ab initio molecular dynamics, conjugate gradient relaxation, and the solid-state nudged-elastic band methods, we predict that AFM $B2$ FeRh becomes unstable at ambient pressure and transforms without a barrier to an AFM(111) orthorhombic (martensitic) ground state below $90\pm 10\mathrm{K}$. We also consider competing structures, in particular, a tetragonal AFM(100) phase that is not the global ground state, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained solution.

The current efforts to find the materials hosting Kitaev model physics have been focused on Mott insulators of $d^5$ pseudospin-1/2 ions ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ru}}^{3+}$ with $t_{2g}^5(S=1/2,L=1)$ electronic configuration. Here we propose that the Kitaev model can be realized in materials based on $d^7$ ions with $t_{2g}^5{e}_g^2(S=3/2,L=1)$ configuration such as ${\mathrm{Co}}^{2+}$, which also host the pseudospin-1/2 magnetism. Considering possible exchange processes, we have derived the $d^7$ pseudospin-1/2 interactions in ${90}^{\circ }$ bonding geometry. The obtained Hamiltonian comprises the bond-directional Kitaev $K$ and isotropic Heisenberg $J$ interactions as in the case of $d^5$ ions. However, we find that the presence of additional spin-active $e_g$ electrons radically changes the balance between Kitaev and Heisenberg couplings. Most remarkably, we show that the exchange processes involving $e_g$ spins are highly sensitive to whether the system is in Mott ($U<\mathrm{\Delta }$) or charge-transfer ($U>\mathrm{\Delta }$) insulating regime. In the latter case, to which many cobalt compounds do actually belong, the antiferromagnetic Heisenberg coupling $J$ is strongly suppressed and spin-liquid phase can be stabilized. The results suggest cobalt-based materials as promising candidates for the realization of the Kitaev model.

In the search for quantum spin liquids, candidate materials for the Kitaev model and its extensions have been intensively explored during the past decade, as the models realize the exact quantum spin liquids in the ground state. Thus far, insulating magnets in the low-spin $d^5$ electron configuration under the strong spin-orbit coupling have been studied for realizing the Kitaev-type bond-dependent anisotropic interactions between the spin-orbital entangled Kramers doublets. To extend the candidates, here we investigate the systems in a high-spin $d^7$ electron configuration, whose ground state is described by the spin-orbital entangled Kramers doublet. By the second-order perturbation in terms of the $t_{2g}-t_{2g}$ and $t_{2g}-e_g$ hoppings, we show that the effective spin model possesses the anisotropic Kitaev interactions as well as the isotropic Heisenberg ones. While the Kitaev interaction is always ferromagnetic, the Heisenberg interaction can become either ferromagnetic or antiferromagnetic depending on the Coulomb interactions and the crystalline electric fields. We also derive the effective model for the low-spin $d^5$ electron configuration within the same perturbation scheme, in which the Kitaev interaction becomes both ferromagnetic and antiferromagnetic, while the Heisenberg one is always ferromagnetic. Referring to the previous study for the Kitaev-Heisenberg model, we find that the quantum spin liquid phase exists in the reasonable parameter region in both $d^7$ and $d^5$ cases, while the former has a richer structure of the phase diagram. We discuss the advantages of the $d^7$ case in comparison with the $d^5$ case. Our results indicate that the high-spin $d^7$ state provides another platform for the Kitaev-type quantum spin liquid.

In antiferromagnets, the Dzyaloshinskii-Moriya interaction lifts the degeneracy of left- and right-circularly polarized spin waves. This relativistic coupling increases the efficiency of spin-wave-induced domain-wall motion and leads to higher drift velocities. We demonstrate that, in biaxial antiferromagnets, the spin-wave helicity controls both the direction and the magnitude of the magnonic force on chiral domain walls. In this case it is shown that the domain-wall velocity is an order of magnitude faster in the presence of the Dzyaloshinskii-Moriya interaction. In uniaxial antiferromagnets, by contrast, the magnonic force is always propulsive, but its strength is still helicity dependent.

The noncentrosymmetric superconductor ${\mathrm{Re}}_{24}{\mathrm{Ti}}_5$, a time-reversal symmetry- (TRS-) breaking candidate with $T_c=6\mathrm{K}$, was studied by means of muon-spin rotation/relaxation ($\mu \mathrm{SR}$) and tunnel-diode oscillator techniques. At the macroscopic level, its bulk superconductivity was investigated via electrical resistivity, magnetic susceptibility, and heat-capacity measurements. The low-temperature penetration depth, superfluid density, and electronic heat capacity all evidence an $s$-wave coupling with an enhanced superconducting gap. The spontaneous magnetic fields revealed by zero-field $\mu \mathrm{SR}$ below $T_c$ indicate a time-reversal symmetry breaking and thus the unconventional nature of superconductivity in ${\mathrm{Re}}_{24}{\mathrm{Ti}}_5$. The concomitant occurrence of TRS breaking also in the isostructural ${\mathrm{Re}}_6(\mathrm{Zr},\mathrm{Hf})$ compounds hints at its common origin in this superconducting family and that an enhanced spin-orbital coupling does not affect pairing symmetry.

An efficient and accurate basis comprised of Volkov states is implemented and tested for time-dependent simulations of interactions between strong laser pulses and crystalline solids. The Volkov states are eigenstates of the free electron Hamiltonian in an electromagnetic field and analytically represent the rapidly oscillating time-dependence of the orbitals, allowing significantly faster time propagation than conventional approaches. The Volkov approach can be readily implemented in plane-wave codes by multiplying the potential energy matrix elements with a simple time-dependent phase factor.

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.05}$, ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{YCu}}_2{\mathrm{O}}_{8+\delta }$, and ${\mathrm{La}}_2{\mathrm{CuO}}_4$ as a function of polarization and excitation energy using samples of the latest generation. From the raw data we derive symmetry-resolved spectra. The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of excitation energy. The spectra agree quantitatively with predictions by field theory [Eur. Phys. J. B 88, 237 (2015)] facilitating the precise extraction of the Heisenberg coupling $J$. In addition, the asymmetric lineshape on the high-energy side is found to be related to amplitude fluctuations of the magnetization. In ${\mathrm{La}}_2{\mathrm{CuO}}_4$ alone, minor contributions from resonance effects may be identified. The spectra in the other symmetries are not universal. The variations may be traced back to weak resonance effects and extrinsic contributions. For all three compounds we find support for the existence of chiral excitations appearing as a continuum in $A_{2g}$ symmetry having an onset slightly below $3J$. In ${\mathrm{La}}_2{\mathrm{CuO}}_4$ an additional isolated excitation appears on top of the $A_{2g}$ continuum.

A chiral soliton lattice stabilized in a monoaxial chiral magnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ is a magnetic superlattice consisting of magnetic kinks with a ferromagnetic background. The magnetic kinks are considered to be topological magnetic solitons (TMSs). Changes in the TMS number yield discretized responses in magnetization and electrical conductivity, and this effect is more prominent in smaller crystals. We demonstrate that, in microprocessed ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ crystals, TMSs are geometrically protected through element-selected micromagnetometry using soft x-ray magnetic circular dichroism (MCD). A series of x-ray MCD data is supported by mean-field and micromagnetic analyses. By designing the microcrystal geometry, TMS numbers can be successfully changed and fixed over a wide range of magnetic fields.

The small-angle neutron scattering is used to measure the spin-wave stiffness in the field-polarized state of the Dzyaloshinskii-Moriya helimagnets ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ with $x=0.03$, 0.06, 0.09, and 0.10. The ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ compounds are helically ordered below $T_c$ and show a helical fluctuation regime above $T_c$ in a wide range up to $T_{\text{DM}}$. The critical temperatures $T_c$ and $T_{\text{DM}}$ decrease with $x$ and tend to 0 at $x=0.11$ and 0.17, respectively. We have found that the spin-wave stiffness $A$ change weakly with temperature for each individual Fe-doped compound. On the other hand, the spin-wave stiffness $A$ decreases with $x$ duplicating the $T_{\text{DM}}$ dependence on $x$, rather than $T_c\left(x\right)$. These findings classify the thermal phase transition in all ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ compounds as an abrupt change in the spin state caused, most probably, by the features of an electronic band structure. Moreover, the criticality in these compounds is not related to the value of the ferromagnetic interaction but demonstrates the remarkable role of the Dzyaloshinskii-Moriya interaction as a factor destabilizing the magnetic order.

We study the topology and symmetry of surface Majorana arcs in superconductors with nonunitary “cyclic” pairing. Cyclic $p$-wave pairing may be realized in a cubic or tetrahedral crystal, while it is a candidate for the interior ${}^3\mathrm{P}_2$ superfluids of neutron stars. The cyclic state is an admixture of full gap and nodal gap with eight Weyl points and the low-energy physics is governed by itinerant Majorana fermions. We here show the evolution of surface states from Majorana cone to Majorana arcs under rotation of surface orientation. The Majorana cone is protected solely by an accidental spin rotation symmetry and fragile against spin-orbit coupling, while the arcs are attributed to two topological invariants: the first Chern number and one-dimensional winding number. Lastly, we discuss how topologically protected surface states inherent to the nonunitary cyclic pairing can be captured from surface probes in candidate compounds, such as ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$. We examine tunneling conductance spectra for two competitive scenarios in ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$—the degenerate $E_u$ scenario and the accidental scenario.

A nodeless superconducting (SC) gap was reported in a recent scanning tunneling spectroscopy experiment of a copper-oxide monolayer grown on a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) substrate [Zhong et al., Sci. Bull. 61, 1239 (2016)], which is in stark contrast to the nodal $d$-wave pairing gap in the bulk cuprates. Motivated by this experiment, we first show with first-principles calculations that the tetragonal CuO (T-CuO) monolayer on the Bi2212 substrate is more stable than the commonly postulated ${\mathrm{CuO}}_2$ structure. The T-CuO monolayer is composed of two ${\mathrm{CuO}}_2$ layers sharing the same O atoms. The band structure is obtained by first-principles calculations, and its strong electron correlation is treated with the renormalized mean-field theory. We argue that one ${\mathrm{CuO}}_2$ sublattice is hole doped while the other sublattice remains half filled and may have antiferromagnetic (AF) order. The doped Cu sublattice can show $d$-wave SC; however, its proximity to the AF Cu sublattice induces a spin-dependent hopping, which splits the Fermi surface and may lead to a full SC gap. Therefore, the nodeless SC gap observed in the experiment could be accounted for by the $d$-wave SC proximity to an AF order, thus it is extrinsic rather than intrinsic to the ${\mathrm{CuO}}_2$ layers.

We study theoretically the electronic band structure of (${\mathrm{PbSe})}_5$(${\mathrm{Bi}}_2{\mathrm{Se}}_3$)${}_6$, which consists of an ordinary insulator PbSe and a topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The first-principles calculations show that this material has a gapped Dirac-cone energy dispersion inside the bulk, which originates from the topological states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layers encapsulated by PbSe layers. Furthermore, we calculate the band structures of (${\mathrm{Bi}}_x{\mathrm{Pb}}_{1-x}\mathrm{Se}$)${}_5$(${\mathrm{Bi}}_2{\mathrm{Se}}_3$)${}_6$ with ${\mathrm{Bi}}_{\mathrm{Pb}}$ antisite defects included in the PbSe layers. The result shows that a high density of ${\mathrm{Bi}}_{\mathrm{Pb}}$ defects can exist in real materials, consistent with the experimentally estimated $x$ of more than 30%. The ${\mathrm{Bi}}_{\mathrm{Pb}}$ defects strongly modify the band alignment between ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and PbSe layers, while the topological interface states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ are kept as a gapped Dirac-cone-like dispersion.