Using the functional renormalization group technique we demonstrate a route for potentially high-temperature odd-parity superconductivity in ferromagnetic materials caused by repulsive electron interactions, where the superconducting pairing is driven by charge-density wave fluctuations. Our model is directly applicable to a lightly cobalt-doped ${\mathrm{LaNiO}}_3$ bilayer grown in the $\left(111\right)$ direction. As the on-site repulsive interaction grows, a charge-density wave state with a charge pattern that respects all point-group symmetries of the bilayer is replaced by a superconducting state with an $f$-wave pairing.

We study hard-core bosons on a class of frustrated lattices with the lowest Bloch band having a degenerate minimum along a closed contour, the moat, in the reciprocal space. We show that at small density the ground state of the system is given by a noncondensed state, which may be viewed as a state of fermions subject to Chern-Simons gauge field. At fixed density of bosons, such a state exhibits domains of incompressible liquids. Their fixed densities are given by fractions of the reciprocal-space area enclosed by the moat.

The temperature dependent exciton transport in the prototypical organic semiconductor di-indeno-perylene (DIP) is investigated by photoluminescence quenching. Analysis by an advanced diffusion model including interference and morphological aspects reveals an exciton diffusion length of about 60 nm at room temperature, which relates to the long-range order induced by the DIP molecular shape. Above 80 K, singlet exciton transport is thermally activated with an energy of 10 to 20 meV. Below 80 K, exciton motion becomes temperature independent and is supported by the crystalline structure of the transport layer in combination with the reduced phonon interaction.

We report magnetization, electron-spin resonance (ESR), and muon-spin relaxation ($\mu$SR) measurements on single crystals of the $S=1/2$ (${\mathrm{Cu}}^{+2}$) kagome compound Cu(1,3-benzendicarboxylate). The $\mu$SR is carried to temperatures as low as 45 mK. The spin-Hamiltonian parameters are determined from the analysis of the magnetization and ESR data. We find that this compound has anisotropic ferromagnetic interactions. Nevertheless, no spin freezing is observed even at temperatures two orders of magnitude lower than the coupling constants. In light of this finding, the relation between persistent spin dynamics and spin liquids on kagome lattices is reexamined.

Structural phase transitions in ${\mathrm{Pr}}_{1\text{−}x}{\mathrm{La}}_x\mathrm{Al}{\mathrm{O}}_3$ (0 ≤ $x$ ≤ 1) single crystals have been studied through heat capacity and high-resolution x-ray scattering measurements. For $\mathrm{Pr}\mathrm{Al}{\mathrm{O}}_3$, the heat capacity shows a sharp first-order peak at the rhombohedral to orthorhombic transition, while a classic mean-field anomaly is observed at the orthorhombic to monoclinic transition at lower temperatures. The transition temperature and the heat capacity anomaly of the two transitions diminish with increasing $x$, and only a single rhombohedral to monoclinic transition is observed for $x$ = 0.8. Although this latter transition is required by group theory to be first order, no such evidence is found in the heat capacity and x-ray scattering measurements. Instead, the results are consistent with mean-field criticality, with a small distribution of transition temperature originating from weak disorder.

A linear-response formulation of the dc spin pumping, i.e., a spin injection from a precessing ferromagnet into an adjacent spin sink, is developed in view of describing many-body effects caused by spin fluctuations in the spin sink. It is shown that when an itinerant ferromagnet near $T_{\mathrm{C}}$ is used as the spin sink, the spin pumping is largely increased owing to the fluctuation enhancement of the spin conductance across the precessing ferromagnet/spin sink interface. As an example, the enhanced spin pumping from yttrium iron garnet into nickel palladium alloy ($T_C\simeq 20$ K) is analyzed by means of a self-consistent renormalization scheme, and it is predicted that the enhancement can be as large as tenfold.

$\mathrm{Pb}{\mathrm{Fe}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_3$ (PFT) belongs to the family of $\mathrm{Pb}{B}_x^{\prime }{B}_{1-x}^{″}{\mathrm{O}}_3$ which have inherent chemical disorder at the $B$ site. Due to this disorder, a complex magnetic phase diagram is expected in the material. In this paper, we report experimental results of magnetic properties of PFT through macroscopic characterization, neutron scattering, and Mössbauer spectroscopy techniques. With these results we show for the first time that $\mathrm{Pb}{\mathrm{Fe}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_3$ behaves very similar to $\mathrm{Pb}{\mathrm{Fe}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_3$; i.e., it undergoes antiferromagnetic transition at 153 K and has a spin-glass transition at 10 K, below which the antiferromagnetism coexists with the spin glass. We suggest that the mechanism which is responsible for such a nontrivial ground state can be explained by a speromagnetlike spin arrangement similar to the one proposed for $\mathrm{Pb}{\mathrm{Fe}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_3$.

Cubic helimagnets such as MnSi and FeGe have provided paradigmatic cases of pressure-induced collapse of band ferromagnetism in metals, accompanied with exotic partial order and chiral spin textures. Isostructural MnGe stands out owing to its much shorter helix pitch and high magnetic moment. By combining resistivity, ac susceptibility, and neutron diffraction measurements under very high pressure, we show that the helical order in MnGe transforms around 6 GPa from a high-spin to a low-spin state, recalling the weak ferromagnetism of MnSi at ambient pressure. Helical order collapses only above 10 GPa. The spin-state transition is supported by ab initio calculations.

$R_2$${\mathrm{RhIn}}_8$ compounds (space group $P4/mmm$, $R$ is a rare-earth element) belong to a large group of structurally related tetragonal materials which involves several heavy-fermion superconductors based on Ce. We have succeeded in growing single crystals of compounds with Nd, Dy, and Er, and following our previous bulk measurements, we performed neutron-diffraction studies to determine their magnetic structures. The Laue diffraction experiment showed that the antiferromagnetic order below the Néel temperature is in all three compounds characterized by the propagation vector $\mathbf{k}=(1/2,1/2,1/2)$. The amplitude and direction of the magnetic moments, as well as the invariance symmetry of the magnetic structure, were determined by subsequent experiments using two- and four-circle diffractometers. The critical exponents were determined from the temperature dependence of the intensities below $T_N$.

We argue that the binding between doubly occupied (doublon) and empty (holon) sites governs incoherent excitations and plays a key role in the Mott transition in strongly correlated Mott-Hubbard systems. We construct a new saddle-point solution with doublon-holon binding in the Kotliar-Ruckenstein slave-boson functional integral formulation of the Hubbard model. On a half-filled honeycomb lattice and square lattice, the ground state is found to exhibit a continuous transition from a paramagnetic semimetal/metal to an antiferromagnetic-ordered Slater insulator with coherent quasiparticles at $U_{c1}$, followed by a Mott transition into an electron-fractionalized AF${}^{*}$ phase without coherent excitations at $U_{c2}$. Such a phase structure appears to be generic for bipartite lattices without frustration. We show that doublon-holon binding unites the three important ideas of strong correlation: coherent quasiparticles, incoherent Hubbard bands, and deconfined Mott insulator.

We elucidate how Chern and topological insulators fulfill an area law for the entanglement entropy. By explicit construction of a family of lattice Hamiltonians, we are able to demonstrate that the area law contribution can be tuned to an arbitrarily small value but is topologically protected from vanishing exactly. We prove this by introducing novel methods to bound entanglement entropies from correlations using perturbation bounds, drawing intuition from ideas of quantum information theory. This rigorous approach is complemented by an intuitive understanding in terms of entanglement edge states. These insights have a number of important consequences: The area law has no universal component, no matter how small, and the entanglement scaling cannot be used as a faithful diagnostic of topological insulators. This holds for all Renyi entropies which uniquely determine the entanglement spectrum, which is hence also nonuniversal. The existence of arbitrarily weakly entangled topological insulators furthermore opens up possibilities of devising correlated topological phases in which the entanglement entropy is small and which are thereby numerically tractable, specifically in tensor network approaches.

We investigate the correlation-induced Mott, magnetic, and topological phase transitions in artificial (111) bilayers of perovskite transition-metal oxides $\mathrm{La}\mathrm{Au}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Ir}{\mathrm{O}}_3$ for which the previous density-functional theory calculations predicted topological insulating states. Using the dynamical-mean-field theory with realistic band structures and Coulomb interactions, $\mathrm{La}\mathrm{Au}{\mathrm{O}}_3$ bilayer is shown to be far away from a Mott insulating regime, and a topological-insulating state is robust. On the other hand, $\mathrm{Sr}\mathrm{Ir}{\mathrm{O}}_3$ bilayer is on the verge of an orbital-selective topological Mott transition and turns to a trivial insulator by an antiferromagnetic ordering. Oxide bilayers thus provide a novel class of topological materials for which the interplay between the spin-orbit coupling and electron-electron interactions is a fundamental ingredient.

We investigate theoretically the fractional quantum Hall effect at half filling in the lowest Landau level observed in asymmetric wide quantum wells. The asymmetry can be achieved by a potential bias applied between the two sides of the well. Within exact-diagonalization calculations in the spherical geometry, we find that the incompressible state is described in terms of a two-component wave function. Its overlap with the ground state can be optimized with the help of a rotation in the space of the pseudospin, which mimics the lowest two electronic subbands.

We report C($1s$) and O($1s$) surface sensitive x-ray photoelectron spectroscopy (XPS) and C and O $K$-edge partial-electron yield near-edge x-ray absorption fine structure (NEXAFS) measurements for (100) and (110) oxidized diamond surfaces, etched by a laser two-photon ultraviolet (UV) desorption process. Etched regions of the (100) surface show increased oxygen coverage with a higher fraction of singly bonded termination species than unetched regions. Similar changes are observed for the (110) but with smaller magnitude. For both surfaces, no major change in $s{p}^2$ bonded carbon is observed. We show that the terminations observed for etched surfaces are consistent with the formation of oxidized {111} facets. For deeply etched samples, atomic force microscopy and scanning electron microscopy confirm the presence of {111}-like facets and reveals the development of nanoscale facetted ridges directed perpendicular to the etching beam polarization. An etching mechanism is proposed involving localized optical absorption by surface electronic states, with the probability for subsequent desorption events varying according to the relative directions of laser polarization and lattice orientation.

We develop a theory for energy and spatially resolved tunneling spectroscopy of topological quantum spin Hall helical states driven out of equilibrium. When a helical liquid is constrained between two superconducting reservoirs transport at the edge is governed by multiple Andreev reflections. The resulting quasiparticle distribution functions of the edge channels exhibit multiple discontinuities at subgap energies with the periodicity of an applied voltage. The combined effect of interactions, disorder, and normal scattering off the superconducting interface leads to the inelastic processes mixing different helicity modes, thus causing smearing of these singularities. If equilibration is strong, then the distribution functions of the edge channels tend to collapse into a Fermi-like function with an effective temperature determined by the superconducting gap, applied voltage, and intraedge interaction parameter. We conclude that mapping out nonequilibrium distribution functions may help to quantify the relative importance of various relevant perturbations that spoil ideally ballistic edge transport.

We show that the combination of a Dresselhaus interaction and a spatially periodic Rashba interaction leads to the formation of a helical liquid in a quantum wire when the electron-electron interaction is weakly screened. The effect is sustained by a helicity-dependent effective band gap which depends on the size of the Dresselhaus and Rashba spin-orbit couplings. We propose a design for a semiconductor device in which the helical liquid can be realized and probed experimentally.

Topological insulators have surface states with a remarkable helical spin structure, with promising prospects for applications in spintronics. Strategies for generating spin-polarized currents, such as the use of magnetic contacts and photoinjection, have been the focus of extensive research. While several optical methods for injecting currents have been explored, they have all focused on one-photon absorption. Here we consider the use of both a fundamental optical field and its second harmonic, which allows the injection of spin-polarized carriers and current by a nonlinear process involving quantum interference between one- and two-photon absorption. General expressions are derived for the injection rates in a generic two-band system, including those for one- and two-photon absorption processes as well as their interference. Results are given for carrier, spin density, and current injection rates on the surface of topological insulators, for both linearly and circularly polarized light. We identify the conditions that would be necessary for experimentally verifying these predictions.

Using time-resolved photoemission spectroscopy, we have investigated the nonequilibrium electronic structures of a ${\mathrm{VO}}_2$ thin film upon photoexcitation. We employed the high-harmonic generation method, which was crucial in obtaining the results. Irradiation by a 170-fs optical pulse at a fluence above $\sim$6 mJ/cm${}^2$ rapidly converts the insulating ${\mathrm{VO}}_2$ thin film into a metallic state, and we show that the transition is accompanied by a spectral-weight redistribution over a 1-eV scale. This observation provides direct spectroscopic evidence of an ultrafast insulator-to-metal transition. The transient metallic state has a unique spectrum that deviates from the static spectrum of the rutile metal phase. We also observe an anomalous spreading of spectral weight up to $\sim$0.4 eV above the Fermi level as soon as the transient metallic state emerges. The temporal evolution of the spectral weight near the Fermi level exhibits an ultrafast increase and a subsequent slower increase over $\sim$3 ps. We suggest that a broadening mechanism related to the excitation of phonon modes is responsible for the spreading of the spectral weight and that the slower increase of the spectral weight is associated with the expansion of the metallic region after photoinduced nucleation. In addition, the initial nucleation of the metallic state appears to be spatially inhomogeneous even near the surface. The results of this study highlight the uniqueness of the nonequilibrium metallic state in comparison to the equilibrium rutile metallic state from an electronic-state perspective.

Majorana bound states can emerge as zero-energy modes at the edge of a two-dimensional topological insulator in proximity to an ordinary $s$-wave superconductor. The presence of an additional ferromagnetic domain close to the superconductor can lead to their localization. We consider both normal-metal–superconductor (N-S) and Josephson (S-N-S) junctions based on helical liquids and study their spectral properties for arbitrary ferromagnetic scatterers in the normal region. Thereby, we explicitly compute Andreev wave functions at zero energy. We show under which conditions these states form localized Majorana bound states in N-S and S-N-S junctions. Interestingly, we can identify Majorana-specific signatures in the transport properties of N-S junctions and the Andreev bound levels of S-N-S junctions that are robust against external perturbations. We illustrate these findings with the example of a ferromagnetic double barrier (i.e., a quantum dot) close to the N-S boundaries.

We report sensitive detection of the nuclear quadrupolar interaction of the ${}^{14}\mathrm{N}$ nuclear spin of the nitrogen-vacancy (NV) center using the electron spin-echo envelope modulation technique. We applied a weak transverse magnetic field to the spin system so that certain forbidden transitions became weakly allowed due to second-order effects involving the nonsecular terms of the hyperfine interaction. The weak transitions cause modulation of the electron spin-echo signal, and a theoretical analysis suggests that the modulation frequency is primarily determined by the nuclear quadrupolar frequency; numerical simulations confirm the analytical results and show excellent quantitative agreement with experiments. This is an experimentally simple method of detecting quadrupolar interactions, and it can be used to study spin systems with an energy structure similar to that of the NV center.