We investigate the chirality of phonon modes in twisted bilayer ${\mathrm{WSe}}_2$ and demonstrate distinct chiral behavior of the $K/K^{\prime }$ valley phonons for twist angles close to $0^{\circ }$ and close to ${60}^{\circ }$. In particular, multiple chiral nondegenerate $K/K^{\prime }$ valley phonons are found for twist angles near ${60}^{\circ }$ whereas no nondegenerate chiral modes are found for twist angles close to $0^{\circ }$. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moiré scale and find similar modes in moiré patterns of strain-engineered bilayers ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in twisted bilayer ${\mathrm{WSe}}_2$. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moiré materials and also for the design of phononic analogues of flat band electrons.

Optical pulling is a counterintuitive phenomenon whereby light can pull particles against its propagation direction, but usually it is necessary to optimize both the incident beam and the manipulated particles. Here, we propose a robust optical pulling scheme using an air waveguide sandwiched between two chiral hyperbolic metamaterials carrying Weyl points. The pulling force is induced by mode conversion between two topologically protected chiral surface arcs supported on the two metamaterial surfaces of the waveguide. We prove that the optical pulling force is totally attributed to the Minkowski-type momentum of light and proportional to the wave-number difference between the two surface-arc waves, thus the pulling force is robust against the material, shape, and size of the particle. Thanks to the backscattering immunity of the surface-arc waves and the in-plane isotropy of the metamaterials, robust optical pulling can be achieved even in a curved waveguide, going beyond standard mechanisms of straight-line pulling.

In this paper, we propose an approach to manipulate a translation-invariant single-band tight-binding system beyond nearest-neighbor coupling. By using a sequence of multiple identical pulsed electric fields with specific strengths and calculated intervals, the propagator of the system can be fully controlled within an infinite space with dimensions given by the configuration of existing nonzero couplings. And if the sequence is repeated, the system evolution can then effectively simulate another system with a different array of hopping energies, provided that these hopping energies are already nonzero in the simulator. Moreover, the effective system's response to a wide range of additional influences, which are not required to be small, is also shown to approach that of the system it simulates in the limit of high kicking frequency. The simulation is therefore robust and the simulator can potentially be indistinguishable from the lattice it simulates. Considering the physical simplicity, this approach cannot only realize flexible experimental platforms for lattices beyond nearest-neighbor coupling, it is also potentially applicable to the manipulation of actual materials.

Motivated by the recent experiments that reported the discovery of vortex Majorana bound states (vMBSs) in iron-based superconductors, we establish a portable scheme to unveil the non-Abelian statistics of vMBSs using normal fermionic modes. The non-Abelian statistics of vMBSs are characterized by the charge flip signal of the fermions that can be easily read out through the charge sensing measurement. The charge flip signal will be significantly suppressed for strong hybridized vMBSs or trivial vortex modes, which efficiently identifies genuine vMBSs. To eliminate the error induced by the unnecessary dynamical evolution of the fermionic modes, we further propose a correction strategy by continually reversing the energy of the fermions, reminiscent of the quantum Zeno effect. Finally, we establish a feasible protocol to perform non-Abelian braiding operations on vMBSs.

Spin excitations of layered copper oxide show various characteristic features, depending on the carrier concentration. In this paper, we conducted inelastic neutron-scattering measurements on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) with $x=0$, 0.075, 0.18, and 0.30 and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ (LSNO) with 1/3 to clarify the origin of the intensity enhancement in the excitation spectrum of LSCO at the energy ($\omega$) of 16–19 meV [Phys. Rev. B 91, 224404 (2015), Phys. Rev. B 93, 094416 (2016)]. We confirmed the presence of a peak structure in the $\omega$ dependence of the local spin susceptibility ${\chi }^{\prime \prime }\left(\omega \right)$ of superconducting (SC) LSCO with a peak energy of 16–19 meV where the spin excitations intersect optical phonon branches. A comparable peak structure is not observed in the insulating ${\mathrm{La}}_2{\mathrm{CuO}}_4$, LSNO, and heavily overdoped LSCO with $x=0.30$. A dome-shaped $x$ dependence of the integrated intensity around the peak energies is revealed for the SC phase by summarizing the present and previously reported results. Furthermore, our phonon calculation on LCO shows the existence of two optical branches at $\sim 19\mathrm{meV}$ that could stabilize stripe alignment of carriers due to out-of-plane vibrations of Cu or O of the ${\mathrm{CuO}}_2$ planes. These results indicate the interplay among spin, charge, and lattice dynamics and suggest that the intensity enhancement is associated with their composite excitations.

The huge axial ratio of supertetragonal perovskites can induce large polarization due to the coupling between the unit cell and ion displacements, which often results in higher Curie temperature or better ferroelectric properties. With the progress of vertical strain engineering, experimentally available supertetragonal ${\mathrm{PbTiO}}_3$ raises much interest in the relation between its intrinsic polarization and structural characteristics, such as the axial ratio and ion displacements that can be obtained with a high-resolution (scanning) transmission electron microscope nowadays. However, the interpretation and use of such obtained structural information are not without peril. Here, employing first-principles calculations, we report on the relationship between the polarization and the structural information for supertetragonal ${\mathrm{PbTiO}}_3$. Our investigation shows that, unlike normal ${\mathrm{PbTiO}}_3$, the strong sublinear correlation raises the difficulty to quantitatively estimate the spontaneous polarization for the axial ratio larger than 1.15. In addition, the relative displacement between cations (i.e., Pb and Ti) proves to be an unsuitable structural parameter to determine the spontaneous polarization, emphasizing the importance of obtaining the positions of anions, albeit presumably more difficult, along with cations.

The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., $\mathrm{Cr}{X}_2$) or the $M\text{−}M$ ($M$ is the metal atom) dimerization (e.g., $\mathrm{Mn}{X}_2$). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, $\mathrm{Cr}{X}_2$ has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in ${\mathrm{CrS}}_2$, leading to significant actuation strain output (i.e., 12.17% and 5.93% along $x$ and $y$ directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.

Motivated by the recent discovery of the anomalously nearest-neighbor attraction arising from the electron-phonon coupling, we quantitatively investigate the enhancing effects of this additional attractive channel on the $d$-wave superconductivity based on dynamic cluster quantum Monte Carlo calculations of a doped two-dimensional extended Hubbard model with nearest-neighbor attraction $-V$. Focusing on the range of $0<-V/t\le 2$, our simulations indicate that the dynamics of $d$-wave projected pairing interaction is attractive at all frequencies and increases with $\left|V\right|$. Moreover, turning on $-V$ attraction enhances the $(\pi ,\pi )$ spin fluctuations but only enhances (suppresses) the charge fluctuations for small (large) momentum transfer. Thus, at $V/t=-1$ relevant to the “holon folding branch”, the charge fluctuations are insufficient to compete with the $d$-wave pairing interaction strengthened by enhanced spin fluctuations. Our work suggests the underlying rich interplay between the spin and charge fluctuations in giving rise to the superconducting properties.

Within density functional theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and antivortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity.

In the Peierls description of a charge density wave (CDW), Fermi surface nesting (FSN)—defined by the divergence of the imaginary part of electronic susceptibility, i.e., $\mathrm{Im}\left\{{\chi }_0\right\}$—leads to divergence of the real part, thus inducing CDW instability at wave vector ${\mathbf{q}}_{\mathrm{CDW}}$. Here we show that the divergence of $\mathrm{Im}\left\{{\chi }_0\right\}$ implying a divergence of $\mathrm{Re}\left\{{\chi }_0\right\}$ at the same ${\mathbf{q}}_{\mathrm{CDW}}$ breaks down for three-dimensional Fermi surfaces and is particularly severe for linearly dispersing electronic bands across the Fermi level ($E_{\mathrm{F}}$), as exemplified by rare-earth tellurides $R{\mathrm{Te}}_n$. By calculating the orbital-, atom-, and momentum-resolved contribution to ${\chi }_0$ of ${\mathrm{EuTe}}_4$, we find that FSN and CDW instability are not driven by the same atoms and orbitals but from different ones. This unique behavior is enabled by linearly dispersing bands across $E_{\mathrm{F}}$ with constant Fermi surface velocity that assists electron-hole pairs to form not only at $E_{\mathrm{F}}$ but also across $E_{\mathrm{F}}$ at ${\mathbf{q}}_{\mathrm{CDW}}$, hence allowing different orbitals to contribute to the divergence of $\mathrm{Re}\left\{{\chi }_0\right\}$ and $\mathrm{Im}\left\{{\chi }_0\right\}$. The above scenario is general and applicable to recent observations of CDWs and spin density waves (SDWs) from linearly dispersing bands in several Dirac and Weyl semimetals and kagome metals. Moreover, as we demonstrate, such component-resolved analysis provides focused input to engineer CDW and SDW states.

A model for ripple formation on liquid surfaces exposed to an external laser or particle beam and a variable ground is developed. The external incident beam is hereby mechanically coupled to the liquid surface due to surface roughness. Starting from the Navier-Stokes equation, the coupled equations for the velocity potential and the surface height are derived in a shallow-water approximation with special attention to viscosity. The resulting equations obey conservation laws for volume and momentum where characteristic potentials for gravitation and surface tension are identified analogously to conservative forces. The approximate solutions are discussed in the context of ripple formation in laser-materials processing involving melting of a surface by a laser beam. Linear stability analysis provides the formation of a damped wave modified by an interplay between the external beam, the viscosity, and the surface tension. The limit of small viscosity leads to damped gravitational and the limit of high viscosity to capillary waves. The resulting wavelengths are in the order of the ripples occurring in laser welding experiments, hinting at the involvement of hydrodynamic processes in their origin. By discussing the response of the system to external periodic excitations with the help of Floquet multipliers, we show that the ripple formation could be triggered by a a periodically modulated external beam, e.g., appropriate repetition rates of an incident laser beam. The weak nonlinear stability analysis provides ranges where hexagonal or stripe structures can appear. The orientation of stripe structures and ripples are shown to be dependent on the incident angle of the laser or particle beam where a minimal angle is reported. Numerical simulations confirm the findings and allow us to describe the influence of variable grounds.

In this paper, we are reporting on the structural and electronic properties of Gd-Pt surface alloys grown on a Pt(111) substrate. Using scanning tunneling microscopy and spectroscopy combined with density functional theory calculations, we are exploring differences between three different surface alloys, identified as single-layer ${\mathrm{GdPt}}_2$, single-layer ${\mathrm{GdPt}}_5$, and double-layer ${\mathrm{GdPt}}_5$. We show that an appropriate choice of substrate temperature as well as surface coverage with Gd atoms allows for selective growth of all observed surface structures.

We construct a sign-problem-free variant of the complex Sachdev-Ye-Kitaev (SYK) model which keeps all the essential properties of the SYK model, including the analytic solvability in the large-$N$ limit and being maximally chaotic. In addition to the number of complex fermions $N$, our model has an additional parameter $M$ controlling the number of terms in the Hamiltonian which we take $M\to \infty$ with keeping $M/N$ constant in the large-$N$ limit. While our model respects global $\text{U}\left(1\right)$ symmetry associated with the fermion number conservation, both the large-$N$ limit and the sign-problem-free nature become explicit in the Majorana representation. We present a detailed analysis on our model, i.e., the random matrix classification based on the symmetry analysis, analytic approach, and the quantum Monte Carlo simulations. All these analyses show that our model exhibits a non-Fermi-liquid physics, a gapless fermionic system lying beyond the conventional Fermi-liquid picture.

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the three-dimensional atomistic Green's function method combined with first-principles calculations, based on the density functional theory, of all interatomic force constants. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps $d$ smaller than $\sim 1$ nm, and follows a $d^{-11.9\pm 1.2}$ power law. It is shown that overlapping electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. The first-principles-based approach proposed in this paper is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.

Nuclear spins in solids exhibit long coherence times due to the small nuclear gyromagnetic ratio. This weak environmental coupling comes at the expense of slow quantum gate operations, which should be as fast as possible for many applications in quantum information processing and sensing. In this work we use nitrogen-vacancy centers in diamond to probe the nuclear spins within dark paramagnetic nitrogen defects (P1 centers) in the diamond lattice. The gyromagnetic ratio of the P1 nuclear spin is augmented by hyperfine coupling to the electron spin, resulting in greatly enhanced coupling to radio frequency control fields. We then demonstrate that this effect can be tuned by variation of an external magnetic field. Our work identifies regimes in which we are able to implement fast quantum control of dark nuclear spins and lays the foundations for further inquiry into rapid control of long-lived spin qubits at room temperature.

We uncover an alternative two-particle Berry phase mechanism to realize exotic corner modes in second-order topological insulators (TIs) and topological superconductors (TSCs) with time-reversal symmetry. We show that the nontrivial pseudospin textures of edge states in two different types of two-dimensional TIs give rise to different two-particle geometric phases in the particle-hole and particle-particle channels, respectively, for which the edge mass domain walls or intrinsic $\pi$ junctions emerge across corners when an external magnetic field or $s$-wave superconductivity is considered, hosting Dirac or Majorana corner modes. Remarkably, with this mechanism we predict the Majorana Kramers pair of corner modes by directly coupling the edge of a type-II time-reversal invariant TI to a uniform $s$-wave SC, in sharp contrast to previous proposals which rely on unconventional SC pairing or a complex setting for a fine-tuned SC $\pi$ junction. We find Au/GaAs(111) to be a realistic material candidate for realizing such a Majorana Kramers pair of corner modes.

We perform measurements of the heat capacity as a function of temperature on ${\mathrm{URu}}_{2-x}{\mathrm{Os}}_x{\mathrm{Si}}_2$ alloys. Our experimental results show that the critical temperature of the second-order phase transition increases while the value of the Sommerfeld coefficient in the ordered state decreases with an increase in osmium concentration. We also observe an increase in the values of the heat capacity at the critical temperature as well as a broadening of the critical fluctuation region with an increase in $x$. We analyze the experimental data using the Haule-Kotliar model which, in particular, identifies the “hidden order” transition in the parent material ${\mathrm{URu}}_2{\mathrm{Si}}_2$ as a transition to a state with a nonzero hexadecapole moment. We demonstrate that our experimental results are consistent with the predictions of that model.

Electronic correlations associated with incipient magnetism have long been recognized as an important factor in stabilizing the largest atomic volume $\delta$ phase of plutonium, yet their strength compared to those in the rare earths and neighboring actinides in the periodic table has largely remained a mystery. We show here using calorimetry measurements, together with prior detailed measurements of the phonon dispersion, that the $5f$ electrons of the $\delta$ phase reside in a pseudogapped state, accompanied by reductions in various physical properties below a characteristic temperature $T^{*}\approx 100\mathrm{K}$. The small characteristic energy scale of the pseudogapped state implies that the $5f$ electrons in plutonium are much closer to the threshold for localization and magnetic order than has been suggested by state-of-the-art electronic structure theory, revealing plutonium to be arguably the most strongly correlated of the elements.

Based on first-principles calculations, we discover a two-dimensional (2D) layered antiferromagnetic (AFM) electride ${\mathrm{Gd}}_2\mathrm{O}$, where anionic excess electrons exist in the interstitial spaces between positively charged cationic layers. It is revealed that each cationic layer composed of three-atom-thick Gd-O-Gd stacks has in-plane ferromagnetic and out-of-plane AFM superexchange interactions between the localized Gd $4f$ spins through O $2p$ orbitals. Interestingly, the interlayer superexchange mediated by the hybridized Gd $5d$ and interstitial-$s$-like states involves intimate couplings between the spin, lattice, and charge degrees of freedom, thereby inducing simultaneous magnetic, structural, and electronic phase transitions. The resulting ground state with the simple hexagonal lattice hosts massless Dirac fermions protected by nonsymmorphic magnetic symmetry, as well as massive Dirac fermions. We thus demonstrate that the anionic excess electrons in AFM ${\mathrm{Gd}}_2\mathrm{O}$ play an important role in the emergence of Dirac semimetal states, therefore offering an intriguing interplay between 2D magnetic electrides and topological physics.