The layered transition metal dichalcogenides (MX2) are an interesting class of materials which exhibit unusual physical properties such as charge density waves (CDW) and suppressed superconductivity due to the lattice instability . In particular, the physical property of "Van der Waals gap" appears due to the repulsive interaction between the anions of layers in the compound. Among these compounds, MoS2 exhibits semiconducting behavior with the direct band gap of 1.8eV at room temperature but becomes an insulator at low temperature. The superconductivity of TC≈4−10K could be induced either by the intercalation of alkaline earth metals such as K+ or by the electrical current applied in the field-effect-transistor (FET) . Among the most reported MX2 compounds, it is hard to tune the superconducting transition temperature (TC) via the cation intercalation because a CDW occurs due to the structural instability . For the compound of PdTe2 with CdI2-type crystal structure, it is interesting and attractive because the Fermi level, EF, is located at the valley of density of states (DOS) , indicating the existence of a low carrier density and it is relatively easy to tune the TC. Hence, it is important to verify if the cation intercalation, such as Cu+ in the parent PdTe2, could effectively modulate the TC. Here, we report the crystal structure and superconductivity in the intercalated system of CuxPdTe2. The resistivity and magnetization measurements were carried out using a SQUID-VSM magnetometer (Quantum Design) under different fields at temperature range between 1.7 and 300K.
High quality single crystals with size of ≈0.5×1.5cm2 were obtained by the self-flux method. The results of the powder and the (00l) XRD patterns for the undoped PdTe2 single crystals are shown in Figs. 1(a) and 1(b), respectively. The crystal structure is identified to be the CdI2-type trigonal structure P-3m1 with lattice parameters of a=4.0401Å and c=5.1417Å, estimated by Rietveld refinement using FullProf program, respectively. The XRD patterns confirm a pure phase of the single crystals obtained. The inset of Fig. 1(b) is the photo of PdTe2 single crystal and the triganol Laue pattern, respectively. A schematic view of the top and side for the crystal structure of PdTe2 is shown in Figs. 1(c) and 1(d), where Pd and Te atoms have 4 coordination numbers. This compound has Van der Waals gap sandwiched between the Cu−Te layers. When Cu is intercalated between the layers, it favors three dimensional (3D) chemical bonding of Te−Cu−Te within Van der Waals gap.
The (00l) XRD patterns for different Cu dopings in PdTe2 single crystals is shown in Fig. 2(a). The (00l) peaks are systematically shifted toward higher angles with the increasing of Cu doping level to 7% and then shifts to low angles from a Cu doping level 10 to 15%. Inset of Fig. 2(a) is the enlarged shift of (001) peaks. Figure 2(b) plots Cu doping dependence of the lattice parameters of c and a, respectively. The shrinkage of c parameters from Cu 3−7% can be attributed to the Cu+ ions incorporated into the octahedral site in between the Te−Te doble layers, forming a new chemical bonding of Te−Cu with the hybridization between Te p- and Cu s-orbitals. The expand of c from Cu 7% to 15% is ascribed to appear the covalent bonding of Pd−Te, indicating that Cu 7% is a critical value of the bonding change from Te−Cu to Pd−Te. The a-axis is observed gradually decreasing with increasing Cu doping content, due to the compressive strain of lattice by Cu intercalation.
The temperature dependence of resistivity for CuxPdTe2 was measured between 1.9K and 300K, as shown in Fig. 3(a). All of the Cu-doped samples are superconducting and TC increases with the Cu doping content to 5% and then decreases from Cu 7% to 15%. The optimal doping level is Cu 5%, showing the highest TC at 2.6K, as shown in Fig. 3(b).
Fig. 3: (a) Temperature dependence of the electric resistivity (ρ) for CuxPdTe2. (b) Electrical resistivity for Cu0.05PdTe2. Inset is the enlarged region of electrical resistivity at near onset superconducting temperature. (c) Magnetic field dependence of electrical resistivity for Cu0.05PdTe2 (d) Temperature dependence of magnetic susceptibility in the zero-field cooling (ZFC) for CuxPdTe2 measured under 10 Oe. Bottom inset is the enlarged region of ZFC at near onset superconducting temperature. Upper inset is the magnetic hysteresis loop for Cu0.05PdTe2 at 1.74 K.[less]
Fig. 3: (a) Temperature dependence of the electric resistivity (ρ) for CuxPdTe2. (b) Electrical resistivity for Cu0.05PdTe2. Inset is the enlarged region of electrical resistivity at near onset superconducting temperature. (c) Magnetic field dependence of electrical resistivity for Cu0.05PdTe2 (d) Temperature dependence of magnetic susceptibility in the zero-field cooling (ZFC) for CuxPdTe2 measured under 10 Oe. Bottom inset is the enlarged region of ZFC at near onset superconducting temperature. Upper inset is the magnetic hysteresis loop for Cu0.05PdTe2 at 1.74 K.
The thermal coefficient α is estimated by the power-law equation ρ(T)=ρ0+ATα (ρ0 is a residual resistivity, A is a constant) and plotted in inset of Fig. 3(a). The variation of α indicates that superconducting transition temperature is sensitive to their carrier density at EF. In general, as the electron density at EF increases, the Fermi-Liquid (FL) state (α≈2) is preferred in the metallic compound but the superconductivity is subsequently suppressed in the excess intercalation region, i.e., inducing excess carries. The superconducting TC at 2.6K with α≈1.0 can be clearly distinguished for the Cu 5% intercalated PdTe2, which gives rise to the optimal TC. Figure 3(c) shows the ρ-T data measured under the various external magnetic fields. The onset superconducting TC drops as the external magnetic field (H) increases and vanishes at H = 300 Oe. The temperature dependence of the magnetic susceptibility measured under an external magnetic field of 10Oe is shown in Fig. 3(d). Data were also collected from the zero field cooling (ZFC) measurement. The shielding volume fraction (SVF) for Cu 5% at 2K was ≈10%. The ZFC curve at around 2K did not saturate because this temperature region is near to the onset transition temperature range. To study the SVF at the saturated temperature region, we measured the magnetization (M) versus magnetic field (H) after decreasing the temperature down to 1.74K using a slow cooling down technique. As shown in the upper inset of Fig.3 (d), the SVF estimated to be ≈111% from the M-H curve at 1.74K, which indicates Cu0.05PdTe2 is the bulk and type-II superconductor under Hc2≈30Oe.
In summary, large single crystals of CuxPdTe2 superconductor with size of 1.5×0.5cm2 were successfully obtained by the conventional self-flux method. The bulk superconductivity in the Cu 5% intercalated PdTe2 is observed with the shielding volume fraction of ≈111%. The superconducting transition temperature TC is systematically tuned by the intercalation of Cu atoms. These results indicate that the CuxPdTe2 is the intercalation-based layered superconductor.