Graphene grain boundaries are known to affect phonon transport and thermal conductivity, suggesting that they may be used to engineer the phononic properties of graphene. Here, the effect of two buckled grain boundaries on long-wavelength flexural acoustic phonons has been investigated as a function of angle of incidence using molecular dynamics. The flexural acoustic mode has been chosen due to its importance to thermal transport. It is found that the transmission through the boundaries is strongly suppressed for incidence angles close to ${35}^{\circ }$. Also, the grain boundaries are found to act as diffraction gratings for the phonons.

We report a hierarchical first-principles investigation on the entangled effects of lattice dimensionality and bond characteristics in the lattice dynamics of silicene and germanene. It is found that bond bending (stretching) negatively (positively) contributes to Grüneisen constant $\gamma$, which results in the negative acoustic (positive optical) $\gamma$. The layer thickening (bond weakening) caused by chemical functionalization tends to increase (decrease) the acoustic (optical) $\gamma$, due to the increased (decreased) bond-stretching effect. The excitation of the negative-$\gamma$ modes results in negative thermal expansion, while mode excitation and thermal expansion compete with each other in thermomechanics. The sensitive structural and electronic responses of silicene and germanene to functionalization help us to derive a generic physical picture for two-dimensional lattice dynamics.

Optimal working points or “sweet spots” have arisen as an important tool for mitigating charge noise in quantum dot logical spin qubits. The exchange-only qubit provides an ideal system for studying this effect because $Z$ rotations are performed directly at the sweet spot, while $X$ rotations are not. Here, we quantify the ability of the sweet spot to mitigate charge noise by treating $X$ and $Z$ rotations on an equal footing. Specifically, we optimize $X$ rotations and determine an upper bound on their fidelity. We find that sweet spots offer a fidelity improvement factor of at least 20 for typical GaAs devices, and more for Si devices.

We ask the question, what are the ideal characteristics of a near-field thermophotovoltaic cell? Our search leads us to a reformulation of near-field radiative heat transfer in terms of the joint density of electronic states of the emitter-absorber pair in the thermophotovoltaic system. This form reveals that semiconducting materials with narrowband absorption spectra are critical to the energy-conversion efficiency. This essential feature is unavailable in conventional bulk semiconductor cells but can be obtained using low-dimensional materials. Our results show that the presence of matched van Hove singularities resulting from quantum confinement in the emitter and absorber of a thermophotovoltaic cell boosts both the magnitude and spectral selectivity of radiative heat transfer, dramatically improving energy-conversion efficiency. We provide a model near-field thermophotovoltaic system design making use of this idea by employing the van Hove singularities present in carbon nanotubes. Shockley-Queisser analysis shows that the predicted heat transfer characteristics of this model device are fundamentally better than existing thermophotovoltaic designs. Our work paves the way for the use of quantum dots, quantum wells, two-dimensional semiconductors, semiconductor nanowires, and carbon nanotubes as future materials for thermophotovoltaic cells.

We present a variant of the recently developed quantum corrected model (QCM) for plasmonic nanoparticles [Nat. Commun. 3, 825 (2012)] using nonlocal boundary conditions. The QCM accounts for electron tunneling in narrow gap regions of coupled metallic nanoparticles, leading to the appearance of new charge-transfer plasmons. Our approach has the advantages that it emphasizes the nonlocal nature of tunneling and introduces only contact resistance, but not ohmic losses through tunneling. Additionally, it can be implemented much more easily in boundary element method (BEM) approaches. We develop the methodology for the QCM using nonlocal boundary conditions and present simulation results of our BEM implementation, which are in good agreement with those of the original QCM.

We demonstrate the generation of a Werner-like state from a single semiconductor quantum dot. The tomographic analysis with temporal gating brings us to a systematic understanding of the relation between the time evolution of quantum correlation and a set of parameters characterizing the exciton states, including fine-structure splitting and cross-dephasing time. The Werner state relates the Bell's parameter in the Clauser, Horne, Shimony, and Holt inequality with a fidelity, which facilitates the evaluation of nonlocality.

We address the tunneling conductance and spin inelastic tunneling spectroscopy of localized paramagnetic moments in a superconducting environment, pertaining to recent measurements on Fe-octaethylporphyrin-chloride using superconducting scanning tunneling microscopy. We demonstrate that the Cooper pair correlations in the tip and substrate generate a finite uniaxial anisotropy field acting on the local spin moment, and we argue that this field may be a source for the observed changes in the conductance spectrum for decreasing distance between the scanning tunneling tip and the local magnetic moment. We make a side-by-side comparison between the superconductor-superconductor junction and normal-metal–superconductor junction, and find qualitative agreement between the two setups while quantitative differences become explicit. When simulating the effects of electron pumping, we obtain additional peaks in the conductance spectrum that can be attributed to excitations between higher-energy spin states. The transverse anisotropy field couples basis states of the local spin which opens for transitions between spin states that are otherwise forbidden by conservation of angular momentum. Finally, we explore the influences of temperature, which tend to enable in-gap transitions, and an external magnetic field, which enables deeper studies of the spin excitation spectrum. We especially notice the appearance of a low and high excitation peak on each side of the main coherence peak as an imprint of transitions between the Zeeman split ground states.

Based on a mode-expansion theory under single-mode approximation, we derived the scatterings parameters for a general one-dimensional photonic grating composed of two different materials, and then established an effective-medium theory for such a composite by equating the obtained scattering parameters to those of a homogeneous medium. Our effective-medium theory well describes the grating structures with general material and geometrical parameters, and recovers two previous formulas, which are valid only at certain limiting conditions. The theory is justified by full-wave simulations and microwave experiments.

Vortex dynamics within arrays of square ferromagnetic nanoelements have been studied by time-resolved scanning Kerr microscopy (TRSKM), while x-ray photoemission electron microscopy has been used to investigate the equilibrium magnetic state of the arrays. An alternating field demagnetization process was found to initialize a distribution of equilibrium states within the individual elements of the array, including quasiuniform states and vortex states of different chirality and core polarization. Repeated initialization revealed some evidence of stochastic behavior during the formation of the equilibrium state. TRSKM with a spatial resolution of ∼300 nm was used to detect vortex gyration within arrays of square nanoelements of 250-nm lateral size. Two arrays were studied consisting of a $9\times 9$ and $5\times 5$ arrangement of nanoelements with 50- and 500-nm element edge-to-edge separation to encourage strong and negligible dipolar interactions, respectively. In the $5\times 5$ element array, TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In the $9\times 9$ array, strong magneto-optical response due to vortex gyration was observed across regions with length equal to either one or two elements. Micromagnetic simulations performed for $2\times 2$ arrays of elements suggest that particular combinations of vortex chirality and polarization in neighboring elements are required to generate the observed magneto-optical contrast.

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems in which the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromagnetic Gd-Gd, Sm-Sm, and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of x-ray magnetic circular dichroism is used in separate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization.

In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. ${\mathrm{LaPt}}_2{\mathrm{Ge}}_2$ undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at $T_{\mathrm{s}}=394$ K, accompanying a double period modulation in the $a$-axis direction, and superconducts at $T_{\mathrm{c}}=0.41$ K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesting. A reduction in the density of states in the monoclinic phase was found in the calculation and confirmed by ${}^{195}\mathrm{Pt}$-NMR. We suggest a CDW as a possible cause for the SPT. By changing the stoichiometry between Pt and Ge, we succeeded in suppressing $T_{\mathrm{s}}$ and increasing $T_{\mathrm{c}}$ in ${\mathrm{LaPt}}_{2-x}{\mathrm{Ge}}_{2+x}$. Comparison of ${}^{139}\mathrm{La}$- and ${}^{195}\mathrm{Pt}$-NMR data reveals moderate fluctuations associated with SPT. From ${}^{139}\mathrm{La}$-NQR measurements at zero field, we found that an isotropic superconducting gap is realized in ${\mathrm{LaPt}}_{2-x}{\mathrm{Ge}}_{2+x}$ ($x=0.20$). We discuss the relationship between superconductivity and the SPT order/fluctuations.

We present inelastic neutron scattering measurements of the antiferromagnetic insulator ${\mathrm{CaMn}}_2{\mathrm{Sb}}_2$, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the $\mathbf{H}$ and $\mathbf{L}$ directions in reciprocal space, with a maximum excitation energy of $\approx 24$ meV. These excitations are well described by spin waves in a Heisenberg model, including first- and second-neighbor exchange interactions $J_1$ and $J_2$ in the Mn plane and also an exchange interaction between planes. The determined ratio $J_2/J_1\approx 1/6$ suggests that ${\mathrm{CaMn}}_2{\mathrm{Sb}}_2$ is an example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the Néel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveals a mean field ordering temperature $\approx 4$ times larger than the reported Néel temperature $T_N=85$ K, suggesting significant frustration arising from proximity to the tricritical point.

The quantization of vortex lines in superfluids requires the introduction of their density $\mathcal{L}(\mathit{r},t)$ in the description of quantum turbulence. The space homogeneous balance equation for $\mathcal{L}\left(t\right)$, proposed by Vinen on the basis of dimensional and physical considerations, allows a number of competing forms for the production term $\mathcal{P}$. Attempts to choose the correct one on the basis of time-dependent homogeneous experiments ended inconclusively. To overcome this difficulty we announce here an approach that employs an inhomogeneous channel flow which is very suitable to distinguish the implications of the various possible forms of the desired equation. We demonstrate that the originally selected form which was extensively used in the literature is in strong contradiction with our data. We therefore present a new form of an inhomogeneous equation for $\mathcal{L}(\mathit{r},t)$ that is in agreement with our data and propose that it should be considered for further studies of superfluid turbulence.

Open quantum systems, when driven by a periodic field, can relax to effective statistical ensembles that resemble their equilibrium counterparts. We consider a class of problems in which a periodically driven quantum system is allowed to exchange both energy and particles with a thermal reservoir. We demonstrate that, even for noninteracting systems, effective equilibration to the grand canonical ensemble requires both fine tuning the system-bath coupling and selecting a sufficiently simple driving protocol. We study a tractable subclass of these problems in which the long-time steady state of the system can be determined analytically, and demonstrate that the system effectively thermalizes with fine tuning, but does not thermalize for general values of the system-bath couplings. When the driven system does not thermalize, it supports a tunable persistent current in the steady state without external bias. We compute this current analytically for two examples of interest: (1) a driven double quantum dot, where the current is interpreted as a dc electrical current, and (2) driven Dirac fermions in graphene, where it is interpreted as a valley current.

Differential resistance measurements are conducted for point contacts (PCs) between the ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ (SRO) single crystal and the tungsten tip approaching along the $c$ axis direction of the crystal. Since the contact is made at liquid helium temperature and the tungsten tip is hard enough to penetrate through the surface layer, consistent superconducting features are observed. First, with the tip pushed towards the crystal, the zero-bias conductance peak (ZBCP) due to Andreev reflection at the normal-superconducting interface increases from 3% to more than 20%, much larger than previously reported, and extends to temperatures higher than the bulk transition temperature. Reproducible ZBCP within 0.2 mV may also help determine the gap value of SRO, on which no consensus has been reached. Second, the logarithmic background can be fitted with the Altshuler-Aronov theory of electron-electron interaction for tunneling into quasi-two-dimensional electron systems. Feasibility of such fitting confirms that spectroscopic information such as density of states is probed, and electronic temperature retrieved from such fitting can be important to analyze the PC spectra. Third, at bias much higher than 0.2 mV there are conductance dips due to the critical current effect. These dips persist up to 6.2 K, possibly due to enhanced superconductivity under uniaxial pressure.

The boundary of symmetry-protected topological states (SPTs) can harbor new quantum anomaly phenomena. In this work, we characterize the bosonic anomalies introduced by the 1+1D non-onsite-symmetric gapless edge modes of (2+1)D bulk bosonic SPTs with a generic finite Abelian group symmetry (isomorphic to $G={\prod }_i{Z}_{N_i}=Z_{N_1}\times Z_{N_2}\times Z_{N_3}\times \cdots )$. We demonstrate that some classes of SPTs (termed “Type II”) trap fractional quantum numbers (such as fractional $Z_N$ charges) at the 0D kink of the symmetry-breaking domain walls, while some classes of SPTs (termed “Type III”) have degenerate zero energy modes (carrying the projective representation protected by the unbroken part of the symmetry), either near the 0D kink of a symmetry-breaking domain wall, or on a symmetry-preserving 1D system dimensionally reduced from a thin 2D tube with a monodromy defect 1D line embedded. More generally, the energy spectrum and conformal dimensions of gapless edge modes under an external gauge flux insertion (or twisted by a branch cut, i.e., a monodromy defect line) through the 1D ring can distinguish many SPT classes. We provide a manifest correspondence from the physical phenomena, the induced fractional quantum number, and the zero energy mode degeneracy to the mathematical concept of cocycles that appears in the group cohomology classification of SPTs, thus achieving a concrete physical materialization of the cocycles. The aforementioned edge properties are formulated in terms of a long wavelength continuum field theory involving scalar chiral bosons, as well as in terms of matrix product operators and discrete quantum lattice models. Our lattice approach yields a regularization with anomalous non-onsite symmetry for the field theory description. We also formulate some bosonic anomalies in terms of the Goldstone-Wilczek formula.

The first-order correlation function, which is accessible experimentally, contains all essential information about the state of a system of noninteracting electrons. Here I discuss how this function can be used to answer the question whether an electronic source working periodically emits a train of single particles or the emitted state is a multiparticle one. Using as an example a train of Lorentzian-shaped single-electron excitations, levitons, I demonstrate that the correlation function in the time domain is able to distinguish clearly single- and multiparticle states, while the correlation function in the energy domain is not. Nevertheless, the correlation function in the energy domain is able to show a transition from a single- to a multiparticle state when some parameter of the source is varied.

We report on a systematic study of the Coulomb-blockade effects in nanofabricated narrow constrictions in thin (Ga,Mn)As films. Different low-temperature transport regimes have been observed for decreasing constriction sizes: the Ohmic, the single-electron tunneling (SET), and a completely insulating regime. In the SET, complex stability diagrams with nested Coulomb diamonds and anomalous conductance suppression in the vicinity of charge degeneracy points have been observed. We rationalize these observations in the SET with a double ferromagnetic island model coupled to ferromagnetic leads. Its transport characteristics are analyzed in terms of a modified orthodox theory of Coulomb blockade which takes into account the energy dependence of the density of states in the metallic islands.

A study of a force detected single-spin magnetic resonance measurement concept with atomic spatial resolution is presented. The method is based upon electrostatic force detection of spin-selection rule controlled single-electron tunneling between two electrically isolated paramagnetic states. Single-spin magnetic resonance detection is possible by measuring the force detected tunneling charge noise on and off spin resonance. Simulation results of this charge noise, based upon physical models of the tunneling and spin physics, are directly compared to measured atomic force microscopy system noise. The results show that the approach could provide single-spin measurement of electrically isolated qubit states with atomic spatial resolution at room temperature.

We present experiments on the dynamics of a two-state parametric fluctuator in a superconducting flux qubit. In spectroscopic measurements, the fluctuator manifests itself as a doublet line. When the qubit is excited in resonance with one of the two doublet lines, the correlation of readout results exhibits an exponential time decay which provides a measure of the fluctuator transition rate. The rate increases with temperature in the interval 40 to 158 mK. Based on the magnitude of the transition rate and the doublet line splitting, we conclude that the fluctuation is induced by quasiparticle tunneling. These results demonstrate the importance of considering quasiparticles as a source of decoherence in flux qubits.