A bimodal size distribution of two-dimensional islands is inferred during interface formation in heteroepitaxial growth of bismuth ferrite on (001) oriented ${\mathrm{SrTiO}}_3$ by sputter deposition. Features observed by in situ x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on ${\mathrm{SrTiO}}_3$. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones.

Although the favored glide planes in hexagonal close-packed Zr are prismatic, screw dislocations can escape their habit plane to glide in either pyramidal or basal planes. Using ab initio calculations within the nudged elastic band method, we show that, surprisingly, both events share the same thermally activated process with an unusual conservative motion of the prismatic stacking fault perpendicularly to itself. Halfway through the migration, the screw dislocation adopts a nonplanar metastable configuration with stacking faults in adjacent prismatic planes joined by a two-layer pyramidal twin.

We investigate collective phenomena with rotationally driven spinners of concave shape. Each spinner experiences a constant internal torque in either a clockwise or counterclockwise direction. Although the spinners are modeled as hard, otherwise noninteracting rigid bodies, their active motion induces an effective interaction that favors rotation in the same direction. With increasing density and activity, phase separation occurs via spinodal decomposition, as well as self-organization into rotating crystals. We observe the emergence of cooperative, superdiffusive motion along interfaces, which can transport inactive test particles. Our results demonstrate novel phase behavior of actively rotated particles that is not possible with linear propulsion or in nondriven, equilibrium systems of identical hard particles.

We employ scaling arguments and optimal fluctuation theory to establish a general relation between quantum Griffiths singularities and the Harris criterion for quantum phase transitions in disordered systems. If a clean critical point violates the Harris criterion, it is destabilized by weak disorder. At the same time, the Griffiths dynamical exponent $z^{\prime }$ diverges upon approaching the transition, suggesting unconventional critical behavior. In contrast, if the Harris criterion is fulfilled, power-law Griffiths singularities can coexist with clean critical behavior, but $z^{\prime }$ saturates at a finite value. We present applications of our theory to a variety of systems including quantum spin chains, classical reaction-diffusion systems and metallic magnets, and we discuss modifications for transitions above the upper critical dimension. Based on these results we propose a unified classification of phase transitions in disordered systems.

An accurate analytical parametrization for the exchange-correlation free energy of the homogeneous electron gas, including interpolation for partial spin polarization, is derived via thermodynamic analysis of recent restricted path integral Monte Carlo (RPIMC) data. This parametrization constitutes the local spin density approximation (LSDA) for the exchange-correlation functional in density functional theory. The new finite-temperature LSDA reproduces the RPIMC data well, satisfies the correct high-density and low- and high-$T$ asymptotic limits, and is well behaved beyond the range of the RPIMC data, suggestive of broad utility.

We report on magnetotransport measurements in InAs nanowires under a large magnetic field (up to 55 T), providing a spectroscopy of the one-dimensional electronic band structure. Large modulations of the conductance mediated by a control of the Fermi energy reveal the Landau fragmentation, carrying the fingerprints of the confined InAs material. Our numerical simulations of the magnetic band structure consistently support the experimental results and reveal key parameters of the electronic confinement.

We study the manipulation of the spin polarization of photoemitted electrons in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by spin- and angle-resolved photoemission spectroscopy. General rules are established that enable controlling the photoelectron spin-polarization. We demonstrate the $\pm 100\%$ reversal of a single component of the measured spin-polarization vector upon the rotation of light polarization, as well as full three-dimensional manipulation by varying experimental configuration and photon energy. While a material-specific density-functional theory analysis is needed for the quantitative description, a minimal yet fully generalized two-atomic-layer model qualitatively accounts for the spin response based on the interplay of optical selection rules, photoelectron interference, and topological surface-state complex structure. It follows that photoelectron spin-polarization control is generically achievable in systems with a layer-dependent, entangled spin-orbital texture.

We propose and analyze the use of hybrid microwave cavities as quantum heat engines. A possible realization consists of two macroscopically separated quantum-dot conductors coupled capacitively to the fundamental mode of a microwave cavity. We demonstrate that an electrical current can be induced in one conductor through cavity-mediated processes by heating up the other conductor. The heat engine can reach Carnot efficiency with optimal conversion of heat to work. When the system delivers the maximum power, the efficiency can be a large fraction of the Carnot efficiency. The heat engine functions even with moderate electronic relaxation and dephasing in the quantum dots. We provide detailed estimates for the electrical current and output power using realistic parameters.

We use a quantitative convergent beam electron diffraction based method to image the valence electron density distribution in $\mathrm{Ba}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$. We show a remarkable increase in both the charge quadrupole of the Fe cations and the charge dipole of the arsenic anions upon Co doping from $x=0$ ($T_c=0\mathrm{K}$) to $x=0.1$ ($T_c=22.5\mathrm{K}$). Our data suggest that an unexpected electronic correlation effect, namely strong coupling of Fe orbital fluctuation and anion electronic polarization, is present in iron-based superconductors.

We investigate the photon statistics, entanglement, and squeezing of a $p\text{−}n$ junction sandwiched between two superconducting leads and show that such an electrically driven photon pump generates correlated and entangled pairs of photons. In particular, we demonstrate that the squeezing of the fluctuations in the quadrature amplitudes of the emitted light can be manipulated by changing the relative phase of the order parameters of the superconductors. This reveals how macroscopic coherence of the superconducting state can be used to tailor the properties of a two-photon state.

A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in local density $\text{approximation}+\mathrm{U}$ electronic structure calculation. Within the mean field approximation, exchange constants can be mapped into a series of total energy calculations by the pair-flip approximation technique. The application to uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but a ferromagnetic one in quadrupoles which is very different from past studies. Further calculation of the spin-lattice interaction indicates it is of the same order with the superexchange and characterizes the overall behavior of the quadrupolar part as a competition between them.

Recently, realizations of Kitaev physics have been sought in the $A_2{\mathrm{IrO}}_3$ family of honeycomb iridates, originating from oxygen-mediated exchange through edge-shared octahedra. However, for the $j_{\text{eff}}=1/2$ Mott insulator in these materials, exchange from direct $d$-orbital overlap is relevant, and it was proposed that a Heisenberg term should be added to the Kitaev model. Here, we provide the generic nearest-neighbor spin Hamiltonian when both oxygen-mediated and direct overlap are present, containing a bond-dependent off-diagonal exchange in addition to Heisenberg and Kitaev terms. We analyze this complete model using a combination of classical techniques and exact diagonalization. Near the Kitaev limit, we find new magnetic phases, 120° and incommensurate spiral order, as well as extended regions of zigzag and stripy order. Possible applications to ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\mathrm{Li}}_2{\mathrm{IrO}}_3$ are discussed.

Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-dependent density functional theory are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered ($1\times 2$) surface of $\mathrm{Ca}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$ ($x=0$, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation ($\sim 0.5\AA$), and the underneath $\mathrm{As}\text{−}{\mathrm{Fe}}_2\text{−}\mathrm{As}$ layer displays a significant buckling. First-principles calculations show that the ($1\times 2$) phase is stabilized by the bulk antiferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap ($\sim 7\text{meV}$ at 7.4 K) is observed to spatially coexist with the ($1\times 2$) phase ($x=0.075$ compound). This implies the coexistence of both superconductivity and AFM ordering at the surface.

Spin-$1/2$ Heisenberg antiferromagnets ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ and ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$, yielding $J/k_B=4.7(2)\mathrm{K}$, $J^{\prime }/k_B=1.42(7)\mathrm{K}$, [$J^{\prime }/J\simeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$, we obtain $J/k_B=14.9(7)\mathrm{K}$, $J^{\prime }/k_B=6.1(3)\mathrm{K}$, [$J^{\prime }/J\simeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

We theoretically study absorption by an undoped graphene layer decorated with arrays of small particles. We discuss periodic and random arrays within a common formalism, which predicts a maximum absorption of 50% for suspended graphene in both cases. The limits of weak and strong scatterers are investigated, and an unusual dependence on particle-graphene separation is found and explained in terms of the effective number of contributing evanescent diffraction orders of the array. Our results can be important to boost absorption by single-layer graphene due to its simple setup with potential applications to light harvesting and photodetection based on energy (Förster) rather than charge transfer.

Using molecular dynamics simulations, we find a reversible transition between the dispersion and aggregation states of solute molecules in aqueous solutions confined in nanoscale geometry, which is not observed in macroscopic systems. The nanoscale confinement also leads to a significant increase of the critical aggregation concentration (CAC). A theoretical model based on Gibbs free energy calculation is developed to describe the simulation results. It indicates that the reversible state transition is attributed to the low free energy barrier (of order $k_{B}T$) in between two energy minima corresponding to the dispersion and aggregation states, and the enhancement of the CAC results from the fact that at lower concentrations the number of solute molecules is not large enough to allow the formation of a stable cluster in the confined systems.

We use the context of dryland vegetation to study a general problem of complex pattern-forming systems: multiple pattern-forming instabilities that are driven by distinct mechanisms but share the same spectral properties. We find that the co-occurrence of two Turing instabilities when the driving mechanisms counteract each other in some region of the parameter space results in the growth of a single mode rather than two interacting modes. The interplay between the two mechanisms compensates for the simpler dynamics of a single mode by inducing a wider variety of patterns, which implies higher biodiversity in dryland ecosystems.

In 1945, Dirac attempted to develop a “formal probability” distribution to describe quantum operators in terms of two noncommuting variables, such as position $x$ and momentum $p$ [Rev. Mod. Phys. 17, 195 (1945)]. The resulting quasiprobability distribution is a complete representation of the quantum state and can be observed directly in experiments. We measure Dirac’s distribution for the quantum state of the transverse degree of freedom of a photon by weakly measuring transverse $x$ so as to not randomize the subsequent $p$ measurement. Furthermore, we show that the distribution has the classical-like feature that it transforms (e.g., propagates) according to Bayes’ law.

We present results on quantum quenches in lattice systems with a fixed number of particles in a much larger number of sites. Both local and global quenches in this limit generically have power-law work distributions (“edge singularities”). We show that this regime allows for large edge singularity exponents beyond that allowed by the constraints of the usual thermodynamic limit. This large-exponent singularity has observable consequences in the time evolution, leading to a distinct intermediate power-law regime in time. We demonstrate these results first using local quantum quenches in a low-density Kondo-like system, and additionally through global and local quenches in Bose-Hubbard, Aubry-Andre, and hard-core boson systems at low densities.

Ultracold neutrons (UCNs) can be bound by the potential of terrestrial gravity and a reflecting mirror. The wave function of the bound state has characteristic modulations. We carried out an experiment to observe the vertical distribution of the UCNs above such a mirror at the Institut Laue-Langevin in 2011. The observed modulation is in good agreement with that prediction by quantum mechanics using the Wigner function. The spatial resolution of the detector system is estimated to be $0.7\mu \mathrm{m}$. This is the first observation of gravitationally bound states of UCNs with submicron spatial resolution.