Motivated by the recent discovery of superconductivity in doped ${\mathrm{NdNiO}}_2$, we study the magnetic exchange interaction $J$ in layered $d^9$ nickelates from first principles. The mother compounds of the high-$T_{\mathrm{c}}$ cuprates belong to the charge-transfer regime in the Zaanen-Sawatzky-Allen diagram and have $J$ larger than 100 meV. While this feature makes the cuprates very different from other transition metal oxides, it is of great interest whether layered $d^9$ nickelates can also have such a large $J$. However, one complexity is that ${\mathrm{NdNiO}}_2$ is not a Mott insulator due to carrier doping from the block layer. To compare the cuprates and $d^9$ nickelates on an equal basis, we study ${\mathrm{RbCa}}_2{\mathrm{NiO}}_3$ and $A_2{\mathrm{NiO}}_2{\mathrm{Br}}_2$ ($A$ denotes a cation with a valence of $2.5+$), which were recently designed theoretically by block-layer engineering. These nickelates are free from the self-doping effect and belong to the Mott-Hubbard regime. We show that these nickelates share a common thread with the high-$T_{\mathrm{c}}$ cuprates in that they also have a significant exchange interaction $J$ as large as about 100 meV.

Although spin is a core property in fermionic systems, its symmetry can be easily violated in a variational simulation, especially when strong correlation plays a vital role therein. In this study, we will demonstrate that the broken spin symmetry can be restored exactly in a quantum computer, with little overhead in circuits, while delivering additional strong correlation energy with the desired spin quantum number. The proposed scheme permits drastic reduction of a potentially large number of measurements required to ensure spin symmetry by employing a superposition of only a few rotated quantum states. Our implementation is universal, simple, and, most importantly, straightforwardly applicable to any Ansatz proposed to date.

The fast diffusion of alkali ions and resulting high ionic conductivity is a defining feature of alkali borate glasses. Here we report impedance spectroscopy and atomic-scale x-ray photon correlation spectroscopy measurements on rubidium borate systems ${\left({\mathrm{Rb}}_2\mathrm{O}\right)}_x{\left({\mathrm{B}}_2{\mathrm{O}}_3\right)}_{100-x}$ with particular focus on $x=30\text{mol}$ %. We find that the coherently scattered intensity does not show temporal fluctuations on timescales corresponding to ionic diffusion. We conclude that the spatial configuration of alkali sites as stepping stones for ionic diffusion evolves on the same timescales as the borate backbone, and that the defect density is low, i.e., at a given instant, nearly all alkali sites are singly occupied.

The computation of molecular excitation energies is essential for predicting photo-induced reactions of chemical and technological interest. While the classical computing resources needed for this task scale poorly, quantum algorithms emerge as promising alternatives. In particular, the extension of the variational quantum eigensolver algorithm to the computation of the excitation energies is an attractive option. However, there is currently a lack of such algorithms for correlated molecular systems that is amenable to near-term, noisy hardware. In this work, we propose an extension of the well-established classical equation of motion approach to a quantum algorithm for the calculation of molecular excitation energies on noisy quantum computers. In particular, we demonstrate the efficiency of this approach in the calculation of the excitation energies of the LiH molecule on an IBM Quantum computer.

We perform nonadiabatic simulations of warm dense aluminum based on the electron-force field (EFF) variant of wave-packet molecular dynamics. Comparison of the static ion-ion structure factor with density functional theory (DFT) is used to validate the technique across a range of temperatures and densities spanning the warm dense matter regime. Focusing on a specific temperature and density (3.5 eV, 5.2 $\mathrm{g}/{\mathrm{cm}}^3$), we report on differences in the dynamic structure factor and dispersion relation across a variety of adiabatic and nonadiabatic techniques. We find the dispersion relation produced with EFF is in close agreement with the more robust and adiabatic Kohn-Sham DFT.

We investigate the Brownian diffusion of particles in one spatial dimension and in the presence of finite regions within which particles can either evaporate or be reset to a given location. For open boundary conditions, we highlight the appearance of a Brownian yet non-Gaussian diffusion: At long times, the particle distribution is non-Gaussian but its variance grows linearly in time. Moreover, we show that the effective diffusion coefficient of the particles in such systems is bounded from below by $1-2/\pi$ times their bare diffusion coefficient. For periodic boundary conditions, i.e., for diffusion on a ring with resetting, we demonstrate a “gauge invariance” of the spatial particle distribution for different choices of the resetting probability currents, in both stationary and nonstationary regimes. Finally, we apply our findings to a stochastic biophysical model for the motion of RNA polymerases during transcriptional pauses, deriving analytically the distribution of the length of cleaved RNA transcripts and the efficiency of RNA cleavage in backtrack recovery.

Mammalian grid cells represent spatial locations in the brain via triangular firing patterns that tessellate the environment. They are regarded as the biological substrate for path integration and to provide an efficient code for space. However, grid cell patterns are strongly influenced by environmental manipulations, in particular, exhibiting local geometrical deformations and defects tied to the shape of the recording enclosure, challenging the view that grid cells constitute a universal code for space. We show that the observed responses to environmental manipulations arise as a natural result under the general framework of feedforward models with spatially unstructured feedback inhibition, which puts the development of triangular patterns in the context of a Turing pattern formation process over physical space. The model produces coherent neuronal populations with equal grid spacing, field size, and orientation.

We develop a theory of edge states based on the Hermiticity of Hamiltonian operators for tight-binding models defined on lattices with boundaries. We describe Hamiltonians using shift operators which serve as differential operators in continuum theories. It turns out that such Hamiltonian operators are not necessarily Hermitian on lattices with boundaries, which is due to the boundary terms associated with the summation by parts. The Hermiticity of Hamiltonian operators leads to natural boundary conditions, and for models with nearest-neighbor (NN) hoppings only, there are reference states that satisfy the Hermiticity and boundary conditions simultaneously. Based on such reference states, we develop a Bloch-type theory for edge states of NN models on a half-plane. This enables us to extract Hamiltonians describing edge states at one end, which are separated from the bulk contributions. It follows that we can describe edge states at the left and right ends separately by distinct Hamiltonians for systems of cylindrical geometry. We show various examples of such edge-state Hamiltonians, including Hofstadter model, graphene model, and higher-order topological insulators.

Strontium ruthenates have a lot of similarities with copper oxide superconductors and are a very interesting object for investigation of mechanisms and conditions which lead to high-temperature superconductivity. We report here on multiple experimental indications of superconductivity with the onset above $40\mathrm{K}$ in strontium ruthenate doped by rhenium and selenium with chlorine used as a flux. The main experimental evidence comes from FTIR spectroscopy of this material followed by the ac and dc magnetization, as well as its heat capacity and magnetoresistance. Structural and morphological studies reveal the heterophase nature of this polycrystalline material as well as changes of lattice parameters relative to the original phases. Experimental data provide evidence of higher critical temperature on the surface compared to that in the bulk of the sample.

A major challenge in network science is to determine parameters governing complex network dynamics from experimental observations and theoretical models. In complex chemical reaction networks, for example, such as those describing processes in internal combustion engines and power generators, rate constant estimates vary significantly across studies despite substantial experimental efforts. Here, we examine the possibility that variability in measured constants can be largely attributed to the impact of missing network information on parameter estimation. Through the numerical simulation of measurements in incomplete chemical reaction networks, we show that unaccountability of network links presumed unimportant (with local sensitivity amounting to less than two percent of that of a measured link) can create apparent rate constant variations as large as one order of magnitude even if no experimental errors are present in the data. Furthermore, the correlation coefficient between the logarithmic deviation of the rate constant estimate and the cumulative relative sensitivity of the neglected reactions was less than 0.5 in all cases. Thus, for dynamical processes on complex networks, iteratively expanding a model by determining new parameters from data collected under specific conditions is unlikely to produce reliable results.

We explore the magnetic-field-driven motion of domain walls with different chiralities in thin ferromagnetic films made of Pt/Co/Pt, Au/Co/Pt, and Pt/Co/Au. From the analysis of domain wall dynamics, we extract parameters characterizing the interaction between domain walls and weak pinning disorder of the films. The variations of domain wall structure, controlled by an in-plane field, are found to modify the characteristic length scale of pinning in strong correlation with the domain wall width, whatever its chirality and the interaction strength between domain walls and pinning defects. These findings should be also relevant for a wide variety of elastic interfaces moving in weak pinning disordered media.

We theoretically study the nonlinear response of interacting neutral bosonic gas in a synthetically driven one-dimensional optical lattice. In particular, we examine the bosonic analog of electronic higher harmonic generation in a strong time-dependent synthetic vector potential manifesting itself as the synthetic electric field. We show that the vector potential can generate reasonably high harmonics in the insulating regime, while the superfluid regime exhibits only a few harmonics. In the insulating regime, the number of harmonics increases with the increase in the strength of the vector potential. This originates primarily due to the field-driven resonant and nonresonant excitations in the neutral Mott state and their recombination with the ground state. If the repulsive interaction between two atoms $\left(U\right)$ is close to the strength of the gauge potential $\left(A_0\right)$, the resonant quasiparticle-quasihole pairs on nearest-neighbor sites, namely, dipole states, are found to a play a dominant role in generating higher harmonics. However, in the strong-field limit $A_0\gg U$, the nonresonant states where quasiparticle-quasihole pairs are not on nearest-neighbor sites give rise to higher harmonics.

Assessing the resilience of a road network is instrumental to improve existing infrastructures and design new ones. Here, we apply the optimal path crack model (OPC) to investigate the mobility of road networks and propose a new proxy for resilience of urban mobility. In contrast to static approaches, the OPC accounts for the dynamics of rerouting as a response to traffic jams. Precisely, one simulates a sequence of failures (cracks) at the most vulnerable segments of the optimal origin-destination paths that are capable to collapse the system. Our results with synthetic and real road networks reveal that their levels of disorder, fractions of unidirectional segments and spatial correlations can drastically affect the vulnerability to traffic congestion. By applying the OPC to downtown Boston and Manhattan, we found that Boston is significantly more vulnerable than Manhattan. This is compatible with the fact that Boston heads the list of American metropolitan areas with the highest average time waste in traffic. Moreover, our analysis discloses that the origin of this difference comes from the intrinsic spatial correlations of each road network. Finally, we argue that, due to their global influence, the most important cracks identified with OPC can be used to pinpoint potential small rerouting and structural changes in road networks that are capable to substantially improve urban mobility.

In some kind of three-dimensional higher-order topological insulators, bulk and surface electronic states are gapped, while there appear gapless hinge states protected by spatial symmetry. Here we show by ab initio calculations that the La apatite electride is a higher-order topological crystalline insulator. It is a one-dimensional electride, in which the one-dimensional interstitial hollows along the $c$ axis support anionic electrons, and the electronic states in these one-dimensional channels are well approximated by the one-dimensional Su-Schrieffer-Heeger model. When the crystal is cleaved into a hexagonal prism, the ${120}^{\circ }$ hinges support gapless hinge states, with their filling quantized to be 2/3. This quantization of the filling comes from a topological origin. We find that the quantized value of the filling depends on the fundamental blocks that constitute the crystal. The apatite consists of the triangular blocks, which is crucial for giving nontrivial fractional charge at the hinge.

Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide an experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this open geometry permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In a nonresonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. In a long-running discussion whether the light is of thermal or electromagnetic origin, these results on graphene nanojunctions clearly favor the thermal picture.

We report on the generation of one- and two-color attosecond hard x-ray pulses at SwissFEL by appropriately tailoring the nonlinear compression of the electron beam. For achieving very short [about 300 attoseconds (as)] and stable single-color pulses, we minimize the collective effects of the longitudinal space-charge forces by strongly compressing the electron beam right before the undulator, utilizing the energy collimator chicane as a third compression stage. For achieving two-color short pulses, we exploit enhanced space-charge forces splitting the beam into two energy components.

Nonadiabatic geometric quantum computation (NGQC) has been developed to realize fast and robust geometric gate. However, the conventional NGQC is that all of the gates are performed with exactly the same amount of time, whether the geometric rotation angle is large or small, due to the limitation of cyclic condition. Here, we propose an unconventional scheme, called nonadiabatic noncyclic geometric quantum computation (NNGQC), that arbitrary single- and two-qubit geometric gate can be constructed via noncyclic non-Abelian geometric phase. Consequently, this scheme makes it possible to accelerate the implemented geometric gates against the effects from the environmental decoherence. Furthermore, this extensible scheme can be applied in various quantum platforms, such as superconducting qubit and Rydberg atoms. Specifically, for single-qubit gate, we make simulations with practical parameters in neutral atom system to show the robustness of NNGQC and also compare with NGQC using the recent experimental parameters to show that the NNGQC can significantly suppress the decoherence error. In addition, we also demonstrate that nontrivial two-qubit geometric gate can be realized via unconventional Rydberg blockade regime within current experimental technologies. Therefore, our scheme provides a promising way for fast and robust neutral-atom-based quantum computation.

In this paper, we make a detailed study of the $D\to V$ helicity form factors (HFFs) within the framework of the QCD light-cone sum rule (LCSR) up to twist-4 accuracy. After extrapolating the LCSR predictions of HFFs to the whole physical $q^2$ region, we get the longitudinal, transverse, and total $|V_{cq}|$-independent decay widths of semileptonic decay $D\to V{\ell }^{+}{\nu }_{\ell }$. Meanwhile, the branching fractions of these decays are also obtained by using the $D^0\left(D^{+}\right)$-meson lifetime, which agrees well with the BESIII results within the error bars. As a further step, we also investigate the differential and mean predictions for charged lepton (vector meson) polarization in the final state $P_{\mathrm{L},\mathrm{T}}^{\ell }$ ($F_{\mathrm{L},\mathrm{T}}^{\ell }$), the forward-backward asymmetry ${\mathcal{A}}_{\mathrm{FB}}^{\ell }$, and the lepton-side convexity parameters ${\mathcal{C}}_{\mathrm{F}}^{\ell }$. Our predictions are consistent with covariant confining quark model results within errors. Thus, we think the LCSR approach for HFFs is applicable for dealing with the $D$-meson semileptonic decays.

Coin flipping is a fundamental cryptographic task where spatially separated Alice and Bob wish to generate a fair coin flip over a communication channel. It is known that ideal coin flipping is impossible in both classical and quantum theory. In this work, we give a short proof that it is also impossible in generalized probabilistic theories under the generalized no-restriction hypothesis. Our proof relies crucially on a formulation of cheating strategies as semi-infinite programs, i.e., cone programs with infinitely many constraints. This introduces a formalism which may be of independent interest to the quantum community.

The structural and electronic properties of twisted bilayer graphene are investigated from first-principles and tight-binding approach as a function of the twist angle (ranging from the first “magic” angle $\theta =1.{08}^{\circ }$ to $\theta =13.{17}^{\circ }$, with the former corresponding to the largest unit cell, comprising 11,164 carbon atoms). By properly taking into account the long-range van der Waals (vdW) interaction, we provide the patterns for the atomic displacements (with respect to the ideal twisted bilayer). The out-of-plane relaxation shows an oscillating (“buckling”) behavior, very evident for the smallest angles, with the atoms around the AA stacking regions interested by the largest displacements. The out-of-plane displacements are accompanied by a significant in-plane relaxation, showing a vortexlike pattern, where the vorticity (intended as curl of the displacement field) is reverted when moving from the top to the bottom plane and vice versa. Overall, the atomic relaxation results in the shrinking of the AA stacking regions in favor of the more energetically favorable AB/BA stacking domains. The measured flat bands emerging at the first magic angle can be accurately described only if the atomic relaxations are taken into account. Quite importantly, the experimental gaps separating the flat-band manifold from the higher and lower energy bands are intimately related to out-of-plane relaxations. The stability of the relaxed bilayer at the first magic angle is estimated to be of the order of 0.5–0.9 meV per atom (or 7–10 K). Our calculations shed light on the importance of an accurate description of the vdW interaction and of the resulting atomic relaxation to envisage the electronic structure of this really peculiar kind of vdW bilayers.