Abstract
The pentatellurides, ZrTe5 and HfTe5 are layered compounds with one dimensional transition-metal chains that show a never understood temperature dependent transition in transport properties as well as recently discovered properties suggesting topological semimetallic behavior. Here we show that these materials are semiconductors and that the electronic transition is due to a combination of bipolar effects and different anisotropies for electrons and holes. We report magneto-transport properties for two kinds of ZrTe5 single crystals grown with the chemical vapor transport (S1) and the flux method (S2), respectively. These have distinct transport properties at zero field: the S1 displays a metallic behavior with a pronounced resistance peak and a sudden sign reversal in thermopower at approximately 130 K, consistent with previous observations of the electronic transition; in strikingly contrast, the S2 exhibits a semiconducting-like behavior at low temperatures and a positive thermopower over the whole temperature range. Refinements on the single-crystal X-ray diffraction and the energy dispersive spectroscopy analysis revealed the presence of noticeable Te-vacancies in the sample S1, confirming that the widely observed anomalous transport behaviors in pentatellurides actually take place in the Te-deficient samples. Electronic structure calculations show narrow gap semiconducting behavior, with different transport anisotropies for holes and electrons. For the degenerately doped n-type samples, our transport calculations can result in a resistivity peak and crossover in thermopower from negative to positive at temperatures close to that observed experimentally. Our present work resolves the longstanding puzzle regarding the anomalous transport behaviors of pentatellurides, and also resolves the electronic structure in favor of a semiconducting state.
Highlights
The pentatellurides ZrTe5 and HfTe5 are layered materials [1,2] with a high concentration of the heavy p-element Te
The results for chemical vapor transport (CVT) and Flux shown here are representative of the single crystals made using different methods and are found to be reproducible by checking several crystals for each method
For the CVT samples, ρðTÞ initially decreases slightly and exhibits a broad peak centered around Tp ≈ 132 K, below which a first demonstrate similar effects of magnetic fields on the (a) typical metallic behavior was recovered; the thermopower attains a large, positive value of about 200 μV=K at room temperature, and it undergoes a dramatic sign change crossing zero at Tp. All these observations in CVT are consistent with previously published results [3,5]
Summary
The pentatellurides ZrTe5 and HfTe5 are layered materials [1,2] with a high concentration of the heavy p-element Te. Possible causes for this behavior were advanced These include density waves [3], inconsistent with the diffraction and high magnetic field data [4]; polaronic models [7], which are, apparently inconsistent with the good low-temperature conduction; a semimetal-semiconductor phase transition [8] or a temperature-induced Lifshitz transition [9,10]. These compounds have become of interest as topological materials whose low-energy electronic structure is controlled by spin-orbit interactions [11]. Our present work may resolve the long-standing puzzle regarding the anomalous transport behaviors and the question of the electronic structure in favor of a semiconducting state
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