Recently, the interplay between the non-Hermitian skin effect (NHSE) and topological phases has attracted a lot of attention. When the NHSE occurs in a topological system, the conventional bulk-boundary correspondence (BBC) based on the Bloch band theory is destroyed. In this work, we investigate a non-Hermitian nodal-ring semimetal with the $s$-wave superconducting (SC) term. We find that the SC term introduces the intrinsic coupling between the different general Brillouin zones (GBZs) of the particle and hole subbands in our model, which drives the total GBZ to the BZ. Furthermore, when the SC term is finite, we show a possibility that the GBZ is the same as the BZ. We also find that the Bloch winding number of the quasi-one-dimensional model can predict the number of Majorana zero states faithfully in this case, which implies that the conventional BBC is reconstructed in our model. The localization strengths of the Majorana zero states are also calculated to show the reconstruction of conventional BBC. The finite-size effect, related experimental regimes in the electric circuit, and the fragility of this reconstructed conventional BBC are also discussed in this work.

We propose a lattice spin model on a cubic lattice that shares many of the properties of the 3D toric code and the X-cube fracton model. The model, made of ${\mathbb{Z}}_3$ degrees of freedom at the links, has the vertex, the cube, and the plaquette terms. Being a stabilizer code the ground states are exactly solved. With only the vertex and the cube terms present, we show that the ground state degeneracy (GSD) is $3^{L^3+3L-1}$, where $L$ is the linear dimension of the cubic lattice. In addition to fractons, there are free vertex excitations we call the freeons. With the addition of the plaquette terms, GSD is vastly reduced to $3^3$, with fracton, fluxon, and freeon excitations, among which only the freeons are deconfined. The model is called the AB model if only the vertex $\left(A_v\right)$ and the cube $\left(B_c\right)$ terms are present and the ABC model if in addition the plaquette terms $\left(C_p\right)$ are included. The AC model consisting of vertex and plaquette terms is the ${\mathbb{Z}}_3$ 3D toric code. The extensive GSD of the AB model derives from the existence of both local and nonlocal logical operators that connect different ground states. The latter operators are identical to the logical operators of the ${\mathbb{Z}}_3$ X-cube model. Fracton excitations are immobile and accompanied by the creation of fluxons—plaquettes having nonzero flux. In the ABC model, such fluxon creation costs energy and ends up confining the fractons. Unlike past models of fractons, vertex excitations are free to move in any direction and pick up a nontrivial statistical phase when passing through a fluxon or a fracton cluster.

We report measurements of the radiative lifetimes and coherence times of the dark and bright excitons in an asymmetric GaAs/AlGaAs quantum dot. The dots, fabricated by partial infilling of asymmetric in situ etched nanoholes, have low symmetry, which leads to significant dark-bright mixing as demonstrated by dark-bright anticrossing in magnetophotoluminescence spectra. Using an orthogonal excitation-detection waveguiding geometry and quasiresonant excitation, we compare the coherence properties, measured by Michelson interferometry, of the dark and bright exciton from the same dot in the absence of an external magnetic field.

The Foldy-Lax equation is generalized for a medium that consists of particles with both electric and magnetic responses. The result is used to compute fields scattered from ensembles of particles. The computational complexity is reduced by hierarchical clustering techniques to enable simulations with on the order of ${10}^{10}$ particles. With so many particles we are able to see the transition to bulk media behavior of the fields. For nonmagnetic materials, the observable index, permittivity, and permeability of the effective bulk medium are in good agreement with the Clausius-Mossotti relation. The fields simulated for particles with both electric and magnetic responses are in good agreement with new analytical results for a generalized effective medium theory.

We study the electronic structure and band topology of fully relaxed twisted multilayer graphene (TMLG). Isolated flat bands emerge in TMLG with the number of layers $[M+N$ with $M$ the layer number of the bottom few-layer graphene (FLG)] up to 10 and with various stacking orders, and most of them are on the hole side. The touched bands of FLGs around the Fermi level are split by the moiré coupling through the electron-hole asymmetry in low-energy bands of FLGs and by the vertical hopping between next-nearest layers. The full structural relaxation leads to global gaps that completely isolate a flat band. For TMLG with given $M$ and $N$, the highest magnitude of Chern numbers ($\left|C\right|$) of the separable flat bands reaches $M+N-1$ and can be hosted by certain isolated bands. The $\left|C\right|=9$ occurs in the isolated flat valence band of several configurations with 10 layers. Such high $\left|C\right|$ originates from the lifting of the band-state degeneracy in the weak regime of moiré coupling or from the topological phase transitions induced by the strong moiré coupling. Moreover, large orbital magnetic moments arise in isolated flat bands with high $\left|C\right|$ and depend on the structural configurations of TMLG.

Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum environment. Transition from weak localization to weak antilocalization with increasing hydrogen adatom density is observed, indicating enhancing Bychkov-Rashba-type SOC in a mirror symmetry broken system. From the low-temperature saturation of phase breaking scattering rate, the existence of spin-flip scattering is identified, which corroborates the existence of magnetic moments in hydrogenated graphene.

Structural domains in ferroic materials can be manipulated to achieve novel properties and functionalities of devices with multiple tunabilities. Herein, we report a study of tunable domain structures in ferroelastic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) films via interfacial engineering. Distinct domain structures are formed in rhombohedral LSMO films depending on the crystallographic orientations of the orthorhombic ${\mathrm{NdGaO}}_3$ (NGO) substrates. A unidirectional lattice modulation is observed in LSMO films grown on $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ substrates, whereas a unidirectional twinned-domain structure is generated by $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates (where Or in the subscript denotes orthorhombic notation). The orientation-dependent domain structures in LSMO films are controlled by anisotropic strain as well as interfacial oxygen octahedral coupling at the heterointerface between LSMO and NGO. The orbital occupancy and in-plane magnetic anisotropy are markedly affected by strain relief induced by the unidirectional structural domains in LSMO films. In addition, the structural results observed in LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ films further demonstrate that anisotropic strain is capable to disturb octahedral connectivity in epitaxial films. Our findings provide future directions to understand and control the domain structures in ferroelastic heterostructures of perovskite materials and open a pathway towards tailoring the desirable physical properties.

The appearance of spontaneous coherence is a fundamental feature of a Bose-Einstein condensate and an essential requirement for possible applications of the condensates for data processing and quantum computing. In the case of a magnon condensate in a magnetic crystal, such computing can be performed even at room temperature. We study the process of the formation of a coherent magnon condensate by direct detection of microwave radiation emitted by magnons in a parametrically driven yttrium iron garnet bulk sample of a special shape. We show that after switching off the parametric pumping, the overpopulated magnon gas evolves to a state, whose coherence is only limited by the natural magnon relaxation into the crystal lattice.

We generalize the concept of topological invariants for mixed states based on the ensemble geometric phase (EGP) to two-dimensional band structures. In contrast to the geometric Uhlmann phase for density matrices, the EGP leads to a proper Chern number for Gaussian, finite-temperature, or nonequilibrium steady states. The Chern number can be expressed as an integral of the ground-state Berry curvature of a fictitious lattice Hamiltonian, constructed from single-particle correlations. For the Chern number to be nonzero the fictitious Hamiltonian has to break time-reversal symmetry.

The influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ${AB\mathrm{O}}_3$ perovskite ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\left({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}\right){\mathrm{O}}_3$ $(0\le x\le 0.5)$. It is found that aliovalent A-site substitution is a viable means to manipulate the magnetically active B-site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary $\mathrm{La}{T}_M{\mathrm{O}}_3$ $(T_M=\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

Quasielastic neutron scattering (QENS) measurements combined with first principles based molecular-dynamics calculations were conducted to study the dynamics of ${\mathrm{Li}}^{+}$ ions in a solid-state electrolyte ${\mathrm{La}}_{2/3-x}{\mathrm{Li}}_{3x}\mathrm{Ti}{\mathrm{O}}_3$ (LLTO) with $x=0.13$. By using a large ${}^7\mathrm{Li}$-enriched single crystal sample, a QENS signal was clearly observed along the three principal axes [110], [111], and [001] at a temperature ($T$) of 600 K. Wave vector dependence of the linewidth of the QENS signal along each direction was explained well using the Chudley-Elliot model for jumps between the $A$ sites of the perovskite lattice through the bottleneck square, which was also supported by molecular dynamics calculations. At $T=600$ K, the estimated self-diffusion coefficient of ${\mathrm{Li}}^{+}$ ($D_{\mathrm{Li}}$) in the $ab$ plane [$D_{\mathrm{Li}}^{ab}=(6.8\pm 0.5)\times {10}^{-6}{\mathrm{cm}}^2$/s] was slightly larger than that along the $c$ axis [$D_{\mathrm{Li}}^c=(4.4\pm 0.3)\times {10}^{-6}{\mathrm{cm}}^2$/s], suggesting quasi-isotropic diffusion, that is, the three-dimensional diffusion of ${\mathrm{Li}}^{+}$ ions. The decrease in $D_{\mathrm{Li}}$ with decreasing $T$ was reasonably explained by a thermal activation process with the activation energy determined from ionic-conductivity measurements. Furthermore, the estimated values of the self-diffusion coefficient of ${\mathrm{Li}}^{+}$ ions are comparable to those in the sulfide-based ${\mathrm{Li}}^{+}$ ion conductor, ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$, although its ionic conductivity is 10 times larger than that for LLTO. The obtained microscopic information on ${\mathrm{Li}}^{+}$ diffusion in LLTO clarifies how to understand the Li conduction mechanism in LLTO and ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$ in a unified manner and can provide a way to increase the ${\mathrm{Li}}^{+}$ ionic conductivity in oxide-based solid electrolytes.

${\mathrm{CaCo}}_{2-y}{\mathrm{As}}_2$ is a unique itinerant system having strong magnetic frustration. Here, we report the effect of electron doping on the physical properties resulting from Ni substitutions for Co. The single crystals of $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_{2-y}{\mathrm{As}}_2$ were characterized by single-crystal x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetization $M$ versus temperature $T$, magnetic field $H$, time $t$, and heat capacity $C_{\mathrm{p}}(H,T)$ measurements. The A-type antiferromagnetic (AFM) transition temperature $T_{\mathrm{N}}=52$ K for $x=0$ decreases to 22 K with only 3% Ni substitution and is completely suppressed for $x>0.16$. For $0.11\le x\le 0.52$ strong ferromagnetic (FM) fluctuations develop as revealed by magnetic susceptibility $\chi \left(T\right)=M\left(T\right)/H$ measurements. For $x=0.11$ and 0.16 competing AFM and FM interactions result in a reentrant spin-glass behavior below $T_{\mathrm{N}}$, as evidenced by the observations of thermomagnetic hysteresis and magnetic relaxation. Enhanced FM fluctuations are also found for the $x=0.21$ and 0.31 crystals, where ${\chi }_c$ increases significantly at low $T$. A large $\chi$ anisotropy in these compositions where ${\chi }_c$ is up to a factor of two larger than ${\chi }_{ab}$ suggests that the FM spin fluctuations are quasi-1D in nature. Weak ferromagnetic contributions to the magnetization are found at $T=2$ K for $x=0.11$–0.31. Heat-capacity $C_{\mathrm{p}}\left(T\right)$ measurements reveal the presence of FM quantum spin fluctuations for $0.11\le x\le 0.52$, where a logarithmic $T$ dependence of $C_{\mathrm{p}}\left(T\right)/T$ is observed at low $T$. The suppression of AFM order by the development of strong FM fluctuations in $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_{2-y}{\mathrm{As}}_2$ crystals suggests the presence of a FM quantum-critical point at $x\approx 0.20$. Our density-functional theory (DFT) calculations confirm that FM fluctuations are enhanced by Ni substitutions for Co in ${\mathrm{CaCo}}_{2-y}{\mathrm{As}}_2$. The Sommerfeld electronic heat-capacity coefficient is enhanced for $x=0$, 0.21, and 0.42 by about a factor of two compared to DFT calculations of the density of states (DOS) at the Fermi energy, suggesting an enhancement of the DOS from electron-phonon and/or electron-electron interactions. The crystals with $x>0.52$ do not exhibit FM spin fluctuations or magnetic order at $T\ge 1.8$ K, which was found from the DFT calculations to arise from a Stoner transition. Superconductivity is not observed above 1.8 K for any of the compositions. Neutron-diffraction studies of crystals with $x=0.11$ and 0.16 in the crossover regime $(0.1\lesssim x\lesssim 0.2)$ show no evidence of A-type ordering as observed in the parent compound with $x=0$. Furthermore, no other common magnetic structures, such as ferromagnetic (FM), helical stacking of in-plane FM layers, or in-plane AFM structure, are found with an ordered moment greater than the uncertainty of $0.05{\mu }_{\mathrm{B}}$ per transition-metal atom.

We investigate magnetic excitations in the $S=1$ easy-plane-type triangular antiferromagnets ${\mathrm{CsFeCl}}_3$ and ${\mathrm{RbFeCl}}_3$ through a combination of inelastic neutron scattering measurements and spin-wave theory calculations based on an established exchange-coupling Hamiltonian. We show that in either material the model falls short of providing an adequate description of the measured intensities and for ${\mathrm{RbFeCl}}_3$ even fails to reproduce the measured dispersion relation. The most striking discrepancy is a very anisotropic azimuthal intensity distribution in the acoustic spin-wave branches in the long-wavelength limit, which is incompatible with spin-wave theory on a fundamental level. The observed anomalies are attributed to long-range dipolar interactions.

I present a formalism to calculate the configuration-averaged lattice thermal conductivity for substitutional random alloys. The method is based on multiple-scattering approach which can capture the effect of disorder-induced configuration fluctuations on single-particle and two-particle phonon Green functions in random alloys. The randomness of the system is dealt within the augmented space theorem. This is combined with a generalized Feynman diagrammatic technique to extract various useful results in the form of mathematical expressions. I show the structure of all possible scattering diagrams up to the fourth order and subsequently illustrate how to obtain Dyson's equation from a resummation of the diagrammatic series. I also study how disorder scattering affects two-particle Green functions associated with thermal response. It was shown explicitly how the disorder scattering renormalizes both the phonon propagators as well as the heat currents. I derive the relation between these renormalized heat currents and the self-energy of the propagators. I have also studied a different class of scattering diagrams which are not related to the self-energy but rather to the vertex corrections. The configuration-averaging scheme is straightforward to apply to other relevant quantities such as joint density of states, thermal diffusivity, etc., for random alloys. The developed formalism is applied to a realistic ${\mathrm{Au}}_{1-x}{\mathrm{Fe}}_x$ binary alloy. The effect of disorder-induced corrections (as compared to the simple virtual crystal approximation) turns out to be appreciable in this alloy. The configuration-averaged lattice thermal conductivity for ${\mathrm{Au}}_{50}{\mathrm{Fe}}_{50}$ shows a quadratic behavior in low-temperature regime ($T\le 30$ K), which increases smoothly to a $T$-independent saturated value at high $T$. Simulated thermal diffusivity $D\left(\nu \right)$ helps to numerically estimate the mobility $\mathrm{edge}\left({\nu }_c\right)$ which in turn evaluates the fraction of localized states. $D\left(\nu \right)$ is found to decrease smoothly (almost linearly) in the high-$\nu$ range, which when fitted to ${({\nu }_c-\nu )}^{\alpha }$ gives the critical $\mathrm{exponent}\left(\alpha \right)$ to be 1.018 for ${\mathrm{Au}}_{50}{\mathrm{Fe}}_{50}$ alloy. This agrees fairly well with the scaling and other theories of Andersen localization.

The band structure of multilayer systems plays a crucial role for the ultrafast hot carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited electrons in a highly ordered Pb monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy with femtosecond visible light pulses, we show that the electron dynamics of the Pb/Ag(111) quantum well system is largely dominated by two types of scattering processes: (i) isotropic intraband scattering processes within the quantum well state (QWS) and (ii) isotropic interband scattering processes from the $p_z$-like QWS into the Pb $p_{x/y}$ band. In the latter case, the Pb QWS acts as an electron source for the momentum space refilling process of the Pb $p_{x/y}$ band. This conclusion is confirmed by the modification of the band structure and the quasiparticle dynamics of the Pb/Ag(111) bilayer film after the adsorption of PTCDA. We find both an adsorption-induced suppression of the QWS itself as well as of the refilling process into the Pb $p_{x/y}$ band. Our study hence demonstrates the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovers a new possibility to selectively tune and control scattering processes occurring in quantum (well) materials by the adsorption of organic molecules.

We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit torque memories. Using first-principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of nonvolatile magnetic random access memories.

We use first-principles calculations based on density functional theory to study how the magnetic properties of an Fe monolayer on a W(001) surface—exhibiting a $c(2\times 2)$ antiferromagnetic ground state—can be modified by an additional $4d$ transition-metal overlayer. To obtain an overview of how the $4d$-band filling influences the exchange interactions in the Fe layer we have calculated the energy dispersion of spin spirals for $4d/\mathrm{Fe}/2\mathrm{W}$ unsupported quadlayers, in which the W(001) substrate is represented by only two atomic layers. Hybridization with the overlayer leads to a reduced ferromagnetic nearest-neighbor exchange interaction and the next-nearest neighbor exchange gains in strength. Surprisingly, we find that the $c(2\times 2)$ antiferromagnetic state is unfavorable for all systems with a $4d$ overlayer. For $4d$ overlayers from the beginning (Nb) or end (Pd) of the series we find a ferromagnetic ground state. As one moves to the center of the series there is a transition via a spin spiral (Mo, Rh) to a $p(2\times 1)$ antiferromagnetic ground state (Tc, Ru). We have studied the Mo, Ru, and Pd overlayer on Fe/W(001) representing the surface by a sufficiently large number of W layers to obtain bulklike properties in its center. The energy dispersions of spin spirals show qualitatively the same results as those from the $4d/\mathrm{Fe}/2\mathrm{W}$ quadlayers. The Dzyaloshinskii-Moriya interaction calculated upon including spin-orbit coupling shows significant strength and considerable frustration effects. The calculated magnetocrystalline anisotropy energy is large as well. All $4d/\mathrm{Fe}/\mathrm{W}\left(001\right)$ films are potential candidates for complex noncollinear spin structures.

Two-dimensional (2D) multiferroic materials, with combined large magnetic anisotropic energy (MAE) and high Curie temperature $\left({\mathit{T}}_C\right)$, have great potential in emerging electronic device applications, such as high-density multistate data storage and fast operation. However, various 2D spintronic materials fabricated in experiment, so far have small MAE and low ${\mathit{T}}_C$. Using first-principles calculations, we proposed two $\mathrm{V}X$ $(X=\mathrm{P}, \mathrm{As})$ monolayers, which are identified to possess robust stability dynamically, thermally, and mechanically. $\mathrm{V}X$ monolayers are half-metals with coexisting ferroelastic and ferromagnetic properties. Moreover, their ground states exhibit significant in-plane MAEs of 101.48 and $262.68\mu \mathrm{eV}$ per V atom for VP and VAs monolayers, respectively. Additionality, the Curie temperatures of VP and VAs monolayers predicted from Heisenberg model are as high as 545 and 890 K, respectively, and the ferromagnetism could survive at room temperature. Our results are beneficial for further research on 2D multiferroics and provide the possibility for future nanodevices.