Photon emission by tunneling electrons can be encouraged by locating a resonator close to the tunnel junction and applying an appropriate voltage bias. However, studies of normal metals show that the resonator also affects how the charges flow, facilitating processes in which correlated tunneling of two charges produces one photon. We develop a theory to analyze this kind of behavior in Josephson junctions by deriving an effective Hamiltonian describing processes where two Cooper pairs generate a single photon. We determine the conditions under which the transport is dominated by incoherent tunneling of two Cooper pairs, while also uncovering a regime of coherent double Cooper-pair tunneling. We show that the system can also display an unusual form of photon blockade and hence could serve as a single-photon source.

Efficient manipulation of magnetization at ultrashort timescales is of particular interest for future technology. Here, we numerically investigate the influence of the so-called field-derivative torque, which was derived earlier based on relativistic Dirac theory [R. Mondal et al., Phys. Rev. B 94, 144419 (2016)], on the spin dynamics triggered by ultrashort laser pulses. We find that only considering the THz Zeeman field can underestimate the spin excitation in antiferromagnetic oxide systems such as, e.g., NiO and CoO. However, accounting for both the THz Zeeman torque and the field-derivative torque, the amplitude of the spin excitation increases significantly. Studying the damping dependence of the field-derivative torque we observe larger effects for materials having larger damping constants.

We have synthesized and studied an iridate, $\mathrm{B}{\mathrm{a}}_{13}\mathrm{I}{\mathrm{r}}_6{\mathrm{O}}_{30}$, with unusual Ir oxidation states: 2/3 $\mathrm{I}{\mathrm{r}}^{6+}\left(5d^3\right)$ ions and 1/3 $\mathrm{I}{\mathrm{r}}^{5+}\left(5d^4\right)$ ions. Its crystal structure features dimers of face-sharing $\mathrm{Ir}{\mathrm{O}}_6$ octahedra, and $\mathrm{Ir}{\mathrm{O}}_6$ monomers that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, $\mathrm{B}{\mathrm{a}}_{13}\mathrm{I}{\mathrm{r}}_6{\mathrm{O}}_{30}$ exhibits two transitions at $T_{\mathrm{N}1}=4.7\mathrm{K}$ and $T_{\mathrm{N}2}=1.6\mathrm{K}$. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K${}^2$ below $T_{\mathrm{N}2}$, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large $J_{\mathrm{eff}}=3/2$ degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with $\mathrm{I}{\mathrm{r}}^{6+}$ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by $\mathrm{I}{\mathrm{r}}^{5+}$ and $\mathrm{I}{\mathrm{r}}^{6+}$ ions. Features displayed by the magnetization and heat capacity suggest that the combination of covalency, SOI, and large effective spins leads to highly frustrated ferrimagnetic ordering, an interesting feature of this high-spin iridate.

We develop a theory of the spin Seebeck effect (SSE) in paramagnets as well as in antiferromagnets at elevated temperatures where the classical limit of the fluctuation-dissipation theorem is applicable. Employing dissipative stochastic models that are valid at these temperatures, we calculate the SSE signal, and we find that both the paramagnetic SSE and the antiferromagnetic SSE are expressed by a single equation that is proportional to the external magnetic field times the spin susceptibility of the magnet. The present result suggests the appearance of a cusp structure at the Néel temperature in the antiferromagnetic SSE signal.

We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong radio-frequency (rf) field, taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. The Usadel equations were solved to obtain the quasiparticle density of states (DOS) and the low-frequency surface resistance $R_s$ as functions of the rf field amplitude $H_0$. It is shown that the interplay of the broadening of the DOS peaks and a decrease of a quasiparticle gap caused by the rf currents produces a minimum in $R_s\left(H_0\right)$ and an extended rise of the quality factor $Q\left(H_0\right)$ with the rf field. Paramagnetic impurities shift the minimum in $R_s\left(H_0\right)$ to lower fields and can reduce $R_s\left(H_0\right)$ in a wide range of $H_0$. Subgap states in the DOS can give rise to a residual surface resistance while reducing $R_s$ at higher temperatures. A proximity-coupled normal layer at the surface can shift the minimum in $R_s\left(H_0\right)$ to either low and high fields and can reduce $R_s$ below that of an ideal surface. The theory shows that the behavior of $R_s\left(H_0\right)$ changes as the temperature and the rf frequency are increased, and the field dependence of $Q\left(H_0\right)$ can be very sensitive to the materials processing. Our results suggest that the nonlinear rf losses can be minimized by tuning pair-breaking effects at the surface using impurity management or surface nanostructuring.

The nanolamellar $M_{2}A\mathrm{C}$ or carbon-based “211 MAX” phases, where $M$ is an early transition metal, $A$ belongs to groups 13–15, and C is carbon, can be described by rigid band models. The same band model applies to all possible $A$ elements belonging to a given group of the periodic table. Changing $M$ for a given $A$ is then equivalent to shifting the Fermi energy $E_F$ through a band structure common to all phases in the group. This is shown by comparing predictions of density functional theory (DFT) to angle-resolved photoemission spectroscopy (ARPES) measurements. In particular, the Fermi surface of a given Al-based MAX phase can be obtained with an acceptable degree of accuracy by simply selecting the appropriate ARPES isoenergy surface of another Al-based phase. In ${\mathrm{V}}_2\mathrm{AlC}$, and in addition to conventional metal energy bands, both DFT and ARPES show the existence of a gapped nodal line located around 0.2 eV below $E_F$ or complex crossing points at $E_F$ with Dirac-like features in some directions. Application of the rigid band model suggests that these topological features as well as others, also predicted by DFT, can be positioned at or close to $E_F$ by an appropriate choice of $M$ and $A$, or by using an appropriate combination of various $M$ and $A$ elements.

The Kohn anomaly and topological change of the Fermi surface in $d$-block metals can occur under high pressure with affiliated significant changes in elastic, mechanical, and transport properties. However, our understanding of their origin and associated physical phenomena remains limited both experimentally and theoretically. Here we study the pressure effect on the Kohn anomaly, electronic topological transition (ETT), and the associated anomalies in physical properties of body-centered cubic (bcc) single-crystal tantalum (Ta). The phonon dispersions of Ta crystal were directly measured up to $\sim 47$ GPa using high-energy resolution inelastic x-ray scattering in a diamond anvil cell with hydrostatic helium medium. A Kohn anomaly in Ta was observed and became significantly stronger at 47.0 GPa at the reduced wave vector of $\sim 0.7$ in the longitudinal acoustic mode along the [ξ,0,0] direction. Our theoretical and experimental results indicate that the electron-phonon coupling and Fermi surface nesting mainly contribute to the Kohn anomaly, and the latter plays a dominant role at high pressures of 17–47 GPa. First-principles calculations further reveal an ETT with a topology change of the Fermi surface to occur at $\sim 100$ GPa in Ta, which causes a softening in the elastic constants ($C_{11}$ and $C_{44}$) and mechanical properties (shear, Young's, and bulk moduli). Our study shows that the $d$-orbital electrons in Ta play a key role in the stability of its electronic topological structure, where electron doping in Ta could significantly depress its ETT and elastic anomaly at high pressures. It is conceivable that our observed Kohn anomaly and ETT in a representative bcc Ta are much more prevalent in $d$-block transition metals under compression than previously thought.

In the present paper we assess the performance of the exact-exchange-only (EXX) Kohn-Sham method to predict band structures of transition metal oxide and metal halide perovskites with the main emphasis on band gaps. For the considered set of prototypical systems with cubic structure, the performance of the EXX method for the prediction of band gaps is comparable with other popular methods for this purpose like density-functional calculations with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional and one-shot $GW$ ($G_0{W}_0$). Comparison with experimental values suggests that in the case of perovskites the EXX method, like HSE and $GW$ methods, should be supplemented by a treatment of electron-phonon effects. The EXX method is computationally more efficient than $G_0{W}_0$ and also provides post-self-consistent access to one-electron wave functions and band structures within the complete Brillouin zone with much lower computational effort than both $G_0{W}_0$ and hybrid DFT methods. We confirm the importance of including the higher-lying semicore shells within the explicitly treated states for halide perovskites which was already noted by earlier studies. Calculations taking into account only the valence space without the semicore states, however, remain still a reasonable option in practice. Spin-orbit coupling effects seem to be slightly underestimated in DFT calculations both with respect to $GW$ and experiment. The source of this underestimation can be of both technical and formal nature, and this point requires more critical consideration. For ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, sizable Rashba spin splittings appear for the structure with the C-N bond of the methylammonium ion not lying along the long diagonal of the cubic unit cell.

The electronic structure of Si(110)“$16\times 2$” double-domain, single-domain, and $1\times 1$ surfaces have been investigated using spin- and angle-resolved photoemission at sample temperatures of $77\mathrm{K}$ and $300\mathrm{K}$. Angle-resolved photoemission was conducted using horizontally and vertically polarized $60\mathrm{eV}$ and $80\mathrm{eV}$ photons. Band-dispersion maps revealed four surface states ($S_1$ to $S_4$) which were assigned to silicon dangling bonds on the basis of measured binding energies and photoemission intensity changes between horizontal and vertical light polarizations. Three surface states ($S_1,S_2$, and $S_4$), observed in the Si(110)“$16\times 2$” reconstruction, were assigned to Si adatoms and Si atoms present at the edges of the corrugated terrace structure. Only one of the four surface states, $S_3$, was observed in both the Si(110)“$16\times 2$” and $1\times 1$ band maps and consequently attributed to the pervasive Si zigzag chains that are components of both the Si(110)“$16\times 2$” and $1\times 1$ surfaces. A state in the bulk-band region was attributed to an in-plane bond. All data were consistent with the adatom-buckling model of the Si(110)“$16\times 2$” surface. Whilst room temperature measurements of $P_y$ and $P_z$ were statistically compatible with zero, $P_x$ measurements of the enantiomorphic A-type and B-type Si(110)“$16\times 2$” surfaces gave small average polarizations of around 1.5% that were opposite in sign. Further measurements at $77\mathrm{K}$ on A-type Si(110)“$16\times 2$” surfaces gave a smaller value of $+0.3$%. An upper limit of $\sim 1\%$ may thus be taken for the longitudinal polarization.

Diffusion Monte Carlo (DMC) calculations were performed for an accurate description of the nature of the ${\mathrm{O}}_2$ adsorption on single layer graphene. We investigated the stable orientation of ${\mathrm{O}}_2$ at a specific adsorption site as well as its equilibrium adsorption energy. At equilibrium adsorption distances, an ${\mathrm{O}}_2$ molecule was found to prefer a horizontal orientation, where the O-O bond is parallel to the graphene surface, to the vertical orientation. However, the vertical orientation is favored at ${\mathrm{O}}_2$-graphene distances shorter than the equilibrium distance, which could be understood by the steric repulsion between O and C atoms. Contrary to previous DFT calculations, our DMC calculations show that the midpoint of a C-C bond (a bridge site) is energetically preferred for the ${\mathrm{O}}_2$ adsorption to a center of a hexagonal ring (a hollow site). The lowest DMC adsorption energy was found at an intermediate point between a hollow site and a bridge site, where the ${\mathrm{O}}_2$ adsorption energy was estimated to be $-0.142\left(4\right)$ eV, which is in very good agreement with the recently reported experimental value. Finally, we have found that ${\mathrm{O}}_2$ is very diffusive on the surface of graphene with the diffusion barrier along a bridge-hollow-bridge path being as small as $\sim 11$ meV.

We present a simple two-layer discontinuous crossed metal-strip array that guides microwaves having very high phase and group indices. The strips are arranged on a square lattice with a two-layer unit cell. The difference in this structure resides in the length of the metal strips, which extend to several unit cells. This work focuses on the isotropic wave dispersion at the lower frequencies. In addition, two of the higher-frequency bands give rise to a very strong negative dispersion, and strong beaming occurs, which can be tailored easily by modifying the relative orientation of the layers.

We study transport across ballistic junctions of materials which host pseudospin-one fermions as emergent low-energy quasiparticles. The effective low-energy Hamiltonians of such fermions are described by integer spin Weyl models. We show that current conservation in such integer spin-$s$ Weyl systems requires continuity across a boundary of only $2s$ (out of $2s+1$) components of the wave function. Using the current conservation conditions, we study the transport between normal metal-barrier-normal metal (NBN) and normal metal-barrier-superconductor (NBS) junctions of such systems in the presence of an applied voltage eV. We show that for a specific value of the barrier potential $U_0$, such NBN junctions act as perfect collimators; any quasiparticle which is incident on the barrier with a nonzero angle of incidence is reflected back with unit probability for any barrier width $d$. We discover an interesting symmetry of this system, namely, the conductance is invariant under $U_0\to 2({\mu }_L\pm \mathit{eV})-U_0$, where ${\mu }_L$ is the chemical potential and the $+$($-$) sign corresponds to particle (hole) mediated transport. For NBS junctions with a proximity-induced $s$-wave pairing potential, which also display such a collimation, we chart out the properties of the subgap tunneling conductance $G$ as a function of the barrier strength and applied voltage. We point out the effect of the collimation on the subgap tunneling conductance of these NBS junctions and discuss experiments which can test our theory.

We show that the physics of deformation in $\alpha$-, $\beta$-, and $6,6,12$-graphyne is, despite their significantly more complex lattice structures, remarkably close to that of graphene, with inhomogeneously strained graphyne described at low energies by an emergent Dirac-Weyl equation augmented by strain induced electric and pseudomagnetic fields. To show this, we develop a continuum theory of deformation in these materials, and consider two versions of this theory: (i) a truncated theory describing only the low-energy degrees of freedom of the conical intersection, which thus is spinor valued as in graphene, and (ii) one describing the full sublattice space. The spinor valued continuum theory agrees very well with the full continuum theory at low energies, showing that the remarkable physics of deformation in graphene generalizes to these more complex carbon architectures. In particular, we find that deformation induced pseudospin polarization and valley current loops, key phenomena in the deformation physics of graphene, both have their counterpart in these more complex carbon materials.

In this work we study the charge exchange process in the scattering of protons by potassium atoms adsorbed on a graphene surface in a low coverage limit. Both, the projected density of states on the alkaline atom site and the final charge states of the hydrogen projectile are calculated by considering the electronic Coulomb repulsion in the $s$-valence orbital. The inner $3p$ and $3s$ states of potassium are included and the local perturbations of the density matrix on the surrounding C atoms are also considered. The interacting systems are described by an Anderson Hamiltonian whose terms are calculated from the chemical properties of the atoms and the extended features of the graphene surface. The positive and negative ion fractions of hydrogen in the collision process are obtained from Keldysh-Green functions, which are calculated by employing the equation of motion method closed up to a second order in the atom-surface coupling term. It is found that the carbon atoms have no possibility of a direct charge exchange process in a frontal collision of the proton with the K adatom, and that the K-$3p$ band, broadened by the interaction with the graphene surface, provides an important source of electrons for the negative ionization of hydrogen, which is also promoted by the presence of a $\mathrm{K}\text{−}3s$ core state. The narrow $4s$ and $3p$ bands of the adsorbed potassium lead to an oscillatory dependence with the projectile incoming energy, of the probability for the three correlated charge states of hydrogen.

Tunneling of spinless electrons from a single-channel emitter into an empty collector through an interacting resonant level of the quantum dot (QD) is studied, when all Coulomb screening of charge variations on the dot is realized by the emitter channel and the system is mapped onto an exactly solvable model of a dissipative qubit. In this model we describe the qubit density matrix evolution with a generalized Lindblad equation, which permits us to count the tunneling electrons and therefore relate the qubit dynamics to the charge transfer statistics. In particular, the coefficients of its generating function equal to the time-dependent probabilities to have the fixed number of electrons tunneled into the collector are expressed through the parameters of a non-Hermitian Hamiltonian evolution of the qubit pure states in-between the successive electron tunneling events. From the leading asymptotics of the cumulant generating function (CGF) linear in time we calculate the Fano factor and the skewness and establish their relation to the extra average and the second cumulants, respectively, of the charge accumulated during the QD evolution from its empty and stationary states, which are defined by the next-to-leading term of the CGF asymptotics. The relation explains the origin of the sub-Poisson and super-Poisson shot noise in this system and shows that the super-Poisson signals existence of a nonmonotonous oscillating transient current and the qubit coherent dynamics. The mechanism is illustrated with particular examples of the generating functions, one of which coincides in the large time limit with the generating function of the $\frac{1}{3}$ fractional Poisson distribution realized without the fractional charge tunneling.

We propose mechanical systems, described by Newton's equation of motion, as suited platforms for symmetry protection of non-Hermitian degeneracies. We point out that in contrast to other systems with gain and loss, fine tuning of parameters is not required to realize symmetry-protected non-Hermitian degeneracies due to an emergent property of mechanical systems. The presence of symmetry-protected exceptional rings with extended chiral symmetry is numerically demonstrated for a mechanical graphene with friction. Furthermore, classification of symmetry-protected non-Hermitian degeneracies is addressed by taking into account the emergent properties of mechanical systems.

The critical behavior of statistical models with long-range interactions exhibits distinct regimes as a function of $\rho$, the power of the interaction strength decay. For large enough $\rho , \rho >{\rho }_{\mathrm{sr}}$, the critical behavior is observed to coincide with that of the short-range model. However, there are controversial aspects regarding this picture, one of which is the value of the short-range threshold ${\rho }_{\mathrm{sr}}$ in the case of the long-range XY model in two dimensions. We study the 2D XY model on the diluted graph, a sparse graph obtained from the 2D lattice by rewiring links with probability decaying with the Euclidean distance of the lattice as ${\left|r\right|}^{-\rho }$, which is expected to feature the same critical behavior of the long-range model. Through Monte Carlo sampling and finite-size analysis of the spontaneous magnetization and of the Binder cumulant, we present numerical evidence that ${\rho }_{\mathrm{sr}}=4$. According to such a result, one expects the model to belong to the Berezinskii-Kosterlitz-Thouless universality class for $\rho \ge 4$, and to present a second-order transition for $\rho <4$.

We present a survey of the structural and magnetic properties of submonolayer transition metal dioxides on the (001) surfaces of the heavy face-centered cubic noble metals Ir and Pt performed by spin-averaged scanning tunneling microscopy (STM) and spin-polarized (SP) STM. Our STM results confirm that deposition of Co, Fe, Mn, and Cr on the $(2\times 1)$ oxygen-reconstructed Ir(001) surface leads to the formation of quasi-one-dimensional chains with a $(3\times 1)$ unit cell. As recently predicted by density functional theory [P. Ferstl et al., Phys. Rev. Lett. 117, 046101 (2016)], our SP-STM images of ${\mathrm{FeO}}_2$ and ${\mathrm{MnO}}_2$ on Ir(001) show a twofold periodicity along the chains which is characteristic for an antiferromagnetic coupling along the chains. In addition, these two materials also exhibit spontaneous, permanent, and long-range magnetic coupling across the chains. Whereas we find a ferromagnetic interchain coupling for ${\mathrm{FeO}}_2$/Ir(001), the magnetic coupling of ${\mathrm{MnO}}_2$ on Ir(001) appears to be a noncollinear ${120}^{\circ }$ spin spiral, resulting in a $(9\times 2)$ magnetic unit cell. On Pt(001), patches of $(3\times 1)$-reconstructed oxide chains could only be prepared by transition metal (Co, Fe, and Mn) deposition onto the cold substrate and subsequent annealing in an oxygen atmosphere. Again SP-STM on ${\mathrm{MnO}}_2$/Pt(001) reveals a very large, $(15\times 2)$ magnetic unit cell which can tentatively be explained by a commensurate ${72}^{\circ }$ spin spiral. Large-scale SP-STM images reveal a long-wavelength spin rotation along the ${\mathrm{MnO}}_2$ chain.

Multiferroic materials that demonstrate magnetically driven ferroelectricity have fascinating properties such as magnetic (electric) field-controlled ferroelectric (magnetic) response that can be used in transformative applications including fast-writing, power-saving, and nondestructive data storage technologies in next-generation computing devices. However, since multiferroicity is typically observed at low temperatures, it is highly desirable to design multiferroic materials that can operate at room temperature. Here we show that $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ is a promising room-temperature multiferroic material, and we unravel in three dimensions (3D) the dynamics of topological defects, strain, and improper ferroelectric domains driven by electric fields in individual $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ nanocrystals. Using Bragg coherent diffractive imaging in combination with group-theoretical analysis, first-principles density functional calculations of phonons, and Landau phase-field theory we uncover in 3D the dynamics of topological defects, strain, and improper ferroelectric domains driven by electric fields in individual $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ nanocrystals. Our results show that $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ is an improper ferroelectric, in contrast to the current paradigm that adheres to the absence of improper ferroelectricity. Moreover, the fine structure of the reconstructed Bragg electron density suggests that $\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$ may be able to harbor novel topological quantum states of matter and a pathway to transform information technologies.

We show that weak measurements can induce a quantum phase transition of interacting many-body systems from an ergodic thermal phase with a large entropy to a nonergodic localized phase with a small entropy, but only if the measurement strength exceeds a critical value. We demonstrate this effect for a one-dimensional quantum circuit evolving under random unitary transformations and generic positive operator-valued measurements of variable strength. As opposed to projective measurements describing a restricted class of open systems, the measuring device is modeled as a continuous Gaussian probe, capturing a large class of environments. By employing data collapse and studying the enhanced fluctuations at the transition, we obtain a consistent phase boundary in the space of the measurement strength and the measurement probability, clearly demonstrating a critical value of the measurement strength below which the system is always ergodic, irrespective of the measurement probability. These findings provide guidance for quantum engineering of many-body systems by controlling their environment.