Observations of topological defects associated with Stone-Wales-type transformations (i.e., bond rotations) in high-resolution transmission electron microscopy (HRTEM) images of carbon nanostructures are at odds with the equilibrium thermodynamics of these systems. Here, by combining aberration-corrected HRTEM experiments and atomistic simulations, we show that such defects can be formed by single electron impacts and, remarkably, at electron energies below the threshold for atomic displacements. We further study the mechanisms of irradiation-driven bond rotations and explain why electron irradiation at moderate electron energies ($~$100 keV) tends to amorphize rather than perforate graphene. We also show via simulations that Stone-Wales defects can appear in curved graphitic structures due to incomplete recombination of irradiation-induced Frenkel defects, similar to formation of Wigner-type defects in silicon.

Pressure-induced superconductivity and structural phase transitions in calcium (Ca) were studied by electrical resistance measurements and angular dispersive x-ray diffraction measurements under pressures above 200 GPa. Other alkaline-earth metals, Sr and Ba, have a high-pressure phase with a “host-guest” structure, and the maximum superconducting transition temperature $T_{\mathrm{c}}$ is observed in this phase. However, the high-pressure phase with the host-guest structure has not yet been observed in Ca. In this work, we found a high-pressure phase (Ca-VII) around 210 GPa with a host-guest structure. Furthermore, the $T_{\mathrm{c}}$ onset at Ca-VII increased continuously, reaching 29 K at 216 GPa.

Due to strong geometric frustration and quantum fluctuation, the $S=1/2$ quantum Heisenberg antiferromagnet on the $\text{kagome}$ lattice has long been considered as an ideal platform to realize a spin liquid (SL), a phase exhibiting fractionalized excitations without any symmetry breaking. A recent numerical study (Yan et al., e-print arXiv:1011.6114) of the Heisenberg $S=1/2,\text{kagome}$ lattice model (HKLM) shows, in contrast to earlier results, that the ground state is a singlet-gapped SL with signatures of ${\mathbb{Z}}_2$ topological order. Motivated by this numerical discovery, we use the projective symmetry group to classify all 20 possible Schwinger fermion mean-field states of ${\mathbb{Z}}_2$ SLs on the $\text{kagome}$ lattice. Among them we found only one gapped ${\mathbb{Z}}_2$ SL (which we call the ${\mathbb{Z}}_2[0,\pi ]\beta$ state) in the neighborhood of the U(1) Dirac SL state. Since its parent state, i.e., the U(1) Dirac SL, was found [Ran et al., Phys. Rev. Lett. 98, 117205 (2007)] to be the lowest among many other candidate U(1) SLs, including the uniform resonating-valence-bond states, we propose this ${\mathbb{Z}}_2[0,\pi ]\beta$ state to be the numerically discovered SL ground state of the HKLM.

Hall probe magnetometry has been used to investigate the magnetization of individual cylindrically shaped Pb nanowires grown by electrocrystallization on a highly oriented pyrolytic graphite electrode. These measurements have been interpreted by comparison with three-dimensional Ginzburg-Landau (GL) calculations for nanowires with our sample parameters. We find that the measured superheating field and the critical field for surface superconductivity are strongly influenced by the temperature-dependent coherence length, $\xi (T)$ and penetration depth $\lambda (T)$ and their relationship to the nanowire diameter. As the temperature is increased toward $T_c$ this drives a change in the superconductor-normal transition from first order irreversible to first order reversible and finally second order reversible. We find that the geometrical flux confinement in our type-I nanowires leads to the formation of a one-dimensional row of single-quantum vortices. While GL calculations show a quite uniform distribution of vortices in thin nanowires, clear vortex bunching is found as the diameter increases, suggesting a transition to a more classical type-I behavior. Subtle changes in minor magnetization loops also indicate that slightly different flux configurations can form with the same vorticity, which depend on the sample history.

Understanding iron-based superconductors requires high-quality impurity-free single crystals. So far they have been elusive for $\beta$-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic-impurity phases. Here, we report synchrotron-clean $\beta$-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect-induced weak ferromagnetism that coexists with superconductivity below $T_c$. In contrast to Fe${}_{1+y}$Te-based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).

We report on a systematic study of the density dependence of mobility in a low-density carbon-doped (100) GaAs two-dimensional hole system (2DHS). At $T=50$ mK, a mobility of 2.6 $\times$ 10${}^6$ cm${}^2$/Vs at a density $p=6.2\times {10}^{10}$ cm${}^{-2}$ was measured. This is the highest mobility reported for a 2DHS to date. Using a backgated sample geometry, the density dependence of mobility was studied from 2.8 $\times$ 10${}^{10}$ cm${}^{-2}$ to 1 $\times$ 10${}^{11}$ cm${}^{-2}$. The mobility vs density cannot be fit to a power law dependence of the form $\mu ~p^{\alpha }$ using a single exponent $\alpha$. Our data indicate a continuous evolution of the power law with $\alpha$ ranging from $~$0.7 at high density and increasing to $~$1.7 at the lowest densities measured. Calculations specific to our structure indicate a crossover of the dominant scattering mechanism from uniform background impurity scattering at high density to remote ionized impurity scattering at low densities. This is the first observation of a carrier density-induced transition from background impurity dominated to remote dopant dominated transport in a single sample.

Previously, the authors reported direct evidence of channel saturation and conductance quantization in atomic-sized gold constrictions through mechanical perturbation studies, and also showed that peaks in conductance histograms are insufficient in evaluating their mechanical stability [Armstrong et al., Phys. Rev. B 82, 195416 (2010)]. In the present study, gold constrictions spanning the range from quantum to semiclassical (Sharvin) conductance regimes are mechanically probed with picolevel resolution in applied force and deformation, along with simultaneous measurements of conductance. While reconfiguration from one constriction size to another is known to occur by apparently random discrete atomic displacements, results reveal a remarkable simplicity—the magnitude of discrete atomic displacements is limited to a small set of values that correspond to elementary slip distances in gold rather than Au-Au interatomic distance. Combined with measurements of the spring constant of constrictions, results reveal two fundamental crossovers in deformation modes with increasing contact diameter—first, from homogeneous shear to defect-mediated deformation at a diameter that is in close agreement with previous predictions [Sørensen et al., Phys. Rev. B 57, 3283 (1998)]; and second, the discovery of another crossover marking surface- to volume-dominated deformation. A remarkable modulus enhancement is observed when the size of the constrictions approaches the Fermi wavelength of the electrons, and in the limit of a single-atom constriction it is at least two times that for bulk gold. Results provide atomistic insight into the stability of these constrictions and an evolutionary trace of deformation modes, beginning with a single-atom contact.

We report an infrared spectroscopy study on ${\mathrm{K}}_{0.83}$Fe${}_{1.53}$Se${}_2$, a semiconducting parent compound of the 245 iron selenide superconductors. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double-peak structure between 4000 and 6000 cm${}^{-1}$ and abundant phonon modes—much more than expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron-diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites.

We analyze the interplay between the absence or presence of nodes in the superconducting gap along electron Fermi surfaces (FSs) in Fe pnictides and $2\Delta /T_c$ along hole FSs ($2{\Delta }_h/T_c$), measured by angle-resolved photoemission spectroscopy (ARPES). We solve the set of coupled gap equations for 4- and 5-pocket models of Fe pnictides and relate the presence of the nodes to $2{\Delta }_h/T_c$ being below a certain threshold. Using ARPES data for $2{\Delta }_h/T_c$, we find that the optimally doped ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ and $\text{Ba}({\text{Fe}}_{1-x}{\text{Co}}_x){}_2{\text{As}}_2$ are likely nodeless, but isovalent ${\text{BaFe}}_2({\text{As}}_{1-x}{\text{P}}_x){}_2$ likely has nodes.

We report a Cu $K$edge resonant inelastic x-ray scattering (RIXS) study of the orbital excitations in the orbital-ordered Mott insulator KCuF${}_3$. By performing the polarization analysis of the scattered photons, the excitation between the $e_g$ orbitals is successfully distinguished from the excitations from $t_{2g}$ to $e_g$. The polarization dependence of the respective excitations is interpreted based on a phenomenological consideration of the symmetry of the RIXS process that yields a necessary condition for observing the excitations. Owing to our polarization analysis, we are able to measure the momentum dependence of the orbital excitation corresponding to the orbital degree of freedom over the whole Brillouin zone. The excitation is found to be dispersionless within our experimental resolution.

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8- and 13-nm antiferromagnetic MnO nanoparticles. For the 8-nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is suppressed compared to that in the bulk material (119 K), while for the 13-nm sample $T_N$ (120 K) is comparable to that in the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of $4.2\pm 0.3$ ${\mu }_B$/Mn for the 8-nm sample and $3.9\pm 0.2$ ${\mu }_B$/Mn for the 13-nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80$\%$ of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.

We present a quantum methodology for the calculation of the inelastic neutron scattering (INS) spectra of an ${\mathrm{H}}_2$ molecule confined in a nanoscale cavity. Our approach incorporates the coupled five-dimensional translation-rotation (TR) energy levels and wave functions of the guest molecule. The computed INS spectra are highly realistic and reflect in full the complexity of the coupled TR dynamics on the anisotropic potential energy surfaces of the confining environment. Utilizing this methodology, we simulate the INS spectra of $p$- and $o$-H${}_2$ in the small cage of the structure II clathrate hydrate and compare them with the experimental data.

We present a study of the effects of simultaneous charge and spin frustration on the two-dimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. Our conclusions are based on exact diagonalization studies that include electron-electron interactions as well as adiabatic electron-phonon coupling terms treated self-consistently. The broken-symmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with in-phase lattice dimerization along one direction and the Wigner crystal state with the checkerboard charge order. For moderate to strong frustration, however, the dominant phase is a novel spin-singlet paired-electron crystal (PEC), consisting of pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC, with coexisting charge order and spin gap in two dimensions, is the quarter-filled band equivalent of the valence bond solid (VBS) that can appear in the frustrated half-filled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of onsite and intersite Coulomb interactions as well as electron-phonon coupling strength. We speculate that the spin-bonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a paired-electron liquid. We discuss the implications of the PEC concept for understanding several classes of quarter-filled band materials that display unconventional superconductivity, focusing, in particular, on organic charge transfer solids. Our work points out the need to go beyond quantum spin liquid concepts for highly frustrated organic charge-transfer solids such as $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ and EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$, which we believe show frustration-induced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and quarter-filled band spinels.

The binding surface of Fe-rich Fe-Pd alloys is explored by means of first-principles calculations in the framework of density functional theory involving unconstrained optimization of the atomic positions within a 108-atom supercell. We find that static displacements arising from geometric optimization provide an important contribution to the total energy, effectively compensating favorable contributions gained from introducing L$1_2$ order in stoichiometric Fe${}_3$Pd. In the concentration range for magnetic shape-memory applications, the energy profile with respect to tetragonal distortion is altered qualitatively, shifting the ground state of the intermixed disordered system from face-centered cubic (fcc) to body-centered tetragonal (bct). From the radial pair distribution function and electronic density of states obtained from a 500-atom supercell calculation we identify the origin of the displacements. These arise from the size-dependent relaxations of the larger Pd atoms, on the fcc side in combination with a Jahn-Teller-like rearrangement of Fe $d$ states at the Fermi level.

We demonstrate that the weak antilocalization effect can serve as a convenient method for detecting decoupled surface transport in topological insulator thin films. In the regime where a bulk Fermi surface coexists with the surface states, the low-field magnetoconductivity is well described by the Hikami-Larkin-Nagaoka equation for single-component transport of noninteracting electrons. When the electron density is lowered, the magnetotransport behavior deviates from the single-component description and strong evidence is found for independent conducting channels at or near the bottom and top surfaces. The magnetic-field-dependent part of corrections to conductivity due to Zeeman energy is shown to be negligible for the fields relevant to the weak antilocalization despite considerable electron-electron interaction effects on the temperature dependence of the conductivity.

We have constructed a pressure-temperature ($P$-$T$) phase diagram of $P$-induced superconductivity in EuFe${}_2$As${}_2$ single crystals via resistivity ($\rho$) measurements up to 3.2 GPa. As hydrostatic pressure is applied, the temperature $T_0$ where an antiferromagnetic (AF) transition of the Fe moments and a structural phase transition occur shifts to lower temperatures, and the corresponding resistive anomaly becomes undetectable for $P⩾2.5$ GPa. This suggests that the critical pressure $P_c$ where $T_0$ becomes zero is about 2.5 GPa. We have found that the AF order of the Eu${}^{2+}$ moments survives up to 3.2 GPa, the highest pressure in the experiments, without significant changes in the AF ordering temperature $T_N$. The superconducting (SC) ground state with a sharp transition to zero resistivity at $T_c~30$ K, indicative of bulk superconductivity, emerges in a pressure range from $P_c~2.5$ to $~3.0$ GPa. At pressures close to but outside the SC phase, the $\rho (T)$ curve shows a partial SC transition (i.e., zero resistivity is not attained) followed by a reentrant-like hump at approximately $T_N$ with decreasing temperature. When nonhydrostatic pressure with a uniaxial-like strain component is applied using a solid pressure medium, the partial superconductivity is continuously observed in a wide pressure range from 1.1 to 3.2 GPa.

We report on the phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in three-layered Ba${}_2$Ca${}_2$Cu${}_3$O${}_6$(F,O)${}_2$ by means of Cu-NMR measurements. It is demonstrated that AFM and SC coexist uniformly in three-layered compounds as well as in four- and five-layered ones. The critical hole density $p_c$ for the long-range AFM order is determined as $p_c\simeq$ 0.075, which is larger than $p_c\simeq$ 0.02 and 0.055 in single- and bilayered compounds, and smaller than $p_c\simeq 0.08$–0.09 and 0.10–0.11 in four- and five-layered compounds, respectively. This variation of $p_c$ is attributed to the magnetic interlayer coupling, which becomes stronger as the stacking number of CuO${}_2$ layers increases; that is, the uniform coexistence of AFM and SC is a universal phenomenon in underdoped regions when a magnetic interlayer coupling is strong enough to stabilize an AFM ordering. In addition, we highlight an unusual pseudogap behavior in three-layered compounds—the gap behavior in low-energy magnetic excitations collapses in an underdoped region where the ground state is the AFM-SC mixed phase.

Using Kubo’s linear response theory, we derive expressions for the frequency-dependent electrical conductivity (Kubo-Greenwood formula), thermopower, and thermal conductivity in a strongly correlated electron system. These are evaluated within ab initio molecular dynamics simulations in order to study the thermoelectric transport coefficients in dense liquid hydrogen, especially near the nonmetal-to-metal transition region. We also observe significant deviations from the widely used Wiedemann-Franz law, which is strictly valid only for degenerate systems, and give an estimate for its valid scope of application toward lower densities.

We report detailed measurements of the de Haas–van Alphen (dHvA) effect in BaFe${}_2$P${}_2$, which is the end member of the superconducting series, BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. Using high-purity samples, we are able to observe dHvA oscillations from all the sheets of the Fermi surface and hence build up a detailed picture of its structure. The results show the existence of a highly warped section of hole surface, which may be the origin of the nodal superconducting gap structure in this series. Importantly, we find that even for this nonsuperconducting end member, one of the hole surfaces almost exactly nests with the inner electron surface. This suggests that improved geometric nesting does not drive the increase in pairing strength with decreasing $x$ in this system.

The implementation of screened hybrid functionals into the wien2k code, which is based on the linearized augmented plane wave (LAPW) basis set, is reported. The Hartree-Fock exchange energy and potential are screened by means of the Yukawa potential as proposed by Bylander and Kleinman [D. M. Bylander and L. Kleinman, Phys. Rev. B 41, 7868 (1990)] for the calculation of the electronic structure of solids with the screened-exchange local density approximation. Details of the formalism, which is based on the method of Massidda, Posternak, and Baldereschi [S. Massidda, M. Posternak, and A. Baldereschi, Phys. Rev. B 48, 5058 (1993)] for the unscreened Hartree-Fock exchange are given. The results for the transition-energy and structural properties of several test cases are presented. The results of calculations of the Cu electric-field gradient in Cu${}_2$O are also presented, and it is shown that the hybrid functionals are much more accurate than the standard local-density or generalized gradient approximations.