Evidence for a Strong Topological Insulator Phase in ZrTe5

Abstract

The complex electronic properties of ZrTe5 have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of ZrTe5, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of ZrTe5.The discovery of topological insulators (TIs), characterized by metallic spin-polarized surface states connecting the bulk valence and conduction bands [1], has stimulated the search for novel topological phases of matter [2][3][4][5][6]. ZrTe 5 has recently emerged as a challenging system with unique, albeit poorly understood, electronic properties [7][8][9][10][11][12][13][14][15][16][17]. Magneto-transport [11], magneto-infrared [13] and optical spectroscopy [14] studies describe ZrTe 5 in terms of a 3D Dirac semimetal. Theoretical calculations have predicted its bulk electronic properties to lie in proximity of a topological phase transition between a strong and a weak TI (STI and WTI, respectively), where only the former displays topologically protected surface states at the experimentally accessible (010) surface [10]. The monolayer is also computed to be a 2D TI [10] and scanning tunneling microscopy/spectroscopy (STM/STS) experiments suggest the existence of topologically protected states at step edges [18,19]. However, the unambiguous identification of the topological phase of ZrTe 5 is still lacking.In this Letter we report on the STI character of the bulk ZrTe 5 by combining ab initio calculations and multiple experimental techniques, at temperature both above and below the one of the resistivity peak, T * ∼ 160 K [7][8][9]15]. Angleresolved photoelectron spectroscopy (ARPES) experiments in the ultraviolet (UV) and soft x-ray (SX) energy ranges reveal the presence of two distinct states at the top of the valence band (VB). On the basis of photon energy dependent studies, we ascribe the origin of these two states to the bulk and crystal surface, respectively. We have performed ab initio calculations of the topological phase diagram of ZrTe 5 , as a function of the interlayer distance b/2. Our measured band dispersion is in agreement with the calculations and it is consistent with the STI case for b/2 = 7.23 ± 0.02Å. This value has been confirmed for our specimen by x-ray diffraction (XRD) measurements. Furthermore, the 3D Dirac semimetal phase is not protected by crystalline symmetries, and it manifests only for the specific b/2 = 7.35Å at the boundar...

Disruption of the Accidental Dirac Semimetal State in ZrTe5 under Hydrostatic Pressure

“…In the presence of the pressure, the Dirac cone is shifted away from the Fermi level and/or opens a finite energy gap. Recent works revealed that the Dirac semimetal state in ZrTe 5 is highly sensitive to the the lattice constants and manifests only at the boundary between the WTI and the STI phases [26,30]. In fact, our results show that it would not be completely destroyed until the pressure up to 2.5 GPa.…”

“…In addition, magnetoinfrared spectroscopy results, such as the linear energy dependence of optical conductivity and Landau level splitting, also support this scenario [24,25]. Very recently, Manzoni et al find out that the 3D Dirac semimetal phase manifests at the boundary between the weak and strong TI phases [26].Applying pressure is known to be a powerful approach to tune the electronic states and lattice structures without introducing disorder or impurity, which has been widely employed in topological materials. Recently, a pressure-induced semimetal to superconductor transition was observed in ZrTe 5 at 6.2 GPa [27].…”

Three-dimensional nature of the band structure of ZrTe5 measured by high-momentum-resolution photoemission spectroscopy

“…The resistivity peaks at 140 K, which is known as the resistivity anomaly 9-12 . Currently the most interesting question regarding ZrTe 5 is whether it is a 3D Dirac semimetal 16,18,19 , a 3D weak TI 21,22 , or a 3D strong TI 15 . The most essential evidence to distinguish these scenarios is to determine whether it has a gapless Dirac cone near Γ, or a gapped state, and to determine the existence of a TSS.…”

Thermodynamic signature of Dirac electrons across a possible topological transition in ZrTe5

“…Hall effect data shows electron-like carriers at low temperature [16], implying some of these additional bands must be populated and Dirac-like or massive Dirac-like features should be present. Note that these features are extremely sensitive to cell volume and strain [17], which may explain the conflicting experimental reports on the electronic properties and topological signatures in ZrTe 5 [4][5][6][7][8][9][10][11][12][13][14].…”

“…In this study we focus on ZrTe 5 , a material whose topological nature is hotly debated; it has been predicted and verified as a Dirac semimetal [4][5][6][7][8], a topological insulator [9][10][11][12] and a trivial semiconductor [13,14]. In addition to its potential topological nature, there is also an unusual anomaly in the temperature dependence of the resistivity that has been conjectured to originate from a Lifshitz transition [15,16].…”