Magnetic behavior of La${}_{0.75}$Pr${}_{0.25}$Co${}_2$P${}_2$ was investigated by a combination of magnetic measurements, magneto-optical imaging, neutron diffraction, and x-ray absorption spectroscopy, including x-ray magnetic circular dichroism. The material crystallizes in the ThCr${}_2$Si${}_2$ structure type and exhibits three consecutive magnetic phase transitions. At 167 K, the Co magnetic moments order ferromagnetically in the $ab$ plane of the tetragonal crystal structure. At 66 K, a ferromagnetic ordering of Pr(4$f$) moments parallel to the $c$ axis causes a rotation of the Co(3$d$) moments towards the $c$ axis in the direction opposite to the Pr moments, thus forming a noncollinear ferrimagnetically ordered structure and switching the direction of the total magnetization from the $ab$ plane to the $c$ axis. The third magnetic transition observed at 35 K is likely associated with the establishment of the collinear ferrimagnetic order along the $c$ axis.

We derive expressions for both parallel and perpendicular components of spin transfer torque (STT) in magnetic tunnel junctions (MTJs), which have several important advantages over the currently available expressions: First they are derived in a more realistic approximation, resulting in excellent agreement with exact results even in the presence of resonant tunneling. Second, we show that they can be expressed in terms of the scattering matrix elements, which gives them a clear physical interpretation. Third, they are given entirely in terms of collinear quantities, which are readily available in existing transport codes. We use these expressions to investigate STT behavior in MTJs with asymmetric barriers at finite bias. The results show that lowering the barrier height in the bulk does not qualitatively change the behavior of STT. The absolute STT increases on account of the overall increase of the barrier transparency; however, the STT efficiency remains in the same range. At the same time, modifications of the interfaces can qualitatively change STT behavior. Thus, interface engineering can be used to control the bias dependence of STT and optimize the performance of STT-based devices.

The magnetization dynamics of coupled and uncoupled spin valves with the structure $\mathrm{NiFe}\left(20\mathrm{nm}\right)/\mathrm{Cu}\left(t_{\mathrm{Cu}}\right)/\mathrm{NiFe}\left(20\mathrm{nm}\right)/\mathrm{IrMn}\left(10\mathrm{nm}\right)$ is probed by broadband ferromagnetic resonance absorption measurements. The coupling intensity between the free and the pinned layers is tailored by varying the Cu thickness $t_{\mathrm{Cu}}$. Broadband spectra exhibited two resonant modes for each value of the applied field. It is observed that the coupling between NiFe layers modifies the amplitude of the absorption peaks and the shape of the dispersion relations for each mode, which becomes particularly distorted in the antiparallel magnetization state. The observed phenomena are well described by applying a semianalytical model that properly takes into account the coupling interactions and allows an efficient numerical calculation of the absorption peak amplitudes and the dispersion-relation shapes.

The absence of inversion symmetry in noncentrosymmetric superconductors leads to magnetoelectric effects that do not exist in centrosymmetric superconductors. One manifestation of this is in the vortex structure in materials with a cubic point group $O$, such as occurs in the superconductors Li${}_2$Pt${}_3$B or Mo${}_3$Al${}_2$C. In particular, a current is predicted to flow parallel to the applied magnetic field in such a vortex. Here we present both analytical and numerical solutions of the Ginzburg-Landau equations that reveal the spatial structure of this current both for a single vortex and in the vortex lattice phase near the upper critical field.

In iron-selenide superconductors, only electronlike Fermi pockets survive, challenging the $S^{\pm }$ pairing based on the quasinesting between the electron and hole Fermi pockets (as in iron arsenides). We performed a functional renormalization group study and showed that an in-phase $S$-wave pairing on the electron pockets ($S_{ee}^{++}$) is realized. The pairing mechanism involves two competing driving forces: The strong $C$-type spin fluctuations cause attractive pair scattering between and within electron pockets via Cooperon excitations on the virtual hole pockets, while the $G$-type spin fluctuations cause repulsive pair scattering. The latter effect is, however, weakened by the hybridization splitting of the electron pockets. The resulting $S_{ee}^{++}$-wave pairing symmetry is consistent with experiments. We further propose that the quasiparticle interference pattern in scanning tunneling microscopy and the Andreev reflection in out-of-plane contact tunneling are efficient probes of in-phase versus antiphase $S$-wave pairing on the electron pockets.

Results are presented for the dynamics arising due to a sudden quench of a boson interaction parameter with the simultaneous switching on of a commensurate periodic potential, the latter providing a source of nonlinearity that can cause inelastic scattering. A quantum kinetic equation is derived perturbatively in the periodic potential and solved within the leading order gradient expansion. A two-particle irreducible formalism is employed to construct the stress-momentum tensor and hence the conserved energy. The dynamics is studied in detail in the phase where the boson spectrum remains gapless. The periodic potential is found to give rise to multiparticle scattering processes that relaxes the boson distribution function. At long times the system is found to thermalize with a thermalization time that depends in a nonmonotonic way on the amount of energy injected into the system due to the quantum quench. This nonmonotonic behavior arises due to the competing effect of an increase of phase space for scattering on the one hand, and an enhancement of the orthogonality catastrophe on the other hand as the quench amplitude is increased. The approach to equilibrium is found to be purely exponential for large quench amplitudes, and more complex for smaller quench amplitudes.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of layered chalcogenide 1$T$-TaS${}_{2-x}$Se${}_x$ to elucidate the electronic states especially relevant to the occurrence of superconductivity. We found a direct evidence for a Ta-5$d$-derived electron pocket associated with the superconductivity, which is fragile against a Mott-gap opening observed in the insulating ground state for S-rich samples. In particular, a strong electron-electron interaction-induced Mott gap driven by a Ta 5$d$ orbital also exists in the metallic ground state for Se-rich samples, while finite ARPES intensity near the Fermi level likely originating from a Se 4$p$ orbital survives, indicative of the orbital-selective nature of the Mott transition. Present results suggest that effective electron correlation and $p$-$d$ hybridization play a crucial role to tune the superconductivity and Mott metal-insulator transition.

We theoretically investigate the influence of lattice distortion effects on possible topological phases in (LaNiO${}_3$)${}_2$/(LaAlO${}_3$)${}_N$ heterostructures grown along the [111] direction. At the Hartree-Fock level, topological phases originate from an interaction-generated effective spin-orbit coupling that opens a gap in the band structure. For the unstrained system, there is a quadratic band touching at the $\Gamma$ point at the Fermi energy for spin-unpolarized electrons and Dirac points at K, K${}^{\prime }$ at the Fermi energy for fully spin-polarized electrons. Using density functional theory we first show that the quadratic band touching and Dirac points are remarkably stable to internal strain-induced out-of-plane distortions and rotations of the oxygen octahedra, which we compute. The lack of a gap opening implies that the mean-field predictions for topological phases for both the polarized and unpolarized systems are little affected by internal strain and lattice relaxations. On the other hand, we also discuss two types of lattice distortions, which have an important effect on the electronic structure. First, an external strain imposed along the [001] cubic direction can open a gap at the $\Gamma$ point, thereby stabilizing a trivial insulating phase in the spin-unpolarized system. However, it leaves the Dirac points intact. As a result, the Hartree-Fock calculation for an effective model using parameters relevant to LaNiO${}_3$ finds that symmetry-breaking strain favors a phase with polarized orbitals and antiferromagnetic spin order, but leaves earlier predictions for a zero-magnetic field topological quantum Hall state essentially unchanged. Second, we identify a possible breathing distortion of the oxygen cages stabilized by correlation effects. Such a distortion opens a gap at the Dirac points and we demonstrate that it would compete with the topological phase in the fully spin-polarized system. Taken together, our results suggest that distortion effects in thin films grown along the [111] direction in perovskites have rather different consequences as compared to those grown along [001].

Tensor networks offer a variational formalism to efficiently represent wave functions of extended quantum many-body systems on a lattice. In a tensor network $\mathcal{N}$, the dimension $\chi$ of the bond indices that connect its tensors controls the number of variational parameters and associated computational costs. In the absence of any symmetry, the minimal bond dimension ${\chi }^{\min }$ required to represent a given many-body wave function $|\Psi \rangle$ leads to the most compact, computationally efficient tensor network description of $|\Psi \rangle$. In the presence of a global, on-site symmetry, one can use a tensor network ${\mathcal{N}}_{\mathrm{sym}}$ made of symmetric tensors. Symmetric tensors allow one to exactly preserve the symmetry and to target specific quantum numbers, while their sparse structure leads to a compact description and lowers computational costs. In this paper we explore the trade-off between using a tensor network $\mathcal{N}$ with minimal bond dimension ${\chi }^{\min }$ and a tensor network ${\mathcal{N}}_{\mathrm{sym}}$ made of symmetric tensors, where the minimal bond dimension ${\chi }_{\mathrm{sym}}^{\min }$ might be larger than ${\chi }^{\min }$. We present two technical results. First, we show that in a tree tensor network, which is the most general tensor network without loops, the minimal bond dimension can always be achieved with symmetric tensors, so that ${\chi }_{\mathrm{sym}}^{\min }={\chi }^{\min }$. Second, we provide explicit examples of tensor networks with loops where replacing tensors with symmetric ones necessarily increases the bond dimension, so that ${\chi }_{\mathrm{sym}}^{\min }>{\chi }^{\min }$. We further argue, however, that in some situations there are important conceptual reasons to prefer a tensor network representation with symmetric tensors (and possibly larger bond dimension) over one with minimal bond dimension.

${}^{125}$Te nuclear magnetic resonance spectra and spin-lattice relaxation of $n$- and $p$-type PbTe, self-doping narrow band-gap semiconductors, have been studied and compared to those of $p$-type GeTe. Spin-lattice relaxation in GeTe can be fit by one component, while that in both PbTe samples must be fit by at least two components, showing electronically homogeneous and inhomogeneous materials, respectively. For PbTe-based materials, the spin-lattice relaxation rate 1/$T_1$ increases linearly with carrier concentration. The data for GeTe fall on the same line and allow us to extend this plot to higher concentrations. Long and short $T_1$ components in both PbTe samples reflect “low,” ∼10${}^{17}$ cm${}^{-3}$, and “high,” ∼10${}^{18}$ cm${}^{-3}$, carrier concentration components. Carrier concentrations in both $n$- and $p$-type PbTe samples obtained from the Hall and Seebeck effects generally match the “high” carrier concentration component, and to some extent, ignore the “low” one. This demonstrates that the Hall and Seebeck effects may have a limited ability for the determination of carrier concentration in complex thermoelectric PbTe-based and other multicomponent materials.

We present a first-principles study of the photocatalytic properties of single-layer SnS${}_2$. First, we calculate the formation energy and the phonon spectrum, verifying static and dynamical stability, respectively. In addition, our calculated energy of solvation suggests that single-layer SnS${}_2$ is stable in aqueous solution. Next, by solving the Bethe-Salpeter equation, we obtain an optical band gap of 2.75 eV, consistent with the measured optical band gap. The resulting exciton binding energy of 0.41 eV is consistent with the Mott-Wannier model. Finally, by aligning the band edges with the redox potentials of water, we find that a bias potential of at least 0.9 V is required to drive the hydrogen evolution and that compressive strains can reduce this bias potential.

We present structural characterization of the wurtzite crystal structure of GaP nanowires, which were recently shown to have a direct electronic band gap. The structural parameters of the wurtzite phase do consist of two lattice parameters and one internal degree of freedom, determining the Ga-P bond length along the $c$ direction. Using density functional theory calculations, we study the influence of the internal degree of freedom on the band structure. By synchrotron x-ray diffraction studies near the Ga-$K$ edge we determine the lattice parameters $a=3.8419$ Å and $c=6.3353$ Å as well as the internal degree of freedom $u=0.37385$ with high accuracy. We find that different Ga-P bond lengths are not equal, in contrast to the case in the zinc blende bulk phase. As a result, a spontaneous polarization is predicted for wurtzite GaP.

A metal film supports the continuum of propagating surface plasmon waves. The interaction of these waves with a dipole (nanoparticle) positioned some distance from the surface of the film can produce well-defined localized plasmon modes whose frequency nonetheless resides inside the continuum. This leads to the resonant enhancement of scattering of surface plasmons off the dipole. The maximum of scattering is found to occur when the distance from the dipole to the surface of the film is equal to one-half of the film thickness. The possibility of controllable plasmon scattering could be advantageous for the field of nanoplasmonics.

We experimentally study the propagation of microwaves in an artificial honeycomb lattice made of dielectric resonators. This evanescent propagation is well described by a tight-binding model, very much like the propagation of electrons in graphene. We measure the density of states, as well as the wave function associated with each eigenfrequency. By changing the distance between the resonators, it is possible to modulate the amplitude of next-(next-)nearest-neighbor hopping parameters and to study their effect on the density of states. The main effect is the density of states becoming dissymmetric and a shift of the energy of the Dirac points. We study the basic elements: an isolated resonator, a two-level system, and a square lattice. Our observations are in good agreement with analytical solutions for corresponding infinite lattice.

We investigate the band dispersion and relevant electronic properties of rubrene single crystals within the $GW$ approximation. Due to the self-energy correction, the dispersion of the highest occupied molecular orbital (HOMO) band increases by 0.10 eV compared to the dispersion of the Kohn-Sham eigenvalues within the generalized gradient approximation, and the effective hole mass consequently decreases. The resulting value of 0.90 times the electron rest mass along the $\Gamma$-$\mathrm{Y}$ direction in the Brillouin zone is closer to experimental measurements than that obtained from density-functional theory. The enhanced bandwidth is explained in terms of the intermolecular hybridization of the HOMO($\mathrm{Y}$) wave function along the stacking direction of the molecules. Overall, our results support the bandlike interpretation of charge-carrier transport in rubrene.

In this theoretical work, we report on voltage-controllable hybridization of electromagnetic modes arising from strong interaction between graphene surface plasmons and molecular vibrations. The interaction strength depends strongly on the volume density of molecular dipoles, the molecular relaxation time, and the molecular layer thickness. Graphene offers much tighter plasmonic field confinement and longer carrier relaxation time compared to noble metals, leading to Rabi splitting and hybridized polaritonic modes at three-orders-of-magnitude lower molecular densities. Electrostatically tunable carrier density in graphene allows for dynamic control over the interaction strength. In addition, the flat dispersion band above the light line arising from the deep confinement of the polaritonic modes gives rise to the omnidirectional excitation. Our approach is promising for practical implementations in infrared sensing and detection.

We propose a Josephson junction setup based on a topological insulator (TI) thin film to detect Majorana states that exploits the unique helical and extended nature of the TI surface state. When the magnetic flux through the junction is close to an integer number of flux quanta, Majorana states, present on both surfaces of the film, give rise to a narrow peak-dip structure in the current-phase relation by hybridizing at the edge of the junction. Remarkably, the maximal Majorana-state contribution to Josephson current takes a (nearly) universal value, approximately equal to the supercurrent capacity of a single quantum channel. These features provide a characteristic signature of Majorana states based entirely on supercurrent.

We demonstrate the use of hydrogen-induced changes in the emission of isoelectric Eu ions, in Mg-doped $p$-type GaN, as a powerful probe to study the dynamics of hydrogen movement under electron-beam irradiation. We identify, experimentally, a two-step process in the dissociation of Mg-H complexes and propose, based on density functional theory, that the presence of minority carriers and the resulting charge states of hydrogen drive this process.

We use angular-resolved photoluminescence excitation spectroscopy to probe energy relaxation towards the lowest lying states ($k_{\parallel }=0$) on the lower polariton branch (LPB) of two different strong-coupled $J$-aggregate organic semiconductor microcavities. We find that states along the upper polariton branch (UPB) are able to undergo efficient relaxation to populate states around $k_{\parallel }=0$ on the LPB; a process that occurs through the rapid relaxation of UPB polaritons to the exciton reservoir. In contrast, LPB states having a photon fraction $>$0.5 are not apparently able to undergo further relaxation towards $k_{\parallel }=0$, constituting an effective relaxation bottleneck.

We present first-principles calculations of piezoelectric coefficients of first and second order for AlN, GaN, InN, and ZnO, along with the composition dependence (bowing) of the linear terms. We show that eight second order coefficients are required and provide a simple description on how to incorporate second order effects in the calculation of the polarization and fields. We demonstrate that the second order effect leads to corrections of the order of 15% for quantum wells and up to 40% for epitaxial quantum dots.