We analyze the possibility of emergent quantum multicritical points (MCPs) with enlarged chiral symmetry, when strongly interacting gapless Dirac fermions acquire a comparable propensity toward the nucleation of Kekulé valence-bond solid (KVBS) and charge-density-wave ($N_b=1$) or $s$-wave pairing ($N_b=2$) or antiferromagnet ($N_b=3$) in a honeycomb lattice, where $N_b$ counts the number of bosonic order-parameter components. Besides the cubic terms present in the order-parameter description of KVBS due to the breaking of a discrete $Z_3$ symmetry, quantum fluctuations generate new cubic vertices near the high-symmetry MCPs. All cubic terms are strongly relevant at the bare level near three spatial dimensions, about which we perform a leading-order renormalization group analysis of coupled Gross-Neveu-Yukawa field theory. We show that due to nontrivial Yukawa interactions among gapless bosonic and fermionic degrees of freedom, all cubic terms ultimately become irrelevant at an $O(2+N_b)$ symmetric MCP, at leas near two spatial dimensions, where $N_b=1,2,3$. Therefore, MCPs with an enlarged $O(2+N_b)$ symmetry near KVBS ordering are stable.

In addition to spin, electrons in many Dirac materials possess an additional pseudospin degree of freedom known as “valley.” In materials where the spin and valley degrees of freedom are weakly coupled, they can be both excited and controlled independently. In this work, we study a model describing the interplay of the spin and valley Hall effects in such two-dimensional materials. We demonstrate the emergence of an additional longitudinal neutral current that is both spin and valley polarized. The additional neutral current allows to control the spin density by tuning the magnitude of the valley Hall effect. In addition, the interplay of the two effects can suppress the Hanle oscillation, that is, the oscillation of the nonlocal resistance of a Hall bar device with in-plane magnetic field. The latter observation provides a possible explanation for the absence of the Hanle oscillation in a number of recent experiments. Our work also opens the possibility to engineer the conversion between the valley and spin degrees of freedom in two-dimensional materials.

Recent static experiments on twist effects in chiral three-dimensional mechanical metamaterials have been discussed in the context of micropolar Eringen continuum mechanics, which is a generalization of linear Cauchy elasticity. For cubic symmetry, Eringen elasticity comprises nine additional parameters with respect to linear Cauchy elasticity, of which three directly influence chiral effects. Here, we discuss the behavior of the static case of an alternative generalization of linear Cauchy elasticity, the Willis equations. We show that in the homogeneous static cubic case, only one additional parameter with respect to linear Cauchy elasticity results, which directly influences chiral effects. We show that the static Willis equations qualitatively describe the experimentally observed chiral twist effects, too. We connect the behavior to a characteristic length scale.

Magnon transport and its efficient manipulation in FIs (ferromagnetic insulators) are important issues in magnon spintronics research, while the relaxation and decay of magnons are still in debates. In this paper, starting from the Hamiltonian of a magnetically ordered system, we derive and solve the general magnon Boltzmann equation. It is found that there exists an “antidiffusion driving effect” in magnon transport, which is caused by the collective interaction of a high density (or strong magnetic field) magnon gas in the limited magnetic background. This effect acts as a “spin pressure” on magnons, resulting in a decrease in magnon propagation length and an extra accumulation near the magnon generation boundary. By applying the derived magnon Boltzmann equation to several cases and comparing with the experiments, it is shown that our theory cannot only describe the influence of the external magnetic field on magnon transport but also confirm the fast drop regime in magnon transport is attributed to the boundary effect. The magnon Boltzmann method also predicts a new pathway for manipulations of magnon transport in FIs.

We show that a synthetic pseudospin-momentum coupling can be used to design quasi-one-dimensional disorder-resistant coupled resonator optical waveguides (CROWs). In this structure, the propagating Bloch waves exhibit a pseudospin-momentum locking at specific momenta where backscattering is suppressed. We quantify this resistance to disorder using two methods. First, we calculate the Anderson localization length $\xi$, obtaining an order of magnitude enhancement compared to conventional CROWs for typical device parameters. Second, we study propagation in the time domain, finding that the loss of wave packet purity in the presence of disorder rapidly saturates, indicating the preservation of phase information before the onset of Anderson localization. Our approach of directly optimizing the bulk Bloch waves is a promising alternative to disorder-robust transport based on higher dimensional topological edge states.

We theoretically study the interaction of an isolated bimeron skyrmion in quasi-two-dimensional easy-plane magnets with a surrounding spin superfluid associated with spontaneously broken U(1) spin-rotational symmetry, revealing that skyrmion energy depends on the local spin current flowing in its background. The finding leads us to propose to manipulate a skyrmion energy landscape via a spin supercurrent, which can be controlled nonlocally by varying the magnitudes of spin-current injection and ejection through the boundaries. Two exemplary cases are discussed: a steady-state motion of a skyrmion induced by a uniform force and a skyrmion motion localized along a one-dimensional racetrack. We envision that a skyrmion interacting with a spin superfluid can serve as a robust pointlike information carrier that can be operated with minimal dissipation.

We theoretically study the Kondo screening of a spin-1/2 magnetic impurity in the bulk of a type-II Weyl semimetal (WSM) by use of the variational wave-function method. We consider a type-II WSM model with two Weyl nodes located on the $k_z$ axis, and the tilting of the Weyl cones are along the $k_x$ direction. Due to coexisting electron and hole pockets, the density of states at the Fermi energy becomes finite, leading to a significant enhancement of the Kondo effect. Consequently, the magnetic impurity and the conduction electrons always form a bound state; this behavior is distinct from that in type-I WSMs, where the bound state is only formed when the hybridization exceeds a critical value. Meanwhile, the spin-orbit coupling and unique geometry of the Fermi surface lead to a strongly anisotropic Kondo screening cloud in coordinate space. The tilting terms break the rotational symmetry of the type-II WSM about the $k_z$ axis, but the system remains invariant under a combined transformation $\mathcal{T}{R}^y\left(\pi \right)$, where $\mathcal{T}$ is the time-reversal operation and $R^y\left(\pi \right)$ is the rotation about the $y$ axis by $\pi$. Largely modified diagonal and off-diagonal components of the spin-spin correlation function on three principal planes reflect this change in band symmetry. Most saliently, on the $x-z$ plane, tilting terms trigger the emergence of nonzero off-diagonal spin-spin correlation function $J_{yz}\left(\mathbf{r}\right)$ and $J_{xy}\left(\mathbf{r}\right)$, while in a type-I WSM they vanish on the $x-z$ plane.

Low-temperature spin relaxation time of electrons localized on iodine donors placed in the middle of a 8-nm CdTe quantum well is studied by using the photoinduced Faraday rotation technique, for donor concentrations in the range from $1\times {10}^9\mathrm{c}{\mathrm{m}}^{-2}$ to $3.6\times {10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$. A maximum relaxation time of the order of 20 ns is found at a doping concentration near the metal-insulator transition. Adapting the theoretical description developed for bulk GaAs [R. I. Dzhioev et al., Phys. Rev. B 66, 245204 (2002)] to a quantum well, we have fitted the experimental results taking as a parameter the spin-orbit constant in CdTe. A satisfactory fit is obtained with ${\alpha }_{so}=0.079\pm 0.011,$ which is also very close to the calculated theoretical value. A comparison of the experimental spin relaxation times measured previously in bulk CdTe to those predicted by the theory is done. Finally, we compare the predicted spin relaxation times in bulk CdTe to those predicted in one of the most studied bulk semiconductor materials: GaAs, and in (less studied) ZnSe.

Monolayer $1T^{\prime }-{\mathrm{WTe}}_2$ attracts rapidly growing interests aiming for promising applications in spintronics, dissipationless transport, and quantum computations. Due to one-dimensional W-W chains, $1T^{\prime }-{\mathrm{WTe}}_2$ exhibits unique anisotropic structure and promising properties, which can be modified by simply applying strains. Based on first-principles calculations, we systematically study the phonon dispersion curves as well as the Raman-active modes of $1T^{\prime }-{\mathrm{WTe}}_2$ under different tensile strains. We find that one branch of acoustic phonon softens at special $\mathit{q}$ points under a critical strain of ${\varepsilon }_a=11.55\%$ along the $a$ axis (with $W-W$ chains) direction, or ${\varepsilon }_b=7.0\%$ along the $b$-axis direction and ${\varepsilon }_{ab}=8.44\%$ along the biaxial direction. Before reaching such critical strains, the Raman frequencies of $A_g^1, A_g^3$, and $A_g^4$ modes, contributing to the main peaks in Raman spectra of $1T^{\prime }-{\mathrm{WTe}}_2$, show anisotropic responses to different strains. The response to biaxial strains is found to be the most sensitive. We find that the frequency shift of $A_g^3$ mode shows parabolic characters of strained $1T^{\prime }-{\mathrm{WTe}}_2$, then we split it into two parts and it shows a Raman-shift transition at $\sim 5$% strains. While for $\mathrm{the}{A}_g^1$ and $A_g^4$ modes, the frequencies change linearly.

$Q$-spoiling is a known phenomenon in wave chaos, where an open chaotic cavity deformed from an integrable one exhibits a significantly reduced $Q$-value. In relativistic quantum mechanics, another mechanism that makes trapping of waves difficult is Klein tunneling. For chaotic scattering of a pseudospin-1 wave from a deformed scalar potential domain, both “leaking” mechanisms are thus present. Surprisingly, we find an energy range in which a pseudospin-1 chaotic cavity is capable of defying both $Q$-spoiling and super-Klein tunneling. The physical origin of this remarkable phenomenon is a peculiar type of unexpectedly robust edge modes that absolutely have no counterpart in nonrelativistic quantum or even in pseudospin-1/2 systems. The phenomenon can be tested experimentally in emerging electronic or photonic metamaterials with pseudospin-1 Dirac cones.

Realization of strong correlation effect in topological materials is very rare due to lack of ideal systems. In this Rapid Communication, we present the magnetotransport properties of a correlated nodal-line semimetal YbCdGe. A valence fluctuating state has been observed through magnetic susceptibility and specific heat data. This compound shows an extremely large, nonsaturating transverse magnetoresistance ($1.14\times {10}^3\%$ at 3 K and 12 T). The cusplike magnetoconductivity at low magnetic field indicates the presence of weak antilocalization. The origin of this phenomenon is further supported from the direction dependent transport measurements. Magnetic field-induced metal-semiconductor-like crossover and a plateau in resistivity are observed at low temperature which are common features of topological semimetals. Hall measurement and electronic band-structure calculation suggest that YbCdGe is a topological nodal-line semimetal with higher carrier (hole) density than a typical Dirac/Weyl semimetal.

We have performed angle-resolved photoemission spectroscopy (ARPES) on layered ternary compounds $\mathrm{ZrGe}{X}_c$ ($X_c=\text{S}$, Se, and Te) with square Ge lattices. ARPES measurements with bulk-sensitive soft-x-ray photons revealed a quasi-two-dimensional bulk-band structure with the bulk nodal loops protected by glide mirror symmetry of the crystal lattice. Moreover, high-resolution ARPES measurements near the Fermi level with vacuum-ultraviolet photons combined with first-principles band-structure calculations elucidated a Dirac-node-arc surface state traversing a tiny spin-orbit gap associated with the nodal loops. We found that this surface state commonly exists in $\mathrm{ZrGe}{X}_c$ despite the difference in the shape of nodal loops. The present results suggest that the spin-orbit coupling and the multiple nodal loops cooperatively play a key role in creating the exotic Dirac-node-arc surface states in this class of topological line-node semimetals.

Long range order in the absence of translational symmetry gives rise to drastic revolution of our common knowledge in condensed-matter physics. Quasicrystals, as such an unconventional system, became a plethora to test our insights and to find exotic states of matter. In particular, electronic properties in quasicrystals have gotten lots of attention along with their experimental realization and controllability in twisted bilayer systems. In this work, we study how quasicrystalline order in bilayer systems can induce unique localization of electrons without any extrinsic disorders. We focus on the dodecagonal quasicrystal that has been demonstrated in twisted bilayer graphene system in recent experiments. In the presence of a small gap, we show the localization generically occurs due to the nonperiodic nature of quasicrystals, which is evidenced by the inverse participation ratio and the energy-level statistics. We understand the origin of such localization by approximating the dodecagonal quasicrystals as an impurity scattering problem.

We report a systematic study on strong enhancement of spin-orbit interaction (SOI) in graphene induced by transition-metal dichalcogenides (TMDs). Low-temperature magnetotoransport measurements of graphene proximitized to different TMDs (monolayer and bulk ${\mathrm{WSe}}_2,{\mathrm{WS}}_2$, and monolayer ${\mathrm{MoS}}_2)$ all exhibit weak antilocalization peaks, a signature of strong SOI induced in graphene. The amplitudes of the induced SOI are different for different materials and thickness, and we find that monolayer ${\mathrm{WSe}}_2$ and ${\mathrm{WS}}_2$ can induce much stronger SOI than bulk ${\mathrm{WSe}}_2,{\mathrm{WS}}_2$, and monolayer ${\mathrm{MoS}}_2$. The estimated spin-orbit (SO) scattering strength for graphene/monolayer ${\mathrm{WSe}}_2$ and graphene/monolayer ${\mathrm{WS}}_2$ reaches $\sim 10$ meV, whereas for graphene/bulk ${\mathrm{WSe}}_2$, graphene/bulk ${\mathrm{WS}}_2$, and graphene/monolayer ${\mathrm{MoS}}_2$, it is around 1 meV or less. We also discuss the symmetry and type of the induced SOI in detail, especially focusing on the identification of intrinsic (Kane-Mele) and valley-Zeeman (VZ) SOI by determining the dominant spin relaxation mechanism. Our findings pave the way for realizing the quantum spin Hall (QSH) state in graphene.

We present experimental and theoretical studies of the anomalous high transmission of light (the Borrmann effect) under the Laue diffraction in a one-dimensional photonic crystal (PhC) characterized by spatial modulation of both refractive index and absorption. We show that a strong modulation of the refractive index along with the large PhC period provide new features of the Borrmann effect as compared to the well-known x-ray Borrmann effect in crystals appearing in PhC wavelength-angular transmission spectra. Namely, the maximal transmission is attained at the Bragg angles of incidence and corresponds alternatively to even or odd orders of the Bragg angles depending on the light wavelength. Second, a dramatic decrease of the angular width of the high transmission areas in the spectrum appear near the diabolic points. According to our description, this effect can be treated as a result of the topological phase transition accompanied by exchange of the parity of spatial distribution of the electromagnetic field of the two eigenmodes experiencing degeneracy. We demonstrate that these peculiarities are inherent to the PhC with the optical losses located in layers with higher refractive index, and disappear if the losses are specific for the PhC layers with lower refractive index. The suggested underlying mechanism involves the contribution of the waveguide PhC modes to the PhC transmission spectra.

We propose a real-space approach explaining and controlling the occurrence of edge-localized gap states between the spectral quasibands of binary tight binding chains with deterministic aperiodic long-range order. The framework is applied to the Fibonacci, Thue-Morse, and Rudin-Shapiro chains, representing different structural classes. Our approach is based on an analysis of the eigenstates at weak intersite coupling, where they are shown to generically localize on locally reflection-symmetric substructures, which we call local resonators. A perturbation theoretical treatment demonstrates the local symmetries of the eigenstates. Depending on the degree of spatial complexity of the chain, the proposed local resonator picture can be used to predict the occurrence of gap-edge states even for stronger couplings. Moreover, we connect the localization behavior of a given eigenstate to its energy, thus providing a quantitative connection between the real-space structure of the chain and its eigenvalue spectrum. This allows for a deeper understanding, based on local symmetries, of how the energy spectra of binary chains are formed. The insights gained allow for a systematic analysis of aperiodic binary chains and offers a pathway to control structurally induced edge states.