We consider the free-carrier dispersion effect in a semiconductor nanocavity in the limit of discrete photoexcited electron-hole pairs. This analysis reveals the possibility of ultrafast, incoherent transduction and gain from a single photon signal to a strong coherent probe field. Homodyne detection of the displaced probe field enables an all-optical method for room-temperature, photon-number-resolving single photon detection. In particular, we estimate that a single photon absorbed within a silicon nanocavity can, within tens of picoseconds, be detected with $\sim 99\%$ efficiency and a dark count rate on the order of kilohertz assuming a mode volume $V_{\text{eff}}\sim {10}^{-2}{(\lambda /n_{\text{Si}})}^3$ for a 4.5-$\mu \mathrm{m}$ probe wavelength and a loaded quality factor $Q$ on the order of ${10}^4$.

Understanding and manipulating the quantum interlayer exciton-phonon coupling in van der Waals heterostructures, especially for infrared active phonons with electromagnetic fields, would set a foundation for realizing exotic quantum phenomena and optoelectronic applications. Here we report experimental observations of strong mutual interactions between infrared active phonons in hexagonal boron nitride (hBN) and excitons in ${\mathrm{WS}}_2$. Our results underscore that the infrared active $A_{2\mathrm{u}}$ mode of hBN becomes Raman active with strong intensities in ${\mathrm{WS}}_2$/hBN heterostructures through resonant coupling to the $B$ exciton of ${\mathrm{WS}}_2$. Moreover, we demonstrate that the activated $A_{2\mathrm{u}}$ phonon of hBN can be tuned by the hBN thickness and harbors a striking anticorrelation intensity modulation, as compared with the optically silent $B_{1\mathrm{g}}$ mode. Our observation of the interlayer exciton-infrared active phonon interactions and their evolution with hBN thickness provide a firm basis for engineering the hyperbolic exciton-phonon polaritons, chiral phonons and fascinating nanophotonics based on van der Waals heterostructures.

Using grazing incidence x-ray diffraction (GIXD) and density functional theory (DFT), we have studied the silicene reconstructions on Ag(111). We propose a comprehensive discussion of all nonequivalent configurations for the $(4\times 4)$, $\left(2\sqrt{3}\times 2\sqrt{3}\right)R{30}^{\circ }$, $\left(\sqrt{13}\times \sqrt{13}\right)13.9^{\circ }$ type-I, and $\left(\sqrt{13}\times \sqrt{13}\right)13.9^{\circ }$ type-II reconstructions, putting in evidence their relative stability. We show that assuming a $p3$ symmetry leads to a total of 12 inequivalent reconstructions, that can be labeled according to an original notation, which specifies the occupation of the $p3$-symmetry Wyckoff positions by the Si atoms of the silicene unit cell and by the Ag atoms of the first three substrate layers. The DFT calculations are in good agreement with the experimental results and indicate that the structures energetically most favorable are those which best reproduce the measured scanning tunneling microscopy images and the GIXD structure factors. These configurations are also those inducing the highest deformation in the substrate, suggesting that the interaction between silicene and Ag can be held to be responsible for the unexpected stability of the two-dimensional silicon layer.

A computational methodology based on ab initio evolutionary algorithms and spin-polarized density functional theory was developed to predict two-dimensional magnetic materials. Its application to a model system borophene reveals an unexpected rich magnetism and polymorphism. A metastable borophene with nonzero thickness is an antiferromagnetic semiconductor from first-principles calculations, and can be further tuned into a half-metal by finite electron doping. In this borophene, the buckling and coupling among three atomic layers are not only responsible for magnetism, but also result in an out-of-plane negative Poisson's ratio under uniaxial tension, making it the first elemental material possessing auxetic and magnetic properties simultaneously.

Atomic diffusion at nanometer length scale may differ significantly from bulk diffusion, and may sometimes even exhibit counterintuitive behavior. In the present work, taking Cu/Ni as a model system, a general phenomenon is reported which results in sharpening of interfaces upon thermal annealing, even in miscible systems. Anomalous x-ray reflectivity from a Cu/Ni multilayer has been used to study the evolution of interfaces with thermal annealing. Annealing at 423 K results in sharpening of interfaces by about 38%. This is the temperature at which no asymmetry exists in the interdiffusivities of Ni and Cu. Thus, the effect is very general in nature, and is different from the one reported in the literature, which requires a large asymmetry in the diffusivities of the two constituents [Erdélyi et al., Science 306, 1913 (2004)]. The general nature of the effect is conclusively demonstrated using isotopic multilayers of ${}^{57}\mathrm{Fe}/{}^{\mathrm{natural}}\mathrm{Fe}$, in which evolution of isotopic interfaces has been observed using nuclear resonance reflectivity. It is found that annealing at suitably low temperature (e.g., 523 K) results in the sharpening of the isotopic interfaces. Since chemically it is a single Fe layer, any effect associated with concentration dependent diffusivity can be ruled out. The results can be understood in terms of fast diffusion along short-circuit paths like triple junctions, which results in an effective sharpening of the interfaces at relatively low temperatures.

Spin accumulation can be generated via the spin Hall effect in a nonmagnetic material. It was previously found that spin accumulation and associated spintronic phenomena are attenuated in metallic bilayer with opposite spin angles. Here, we investigate the spin Hall magnetoresistance (SMR) in $\mathrm{Pt}/{\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) with a thin W capping layer, in which W and Pt have opposite spin Hall angles. We find an increase of the SMR ratio with a W capping layer thinner than 1.0 nm. According to theoretical simulation, we attribute this phenomenon to the enhancement of the spin accumulation at the Pt/YIG interface, opposite to previous observations. Our findings provide a new approach for generating spin accumulation and associated pure spin current intensity for spintronic applications.

We report that atomic-layer alloying (intermixing) at a Pt/Co interface can increase, by approximately 30%, rather than degrade the interfacial spin transparency, and thereby strengthen the efficiency of the dampinglike spin-orbit torque arising from the spin Hall effect in the Pt. At the same time, this interfacial alloying substantially reduces fieldlike spin-orbit torque. Insertion of an ultrathin magnetic alloy layer at heavy-metal/ferromagnet interfaces represents an effective approach for improving interfacial spin transparency that may enhance not only spin-orbit torques but also the spin detection efficiency in inverse spin Hall experiments.

A theoretical formulation for spin transport through an antiferromagnetic (AF) insulator is presented in a case that is driven/detected by the direct/inverse spin Hall effect in two heavy-metal contacts. The spin signal is shown to be transferred by the ferromagnetic correlation function of the antiferromagnet, which is calculated based on a magnon representation. To cover the high-temperature regimes, we include an auxiliary field representing short AF correlations and a temperature-dependent damping due to magnon scattering. The diffusion length for spin is long, close to the degeneracy of the two AF magnons, and has a maximum as a function of temperature near the Néel transition.

We extend the Anderson impurity model to a large-spin Fermi system with spin $f=\frac{3}{2}$, stimulated by the realization of large-spin ultracold Fermi atoms. The condition required for the spontaneous formation of local magnetic moments is examined and the ground-state mean-field magnetic phase diagram is explored carefully. We find that the spin-$\frac{3}{2}$ Fermi system involves magnetic phase regions I, II, and III that correspond to one, two, and three particle/hole occupation, respectively. In addition, it is observed that all the three magnetic phases have fourfold-degenerate ground states. Finally, the phase transition between the three phases seems to be of the first order.

Motivated by recent studies on current-driven domain-wall motion we have explored the dispersion of spin waves (magnons) in ultrathin nickel/cobalt multilayers. The layers are grown epitaxially on Cu(100) surfaces and consist of $n$ nickel single-atom layers, each topped by a cobalt bilayer with $n=2$, 3, and $4\left(n\times \mathrm{N}{\mathrm{i}}_1\mathrm{C}{\mathrm{o}}_2\right)$. Layers of the type $2\times {\mathrm{Ni}}_2{\mathrm{Co}}_1$ are also studied. While parallel to the film plane the magnon dispersion is nearly equal to that of pure cobalt films, magnons with wave vectors perpendicular to the film plane (standing waves) are considerably softened. The softening is attributed to a reduction of the effective interlayer exchange coupling between the two layers next to the interface with the Cu(100) substrate.

We investigate and compare the electronic structure of a bulk single crystal of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ garnet [YIG, a high-$T_C$ (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature $T_C=380\mathrm{K}$, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe $2p$ HAXPES spectrum of the bulk single crystal exhibits a purely trivalent ${\mathrm{Fe}}^{3+}$ state for octahedral and tetrahedral sites. The Fe $3s$ spectrum shows a clear splitting which allows us to estimate the on-site Fe $3s\text{−}3d$ exchange interaction energy. The valence band HAXPES spectrum shows Fe $3d$, O $2p$, and Fe $4s$ derived features and a band gap of $\sim 2.3\mathrm{eV}$ in the occupied density of states, consistent with the known optical band gap of $\sim 2.7\mathrm{eV}$. Fe $L$-edge XAS identifies the octahedral ${\mathrm{Fe}}^{3+}$ and tetrahedral ${\mathrm{Fe}}^{3+}$ site features. XMCD spectra at the Fe $L_{2,3}$ edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral- and tetrahedral-site spins. The calculated Fe $2p$ HAXPES, Fe $L$-edge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite ${\mathrm{Fe}}^{2+}$ contribution and a reduced ${\mathrm{Fe}}^{3+}$ local spin moment. The ${\mathrm{Fe}}^{2+}$ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of $\sim 6$ nm. The presence of ${\mathrm{Fe}}^{2+}$ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.

We report the enhancement of the spin Hall effect in a (111)-oriented epitaxial $\alpha$-Ta/CoFeB bilayer system, which was previously believed to show the small spin Hall angle (SHA). The SHA of the epitaxial $\alpha$-Ta/CoFeB structure estimated by spin Hall magnetoresistance measurement is 0.15, a value that is 1.5 times larger than the conventional amorphous-Ta/CoFeB structure. Effective SHA estimated by harmonic Hall measurement qualitatively supports the SMR result, whereas the strength of the field-like field of epitaxial $\alpha$-Ta samples is similar to that of amorphous Ta samples. According to the resistivity dependence on the SHA, the origin of the enhanced SHA in epitaxial $\alpha$-Ta is due to the extrinsic contribution to the SHA.

Landau's theory of the Fermi liquid is adapted to analyze the impact of electron-electron (interactions on the deficit of the superfluid density ${\rho }_{s0}={\rho }_s(T=0)$ in dirty superconducting electron systems in which the damping $\gamma$ of single-particle excitations exceeds the zero-temperature BCS gap ${\mathrm{\Delta }}_0$. In the dirty strong-coupling limit $\gamma /{\mathrm{\Delta }}_0\gg 1,m^{*}/m_e\gg 1$, the formula derived for ${\rho }_{s0}$ is shown to coincide with the well-known empirical Uemura relation provided pair-breaking contributions are nonexistent. The roles of the crystal lattice and magnetic pair-breaking effects in the observed decline of the zero-temperature superfluid density ${\rho }_{s0}$ in overdoped ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ compounds are also discussed, and our procedure is applied to elucidation of results from the pioneering experimental studies performed recently by Bozovic and collaborators in these compounds.

The Kondo and periodic Anderson model (PAM) are known to provide a microscopic picture of many of the fundamental properties of heavy-fermion materials and, more generally, a variety of strong correlation phenomena in $4f$ and $5f$ systems. In this paper, we apply the determinant quantum Monte Carlo method to include disorder in the PAM, specifically the removal of a fraction $x$ of the localized orbitals. We determine the evolution of the coherence temperature $T^{*}$, where the local moments and conduction electrons become entwined in a heavy-fermion fluid, with $x$ and with the hybridization $V$ between localized and conduction orbitals. We recover several of the principal observed trends in $T^{*}$ of doped heavy fermions, and we also show that, within this theoretical framework, the calculated nuclear magnetic resonance relaxation rate tracks the experimentally measured behavior in pure and doped ${\mathrm{CeCoIn}}_5$. Our results contribute to important issues in the interpretation of local probes of disordered, strongly correlated systems.

In this paper we report low-temperature magnetic properties of the rare-earth perovskite material ${\mathrm{YbAlO}}_3$. Results of elastic and inelastic neutron scattering experiment, magnetization measurements along with the crystalline electrical field (CEF) calculations, suggest that the ground state of Yb moments is a strongly anisotropic Kramers doublet, and the moments are confined in the $ab$ plane, pointing at an angle of $\phi =\pm 23.5^{\circ }$ to the $a$ axis. With temperature decreasing below $T_{\mathrm{N}}=0.88$ K, Yb moments order into the coplanar but noncollinear antiferromagnetic (AFM) structure $AxGy$, where the moments are pointed along their easy axes. In addition, we highlight the importance of the dipole-dipole interaction, which selects the type of magnetic ordering and may be crucial for understanding magnetic properties of other rare-earth orthorhombic perovskites. Further analysis of the broad diffuse neutron scattering shows that one-dimensional interaction along the $c$ axis is dominant and suggests ${\mathrm{YbAlO}}_3$ as a new member of one-dimensional quantum magnets.

The Fe electronic structure and magnetism in (i) monoclinic ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ with a metal-insulator transition at $T_{MI}\sim 140\mathrm{K}$ and (ii) quasicubic half-metallic ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe $L_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ and intermediate between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$) despite being related by an isoelectronic (${\mathrm{Ca}}^{2+}/{\mathrm{Ba}}^{2+}$) substitution. For ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$, the powder-averaged Fe spin moment along the field direction ($B=5\mathrm{T}$), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For $B=1\mathrm{T}$, the unsaturated XMCD signal is reduced below $T_{MI}$ consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$, the field-aligned Fe spins are larger than for ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ ($B=5\mathrm{T}$) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at $T_C^{Ba}=305\mathrm{K}$. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re $5d$ electrons on the Fe $3d$ local magnetism of these Fe/Re double perovskites.

We study the quantum critical phenomena emerging at the transition from triple-Weyl semimetal to band insulator, which is a topological phase transition described by the change of topological invariant. The critical point realizes a new type of semimetal state in which the fermion dispersion is cubic along two directions and quadratic along the third. Our renormalization group analysis reveals that the Coulomb interaction is marginal at low energies and even arbitrarily weak Coulomb interaction suffices to induce an infrared fixed point. We compute a number of observable quantities, and show that they all exhibit non-Fermi-liquid behaviors at the fixed point. When the interplay between the Coulomb and short-range four-fermion interactions is considered, the system becomes unstable below a finite-energy scale. The system undergoes a first-order topological transition when the fermion flavor $N$ is small, and enters into a nematic phase if $N$ is large enough. Non-Fermi-liquid behaviors are hidden by the instability at low temperatures, but can still be observed at higher temperatures. Experimental detection of the predicted phenomena is discussed.

We study theoretically a lattice of locally bistable driven-dissipative nonlinear cavities. The system is found to resemble the classical Ising model and enables its effective simulation. First, we benchmark the performance of driven-dissipative nonlinear cavities for spin-glass problems, and study the scaling of the ground-state-energy deviation and success probability as a function of system size. Next, we show how an effective bias field can be included in an optical model and use it for probabilistic solving of optimization problems. As particular examples we consider NP-hard problems embedded in the Ising model, namely graph partitioning and the knapsack problem. Finally, we confirm that locally bistable polariton networks act as classical optimizers and can potentially provide an improvement within the exponential complexity class.

We introduce a photonic nanostructure made of two-dimensional materials that can lead to non-Markovian dynamics in the spontaneous emission of a nearby quantum emitter. Specifically, we investigate the spontaneous emission dynamics of a two-level quantum emitter with picosecond free-space decay time and modeling $J$ aggregates close to a ${\mathrm{MoS}}_2$ nanodisk. Reversible population dynamics in the quantum emitter is obtained when the emitter's frequency matches the frequency of an exciton-polariton resonance created by the nanodisk. When such isolated resonances exist, decaying Rabi oscillations may occur. The overlapping of exciton-polariton resonances also affects strongly the decay dynamics at close distances to the nanodisk, giving rise to complex decaying population oscillations. At very close distances of the emitter to the nanodisk, the ultra-strong-coupling regime appears where, after a very fast oscillatory partial decay of the initial population, the emitter rest population remains constant over long times and population trapping occurs. The size and material quality of the nanodisk is shown to be of lesser influence on the above results.

Metasurfaces allow for agile manipulation of incoming light using a single layer of resonators. Despite recent progress, it remains difficult to generate new spectral components using nonlinear surfaces, because of the limited interaction length of a pulse traveling through a single surface. Time-dependent surfaces offer an exciting alternative to overcome this limitation, but a self-consistent framework to describe this mechanism is lacking. Based on an analytical model and finite-difference time-domain numerical simulations, we obtain physical insight into the frequency-shifting process that occurs when broadband electromagnetic pulses interact with time-varying surfaces. In particular, we find that there is an intriguing relationship between the bandwidth of the incident pulse, the targeted frequency shift, and the number of Lorentzian resonators that need to be implemented. We also demonstrate that in certain parameter regimes, pulse distortion and a deviation of pulse amplitude cannot be avoided. These results are independent of the mechanism that generates the time dependence. They are also independent of the frequency, the geometry, size, and material of the unit cell, but they are rather a direct result of the subtle interplay between time-dependent Lorentzian resonances.