Interfaces between a bulk-insulating topological insulator (TI) and metallic adatoms have been studied using high-resolution, angle-resolved, and core-level photoemission. Fe, Nb, and Ag were evaporated onto ${\mathrm{Bi}}_{\mathbf{1}.\mathbf{5}}{\mathrm{Sb}}_{\mathbf{0}.\mathbf{5}}{\mathrm{Te}}_{\mathbf{1}.\mathbf{7}}{\mathrm{Se}}_{\mathbf{1}.\mathbf{3}}$ (BSTS) surfaces both at room temperature and 38 K. The coverage and temperature dependence of the adsorption and interfacial formation process have been investigated, highlighting the effects of the overlayer growth on the occupied electronic structure of the TI. For all coverages at room temperature and for those equivalent to less than 0.2 monolayer at low temperature all three metals lead to a downward shift of the TI bands with respect to the Fermi level. At room temperature Ag appears to intercalate efficiently into the van der Waals gap of BSTS, accompanied by low-level substitution for the Te/Se atoms of the termination layer of the crystal. This Te/Se substitution with silver increases significantly for low temperature adsorption, and can even dominate the electrostatic environment of the Bi/Sb atoms in the BSTS near-surface region. On the other hand, Fe and Nb evaporants remain close to the termination layer of the crystal. On room temperature deposition, they initially substitute isoelectronically for Bi as a function of coverage, before substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too immobile for substitution processes and show a behavior consistent with clustering on the surface. For both Ag and Fe/Nb, these differing adsorption pathways still lead to the qualitatively similar and remarkable behavior for low temperature deposition that the chemical potential first moves downward ($p$-type dopant behavior) and then upward ($n$-type behavior) on increasing coverage.

One of the challenges in valleytronics is how to effectively manipulate the valley degree of freedom of electrons in graphene. We propose to use a pseudovalley exchange field to rotate the valley, which arises in a graphene superlattice (GS) structure due to the two inequivalent $K$ and $K^{\prime }$ valleys folded and coupled together. The valley is shown to precess periodically in real space, but the precession itself has a spatial anisotropy unless the GS structure still possesses a rotational symmetry. The pseudovalley exchange field exclusively determines the valley precession periodicity when the electron energy is much larger than the possible GS energy gap. Our findings provide a practical way to control and manipulate the valley degree of freedom in graphene.

For spin-polarized low-energy electrons impinging on a crystalline surface, an important reaction channel is the collision with a bound valence electron of opposite spin, followed by the emission of a correlated electron pair with antiparallel spins. While primary and valence electrons are not entangled, the screened Coulomb interaction generates spin entanglement between the two outgoing electrons. As a quantitative measure of this entanglement, we calculated a modified von Neumann entropy in terms of direct and exchange transition matrix elements. For coplanar symmetric setups with equal energies of antiparallel-spin electrons, maximal entanglement is analytically shown to occur quite universally, irrespective of the choice of the primary electron energy, the outgoing electron energy, and polar emission angle, and even of the choice of the surface system. Numerical results for Fe(110) and Cu(111) demonstrate first that strong entanglement can persist for unequal energies and second that an overall entanglement reduction due to nonentangled parallel-spin electrons can be avoided for ferromagnetic and even for nonmagnetic surfaces.

We show that the effective hyperfine interaction for heavy-hole states (or any particle described with a $p$-like Bloch function) can induce nontrivial dynamics of nuclear spins. Experimental evidence can be found, e.g., in self-assembled quantum dots by measuring the saturation of nuclear spin polarization with different orientations of an external magnetic field.

The Bethe-Salpeter equation (BSE) is currently the state of the art in the description of neutral electronic excitations in both solids and large finite systems. It is capable of accurately treating charge-transfer excitations that present difficulties for simpler approaches. We present a local basis set formulation of the BSE for molecules where the optical spectrum is computed with the iterative Haydock recursion scheme, leading to a low computational complexity and memory footprint. Using a variant of the algorithm we can go beyond the Tamm-Dancoff approximation. We rederive the recursion relations for general matrix elements of a resolvent, show how they translate into continued fractions, and study the convergence of the method with the number of recursion coefficients and the role of different terminators. Due to the locality of the basis functions the computational cost of each iteration scales asymptotically as $O\left(N^3\right)$ with the number of atoms, while the number of iterations typically is much lower than the size of the underlying electron-hole basis. In practice we see that, even for systems with thousands of orbitals, the runtime will be dominated by the $O\left(N^2\right)$ operation of applying the Coulomb kernel in the atomic orbital representation.

We show that double-layer graphene (DLG), where an external potential induces a charge imbalance between $n$- and $p$-type layers, is a promising candidate to realize an exciton condensate in equilibrium. To prove this phenomenon experimentally, we suggest coupling two DLG systems, separated by a thin insulating barrier, and measuring the excitonic Josephson effect. For this purpose we calculate the ac and dc Josephson currents induced by tunneling excitons and show that the former only occurs when the gate potentials of the DLG systems differ, irrespective of the phase relationship of their excitonic order parameters. A dc Josephson current develops if a finite order-parameter phase difference exists between two coupled DLG systems with identical gate potentials.

The local-density approximation combined with the dynamical mean-field theory $\left(\text{LDA}+\text{DMFT}\right)$ is applied to the paramagnetic phase of light actinide dioxides: ${\mathrm{UO}}_2,{\mathrm{NpO}}_2$, and ${\mathrm{PuO}}_2$. The calculated band gaps and the valence-band electronic structure are in very good agreement with the optical absorption experiments as well as with the photoemission spectra. The hybridization of the actinide $5f$ shell with the $2p$ states of oxygen is found to be relatively large; it increases the filling of the $5f$ orbitals from the nominal ionic configurations with two, three, and four electrons to nearly half-integer values 2.5, 3.4, and 4.4. The large hybridization leaves an imprint also on the core-level photoemission spectra in the form of satellite peaks. It is demonstrated that these satellites are accurately reproduced by the $\text{LDA}+\text{DMFT}$ calculations.

We show that the local in-gap Green's function of a band insulator ${\mathbf{G}}_0(\epsilon ,{\mathbf{k}}_{\parallel },{\mathbf{r}}_{\perp }=0)$, with ${\mathbf{r}}_{\perp }$ the position perpendicular to a codimension-1 or codimension-2 impurity, reveals the topological nature of the phase. For a topological insulator, the eigenvalues of this Green's function attain zeros in the gap, whereas for a trivial insulator the eigenvalues remain nonzero. This topological classification is related to the existence of in-gap bound states along codimension-1 and codimension-2 impurities. Whereas codimension-1 impurities can be viewed as soft edges, the result for codimension-2 impurities is nontrivial and allows for a direct experimental measurement of the topological nature of two-dimensional insulators.

Using density-functional theory, we have studied the electronic and magnetic properties of two promising compounds that can be used as cathode materials, namely, ${\mathrm{Na}}_2{\mathrm{Fe}}_2{\mathrm{P}}_3{\mathrm{O}}_9\mathrm{N}$ and ${\mathrm{Na}}_3{\mathrm{TiP}}_3{\mathrm{O}}_9\mathrm{N}$. When Na is extracted, we found the volume change to be quite small, with values of $\sim -0.6\%$ for ${\mathrm{Na}}_3{\mathrm{TiP}}_3{\mathrm{O}}_9\mathrm{N}$ and $-5\%$ for ${\mathrm{Na}}_2{\mathrm{Fe}}_2{\mathrm{P}}_3{\mathrm{O}}_9\mathrm{N}$. Our calculated voltages with the Hubbard-type correction (GGA+U) approximation are 2.93 V for ${\mathrm{Na}}_3{\mathrm{TiP}}_3{\mathrm{O}}_9\mathrm{N}/{\mathrm{Na}}_2{\mathrm{TiP}}_3{\mathrm{O}}_9\mathrm{N}$ and 2.68 V for ${\mathrm{Na}}_2{\mathrm{Fe}}_2{\mathrm{P}}_3{\mathrm{O}}_9\mathrm{N}/{\mathrm{NaFe}}_2{\mathrm{P}}_3{\mathrm{O}}_9\mathrm{N}$, in good agreement with the experimental data. Our results confirm that these compounds are very promising for rechargeable Na-ion batteries.

The single-particle spectral properties of the two-dimensional $t\text{−}J$ model in the parameter regime relevant to cuprate high-temperature superconductors are investigated using cluster perturbation theory. Various anomalous features observed in cuprate high-temperature superconductors are collectively explained in terms of the dominant modes near the Mott transition in this model. Although the behavior of the dominant modes in the low-energy regime is similar to that in the two-dimensional Hubbard model, significant differences appear near the Mott transition for the high-energy electron removal excitations which can be considered to primarily originate from holon modes in one dimension. The overall spectral features are confirmed to remain almost unchanged as the cluster size is increased from $4\times 4$ to $6\times 6$ sites by using a combined method of the non-Abelian dynamical density-matrix renormalization group method and cluster perturbation theory.

In strongly correlated electron systems, the emergence of states in the Mott gap in the single-particle spectrum following the doping of the Mott insulator is a remarkable feature that cannot be explained in a conventional rigid-band picture. Here, based on an analysis of the quantum numbers and the overlaps of relevant states, as well as through a demonstration using the ladder and bilayer $t\text{−}J$ models, it is shown that in a continuous Mott transition due to hole doping, the magnetically excited states of the Mott insulator generally emerge in the electron-addition spectrum with the dispersion relation shifted by the Fermi momentum in the momentum region where the lower Hubbard band is not completely filled. This implies that the dispersion relation of a free-electron-like mode in the electron-addition spectrum eventually transforms into essentially the momentum-shifted magnetic dispersion relation of the Mott insulator, while its spectral weight gradually disappears toward the Mott transition. This feature reflects the spin-charge separation of the Mott insulator.

We present an approach for controlling quantum coherences in condensed matter by interaction with a nonresonant optical control field. Coherent intersubband (IS) excitations of electrons in GaAs/AlGaAs quantum wells are manipulated by a strong nonresonant terahertz (THz) field as demonstrated by phase-resolved two-color two-dimensional spectroscopy. In the linear regime of IS response, we observe a THz-induced enhancement of the midinfrared (MIR) IS absorption and a dispersive perturbed free induction decay caused by a THz-induced blueshift of the IS polarization. In the regime of IS Rabi oscillations, the THz field causes pronounced phase shifts of the coherently emitted MIR field, while the IS Rabi frequency remains unaffected. Such behavior is accounted for by a full solution of the Maxwell-Bloch equations, treating the THz and MIR fields without approximations. Our control scheme paves the way for THz control of IS emitters and holds potential for an extension to other systems.

Freestanding ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single-crystal flakes of variable thicknesses are grown using a catalyst-free vapor-solid synthesis and are subsequently transferred onto a clean ${\mathrm{Si}}^{++}/{\mathrm{SiO}}_2$ substrate where the flakes are contacted in Hall bar geometry. Low-temperature magnetoresistance measurements are presented which show a linear magnetoresistance for high magnetic fields and weak antilocalization (WAL) at low fields. Despite an overall strong charge-carrier tunability for thinner devices, we find that electron transport is dominated by bulk contributions for all devices. Phase-coherence lengths $l_{\varphi }$ as extracted from WAL measurements increase linearly with increasing electron density exceeding $1\mu \mathrm{m}$ at 1.7 K. Although $l_{\varphi }$ is in qualitative agreement with electron-electron interaction-induced dephasing, we find that spin-flip scattering processes limit $l_{\varphi }$ at low temperatures.

We consider theoretically a wide graphene ribbon that is attached on both ends to electronic reservoirs, which generally have different temperatures. The graphene ribbon is assumed to be deposited on a substrate that leads to a spin-orbit coupling of the Rashba type. We calculate the thermally induced charge current in the ballistic transport regime as well as the thermoelectric voltage (Seebeck effect). Apart from this, we also consider thermally induced spin current and spin polarization of the graphene ribbon. The spin currents are shown to have generally two components; one parallel to the temperature gradient and the other perpendicular to this gradient. The latter corresponds to the spin current due to the spin Nernst effect. Additionally, we also consider the heat current between the reservoirs due to transfer of electrons.

Motivated by recent experiments on the Heisenberg $S=1/2$ quantum spin liquid candidate material kapellasite, we classify all possible chiral (time-reversal symmetry breaking) spin liquids with fermionic spinons on the kagome lattice. We obtain the phase diagram for the physically relevant extended Heisenberg model, comparing the energies of a wide range of microscopic variational wave functions. We propose that, at low temperature, kapellasite exhibits a gapless chiral spin liquid phase with spinon Fermi surfaces. This two-dimensional state inherits many properties of the nearby one-dimensional phase of decoupled antiferromagnetic spin chains, but also shows some remarkable differences. We discuss the spin structure factors and other physical properties.

We study the fate of the so-called ${\mathrm{\Theta }}_{II}$-loop-current order that breaks both time-reversal and parity symmetries in a two-dimensional hot spot model with antiferromagnetically mediated interactions, using Fermi surfaces relevant to the phenomenology of the cuprate superconductors. We start from a three-band Emery model describing the hopping of holes in the ${\mathrm{CuO}}_2$ plane that includes two hopping parameters $t_{pp}$ and $t_{pd}$, local onsite Coulomb interactions $U_d$ and $U_p$, and nearest-neighbor $V_{pd}$ couplings between the fermions in the copper $\left[\mathrm{Cu}\left(3d_{x^2-y^2}\right)\right]$ and oxygen $[\mathrm{O}\left(2p_x\right)$ and $\mathrm{O}\left(2p_y\right)]$ orbitals. By focusing on the lowest-energy band, we proceed to decouple the local interaction $U_d$ of the Cu orbital in the spin channel using a Hubbard-Stratonovich transformation to arrive at the interacting part of the so-called spin-fermion model. We also decouple the nearest-neighbor interaction $V_{pd}$ to introduce the order parameter of the ${\mathrm{\Theta }}_{II}$-loop-current order. In this way, we are able to construct a consistent mean-field theory that describes the strong competition between the composite order parameter made of a quadrupole-density wave and $d$-wave pairing fluctuations proposed in Efetov et al. [Nat. Phys. 9, 442 (2013)] with the ${\mathrm{\Theta }}_{II}$-loop-current order parameter that is argued to be relevant for explaining important aspects of the physics of the pseudogap phase displayed in the underdoped cuprates.