A fully self-contained study of the thermodynamic and electronic properties of intrinsic point defects in the solar absorber materials CuInSe${}_2$ and CuGaSe${}_2$ based on screened-exchange hybrid density functional theory is presented. The results are partly at odds with data obtained within local density functional theory in former studies. ${\mathrm{Ga}}_{\mathrm{Cu}}$ electron traps as well as ${\mathrm{Cu}}_{\mathrm{In}}$ and ${\mathrm{Cu}}_{\mathrm{Ga}}$ hole traps are found to be the dominant intrinsic recombination centers. In contrast to the accepted view, complex formation of antisites with copper vacancies is not decisive for explaining the favorable properties of CuInSe${}_2$, since ${\mathrm{In}}_{\mathrm{Cu}}$ is already a shallow defect by itself. The localization of holes is observed on ${\mathrm{Cu}}_{\mathrm{In}}$ and ${\mathrm{Cu}}_{\mathrm{Ga}}$ as well as on ${\mathrm{V}}_{\mathrm{In}}$ and ${\mathrm{V}}_{\mathrm{Ga}}$ when supercells of 216 atoms are used. Furthermore, the results raise doubts about the relevance of selenium vacancies and DX centers for experimentally observed metastabilities. Finally, a guide to the optimal preparation conditions in terms of the point defect physics of CuInSe${}_2$ and CuGaSe${}_2$ for their application as solar cell absorbers is provided.

We introduce the concept of nearly hyperuniform network (NHN) structures as alternatives to the conventional continuous random network (CRN) models for amorphous tetrahedrally coordinated solids, such as amorphous silicon (a-Si). A hyperuniform solid has a structure factor $S\left(k\right)$ that approaches zero as the wavenumber $k\to 0$. We define a NHN as an amorphous network whose structure factor $S(k\to 0)$ is smaller than the liquid value at the melting temperature. Using a novel implementation of the Stillinger-Weber potential for the interatomic interactions, we show that the energy landscape for a spectrum of NHNs includes a sequence of local minima with an increasing degree of hyperuniformity [smaller $S(k\to 0)$] that is significantly below the frozen-liquid value and that correlates with other measurable features in $S\left(k\right)$ at intermediate and large $k$ and with the width of the electronic band gap.

Silver orthophosphate (Ag${}_3$PO${}_4$) exhibits extremely high photocatalytic activity under visible light. Gaining insight into the behavior of point defects would open opportunities for new applications. Our hybrid density-functional calculations show that intrinsic point defects are unlikely to cause carrier trapping. Charge neutrality analysis suggests that Ag${}_3$PO${}_4$ possibly behaves as a weak $n$-type semiconductor under the O-poor condition and as a weak $p$-type semiconductor under the O-rich condition. Unlikely to other oxides, unintentionally incorporated hydrogen does not necessarily lead to strong $n$-type conductivity. Migration properties of some Ag-related defects and hydrogen are also discussed.

We examine and compare the diagonal magnetoresistance, $R_{xx}$, and the photovoltage induced by microwave ($42\le f<300$ GHz) and terahertz ($f\ge 300$ GHz) photoexcitation in the high mobility quasi-two-dimensional GaAs/AlGaAs system. The data demonstrate strong radiation-induced magnetoresistance oscillations in $R_{xx}$ to 360 GHz. In addition, cyclotron resonance is observed in the photovoltage to 725 GHz. These results show that our high-mobility GaAs/AlGaAs two-dimensional electron system (2DES) specimens remain photoactive in magnetotransport into the terahertz band.

Selenium adsorption on molybdenum surfaces is a relevant process in the production of thin-film solar cells, in particular as far as the formation of the layered compound MoSe${}_2$ is concerned. In this paper we investigate the energetics of Se adsorption on the (110) surface of molybdenum using first-principles calculations in the two limiting cases of low and high coverage, and we establish a comparison with the more extensively investigated case of sulfur adsorption at submonolayer coverage. The studied system provides the opportunity for testing the most crucial approximations, namely, the choice of the exchange-correlation functional and the pseudopotential generation. We find that semicore states of molybdenum have an influence on calculated surface energies and, to a lesser extent, on adsorption energies. We compare some more or less popular semilocal exchange-correlation functionals, including one recently proposed as an improvement for quasi-two-dimensional systems. The results show that the preferred adsorption site changes with coverage and suggest a strong variation of the adsorption energy with coverage.

We study the intravalley spin-orbit-mediated spin relaxation in monolayers of MoS${}_2$ within a two bands effective Hamiltonian. The intrinsic spin splitting of the valence band as well as a Rashba-like coupling due to the breaking of the out-of-plane inversion symmetry are considered. We show that, in the hole doped regime, the out-of-plane spin relaxation is not very efficient since the spin splitting of the valence band tends to stabilize the spin polarization in this direction. We obtain spin lifetimes larger than nanoseconds, in agreement with recent valley polarization experiments.

The paper proposes a self-consistent Green function description of the induced surface superconductivity in a disordered three-dimensional topological insulator (TI) coupled to an $s$-wave superconductor. We recover earlier results regarding the induced spin-triplet $p$-wave pairing, showing that a mixture of $p$- and $s$-wave pair correlations appears as a result of broken spin-rotation symmetry on the helical surface of the TI. Unlike the $s$-wave pairing, the $p$-wave component is found to be suppressed in dirty TIs in which the elastic mean-free path is much smaller than the superconducting coherence length. The suppression is due to the generic nonlocality of the spin-triplet correlations, which makes them strongly dependent on the mean-free path in a disordered system. In dirty TIs the induced superconductivity is predicted to be predominantly $s$-wave like. In cleaner TIs, however, the $p$-wave component may reach a magnitude comparable to (but not larger than) the $s$-wave pairing.

We report a combined experimental and theoretical study of the spin $S=\frac{1}{2}$ nanomagnet Cu${}_5$(OH)${}_2$(NIPA)${}_4·$10H${}_2$O (Cu${}_5$-NIPA). Using thermodynamic, electron spin resonance, and ${}^1$H nuclear magnetic resonance measurements on one hand, and ab initio density-functional band-structure calculations, exact diagonalizations, and a strong-coupling theory on the other, we derive a microscopic magnetic model of Cu${}_5$-NIPA and characterize the spin dynamics of this system. The elementary fivefold Cu${}^{2+}$ unit features an hourglass structure of two corner-sharing scalene triangles related by inversion symmetry. Our microscopic Heisenberg model comprises one ferromagnetic and two antiferromagnetic exchange couplings in each triangle, stabilizing a single spin $S=\frac{1}{2}$ doublet ground state (GS), with an exactly vanishing zero-field splitting (by Kramers' theorem), and a very large excitation gap of $\Delta \simeq 68$ K. Thus, Cu${}_5$-NIPA is a good candidate for achieving long electronic spin relaxation ($T_1$) and coherence ($T_2$) times at low temperatures, in analogy to other nanomagnets with low-spin GS's. Of particular interest is the strongly inhomogeneous distribution of the GS magnetic moment over the five Cu${}^{2+}$ spins. This is a purely quantum-mechanical effect since, despite the nonfrustrated nature of the magnetic couplings, the GS is far from the classical collinear ferrimagnetic configuration. Finally, Cu${}_5$-NIPA is a rare example of a $S=\frac{1}{2}$ nanomagnet showing an enhancement in the nuclear spin-lattice relaxation rate $1/T_1$ at intermediate temperatures.

We report the observation of superconductivity in the CuIr${}_2$Se${}_4$ spinel induced by partial substitution of Pt for Ir. The optimal doping level for superconductivity in Cu(Ir${}_{1-x}$Pt${}_x$)${}_2$Se${}_4$ is $x$ $=$ 0.2, where $T_c$ is 1.76 K. A superconducting $T_c$ vs composition dome is established between the metallic, normal conductor CuIr${}_2$Se${}_4$ and semiconducting CuIrPtSe${}_4$. Electronic structure calculations show that the optimal $T_c$ occurs near the electron count of a large peak in the calculated electronic density of states and that CuIrPtSe${}_4$ is a band-filled insulator. Characterization of the superconducting state in this heavy metal spinel through determination of $\Delta C/\gamma T_c$ indicates that it is BCS-like. The relatively high upper critical field at the optimal superconducting composition [$H_c$${}_2$(0) $=$ 3.2 T] is much larger than that reported for analogous rhodium spinels and is comparable to or exceeds the Pauli field (${\mu }_0{H}_P$), suggesting that strong spin-orbit coupling may influence the superconducting state. Further, comparison to doped CuIr${}_2$S${}_4$ suggests that superconductivity in iridium spinels is not necessarily associated with the destabilization of a charge-ordered spin-paired state through doping.

The use of high-performance perovskite-structure epitaxial relaxor ferroelectric films [S. H. Baek et al., Science 334, 958 (2011)] is hindered by the lack of knowledge of epitaxial effects therein. It is experimentally shown here that a biaxial epitaxial compression can favor the relaxor state over ferroelectricity. The absence of the ferroelectric transition and the existence of the low-temperature relaxor state are evidenced by a combination of x-ray diffraction, dielectric, polarization, and optical studies of PbSc${}_{0.5}$Nb${}_{0.5}$O${}_3$ films epitaxially grown on La${}_{0.5}$Sr${}_{0.5}$CoO${}_3$/MgO(001). This finding is beyond existing models of polarization in perovskite-structure epitaxial films and beyond the established ability to induce ferroelectricity by an epitaxial strain.

In the effective Ising spin-1/2 antiferromagnetic chain system BaCo${}_{\text{2}}$V${}_{\text{2}}$O${}_{\text{8}}$ the magnetic-field influence is highly anisotropic. For magnetic fields along the easy axis $c$, the Néel order is strongly suppressed already at low fields and an incommensurate order is entered above 4 T. We present a detailed study of the magnetic phase diagrams for different magnetic field directions, which are derived from magnetization data, high-resolution thermal expansion, and magnetostriction measurements as well as from the thermal conductivity. Zero-field thermal-expansion data reveal that the magnetic transition is accompanied by an orthorhombic distortion within the $ab$ plane. Under ambient conditions the crystals are heavily twinned, but the domain orientation can be influenced either by applying uniaxial pressure or a magnetic field along the [100] direction. In addition, our data reveal a pronounced in-plane magnetic anisotropy for fields applied within the $ab$ plane. For $H\parallel \left[110\right]$, the magnetic field influence on $T_{\text{N}}$ is weak, whereas for magnetic fields applied along [100], $T_{\text{N}}$ vanishes at about 10 T and the zero-field Néel order is completely suppressed as is confirmed by neutron diffraction data. The second-order phase transition strongly suggests a quantum critical point being present at $H\simeq 10$ T parallel [100].

Single crystals of the type-I clathrate series Sr${}_{8-x}$Eu${}_x$Ga${}_{16}$Ge${}_{30}$ ($x=0.01$–8) were grown by Ga flux and systematically studied by magnetic susceptibility, electrical resistivity, and electron spin resonance (ESR) measurements. Both magnetic transition temperatures ($T_c$ and $T^{*}$) were mapped as a function of $x$. We found that the Curie temperature ($T_c$) and the magnetic anomaly temperature ($T^{*}$) decrease systematically as Eu is substituted by Sr until no ordering is observed above 2 K for $x=1.0$. For samples where the rattling Einstein temperature of ${\theta }_E\approx 25$–60 K lies above $T_c$, we have observed no features in the ESR data, suggesting that the Eu${}^{2+}$ 4$f$ spin dynamics is weakly coupled to the rattling modes. As the temperature is lowered and the magnetic field is increased, the ESR linewidth ($\Delta H$) behavior points to the formation of magnetic polarons.

An explanation is given for the ultrafast insulator-to-metal transition in VO${}_2$ following photoexcitation. The photoinduced orbital imbalance induces a coherent motion of the V-V dimers affecting the electronic structure. After the closing of the gap, Boltzmann scattering equilibrates the electron densities. If the electron density exceeds a critical value, a phase transition occurs to the metallic state. The model explains several key features, such as a structural bottleneck, coherent structural motion combined with phase shifts in the oscillation, the absence of ultrafast metal-to-insulator transitions, and the need for a critical fluency.

We have investigated hydrogenated single-wall armchair ($n$,$n$) carbon nanotubes (CNTs) with $4\le n\le 12$ by using density functional theory calculations. Hydrogen adsorption may transform the CNTs into a horseshoe shape. Nonmagnetic edge-localized electron states have been identified in CNTs of horseshoe shape in this work. The $\sigma$-bond-like orbital overlaps across the edge-localized electrons, which sustain the horseshoe shape of the CNTs and hinder spontaneous unravelling into a graphene nanoribbon, cause the horseshoe-shape CNTs to become nonmagnetic metal. The bonding energy between the hydrogenated edges increases with the CNT diameter, approaching a limit of $\sim$1 eV.

Using electron energy loss spectroscopy, we have probed the spin waves of fcc(100) cobalt after capping with 1–3 atom layers of pseudomorphic nickel or up to 12 atom layers of copper. The intensity decay of the spin wave signal is quantitatively described by the mean-free path of the incident and the scattered electron within the capping layer. The observed spin waves are therefore localized at the cobalt side of the Co/Ni and Co/Cu interfaces. Compared to the free cobalt surface, the interface spin waves are downshifted in frequency. The effect is attributed to a reduced exchange interaction between cobalt atoms at the interface.

We study the effects of strain on the electronic properties and persistent current characteristics of a graphene ring using the Dirac representation. For a slightly deformed graphene ring flake, one obtains sizable pseudomagnetic (gauge) fields that may effectively reduce or enhance locally the applied magnetic flux through the ring. Flux-induced persistent currents in a flat ring have full rotational symmetry throughout the structure; in contrast, we show that currents in the presence of a circularly symmetric deformation are strongly inhomogeneous, due to the underlying symmetries of graphene. This result illustrates the inherent competition between the “real” magnetic field and the “pseudo” field arising from strains, and suggests an alternative way to probe the strength and symmetries of pseudomagnetic fields on graphene systems.

Transport measurements were performed on the organic layered compound $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$ under hydrostatic pressure. The carrier types, densities, and mobilities are determined from the magnetoconductance of $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$. While evidence of high-mobility massless Dirac carriers has already been given, we report here their coexistence with low-mobility massive holes. This coexistence seems robust as it has been found up to the highest studied pressure. Our results are in agreement with recent DFT calculations of the band structure of this system under hydrostatic pressure. A comparison with graphene Dirac carriers has also been done.

A reversible optomechanical molecular switch based on a single azobenzene molecule suspended via thiolate links between realistic models of gold tips is investigated. Using a combination of the transfer-matrix technique and density functional theory, we focus on the conductance of the nanodevice in the two (meta)stable cis- and trans-junction conformations. We find the conductance of both conformations to be broadly similar. In qualitative agreement with related experiments, we find that the same nanodevice with one/two methylene linker group(s) inserted on one/both ends of the azobenzene molecule is driven into the tunneling regime and reduces the conductances by up to 2 orders of magnitude, again, almost uniformly for both conformations. These results clarify the huge differences in switching ratios found previously and indicate that this nanodevice is not particularly suited for use as a molecular switch based on conductance change.

We study parametric quantum pumping in a two-dimensional topological insulator bar in the presence of electron interactions described by a helical Luttinger liquid. The pumping current is generated by two point contacts whose tunneling amplitudes are modulated in time. The helical nature of the edge states of the system ensures the generation of a pumped spin current that is determined by interference effects related to spin-flipping or spin-preserving tunneling at the quantum point contacts and which can be controlled by all electrical means. We show that the period of oscillation and the position of the zeros of the spin current depend on the strength of the electron interactions, giving the opportunity to directly extract information about them when measured.

We present lattice constants and cohesive energies of alkali, alkaline earth, and transition metals using the correlation energy evaluated within the adiabatic-connection fluctuation-dissipation (ACFD) framework in the random phase approximation (RPA) and compare our findings to results obtained with the meta-GGA functional revTPSS and the gradient corrected PBE (Perdew-Burke-Ernzerhof) functional and the PBEsol functional (PBE reparametrized for solids), as well as a van der Waals (vdW) corrected functional optB88-vdW. Generally, the RPA reduces the mean absolute error in the lattice constants by about a factor 2 compared to the other functionals. Atomization energies are also on par with the PBE functional, and about a factor 2 better than with the other functionals. The study confirms that the RPA describes all bonding situations equally well including van der Waals, covalent, and metallic bonding.