Operator spreading under unitary time evolution has attracted a lot of attention recently as a way to probe many-body quantum chaos. While quantities such as out-of-time-ordered correlators (OTOCs) do distinguish interacting from noninteracting systems, it has remained unclear to what extent they can truly diagnose chaotic vs integrable dynamics in many-body quantum systems. Here, we analyze operator spreading in generic one-dimensional many-body quantum systems using a combination of matrix product operator (MPO) and analytical techniques, focusing on the operator right weight. First, we show that while small bond dimension MPOs allow one to capture the exponentially decaying tail of the operator front, in agreement with earlier results, they lead to significant quantitative and qualitative errors for the actual front—defined by the maximum of the right weight. We find that while the operator front broadens diffusively in both integrable and chaotic interacting spin chains, the precise shape and scaling of the height of the front in integrable systems is anomalous for all accessible times. We interpret these results using a quasiparticle picture. This provides a sharp, although rather subtle, signature of many-body quantum chaos in the operator front.

The physical model of nanosecond laser ablation of semi-insulating 4H-SiC irradiated by KrF excimer laser with a wavelength of 248 nm was studied. The etching depth was tested by a stylus surface profiler. The morphology of the ablation pit and the thickness of the damaged layer were observed through scanning electron microscope. The phases at the laser irradiated surface were analyzed by Raman spectroscopy. In situ x-ray photoelectron spectroscopy was used to obtain component distribution of the damaged layer and a reasonable thermophysical model was constructed. The temperature distribution of the substrate after the laser irradiation was calculated according to this model. It was found that the etching depth had a linear relationship with the number of laser pulses and the thickness of the uniform damaged layer was independent of pulse number $(>10)$. The thickness of the ablated layer and the newly generated damaged layer are equivalent for each laser irradiation. The established laser ablation model deepens the understanding of physical process and mechanism of nanosecond laser etching of SiC and provides a theoretical guidance for laser processing of SiC.

We study a model of $N$ fermions in a quantum dot, coupled to $M$ bosons by a disorder-induced complex Yukawa coupling [Yukawa Sachdev-Ye-Kitaev (SYK) model], in order to explore the interplay between non-Fermi liquid and superconductivity in a strongly coupled, (quantum-)critical environment. We analyze the phase diagram of the model for an arbitrary complex interaction and arbitrary ratio of $N/M$, with special focus on the two regimes of non-Fermi-liquid behavior: an SYK-like behavior with a power-law frequency dependence of the fermionic self-energy and an impuritylike behavior with frequency independent self-energy. We show that the crossover between the two can be reached by varying either the strength of the fermion-boson coupling or the ratio $M/N$. We next argue that in both regimes the system is unstable to superconductivity if the strength of time-reversal-symmetry-breaking disorder is below a certain threshold. We show how the corresponding onset temperatures vary between the two regimes. We argue that the superconducting state is highly unconventional with an infinite set of minima of the condensation energy at $T=0$, corresponding to topologically different gap functions. We discuss in detail similarities and differences between this model and the model of dispersion-full fermions tuned to a metallic quantum-critical point, with an effective singular dynamical interaction $V\left(\mathrm{\Omega }\right)\propto 1/{\left|\mathrm{\Omega }\right|}^{\gamma }$ (the $\gamma$ model).

Superconductivity in the cuprates is characterized by an anisotropic electronic gap of $d$-wave symmetry. The aim of this work is to understand how this anisotropy affects the nonequilibrium electronic response of high-$T_c$ superconductors. Here we use a polarization selective time domain experiment to address the dynamics of electronic excitation of different symmetry in optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Y}}_{0.08}{\mathrm{Ca}}_{0.92}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ and measure the nodal and antinodal nonequilibrium response resulting from photoexcitations with ultrashort pulses with photon energy comparable to the superconducting gap. The response to long wavelength photoexcitation with pump polarization along the Cu-Cu axis of the sample is discussed with the support of an effective $d$-wave BCS model which suggests that such transient response could be ascribed to an increase of pair coherence in the antinodal region.

We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy ${\mathrm{Co}}_2\mathrm{FeAl}$, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (${\tau }_M$) in films of ${\mathrm{Co}}_2\mathrm{FeAl}$ is almost independent of varying structural order, and that it is similar to that in elemental $3d$ ferromagnets. In contrast, the slower process of magnetization recovery, specified by ${\tau }_R$, is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter ($\alpha$). Based on these results we argue that for ${\mathrm{Co}}_2\mathrm{FeAl}$ the remagnetization process is dominated by magnon dynamics, something which might have general applicability.

Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet $\mathrm{U}{\mathrm{Te}}_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wave-vector ${\mathbf{k}}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $\mu \parallel \mathbf{a}$ with a sine-wave modulation of wave-vector ${\mathbf{k}}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction $\mathbf{c}$. These fluctuations saturate below the temperature $T_1^{*}\simeq 15\mathrm{K}$, in possible relation with anomalies observed in thermodynamic, electrical-transport, and nuclear-magnetic-resonance measurements. The absence or weakness of ferromagnetic fluctuations in our data collected at temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV is emphasized. These results constitute constraints for models of magnetically mediated superconductivity in $\mathrm{U}{\mathrm{Te}}_2$.

Despite the structural resemblance of certain cuprate and nickelate parent compounds there is a striking spread of $T_c$ among such transition metal oxide superconductors. We adopt a minimal two-orbital $e_g$ model which covers cuprates and nickelate heterostructures in different parametric limits, and analyze its superconducting instabilities. The joint consideration of interactions, doping, fermiology, and in particular the $e_g$ orbital splitting allows us to explain the strongly differing pairing propensities in cuprate and nickelate superconductors.

The discovery of high-temperature superconductivity in hydrogen-rich compounds has fueled the enthusiasm for finding materials with more promising superconducting properties among hydrides. However, the ultrahigh pressure needed to synthesize and maintain high-temperature hydrogen-rich superconductors hinders the experimental investigation of these materials. For practical applications, it is also highly desired to find more hydrogen-rich materials that superconduct at high temperatures but under relatively lower pressures. Based on first-principles density functional theory, we calculate the electronic and phonon band structures for a ternary borohydride formed by intercalating ${\mathrm{BH}}_4$ tetrahedrons into a fcc potassium lattice, ${\mathrm{KB}}_2{\mathrm{H}}_8$. Remarkably, we find that this material is dynamically stable and one of its $s{p}^3$-hybridized $\sigma$-bonding bands is metallized (i.e., partially filled) above a moderate high pressure. This metallized $\sigma$-bonding band couples strongly with phonons, giving rise to a strong superconducting pairing potential. By solving the anisotropic Eliashberg equations, we predict that the superconducting transition temperature of this compound is 134–146 K around 12 GPa.

We identify a valley-polarized Chern insulator in a van der Waals heterostructure, monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$/monolayer ${\mathrm{CrI}}_3$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic coupling nearly closes the band gap of monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$, and the strong intralayer spin-orbit coupling further lifts the valley degeneracy by over 100 meV, leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding a hexagonal boron nitride ($h$-BN) layer. We find large orbital magnetization in the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer that is much larger than spin, which can induce a measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest-neighbor ${\mathrm{CrI}}_3$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase, allowing the construction of superlattices with valley, spin, and layer degrees of freedom.

The complex dynamics of an increasing number of systems is attributed to the emergence of a rugged energy landscape with an exponential number of metastable states. To develop this picture into a predictive dynamical theory, I discuss how to compute the exponentially small probability of a jump from one metastable state to another. This is expressed as a path integral that can be evaluated by saddle-point methods in mean-field models, leading to a boundary value problem. The resulting dynamical equations are solved numerically by means of a Newton-Krylov algorithm in the paradigmatic spherical $p$-spin glass model that is invoked in diverse contexts from supercooled liquids to machine-learning algorithms. I discuss the solutions in the asymptotic regime of large times and the physical implications on the nature of the ergodicity-restoring processes.

A spin-1/2 Heisenberg model on a honeycomb lattice is investigated by doing triplon analysis and quantum Monte Carlo calculations. This model, inspired by ${\mathrm{Cu}}_2{\left(\mathrm{pymca}\right)}_3({\mathrm{ClO}}_4$), has three different antiferromagnetic exchange interactions ($J_A, J_B, J_C$) on three different sets of nearest-neighbor bonds which form a kagome superlattice. While the model is bipartite and unfrustrated, its quantum phase diagram is found to be dominated by a quantum paramagnetic phase that is best described as a spin-gapped hexagonal-singlet state. The Néel antiferromagnetic order survives only in a small region around $J_A=J_B=J_C$. The magnetization produced by the external magnetic field is found to exhibit plateaus at 1/3 and 2/3 of the saturation value, or at 1/3 alone, or no plateaus. Notably, the plateaus exist only inside a bounded region within the hexagonal-singlet phase. This study provides a clear understanding of the spin-gapped behavior and magnetization plateaus observed in ${\mathrm{Cu}}_2{\left(\mathrm{pymca}\right)}_3({\mathrm{ClO}}_4$), and also predicts the possible disappearance of 2/3 plateau under pressure.

We investigated chemical pressure effects in electron-doped (e-doped) FeSe by fabricating an electric-double-layer structure with single-crystalline FeSe films on ${\mathrm{LaAlO}}_3$ with Se substituted by isovalent Te and S. Our method enables transport measurements of e-doped FeSe. Electron doping by applying a gate voltage of 5 V increases $T_{\mathrm{c}}$ of the ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ and ${\mathrm{FeSe}}_{1-y}{\mathrm{S}}_y$ films with $0\le x\le 0.4$ and $0\le y\le 0.25$. Applying both positive and negative chemical pressures monotonically suppresses $T_{\mathrm{c}}$ of the e-doped FeSe. $T_{\mathrm{c}}$ and its evolution with chemical pressure is very different between undoped and e-doped FeSe, which is considered to be related to the difference in change of density of states at Fermi energy as a function of chemical pressure in the two systems. These results might be one of the manifestations of the possible difference of superconductivity mechanisms in these systems.

We perform a detailed calculation of the various contributions to the fluctuation conductivity of a granular metal close to its superconducting transition. We find three distinct regions of power law behavior in reduced temperature, $\eta =(T-T_c)/T_c$, with crossovers at $\mathrm{\Gamma }/T_c$ and $E_{\text{Th}}/T_c$, where $\mathrm{\Gamma }$ is the electron tunneling rate, and $E_{\text{Th}}$ is the Thouless energy of a grain. The calculation includes both intergrain and intragrain degrees of freedom. This complete theory of the fluctuation region in granular superconductors is then compared to experimental results from boron-doped nanocrystalline diamond, using the assumption of a constant phase breaking rate ${\tau }_{\varphi }^{-1}$. We find a semiquantitative agreement between the theoretical and experimental results only in the case of large phase breaking. We argue that there may be a phase breaking mechanism in granular metals worthy of further experimental and theoretical investigation.

Correlated-electron theories of superconductivity in layered cuprates often start from the premise of a gapped spin-liquid phase proximate to the superconducting state. This assumption is justified based on analytical and numerical demonstrations of a superconducting Luther-Emery phase in the doped 2-leg one-band Hubbard ladder, and the perceived analogy between coupled ladders and the two-dimensional ${\mathrm{CuO}}_2$ layer. We demonstrate from accurate density matrix renormalization group studies the absence of the superconducting Luther-Emery phase in the doped 2-leg three-band ladder consisting of both copper and oxygen, even as the spin gap is large in the undoped three-band ladder. For realistic oxygen-oxygen hopping and Hubbard repulsion on the oxygen atoms, density-density rather than pairing correlations are dominant at long range. This result is equally valid whether or not the oxygens outside the ladder proper, over and above the rung and leg oxygens, are included in the computation. These results demonstrate the critical importance of oxygen orbitals, and raise disturbing questions about the applicability of many of the existing correlated-electron theories of superconductivity.

We study the quench dynamics of a topological $p$-wave superfluid with two competing order parameters, ${\mathrm{\Delta }}_{\pm }\left(t\right)$. When the system is prepared in the $p+ip$ ground state and the interaction strength is quenched, only ${\mathrm{\Delta }}_{+}\left(t\right)$ is nonzero. However, we show that fluctuations in the initial conditions result in the growth of ${\mathrm{\Delta }}_{-}\left(t\right)$ and chaotic oscillations of both order parameters. We term this behavior phase III'. In addition, there are two other types of late-time dynamics—phase I where both order parameters decay to zero and phase II where ${\mathrm{\Delta }}_{+}\left(t\right)$ asymptotes to a nonzero constant while ${\mathrm{\Delta }}_{-}\left(t\right)$ oscillates near zero. Although the model is nonintegrable, we are able to map out the exact phase boundaries in parameter space. Interestingly, we find phase III' is unstable with respect to breaking the time-reversal symmetry of the interaction. When one of the order parameters is favored in the Hamiltonian, the other one rapidly vanishes and the previously chaotic phase III' is replaced by the Floquet topological phase III that is seen in the integrable chiral $p$-wave model.

We study the spectrum of a bilayer of Kitaev magnets on the honeycomb lattice, coupled by Heisenberg exchange in the quantum-dimer phase at strong interlayer coupling. Using the perturbative continuous unitary transformation we perform series expansion starting from the fully dimerized limit, to evaluate the elementary excitations, reaching up to and focusing on the two-triplon sector. In stark contrast to conventional bilayer quantum magnets, and because of the broken $SU\left(2\right)$ invariance, encoded in the intralayer directional compass exchange, the bilayer Kitaev magnet is shown to exhibit a rich structure of two-triplon scattering-state continua, as well as several collective two-triplon (anti)bound states. Direct physical pictures for the occurrence of the latter are provided and the (anti)bound states are studied versus the stacking type, the spin components, and the exchange parameters. In addition to the two-triplon spectra, we investigate a corresponding experimental probe by evaluating the magnetic Raman-scattering intensity. We find a very strong sensitivity of this intensity on the two-triplon interactions and the scattering geometry, however, a signal from the (anti)bound states appears only in very close proximity to the continuum.

We calculate the phase diagram of a model for topological superconducting wires with local $s$-wave pairing, spin-orbit coupling $\vec{\lambda }$, and magnetic field $\vec{B}$ with arbitrary orientations. This model is a generalized lattice version of the one proposed by Lutchyn et al. [Phys. Rev. Lett. 105, 077001 (2010)] and Oreg et al. [Phys. Rev. Lett. 105, 177002 (2010)], who considered $\vec{\lambda }$ perpendicular to $\vec{B}$. The model has a topological gapped phase with Majorana zero modes localized at the ends of the wires. We determine analytically the boundary of this phase. When the directions of the spin-orbit coupling and magnetic field are not perpendicular, in addition to the topological phase and the gapped nontopological phase, a gapless superconducting phase appears.

As important progress for simulating realistic device materials, we report the first-principles nonequilibrium dynamical cluster theory for simulating the quantum transport properties of nanoelectronics with inevitable disordered defects or dopants. In this method, we formulate the nonequilibrium dynamical cluster theory in Keldysh's Green's function representation, and implement it with the exact muffin-tin orbital based density functional theory. With this method, the important correlation effects of disordered scattering and short-range order effects can be effectively treated for the nonequilibrium electronic structure and quantum transport calculations of devices under finite bias. Moreover, a double-energy-contour technique is devised to considerably improve the numerical convergence in the nonequilibrium electron structure calculation. As the demonstration, the first-principles nonequilibrium dynamical cluster theory is applied to calculate Cu/Co junction with disordered interface and Fe/vacuum/Fe magnetic tunnel junction with surface roughness. We find that a sizable transmission decrease can be induced by including the correlation effects of disorders of few layers in the Cu/Co junction, presenting the important transport channel closing due to disordered quantum interference. For Fe/vacuum/Fe junction, we find that short-range order of surface roughness, with the important clustering and anticlustering tendencies, can dramatically change the transmission properties compared to the case of (or close to) complete randomness. The development of first-principles nonequilibrium dynamical cluster theory provides an important approach for analyzing the process-dependent device performance, extending the capability of first-principles quantum transport simulation.

The decoherence of quantum states defines the transition between the quantum world and classical physics. Decoherence or, analogously, quantum mechanical collapse events pose fundamental questions regarding the interpretation of quantum mechanics and are technologically relevant because they limit the coherent information processing performed by quantum computers. We have discovered that the transition regime enables a novel type of matter transport. Applying this discovery, we present nanoscale devices in which decoherence, modeled by random quantum jumps, produces fundamentally novel phenomena by interrupting the unitary dynamics of electron wave packets. Noncentrosymmetric conductors with mesoscopic length scales act as two-terminal rectifiers with unique properties. In these devices, the inelastic interaction of itinerant electrons with impurities acting as electron trapping centers leads to a novel steady state characterized by partial charge separation between the two leads, or, in closed circuits to the generation of persistent currents. The interface between the quantum and the classical worlds therefore provides a novel transport regime of value for the realization of a new category of mesoscopic electronic devices.

We numerically investigate the localization mechanisms of the quantum anomalous Hall effect (QAHE) with a large Chern number $\mathcal{C}$ in bilayer graphene and magnetic topological insulator thin films doped with nonmagnetic or spin-flip (magnetic) disorder. By calculating the modified Berry curvature in real space, we demonstrate that QAHEs in both systems turn into Anderson insulators when the disorder strength is large enough in the presence of nonmagnetic disorder. However, in the presence of spin-flip disorder, the localization mechanisms in the two systems are completely distinct. For ferromagnetic bilayer graphene with Rashba spin-orbit coupling, the QAHE with $\mathcal{C}=4$ first enters a metallic phase and then turns into an Anderson insulator with the increase of disorder strength, whereas for magnetic topological insulator thin films, the QAHE with $\mathcal{C}=-\mathcal{N}$ first enters a metallic phase, then turns into another QAHE with $\mathcal{C}=-(\mathcal{N}-1)$ as disorder strength increases, and finally turns into an Anderson insulator after $\mathcal{N}-1$ cycles between QAHE and metallic phases. The phase transitions in the two systems originate from the exchange of Berry curvature between conduction and valence bands. In the end, we provide a phenomenological picture related to the topological charges to help understand the underlying physical origins of the two different phase-transition mechanisms.