After an initial transient period, the conduction regime in a one-dimensional superconducting wire that carries a fixed current is either normal, periodic, or stationary. The phase diagram for these possibilities was studied in Phys. Rev. Lett. 99, 167003 (2007) for particular values of the length and the material parameters. We have extended this study to arbitrary length and to a range of material parameters that includes realistic values. Variation of the length leads to scaling laws for the phase diagram. Variation of the material parameters leads to new qualitative features and new phases, including a parameter region in which all three regimes are possible.

We investigate the superconducting lifetime of long current-biased Josephson junctions, in the presence of Gaussian and non-Gaussian noise sources. In particular, we analyze the dynamics of a Josephson junction as a function of the noise signal intensity, for different values of the parameters of the system and external driving currents. We find that the mean lifetime of the superconductive state is characterized by nonmonotonic behavior as a function of noise intensity, driving frequency, and junction length. We observe that these nonmonotonic behaviors are connected with the dynamics of the junction phase string during the switching towards the resistive state. An important role is played by the formation and propagation of solitons, with two different dynamical regimes characterizing the dynamics of the phase string. Our analysis allows to evidence the effects of different bias current densities, that is a simple spatially homogeneous distribution and a more realistic inhomogeneous distribution with high current values at the edges. Stochastic resonant activation, noise-enhanced stability, and temporary trapping phenomena are observed in the system investigated.

Following the field theoretic approach of Basko et al. [Ann. Phys. 321, 1126 (2006)], we study in detail the real-time dynamics of a system expected to exhibit many-body localization. In particular, for time scales inaccessible to exact methods, we demonstrate that within the second-Born approximation the temporal decay of the density-density correlation function is consistent with a finite value for $t\to \infty$, as expected in a nonergodic state. This behavior persists over a wide range of disorder and interaction strengths and suggests that the nonergodic phase is surprisingly robust compared to the prediction of Basko et al. We argue that this discrepancy may be resolved by the presence of a nonergodic, but metallic, phase.

Neutron diffraction studies have revealed an uncommon commensurate-incommensurate magnetic phase transition with decreasing temperature in the (∼5%) Fe-doped bilayer ruthenate ${\mathrm{Ca}}_3(\mathrm{Ru},\mathrm{Fe}{)}_2{\mathrm{O}}_7$. An incommensurate phase formed of a cycloidal spiral spin structure coexists with a commensurate one below the phase transition at 42 K and persists down to the lowest temperature, accompanied by higher-order magnetic satellite peaks which indicate the formation of a magnetic soliton lattice. We ascribe these findings to the competing magnetic interactions in this system. This study demonstrates an effective approach to tune novel magnetic and electronic properties of ruthenates via $3d$ magnetic transition-metal substitution.

While the absorption of light is ubiquitous in nature and in applications, the question remains how absorption modifies the transmission channels in random media. We present a numerical study on the effects of optical absorption on the maximal transmission and minimal reflection channels in a two-dimensional disordered waveguide. In the weak absorption regime, where the system length is less than the diffusive absorption length, the maximal transmission channel is dominated by diffusive transport and it is equivalent to the minimal reflection channel. Its frequency bandwidth is determined by the underlying quasimode width. However, when the absorption is strong, light transport in the maximal transmission channel undergoes a sharp transition and exhibits ballistic-like transport. Its frequency bandwidth increases with absorption, and the exact scaling varies with the sample's realization. The minimal reflection channel becomes different from the maximal transmission channel and becomes dominated by absorption. Counterintuitively, we observe in some samples that the minimum reflection eigenvalue increases with absorption. Our results show that strong absorption turns open channels in random media from diffusive to ballistic-like transport.

We study theoretically quantum dynamics of interacting bosons in artificial magnetic fields as engineered in recent ultracold atomic experiments, where quantum cyclotron orbital motion has been observed. With exact numerical simulations and perturbative analyses, we find that interactions induce damping in the cyclotron motion. The damping time is found to be dependent on interaction and tunneling strengths monotonically, while its dependence on magnetic flux is nonmonotonic. Sufficiently strong interactions would render bosons dynamically localized, inhibiting the cyclotron motion. The damping predicted by us can be construed as an interaction-induced quantum decoherence of the cyclotron motion.

This predictive theoretical contribution demonstrates that spin-crossover (SC) solids, under light in the regime of the thermally induced hysteresis loop, contain all necessary ingredients for observing autocatalytic behaviors. The sustained oscillations of the high-spin (HS) fraction emerge spontaneously as a result of a subtle balance between the nonlinear relaxation of the HS fraction and the photothermal effect caused by a steady light excitation. Here, we perform a detailed analytical study based on linear stability investigations to clarify the conditions of emergence of such phenomena with respect to the system's parameter in SC solids. Very accurate predictions were obtained and confirmed by the numerical investigations. The treatment developed here should generate experimental results on the nonlinear dynamics and self-induced oscillations in SC solids, and furthermore it should open the perspective of future observations of dissipative structures and pattern formation in all kinds of switchable molecular solids.

In this paper, we present a model for the breathing crystal field of spin-crossover solids. Using Monte Carlo simulation, we have systematically studied the thermal transition of spin-crossover solids for local random fields with different statistical characteristics. It is found that the breathing of crystal field provokes the broadening of the hysteresis loop, but for strong colored fluctuations its power spectrum is diminished and the width of the hysteresis loop approaches again to the deterministic one.

We study the structural, magnetic, and transport properties of ${\mathrm{LaAlO}}_3$/${\mathrm{EuTiO}}_3$/${\mathrm{SrTiO}}_3$ heterostructures grown by pulsed laser deposition. The samples are characterized in situ by electron diffraction and scanning probe microscopy and ex situ by transport measurements and x-ray absorption spectroscopy. ${\mathrm{LaAlO}}_3$/${\mathrm{EuTiO}}_3$/${\mathrm{SrTiO}}_3$ films show a ferromagnetic transition at $T\le 7.5$ K, related to the ordering of ${\mathrm{Eu}}^{2+}$ spins, even in samples characterized by just two ${\mathrm{EuTiO}}_3$ unit cells. A finite metallic conductivity is observed only in the case of samples composed by one or two ${\mathrm{EuTiO}}_3$ unit cells and, simultaneously, by a ${\mathrm{LaAlO}}_3$ thickness greater than or equal to four unit cells. The effect of ferromagnetic ${\mathrm{EuTiO}}_3$ on the transport properties of $\delta$-doped ${\mathrm{LaAlO}}_3$/${\mathrm{EuTiO}}_3$/${\mathrm{SrTiO}}_3$ is critically discussed.

We present the temporal evolution of the longitudinal spin Seebeck effect in a YIG$|$Pt bilayer system. Our findings reveal that this effect is a submicrosecond fast phenomenon governed by the thermal-magnon diffusion along the thermal gradient inside the magnetic material. A comparison of experimental results with the thermal-driven magnon-diffusion model demonstrates that the temporal behavior of this effect depends on the time development of the temperature gradient in the magnetic material close to the interface. The effective thermal-magnon diffusion length for the YIG$|$Pt system is estimated to be around 500 nm.

We calculate the magnon lifetime in an easy-plane ferromagnet on a tetragonal lattice in a transverse magnetic field. At zero temperature magnons are unstable with respect to spontaneous decay into two other magnons. Varying the ratio of intrachain to interchain exchanges in this model, we consider the effect of dimensionality on spontaneous magnon decay. The strongest magnon damping is found in the quasi-one-dimensional case for momenta near the Brillouin zone boundary. The sign of a weak interchain coupling has little effect on the magnon decay rate. The obtained theoretical results suggest the possibility of an experimental observation of spontaneous magnon decay in a quasi-one-dimensional ferromagnet $\mathrm{Cs}\mathrm{Ni}{\mathrm{F}}_3$. We also find an interesting enhancement of the magnon decay rate for a three-dimensional ferromagnet. The effect is present only for the nearest-neighbor model and is related to an effective dimensionality reduction in the two-magnon continuum.

In this Comment on the paper by S. Lei et al. [Phys. Rev. B 86, 134115 (2012)] we discuss in detail the quality and reliability of the experimental results the authors rely on for their statements. Indeed do we believe that the experimental data do not allow the conclusions drawn by the authors. In order to support our assessment we present our own newly recorded experimental data. Furthermore, we question the validity of the authors' finite element modeling, the outcome of which being, at least partially, contradictory to the experimental results.

In response to the preceding Comment of Jungk et al. [Phys. Rev. B 89, 226101 (2014)] regarding the validity of our finite element method (FEM) modeling in quantitatively predicting the piezoelectric response behavior in piezoresponse force microscopy (PFM), particularly the lateral response, we present here our latest experiment results that show quantitative agreement between FEM predictions and experiments. An approach to decouple the vertical PFM signal into cantilever vertical deflection and buckling contribution is proposed. Combined with simultaneously obtained lateral PFM, we show the possibility of performing quantitative three-dimensional PFM measurements (two lateral and one vertical response) from one single PFM image. A method to perform background subtraction for lateral signals is now presented that eliminates the spurious background issues seen in the past experimental works cited previously by Lei et al. [Phys. Rev. B 86, 134115 (2012)], as well as in the new results presented in the Comment of Jungk et al. The quantitative agreement between experiments and FEM then becomes clear with such a subtraction scheme.

Metallic transition-metal oxides undergo a metal-to-insulator transition (MIT) as the film thickness decreases across a critical thickness of several monolayers (MLs), but its driving mechanism remains controversial. We have studied the thickness-dependent MIT of the ferromagnetic metal ${\mathrm{La}}_{0.6}$${\mathrm{Sr}}_{0.4}$${\mathrm{MnO}}_3$ by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. As the film thickness was decreased across the critical thickness of the MIT (6–8 ML), a gradual decrease of the ferromagnetic signals and a concomitant increase of paramagnetic signals were observed, while the Mn valence abruptly decreased towards ${\mathrm{Mn}}^{3+}$. These observations suggest that the ferromagnetic phase gradually and most likely inhomogeneously turns into the paramagnetic phase and both phases abruptly become insulating at the critical thickness.

The position of the first satellite valley in wurtzite GaN is directly determined by near-band-gap photoemission spectroscopy of $p$-doped GaN activated to negative electron affinity. The photoemission spectra exhibit two structures, with fixed energy position, which originate from electrons accumulated in the conduction band valleys of the bulk material. We assigned the two observed features respectively to $\Gamma$ and L valleys and obtain an intervalley energy separation of $0.90\pm 0.08$ eV, well below the theoretical values of the lowest subsidiary valley energy provided by ab initio calculations.

We report on a systematic study of the structural, magnetic, and transport properties of high-purity $1T$-${\mathrm{VS}}_2$ powder samples prepared under high pressure. The results differ notably from those previously obtained by deintercalating Li from $\mathrm{Li}{\mathrm{VS}}_2$. First, no charge-density wave (CDW) is found by transmission electron microscopy down to 94 K, though ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector ${\mathbf{q}}_{\text{CDW}}\approx (0.21,0.21,0)$ previously reported in deintercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of nonmetallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and nonmetallic domains in the nm scale. We conclude that $1T$-${\mathrm{VS}}_2$ is at the verge of a CDW transition and suggest that residual electronic doping in Li deintercalated samples stabilizes a uniform CDW phase with metallic properties.

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states—itinerant ${\mathrm{Ce}}^{4+}$ and localized ${\mathrm{Ce}}^{3+}$. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an “alloy” problem involving two valences with competing and numerous site-occupancy configurations. We use density-functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling the valence by pseudoternary alloying. For Ce and its compounds, such as $(\mathrm{Ce},\mathrm{La}{)}_2(\mathrm{Fe},\mathrm{Co}{)}_{14}\mathrm{B}$ permanent magnets, we find a stable mixed-valent $\alpha$ state near the spectroscopic value of ${\nu }_s=3.53$. Ce valency in compounds depends on its steric volume and local chemistry. For La doping, Ce valency shifts towards $\gamma$-like ${\mathrm{Ce}}^{3+}$, as expected from steric volume; for Co doping, valency depends on local Ce-site chemistry and steric volume. Our approach captures the key origins of anomalous valency and site-preference chemistry in complex compounds.

The effective Hamiltonian that describes magnetic exchange interaction in a topological insulator is calculated, projecting bulk contact $s$-$d$ interaction onto surface electron states. This model consistently describes the indirect exchange between magnetic atoms mediated either by massless Dirac fermions in thick films or by massive excitations in a thin slab with tunneling between opposite surfaces. It is shown that an effective $s$-$d$ exchange is sensitive to the symmetry of surface wave functions and depends on the position of an impurity spin relative to slab surfaces. Under a vertical bias the indirect exchange between localized spins is switchable between opposite surfaces, and the oscillating range function reveals zero magnetic field beating as a signature of Rashba spin splitting.

Hexagonal boron nitride (h-BN) is a prominent member in the growing family of two-dimensional materials with potential applications ranging from being an atomically smooth support for other two-dimensional materials to templating growth of molecular layers. We have studied the structure of monolayer h-BN grown by chemical vapor deposition on Ir(111) by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art density functional theory (DFT) calculations. The lattice mismatch between the h-BN and Ir(111) surface results in the formation of a moiré superstructure with a periodicity of $\sim$29 Å and a corrugation of $\sim$0.4 Å. By measuring the field emission resonances above the h-BN layer, we find a modulation of the work function within the moiré unit cell of $\sim$0.5 eV. DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental findings and allow us to relate the change in the work function to the subtle changes in the interaction between boron and nitrogen atoms and the underlying substrate atoms within the moiré unit cell. Hexagonal boron nitride on Ir(111) combines weak topographic corrugation with a strong work function modulation over the moiré unit cell. This makes h-BN/Ir(111) a potential substrate for electronically modulated thin film and heterosandwich structures.

Bernal- and non-Bernal-stacked graphene layers have been systematically studied by Raman imaging and spectroscopy. Two dominant Raman modes, $G$ and $G^{\prime }$ (or 2$D$), of folded graphene layers exhibit three types of spectral features when interlayer lattice mismatches, defined by a rotational angle varies. Among these folded graphene layers, the most interesting one is the folded graphene layers that present an extremely strong $G$ mode enhanced by a twist-induced Van Hove singularity. The evolution of Raman $G$ and $G^{\prime }$ modes of such folded graphene layers are probed by changing the excitation photon energies. In this paper, doublet splitting of the $G^{\prime }$ mode in a folded double-layer (1 + 1) and of the $G$ mode in a folded tetralayer (2 + 2) graphene are observed and discussed. The $G^{\prime }$ mode splitting in folded double-layer graphene is attributed to the coexistence of inner and outer scattering processes and the trigonal warping effect as well as further downward bending of the inner dispersion branch at a visible excitation energy. The two peaks of the $G$ mode in folded tetralayer graphene are assigned to Raman-active mode ($E_{2\mathrm{g}}$) and lattice mismatch activated infrared-active mode ($E_{1\mathrm{u}}$), which is further verified by the temperature-dependent Raman measurements. Our study provides a summary and discussion of Raman spectra of Bernal- and non-Bernal-stacked graphene layers and further demonstrates the versatility of Raman spectroscopy for exploiting electronic band structures of graphene layers.