We present a comparative, theoretical study of the doping dependence of the critical temperature $T_C$ of the ferromagnetic insulator-metal transitions in Gd-doped and O-deficient EuO, respectively. The strong $T_C$ enhancement in ${\text{Eu}}_{1-x}{\text{Gd}}_x\text{O}$ is due to Kondo-like spin fluctuations on the Gd sites, which are absent in ${\text{EuO}}_{1-x}$. Moreover, we find that the $T_C$ saturation in ${\text{Eu}}_{1-x}{\text{Gd}}_x\text{O}$ for large $x$ is due to a reduced activation of dopant electrons into the conduction band, in agreement with experiments, rather than antiferromagnetic long-range contributions of the RKKY interaction. The results shed light on possibilities for further increasing $T_C$.

We study the quantum dynamics of two quantum dots (QDs) or artificial atoms coupled through the fundamental localized plasmon of a gold nanorod resonator. We derive an intuitive and efficient time-local master equation, in which the effect of the metal nanorod is taken into consideration self-consistently using a quasinormal mode (QNM) expansion technique of the photon Green function. Our efficient QNM technique offers an alternative and more powerful approach over the standard Jaynes-Cummings model, where the radiative decay, nonradiative decay, and spectral reshaping effect of the electromagnetic environment is rigorously included in a clear and transparent way. We also show how one can use our approach to compliment the approximate Jaynes-Cummings model in certain spatial regimes where it is deemed to be valid. We then present a study of the quantum dynamics and photoluminescence spectra of the two plasmon-coupled QDs. We first explore the non-Markovian regime, which is found to be important only on the ultrashort time scale of the plasmon mode which is about 40 fs. For the field free evolution case of excited QDs near the nanorod, we demonstrate how spatially separated QDs can be effectively coupled through the plasmon resonance and we show how frequencies away from the plasmon resonance can be more effective for coherently coupling the QDs. Despite the strong inherent dissipation of gold nanoresonators, we show that qubit entanglements as large as 0.7 can be achieved from an initially separate state, which has been limited to less than 0.5 in previous work for weakly coupled reservoirs. We also study the superradiance and subradiance decay dynamics of the QD pair. Finally, we investigate the rich quantum dynamics of QDs that are incoherently pumped, and study the polarization dependent behavior of the emitted photoluminescence spectrum where a double-resonance structure is observed due to the strong photon exchange interactions. Our general quantum plasmonics formalism can easily be extended to include multiple QDs interacting through the QNMs of metallic resonator structures, fully accounting for radiative and nonradiative coupling, as well as nonlinear light-matter interaction processes.

The analytical solution for the low-temperature $1/f$ noise in the microwave dielectric constant of amorphous films at frequency ${\nu }_0\sim 5\mathrm{GHz}$ due to tunneling two-level systems (TLSs) is derived within the standard tunneling model including the weak dipolar or elastic TLS-TLS interactions. The $1/f$ frequency dependence is caused by TLS spectral diffusion characterized by the width growing logarithmically with time. Temperature and field dependencies are predicted for the noise spectral density in typical glasses with universal TLSs. The satisfactory interpretation of the recent experiment by J. Burnett et al. [Nat. Commun. 5, 4119 (2014)] in Pt-capped Nb superconducting resonators is attained by assuming a smaller density of TLSs compared to ordinary glasses, which is consistent with the very high internal quality factor in those samples.

We report the investigation of the generation and detection of GHz coherent acoustic phonons in plasmonic gold nanoparticle superlattices (NPSs). The experiments have been performed with an optical femtosecond pump-probe scheme across the optical plasmon resonance of the superlattice. Our experiments allow us to estimate first the fundamental mechanical parameters such as the collective elastic response (sound velocity) of the NPS and the nanocontact elastic stiffness. Furthermore, it appears that the light-induced coherent acoustic-phonon pulse has a typical in-depth spatial extension of about 45 nm which is roughly four times the optical skin depth in gold. The modeling of the transient optical reflectivity indicates that the mechanism of phonons generation is achieved through ultrafast heating of the NPS assisted by light excitation of the volume plasmon polariton. Based on these results, we demonstrate that it is possible to map the photon-electron-phonon interaction in subwavelength nanostructures which, in particular, provides insights on the fundamental properties of these nanometamaterials.

Recently, topological superconducting states have attracted much interest. In this paper, we consider a topological superconductor with $Z_2$ topological mirror order [Y.-Y. Tai et al., Phys. Rev. B 91, 041111(R) (2015)] and $s_{\pm }$-wave superconducting pairing symmetry, within a two-orbital model originally designed for iron-based superconductivity [Y.-Y. Tai  et al., Europhys. Lett. 103, 67001 (2013)]. We predict the existence of gapless edge states. We also study the local electronic structure around an adsorbed interstitial magnetic impurity in the system, and find the existence of low-energy in-gap bound states even with a weak spin polarization on the impurity. We also discuss the relevance of our results to a recent scanning tunneling microscopy experiment on a Fe(Te,Se) compound with an adsorbed Fe impurity [J.-X. Yin et al., Nat. Phys. 11, 543 (2015)], for which our density functional calculations show the Fe impurity is spin polarized.

We calculate a supercurrent across a two-dimensional topological insulator subjected to an external magnetic field. When the edge states of a narrow two-dimensional topological insulator are hybridized, an external magnetic field can close the hybridization gap, thus driving a quantum phase transition from insulator to semimetal states of the topological insulator. We find a sign reversal of the supercurrent at the quantum phase transition revealing intrinsic properties of topological insulators via the Josephson effect.

We determine, using a self-consistent method, the charge and spin currents in ballistic Josephson junctions consisting of several ferromagnetic ($F$) layers sandwiched between superconducting ($S$) electrodes (SFS-type junctions). When there are two $F$ layers, we also consider the experimentally relevant configuration where a normal ($N$) nonmagnetic spacer layer separates them. We study the current-phase relationships as functions of geometrical parameters that are accessible experimentally including particularly the angles that characterize the relative orientation of the magnetization in the $F$ layers. Our self-consistent method ensures that the proper charge conservation laws are satisfied, and that important proximity effects are fully and properly accounted for. We find that as we vary the phase difference $\mathrm{\Delta }\phi$ between the two outer $S$ electrodes, multiple harmonics in the current-phase relations emerge, the extent of which depends on the interface scattering strength and on the relative $F$ layer widths and magnetization orientations. By manipulating the relative $F$ layer magnetization orientations, we find that the charge supercurrent can reverse directions or vanish altogether. These findings are discussed in the context of the generation and long-range nature of triplet pair correlation within these structures. We also investigate the spin currents and associated spin-transfer torques throughout the entire junction regions. For noncollinear relative magnetizations, the nonconserved spin currents in a given $F$ region give rise to net torques that can switch directions at particular magnetic configurations or $\mathrm{\Delta }\phi$ values. The details of the spin current behavior are shown to depend strongly on the degree of magnetic inhomogeneity in the system, including the number of $F$ layers and the relative widths of the $F$ and $N$ layers.

We have studied orbital dynamics in the spin liquid candidate $\mathrm{B}{\mathrm{a}}_3\mathrm{CuS}{\mathrm{b}}_2{\mathrm{O}}_9$ using multifrequency electron-spin resonance. We prepared two high-quality single crystals. The crystal with a slight copper deficiency shows a structural phase transition at around 200 K due to the cooperative Jahn-Teller effect, accompanied with orbital ordering. In contrast, the crystal with almost perfect stoichiometry shows no orbital ordering down to the lowest temperature of 1.5 K. Dramatic change in the $g$ factor anisotropy as a function of frequency and temperature demonstrates orbital quantum fluctuations at a nearly constant time scale of $\sim 100\mathrm{ps}$ below 20 K, evidencing the emergence of an orbital liquid state in this quantum spin liquid compound.

We investigate temperature- and composition-dependent structural and dynamical properties of Al-Au melts. Experiments are performed to obtain accurate density and viscosity data. The system shows a strong negative excess volume, similar to other Al-based binary alloys. We develop a molecular-dynamics (MD) model of the melt based on the embedded-atom method (EAM), gauged against the available experimental liquid-state data. A rescaling of previous EAM potentials for solid-state Au and Al improves the quantitative agreement with experimental data in the melt. In the MD simulation, the admixture of Au to Al can be interpreted as causing a local compression of the less dense Al system, driven by less soft Au–Au interactions. This local compression provides a microscopic mechanism explaining the strong negative excess volume of the melt. We further discuss the concentration dependence of self- and interdiffusion and viscosity in the MD model. Al atoms are more mobile than Au, and their increased mobility is linked to a lower viscosity of the melt.

We investigate the dynamical properties of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. For five effective models proposed for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, we numerically calculate dynamical structure factors (DSFs) with an exact diagonalization method. An effective model obtained from ab initio calculations explains inelastic neutron scattering experiments adequately. We further calculate excitation modes based on linearized spin-wave theory. The spin-wave excitation of the effective models obtained by ab initio calculations disagrees with the low-lying excitation of DSFs. We attribute this discrepancy to the location of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ in a parameter space close to the phase boundary with the Kitaev spin-liquid phase.

We performed inelastic neutron scattering experiments on ${\mathrm{Cu}}_2{}^{114}\mathrm{Cd}{}^{11}{\text{B}}_2{\mathrm{O}}_6$ powder. The magnetic excitations at low temperatures are similar to those of the interacting spin-$\frac{1}{2}$ tetramers in the ordered state. The weak excitations existing above 3 meV suggest that the Higgs mode appears in ${\mathrm{Cu}}_2{\mathrm{CdB}}_2{\mathrm{O}}_6$ at ambient pressure and zero magnetic field. We evaluated $J_1=27.3\pm 1.0$ and $J_2=-14.0\pm 1.4$ meV for the intratetramer interactions and $J_3=-0.4\pm 0.2$ and $J_4=1.4\pm 0.2$ meV for the intertetramer interactions. The spin gap in the isolated spin tetramer was calculated to be 1.6 meV, which is less than the effective intertetramer interaction value $(3.6\pm 0.8$ meV). Therefore, antiferromagnetic long-range order is possible, although the ground state of the isolated spin tetramer is the spin-singlet state. We discuss the temperature dependence of the magnetic excitations.

We report on the anisotropic properties of Pauli-limited superconductivity (SC) and antiferromagnetism (AFM) in the solid solutions $\text{CeCo}{\left({\text{In}}_{1-x}{\text{Zn}}_x\right)}_5$ $\left(x\le 0.07\right)$. In $\text{CeCo}{\left({\text{In}}_{1-x}{\text{Zn}}_x\right)}_5$, the SC transition temperature $T_c$ is continuously reduced from 2.3 K $\left(x=0\right)$ to $\sim 1.4$ K $\left(x=0.07\right)$ by doping Zn, and then the AFM order with the transition temperature of $T_N\sim 2.2\text{K}$ develops for $x$ larger than $\sim 0.05$. The present thermal, transport, and magnetic measurements under magnetic field $B$ reveal that the substitution of Zn for In yields little change of low-temperature upper critical field ${\mu }_0{H}_{c2}$ for both the tetragonal $a$ and $c$ axes, while it monotonically reduces the SC transition temperature $T_c$. In particular, the magnitudes of ${\mu }_0{H}_{c2}$ at the nominal Zn concentration of $x=0.05$ (measured Zn amount of $\sim 0.019)$ are 11.8 T for $B\left|\right|a$ and 4.8 T for $B\left|\right|c$, which are as large as those of pure compound though $T_c$ is reduced to 80% of that for $x=0$. We consider that this feature originates from a combination of both an enhanced AFM correlation and a reduced SC condensation energy in these alloys. It is also clarified that the AFM order differently responds to the magnetic field, depending on the field directions. For $B\left|\right|c$, the clear anomaly due to the AFM transition is observed up to the AFM critical field of $\sim 5$ T in the thermodynamic quantities, whereas it is rapidly damped with increasing $B$ for $B\left|\right|a$. We discuss this anisotropic response on the basis of a rich variety of AFM modulations involved in the Ce115 compounds.

The radiation dynamics of optical emitters can be manipulated by properly designed material structures modifying local density of photonic states, a phenomenon often referred to as the Purcell effect. Plasmonic nanorod metamaterials with hyperbolic dispersion of electromagnetic modes are believed to deliver a significant Purcell enhancement with both broadband and nonresonant nature. Here, we have investigated finite-size resonators formed by nanorod metamaterials and shown that the main mechanism of the Purcell effect in such resonators originates from the supported hyperbolic modes, which stem from the interacting cylindrical surface plasmon modes of the finite number of nanorods forming the resonator. The Purcell factors delivered by these resonator modes reach several hundreds, which is up to 5 times larger than those in the ε-near-zero regime. It is shown that while the Purcell factor delivered by the Fabry-Pérot modes depends on the resonator size, the decay rate in the ε-near-zero regime is almost insensitive to geometry. The presented analysis shows a possibility to engineer emission properties in structured metamaterials, taking into account their internal composition.

We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe pnictides $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ $(A=\mathrm{K},\text{Rb},\text{Cs})$ as well as the fictitious ${\text{FrFe}}_2{\text{As}}_2$ and $a$-axis stretched ${\text{CsFe}}_2{\text{As}}_2$. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling $J_H$ and on-site Coulomb repulsion $U$ in multiorbital Fe pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a nontrivial change in the iron $3d$ hoppings, an increase of orbitally-selective correlations, and the presence of incoherent weight at high binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund's metal. This is especially prominent in $ab$-stretched ${\text{CsFe}}_2{\text{As}}_2$. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on $J_H$, and we provide estimates of the coherence scale $T^{*}$. We discuss these results in the framework of reported experimental observations.

The electronic structure of (group-III) metal-monochalcogenide monolayers exhibits many unusual features. Some, such as the unusually distorted upper valence-band dispersion we describe as a “caldera,” are primarily the result of purely orbital interactions. Others, including spin splitting and wave-function spin mixing, are directly driven by spin-orbit coupling. We employ elementary group theory to explain the origin of these properties, and use a tight-binding model to calculate the phenomena enabled by them, such as the band-edge carrier effective $g$ factors, optical absorption spectrum, conduction electron spin orientation, and a relaxation-induced upper-valence-band population inversion and spin polarization mechanism.

$\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ is a transparent wide-band-gap semiconductor that has attracted considerable interest in recent years due to its suitable electrical conductivity and transparency in the ultraviolet spectral region. In this work we investigate the electronic properties of the near valence-band-edge region for semiconducting $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ (100) bulk single crystals using core-level photoelectron spectroscopy and ab initio theory within the framework of density functional theory and the $\mathit{GW}$ approach. We find good agreement between the experimental results and the theoretical calculations. This is explained by the hybridization of the Ga $3d$ and O $2s$ states, similar as for ${\mathrm{In}}_2{\mathrm{O}}_3$.

Shot noise of electrons that are transmitted with probability $T$ through a quantum point contact (biased at a voltage $V_0$) leads to a fluctuating current that in turn emits radiation in the microwave regime. By calculating the Fano factor $F$ for the case where only a single channel contributes to the transport, it has been shown that the radiation produced at finite frequency ${\omega }_0$ close to $e{V}_0/\hslash$ and at low temperatures is nonclassical with sub-Poissonian statistics ($F<1$). The origin of this effect is the fermionic nature of the electrons producing the radiation, which reduces the probability of simultaneous emission of two or more photons. However, the Fano factor, being a time-averaged quantity, offers only limited information about the system. Here, we calculate the second-order coherence $g^{\left(2\right)}\left(\tau \right)$ for this source of radiation. We show that due to the interference of two contributions, two photon processes (leading to bunching) are completely absent at zero temperature for $T=50\%$. At low temperatures, we find a competition of the contribution due to Gaussian current-current fluctuations (leading to bunching) with the one due to non-Gaussian fluctuations (leading to antibunching). At slightly elevated temperatures, the non-Gaussian contribution becomes suppressed, whereas the Gaussian contributions remain largely independent of temperature. We show that the competition of the two contributions leads to a nonmonotonic behavior of the second-order coherence as a function of time. As a result, $g^{\left(2\right)}\left(\tau \right)$ obtains a minimal value for times ${\tau }^{*}\simeq {\omega }_0^{-1}$. Close to this time, the second-order coherence remains below 1 at temperatures where the Fano factor is already above 1. We identify realistic experimental parameters that can be used to test the sub-Poissonian nature of the radiation.

We observe exceptionally high diffusion of excitons in diamond using time-resolved photoluminescence imaging. The diffusivity is found to increase with decreasing temperature more rapidly than well-established predictions. The highest diffusion constant, $9.2\times {10}^3{\mathrm{cm}}^2/\text{s}$, measured for thermalized excitons is 150 times that recorded previously in diamond. We elucidate the momentum relaxation mechanisms determining transport: intraband and interband scattering by acoustic phonons in the exciton fine-structure levels. The enhanced diffusivity above 100 K is explained by a free-carrier contribution.