We demonstrate the real-time detection of single photogenerated electrons in lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells with either a thin (20 nm) or a thick (100 nm) AlGaAs barrier layer. The observed photon energy and power dependencies of the photoelectron detection efficiency both indicate that the trapped photoelectrons are predominantly generated in the buffer layer followed by tunneling into one of the two dots for the thin barrier sample, whereas they are directly generated in the well in the thick barrier sample. Single photoelectron detection after selective excitation of the heavy- and light-hole state in the dot is well resolved in the latter case. This ensures the applicability of our quantum well-based quantum dot systems for the coherent projection from single photon polarization to single electron spin states.

We have investigated the accuracy dependence of a single-electron pump on the confinement-potential shape of a quantum dot (QD) pump. A uniquely designed QD, which employs multiple gates to control the shape of the QD potential well, is utilized for electron pumping. It has been observed that the accuracy of the pump can be dramatically enhanced by achieving smaller QD size and greater decoupling from the electrodes, which is supported by the so-called decay-cascade model. The accuracy of the pump current is estimated, based on the decay-cascade model, to be close to 0.1 ppm for 80-pA pump current when the confinement-potential shape is optimally tuned. This is the highest reported accuracy, although it is a theoretically estimated value, among QD-based pumps measured at 4.2 K in the absence of an external magnetic field. Our numerical calculations show that the enhancement of the estimated accuracy is mainly due to the increase of the electron addition energy in the QD as well as the increase of the height and thickness of the QD potential barrier.

Dimensional evolution between one- (1D) and two-dimensional (2D) topological phases is investigated systematically. The crossover from a 2D topological insulator to its 1D limit shows oscillating behavior between a 1D ordinary insulator and a 1D topological insulator. By constructing a 2D topological system from a 1D topological insulator, it is shown that there exist possibly weak topological phases in 2D time-reversal invariant band insulators, one of which can be realized in anisotropic systems. The topological invariant of the phase is $Z_2=0$. However, the edge states may appear along specific boundaries. It can be interpreted as arranged 1D topological phases, and have a symmetry-protecting nature as does the corresponding 1D topological phase. Robust edge states can exist under specific conditions. These results provide further understanding of 2D time-reversal invariant insulators, and can be realized experimentally.

Using a realistic quantum master equation, we show that the resonance fluorescence spectra of a two-level artificial atom (quantum dot) can be tuned by adjusting its photonic local density of states via biasing one or more graphene monolayers. The structured photon reservoir is included using a photon Green function theory which fully accounts for the loss and dispersion. The field-driven Mollow triplet spectrum can be actively controlled by the graphene bias in the THz frequency regime. We also consider the effect of a dielectric support environment and multiple graphene layers on the emitted fluorescence. Finally, thermal bath effects are considered and are shown to be important for low THz frequencies.

Extensive two- and three-dimensional periodic first-principles simulations have been carried out to investigate the mechanical response of graphite to hydrostatic and nonhydrostatic stress conditions. Our results show a clear analogy between uniaxial $\left({\sigma }_z\right)$ stress and hydrostatic pressure as far as structural changes in the unit cell are concerned. For intralayer C-C distances and in-plane graphite vibrational frequencies, the similarity with hydrostatic conditions is however found under biaxial $\left({\sigma }_x={\sigma }_y\right)$ stresses. The calculated uniaxial equation of state is further used to investigate sliding mechanisms of a graphite layer in graphite at different interlayer separations, thus providing insight at an atomic level on the origin of the low static friction coefficient of graphite.

We investigated film thickness dependence of domain size and transport property in ${\mathrm{VO}}_2$ thin films on rutile ${\mathrm{TiO}}_2$ (001) substrates and identified formation mechanism of the microscaled domain. It was found that domain size decreased with increasing film thickness and the domain boundary consisted of cracks and dislocations, clarified by high-resolution transmission electron microscopy. The detailed images showed, the tensile-strained ${\mathrm{VO}}_2$ lattices received by ${\mathrm{TiO}}_2$ (001) were partially relaxed around the cracks and dislocations. The relaxed lattice is likely to return the original metal-insulator transition temperature of 340 K, whereas the tensile-strained lattice has the transition at 300 K in a ${\mathrm{VO}}_2/{\mathrm{TiO}}_2$ (001) system. Thus, the mixed states of strained and relaxed crystal lattice and the increase in dislocation density in thicker films cause the overly broad resistance behavior against temperature. Furthermore, the origin of the dislocations and the thickness dependence of the domain size could be explained by the energy release of shear stress generated by competition between the pinning layers at near-interface ${\mathrm{VO}}_2$ layers holding the tetragonal structure and the near-surface layers separated from the substrate attempting the lattice transformation to a monoclinic structure. This understanding enables us to more precisely design the size and configuration of these domains and their transport properties.

Heat transport in nanoscale systems is both hard to measure microscopically, and hard to interpret. Ballistic and diffusive heat flow coexist, adding confusion. This paper looks at a very simple case: a nanoscale crystal repeated periodically. This is a popular model for simulation of bulk heat transport using classical molecular dynamics (MD), and is related to transient thermal grating experiments. Nanoscale effects are seen in perhaps their simplest form. The model is solved by an extension of standard quasiparticle gas theory of bulk solids. Both structure and heat flow are constrained by periodic boundary conditions. Diffusive transport is fully included, while ballistic transport by phonons of a long mean free path is diminished in a specific way. Heat current $J\left(x\right)$ and temperature gradient $\nabla T\left(x^{\prime }\right)$ have a nonlocal relationship, via $\kappa (x-x^{\prime })$, over a distance $|x-x^{\prime }|$ determined by phonon mean free paths. In MD modeling of bulk conductivity, finite computer resources limit system size. Long mean free paths, comparable to the scale of heating and cooling, cause undesired finite-size effects that have to be removed by extrapolation. The present model allows this extrapolation to be quantified. Calculations based on the Peierls-Boltzmann equation, using a generalized Debye model, show that extrapolation involves fractional powers of $1/L$. It is also argued that heating and cooling should be distributed sinusoidally $[\dot{e}\propto cos(2\pi x/L)]$ to improve convergence of numerics.

Using density functional calculations, we find that the newly synthesized ${\mathrm{Ca}}_2{\mathrm{FeOsO}}_6$ has the high-spin ${\mathrm{Fe}}^{3+}$ $\left(3d^5\right)-{\mathrm{Os}}^{5+}$ $\left(5d^3\right)$ state. The octahedral ${\mathrm{Os}}^{5+}$ ion has a large intrinsic exchange splitting, and its $t_{2g\uparrow }^3$ configuration makes the spin-orbit coupling ineffective. Moreover, there is a strong antiferromagnetic (AF) coupling between the neighboring ${\mathrm{Fe}}^{3+}$ $(S=\mathrm{5/2})$ and ${\mathrm{Os}}^{5+}$ $(S=–\mathrm{3/2})$, but the AF couplings within both the fcc ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Os}}^{5+}$ sublattices are one order of magnitude weaker. Therefore a magnetic frustration is suppressed and a stable ferrimagnetic ground state appears. This ferrimagnetic order is due to the virtual hopping of the $t_{2g}$ electrons from ${\mathrm{Os}}^{5+}$ $\left(t_{2g\downarrow }^3\right)$ to ${\mathrm{Fe}}^{3+}$ $\left(t_{2g\uparrow }^3{e}_{g\uparrow }^2\right)$. However, if the experimental bended ${\mathrm{Fe}}^{3+}-{\mathrm{O}}^{2-}-{\mathrm{Os}}^{5+}$ exchange path gets straight, the $e_g$ hopping from ${\mathrm{Fe}}^{3+}$ $\left(t_{2g\uparrow }^3{e}_{g\uparrow }^2\right)$ to ${\mathrm{Os}}^{5+}$ $\left(t_{2g\uparrow }^3\right)$ would be facilitated and then a ferromagnetic (FM) coupling would occur.

We report on the genuine origin of the anisotropy of the magnetization $M$ in rare-earth (RE) metals and RE-based alloys. Taking Ho-based layered nanostructures as testing ground, we prove that the anisotropy of M is substantial despite that the sixfold magnetic anisotropy constant $K_6^6$ vanishes, which contradicts the established wisdom [E. R. Callen and H. B. Callen, J. Phys. Chem. Solids 16, 310 (1960)]. Furthermore, we show that the symmetric anisotropic contributions to $M$ and $K_6^6$ vary with temperature distinctively from one another, which indicates that both anisotropic effects are unrelated and stem from dissimilar microscopic sources. Our findings are discussed according to the theory [R. J. Elliott and M. F. Thorpe, J. Appl. Phys. 39, 802 (1968)] that predicts the emergence of symmetric anisotropic indirect-exchange terms under the presence of orbital moments. We show evidence that the anisotropy of $M$ is caused by the indirect-exchange coupling among localized $4f$ magnetic moments mediated by spin-orbit coupled conduction electrons, which ultimately generates a spatially nonuniform spin polarization that replicates the lattice symmetry.

We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound ${\mathrm{NpNi}}_5$. By combining macroscopic and shell-specific techniques, we determine the $5f$-shell occupation number $n_f$ close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below $T_C=16$ K $({\mu }_S=-1.88$ ${\mu }_B$ and ${\mu }_L=3.91$ ${\mu }_B)$. The apparent coexistence of ordered and disordered phases observed in the Mössbauer spectra is explained in terms of slow relaxation between the components of a quasitriplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment $\left(3\langle T_z\rangle /\langle S_z\rangle =+1.43\right)$ is positive and large, suggesting a localized character of the $5f$ electrons. The angular part of the spin-orbit coupling $\left(\langle \vec{\ell }·\vec{s}\rangle =-5.55\right)$ is close to the value of $-6.25$ calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.

We present an optical spectroscopy study on F-substituted ${\mathrm{NdOBiS}}_2$ superconducting single crystals grown using the KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 ${\mathrm{cm}}^{-1}$ . The plasma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of $x=0.5$, but very close to the value based on a doping level of $7\%$ of itinerant electrons per Bi site as determined by the angle-resolved photoemission spectroscopy (ARPES) experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a charge-density wave (CDW) instability as being widely discussed for a doping level of $x=0.5$.

We propose a general mechanism of multiple-$Q$ ordering with noncollinear and noncoplanar spin textures in itinerant magnets. By analyzing the fourth-order perturbation with respect to the spin-charge coupling, we find that the instability toward multiple-$Q$ ordering is caused by $\left(d-2\right)$-dimensional connections of the Fermi surfaces in the extended Brillouin zone by the multiple-$Q$ wave vectors: zero(one)-dimensional point (line) connections in the two(three)-dimensional systems. The instabilities are obtained as the “fixed points” in the two-parameter flow diagram, irrespective of lattice structures. The hidden Fermi surface instability provides a universal origin of noncollinear and noncoplanar instabilities common to frustrated and unfrustrated lattice systems.

We examine the role of neutral divacancies on the electronic and atomic structure at ${\mathrm{SrTiO}}_3$ (001) surfaces using a density functional theory + $U$ approach. Our results show that the interactions between divacancies are significantly less repulsive at the SrO-terminated surface (0.05 eV) than at the ${\mathrm{TiO}}_2$-terminated one (0.38 eV), mainly due to the increased electrostatic screening at the ionic SrO layer compared to the covalently bonded ${\mathrm{TiO}}_2$ layer. The interaction energies are a nonmonotonic function of distance, with the fourth-nearest-neighbor oxygen-oxygen divacancy showing a significantly reduced repulsion at 0 K on the ${\mathrm{TiO}}_2$-terminated surface where the defects are in the equatorial oxygen plane. This enhanced reduction in the repulsive interaction is a consequence of the much larger reduction in local symmetry relative to other divacancy arrangements arising from strong coupling with in-plane octahedral distortions. On the SrO-terminated surface, due to increased electrostatic screening, the interaction energy begins to decrease beyond the third-nearest neighbor. On both surfaces, the reduced repulsion (0.05 and 0.38 eV) should permit oxygen vacancy ordering at finite temperatures. Finally, we discuss the emergence of a two-dimensional electron gas due to oxygen divacancies at both the ${\mathrm{TiO}}_2$- and SrO-terminated ${\mathrm{SrTiO}}_3$ (001) surfaces and contrast them with the case of a single oxygen vacancy. Neutral oxygen vacancies on the SrO termination lead to more electron localization than on the ${\mathrm{TiO}}_2$ surface. These results suggest an explanation for the local ordering observed in experiment, thereby highlighting the importance of ordering both for enhanced conductivity and carrier densities at oxide surfaces and at heterostructure interfaces.

The dynamics of individual magnetic skyrmions confined in ultrathin film dots is studied theoretically. The systems considered are transition-metal ferromagnets possessing perpendicular magnetic anisotropy and particular attention is given to the dynamic response of the skyrmions to perpendicular ac fields. By using micromagnetics simulations, it is shown that breathing modes can hybridize with geometrically quantized spin wave eigenmodes of the circular dots, leading to distinct features in the power spectrum that differ from the behavior expected for uniformly magnetized systems. The static field dependence of the breathing mode frequency offers a direct means of detecting and characterizing such skyrmion states in experiment.

Magnetic frustration effects in artificial kagome arrays of nanomagnets with out-of-plane magnetization are investigated using magnetic force microscopy and Monte Carlo simulations. Experimental and theoretical results are compared to those found for the artificial kagome spin ice in which the nanomagnets have in-plane magnetization. In contrast with what has been recently reported, we demonstrate that long-range (i.e., beyond nearest-neighbor) dipolar interactions between the nanomagnets cannot be neglected when describing the magnetic configurations observed after demagnetizing the arrays using a field protocol. As a consequence, there are clear limits to any universality in the behavior of these two artificial frustrated spin systems. We provide arguments to explain why these two systems show striking similarities at first sight in the development of pairwise spin correlations.

A novel physical phenomenon has been observed that resembles a large asymmetric switchinglike magnetoresistance at low applied fields in bulk metamagnetic Heusler alloys. A thermally activated isothermal forward metamagnetic transition with a signature of a pronounced time-dependent relaxation was observed in a bulk B-substituted off-stoichiometric Ni-Mn-In Heusler alloy, whereas the reverse metamagnetic transition exhibited the usual athermal behavior with no thermal activation. The asymmetry between the forward and reverse metamagnetic transitions resulted in a large switchinglike, low-field magnetoresistance (∼16% for a field change of $B$ = 0 → 0.25 T at $T$ = 304 K) in the bulk Heusler alloys, ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{In}}_{15-x}{\mathrm{B}}_x$ (1 ⩽ $x$ ⩽ 2).

The resonant period drop observed at low temperatures in torsional oscillators containing solid helium had been interpreted as a signature of a supersolid. However, it was found that the shear modulus increase found in solid helium at the same low temperature could also decrease the resonant period of the torsional oscillator. We report the results of a study in two different torsional oscillators that were specially designed to minimize the shear modulus effect and maximize any possible supersolid response. We were able to place an upper limit on the nonclassical rotational inertia or supersolid fraction of $4\times {10}^{-6}$. Moreover, we have repeated an earlier experiment on the hcp ${}^3\mathrm{He}$ solid, which shows similar low-temperature stiffening to that in hcp ${}^4\mathrm{He}$. We found that the small drop of the resonant period measured in the hcp ${}^3\mathrm{He}$ sample is comparable in size to that found in the hcp ${}^4\mathrm{He}$ samples. These results strongly suggest that the resonant period drop reported in most torsional oscillator studies in the last decade is primarily a consequence of the shear modulus stiffening effect.

P/As substitution effects on the transport properties of polycrystalline ${\mathrm{LaFeP}}_{1\text{−}x}{\mathrm{As}}_x{\mathrm{O}}_{1\text{−}y}{\mathrm{F}}_y$ with $x$ = 0–1.0 and $y$ = 0–0.1 have been studied. In the F-free samples $(y$ = 0), a superconducting (SC) dome with a maximum $T_c$ of 12 K is observed around $x$ = 0–0.3. This is separated from another SC dome with $T_c$ $\sim$ 10 K at $x$ = 0.6–0.8 by an antiferromagnetic region $(x$ = 0.3–0.6), giving a two-dome feature in the $T_c-x$ phase diagram. As $y$ increases, the two SC domes merge together, changing to a double-peak structure at $y$ = 0.05 and a single dome at $y$ = 0.1. This proves the presence of two different Fermi surface states in this system.

We introduce a microwave bolometer aimed at high-quantum-efficiency detection of wave packet energy within the framework of circuit quantum electrodynamics, the ultimate goal being single microwave photon detection. We measure the differential thermal conductance between the detector and its heat bath, obtaining values as low as $5\text{fW}/\mathrm{K}$ at $50\text{mK}$. This is one tenth of the thermal conductance quantum and corresponds to a theoretical lower bound on noise-equivalent power of order ${10}^{-20}\text{W}/\sqrt{\text{Hz}}$ at $50\text{mK}$. By measuring the differential thermal conductance of the same bolometer design in substantially different environments and materials, we determine that electron-photon coupling dominates the thermalization of our nanobolometer.

We investigate the electronic conductance and thermopower of a single-molecule junction consisting of bis-(4-aminophenyl) acetylene (B4APA) connected to gold electrodes. We use nonequilibrium Green's function methods in combination with density-functional theory (DFT) and the many-body $GW$ approximation. To simulate recent break junction experiments, we calculate the transport properties of the junction as it is pulled apart. For all junction configurations, DFT with a standard semilocal functional overestimates the conductance by almost an order of magnitude, while the thermopower is underestimated by up to a factor of 3, except for the most highly stretched junction configurations. In contrast, the $GW$ results for both conductance and thermopower are in excellent agreement with experiments for a wide range of electrode separations. We show that the $GW$ self-energy not only renormalizes the molecular energy levels but also the coupling strength. The latter is a consequence of the finite response time associated with the electronic screening in the metal electrodes.