ZrTe5 has recently emerged as a challenging system with unique, albeit poorly understood, electronic properties. Magneto-transport, magneto-infrared and optical spectroscopy studies describe ZrTe5 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. The monolayer is computed to be a quantum spin hall insulator (QSHI) and scanning tunnelling microscopy/spectroscopy (STM/STS) experiments suggest the existence of topologically protected states at step edges. However, an unambiguous identification of the topological phase of ZrTe5 is still lacking.
In this work we have performed a comprehensive experimental and theoretical investigation of the electronic and structural properties of the bulk ZrTe5 in order to answer to this open question.
UV-ARPES measurements have been performed at the APE beamline at Elettra which provides a state-of-the art ARPES endstation, offering high energy (~6 meV) and angular (~0.2°) resolutions. Moreover, the sample dimensions and the presence of multi-domains regions on the surface have made essential the use of the DA30 analyzer which avoids moving the sample during measurements.
UV- (20-36 eV) and SX-ARPES (310-510 eV) measurements reveal the presence of two distinct states at the top of the valence band (VB). On the basis of photon energy and photon polarization dependent studies, we have ascribed the origin of these two states to the bulk and crystal surface, respectively.
Figure 1 shows UV-ARPES data taken at different photon energies. Two states are clearly resolved in proximity of EF. One of them has been recognized as a surface state (SS) since it is not dispersing with the change of photon energy, while the other (which shows a M-like shape at 22 eV and 32 eV, while at 23.5 eV it is nearly degenerate to the SS) has been attributed to a bulk valence band state.
Figure 1. (a) UV ARPES measurement performed at 165 K and photon energy 22 eV along the ka direction. Two states at the top of VB are clearly resolved. (b) and (c) UV ARPES measurements along the ka,direction for 23.5 eV (ΓX) and 29 eV (YX1), respectively. At Y two states are observed, one crossing EF and the other forming a M-like shape reaching its maximum below EF.
Scanning tunneling spectroscopy (STS) reports a gapless density of state for energy above and below EF. Furthermore, the observation of standing waves in the dI/dV conductance maps in proximity of step regions suggests that the metallic density of states at EF arises from a two-dimensional surface state.
DFT calculations have been also performed on ZrTe5. The material lies near a topological phase transition from the STI to the WTI phase that can be triggered by a change of the interlayer distance, b/2, as shown in figure 2. The topological phase transition takes place at a b/2 value of 7.35 Å, which is the unique value in which the 3D Dirac semimetal phase is allowed. In order to find the crystallographic interlayer lattice distance b/2 of the ZrTe5 samples, X-ray diffraction measurements have been performed.
Figure 2. Topological Phase diagram of ZrTe5. Bulk band gap evolution at Γ as a function of the interlayer distance d (black line), X-ray diffraction intensity of the b/2 peak measured at 300 K (blue line).
The XRD experiment has been carried out at the XRD1 Beamline at the Elettra Synchrotron which provides a state-of-the art XRD endstation. The sample investigated in this study shows a value of b/2 = 7.23 Å at room temperature (300 K).
All together, the presence of a linear surface state revealed by UV- and SX-ARPES, the gapless density of states measured with STS, the theoretical prediction of a topological phase transition by changing the interlayer lattice distance and its experimental estimation with the XRD experiment univocally indicate that ZrTe5 lies in the STI phase, in proximity of a topological phase transition.
G. Manzoni,1 L. Gragnaniello,2 G. Autès,3, 4 T. Kuhn,2 A. Sterzi,1 F. Cilento,5 M. Zacchigna,6 V. Enenkel,2 I. Vobornik,6 L. Barba,7 F. Bisti,8 Ph. Bugnon,3 A. Magrez,3 V. N. Strocov,8 H. Berger,3 O. V. Yazyev,3,4 M. Fonin,2 F. Parmigiani,1,5,9 and A. Crepaldi3,5
1 Università degli Studi di Trieste, Trieste, Italy
2 University of Konstanz, Konstanz, Germany
3 Institute of Physics, Ecole Polytechnique Fèdèrale de Lausanne, Lausanne, Switzerland
4 National Center for Computational Design and Discovery of Novel Materials MARVEL, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
5 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
6 C.N.R. - I.O.M., Trieste, Italy
7 CNR - Institute of Crystallography, Trieste, Italy
8 Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland
9 International Faculty - University of Köln, Köln, Germany
Fulvio Parmigiani email: email@example.com
G. Manzoni, L. Gragnaniello, G. Autès, T. Kuhn, A. Sterzi, F. Cilento, M. Zacchigna, V. Enenkel, I. Vobornik, L. Barba, F. Bisti, Ph. Bugnon, A. Magrez, V. N. Strocov, H. Berger, O. V. Yazyev, M. Fonin, F. Parmigiani and A. Crepaldi, Evidence for a Strong Topological Insulator Phase in ZrTe5, Phys. Rev. Lett. 117, 237601 (2016), DOI: https://doi.org/10.1103/PhysRevLett.117.237601