arXiv subjects

H. M. Weng

Publications and source records attributed to H. M. Weng.

At least 19 recordsLinked to original sources

arXiv2024

Intrinsic second-order topological insulators in two-dimensional polymorphic graphyne with sublattice approximation

In two dimensions, intrinsic second-order topological insulators (SOTIs) are characterized by topological corner states that emerge at the intersections of distinct edges with reversed mass signs, enforced by spatial symmetries. Here, we present a comprehensive investigation within the class BDI to clarify the symmetry conditions ensuring the presence of intrinsic SOTIs in two dimensions. We reveal that the (anti-)commutation relationship between spatial symmetries and chiral symmetry is a reliable indicator of intrinsic corner states. Through first-principles calculations, we identify several ideal candidates within carbon-based polymorphic graphyne structures for realizing intrinsic SOTIs under sublattice approximation. Furthermore, we show that the corner states in these materials persist even in the absence of sublattice approximation. Our findings not only deepen the understanding of higher-order topological phases but also open new pathways for realizing topological corner states that are readily observable.

cond-mat.mtrl-sci↗

arXiv2022

Linear-in-Frequency Optical Conductivity over a broad range in the three-dimensional Dirac semimetal candidate Ir$_2$In$_8$Se

The optical conductivity of the new Dirac semimetal candidate Ir$_2$In$_8$Se is measured in a frequency range from 40 to 30000 cm$^{-1}$ at temperatures from 300 K down to 10 K. The measurement reveals that the compound is a low carrier density metal. We find that the real part of the conductivity $σ_1(ω)$ is linear in frequency over a broad range from 500 to 4000 cm$^{-1}$ at 300 K and varies slightly with cooling. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings near the Fermi level. Band structure calculations indicate the presence of type-II Dirac points. By comparing our data with the optical conductivity computed from the band structure, we conclude that the observed linear dependence mainly originates from the Dirac cones and the transition between the Dirac cones and the next lower bands. In addition, a weak energy gap feature is resolved below the charge density wave phase transition temperature in reflectivity spectra. An enhanced structure arising from the imperfect Fermi surface nesting is identified in the electronic susceptibility function, suggesting a Fermi surface nesting driven instability.

cond-mat.str-el↗

arXiv2022

Robust Dirac lines against Ge vacancy and possible spin-orbit Dirac points in nonsymmorphic HfGe0.92Te

Looking for new materials with Dirac points has been a fascinating subject of research. Here we report the growth, crystal structure, and band structure of HfGe0.92Te single crystals, featuring three different types of Dirac points. HfGe0.92Te crystalizes in a nonsymmorphic tetragonal space group P4/nmm (No. 129), having square Ge-atom plane with vacancies about 8%. Despite the vacancies on Ge site, the Dirac nodal line composed of conventional Dirac points vulnerable to spin-orbit coupling (SOC) is observed using angle-resolved photoemission spectroscopy, accompanied with the robust Dirac line protected by the nonsymmorphic symmetry against both SOC and vacancies. Specially, spin-orbit Dirac points (SDPs) originated from the surface formed under SOC are hinted to exist according to our experiments and calculations. Quasi-two-dimensional (quasi-2D) characters are observed and further confirmed by angular-resolved magnetoresistance. HfGe0.92Te is a good candidate to explore exotic topological phases or topological properties with three different types of Dirac points and a promising candidate to realize 2D SDPs.

cond-mat.mtrl-sci↗

arXiv2021

Spontaneous ferromagnetism induced topological phase transition in EuB6

The interplay between various symmetries and electronic bands topology is one of the core issues for topological quantum materials. Spontaneous magnetism, which leads to the breaking of time-reversal symmetry, has been proven to be a powerful approach to trigger various exotic topological phases. In this work, utilizing the combination of angle-resolved photoemission spectroscopy, magneto-optical Kerr effect microscopy and first-principles calculations, we present the direct evidence on the realization of the long-sought spontaneous time-reversal symmetry breaking induced topological phase transition in soft ferromagnetic EuB$_6$. We successfully disentangle the bulk band structure from complicated surface states, and reveal the hallmark band inversion occurring between two opposite-parity bulk bands below the phase transition temperature. Besides, our magneto-optical Kerr effect microscopy result confirms the simultaneous formation of magnetic domains in EuB$_6$, implying the intimate link between the topological phase transition and broken time-reversal symmetry therein. Our results demonstrate that EuB$_6$ provides a potential platform to study the interplay between the topological phases and tunable magnetic orders.

cond-mat.mtrl-sci↗

arXiv2020

Magnetically driven loss of centrosymmetry in metallic Pb2CoOsO6

We report magnetic, transport, neutron diffraction, and muon spin rotation data showing that Pb$_2$CoOsO$_6$, a newly synthesized metallic double-perovskite with a centrosymmetric space group at room temperature, exhibits a continuous second-order phase transition at 45 K to a magnetically ordered state with a non-centrosymmetric space group. The absence of inversion symmetry is very uncommon in metals, particularly metallic oxides. In contrast to the recently reported ferroelectric-like structural transition in LiOsO$_3$, the phase transition in Pb$_2$CoOsO$_6$ is driven by a long-range collinear antiferromagnetic order, with propagation vector $\textbf{k} = (\frac{1}{2},0,\frac{1}{2})$, which relieves the frustration associated with the symmetry of themagnetic exchanges. This magnetically-driven loss of inversion symmetry represents a new frontier in the search for novel metallic behavior.

cond-mat.str-el↗

arXiv2019

Phononic Helical Nodal Lines with $\mathcal{PT}$ Protection in MoB$_{2}$

While condensed matter systems host both Fermionic and Bosonic quasi-particles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such a phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine first-principles calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal ($\mathcal{PT}$) symmetry protected helical nodal lines in the phonon spectrum of MoB$_{2}$. This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.

cond-mat.mtrl-sci↗

arXiv2018

Observation of Double Weyl Phonons in Parity-Breaking FeSi

Condensed matter systems have now become a fertile ground to discover emerging topological quasi-particles with symmetry protected modes. While many studies have focused on Fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by the recent theoretical prediction of double-Weyl phonons in transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we directly measured the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double-Weyl points in FeSi, which will be an ideal material to explore emerging Bosonic excitations and its topologically non-trivial properties.

cond-mat.mtrl-sci↗

arXiv2017

Observation of Open-Orbit Fermi Surface Topology in Extremely Large Magnetoresistance Semimetal MoAs$_2$

While recent advances in band theory and sample growth have expanded the series of extremely large magnetoresistance (XMR) semimetals in transition metal dipnictides $TmPn_2$ ($Tm$ = Ta, Nb; $Pn$ = 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 MoAs$_2$, which is isostructural to the $TmPn_2$ 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 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 MoAs$_2$ to the carriers motion on the FSs with dominant open-orbit topology, innovating in the understanding of quadratic XMR in semimetals.

cond-mat.mtrl-sci↗

arXiv2017

Evidence of topological insulator state in the semimetal LaBi

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 [Phys. Rev. Lett. 117, 127204 (2016)], a topologically trivial XMR semimetal, except the existence of a band inversion along the $Γ$-$X$ direction, with one massless and one gapped Dirac-like surface state at the $X$ and $Γ$ 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.

cond-mat.mtrl-sci↗

arXiv2016

Compensated semimetal LaSb with unsaturated magnetoresistance

By combining angle-resolved photoemission spectroscopy and quantum oscillation measurements, we performed a comprehensive investigation on the electronic structure of LaSb, which exhibits near-quadratic extremely large magnetoresistance (XMR) without any sign of saturation at magnetic fields as high as 40 T. We clearly resolve one spherical and one intersecting-ellipsoidal hole Fermi surfaces (FSs) at the Brillouin zone (BZ) center $Γ$ and one ellipsoidal electron FS at the BZ boundary $X$. The hole and electron carriers calculated from the enclosed FS volumes are perfectly compensated, and the carrier compensation is unaffected by temperature. We further reveal that LaSb is topologically trivial but share many similarities with the Weyl semimetal TaAs family in the bulk electronic structure. Based on these results, we have examined the mechanisms that have been proposed so far to explain the near-quadratic XMR in semimetals.

cond-mat.mes-hall↗

arXiv2016

Observation of Weyl nodes and Fermi arcs in TaP

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with nonzero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in HfTe5 Induced via Pressures

Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising layered topological insulators since they are both interlayer weakly bonded materials and also with a large bulk gap in the single layer. It paves a new way for the study of novel topological quantum phenomenon tuned via external parameters. Here, we report the discovery of superconductivity and properties evolution in HfTe5 single crystal induced via pressures. Our experiments indicated that anomaly resistance peak moves to low temperature first before reverses to high temperature followed by disappearance which is opposite to the low pressure effect on ZrTe5. HfTe5 became superconductive above ~5.5 GPa up to at least 35 GPa in the measured range. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. High pressure Raman revealed that new modes appeared around pressure where superconductivity occurs. Crystal structure studies shown that the superconductivity is related to the phase transition from Cmcm structure to monoclinic C2/m structure. The second phase transition from C2/m to P-1 structure occurs at 12 GPa. The combination of transport, structure measurement and theoretical calculations enable a completely phase diagram of HfTe5 at high pressures.

cond-mat.supr-con↗

arXiv2015

Topological Critical Point and Resistivity Anomaly in HfTe5

There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, like 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. We find 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.

cond-mat.mtrl-sci↗

arXiv2015

Observation of Fermi arc spin texture in TaAs

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin-polarized. The measured spin texture fulfills the requirement of mirror and time reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2015

Observation of Weyl nodes in TaAs

In 1929, H. Weyl proposed that the massless solution of Dirac equation represents a pair of new type particles, the so-called Weyl fermions [1]. However the existence of them in particle physics remains elusive for more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of Fermi energy (called as Weyl nodes), the low energy excitation behaves exactly the same as Weyl fermions. Here, by performing soft x-ray angle-resolved photoemission spectroscopy measurements which mainly probe bulk band structure, we directly observe the long-sought-after Weyl nodes for the first time in TaAs, whose projected locations on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence of existence of Weyl fermion quasiparticles in TaAs.

cond-mat.mtrl-sci↗

arXiv2015

Experimental discovery of Weyl semimetal TaAs

Weyl semimetals are a class of materials that can be regarded as three-dimensional analogs of graphene breaking time reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, matching remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2014

Optical spectroscopy study of Nd(O,F)BiS2 single crystals

We present an optical spectroscopy study on F-substituted NdOBiS$_2$ superconducting single crystals grown using KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 \cm. The plasma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of x=0.5, but very close to the value based on a doping level of 7$\%$ of itinerant electrons per Bi site as determined by ARPES experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a CDW instability as being widely discussed for a doping level of x=0.5.

cond-mat.supr-con↗

arXiv2014

Observation of anomalous temperature dependence of spectrum on small Fermi surfaces in a BiS2-based superconductor

We performed an angle-resolved photoemission spectroscopy study of the BiS2-based superconductor Nd(O,F)BiS2. Two small electron-like Fermi surfaces around X (pi, 0) are observed, which enclose 2.4% and 1.1% of the Brillouin zone area, respectively, corresponding to an electron doping of 7% per Bi site. The low-energy spectrum consists of a weakly-dispersing broad hump and a dispersive branch, which follows well the calculated band dispersion. This hump is drastically suppressed with increasing temperature, while the dispersive branch is essentially unaffected. The anomalous thermal effect indicates a highly interacting electronic state, in which the superconducting pairing develops.

cond-mat.supr-con↗