We develop a renormalization group (RG) description of the localization properties of one-dimensional (1D) quasiperiodic lattice models. The RG flow is induced by increasing the unit cell of subsequent commensurate approximants. Phases of quasiperiodic systems are characterized by RG fixed points associated with renormalized single-band models. We identify fixed points that include many previously reported exactly solvable quasiperiodic models. By classifying relevant and irrelevant perturbations, we show that the phase boundaries of more generic models can be determined with exponential accuracy in the approximant's unit cell size, and in some cases analytically. Our findings provide a unified understanding of widely different classes of 1D quasiperiodic systems.

The salient property of the electronic band structure of twisted bilayer graphene (TBG), at the so-called magic angle (MA), is the emergence of flat bands around the charge neutrality point. These bands are associated with the observed superconducting phases and the correlated insulating states. Scanning tunneling microscopy combined with angle resolved photoemission spectroscopy are usually used to visualize the flatness of the band structure of TBG at the MA. Here, we theoretically argue that spin pumping (SP) provides a direct probe of the flat bands of TBG and an accurate determination of the MA. We consider a junction separating a ferromagnetic insulator and a heterostructure of TBG adjacent to a monolayer of a transition metal dichalcogenide. We show that the Gilbert damping of the ferromagnetic resonance experiment, through this junction, depends on the twist angle of TBG, and exhibits a sharp drop at the MA. We discuss the experimental realization of our results which open the way to a twist switchable spintronics in twisted van der Waals heterostructures.

Strain-induced transitions of polarization reversal in thin films of a ferrielectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ (CIPS) with ideally conductive electrodes are explored using the Landau-Ginzburg-Devonshire approach with an eighth-order free energy expansion in polarization powers. Due to multiple potential wells, the height and position of which are temperature and strain dependent, the energy profiles of CIPS can flatten in the vicinity of the nonzero polarization states. Thereby, we reveal an unusually strong effect of a mismatch strain on the out-of-plane polarization reversal, hysteresis loop shape, dielectric susceptibility, and piezoelectric response of CIPS films. By varying the sign of the mismatch strain and its magnitude in a fairly narrow range, quasistatic hysteresisless paraelectric curves can transform into double, triple, and other types of pinched and single hysteresis loops. The strain effect on the polarization reversal is opposite, i.e., anomalous, in comparison with many other ferroelectric films, for which the out-of-plane remanent polarization and coercive field increase strongly for tensile strains and decrease or vanish for compressive strains. For definite values of temperature and mismatch strain, the low-frequency hysteresis loops of polarization may exhibit negative slope in a relatively narrow range of external field amplitude and frequency. The low-frequency susceptibility hysteresis loops, which correspond to the negative slope of polarization loops, contain only positive values, which can be giant in the entire range of field changes. The corresponding piezoresponse also reaches giant values, being maximal near coercive fields.

We prepared high-quality pure and copper-doped ZnO films by the standard spray pyrolysis method and systematically investigated the correlations between their magnetic ordering and the existing native defect states in the ZnO host lattice. The magnetization curves showed a paramagnetic phase, which coexists with the ferromagnetic phase at room temperature in all doped films. The results indicated that the detected ferromagnetic ordering is intrinsic and induced by the Cu dopants and correlated with the existing defect states and vacancies in the lattice. By increasing the Cu dopant in the films, we detected a clear improvement in the ferromagnetic ordering at room temperature. This is correlated with an obvious enhancement in the oxygen vacancy ($V_{\mathrm{O}}$) and zinc interstitial (${\mathrm{Zn}}_{\mathrm{i}}$) concentrations and a slight reduction in the concentration of zinc vacancies and oxygen interstitial levels. The magnetic ordering mechanism in the doped films is associated with the long-range ferromagnetic coupling between Cu ions mediated by the intrinsic defects of $V_{\mathrm{O}}$ and ${\mathrm{Zn}}_{\mathrm{i}}$, through the reaction defect complexes networks of ${\mathrm{Cu}}^{+2}\text{−}{\mathrm{V}}_{\mathrm{O}}\text{−}{\mathrm{Cu}}^{+}$ and ${\mathrm{Cu}}^{+2}\text{−}{\mathrm{Zn}}_{\mathrm{i}}\text{−}{\mathrm{Cu}}^{+}$. The bound-magnetic polarons (BMPs) are formed through these indirect exchange interactions. The magnetization curves are closely fitted with the BMPs model and the concentration of these BMPs is found to be above the reported percolation threshold of BMPs to construct long-range ferromagnetic coupling in Cu-ZnO.

The helimagnetic compounds ${\mathrm{MnSi}}_{1-x}{\mathrm{Ge}}_x$ show the three-dimensional multiple-$q$ order referred to as hedgehog–anti-hedgehog spin lattice (HSL) states. Two representative forms of HSL are the cubic-$3q$ spin lattice with $q\parallel \langle 100\rangle$ and the tetrahedral-$4q$ spin lattice with $q\parallel \langle 111\rangle$, which show up typically for $x=1.0–0.8$ and 0.6, respectively. Here, we have investigated the spin fluctuations in the ${\mathrm{MnSi}}_{1-x}{\mathrm{Ge}}_x$ polycrystalline samples with $x=0.6$ and 0.8 by using the time-of-flight neutron inelastic scattering and modulated intensity with zero effort type of neutron spin-echo techniques to elucidate the microscopic origin of the unconventional Hall effect in the HSL states. This research is motivated by the observation of a sign change in the unconventional Hall resistivity as a function of temperature [Y. Fujishiro, N. Kanazawa, T. Nakajima, X. Z. Yu, K. Ohishi, Y. Kawamura, K. Kakurai, T. Arima, H. Mitamura, A. Miyake, K. Akiba, M. Tokunaga, A. Matsuo, K. Kindo, T. Koretsune, R. Arita, and Y. Tokura, Nat. Commun. 10, 1059 (2019)]. The present results reveal the correspondences between the temperature ranges where the positive Hall resistivity and spin fluctuations are observed. These results agree well with the theoretical model of the conduction electrons scattered by the fluctuating spin clusters with a nonzero average of sign-biased scalar spin chirality as a mechanism of the positive Hall resistivity [H. Ishizuka and N. Nagaosa, Sci. Adv. 4, eaap9962 (2018)].

Magnetic van der Waals materials offer a physical paradigm for studying two-dimensional (2D) magnetic systems. 2D antiferromagnets (AFMs) are of great interest in the emerging field of AFM spintronics where interaction between local AFM moments and a spin current is fully utilized. Here, we report on spin Hall magnetoresistance (SMR) in the ${\mathrm{NiPS}}_3/\mathrm{Pt}$ system. The magnetic field and temperature dependence of the resistivity change unambiguously revealed the magnetic properties of ${\mathrm{NiPS}}_3$, such as the easy-plane anisotropy and the Néel temperature. As SMR is a manifestation of the magnetic moments interacting with the spin current that is an essential requirement in spintronics, our results open an avenue for 2D AFM spintronics.

When pressurized, the heavy fermion compound ${\mathrm{CeNiC}}_2$ reveals a rich electronic phase diagram and shows unconventional superconductivity with a transition temperature $T_c\sim 3.7$ K, the highest among Ce-based heavy fermion superconductors [Katano et al., Phys. Rev. B 99, 100501(R) (2019)]. Understanding of this appearance of superconductivity in the vicinity of a magnetic quantum critical point is still lacking. Given that physical properties of ${\mathrm{CeNiC}}_2$ are sensitive to subtle changes in the interatomic distances, information on atomic coordinates may offer essential insights into the local lattice arrangements, and thus the mechanisms behind the exotic phases and phase transitions. However, extraction of precise information on the atomic coordinates under pressure remains a challenge. To find a correlation between the local lattice environments and exotic physical properties in ${\mathrm{CeNiC}}_2$, we investigate its crystal structure from ambient pressure to 18.6 GPa via single crystal x-ray diffraction. The pressure dependence of unit-cell parameters reveals anisotropic linear compressibility $k$, following the relationship $|k_a|(3.70\times {10}^{-3}\mathrm{G}{\mathrm{Pa}}^{-1})>|k_c|(1.97\times {10}^{-3}\mathrm{G}{\mathrm{Pa}}^{-1})>\left|k_{\mathrm{b}}\right|(1.39\times {10}^{-3}\mathrm{G}{\mathrm{Pa}}^{-1})$, and a large bulk modulus, $B_0\sim 134\pm 3$ GPa. Although the atomic coordinates between Ce and Ni remain unchanged under applied pressure, the directions of the first nearest and the second nearest neighbors between both the Ce-Ce and Ni-Ni atoms switch at $\sim 7$ GPa. Notably, near this pressure the antiferromagnetic ordering temperature reaches maximum in the pressure-temperature phase diagram of ${\mathrm{CeNiC}}_2$. Our results suggest that the direction of the nearest neighbors interchange might play a key role in the suppression of magnetic order and the enhancement of the Kondo effect.

Magnetotransport measurements of the centrosymmetric square net ${{\mathrm{Eu}(\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x)}_4$ compounds reveal evidence of both reciprocal- and real-space topology. For compositions $0.50\le x\le 0.90$, several intermediate field phases are found by magnetization measurements when $H\parallel c$, where a maximum in the topological Hall effect is observed, pointing to the existence of topological (real-space topology) or noncoplanar spin textures. For $0.25\le x\le 0.39$, magnetization measurements reveal an intermediate field state, but no transition is visible in the Hall measurements. For $x=0.15$, only one magnetic transition occurs below the Néel temperature $T_N$, and no intermediate field spin reorientations are observed. The Hall effect varies smoothly before the spin-polarized (SP) state. However, in the SP state, Hall measurements reveal a large anomalous Hall effect (AHE) for all compositions, a consequence of reciprocal-space topology. Density functional theory calculations in the paramagnetic state indeed reveal a Dirac point that lies very near the Fermi energy, which is expected to split into Weyl nodes in the SP state, thereby explaining the observed AHE. These results suggest the ${{\mathrm{Eu}(\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x)}_4$ family is a rare material platform where real- and reciprocal-space topology exist in a single material platform.

Due to its incommensurate nature, moiré superlattices host not only acoustic phonons but also another type of soft collective modes called phasons. Here, we investigate the impact of electron-phason scattering on the transport properties of moiré systems. We show that the resistivity can scale linearly with temperature down to temperatures much lower than the Bloch-Grüneisen scale defined by electron kinematics on the Fermi surface. This result stems from the friction between layers, which transfers phason spectral weight to a broad diffusive low-energy peak in the mechanical response of the system. As a result, phason scattering becomes a very efficient channel for entropy production at low temperatures. We also consider the contributions of phasons to thermodynamic properties at low temperatures and find a “metallic-like” linear-in-$T$ behavior for the specific heat, despite the fact that this behavior is due to mechanical and not electronic degrees of freedom. We discuss the implications of this finding to reports of linear-in-$T$ resistivity in the phase diagram of twisted bilayer graphene.

We investigate the imaging properties of copper-based superlens surrounded by copper oxide $({\mathrm{Cu}}_2\mathrm{O})$. A subwavelength image resolution of the order $\lambda /9$ is demonstrated theoretically and verified in numerical simulations. It is shown that the existence of excitons in ${\mathrm{Cu}}_2\mathrm{O}$ influences the static and dynamical optical properties of the lens, providing an unique way of optically controlling the lens properties. In particular, an improvement of image quality caused by absorption in the spectral region of excitonic resonances is investigated. The plasmon-exciton interaction in the system may pave the way to tunable, highly nonlinear superlens designs.

One of the intrinsic drift velocity limits of the quantum Hall effect is the collective magnetoexciton (ME) instability. It has been demonstrated in bilayer graphene (BLG) using noise measurements [W. Yang et al., Phys. Rev. Lett. 121, 136804 (2018)]. We reproduce this experiment in monolayer graphene (MLG), and show that the same mechanism carries a direct relativistic signature on the breakdown velocity. Based on theoretical calculations of MLG- and BLG-ME spectra, we show that Doppler-induced instabilities manifest for a ME phase velocity determined by a universal value of the ME conductivity, set by the Hall conductance.

The concept of topology has been widely applied to condensed matter, going beyond the band crossover in reciprocal spaces. A recent breakthrough suggested unconventional topological physics in a quadruple perovskite ${\mathrm{TbMn}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$, whose magnetism-induced polarization manifests a unique Roman surface topology [G. Liu et al., Nat. Commun. 13, 2373 (2022)]. However, the available experimental evidence based on tiny polarizations of polycrystalline samples could be strengthened. Here, this topological multiferroicity is demonstrated by using density functional theory calculations, which ideally confirms the Roman surface trajectory of magnetism-induced polarization. In addition, an alternative material in this category is proposed to systematically enhance the performance, by promoting its magnetism-induced polarization to an easily detectable level.

Despite the intensive interest in copper- and nickel-based superconductivity in infinite-layer structures, the physical properties of many other infinite-layer transition-metal oxides remain largely unknown. Here we unveil, by the first-principles calculations, the electronic correlation (EC)-driven series of quantum-phase transitions (QPTs) in infinite-layer ${\mathrm{SrMnO}}_2$, where spin and charge orders are strongly interwoven. At a weak EC region, ${\mathrm{SrMnO}}_2$ is a ferromagnetic metal with anisotropic spin transportation, as a promising spin valve under room temperature. When EC is slightly increased, a structural transition accompanied by charge/bond disproportion occurs as a consequence of Fermi surface nesting, resulting in a ferromagnetic insulator with reduced Curie temperature. When EC is further enhanced, another structural transition occurs that drives the system into a different-type multiferroic phase with piezoelectricity-tunable antiferromagnetic orders. Therefore, infinite-layer ${\mathrm{SrMnO}}_2$ is possibly a unique system on the quantum critical point, where small perturbations of EC can realize a series of QPTs.

We investigate the effect of disorder on topologically nontrivial states in a two-dimensional (2D) mechanical system. We first propose a quantum spin Hall (QSH) insulator based on an out-of-plane spring-mass model and analytically study the interplay between the disorder and topology in both topologically trivial and nontrivial systems. We adopt the spin Bott index to characterize the topological property in disordered mechanical systems. By tracking the evolution of the spin Bott index with the increase of disorders, we quantitatively demonstrate the disorder induced transition from a topologically nontrivial QSH insulator to a trivial insulator. We then validate the topological phase transition through transient analysis in discrete lattices. Finally, we design a phononic crystal based on the discrete spring-mass model and numerically verify the topologically protected states along the boundary between the trivial insulator and disordered topological QSH insulator in a continuous system. This work puts a step forward in understanding the role of disorder in a 2D topological classical system.

We investigate a non-Hermitian model featuring nonreciprocal gradient hoppings. Through an in-depth analysis of the Liouvillian spectrum and dynamics, we confirm the emergence of the Liouvillian skin effect resulting from the nonreciprocal nature of hoppings in this model. Furthermore, we observe that the presence of gradient hopping strength leads to an accelerated relaxation time for the system. Through numerical investigations of the Liouvillian gap, relaxation time, and steady-state localization length, we discover that the relaxation time in this model cannot be explained by the currently established relationship associated with the Liouvillian skin effect. This discrepancy highlights the need for further exploration and theoretical advancements to fully comprehend the intricate mechanisms underlying quantum relaxation processes. Motivated by these findings, we propose a theoretical approach to realize this non-Hermitian model in an atomic system with a sideband structure by employing an adiabatic elimination technique. These results contribute to our deeper comprehension of quantum relaxation dynamics and provide theoretical backing for the development of techniques aimed at controlling quantum relaxation processes.

The interplay between spin-orbit coupling, anisotropic magnetic interaction, frustration-induced quantum fluctuations, and spin correlations can lead to novel quantum states with exotic excitations in rare-earth-based quantum magnets. Herein, we present the crystal structure, magnetization, electron spin resonance (ESR), specific heat, and nuclear magnetic resonance (NMR) experiments on the polycrystalline samples of ${\mathrm{Ba}}_9{\mathrm{Yb}}_2{\mathrm{Si}}_6{\mathrm{O}}_{24}$, in which ${\mathrm{Yb}}^{3+}$ ions form a perfect honeycomb lattice without detectable antisite disorder. The magnetization data reveal antiferromagnetically coupled spin-orbit entangled $J_{\mathrm{eff}}=\frac{1}{2}$ degrees of freedom of ${\mathrm{Yb}}^{3+}$ ions in the Kramers doublet state. The ESR measurements reveal that the first excited Kramers doublet is 32.3(7) meV above the ground state. The specific heat results suggest the absence of any long-range magnetic order in the measured temperature range. Furthermore, the ${}^{29}\mathrm{Si}$ NMR results do not indicate any signature of magnetic ordering down to 1.6 K, and the spin-lattice relaxation rate reveals the presence of a field-induced gap that is attributed to the Zeeman splitting of the Kramers doublet state in this quantum material. Our experiments detect neither spin freezing nor long-range magnetic ordering down to 1.6 K. The current results suggest the presence of short-range spin correlations in this spin-orbit entangled $J_{\mathrm{eff}}=\frac{1}{2}$ rare-earth magnet on a honeycomb lattice.

A generalization of the Aubry-André model, the noninteracting Ganeshan-Pixley-Das Sarma model (GPD) introduced by Ganeshan et al. [Phys. Rev. Lett. 114, 146601 (2015)], is known analytically to possess a mobility edge, allowing both extended and localized eigenstates to coexist. This mobility edge has been hypothesized to survive in closed many-body interacting systems, giving rise to a new nonergodic metallic phase. In this work, coupling the interacting GPD model to a thermal bath, we provide direct numerical evidence for multiple qualitative behaviors in the parameter space of disorder strength and energy level. In particular, we look at the bath-induced saturation of entanglement entropy to classify three behaviors: thermalized, nonergodic extended, and localized. We also extract the localization length in the localized phase using the long-time dynamics of the entanglement entropy and the spin imbalance. Our work demonstrates the rich localization landscape of generalized Aubry-André models containing mobility edges in contrast to the simple Aubry-André model with no mobility edge.

${\mathrm{BaCo}}_2\left({\mathrm{AsO}}_4\right){}_2$ (BCAO) has seen extensive study since its initial identification as a proximate Kitaev quantum spin liquid candidate. Thought to be described by the highly anisotropic XXZ-$J_1\text{−}{J}_3$ model, the ease with which magnetic order is suppressed in the system indicates proximity to a spin liquid phase. Upon chemical tuning via partial arsenic substitution with vanadium, we show an initial suppression of long-range incommensurate order in the BCAO system to $T\approx 3.0$ K, followed by increased spin freezing at higher substitution levels. Between these two regions, at around 10% substitution, the system is shown to pass through a critical point where the competing $J_1/J_3$ exchange interactions become more balanced, producing a more complex magnetic ground state, likely stabilized by quantum fluctuations. This state shows how slight compositional change in magnetically frustrated systems may be leveraged to tune ground state degeneracies and potentially realize a quantum spin liquid state.