We report force mapping experiments on Si(111)-(7$\times$7) surfaces with adsorbed hydrogen, using atomic force microscopy at room temperature supported by density functional theory (DFT) simulations. On the basis of noncontact atomic force microscopy (NC-AFM) images as well as force versus distance curves measured over both hydrogen-passivated and bare Si adatoms, we identified two types of tip termination, which result in different modes of interaction with the surface. The statistics of the tip dependence of the measured forces, which are effectuated using various tip states with different cantilevers, reveal the typical values of the force and their distribution in the two characteristic interaction modes. The experimental results are corroborated by DFT calculations performed for different tip structures. As a reactive tip, the dimer-terminated Si tip yields results in satisfactory agreement with experimental force curves for hydrogen-passivated and nonpassivated Si adatom sites. An oxidized Si dimer tip that bears a hydroxyl group on its apex reproduces well the experimental force curves acquired by nonreactive tips. This tip model could thus be used to interpret the experimentally obtained weak image contrast for the Si(111)-(7$\times$7) surface. The forces are thought to arise as a result of a weak electrostatic interaction involving a permanent dipole at the tip apex enhanced by the charge density redistribution due to the interaction with surface adatoms.

Recent advances in nanofabrication techniques allow for the manufacture of optical metamaterials, bringing their unique and extraordinary properties to the visible regime and beyond. However, an analytical description of optical nanoplasmonic metamaterials is challenging due to the characteristic optical behavior of metals. Here we present an analytical theory that allows us to bring established microwave metamaterials models to optical wavelengths. This method is implemented for nanoscaled plasmonic wire-mesh and trihelical metamaterials, and we obtain an accurate prediction for their dispersive behavior at optical and near-IR wavelengths.

The double perovskite material Sr${}_2$FeMoO${}_6$ has the rare and desirable combination of a half-metallic ground state with 100$\%$ spin polarization and ferrimagnetic $T_{\mathrm{c}}\simeq 420$ K, well above room temperature. In this two-part paper, we present a comprehensive theoretical study of the magnetic and electronic properties of half-metallic double perovskites. In this paper we present exact diagonalization calculations of the “fast” Mo electronic degrees coupled to “slow” Fe core spin fluctuations treated by classical Monte Carlo techniques. From the temperature dependence of the spin-resolved density of states, we show that the electronic polarization at the chemical potential is proportional to magnetization as a function of temperature. We also consider the effects of disorder and show that excess Fe leaves the ground state half-metallic while antisite disorder greatly reduces the polarization. In the companion paper [Phys. Rev. B 87, 165105 (2013)] we will derive an effective classical spin Hamiltonian that provides a new framework for understanding the magnetic properties of half-metallic double perovskites including the effects of disorder. Our results on the dependence of the spin polarization on temperature and disorder have important implications for spintronics.

Double perovskites such as Sr${}_2$FeMoO${}_6$ are materials with half-metallic ground states and ferrimagnetic $T_c$'s well above room temperature. This paper is the second of our comprehensive theory for half-metallic double perovskites. Here we derive an effective Hamiltonian for the Fe core spins by “integrating out” the itinerant Mo electrons and obtain an unusual double-square-root form of the spin-spin interaction. We validate the classical spin Hamiltonian by comparing its results with those of the full quantum treatment presented in a companion paper [O. N. Meetei et al., Phys. Rev. B 87, 165104 (2013)]. We then use the effective Hamiltonian to compute magnetic properties as a function of temperature and disorder and discuss the effect of excess Mo, excess Fe, and antisite disorder on the magnetization and $T_c$. We conclude with a proposal to increase $T_c$ without sacrificing carrier polarization.

We consider the time evolution after sudden quenches of global parameters in translational invariant Hamiltonians and study the time average expectation values and entanglement entropies in finite chains. We show that in noninteracting models the time average of spin correlation functions is asymptotically equal to the infinite time limit in the infinite chain, which is known to be described by a generalized Gibbs ensemble. The equivalence breaks down considering nonlocal operators, and we establish that this can be traced back to the existence of conservation laws common to the Hamiltonian before and after the quench. We develop a method to compute the leading finite-size corrections. We find that the finite-size corrections are generally large in observables with large relaxation timescales.

We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with experimental results that can describe and predict the mechanisms for the formation of various types of nanostructures observed during droplet epitaxy. The crucial features of the model include the explicit and independent representation of atoms with different species and the ability to treat solid and liquid phases independently. Using this model, we examine nanostructural evolution in droplet epitaxy. The model faithfully captures several of the experimentally observed structures, including compact islands and nanorings. Moreover, simulations show the presence of Ga/GaAs core-shell structures that we validate experimentally. A fully analytical model of droplet epitaxy that explains the relationship between growth conditions and the resulting nanostructures is presented, yielding key insight into the mechanisms of droplet epitaxy.

The spin and orbital configuration of magnetic metal phthalocyanines (MPcs) deposited on metallic substrates are strongly influenced by the rehybridization of the molecular states with the underlying metal. FePc, CoPc, and CuPc isolated molecules are archetypal systems to investigate the interrelationship between magnetic moments and orbital symmetry after deposition on a metallic substrate. MPcs form long-range ordered chains self-assembled along the reconstructed channels of the Au(110) surface. X-ray magnetic circular dichroism from the $L_{2,3}$ absorption edges of Fe, Co, and Cu shows that the orbital and spin configuration are strongly modified upon adsorption on the Au(110) surface if the orbitals responsible of the magnetic moment are involved in the interaction process. The magnetic moment for a single layer of molecular chains is completely quenched for the CoPc molecules, fully preserved for the CuPc and reduced for the FePc ones. The modified magnetic configuration is confined to the very interface layer, i.e., to the MPc molecules bound to the metal substrate up to the compact packing of the single layer. The different response can be rationalized in terms of the symmetry/orientation of the metal-ion $d$ states interacting with the substrate states, as indicated by density functional theory calculations in agreement with experimental findings.

We investigated the H desorption process and the surface structure after H desorption in quasi-free-standing monolayer graphene (QFMLG) on silicon carbide (SiC). In situ scanning tunneling microscopy observations revealed that H adatoms preferentially desorb at SiC step edges. We found self-organization of a hexagonal network of QFMLG nanoribbons during H desorption. The surface structure on the nanoribbons indicates that electron scattering occurs at the boundary between the QFMLG and H-desorbed regions. The configuration of the network is determined by the formation energy of the boundary and by the surface stress energy.

The standard approach for path-integral Monte Carlo simulations of open quantum systems is extended as an efficient tool to monitor the time evolution of coherences (off-diagonal elements of the reduced density matrix) also for strong coupling to environments. Specific simulations are performed for two-level systems embedded in Ohmic and sub-Ohmic reservoirs in the domains of coherent and incoherent dynamics of the polarization. In the latter regime, the notorious difficulty to access the long-time regime is overcome by combining simulations on moderate time scales with iteratively calculated initial densities. This allows us to extract relaxation rates for sub-Ohmic environments at finite temperatures and over a broad range of couplings and to compare them to analytical predictions. The time evolution of the von Neumann entropy provides insight into the quantum phase transition at thermal equilibrium from a delocalized to a localized state at zero temperature.

We theoretically discover that unique eigenmodes of skyrmion crystal (SkX) are not only magnetically active to an ac magnetic field (${\mathit{H}}^{\omega }$) but also electrically active to an ac electric field (${\mathit{E}}^{\omega }$) in a multiferroic chiral-lattice magnet Cu${}_2$OSeO${}_3$, which amplifies the dynamical magnetoelectric coupling between ${\mathit{E}}^{\omega }$ and the spin texture. The resulting intense interference between their electric and their magnetic activation processes can lead to an unprecedentedly large diode effect on the microwave, i.e., its absorption by SkX changes up to $\sim$20$\%$ when the incident direction is reversed. Our results demonstrate that the skyrmion could be a promising building block for microwave devices.

We present measurements of the superconducting critical temperature $T_c$ and upper critical field $H_{c2}$ as a function of pressure in the transition metal dichalcogenide 2$H$-NbS${}_2$ up to 20 GPa. We observe that $T_c$ increases smoothly from 6 K at ambient pressure to about $8.9$ K at 20 GPa. This range of increase is comparable to the one found previously in 2$H$-NbSe${}_2$. The temperature dependence of the upper critical field $H_{c2}\left(T\right)$ of 2$H$-NbS${}_2$ varies considerably when increasing the pressure. At low pressures, $H_{c2}\left(0\right)$ decreases, and at higher pressures both $T_c$ and $H_{c2}\left(0\right)$ increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two-band approach.

While two-level systems (TLSs) are ubiqitous in solid state systems, microscopic understanding of their nature remains an outstanding problem. Conflicting phenomenological models are used to describe TLSs in seemingly similar materials when probed with different experimental techniques. Specifically, bulk measurements in amorphous solids have been interpreted using the model of a tunneling atom or group of atoms, whereas TLSs observed in the insulating barriers of Josephson junction qubits have been understood in terms of tunneling of individual electrons. Motivated by recent experiments studying TLSs in Josephson junctions, especially the effects of elastic strain on TLS properties, we analyze the interaction of the electronic TLS with phonons. We demonstrate that strong polaronic effects lead to dramatic changes in TLS properties. Our model gives a quantitative understanding of the TLS relaxation and dephasing as probed in Josephson junction qubits, while providing an alternative interpretation of bulk experiments. We demonstrate that a model of polaron dressed electronic TLS leads to estimates for the density and distribution of parameters of TLSs consistent with bulk experiments in amorphous solids. This model explains such surprising observations of recent experiments as the existence of minima in the energy of some TLSs as a function of strain and makes concrete predictions for the character of TLS dephasing near such minima. We argue that better understanding of the microscopic nature of TLSs can be used to improve properties of quantum devices, from an enhancement of relaxation time of TLSs to creating new types of strongly interacting optomechanical systems.

An interesting and novel effect has been observed in the form of a high-resistance state below the superconducting transition temperature of Al nanorings. The resistance exceeded 15 times the value of the normal-state resistance even though the ring was superconducting. This counterintuitive effect can be explained in terms of charge imbalance caused by the nonequilibrium accumulation of quasiparticles near tilted normal-superconductor interfaces that occur naturally in highly constricted nanostructures. Simulations using finite-element analysis have been performed which support the experimental results. Similar results have been observed in wires and disks, but the results of this work clarify and extend this previous work.

We present a method to build magnetic models of insulators based on high-temperature expansions by fitting both the magnetic susceptibility and the low-temperature specific heat data. It is applied to the frustrated magnet kapellasite [Cu${}_3$Zn(OH)${}_6$Cl${}_2$] with the $J_1-J_2-J_d$-Heisenberg model on the kagome lattice. Experimental data are reproduced with a set of competing exchange energies centered at $J_1=-12,J_2=-4$, and $J_d=15.6\mathrm{K}$, where $J_d$ is the third-neighbor exchange energy across the hexagon. Strong constraints between these exchange energies are established. These values confirm the results of Fåk et al. [Phys. Rev. Lett. 109, 037208 (2012)] regarding the location of kapellasite in the cuboc2 phase of the Heisenberg model. The quality and limits of this modeling are discussed.

Using first-principles density functional theory calculations, in conjunction with the Mahan–Nozières–de Dominicis theory, we calculate the x-ray absorption spectra of the alloys of graphene and monolayer hexagonal boron nitride on a Ni (111) substrate. The chemical neighborhood of the constituent atoms (B, C, and N) inside the alloy differs from that of the parent phases. In a systematic way, we capture the change in the $K$-edge spectral shape, depending on the chemical neighborhood of B, C, and N. Our work also reiterates the importance of the dynamical core-hole screening for a proper description of the x-ray absorption process in $\mathit{sp}$${}^2$-bonded layered materials.

Based on a careful perusal of data, we revisit the longstanding issues of transport anisotropy and incoherence-coherence crossovers in graphite. Using a realistic many-body approach, we unearth surprising but distinctive signatures of band-selective Kondoesque physics in graphite. The intricate interplay between local multiband Coulomb and effectively enhanced residual spin-orbit interactions comprehensively rationalizes these anomalies in terms of a manifestation of a multiband Kondo effect with a small Fermi liquid coherence scale.

Periodic and cluster density functional theory (DFT) calculations, including DFT+$U$ and hybrid functionals, are applied to study magnetostructural correlations in spin-$\frac{1}{2}$ frustrated chain compounds Cu$X$${}_2$: CuCl${}_2$, CuBr${}_2$, and a fictitious chain structure of CuF${}_2$. The nearest-neighbor and second-neighbor exchange integrals $J_1$ and $J_2$ are evaluated as a function of the Cu–$X$–Cu bridging angle $\theta$ in the physically relevant range 80${}^{\circ }$–110${}^{\circ }$. In the ionic CuF${}_2$, $J_1$ is ferromagnetic for $\theta \le {100}^{\circ }$. For larger angles, the antiferromagnetic superexchange contribution becomes dominant, in accord with the Goodenough-Kanamori-Anderson rules. However, both CuCl${}_2$ and CuBr${}_2$ feature ferromagnetic $J_1$ in the whole angular range studied. This surprising behavior is ascribed to the increased covalency in the Cl and Br compounds, which amplifies the contribution from Hund's exchange on the ligand atoms and renders $J_1$ ferromagnetic. At the same time, the larger spatial extent of $X$ orbitals enhances the antiferromagnetic $J_2$, which is realized via the long-range Cu–$X$–$X$–Cu paths. Both periodic and cluster approaches supply a consistent description of the magnetic behavior which is in good agreement with the experimental data for CuCl${}_2$ and CuBr${}_2$. Thus, owing to their simplicity, cluster calculations have excellent potential to study magnetic correlations in more involved spin lattices, and facilitate application of quantum-chemical methods.

The magnetization dynamics induced by a pulsed magnetic field is investigated by time- and polarization-resolved photoluminescene measurements in (Cd,Mn)Te/(Cd,Mg)Te quantum wells. The magnetization dynamics of Mn${}^{2+}$ ions is found to be strongly dependent on the external static magnetic field. A dynamical response of the magnetization on a subnanosecond time scale is observed at zero static magnetic field, while it drastically slows down and approaches the spin-lattice relaxation time constant for a nonzero static field. Theoretical calculations emphasize the importance of local spin interactions that interplay with the Zeeman interaction for the observed magnetization dynamics.

${}^{23}$Na NMR studies of the Na-K eutectic alloy embedded into porous glass with 7-nm pores showed that melting of Na${}_2$K confined nanoparticles is a continuous process with smooth changes in the Knight shift of a narrow resonance line and nuclear spin relaxation between those in the crystalline and liquid states. The intermediate state, which occurs upon melting is stable and more favorable than the liquid state. The inverse freezing transformation can be sharp as at a first-order transition or continuous depending on the initial temperature of cooling. The results suggest revision of theoretical predictions for the melting and freezing transitions in confined geometry.

Understanding the effects of electron-electron interactions in half-metallic magnets (HMs), which have band structures with one gapped spin channel and one metallic channel, poses fundamental theoretical issues as well as having importance for their potential applications. Here we use determinant quantum Monte Carlo to study the impacts of an on-site Hubbard interaction $U$, finite temperature, and an external (Zeeman) magnetic field on a bilayer tight-binding model which is a half-metal in the absence of interactions, by calculating the spectral density, conductivity, spin polarization of carriers, and local magnetic properties. We quantify the effect of $U$ on the degree of thermal depolarization, and follow relative band shifts and monitor when significant gap states appear, each of which can degrade the HM character. For this model, Zeeman coupling induces, at fixed particle number, two successive transitions: compensated half-metal with spin-down band gap $\to$ metallic ferromagnet $\to$ saturated ferromagnetic insulator. However, over much of the more relevant parameter regime, the half-metallic properties are rather robust to $U$.