Whereas electromagnetic surface waves are confined to a planar interface between two media, line waves exist at the one-dimensional interface between three materials. Here we derive a nonlocal integral equation for computing the properties of line waves, valid for surfaces characterized in terms of a general tensorial impedance. We find a good approximation—in many cases—is to approximate this as a local differential equation, where line waves are one-dimensional analogs of surface plasmons bound to a spatially dispersive metal. For anisotropic surfaces we find the oscillating decay of recently discovered “ghost” line waves can be explained in terms of an effective gauge field induced by the surface anisotropy. These findings are validated using finite-element simulations.

We have investigated the structure of hydrogen-intercalated quasifreestanding monolayer graphene (QFMLG) grown on $6H$-SiC(0001) by employing total-reflection high-energy positron diffraction. At least nine diffraction spots of the zeroth-order Laue zone were resolved along $\langle 11\overline{2}0\rangle$ and three along $\langle 1\overline{1}00\rangle$, which are assigned to graphene, SiC, and higher-order spots from multiple diffraction on both lattices. We further performed a rocking curve analysis based on the full dynamical diffraction theory to precisely determine the spacing between QFMLG and the SiC substrate. Our study yields a spacing of $d_{\mathrm{QFMLG}}=4.18\left(6\right)\text{Å}$ that is in excellent agreement with the results from density-functional theory calculations published previously.

We numerically study the two-dimensional coherent spectrum (2DCS) of the Tomonaga-Luttinger liquid hosted by the $S=\frac{1}{2}$ XY spin chain. The 2DCS characterizes the system's third-order nonlinear magnetic response triggered by three pulses, separated successively by the delay time and the waiting time. It exhibits a photon echo signal resulting from a lensing process of the fractional excitations: Two photon-excited fractional excitations, initially moving apart, reverse their directions of motion and annihilate each other. In the XY chain, the nonlinearity in the dispersion relation of the Jordan-Wigner fermions leads to the dispersion of the fractional excitation wave packets and thereby suppresses lensing. The magnitude of the echo signal decreases exponentially with increasing delay time. The decay rate scales with the temperature $T$ as $T^n$ at low temperature, where $n$ is the leading order of the Jordan-Wigner fermion dispersion, and as $T$ at high temperature. By contrast, as the waiting time increases, the magnitude of the echo signal saturates, reflecting the integrability of the system. Our results illustrate the effectiveness of the 2DCS in detecting subtle dynamical properties of optical excitations in spin chains.

We investigate magnetic properties of quantum dot arrays of moiré triangular superlattices. Starting from a reciprocal space model, we use the projection technique to obtain maximally localized Wannier functions and determine generalized Hubbard model parameters. The many-body Hamiltonian is solved using the exact diagonalization method as a function of the number of electrons in differently shaped quantum dots arrays. Finite spin polarization is observed within a wide range of filling factors for small twist angles and sufficiently strong interactions in most of the studied structures. The prospect for a magnetization controlled by applying a displacement field is presented. In the vicinity of half-filling, signatures of Nagaoka ferromagnetism in moiré materials are seen, which we demonstrate by comparing results with the corresponding on-site Hubbard model.

We report the elastic properties of the Kondo semimetal CeBi by resonant ultrasound spectroscopy measurements at zero magnetic field. Clear elastic softening is found in bulk modulus $C_B$ below $\sim 60$ K. Such a softening in $C_B$, in addition to the anomalous temperature dependent Poisson's ratio, is hardly attributable to multipolar response for a stable localized $4f$ orbital, but can be well described by a two-band model arising from the hybridization between conduction and $4f$ electrons. These results probably are consequences of the valence fluctuations in this Kondo semimetal as originally suggested by a Fermi-surface expansion observed in a previous angle-resolved photoemission spectroscopy study [P. Li et al., Phys. Rev. B 100, 155110 (2019)].

Half-Heusler systems host a plethora of different ground states, especially with nontrivial topology. However, there is still a lack of spectroscopic insight into the corresponding band inversion in this family. In this work, we locally explore the half-Heuslers $\mathrm{Y}T\mathrm{Bi}$ ($T=\text{Pt}$ and Pd) by means of scanning tunneling microscopy/spectroscopy. From our analysis of the (120) surface plane, we infer that the increase of the spin-orbit coupling upon going from Pd to Pt is the main player in tuning the surface states from trivial to topologically nontrivial. Our measurements unveil a ($2\times 1$) reconstruction of the (120) surface of both systems. Using density functional theory calculations, we show that the observed different behavior of the local density of states near the Fermi level in these two materials is directly related to the presence of metallic surface states. Our work sheds new light on a well known tunable family of materials and opens new routes to explore the presence of topological states of matter in half-Heusler systems and its microscopic observation.

We find that, in the presence of weak incoherent effects from surrounding environments, the low-temperature conductance of nearest-neighbor tight-binding fermionic chains exhibits a counter-intuitive monotonic growth with system length when Fermi energy is near the band edges, indicating a superballistic scaling. This fascinating environment-assisted superballistic scaling of conductance occurs over a finite but extended regime of system lengths. This regime can be systematically expanded by decreasing the coupling to the surrounding environments and by reducing temperature. This behavior is robust against weak disorder and slight shifts from band edge, although the extent of the superballistic scaling regime is affected by them. We give precise predictions of how the superballistic scaling regime depends on coupling to surrounding environments, disorder strength, shifts from band edge, and temperature. There is no corresponding analog of this behavior in isolated systems. The superballistic scaling stems from an intricate interplay of incoherent effects from surrounding environments and exceptional points of the system's transfer matrix that occur at every band edge.

We consider macroscopically large 3-partitions $(A,B,C)$ of connected subsystems $A\cup B\cup C$ in infinite quantum spin chains and study the Rényi-$\alpha$ tripartite information $I_3^{\left(\alpha \right)}(A,B,C)$. At equilibrium in clean 1D systems with local Hamiltonians it generally vanishes. A notable exception is the ground state of conformal critical systems, in which $I_3^{\left(\alpha \right)}(A,B,C)$ is known to be a universal function of the cross ratio $x=\left|A\right|\left|C\right|/\left[\right(\left|A\right|+\left|B\right|\left)\right(\left|C\right|+\left|B\right|\left)\right]$, where $\left|A\right|$ denotes $A$'s length. We identify different classes of states that, under time evolution with translationally invariant Hamiltonians, locally relax to states with a nonzero (Rényi) tripartite information, which furthermore exhibits a universal dependency on $x$. We report a numerical study of $I_3^{\left(\alpha \right)}$ in systems that are dual to free fermions, propose a field-theory description, and work out their asymptotic behavior for $\alpha =2$ in general and for generic $\alpha$ in a subclass of systems. This allows us to infer the value of $I_3^{\left(\alpha \right)}$ in the scaling limit $x\to 1^{-}$, which we call “residual tripartite information”. If nonzero, our analysis points to a universal residual value $-log2$ independently of the Rényi index $\alpha$, and hence applies also to the genuine (von Neumann) tripartite information.

The emergent electric field (EEF) is a fictitious electric field acting on conduction electrons through the Berry phase mechanism. The EEF is generated by the dynamics of noncollinear spin configurations, and it becomes nonzero even in one dimension. Although the EEF has been studied for several one-dimensional chiral magnets, most of the theoretical studies were performed in limited situations with respect to the strength and direction of the magnetic fields. Furthermore, the effect of the edges of the system has not been clarified, whereas it can be crucial in nanoscale and microscale samples. Here, we perform a comprehensive theoretical study on the momentum-frequency profile of the EEF in a one-dimensional chiral magnet using the Landau-Lifshitz-Gilbert equation while changing the strength and direction of the static and ac magnetic fields for both bulk and finite-size chains with edges. From the bulk calculations under the periodic boundary condition, we find that the EEF is resonantly enhanced at the magnetic resonance frequencies; interestingly, the higher resonance modes are more clearly visible in the frequency profile of the EEF response than in the magnetic one. Furthermore, we show that the EEF is amplified along with the solitonic feature of the spin texture introduced by the static magnetic field perpendicular to the chiral axis. We also show that the static magnetic field parallel to the chiral axis drives the EEF in the field direction, in addition to much slower drift motion in the opposite direction associated with the Archimedean screw dynamics, suggesting a dc electric current generation. For the finite-size chains under the open boundary condition, we find additional resonance modes localized at the edges of the system that are also more clearly visible in the EEF response than the magnetic one. Moreover, we show that a substantial EEF is generated from the edges even in the forced-ferromagnetic phase, although it is absent in the bulk case. Our results reveal that the emergent electric phenomena in one-dimensional chiral magnets can be tuned by the magnetic field and the sample size, and they provide not only a good probe of the magnetic resonances but also a platform for the applications to electronic devices.

The effect of random competing single-ion anisotropies in antiferromagnets was studied using epitaxial ${\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_2$ antiferromagnetic thin film alloys grown via molecular beam epitaxy. The crystal structure of this material is tetragonal for all values of $x$, and the Mn sites have a magnetic easy-axis single-ion anisotropy while the Ni sites have an easy-plane anisotropy perpendicular to the Mn easy axis. Crystallographic and magnetization measurements demonstrated that the thin film alloys were homogeneously mixed and did not phase separate into their constituent parts. Pure ${\mathrm{MnF}}_2$ thin films epitaxially grown on ${\mathrm{MgF}}_2$ exhibited compressive strain along all three crystallographic axes which resulted in piezomagnetic effects. The piezomagnetism disappeared if the film was grown on a $\left(\mathrm{MnNi}\right){\mathrm{F}}_2$ graded buffer layer. A mean-field theory fit to the transition temperature as a function of the Mn concentration $x$, which takes into account piezomagnetic effects, gave a magnetic exchange constant between Mn and Ni ions of $J_{\text{MnNi}}=0.305\pm 0.003\mathrm{meV}$. Mean-field theory calculations also predicted the existence of an oblique antiferromagnetic phase in the ${\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_2$ alloy which agreed with the experimental data. A magnetic phase diagram for ${\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_2$ thin film alloys was constructed and showed evidence for the existence of two unique magnetic phases, in addition to the ordinary antiferromagnetic and paramagnetic phases: an oblique antiferromagnetic phase, and an emergent magnetic phase proposed to be either a magnetic glassy phase or a helical phase. The phase diagram is quantitatively different from that of ${\mathrm{Fe}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_2$ because of the much larger single-ion anisotropy of ${\mathrm{Fe}}^{2+}$ compared to ${\mathrm{Mn}}^{2+}$.

Two-dimensional materials with a negative Poisson ratio, which exhibit unique mechanical behavior that expands laterally when stretched, have attracted considerable attention for their practical applications in sensors, biomedicine, and other fields. Here, using the elastic solid theory and first-principles calculations, we screen out a series of two-dimensional (2D) auxetic magnets ${MX}_2$ protected by $P\overline{4}m2$ crystal symmetry. We show that various types of chiral antiferromagnetic spin configurations can be achieved. Specifically, the elliptical antiferromagnetic antiskyrmion can be stabilized in $\mathrm{Mn}{\mathrm{I}}_2$ and $\mathrm{Co}{\mathrm{Cl}}_2$ monolayers under the external strain along the $x$-direction due to the coexistence of a negative Poisson ratio and anisotropic Dzyaloshinskii-Moriya interaction. Our work thus enriches the family of 2D auxetic magnets by introducing intrinsic long-range magnetic order, and it provides a platform for investigating strain-tunable topological antiferromagnetism.

We theoretically show that in a current-carrying hybrid superconductor (S)/normal metal (N) bridge there is a state with spatial oscillations of modulus of superconducting order parameter $\mathrm{\Delta }$ along the bridge and zero vorticity. This stationary state is realized at large currents, when proximity-induced superconductivity in the N layer is suppressed. With an increase of the current the number of oscillations of $\mathrm{\Delta }$ increases, which leads to oscillations of differential resistance or kinetic inductance of an SN bridge with normal or superconducting leads. At current the exceeding critical current $I_{\mathrm{c}}$, this spatially oscillating state transforms to periodically in time moving the vortex chain across the SN bridge. Because of these properties, we call such a state a nascent vortex state.

Extraordinary transmission in wave physics, such as in optics or acoustics, is related to the theory of metamaterials in which resonances enhance wave transmission through apertures. Near-perfect acoustic transmission can, however, exist for subwavelength apertures in a thin wall without resonance, strongly contrasting to the situation in optics. We demonstrate this by experiments on airborne acoustic transmission through metal plates perforated with a variable number of circular holes at constant filling fraction $\alpha \sim 0.12$ in a waveguide. Dissipative theory, including interactions between holes, shows that, at near-unity transmittance, the holes act as a low-inertance metasurface, which is verified by simulations. We also present a simple equation for the lossless, thin-wall case. Calculated enhancements in power transmission and acoustic energy density, which agree with the maximum measured values $\sim 7$ and 27 for constant $\alpha$, are close to the lossless values 1/$\alpha$ and $1/\left(2{\alpha }^2\right)$, respectively, and are also close to simulations, obtained at 1 kHz for one hundred 3.5 mm diameter holes in a 1 mm thick wall. Conditions for the maximum transmission enhancement in the general case are also obtained, demonstrating the counterintuitive result that the enhancement is optimal at $\sim 50\%$ power transmission.

Disorder is ubiquitous in solid-state systems and its crucial influence on transport properties was revealed by the discovery of Anderson localization. Generally speaking, all bulk states will be exponentially localized in the strong disorder limit, but whether an Anderson transition takes place depends on the dimension and symmetries of the system. The scaling theory and symmetry classes are at the heart of the study of the Anderson transition, and the critical exponent $\nu$ characterizing the power-law divergence of localization length is of particular interest. In contrast with the well-established lower critical dimension $d_l=2$ of the Anderson transition, the upper critical dimension $d_u$, above which the disordered system can be described by mean-field theory, remains uncertain and precise numerical evaluations of the critical exponent in higher dimensions are needed. In this study, we apply the Borel-Padé resummation method to the known perturbative results of the nonlinear sigma model to estimate the critical exponents of the Bogoliubov–de Gennes classes. We also report numerical simulations of class DIII in three dimensions, and classes C and CI in four dimensions, and compare the results of the resummation method with these and previously published work. Our results may be experimentally tested in realizations of quantum kicked rotor models in atomic-optic systems, where the critical behavior of dynamical localization in higher dimensions can be measured.

We present small-angle neutron scattering studies of the magnetic vortex lattice (VL) in ${\mathrm{Ca}}_{0.04}{\mathrm{Y}}_{0.96}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ up to a field of 16.7 T and ${\mathrm{Ca}}_{0.15}{\mathrm{Y}}_{0.85}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ up to 25 T to investigate the general behavior of the superconducting gap in YBCO-based compounds at high magnetic field. We find in these overdoped compounds that the series of VL structure transitions have shifted down in field relative to those reported for the undoped compound. The hole doping by calcium is expected to alter the Fermi velocity and it reduces the upper critical field of the system. However, we attribute the VL changes mainly to the weakening of the 1D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the VL is similar to recent measurements on the parent compound in even higher fields of 25 T, which indicates that the fundamental $d$-wave nature of the superconducting gap is unchanged by calcium doping. This is corroborated by the temperature dependence of the VL form factor, which also shows the same $d$-wave behavior as observed in other cuprates. We also argue that they might be the explanation of Pauli paramagnetic effects in the field dependence of the VL form factor.

The hyperfine interactions between the electronic spin of the nitrogen-vacancy (NV) center in diamond and its surrounding nuclear spins are essential for quantum sensing and information processing. However, these interactions can be influenced by strains and lattice vibrations, causing changes in the resonant frequencies and thus the decoherence of multispin system. We used first-principles calculations to obtain the linear response of the hyperfine tensors to all the independent strain components and then converted it to stress response using the elastic tensor, which was obtained via first principles calculation as well. Additionally, we obtained the thermal expansion relation $V(T,P)$ for the NV center from first-principles calculations, rather than relying on experimental data for pristine diamond. This enabled us to calculate the hyperfine tensors as a function of thermal state variables $T$ and $P$, i.e., $\mathit{A}(T,P)$, with the previously overlooked volume-dependence of the vibrational contribution included. For the ${}^{14}\mathrm{N}$ nucleus, the hyperfine interaction variation is mainly due to the vibrational contribution ${\mathit{A}}_{\text{ph}}$, which is insensitive to volume changes due to the near cancellation of two volume-dependent terms. For the surrounding ${}^{13}\mathrm{C}$ nuclei, our calculations confirm previous findings that both the change of the static term ${\mathit{A}}_0$ by thermal expansion and the vibration term ${\mathit{A}}_{\text{ph}}$ are important. Our results reveal a complex interplay among structural, electronic, and vibrational properties of the NV center system and present a comprehensive method for calculating small variations of physical quantities due to external perturbation.

The generalized Drude formula is used to study conductivity properties of the two-dimensional weakly doped Holstein model with a free-electron-like dispersion. The relaxation processes associated with the scattering of conduction electrons by optical phonons are described in terms of a frequency- and temperature-dependent memory function. The imaginary and real parts of the memory function are analyzed in detail in the regime when characteristic energy scales of the problem, i.e., electron Fermi energy and optical phonon energy, are comparable in size. Results obtained at zero temperature and at finite temperatures are used to determine temperature effects in the real part of the dynamical conductivity as well as the frequency dependence of the optical electron mass and the electron relaxation rate. Finally, the characteristic fingerprints of a Holstein system with multiple phonon branches are identified in the dynamical electron conductivity.

${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$ solid solutions show enhanced thermoelectric performance due to band convergence, especially when the x is approximately 0.7. The complex band edges arising from crossover of two conduction bands have been discussed from various aspects, but the temperature-induced band edges renormalization have been neglected. We report the electron-phonon renormalization in ${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$ and find different responses of band gap with temperature. The contributions from lattice expansion and temperature induced atomic vibrations are both considered. The band gaps of the end-point compounds are found to have opposite temperature dependences (e.g., from 0 to 800 K, the ${\mathrm{Mg}}_2\mathrm{Si}$ gap decreases by $\sim 0.12\mathrm{eV}$ while the ${\mathrm{Mg}}_2\mathrm{Sn}$ gap increases by $\sim 0.02\mathrm{eV}$). The remarkably different temperature dependences are due to the different chemical characters of the two conduction bands (light and heavy bands) in ${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$. Our work reveals the origin of relationship between temperature and electronic structure of the band edges in ${\mathrm{Mg}}_2\mathrm{Si}$ and ${\mathrm{Mg}}_2\mathrm{Sn}$. This behavior extends to the alloy system. The temperature effect does not affect the result that there is band convergence of the two carrier pockets near the conduction band minimum in the ${\mathrm{Mg}}_2\left(\mathrm{Si},\mathrm{Sn}\right)$ alloy thermoelectric system.

In this work we theoretically investigate the radiation from electrons passing along a periodic chain of dimers—the particles consisting of two coupled subwavelength parts. High-quality resonances can be excited in dimers due to interaction between the constituent particles, making quasibound states in the continuum. We show that at the resonant frequencies, the intensity of Smith-Purcell radiation increases sharply. Although numerical analysis is performed for the dimers consisting of two equal-coupled particles, the theory constructed is valid for more general binary objects consisting of two different subwavelength-coupled particles of arbitrary form and dielectric properties, and we have formulated the analytical conditions describing this effect. We prove that there is a wide range of the parameters for which Smith-Purcell radiation can be boosted by up to two orders of magnitude. Apparently, the effect can be even stronger with an appropriate choice of parameters. A resonant mechanism of enhancement of Smith-Purcell radiation paves the way to the novel and very effective radiation source based on metasurfaces made of resonant elements.