Memory is a ubiquitous characteristic of complex systems, and critical phenomena are one of the most intriguing phenomena in nature. Here, we propose an Ising model with memory, develop a corresponding theory of critical phenomena with memory for complex systems, and discover a series of surprising novel results. We show that a naive theory of a usual Hamiltonian with a direct inclusion of a power-law decaying long-range temporal interaction violates radically a hyperscaling law for all spatial dimensions even at and below the upper critical dimension. This entails both indispensable consideration of the Hamiltonian for dynamics, rather than the usual practice of just focusing on the corresponding dynamic Lagrangian alone, and transformations that result in a correct theory in which space and time are inextricably interwoven, leading to an effective spatial dimension that repairs the hyperscaling law. The theory gives rise to a set of novel mean-field critical exponents, which are different from the usual Landau ones, as well as new universality classes. These exponents are verified by numerical simulations of the Ising model with memory in two and three spatial dimensions.

We develop deep autoregressive networks with multi channels to compute many-body systems with continuous spin degrees of freedom directly. As a concrete example, we demonstrate the two-dimensional XY model with the continuous-mixture networks and rediscover the Kosterlitz–Thouless (KT) phase transition on a periodic square lattice. Vortices characterizing the quasi-long range order are accurately detected by the generative model. By learning the microscopic probability distributions from the macroscopic thermal distribution, the networks are trained as an efficient physical sampler which can approximate the free energy and estimate thermodynamic observables unbiasedly with importance sampling. As a more precise evaluation, we compute the helicity modulus to determine the KT transition temperature. Although the training process becomes more time-consuming with larger lattice sizes, the training time remains unchanged around the KT transition temperature. The continuous-mixture autoregressive networks we developed thus can be potentially used to study other many-body systems with continuous degrees of freedom.

We introduce and demonstrate a new approach to measure the electron-hole dynamics and coherence induced by strong-field ionization using hole-assisted high-harmonic spectroscopy. The coherent driving of the infrared and XUV pulses correlates the dynamics of the core-hole and the valence-hole by coupling multiple continua, which leads to the otherwise forbidden absorption and emission of high harmonics. An analytical model is developed based on the strong-field approximation by taking into account the essential multielectron configurations. The emission spectra from the core-valence transition and the core-hole recombination are found to modulate strongly as functions of the time delay between the two pulses, suggesting that the coherent electron wave packets in multiple continua can be utilized to temporally resolve the core-valence transition in attoseconds.

Optical gain and loss of microcavity greatly affect the quality of lasing, how to improve optical gain and decrease optical loss is of great significance for the preparation of laser. In this study, four types standard microdisks with different thicknesses of 2.2 µm, 1.9 µm, 1.7 µm, and 1.45 µm were fabricated by micromachining technology process to modulate optical gain and loss of microdisk lasing. The whispering gallery mode lasing in the ultraviolet range of GaN microdisk devices was investigated for these devices in order to clarify the effect of microdisk thickness on device characteristics. The quality factor $Q$ and lasing mode number for different thicknesses are calculated from the stimulated spectra. The lifetimes of the exciton combination properties of the devices were observed using time-resolved PL spectroscopy. The lasing modes are modulated, and the lifetime decreases, while the $Q$ factor of the devices first increases and then decreases with decreasing thickness. All these results are induced by optical gain and loss competition.

Electronic charge of molecules can move on time scales when the nuclei stand practically still, from few hundreds of attoseconds to few femtoseconds. This ultrafast process is called “charge migration”. A typical consequence is rapid change of electronic dipole, which points to the center of charge. Corresponding linear (one-dimensional, 1D) and planar (2D) dipolar motions have already been well documented. Here we construct the first case of charge migration which causes chiral 3D dipolar motion, specifically along a helix about oriented iodo-acetylene (HCCI). Quantum dynamics simulations show that this can be induced by well-designed laser pulses.

On-chip optical nonreciprocal devices are vital components for integrated photonic systems and scalable quantum information processing. Nonlinear optical isolators and circulators have attracted considerable attention because of their fundamental interest and their important advantages in integrated photonic circuits. However, optical nonreciprocal devices based on Kerr or Kerr-like nonlinearity are subject to dynamical reciprocity when the forward and backward signals coexist simultaneously in a nonlinear system. Here, we theoretically propose a method for realizing on-chip nonlinear isolators and circulators with dynamic nonreciprocity. Dynamic nonreciprocity is achieved via the chiral modulation on the resonance frequency due to coexisting self- and cross-Kerr nonlinearities in an optical ring resonator. This work showing dynamic nonreciprocity with a Kerr nonlinear resonator can be an essential step toward integrated optical isolation.

We theoretically study optomechanically induced transparency in a spinning Kerr-nonlinear resonator. The interplay between the optical Kerr effect and the Sagnac effect provides a flexible tool for modifying the optomechanically induced transparency windows of the signal comparing to the system of a single spinning resonator. It is found that the system will exhibit distinct transparency phenomenon and fast-to-slow light effects. More importantly, a symmetric transparency window appears by adjusting the rotation-induced Sagnac frequency shift to compensate for the Kerr-induced frequency shift. These results open up a new way to explore novel light propagation of optomechanically induced transparency devices in spinning resonators with Kerr nonlinearity.

This work is mainly based on postgraduate lectures at Tongji University since 2020 spring. We firstly revisit the elastic spin and orbital angular momentum [Proc. Natl. Acad. Sci. USA 115, 9951 (2018)] but more general for anisotropic systems by applying Noether's theorem to the elastic Lagrangian and by applying the symmetry argument in the field theory. Then, fundamental relations between elastic energy flux and elastic spin are uncovered. In particular cases, the wave spin is closely related to the vorticity of energy flux and momentum. Secondly, we move forward from the elastic spin to revisit the phonon spin [Fizika Tverdogo Tela 3, 2160 (1961)] by applying the second quantization to elastic fields. We show that the uncovered phonon spin, a polarized elastic-vibration quanta, is generally not restricted to transverse phonon modes, but applying to general phonon modes, such as the longitudinal phonon modes, surface phonon modes, and hybridized phonon modes, regarded as a consequence of mode interferences. The elastic spin and phonon spin originate from the local rotating of the field polarization in time domain, not the local circulation (vorticity) of displacement or velocity in space domain. It is hopeful that the present results could advance the fundamental understanding of phonon spin and elastic spin, and promote the spin phononics for hybrid quantum sensing and technology with multiple degrees of freedom.

Dipoles in ferroelectric copolymer P(VDF-TrFE) can be driven by electric field, introducing phonon transport modulations via polarizing molecular chains. The thermal conductivity in single 75/25 P(VDF-TrFE) nanofibers is found to increase with electric field related phonon renormalization, resulted from change in vibrational assignment excited by polarization process. This is evidenced by a direct change of bond energy and bond length in 75/25 P(VDF-TrFE) nanofibers from Raman characterization under polarization electric field. The experimental results provide further intuitive evidences that the size of ferroelectric polymers could directly affect the ferroelectricity from the size-dependent thermal transport measurement.

Van der Waals (vdW) layered two-dimensional (2D) materials, which may have high carrier mobility, valley polarization, excellent mechanical properties and air stability, have been widely investigated before. We explore the possibility of producing a spin-polarized two-dimensional electron gas (2DEG) in the heterojunction composed of insulators MoSi$_{2}$N$_{4}$ and VSi$_{2}$N$_{4}$ by using first-principles calculations. Due to the charge transfer effect, the 2DEG at the interface of the MoSi$_{2}$N$_{4}$/VSi$_{2}$N$_{4}$ heterojunction is found. Further, for different kinds of stacking of heterojunctions, lattice strain and electric fields can effectively tune the electronic structures and lead to metal-to-semiconductor transition. Under compressive strain or electric field parallel to $c$ axis, the 2DEG disappears and band gap opening occurs. On the contrary, interlayer electron transfer enforces the system to become metallic under the condition of tensile strain or electric field anti-parallel to $c$ axis. These changes are mainly attributed to electronic redistribution and orbitals' reconstruction. In addition, we reveal that MoSi$_{2}$N$_{4}$/VSi$_{2}$N$_{4}$ lateral heterojunctions of armchair and zigzag edges exhibit different electronic properties, such as a large band gap semiconductor and a metallic state. Our findings provide insights into electronic band engineering of MoSi$_{2}$N$_{4}$/VSi$_{2}$N$_{4}$ heterojunctions and pave the way for future spintronics applications.

We investigate high-quality single-domain crystals of YbB$_{12}$ using the precise x-ray diffraction technique in combination with the low-temperature polarized THz–infrared spectroscopy and accurate magnetotransport measurements. It is shown for the first time that this archetypal strongly correlated system with a metal-insulator transition to a mysterious dielectric ground state with a metal Fermi surface [Science 362, 65 (2018) and ibid 362, 32 (2018)] is actually a heterogeneous compound in the regime of electronic phase separation. Changes in the configuration of the discovered dynamic charge stripes are proposed upon cooling. As a result, a conclusion is drawn in favor of a crossover between different patterns of the filamentary electronic structure penetrating the semiconducting matrix of YbB$_{12}$. We argue that the discovery of stripes in YbB$_{12}$ is fundamental, elucidating the nature of exotic dielectric state in Kondo insulators.

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe] with superconducting transition temperatures ($T_{\rm c}$) above 40 K have been discovered. Due to the large interlayer distance ($\sim $15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above $T_{\rm c}$ is revealed by nuclear magnetic resonance (NMR) measurements on $^{77}$Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY)$_{x}$FeSe, with a reduced interlayer distance ($\sim $10 Å) compared to (TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe. By performing $^{77}$Se NMR and transport measurements, we observe a similar pseudogap behavior well above $T_{\rm c}$ of $\sim $40 K and a large resistivity anisotropy of $\sim$$10^{\boldsymbol{4}}$ in (PY)$_{x}$FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.

The intercalated iron selenide (Li,Fe)OHFeSe has a strongly layered structure analogous to the quasi-two-dimensional (2D) bismuth cuprate superconductors, and exhibits both high-temperature ($T_{\rm c}$) and topological superconductivity. However, the issue of its superconductivity dimensionality has not yet been fully investigated so far. Here we report that the quasi-2D superconductivity features, including the high anisotropy $\gamma = 151$ and the associated quasi-2D vortices, are also revealed for (Li,Fe)OHFeSe, based on systematic experiments of the electrical transport and magnetization and model fittings. Thus, we establish a new vortex phase diagram for (Li,Fe)OHFeSe, which delineates an emergent quasi-2D vortex-liquid state, and a subsequent vortex-solid dimensional crossover from a pancake-like to a three-dimensional state with decreasing temperature and magnetic field. Furthermore, we find that all the quasi-2D characteristics revealed here for the high-$T_{\rm c}$ iron selenide superconductor are very similar to those reported for high-$T_{\rm c}$ bismuth cuprate superconductors.

Hydrides offer an opportunity to study high critical temperature (high-$T_{\rm c}$) superconductivity at experimentally achievable pressures. However, the pressure needed remains extremely high. Using density functional theory calculations, herein we demonstrate that a new rare earth hydride ErH$_{2}$ could be superconducting with $T_{\rm c} \sim 80$ K at 14.5 GPa, the lowest reported value for compressed hydrides to date. Intriguingly, due to Kondo destruction, superconductivity was prone to exist at 15 GPa. We also reveal an energy gap at 20 GPa on the background of normal metallic states. At 20 GPa, this compressed system could act as a host of superconductor judged from a sharp jump of spontaneous magnetic susceptibility with an evanescent spin density of state at Fermi level. Finally, electron pairing glue for ErH$_{2}$ at these three typical pressures was attributed to the antiferromagnetic spin fluctuation.

The discovery of pressure-induced superconductivity in helimagnets (CrAs, MnP) has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity. However, the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear. Here, we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure. Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV, which can be effectively described by the Heisenberg model with nearest neighbor exchange interactions. Most surprisingly, the spin excitations are largely quenched above the Néel temperature, in contrast to cuprates and iron pnictides where the spectral weight is mostly preserved in the paramagnetic state. Our results suggest that the helimagnetic order is driven by strongly frustrated exchange interactions, and that CrAs is at the verge of itinerant and correlation-induced localized states, which is therefore highly pressure-tunable and favorable for superconductivity.

We present $dC/dV$ analysis based on the capacitance-voltage ($C$–$V$) measurement of quantum-dot light-emitting diodes (QLEDs), and find that some key device operating parameters (electrical and optical turn-on voltage, peak capacitance, maximum efficiency) can be directly related to the turning points and maximum/minimum of the $dC/dV$ (versus voltage) curve. By the $dC/dV$ study, the behaviors such as low turn-on voltage, simultaneous electrical and optical turn-on process, and carrier accumulation during the device aging can be well explained. Moreover, we perform the $C$–$V$ and $dC/dV$ measurement of aged devices, and confirm that our $dC/dV$ analysis is correct for them. Thus, our $dC/dV$ analysis method can be applied universally for QLED devices. It provides an in-depth understanding of carrier dynamics in QLEDs through simple $C$–$V$ measurement.

A magnetic tunnel junction (MTJ) is the core component in memory technologies, such as the magnetic random-access memory, magnetic sensors and programmable logic devices. In particular, MTJs based on two-dimensional van der Waals (vdW) heterostructures offer unprecedented opportunities for low power consumption and miniaturization of spintronic devices. However, their operation at room temperature remains a challenge. Here, we report a large tunnel magnetoresistance (TMR) of up to 85% at room temperature ($T = 300$ K) in vdW MTJs based on a thin ($ < 10$ nm) semiconductor spacer WSe$_{2}$ layer embedded between two Fe$_{3}$GaTe$_{2}$ electrodes with intrinsic above-room-temperature ferromagnetism. The TMR in the MTJ increases with decreasing temperature up to 164% at $T = 10$ K. The demonstration of TMR in ultra-thin MTJs at room temperature opens a realistic and promising route for next-generation spintronic applications beyond the current state of the art.