Delafossites represent natural heterostructures which can host rather different electronic characteristics in their constituting layers. The design of heterostructure architectures highlighting the competition between such varying layer properties is promising from the viewpoint of basic research as well as for future technological applications. By means of the combination of density functional theory and dynamical mean-field theory, we here unveil the formation of highly correlated electron states in delafossite heterostructures built from metallic ${\mathrm{PdCrO}}_2$ and insulating ${\mathrm{AgCrO}}_2$. Due to the sophisticated coupling between layers of strong and of weak internal electron-electron interaction, correlation-induced semimetals at ambient temperature and doped Mott insulators at lower temperature are predicted. The unique electronic structure of delafossite heterostructures opens a door to research on challenging quantum matter.

The topological $p$-wave pairing of composite fermions, believed to be responsible for the 5/2 fractional quantum Hall effect (FQHE), has generated much exciting physics. Motivated by the parton theory of the FQHE, we consider the possibility of a new kind of emergent “superconductivity” in the 1/3 FQHE, which involves condensation of clusters of $n$ composite bosons. From a microscopic perspective, the state is described by the $n\overline{n}111$ parton wave function ${\mathcal{P}}_{\mathrm{LLL}}{\mathrm{\Phi }}_n{\mathrm{\Phi }}_n^{*}{\mathrm{\Phi }}_1^3$, where ${\mathrm{\Phi }}_n$ is the wave function of the integer quantum Hall state with $n$ filled Landau levels and ${\mathcal{P}}_{\mathrm{LLL}}$ is the lowest-Landau-level projection operator. It represents a ${\mathbb{Z}}_n$ superconductor of composite bosons, because the factor ${\mathrm{\Phi }}_1^3\sim {\prod }_{j<k}{(z_j-z_k)}^3$, where $z_j=x_j-i{y}_j$ is the coordinate of the $j\mathrm{th}$ electron, binds three vortices to electrons to convert them into composite bosons, which then condense into the ${\mathbb{Z}}_n$ superconducting state $|{\mathrm{\Phi }}_n{|}^2$. From a field theoretical perspective, this state can be understood by starting with the usual Laughlin theory and gauging a ${\mathbb{Z}}_n$ subgroup of the U(1) charge conservation symmetry. We find from detailed quantitative calculations that the $2\overline{2}111$ and $3\overline{3}111$ states are at least as plausible as the Laughlin wave function for the exact Coulomb ground state at filling $\nu =7/3$, suggesting that this physics is possibly relevant for the 7/3 FQHE. The ${\mathbb{Z}}_n$ order leads to several observable consequences, including quasiparticles with fractionally quantized charges of magnitude $e/\left(3n\right)$ and the existence of multiple neutral collective modes. It is interesting that the FQHE may be a promising venue for the realization of exotic ${\mathbb{Z}}_n$ superconductivity.

Electrical switching and readout of antiferromagnets allows to exploit the unique properties of antiferromagnetic materials in nanoscopic electronic devices. Here we report experiments on the spin-orbit torque induced electrical switching of a polycrystalline, metallic antiferromagnet with low anisotropy and high Néel temperature. We demonstrate the switching in a Ta/MnN/Pt trilayer system, deposited by (reactive) magnetron sputtering. The dependence of switching amplitude, efficiency, and relaxation are studied with respect to the MnN film thickness, sample temperature, and current density. Our findings are consistent with a thermal activation model and resemble to a large extent previous measurements on CuMnAs and ${\mathrm{Mn}}_2\mathrm{Au}$, which exhibit similar switching characteristics due to an intrinsic spin-orbit torque.

Much has been learned about universal properties of entanglement entropy (EE) and participation ration (PR) for Anderson localization. We find a new subextensive scaling with system size of the above measures for algebraic localization as noticed in one-dimensional long-range hopping models in the presence of uncorrelated disorder. While the scaling exponent of EE seems to vary universally with the long distance localization exponent of single particle states (SPSs), PR does not show such university as it also depends on the short-range correlations of SPSs. On the other hand, in the presence of correlated disorder, an admixture of two species of SPSs (ergodic delocalized and nonergodic multifractal or localized) are observed, which leads to extensive (subextensive) scaling of EE (PR). Considering typical many-body eigenstates, we obtain above results that are further corroborated with the asymptotic dynamics. Additionally, a finite time secondary slow growth in EE is witnessed only for correlated case while for the uncorrelated case there exists only primary growth followed by the saturation. We believe that our findings from the typical many-body eigenstate would remain unaltered even in the weakly interacting limit.

Motivated by recent progress on field-induced phase transitions in quasi-one-dimensional quantum antiferromagnets, we study the phase diagram of spin-$1/2$ Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations. The interchain interaction is shown to enhance longitudinal spin correlations and stabilize an incommensurate longitudinal spin density wave order at intermediate fields. With increasing field, the ground state changes to a canted antiferromagnetic order and the magnetization fully saturates above a quantum critical point, which is shown to fall into the $(3+2)\mathrm{D}$ $XY$ universality. In the quantum critical regime, the system experiences a fascinating dimensional crossover to a universal Tomonaga-Luttinger liquid (TLL) as demonstrated by the continuous change of the critical exponent of the specific heat. The TLL behavior extends to a broad field and temperature regime, as characterized by the calculated NMR relaxation rate $1/T_1$. Our results not only explain a number of puzzling observations in a quasi-one-dimensional antiferromagnet ${\mathrm{YbAlO}}_3$ but also lay down a concrete ground to the study on these materials in general.

We show that, although the equilibrium band dispersion of the Shockley-type surface state of two-dimensional Au(111) quantum films grown on W(110) does not deviate from the expected free-electron-like behavior, its nonequilibrium energy-momentum dispersion probed by time- and angle-resolved photoemission exhibits a remarkable kink above the Fermi level due to a significant enhancement of the effective mass. The kink is pronounced for certain thicknesses of the Au quantum well and vanishes in the very thin limit. We identify the kink as induced by the coupling between the Au(111) surface state and emergent quantum-well states which probe directly the buried gold-tungsten interface. The signatures of the coupling are further revealed by our time-resolved measurements which show that surface state and quantum-well states thermalize together behaving as dynamically locked electron populations. In particular, relaxation of hot carriers following laser excitation is similar for both surface state and quantum-well states and much slower than expected for a bulk metallic system. The influence of quantum confinement on the interplay between elementary scattering processes of the electrons at the surface and ultrafast carrier transport in the direction perpendicular to the surface is shown to be the reason for the slow electron dynamics.

We theoretically calculate the impurity-scattering-induced resistivity of twisted bilayer graphene at low twist angles where the graphene Fermi velocity is strongly suppressed. We consider, as a function of carrier density, twist angle, and temperature, both long-ranged Coulomb scattering and short-ranged defect scattering within a Boltzmann theory relaxation time approach. For experimentally relevant disorder, impurity scattering contributes a resistivity comparable to (much larger than) the phonon scattering contribution at high (low) temperatures. Decreasing twist angle leads to larger resistivity, and in general, the resistivity increases (decreases) with increasing temperature (carrier density). Inclusion of the van Hove singularity in the theory leads to a strong increase in the resistivity at higher densities, where the chemical potential is close to a van Hove singularity, leading to an apparent density-dependent plateau-type structure in the resistivity, which has been observed in recent transport experiments. We also show that the Matthiessen's rule is strongly violated in twisted bilayer graphene at low twist angles.

We expand the Cooper problem by including a third electron in an otherwise empty band. We demonstrate the formation of a trimer state of two electrons above the Fermi sea and the third electron, for sufficiently strong interband attractive interaction. We show that the critical interaction strength is the lowest for small Fermi velocities, large masses of the additional electron, and large Debye energy. This trimer state competes with the formation of the two-electron Cooper pair, and can be created transiently via optical pumping.

We develop a nonperturbative approach for calculating the superconducting transition temperatures ($T_c$'s) of liquids. The electron-electron scattering amplitude induced by electron-phonon coupling (EPC), from which an effective pairing interaction can be inferred, is related to the fluctuation of the $T$ matrix of electron scattering induced by ions. By applying the relation, EPC parameters can be extracted from a path-integral molecular dynamics simulation. For determining $T_c$, the linearized Eliashberg equations are reestablished nonperturbatively. We apply the approach to estimate $T_c$'s of metallic hydrogen liquids. It indicates that metallic hydrogen liquids in the pressure regime from $0.5$ to $1.5\mathrm{TPa}$ have $T_c$'s well above their melting temperatures and therefore are superconducting liquids.

Real-space separations of countermoving states to opposite surfaces or edges are associated with different types of Hall effects, such as the quantum, spin, or the anomalous Hall effect. Some systems provide the possibility to separate a fraction of countermovers in a completely different fashion: surface states propagating all in the same direction, balanced by countermoving bulk states, realized, e.g., in Weyl metals with intrinsically or extrinsically broken inversion and time-reversal symmetries. In this Rapid Communication we show that these copropagating surface modes are associated with a specific Hall effect—a parabolic potential profile in the direction perpendicular to and in its magnitude linear in the applied field. While in two-dimensional (2D) systems the parabolic potential profile is directly measurable, in 3D the resulting voltage between the bulk and surface is measurable in the geometry of a hollow cylinder. Moreover, the parabolic Hall effect leads to characteristic signatures in the longitudinal conductivity.

Motivated by a recent experiment [A. Marguerite et al., Nature (London) 575, 628 (2019)] on imaging in graphene samples, we investigate theoretically the dissipation induced by resonant impurities in the quantum Hall regime. The impurity-induced forward scattering of electrons at quantum Hall edges leads to an enhanced phonon emission, which reaches its maximum when the impurity state is tuned to resonance by a scanning tip voltage. Our analysis of the effect of the tip potential on the dissipation reveals peculiar thermal rings around the impurities, consistent with experimental observations. Remarkably, this impurity-induced dissipation reveals nontrivial features that are unique for chiral one-dimensional systems such as quantum Hall edges. First, the dissipation is not accompanied by the generation of resistance. Second, this type of dissipation is highly nonlocal: A single impurity induces heat transfer to phonons along the whole edge.

Ultrafast techniques have emerged as promising methods to study and control quantum materials. To maintain the quantum nature of the systems under study, excess heating must be avoided. In this paper, we demonstrate a method that employs the nonequilibrium laser excitation of planar-stretching optical phonons in tetragonal systems to quench an electronic nematic state across a quantum phase transition. Appropriately tuned off-resonant pulses can perform a quantum quench of the system either into the nematic phase (red detuning) or out of it (blue detuning). The nonlinear coupling of this phonon mode to nematicity not only mediates interactions in the nematic channel, but it also suppresses heating effects. We illustrate the applicability of our general results by considering the microscopic parameters of the nematic unconventional superconductor FeSe.

The tunability of the interlayer coupling by twisting one layer with respect to another layer of two-dimensional materials provides a unique way to manipulate the phonons and related properties. We refer to this engineering of phononic properties as twistnonics. We study the effects of twisting on low-frequency shear modes (SMs) and layer breathing modes in a transition-metal dichalcogenide (TMD) bilayer using atomistic classical simulations. We show that these low-frequency modes are extremely sensitive to twisting and can be used to infer the twist angle. We find ultrasoft phason modes (frequency $\lesssim 1{\mathrm{cm}}^{-1}$, comparable to acoustic modes) for any nonzero twist, corresponding to an effective translation of the moiré lattice by relative displacement of the constituent layers in a nontrivial way. Unlike the acoustic modes, the velocity of the phason modes are quite sensitive to the twist angle. Also, high-frequency SMs appear for small twist angles, identical to those in stable bilayer TMD $(\theta =0^{\circ }$ or ${60}^{\circ })$, due to the overwhelming growth of stable stacking regions in relaxed twisted structures. Our study reveals the possibility of an intriguing $\theta$-dependent superlubric to pinning behavior and of the existence of ultrasoft modes in all two-dimensional materials.

We point out the experimental relevance and the detection scheme of symmetry-enriched U(1) quantum spin liquids (QSLs) outside the perturbative spin-ice regime. Recent experiments on Ce-based pyrochlore QSL materials suggest that the candidate QSL may not be proximate to the well-known spin-ice regime, and thus differs fundamentally from other pyrochlore QSL materials. We consider the possibility of the $\pi$-flux U(1) QSL favored by frustrated transverse exchange interactions rather than the usual quantum spin ice. It was previously suggested that both dipolar U(1) QSL and octupolar U(1) QSL can be realized for the generic spin model for the dipole-octupole doublets of the ${\mathrm{Ce}}^{3+}$ local moments on the pyrochlore magnets ${\mathrm{Ce}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ and ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. We explain and predict the experimental signatures, especially the magnetic field response of the octupolar $\pi$-flux U(1) QSL. Fundamentally, this remarkable state is a mixture of symmetry enrichments from point-group symmetry and from translational symmetry. We discuss the relevant experiments for pyrochlore U(1) QSLs and further provide some insights to the pyrochlore Heisenberg model.

Active A-site cations like Bi or Pb in ${\mathrm{ABO}}_3$ perovskites are known for their valence-skipping nature as well as for the stereochemical activities connected to lone pairs. While the former gives rise to breathing distortions in compounds like ${\mathrm{BiNiO}}_3$ or ${\mathrm{PbCrO}}_3$, the latter has been held responsible for ferroelectric, polar distortions in compounds like ${\mathrm{BiFeO}}_3$ or ${\mathrm{PbVO}}_3$. The microscopic origins of both distortions have been argued to stem from the hybridization between Bi(Pb) and O. Employing first-principles calculations, together with a variational solution of the first-principles-inspired model Hamiltonian, we investigate the interplay of two types of distortion instabilities in Bi/Pb-based transition metal perovskites. Our study reveals that in the absence of orbital degeneracy of the B site, the preference of one over the other is dictated by the relative positioning of O $2p$ level with respect to the A-site $6p$ level. Closeness of the two levels favors polar distortion over the breathing and vice versa, level positioning of O $2p$ being dictated by the strength of B-O hybridization.

The hitherto-studied topological nature in Ce monopnictides ($\mathrm{Ce}Pn$) has been discussed based solely on their $p\text{−}d$ band inversion, despite the existence of $f$ electrons. Interestingly, however, we have found that CeN, the lightest $\mathrm{Ce}Pn$, has nontrivial $Z_2$ topology originating from $f\text{−}d$ band inversion. Through density-functional theory and dynamical mean-field theory calculations, we have demonstrated that, among the correlated $f$-electron systems of $\mathrm{Ce}Pn$, the lightest CeN is a coherent narrow $f$-band system having an $f\text{−}d$ band-inverted topological Kondo insulator nature, whereas the heaviest CeBi is a strongly localized $f$-electron system having a conventional $p\text{−}d$ band-inverted topological insulator nature. For CeN, topological surface states (TSSs) are identified clearly on three different (001), (110), and (111) surfaces. For CeBi, however, TSSs are identified only on the (111) surface. Furthermore, intriguing topological-crystalline insulator-type TSSs are identified on the (110) surfaces of both CeN and CeBi.

A gel consists of a network of particles or molecules formed for example using the sol-gel process, by which a solution transforms into a porous solid. Particles or molecules in a gel are mainly organized on a scaffold that makes up a porous system. Quantized vortices in type-II superconductors mostly form spatially homogeneous ordered or amorphous solids. Here we present high-resolution imaging of the vortex lattice displaying dense vortex clusters separated by sparse or entirely vortex-free regions in $\beta \text{−}{\mathrm{Bi}}_2\mathrm{Pd}$ superconductor. We find that the intervortex distance diverges upon decreasing the magnetic field and that vortex lattice images follow a multifractal behavior. These properties, characteristic of gels, establish the presence of a novel vortex distribution, distinctly different from the well-studied disordered and glassy phases observed in high-temperature and conventional superconductors. The observed behavior is caused by a scaffold of one-dimensional structural defects with enhanced stress close to the defects. The vortex gel might often occur in type-II superconductors at low magnetic fields. Such vortex distributions should allow to considerably simplify control over vortex positions and manipulation of quantum vortex states.

The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable $\mathrm{Z}{\mathrm{r}}_{1-x}\mathrm{A}{\mathrm{l}}_x\mathrm{N}(x=0.0$, 0.12, 0.26, 0.40) are investigated by resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to first principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first principles all-electron full-potential calculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity and element selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develops from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.

Quasiparticle states in Dirac systems with complex impurity potentials are investigated. It is shown that an impurity site with loss leads to a nontrivial distribution of the local density of states (LDOS). While the real part of defect potential induces a well-pronounced peak in the density of states (DOS), the DOS is either weakly enhanced at small frequencies or even forms a peak at the zero frequency for a lattice in the case of non-Hermitian impurity. As for the spatial distribution of the LDOS, it is enhanced in the vicinity of impurity but shows a dip at a defect itself when the potential is sufficiently strong. The results for a two-dimensional hexagonal lattice demonstrate the characteristic trigonal-shaped profile for the LDOS. The latter acquires a double-trigonal pattern in the case of two defects placed at neighboring sites. The effects of non-Hermitian impurities could be tested both in photonic lattices and certain condensed matter setups.

We present a quantum field theoretical method for the characterization of disordered complex media with short laser pulses in an optical coherence tomography setup (OCT). We solve this scheme of coherent transport in space and time with weighted essentially nonoscillatory methods (WENO). WENO is preferentially used for the determination of highly nonlinear and discontinuous processes including interference effects and phase transitions like Anderson localization of light. The theory determines spatiotemporal characteristics of the scattering mean free path and the transmission cross section that are directly measurable in time-of-flight (ToF) and pump-probe experiments. The results are a measure of the coherence of multiple scattering photons in passive as well as in optically soft random media. Our theoretical results of ToF are instructive in spectral regions where material characteristics such as the scattering mean free path and the diffusion coefficient are methodologically almost insensitive to gain or absorption and to higher-order nonlinear effects. Our method is applicable to OCT and other advanced spectroscopy setups including samples of strongly scattering mono- and polydisperse complex nano- and microresonators.