The magnetic properties of Fe chains and clusters on Pt(111) are investigated in the framework of a functional-integral theory of itinerant magnetism. The considered nanostructures show a ferromagnetic (FM) ground state with nearly saturated Fe local magnetic moments ${\mu }_{\text{Fe}}^0\simeq 3.15{\mu }_{\text{B}}$. In addition, small moments ${\mu }_{\text{Pt}}^0\simeq 0.1–0.3{\mu }_{\text{B}}$ are induced at the Pt substrate, which depend sensitively on the number of Fe atoms in their nearest-neighbor (NN) shell. The spin-fluctuation (SF) energies $\mathrm{\Delta }{F}_l\left(\xi \right)$ at the different atoms $l$ are calculated as a function of the local exchange fields ${\xi }_l$, by using a real-space recursive expansion of the local Green's functions. Results for the temperature dependence of the average magnetization per atom ${\overline{\mu }}_N$, local magnetic moments ${\mu }_l$, and spin correlation functions ${\gamma }_{lk}$ are derived. At the Fe atoms the dominant magnetic excitations are fluctuations of the local-moment orientations. The spin-flip energies $\mathrm{\Delta }{F}_l\left(\xi \right)$ in the deposited Fe clusters are found to be about $50\%$ smaller than in free-standing clusters of comparable size. This results in flatter SF-energy landscapes and in a weaker stability of the FM order at $T>0$. The effective exchange interactions between the Fe local moments, which are derived from the electronic calculations, reveal competing FM and antiferromagnetic couplings at different distances. In contrast to Fe, the main spin excitations at the Pt atoms are fluctuations of the size of the induced local magnetic moments. The interplay between the different types of spin excitations and their effect on the temperature-dependent magnetic properties is discussed.

In classical turbulence the kinematic viscosity $\nu$ is involved in two phenomena. The first is the energy dissipation and the second is the mechanical momentum flux toward the wall. In superfluid turbulence the mechanism of energy dissipation is different, and it is determined by an effective viscosity which was introduced by Vinen and is denoted as ${\nu }^{\prime }$. In this paper we show that in superfluid turbulence the transfer of mechanical momentum to the wall is caused by the presence of a quantum vortex tangle, giving rise to another effective “momentum” viscosity that we denote as ${\nu }_{\text{m}}\left(T\right)$. The temperature dependence of the second effective viscosity is markedly different from Vinen's effective viscosity ${\nu }^{\prime }\left(T\right)$. We show that the notion of vortex-tension force, playing an important role in the theory of quantum turbulence, can be understood as the gradient of the Reynolds-stress tensor, which is, in fact, determined by the second newly defined kinematic viscosity ${\nu }_{\text{m}}\left(T\right)$.

Inverse magnetostriction is the effect by which magnetization can be changed upon application of stress/strain. A strain modulation may be created electrically by exciting interdigitated transducers to generate surface acoustic waves (SAWs). Hence SAWs appear as a possible route towards induction-free undulatory magnetic data manipulation. Here we demonstrate experimentally on an out-of-plane magnetostrictive layer a reduction of the coercive field of up to $60\%$ by a SAW, over millimetric distances. A simple model shows that this spectacular effect can be partly explained by the periodic lowering of the strain-dependent domain nucleation energy by the SAW. This proof of concept was done on (Ga,Mn)(As,P), a magnetic semiconductor in which the out-of-plane magnetic anisotropy can be made very weak by epitaxial growth; it should guide material engineering for all-acoustic magnetization switching.

We study a chain of magnetic moments exchange coupled to a conventional three-dimensional superconductor. In the normal state the chain orders into a collinear configuration, while in the superconducting phase we find that ferromagnetism is unstable to the formation of a magnetic spiral state. Beyond weak exchange coupling the spiral wave vector greatly exceeds the inverse superconducting coherence length as a result of the strong spin-spin interaction mediated through the subgap band of Yu-Shiba-Rusinov states. Moreover, the simple spin-spin exchange description breaks down as the subgap band crosses the Fermi energy, wherein the spiral phase becomes stabilized by the spontaneous opening of a $p$-wave superconducting gap within the band. This leads to the possibility of electron-driven topological superconductivity with Majorana boundary modes using magnetic atoms on superconducting surfaces.

Simultaneous thermal, acoustic, and magnetic emission (AE and ME) measurements during thermally induced martensitic transformation in ${\text{Ni}}_2\text{MnGa}$ single crystals demonstrate that all three types of the above noises display many coincident peaks and the same start and finish temperatures. The amplitude and energy distribution functions for AE and ME avalanches satisfy power-law behavior, corresponding to the symmetry of the martensite. At zero external magnetic field asymmetry in the exponents was obtained: their value was larger for heating than for cooling. Application of constant, external magnetic fields (up to $B=722$ mT) leads to the disappearance of the above asymmetry, due to the decrease of the multiplicity of the martensite variants. Time correlations (i.e., the existence of nonhomogeneous temporal processes) within AE as well as ME emission events are demonstrated by deviations from the uncorrelated behavior on probability distributions of waiting times as well as of a sequence of number of events. It is shown that the above functions collapse on universal master curves for cooling and heating as well as for AE and ME noises. The analysis of the existence of temporal correlations between AE and ME events revealed that at short times the acoustic signals show a time delay relative to the magnetic one, due to the time necessary for the propagation of the ultrasound. At intermediate times, as expected, the magnetic signal is delayed, i.e., the magnetic domain rearrangement followed the steps of structural transformation. At much longer times the deviation from an uncorrelated (Poisson-type) behavior is attributed to the nonhomogeneity of the avalanche statistics.

An interatomic potential model that can simultaneously describe metallic, covalent, and ionic bonding is suggested by combining the second nearest-neighbor modified embedded-atom method (2NNMEAM) and the charge equilibration (Qeq) method, as a further improvement of a series of existing models. Paying special attention to the removal of known problems found in the original Qeq model, a mathematical form for the atomic energy is newly developed, and carefully selected computational techniques are adapted for energy minimization, summation of Coulomb interaction, and charge representation. The model is applied to the Ti-O and Si-O binary systems selected as representative oxide systems for a metallic element and a covalent element. The reliability of the present $2\mathrm{NNMEAM}+\mathrm{Qeq}$ potential is evaluated by calculating the fundamental physical properties of a wide range of titanium and silicon oxides and comparing them with experimental data, density functional theory calculations, and other calculations based on (semi-)empirical potential models.

One of the key impediments to the development of ${\mathrm{BaTiO}}_3$-based materials as candidates to replace toxic-Pb-based solid solutions is their relatively low ferroelectric Curie temperature ($T_C$). Among many potential routes that are available to modify $T_C$, ionic substitutions at the Ba and Ti sites remain the most common approach. Here, we perform density functional theory (DFT) calculations on a series of $A{\mathrm{TiO}}_3$ and $\mathrm{Ba}B{\mathrm{O}}_3$ perovskites, where $A=\mathrm{Ba}$, Ca, Sr, Pb, Cd, Sn, and Mg and $B=\mathrm{Ti}$, Zr, Hf, and Sn. Our objective is to study the relative role of $A$ and $B$ cations in impacting the $T_C$ of the tetragonal ($P4mm$) and rhombohedral ($R3m$) ferroelectric phases in ${\mathrm{BaTiO}}_3$-based solid solutions, respectively. Using symmetry-mode analysis, we obtain a quantitative description of the relative contributions of various divalent ($A$) and tetravalent ($B$) cations to the ferroelectric distortions. Our results show that Ca, Pb, Cd, Sn, and Mg have large mode amplitudes for ferroelectric distortion in the tetragonal phase relative to Ba, whereas Sr suppresses the distortions. On the other hand, Zr, Hf, and Sn tetravalent cations severely suppress the ferroelectric distortion in the rhombohedral phase relative to Ti. In addition to symmetry modes, our calculated unit-cell volume also agrees with the experimental trends. We subsequently utilize the symmetry modes and unit-cell volumes as features within a machine learning approach to learn $T_C$ via an inference model and uncover trends that provide insights into the design of new high-$T_C{\mathrm{BaTiO}}_3$-based ferroelectrics. The inference model predicts ${\mathrm{CdTiO}}_3-{\mathrm{BaTiO}}_3$ solid solutions to have a higher $T_C$ and, therefore, we experimentally synthesized these solid solutions and measured their $T_C$. Although the calculated mode strength for ${\mathrm{CdTiO}}_3$ in the tetragonal phase is even larger than that for ${\mathrm{PbTiO}}_3$, the $T_C$ of ${\mathrm{CdTiO}}_3-{\mathrm{BaTiO}}_3$ solid solutions in the tetragonal phase does not show any appreciable enhancement. Thus, ${\mathrm{CdTiO}}_3-{\mathrm{BaTiO}}_3$ does not follow the inference model, which is based on established data and trends for $A{\mathrm{TiO}}_3$. Rather, our experimental phase diagram for ${\mathrm{CdTiO}}_3-{\mathrm{BaTiO}}_3$ suggests that it behaves markedly differently from any other ${\mathrm{BaTiO}}_3$-based systems studied so far.

The nonlinear dynamics of a transverse domain wall (TDW) in permalloy and nickel nanostrips with two artificially patterned pinning centers is studied numerically up to rf frequencies. The phase diagram frequency-driving amplitude shows a rich variety of dynamical behaviors depending on the material parameters and the type and shape of pinning centers. We find that T-shaped traps (antinotches) create a classical double well Duffing potential that leads to a small chaotic region in the case of nickel and a large one for Py. In contrast, the rectangular constrictions (notches) create an exponential potential that leads to larger chaotic regions interspersed with periodic windows for both Py and Ni. The influence of temperature manifests itself by enlarging the chaotic region and activating thermal jumps between the pinning sites while reducing the depinning field at low frequency in the notched strips.

We present an algorithm for the efficient simulation of the half-filled spinless $t\text{−}V$ model on bipartite lattices, which combines the stochastic series expansion method with determinantal quantum Monte Carlo techniques widely used in fermionic simulations. The algorithm scales linearly in the inverse temperature, cubically with the system size, and is free from the time-discretization error. We use it to map out the finite-temperature phase diagram of the spinless $t\text{−}V$ model on the honeycomb lattice and observe a suppression of the critical temperature of the charge-density-wave phase in the vicinity of a fermionic quantum critical point.

A computational study within the framework of density functional theory in the local density approximation (LDA) is presented for native defects and doping in ${\mathrm{ZnGeN}}_2$. Gap corrections are taken into account using an LDA+$U$ approach and finite size corrections for charged defects are evaluated in terms of an effective charge model, introduced in this paper. The donor or acceptor characteristics of each of the cation and N vacancies and the two cation antisite defects are determined as well as their energies of formation under different chemical potential conditions. These are then used to determine defect concentrations and Fermi level pinning self-consistently. The cation antisite defects are found to have significantly lower formation energy than the cation vacancies. At a typical growth temperature of 1200 K, the charge neutrality condition pins the Fermi level close to the crossing of the formation energies of the ${\mathrm{Zn}}_{\mathrm{Ge}}^{-1}$ acceptor with the ${\mathrm{Ge}}_{\mathrm{Zn}}^{2+}$ shallow donor. Since this point lies closer to the valence-band maximum (VBM), intrinsic $p$-type doping would result at the growth temperature and will persist at room temperature if the defect concentrations are frozen in. It is the highest and of order ${10}^{16}{\mathrm{cm}}^{-3}$ for the most Ge-poor condition. On the other hand, for the most Ge-poor condition, it drops to ${10}^{13}{\mathrm{cm}}^{-3}$ at 1200 K and to almost zero at 300 K because then the Fermi level is too close to the middle of the gap. Oxygen impurities are found to strongly prefer the ${\mathrm{O}}_{\mathrm{N}}$ substitutional site and are found to be shallow donors with a very low energy of formation. It can only be suppressed by strongly reducing the oxygen partial pressure relative to that of nitrogen. At high temperatures, however, introduction of oxygen will be accompanied by compensating ${\mathrm{Zn}}_{\mathrm{Ge}}^{-2}$ acceptors and would lead to negligible net doping. The prospects for Ga base $p$-type doping are evaluated. While good solubility is expected, site competition between Zn and Ge sites is found to lead to a compensation problem similar to that of the two antisites and leads to $p$-type doping of the same level of ${10}^{16}{\mathrm{cm}}^{-3}$.

Predictive modeling of the thermal conductivity of nanostructures remains a large challenge. Ab initio studies rely on parametrized models of scattering rates of the nanostructure elements. Simplified interatomic potentials, on the other hand, often fail in the quantitative prediction of thermal conductivity. We have developed a simple and short-ranged orthogonal tight-binding model for the thermal properties of Si. We systematically introduce separate handles to tune the value and slope of the potential energy with respect to interatomic distance. Furthermore, we add an embedding potential in our model to capture the correct structural stability trend in Si. The model shows excellent transferability to the thermal properties of diamond Si along with good reproduction of the relative stabilities of different structures. We demonstrate the improvement over earlier tight-binding models and contrast the transferability of our model with simple interatomic potentials.

We demonstrate the dynamics and optical control of a large quantum mechanical solid state spin system consisting of a donor electron spin strongly coupled to the 9/2 nuclear spin of ${}^{115}\mathrm{In}$ in the semiconductor ZnO. Comparison of electron spin dynamics observed by time-resolved pump-probe spectroscopy with density matrix theory reveals nuclear spin pumping via optically oriented electron spins, coherent spin-spin interaction, and quantization effects of the ten nuclear spin levels. Modulation of the optical electron spin orientation at frequencies above 1 MHz gives evidence for fast optical manipulation of the nuclear spin state.

Energy spectra both of the conduction and valence bands of the HgTe quantum wells with a width close to the Dirac point were studied experimentally. Simultaneous analysis of the Shubnikov–de Haas oscillations and the Hall effect over a wide range of electron and hole densities yields surprising results: the top of the valence band is strongly split by spin-orbit interaction while the splitting of the conduction band is absent, within experimental accuracy. This holds true for the structures with normal and inverted band ordering. The results obtained are inconsistent with the results of $\mathit{kP}$ calculations, in which the smooth electric field across the quantum well is only reckoned in. It is shown that taking into account the asymmetry of the quantum-well interfaces within a tight-binding method gives reasonable agreement with the experimental data.

We investigate van der Waals (vdW) heterostructures made of germanene, stanene, or silicene with hexagonal boron nitride (h-BN). The intriguing topological properties of these buckled honeycomb materials can be maintained and further engineered in the heterostructures, where the competition between the substrate effect and external electric fields can be used to control the tunable topological phase transitions. Using such heterostructures as building blocks, various vdW topological domain walls (DW) are designed, along which there exist valley polarized quantum spin Hall edge states or valley-contrasting edge states which are protected by valley(spin)- resolved topological charges and can be tailored by the patterning of the heterojunctions and by external fields.

We show that the electron mobility in ideal, free-standing two-dimensional “buckled” crystals with broken horizontal mirror (${\sigma }_{\mathrm{h}}$) symmetry and Dirac-like dispersion (such as silicene and germanene) is dramatically affected by scattering with the acoustic flexural modes (ZA phonons). This is caused both by the broken ${\sigma }_{\mathrm{h}}$ symmetry and by the diverging number of long-wavelength ZA phonons, consistent with the Mermin-Wagner theorem. Non-${\sigma }_{\mathrm{h}}$-symmetric, “gapped” 2D crystals (such as semiconducting transition-metal dichalcogenides with a tetragonal crystal structure) are affected less severely by the broken ${\sigma }_{\mathrm{h}}$ symmetry, but equally seriously by the large population of the acoustic flexural modes. We speculate that reasonable long-wavelength cutoffs needed to stabilize the structure (finite sample size, grain size, wrinkles, defects) or the anharmonic coupling between flexural and in-plane acoustic modes (shown to be effective in mirror-symmetric crystals, like free-standing graphene) may not be sufficient to raise the electron mobility to satisfactory values. Additional effects (such as clamping and phonon stiffening by the substrate and/or gate insulator) may be required.

We present a generalized model to describe the lattice thermal conductivity of low-dimensional (low-D) and disordered systems. The model is a straightforward generalization of the Debye-Peierls and Allen-Feldman schemes to arbitrary dimensions, accounting for low-D effects such as differences in dispersion, density of states, and scattering. Similar in spirit to the Allen-Feldman approach, heat carriers are categorized according to their transporting capacity as propagons, diffusons, and locons. The results of the generalized model are compared to experimental results when available, and equilibrium molecular dynamics simulations otherwise. The results are in very good agreement with our analysis of phonon localization in disordered low-D systems, such as amorphous graphene and glassy diamond nanothreads. Several unique aspects of thermal transport in low-D and disordered systems, such as milder suppression of thermal conductivity and negligible diffuson contributions, are captured by the approach.

Despite fermion doubling, a two-dimensional quasi-relativistic spin-1/2 system can still lead to true fractionalization of electrical charge, when a massive ordered phase supports a “half-vortex”. Such topological defect is possible when the order parameter in form of Dirac mass is described by two $U\left(1\right)$ angles, and each of them winds by an angle $\pi$ around a point. We demonstrate that such a mass configuration in an eight-dimensional Dirac Hamiltonian exhibits only a single bound zero mode, and therefore binds the charge of $e/2$. In graphene, for example, such an ordered phase is provided by the easy-plane spin-triplet Kekule valence bond solid. We argue that an application of an in-plane magnetic field can cause an excitonic instability toward such ordered phase, even for weak repulsion, when the on-site, nearest-neighbor, and second neighbor components of it are of comparable strengths.