BaIrO${}_3$ adopts the 9R polytype structure if it is synthesized at ambient pressure. High-pressure and high-temperature treatments up to 10 GPa have led to other 5H and 6H polytype phases, which are quenchable to the ambient condition. However, the single-phase 3C perovskite, the end member of polytypes, has not been obtained so far. Here we report the high-pressure synthesis of BaIrO${}_3$ perovskite under 25 GPa and 1150 °C. Rietveld refinement on the powder x-ray diffraction pattern revealed that this new compound crystallizes in a tetragonal $I$4/mcm structure rather than the expected cubic Pm-3$m$ structure. The new perovskite is a Pauli paramagnetic metal with a Fermi liquid ground state, i.e., ρ ∝ $T^2$ below 6 K. The availability of BaIrO${}_3$ perovskite also allows completion of the structure-physical property evolution in the $A$IrO${}_3$ perovskites ($A$ = Ca, Sr, Ba). The Fermi energy located in bands originated from Ir: 5$d$ and O: 2p electrons are found to be very sensitive to the change of crystal structure.

We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Cross-sectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wave function is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tight-binding model of Mn acceptors in GaAs. For Mn acceptors on the (110) surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantly the magnetic anisotropy landscape. However, the acceptor hole wave function is strongly localized around the Mn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed on deeper layers below the surface, the acceptor hole wave function is more delocalized and the corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However, the magnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis. We predict that substantially larger magnetic fields are required to observe a significant field dependence of the tunneling current for impurities located several layers below the GaAs surface.

We investigate the process of exciton dissociation in ordered and disordered model donor/acceptor systems and describe a method to calculate exciton dissociation rates. We consider a one-dimensional system with Frenkel states in the donor material and states where charge transfer has taken place between donor and acceptor. We introduce a Green's function approach to calculate the generation rates of charge-transfer states. For disorder in the Frenkel states we find a clear exponential dependence of charge dissociation rates with exciton-interface distance, with a distance decay constant $\beta$ that increases linearly with the amount of disorder. Disorder in the parameters that describe (final) charge-transfer states has little effect on the rates. Exciton dissociation invariably leads to partially separated charges. In all cases final states are “hot” charge-transfer states, with electron and hole located far from the interface.

The electronic and magnetic properties of hydrogenated monolayer MoS${}_2$ subject to equibiaxial tensile strain are systematically investigated using first-principles calculations. It is shown that at the strain-free state, the hydrogenated MoS${}_2$ is an $n$-type semiconductor with no spontaneous magnetism. As the tensile strain increases, however, a ground-state transition from nonmagnetism to ferromagnetism (FM) occurs with simultaneously enhanced magnetic moment and stability. The maximum FM state is achieved at a strain of 6.6$\%$, which corresponds to Curie temperature $T_c$ of 232 K. As the strain increases further, both strength and stability of the ferromagnetic state weaken and eventually vanish due to the enhanced ionic character in Mo-S bonds and the resulting delocalization of Mo $d$ orbitals. Similar behavior is also predicted in hydrogenated monolayer MoSe${}_2$. The present work by combining chemical functionalization and strain engineering provides a route to harness the magnetic properties of two-dimensional transition metal dichalcogenides for spintronic applications.

Conductance fluctuations produced by the presence of disorder in zigzag and armchair graphene nanoribbons are studied. We show that quantum transport in zigzag nanoribbons takes place via edge states which are exponentially localized, as in the standard Anderson localization problem, whereas for armchair nanoribbons the symmetry of the graphene sublattices produces anomalous localization, or delocalization. We show that these two different electron localizations lead to significant differences of the conductance statistics between zigzag and armchair nanoribbons. In particular, armchair nanoribbons show nonconventional large conductance fluctuations relative to those of Anderson-localized electrons. We calculate analytically the complete distribution of conductances for both types of ribbons. Without free fitting parameters, we verify our theoretical results by performing numerical simulations of disordered zigzag and armchair nanoribbons of experimentally achievable lengths and widths.

The bulk sensitivity of hard x-ray photoelectron spectroscopy in combination with circularly polarized radiation of the P09 beamline at PETRA III enables the investigation of the magnetic properties of capped films. We have determined the temperature dependence of the magnetic circular dichroism in the Fe 2$p$ and in the Gd 3$d$ states in amorphous GdFe films. The magnetic dichroism reflects the stronger temperature dependence of Gd moments compared to Fe moments in agreement with mean-field models. We resolved the exchange split Gd 3$d_{5/2}$ substates and found a significant temperature dependence of the splitting which is attributed to a temperature dependent part of the exchange energy caused by Stoner-like itinerant states. A comparison of experimental and simulated x-ray photoemission spectra with corresponding data for x-ray absorption spectroscopy exciting the identical core holes clearly demonstrates that the core hole exchange interaction is the origin of the observed multiplet features for Gd.

The dependence of the superconducting gaps in epitaxial Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ thin films on the nominal doping $x$ ($0.04⩽x⩽0.15$) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps—although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps ${\Delta }_{\mathrm{S}}$ and ${\Delta }_{\mathrm{L}}$ show similar monotonic trends as a function of the local critical temperature $T_{\mathrm{c}}^{\mathrm{A}}$ (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on $x$ is well correlated to the trend of the critical temperature, i.e., to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with $T_{\mathrm{c}}$ according to the empirical law ${\Omega }_0=4.65k_{\mathrm{B}}{T}_{\mathrm{c}}$, and with a total electron-boson coupling strength ${\lambda }_{\mathrm{tot}}=2.22$ for $x⩽0.10$ (i.e., up to optimal doping) that slightly decreases to ${\lambda }_{\mathrm{tot}}=1.82$ in the overdoped samples ($x=0.15$).

We consider a voltage-biased normal metal-insulator-superconductor (NIS) tunnel junction, connected to a high-temperature external electromagnetic environment. This model system features the commonly observed subgap leakage current in NIS junctions through photon-assisted tunneling which is detrimental for applications. We first consider a NIS junction directly coupled to the environment and analyze the subgap leakage current both analytically and numerically; we discuss the link with the phenomenological Dynes parameter. Then, we focus on a circuit where a low-temperature lossy transmission line is inserted between the NIS junction and the environment. We show that the amplitude of the transmitted frequencies relevant for the photon-assisted tunneling is exponentially suppressed as the length $\ell$ and the resistance per unit length $R_0$ of the line are increased. Consequently, the subgap current is reduced exponentially as well. This property can not be obtained by means of lumped circuit elements. We finally discuss our results in view of the performance of NIS junctions in applications.

We employ internal photoemission spectroscopy to directly measure the valence-band Van Hove singularity, and identify phonons participating in indirect intervalence-band optical transitions. Photoemission of holes photoexcited through transitions between valence bands displays a clear and resolvable threshold, unlike previous reports of interband critical points which become obscure in doped materials. We also demonstrate the enhancement of optical phonon-assisted features primarily contributing to the photoemission yield. This result is evidence of the relaxation of photoexcited hot holes through intravalence-band scatterings in heterojunctions, which contrast with intervalence-band scatterings in bulks.

Recent electron quantum optics experiments performed with on-demand single electron sources call for a mixed time/frequency approach to electronic quantum coherence. Here, we present a Wigner function representation of first-order electronic coherence and show that it provides a natural visualization of the excitations emitted by recently demonstrated single electron sources. It also gives a unified perspective on single particle and two particle interferometry experiments. In particular, we introduce a nonclassicality criterion for single electron coherence and discuss it in the context of Mach-Zehnder interferometry. Finally, the electronic Hanbury Brown and Twiss and the Hong-Ou-Mandel experiments are interpreted in terms of overlap of Wigner function, thus connecting them to signal processing.

A long phase coherent normal (N) wire between superconductors (S) is characterized by a dense phase-dependent Andreev spectrum. We investigate the current response of Andreev states of an NS ring to a time-dependent Aharonov Bohm flux superimposed to a dc one. The ring is modeled with a tight-binding Hamiltonian including a superconducting region with a BCS coupling between electron and hole states, in contact with a normal region with on-site disorder. Both dc and ac currents are determined from the computed eigenstates and energies using a Kubo formula approach. Beside the well-known Josephson current, we identify different contributions to the ac response: a low-frequency one related to the dynamics of the thermal occupations of the Andreev states and a higher-frequency one related to microwave induced transitions between levels. Both are characterized by phase dependencies with a high-harmonic content, opposite to one another. Our findings are successfully compared to the results of recent experiments.

We show that the junction design in the laterally configured ferromagnetic/nonmagnetic (FM/NM) hybrid structure significantly affects the spin transport under high-bias current. The thermal conductivity mismatch between FM and NM and the inhomogeneous current distribution at the interface produce a reversed temperature gradient, which drastically reduces the thermal spin injection efficiency. The homogeneity in the temperature gradient at the interface is a crucial factor in determining the efficiency of the thermal spin current generation. The experimental results show excellent agreement with the theoretical model [Y. Takezoe et al., Phys. Rev. B 82, 094451 (2010)] that the spin relaxation is proportional to the second spatial derivative of the temperature.

We study nonequilibrium polariton condensates in a harmonic potential trap. After finding the steady-state density and supercurrent distributions of nonequilibrium condensates, we calculate the collective excitations of the system. From collective excitation spectra, we can identify the bifurcation of the stable and unstable modes in terms of the pumping spot size and power. There exist Nambu-Goldstone modes that reveal the pattern of the spontaneous symmetry breaking of the system. Moreover, the unstable mechanism associated with the inward supercurrent flow is characterized by the existence of a supersonic region within the condensate.

We consider a spin-$1/2$ Bose-Einstein condensate initially prepared in a single-spin projection. The two channels of dynamical excitations existing in such a system (density and spin waves) are discussed and we show how pure spin waves can be excited in the presence of local magnetic defects. We analyze the role played by spin excitations on the Landau superfluidity criterion and demonstrate the absence of absolute superfluidity for the antiferromagnetic condensate. In the ferromagnetic case, we identify two critical velocities for the breakdown of superfluidity.

In type-II superconductors, the flux-free state (Meissner state) may be invaded by vortices bearing quantized flux once $H$ is above the lower critical field $H_{c1}$. However, the actual first flux penetration does not occur at $H_{c1}$ due to the presence of a surface barrier and the fact that the Meissner state may also exist as a metastable state up to a larger (superheating) field. In this work we determine the field for the first vortex penetration in superconductors by directly imaging the first vortex threading a superconducting Pb film with antidots. We find that the first vortex penetration occurs when the surface superconducting currents reach the depairing current, locally breaking superconductivity and allowing a vortex to nucleate.

Thorough resistivity measurements on single crystals of CeZn${}_{11}$ under pressure $p$ and magnetic field $H$ are presented. At ambient pressure, CeZn${}_{11}$ orders antiferromagnetically at $T_N=2$ K. The pressure dependence of the resistivity reveals an increase of the Kondo effect. We determine the pressure evolution of the magnetic exchange interaction between conduction and localized $4f$ electrons. It qualitatively reproduces the pressure evolution of the magnetic ordering temperature $T_{O_1}$ (with $T_{O_1}=T_N$ at ambient pressure). In addition to $T_{O_1}$, a new anomaly $T_{O_2}$ appears under pressure. Both anomalies are found to increase with applied pressure up to $4.9$ GPa, indicating that CeZn${}_{11}$ is far from a pressure induced quantum critical point. Complex $T$-$H$ phase diagrams are obtained under pressure which reveal the instability of the ground state in this compound.

We calculate the exact Kohn-Sham (KS) scalar and vector potentials that reproduce, within current density functional theory, the steady-state density and current density corresponding to an electron quasiparticle added to the ground state of a model quantum wire. Our results show that, even in the absence of an external magnetic field, a KS description of a steady-state system in general requires a nonzero exchange-correlation magnetic field that is purely mechanical in origin. The KS paramagnetic current density is not, in general, that of the interacting system in any gauge.

The effective crystal field in multiorbital correlated materials can be either enhanced or reduced by electronic correlations with crucial consequences for the topology of the Fermi surface and, hence, on the physical properties of these systems. In this respect, recent local density approximation plus dynamical mean-field theory studies of Ni-based heterostructure have shown contradicting results, depending on whether the less correlated $p$ orbitals are included or not. We investigate the origin of this problem and identify the key parameters controlling the Fermi surface properties of these systems. Without the $p$ orbitals, the model is quarter-filled, while the $d$ manifold moves rapidly towards half-filling when the $p$ orbitals are included. This implies that the local Hund's exchange, while rather unimportant for the former case, can play a predominant role in controlling the orbital polarization for the extended basis set by favoring the formation of a larger local magnetic moment.

The $p$-type semiconductor Cu${}_3$PSe${}_4$ has recently been established to have a direct band gap of 1.4 eV and an optical absorption spectrum similar to GaAs [Foster et al., Appl. Phys. Lett. 99, 181903 (2011)], suggesting a possible application as a solar photovoltaic absorber. Here we calculate the thermodynamic stability, defect energies and concentrations, and several material properties of Cu${}_3$PSe${}_4$ using a wholly GGA+$U$ method (the generalized gradient approximation of density functional theory with a Hubbard $U$ term included for the Cu-$d$ orbitals). We find that two low energy acceptor defects, the copper vacancy V${}_{\text{Cu}}$ and the phosphorus-on-selenium antisite P${}_{\text{Se}}$, establish the $p$-type behavior and likely prevent any $n$-type doping near thermal equilibrium. The GGA+$U$ defect calculation method is shown to yield more accurate results than the more standard method of applying post-calculation GGA+$U$-based band-gap corrections to strictly GGA defect calculations.