We demonstrate that quantum-critical spin dynamics can be probed in high magnetic fields using muon-spin relaxation (${\mu }^{+}\mathrm{SR}$). Our model system is the strong-leg spin ladder bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). In the gapless Tomonaga-Luttinger liquid phase we observe finite-temperature scaling of the ${\mu }^{+}\mathrm{SR}$ $1/T_1$ relaxation rate which allows us to determine the Luttinger parameter $K$. We discuss the benefits and limitations of local probes compared with inelastic neutron scattering.

Recent neutron scattering observations by [Plumb et al., Phys. Rev. X 6, 041055 (2016)] reveal that the ground state of ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ is magnetic with two distinct Fe environments, instead of a quantum spin liquid as had been previously thought. Starting with the relevant $O\left(N\right)$-symmetric vector model of ${\mathrm{FeSc}}_2{\mathrm{S}}_4$, we study how the discrete $\left(Z_2\right)$ and continuous rotational symmetries are successively broken, yielding nematic and ordered phases. At high temperatures, we find that the nematic order parameter falls as $T^{-\gamma }$ $\left(\gamma >0\right)$, and therefore, ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ lacks any distinct nematic ordering temperature. This feature indicates that the three-dimensional diamond lattice of ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ is highly susceptible to the breaking of Ising symmetries, and explains the two distinct Fe environments that are present even at high temperatures, as seen by Mössbauer and far-infrared optical spectroscopy.

The occurrence of so-called temperature-induced neutral-ionic transitions (TINIT) in mixed-stack charge-transfer crystals is quite rare. Here we reinvestigate one of the crystals which has been claimed to undergo such a transition, $3,3^{\prime },5,5^{\prime }$-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). Extensive optical data allow us to conclude that the transition should be classified as a valence instability, and not as a “true” TINIT, as the $\sim 0.5$ neutral-ionic borderline is not crossed. The ionicity $\varrho$, or average charge at the molecular sites, indeed changes very little at the transition, from about 0.3 to about 0.4, and is accompanied by stack dimerization. The transition is first order with large hysteresis, and the crystal may crack or break. For this reason we have been unable to collect x-ray structural data on the low-temperature phase, but with the help of semiempirical calculations we are able to assess a plausible scenario for this peculiar phase transition and its mechanism.

Olivine $\mathrm{LiMnP}{\mathrm{O}}_4$ is a promising cathode material for Li-ion batteries. One drawback of this material is the propensity of its delithiated phase, $\mathrm{MnP}{\mathrm{O}}_4$, to evolve oxygen gas above approximately 200 °C. During thermal runaway of cells, this oxygen gas can burn the electrolyte and other cell components and thereby jeopardize safety. Partial substitution of Mn with $M=\mathrm{Fe}$, Ni, Al, or Mg has been used to improve the lithium intercalation kinetics of $\mathrm{L}{\mathrm{i}}_x\mathrm{MnP}{\mathrm{O}}_4$; however, the effect of these substitutions on oxygen evolution is not fully documented. In this paper, we calculate phase diagrams and oxygen evolution diagrams for these $\mathrm{M}{\mathrm{n}}_{1-x}{M}_{\mathrm{x}}\mathrm{P}{\mathrm{O}}_4$ delithiated cathode materials. To generate the phase diagrams, we use subregular solid-solution models and fit the energetic parameters of these models to density functional theory calculations of special quasirandom structures. The resulting thermodynamic models describe the effect of mixing on the initial temperature of oxygen evolution and on the cumulative amount of oxygen evolution at elevated temperatures. We find that addition of Fe increases the initial temperature and decreases the cumulative amount of oxygen evolution. $\mathrm{M}{\mathrm{n}}_{0.5}\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{P}{\mathrm{O}}_4$ exhibits an initial temperature 50 °C higher than $\mathrm{MnP}{\mathrm{O}}_4$ and releases 70% less oxygen gas at 300 °C. Al is insoluble in $\mathrm{MnP}{\mathrm{O}}_4$, so addition of Al has no affect on the initial temperature. However, Al addition does slightly decrease the amount of oxygen evolution due to an inactive $\mathrm{AlP}{\mathrm{O}}_4$ component. Mg and Ni both decrease the initial temperature of oxygen evolution, and therefore may worsen the safety of $\mathrm{MnP}{\mathrm{O}}_4$.

We report a comprehensive investigation of the crystal structure and magnetic behavior of the $S=1$ compound ${\mathrm{Ba}}_2\mathrm{Ni}{\left({\mathrm{PO}}_4\right)}_2$ with a honeycomblike topology of the spin lattice. Magnetic susceptibility and specific-heat data reveal two successive transitions at $T_{N1}=5\mathrm{K}$ and $T_{N2}=4.6\mathrm{K}$. Additionally, these data demonstrate a broad peak at $T_{max}\sim 8\mathrm{K}$, indicative of the short-range magnetic order above $T_{N1}$, whereas below $T_{N1}$ field-induced transitions around 4 and 10 T are identified from the magnetization measurements. Neutron diffraction in zero field establishes stripe antiferromagnetic order below $T_{N2}$ with the ordered moment of $1.75\left(8\right){\mu }_B/{\mathrm{Ni}}^{2+}$ at 1.5 K. Density-functional band-structure calculations reveal the leading interaction $J_3=3.5\mathrm{K}$ running perpendicular to the honeycomb planes, and weaker interactions $J_1=0.5\mathrm{K}$ and $J_4=1.8\mathrm{K}$ within the honeycomb planes, whereas the stripe order is stabilized by the diagonal interlayer interaction $J_2=1.3\mathrm{K}$ that frustrates $J_1$. This is in contrast to the usually expected scenario where the competing second- and third-neighbor interactions on the honeycomb lattice stabilize the stripe order. The ${\mathrm{Ni}}^{2+}$ ions feature a sizable easy-plane anisotropy $A\simeq 10.5\mathrm{K}$, but the position of the easy plane changes from one atom to another, thus amplifying magnetic frustration.

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature ${\mathrm{T}}_{\mathrm{C}}$. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$. The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time scales. The onset of a superconducting precursor state was found at a temperature ${\mathrm{T}}_{\mathrm{ONS}}>{\mathrm{T}}_{\mathrm{C}}$, consistent with the temperature scale identified via static optical spectroscopy. Furthermore, the temperature evolution of the coherent vibration of the Ba ion, strongly renormalized by the onset of superconductivity, revealed a pronounced anomaly at the same temperature ${\mathrm{T}}_{\mathrm{ONS}}$. The microscopic nature of such a precursor state is discussed in terms of preformed pairs and enhanced bilayer coherence.

A self-consistent Bogoliubov–de Gennes theory of the vortex lattice state in a 2D strong type-II superconductor at high magnetic fields reveals a novel quantum mixed state around the semiclassical ${\mathrm{H}}_{c2}$, characterized by a well-defined Landau-Bloch band structure in the quasiparticle spectrum and suppressed order-parameter amplitude, which sharply crossover into the well-known semiclassical (Helfand-Werthamer) results upon decreasing magnetic field. Application to the 2D superconducting state observed recently on the surface of the topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ accounts well for the experimental data, revealing a strong type-II superconductor, with unusually low carrier density and very small cyclotron mass, which can be realized only in the strong coupling superconductor limit.

Superconductivity is shown to be completely destroyed in thin mesoscopic or nanoscopic rings closed by the junction with a noticeable interfacial pair breaking and/or a Josephson coupling, if a ring's radius $r$ is less than the minimum radius $r_{\text{min}}$. The quantity $r_{\text{min}}$ depends on the phase difference $\chi$ across the junction, or on the magnetic flux that controls $\chi$ in the flux-biased ring. It also depends on the Josephson and interfacial effective coupling constants, and in particular, on whether the ring is closed by the 0 or the $\pi$ junction. The current-phase relation is substantially modified when the ring's radius exceeds $r_{\text{min}}$ for some of the phase difference values, or slightly goes beyond its maximum. The modified critical temperature $T_c$ and the temperature-dependent supercurrent near $T_c$ are identified here as functions of the ring's radius and the magnetic flux.

We have succeeded in growing large single crystals of double-perovskite ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ with the $c$ axis aligned, and carried out a crystal structure analysis and measurements of resistivity and magnetization. A first-order metal-insulator transition at $T_{MI}\simeq 290$ K is accompanied by a large jump of magnetization and lattice constants. The distortion of ${\mathrm{MnO}}_6$ octahedra is consistent with a ferroic orbital order of $x^2-y^2$ type, although other orbital orders cannot be completely discarded. Néel temperature $T_N$ is determined to be 235 K, which is fairly lower than $T_{MI}$, on the basis of the onset of a large magnetic anisotropy. Another structural phase transition from tetragonal to orthorhombic is found at 370 K, which arises from staggered tilting of ${\mathrm{MnO}}_6$ octahedra and little affects the transport and magnetic properties.

Topological Dirac semimetals (DSMs) exhibit nodal points through which energy bands disperse linearly in three-dimensional (3D) momentum space, a 3D analog of graphene. The first experimentally confirmed DSMs with a pair of Dirac points (DPs), ${\mathrm{Na}}_3\mathrm{Bi}$ and ${\mathrm{Cd}}_3{\mathrm{As}}_2$, show topological surface Fermi arc states and exotic magnetotransport properties, boosting the interest in the search for stable and nontoxic DSM materials. Based on density-functional theory and dynamical mean-field theory calculations, we predict a family of palladium and platinum oxides to be robust 3D DSMs with three pairs of Dirac points that are well separated from bulk bands. The Fermi arcs at the surface display a Lifshitz transition upon a continuous change of the chemical potential. Corresponding oxides are already available as high-quality single crystals, an excellent precondition for the verification of our predictions by photoemission and magnetotransport experiments, extending DSMs to the versatile family of transition-metal oxides.

The concept of a Fermi surface, the constant-energy surface containing all the occupied electron states in momentum, or wave-vector $\left(k\right)$, space plays a key role in determining electronic properties of conductors. In two-dimensional (2D) carrier systems, the Fermi surface becomes a contour which, in the simplest case, encircles the occupied states. In this case, the area enclosed by the contour, which we refer to as the Fermi sea (FS), is a simple disk. Here we report the observation of an FS with a new topology, namely, an FS in the shape of an annulus. Such an FS is expected in a variety of 2D systems where the energy band dispersion supports a ring of extrema at finite $k$, but its experimental observation has been elusive. Our study provides (1) theoretical evidence for the presence of an annular FS in 2D hole systems confined to wide GaAs quantum wells and (2) experimental signatures of the onset of its occupation as an abrupt rise in the sample resistance, accompanied by a sudden appearance of Shubnikov–de Haas oscillations at an unexpectedly high frequency whose value does not simply correspond to the (negligible) density of holes contained within the annular FS.

We have designed and fabricated a metamaterial consisting of planar metamolecules which exhibit unusual, almost perfect anapole behavior in the sense that the electric dipole radiation is almost canceled by the toroidal dipole one, producing thus an extremely high $Q$-factor at the resonance frequency. Thus we have demonstrated theoretically and experimentally that metamaterials approaching ideal anapole behavior have very high $Q$-factor. The size of the system, at the millimeter range, and the parasitic magnetic quadrupole radiation are the factors limiting the size of the $Q$-factor. In spite of the very low radiation losses the estimated local fields at the metamolecules are extremely high, of the order of ${10}^4$ higher than the external incoming field.

Optical force and torque provide unprecedented control on the spatial motion of small particles. A valid scientific question, that has many practical implications, concerns the existence of fundamental upper bounds for the achievable force and torque exerted by a plane wave illumination with a given intensity. Here, while studying isotropic particles, we show that different light-matter interaction channels contribute to the exerted force and torque, and analytically derive upper bounds for each of the contributions. Specific examples for particles that achieve those upper bounds are provided. We study how and to which extent different contributions can add up to result in the maximum optical force and torque. Our insights are important for applications ranging from molecular sorting, particle manipulation, and nanorobotics up to ambitious projects such as laser-propelled spaceships.

We study the antiferromagnetic and paramagnetic Kondo insulator phases of the Kondo lattice model on the cubic lattice at half filling using the cellular dynamical mean-field theory (CDMFT) with the numerical renormalization group (NRG) as the impurity solver, focusing on the fine details of the spectral function and self-energy. We find that the nonlocal correlations increase the gap in both the antiferromagnetic and Kondo insulator phases and shrink the extent of the antiferromagnetic phase in the phase diagram but do not alter any properties qualitatively. The agreement between the numerical CDMFT results and those within a simple hybridization picture, which adequately describes the overall band structure of the system but neglects all effects on the inelastic-scattering processes, is similar to that of the single-site DMFT results; there are deviations that are responsible for the additional fine structure, in particular for the asymmetric spectral resonances or dips that become more pronounced in the strong-coupling regime close to the antiferromagnet-paramagnetic quantum phase transition. These features appear broader in the CDMFT mostly due to numerical artifacts linked to more aggressive state truncation required in the NRG.

We study spinless fermions with nearest-neighbor repulsive interactions ($t\text{−}V$ model) on the two-dimensional three-band Lieb lattice. At half-filling, the free electronic band structure consists of a flat band at zero energy and a single cone with linear dispersion. The flat band is expected to be unstable upon inclusion of electronic correlations, and a natural channel is charge order. However, due to the three-orbital unit cell, commensurate charge order implies an imbalance of electron and hole densities and therefore doping away from half-filling. Our numerical results show that below a finite-temperature Ising transition a charge density wave with one electron and two holes per unit cell and its partner under particle-hole transformation are spontaneously generated. Our calculations are based on recent advances in auxiliary-field and continuous-time quantum Monte Carlo simulations that allow sign-free simulations of spinless fermions at half-filling. It is argued that particle-hole symmetry breaking provides a route to access levels of finite doping, without introducing a sign problem.

First-principles calculations based on many-electron perturbation theory methods, such as the ab initio $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) approach, are reliable ways to predict quasiparticle and optical properties of materials, respectively. However, these methods involve more care in treating the electron-electron interaction and are considerably more computationally demanding when applied to systems with reduced dimensionality, since the electronic confinement leads to a slower convergence of sums over the Brillouin zone due to a much more complicated screening environment that manifests in the “head” and “neck” elements of the dielectric matrix. Here we present two schemes to sample the Brillouin zone for $GW$ and $GW$-BSE calculations: the nonuniform neck subsampling method and the clustered sampling interpolation method, which can respectively be used for a family of single-particle problems, such as $GW$ calculations, and for problems involving the scattering of two-particle states, such as when solving the BSE. We tested these methods on several few-layer semiconductors and graphene and show that they perform a much more efficient sampling of the Brillouin zone and yield two to three orders of magnitude reduction in the computer time. These two methods can be readily incorporated into several ab initio packages that compute electronic and optical properties through the $GW$ and $GW$-BSE approaches.

Cu-based materials are potential candidates for commercial thermoelectric materials due to their abundance, nontoxicity, and high performance. We incorporate the multiband Boltzmann transport equations with first-principles calculations to theoretically investigate the thermoelectric properties of Cu-based chalcopyrite compounds. As a demonstration of our method, the thermoelectric properties of quaternary compounds ${\mathrm{Cu}}_2{\mathrm{ZnSnX}}_4$ (X = S, Se) and ternary compounds ${\mathrm{CuBTe}}_2$ (B = Ga, In) are studied. We systematically calculate the electrical conductivity, the Seebeck coefficient, and the power factor of the four materials above based on parameters obtained from first-principles calculations and using several other fitting parameters. For quaternary compounds, our results reveal that ${\mathrm{Cu}}_2{\mathrm{ZnSnSe}}_4$ is better than ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ and its optimal hole concentration is around $5\times {10}^{19}{\mathrm{cm}}^{-3}$ with the peak power factor 4.7 $\mu {\mathrm{W}/\mathrm{cm}\mathrm{K}}^2$ at 600 K. For ternary compounds, we find that their optimal hole concentrations are around $1\times {10}^{20}{\mathrm{cm}}^{-3}$ with the peak power factors over 26 $\mu {\mathrm{W}/\mathrm{cm}\mathrm{K}}^2$ at 800 K.

Two of the most striking experimental findings when investigating exciton spectra in cuprous oxide using high-resolution spectroscopy are the observability and the fine structure splitting of $F$ excitons reported by J. Thewes et al. [Phys. Rev. Lett. 115, 027402 (2015)]. These findings show that it is indispensable to account for the complex valence band structure and the cubic symmetry of the solid in the theory of excitons. This is all the more important for magnetoexcitons, where the external magnetic field reduces the symmetry of the system even further. We present the theory of excitons in ${\mathrm{Cu}}_2\mathrm{O}$ in an external magnetic field and especially discuss the dependence of the spectra on the direction of the external magnetic field, which cannot be understood from a simple hydrogenlike model. Using high-resolution spectroscopy, we also present the corresponding experimental spectra for cuprous oxide in Faraday configuration. The theoretical results and experimental spectra are in excellent agreement as regards not only the energies but also the relative oscillator strengths. Furthermore, this comparison allows for the determination of the fourth Luttinger parameter $\kappa$ of this semiconductor.

We address the electromagnetic properties of two-dimensional electron gas confined by a dielectric environment in the presence of both Rashba and Dresselhaus spin-orbit interactions. It is demonstrated that off-diagonal components of the conductivity tensor resulting from a delicate interplay between Rashba and Dresselhaus couplings lead to the hybridization of transverse electric and transverse magnetic surface electromagnetic modes localized at the interface. We show that the characteristics of these hybrid surface waves can be controlled by additional intense external off-resonant coherent pumping.