We theoretically investigate the impurity levels and exchange interaction between magnetic impurities in graphene driven by an off-resonant circularly polarized light field. Our analysis captures the nonperturbative effects resulting from scattering with magnetic impurities with a strong on-site potential. Under irradiation, a dynamical band gap opens up at the Dirac point, allowing impurity levels to exist inside the gap. These impurity levels are shown to give rise to a resonance feature in the exchange energy for impurities located either at the same sublattice or at different sublattices. The exchange interaction also shows a wider spatial range of antiferromagnetic behavior due to irradiation. Our work demonstrates that the exchange energy of magnetic impurities in graphene is extensively tunable by light irradiation in the presence of strong potential scattering.

Compared to monolayer graphene, electrons in Bernal-stacked bilayer graphene (BLG) have an additional layer degree of freedom, offering a platform for developing layered spintronics with the help of proximity-induced magnetism. Based on an effective phenomenological model, we systematically study the effect of this magnetism on the spin-dependent band structure near the Fermi energy and identify the magnetic phases induced in BLG by proximity with magnets. We show that spin polarization can develop in BLG due to this proximity effect. This spin polarization depends strongly on the layer distribution of magnetism, and can always be controlled by gate voltage which shifts spin-dependent band edges and modifies the total band gap. We further show that the band spin polarization can be modified by the proximity-induced staggered sublattice potential. By taking full advantage of layer-dependent magnetism in BLG, we propose that spintronic devices such as a spin filter, a giant magnetoresistance device, and a spin diode can operate under fully electric control, which is easier than the common magnetic field control.

The study of novel 2D platforms implementing magnetism in tunable van der Waals (vdW) homo- and heterostructures paves the way to innovative spintronics and magnetic devices. In this study, we unravel the intriguing properties of few-layer ${\mathrm{Nb}}_3{\mathrm{I}}_8$ vdW nanofilms from first principles, showing how and to what extent specific magnetic orderings can be tuned using several degrees of freedom, such as film thickness, stacking geometry, and strain or even a combination of them. All these aspects are explored here, giving a comprehensive view of this novel and promising magnetic material.

Inelastic electron tunneling in a scanning tunneling microscope is used to generate excitons in monolayer tungsten disulfide (${\mathrm{WS}}_2$). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments.

In a first-order topological phase with sublattice degrees of freedom, a change in the boundary sublattice termination has no effect on the existence of gapless boundary states in dimensions higher than one. However, such a change may strongly affect the physical properties of those boundary states. Motivated by this observation, we perform a systematic study of the impact of sublattice terminations on the boundary physics on the two-dimensional kagome lattice. We find that the energies of the Dirac points of helical edge states in two-dimensional first-order topological kagome insulators sensitively depend on the terminating sublattices at the edge. Remarkably, this property admits the realization of a time-reversal invariant second-order topological superconducting phase with highly controllable Majorana Kramers pairs at the corners and sublattice domain walls by putting the topological kagome insulator in proximity to a $d$-wave superconductor. Moreover, substituting the $d$-wave superconductor with a conventional $s$-wave superconductor, we find that highly controllable Majorana zero modes can also be realized at the corners and sublattice domain walls if an in-plane Zeeman field is additionally applied. Our study reveals promising platforms to implement highly controllable Majorana zero modes.

Ultrafast spectroscopy of coherent acoustic phonon (CAP) dynamics has recently been proposed as a method to characterize acoustic deformation potential (ADP), a key standard to quantify carrier–acoustic phonon coupling in semiconductors. In this Letter, we illustrate the importance of addressing the diffusion effect in ADP characterization by this method, using Ge as the demonstration system. It is found that the ADP mechanism and the thermoelastic effect have comparable contributions to CAP generation in Ge. Due to the different dependences on pump photon energies, the roles of these two mechanisms were assessed by varying pump wavelengths, based on which the ADP coupling constant of Ge was obtained. The analysis reveals that the carrier diffusion has a considerable impact on the shape of the CAP wave packet and must be processed cautiously for the ADP characterization for Ge.

Periodically driven (Floquet) phases are attractive due to their ability to host unique physical phenomena with no static counterparts. We propose a general approach in nontrivially devising a square-root version of existing Floquet phases, applicable both in noninteracting and in interacting setting. The resulting systems are found to yield richer physics that is otherwise absent in the original counterparts and is robust against parameter imperfection. These include the emergence of Floquet topological superconductors with arbitrarily many zero, $\pi$, and $\pi /2$ edge modes, as well as $4T$-period Floquet time crystals in disordered and disorder-free systems ($T$ being the driving period). Remarkably, our approach can be repeated indefinitely to obtain a $2^n\mathrm{th}$-root version of any periodically driven system, thus, allowing for the discovery and systematic construction of exotic Floquet phases.

I discuss the quantum dynamics of strongly disordered quantum systems with critically long range interactions, decaying as $1/r^{2d}$ in $d$ spatial dimensions. I argue that, contrary to expectations, localization in such systems is stable at low orders in perturbation theory, giving rise to an unusual “critically many-body localized (MBL) regime.” I discuss the phenomenology of this critical MBL regime, which includes distinctive signatures in entanglement, charge statistics, noise, and transport. Experimentally, such a critically localized regime can be realized in three-dimensional systems with van der Waals interactions, such as Rydberg atoms, and in one-dimensional systems with $1/r^2$ interactions, such as trapped ions. I estimate timescales on which high-order perturbative and nonperturbative (avalanche) phenomena may destabilize this critically MBL regime and conclude that the avalanche sets the limiting timescale, in the limit of strong disorder or weak interactions.

Inorganic electrides are a new class of compounds catering to the interest of scientists due to the multiple usages exhibited by interstitial electrons in the lattice. However, the influence of the shape and distribution of interstitial electrons on physical properties and new forms of physical states is still unknown. In this work, crystal structure search algorithms are employed to explore the possibility of forming unique electrides in the As-Li system, where interstitial electrons behave as one-dimensional (1D) electron chains (1D electride) in the $Pmmm$ phase of $\mathrm{As}{\mathrm{Li}}_7$ and transform into zero-dimensional (0D) electron clusters (0D electride) in the $P6/mmm$ phase at 80 GPa. The $P6/mmm$ phase has relatively high superconductivity at 150 GPa $\left(T_c=38.4\mathrm{K}\right)$ compared to classical electrides, even at moderate pressure with $T_c=16.6\mathrm{K}$. In addition, a Dirac cone in the band has been observed, expanding the sources of Dirac materials. The survival of $\mathrm{As}{\mathrm{Li}}_7$ at room temperature is confirmed by molecular dynamics simulation at 300 K. At 1000 K, the As atoms in the system act like a solid, while a portion of the Li atoms cycle around the As atoms, and another portion of the Li atoms flow freely like a liquid, showing the unusual physical phenomenon of the coexistence of the plastic and superionic states. This suggests that the superionic and plastic states cannot only be found in hydrides but also in the electride. Our results indicate that superconducting electride $\mathrm{As}{\mathrm{Li}}_7$ with superionic and plastic states can exist in Earth's interior.

The complexity of electronic band structures in quantum materials offers new charge-neutral degrees of freedom stable for transport, a promising example being the valley (axial) degree of freedom in Weyl semimetals (WSMs). A noninvasive probe of their transport properties is possible by exploiting the frequency dependence of the magnetic noise generated in the vicinity of the material. In this Letter, we investigate the magnetic noise generically associated with diffusive transport using a systematic Langevin approach. Taking a minimal model of magnetic WSMs for demonstration, we show that thermal fluctuations of the charge current, the valley current, and the magnetic order can give rise to magnetic noise with distinctively different spectral characters, which provide a theoretical guidance to separate their contributions. Our approach is extendable to the study of magnetic noise and its spectral features arising from other transport degrees of freedom in quantum materials.

Recent experimental observation of inertial spin dynamics calls upon holistic reevaluation of the theoretical framework of magnetic resonance in ferromagnets. Here, we derive the secular equation of an inertial spin system in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency of precessional ferromagnetic resonances is decreased as compared to the noninertial case. We also find that the frequency of nutational resonances is generally increased due to the presence of magnetic anisotropy and applied magnetic field. We obtain exact solutions of the secular equation and approximations that employ the terminology of noninertial theory and thus allow for convenient estimates of the inertial effects.

Despite many experimental and theoretical efforts, the pairing symmetry of superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ remains undecided. The accidentally degenerate $d_{x^2-y^2}+ig$ is consistent with most current experiments and seems to be one of the most probable candidates, but we still lack a satisfactory theoretical mechanism for its appearance. Here we construct a phenomenological model combining realistic electronic band structures and all symmetry-allowed multipole fluctuations as potential pairing glues and make a systematic survey of major pairing states within the Eliashberg framework. Our calculations show that $d_{x^2-y^2}+ig$ can arise naturally from the interplay of antiferromagnetic, ferromagnetic, and electric multipole fluctuations whose coexistence is manifested in previous experiments and calculations. Our work provides a physically reasonable basis supporting the possibility of $d_{x^2-y^2}+ig$ pairing in superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Experimental identification of quantum spin ice (QSI), a U(1) quantum spin liquid on the pyrochlore lattice hosting emergent photons, is a major challenge in frustrated magnets. In this work, we propose ultrasound measurements as a tool for probing the emergent photons of various QSI phases. Our analysis includes QSI phases in non-Kramers doublet compounds such as ${\mathrm{Pr}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ as well as dipolar-octupolar Kramers doublet compounds such as ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$. The latter may host emergent photons associated with an octupolar component which renders them difficult to detect with inelastic neutron scattering. We demonstrate theoretically how the speed of the emergent photons can be obtained from the renormalization of the phonon spectrum and show that ultrasound measurements provide a means of distinguishing the dipolar from the octupolar QSI phase in dipolar-octupolar materials.

Whereas a quantum spin liquid (QSL) phase has been identified in the $J_1\text{−}{J}_2$ Heisenberg model on a triangular lattice via numerical calculations, debate persists about whether or not such a QSL is gapped or gapless with contradictory conclusions from different techniques. Moreover, information about excitations and dynamics is crucial for the experimental detection of such a phase. In this paper, we use exact diagonalization to characterize signatures of a QSL phase on the triangular lattice through the dynamical spin structure factor $\mathcal{S}(q,\omega )$ and Raman susceptibility $\chi \left(\omega \right)$. We find that spectra for the QSL phase show distinct features compared to those of neighboring phases; and both the Raman spectra and spin structure factor show gapped behavior in the QSL phase. Interestingly, there is a prominent excitation mode in the Raman $A_2$ channel, indicating a strong subleading tendency toward a chiral spin liquid phase.