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YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-II Weyl semimetal. However, as a representation of the new category of topological matters, the scanning tunneling microcopy (STM) results on such important material are still absent. Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples. A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure. Analysis of STM images yields the information of the index of the vicinal surface. Our results here lay a playground of future atomic scale research on YbMnBi2.
In 1929, H. Weyl found that a massless Dirac fermion could be decomposed into one pair of relativistic particles with opposite chiralities, the Weyl fermion. To date, Weyl fermions have not been discovered in high-energy particle physics. Recently, condensed matter physicists discovered that in certain crystals lacking of space inversion or time reversal symmetry, their low-energy quasiparticle excitations could be described by the Weyl equation.[1–45] Hence such a crystal is named as Weyl semimetal. In a Weyl semimetal, lots of novel phenomena, including nonlocal transport and the chiral anomaly effect, have been proposed.[1–4] More interestingly, scientists found that a Weyl cone in the bulk energy bands of a Weyl semimetal carries a non-zero Chern number and therefore the Weyl semimetal is a new generation of topological phase of condensed matter beyond topological insulators. The topology in a Weyl semimetal manifests itself as the exotic disrupted surface band, which terminates at bulk Weyl nodes, the Fermi arc.[1–8] The first Weyl semimetal material was discovered in the TaAs class of crystals.[6–9] Many important predictions including bulk Weyl cones, surface Fermi arc band structure, negative magnetoresistance (chiral anomaly) phenomenon, and novel quasiparticle interference (QPI) were experimentally observed in this class of materials.[1–5] Research on Weyl semimetals has already become a new frontier of condensed matter physics, materials science, and nanotechnology.
According to whether the Lorentz symmetry is conserved, Weyl semimetals can be categorized into two classes. The TaAs class is named as type-I Weyl semimetal, which is Lorentz invariant and obeys the relativistic quantum filed theory. Additionally, a new type (type-II) of Weyl quasiparticle was discovered in WTe2 class of materials.[39–43] Type-II Weyl fermions break the Lorentz symmetry, and thus can only emerge as low-energy excitations in a condensed matter system. Therefore, the type-II Weyl fermion is a unique phenomenon of crystal. Another important symmetry is time-reversal symmetry. TaAs and WTe2 classes of Weyl semimetals are both time-reversal-symmetry invariant. However, a time-reversal-symmetry breaking Weyl semimetal is also predicted to host several novel effects, e.g., chiral magnetic effect and unconventional superconductivity, which are not possible in the time-reversal-symmetry invariant semimetals.[1–5] Recently, YbMnBi2 was discovered as a type-II time-reversal-symmetry breaking Weyl semimetal.[46] It breaks both time-reversal and Lorentz symmetries, and thus has become a research focus recently.
Scanning tunneling microscope/spectroscope (STM/STS) is an experimental tool with high spatial, energy, and quasi-momentum resolutions. It has been successfully applied to investigate the atomic structure, ionization effect, and quasiparticle interference in many important materials.[47–61] However, for YbMnBi2 samples, which are of importance in both fundamental research and potential electronic applications, no STM results have been reported up to date. In this article, we employed STM to study the morphology on the surface of an YbMnBi2 single crystal.
Our YbMnBi2 samples with a size of about 1 mm×1 mm×0.5 mm were grown by the standard flux method. The samples mounted on the sample plate with their (100) surface exposed were cleaved in a ultra-high vacuum system at about 80 K, and then immediately loaded into a commercial STM (Unisoku 1600) for further characterization at 4.8 K. Electrochemically etched tungsten wires were used as the STM tips.
Figure
Having the morphology in mind, we next seek to understand the different facets. Figure
QPI measurement is a powerful method to detect the scattering property of the charge carriers on a surface of a topological material.[56–60] A QPI map requires the surface electron to form a certain type of two-dimensional standing wave, usually around point defects. However, unfortunately on our samples, the (001) facet only occurs in a small area. We have not observe any defects on the (001) facets. On the vicinal surface, we indeed detect a area with atomically flat morphology with two point defects on it. As demonstrated in Fig.
In our STM experiment on vacuum cleaved YbMnBi2 samples, we find that our samples possess very few amount of defects, thus are of high quality. But the morphology after cleaving is very rough, which renders the QPI type of measurement. We determine that the terrace on the surface is (001) oriented and the incline is (015) facets. The hill and valley morphology is believed to come from the non-layer nature of the crystal structure. Our results provide some guide for the future investigation of the type-II magnetic Weyl semimetal YbMnBi2. In order to obtain a large area of flat (100) surface, a more complicated surface technology, such as sputtering and annealing, is required.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] | |
[34] | |
[35] | |
[36] | |
[37] | |
[38] | |
[39] | |
[40] | |
[41] | |
[42] | |
[43] | |
[44] | |
[45] | |
[46] | |
[47] | |
[48] | |
[49] | |
[50] | |
[51] | |
[52] | |
[53] | |
[54] | |
[55] | |
[56] | |
[57] | |
[58] | |
[59] | |
[60] | |
[61] |