NbSe2 Single Crystals Research Articles

Unravelling NbSe2 single crystal: First principle insights, optical properties, synthesis and X-ray diffraction profile investigation

Niobium diselenide (NbSe2) crystals were synthesized using the chemical vapour transport (CVT) technique. The Generalized Gradient Approximation (GGA) method was used in the density functional theory (DFT) calculation. The Perdew-Burke-Ernzerhof (PBE) pseudopotential function was utilised to emphasise the structural, electrical, elastic constant, and optical characteristics of the hexagonal P63/mmc symmetric structured crystalline phase of NbSe2. Additionally, the density of states (DOS) revealed that, at the Fermi level, the Nb (3d) states predominated, with the Se (5p) and Se (5 s) states playing a minor role. The band structure confirmed the crystal's metallic nature, which showed an overlap between the conduction and valence bands. Scherrer method, Williamson-Hall (WH) analysis, Size-Strain Plot (SSP), and Halder-Wagner (HW) plot utilising uniform deformation model (UDM), uniform stress deformation model (USDM), and uniform deformation energy density model (UDEDM) models were used to investigate the X-ray diffraction (XRD) profile. The crystallite size, inherent stress, strain, and energy density parameters were estimated. Optical microscopy and transmission electron microscopy (TEM) had both been used to study surface morphology.

X-ray photoelectron diffraction as a modern tool for determining surface stacking sequence in layered materials

We investigated the surface structure of a NbSe2 single crystal at room temperature, using angle-scanned x-ray photoelectron diffraction (XPD) combined with multiple scattering calculations. Different stacking sequences were tested (1T, 2H a , 2H c , and 3R), including possible stacking faults and a mixed 2H–3R stacking proposed earlier in the literature. We confirm the capability of XPD to distinguish different proposed structural models and, unambiguously, determine the true surface structure. Also, our findings provide reliable in-plane and interlayer distances. We observed expansions of the perpendicular distances between atomic planes within the monolayer and between monolayers of 3%–5%. These results are important as accurate experimental input for the development of theoretical methods that involve a quantitative description of van der Waals systems.

Peak Effects in 2H-NbSe2 Single Crystals Introduced with Artificial Defects by Particle Irradiation

Effects of splayed columnar defects in NbSe2 single crystals introduced by 800 MeV Xe and 320 MeV Au irradiations were studied. Peak effects have been observed for both samples. For 320 MeV Au irradiated samples, the peak field is found to increase with increasing the splay angle, while in samples irradiated by 800 MeV Xe the peak field shows an opposite dependence. The temperature dependence of the peak field for these two samples are also different. Based on the analysis of pinning force density, this different temperature dependence of the peak field may originate from the different pinning mechanisms.

Peak Effects in 2H-NbSe2 Single Crystals Induced by Particle Irradiations

Effects of three kinds of columnar defects in NbSe2 single crystals introduced by 320 MeV Au irradiation with a dose equivalent matching field B Φ up to 4 T were studied. Through the magnetization measurement using a SQUID magnetometer, two pronounced peak effects in the external field dependence of critical current density (J c) were observed in crystals introduced with splayed and tilted columnar defects. In crystals with splayed columnar defects, J c at the peak field reaches the maximum for splay angle θ CD = ±10°, which is a little different from that found in YBa2Cu3O7 and iron-based superconductors, where optimal θ CD is equal to ±5°.

Moiré superlattice modulations in single-unit-cell FeTe films grown on NbSe2 single crystals* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0302400, 2016YFA0300602, and 2017YFA0302903), the National Natural Science Foundation of China (Grant No. 11227903), the Beijing Municipal Science and Technology Commission, China (Grant Nos. Z181100004218007 and Z191100007219011), the National Basic Research Program of China

Interface can be a fertile ground for exotic quantum states, including topological superconductivity, Majorana mode, fractal quantum Hall effect, unconventional superconductivity, Mott insulator, etc. Here we grow single-unit-cell (1UC) FeTe film on NbSe2 single crystal by molecular beam epitaxy (MBE) and investigate the film in-situ with a home-made cryogenic scanning tunneling microscopy (STM) and non-contact atomic force microscopy (AFM) combined system. We find different stripe-like superlattice modulations on grown FeTe film with different misorientation angles with respect to NbSe2 substrate. We show that these stripe-like superlattice modulations can be understood as moiré pattern forming between FeTe film and NbSe2 substrate. Our results indicate that the interface between FeTe and NbSe2 is atomically sharp. By STM–AFM combined measurement, we suggest that the moiré superlattice modulations have an electronic origin when the misorientation angle is relatively small (≤ 3°) and have structural relaxation when the misorientation angle is relatively large (≥ 10°).

Disordering Induced Second Magnetization Peak Effect and the Vortex Pinning Mechanism in V0.0007NbSe2 Single Crystal

The consequences of the random pinning in the presence of weak disorder are studied systematically based on the appearance of the second magnetization peak (SMP) and peak effect in the magnetic measurement. Here, we have observed the system dependency of the SMP effect. We have analysed the dc magnetization curves and the associated vortex pinning mechanisms of V0.0007NbSe2 single crystal at different temperatures and magnetic fields to estimate its pinning strengths in the external magnetic field H oriented parallel to the crystallographic ab plane. The characteristics of critical current density result in the presence of weak collective pinning near zero-magnetic field and the large bundle collective pinning along with SMP effect at the intermediate magnetic fields. The field dependence characteristics of the pinning force density highlights the presence of Bean-Livingstone’s zero-field peak with the presence of intermediate peak and high-field peak effect phenomena. The temperature dependence of the critical current density shows the dominating nature of δl pinning in the intermediate fields region. The vortex phase diagram of V0.0007NbSe2 is drawn showing different vortex phases at altered temperatures and magnetic fields. Thus, this study depicts the SMP effect and the diverse pinning characteristics in the quasi-two-dimensional superconductor associated with the external defects.

Second magnetization peak effect and the vortex phase diagram of V0.0015NbSe2 single crystal

Effect of the weak point disorder on vortex matter phase diagram is studied by incorporation of V atoms through magnetic and magnetoresistance measurement in layered NbSe2 single crystal. We observe that the point disorder introduces fishtail effect and the second magnetization peak (SMP) in the M-H curve of V0.0015NbSe2 at magnetic field far below upper critical field (Hc2). 3D Collective creep of elastic flux line lattice (FLL) support the vortex motion below the onset of SMP. However, the crossover from collective to plastic deformation of FLL support the occurrence of SMP effect. Presence of the peak effect in pinning force density near irreversible magnetic field (Hirr) indicates the 2D to 3D distortion of FLL due to lattice softening associated with the rapid reduction of elastic modulus. Magneto transport measurement shows the glassy transition near the zero resistance region. In the glassy region, the flux line shows 2D characteristics. Above zero resistive region, thermally activated flux flow region is well described by Arrhenius relation. It is seen that the activation energy is decreased significantly by the incorporation of V atoms in NbSe2. The magnetic field dependence of activation energy follows a power law of U0(H) ~ H-α where the exponent α changed from 0.5 to a 0.9 at a crossover field of Hcr = ~ 0.7 T, indicating the transition from weak plastic deformation of FLL to the strong entangled state in vortex liquid phase. Finally a vortex phase diagram is constructed demonstrating all the different phases of vortex lattice of V0.0015NbSe2 at different magnetic fields and temperatures.

Structural and electroresistive properties of layered compounds based on the 1-2-3 HTSC system and transition metal dichalcogenides under extreme external influences (review)

The problem of the influence of extreme external influences (high pressure, sharp temperature drops, structural relaxation, and strong magnetic fields) on various mechanisms of electric transport of HTSC compounds Re1Ba2Cu3O7-δ (Re = Y or another rare-earth ion) and dichalcogenides of transition metals are considered. The features of the crystal structure and the effect of structural defects of various morphologies on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states are discussed. A review of the experimental data obtained in the study of the effect of high hydrostatic pressure and other extreme effects on various mechanisms of electric transport of Re1Ba2Cu3O7-δ compounds of various compositions and transition metal dichalcogenides of various technological backgrounds is carried out. Various theoretical models devoted to the effect of high pressure on the electrical conductivity of HTSC compounds of the 1-2-3 system and transition metal dichalcogenides are discussed, and a comprehensive comparative analysis of their magnetoresistive characteristics under extreme external influences is performed. In particular, it was shown, that the relatively weak effect of pressure on the Tc value of optimally doped samples can be explained within the framework of a model assuming the presence of a Van Hove singularity in the spectrum of charge carriers which is characteristic of strongly coupled lattices. This is confirmed by the observation similar features of the behavior of the baric derivatives dTc/dP depending on the change composition in NbSe2 single crystals, which also belong to systems of two-dimensional lattices and have a similar anisotropy parameter. Nevertheless, it is still possible to formulate a number of questions that have not yet found a final experimental and theoretical solution. Namely, what is the role of the crystal lattice and structural defects and, in particular, twinning planes? What is the reason for the broadening of the resistive transitions of HTSC compounds into the superconducting state under pressure, and what is the relationship between this broadening and charge transfer and the nature of the redistribution of the vacancy subsystem? What is the role of phase separation in the implementation of different modes of longitudinal and transverse transport? Obviously, more research, both experimental and theoretical, is needed to answer these questions.

Influence of pressure on the transport, magnetic, and structural properties of superconducting Cr0.0009NbSe2 single crystal.

We investigate the superconducting critical current density (Jc), transition temperature (Tc), and flux pinning properties under hydrostatic pressure (P) for Cr0.0009NbSe2 single crystal. The application of P enhances Tc in both electrical resistivity (∼0.38 K GPa−1: 0 ≤ P ≤ 2.5 GPa) and magnetization (∼0.98 K GPa−1: 0 ≤ P ≤ 1 GPa) measurements, which leads to a monotonic increase in Jc and flux pinning properties. The field-dependent Jc at various temperatures under P is analyzed within the collecting pinning theory and it shows that δTc pinning is the crossover to δl pinning above the critical pressure (Pc ∼0.3 GPa). Our systematic analysis of the flux pinning mechanism indicates that both the density of pinning centers and pinning forces greatly increase with the application of P, which leads to an enhancement in the vortex state. Structural studies using synchrotron X-ray diffraction under pressure illustrate a stable hexagonal phase without any significant impurity phase and lattice parameter reduction with P shows highly anisotropic nature.

Study of transport properties in Se-deficient and Fe-intercalated NbSe2 single crystals: experiment and theory

In this study, the magnetoresistance measurements of Se-deficient (i.e., NbSe1.85) as well as Fe-incorporated NbSe2 (Fe0.0015NbSe2) were performed to observe the effect of both intrinsic and extrinsic defect in the thermally activated flux flow region (TAFF) of NbSe2. In TAFF region, NbSe1.85 shows nonlinear response of thermal activation energy (TAE) with temperature following the modified TAFF method. For NbSe2 and Fe0.0015NbSe2, TAE depends linearly on temperature and hence was evaluated using Arrhenius relation. NbSe1.85 can be considered as the 2D-like system in the TAFF region. The magnetic field dependence of TAE shows parabolic nature in Fe0.0015NbSe2 in contrast to the power-law dependence of TAE in NbSe1.85. The power-law dependence of TAE in NbSe1.85 indicates the plastic deformation flux lines. The parabolic dependence indicates the elastic deformation of flux lines in pure as well as in Fe0.0015NbSe2. The band structures and density of states (DOS) of the above mentioned two cases were calculated using first-principle density functional theory. The number of bands and the DOS at the Fermi level decreases remarkably for both Se vacancy and Fe doping cases, indicating to the degradation of superconductivity. A peak shift in the partial density of state of Nb was observed at the Fermi level of Fe0.0015NbSe2. Spin-polarized optimization of first-principle calculations implies large Fe–Se overlaps and contradicts the Kondo mechanism due to the low concentration of Fe atoms. The spin polarization calculation indicates the negligible effect of magnetism of Fe atoms in Fe0.0015NbSe2.

Coexistence of superconductivity and ferromagnetism in defect-induced NbSe2 single crystals

Here, we report the coexistence of superconductivity (SC) and ferromagnetism (FM), which are considered as two exclusive phenomena. We have grown NbSe2 single crystals by chemical vapor transport method. In NbSe2, we have intentionally created Se deficiency (i.e., NbSe1.85) by substitution of Fe atoms (i.e., Fe0.0015NbSe-2). The temperature-dependent resistance measurements confirm that both Se and Fe defects decrease the superconducting transition temperature (Tc) due to the presence of disorder as well as enhancement of scattering phenomena of charge carriers. Subsequently, the temperature-dependent magnetization measurement gives the signature of coexistence of SC and FM in NbSe1.85. Interestingly, there is no such evidence in case of Fe0.0015NbSe2. The Tc and TFM are estimated from the derivative plot of DC susceptibility curves. It reveals that the bulk SC with FM in NbSe1.85 is obtained up to 0.05 T. Further, the M–H plot confirms the coexistence of SC and FM in NbSe1.85 at 2 K. In addition, the upper limit of superconductivity at different temperatures for NbSe1.85 and Fe0.0015NbSe2 is calculated which reflects the superposition region of SC and FM in NbSe1.85 and the suppression of the SC in Fe0.0015NbSe2 as compared to NbSe2. This study not only provides the role of defects in SC and FM properties but also encourages to study the effect of defects on other types of superconducting materials.

Enhancement of superconducting properties and flux pinning mechanism on Cr0.0005NbSe2 single crystal under Hydrostatic pressure

Superconducting properties of Cr0.0005NbSe2 (Tc~6.64 K) single crystals have been investigated through the temperature dependent resistivity (~8 GPa) and DC magnetization (~1 GPa) measurements. Further, the critical current density (Jc) as a function of applied magnetic field has been studied from magnetic isotherms. The vortex pinning mechanisms have also been systematically analyzed using weak collective pinning theory as a function of pressure. The Jc corresponds to the flux flow enhanced by the application of pressure due to increase of Tc and vortex changes. We found that the pressure is responsible for the spatial variations in the charge carrier mean free path (δl pinning). We find that core point pinning is more dominant than surface pinning which is caused by the application of pressure. In addition, Jc(H = 0) increases from 3.9 × 105 (0 GPa) to 1.3 × 106 (1.02 GPa) A/cm2 at 2 K as the pressure is increased from normal pressure to 1.02 GPa. The pressure dependence of Tc (dTc/dP) becomes 0.91 K/GPa and 0.75 K/GPa from magnetization and resistivity measurements respectively. We found that the pressure promotes the anisotropy nature, and decrease of coherence length and resulting in pathetic interface of the vortex core with pinning centers.

Pressure assisted enhancement in superconducting properties of Fe substituted NbSe2 single crystal

The impact of hydrostatic pressure (P) up to 1 GPa on Tc, Jc and the nature of the pinning mechanism in FexNbSe2 single crystals have been investigated within the framework of the collective theory. We found that the pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature (δTc) to the regime controlled by spatial variation in the mean free path (δℓ). Furthermore, Tc and low field Jc are slightly induced, although the Jc drops more rapidly at high fields than at ambient P. The pressure effect enhances the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centers. Moreover, the P can induce the density of states, which, in turn, leads to enhance in Tc with increasing P. P enhances the Tc with the rates of dTc/dP of 0.86, 1.35 and 1.47 K/GPa for FexNbSe2, respectively. The magnetization data are used to establish a vortex phase diagram. The nature of the vortices has been determined from the scaling behaviour of the pinning force density extracted from the Jc–H isotherms and demonstrates the point pinning mechanism.

Proximity-effect-induced Superconducting Gap in Topological Surface States \u2013 A Point Contact Spectroscopy Study of NbSe2/Bi2Se3 Superconductor-Topological Insulator Heterostructures

Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi2Se3 thin films grown on superconducting NbSe2 single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi2Se3 was observed in the spectra, which decreased with increasing Bi2Se3 layer thickness, consistent with the proximity effect in the bulk states of Bi2Se3 induced by NbSe2. At very low temperatures, an extra point contact feature which may correspond to a second energy gap appeared in the spectrum. For a 16 quintuple layer Bi2Se3 on NbSe2 sample, the bulk state gap value near the top surface is ~159 μeV, while the second gap value is ~120 μeV at 40 mK. The second gap value decreased with increasing Bi2Se3 layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi2Se3 thicknesses. It is plausible that this is due to superconductivity in Bi2Se3 topological surface states induced through the bulk states. The two induced gaps in the PCS measurement are consistent with the three-dimensional bulk state and the two-dimensional surface state superconducting gaps observed in the angle-resolved photoemission spectroscopy (ARPES) measurement.

Electrostatic and electrochemical tuning of superconductivity in two-dimensional NbSe2 crystals

We report modulation of the superconducting critical temperature (Tc) of ultrathin niobium diselenide (NbSe2) single crystals by gating an electric double-layer transistor. We realized reversible and irreversible changes of the Tc by adjusting the operating range of the voltage. The reversible and irreversible responses correspond to the electrostatic carrier doping and the electrochemical etching of the crystal, respectively. The results suggest that electric double-layer gating provides opportunities to control and functionalize collective electronic phenomena in two-dimensional crystals.

Orientational coupling between the vortex lattice and the crystalline lattice in a weakly pinned Co0.0075NbSe2 single crystal

We report experimental evidence of strong orientational coupling between the crystal lattice and the vortex lattice in a weakly pinned Co-doped NbSe2 single crystal through direct imaging using low temperature scanning tunneling microscopy/spectroscopy. When the magnetic field is applied along the six-fold symmetric c-axis of the NbSe2 crystal, the basis vectors of the vortex lattice are preferentially aligned along the basis vectors of the crystal lattice. The orientational coupling between the vortex lattice and crystal lattice becomes more pronounced as the magnetic field is increased. This orientational coupling enhances the stability of the orientational order of the vortex lattice, which persists even in the disordered state at high fields where dislocations and disclinations have destroyed the topological order. Our results underpin the importance of crystal lattice symmetry on the vortex state phase diagram of weakly pinned type II superconductors.

Erratum: Disordering of the vortex lattice through successive destruction of positional and orientational order in a weakly pinned Co0.0075NbSe2 single crystal

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that the vortex lattice in a 3-dimensional superconductor disorders through successive destruction of positional and orientational order, as the magnetic field is increased across the peak effect. At the onset of the peak effect, the equilibrium quasi-long range ordered state transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

Optical analysis for few TMDC materials

The transition metal dichalcogenides possess layered structure of Se-M-Se (M = Nb, Mo, Ta and W) sandwich interact with each other by van der Waal forces and can also provide sites for intercalation. Because of their technological importance, lubricants, catalysts, battery cathodes and electrodes in the photoelectrochemical solar cells, much attention has gone in the studies of growth of these materials in crystalline and nanocrystalline forms. In the present work we report the growth of NbSe2, MoSe2, TaSe2 and WSe2 single crystals and determine the optical bandgap using optical absorption. The optical absorption of as-grown crystals has been measured at room temperature near the fundamental absorption edge. Both direct and indirect transitions are involved in the absorption process. The indirect transition was found to be allowed with two phonons involved in the process. The direct and indirect energy gaps and phonon energies for all crystals have been estimated. The results obtained are discussed in detail.

Disordering of the vortex lattice through successive destruction of positional and orientational order in a weakly pinned Co0.0075NbSe2 single crystal.

The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that the vortex lattice in a 3-dimensional superconductor disorders through successive destruction of positional and orientational order, as the magnetic field is increased across the peak effect. At the onset of the peak effect, the equilibrium quasi-long range ordered state transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.

A 350 mK, 9 T scanning tunneling microscope for the study of superconducting thin films on insulating substrates and single crystals

We report the construction and performance of a low temperature, high field scanning tunneling microscope (STM) operating down to 350 mK and in magnetic fields up to 9 T, with thin film deposition and in situ single crystal cleaving capabilities. The main focus lies on the simple design of STM head and a sample holder design that allows us to get spectroscopic data on superconducting thin films grown in situ on insulating substrates. Other design details on sample transport, sample preparation chamber, and vibration isolation schemes are also described. We demonstrate the capability of our instrument through the atomic resolution imaging and spectroscopy on NbSe2 single crystal and spectroscopic maps obtained on homogeneously disordered NbN thin film.