The superconducting diode effect has recently been reported in a variety of systems and different symmetry-breaking mechanisms have been examined. However, the frequency range of these potentially important devices still remains obscure. We investigated superconducting microbridges of ${\mathrm{Nb}}_3\mathrm{Sn}$ in out-of-plane magnetic fields; optimum magnetic fields of $\sim$10 mT generate $\sim 10\%$ diode efficiency, while higher fields of $\sim$15–20 mT quench the effect. The diode changes its polarity with magnetic field reversal. We documented superconductive diode rectification at frequencies up to 100 kHz, the highest reported as of today. Interestingly, the bridge resistance during diode operation reaches a value that is a factor of two smaller than in its normal state, which is compatible with the vortex-caused mechanism of resistivity. This is confirmed by finite-element modeling based on time-dependent Ginzburg-Landau equations. To explain experimental findings, no assumption of lattice thermal inequilibrium has been required. Dissimilar edges of the superconductor strip can be responsible for the inversion symmetry breaking by the vortex penetration barrier; visual evidence of this opportunity was revealed by scanning electron microscopy. Estimates are in favor of a much higher (GHz) range of frequencies for this type of diode.

Two recent papers explore the idea of “pairons” (incoherent bound pairs above $T_c$) as a model for the pseudogap in cuprate high-$T_c$ superconductors. From this model the authors calculate the resultant electronic specific heat and static magnetic susceptibility. At elevated temperature the pairons thermally unbind causing a broad peak in the specific heat, additional to the second-order peak at $T_c$ where the pairons coherently condense. With this unbinding the electronic entropy recovers to its bare quasiparticle linear-in-$T$ value. We show that this is inconsistent with the measured specific heat which reveals an entropy which never recovers to the highest temperature investigated (about 400 K). In the case of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ there is a broad peak above $T_c$ in the specific heat coefficient, $\gamma$, but this arises from the nearby van Hove singularity (vHs) which is more distant in the case of ${\mathrm{Y}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ and therefore not discernible. We propose a number of further critical tests. In the pairon model the BCS ratios are not satisfied until vanishingly near where superconductivity disappears in the heavily overdoped region whereas, experimentally, these mean-field ratios are sustained across the overdoped regime once the pseudogap has closed at critical doping.

We report on measurements of dissipation and frequency noise at millikelvin temperatures of nanomechanical devices covered with aluminum. A clear excess damping is observed after switching the metallic layer from superconducting to the normal state with a magnetic field. Beyond the standard model of internal tunneling systems coupled to the phonon bath, here we consider the relaxation to the conduction electrons together with the nature of the mechanical dispersion laws for stressed/unstressed devices. With these key ingredients, a model describing the relaxation of two-level systems inside the structure due to interactions with electrons and phonons with well-separated timescales captures the data. In addition, we measure an excess $1/f$-type frequency noise in the normal state, which further emphasizes the impact of conduction electrons.

In-plane uniaxial pressure has been shown to strongly tune the superconducting state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ by approaching a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At the VHS, $T_{\text{c}}$ and the in- and out-of-plane upper critical fields are all strongly enhanced, and the latter has changed its curvature as a function of temperature from convex to concave. However, due to strain inhomogeneity it has not been possible so far to determine how the upper critical fields change with strain. Here, we show the strain dependence of both upper critical fields, which was achieved due to an improved sample preparation. We find that the in-plane upper critical field is mostly linear in $T_{\text{c}}$. On the other hand, the out-of-plane upper critical field varies with a higher power in $T_{\text{c}}$, and peaks strongly at the VHS. The strong increase in magnitude and the change in form of $H_{\mathrm{c}2\left|\right|\mathrm{c}}$ occur very close to the Van Hove strain, and point to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition.

We present a binary classifier to detect gapped quantum phases based on neural networks. By considering the errors on top of a suitable reference state describing the gapped phase, we show that a neural network trained on the errors can capture the correlation between the errors and can be used to detect the phase boundaries of the gapped quantum phase. We demonstrate the application of the method for matrix product state calculations for different quantum phases exhibiting local symmetry-breaking order, symmetry-protected topological order, and intrinsic topological order.

Inspired by the advancements in large language models based on transformers, we introduce the transformer quantum state (TQS): a versatile machine learning model for quantum many-body problems. In sharp contrast to Hamiltonian/task specific models, TQS can generate the entire phase diagram, predict field strengths with experimental measurements, and transfer such a knowledge to new systems it has never been trained on before, all within a single model. With specific tasks, fine-tuning the TQS produces accurate results with small computational cost. Versatile by design, TQS can be easily adapted to new tasks, thereby pointing towards a general-purpose model for various challenging quantum problems.

Stacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological edge states, imaging them has been instrumental in determining local variations in twisted bilayer graphene. Here, we analyze the mechanisms causing stacking domain boundary contrast in bright field low-energy electron microscopy (BF-LEEM) both for graphene on SiC, where domain boundaries are caused by strain, and for twisted few-layer graphene. We show that when domain boundaries are between the top two graphene layers, BF-LEEM contrast is observed due to amplitude contrast and corresponds well to calculations of the contrast based purely on the local stacking in the domain boundary. Conversely, for deeper-lying domain boundaries, amplitude contrast only provides a weak distinction between the inequivalent stackings in the domains themselves. However, for small domains, electrons from different parts of the unit cell interfere, causing a very strong phase contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials.

The massless Dirac equation is studied in curved space-time on the ($2+1$)-dimensional graphene sheet in time-dependent geometries. Emergent pseudogauge fields are found both in the adiabatic regime and, for high-frequency periodic geometries, in the nonadiabatic regime for a generic Friedmann-Lemaître-Robertson-Walker metric in Fermi normal coordinates. The former extends the conventionally understood homogeneous pseudogauge field to include weak temporal inhomogeneities. The latter, through the usage of Floquet theory, represents another class of emergent pseudogauge field and is argued to potentially provide a condensed matter realization of cosmological high-frequency geometries.

We grew needle-shaped single crystals of GdAgGe, which crystallizes in a noncentrosymmetric hexagonal crystal structure with space group $P\overline{6}$ $2m$ (189). The magnetic susceptibility data for $H \perp c$ reveal two pronounced antiferromagnetic transitions at $T_{N1}$ = 20 K and $T_{N2}$ = 14.5 K. The magnetic susceptibility anomalies are less prominent for $H \parallel c$. The transition at $T_{N1}$ is accompanied by a pronounced heat capacity anomaly confirming the bulk nature of the magnetic transition. Below $T_{N1}$, the electrical resistivity data follows a $T^{3/2}$ dependence. In the magnetically ordered state, GdAgGe shows positive transverse magnetoresistance, which increases with decreasing temperature and increasing field, reaching a value of $\sim 27\%$ at 9 T and 10 K. The Hall resistivity data and electronic band structure calculations suggest that both the hole and the electron charge carriers contribute to the transport properties. The electronic band structure displays linear band crossings near the Fermi level. The calculations reveal that GdAgGe has a nodal line with drumhead surface states coupled with a nonzero Berry phase, making it a nontrivial nodal-line semimetal.

We study the edge spectrum of twisted sheets of single layer and bilayer graphene in cases where the continuum model predicts a valley Chern insulator—an insulating state in which the occupied moiré mini-bands from each valley have a net Chern number, but both valleys together have no net Chern number, as required by time-reversal symmetry. In a simple picture, such a state might be expected to have chiral valley polarized counterpropagating edge states. We present results from exact diagonalization of the tight-binding model of commensurate structures in the ribbon geometry. We find that for both the single-layer and bilayer moiré ribbons robust edge modes are generically absent. We attribute this lack of edge modes to the fact that the edge induces valley mixing.

A nodal-line semimetal is a topological gapless phase containing one-dimensional degeneracies called nodal lines. The nodal lines are deformed by a continuous change of the system such as pressure and they can even change their topology, but it is not systematically understood what kind of changes of topology of nodal lines are possible. In this paper, we classify the events of topology change of nodal lines by the Morse theory and reveal that only three types of topology changes of nodal lines, i.e., creation, reconnection, and annihilation, are possible in the spinless nodal-line semimetal protected by inversion and time-reversal symmetries. They are characterized by an index having the values 0, 1, and 2 for the above three types in the Morse theory. Moreover, we extend our theory to systems with rotational symmetries and mirror symmetry and disclose the possible events of topology change of nodal lines under each symmetry.

We study the low-energy collective properties of a one-dimensional spin-1 Bose gas using bosonization. After giving an overview of the technique, emphasizing the physical aspects, we apply it to the $S=1$ Bose-Hubbard Hamiltonian and find a separation of the quadrupole-spin-charge sectors, confirmed by time–matrix-product states (time-MPS) numerical simulations. Additionally, through the single-particle spectrum, we show the existence of the superfluid–Mott-insulator transition and the point at which the physics are described by a Heisenberg-like Hamiltonian. The magnetic phase diagrams are found for both the superfluid and insulating regimes; the latter is determined by decomposing the complete Heisenberg bilinear-biquadratic Hamiltonian, which describes the Mott insulator, into simpler, effective Hamiltonians. This allows us to keep our methods flexible and transferable to other interesting interacting condensed matter systems.

The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction mediates collinear magnetic interactions via the conduction electrons of a nonmagnetic spacer, resulting in a ferro- or antiferromagnetic magnetization in magnetic multilayers. Recently it has been discovered that heavy nonmagnetic spacers are able to mediate an indirect magnetic coupling that is noncollinear and chiral. This Dzyaloshinskii-Moriya-enhanced RKKY interaction causes the emergence of a variety of interesting magnetic structures, such as skyrmions and spin spirals. Here, we show by spin-polarized scanning tunneling microscopy that the interchain coupling between manganese oxide chains on Ir(001) can reproducibly be switched from chiral to collinear antiferromagnetic by increasing the oxidation state of ${\mathrm{MnO}}_2$, while the reverse process can be induced by thermal reduction. The underlying structure–property relationship is revealed by low-energy electron diffraction intensity analysis. Density functional theory calculations suggest that the magnetic transition may be caused by a significant increase of the Heisenberg exchange which overrides the Dzyaloshinskii-Moriya interaction upon oxidation.

Interplay between magnetism and electronic band topology is of central current interest in topological matter research. We use quantum oscillations as a powerful tool to probe the evolution of band topology in the Dirac semimetal ${\mathrm{EuMnSb}}_2$. The Eu local $4f$ magnetic moments display different antiferromagnetic states below 25 K and a field-polarized phase above 16 T. Upon cooling from 65 K into the field-polarized state, an exotic temperature-dependent shift of oscillation peaks arises, accompanied by the development of nonzero Berry phase and a huge unconventional splitting of the oscillations. Band-structure calculations confirm the change from trivial to nontrivial band topology induced by the ferromagnetic Eu state, classifying ${\mathrm{EuMnSb}}_2$ as a unique magnetic topological semimetal.

Correlated-electron perovskite ${\mathrm{CaIrO}}_3$ shows the topologically protected line node near the Fermi level, wherein the high-mobility Dirac electrons reach the quantum limit (QL) with the one-dimensionally (1D) dispersive $n=0$ Landau level (LL) state at a moderate magnetic field. In the QL, the longitudinal magnetoresistance (MR) shows an extreme anisotropy against the field $\left(B\right)$ direction in spite of nearly isotropic Dirac band dispersions. The resistivity for $B\parallel a$ shows an insulating behavior with the MR ratio exceeding $2000\%$ around 18 T, whereas the $B\parallel c$ MR always remains metallic. This is accounted for in terms of the large difference of Fermi velocity of the $n=0$ LL between $B\parallel a$ and $B\parallel c$ due to the $B$ direction dependent $5d$ orbital electron hopping, which triggers the different instability toward the charge density wave formation for the 1D $(\parallel B)$ dispersive $n=0$ LL.

We investigate the possibility of a striped inhomogeneous phase occurring as an electronic system with an order parameter linearly coupled to the elastic degrees of freedom is tuned through the electronic phase transition. We find that in finite systems where boundary conditions create an elastic incompatibility, a stripe pattern may emerge in the vicinity of the electronic transition. Stripes are stabilized when the gained elastic energy overcomes the typical electronic and domain-wall energy costs. In thin-film geometries, the stripes are found to extend across the entire depth of the system. We also study the behavior of the phase fraction across the spinodal region of a first-order phase transition. The orientation of stripes and the possibility of more complicated patterns are also discussed.

Using local density correlation functions for a one-dimensional spin system, we introduce a correlation function difference (CFD) which compares correlations on a given site between a full system of size $L$ and its restriction to $\ell <L$ sites. We show that CFD provides useful information on transfer of information in quantum many-body systems by considering the examples of ergodic, Anderson, and many-body localized regimes in a disordered XXZ spin chain. In the ergodic phase, we find that the propagation of CFD is asymptotically faster than the spin transport but slower than the ballistic propagation implied by the Lieb-Robinson bound. In contrast, in the localized cases, we unravel an exponentially slow relaxation of CFD. Connections between CFD and other observables detecting nonlocal correlations in the system are discussed.