We study the localization properties and the Anderson transition in the three-dimensional Lieb lattice ${\mathcal{L}}_3\left(1\right)$ and its extensions ${\mathcal{L}}_3\left(n\right)$ in the presence of disorder. We compute the positions of the flatbands, the disorder-broadened density of states, and the energy-disorder phase diagrams for up to $n=4$. Via finite-size scaling, we obtain the critical properties such as critical disorders and energies as well as the universal localization lengths exponent $\nu$. We find that the critical disorder $W_c$ decreases from $\sim 16.5$ for the cubic lattice, to $\sim 8.6$ for ${\mathcal{L}}_3\left(1\right), \sim 5.9$ for ${\mathcal{L}}_3\left(2\right)$, and $\sim 4.8$ for ${\mathcal{L}}_3\left(3\right)$. Nevertheless, the value of the critical exponent $\nu$ for all Lieb lattices studied here and across various disorder and energy transitions agrees within error bars with the generally accepted universal value $\nu =1.590(1.579,1.602)$.

Although many effective-medium theories have been proposed for studying metamaterials, most of them do not work well for multilayer metamaterials with small interlayer distances. Based on rigorous mode-expansion analyses on a model system consisting of multiple layers of subwavelength gratings, we identify that the failures of conventional effective-medium theories are caused by neglecting strong near-field couplings in homogenizing such systems. These understandings motive us to propose an alternative homogenization approach for strongly coupled multilayer metamaterials, in which predominant near-field corrections have been considered automatically. Our effective-medium theory can well describe multilayer metamaterials with arbitrary interlayer distances including particularly those systems for which conventional effective-medium theories fail. We finally extend our theory to multilayer metamaterials with complicated microstructures and validate the theory by full-wave simulations as well as microwave experiments. Our theory not only well complements the available effective-medium theory formalisms, but also provides a powerful tool to study the properties of strongly coupled metamaterials, which may find many applications in practice.

Compared to a space group, a magnetic space group is much more complex, as both magnetic structure and magnetic moment direction can change the original symmetries of materials. The interplay between space symmetry and magnetism can generate versatile novel quantum states. However, detecting these topological phases experimentally, achieved through manipulating the magnetic configuration, has been restricted. It is mainly because the intrinsic link between the theory and the available experimental technique remains elusive. Here, we show that high harmonic generation (HHG) can identify these topological quantum states. We use rhombohedral ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ film as an example, and analyze the symmetry-dependent harmonic order and signal by combining first-principles calculations and time-dependent Liouville equation numerical computations. Our results provide a fundamental basis for using HHG to study the topological quantum phases mediated by various magnetic configurations that can be easily realized by applying an external magnetic field.

Phononic metamaterial which consists of two (or several) nanolayers separated by a planar defect of atomic-scale thickness is studied, under the assumption that the two-channel phonon interference mechanism of the transverse (cross-plane) heat flux control is dominant at high temperatures. An analytically exactly solvable discrete three-dimensional (3D) model of the multilayer interface between two semi-infinite bcc-lattice crystals is used to simulate phononic metamirror, metafilter, and meta-absorber. Two options of the general model in which two-path phonon interference reveals itself as double-humped resonance in the interface phonon absorption at weak dissipation or as total phonon transmission and total phonon reflection in the lossless limit are considered. An analogy is discussed between doubly resonant dissipative vibration systems and earlier investigated doubly resonant electromagnetic structure exhibiting both types of behavior: induced transparency and superscattering. It is shown that triple-peaked absorption resonance may arise from superposition of two-path phonon interference and Fabry-Pérot-type interference in the system with triple defect layer. The existence conditions of double-peaked and triple-peaked resonances as well as total interface absorption are analyzed in terms of nondissipative phonon scattering properties and dissipative parameters. Also additional peculiarities relevant to the thermal interface resistance problem are described. The study provides insight into heat management in phononic nanostructures and metamaterials like metamirrors, metafilters, and meta-absorbers.

Ferrimagnetic insulators capped with a heavy metal are becoming an increasingly interesting materials system in spintronics due to their unique ability for electrical manipulation and detection of magnetic states and spin textures via spin-orbit torques. The ability to engineer magnetic anisotropy is a powerful tool for tuning the recently discovered phenomena in these bilayers such as electrical switching or the stabilization of topological magnetic textures. We observe large shifts in the magnetic anisotropy in ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ thin films due to heavy-metal capping layers, which strongly depends on the orientation of the substrate and therefore the orientation of the epitaxial films. This work suggests large Rashba spin-orbit coupling at the metal/ferrimagnetic-insulator interface, which can be engineered in spintronic devices that utilize spin-orbit torques for electrical control of the magnetization in magnetic insulators.

The interface between ferromagnetic (FM) and nonmagnetic (NM) metal plays a fundamental role in the generation, transmission, detection of spin current, and spin-based devices. The impact of physical properties on the interface such as roughness, crystallization conditions, and even impedance matching has been widely investigated. In this work, the effects of interfacial chemical state at the interface of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and Pt (YIG/Pt) are systematically investigated through spin injection. Prior to Pt deposition, the spin mixing conductance and inverse spin Hall voltage of YIG/Pt hybrids increased up to 300%, using surface chemical processing on YIG. The interfacial chemical state is identified from x-ray photoelectron spectroscopy spectra and peak decomposition. Both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations appear at the interface and higher content of ${\mathrm{Fe}}^{3+}$ enhances the interface magnetic moment which consequently plays a crucial role in spin injection efficiency and inverse spin Hall voltage. In order to prevent the influence of magnetic proximity effect of Pt layers close to the FM/NM interface, a Cu spacer is inserted. It is found that the improved spin injection efficiency due to iron cations still exists in the YIG/Cu/Pt structure, which confirms the universal influence of the interfacial chemical state on spin pumping efficiency. This discovery has potential implications for spin generation, detection, and even optimization of spintronic devices.

We experimentally explore the orientation dependence of the magnetization in single-crystal ${\mathrm{Co}}_{0.77}{\mathrm{Ru}}_{0.23}$ alloy films and demonstrate that the material remains magnetically anisotropic near and even above the Curie temperature $T_C$, which is incompatible with the conventional description of anisotropy via temperature-dependent anisotropy constants. To accomplish an appropriate description of magnetic anisotropy at all temperatures, we derive an improved, but still simple free-energy description based upon the anisotropic Heisenberg model that is in excellent agreement with our experimental data.

We systematically investigate the electronic structure, magnetism, and high-temperature superconductivity (SC) in multilayer octagraphene and octagraphite (bulk octagraphene). A tight-binding model is used to fit the electronic structures of single-layer and multilayer octagraphenes and octagraphite. We find that multilayer octagraphene and octagraphite follow a simple A-A stacking structure from the energy analysis. The van der Waals interaction induces $t_{\perp }\approx 0.25$ eV and the hopping integral within each layer changes little when the layer number $n$ increases. There is a well Fermi-surface nesting with nesting vector $\mathbf{Q}=(\pi ,\pi )$ for single-layer octagraphene at half-filling, which can induce a two-dimensional Néel antiferromagnetic order. With increasing layer number $n\to \infty$, the Fermi-surface nesting transforms to three-dimensional (3D) with nesting vector $\mathbf{Q}=(\pi ,\pi ,\pi )$ and shows that the system has a 3D Néel antiferromagnetic order. Upon doping, multilayer octagraphene and octagraphite can enter a high-temperature $s^{\pm }$ SC driven by spin fluctuation. We evaluate the superconducting transition temperature $T_c$ by using the random-phase approximation, which yields a high $T_c$ even if the layer number $n\ge 3$. Our study shows that multilayer octagraphene and octagraphite are promising candidates for realizing high-temperature SC.

In spin chains with local unitary evolution preserving the magnetization $S^{\mathrm{z}}$, the domain-wall state $\left|\cdots \uparrow \uparrow \uparrow \uparrow \uparrow \downarrow \downarrow \downarrow \downarrow \downarrow \cdots \right.〉$ typically “melts.” At large times, a nontrivial magnetization profile develops in an expanding region around the initial position of the domain wall. For nonintegrable dynamics, the melting is diffusive, with entropy production within a melted region of size $\sqrt{t}$. In contrast, when the evolution is integrable, ballistic transport dominates and results in a melted region growing linearly in time, with no extensive entropy production: The spin chain remains locally in states of zero entropy at any time. Here we show that, for the integrable spin-$1/2$ XXZ chain, low-energy quantum fluctuations in the melted region give rise to an emergent Luttinger liquid which, remarkably, differs from the equilibrium one. The striking feature of this emergent Luttinger liquid is its quasiparticle charge (or Luttinger parameter $K$), which acquires a fractal dependence on the XXZ chain anisotropy parameter $\mathrm{\Delta }$.

Cavity spintronics recently heralded nonlocal magnonic signal transfer between magnetic samples. Here we show that by including superconductors in the cavity, we can make use of these principles to bring composite superconductor-ferromagnet systems to the macroscale. We analyze how a superconductor's ac conductivity influences the spin dynamics of a spatially separated magnet, and we discuss the potential impact on spintronic applications.

The superconductor-insulator transition in a transverse magnetic field is studied in a highly disordered MoC film with the product of the Fermi momentum and the mean free path $k_{F}l$ close to unity. Surprisingly, the Zeeman paramagnetic effects dominate over orbital coupling on both sides of the transition. In the superconducting state it is evidenced by a high upper critical magnetic field $B_{c2}$, by its square-root dependence on temperature, as well as by the Zeeman splitting of the quasiparticle density of states (DOS) measured by scanning tunneling microscopy. At $B_{c2}$ a logarithmic anomaly in DOS is observed. This anomaly is further enhanced in an increasing magnetic field, which is explained by the Zeeman splitting of the Altshuler-Aronov DOS driving the system into a more insulating or resistive state. A spin-dependent Altshuler-Aronov correction is also needed to explain the transport behavior above $B_{c2}$.

The development of defect populations after proton irradiation of $n$-type $4H$-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below $1000{}^{\circ }\mathrm{C}$. The model proposes the ${\mathrm{EH}}_4$ and ${\mathrm{EH}}_5$ traps frequently found by DLTS to originate from the ($+$/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the transformation channel between the silicon vacancy (${\mathrm{V}}_{\mathrm{Si}}$) and CAV is effectively blocked under $n$-type conditions, but becomes available in samples where the Fermi level has moved towards the center of the band gap due to irradiation-induced donor compensation. The annealing of ${\mathrm{V}}_{\mathrm{Si}}$ and the carbon vacancy (${\mathrm{V}}_{\mathrm{C}}$) is shown to be dominated by recombination with residual self-interstitials at temperatures of up to $400{}^{\circ }\mathrm{C}$. Going to higher temperatures, a decay of the CAV pair density is reported which is closely correlated to a renewed increase of ${\mathrm{V}}_{\mathrm{C}}$ concentration. A conceivable explanation for this process is the dissociation of the CAV pair into separate carbon anitisites and ${\mathrm{V}}_{\mathrm{C}}$ defects. Lastly, the presented data supports the claim that the removal of free carriers in irradiated SiC is due to introduced compensating defects and not passivation of shallow nitrogen donors.

The lattice dynamic response of body-centered tetragonal $\beta \text{−}\mathrm{Sn}$ $(I{4}_1/amd)$ under high-pressure and -temperature conditions is determined using experimental optical vibration modes. Raman scattering is used to map the phase stability region of $\beta \text{−}\mathrm{Sn}$ to perform mode Grüneisen analysis, and we demonstrate the necessity of an optical intensity calibration for Raman thermometry. The Grüneisen tensor is evaluated along a set of isotherms to address shortcomings of single-mode Grüneisen parameters with respect to anisotropic deformations of this tetragonal structured soft metal. The changes observed here in the Grüneisen tensor as a function of temperature are related to anharmonicity and denote potential criteria for the onset of premelting.

The complex interplay of local disorder with the structure and dynamics and their role in enhancing the electromechanical response makes relaxor ferroelectric materials fascinating both from the scientific and technological standpoints. It is generally believed that in chemically disordered ferroelectric solid solutions, the normal ferroelectric state gradually yields to a relaxor ground state on increasing the concentration of localized (point-defect-like) random-field centers. That a different kind of process can spontaneously stabilize a relaxor ground state is less known, especially in the family of ferroelectric perovskites. We demonstrate the occurrence of this less-known phenomenon in $(1-x){\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3\text{−}\left(x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3[(1-x)\mathrm{KBT}-\left(x\right)\mathrm{NBT}]$. Unlike the gradual evolution common to most ferroelectric solid solutions, KBT-xNBT exhibits an abrupt crossover from a normal ferroelectric ground state to a full-blown relaxor ground state for $x>0.58$. We show that this abrupt crossover to the relaxor state is caused by stabilization of an incommensurate-like/highly disordered $M$-point ferroelastic distortion in the ergodic/paraelectric temperature regime. The abrupt crossover manifests as composition driven anomalous changes in several important properties such as dielectric response, electromechanical properties, tetragonality, and coercive field, electrostrain, and mimics a scenario often encountered in composition driven interferroelectric transformations.

Electronic nematic behavior has been identified and studied in iron-based superconductors for some time, particularly in the well-known ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system, where it is well known to compete with superconductivity. On the other hand, it has been shown recently that FeSe displays a negligible effect of nematicity on superconductivity near the superconducting transition, and actual cooperation between the two orders when the system is doped with S. Recently it has also been proposed that LiFeAs undergoes a nematic transition in the superconducting state itself. Generally, we expect superconductivity to be anisotropic when it coexists with nematic order, but it is not clear under what circumstances the two orders compete or cooperate, nor how the anisotropy of the superconducting state correlates with that in the nematic state. To address this, we study a simple mean-field model of a $d$-wave Pomeranchuk instability together with a mixed $s,d$ pairing interaction, and identify when nematicity is enhanced or suppressed by superconductivity. We show that the competition or cooperation depends significantly on the distortion of the Fermi surface due to nematicity relative to the anisotropy of the superconducting gap function. Further, we discuss the implications of our results for FeSe and LiFeAs.

We performed scanning tunneling microscopy/spectroscopy measurements in the superconducting state of underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+x}$, and demonstrated that the checkerboard (CB) and Bogoliubov quasiparticle interference (QPI) modulations, which are considered to be characteristic features for the low-energy states around the antinodal and nodal regions of the Fermi surface, respectively, coexist in identical regions of the Cu-O plane. In the present work, these electronic superstructures are successfully separated from each other, and the local amplitude and phase are evaluated for the CB modulation and the ${\mathit{q}}_1$ component of QPI, which are characterized by similar wave vectors. The ${\mathit{q}}_1$-QPI component tends to be in-phase and antiphase with the CB modulation for positive and negative bias voltages, respectively. The significant spatial phase relationship is confirmed in regions where the local amplitude of CB modulation is comparatively large. This finding implies an interplay between the QPI and CB modulations.

The anisotropic London equations taking into account the normal currents are derived and applied to the problem of the surface impedance in the Meissner state of anisotropic materials. It is shown that the complex susceptibility of the anisotropic slab depends on the orientation of the applied microwave field relative to the crystal axes. In particular, the anisotropic sample in the microwave field is subject to a torque, unless the field is directed along one of the crystal principle axes.

A recent experiment reported that robust superconductivity appears in NbTi alloys under ultrahigh pressures with an almost constant superconducting $T_c$ of $\sim 19$ K from 120 to 261.7 GPa [J. Guo et al., Adv. Mater. 31, 1807240 (2019)], which is very rare among the known superconductors. We investigate the origin of this novel superconducting behavior in NbTi alloys based on density functional theory and density functional perturbation theory calculations. Our results indicate that the pressure tends to transform NbTi alloys from a random phase to a uniformly ordered crystal phase, and the exotic robust superconductivity of NbTi alloys can still be understood in the framework of BCS theory. The Nb element in NbTi alloys plays a dominant role in the superconductivity at low pressure, while the NbTi crystal with an alternative and uniform Nb and Ti atomic arrangement may be responsible for the stable superconductivity under high pressures. The robust superconducting transition temperature of NbTi under ultrahigh pressure can be explained by a synergistic effect of the enhanced phonon frequency, the modestly reduced total electron-phonon coupling, and the pressure-dependent screened Coulomb repulsion.

Recently, the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ material has been proposed as the first intrinsic antiferromagnetic topological insulator (AFMTI), where the interplay between magnetism and topology induces several fascinating topological phases, such as the quantum anomalous Hall effect, Majorana fermions, and axion electrodynamics. However, an exactly solvable model being capable of capturing the essential physics of the interplay between magnetism and topology is still absent. Here, inspired by the Ising-like nature [B. Li et al. Phys. Rev. Lett. 124, 167204 (2020)] and the topological property of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$, we propose a topological Ising-Kondo lattice (TIKL) model to study its ground state property in an analytical way at zero temperature. The resultant phase diagram includes rich topological and magnetic states, which emerge in the model proposed in a natural and consistent way for the intrinsic magnetic topological insulator. With Monte Carlo simulation, we extend the AFMTI ground state to finite temperature. It reveals that topological properties do sustain at high temperature, which even can be restored by elevated temperature at suitable correlation strength. The results demonstrate that TIKL may offer an insight for future experimental research, with which magnetism and transport properties could be fine-tuned to achieve more stable and exotic magnetic topological quantum states.