The behavior of cation substitutional hole doping in GaN and ZnO is investigated using hybrid density functional calculations. Our results reveal that Mg substitution for Ga $\left({\mathrm{Mg}}_{\mathrm{Ga}}\right)$ in GaN can assume three different configurations. Two of the configurations are characterized by the formation of defect-bound small polaron (i.e., a large structural distortion accompanied by hole localization on one of the neighboring N atoms). The third one has a relatively small but significant distortion that is characterized by highly anisotropic polaron localization. In this third configuration, ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits both effective-mass-like and noneffective-mass-like characters. In contrast, a similar defect in ZnO, ${\mathrm{Li}}_{\mathrm{Zn}}$, cannot sustain the anisotropic polaron in the hybrid functional calculation, but undergoes spontaneous breaking of a mirror symmetry through a mechanism driven by the hole localization. Finally, using ${\mathrm{Na}}_{\mathrm{Zn}}$ in ZnO as an example, we show that the deep acceptor levels of the small-polaron defects could be made shallower by applying compressive strain to the material.

We study theoretically the performance of electrically pumped self-organized quantum dots as a gain material in the mid-IR range at room temperature. We analyze an AlGaAs/InGaAs based structure composed of dots-in-a-well sandwiched between two quantum wells. We numerically analyze a comprehensive model by combining a many-particle approach for electronic dynamics with a realistic modeling of the electronic states in the whole structure. We investigate the gain both for quasiequilibrium conditions and current injection. Comparing different structures, we find that steady-state gain can only be realized by an efficient extraction process, which prevents an accumulation of electrons in continuum states, that make the available scattering pathways through the quantum dot active region too fast to sustain inversion. The tradeoff between different extraction/injection pathways is discussed. Comparing the modal gain to a standard quantum-well structure as used in quantum cascade lasers, our calculations predict reduced threshold current densities of the quantum dot structure for comparable modal gain. Such a comparable modal gain can, however, only be achieved for an inhomogeneous broadening of a quantum-dot ensemble that is close to the lower limit achievable today using self-organized growth.

By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance–area products $\left(R_{c}A\right)$ of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential $dV\text{−}dI$ curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small $R_{c}A$ values. With increasing $R_{c}A$ values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.

Graphene flakes placed on hexagonal boron nitride feature in the presence of a magnetic field a complex electronic structure due to a hexagonal moiré potential resulting from the van der Waals interaction with the substrate. The slight lattice mismatch gives rise to a periodic supercell potential. Zone folding is expected to create replicas of the original Dirac cone and Hofstadter butterflies. Our large-scale tight-binding simulation reveals an unexpected coexistence of a relativistic and nonrelativistic Landau level structure. The presence of the zeroth Landau level and its associated butterfly is shown to be the unambiguous signature for the occurrence of the Dirac cone replica.

We propose a new quantization method for superconducting electronic circuits involving a Josephson-junction device coupled to a linear microwave environment. The method is based on an exact impedance synthesis of the microwave environment considered as a blackbox with impedance function $Z\left(s\right)$. The synthesized circuit captures dissipative dynamics of the system with resistors coupled to the reactive part of the circuit in a nontrivial way. We quantize the circuit and compute relaxation rates following previous formalisms for lumped element circuit quantization. Up to the errors in the fit our method gives an exact description of the system and its losses.

We have prepared polycrystalline samples of ${\mathrm{LaSrRh}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_4$ and ${\mathrm{LaSr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{RhO}}_4$, and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization, and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the $g$ factor to be $g=2.3$ from the electron spin resonance, and the spin number to be $S=1$ from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the $S=1$ spins is 2%–5%, which depends on the degree of disorder in the La/Sr/Ca site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic ${\mathrm{Rh}}^{3+}$ ions is discussed.

Classical spin-wave calculations demonstrate that the macroscopic continuous degeneracy associated with the two-dimensional kagome Heisenberg spin lattice persists in the case of the stacked fcc structure, giving rise to zero-energy modes in three dimensions. The addition of an effective local cubic anisotropy is shown to remove this continuous degeneracy and introduce a gap in the spectrum as well as modify the inelastic scattering function $S(\mathbf{q},\omega )$. This scenario supports earlier Monte Carlo simulations which indicate that the phase transition to long-range $q=0$ magnetic order is driven to be discontinuous by critical fluctuations associated with the large degeneracy in the absence of anisotropy but becomes continuous with the addition of anisotropy. The results are relevant to Ir-Mn alloys, which are widely used in the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in spin-valve structures.

Nonresonant circularly polarized electromagnetic radiation can exert torques on magnetizations by the inverse Faraday effect (IFE). Here, we discuss the enhancement of IFE by spin-orbit interactions. We illustrate the principle by studying a simple generic model system, i.e., the quasi-one-dimensional ring in the presence of linear/cubic Rashba and Dresselhaus interactions. We combine the classical IFE in electron plasmas that is known to cause persistent currents in the plane perpendicular to the direction of the propagation of light with the concept of current and spin-orbit-induced spin transfer torques. We calculate light-induced spin polarization that in ferromagnets might give rise to magnetization switching.

We present ab initio calculations to investigate second harmonic generation (SHG) response of single wall zigzag pristine boron-nitride nanotubes (BNNTs) and BNNTs modified by the molecular hydrogen adsorption. Calculations have been performed using density functional theory (DFT) within the local-density approximation (LDA) together with the GW Green function method to determine the band gap. A length gauge approach has been used to calculate the nonlinear optical response with the scissors correction to obtain the nonlinear susceptibility ${\chi }^{zzz}(-2\omega ;\omega ,\omega )$ of the zigzag BNNTs. We have found that, contrary to reports in the literature, the (5,0) and (9,0) BNNTs have a nonvanishing SHG response. We have also found that SHG intensity decreases with the increase of the molecular hydrogen coverage.

We use numerical simulations to predict peculiar magnetotransport fingerprints in polycrystalline graphene, driven by the presence of grain boundaries of varying size and orientation. The formation of Landau levels is shown to be restricted by the polycrystalline morphology, requiring the magnetic length to be smaller than the average grain radius. The nature of localization is also found to be unusual, with strongly localized states at the center of Landau levels (including the usually highly robust zero-energy state) and extended electronic states lying between Landau levels. These extended states percolate along the network of grain boundaries, resulting in a finite value for the bulk dissipative conductivity and suppression of the quantized Hall conductance. Such breakdown of the quantum Hall regime provoked by extended structural defects is also illustrated through two-terminal Landauer-Büttiker conductance calculations, indicating how a single grain boundary induces cross linking between edge states lying at opposite sides of a ribbon geometry.

We study bilayer fermionic cold-atom systems with dipolar interactions, as well as a two-component tunable pseudopotential (TCTP) model which keeps only the zeroth- and first-order Haldane pseudopotentials, at a total Landau-level filling factor of 1/2. Our numerical results on the TCTP model indicate that a Haldane-Rezayi state describes the critical point between strong and weak $d$-wave pairing quantum Hall phases. Upon further increasing the attractive zeroth-order pseudopotentials, the system transits from the strong-pairing phase to a stripe phase and then to a cluster phase (or phase separation). The dipolar interaction can be mapped onto the TCTP model in the strong-pairing phase if high-order pseudopotentials are ignored. Our numerical results show that this is indeed the case, so the strong-pairing phase exists in the cold-atom system.

A quantum field theory approach is applied to investigate the dynamics of flexural phonons in a metallic membrane like graphene, looking for the effects deriving from the strong interaction between the electronic excitations and elastic deformations. Relying on a self-consistent screening approximation to the phonon self-energy, we show that the theory has a critical point characterized by the vanishing of the effective bending rigidity of the membrane at low momentum. We also check that the instability in the sector of flexural phonons takes place without the development of an in-plane static distortion, which is avoided due to the significant reduction of the electron-phonon couplings for in-plane phonons at large momenta. Furthermore, we analyze the scaling properties of the many-body theory to identify the order parameter that opens up at the point of the transition. We find that the vanishing of the effective bending rigidity and the onset of a nonzero expectation value of the mean curvature are concurrent manifestations of the critical behavior. The results presented here imply that, even in the absence of tension, the theory has a critical point at which the flat geometry becomes an unstable configuration of the metallic membrane, with a condensation of the mean curvature field that may well reproduce the smooth distribution of ripples observed in free-standing graphene.

Based on a minimal two-orbital model [Tai et al., Europhys. Lett. 103, 67001 (2013)], which captures the canonical electron-hole-doping phase diagram of the iron-pnictide ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, we study the evolution of quasiparticle states as a function of doping using the Bogoliubov–de Gennes equations with and without a single impurity. Analyzing the density of states of uniformly doped samples, we are able to identify the origin of the two superconducting gaps observed in optimally hole- or electron-doped systems. The local density of states (LDOS) is then examined near a single impurity in samples without antiferromagnetic order. The qualitative features of our results near the single impurity are consistent with a work based on a five-orbital model [T. Kariyado et al., J. Phys. Soc. Jpn. 79, 083704 (2010)]. Some of the results are consistent with recent angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy experiments. This further supports the validity of our two-orbital model in dealing with LDOS in the single-impurity problem. Finally, we investigate the evolution of the LDOS with doping near a single impurity in the unitary or strong scattering limit, such as Zn replacing Fe. The positions of the in-gap resonance peaks exhibited in our LDOS may indirectly reflect the evolution of the Fermi surface topology according to the phase diagram. Our prediction of in-gap states and the evolution of the LDOS near a strong scattering single impurity can be validated by further experiments probing the local quasiparticle spectrum.

We argue that spin-sensitive quasiparticle scattering may generate electron-hole imbalance in superconducting structures, such as, e.g., superconducting-normal hybrids with spin-active interfaces. We elucidate a transparent physical mechanism for this effect demonstrating that scattering rates for electrons and holes at such interfaces differ from each other. Explicitly evaluating the wave functions of electronlike and holelike excitations in superconducting-normal bilayers we derive a general expression for the thermoelectric current and show that—in the presence of electron-hole imbalance—this current can reach maximum values as high as the critical current of a superconductor.

We review the concept of superfluidity and, based on real and thought experiments, we use the formalism of second quantization to derive expressions that allow the calculation of the superfluid density for general Hamiltonians with path-integral methods. It is well known that the superfluid density can be related to the response of the free energy to a boundary phase twist, or to the fluctuations of the winding number. However, we show that this is true only for a particular class of Hamiltonians. In order to treat other classes, we derive general expressions of the superfluid density that are valid for various Hamiltonians. While the winding number is undefined when the number of particles is not conserved, our general expressions allow us to calculate the superfluid density in all cases. We also provide expressions of the superfluid densities associated to the individual components of multispecies Hamiltonians, which remain valid when interspecies conversions occur. The cases of continuous and discrete spaces are discussed, and we emphasize common mistakes that occur when considering lattices with nonorthonormal primitive vectors.

We investigate the pressure and temperature dependence of the lattice dynamics of the underdoped, stoichiometric, high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ by means of Raman spectroscopy and ab initio calculations. This system undergoes a reversible pressure-induced structural phase transition around 10 GPa to a collapsed orthorhombic structure that is well reproduced by the calculation. The coupling of the $B_{1g}$-like buckling phonon mode to the electronic continuum is used to probe superconductivity. In the low pressure phase, self-energy effects through the superconducting transition renormalize this phonon, and the amplitude of this renormalization strongly increases with pressure. Whereas our calculation indicates that this mode's coupling to the electronic system is only marginally affected by the structural phase transition, the aforementioned renormalization is completely suppressed in the high pressure phase, demonstrating that under hydrostatic pressures higher than 10 GPa, superconductivity in ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ is greatly weakened or obliterated.

We use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ (C15) with static antiferromagnetic (AF) order along the $a$ axis of the orthorhombic structure and ${\mathrm{NaFe}}_{0.935}{\mathrm{Co}}_{0.045}\mathrm{As}$ (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near $E_{r1}=3.25$ meV and a broad dispersionless mode at $E_{r2}=6$ meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the $a$ and $c$ axes, while the dispersionless mode is isotropic similar to that of C45. Since the $a$-axis polarized spin excitations of the anisotropic resonance appear below $T_c$, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity.

We have studied the MnGe chiral magnet below $T_{\text{N}}=170$ K, by magnetic measurements, Mössbauer spectroscopy, and by neutron diffraction at ambient and under nonhydrostatic pressure. At ambient pressure, we observe the coexistence of two magnetic phases belonging to the same crystal phase in a large temperature range (down to 100 K) below $T_{\mathrm{N}}$: ferromagnetically correlated rapidly fluctuating spins coexist with frozen spins involved in the helical order. Applying a uniaxial pressure component induces a strong magnetic texture, where most of the helical axes reorient along the stress axis. The magnetic texture persists in the fluctuating chiral state up to $T_{\mathrm{N}}$. Our results suggest that the zero field ground state at ambient pressure is a multidomain state consisting of helical domains with random orientations rather than a three-dimensional skyrmion lattice. They show the presence of an unusually broad transition to paramagnetism with a dynamical phase separation triggered by temperature.

Photoluminescence excitation measurements in semiconducting carbon nanotubes show a systematic nonresonant contribution between the well-known excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species, including its tiny nonresonant component. By comparison with the recently reported excitonic absorption cross section on the $S_{22}$ resonance, we found a universal nonresonant absorbance which turns out to be of the order of one-half of that of an equivalent graphene sheet. This value, as well as the absorption line shape in the nonresonant window, is in excellent agreement with microscopic calculations based on the density-matrix formalism. This nonresonant absorption of semiconducting nanotubes is essentially frequency independent over 0.5-eV-wide windows and reaches approximately the same value between the $S_{11}$ and $S_{22}$ resonances and between the $S_{22}$ and $S_{33}$ resonances. In addition, the nonresonant absorption cross section turns out to be the same for all the chiral species we measured in this study. From a practical point of view, this study provides a solid framework for sample content analysis based on photoluminescence studies by targeting specific excitation wavelengths that lead to almost uniform excitation of all the chiral species of a sample within a given diameter range.

In our work we combined experimental and theoretical investigations of the relaxation dynamics of the single wall carbon nanotubes (SW-CNTs) in solution samples with enriched chiralities of $(7,5)$ and $(7,6)$ species. In two-color pump-probe studies we observe three-exponential decay in the differential transmission spectra in the range of few picoseconds, tens of picoseconds, and hundreds of picoseconds. Decay curves are very similar for both SW-CNT chiralities under resonant excitation and probing of excited and ground state transition energies, respectively. Both types of tubes exhibit no changes in decay for the different excitation energies in the range $\pm 50\mathrm{meV}$ around the excited state. By tuning the probe pulse towards energies higher then ground state (up to $+350\mathrm{meV}$) we observe acceleration of the first decay component from $5.8\mathrm{ps}$ down to $1.6\mathrm{ps}$. Our experimental results are supported by time resolved microscopic calculations based on carbon nanotube Bloch equations proving the fast decay component behavior being dominated through scattering with acoustic phonons.