In the development of first-principles high-throughput searches for materials with desirable functional properties, there is a clear need for an efficient method to determine the ground-state and low-energy alternative structures of superlattices. A method based on a simple strategy—to generate starting structures based on low-energy structures of the constituent compounds, which are then optimized via structural relaxation calculations—is proposed. This “stacking method” is demonstrated on the 2:2 $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$/$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ superlattice, which has been the subject of recent experimental and theoretical interest. Considerations relevant to wider use of the method are discussed.

We study the zero-temperature ground-state (GS) phase diagram of a spin-half $J_1$–$J_2$ $XY$ model on the honeycomb lattice with nearest-neighbor exchange coupling $J_1>0$ and frustrating next-nearest-neighbor exchange coupling $J_2\equiv \kappa J_1>0$, where both bonds are of the isotropic $XY$ type, using the coupled cluster method. Results are presented for the GS energy per spin, magnetic order parameter, and staggered dimer valence-bond crystalline (SDVBC) susceptibility, for values of the frustration parameter in the range $0\le \kappa \le 1$. In this range, we find phases exhibiting, respectively, Néel $xy$ planar [N(p)], Néel $z$-aligned [N($z$)], SDVBC, and Néel-II $xy$ planar [N-II(p)] orderings. The Néel-II states, which break the lattice rotational symmetry, are ones in which the spins of nearest-neighbor pairs along one of the three equivalent honeycomb directions are parallel, while those in the other two directions are antiparallel. The N(p) state, which is stable for the classical version of the model in the range $0\le \kappa \le \frac{1}{6}$, is found to form the GS phase out to a first quantum critical point at ${\kappa }_{c_1}=0.216\left(5\right)$, beyond which the stable GS phase has N($z$) order over the range ${\kappa }_{c_1}<\kappa <{\kappa }_{c_2}=0.355\left(5\right)$. For values $\kappa >{\kappa }_{c_2}$, we find a strong competition to form the GS phase between states with N-II(p) and SDVBC forms of order. Our best estimate, however, is that the stable GS phase over the range ${\kappa }_{c_2}<\kappa <{\kappa }_{c_3}\approx 0.52\left(3\right)$ is a mixed state with both SDVBC and N-II(p) forms of order; and for values $\kappa >{\kappa }_{c_3}$ is the N-II(p) state, which is stable at the classical level only at the highly degenerate point $\kappa =\frac{1}{2}$. Over the range $0\le \kappa \le 1$, we find no evidence for any of the spiral phases that are present classically for all values $\kappa >\frac{1}{6}$, nor for any quantum spin-liquid state.

Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with $s$-wave on-site pairing) of copper-doped bismuth-selenium compounds. Using a self-consistent $T$-matrix approach for impurity scattering, we examine in-gap states in density of states. We find that the results are well characterized by a single material variable, which measures relativistic effects in the Dirac Hamiltonian. In the nonrelativistic regime, an odd-parity superconducting state is fragile against nonmagnetic impurities. We show that this behavior is caused by a $p$-wave character involved in the topological superconducting state. In contrast, we show that in the relativistic regime the superconductivity is robust against nonmagnetic impurities, owing to an $s$-wave character. To summarize, the system has two aspects, $p$- and $s$-wave features, depending on the weight of relativistic effects.

We report an unusual coexistence of ferromagnetism and ferrimagnetism, and metamagnetism in ${\text{Pr}}_{0.6}$${\text{Er}}_{0.4}$${\text{Al}}_2$. In addition, this compound retains a clear Griffiths phase behavior even at 1 kOe magnetic field and shows a large exchange bias after field cooling from the paramagnetic state. The crystal-field excitations and opposite exchange interactions between nearest-neighbor and next-nearest-neighbor rare earth sites explain these behaviors.

We propose a method to detect a signature of non-Abelian statistics in a one-dimensional (1D) superconducting wire by tuning the effective coupling between a pair of Majorana particles. Our experiment requires a single wire with two segments in the topological superconducting phase and a total of four Majorana particles. We show that an appropriately timed “pulse” in the coupling between the two middle Majoranas leads to a coherent rotation of the two qubits associated with the pairs of Majoranas in the two topological superconducting segments, in much the same way that an appropriate-length pulse of a transverse magnetic field can be used to rotate spins in nuclear magnetic resonance. This can be exploited both to probe the correlations of the Majoranas and to manipulate the Majorana qubits in these 1D wires. We discuss the experimental requirements for such a coherent rotation to be observable when the wire system is coupled to a fermionic bath.

Neutron diffraction and inelastic spectroscopy is used to characterize the magnetic Hamiltonian of ${\mathrm{SrHo}}_2{\mathrm{O}}_4$ and ${\mathrm{SrDy}}_2{\mathrm{O}}_4$. Through a detailed computation of the crystal-field levels we find site-dependent anisotropic single-ion magnetism in both materials, and diffraction measurements show the presence of strong one-dimensional spin correlations. Our measurements indicate that competing interactions of the zigzag chain, combined with frustrated interchain interactions, play a crucial role in stabilizing spin-liquid type correlations in this series.

We study the electronic structure of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce $3d\text{−}4f$ and Cu $2p\text{−}3d$ thresholds to investigate the role of Kondo screening in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$. The Ce $3d$ core-level PES shows the $f^0$, $f^1$, and $f^2$ features, and the Ce $3d\text{−}4f$ XAS shows corresponding features due to transitions into the $f^1$ and $f^2$ states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce $3d\text{−}4f$ threshold show an intense Ce $4f^1$ resonance just below the Fermi level, while the Ce $4f^0$ feature is observed at a binding energy of 2.5 eV. The obtained $f$ partial density of states shows an unusual pseudogaplike dip at the Fermi level in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$. The results indicate the importance of Kondo screening and a pseudogap in the $f$ partial density of states for the anomalous transport and magnetic properties of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_2$.

We here present inelastic neutron scattering results on the strongly correlated cubic superconductor $\mathrm{U}{\mathrm{Be}}_{13}$ ($T_c$ = 0.85 K) obtained on a large single crystal by high-resolution cold neutron three-axis spectroscopy. We observed spin dynamics at a unique momentum space position building up below $T\sim 50$ K and changing significantly on entering the superconducting state. The observed short-range longitudinal character of the correlations can be understood as a result of competing magnetic interactions. The energy dependence in the normal state reflects the energy scales determined from specific heat, whereas the low-temperature data suggest the opening of a superconducting gap. Our findings are consistent with a superconducting order parameter exhibiting $s\pm$ or $d$-wave symmetry and placing pure UBe${}_{13}$ in the strong coupling regime.

We present a time-resolved photoluminescence (PL) study in real and momentum space of a polariton condensate switch in a quasi-one-dimensional semiconductor microcavity. The polariton flow across the ridge is gated by excitons inducing a barrier potential due to repulsive interactions. A study of the device operation dependence on the power of the pulsed gate beam obtains a satisfactory compromise for the on-off signal ratio and switching time of the order of 0.3 and $\sim 50$ ps, respectively. The opposite transition is governed by the long-lived gate excitons, consequently, the off-on switching time is $\sim 200$ ps, limiting the overall operation speed of the device to $\sim 3$ GHz. The experimental results are compared to numerical simulations based on a generalized Gross-Pitaevskii equation, taking into account incoherent pumping, decay, and energy relaxation within the condensate.

We analyze the effect of a strong Zeeman field on the spectrum of collective excitations of monolayer transition metal dichalcogenides. The combination of the Dresselhaus-type spin-orbit coupling and an external Zeeman field results in the lifting of the valley degeneracy in the valence band of these crystals. We show that this lifting of the valley degeneracy manifests in the appearance of an additional plasmon mode with linear in wave-number dispersion along with the standard square root in wave-number mode. Despite this novel mode being subject to Landau damping, it corresponds to a well-defined quasiparticle peak in the spectral function of the electron gas.

Lattice thermal conductivity, ${\kappa }_p$, of suspended and supported graphene nanoribbons (GNRs) is studied over a wide temperature range, taking into account the dispersive nature of confined acoustic phonon modes. Employing a modified Callaway model, an expression for ${\kappa }_p$ is developed, considering the explicit contributions from in-plane longitudinal, transverse, and torsional acoustic, and out-of-plane flexural acoustic phonon modes. Numerical calculations of ${\kappa }_p$($T$) are presented assuming the confined acoustic phonons to be scattered by sample boundaries, impurities, and other phonons via both normal and umklapp processes. The effect of phonon confinement is to modify the phonon group velocities and the temperature dependence of ${\kappa }_p$. In a suspended 5-nm-wide GNR at room temperature, a decrease in ${\kappa }_p$ by ∼70% is predicted. Our study brings out the relative importance of the contributing phonon modes and reveals the influence of flexural phonons on ${\kappa }_p$ as a marked shoulder at low temperatures. The role of the various sample-dependent scattering mechanisms is examined. The substrate, in supported GNRs, is shown to curtail the phonon mean free path and suppress the low-temperature ${\kappa }_p$. Our results are in good agreement with recent experimental data of Bae et al. [M. H. Bae, Z. Li, Z. Aksamija, P. N. Martin, F. Xiong, Z. Y. Ong, I. Knezevic, and E. Pop, Nat. Commun. 4, 1734 (2013)] for supported GNRs.

Weyl semimetals are a three-dimensional topological phase of matter with isolated band touchings in the Brillouin zone. These points have an associated chirality, and many of the proposals to detect the Weyl semimetal state rely on the chiral anomaly. A consequence of the chiral anomaly is that under the application of an $\mathbf{E}·\mathbf{B}$ field, charge is transferred between points of opposite chirality. In this paper we propose an optical absorption experiment that provides evidence for the chiral anomaly. We use the Kubo formula, and find that an applied $\mathbf{E}·\mathbf{B}$ induces the formation of steplike features at finite frequency in the interband optical conductivity. We study the effect of scattering and finite temperatures on this feature and find that it should be observable at low temperatures in pure samples. Finally we discuss how the application of an $\mathbf{E}·\mathbf{B}$ field can be used to map out the frequency dependence of the scattering rate.

Using a density-functional scheme, we study the migration of a single O atom in a (110) plane between two adjacent bond-center sites in bulk Si. The minimum energy migration path is found through the nudged elastic band method within a generalized gradient approximation for the electronic structure. The energy barrier is then also evaluated within a hybrid functional scheme. We achieve for the transition barrier a best estimate of 2.3 eV in the generalized gradient approximation and of 2.7 eV in the hybrid functional scheme, both in fair agreement with the commonly accepted experimental value of 2.53 eV. The transition is characterized by a saddle point which does not occur at the midpoint between the two bond-center sites and by a pattern of displacements extending up to the second nearest-neighbor Si atoms. The atomistic mechanism of oxygen migration is analyzed from three complementary viewpoints involving the evolution of the structure, the Wannier centers, and the single-particle energies and wave functions. The diffusion process can be separated into two distinct parts. In one part, the exchange of the Si atoms in the first-neighbor shell of the diffusing O atom occurs through the formation of a threefold coordinated O center and an overcoordinated Si atom. In the other part, the Si–Si bond flips its position through the creation of occupied and unoccupied Si dangling bonds which give rise to states in the band gap.

We study Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units. Two models of short-range disorder, namely, the Anderson on-site disorder model and the vacancy defect model, are considered. For graphene with Anderson disorder, localization lengths of quasi-one-dimensional systems with various disorder strengths, edge symmetries, and boundary conditions are calculated using the real-space Kubo-Greenwood formalism, showing excellent agreement with independent transfer matrix calculations and superior computational efficiency. Using these data, we demonstrate the applicability of the one-parameter scaling theory of localization length and propose an analytical expression for the scaling function, which provides a reliable method of computing the two-dimensional localization length. This method is found to be consistent with another widely used method which relates the two-dimensional localization length to the elastic mean free path and the semiclassical conductivity. Abnormal behavior at the charge neutrality point is identified and interpreted to be caused by finite-size effects when the system width is comparable to or smaller than the elastic mean free path. We also demonstrate the finite-size effect when calculating the two-dimensional conductivity in the localized regime and show that a renormalization group $\beta$ function consistent with the one-parameter scaling theory can be extracted numerically. For graphene with vacancy disorder, we show that the proposed scaling function of localization length also applies. Last, we discuss some ambiguities in calculating the semiclassical conductivity around the charge neutrality point due to the presence of resonant states.

We have studied the electronic structure of CdS/ZnSe coupled quantum dots, a novel heterostructure at the nanoscale. Our calculations reveal CdS/ZnSe coupled quantum dots are type II in nature where the anion $p$ states play an important role in deciding the band offset for the highest occupied molecular orbitals (HOMO). We show that the offsets of HOMO as well as the lowest unoccupied molecular orbitals (LUMO) can be tuned by changing the sizes of the components of the coupled quantum dots, thereby providing an additional control parameter to tune the band gap and the optical properties. Our investigations also suggest that the formation of an alloy near the interface has very little influence on the band offsets, although it affects the spatial localization of the quantum states from the individual components. Comparing the influence of strain on coupled quantum dots and core-shell nanowires, we find strain practically has no role in the electronic structure of coupled quantum dots as the small effective area of the interface in a coupled quantum dot helps a large part of the structure remain free from any substantial strain. We argue that in contrast to core-shell nanowires, quantum confinement is the key parameter that controls the electronic properties of coupled quantum dots and should therefore be an ideal candidate for the design of a quantum device.

We develop a graph theoretical formalism to account for the fact that $s{p}^2$ carbon can become spin ordered or generate free radicals for purely topological reasons. While this phenomenon has been previously considered a binary operator, we here show a quantification in discrete units of frustrations. The graph theory method is combined with open density functional theory calculations to establish the existence of an energy of frustration that is shown to greatly improve the description of carbon nanostructure energetics using classical force fields. The methodology is illustrated for a number of systems and, owing to the small computational overhead associated with its evaluation, is expected to be easily integrable into any modeling approach based on a structure's adjacency matrix.

The structure of the orthorhombic (Pbnm) polytype of ${\text{SrIrO}}_3$ has been investigated between 3 and 1100 K using a combination of synchrotron and neutron diffraction methods. The orthorhombic structure persists to 1100 K, the highest temperature available in this work. This is a consequence of the larger than expected octahedral tilting estimated from the neutron diffraction studies. We postulate that the strong spin-orbit coupling of the Ir${}^{4+}$ cation, which splits the $t_{2g}$ band, introduces additional strain on the lattice. This introduces unusual thermal expansion of the cell. ${\text{SrIrO}}_3$ was characterized by resistivity, magnetization, and specific heat measurements. Metallic conductivity was observed between 2 and 300 K without indication of the previously reported metal-insulator transition. The Sommerfeld constant γ was 3.12(2) mJ mol${}^{-1}$ K${}^{-2}$, and a Fermi-liquid behavior was observed between 2 and 30 K with positive magnetoresistence of up to 2% (at 70 kOe and between 2 and 50 K).

Intermetallic clathrates are promising candidates for thermoelectric applications. For theoretical investigations of their lattice thermal conductivity, effective potentials for silicon- and germanium-based structures have been developed. To stabilize the fourfold coordinated cage framework, angular dependent potentials are necessary. The phononic properties obtained from these potentials are in good agreement with ab initio calculated and experimental data. Molecular dynamics simulations using the Green-Kubo method are used to calculate the thermal conductivity. Type-I clathrate structures with different degrees of complexity have been studied. The influence of the rattling modes of the guest atoms inside the cages is identified as one source of the low thermal conductivity. However, the vacancies in the host framework also reduce the lattice thermal conductivity of clathrate systems. The results are compared to experimental data where available.

A set of $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3$ (PMN) relaxor ceramics with different porosities (66–96% of theoretical density) was processed from the pure perovskite PMN powder by varying the sintering temperature (930–980 K) and isostatic pressure. The effective room temperature permittivity at 100 Hz varied correspondingly from ∼4000 to ∼15 000. We measured and fitted the room temperature infrared (IR) reflectivity of corresponding polished ceramics, from which the effective dielectric function was obtained. The results were compared with the low-frequency permittivity data and discussed using several effective medium models. The best results were obtained using the Lichtenecker model (with the exponent α ≈ 0.2), which fits quite well the IR spectra as well as the static permittivity for all the porosity values. Also the simulation using the finite element method supported the same model. It allows us to conclude that the samples show quite complex shape and topology of the pores, but all of the pores show similarity with a weak nonzero percolation (open porosity).

Possible anisotropic spin-singlet pairings in ${\mathrm{Bi}}_{2}X{}_3$ ($X$ is Se or Te) are studied. Among six pairings compatible with the crystal symmetry, two novel pairings show nontrivial surface Andreev bound states, which form flat bands and could produce zero-bias conductance peak in measurements such as point-contact spectroscopy. By considering purely repulsive short-range Coulomb interaction as the pairing mechanism, the dominant superexchange terms are all antiferromagnetic, which would usually favor spin-singlet pairing in ${\mathrm{Bi}}_{2}X{}_3$. Mean field analyses show that the interorbital pairing interaction favors a mixed spatial-parity anisotropic pairing state, and one pairing channel with zero-energy surface states has a sizable component. The results provide important information for future experiments.