Halide perovskites have emerged as a potential photoconducting material for photovoltaics and hard radiation detection. We investigate the nature of charge transport in the semi-insulating chalcohalide ${\mathrm{Hg}}_3{\mathrm{Se}}_2{\mathrm{I}}_2$ compound using the temperature dependence of dark current, thermally stimulated current (TSC) spectroscopy, and photoconductivity measurements as well as first-principles density functional theory (DFT) calculations. Dark conductivity measurements and TSC spectroscopy indicate the presence of multiple shallow and deep level traps that have relatively low concentrations of the order of ${10}^{13}-{10}^{15}\mathrm{c}{\mathrm{m}}^{-3}$ and capture cross sections of $\sim {10}^{-16}\mathrm{c}{\mathrm{m}}^2$. A distinct persistent photoconductivity is observed at both low temperatures ($<170\mathrm{K}$) and high temperatures ($>230$ K), with major implications for room-temperature compound semiconductor radiation detection. From preliminary DFT calculations, the origin of the traps is attributed to intrinsic vacancy defects ($V_{\mathrm{Hg}}, V_{\mathrm{Se}}$, and $V_{\mathrm{I}}$) and interstitials (${\mathrm{Se}}_{\mathrm{int}}$) or other extrinsic impurities. The results point the way for future improvements in crystal quality and detector performance.

We consider two-dimensional (2D) Dirac quantum ring systems formed by the infinite mass constraint. When an Aharonov-Bohm magnetic flux is present, e.g., through the center of the ring domain, persistent currents, i.e., permanent currents without dissipation, can arise. In real materials, impurities and defects are inevitable, raising the issue of robustness of the persistent currents. Using localized random potential to simulate the disorder, we investigate how the ensemble-averaged current magnitude varies with the disorder density. For comparison, we study the nonrelativistic quantum counterpart by analyzing the solutions of the Schrödinger equation under the same geometrical and disorder settings. We find that, for the Dirac ring system, as the disorder density is gradually increased, the average current decreases slowly initially and then plateaus at a finite nonzero value, indicating remarkable robustness of the persistent currents. The physical mechanism responsible for the robustness is the emergence of a class of boundary states—whispering gallery modes. In contrast, in the Schrödinger ring system, such boundary states cannot form, and the currents diminish rapidly to zero with an increase in the disorder density. We develop a physical theory based on a quasi-one-dimensional approximation to understand the striking contrast in the behaviors of the persistent currents in the Dirac and Schrödinger rings. Our 2D Dirac ring systems with disorder can be experimentally realized, e.g., on the surface of a topological insulator with natural or deliberately added impurities from the fabrication process.

We report the interplay between Cooper pairs in superconductor NbN and topological surface states in $\mathrm{Sm}{\mathrm{B}}_6$ nanowire through magnetotransport measurements. The NbN electrodes are superconductive below 13.5 K and the $\mathrm{Sm}{\mathrm{B}}_6$ nanowire shows the surface states dominate transport below 6 K. The tunneling between the topological surface states in $\mathrm{Sm}{\mathrm{B}}_6$ and the Cooper pairs in NbN gives rise to a transition from positive to negative magnetoresistance between 0 and 14 T.

The interplay between dissipation, interactions, and gauge fields opens the possibility to rich emerging physics. Here we focus on a setup in which the system is coupled at its extremities to two different baths which impose a current. We then study the system's response to a gauge field depending on the filling. We show that the current induced by the baths has a marked dependence on the magnetic field at low fillings which is significantly reduced at larger fillings. We explain the interplay between interactions, gauge field, and dissipation by studying the system's energy spectrum at the different fillings. This interplay also results in the emergence of negative differential conductivity. For this study we have developed a number-conserving treatment which allows a numerical exact treatment of fairly large system sizes, and which can be extended to a large class of systems.

We investigate the vibrational properties of Pt- and Pd-phthalocyanine (PtPc and PdPc) molecules on Ag(111) with high-resolution electron energy loss spectroscopy (HREELS). In the monolayer regime, both molecules exhibit long-range order. The vibrational spectra prove a flat adsorption geometry. The redshift of specific vibrational modes suggests a moderate interaction of the molecules with the substrate. The presence of asymmetric vibrational peaks indicates an interfacial dynamical charge transfer (IDCT). The molecular orbital that is involved in IDCT is the former $E_{\mathrm{g}}$ lowest unoccupied molecular orbital (LUMO) of the molecules that becomes partially occupied upon adsorption. A group-theoretical analysis of the IDCT modes, based on calculated vibrational frequencies and line shape fits, provides proof for the reduction of the symmetry of the molecule-substrate complex from fourfold $D_{4h}$ to $C_{2v}\left({\sigma }_v\right), C_s\left({\sigma }_v\right)$, or $C_2$ and the ensuing lifting of the degeneracy of the former LUMO of the molecule. The vibration-based analysis of orbital degeneracies, as carried out here for PtPc/Ag(111) and PdPc/Ag(111), is particularly useful whenever the presence of multiple molecular in-plane orientations at the interface makes the analysis of orbital degeneracies with angle-resolved photoemission spectroscopy difficult.

By using the density functional theory plus $U$ (DFT+$U$) method and taking spin-orbit coupling into account, we investigate the electronic and phonon properties of ${\mathrm{Pu}}_3\mathrm{Ga}$. Most interestingly, we find that there is a significant phonon softening in the transverse acoustic branch at the $L$ point in the Brillouin zone. Via bond strength analysis, we reveal that the nearest-neighboring bond strengths in the face-centered-cubic lattice are weaker around Ga atoms, which is responsible for the observed phonon softening.

All-optical spin switching is a potential trailblazer for information storage and communication at an unprecedented fast rate free of magnetic fields. However, the current wisdom is largely based on semiempirical models of effective magnetic fields and heat pulses, so it is difficult to provide high-speed design protocols for actual devices. Here, we carry out a massively parallel first-principles and model calculation for 13 spin systems and magnetic layers, free of any effective field, to establish a simpler and alternative paradigm of laser-induced ultrafast spin reversal and to point out a path to a full-integrated photospintronic device. It is the interplay of the optical selection rule and sublattice spin orderings that underlines seemingly irreconcilable helicity-dependent and -independent switchings. Using realistic experimental parameters, we predict that strong ferrimagnets, in particular, Laves phase C15 rare-earth alloys, meet the telecommunication energy requirement of 10 fJ, thus allowing a cost-effective subpicosecond laser to switch spin in the gigahertz region.

We report a systematic study of anisotropy resistivity, magnetoresistance, and Hall effect of ${\mathrm{Li}}_{0.32}$(${\mathrm{NH}}_3$)${}_y{\mathrm{Fe}}_2{\mathrm{Te}}_{1.2}{\mathrm{Se}}_{0.8}$ single crystals. When compared to the parent compound ${\mathrm{FeTe}}_{0.6}{\mathrm{Se}}_{0.4}$, the Li-${\mathrm{NH}}_3$ intercalation not only increases the superconducting transition temperature but also enhances the electronic anisotropy in both normal and superconducting states. Moreover, in contrast to the parent compound, the Hall coefficient $R_{\mathrm{H}}$ becomes negative at low temperature, indicating electron-type carriers are dominant due to Li doping. On the other hand, the sign reverse of $R_{\mathrm{H}}$ at high temperature and the failure of scaling behavior of magnetoresistance imply that hole pockets may be still crossing or just below the Fermi energy level, leading to the multiband behavior in ${\mathrm{Li}}_{0.32}$(${\mathrm{NH}}_3$)${}_y{\mathrm{Fe}}_2{\mathrm{Te}}_{1.2}{\mathrm{Se}}_{0.8}$.

Guided elastic waves in homogeneous isotropic plates exhibit conical dispersion at zero wave number for discrete values of Poisson's ratio where accidental degeneracy between longitudinal and transverse thickness resonances occur. Waves excited at the coincidence frequency have the infinite phase velocity associated with thickness mode resonances, but the group velocity remains finite. This leads to propagating waves that oscillate in time but are spatially uniform over the plate surface. Here, accidental degeneracy is induced in an aluminum plate by cooling the plate to tune the Poisson's ratio through the degenerate point. We measure linear dispersion in the transition from forward to backward propagating waves near zero wave number. In addition, we demonstrate that waves generated near the coincidence frequency exhibit spatially uniform phase over the plate surface, and show angle independent mode conversion upon encountering the free edge of the plate. We propose that elastic plates offer a simple system to explore the physics of conical dispersion at zero wave number, and that this phenomenon may find application in acoustic devices and nondestructive testing.

Hybrid quantum/classical techniques can flexibly couple ab initio simulations to an empirical or elastic medium to model materials systems that cannot be contained in small periodic supercells. However, due to electronic nonlocality, a total energy cannot be defined, meaning energy barriers cannot be calculated. We provide a general solution using the principle of virtual work in a modified nudged elastic band algorithm. Our method enables ab initio calculations of the kink formation energy for $\langle 100\rangle$ edge dislocations in molybdenum and lattice trapping barriers to brittle fracture in silicon.

We report on a single-crystal neutron-diffraction study of the evolution of the antiferromagnetic order in the heavy-fermion compound ${\mathrm{CePd}}_{1-x}{\mathrm{Ni}}_x\mathrm{Al}$ which exhibits partial geometric frustration due to its distorted kagome structure. The magnetic structure is found to be unchanged with a propagation vector $Q_{\mathrm{AF}}\approx \left(0.500.35\right)$ for all Ni concentrations $x$ up to $x_c\approx 0.14$. Upon approaching the quantum critical concentration $x_c$, the ordered moment vanishes linearly with Néel temperature $T_{\mathrm{N}}$, in good agreement with CePdAl under hydrostatic pressure. For all Ni concentrations, substantial short-range magnetic correlations are observed above $T_{\mathrm{N}}$ as a result of frustration.

The polar Kerr effect arises in states with broken time-reversal symmetry and has recently been observed in a series of unconventional superconductors. In the normal state, the Kerr effect is driven by time reversal symmetry breaking of the spin system in conjunction with spin-orbit coupling. In contrast for the superconducting state the effect may arise from a chiral gap structure breaking time reversal symmetry within the orbital degree of freedom. Here, we study the interplay of both mechanisms being present simultaneously in the chiral superconducting phase of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ including spin-orbit coupling. It was found that the introduction of spin-orbit coupling induces significant orbital mixing within the band structure. This has a profound influence on calculations of anomalous Hall transport and thus the Kerr angle. We also compare our 3D model of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ to a recent 2D model and analyze in detail which parts of the Brillouin zone predominantly contribute to the effect in both models.

Weyl points in three spatial dimensions are characterized by a $\mathbb{Z}$-valued charge—the Chern number—which makes them stable against a wide range of perturbations. A set of Weyl points can mutually annihilate only if their net charge vanishes, a property we refer to as robustness. While nodal loops are usually not robust in this sense, it has recently been shown using homotopy arguments that in the centrosymmetric extension of the $\mathrm{AI}$ symmetry class they nevertheless develop a ${\mathbb{Z}}_2$ charge analogous to the Chern number. Nodal loops carrying a nontrivial value of this ${\mathbb{Z}}_2$ charge are robust, i.e., they can be gapped out only by a pairwise annihilation and not on their own. As this is an additional charge independent of the Berry $\pi$-phase flowing along the band degeneracy, such nodal loops are, in fact, doubly charged. In this manuscript, we generalize the homotopy discussion to the centrosymmetric extensions of all Atland-Zirnbauer classes. We develop a tailored mathematical framework dubbed the $\mathrm{AZ}+\mathcal{I}$ classification and show that in three spatial dimensions such robust and multiply charged nodes appear in four of such centrosymmetric extensions, namely, $\mathrm{AZ}+\mathcal{I}$ classes $\mathrm{CI}$ and $\mathrm{AI}$ lead to doubly charged nodal lines, while $\mathrm{D}$ and $\mathrm{BDI}$ support doubly charged nodal surfaces. We remark that no further crystalline symmetries apart from the spatial inversion are necessary for their stability. We provide a description of the corresponding topological charges, and develop simple tight-binding models of various semimetallic and superconducting phases that exhibit these nodes. We also indicate how the concept of robust and multiply charged nodes generalizes to other spatial dimensions.

Inverted HgTe/CdTe quantum wells have been used as a platform for the realization of two-dimensional topological insulators, bulk insulator materials with spin-helical metallic edge states protected by time-reversal symmetry. This paper investigates the spectrum and the charge transport in HgTe/CdTe quantum well junctions both in the topological regime and in the absence of time-reversal symmetry. We model the system using the Bernevig-Hughes-Zhang effective Hamiltonian and compute the transport properties using recursive Green's functions with a finite differences' method. Specifically, we have studied the material's spatially resolved conductance in a setup with a gated central region, forming monopolar ($n-n^{\prime }-n$) and heteropolar ($n-p-n, n\text{-TI-}n$) double junctions, which have been recently realized in experiments. We find regimes in which the edge states carry spin-polarized currents in the central region even in the presence of a small magnetic field, which breaks time-reversal symmetry. More interestingly, the conductance displays spin-dependent, Fabry-Perót-like oscillations as a function of the central gate voltage producing tunable, fully spin-polarized currents through the device.

We study transport properties and the charge quantization phenomenon in a small metallic island connected to the leads through two quantum point contacts (QPCs). The linear conductance is calculated perturbatively with respect to weak tunneling and weak backscattering at QPCs as a function of the temperature $T$ and gate voltage. The conductance shows Coulomb blockade (CB) oscillations as a function of the gate voltage that decay with the temperature as a result of thermally activated fluctuations of the charge in the island. The regimes of quantum $T\ll E_C$ and thermal $T\gg E_C$ fluctuations are considered, where $E_C$ is the charging energy of an isolated island. Our predictions for CB oscillations in the quantum regime coincide with previous findings by Furusaki and Matveev [Phys. Rev. B 52, 16676 (1995)]. In the thermal regime the visibility of Coulomb blockade oscillations decays with the temperature as $\sqrt{T/E_C}exp(-{\pi }^{2}T/E_C)$, where the exponential dependence originates from the thermal averaging over the instant charge fluctuations, while the prefactor has a quantum origin. This dependence does not depend on the strength of couplings to the leads. The differential capacitance, calculated in the case of a single tunnel junction, shows the same exponential decay, however the prefactor is linear in the temperature. This difference can be attributed to the nonlocality of the quantum effects. Our results agree with the recent experiment [Nature (London) 536, 58 (2016)] in the whole range of the parameter $T/E_C$.

Nonreciprocal electromagnetic devices play a crucial role in modern microwave and optical technologies. Conventional methods for realizing such systems are incompatible with integrated circuits. With recent advances in integrated photonics, the need for efficient on-chip magnetless nonreciprocal devices has become more pressing than ever. This paper leverages space-time engineered asymmetric photonic band gaps to generate optical isolation. It shows that a properly designed space-time modulated slab is highly reflective/transparent for opposite directions of propagation. The corresponding design is magnetless, accommodates low modulation frequencies, and can achieve very high isolation levels. An experimental proof of concept at microwave frequencies is provided.

Zaghoo et al. [Phys. Rev. B 93, 155128 (2016)] report on the observation of a first-order phase transition to metallic hydrogen at pressures of 100–170 GPa and high temperatures of 1100–1800 K. Here, based on the analysis of their optical spectroscopy data and finite element calculations, we show that the presented data do not support the existence of such a transition at their claimed $T_c$ and are likely related to a continuous band-gap reduction.

A recent article by Zaghoo et al. [Phys. Rev. B 93, 155128 (2016)] presented high-pressure high-temperature optical experiments claiming the observation of a first-order transition to atomic liquid metallic hydrogen. Here, we demonstrate that the experimental evidence presented is unsubstantial for such a claim. Furthermore, the claimed results and conclusions contradict previously published works, including those by the same research group.

Using a wave function Dirac Bogoliubov–de Gennes method, we demonstrate that the tunable Fermi level of a graphene layer in the presence of Rashba spin-orbit coupling (RSOC) allows for producing an anomalous nonlocal Andreev reflection and equal spin superconducting triplet pairing. We consider a graphene nanojunction of a ferromagnet-RSOC-superconductor-ferromagnet configuration and study scattering processes, the appearance of spin triplet correlations, and charge conductance in this structure. We show that the anomalous crossed Andreev reflection is linked to the equal spin triplet pairing. Moreover, by calculating current cross-correlations, our results reveal that this phenomenon causes negative charge conductance at weak voltages and can be revealed in a spectroscopy experiment, and may provide a tool for detecting the entanglement of the equal spin superconducting pair correlations in hybrid structures.

Electronic metal-support interactions (EMSI) in catalysis are commonly rationalized in terms of an electron transfer between support material and supported metal catalyst particles. This general perspective, however, cannot fully explain experimentally observed EMSI for metallic nanoparticulate catalysts, because the strong charge screening of metals should locally confine effects of direct electronic interaction with the support to the catalyst-support interface (CSI), which, apart from the perimeter, is largely inaccessible for catalysis reactants. The concept of capacitive EMSI is proposed here for catalyst particles at the nanometer scale, where electronic equilibration results in a long-range charging of the catalytically active outer surface (CAOS) bypassing the expected strong metallic charge screening, which is confirmed and quantified by electrostatic and density functional theory simulations revealing a strong dependence on the coverage of the support surface with catalyst particles. This long-range charge transfer leads to a shift of the local work function at the CAOS. In order to describe the catalytic consequences, an amendment of $d$-band theory in terms of ‘$d$-band + work function’ is proposed. Furthermore, the charging of remote catalytic sites at the CAOS scales with the relative dielectric constant of the surrounding medium, and it is concluded that EMSI can have surprisingly strong influence especially in the presence of a strongly polarizable dielectric.