Doping of metal into transition metal dichalcogenides PdTe2

Rh-doped PdTe2 single crystals were successfully prepared using the solid-state reaction. The change of lattice constants and T c with increasing Rh content were studied and investigated. It was found that there is a very slight increase of lattice constant c with increasing Rh content. The values of T c were above 2 K when Rh content is equal to 0.05 and 0.1. The largest T c was~2.7 K for Rh0.05PdTe2 single crystal. T c does not increase with increasing x from 0.05 to 0.1. M-T measurement of Rh0.05PdTe2 single crystal with applied magnetic field H perpendicular to the c axis and along and c axis gives evidence of type II superconducting behavior. This study shows Rh doping is a promising method to increase T c and investigate SC in PdTe2.


Introduction
More and more novel properties have been found in transition metal dichalcogenides (TMDs) in condensed matter physics and technological applications [1][2][3] .Transition metal dichalcogenides range from metals such as VSe 2 , superconductors such as NbSe 2 , semimetals such as TiSe 2 , and cs such as MoSe 2 , to insulators such as HfS 2 [4-8]   .Among these transition metal dichalcogenides, PdTe 2 has attracted great interest from many research groups in recent years owing to the evidence of unconventional superconducting properties [9- 10] .PdTe 2 is expected to show evidence of topological superconductivity as it is found to be a type-II Dirac material and the superconductivity (SC) of the surface in PdTe 2 is possible to be non-trivial topological [11] .Some experiments found evidence of type I superconducting behavior in PdTe 2 while other experiments showed type II superconducting behavior [12][13] .All these novel and contradictory behaviors of PdTe 2 inspired us to investigate more about SC in PdTe 2 .
PdTe 2 crystallizes in the trigonal CdI 2 structure and its transition temperature T c is about 1.7 K [14] .Due to a very low critical temperature (T c ) and the possibility of doping, metal doping has been tried to enhance SC in the layered PdTe 2 superconductor by cation doping such as Cu + .For the intercalation level of Cu 5%, it shows T c = 2.4 K in Cu intercalated PdTe 2 , which is higher than the T c in pristine PdTe 2 [15-16]   .People tried Angle-Resolved Photoemission Spectroscopy (ARPES) measurements of Cu 0.05 PdTe 2 single crystals have been tried to get something about the electronic structure combined with band structure calculations, the results reveal that the band structure shifts downward because of Cu-intercalation as electron-doping [17] .As Cu doping in PdTe 2 could enhance SC, there is the possibility to achieve higher T c by the doping of other metals.
In this report, we studied a new doped system Rh x PdTe 2 to learn more about the doping in PdTe 2 .Rh x PdTe 2 (x=0,0.03,0.05,0.1)were prepared and we cleaved some single crystals to study SC.Single crystals were obtained by a solid-state reaction method and enhanced T c up to 2.7 K for x=0.5 was observed.With further doping from x=0.5 to x=1, increasement of T c wasn't observed.We also tried the M-H measurement for Rh 0.05 PdTe 2 single crystal, which shows a type Ⅱ superconducting behavior.

Experimental details
Polycrystalline Rh x PdTe 2 (x=0, 0.03, 0.05, 0.1) compounds were synthesized via solid-state reaction.Powders of Rh, Pd, and Te were mixed in a stoichiometry ratio and then pelletized by applying uniaxial stress with a tablet press.The pellet was first placed in a quartz tube, then the quartz tube was vacuumed, as shown in Figure 1.Vacuum environment 10 -5 Torr is achieved by vacuum packaging equipment.The whole configuration of the sealed quartz tube was heated from room temperature (RM) to 780 ℃ with a rate of 50 ℃/min in a box furnace and held at 780 ℃ for no less than 10 h, after that the furnace naturally cooled to RM, as shown in Figure 2. The silver chunks were obtained after the heat treatment and the chunks could be ground to dark grey powder.Firstly, the PdTe 2 sample was broken into some pieces and finely ground to get the structural characterization.The structural characterization was done with a Rigaku cometer (RINT 1100) using Cu Kα radiation.After determining the pure PdTe 2 phase, we tried Rh doping and cleaved some single crystals from the Rh x PdTe 2 polycrystalline samples.Then, the diffraction peaks of Rh x PdTe 2 single crystals were obtained via the XRD diffraction technique.Le Bail fitting was tried to fit the XRD pattern of PdTe 2 using the unique GSAS project [18] .Zero field cooled magnetization as a function of T for all the samples has been experimentally measured using a Quantum Design Magnetic Property Measurement System.

XRD patterns of Rh x PdTe 2
The XRD patterns of PdTe 2 are shown in Figure 3.All peaks are indexed with the space group P3̅ -m1 and no impurity peaks are observed.The lattice constants a, b, and c were determined by fitting the XRD pattern with Le Bail fitting.The obtained lattice constants, a and c, were found to be 4.040(6) and 5.13(3) Å, which agrees with [16] .Figure 4 shows the schematic side view of the structure of PdTe 2 using VESTA software.PdTe 2 shows a layered two-dimensional structure.For each layer, the palladium atoms array is sandwiched between two tellurium atoms up and down [16] .The gap between the two layers gives the possibility for doping in this compound.Figure 5 shows XRD patterns of Rh x PdTe 2 (x=0, 0.03, 0.05, 0.1) single crystal.All the peaks are perpendicular to the ab plane direction, and (00l) peaks shift to the lower angle as a function of Rh content.We calculated the f c lattice constant for each Rh content as shown in Figure 6.It shows a slight increase in the c lattice constant with increasing Rh content.

Temperature dependence of susceptibility for Rh x PdTe 2
Figure 7 shows the temperature dependence of susceptibility in ZFC for single crystals of Rh x PdTe 2 (x=0,0.03,0.05,0.1).For x=0 and x=0.3,SC wasn't found in the samples for the measurement above 2 dependence of T c is shown in Figure 8. T c didn't increase with the increase of x from 0.05 to 0.1.This suggests that the enhanced superconductivity does not depend on the charge transfer from the doping.

Conclusion
Rh x PdTe 2 (x=0,0.03,0.05,0.1)single crystals were successfully prepared in this study.For x=0.05 and 0.1, SC could be found for the measurement above 2 K.This suggests SC enhancement could be achieved by Rh doping.T c does not increase with increasing x from 0.05 to 0.1, which shows SC does not depend on the charge transfer from the doping.M-T measurement of Rh 0.05 PdTe 2 single crystal with applied magnetic field along the ab plane direction and c axis shows type II superconducting behavior.This study gives us a new way to enhance SC in PdTe 2 that can help us to learn more about PdTe 2 above 2 K .This study shows the possibility of doping of other metals besides Cu, which provides more chance to study the doping in PdTe 2 .This study also enriches the field of transition metal dichalcogenides.

Figure 1 .
Figure 1.Configuration of the sample.

Figure 3 .
Figure 3. Le Bail refinement plot of PdTe 2 .The observed data are shown by the black crosses.The positions of Bragg reflections are represented using green short vertical lines.The data after refinement are shown as red lines.The blue lines denote the difference between the refined and observed patterns.

Figure 4 .
Figure 4.The schematic side view of the structure of PdTe 2 .

Figure 6 .
Figure 6.The increase of c lattice constant as increasing Rh content.

Figure 7 .
Figure 7. Temperature dependence of susceptibility for Rh x PdTe 2 single crystals in zero field cooling when the applied magnetic field H equals 10 Oe.

Figure 8 .
Figure 8. Rh content dependence of T c for Rh x PdTe 2 single crystals.

3. 3 .
The field dependence of the magnetization of Rh 0.05 PdTe 2 single crystal As we mentioned above, some experiments found evidence of type I superconducting behavior in PdTe 2 when other experiments showed type II superconducting behavior.To learn more about the type I or type II superconducting nature, change of magnetization as a function of the external magnetic field of Rh 0.05 PdTe 2 single crystal was studied, as shown in Figures 9 and 10, with the applied magnetic field H along the ab plane direction plane and c axis.Both in Figures 8 and 9, M-H measurement was tried at different temperatures.The significantly different behaviors for the ab-basal plane and c-axis show a type II superconducting nature of Rh 0.05 PdTe 2 single crystal.

Figure 9 .
Figure 9. M -T of Rh 0.05 PdTe 2 single crystal with external magnetic field along the ab-basal.

Figure 10 .
Figure 10.M -T of Rh 0.05 PdTe 2 single crystal with external magnetic field along the c axis.