Formation of metallic NbSe2 nanotubes and nanofibers

We succeed in synthesizing NbSe₂ nanotubes along with nanofibers by chemical vapor transportation. They are stable crystalline systems and can be synthesized reproducibly in a nearly equilibrium reacting process. We have investigated these nanosize structures of NbSe₂ by transmission electron microscopy and electron diffraction. Both of the structures have a similar size of 100–200 nm in diameter. While nanotubes consist of rolled-up NbSe₂ layers, nanofibers are a pile of thin flat layers. We propose a mechanism of the formation of NbSe₂ nanotubes and nanofibers on the basis of deseleniditive transition from a NbSe₃ fiber-shaped crystal. We also measured electrical resistance of the nanofibers with conductive atomic force microscopy and demonstrated that the material show metallic behavior at room temperature.     

Magnetic susceptibility of NbSe3 and TaSe3

The magnetic susceptibility of NbSe₃ shows a decrease beginning slightly above its upper charge density wave transition (CDW) of 144 K, but no change within our resolution near the 59 K transition. The change in the density of states at the Fermi level due to the upper transition is 0.14 states-eV/Nb. TaSe₃ on the other hand has a temperature independent susceptibility. In some cases the trichalcogenides are contaminated with their corresponding dichalcogenide. Such contamination can be observed by susceptibility measurements in the case of 2HTaSe₂ but not of 2HNbSe₂. We also report an anomaly in the susceptibility of 4HaNbSe₂, which suggests a CDW transition at 45 K.     

Elastic constants of 2H-MoS2 and 2H-NbSe2 extracted from measured dispersion curves and linear compressibilities

Recent neutron data on the dispersion curves and X-ray measurements of the linear compressibilities of the 2H polytypes of MoS₂ and NbSe₂ have been used to obtain approximate values of the five independent elastic constants of these materials. In the case of NbSe₂ sufficient information is available to over-determine the elastic constants and the results are self consistent within estimated uncertainties, although the uncertainties are especially large for c₃₃ and c₁₁. Additional related considerations such as Debye temperatures and model calculations of c₃₃, and c₄₄ are also made. It is found that there is significant and unexplained disagreement between the value of the low temperature specific heat Debye temperature of NbSe₂ and the value determined on the basis of the elastic constants, but that the model predictions of c₃₃ and c₄₄ are in satisfactory agreement with the values extracted from the neutron data for both MoS₂, and NbSe₂.     

Phenomenological theory of the increase in the superconducting transition temperature in thin film layered sandwiches

A phenomenological theory is presented to account for the observed increase in the critical temperature, as a function of the number of layers, of multilayer sandwiches composed of alternating superconducting and insulating layers. The transition temperature of the layered compound NbSe₂ when cleaved down to a small number of layers and the pressure dependence of the transition temperature of bulk NbSe₂ can also be fitted into this scheme.     

Low temperature phase transition in the chain-like compounds NbSe3 and TaSe3

New transitions have been observed at low temperature by electrical measurements on the linear trichalcogenides NbSe₃ and TaSe₃. The resistivity of TaSe₃ drops near zero below 1.5 K which may indicate a superconducting transition. The resistivity of NbSe₃ drops below 2.2 K by 30 to 75% of its residual value at 4.2 K. However for TaSe₃ and NbSe₃ initial susceptibility measurements do not show any increase in diamagnetism indicating a flux expulsion. In addition the transitions below the critical temperatures are strongly non-linear with current density higher than 10^-3 A mm^-2.     

Optical transitions in group V transition metal dichalcogenides

A comparison of the correspondent structures of thermoreflectance spectra in NbSe₂, TaSe₂, NbTe₂ gives indications in favour d-d transitions as responsible of the main features of the optical spectra at relatively low energies. Besides, a temperature broadened saddle point in the joint density of states in NbSe₂ is identified.     

Ultrasonic-assisted top-down preparation of NbSe2 micro/nanoparticles and hybrid material as solid lubricant for sliding electrical contact

2H-NbSe₂ single crystal flake (ca. 2 × 2 × 0.5 mm in size) by chemical vapor transport is employed as the precursor for a top-down preparation of NbSe₂ micro/nanoparticles by two kinds of processes, i.e. (1) mechanical exfoliation; (2) ultrasonic-assisted exfoliation in ethanol without ageing and with ageing for 210 days. NbSe₂ micro/nanoparticles are applied on top of a Cu disk by a drop-casting process and the tribological property in sliding against a Cu pin under sliding electrical contact is investigated at room temperature. Mechanical exfoliation produces NbSe₂ microplatets with typical sizes of 1 μm to 30 μ m with a thickness less than 2 μm. Ultrasonic-assisted exfoliation without aging facilitates the formation of NbSe₂ micro/nanoplatets with sizes of 0.1 μm to 25 μm and nano-whiskers with 100 nm in diameter and 1 ~ 3 μm in length, but Nb₂O₅ and Se are also found on the basis of XPS results. Prolonged aging of the suspensions modifies the morphology by converting platets and whiskers into corrugated floccules (hybrid material), which are composed of Nb₂O₅, Se, NbSe₂, and graphene. Notably, NbSe₂ micro/nanoparticles by ultrasonic-assisted exfoliation without ageing exhibit an excellent lubricating property with low friction coefficient (0.3), mild wear, and longer wear lifetime (120 min) than that of mechanical exfoliated NbSe₂ microplatets (10 min). The wear lifetime for the aged NbSe₂ micro/nanoparticles can be as long as 504 min and are 4.2 times of the sample without aging, which can be a good solid lubricant for sliding electrical contact     

Raman spectrum of metallic layered compound NbSe2

Raman spectrum of layer-type compound NbSe₂ has been obtained at liquid nitrogen temperature. The frequencies of the Raman active modes E²_2g, A_1g and E¹_2g are measured to be 29.6 cm^?1, 230.9 cm^?1 and 238.3 cm^?1 respectively. The observed second order Raman spectrum of NbSe₂ is very different from the corresponding spectrum of MoS₂. These results show that the force constants of NbSe₂ are less anisotropic than those of MoS₂.     

超导材料NbS2上临界磁场的理论分析

根据两带Ginzburg-Landau 理论计算了层状超导材料NbS₂的上临界磁场, 以及上临界磁场各向异性参数随温度的变化情况, 并将NbS₂与MgB₂, NbSe₂上临界磁场的各向异性进行比较. 所得计算结果与已有实验数据符合得很好, 充分说明了NbS₂的超导电性具有两能隙特征. NbS₂上临界磁场各向异性参数在5.0 K附近逐渐变小, 这与MgB₂和NbSe₂有相似之处. 但NbS₂的上临界磁场各向异性参数大约为7.3, 明显大于MgB₂和NbSe₂. 计算结果还表明, NbS₂较大能隙所对应能带的有效质量比约为54, 另一能带的有效质量基本为各向同性.     

Superconductivity in Nb.95Ta.05Se3

We have measured the resistivity of NbSe₃ doped with 5% Ta from room temperature down to 0.5K and compared our results with similar measurements on pure NbSe₃. The pure sample remains normal to the lowest temperature (0.5K), whereas the doped sample has a sharp transition to the superconducting state with T_c = 1.5 ± 0.2 K. Measurements of the critical magnetic field indicate that the Ta doped samples are homogeneous, anisotropic three dimensional superconductors.     

Effect of charge density wave on thermoreflectance spectra of 2HNbSe2

Thermoreflectance curves of 2HNbSe₂ below and above the ICDW transition temperature are presented. Modifications through the transition temperature are negligible and this is attributed to a smaller value of the CDW gap with respect to the one in 2HTaSe₂.     

Planar graphene Josephson coupling via van der Waals superconducting contacts

We report on the fabrication and transport characteristics of van der Waals (vdW)-contacted planar Josephson junctions. In a device, two pieces of cleaved 2H-NbSe₂ superconducting flakes and a monolayer graphene sheet serve as the superconducting electrodes and the normal-conducting spacer, respectively. A stack of NbSe₂?graphene?hexagonal-boron-nitride (hBN) heterostructure with clean and flat interfaces was prepared by a dry transfer technique. The outermost hBN layer protected the NbSe₂?graphene?NbSe₂ Josephson junction from chemical contamination during the fabrication processes. The Josephson coupling was confirmed by a periodic modulation of the junction critical current $I_{\text{c}}$ in a perpendicular magnetic field. The temperature dependence of $I_{\text{c}}$ showed long and diffusive Josephson coupling characteristics. The temperature dependence of the superconducting gap, obtained from the multiple Andreev reflection features, followed the Bardeen?Cooper?Schrieffer (BCS) prediction.     

Inhomogeneous superconducting state in the overdoped regime of La2−xSrxCuO4: Comparison with the superconducting state of NbSe2

We have measured the temperature dependence of the magnetic susceptibility, χ vs. T, and the magnetization curve, M vs. H, for NbSe₂ single crystals, in order to compare the superconducting (SC) state in the overdoped regime of La_2−xSr_xCuO₄ (LSCO) with the SC state of the layered conventional superconductor NbSe₂. While a plateau in χ vs. T in a moderate magnetic field and a so-called second peak in M vs. H, which is due to the marked enhancement of vortex pinning, have been observed in the overdoped regime of LSCO, these behaviors have not been observed in NbSe₂. The present results indicate that the anomalously marked enhancement of vortex pinning is a characteristic feature in the overdoped LSCO where a microscopic phase separation into SC and normal-state regions takes place.     

Mass production of graphene-like single-crystalline NbSe2 (004) nanosheets via intercalant-assisted thermal cleavage

The author reports the mass production of dense inorganic graphene-like exfoliated single-crystalline niobium diselenide (NbSe₂) (004) multilayered nanosheets standing on quartz substrate. Similar to the most-used micromechanical cleavage to exfoliated NbSe₂ nanosheets, the synthesis was performed starting directly from bulk powdered counterparts, but using a novel type of substrate-irrelevant vapor-phase exfoliation. A possible intercalant-assisted thermal cleavage growth mechanism for the NbSe₂-derived nanosheets, including a combination of intercalant injection-atomic delamination-thermal exfoliation-lateral growth, is discussed. This two-component nanosheet-forming technique appears to potentially work on homologous series of sandwich-type inorganic layered 2H transition-metal dichalcogenide compounds under proper experimental conditions as well.     

Anomalous resistivity of iodine-free 2H-NbSe2

Resistance ratios of crystals of 2H-NbSe₂ grown with and without iodine were measured over the temperature range 6K–40K. Iodine-free crystals had higher ratios than any NbSe₂ previously reported and exhibited an abrupt anomaly at the onset of the charge density wave phase. All crystals displayed a cubic temperature dependence of their resistivities below 18K.     

Phase transitions in NbSe3

We describe the physical properties of the transition metal trichalcogenide NbSe₃ including electrical resistivity, magnetic susceptibility, and heat capacity. NbSe₃ undergoes phase transitions at 145 and 59 K. The effect of pressure on these transitions is also reported. We try to explain the physical properties of NbSe₃ in terms of the formation of charge density waves.     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.      

Incommensurate superlattice reflections in quasi one dimensional conductors, (MSe4)2I (M=Ta and Nb)

X-ray diffraction and conductivity measurements have been made on (MSE₄)₂I, (M=Ta and Nb). A phase transition of (NbSe₄)₂I was found and the transition temperature T_c was determined to be about 210K by the conductivity measurement. For (TaSe₄)₂I, T_c was about 260K as reported previously. Superlattice reflections due to the formation of charge density wave (CDW) in (TaSe₄)₂I were observed at points $\text{q}\text{?}\text{=(±0.05, ±0.05, ±0.085)}$. The type of CDW of (NbSe₄)₂I is known to be identical to that of (TaSe₄)₂I. Temperature dependence of the threshold electric field of nonlinear conductivity is consistent with the result in previous reports.     

Charge density wave transport in (TaSe4)2I

We report non linear transport properties below the metal-insulat transition temperature T_° = 263 K in the halogened metal transition tetrachalcogenide (TaSe₄)₂I. These non linear properties are similar to those of NbSe₃ and TaS₃ and indicate that (TaSe₄)₂I is a new compound exhibiting charge density wave transport.     

Photoemission studies of Cu intercalated NbSe2

We report growth and angle resolved photoemission studies of Cu intercalated NbSe₂. Cu_xNbSe₂ with x=0, 0.04 and 0.08 were grown by iodine vapor transport method. Intercalation and doping was confirmed by the change in the c-axis lattice constant and the change in the electronic structure, respectively. To investigate the nesting condition, we performed auto-correlation analysis on the data. We find that the magnitude of the nesting vector decreases with doping but the direction remains unchanged. Our observation is consistent with the nesting coming from intra-pocket transition.