Gadolinium has long been believed to undergo a high-pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended $s$-orbital to the compact $f$-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the $f$-electrons [Fabbris et al., Phys. Rev. B 88, 245103 (2013)]. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high-pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using x-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes $hcp\to 9R\to \mathit{dhcp}\to \mathit{fcc}\to \mathit{d}-\mathit{fcc}$ is reproduced. The so-called “volume-collapsed” high-pressure phase is shown to be an unusual stacking of close-packed planes, with $\mathit{Fddd}$ symmetry and a density change of less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the $f$-electrons. The key result is that there is no XRD evidence for volume collapse in gadolinium. The sequence of phase transitions is well described by standard density functional theory. There is no need for special treatment of the $f$-electrons or evidence of $f$-electron bonding. Noting that in previous spectroscopic evidence there is no change in the $f$-electrons we conclude that high-pressure gadolinium has no complicated $f$-electron physics such as Mott-Hubbard, Kondo, or valence transitions.

The alternating current (ac) Josephson effect in a time-independent spatially inhomogeneous setting is manifested by the occurrence of Josephson oscillations—periodic macroscopic phase-induced collective motions of the quantum condensate. So far, this phenomenon was observed at cryogenic temperatures in superconductors, in superfluid helium, and in Bose-Einstein condensates (BECs) of trapped atoms. Here, we report on the discovery of the ac Josephson effect in a magnon BEC carried by a room-temperature ferrimagnetic film. The BEC is formed in a parametrically populated magnon gas in the spatial vicinity of a magnetic trench created by a dc electric current. The appearance of the Josephson effect is manifested by oscillations of the magnon BEC density in the trench, caused by a coherent phase shift between this BEC and the BEC in the nearby regions. Our findings advance the physics of room-temperature macroscopic quantum phenomena and will allow for their application for data processing in magnon spintronics devices.

Modification of the electronic and crystal structure of zero-layer graphene grown on $6H$-SiC(0001) after Co intercalation is reported. Using a wide range of techniques including angle-resolved photoelectron spectroscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, low-energy electron diffraction, we found that zero-layer graphene on SiC transforms into graphene monolayer as a result of cobalt intercalation. The Dirac cone of $\pi$ band characteristic of quasi-freestanding graphene is observed. In combination with high-resolution transmission electron microscopy and atomic force microscopy data, we conclude that ultrathin silicide $\mathrm{CoSi}\text{/}{\mathrm{CoSi}}_2$ structure is formed between graphene and SiC substrate. Investigation of magnetic properties reveals ferromagnetic behavior with open hysteresis loop. The results of this work are the basis for further implementation of magneto-spin-orbit graphene on a semiconducting substrate and are important for the future application of such graphene in spintronics.

Inelastic Cooper pair tunneling across a voltage-biased Josephson junction in series with one or more microwave cavities can generate photons via resonant processes in which the energy lost by the Cooper pair matches that of the photon(s) produced. We generalize previous theoretical treatments of such systems to analyze cases where two or more different photon generation processes are resonant simultaneously. We also explore in detail a specific case where the generation of a single photon in one cavity mode is simultaneously resonant with the generation of two photons in a second mode. We find that the coexistence of the two resonances leads to effective couplings between the modes which in turn generate entanglement.

We combined photoelectron spectroscopy with first-principles calculations to investigate electronic properties of ${\mathrm{SrTiO}}_3$ doped with Ni impurities. High-quality epitaxial pristine ${\mathrm{SrTiO}}_3$ and ${\mathrm{SrTiO}}_3$:${\mathrm{Ni}}_x$ films with $x=0.06$ and 0.12 were prepared by pulsed laser deposition. Electronic band structure calculations for the ground state, as well as one-step model photoemission calculations, which were obtained by means of the Korringa-Kohn-Rostoker Green's function method, predict the formation of localized $3d$-impurity bands in the band gap of ${\mathrm{SrTiO}}_3$ close to the valence band maxima. The measured valence bands at the resonance Ni $2p$ excitation and band dispersion confirm these findings.

Unconventional superconductivity recently observed in twisted bilayer graphene is associated with the presence of van-Hove singularities very close to the Fermi level reflecting the flattening of bands for a set of magic twist angles. In this paper, we address a stack of two identical quasi-one-dimensional layers, each one composed of a set of chains with $p$-wave orbitals at each site. When the layers are stacked with a ${90}^{\circ }$ relative angle, the bilayer system resembles the Mielke lattice (which admits one exact flat band in the one-body tight-binding model for particular values of the hopping parameters). When a small rotation is applied to one of the layers, regions with different layer stacking appear that may be characterized as one-dimensional or two-dimensional regions according to the most relevant hopping integrals between layers. The system, for sizes smaller or of the order of the Moiré pattern unit cell, can be qualitatively described: (i) addressing individually each region, for example in what concerns the density of states, (ii) interpreting the full lattice as a coupled system of these regions. This generates a $n$-band model, where each band is associated to a particular region of the lattice. We address the role of these different regions on the upper critical field transition curve of a superconducting phase.

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field B${}_{\mathrm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim 0.5$ K, and magnetic fields, which went up to $\sim 38$ T along the $c$ axis and in the ab plane. In the high-field region parallel to the ab plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state.

We show that in inverted HgTe quantum wells, the application of a strong in-plane magnetic field restores the normal sequence of the subbands, thus transforming the system from a topological insulator into a normal insulator phase. The transformation consists of two consecutive quantum phase transitions due to the large Zeeman splitting of electron states. On the other hand, due to the negligible Zeeman splitting of hole states, the intermediate phase between the transitions is semimetallic. Both phase transitions and the semimetallic phase have already been observed, and our theory allows for a consistent interpretation of the experimental results.

We study heating dynamics in isolated quantum many-body systems driven periodically at high frequency and large amplitude. Combining the high-frequency expansion for the Floquet Hamiltonian with Fermi's golden rule (FGR), we develop a master equation termed the Floquet FGR. Unlike the conventional one, the Floquet FGR correctly describes heating dynamics, including the prethermalization regime, even for strong drives, under which the Floquet Hamiltonian is significantly dressed, and nontrivial Floquet engineering is present. The Floquet FGR depends on system size only weakly, enabling us to analyze the thermodynamic limit with small-system calculations. Our results also indicate that, during heating, the system approximately stays in the thermal state for the Floquet Hamiltonian with a gradually rising temperature.

The recent scanning tunneling microscopy (STM) observation of $\mathsf{U}$-shaped and $\mathsf{V}$-shaped spectra (and their mixture) in superconducting ${\mathrm{Nd}}_{1\text{−}x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films has been interpreted as the presence of two distinct gap symmetries in this nickelate superconductor [Gu et al., Nat. Commun. 11, 6027 (2020)]. Here, using a two-band model of nickelates capturing dominant contributions from Ni-$3d_{x^2\text{−}{y}^2}$ and rare-earth (R)-$5d_{3z^2\text{−}{r}^2}$ orbitals, we show that the experimental observation can be simply explained within a pairing scenario characterized by a conventional $d_{x^2\text{−}{y}^2}$-wave gap structure with the lowest harmonic on the Ni band and a $d_{x^2\text{−}{y}^2}$-wave gap with higher harmonics on the R band. We perform realistic simulations of STM spectra employing first-principles Wannier functions to properly account for the tunneling processes and obtain $\mathsf{V},\mathsf{U},$ and mixed spectral line shapes depending on the position of the STM tip within the unit cell. The $\mathsf{V}$- and $\mathsf{U}$-shaped spectra are contributed by Ni and R bands, respectively, and Wannier functions, in essence, provide position-dependent weighting factors, determining the spectral line shape at a given intra-unit-cell position. We propose a phase-sensitive experiment to distinguish between the proposed $d$-wave gap structure and the time-reversal symmetry-breaking $d+is$ gap which yields very similar intra-unit-cell spectra.

The chirality-induced spin selectivity (CISS) effect enables the detection of chirality as electrical charge signals. It is often studied using a two-terminal circuit geometry where a ferromagnet is connected to a chiral component, and a change of electrical resistance is reported upon magnetization reversal. This is however not expected in the linear response regime because of compensating reciprocal processes, limiting the interpretation of experimental results. Here we show that magnetoresistance effects can indeed appear even in the linear response regime, either by changing the magnitude or the direction of the magnetization or an applied magnetic field. We illustrate this in a spin-valve device and in a chiral thin film as the CISS-induced Hanle magnetoresistance (CHMR) effect. This effect helps to distinguish spin-transport-related effects from other effects, and can thereby provide further insight into the origin of CISS.

Topology-driven nonlinear light-matter effects open up new paradigms for both topological photonics and nonlinear optics. Here, we propose to achieve high-efficiency second harmonic generation in a second-order photonic topological insulator. Such a system hosts highly localized topological corner states with large quality factors for both fundamental and second harmonic waves, which could be matched perfectly in frequency by simply tuning the structural parameters. Through the nonlinear interaction of the doubly resonant topological corner states, unprecedented frequency conversion efficiency is predicted. In addition, the robustness of the nonlinear process against defects is also demonstrated. Our work opens up avenues toward topologically protected nonlinear frequency conversion using high-order photonic topological modes.

We analyze the conductance of a one-dimensional topological superconductor periodically driven to host Floquet Majorana zero modes for different configurations of coupling to external leads. We compare the conductance of constantly coupled leads, as in standard transport experiments, with the stroboscopic conductance of pulsed coupling to leads used to identify a scattering matrix topological index for periodically driven systems. We find that the sum of the DC conductance at voltages corresponding to integer multiples of the driving frequency is quantitatively close to the stroboscopic conductance at all voltage biases. This is consistent with previous work which indicated that the summed conductance at $\mathrm{zero}/\pi$ resonances is quantized. We quantify the difference between the two in terms of the widths of their respective resonances and analyze that difference for two different stroboscopic driving protocols of the Kitaev chain. While the quantitative differences are protocol dependent, we find that generically the discrepancy is larger when the zero-mode weight at the end of the chain depends strongly on the offset time between the driving cycle and the pulsed coupling period.

Novel compounds with two-dimensional square lattices formed by group IV or V elements (Si, Sn, Ge, Bi, and Sb) have been attracting a great deal of attention in recent times, mainly because of the possible emergence of various topological phases therein. Here, we successfully grow the single crystals of Sn-square-net based material $\mathrm{Sc}{\mathrm{Sn}}_2$, and systematically perform their magnetization and magnetotransport measurements. Clear quantum oscillations emerge in the magnetization isotherms along different field orientations, from which nonzero Berry phases are extracted, implying that $\mathrm{Sc}{\mathrm{Sn}}_2$ harbors three-dimensional Fermi surfaces and nontrivial electronic states. Similar to many other topological semimetals with extremely large magnetoresistance, $\mathrm{Sc}{\mathrm{Sn}}_2$ shows field-induced resistivity enhancement as well, which has been proven to be not of a gap opening origin. Besides, at low temperature, large magnetoresistance with a quasilinear field dependence is observed. Our analysis of the magnetotransport data finds that the quasilinear magnetoresistance in $\mathrm{Sc}{\mathrm{Sn}}_2$ cannot be understood by several familiar mechanisms proposed in the literature. These findings are expected to have far reaching implications for both the fundamental understanding and magnetoresistance device application of topological semimetal materials.

Floquet engineering of isolated systems is often based on the concept of the effective time-independent Floquet Hamiltonian, which describes the stroboscopic evolution of a periodically driven quantum system in steps of the driving period and which is routinely obtained analytically using high-frequency expansions. The generalization of these concepts to open quantum systems described by a Markovian master equation of Lindblad type turns out to be nontrivial: On the one hand, already for a two-level system two different phases can be distinguished, where the effective time-independent Floquet generator (describing the stroboscopic evolution) is either again Markovian and of Lindblad type or not. On the other hand, even though in the high-frequency regime a Lindbladian Floquet generator (Floquet Lindbladian) is numerically found to exist, this behavior is, curiously, not correctly reproduced within analytical high-frequency expansions. Here, we demonstrate that a proper Floquet Lindbladian can still be obtained from a high-frequency expansion, when treating the problem in a suitably chosen rotating frame. Within this approach, we can then also describe the transition to a phase at lower driving frequencies, where no Floquet Lindbladian exists, and show that the emerging non-Markovianity of the Floquet generator can entirely be attributed to the micromotion of the open driven system.

We present a diagrammatic Monte Carlo study of a lattice polaron interacting with an acoustic phonon branch through the deformation potential. Weak and strong coupling regimes are separated by a self-trapping region where quantum resonance between various possible lattice deformations is seen in the ground-state properties, spectral function, and optical conductivity. This study shows that the acoustic lattice polaron represents a distinct quantum object with unique features, markedly different from any previously considered polaron model. In particular, the acoustic lattice polaron exhibits an interplay between long- and short wavelength acoustic vibrations, resulting in a composite phonon cloud which leads to the formation of multiple competing polaron states with a complex spectral response.