New Superstructure on the Surface of 2H-NbSe<b><sub>2</sub></b>and Tunneling Spectra at 4.2 K

Komori, Fumio; Iwaki, Takao; Hattori, Ken; Shiino, O.; Hasegawa, Tetsuya

doi:10.1143/jpsj.66.298

Cited by 8 publications

(15 citation statements)

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“…The former islands showed characteristic V-shaped density of states with a finite density of states at zero-bias voltage in the dI/dV curve at T = 6 K, a result reminiscent of the spectral features of bulk 2H-NbSe 2 . 18 This island type is probably made of 2H domains. However, the dI/dV curve of type-B islands showed markedly different characteristics; the density of states vanished in a wide energy region of ± 0.2 eV with respect to E F , indicating an insulating nature with an energy gap of~0.4 eV.…”

Section: Resultsmentioning

confidence: 97%

“…[11][12][13][14] In contrast to the intensively studied 2H-NbSe 2 , 15,16 the 1T counterpart has not yet been synthesized either in bulk or as an ultrathin film. Although a 1T-like phase has been observed locally around defects [17][18][19] in thermally treated bulk samples, 20 Ag x Nb 2 Se 3 (Koslowski et al 21 ) and Nb 1 − x Ti x Se 2 (Di Salvo et al 22 ), it has been difficult to selectively fabricate a pristine sample of a single phase for precise physical measurements as well as for device applications. It is thus of great importance to fabricate an ultrathin film of pristine 1T-NbSe 2 and to investigate its electronic states to reveal as-yet-unknown electronic properties.…”

Section: Introductionmentioning

confidence: 99%

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Monolayer 1T-NbSe2 as a Mott insulator

et al. 2016

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The emergence of exotic quantum phenomena is often triggered by a subtle change in the crystal phase. Transition metal dichalcogenides (TMDs) exhibit a wide variety of novel properties, depending on their crystal phases, which can be trigonal prismatic (2H) or octahedral (1T). Bulk NbSe 2 crystallizes into the 2H phase, and the charge density wave and the superconductivity emerge simultaneously and interact with each other, thereby creating various anomalous properties. However, these properties and their interplay in another polymorph, 1T-NbSe 2 , have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1T-NbSe 2 crystal grown epitaxially on bilayer graphene. In contrast with 2H-NbSe 2 , monolayer 1T-NbSe 2 was found to be a Mott insulator, with an energy gap of 0.4 eV. We also found that the insulating 1T and metallic 2H phases can be selectively fabricated by simply controlling the substrate temperature during epitaxy. The present results open a path to crystal-phase engineering based on TMDs.

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Section: Resultsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Monolayer 1T-NbSe2 as a Mott insulator

et al. 2016

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“…At present, it is difficult to estimate quantitatively the anomaly of the density of the states due to the √ 13 × √ 13 CDW from the shape of the tunneling spectra, because the observed shape depends on the distance between the measured point in the domain and its boundary with the intrinsic surface or with the neighboring disordered areas. We found [6] that the asymmetry of the tunneling gap tends to increase with increasing distance from the boundary to the intrinsic surface. As for the proximity of the superconductivity to the domain, the amplitude of the superconducting gap on a domain was found to decrease greatly at a distance of 6 nm from the boundary.…”

Section: Resultsmentioning

confidence: 74%

“…On the intrinsic surface, the spectrum is quantitatively similar to those reported in [8] and [9], and has the expected CDW gap. The superconducting gap was also observed on the intrinsic 2H-surface [6]. On the highly modulated surface, the gap shape is asymmetric, and its width and depth shown in Fig.…”

Section: Resultsmentioning

confidence: 78%

“…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.…”

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confidence: 69%

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

Komori

Iwaki

Hattori

et al. 1998

Applied Physics A: Materials Science & Processing

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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...

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