We present extensive ab initio path integral Monte Carlo (PIMC) results for the spin-resolved density response of the uniform electron gas at warm dense matter conditions. This allows us to unambiguously assess the accuracy of previous theoretical approximations, thereby providing valuable insights for the future development of dielectric schemes. From a physical perspective, we observe a nontrivial manifestation of an effective electron-electron attraction that emerges in the spin-offdiagonal static density response function at strong coupling, $r_s\gtrsim 5$. All PIMC results are freely available online and can be used to benchmark approximations and simulation schemes.

Buoyancy is a well-known effect in immiscible binary Bose-Einstein condensates. Depending on the differential confinement experienced by the two components, a bubble of one component sitting at the center of the other eventually floats to the surface, around which it spreads either totally or partially. We discuss how quantum fluctuations may significantly change the volume and position of immiscible bubbles. We consider the particular case of two miscible components, forming a pseudoscalar bubble condensate with enhanced quantum fluctuations (quantum bubble), immersed in a bath provided by a third component, with which they are immiscible. We show that in such a peculiar effective binary mixture, quantum fluctuations change the equilibrium of pressures that define the bubble volume and modify as well the criterion for buoyancy. Once buoyancy sets in, in contrast to the mean-field case, quantum fluctuations may place the bubble at an intermediate position between the center and the surface. At the surface, the quantum bubble may transition into a floating self-bound droplet.

Direct and indirect excitons in Rydberg states in transition metal trichalcogenide (TMTC) monolayers, bilayers, and van der Waals (vdW) heterostructures in an external magnetic field are studied within the framework of the effective mass approximation. Binding energies of magnetoexcitons are calculated using the Rytova-Keldysh potential for direct magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect magnetoexcitons. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients for magnetoexcitons that depend strongly on the effective mass anisotropy of electrons and holes. The comparative study of TMTCs and phosphorene is given. In ${\mathrm{TiS}}_3$, ${\mathrm{TiSe}}_3$, and ${\mathrm{ZrSe}}_3$ the excitonic binding energies and diamagnetic coefficients demonstrate the same kind of anisotropy as in phosphorene. In contrast, ${\mathrm{ZrS}}_3$ has the opposite anisotropy to phosphorene. The tunability of the binding energy of direct and indirect magnetoexcitons by the external magnetic field and the possibility to control the binding energy of magnetoexcitons in vdW heterostructures by manipulation of numbers of hBN monolayers are shown.

High-gradient and high-efficiency acceleration in plasma-based accelerators has been demonstrated, showing its potential as the building block for a future collider operating at the energy frontier of particle physics. However, generating and accelerating the required spin-polarized beams in such a collider using plasma-based accelerators have been a long-standing challenge. Here we show that the passage of a highly relativistic, high-current electron beam through a single-species (ytterbium) vapor excites a nonlinear plasma wake by primarily ionizing the two outer $6s$ electrons. Further photoionization of the resultant ${\mathrm{Yb}}^{2+}$ ions by a circularly polarized laser injects the $4f^{14}$ electrons into this wake, generating a highly spin-polarized beam. Combining time-dependent Schrödinger equation simulations with particle-in-cell simulations, we show that a subfemtosecond, high-current (4 kA) electron beam with up to 56% net spin polarization can be generated and accelerated to 15 GeV in just 41 cm. This relatively simple scheme solves the perplexing problem of producing spin-polarized relativistic electrons in plasma-based accelerators.

We study the dynamics induced by quenching an ultracold quantum many-body system between two supersymmetric Hamiltonians. Such a quench can be created by carefully changing the external trapping potential and leads to a situation where the eigenspectra before and after the quench are nearly identical. We show that the dynamics originating from this can be conveniently described using knowledge about the initial state only and apply this insight to the specific example of a fermionic gas that is initially trapped in an infinite box potential. When quenching to different, higher order supersymmetric partners potentials we observe the appearance of many-body revivals in the survival probability and show that some of these are robust at finite temperatures. This is in contrast to the well-known Talbot effect, which is the standard example for quenching into a system with a quadratic spectrum.

Exploring the origin of the metal-to-insulator transition (MIT) in transition metal oxide heterostructures is of high interest in current condensed matter physics research. Here based on density functional theory calculations with the meta-GGA exchange correlation functional SCAN, we find distinct mechanisms of MIT in $({\mathrm{SrVO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)$ superlattices (SLs). The SCAN functional is sufficient to determine the ground state structure and possible symmetry breaking at a given lateral lattice constant and best describes the electronic and magnetic properties of the weakly correlated $({\mathrm{SrRuO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)$ SL and its constituents by minimizing the self-interaction error. However, an additional Hubbard $U$ term is necessary for the strongly correlated $({\mathrm{SrVO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)\mathrm{SLs}$. We show that SCAN $+$ $U$ always favors the monoclinic $({P2}_1/c)$ symmetry in $({\mathrm{Sr}X\mathrm{O}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)\mathrm{SLs}$, $X=\mathrm{V} \mathrm{and} \mathrm{Ru}$, irrespective of the in-plane lattice constant and $X$. For the orthorhombic $({\mathrm{SrVO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)\mathrm{SL}\left(Cmmm\right)$ at $a_{\mathrm{STO}}$ (tensile strain $+1.7\%$), we report strong correlation and confinement driven Mott-Hubbard type MIT via long-range stripe antiferromagnetic ordering, whereas, under compressive strain $(-3.6\%)$ at $a_{\mathrm{YAO}}$, the interplay of confinement, correlation, and finite octahedral tilts and rotations lead to monoclinic $({P2}_1/c)$ symmetry, which drives an orbital reconstruction and a concomitant MIT with ferromagnetic spin alignment. Lastly, using Boltzmann transport theory within the constant relaxation time approximation, for $({\mathrm{SrVO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)\mathrm{SL}$ at $a_{\mathrm{STO}}$, we obtain large $n$-type Seebeck coefficients $S$ of $-566$ (in-plane) and $-454$ $\mu \mathrm{V}/\mathrm{K}$ (cross-plane), respectively, along with an in-plane (cross-plane) power factor of 31.4 (8.5) $\mu \mathrm{W}{\mathrm{K}}^{-2}{\mathrm{cm}}^{-1}$ (assuming $\tau =4$ fs) at 300 K. These values directly categorize $({\mathrm{SrVO}}_3{)}_1\text{/}{\left({\mathrm{SrTiO}}_3\right)}_1\left(001\right)\mathrm{SL}$ as a promising oxide thermoelectric material.

Variational quantum algorithms are considered to be appealing applications of near-term quantum computers. However, it has been unclear whether they can outperform classical algorithms or not. To reveal their limitations, we must seek a technique to benchmark them on large-scale problems. Here we propose a perturbative approach for efficient benchmarking of variational quantum algorithms. The proposed technique performs perturbative expansion of a circuit consisting of Clifford and Pauli rotation gates, which is enabled by exploiting the classical simulatability of Clifford circuits. Our method can be applied to a wide family of parameterized quantum circuits consisting of Clifford gates and single-qubit rotation gates. The approximate optimal parameter obtained by the method can also serve as an initial guess for further optimizations on a quantum device. As the first application of the method, we perform a benchmark of so-called hardware-efficient-type ansatzes when they are applied to the variational quantum eigensolver (VQE) of one-dimensional hydrogen chains up to ${\mathrm{H}}_{24}$, which corresponds to a 48-qubit system using a standard workstation. This is the largest scale benchmark of the VQE to the best of our knowledge and reveals the limitation of hardware-efficient-type ansatzes.

The unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ has been the subject of enormous interest over more than two decades, but until now the form of its order parameter has not been explicitly determined. Since groundbreaking NMR experiments revealed recently that the pairs are of dominant spin-singlet character, attention has focused on time-reversal symmetry breaking linear combinations of $s$-, $d$-, and $g$-wave one-dimensional (1D) irreducible representations. However, a state of the form $d_{xz}+i{d}_{yz}$ corresponding to the two-dimensional representation $E_g$ has also been proposed based on some experiments. We present a systematic study of the stability of various superconducting candidate states, assuming that pairing is driven by the fluctuation exchange mechanism, including a realistic three-dimensional Fermi surface, full treatment of both local and nonlocal spin-orbit couplings, and a wide range of Hubbard-Kanamori interaction parameters $U,J,U^{\prime },J^{\prime }$. The leading superconducting instabilities are found to exhibit nodal even-parity $A_{1g}\left(s^{\prime }\right)$ or $B_{1g}\left(d_{x^2-y^2}\right)$ symmetries, similar to the findings in two-dimensional models without longer-range Coulomb interaction which tends to favor $d_{xy}$ over $d_{x^2-y^2}$. Within the so-called Hund's coupling mean-field pairing scenario, the $E_g(d_{xz}/d_{yz})$ solution can be stabilized for large $J$ and specific forms of the spin-orbit coupling, but for all cases studied here the eigenvalues of other superconducting solutions are significantly larger when the full fluctuation exchange vertex is included in the pairing kernel. Additionally, we compute the spin susceptibility in relevant superconducting candidate phases and compare to recent neutron scattering and nuclear magnetic resonance (NMR) Knight shift measurements. It is found that $d_{xz}+i{d}_{yz}$ order supports a neutron resonance in its superconducting phase, in contrast to a recent experiment [K. Jenni et al., Phys. Rev. B 103, 104511 (2021)], whereas $s^{\prime }+i{d}_{x^2-y^2}$ does not. Furthermore, comparison of the Knight shift reveals that $s^{\prime }+i{d}_{x^2-y^2}$ exhibits a larger low-temperature shift than $d_{xz}+i{d}_{yz}$.

Free quantum field theory in flat space-time is often believed to be well established, holding no surprises. We hope, by the end of this paper, to have demonstrated that surprises still exist. For example, we will show that a uniformly accelerated atom in Minkowski space-time emits entangled photon pairs in a squeezed state which mimics entanglement of the Minkowski vacuum in that the entanglement is between Minkowski modes which are dominantly in opposite causal wedges of the space-time. Similar emission of photon pairs occurs if an atom is held above the black hole event horizon. Namely, a ground-state atom becomes excited by emitting a “negative” -energy photon under the horizon and then spontaneously decays back to the ground state by emitting a positive-energy photon outside the horizon, which propagates away from the black hole.

We give evidence that consecutive explosive transitions may occur in two-layered networks, when a dynamical layer made of an ensemble of networking phase oscillators interacts with an environmental layer of oscillators which are in a state of approximate synchronization. Under these conditions, the interlayer coupling induces two consecutive explosive transitions in the dynamical layer, each one associated with a hysteresis loop. We also show that the same phenomenon can be observed when the environmental layer is simplified into a single node with phase lag. Theoretical arguments unveil that the mechanisms at the basis of the two transitions are in fact different, with the former originating from a coupling-amplified disorder and the latter originating from a coupling-induced synchronization. We discuss the relevance of the observed state in brain dynamics and show how it may emerge in a real brain network.

The realization of triply degenerate points (TDPs) with exotic fermionic excitations has opened a new perspective for the understanding of our nature. Here we explore the coexistence of single unpaired TDP and multiple twofold Weyl points (WPs) and propose an experimental scheme with ultracold pseudospin-1 atomic gases trapped in optical lattices. We show that the predicted single unpaired TDP emerged by the interplay of quadratic spin-vector- and spin-tensor-momentum-coupling could possess a topological nontrivial middle band. This exotic TDP with mirror symmetry breaking is essential different from the recently observed TDPs that must appear in pairs due to the Nielsen-Ninomiya theorem and host the topological trivial middle band. Strikingly, the topologically protected Fermi-arc states directly connect the unpaired TDP with additional WPs, in contrast to the conventional Fermi-arc states that connect the same degeneracies of band degenerate points with opposite chirality. Furthermore, the different types of TDPs with unique linking structure of Fermi arcs can be readily distinguished by measuring spin texture along high-symmetry lines of the system. Our scheme provides a platform for emerging new fermions with exotic physical phenomena and versatile device applications.

Accelerating computation with quantum resources is limited by the challenges of high-fidelity control of quantum systems. Reservoir computing presents an attractive alternative, as precise control and full calibration of system dynamics are not required. Instead, complex internal trajectories in a large state space are leveraged as a computational resource. Quantum systems offer a unique venue for reservoir computing, given the presence of interactions unavailable in classical systems and a potentially exponentially-larger computational space. With a reservoir comprised of a single $d$-dimensional quantum system, we demonstrate clear performance improvement with Hilbert space dimension at two benchmark tasks and advantage over the physically analogous classical reservoir. Quantum reservoirs as realized by current-era quantum hardware offer immediate practical implementation and a promising outlook for increased performance in larger systems.

Rare-earth chalcohalide $\mathrm{RECh}X$ (RE = rare earth; Ch = O, S, Se, Te; $X$ = F, Cl, Br, I) is a newly reported family of Kitaev spin liquid candidates. The family offers a platform where a strong spin-orbit coupling meets a van der Waals layered and undistorted honeycomb spin lattice, which outputs highly anisotropic exchange couplings required by the Kitaev model. YbOCl is the first single crystal of the family we grew, with a size up to $\sim 15\mathrm{mm}$. We have performed magnetization and high magnetic field electron spin resonance measurements from 2 to 300 K. We develop the mean-field scenario for the anisotropic spin system, with which we are able to well describe the experiments and reliably determine the fundamental parameters. The self-consistent simulations give the anisotropic spin-exchange interactions of $J_{\pm }$ ($\sim -0.3\mathrm{K}$) and $J_{zz}$ ($\sim 1.6\mathrm{K}$), and $g$ factors of $g_{ab}$ ($\sim 3.4$) and $g_c$ ($\sim 2.9$). Based on the spin-exchange interactions, we employ the exact diagonalization method to work out the ground state phase diagram of YbOCl in terms of the off-diagonal exchange couplings. The phase diagram hosting rich magnetic phases including the spin-disordered one, sheds light on the novel magnetic properties of the family, particularly the Kitaev physics.

We propose a protocol that achieves fast adiabatic transfer between two orthogonal states of a qubit by coupling with an ancilla. The qubit undergoes Landau-Zener dynamics, whereas the coupling realizes a time-dependent Hamiltonian, which is diagonal in the spin's instantaneous Landau-Zener eigenstates. The ancilla (or meter), in turn, couples to a thermal bath such that the overall dynamics is incoherent. We analyze the protocol's fidelity as a function of the strength of the coupling and of the relaxation rate of the meter. When the meter's decay rate is the largest frequency scale of the dynamics, the spin dynamics is encompassed by a master equation describing dephasing of the spin in the instantaneous eigenbasis. In this regime, the fidelity of adiabatic transfer improves as the bath temperature is increased. Surprisingly, the adiabatic transfer is significantly more efficient in the opposite regime, where the timescale of the ancilla dynamics is comparable to the characteristic spin timescale. Here, for low temperatures the coupling with the ancilla tends to suppress diabatic transitions via effective cooling. The protocol can be efficiently implemented by means of a pulsed, stroboscopic coupling with the ancilla and is robust against moderate fluctuations of the experimental parameters.

Bulk charge density waves are now reported in nearly all high-temperature superconducting cuprates, with the noticeable exception of one particular family: the copper oxychlorides. Here, we used resonant inelastic x-ray scattering to reveal a bulk charge density waves in these materials. Combining resonant inelastic x-ray scattering with nonresonant inelastic x-ray scattering, we investigate the interplay between the lattice excitations and the charge density wave, and evidence the phonon anomalies of the Cu-O bond-stretching mode at the charge density wave wave vector. We propose that such electron-phonon anomalies occur in the presence of dispersive charge excitations emanating from the charge density wave and interacting with the Cu-O bond-stretching phonon. Our results pave the way for future studies, combining both bulk and surface probes, to investigate the static and dynamical properties of the charge density wave in the copper oxychloride family.