Direct dynamic nuclear polarization of ${}^1$H nuclei in frozen water and water-ethanol mixtures is demonstrated using silicon nanoparticles as the polarizing agent. Electron spins at dangling-bond sites near the silicon surface are identified as the source of the nuclear hyperpolarization. This polarization method opens avenues for the fabrication of surface engineered nanostructures to create high nuclear spin polarized solutions without introducing contaminating radicals, and for the study of molecules adsorbed onto surfaces.

Charge order is a commonly observed phenomenon in strongly correlated materials. However, most theories are based on a repulsive intersite Coulomb interaction in order to explain charge order. Here we show that only due to local interactions, charge order is favorable in heavy fermion systems at quarter filling. The driving force is the Kondo effect, which leads to a nonlinear dependence on the filling of the lattice site for the energy gain. We use dynamical mean field theory combined with numerical renormalization group to demonstrate the existence of charge order at quarter filling for a Kondo lattice model.

An equation-of-motion based theory for the description of light emission from multilevel semiconductor quantum dots (QDs) is presented. It accounts for electronic excitations in the presence of Coulomb interaction, leading to multiexciton states, and their coupling to the quantized electromagnetic field. The two key aspects of this work concern (i) the combination of an exact treatment of the electronic degrees of freedom with an approximate approach for the photonic degrees of freedom that is based on the cluster expansion technique and (ii) the consistent incorporation of scattering and dephasing due to the coupling to delocalized electronic states and phonons into the equations of motion via Lindblad terms. Differences to previously used theories are discussed and results of the theory are shown for free-space emission, where multiexciton spectra are shown, and for emission into a single high-$Q$ cavity mode. In the latter case, a full solution of the von-Neumann equation is used to benchmark the proposed theory, which we term “finite-size hierarchy” (FSH) method.

The energetic stability, electronic, and structural properties of graphene adsorbed on the amorphous HfO${}_2$ surface (G/HfO${}_2$) have been examined through ab initio theoretical investigations. We have considered the graphene adsorption on (i) defect-free (pristine) and defective HfO${}_2$ surfaces, (ii) oxygen vacancy, and (iii) interstitial oxygen atoms. We find that the formation of G/HfO${}_2$ is an exothermic process, ruled by van der Waals interactions. In (i) and (iii) there is no net charge transfer between the graphene and the HfO${}_2$ surface. In contrast, upon the presence of oxygen vacancy, the adsorbed graphene sheet becomes $n$-type doped, due to a donor level lying above the Dirac point of the graphene. The absence of G–HfO${}_2$ chemical bonds has been maintained, however, the graphene adsorption energy increases when compared with (i) and (iii). Finally, in (ii) we find that HfO${}_2$ surface potential becomes more inhomogeneous, strengthening the formation of electron- and hole-rich regions on the graphene sheet.

We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)${}_2$As${}_2$ and Ba(Fe,TM)${}_2$As${}_2$ with transition metal TM = Ni or Cu. The valence band photoemission data show directly that the TM states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the $3d^8$ final state of the $LVV$ Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is rather robust against impurity scattering.

We show that in the regime when strong disorder is more relevant than field quantization, the superfluid-to-Bose-glass criticality of one-dimensional bosons is preceded by the prolonged logarithmically slow classical-field renormalization flow of the superfluid stiffness at mesoscopic scales. With the system compressibility remaining constant, the quantum nature of the system manifests itself only in the renormalization of dilute weak links. On the insulating side, the flow ultimately reaches a value of the Luttinger parameter at which the instanton–anti-instanton pairs start to proliferate, in accordance with the universal quantum scenario. This happens first at astronomical system sizes because of the suppressed instanton fugacity. We illustrate our result by first-principles simulations.

BaMn${}_2$As${}_2$ is a local-moment antiferromagnetic insulator with a Néel temperature $T_{\mathrm{N}}$ of 625 K and a large ordered moment of 3.9 ${\mu }_{\mathrm{B}}$/Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6$\%$ K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ remains robust up to $x=0.4$. The ordered moment is nearly independent of $x$ for 0 $\le x\le$ 0.4 and $T_{\mathrm{N}}$ decreases to 480 K at $x=0.4$.

We theoretically study the magnetic excitations in a frustrated two-leg spin-ladder system, in which antiferromagnetic exchange interactions act on the nearest-neighbor and next-nearest-neighbor bonds in the leg direction, and on the nearest-neighbor bonds in the rung direction. A dynamical spin correlation function at zero temperature is calculated using the dynamical density-matrix renormalization-group method for possible magnetic phases, i.e., columnar-dimer and rung-singlet phases. The columnar-dimer phase is characterized by multispinon excitations with a spin gap, whereas the rung-singlet phase is dominated by the triplet excitation in the rung direction. One major difference found between these two phases appears in the spectral weight of magnetic excitations, in particular, the bonding and antibonding modes in the rung direction. Therefore, we can distinguish one phase from the other by the difference in the spectral weight. Furthermore, we examine the effect of frustration on both modes in the rung-singlet phase with a perturbation theory from the strong coupling limit. The antibonding mode is shown to be stable against frustration, and a wave number with minimum excitation energy is shifted from being commensurate to being incommensurate. In contrast, the bonding mode is merged into the continuum excitation of multiple triplet excitations by increasing frustration. By comparing our results with inelastic neutron scattering experiments for BiCu${}_2$PO${}_6$, the magnitude of the magnetic exchange interactions and the ground state will be determined.

We propose a new approach to identifying and rationalizing the contribution of core electron polarization to dielectric screening, based on ab initio calculations of the dielectric matrix in its eigenpotential basis. We also present calculations of phonon frequencies, dielectric constants, and quasiparticle energies of several systems, and we discuss the quantitative effect of including core polarization. Our findings illustrate efficient ways of approximating the spectral decomposition of dielectric matrices used, e.g., in many-body perturbation theory and dielectric constant calculations, with substantial computational gains for large systems composed of heavy atoms.

Here we report on interface reconstruction in the recently discovered superconducting artificial superlattices based on insulating ${\mathrm{CaCuO}}_2$ and ${\mathrm{SrTiO}}_3$ blocks. Hard x-ray photoelectron spectroscopy shows that the valence-band alignment prevents any electronic reconstruction by direct charge transfer between the two blocks. We demonstrate that the electrostatic built-in potential is suppressed by oxygen redistribution in the alkaline-earth-metal interface planes. By using highly oxidizing growth conditions, the oxygen coordination in the reconstructed interfaces may be increased, resulting in the hole doping of the cuprate block and thus in the appearance of superconductivity.

A practical numerical method for the rapid construction of nanotube caps is proposed. Founded upon the notion of lattice duality, the algorithm considers the face dual representation of a given nanotube which is used to solve an energy minimization problem analogous to The Thomson Problem. Not only does this produce caps for nanotubes of arbitrary chirality, but the caps generated will be physically sensible and in most cases the lowest energy structure. To demonstrate the applicability of the technique, caps of the (5,5) and the (10,0) nanotubes are investigated by means of density-functional tight binding (DFTB). The calculation of cap energies highlights the ability of the algorithm to produce lowest energy caps. Due to the preferential construction of spherical caps, the technique is particularly well suited for the construction of capped multiwall nanotubes (MWNTs). To validate this proposal and the overall robustness of the algorithm, a MWNT is constructed containing the chiralities (9,2)@(15,6)@(16,16). The algorithm presented paves the way for future computational investigations into the physics and chemistry of capped nanotubes.

The low-temperature electrical conductance through correlated quantum dots provides a sensitive probe of the physics (e.g., of Fermi-liquid versus non-Fermi-liquid behavior) of such systems. Here, we investigate the role of level asymmetry (gate voltage) and local Coulomb repulsion (charging energy) on the low-temperature and low-field scaling properties of the linear conductance of a quantum dot described by the single-level Anderson impurity model. We use the numerical renormalization group to quantify the regime of gate voltages and charging energies where universal Kondo scaling may be observed and also quantify the deviations from this universal behavior with increasing gate voltage away from the Kondo regime and with decreasing charging energy. We also compare our results with those from a recently developed method for linear and nonlinear transport, which is based on renormalized perturbation theory using dual fermions, finding excellent agreement at particle-hole symmetry and for all charging energies and reasonable agreement at small finite level asymmetry. Our results could be a useful guide for detailed experiments on conductance scaling in semiconductor and molecular quantum dots exhibiting the Kondo effect.

We demonstrate that the combination of an external magnetic field and the intrinsic spin-orbit interaction results in nonadiabatic precession of the electron spin after transmission through a quantum point contact (QPC). We suggest that this precession may be observed in a device consisting of two QPCs situated in series. The pattern of resonant peaks in the transmission is strongly influenced by the non-Abelian phase resulting from this precession. Moreover, an unusual type of resonance which is associated with suppressed, rather than enhanced, transmission (i.e., antiresonance) emerges in the strongly nonadiabatic regime. The shift in the resonant transmission peaks is dependent on the spin-orbit interaction and therefore offers a way to directly measure these interactions in a ballistic one-dimensional system.

Bimetallic nanoparticles (often known as nanoalloys) with core-shell arrangement are of special interest in several applications, such as in optics, catalysis, magnetism, and biomedicine. Despite wide interest in applications, the physical factors stabilizing the structures of these nanoparticles are still unclear to a great extent, especially for what concerns the relationship between geometric structure and chemical ordering patterns. Here global-optimization searches are performed in order to single out the most stable chemical ordering patterns corresponding to the most important geometric structures, for a series of weakly miscible systems, including AgCu, AgNi, AgCo, and AuCo. The calculations show that (i) the overall geometric structure of the nanoalloy and the shape and placement of its inner core are strictly correlated; (ii) centered cores can be obtained in icosahedral nanoparticles but not in crystalline or decahedral ones, in which asymmetric quasi-Janus morphologies form; (iii) in icosahedral nanoparticles, when the core exceeds a critical size, a new type of morphological instability develops, making the core asymmetric and extending it towards the nanoparticle surface; (iv) multicenter patterns can be obtained in polyicosahedral nanoalloys. Analogies and differences between the instability of the core in icosahedral nanoalloys and the Stranski-Krastanov instability occurring in thin-film growth are discussed. All these issues are crucial for designing strategies to achieve effective coatings of the cores.

Defects on insulating surfaces have been studied much less compared to conductors due to experimental difficulties. Here we report quantitative structure analysis of atomic steps on the MgO(100) surface combining aberration-corrected high resolution transmission electron microscopy and density function theory calculations. While the broad faces show little relaxation or rumpling, the atoms at the step sites have significant displacements, depending on the atomic coordination. The general trend is to smooth the steps and lower their formation energy. The angles at the upper and lower corners of the mono-atomic step increase from unrelaxed 90° to 94° and 101°, respectively. The experimentally measured positions of the atoms at the step sites match the density function theory results within several picometers.

The paper reassesses the old but still controversial problem of the transverse force on a vortex and the vortex mass. The transverse force from free bulk quasiparticles on the vortex, both in the Bose and the Fermi liquids, originates from the Aharonov-Bohm effect. However, in the Fermi liquid, one should take into account peculiarities of the Aharonov-Bohm effect for BCS quasiparticles described by two-component spinor wave functions. There is no connection between the transverse force (either from free bulk quasiparticles or from vortex-core bound quasiparticles) and the spectral flow in the vortex core in superfluid Fermi liquid, in contrast to widely known claims. In fact, there is no steady spectral flow in the core of the moving vortex, and the analogy with the Andreev bound states in the superconductor–normal-metal–superconductor junction, where the spectral flow is really possible, is not valid in this respect. The role of the backflow on the vortex mass is clarified. The backflow is an inevitable consequence of a mismatch between the currents inside and outside the vortex core and restores the conservation of the particle number (charge) violated by this mismatch. In the Fermi liquid, the backflow compensates the current through the core bound states, which is a source of the vortex mass (the Kopnin mass). This results in renormalization of the Kopnin vortex mass by a numerical factor.

We report detailed investigations of the properties of a superconductor obtained by substituting In at the Sn site in the topological crystalline insulator (TCI), SnTe. Transport, magnetization, and heat capacity measurements have been performed on crystals of Sn${}_{0.6}$In${}_{0.4}$Te, which is shown to be a bulk superconductor with $T_c^{\mathrm{onset}}$ at $\sim$$4.70\left(5\right)$ K and $T_c^{\mathrm{zero}}$ at $\sim$$3.50\left(5\right)$ K. The upper and lower critical fields are estimated to be ${\mu }_0{H}_{c2}\left(0\right)=1.42\left(3\right)$ T and ${\mu }_0{H}_{c1}\left(0\right)=0.90\left(3\right)$ mT, respectively, while $\kappa =56.4\left(8\right)$ indicates this material is a strongly type-II superconductor.

The thermally activated glide of screw dislocations in bcc crystals proceeds through the formation of kinks by pairs, a mechanism named after Peierls for his early work on dislocation theory. A method is proposed to compute the dislocation kink-pair formation enthalpy from density functional theory (DFT) calculations. This method consists of properly adjusting the parameters of a one-dimensional line tension model from atomistic calculations performed in small simulation cells. This model is applied to bcc iron to determine the kink-pair formation enthalpy at different applied stresses from DFT calculations.

$a$-Nb${}_x$Si${}_{1-x}$ thin films with thicknesses down to 25 Å have been structurally characterized by transmission electron microscopy measurements. As-deposited or annealed films are shown to be continuous and homogeneous in composition and thickness, up to an annealing temperature of 500 ${}^{\circ }\mathrm{C}$. We have carried out low-temperature transport measurements on these films close to the superconductor-to-insulator transition (SIT) and shown a qualitative difference between the effect of annealing or composition and a reduction of the film thickness on the superconducting properties of $a$-NbSi. These results question the pertinence of the sheet resistance $R_{\square }$ as the relevant parameter to describe the SIT.

We present a general scheme based on nonlinear response theory to calculate the expansion of correlation functions such as the pair-correlation function or the exchange-correlation hole of an inhomogeneous many-particle system in terms of density derivatives of arbitrary order. We further derive a consistency condition that is necessary for the existence of the gradient expansion. This condition is used to carry out an infinite summation of terms involving response functions up to infinite order from which it follows that the coefficient functions of the gradient expansion can be expressed in terms of the local density profile rather than the background density around which the expansion is carried out. We apply the method to the calculation of the gradient expansion of the one-particle density matrix to second order in the density gradients and recover in an alternative manner the result of Gross and Dreizler [Gross and Dreizler, Z. Phys. A 302, 103 (1981)], which was derived using the Kirzhnits method. The nonlinear response method is more general and avoids the turning point problem of the Kirzhnits expansion. We further give a description of the exchange hole in momentum space and confirm the wave vector analysis of Langreth and Perdew [Langreth and Perdew, Phys. Rev. B 21, 5469 (1980)] for this case. This is used to derive that the second-order gradient expansion of the system averaged exchange hole satisfies the hole sum rule and to calculate the gradient coefficient of the exchange energy without the need to regularize divergent integrals.