We use time- and angle-resolved photoemission spectroscopy to characterize the dynamics of the energy gap in superconducting ${\text{Bi}}_2$${\text{Sr}}_2$${\text{CaCu}}_2$${\text{O}}_{8+\delta }$ (Bi2212). Photoexcitation drives the system into a nonequilibrium pseudogap state: Near the Brillouin zone diagonal (inside the normal-state Fermi arc), the gap completely closes for a pump fluence beyond $F\approx 15$ $\mu$J/cm${}^2$; toward the Brillouin zone face (outside the Fermi arc), it remains open to at least 24 $\mu$J/cm${}^2$. This strongly anisotropic gap response may indicate multiple competing ordering tendencies in Bi2212. Despite these contrasts, the gap recovers with relatively momentum-independent dynamics at all probed momenta, which shows the persistent influence of superconductivity both inside and outside the Fermi arc.

We present an ab initio calculation of the $\vec{k}$- and spin-resolved electronic lifetimes in the half-metallic Heusler compounds ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$ and ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$. We determine the spin-flip and spin-conserving contributions to the lifetimes and study in detail the behavior of the lifetimes around states that are strongly spin mixed by spin-orbit coupling. We find that for nondegenerate bands the spin mixing alone does not determine the energy dependence of the (spin-flip) lifetimes. Qualitatively, the lifetimes reflect the lineup of electron and hole bands. We predict that different excitation conditions lead to drastically different spin-flip dynamics of excited electrons and may even give rise to an enhancement of the nonequilibrium spin polarization.

The relaxation and transport dynamics of photoexcited carriers in $p$-type stoichiometric polycrystalline CuInSe${}_2$ is investigated by optical-pump terahertz-probe spectroscopy. For all time delays and temperatures studied, the optically measured photoconductivity exhibits a characteristic free carrier Drude response, which allows analyzing the carrier scattering and relaxation phenomena in detail. The hot carrier distribution initially present after photoexcitation is found to relax within the first 200 ps by electron-phonon interaction with polar longitudinal optical (LO) phonons. The relaxed carrier distribution found after 200 ps indicates room temperature minority carrier mobilities close to 1000 cm${}^2$/Vs, in excellent agreement with Hall effect carrier mobilities previously determined for $n$-type single crystals. Analysis of the temperature dependence shows that the mobility at low temperatures is limited by ionized impurity scattering, while at room temperature the scattering of electrons with polar LO phonons dominates.

We propose two-dimensional (2D) topological insulators (TIs) in functionalized germanenes (GeX, X$=$H, F, Cl, Br, or I) using first-principles calculations. We find GeI is a 2D TI with a bulk gap of about 0.3 eV, while GeH, GeF, GeCl, and GeBr can be transformed into TIs with sizable gaps under achievable tensile strains. A unique mechanism is revealed to be responsible for the large topologically nontrivial gap obtained: due to the functionalization, the $\sigma$ orbitals with stronger spin-orbit coupling (SOC) dominate the states around the Fermi level, instead of original $\pi$ orbitals with weaker SOC. Thereinto, the coupling of the $p_{xy}$ orbitals of Ge and heavy halogens in forming the $\sigma$ orbitals also plays a key role in the further enlargement of the gaps in halogenated germanenes. Our results suggest a realistic possibility for the utilization of topological effects at room temperature.

We consider one-dimensional topological insulators hosting fractionally charged midgap states in the presence and absence of induced superconductivity pairing. Under the protection of a discrete symmetry, relating positive and negative energy states, the solitonic midgap states remain pinned at zero energy when superconducting correlations are induced by proximity effect. When the superconducting pairing dominates the initial insulating gap, Majorana fermion phases develop for a class of insulators. As a concrete example, we study the Creutz model with induced $s$-wave superconductivity and repulsive Hubbard-type interactions. For a finite wire, without interactions, the solitonic modes originating from the nonsuperconducting model survive at zero energy, revealing a fourfold-degenerate ground state. However, interactions break the aforementioned discrete symmetry and completely remove this degeneracy, thereby producing a unique ground state which is characterized by a topological bulk invariant with respect to the product of fermion parity and bond inversion. In contrast, the Majorana edge modes are globally robust to interactions. Moreover, the parameter range for which a topological Majorana phase is stabilized expands when increasing the repulsive Hubbard interaction. The topological phase diagram of the interacting model is obtained using a combination of mean-field theory and density matrix renormalization group techniques.

We present genuine bulk Ir $5d$ $j_{\mathrm{eff}}$ states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with $s$- and $p$-polarized lights. HAXPES spectra of Sr${}_2$IrO${}_4$ and Ba${}_2$IrO${}_4$ are well reproduced by the quasiparticle density of states calculated by the local density approximation with dynamical mean-field theory ($\text{LDA}+\text{DMFT}$). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals “bad metallic” states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Néel temperature as predicted by the $\text{LDA}+\text{DMFT}$.

Optically excited electron-hole pairs, driven by a strong terahertz (THz) field, create high sidebands in the optical spectrum. The sideband spectrum exhibits a “plateau” up to a cutoff of $3.17U_p$, where $U_p$ is the ponderomotive energy. This cutoff is determined, semiclassically, from the maximum kinetic energy an electron-hole pair can gain from the THz field along a closed trajectory. A full quantum treatment reveals a second, classically forbidden, plateau with a cutoff of $8U_p$, the maximum kinetic energy an electron-hole pair can gain from the THz field along an open trajectory. The second plateau appears because a spatially separated electron and hole can still recombine if the classical excursion is less than the width of the electron-hole wave function, which occurs when the coherence time is longer than the excursion time (half the THz field period). This effect broadens the range of materials and excitation conditions where high-sideband generations can occur, thereby providing a wealth of systems for ultrafast electro-optical applications.

A boundary transfer matrix formulation allows to calculate the Loschmidt echo for one-dimensional quantum systems in the thermodynamic limit. We show that nonanalyticities in the Loschmidt echo and zeros for the Loschmidt amplitude in the complex plane (Fisher zeros) are caused by a crossing of eigenvalues in the spectrum of the transfer matrix. Using a density-matrix renormalization group algorithm applied to these transfer matrices, we numerically investigate the Loschmidt echo and the Fisher zeros for quantum quenches in the XXZ model with a uniform and a staggered magnetic field. We give examples—both in the integrable and the nonintegrable cases—where the Loschmidt echo does not show nonanalyticities although the quench leads across an equilibrium phase transition, and examples where nonanalyticities appear for quenches within the same phase. For a quench to the free fermion point, we analytically show that the Fisher zeros sensitively depend on the initial state and can lie exactly on the real axis already for finite system size. Furthermore, we use bosonization to analyze our numerical results for quenches within the Luttinger liquid phase.

We investigate by broadband femtosecond pump-probe spectroscopy the transient optical response of a metal-dielectric Bragg filter (MDBF) made of thin layers of gold and fused silica. A quantitative comparison of the experimental results with a semiclassical model with no fitting parameters allows us to disentangle all the different contributions to the transient optical response of the MDBF, which include the ultrafast dynamics of thermalized and nonthermalized electrons as well as of lattice phonons excited in the metal layers. Furthermore, we provide a quantitative comparison of the optical response of the MDBF with that of a single gold layer under comparable excitation conditions. The results clearly show the advantages and disadvantages of the multilayer configuration in terms of all-optical-modulation capability.

We present a linear-response formalism for a system of correlated electrons out of equilibrium, as relevant for the probe optical absorption in pump-probe experiments. We consider the time dependent optical conductivity $\sigma (\omega ,t)$ and its nonequilibrium properties. As an application we numerically study a single highly excited charged particle in the spin background, as described within the two-dimensional $t$-$J$ model. Our results show that the optical sum rule approaches the equilibriumlike one very fast; however, the time evolution and the final asymptotic behavior of the absorption spectra in the finite systems considered still reveal dependence on the type of initial pump perturbation. This is observed in the evolution of its main features: the midinfrared peak and the Drude weight.

As a model for describing finite-size effects in topological insulator thin films, we study a one-dimensional (1D) effective model of a topological insulator (TI). Using this effective 1D model, we reveal the precise correspondence between the spatial profile of the surface wave function, and the dependence of the finite-size energy gap on the thickness ($L_x$) of the film. We solve the boundary problem both in the semi-infinite and slab geometries to show that the $L_x$ dependence of the size gap is a direct measure of the amplitude of the surface wave function $\psi \left(x\right)$ at the depth of $x=L_x+1$ [here, the boundary condition is chosen such that $\psi \left(0\right)=\mathbf{0}$]. Depending on the parameters, the edge state function shows either a damped oscillation (in the “TI-oscillatory” region of Fig. 2), or becomes overdamped (in the “TI-overdamped” phase of Fig. 2). In the original 3D bulk TI, an asymmetry in the spectrum of valence and conduction bands is omnipresent. Here, we demonstrate that by tuning this asymmetry one can drive a crossover from the TI oscillatory to the TI-overdamped phase.

We study THz pulses generated from plasmonic metal nanostructures under femtosecond illumination of near-IR light. We find two regimes of excitation, according to the order of the dependence of the THz fluence on the incident near-IR intensity: less then second order at low intensities, changing to approximately fourth order for higher intensities. These regimes are most likely associated with two THz generation mechanisms: optical rectification, and the ponderomotive acceleration of ejected electrons. These data provide evidence that both mechanisms can be at work in the same experiment.

We investigate the excitonic dynamics in MoSe${}_2$ monolayer and bulk samples by femtosecond transient absorption. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by measuring a differential reflection of a probe pulse tuned in the range 790–820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in a bulk sample under comparable conditions. We also observe the saturated absorption induced by excitons in both monolayers and the bulk in the differential reflection measurements, which indicates their potential applications as saturable absorbers.

We consider a model of Dirac fermions coupled to flexural phonons to describe a graphene sheet fluctuating in dimension $2+d$. We derive the self-consistent screening equations for the quantum problem, exact in the limit of large $d$. We first treat the membrane alone, and work out the quantum to classical and harmonic to anharmonic crossovers. For the coupled electron-membrane problem, we calculate the dressed two-particle propagators of the elastic and electron interactions and find that it exhibits a collective mode which becomes unstable at some wave vector $q_{\mathrm{c}}$ for large enough coupling $g$. The saddle-point analysis, exact at large $d$, indicates that this instability corresponds to spontaneous and simultaneous appearance of Gaussian curvature and electron puddles. The relevance to ripples in graphene is discussed.

The piezoelectric properties of tetragonal perovskites $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$, $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$, and $\mathrm{K}\mathrm{Nb}{\mathrm{O}}_3$ under uniaxial strain along the [001] direction are calculated from first principles. In order to study the piezoelectric response of individual atomic sublattices, the total polarization $P$ and piezoelectric constant $d_{33}$ are decomposed into the contributions of $A$-site and $B$-site atoms. These three materials show different behaviors at the atomic scale. In $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$, the Pb and Ti contribute almost equally to the polarization and $d_{33}$. In $\mathrm{K}\mathrm{Nb}{\mathrm{O}}_3$, most of the polarization and $d_{33}$ are contributed by the ferroelectrically active Nb. But in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$, as the $c$ axis increases, the contribution of Ba to $d_{33}$ rises rapidly, reaching 50% of the total $d_{33}$ at $c$ = 4.30 Å. This means that the covalent bonding between Ba and O is significant and has an important impact on the piezoelectric response of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$.

The distribution of magnetic moments in the iron-rich intermetallic compound ${\mathrm{Lu}}_2{\mathrm{Fe}}_{17}$ with Fe partly substituted by Ru has been investigated on ${\mathrm{Lu}}_2{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ by using several techniques. The crystal structure and temperature evolution of the magnetic structure was studied using high resolution and high flux neutron powder diffractometers. In contrast to the parent ${\mathrm{Lu}}_2{\mathrm{Fe}}_{17}$, which transforms on cooling from an antiferromagnetic to a ferromagnetic state, ${\mathrm{Lu}}_2{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ is an antiferromagnet (Néel temperature $T_{\mathrm{N}}$ = 208 K) down to the lowest temperatures. The magnetic polarization at the lutetium $L_{2,3}$ absorption edges, iron $K$ and ruthenium $L_2$ edges was studied by x-ray magnetic circular dichroism (XMCD) on a single crystal of ${\mathrm{Lu}}_2{\mathrm{Fe}}_{16.5}{\mathrm{Ru}}_{0.5}$ in a magnetic field of 3 T, i.e., above the field-induced metamagnetic transition. The Ru XMCD signal proves the existence of induced magnetic polarization parallel to the dominant Fe sublattice magnetization. A corresponding theoretical analysis in the framework of the local spin density formalism is provided in order to discern the structural disorder effects and magnetism in the system.

Time-resolved scanning Kerr microscopy (TRSKM) has been used to examine the effect of nonlocal spin transfer torque (NL-STT) within a two-terminal current perpendicular-to-plane nonlocal spin valve. A combination of Oersted field and NL-STT was used to excite and control the magnetization dynamics. By comparison with a macrospin model, the strength of the NL-STT was quantified and found to be comparable to that achieved by direct injection of spin-polarized current. The sensitivity of the magneto-optical probe to small out-of-plane deflections of the magnetization also allows the NL-STT to be quantified in quasi-dc measurements, greatly simplifying the investigation of STT phenomena.