Various technical developments have extended the potential of angle-resolved photoemission spectroscopy (ARPES) tremendously over the last 20 years. In particular improved momentum, energy, and spin resolution as well as the use of photon energies from a few eV up to several keV make ARPES a rather unique tool to investigate the electronic properties of solids and surfaces. With our work we present a generalization of the state-of-the-art description of the photoemission process, the so-called one-step model that describes excitation, transport to the surface, and escape into the vacuum in a coherent way. In particular, we present a theoretical description of temperature-dependent ARPES with a special emphasis on spin fluctuations. Finite-temperature effects are included within the so-called alloy analogy model which is based on the coherent potential approximation, and this method allows us to describe uncorrelated lattice vibrations in combination with spin fluctuations quantitatively on the same level of accuracy. To demonstrate the applicability of our approach a corresponding numerical analysis has been applied to spin- and angle-resolved photoemission of Fe(100) at finite temperatures.

We theoretically demonstrate an approach for designing absorbers with strongly intensity-dependent absorption. The proposed absorbers consist of a spacer layer between a top resistive sheet and an underlying metallic substrate, akin to the traditional Salisbury screen, except for the use of an epsilon-near-zero slab with a nonlinear dielectric inclusion as the spacer layer. Such absorbers may be designed to exhibit highly tailorable absorption characteristics, including either saturable or reverse saturable absorption. In addition, the proposed nonlinear absorbers include interesting features such as high angular selectivity, insensitivity with respect to the absorber thickness, bandwidth tunability, and the possibility of operating with or without hysteresis. The proposed nonlinear absorbers may be appealing for several applications and nonlinear devices, such as optical limiters.

We report the implementation of the fully relativistic exact muffin-tin orbital (EMTO) method for both first-principles electronic structure and quantum transport simulation of magnetic and nonmagnetic device materials. We consider a device-material system containing the inevitable atomic disorders in contact with different electrode materials. The Kohn-Sham Dirac equations for both cases with and without spin polarization are self-consistently solved for the central device-material system with the Green's function method. The fully relativistic charge-current density, conventional Pauli spin current density, and transmission coefficient are formulated with the nonequilibrium Green's function technique. To treat the influence of disordered defects/impurities, we combine the nonequilibrium Green's function in the Keldysh space with the coherent potential approximation, and account for the multiple disorder scattering by vertex corrections to a two-Green's-function correlator to calculate the disorder-averaged charge and spin current density. As a demonstration of the present implementation, we calculate the electronic structure of the bulk Pt, Co, and HgTe and Rashba-type surface states of Au and $\mathrm{Ag}/{\mathrm{Ag}}_2{\mathrm{Bi}}_1$ alloy surfaces. We find that the EMTO electronic structures of all the calculated systems agree well with the results of the projector-augmented wave method. The electronic charge and spin transport implementations are tested with perfect and disordered Cu/Co/Pt/Cu junctions. The important effects of interface and atomic disorders are illustrated for the spin transport in the presence of relativistic effects. The implementation of the fully relativistic EMTO-based device-material simulation provides an important tool for analyzing both the charge and spin transport through nanostructures and materials, significantly extending the capability of first-principles material design for spintronic device applications.

We report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. For the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. These results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay times measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems.

We studied the electronic band structure of pulsed laser deposition (PLD) grown (111)-oriented ${\mathrm{SrRuO}}_3$ thin films using in situ angle-resolved photoemission spectroscopy technique. We observed light bands with a renormalized quasiparticle effective mass of about $0.8m_e$. The electron-phonon coupling underlying this mass renormalization yields a characteristic “kink” in the band dispersion. The self-energy analysis using the Einstein model suggests five optical phonon modes covering an energy range of 44–90 meV contribute to the coupling. In addition, we show that the quasiparticle spectral intensity at the Fermi level is considerably suppressed, and two prominent peaks appear in the valance band spectrum at binding energies of 0.8 and 1.4 eV, respectively. We discuss the possible implications of these observations. Overall, our work demonstrates that high-quality thin films of oxides with large spin-orbit coupling can be grown along the polar (111) orientation by the PLD technique, enabling in situ electronic band structure study. This could allow for characterizing the thickness-dependent evolution of band structure of (111) heterostructures—a prerequisite for exploring possible topological quantum states in the bilayer limit.

Weyl semimetals are phases of matter with excitations effectively described by massless Dirac fermions. Their critical nature makes unclear the persistence of such a phase in the presence of disorder. We present a theorem ensuring the stability of the semimetallic phase in the presence of weak quasiperiodic disorder. The proof relies on the subtle interplay of the relativistic quantum field theory description combined with number-theoretical properties used in Kolmogorov-Arnold-Moser theory.

We study models with fractonlike order based on ${\mathbb{Z}}_2$ lattice gauge theories with subsystem symmetries in two and three spatial dimensions. The three-dimensional (3D) model reduces to the 3D toric code when subsystem symmetry is broken, giving an example of a subsystem symmetry enriched topological phase. Although not topologically protected, its ground-state degeneracy has as leading contribution a term which grows exponentially with the square of the linear size of the system. Also, there are completely mobile gauge charges living along with immobile fractons. Our method shows that fractonlike phases are also present in more usual lattice gauge theories. We calculate the entanglement entropy $S_A$ of these models in a subregion $A$ of the lattice and show that it is equal to the logarithm of the ground-state degeneracy of a particular restriction of the full model to $A$.

We report on electron transport in gate-tunable Corbino-geometry Josephson junctions (CJJs) fabricated from an epitaxially grown Aluminum (Al) superconductor on an InAs quantum well. We observed that a supercurrent can be tuned by the gate voltage and sweeping an out-of-plane magnetic field initiates a sudden rise in the electrical resistance. From the analysis using a theoretical model for flow resistance due to Josephson vortices in CJJs, we evaluate the vortex core size, which substantially depends on the carrier density and temperature. These results will be subsequently utilized for future operations of the Josephson vortices in CJJs.

Resonant tunneling of electrons between two ferromagnets and a quantum dot in the presence of an externally applied magnetic field reveals a strong gate dependence in the linear and nonlinear bias regime. This gate dependence originates from the interplay between Coulomb interactions and spin-dependent hybridization between the quantum dot and the leads. To take into account Coulomb interaction strengths of the same order of magnitude as the external magnetic field and the hybridization strength we adopt the numerically exact iterative summation of path integrals.

Two-photon absorption holds potential for many practical applications. We theoretically investigate the onset of this phenomenon in a semiconductor quantum dot–metallic nanoshell composite subjected to a resonant CW excitation. Two-photon absorption in this system may occur in two ways: incoherent—due to a consecutive ground-to-one-exciton-to-biexciton transition—and coherent—due to a coherent two-photon process, involving the direct ground-to-biexciton transition in the quantum dot. The presence of the nanoshell nearby the quantum dot gives rise to two principal effects: (i) renormalization of the applied field amplitude and (ii) renormalization of the resonance frequencies and radiation relaxation rates of the quantum dot, both depending on the quantum dot level populations. We show that in the perturbation regime, when the excitonic levels are only slightly populated, each of these factors may give rise to either suppression or enhancement of the two-photon absorption. The complicated interplay of the two determines the final effect. Beyond the perturbation regime, it is found that the two-photon absorption experiences a drastic enhancement, which occurs independently of the type of excitation, either into the one-exciton resonance or into the two-photon resonance. Other characteristic features of the two-photon absorption of the composite, emerging from the coupling between both nanoparticles, are bistability and self-oscillations.

We study the ergodic side of the many-body localization transition in its standard model, the disordered Heisenberg quantum spin chain. We show that the Thouless energy, extracted from long-range spectral statistics and the power-spectrum of the full momentum distribution fluctuations, is not large enough to guarantee thermalization. We find that both estimates coincide and behave nonmonotonically, exhibiting a strong peak at an intermediate value of the disorder. Furthermore, we show that nonthermalizing initial conditions occur well within the ergodic phase with larger probability than expected. Finally, we propose a mechanism, driven by the Thouless energy and the presence of anomalous events, for the transition to the localized phase.

We study the unitary dynamics of randomly or quasiperiodically driven tilted Bose-Hubbard (tBH) model in one dimension deep inside its Mott phase starting from a ${\mathbb{Z}}_2$ symmetry-broken state. The randomness is implemented via a telegraph noise protocol in the drive period while the quasiperiodic drive is chosen to correspond to a Thue-Morse sequence. The periodically driven tBH model (with a square pulse protocol characterized by a time period $T$) is known to exhibit transitions from dynamical regimes with long-time coherent oscillations to those with rapid thermalization. Here we show that starting from a regime where the periodic drive leads to rapid thermalization, a random drive, which consists of a random sequence of square pulses with period $T+\alpha dT$, where $\alpha =\pm 1$ is a random number and $dT$ is the amplitude of the noise, restores long-time coherent oscillations for special values of $dT$. A similar phenomenon can be seen for a quasiperiodic drive following a Thue-Morse sequence where such coherent behavior is shown to occur for a larger number of points in the $(T,dT)$ plane due to the additional structure of the drive protocol. We chart out the dynamics of the system in the presence of such aperiodic drives, provide a qualitative analytical understanding of this phenomenon, point out the role of quantum scars behind it, and discuss experiments which can test our theory.

Motivated by recent observations of unconventional out-of-plane dampinglike torque in ${\mathrm{WTe}}_2$/Permalloy bilayer systems, we calculate the spin-orbit torque generated in two-dimensional transition metal dichalcogenide (TMD)–ferromagnet heterostructures using first-principles methods and linear response theory. Our numerical calculation of spin-orbit torques in ${\mathrm{WTe}}_2/\mathrm{Co}$ and ${\mathrm{MoTe}}_2/\mathrm{Co}$ heterostructures shows both conventional and novel dampinglike torkances (torque per electric field) with comparable magnitude, around $100\hslash /2e{\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$, for an electric field applied perpendicular to the mirror plane of the TMD layer. To gain further insight into the source of dampinglike torque, we compute the spin current flux between the TMD and Co layers and find good agreement between the two quantities. This indicates that the conventional picture of dampinglike spin-orbit torque, whereby the torque results from the spin Hall effect plus spin transfer torque, largely applies to TMD/Co bilayer systems.

In this work, we report a mechanism of stop-band formation in a multilayer array of germanium nanoparticles embedded in a crystalline silicon matrix. When only a single layer of nanoparticles is embedded, the local resonance, induced by the destructive interference between two different phonon wave paths, gives rise to several sharp and significant transmittance dips. On the other hand, when the number of the layers of embedded nanoparticles further increases to ten, a stop band with complete phonon reflection is formed due to the two-path resonance Bragg-type phonon interference. The wave packet simulations further uncover that the stop band originates from the collective phonon resonances in the embedded nanoparticles layers. Compared with the traditional stop-band formation mechanism that is the single-path Bragg reflection, the two-path phonon-interference resonance mechanism has a significant advantage in not requiring the strict periodicity in the embedded nanoparticles multilayer array. We also demonstrate that the stop band can significantly suppress thermal conductance in the low-frequency regime. Our work provides a robust, scalable, and easily modulable stop-band formation mechanism, which opens a degree of freedom for phononics-related heat control.

In recent years, transient electric field is becoming a means for the generation of hidden states or phase transitions. In this study, the spatial-temporal distribution of picosecond surface electric fields, associated with the femtosecond-laser-induced charge emission from ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) single-crystal films, were interrogated by subpicosecond electron pulses and reconstructed by a three-layer theoretical model. A strong picosecond surface electric field, on the order of 100 kV/m, was generated on LSMO films. Such self-induced transient electric fields may be a promising means for controlling ultrafast processes in materials.

Recently superconductivity has been discovered at around 200 K in a hydrogen sulfide system and around 260 K in a lanthanum hydride system, both under pressures of about 200 GPa. These record-breaking transition temperatures bring within reach the long-term goal of obtaining room-temperature superconductivity. We have used first-principles calculations based on density functional theory along with Migdal-Eliashberg theory to investigate the electron-phonon mechanism for superconductivity in the $Fm\overline{3}m$ phase proposed for the ${\mathrm{LaH}}_{10}$ superconductor. We show that the very high transition temperature $T_c$ results from a highly optimized electron-phonon interaction that favors coupling to high-frequency hydrogen phonons. Various superconducting properties are calculated, such as the energy gap, the isotope effect, the specific heat jump at $T_c$, the thermodynamic critical field, and the temperature-dependent penetration depth. However, our main emphasis is on the finite-frequency optical properties, measurement of which may allow for an independent determination of $T_c$ and also a confirmation of the mechanism for superconductivity.

The Hopf insulator is a three-dimensional topological insulator outside the standard classification of topological insulators. Here we consider two types of non-Hermitian Hopf insulators, one without and one with the non-Hermitian skin effect. The isolated gapless points of the Hermitian model are broadened into finite regimes in the non-Hermitian models. However, the modulus of the Hopf index remains quantized in the gapped regimes. The model without the non-Hermitian skin effect allows an accurate evaluation of its generalized Hopf index and energy spectrum, showing an agreement between the gapless-regime estimations from the systems with periodic and open boundary conditions. Near the zero-energy plane, Fermi rings can be observed whenever the Hopf index is quantized at nonzero values, and there is a bulk-boundary correspondence between the modulus of the Hopf index and the number of Fermi rings. The other model manifests the non-Hermitian skin effect in the generalized Brillouin zone and shows the skewed profiles of the bulk states. The Hopf index and energy spectrum are shown to be sensitive to the boundary condition in the presence of the non-Hermitian skin effect.

We attempted to assign the spin structure of a layered organic antiferromagnet, $\kappa$-(d8-BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$, which is a key material located closest to the Mott boundary at ambient pressure among this family of compounds, by investigating its macroscopic magnetization thoroughly, motivated by a recent successful assignment of the spin structure of an isostructural material, $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ [BEDT-TTF and d8-BEDT-TTF are bis(ethylenedithio)tetrathiafulvalene and its deuterated molecule, respectively]. We measured the isothermal magnetization after careful choice of the measurement temperatures and cooling speed at around 80 K, so that the magnetism of the antiferromagnetic phase can be effectively extracted. Consequently, we observed hysteresis loops signifying ferromagnetism and steplike behavior when the magnetic field applied parallel to the crystallographic $b$ and $a$ axes was swept, respectively. The possible spin structure consistent with these results was discussed in terms of probable interactions between the spins, such as exchange interactions and the Dzyaloshinskii-Moriya interaction. Eventually, we asserted that $\kappa$-(d8-BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ has a spin structure with the easy axis being the $c$ axis and the net canted moment parallel to the $b$ axis, which is surprisingly different from that of $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$. We suggested that this difference originates from the difference of the sign of the interlayer interaction between the two materials. We also elucidated the overall picture of the magnetization processes of this material under the magnetic fields parallel to the three principle axes, which are also in contrast to those of $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$. In particular, the spin-reverse transition at which half of the spins rotate by ${180}^{\circ }$ was not induced by the $b$-axis magnetic field, as in the case of $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$, but by the $a$-axis magnetic field. Finally, numerical simulations and magnetic symmetry analysis enabled us to confirm the validity of the spin structures proposed for the two antiferromagnets under zero and high magnetic fields.

We analyze a system in which a topological Majorana zero mode (tMZM) combines with a Majorana produced by quantum frustration (fMZM) to produce a novel ground state. The system that we study combines two parts, a grounded topological superconducting wire that hosts two tMZMs at its ends and an on-resonant quantum dot connected to two dissipative leads. The quantum dot with dissipative leads creates an effective two-channel Kondo (2CK) state in which quantum frustration yields an isolated fMZM at the dot. We find that coupling the dot to one end of the topological wire stabilizes the tMZM at the other end. Three routes are used to obtain these results: (i) calculation of the conductance through an auxiliary detector quantum dot, (ii) renormalization group (RG) arguments and the $g$ theorem, and (iii) a fully nonequilibrium calculation of the $I\left(V\right)$ curve and shot noise $S\left(V\right)$ through the detector dot. In addition to providing a route to achieving an unpaired Majorana zero mode, this scheme provides a clear signature of the presence of the 2CK frustration-induced Majorana.