Showing papers by "Hongming Weng published in 2017"

Observation of three-component fermions in the topological semimetal molybdenum phosphide

Abstract: In quantum field theory, Lorentz invariance leads to three types of fermion-Dirac, Weyl and Majorana. Although the existence of Weyl and Majorana fermions as elementary particles in high-energy physics is debated, all three types of fermion have been proposed to exist as low-energy, long-wavelength quasiparticle excitations in condensed-matter systems. The existence of Dirac and Weyl fermions in condensed-matter systems has been confirmed experimentally, and that of Majorana fermions is supported by various experiments. However, in condensed-matter systems, fermions in crystals are constrained by the symmetries of the 230 crystal space groups rather than by Lorentz invariance, giving rise to the possibility of finding other types of fermionic excitation that have no counterparts in high-energy physics. Here we use angle-resolved photoemission spectroscopy to demonstrate the existence of a triply degenerate point in the electronic structure of crystalline molybdenum phosphide. Quasiparticle excitations near a triply degenerate point are three-component fermions, beyond the conventional Dirac-Weyl-Majorana classification, which attributes Dirac and Weyl fermions to four- and two-fold degenerate points, respectively. We also observe pairs of Weyl points in the bulk electronic structure of the crystal that coexist with the three-component fermions. This material thus represents a platform for studying the interplay between different types of fermions. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.

Large intrinsic anomalous Hall effect in half-metallic ferromagnet Co3Sn2S2 with magnetic Weyl fermions

Abstract: The origin of anomalous Hall effect (AHE) in magnetic materials is one of the most intriguing aspect in condensed matter physics and has been controversial for a long time. Recent studies indicate that the intrinsic AHE is closely related to the Berry curvature of occupied electronic states. In a magnetic Weyl semimetal with broken time-reversal symmetry, there are significant contributions on Berry curvature around Weyl nodes, which would lead to a large intrinsic AHE. Here, we report the large intrinsic AHE in the half-metallic ferromagnet Co3Sn2S2 single crystal. By systematically mapping out the electronic structure of Co3Sn2S2 theoretically and experimentally, the large intrinsic AHE should originate from the Weyl fermions near the Fermi energy. Furthermore, the intrinsic anomalous Hall conductivity depends linearly on the magnetization and this can be attributed to the sharp decrease of magnetization and the change of topological characteristics.

Electronic evidence of temperature-induced Lifshitz transition and topological nature in ZrTe5.

Abstract: The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced. To understand the anomalous electronic transport properties of ZrTe5 remains an elusive puzzle. Here, Zhang et al. report direct electronic evidence to the origin of the resistivity anomaly and temperature induced Lifshitz transition in ZrTe5, indicating it being a weak topological insulator.

Topological nodal line semimetals in the CaP 3 family of materials

Abstract: By using first-principles calculations and a $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model analysis, we propose that the three-dimensional topological nodal line semimetal state can be realized in the ${\mathrm{CaP}}_{3}$ family of materials, which includes ${\mathrm{CaP}}_{3},{\mathrm{CaAs}}_{3},{\mathrm{SrP}}_{3},{\mathrm{SrAs}}_{3}$, and ${\mathrm{BaAs}}_{3}$, when spin-orbit coupling (SOC) is ignored. The closed topological nodal line near the Fermi energy is protected by time reversal symmetry and spatial inversion symmetry. Moreover, drumheadlike two-dimensional surface states are also obtained on the $c$-direction surface of these materials. When SOC is included, the gaps open along the nodal line and these materials become strong topological insulators with ${Z}_{2}$ indices as $(1;010)$.

Topological nodal line semimetals predicted from first-principles calculations

Abstract: Topological semimetals are newly discovered states of quantum matter, which have extended the concept of topological states from insulators to metals and attracted great research interest in recent years. In general, there are three kinds of topological semimetals, namely Dirac semimetals, Weyl semimetals, and nodal line semimetals. Nodal line semimetals can be considered as precursor states for other topological states. For example, starting from such nodal line states, the nodal line structure might evolve into Weyl points, convert into Dirac points, or become a topological insulator by introducing the spin–orbit coupling (SOC) or mass term. In this review paper, we introduce theoretical materials that show the nodal line semimetal state, including the all-carbon Mackay–Terrones crystal (MTC), anti-perovskite Cu3PdN, pressed black phosphorus, and the CaP3 family of materials, and we present the design principles for obtaining such novel states of matter.

From Nodal Chain Semimetal to Weyl Semimetal in HfC.

Abstract: Based on first-principles calculations and effective model analysis, we propose that the WC-type HfC, in the absence of spin-orbit coupling (SOC), can host a three-dimensional nodal chain semimetal state. Distinguished from the previous material IrF_{4} [T. Bzdusek et al., Nature 538, 75 (2016)], the nodal chain here is protected by mirror reflection symmetries of a simple space group, while in IrF_{4} the nonsymmorphic space group with a glide plane is a necessity. Moreover, in the presence of SOC, the nodal chain in WC-type HfC evolves into Weyl points. In the Brillouin zone, a total of 30 pairs of Weyl points in three types are obtained through the first-principles calculations. Besides, the surface states and the pattern of the surface Fermi arcs connecting these Weyl points are studied, which may be measured by future experiments.

Experimental evidence of hourglass fermion in the candidate nonsymmorphic topological insulator KHgSb

Abstract: Topological insulators (TIs) host novel states of quantum matter characterized by nontrivial conducting boundary states connecting valence and conduction bulk bands. All TIs discovered experimentally so far rely on either time-reversal or mirror crystal symmorphic symmetry to protect massless Dirac-like boundary states. Several materials were recently proposed to be TIs with nonsymmorphic symmetry, where a glide mirror protects exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of this new fermion is missing. Using angle-resolved photoemission spectroscopy, we provide experimental evidence of hourglass fermions on the (010) surface of crystalline KHgSb, whereas the (001) surface has no boundary state, in agreement with first-principles calculations. Our study will stimulate further research activities of topological properties of nonsymmorphic materials.

Three-component fermions with surface Fermi arcs in topological semimetal tungsten carbide

Abstract: Topological Dirac and Weyl semimetals not only host quasiparticles analogous to the elementary fermionic particles in high-energy physics, but also have nontrivial band topology manifested by exotic Fermi arcs on the surface. Recent advances suggest new types of topological semimetals, in which spatial symmetries protect gapless electronic excitations without high-energy analogy. Here we observe triply-degenerate nodal points (TPs) near the Fermi level of WC, in which the low-energy quasiparticles are described as three-component fermions distinct from Dirac and Weyl fermions. We further observe the surface states whose constant energy contours are pairs of Fermi arcs connecting the surface projection of the TPs, proving the nontrivial topology of the newly identified semimetal state.

Theoretical prediction of two-dimensional functionalized MXene nitrides as topological insulators

Abstract: Recently, two-dimensional (2D) transition-metal carbides and nitrides, namely, MXenes have attracted a lot of attention for electronic and energy storage applications. Due to a large spin-orbit coupling and the existence of a Dirac-like band at the Fermi energy, it has been theoretically proposed that some of the MXenes will be topological insulators (TIs). Up to now, all of the predicted TI MXenes belong to transition-metal carbides, whose transition-metal atom is W, Mo, or Cr. Here, on the basis of first-principles and ${\mathbb{Z}}_{2}$ index calculations, we demonstrate that some of the MXene nitrides can also be TIs. We find that ${\mathrm{Ti}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ is a 2D TI, whereas ${\mathrm{Zr}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ is a semimetal with nontrivial band topology and can be turned into a 2D TI when the lattice is stretched. We also find that the tensile strain can convert ${\mathrm{Hf}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ semiconductor into a 2D TI. Since Ti is one of the most used transition-metal elements in the synthesized MXenes, we expect that our prediction can advance the future application of MXenes as TI devices.

Evidence of topological insulator state in the semimetal LaBi

Abstract: By employing angle-resolved photoemission spectroscopy combined with first-principles calculations, we performed a systematic investigation on the electronic structure of LaBi, which exhibits extremely large magnetoresistance (XMR), and is theoretically predicted to possess band anticrossing with nontrivial topological properties. Here, the observations of the Fermi-surface topology and band dispersions are similar to previous studies on LaSb [L.-K. Zeng, R. Lou, D.-S. Wu, Q. N. Xu, P.-J. Guo, L.-Y. Kong, Y.-G. Zhong, J.-Z. Ma, B.-B. Fu, P. Richard, P. Wang, G. T. Liu, L. Lu, Y.-B. Huang, C. Fang, S.-S. Sun, Q. Wang, L. Wang, Y.-G. Shi, H. M. Weng, H.-C. Lei, K. Liu, S.-C. Wang, T. Qian, J.-L. Luo, and H. Ding, Phys. Rev. Lett. 117, 127204 (2016)], a topologically trivial XMR semimetal, except the existence of a band inversion along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}X$ direction, with one massless and one gapped Dirac-like surface state at the $X$ and $\mathrm{\ensuremath{\Gamma}}$ points, respectively. The odd number of massless Dirac cones suggests that LaBi is analogous to the time-reversal ${Z}_{2}$ nontrivial topological insulator. These findings open up a new series for exploring novel topological states and investigating their evolution from the perspective of topological phase transition within the family of rare-earth monopnictides.

d Orbital Topological Insulator and Semimetal in the Antifluorite Cu2S Family: Contrasting Spin Helicities, Nodal Box, and Hybrid Surface States.

Abstract: We reveal a class of three-dimensional d orbital topological materials in the antifluorite Cu2S family. Derived from the unique properties of low-energy t2g states, their phases are solely determined by the sign of the spin-orbit coupling (SOC): topological insulator (TI) for negative SOC and topological semimetal for positive SOC, both having Dirac cone surface states but with contrasting helicities. With broken inversion symmetry, the semimetal becomes one with a nodal box consisting of butterfly-shaped nodal lines that are robust against SOC. Further breaking the tetrahedral symmetry by strain leads to an ideal Weyl semimetal with four pairs of Weyl points. Interestingly, the Fermi arcs coexist with a surface Dirac cone on the (010) surface, as required by a [Formula: see text] invariant.

Heavy Weyl Fermion State in CeRu 4 Sn 6

Abstract: Weyl semimetals have highly mobile charged particles that may make them useful in electronic devices, but only a few of them have been identified thus far. Computational analysis reveals that the compound CeRu${}_{4}$Sn${}_{6}$ exhibits Weyl-like behavior and may be a new type of material known as a heavy Weyl fermion state.

Superconductivity in HfTe5 across weak to strong topological insulator transition induced via pressures.

Abstract: Recently, theoretical studies show that layered HfTe5 is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe5 are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe5 crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.

Pressure-induced topological phase transitions and strongly anisotropic magnetoresistance in bulk black phosphorus

Abstract: We report the anisotropic magnetotransport measurement on a noncompound band semiconductor black phosphorus (BP) with magnetic field $\mathbit{B}$ up to 16 Tesla applied in both perpendicular and parallel to electric current $\mathbit{I}$ under hydrostatic pressures. The BP undergoes a topological Lifshitz transition from band semiconductor to a zero-gap Dirac semimetal state at a critical pressure ${P}_{c}$, characterized by a weak localization-weak antilocalization transition at low magnetic fields and the emergence of a nontrivial Berry phase of $\ensuremath{\pi}$ detected by SdH magneto-oscillations in magnetoresistance curves. In the transition region, we observe a pressure-dependent negative MR only in the $\mathbit{B}\ensuremath{\parallel}\mathbit{I}$ configuration. This negative longitudinal MR is attributed to the Adler-Bell-Jackiw anomaly (topological $\mathbit{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{B}$ term) in the presence of weak antilocalization corrections.

Experimental Observation of Three-Component New Fermions in Topological Semimetal MoP

Abstract: Condensed matter systems can host quasiparticle excitations that are analogues to elementary particles such as Majorana, Weyl, and Dirac fermions. Recent advances in band theory have expanded the classification of fermions in crystals, and revealed crystal symmetry-protected electron excitations that have no high-energy counterparts. Here, using angle-resolved photoemission spectroscopy, we demonstrate the existence of a triply degenerate point in the electronic structure of MoP crystal, where the quasiparticle excitations are beyond the Majorana-Weyl-Dirac classification. Furthermore, we observe pairs of Weyl points in the bulk electronic structure coexisting with the 'new fermions', thus introducing a platform for studying the interplay between different types of fermions.

Electronic structure of SrSn 2 As 2 near the topological critical point

Abstract: Topological materials with exotic quantum properties are promising candidates for quantum spin electronics. Different classes of topological materials, including Weyl semimetal, topological superconductor, topological insulator and Axion insulator, etc., can be connected to each other via quantum phase transition. For example, it is believed that a trivial band insulator can be twisted into topological phase by increasing spin-orbital coupling or changing the parameters of crystal lattice. With the results of LDA calculation and measurement by angle-resolved photoemission spectroscopy (ARPES), we demonstrate in this work that the electronic structure of SrSn2As2 single crystal has the texture of band inversion near the critical point. The results indicate the possibility of realizing topological quantum phase transition in SrSn2As2 single crystal and obtaining different exotic quantum states.

Anomalous Magneto-Transport Behavior in Transition Metal Pentatelluride HfTe 5

Abstract: There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, such as the formation of charge density wave or polaron, have been proposed, but so far no conclusive evidence has been presented. In this work, we investigate the unusual temperature dependence of magneto-transport properties in HfTe5. It is found that a three-dimensional topological Dirac semimetal state emerges only at around Tp (at which the resistivity shows a pronounced peak), as manifested by a large negative magnetoresistance. This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe5. Our work not only provides the first evidence of a temperature-induced critical topological phase transition in HfTe5 but also gives a reasonable explanation on the long-lasting question.

Noncollinear Magnetic Structure and Multipolar Order in Eu 2 Ir 2 O 7

Abstract: The magnetic properties of the pyrochlore iridate material ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ ($5{d}^{5}$) have been studied based on first principles calculations, where the crystal field splitting $\mathrm{\ensuremath{\Delta}}$, spin-orbit coupling (SOC) $\ensuremath{\lambda}$, and Coulomb interaction $U$ within Ir $5d$ orbitals all play significant roles. The ground state phase diagram has been obtained with respect to the strength of SOC and Coulomb interaction $U$, where a stable antiferromagnetic ground state with all-in--all-out (AIAO) spin structure has been found. In addition, another antiferromagnetic state with energy close to AIAO has also been found to be stable. The calculated nonlinear magnetization of the two stable states both have the $d$-wave pattern but with a $\ensuremath{\pi}/4$ phase difference, which can perfectly explain the experimentally observed nonlinear magnetization pattern. Compared with the results of the nondistorted structure, it turns out that the trigonal lattice distortion is crucial for stabilizing the AIAO state in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. Furthermore, besides large dipolar moments, we also find considerable octupolar moments in the magnetic states.

Topologically Entangled Rashba-Split Shockley States on the Surface of Grey Arsenic.

Abstract: Japan Society for the Promotion of Science (KAKENHI Grants 25220707, 26800165, and 15K17675)

Observation of open-orbit Fermi surface topology in the extremely large magnetoresistance semimetal MoAs 2

Abstract: While recent advances in band theory and sample growth have expanded the series of extremely large magnetoresistance (XMR) semimetals in transition-metal dipnictides $TmP{n}_{2}$ ($Tm=\mathrm{Ta}$, Nb; $Pn=\mathrm{P}$, As, Sb), the experimental study on their electronic structure and the origin of XMR is still absent. Here, using angle-resolved photoemission spectroscopy combined with first-principles calculations and magnetotransport measurements, we performed a comprehensive investigation on ${\mathrm{MoAs}}_{2}$, which is isostructural to the $TmP{n}_{2}$ family and also exhibits quadratic XMR. We resolve a clear band structure well agreeing with the predictions. Intriguingly, the unambiguously observed Fermi surfaces (FSs) are dominated by an open-orbit topology extending along both the [100] and [001] directions in the three-dimensional Brillouin zone. We further reveal the trivial topological nature of ${\mathrm{MoAs}}_{2}$ by bulk parity analysis. Based on these results, we examine the proposed XMR mechanisms in other semimetals, and conclusively ascribe the origin of quadratic XMR in ${\mathrm{MoAs}}_{2}$ to the carriers motion on the FSs with dominant open-orbit topology, innovating in the understanding of quadratic XMR in semimetals.

Heavy Weyl fermion state in CeRu$_4$Sn$_6$

Abstract: A new type of topological state in strongly corrected condensed matter systems, heavy Weyl fermion state, has been found in a heavy fermion material CeRu$_4$Sn$_6$, which has no inversion symmetry. Both two different types of Weyl points, type I and II, can be found in the quasi-particle band structure obtained by the LDA+Guztwiller calculations, which can treat the strong correlation effects among the f-electrons from Cerium atoms. The surface calculations indicate that the topologically protected Fermi arc states exist on the (010) but not on the (001) surfaces.

Interaction-driven quantum anomalous Hall effect in halogenated hematite nanosheets

Abstract: Based on first-principle calculations and $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model analysis, we show that the quantum anomalous Hall (QAH) insulating phase can be realized in the functionalized hematite (or $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ nanosheet, and the obtained topological gap can be as large as $\ensuremath{\sim}300\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. The driving force of the topological phase is the strong interactions of localized Fe $3d$ electrons operating on the quadratic band crossing point of the noninteracting band structures. Such an interaction driven QAH insulator is different from the single particle band topology mechanism in the experimentally realized QAH insulator, the magnetic ion doped topological insulator film. Depending on the thickness of the nanosheet, a topological insulating state with helical-like or chiral edge states can be realized. Our work provides a realization of the interaction-driven QAH insulating state in a realistic material.

Ultra-broadband photodetection of Weyl semimetal TaAs up to infrared 10 {\mu}m range at room temperature

Abstract: Photodetectors with broadband optical response have promising applications in many advanced optoelectronic and photonic devices Especially, those with the detection range up to mid-infrared at room temperature are very challenging and highly desired Recently, Weyl semimetal has been discovered and proposed to be favorable for photodetection since in general it breaks Lorentz invariance to have tilted chiral Weyl cones around Fermi level, which leads to chirality dependent photocurrents at arbitrarily long wavelength Furthermore, the linear dispersion bands in Weyl cones result in very high carrier mobility and much reduced thermal carrier compared with the parabolic ones in narrow-gap semiconductors Here, we report that the Weyl semimetal TaAs based photodetector can operate at room temperature with spectral range from blue (4385 nm) to mid-infrared (1029 {\mu}m) light wavelengths and the responsibility and detectivity is more than 78 uA W-1 and 188*107 Jones, respectively This is the first photodetector made by a Weyl semimetal and shows its promising in room-temperature mid-infrared photodetection

Robustness of topological states with respect to lattice instability in the nonsymmorphic topological insulator KHgSb

Abstract: We report a polarized Raman scattering study of nonsymmorphic topological insulator KHgSb with hourglasslike electronic dispersion Supported by theoretical calculations, we show that the lattice of the previously assigned space group $P{6}_{3}/mmc$ (No 194) is unstable in KHgSb While we observe one of two calculated Raman active ${\mathrm{E}}_{2g}$ phonons of space group $P{6}_{3}/mmc$ at room temperature, an additional ${\mathrm{A}}_{1g}$ peak appears at 995 ${\mathrm{cm}}^{\ensuremath{-}1}$ upon cooling below ${T}^{*}=150$ K, which suggests a lattice distortion Several weak peaks associated with two-phonon excitations emerge with this lattice instability We also show that the sample is very sensitive to high temperature and high laser power, conditions under which it quickly decomposes, leading to the formation of Sb Our first-principles calculations indicate that space group $P{6}_{3}mc$ (No 186), corresponding to a vertical displacement of the Sb atoms with respect to the Hg atoms that breaks the inversion symmetry, is lower in energy than the presumed $P{6}_{3}/mmc$ structure and preserves the glide-plane symmetry necessary to the formation of hourglass fermions

Weyl Ferroelectric Semimetal

Abstract: The recent discoveries of ferroelectric metal and Weyl semimetal (WSM) have stimulated a natural question: whether these two exotic states of matter can coexist in a single material or not. These two discoveries ensure us that physically it is possible since both of them share the same necessary condition, the broken inversion symmetry. Here, by using first-principles calculations, we demonstrate that the experimentally synthesized nonmagnetic HgPbO$_3$ represents a unique example of such hybrid \"\\emph{Weyl ferroelectric semimetal}\". Its centrosymmetric $R\\bar{3}c$ phase will undergo a ferroelectric phase transition to the ferroelectric $R3c$ structure. Both phases are metallic and the ferroelectric phase owns a spontaneous polarization of 33 $\\mu$C/cm$^2$. Most importantly, it also harbors six pairs of chiral Weyl nodes around the Fermi level to be an oxide WSM. The structural symmetry broken phase transition induces a topological phase transition. The coexistence of ferroelectricity and Weyl nodes in HgPbO$_3$ is an ideal platform for exploring multiphase interaction and mutual control. The Weyl nodes can be tuned by external pulse electric field, which is promising for potential applications of integrated topotronic and ferroelectric devices.

Topologically entangled Rashba-split Shockley states on the surface of grey arsenic

Abstract: We discover a pair of spin-polarized surface bands on the (111) face of grey arsenic by using angle-resolved photoemission spectroscopy (ARPES). In the occupied side, the pair resembles typical nearly-free-electron Shockley states observed on noble-metal surfaces. However, pump-probe ARPES reveals that the spin-polarized pair traverses the bulk band gap and that the crossing of the pair at Γ[over ¯] is topologically unavoidable. First-principles calculations well reproduce the bands and their nontrivial topology; the calculations also support that the surface states are of Shockley type because they arise from a band inversion caused by crystal field. The results provide compelling evidence that topological Shockley states are realized on As(111).

Observation of bulk nodal lines in topological semimetal ZrSiS

Abstract: ZrSiS is the most intensively studied topological nodal-line semimetal candidate, which is proposed to host multiple nodal lines in its bulk electronic structure. However, previous angle-resolved photoemission spectroscopy (ARPES) experiments with vacuum ultraviolet lights mainly probed the surface states. Here using bulk-sensitive soft X-ray ARPES, we acquire the bulk electronic states of ZrSiS without any interference from surface states. Our results clearly show two groups of three-dimensional bulk nodal lines located on high-symmetry planes and along high-symmetry lines in the bulk Brillouin zone, respectively. The nodal lines on high-symmetry planes are enforced to pin at the Fermi level by carrier compensation and constitute the whole Fermi surfaces. This means that the carriers in ZrSiS are entirely contributed by nodal-line fermions, suggesting that ZrSiS is a remarkable platform for studying physical properties related to nodal lines.

Realization of low-energy Dirac fermions in (Ir$_{1-x}$Pt$_x$)Te$_2$ superconductors

Abstract: Superconducting and topological states are two distinct quantum states of matter. Superconductors with topologically nontrivial electronic states provide a platform to study the interplay between them. Recent study revealed that a bulk superconductor PdTe2 is a topological Dirac semimetal with type-II Dirac fermions. However, the Dirac fermions in PdTe2 have no contribution to the superconducting pair because the Dirac points reside far below the Fermi level (EF). Here, we show that in the isostructural compounds (Ir1-xPtx)Te2, the type-II Dirac points can be adjusted to EF by element substitution at x ~ 0.1, for which the bulk superconductivity appears near 2 K. The (Ir1-xPtx)Te2 superconductor with the Dirac points at EF paves the way for studying the relevant exotic physical phenomena such as topological superconductivity in Dirac semimetals.