In-gap States Research Articles

Ultrafast nonlinear absorption of TMDC quantum dots

We study theoretically absorption properties of MoS 2 quantum dots placed in an ultrashort and strong optical pulse with the duration of a few femtoseconds and the amplitude up to 1.5 V/Å. While the monolayer of MoS 2 has a bandgap of ≈ 1 . 8 eV , the quantum dots of MoS 2 have in-gap edge states, which open additional absorption channels. The absorbance of the system strongly depends on the amplitude of the pulse. At small amplitude, less than 0.5 V/Å, the absorbance varies by more than 20% with the frequency of the pulse. The system shows saturable absorption if the frequency of the pulse is larger than the bulk bandgap and reverse saturable absorption if the frequency of the pulse is in the bandgap. At large amplitudes, > 0 . 5 V/Å, of the pulse, the ultrafast absorbance of a quantum dot has a weak dependence on the pulse frequency. If the frequency of the pulse is less than the corresponding bulk bandgap, the absorbance has a nonmonotonic dependence on the pulse amplitude with the maximum at ≈ 0 . 5 V/Å. We also present the results for QDs of different TMDC materials, which show that at large field amplitude, > 0 . 5 V/Å, the absorbance is universal and has weak dependence on the type of TMDC. • Interaction of strong optical pulses with TMDC quantum dots. • Nonlinear ultrafast electron dynamics in TMDC quantum dots. • Universal high field behavior of quantum dot absorbance. • Controllable energy storage in quantum dots.

Study of intrinsic point defects in γ-In2Se3 based on first principles calculations for the realization of an efficient UV photodetector

In 2 Se 3 have attracted researcher’s attention due to its potential optoelectronic and memory device applications, having various polymorphs with few of them having layered structure. There exists a non-layered phase with vacancy ordered in screw form known as γ-In 2 Se 3 , whose properties are found to get affected majorly by the presence of defects. Here, the first principles calculations based on the density functional theory are performed to study the intrinsic point defects in defect wurtzite structure of γ-In 2 Se 3 using GGA-PBE approximations. Thermodynamic charge transitions based on the formation energy of the defects are determined for all native defects, V In , V Se , In i , Se i , Se In , In Se , under In-rich and Se-rich experimental growth conditions. The charge transition remains the same but transition energies differ for possible coordination of Se vacancies, whereas the charge transition as well as transition energy vary for different coordination sites of In vacancies. The In interstitial, In i is the most favourable defect site under In-rich conditions whereas Se In at the VBM and V In1 near CBM are dominating under Se-rich conditions. The origin of n-type conductivity of γ-In 2 Se 3 is found to be the presence of In i interstitials and In Se antisite in the defect wurtzite γ-In 2 Se 3 . Further, the films grown using PLD shows an n-type conductivity under In-rich conditions leading to a good performance with photo responsivity and specific detectivity of 5.22 × 10 2 A/W and 5.08 × 10 13 Jones. The understanding of these vacancies determines their thermodynamic behaviour which can help for the controlled growth of γ-In 2 Se 3 which leads to better device performance. • Thermodynamic transition states are predicted for γ-In 2 Se 3 under In-rich and Se-rich growth conditions using first principles study. • Origin of n-type conductivity of γ-In 2 Se 3 is found to be the presence of In i and In Se and V Se is observed to introduce the in-gap states. • Highly crystalline n-type γ-In 2 Se 3 thin films are optimized under the In-rich growth conditions using PLD technique. • High concentrations of In i and In Se are identified using PL studies. • High responsivity and detectivity is achieved for the prepared device indicating low trap states due to V Se .

Orbital Shift-Induced Boundary Obstructed Topological Materials with a Large Energy Gap.

Boundary obstructed topological phases caused by Wannier orbital shift between ordinary atomic sites are proposed, which, however, cannot be indicated by symmetry eigenvalues at high symmetry momenta (symmetry indicators) in bulk. On the open boundary, Wannier charge centers can shift to different atoms from those in bulk, leading to in‐gap surface states, higher‐order hinge states or corner states. To demonstrate such orbital shift‐induced boundary obstructed topological insulators, eight material candidates are predicted, all of which are overlooked in the present topological databases. Metallic surface states, hinge states, or corner states cover the large bulk energy gap (e.g., more than 1 eV in TlGaTe2) at related boundary, which are ready for experimental detection. Additionally, these materials are also fragile topological insulators with hourglass‐like surface states.

Second-order and real Chern topological insulator in twisted bilayer α -graphyne

The study of higher-order and real topological states as well as the material realization have become a research forefront of topological condensed matter physics in recent years. Twisted bilayer graphene (tbG) is proved to have higher-order and real topology. However, whether this conclusion can be extended to other two-dimensional twisted bilayer carbon materials and the mechanism behind it lack explorations. In this paper, we identify the twisted bilayer $\ensuremath{\alpha}$-graphyne (tbGPY) at large twisting angle as a real Chern insulator (also known as Stiefel-Whitney insulator) and a second-order topological insulator. Our first-principles calculations suggest that the tbGPY at $21.{78}^{\ensuremath{\circ}}$ is stable at 100 K with a larger bulk gap than the tbG. The nontrivial topological indicators, including the real Chern number and a fractional charge, and the localized in-gap corner states are demonstrated from first-principles and tight-binding calculations. Moreover, with ${\mathcal{C}}_{6z}$ symmetry, we prove the equivalence between the two indicators, and explain the existence of the corner states. To decipher the real and higher-order topology inherited from the moir\'e heterostructure, we construct an effective four-band tight-binding model capturing the topology and dispersion of the tbGPY at large twisting angle. A topological phase transition to a trivial insulator is demonstrated by breaking the ${\mathcal{C}}_{2y}$ symmetry of the effective model, which gives insights on the trivialization of the tbGPY as reducing the twisting angle to $9.{43}^{\ensuremath{\circ}}$ suggested by our first-principles calculations.

Evolution of the Electronic Structure of Ultrathin MnBi2Te4 Films.

Ultrathin films of intrinsic magnetic topological insulator MnBi2Te4 exhibit fascinating quantum properties such as the quantum anomalous Hall effect and the axion insulator state. In this work, we systematically investigate the evolution of the electronic structure of MnBi2Te4 thin films. With increasing film thickness, the electronic structure changes from an insulator type with a large energy gap to one with in-gap topological surface states, which is, however, still in drastic contrast to the bulk material. By surface doping of alkali-metal atoms, a Rashba split band gradually emerges and hybridizes with topological surface states, which not only reconciles the puzzling difference between the electronic structures of the bulk and thin-film MnBi2Te4 but also provides an interesting platform to establish Rashba ferromagnet that is attractive for (quantum) anomalous Hall effect. Our results provide important insights into the understanding and engineering of the intriguing quantum properties of MnBi2Te4 thin films.

Topological surface superconductivity in FeSe0.45Te0.55

The engineering of Majorana zero modes in topological superconductors, a paradigm for the realization of topological quantum computing and topology-based devices, has been hampered by the absence of materials with sufficiently large superconducting gaps. Recent experiments, however, have provided enthralling evidence for the existence of topological surface superconductivity in the iron-based superconductor FeSe0.45Te0.55 possessing a full s±-wave gap of a few meV. Here, we propose a mechanism for the emergence of topological superconductivity on the surface of FeSe0.45Te0.55 by demonstrating that the interplay between the s±-wave symmetry of the superconducting gap, surface magnetism, and a Rashba spin–orbit interaction gives rise to robust topological superconducting phases. Moreover, the proposed mechanism explains a series of experimentally observed hallmarks of topological superconductivity, such as the emergence of Majorana zero modes in the center of vortex cores and at the end of line defects, as well as of chiral Majorana edge modes along domain walls. We also propose that the spatial distribution of supercurrents near a domain wall is a characteristic signature measurable via a scanning superconducting quantum interference device that can distinguish between chiral Majorana edge modes and trivial in-gap states.

Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator

Room-temperature realization of macroscopic quantum phases is one of the major pursuits in fundamental physics1,2. The quantum spin Hall phase3-6 is a topological quantum phase that features a two-dimensional insulating bulk and a helical edge state. Here we use vector magnetic field and variable temperature based scanning tunnelling microscopy to provide micro-spectroscopic evidence for a room-temperature quantum spin Hall edge state on the surface of the higher-order topological insulator Bi4Br4. We find that the atomically resolved lattice exhibits a large insulating gap of over 200 meV, and an atomically sharp monolayer step edge hosts an in-gap gapless state, suggesting topological bulk-boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection inherent in the underlying band topology. We further identify the geometrical hybridization of such edge states, which not only supports the Z2 topology of the quantum spin Hall state but also visualizes the building blocks of the higher-order topological insulator phase. Our results further encourage the exploration of high-temperature transport quantization of the putative topological phase reported here.

Reentrant localization transition in the Su-Schrieffer-Heeger model with random-dimer disorder

The question of whether the topological insulators can host a stable nontrivial phase in the presence of spatially correlated disorder is a fundamental question of interest. Based on theoretical investigations, we analyze the effect of random-dimer disorder on the quantum phase transitions of the Su-Schrieffer-Heeger model. We explicitly demonstrate that, due to the absence of symmetry, there are no in-gap edge states in certain disordered topological nontrivial gapped phase and the bulk-boundary correspondence breaks down. However, the fractionalized end charges still appear at the ends of the chain. The energy distribution possibility of the in-gap edge states and the possible values of fractionalized end charges are dependent on the concentration of random-dimer disorder. On the other hand, random-dimer disorder and dimer hopping intertwine in an interesting manner. Reentrant localization transition behavior appears, which is evidenced by the fingerprint of the inverse participation ratio, normalized participation ratio, and tunneling conductivity.

Atomic-scale visualization of the p−d hybridization in III-V semiconductor surfaces doped with transition metal impurities

p-d hybridization of transition metal impurities in a semiconductor host is the mechanism that couples valence-band electrons and localized spins. We use scanning tunneling microscopy and spectroscopy combined with density functional theory to probe at the atomic scale hybridization of Cr single impurities with GaAs host. Combining spatial density of states mapping and in-gap states spectroscopy of the Cr substituted at the surface of the semiconductor, we give a detailed picture of the spatial extension and the electronic structure of the strongly anisotropic wave function of Cr on GaAs(110). First principles calculations allow to identify electronic character and origin of each states and show that the main resonance peaks and the wave function with "drop-eyes" lobes experimentally observed for 3d metal impurities in III-V semiconductor are direct local evidences of the p-d hybridization.

Prediction of half-metallic gap formation and Fermi level position in Co-based Heusler alloy epitaxial thin films through anisotropic magnetoresistance effect

We have investigated the temperature $(T)$ dependence of the anisotropic magnetoresistance (AMR) effect of ${\mathrm{Co}}_{2}{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) epitaxial thin films having different compositions and atomic orders to examine the relation between AMR and the half-metallic electronic structure based on a developed theoretical model. The $T$ dependence of the resistance change (\ensuremath{\Delta}\ensuremath{\rho}) of the AMR normalized at 10 K is minimal in the CFGG films having a standard composition and high atomic order. In contrast, the films having a large atomic disorder due to the Co-rich composition exhibit a large reduction of \ensuremath{\Delta}\ensuremath{\rho} with $T$. Our theoretical model of AMR well explains this behavior; namely, the half-metallic ferromagnets having the Fermi level $({E}_{\mathrm{F}})$ around the center of the half-metallic gap are predicted to show a small $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} because of no/few localized $d\ensuremath{\downarrow}$ states at ${E}_{\mathrm{F}}$. On the contrary, a large change of \ensuremath{\Delta}\ensuremath{\rho} with $T$ is expected for the half-metallic materials having ${E}_{\mathrm{F}}$ close to gap edges or large in-gap states because of the contribution of thermally excited s-d scattering involving the $d\ensuremath{\downarrow}$ states. Moreover, we have also investigated the variation of \ensuremath{\Delta}\ensuremath{\rho} with $T$ for the thin films of various half-metallic Co-based Heusler alloys and found the behavior that agrees with our theoretical prediction. The present study proves that the formation of a half-metallic gap and the position of ${E}_{\mathrm{F}}$ in a half-metallic material are readily predictable from the $T$ dependence of \ensuremath{\Delta}\ensuremath{\rho} of the AMR effect, which can be a facile way for efficient screening of half-metallic materials.

Experimental realization of boundary-obstructed topological insulators using acoustic two-dimensional Su–Schrieffer–Heeger network

Topological insulators that can host special symmetry-protected boundary states and corner states have attracted increasing intention in acoustic engineering. Recently, the concept of the boundary-obstructed topological (BOT) phases has defined a class of topological phases without bulk energy band closing around zero energy, which greatly broadens the applications of the topological states. In this work, based on the two-dimensional Su–Schrieffer–Heeger network, we show that the band degeneracies around zero energy can be removed to open a complete bandgap by judiciously tuning the hopping terms to break C4v symmetry down to C2v symmetry but with the topological phase invariant, which can be directly characterized by the BOT phase. Furthermore, we experimentally propose a rigorous acoustic sample to visualize the hierarchy of the in-gap higher-order topological states exactly. Crucially, by designedly connecting the lattice with outside environment, we show that these spectrally isolated states still response to the specific frequencies robustly. Our results are expected to be helpful for manipulating wave propagation and sound energy harvesting.

Ultrafast Momentum-Resolved Hot Electron Dynamics in the Two-Dimensional Topological Insulator Bismuthene.

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than those in conventional semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.

Yu-Shiba-Rusinov States in a Superconductor with Topological Z_{2} Bands.

A Yu-Shiba-Rusinov (YSR) state is a localized in-gap state induced by a magnetic impurity in a superconductor. Recent experiments used an STM tip to manipulate the exchange coupling between an Fe adatom and the FeTe_{0.55}Se_{0.45} superconductor possessing a Z_{2} nontrivial band structure with topological surface states. As the tip moves close to the single Fe adatom, the energy of the in-gap state modulates and exhibits a zero-energy crossing followed by an unusual return to zero energy, which cannot be understood by coupling the magnetic impurity to the superconducting topological surface Dirac cone. Here, we numerically and analytically study the YSR states in superconductors with nontrivial Z_{2} bands and show the emergence of the two zero-energy crossings as a function of the exchange coupling between the magnetic impurity and the bulk states. We analyze the role of the topological surface states and compare in-gap states to systems with trivial Z_{2} bands. The spin polarization of the YSR states is further studied for future experimental measurement.

Closing the hybridization charge gap in the Kondo semiconductor SmB6 with an ultrahigh magnetic field

We have investigated the high-frequency magnetoresistance of Kondo semiconductor ${\mathrm{SmB}}_{6}$ under ultrahigh magnetic fields generated by the electromagnetic flux-compression technique. The semiconductor-metal transition due to closing of the hybridization charge gap was observed at approximately 180 T. The critical magnetic field observed is substantially higher than that reported previously. At temperatures below around 10 K, another transition was observed at a lower field (approximately 80 T). We suggest that the observed 80-T transition relates with the energy scale of quasiparticles such as the spin polaron in the in-gap state.

Electronic Properties of Defective Janus MoSSe Monolayer.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) hold great promise in electronics and optoelectronics due to their novel electronic and optical properties. In TMDCs, structural defects are inevitable and might play a decisive role in device performance. In this work, point defects, line vacancies, and 60° grain boundaries (GBs) are explored in 2D Janus MoSSe, a new member to the family of TMDCs, by means of the first-principles calculations. S and Se vacancies are found to be the most favorable point defects, and they tend to aggregate along the zigzag direction to form line vacancies. Comparing with isolated point defects, line vacancies induced in-gap states are more dispersive. In particular, 60° GBs behave as one-dimensional metallic quantum wires, as a consequence of the polar discontinuity. Thus, effectively controlling the formation of defects at nanoscale brings new electronic characteristics, providing new opportunities to broaden the applications of 2D TMDCs.

Manipulation of acoustic vortex with topological dislocation states

Higher-order topological insulators as an exotic type of topological phases harboring fascinating topological corner or hinge states have attracted extensive attention recently. Dislocations are crystallinity-breaking defects in lattices that cannot be removed by local deformations due to nontrivial real-space topology. It is recently realized that dislocations can be used as a probe for higher-order topology. In this work, we propose a scheme to obtain acoustic dislocation states by introducing screw dislocations into higher-order topological insulators in a Kagome lattice. The topological dislocation states carry nonzero orbital angular momentum, which are locked to their propagation direction. We show that the screw dislocation states exist for both the tight binding model and the waveguide model as long as the system symmetry is preserved. By delicately designing the dislocation core, the dislocation states with selective angular momentum can be shifted into the bulk bandgap. Based on this in-gap dislocation states, filtering of acoustic vortex with a selective angular momentum is well achieved.

Non-Hermitian second-order topology induced by resistances in electric circuits

The study of topological states in electric circuits is developing rapidly recently and forming the field of topoelectrical circuits. As both the loss and gain can be easily introduced and tuned in electric circuits, it permits flexible and accurate fabrications of non-Hermitian systems. Loss is generally considered negative in electric circuits, as it weakens the electrical signals. However, we show here that a non-Hermitian second-order topological phase can be realized in electric circuits by taking advantage of loss. We observe the gapped edge states and in-gap corner states in experiment. Our system provides a readily accessible platform to explore the non-Hermitian-induced topological phases.

Modification of the synthesis of layered titanium chloride nitride

Layered titanium nitride chloride (TiNCl) exhibits a relatively high superconducting transition of ∼16 K, and it is a candidate for the unconventional superconductor. To better understand the superconductivity, we modified the synthesis process for TiNCl. Highly crystalline TiNCl was prepared by chemical vapor transport at different transport temperatures and intercalated with sodium (Na) metal. Although the crystallographic parameters obtained through Rietveld refinement are similar, the superconducting properties strongly depend on the treatment temperature of pristine TiNCl. Optical and photoemission spectroscopy revealed an in-gap state below the Fermi level, indicating hydrogen incorporation, similar to the case of β−ZrNCl. Hydrogen temperature-programmed desorption revealed hydrogen emission against the heat treatment of the obtained pristine TiNCl. The in-gap state of hydrogen inhibits the introduction of electron carriers from the intercalated Na metal.

Alumina Graphene Catalytic Condenser for Programmable Solid Acids

Precise control of electron density at catalyst active sites enables regulation of surface chemistry for the optimal rate and selectivity to products. Here, an ultrathin catalytic film of amorphous alumina (4 nm) was integrated into a catalytic condenser device that enabled tunable electron depletion from the alumina active layer and correspondingly stronger Lewis acidity. The catalytic condenser had the following structure: amorphous alumina/graphene/HfO2 dielectric (70 nm)/p-type Si. Application of positive voltages up to +3 V between graphene and the p-type Si resulted in electrons flowing out of the alumina; positive charge accumulated in the catalyst. Temperature-programmed surface reaction of thermocatalytic isopropanol (IPA) dehydration to propene on the charged alumina surface revealed a shift in the propene formation peak temperature of up to ΔT peak∼50 °C relative to the uncharged film, consistent with a 16 kJ mol-1 (0.17 eV) reduction in the apparent activation energy. Electrical characterization of the thin amorphous alumina film by ultraviolet photoelectron spectroscopy and scanning tunneling microscopy indicates that the film is a defective semiconductor with an appreciable density of in-gap electronic states. Density functional theory calculations of IPA binding on the pentacoordinate aluminum active sites indicate significant binding energy changes (ΔBE) up to 60 kJ mol-1 (0.62 eV) for 0.125 e- depletion per active site, supporting the experimental findings. Overall, the results indicate that continuous and fast electronic control of thermocatalysis can be achieved with the catalytic condenser device.

Compositional Variation in FAPb1-xSnxI3 and Its Impact on the Electronic Structure: A Combined Density Functional Theory and Experimental Study.

Given their comparatively narrow band gap, mixed Pb–Sn iodide perovskites are interesting candidates for bottom cells in all-perovskite tandems or single junction solar cells, and their luminescence around 900 nm offers great potential for near-infrared optoelectronics. Here, we investigate mixed FAPb1–xSnxI3 offering the first accurate determination of the crystal structure over a temperature range from 293 to 100 K. We demonstrate that all compositions exhibit a cubic structure at room temperature and undergo at least two transitions to lower symmetry tetragonal phases upon cooling. Using density functional theory (DFT) calculations based on these structures, we subsequently reveal that the main impact on the band gap bowing is the different energy of the s and p orbital levels derived from Pb and Sn. In addition, this energy mismatch results in strongly composition-dependent luminescence characteristics. Whereas neat and Sn-rich compounds exhibit bright and narrow emission with a clean band gap, Sn-poor compounds intrinsically suffer from increased carrier recombination mediated by in-gap states, as evidenced by the appearance of pronounced low-energy photoluminescence upon cooling. This study is the first to link experimentally determined structures of FAPb1–xSnxI3 with the electronic properties, and we demonstrate that optoelectronic applications based on Pb–Sn iodide compounds should employ Sn-rich compositions.