Nonlinear optics has regained attention in recent years, especially in the context of optospintronics and topological materials. Nonlinear responses involved in various degrees of freedom manifest their intricacy more pronounced than linear responses. However, for a certain class of nonlinear responses, a connection can be established with linear-response coefficients, enabling the exploration of diverse nonlinear-response functionality in terms of the linear-response counterpart. Our study quantum mechanically elucidates the relation between such nonlinear and linear responses we call the Pitaevskii relation and identifies the condition for the relation to hold. Following the obtained general formulation, we systematically identify the Pitaevskii relations such as the inverse magnetoelectric effect and inverse natural optical activity unique to systems manifesting the space-inversion-symmetry breaking. These results provide a systematic understanding of intricate nonlinear responses and may offer further implications to ultrafast spintronics.

We explore the possibility to control the superconducting transition temperature at optimal hole doping $T_c^{\mathrm{opt}}$ in cuprates by tuning the chemical formula (CF). $T_c^{\mathrm{opt}}$ can be theoretically predicted from the parameters of the ab initio low-energy effective Hamiltonian with one antibonding (AB) $\mathrm{Cu}3d_{x^2-y^2}/\mathrm{O}2p_{\sigma }$ orbital per Cu atom in the ${\mathrm{CuO}}_2$ plane, notably the nearest-neighbor hopping amplitude $|t_1|$ and the ratio $u=U/|t_1|$, where $U$ is the onsite effective Coulomb repulsion. However, the CF dependence of $|t_1|$ and $u$ is a highly nontrivial question. In this paper, we propose the universal dependence of $|t_1|$ and $u$ on the CF and structural features in hole doped cuprates with a single ${\mathrm{CuO}}_2$ layer sandwiched between block layers. To do so, we perform extensive ab initio calculations of $|t_1|$ and $u$ and analyze the results by employing a machine-learning method called hierarchical dependence extraction (HDE). The main results are (a) $|t_1|$ has a main-order dependence on the radii $R_{\mathrm{X}}$ and $R_{\mathrm{A}}$ of the apical anion X and cation A in the block layer. ($|t_1|$ increases when $R_{\mathrm{X}}$ or $R_{\mathrm{A}}$ decreases.) (b) $u$ has a main-order dependence on the ionic charge $Z_{\mathrm{X}}$ of X and the hole doping $\delta$ of the AB orbital. ($u$ decreases when $|Z_{\mathrm{X}}|$ increases or $\delta$ increases.) We elucidate and discuss the microscopic mechanism of items (a) and (b). We demonstrate the predictive power of the HDE by showing the consistency between items (a) and (b) and results from previous works. The present results provide a basis for optimizing superconducting properties in cuprates and possibly akin materials. Also, the HDE method offers a general platform to identify dependencies between physical quantities.

Recently, the discovery of superconductivity in Ruddlesden-Popper (RP) ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ under pressure has further expanded the realm of nickelate-based superconductor family. In this paper, we perform a first-principles study of ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ for both the $P{2}_1/a$ phase at ambient pressure and $I4/mmm$ phase at high pressure, with $U=0$, 3.5 eV. Our results confirm the characteristic upward shift of a $\mathrm{Ni}\text{−}{d}_{z^2}$ bonding band under pressure. Moreover, our analysis of electronic spectrum and orbital occupancy unveil the dynamic mechanism of electronic reconstructions under pressure, embedded in a critical dual effect. Based on our results, we further propose a trilayer two-orbital model by performing Wannier downfolding on $\mathrm{Ni}\text{−}{e}_g$ orbitals. Our model reveals four Fermi surface sheets with $\alpha ,\beta ,{\beta }^{\prime },\gamma$ pockets, bearing resemblance to that of bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. According to the model, our calculated spin susceptibility under random phase approximation shows that the $d_{x^2-y^2}$ orbital is also important for the magnetic fluctuation in the RP series. Finally, a high energy 16-orbital model with direct $dp,pp$ hoppings is proposed, which implies that ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ also lies in the charge-transfer picture within the Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and multiorbital models shed light on theoretical electronic correlation study and experimental exploration of lower pressure superconductors in the RP series.

We propose a protocol based only on local conductance measurements for distinguishing trivial from topological phases in realistic three-terminal superconducting nanowires coupled to normal leads, capable of hosting Majorana zero modes (MZMs). By using Green's functions and the scattering matrix approach, we calculate the conductance matrix and the local density of states (LDOS) as functions of the asymmetry in the couplings to the left $\left({\mathrm{\Gamma }}_L\right)$ and right $\left({\mathrm{\Gamma }}_R\right)$ leads. In the trivial phase, we find that the zero-bias local conductances are distinctively affected by variations in ${\mathrm{\Gamma }}_R$ (for fixed ${\mathrm{\Gamma }}_L$): while $G_{LL}$ is mostly constant, $G_{RR}$ decays exponentially as ${\mathrm{\Gamma }}_R$ is decreased. In the topological phase, surprisingly, $G_{LL}$ and $G_{RR}$ are both suppressed with $G_{LL}\sim G_{RR}$. This nonlocal suppression of $G_{LL}$ with ${\mathrm{\Gamma }}_R$ scales with the MZM hybridization energy ${\varepsilon }_m$ and arises from the emergence of a dip in the LDOS near zero energy at the left end of the wire, which affects the local Andreev reflection. We further exploit this nonlocality of the local Andreev processes and the gate-controlled suppression of the LDOS by proposing a Majorana-based transistor. Our results hold for zero and low electron temperatures $T<20\mathrm{mK}$. For $T=30,40\mathrm{mK}$, $G_{LL}$ and $G_{RR}$ become less correlated. As an additional nonlocal fingerprint of the topological phase at higher $T^{\prime }\mathrm{s}$, we predict modulations in our asymmetric conductance deviation $\delta G_{LL}^{\mathrm{asym}}=G_{LL}^{{\mathrm{\Gamma }}_R={\mathrm{\Gamma }}_L}-G_{LL}^{{\mathrm{\Gamma }}_R\ll {\mathrm{\Gamma }}_L}$ that remains commensurate with the Majorana oscillations in ${\varepsilon }_m$ over the range $30<T<150\mathrm{mK}$.

Local counterdiabatic driving (CD) provides a feasible approach for realizing approximate adiabatic or reversible processes like quantum state preparation using only local controls and without demanding excessively long protocol times. However, in many instances getting high accuracy of such CD protocols requires engineering very complicated new controls or pulse sequences. In this work, we describe a systematic method for altering the adiabatic path by adding extra local controls along which performance of local CD protocols is enhanced, both close to and far from the adiabatic limit. We also identify an iterative procedure to improve the performance of local counterdiabatic driving further without any knowledge of the quantum wave function. We then show that these methods provides dramatic improvement in the preparation of nontrivial Greenberger-Horne-Zeilinger ground states of several different spin systems with both short-range and long-range interactions.

We explore the contributions of adiabatic and nonadiabatic spin-transfer torques (STT) of a spin-polarized current to the thermally activated creep motion of domain walls in a thin (Ga,Mn)(As,P) film with perpendicular anisotropy. The nonadiabatic STT is found to compensate linearly an external magnetic field, with a slope compatible with a steady domain-wall internal structure. Close to the compensation, the adiabatic contribution is strongly enhanced and may both increase or reduce domain-wall velocity, which we associate to variations of creep pinning energy barrier with domain-wall internal structure. Far from compensation, the effects of adiabatic STT become negligible and so field and current driven domain-wall motion presents common universal behaviors described by the quenched Edwards Wilkinson universality class.

Both the Haldane spin chain and a topologically dimerized chain feature topologically protected edge states that are expected to be robust against some kind of noise. To elucidate whether it might be feasible to create such edge states in dimerized chains in a controlled manner in solid states environments, e.g., as spin chains on surfaces, as has already been successfully achieved with the Haldane chain, we investigate their robustness with respect to long-range coupling, anisotropies, and finite chain length. The theoretical investigation is based on an alternating Heisenberg spin chain with spin-$1/2$, which is investigated using exact diagonalization. We find that dimerized chains and Haldane chains have robustness against long-range coupling and anisotropies. In particular, dimerized spin chains are significantly more robust than Haldane chains.

We study the ground-state phase diagram of the half-filled extended Haldane-Hubbard model on the honeycomb lattice with sublattice-dependent on-site repulsion ($U_{\text{A/B}}$) using the exact diagonalization (ED) and mean-field (MF) methods. The resulting phase diagram shows that there is a topologically nontrivial phase with the Chern number $C=1$, emerging via the development of the imbalance between $U_{\text{A}}$ and $U_{\text{B}}$. In this phase, the antiferromagnetic correlations are observed in the ED calculation, in line with the finite antiferromagnetic order obtained by the MF method. The spontaneous symmetry breaking of SU(2) spin rotation in the phase is also identified in the MF level. Distinct from previous studies in which the exotic $C=1$ phase relies on the interplay between sublattice-dependent potentials and electronic interactions, our paper presents an alternative way by solely tuning the on-site interactions.

The difference in the thermal vibration of oxygen in the cubic perovskites $\mathrm{PbTi}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ was explored by associating it with their phase transitions at low temperatures. The thermal vibration factor for oxygen in cubic $\mathrm{PbTi}{\mathrm{O}}_3$ was estimated by employing an indirect approach that combines experimental Ti $L_{2,3}$-edge electron energy-loss spectroscopy and crystal field multiplet calculations incorporating atomic vibrations. The results uncovered relatively isotropic vibration behavior in $\mathrm{PbTi}{\mathrm{O}}_3$ and strong anisotropy in $\mathrm{SrTi}{\mathrm{O}}_3$, despite their identical lattice parameters at different temperatures. Specifically, similar vibration factors along the Ti-O bond were estimated for both materials, whereas the amplitude of oxygen vibration in the $A$-O plane for $\mathrm{PbTi}{\mathrm{O}}_3$ was significantly smaller than that for $\mathrm{SrTi}{\mathrm{O}}_3$. This larger oxygen vibration in the $A$-O plane of $\mathrm{SrTi}{\mathrm{O}}_3$ can be attributed to the existence of the rotational phonon mode, contributing to a $\mathrm{Ti}{\mathrm{O}}_6$ octahedron rotational phase transition at low temperature due to softening of the $R_{25}$ mode. Therefore, the smaller vibration amplitude for oxygen in $\mathrm{PbTi}{\mathrm{O}}_3$ could possibly be attributed to the intrinsic small rotational phonon mode, leading to a zone-center displacement-type phase transition induced by a softened ${\mathrm{\Gamma }}_{15}$ mode. Further, the charge density map obtained by density functional theory calculation proposes a broader valence electron distribution for $\mathrm{P}{\mathrm{b}}^{2+}$ compared to $\mathrm{S}{\mathrm{r}}^{2+}$ and greater electron repulsion between Pb and O, potentially contributing to smaller oxygen vibration in the Pb-O plane of $\mathrm{PbTi}{\mathrm{O}}_3$. This research provides valuable insights into the role of vibrational behavior in phase transition mechanisms, and offers the potential to predict the phase transition types for perovskites in the high-temperature cubic paraelectric phase.

Exciton binding energies are fundamental to understanding excitonic materials, especially those with the potential for ground-state exciton condensation. However, these energies are typically defined with significant limitations in their consideration of electron correlation. Here we present a variational theory for computing exciton binding energies in ground-state exciton condensates in which we define the binding as the energy difference between fully correlated many-electron systems with $M$ and $M-1$ excitons, respectively. The $(M-1)$ system is obtained by adding a constraint to the ground-state energy minimization that removes an exciton while allowing all other electronic degrees of freedom to relax. We perform the energy minimizations with variational calculations of the two-electron reduced density matrix (2-RDM) in which the additional constraint is treated along with the $N$-representability conditions—necessary constraints for the 2-RDM to represent an $N$-electron system—by semidefinite programming. We demonstrate the theory first in the Lipkin model and then in several stacked organic and inorganic systems that exhibit the beginnings of exciton condensation. We find that in the Lipkin model the traditional exciton binding model overbinds relative to the constrained approach. This has significant implications for theoretical characterizations of exciton condensates which rely on exciton binding energy to make predictions regarding condensate stability and critical temperatures. This correlated approach to defining and computing exciton binding energies may therefore have important applications for understanding the relationship between binding and condensation, especially for the BCS-BEC crossover.

The discovery of the quantum spin Hall effect led to the exploration of the electronic transport for spintronic devices. We theoretically investigated the electronic conductance in large-gap realistic quantum spin Hall system ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ nanoribbons. By an ab initio approach, we found that the edge states present a penetration depth of about 0.9 nm, much smaller than those predicted in other two-dimensional topological systems, thus, suggesting that ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ allows the exploitation of topological transport properties in narrow ribbons. Using nonequilibrium Green's function calculations, we have examined the electron conductivity upon the presence of $\mathrm{Se}\leftrightarrow \mathrm{Hg}$ antistructure defects randomly distributed in the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ scattering region. By considering scattering lengths up to 109 nm, we found localization lengths that can surpass micrometer sizes for narrow nanoribbons ($<9$ nm). These findings can contribute to further understanding the behavior of topological insulators under realistic conditions and their integration within electronic spintronic devices.

The upconversion luminescence (UCL) assisted by inter-ion energy transfer has received widespread attention. The inter-ion energy transfer microparameters are the fundamental physical quantities determining the UCL process. However, the experimental exploration of the microparameters has not been reported due to challenges in dynamics models. Here, we report on a universal periodic lattice microscopic rate equation model for upconversion dynamics and determination of the microparameters of energy transfer between ions. In periodic lattices, we propose a processing method for ionic interactions, improving the accuracy of the microscopic model. The random doping of luminescent ions in host lattices often leads to randomness in lattice geometric modeling. Therefore, based on the proposed evaluation criterion for the randomness of lattice modeling, we develop a multiple modeling method for periodic small-scale lattices to replace single modeling of large-scale lattices. This eliminates the influence of randomness in lattice geometric modeling on microscopic model calculations, significantly reducing the computational complexity of the microscopic model (reducing computational time by three orders of magnitude) while ensuring model accuracy. Accurate correlation is actualized from micro-ion interactions to macroluminescence. Using this microscopic model and combined with experimental data on luminescence decay curves at multiple energy levels and ion doping concentrations, 19 inter-ion energy transfer microparameters of $\mathrm{E}{\mathrm{r}}^{3+}$ ions in β-$\mathrm{NaY}{\mathrm{F}}_4$:$\mathrm{E}{\mathrm{r}}^{3+}$ are determined.

Lead-free inorganic $A_{2}M\left(\mathrm{I}\right)M\left(\mathrm{III}\right)X_6$ halide double perovskites (HDPs) are potentially benign materials with improved chemical stability as compared to organic lead halide perovskites. However, achieving comparable efficiency in optoelectronic devices requires careful exploration of suitable HDPs with favorable electronic, optical, and transport properties. In these efforts, we comprehensively studied ${\mathrm{Cs}}_2{\mathrm{AgSbBr}}_{6\text{−}x}{\mathrm{Cl}}_x$ $(x=0–6)$ mixed HDPs by employing state-of-the-art first-principles-based density functional theory, density functional perturbation theory, and many-body perturbation theory, namely $G_0{W}_0$ and the Bethe-Salpeter equation (BSE). Our findings demonstrate the thermodynamic and mechanical stability of these materials. Furthermore, the structural polymorphs for $x=2$, 3, and 4 exhibit nearly identical energies, indicating remarkably similar structural stability. The electronic band gap (indirect), calculated through HSE06 and $G_0{W}_0$, ranges from 1.68 to 2.49 eV. Meanwhile, the exciton binding energy, determined via standard BSE calculations, rises from 0.18 to 0.37 eV with increasing Cl concentration. The Fröhlich model is also used to account for the impact of electron (hole)–phonon coupling, and electron mobility is found to increase with a decrease in Cl concentration from 5.07 to $28.07{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, while hole mobility shows an oscillating behavior. Overall, the meritorious tunable properties of mixed HDPs underscore their potential as stable, nontoxic materials with favorable attributes for optoelectronic applications.

We propose a construction of kinetically constrained models using the Markovian quantum dynamics under strong dissipation. Engineering the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) equation through classical noise, we show that strong dissipation leads to the emergent decoherence-free subspaces, within which constrained quantum many-body unitary dynamics can take place. We argue that the unitary dynamics constructed by the GKSL dynamics is more tightly constrained than that constructed by the strongly interacting Hamiltonian, where the interactions have the same form with the GKSL jump operators. As an example, we demonstrate that a one-dimensional spin system with two-site dissipation leads to the kinetically constrained “PXQ” model, which exhibits the free domain-wall motion with an additional frozen-block structure. Under a uniform magnetic field, the PXQ model shows the domain-wall localization, similar to the Wannier-Stark localization. We then couple two PXQ chains with the magnetic field by an interchain interaction. We discover that, while localization of the domain walls persists despite the interactions for typical parameter regimes, a nontrivial partial delocalization appears for a certain parameter line.

The dynamical response of magnetic domain walls in ultrathin magnetic films to external magnetic fields is determined not only by the composition and thickness of the layers but also by the growth conditions. Growth conditions can induce significant structural changes to the layers and at the interfaces between them, affecting in particular the dynamics of domain walls, their mobility, elastic tension, and the pinning forces acting on them. In this work, we focus specifically on the effect of Ir layer growth pressure in Ta/Pt/Co/Ir/Ta ultrathin multilayered films. Measurements of the DC magnetic properties, domain-wall velocity, and domain morphology in the creep regime for both constant and alternating field pulses were performed for a batch of samples where the Ir layer was grown at different pressures. We find that the saturation magnetization, the effective anisotropy constant, and the domain-wall surface tension grow with increasing pressure and saturate at a threshold pressure, while the Dzyaloshinskii-Moriya field and the strength of the disorder remain practically unaltered over the range of pressures considered.

Non-Hermitian spin systems have attracted extensive interest due to their unconventional magnetic properties, rich entanglement resources, and unusual quantum criticality phenomena. In this paper, based on the exact solution of a one-dimensional non-Hermitian spin-1/2 XY model with rotation-time-reversal ($\mathcal{RT}$) symmetry [X. Z. Zhang et al., Phys. Rev. A 87, 012114 (2013)], we study the ground-state energy density, magnetization, correlation functions, quantum entanglement, and correlation in $\mathcal{RT}$-symmetric and $\mathcal{RT}$-broken phases as well as their characteristics at the exceptional point. We find that the energy density rises faster with the non-Hermitian parameter $\left|\gamma \right|$ in the symmetric region than in the broken one and is elevated rapidly at the exceptional point, and the decay of magnetization has similar results. We analyze the effect of $\gamma$ on the system and find that the energy density decreases linearly with the external magnetic field $h$ for $\gamma >0,$ while it is bifurcated when $\gamma =0$. In addition, the behaviors of the energy density indicate that the phase transition caused by symmetry breaking is second order, which is further demonstrated by the magnetization, quantum entanglement, etc. The numerical results of the correlation functions surprisingly indicate that the $\mathcal{RT}$-broken phase has quasi-long-range order, which is quite different from the Hermitian XY model. Especially, the crossover behavior of the ground-state entanglement shows that it increases with $\gamma$ in the symmetric region, which is opposite to the case of the broken one, and its maximum always appears at the exceptional point. The above behaviors at the phase boundary are actually ascribed to the fierce competition between $\gamma$ and $h$, which results in a rapid decline of magnetization and the appearance of the maximum of entanglement.