Cubic crystal structure and regular octahedral environment of ${\mathrm{Ir}}^{4+}$ render antifluorite-type ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $\mathrm{\Delta }\simeq 0.7\mathrm{eV}$ for charge transport, the antiferromagnetic Curie-Weiss temperature of ${\theta }_{\mathrm{CW}}\simeq -43\mathrm{K}$, and the extrapolated saturation field of $H_s\simeq 87\mathrm{T}$. All these parameters are well reproduced ab initio using $U_{\mathrm{eff}}=2.2\mathrm{eV}$ as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5\mathrm{K}$ is about one half of the Heisenberg term $J\simeq 13\mathrm{K}$. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2\mathrm{K}$. Our results suggest that ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ may offer the best possible cubic conditions for ${\mathrm{Ir}}^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.

When the excitation of carriers in real space is focused down to the nanometer scale, the carrier system can no longer be viewed as homogeneous, and ultrafast transport of the excited carrier wave packets occurs. In state-of-the-art semiconductor structures such as low-dimensional heterostructures or monolayers of transition-metal dichalcogenides, the Coulomb interaction between excited carriers becomes stronger due to confinement or reduced screening. This demands a fundamental understanding of strongly interacting electrons and holes and the influence of Coulomb correlations. To study the corresponding particle dynamics in a controlled way, we consider a system of up to two electron-hole pairs exactly within a wave-function approach. We show that the excited wave packets contain a nontrivial mixture of free-particle and excitonic states. We further scrutinize the influence of the Coulomb interaction on the wave-packet dynamics, revealing its different role for below- and above-band-gap excitation.

The electromagnetic modes and the resonances of homogeneous, finite size, two-dimensional bodies are examined in the frequency domain by a rigorous full wave approach based on an integro-differential formulation of the electromagnetic scattering problem. Using a modal expansion for the current density that disentangles the geometric and material properties of the body the integro-differential equation for the induced surface (free or polarization) current density field is solved. The current modes and the corresponding resonant values of the surface conductivity (eigenconductivities) are evaluated by solving a linear eigenvalue problem with a non-Hermitian operator. They are inherent properties of the body geometry and do not depend on the body material. The material only determines the coefficients of the modal expansion and hence the frequencies at which their amplitudes are maximum (resonance frequencies). The eigenconductivities and the current modes are studied in detail as the frequency, and the shape and the size of the body vary. Open and closed surfaces are considered. The presence of vortex current modes, in addition to the source-sink current modes (no whirling modes), which characterize plasmonic oscillations, is shown. Important topological features of the current modes, such as the number of sources and sinks, the number of vortices, and the direction of the vortices are preserved as the size of the body and the frequency vary. Unlike the source-sink current modes, in open surfaces the vortex current modes can be resonantly excited only in materials with a positive imaginary part of the surface conductivity. Eventually, as examples, the scattering by two-dimensional bodies with either a positive or negative imaginary part of the surface conductivity is analyzed and the contributions of the different modes are examined.

Whether the surface states in ${\mathrm{SmB}}_6$ are topological is still a critical issue in the field of topological Kondo insulators. In the magneto-transport study of single crystalline ${\mathrm{SmB}}_6$ microribbons, we have revealed a positive planar Hall effect (PHE), the amplitude of which increases dramatically with decreasing temperatures but saturates below 5 K. This positive PHE is ascribed to the surface states of ${\mathrm{SmB}}_6$ and is expected to arise from the anisotropy in lifting the topological protection from backscattering by the in-plane magnetic field, thus suggesting the topological nature of surface states in ${\mathrm{SmB}}_6$. On the contrary, a negative PHE is observed for the bulk states at high temperatures, which is almost three orders of magnitudes weaker than the surface-induced positive PHE.

Highly confined surface waves present unique opportunities to enhance light interactions with localized emitters or molecules. Hyperbolic dispersion in metasurfaces allows us to tailor and manipulate surface waves, enhancing the local density of states over broad bandwidths. So far, propagation on this platform was mainly studied in planar geometries, which facilitates the analysis but somehow limits the realm of possibilities. Here we show that “wrapping” hyperbolic metasurfaces into tubes may greatly enrich the wave propagation dynamics along their axis. This system shows strong interaction with fields and sources carrying optical angular momentum and pronounced field asymmetries, and opens pathways to valley-specific excitation and routing. In addition, we demonstrate that various parameter regimes enable strong spin/helicity momentum locking.

Electrical and thermal conductances of a quantum Hall bar reflect the topological structure of the incompressible bulk phase. Here we show that noise of the electrical current carried through the edge is evidence of the interplay between these two topological observables. Transport through a structured edge is modeled by a voltage-biased line junction made up of two counterpropagating modes associated with respective filling factors. Specifically, we focus on the edge of a $\nu =2/3$ fractional quantum Hall state. Noise is generated at a point distinctly separated from the hot spot (where most of the Ohmic dissipation takes place) and reflects the competition between ballistically carried downstream current and diffusively carried heat (which can propagate also upstream). We propose specific setups where our predictions can be measured.

Responses of materials at high pressures can reveal fundamental insights into properties of condensed matter that are otherwise masked under ambient conditions. Using first-principles calculations to determine the electron-phonon interactions of free-electron metals at high pressures, we show that the electrical and thermal transport properties of aluminum, gold, and silver can be significantly enhanced with the application of high pressures. This is partly attributed to the decrease in electron-phonon coupling with increasing pressures. At elevated temperatures, under these high-pressure conditions, the thermal conductivity of aluminum is amongst the highest of any known material, surpassing the thermal conductivity of diamond at ambient pressure. Moreover, through calculation of the spectrally resolved electron-phonon coupling parameter at high pressures, we show an increased contribution to electron-phonon coupling from high-frequency longitudinal phonon modes scattering with electrons in aluminum. Using the same analysis with gold and silver, we show that low frequency acoustic phonon scattering with electrons is increased at high pressures. This work provides valuable insights into the electron-phonon scattering processes occurring under high pressures, which gives rise to new regimes of electrical and thermal transport properties in metals.

Motivated by intertwined crystal symmetries and topological phases, we study the possible realization of topological insulators in nonsymmorphic crystals at integer fillings. In particular, we consider spin-orbit-coupled electronic systems of two-dimensional crystal Shastry-Sutherland lattices at integer filling where the gapless line degeneracy is protected by glide reflection symmetry. Based on a simple tight-binding model, we investigate how the topological insulating phase is stabilized by breaking nonsymmorphic symmetries but in the presence of time reversal symmetry and inversion symmetry. In addition, we also discuss the regime where Dirac semimetal is stabilized, having nontrivial $Z_2$ invariants even without spin-orbit coupling. Our study can be extended to more general cases where all lattice symmetries are broken and we also discuss possible application to topological Kondo insulators in nonsymmorphic crystals where crystal symmetries can be spontaneously broken as a function of the Kondo coupling.

We theoretically study the topological properties of the tight-binding model on the breathing kagome lattice with antisymmetric spin-orbit coupling (SOC) between nearest neighbors. We show that the system hosts nontrivial topological phases even without second-nearest-neighbor hopping and that the weakly dispersing band of the kagome lattice can become topological. The main results are presented in the form of phase diagrams, where the ${\mathbb{Z}}_2$ topological index is shown as a function of SOC (intrinsically allowed and Rashba) and lattice trimerization. In addition, exact diagonalization is compared with effective low-energy theories around the high-symmetry points. We find that the weakly dispersing band has a very robust topological property associated with it. Moreover, the Rashba SOC can produce a topological phase rather than hinder it, in contrast to the honeycomb lattice. Finally, we consider the case of a fully spin polarized (ferromagnetic) system, breaking time-reversal symmetry. We find a phase diagram that includes systems with finite Chern numbers. In this case too, the weakly dispersing band is topologically robust to trimerization.

We realize a potential platform for an efficient spin-photon interface, namely negatively-charged silicon-vacancy centers in a diamond membrane coupled to the mode of a fully-tunable, fiber-based, optical resonator. We demonstrate that introducing the thin ($\sim 200\text{nm}$), single crystal diamond membrane into the mode of the resonator does not change the cavity properties, which is one of the crucial points for an efficient spin-photon interface. In particular, we observe constantly high Finesse values of up to 3000 and a linear dispersion in the presence of the membrane. We observe cavity-coupled fluorescence from an ensemble of ${\mathrm{SiV}}^{-}$ centers with an enhancement factor of $\sim 1.9$. Furthermore from our investigations we extract the ensemble absorption and extrapolate an absorption cross section of $(2.9\pm 2)\times {10}^{-12}{\text{cm}}^2$ for a single ${\mathrm{SiV}}^{-}$ center, much higher than previously reported.

The spin and intensity of optically trapped polariton condensates are studied under steady-state elliptically polarized nonresonant pumping. Three distinct effects are observed: (1) spin inversion where condensation occurs in the opposite handedness from the pump, (2) spin and intensity hysteresis as the pump power is scanned, and (3) a sharp “spin collapse” transition in the condensate spin as a function of the pump ellipticity. We show these effects are strongly dependent on trap size and sample position and are linked to small counterintuitive energy differences between the condensate spin components. Our results, which fail to be fully described within the commonly used nonlinear equations for polariton condensates, show that a more accurate microscopic picture is needed to unify these phenomena in a two-dimensional condensate theory.

It is shown that truncating the cluster expansion of the energy of alloys gives rise to renormalized effective cluster interactions that are explicit functions of the configurational variables. Such a dependence of the renormalized cluster interactions is in addition to their dependence on the volume of the alloy and on other structural parameters. The physical picture that emerges is different from the commonly used representation of the configurational energy by means of a generalized Ising-like model, which follows from the assumption that the contributions of the effective cluster interactions can be neglected beyond a relatively small cluster size. The physical picture is one in which the sum of the effective interactions contributes over long distances but the expected “near-sightedness” of the energy is preserved by the renormalized interactions. Furthermore, the cluster expansion is implemented by simultaneously fitting the volume- and configuration-dependent energy function to the zero-pressure values of the energies of formation, volumes, bulk moduli, and pressure derivatives of the bulk modulus of a set of ordered compounds. As an example of this formulation of the cluster expansion, we apply the methodology to the Cu-Au system for different types of cell-internal and cell-external relaxations.

We propose a scheme for investigating the nonequilibrium aspects of small-polaron physics using an array of superconducting qubits and microwave resonators. This system, which can be realized with transmon or gatemon qubits, serves as an analog simulator for a lattice model describing a nonlocal coupling of a quantum particle (excitation) to dispersionless phonons. We study its dynamics following an excitation-phonon (qubit-resonator) interaction quench using a numerically exact approach based on a Chebyshev-moment expansion of the time-evolution operator of the system. We thereby glean heretofore unavailable insights into the process of the small-polaron formation resulting from strongly momentum-dependent excitation-phonon interactions, most prominently about its inherent dynamical timescale. To further characterize this complex process, we evaluate the excitation-phonon entanglement entropy and show that initially prepared bare-excitation Bloch states here dynamically evolve into small-polaron states that are close to being maximally entangled. Finally, by computing the dynamical variances of the phonon position and momentum quadratures, we demonstrate a pronounced non-Gaussian character of the latter states, with a strong antisqueezing in both quadratures.

We report anisotropic magnetization plateaus in the skew-chain antiferromagnet ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ with $S_{\mathrm{eff}}=1/2$ and a large $g$ factor. When the field is applied along the easy $b$ axis, a 1/2-like magnetization plateau is observed within $5.4\sim 11.6\mathrm{T}$, followed by a 3/4 plateau at 15.7 T. For the hard $a$ and $c$ axes, however, there is only a sign of 1/2-like magnetization plateau. These observations are quite different from the isostructural ${\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ [Z. W. Ouyang et al., Phys. Rev. B 97, 144406 (2018)]. Theoretically, the first-principles calculations reveal the large interchain coupling, which leads to classical antiferromagnetic ordering and spin-flop-like transition. With this large interchain coupling, the exact diagonalization analysis yields 1/2 and 3/4 plateaus, showing the quantum origin of magnetization plateaus. Thus, ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ is an interchain-coupled system showing both classical and quantum behaviors.

Tuning magnetic order in magnetic semiconductors is a long-sought goal. Here, we propose that a proper mix of acceptors with magnetic dopants may quench local magnetic order in favor of the long-range one. Using Mn and acceptor codoped LiZnAs as an example, we show, by first-principles calculations, the emergence of a long-range magnetic order. This intriguing observation may be understood based on a crossover between an acceptor-free magnetism and a band-coupling magnetism. Our findings pave the way for a precise control of magnetic order in future spintronic devices.

We present the analysis of the spin signals obtained in NiFe based metallic lateral spin valves. We exploit the spin-dependent diffusive equations in both the conventional one-dimensional (1D) analytic modeling and in 3D finite element method simulations. These approaches are used for extracting the spin diffusion length and the effective spin polarization in Py/Al, Py/Cu, and Py/Au based lateral nanostructures at both 300 K and 77 K. Both the analytic modeling and 3D finite element method simulations give consistent results. The combination of these models provides a powerful tool for reliable spin transport characterization in all metallic spin valves and gives an insight into the spin/charge current and spin accumulations 3D distributions in these devices. We provide the necessary ingredients to develop the 3D finite element modeling of diffusive spin transport.

We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band $s_{\pm }$-wave Eliashberg approach, able to account for all the main observed features in the different properties. Besides the validation of the model itself, the comparison between experiment and theory allows discussing the possible role of the Fe-As planes in defining the superconducting properties of these compounds, the relevance of coherence effects, and the presence of nodes in the superconducting order parameter.

We investigate the connection between highly frustrated kagome based Hamiltonians and a recently synthesized family of materials containing ${\mathrm{Ti}}^{3+}S=\frac{1}{2}$ ions. Employing a combination of all electron density functional theory and numerical diagonalization techniques, we establish the Heisenberg Hamiltonians for the distorted kagome antiferromagnets ${\mathrm{Rb}}_2{\mathrm{NaTi}}_3{\mathrm{F}}_{12},{\mathrm{Cs}}_2{\mathrm{NaTi}}_3{\mathrm{F}}_{12}$, and ${\mathrm{Cs}}_2{\mathrm{KTi}}_3{\mathrm{F}}_{12}$. We determine magnetization curves in excellent agreement with experimental observations. Our calculations successfully clarify the relationship between the experimental observations and the magnetization-plateau behavior at $\frac{1}{3}$ height of the saturation and predict characteristic behaviors under fields that are higher than the experimentally measured region. We demonstrate that the studied Ti(III) family of materials interpolates between the kagome strip and kagome lattice.

Single crystals of ${\mathrm{Sr}}_2{\mathrm{Mn}}_3{\mathrm{As}}_2{\mathrm{O}}_2$ have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of ${\mathrm{MnO}}_2$ and MnAs layers, analogously to the active ${\mathrm{CuO}}_2$ and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on ${\mathrm{Sr}}_2{\mathrm{Mn}}_3{\mathrm{As}}_2{\mathrm{O}}_2$ single crystals converge toward a picture of independent magnetic order at $T_1\sim 79$ K and $T_2\sim 360$ K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of $SJ\sim 3.7$ meV. This is much smaller than those for the Mn(2) sublattice.