The phase diagram of a half-filled hard-core boson two-leg ladder in a flux is investigated by means of numerical simulations based on the density matrix renormalization group (DMRG) algorithm and bosonization. We calculate experimentally accessible observables such as the momentum distribution, as well as rung current, density wave, and bond-order wave correlation functions, allowing us to identify the Mott Meissner and Mott vortex states. We follow the transition from commensurate Meissner to incommensurate vortex state at increasing interchain hopping until the critical value [Piraud et al. Phys. Rev. B 91, 140406 (2015)] above which the Meissner state is stable at any flux. For flux close to $\pi$, and below the critical hopping, we observe the formation of a second incommensuration in the Mott vortex state that could be detectable in current experiments.

We show that confinement in small volumes affects the interplay of exchange and dipolar interactions and the magnetic phases of hard and soft spherical core-shell nanoparticles. Large variations in the magnetization of thin shells may occur due to the core dipolar field gradient within the shell. The reversal field is tunable by the trends imposed by the dipolar and core-shell interface exchange energies. We show, for instance, that the reversal field of a ${\mathrm{CoFe}}_2{\mathrm{O}}_4$(30 nm)@${\mathrm{MnFe}}_2{\mathrm{O}}_4$(6 nm) particle ranges from 15.5 kOe for antiferromagnetic coupling down to 2.5 kOe for ferromagnetic coupling.

The dynamic behavior of magnetic antivortices stabilized in patterned pound-key-like microstructures was studied using microfocus Brillouin light scattering (micro-BLS) at frequencies above the gyrotropic mode $\left(>1\mathrm{GHz}\right)$. Micro-BLS spectra obtained as a function of the frequency of a driving microwave field show an intricate spectrum for the antivortex state for an in-plane driving field. Spatial mode profiles for the strongest antivortex resonance frequencies, obtained for samples in the antivortex as well as the single domain states, show that while the symmetry of one of the observed resonances is relatively insensitive to the spin configuration, the antivortex exhibits a unique mode profile for the other. A comparison with micromagnetic simulations shows that the frequency and symmetry of the latter are consistent with one of the antivortex azimuthal modes. Furthermore, the simulations show that this mode involves coupling between the antivortex spin excitations and propagating spin waves in the structure legs, which may be useful for high-wave-number spin-wave generation.

In this paper, we study a topological phase transition in a wire medium operating at infrared frequencies. This transition occurs in the reciprocal space between the indefinite (open-surface) regime of the metamaterial and its dielectric (closed-surface) regime. Due to the spatial dispersion inherent to a wire medium, a hybrid regime turns out to be possible at the transition frequency. Both such surfaces exist at the same frequency and touch one another. At this frequency, all values of the parallel wave vector correspond to propagating spatial harmonics. The implication of this regime is the overwhelming radiation enhancement. We numerically investigate the gain in radiated power for a subwavelength dipole source submerged into such medium. In contrast to previous works, this gain (called the Purcell factor) turns out to be higher for a parallel dipole than for a perpendicular one.

The zero-temperature phase diagram of a $p$-orbital two-component fermionic system loaded into a one-dimensional optical lattice is mapped out by means of analytical and numerical techniques. It is shown that the $p$-band model away from half-filling hosts various competing superconducting phases for attractive and repulsive interactions. At quarter-filling, we analyze the possible formation of incompressible Mott phases and in particular for repulsive interactions, we find the occurrence of a Mott transition with the formation of fully gapped bond-ordering waves.

We consider hard-core bosons on the kagome lattice in the presence of short-range repulsive interactions and focus particularly on the filling factor $1/3$. In the strongly interacting limit, the low-energy excitations can be described by the quantum fully packed loop coverings on the triangular lattice. Using a combination of tensor product state based methods and exact diagonalization techniques, we show that the system has an extended ${\mathbb{Z}}_2$ topological liquid phase as well as a latti ce nematic phase. The latter breaks lattice rotational symmetry. By tuning appropriate parameters in the model, we study the quantum phase transition between the topological and the symmetry broken phases. We construct the critical theory for this transition using a mapping to an Ising gauge theory that predicts the transition to belong to the $O\left(3\right)$ universality class.

The adsorption of Ni, Co, Mn, Ti, and Zr at the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$-reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a $\left(1\times 1\right)$ LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite $\left({\mathrm{XFe}}_2{\mathrm{O}}_4\right)$ compounds, similar behavior is likely to be typical for elements forming a solid solution with ${\mathrm{Fe}}_3{\mathrm{O}}_4$.

A general expression for the thermodynamic potential of the model of semi-infinite jellium is obtained. By using this expression, the surface energy for the infinite barrier model is calculated. The behavior of the surface energy and of the chemical potential as functions of the Wigner-Seitz radius and the influence of the Coulomb interaction between electrons on the calculated values is studied. It is shown that taking into account the Coulomb interaction between electrons leads to growth of the surface energy. The surface energy is positive in the entire area of the Wigner-Seitz radius. It is shown that taking into account the Coulomb interaction between electrons leads to a decrease of the chemical potential.

We present a framework for obtaining reliable solid-state charge and optical excitations and spectra from optimally tuned range-separated hybrid density functional theory. The approach, which is fully couched within the formal framework of generalized Kohn-Sham theory, allows for the accurate prediction of exciton binding energies. We demonstrate our approach through first principles calculations of one- and two-particle excitations in pentacene, a molecular semiconducting crystal, where our work is in excellent agreement with experiments and prior computations. We further show that with one adjustable parameter, set to produce the known band gap, this method accurately predicts band structures and optical spectra of silicon and lithium fluoride, prototypical covalent and ionic solids. Our findings indicate that for a broad range of extended bulk systems, this method may provide a computationally inexpensive alternative to many-body perturbation theory, opening the door to studies of materials of increasing size and complexity.

We investigate the phase diagram of spinless fermions with nearest- and next-nearest-neighbor density-density interactions on the honeycomb lattice at half-filling. Using exact diagonalization techniques of the full Hamiltonian and constrained subspaces, combined with a careful choice of finite-size clusters, we determine the different charge orderings that occur for large interactions. In this regime, we find a two-sublattice Néel-like state, a charge modulated state with a tripling of the unit cell, a zigzag phase, and a charge ordered state with a 12-site unit cell we call Néel domain wall crystal, as well as a region of phase separation for attractive interactions. A sizable region of the phase diagram is classically degenerate, but it remains unclear whether an order-by-disorder mechanism will lift the degeneracy. For intermediate repulsion, we find evidence for a Kekulé or plaquette bond-order wave phase. We also investigate the possibility of a spontaneous Chern insulator phase (dubbed topological Mott insulator), as previously put forward by several mean-field studies. Although we are unable to detect convincing evidence for this phase based on energy spectra and order parameters, we find an enhancement of current-current correlations with the expected spatial structure compared to the noninteracting situation. While for the studied $t-V_1-V_2$ model, the phase transition to the putative topological Mott insulator is preempted by the phase transitions to the various ordered states, our findings might hint at the possibility for a topological Mott insulator in an enlarged Hamiltonian parameter space, where the competing phases are suppressed.

The emergence of the Haldane Chern insulator state due to strong short-range repulsive interactions in the half-filled fermionic spinless honeycomb lattice model has been proposed and challenged with different methods and yet it still remains controversial. In this work we revisit the problem using the infinite density matrix renormalization group method and report numerical evidence supporting (i) the absence of the Chern insulator state, (ii) two previously unnoticed charge ordered phases, and (iii) the existence and stability of all the nontopological competing orders that were found previously within mean field. In addition, we discuss the nature of the corresponding phase transitions based on our numerical data. Our work establishes the phase diagram of the half-filled honeycomb lattice model, tilting the balance towards the absence of a Chern insulator phase for this model.

Measurements of the electrical resistivity, from 1.5 to 300 K, and of the low temperature magnetoresistance of highly conductive carbon nanotube (CNT) fibers, obtained by wet-spinning from liquid crystalline phase (LCP), are reported. At high temperature the results obtained on the raw CNT fibers show a typical metallic behavior and the resistivity levels without postdoping process were found to be only one order of magnitude higher than the best electrical conductors, with the specific conductivity (conductivity per unit weight) comparable to that of pure copper. At low temperature a logarithmic dependence of the resistivity and the temperature dependence of the negative magnetoresistance are consistent with a two-dimensional quantum charge transport—weak localization and Coulomb interaction—in the few-walled CNT fibers. The temperature dependence of the phase-breaking scattering rate has also been determined from magnetoresistance measurements. In the temperature range $T<100\mathrm{K}$, electron-electron scattering is found to be the dominant source of dephasing in these highly conductive CNT fibers. While quantum effects demonstrate the two-dimensional aspect of conduction in the fibers, the fact that it was found that their resistance is mainly determined by the intrinsic resistivity of the CNTs—and not by intertube resistances—suggests that better practical conductors could be obtained by improving the quality of the CNTs and the fiber morphology.

Using x-ray photoelectron spectroscopy, thermal desorption spectroscopy, and scanning tunneling microscopy, we show that upon keV ${\mathrm{Xe}}^{+}$ irradiation of graphene on Ir(111), Xe atoms are trapped under the graphene. Upon annealing, aggregation of Xe leads to graphene bulges and blisters. The efficient trapping is an unexpected and remarkable phenomenon given the absence of chemical binding of Xe to Ir and to graphene, the weak interaction of a perfect graphene layer with Ir(111), as well as the substantial damage to graphene due to irradiation. By combining molecular dynamics simulations and density functional theory calculations with our experiments, we uncover the mechanism of trapping. We describe ways to avoid blister formation during graphene growth, and also demonstrate how ion implantation can be used to intentionally create blisters without introducing damage to the graphene layer. Our approach may provide a pathway to synthesize new materials at a substrate—2D material interface or to enable confined reactions at high pressures and temperatures.

Inelastic x-ray scattering measurements on liquid Bi were carried out. Prominent acoustic mode excitations were observed in the dynamic structure factor to beyond $12{\mathrm{nm}}^{-1}$, which resolves previously conflicting results as to their presence beyond $6{\mathrm{nm}}^{-1}$. We find the dispersion curve of the excitation energy with momentum transfer is nearly flat from 7 to $15{\mathrm{nm}}^{-1}$ consistent with ab initio calculations of liquid Bi [J. Souto et al., Phys. Rev. B 81, 134201 (2010)]. Our ab initio and classical molecular dynamics simulations suggest that a long-range force is needed to reproduce the flatness of the dispersion curve, and the long-range force is correlated with a local structure consisting of shorter and longer bonds in the liquid.

The structural and chemical order are the most important parameters governing the physical properties of the Heusler compounds. Here, we give a comprehensive overview of the correlations between structural and chemical order, electronic transport (longitudinal and transverse) and magnetic (static and dynamic) properties of ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ Heusler alloy epitaxial thin films grown on MgO(001) single-crystal substrates. X-ray diffraction measurements indicated that depending on the annealing temperature the films show $B2$ or $L{2}_1$ chemical ordering. Longitudinal magnetoresistivity experiments revealed that for the best $L{2}_1$ ordered film, at temperatures bellow $125\text{K}$, the magnon assisted electronic scattering is quenched indicating the appearance of half-metallicity. The presence of quantum correction in resistivity, whose strength is dependent on the structural ordering, was evidenced at low temperatures. Anomalous Hall experiments indicated that the intrinsic band structure contribution has an opposite sign and that is dominant over the extrinsic skew scattering mechanism. The presence of a small uniaxial magnetic anisotropy contribution superimposed on a larger biaxial one was evidenced via ferromagnetic resonance microstrip line measurements. The biaxial term is well correlated with chemical ordering, having a minimum value for the optimum $L{2}_1$ ordered film. The damping parameter was evaluated from ferromagnetic resonance linewidth measurements, and a coefficient as low as $1.9\times {10}^{-3}$ was found for the $L{2}_1$ phase.

Magnetization dynamics of a bilayer structure in the presence of a spin-transfer torque is studied using an atomistic model coupled with a model of spin accumulation. The spin-transfer torque is decomposed into two components: adiabatic and nonadiabatic torques, expressed in terms of the spin accumulation, which is introduced into the atomistic model as an additional field. The evolution of the magnetization and the spin accumulation are calculated self-consistently. We introduce a spin-polarized current into a material containing a domain wall whose width is varied by changing the anisotropy constant. It is found that the adiabatic spin torque tends to develop in the direction of the magnetization whereas the nonadiabatic spin torque arising from the mistracking of conduction electrons and local magnetization results in out-of-plane magnetization components. However, the adiabatic spin torque significantly dominates the dynamics of the magnetization. The total spin-transfer torque acting on the magnetization increases with the anisotropy constant due to the increasing magnetization gradient.

The effect of fluctuations on the nuclear magnetic resonance (NMR) relaxation rate $W=T_1^{-1}$ is studied in a complete phase diagram of a two-dimensional superconductor above the upper critical field line $H_{\mathrm{c}2}\left(T\right)$. In the region of relatively high temperatures and low magnetic fields, the relaxation rate $W$ is determined by two competing effects. The first one is its decrease in the result of suppression of the quasiparticle density of states (DOS) due to formation of fluctuation Cooper pairs (FCPs). The second one is a specific, purely quantum relaxation process of the Maki-Thompson (MT) type, which for low field leads to an increase of the relaxation rate. The latter describes particular fluctuation processes involving self-pairing of a single electron on self-intersecting trajectories of a size up to phase-breaking length ${\ell }_{\varphi }$ which becomes possible due to an electron spin-flip scattering event at a nucleus. As a result, different scenarios with either growth or decrease of the NMR relaxation rate are possible upon approaching the normal-metal–type-II superconductor transition. The character of fluctuations changes along the line $H_{\mathrm{c}2}\left(T\right)$ from the thermal long-wavelength type in weak magnetic fields to the clusters of rotating FCPs in fields comparable to $H_{\mathrm{c}2}\left(0\right)$. We find that below the well-defined temperature $T_0^{*}\approx 0.6T_{\mathrm{c}0}$, the MT process becomes ineffective even in the absence of intrinsic pair breaking. The small scale of the FCP rotations ${\xi }_{\mathrm{xy}}$ in such high fields impedes formation of long $(\lesssim {\ell }_{\varphi })$ self-intersecting trajectories, causing the corresponding relaxation mechanism to lose its efficiency. This reduces the effect of superconducting fluctuations in the domain of high fields and low temperatures to just the suppression of quasiparticle DOS, analogous to the Abrikosov vortex phase below the $H_{\mathrm{c}2}\left(T\right)$ line.

We model a superconducting $p-n$ junction in which the $n$ and the $p$ sides are contacted through two optical quantum dots (QDs), each embedded into a photonic nanocavity. Whenever a Cooper pair is transferred from the $n$ side to the $p$ side, two photons are emitted. When the two electrons of a Cooper pair are transported through different QDs, polarization-entangled photons are created, provided that the Cooper pairs retain their spin singlet character while being spatially separated on the two QDs. We show that a Clauser-Holt-Shimony-Horne (CHSH) Bell-type measurement is able to detect the entanglement of the photons over a broad range of microscopic parameters, even in the presence of parasitic processes and imperfections.

The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce fingerlike patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the nonadiabatic contribution of the spin transfer torque mechanism.

We use inelastic neutron scattering to measure magnetic fluctuations as a function of transverse magnetic field and temperature in a single crystal of the coupled Haldane chain compound $\mathrm{SrNi}{}_2{\text{V}}_2{\text{O}}_8$ with uniaxial anisotropy. At the base temperature (2 K), spin excitations confirm a field-induced quantum phase transition from the disordered Haldane (gapped singlet) state to a gapped long-range ordered state through a quantum critical point (QCP) (${\mu }_0{H}_c\approx 11.5$ T). At elevated temperatures in the vicinity of the QCP, both the energy and linewidth of the lowest triplet mode scale linearly with temperature, in accordance with quantum criticality. We also demonstrate that the experimental field dependence of the triplet excitation modes at the base temperature is in agreement with the bosonic and macroscopic models, but is in contrast to the fermionic and perturbative models.