We investigate minimal excitation states for heat transport into a fractional quantum Hall system driven out of equilibrium by means of time-periodic voltage pulses. A quantum point contact allows for tunneling of fractional quasiparticles between opposite edge states, thus acting as a beam splitter in the framework of the electron quantum optics. Excitations are then studied through heat and mixed noise generated by the random partitioning at the barrier. It is shown that levitons, the single-particle excitations of a filled Fermi sea recently observed in experiments, represent the cleanest states for heat transport since excess heat and mixed shot noise both vanish only when Lorentzian voltage pulses carrying integer electric charge are applied to the conductor. This happens in the integer quantum Hall regime and for Laughlin fractional states as well, with no influence of fractional physics on the conditions for clean energy pulses. In addition, we demonstrate the robustness of such excitations to the overlap of Lorentzian wave packets. Even though mixed and heat noise have nonlinear dependence on the voltage bias, and despite the noninteger power-law behavior arising from the fractional quantum Hall physics, an arbitrary superposition of levitons always generates minimal excitation states.

Palladium normally does not easily substitute for Ti or Zr in perovskite oxides. Moreover, Pd is not normally magnetic (but becomes ferromagnetic under applied uniaxial stress or electric fields). Despite these two great obstacles, we have succeeded in fabricating lead zirconate titanate with 30% Pd substitution. For 20:80 Zr:Ti, the ceramics are generally single-phase perovskites (>99%) but sometimes exhibit 1% PdO, which is magnetic at room temperature. The resulting material is multiferroic (ferroelectric-ferromagnetic) at room temperature. The processing is slightly unusual (>8 h in high-energy ball-milling in Zr balls), and the density functional theory provided shows that it occurs because of $\mathrm{P}{\mathrm{d}}^{+4}$ in the oversized $\mathrm{P}{\mathrm{b}}^{+2}$ site; if all $\mathrm{P}{\mathrm{d}}^{+4}$ were to go into the $\mathrm{T}{\mathrm{i}}^{+4}$ perovskite B site, only a small moment of 0.1 Bohr magnetons would result.

The ordinary and the extraordinary Hall effects were studied in gradually oxidized amorphous CoFeB ferromagnets over six orders of resistivity from the metallic to the strongly insulating regime. Polarity of the extraordinary Hall effect reverses, and the amplitude of both the ordinary and the extraordinary Hall effects increases quadratically with resistivity when resistance exceeds the quantum resistance threshold $h/2e^2.$ The absolute value of the extraordinary Hall effect scales linearly with the ordinary one in the entire range over eight orders of magnitude between the metallic and the insulating states. The behavior differs qualitatively and quantitatively from the theoretically predicted and experimentally known behavior in other materials.

Using an extension of the full elastodynamic layer-multiple-scattering method to structures of fluid-saturated poroelastic spherical bodies, a comprehensive theoretical study of the acoustic response of double-porosity submerged liquid-saturated granular polymeric materials of specific morphology consisting of touching porous polymer spheres arranged in a fcc lattice, beyond the long-wavelength effective-medium description, is presented. Calculated transmission and absorption spectra of finite slabs of these materials are analyzed by reference to the acoustic modes of the constituent porous spherical grains as well as to relevant dispersion diagrams of corresponding infinite crystals, and a consistent interpretation of the results is provided. In particular, it is shown that resonant modes with very long lifetime, localized in the spheres in the form of slow longitudinal waves, which are peculiar to poroelastic materials, are formed when the viscous length is much shorter than the radius of the inner pores of the spheres. These modes, which can be easily tuned in frequency by adjusting the intrinsic porosity of the spheres, induce some remarkable features in the acoustic behavior of these double-porosity materials, such as narrow dispersionless absorption bands and directional transmission gaps.

The finite-temperature phase diagram is obtained for an infinite honeycomb lattice with spin-$1/2$ Ising interaction $J$ by using thermal-state fidelity and the von Neumann entropy based on the infinite projected entangled pair state algorithm with ancillas. The tensor network representation of the fidelity, which is defined as an overlap measurement between two thermal states, is presented for thermal states on the honeycomb lattice. We show that the fidelity per lattice site and the von Neumann entropy can capture the phase transition temperatures for an applied magnetic field, consistent with the transition temperatures obtained via the transverse magnetizations, which indicates that a continuous phase transition occurs in the system. In the temperature-magnetic field plane, the phase boundary for finite temperature is found to be well approximated by the functional form ${\left(k_B{T}_c\right)}^2+h_c^2/2=a{J}^2$ with a single numerical fitting coefficient $a=2.298\left(7\right)$, where $T_c$ and $h_c$ are the critical temperature and field with Boltzmann constant $k_B$. The critical temperature in the absence of magnetic field is estimated as $k_B{T}_c/J=\sqrt{a}\simeq 1.516\left(2\right)$, compared with the exact result $k_B{T}_c/J=1.51865\cdots$. For the quantum state at zero temperature, this phase boundary function gives the critical field estimate $h_c/J=\sqrt{2a}\simeq 2.144\left(3\right)$, compared to the known value $h_c/J=2.13250\left(4\right)$ calculated from a cluster Monte Carlo approach.

We study numerically the entanglement entropy and spatial correlations of the one-dimensional transverse-field Ising model with three different perturbations. First, we focus on the out-of-equilibrium steady state with an energy current passing through the system. By employing a variety of matrix-product state based methods, we confirm the phase diagram and compute the entanglement entropy. Second, we consider a small perturbation that takes the system away from integrability and calculate the correlations, the central charge, and the entanglement entropy. Third, we consider periodically weakened bonds, exploring the phase diagram and entanglement properties first in the situation when the weak and strong bonds alternate (period two bonds) and then the general situation of a period of $n$ bonds. In the latter case we find a critical weak bond that scales with the transverse field as $J_c^{\prime }/J={(h/J)}^n$, where $J$ is the strength of the strong bond, $J^{\prime }$ is that of the weak bond, and $h$ is the transverse field. We explicitly show that the energy current is not a conserved quantity in this case.

We present an experimental realization of a quantum critical point in an itinerant antiferromagnet composed of nonmagnetic constituents, TiAu. By partially substituting Ti with Sc in ${\mathrm{Ti}}_{1-x}{\mathrm{Sc}}_x\mathrm{Au}$, a doping amount of $x_c=0.13\pm 0.01$ induces a quantum critical point with minimal disorder effects. The accompanying non-Fermi liquid behavior is observed in both the resistivity $\rho \propto T$ and specific heat $C_p/T\propto -\ln T$, characteristic of a two-dimensional antiferromagnet. The quantum critical point is accompanied by an enhancement of the spin fluctuations, as indicated by the diverging Sommerfeld coefficient $\gamma$ at $x=x_c$.

We study the Landau-Zener tunneling problem for particles bound in periodic lattice insulators. To this end, we construct the path integral based on the Bloch and Wannier functions in the presence with an external force, and the transition amplitude is calculated for the Su-Schrieffer-Heeger model. We find that the tunneling probability in bulk periodic systems becomes drastically larger than that by the Landau-Zener formula. This enhancement is prominent for small values of the external field or small hopping integral compared with the gap and comes from the difference between the Dirac and the periodic dispersions. In addition, when the lattice effect is strong, another analytical formula of the tunneling probability is given with a different behavior from the Landau-Zener formula. Finally, we discuss the observation scheme for the lattice effect.

Growth, electronic, and magnetic properties of ${\gamma }^{\prime }\text{−}{\mathrm{Fe}}_4\mathrm{N}$ atomic layers on Cu(001) are studied by scanning tunneling microscopy/spectroscopy and x-ray absorption spectroscopy/magnetic circular dichroism. A continuous film of ordered trilayer ${\gamma }^{\prime }\text{−}{\mathrm{Fe}}_4\mathrm{N}$ is obtained by Fe deposition under ${\mathrm{N}}_2$ atmosphere onto monolayer ${\mathrm{Fe}}_2\mathrm{N}/\mathrm{Cu}\left(001\right)$, while the repetition of a bombardment with 0.5 keV ${\mathrm{N}}^{+}$ ions during growth cycles results in imperfect bilayer ${\gamma }^{\prime }\text{−}{\mathrm{Fe}}_4\mathrm{N}$. The increase in the sample thickness causes the change in the surface electronic structure, as well as the enhancement in the spin magnetic moment of Fe atoms reaching $\sim 1.4{\mu }_{\mathrm{B}}/\mathrm{atom}$ in the trilayer sample. The observed thickness-dependent properties of the system are well interpreted by the layer-resolved density of states calculated using first principles, which demonstrates the strongly layer dependent electronic states within each surface, subsurface, and interfacial plane of the ${\gamma }^{\prime }\text{−}{\mathrm{Fe}}_4\mathrm{N}$ atomic layers on Cu(001).

In the last few years, charge density waves (CDWs) have been ubiquitously observed in high-temperature superconducting cuprates and are now the most investigated among the competing orders in the still hot debate on these systems. A wealth of new experimental data raises several fundamental issues that challenge the various theoretical proposals. We here relate our mean-field instability line $T_{\mathrm{CDW}}^0$ of a strongly correlated Fermi liquid to the pseudogap $T^{*}\left(p\right)$ line, marking in this way the onset of CDW-fluctuations. These fluctuations reduce strongly the mean-field critical line. Controlling this reduction via an infrared frequency cutoff related to the characteristic time of the probes, we account for the complex experimental temperature versus doping phase diagram. We provide a coherent scenario explaining why different CDW onset curves are observed by different experimental probes and seem to extrapolate at zero temperature into seemingly different quantum critical points (QCPs) in the intermediate and overdoped region. The nearly singular anisotropic scattering mediated by these fluctuations also accounts for the rapid changes of the Hall number seen in experiments and provides the first necessary step for a possible Fermi surface reconstruction fully establishing at lower doping. Finally, we show that phase fluctuations of the CDWs, which are enhanced in the presence of strong correlations near the Mott insulating phase, naturally account for the disappearance of the CDWs at low doping with yet another QCP as seen by the experiments.

We present a correction scheme for periodic $\mathrm{\Gamma }$-only $GW$ calculations in a Gaussian basis. For four benchmark systems, the dependence of the corrected $GW$ quasiparticle levels on the cell size is reduced by a factor of three to ten compared to $GW$ calculations without correction. The correction scheme comes along with negligible computational cost and enables $GW$ calculations for supercells containing hundreds of atoms with Gaussian basis functions.

We have investigated the electronic and optical properties of ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\right)}_2\mathrm{Ir}{\mathrm{O}}_4$ ($x=0–0.375$) and ${\left(\mathrm{S}{\mathrm{r}}_{1-y}\mathrm{B}{\mathrm{a}}_y\right)}_2\mathrm{Ir}{\mathrm{O}}_4$ ($y=0–0.375$) epitaxial thin films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the $J_{\mathrm{eff}}=1/2$ spin-orbit Mott insulator $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$. As the average A-site ionic radius increases from ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{C}{\mathrm{a}}_x\right)}_2\mathrm{Ir}{\mathrm{O}}_4$ to ${\left(\mathrm{S}{\mathrm{r}}_{1-y}\mathrm{B}{\mathrm{a}}_y\right)}_2\mathrm{Ir}{\mathrm{O}}_4$, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated $J_{\mathrm{eff}}=1/2$ and $J_{\mathrm{eff}}=3/2$ bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically forbidden spin-orbit exciton and the intersite optical transitions within the $J_{\mathrm{eff}}=1/2$ band. Our experimental results are consistent with this interpretation as implemented by a multiorbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and intersite $d\text{−}d$ transitions within the $J_{\mathrm{eff}}=1/2$ band.

Photonic crystal cavities enable the realization of high Q-factor and low mode-volume resonators, with typical architectures consisting of a thin suspended periodically patterned layer to maximize confinement of light by strong index guiding. We investigate a heterostructure-based approach comprising a high refractive index core and lower refractive index cladding layers. While confinement typically decreases with decreasing index contrast between the core and cladding layers, we show that, counterintuitively, due to the confinement provided by the photonic band structure in the cladding layers, it becomes possible to achieve Q factors $>{10}^4$ with only a small refractive index contrast. This opens up opportunities for implementing high-Q factor cavities in conventional semiconductor heterostructures, with direct applications to the design of electrically pumped nanocavity lasers using conventional fabrication approaches.

Transition-metal nanoparticles adsorbed on graphene are of great interest due to the unique catalytic and magnetic properties resulting from nanoparticles-graphene interactions. Comparison between the physical properties of such systems and those of the same nanoparticles in the gas phase is especially important. Here we report a systematic density functional investigation of the structural, energetic, and magnetic properties of small ${\mathrm{Ni}}_n$, ${\mathrm{Pd}}_n$, and ${\mathrm{Pt}}_n$ clusters, comprising from $n=1$ to 6 atoms, in the gas phase and adsorbed on a graphene monolayer. Our results show that the $\mathrm{Ni}$ adatom binds to the graphene hollow site, with $-1.47\text{−}\mathrm{meV}$ adsorption energy, while $\mathrm{Pd}$ and $\mathrm{Pt}$ prefer the bridge sites, with $-1.14$- and $-1.62\text{−}\mathrm{meV}$ adsorption energies, respectively. This difference is determined by a competition between quantum and classical forces. ${\mathrm{Ni}}_2$ and ${\mathrm{Pt}}_2$ dimers bind perpendicularly on hollow and bridge sites, respectively, while ${\mathrm{Pd}}_2$ lies parallel to the graphene sheet, with each adatom on a bridge site. For larger ${\mathrm{TM}}_n$ ($\mathrm{TM}$ = $\mathrm{Ni}$, $\mathrm{Pd}$, $\mathrm{Pt}$; $n=3–6$) clusters, either two or three atoms bind to bridge graphene sites. In almost all cases the adsorbed clusters retain their gas-phase structures. The exceptions are ${\mathrm{Ni}}_5$ and ${\mathrm{Pt}}_4$, which acquire more compact structures with effective coordination number 12 and $19\%$ larger than in the gas phase, respectively. As the number of atoms grows, the cluster binds more weakly to the graphene, while its binding energy mounts up. Van der Waals corrections to the plain density functional theory (DFT) total energy raise the adsorption energy, but leave the cluster structure unchanged, in the gas phase or upon adsorption. Bader charge analysis shows that adsorption causes minor charge redistribution: the $\mathrm{TM}$ atoms bound to $\mathrm{C}$ atoms become positively charged, while the remaining metal atoms acquire negative charge. We have derived an approximate analytical expression for the local densities of states for the $d$ orbitals of $\mathrm{Ni}$, $\mathrm{Pd}$, and $\mathrm{Pt}$ adatoms, on the basis of an extended Anderson-Newns model. Comparison with the DFT local densities of states for adsorption at hollow sites has identified interference among the wave functions responsible for the binding of distinct $d$ levels to the $\mathrm{C}$ atoms. No such interference has become visible for adsorption at bridge sites.

Surface sensitive electric current measurements are important experimental tools poorly corroborated by theoretical models. We show that the drift-diffusion equations offer a framework for a consistent description of such experiments. The current flow is calculated as a perturbation of an equilibrium solution depicting the space charge layer. We investigate the accumulation and inversion layers in great detail. Relying on numerical findings, we identify the proper length parameter, the relationship of which with the length of the space charge layer is not simple. If the length parameter is large enough, long-ranged modes dominate the Green's function of the current equation, leading to two-dimensional currents. In addition, we demonstrate that the surface behavior of the currents is ruled by only a few parameters. This explains the fact that simplistic conductivity models have proven effective but makes reconstructions of conductance profiles from surface currents rather questionable.

We probe the interconversion of spin permutation symmetry of weakly bound electron-hole carrier pairs in an organic light-emitting diode by monitoring the changes in yield of recombinant species—singlet and triplet excitons—through fluorescence and phosphorescence, respectively. Spin mixing occurs by spin precession in local hyperfine fields and is suppressed by an external magnetic field, leading to an anticorrelation of fluorescence and phosphorescence yield, which follows the same functionality as magnetoresistance. A resonant radio-frequency field reverses this effect, enhancing spin mixing to raise the phosphorescence and lower the fluorescence. The experiment offers a direct simultaneous optical probe of the two interconverting spin states in the radical-pair mechanism, which features prominently in models of biological magnetoception.

Topological semimetals are characterized by protected crossings between conduction and valence bands. These materials have recently attracted significant interest because of the deep connections to high-energy physics, the novel topological surface states, and the unusual transport phenomena. While Dirac and Weyl semimetals have been extensively studied, the nodal-line semimetal remains largely unexplored due to the lack of an ideal material platform. In this paper, we report the magnetotransport properties of the two nodal-line semimetal candidates CaAgAs and CaCdGe. First, the transport properties of our single crystalline CaAgAs agree with those of CaAgAs polycrystals. They can be explained by the single-band model, consistent with the theoretical proposal that only nontrivial Fermi pockets linked by the topological nodal-line are present at the Fermi level. Second, our CaCdGe sample provides an ideal platform to perform comparative studies because the theoretical calculation shows that it features the same topological nodal line but has a more complicated Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through our systematic magnetotransport and first-principles band structure calculations, we show that our CaTX compounds can be used to study, isolate, and control the novel topological nodal-line physics in real materials.

One-dimensional (1D) electron systems in the presence of Coulomb interaction are described by Luttinger liquid theory. The strength of Coulomb interaction in the Luttinger liquid, as parametrized by the Luttinger parameter $K$, is in general difficult to measure. This is because $K$ is usually hidden in power-law dependencies of observables as a function of temperature or applied bias. We propose a dynamical way to measure $K$ on the basis of an electronic time-of-flight experiment. We argue that the helical Luttinger liquid at the edge of a 2D topological insulator constitutes a preeminently suited realization of a 1D system to test our proposal. This is based on the robustness of helical liquids against elastic backscattering in the presence of time-reversal symmetry.

Circuit quantum electrodynamics (QED) studies the interaction of artificial atoms, open transmission lines, and electromagnetic resonators fabricated from superconducting electronics. While the theory of an artificial atom coupled to one mode of a resonator is well studied, considering multiple modes leads to divergences which are not well understood. Here, we introduce a first-principles model of a multimode resonator coupled to a Josephson junction atom. Studying the model in the absence of any cutoff, in which the coupling rate to mode number $n$ scales as $\sqrt{n}$ for $n$ up to $\infty$, we find that quantities such as the Lamb shift do not diverge due to a natural rescaling of the bare atomic parameters that arises directly from the circuit analysis. Introducing a cutoff in the coupling from a nonzero capacitance of the Josephson junction, we provide a physical interpretation of the decoupling of higher modes in the context of circuit analysis. In addition to explaining the convergence of the quantum Rabi model with no cutoff, our work also provides a useful framework for analyzing the ultrastrong coupling regime of a multimode circuit QED.

Engineering the electron dispersion of graphene to be spin-dependent is crucial for the realization of spin-based logic devices. Enhancing spin-orbit coupling in graphene can induce spin Hall effect, which can be adapted to generate or detect a spin current without a ferromagnet. Recently, both chemically and physically decorated graphenes have shown to exhibit large nonlocal resistance via the spin Hall and its inverse effects. However, these nonlocal transport results have raised critical debates due to the absence of field dependent Hanle curve in subsequent studies. Here, we introduce Au clusters on graphene to enhance spin-orbit coupling and employ a nonlocal geometry to study the spin Hall induced nonlocal resistance. Our results show that the nonlocal resistance highly depends on the applied gate voltage due to various current channels. However, the spin Hall induced nonlocal resistance becomes dominant at a particular carrier concentration, which is further confirmed through Hanle curves. The obtained spin Hall angle is as high as $\sim 0.09$ at 2 K. Temperature dependence of spin relaxation time is governed by the symmetry of spin-orbit coupling, which also depends on the gate voltage: asymmetric near the charge neutral point and symmetric at high carrier concentration. These results inspire an effective method for generating spin currents in graphene and provide important insights for the spin Hall effect as well as the symmetry of spin scattering in physically decorated graphene.