Gap Opening Mechanism Research Articles

Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials

Despite serious effort, the nature of the magnetic interactions and the role of electron-correlation effects in magnetic two-dimensional (2D) van der Waals materials remains elusive. Using CrI3 as a model system, we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron-scattering measurements. Remarkably, this approach identifies an unreported correlation-enhanced interlayer super-superexchange, which rotates the magnon Dirac lines off, and introduces a gap along the high-symmetry Γ-K-M path. This discovery provides a different perspective on the gap-opening mechanism observed in CrI3, which was previously associated with spin–orbit coupling through the Dzyaloshinskii–Moriya interaction or Kitaev interaction. Our observation elucidates the critical role of electron correlations on the spin ordering and spin dynamics in magnetic van der Waals materials and demonstrates the necessity of explicit treatment of electron correlations in the broad family of 2D magnetic materials.

Impact of novel electron-phonon coupling mechanisms on valley physics in two-dimensional materials

We systematically study the impact of various electron-acoustic-phonon coupling mechanisms on valley physics in two-dimensional materials. In the static strain limit, we find that Dirac cone tilt and deformation potential have analogous valley Hall response since they fall into the same universality class of pseudospin structure. However, such argument fails for the coupling mechanism with position-dependent Fermi velocity. For the isotropic case, a significant valley Hall effect occurs near charge neutrality similar to the bond-length change, whereas for the anisotropic case, the geometric valley transport is suppressed, akin to the deformation potential. Gap opening mechanism by nonuniform strain is found to totally inhibit the valley Hall transport, even if the dynamics of strains are introduced. By varying gate voltage, a tunable phonon-assisted valley Hall response can be realized, which paves a way toward rich phenomena and new functionalities of valley acoustoelectronics.

Improved thermoelectric performance in PbSe–AgSbSe2 by manipulating the spin-orbit coupling effects

Abstract The thermoelectric performance of a semiconductor is intrinsically limited by the coupling of the transport factors, originating in the mechanism of band gap opening. Here we report the anomalous semiconductor AgSbSe2-alloyed PbSe, in which the spin-orbit coupling (SOC) induced band gap allows the decoupling of electrical conductivity and Seebeck coefficient. In such a SOC semiconductor, the decrease of lattice constant increases the band gap from 0.23 eV up to 0.5 eV, seemingly in contrast to the previously measured negative pressure coefficient of band gap. Very different thermoelectric behaviors from the pristine PbSe were observed in PbSe–AgSbSe2, and finally a high peak ZT of 1.65 at 858 K and a remarkable average ZT (300–858 K) of 0.86 were obtained. This study reveals the novel relativistic effects in PbSe–AgSbSe2 system, and its thermoelectric performance is believed to have a large potential to be further improved.

Enhancement of Deep-Subwavelength Band Gaps in Flat Spiral-Based Phononic Metamaterials Using the Trampoline Phenomena

Abstract Elastic and acoustic metamaterials can sculpt dispersion of waves through resonances. In turn, resonances can give rise to negative effective properties, usually localized around the resonance frequencies, which support band gaps at subwavelength frequencies (i.e., below the Bragg-scattering limit). However, the band gaps width correlates strongly with the resonators’ mass and volume, which limits their functionality in applications. Trampoline phenomena have been numerically and experimentally shown to broaden the operational frequency ranges of two-dimensional, pillar-based metamaterials through perforation. In this work, we demonstrate trampoline phenomena in lightweight and planar lattices consisting of arrays of Archimedean spirals in unit cells. Spiral-based metamaterials have been shown to support different band gap opening mechanisms, namely, Bragg-scattering, local resonances and inertia amplification. Here, we numerically analyze and experimentally realize trampoline phenomena in planar metasurfaces for different lattice tessellations. Finally, we carry out a comparative study between trampoline pillars and spirals and show that trampoline spirals outperform the pillars in lightweight, compactness and operational bandwidth.

Seismic design and performance of self-centering-beam moment-frames

A self-centering beam (SCB) is an innovative shop-fabricated structural member incorporating a gap-opening mechanism into its body. This beam can exhibit self-centering characteristics by using high-strength post-tensioning strands and by dissipating seismic energy by combining pretensioned bolt frictional devices. This paper describes a step-by-step seismic design of self-centering-beam moment frames (SCB-MFs). A series of 3-, 6-, 9- and 12-storey steel frames is designed using this procedure. Fabrication tolerances are included in the model to investigate the effects of beam length tolerance on the seismic responses of SCB-MFs. Nonlinear time history analyses are conducted on these frames under an ensemble of 20 historical earthquakes. The same buildings are designed as ‘special moment frames’ according to ASCE 7–16 for seismic comparison. The results suggest that the proposed SCB-MFs with fabrication tolerances can achieve comparable maximum seismic performance and eliminate residual drifts compared with traditional moment frames of an equivalent beam size and having fully restrained connections.

Sizable Band Gap in Epitaxial Bilayer Graphene Induced by Silicene Intercalation.

Opening a band gap in bilayer graphene (BLG) is of significance for potential applications in graphene-based electronic and photonic devices. Here, we report the generation of a sizable band gap in BLG by intercalating silicene between BLG and Ru substrate. We first grow high-quality Bernal-stacked BLG on Ru(0001) and then intercalate silicene to the interface between the BLG and Ru, which is confirmed by low-energy electron diffraction and scanning tunneling microscopy. Raman spectroscopy shows that the G and 2D peaks of the intercalated BLG are restored to the freestanding-BLG features. Angle-resolved photoelectron spectroscopy measurements show that a band gap of about 0.2 eV opens in the BLG. Density functional theory calculations indicate that the large-gap opening results from a cooperative contribution of the doping and rippling/strain in the BLG. This work provides insightful understanding on the mechanism of band gap opening in BLG and enhances the potential of graphene-based device development.

Gap Opening Mechanism at the Dirac Point in the Electronic Spectrum of Gd-Doped Topological Insulator

The electronic structure of magnetically doped topological insulator Bi1.09Gd0.06Sb0.85Te3 is studied in the vicinity of the Dirac point at various temperatures (above and below the Neel temperature, 1–35 K) and synchrotron radiation polarizations using angle-resolved photoelectron spectroscopy. It is shown that the energy gap exists in photoemission spectra at the Dirac point, which remains open above the long-range magnetic ordering temperature TN. Measurements of magnetic properties by the superconducting magnetometry method show antiferromagnetic ordering with the paramagnetic transition temperature of 8.3 K. The studies of the temperature dependence of the Dirac cone state intensity by photoelectron spectroscopy confirm the existence of the magnetic transition and show the possibility of its indication directly from photoemission spectra. A more detailed analysis of the splitting between states of upper and lower Dirac cones (i.e., the energy gap) at the Dirac point in the photoelectron spectra shows the dependence of the gap at the Dirac point on the synchrotron radiation polarization type (28–30 meV for p-polarization and 22–25 meV for circularly polarized radiation of opposite chirality). The gap opening mechanism at the Dirac point above TN due to “coupling” of Dirac fermions with opposite momenta and spin orientations due to their interaction with the spin texture formed immediately during photoemission in the region of the photoemission hole at the magnetic impurity atom (Gd). It is shown that the gap at the Dirac point, measured above TN, is dynamic and is formed immediately during photoemission. In this case, the gap nature remains magnetic (even in the absence of long-range magnetic ordering) and is caused by properties of magnetic topological insulator, which does control the gap invariability when passing through TN. The dynamic nature of the generated gap is confirmed by its dependence on synchrotron radiation polarization.

Current-Induced Gap Opening in Interacting Topological Insulator Surfaces.

Two-dimensional topological insulators (TIs) host gapless helical edge states that are predicted to support a quantized two-terminal conductance. Quantization is protected by time-reversal symmetry, which forbids elastic backscattering. Paradoxically, the current-carrying state itself breaks the time-reversal symmetry that protects it. Here we show that the combination of electron-electron interactions and momentum-dependent spin polarization in helical edge states gives rise to feedback through which an applied current opens a gap in the edge state dispersion, thereby breaking the protection against elastic backscattering. Current-induced gap opening is manifested via a nonlinear contribution to the system's I-V characteristic, which persists down to zero temperature. We discuss prospects for realizations in recently discovered large bulk band gap TIs, and an analogous current-induced gap opening mechanism for the surface states of three-dimensional TIs.

Initial stiffness of self-centering systems and application to self-centering-beam moment-frames

There has been considerable development of self-centering seismic force resisting systems in the past few decades, many of which incorporate gap opening mechanisms. Experiments on these self-centering systems often result in an initial stiffness that is less than the predicted theoretical stiffness. While the discrepancy has often been attributed to uneven bearing surfaces at the gap opening mechanism, the cause of the low stiffness is not well understood and can have critical importance for drift-controlled systems such as moment frames.Concepts related to the initial stiffness of self-centering systems are investigated and the difference between experimental and theoretical initial stiffness is evaluated for a range of previous testing programs on different types of self-centering systems. Seven sources of added flexibility are identified. Then, the concepts for improving initial stiffness are applied to one specific system, the self-centering beam moment frame (SCB-MF). The self-centering beam consists of a two-part beam (upper and lower parts), post-tensioning strands to keep the parts aligned, and bolted friction elements to dissipate seismic energy. The initial stiffness of the SCB-MF is predicted using derived equations and finite element analyses and then the predictions are evaluated against experimental results. It is shown that if the added sources of flexibility are neglected (which is typical practice), the ratio of the average initial secant stiffness from five experiments to the predicted value is 38%, but that if the sources of flexibility are considered, the initial stiffness can be captured. The average ratio of measured to predicted stiffness for the theoretical model including pin flexibility is 59%, while the average ratio for finite element analyses including pin flexibility and beam length tolerance is 102%, demonstrating a significant improvement in accuracy. Using this new understanding about the sources of flexibility in self-centering systems, methods for increasing the stiffness of the SCB-MF are proposed and evaluated.

Chern and Z2 topological insulating phases in perovskite-derived 4d and 5d oxide buckled honeycomb lattices

Based on density functional theory calculations including a Coulomb repulsion parameter U, we explore the topological properties of (LaXO3)2/(LaAlO3)4 (111) with X = 4d and 5d cations. The metastable ferromagnetic phases of LaTcO3 and LaPtO3 with preserved P321 symmetry emerge as Chern insulators (CI) with C = 2 and 1 and band gaps of 41 and 38 meV at the lateral lattice constant of LaAlO3, respectively. Berry curvatures, spin textures as well as edge states provide additional insight into the nature of the CI states. While for X = Tc the CI phase is further stabilized under tensile strain, for X = Pd and Pt a site disproportionation takes place when increasing the lateral lattice constant from aLAO to aLNO. The CI phase of X = Pt shows a strong dependence on the Hubbard U parameter with sign reversal for higher values associated with the change of band gap opening mechanism. Parallels to the previously studied (X2O3)1/(Al2O3)5 (0001) honeycomb corundum layers are discussed. Additionally, non-magnetic systems with X = Mo and W are identified as potential candidates for Z2 topological insulators at aLAO with band gaps of 26 and 60 meV, respectively. The computed edge states and Z2 invariants underpin the non-trivial topological properties.

Probing planet formation and disk substructures in the inner disk of Herbig Ae stars with CO rovibrational emission

Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk. Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars. Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data. Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (NCO < 1018 cm−2); no CO is detected within or at the inner dust rim, due to dissociation at high temperatures. In contrast, explaining the narrow lines with high vibrational ratios requires an inner cavity of a least 5 AU in both dust and gas, followed by a cool (100–300 K) molecular gas reservoir with gas-to-dust ratios greater than 10 000 (NCO > 1018 cm−2) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (Tgas ~ Tdust). Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future.

Topological-magnetic proximity effect in Sb2Te3/CrI3 heterostructures

The topological-magnetic proximity effect between Sb2Te3 and a CrI3 monolayer is studied using a first-principles method. According to the results, a compressive strain of the CrI3 monolayer leads to a global energy gap at the topological surface states of Sb2Te3. The gap size is sensitive to the separation distance between Sb2Te3 and CrI3, and the gap opening mechanism is attributed to the exchange field from CrI3. This study is of significance to design systems of topological magnetoelectric effect as well as the understanding and control of the electronic behaviors in Sb2Te3/CrI3 and other topological insulator/magnetic insulator heterostructures alike.

Experimental study on spatial prefabricated self-centering steel frame with beam-column connections containing bolted web friction devices

The structural system of prefabricated self-centering steel frame (PSCF) was proposed previously by the author. Experimental studies focusing on PSCF connections, plane frame, overall structures as well as the numerical simulations were conducted. Compared with the self-centering steel frame (SCF), the PSCF has avoided the on-site aerial tension of steel strands and simultaneously achieves similar seismic performance as well as self-centering capacity. While the self-centering function of both PSCF and SCF may be restrained by conventional floor systems. Based on this thesis, a spatial PSCF with a new type of floor system containing sliding secondary beams was proposed in this paper to enable the frame expansion, and pseudo-dynamic and quasi-static tests toward it were conducted. The test results indicated that the proposed new floor system was reliable and feasible in accommodating the frame expansion. Meanwhile, the spatial PSCF with the new floor system has a favorable self-centering capacity, reliable gap-opening mechanism, superior seismic performance and enough redundancy to withstand multiple aftershocks.

Behavior and design of top flange-rotated self-centering steel connections equipped with SMA ring spring dampers

Existing self-centering steel connections developed based on either posttensioned (PT) tendons or shape memory alloy (SMA) elements are usually symmetric ones, in which case the connections rotate via a gap-opening mechanism over the beam-to-column interface and as a result causes unwanted “frame-expansion” (also called beam growth) effect and severe slab damage. This paper presents a novel type of asymmetric self-centering steel connection, which rotates around the top flange of the beam. The bending moment capacity and restoring force are provided by a couple of SMA dampers attached beneath the bottom flange of the beam. The working principles of the damper devices and the beam-to-column connections are discussed in detail, and a comprehensive experimental study on individual SMA rings, individual SMA damper devices, as well as two proof-of-concept beam-to-column steel connections, is conducted. The test results show that this type of connection generally exhibits typical flag-shaped hysteretic behavior with satisfactory energy dissipation and excellent self-centering capability. An extra ‘training’ process with an adequate pre-deformation to the SMA ring spring system can be adopted to effectively mitigate the degradation effect of the connections under repeated loading. Based on the test observations, a preliminary design recommendation is also proposed.

Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization

The quantum anomalous Hall effect (QAHE), characterized by dissipationless quantized edge transport, relies crucially on a non-trivial topology of the electronic bulk bandstructure and a robust ferromagnetic order that breaks time-reversal symmetry. Magnetically-doped topological insulators (TIs) satisfy both these criteria, and are the most promising quantum materials for realizing the QAHE. Because the spin of the surface electrons aligns along the direction of magnetic-impurity exchange field, only magnetic TIs with an out-of-plane magnetization are thought to open a gap at the Dirac point (DP) of the surface states, resulting in the QAHE. Using a continuum model supported by atomistic tight-binding and first-principles calculations of transition-metal doped Bi$_2$Se$_3$, we show that a surface-impurity potential generates an additional effective magnetic field which spin-polarizes the surface electrons along the direction perpendicular to the surface. The predicted gap-opening mechanism results from the interplay of this additional field and the in-plane magnetization that shifts the position of the DP away from the $\Gamma$ point. This effect is similar to the one originating from the hexagonal warping correction of the bandstructure but is one order of magnitude stronger. Our calculations show that in a doped TI with in-plane magnetization the impurity-potential-induced gap at the DP is comparable to the one opened by an out-of-plane magnetization.

Exciton Peierls mechanism and universal many-body gaps in carbon nanotubes

"Metallic" carbon nanotubes exhibit quasiparticle gaps when isolated from a screening environment. The gap-opening mechanism is expected to be of electronic origin, but the precise nature is debated. In this work, we show that hybrid density functional theory predicts a set of excitonic instabilities capable of opening gaps of the size found in experiment. The excitonic instabilities are coupled to vibrational modes and, in particular, the modes associated with the ${\bf \Gamma}-E_{2g}$ and ${\bf K}-A'_1$ Kohn anomalies of graphene, inducing Peierls lattice distortions with a strong electron-phonon coupling. In the larger tubes, the longitudinal optical phonon mode becomes a purely electronic dimerization that is fully symmetry conserving in the zigzag and chiral tubes, but breaks the symmetry in the armchair tubes. The resulting gaps are universal (i.e., independent of chirality) and scale as 1/R with tube radius.

Probing Signatures of a Distant Planet around the Young T-Tauri Star CI Tau Hosting a Possible Hot Jupiter

Abstract We search for signatures of a distant planet around the two million-year-old classical T-Tauri star CI Tau hosting a hot-Jupiter candidate ( ) in an eccentric orbit (e ∼ 0.3). To probe the existence of an outer perturber, we reanalyzed 1.3 mm dust continuum observations of the protoplanetary disk around CI Tau obtained by the Atacama Large Millimeter/submillimeter Array (ALMA). We found a gap structure at ∼0.″8 in CI Tau’s disk. Our visibility fitting assuming an axisymmetric surface brightness profile suggested that the gap is located at a deprojected radius of 104.5 ± 1.6 au and has a width of 36.9 ± 2.9 au. The brightness temperature around the gap was calculated to be ∼2.3 K lower than that of the ambient disk. Gap-opening mechanisms such as secular gravitational instability (GI) and dust trapping can explain the gap morphology in the CI Tau disk. The scenario that an unseen planet created the observed gap structure cannot be ruled out, although the coexistence of an eccentric hot Jupiter and a distant planet around the young CI Tau would be challenging for gravitational scattering scenarios. The mass of the planet was estimated to be between ∼0.25 M Jupiter and ∼0.8 M Jupiter from the gap width and depth ( ) in the modeled surface brightness image, which is lower than the current detection limits of high-contrast direct imaging. The young classical T-Tauri CI Tau may be a unique system for exploring the existence of a potential distant planet as well as the origin of an eccentric hot Jupiter.

Competing Gap Opening Mechanisms of Monolayer Graphene and Graphene Nanoribbons on Strong Topological Insulators.

Graphene is a promising material for designing next-generation electronic and valleytronic devices, which often demand the opening of a bandgap in the otherwise gapless pristine graphene. To date, several conceptually different mechanisms have been extensively exploited to induce bandgaps in graphene, including spin-orbit coupling and inversion symmetry breaking for monolayer graphene, and quantum confinement for graphene nanoribbons (GNRs). Here, we present a multiscale study of the competing gap opening mechanisms in a graphene overlayer and GNRs proximity-coupled to topological insulators (TIs). We obtain sizable graphene bandgaps even without inversion symmetry breaking and identify the Kekulé lattice distortions caused by the TI substrates to be the dominant gap opening mechanism. Furthermore, Kekulé distorted armchair GNRs display intriguing nonmonotonous gap dependence on the nanoribbon width, resulting from the coexistence of quantum confinement, edge passivation, and Kekulé distortions. The present study offers viable new approaches for tunable bandgap engineering in graphene and GNRs.

GAS GIANT PLANET FORMATION IN THE PHOTOEVAPORATING DISK. I. GAP FORMATION

ABSTRACT Planet formation and photoevaporation have both been considered as gap opening mechanisms in protoplanetary disks. We have studied giant planet formation in a photoevaporating disk with long-term evolution. Our calculations suggest that the core accretion rate of a protoplanet declines and the trigger of the runaway gas accretion for a giant planet is delayed under the action of photoevaporation. We find that the final mass of a giant planet characterized by the “gap-limiting” case is not influenced by photoevaporation but the final mass of a giant planet characterized by the “diffusion-limiting” case is greatly influenced by photoevaporation. Considering the formation process of giant planets, we suggest that the locations of the gaps opened by giant planets are within 30–40 au and the gap width in the “gap-limiting” case is wider than that in the “diffusion-limiting” case. We also find that gaps in photoevaporating disks are wider than those in non-photoevaporating disks. Our calculations suggest that the origins of multiple gaps in a disk can be diverse depending on their formation locations. In the formation region of giant planets, gaps are opened by giant planets. The outer gap beyond the giant planet formation region may be opened under the action of photoevaporation. A gap may also be opened at 1–3 au under the actions of photoevaporating dissipation and gas accretion of the outer giant planets.

Topological polaritons from photonic Dirac cones coupled to excitons in a magnetic field

We introduce an alternative scheme for creating topological polaritons (topolaritons) by exploiting the presence of photonic Dirac cones in photonic crystals with triangular lattice symmetry. As recently proposed, topolariton states can emerge from a coupling between photons and excitons combined with a periodic exciton potential and a magnetic field to open up a topological gap. We show that in photonic crystals the opening of the gap can be substantially simplified close to photonic Dirac points. Coupling to Zeeman-split excitons breaks time reversal symmetry and allows to gap out the Dirac cones in a non-trival way, leading to a topological gap similar to the strength of the periodic exciton potential. Compared to the original topolariton proposal [Karzig {\em et al}, PRX {\bf 5}, 031001 (2015)], this scheme significantly increases the size of the topological gap over a wide range of parameters. Moreover, the gap opening mechanism highlights an interesting connection between topolaritons and the Haldane and Raghu scheme [Haldane and Raghu, PRL {\bf 100}, 013904 (2008)] to create topological photons in magneto-optically active materials.