We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on-demand single-spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two-dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several spin pairs can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on-demand single-spin emitter or single-charge emitter. Again, due to the helicity of the topological insulator, the single-spin emitter or charge emitter is dissipationless and immune to disorder. The proposed single-spin emitter can be an important building block of future spintronic devices.

We studied experimentally the coherent exchange interaction between two antiparallel electron spins in a double quantum dot with an increasing magnetic field gradient between the two dots. We observed an increase of the oscillation frequency and a reduction of the oscillation contrast. Both observations are consistent with a simple model of two spins interacting via exchange interaction. The demonstrated control of the two-electron spin states is interesting to produce gates for individual spin qubits.

The emission cascade of a single quantum dot is a promising source of entangled photons. A prerequisite for this source is the use of a symmetric dot analogous to an atom in a vacuum, but the simultaneous achievement of structural symmetry and emission in a telecom band poses a challenge. Here we report the growth and characterization of highly symmetric InAs/InAlAs quantum dots self-assembled on $C_{3v}$ symmetric InP(111)A. The broad emission spectra cover the O $\left(\lambda \sim 1.3 \mu \mathrm{m}\right)$, C $\left(\lambda \sim 1.55 \mu \mathrm{m}\right)$, and $L \left(\lambda \sim 1.6 \mu \mathrm{m}\right)$ telecom bands. The distribution of the fine-structure splittings is considerably smaller than those reported in previous works on dots at similar wavelengths. The presence of dots with degenerate exciton lines is further confirmed by the optical orientation technique. Thus, our dot systems are expected to serve as efficient entangled photon emitters for long-distance fiber-based quantum key distribution.

We analyze a one-dimensional ring structure composed of many two-level systems, in the limit where only one excitation is present. The two-level systems are coupled to a common environment, where the excitation can be lost, which induces super- and subradiant behavior. Moreover, each two-level system is coupled to another independent environment, modeled by a classical white noise, simulating a dephasing bath and described by the Haken-Strobl master equation. Single-exciton superradiance, an example of a cooperative quantum coherent effect, is destroyed at a critical dephasing strength proportional to the system size, showing robustness of cooperativity to the action of the dephasing environment. We also show that the coupling to a common decay channel contrasts with the action of dephasing, driving the entanglement decay to slow down on increasing the system size. Moreover, after a projective measurement which finds the excitation in the system, the entanglement reaches a stationary value, independent of the initial conditions.

We have investigated the evolution of electronic structure in the Fe-based ladder compound Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{Se}}_3$ ($0\le x\le 0.15$) by means of resistivity, magnetic susceptibility, and reflectivity measurements. Although the Co substitution into ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ greatly decreases the resistivity, the compound still shows an insulating behavior at $x=0.15$. Moreover, the insulating state at $x=0.15$ is robust even under the high pressure of 8 GPa. The antiferromagnetic transition temperatures are gradually suppressed by substituting Co atoms at $x\le 0.10$, while the spin glass state is observed at $x\ge 0.125$. Our finding revealed that the substituted Co atoms play a dual effect: electron-carrier dopant and localized moment, providing a supplemental insight into the doping effect in the Fe-based superconductors.

We have investigated the physical properties of a new layered oxyselenide ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$, which crystallizes in an unusual intergrowth structure with ${\mathrm{Cu}}_2{\mathrm{Se}}_2$ and ${\mathrm{Bi}}_2{\mathrm{YO}}_4$ layers. Electric transport measurement indicates that ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is holelike and it may be a potential thermoelectric material at high temperatures. First-principles calculations are in agreement with experimental results and show that ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ is a quasi-two-dimensional metal. Further theoretical investigation suggests the ground states of the ${\mathrm{Bi}}_2{\mathrm{YO}}_4{\mathrm{Cu}}_2{\mathrm{Se}}_2$ type can be tuned by designing the blocking layers, which will enrich the physical properties of these compounds.

Motivated by the experimental discovery of the quantum anomalous Hall effect and the interest in quantum phase transitions of correlated electrons, we consider interaction effects at a quantum anomalous Hall critical point. We study a microscopic lattice model of spinful fermions on the triangular lattice which exhibits a $C=2$ Chern insulator (CI) phase with a quantized anomalous Hall effect, sandwiched between two normal insulator (NI) phases. The first NI-CI quantum phase transition is driven by simultaneous mass inversion of a pair of Dirac fermions, with short range interactions being perturbatively irrelevant at the transition. The second CI-NI transition is driven by a quadratic band touching point protected by momentum space topology and $C_6$ lattice symmetry. A one-loop renormalization group analysis shows that short range interactions lead to a single marginally relevant perturbation at this transition. We obtain the mean field phase diagram of this model incorporating weak repulsive Hubbard interactions, finding an emergent dome of nematic order around this CI-NI topological critical point. We discuss the crossovers in the Hall conductivity at nonzero temperature, and the Landau theory of the quantum and thermal transitions out of the nematic phase. Our results may be relevant to ferromagnetic double perovskite films with spin-orbit coupling which have been proposed to host such a Chern transition. Our work provides perhaps the simplest example of an emergent phase near a quantum critical point.

The electronic structure of stacking faults and surfaces without and with an additional displaced layer is calculated for the case of rhombohedral (ABC) graphite. The full-potential local-orbital code and the generalized gradient approximation to density functional theory are used. All considered surfaces and interfaces induce surface/interface bands. All discovered surface and interface bands are restricted to the vicinity of the symmetry line $K-M$ in the two-dimensional Brillouin zone. There are groups of localized band pairs around $\pm 0$, $\pm 0.2$, and $\pm 0.6$ eV for one of the two considered types of stacking faults; $\pm 0$ and $\pm 0.5$ eV for the other type and for a displaced surface layer. At the $K$ point in the Brillouin zone, there is a one-to-one correspondence between these localized bands and the eigenvalues of those linear atomic clusters, which are produced by the perturbation of periodicity due to the displaced surface layer or due to the stacking faults. Some of the localized bands produce strong van Hove singularities in the local density of states near the surface or interface at energies up to several 0.1 eV. It is suggested to check these findings experimentally by appropriate spectroscopic methods. Undisturbed bulk (ABC) graphite is virtually a zero-gap semiconductor with a minute density of states at the Fermi energy. Both the surface and any of the considered stacking faults produce sharp peaks in the local density of states near the perturbation at energies of about 10 meV around the Fermi energy. This should provide a considerable contribution to the conductivity and its temperature dependence for samples with stacking faults or large surface-to-volume fraction.

We propose a finite-size correction scheme for the formation energy of charged defects and impurities in one-dimensional systems within density functional theory. The energy correction in a supercell geometry is obtained by solving the Poisson equation in a continuum model which is described by an anisotropic permittivity tensor, with the defect charge distribution derived from first-principles calculations. We implement our scheme to study impurities and dangling bonds in silicon nanowires and demonstrate that the formation energy of charged defects rapidly converges with the supercell size.

We systematically study the coherent transport (Josephson tunneling and counterflow current) and its breakdown which leads to incoherent charge flow in the excitonic BCS condensate formed in GaAs bilayers at ${\nu }_{\mathrm{tot}}=1/2+1/2$. The Josephson currents in samples with three different interlayer distances vary by four orders of magnitude. In contrast, the breakdown thresholds for the ${\nu }_{\mathrm{tot}}=1$ quantum Hall state are comparable. Furthermore, Coulomb drag in a Corbino ring reveals that the coherent counterflow current coexists with the dissipative charge transport.

The evolution in structure of the prototypical network-forming glass ${\mathrm{GeSe}}_2$ is investigated at pressures up to $\sim 16$ GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The neutron diffraction work at pressures $\le 8.2$ GPa employed the method of isotope substitution, and the molecular dynamics simulations were performed with two different exchange-correlation functionals, the Becke-Lee-Yang-Parr (BLYP) and the hybrid Heyd-Scuseria-Ernzerhof HSE06. The results show density-driven structural transformations that differ substantially from those observed in common oxide glasses such as ${\mathrm{SiO}}_2$ and ${\mathrm{GeO}}_2$. Edge-sharing tetrahedra persist as important structural motifs until a threshold pressure of $\sim 8.5$ GPa is attained, whereupon a mediating role is found for homopolar bonds in the appearance of higher coordinated Ge-centered polyhedra. These mechanisms of network transformation are likely to be generic for the class of glass-forming materials where homopolar bonds and fragility-promoting edge-sharing motifs are prevalent in the ambient-pressure network.

In the molecular nanomagnet ${\mathrm{Fe}}_8$ tunneling can occur from a metastable state to an excited state followed by a transition to the ground state. This transition is accompanied by an energy release with an equivalent frequency of $109.5$ GHz. We constructed an experimental setup to measure whether this energy is released thermally or in the form of electromagnetic radiation. Contrary to a previous publication we find no evidence for release of electromagnetic radiation and place an upper limit of 5% on the total radiative energy released during the transition. The transitions between the first and second excited states to the ground state are consistent with a release of thermal energy alone. We also observe that the energy release extends for a longer time for the second excited state than for the first excited state.

Temperature dependence of the magnetocrystalline anisotropy energy and magnetization of the prototypical rare-earth magnet ${\mathrm{YCo}}_5$ is calculated from first principles, utilizing the relativistic disordered local-moment approach. We discuss a strategy to enhance the finite-temperature anisotropy field by hole doping, paving the way for an improvement of the coercivity near room temperature or higher.

We report a complete characterization of the anomalous Nernst effect (ANE) and its relationship with the anomalous Hall effect (AHE) in ${\mathrm{Fe}}_3{\mathrm{O}}_4$. By combining full thermoelectric and electric transport measurements as a function of temperature, we have verified that the universal scaling relation between the anomalous Hall and diagonal conductivities (${\sigma }_{zy}\propto {\sigma }_{zz}^{1.6}$), observed in materials with bad-metal-hopping type of conduction, is also applicable to the thermoelectric transport. We further show that the ANE and AHE are commonly related through the Mott relation, therefore demonstrating its validity for anomalous transport phenomena in materials with conduction in the the dirty regime.

We have performed polarized Raman scattering measurements on the newly discovered superconductor Nd(O,F)${\mathrm{BiS}}_2$ ($T_c=4$ K). We observe 2 Raman-active modes, with frequencies in accordance with first-principles calculations. One $A_{1g}$ phonon mode at $112.4 \mathrm{cm}{}^{-1}$ exhibits a Fano line shape due to electron-phonon coupling. We find a resonance for this mode at 2.45 eV excitation energy. We estimate a 0.68 contribution of this mode to the electron-phonon coupling constant $\lambda$. Our Raman results suggest that the ${\mathrm{BiS}}_2$-based superconductors are possibly phonon-mediated BCS superconductors.

Examples of materials where an “order by disorder” mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ reveals a most convincing case of this mechanism, with the observation of a spin gap at zone centers having recently been interpreted as a corroboration of this physics. Here we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 $\mu \mathrm{eV}$, which is of the same order of magnitude as the one observed experimentally. We report high-resolution inelastic neutron scattering data which can be well understood within this scenario.

We report neutron diffraction measurement results for an Ising antiferromagnet ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ under uniaxial pressure along the geometrically frustrated isosceles-triangular-lattice direction. We find that an onset incommensurate wave number at the Néel temperature increases with pressure from 0.378 to 0.411 at 400 MPa. The observations suggest that the anisotropic deformation of the lattice by the uniaxial pressure significantly modifies the spin frustration, leading to an increase in the nearest-neighbor to next-nearest-neighbor interaction ratio from 1.33 to 1.81.

We have resolved long-standing discrepancies between the theoretical and experimental crystal structures of boron carbide ${\mathrm{B}}_{13}{\mathrm{C}}_2$. Theoretical studies predict that ${\mathrm{B}}_{13}{\mathrm{C}}_2$ should be stoichiometric and have the highest symmetry of the boron carbides. Experimentally, ${\mathrm{B}}_{13}{\mathrm{C}}_2$ is a semiconductor and many defect states have been reported, particularly in the CBC chain. Reconciling the disordered states of the chain, the chemical composition, and the lowest-energy state is problematic. We have solved this problem by constructing a structural model where approximately three-quarters of the unit cells contain $\left({\mathrm{B}}_{11}\mathrm{C}\right)$(CBC) and one-quarter of them contain $\left({\mathrm{B}}_{12}\right)\left({\mathrm{B}}_4\right)$. This structural model explains many experimental results, such as the large thermal factors in x-ray diffraction and the broadening of the Raman spectra, without introducing unstable CBB chains. The model also solves the energy-gap problem. We show that there are many arrangements of these two types of unit cells, which are energetically almost degenerate. This demonstrates that boron carbides are well described by a geometrically frustrated system, similar to that proposed for $\beta$-rhombohedral boron.

The ferroelectric phase transition of uniaxial ${\mathrm{Ca}}_{0.28}{\mathrm{Ba}}_{0.72}{\mathrm{Nb}}_2{\mathrm{O}}_6$ single crystals with a moderate effective charge disorder was investigated by Brillouin scattering to clarify the dynamic properties. In the tetragonal paraelectric phase a remarkable softening of the sound velocity of the longitudinal acoustic mode and a significant increase in the sound attenuation were observed close to the Curie temperature $T_{\mathrm{C}}=527\mathrm{K}$. The intermediate temperature $T^{*}$ $\sim 640\mathrm{K}$ and the Burns temperature $T_{\mathrm{B}}$ $\sim 790\mathrm{K}$ were determined from the temperature variation in the sound attenuation. The intense broad central peak (CP) caused by polarization and strain fluctuations due to polar nanoregions was clearly observed in the vicinity of $T_{\mathrm{C}}$. The relaxation time determined by the CP width clearly shows critical slowing down towards $T_{\mathrm{C}}$, reflecting a weakly first-order phase transition under weak random fields.

Inelastic neutron scattering was employed to study the crystal-field interaction in the strontium-doped rare-earth compounds $R_x{\mathrm{Sr}}_{1-x}{\mathrm{CoO}}_{3-z}$ ($R=\mathrm{Pr}$, Nd, Ho, and Er). Particular emphasis is laid on the effect of oxygen deficiencies that naturally occur in the synthesis of these compounds. The observed energy spectra are found to be the result of a superposition of crystal fields with different nearest-neighbor oxygen coordination at the $R$ sites. The experimental data are interpreted in terms of crystal-field parameters, which behave in a consistent manner through the rare-earth series, thereby allowing a reliable extrapolation for rare-earth ions not considered in the present work.