The impressive progress in fabricating and controlling superconducting devices for quantum information processing has reached a level where reliable theoretical predictions need to account for quantum correlations that are not captured by the conventional modeling of contemporary quantum computers. This applies particularly to the qubit initialization as the process which crucially limits typical operation times. Here, we employ numerically exact methods to study realistic implementations of a transmon qubit embedded in electromagnetic environments focusing on the most important system-reservoir correlation effects such as the Lamb shift and entanglement. For the qubit initialization we find a fundamental trade-off between speed and accuracy which sets intrinsic constraints in the optimization of future reset protocols. Instead, the fidelities of quantum logic gates can be sufficiently accurately predicted by standard treatments. Our results can be used to accurately predict the performance of specific setups and also to guide future experiments in probing low-temperature properties of qubit reservoirs.

The soft ferroelectric phonon in ${\mathrm{SrTiO}}_3$ observed with optical spectroscopy has an extraordinarily strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. This “charged phonon” effect in ${\mathrm{SrTiO}}_3$ is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferroelectric phonon and the interband transitions across the 3-eV gap of ${\mathrm{SrTiO}}_3$. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the biphonon mediated pairing interaction in the electron-doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature. Biphonon exchange is therefore an important pairing mechanism at low doping, and may be the key to understanding the occurrence of superconductivity in doped ${\mathrm{SrTiO}}_3$ and other low electron density materials.

We show that the mesoscopic incommensurate ${\mathbb{Z}}_2$ vortex crystals proposed for layered triangular anisotropic magnets can be most saliently identified by two distinctive signatures in dynamical spin response experiments: the presence of pseudo-Goldstone phononlike modes at low frequencies $\omega$, associated with the collective vibrations of the vortex cores, and a characteristic multiscattered intensity profile at higher $\omega$, arising from a large number of Bragg reflections and magnon band gaps. These are direct fingerprints of the large vortex sizes and magnetic unit cells and the solitonic spin profile around the vortex cores.

We introduce a complete physical model for the single-particle electronic structure of twisted bilayer graphene (TBLG), which incorporates the crucial role of lattice relaxation. Our model, based on $k·p$ perturbation theory and openly available, combines the accuracy of density functional theory calculations through effective tight-binding Hamiltonians with the computational efficiency and complete control of the twist angle offered by continuum models. The inclusion of relaxation significantly changes the band structure at the first magic-angle twist corresponding to flat bands near the Fermi level (the “low-energy” states), and eliminates the appearance of a second magic-angle twist. We show that minimal models for the low-energy states of TBLG can be easily modified to capture the changes in electronic states as a function of twist angle.

In a weakly collisional, low-electron-beta plasma, large-scale Alfvén turbulence transforms into inertial kinetic-Alfvén turbulence at scales smaller than the ion microscale (gyroscale or inertial scale). We propose that at such kinetic scales, the nonlinear dynamics tends to organize turbulent eddies into thin current sheets, consistent with the existence of two conserved integrals of the ideal equations, energy and helicity. The formation of strongly anisotropic structures is arrested by the tearing instability that sets a critical aspect ratio of the eddies at each scale $a$ in the plane perpendicular to the guide field. This aspect ratio is defined by the balance of the eddy turnover rate and the tearing rate, and varies from ${(d_e/a)}^{1/2}$ to $d_e/a$ depending on the assumed profile of the current sheets. The energy spectrum of the resulting turbulence varies from $k^{-8/3}$ to $k^{-3}$, and the corresponding spectral anisotropy with respect to the strong background magnetic field from $k_z\lesssim k_{\perp }^{2/3}$ to $k_z\lesssim k_{\perp }$.

We present femtosecond hard x-ray diffraction experiments that study the effect of photodoping on the perovskite ${\mathrm{EuTiO}}_3$. We observe an ultrafast transient increase of reflection intensities that are directly related to the antiferrodistortive rotation of the oxygen octahedra. This increase relates directly to an increase of the order parameter associated with a purely structural phase transition that is opposite to the behavior of increasing the materials temperature. First-principles calculations show that the creation of electron-hole pairs leads to changes in the vibrational potential that is consistent with an increase of the order parameter. The order parameter increase can be understood from an ultrafast charge-transfer-induced reduction of the Goldschmidt tolerance factor, which is a fundamental control parameter for the properties of perovskites.

Magnetic energy transfer from small to large scales due to successive magnetic island coalescence is investigated. A solvable analytical model is introduced and shown to correctly capture the evolution of the main quantities of interest, as borne out by direct numerical simulations. Magnetic reconnection is identified as the key mechanism enabling the inverse transfer, and setting its properties: Magnetic energy decays as ${\tilde{t}}^{-1}$, where $\tilde{t}$ is time normalized to the (appropriately defined) reconnection timescale, and the correlation length of the field grows as ${\tilde{t}}^{1/2}$. The magnetic energy spectrum is self-similar, and evolves as $\propto {\tilde{t}}^{-3/2}{k}^{-2}$, where the $k$ dependence is imparted by the formation of thin current sheets.

We introduce and investigate material junctions as a generic and tunable electronic platform for the realization of exotic non-Hermitian (NH) topological states of matter, where the NH character is induced by the surface self-energy of a thermal reservoir. As a conceptually rich and immediately experimentally realizable example, we consider a three-dimensional topological insulator (TI) coupled to a ferromagnetic lead. Remarkably, the symmetry-protected TI is promoted in a dissipative fashion to a non-symmetry-protected NH Weyl phase with no direct Hermitian counterpart and which exhibits robustness against any perturbation. The transition between a gapped phase and the NH Weyl phase may be readily tuned experimentally with the magnetization direction of the ferromagnetic lead. Given the robustness of this exotic nodal phase, our general analysis also applies to, e.g., a two-dimensional electron gas close to criticality in proximity to a ferromagnetic lead. There, the predicted bulk Fermi arcs are directly amenable to surface spectroscopy methods such as angle-resolved photoemission spectroscopy.

Using discrete element simulations, we demonstrate that critical behavior for yielding in soft disk and sphere packings is independent of the distance to isostaticity over a wide range of dimensionless pressures. Jammed states are explored via quasistatic shear at fixed pressure, and the statistics of the dimensionless shear stress $\mu$ of these states obey a scaling description with a diverging length scale $\xi \propto |\mu -{\mu }_c{|}^{-\nu }$. The critical scaling functions and values of the scaling exponents are nearly independent of distance to isostaticity despite the large range of pressures studied. Our results demonstrate that yielding of jammed systems represents a distinct nonequilibrium critical transition from the isostatic critical transition which has been demonstrated by previous studies. Our results may also be useful in deriving nonlocal rheological descriptions of granular materials, foams, emulsions, and other soft particulate materials.

Based on first-principles calculations and a symmetry-based indicator analysis, we find a class of topological crystalline insulators (TCIs) with a $C_2$ rotation anomaly in a family of Zintl compounds, including ${\mathrm{Ba}}_3{\mathrm{Cd}}_2{\mathrm{As}}_4$, ${\mathrm{Ba}}_3{\mathrm{Zn}}_2{\mathrm{As}}_4$, and ${\mathrm{Ba}}_3{\mathrm{Cd}}_2{\mathrm{Sb}}_4$. The nontrivial band topology protected by the coexistence of $C_2$ rotation symmetry and time-reversal symmetry $T$ leads to two surface Dirac cones at generic momenta on both top and bottom surfaces perpendicular to the rotation axis. In addition, $(d-2)$-dimensional helical hinge states are also protected along the hinge formed by two side surfaces parallel to the rotation axis. We develop a method based on the nested Wilson loop technique to prove the existence of these surface Dirac cones due to a $C_2$ anomaly and precisely locate them as demonstrated in studying these TCIs. The helical hinge states are also calculated. Finally, we show that external strain can be used to tune the topological phase transitions among TCIs, strong $Z_2$ topological insulators, and trivial insulators.