Xiaoting Zhou - Academia.edu (original) (raw)

Papers by Xiaoting Zhou

Saddle-point van Hove singularities in the topological surface states are interesting because the... more Saddle-point van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a k · p model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (Pt_2HgSe_3) harbours saddle-like topological surface states with associated van Hove singularities. Pt_2HgSe_3 is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC) which are protected by combined inversion (I) and time-reversal (T) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological insulator state with nonzero weak topological invariant Z_2=(0;001) and mirror Chern number n_M=2. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddle-like energy dispersion with type II van Hove singularities. We also discuss how modulatin...

We use photoemission spectroscopy to discover the first topological magnet in three dimensions, t... more We use photoemission spectroscopy to discover the first topological magnet in three dimensions, the material Co_2MnGa.

arXiv: Materials Science, 2020

Most topological insulators (TIs) discovered today in spinful systems can be transformed to topol... more Most topological insulators (TIs) discovered today in spinful systems can be transformed to topological semimetals (TSMs) with vanishing bulk gap via reduction of the spin-orbit coupling (SOC), which manifests the intrinsic links between the gapped TI phases and the gapless TSMs. Recently, we have proposed a novel family of TSMs in time-reversal invariant spinless systems, which host butterfly-like nodal-lines (NLs) consisting of a pair of identical concentric intersecting coplanar ellipses (CICE), protected by spatial symmetries, including a pair of glide symmetries. In addition, we have developed a minimal tight-binding model exhibiting CICE in space group (SG) PbamPbamPbam (No. 55), one of the nine identified feasible SGs. Here, we generalize the model by including the effect of SOC, and demonstrate that, with substantial SOC, the system undergoes a transition from the CICE TSM to a mathbbZ4\mathbb{Z}_{4}mathbbZ4 = 2 topological crystalline insulator (TCI). This TCI supports in turn a double-hourgla...

Physical Review B

In recent years, the exotic properties of topological semimetals (TSMs) have attracted great atte... more In recent years, the exotic properties of topological semimetals (TSMs) have attracted great attention and significant efforts have been made in seeking new topological phases and material realization. In this work, we propose a family of TSMs which harbors an unprecedented nodal line (NL) landscape consisting of a pair of concentric intersecting coplanar ellipses (CICEs) at half-filling. Meanwhile, the CICE at half-filling guarantees the presence of a second pair of CICEs beyond half-filling. Both CICEs are linked at fourfold degenerate points at zone boundaries. In addition, we identify the generic criteria for the existence of the CICE in a time-reversal-invariant spinless fermion system or a spinful system with negligible spin-orbital coupling. Consequently, 9 out of 230 space groups (SGs) are feasible for hosting CICEs whose location centers in the first Brillouin zone (BZ) are identified. We provide a simple model with SG Pbam (No. 55) which exhibits CICEs, and the exotic intertwined drumhead surface states, induced by double band inversions. Finally, we propose a series of material candidates that host butterflylike CICE NLs, such as ZrX 2 (X = P, As), Tl 2 GeTe 5 , CYB 2 , and Al 2 Y 3 .

Physical Review B

We explore the topological properties of a modified Haldane model (MHM) in which the strength of ... more We explore the topological properties of a modified Haldane model (MHM) in which the strength of the nearest-neighbor and next-nearest-neighbor hopping terms is made unequal and the threefold rotational symmetry C 3 is broken by introducing a dimerization term (|t 1w(2w) | < t 1s(2s)) in the Hamiltonian. Using the parameter η = t 1w /t 1s = t 2w /t 2s , we show that this MHM supports a transition from the quantum anomalous Hall insulator to a higher-order topological insulator (HOTI) phase at η = ±0.5. It also hosts a zero-energy corner mode on a nanodisk that can transition to a trivial insulator without gap closing when the inversion symmetry is broken. The gap-closing critical states are found to be magnetic semimetals with a single Dirac node which, unlike the classic Haldane model, can move along the high-symmetry lines in the Brillouin zone. Our MHM offers a rich tapestry of HOTIs and other topological and nontopological phases.

Physical Review B

Relativistic massless Weyl and Dirac fermions exhibit the isotropic and linear dispersion relatio... more Relativistic massless Weyl and Dirac fermions exhibit the isotropic and linear dispersion relations to preserve the Pioncaré symmetry, the most fundamental symmetry in high energy physics. In solids, the counterparts of the Pioncaré symmetry are crystallographic symmetries, and hence, it is natural to explore generalizations of Dirac and Weyl fermions compatible with their crystallographic symmetries and then the new physics coming along with them. Here, we study an important kind of generalization, namely massless Dirac fermions with higher-order dispersion relations protected by crystallographic symmetries in three-dimensional nonmagnetic systems. We perform a systematic search over all 230 space groups with time-reversal symmetry and spin-orbit coupling considered. We find that the order of dispersion cannot be higher than three, i.e., only the quadratic and cubic Dirac points (QDPs and CDPs) are possible. We discover previously unknown classes of higher-order Dirac points, including the chiral QDPs with Chern numbers of ±4 and the QDPs/CDPs without centrosymmetry. Especially the chiral QDPs feature four extensive surface Fermi arcs and four chiral Landau bands and hence leads to observable signatures in spectroscopic and transport experiments. We further show that these higherorder Dirac points represent parent phases for other exotic topological structures. Via controlled symmetry breaking, QDPs and CDPs can be transformed into double Weyl points, triple Weyl points, charge-2 Dirac points or Weyl loops. Using first-principles calculations, we also identify possible material candidates, including α-TeO 2 and YRu 4 B 4 , which realize the predicted nodal structures.

Physical Review B

Saddle-point van Hove singularities in the topological surface states are interesting because the... more Saddle-point van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a k • p model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (Pt2HgSe3) harbors saddle-like topological surface states with associated van Hove singularities. Pt2HgSe3 is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC) which are protected by combined inversion (I) and timereversal (T) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological state with nonzero weak topological invariant Z2 = (0; 001) and mirror Chern number nM = −2. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddle-like energy dispersion with type II van Hove singularities. We also discuss how modulating the crystal structure can drive Pt2HgSe3 into a Dirac semimetal state with a pair of Dirac points. Our results indicate that Pt2HgSe3 is an ideal candidate material for exploring the properties of topological insulators with saddle-like surface states.

Science

Topological matter is known to exhibit unconventional surface states and anomalous transport owin... more Topological matter is known to exhibit unconventional surface states and anomalous transport owing to unusual bulk electronic topology. In this study, we use photoemission spectroscopy and quantum transport to elucidate the topology of the room temperature magnet Co2MnGa. We observe sharp bulk Weyl fermion line dispersions indicative of nontrivial topological invariants present in the magnetic phase. On the surface of the magnet, we observe electronic wave functions that take the form of drumheads, enabling us to directly visualize the crucial components of the bulk-boundary topological correspondence. By considering the Berry curvature field associated with the observed topological Weyl fermion lines, we quantitatively account for the giant anomalous Hall response observed in this magnet. Our experimental results suggest a rich interplay of strongly interacting electrons and topology in quantum matter.

Proceedings of the National Academy of Sciences

Bismuth-based materials have been instrumental in the development of topological physics, even th... more Bismuth-based materials have been instrumental in the development of topological physics, even though bulk bismuth itself has been long thought to be topologically trivial. A recent study has, however, shown that bismuth is in fact a higher-order topological insulator featuring one-dimensional (1D) topological hinge states protected by threefold rotational and inversion symmetries. In this paper, we uncover another hidden facet of the band topology of bismuth by showing that bismuth is also a first-order topological crystalline insulator protected by a twofold rotational symmetry. As a result, its (11¯0) surface exhibits a pair of gapless Dirac surface states. Remarkably, these surface Dirac cones are “unpinned” in the sense that they are not restricted to locate at specific k points in the (11¯0) surface Brillouin zone. These unpinned 2D Dirac surface states could be probed directly via various spectroscopic techniques. Our analysis also reveals the presence of a distinct, previous...

2D Materials

Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host uniq... more Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host unique boundary modes, in that the surface normal to the rotational axis can feature surface states with 'unpinned' Dirac points, which are not constrained to lie on high symmetry points or lines, but can lie at any general k point in the Brillouin zone. Also, as a higher order bulk boundary correspondence is involved here, a three-dimensional (3D) TCI can support one-dimensional (1D) helical edge states. Using first-principles band structure calculations, we identify the van der Waals material α-Bi 4 Br 4 as a purely rotation symmetry protected TCI. We show that the (0 1 0) surface of Bi 4 Br 4 exhibits a pair of unpinned topological Dirac fermions which are related to the presence of a twofold rotation axis. These unpinned Dirac fermions possess an exotic spin texture which will be highly favorable for spin transport, and a band structure that consists of van Hove singularities due to a Lifshitz transition. We also identify 1D topological hinge states along the edges of an α-Bi 4 Br 4 rod. We comment on how the predicted topological features in α-Bi 4 Br 4 could be accessed experimentally. LETTER Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Physical Review B

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivi... more Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have proposed new TCI states protected by rotational symmetries and provided powerful guidelines to search for TCIs in real materials. Building upon recent theoretical works, we demonstrate a feasible method to identify new TCI states based on first-principles calculations. We systematically unveil the topological properties of the TCI states in Ca2As. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca2Sb, Ca2Bi and Sr2Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca1−xSrx)2As and Ca2AsxSb1−x. Our work reveals rich and exotic TCI physics across the Ca2As family of materials and demonstrates a complete roadmap for uncovering novel TCIs topological nature based on first-principle calculations. Such a method can be broadly applied in searching for new TCIs.

Physical Review Materials

Crystalline symmetries can generate exotic band-crossing features, which can lead to unconvention... more Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point-a four-fold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction-can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the non-ferroelectric phase of LiOsO3. Upon lowering temperature, LiOsO3 has been shown experimentally to undergo a structural phase transition from the non-ferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually-crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of LiOsO3, each of which is transformed into a single nodal ring across the phase transition.

Physical Review B

Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting n... more Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting nodal lines in their bulk electronic spectrum and nontrivial drumhead states on the surface. Based on first-principles calculations and an effective model analysis, we identify the presence of topological nodal-line semimetal states in the low crystalline symmetric TT'X family of compounds (T, T' = transition metal, X= Si, or Ge) in the absence of spin-orbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to small lattice symmetry this material harbors a single nodal line on the k y = 0 plane with large energy dispersion and unique drumhead surface state with a saddle-like energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z 2 = (1; 000). The topological surface state evolves from the drumhead surface state via the sharing of its saddlelike energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as Bi 2 Se 3 with Dirac-cone-like energy dispersions.

Physical Review B

In this work we theoretically study, using Floquet-Bloch theory, the influence of circularly and ... more In this work we theoretically study, using Floquet-Bloch theory, the influence of circularly and linearly polarized light on two-dimensional band structures with Dirac and quadratic band touching points, and flat bands, taking the nearest neighbor hopping model on the kagome lattice as an example. We find circularly polarized light can invert the ordering of this three band model, while leaving the flat-band dispersionless. We find a small gap is also opened at the quadratic band touching point by 2-photon and higher order processes. By contrast, linearly polarized light splits the quadratic band touching point (into two Dirac points) by an amount that depends only on the amplitude and polarization direction of the light, independent of the frequency, and generally renders dispersion to the flat band. The splitting is perpendicular to the direction of the polarization of the light. We derive an effective low-energy theory that captures these key results. Finally, we compute the frequency dependence of the optical conductivity for this 3-band model and analyze the various interband contributions of the Floquet modes. Our results suggest strategies for optically controlling band structure and interaction strength in real systems.

Physical review letters, Jan 13, 2017

Topological semimetals can be classified by the connectivity and dimensionality of the band cross... more Topological semimetals can be classified by the connectivity and dimensionality of the band crossings in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimetal are zero-dimensional (0D) points, whereas the band crossings of a nodal-line semimetal are one-dimensional (1D) closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect three-dimensional (3D) band crossings characterized by nontrivial links such as a Hopf link or a coupled chain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topological surface states distinct from those in previously known topological semimetals with a vanishing spin-orbit interaction. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using first-principles band structure calculations, we predict the ferromagnetic ful...

Scientific reports, May 9, 2017

Whole-exome sequencing (WES), one of the next-generation sequencing (NGS), has become a powerful ... more Whole-exome sequencing (WES), one of the next-generation sequencing (NGS), has become a powerful tool to identify exonic variants. Investigating causality of the sequence variants in human disease becomes an important part in NGS for the research and clinical applications. Recently, important guidelines on them have been published and will keep on updating. In our study, two Chinese families, with the clinical diagnosis of "Epilepsy", which presented with seizures, psychomotor retardation, hypotonia and etc. features, were sequenced by Trio-WES (including the proband and the unaffected parents), and a standard interpretation of the identified variants was performed referring to the recently updated guidelines. Finally, we identified three novel mutations (c.71 C > T, p.P24L; c.1387-1389delGAG, p.E463-; c.134 G > A, p.W45*; NM_000026) in ADSL in the two Chinese families, and confirmed them as the causal variants to the disease-Adenylosuccinate Lyase Deficiency. Previo...

ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can b... more ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can be spin-polarized and similar to a 2D quantum anomalous Hall state. If an s-wave superconductivity can be induced by proximity effect on such a surface, a 2D topological superconducting phase is obtained. If we consider a TI-ferromagnet(FM)-superconductor(SC) heterostructure, a 2D time-reversal symmetry breaking topological superconducting (TSC) phases with Majorana edge mode(s) will be realized. We demonstrate that the existence of the edge modes critically depend on the combination of the directions and magnitudes of spin polarization on all surfaces, and that a model describing the states on only one surface is insufficient. We find that the number, the positions and the chirality of these edge modes corresponding to various TSC phases can be engineered by controlling the ferromagnetism on different surfaces. Our results are obtained by self-consistently solving for the edge modes in a 3D lattice model for topological insulator in contact with an s-wave BCS superconductor. We also offer an analysis to illustrate the underlying physics, using an effective 2D theory for the surface states.

ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can b... more ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can be spin-polarized and similar to a 2D quantum anomalous Hall state. If an s-wave superconductivity can be induced by proximity effect on such a surface, a 2D topological superconducting phase is obtained. If we consider a TI-ferromagnet(FM)-superconductor(SC) heterostructure, a 2D time-reversal symmetry breaking topological superconducting (TSC) phases with Majorana edge mode(s) will be realized. We demonstrate that the existence of the edge modes critically depend on the combination of the directions and magnitudes of spin polarization on all surfaces, and that a model describing the states on only one surface is insufficient. We find that the number, the positions and the chirality of these edge modes corresponding to various TSC phases can be engineered by controlling the ferromagnetism on different surfaces. Our results are obtained by self-consistently solving for the edge modes in a 3D lattice model for topological insulator in contact with an s-wave BCS superconductor. We also offer an analysis to illustrate the underlying physics, using an effective 2D theory for the surface states.

We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magn... more We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed.

Saddle-point van Hove singularities in the topological surface states are interesting because the... more Saddle-point van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a k · p model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (Pt_2HgSe_3) harbours saddle-like topological surface states with associated van Hove singularities. Pt_2HgSe_3 is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC) which are protected by combined inversion (I) and time-reversal (T) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological insulator state with nonzero weak topological invariant Z_2=(0;001) and mirror Chern number n_M=2. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddle-like energy dispersion with type II van Hove singularities. We also discuss how modulatin...

We use photoemission spectroscopy to discover the first topological magnet in three dimensions, t... more We use photoemission spectroscopy to discover the first topological magnet in three dimensions, the material Co_2MnGa.

arXiv: Materials Science, 2020

Most topological insulators (TIs) discovered today in spinful systems can be transformed to topol... more Most topological insulators (TIs) discovered today in spinful systems can be transformed to topological semimetals (TSMs) with vanishing bulk gap via reduction of the spin-orbit coupling (SOC), which manifests the intrinsic links between the gapped TI phases and the gapless TSMs. Recently, we have proposed a novel family of TSMs in time-reversal invariant spinless systems, which host butterfly-like nodal-lines (NLs) consisting of a pair of identical concentric intersecting coplanar ellipses (CICE), protected by spatial symmetries, including a pair of glide symmetries. In addition, we have developed a minimal tight-binding model exhibiting CICE in space group (SG) PbamPbamPbam (No. 55), one of the nine identified feasible SGs. Here, we generalize the model by including the effect of SOC, and demonstrate that, with substantial SOC, the system undergoes a transition from the CICE TSM to a mathbbZ4\mathbb{Z}_{4}mathbbZ4 = 2 topological crystalline insulator (TCI). This TCI supports in turn a double-hourgla...

Physical Review B

In recent years, the exotic properties of topological semimetals (TSMs) have attracted great atte... more In recent years, the exotic properties of topological semimetals (TSMs) have attracted great attention and significant efforts have been made in seeking new topological phases and material realization. In this work, we propose a family of TSMs which harbors an unprecedented nodal line (NL) landscape consisting of a pair of concentric intersecting coplanar ellipses (CICEs) at half-filling. Meanwhile, the CICE at half-filling guarantees the presence of a second pair of CICEs beyond half-filling. Both CICEs are linked at fourfold degenerate points at zone boundaries. In addition, we identify the generic criteria for the existence of the CICE in a time-reversal-invariant spinless fermion system or a spinful system with negligible spin-orbital coupling. Consequently, 9 out of 230 space groups (SGs) are feasible for hosting CICEs whose location centers in the first Brillouin zone (BZ) are identified. We provide a simple model with SG Pbam (No. 55) which exhibits CICEs, and the exotic intertwined drumhead surface states, induced by double band inversions. Finally, we propose a series of material candidates that host butterflylike CICE NLs, such as ZrX 2 (X = P, As), Tl 2 GeTe 5 , CYB 2 , and Al 2 Y 3 .

Physical Review B

We explore the topological properties of a modified Haldane model (MHM) in which the strength of ... more We explore the topological properties of a modified Haldane model (MHM) in which the strength of the nearest-neighbor and next-nearest-neighbor hopping terms is made unequal and the threefold rotational symmetry C 3 is broken by introducing a dimerization term (|t 1w(2w) | < t 1s(2s)) in the Hamiltonian. Using the parameter η = t 1w /t 1s = t 2w /t 2s , we show that this MHM supports a transition from the quantum anomalous Hall insulator to a higher-order topological insulator (HOTI) phase at η = ±0.5. It also hosts a zero-energy corner mode on a nanodisk that can transition to a trivial insulator without gap closing when the inversion symmetry is broken. The gap-closing critical states are found to be magnetic semimetals with a single Dirac node which, unlike the classic Haldane model, can move along the high-symmetry lines in the Brillouin zone. Our MHM offers a rich tapestry of HOTIs and other topological and nontopological phases.

Physical Review B

Relativistic massless Weyl and Dirac fermions exhibit the isotropic and linear dispersion relatio... more Relativistic massless Weyl and Dirac fermions exhibit the isotropic and linear dispersion relations to preserve the Pioncaré symmetry, the most fundamental symmetry in high energy physics. In solids, the counterparts of the Pioncaré symmetry are crystallographic symmetries, and hence, it is natural to explore generalizations of Dirac and Weyl fermions compatible with their crystallographic symmetries and then the new physics coming along with them. Here, we study an important kind of generalization, namely massless Dirac fermions with higher-order dispersion relations protected by crystallographic symmetries in three-dimensional nonmagnetic systems. We perform a systematic search over all 230 space groups with time-reversal symmetry and spin-orbit coupling considered. We find that the order of dispersion cannot be higher than three, i.e., only the quadratic and cubic Dirac points (QDPs and CDPs) are possible. We discover previously unknown classes of higher-order Dirac points, including the chiral QDPs with Chern numbers of ±4 and the QDPs/CDPs without centrosymmetry. Especially the chiral QDPs feature four extensive surface Fermi arcs and four chiral Landau bands and hence leads to observable signatures in spectroscopic and transport experiments. We further show that these higherorder Dirac points represent parent phases for other exotic topological structures. Via controlled symmetry breaking, QDPs and CDPs can be transformed into double Weyl points, triple Weyl points, charge-2 Dirac points or Weyl loops. Using first-principles calculations, we also identify possible material candidates, including α-TeO 2 and YRu 4 B 4 , which realize the predicted nodal structures.

Physical Review B

Saddle-point van Hove singularities in the topological surface states are interesting because the... more Saddle-point van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a k • p model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (Pt2HgSe3) harbors saddle-like topological surface states with associated van Hove singularities. Pt2HgSe3 is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC) which are protected by combined inversion (I) and timereversal (T) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological state with nonzero weak topological invariant Z2 = (0; 001) and mirror Chern number nM = −2. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddle-like energy dispersion with type II van Hove singularities. We also discuss how modulating the crystal structure can drive Pt2HgSe3 into a Dirac semimetal state with a pair of Dirac points. Our results indicate that Pt2HgSe3 is an ideal candidate material for exploring the properties of topological insulators with saddle-like surface states.

Science

Topological matter is known to exhibit unconventional surface states and anomalous transport owin... more Topological matter is known to exhibit unconventional surface states and anomalous transport owing to unusual bulk electronic topology. In this study, we use photoemission spectroscopy and quantum transport to elucidate the topology of the room temperature magnet Co2MnGa. We observe sharp bulk Weyl fermion line dispersions indicative of nontrivial topological invariants present in the magnetic phase. On the surface of the magnet, we observe electronic wave functions that take the form of drumheads, enabling us to directly visualize the crucial components of the bulk-boundary topological correspondence. By considering the Berry curvature field associated with the observed topological Weyl fermion lines, we quantitatively account for the giant anomalous Hall response observed in this magnet. Our experimental results suggest a rich interplay of strongly interacting electrons and topology in quantum matter.

Proceedings of the National Academy of Sciences

Bismuth-based materials have been instrumental in the development of topological physics, even th... more Bismuth-based materials have been instrumental in the development of topological physics, even though bulk bismuth itself has been long thought to be topologically trivial. A recent study has, however, shown that bismuth is in fact a higher-order topological insulator featuring one-dimensional (1D) topological hinge states protected by threefold rotational and inversion symmetries. In this paper, we uncover another hidden facet of the band topology of bismuth by showing that bismuth is also a first-order topological crystalline insulator protected by a twofold rotational symmetry. As a result, its (11¯0) surface exhibits a pair of gapless Dirac surface states. Remarkably, these surface Dirac cones are “unpinned” in the sense that they are not restricted to locate at specific k points in the (11¯0) surface Brillouin zone. These unpinned 2D Dirac surface states could be probed directly via various spectroscopic techniques. Our analysis also reveals the presence of a distinct, previous...

2D Materials

Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host uniq... more Rotational-symmetry-protected topological crystalline insulators (TCIs) are expected to host unique boundary modes, in that the surface normal to the rotational axis can feature surface states with 'unpinned' Dirac points, which are not constrained to lie on high symmetry points or lines, but can lie at any general k point in the Brillouin zone. Also, as a higher order bulk boundary correspondence is involved here, a three-dimensional (3D) TCI can support one-dimensional (1D) helical edge states. Using first-principles band structure calculations, we identify the van der Waals material α-Bi 4 Br 4 as a purely rotation symmetry protected TCI. We show that the (0 1 0) surface of Bi 4 Br 4 exhibits a pair of unpinned topological Dirac fermions which are related to the presence of a twofold rotation axis. These unpinned Dirac fermions possess an exotic spin texture which will be highly favorable for spin transport, and a band structure that consists of van Hove singularities due to a Lifshitz transition. We also identify 1D topological hinge states along the edges of an α-Bi 4 Br 4 rod. We comment on how the predicted topological features in α-Bi 4 Br 4 could be accessed experimentally. LETTER Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Physical Review B

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivi... more Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have proposed new TCI states protected by rotational symmetries and provided powerful guidelines to search for TCIs in real materials. Building upon recent theoretical works, we demonstrate a feasible method to identify new TCI states based on first-principles calculations. We systematically unveil the topological properties of the TCI states in Ca2As. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca2Sb, Ca2Bi and Sr2Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca1−xSrx)2As and Ca2AsxSb1−x. Our work reveals rich and exotic TCI physics across the Ca2As family of materials and demonstrates a complete roadmap for uncovering novel TCIs topological nature based on first-principle calculations. Such a method can be broadly applied in searching for new TCIs.

Physical Review Materials

Crystalline symmetries can generate exotic band-crossing features, which can lead to unconvention... more Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point-a four-fold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction-can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the non-ferroelectric phase of LiOsO3. Upon lowering temperature, LiOsO3 has been shown experimentally to undergo a structural phase transition from the non-ferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually-crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of LiOsO3, each of which is transformed into a single nodal ring across the phase transition.

Physical Review B

Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting n... more Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting nodal lines in their bulk electronic spectrum and nontrivial drumhead states on the surface. Based on first-principles calculations and an effective model analysis, we identify the presence of topological nodal-line semimetal states in the low crystalline symmetric TT'X family of compounds (T, T' = transition metal, X= Si, or Ge) in the absence of spin-orbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to small lattice symmetry this material harbors a single nodal line on the k y = 0 plane with large energy dispersion and unique drumhead surface state with a saddle-like energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z 2 = (1; 000). The topological surface state evolves from the drumhead surface state via the sharing of its saddlelike energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as Bi 2 Se 3 with Dirac-cone-like energy dispersions.

Physical Review B

In this work we theoretically study, using Floquet-Bloch theory, the influence of circularly and ... more In this work we theoretically study, using Floquet-Bloch theory, the influence of circularly and linearly polarized light on two-dimensional band structures with Dirac and quadratic band touching points, and flat bands, taking the nearest neighbor hopping model on the kagome lattice as an example. We find circularly polarized light can invert the ordering of this three band model, while leaving the flat-band dispersionless. We find a small gap is also opened at the quadratic band touching point by 2-photon and higher order processes. By contrast, linearly polarized light splits the quadratic band touching point (into two Dirac points) by an amount that depends only on the amplitude and polarization direction of the light, independent of the frequency, and generally renders dispersion to the flat band. The splitting is perpendicular to the direction of the polarization of the light. We derive an effective low-energy theory that captures these key results. Finally, we compute the frequency dependence of the optical conductivity for this 3-band model and analyze the various interband contributions of the Floquet modes. Our results suggest strategies for optically controlling band structure and interaction strength in real systems.

Physical review letters, Jan 13, 2017

Topological semimetals can be classified by the connectivity and dimensionality of the band cross... more Topological semimetals can be classified by the connectivity and dimensionality of the band crossings in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimetal are zero-dimensional (0D) points, whereas the band crossings of a nodal-line semimetal are one-dimensional (1D) closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect three-dimensional (3D) band crossings characterized by nontrivial links such as a Hopf link or a coupled chain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topological surface states distinct from those in previously known topological semimetals with a vanishing spin-orbit interaction. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using first-principles band structure calculations, we predict the ferromagnetic ful...

Scientific reports, May 9, 2017

Whole-exome sequencing (WES), one of the next-generation sequencing (NGS), has become a powerful ... more Whole-exome sequencing (WES), one of the next-generation sequencing (NGS), has become a powerful tool to identify exonic variants. Investigating causality of the sequence variants in human disease becomes an important part in NGS for the research and clinical applications. Recently, important guidelines on them have been published and will keep on updating. In our study, two Chinese families, with the clinical diagnosis of "Epilepsy", which presented with seizures, psychomotor retardation, hypotonia and etc. features, were sequenced by Trio-WES (including the proband and the unaffected parents), and a standard interpretation of the identified variants was performed referring to the recently updated guidelines. Finally, we identified three novel mutations (c.71 C > T, p.P24L; c.1387-1389delGAG, p.E463-; c.134 G > A, p.W45*; NM_000026) in ADSL in the two Chinese families, and confirmed them as the causal variants to the disease-Adenylosuccinate Lyase Deficiency. Previo...

ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can b... more ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can be spin-polarized and similar to a 2D quantum anomalous Hall state. If an s-wave superconductivity can be induced by proximity effect on such a surface, a 2D topological superconducting phase is obtained. If we consider a TI-ferromagnet(FM)-superconductor(SC) heterostructure, a 2D time-reversal symmetry breaking topological superconducting (TSC) phases with Majorana edge mode(s) will be realized. We demonstrate that the existence of the edge modes critically depend on the combination of the directions and magnitudes of spin polarization on all surfaces, and that a model describing the states on only one surface is insufficient. We find that the number, the positions and the chirality of these edge modes corresponding to various TSC phases can be engineered by controlling the ferromagnetism on different surfaces. Our results are obtained by self-consistently solving for the edge modes in a 3D lattice model for topological insulator in contact with an s-wave BCS superconductor. We also offer an analysis to illustrate the underlying physics, using an effective 2D theory for the surface states.

ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can b... more ABSTRACT A surface of a strong 3D topological insulator (TI) doped with ferromagnetic atoms can be spin-polarized and similar to a 2D quantum anomalous Hall state. If an s-wave superconductivity can be induced by proximity effect on such a surface, a 2D topological superconducting phase is obtained. If we consider a TI-ferromagnet(FM)-superconductor(SC) heterostructure, a 2D time-reversal symmetry breaking topological superconducting (TSC) phases with Majorana edge mode(s) will be realized. We demonstrate that the existence of the edge modes critically depend on the combination of the directions and magnitudes of spin polarization on all surfaces, and that a model describing the states on only one surface is insufficient. We find that the number, the positions and the chirality of these edge modes corresponding to various TSC phases can be engineered by controlling the ferromagnetism on different surfaces. Our results are obtained by self-consistently solving for the edge modes in a 3D lattice model for topological insulator in contact with an s-wave BCS superconductor. We also offer an analysis to illustrate the underlying physics, using an effective 2D theory for the surface states.

We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magn... more We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed.