We report on transmission electron microscope beam-induced ferroelectric domain nucleation and motion. While previous observations of this phenomenon have been reported, a consistent theory explaining induced domain response is lacking, and little control over domain behavior has been demonstrated. We identify positive sample charging, a result of Auger and secondary electron emission, as the underlying mechanism driving domain behavior. By converging the electron beam to a focused probe, we demonstrate controlled nucleation of nanoscale domains. Molecular dynamics simulations performed are consistent with experimental results, confirming positive sample charging and reproducing the result of controlled domain nucleation. Furthermore, we discuss the effects of sample geometry and electron irradiation conditions on induced domain response. These findings elucidate past reports of electron beam-induced domain behavior in the transmission electron microscope and provide a path towards more predictive, deterministic domain patterning through electron irradiation.

We analyze theoretically the finite-temperature polarization dynamic in displacive-type ferroelectrics. In particular we consider the thermally activated switching time of a single-domain ferroelectric polarization studied by means of the Landau-Khalatnikov equation, extended to capture thermal fluctuations. The results are compared with the switching time formula that follows from the analytical solution of Pauli master equations. In a second step we focus on the phase diagram of a prototypical ferroelectric as described by the temperature-dependent Landau-Devonshire model including thermal fluctuations. Our simulations show the emergence of phase instability at reduced sizes which we attribute to thermal fluctuations in the order parameter in the respective phase. We conclude that, along with the temperature-dependent potential coefficients, thermal fluctuations should be taken into account to achieve a comprehensive description of the thermal behavior of reduced-size ferroelectrics. To support our conclusions, we simulated the electric-field activated switching time for a multidomain system and compared the results to the predictions of well-established models such as Kolmogorov-Avrami-Ishibashi.

We report complementary experimental, numerical, and theoretical study of turbulent coflow, counterflow, and pure superflow of superfluid ${}^4\mathrm{He}$ in a channel, resulting in a physically transparent and relatively simple model of decaying quantum turbulence that accounts for interactions of coexisting quantum and classical components of turbulent superfluid ${}^4\mathrm{He}$. We further offer an analytical theory of the energy spectra of steady-state quantum turbulence in the counterflow and pure superflow, based on algebraic approximation for the energy fluxes over scales. The resulting spectra are not of the classic Kolmogorov form, but strongly suppressed by the mutual friction, leading to the energy dissipation at all scales, enhanced by the counterflow-induced decoupling of the normal and superfluid velocity fluctuations.

We demonstrate theoretically that proximity-induced superconductivity in silicene offers the possibility to exert strong quantum ground state control. We show that electrically controlled 0-$\pi$ transitions occur in Josephson junctions in the presence of an exchange field due to the buckling of the silicene lattice. We also discover that zigzag-oriented interfaces, featuring intervalley scattering, cause a ${\phi }_0$ state with an applied electric field. Finally, we demonstrate that Majorana bound states along the silicene edge are tunable via the edge orientation, electric, and in-plane spin exchange fields.

This paper reports on symmetry breaking in the aperiodic inclusion compound $n\text{-octadecane/urea}$ and its isotopomer $n\text{-octadecane/urea-}{d}_4$. The high-symmetry phase is described by a hexagonal rank-4 superspace group. Pretransitional phenomena in this crystallographic superspace reveal competing short-range-ordering phenomena within the high-symmetry phase. Very high-resolution diffraction data show that critical scattering appears at inequivalent points within the four-dimensional Brillouin zone, although the first phase transition at ${T_c}_1$ near 158 K implies the condensation at only one of those points. The resulting superspace group remains of dimension 4. Two other phase transitions are reported at ${T_c}_2=152.8\left(4\right)\mathrm{K}$ and ${T_c}_3=109\left(4\right)\mathrm{K}$ in $n\text{-octadecane/urea-}{d}_4$. The two low-symmetry phases that arise are described by rank-5 superspace groups.

We investigated the crystal structures and dielectric properties of an improper ferroelectric ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Al}}_2{\mathrm{O}}_4 (x\le 0.1)$ and revealed that suppressing the condensation of the $M$-point soft mode involves the emergence of a “fluctuating” state. In the low-Sr-concentration region of $x\le 0.06$, crystals exhibit a ferroelectric phase transition at $T_{\mathrm{C}}$ from a paraelectric phase with a space group $P{6}_{3}22$ (PE phase) to a low-temperature ferroelectric phase with a $P{6}_3$ structure with doubled $a$ and $b$ axes (FE phase). Additionally, the temperature dependence of the dielectric constant ${\varepsilon }^{\prime }$ exhibits a peak at $T_{\mathrm{C}}$. As $x$ increases, $T_{\mathrm{C}}$ decreases substantially, and the peak at $T_{\mathrm{C}}$ becomes small. For $x\ge 0.07$, the peak is barely noticeable and becomes an anomaly at $T^{*}\approx 200$ K, indicating that the system possesses another state (FL state) below the $T^{*}$. The PE phase has been reported to possess two energetically competing soft modes at the $M$ and $K$ points. Electron diffraction (ED) experiments revealed that the superlattice reflections of the FE phase become diffuse scatterings originating from the $M$-point soft mode as the FE-FL boundary is approached. The $K$-point soft mode disappears in the FL state, whereas the $M$-point soft mode survives and fluctuates without condensation. Dark-field (DF) images revealed that the $M$-point soft mode exhibits coherent motion in nanodomains with sizes of approximately 10 nm in the FL state. The emergence of the FL state is ascribed to enhanced vibration of the ${\mathrm{AlO}}_4$ tetrahedra resulting from the substitution of Sr, which has a smaller ionic radius than Ba.

Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with a total average diameter around $\sim 27$ nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of ${\mathrm{Co}}_3{\mathrm{O}}_4$ crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias $H_E$ and coercive $H_C$ fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both phases. The EB effects persist up to temperatures above the ordering temperature of the oxide shell. Results of our atomistic Monte Carlo simulations of particles with the same size and composition as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal, giving rise to the observed maximum in $H_E$ and $H_C$.

Spin-wave eigenmodes in a disk-shaped FeB nanomagnet were investigated using magnetic tunnel junctions with a FeB magnetic layer and MgO barrier. Noise spectrum measurements for a perpendicularly magnetized junction of 120 nm in diameter showed thermal excitation of distinct spin-wave modes. The measured spectra were compared with analytical calculations and numerical simulations. As a result, the observed modes were classified as spin-wave eigenmodes with various nodal circles and nodal diameter modes by taking into account splitting between directional spin-wave modes originating from dynamic dipolar interaction instead of Dzyaloshinskii-Moriya interaction. These results provide a fundamental understanding of spin dynamics in nanosize spintronic devices.

We report unusual magnetization switching processes and angular-dependent exchange bias effects in fully epitaxial ${\mathrm{Co}}_3\mathrm{FeN}$/MnN bilayers, where magnetocrystalline anisotropy and exchange coupling compete, probed by longitudinal and transverse magneto-optic Kerr effect (MOKE) magnetometry. The MOKE loops show multistep jumps corresponding to the nucleation and propagation of ${90}^{\circ }$ domain walls in as-grown bilayers. By inducing exchange coupling, we confirm changes of the magnetization switching process due to the unidirectional anisotropy field of the exchange coupling. Taking into account the experimentally obtained values of the fourfold magnetocrystalline anisotropy, the unidirectional anisotropy field, the exchange-coupling constant, and the uniaxial anisotropy including its direction, the calculated angular-dependent exchange bias reproduces the experimental results. These results demonstrate the essential role of the competition between magnetocrystalline anisotropy and exchange coupling for understanding and tailoring exchange-coupling phenomena usable for engineering switching in fully epitaxial bilayers made of tailored materials.

Systematic measurements of temperature-dependent magnetization, resistivity, and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 GPa were conducted on single crystals of ${\mathrm{CrAuTe}}_4$. Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic $c$ axis below $T_{\mathrm{N}}=255$ K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The Néel temperature decreases monotonically under pressure, decreasing to $T_{\mathrm{N}}=236$ K at 5.22 GPa. The pressure dependencies of (i) $T_{\mathrm{N}}$, (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low-temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states.

We present magnetometry and muon spin rotation $\left(\mu \mathrm{SR}\right)$ measurements of the superconducting dichalcogenide ${\mathrm{Ir}}_{0.95}{\mathrm{Pt}}_{0.05}{\mathrm{Te}}_2$. From both sets of measurements, we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional $s$-wave model and yield fitting parameters consistent with a BCS weak coupling superconductor. We therefore see no evidence for exotic superconductivity in ${\mathrm{Ir}}_{0.95}{\mathrm{Pt}}_{0.05}{\mathrm{Te}}_2$.

Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles study based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. The study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.

In this paper, the surface effects of individual heavy ions impacting thin polymerlike films were investigated, using molecular dynamics simulations with the finite extensible nonlinear elastic (FENE) potential to describe the molecular chains. The perturbation introduced by the ions in the lattice was modeled assuming that the initial excitation energy in the ion track is converted into an effective temperature, as in a thermal spike. The track was heated only within the film thickness $h$, leaving a nonexcited substrate below. The effect of decreasing thickness on cratering and sputtering was evaluated. The results were compared to experimental data of thin polymer films bombarded by MeV–GeV ions and to simulations performed with the Lennard-Jones potential. While several qualitative results observed in the experiments were also seen in the simulations, irrespective of the potential used, there are important differences observed on FENE films. Crater dimensions, rim volume, and sputtering yields are substantially reduced, and a threshold thickness for molecular ejection appears in FENE simulations. This is attributed to the additional restrictions on mass transport out of the excited track region imposed by interchain interactions (entanglements) and by the low mobility of the molten phase induced by the spike.

The total spontaneous emission rate of a quantum emitter in the presence of an infinite ${\mathrm{MoS}}_2$ monolayer is enhanced by several orders of magnitude, compared to its free-space value, due to the excitation of surface exciton polariton modes and lossy modes. The spectral and distance dependence of the spontaneous emission rate are analyzed and the lossy surface wave, surface exciton polariton mode and radiative contributions are identified. The transverse magnetic and transverse electric exciton polariton modes can be excited for different emission frequencies of the quantum emitter, and their contributions to the total spontaneous emission rate are different. To calculate these different decay rates we use the non-Hermitian description of light-matter interactions, employing a Green's tensor formalism. The distance dependence follows different trends depending on the emission energy of the quantum emitter. For the case of the lossy surface waves, the distance dependence follows a $z^{-n},n=2,3,4$, trend. When transverse magnetic exciton polariton modes are excited, they dominate and characterize the distance dependence of the spontaneous emission rate of a quantum emitter in the presence of the ${\mathrm{MoS}}_2$ layers. The interaction between a quantum emitter and a ${\mathrm{MoS}}_2$ superlattice is investigated, and we observe a splitting of the modes supported by the superlattice. Moreover, a blueshift of the peak values of the spontaneous emission rate of a quantum emitter is observed as the number of layers is increased. The field distribution profiles, created by a quantum emitter, are used to explain this behavior.

The presence of $d_{xz}\text{−}{d}_{yz}$ orbital ordering is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but the details of the order parameter remain controversial. Here, we report a high-resolution angle-resolved photoemission spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90 K, but, in contrast, the changes to the electron pockets do not require fourfold symmetry breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands, which themselves are not split within resolution. These observations lead us to propose a scenario of “unidirectional nematic bond ordering” to describe the low-temperature electronic structure of FeSe, supported by good agreement with ten-orbital tight-binding model calculations.

The doping efficiency of lithium deposited at cryogenic temperatures on epitaxial and chemical vapor deposition monolayer graphene has been investigated under ultrahigh-vacuum conditions. Change of charge-carrier density was monitored by gate voltage shift of the Dirac point and by Hall measurements in low and high doping regimes. It was found that preannealing the graphene greatly enhanced the maximum levels of doping that could be achieved: doping saturated at $\mathrm{\Delta }n=2\times {10}^{13}{e}^{-}/{\mathrm{cm}}^2$ without annealing, independent of sample type or previous processing; after a 900 K anneal, the saturated doping rose one order of magnitude to $\mathrm{\Delta }n=2\times {10}^{14}{e}^{-}/{\mathrm{cm}}^2$.

We report the electrical transport properties for Weyl semimetal TaAs in an intense magnetic field. Strong temperature-dependent anomalies occur in the Hall signals and longitudinal magnetoresistance at low temperatures when the Weyl electrons are confined into the lowest Landau level. The temperature-dependent behaviors indicate that electron-hole pairing instability may be the origin of the anomalies. Our measurements show that the ultraquantum regime for the Weyl semimetals is not featureless.