Semiconductor quantum dots are promising sources for polarization-entangled photons. As an alternative to the usual cascaded biexciton-exciton emission, direct two-photon emission from the biexciton can be used. With a high-quality optical resonator tuned to half the biexciton energy, a large proportion of the photons can be steered into the two-photon emission channel. In this case the degree of polarization entanglement is inherently insensitive to the exciton fine-structure splitting. In the present work we analyze the biexciton emission with particular emphasis on the influence of coupling of the quantum-dot cavity system to its environment. Especially for a high-quality cavity, the coupling to the surrounding semiconductor material can open up additional phonon-assisted decay channels. Our analysis demonstrates that with the cavity tuned to half the biexciton energy, the potentially detrimental influence of the phonons on the polarization entanglement is strongly suppressed—high degrees of entanglement can still be achieved. We further discuss spectral properties and statistics of the emitted twin photons.

We present a polarization resolved Raman scattering study of surface vibration modes in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first-principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 ${\mathrm{cm}}^{-1}$ are associated with Raman active $A_{1g}$ bulk phonon modes, the other two at 136 and 158 ${\mathrm{cm}}^{-1}$ are associated with infrared active bulk phonons with $A_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from $D_{3d}$ in the bulk to $C_{3v}$ on the crystal surface. In particular, the 158 ${\mathrm{cm}}^{-1}$ surface phonon mode shows a Fano line shape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

Hybrid structures of quantum spin Hall insulators (QSHIs) and superconductors (Ss) present a unique opportunity to access dissipationless topological states of matter, which, however, is frequently hindered by the lack of control over the spin polarization in QSHIs. We propose a very efficient spin-polarization mechanism based on the magnetoelectric (Edelstein) effect in superconducting QSHI structures. It acts akin to the Zeeman splitting in an external magnetic field, but with an effective $g$ factor of order of 1000, resulting in an unprecedented spin splitting effect. It allows a magnetic control of the QSHI/S hybrids without destroying superconductivity. As an example, we demonstrate a recurrent crossover from ${\mathrm{\Phi }}_0$- to (${\mathrm{\Phi }}_0/2$)-periodic oscillations of the Josephson current in an rf superconducting quantum interference device (${\mathrm{\Phi }}_0=h/2e$ is the magnetic flux quantum). The predicted period halving is a striking manifestation of $0-\pi$ Josephson transitions with a superharmonic $\pi$-periodic current-phase relationship at the transition. Such controllable $0-\pi$ transitions may offer new perspectives for dissipationless spintronics and engineering flux qubits.

Searching for ferromagnetic (FM) van der Waals (vdW) heterostructures with high Curie temperature and multiple-band alignments is crucial to develop next-generation spintronic and optoelectronic nanodevices. Here, through first-principles methods, the FM vdW heterostructure is found for the first time in the Mg(${\mathrm{OH})}_2/{\mathrm{VS}}_2$ heterobilayer with high Curie temperature (385 K) and type-II alignment. The negative electric field induces the transition from type-II to type-III alignment in the spin-up channel. However, under the positive electric field, the type-II to type-I alignment transition occurs in the spin-up and spin-down channels. This work provides the possibilities of realizing the high-temperature FM vdW heterostructures and electrostatic gating dependent multiple-band alignments in practice.

Using density functional theory combined with orbital-selective band unfolding techniques, we study the effective band structure of silicene $\left(3\times 3\right)$/Ag(111) $\left(4\times 4\right)$ structure. Consistent with the ARPES spectra recently obtained by [Feng et al. Proc. Natl. Acad. Sci. USA 113, 14656 (2016)], we observe six pairs of Dirac cones near the boundary of the Brillouin zone (BZ) of $\mathrm{Ag}(1\times 1)$, while no Dirac cone is observed inside the BZ. Furthermore, we find that these Dirac cones are induced by the interfacial Si-Ag hybridization, mainly composed of Si $p_z$ orbitals and Ag $sp$ bands, which is intrinsically different from the Dirac cones in free-standing silicene.

We present a novel scheme to accurately predict atomic forces as vector quantities, rather than sets of scalar components, by Gaussian process (GP) regression. This is based on matrix-valued kernel functions, on which we impose the requirements that the predicted force rotates with the target configuration and is independent of any rotations applied to the configuration database entries. We show that such covariant GP kernels can be obtained by integration over the elements of the rotation group $\mathit{SO}\left(d\right)$ for the relevant dimensionality $d$. Remarkably, in specific cases the integration can be carried out analytically and yields a conservative force field that can be recast into a pair interaction form. Finally, we show that restricting the integration to a summation over the elements of a finite point group relevant to the target system is sufficient to recover an accurate GP. The accuracy of our kernels in predicting quantum-mechanical forces in real materials is investigated by tests on pure and defective Ni, Fe, and Si crystalline systems.

Motivated by recent experiments on spin chain materials ${\mathrm{K}}_2{\mathrm{CuSO}}_4{\mathrm{Cl}}_2$ and ${\mathrm{K}}_2{\mathrm{CuSO}}_4{\mathrm{Br}}_2$, we theoretically investigate the problem of weakly coupled spin chains (chain exchange $J$, interchain $J^{\prime }$) subject to a staggered between chains, but uniform within a given chain, Dzyaloshinskii-Moriya (DM) interaction of magnitude $D$. In the experimentally relevant limit $J^{\prime }\ll D\ll J$ of the strong DM interaction the spins on the neighboring chains are forced to rotate in opposite directions, effectively resulting in a cancellation of the interchain interaction between components of spins in the plane normal to the vector $\mathit{D}$. This has the effect of promoting a two-dimensional collinear spin density wave state, which preserves U(1) symmetry of rotations about the $\mathit{D}$ axis. We also investigate response of this interesting system to an external magnetic field $\mathit{h}$ and obtain the h-D phase diagrams for the two important configurations $\mathit{h}\parallel \mathit{D}$ and $\mathit{h}\perp \mathit{D}$.

We present a novel energetic model that takes into account atomistic relaxations to describe the thermodynamic properties of $A_c{B}_{1-c}$ binary alloys. It requires the calculation of the energies on each site of a random solid solution after relaxation as a function of both the local composition and the nominal concentration. These site energies are obtained by molecular static simulations using $N$-body interatomic potentials derived from the second-moment approximation (SMA) of the tight-binding scheme. This new model allows us to determine the effective pair interactions (EPIs) that drive the short-range order (SRO) and to analyze the relative role of the EPIs’ contribution to the mixing enthalpy, with respect to the contribution due to the lattice mismatch between the constituents. We apply this formalism to Au-Ni and Ag-Cu alloys, both of them tending to phase separate in the bulk and exhibiting a large size mismatch. Rigid-lattice Monte Carlo (MC) simulations lead to phase diagrams that are in good agreement with both those obtained by off-lattice SMA-MC simulations and the experimental ones. While the phase diagrams of Au-Ni and Ag-Cu alloys are very similar, we show that phase separation is mainly driven by the elastic contribution for Au-Ni and by the EPIs’ contribution for Ag-Cu. Furthermore, for Au-Ni, the analysis of the SRO shows an inversion between the tendency to order and the tendency to phase separate as a function of the concentration.

The focusing of spin waves in magnetic materials has a number of important advantages for directing energy and thus information. In contrast to earlier works, we theoretically calculate the focusing of short-wavelength spin waves. We show that strong focusing of large wave-vector spin waves naturally occurs about halfway out in the Brillouin zone with no need for a magnetocrystalline anisotropy, dipolar effects, or an external magnetic field. The fact that the wavelength is on the order of the lattice constant leads to a form of lattice-induced anisotropy in the wave propagation. We also explore the tunability that is achieved by the application of both an external field and a uniaxial anisotropy. In this case there is a rotation of the focusing pattern. The rotation angle can be changed by varying the field strength, demonstrating tunability.

We present muon spin rotation and relaxation ($\mu \mathrm{SR}$) measurements on the noncentrosymmetric superconductor ${\mathrm{PbTaSe}}_2$. From measurements in an applied transverse field between $H_{c1}$ and $H_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. These data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero-field measurements show no evidence for a time-reversal symmetry-breaking field greater than 0.05 G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that ${\mathrm{PbTaSe}}_2$ is characterized by conventional BCS $s$-wave superconductivity in multiple bands.

We report resonant inelastic x-ray scattering (RIXS) measurements on ordered double-perovskite samples containing ${\mathrm{Re}}^{5+}$ and ${\mathrm{Ir}}^{5+}$ with $5d^2$ and $5d^4$ electronic configurations, respectively. In particular, the observed RIXS spectra of ${\mathrm{Ba}}_2{\mathrm{YReO}}_6$ and ${\mathrm{Sr}}_{2}M{\mathrm{IrO}}_6$ ($M$ = Y, Gd) show sharp intra-$t_{2g}$ transitions, which can be quantitatively understood using a minimal “atomic” Hamiltonian incorporating spin-orbit coupling $\lambda$ and Hund's coupling $J_H$. Our analysis yields $\lambda =0.38\left(2\right)\mathrm{eV}$ with $J_H=0.26\left(2\right)\mathrm{eV}$ for ${\mathrm{Re}}^{5+}$ and $\lambda =0.42\left(2\right)\mathrm{eV}$ with $J_H=0.25\left(4\right)\mathrm{eV}$ for ${\mathrm{Ir}}^{5+}$. Our results provide sharp estimates for Hund's coupling in $5d$ oxides and suggest that it should be treated on equal footing with spin-orbit interaction in multiorbital $5d$ transition-metal compounds.

We study how the synergetic effect of spin-orbit coupling (SOC) and Zeeman splitting (ZS) affects the optical conductivity in the one-dimensional Hubbard model using the Kubo formula. We focus on two phenomena: (1) the electric dipole spin resonance (EDSR) in the metallic regime and (2) the optical conductivity in the Mott-insulating phase above the optical gap. In both cases, we calculate qualitatively the effects of SOC and ZS and how they depend on the relative angle between the SOC vector and the magnetic field direction. First, we investigate the spin resonance without electron correlation (the Hubbard parameter $U=0$). Although, neither SOC nor ZS causes any resonance by itself in the optical conductivity, the EDSR becomes possible when both of them exist. The resulting contribution to the optical conductivity is analyzed analytically. The effect of $U$ on the spin resonance is also studied with a numerical method. It is found that at half filling, the resonance is first enhanced for small $U$ and then suppressed when the optical gap is large enough. In the strong coupling limit $U\to \infty$ at half filling, we also refer to the resonance between the lower and upper Hubbard bands appearing at $\omega \sim U$, above the optical gap. A large magnetic field tends to suppress the signal while it is recovered thanks to SOC depending on the relative angle of the magnetic field.

Topological metals with protected band-crossing points have been attracting great interest. Here we report topological band features in a family of metal diboride materials. Using first-principles calculations, we show that these materials are metallic, and close to Fermi level, there appears coexistence of one pair of nodal rings and one pair of triply degenerate nodal points (TNPs). The nodal ring here is distinct from the previously studied ones in that its formation requires four entangled bands, not just two as in previous cases, hence it is termed as a four-band nodal ring (FNR). Remarkably, we show that FNR features Dirac-cone-like surface states, in contrast to the usual drumhead surface states for two-band nodal rings. Due to the presence of inversion symmetry, the TNP here is also different from those discussed previously in inversion-asymmetric systems. Especially, when spin-orbit coupling is included, the TNP here transforms into a novel Dirac point that is close to the borderline between the type-I and type-II Dirac point categories. We discuss their respective symmetry protections, and construct effective models for their characterization. The large linear energy range ($>2$ eV) in these materials should facilitate the experimental detection of the signatures of these nontrivial band crossings.

We formulate a general theory to study the time-dependent charge and energy transport of an adiabatically driven interacting quantum dot in contact with a reservoir for arbitrary amplitudes of the driving potential. We study within this framework the Anderson impurity model with a local ac gate voltage. We show that the exact adiabatic quantum dynamics of this system is fully determined by the behavior of the charge susceptibility of the frozen problem. At $T=0$, we evaluate the dynamic response functions with the numerical renormalization group (NRG). The time-resolved heat production exhibits a pronounced feature described by an instantaneous Joule law characterized by a universal Büttiker resistance quantum $R_0=h/\left(2e^2\right)$ for each spin channel. We show that this law holds in the noninteracting as well as in the interacting system and also when the system is spin polarized. In addition, in the presence of a static magnetic field, the interplay between many-body interactions and spin polarization leads to a nontrivial energy exchange between electrons with different spin components.

Assuming that the nuclear magnetic resonance (NMR) signal from a ${}^{13}\mathrm{C}$-isotope-enriched layer of graphene can be made sufficiently intense to be measured, we compute the NMR line shape of the different crystals' ground states that are expected to occur in graphene in a strong magnetic field. We first show that in nonuniform states there is, in addition to the frequency shift due to the spin hyperfine interaction, a second contribution of equal importance from the coupling between the orbital motion of the electrons and the nuclei. We then show that if the linewidth of the bare signal can be made sufficiently small, the Wigner and bubble crystals have line shapes that differ qualitatively from that of the uniform state at the same density while crystal states that have spin or valley pseudospin textures do not. Finally, we find that a relatively small value of the bare linewidth is sufficient to wash out the distinctive signature of the crystal states in the NMR line shape.

A fully analytical semiclassical theory of cooperative optical processes which occur in an ensemble of molecules embedded in a spherical core-shell nanoparticle is developed from first principles. Both the plasmonic Dicke effect and spaser generation are investigated for the designs in which a shell/core contains an arbitrarily large number of active molecules in the vicinity of a metallic core/shell. An essential aspect of the theory is an ab initio account of the feedback from the core/shell boundaries which significantly modifies the molecular dynamics. The theory provides rigorous, albeit simple and physically transparent, criteria for both plasmonic superradiance and surface plasmon generation.

Transition-metal dichalcogenides (${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$) have recently drawn much attention, because of the nonsaturating extremely large magnetoresistance (XMR) observed in these compounds in addition to the predictions of likely type-II Weyl semimetals. Contrary to the topological insulators or Dirac semimetals where XMR is linearly dependent on the field, in ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$ the XMR is nonlinearly dependent on the field, suggesting an entirely different mechanism. Electron-hole compensation has been proposed as a mechanism of this nonsaturating XMR in ${\mathrm{WTe}}_2$, while it is yet to be clear in the case of ${\mathrm{MoTe}}_2$ which has an identical crystal structure of ${\mathrm{WTe}}_2$ at low temperatures. In this Rapid Communication, we report low-energy electronic structure and Fermi surface topology of ${\mathrm{MoTe}}_2$ using angle-resolved photoemission spectrometry (ARPES) technique and first-principles calculations, and compare them with that of ${\mathrm{WTe}}_2$ to understand the mechanism of XMR. Our measurements demonstrate that ${\mathrm{MoTe}}_2$ is an uncompensated semimetal, contrary to ${\mathrm{WTe}}_2$ in which compensated electron-hole pockets have been identified, ruling out the applicability of charge compensation theory for the nonsaturating XMR in ${\mathrm{MoTe}}_2$. In this context, we also discuss the applicability of other existing conjectures on the XMR of these compounds.

We use exact diagonalization to study the quantum phases and phase transitions when a single species of fermionic atoms at a Landau level filling factor ${\nu }_f=1$ in a rotating trap interact through a $p$-wave Feshbach resonance. We show that under a weak pairing interaction, the system undergoes a second-order quantum phase transition from a ${\nu }_f=1$ fermionic integer quantum Hall (FIQH) state at positive detuning, to a ${\nu }_b=\frac{1}{4}$ bosonic fractional quantum Hall (BFQH) state at negative detuning. However, when the pairing interaction increases, a new phase between them emerges, corresponding to a fraction of fermionic atoms staying in a coherent superposition of a bosonic molecule state and an unbound pair. The phase transition from the FIQH phase to the new phase is of second order and that from the new phase to BFQH phase is of first order.

We study the antiferromagnetic quantum critical metal in $3-\epsilon$ space dimensions by extending the earlier one-loop analysis [Sur and Lee, Phys. Rev. B 91, 125136 (2015)] to higher-loop orders. We show that the $\epsilon$ expansion is not organized by the standard loop expansion, and a two-loop graph becomes as important as one-loop graphs due to an infrared singularity caused by an emergent quasilocality. This qualitatively changes the nature of the infrared fixed point, and the $\epsilon$ expansion is controlled only after the two-loop effect is taken into account. Furthermore, we show that a ratio between velocities emerges as a small parameter, which suppresses a large class of diagrams. We show that the critical exponents do not receive corrections beyond the linear order in $\epsilon$ in the limit that the ratio of velocities vanishes. The $\epsilon$ expansion gives critical exponents which are consistent with the exact solution obtained in $0<\epsilon \le 1$.