The dimensional crossover induces varieties of quantum phenomena. In this paper, we demonstrate the quantum critical dynamics under dimensional crossover involving many-body phase transitions by continuously suppressing correlations and tunnelings along one direction of bulk materials, which provides a smooth connection from higher dimensions to lower ones based on the intrinsic correlations rather than geometry tailoring. By measuring the nonadiabatic excitations, the critical scaling laws in both three and two dimensions are observed and are consistent with predictions. In addition, we find scaling behaviors for intermediate regimes with noninteger dimensions. This study provides insights to extend the descriptions of critical exponents into more general or complex scenarios.

Most of our current understanding of phase separation is based on ideas that disregard correlations. Here we illuminate unexpected effects of correlations on the structure and thermodynamics of interfaces and in turn phase separation, which are decisive in systems with strong interactions. Evaluating the continuum limit of the Ising model on the Bethe-Guggenheim level, we derive a Cahn-Hilliard free energy that takes into account pair correlations. For a one-dimensional interface in a strip geometry, these are shown to give rise to an effective interface broadening at interaction strengths near and above the thermal energy, which is verified in the Ising model. Interface broadening is the result of an entropy-driven interface delocalization, which is not accounted for in the widely adopted mean field theory. Pair correlations are required for thermodynamic consistency as they enforce a thermodynamically optimal local configuration of defects and profoundly affect nucleation and spinodal decomposition at strong coupling.

When a broadband signal propagates through a dispersive medium, some frequency components move faster than the center of the pulse. This leads to the appearance of precursors, transient signals that emerge from the medium earlier than the main part of the pulse and seem to propagate superluminally. Here, we investigate the microscopic origin of precursors in a minimal setup: an array of qubits coupled to a waveguide. The linear transmission function only converges to that of a continuous medium for large qubit numbers. Nevertheless, the dispersion produced by only two qubits is enough to produce oscillatory transients. Precursors are best observed under conditions of electromagnetically induced transparency, as the center of the pulse is significantly delayed. Under these conditions, just a single qutrit is enough to generate a precursor. Our results pave the way towards dispersion engineering of light with just a few qubits and can be realized with superconducting qubits coupled to transmission lines or atoms coupled to optical waveguides.

We apply continuous similarity transformations (CSTs) to the easy-axis antiferromagnetic XXZ model on the square lattice. The CST flow equations are truncated in momentum space by the scaling dimension $d$ so that all contributions with $d\le 2$ are taken into account. The resulting quartic magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and two-magnon sector. In this way, a quantitative description of the ground-state energy, the one-magnon dispersion, and its gap as well as of two-magnon bound states is gained for anisotropies ranging from the gapped Ising model to the gapless Heisenberg model. We discuss the critical properties of the gap closing as well as the evolution of the one-magnon roton mininum. The excitation energies of two-magnon bound states are calculated, and their decay into the two-magnon continuum is determined via the inverse participation ratio.

In this work we study the localization of flexural waves in highly symmetric clusters of scatterers. It is shown that when the scatterers are placed regularly in the perimeter of a circumference, the quality factor of the resonances strongly increases with the number of scatterers in the cluster. It is also found that in the continuous limit, that is to say, when the number of scatterers tends to infinite, the quality factor is infinite so that the modes belong to the class of the so-called bound states in the continuum or BICs, and an analytical expression for the resonant frequency is found. These modes have different multipolar symmetries, and we show that for high multipolar orders the modes tend to localize at the border of the circumference, therefore forming a whispering gallery mode with an extraordinarily high quality factor. Numerical experiments are performed to check the robustness of these modes under different types of disorder and also to study their excitation from the far field. Although we have focused our study on flexural waves, the methodology presented in this work can be applied to other classical waves, like electromagnetic or acoustic waves, being a promising approach for the design of high-quality resonators based on finite clusters of scatterers.

As a frontier issue of physics and material, the structures and related properties of borides have been extensively investigated in fundamental science. The search for pressure-induced stable compounds has become a feasible approach to acquire borides that are inaccessible at atmospheric pressure. Combined with state-of-the-art swarm intelligence structure prediction and first-principles calculations, we systematically explored the Rb-B system and uncovered a series of unprecedented RbB, ${\mathrm{Rb}}_2{\mathrm{B}}_3$, ${\mathrm{RbB}}_3$, ${\mathrm{RbB}}_6$, ${\mathrm{RbB}}_8$, and ${\mathrm{RbB}}_{10}$ under high pressure. It is found that the catenation of boron evolves from linear chain to layered sheets, clusterlike units, and further to three-dimensional tunnel structures with increasing boron content. Among them, ${\mathrm{RbB}}_6$ and ${\mathrm{RbB}}_8$ are expected phonon-mediated superconductors with $T_c$ of ∼12 K and superhard material with a hardness of ∼37 GPa at ambient pressure, respectively. Additionally, ${\mathrm{RbB}}_8$ is a suitable precursor for obtaining the superconducting $o\text{−}{\mathrm{B}}_{16}$ boron allotrope by removing Rb due to its better stability than isomorphic ${\mathrm{SrB}}_8$. The current results provide insights into the design of unforeseen borides and illustrate intriguing B-B bonding features originating from Rb → B charge transfer under pressures.

A physical system connected to two thermal reservoirs at different temperatures is said to act as a heat rectifier when it is able to bias the heat current in a given direction, similarly to an electronic diode. We propose to quantify the performance of a heat rectifier by mapping out the tradeoff between heat currents and rectification. By optimizing over the system's parameters, we obtain Pareto fronts, which can be efficiently computed using general coefficients of performance. This approach naturally highlights the fundamental tradeoff between heat rectification and conduction, and allows for a meaningful comparison between different devices for heat rectification. We illustrate the practical relevance of these ideas on three minimal models for spin-boson nanoscale rectifiers, i.e., systems consisting of one or two interacting qubits coupled to bosonic reservoirs biased in temperature. Our results demonstrate the superiority of two strongly interacting qubits for heat rectification.

We report here on results of systematic experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the band gap for the smallest dots, whereas the intensity of below band gap harmonics remains weakly affected by the dot size. In addition, it is observed that the suppression of the above gap harmonics is enhanced with increasing the driving wavelength. These systematic investigations allow us to develop a simple physical picture explaining the observed suppression of the highest harmonics: the discretization of electronic energy levels seems to be not the predominant contribution to the observed suppression but rather the confined dot size itself, causing field-driven electrons to scatter off the dot's walls. The reduction in the dot size below the classical electron oscillatory radius and the corresponding scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single-molecule response, and are crucial for nanophotonic platforms aiming at control over high-order harmonic properties and efficiency.

Understanding the mechanism through which an open quantum system exchanges information with an environment is central to the creation and stabilization of quantum states. This theme has been explored recently, with attention mostly focused on system control or environment engineering. Here, we bring these ideas together to describe the many-body dynamics of an extended atomic array coupled to a squeezed vacuum. We show that fluctuations can drive the array into a pure dark state decoupled from the environment. The dark state is obtained for an even number of atoms and consists of maximally entangled atomic pairs, or dimers, that mimic the behavior of the squeezed field. Each pair displays reduced fluctuations in one polarization quadrature and amplified in another. This dissipation-induced stabilization relies on an efficient transfer of correlations between pairs of photons and atoms. It uncovers the mechanism through which squeezed light causes an atomic array to self-organize and illustrates the increasing importance of spatial correlations in modern quantum technologies where many-body effects play a central role.

We consider interacting bosonic particles on a two-leg triangular ladder in the presence of an artificial gauge field. We employ density matrix renormalization group numerical simulations and analytical bosonization calculations to study the rich phase diagram of this system. We show that the interplay between the frustration induced by the triangular lattice geometry and the interactions gives rise to multiple chiral quantum phases. Phase transitions between superfluid to Mott-insulating states occur, which can have Meissner or vortex character. Furthermore, a state that explicitly breaks the symmetry between the two legs of the ladder, the biased chiral superfluid, is found for values of the flux close to $\pi$. In the regime of hard-core bosons, we show that the extension of the bond order insulator beyond the case of the fully frustrated ladder exhibits Meissner-type chiral currents. We discuss the consequences of our findings for experiments in cold atomic systems.

We propose an alternative route to stabilize magnetic skyrmion textures which does not require Dzyaloshinkii-Moriya interactions, magnetic anisotropy, or an external Zeeman field. Instead, it solely relies on the emergence of flux in the system's ground state. We discuss scenarios that lead to a nonzero flux and identify the magnetic skyrmion ground states which become accessible in its presence. Moreover, we explore the chiral superconductors obtained for the surface states of a topological crystalline insulator when two types of magnetic skyrmion crystals coexist with a pairing gap. Our work opens perspectives for engineering topological superconductivity in a minimal fashion and promises to unearth functional topological materials and devices which may be more compatible with electrostatic control than the currently explored skyrmion-Majorana platforms.

We report on the strong field ionization of ${\mathrm{H}}_2$ by a corotating two-color laser field. We measure the electron momentum distribution in coincidence with the kinetic energy release (KER) of dissociating hydrogen molecules. In addition to a characteristic half-moon structure, we observe a low-energy structure in the electron momentum distribution at a KER of about 3.5 eV. We speculate that the outgoing electron interacts with the molecular ion, despite the absence of classical recollisions under these conditions. Time-dependent density functional theory simulations support our conclusions.

“When in a difficult situation, it is sometimes better to give up and start all over again.” While this empirical truth has been regularly observed in a wide range of circumstances, quantifying the effectiveness of such a heuristic strategy remains an open challenge. In this paper, we combine the notions of optimal control and stochastic resetting to address this problem. The emerging analytical framework allows one not only to measure the performance of a given restarting policy, but also to obtain the optimal strategy for a wide class of dynamical systems. We apply our technique to a system with a final reward and show that the reward value must be larger than a critical threshold for resetting to be effective. Our approach, analogous to the celebrated Hamilton-Jacobi-Bellman paradigm, provides the basis for the investigation of realistic restarting strategies across disciplines. As an application, we show that the framework can be applied to an epidemic model to predict the optimal lockdown policy.

Elastic softening in the paraelastic phases of ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3$ is largest near the transition temperatures and decreases on heating smoothly over extended temperature ranges. Softening extends to the highest measured temperature (850 K) for Ba-rich compounds. The temperature evolution of the excess compliance of the precursor softening follows a power law $\delta S\propto |T-T_{\mathrm{C}}{|}^{-\kappa }$ with a characteristic exponent $\kappa$ ranging between 1.5 in ${\mathrm{SrTiO}}_3$ and 0.2 in ${\mathrm{BaTiO}}_3$. The latter value is below the estimated lower bounds of displacive systems with three orthogonal soft phonon branches (0.5). An alternative Vogel-Fulcher analysis shows that the softening is described by extremely low Vogel-Fulcher energies $E_a$, which increase from ${\mathrm{SrTiO}}_3$ to ${\mathrm{BaTiO}}_3$ indicating a change from a displacive to a weakly order-disorder character of the elastic precursor. Mixed crystals of ${\mathrm{Ba}}_x{\mathrm{Sr}}_{1-x}{\mathrm{TiO}}_3$ possess intermediate behavior. The amplitude of the precursor elastic softening increases continuously from ${\mathrm{SrTiO}}_3$ to ${\mathrm{BaTiO}}_3$. Using power-law fittings reveals that the elastic softening is still $33\%$ of the unsoftened Young's modulus at temperatures as high as 750 K in ${\mathrm{BaTiO}}_3$ with $\kappa \simeq$ 0.2. This proves that the high-temperature elastic properties of these materials are drastically affected by precursor softening.

We report an experimental observation of mesoscopic fluctuations underlying the thermodynamic anomalies of ambient liquid water. The combination of two sound-velocity-measurement methods with largely different frequencies, namely inelastic x-ray scattering (IXS) in the terahertz band and ultrasonic (US) in the megahertz band, allows us to investigate a relaxation phenomenon that has a characteristic frequency between the two aforementioned frequencies. We performed IXS measurements to obtain the IXS sound velocity of liquid water from the ambient conditions to the supercritical region of liquid–gas phase transition (LGT) and compared the results with the US sound velocity reported in the literature. We found that the ratio of the sound velocities, $S_f$, which corresponds to the relaxation strength, obtained using these two methods exhibits a simple but significant change. Two distinct rises were observed in the high- and low-temperature regions, implying that two relaxation phenomena exist. In the high-temperature region, a peak was observed near the LGT critical ridge line, which was linked to changes in the magnitude of density fluctuation and isochoric and isobaric-specific heat capacities. This result indicates that the high-temperature relaxation originates from the LGT critical fluctuation, proving that this method is effective for observing such mesoscopic fluctuations. Meanwhile, in the low-temperature region, $S_f$ increased from 550 K toward the low-temperature region and reached a high value, attaining the “fast sound” state under ambient conditions. This result indicates that another mechanism of relaxation exists, which causes the sound velocity anomaly, including the “fast sound” phenomenon of liquid water under ambient conditions. The change in $S_f$ in the low-temperature region is linked to the change in the isochoric heat capacity, which identified that this relaxation causes the well-known heat capacity anomaly of liquid water. This low-temperature relaxation corresponds to the critical fluctuation of the liquid–liquid phase transition (LLT) that is speculated to exist in the supercooled region. In this study, both LGT and LLT critical fluctuations were observed, and the relationship between thermodynamics and the critical fluctuations was comprehensively discussed by analyzing the similarities and differences between the two phase transitions.

In continuous starvation, bacteria will enter an extended stationary phase after the death phase. Growth-advantaged genetic mutations gradually emerge in the long-term stationary phase. The different competition patterns between younger and aged cultures, and the constant density and heterogeneity of living cells in very-aged cultures indicate the complexity of population dynamics under prolonged starvation. Based on resource conservation and protein allocation, here we constructed a simple growth-death-recycling model to describe the population dynamics of one or two bacterial strains during long-term starvation. Through analytical derivation or numerical simulation, we produced the survival-extinction phase diagram of one strain at steady state, and that of two strains at steady state, in a limited detection time, or consuming different nutrients. By relating allocated proteomic fractions to growth, death, and recycling rates, we showed the effects of proteome allocation on the phase diagrams. The constraints and optimization of protein allocation lead to the tradeoff between processes for cell growth, maintenance (repair), and nutrient recycling. The results reveal multiple strategies of bacterial survival and coexistence during long-term starvation.