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The rare-earth ditellurides are known to form two-dimensional square lattice where the strong Fermi surface nesting leads to structural modulation. In contrast to charge density waves, the supercell modulation is accompanied by the formation of the periodic Te vacancy network, where the Te deficiency affects the nesting vector (i.e. the supercell size) via tuning the chemical potential. In this work, first principles electronic structure calculations for the $\sqrt{5}\times\sqrt{5}$ supercell, that commonly appears in this family of tellurides, unveil interesting electronic, transport, and migration properties of the Te defect lattice in DyTe$_{1.8}$. The reconstruction of the Te-deficient square lattice, consisting of a single Te-dimer and a pair Te-trimers per unit cell, gives rise to an out-of-plane polarization, whose direction depends on the position of the dimer. This results in various close-in-energy parallel and antiparallel polarization configurations of successive Te layers depending on the dimer positions. We predict that the orientation of the Te dimers, and hence the corresponding structural motifs, can be reversibly switched between two in-plane perpendicular directions under tensile epitaxial strain via a piezoelectric substrate, resulting in a colossal conductivity switching. Furthermore, the Te-dimer orientations result in asymmetric Fermi surface which can be confirmed by quantum oscillations measurements. Finally, we present numerical results for the migration paths and energy landscape through various divacancy configurations in the presence or absence of epitaxial strain.

Rare-earth diantimondes exhibit coupling between structural and electronic orders which are tunable under pressure and temperature. Here we present the discovery of a new polymorph of LaSb$_2$ stabilized in thin films synthesized using molecular beam epitaxy. Using diffraction, electron microscopy, and first principles calculations we identify a YbSb$_2$-type monoclinic lattice as a yet-uncharacterized stacking configuration. The material hosts superconductivity with a $T_\mathrm{c}$ = 2 K, which is enhanced relative to the bulk ambient phase, and a long superconducting coherence length of 140 nm. This result highlights the potential thin film growth has in stabilizing novel stacking configurations in quasi-two dimensional compounds with competing layered structures.

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co$_{1/3}$NbS$_2$, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up a new avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals., Comment: 4 figures

Antiferromagnetic (AFM) materials are a pathway to spintronic memory and computing devices with unprecedented speed, energy efficiency, and bit density. Realizing this potential requires AFM devices with simultaneous electrical writing and reading of information, which are also compatible with established silicon-based manufacturing. Recent experiments have shown tunneling magnetoresistance (TMR) readout in epitaxial AFM tunnel junctions. However, these TMR structures were not grown using a silicon-compatible deposition process, and controlling their AFM order required external magnetic fields. Here we show three-terminal AFM tunnel junctions based on the noncollinear antiferromagnet PtMn3, sputter-deposited on silicon. The devices simultaneously exhibit electrical switching using electric currents, and electrical readout by a large room-temperature TMR effect. First-principles calculations explain the TMR in terms of the momentum-resolved spin-dependent tunneling conduction in tunnel junctions with noncollinear AFM electrodes.

This paper presents the DaG LLM (David and Goliath Large Language Model), a language model specialized for Korean and fine-tuned through Instruction Tuning across 41 tasks within 13 distinct categories.

As an emerging non-volatile memory technology, magnetic random access memory (MRAM) has key features and advantages including non-volatility, high speed, endurance, low power consumption and radiation tolerance. Conventional MRAM utilizes magnetic tunnel junctions (MTJs), which consist of two ferromagnetic layers separated by an insulating tunnel barrier. The orientation of the magnetic layers represents the binary data (0 or 1), and electrical resistance changes depending on the relative orientation of these magnetic layers. Despite these advancements, the quest for a swifter, more stable magneto-resistive random-access memory paradigm persists. In this vein, we present a groundbreaking development: room-temperature antiferromagnetic tunnel junctions devoid of any net magnetic moment. Over 200% tunneling altermagnetic resistance (TAR) ratio was measured at RuO2 (110)/MgO/RuO2 (110)/W structure, which is achieved by changing the antiferromagnetic Neel vector of RuO2 with an ultralow current density 2 MA*cm-2., Comment: Modification required

Square net tellurides host an array of electronic ground states and commonly exhibit charge-density-wave ordering. Here we report the epitaxy of DyTe$_{2-\delta}$ on atomically flat MgO (001) using molecular beam epitaxy. The films are single phase and highly oriented as evidenced by transmission electron microscopy and X-ray diffraction measurements. Epitaxial strain is evident in films and is relieved as the thickness increases up to a value of approximately 20 unit cells. Diffraction features associated with a supercell in the films are resolved which is coupled with Te-deficiency. First principles calculations attribute the formation of this defect lattice to nesting conditions in the Fermi surface, which produce a periodic occupancy of the conducting Te square-net, and opens a band gap at the chemical potential. This work establishes the groundwork for exploring the role of strain in tuning electronic and structural phases of epitaxial square-net tellurides and related compounds.

As the nascent field of quantum computing develops, an increasing number of quantum hardware modalities, such as superconducting electronic circuits, semiconducting spins, trapped ions, and neutral atoms, have become available for performing quantum computations. These quantum hardware modalities exhibit varying characteristics and implement different universal quantum gate sets that may e.g. contain several distinct two-qubit quantum gates. Adapting a quantum circuit from a, possibly hardware-agnostic, universal quantum gate set to the quantum gate set of a target hardware modality has a crucial impact on the fidelity and duration of the intended quantum computation. However, current quantum circuit adaptation techniques only apply a specific decomposition or allow only for local improvements to the target quantum circuit potentially resulting in a quantum computation with less fidelity or more qubit idle time than necessary. These issues are further aggravated by the multiple options of hardware-native quantum gates rendering multiple universal quantum gates sets accessible to a hardware modality. In this work, we developed a satisfiability modulo theories model that determines an optimized quantum circuit adaptation given a set of allowed substitutions and decompositions, a target hardware modality and the quantum circuit to be adapted. We further discuss the physics of the semiconducting spins hardware modality, show possible implementations of distinct two-qubit quantum gates, and evaluate the developed model on the semiconducting spins hardware modality. Using the developed quantum circuit adaptation method on a noisy simulator, we show the Hellinger fidelity could be improved by up to 40% and the qubit idle time could be decreased by up to 87% compared to alternative quantum circuit adaptation techniques., Comment: to appear in DATE'23

The Su, Schrieffer and Heeger (SSH) model, describing the soliton excitations in polyacetylene due to the formation of antiphase domain walls (DW) from the alternating bond pattern, has served as a paradigmatic example of one-dimensional (1D) chiral topological insulators. While the SSH model has been realized in photonic and plasmonic systems, there have been limited analogues in three-dimensional (3D) electronic systems, especially regarding the formation of antiphase DWs. Here, we propose that pristine bulk Bi, in which the dimerization of $(111)$ atomic layers renders alternating covalent and van der Waals bonding within and between successive $(111)$ bilayers, respectively, serves as a 3D analogue of the SSH model. First, we confirm that the two dimerized Bi structures belong to different Zak phases of 0 and $\pi$ by considering the parity eigenvalues and Wannier charge centers, while the previously reported bulk topological phases of Bi remain invariant under the dimerization reversal. Next, we demonstrate the existence of topologically non-trivial $(111)$ and trivial $(11\bar{2})$ DWs in which the number of in-gap DW states (ignoring spin) is odd and even respectively, and show how this controls the interlinking of the Zak phases of the two adjacent domains. Finally, we derive general criteria specifying when a DW of arbitrary orientation exhibits a $\pi$ Zak phase based on the flip of parity eigenvalues. An experimental realization of dimerization in Bi and the formation of DWs may be achieved via intense femtosecond laser excitations that can alter the interatomic forces and bond lengths.

We investigate the integer quantum Hall effect in Cd3As2 thin films under conditions of strong to moderate quantum confinement (thicknesses of 10 nm, 12 nm, 15 nm). In all the films, we observe the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor {\nu} = 1) and at spin-degenerate higher index Landau levels with even filling factors ({\nu} = 2,4,6). With increasing quantum confinement, we also observe a lifting of the Landau level spin degeneracy at {\nu} = 3, manifest as the emergence of an anomaly in the longitudinal and Hall resistivity. Tight-binding calculations show that the enhanced g-factor likely arises from a combination of quantum confinement and corrections from nearby subbands. We also comment on the magnetic field induced transition from an insulator to a quantum Hall liquid when the chemical potential is near the charge neutrality point.

We report direct visualization of spin-flip transition of the surface layer in antiferromagnet MnBi4Te7, a natural superlattice of alternating MnBi2Te4 and Bi2Te3 layers, using cryogenic magnetic force microscopy (MFM). The observation of magnetic contrast across domain walls and step edges confirms that the antiferromagnetic order persists to the surface layers. The magnetic field dependence of the MFM images reveals that the surface magnetic layer undergoes a first-order spin-flip transition at a magnetic field that is lower than the bulk transition, in excellent agreement with a revised Mills' model. Our analysis indicates an enhancement of the order parameter in the surface magnetic layer, implying robust ferromagnetism in the single-layer limit. The direct visualization of surface spin-flip transition not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultra-thin films of MnBi4Te7 and other natural superlattice topological magnets., Comment: 6 pages, 4 figures

With the development of IoT, the sensor usage has been elevated to a new level, and it becomes more crucial to maintain reliable sensor networks. In this paper, we provide how to efficiently and reliably manage the sensor monitoring system for securing fresh data at the data center (DC). A sensor transmits its sensing information regularly to the DC, and the freshness of the information at the DC is characterized by the age of information (AoI) that quantifies the timeliness of information. By considering the effect of the AoI and the spatial distance from the sensor on the information error at the DC, we newly define an error-tolerable sensing (ETS) coverage as the area that the estimated information is with smaller error than the target value. We then derive the average AoI and the AoI violation probability of the sensor monitoring system, and finally present the {\eta}-coverage probability, which is the probability that the ETS coverage is greater than {\eta} ratio of the maximum sensor coverage. We also provide the optimal transmission power of the sensor, which minimizes the average energy consumption while guaranteeing certain level of the {\eta}-coverage probability. Numerical results validate the theoretical analysis and show the tendency of the optimal transmission power according to the maximum number of retransmissions. This paper can pave the way to efficient design of the AoI-sensitive sensor networks for IoT.

Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi$_2$Te$_4$ single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi$_2$Te$_4$, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets., Comment: 11 pages, 4 figures

MoTe$_2$ has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe$_2$, and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory (DFT+U) calculations, we investigate the structural phase transition between the polar T$_d$ and nonpolar 1T$'$ phases of MoTe$_2$ in reference to a hypothetical high-symmetry T$_0$ phase that exhibits higher-order topological features. In the T$_d$ phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T$_d$-MoTe$_2$, and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

Much of the dramatic growth in research on topological materials has focused on topologically protected surface states. While the domain walls of topological materials such as Weyl semimetals with broken inversion or time-reversal symmetry can provide a hunting ground for exploring topological interfacial states, such investigations have received little attention to date. Here, utilizing in-situ cryogenic transmission electron microscopy combined with first-principles calculations, we discover intriguing domain-wall structures in MoTe2, both between polar variants of the low-temperature(T) Weyl phase, and between this and the high-T high-order topological phase. We demonstrate how polar domain walls can be manipulated with electron beams and show that phase domain walls tend to form superlattice-like structures along the c axis. Scanning tunneling microscopy indicates a possible signature of a conducting hinge state at phase domain walls. Our results open avenues for investigating topological interfacial states and unveiling multifunctional aspects of domain walls in topological materials., Comment: 6 figures

The polarization and piezoelectric response of the BiTe$X$ ($X$=Cl, Br, and I) layered tellurohalides are computed from first principles. The results confirm a mixed ionic-covalent character of the bonding, and demonstrate that the internal structure within each triple layer is only weakly affected by the external stress, while the changes in the charge distribution with stress produce a substantial negative piezoelectric response. This suggests a new mechanism for negative piezoelectric response that should remain robust even in ultra-thin film form in this class of materials.

The anomalous Hall effect (AHE) is a non-linear Hall effect appearing in magnetic conductors, boosted by internal magnetism beyond what is expected from the ordinary Hall effect. With the recent discovery of the quantized version of the AHE, the quantum anomalous Hall effect (QAHE), in Cr- or V-doped topological insulator (TI) (Sb,Bi)$_2$Te$_3$ thin films, the AHE in magnetic TIs has been attracting significant interest. However, one of the puzzles in this system has been that while Cr- or V-doped (Sb,Bi)$_2$Te$_3$ and V-doped Bi$_2$Se$_3$ exhibit AHE, Cr-doped Bi$_2$Se$_3$ has failed to exhibit even ferromagnetic AHE, the expected predecessor to the QAHE, though it is the first material predicted to exhibit the QAHE. Here, we have successfully implemented ferromagnetic AHE in Cr-doped Bi$_2$Se$_3$ thin films by utilizing a surface state engineering scheme. Surprisingly, the observed ferromagnetic AHE in the Cr-doped Bi$_2$Se$_3$ thin films exhibited only positive slope regardless of the carrier type. We show that this sign problem can be explained by the intrinsic Berry curvature of the system as calculated from a tight-binding model combined with a first-principles method., Comment: 24 pages, 5 figures

Many hyperparameter optimization (HyperOpt) methods assume restricted computing resources and mainly focus on enhancing performance. Here we propose a novel cloud-based HyperOpt (CHOPT) framework which can efficiently utilize shared computing resources while supporting various HyperOpt algorithms. We incorporate convenient web-based user interfaces, visualization, and analysis tools, enabling users to easily control optimization procedures and build up valuable insights with an iterative analysis procedure. Furthermore, our framework can be incorporated with any cloud platform, thus complementarily increasing the efficiency of conventional deep learning frameworks. We demonstrate applications of CHOPT with tasks such as image recognition and question-answering, showing that our framework can find hyperparameter configurations competitive with previous work. We also show CHOPT is capable of providing interesting observations through its analysing tools, Comment: 10 pages, 9 figures. arXiv admin note: text overlap with arXiv:1807.01774 by other authors

The boom of deep learning induced many industries and academies to introduce machine learning based approaches into their concern, competitively. However, existing machine learning frameworks are limited to sufficiently fulfill the collaboration and management for both data and models. We proposed NSML, a machine learning as a service (MLaaS) platform, to meet these demands. NSML helps machine learning work be easily launched on a NSML cluster and provides a collaborative environment which can afford development at enterprise scale. Finally, NSML users can deploy their own commercial services with NSML cluster. In addition, NSML furnishes convenient visualization tools which assist the users in analyzing their work. To verify the usefulness and accessibility of NSML, we performed some experiments with common examples. Furthermore, we examined the collaborative advantages of NSML through three competitions with real-world use cases.

Recent developments in topological semimetals open a way to realize relativistic dispersions in condensed matter systems. One recently studied type of topological feature is the "triple nodal point" where three bands become degenerate. In contrast to Weyl and Dirac nodes, triple nodal points, which are protected by a rotational symmetry, have nodal lines attached, so that a characterization in terms of a chirality is not possible. Previous studies of triple nodal points considered nonmagnetic systems, although an artificial Zeeman splitting was used to probe the topological nature. Here instead we treat a ferromagnetic material, half-metallic GdN, where the splitting of the triple nodal points comes from the spin-orbit coupling. The size of the splitting ranges from 15 to 150 meV depending on the magnetization orientation, enabling a transition between a Weyl-point phase and a "nearly triple nodal point" phase that exhibits very similar surface spectra and transport properties compared to a true triple-node system. The rich topological surface states, manipulable via the orientation of the magnetization, make half-metallic GdN a promising platform for future investigations and applications.

Exotic links and chains attract interests across various disciplines including mathematics, biology, chemistry and physics. Here, we propose that topological Hopf-chain networks, consisting of one-, two- and three-dimensional (3D) Hopf chains, can be found in the momentum space. These networks can be evolved from a 3D triple-points phase by varying symmetries of a four-band model. Moreover, we identify that the Hopf-chain networks exist in a family of crystals Sc3XC (X = Al, Ga, In, Tl). The crystals are 3D triple-points metals, and transit to topological metals with Hopf-chain networks under strains. These novel Hopf networks exhibit unique Landau levels and magneto-transport properties., Comment: Phys. Rev. B (accepted) (2019)

A key challenge in manipulating the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque is to identify materials that exhibit an efficient charge-to-spin current conversion. Ab initio electronic structure calculations reveal that the intrinsic spin Hall conductivity (SHC) for pristine $\beta$-W is about sixty percent larger than that of $\alpha$-W. More importantly, we demonstrate that the SHC of $\beta$-W can be enhanced via Ta alloying. This is corroborated by spin Berry curvature calculations of W$_{1-x}$Ta$_x$ ($x$ $\sim$ 12.5%) alloys which show a giant enhancement of spin Hall angle of up to $\approx$ $-0.5$. The underlying mechanism is the synergistic behavior of the SHC and longitudinal conductivity with Fermi level position. These findings, not only pave the way for enhancing the intrinsic spin Hall effect in $\beta$-W, but also provide new guidelines to exploit substitutional alloying to tailor the spin Hall effect in various materials.

A fully quantum mechanical description of the precessional damping of Pt/Co bilayer is presented in the framework of the Keldysh Green function approach using {\it ab initio} electronic structure calculations. In contrast to previous calculations of classical Gilbert damping ($\alpha_{GD}$), we demonstrate that $\alpha_{GD}$ in the quantum case does not diverge in the ballistic regime due to the finite size of the total spin, $S$. In the limit of $S\rightarrow\infty$ we show that the formalism recovers the torque correlation expression for $\alpha_{GD}$ which we decompose into spin-pumping and spin-orbital torque correlation contributions. The formalism is generalized to take into account a self consistently determined dephasing mechanism which preserves the conservation laws and allows the investigation of the effect of disorder. The dependence of $\alpha_{GD}$ on Pt thickness and disorder strength is calculated and the spin diffusion length of Pt and spin mixing conductance of the bilayer are determined and compared with experiments., Comment: 8 pages, 5 figures

The interfacial phase-change memory (iPCM) GeTe/Sb2Te3, promising candidates for the next generation non-volatile random-access memories, exhibits fascinating topological properties. Depending on the atomic-layer-stacking sequence of the GeTe block, the iPCM can be either in the SET (Ge-Te-Ge-Te) or RESET (Te-Ge-Ge-Te) states, where the former exhibits ferroelectric polarization and electrical conductivity two orders of magnitude larger than that of the RESET state. But, its origin remains elusive. Here, we predict the emergence of a Weyl semimetal phase in the SET state induced by the ferroelectric polarization which breaks the crystal inversion symmetry. We show that the giant conductivity enhancement of the SET phase is due to the appearance of gapless Weyl nodes. The Ge-Te- or Sb-Te-terminated surfaces of Weyl semimetal iPCM produce surface states with completely distinctive topology, where the former consists solely of Fermi arcs while the latter consists of both closed Fermi surface and open Fermi arcs. The iPCM with van der Waals interfaces offers an ideal platform for exploiting the exotic Weyl properties for future memory device applications., Comment: 9 pages, 5 figures

${\it Ab}$ ${\it initio}$ electronic structure calculations reveal that tetragonal distortion has a dramatic effect on the relative stability of the various magnetic structures (C-, A-, G-, A$'$-AFM, and FM) of FeRh giving rise to a wide range of novel stable/metastable structures and magnetic phase transitions between these states. We predict that the ${\it cubic}$ G-AFM structure, which was believed thus far to be the ground state, is metastable and that the ${\it tetragonally}$ expanded G-AFM is the stable structure. The low energy barrier separating these states suggests phase coexistence at room temperature. We propose a novel A$'$-AFM phase to be the ${\it global}$ ground state among all magnetic phases which arises from the strain-induced tuning of the exchange interactions. The results elucidate the underlying mechanism for the recent experimental findings of electric-field control of magnetic phase transition driven via tetragonal strain. The novel magnetic phase transitions open interesting prospects for exploiting strain engineering for the next-generation memory devices.

The picture of how a gap closes in a semiconductor has been radically transformed by topological concepts. Instead of the gap closing and immediately re-opening, topological arguments predict that, in the absence of inversion symmetry, a metallic phase protected by Weyl nodes persists over a finite interval of the tuning parameter (e.g. pressure $P$) . The gap re-appears when the Weyl nodes mutually annihilate. We report evidence that Pb$_{1-x}$Sn$_x$Te exhibits this topological metallic phase. Using pressure to tune the gap, we have tracked the nucleation of a Fermi surface droplet that rapidly grows in volume with $P$. In the metallic state we observe a large Berry curvature which dominates the Hall effect. Moreover, a giant negative magnetoresistance is observed in the insulating side of phase boundaries, in accord with \emph{ab initio} calculations. The results confirm the existence of a topological metallic phase over a finite pressure interval., Comment: 16 pages, 11 figures, 1 table

Using first-principles calculations, we predict a Chern insulating phase in thin films of the ferromagnetic semi-metal GdN. In contrast to previously proposed Chern insulator candidates, which mostly rely on honeycomb lattices, this system affords a great chance to realize the quantum anomalous Hall Effect on a square lattice without either a magnetic substrate or transition metal doping, making synthesis easier. The band inversion between 5d-orbitals of Gd and 2p-orbitals of N is verified by first-principles calculation based on density functional theory, and the band gap can be as large as 100 meV within GdN trilayer. With further increase of film thickness, the band gap tends to close and the metallic bulk property becomes obvious., Comment: accepted by PRB RC

Employing ab initio electronic calculations, we propose a new type of two-dimensional (2D) topological insulator (TI), monolayer (ML) low buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gaps. Monolayer LB HgTe undergoes a transition to a topological nontrivial phase under the appropriate in-plane tensile strain ({\epsilon} > 2.6%) due to the combination effects of strain and spin orbital coupling (SOC). Under the 2.6%< {\epsilon} <4.2% tensile strain, the band inversion and topological nontrivial gap are induced by the SOC. For {\epsilon} >4.2%, the band inversion is already realized by strain but the topological gap is induced by SOC. The band gap of monolayer LB HgTe TI phase can be tuned over a wide range from 0 eV to 0.20 eV as the tensile strain increases from 2.6% to 7.4%. Similarly, the topological phase transition of monolayer LB HgSe is induced by strain and SOC as the strain {\epsilon} >3.1%. The topological band gap can be 0.05 eV as the strain increases to about 4.6%. The large band gap of 2D LB HgTe and HgSe monolayers make this type of material suitable for practical applications at room-temperature.

Development of topological insulating phases in IV-VI compounds under dynamic lattice deformations is studied using first-principles methods. Unlike the static state of topological phases at equilibrium conditions, we show that non-trivial topological phases are induced in the compounds by the dynamic lattice deformations from selective phonon modes. Calculations of the time-reversal polarization show that the Z2 invariant of the compounds is flipped by the atomic displacements of selective phonon modes and that the compounds exhibit oscillating topological phases upon dynamic lattice deformations. Our results indicate that the elementary excitations in solids can trigger topological phases in trivial band insulators., Comment: 15 pages, 4 figures

Materials with non-trivial topology in their electronic structures enforce the existence of helical Dirac fermionic surface states. We discovered emergent topological phases in the stacked structures of topological insulator and band insulator layers where the surface Dirac fermions interact to each other with particular helicity ordering. Using first-principles calculations and a model Lagrangian, we explicitly demonstrated that such helicity ordering occurs in real materials of ternary chalcogen compounds and determines their topological insulating phase. Our results reveal the rich collective nature of interacting surface Dirac fermions, and pave the way for utilizing topological phases for technological devices such as non-volatile memories.

We report a discovery, through first-principles calculations, that crystalline Ge-Sb-Te (GST) phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surface-like interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb2Te3 layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs., Comment: 13 pages, 4 figures