We investigate a multiqubit quantum battery-charger model, focusing on its potential emulation on a superconducting qubit chip. Using a large-spin representation, we first obtain the analytical form of the energy $E_B\left(t\right)$ and power $P_B\left(t\right)$, and their maximum values $E_B^{\max }$ and $P_B^{\max }$, of the battery part by means of the antiferromagnetic Holstein-Primakoff transformation within the low-energy approximation. In this case, our results show that superextensive scaling behavior of $P_B^{\max }$ ensues. By further combining these with the ones obtained via exact diagonalization, we classify the dynamics of various physical quantities, including the entanglement between the battery and charger parts for system sizes encompassing over 10 000 qubits. Finally, by checking a diverse set of system configurations, including either a fixed battery size with a growing number of charger qubits or when both parts simultaneously grow, we classify the system size scalings of $E_B^{\max }$ and $P_B^{\max }$, relating it with the entanglement entropy in the system. In agreement with the analytical results, robust superextensive behavior of $P_B^{\max }$ is also observed in this case. Our work provides an overall guide for expected features in experiments of quantum batteries emulated in superconducting qubit platforms, in particular ones that exhibit long-range couplings.

We provide a detailed comparison between the dynamics of high-temperature spatiotemporal correlation functions in quantum and classical spin models. In the quantum case, our large-scale numerics are based on the concept of quantum typicality, which exploits the fact that random pure quantum states can faithfully approximate ensemble averages, allowing the simulation of spin-$1/2$ systems with up to 40 lattice sites. Due to the exponentially growing Hilbert space, we find that for such system sizes even a single random state is sufficient to yield results with extremely low noise that is negligible for most practical purposes. In contrast, a classical analog of typicality is missing. In particular, we demonstrate that to obtain data with a similar level of noise in the classical case, extensive averaging over classical trajectories is required, no matter how large the system size. Focusing on (quasi-)one-dimensional spin chains and ladders, we find remarkably good agreement between quantum and classical dynamics. This applies not only to cases where both the quantum and classical models are nonintegrable but also to cases where the quantum spin-$1/2$ model is integrable and the corresponding classical $s\to \infty$ model is not. Our analysis is based on the comparison of space-time profiles of the spin and energy correlation functions, where the agreement is found to hold not only in the bulk but also in the tails of the resulting density distribution. The mean-squared displacement of the density profiles reflects the nature of emerging hydrodynamics and is found to exhibit similar scaling for quantum and classical models.

Magic-angle twisted bilayer graphene exhibits quasiflat low-energy bands with Van Hove singularities close to the Fermi level. These singularities play an important role in the exotic phenomena observed in this material, such as superconductivity and magnetism, by amplifying electronic correlation effects. In this work, we study the correspondence of four-terminal conductance and the Fermi surface topology as a function of the twist angle, pressure, and energy in mesoscopic, ballistic samples of small-angle twisted bilayer graphene. We establish a correspondence between features in the wide-junction conductance and the presence of Van Hove singularities in the density of states. Moreover, we identify additional transport features, such as a large, pressure-tunable minimal conductance, conductance peaks coinciding with nonsingular band crossings, and unusually large conductance oscillations as a function of the system size. Our results suggest that twisted bilayer graphene close the magic angle is a unique system featuring simultaneously large conductance due to the quasiflat bands, strong quantum nonlinearity due to the Van Hove singularities, and high sensitivity to external parameters, which could be utilized in high-frequency device applications and sensitive detectors.

${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ represents a unique example in the family of B20 cubic helimagnets with a tilted spiral and a low-temperature skyrmion phase arising for magnetic fields applied along the easy crystallographic $\langle 100\rangle$ axes. Although the stabilization mechanism of these phases can be accounted for by cubic magnetic anisotropy, the skyrmion nucleation process is still an open question, since the stability region of the skyrmion phase displays strongly hysteretic behavior with different phase boundaries for increasing and decreasing magnetic fields. Here, we address this important point using micromagnetic simulations and come to the conclusion that skyrmion nucleation is underpinned by the reorientation of spiral domains occurring near the critical magnetic fields of the phase diagrams: $H_{C1}$, the critical field of the transition between the helical and conical/tiled spiral phase, and $H_{C2}$, the critical field between the conical/tiled spiral and the homogenous phase. By studying a wide variety of cases we show that domain walls may have a 3D structure. Moreover, they can carry a finite topological charge stemming from half-skyrmions (merons) also permitting along-the-field and perpendicular-to-the-field orientation. Thus, domain walls may be envisioned as nucleation source of skyrmions that can form thermodynamically stable and metastable lattices as well as skyrmion networks with misaligned skyrmion tubes. The results of numerical simulations are discussed in view of recent experimental data on chiral magnets, in particular, for the bulk cubic helimagnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$.

We demonstrate that the three-dimensional $e_g$ orbital Hubbard model can be generalized to arbitrary dimension $d$, and that the form of the result is determined uniquely by the requirements that (i) the twofold degeneracy of the $e_g$ orbital be retained, and (ii) the cubic lattice be turned into a hypercubic lattice. While the local Coulomb interaction $U$ is invariant for each basis of orthogonal orbitals, the form of the kinetic energy depends on the orbital basis and takes the most symmetric form for the so-called complex-orbital basis. Characteristically, with respect to this basis, the model has two hopping channels: one that is orbital-flavor conserving, and a second one that is orbital-flavor nonconserving. We show that the noninteracting electronic structure consists of two nondegenerate bands of plane-wave real-orbital single-particle states for which the orbital depends on the wave vector. Due to the latter feature each band is unpolarized at any filling, and has a non-Gaussian density of states at $d=\infty$. The orbital liquid state is obtained by filling these two bands up to the same Fermi energy. We investigate the $e_g$ orbital Hubbard model in the limit $d\to \infty$, treating the on-site Coulomb interaction $U$ within the Gutzwiller approximation, thus determining the correlation energy of the orbital liquid and the (disordered) paraorbital states. In perfect analogy with the case of the spin Hubbard model, the Gutzwiller approximation is demonstrated to be exact at $d=\infty$ for the orbital Hubbard model, because of the collapse of electron correlations to a single site. At half-filling (one electron per site on average, $n=1$) one finds a Brinkman-Rice type “metal-insulator” transition in the orbital liquid, which is analogous to the transition for a paramagnetic state in the spin model, but occurs at stronger Hubbard interaction $U$ due to the enhanced kinetic energy provided by the nonconserving hopping channel. We show that the orbital liquid is the ground state everywhere in the $(n,U)$ phase diagram except close to half-filling at sufficiently large $U$, where ferro-orbital order with real orbitals occupied is favored. The latter feature is shown to be specific for $d=\infty$, being of mathematical nature due to the exponential tails in the density of states.

We revisit the lattice formulation of the Schwinger model using the Kogut-Susskind Hamiltonian approach with staggered fermions. This model, introduced by Banks et al. Phys. Rev. D 13, 1043 (1976), contains the mass term $m_{\mathrm{lat}}{\sum }_n{(-1)}^n{\chi }_n^{†}{\chi }_n$, and setting it to zero is often assumed to provide the lattice regularization of the massless Schwinger model. We instead argue that the relation between the lattice and continuum mass parameters should be taken as $m_{\mathrm{lat}}=m-\frac{1}{8}{e}^{2}a$. The model with $m=0$ is shown to possess a discrete chiral symmetry that is generated by the unit lattice translation accompanied by the shift of the $\theta$ angle by $\pi$. While the mass shift vanishes as the lattice spacing $a$ approaches zero, we find that including this shift greatly improves the rate of convergence to the continuum limit. We demonstrate the faster convergence using both numerical diagonalizations of finite lattice systems, as well as extrapolations of the lattice strong coupling expansions.

We present a density-functional theory (DFT) study of the structural, electronic, and chemical bonding behavior in germanium (Ge)-doped vanadium dioxide $\left({\mathrm{VO}}_2\right)$. Our motivation is to explain the reported increase of the metal-insulator transition temperature under Ge doping and to understand how much of the fundamental physics and chemistry behind it can be captured at the conventional DFT level. We model doping using a supercell approach, with various concentrations and different spatial distributions of Ge atoms in ${\mathrm{VO}}_2$. Our results suggest that the addition of Ge atoms strongly perturbs the high-symmetry metallic rutile phase and induces structural distortions that partially resemble the dimerization of the experimental insulating structure. Our work, therefore, hints at a possible explanation of the observed increase in transition temperature under Ge doping, motivating further studies into understanding the interplay of structural and electronic transitions in ${\mathrm{VO}}_2$.

We study the electronic structure of hole-doped transition metal dichalcogenides for small twist angles, where the on-site repulsion is extremely strong. Using unbiased diagrammatic Monte Carlo simulations, we find evidence for magnetic correlations which are driven by delocalization, and can be controlled in situ via the dielectric environment. For weak spin-orbit coupling we find that the moderately doped system becomes antiferromagnetic, while the regime of strong spin-orbit coupling features ferromagnetic correlations. We show that this behavior is accurately predicted by an analytical theory based on moment expansion of the Hamiltonian, and an analysis of corresponding particle trajectories.

Superconductivity in FeSe is strongly enhanced under applied pressure and it is proposed to emerge from anomalously coupled structural and magnetic phases. Small impurities inside the Fe plane can strongly disrupt the pair formation in FeSe at ambient pressure and can also reveal the interplay between normal and superconducting phases. Here, we investigate how an impurity inside the Fe plane induced by the Cu substitution can alter the balance between competing electronic phases of FeSe at high pressures. In the absence of an applied magnetic field, at low pressures the nematic and superconducting phases are suppressed by a similar factor. On the other hand, at high pressures, above 10 kbar, the superconductivity remains unaltered despite the lack of any signature in transport associated to a magnetic phase in zero-magnetic field. However, by applying a magnetic field, the resistivity displays an anomaly preceding the activated behavior in temperature, assigned to a magnetic anomaly. We find that the high-pressure superconducting phase of FeSe is robust and remains enhanced in the presence of Cu impurity, whereas the magnetic phase is not. This could suggest that high-$T_{\mathrm{c}}$ superconductivity has a sign-preserving order parameter in the presence of a rather glassy magnetic phase.

Skyrmions are topological magnetic textures that can arise in noncentrosymmetric ferromagnetic materials. In most systems experimentally investigated to date, skyrmions emerge as classical objects. However, the discovery of skyrmions with nanometer length scales has sparked interest in their quantum properties. Here, we simulate the ground states of two-dimensional spin-$1/2$ Heisenberg lattices with Dzyaloshinskii-Moriya interactions and discover a broad region in the zero-temperature phase diagram which hosts quantum skyrmion lattices. We argue that the quantum skyrmion lattice phase can be detected experimentally in the magnetization profile via local magnetic polarization measurements as well as in the spin structure factor measurable via neutron scattering experiments. Finally, we explore the resulting quantum skyrmion state, analyze its real-space polarization profile and show that it is a nonclassical state featuring entanglement between quasiparticle and environment mainly localized near the boundary spins of the skyrmion.

The eigennmodes in a non-Hermitian system containing nonreciprocal couplings may exhibit an extreme localization, also known as non-Hermitian skin effect (NHSE), under open boundary conditions. Here, we ex- plore unconventional scenarios where the interplay between multiple asymmetric couplings can cause the NHSE to vanish, unlike in known models with nonreciprocal couplings, where the NHSE vanishes only when the non-Hermiticity is turned off. We derive general conditions for the NHSE in a non-Hermitian superlattice in which the overall eigenmode localization is determined by the geometric mean of the cumulative contributions of all asymmetric coupling segments. In the limit of large unit cells, our results provide a route towards the NHSE caused by asymmetric hopping textures, rather than single asymmetric hoppings. We provide explicit electrical circuit setups for realizing our observations, which also extend to other established platforms such as photonics, mechanics, and optics systems, and quantum circuits.

Clean two-dimensional Dirac systems have received a great deal of attention for being a prime candidate to observe hydrodynamical transport behavior in interacting electronic systems. This is mostly due to recent advances in the preparation of ultrapure samples with sufficiently strong interactions. In this paper we investigate the role of collective modes in the thermoelectric transport properties of those systems. We find that dynamical particle-hole pairs, plasmons, make a sizable contribution to the thermal conductivity. While the increase at the Dirac point is moderate, it becomes large towards larger doping. We suspect that this is a generic feature of ultraclean two-dimensional electronic systems, also applicable to degenerate systems.

We present a method for solving eigenvalue problems on a quantum computer based on spectroscopy. The method works by coupling a “probe” qubit to a set of system simulation qubits and then time evolving both the probe and the system under Hamiltonian dynamics. In this way, we simulate spectroscopy on a quantum computer. We test our method on the IBM quantum hardware for a simple single-spin model and an interacting Kitaev chain model. For the Kitaev chain, we trace out the pseudotopological phase boundary for a two-site model.

We show that the dynamical instability of quantum vortices with more than a single unit of circulation results from a superradiant bound state inside the vortex core. We then reveal a striking behavior of the system in the nonlinear regime. Counter to the expectation that the instability should produce well-separated vortices, the system evolves into a bound state of corotating singularities orbiting within a few healing lengths of each other, undergoing modulations in separation. During these modulations, kinetic energy is traded back and forth between sound waves and vortical degrees of freedom.