Spin-polarized quasiparticles can be easily created during spin-filtering through a ferromagnetic insulator (FI) in contact with a superconductor due to pair breaking effects at the interface. A combination FI-N-FI sandwiched between two superconductors can be used to create and analyze such spin-polarized quasiparticles through their nonequilibrium accumulation in the middle metallic (N) layer. We report spin-polarized quasiparticle regulation in a double spin-filter tunnel junction in the configuration NbN-GdN1-Ti-GdN2-NbN. The middle Ti layer provides magnetic decoupling between two ferromagnetic GdN and a place for nonequilibrium quasiparticle accumulation. The two GdN(1,2) layers were deposited under different conditions to introduce coercive contrast. The quasiparticle tunneling spectra has been measured at different temperatures to understand the tunneling mechanism in these double spin-filter junctions. The conductance spectra were found to be comparable to an asymmetric SINI'S-type tunnel junction. A hysteretic R-H loop with higher resistance for the antiparallel configuration compared to parallel state was observed, asserting the spin-polarized nature of quasiparticles. The hysteresis in the R-H loop was found to disappear for subgap bias current. This difference can be understood by considering modification of the interlayer coupling due to nonequilibrium spin-polarized quasiparticle accumulation in the Ti layer.

We elaborate a theoretical description of large Josephson junctions which is based on Werthamer's microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is particularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the time integrals at each time step while taking into account all the memory effects. To validate the obtained microscopic model of large Josephson junction we focus our attention on the Josephson flux-flow oscillator. The proposed microscopic model of flux-flow oscillator does not involve the phenomenological damping parameter, rather the damping is taken into account naturally in the tunnel current amplitudes calculated at a given temperature. The theoretically calculated current-voltage characteristics is compared to our experimental results obtained for a set of fabricated flux-flow oscillators of different lengths.

Originating from a broken spatial inversion symmetry, ferroelectricity is a functionality of materials with an electric dipole that can be switched by external electric fields. Spontaneous polarization is a crucial ferroelectric property, and its amplitude is determined by the strength of polar structural distortions. Density functional theory (DFT) is one of the most widely used theoretical methods to study ferroelectric properties, yet it is limited by the levels of approximations in electron exchange-correlation. On the one hand, the local density approximation (LDA) is considered to be more accurate for the conventional perovskite ferroelectrics such as $\mathrm{BaTi}{\mathrm{O}}_3$ and $\mathrm{PbTi}{\mathrm{O}}_3$ than the generalized gradient approximation (GGA), which suffers from the so-called super-tetragonality error. On the other hand, GGA is more suitable for hydrogen-bonded ferroelectrics than LDA, which largely overestimates the strength of hydrogen bonding in general. We show here that the recently developed general-purpose strongly constrained and appropriately normed (SCAN) meta-GGA functional significantly improves over the traditional LDA/GGA for structural, electric, and energetic properties of diversely bonded ferroelectric materials with a comparable computational effort and thus enhances largely the predictive power of DFT in studies of ferroelectric materials. We also address the observed system-dependent performances of LDA and GGA for ferroelectrics from a chemical bonding point of view.

Iron telluride (FeTe) is known to display bicollinear magnetic order at low temperatures together with a monoclinic lattice distortion. Because the bicollinear order can involve two different wave vectors $(\pi /2,\pi /2)$ and $(\pi /2,-\pi /2)$, symmetry considerations allow for the possible stabilization of a nematic state with short-range bicollinear order coupled to monoclinic lattice distortions at a $T_S$ higher than the temperature $T_N$ where long-range bicollinear order fully develops. As a concrete example, the three-orbital spin-fermion model for iron telluride is studied with an additional coupling ${\tilde{\lambda }}_{12}$ between the monoclinic lattice strain and an orbital-nematic order parameter with $B_{2g}$ symmetry. Monte Carlo simulations show that with increasing ${\tilde{\lambda }}_{12}$ the first-order transition characteristic of FeTe splits and bicollinear nematicity is stabilized in a (narrow) temperature range. In this new regime, the lattice is monoclinically distorted and short-range spin and orbital order breaks rotational invariance. A discussion of possible realizations of this exotic state is provided.

Motivated by the discovery of more and more phosphates with relatively strong nonlinear optic effect, we studied the mechanism of the second-harmonic generation (SHG) effect in several phosphates by band model and first-principles calculations. When the energy of an incident photon is much smaller than the band gap of material, the SHG is almost frequency independent and determined by the combination of Berry connection and a symmetric tensor. The SHG effect in phosphates can be enhanced by the enhancement of orbital hybridization or the reduction of charge-transfer energy, which results in widened bandwidth of occupied state and reduced band gap in the electronic structure, respectively. By the first-principles calculation on the electronic structures of several phosphates—${\mathrm{BPO}}_4, {\mathrm{LiCs}}_2{\mathrm{PO}}_4, \beta -{\mathrm{Li}}_3{\mathrm{VO}}_4$, and $\beta -{\mathrm{Li}}_3{\mathrm{PO}}_4$—we interpreted the relatively strong SHG effect in ${\mathrm{LiCs}}_2{\mathrm{PO}}_4$ and $\beta -{\mathrm{Li}}_3{\mathrm{VO}}_4$ as the consequence of the reduced charge-transfer energy compared to their parent $\beta -{\mathrm{Li}}_3{\mathrm{PO}}_4$, while the enhanced SHG in ${\mathrm{BPO}}_4$ is resulting from enhanced orbital hybridization.

Using semianalytical models we calculate the energy, effective Bohr radius, and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L\gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with super-radiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L<20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, a different response is observed for core-shell and core-crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g., in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching microseconds.

Cathodoluminescence spectroscopy is a key analysis technique in nanophotonics research and technology, yet many aspects of its fundamental excitation mechanisms are not well understood on the single-electron and single-photon level. Here, we determine the cathodoluminescence emission statistics of InGaN quantum wells embedded in GaN under 6–30-keV electron excitation and find that the light emission rate varies strongly from electron to electron. Strong photon bunching is observed for the InGaN quantum well emission at 2.77 eV due to the generation of multiple quantum well excitations by a single primary electron. The bunching effect, measured by the $g^{\left(2\right)}\left(t\right)$ autocorrelation function, decreases with increasing beam current in the range 3–350 pA. Under pulsed excitation ($p=2–100\mathrm{ns}$; 0.13–6 electrons per pulse), the bunching effect strongly increases. A model based on Monte Carlo simulations is developed that assumes a fraction $\gamma$ of the primary electrons generates electron-hole pairs that create multiple photons in the quantum wells. At a fixed primary electron energy (10 keV) the model explains all $g^{\left(2\right)}$ measurements for different beam currents and pulse durations using a single value for $\gamma =0.5$. At lower energies, when electrons cause mostly near-surface excitations, $\gamma$ is reduced ($\gamma =0.01$ at 6 keV), which is explained by the presence of a AlGaN barrier layer that inhibits carrier diffusion to the buried quantum wells. The combination of $g^{\left(2\right)}$ measurements in pulsed and continuous mode with spectral analysis provides a powerful tool to study optoelectronic properties and may find application in many other optically active systems and devices.

The formalism of the reduced density matrix is pursued in both length and velocity gauges of the perturbation to the crystal Hamiltonian. The covariant derivative is introduced as a convenient representation of the position operator. This allow us to write compact expressions for the reduced density matrix in any order of the perturbation which simplifies the calculations of nonlinear optical responses; as an example, we compute the first- and third-order contributions of the monolayer graphene. Expressions obtained in both gauges share the same formal structure, allowing a comparison of the effects of truncation to a finite set of bands. This truncation breaks the equivalence between the two approaches: its proper implementation can be done directly in the expressions derived in the length gauge, but requires a revision of the equations of motion of the reduced density matrix in the velocity gauge.

Monolayer structures of Pb on Si(111) attracted recently considerable interest as superconductivity was found in these truly two-dimensional (2D) structures. In this study, we analyzed the electronic surface band structure of the so-called striped incommensurate Pb phase with $\frac{4}{3}$ ML coverage by means of spin-resolved photoemission spectroscopy. Our results fully agree with density functional theory calculations done by Ren et al. [Phys. Rev. B 94, 075436 (2016)]. We observe a local Zeeman-type splitting of a fully occupied and spin-polarized surface band at the ${\overline{\text{K}}}_{\sqrt{3}}$ points. The growth of this densely packed Pb structure results in the formation of imbalanced rotational domains, which triggered the detection of $C_{3v}$ symmetry forbidden spin components for surface states around the Fermi energy. Moreover, the Fermi surface of the metallic surface state of this phase is Rashba spin split and revealed a pronounced warping. However, the 2D nesting vectors are incommensurate with the atomic structure, thus keeping this system rather immune against charge density wave formation and possibly enabling a superconducting behavior.

Long-lived and ultraconfined plasmons in two-dimensional (2D) electron systems may provide a subwavelength diagnostic tool to investigate localized dielectric, electromagnetic, and pseudo-electromagnetic perturbations. In this article, we present a general theoretical framework to study the scattering of 2D plasmons against such perturbations in the nonretarded limit. We discuss both parabolic-band and massless Dirac fermion 2D electron systems. Our theory starts from a Lippmann-Schwinger equation for the screened potential in an inhomogeneous 2D electron system and utilizes as inputs analytical long-wavelength expressions for the density-density response function, going beyond the local approximation. We present illustrative results for the scattering of 2D plasmons against a pointlike charged impurity and a one-dimensional electrostatic barrier due to a line of charges. Exact numerical results obtained from the solution of the Lippmann-Schwinger equation are compared with approximate results based on the Born and eikonal approximations. The importance of nonlocal effects is finally emphasized.

We study charge transport through a floating mesoscopic superconductor coupled to counterpropagating fractional quantum Hall edges at filling fraction $\nu =2/3$. We consider a superconducting island with finite charging energy and investigate its effect on transport through the device. We calculate conductance through such a system as a function of temperature and gate voltage applied to the superconducting island. We show that transport is strongly affected by the presence of parafermionic zero modes, leading at zero temperature to a zero-bias conductance quantized in units of $\nu e^2/h$ independent of the applied gate voltage.

Applying four-wave mixing spectroscopy to a high-quality GaN/AlGaN single quantum well, we report on the experimental determination of excitonic dephasing times at different temperatures and exciton densities in III-nitride heterostructures. By comparing the evolution with the temperature of the dephasing and the spin-relaxation rate, we conclude that both processes are related to the rate of excitonic collisions. When spin relaxation occurs in the motional-narrowing regime, it remains constant over a large temperature range as the spin-precession frequency increases linearly with temperature, hence compensating for the observed decrease in the dephasing time. From those measurements, a value of the electron-hole exchange interaction strength of 0.45 meV at $T=10\mathrm{K}$ is inferred.

The hallmark of a two-dimensional (2d) topologically ordered phase is the existence of deconfined “anyon” excitations that have exotic braiding and exchange statistics, different from those of ordinary bosons or fermions. As opposed to conventional Landau-Ginzburg-Wilson phases, which are classified on the basis of the spontaneous breaking of an underlying symmetry, topologically ordered phases, such as those occurring in the fractional quantum Hall effect, are absolutely stable, not requiring any such symmetry. Recently, though, it has been realized that symmetries, which may still be present in such systems, can give rise to a host of new, distinct, many-body phases, all of which share the same underlying topological order. These “symmetry enriched” topological (SET) phases are distinguished not on the basis of anyon braiding statistics alone, but also by the symmetry properties of the anyons, such as their fractional charges, or the way that different anyons are permuted by the symmetry. Thus a useful approach to classifying SETs is to determine all possible such symmetry actions on the anyons that are algebraically consistent with the anyon statistics. Remarkably, however, there exist symmetry actions that, despite being algebraically consistent, cannot be realized in any physical system, and hence do not lead to valid 2d SETs. One class of such “anomalous” SETs, characterized by certain disallowed symmetry fractionalization patterns, finds a physical interpretation as an allowed surface state of certain three-dimensional (3d) short-range entangled phases, but another, characterized by some seemingly valid but anomalous permutation actions of the symmetry on the anyons and encoded in an $H^3(G,\mathcal{A})$ group cohomology class, has so far eluded a physical interpretation. In this work, we find a way to physically realize these anomalously permuting SETs at the surfaces of certain 3d long-range entangled phases, expanding our understanding of general anomalous SETs in two dimensions.

The existence of large instantaneous local magnetic moments in paramagnetic phases is a direct signature of Mott localization. In order to track the doping evolution of fluctuating local moments in iron-based superconductors, we jointly use two fast probes, x-ray emission and absorption spectroscopies. Exploring K- and Cr-hole-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$, we find a systematic increase in the local moment with hole-doping, in contrast with inelastic neutron scattering measurements, which suggest an opposite trend. Our results support the theoretical scenario in which a Mott insulating state that would be realized for half-filled conduction bands has an influence throughout the phase diagram of these iron pnictides.

We investigate theoretically an interacting metallic wire with a strong magnetic field directed along its length and show that it is a highly tunable one-dimensional system. By considering a suitable change in spatial geometry, we build an analogy between the problem in the zeroth Landau level with Landau level degeneracy $N$ to one-dimensional fermions with an $N$-component pseudospin degree of freedom and $SU\left(2\right)$-symmetric interactions. This analogy allows us to establish the phase diagram as a function of the interactions for small $N$ (and make conjectures for large $N$) using renormalization group and bosonization techniques. We find pseudospin-charge separation with a gapless $U\left(1\right)$ charge sector and several possible strong-coupling phases in the pseudospin sector. For odd $N$, we find a fluctuating pseudospin-singlet charge density wave phase and a fluctuating pseudospin-singlet superconducting phase which are topologically distinct. For even $N>2$, similar phases exist, although they are not topologically distinct, and an additional novel pseudospin-gapless phase appears. We discuss experimental conditions for observing our proposals.

The zero-bias peak (ZBP) is understood as the definite signature of a Majorana bound state (MBS) when attached to a semi-infinite Kitaev nanowire (KNW) nearby zero temperature. However, such characteristics concerning the realization of the KNW constitute a profound experimental challenge. We explore theoretically a QD connected to a topological KNW of finite size at nonzero temperatures and show that overlapped MBSs of the wire edges can become effectively decoupled from each other and the characteristic ZBP can be fully recovered if one tunes the system into the leaked Majorana fermion fixed point. At very low temperatures, the MBSs become strongly coupled. We derive universal features of the conductance as a function of the temperature and the relevant crossover temperatures. Our findings offer additional guides to identify signatures of MBSs in solid state setups.

We study the effect of trigonal warping on the focusing of electrons by n-p junctions in graphene. We find that perfect focusing, which was predicted for massless Dirac fermions, is only preserved for one specific lattice orientation. In the general case, trigonal warping leads to the formation of cusp caustics, with a different position of the focus for graphene's two valleys. We develop a semiclassical theory to compute these positions and find very good agreement with tight-binding simulations. Considering the transmission as a function of potential strength, we find that trigonal warping splits the single Dirac peak into two distinct peaks, leading to valley polarization. We obtain the transmission curves from tight-binding simulations and find that they are in very good agreement with the results of a billiard model that incorporates trigonal warping. Furthermore, the positions of the transmission maxima and the scaling of the peak width are accurately predicted by our semiclassical theory. Our semiclassical analysis can easily be carried over to other Dirac materials, which generally have different Fermi surface distortions.