Thin Mn germanide films with nanoscale thicknesses on Ge(111) have been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), core-level spectroscopy (CLS), and x-ray magnetic circular dichroism. The 260 ${}^{\circ }$C annealing of 16 monolayers of Mn deposited on Ge(111)$c$($2\times 8$) resulted in a uniform film with intense threefold split $\sqrt{3}\times \sqrt{3}$ LEED spots and Moiré patterns in the STM images. This ultrathin film shows a clear ferromagnetism with a Curie temperature of $\sim$250 K. High-resolution Ge 3$d$ CLS spectra were recorded with photon energies between 50 and 90 eV at normal and 60${}^{\circ }$ emission angle. To achieve a consistent fit over the energy and angular range three components were used in the line-shape analysis. The low temperature (260 ${}^{\circ }$C) annealed film shows significant differences in terms of electronic structure and magnetism in contrast to the high temperature (330 ${}^{\circ }$C or above) annealed ones. Our results indicate that the annealing temperature and the Mn coverage play important roles in the formation of a thin magnetic Mn germanide film.

Based on first-principles calculations, we provide a map of the energetic stability and the electronic behavior of the topologically protected surface states of the topological insulator (TI), Bi${}_2$Se${}_3$, doped with transition metals (TMs), TM/Bi${}_2$Se${}_3$(111). We find that Fe, Mn, and Cr impurities are energetically more stable at the Bi substitutional sites, whereas we may find energetically stable substitutional as well as interstitial configurations for Co and Ni impurity atoms. Through scanning tunneling microscopy simulations, we verify that each TM atomic species and its position in the Bi${}_2$Se${}_3$(111) surface can be identified. The substitutional Fe and Cr impurities exhibit an energetic preference for the out-of-plane net magnetic moment, giving rise to a small energy gap at the Dirac point (DP), whereas the in-plane magnetic moment of substitutional Mn/Bi${}_2$Se${}_3$(111) promotes a shift of the DP from the center of the surface Brillouin zone, opening up a small energy gap. For the substitutional impurities, the shapes of the metallic bands are somewhat preserved compared with the energy bands of the pristine Bi${}_2$Se${}_3$(111) surface. Interstitial Co atoms also present an in-plane net magnetic moment, where we find the formation of metallic bands, suppressing the presence of the DP. For the Ni/Bi${}_2$Se${}_3$(111) impurity there is not a net magnetic moment, and therefore, the DP is preserved. Further formation energy results indicate other plausible (meta)stable impurity configurations, giving rise to quite different scenarios for the topological properties of TM/Bi${}_2$Se${}_3$(111), even for the same TM impurity.

One-dimensional crystals have long range translational invariance which manifests as long exciton diffusion lengths, but such intrinsic properties are often obscured by environmental perturbations. We use a first-passage approach to model single-walled carbon nanotube (SWCNT) exciton dynamics (including exciton-exciton annihilation and end effects) and compare it to results from both continuous-wave and multipulse ultrafast excitation experiments to extract intrinsic SWCNT properties. Excitons in suspended SWCNTs experience macroscopic diffusion lengths, on the order of the SWCNT length (1.3–4.7 $\mu$m), in sharp contrast to encapsulated samples. For these pristine samples, our model reveals intrinsic lifetimes (350–750 ps), diffusion constants (130–350 cm${}^2$/s), and absorption cross sections (2.1–$3.6\times {10}^{-17}$ cm${}^2$/atom) among the highest previously reported.

Active control of heat flow is challenging. We demonstrate that molecular spin valves offer a unique opportunity for achieving this goal. Our first-principles calculations of the transport of electrons and phonons in nickel-benzenedithiol-nickel junctions show that when the magnetization direction of the electrodes is changed from parallel to antiparallel the junctions become thermally insulating. Our findings, therefore, suggest a novel avenue for actively controlling thermal transport via the spin degree of freedom.

We present frictional measurements involving controlled lateral manipulation of antimony nanoparticles on graphite featuring atomically smooth particle-substrate interfaces via tapping- and contact-mode atomic force microscopy. As expected from earlier studies, the power required for lateral manipulation as well as the frictional forces recorded during the manipulation events exhibit a linear dependence on the contact area over a wide size range from 2000 nm${}^2$ to 120 000 nm${}^2$. However, we observe a significant and abrupt increase in frictional force and dissipated power per contact area at a value of about 20 000 nm${}^2$, coinciding with a phase transition from amorphous to crystalline within the antimony particles. Our results suggest that variations in the structural arrangement and stoichiometry of antimony oxide at the interface between the particles and the substrate may be responsible for the observed effect.

We present a detailed ab initio study of the electronic structure and magnetic order of an Fe monolayer on the Ir(001) surface covered by adsorbed oxygen and hydrogen. The results are compared to the clean Fe/Ir(001) system, where recent intensive studies indicated a strong tendency towards an antiferromagnetic order and complex magnetic structures. The adsorption of an oxygen overlayer significantly increases interlayer distance between the Fe layer and the Ir substrate, while the effect of hydrogen is much weaker. We show that the adsorption of oxygen (and also of hydrogen) leads to a $p(2\times 1)$ antiferromagnetic order of the Fe moments, which is also supported by an investigation based on a disordered local moment state. Simulated scanning tunneling images using the simple Tersoff-Hamann model hint that the proposed $p(2\times 1)$ antiferromagnetic order could be detected even by nonmagnetic tips.

We use a first-principles based kinetic Monte Carlo simulation to study the movement of a solid iron nanocrystal inside a carbon nanotube driven by the electrical current. The origin of the iron nanocrystal movement is the electromigration force. Even though the iron nanocrystal appears to be moving as a whole, we find that the core atoms of the nanocrystal are completely stationary, and only the surface atoms are moving. Movement in the contact region with the carbon nanotube is driven by electromigration forces, and the movement on the remaining surfaces is driven by diffusion. Results of our calculations also provide a simple model, which can predict the center of mass speed of the iron nanocrystal over a wide range of parameters. We find both qualitative and quantitative agreement of the iron nanocrystal center of mass speed with experimental data.

We report the results of a neutron diffraction study of structural evolution in sodium niobate, NaNbO${}_3$, which is the parent compound for lead-free ferroelectric materials, as a function of temperature from 15 to 930 K over six phases. The Rietveld analysis of the high-resolution powder neutron diffraction data shows the variation in the structure from cubic to rhombohedral ferroelectric structures. However, the refinements on local structure by the pair distribution function (PDF) method indicates that there are only three basic patterns of the local structure, and the ground states of NaNbO${}_3$ in the low-temperature antiferroelectric and ferroelectric phases have the R3c symmetry, even though in the long range the system shows the Pbcm symmetry or the coexistence of two phases. The origin of the complex phase behavior and its implications on the performance as lead-free ferroelectrics are discussed.

Our experiments show that in ${}^4$He crystals, the binding of ${}^3$He impurities to dislocations does not necessarily imply their pinning. Indeed, in these crystals, there are two different regimes of the motion of dislocations when impurities bind to them. At low driving strain $\epsilon$ and frequency $\omega$, where the dislocation speed is less than a critical value (45 $\mu$m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of $\epsilon \omega$. The critical speed separating the two regimes is two orders of magnitude smaller than the average speed of free ${}^3$He impurities in the bulk crystal lattice. We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results solve an apparent contradiction between some experiments, which showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed.

In the search for MgB${}_2$-like phonon-mediated superconductors we have carried out a systematic density functional theory study of the Ca-B system, isoelectronic to Mg-B, at ambient and gigapascal pressures. A remarkable variety of candidate high-pressure ground states have been identified with an evolutionary crystal structure search, including a stable alkaline-earth monoboride oI8-CaB, a superconductor with an expected critical temperature (T${}_c$) of 5.5 K. We have extended our previous study of CaB${}_6$ [Phys. Rev. Lett. 108, 102501 (2012)] to nearby stoichiometries of CaB${}_{6+x}$, finding that extra boron further stabilizes the proposed B${}_{24}$ units. Here an explanation is given for the transformation of cP7-CaB${}_6$ into the more complex oS56 and tI56 polymorphs at high pressure. The stability of the known metallic tP20 phase of CaB${}_4$ at ambient pressure is explained from a crystal structure and chemical bonding point of view. The tP20 structure is shown to destabilize at 19 GPa relative to a semiconducting MgB${}_4$-like structure due to chemical pressure from the metal ion. The hypothetical AlB${}_2$-type structure of CaB${}_2$, previously shown to have favorable superconducting features, is demonstrated here to be unstable at all pressures; two new metallic CaB${}_2$ polymorphs with unusual boron networks stabilize at elevated pressures above 8 GPa but are found to have very low critical temperatures (T${}_c$ $\sim$ 1 K). The stability of all structures has been rationalized through comparison with alkaline-earth analogs, emphasizing the importance of the size of the metal ion for the stability of borides. Our study illustrates the inverse correlation between the thermodynamic stability and superconducting properties and the necessity to carefully examine both in the design of new synthesizable superconducting materials.

We present neutron-scattering measurements on single crystals of lightly doped La${}_{2-x}$Ba${}_x$CuO${}_4$, with $0\le x\le 0.035$. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three-dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high $T_N$, to a two-dimensional (2D) incommensurate (IC) ground state with finite-ranged static correlations, which appear below a relatively low effective $T_N$. At low temperatures, the 2D IC magnetism coexists with the 3D C magnetism for doping concentrations as low as $\sim$0.0125. We find no signal of a 3D C magnetic ground state by $x\sim 0.025$, consistent with the upper limit of $x\sim 0.02$ observed in the sister family of doped Mott insulators, La${}_{2-x}$Sr${}_x$CuO${}_4$. The 2D IC ground states observed for $0.0125\le x\le 0.035$ are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: La${}_{2-x}$Ba${}_x$CuO${}_4$, with $0.05\le x\le 0.095$. We construct a phase diagram based solely on magnetic order-parameter measurements, which displays much of the complexity of standard high-temperature superconductivity phase diagrams discussed in the literature. Analysis of high-energy resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low-energy dynamic magnetic susceptibility as $x$ increases from 0.0125 to 0.035. This low-energy, dynamic susceptibility falls off with increasing temperature on a scale much higher than the effective 2D IC $T_N$ appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the “pseudogap” regime of the phase diagram.

Using ${}^{63}$Cu NMR, we establish that the enhancement of spin order by a magnetic field $H$ in YBa${}_2$Cu${}_3$O${}_{6.45}$ arises from a competition with superconductivity because the effect occurs for $H$ perpendicular, but not parallel, to the CuO${}_2$ planes, and it persists up to field values comparable to $H_{c2}$. We also find that the spin freezing has a glassy nature and that the frozen state onsets at a temperature which is independent of the magnitude of $H$. These results, together with the presence of a competing charge-ordering instability at nearby doping levels, are strikingly parallel to those previously obtained in La-214. This suggests a universal interpretation of magnetic field effects in underdoped cuprates where the enhancement of spin order by the field may not be the primary phenomenon but rather a byproduct of the competition between superconductivity and charge order. We also observe that low-energy spin fluctuations are manifested up to relatively high temperatures where they partially mask the signature of the pseudogap in $1/T_1$ data of planar Cu sites.

We report the temperature-pressure ($T$-$P$) phase diagram of CePt${}_2$In${}_7$ single crystals, especially the pressure evolution of the antiferromagnetic order and the emergence of superconductivity, which have been studied by electrical resistivity and ac calorimetry under nearly hydrostatic environments. Compared with its polycrystalline counterpart, bulk superconductivity coexists with antiferromagnetism in a much narrower pressure region. The possible existence of textured superconductivity and local quantum criticality also are observed in CePt${}_2$In${}_7$, exhibiting a remarkable similarity with CeRhIn${}_5$.

We study the thermodynamic stability at low temperatures of a series of alkali-metal–zinc double-cation borohydrides, including LiZn(BH${}_4$)${}_3$, LiZn${}_2$(BH${}_4$)${}_5$, NaZn(BH${}_4$)${}_3$, NaZn${}_2$(BH${}_4$)${}_5$, KZn(BH${}_4$)${}_3$, and KZn${}_2$(BH${}_4$)${}_5$. While LiZn${}_2$(BH${}_4$)${}_5$, NaZn(BH${}_4$)${}_3$, NaZn${}_2$(BH${}_4$)${}_5$, and KZn(BH${}_4$)${}_3$ were recently synthesized, LiZn(BH${}_4$)${}_3$ and KZn${}_2$(BH${}_4$)${}_5$ are hypothetical compounds. Using the minima-hopping method, we discover two new lowest-energy structures for NaZn(BH${}_4$)${}_3$ and KZn${}_2$(BH${}_4$)${}_5$ which belong to the $C2/c$ and $P2$ space groups, respectively. These structures are predicted to be both thermodynamically stable and dynamically stable, implying that their existence may be possible. On the other hand, we predict that the lowest-energy $P1$ structure of LiZn(BH${}_4$)${}_3$ is unstable, suggesting a possible reason elucidating why this compound has not been experimentally identified. In exploring the low-energy structures of these compounds, we find that their energetic ordering is sensitive to the inclusion of the van der Waals interactions. We also find that a proper treatment of these interactions, e.g., as given by a nonlocal density functional such as vdW-DF2, is necessary to address the stability of the low-energy structures of these compounds.

In the temperature dependence of magnetization, the Heusler compound Ru${}_{1.9}$Fe${}_{0.1}$CrSi exhibits a peak at a temperature which is defined as $T_N^{*}$. Below that temperature strong irreversibility occurs, the onset temperature of which is defined as $T_g$. However, no evidence of long-range order has been found. In this study the magnetic properties of these anomalies were investigated using zero-field (ZF) and longitudinal-magnetic-field (LF) muon-spin-relaxation ($\mu$SR) measurements. In the temperature dependence of the relaxation rate of ZF-$\mu$SR, a peak at $\sim$16 K was observed, which agrees with $T_g$. LF-$\mu$SR measurements as a function of magnetic field reveal the existence of a static internal magnetic field at 0.3 K. Around $T_N^{*}\sim 30$ K, we detected no anomalies that can be associated with a magnetic phase transition in the temperature dependence of the relaxation rate of $\mu$SR, but a large decrease in the initial asymmetry was observed. LF-$\mu$SR measurements suggest that the internal magnetic field appears even around $T_N^{*}$. These results suggest that around $T_N^{*}$ independent spin-frozen regions form inhomogeneously. With decreasing temperature these regions gradually develop, and eventually, at $T_g$ spin-glass freezing occurs with correlations over the whole sample.

The ground-state properties of spin-polarized deuterium (D$\downarrow$) at zero temperature are obtained by means of diffusion Monte Carlo calculations within the fixed-node approximation. Three D$\downarrow$ species have been investigated (D${\downarrow }_1$, D${\downarrow }_2$, D${\downarrow }_3$), corresponding respectively to one, two, and three equally occupied nuclear-spin states. The influence of the backflow correlations on the ground-state energy of the systems is explored. The equations of state of liquid D${\downarrow }_2$ and D${\downarrow }_3$ are obtained and compared with the ones obtained in previous approximate predictions. The density and pressure at which D${\downarrow }_1$ experiences a gas-liquid transition at $T=0$ are obtained.

A burning question in the emerging field of spin-orbit driven insulating iridates, such as Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$, is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic $j_{\mathrm{eff}}=1/2$ band or as a band insulator where the gap is assisted by spin-orbit interaction or Coulomb correlations or both. The difference between these two interpretations is that only for the former strong spin-orbit coupling ($\lambda \gtrsim W,$ where $W$ is the bandwidth) is essential. We have synthesized the isostructural and isoelectronic Li${}_2$RhO${}_3$ and report its electrical resistivity and magnetic susceptibility. Remarkably, it shows insulating behavior together with fluctuating effective $S=1/2$ moments, similar to Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$, although in Rh${}^{4+}$ (4$d^5$) the spin-orbit coupling is greatly reduced. We show that this behavior has a nonrelativistic one-electron origin (although Coulomb correlations assist in opening the gap) and can be traced to the formation of quasimolecular orbitals, similar to those in Na${}_2$IrO${}_3$.

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of nonlocal collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.

We model the thermal, irradiation-induced fusion of single-wall carbon nanotubes into $X$ junctions utilizing a Metropolis Monte Carlo algorithm enacting the most commonly observed transformations in sp${}^2$ carbon. We introduce the concept of the sp${}^2$ to sp${}^3$ parallel defect to model early stages of welding. Quantum mechanical tight-binding calculations are used in conjunction with the Landauer transport formalism to yield transport spectra and density of states at every step of the fusion process in metal/metal, semiconductor/metal, and semiconductor/semiconductor junctions. Insight into the coalescence transformation pathway is gained, and numerical simulations reveal the process as a series of five distinct phases based on the evolving Euler characteristic of the structure. We also highlight the use of the studied $X$ junctions in device applications such as current/voltage dividers, voltage monitors, and logic gates.