We carry out first-principles calculations for ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4/$${\mathrm{Sr}}_3$${\mathrm{Ru}}_2$${\mathrm{O}}_7$ superlattices. We show that such systems develop a significant structural rearrangement within the superlattice, which leads to a modification of the electronic structure close to the Fermi level. Compared with the pure Sr${}_2$RuO${}_4$ and Sr${}_3$Ru${}_2$O${}_7$ phases, we find that the positions of the peaks in the density of states close to the Fermi levels get shifted and renormalized in the spectral weight. Then, by means of the maximally localized Wannier functions approach, we determine the effective tight-binding parameters for Ru bands and used them to discuss the modification of the electronic structure and the collective behavior of superlattice with respect to the bulk phases.

We consider a system consisting of a topological insulator with an array of magnetic adatoms interacting with the surface states of electrons. We find that the indirect coupling of the magnetic impurities results in a ferromagnetic ordering of the magnetic moments and is also responsible for the unusual linear dispersion of the surface magnons. We also show that the interaction of magnons with surface electrons essentially renormalizes the electron energy spectrum. The renormalized spectrum is nonlinear and can be characterized by a negative effective mass of electrons and holes for any $k\ne 0$. The electron velocity near the Dirac point depends on the electron-magnon coupling.

In order to gain new insight in the ZnO system, we performed crystal structure prediction using simulated annealing with an empirical potential and local optimization on ab initio level, both at standard and elevated pressure. We have found the experimentally observed structure types [wurtzite (B4), sphalerite (B3), and rock salt (B1)] in agreement with previous research. In addition, many new interesting modifications were found in different regions of the energy landscape, such as the β-BeO type, the GeP type, the NiAs type, and the so-called “5-5” type modification. At extreme conditions (>150 GPa), we observe a CsCl (B2) type of structure, and as a possible intermediate phase along the NaCl (B1) → CsCl (B2) transition route, we suggest the α-WC $\left(B_h\right)$ modification. Furthermore, we have investigated the electronic properties of ZnO structures. Our investigations offer new possibilities of tuning the band gap with pure zinc oxide by employing modifications with different structural arrangements.

The magnitude of spin accumulation created in semiconductors by electrical injection of spin-polarized electrons from a ferromagnetic tunnel contact is investigated, focusing on how the spin signal detected in a Hanle measurement varies with the thickness of the tunnel oxide. An extensive set of spin-transport data for Si and Ge magnetic tunnel devices reveals a scaling with the tunnel resistance that violates the core feature of available theories, namely, the linear proportionality of the spin voltage to the injected spin current density. Instead, an anomalous scaling of the spin signal with the tunnel resistance is observed, following a power law with an exponent between 0.75 and 1 over 6 decades. The scaling extends to tunnel resistance values larger than 10${}^9$ $\Omega \mu {\text{m}}^2$, far beyond the regime where the classical impedance mismatch or back flow into the ferromagnet play a role. This scaling is incompatible with existing theory for direct tunnel injection of spins into the semiconductor. It also demonstrates conclusively that the large spin signal does not originate from two-step tunneling via localized states near the oxide/semiconductor interface. Control experiments show that spin accumulation in localized states within the tunnel barrier or artifacts are also not responsible. Altogether, the scaling results suggest that, contrary to all existing descriptions, the spin signal is proportional to the applied bias voltage, rather than the (spin) current.

We study a magnetic impurity intercalated in bilayer graphene. A representative configuration generates a hybridization function with strong dependence on the conduction-electron energy, including a full gap with one hard and one soft edge. Shifts of the chemical potential via gating or doping drive the system between non-Kondo (free-moment) and Kondo-screened phases, with strong variation of the Kondo scale. Quantum phase transitions near the soft edge are of Kosterlitz-Thouless type, while others are first order. Near the hard edge, a bound-state singlet appears inside the gap; although of single-particle character, its signatures in scanning tunneling spectroscopy are very similar to those arising from a many-body Kondo resonance.

We study second harmonic generation in $h$-BN and MoS${}_2$ monolayers using an ab initio approach based on many-body theory. We show that electron-hole interaction doubles the signal intensity at the excitonic resonances with respect to the contribution from independent electronic transitions. This implies that electron-hole interaction is essential to describe second harmonic generation in those materials. We argue that this finding is general for nonlinear optical properties in nanostructures and that the present methodology is the key to disclose these effects.

The electrostatic behavior of a prototypical three-dimensional topological insulator, Bi${}_2$Se${}_3$(111), is investigated by a scanning tunneling microscopy (STM) study of the distribution of Rb atoms adsorbed on the surface. The positively charged ions are screened by both free electrons residing in the topological surface state as well as band bending induced quantum well states of the conduction band, leading to a surprisingly short screening length. Combining a theoretical description of the potential energy with STM-based atomic manipulation, we demonstrate the ability to create tailored electronic potential landscapes on topological surfaces with atomic-scale control.

The phase diagram of a quantum paraelectric antiferromagnet EuTiO${}_3$ under an external electric field is calculated using Landau-Ginzburg-Devonshire theory. The application of an electric field $E$ in the absence of strain leads to the appearance of a ferromagnetic (FM) phase due to the magnetoelectric (ME) coupling. At an electric field greater than a critical field, $E_{cr}$, the antiferromagnetic (AFM) phase disappears for all considered temperatures, and FM becomes the only stable magnetic phase. The calculated value of the critical field is close to the values reported recently by Ryan et al. [Nat. Commun. 4, 1334 (2013)] for EuTiO${}_3$ film under a compressive strain. The FM phase can also be induced by an $E$-field in other paraelectric antiferromagnetic oxides with a positive AFM-type ME coupling coefficient and a negative FM-type ME coupling coefficient. The results show the possibility of controlling multiferroicity, including the FM and AFM phases, with help of an electric field application.

A special version of the Landau theory of phase transitions has been developed, capturing the order-disorder phase transition in the bulk and elastically strained vanadium-hydrogen system. The equations for the solubility isotherms are obtained and the influence of biaxial strain on the critical temperature is determined. The critical temperature is found to change exponentially with strain. The calculated solubility isotherms agree well with experimental data for temperatures below, as well as above, the critical temperature. The influence of elastic strain on the solubility isotherm is analyzed and noticeable deviations from Sieverts’ law, even at the extremely low concentrations, are obtained.

We consider Gaussian fluctuations about domain walls embedded in one- or two-dimensional spin lattices. Analytic expressions for the free energy of one domain wall are obtained. From these, the temperature dependence of experimentally relevant spatial scales—i.e., the correlation length for spin chains and the size of magnetic domains for thin films magnetized out of plane—are deduced. Stability of chiral order inside domain walls against thermal fluctuations is also discussed.

The phase difference between the banks of an edge-type planar Josephson junction crossing the narrow thin-film strip depends on wether or not vortices are present in the junction banks. For a vortex close to the junction this effect has been seen by Golod, Rydh, and Krasnov [Phys. Rev. Lett. 104, 227003 (2010)], who showed that the vortex may turn the junction into $\pi$ type. It is shown here that even if the vortex is far away from the junction, it still changes the 0 junction to a $\pi$ junction when situated close to the strip edges. Within the approximation used, the effect is independent of the vortex-junction separation, a manifestation of the topology of the vortex phase which extends to macroscopic distances of superconducting coherence.

The crystal structure, resistivity, and magnetic susceptibility of the Li${}_2$Ir${}_{1-x}$Ru${}_x$O${}_3$ ($x=0$–1) polycrystals have been investigated. We found that the parent antiferromagnetic phase disappears for $x>0.2$ and bond dimers appear in the averaged structure for $x>0.5$ and likely fluctuate for much smaller $x$. Unexpectedly, this system remains insulating for all the doping levels, contrary to the predictions based on the one-band $j_{\mathrm{eff}}=1/2$ Kitaev-Heisenberg model. These results suggest that the honeycomb iridates doped with ruthenium are a unique $5d$-orbital-based platform for studying the interplay of the charge, orbital, spin, and lattice degrees of freedom.

We use angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of CaFe${}_2$As${}_2$ in the collapsed tetragonal (CT) phase. This unusual phase of iron arsenic high-temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the postgrowth thermal treatment of single crystals, we were able to stabilize the CT phase at ambient pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase, along with an apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.

We present evidence for weak ferromagnetism in both the rhombohedral and cubic compositions of BF-$x$BT solid solutions for $x$ < 0.55. Rietveld refinement of nuclear and magnetic structures reveals that the $G$-type antiferromagnetic ordering of the Fe${}^{3+}$ magnetic sublattice survives up to $x$ ∼ 0.50. We address the issue of weak ferromagnetism due to spin canting, which is allowed by the symmetry in the $R$3$c$ space group but not in the cubic $Pm\overline{3}m$ space group. It is shown that the local symmetry of the average cubic compositions of BF-$x$BT for 0.35 < $x$ < 0.55 is broken due to off-centering of Bi${}^{3+}$ in the (1-10) plane and O${}^{2-}$ along the $\langle 110\rangle$ direction from their special Wyckoff positions at (0,0,0) and (1/2,1/2,0), respectively. The local O${}^{2-}$ disorder is shown to be equivalent to local antiferrodistortive rotation, leading to deviation of the Fe${}^{3+}$-O${}^{2-}$-Fe${}^{3+}$ bond angle from 180° that allows spin canting due to Dzyaloshinskii-Moriya interaction ${\vec{D}}_{i,j}·({\vec{S}}_i\times {\vec{S}}_j)$, which is otherwise irreconcilable with the ideal cubic symmetry. The magnetization and neutron powder diffraction measurements confirm the absence of magnetic ordering at room temperature for $x$ ≳ 0.55.

We investigate the properties of the $XY$ pyrochlore antiferromagnet with infinite local $\langle 111\rangle$ planar anisotropy. We identify the ground states and show that the configurational ground state entropy is subextensive. By computing the free energy due to harmonic fluctuations and by carrying out Monte Carlo simulations, we find that the model exhibits thermal order-by-disorder leading to low-temperature long-range order consisting of discrete magnetic domains. In doing so, we set aside doubts that order-by-disorder survives in the thermodynamic limit in this model. We compute the spin wave spectrum and show that thermal and quantum fluctuations select the same magnetic structure. With a previously unreported finite-size scaling analysis of Monte Carlo data, we confirm that the transition is first order for the $XY$ model. Using Monte Carlo simulations, we find that the state selected by thermal fluctuations in this $XY$ pyrochlore antiferromagnet can survive the addition of sufficiently weak nearest-neighbor pseudo-dipolar interactions or long-range dipolar interactions to the spin Hamiltonian. Quite interestingly, the resulting state selected by thermal order-by-disorder is metastable below some temperature. We discuss our results in relation to the Er${}_2$Ti${}_2$O${}_7$ and Er${}_2$Sn${}_2$O${}_7$ pyrochlore antiferromagnets.

We report bulk superconductivity of ternary equiatomic pnictides LaMP ($M$ = Ir and Rh; P = P and As) along with the crystal structure and physical properties (resistivity, magnetization, and heat capacity). It crystallized in a noncentrosymmetric structure type (space group $I$4${}_1$md, $Z$ = 4), which is an ordered ternary derivative of the centrosymmetric α-ThSi${}_2$ structure. Three ternary pnictides showed superconductivity at low temperatures, and the highest critical temperature, $T_c$ = 5.3 K, was observed for LaIrP. The experimental and density functional calculations results suggested that LaMP could be categorized as a Bardeen–Cooper–Schrieffer superconductor with intermediate coupling. Considering the similar Sommerfeld value and Debye temperature, the differences in $T_c$ were attributed to the difference in the electron-phonon coupling strength in the compounds.

The synthesis and structural and physical properties of iron lanthanide oxypnictide superconductors, $L_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{1-x}{\mathrm{O}}_4$ $(L=\mathrm{Pr},\mathrm{Sm},\mathrm{Gd})$, with transition temperature at ∼25 K are reported. Single crystals have been grown at high pressure using the cubic anvil technique. The crystal structure consists of layers of $L_2{\mathrm{O}}_2$ tetrahedra separated by alternating layers of chains of Te and of ${\mathrm{Fe}}_2{\mathrm{As}}_2$ tetrahedra: $-L_2{\mathrm{O}}_2-\mathrm{Te}-L_2{\mathrm{O}}_2-{\mathrm{Fe}}_2{\mathrm{As}}_2-L_2{\mathrm{O}}_2-\mathrm{Te}-L_2{\mathrm{O}}_2-$ (space group: $I\mathrm{4/}\mathit{mmm}$, $a\sim 4.0$, $c\sim 29.6\AA )$. Substitution of oxygen by fluorine increases the critical temperature, e.g., in ${\mathrm{Gd}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{1-x}{\mathrm{O}}_{4-y}{\mathrm{F}}_y$ up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of ∼31.

Recently, it was realized that quantum states of matter can be classified as long-range entangled states (i.e., with nontrivial topological order) and short-range entangled states (i.e., with trivial topological order). We can use group cohomology class ${\mathcal{H}}^d(SG,\mathbb{R}/\mathbb{Z})$ to systematically describe the SRE states with a symmetry $SG$ [referred as symmetry-protected trivial (SPT) or symmetry-protected topological (SPT) states] in $d$-dimensional space-time. In this paper, we study the physical properties of those SPT states, such as the fractionalization of the quantum numbers of the global symmetry on some designed point defects and the appearance of fractionalized SPT states on some designed defect lines/membranes. Those physical properties are SPT invariants of the SPT states which allow us to experimentally or numerically detect those SPT states, i.e., to measure the elements in ${\mathcal{H}}^d(G,\mathbb{R}/\mathbb{Z})$ that label different SPT states. For example, $2+1$-dimensional bosonic SPT states with $Z_n$ symmetry are classified by a ${\mathbb{Z}}_n$ integer $m\in {\mathcal{H}}^3(Z_n,\mathbb{R}/\mathbb{Z})={\mathbb{Z}}_n$. We find that $n$ identical monodromy defects, in a $Z_n$ SPT state labeled by $m$, carry a total $Z_n$ charge $2m$ (which is not a multiple of $n$ in general).

The nonequilibrium Dyson (or Kadanoff-Baym) equation, which is an equation of motion with a long-range memory kernel for real-time Green functions, underlies many numerical approaches based on the Keldysh formalism. In this paper we map the problem of solving the Dyson equation in real time onto a noninteracting auxiliary Hamiltonian with additional bath degrees of freedom. The solution of the auxiliary model does not require the evaluation of a memory kernel and can thus be implemented in a very memory efficient way. The mapping is derived for a self-energy which is local in space and is thus directly applicable within nonequilibrium dynamical mean-field theory (DMFT). We apply the method to study the interaction quench in the Hubbard model for an optical lattice with a narrow confinement, using inhomogeneous DMFT in combination with second-order weak-coupling perturbation theory. We find that, although the quench excites pronounced density oscillations, signatures of the two-stage relaxation similar to the homogeneous system can be observed by looking at the time-dependent occupations of natural orbitals.