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The relation between projective measurements and generalized quantum measurements is a fundamental problem in quantum physics, and clarifying this issue is also important to quantum technologies. While it has been intuitively known that projective measurements can be constructed from sequential generalized or weak measurements, there is still lack of a proof of this hypothesis in general cases. Here we prove it from the perspective of quantum channels. We show that projective measurements naturally arise from sequential generalized measurements in the asymptotic limit. Specifically, a selective projective measurement arises from a set of typical sequences of selective generalized measurements. We provide an explicit scheme to construct projective measurements of a quantum system with sequential generalized measurements. Remarkably, a single ancilla qubit is sufficient to mediate sequential generalized measurements for constructing arbitrary projective measurements of a generic system., Comment: 8 pages, 2 figures (+6 pages supplement)

In stark contrast to the conventional charge density wave (CDW) materials, the one-dimensional CDW on the In/Si(111) surface exhibits immediate damping of the CDW oscillation during the photoinduced phase transition. Here, by successfully reproducing the experimentally observed photoinduced CDW transition on the In/Si(111) surface by performing real-time time-dependent density functional theory (rt-TDDFT) simulations, we demonstrate that photoexcitation promotes valence electrons from Si substrate to empty surface bands composed primarily of the covalent p-p bonding states of the long In-In bonds, generating interatomic forces to shorten the long bonds and in turn drives coherently the structural transition. We illustrate that after the structural transition, the component of these surface bands occurs a switch among different covalent In bonds, causing a rotation of the interatomic forces by about {\pi}/6 and thus quickly damping the oscillations in feature CDW modes. These findings provide a deeper understanding of photoinduced phase transitions., Comment: 11 pages,3 figures

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory method, here we track the motion of individual vanadium (V) ions during PIPT in VO

Laser-induced nonthermal melting in semiconductors has been studied over the last four decades, but the underlying mechanism is still under debate. Here, by utilizing an advanced real-time time-dependent density functional theory simulation, we reveal that the photoexcitation-induced ultrafast nonthermal melting in silicon occurs via homogeneous nucleation with random seeds originating from a self-amplified local dynamic instability at the photoexcited states rather than by simultaneously breaking of all bonds, as suggested by the inertial model, phonon instability, or Coulombic repulsion mechanisms. Due to this local dynamic instability, any initial small random thermal displacements of atoms can be amplified by a charge transfer of photoexcited carriers, which in turn creates a local self-trapping center for the excited carriers and yields the random nucleation seeds. This finding provides fresh insights into photoinduced ultrafast nonthermal melting., 27 pages,8 figures

Photoinduced phase transition (PIPT) is always treated as a coherent process, but ultrafast disordering in PIPT is observed in recent experiments. Utilizing the real-time time-dependent density functional theory (rt-TDDFT) method, here, we track the motion of individual vanadium (V) ions during PIPT in VO2 and uncover that their coherent or disordered dynamics can be manipulated by tuning the laser fluence. We find that the photoexcited holes generate a force on each V-V dimer to drive their collective coherent motion, in competing with the thermal-induced vibrations. If the laser fluence is so weak that the photoexcited hole density is too low to drive the phase transition alone, the PIPT is a disordered process due to the interference of thermal phonons. We also reveal that the photoexcited holes populated by the V-V dimerized bonding states will become saturated if the laser fluence is too strong, limiting the timescale of photoinduced phase transition., Comment: 23 pages,10 figures

Ultrafast photoexcitation can induce a nonequilibrium dynamic with electron-lattice interaction, offering an effective way to study photoinduced phase transitions (PIPTs) in solids. The issue that atomic displacements after photoexcitation belong to coherent change or disordered process, has become a controversy in the PIPT community. Using real-time time-dependent density functional theory (rt-TDDFT) simulations, we obtain both the coherent and the disordered PIPTs (dimer dissociation) in IrTe2 with the different electronic occupations. More importantly, we found that in the disordered phase transition, there exists a local correlation between different dimers regarding their dissociation status. One can define vertical groups across the layers. The dimers in the same group will dissociate in a correlated fashion: they either all dissociate, or all not dissociate. On the other hand, the dimers in neighboring groups will have an anti-correlation: if the dimers in one group dissociate, the dimers in the neighboring group tend not to be dissociated, and vice versus., Comment: 13 pages,5 figures

Path-independent (PI) quantum control has recently been proposed to integrate quantum error correction and quantum control [Phys. Rev. Lett. 125, 110503 (2020)], achieving fault-tolerant quantum gates against ancilla errors. Here we reveal the underlying algebraic structure of PI quantum control. The PI Hamiltonians and propagators turn out to lie in an algebra isomorphic to the ordinary matrix algebra, which we call the PI matrix algebra. The PI matrix algebra, defined on the Hilbert space of a composite system (including an ancilla system and a central system), is isomorphic to the matrix algebra defined on the Hilbert space of the ancilla system. By extending the PI matrix algebra to the Hilbert-Schmidt space of the composite system, we provide an exact and unifying condition for PI quantum control against ancilla noise., Comment: 9 pages, 2 figures

A design of n-type InAs piezoelectric tunnel FETs (PE-TFETs), which is worth further experimental verification, is proposed for steep sub-threshold slope and studied using TCAD simulations. Piezoelectric layers (PE-layer) are inserted in gate stacks to induce dynamic compressive strain and to modulate the band gap during switching. The non-equilibrium Green’s function (NEGF) approach with strained 8-band ${k}\cdot {p}$ Hamiltonian is used to investigate the PE-TFETs performance. Our results suggest that PE-TFETs are compressively strained perpendicular to the transport direction at OFF state, while PE-TFETs feel no strain at ON state. This compressive strain enlarges the channel band gap and meanwhile reduces the tunneling energy window, which greatly suppresses OFF current. In this work, the n-type InAs PE-TFETs achieve 50 times lower OFF current than conventional TFETs, and the minimum ${SS}$ of PE-TFETs is $48~{mV/dec}$ with $0.5~{V}$ supply voltage. Moreover, the proposed design is helpful to suppress the ambipolar current.

The hot carrier cooling occurs in most photoexcitation-induced phase transitions (PIPTs), but its role has often been neglected in many theoretical simulations as well as in proposed mechanisms. Here, by including the previously ignored hot carrier cooling in real-time time-dependent density functional theory (rt-TDDFT) simulations, we investigated the role of hot carrier cooling in PIPTs. Taking IrTe2 as an example, we reveal that the cooling of hot electrons from the higher energy levels of spatially extended states to the lower energy levels of the localized Ir-Ir dimer antibonding states strengthens remarkably the atomic driving forces and enhances atomic kinetic energy. These two factors combine to dissolute the Ir-Ir dimers on a timescale near the limit of atomic motions, thus initiating a deterministic kinetic phase transition. We further demonstrate that the subsequent cooling induces nonradiative recombination of photoexcited electrons and holes, leading to the ultrafast recovery of the Ir-Ir dimers observed experimentally. These findings provide a complete picture of the atomic dynamics in optically excited structural phase transitions., Comment: 20 pages, 8 figures

Enhancing valley splitting in SiGe heterostructures is a crucial task for developing silicon spin qubits. Complex SiGe heterostructures, sharing a common feature of four-monolayer (4ML) Ge layer next to the silicon quantum well (QW), have been computationally designed to have giant valley splitting approaching 9 meV. However, none of them has been fabricated may due to their complexity. Here, we remarkably simplify the original designed complex SiGe heterostructures by laying out the Si QW directly on the Ge substrate followed by capping a (Ge4Si4)n superlattice(SL) barrier with a small sacrifice on VS as it is reduced from a maximum of 8.7 meV to 5.2 meV. Even the smallest number of periods (n = 1) will also give a sizable VS of 1.6 meV, which is large enough for developing stable spin qubits. We also develop an effective Hamiltonian model to reveal the underlying microscopic physics of enhanced valley splitting by (Ge4Si4)n SL barriers. We find that the presence of the SL barrier will reduce the VS instead of enhancing it. Only the (Ge4Si4)n SL barriers with an extremely strong coupling with Si QW valley states provide a remarkable enhancement in VS. These findings lay a solid theoretical foundation for the realization of sufficiently large VS for Si qubits.

Carrier induced defect creation at the semiconductor-oxide interface has been known as the origin of electronic device degradation for a long time, but how exactly the interface lattice can be damaged by carriers (especially low-energy ones) remains unclear. Here we carry out real-time time-dependent density functional theory simulations on concrete $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_{2}$ interfaces to study the interaction between excited electrons and interface bonds. We show that the normal interface Si-H bonds are generally resistant to electrons due to the delocalized nature and high energy level of the Si-H antibonding states, and due to the high-energy barrier to break the Si-H bond. However, if an additional hydrogen atom exists by attaching to a nearby oxygen atom (forming a ``Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O'' complex), the Si-H bond will be greatly weakened, including the reduction of energy barrier for bond breaking, and the lowering of the antibonding state energy level which favors electron injection. Together with the multiple vibrational excitation process, the corresponding Si-H bond can be broken much more easily. Thus we propose that the Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O complex will be the center for defect creation and device degradation. We also explain why such a center might be relatively easy to form during the hydrogen annealing process.

We propose two different designs of p-type piezoelectric (PE) FinFETs (PE-FinFETs) covering low-power (LP) and high-performance (HP) operation modes. LP mode PE-FinFETs achieve a lower OFF-current ( ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) and HP mode PE-FinFETs result in a larger ON-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ). These two different modes are achieved by simply changing the outer PE-gates bias. The advanced nonequilibrium Green’s function (NEGF) approach self-consistent with six-band ${k}\cdot {p}$ method including phonon scattering is used to investigate the device performance. The gate voltage-controlled strain is analytically derived from the principle of PE effect. The HP and LP PE-FinFETs are studied, respectively, and the impacts of channel material, device orientation, and phonon scattering on both HP and LP PE-FinFETs are comprehensively investigated. The simulation results show that Ge is superior to Si for both LP and HP PE-FinFETs. With a supply voltage of 0.5 V, ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ of Ge LP mode PE-FinFETs is reduced by 18 times, and Ge HP mode PE-FinFETs obtain 50% ON-current enhancement. ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ( ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ ) is unchanged for LP (HP) mode PE-FinFETs. Phonon scattering not only causes a large ${I}_{ \mathrm{\scriptscriptstyle ON}}$ degradation but also changes the optimal device orientation for both Si and Ge HP mode PE-FinFETs compared to their counterparts without phonon scattering.

We theoretically investigate quantum transport properties of quantum anomalous Hall bilayers, with arbitrary ratio of lattice constants, i.e., with lattice mismatch. In the simplest case of ratio 1 (but with different model parameters in two layers), the inter-layer coupling results in resonant traversing between forward propagating waves in two layers. In the case of generic ratios, there is a quantized conductance plateau originated from two Chern numbers associated with two layers. However, the phase boundary of this quantization plateau consists of a fractal transitional region (instead of a clear transition line) of interpenetrating edge states (with quantized conductance) and bulk states (with unquantized conductance). We attribute these bulk states as mismatch induced in-gap bulk states. Different from in-gap localized states induced by random disorder, these in-gap bulk states are extended in the limit of vanishing random disorder. However, the detailed fine structure of this transitional region is sensitive to disorder, lattice structure, sample size, and even the configuration of leads connecting to it, due to the bulk and topologically trivial nature of these in-gap bulk states., Comment: 16 pages, 15 figures

The electric dipole spin resonance (EDSR) combining strong spin-orbit coupling (SOC) and electric-dipole transitions facilitates fast spin control in a scalable way, which is the critical aspect of the rapid progress made recently in germanium (Ge) hole-spin qubits. However, a puzzle is raised because centrosymmetric Ge lacks the Dresselhaus SOC, a key element in the initial proposal of the hole-based EDSR. Here, we demonstrate that the recently uncovered finite k-linear Rashba SOC of 2D holes offers fast hole spin control via EDSR with Rabi frequencies in excellent agreement with experimental results over a wide range of driving fields. We also suggest that the Rabi frequency can reach 500 MHz under a higher gate electric field or multiple GHz in a replacement by [110]oriented wells. These findings bring a deeper understanding for hole-spin qubit manipulation and offer design principles to boost the gate speed., Comment: 11 pages, 3 figures

Exploring hidden effects that have been overlooked given the nominal global crystal symmetry but are indeed visible in solid-state materials has been a fascinating subject of research recently. Here, we introduce a novel hidden Zeeman-type spin polarization (HZSP) in nonmagnetic bulk crystals with sublattice structures. In the momentum space of these crystals, the doubly degenerate bands formed in a certain plane can exhibit a uniform spin configuration with opposite spin orientations perpendicular to this plane, whereas such degenerate states are spatially separated in a pair of real-space sectors. Interestingly, we find that HZSP can manifest itself in both centrosymmetric and non-centrosymmetric materials. We further demonstrate the important role of nonsymmorphic twofold screw-rotational symmetry played in the formation of HZSP. Moreover, two representative material examples, i.e., centrosymmetric WSe$_2$ and noncentrosymmetric BaBi$_4$O$_7$, are identified to show HZSP via first-principles calculations. Our finding thus not only opens new perspectives for hidden spin polarization research but also significantly broadens the range of materials towards spintronics applications., Comment: 5 pages, 3 figures

2D layered chalcogenide semiconductors have been proposed as a promising class of materials for low-dimensional electronic, optoelectronic, and spintronic devices. Here, all-2D van der Waals vertical spin-valve devices, that combine the 2D layered semiconductor InSe as a spacer with the 2D layered ferromagnetic metal Fe3 GeTe2 as spin injection and detection electrodes, are reported. Two distinct transport behaviors are observed: tunneling and metallic, which are assigned to the formation of a pinhole-free tunnel barrier at the Fe3 GeTe2 /InSe interface and pinholes in the InSe spacer layer, respectively. For the tunneling device, a large magnetoresistance (MR) of 41% is obtained under an applied bias current of 0.1 µA at 10 K, which is about three times larger than that of the metallic device. Moreover, the tunneling device exhibits a lower operating bias current but a more sensitive bias current dependence than the metallic device. The MR and spin polarization of both the metallic and tunneling devices decrease with increasing temperature, which can be fitted well by Bloch's law. These findings reveal the critical role of pinholes in the MR of all-2D van der Waals ferromagnet/semiconductor heterojunction devices.

Author(s): Liu, WH; Luo, JW; Li, SS; Wang, LW | Abstract: Photoinduced phase transitions can have complex and intriguing behaviors more than material ground-state dynamics. Understanding the underlying mechanism can help us to design new ways to manipulate the materials. A variety of mechanisms has been proposed to explain the photoinduced phase transitions of IrTe2, but a consensus has yet to be reached. Here, we study the photo-induced phase transitions of IrTe2 by performing the real-time time-dependent density functional theory (rt-TDDFT) simulations in combination with the occupation constrained DFT method. We reveal that the microscopic driving force for the photo-induced phase transitions arises from the tendency to lower the energy levels occupied by the excited carriers, through the increase or decrease of the associated atomic pair distances, depending on whether the newly occupied states are antibonding or bonding states, respectively. The geometric constraints between different bonds represented by the Poisson ratio can bring together different tendencies from different atomic pairs, thus forming a complex intriguing dynamic picture depending on the intensity of the excitation. We also find that phonons don't play a primary role, but can assist the phase transition. These findings imply that one can control the structural phase transitions by selectively exciting photocarriers into designated atomic states using appropriate photon sources.

Degrees of freedom like charge, spin, and valley in condensed matter have drawn much attention in the past decades. Recently, several kinds of nodal line semimetals have been discovered for which the conduction and valence bands cross along one-dimensional lines, usually forming loops. These distinct loops, similar to valleys, can be used as a possible carrier of information. Here we investigate the loop polarization during quantum transports, i.e., barrier tunneling as well as disorder scattering, in Hopf semimetals with two disconnected loops. We find that in both cases, the loop polarization is mostly dominated by the performance of normal incidence, due to the mirror symmetry of two loop planes and the finite size effect along one direction. After scattered by nonmagnetic disordered potentials, the opposite polarizations can intrinsically reveal the topological structure of two loops, linked or unlinked. The effects from smooth barriers and disorders are also investigated. We find that a long range smooth barrier leads to an even clearer and purer picture of scattering and loop polarization, due to the suppression of large momentum transfer. These effects can be used as a controllable loop filter and detector.

Nonradiative carrier recombinations at deep centers in semiconductors are of great importance for both fundamental physics and device engineering. In this article, we provide a revised analysis of Huang’s original nonradiative multi-phonon (NMP) theory with ab initio calculations. First, we confirmed at the first-principles level that Huang’s concise formula gives the same results as the matrix-based formula, and that Huang’s high-temperature formula provides an analytical expression for the coupling constant in Marcus theory. Secondly, we correct for anharmonic effects by taking into account local phonon-mode variations for different charge states of a defect. The corrected capture rates for defects in GaN and SiC agree well with experiments.

The modern theory of orbital magnetization (OM) was developed by using Wannier function method, which has a formalism similar with the Berry phase. In this manuscript, we perform a numerical study on the fate of the OM under disorder, by using this method on the Haldane model in two dimensions, which can be tuned between a normal insulator or a Chern insulator at half filling. The effects of increasing disorder on OM for both cases are simulated. Energy renormalization shifts are observed in the weak disorder regime and topologically trivial case, which was predicted by a self-consistent T-matrix approximation. Besides this, two other phenomena can be seen. One is the localization trend of the band orbital magnetization. The other is the remarkable contribution from topological chiral states arising from nonzero Chern number or large value of integrated Berry curvature. If the fermi energy is fixed at the gap center of the clean system, there is an enhancement of |M| at the intermediate disorder, for both cases of normal and Chern insulators, which can be attributed to the disorder induced topological metal state before localization., Comment: 14 pages, 6 figures

Charged-defect calculation using a periodic supercell is a significant class of problems in solid state physics. However, the finite supercell size induces an undesirable long-range image charge Coulomb interaction. Although a variety of methods have been proposed to eliminate such image Coulomb interaction, most of the previous schemes are based on a rough approximation of the defect charge screening. In this work, we present a rigorous derivation of the image charge interaction with a new defect screening model where the use of bulk macroscopic dielectric constant can be avoided. We have verified this approach in comparison with a widely used approach for 12 different defects. Our correction scheme offers a much faster convergence concerning the supercell size for cases with considerable image charge interactions. In those cases, we also found that the nonlinear dielectric screening might play an important role. The proposed new defect screening model will also shed new light on understanding the defect screening properties and can be applied to other defect systems.

Author(s): Liu, YY; Liu, F; Wang, R; Luo, JW; Jiang, X; Huang, R; Li, SS; Wang, LW | Abstract: The complexity of charge trapping in semiconductor devices, such as high-κ MOSFETs, is increasing as the devices themselves become more complicated. To facilitate research into such charge-trapping issues, here we propose an optimized simulation framework that is composed of density-functional theory (DFT) for electronic structure calculation and Marcus theory for the calculation of charge-trapping rates. The DFT simulations are either carried out or corrected by using the Heyd-Scuseria-Ernzerhof hybrid functional. Using this framework, the hole-trapping characteristics along multiple paths in Si/SiO2/HfO2 stacks are investigated, and the relative importance of each path is revealed by calculating its exact hole-trapping rate. Besides the study on crystalline stacks, we also create an amorphous stack, which is more realistic compared with experiments and real devices, to reveal more active trapping centers and to study the statistical feature of charge trapping induced by structural disorder. In addition, to seek effective measures for relieving these charge-trapping problems, the effects of hydrogen and fluorine passivations are discussed, and physical insights for improving the performance of high-κ MOSFETs are provided.

We present a fully ab initio direct quantum transport simulation of nano-interconnect containing a record number of atoms (~5000). Various defects of the nanosized copper (Cu) interconnects, including different wire-neck shapes, twin boundaries, point defects, cobalt (Co) interstitial layers, as well as the thermal effects, are investigated with an aim to reveal the largest effects in such imperfections. We also found that the classical picture of the interconnect resistance, e.g. Matthiessen's rule, might not be valid at nanoscale, due to strong quantum interferences.

The theoretical limit of sub-threshold swing ( SS ) in the n-type piezoelectric FinFET (Piezo-FinFET) is thoroughly investigated. Gate voltage modulated strain, conduction band edge shift, and the SS are evaluated utilizing physics-based modeling and numerical simulation, indicating a strong dependence of SS on the fin width, crystalline orientation, and physical properties of the piezo-material used in the gate stack. For [001] Si and [111] Ge Piezo-FinFETs with 5-nm fin width and 3-nm PZT-5H thickness, SS values down to 40 mV/decade are achievable after design optimizations. A figure of merit, ${E}_{\text{cr}} {d}_{z3}/{S}_{\text{pie}, 33}$ , is proposed for the piezo-materials in terms of the critical electric field ( ${E}_\text{cr}$ ), the piezoelectric strain constant ( ${d}_{z3}$ ), and the elastic compliance constant ( ${S}_{\text{pie}, 33}$ ), to guide the Piezo-FinFET designs.

Doping is a key technology in modern semiconductor industry. With reduced size of semiconductor devices, quantum effect becomes significant, and the classic theory for device design will no longer be valid. Thus the traditional doping technology is facing a great challenge. On the other hand, the third generation wide-gap semiconductors like GaN, ZnO and TiO2 have great application potential in optoelectronics and microelectronics due to their wide band gap, direct gap character and good stability. However, there is a doping bottleneck on the p-type doping of these materials. In the field of application of semiconductor technology, doping mechanism in photocatalysis, spintronic materials, and novel two-dimensional (2D) semiconductors are also the frontier of research. The research in this project focuses on the above key topics. The primary discoveries include the follows: (i) Based on first-principles band structure calculations and analysis of the band-edge wave function characters, we proposed that passivated (Mo+C)-doped TiO2 is a strong candidate for hydrogen production through water splitting. The results received highly attention from the experts in the field of photocatalysis, and the feasibility was confirmed by recent experimental studies. (ii) We discovered that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d0 semiconductors. The origin and enhancement of such hole-induced ferromagnetism is studied, which suggested new possibility of preparing non-magnetically doped spintronic semiconductor devices. (iii) We systematically studied the doping mechanism and bottleneck of donor and acceptor impurities in quantum dots and quantum wires. Due to the effect of quantum confinement effect, the energy of the conduction band minimum (CBM) increases whereas that of the valence and maximum (VBM) decreases. We found that defect formation energy and transition energy level increase when the size of the quantum dot (QD) decreases, leading to the self-purification effect and degrade the performance of devices. Focused on this problem, we proposed the approach of overcoming the doping bottleneck in nanostructures, and developed a model to describe the quantum Stark effect in hole impurity state of quantum dots. These works promoted the research in the application of nano-devices. (iv) For the first time, the band alignment between two-dimensional transition-metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) is calculated. We predicted the type-II band alignment in MoX2/WX2 heterostructures. In addition, direct-indirect gap transition and semiconductor-metal transition in strained MoS2 is observed. Moreover, we studied the mechanical, optical properties and band gap modulation of graphyne, a new allotrope of carbon, and discovered that Ca-doped graphyne had high capacity of hydrogen storage. The project has attracted great attentions, and the related publications are highly cited by researchers all over the world. These works bring better understanding of semiconductor doping theory, and have important scientific value for the device design and property prediction of new generation semiconductors and nano-devices.

As the high throughout calculations and material genome approaches become more and more popular in material science, the search for optimal ways to predict atomic global minimum structure is a high research priority. This paper presents a fast method for global search of atomic structures at ab initio level. The structures global optimization (SGO) engine consists of a high-efficiency differential evolution algorithm, accelerated local relaxation methods and a plane-wave density functional theory code running on GPU machines. The purpose is to show what can be achieved by combining the superior algorithms at the different levels of the searching scheme. SGO can search the global-minimum configurations of crystals, two-dimensional materials and quantum clusters without prior symmetry restriction in a relatively short time (half or several hours for systems with less than 25 atoms), thus making such a task a routine calculation. Comparisons with other existing methods such as minima hopping and genetic algorithm are provided. One motivation of our study is to investigate the properties of magnetic systems in different phases. The SGO engine is capable of surveying the local minima surrounding the global minimum, which provides the information for the overall energy landscape of a given system. Using this capability we have found several new configurations for testing systems, explored their energy landscape, and demonstrated that the magnetic moment of metal clusters fluctuates strongly in different local minima.

Germanium selenide (GeSe) is an isoelectronic analogue of phosphorene, which has been studied widely in recent experiments. In this paper, we have investigated tunable electronic structures and transport properties of 2D and quasi-1D GeSe by using a self-consistent ab initio approach. The calculated band structures show stretching and compression in the zigzag direction and stretching in the armchair direction, and all can enlarge the band gap of 2D GeSe nanosheet. However, the compression in the armchair direction will reduce the band gap of the 2D GeSe nanosheet. In addition, appropriate compressions in both directions can change the 2D GeSe nanosheet from indirect band gap to direct band gap. When the 2D GeSe nanosheet is cut into a quasi-1D nanoribbon, the band structures can be modulated by the ribbon width and the passivation. The unpassivated zigzag GeSe nanoribbons are metals regardless of the ribbon width. The H-passivated zigzag GeSe nanoribbons are semiconductors with direct band gaps, and the ...

Pt nanoclusters play an important role in catalysis-related applications. Essential to their activities are their geometries and energy landscapes. In this work, we studied the energy landscapes of Pt clusters using a parallel differential evolution optimization algorithm and an accelerated ab initio atomic relaxation method, which allowed us to explore unprecedentedly large numbers of geometry local minima at ab initio level. We found many lower-energy isomers with low symmetry in their geometry. The energy landscapes were demonstrated to be glass-like with a large number of local minimum structures close to the global minimum. The electronic and magnetic properties of most glass-like local minima were dramatically different from the global minimum, and they should be observed in the experimental measurements. The connections between these local minima were further analyzed using data mining techniques.

We investigate the transport properties of Schottky contact structure based on zigzag phosphorene nanoribbon (ZPNR) by using the non-equilibrium Green's function formalism with density functional theory. The calculated band structures show the unpassivated ZPNR is metal and the H-passivated ZPNR is semiconductor. The in-plane contact structure of unpassivated ZPNR and H-passivated ZPNR leads to the formation of a Schottky barrier, which results in rectifying current-voltage characteristics. The rectification ratio (RR) can reach to 10 3 in the bias region from 0.3 V to 0.5 V. By extending the length of H-passivated ZPNR in Schottky contact structure, the currents of the Schottky contact structure are decreased, but the RR can be enlarged obviously. When the length of H-passivated ZPNR is four times than that of the unpassivated ZPNR, the average RR increases to 10 6 in the bias region from 0.3 V to 0.5 V and the maximal RR is boosted up to 10 7 at 0.45 V.

Molecular spintronics is a new and emergent sub-area of spintronics that has the potential use in future information storage, magnetic sensing and quantum computing. We investigate the spin transport properties of a single benzene molecule connected to zigzag graphene nanoribbons (ZGNRs) by using a self-consistent ab initio approach which combines the non-equilibrium Green's function (NEGF) formalism with density functional theory (DFT). The spin-resolved current-voltage characteristics of the single benzene molecule at finite biases are different while the left and right zigzag graphene electrodes with the parallel (P) and anti-parallel (AP) magnetism configurations. The perfect (100%) spin polarization in a large bias region can be realized with both P and AP magnetism configuration. However, the spin-resolved rectifications are only found with AP magnetism configuration. More importantly, both of the α-spin and β-spin currents would drop remarkably when one ZGNR electrode is twisted. Especially the α-spin currents with P magnetism configuration will decrease by up to 8 orders of magnitude when the twisted angle reaches 90°. The above results demonstrate that this junction holds promise in the design of a high-performance multifunctional single-molecule spintronic device.

Low-energy impurity diffusion in a host material is often regarded as an adiabatic process, characterized by its adiabatic potential energy barrier. Here we show that the diffusion process in semiconductors can involve nonadiabatic electron excitations, rending it to be a more complicated process. Impurity diffusion in a device at working temperature can pump one electron up from localized impurity state into the host conduction band and causes the impurity to be a dynamic donor since it temporarily loses its electron to the host. This nonadiabatic process, against a common belief, fundamentally change the diffusion behavior, including its barrier height and diffusion path. Although we mainly demonstrate this process with Au metal impurity in bulk Si through time-dependent density functional theory simulations, we believe this could be a rather common phenomenon as it is shown that the similar phenomena also exist in Zn, Cd impurities diffusion in bulk Si, and Ti diffusion in $\mathrm{Ti}{\mathrm{O}}_{2}$. We believe this study can open up a new direction of inquiry for such diffusion behavior in semiconductor.

Author(s): Liu, YY; Zheng, F; Jiang, X; Luo, JW; Li, SS; Wang, LW | Abstract: Accurate microscopic description of the charge-trapping process from semiconductor to defects in the dielectric-oxide layer is of paramount importance for understanding many microelectronic devices such as complementary metal-oxide-semiconductor (CMOS) transistors, as well as electrochemical reactions. Unfortunately, most current microscopic descriptions of such processes are based on empirical models with parameters fitted to experimental device performance results or simplified approximations like the Wentzel-Kramers-Brillouin (WKB) method. Some critical questions are still unanswered, including: What controls the charge-hopping rate, the coupling strength between the defect level to semiconductor level, or the energy difference? How does the hopping rate decay with defect-semiconductor distance? What is the fluctuation of the defect level, especially in amorphous dielectrics? Many of these questions can be answered by ab initio calculations. However, to date, there are few ab initio studies for this problem mainly due to technical challenges from atomic-structure construction to large-system calculations. Here, using the latest advances in calculation methods and codes, we study the carrier-trapping problem using density-functional theory (DFT) based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional. The valence bond random-switching method is used to construct the crystalline-Si-amorphous-SiO2 (c-Si/a-SiO2) interfacial atomic structure, and the HSE yields a band offset that agrees well with experiments. The hopping rate is calculated with the Marcus theory, and the hopping-rate dependences on the gate potential and defect distances are revealed, as well as the range of fluctuation results from amorphous structural variation. We also analyze the result with the simple WKB model and find a major difference in the description of the coupling constant decay with the defect-semiconductor distance. Our results provide the ab initio simulation insights for this important carrier-trapping process for device operation.

Hidden Rashba and Dresselhaus spin splittings in centrosymmetric crystals with subunits/sectors having non-centrosymmetric symmetries (the R-2 and D-2 effects) have been predicted theoretically and then observed experimentally, but the microscopic mechanism remains unclear. Here we demonstrate that the spin splitting in the R-2 effect is enforced by specific symmetries, such as non-symmorphic symmetry in the present example, which ensures that the pertinent spin wavefunctions segregate spatially on just one of the two inversion-partner sectors and thus avoid compensation. We further show that the effective Hamiltonian for the conventional Rashba (R-1) effect is also applicable for the R-2 effect, but applying a symmetry-breaking electric field to a R-2 compound produces a different spin-splitting pattern than applying a field to a trivial, non-R-2, centrosymmetric compound. This finding establishes a common fundamental source for the R-1 effect and the R-2 effect, both originating from local sector symmetries rather than from the global crystal symmetry per se., The Dresselhaus and Rashba effects have traditionally been expected only in non-centrosymmetric systems but recent work has shown that they can exist in some centrosymmetric materials. Here the authors show that the so-called hidden Rashba effect originates from wavefunction segregation enforced by local symmetries.

First-principles calculations of charged defects have become a cornerstone of research in semiconductors and insulators by providing insights into their fundamental physical properties. But, current standard approach using the so-called ``jellium model'' has encountered both conceptual ambiguity and computational difficulty, especially for low-dimensional semiconducting materials. In this paper, we propose a more physical and straightforward ``transfer to real state'' model to calculate the formation energies of charged defects in both three-dimensional (3D) bulk and low-dimensional semiconductors. Within this universal model, the ionized electrons or holes are placed on the realistic host band-edge states instead of the virtual jellium state; therefore, rendering it not only naturally keeps the supercell charge neutral, but also has clear physical meaning, thus can be easily extended to study other physical properties such as exciton binding energy, free carrier bound to the defect in low-dimensional systems. This realistic model reproduces the same accuracy as the traditional jellium model for most of the 3D semiconducting materials, and remarkably, for the low-dimensional structures, it can cure the divergence caused by the artificial long-range electrostatic energy encountered in the jellium model, and hence gives meaningful formation energies of defects in charged-state and transition-energy levels of the corresponding defects. Our realistic method, therefore, will have significant impact for the study of defect physics in all low-dimensional systems.

Although the direct or indirect nature of the bandgap transition is an essential parameter of semiconductors for optoelectronic applications, the understanding why some of the conventional semiconductors have direct or indirect bandgaps remains ambiguous. In this Letter, we revealed that the existence of the occupied cation d bands is a prime element in determining the directness of the bandgap of semiconductors through the s-d and p-d couplings, which push the conduction band energy levels at the X- and L-valley up, but leaves the {\Gamma}-valley conduction state unchanged. This unified theory unambiguously explains why Diamond, Si, Ge, and Al-containing group III-V semiconductors, which do not have active occupied d bands, have indirect bandgaps and remaining common semiconductors, except GaP, have direct bandgaps. Besides s-d and p-d couplings, bond length and electronegativity of anions are two remaining factors regulating the energy ordering of the {\Gamma}-, X-, and L-valley of the conduction band, and are responsible for the anomalous bandgap behaviors in GaN, GaP, and GaAs that have direct, indirect, and direct bandgaps, respectively, despite the fact that N, P, and As are in ascending order of the atomic number. This understanding will shed light on the design of new direct bandgap light-emitting materials., Comment: 15 pages, 4 figures

As the characteristic lengths of advanced electronic devices are approaching the atomic scale, ab initio simulation method, with full consideration of quantum mechanical effects, becomes essential to study the quantum transport phenomenon in them. The widely used non-equilibrium Green’s function (NEGF) combined with the density functional theory (DFT) approach prefers a localized basis set. As many states of the art DFT calculations for solid state systems are carried out in plane waves, it is thus worth to investigate the feasibility of using the plane wave basis set for large-scale quantum transport calculations. Here we present a plane wave method for large-scale transport calculations based on a previously developed scattering state calculation approach (Wang, 2005). We address the unique computational challenges of applying that approach for large-scale systems where it is too expensive to calculate all the occupied eigenstates of the system as in conventional DFT calculations. By applying several high-efficiency parallel algorithms, including linear-scaling DFT algorithm, folded spectrum method, and Chebyshev filter technique, we demonstrate that it is possible to use this approach to simulate a system with several thousand atoms on high performance computing platforms. This method is not only used to study nanowire interconnects, showing how the shape and point defect affects their transport properties, but also used to study nanoscale Si transistor. Such quantum transport simulation method will be useful for investigating and designing nanoscale devices.

In dye-sensitized solar cells (DSCs), the electron transfer from photoexcited dye molecules to semiconductor substrates remains a major bottleneck. Replacing TiO2 with ZnO is expected to enhance the efficiency of DSCs, owing to the latter possesses a much larger electron mobility, but similar bandgap and band positions as TiO2 remain. However, the record efficiency of ZnO-based DSCs is only 7% compared with 13% of TiO2-based DSCs due to the even slower electron-transfer rate in ZnO-based DSCs, which becomes a long-standing puzzle. Here, we computationally investigate the electron transfer from the dye molecule into ZnO and TiO2, respectively, by performing the first-principles calculations within the frame of the Marcus theory. The predicted electron-transfer rate in the TiO2-based DSC is about 1.15 × 10(9) s(-1), a factor of 15 faster than that of the ZnO-based DSC, which is in good agreement with experimental data. We find that the much larger density of states of the TiO2 compared with ZnO near the conduction band edge is the dominant factor, which is responsible for the faster electron-transfer rate in TiO2-based DSCs. These denser states provide additional efficient channels for the electron transfer. We also provide design principles to boost the efficiency of DSCs through surface engineering of high mobility photoanode semiconductors.

We study the decoherence of a coupled quantum system consisting of a central spin and its correlated environment described by a general X Y spin-chain model. We make it clear that the evolution of the coherence factor sensitively depends on the initial state of the environment spin-chain. Specially, the dynamical evolution of the coherence factor of the central spin is numerically and analytically investigated in both weak and strong coupling cases with initial state set as the ground state of the initial Hamiltonian H E ( λ i ) at time t = 0 which may be different from the evolving Hamiltonian H E ( λ e ) for time t > 0, as well as set as the thermal equilibrium state. In both weak and strong coupling cases, we show that the evolution of the coherence factor with initial state being the ground state of the initial Hamiltonian can be approximated by a Gaussian. Particularly, in the strong coupling regime the coherence factor oscillates rapidly under a Gaussian envelope. The width of the Gaussian decay (envelope) is presented in detail.

Si quantum dots (QDs) have a significant improvement in luminous efficiency compared with bulk Si, achieved by alleviating the forbiddance of no-phonon Γ–Γ radiative transition determined by the law of momentum conservation. Two divergent mechanisms have been proposed to account for the breakdown of momentum conservation in Si QDs, one is due to the space-confinement-induced spread of k-space wave functions associated with Heisenberg uncertainty principle Δr · Δk > 1/2, and the other is due to the interface-effect-induced intervalley mixing between indirect and direct bandgap states. Both mechanisms could cause a small overlap of the electron and hole wave functions in k-space and make vertical transitions allowed, which leads to the zero-phonon light emission. In this work, we unravel the hierarchical relationship between these two primary mechanisms in the process of zero-phonon light emission from indirect bandgap QDs, by performing semiempirical pseudopotential calculation including many-body interaction on the room-temperature luminescent properties of a series of Si, Ge, and Ge/Si core/shell QDs. We show that the space confinement mechanism is dominant in both Si and Ge indirect bandgap QDs, and the interface-induced intervalley coupling mechanism plays a minor role. While in Ge/Si core/shell QDs, the interface-induced intervalley coupling mechanism has a more pronounced contribution to enhanced light emission, implying one can further enhance light emission via engineering interface based on the intervalley coupling mechanism. Given this, we further engineer the Ge QD interface by bringing four motifs of Si/Ge multiple layers from previously inverse designed Si/Ge superlattices and core/shell nanowires for light emitters. We show that two out of four motifs always give rise to two orders of magnitude enhancement in light emission relative to the Ge and Si QDs. We demonstrate that the interface engineering can enhance light emission in indirect bandgap QDs substantially and regulate the intervalley coupling mechanism as the primary factor over the space confinement mechanism in breaking the momentum conservation law.

Landauer-B\"uttiker formula describes the electronic quantum transports in nanostructures and molecules. It will be numerically demanding for simulations of complex or large size systems due to, for example, matrix inversion calculations. Recently, Chebyshev polynomial method has attracted intense interests in numerical simulations of quantum systems due to the high efficiency in parallelization, because the only matrix operation it involves is just the product of sparse matrices and vectors. Many progresses have been made on the Chebyshev polynomial representations of physical quantities for isolated or bulk quantum structures. Here we present the Chebyshev polynomial method to the typical electronic scattering problem, the Landauer-B\"uttiker formula for the conductance of quantum transports in nanostructures. We first describe the full algorithm based on the standard bath kernel polynomial method (KPM). Then, we present two simple butefficient improvements. One of them has a time consumption remarkably less than the direct matrix calculation without KPM. Some typical examples are also presented to illustrate the numerical effectiveness., Comment: 10 pages, 8 figures

The self-assembled quantum dots (QDs) provide an ideal platform for realization of quantum information technology because it provides on demand single photons, entangled photon pairs from biexciton cascade pro- cess, single spin qubits, and so on. The fine structure splitting (FSS) of exciton is a fundamental property of QDs for thees applications. From the symmetry point of view, since the two bright exciton states belong to two different representations for QDs with C2v symmetry, they should not only have different energies, but also have different lifetimes, which is termed exciton lifetime asymmetry. In contrast to extensively studied FSS, the investigation of the exciton lifetime asymmetry is still missed in literature. In this work, we carried out the first investigation of the exciton lifetime asymmetry in self-assembled QDs and presented a theory to deduce lifetime asymmetry indirectly from measurable qualities of QDs. We further revealed that intrinsic lifetimes and their asymmetry are fundamental quantities of QDs, which determine the bound of the extrinsic lifetime asymmetries, polarization angles, FSSs, and their evolution under uniaxial external forces. Our findings provide an important basis to deeply understanding properties of QDs., 6 figures, 9 pages. Comments are welcome

Upon heating, almost all zinc-blende (ZB) and diamond-like semiconductors undergo volume contraction at low temperature, i.e. negative thermal expansion (NTE), instead of commonly expected expansion. Specifically, CuCl has the largest NTE among these semiconductors with a coefficient comparable with the record value of ZrW2O8. So far, underlying physical mechanism remains ambiguous. Here, we present a systematic and quantitative study of the NTE in ZB and diamond-like semiconductors using first-principles calculations. We clarified that the material ionicity, which renders the softening of the bond-angle-bending and thus, the enhancement of excitation of the transverse acoustic (TA) phonon, is responsible for the NTE of ZB and diamond-like semiconductors. With the increase in the ionicity from the groups IV, III-V, IIB-VI to IB-VII ZB semiconductors, the coefficient of the maximum NTE increases due to the weakness in bond-rotation effect, which makes the relative motion between cation and anion transverse to the direction of the bond more feasible and the mode Grüneisen parameters of the TA modes more negative. Since CuCl has the highest ionicity among all ZB and diamond-like semiconductors, it is expected to have the largest NTE, in good agreement with the experimental observation. This understanding would be beneficial for tetrahedral materials with specific applications.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2D layered materials have emerged in recent years as a new platform to host novel electronic, optical, or excitonic physics and develop unprecedented nanoelectronic and energy applications. By definition, these materials are strongly anisotropic between the basal plane and cross the plane. The structural and property anisotropies inside their basal plane, however, are much less investigated. Black phosphorus, for example, is a 2D material that has such in-plane anisotropy. Here, a rare chemical form of arsenic, called black-arsenic (b-As), is reported as a cousin of black phosphorus, as an extremely anisotropic layered semiconductor. Systematic characterization of the structural, electronic, thermal, and electrical properties of b-As single crystals is performed, with particular focus on its anisotropies along two in-plane principle axes, armchair (AC) and zigzag (ZZ). The analysis shows that b-As exhibits higher or comparable electronic, thermal, and electric transport anisotropies between the AC and ZZ directions than any other known 2D crystals. Such extreme in-plane anisotropies can potentially implement novel ideas for scientific research and device applications.