We present calculated valence and C $1s$ near-edge excitation spectra of solid ${\mathrm{C}}_{60}$ and experimental results measured with high-resolution electron energy-loss spectroscopy. The near-edge calculations are carried out using three different methods: solution of the Bethe-Salpeter equation (BSE) as implemented in the ocean suite (Obtaining Core Excitations with Ab Initio methods and the NIST BSE solver), the excited-electron core-hole approach, and the constrained-occupancy method using the Stockholm-Berlin core excitation code, StoBe. The three methods give similar results and are in good agreement with experiment, though the BSE results are the most accurate. The BSE formalism is also used to carry out valence level calculations using the NIST BSE solver. Theoretical results include self-energy corrections to the band gap and bandwidths, lifetime-damping effects, and Debye-Waller effects in the core excitation case. A comparison of spectral features to those observed experimentally illustrates the sensitivity of certain features to computational details, such as self-energy corrections to the band structure and core-hole screening.

We use x-ray absorption spectroscopy as well as its linear and circular magnetic dichroisms to characterize relevant interactions and energy scales in the surface intermetallic ${\mathrm{CePt}}_5/\text{Pt}\left(111\right)$. The experiments provide insight into crystal field splitting, effective paramagnetic moments, their Kondo screening and mutual interactions, and thus into many aspects which typically determine the low-temperature behavior of correlated rare-earth compounds. Exploiting the tunability of Ce valence through the thickness-dependent epitaxial strain at the ${\mathrm{CePt}}_5/\text{Pt}\left(111\right)$ interface, we are able to systematically investigate the impact of hybridization strength on these interactions. Considerable Kondo screening is indeed observed at all ${\mathrm{CePt}}_5$ thicknesses, and found to be strongest in case of strongest hybridization. While the magnetic response is commensurate with an impurity Kondo scale of $T_K\gtrsim {10}^2$ K for specimen temperatures $T\gtrsim 30$ K, this is no longer the case at lower temperature. Its detailed study by x-ray circular magnetic dichroism (XMCD) at one specific thickness of ${\mathrm{CePt}}_5$ reveals an anomaly of the susceptibility at $T^{*}\approx 25$ K instead, which we tentatively associate with the onset of lattice coherence. At lowest temperature we observe paramagnetic saturation with a small Ce $4f$ saturation magnetization. Within the framework of itinerant $4f$ electrons, saturation is due to a field-induced Lifshitz transition involving a very heavy band with correspondingly small degeneracy temperature of $T_F\approx 7$ K. This small energy scale results in the persistence of Curie-Weiss behavior across the entire range of experimentally accessible temperatures $(T\gtrsim 2$ K). Our work highlights the potential of magnetic circular dichroism studies in particular for Kondo and heavy-fermion materials, which so far has remained largely unexplored.

We show that a cylindrical lensing system composed of two metasurfaces with suitably tailored non-Hermitian (i.e., with distributed gain and loss) and nonlocal (i.e., spatially dispersive) properties can perform magnified imaging with reduced aberrations. More specifically, we analytically derive the idealized surface-impedance values that are required for “perfect” magnification and imaging and elucidate the role and implications of non-Hermiticity and nonlocality in terms of spatial resolution and practical implementation. For a basic demonstration, we explore some proof-of-principle quasilocal and multilayered implementations and independently validate the outcomes via full-wave numerical simulations. We also show that the metasurface frequency-dispersion laws can be chosen so as to ensure unconditional stability with respect to arbitrary temporal excitations. These results, which extend previous studies on planar configurations, may open intriguing venues in the design of metastructures for field imaging and processing.

Quantum corrections to Thomas-Fermi (TF) theory are investigated for noninteracting one-dimensional fermions with known uniform semiclassical approximations to the density and kinetic energy. Their structure is analyzed, and contributions from distinct phase space regions (classically-allowed versus forbidden at the Fermi energy) are derived analytically. Universal formulas are derived for both particle numbers and energy components in each region. For example, in the semiclassical limit, exactly ${\left(6\pi \sqrt{3}\right)}^{-1}$ of a particle leaks into the evanescent region beyond a turning point. The correct normalization of semiclassical densities is proven analytically in the semiclassical limit. Energies and densities are tested numerically in a variety of one-dimensional potentials, especially in the limit where TF theory becomes exact. The subtle relation between the pointwise accuracy of the semiclassical approximation and integrated expectation values is explored. The limitations of the semiclassical formulas are also investigated when the potential varies too rapidly. The approximations are shown to work for multiple wells, except right at the mid-phase point of the evanescent regions. The implications for density functional approximations are discussed.

We use coupled-cluster quantum chemical methods to calculate the energetics of molecular clusters cut out of periodic molecular hydrogen structures that model observed phases of solid hydrogen. The hydrogen structures are obtained from Kohn-Sham density functional theory (DFT) calculations at pressures of 150, 250, and 350 GPa, which are within the pressure range in which phases II, III, and IV are found to be stable. The calculated deviations in the DFT energies from the coupled-cluster data are reported for different functionals, and optimized functionals are generated which provide reduced errors. We give recommendations for semilocal and hybrid density functionals that are expected to provide an accurate description of hydrogen at high pressures.

We consider the transport properties of a two-dimensional topological insulator in a double quantum point contact geometry in the presence of a time-dependent external field. In the proposed setup an external gate is placed above a single constriction and it couples only with electrons belonging to the top edge. This asymmetric configuration and the presence of an ac signal allow for a quantum pumping mechanism, which, in turn, can generate finite heat and charge currents in an unbiased device configuration. A microscopic model for coupling with the external time-dependent gate potential is developed and the induced finite heat and charge currents are investigated. We demonstrate that in the noninteracting case, heat flow is associated with a single spin component, due to the helical nature of the edge states, and therefore a finite and polarized heat current is obtained in this configuration. The presence of $e\text{−}e$ interchannel interactions strongly affects the current signal, lowering the degree of polarization of the system. Finally, we also show that separate heat and charge flows can be achieved, varying the amplitude of the external gate.

The spin-reorientation transition of an ultrathin film from perpendicular to in-plane magnetization is driven by a competition between dipole and anisotropy energies. In situ measurements of the magnetic susceptibility of Fe/2 monolayers (ML) Ni/W(110) films as a function of Fe coverage, made as the films are deposited at constant temperature, show two clear peaks that are described quantitatively as a long-range and a local realization of the transition. In the long-range realization, the susceptibility probes the striped domain pattern that is formed in response to the balance of energetics on a mesoscopic scale. Here the reorientation transition occurs at a noninteger layer thickness. In the local realization, the susceptibility probes the response of small islands with in-plane anisotropy in the third atomic Fe layer that are grown on the second atomic Fe layer, which has perpendicular anisotropy. It is a response to the local finite-size, metastable energetics due to discrete steps in thickness. An excellent quantitative description of the susceptibility data is obtained when both local and long-range aspects of the spin-reorientation transition are included.

SmO (001) epitaxial thin films were grown on $\mathrm{YAl}{\mathrm{O}}_3$ (110) substrates by pulsed laser deposition method. X-ray photoemission spectroscopy indicated coexistence of $\mathrm{S}{\mathrm{m}}^{2+}$ and $\mathrm{S}{\mathrm{m}}^{3+}$, suggesting a valence fluctuating state. A SmO thin film showed a metallic conductionlike SmO polycrystal in a previous study. However, a SmO thin film showed nonmonotonical temperature dependence at low temperature in contrast with the polycrystal. A local resistivity minimum was observed at 16 K, probably caused by the dense Kondo effect, and the resistivity below 2 K was proportional to $T^2$. These features suggest the heavy-fermionic nature of SmO.

We consider finite-sized interfaces of a Weyl semimetal and show that the corresponding confinement potential is similar to the application of a magnetic field. Among the numerous states, which can be labeled by indices $n$ like in Landau levels, the $n=0$ surface state describes the Weyl semimetal Fermi arc at a given chemical potential. Moreover, the analogy with a magnetic field shows that an external in-plane magnetic field can be used to distort the Fermi arc and would explain some features of magnetotransport in Weyl semimetals. We derive the Fermi arc for type-I and type-II Weyl semimetals where we deal with the tilt anisotropy by the use of Lorentz boosts. In the case of type-II Weyl semimetals, this leads to many additional topologically trivial surface states at low energy. Finally, we extend the Aharonov-Casher argument and demonstrate the stability of the Fermi arc over fluctuations of the surface potential.

In this paper we introduce and analyze a system design for quantum-dot-based qubits that simultaneously supports scalable one-qubit and two-qubit gates, and single-shot qubit measurement. All three key processes (one-qubit gates, two-qubit gates, and qubit measurement) rely on the interaction between the electron in each quantum dot and exciton polaritons formed in a quantum well situated near the quantum dots. A key feature of our proposed system is the use of polariton traps, which we show enhances the quantum-dot–quantum-well interaction by a factor of 10 and consequently results in $100\times$ faster two-qubit gates. We also introduce a one-qubit gate that is based on a combination of optical and microwave control, which is supported in the same device and system configuration as the other operations, in contrast to the conventional one-qubit gate that is based on all-optical control.

The many-body radiative heat transfer theory [P. Ben-Abdallah, S.-A. Biehs, and K. Joulain, Phys. Rev. Lett. 107, 114301 (2011)] considered only the contribution from the electric dipole moment. For metal particles, however, the magnetic dipole moment due to eddy current plays an important role, which can further couple with the electric dipole moment to introduce crossed terms. In this paper, we develop the coupled electric and magnetic dipole (CEMD) approach for the radiative heat transfer in a collection of objects in mutual interaction. Due to the coupled electric and magnetic interactions, four terms, namely the electric-electric, the electric-magnetic, the magnetic-electric, and the magnetic-magnetic terms, contribute to the radiative heat flux and the local energy density. The CEMD is applied to study the radiative heat transfer between various dimers of nanoparticles. It is found that each of the four terms can dominate the radiative heat transfer depending on the position and composition of particles. Moreover, near-field many-body interactions are studied by CEMD considering both dielectric and metallic nanoparticles. The near-field radiative heat flux and local energy density can be greatly increased when the particles are in coupled resonances. Surface plasmon polariton and surface phonon polariton can be coupled to enhance the radiative heat flux.

We analyze the dynamics of two strongly interacting fermions moving in two-dimensional lattices under the action of a periodic electric field, both with and without a magnetic flux. Due to the interaction, these particles bind together forming a doublon. We derive an effective Hamiltonian that allows us to understand the interplay between the interaction and the driving, revealing surprising effects that constrain the movement of the doublons. We show that it is possible to confine doublons to just the edges of the lattice and to a particular sublattice if different sites in the unit cell have different coordination numbers. Contrary to what happens in one-dimensional systems, here we observe the coexistence of both topological and Shockley-like edge states when the system is in a nontrivial phase.

We investigate the photoluminescence (PL) dynamics of self-trapped excitons (STEs) in a $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ single crystal from the viewpoint of the transition process from the free exciton to the STE. We succeed in measuring the PL rise time ($\sim 24$ ps) at 8 K corresponding to the tunneling time through the barrier between the free exciton and STE states in the adiabatic potential. From the analysis of the PL rise time of the STE based on perturbation theory for the tunneling time considering exciton-phonon interactions, we obtain the following results. Acoustic phonons near the Brillouin zone center contribute to the tunneling process. This suggests that the wave function of the STE is still spatially extended at the final state in the tunneling process. Furthermore, we investigate temperature dependence of the PL rise time of the STE. It is found that the PL rise time decreases with increasing temperature. The PL rise times in the temperature range from 8 to 100 K can be quantitatively explained by an adiabatic theory for the tunneling process. Consequently, the self-trapping process is dominated by the tunneling process at low temperatures.

A novel mechanism of spin-induced ferroelectricity is unraveled in the alternating longitudinal conical (ALC) magnetic structure. Because the noncollinear ALC structure possesses a $c$-axis component with collinear ↑–↑–↓–↓ spin order, spin-driven ferroelectricity along the $c$ axis due to the exchange striction mechanism is predicted. Our experiments verify this prediction in the Y-type hexaferrite $\mathrm{B}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{1.7}\mathrm{C}{\mathrm{o}}_2\mathrm{F}{\mathrm{e}}_{11}\mathrm{Al}{\mathrm{O}}_{22}$, where ferroelectricity along the $c$ axis is observed up to room temperature. Neutron diffraction data clearly reveal the ALC phase and its evolution with magnetic fields. The $c$-axis electric polarization can be well modulated by applying either $ab$-plane or $c$-axis magnetic fields, even at 305 K. This kind of spin-induced ferroelectricity associated with the ALC magnetic structure provides a new resource of type II multiferroics.

We propose a compact and highly efficient scheme for complete Bell-state analysis using two-photon absorption in a superconducting proximity region of a semiconductor avalanche photodiode. One-photon transitions to the superconducting Cooper-pair based condensate in the conduction band are forbidden, whereas two-photon transitions are allowed and are strongly enhanced by superconductivity. This Cooper-pair based two-photon absorption results in a strong detection preference of a specified entangled state. Our analysis shows high detection purity of the desired Bell state with negligible false detection probability. The theoretically demonstrated concept can pave the way towards practical realizations of advanced quantum information schemes.

We offer an explanation for the recently observed pressure-induced magnetic state in the iron-chalcogenide FeSe based on ab initio estimates for the pressure evolution of the most important Coulomb interaction parameters. We find that an increase of pressure leads to an overall decrease mostly in the nearest-neighbor Coulomb repulsion, which in turn leads to a reduction of the nematic order and the generation of magnetic stripe order. We treat the concomitant effects of band renormalization and the induced interplay of nematic and magnetic order in a self-consistent way and determine the generic topology of the temperature-pressure phase diagram and find qualitative agreement with the experimentally determined phase diagram.

The relation between the polar structural instability and superconductivity in a Weyl semimetal candidate ${\mathrm{MoTe}}_2$ has been clarified by finely controlled physical and chemical pressure. The physical pressure as well as the chemical pressure, i.e., the Se substitution for Te, enhances the superconducting transition temperature $T_{\mathrm{c}}$ at around the critical pressure where the polar structure transition disappears. From the heat capacity and thermopower measurements, we ascribe the significant enhancement of $T_{\mathrm{c}}$ at the critical pressure to a subtle modification of the phonon dispersion or the semimetallic band structure upon the polar-to-nonpolar transition. On the other hand, the physical pressure, which strongly reduces the interlayer distance, is more effective on the suppression of the polar structural transition and the enhancement of $T_{\mathrm{c}}$ as compared with the chemical pressure, which emphasizes the importance of the interlayer coupling on the structural and superconducting instability in ${\mathrm{MoTe}}_2$.

We theoretically analyze contributions from the Kondo effect to the spin polarization and spin diffusion length in all-metal nonlocal spin valves. Interdiffusion of ferromagnetic atoms into the normal metal layer creates a region in which Kondo physics plays a significant role, giving discrepancies between experiment and existing theory. We start from a simple model and construct a modified spin drift-diffusion equation which clearly demonstrates how the Kondo physics not only suppresses the electrical conductivity but even more strongly reduces the spin diffusion length. We also present an explicit expression for the suppression of spin polarization due to Kondo physics in an illustrative regime. We compare this theory to previous experimental data to extract an estimate of the Elliot-Yafet probability for Kondo spin flip scattering of $0.7\pm 0.4$, in good agreement with the value of 2/3 derived in the original theory of Kondo.

The competition between spin-orbit coupling, bandwidth $\left(W\right)$, and electron-electron interaction $\left(U\right)$ makes iridates highly susceptible to small external perturbations, which can trigger the onset of novel types of electronic and magnetic states. Here we employ first principles calculations based on density functional theory and on the constrained random phase approximation to study how dimensionality and strain affect the strength of $U$ and $W$ in ${\left({\mathrm{SrIrO}}_3\right)}_m/\left({\mathrm{SrTiO}}_3\right)$ superlattices. The result is a phase diagram explaining two different types of controllable magnetic and electronic transitions, spin-flop and insulator-to-metal, connected with the disruption of the $J_{\mathrm{eff}}=1/2$ state which cannot be understood within a simplified local picture.