Room Temperature Evidence of PtTe 2 Topological Semimetal Character

Recent Advances in Topological Quantum Materials by Angle-Resolved Photoemission Spectroscopy

Topological semimetal represents a novel quantum phase of matter, which exhibits a variety of fascinating quantum phenomena. This class of materials not only have potential applications in electronic devices, but also represent one of the hottest topics in the field of quantum materials. According to the band structure of these materials in the three-dimensional momentum space, topological semimetals can be classified into Dirac semimetals, Weyl semimetals and nodal-line semimetals. Extensive studies on these materials have been conducted using various techniques. For example, angle-resolved photoemission spectroscopy (ARPES) has directly observed the Fermi arc that connects two Weyl points with opposite chiralities in the surface states of Weyl semimetals; the Dirac points, Weyl points as well as the Dirac nodal line in the bulk states have also been revealed by soft X-ray ARPES; the observation of negative magnetoresistance in transport measurements has been taken as the evidence for the chiral anomaly in Weyl and Dirac semimetals; the chirality of the Weyl fermions have been detected by measuring the photocurrent in response of circularly polarized light; in addition, strong second harmonic generation and THz emission have been observed, indicating strong non-linear effects of Weyl semimetals. Infrared spectroscopy is a bulk-sensitive technique, which not only covers a very broad energy range (meV to several eV), but also has very high energy resolution (dozens of µeV). Investigations into the optical response of these materials not only help understand the physics of the topological phase and explore novel quantum phenomena, but also pave the way for future studies and applications in optics. In this article, we introduce the optical studies on several topological semimetals, including Dirac, Weyl and nodal-line semimetals.

Magnetic topological semimetals have been at the forefront of condensed matter physics due to their ability to exhibit exotic transport phenomena. Investigating the interplay between magnetic and topological orders in systems with broken time-reversal symmetry is crucial for realizing non-trivial quantum effects. We delve into the electronic structure of the rare-earth-based antiferromagnetic Dirac semimetal EuMg2Bi2 using first-principles calculations and angle-resolved photoemission spectroscopy. Our calculations reveal that the spin–orbit coupling (SOC) in EuMg2Bi2 prompts an insulator to topological semimetal transition, with the Dirac bands protected by crystal symmetries. The linearly dispersive states near the Fermi level, primarily originating from Bi 6p orbitals, are observed on both the (001) and (100) surfaces, confirming that EuMg2Bi2 is a three-dimensional topological Dirac semimetal. This research offers pivotal insights into the interplay between magnetism, SOC and topological phase transitions in spintronics applications.

The interplay between time-reversal symmetry (TRS) and band topology plays a crucial role in topological states of quantum matter. In time-reversal-invariant (TRI) systems, the inversion of spin-degenerate bands with opposite parity leads to nontrivial topological states, such as topological insulators and Dirac semimetals. When the TRS is broken, the exchange field induces spin splitting of the bands. The inversion of a pair of spin-splitting subbands can generate more exotic topological states, such as quantum anomalous Hall insulators and magnetic Weyl semimetals. So far, such topological phase transitions driven by the TRS breaking have not been visualized. In this work, using angle-resolved photoemission spectroscopy, we have demonstrated that the TRS breaking induces a band inversion of a pair of spin-splitting subbands at the TRI points of Brillouin zone in EuB$_6$, when a long-range ferromagnetic order is developed. The dramatic changes in the electronic structure result in a topological phase transition from a TRI ordinary insulator state to a TRS-broken topological semimetal (TSM) state. Remarkably, the magnetic TSM state has an ideal electronic structure, in which the band crossings are located at the Fermi level without any interference from other bands. Our findings not only reveal the topological phase transition driven by the TRS breaking, but also provide an excellent platform to explore novel physical behavior in the magnetic topological states of quantum matter.

We review recent theoretical progress in the understanding and prediction of novel topological semimetals. Topological semimetals define a class of gapless electronic phases exhibiting topologically stable crossings of energy bands. Different types of topological semimetals can be distinguished on the basis of the degeneracy of the band crossings, their codimension (e.g., point or line nodes), and the crystal space group symmetries on which the protection of stable band crossings relies. The dispersion near the band crossing is a further discriminating characteristic. These properties give rise to a wide range of distinct semimetal phases such as Dirac or Weyl semimetals, point or line node semimetals, and type I or type II semimetals. In this review we give a general description of various families of topological semimetals, with an emphasis on proposed material realizations from first-principles calculations. The conceptual framework for studying topological gapless electronic phases is reviewed, with a particular focus on the symmetry requirements of energy band crossings, and the relation between the different families of topological semimetals is elucidated. In addition to the paradigmatic Dirac and Weyl semimetals, we pay particular attention to more recent examples of topological semimetals, which include nodal line semimetals, multifold fermion semimetals, and triple-point semimetals. Less emphasis is placed on their surface state properties, their responses to external probes, and recent experimental developments.

Topological materials hosting kagome lattices have drawn considerable attention due to the interplay between topology, magnetism, and electronic correlations. The (Fe$_{1-x}$Co$_x$)Sn system not only hosts a kagome lattice but has a tunable symmetry breaking magnetic moment with temperature and doping. In this study, angle resolved photoemission spectroscopy and first principles calculations are used to investigate the interplay between the topological electronic structure and varying magnetic moment from the planar to axial antiferromagnetic phases. A theoretically predicted gap at the Dirac point is revealed in the low temperature axial phase but no gap opening is observed across a temperature dependent magnetic phase transition. However, topological surface bands are observed to shift in energy as the surface magnetic moment is reduced or becomes disordered over time during experimental measurements. The shifting surface bands may preclude the determination of a temperature dependent bulk gap but highlights the intricate connections between magnetism and topology with a surface/bulk dichotomy that can affect material properties and their interrogation.

Angle-resolved photoemission spectroscopy (ARPES), an experimental technique based on the photoelectric effect, is arguably the most powerful method for probing the electronic structure of solids. The past decade has witnessed notable progress in ARPES, including the rapid development of soft-X-ray ARPES, time-resolved ARPES, spin-resolved ARPES and spatially resolved ARPES, as well as considerable improvements in energy and momentum resolution. Consequently , ARPES has emerged as an indispensable experimental probe in the study of topological materials, which have characteristic non-trivial bulk and surface electronic structures that can be directly detected by ARPES. Over the past few years, ARPES has had a crucial role in several landmark discoveries in topological materials, including the identification of topological insulators and topological Dirac and Weyl semimetals. In this Technical Review , we assess the latest developments in different ARPES techniques and illustrate the capabilities of these techniques with applications in the study of topological materials.

Topological 3D Dirac semimetals are characterized by bulk Dirac cone band crossings and nontrivial topological surface states, giving rise to a wealth of exotic physical properties and attracting considerable attention in recent years. Understanding the nonequilibrium dynamics of Dirac semimetals in micro-size provides critical guidance for the design of micro- and nanoscale optoelectronic and ultrafast photonic devices. In this work, we employ time-resolved microscopic transient spectroscopy to investigate the nonequilibrium photocarrier and lattice dynamics in microcrystalline Dirac semimetal NiTe2, a prototypical 3D Dirac semimetal. Following photoexcitation at 390 nm, the transient reflectivity kinetics of NiTe2 can be well described with a triple-exponential decay function. The fastest relaxation component occurs on a sub-picosecond timescale and increases with pump fluence, which originates from electron-optical phonon coupling. An intermediate relaxation process with a characteristic time of ~8 ps is attributed to electron–hole recombination, while a slower decay component on the order of ~20–30 ps can be assigned to the anharmonic decay of optical phonons into acoustic phonons. Polarization-resolved measurements reveal nearly in-plane isotropic transient responses, which are insensitive to the polarization of probe light. These findings contribute to the physical insights for the development of future photonics and optoelectronic devices based on topological Dirac semimetals.

Topological semimetal, known as a type of topological quantum materials without energy gap, has attracted lots of research interests due to its unique physical properties such as novel quasiparticles, giant magnetoresistance and large carrier mobility. Topological semimetal can be further classified into topological Dirac semimetal, topological Weyl semimetal, topological nodal-line semimetal and topological semimetals with " new fermions”. The high-resolution angle-resolved photoemission spectroscopy (ARPES) has emerged as a powerful experimental technique to directly visualize the electronic structure and identify the characteristic topological electronic states in topological semimetals. Here we would briefly introduce the ARPES technique and review some of the recent progress of ARPES study on the electronic structures of typical topological semimetals. We would focus mostly on the physics origin and ARPES signature of topological electronic structures and hope the readers would find it interesting and useful in the understanding of this material class which both is important in physics and has promising application potentials.

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 full Heusler compound Co_{2}MnGa as a candidate. Both Hopf link and chainlike bulk band crossings and unconventional topological surface states are identified.

Magnetotransport and ARPES studies of large-area Sb2Te3 and Bi2Te3 topological insulators grown by MOCVD on Si

Recently, the Dirac and Weyl semimetals have attracted extensive attention in condensed matter physics due to both the fundamental interest and the potential application of a new generation of electronic devices. Here we review the exotic electrical transport phenomena in Dirac and Weyl semimetals. Section is a brief introduction to the topological semimetals (TSMs). In Section and Section , the intriguing transport phenomena in Dirac semimetals (DSMs) and Weyl semimetals (WSMs) are reviewed, respectively. The most widely studied Cd3As2 and the TaAs family are selected as representatives to show the typical properties of DSMs and WSMs, respectively. Beyond these systems, the advances in other TSM materials, such as ZrTe5 and the MoTe2 family, are also introduced. In Section , we provide perspectives on the study of TSMs especially on the magnetotransport investigations.

Dirac and Weyl semimetals both exhibit arc-like surface states. However, whereas the surface Fermi arcs in Weyl semimetals are topological consequences of the Weyl points themselves, the surface Fermi arcs in Dirac semimetals are not directly related to the bulk Dirac points, raising the question of whether there exists a topological bulk-boundary correspondence for Dirac semimetals. In this work, we discover that strong and fragile topological Dirac semimetals exhibit one-dimensional (1D) higher-order hinge Fermi arcs (HOFAs) as universal, direct consequences of their bulk 3D Dirac points. To predict HOFAs coexisting with topological surface states in solid-state Dirac semimetals, we introduce and layer a spinful model of an s–d-hybridized quadrupole insulator (QI). We develop a rigorous nested Jackiw–Rebbi formulation of QIs and HOFA states. Employing ab initio calculations, we demonstrate HOFAs in both the room- (α) and intermediate-temperature (α″) phases of Cd3As2, KMgBi, and rutile-structure ( beta ^{prime} -) PtO2.

We elucidate the transport properties and electronic structures of distorted rutile-type WO2. Electrical resistivity and Hall effect measurements of high-quality single crystals revealed the transport property characteristics of topological materials; these characteristics included an extremely large magnetoresistance of 13 200% (2 K and 9 T) and a very high carrier mobility of 25 700 cm2 V−1 s−1 (5 K). First-principles calculations revealed Dirac nodal lines (DNLs) near the Fermi energy in the electronic structure when spin–orbit interactions (SOIs) were absent. Although these DNLs mostly disappeared in the presence of SOIs, band crossings at high-symmetry points in the reciprocal space existed as Dirac points. Furthermore, DNLs protected by nonsymmorphic symmetry persisted on the ky = π/b plane. The unique transport properties originating from the topological electronic structure of chemically and thermally stable WO2 could represent an opportunity to investigate the potential electronic applications of the material.

The electronic structure of magnetically-doped TI with stoichiometry Bi1.09Gd0.06Sb0.85Te3 in the region of the Dirac point has been studied in detail by angle-resolved photoelectron spectroscopy (ARPES) at various temperatures (above and below the Néel temperature, 1-35 K) and different polarizations of synchrotron radiation. It has shown that the energy gap in photoemission spectra opens at the Dirac point and remains open above the temperature of the long-range magnetic ordering, Tn. Measurements of magnetic properties by the superconducting magnetometry method (SQUID) have shown antiferromagnetic ordering with a transition temperature to the paramagnetic phase equal to 8.3 K. Study of the temperature dependence of the Dirac cone state intensity at the Г point by ARPES has confirmed the magnetic transition and has shown a possibility of its indication directly from photoemission spectra. A more detailed analysis of the splitting between the upper and lower Dirac cone states (i.e. the energy gap) at the Dirac point in the photoelectron spectra has shown the dependence of the measured gap on the synchrotron radiation polarization (about 28-30 meV for p-polarization and 22-25 meV for circularly polarized radiation of opposite chirality). The mechanism of opening the gap at a Dirac point above the Tn was proposed due to the “pairing” of the Dirac fermions with opposite momentum and spin orientation as a result of their interaction with the spin texture generated by photoemission in the region of the photoemission hole on a magnetic impurity atom (Gd). It was shown that the gap at the Dirac point, measured above Tn, is dynamic and is formed directly during photoemission process. At the same time, the origin of the gap remains magnetic (even when the long-range magnetic ordering is destroyed) and is associated with the properties of the magnetic topological insulator that determines a practically unchanged size of the gap above Tn. The dynamic origin of the generated gap is confirmed by the dependence of its magnitude on the polarization of synchrotron radiation.