To describe vibrationally mediated configuration changes of adsorbates on surfaces we have developed a theory to calculate both reaction rates and pathways. The method uses the T-matrix to describe excitations of vibrational states by the electrons of the substrate, adsorbate, and tunneling electrons from a scanning tunneling probe. In addition to reaction rates, the theory also provides the reaction pathways by going beyond the harmonic approximation and using the full potential energy surface of the adsorbate which contains local minima corresponding to the adsorbates different configurations. To describe the theory, we reproduce the experimental results in [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)], where the hydrogen/deuterium atom of an adsorbed hydroxyl (OH/OD) exhibits back and forth flipping between two equivalent configurations on a Cu(110) surface at $T=6$ K. We estimate the potential energy surface and the reaction barrier, $\sim 160$ meV, from DFT calculations. The calculated flipping processes arise from (i) at low bias, tunneling of the hydrogen through the barrier, (ii) intermediate bias, tunneling electrons excite the vibrations increasing the reaction rate although over the barrier processes are rare, and (iii) higher bias, overtone excitations increase the reaction rate further.

Both spin-liquid and magnetically ordered phases of both half-integer and integer low-spin quantum magnets are of interest, since the magnetic structures found in the latter case usually have no classical counterparts. Such a magnetic structure was found in a combined experimental and theoretical study of the integer spin system $\mathrm{Ni}{(\mathrm{NO}}_3{)}_2$. Our thermodynamic measurements have revealed a magnetically ordered phase with small spontaneous magnetization at $T_{\mathrm{C}} = 5.5\mathrm{K}$. The magnetization saturation of about $2{\mu }_{\mathrm{B}}$ at low temperatures corresponds to the high-spin state ($S = 1$) of ${\mathrm{Ni}}^{2+}$ ions evidenced in $L_{2,3}$ edges in x-ray absorption spectroscopy spectra. We show that a consistent description of the available data is possible within a noncollinear umbrella-type ferrimagnetic ground state model for which both intra- and interlayer magnetic interactions should be antiferromagnetic. Such a scenario is suggested by the first-principles and model calculations.

The electronic, thermodynamical, and transport properties of ordered ${\mathrm{Fe}}_{3}X$ ($X=\text{Al},\mathrm{Si}$) alloys are studied from first principles. We present here a unified approach to the phase stability, the estimate of the Curie temperature, the temperature dependence of sublattice magnetizations, magnon spectra, the spin-stiffnesses, and residual resistivities. An important feature of the present study is that all calculated physical properties are determined in the framework of the same first-principles electronic structure model combined with the effective Ising and Heisenberg Hamiltonians used for study of the thermodynamical properties of alloys. Curie temperatures, spin-stiffnesses, and magnon spectra are determined using the same calculated exchange integrals. Finally, the transport properties are calculated using the linear-response theory. Our theoretical estimates compare well with available experimental data. In particular, calculations predict (in agreement with experiment) the ordered $D{0}_3$ phase as the ground-state alloy structure, demonstrate that a correct relation of Curie temperatures of ${\mathrm{Fe}}_3\mathrm{Al}/{\mathrm{Fe}}_3\mathrm{Si}$ alloys can be obtained only by going beyond a simple mean-field approximation, provide reasonable estimates of spin-stiffnesses, and give resistivities compatible with structural disorder observed in the experiment. Although the calculated temperature dependences of the Fe magnetization on different sublattices are similar, they nevertheless deviate more than in the experiment, and we discuss a possible origin.

An unconventional pairing mechanism in the heavy-fermion material ${\mathrm{URu}}_2{\mathrm{Si}}_2$ is studied. We propose a mixed singlet-triplet $d$-density wave to be the hidden-order state in ${\mathrm{URu}}_2{\mathrm{Si}}_2$. The exotic order is topologically nontrivial and supports a charge $2e$ skyrmionic spin texture, which is assumed to fractionalize into merons and antimerons at the deconfined quantum critical point. The interaction between these fractional particles results in a (pseudo)spin-singlet chiral $d$-wave superconducting state, which breaks time-reversal symmetry. Therefore, it is highly likely to produce a nonzero signal of the polar Kerr effect at the onset of the superconductivity, consistent with recent experiments. In addition, the nodal structures of the possible pairing functions in our model are consistent with the thermodynamic experiments in ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

Recent experiments suggest that Bi impurities segregate to form bilayer films on Ni and Cu grain boundaries (GBs) but do not segregate in Fe. To explain these phenomena, we study the total energies of Bi films on transition-metal (TM) $\Sigma 3\left(111\right)$ and $\Sigma 5\left(012\right)$ GBs using density functional theory. Our results agree with the observed stabilities. We propose a model to predict Bi bilayer stability at Ni GBs which suggests that Bi bilayer is not stable on (111) twist CSL GBs but is stable in most (100) twist CSL GBs. We investigate the interaction and bonding character between Bi and TMs to explain the differences among TMs based on localization of orbitals and magnetism, as well as evaluating the contribution of interfacial phonons at high temperature.

We studied the effect of dopants (Al, Ti, Zr) on the thermal stability of iron-nitride thin films prepared using a dc magnetron sputtering technique. Structure and magnetic characterization of deposited samples reveal that the thermal stability together with soft magnetic properties of iron-nitride thin films get significantly improved with doping. To understand the observed results, detailed Fe and N self-diffusion measurements were performed. It was observed that N self-diffusion gets suppressed with Al doping, whereas Ti or Zr doping results in somewhat faster N diffusion. On the other hand, Fe self-diffusion seems to get suppressed with any dopant of which the heat of nitride formation is significantly smaller than that of iron nitride. Importantly, it was observed that N self-diffusion plays only a trivial role, as compared to Fe self-diffusion, in affecting the thermal stability of iron-nitride thin films. Based on the obtained results, the effect of dopants on the self-diffusion process is discussed.

Using first-principles calculation, we have investigated the structural and magnetic properties of ${\mathrm{Mn}}_x{\mathrm{Ga}}_{1-x}$ alloys with Mn content ($x$) changing from 0.5 to 0.75 in step of $x=0.125$. The ${\mathrm{Mn}}_x{\mathrm{Ga}}_{1-x}$ systems crystallize in tetragonal structures, and the tetragonal ratio ($c/a$) is strongly influenced by chemical composition. The in-plane lattice constants ($a$) show very weak concentration dependence, but the lattice parameters along the $z$ axis ($c$) are significantly modified by $x$. We have found that the magnetic properties of ${\mathrm{Mn}}_x{\mathrm{Ga}}_{1-x}$ alloys are strongly affected by the Mn content and the epitaxial strain. Very interestingly, the magnetocrystalline anisotropy (MCA) of ${\mathrm{Mn}}_x{\mathrm{Ga}}_{1-x}$ alloys follows totally different behaviors with $x$ and epitaxial strain. An especially wide range of MCA values are obtained by straining the ${\mathrm{Mn}}_{0.5}{\mathrm{Ga}}_{0.5}$ alloy. The spin polarization (at the Fermi level) of ${\mathrm{Mn}}_{0.75}{\mathrm{Ga}}_{0.25}$ has been calculated from 44% to 57% with epitaxial strain.

Magnetization and muon spin rotation and relaxation ($\mu \mathrm{SR}$) measurements have been performed to study the superconducting and magnetic properties of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$. From magnetization measurements the lower and upper critical fields $T=0\mathrm{K}$ are found to be 8.5(1) mT and 1.44(2) T, respectively. Zero-field $\mu \mathrm{SR}$ data show no sign of any magnetic ordering or weak magnetism. Transverse-field $\mu \mathrm{SR}$ measurements in the vortex state provide the temperature dependence of the magnetic penetration depth $\lambda$. The temperature dependence of ${\lambda }^{-2}$ is consistent with the existence of a single $s$-wave energy gap in the superconducting state of ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ with a gap value of 0.82(2) meV at absolute zero temperature. However, a two-gap $s+s$-wave model fit with gap values of 0.91(4) and 0.14(7) meV cannot be ruled out completely. The magnetic penetration depth at zero temperature $\lambda \left(0\right)$ is 291(3) nm. The ratio $\Delta \left(0\right)/k_{\mathrm{B}}{T}_{\mathrm{c}}=2.1\left(1\right)$ indicates that ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ should be considered a strong-coupling superconductor.

We calculate the bipartite Rényi entanglement entropy of an $L\times L\times 2$ bilayer Hubbard model using a determinantal quantum Monte Carlo method recently proposed by Grover [Phys. Rev. Lett. 111, 130402 (2013)]. Two types of bipartition are studied: (i) one that divides the lattice into two $L\times L$ planes and (ii) one that divides the lattice into two equal-size ($L\times L/2\times 2$) bilayers. We compare our calculations with those for the tight-binding model studied by the correlation matrix method. As expected, the entropy for bipartition (i) scales as $L^2$, while the latter scales with $L$ with possible logarithmic corrections. The onset of the antiferromagnet to singlet transition shows up by a saturation of the former to a maximal value and the latter to a small value in the singlet phase. We comment on the large uncertainties in the numerical results with increasing $U$, which would have to be overcome before the critical behavior and logarithmic corrections could be quantified.

Motivated by recent experimental evidence of charge order in the pseudogap phase of cuprates, we perform a variational analysis of spin-singlet density wave ordering in metals with antiferromagnetic interactions on the square lattice, using a wave function with double occupancy projected out. We examine ordering with and without time-reversal symmetry, with an arbitrary wave vector and a tunable form factor. Depending on parameters, we find $d$-form factor density wave ordering, with a wave vector either parallel to the lattice generators or diagonally oriented, or a ground state which carries a time-reversal breaking pattern of spontaneous currents.

Size effect in ferroelectrics has been extensively investigated in the past with the general consensus that the long-range ferroelectric ordering gradually disappears with decreasing crystallite size, eventually leading to the paraelectric state. In this paper, we show that the compositions exhibiting giant tetragonality ($c/a\sim 1.18$) of the ferroelectric alloy system ${\mathrm{BiFeO}}_3\text{−}{\mathrm{PbTiO}}_3$ transform from a pure tetragonal phase to a state comprising tetragonal and rhombohedral phases as the average crystallite size is reduced from $\sim 10$ to $\sim 1$ μm. It is argued that the increased surface energy in the smaller sized crystallites creates an equivalent compressive stress that drives the system towards tetragonal-rhombohedral criticality.

The reflection and transmission coefficients of a surface phonon-polariton propagating along the surface of a thin film of ${\mathrm{SiO}}_2$ and crossing the interface of two dielectric media are analytically determined. Based on the expansion of the electrical and magnetic fields in terms of normal modes, explicit expressions for the reflectivity and transmissivity of the radiation fields generated at the dielectric interface are also obtained. Symmetrical and simple Fresnel-like formulas are derived for nanofilms. For the dielectric interfaces of $\mathrm{air}/{\mathrm{BaF}}_2$ and $\mathrm{air}/{\mathrm{Al}}_2{\mathrm{O}}_3$, it is shown that: (i) The polariton reflectivity (transmissivity) decreases (increases) as the film thickness increases, while its radiation equivalent follows the opposite behavior. (ii) In the polariton and radiation fields, the transmissivity is significantly more sensitive than the reflectivity to the changes on the permittivity mismatch of the dielectric interface. For a 143-nm-thick film, the polariton transmissivity (reflectivity) changes $13.2\%$ ($1.9\%$), when this mismatch varies by $50\%$. (iii) The reflectivity and transmissivity of the radiation fields are smaller than their polariton counterparts, which together account for around $82\%$ of the total energy. The proposed formalism accurately fulfills the principle of conservation of energy for describing the reflection and transmission of both the polariton and radiation fields generated at a dielectric interface.

A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force ($F_g$) and the scattering force ($F_{sc}$). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30–60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a $z^{-4}$ dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant $F_{sc}$ and the spatially dependent $F_g$ give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.

The degeneracy and spatial support of pseudo-Landau levels (pLLs) in strained honeycomb lattices systematically depends on the geometry; for instance, in hexagonal and rectangular flakes the zeroth pLL displays a twofold increased degeneracy, while the characteristic sublattice polarization of the zeroth pLL is only fully realized in a zigzag-terminated triangle. These features are dictated by algebraic constraints in the atomistic theory and signify a departure from the standard picture in which all qualitative differences between pLLs and Landau levels induced by a magnetic field trace back to the valley antisymmetry of the pseudomagnetic field.

Surface plasmon resonances in metal nanoparticles are an emerging technology platform for nano-optics applications from sensing to solar energy conversion. The electromagnetic near field associated with these resonances arises from modes determined by the shape, size, and composition of the metal nanoparticle. When coupled in the near field, multiple resonant modes can interact to give rise to interference effects offering fine control of both the spectral response and spatial distribution of fields near the particle. Here, we present an examination of experimental electron energy loss spectroscopy (EELS) of silver nanorod monomer surface plasmon modes and present an explanation of observed spatial amplitude modulation of the Fabry-Pérot resonance modes of these silver nanorods using electrodynamics simulations. For these simulations, we identify differences in spectral peak symmetry in light scattering and electron spectroscopies (EELS and cathodoluminescence) and analyze the distinct near-field responses of silver nanorods to plane-wave light and electron beam excitation in terms of a coupled oscillator model. Effects of properties of the material and the incident field are evaluated, and the spatially resolved EELS signals are shown to provide a signature for assessing Fano-like interference effects in silver nanorods. These findings outline key considerations and challenges for interpreting electron microscopy data on plasmonic nanoparticles for understanding nanoscale optics and for characterization and design of photonic devices.

We study the Ruderman-Kittel-Kasuya-Yosida interaction in $p$-doped transition-metal dichalcogenides such as ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$. We consider magnetic impurities hybridized to the Mo $d$ orbitals characteristic of the valence bands. Using the Matsubara Green's-function formalism, we obtain the two-impurity interaction vs their separation and chemical potential of the system, accounting for the important angular dependence which reflects the underlying triangular lattice symmetry. The inclusion of the valence-band valley at the $\Gamma$ point results in a strong enhancement of the interaction. Electron-scattering processes transferring momentum between valleys at different symmetry points give rise to complex spatial oscillation patterns. Variable doping would allow the exploration of rather interesting behavior in the interaction of magnetic impurities on the surfaces of these materials, including the control of the interaction symmetry, which can be directly probed in scanning tunneling microscopy experiments.

We study entanglement via the subsystem purity relative to bipartitions of arbitrary excited states in (1+1)-dimensional conformal field theory, equivalent to the scaling limit of one-dimensional quantum critical systems. We compute the exact subpurity as a function of the relative subsystem size for numerous excited states in the Ising and three-state Potts models. We find that it decays exponentially when the system and the subsystem sizes are comparable until a saturation limit is reached near half-partitioning, signaling that excited states are maximally entangled. The exponential behavior translates into extensivity for the second Rényi entropy. Since the coefficient of this linear law depends only on the excitation energy, this result shows an interesting relationship between energy and quantum information and elucidates the role of microscopic details.

We employ $\mathit{ab}\mathit{initio}$ simulations based on density functional theory (DFT) to calculate the electronic transport coefficients (electrical conductivity, thermal conductivity, and thermopower) of molybdenum over a broad range of thermodynamic states. By comparing to available experimental data, we show that DFT is able to describe the desired transport properties of this refractory metal with high accuracy. Most noteworthy, both the positive sign and the quantitative values of the thermopower of solid molybdenum are reproduced very well. We calculate the electrical and thermal conductivity in the solid and the fluid phase between 1000 and 20 000 K and a wide span in density and develop empirical fit formulas for direct use in practical applications, such as magneto-hydrodynamics simulations. The influence of thermal expansion in conductivity measurements at constant pressure is also discussed in some detail.