We have developed a nanomechanical resonator, for which the motional degree of freedom is a superfluid ${}^4\mathrm{He}$ oscillating flow confined to precisely defined nanofluidic channels. It is composed of an in-cavity capacitor measuring the dielectric constant, which is coupled to a superfluid Helmholtz resonance within nanoscale channels, and it enables sensitive detection of nanofluidic quantum flow. We present a model to interpret the dynamics of our superfluid nanomechanical resonator, and we show how it can be used for probing confined geometry effects on thermodynamic functions. We report isobaric measurements of the superfluid fraction in liquid ${}^4\mathrm{He}$ at various pressures, and the onset of quantum turbulence in restricted geometry.

This paper describes experiments utilizing a unique property of electron glasses to gain information on the fundamental nature of the interacting Anderson-localized phase. The methodology is based on measuring the energy absorbed by the electronic system from alternating electromagnetic fields as a function of their frequency. Experiments on three-dimensional (3D) amorphous indium-oxide films suggest that, in the strongly localized regime, the energy spectrum is discrete and inelastic electron-electron events are strongly suppressed. These results imply that, at low temperatures, electron thermalization and finite conductivity depend on coupling to the phonon bath. The situation is different for samples nearing the metal-insulator transition; in insulating samples that are close to the mobility edge, energy absorption persists to much higher frequencies. Comparing these results with previously studied 2D samples [Ovadyahu, Phys. Rev. Lett. 108, 156602 (2012)] demonstrates that the mean-level spacing (on a single-particle basis) is not the only relevant scale in this problem. The possibility of delocalization by many-body effects and the relevance of a nearby mobility edge (which may be a many-body edge) are discussed.

Weyl semimetals are one kind of three-dimensional gapless semimetal with nontrivial topology in the momentum space. The chiral anomaly in Weyl semimetals manifests as a charge imbalance between the Weyl nodes of opposite chiralities induced by parallel electric and magnetic fields. We investigate the chiral anomaly effect on the plasmon mode in both intrinsic and doped Weyl semimetals within the random phase approximation. We prove that the chiral anomaly gives rise to a different plasmon mode in intrinsic Weyl semimetals. We also find the chiral anomaly leads to some exotic properties in the plasmon dispersion in doped Weyl semimetals. Consequently, the unconventional plasmon mode acts as a signature of the chiral anomaly in Weyl semimetals, by which the spectrum of plasmon provides a proper way to detect the Lifshitz transition.

An interesting route to the realization of topological Chern bands in ultracold atomic gases is through the use of optical flux lattices. These models differ from the tight-binding real-space lattice models of Chern insulators that are conventionally studied in solid-state contexts. Instead, they involve the coherent coupling of internal atomic (spin) states, and can be viewed as tight-binding models in reciprocal space. By changing the form of the coupling and the number $N$ of internal spin states, they give rise to Chern bands with controllable Chern number and with nearly flat energy dispersion. We investigate in detail how interactions between bosons occupying these bands can lead to the emergence of fractional quantum Hall states, such as the Laughlin and Moore-Read states. In order to test the experimental realization of these phases, we study their stability with respect to band dispersion and band mixing. We also probe interesting topological phases that emerge in these systems when the Chern number is greater than 1.

We experimentally and theoretically demonstrate subwavelength scale localization of spoof surface plasmon polaritons at a point defect in a two-dimensional groove metal array. An analytical expression for dispersion relation of spoof surface plasmon polaritons substantiates the existence of a band gap where a defect mode can be introduced. A waveguide coupling method allows us to excite localized spoof surface plasmon polariton modes and measure their resonance frequencies. Numerical calculations confirm that localized modes can have a very small modal volume and a high $Q$ factor both of which are essential in enhancing light-matter interactions. Interestingly, we find that the localized spoof surface plasmon polariton has a significant toroidal dipole moment, which is responsible for the high $Q$ factor, as well as an electric quadrupole moment. In addition, the dispersion properties of spoof surface plasmon polaritons are analyzed using a modal expansion method and numerical calculations.

A numerically exact solution to the many emitter–cavity problem as an open many body system is presented. The solution gives access to the full, nonperturbative density matrix and thus the full quantum statistics and quantum correlations. The numerical effort scales with the third power in the number of emitters. Notably the solution requires none of the common approximations such as good/bad cavity limit. As a first application the recently discussed concept of coherent surface plasmon amplification—spaser—is addressed: A spaser consists of a plasmonic nanostructure that is driven by a set of quantum emitters. In the context of laser theory it is a laser in the (very) bad cavity limit with an extremely high light matter interaction strength. The method allows us to answer the question of spasing with a fully quantized theory.

Interaction between K adatoms on graphene is investigated by first-principles calculations based on density function theory and analytical analyses based on the $\mathbf{k}·\mathbf{p}$ perturbation theory. The calculation shows that there is a strong repulsion between K adatoms. The main origin of this strong repulsion is not from the dipole-dipole interaction as suggested for K adatoms on graphite surface, but comes from the screened Coulomb interaction. Potassium adatom on graphene donates its $s$ electron and becomes ${\mathrm{K}}^{+}$. The positively charged K adatom induces electron density oscillation on graphene which is responsible for the screened Coulomb repulsion between the K adatoms.

In the sample size domain so far explored, the thermal conductivity of graphene shows an intriguing dependence on the sample length $L_z$ along the heat flux direction. An extrapolated infinite value for such a thermal conductivity is sometimes suggested for infinite samples, while other investigations predict anyway an upper limit for it. We address this issue by avoiding any guess or approximation on the underlying microscopic heat transport mechanisms; we rather perform direct atomistic simulations aimed at estimating thermal conductivity in samples with increasing size up to the unprecedented value of $L_z=0.1$ mm. Our results provide evidence that thermal conductivity in graphene is definitely upper limited in samples long enough to allow a diffusive transport regime for both single and collective phonon excitations.

We report the coexistence of charge puddles and topographic ripples in graphene decoupled from the Ir(111) substrate it was grown on. We show the topographic and the charge disorder to be locally correlated as a result of the intercalation of molecular species. From the analysis of quasiparticle scattering interferences, we find a linear dispersion relation, demonstrating that graphene on a metal can recover its intrinsic electronic properties. The measured Fermi velocity $v_F=0.9\pm 0.04\times {10}^6$ m/s is lower than in graphene on dielectric substrates, pointing to a strong screening of electron-electron interactions in graphene by the nearby metallic substrate.

We propose a first-principles effective medium formalism to study the propagation of electron waves in semiconductor heterostructures with a zero band gap. Our theory confirms that near the $K$ point the dynamics of a two-dimensional electron gas modulated by an external electrostatic potential with honeycomb symmetry is described by the same pseudospinor formalism and Dirac massless equation as a graphene monolayer. Furthermore, we highlight that even though other superlattices based on semiconductors with a zincblende-type structure can have a zero band-gap and a linear energy-momentum dispersion, the corresponding effective medium Hamiltonian is rather different from that of graphene, and can be based on a single-component wave function.

We have investigated the electronic, magnetic, and optical properties of hydrogenated diamond nanowires (DNWs) with nitrogen-vacancy (NV) centers using density functional theory. The strong localization of defect states results in the formation of local magnetic moments with a spin-polarization energy that is close to those for transition-metal atoms. Such spin-polarized defect states are found to be stable well above room temperature, in agreement with previous experimental reports. In addition, we find that a semiconductor-metal transition can be triggered upon applying a transverse electric field. Furthermore, an enhanced optical absorption in the visible-light region is predicted in DNWs with NV centers. The strength and the position of the absorption can be tuned or optimized by an external electric field and/or the nanowire diameter.

We present a stochastic formulation of the Kadanoff-Baym or Keldysh theory to calculate the conductance of a finite Kitaev chain coupled to two electron reservoirs. We study the dependence of the conductance on the number of sites in the chain and find that only for sufficiently long chains and in the regime that the chain is a topological superconductor the conductance at both ends tends to the universal value $2e^2/h$, as expected on the basis of the contact resistance of a single conducting channel provided by the Majorana zero mode. In this topologically nontrivial case we find an exponential decay of the current inside the chain and a simple analytical expression for the decay length. Finally, we also study the differential conductance at nonzero bias and the full current-voltage curves. We find a nonmonotonic behavior of the maximal current through the Kitaev chain as a function of the coupling strength with the reservoirs.

The equivalence between different physical systems permits us to transfer knowledge between them and to characterize the universal nature of their dynamics. We demonstrate that a nanopillar driven by a spin-transfer torque is equivalent to a rotating magnetic plate, which permits us to consider the nanopillar as a macroscopic system under a time-modulated injection of energy, that is, a simple parametric resonator. This equivalence allows us to characterize the phases diagram and to predict magnetic states and dynamical behaviors, such as solitons, stationary textures, and oscillatory localized states, among others. Numerical simulations confirm these predictions.

The combination of high-pressure ferromagnetic resonance (FMR) and an ab-initio scheme suitable for calculation of spin-orbit mediated anisotropic exchange interactions in molecular materials provides insights into the role of spin-orbit coupling (SOC) in a Se-based organic ferromagnet. FMR measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure (up to 2.2 GPa), in excellent agreement with calculations based on the known pressure dependence of the structure. The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of SOC in a wide range of organics where this effect is usually considered to be small.

We have investigated electronic and magnetic properties of iron pnictide $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$, which is isostructural to iron pnictide superconductors $\mathrm{LiFeAs}$ and $\mathrm{NaFeAs}$, using polycrystalline and single-crystal samples. We found that $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ is characterized by strong electron correlations and shows weak ferromagnetic behavior with the Curie temperature $T_{\mathrm{c}}\sim 42\mathrm{K}$, in contrast with the superconductivity in $\mathrm{LiFeAs}$ and $\mathrm{NaFeAs}$. Its electronic transport properties exhibit quasi-two-dimensional characteristics: While the in-plane resistivity follows quadratic temperature dependence for $T<10\mathrm{K}$, the $c$-axis resistivity shows nonmetal-to-metal crossover near 20 K. In addition, we have studied the Co-doping effect on $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ and found that Co doping enhances the ferromagnetism with $T_{\mathrm{c}}$ increasing up to 83 K for $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{As}$. From these results, we discussed the origin of the ferromagnetism of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1+y}\mathrm{As}$ and its relevance with the magnetism of iron pnictide superconductor parent compounds.

In this paper, molecular dynamics (MD) simulations based on the modified-embedded atom method (MEAM) and a phase-field crystal (PFC) model are utilized to quantitatively investigate the solid-liquid properties of Fe. A set of second nearest-neighbor MEAM parameters for high-temperature applications are developed for Fe, and the solid-liquid coexisting approach is utilized in MD simulations to accurately calculate the melting point, expansion in melting, latent heat, and solid-liquid interface free energy, and surface anisotropy. The required input properties to determine the PFC model parameters, such as liquid structure factor and fluctuations of atoms in the solid, are also calculated from MD simulations. The PFC parameters are calculated utilizing an iterative procedure from the inputs of MD simulations. The solid-liquid interface free energy and surface anisotropy are calculated using the PFC simulations. Very good agreement is observed between the results of our calculations from MEAM-MD and PFC simulations and the available modeling and experimental results in the literature. As an application of the developed model, the grain boundary free energy of Fe is calculated using the PFC model and the results are compared against experiments.

The magnetic phases of the ideal spin-$\frac{1}{2}$ triangular-lattice antiferromagnet ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ are identified and studied using ${}^{135,137}\mathrm{Ba}$ nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30 T, oriented parallel and near perpendicular to the crystallographic $ab$ plane. For both directions, the saturation field is approximately 33 T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to a $\frac{1}{3}$ magnetization plateau ($M_{\mathrm{sat}}/3$), as well as for a higher-field phase transition near to $\sim \left(\frac{3}{5}\right)M_{\mathrm{sat}}$ for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semiclassical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the $c$ axis.