Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe₅

Abstract

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980's. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculation. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with reduced thickness. Low-temperature Hall experiment sheds lights on more intrinsic anisotropic electrical transport, with hole mobility of 3,000 and 1,500 cm 2 /V· s along a-axis and c-axis respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with highest electron mobility up to 44,000 cm 2 /V· s.Keywords: ZrTe5 Single crystal, 2D material, optical anisotropy, electrical anisotropy, quantum oscillations 3The discovery of graphene 1 began a new era of condensed-matter research because of its unique two-dimensional Dirac band structure, which hosts many profound physical phenomena such as the anomalous integer quantum Hall effect (IQHE) 2 . Since then great efforts have been made towards expanding the spectrum of topological materials and bringing many conceptual materials into reality. Transition metal pentatellurides such as ZrTe5 and HfTe5 have been widely studied in bulk form since early 1980's due to their anomalous resistivity peak and X-ray diffraction intensity peak at low temperature 3,4 , large thermoelectric power 5 , pressure-induced superconductivity 6,7 , absence of a structural phase transition corresponding to resistivity anomaly 8 , and chiral magnetic effect 9 . In recent years, ZrTe5 research has been revived because of its non-trivial topological properties. Some theoretical predictions and experimental results 10,11 indicate that it is a 3D Dirac semimetal, a mimic of graphene with linear energy dispersion in all three directions. On the other hand, its monolayer form is also claimed to be a candidate of quantum spin Hall insulator 12,13 , which is very rare among the natural compounds 14 . Shubnikov-de Haas oscillations 10,15,16 , Zeeman Splitting 17,18 , and fractional quantum Hall effect 19 were also observed in bulk ZrTe5.Meanwhile in recent years the 2D family has been expanded to a wide range of mat...

Bipolar Conduction as the Possible Origin of the Electronic Transition in Pentatellurides: Metallic vs Semiconducting Behavior

“…Importantly, the high-conductivity directions for electrons and holes are opposite, and the hole conductivity is higher than the electron conductivity for transport along the usually measured a axis (x). Specifically, the hole conductivity is highest along a, while the electron conductivity is highest along c. This is in accordance with transport data for few-layer, p-type ZrTe 5 [35], where a hole mobility approximately 2 times larger was measured along the a axis compared with the c axis. This has important implications.…”

Ultrathin 2D GeSe2 Rhombic Flakes with High Anisotropy Realized by Van der Waals Epitaxy

“…The incident light was polarized along the horizontal direction, which is parallel to the y-axis shown in Figure 4a. The Raman scattering intensity (I) of the observed Raman vibrational modes can be identified by: [40][41][42] i s These three peaks achieve their maximum intensities at 0° and 180°, while their minimum is at 90° (Figure 4d; Figure S4, SI).…”

Lifshitz transition mediated electronic transport anomaly in bulk ZrTe5

“…The observation of CME in ZrTe 5 [9] kicked off intensive transport studies in DSM (such as Na 3 Bi [24], Cd 3 As 2 [25], ZrTe 5 [11,[30][31][32][33][34][35][36][37] and HfTe 5 [38][39][40][41]) and WSM (e.g., TaAs [42], NbAs [43], NbP [44], and TaP [45]) materials. The readily accessible quantum limit in ZrTe 5 (only several Tesla [10,37], instead of above 40 T for Cd 3 As 2 [46]), along with its chemical stability, advocates unique strategic advantages of exploring ZrTe 5based materials as a unique topological platform [8,47].…”