The effect of temperature on the formation and growth of monoatomic chains is investigated by extensive molecular dynamics simulations using a semiempirical potential based on the second-moment approximation to the tight-binding Hamiltonian. Gold nanowires, with an aspect ratio of $\sim$13 and a cross section of $\sim$1 $\mathrm{n}{\mathrm{m}}^2$, are stretched at a rate of 3 $\mathrm{m}/\mathrm{s}$ in the range of temperatures 5–600 K with 50 initial configurations per temperature. A detailed study on the probability to form monoatomic chains (MACs) is presented. Two domains are apparent in our simulations: one at $T<100$ K, where MACs develop from crystalline disorder at the constriction, and the other at $T>100$ K, where MACs form as a consequence of plastic deformation of the nanowire. Our results show that the average length of the formed MACs maximizes at $T=$150 K, which is supported by simple energy arguments.

We propose an approach allowing a systematic optimization of lenses based on hyperbolic metamaterials. The lensing properties of these highly anisotropic materials are summed up in a complex effective index extracted from the complex dispersion relation. The analytical expression of this effective index in the homogenization regime or its direct computation from the Bloch band diagram in the resonant regime leads to hyperbolic metamaterials that outperform the state-of-art flat lenses. We show that feasible metal-dielectric multilayers provide superresolved images for visible light (around 400 nm) even when fully taking absorption into account.

The origin of the two-dimensional surface states localized in subsurface regions of the Ge(111) substrate has been studied by density-functional-theory calculations, which were compared with the experimental results of angle-resolved photoelectron spectroscopy. For the Bi/Ge(111)-$(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$, Br/Ge(111)-(1$\times$1), and Tl/Ge(111)-(1$\times$1) surfaces, we found that the surface states are classified into three groups. The energy dispersion and the orbital character for each band implies the relationship between the subsurface states and the bulk heavy-hole, light-hole, and spin-orbit split-off bands. These results indicate that the subsurface states originate from the bulk bands that are perturbed due to the truncation of the three-dimensional periodicity at the surface.

We study the dependence of the quantum yield of photoluminescence of a dense, periodic array of semiconductor nanocrystals (NCs) on the level of doping and NC size. Electrons introduced to NCs via doping quench photoluminescence by the Auger process, so that practically only NCs without electrons contribute to the photoluminescence. Computer simulation and analytical theory are used to find a fraction of such empty NCs as a function of the average number of donors per NC and NC size. For an array of small spherical NCs, the quantization gap between $1S$ and $1P$ levels leads to transfer of electrons from NCs with large number of donors to those without donors. As a result, empty NCs become extinct, and photoluminescence is quenched abruptly at an average number of donors per NC close to 1.8. The relative intensity of photoluminescence is shown to correlate with the type of hopping conductivity of an array of NCs.

Control of octahedral rotations in the ABO${}_3$ perovskite oxides has been of great interest due to its potential in rationally discovering and designing new multifunctional phases. In this study, we show that octahedral rotations of the SrRuO${}_3$ films can be controlled by oxygen vacancies as well as by interfacial coupling, which further determines the physical properties. Half-integer reflections using high-resolution synchrotron x-ray diffraction were carried out to determine the octahedral rotation pattern of SrRuO${}_3$ films on SrTiO${}_3$ substrates. The transition of RuO${}_6$ rotation pattern accompanied by the structural change from monoclinic $P$2${}_1/m$ to tetragonal $F$4/mmc can be understood from the preference of oxygen vacancies in the SrO atomic plane and the coupling of octahedra across the interface between film and substrate. The field angle dependence of magnetoresistance further confirmed the structural phase transition with changes in octahedral rotations. The monoclinic phase has the uniaxial magnetic easy axis 30° away from the [001] direction towards the [010] direction while the tetragonal phase has uniaxial magnetic easy axis along the fourfold axis which is perpendicular to the film surface. This study demonstrates the ability to control the octahedral rotations in perovskite films and its importance when designing thin films and multilayers with desired functional property.

The properties of ferroelectric materials, such as lead zirconate titanate (PZT), are heavily influenced by the interaction of defects with domain walls. These defects are either intrinsic or are induced by the addition of dopants. We study here PbTiO${}_3$ (the end member of a key family of solid solutions) in the presence of acceptor (Fe) and donor (Nb) dopants, and the interactions of the different defects and defect associates with the domain walls. For the case of iron acceptors, the calculations point to the formation of defect associates involving an iron substitutional defect and a charged oxygen vacancy (Fe${}_{\text{Ti}}^{\prime }$-V${}_{\mathrm{O}}^{{}^{..}}$). This associate exhibits a strong tendency to align in the direction of the bulk polarization; in fact, ordering of defects is also observed in pure PbTiO${}_3$ in the form of lead-oxygen divacancies. Conversely, calculations on donor-doped PbTiO${}_3$ do not indicate the formation of polar defect complexes involving donor substitutions. Last, it is observed that both isolated defects in donor-doped materials and defect associates in acceptor-doped materials are more stable at 180${}^{\circ }$ domain walls. However, polar defect complexes lead to asymmetric potentials at domain walls due to the interaction of the defect polarization with the bulk polarization. The relative pinning characteristics of different defects are then compared, to develop an understanding of defect–domain-wall interactions in both doped and pure PbTiO${}_3$. These results may also help in understanding hardening and softening mechanisms in PZT.

We investigate the electronic structure of defects in LaAlO${}_3$ (LAO) and their effects on electronic properties of bulk and heterostructures. Our calculations indicate that vacancies have lower formation energies than interstitials and antisites. The La vacancy ($V_{\mathrm{La}}$) and the Al vacancy ($V_{\mathrm{Al}}$) are deep acceptors, while the oxygen vacancy ($V_{\mathrm{O}}$) is a deep donor. The impact of these defects on the performance of metal-oxide-semiconductor devices is analyzed by placing the LAO band edges and defect levels with respect to the band edges of GaN, InGaAs, and Si. $V_{\mathrm{O}}$ introduces levels in the gap or in the vicinity of the semiconductor conduction band, resulting in carrier traps and/or leakage current through the gate oxide, while $V_{\mathrm{La}}$ and $V_{\mathrm{Al}}$ are sources of negative fixed charges. We also discuss how oxygen vacancies in LAO can influence the observed two-dimensional electron gas (2DEG) in LaAlO${}_3$/SrTiO${}_3$ heterostructures. We conclude that $V_{\mathrm{O}}$ in the LAO layer may provide electrons that fill compensating surface states, resulting in higher 2DEG densities, at least for modest LAO layer thicknesses.

Magnetization studies have been performed on a polycrystalline sample of Dy${}_5$Si${}_3$Ge as a function of an applied magnetic field (up to 50 kOe) and hydrostatic pressure (up to 10 kbar) in the 5–300 K temperature range. The anomalous behavior of the magnetic susceptibility indicates that a Griffiths-like phase exists at low magnetic fields and pressures up to 10 kbar. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order crystallographic transformation into a single first-order magnetic-crystallographic transformation using a magnetic field and hydrostatic pressure as tuning parameters. The effect of pressure on the Griffiths-like phase is reported and analyzed in the framework of the complex competition between the interslab and intraslab magnetic interactions.

We have used first-principles methods to investigate the phase diagram of multiferroic bismuth ferrite (BiFeO${}_3$ or BFO), revealing the energetic and vibrational features that control the occurrence of various relevant structures. More precisely, we have studied the relative stability of four low-energy BFO polymorphs by computing their free energies within the quasiharmonic approximation, introducing a practical scheme that allows us to account for the main effects of spin disorder. As expected, we find that the ferroelectric ground state of the material (with $R3c$ space group) transforms into an orthorhombic paraelectric phase ($Pnma$) upon heating. We show that this transition is not significantly affected by magnetic disorder, and that the occurrence of the $Pnma$ structure relies on its being vibrationally (although not elastically) softer than the $R3c$ phase. We also investigate a representative member of the family of nanotwinned polymorphs recently predicted for BFO [S. Prosandeev et al., Adv. Funct. Mater. 23, 234 (2013)] and discuss their possible stabilization at the boundaries separating the $R3c$ and $Pnma$ regions in the corresponding pressure-temperature phase diagram. Finally, we elucidate the intriguing case of the so-called supertetragonal phases of BFO: Our results explain why such structures have never been observed in the bulk material, despite their being stable polymorphs of very low energy. Quantitative comparison with experiment is provided whenever possible, and the relative importance of various physical effects (zero-point motion, spin fluctuations, thermal expansion) and technical features (employed exchange-correlation energy density functional) is discussed. Our work attests the validity and usefulness of the quasiharmonic scheme to investigate the phase diagram of this complex oxide, and prospective applications are discussed.

We measure the anomalous Hall effect (AHE) in several patterns of the itinerant ferromagnet ${\text{SrRuO}}_3$ before and after the patterns are irradiated with electrons. The irradiation increases the resistivity of the patterns due to the introduction of point defects and we find that the AHE coefficient $R_s$ scales with the total resistivity before and after irradiation which indicates that the AHE is determined by the total resistivity. We discuss possible origins of slight deviations from scaling that are observed at low temperature, particularly below 70 K.

The spin Hall effect (SHE) has recently turned out to be an interesting tool for the conversion between charge and spin currents, the conversion factor being characterized by the spin Hall angle ${\Phi }_H$. Large spin Hall angles have been now measured in heavy metals like W (${\Phi }_H=-0.33$) and Cu doped with Bi impurities (${\Phi }_H=-0.24$). In this article we express the contributions to the SHE induced by skew scattering and scattering with side-jump from Bi impurities in Cu, and we use ab initio calculations of the electronic structure of CuBi alloys to estimate the values of these two contributions. The predominant effect comes from skew scattering; the spin Hall angle is negative in agreement with experiments, but the calculated amplitude is smaller.

We report on the Cherenkov-type excitation of spin waves (SWs) in ferromagnets. Our micromagnetic simulations show that a localized magnetic field pulse moving sufficiently fast along the surface of a ferromagnet generates a SW boom, with a Mach-type cone of propagating wave fronts. The SWs are formed when the velocity of the source exceeds the propagation speed of SWs. Unlike the single cone of the usual Cherenkov effect, we find that the magnetic Mach cone consists of two wave fronts with different wave numbers. In patterned thin strips, this magnetic analog of the Cherenkov effect should enable the excitation of SWs with well-defined and velocity-dependent frequency. It thereby provides a promising route towards tunable SW generation, with important potential for applications in magnonic devices.

Na${}_4$Ir${}_3$O${}_8$ is a candidate material for a three-dimensional quantum spin liquid on the hyperkagome lattice. We present thermodynamic measurements of heat capacity $C$ and thermal conductivity $\kappa$ on high-quality polycrystalline samples of Na${}_4$Ir${}_3$O${}_8$ down to $T=500$ and 75 mK, respectively. Absence of long-range magnetic order down to $T=75$ mK strongly supports claims of a spin-liquid ground state. The constant magnetic susceptibility $\chi$ below $T\approx 25$ K and the presence of a small but finite linear-$T$ term in $C\left(T\right)$ suggest the presence of gapless spin excitations. Additionally, the magnetic Gr$\ddot{\mathrm{u}}$neisen ratio shows a divergence as $T\to 0$ K and a scaling behavior, which clearly demonstrates that Na${}_4$Ir${}_3$O${}_8$ is situated close to a zero-field QCP.

We discuss the phase diagram and phase transitions in $\text{U}\left(1\right)\times {\mathbb{Z}}_2$ three-band superconductors with broken time reversal symmetry. We find that beyond mean-field approximation and for sufficiently strong frustration of interband interactions there appears an unusual metallic state precursory to a superconducting phase transition. In that state, the system is not superconducting. Nonetheless, it features a spontaneously broken ${\mathbb{Z}}_2$ time reversal symmetry. By contrast, for weak frustration of interband coupling the energy of a domain wall between different ${\mathbb{Z}}_2$ states is low and thus fluctuations restore broken time reversal symmetry in the superconducting state at low temperatures.

Quantum criticality provides an important route to revealing universal nonequilibrium behavior. A canonical example of a critical point is the Bose-Hubbard model, which we study under the application of an electric field. A Boltzmann transport formalism and $\epsilon$ expansion are used to obtain the nonequilibrium conductivity and current noise. This approach allows us to explicitly identify how a universal nonequilibrium steady state is maintained, by identifying the rate-limiting step in balancing Joule heating and dissipation to a heat bath. It also reveals that the nonequilibrium distribution function is very far from a thermal distribution.

A new method for assessing the similarity of material compositions is described. A similarity measure is important for the classification and clustering of compositions. The similarity of the material compositions is calculated utilizing a data-mined ionic substitutional similarity based upon the probability with which two ions will substitute for each other within the same structure prototype. The method is validated via the prediction of crystal structure prototypes for oxides from the Inorganic Crystal Structure Database, selecting the correct prototype from a list of known prototypes within five guesses 75% of the time. It performs particularly well on the quaternary oxides, selecting the correct prototype from a list of known prototypes on the first guess 65% of the time.

We report the design and the characterization of artificial structures made of periodical distributions of structured cylindrical scatterers embedded in a two-dimensional (2D) waveguide. For certain values of their geometrical parameters they show simultaneously negative effective bulk modulus and negative effective mass density. Here our analysis is focused on the frequencies where they behave like materials with negative density or density near zero (DNZ). The scattering units consist of a rigid cylindrical core surrounded by an anisotropic shell divided in angular sectors. The units are embedded in a 2D waveguide whose height is smaller than the length of the cylinders, which makes the structure quasi-2D. We have obtained the dispersion relation of the surface acoustic waves excited at frequencies with negative effective density. Also, we report phenomena associated with their DNZ behavior, such as tunneling through narrow channels, control of the radiation field, perfect transmission through sharp corners, and power splitting. Preliminary experiments performed on samples with millimeter-scale dimensions demonstrated their single-negative behavior, with the main drawback being the strong losses measured at the frequencies where the negative behavior is observed.

The coupling of submonolayer coverages of Fe-phthalocyanine molecules on bare and oxygen-covered ferromagnetic Co(001) films was studied by x-ray-absorption spectroscopy, especially the x-ray magnetic circular dichroism, in combination with density functional theory. We observe that the magnetic moments of the paramagnetic molecules are aligned even at room temperature, resulting from a magnetic coupling to the substrate. While the magnetization of the Fe ions directly adsorbed on the Co surface is parallel to the magnetization of the Co film, the introduction of an oxygen interlayer leads to an antiparallel alignment. As confirmed by theory, the coupling strength is larger for the system FePc/Co than for FePc/O/Co, causing a stronger temperature dependence of the Fe magnetization for the latter system. Furthermore, the calculations reveal that the coupling mechanism changes due to the O layer from mostly direct exchange to Co of the bare surface to a 180${}^{\circ }$ antiferromagnetic superexchange via the O atoms. Finally, by comparing the experimental x-ray-absorption spectra at the N $K$ edge with the corresponding calculations, the contribution of the individual orbitals has been determined and the two inequivalent N atoms of the molecules could be distinguished.

We study the polarization-state formation in magnonic vortex crystals via scanning transmission x-ray microscopy. Self-organized state formation is observed by adiabatic reduction of a high-frequency field excitation. The emerging polarization patterns are shown to depend on the frequency of excitation and the strength of the dipolar interaction between the elements. In spite of the complexity of the investigated system, global order caused by local interactions creates polarization states with a high degree of symmetry. A fundamental dipole model and coupled equations of motion are adopted to analytically describe the experimental results. The emerging states can be predicted by a fundamental stability criterion based on the excitability of eigenmodes in the crystal. Micromagnetic simulations give additional insight into the underlying processes.

Using a simple two-band model for Fe-based pnictides and the generalized Eilenberger equation, we present a microscopic derivation of a time-dependent equation for the amplitude of the spin-density wave near the quantum critical point where it turns to zero. This equation describes the dynamics of the magnetic ($m$), as well as the superconducting order parameter ($\Delta$). It is valid at low temperatures $T$ and small $m$ ($T,m\ll \Delta$) in a region of coexistence of both order parameters, $m$ and $\Delta$. The boundary of this region is found in the space of the nesting parameter $\{{\mu }_0,{\mu }_{\varphi }\}$, where ${\mu }_0$ describes the relative position of the electron and the hole pockets on the energy scale and ${\mu }_{\varphi }$ accounts for the ellipticity of the electron pocket. At low $T$ the number of quasiparticles is small due to the presence of the energy gap $\Delta$, and therefore the quasiparticles do not play a role in the relaxation of $m$. This circumstance allows one to derive the time-dependent equation for $m$ in contrast to the case of conventional superconductors for which the time-dependent Ginzburg-Landau equation can be derived near $T_{\mathrm{c}}$ only in some special cases (high concentration of paramagnetic impurities) [L. P. Gor'kov and G. M. Eliashberg, Sov. Phys. JETP 27, 328 (1968)]. In the stationary case the derived equation is valid at arbitrary temperatures. We find a solution of the stationary equation which describes a domain wall in the magnetic structure. In the center of the domain wall the superconducting order parameter has a maximum, which means a local enhancement of superconductivity. Using the derived time-dependent equation for $m$, we investgate also the stability of a uniform commensurate spin-density wave (SDW) and obtain the values of $\{{\mu }_0,{\mu }_{\varphi }\}$ at which the first-order transition into the state with $m=0$ takes place or the transition to the state with an inhomogeneous SDW occurs.