The structural and magnetic properties of two mixed-valence cobaltites with a formal population of 0.30 Co${}^{4+}$ ions per f.u., (Pr${}_{1-y}$Y${}_y$)${}_{0.7}$Ca${}_{0.3}$CoO${}_3$ ($y=0$ and 0.15), have been studied down to very low temperatures by means of high-resolution neutron diffraction, SQUID magnetometry, and heat-capacity measurements. The results are interpreted within the scenario of the spin-state crossover from a room-temperature mixture of the intermediate-spin Co${}^{3+}$ and low-spin Co${}^{4+}$ (IS/LS) to the LS/LS mixture in the sample ground states. In contrast to the yttrium-free $y=0$ that retains the metallic-like character and exhibits ferromagnetic (FM) ordering below 55 K, the doped system $y=0.15$ undergoes a first-order metal-insulator transition at 132 K, during which not only the crossover to low-spin states but also a partial electron transfer from Pr${}^{3+}$ $4f$ to cobalt 3d states takes place simultaneously. Taking into account the nonmagnetic character of LS Co${}^{3+}$, such a valence shift electronic transition causes a magnetic dilution, formally to 0.12 LS Co${}^{4+}$ or 0.12 $t_{2g}$ hole spins per f.u., which is the reason for an insulating, highly nonuniform magnetic ground state without long-range order. Nevertheless, even in that case there exists a relatively strong molecular field distributed over all the crystal lattice. It is argued that the spontaneous FM order in $y=0$ and the existence of strong FM correlations in $y=0.15$ apparently contradict the single $t_{2g}$ band character of LS/LS phase. The explanation we suggest relies on a model of the defect-induced, itinerant hole-mediated magnetism, where the defects are identified with the magnetic high-spin Co${}^{3+}$ species stabilized near oxygen vacancies.

We present an ab initio real-time-based computational approach to study nonlinear optical properties in condensed matter systems that is especially suitable for crystalline solids and periodic nanostructures. The equations of motion and the coupling of the electrons with the external electric field are derived from the Berry-phase formulation of the dynamical polarization [Souza et al., Phys. Rev. B 69, 085106 (2004)]. Many-body effects are introduced by adding single-particle operators to the independent-particle Hamiltonian. We add a Hartree operator to account for crystal local effects and a scissor operator to correct the independent particle band structure for quasiparticle effects. We also discuss the possibility of accurately treating excitonic effects by adding a screened Hartree-Fock self-energy operator. The approach is validated by calculating the second-harmonic generation of SiC and AlAs bulk semiconductors: an excellent agreement is obtained with existing ab initio calculations from response theory in frequency domain [Luppi et al., Phys. Rev. B 82, 235201 (2010)]. We finally show applications to the second-harmonic generation of CdTe and the third-harmonic generation of Si.

We present experimental and theoretical studies of the magneto-optical properties of $p$-type In${}_{1-x}$Mn${}_x$As and In${}_{1-x}$Mn${}_x$Sb ferromagnetic semiconductor films in ultrahigh magnetic fields oriented along [001]. Samples were fabricated by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). To model the results, we used an 8-band Pidgeon-Brown model generalized to include the wave vector dependence of the elec-tronic states along $k_z$ as well as the s-d and p-d exchange interactions with the localized Mn $d$ electrons. The Curie temperature is taken as an input parameter and the average Mn spin is treated in mean-field theory. We compared Landau level and band structure calculations with observed cyclotron resonance (CR) measurements. While differences between the CR measurements are seen for MBE and MOVPE samples, our calculations indicate that they arise from differences in the carrier densities. In addition, the difference in the carrier densities suggests significantly larger average spin for the MOVPE structures; this fact could be responsible for higher Curie temperatures in this material system.

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C${}_{60}$ we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current $\Delta E$ and the CT energy $E_{\mathrm{CT}}$ may be regarded as identical. The temperature-dependent analysis of open-circuit voltage $V_{\mathrm{OC}}$ and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of $\Delta E$ and $E_{\mathrm{CT}}$ is again found for all material systems with the exception of copper phthalocyanine/C${}_{60}$. The general rule of thumb for organic semiconductor heterojunctions, that $V_{\mathrm{OC}}$ at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.

We present a theoretical study of the spin-valley blockade transport effect in a double quantum dot defined in a straight carbon nanotube. We find that intervalley scattering due to short-range impurities completely lifts the spin-valley blockade and induces a large leakage current in a certain confined range of the external magnetic field vector. This current hot spot emerges due to different effective magnetic fields acting on the spin-valley qubit states of the two quantum dots. Our predictions are compared to a recent measurement [F. Pei et al., Nat. Nanotech. 7, 630 (2012)]. We discuss the implications for blockade-based schemes for qubit initialization/readout and motion sensing of nanotube-based mechanical resonators.

We study a Hanbury Brown and Twiss (HBT) interferometer formed with chiral edge channels of a quantum Hall system. HBT cross correlations are calculated for a device operating both in the integer and fractional quantum Hall regimes, the latter at Laughlin filling fractions. We find that in both cases, when the current is dominated by electron tunneling, current-current correlations show antibunching, characteristic of fermionic correlations. When the current-current correlations are dominated by quasiparticle tunneling, the correlations reveal bunching, characteristic of bosons. For electron tunneling, we use the Keldysh technique, and show that the result for fractional filling factors can be obtained in a simple way from the results of the integer case. It is shown that quasiparticle-dominated cross-current correlations can be analyzed by means of a quantum master-equation approach. We present here a detailed derivation of the results [Campagnano et al., Phys. Rev. Lett. 109, 106802 (2012)] and generalize them to all Laughlin fractions.

Topological crystalline insulators (TCIs) are new states of matter whose topological distinction relies on the crystal symmetry of periodic solids. The first material realization of TCIs has recently been predicted and observed in IV-VI semiconductor SnTe and related alloys. By combining $k·p$ theory and band structure calculation, we present a unified approach to study topological surface states on various crystal surfaces of these TCI materials based on the electronic structure of the bulk. Depending on the surface orientation, we find two types of surface states with qualitatively different properties. In particular, the (111) surface states consist of four Dirac cones centered at $\overline{\Gamma }$ and $\overline{M}$, while Dirac cones on (001) and (110) surfaces are located at non-time-reversal-invariant momenta. The latter types of surface states exhibit a Lifshitz transition as a function of Fermi energy, which is accompanied by a Van Hove singularity in the density of states arising from saddle points in the band structure.

We present a completely unbiased and controlled numerical method to solve quantum impurity problems in $d$-dimensional lattices. This approach is based on a canonical transformation, of the Lanczos form, where the complete lattice Hamiltonian is exactly mapped onto an equivalent one-dimensional system, in the same spirit as Wilson's numerical renormalization, and Haydock's recursion method. We introduce many-body interactions in the form of a Kondo or Anderson impurity and we solve the low-dimensional problem using the density matrix renormalization group. The technique is particularly suited to study systems that are inhomogeneous, and/or have a boundary. The resulting dimensional reduction translates into a reduction of the scaling of the entanglement entropy by a factor $L^{d-1}$, where $L$ is the linear dimension of the original $d$-dimensional lattice. This allows one to calculate the ground state of a magnetic impurity attached to an $L\times L$ square lattice and an $L\times L\times L$ cubic lattice with $L$ up to 140 sites. We also study the localized edge states in graphene nanoribbons by attaching a magnetic impurity to the edge or the center of the system. For armchair metallic nanoribbons we find a slow decay of the spin correlations as a consequence of the delocalized metallic states. In the case of zigzag ribbons, the decay of the spin correlations depends on the position of the impurity. If the impurity is situated in the bulk of the ribbon, the decay is slow as in the metallic case. On the other hand, if the adatom is attached to the edge, the decay is fast, within few sites of the impurity, as a consequence of the localized edge states, and the short correlation length. The mapping can be combined with ab initio band structure calculations to model the system, and to understand correlation effects in quantum impurity problems starting from first principles.

We investigate the real-time dynamics of a critical spin-1/2 chain (XXZ model) prepared in an inhomogeneous initial state with different magnetizations on the left and right halves. Using the time-evolving block decimation method, we follow the front propagation by measuring the magnetization and entanglement entropy profiles. At long times, as in the free fermion case [Antal et al., Phys. Rev. E 59, 4912 (1999)], a large central region develops where correlations become time independent and translation invariant. The shape and speed of the fronts is studied numerically and we evaluate the stationary current as a function of initial magnetic field and as a function of the anisotropy $\Delta$. We compare the results with the conductance of a Tomonaga-Luttinger liquid, and with the exact free-fermion solution at $\Delta =0$. We also investigate the two-point correlations in the stationary region and find a good agreement with the “twisted” form obtained by Lancaster and Mitra [Phys. Rev. E 81, 061134 (2010)] using bosonization. Some deviations are nevertheless observed for strong currents.

We examine the properties of edge states in a two-dimensional topological insulator. Based on the Kane-Mele model, we derive coupled equations for the energy and the effective width of the edge states at a given crystal momentum in a semi-infinite honeycomb lattice with a zigzag boundary. It is revealed that, in a one-dimensional Brillouin zone, the edge states merge into continuous bands of the bulk states through a bifurcation of the edge-state width. We discuss the implications of the results for experiments in monolayer or thin-film topological insulators.

We use group or representation theory and scattering matrix calculations to derive analytical results for the band structure topology and the scattering parameters, applicable to any chiral photonic crystal with body-centered-cubic symmetry $I432$ for circularly polarized incident light. We demonstrate in particular that all bands along the cubic $\left[100\right]$ direction can be identified with the irreducible representations $E_{\pm }$, $A$, and $B$ of the $C_4$ point group. $E_{+}$ and $E_{-}$ modes represent the only transmission channels for plane waves with wave vector along the $\Delta$ line, and $E_{-}$ and $E_{+}$ are identified as noninteracting transmission channels for right- and left-circularly polarized light, respectively. Scattering matrix calculations provide explicit relationships for the transmission and reflectance amplitudes through a finite slab which guarantee equal transmission rates for both polarizations and vanishing ellipticity below a critical frequency, yet allowing for finite rotation of the polarization plane. All results are verified numerically for the so-called 8-srs geometry, consisting of eight interwoven equal-handed dielectric gyroid networks embedded in air. The combination of vanishing losses, vanishing ellipticity, near-perfect transmission, and optical activity comparable to that of metallic metamaterials makes this geometry an attractive design for nanofabricated photonic materials.

We investigate the problem of local rigidity of Si nanocrystals embedded in amorphous silica. By analyzing the elastic (bulk) modulus field into atomic contributions, we show that it is highly inhomogeneous. It consists of a hard region in the interior of the nanocrystals, with moduli $\sim 105$ GPa, compared to 98 GPa for bulk Si, and of “superhard” ($\sim 120$ GPa) and “supersoft” ($\sim 80$ GPa) regions in the outer parts. Overall, the nanocrystal bulk modulus is significantly enhanced compared to the bulk, and its variation with size accurately follows a power-law dependence on the average bond length. The bulk modulus of the oxide matrix and of the interface region is nearly constant with size, with values 60 and 70 GPa, respectively. The average optical (homopolar) gap is directly linked to the elastic and bond-length variations.

We report an experimental investigation of the fractional quantum Hall effect (FQHE) at the even-denominator Landau-level filling factor $\nu =1/2$ in very-high-quality wide GaAs quantum wells and at very high magnetic fields up to 45 T. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths $W$ ranging from 41 to 96 nm and have variable densities in the range of $\simeq$$4\times {10}^{11}$ to $\simeq$$4\times {10}^{10}$ cm${}^{-2}$. We present several experimental phase diagrams for the stability of the $\nu =1/2$ FQHE in these quantum wells. In general, for a given $W$, the 1/2 FQHE is stable in a limited range of intermediate densities where it has a bilayerlike charge distribution; it makes a transition to a compressible phase at low densities and to an insulating phase at high densities. The densities at which the $\nu =1/2$ FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the $\nu =1/2$ FQHE and turns the electron system into a compressible state. We also present a plot of the symmetric-to-antisymmetric subband separation (${\Delta }_{\mathrm{SAS}}$), which characterizes the interlayer tunneling, vs density for various $W$. This plot reveals that ${\Delta }_{\mathrm{SAS}}$ at the boundary between the compressible and FQHE phases increases linearly with density for all the samples. There is no theoretical explanation for such a simple dependence. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the interlayer and intralayer Coulomb interactions and ${\Delta }_{\mathrm{SAS}}$. We conclude that consistent with the conclusions of some of the previous studies, the $\nu =1/2$ FQHE observed in wide GaAs quantum wells with symmetric charge distribution is stabilized by a delicate balance between the interlayer and intralayer interactions and is very likely described by a two-component (${\Psi }_{331}$) state.

We report the results of photoemission spectroscopy on the potassium concentration ($x$) dependence of K${}_x$ picene films on graphite. The results show a systematic shift of a whole valence band and the appearance of a clear Fermi edge for a certain value of $x$, with additional structures at higher binding energies. Observed spectral evolutions are reminiscent to those of doped C${}_{60}$, implying that the K${}_x$ picene film has multiple phases depending on the K concentration. Appearance of a clear Fermi edge, in contrast to multilayer K${}_x$ picene films on Au, SiO${}_2$, and Ag(111) substrates, where no clear Fermi edges are observed, suggests that the emergence of the metallic states for K${}_x$ picene films depends on the specific molecular orientations in the films on different substrates.

We have investigated shot noise and conductance of multiterminal graphene nanoribbon devices at temperatures down to 50 mK. Away from the charge neutrality point, we find a Fano factor $F\approx 0.4$, nearly independent of the charge density. Our shot noise results are consistent with theoretical models for disordered graphene ribbons with a dimensionless scattering strength $K_0\approx 10$ corresponding to rather strong disorder. Close to charge neutrality, an increase in $F$ up to $\sim$0.7 is found, which indicates the presence of a dominant Coulomb gap possibly due to a single quantum dot in the transport gap.

Spectra of the C1s core hole, created in x-ray photoemission and screened by electronic excitations in pristine and doped graphene are calculated and discussed. We find that singular effects in the lineshapes are not possible in the pristine graphene, and their observation should be connected with the doping. However, the structure of the low-energy excitation spectrum in the region where the singular behavior is expected leads to asymmetries in the core-hole lineshapes in pristine graphene similar to those in doped graphene. This makes the analysis more complex than in the case of metals and may lead to an incorrect or incomplete interpretation of the experimental results.

We theoretically demonstrate that the transport inefficiency recently found experimentally for branched-out mesoscopic networks can also be observed in a quantum ring of finite width with an attached central horizontal branch. This is done by investigating the time evolution of an electron wave packet in such a system. Our numerical results show that the conductivity of the ring does not necessary improve if one adds an extra channel. This ensures that there exists a quantum analog of the Braess paradox, originating from quantum scattering and interference.

The monoclinic (M) phase reported recently in ferroelectric solid solutions having compositions at the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases could be responsible for the extraordinary piezoelectric properties of these materials, but its very existence is under serious debate. In this article we show by computer simulation that using a sixth order Landau free energy polynomial with small polarization anisotropy at the MPB and taking into account the electrostatic and elastic interactions, a hierarchical structure consisting of M phase nanodomains is formed within a composition or temperature range enclosing the MPB. It is found that the M phase, even though unstable according to the Landau free energy, is stabilized by the long-range elastic and electrostatic interactions. A new phase diagram containing the M phase is established based on the simulation results.

We demonstrate the displacement of domain walls via pure diffusive spin currents in a nonlocal spin valve geometry, without any externally applied fields. We implement a localized detection of the domain wall position by simultaneous nonlocal spin signal measurements using contacts on both sides of the spin current conduit, which allows us to determine the domain wall position even underneath the spin current conduit. Using this detection method, we probe the domain wall position as it moves across the spin current conduit when sweeping a field or on current application. Injecting pure spin currents in the nonlocal architecture, we find that in our optimized geometry we can displace a transverse head-to-head or tail-to-tail domain wall without any externally applied fields at effective spin currents $<$${10}^{10}$ A/m${}^2$, showing that this method can be a viable avenue to low-power domain wall manipulation.