The effect of the interface roughness on the spin injection from a ferromagnet into a semiconductor is studied theoretically. Even a small interface irregularity can lead to a significant enhancement of the injection efficiency. When a typical size of the irregularity, $a$, is within a domain ${\lambda }_F\ll a\ll {\lambda }_N$, where ${\lambda }_F$ and ${\lambda }_N$ are the spin-diffusion lengths in the ferromagnet and semiconductor, respectively, the geometrical enhancement factor is $\sim {\lambda }_N/a$. The origin of the enhancement is the modification of the local electric field on small scales $\sim a$ near the interface. We demonstrate the effect of enhancement by considering a number of analytically solvable examples of injection through curved ferromagnet-semiconductor interfaces. For a generic curved interface, the enhancement factor is $\sim {\lambda }_N/R$, where $R$ is the local radius of curvature.

Based on an energy variational approach, we propose a sharp interface model for simulating solid-state dewetting of thin films with (weakly) anisotropic surface energies. The morphology evolution of thin films is governed by surface diffusion and contact line migration. For the contact line migration, we introduce a relaxation kinetics with a finite contact line mobility by energy gradient flow method. We implement the mathematical model in an explicit finite-difference scheme with cubic spline interpolation for evolving marker points. Following validation of the mathematical and numerical approaches, we simulate the evolution of thin-film islands, semi-infinite films, and films with holes as a function of film dimensions, isotropic Young angle ${\theta }_i$, anisotropy strength and crystal symmetry, and film crystal orientation relative to the substrate normal. We find that in addition to classical wetting (where holes in a film heal) and dewetting (where holes in a film grow), we observe cases where a hole through the film heals but leaves a finite-size hole/bubble between the continuous film and substrate or where the hole heals leaving a continuous film that is not bonded to the substrate. Surface energy anisotropy (i) increases the instability that leads to island breakup into multiple islands, (ii) enhances hole healing, and (iii) leads to finite island size even under some conditions where the isotropic Young angle ${\theta }_i$ suggests that the film wets the substrate. The numerical results presented in the paper capture many of the complexities associated with solid-state dewetting experiments.

Ripples in pristine freestanding graphene naturally orient themselves in an array that is alternately curved-up and curved-down; maintaining an average height of zero. Using scanning tunneling microscopy (STM) to apply a local force, the graphene sheet will reversibly rise and fall in height until the height reaches 60%–70% of its maximum at which point a sudden, permanent jump occurs. We successfully model the ripples as a spin-half Ising magnetic system, where the height of the graphene plays the role of the spin. The permanent jump in height, controlled by the tunneling current, is found to be equivalent to an antiferromagnetic-to-ferromagnetic phase transition. The thermal load underneath the STM tip alters the local tension and is identified as the responsible mechanism for the phase transition. Four universal critical exponents are measured from our STM data, and the model provides insight into the statistical role of graphene's unusual negative thermal expansion coefficient.

Raman scattering (RS) spectra and current-voltage characteristics at room temperature were measured in six series of small samples fabricated by means of electron-beam lithography on the surface of a large size $(5\times 5\text{mm)}$ industrial monolayer graphene film. Samples were irradiated by different doses of $\mathrm{C}{}^{+}$ ion beam up to ${10}^{15}$ $\mathrm{cm}{}^{-2}$. It was observed that at the utmost degree of disorder, the Raman spectra lines disappear which is accompanied by the exponential increase of resistance and change in the current-voltage characteristics. These effects are explained by suggestion that highly disordered graphene film ceases to be continuous and splits into separate fragments. The relationship between structure (intensity of RS lines) and sample resistance is defined. It is shown that the maximal resistance of the continuous film is of the order of reciprocal value of the minimal graphene conductivity $\pi h/4e^2\approx 20$ kOhm.

We present an approach to the melting of graphene based on nucleation theory for a first order phase transition from the two-dimensional (2D) solid to the 3D liquid via an intermediate quasi-2D liquid. The applicability of nucleation theory, supported by the results of systematic atomistic Monte Carlo simulations, provides an intrinsic definition of the melting temperature of graphene, $T_m$, and allows us to determine it. We find $T_m\simeq 4510$ K, about 250 K higher than that of graphite using the same interatomic interaction model. The found melting temperature is shown to be in good agreement with the asymptotic results of melting simulations for finite disks and ribbons of graphene. Our results strongly suggest that graphene is the most refractory of all known materials.

The ability of the phase-field-crystal (PFC) model to quantitatively predict atomistic defect structures in crystalline solids is addressed. First, general aspects of the PFC model are discussed within the context of obtaining quantitative results in solid materials. Then a specific example is used to illustrate major points. Specifically, accelerated molecular dynamics is used to compute the one-particle probability density ${\rho }^{\left(1\right)}\left(\mathit{r}\right)$ in a complex atomistic defect consisting of a Lomer dislocation with an equilibrium distribution of vacancies in the core, and the results are considered within the general framework of the PFC model. As expected, ${\rho }^{\left(1\right)}\left(\mathit{r}\right)$ shows numerous spatially localized peaks with integrated densities smaller than unity, as would arise in a PFC computation. However, the ${\rho }^{\left(1\right)}\left(\mathit{r}\right)$ actually corresponds to a time-averaged superposition of a few well-defined atomic configurations each having a well-defined energy. The deconvolution of ${\rho }^{\left(1\right)}\left(\mathit{r}\right)$ to obtain the actual distinct atomic configurations is not feasible. Using a potential energy functional that accurately computes the energies of distinct configurations, the potential energy computed using ${\rho }^{\left(1\right)}\left(\mathit{r}\right)$ differs from the actual average atomistic energy by $\sim 50$ eV divided among approximately 46 atoms in the core of the defect. Attempts to rectify this deviation by introducing correlations cannot significantly reduce this error. The simulations show energy barriers between distinct configurations varying by up to 0.5 eV, indicating that the simple kinetic evolution law used in PFC cannot accurately capture the true time evolution in this problem. Overall, these results demonstrate, in one nontrivial case, that the PFC model is probably unable to predict atomistic defect structures, energies, or kinetic barriers at the quantitative levels needed for application to problems in materials science.

The magnetization behavior, magnetic anisotropy, and domain configurations of Co/Pd multilayers with perpendicular magnetic anisotropy capped with permalloy is investigated using magnetometry, magnetic force microscopy, and ferromagnetic resonance. The thickness of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ layer in ${\left[\mathrm{Co}/\mathrm{Pd}\right]}_5/\mathrm{NiFe} \left(t\right)$ was varied from $t=0$ to 80 nm in order to study the interplay between the anisotropy and magnetization directions of Co/Pd and NiFe. By varying the thickness of the NiFe layer, the net anisotropy changes sign, but domains with plane-normal magnetization are present even for the thickest NiFe. Ferromagnetic resonance measurements show a decrease in damping with increasing NiFe thickness. The results demonstrate how the magnetic behavior of mixed-anisotropy thin films can be controlled.

Using spin density functional theory (SDFT) calculations, we have studied the magnetic states, including collinear and noncollinear magnetic interlayer coupling, of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ ultrathin films sandwiching Rh(001) layers. We found very large values for the interlayer exchange coupling (IEC) in $\mathrm{Co}/{\mathrm{Rh}}_n/\mathrm{Co}$ or ${\left(\mathrm{FeCo}\right)}_m/{\mathrm{Rh}}_n/\mathrm{Co}$ structures as compared to, e.g., Ag or Au spacer layers. The IEC oscillates with the Rh spacer thickness showing a transition between strong antiferromagnetic and ferromagnetic coupling between five- and seven-layer thickness of the Rh film. Moreover, depending on the thickness of the FeCo film, a reorientation transition between in-plane and out-of-plane easy axis was found when spin-orbit coupling is considered in the calculations. This result suggests that, for specific arrangements such as ${\left(\mathrm{FeCo}\right)}_2/{\mathrm{Rh}}_5/\mathrm{Co}$ structures, a competition between IEC and magnetic anisotropy of coupled films may result in noncollinear ordering. This possibility was studied with constrained, noncollinear SDFT calculations and the results were mapped onto a classical spin model to explore the richness of spin structures that can arise in these multilayer systems.

The discovery in 2001 of superconductivity in some heavy fermion compounds of the $RM{\mathrm{In}}_5$ $(R=4f$ or $5f$ elements; $M=\text{Co}$, Rh, Ir) family, has triggered an enormous amount of research into understanding the physical origin of superconductivity and its relation with magnetism. Although many properties have been clarified, there are still crucial questions that remain unanswered. One of these questions is the particular role of the transition metal in determining the value of critical superconducting temperature $\left(T_C\right)$. In this work, we analyze an interesting regularity that is experimentally observed in this family of compounds, where the lowest Néel temperatures are obtained in the Co-based materials. We focus our analysis on the $\mathrm{Gd}M{\mathrm{In}}_5$ compounds and perform density-functional-theory-based total-energy calculations to obtain the parameters for the exchange coupling interactions between the magnetic moments located at the ${\mathrm{Gd}}^{3+}$ ions. Our calculations indicate that the ground state of the three compounds is a $C$-type antiferromagnet determined by the competition between the first- and second-neighbor exchange couplings inside ${\mathrm{GdIn}}_3$ planes and stabilized by the couplings across $M{\mathrm{In}}_2$ planes. We then solve a model with these magnetic interactions using a mean-field approximation and quantum Monte Carlo simulations. The results obtained for the calculated Néel and Curie-Weiss temperatures, the specific heat, and the magnetic susceptibility are in very good agreement with the existent experimental data. Remarkably, we show that the first-neighbor interplane exchange coupling in the Co-based material is much smaller than in the Rh and Ir analogs which leads to a more two-dimensional magnetic behavior in the former. This result explains the observed lower Néel temperature in Co-115 systems and may shed light on the fact that the Co-based 115 superconductors present the highest $T_C$.

We report variational Monte Carlo calculations for the spin-$\frac{1}{2}$ Heisenberg model on the kagome lattice in the presence of both nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ antiferromagnetic superexchange couplings. Our approach is based upon Gutzwiller projected fermionic states that represent a flexible tool to describe quantum spin liquids with different properties (e.g., gapless and gapped). We show that, on finite clusters, a gapped ${\mathbb{Z}}_2$ spin liquid can be stabilized in the presence of a finite $J_2$ superexchange, with a substantial energy gain with respect to the gapless $U\left(1\right)$ Dirac spin liquid. However, this energy gain vanishes in the thermodynamic limit, implying that, at least within this approach, the $U\left(1\right)$ Dirac spin liquid remains stable in a relatively large region of the phase diagram. For $J_2/J_1\gtrsim 0.3$, we find that a magnetically ordered state with $\mathbf{q}=\mathbf{0}$ overcomes the magnetically disordered wave functions, suggesting the end of the putative gapless spin-liquid phase.

We propose and demonstrate spin manipulation by magnetically controlled modulation of pure spin currents in cobalt/copper lateral spin valves, fabricated on top of the magnetic insulator ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG). The direction of the YIG magnetization can be controlled by a small magnetic field. We observe a clear modulation of the nonlocal resistance as a function of the orientation of the YIG magnetization with respect to the polarization of the spin current. Such a modulation can only be explained by assuming a finite spin-mixing conductance at the Cu/YIG interface, as it follows from the solution of the spin-diffusion equation. These results open a path towards the development of spin logics.

Ferroelectric-paraelectric (FE-PE) phase transitions have been primarily explained by the phenomenological Landau-Devonshire theory and a soft-zone-center mode of vibration in the literature. In this work, we study the atomic structure and polarization evolution of $\mathrm{PbTi}{\mathrm{O}}_3$ as a function of temperature using ab initio molecular dynamics simulations. In contrast to conventional molecular dynamics analyses where results are averaged over time, we categorize the atomic configurations as a function of time in terms of Ti-O bond lengths in the nearest-neighboring shell. We show that an appreciable amount of cubic configuration exists at temperatures about 300 K below its FE-PE phase transition temperature of 763 K, even though the time-averaged overall atomic configuration is tetragonal. The quantitative results depict that as the temperature increases the population of the cubic configuration increases and that of the tetragonal configuration decreases, signifying that the FE-PE phase transition is intrinsically second order. It reveals that the thermal fluctuation of the cubic configurations in the tetragonal matrix makes a significant contribution to the negative thermal expansion in the FE phase region because the cubic configuration has smaller volume and higher entropy than the tetragonal matrix.

We report the phase diagram of the doping series ${\mathrm{U}}_2{\mathrm{Pt}}_x{\mathrm{Rh}}_{1-x}{\mathrm{C}}_2$, studied through measurements of resistivity, specific heat, and magnetic susceptibility. The Néel temperature of ${\mathrm{U}}_2{\mathrm{RhC}}_2$ of $\sim 22$ K is suppressed with increasing Pt content, reaching zero temperature close to $x=0.7$, where we observed signatures of increased quantum fluctuations. In addition, evidence is presented that the antiferromagnetic state undergoes a spin-reorientation transition upon application of an applied magnetic field. This transition shows nonmonotonic behavior as a function of $x$, peaking at around $x=0.3$. Superconductivity is observed for $x\ge 0.9$, with $T_{\mathrm{c}}$ increasing with increasing $x$. The reduction in $T_{\mathrm{c}}$ and increase in residual resistivity with decreasing Pt content is inconsistent with the extension of the Abrikosov-Gor'kov theory to unconventional superconductivity.

Synchrotron soft x-ray magnetic circular and linear dichroism (XMCD and XMLD) spectroscopies at the Dy $M_{4,5}$ and Co $L_{2,3}$ edges are reported to investigate the temperature-dependent magnetic properties of rare earth–transition metal ${\mathrm{DyCo}}_3$ alloy films. In a temperature region between 4.4 K and 300 K, the two sites of Co and Dy are ferrimagnetic alloy coupled, with the Dy magnetization aligned to the applied field as the dominant site. The combined method of XMCD and XMLD and the application of the sum rules allows one to monitor the detailed changes in the magnetic contribution from both sites separately. It reveals that Dy possesses a very large moment of $8.2{\mu }_{\mathrm{B}}$/atom at low temperatures. At higher temperatures, the reduction of the total magnetic moment is dominated by the behavior of the Dy magnetic moments which exhibit a steeper decrease, especially in the temperature regime between 150 K and 200 K. The reoccupation of the Zeeman split levels of the $4f$ sites of Dy indicates that thermal fluctuations are the driving force.

Orthorhombic $\mathrm{SrIr}{\mathrm{O}}_3$ is a typical spin-orbit-coupled correlated metal that shows diversified physical properties under external stimuli. Here the nonlinear Hall effect and weakly temperature-dependent resistance are observed in a $\mathrm{SrIr}{\mathrm{O}}_3$ film epitaxially grown on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. It infers that orthorhombic $\mathrm{SrIr}{\mathrm{O}}_3$ is a semimetal oxide. However, a linear Hall effect and insensitive-temperature-dependent resistance are observed in $\mathrm{SrIr}{\mathrm{O}}_3$ films grown on $(\mathrm{La},\mathrm{Sr})(\mathrm{Al},\mathrm{Ta}){\mathrm{O}}_3$ (LSAT) substrates, suggesting a tunable semimetallic state due to a band structure change in $\mathrm{SrIr}{\mathrm{O}}_3$ films under different compressive strains. The mechanism of this evolution is explored in detail through a strain-state analysis by reciprocal space mapping and electron diffraction, carrier density and mobility calculations, as well as electronic band structure evolution under compressive strain (predicted by a tight-binding approximation). It might suggest that the strain-induced band shift leads to semimetallic tuning in the $\mathrm{SrIr}{\mathrm{O}}_3$ film grown on $\mathrm{SrTi}{\mathrm{O}}_3$ to LSAT substrates. Our findings illustrate the tunability of $\mathrm{SrIr}{\mathrm{O}}_3$ properties and pave the way to induce different physical states in $\mathrm{SrIr}{\mathrm{O}}_3$, such as the proposed topological insulator state in heterostructures.

By employing three different measurement methods, we rigorously show that micron-scale ballistic thermal conduction can be found in Si-Ge heterogeneously interfaced nanowires exhibiting low thermal conductivities. The heterogeneous interfaces localize most high-frequency phonons and suppress the total thermal conductivity below that of Si or Ge. Remarkably, the suppressed thermal conductivity is accompanied with an elongation of phonon mean free paths over 5 μm at room temperature, which is not only more than 25 times longer than that of Si or Ge but also longer than those of the best thermal conductors like diamond or graphene. We estimate that only 0.1% of the excited phonons carry out the heat transfer process, and, unlike phonon transport in Si or Ge, the low-frequency phonons in Si-Ge core-shell nanowires are found to be insensitive to twin boundaries, defects, and local strain. The ballistic thermal conduction persisting over 5 μm, along with the suppressed thermal conductivity, will enable wave engineering of phonons at room temperature and inspire new improvements of thermoelectric devices.

Monte Carlo simulations within a tight-binding Ising model (TBIM) have been performed on bulk, surfaces, and nanoclusters of ${\mathrm{Co}}_{1\text{−}c}\text{−}{\mathrm{Pt}}_c$ alloys in order to describe and understand the competition or synergy between surface segregation and chemical ordering phenomena in nanoalloys. Considering effective pair interactions (EPIs) up to the third neighbors, we put in evidence new ordered phases at low temperature in the Co-Pt bulk phase diagram. On the infinite (100) and (111) surfaces, the Pt surface segregation leads to select the Pt-rich plan at the surface without modification of the bulk ordering in the (100) orientation but with an extension of the ordering on a larger composition range in the (111) orientation as compared to the bulk. The truncated octahedron clusters of 405 and 1289 atoms are studied. Their chemical structure is compared in their core with the bulk phase diagram and in their facets with the (111) and (100) infinite surfaces segregation isotherms. The cluster core presents an asymmetry as compared to the bulk phase diagram. The (111) facets are similar to the (111) surface, whereas the (100) facets present geometrical frustrations for the segregation versus core ordering.