Detailed quantitative measurement of surface dynamics during thin film growth is a major experimental challenge. Here x-ray photon correlation spectroscopy with coherent hard x rays is used in a grazing-incidence small-angle x-ray scattering (i.e., Co-GISAXS) geometry as a tool to investigate nanoscale surface dynamics during sputter deposition of a-Si and a-$\mathrm{WS}{\mathrm{i}}_2$ thin films. For both films, kinetic roughening during surface growth reaches a dynamic steady state at late times in which the intensity autocorrelation function $g_2\left(q,t\right)$ becomes stationary. The $g_2\left(q,t\right)$ functions exhibit compressed exponential behavior at all wavenumbers studied. The overall dynamics are complex, but the most surface sensitive sections of the structure factor and correlation time exhibit power law behaviors consistent with dynamical scaling.

We use first-principles calculations to investigate the lattice properties of group-IV monochalcogenides. These include static dielectric permittivity, elastic and piezoelectric tensors. For the monolayer, it is found that the static permittivity, besides acquiring a dependence on the interlayer distance, is comparatively higher than in the 3D system. In contrast, it is found that elastic properties are little changed by the lower dimensionality. Poisson ratios relating in-plane deformations are close to zero, and the existence of a negative Poisson ratio is also predicted for the GeS compound. Finally, the monolayer shows piezoelectricity, with piezoelectric constants higher than those recently predicted to occur in other 2D systems, such as hexagonal BN and transition-metal dichalcogenide monolayers.

Hybrid pumping appears as a promising compromise in order to reach the much coveted goal of an electrically pumped organic laser. In such configuration the organic material is optically pumped by an electrically pumped inorganic device on a chip. This engineering solution requires therefore an optimization of the organic gain medium under optical pumping. Here, we report a detailed study of the gain features of dye-doped polymer thin films. In particular we introduce the gain efficiency $K$, in order to facilitate comparison between different materials and experimental conditions. The gain efficiency was measured with a variety of experimental methods (pump-probe amplification, variable stripe length method, laser thresholds) in order to study several factors which modify the actual gain of a layer, namely the confinement factor, the pump polarization, the molecular anisotropy, and the re-absorption. For instance, for a 600-nm-thick 5-wt % DCM doped poly(methyl methacrylate) (PMMA) layer, the different experimental approaches give a consistent value of $K\simeq 80\text{−}\mathrm{cm}{\mathrm{MW}}^{-1}$. On the contrary, the usual model predicting the gain from the characteristics of the material leads to an overestimation by two orders of magnitude, which raises a serious problem in the design of actual devices. In this context, we demonstrate the feasibility to infer the gain efficiency from the laser threshold of well-calibrated devices. Temporal measurements at the picosecond scale were carried out to support the analysis.

We present measurements of thermal and electrical conductivity of polycrystalline permalloy (Ni-Fe), aluminum, copper, cobalt, and nickel thin films with thickness $<200$ nm. A micromachined silicon-nitride membrane thermal-isolation platform allows measurements of both transport properties on a single film and an accurate probe of the Wiedemann–Franz (WF) law expected to relate the two. Through careful elimination of possible effects of surface scattering of phonons in the supporting membrane, we find excellent agreement with WF in a thin Ni-Fe film over nearly the entire temperature range from 77 to 325 K. All other materials studied here deviate somewhat from the WF prediction of electronic thermal conductivity with a Lorenz number, $L$, suppressed from the free-electron value by $10\%\text{to}20\%$. For Al and Cu we compare the results to predictions of the theoretical expression for the Lorenz number as a function of $T$. This comparison indicates two different types of deviation from expected behavior. In the Cu film, a higher than expected $L$ at lower $T$ indicates an additional thermal conduction mechanism, while at higher $T$ lower than expected values suggests an additional inelastic scattering mechanism for electrons. We suggest the additional low-$TL$ indicates a phonon contribution to thermal conductivity and consider increased electron-phonon scattering at grain boundaries or surfaces to explain the high-$T$ reduction in $L$.

$\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ is a spin-orbit-coupled insulator with an antiferromagnetic (AFM) transition at $T_N=240\mathrm{K}$. We report results of a comprehensive study of single-crystal $\mathrm{S}{\mathrm{r}}_2\mathrm{I}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{b}}_x{\mathrm{O}}_4\left(0\le x\le 0.03\right)$. This study found that a mere 3% $\left(x=0.03\right)$ of tetravalent $\mathrm{T}{\mathrm{b}}^{4+}\left(4{\mathrm{f}}^7\right)$ substituting for $\mathrm{I}{\mathrm{r}}^{4+}$ (rather than $\mathrm{S}{\mathrm{r}}^{2+}$) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of the magnetic interactions and charge gap. The insulating state at $x=0.03$ is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95,0,0) and (0,0.95,0) in the neutron diffraction, suggesting a spiral or spin-density-wave order. It is apparent that Tb doping effectively changes the relative strength of the spin-orbit interaction (SOI) and the tetragonal crystal electric field and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM is accompanied by no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and causes a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.

The recent phase-sensitive measurements in the superconducting $B$ phase of ${\mathrm{UPt}}_3$ provide strong evidence for the triplet, chiral $k_z{(k_x\pm i{k}_y)}^2$ pairing symmetries, which endow the Cooper pairs with orbital angular momentum projections $L_z=\pm 2$ along the $c$ axis. In the absence of disorder such pairing can support both line and point nodes, and both types of nodal quasiparticles exhibit nontrivial topology in the momentum space. The point nodes, located at the intersections of the closed Fermi surfaces with the $c$ axis, act as the double monopoles and the antimonopoles of the Berry curvature, and generalize the notion of Weyl quasiparticles. Consequently, the $B$ phase should support an anomalous thermal Hall effect, the polar Kerr effect, in addition to the protected Fermi arcs on the $(1,0,0)$ and the $(0,1,0)$ surfaces. The line node at the Fermi surface equator acts as a vortex loop in the momentum space and gives rise to the zero-energy, dispersionless Andreev bound states on the $(0,0,1)$ surface. At the transition from the $B$ phase to the $A$ phase, the time-reversal symmetry is restored, and only the line node survives inside the $A$ phase. As both line and double-Weyl point nodes possess linearly vanishing density of states, we show that weak disorder acts as a marginally relevant perturbation. Consequently, an infinitesimal amount of disorder destroys the ballistic quasiparticle pole, while giving rise to a diffusive phase with a finite density of states at the zero energy. The resulting diffusive phase exhibits $T$-linear specific heat, and an anomalous thermal Hall effect. We predict that the low-temperature thermodynamic and transport properties display a crossover between a ballistic thermal Hall semimetal and a diffusive thermal Hall metal. By contrast, the diffusive phase obtained from a time-reversal-invariant pairing exhibits only the $T$-linear specific heat without any anomalous thermal Hall effect.

We provide a general procedure to calculate the current-induced spin-transfer torque which acts on a general steep magnetic texture due to the exchange interaction with an applied spin-polarized current. As an example, we consider a one-dimensional ferromagnetic quantum wire and also include a Rashba spin-orbit interaction. The spin-transfer torque becomes generally spatially nonlocal. Likewise, the Rashba spin-orbit interaction induces a spatially nonlocal fieldlike nonequilibrium spin-transfer torque. We also find a spatially varying nonadiabaticity parameter and markedly different domain wall dynamics for very steep textures as compared to wide domain walls.

We present high-energy x-ray diffraction data under applied pressures up to $p=29\mathrm{GPa}$, neutron diffraction measurements up to $p=1.1\mathrm{GPa}$, and electrical resistance measurements up to $p=5.9\mathrm{GPa}$, on ${\mathrm{SrCo}}_2{\mathrm{As}}_2$. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at $T=7\mathrm{K}$. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the $a$ axis is the same for the T and cT phases, whereas, along the $c$ axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ for $p\le 5.9$ GPa and $T\ge 1.8$ K. The resistivity data also show signatures consistent with a pressure-induced phase transition for $p \gtrsim 5.5$ GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a $\frac{c}{a}$ ratio of 2.54. Furthermore, these calculations indicate that the cT phase of ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ should manifest either A-type antiferromagnetic or ferromagnetic order.

${\mathrm{TaS}}_2$ is a transition-metal dichalcogenide having an exceptionally rich phase diagram, which includes exotic phenomena such as a charge density wave. We analyzed the structure, bonding, ground state, and dynamical stability of $1T, 2H$, and $3R$ phases of ${\mathrm{TaS}}_2$, and a commensurate charge density wave phase from the first principles. Van der Waals interaction among layers and strong electron-electron interactions were included by using the exact exchange plus random phase approximation, a high-level quantum mechanical approach. The calculated structural parameters agree well with the available experimental data. The individual sheets of ${\mathrm{TaS}}_2$ are bound by dispersive forces, which are stronger than dispersive forces in graphite and fluorographite. $1T-{\mathrm{TaS}}_2$ is dynamically unstable at low temperature, which leads to the formation of charge density wave and opening of the in-plane band gap. Anharmonic phonon-phonon interactions stabilize the $1T$ structure at elevated temperatures. The calculated phase diagram of ${\mathrm{TaS}}_2$ reveals that the $1T$ phase is the ground state at temperatures above 1300 K, $2H$ below this point, and the charge density wave phase becomes more stable than the perfect $1T$ structure below 480 K.

The in-plane rotation of magnetic stripe domains in a 65-nm magnetostrictive ${\mathrm{Fe}}_{0.8}{\mathrm{Ga}}_{0.2}$ epitaxial film was investigated combining magnetic force microscopy, vibration sample magnetometry, and x-ray resonant magnetic scattering measurements. We analyzed the behavior of the stripe pattern under the application of a bias magnetic field along the in-plane direction perpendicular to the stripe axis, and made a comparison with the analogous behavior at remanence. The experimental results have been explained by means of micromagnetic simulations, supported by energy balance considerations. Fields smaller than $\sim 400$ Oe do not induce any stripe rotation; rather, a deformation of the closure domains pattern was evidenced. Larger fields produce a sudden rotation of the stripe structure.

The magnetic-field-induced ${90}^{\circ }$ flop of ferroelectric polarization $\mathit{P}$ in a spin-spiral multiferroic material ${\mathrm{TbMnO}}_3$ is theoretically studied based on a microscopic spin model. I find that the direction of the $\mathit{P}$ flop or the choice of $+{\mathit{P}}_a$ or $-{\mathit{P}}_a$ after the flop is governed by magnetic torques produced by the applied magnetic field $\mathit{H}$ acting on the Mn spins and thus is selected in a deterministic way, in contradistinction to the naively anticipated probabilistic flop. This mechanism resolves a puzzle of the previously reported memory effect in the $\mathit{P}$ direction depending on the history of the magnetic-field sweep, and enables controlled switching of multiferroic domains by externally applied magnetic fields. My Monte-Carlo analysis also uncovers that the magnetic structure in the $\mathit{P}\parallel \mathit{a}$ phase under $\mathit{H}\parallel \mathit{b}$ is not a previously anticipated simple $ab$-plane spin cycloid but a conical spin structure.

We investigate the complex conductivity of a highly disordered MoC superconducting film with $k_{F}l\approx 1$, where $k_F$ is the Fermi wave number and $l$ is the mean free path, derived from experimental transmission characteristics of coplanar waveguide resonators in a wide temperature range below the superconducting transition temperature $T_c$. We find that the original Mattis-Bardeen model with a finite quasiparticle lifetime, $\tau$, offers a perfect description of the experimentally observed complex conductivity. We show that $\tau$ is appreciably reduced by scattering effects. Characteristics of the scattering centers are independently found by scanning tunneling spectroscopy and agree with those determined from the complex conductivity.

The photoluminescence (PL) spectra of ${\mathrm{Ar}}^{+}$-ion irradiated single crystals of ${\mathrm{SrTiO}}_3$ (STO) excited by the 325 nm line of a He-Cd laser are compared with those of pristine crystals, epitaxial films, and amorphous layers of STO at several temperatures down to 20 K. The 550 eV ${\mathrm{Ar}}^{+}$-beam irradiation activates three distinctly visible PL peaks: blue ($\sim 430$ nm), green ($\sim 550$ nm), and infrared ($\sim 820$ nm) at room temperature, making the photoluminescence multicolored. The abrupt changes in PL properties below $\approx 100$ K are discussed in relation with the antiferrodistortive structural phase transition in ${\mathrm{SrTiO}}_3$ from cubic to tetragonal symmetry, which makes it a direct bandgap semiconductor. The photoluminescence spectra are also tuned by an electrostatic gate field in a field-effect transistor geometry. At 20 K, we observed a maximum increase of $\sim 20\%$ in PL intensity under back gating of ${\mathrm{SrTiO}}_3$.

We study the properties of the ground states of the one- and two-dimensional Hubbard models at half-filling and moderate doping using entanglement-based measures, which we calculate numerically using the momentum-space density matrix renormalization group (DMRG). In particular, we investigate quantities such as the single-site entropy and two-site mutual information of single-particle momentum states as well as the behavior of the bipartite subsystem entropy for partitions in momentum space. The distribution of these quantities in momentum space gives insight into the fundamental nature of the ground state, insight that can be used to make contact with weak-coupling-based analytic approaches and to optimize numerical methods, the momentum-space DMRG in particular. We study the site and subsystem entropies as a function of interaction strength $U$ and system size. In both the one- and two-dimensional cases, we find that the subsystem entropy scales proportionally to $U^2$ for weak $U$ and proportionally to volume. Nevertheless, the optimized momentum-space DMRG can provide variationally accurate results for the two-dimensional Hubbard model at weak coupling for moderate system sizes.

We introduce a set of generalized slave-particle models for extended Hubbard models that treat localized electronic correlations using slave-boson decompositions. Our models automatically include two slave-particle methods of recent interest, the slave-rotor and slave-spin methods, as well as a ladder of new intermediate models where one can choose which of the electronic degrees of freedom (e.g., spin or orbital labels) are treated as correlated degrees of freedom by the slave bosons. In addition, our method removes the aberrant behavior of the slave-rotor model, where it systematically overestimates the importance of electronic correlation effects for weak interaction strength, by removing the contribution of unphysical states from the bosonic Hilbert space. The flexibility of our formalism permits one to separate and isolate the effect of correlations on the key degrees of freedom.

With the improvement of the experimental techniques, many new phenomena in which the photon degrees of freedom are involved have been discovered. Typical examples are the exciton-polariton quasiparticles, excitons strongly coupled to photons. A correct description of these systems requires a full account of the photon's dynamics, including the vector gauge degrees of freedom. In order to include this contribution in ab initio many-body perturbation theories, here we present a generalization of the Hedin equations, originally derived for a many-electron system with Coulomb interaction. These equations are now derived to take into account the strong-coupling physics of the electrons and the transverse photon's degrees of freedom.

We theoretically study the atomic structure and energetics of silicon and silicon-nitrogen impurities in graphene. Using density-functional theory, we get insight into the atomic structures of the impurities, evaluate their formation energies and assess their abundance in realistic samples. We find that nitrogen, as well as oxygen and hydrogen, are trapped at silicon impurities, considerably altering the electronic properties of the system. Furthermore, we show that nitrogen doping can induce local magnetic moments resulting in spin-dependent transport properties, even though neither silicon nor nitrogen impurities are magnetic by themselves. To simulate large systems with many randomly distributed impurities, we derive tight-binding models that describe the effects of the impurities on graphene $\pi$ electron structure. Then by using the linear-scaling real-space Kubo-Greenwood method, we evaluate the transport properties of large-scale systems with random distribution of impurities, and find the fingerprintlike scattering cross sections for each impurity type. The transport properties vary widely, and our results indicate that some of the impurities can even induce strong localization in realistic graphene samples.

We have studied the superlattices with alternating block layers (BLs) of heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ and conventional-metal ${\mathrm{YbCoIn}}_5$ by site-selective nuclear magnetic resonance spectroscopy, which uniquely offers spatially resolved dynamical magnetic information. We find that the presence of antiferromagnetic fluctuations is confined to the Ce BLs, indicating that magnetic degrees of freedom of $f\mathrm{electrons}$ are quenched inside the Yb BLs. Contrary to simple expectations that the two dimensionalization enhances fluctuations, we observe that antiferromagnetic fluctuations are rapidly suppressed with decreasing Ce BL thickness. Moreover, the suppression is more prominent near the interfaces between the BLs. These results imply significant effects of local inversion symmetry breaking at the interfaces.