We show that electric currents are induced in inversion asymmetric ferromagnets if the exchange splitting varies in time after excitation by laser pulses. We interpret this phenomenon as the magnetic variant of the inverse Edelstein effect. Based on ab initio calculations we determine the size of this effect in Co/Pt(111) and Mn/W(001) magnetic bilayer systems and investigate its dependence on the magnetization direction. The comparison of our theoretical results to experiments measuring the THz signal emitted after laser excitation suggests that ultrafast demagnetization in $3d$ transition metal ferromagnets is dominated by transverse spin fluctuations rather than by a reduction of the exchange splitting.

A three-orbital-model approach for studying spin-wave excitations in the strongly spin-orbit coupled layered perovskite iridates is presented which provides a unified description of magnetic excitations as well as the electronic band structure. The calculated spin-wave dispersions with realistic three-band parameters are in excellent agreement with the resonant inelastic x-ray scattering (RIXS) data for iridates, including the strong antiferromagnet zone-boundary dispersion in the single-layer compound ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and the large anisotropy gap in the bilayer compound ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$. The RIXS spin-wave data are shown to provide evidence of mixing between the $J=\frac{1}{2}$ and $\frac{3}{2}$ sectors in both compounds.

We consider the current-phase relation (CPR) in Josephson junctions with complex insulator-superconductor-ferromagnetic interlayers in the vicinity of the 0-$\pi$ transition. We find a strong impact of the second harmonic on the CPR of the junctions. It is shown that the critical current can be kept constant in the region of 0-$\pi$ transition, while the CPR transforms through multivalued hysteretic states depending on the relative values of tunnel transparency and magnetic thickness. Moreover, the CPR in the transition region has multiple branches with distinct ground states.

Ubiquitous on Earth, α-quartz plays an important role in modern science and technology. However, despite extensive research in the past, the mechanism of the polymorphic transitions of α-quartz at high pressures remains poorly understood. Here, combining in situ single-crystal x-ray diffraction experiment and advanced ab initio modeling, we report two stability limits and competing transition pathways of α-quartz under high pressure. Under near-equilibrium compression conditions at room temperature, α-quartz transits to a new $P2/c$ silica phase via a structural intermediate. If the thermally activated transition is kinetically suppressed, the ultimate stability of α-quartz is controlled by its phonon instability and α-quartz collapses into a different crystalline phase. Our studies reveal that pressure-induced solid-state transformation of α-quartz undergoes a succession of structural stability limits, due to thermodynamic and mechanical catastrophes, and exhibits a hierarchy of transition pathways contingent upon kinetic conditions.

The thermal conductivity of cylindrical and smooth silicon nanowires is systematically studied as a function of diameter and length by fully atomistic simulations. A transient thermal regime is created and monitored by molecular dynamics. This approach-to-equilibrium methodology, already proven to be very efficient for bulk systems, is here applied to nanowires of length up to 1.2 micron, and diameter in the range 1 to 14 nm. It is shown that the temperature profile along the nanowire axis and its temporal evolution comply with the heat equation. A one-dimensional thermal conductivity is extracted from the characteristic decay time according to the time-dependent solution of the heat equation. Like for the bulk using the same method, the thermal conductivity exhibits a length dependence due to the slowly growing cumulative distribution of the phonon mean free paths. Unlike the bulk, the infinite-length saturation of the conductivity value can be observed in our simulations, thanks to a reduction of the phonon mean free path in the nanowires, estimated to be a few hundred nanometers. A finite thermal conductivity can be extracted for each diameter, and no divergence at long length is observed.

We present a comprehensive set of elastic and inelastic neutron scattering measurements on a range of Fe-doped samples of $\mathrm{U}{\left({\mathrm{Ru}}_{1-x}{\mathrm{Fe}}_x\right)}_2{\mathrm{Si}}_2$ with $0.01\le x\le 0.15$. All of the samples measured exhibit long-range antiferromagnetic order, with the size of the magnetic moment quickly increasing to 0.$51{\mu }_B$ at 2.5% doping and continuing to increase monotonically with doping, reaching 0.$69{\mu }_B$ at 15% doping. Time-of-flight and inelastic triple-axis measurements show the existence of excitations at (1 0 0) and (1.4 0 0) in all samples, which are also observed in the parent compound. While the excitations in the 1% doping are quantitatively identical to the parent material, the gap and width of the excitations change rapidly at 2.5% Fe doping and above. The 1% doped sample shows evidence for a separation in temperature between the hidden order and antiferromagnetic transitions, suggesting that the antiferromagnetic state emerges at very low Fe dopings. The combined neutron scattering data suggest not only discontinuous changes in the magnetic moment and excitations between the hidden order and antiferromagnetic phases, but that these changes continue to evolve up to at least $x=0.15$.

In the field of spin caloritronics recent theoretical models suggested a significant influence of the interfaces of the magnetic tunnel junction (MTJ) on the thermal transport. In this work magnetothermopower measurements are carried out on CoFeB/MgO/CoFeB nanopillars and an unexpected increase of the thermovoltage with the diameter of the nanopillars is observed. To understand this behavior the thermal profiles are computed by finite element simulations. The observed behavior with the pillar diameter could only be reproduced in simulations by considering a far lower effective thermal conductivity of the MgO than the intrinsic thin-film value. In agreement with theoretical predictions, a finite thermal conductivity of the MgO/CoFeB interface can explain this observation. This is experimental evidence of the influence of the thermal resistance of the MgO/CoFeB interfaces on magnetothermovoltage measurements and is in agreement with recent theoretical predictions. The measured magnetothermovoltage is around 4.5 μV and the simulated temperature difference is about 2 K across the tunnel barrier, which resulted in a magnetic contribution of the thermopower of $\mathrm{\Delta }{S}_{\mathrm{MTJ}}\approx -2.25\mu \mathrm{V}{\mathrm{K}}^{-1}$. This value was about 20 times smaller than the result obtained by the typically used thermal conductivity of MgO thin films.

The development of short- and long-range magnetic order induced in a frustrated zigzag ladder compound ${\mathrm{SrDy}}_2{\mathrm{O}}_4$ by an applied field is studied using neutron-diffraction techniques. In zero field, ${\mathrm{SrDy}}_2{\mathrm{O}}_4$ lacks long-range magnetic order down to temperatures as low as 60 mK, and the observed powder-neutron-diffraction (PND) patterns are dominated by very broad diffuse scattering peaks. Single-crystal neutron diffraction reveals that the zero-field magnetic structure consists of a collection of antiferromagnetic chains running along the $c$ axis and that there is very little correlation between the chains in the $ab$ plane. In an applied magnetic field, the broad diffuse scattering features in PND are gradually replaced by much sharper peaks, however, the pattern remains rather complex, reflecting the highly anisotropic nature of ${\mathrm{SrDy}}_2{\mathrm{O}}_4$. Single-crystal neutron diffraction shows that a moderate field applied along the $b$ axis induces an up-up-down magnetic order associated with a $\frac{1}{3}$-magnetization plateau, in which magnetic correlation length in the $ab$ plane is significantly increased, but it nevertheless remains finite. The resolution-limited $\mathbf{k}=0$ peaks associated with a ferromagnetic arrangement appear in powder and single-crystal neutron-diffraction patterns in fields of 2.5 T and above.

It has previously been found that a magnetic impurity in a conventional $s$-wave superconductor can give rise to a local $\pi$-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the $\pi$-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When nonresonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a $\pi$-phase shift. We also show that for a finite spin-orbit coupling, the $\pi$-phase shift is preserved and occurs in a large portion of the topologically nontrivial phase.

The recent discovery of superconductivity in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ [H. Takahashi et al., Nat. Mater. 14, 1008 (2015)] has stimulated considerable interest in 123-type iron chalcogenides. This material is the first reported iron-based two-leg ladder superconductor, as opposed to the prevailing two-dimensional layered structures of the iron superconductor family. Once the hydrostatic pressure exceeds 11 GPa, ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ changes from a semiconductor to a superconductor below 24 K. Although previous calculations correctly explained its ground-state magnetic state and electronic structure, the pressure-induced phase transition was not successfully reproduced. In this work, our first-principles calculations show that with increasing pressure the lattice constants as well as local magnetic moments are gradually suppressed, followed by a first-order magnetic transition at a critical pressure, with local magnetic moments dropping to zero suddenly. Our calculations suggest that the self-doping caused by electrons transferred from S to Fe may play a key role in this transition. The development of a nonmagnetic metallic phase at high pressure may pave the way to superconductivity. As extensions of this effort, two other 123-type iron chalcogenides, ${\mathrm{KFe}}_2{\mathrm{S}}_3$ and ${\mathrm{KFe}}_2{\mathrm{Se}}_3$, have also been investigated. ${\mathrm{KFe}}_2{\mathrm{S}}_3$ also displays a first-order transition with increasing pressure, but ${\mathrm{KFe}}_2{\mathrm{Se}}_3$ shows instead a second-order or weakly first-order transition. The required pressures for ${\mathrm{KFe}}_2{\mathrm{S}}_3$ and ${\mathrm{KFe}}_2{\mathrm{Se}}_3$ to quench the magnetism are higher than for ${\mathrm{BaFe}}_2{\mathrm{S}}_3$. Further experiments could confirm the predicted first-order nature of the transition in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{KFe}}_2{\mathrm{S}}_3$, as well as the possible metallic/superconductivity state in other 123-type iron chalcogenides under high pressure.

In this study, we investigate on the basis of ab initio calculations how the morphology, molecular on-site energies, reorganization energies, and charge transfer integral distribution affect the hopping charge transport and the exciton formation process in disordered organic semiconductors. We focus on three materials applied frequently in organic light-emitting diodes: $\alpha \text{−}\mathrm{NPD}$, TCTA, and Spiro-DPVBi. Spatially correlated disorder and, more importantly, superexchange contributions to the transfer integrals, are found to give rise to a significant increase of the electric field dependence of the electron and hole mobility. Furthermore, a material-specific correlation is found between the HOMO and LUMO energy on each specific molecular site. For $\alpha \text{−}\mathrm{NPD}$ and TCTA, we find a positive correlation between the HOMO and LUMO energies, dominated by a Coulombic contribution to the energies. In contrast, Spiro-DPVBi shows a negative correlation, dominated by a conformational contribution. The size and sign of this correlation have a strong influence on the exciton formation rate.

Understanding phonon transport across interfaces serves as a major tool to advance a diverse spectrum of fundamental and applied research. Unlike bulk materials, where the three-phonon scattering process is relatively straightforward to investigate, little research has been dedicated to the detailed analysis of the three-phonon scattering process at interfaces due to the complexity of interfaces and the mismatch of phonon dispersions of the two connecting parts. Based on the nonequilibrium molecular dynamics simulation, which is one of the most popular approaches to investigate the thermal conductance, we develop an explicit theoretical framework by considering the full third-order force constants field to quantify the two- and three-phonon scattering at interfaces. Bulk Ar is used as a benchmark to validate the computational scheme by comparing the results with those using the all-order phonon scattering method [frequency-dependent directly decomposed method; Y. Zhou and M. Hu, Phys. Rev. B 92, 195205 (2015)]. Then, Ar–heavy Ar and Si-Ge interfaces are studied and the respective role of two- and three-phonon scattering processes is quantitatively characterized at different temperatures. Moreover, all four different types of the three-phonon scattering process are explicitly evaluated. The method developed herein for splitting the two- and three-phonon scattering processes in the interfacial heat transport is expected to advance our understanding of the phonon process at interfaces, and will facilitate designing high-performance interfacial structures in terms of efficient thermal management.

Hotspots of surface-enhanced resonance Raman scattering (SERRS) are localized within 1 nm at gaps or crevices of plasmonic nanoparticle dimers. We demonstrate SERRS hotspots with volumes that are extended in one dimension tens of thousand times compared to standard zero-dimensional hotspots using crevices of plasmonic nanowire dimers. According to the polarization measurements, a plasmon resonance in the direction along the dimer width generates SERRS hotspots. SERRS images show oscillating patterns between edges of the hotspot. The SERRS intensity becomes the strongest at the edges, indicating that Fabry-Perot-type resonance of surface plasmons is involved in the Raman enhancement. These optical properties of the SERRS hotspots are quantitatively reproduced by numerical calculations based on the electromagnetic (EM) mechanism. EM coupling energy between dye molecule excitons and plasmons is evaluated using spectral changes in plasmon resonance reflected in a loss of SERRS activity at the hotspots. The coupling energies are consistent with the calculated EM enhancement factors.

We used high-spatial-resolution, low-temperature near-field scanning optical microscopy (NSOM) operating at magnetic fields $B=0–10\mathrm{T}$ to study the effects of Wigner localization (WL) on emission spectra of single self-organized InP/GaInP quantum dots (QDs) and investigate the stability of singly (trion) and doubly (tetron) charged exciton complexes in the weak quantum confinement regime. Using NSOM measurements together with configuration interaction calculations, we identify the dots having different electron population $\mathrm{N}(\mathrm{N}=1–12)$, quantum confinement $(\hslash {\omega }_0=0.6–8\mathrm{meV})$ and size $D(D=70–170\mathrm{nm})$. For $\mathrm{N}=2$, we observed a magnetic-field-induced molecular-droplet transition, accompanied by the decomposition of the tetron into a Wigner molecule complex (WMC), and the activation of rotovibronic structure. For $\mathrm{N}=1$, unusually strong vibronic structure resulting from a trion-type WMC was observed. We have shown that magnetic-field-induced shifts of this structure allow measurement of single particle Fock-Darwin levels and angular momentum transitions of the WMC. In addition, we demonstrated the use of NSOM imaging to probe the charge density distribution and observed anomalous dependence of the image size on the quantum confinement, implying a pairing of electrons or formation of whispering gallery modes in the QD. We demonstrated that InP/GaInP QDs, provide a Wigner-Seitz radius $\left(r_s\right)$ up to 13, and that the measurements of NSOM magneto-optical spectroscopy using these dots makes it possible to study effects arising from strong Coulomb interaction of a few confined electrons (holes).

We address the effect of nearest-neighbor (NN) interactions on the topological properties of the Su-Schrieffer-Heeger (SSH) chain, with alternating hopping amplitudes $t_1$ and $t_2$. Both numerically and analytically, we show that the presence of interactions induces phase transitions between topologically different regimes. In the particular case of one-hole excitations in a half-filled SSH chain, the $V/t_2$ versus $t_1/t_2$ phase diagram has topological phases at diagonal regions of the phase plane. The interaction acts in this case as a passivation potential. For general filling of the SSH chain, different eigensubspaces of the SSH Hamiltonian may be classified as topologically trivial and nontrivial. The two-hole case is studied in detail in the large interaction limit, and we show that a mapping can be constructed of the two-hole SSH eigensubspaces into one-particle states of a noninteracting one-dimensional (1D) tight-binding model, with interfaces between regions with different hopping constants and local potentials. The presence of edge states of topological origin in the equivalent chain can be readily identified, as well as their correspondence to the original two-hole states. Of these states only some, identified by us, are protected and, therefore, truly topological. Furthermore, we found that the presence of the NN interaction generates a state where two holes occupy two consecutive edge states. Such many-body states should also occur for arbitrary filling leading to the possibility of a macroscopic hole gathering at the surface (at consecutive edge states).

Approximating the frequency dispersion of the permittivity of materials with simple analytical functions is of fundamental importance for understanding and modeling the optical response of materials and resulting structures. In the generalized Drude-Lorentz model, the permittivity is described in the complex frequency plane by a number of simple poles having complex weights, which is a physically relevant and mathematically simple approach: By construction, it respects causality, represents physical resonances of the material, and can be implemented easily in numerical simulations. We report here an efficient method of optimizing the fit of measured data with the Drude-Lorentz model having an arbitrary number of poles. We show examples of such optimizations for gold, silver, and copper, for different frequency ranges and up to four pairs of Lorentz poles taken into account. We also provide a program implementing the method for general use.

Dynamical mean-field theory is used to study the quantum critical point (QCP) in the doped Hubbard model on a square lattice. The QCP is characterized by a universal scaling form of the self-energy and a spin density wave instability at an incommensurate wave vector. The scaling form unifies the low-energy kink and the high-energy waterfall feature in the spectral function, while the spin dynamics includes both the critical incommensurate and high-energy antiferromagnetic paramagnons. We use the frequency-dependent four-point correlation function of spin operators to calculate the momentum-dependent correction to the electron self-energy. By comparing with the calculations based on the spin-fermion model, our results indicate the frequency dependence of the quasiparticle-paramagnon vertices is an important factor to capture the momentum dependence in quasiparticle scattering.

The geometrical intrinsic contribution to the anomalous Hall conductivity (AHC) of a metal is commonly expressed as a reciprocal-space integral: as such, it only addresses unbounded and macroscopically homogeneous samples. Here we show that the geometrical AHC has an equivalent expression as a local property. We define a “geometrical marker” which actually probes the AHC in inhomogeneous systems (e.g., heterojunctions), as well as in bounded samples. The marker may even include extrinsic contributions of geometrical nature.

When constructing a Wilson chain to represent a quantum impurity model, the effects of truncated bath modes are neglected. We show that their influence can be kept track of systematically by constructing an “open Wilson chain” in which each site is coupled to a separate effective bath of its own. As a first application, we use the method to cure the so-called mass-flow problem that can arise when using standard Wilson chains to treat impurity models with asymmetric bath spectral functions at finite temperature. We demonstrate this for the strongly sub-Ohmic spin-boson model at quantum criticality where we directly observe the flow towards a Gaussian critical fixed point.