Graphene subject to a strong, tilted magnetic field exhibits an insulator-metal transition tunable by tilt angle, attributed to the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at filling factor $\nu =0$. We develop a theoretical description for the spin and valley edge textures in the two phases, and the implied evolution in the nature of edge modes through the transition. In particular, we show that the CAF has gapless neutral modes in the bulk, but supports gapped charged edge modes. At the transition to the FM state the charged edge modes become gapless and are smoothly connected to the helical edge modes of the FM state. Possible experimental consequences are discussed.

We derive the asymptotics of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in the graphene Bernal bilayer using a four-band model of the bilayer spectrum. We find three distinct regimes depending on the position of the Fermi energy in the bilayer spectrum: in the bonding-antibonding gap, at the gap edge, and outside the gap. In particular, for impurities on the bilayer bonding sublattice (the “$A$” sublattice) and Fermi energies close to the bonding-antibonding gap edge $E_g$, we identify a (integrable) logarithmic divergence of the integrand of the RKKY exchange integral. This divergence drives a number of novel RKKY effects for impurities on the $A$ sublattice: (i) an asymptotic $R^{-5/2}$ term at the gap edge and (ii) a derivative discontinuity in RKKY interaction as a function of the Fermi energy at the gap edge. In the case of intercalated impurities (impurities between the two layers of the bilayer), we find a remarkable discontinuity in the period of the RKKY oscillation at the gap edge. The period of the oscillation limits to $\lambda =\sqrt{2}\pi \hslash v_F/t_{\perp }$ as $E_F\to E_g$ from below the gap edge, while it limits to $\lambda \to \infty$ if $E_F\to E_g$ from above the gap edge $(t_{\perp }$ is the interlayer coupling, $v_F$ the Fermi velocity of graphene). The origin of this discontinuity we attribute to (i) the $A$ sublattice divergence and (ii) interference effects driven by the intrinsic valley degree of freedom of graphene. On this basis, we predict that the magnetic response of intercalated bilayer graphene will show a profound sensitivity to doping for Fermi energies near the bonding-antibonding gap edge.

Graphene superlattices have been extensively investigated and exhibit various emergent single-particle properties in addition to those of pristine graphene, yet to date, the collective plasmonic behaviors of the Dirac electrons in such systems remain largely unexplored. Here we use a microscopic description to explore the quantum plasmonic properties of one-dimensional graphene superlattices under physically realistic conditions. The emerging additional Dirac points at nonzero electron energies caused by the chiral nature of graphene carriers lead to interband excitations between the induced minibands. Such segments of the electron-hole pair continuum are strongly coupled to the plasmon modes along the superlattice direction. As a result, there exist a spectrum of emergent quantum plasmonic effects, including enhanced damping, multiple plasmon modes at the same wave vector, and oscillatory plasmon energies with respect to the sweeping Fermi level. In contrast, the coupling along the perpendicular direction is much weaker, and the plasmonic dispersion and damping remain largely intact as those of pristine graphene. These findings of fundamental nature may also offer additional technological potentials in graphene optoelectronics, such as plasmonic waveguides.

High pressure x-ray powder-diffraction experiments were performed on europium metal up to $\sim 70\mathrm{GPa}$. Above 38 GPa, europium transforms from the incommensurately modulated Eu-IV phase to a second phase with an incommensurately modulated crystal structure, Eu-V. This is a previously unseen incommensurately modulated to incommensurately modulated transition in the elements at high pressure. High-pressure high-temperature experiments were also performed up to 449 K in order to make an initial estimate of the positions of the phase boundaries of the incommensurate phases.

A scanning tunneling microscopy study of a planar close-packed ${\mathrm{C}}_{60}$ hexagonal molecular layer on a ${\mathrm{WO}}_2/\mathrm{W}\left(110\right)$ substrate reveals the existence of ${\mathrm{C}}_{60}$ domains exhibiting two preferred orientations at an angle with an underlying periodic groove structure in the substrate. An analysis of the van der Waals interactions between substrate and layer retrieves the observed misorientations as those corresponding to minima in the interaction energy of the substrate-layer system. The misorientation between two ${\mathrm{C}}_{60}$ domains is accommodated in a tilt boundary by a linear array of molecular structural units identified as disclination dipoles, i.e., rotational defects in the hexagonal structure of the layer. A field theory of disclinations and dislocations is used to construct maps of the elastic energy, strains, curvatures, and stresses induced by the lattice defects over the layer. The predicted regions of high compression are found to overlap with those where the fullerene molecules do not undergo rotation.

We propose a model ferromagnet based on the Dirac Hamiltonian in three spatial dimensions, and study its transport properties which include anisotropic magnetoresistance (AMR) and anomalous Hall (AH) effect. This relativistic extension allows two kinds of ferromagnetic order parameters, denoted by $\mathit{M}$ and $\mathit{S}$, which are distinguished by the relative sign between the positive- and negative-energy states (at zero momentum) and become degenerate in the nonrelativistic limit. Because of the relativistic coupling between the spin and the orbital motion, both $\mathit{M}$ and $\mathit{S}$ induce anisotropic deformations of the energy dispersion (and the Fermi surfaces) but in mutually opposite ways. The AMR is determined primarily by the anisotropy of the Fermi surface (group velocity), and secondarily by the anisotropy of the damping; the latter becomes important for $\mathit{M}=\pm \mathit{S}$, where the Fermi surfaces are isotropic. Even when the chemical potential lies in the gap, the AH conductivity is found to take a finite nonquantized value ${\sigma }_{ij}=-(\alpha /3{\pi }^2\hslash ){\epsilon }_{ijk}{S}_k$, where $\alpha$ is the (effective) fine structure constant. This offers an example of Hall insulator in three spatial dimensions.

We measure the low-frequency thermal fluctuations of pure spin current in a platinum film deposited on yttrium iron garnet via the inverse spin Hall effect (ISHE)-mediated voltage noise as a function of the angle $\alpha$ between the magnetization and the transport direction. The results are consistent with the fluctuation-dissipation theorem in terms of the recently discovered spin Hall magnetoresistance (SMR). We present a microscopic description of the $\alpha$ dependence of the voltage noise in terms of spin-current fluctuations and ISHE.

Much interest in the superconducting proximity effect in three-dimensional (3D) topological insulators (TIs) has been driven by the potential to induce Majorana bound states at the interface. Most candidate materials for 3D TI, however, are bulk metals, with bulk states at the Fermi level coexisting with well-defined surface states exhibiting spin-momentum locking. In such topological metals, the proximity effect can differ qualitatively from that in TIs. By studying a model topological metal-superconductor (TM-SC) heterostructure within the Bogoliubov–de Gennes formalism, we show that the pair amplitude reaches the naked surface, unlike in a topological insulator-superconductor (TI-SC) heterostructure where it is confined to the interface. Furthermore, we predict vortex-bound-state spectra to contain a Majorana zero mode localized at the naked surface, separated from the bulk vortex-bound-state spectra by a finite energy gap in such a TM-SC heterostructure. These naked-surface-bound modes are amenable to experimental observation and manipulation, presenting advantages of TM-SC over TI-SC.

Superconducting fluctuations were studied through angular-dependent torque measurements on a series of ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_{6+\delta }$ (BSLCO) single crystals. Two distinct superconducting fluctuation diamagnetisms were observed: one is the superconducting thermal fluctuation, with a boundary close to the superconducting phase boundary, while the other one, up to a temperature as high as about 180 K, showing maximum signal in the sample with hole carrier density $p=0.125$, could be due to preformed pairs. In addition, we observed linearly temperature-dependent paramagnetic torque signals in BSLCO samples, possibly a result of quantum criticality from a quantum critical point at the optimal doping.

We have studied ab initio the phase transition in ${\mathrm{PbZrO}}_3$, a perovskite oxide usually presented as the prototypic antiferroelectric material. Our work reveals the crucial role that antiferrodistortive modes—involving concerted rotations of the oxygen octahedra in the structure—play in the transformation, as they select the observed antiferroelectric phase, among competing structural variants, via a cooperative trilinear coupling.

We present a perturbative treatment of the subohmic spin-boson model which remedies a crucial flaw in previous treatments. The problem is traced back to the incorrect application of a Markov type approximation to specific terms in the temporal evolution of the reduced density matrix. The modified solution is consistent both with numerical simulations and the exact solution obtained when the bath-coupling spin-space direction is parallel to the qubit energy-basis spin. We therefore demonstrate that the subohmic spin-boson model is capable of describing arbitrarily small ratios of the $T_2$ and $T_1$ decoherence times, associated with the decay of the off-diagonal and diagonal reduced density-matrix elements, respectively. An analytical formula for $T_2/T_1$ at the absolute zero of temperature is provided in the limit of a subohmic bath with vanishing spectral power law exponent. Small ratios closely mimic the experimental results for solid state (flux) qubits, which are subject predominantly to low-frequency electromagnetic noise, and we suggest a reanalysis of the corresponding experimental data in terms of a nonanalytic decay of off-diagonal coherence.

Bulk magnetization, transport, and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, ${\mathrm{CeO}}_{0.3}{\mathrm{F}}_{0.7}{\mathrm{BiS}}_2$. Ferromagnetism develops below $T_{FM}=6.54\left(8\right)\mathrm{K}$ and superconductivity is found to coexist with the ferromagnetic state below $T_{SC}\sim 4.5\mathrm{K}$. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the $c$ axis to the $ab$ plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.

We studied the thermal conductivity of various transition-metal oxides with the spinel structure $({\mathrm{MnV}}_2{\mathrm{O}}_4$, ${\mathrm{FeV}}_2{\mathrm{O}}_4$, ${\mathrm{CoV}}_2{\mathrm{O}}_4$, and ${\mathrm{Mn}}_3{\mathrm{O}}_4)$ upon varying the temperature and magnetic field. We found that for the spinel oxides having ${\mathrm{V}}^{3+}$ ions (two electrons in the $t_{2g}$ states) at the octahedral site, the orbital ordering suppresses the thermal resistivity (inverse thermal conductivity) in contrast to purely magnetic ordering, indicating that the orbital fluctuation of the V $t_{2g}$ states is the main factor affecting the thermal conductivity. On the other hand, for ${\mathrm{Mn}}_3{\mathrm{O}}_4$, which has ${\mathrm{Mn}}^{3+}$ ions (one electron in the $e_g$ states) at the octahedral site, the thermal resistivity is suppressed in association with the successive magnetic phase transitions with decreasing temperature and increasing magnetic field. This can be explained by the frustration of ${\mathrm{Mn}}^{3+}$ spins and the fluctuation of ${\mathrm{Mn}}^{3+}$ $e_g$ orbitals.

We present a detailed study of the phase diagram surrounding the skyrmion lattice (SkL) phase of ${\mathrm{Cu}}_2{\mathrm{OSe}}_2{\mathrm{O}}_3$ using high-precision magnetic ac susceptibility measurements. An extensive investigation of transition dynamics around the SkL phase using the imaginary component of the susceptibility revealed that at the conical-to-SkL transition a broad dissipation region exists with a complex frequency dependence. The analysis of the observed behavior within the SkL phase indicates a distribution of relaxation times intrinsically related to SkL. At the SkL-to-paramagnet transition a narrow first-order peak is found that exhibits a strong frequency and magnetic field dependence. Surprisingly, very similar dependence has been discovered for the first-order transition below the SkL phase, i.e., where the system enters the helical and conical state(s), indicating similar processes across the order-disorder transition.

The oxypnictides ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{MnAsO}$ have been successfully synthesized with $x$ up to 0.1. A synchrotron x-ray diffraction study demonstrates that there is no change in crystal symmetry upon doping with Sr. An expansion of the interlayer distance between Nd-O-Nd and As-Mn-As blocks is observed with increasing $x$. Results from variable temperature neutron diffraction and resistivity measurements show that the local moment antiferromagnetic order of the Mn spins is preserved as the ${\left[\mathrm{MnAs}\right]}^{-}$ layers are hole doped and the materials are driven metallic for $x\ge$ 0.05. A sizable positive magnetoresistance is observed at low temperature which demonstrates that multiple MR mechanisms are possible in LnMnAsO oxypnictides.

The characterization and applications of topological insulators depend critically on their protected surface states, which, however, can be obscured by the presence of trivial dangling bond states. Our first-principle calculations show that this is the case for the pristine $\left(111\right)$ surface of SnTe. Yet, the predicted surface states unfold when the dangling bond states are passivated in proper chemisorption. We further extract the anisotropic Fermi velocities, penetration lengths, and anisotropic spin textures of the unfolded $\overline{\Gamma }$- and $\overline{M}$-surface states, which are consistent with the theory in Zhang et al. [Phys. Rev. B 86, 081303 (2012)]. More importantly, this chemisorption scheme provides an external control of the relative energies of different Dirac nodes, which is particularly desirable in multivalley transport.

We report high-resolution angle-resolved photoelectron spectroscopy (ARPES) results on the (001) cleavage surface of ${\mathrm{YbB}}_6$, a rare-earth compound that has been recently predicted to host surface electronic states with topological character. We observe two types of well-resolved metallic states, whose Fermi contours encircle the time-reversal invariant momenta of the ${\mathrm{YbB}}_6\left(001\right)$ surface Brillouin zone, and whose full $E\left(k\right)$ dispersion relation can be measured wholly unmasked by states from the rest of the electronic structure. Although the two-dimensional character of these metallic states is confirmed by their lack of out-of-plane dispersion, our work reveals two aspects which were not observed in previous experiments. First, these states do not resemble two branches of opposite, linear velocity that cross at a Dirac point, but rather straightforward parabolas that terminate to high binding energy with a clear band bottom. Secondly, these states are sensitive to time-dependent changes of the ${\mathrm{YbB}}_6$ surface under ultrahigh-vacuum conditions. Adding the fact that these data from cleaved ${\mathrm{YbB}}_6$ surfaces also display spatial variations in the electronic structure, it appears there is little in common between the theoretical expectations for an idealized ${\mathrm{YbB}}_6\left(001\right)$ crystal truncation on the one hand, and these ARPES data from real cleavage surfaces on the other.

The theoretical models of radiative recombinations in both ${\mathrm{CuIn}}_{1-x}{\mathrm{Ga}}_x{\mathrm{Se}}_2$ chalcopyrite and ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ kesterite, and related compounds, were revised. For heavily doped materials, electrons are free or bound to large donor agglomerates which hinders the involvement of single donors in the radiative recombination channels. In this work, we investigated the temperature and excitation power dependencies of the photoluminescence of ${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$-based solar cells in which the absorber layer was grown through sulphurization of multiperiod structures of precursor layers. For both samples the luminescence is dominated by an asymmetric band with peak energy at $\sim 1.22$ eV, which is influenced by fluctuating potentials in both conduction and valence bands. A value of $\sim 60$ meV was estimated for the root-mean-square depth of the tails in the conduction band. The radiative transitions involve the recombination of electrons captured by localized states in tails of the conduction band with holes localized in neighboring acceptors that follow the fluctuations in the valence band. The same acceptor level with an ionization energy of $\sim 280$ meV was identified in both absorber layers. The influence of fluctuating potentials in the electrical performance of the solar cells was discussed.

Magnetic interactions among substitutional dopant impurities in silicon nanowires are investigated theoretically using density functional calculations. Our results show that while dopant impurities in silicon nanowires have no magnetic ordering in the ground state, a magnetic moment imposed at an impurity by applying an effective local magnetic field induces a magnetic moment, smaller in magnitude and opposite in sign, at an adjacent impurity, demonstrating an antiferromagnetic coupling between the impurity spins. The sign of the calculated Heisenberg exchange parameter $J$ between the impurity spins also corresponds to the antiferromagnetic coupling and its magnitude decreases monotonically as the distance between impurities increases. Our results suggest that, while there is no static magnetic moment on dopant impurity atoms, a direct exchange interaction between impurity states of the dopants may result in an instantaneous short-range antiferromagnetic correlation between the impurity spins, confirming the suggestion from a recent experimental work observing the Kondo-like temperature dependence of the electrical resistance of doped silicon nanowires.