A multiterminal device based on a carbon nanotube quantum dot was used at very low temperature to probe a single electronic and nuclear spin embedded in a bis-(phthalocyaninato) terbium (III) complex (TbPc${}_2$). A spin-valve signature with large conductance jumps was found when two molecules were strongly coupled to the nanotube. The application of a transverse field separated the magnetic signal of both molecules and enabled single-shot read-out of the terbium nuclear spin. The Landau-Zener (LZ) quantum tunneling probability was studied as a function of field sweep rate, establishing a good agreement with the LZ equation and yielding the tunnel splitting $\Delta$. It was found that $\Delta$ increased linearly as a function of the transverse field. These studies are an essential prerequisite for the coherent manipulation of a single nuclear spin in TbPc${}_2$.

The ground state of bilayer graphene is investigated using density functional calculations with a local spin-density approximation. We find a ground state with layer antiferromagnetic ordering, which is one of the candidate states suggested by former studies based on a simplified model. The calculations prove that the layer antiferromagnetic state is stable even if the nonlocal Coulomb interaction and the remote hopping are included. The gap of the correlated ground state is about $1.8$ meV, in reasonable agreement with the experimental value. The surface magnetism in the bilayer graphene is of the order of ${10}^{-2}{\mu }_B/$nm${}^2$.

We study a single dark N2 electron spin defect in diamond, which is magnetically coupled to a nearby nitrogen-vacancy (NV) center. We perform pulsed electron spin resonance on this single spin by mapping its state to the NV center spin and optically reading out the latter. Moreover, we show that the NV center's spin polarization can be transferred to the electron spin by combined two decoupling control-NOT gates. These two results allow us to extend the NV center's two key properties—optical spin polarization and detection—to any electron spin in its vicinity. This enables dark electron spins to be used as local quantum registers and engineerable memories.

We report a surface x-ray diffraction study of the structure of In films grown on $\mathrm{Si}\left(111\right)-\left(7\times 7\right)$ and $\mathrm{Si}\left(111\right)-\left(\sqrt{3}\times \sqrt{3}\right)-\mathrm{In}$ substrates at a low temperature (135 K). The $\left(7\times 7\right)$ reconstruction of the clean Si(111) surface is found to persist upon burial by the In. X-ray reflectivity measurements yield patterns that deviate strongly from the ideal case; the results suggest a complex In film structure, possibly distorted by the corrugated interfacial reconstruction. By contrast, In films grown on the $\mathrm{Si}\left(111\right)-\left(\sqrt{3}\times \sqrt{3}\right)-\mathrm{In}$ surface exhibit reflectivity data that are much closer to the ideal case. The films are found to grow approximately layer by layer, resulting in a relatively small roughness. Upon annealing, the films develop preferred thicknesses at 10, 13, and 16 monolayers (MLs). Previous photoemission studies revealed preferred thicknesses at 4 and 7 MLs. Putting these results together, the preferred thickness sequence, 4, 7, 10, 13, and 16 ML, establishes a trilayer oscillation period. This period is expected from the known electronic structure of In, and arises from quantum confinement of the In valence electrons. This is the second example, after the well-known bilayer period in Pb, which shows quantum oscillations over a wide range of film thickness.

The higher Rydberg states of the $E_{11}$ exciton in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) were studied using one- and two-photon photoluminescence excitation spectroscopy. Increasing the hole-dopant concentration resulted in a redshift of the first excited state (2$g$) and a blueshift of the ground state (1$u$) of the $E_{11}$ exciton. From the redshift of higher Rydberg states, we found that a reduction of the band-gap energy occurs in hole-doped SWCNTs. These findings show that the exciton and electronic band structures are modified by the presence of a small number of holes in the SWCNT one-dimensional structures.

Results are presented for a two-point correlation function of a spin chain after a quantum quench for an intermediate time regime where inelastic effects are weak. A Callan-Symanzik-like equation for the correlation function is explicitly constructed which is used to show the appearance of three distinct scaling regimes. One is for spatial separations within a light cone, the second is for spatial separations on the light cone, and the third is for spatial separations outside the light cone. In these three regimes, the correlation function is found to decay with power laws with nonequilibrium exponents that differ from those in equilibrium, as well as from those obtained from quenches in a quadratic Luttinger liquid theory. A detailed discussion is presented on how the existence of scaling depends on the properties of the initial state before the quench.

We introduce an approach that allows us to significantly reduce the computational cost for the evaluation of the Fock exchange operator when it is applied to an arbitrary periodic wave function in the case of large simulation cells. This method is based on the exponential decay properties of the density matrix and can be applied to systems exhibiting a finite electronic gap. Our scheme does not require the evaluation of Wannier's functions and can be easily implemented in density-functional theory codes based on plane waves. We first validate and benchmark the approach on the isolated benzene molecule, and then we apply it to the isolated tetraphenylporphyrin molecule (C${}_{44}$H${}_{30}$N${}_4$) and to a model of bulk Si comprising 512 atoms.

We consider a SU(3) model with antiferromagnetic three-site ring exchanges, in addition to two-site exchanges, on the triangular lattice. We first present numerical site-factorized state studies on the magnetic ordered states, which show two different three-sublattice-ordered states—the antiferroquadrupolar phase and the standard ${120}^{\circ }$ antiferromagnetic phase—along the axis of the strength of the three-site ring exchanges. We further study the model using slave-fermion mean field approaches in which we rewrite the exchange operators in terms of three flavors of fermions. At the mean-field level, we find that the main competing trial states are the trimer state (triangular plaquette state), gapped uniform $\pi /3$-flux chiral spin liquid, and gapped uniform $2\pi /3$-flux chiral spin liquid. The filled band of the $\pi /3$-flux chiral spin liquid has Chern number $+1$, and that of the $2\pi /3$-flux chiral spin liquid has Chern number $+2$. We also give the effective Chern-Simons theory for each chiral spin liquid at the mean-field level.

We present theoretical evidence for the phase dependence of hole localization and transport in bismuth vanadate (BiVO${}_4$). Our hybrid density-functional theory calculations predict that, in the tetragonal phase [tetragonal scheelite BiVO${}_4$ ($ts$-BiVO${}_4$)], an excess hole tends to localize around a BiO${}_8$ polyhedron with strong lattice distortion, whereas, in the monoclinic phase [monoclinic scheelite BiVO${}_4$ ($ms$-BiVO${}_4$)], it spreads over many lattice sites. The phase-dependent behavior is likely related to the higher structural stability of $ms$-BiVO${}_4$ than $ts$-BiVO${}_4$, which may suppress hole-induced lattice distortions. Our study also demonstrates that the relatively weakly localized hole in $ms$-BiVO${}_4$ undergoes faster diffusion compared to the case of $ts$-BiVO${}_4$, irrespective of the fact that the degrees of localization and mobility vary depending on the choice of exchange-correlation functional. The mobility difference may provide an explanation for the enhanced photocatalytic activity of $ms$-BiVO${}_4$ over $ts$-BiVO${}_4$ for water oxidation, considering that the increased mobility would lead to an increase in the hole current to the catalyst surface.

We report on spatially resolved two-color pump-probe Hanle–magneto-optical Kerr effect studies of low-temperature electron spin diffusion in bulk $n$-type GaAs ($n_D=1.4\times {10}^{16}{\mathrm{cm}}^{-3}$). The influence of the lattice temperature on electron spin diffusion is investigated and it is shown that simple drift-diffusion models do not describe the spin diffusion for above-band-gap optical spin injection correctly. Variation of lattice temperature and excitation energy allow us to trace this discrepancy to the influence of the pump-induced local overheating of the electron system with respect to the lattice which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments.

Designing materials for desired thermal conduction can be achieved via extended defects. We theoretically demonstrate the concept by investigating thermal transport in graphene nanoribbons (GNRs) with the extended line defects observed by recent experiments. Our nonequilibrium Green's function study excluding phonon-phonon interactions finds that thermal conductance can be tuned over wide ranges (more than 50$\%$ at room temperature), by controlling the orientation and the bond configuration of the embedded extended defect. Further transmission analysis reveals that the thermal-conduction tuning is attributed to two fundamentally different mechanisms, via modifying the phonon dispersion and/or tailoring the strength of defect scattering. The finding, applicable to other materials, provides useful guidance for designing materials with desired thermal conduction.

We study the structure of the ground-state wave functions of bosonic symmetry protected topological (SPT) insulators in three space dimensions. We demonstrate that the differences with conventional insulators are captured simply in a dual vortex description. As an example, we show that a previously studied bosonic topological insulator with both global U(1) and time-reversal symmetry can be described by a rather simple wave function written in terms of dual “vortex ribbons.” The wave function is a superposition of all the vortex-ribbon configurations of the boson, and a factor $(-1)$ is associated with each self-linking of the vortex ribbons. This wave function can be conveniently derived using an effective field theory of the SPT phase in the strong-coupling limit, and it naturally explains all the phenomena of this SPT phase discussed previously. The ground-state structure for other three-dimensional (3D) bosonic SPT phases are also discussed similarly in terms of vortex loop gas wave functions. We show that our methods reproduce known results on the ground-state structure of some 2D SPT phases.

The exact Hamiltonians for Rashba and cubic Dresselhaus spin-orbit couplings on a curved surface with an arbitrary shape are rigorously derived. Two orthogonal principal curvatures dominate the electronic spin transport, and the asymptotic behavior of the normal confined potential on a curved surface is insignificant. For a curved surface with a large curvature, the higher order momentum terms play an important role in controlling spin transport. The Rashba spin-orbit coupling on a curved surface only induces the extra pseudopotential term, and the cubic Dresselhaus spin-orbit coupling on a curved surface can induce the extra pseudokinetic and pseudomomentum terms. Because of the extra curvature-induced terms and the associated pseudomagnetic fields, spin transport on a curved surface is very different from that on a flat surface. The Hamiltonians on both cylindrical and spherical surfaces are explicitly derived here, and the associated physical properties of electrons are studied in detail.

The London penetration depth was measured in optimally doped Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$ crystals, with and without columnar defects produced by 1.4 GeV ${}^{208}$Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap $\Delta {}_{\min }/k_B{T}_C\approx 1$. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a $T^2$ power law was observed below $T_C/3$, most likely due to elevated scattering. Neither the location nor the sharpness of the superconducting transition was affected by irradiation. The data provide evidence for an $s_{+-}$ pairing state.

We show that a magnetic vortex is the ground state of an array of magnetic particles shaped as a hexagonal fragment of a triangular lattice, even for a small number of particles in the array $N\le 100$. The vortex core appears and the symmetry of the vortex state changes with the increase of the intrinsic magnetic anisotropy of the particle $\beta$; the further increase of $\beta$ leads to the destruction of the vortex state. Such vortices can be present in arrays as small in size as a dozen nanometers.

We report an experimental and computational study of single-crystal NdFeO${}_3$, which features two inequivalent magnetic sublattices, namely, Fe and Nd sublattices that are coupled in an antiparallel fashion. This paper reveals that a strong interaction between 3$d$ and 4$f$ electrons of the two sublattices along with a spin-lattice coupling drives an extremely interesting magnetic state that is highly sensitive to the orientation and history of weak magnetic field. The following phenomena are particularly remarkable: (1) sharply contrasting magnetization $M$($T$) along the $a$ and $c$ axes; (2) a first-order spin switching along the $a$ axis below 29 K when the system is zero-field-cooled; and (3) a progressive magnetization reversal when the system is field-cooled. The intriguing magnetic behavior is captured in our first-principles density functional theory calculations.

The vortex-state electrodynamics of $s$-wave superconductors has been studied by infrared spectroscopy. Far-infrared transmission and reflection spectra of superconducting Nb${}_{0.5}$Ti${}_{0.5}$N and NbN thin films were measured in a magnetic field perpendicular to the film surface, and the optical conductivity was extracted. The data show clear reduction of superconducting signature. We consider the vortex state as a two-component effective medium of normal cores embedded in a BCS superconductor. The spectral features are well explained by the Maxwell Garnett theory. Our analysis supports the presence of magnetic-field-induced pair-breaking effects in the superconducting component outside of the vortex cores.

Ultrathin AlO${}_x$ layers are nowadays widely employed to make tunneling junctions and, as a common practice, experimental transport data are often rationalized in terms of analytical models invoking effective electronic and geometric properties of the oxide layer. In this paper we examine the reliability of such models by performing first-principles simulations of the transport properties of Al/AlO${}_x$/Al junctions. The band gap, effective mass, and interface width obtained from ground state density-functional calculations are used within a potential barrier model, known also as the Simmons model, and its predictions of the conductance are compared with first-principles results. We also propose an analytical expression for the conductance based on a tight-binding model of the interface oxide. We show that the success of the potential barrier model in fitting experimental transport measurements rests on its formal similarity with the tight binding model which, in contrast to the former, is directly related to the realistic electronic structure of the interface.

We investigate the dynamical and steady-state spin response of the nonequilibrium Anderson model to magnetic fields, bias voltage, and temperature using a numerically exact method combining a bold-line quantum Monte Carlo technique with a memory function formalism. We obtain converged results in a range of previously inaccessible regimes, in particular calculating the spin dynamics for a range of temperatures down to the crossover to the Kondo domain. We provide predictions for nonequilibrium phenomena, including nonmonotonic temperature dependence of observables at high bias voltage and oscillatory quench dynamics at high magnetic fields.

Motivated by the recent discovery of the $Z_2$ quantum-spin liquid state in the nearest-neighbor Heisenberg model on the kagome lattice, we investigate the “even-odd” effect occurring when this state is confined to infinitely long cylinders of finite circumference. We pursue a dual analysis, where we map the effective $Z_2$ gauge theory from the kagome lattice to a frustrated Ising model on the dice lattice. Unexpectedly, we find that the latter theory, if restricted to nearest-neighbor interactions, is insufficient to capture this effect. We provide an explanation of why further neighbor interactions are needed via a high-temperature expansion of the effective Hamiltonian. We then carry out projective symmetry group analysis to understand which second-neighbor interactions can be introduced while respecting the lattice symmetries. Finally, we qualitatively compare our results to numerics by computing the dimerization operator within our theory. Systems with odd circumferences exhibit a nonvanishing dimerization that decays exponentially with circumference.