Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the “moiré pattern” to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide homo- and heterobilayers. However, a largely unexplored factor is the interlayer distance $d$, which can impact the interlayer coupling strength exponentially. In this paper, we quantitatively determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer $\mathrm{Mo}{\mathrm{S}}_2$. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of $K\text{−}\mathrm{valley}$ coupling strength with an 8% reduction of the vdW gap, pointing to a strategy for designing a unique electronic system in vdW bilayers.

To date, second-harmonic generation (SHG) at nanoscale has concentrated on employing high-refractive-index nanostructures, owing to the strong field confinement at deep subwavelength scales based on optically resonant effects. However, nanostructures with lower index contrast between the structures and the surroundings generally exhibit weaker resonant effects and lower field confinement. To address this issue, by harnessing the large nonlinearity of ${\mathrm{LiNbO}}_3$, we propose an approach to employ guided resonances and bound states in the continuum (BICs) with a ${\mathrm{LiNbO}}_3$ metasurface consisting of a ${\mathrm{LiNbO}}_3$ disk array sitting on a ${\mathrm{LiNbO}}_3$ thin film. Such a system can transform the guided modes supported by a ${\mathrm{LiNbO}}_3$ thin film into high-quality guided resonances which can be excited directly under plane-wave illumination. Importantly, we further demonstrate strong field confinement inside a ${\mathrm{LiNbO}}_3$ thin film with a tailorable quality factor ($Q$ factor) by realizing a Friedrich-Wintgen BIC. Such a unique mode engineering enables a record-high SHG efficiency of 5% under a pump intensity as low as 0.4 $\mathrm{MW}/{\mathrm{cm}}^2$. Moreover, we reveal the influence of nonlinear resonances and cross coupling on the SHG by showing the anomalous SHG and efficiency tuning with the rotation of the crystal axis. Our work offers a route to construct enhanced SHG based on high-$Q$ guided resonances and BICs, including low-index and high-index nonlinear materials.

For a wide range of nonclassical magnonic states that have been proposed and demonstrated recently, a new time scale besides the magnon lifetime—the magnon dephasing time—becomes important, but this time scale is rarely studied. Considering exchange interaction and spin-phonon coupling, we evaluate the pure magnon dephasing time and find it to be smaller than the magnon lifetime at temperatures of a few kelvins. By examining a magnonic cat state as an example, we show how pure dephasing of magnons destroys and limits the survival of quantum superpositions. Thus it will be critical to perform quantum operations within the pure dephasing time. We further derive the master equation for the density matrix describing such magnonic quantum states taking into account the role of pure dephasing. This methodology can be generalized to include additional dephasing channels that experiments are likely to encounter in the future. Our findings enable one to design and manipulate robust quantum states of magnons for information processing.

We report the temperature-dependent band structure and energy dependence of the electron-phonon interaction of the topological surface state (TSS) on the three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ using angle-resolved photoemission spectroscopy. Despite the nonrigid-band-like band modification as the temperature increases, the degeneracy at the Dirac point (DP) was robust, indicating topological protection. The electron-phonon coupling parameter depends on the initial-state electron energy, varying from 0.13 (at the DP) to 0.02 (100 meV above the DP), in agreement with reported first-principles calculations. Our findings provide insight into the physical properties derived from TSSs at finite temperatures.

In this Letter we discuss the classical three-dimensional XY model whose nearest-neighbor interaction is a mixture of $cos({\theta }_i-{\theta }_j)$ (ferromagnetic) and $cos2({\theta }_i-{\theta }_j)$ (nematic). This model is dual to a theory with integer and half-integer vortices. While both types of vortices interact with a noncompact $U\left(1\right)$ gauge field, the half-integer vortices interact with an extra interaction mediated by a ${\mathbb{Z}}_2$ gauge field. We shall discuss the confinement-deconfinement transition of the half-integer vortices, the Wilson and the ‘t Hooft loops, and their mutual statistics in path integral language. In addition, we shall present a quantum version of the classical model which exhibits these physics.

The tight-binding method is an effective way to study topological insulators. However, an intrinsic element, i.e., the acoustic sum rule (ASR) in phonon systems, has often been overlooked when constructing the Wannier tight-binding phonon Hamiltonian, which results in edge states of acoustic modes rarely observed in previous works. In the present work, we show that the topological acoustic edge states in graphene need to be assessed by ASR. Interestingly, when the inversion-time-reversal symmetry is broken, the distributions of some edge states will mutate from two edges of the nanoribbon to one edge. Moreover, some other topological phonon modes, which are different from previous reports, have also been observed. Our results would shed light on the method for searching topological phonon states, and provide effective strategies for designing phononic devices.

Spin-polarized currents originate a spin-transfer torque that enables the manipulation of magnetic textures. Here we theoretically study the effect of a spin-polarized current on the magnetic texture corresponding to a chiral soliton lattice in a monoaxial helimagnet under a transverse magnetic field. At sufficiently small current density the chiral soliton lattice reaches a steady motion state with a velocity proportional to the intensity of the applied current, the mobility being independent of the density of solitons and the magnetic field. This motion is accompanied with a small conical distortion of the chiral soliton lattice. At large current density the spin-transfer torque destabilizes the chiral soliton lattice, driving the system to a ferromagnetic state parallel to the magnetic field. We analyze how the deformation of the chiral soliton lattice depends on the applied current density. The destruction of the chiral soliton lattice under current could serve as a possible erasure mechanisms for spintronic applications.

Two-leg spin-$1/2$ ladders with anisotropy and two different dimerization patterns are analyzed at zero temperature. This model is equivalent to a modulated interacting (Kitaev) ladder. The Hartree-Fock mean-field approximation reduces the model to a sum of two quadratic effective Majorana Hamiltonians, which are dual to two quantum transverse XY chains. The mapping between the effective Hamiltonian of the ladder and a pair of chains considerably simplifies calculations of the order parameters and analysis of the hidden symmetry breaking. The ground-state phase diagram of the staggered ladder contains nine phases, four of them are conventional antiferromagnets, while the other five possess nonlocal brane orders. Using the dualities and the newly found exact results for the local and string order parameters of the transverse XY chains, we were able to find analytically all the magnetizations and the brane order parameters for the staggered case, as functions of the renormalized couplings of the effective Hamiltonian. The columnar ladder has three ground-state phases and does not possess magnetic long-ranged order. The brane order parameters for these three phases are calculated numerically from the Toeplitz determinants. All brane-ordered phases are spin liquids with identified distinct order parameters, winding numbers, and sets of the Majorana edge modes. Disorder lines and the special points of disentanglement are found in both dimerization patterns. We expect this study to motivate the search for the real spin-Peierls anisotropic ladder compounds which manifest predicted properties.

We report a comprehensive investigation of the effects of quantum turbulence and quantized vorticity in superfluid ${}^4\mathrm{He}$ at ultralow temperatures on a nonlinear microelectromechanical systems (MEMS) resonator which is uniquely sensitive to fluctuations of quantum turbulence. We observe that the phase noise of the MEMS is significantly increased in the presence of turbulence, while the noise in the amplitude channel is negligible in comparison. We formulated a model based on a fluctuating number of vortices pinned on the device to explain the observed noise. We also present measurements of the noise statistics and power spectra, providing insight into the dissipation mechanism of vortices at the micrometer scale in the ultraquantum regime. The high degree of customizability of MEMS oscillators coupled with their sensitivity to a small number of vortices lays the foundation for a more complete understanding of the interaction between quantized vortices and oscillating structures, and also offers a window into the dynamics of single vortices and small-scale fluctuations of quantum turbulence.

We propose a method for the generation of the transverse valley currents, which is based on the pure crossed Andreev reflection (pCAR) in the superconducting hybrid junctions composed of the gapped $\alpha -{\mathcal{T}}_3$ lattices with ferromagnet-induced exchange interaction. The angle-resolved pCAR probability is asymmetric for a given valley, resulting in the transverse valley currents with zero net charge. This pCAR assisted charge-valley conversion is highly efficient with the valley Hall angle reaching an order of unity, suggesting potential applications for valleytronic devices.

Despite recent experimental progress toward observing large zero-bias conductance peaks (ZBCPs) as signatures of Majorana modes, confusion remains about whether Majorana modes have been observed. This is in part due to the theoretical prediction of fine-tuned trivial (i.e., nontopological) zero-bias peaks that occur because of uncontrolled quantum dots or disorder potentials. While many aspects of the topological phase can be somewhat fine-tuned because the topological phase space is often small, the quantized height of the ZBCP associated with a Majorana mode is known to be robust at sufficiently low temperatures even as the tunnel barrier is pinched off to vanishingly small normal-state conductance. The key shortcoming of the existing experimental works is an acute lack of stability of the putative Majorana mode features, indicating the probable absence of a topological phase, and the current paper suggests specific experimentally accessible measures for a careful quantitative analysis of the measured ZBCP stability. In this paper, we study how the counterintuitive robustness of the ZBCP height to the tunnel barrier strength can be used to distinguish Majorana modes from nontopological ZBCPs. To this end, we introduce a dimensionless quality factor $F$ to quantify the robustness of the ZBCP height based on the range of normal-state (i.e. above-gap) conductance (which depends crucially on the tunnel barrier height) over which the ZBCP height remains within a prespecified range of quantization. By computing this quality factor $F$ together with the topological characteristics for a wide range of models and parameters, we find that Majoranas are significantly more robust (i.e., have a higher value of $F$) compared with nontopological ZBCPs in the ideal low-temperature limit. Even at a temperature as high as the experimentally used 20 mK, we find that we can set a threshold value of $F\sim 2.5$ (for $\epsilon =0.1$) so ZBCPs associated with a quality factor $F>2.5$ are likely topological and $F\ll 2.5$ are topologically trivial. More precisely, the value of $F$ is operationally related to the degree of separation of the Majorana modes in the system, although $F$ uses only the experimentally measured tunnel conductance properties. Finally, we discuss how the quality factor $F$ measured in a transport setup can help estimate the quality of topological qubits made from Majorana modes. In particular, we show that if the induced gap can be enhanced somehow beyond the currently available $\sim 30\mu \mathrm{eV}$ in InAs/Al samples, large (small) values of $F$ could easily distinguish between stable topological (unstable trivial) ZBCPs with the quantum dot induced quasi-Majorana bound states occasionally behaving similar to topological Majorana modes in short wires.

In recent years, photosensitive materials have been in huge demand because of their fascinating ability to convert absorbed photon energy to generate strain and henceforth tuning physical properties. In this work we report the photosensitive activity of ${\mathrm{SrMnO}}_3$. Using a power-dependent and temperature-dependent Raman study with different laser sources having wavelengths across the optical band gap of ${\mathrm{SrMnO}}_3$, we divulge the photosensitive character of ${\mathrm{SrMnO}}_3$ thin film. Upon laser light illumination, Raman modes soften, and softening increases further with an increase in laser power. A similar kind of mode variation is observed with increasing temperature at fixed laser power. A soft x-ray absorption near-edge structure in the presence of laser illumination reveals the change in crystal field splitting associated with mode softening.

Frustration-induced strong quantum fluctuations, spin correlations, and interplay between competing degrees of freedom are some of the key ingredients that underlie exotic states with fractional excitations in quantum materials. Rare-earth-based two-dimensional magnetic lattices possessing a crystal electric field, spin-orbit coupling, anisotropy, and electron correlation between rare-earth moments offer a new paradigm in this context. Herein, we present crystal structure, magnetic susceptibility, and specific heat results accompanied by crystal electric field calculations on polycrystalline samples of ${\mathrm{Ba}}_3{R\mathrm{B}}_9{\mathrm{O}}_{18}$ $(R=\mathrm{Yb}, \mathrm{Er})$, in which $R^{3+}$ ions form a perfect triangular lattice. The localized $R^{3+}$ spins show neither long-range magnetic order nor spin-glass behavior down to 1.9 K in ${\mathrm{Ba}}_3{R\mathrm{B}}_9{\mathrm{O}}_{18}$. Magnetization data reveal pseudospin $J_{\mathrm{eff}}=1/2$ $\left({\mathrm{Yb}}^{3+}\right)$ degrees of freedom in the Kramers doublet state and a weak antiferromagnetic interaction between $J_{\mathrm{eff}}=1/2$ moments in the Yb variant. On the other hand, the effective moment ${\mu }_{\mathrm{eff}}=8.8{\mu }_B$ was obtained from the Curie-Weiss fit of the low-temperature susceptibility data in ${\mathrm{Ba}}_3{\mathrm{ErB}}_9{\mathrm{O}}_{18}$, which suggests the admixture of higher-crystal-electric-field states with the ground state. The Curie-Weiss fit of low-temperature susceptibility data for the Er system unveils the presence of a bit stronger antiferromagnetic interaction between ${\mathrm{Er}}^{3+}$ moments compared with its ${\mathrm{Yb}}^{3+}$ analog. ${\mathrm{Ba}}_3{\mathrm{ErB}}_9{\mathrm{O}}_{18}$ does not show long-range magnetic order down to 500 mK. Furthermore, our crystal electric field calculations based on the thermodynamic data suggest the presence of a small gap between the ground and first excited Kramers doublets. The broad maximum around 4 K in the specific heat at zero field is attributed to the thermal population of the first crystal electric field excited state in ${\mathrm{Ba}}_3{\mathrm{ErB}}_9{\mathrm{O}}_{18}$.

We theoretically study the contribution of superconducting fluctuation to extrinsic spin Hall effects in two- and three-dimensional electron gas and intrinsic spin Hall effects in two-dimensional electron gas with Rashba-type spin-orbit interaction. The Aslamazov-Larkin, density of states, and Maki-Thompson terms have logarithmic divergence $ln\epsilon$ in the limit $\epsilon =(T-T_{\mathrm{c}})/T_{\mathrm{c}}\to +0$ in two-dimensional systems for both extrinsic and intrinsic spin Hall effects except the Maki-Thompson terms in extrinsic effect, which are proportional to ${(\epsilon -{\gamma }_{\phi })}^{-1}ln\epsilon$ with a cutoff ${\gamma }_{\phi }$ in two-dimensional systems. We found that the fluctuation effects on the extrinsic spin Hall effect have an opposite sign to that in the normal state and thus suppress the spin Hall conductivity. In contrast, those on the intrinsic spin Hall effect have the same sign and enhance it.

Twisting two monolayers of a high-$T_c$ cuprate superconductor can engender a chiral topological state with spontaneously broken time-reversal symmetry $\mathcal{T}$. A crucial ingredient required for the emergence of a gapped topological phase is electron tunneling between the ${\mathrm{CuO}}_2$ planes, whose explicit form (in an ideal sample) is dictated by the symmetry of the atomic orbitals. However, a large body of work on interlayer transport in cuprates indicates the importance of disorder-mediated incoherent tunneling, which evades symmetry constraints present in an idealized crystal. This arises even in the cleanest single-crystal samples through oxygen vacancies, in layers separating the ${\mathrm{CuO}}_2$ planes, introduced to achieve the hole doping necessary for superconductivity. Here we assess the influence of incoherent tunneling on the phase diagram of a twisted bilayer and show that the model continues to support a fully gapped topological phase with broken $\mathcal{T}$. Compared to the model with a constant, momentum conserving interlayer coupling, the extent of the topological phase around the ${45}^{\circ }$ twist decreases with increasing incoherence, but remains robustly present for parameters likely relevant to ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$.

The observation of hydrodynamic transport in the metallic delafossite ${\mathrm{PdCoO}}_2$ has increased interest in this family of highly conductive oxides, but experimental studies so far have mostly been confined to bulk crystals. In this paper, the development of high-quality epitaxial thin films of ${\mathrm{PdCoO}}_2$ has enabled a thorough study of the conductivity as a function of film thickness using both dc transport and time-domain terahertz (THz) spectroscopy. As film thickness increases from 12 to 102 nm, the residual resistivity decreases, and we observe a large deviation from Matthiessen's rule (DMR) in the temperature dependence of the resistivity. We find that the complex THz conductivity is well fit by a single Drude term. We fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be nearly independent of the residual resistivity and cannot be the primary mechanism for the observed DMR. Rather, we observe large changes in the spectral weight as a function of disorder, changing by a factor of 1.5 from the most disordered to least disordered films. We believe this corresponds to a mass enhancement of $\ge 2$ times the value of the bulk effective mass which increases with residual disorder. This suggests that the mechanism behind the DMR observed in dc resistivity is primarily driven by changes in the electron mass. We discuss the possible origins of this behavior including the possibility of disorder-enhanced electron-phonon scattering, which can be systematically tuned by film thickness.

Iron-based superconductors offer an ideal platform for studying topological superconductivity and Majorana fermions. In this paper, we carry out a comprehensive study of the band topology and topological surface states of a number of iron-based superconductors using a combination of density functional theory (DFT) and dynamical mean-field theory. We find that the strong electronic correlation of Fe $3d$ electrons plays a crucial role in determining the band topology and topological surface states of iron-based superconductors. Electronic correlation not only strongly renormalizes the bandwidth of Fe $3d$ electrons, but also shifts the band positions of both Fe $3d$ and As/Se $p$ electrons. As a result, electronic correlation moves the DFT-calculated topological surface states of many iron-based superconductors much closer to the Fermi level, which is crucial for realizing topological superconducting surface states and observing Majorana zero modes as well as achieving practical applications, such as quantum computation. More importantly, electronic correlation can change the band topology and make some iron-based superconductors topologically nontrivial with topological surface states whereas they have trivial band topology and no topological surface states in DFT calculations. Our paper demonstrates that it is important to take into account electronic correlation effects in order to accurately determine the band topology and topological surface states of iron-based superconductors and other strongly correlated materials.

Resonant inelastic x-ray scattering (RIXS), which probes the occupied and unoccupied electronic subspaces in an interrelated fashion, is one of the most detailed, complex, and information-rich experimental techniques employed in the investigation of electronic structure across physics, chemistry, and materials science. We introduce an ab initio, accurate, and efficient computational framework for simulation and analysis of RIXS spectra, which combines two diverse and established approaches to modeling electronic excited states. The core-hole linear-response RIXS (CleaRIXS) method not only ensures accurate incorporation of the interaction of electrons with core and valence holes, but also automatically maps the salient RIXS features to the relevant electronic excitations and deexcitations. Through direct comparison with previous methanol C $K$-edge RIXS measurements [A. Benkert et al. J. Phys. Chem. A 120, 2260 (2016).], we show the efficacy of the formalism in modeling different regions of the RIXS spectrum and in gaining physical insight regarding their origins. The importance of including the electron-hole interactions outside the core region is explored, in addition to the connection between CleaRIXS and determinant-based approaches for simulating x-ray absorption and nonresonant x-ray emission. CleaRIXS provides a robust and extendable framework for prediction and interpretation of RIXS processes and for the simulation of complex electronic excited states in general.

In this paper, we propose a general implementation of the Virasoro generators and Kac-Moody currents in generic tensor network representations of two-dimensional (2D) critical lattice models. Our proposal works even when a quantum Hamiltonian of the lattice model is not available, which is the case in many numerical computations involving numerical blockings. We tested our proposal on the 2D Ising model, and also the dimer model, which works to high accuracy even with a fairly small system size. Our method makes use of eigenstates of a small cylinder to generate descendant states in a larger cylinder, suggesting some intricate algebraic relations between lattices of different sizes.