Highly excited states of quantum many-body systems are central objects in the study of quantum dynamics and thermalization that challenge classical computational methods due to their volume-law entanglement content. In this work, we explore the potential of variational quantum algorithms to approximate such states. We propose an adaptive variational quantum eigensolver (VQE) for excited states (X) that self-generates a variational ansatz for arbitrary eigenstates of a many-body Hamiltonian $H$ by attempting to minimize the energy variance with respect to $H$. We benchmark the method by applying it to an Ising spin chain with integrable and nonintegrable regimes, where we calculate various quantities of interest, including the total energy, magnetization density, and entanglement entropy. We also compare the performance of adaptive VQE-X to an adaptive variant of the folded-spectrum method. For both methods, we find a strong dependence of the algorithm's performance on the choice of operator pool used for the adaptive construction of the ansatz. In particular, an operator pool including long-range two-body gates accelerates the convergence of both algorithms in the nonintegrable regime. We also study the scaling of the number of variational parameters with system size, finding that an exponentially large number of parameters may be necessary to approximate individual highly excited states. Nevertheless, we argue that these methods lay a foundation for the use of quantum algorithms to study finite-energy-density properties of many-body systems.

Two-dimensional (2D) angular correlation of annihilation radiation and Compton scattering are both powerful techniques to investigate the bulk electronic structure of crystalline solids through the momentum density of the electrons. Here we apply both methods to a single crystal of Pd to study the electron momentum density and the occupancy in the first Brillouin zone and to point out the complementary nature of the two techniques. To retrieve the 2D spectra from one-dimensional Compton profiles, a direct inversion method is implemented and benchmarked against the well-established Cormack's method. The comparison of experimental spectra with first-principles density functional theory calculations of the electron momentum density and the two photon momentum density clearly reveals the importance of positron probing effects on the determination of the electronic structure. While the calculations are in good agreement with the experimental data, our results highlight some significant discrepancies.

Nonlinear optical studies of the yellow and green exciton series in ${\mathrm{Cu}}_2\mathrm{O}$ have been reported for more than 40 years. Because of the band structure (the two highest even-parity valence and lowest conduction bands), the $S$ excitons of the two lowest exciton series are dipole-forbidden for one-photon absorption and thus dipole-allowed for two-photon absorption. There is an odd-parity higher conduction band that leads with the two even-parity valence bands to the blue and violet exciton series. We report on second-harmonic generation (SHG) of the blue exciton series. The odd-parity $S$-exciton SHG is due to a dipole-quadrupole excitation and a dipole emission process. Because of their high oscillator strength density, polariton effects have to be taken into account, since resonances might be shifted to higher energies by up to $10\mathrm{me}\mathrm{V}$ compared to the transverse exciton energies. The polariton dispersion for the blue excitons up to $n=4$ is calculated and compared to the experimental results. In magnetic fields up to $10\mathrm{T}$ applied in a Voigt configuration ($\mathbf{B}\perp \mathbf{k}$), SHG of $S$ excitons by a dipole-dipole excitation is observed, which is due to the admixture of dipole-dipole excited $P$ excitons by the effective electric field from the magneto-Stark effect (MSE). From the analysis of the diamagnetic shift and the MSE interaction of the three-level system of $1S, 2S$, and $2P$ excitons, we derive experimental results for the ratio $\langle r_{n,l}^2\rangle /{\mu }_X$ between the average of radius squared for the three states and the reduced exciton mass. For higher principal quantum number states, we observe magnetoexcitons up to $n=8$. We analyze their magnetic field dependence and derive the electron effective-mass values for the crystalline orientations [111], $\left[1\overline{1}0\right]$, and [001].

The transverse field in the quantum Ising chain is linearly ramped from the para- to the ferromagnetic phase across the quantum critical point at a rate characterized by a quench time ${\tau }_Q$. We calculate a connected kink-kink correlator in the final state at zero transverse field. The correlator is a sum of two terms: a negative (antibunching) Gaussian that depends on the Kibble-Zurek correlation length only and a positive term that depends on a second longer scale of length. The second length is made longer by dephasing of the state excited near the critical point during the following ramp across the ferromagnetic phase. This interpretation is corroborated by considering a linear ramp that is halted in the ferromagnetic phase for a finite waiting time and then continued at the same rate as before the halt. The extra time available for dephasing increases the second scale of length that asymptotically grows linearly with the waiting time. The dephasing also suppresses magnitude of the second term, making it negligible for waiting times much longer than ${\tau }_Q$. The same dephasing can be obtained with a smooth ramp that slows down in the ferromagnetic phase. Assuming sufficient dephasing, we also obtain higher order kink correlators and the ferromagnetic correlation function.

The two-dimensional material $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, which possesses both out-of-plane ferroelectricity and piezoelectricity in monolayer, opens opportunity for the smart material integration in a micro-electro-mechanical system. However, the piezoelectric response decreases with the decreasing thickness of $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$, hindering the application of few-layer $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ in electromechanical transformation. In this work, we report a strategy to enhance the out-of-plane piezoelectric coefficients by constructing ${\mathrm{In}}_2{\mathrm{Se}}_3$/transition metal dichalcogenides (TMDs) heterostructures. Such a strategy shows great piezoelectric enhancement compared with monolayer ${\mathrm{In}}_2{\mathrm{Se}}_3$, and the largest $e_{31}$ of $2.9\times {10}^{-10}\mathrm{C}/\mathrm{m}$ is achieved in $\mathrm{Mo}{\mathrm{S}}_2/{\mathrm{In}}_2{\mathrm{Se}}_3$. A phenomenological model is further constructed to connect the piezoelectric coefficients with the polarizations of the heterostructures. Furthermore, the induced dipole moment strongly affects the band structure of heterostructures, yielding tunable light absorption. The proposed ${\mathrm{In}}_2{\mathrm{Se}}_3$/TMDs heterostructures represent an innovative strategy for the development of future efficient piezoelectric and are attractive for deployment in electromechanical systems.

The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-principles input. A unified analysis of the size scaling of the thermal conductivity in the longitudinal and transverse directions of the system is made within the computational framework complemented with a macroscopic hydrodynamic approach. As a result, we demonstrate a strong end effect on the phonon Knudsen minimum, as a hallmark of the transition from ballistic to hydrodynamic heat transports, along a rectangular graphite ribbon with finite length and width. The phonon Knudsen minimum is found to take place only when the ribbon length is ∼5–10 times the upper limit of the width range in the hydrodynamic regime. This paper contributes to a unique methodology with high efficiency and a deeper understanding of the size effect on phonon hydrodynamics, which would open opportunities for its theoretical and experimental investigation in graphitic micro- and nanostructures.

Katsnelson has shown that, within the Fermi-liquid approach, the optical conductivity of Dirac electrons in graphene is not affected by many-body interactions [M. I. Katsnelson, Eur. Phys. Lett. 84, 37001 (2008)]. We show that, when the Dirac cone is tilted, the Fermi-liquid corrections arise in the optical conductivity in a manner that the correction depends on the angle between the electric field of the incident light and the tilt direction. Therefore the mapping of the optical conductivity for various directions of the incident light enables a determination of the many-body effect in the optical conductivity spectrum of the two-dimensional tilted Dirac cone materials.

Topological insulators (TIs) are emergent materials with unique band structure, which allow the study of quantum effect in solids, as well as contribute to high-performance quantum devices. To achieve the better performance of TIs, here, we present a codoping strategy using synergistic rare-earth (RE) Sm and transition-metal Fe dopants in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals, which combine the advantages of both a transition-metal-doped TI [high ferromagnetic ordering temperature and observed quantum anomalous Hall effect (QAHE)], and a RE doped TI (large magnetic moments and significant spin-orbit coupling). In the as-grown single crystals, clear evidences of ferromagnetic ordering were observed. The angle-resolve photoemission spectroscopy indicates the ferromagnetism opens a ∼44 meV band gap at the surface Dirac point. Moreover, the mobility of the carriers at 3 K is $\sim 7400\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$, and we thus observed an ultra-strong Shubnikov–de Haas oscillation in the longitudinal resistivity, as well as the Hall steps in transverse resistivity <14 T. Our transport and angular-resolved photoemission spectroscopy results suggest that the RE and transition metal codoping in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ system is a promising avenue to implement the QAHE, as well as harnessing the massive Dirac fermion in electrical devices.

A method is developed to calculate the ligand field (LF) parameters and the multiplet spectra of local magnetic centers with open $d$ and $f$ shells in solids in a parameter-free way. This method proceeds from density functional theory and employs Wannier projections of nonmagnetic band structures onto local $d$ or $f$ orbitals. Energies of multiplets and optical, as well as x-ray spectra are determined by exact numerical diagonalization of a local Hamiltonian describing Coulomb, LF, and spin-orbit interactions. The method is tested for several $3d$ and $5f$ compounds for which the LF parameters and multiplet spectra are experimentally well known. In this way, we obtain good agreement with the experiments for ${\mathrm{La}}_2{\mathrm{NiO}}_4, {\mathrm{CaCuO}}_2, {\mathrm{Li}}_2{\mathrm{CuO}}_2$, ZnO:Co, and ${\mathrm{UO}}_2$.

We develop a valley-dependent envelope function theory that can describe the effects of arbitrary configurations of interface steps and miscuts on the qubit relaxation time. For a given interface roughness, we show how our theory can be used to find the valley-dependent dipole matrix elements, the valley splitting, and the spin-valley coupling as a function of the electromagnetic fields in a Si/SiGe quantum dot spin qubit. We demonstrate that our theory can quantitatively reproduce and explain the result of experimental measurements for the spin relaxation time with only a minimal set of free parameters. Investigating the sample dependence of spin relaxation, we find that at certain conditions for a disordered quantum dot, the spin-valley coupling vanishes. This, in turn, completely blocks the valley-induced qubit decay. We show that the presence of interface steps can in general give rise to a strongly anisotropic behavior of the spin relaxation time. Remarkably, by properly tuning the gate-induced out-of-plane electric field, it is possible to turn the spin-valley hot spot into a “cold spot” at which the relaxation time is significantly prolonged and where the spin relaxation time is additionally first-order insensitive to the fluctuations of the magnetic field. This electrical tunability enables on-demand fast qubit reset and initialization that is critical for many quantum algorithms and error correction schemes. We therefore argue that the valley degree of freedom can be used as an advantage for Si spin qubits.

Diffuse phonon scattering strongly affects the phonon transport through a disordered interface. The often-used diffuse mismatch model assumes that phonons lose memory of their origin after being scattered by the interface. Using mode-resolved atomistic Green's function simulation, we demonstrate that diffuse phonon scattering by a single disordered interface cannot make a phonon lose its memory and thus the applicability of the diffusive mismatch model is limited. An analytical expression for diffuse scattering probability based on the continuum approximation is also derived and shown to work reasonably well at low frequencies.

Studies of the current-induced spin polarization (CISP) have been recently reinvigorated due to the discoveries of CISP in some burgeoning materials such as oxide interfaces, van der Waals, and topological quantum materials. Here, we investigate the CISP in two-dimensional systems for different types of spin-orbit coupling (SOC) using the Boltzmann transport theory. We find an anisotropic response of CISP to the current direction which strongly depends on the type of SOC. We demonstrate that the CISP is nonlinear with respect to the SOC magnitude, depends on the Fermi energy, and exhibits two different transport regimes for low or high carrier density. Finally, we propose a magnetoresistance device which can exploit the predicted CISP anisotropy.

Fluctuations associated with relaxations in the far-from-equilibrium regime is of fundamental interest for a large variety of systems within broad scales. Recent advances in techniques such as spectroscopy have generated the possibility for measuring the fluctuations of the mesoscopic systems in connection to the relaxation processes when driving the underlying quantum systems far from equilibrium. We present a general nonequilibrium fluctuation-dissipation theorem (FDT) for quantum Markovian processes where the detailed-balance condition is violated. Apart from the fluctuations, the relaxation involves extra correlation that is governed by the quantum curl flux emerged in the far-from-equilibrium regime. Such a contribution vanishes for the thermal equilibrium, so that the conventional FDT is recovered. We finally apply the nonequilibrium FDT to the molecular junctions, elaborating the detailed-balance-breaking effects on the optical transmission spectrum. Our results have the advantage of and exceed the scope of the fluctuation-dissipation relation in the perturbative and near equilibrium regimes, and is of broad interest for the study of quantum thermodynamics.

We study the level-spacing statistics for noninteracting Hamiltonians defined on the two-dimensional quasiperiodic Ammann-Beenker (AB) tiling. When applying the numerical procedure of “unfolding,” these spectral properties in each irreducible sector are known to be well described by the universal Gaussian orthogonal random matrix ensemble. However, the validity and numerical stability of the unfolding procedure has occasionally been questioned due to the fractal self-similarity in the density of states for such quasiperiodic systems. Here, using the so-called $r$-value statistics for random matrices, $P\left(r\right)$, for which no unfolding is needed, we show that the Gaussian orthogonal ensemble again emerges as the most convincing level statistics for each irreducible sector. The results are extended to random-AB tilings where random flips of vertex connections lead to the irreducibility.

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density-functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states.

The effect of electronic Coulomb interaction on the phase formation of fcc Ce lattice is investigated by full ab initio calculations without adjustable Coulomb $U$ and $J$ parameters using the Gutzwiller wavefunction-based correlation matrix renormalization theory (CMRT). Its total energy and pressure as a function of volume agree reasonably well with existing $\text{DFT}+\text{Gutzwiller}$ calculations and experiments, indicating correct capture of electronic correlation and screening effects within the CMRT formalism. A stable phase is found in line with the experimental $\alpha$-Ce phase, and a lurking phase is identified supposedly linked with the experimental $\gamma$-Ce phase. A criterion based on the local $4f$ electron charge fluctuation is introduced to confirm the distinct electronic correlation natures of both phases.

The discovery of Berry curvature (BC) has spurred a tremendous surge of research into various quantum phenomena such as the anomalous transport of electrons and the topological phases of matter. In two-dimensional crystalline systems, the conventional definition of the BC lacks the in-plane components and thus it cannot explain the transverse transport along the plane-normal direction. Here, we modify the BC to provide in-plane components in two dimensions, giving rise to the vertical Hall effects that describe out-of-plane transports in response to in-plane perturbations and their Onsager reciprocity. Our first-principles calculations show that a large in-plane BC can appear even in an atomic-thick $\mathrm{Gd}{\mathrm{Ag}}_2$ monolayer, and a hexagonal $\mathrm{Bi}{\mathrm{Ag}}_2$ monolayer can host a large BC dipole known to vanish in the conventional BC. The quantum transports driven by the hitherto-hidden BC will become more significant in recently emerging two-dimensional platforms, including van der Waals heterostructures.

We study the band structure modulation and spin-Hall effect of an irradiated two-dimensional heavy-hole system with $k^3$-Rashba spin-orbit coupling. We find that the band structure becomes anisotropic under the illumination by the light. Most remarkably, in the presence of linearly polarized light a pair of additional spin-degeneracy points, apart from the $\mathrm{\Gamma }$ point, emerge in the energy dispersion. The locations of these points are solely determined by the amplitude of the incident light. If this degeneracy occurs around the Fermi level, the spin-Hall conductivity exhibits a resonance. Away from the degeneracy points, the light rotates the average spin polarization. The possible effects of $k^3$-Dresselhaus spin-orbit coupling are also discussed.

We explore the significance of laser-induced correlation effects for high-order harmonic generation (HHG) in a linear chain model of a generic band gap material for both single-IR-pulse and VUV-pump–IR-probe HHG schemes. We examine various pump-pulse excitations, and categorize signatures of laser-induced correlation effects into two classes related to edge-state and bulklike effects. The relative importance of the edge-state ef- fects decreases with increasing system size, while the bulklike effects remain. We identify regimes where these effects may alter the harmonic yield by an order of magnitude, but also regions where they may be neglected. We characterize the underlying laser-induced dynamics of the correlational mechanisms, using both an electronic population description in a static field-free basis and a comparison to a band-structure picture with laser-induced time-dependent energy shifts. From this we provide a guideline on when to account for laser-induced electron correlations and give an estimate of the timescale in which the laser-induced electron-electron interactions can cause correlational changes in the harmonic spectra. We find this timescale to be a few tens of femtoseconds.

The ultimate fate of the spin moment in ultrafast demagnetization is to appear as a gain in momentum of the lattice. However, the route by which it occurs remains to be conclusively demonstrated. Employing state-of-the-art time-dependent density functional theory we show that spin-orbit coupling at femtosecond timescales drives a transfer of spin angular momentum into orbital angular momentum, which via the Coulomb term transfers from this orbital angular momentum instantaneously to the lattice. The rate of change of angular and spin momenta have, respectively, predominantly extrinsic (the pulse duration) and intrinsic (spin-orbit coupling) timescales. This facilitates the design of pulses to clearly disentangle the physics of angular momentum redistribution. While experiments predominantly use bulk elemental Ni, we propose Co in thin film geometry with an experimentally realizable time resolution for ultrashort pulses will allow clear observation of the ultrafast transfer of momentum via orbital degrees of freedom to the lattice.