Study On Transport Properties Of Several Niobium-based Van Der Waals Compounds

Due to the weak interaction between layers of van der Waals materials,thin layer films can be obtained by mechanical exfoliation from their counterpart bulk samples with high quality.The isolated layer units can be designed and constructed in various superlattices or two-dimensional devices,thus this kind of material has attracted extensive attention.In addition,single-crystal thin-films of the materials that are not easily obtained by other growth methods,such as molecular beam epitaxy or atomic layer deposition,can be obtained by mechanical exfoliation,which greatly expands the research scope of thin film materials.In this paper,the physical properties of Nb-based van der Waals materials,such as Nb Se₂ and Nb₂Si Te₄,are studied in detail.Meanwhile,chemical doping and gate-tuning technology are used to tune their physical properties.The main research results are as follows:1.Using the newly-developed solid ionic gating technique,we measured the electrical transport properties of a thin-flake Nb Se₂ superconductor(T_c≈6.67 K and T_CDW≈33 K)under continuous Li-ion intercalation,which resulted in an electron doping effect.It is found that the charge density wave transition is suppressed with the Li-ion doping,and concurrently hole carrier density decreases from 7×10¹⁴ cm^-2 to2×10¹⁴ cm^-2.The tunable ability in carrier density is about 70%,which is 5 times larger than that in the liquid ionic gating method.Meanwhile,we find that the scattering-type of conduction carriers transits from the electron-electron scattering to the s-d scattering process,which may be caused by the change of the occupied states belonging to 4d-orbitals in Nb atoms under Li intercalation.Simultaneously,we observe a certain decrement of electron-phonon coupling based on the electron-phonon scattering model at the high temperature range.Based on the data from in situ measurements,we construct a full phase diagram of carrier density,electron-phonon coupling,and superconducting critical temperature T_c derived from the intercalated Nb Se₂ material and qualitatively explain the variation of T_c within the Bardeen-Cooper-Schrieffer theory framework.We consider that the in situ solid ionic gating method provides a direct way for revealing the relationship between carrier density and superconductivity,which is helpful in clear understanding the electronic phase competition that existed in transition metal dichalcogenides.2.Two-dimensional van der Waals material is a hot issue because of its potential widespread application in optoelectronics,memories,and magnetics.The ternary compound Nb₂Si Te₄ is a van der Waals semiconductor with excellent air stability and small cleavage energy,which is suitable for preparing a few layers counterpart to explore novel properties.Here,properties of bulk Nb₂Si Te₄ with ambipolar carriers transport and large in-plane electrical anisotropy are demonstrated.It is found that hole carriers dominate at a temperature above 55 K with a carrier active energy of 31.3 me V.Below 35 K,electronic transport behavior gradually appears.The carrier mobility measured at 100 K is about 213 cm² V^-1 s^-1 in bulk Nb₂Si Te₄.In a thin flake Nb₂Si Te₄,the resistivity ratio between the crystalline axes of a and b is reaching about 47.3 at 2.5K,indicating a large anisotropic transport behavior,consistent with their great difference in carrier effective mass along the two directions predicted by calculation.These transport properties are demonstrated here to provide accurate information for modulating or utilizing the ambipolar carrier properties of Nb₂Si Te₄ for applications.3.To search for high electron mobility materials in the Nb₂Si Te₄ based material system,Mo-doping into Nb₂Si Te₄ single crystal was performed.We expect that the electron carriers dominated semiconductors can be obtained through the slightly up-shift of the Fermi level.Nb_2-xMo_xSi Te₄（x=0.07,0.15）single crystals were fabricated by flux method.The analysis of X-ray photoelectron spectroscopy shows that the doped Mo atoms can effectively reduce the proportion of the highly oxidized state of Nb⁵⁺,indicating that the doped Mo atoms may replace the position of the Nb atoms.At the same time,the resistivities of two kinds of Mo-doped single crystal samples were measured.It was found that with the increase of Mo content,the conduction behavior quickly changed from semiconducting to a metallic behavior.Furthermore,compared with the Nb₂Si Te₄,the main carriers in Mo-doped samples are electrons,and the carrier concentration is about 10¹⁸ cm^-3.In addition,we also try to use the gate-tunable method to intercalate Li-ions into Nb₂Si Te₄ thin-flakes to tune the Fermi level of Nb₂Si Te₄ in a wide range.It is found the resistivity of Nb₂Si Te₄ thin-flakes also changed from semiconducting to metallic behavior,covering the possible doping range of samples with high electron mobility.