Cluster-assembled materials have long been pursued as they provide unprecedented opportunities to integrate desired functionalities in technological devices. However, patterning clusters in ordered phases in avoid of agglomeration remains challenging. Here we propose a strategy to construct stable two-dimensional (2D) superlattices by using tetrahedral clusters covalently linked by oxygen atoms. When satisfying the closed shells of both molecular and atomic orbitals for the clusters and oxygen linkers, respectively, the assembled 2D superlattices exhibit outstanding energetic, dynamic, and thermal stabilities. Following these criteria, monolayers of ${\mathrm{B}}_4{\mathrm{O}}_2$, ${\mathrm{Al}}_4{\mathrm{O}}_2$, ${\mathrm{Ga}}_4{\mathrm{O}}_2$, ${\mathrm{In}}_4{\mathrm{O}}_2$, ${\mathrm{Cr}}_4{\mathrm{O}}_2$, ${\mathrm{Mo}}_4{\mathrm{O}}_2$, and ${\mathrm{Te}}_4{\mathrm{O}}_2$ were assembled, which possess application-desired physical properties, such as moderate band gaps, small carrier effective masses, and strong optical absorption in the visible regime. These critical insights pave a new avenue to fabricate new materials and devices using cluster building blocks with precise structures and functionalities.

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$ showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of canted spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit ${\lambda }_s{\theta }_{xy}\approx 1–50$ nm in ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$ monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (${\lambda }_s$) and charge-to-spin interconversion efficiency as large as ${\theta }_{xy}\approx 80$%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities.

The shear-induced self-organization of active rotors into stripy aggregates is studied by carrying out computational simulations. The rotors, modeled by monolayers of frictional spheres, develop to stripy microstructures only when they counterrotate with respect to the vorticity of the imposed shear flow. The average width of the stripes is demonstrated to be linearly dependent on the relative intensity of active torque to the shear rate. By giving insight into three collective particle behaviors, i.e., shear-induced diffusion, rotation-induced rearrangement, and edge flows, we explain the mechanisms of formation of the particle stripes. Additionally, the rheological result shows the dependence of shear and rotational viscosities on the active torque direction and the oddness of the normal stress response. By exhibiting a collective phenomenon of active rotors, our study paves the way to understanding chiral active matter.

We develop an approach to understanding intrinsic mechanisms that cause the $T_1$-decay rate of a multilevel superconducting qubit to depend on the photonic population of a coupled, detuned cavity. Our method yields simple analytic expressions for both the coherently driven or thermally excited cases, which are in good agreement with full master equation numerics, and also facilitates direct physical intuition. It also predicts several interesting phenomena. In particular, we find that in a wide range of settings, the cavity-qubit detuning controls whether a nonzero photonic population increases or decreases qubit Purcell decay. Our method combines insights from a Keldysh treatment of the system, and Lindblad perturbation theory.

We study the dynamics of the quasi-one-dimensional Ising-Heisenberg antiferromagnet ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ under a transverse magnetic field. Combining inelastic neutron scattering experiments and theoretical analyses by field theories and numerical simulations, we mainly elucidate the structure of the spin excitation spectrum in the high-field phase, appearing above the quantum phase transition point ${\mu }_0{H}_c\approx 10\mathrm{T}$. We find that it is characterized by collective solitonic excitations superimposed on a continuum. These solitons are strongly bound in pairs due to the effective staggered field induced by the nondiagonal $g$ tensor of the compound and are topologically different from the fractionalized spinons in the weak-field region. The dynamical susceptibility numerically calculated with the infinite time-evolving block decimation method shows an excellent agreement with the measured spectra, which enables us to identify the dispersion branches with elementary excitations. The lowest-energy dispersion has an incommensurate nature and has a local minimum at an irrational wave number due to the applied transverse field.

In this paper, we begin with a model of a $\mathrm{\Lambda }$-type atom whose both transitions are chirally coupled to a wave guide and then extend the model to its giant-atom version. We investigate the single-photon scatterings of the giant-atom model in both the Markovian and the non-Markovian regimes. It is shown that the chiral atom-wave-guide couplings enable nonreciprocal, reflectionless, and efficient frequency conversion, while the giant-atom structure introduces intriguing interference effects to the scattering behaviors, such as ultranarrow scattering windows. The chiral giant-atom model exhibits quite different scattering spectra in the two regimes and, in particular, demonstrates non-Markovicity-induced nonreciprocity under specific conditions. These phenomena can be understood from the effective detuning and decay rate of the giant-atom model. We believe that our results have potential applications in integrated photonics and quantum network engineering.

Two-body dissipation usually gives rise to a complex interaction. Here we study the effect of two-body dissipation on few-body physics, including the fundamental two-body effective scattering and the three-body Efimov physics. By employing a two-channel model that incorporates the decay of closed-channel molecules (generating the two-body dissipation), we explicitly relate the real and imaginary part of the inverse scattering length ($a_s^{-1}$) to closed-channel detuning and decay rate. In particular, we show that the imaginary part of $a_s^{-1}$ is given by the product of the molecule decay rate and the effective range. Such complex scattering length is found to generate an additional imaginary Coulomb potential when three atoms come close to each other, thereby suppressing the formation of trimer bound states and modifying the conventional discrete scaling in Efimov physics.

The molecular solids ${\beta }^{\prime }\text{−}\text{X}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ (where X represents a cation) are typical compounds whose electronic structures are described by single-orbital Hubbard-type Hamiltonians with geometrical frustration. Using the ab initio downfolding method, we derive the low-energy effective Hamiltonians for ${\beta }^{\prime }\text{−}\text{X}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ with available room- and low-temperature structures. We find that the amplitudes of the Coulomb interactions and the anisotropy of the hopping parameters in the effective Hamiltonians are sensitive to the changes in the lattice constants induced by lowering the temperature. The obtained effective Hamiltonians are analyzed using the exact diagonalization method with the boundary condition average. We find that a significant reduction of the antiferromagnetic ordered moment occurs in the effective Hamiltonian of ${\beta }^{\prime }\text{−}{\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ with the low-temperature structure. The reduction is consistent with the quantum spin liquid behavior observed in experiments. The comprehensive derivations of the effective Hamiltonians and exact-diagonalization analyses of them will clarify the microscopic origins of the exotic quantum states of matter found in ${\beta }^{\prime }\text{−}\text{X}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ such as the quantum spin liquid behavior.

The pseudomagnetic field created by a nonuniform uniaxial strain is introduced into the antiferromagnetic honeycomb nanoribbons. The formation of magnon pseudo-Landau levels, which appear from the upper end of the spectrum and whose level spacings are proportional to the square root of the level index, is revealed by the linear spin-wave theory. The antiferromagnetic order is gradually weakened along the $y$-direction by the strain. At large enough strength, the system is decoupled into isolated zigzag chains near the upper boundary and demonstrates one-dimensional magnetic property there. While the quantum Monte Carlo simulations also predict such a transition, this exact method gives a critical point deeper in the bulk. We also investigate the $XY$ antiferromagnetic honeycomb nanoribbons and find similar pseudo-Landau levels and antiferromagnetic evolution. Our results unveil the effect of a nonuniform uniaxial strain on the spin excitations and may be realized experimentally based on two-dimensional quantum magnetic materials.

We present a combined experimental and theoretical study of the transient absorption spectroscopy of ${\mathrm{N}}_2\mathrm{O}$ at the N $K$ edge (400 eV) by employing an attosecond soft x-ray pulse and an overlapping intense femtosecond infrared pulse. Our experiment demonstrates that tunnel ionization of the core-excited states of neutral ${\mathrm{N}}_2\mathrm{O}$ causes prominent half-cycle oscillations in the absorption signal. In addition, we observe the appearance of the ${\mathrm{N}}_2{\mathrm{O}}^{+}$ signal with increasing delay as a result of the tunneling ionization of ${\mathrm{N}}_2\mathrm{O}$, thereby allowing us to follow the tunneling process in real time. We have developed theoretical models that allow us to interpret and reproduce the main trends of the experimental data.

Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and ${\mathrm{CaF}}_2$. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the “symmetry-forbidden” piezoelectric effect ranges from $\sim 1$ to ${10}^{-5}$ pm/V ($\sim 0.5\%$ to $\sim 2\times {10}^{-5}\%$ of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon.

We complete the task of optimal probabilistic coherence distillation protocol, whose aim is to transform a general state into a set of $n$-level maximally coherent states via strictly incoherent operations (SIO). Specifically, we present the necessary and sufficient condition for the transformation from a mixed state into a pure-state ensemble via SIO. With the aid of this condition and the simplex algorithm, we can accomplish the probabilistic distillation protocol by presenting an analytical expression of the maximal average distillable coherence of a general state and constructing the corresponding operation achieving this bound. Our protocol is a universal protocol since it can be applied to any coherence measure.

${\mathrm{Cu}}_2\mathrm{O}$ has appealing properties as an electrode for photoelectrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies ($V_{\mathrm{O}}$) in the experimentally observed ($\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ reconstruction of the dominant (111) surface. Our results show that these $V_{\mathrm{O}}$ are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged $V_{\mathrm{O}}$ plays a crucial role in the nonradiative electron capture process with a capture coefficient of about ${10}^{-9}{\mathrm{cm}}^3$/s and a lifetime of 0.04 ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface $V_{\mathrm{O}}$ chemistry will be a crucial step in optimizing ${\mathrm{Cu}}_2\mathrm{O}$ for photoelectrode applications.

This paper reports a novel time-domain expression for the current-response kernels of a Josephson tunnel junction between BCS superconductors with in general different energy gaps, and its use to simulate the classical dynamics of such junctions. The simulations show a dynamic regime characterized by the resonance between the Josephson oscillations and the gap oscillations in the asymptotics of the current kernels that has not been studied previously. The resonance manifests itself as a hysteresis in the dc current-voltage characteristics of the current-biased junctions in the voltage range above the energy gap, in addition to the usual “inertial” hysteresis characteristic for tunnel junctions at voltages below the energy gap. Features of the IV curves related to the gap resonance, including the above-the-gap hysteresis, should manifest themselves in many structures and devices utilizing high-quality Josephson tunnel junctions with relatively large, but achievable current densities.

van der Waals heterostructures consisting of vertically stacked transition-metal dichalcogenides (TMDs) exhibit a rich landscape of bright and dark intra- and interlayer excitons. In spite of a growing literature in this field of research, the type of excitons dominating optical spectra in different van der Waals heterostructures has not yet been well established. The spectral position of exciton states depends strongly on the strength of hybridization and energy renormalization due to the periodic moiré potential. Combining exciton density-matrix formalism and density-functional theory, we shed light on the exciton landscape in TMD homo- and heterobilayers at different stackings. This allows us to identify on a microscopic footing the energetically lowest-lying exciton state for each material and stacking. Furthermore, we disentangle the contribution of hybridization and layer polarization-induced alignment shifts of dark and bright excitons in photoluminescence spectra. By revealing the exciton landscape in van der Waals heterostructures, our work provides the basis for further studies of the optical, dynamical, and transport properties of this technologically promising class of nanomaterials.

Excitation of chaotic magnetization dynamics in nanomagnets is of great interest because it bridges the condensed matter physics and nonlinear science and has a potential to emerging technologies such as neuromorphic computing. However, it has been difficult to observe and identify chaos in spintronics devices because the excitation of chaos requires dynamics in a large-dimensional phase space, according to the Poincaré-Bendixson theorem. An efficient way to overcome this issue is using feedback, which enables the dynamical degrees of freedom to be increased even in a single device. Here, we experimentally demonstrate the excitation of chaos in a vortex spin-torque oscillator by utilizing a feedback circuit. The radio-frequency current emitted by the oscillator flows in the feedback circuit and is converted into an oscillating magnetic field. The oscillating field generates a torque acting on the vortex and modulates its dynamics, resulting in chaotic dynamics which can be tuned by electrical means.

Excited-state quantum phase transitions depend on and reveal the structure of the whole spectrum of many-body systems. While they are theoretically well understood, finding suitable signatures and detecting them in actual experiments remains challenging. For instance, in spinor gases, excited-state phases have been identified and characterized through a topological order parameter that is challenging to measure in experiments. Here we propose the Raman-dressed spin-orbit-coupled gas as a novel platform to explore excited-state quantum phase transitions. In a weakly coupled regime, the dressed system is equivalent to a spinor gas with tunable spin-spin interactions. Through this equivalence we are able to identify excited-state phases in the Raman-dressed Bose gas. The phases are characterized by the behavior of the spatial density modulations, or stripes, induced by spin-orbit coupling, and can in principle be measured in current state-of-the-art experiments with ultracold atoms. Conversely, we show that the properties of the excited phase can be exploited to prepare stripe states with large and stable density modulations.

We theoretically study the low-energy scattering of ultracold atoms by a dielectric nanosphere of silica glass levitated in a vacuum. The atom and dielectric surface interact via dispersion force of which strength sensitively depends on the polarizability, dielectric function, and geometry. For cesium and rubidium atoms, respectively, we compute the atom-surface interaction strength and characterize the stationary scattering states by taking adsorption of the atoms onto the surface into account. As the energy of the incoming atoms is lowered, we find that differences between quantum and classical scatterings emerge in two steps. First, the quantum-mechanical differential cross section of the elastic scattering starts to deviate from the classical one at an energy scale comparable to a few microkelvin in units of temperature due to the de Broglie matter-wave diffraction. Second, the differences are found in the cross sections in a regime lower than a few nanokelvin, where the classically forbidden reflection occurs associated with the $s$-wave scattering and the discrete nature of angular momentum. We also study the dependencies of quantum and classical scattering properties on the radius of the nanosphere. This paper paves the way to identify the quantum regime and to understand the physical origin of quantum effects in the collisions between a nanoparticle and environmental gas over various temperatures.