Nanomagnetometry using the nitrogen-vacancy (NV) center in diamond has attracted a great deal of interest due to its unique combination of room temperature operation, nanoscale resolution, and high sensitivity. One of the important goals for nanomagnetometry is to be able to detect nanoscale nuclear magnetic resonance (NMR) in individual molecules. Our theoretical analysis details a method by which a single molecule on the surface of diamond, with characteristic NMR frequencies, can be detected using a proximate NV center on a time scale of an order of seconds with nanometer precision. We perform spatiotemporal resolution optimization and subsequently outline paths to greater sensitivity. Our method is suitable for application in low and relatively inhomogeneous background magnetic fields in contrast to both conventional liquid and solid state NMR spectroscopy.

The spin-glass behavior of Y${}_2$Mo${}_2$O${}_7$ has remained a puzzle for nearly three decades. Free of bulk disorder within the resolution of powder diffraction methods, it is thought that this material is a rare realization of a spin glass resulting from weak disorder such as bond disorder or local lattice distortions. Here we report on the single-crystal growth of Y${}_2$Mo${}_2$O${}_7$. Using neutron scattering, we present isotropic magnetic diffuse scattering occurring below the spin-glass transition. Our attempts to model the diffuse scattering using a computationally exhaustive search of a class of simple spin Hamiltonians show no agreement with the experimentally observed energy-integrated (diffuse) neutron scattering. This suggests that spin degrees of freedom are insufficient to describe this system. Indeed, a $T^2$ temperature dependence in the heat capacity and density functional theory calculations hint at the presence of a significant frozen degeneracy in both the spin and orbital degrees of freedom resulting from spin-orbital coupling (Kugel-Khomskii type) and random fluctuations in the Mo environment at the local level.

Skyrmions are topologically protected field structures with particlelike characteristics that play important roles in several areas of science. Recently, skyrmions have been directly observed in chiral magnets. Here, we investigate the effects of pointlike nonmagnetic impurities on the distinct initial states (random or helical ones) and on the formation of the skyrmion crystal in a discrete lattice. Using Monte Carlo techniques, we have found that even a small percentage of spin vacancies present in the chiral magnetic thin film considerably affects the skyrmion order. The main effects of impurities are somewhat similar to thermal effects. The presence of these spin vacancies also induces the formation of bimerons in both the helical and skyrmion states. We also investigate how adjacent impurities forming a hole affect the skyrmion crystal.

We present a first-principles investigation of the electronic structure of liquid water based on many-body perturbation theory (MBPT), within the $G_0{W}_0$ approximation. The liquid quasiparticle band gap and the position of its valence band maximum and conduction band minimum with respect to vacuum were computed and it is shown that the use of MBPT is crucial to obtain results that are in good agreement with experiment. We found that the level of theory chosen to generate molecular dynamics trajectories may substantially affect the electronic structure of the liquid, in particular, the relative position of its band edges and redox potentials. Our results represent an essential step in establishing a predictive framework for computing the relative position of water redox potentials and the band edges of semiconductors and insulators.

Paramagnetic atoms inside nanometer sized fullerenes realize robust, and chemically protected, spin systems. Changing the stoichiometry of the endohedral clusters results in a variety of magnetic ground states, as it is demonstrated for ${\mathrm{Dy}}_n{\mathrm{Sc}}_{3-n}\mathrm{N@}{\mathrm{C}}_{80}$ ($n=1,2,3$). All three exhibit distinct hysteresis and qualify as single-molecule magnets. In zero field the magnetization of $n=1$ decays via quantum tunneling, while ferromagnetic coupling of the individual dysprosium moments results in remanence for ${\mathrm{Dy}}_2\mathrm{ScN@}{\mathrm{C}}_{80}$ and in a frustrated ground state for $n=3$. The latter ground state turns out to be one of the simplest realizations of a frustrated, ferromagnetically coupled, system.

The ferropnictidesuperconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in $\approx 40$ Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to $\approx$0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons.

We study two anisotropically interacting spins coupled to optical phonons; we restrict our analysis to the regime of strong coupling to the environment, to the antiadiabatic region, and to the subspace with zero value for $S_T^z$ (the $z$ component of the total spin). In the case where each spin is coupled to a different phonon bath, we assume that the system and the environment are initially uncorrelated (and form a simply separable state) in the polaronic frame of reference. By analyzing the polaron dynamics through a non-Markovian quantum master equation, we find that the system manifests a small amount of decoherence that decreases both with increasing nonadiabaticity and with enhancing strength of coupling; whereas, under the Markovian approximation, the polaronic system exhibits a decoherence-free behavior. For the situation where both spins are coupled to the same phonon bath, we also show that the system is decoherence-free in the subspace where $S_T^z$ is fixed. To suppress decoherence through quantum control, we employ a train of $\pi$ pulses and demonstrate that unitary evolution of the system can be retained. We propose realization of a weakly decohering charge qubit from an electron in an oxide-based (tunnel-coupled) double-quantum-dot system.

We study skyrmions in magnetic thin films with structural inversion asymmetry perpendicular to the film plane. We construct a phenomenological model that describes the interaction between the motion of skyrmions and electric currents to lowest order in spin-orbit coupling. Based on this model, we estimate the experimental verifiable velocities for current-driven motion of skyrmion textures based on available results obtained from domain-wall dynamics.

The length scale of the magnetization gradients in chiral magnets is determined by the relativistic Dzyaloshinskii-Moriya interaction. Thus, even conventional spin-transfer torques are controlled by the relativistic spin-orbit coupling in these systems, and additional relativistic corrections to the current-induced torques and magnetization damping become important for a complete understanding of the current-driven magnetization dynamics. We theoretically study the effects of reactive and dissipative homogeneous spin-orbit torques and anisotropic damping on the current-driven skyrmion dynamics in cubic chiral magnets. Our results demonstrate that spin-orbit torques play a significant role in the current-induced skyrmion velocity. The dissipative spin-orbit torque generates a relativistic Magnus force on the skyrmions, whereas the reactive spin-orbit torque yields a correction to both the drift velocity along the current direction and the transverse velocity associated with the Magnus force. The spin-orbit torque corrections to the velocity scale linearly with the skyrmion size, which is inversely proportional to the spin-orbit coupling. Consequently, the reactive spin-orbit torque correction can be the same order of magnitude as the nonrelativistic contribution. More importantly, the dissipative spin-orbit torque can be the dominant force that causes a deflected motion of the skyrmions if the torque exhibits a linear or quadratic relationship with the spin-orbit coupling. In addition, we demonstrate that the skyrmion velocity is determined by anisotropic magnetization damping parameters governed by the skyrmion size.

Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe${}_2$As${}_2$, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron $3d_{xz,yz}$ orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.

We derive equations of motion for calculating the near-edge x-ray absorption spectrum in molecules and condensed matter, based on a two-determinant approximation and Dirac's variational principle. The theory provides an exact solution for the linear response when the Hamiltonian or energy functional has only diagonal interactions in some basis. We numerically solve the equations to compare with the Mahan–Nozières–De Dominicis theory of the edge singularity in metallic conductors. Our extracted power-law exponents are similar to those of the analytic theory, but are not in quantitative agreement. The calculational method can be readily generalized to treat Kohn-Sham Hamiltonians with electron-electron interactions derived from correlation-exchange potentials.

Non-Hermitian Hamiltonians arising from parity-time (PT)-symmetric potentials have been extensively explored in optical systems, owing to their ability to generate asymmetric and even nonreciprocal light propagation. In this paper, we investigate such PT potentials in plasmonic systems, demonstrating asymmetric optical propagation in deeply subwavelength waveguides. In particular, we investigate a five layer plasmonic waveguide composed of metallic layers separated by dielectric media containing either loss or gain in equal quantities. Through an analytic solution of Maxwell's equations, we identify the four lowest order modes of the waveguide, including two positive index modes and two negative index modes, and investigate their evolution with increasing but balanced gain and loss, $\kappa$. Both the exact analytic approach and an approximate one based on Rayleigh-Schrodinger perturbation theory demonstrate eigenvalue merging and state coalescence with increasing $\kappa$, unlike the familiar energy-level splitting observed in conventional coupled systems. The state coalescence always occurs between modes of opposite parity. Also, by changing the coupling between the waveguide layers, state coalescence can occur between modes with opposite refractive indices, resulting in the merging of a positive index mode with a negative index mode at the exceptional point. We use dispersion diagrams and field profiles to illustrate the asymmetric plasmon propagation properties with increasing $\kappa$. We also show that at the system's exceptional point, the modal power varies quadratically along the waveguide. This study represents a spectral analysis of deeply subwavelength PT-symmetric plasmonic and multimodal photonic waveguides and provides a foundation for designing asymmetric and unidirectional nanophotonic devices.

Recently, it was shown that the topological properties of 2D and 3D topological insulators are captured by a $Z_2$ chiral anomaly in the boundary field theory. It remained, however, unclear whether the anomaly survives electron-electron interactions. We show that this is, indeed, the case, thereby providing an alternative formalism for treating topological insulators in the interacting regime. We apply this formalism to fractional topological insulators (FTI) via projective/parton constructions and use it to test the robustness of all fractional topological insulators that can be described in this way. The stability criterion we develop is easy to check and based on the pair switching behavior of the noninteracting partons. In particular, we find that FTIs based on bosonic Laughlin states and the $M=0$ bosonic Read-Rezayi states are fragile and may have a completely gapped and nondegenerate edge spectrum in each topological sector. In contrast, FTIs based on fermionic $Z_k$ Read-Rezayi states with $M=1$ and odd $k$ and the bosonic 3D topological insulator with a $\pi /4$ fractional $\theta$ term are topologically stable.

A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room-temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature-dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon-mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.

Molecular monolayers of the charge-transfer salt dithienobenzodithiophene-tetrafluorotetracyanoquinodimethane (DTBDT-F${}_4$TCNQ) have been deposited on C(R$15\times 3$)/W(110), Co/W(110), and hcp Co(0001) using molecular beam epitaxy in an ultrahigh vacuum. The integrity of the deposited molecules has been confirmed by scanning tunneling microscopy. Scanning tunneling spectroscopy has been used to determine the energetic positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital of acceptor and donor in the pure and in the mixed phase. The mixed charge transfer phase exhibits a new HOMO close to the Fermi edge depicting a charge transfer of less than one elementary charge. The total amount of the transferred charge depends on the underlying metal substrate. The binding energy of the HOMO decreases from $0.4$ eV for C(R$15\times 3$)/W(110) to $0.16$ eV for Co(0001), thus revealing the tunability of the hole injection barrier. The energy gap between HOMO and LUMO decreases from $0.96$ eV ($0.81$ eV) for C(R$15\times 3$)/W(110) [ps-ML Co/W(110)] to $0.66$ eV for Co(0001), demonstrating a strong reduction compared to the value of the pure moieties.

We present a first-principles study of spin-dependent electron transport through gold nanochains suspended between cobalt electrodes aimed at elucidating the electronic origin of the high magnetoresistance recently observed experimentally in such nanojunctions. Our nonequilibrium Green's function based calculations confirm the occurrence of high spin polarization of conductance in Co/Au/Co nanocontacts, which in some cases reaches 90%. Our analysis allows to relate such behavior to the hybridization of electronic orbitals of neighboring cobalt and gold atoms of the nanocontact. The results obtained give clear evidence of the presence of spin injection from Co electrodes into paramagnetic gold contacts.

The honeycomb lattice material ${\mathrm{Li}}_2\mathrm{Ru}{\mathrm{O}}_3$ undergoes a dimerization of Ru${}^{4+}$ cations on cooling below $270{}^{\circ }$C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a valence bond crystal, with a strong structural bond disproportionation. On warming, x-ray diffraction shows that discrete threefold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x rays show that disordered dimers survive at the nanoscale up to at least 650 ${}^{\circ }$C. The high-temperature phase of ${\mathrm{Li}}_2\mathrm{Ru}{\mathrm{O}}_3$ is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analog of the resonating valence bond state often discussed in connection with high-$T_c$ cuprates.

We present the implementation of linear-response time-dependent density functional theory based on the self-consistent Sternheimer equation and employing a basis set of numerical pseudo-atomic orbitals. We demonstrate this method by presenting test calculations on systems of increasing size ranging from benzene to chlorophyll $a$, and by comparing our results with those obtained within Casida's formalism and with previous calculations. We provide a detailed assessment of the accuracy of this method, both in relation to the use of local orbitals for describing electronic excitations and to the handling of the frequency response using Padé approximants. We establish a simple criterion for estimating a priori the accuracy of the basis set in the calculation of optical spectra. We show that the computational cost of this method scales quadratically with the system size.