Phonon softening is a common identifier of strong coupling between electronic and vibrational degrees of freedom in materials, which is responsible for many of the exotic phenomena observed in scientifically interesting and technologically relevant materials. Two cubic rare-earth sesquioxides (RESO), ${\mathrm{Eu}}_2{\mathrm{O}}_3$ and ${\mathrm{Yb}}_2{\mathrm{O}}_3$, display anomalously soft phonons compared to trends in phonon energy exhibited by other cubic RESOs. In an effort to elucidate the mechanisms responsible for the anomalous mode softening in ${\mathrm{Eu}}_2{\mathrm{O}}_3$ and ${\mathrm{Yb}}_2{\mathrm{O}}_3$, we performed temperature- and magnetic field-dependent Raman scattering studies of polycrystalline samples of these materials. Our data suggest that two different underlying phenomena are responsible for these anomalies. While we observe crystal electric field (CEF) electronic transitions in both materials, only in ${\mathrm{Yb}}_2{\mathrm{O}}_3$ does the energetic proximity of CEF excitations to phonons appear to lead to significant electron-phonon coupling in the form of CEF-phonon coupling. We find evidence that defects—rather than interactions between vibrational and electronic excitations—are responsible for the soft phonons in ${\mathrm{Eu}}_2{\mathrm{O}}_3$. These results place constraints on proposed phonon mode softening mechanisms in rare-earth sesquioxides and illustrate the varied effects of electron-phonon coupling in these materials.

The presence of flat bands is a source of localization in lattice systems. While flat bands are often unstable with respect to interactions between the particles, they can persist in certain cases. We consider a diamond ladder with transverse hopping that possesses such stable flat bands and show that many-body localization appears in the presence of interactions without quenched disorder. We numerically demonstrate that the eigenstate thermalization hypothesis is violated. We show that the influence of degenerate flat band states spreads across the full energy spectrum and grows extensively in the thermodynamic limit. Furthermore, we verify localization in terms of time evolution of some local observable, revival probability, participation ratio, and entanglement entropy.

The penetration of atomic hydrogen through defect-free graphene was generally predicted to have a barrier of at least several eV, which is much higher than the 1-eV barrier measured for hydrogen-gas permeation through pristine graphene membranes. Herein, our density functional theory calculations show that ripples, which are ubiquitous in atomically thin crystals and mostly overlooked in the previous simulations, can significantly reduce the barriers for all steps constituting the mechanism of hydrogen-gas permeation through graphene membranes, including dissociation of hydrogen molecules, reconstruction of the dissociated hydrogen atoms and their flipping across graphene. Especially, the flipping barrier of hydrogen atoms from a cluster configuration is found to decrease rapidly down to $<1\mathrm{eV}$ with increasing ripples' curvature. The estimated hydrogen permeation rates by fully considering the distribution of ripples with all realistic curvatures and the major reaction steps that occurred on them are quite close to the experimental measurements. Our work provides insights into the fundamental understanding of hydrogen-gas permeation through graphene membranes and emphasizes the importance of nanoscale nonflatness (ripples) in explaining many surface and transport phenomena (for example, functionalization, corrosion and separation) in graphene and other two-dimensional materials.

Using many-body perturbation theory and density functional perturbation theory, we study the vibrational spectra of the internal stretch (IS) mode of CO on Pd(111) for the bridge and hollow adsorption structures that are experimentally identified at 0.5 ML coverage. Our theoretical treatment allows us to determine the temperature dependence of the IS vibrational spectra under thermal conditions as well as the time evolution of the nonequilibrium transient spectra induced by femtosecond laser pulses. Under thermal conditions (i.e., for equal electronic $T_e$ and phononic $T_l$ temperatures), the calculated lifetimes at 10–150 K are mostly due to nonadiabatic couplings (NCs), i.e., first-order electronic excitations. As temperature increases, also the contribution of the second-order electron-mediated phonon-phonon couplings (EMPPCs) progressively increases from 25% at low temperatures to 50% at 300 K. Our calculations for the laser-induced nonequilibrium conditions comprise experimental absorbed fluences of $6–130\mathrm{J}/{\mathrm{m}}^2$. For fluences for which $T_e>2000\mathrm{K}$, the transient vibrational spectra are characterized by two different regimes that follow the distinct time evolution of $T_e$ and $T_l$ and are respectively dominated by NC and EMPPC processes. At lower fluences, the initial fast regime becomes progressively negligible as $T_e$ decreases and only the steady second regime remains visible. Qualitatively, all these spectral properties are common to both of the adsorption structures studied here.

In the recently discovered heavy fermion superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, a magnetic-field-induced phase transition has been observed inside the superconducting state, which is proposed to be a transition from an even- to an odd-parity superconducting state. The odd-parity superconducting state and its large upper critical field has been explained by local inversion symmetry breaking and consequent Rashba spin-orbit coupling. Here, we report the experimental tuning of the superconductivity in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ via applied pressure. Superconductivity is continuously suppressed up to 2.5 GPa. The kink in the upper critical field, which has been used to indicated the even- to odd-parity transition, is also suppressed, indicating the odd-parity state is suppressed faster than the even-parity state. Above 2.5 GPa, there might be a second dome of superconductivity, which requires further investigation.

We investigate a semiconductor nanowire-based gatemon qubit with epitaxial Al on two facets of the nanowire, allowing gate control of wire density. Two segments have the Al removed, one forming a Josephson junction, and the other operating as a transistor, providing in situ switching between dc transport and qubit operation. Gating the nanowire (NW) changes the rate of decay of qubit frequency in magnetic field applied parallel to the NW. Gating the Josephson junction can lead additionally to nonmonotonic behavior of the qubit frequency in parallel magnetic field. A detailed model of the wire and junction yields behavior consistent with experiment, and allows us to propose that gating the bulk wire affects the rate of frequency decay in fields due to changes in the bulk potential distribution, while nonmonotonic behavior of the frequency is due to the interference of Andreev bound states in the junction. Gating the junction further allows access to the multimode regime, where fluctuations in qubit frequency are measured to be considerably smaller than the theoretical “universal” value, also smaller than numerics, and consistent with previous measurements of fluctuating critical currents.

Two-dimensional Dirac semimetals with a single massless Dirac cone exhibit a parity anomaly. Usually, such a kind of anomalous topological semimetallic phase in real materials is unstable where any amount of disorder can drive it into a diffusive metal and destroy the half-integer quantized Hall conductivity as an indicator of parity anomaly. Here, based on a low-energy effective model, we propose an anisotropic Dirac semimetal which explicitly breaks time-reversal symmetry and carries half-integer quantized Hall conductivity. This topological semimetallic phase can be realized on a deformed honeycomb lattice subjected to a magnetic flux. Moreover, we perceptively investigate the disorder correction to the Hall conductivity. The results show that the effects of disorder can be strongly suppressed and thereby the nearly half-integer quantization of Hall conductivity can exist in a wide region of disorder, indicating that our proposed anisotropic Dirac semimetal is an exciting platform to investigate the parity anomaly phenomena.

Efficient numerical methods are required for the design of optimized devices. In magnonics, the primary computational tool is micromagnetic simulations, which solve the Landau-Lifshitz equation discretized in time and space. However, their computational cost is high, and the complexity of their output hinders insight into the physics of the simulated system, especially in the case of multimode propagating-wave-based devices. We propose a finite-element modal method allowing an efficient solution of the scattering problem for dipole-exchange spin waves propagating perpendicularly to the magnetization direction. The method gives direct access to the scattering matrix of the whole system and its components. We extend the formula for the power carried by a magnetostatic mode in the Damon-Eshbach configuration to the case with exchange, allowing the scattering coefficients to be normalized to represent the fraction of the input power transferred to each output channel. We apply the method to the analysis of spin wave scattering on a basic functional block of magnonic circuits, consisting of a resonator dynamically coupled to a thin film. The results and the method are validated by comparison with micromagnetic simulations.

Layered superconductors like high-$T_c$ cuprates display out-of-plane plasma oscillations between layers sustained by the weak Josephson coupling among the superconducting sheets, the so-called Josephson plasmons. Bilayer cuprates hosts two of such modes. However, due to the anisotropy of the electronic response, their description at generic wavevector cannot be separated from that of the in-plane oscillations. In this paper, we provide an analytical theoretical framework to describe the dispersions and the polarizations of the generalized plasma modes of such systems, an aspect partly addressed in previous literature. We employ this framework to explain the peculiar characteristics of their linear optical response, by providing a fully microscopic explanation for the appearance of a finite-frequency peak in the real part of the optical conductivity. From a wider perspective, the complete characterization of the Josephson plasma modes provided by our approach represents a groundwork to address open issues raised by recent experiments with strong THz pulses, able to drive them beyond the linear-response regime.

We investigate the effect of Dzyaloshinskii-Moriya (DM) interactions in the planar pyrochlore (checkerboard) antiferromagnet, one of the paradigmatic models of spin frustration, and establish the classical phase diagram using a combination of analytical and numerical approaches. While anisotropic interactions generally tend to remove the frustration, here we show that a high degree of frustration survives in a large region of the phase diagram. In conjunction with the fixed handedness introduced by the DM anisotropy, this spawns a cascade of incommensurate and double-twisted multidomain phases, consisting of spatially intertwined domains of the underlying competing phases of the isotropic frustrated point. The results underpin an interesting mechanism of generating multi-$Q$ phases in systems that combine high degree of frustration and chiral interactions.

We propose a general procedure for extracting the running coupling constants of the underlying field theory of a given classical statistical model on a two-dimensional lattice, combining tensor network renormalization (TNR) and the finite-size scaling theory of conformal field theory. By tracking the coupling constants at each scale, we are able to visualize the renormalization group flow and demonstrate it with the classical Ising and three-state Potts models. Furthermore, utilizing this methodology, we reveal the limitations due to finite bond dimension $D$ on TNR applied to critical systems. We find that a finite correlation length is imposed by the finite bond dimension in TNR, and it can be attributed to an emergent relevant perturbation that respects the symmetries of the system. The correlation length shows the same power-law dependence on $D$ as the “finite entanglement scaling” of the matrix product states.

We propose a mesoscopic thermometer for ultrasensitive detection based on the proximity effect in superconductor–normal metal (SN) heterostructures. The device is based on the zero-bias anomaly due to the inelastic Cooper-pair tunneling in an SNIS junction (I stands for an insulator) coupled to an ohmic electromagnetic (EM) environment. The theoretical model is done in the framework of the quasiclassical Usadel Green's formalism and the dynamical Coulomb blockade. The usage of an ohmic EM environment makes the thermometer highly sensitive down to very low temperatures, $T\lesssim 5\mathrm{mK}$. Moreover, defined in this way, the thermometer is stable against small but nonvanishing voltage amplitudes typically used for measuring the zero-bias differential conductance in experiments. Finally, we propose a simplified view, based on an analytic treatment, which is in very good agreement with numerical results and can serve as a tool for the development, calibration, and optimization of such devices in future experiments in quantum calorimetry.

We address the out-of-equilibrium dynamics arising from quantum-quench (QQ) protocols (instantaneous changes of the Hamiltonian parameters) in many-body systems within their quantum critical regime and in contact with (homogeneously coupled) thermal baths. We consider two classes of QQ protocols. In one of them, the thermal bath is used to prepare the initial finite-temperature Gibbs state; then, after quenching, the thermal bath is removed and the dynamics of the system is unitary. We also address a more complex QQ protocol where the thermal bath is not removed after quenching, thus the quantum evolution is also driven by the interaction with the bath, which may be described by appropriate master equations for the density matrix of the system, where a further relevant time scale, or inverse decay rate, characterizes the system-bath coupling. Under these QQ protocols, the critical system develops out-of-equilibrium scaling behaviors, which extend those for isolated critical systems, by introducing further scaling variables proportional to the temperature of the thermal bath and the decay rate of the system-bath interactions. These out-of-equilibrium scaling behaviors are checked by analyzing QQ protocols within fermionic Kitaev wires, or equivalently quantum Ising chains, supplemented with a particular modelization of thermal bath that guarantees the asymptotic thermalization within the Lindblad master equation for the dynamics of open systems.

Bipolar ($n$- and $p$-type) electric conductivity control is at the heart of semiconductor technologies. However, achieving such control in ultrawide-band-gap semiconductors has been a major challenge because of the very high donor and/or acceptor binding energies of these materials. In the case of aluminum nitride (AlN), which is an ultrawide-band-gap semiconductor and one of the first candidate materials for solid-state deep-ultraviolet emitters, the substitutional magnesium (Mg) acceptor binding energy has been reported to be at least 500 meV; thus, $p$-type electric conductivity control in AlN by Mg doping is believed to be unfeasible. Here, we experimentally and theoretically revisit the substitutional Mg acceptor binding energy of AlN. Our bound exciton luminescence and impurity-related transition spectroscopic studies indicate that the substitutional Mg acceptor binding energy of AlN is well below 500 meV. This statement is supported by variational calculations using anisotropic hole effective masses derived from first-principles calculations. The three independent approaches estimate the substitutional Mg acceptor binding energy of AlN to be $330\pm 80\mathrm{meV}$. We find that considering electron-hole exchange interaction, hole anisotropy, and carrier-phonon coupling of AlN leads to a more realistic substitutional Mg acceptor binding energy.

Exploring topological phases in non-Hermitian systems has attracted significant recent attention. One intriguing question is how topological edge states compete with the non-Hermitian skin effect. Here we report the experimental observation of corner states in a two-dimensional nonreciprocal rhombus honeycomb electric circuit. We construct nonreciprocal and non-Hermitian circuits by introducing current-direction resolved capacitance between two nodes depending on the current direction. A skin effect thus emerges due to the nonreciprocity and prevails in dragging the corner state into the bulk. The non-Bloch winding number defined in the generalized Brillouin zone is adopted to characterize the topological phase transition. Interestingly, we find that the non-Bloch $Z_2$ Berry phase can serve as an invariant to describe the non-Hermitian topology. By tuning the nonreciprocal parameter, we observe unbalanced distribution of corner states emerging on two acute angles of the rhombus lattice, with the localization length of the left corner state increasing exponentially with the degree of nonreciprocity.

Metallic Kondo lattice systems that have been prepared to exhibit a competition between ordering of magnetic moments and shielding of those moments by the conduction electrons down to absolute zero display unusual low-temperature responses. Here we show that the dominant response of such systems is caused by two quantum effects: zero-point motion of the ions and the size of the system restricting the allowed wavelengths of fluctuations. This zero-point motion of the ions induces a broad distribution in Kondo shielding temperatures that renders the assumption of a uniform heavy-fermion ground state untenable. However, letting go of this assumption and instead incorporating these two quantum effects leads to percolation physics that quantitatively captures the non-Fermi liquid response in both stoichiometric and doped quantum critical compounds, allowing for a unified description of all quantum critical systems.

The pseudogap metal phase of hole-doped cuprates can be described by small Fermi surfaces of electronlike quasiparticles, which enclose a volume violating the Luttinger relation. This violation requires the existence of additional fractionalized excitations which can be viewed as fractionalized remnants of the paramagnon. We fractionalize the paramagnon into the bosonic spinons of the spin liquid described by the ${\mathbb{CP}}^1$ U(1) gauge theory, and we present a gauge theory of the bosonic spinons, a Higgs field, and an ancilla layer of fermions coupled to the original electrons. Along with the small Fermi surface pseudogap metal, this theory displays conventional phases: the large Fermi surface Fermi liquid with a low-energy paramagnon mode, and phases with spin density wave order. We describe the evolution of the electronic photoemission spectrum across these quantum phase transitions. We consider both the two-sublattice Néel and incommensurate spin density wave phases, and we find that the latter has spiral spin correlations.

We investigate quantum emitters (QEs) interacting with a photonic structured bath made of bilayer square lattices, where the resonance anticrossing between the energy bands opens a symmetric middle energy gap. Due to the intrinsic chiral symmetry of the bath and interactions with the squarelike band edges, the QE-photon dressed states generated in this inner band gap are odd neighbor, robust, and anisotropic, when the emitters' transition frequencies lie in the middle of the band gap. We also use giant artificial atoms to engineer and modify the dressed states' patterns. Exotic bound states can lead to spin models with symmetry protection, resulting in fascinating many-body phases. As an example, we show that this proposal can be used to generate both edge states and corner states in the generalized two-dimensional Su-Schrieffer-Heeger model. This work opens up different avenues for research into innovative quantum many-body physics and quantum simulations with photonic or phononic multilayer structures.

Non-Hermitian quasicrystal forms a unique class of matter with symmetry-breaking, localization and topological transitions induced by gain and loss or nonreciprocal effects. In this work, we introduce a non-Abelian generalization of non-Hermitian quasicrystal, in which the interplay between non-Hermitian effects and non-Abelian quasiperiodic potentials create mobility edges and rich transitions among extended, critical and localized phases. These generic features are demonstrated by investigating three non-Abelian variants of the non-Hermitian Aubry-André-Harper model. A unified characterization is given to their spectrum, localization, entanglement and topological properties. Our findings thus add new members to the family of non-Hermitian quasicrystal and uncover unique physics that can be triggered by non-Abelian effects in non-Hermitian systems.