We theoretically study the impact of changes in surroundings on the electron ground-state effective $g$ factor in defect-free GaAs/AlGaAs nanodisks. To perform the study, we formulate and deploy a computational efficient full three-dimensional model to describe the effective $g$-factor tensor in semiconductor nano-objects of complex geometry and material content. This model is based on an effective $2\times 2$ conduction-band Hamiltonian which includes the Rashba and Dresselhaus spin-orbit couplings. The description is suited to clarify the important question of the controllability of the electron effective $g$ factor in semiconductor nano-objects. The results of this theoretical study suggest that in the defect-free GaAs/AlGaAs nanodisks, the effective $g$ factor can be tuned within a wide range by proper design of the nanodisk environment. The $zz$ components of the electron effective $g$-factor tensor obtained in our simulation are in good agreement with some recent experimental observations.

Theory predicts that graphene under uniaxial compressive strain in an armchair direction should undergo a topological phase transition from a semimetal into an insulator. Due to the change of the hopping integrals under compression, both Dirac points shift away from the corners of the Brillouin zone towards each other. For sufficiently large strain, the Dirac points merge and an energy gap appears. However, such a topological phase transition has not yet been observed in normal graphene (due to its large stiffness) neither in any other electronic system. We show numerically and analytically that such a merging of the Dirac points can be observed in electronic artificial graphene created from a two-dimensional electron gas by application of a triangular lattice of repulsive antidots. Here, the effect of strain is modeled by tuning the distance between the repulsive potentials along the armchair direction. Our results show that the merging of the Dirac points should be observable in a recent experiment with molecular graphene.

The complexity associated with local structures continues to pose challenges with regard to the understanding of the structure-property relationship in $\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3$-based lead-free piezoceramics. $\left(1-x\right)\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3-\left(x\right)\mathrm{BaTi}{\mathrm{O}}_3$ is an extensively studied system because of its interesting piezoelectric properties. Recently, a room temperature phase boundary was reported at $x=0.03$ in this system [Ma et al., Adv. Funct. Mater. 23, 5261 (2013)]. In the present work we have examined this subtle phase boundary using x-ray diffraction, neutron diffraction, dielectric measurements as a function of composition $(x<0.06)$, temperature, and electric field. Our results show that this boundary separates an $R3c+Cc$-like structural state for $x<0.03$ from an $R3c+$ cubiclike structural state for $0.03\le x\le 0.05$ in the unpoled specimens. This phase boundary is characterized by an anomalous reduction in the depolarization temperature, and a suppression of the tetragonal distortion of the high temperature $P4bm$ phase. Our results also provide the clue to understand the pathway leading to the cubiclike structure of the critical composition $x=0.06$, known for its highest piezoelectric response.

Design of novel artificial materials based on ferroelectric perovskites relies on the basic principles of electrostatic coupling and in-plane lattice matching. These rules state that the out-of-plane component of the electric displacement field and the in-plane components of the strain are preserved across a layered superlattice, provided that certain growth conditions are respected. Intense research is currently directed at optimizing materials functionalities based on these guidelines, often with remarkable success. Such principles, however, are of limited practical use unless one disposes of reliable data on how a given material behaves under arbitrary electrical and mechanical boundary conditions. Here we demonstrate, by focusing on the prototypical ferroelectrics ${\mathrm{PbTiO}}_3$ and ${\mathrm{BiFeO}}_3$ as test cases, how such information can be calculated from first principles in a systematic and efficient way. In particular, we construct a series of two-dimensional maps that describe the behavior of either compound (e.g., concerning the ferroelectric polarization and antiferrodistortive instabilities) at any conceivable choice of the in-plane lattice parameter, $a$, and out-of-plane electric displacement, $D$. In addition to being of immediate practical applicability to superlattice design, our results bring new insight into the complex interplay of competing degrees of freedom in perovskite materials and reveal some notable instances where the behavior of these materials depart from what naively is expected.

Temperature changes in thin-film Ge:Mn spin-glass dynamics are presented that exhibit temperature chaos (TC) when the spin-glass correlation length $\xi (t,T)$ grows to its thickness $\mathcal{L}$. For small $\mathcal{L}\approx 15.5$ nm, the transition to chaos takes place over a temperature range $\mathrm{\Delta }T$ sufficiently large to exhibit both reversible and chaotic behavior. The value of $\mathrm{\Delta }T$ can be related to the critical exponent for TC, $\zeta$. Experimentally, $\zeta$ is found to be $\approx 1.06$, in the range of recent simulations. The presence of a specific length scale $\mathcal{L}$ allows the transition to chaos to be examined over measurable laboratory temperature changes. The transition is found to be abrupt. Bulk materials, with a distribution of crystallite sizes, will smear out the transition, resulting in a very slow crossover. The abruptness of the transition and its nature are compared with recent theoretical calculations.

We report on the strain-induced switch between ferromagnetic (FM) and antiferromagnetic (AFM) orderings in $1T\text{−}\mathrm{Cr}{X}_2$ ($X=\text{Se}$, Te) monolayers based on first-principles calculations. The ${\mathrm{CrSe}}_2$ and ${\mathrm{CrTe}}_2$ monolayers without strains are found to be AFM and FM, respectively. Under biaxial tensile strain, the ${\mathrm{CrSe}}_2$ monolayer tends to be FM when the strain is larger than 2%. The FM state is further stabilized when the strain is increased. Moreover, the ${\mathrm{CrSe}}_2$ monolayer becomes half metallic when the tensile strain is larger than 10%, while for the ${\mathrm{CrTe}}_2$ monolayer, the critical strain at which the transition between the FM and AFM states occurs is compressive, of $-1\%$. Relatively small tensile strains of 4% and 2%, respectively, can enhance the Curie temperature of the ${\mathrm{CrSe}}_2$ and ${\mathrm{CrTe}}_2$ monolayers above room temperature. The strain-induced switch between the FM and AFM states in a ${\mathrm{CrSe}}_2 \left({\mathrm{CrTe}}_2\right)$ monolayer can be understood by the competition between the AFM Cr-Cr direct exchange interaction and the FM Cr-Se(Te)-Cr superexchange interaction. Tunable and attractive magnetic and electronic properties controlled by flexible strain are desirable for future nanoelectronic applications.

Rectangular nanowires may have domain walls nucleated and moved through them to realize many devices. It has been shown that at a particular width and thickness of a nanowire with perpendicular anisotropy, there is a switch from the domain wall being of Bloch-type to it being of Néel-type. This critical shape can be found through micromagnetics simulations, but here we present an analytic calculation for the energies of both wall types involving approximations for the demagnetizing energy density. The approximations implemented include a one-dimensional model of the magnetization and a simplified initial magnetization configuration whereby the demagnetizing energy density is solved through an iterative process. The expressions developed are long, but have the advantage that by simply inputting material parameters for the magnetic material, the critical shape can be found using a calculator in a matter of seconds. We compare our results to those found using micromagnetics and those found experimentally and the agreement is good.

A unique property of metal nanoclusters is the “superatom” shell structure of their delocalized electrons. The electronic shell levels are highly degenerate and therefore represent sharp peaks in the density of states. This can enable exceptionally strong electron pairing in certain clusters composed of tens to hundreds of atoms. In a finite system, such as a free nanocluster or a nucleus, pairing is observed most clearly via its effect on the energy spectrum of the constituent fermions. Accordingly, we performed a photoionization spectroscopy study of size-resolved aluminum nanoclusters and observed a rapid rise in the near-threshold density of states of several clusters $\left(\mathrm{A}{\mathrm{l}}_{37,44,66,68}\right)$ with decreasing temperature. The characteristics of this behavior are consistent with compression of the density of states by a pairing transition into a high-temperature superconducting state with $T_c\gtrsim 100\text{K}$. This value exceeds that of bulk aluminum by two orders of magnitude. These results highlight the potential of novel pairing effects in size-quantized systems and the possibility to attain even higher critical temperatures by optimizing the particles' size and composition. As a new class of high-temperature superconductors, such metal nanocluster particles are promising building blocks for high-$T_c$ materials, devices, and networks.

We present a variational ansatz for the ground state of a strongly correlated Bose system. This ansatz goes beyond the Jastrow-Feenberg functional form and explicitly enforces coordination shells in the structure of the wave function. We apply this ansatz to liquid helium-4 with a simple three-variable parametrization of the pair functions. The optimized wave function is found to give an excellent description of the mid-range correlations in the fluid. We also demonstrate the possibility to use this ansatz to study inhomogeneous systems. The phase separation and free surface emerge naturally in this wave function, even though it is constructed of short-range two-body functions and does not contain one-body terms. Because no explicit description of the surface is necessary, this provides a powerful description tool for cluster states.

${\mathrm{Ba}}_{6-x}{\mathrm{Sr}}_x{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ solid solution with $0\le$ $x$ $\le 6$ forms the filled tetragonal tungsten bronze (TTB) structure. The Ba-end member crystallizes in the highest symmetry $P4/mbm$ space group $(a=b=12.5842(18)\AA$ and $c=3.9995\left(8\right)\AA )$ and so do all the compositions with $0\le$ $x$ $\le 5$. The Sr-end member of the solid solution crystallizes in the tentatively assigned $Amam$ space group $(a{}^{*}=17.506(4)\AA$, $b{}^{*}=34.932\left(7\right)\AA$, and $c{}^{*}=7.7777\left(2\right)\AA )$. The latter space group is related to the parent $P4/mbm$ TTB structure as $a{}^{*}$ $\approx$ $\sqrt{2}a,b{}^{*}$ $\approx 2\sqrt{2}a,c{}^{*}=2c$. Low-temperature specific heat measurements indicate that the Ba-rich compositions with $x$ $\le 2$ are conventional BCS superconductors with $T_C$ $\le 1.6$ K and superconducting energy gaps of $\le 0.38$ meV. The values of the $T_C$ in the cation-filled Nb-based TTBs reported here are comparable with those of the unfilled ${\mathrm{K}}_x{\mathrm{WO}}_3$ and ${\mathrm{Na}}_x{\mathrm{WO}}_3$ TTBs having large alkali ion deficiency. As the unit cell volume decreases with increasing $x$, an unexpected metal-insulator transition (MIT) in ${\mathrm{Ba}}_{6-x}{\mathrm{Sr}}_x{\mathrm{Nb}}_{10}{\mathrm{O}}_{30}$ occurs at $x$ $\ge 3$. We discuss the possible origins of the MIT in terms of the carrier concentration, symmetry break, and Anderson localization.

In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal nonuniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.

We studied the physical properties of hollandite titanates, ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$, which have double chains of edge-sharing ${\mathrm{TiO}}_6$ octahedra with $d$ electrons in the $t_{2g}$ states. We found that there is an electronic phase transition at $\sim 220$ K, at which various properties exhibit anomalies. This phase transition is characterized by a modulation in the ${\mathrm{TiO}}_6$ chains and a spectral weight transfer of over 2 eV in the optical conductivity spectrum, which are presumably caused by charge and orbital ordering of the Ti $t_{2g}$ electrons.

We propose thermally populated domain walls (DWs) in an easy-plane ferromagnetic insulator as robust spin carriers between two metals. The chirality of a DW, which serves as a topological charge, couples to the metal spin accumulation via spin-transfer torque and results in the chirality-dependent thermal nucleation rates of DWs at the interface. After overpopulated DWs of a particular (net) chirality diffuse and leave the ferromagnet at the other interface, they reemit the spin current by spin pumping. The conservation of the topological charge supports an algebraic decay of spin transport as the length of the ferromagnet increases; this is analogous to the decaying behavior of superfluid spin transport but contrasts with the exponential decay of magnon spin transport. We envision that similar spin transport with algebraic decay may be implemented in materials with exotic spin phases by exploiting topological characteristics and the associated conserved quantities of their excitations.

We demonstrate that ferromagnetic and antiferromagnetic excitations can be triggered by the dynamical spin accumulations induced by the bulk and surface contributions of the spin Hall effect. Due to the spin-orbit interaction, a time-dependent spin density is generated by an oscillatory electric field applied parallel to the atomic planes of Fe/W(110) multilayers. For symmetric trilayers of Fe/W/Fe in which the Fe layers are ferromagnetically coupled, we demonstrate that only the collective out-of-phase precession mode is excited, while the uniform (in-phase) mode remains silent. When they are antiferromagnetically coupled, the oscillatory electric field sets the Fe magnetizations into elliptical precession motions with opposite angular velocities. The manipulation of different collective spin-wave dynamical modes through the engineering of the multilayers and their thicknesses may be used to develop ultrafast spintronics devices. Our work provides a general framework that probes the realistic responses of materials in the time or frequency domain.

Experiments have confirmed that double monolayer graphene does not generate finite-temperature electron-hole superfluidity, because of very strong screening of the pairing attraction. The linear dispersing energy bands in monolayer graphene block any attempt to reduce the strength of the screening. We propose a hybrid device with two sheets of monolayer graphene in a modulated periodic perpendicular magnetic field. The field preserves the isotropic Dirac cones of the original monolayers but reduces the slope of the cones, making the monolayer Fermi velocity $v_F$ smaller. We demonstrate that with current experimental techniques, the reduction in $v_F$ can weaken the screening sufficiently to allow electron-hole superfluidity at measurable temperatures.

Epitaxial films of $\mathrm{C}{\mathrm{o}}_x\mathrm{M}{\mathrm{n}}_y\mathrm{G}{\mathrm{e}}_z$ grown on Ge (111) substrates by molecular-beam-epitaxy techniques have been investigated as a continuous function of composition using combinatorial synchrotron x-ray diffraction (XRD) and x-ray fluorescence (XRF) spectroscopy techniques. A high-resolution ternary epitaxial phase diagram is obtained, revealing a small number of structural phases stabilized over large compositional regions. Ordering of the constituent elements in the compositional region near the full Heusler alloy $\mathrm{C}{\mathrm{o}}_2\mathrm{MnGe}$ has been examined in detail using both traditional XRD and a new multiple-edge anomalous diffraction (MEAD) technique. Multiple-edge anomalous diffraction involves analyzing the energy dependence of multiple reflections across each constituent absorption edge in order to detect and quantify the elemental distribution of occupation in specific lattice sites. Results of this paper show that structural and chemical ordering are very sensitive to the Co : Mn atomic ratio, such that the ordering is the highest at an atomic ratio of 2 but significantly reduced even a few percent off this ratio. The in-plane lattice is nearly coherent with that of the Ge substrate, while the approximately 2% lattice mismatch is accommodated by the out-of-plane tetragonal strain. The quantitative MEAD analysis further reveals no detectable amount (<0.5%) of Co-Mn site swapping, but instead high levels (26%) of Mn-Ge site swapping. Increasing Ge concentration above the Heusler stoichiometry $(\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.25}\mathrm{G}{\mathrm{e}}_{0.25})$ is shown to correlate with increased lattice vacancies, antisites, and stacking faults, but reduced lattice relaxation. The highest degree of chemical ordering is observed off the Heusler stoichiometry with a Ge enrichment of 5 at.%.

In this paper, we present a systematic study of the effects of Fe doping on the electronic and magnetic structures of spinel ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ by ab initio density functional theory and atomistic spin dynamics calculations. Our calculated magnetic structure for pristine ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ correctly reproduces the experimental one with a q-vector of (0.67, 0.67,0.0), establishing the accuracy of the calculated interatomic exchange interactions. We show that the noncollinear spin structure with a nonzero q-vector in the spinel structure is driven towards collinearity by Fe doping by a complex interplay between interatomic exchange interactions. In the inverse spinel structure with 100% Fe doping, a collinear antiferromagnetic order develops along with a half-metallic electronic structure, which evolves due to the chemical disorder between Fe and Co in the B sites described by the coherent potential approximation. This is a comprehensive theoretical study to understand the evolution of magnetic and electronic properties of multiferroic ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ doped with Fe.