An unusual form of superconductivity, called Ising superconductivity, has recently been uncovered in mono- and few-layered transition metal dichalcogenides. This 2D superconducting state is characterized by the so-called Ising spin-orbit coupling (SOC), which produces strong oppositely oriented effective Zeeman fields perpendicular to the 2D layer in opposite momentum space valleys. We examine the Yu-Shiba-Rusinov (YSR) bound states localized at magnetic impurities in Ising superconductors and show that the unusual SOC manifests itself in unusually strong anisotropy in magnetic field response observable in STM experiments. For a chain of magnetic impurities with moments parallel to the plane of Ising superconductors we show that the low energy YSR band hosts topological superconductivity and Majorana excitations as a direct manifestation of topological effects induced by Ising spin-orbit coupling.

We develop a method to study the effect of the superconducting transition on the resonant inelastic x-ray scattering (RIXS) signal in superconductors with an order parameter with an arbitrary symmetry within a quasiparticle approach. As an example, we compare the direct RIXS signal below and above the superconducting transition for $p$-wave-type order parameters. For a $p$-wave order parameter with a nodal line, we show that, counterintuitively, the effect of the gap is most noticeable for momentum transfers in the nodal direction. This phenomenon may be naturally explained as a type of nesting effect.

Multiple relaxation times detected in the ac magnetic susceptibility of several single-molecule magnets have been always assigned to extrinsic factors, such as nonequivalent magnetic centers or effects of intermolecular interactions in the crystal. By solving quantum relaxation equations, we prove that the observed multiple relaxation times can be of intramolecular origin and can show up even in single-ion metal complexes. For the latter a remarkably good description of the coexistent two relaxation times is demonstrated on several experimental examples. This proves the relevance of the intramolecular mechanism of multiple relaxation times in such systems, which is even easier justified in polynuclear magnetic complexes.

We performed a comprehensive angle-resolved photoemission spectroscopy study of the electronic band structure of LaOFeAs single crystals. We found that samples cleaved at low temperature show an unstable and highly complicated band structure, whereas samples cleaved at high temperature exhibit a stable and clearer electronic structure. Using in situ surface doping with K and supported by first-principles calculations, we identify both surface and bulk bands. Our assignments are confirmed by the difference in the temperature dependence of the bulk and surface states.

We study terahertz photoconductivity of a rutile single crystal between 10 and 300 K under strong photoexcitation by femtosecond pulses at 266 nm. A marked dependence of the carrier mobility on the carrier density is observed leading to highly complex transport phenomena on a picosecond time scale. We develop a general model of carrier photoconductive response in the case of time dependent inhomogeneous distribution of carrier density and mobility. This allows us to assess an important role of both electrons and holes in the response of photoexcited rutile. At low temperatures, the carrier mobility is initially reduced due to the electron-hole scattering and increases by one order of magnitude upon ambipolar diffusion of the carriers into deeper regions of the sample. At room temperature, contributions of transient hot optical phonons and/or of midinfrared polaron excitations with charge-density-dependent dielectric strength emerge in the photoconductivity spectra.

We find an unambiguous relationship between disorder-driven features in the temperature dependence of the resistance and the behavior, as functions of the temperature, of the parameters necessary to describe some of the relaxation processes in the photoinduced differential time-resolved reflectivity of three samples of ${\text{Nd}}_{1.83}{\text{Ce}}_{0.17}{\text{CuO}}_{4\pm \delta }$. The latter, sharing the same $\text{Ce}$ content, have been fabricated and annealed ad hoc in order to differ only for the degree of disorder, mainly related to oxygen content and location, and, consequently, for the temperature dependence of the resistance: Two of them present a minimum in the resistance and behave as a superconductor and as a metal, respectively; the third behaves as an insulator. The systematic coherence between the resistance and the relaxation parameters behaviors in temperature for all three samples is absolutely remarkable and shows that pump-probe measurements can be extremely sensitive to disorder as it drives the emergence of new excitations and of the related relaxation channels as in this paradigmatic case.

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula $AM{X}_3$, where the $A$ cations occupy the hollows formed by the $M{X}_3$ octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite $\left({\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3\right)$ with direct nonequilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster $\left({\mathrm{CH}}_3{\mathrm{NH}}_3^{+}\right)$ and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. We also reveal the strongly localized phonons associated with the internal motions of the ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cluster, which contribute little to the total thermal conductivity. The importance of the ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

There is strong interest in discovering or designing wide-gap Chern insulators. Here we follow a Chern insulator to trivial Mott insulator transition versus interaction strength $U$ in a honeycomb-lattice Fe-based transition-metal oxide, discovering that a spin-orbit coupling energy scale $\xi =40$ meV can produce and maintain a topologically entangled Chern insulating state against large band structure changes arising from an interaction strength $U$ up to 60 times as large. Within the Chern phase the minimum gap switches from the zone corner $K$ to the zone center $\mathrm{\Gamma }$ while maintaining the topological structure. At a critical strength $U_c$, the continuous evolution of the electronic structure encounters a gap closing then reopening, upon which the system reverts to a trivial Mott insulating phase. This Chern insulator phase of honeycomb lattice ${\mathrm{Fe}}^{2+}{\mathrm{BaFe}}_2$(${\mathrm{PO}}_4$)${}_2$ corresponds to a large Chern number $\mathcal{C}=-3$ that will provide enhanced anomalous Hall conductivity due to the associated three edge states threading through the bulk gap of 80 meV.

The interplay of electronic band structures and electron-electron interactions is known to develop new phases in condensed matter. In this paper, we investigate the thermodynamic phases and corresponding electronic structures of a Weyl semimetal in a strong on-site Coulomb interaction limit. Based on a minimum model of a Weyl semimetal with two linear Weyl nodes, it is shown that generically the Weyl semimetal becomes magnetic in the presence of interactions. In particular, it is shown that the Dzyaloshinskii-Moriya exchange interaction is generally induced so that the A-type antiferromagnetic (A-AFM) phase and the spiral spin density wave (SSDW) states are two generic phases. Furthermore, we find that Weyl nodes proliferate and it is possible to doubly enhance the unusual properties of noninteracting Weyl semimetals through the realization of double-Weyl nodes in a strong correlation limit. Specifically, it is shown that in the SSDW phase, linear Weyl nodes are tuned into double-Weyl nodes with the corresponding charges being $\pm 2$. As the spin-orbit coupling increases, a quantum phase transition occurs with the SSDW phase being turned into an A-AFM phase and, at the same time, double-Weyl nodes are disintegrated into two pairs of linear Weyl nodes. Our results reveal the unusual interplay between the topology of electronic structures and magnetism in strongly correlated phases of Weyl semimetals.

Materials with a perfect kagome lattice structure of magnetic ions are intensively sought after, because they may exhibit exotic ground states like a quantum spin liquid phase. Barlowite is a natural mineral that features perfect kagome layers of copper ions. However, in barlowite there are also copper ions between the kagome layers, which mediate strong interkagome couplings and lead to an ordered ground state. Using ab initio density functional theory calculations we investigate whether selective isoelectronic substitution of the interlayer copper ions is feasible. After identifying several promising candidates for substitution we calculate the magnetic exchange couplings based on crystal structures predicted from first-principles calculations. We find that isoelectronic substitution with nonmagnetic ions significantly reduces the interkagome exchange coupling. As a consequence, interlayer-substituted barlowite can be described by a simple two-parameter Heisenberg Hamiltonian, for which a quantum spin liquid ground state has been predicted.

We study the temperature-dependent electrical and optical properties of hybrid charge transfer excitons (HCTEs) at a ZnO/4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) organic/inorganic semiconductor heterojunction (OI-HJ). The electroluminescence spectrum of the HCTE formed by recombination of injected charge undergoes a hypsochromic shift from 1.65 ± 0.01 eV to 2.05 ± 0.01 eV with decreasing temperature from $T=300\mathrm{K}$ to 30 K at a current density of $J=100\mathrm{mA}\mathrm{c}{\mathrm{m}}^{-2}$, and with increasing voltage from $V=1.5\mathrm{V}$ to 4.5 V. We observe an external quantum efficiency of 6.0 ± 0.2% at a wavelength of 332 nm for excitons generated in CBP, indicating exciton to charge conversion via HCTEs. However, no HCTE photoluminescence is observed at temperatures as low as $T=25\mathrm{K}$. A quantum mechanical model based on the coexistence of both free and trapped singlet HCTE states at the OI-HJ describes these observations. The free HCTE consists of a hole in CPB Coulombically bound with an energy of 9 meV to a delocalized electron in ZnO, and it is the precursor to photocurrent generation via CBP Frenkel excitons. The observed electroluminescence is due to electron injection into localized defect states at the ZnO surface that are bound to holes in CBP forming a trapped HCTE with binding energies of 60–430 meV and oscillator strength that is four orders of magnitude higher compared to the free HCTE.

The formation of nonlinear Bloch states in open driven-dissipative systems of exciton-polaritons loaded into a weak-contrast 1D periodic lattice is studied numerically and analytically. The condensate is described within the framework of mean-field theory by the coupled equations for the order parameter and for the density of incoherent excitons. The stationary nonlinear solutions having the structure of Bloch waves are studied in detail. It is shown that there is a bifurcation leading to the appearance of a family of essentially nonlinear states. The special feature of these solutions is that its current does not vanish when the quasimomentum of the state approaches the values equal to half of the lattice constant. To explain the bifurcations found in numerical simulations a simple perturbative approach is developed. The stability of the nonlinear states is examined by linear spectral analysis and by direct numerical simulations. An experimental scheme allowing the observation of the discussed nonlinear current states is suggested and studied by numerical simulations.

A magnetic skyrmionium is a nontopological soliton, which has a doughnut-like out-of-plane spin texture in thin films, and can be phenomenologically viewed as a coalition of two topological magnetic skyrmions with opposite topological numbers. Due to its zero topological number ($Q=0$) and doughnut-like structure, the skyrmionium has its distinctive characteristics as compared to the skyrmion with $Q=\pm 1$. Here we systematically study the generation, manipulation, and motion of a skyrmionium in ultrathin magnetic nanostructures by applying a magnetic field or a spin-polarized current. It is found that the skyrmionium moves faster than the skyrmion when they are driven by the out-of-plane current, and their velocity difference is proportional to the driving force. However, the skyrmionium and skyrmion exhibit an identical current-velocity relation when they are driven by the in-plane current. It is also found that a moving skyrmionium is less deformed in the current-in-plane geometry compared with the skyrmionium in the current-perpendicular-to-plane geometry. Furthermore we demonstrate the transformation of a skyrmionium with $Q=0$ into two skyrmions with $Q=+1$ in a nanotrack driven by a spin-polarized current, which can be seen as the unzipping process of a skyrmionium. We illustrate the energy and spin structure variations during the skyrmionium unzipping process, where linear relations between the spin structure and energies are found. These results could have technological implications in the emerging field of skyrmionics.

The flow-equation method was proposed by Wegner as a technique for studying interacting systems in one dimension. Here, we apply this method to a disordered one-dimensional model with power-law decaying hoppings. This model presents a transition as function of the decaying exponent $\alpha$. We derive the flow equations and the evolution of single-particle operators. The flow equation reveals the delocalized nature of the states for $\alpha <\frac{1}{2}$. Additionally, in the regime $\alpha >\frac{1}{2}$, we present a strong-bond renormalization group structure based on iterating the three-site clusters, where we solve the flow equations perturbatively. This renormalization group approach allows us to probe the critical point $\left(\alpha =1\right)$. This method correctly reproduces the critical level-spacing statistics and the fractal dimensionality of the eigenfunctions.

The response of the crystal lattice to an electric shift current induced via the two-photon bulk-photovoltaic effect in a lithium niobate (${\mathrm{LiNbO}}_{\text{3}}$) crystal is directly mapped by femtosecond x-ray diffraction. Acoustic strain waves of large amplitude are generated by piezoelectric coupling to the current-related polarization while other mechanisms such as anharmonic phonon-phonon couplings and electron-phonon coupling through deformation potentials play a minor role. A striking variation of the strain wave speed occurs as a function of the relative orientation between the crystal's $c$-axis, the direction of the current flow, and the polarization of the incident pump pulse. The observed behavior is relevant for a large class of ferroelectrics.

${\mathrm{CuCrO}}_2$ is a manifestation of a two-dimensional triangular antiferromagnet which exhibits an incommensurate noncollinear magnetic structure similar to a classical ${120}^{\circ }$ ordering. Using the inelastic neutron scattering technique, direct evidence of a magnon-phonon coupling in ${\mathrm{CuCrO}}_2$ is revealed via the mixed magnon-phonon character of the excitation mode at 12.5 meV as well as a minimum at the zone boundary. A simple model Hamiltonian that incorporates an exchange-striction type magnon-phonon coupling accurately reproduces the observed features. Also, continuum excitations originating from the interaction between quasiparticles are observed with strong intensity at the zone boundary. These features of the magnetic excitations are key to an understanding of the emergent excitations in noncollinear antiferromagnetic compounds.

We report single-crystal x-ray-diffraction measurements on ${\mathrm{Na}}_2{\mathrm{Ti}}_{2}P{n}_2\mathrm{O}(Pn=\mathrm{As},\mathrm{Sb})$ which reveal a charge superstructure that appears below the density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements we establish that the associated distortion mode can be described by two propagation vectors ${\mathbf{q}}_1=(1/2,0,l)$ and ${\mathbf{q}}_2=(0,1/2,l)$ with $l=0$ (Sb) or $l=1/2$ (As) and primarily involves in-plane displacements of the Ti atoms perpendicular to the Ti-O bonds. We also present angle-resolved photoemission spectroscopy measurements, which show band folding and backbending consistent with a density wave with the same wave-vectors ${\mathbf{q}}_1$ and ${\mathbf{q}}_2$ associated with Fermi-surface nesting, and muon-spin relaxation data, which show no indication of spin density wave order. The results provide direct evidence for phonon-assisted charge density wave order in ${\mathrm{Na}}_2{\mathrm{Ti}}_{2}P{n}_2\mathrm{O}$ and fully characterize a proximate ordered phase that could compete with superconductivity in doped ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$.

We show by means of ab initio calculations and tight-binding modeling that an oxide system based on a honeycomb lattice can sustain topologically nontrivial states if a single orbital dominates the spectrum close to the Fermi level. In such a situation, the low-energy spectrum is described by two Dirac equations that become nontrivially gapped when spin-orbit coupling (SOC) is switched on. We provide one specific example but the recipe is general. We discuss a realization of this starting from a conventional spin-$\frac{1}{2}$ honeycomb antiferromagnet whose states close to the Fermi energy are $d_{z^2}$ orbitals. Switching off magnetism by atomic substitution and ensuring that the electronic structure becomes two-dimensional is sufficient for topologicality to arise in such a system. By deriving a tight-binding Wannier Hamiltonian, we find that the gap in such a model scales linearly with SOC, opposed to other oxide-based topological insulators, where smaller gaps tend to appear by construction of the lattice. We show that the quantum spin Hall state in this system survives in the presence of off-plane magnetism and the orbital magnetic field and we discuss its Landau level spectra, showing that our recipe provides a $d_{z^2}$ realization of the Kane-Mele model.

The effects of nonequilibrium constraints and dephasing on the circulating currents in molecular junctions are analyzed. Circulating currents are manifestations of quantum effects and can be induced either by externally applied bias or an external magnetic field through the molecular system. In a symmetric Aharonov-Bohm ring, bond currents have two contributions, bias driven and magnetic field driven. We analyze the competition between these two contributions and show that, as a consequence, current through one of the branches can be completely suppressed. We then study the effect of asymmetry (as a result of chemical substitution) on the current pathways inside the molecule and study asymmetry-induced circulating currents (without magnetic field) by tuning the coupling strength of the substituent (at finite bias).