Tunneling spectroscopy around the boundary of a small impurity phase on the surface of 2H-NbSe 2

Abstract

The structure and tunneling spectra of a cleaved surface of a 2H−NbSe 2 single crystal were studied at 4.2 K in an ultra-high vacuum using scanning tunneling microscopy. Small domains with √ 13 × √ 13 surface modulation were found adjacent to the flat 2H-type surface with the 3 × 3 intrinsic charge density wave (CDW). Using tunneling spectroscopy, density of states around the Fermi energy at the domains was found to be lower than that at the 2H-type surface. The √ 13 × √ 13 modulation is considered to originate from another type of CDW. The observed proximity of superconductivity of 2H-crystal to the highly modulated domain was very small. These results suggest that the crystal structure of the highly-modulated domain is the 4Hd polytype.Charge density waves (CDWs) in transition metal dichalcogenides have been studied for over three decades. Different types of crystal structure have been found, depending on the growth conditions and the materials. Superstructures due to CDW appear at various temperatures and have been investigated mainly by X-ray diffraction [1], and recently by scanning tunneling microscopy (STM) [2]. Among the various transition metal dichalcogenides, the electronic properties of 2H−NbSe 2 have attracted much attention at low temperature because superconductivity with transition temperature T c of 7.2 K coexists with a CDW (T c = 33 K). CDW has not yet been studied in the other polytypes of NbSe 2 , however, since single crystals of these materials could not be obtained at room temperature.A large single crystal of 2H-NbSe 2 is usually grown using an iodine transport method with the growth temperature T growth of about 750 • C. When T growth is increased, crystals with the other polytypes of 4Hd−NbSe 2 (910 • C < T growth < 950 • C) or of 1T−NbSe 2 (T growth > 980 • C) are grown. However, it is very difficult to quench these single crystals to room temperature, and their structures change to the 2H-type [3]. Gavarri et al. [4] reported the existence of 4Ha-type small clusters in a single crystal of 2H−NbSe 2 at room temperature, as determined by analysis of spot profiles of X-ray diffraction. So far, electronic properties of 4Ha−NbSe 2 have been studied only in powder samples, and it is a superconductor with T c of 6.3 K [5].Recently, we have measured the surface of a single crystal of 2H−NbSe 2 using STM, and found small crystals of NbSe 2 whose CDW is stronger than that of the intrinsic 2H−NbSe 2 [6]. In the present paper, we report further STM results including tunneling spectroscopy, and discuss the superstructure due to CDW and electronic properties of small crystals with structure other than 2H-type. MethodsA single crystal of 2H−NbSe 2 was prepared by an iodine vapor transport method in a quartz tube. The growth temperature was in the range of 740-760 • C with a temperature gradient of about 4 • C/cm. This growth condition is almost the same as that of specimens used for the precise structural analysis [4]. We confirmed 2H-type atomic structure by X-ray diffractometry, and sharp superco...