The morphological evolution of Si(001) is investigated for normal incidence 2 keV ${\text{Kr}}^{+}$ ion irradiation under ultra-high vacuum conditions as a function of temperature and ion fluence through scanning tunneling microscopy and low energy electron diffraction. Under the conditions chosen, the selvage of Si(001) amorphizes below the critical temperature $T_c$ of 670 K, while above it remains crystalline. Below $T_c$ the sample remains flat, irrespective of the ion fluence. Above $T_c$, the crystalline sample displays for fixed ion fluence and as a function of sample temperature a pronounced roughness maximum at 700 K. Around this temperature, with increasing ion fluence a strong increase of roughness as well as coarsening are observed. Pyramidal pits and mounds develop, with facets formed by Si steps and narrow reconstructed terraces. Most exciting, with increasing ion fluence the pattern reorients from pits and mounds with edges along the $\langle 110\rangle$ directions to ridges and valleys rotated $\approx {45}^{\circ }$ to the $\langle 110\rangle$ directions.

Elemental two-dimensional (2D) materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity of $30.4$ and $7.8$ W/mK along the zigzag and armchair directions, respectively, at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and $1\mu \mathrm{m}$ provide the main contribution to the large thermal conductivity in the zigzag direction; mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nanoelectronic applications and thermal management.

We investigate the properties of a phononic crystal plate with hollow pillars and introduce the existence of whispering-gallery modes (WGMs). We show that by tuning the inner radius of the hollow pillar, these modes can merge inside both Bragg and low frequency band gaps, deserving phononic crystal and acoustic metamaterial applications. These modes can be used as narrow pass bands for which the quality factor can be greatly enhanced by the introduction of an additional cylinder between the hollow cylinder and the plate. We discuss some functionalities of these confined WGM in both Bragg and low frequency gaps for wavelength division in multiplexer devices using heteroradii pillars introduced into waveguide and cavity structures.

A theoretical model of Gd/Fe multilayers is constructed using the atomistic spin dynamics formalism. By varying the thicknesses and number of layers we have shown that a strong dependence of the energy required for thermally induced magnetization switching (TIMS) is present; with a larger number of interfaces, lower energy is required. The results of the layer resolved dynamics show that the reversal process of the multilayered structures, similar to that of a GdFeCo alloy, is driven by the antiferromagnetic interaction between the transition-metal and rare-earth components. Finally, while the presence of the interface drives the reversal process, we show here that the switching process does not initiate at the surface but from the layers furthest from it, a departure from the alloy behavior which expands the classes of material types exhibiting TIMS.

We study resonant translational, breathing, and twisting modes of transverse magnetic domain walls pinned at notches in ferromagnetic nanostrips. We demonstrate that a mode's sensitivity to notches depends strongly on the mode's characteristics. For example, the frequencies of modes that involve lateral motion of the wall are the most sensitive to changes in the notch intrusion depth, especially at the narrow, more strongly confined end of the domain wall. In contrast, the breathing mode, whose dynamics are concentrated away from the notches is relatively insensitive to changes in the notches' sizes. We also demonstrate a sharp drop in the translational mode's frequency towards zero when approaching depinning which is confirmed, using a harmonic oscillator model, to be consistent with a reduction in the local slope of the notch-induced confining potential at its edge.

To explore the origin of the unusual nonbulk superconductivity with a $T_{\mathrm{c}}$ up to 49 K reported in the rare-earth-doped $\mathrm{CaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$, the chemical composition, magnetization, specific heat, resistivity, and annealing effect are systematically investigated on nominal $\left(\mathrm{C}{\mathrm{a}}_{1-x}{R}_x\right)\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ single crystals with different $x$ and $R=\mathrm{La}$, Ce, Pr, and Nd. All display a doping-independent $T_{\mathrm{c}}$ once superconductivity is induced, a doping-dependent low field superconducting volume fraction $f$, and a large magnetic anisotropy η in the superconducting state, suggesting a rather inhomogeneous superconducting state in an otherwise microscale homogenous superconductor. The wavelength dispersive spectroscopy and specific heat show the presence of defects that are closely related to $f$, regardless of the $R$ involved. The magnetism further reveals that the defects are mainly superparamagnetic clusters for $R=\mathrm{Ce}$, Pr, and Nd with strong intercluster interactions, implying that defects are locally self-organized. Annealing at 500 °C, without varying the doping level $x$, suppresses $f$ profoundly but not the $T_{\mathrm{c}}$. The above observations provide evidence for the crucial role of defects in the occurrence of the unusually high $T_{\mathrm{c}}\sim 49\mathrm{K}$ in $\left(\mathrm{C}{\mathrm{a}}_{1-x}{R}_x\right)\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ and are consistent with the interface-enhanced superconductivity recently proposed.

In this work we report a scanning tunneling microscopy investigation of lithographically defined superconducting nanosquares. The obtained spectroscopic maps reveal the spatial evolution of both the superconducting condensate and the screening currents as a function of the applied magnetic field. The symmetry of the nanostructure is imposed on the condensate and it controls the distribution of the vortices inside the nanosquare. Our local study allows exploring the impact of small structural defects, omnipresent in these kind of structures, on both the supercurrent and vortex distribution. As a result, direct experimental evidence of vortex pinning and current crowding at the nanoscale has been obtained.

Spin-based electronic devices such as magnetic memory and spin logic rely on spin information transport. Conduction electrons, due to their intrinsic spin angular momentum, become an obvious choice for spin information carriers. Here, we experimentally demonstrate that magnons, quasiparticles representing low-energy excitations of ferromagnetic materials, can serve as effective spin information carriers as well. Specifically, we consider two nonmagnetic heavy metals (HMs) that are separated by an electric leak-free ferrimagnetic insulator. When an electric current is applied in one of the HM layers, magnons in the ferrimagnetic insulator are excited and become an effective medium to couple the spin currents in two HMs. As a result, the charge/spin current in one HM layer can drag a charge/spin current in the other HM layer. This work provides a route for spin-based electronic devices where the spin transport is carried by quasiparticles other than electrons.

Observation of replica bands in the angle-resolved photoemission spectroscopy (ARPES) spectra of single-layer FeSe on a strontium titanate substrate revealed a phonon component contribution to mechanisms behind its high-$T_c$ superconductivity. We study the interaction of the in-layer FeSe electrons with the electric potential of the longitudinal (LO) modes at the surface of bulk $\mathrm{SrTi}{\mathrm{O}}_3$. A two-dimensional system of charges at the $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3$ interface includes both the itinerant and the immobile electrons. The latter significantly change the interface characteristics, increasing screening at the substrate surface and thereby reducing the strength of the electron-LO-phonon interaction. In what follows, the dielectric constant serves as a free parameter and is determined using the ARPES measurements of the replicas. Two-dimensional Coulomb screening is accounted for in the random-phase approximation. It is shown that the model is applicable over the entire range of the parameters typical for current experiments. The estimates from this model make possible the conclusion that the LO-phonon-mediated pairing alone cannot account for the temperatures of the superconducting transitions $T_c$ in the single-layer $\mathrm{FeSe}/\mathrm{SrTi}{\mathrm{O}}_3$ reported in these experiments. This does not exclude that the LO-phonon mechanisms can become more significant in differently and better prepared single-layer FeSe films. Available experiments are briefly discussed. Thus far no measurements exist on the dependence of $T_c$ on the concentration of electrons doped into the in-layer FeSe band.

A first-principles-based effective Hamiltonian approach is used to predict the existence of a highly tunable piezocaloric effect in antiferroelectric ${\mathrm{PbZrO}}_3$. The high tunability originates from a strong dependence of both the magnitude and sign of the piezocaloric temperature change on the initial temperature and the nature of the stress. The linearity of the temperature response to the applied stress allows for the doubling of the efficiency of the basic solid state refrigeration cycle. The large values and high tunability of the piezocaloric effect in antiferroelectrics is traced to the strong coupling between the multiple order parameters that coexist in such materials. An experimental setup for the demonstration of such an unusual effect is proposed.

We studied high-density amorphous ices between 0.004 and 1.6 GPa by isobaric in situ volumetry and by subsequent ex situ x-ray diffraction and differential scanning calorimetry at 1 bar. Our observations indicate two processes, namely, relaxation in the amorphous matrix and crystallization, taking place at well-separated time scales. For this reason, we are able to report rate constants of crystallization $k_X$ and glass-transition temperatures $T_g$ in an unprecedented pressure range. $T_g$’s agree within ±3 K with earlier work in the small pressure range where there is overlap. Both $T_g$ and $k_X$ show a pressure anomaly between 0.7 and 1.1 GPa, namely, a $k_X$ minimum and a $T_g$ maximum. This anomalous pressure dependence suggests a continuous phase transition from high- (HDA) to very-high-density amorphous ice (VHDA) and faster hydrogen bond dynamics in VHDA. We speculate this phenomenology can be rationalized by invoking the crossing of a Widom line between 0.7 and 1.1 GPa emanating from a low-lying HDA-VHDA critical point. Furthermore, we interpret the volumetric relaxation of the amorphous matrix to be accompanied by viscosity change to explain the findings such that the liquid state can be accessed prior to the crystallization temperature $T_X$ at <0.4 GPa and >0.8 GPa.

The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic ${\mathrm{BiFeO}}_3$ (BFO) and superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) grown on a (001) ${\mathrm{SrTiO}}_3$ substrate is studied by a time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCO interface as observed in magnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.

We demonstrate a nonradiative flat band for spoof surface plasmon polaritons bounded on a structured surface with Lieb lattice symmetry in the terahertz regime. First, we theoretically derive the dispersion relation of spoof plasmons in a metallic Lieb lattice based on the electrical circuit model. We obtain three bands, one of which is independent of wave vector. To confirm the theoretical result, we numerically and experimentally observe the flat band in transmission and attenuated total reflection configurations. We reveal that the quality factor of the nonradiative flat-band mode decoupled from the propagating wave is higher than that of the radiative flat-band mode. This indicates that the nonradiative flat-band mode is three-dimensionally confined in the lattice.

Luttinger liquids (LLs) are one-dimensional systems with well-understood instabilities due to Umklapp or backscattering. We study a generalization of the Luttinger model, which incorporates a time reversal symmetry breaking interaction producing a complex forward scattering amplitude ($g_2$ process). The resulting low energy state is still a LL and belongs to the family of interacting Schulz-Shastry models. Remarkably, it becomes increasingly robust against additional perturbations—for purely imaginary $g_2$, both Umklapp and local backscattering are always irrelevant. Changing the phase of the interaction generates a nontrivial Berry phase, with a universal geometric phase difference between ground and a one boson excited state depending only on the LL parameter.

In order to investigate the mechanism of photoexcited precession of magnetization in ferromagnetic $\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{As}$, magneto-optical (MO) and differential reflectivity ($\mathrm{\Delta }R/R$; DR) temporal profiles are studied at relatively long (picosecond to nanosecond) and ultrashort (1 ps or less) time scales for samples with different Mn content $(x=0.01-0.11)$. As to the oscillatory MO profiles observed in the long time scale, simulation based on the Landau-Lifshitz-Gilbert equation combined with two different MO effects confirms photoinducement of the perpendicular anisotropy component $\mathrm{\Delta }{H}_{\mathrm{eff},\perp }$. As for the profiles observed in the ultrashort time scale, they are consistently explained in terms of the dynamics of photogenerated carriers, but not by the sudden reduction in magnetization (the ultrafast demagnetization). In light of these experimental results and analyses, a mechanism that accounts for the photoinduced $\mathrm{\Delta }{H}_{\mathrm{eff},\perp }$ is addressed: namely, photoionizationlike excitation of $\mathrm{M}{\mathrm{n}}^{2+}, \mathrm{M}{\mathrm{n}}^{2+}+h\nu \to \mathrm{M}{\mathrm{n}}^{2+,*}=\mathrm{M}{{\mathrm{n}}^3}^{+}+e^{-}$. That such excitation tips magnetic anisotropy toward the out-of-plane direction through the inducement of orbital angular momentum and the gradient $\partial \left(\mathrm{M}{\mathrm{n}}^{2+,*}\right)/\partial z$ is discussed. The validity of the proposed mechanism is examined by estimating the efficiency of excitation on the basis of the Lambert-Beer law and the experimental $\mathrm{\Delta }{H}_{\mathrm{eff},\perp }$ values, through which an efficiency of 1–10 ppm with a nominal optical cross section of around $5\times {10}^{-12}{\mathrm{m}}^2$ is obtained.

Engineering and studying few-electron states in ballistic conductors is a key step towards understanding entanglement in quantum electronic systems. In this Rapid Communication, we introduce the intrinsic two-electron coherence of an electronic source in quantum Hall edge channels and relate it to two-electron wave functions and to current noise in a Hanbury Brown–Twiss interferometer. Inspired by the analogy with photon quantum optics, we propose to measure the intrinsic two-electron coherence of a source using low-frequency current correlation measurements at the output of a Franson interferometer. To illustrate this protocol, we discuss how it can distinguish between a time-bin-entangled pure state and a statistical mixture of time-shifted electron pairs.

The transition-metal dichalcogenides have attracted a lot of attention as a possible stepping-stone toward atomically thin and flexible field-effect transistors. One key parameter to describe the charge transport is the time between two successive scattering events—the transport scattering time. In a recent report, we have shown that it is possible to use density functional theory to obtain the band structure of two-dimensional semiconductors in the presence of field effect doping. Here, we report a simple method to extract the scattering time from the experimental conductivity and from the knowledge of the band structure. We apply our approach to monolayers and multilayers of ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{MoTe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ in the presence of a gate. In ${\mathrm{WS}}_2$, for which accurate measurements of mobility have been published, we find that the scattering time is inversely proportional to the density of states at the Fermi level. Finally, we show that it is possible to identify the critical doping at which different valleys start to be occupied from the doping dependence of the conductivity.

At a quantum critical point, bipartite entanglement entropies have universal quantities which are subleading to the ubiquitous area law. For Renyi entropies, these terms are known to be similar to the von Neumann entropy, while being much more amenable to numerical and even experimental measurement. We show here that when calculating universal properties of Renyi entropies, it is important to account for unusual corrections to scaling that arise from relevant local operators present at the conical singularity in the multisheeted Riemann surface. These corrections grow in importance with increasing Renyi index. We present studies of Renyi correlation functions in the $1+1$ transverse-field Ising model (TFIM) using conformal field theory, mapping to free fermions, and series expansions, and the logarithmic entropy singularity at a corner in $2+1$ for both free bosonic field theory and the TFIM, using numerical linked cluster expansions. In all numerical studies, accurate results are only obtained when unusual corrections to scaling are taken into account. In the worst case, an analysis ignoring these corrections can get qualitatively incorrect answers, such as predicting a decrease in critical exponents with the Renyi index, when they are actually increasing. We discuss a two-step extrapolation procedure that can be used to account for the unusual corrections to scaling.