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The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling., Comment: accepted by PRL

Recent classification efforts encompassing crystalline symmetries have revealed rich possibilities for solid-state systems to support a tapestry of exotic topological states. However, finding materials that realize such states remains a daunting challenge. Here we show how the interplay of topology, symmetry, and magnetism combined with doping and external electric and magnetic field controls can be used to drive the previously unreported SrIn$_2$As$_2$ materials family into a variety of topological phases. Our first-principles calculations and symmetry analysis reveal that SrIn$_2$As$_2$ is a dual topological insulator with $Z_2=(1;000)$ and mirror Chern number $C_M= -1$. Its isostructural and isovalent antiferromagnetic cousin EuIn$_2$As$_2$ is found to be an axion insulator with $Z_4= 2$. The broken time-reversal symmetry via Eu doping in Sr$_{1-x}$Eu$_x$In$_2$As$_2$ results in a higher-order or topological crystalline insulator state depending on the orientation of the magnetic easy axis. We also find that antiferromagnetic EuIn$_2$P$_2$ is a trivial insulator with $Z_4= 0$, and that it undergoes a magnetic field-driven transition to an ideal Weyl fermion or nodal fermion state with $Z_4= 1$ with applied magnetic field. Our study identifies Sr$_{1-x}$Eu$_x$In$_2$(As, P)$_2$ as a new tunable materials platform for investigating the physics and applications of Weyl and nodal fermions in the scaffolding of crystalline and axion insulator states., Comment: 8 Pages, 4 Figures, SM not included

Kagome materials often host exotic quantum phases, including spin liquids, Chern gap, charge density wave, and superconductivity. Existing scanning microscopy studies of the kagome charge order have been limited to nonkagome surface layers. Here, we tunnel into the kagome lattice of FeGe to uncover features of the charge order. Our spectroscopic imaging identifies a 2×2 charge order in the magnetic kagome lattice, resembling that discovered in kagome superconductors. Spin mapping across steps of unit cell height demonstrates the existence of spin-polarized electrons with an antiferromagnetic stacking order. We further uncover the correlation between antiferromagnetism and charge order anisotropy, highlighting the unusual magnetic coupling of the charge order. Finally, we detect a pronounced edge state within the charge order energy gap, which is robust against the irregular shape fluctuations of the kagome lattice edges. We discuss our results with the theoretically considered topological features of the kagome charge order including unconventional magnetism and bulk-boundary correspondence. National Research Foundation (NRF) Submitted/Accepted version M.Z.H. acknowledges support from the US Department of Energy, Office of Science, National Quantum Information Science Research Centers, Quantum Science Center and Princeton University. M.Z.H. acknowledges visiting scientist support at Berkeley Lab (Lawrence Berkeley National Laboratory) during the early phases of this work. Theoretical and STM works at Princeton University was supported by the Gordon and Betty Moore Foundation (GBMF9461; GBMF4547; M.Z.H.). The theoretical work including ARPES were supported by the US DOE under the Basic Energy Sciences program (grant number DOE/BES DE-FG-02-05ER46200; M.Z.H.). T.-R.C. was supported by the Young Scholar Fellowship Program under a MOST grant for the Columbus Program, MOST111-2636-M-006-014, the Higher Education Sprout Project, Ministry of Education to the Headquarters of University Advancement at the National Cheng Kung University (NCKU), the National Center for Theoretical Sciences (Taiwan). The work at Rice was supported by US NSF-DMR-2100741, the Robert A. Welch Foundation under grant no. C-1839, C-2024, and No. C-1411, the U.S. Department Of Energy (DOE) grant no. DE-SC0021421 and No. DE-SC0018197, and the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant no. GBMF9470. The work at Nanyang Technological University was supported by the National Research Foundation, Singa- pore under its Fellowship Award (NRF-NRFF13-2021-0010). T.N. acknowledges support from the European Union’s Horizon 2020 research and innovation programme (ERC-StG-Neupert-757867-PARATOP). G. X. acknowledges support from the National Key Research and Development Program of China (2018YFA0307000), and the National Natural Science Foundation of China (11874022).

Topological insulators with intrinsic magnetic order are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. To study these systems and their physics, people have proposed a variety of magnetic topological insulator systems, including HoSbTe, an antiferromagnetic weak topological insulator candidate. In this work, we use scanning tunneling microscopy to probe the electronic structure of HoSbTe with antiferromagnetic and ferromagnetic orders that are tuned by applying an external magnetic field. Although around the Fermi energy, we find minor differences between the quasi-particle interferences under the ferromagnetic and antiferromagnetic orders, deep inside the valance region, a new quasi-particle interference signal emerges with ferromagnetism. This observation is consistent with our first-principles calculations indicating the magnetism-driven transition of the electronic states in this spin-orbit coupled topological magnet., Comment: 13 pages, 7 figures

The non-trivial magnetic texture in real space gives rise to the intriguing phenomenon of topological Hall effect (THE), which is relatively less explored in topological semimetals. Here, we report large THE in the antiferromagnetic (AFM) state in single crystals of EuAgAs, an AFM Dirac semimetal. EuAgAs hosts AFM ground state below $T_N$ = 12 K with a weak ferromagnetic component. The in-plane isothermal magnetization below $T_N$ exhibits a weak metamagnetic transition. We also observe chiral anomaly induced positive longitudinal magnetoconductivity which indicates a Weyl fermion state under applied magnetic field. The first-principles calculations reveal that EuAgAs is an AFM Dirac semimetal with a pair of Dirac cones, and therefore, a Weyl semimetailic state can be realized under time-reversal symmetry breaking via an applied magnetic field. Our study establishes that EuAgAs is a novel system for exploiting the interplay of band topology and the topology of the magnetic texture., Accepted for publication in Phys. Rev. B as a Letter

NbGe${}_{2}$ is a type-I full-gap bulk chiral superconductor with Kramers-Weyl fermions and an enhanced critical field from filamentary superconductivity. Quantum oscillations and first-principles calculations reveal two hole Fermi pockets centered around the Weyl $M$ point and one trivial hole pocket near the $H$ point in the Brillouin zone. The authors attribute the electron-phonon mediated superconductivity to the Van Hove singularity arising from the saddle-point-like dispersion at the $M$ and $H$ points near the Fermi level.

Kagome-net, appearing in areas of fundamental physics, materials, photonic and cold-atom systems, hosts frustrated fermionic and bosonic excitations. However, it is extremely rare to find a system to study both fermionic and bosonic modes to gain insights into their many-body interplay. Here we use state-of-the-art scanning tunneling microscopy and spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet. Our results reveal the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through a combination of self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice analog of kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials., Comment: To appear in Nature Communications (2020)

Topological nodal-line semimetals support protected band crossings which form nodal lines or nodal loops between the valence and conduction bands and exhibit novel transport phenomena. Here we address the topological state of the nodal-line semimetal candidate material, CaCdSn, and report magnetotransport properties of its single crystals grown by the self-flux method. Our first-principles calculations show that the electronic structure of CaCdSn harbors a single nodal loop around the $\Gamma$ point in the absence of spin-orbit coupling (SOC) effects. The nodal crossings in CaCdSn are found to lie above the Fermi level and yield a Fermi surface that consists of both electron and hole pockets. CaCdSn exhibits high mobility ($\mu \approx 3.44\times 10^4$ cm$^2$V$^{-1}$s$^{-1}$) and displays a field-induced metal-semiconductor like crossover with a plateau in resistivity at low temperature. We observe an extremely large and quasilinear non-saturating transverse as well as longitudinal magnetoresistance (MR) at low temperatures ($\approx 7.44\times 10^3 \%$ and $\approx 1.71\times 10^3\%$, respectively, at 4K). We also briefly discuss possible reasons behind such a large quasilinear magnetoresistance and its connection with the nontrivial band structure of CaCdSn., Comment: 7 pages, 5 figures, Accepted in PRB

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 $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$) harbors saddlelike topological surface states with associated Van Hove singularities. ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC), which are protected by combined inversion ($\mathcal{I}$) and time-reversal ($\mathcal{T}$) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological state with nonzero weak topological invariant ${Z}_{2}=(0;001)$ and mirror Chern number ${n}_{M}=\ensuremath{-}2$. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddlelike energy dispersion with type II Van Hove singularities. We also discuss how modulating the crystal structure can drive ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ into a Dirac semimetal state with a pair of Dirac points. Our results indicate that ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ is an ideal candidate material for exploring the properties of topological insulators with saddlelike surface states.

Materials with triply-degenerate nodal points in their low-energy electronic spectrum produce crystalline-symmetry-enforced three-fold fermions, which conceptually lie between the two-fold Weyl and four-fold Dirac fermions. Here we show how a silver-based Dirac semimetal BaAgAs realizes three-fold fermions through our first-principles calculations combined with a low-energy effective $\mathbf{k.p}$ model Hamiltonian analysis. BaAgAs is shown to harbor triply-degenerate nodal points, which lie on its $C_{3}$ rotation axis, and are protected by the $C_{6v}$($C_2\otimes C_{3v}$) point-group symmetry in the absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs transitions into a nearly-ideal Dirac semimetal state with a pair of Dirac nodes lying on the $C_{3}$ rotation axis. We show that breaking inversion symmetry in the BaAgAs$_{1-x}$P$_x$ alloy yields a clean and tunable three-fold fermion semimetal. Systematic relaxation of other symmetries in BaAgAs generates a series of other topological phases. BaAgAs materials thus provide an ideal platform for exploring tunable topological properties associated with a variety of different fermionic excitations., Comment: 6 pages, 4 figures; Published in Phys. Rev. Materials (R)

We report a metal-insulator like transition in single-crystalline 3D topological insulator Bi2Te3 at a temperature of 230 K in the presence of an external magnetic field applied normal to the surface. This transition becomes more prominent at larger magnetic field strength with the residual resistance value increasing linearly with the magnetic field. At low temperature, the magnetic field dependence of the magnetoresistance shows a transition from logarithmic to linear behavior and the onset magnetic field value for this transition decreases with increasing temperature. The logarithmic magnetoresistance indicates the weak anti-localization of the surface Dirac electrons while the high temperature behavior originates from the bulk carriers due to intrinsic impurities. At even higher temperatures beyond ∼230 K, a completely classical Lorentz model type quadratic behavior of the magnetoresistance is observed. We also show that the experimentally observed anomalies at ∼230 K in the magneto-transport properties do not originate from any stacking fault in Bi2Te3.

Two dimensional magnetic materials, with tunable electronic properties could lead to new spintronic, magnetic and magneto-optic applications. Here, we explore intrinsic magnetic ordering in two dimensional monolayers of transition metal tri-halides (MX$_3$, M = V, Cr, Mn, Fe and Ni, and X = F, Cl, Br and I), using density functional theory. We find that other than FeX$_3$ family which has an anti-ferromagnetic ground state, rest of the trihalides are ferromagnetic. Amongst these the VX$_3$ and NiX$_3$ family are found to have the highest magnetic transition temperature, beyond the room temperature. In terms of electronic properties, the tri-halides of Mn and Ni are either half metals or Dirac half metals, while the tri-halides of V, Fe and Cr are insulators. Among all the trihalides studied in this paper, we find the existence of very clean spin polarized Dirac half metallic state in MnF$_3$, MnCl$_3$, MnBr$_3$, NiF$_3$ and NiCl$_3$. These spin polarized Dirac half metals will be immensely useful for spin-current generation and other spintronic applications.

Band crossings that occur on a mirror plane are compelled to form nodal loops in the momentum space in the absence of spin-orbit coupling (SOC). When other equivalent mirror planes are present, multiple such nodal loops can combine to form interesting topological structures involving crossed nodal lines. Here, based on first-principles calculations and an effective $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model analysis, we show that CaAuAs hosts a unique starfruitlike crossed-nodal-line structure in the bulk electronic dispersion, which is comprised of three nodal loops that cross each other at the time-reversal-invariant momentum point $A$. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}A$ direction in the Brillouin zone. These Dirac points are protected by the combination of time-reversal, inversion, and ${C}_{3}$ rotation symmetries. We discuss how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.

In a finite time quantum quench of the Haldane model, the Chern number determining the topology of the bulk remains invariant, as long as the dynamics is unitary. Nonetheless, the corresponding boundary attribute, the edge current, displays interesting dynamics. For the case of sudden and adiabatic quenches the post quench edge current is solely determined by the initial and the final Hamiltonians, respectively. However for a finite time ($\tau$) linear quench in a Haldane nano ribbon, we show that the evolution of the edge current from the sudden to the adiabatic limit is not monotonic in $\tau$, and has a turning point at a characteristic time scale $\tau=\tau_c$. For small $\tau$, the excited states lead to a huge unidirectional surge in the edge current of both the edges. On the other hand, in the limit of large $\tau$, the edge current saturates to its expected equilibrium ground state value. This competition between the two limits lead to the observed non-monotonic behavior. Interestingly, $\tau_c$ seems to depend only on the Semenoff mass and the Haldane flux. A similar dynamics for the edge current is also expected in other systems with topological phases., Comment: 9 pages with 9 figures. Comments, suggestions and criticism are welcome

We propose four different thermodynamically stable structural phases of arsenic monolayers based on ab-initio density functional theory calculations all of which undergo a topological phase transition on application of a perpendicular electric field. All the four arsenic monolayer allotropes have a wide band gap, varying from 1.21 eV to 3.0 eV (based on GW calculations), and in general they undergo a metal-insulator quantum phase transition on application of uniaxial in-layer strain. Additionally an increasing transverse electric field induces band-inversion at the {\Gamma} point in all four monolayer allotropes, leading to a nontrivial topological phase (insulating for three and metallic for one allotrope), characterized by the switching of the Z2 index, from 0 (before band inversion) to 1 (after band inversion). The topological phase tuned by the transverse electric field, should support spin-separated gapless edge states which should manifest in quantum spin Hall effect., Comment: 8 pages, 5 figures; Revised version with phonon dispersion; To appear in PRB

We calculate the tunneling density of states (TDOS) in a three wire junction of interacting spin-1/2 electrons, and find an anomalous enhancement of the TDOS in the zero bias limit, even for repulsive interactions for several bosonic fixed points. This enhancement is physically related to the reflection of holes from the junction for incident electrons, and it occurs only in the vicinity of the junction ($x < v_{\rm min}/2\omega$ where $v_{\rm min}$ is the minimum of the velocity of charge or spin excitations and $\omega$ is the bias frequency), crossing over to the bulk value which is always suppressed, at larger distances. The TDOS exponent can be directly probed in a STM experiment by measuring the differential tunneling conductance as a function of either the bias voltage or temperature as done in C. Blumenstein et al. Nature Physics 7, 776 (2011)., Comment: 10 pages, 3 figures and 2 tables; final version; to appear in PRB

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane N\'eel vector direction below a critical thickness of MnTe, which is the ground state amongst all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic two-fold rotational symmetry $C_2{\cdot}T$. The axion insulator phase evolves into a trivial insulator as we increase the thickness of the magnetic layers. By switching the N\'eel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the N\'eel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice., Comment: 14 pages, 13 figures