Transport measurements are readily used to probe different phases in disordered topological insulators (TIs), where determining topological invariants explicitly is challenging. On that note, universal conductance fluctuations (UCF) theory asserts the conductance $G$ for an ensemble has a Gaussian distribution, and that standard deviation $\delta G$ depends solely on the symmetries and dimensions of the system. Using a real-space tight-binding Hamiltonian on a system with Anderson disorder, we explore conductance fluctuations in a thin ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film and demonstrate the agreement of their behavior with UCF hypotheses. We further show that magnetic field applied out-of-plane breaks the time-reversal symmetry and transforms the system's Wigner-Dyson class from symplectic to unitary, increasing $\delta G$ by $\sqrt{2}$. Finally, we reveal that while ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is a strong TI, weak TI and metallic phases can be stabilized in presence of strain and disorder, and detected by monitoring the conductance fluctuations.

Generalized string-net models have been proposed recently in order to enlarge the set of possible topological quantum phases emerging from the original string-net construction. In the present work we do not consider vertex excitations, and we restrict ourselves to plaquette excitations, or fluxons, that satisfy important identities. We explain how to compute the energy-level degeneracies of the generalized string-net Hamiltonian associated with an arbitrary unitary fusion category. In contrast to the degeneracy of the ground state, which is purely topological, the degeneracy of excited energy levels depends not only on the Drinfeld center of the category, but also on internal multiplicities obtained from the tube algebra defined from the category. For a noncommutative category, these internal multiplicities result in extra nontopological degeneracies. Our results are valid for any trivalent graph and any orientable surface. We illustrate our findings with nontrivial examples.

A study of the anharmonic phonon-scattering processes in Yb-filled ${\mathrm{CoSb}}_3$ skutterudite with low lattice thermal conductivity is carried out by combining temperature-dependent Raman-scattering measurements and first-principles calculations. The softening of the phonon frequency reveals the phonon anharmonicity. The two selected phonon modes exhibit strong cubic and quartic anharmonicity. The calculated weighted phase space and scattering rates of three- and four-phonon scattering further support the anharmonicity. At high temperatures, four-phonon scattering dominates the phonon-scattering processes at low frequencies. Fermi resonances among low-frequency phonons provide strong four-phonon scattering, leading to low lattice thermal conductivity. This study provides insight into the nature of anharmonic processes in highly efficient Yb-filled skutterudites and demonstrates the important role of the high-order phonon-phonon interactions in lowering lattice thermal conductivity.

Here, we quantitatively estimate the impact of the inevitable Si surface passivation prior to III-V/Si heteroepitaxy on the surface energy of the Si initial substrate, and explore its consequences for the description of wetting properties. Density functional theory is used to determine absolute surface energies of P- and Ga-passivated Si surfaces and their dependences with the chemical potential. Especially, we show that, while a $\approx 90\mathrm{meV}/{\AA }^2$ surface energy is usually considered for the nude Si surface, surface passivation by Ga- or P- atoms leads to a strong stabilization of the surface, with a surface energy in the [50–75 $\mathrm{meV}/{\AA }^2$] range. The all ab initio analysis of the wetting properties indicate that a complete wetting situation would become possible only if the initial passivated Si surface could be destabilized by at least $15\mathrm{meV}/{\AA }^2$ or if the III-V (001) surface could be stabilized by the same amount.

We present a microscopic many-body theory of the recently measured two-dimensional coherent spectroscopy (2DCS) of excitons and trions in monolayer ${\mathrm{MoSe}}_2$ materials [K. Hao et al., Nano Lett. 16, 5109 (2016)], where excitons and trions can be well interpreted as repulsive and attractive polarons, respectively, in the dilute limit of exciton density. We derive a simple relation for the 2DCS spectrum in terms of a modified, mixing time-dependent polaron Green function, which is valid in the single-exciton limit. Our simulated spectra are in excellent qualitative agreement with experiments without introducing any phenomenological parameters such as decoherence rates. In particular, quantum beats between the off-diagonal cross peaks in the experimental 2DCS spectra are well reproduced. Our work, therefore, clarifies the microscopic principle that underlies the observed optical signals of exciton-trion coherence. We find that there are two quantitative discrepancies between theory and experiment: the smaller than expected cross-peak strength and the slightly unsynchronized quantum beats at different cross peaks. Tentatively, we attribute these residual discrepancies to the finite exciton density and the resultant polaron-polaron interaction, which is not taken into account in our theory.

From the low-energy model, the topological field theory indicates that the surface magnetization can open a surface gap in three-dimensional (3D) topological insulators (TIs), resulting in a half-quantized Hall conductance. Here, by employing the realistic lattice model, we show the occurrence of the surface phase transitions, accompanied by the sharp changes of the surface Chern number from $\frac{1}{2}$ to $-\frac{3}{2}$, and finally to $-\frac{1}{2}$, in 3D TIs induced by surface magnetization. These surface phase transitions lead to the sudden jumps in the magneto-electric coefficient and the quantum Hall conductance, which are experimentally observable. Furthermore, we present the phase diagram that elucidates the behavior of the 3D TI surface Chern numbers under surface magnetization for different $Z_4$ topological numbers. Our study highlights the presence of the new phases with broken bulk-boundary correspondence and enriches the understanding of the properties of TIs.

A number of interesting physical phenomena have been discovered in magic-angle twisted bilayer graphene (MATBG), such as superconductivity, correlated gapped and gapless phases, etc. The gapped phases are believed to be symmetry-breaking states described by mean-field theories, whereas gapless phases exhibit features not explained by mean-field theories. This work, using a combination of poor man's scaling, numerical renormalization group, and dynamic mean-field theory, demonstrates that the gapless phases are the heavy-Fermi-liquid state, where some symmetries might be broken while the others are preserved. We adopt the recently proposed topological heavy-fermion model for MATBG, where effective local orbitals around AA-stacking regions and Dirac fermions surrounding them play the roles of local moments (LM's) and itinerant electrons, respectively. At zero temperature and most noninteger fillings, the ground states are found to be heavy Fermi liquids and exhibit Kondo resonance peaks. The Kondo temperature $T_{\mathrm{K}}$ is found at the order of 1 meV. A higher temperature than $T_{\mathrm{K}}$ will drive the system into a metallic LM phase where disordered LM's, obeying Curie's law, and a Fermi liquid formed by itinerant electrons coexist. At integer fillings $\pm 1,\pm 2, T_{\mathrm{K}}$ is suppressed to zero or a value weaker than the Ruderman-Kittel-Kasuya-Yoshida interaction, leading to Mott insulators or symmetry-breaking states. Remarkably, this theory offers a unified explanation for several experimental observations, such as zero-energy peaks and quantum-dot-like behaviors in STM, the so-called Pomeranchuk effect, and the sawtooth feature of inverse compressibility, etc. For future experimental verification, we predict that the Fermi surface in the gapless phase will shrink upon heating, as a characteristic of the heavy Fermi liquid. We also conjecture that the heavy Fermi liquid is the parent state of the observed unconventional superconductivity because the Kondo screening reduces the overwhelming Coulomb interaction ($U\sim 60\mathrm{meV}$) to a rather small residual effective interaction ($U^{*}\sim 1\mathrm{meV}$) that is comparable to possible weak attractive interactions.

Since the theoretical and experimental demonstration of the quasibonding (QB) interactions between van der Waals (vdW) layers of two-dimensional (2D) materials, diverse effects and applications of interlayer and interface QB in vdW homo- and heterostructures have been reported. However, for vdW heterostructures (vdWHs) composed of magnetic and nonmagnetic 2D layers, the principles of interlayer QB interaction in these systems and their potential application remain to be elucidated. In the current work, by density functional theory calculations combined with crystal orbital Hamiltonian population analysis, the spin-dependent and multilevel interlayer QB (namely, orbital hybridization) in a $H-{\mathrm{MoSe}}_2/H-\mathrm{V}{\mathrm{Se}}_2$ nonmagnetic/magnetic vdWH is established, and its multifunctional properties for spintronics and spin-dependent optoelectronics are revealed. In particular, (1) the two spin channels have distinct interlayer interactions: spin-up is a two-level interaction while three energy levels are involved in spin-down; for each level, multiorbitals (mainly $s, p_z$, and $d_{z^2}$ orbitals) make contributions to the interlayer interaction; (2) distinct band alignment types ranging from type I and type II to a mixed type appear; and (3) the interlayer interaction makes the $\mathrm{V}{\mathrm{Se}}_2$ layer have better bipolar magnetic semiconductor properties. Our current work demonstrates the orbital-resolved multilevel analysis as a powerful tool for understanding the interlayer QB interaction and indicates the potential of the ${\mathrm{MoSe}}_2/\mathrm{V}{\mathrm{Se}}_2$ vdWH for multifunctional applications.

Recently, ferroelectricity has been demonstrated in the family of halide perovskites $\mathrm{CsGe}{X}_3$ $(X=\text{Cl}, \mathrm{Br}, \mathrm{I})$. We develop a first-principles-based computational approach to probe the finite-temperature properties of ${\mathrm{CsGeBr}}_3$. Our computations reveal the existence of a dynamic antipolar phase above the Curie temperature. The phase can be stabilized below the Curie temperature through quenching. Furthermore, competition between the polar and antipolar phases results in the formation of rare polar-antipolar domains. We demonstrate that such polar-antipolar domains can result in the formation of unusual polarization fields with head-to-head and tail-to-tail dipole arrangements, which not only offers an explanation for the recent experimental findings but could also lead to the discovery of unusual topological polar structures.

Mott-insulating transition metal oxides containing $t_{2g}^4$ ions with strong spin-orbit coupling were recently demonstrated to display unusual magnetism due to a dynamical mixing of the low-energy multiplet states via exchange processes. Here we derive exchange interactions in the situation where a tetragonal or trigonal crystal field selects a single relevant excited state on top of the singlet ionic ground state, producing an effective spin-$\frac{1}{2}$. We show that these moments universally obey antiferromagnetic transverse-field Ising model (TFIM) with an intrinsic transverse field generated by the splitting of the two ionic singlets. Using ${\mathrm{Ru}}^{4+}$ as a example ion, we provide quantitative estimates of the exchange and illustrate the emergent TFIM physics based on phase diagrams and excitation spectra obtained for several 2D lattices—square, honeycomb, as well as for the frustrated triangular lattice.

Centrosymmetric antiferromagnetic semiconductors, although abundant in nature, appear less favorable in spintronics owing to the lack of inherent spin polarization and magnetization. We unveil hidden Zeeman-type spin splitting in layered centrosymmetric antiferromagnets with asymmetric sublayer structures through first-principles simulations and symmetry analysis. Taking the bilayer counterpart of recently synthesized monolayer MnSe, we demonstrate that the degenerate states around specific $\mathit{k}$ points spatially segregate on different sublayers forming $\mathcal{PT}$-symmetric pairs. Furthermore, degenerate states exhibit uniform in-plane spin configurations with opposite orientations enforced by mirror symmetry. Bands are locally Zeeman split up to order of $\sim 70\mathrm{meV}$. Strikingly, a tiny electric field of a few $\mathrm{mV}{\AA }^{-1}$ along the $z$ direction breaks the double degeneracy forming additional Zeeman pairs. The design principle to obtain Zeeman-type splitting in centrosymmetric antiferromagnets established here expands the range of materials among which to look for spintronics.

Elucidating the interplay between nuclear and electronic degrees of freedom that govern the complex dielectric behavior of materials under intense photoexcitation is essential for tailoring optical properties on demand. However, conventional transient reflectivity experiments have been unable to differentiate between real and imaginary components of the dielectric response, omitting crucial electron-lattice interactions. Utilizing thin film interference we unambiguously determine the photoinduced change in the complex dielectric function in the Peierls semimetal bismuth and examine its dependence on the excitation density and nuclear motion of the $A_{1g}$ phonon. Our modeled transient reflectivity data reveal a progressive broadening and redshift of Lorentz oscillators with increasing excitation density and underscores the importance of both electronic and nuclear coordinates in the renormalization of interband transitions.

Magnons, as fundamental quasiparticles that emerge in elementary spin excitations, hold a big promise for innovating quantum technologies in information coding and processing. By establishing the exact relation between Fisher information and entanglement in partially accessible metrological schemes, we rigorously prove that bipartite entanglement plays a crucial role during the dynamical encoding process. However, the presence of an entanglement during the measurement process unavoidably reduces the ultimate measurement precision. These findings are verified in an experimentally feasible cavity magnonic system engineered for detecting a weak magnetic field by performing precision measurements through the cavity field. Moreover, we further demonstrate that, within a weak-coupling region, measurement precision can reach the Heisenberg limit. Additionally, quantum criticality also enables us to enhance measurement precision in a strong-coupling region.

Spectral singularities (SSs) emerge at isolated real frequencies when the scattering coefficients of a system diverge, producing scattering anomalies in non-Hermitian systems. Here, based on parity-time symmetry, we introduce SSs for evanescent waves, and explore their exotic features. We show that evanescent wave SSs can realize highly reconfigurable unidirectional lasers and absorbers, and offer the opportunity to observe extreme scattering anomalies associated with SSs in fully passive platforms, decoupling their extreme scattering responses from energy considerations. More broadly, our study opens avenues for non-Hermitian wave engineering, showcasing a link between non-Hermitian physics and structured waves, with implications in nano-optics for extreme wave-matter interactions and functional devices.

We study the tuning effect of a transverse magnetic field on the confinement of spinons in the infinite XXZ spin-1/2 chain. The spinon confinement in this model takes place in the gapped antiferromagnetic phase upon application of a staggered longitudinal magnetic field. The tuning transverse magnetic field has mutually orthogonal uniform and staggered components. The energy spectra of the two-spinon bound states (the “mesons”) in the confinement regime are analytically calculated in this model using two different perturbative schemes. The first one applies in the extreme anisotropic (Ising) limit and employs the inverse anisotropy constant as a small parameter. The second perturbative scheme, which applies at any anisotropy in the gapped antiferromagnetic domain, exploits the integrability of the XXZ spin chain at zero magnetic field. The small parameters in the second technique are the components of the transverse, and staggered longitudinal magnetic fields. It is shown that the weak transverse magnetic field mixes the transverse and longitudinal meson modes, and leads to an avoided crossing of their energies upon increase of its strength. The explicit formulas for the two-spinon contribution to the dynamical structure factors of local spin operators are obtained as well in this model in the weak confinement regime for wave vectors close to the points $k=0$ and $\pi$.

We study $N$ qubits having infinite-range Ising interaction and subjected to a periodic pulse of an external magnetic field. We analytically solve the cases of $N=5$ to 11 qubits, finding its eigensystem, the dynamics of the entanglement for various initial states, and the unitary evolution operator. These quantities shows signatures of quantum integrability. For the general case of $N>11$ qubits, we provide a conjecture on quantum integrability based on the numerical evidence such as degenerate spectrum, and the exact periodic nature of the time-evolved unitary evolution operator and the entanglement dynamics. Using linear entropy, we show that for the class of initial unentangled state, the entanglement periodically displays maximum and zero values.

By simultaneous Co and Ca doping we were able to obtain an ${\mathrm{EuFe}}_2{\mathrm{As}}_2$-based compound with superconductivity appearing above the antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ magnetic moments. However, as soon as the antiferromagnetic order appears, a reentrance behavior is observed—instead of zero resistivity and diamagnetic signal down to the temperature of 2 K. By investigating magnetization, ac susceptibility, and electrical transport properties of ${\mathrm{Eu}}_{0.73}{\mathrm{Ca}}_{0.27}{\left({\mathrm{Fe}}_{0.87}{\mathrm{Co}}_{0.13}\right)}_2{\mathrm{As}}_2$, and comparing them to previously studied Mössbauer effect and neutron scattering measurements of this and similar compounds, an explanation of such behavior is proposed.

Understanding the origin of damping mechanisms in the magnetization dynamics of metallic ferromagnets is a fundamental problem for nonequilibrium many-body physics of systems in which quantum conduction electrons interact with localized spins assumed to be governed by the classical Landau-Lifshitz-Gilbert (LLG) equation. It is also of critical importance for applications because damping affects energy consumption and the speed of spintronic and magnonic devices. Since the 1970s, a variety of linear-response and scattering theory approaches have been developed to produce widely used formulas for computation of the spatially independent Gilbert scalar parameter as the magnitude of the Gilbert damping term in the LLG equation. The Schwinger-Keldysh field theory (SKFT), largely unexploited for this purpose, offers additional possibilities, such as to rigorously derive an extended LLG equation by integrating quantum electrons out. Here we derive such an equation whose Gilbert damping for metallic ferromagnets is nonlocal, i.e., dependent on all localized spins at a given time, and nonuniform, even if all localized spins are collinear and spin-orbit coupling (SOC) is absent. This is in sharp contrast to standard lore, in which nonlocal damping is considered to emerge only if localized spins are noncollinear—for such situations, direct comparison using the example of a magnetic domain wall shows that SKFT-derived nonlocal damping is an order of magnitude larger than the previously considered one. Switching on SOC makes such nonlocal damping anisotropic, in contrast to standard lore, in which SOC is usually necessary to obtain a nonzero Gilbert damping scalar parameter. Our analytical formulas, with their nonlocality being more prominent in low spatial dimensions, are fully corroborated by numerically exact quantum-classical simulations.

$W$-type ${\mathrm{SrFe}}_{18}{\mathrm{O}}_{27}$ hexaferrite is emerging as a potential material for permanent magnet applications. Despite this, theoretical modeling on $W$-type hexaferrites is still lacking, leaving only experimental findings to date. Employing density functional theory, we conduct a detailed analysis of pure $W$-type ${\mathrm{SrFe}}_{18}{\mathrm{O}}_{27}$ hexaferrite and its compositions with Ni/Zn substitutions, to explore their intrinsic magnetic properties and assess their viability as potential permanent magnets. We found no significant effect of Ni/Zn substitution on the magnetocrystalline anisotropy energy (MAE); however, Zn substitution is particularly helpful for improving the magnetization ${\mu }_0{M}_s$. The calculated MAE constant $K_u$ values indicate that the compounds are uniaxial with easy axis along the (001) direction. The origin of the predicted MAE is investigated using second-order perturbation theory analysis and the electronic structure. We found that different Fe sublattices contribute differently to the MAE, providing a unique way to enhance the MAE with small site-specific substitutions. The results highlight the challenge of simultaneously enhancing $K_u$ and ${\mu }_0{M}_s$ in $W$-type hexaferrite compounds. However, the compounds show intriguing properties with moderate $K_u$ and high ${\mu }_0{M}_s$, which may outperform the conventional $M$-type ferrite magnets in some applications.

Here we report recent data on the phenomenon of total nonreflection of spin waves (NRSWs). Using space- and phase-resolved Brillouin light scattering we experimentally studied the amplitude, direction, and phase of the NRSW in yttrium iron garnet films with artificial defects—an antidot array. An NRSW in the form of a focused beam of high intensity with the group velocity pointing along the line of defects is observed only for a critical angle between spin-wave phase velocity and an antidot array direction in a magnetic field. It is shown that the NRSW is an independent noncollinear spin wave arising as a result of the anisotropic nature of the propagation of surface magnetostatic spin waves in a magnetic medium. The numerical simulations agree well with experimental data. Micromagnetic simulations performed let us elucidate the phenomenon of the NRSW and map the spin-wave profile of dynamic magnetization inside the NRSW. The estimated mean free path of the NRSW reaches about 1 mm in the yttrium iron garnet film with a thickness of a few micrometers. These results demonstrate a unique approach to focusing and enhancing spin waves in magnetics through the use of artificial defects.