We provide evidence for the coherence and Rabi oscillations of spin solitons pinned by the local breaking of translational symmetry in isotropic Heisenberg chains (simple antiferromagnetic Néel or spin Peierls). We show that these correlated spin systems that are made of hundreds of coupled spins bear an overall spin $S=1/2$ and can be manipulated as a single spin. This is clearly contrary to known spin qubits which are paramagnetic centers, highly diluted to prevent decoherence. These results offer an alternative approach for spin qubits, paving the way for the implementation of a different type of quantum computer.

Using Brillouin spectroscopy, an observation has been made of the band structures of nanostructured defect magnonic crystals. The samples are otherwise one-dimensional periodic arrays of equal-width ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ and cobalt nanostripes, where the defects are stripes of a different width. A dispersionless defect branch emerges within the band gap with a frequency tunable by varying the defect stripe width, while the other branches observed are similar to those of a defect-free crystal. Micromagnetic and finite-element simulations performed unveil additional tiny band gaps and the frequency-dependent localization of the defect mode in the vicinity of the defects.

We derive an analytical solution to the Boltzmann transport equation (BTE) to relate nondiffusive thermal conductivity measurements by thermoreflectance techniques to the bulk thermal conductivity accumulation function, which quantifies cumulative contributions to thermal conductivity from different mean free path energy carriers (here, phonons). Our solution incorporates two experimentally defined length scales: thermal penetration depth and heating laser spot radius. We identify two thermal resistances based on the predicted spatial temperature and heat flux profiles. The first resistance is associated with the interaction between energy carriers and the surface of the solution domain. The second resistance accounts for transport of energy carriers through the solution domain and is affected by the experimentally defined length scales. Comparison of the BTE result with that from conventional heat diffusion theory enables a mapping of mean-free-path-specific contributions to the measured thermal conductivity based on the experimental length scales. In general, the measured thermal conductivity will be influenced by the smaller of the two length scales and the surface properties of the system. The result is used to compare nondiffusive thermal conductivity measurements of silicon with first-principles-based calculations of its thermal conductivity accumulation function.

Phonon densities of states (DOS) of bcc $\alpha \text{−}{}^{57}\mathrm{Fe}$ were measured from room temperature through the 1044 K Curie transition and the 1185 K fcc $\gamma$-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700 K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born–von Kármán model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the $\alpha \text{−}\gamma$ structural phase transformation is also reported.

Measurements of magnetothermopower $[S(H,T)]$ of interfacial $\delta$-doped ${\mathrm{LaTiO}}_3$/${\mathrm{SrTiO}}_3$ (LTO/STO) heterostructure by an isostructural antiferromagnetic perovskite ${\mathrm{LaCrO}}_3$ are reported. The thermoelectric power of the pure LTO/STO interface at 300 K is $\approx 118$ $\mu V/K$, but increases dramatically on $\delta$ doping. The observed linear temperature dependence of $S\left(T\right)$ over the temperature range 100 to 300 K is in agreement with the theory of diffusion thermopower of a two-dimensional electron gas. The $S\left(T\right)$ displays a distinct enhancement in the temperature range ($T$ $<$ 100 K) where the sheet resistance shows a Kondo-type minimum. We attributed this maximum in $S\left(T\right)$ to Kondo scattering of conduction electron by localized impurity spins at the interface. The suppression of $S$ by a magnetic field and the isotropic nature of the suppression in out-of-plane and in-plane field geometries further strengthen the Kondo-model-based interpretation of $S(H,T)$.

The crystal structure and charge and orbital order of magnetite below the Verwey temperature are calculated using a first-principles hybrid density functional theory (DFT) method. The initial atomic positions in the crystal-structure calculation are those recently refined from x-ray diffraction data for the $Cc$ space-group unit cell [Senn, Wright, and Attfield, Nature (London) 481, 173 (2012)]. Fermi contact and magnetic dipolar contributions to hyperfine fields at ${}^{57}\mathrm{Fe}$ nuclei obtained from hybrid DFT calculations are used to obtain NMR resonance frequencies for magnetite for a range of external magnetic field directions in a relatively weak field. NMR frequencies from hybrid density functional theory calculations are compared to NMR data [M. Mizoguchi, J. Phys. Soc. Jpn. 70, 2333 (2001)] for a range of applied magnetic field directions. NMR resonance frequencies of $B$-site Fe ions show large relative variations with applied field direction owing to anisotropic hyperfine fields from charge and orbital ordered Fe $3d$ minority-spin electrons at those sites. Good agreement between computed and measured NMR resonance frequencies confirms the pattern of charge and orbital order obtained from calculations. The charge and orbital order of magne-tite in its low-temperature phase obtained from hybrid DFT calculations is analyzed in terms of one-electron bonds between Fe ions. The Verwey transition in magnetite therefore resembles Mott-Peierls transitions in vanadium oxides which undergo symmetry-breaking transitions owing to electron-pair bond formation.

Koopmans-compliant functionals emerge naturally from extending the constraint of piecewise linearity of the total energy as a function of the number of electrons to each fractional orbital occupation. When applied to approximate density-functional theory, these corrections give rise to orbital-density-dependent functionals and potentials. We show that the simplest implementations of Koopmans' compliance provide accurate estimates for the quasiparticle excitations and leave the total energy functional almost or exactly intact, i.e., they describe correctly electron removals or additions, but do not necessarily alter the electronic charge density distribution within the system. Additional Koopmans-compliant functionals can be constructed that modify the potential energy surface, starting, e.g., from Perdew-Zunger corrections. These functionals become exactly one-electron self-interaction free and, as all Koopmans-compliant functionals, are approximately many-electron self-interaction free. We discuss in detail these different formulations, and provide extensive benchmarks for the 55 molecules in the reference G2-1 set, using Koopmans-compliant functionals constructed from local-density or generalized-gradient approximations. In all cases, we find excellent performance in the electronic properties, comparable or improved with respect to that of many-body perturbation theories, such as ${\mathrm{G}}_0\mathrm{W}{}_0$ and self-consistent GW, at a fraction of the cost and in a variational framework that also delivers energy derivatives. Structural properties and atomization energies preserve or slightly improve the accuracy of the underlying density-functional approximations.

Ge heteroepitaxy on Si (1 1 10) substrates induces the formation of prism-shaped in-plane nanowires bounded with $\left\{105\right\}$ facets. In this work, in-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si (1 1 10) templates oriented in the $\left[55\overline{1}\right]$ direction. Atomic force microscopy (AFM) reveals that a self-elongation of the nanowires occurs, resembling the phenomena observed on rib-patterned Si (0 0 1) templates, which indicates that this is a universal effect for nanowires grown on rib patterns. Finite-element simulations, performed with input from the latest ab initio calculations, reveal that the mechanism behind these phenomena is the minimization of the total energy density of the epilayer under rib-dominated geometry. Ge surface diffusion leads to a broadening of the Ge nanowires at the rib shoulder sites, which is proved to be an effective route to reduce the total energy density. Our results provide a straightforward solution for the realization of a single or a few Ge nanowires for potential device applications.

We studied plasmonlike resonances in one-dimensional (1D) atomic chain systems by using time-dependent density-functional theory (TDDFT) and local density functional theory. Recent TDDFT studies have shown the coexistence of longitudinal and transverses collective plasmonlike resonances in the atomic chains of simple and noble metals. Such atomic chains contain only a few atoms. The induced polarization occurs along the atomic chain in longitudinal mode and perpendicular to the atomic chain in transverse mode. To understand the emergence of plasmonlike resonance in 1D atomic chains better, we studied carbon chains in which plasmonic resonances are not expected to occur. We used TDDFT to study the emergence of collective resonances in various forms of carbon chains, cumulenes ${\mathrm{C}}_n{\mathrm{H}}_4$, polyynes ${\mathrm{C}}_n{\mathrm{H}}_2$, and alkenes ${\mathrm{C}}_n{\mathrm{H}}_{n+2}$. The excitation energy and dipole oscillation strengths of these systems were determined through TDDFT by using the turbomole package. We determined how collective plasmonlike resonances arise from single-electron excitations when the number of electrons increases as the carbon chain lengthens. The collective excitation behavior is then compared with that of metallic atomic chains. Our TDDFT results showed longitudinal collective modes for cumulenes and polyynes, as well as for finite-length chains. These collective excitations exhibit the same behavior as that of longitudinal “plasmon” previously identified in sodium and silver chains, although polyynes are gapped in the long chain limit. Such longitudinal excitations are absent in alkenes. However, unlike metal atomic chains, carbon chains exhibited no transverse collective mode. The band structure of periodic atomic chains was calculated by using the standard local density functional method. These structures were used to interpret the results and to relate the single-electron excitation to the collective plasmonlike response. Within the one-particle quantum-well picture, the longitudinal mode in the linear atomic chain arises from intraband transition with $\Delta q=1$, where $q$ is the quantum number of quantum wells. $\Delta q=1$ intraband transitions can be found in metallic (e.g., Na) chains and in carbon chains (cumulenes and polyynes), such longitudinal collective mode is rather “generic”. Meanwhile, the transverse modes of the sodium chains are attributed to interband transitions with an even $\Delta q$ (dominated by $\Delta q=0$), and such transverse collective excitations only form if the allowed $\Delta q=0$ transitions occur between bands that are parallel to each other. Such bands can be found in simple metals, but not in carbon chains.

We propose an all-superconducting three-terminal setup consisting in a carbon nanotube (or semiconducting nanowire) contacted to three superconducting leads. The resulting device, referred to as a “biSQUID,” is made of four quantum dots arranged in two loops of different surface area. We show how this biSQUID can prove a useful tool to probe nonlocal quantum phenomena in an interferometry setup. We study the measured critical current as a function of the applied magnetic field, which shows peaks in its Fourier spectrum, providing clear signatures of multipair Josephson processes. The device does not require any specific fine-tuning as these features are observed for a wide range of microscopic parameters—albeit with a nontrivial dependence. Competing effects which may play a significant role in actual experimental realizations are also explored.

Nonresonant Raman spectroscopy in the hard x-ray regime has been used to explore the electronic structure of the first two members of the Ruddlesden-Popper series ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}$ of iridates. By tuning the photon energy transfer around 530 eV, we have been able to explore the oxygen $K$-near-edge structure with bulk sensitivity. The angular dependence of the spectra has been exploited to assign features in the 528–535 eV energy range to specific transitions involving the Ir $5d$ orbitals. This has allowed us to extract reliable values for both the $t_{2g}$–$e_g$ splitting arising from the cubic component of the crystal field $\left(10Dq\right)$, in addition to the splitting of the $e_g$ orbitals due to tetragonal distortions. The values we obtain are (3.8, 1.6) eV and (3.55, 1.9) eV for ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, respectively.

Single-phase films of the full-Heusler compound ${\mathrm{Fe}}_2\mathrm{TiSi}$ have been prepared by magnetron sputtering. The compound is found to be a semiconductor with a gap of 0.4 eV. The electrical resistivity has a logarithmic temperature dependence up to room temperature due to Kondo scattering of a dilute free electron gas off superparamagnetic impurities. The origin of the electron gas is extrinsic due to residual off-stoichiometry. Density functional theory calculations of the electronic structure are in excellent agreement with electron energy loss, optical, and x-ray absorption experiments. ${\mathrm{Fe}}_2\mathrm{TiSi}$ may find applications as a thermoelectric material.

We probe the near Fermi-level electronic structure of tunable topological-insulator $\left({\mathrm{Bi}}_2{\mathrm{Se}}_3\right)$/cuprate-superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ $(T_{\mathrm{c}}\simeq 91\mathrm{K})$ heterostructures in their proximity-induced superconductivity regime. Our careful momentum space imaging provides clear evidence for a two-phase coexistence and a striking lack of any strong $d$-wave proximity effect expected in this system. Our Fermi surface imaging data identify key contributors in reducing the proximity-induced gap below the 5 meV or to a lower energy range $(\ll {\Delta }_{\mathrm{BSCCO}})$. These results correlate with our observation of momentum space separation between the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ Fermi surface topologies and mismatch of lattice symmetries in addition to the presence of a small coherence length. These studies not only provide critical momentum space insights into the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ heterostructures, but also set an upper bound on the proximity-induced gap for realizing a much sought out Majorana fermion condition in this system.

We derive the shape of the high-energy features due to a weakly coupled boson in cuprate superconductors, as seen experimentally in ${\text{Bi}}_2{\text{Sr}}_2{\text{Ca}}_1{\text{Cu}}_2{\text{O}}_{8+x}$ (BSCCO) by Lee et al. [Nature (London) 442, 546 (2006)]. A simplified model is used of $d$-wave Bogoliubov quasiparticles coupled to Einstein oscillators with a momentum-independent electron-boson coupling and an analytic fitting form is derived, which allows us (a) to extract the boson mode's frequency and (b) to estimate the electron-boson coupling strength. We further calculate the maximum possible superconducting gap due to an Einstein oscillator with the extracted electron-boson coupling strength, which is found to be less than 0.2 times of the observed gap indicating at the observed boson's nondominant role in the superconductivity's mechanism. The extracted momentum-independent electron-boson coupling parameter (that we show a posteriori to indeed be in the weak-coupling regime) is then to be interpreted as an (band-structure detail dependent weighted) average over the Brillouin zone of the actual momentum-dependent electron-boson coupling in BSCCO.

We report on hot electron induced impact ionization and large room-temperature magnetoresistance (MR) in micron-sized channels of $n$-type high-mobility InAs $(\mu =3.3{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at $T=300\mathrm{K})$: the MR reaches values of up to 450% in magnetic fields of 1 T and applied voltages of ∼1 V and is weakly dependent on temperature. We present Monte Carlo simulations of the hot electron dynamics to account for the large MR and its dependence on the sample geometry and applied electric and magnetic fields. Our work demonstrates that the impact ionization of electrons at room temperature, under small applied magnetic fields (<1 T) and small voltages (<1 V), can provide an extremely sensitive mechanism for controlling the electrical resistance of high-mobility semiconductors.

We show that charge doping can induce transitions between three distinct adsorbate phases in hydrogenated and fluorinated graphene. By combining ab initio, approximate density functional theory and tight-binding calculations we identify a transition from islands of ${\mathrm{C}}_8{\mathrm{H}}_2$ and ${\mathrm{C}}_8{\mathrm{F}}_2$ to random adsorbate distributions around a doping level of $\pm 0.05$ $e$/C atom. Furthermore, in situations with random adsorbate coverage, charge doping is shown to trigger a sublattice ordering transition when the doping level exceeds the adsorbate concentration. Rehybridization and lattice distortion energies make graphene, which is covalently functionalized from one side only, most susceptible to these two kinds of phase transitions. The energy gains associated with the clustering and ordering transitions can exceed room temperature thermal energies.

Using surface x-ray diffraction in combination with ab initio calculations we have studied the atomic structure of ultra-thin ZnO films deposited on Fe(110). In contrast to expectation that ZnO adopts the “graphitic” hexagonal Boron-nitride structure to the Wurtzite (WZ) structure is observed. Its formation is related to oxygen impurities in Fe(110) hollow sites inducing an anisotropic charge redistribution within the film which is characterized by a metallic surface. Our results provide a deeper understanding of depolarization mechanisms in ultrathin polar films at the atomic scale.

We present a density functional theory study of the thermodynamic and electronic properties of phosphorene nanoribbons. We consider a variety of terminations and reconstructions of ribbon edges, both with and without hydrogen passivation, and calculate an ab intio phase diagram that identifies energetically preferred edges as a function of temperature and hydrogen partial pressure. These studies are also accompanied by detailed electronic structure calculations from which we find that ribbons with hydrogenated edges are typically direct gap semiconductors with fundamental gaps that are in excess of phosphorene, the gaps varying inversely with ribbon width. In contrast, ribbons with bare or partially passivated edges either have metallic edges or are semiconducting with band gaps that are smaller than those of their hydrogenated counterparts due to the appearance of midgap edge states. Overall, our studies provide a basis for tailoring the electronic properties of phosphorene nanoribbons by controlling the edge termination via processing conditions (temperature and hydrogen partial pressure) as well as by confinement of carriers via control over ribbon width.

We employ a combination of pulsed- and continuous-wave polarized terahertz spectroscopy techniques to probe temperature-dependent spin waves in the antiferromagnet $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_3$. Our optical data span 1.6–467 K and reveal a conspicuous spin reorientation between 110 and 170 K, during which the lower-energy mode softens completely. Complementary inelastic neutron scattering reveals that the frequencies of the optically excited spin waves are consistent with a temperature-variable spin gap in the low-energy spin-wave dispersion of $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_3$. The result links the temperature dependence of the spin waves to a dynamic in-plane anisotropy. The magnetic anisotropy is calculated based on the results of the optical measurements. The change observed in the anisotropy energy along the $a$ and $c$ crystal axes suggests that the spin reorientation evident in $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_3$ is driven by temperature-dependent in-plane anisotropy.