Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson-junction-based artificial atoms, while maintaining superior coherence. We demonstrate a superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in Josephson-junction-based quantum circuits.

Although defect clusters are detrimental to the electronic and mechanical properties of semiconductor materials, annihilation of such clusters is limited by their lack of thermal mobility due to high migration barriers. Here, we find that small clusters in bulk SiC (a covalent material of importance for both electronic and nuclear applications) can become mobile at room temperature under the influence of electron radiation. So far, direct observation of radiation-induced diffusion of defect clusters in bulk materials has not yet been demonstrated. This finding was made possible by low-angle annular dark-field scanning transmission electron microscopy combined with a nonrigid registration technique to remove sample instability, which enables atomic resolution imaging of small migrating defect clusters. We show that the underlying mechanism of this athermal diffusion is a ballistic collision between incoming electrons and cluster atoms. Our findings suggest that defect clusters may be mobile under certain irradiation conditions, changing the current understanding of the cluster annealing process in irradiated covalent materials.

In this paper, we study the doping effects on the magnetic states of $\mathrm{CuCr}{\mathrm{O}}_2$ based on the classical frustrated spin model [Lin et al., Phys. Rev. B 89, 220405(R) (2014)]. Several experimental observations can be well reproduced by the Monte Carlo simulations of the modified spin models. Our paper suggests that the disorder induced by V/Al doping cooperating with the frustration in the system may contribute to the emergence of the spin glass state. Furthermore, the hole doping by $\mathrm{M}{\mathrm{g}}^{2+}$ substituting $\mathrm{C}{\mathrm{r}}^{3+}$ enhances the quantum fluctuations and bond disorder which modulate the biquadratic exchanges and in turn results in the promotion of the spiral phase, consistent with the experimental report.

Paramagnetic impurities in a quantum spin liquid give rise to Kondo effects with highly unusual properties. We have studied the effect of locally coupling a paramagnetic impurity with the spin-$\frac{1}{2}$ honeycomb Kitaev model in its gapless spin-liquid phase. The (impurity) scaling equations are found to be insensitive to the sign of the coupling. The weak and strong coupling fixed points are stable, with the latter corresponding to a noninteracting vacancy and an interacting, spin-1 defect for the antiferromagnetic and ferromagnetic cases, respectively. The ground state in the strong coupling limit in both cases has a nontrivial topology associated with a finite $Z_2$ flux at the impurity site. For the antiferromagnetic case, this result has been obtained straightforwardly owing to the integrability of the Kitaev model with a vacancy. The strong-coupling limit of the ferromagnetic case is, however, nonintegrable, and we address this problem through exact-diagonalization calculations with finite Kitaev fragments. Our exact diagonalization calculations indicate that the weak-to-strong coupling transition and the topological phase transition occur rather close to each other and are possibly coincident. We also find an intriguing similarity between the magnetic response of the defect and the impurity susceptibility in the two-channel Kondo problem.

We demonstrate that the induced toroidal dipole, represented by currents flowing on the surface of a torus, makes a distinct and indispensable contribution to circular dichroism. We show that toroidal circular dichroism supplements the well-known mechanism involving electric dipole and magnetic dipole transitions. We illustrate this with rigorous analysis of the experimentally measured polarization-sensitive transmission spectra of an artificial metamaterial, constructed from elements of toroidal symmetry. We argue that toroidal circular dichroism will be found in large biomolecules with elements of toroidal symmetry and should be taken into account in the interpretation of circular dichroism spectra of organics.

The layered transition metal dichalcogenide vanadium disulfide (${\mathrm{VS}}_2$), which nominally has one electron in the $3d$ shell, is potent for strong-correlation physics and is possibly another realization of an effective one-band model beyond the cuprates. Here monolayer ${\mathrm{VS}}_2$ in both the trigonal prismatic and the octahedral phases is investigated using density functional theory plus Hubbard $U$ ($\mathrm{DFT}+U$) calculations. Trigonal prismatic ${\mathrm{VS}}_2$ has an isolated low-energy band that emerges from a confluence of crystal-field splitting and direct V-V hopping. Within spin density functional theory, ferromagnetism splits the isolated band of the trigonal prismatic structure, leading to a low-band-gap, $S=\frac{1}{2}$, ferromagnetic Stoner insulator; the octahedral phase is higher in energy. Including the on-site interaction $U$ increases the band gap, leads to Mott insulating behavior, and, for sufficiently high values, stabilizes the ferromagnetic octahedral phase. The validity of DFT and $\mathrm{DFT}+U$ for these two-dimensional materials with potential for strong electronic correlations is discussed. A clear benchmark is given by examining the experimentally observed charge density wave in octahedral ${\mathrm{VS}}_2$, for which DFT grossly overestimates the bond length differences compared to known experiments; the presence of charge density waves is also probed for the trigonal prismatic phase. Finally, we investigate why only the octahedral phase has been observed in experiments and discuss the possibility of realizing the trigonal prismatic phase. Our work suggests that trigonal prismatic ${\mathrm{VS}}_2$ is a promising candidate for strongly correlated electron physics that, if realized, could be experimentally probed in an unprecedented fashion due to its monolayer nature.

We report transport measurements in a single edge channel of an InAs/GaSb quantum spin Hall insulator, where the conduction occurs through only one pair of counterpropagating edge modes. By using a specific sample design involving highly asymmetric current paths, we electrically isolate a single edge channel of the two-dimensional topological insulator from the other edge. This enables us to probe a single edge by multiterminal measurements. Both two-terminal and four-terminal resistances show a nearly quantized plateau around $h/e^2$ for a 4-$\mu \mathrm{m}$-long edge, indicating quasiballistic transport. Our approach is advantageous in that it allows us to gain insight into a microscopic region from local measurements.

Optical forces can be used to manipulate small particles through various mechanisms. In this paper, we present a comprehensive analysis of optical forces acting on the nanoparticles located over a substrate using different manipulation techniques, as well as the conditions of the optimization of these forces. In particular, we study optical trapping, acceleration, and binding. Calculations are carried out using the exact multipole expansion method combined with Maxwell stress tensor formalism, providing a general framework to study optical forces on particles for arbitrary incident fields using closed-form expressions. The method takes into account multiple scattering between the particles and substrate and allows clear predictive abilities well beyond the dipole model. We consider the interaction of dielectric and metallic nanoparticles with various substrates. The presence of substrate is shown to have a significant impact on the nanoparticles’ resonances and provides an additional degree of freedom in tailoring the optical forces. We explore different physical processes contributing to the optical force and their interplay on the mobility of the particle. It is established that engineering layered substrates can broaden the scope of trapping and acceleration and enhance the binding forces. It can also provide a high tunability of the acceleration direction. The analysis presented in this paper provides key physical insights to identify optimum setup for nanoparticles manipulation in various applications.

We explain how the electrical current flow in a molecular junction can modify the vibrational spectrum of the molecule by renormalizing its normal modes of oscillations. This is demonstrated with first-principles self-consistent transport theory, where the current-induced forces are evaluated from the expectation value of the ionic momentum operator. We explore here the case of ${\mathrm{H}}_2$ sandwiched between two Au electrodes and show that the current produces stiffening of the transverse translational and rotational modes and softening of the stretching modes along the current direction. Such behavior is understood in terms of charge redistribution, potential drop, and elasticity changes as a function of the current.

We demonstrate structural stability of monolayer zigzag and armchair blue phosphorus nanotubes by means of molecular dynamics simulations. The vibrational spectrum and electronic band structure are determined and analyzed as functions of the tube diameter and axial strain. The nanotubes are found to be semiconductors with a sensitive indirect band gap that allows flexible tuning.

The Ruddlesden-Popper (RP) homologous series ${\mathrm{Sr}}_{n+1}{\mathrm{Ti}}_n{\mathrm{O}}_{3n+1}$ provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide ${\mathrm{SrTiO}}_3$. We use ab initio many-body perturbation theory within the $GW$ approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectra of ${\mathrm{Sr}}_{n+1}{\mathrm{Ti}}_n{\mathrm{O}}_{3n+1}$ for $n=1–5$ and $\infty$. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small $n$ and reach a value of 330 meV for $n=1$, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of ${\mathrm{Sr}}_2{\mathrm{TiO}}_4$ ($n=1$) localizes in the two-dimensional plane defined by the ${\mathrm{TiO}}_2$ layer, and we explain the origin of its localization.

We investigate the cascaded emission of photons from low-density InGaAs/GaAs quantum dots grown by metal-organic vapor-phase epitaxy that are intentionally redshifted toward telecommunication wavelengths. We observe multiple radiative cascades within a single quantum dot and attribute these to neutral and charged excited configurations. The corresponding transitions are identified by combining microphotoluminescence and photon correlation measurements. Full-configuration interaction calculations further support the identification of the emission lines and provide additional information about the confinement of electron and hole wave functions. We apply a Monte Carlo simulation to estimate the effective spin scattering rates between excited triplet and singlet ground states of the negatively charged trion. These spin-flip processes directly affect the observed radiative cascade.

We present an experimental study of the scattering mechanisms in a two-dimensional electron system which is either fully induced by the field effect or resulting from remote doping. The quality criteria—the electron mobility, the quantum scattering time, and the number and development of certain fractional quantum Hall states—are analyzed and compared. By eliminating the scattering off remote ionized impurities (RI) in undoped systems, we can identify the density regimes most susceptible to RI scattering and their impact on the formation of fractional quantum Hall states.

A generalized tight-binding model, which is based on the subenvelope functions of the different sublattices, is developed to explore the novel magnetic quantization in monolayer gray tin (tinene). The effects due to the $s{p}^3$ bonding, the spin-orbital coupling, the magnetic field, and the electric field are simultaneously taken into consideration. The unique magnetoelectronic properties lie in two groups of low-lying Landau levels, with different orbital components, localization centers, state degeneracy, spin configurations, and magnetic- and electric-field dependencies. The first and second groups mainly come from the $5p_z$ and ($5p_x,5p_y$) orbitals, respectively. Their Landau-level splittings are, respectively, induced by the electric field and spin-orbital interactions. The intragroup anticrossings are only revealed in the former. The unique tinene Landau levels are absent in graphene, silicene, and germanene.

The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and $\mathit{k}$ space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal $g$ factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms.

The electronic structure of hydrogen impurity in ${\mathrm{Lu}}_2{\mathrm{O}}_3$ was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in $f$-electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a $\text{DFT}+U$ approach which accounts for the on-site correlation of the $4f$ electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within $\text{DFT}+U$ the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in ${\mathrm{Lu}}_2{\mathrm{O}}_3$ if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei).

We analyze the thermalization of a photoexcited charge carrier coupled to a single branch of quantum phonons within the Holstein model. To this end, we calculate the far-from-equilibrium time evolution of a pure many-body state and compare it with predictions of the thermal Gibbs ensemble. We show that at strong enough carrier excitation, the nonequilibrium system evolves towards a thermal steady state. Our analysis is based on two classes of observables. First, the occupations of fermionic momenta, which are the eigenvalues of the one-particle density matrix, match in the steady state the values in the corresponding Gibbs ensemble. This indicates thermalization of static fermionic correlations on the entire lattice. Second, the dynamic current-current correlations, including the time-resolved optical conductivity, also take the form of their thermal counterparts. Remarkably, both static and dynamic fermionic correlations thermalize with identical temperatures. Our results suggest that the subsequent relaxation processes, observed in time-resolved ultrafast spectroscopy, may be efficiently described by applying quasithermal approaches, e.g., multitemperature models.

The paper studies the spatial variation of the magnetization in a nonconducting magnetic sample with an excess number of magnons in comparison to the equilibrium. The phenomenon is considered using the Landau-Lifshits equation with additional terms describing the longitudinal relaxation of the magnetization, the magnon diffusion, and the magnon creation by external pumping. The free energy of the system is presented in the mean-field approximation. It is shown that, if the pumping exceeds some critical value, regions of a new phase arise where the magnetic moments are oriented opposite to the magnetization of the magnetic sample. The phenomenon is similar to the appearance of droplets of condensed phase in a supersaturated vapor. The appearance of a new phase either in the form of a single domain or a periodical lattice is demonstrated. The studied process is a competitor to the process of the Bose-Einstein condensation of magnons.

The magnetic nature of a metallic two-dimensional triangular compound, ${\mathrm{CrSe}}_2$, has been investigated by muon spin rotation and relaxation ($\mu {}^{+}\mathrm{SR}$) measurements using both powder and single crystal samples. It is found that ${\mathrm{CrSe}}_2$ enters into a static antiferromagnetic (AF) ordered state below 157 K ($=T_{\mathrm{N}}$). Furthermore, the AF state is slightly changed below around 20 K ($=T_{\mathrm{N}2}$). Based on the analysis of the internal magnetic fields at the muon sites predicted with DFT calculations, collinear AF and helical ${120}^{\circ }$ AF are clearly eliminated for the ground state of ${\mathrm{CrSe}}_2$. The most probable one is an incommensurate spin density wave order.