One of the exciting features of two-dimensional (2D) materials is their electronic and optical tunability through strain engineering. Previously, we found a class of 2D ferroelectric Rashba semiconductors $\mathrm{Pb}X$ ($X=\text{S}$, Se, Te) with tunable spin-orbital properties. In this work, based on our previous tight-binding (TB) results, we derive an effective low-energy Hamiltonian around the symmetry points that captures the effects of strain on the electronic properties of $\mathrm{Pb}X$. We find that strains induce gauge fields which shift the Rashba point and modify the Rashba parameter. This effect is equivalent to the application of in-plane magnetic fields. The out-of-plane strain, which is proportional to the electric polarization, is also shown to modify the Rashba parameter. Overall, our theory connects strain and spin splitting in ferroelectric Rashba materials, which will be important to understand the strain-induced variations in local Rashba parameters that will occur in practical applications.

The Raman frequency of the intersubband charge-density excitation plasmon in type-I and type-II quantum wells is known to be very sensitively controlled via a low-power optical pump signal. We find that, above a threshold electron density of approximately $4\pm 2\times {10}^{10}\mathrm{c}{\mathrm{m}}^{-2}$, the charge density plasmon mode splits first into two, and then with increasing density into three closely spaced frequencies. A similar splitting occurs in the spin-density wave plasmon associated with the same intersubband transition. We analyze the results including the coupling to the longitudinal optical phonon and hypothesize that the splittings arise from the special situation of a structure of spatially separated bipolar plasmons.

The ubiquitous van der Waals (vdW) interaction determines the fundamental properties of material surfaces. Here, we show that graphene can partly screen the vdW energy by up to 53%, as tested in a graphene trilayer and a sandwiched BN/graphene/BN multilayer using the random phase approximation and density functional theory based many-body-dispersion method. The vdW screening turns weaker in semiconducting and insulating graphenelike monoatomic layers, displaying a strong correlation with the band gap and the dielectric constant of materials, but depends less on the screened materials. The revealed relationship between the vdW screening and the fundamental electronic structure of the graphenelike material will be instrumental in the rational control of surface interactions.

A key feature of topological insulators (TIs) is symplectic symmetry of the Hamiltonian which changes to unitary when time-reversal symmetry is lifted and a topological phase transition occurs. However, such a crossover has yet to be explicitly observed by directly probing the symmetry class of the Hamiltonian. In this Rapid Communication, we have probed the symmetry class of topological insulators by measuring the mesoscopic conductance fluctuations in the TI ${\mathrm{Bi}}_{1.6}{\mathrm{Sb}}_{0.4}{\mathrm{Te}}_2\mathrm{Se}$, which shows an exact factor of 2 reduction on application of a magnetic field due to a crossover from symplectic to unitary symmetry classes. The reduction provides an unambiguous proof that the fluctuations arise from the universal conductance fluctuations (UCFs), due to quantum interference, and persists from $T\sim 22\mathrm{mK}$ to $4.2\mathrm{K}$. We have also compared the phase breaking length $l_{\varphi }$ extracted from both magnetoconductance and UCFs which agree well within a factor of 2 in the entire temperature and gate voltage range. Our experiment confirms UCF as the major source of fluctuations in mesoscopic disordered topological insulators, and the intrinsic preservation of time-reversal symmetry in these systems.

A heavy hole confined to an InGaAs quantum dot promises the union of a stable spin and optical coherence to form a near perfect, high-bandwidth spin-photon interface. Despite theoretical predictions and encouraging preliminary measurements, the dynamic processes determining the coherence of the hole spin are yet to be understood. Here, we establish the regimes that allow for a highly coherent hole spin in these systems, recovering a crossover from hyperfine to electrical-noise dominated decoherence with a few-Tesla external magnetic field. Dynamic decoupling allows us to reach the longest ground-state coherence time, $T_2$, of $4.0\pm 0.2\mu \mathrm{s}$, observed in this system. The improvement of coherence we measure is quantitatively supported by an independent analysis of the local electrical environment.

Topological semimetals are a fascinating class of quantum materials that possess extraordinary electronic and transport properties. These materials have attracted great interest in recent years for their fundamental significance and potential device applications. Currently a major focus in this research field is to theoretically explore and predict and experimentally verify and realize material systems that exhibit a rich variety of topological semimetallic behavior, which would allow a comprehensive characterization of the intriguing properties and a full understanding of the underlying mechanisms. In this paper, we report on ab initio calculations that identify a carbon allotrope with simple orthorhombic crystal structure in $Pbcm$ ($D_{2h}^{11}$) symmetry. This carbon allotrope can be constructed by inserting zigzag carbon chains between the graphene layers in graphite or by a crystalline modification of a (3,3) carbon nanotube with a double cell reconstruction mechanism. Its dynamical stability has been confirmed by phonon and molecular dynamics simulations. Electronic band calculations indicate that it is a nodal-line semimetal comprising two nodal lines that go through the whole Brillouin zone in bulk and a projected surface flat band around the Fermi level. The present findings establish an additional topological semimetal system in the nanostructured carbon allotropes family and offer insights into its outstanding structural and electronic properties.

Nodal-loop semimetals are materials in which the conduction and valence bands cross on a one-dimensional loop in the reciprocal space. For the nodal-loop character to manifest in physical properties, it is desired that the loop is close to the Fermi level, relatively flat in energy, simple in its shape, and not coexisting with other extraneous bands. Here, based on the first-principles calculations, we show that monoclinic ${\mathrm{CuTeO}}_3$ is a realistic nodal-loop semimetal that satisfies all these requirements. The material features only a single nodal loop around the Fermi level, protected by either of the two independent symmetries: the $\mathcal{PT}$ symmetry and the glide mirror symmetry. The size of the loop can be effectively tuned by strain, and the loop can even be annihilated under stain, making a topological phase transition to a trivial insulator phase. Including the spin-orbit coupling opens a tiny gap in the loop, and the system becomes a ${\mathbb{Z}}_2$ topological semimetal with a nontrivial bulk ${\mathbb{Z}}_2$ invariant but no global band gap. The corresponding topological surface states have been identified. We also construct a low-energy effective model to describe the nodal loop and the effect of spin-orbit coupling.

The linear response of a two-dimensional electron gas in a perpendicular magnetic field in the presence of a spatially dependent classically smooth electrostatic potential is studied theoretically, by application of the Kubo formula for a nonlocal conductivity tensor. In the classical transport regime, a general expression for the conductivity tensor through the correlation functions of the homogeneous electron gas is derived. The quantum transport regime, when Landau quantization is essential, is studied for the case of unidirectional periodic potential modulation. Apart from the Shubnikov–de Haas oscillations, the resistivity can demonstrate quantum oscillations with larger periods and smaller amplitudes, which survive when temperature increases. These oscillations exist when the modulation amplitude considerably exceeds the cyclotron energy so the Landau subbands, formed out of the Landau levels by the modulation potential, overlap in the energy domain. Both diagonal components of the resistivity tensor demonstrate oscillations related to modification of the density of states by the modulation. In addition, the resistivity component perpendicular to the modulation axis, which is caused by the scattering-assisted hopping transport, shows another kind of oscillation related to enhancement of the hopping probability when the guiding center of cyclotron orbit shifts by the doubled cyclotron radius. It is suggested that such high-temperature oscillations can be detected under conditions when the modulation period considerably exceeds the cyclotron radius.

By combining experimental photoluminescence excitation spectroscopy and calculations based on density functional theory and many-body Green's functions, the most efficient excitation channels of infrared (IR) emission from Mn-${\mathrm{Mg}}_k$ paramagnetic complexes stabilized in GaN:(Mn,Mg) are here identified. Moreover, a Tanabe-Sugano energy diagram for $3d^2{\mathrm{Mn}}^{5+}$ is reconstructed and Mn-${\mathrm{Mg}}_3$ are singled out as the predominant configurations responsible for the IR emission. The correlation of intensity of the individual emission lines as a function of temperature and excitation energy allows assigning them to well-defined and specific optical transitions.

Motivated by the recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)] concerning possible thermoelectric signatures of Dirac composite fermions, we perform a systematic experimental study of thermoelectric transport of an ultrahigh-mobility $\mathrm{GaAs}/\mathrm{A}{\mathrm{l}}_x\mathrm{G}{\mathrm{a}}_{1\text{−}x}\mathrm{As}$ two-dimensional electron system at filling factor $v=1/2$. We demonstrate that the thermopower $S_{xx}$ and Nernst $S_{xy}$ are symmetric and antisymmetric with respect to $B=0$ T, respectively. The measured properties of thermopower $S_{xx}$ at $v=1/2$ are consistent with previous experimental results. The Nernst signals $S_{xy}$ of $v=1/2$, which have not been reported previously, are nonzero and show a power-law relation with temperature in the phonon-drag dominant region. In the electron-diffusion dominant region, the Nernst signals $S_{xy}$ of $v=1/2$ are found to be significantly smaller than the linear temperature dependent values predicted by Potter et al., and decreasing with temperature faster than linear dependence.

Subwavelength spatial control of phase using metasurfaces is attractive for practical applications such as flat lens, vector beam generator, ultrathin wave-plate and high-resolution holography. All-dielectric metasurfaces provide two phase modulation mechanisms, which are propagation phase dependent on the dimension of meta-atoms and geometric phase dependent on the orientation of meta-atoms. Recently, it was proposed that by combining geometric phase and propagation phase, arbitrary and independent phase profiles can be imposed on a pair of orthogonal polarizations. However, a succinct expression of phase change introduced during an arbitrary polarization conversion process has not been presented. Here, we derive the geometric phase for the general case of nonorthogonal polarization conversions. Through the numerical simulations of a metasurface orbital angular momentum generator under both circular and elliptical polarization illuminations, we verify our theoretical analyses. This work presents the mechanism of controlling polarization and phase of arbitrary polarizations using both geometric phase and propagation phase, which exhibits the potential to enrich the capacity of polarization multiplexed metasurfaces.

The microwave absorption and noise caused by quantum two-level systems (TLSs) dramatically suppress the coherence in Josephson junction qubits that are promising candidates for quantum information applications. It is a challenge to understand microwave absorption by TLSs because of the spectral diffusion resulting from fluctuations in their resonant frequencies induced by their long-range interactions. Here we treat the spectral diffusion explicitly using the generalized master equation formalism. The proposed theory predicts that the linear absorption regime holds while a TLS Rabi frequency is smaller than their phase decoherence rate. At higher external fields, a nonlinear absorption regime is found with the loss tangent inversely proportional to the intensity of the field. The theory can be generalized to acoustic absorption and lower dimensions realized in superconducting qubits.

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analog of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including ${}^{63,65}\mathrm{Cu}$ nuclear quadrupole resonance, ${}^{13}\mathrm{C}$ nuclear magnetic resonance, and muon spin rotation to show that the magnetic ground state of the ${\mathrm{Cu}}^{2+}$ ($S=1/2$) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish the onset of an intrinsically inhomogeneous magnetic state at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexists with the remnant high-temperature dynamical component down to $T_l=20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components, we conclude that the two components coexist on a microscopic level.

We investigate a spin-$1/2$ two-leg honeycomb ladder with frustrating next-nearest-neighbor (NNN) coupling along the legs, which is equivalent to two $J_1\text{−}{J}_2$ spin chains coupled with $J_{\perp }$ at odd rungs. The full parameter region of the model is systematically studied using conventional and infinite density-matrix renormalization group as well as bosonization. The rich phase diagram consists of five distinct phases: a Haldane phase, a NNN-Haldane phase, and a staggered dimer phase when $J_{\perp }<0$; a rung singlet phase and a columnar dimer phase when $J_{\perp }>0$. An interesting reentrant behavior from the dimerized phase into the Haldane phase is found as the frustration $J_2$ increases. The universalities of the critical phase transitions are fully analyzed. Phase transitions between dimerized and disordered phases belong to the two-dimensional Ising class with a central charge $c=1/2$. The transition from the Haldane phase to NNN-Haldane phase is of a weak topological first order, while the continuous transition between the Haldane phase and rung singlet phase has a central charge $c=2$.

A major challenge in antiferromagnetic (AFM) spintronics is to find an efficient way to manipulate AFM states, which are inert to a uniform magnetic field due to a vanishingly small net magnetization. The question is, how does an AFM respond to an inhomogeneous field? Here, we present a complete classification of the magnetic forces on an AFM domain wall (DW): (i) The tiny net magnetization can respond to the field gradient. (ii) The staggered magnetization is sensitive to the field difference of two sublattices. (iii) DW energy has a quadratic dependence on the magnetic field due to its noncollinear structure. Interestingly, the first two forces drive the DW to propagate to the opposite directions in a nanowire, but the third effect tends to push the DW to the high-field region. Consequently, the competition between these three forces can be used to understand the seemingly contradictory results on AFM-DW motion reported in the literature. Additionally, our results provide a route for a speedy manipulating AFM-DW; our numerical simulation indicated that, for antiferromagnetically coupled ferromagnetic layers, the DW propagating speed can reach tens of kilometers per second, an order of magnitude higher than that driven by an electric current.

There is a general perception that a large piezoelectric response in ferroelectric solid solutions requires a morphotropic/polymorphic phase boundary (MPB/PPB), i.e., a composition driven interferroelectric instability. This correlation has received theoretical support from models which emphasize field driven polarization rotation and/or interferroelectric transformations. Here, we show that the ferroelectric system $\left(1-x\right)\mathrm{PbTi}{\mathrm{O}}_3-\left(x\right)\mathrm{Bi}\left({\mathrm{Zr}}_{1/2}{\mathrm{Ni}}_{1/2}\right){\mathrm{O}}_3$ (PT-BNZ), which shows $d_{33}$ ($\sim 400\mathrm{p}\mathrm{C}/\mathrm{N}$) comparable to the conventional MPB/PPB systems, does not belong to this category. In the unpoled state the compositions of PT-BNZ showing large $d_{33}$ exhibit a coexistence of tetragonal and cubiclike (CL) phases on the global length scale. A careful examination of the domain strucures and global structures (both in the unpoled and poled states) revealed that the CL phase has no symptom of average rhombohedral distortion even on the local scale. The CL phase is rather a manifestation of tetragonal regions of short coherence length. Poling increases the coherence length irreversibly which manifests as poling induced $\mathrm{CL}\to P4mm$ transformation on the global scale. PT-BNZ is therefore qualitatively different from the conventional MPB piezoelectrics. In the absence of the composition and temperature driven interferroelectric instability in this system, polarization rotation and interferroelectric transformation are no longer plausible mechanisms to explain the large electromechanical response. The large piezoelectricity is rather associated with the increased structural-polar heterogeneity due to domain miniaturization without the system undergoing a symmetry change. Our study proves that attainment of large piezoelectricity does not necessarily require interferroelectric instability (and hence morphotropic/polymorphic phase boundary) as a criterion.

We explore superconducting instability for a clean two-band layered superconductor with deep and shallow bands in the magnetic field applied perpendicular to the layers. In the shallow band, the quasiclassical approximation is not applicable, and Landau quantization has to be accounted for exactly. The electronic spectrum of this band in the magnetic field is composed of the one-dimensional Landau-level minibands. With increasing magnetic field the system experiences a series of Lifshitz transitions when the chemical potential enters and exits the minibands. These transitions profoundly influence the shape of the upper critical field at low temperatures. In addition, the Zeeman spin splitting may cause the nonuniform state with interlayer modulation of the superconducting order parameter [Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state]. Typically, the quantization effects in the shallow band strongly promote the formation of this state. The uniform state remains favorable only in the exceptional resonance cases when the spin-splitting energy exactly matches the Landau-level spacing. Furthermore, for specific relations between electronic spectrum parameters, the alternating FFLO state may realize, in which the order parameter changes sign between the neighboring layers. For all above cases, the reentrant high-field superconducting states may emerge at low temperatures if the shallow band has significant contribution to the Cooper pairing.

The interaction between electrons and lattice vibrations determines key physical properties of materials, including their electrical and heat transport, excited electron dynamics, phase transitions, and superconductivity. We present an ab initio method that employs atomic orbital (AO) wave functions to compute the electron-phonon (e-ph) interactions in materials and interpolate the e-ph coupling matrix elements to fine Brillouin zone grids. We detail the numerical implementation of such AO-based e-ph calculations, and benchmark them against direct density functional theory calculations and Wannier function (WF) interpolation. The key advantages of AOs over WFs for e-ph calculations are outlined. Since AOs are fixed basis functions associated with the atoms, they circumvent the need to generate a material-specific localized basis set with a trial-and-error approach, as is needed in WFs. Therefore, AOs are ideal to compute e-ph interactions in chemically and structurally complex materials for which WFs are challenging to generate, and are also promising for high-throughput materials discovery. While our results focus on AOs, the formalism we present generalizes e-ph calculations to arbitrary localized basis sets, with WFs recovered as a special case.

The electronic and magnetic properties of $d^{3.5}$ iridate ${\mathrm{Ba}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_9$ have been studied using first-principles electronic structure calculations. The results of the calculations reveal that the system lies in an intermediate spin-orbit coupling (SOC) regime. There is strong covalency of $\mathrm{Ir}-5d$ and $\text{O}-2p$ orbitals. SOC, together with covalency, conspires to reduce the magnetic moment at the Ir site. By calculating the hopping interactions and exchange interactions, it is found that there is strong antiferromagnetic intradimer coupling within an ${\mathrm{Ir}}_2{\mathrm{O}}_9$ unit and other antiferromagnetic interdimer interactions make the system frustrated. The anisotropic magnetic interactions are also calculated. The calculations reveal that the magnitude of the Dzyaloshinskii-Moriya interactions parameter is small for this system. The magnetocrystalline anisotropy energy is large for this system and the easy axis lies on the $ab$ plane.