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Electronic transport and specific heat of 1T- VSe2

The results of low temperature thermoelectric power and the specific heat of 1T-VSe2 (Vanadium diselenide) have been reported along with the electrical resistivity, and Hall coefficient of the compound. The Charge Density Wave (CDW) transition is observed near 110K temperature in all these properties. The Thermoelectric power has been measured from 15K to 300K spanning the incommensurate and commensurate CDW regions. We observed a weak anomaly at the CDW transition for the first time in the specific heat of VSe2. The linear temperature dependence of resistivity and thermoelectric power at higher temperatures suggests a normal metallic behavior and electron-phonon scattering above the CDW transition. The positive thermoelectric power and negative Hall coefficient along with strongly temperature dependent behavior in the CDW phase suggest a mixed conduction related to the strongly hybridized s-p-d bands in this compound.


Introduction
Among the layered compounds of transition metal dichalcogenides, 1T-VSe 2 possesses some peculiar properties. The magnetic and thermal properties at low temperatures suggest a narrow band with exchange enhanced susceptibility [1]. But the conductivity behavior and a transition at 110K in 1T-VSe 2 have similarity to the CDW transition in more covalent polytypes of 2Hvariety of NbSe 2 , TaS 2 and TaSe 2 . [1,2,3,4,5] Hence it suggests that in this layered compound 110K -transition is relatively insensitive to the Fermi surface topology and nesting vectors.
Moreover, the detailed characterization of CDW-related lattice distortion, its phasing in different lattice directions, the location of the onset and commensurate-distortions temperatures T 0 and T d etc., have been controversial in VSe 2 . The successions of incommensurate superlattice were observed much above 110K in an electron diffraction study [6], and a drop in the magnetic susceptibility with hysteresis much above 110K was also subsequently reported [7]. The Nuclear magnetic resonance (NMR) study of CDW state showed a clear pre-transitional broadening of the 51 V quadrupole satellites and 77 Se resonance line [8,9]. A detailed account of the lattice distortions observed in different studies has been given by K. Tsutsumi et al and they reported for the first time a new unusual superstructure at the onset temperature 110K that is commensurate (4a 4a) in the plane of layers, but incommensurate perpendicular to it [10,11].
Up to 1.5% excess of V in the van der Waals sites, does not affect the CDW transition. Similar results have also been reported by us where up to 5% intercalation of Fe does not affect the transition [12]. Although the anomalies in resistance and Hall coefficient are quite clear, these anomalies become rather weak in the magnetic susceptibility, and are controversial in the structural properties [1,2]. On the other hand, before this report, no appreciable anomaly was reported in the specific heat. [13] The application of external pressure enhances the CDW 3 anomalies and transition temperature shifts to higher temperature. This has been explained by Friend and Jerome, as due to the broadening of d-band of V, and the reduction in the coulomb repulsion effects on CDW [14]. The substitutional doping of Nb also induces the same effects, and the magnetic susceptibility results show an increase in transition temperature to 220K for 1.5% of Nb [15].
In the present study, we have investigated the thermoelectric power (TEP), and the Specific heat, of the VSe 2 . The Thermopower results were found to be very appealing and contrary to earlier report [12], show linear temperature dependence above the CDW transition. A weak anomaly in the specific heat at CDW transition has been observed and the electronic and phonon contribution to specific heat have been reported for the first time. We have further explored the role of the strains/impurities on the electrical resistivity of the compound. The low temperature Hall coefficient behavior is also presented to show the mixed conduction behavior in the compound.

Experimental details
The compound was prepared by reacting V chips (99.7%) and 5% excess Se granules (99.999) over required composition in a sealed quartz ampoule at relatively low temperature of 650 0 C in order to reduce self intercalation of VSe 2 . Single crystal flakes were obtained by vapor transport in over 15 days after keeping a pelletized VSe 2 at 700 0 C inside a quartz tube. A large quantity of single crystals of different sizes (some of them as big as 10 × 8 mm 2 ), were obtained between 600 to 650 0 C at the lower temperature end of the tube. The Rietveld refinement of the X-ray diffraction data of the powder sample done using Philips X'pert-PRO Diffractometer, shows the compound to form in the single trigonal phase (Space group: P3 m1) with the lattice parameters a = 3.356Å and c = 6.104 Å. 4 The electrical resistivity, thermoelectric power and Hall coefficient were measured from 10-300K using a APD Close cycle Helium refrigerator unit. Specific heat of the compound was measured from 2-300K by relaxation method using a Quantum Design PPMS (Physical Properties Measurement System).

Thermoelectric Power
The thermoelectric power of the VSe 2 single crystal as well as the polycrystalline sample from 10-300K, is shown in the figure 1. The TEP of the compound shows a broad peak in temperature regime 70-110K below the CDW transition at 110K. Previous studies have attributed this region to be of incommensurate CDW 110K to commensurate CDW state at 70K [1,2,15,16,17]. This anomaly observed in TEP is sharper compared to that observed in the electrical resistivity and the magnetic susceptibility. Like the Hall coefficient, the CDW of the compound is more Surface at the zone corners [18,19,20]. However a positive thermoelectric power can also occur if the electrons of higher energy in the d band (which is narrow) of vanadium are scattered more (or have low smaller relaxation time), compared to the low energy electrons.
As seen from the figure, the TEP of the single crystal sample shows higher peak at the transition compared to the polycrystalline sample, and also the value of TEP at room temperature for single crystal was found to be larger by ~1µV/K. This difference in TEP values for single crystal and 5 polycrystalline sample is rather very small and may vary with the sample quality. The TEP of both the single crystal and polycrystalline samples show linear temperature dependence (slope dS/dT ~ 0.042µV/K 2 ) above the CDW transition. The TEP of VSe 2 measured down to 70K by Bruggen et al for their crystal flake and polycrystalline pellet also shows similar values at room temperature but shows a wave like behavior above the CDW transition [21]. This behavior cannot be accepted as there is no other transition reported in this temperature regime, so their result arises questions about the quality their sample. Compared to their data, our result measured on the good quality crystals shows sharper variation at CDW transition and linear temperature dependence is observed from 110K to 300K. We have already reported similar linear dependence of TEP down to 30K for the Fe intercalated VSe 2 single crystal, where the CDW transition gets suppressed for more than 5% Fe intercalation [12]. which is also a layered system.

Specific Heat
The temperature dependence of the specific heat (heat capacity) of the VSe 2 from 2-300K is shown in the figure 2, which shows a weak but noticeable anomaly at the CDW transition. It should be mentioned here that this anomaly was not yet observed in earlier reports. The inset of 6 the figure shows the enlarged view of the specific heat anomaly. From the data we were able to get the electronic and phonon contribution parameters.
The total heat capacity C p , in the absence of any magnetic contribution is given by C p = C e + C lat = T + T 3 , where C e and C p are the electronic and lattice contribution to the specific heat and and are respective coefficients. The values of , and as determined from the intercept and slope of the C p /T versus T 2 graph in the temperature below 7K, were found to be 7 mJ/mol-K 2 and 0.604 mJ/mol-K 4 respectively. In this low temperature range (T < D /40) C p can be believed to be following the Debye theory of specific heat. The values of , and are well within the range as observed for other transition metal chalcogenides [13].
The Debye temperature D was calculated from the using the relation Where n is the number of atoms per molecules, R is the gas constant (8.31 J/mole-K). This gives a value of D to be 213K, which agrees well with the value 220 5K determined by Kamarchuk et al using the Debye frequency calculation from the point contact spectroscopy [23].

Electrical Resistivity
Resistivity and impurities inside the crystal play a significant role. We believe that these strains also have some temperature dependence, and hence Mattheissen rule is not obeyed in these circumstances.

Hall Coefficient
Hall coefficient was measured on the 8 m thick, crystal flake of VSe 2 , in the magnetic field H sign for R H below the room temperature indicates towards the presence of mixed conduction carriers in the system. However it is not easy to answer and need to be explored further.

Conclusions
The