We investigate a periodic array of aluminum nanoantennas embedded in a light-emitting slab waveguide. By varying the waveguide thickness, we demonstrate the transition from weak to strong coupling between localized surface plasmons in the nanoantennas and refractive index guided modes in the waveguide. We experimentally observe a nontrivial relationship between extinction and emission dispersion diagrams across the weak- to strong-coupling transition. These results have implications for a broad class of photonic structures where sources are embedded within coupled resonators. For nanoantenna arrays, strong versus weak coupling leads to drastic modifications of radiation patterns without modifying the nanoantennas themselves, thereby representing an unprecedented design strategy for nanoscale light sources.

The structure of 0.8 ML In coverage on the Si(111)-$\sqrt{3}\times \sqrt{3}$-Au surface is investigated by scanning tunneling microscopy/spectroscopy and low energy electron diffraction. By depositing 1.2 ML In at room temperature followed by annealing at $500{}^{\circ }\mathrm{C}$, the surface reveals the $\sqrt{7}\times \sqrt{3}$ structure with 0.8 ML In coverage. The detailed atomic structure is studied by both the topography and $dI/dV$ images, and the results are further supported by the density functional theory. The observed structure has higher In coverage than the previous experiments and has not been proposed in the previous calculations.

We consider a large quantum system with spins $\frac{1}{2}$ whose dynamics is driven entirely by measurements of the total spin of spin pairs. This gives rise to a dissipative coupling to the environment. When one averages over the measurement results, the corresponding real-time path integral does not suffer from a sign problem. Using an efficient cluster algorithm, we study the real-time evolution from an initial antiferromagnetic state of the two-dimensional Heisenberg model, which is driven to a disordered phase, not by a Hamiltonian, but by sporadic measurements or by continuous Lindblad evolution.

Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area $(\ge 10\pi )$ near resonant optical pulses, we experimentally demonstrate how phonon-mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity $\ge 70\%$. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher-fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition, we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon-mediated processes are shown to facilitate direct biexciton preparation via two-photon biexciton absorption, with fidelities $>80\%$. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot–phonon bath interactions with Bloch-Redfield theory.

Recent experiments provide evidence for photocurrent generation in Dirac systems such as topological-insulator surface states and graphene. Within the simplest picture, the magnitude of the photocurrents is governed by the competition between photoexcitation of particle-hole pairs and current relaxation by scattering. Here, we study the relaxation of photocurrents by electron-electron $(e-e)$ collisions, which should dominate in clean systems. We compute the current relaxation rate as a function of the initial energies of the photoexcited carriers and the Fermi energy. For a positive Fermi energy, we find that collisions of a single excited electron with the Fermi sea can substantially increase the current, while for a single excited hole the current initially decreases. Together these processes partially cancel leading to a relative suppression of the relaxation of the total photocurrent carried by an electron-hole pair. We also analyze the limit of many scattering events and find that while $e-e$ collisions initially reduce the current associated with a single hole, the current eventually reverses sign and becomes as large in magnitude as in the electron case. Thus, for photoexcited electron-hole pairs, the current ultimately relaxes to zero. We discuss schemes which may allow one to probe the nontrivial current amplification physics for individual carriers in experiment.

Carbon-based molecular semiconductors are explored for application in spintronics because their small spin-orbit coupling promises long spin lifetimes. We calculate the electronic transport from first principles through spin valves comprising bi- and tri-layers of the fullerene molecules ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, sandwiched between two Fe electrodes. The spin polarization of the current, and the magnetoresistance depend sensitively on the interactions at the interfaces between the molecules and the metal surfaces. They are much less affected by the thickness of the molecular layers. A high current polarization $(\mathrm{CP}>90\%)$ and magnetoresistance $(\mathrm{MR}>100\%)$ at small bias can be attained using ${\mathrm{C}}_{70}$ layers. In contrast, the current polarization and the magnetoresistance at small bias are vanishingly small for ${\mathrm{C}}_{60}$ layers. Exploiting a generalized Jullière model we can trace the differences in spin-dependent transport between ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ layers to differences between the molecule-metal interface states. These states also allow one to interpret the current polarization and the magnetoresistance as a function of the applied bias voltage.

Using accurate dynamic polarizabilities of the Li, Na, K, and Rb atoms, we scrutinize the thermal Casimir-Polder interactions of these atoms with a single-layered graphene. Considering the modified Lifshitz theory for material interactions, we reanalyze the dispersion coefficient $\left(C_3\right)$ values of the above atoms with graphene as functions of separation distance, gap parameter, and temperature, among which some of them were earlier studied by estimating the dynamic polarizabilities of the above atoms using the single oscillator model. All these $C_3$ coefficients have been evaluated in the framework of the Dirac model. The interactions are described for a wide range of distances and temperatures to demonstrate the changes in behavior with the varying conditions of the system. Also, sensitivities of the interactions with the gap parameters are analyzed explicitly. From these analyses, we find a suitable region where measurements of interaction potentials can be carried out so that a justifiable value for the gap parameter can be surmised.

$\mathrm{SrMn}{\mathrm{O}}_3$ is a perovskite compound which, unlike most perovskites, can be synthesized in three different but closely related polymorphs. In this paper, an experimental equation of state of the 6H polymorph is reported. The experimentally determined bulk modulus of $\mathrm{SrMn}{\mathrm{O}}_3$ increases from 115.6(11) GPa in the 4H polymorph to 143.7(17) GPa in the 6H polymorph, while density functional theory (DFT) calculations predict a further increase to 172.5(4) GPa in the 3C polymorph. In situ observations of transformations between the three known polymorphs, under high pressure and high temperature conditions, are also reported. The results are compared with extensive DFT calculations and literature, and it is demonstrated that the 6H polymorph is the thermodynamically stable phase between 5.9(3) and 18.1(2) GPa at 0 K. The effect of possible oxygen substoichiometry is also explored, using DFT. Finally, the findings are combined with the existing knowledge of the phase behavior in this system to outline where further knowledge needs to be collected before a pressure/temperature (PT) phase diagram can be constructed for this system.

Decoherence of the 795 nm ${}^3\mathrm{H}{}_6$ to ${}^3\mathrm{H}{}_4$ transition in $1\%{\mathrm{Tm}}^{3+}:{\mathrm{Y}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ (Tm:YGG) is studied at temperatures as low as 1.2 K. The temperature, magnetic field, frequency, and time scale (spectral diffusion) dependence of the optical coherence lifetime is measured. Our results show that the coherence lifetime is impacted less by spectral diffusion than other known thulium-doped materials. Photon echo excitation and spectral hole burning methods reveal uniform decoherence properties and the possibility to produce full transparency for persistent spectral holes across the entire 56 GHz inhomogeneous bandwidth of the optical transition. Temperature-dependent decoherence is well described by elastic Raman scattering of phonons with an additional weaker component that may arise from a low density of glass-like dynamic disorder modes (two-level systems). Analysis of the observed behavior suggests that an optical coherence lifetime approaching 1 ms may be possible in this system at temperatures below 1 K for crystals grown with optimized properties. Overall, we find that Tm:YGG has superior decoherence properties compared to other Tm-doped crystals and is a promising candidate for applications that rely on long coherence lifetimes, such as optical quantum memories and photonic signal processing.

Efficient manipulation of nuclear spins is important for utilizing them as qubits for quantum computing. In this work we report record high polarizations of ${}^{31}\mathrm{P}$ and ${}^{29}\mathrm{Si}$ nuclear spins in P-doped silicon in a strong magnetic field (4.6 T) and at temperatures below 1 K. We reached ${}^{31}\mathrm{P}$ nuclear polarization values exceeding $98\%$ after 20 min of pumping the high-field electron spin resonance (ESR) line with a very small microwave power of 0.4 $\mu \mathrm{W}$. We evaluate that the ratio of the hyperfine-state populations increases by three orders of magnitude after 2 hours of pumping, and an extremely pure nuclear spin state can be created, with less than 0.01 ppb impurities. A negative dynamic nuclear polarization has been observed by pumping the low-field ESR line of ${}^{31}\mathrm{P}$ followed by the flip-flip cross relaxation, the transition which is fully forbidden for isolated donors. We estimate that while pumping the ESR transitions of ${}^{31}\mathrm{P}$ also the nuclei of ${}^{29}\mathrm{Si}$ get polarized, and polarization exceeding $60\%$ has been obtained. We performed measurements of relaxation rates of flip-flop and flip-flip transitions which turned out to be nearly temperature independent. Temperature dependence of the ${}^{31}\mathrm{P}$ nuclear relaxation was studied down to 0.75 K, below which the relaxation time became too long to be measured. We found that the polarization evolution under pumping and during relaxation deviates substantially from a simple exponential function of time. We suggest that the nonexponential polarization dynamics of ${}^{31}\mathrm{P}$ donors is mediated by the orientation of ${}^{29}\mathrm{Si}$ nuclei, which affect the transition probabilities of the forbidden cross-relaxation processes.

We investigated the training effect and magnetization reversal in CoO/Co bilayer films grown epitaxially on MgO (001) substrates. The asymmetry of the magnetization reversal, which appears due to the exchange bias after field cooling, survives after training, in contrast to the case of polycrystalline bilayers. By applying hysteresis loops with the magnetic field perpendicular to the cooling field, we are able to modify the orientation of the average uncompensated magnetization of the antiferromagnetic CoO. Subsequently, the untrained state can be partially restored, and more importantly the magnetization reversal asymmetry can be inverted by starting the perpendicular loop with the appropriate field polarity. Consequently, we succeeded in manipulating the magnetization reversal asymmetry and even in achieving opposite reversal asymmetries in the same exchange bias system.

The anisotropy of the Fe orbital moment is investigated in epitaxial ultrathin ($<2\mathrm{nm}$) FePt layers by angular dependent x-ray magnetic circular dichroism (XMCD) for two distinct epitaxial systems, CoO/FePt/Pt(001) and Pt/FePt/MnPt/Pt(001). Element selective hysteresis loops obtained from the XMCD signal at the Fe $L_3$ edge are used to quantify the anisotropy constants, which are consistent with the corresponding chemical order parameter. A careful procedure for XMCD sum rules data analysis is set up to avoid experimental overestimation of the orbital moment. The measured orbital moment of Fe is found comparable to theoretical values and shows a significant anisotropy. This orbital moment anisotropy is discussed in the light of the current theoretical understanding of magnetocrystalline anisotropy in ordered alloys.

We derive an effective spin-$\frac{1}{2}$ exchange model for non-Kramers ${\mathrm{Tb}}^{3+}$ states in the pyrochlore ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. The four anisotropic nearest-neighbor exchange constants, as well as next-neighbor exchange constants, are derived for the effective model. This work goes beyond the independent tetrahedra model by considering all nearest-neighbor exchange paths on the pyrochlore lattice. Estimates of the exchange constants reveal that ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is described by a quantum spin ice Hamiltonian.

The magnetism of the A-site-ordered perovskite $\mathrm{LaM}{\mathrm{n}}_3{\mathrm{V}}_4{\mathrm{O}}_{12}$ is studied comprehensively by means of neutron powder diffraction experiments and theoretical calculations. Magnetic neutron diffraction results show that a rhombohedral 60° spin structure emerges on the cubic lattice below a 44-K Néel transition. Ab initio electronic structure calculations confirm that high-spin ${\mathrm{Mn}}^{2+}$ moments are localized while V $3d$-band states are itinerant, and that the noncollinear 60° spin structure is more stable than collinear ferromagnetic or G-type antiferromagnetic alternatives. Effective Heisenberg model calculations reveal that the appearance of such a nontrivial spin structure can be attributed to significant next-nearest-neighbor and third-nearest-neighbor magnetic interactions.

${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ epitaxial layers were grown by molecular beam epitaxy and controlled using in situ characterization tools (electron diffraction, x-ray photoelectron, and Auger spectroscopies) in order to use these alloys in magnetic tunnel junctions. Thick films are shown to be BCC up to $x=70$%, whereas thin films grown on Fe(001) are BCC over the whole Co concentration range up to a critical thickness. Fe/Co intermixing is also examined in detail. The tunnel magnetoresistance and conductance of MgO-based magnetic tunnel junctions built with BCC ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ electrodes show peculiar voltage dependencies. Their interpretation relies on spin- and symmetry-resolved photoemission measurements and ab initio calculations. The bulk and interfacial occupied states detected by photoemission are compared to the calculated ones. Experimental results and ab initio calculations are in good agreement. Finally, the experimental conductance curves are compared to the autoconvolution of the $\Delta 1$ density of states in which both bulk and interface states are taken into account.

Based on spin-dependent transport theory and thermodynamics, we develop a generalized theory of the Joule heating in the presence of a spin current. Along with the conventional Joule heating consisting of an electric current and electrochemical potential, it is found that the spin current and spin accumulation give an additional dissipation because the spin-dependent scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a change of entropy. The theory is applied to investigate the dissipation due to pure spin-current generated by spin pumping across a ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from an interface because the spin pumping is a transfer of both the spin angular momentum and the energy from the ferromagnet to conduction electrons near the interface. It is found that the dissipation is proportional to the enhancement of the Gilbert damping constant by spin pumping.

We present a theoretical study aimed at elucidating the origin of the inverse symmetry-breaking transition observed in ultrathin magnetic films with perpendicular anisotropy. We study the behavior of the dipolar frustrated Ising model in a mean field approximation as well as two other models with simple domain walls. By a numerical analysis we show that the internal degrees of freedom of the domain walls are decisive for the presence of the inverse symmetry-breaking transition. In particular, we show that in a sharp domain wall model the inverse transition is absent. At high temperatures the additional degrees of freedom of the extended domain walls increase the entropy of the system leading to a reduction of the free energy of the stripe phase. Upon lowering the temperature the domain walls become narrow, and the reduction of the number of degrees of freedom associated with them manifests in a reduction of entropy which eventually induces an inverse transition to the competing homogenous phase. We also show that, for a growing external field at constant temperature, the stripe width grows strongly when approaching the critical field line and diverges at the transition. These results indicate that the inverse transition is a continuous phase transition and that the domain wall profiles and the temperature have little effect on the critical behavior of the period of the domain as a function of the applied field.

In this paper we address the ratio of the shear viscosity to entropy density $\eta /s$ in bosonic and fermionic superfluids. A small $\eta /s$ is associated with nearly perfect fluidity, and more general measures of the fluidity perfection/imperfection are of wide interest to a number of communities. We use a Kubo approach to concretely address this ratio via low-temperature transport associated with the quasiparticles. Our analysis for bosonic superfluids utilizes the framework of the one-loop Bogoliubov approximation, whereas for fermionic superfluids we apply BCS theory and its BCS-BEC extension. Interestingly, we find that the transport properties of strict BCS and Bogoliubov superfluids have very similar structures, albeit with different quasiparticle dispersion relations. While there is a dramatic contrast between the power law and exponential temperature dependence for $\eta$ alone, the ratio $\eta /s$ for both systems is more similar. Specifically, we find the same linear dependence (on the ratio of temperature $T$ to inverse lifetime $\gamma \left(T\right))$ with $\eta /s\propto T/\gamma \left(T\right)$, corresponding to imperfect fluidity. By contrast, near the unitary limit of BCS-BEC superfluids a very different behavior results, which is more consistent with near-perfect fluidity.

We propose a simple approach to realize a topological phase transition using a spatial periodic potential. As an example, we examine the electronic structures of HgTe/CdTe quantum wells, and demonstrate that their band structures can be effectively manipulated by the periodic potential. At a critical potential, we find that a conventional band insulator undergoes a topological phase transition into a quantum spin Hall system, which is characterized by an abrupt change of the spin Chern number and emerging edge states. Our proposal provides an interesting way to dynamically turn on or off topologically protected edge states for application in switching devices.