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Hydrogenation of the Transition Metal Dichalcogenide Superconductors


The impact of hydrogen on superconductivity in the transition metals bears some resemblance to the effect hydrogen has on the KH-GIC's. To be convinced of this, compare Figure gif a) with the Tc versus x plot in Figure gif . In both figures Tc at first rises with increasing H content, reaching its maximum near x 0.1, and then falls to an unmeasurably low level at high x. ( Tc ;SPMlt; 0.5 K for H0.87TaS2.[179]. This data point is not shown in Figure gif.) Tc also increases slightly for NbSe2 when a small amount of hydrogen is added.[179]

Figure: Tc increase in TaS2 induced by a) hydrogenation and b) pressure. a) From Ref. [179]. The error bars represent the transition width, while the circles are the volume % superconducting. This experiment was performed on a powder sample. At a hydrogen concentration ofu.87, Tc ;SPMlt; 0.5 K (not shown). b) From Ref. [90]. TCDW is the CDW onset temperature, while Tc is the usual superconducting transition temperature. 4H b and 2H are TaS2 polytypes with different crystal structures.

The resemblance between the C8KHx and the TMDC data turns out to be mostly coincidence since the causes are probably quite different. The hydrogen-induced Tc enhancement in the TMDC's TaS2 and NbSe2 is now known to be due to suppression of a charge-density wave transition that occurs in the unhydrogenated materials.[179,90] The charge-density wave can also be suppressed (and consequently Tc can be increased) by intercalation, pressure, or dopants beside hydrogen.[90] CDW destruction by intercalation was briefly mentioned in Section gif. CDW suppression by pressure is illustrated in Figure gif b) for TaS2. CDW suppression by pressure is a general phenomenon for the CDW's of the layered TMDC's.[90] CDW destruction by impurities is exemplified in the system Nb1-xTixSe3. NbSe3 has Tc < 50 mK, whereas the compound with x = 0.001 has Tc 2.1 K.[91] The extreme sensitivity of the CDW to impurities has understandably resulted in great reproducibility problems with superconductivity in the TMDC's.[91]

The CDW-based explanation for Tc enhancement in the TMDC's is fairly well-established because of the observation of a CDW in several different experiments. For example, Figure gif show discontinuities in the resistivity of TaSe2 due to a CDW transition.[264] There can be no doubt that these high-temperature resistivity anomalies are due to CDW formation since the associated periodic lattice distortion has been directly observed using TEM, neutron scattering, and x-ray diffraction.[90,264]

Figure: In-plane resistivity discontinuities in TaSe2 associated with CDW formation. From Ref. [264]. 1T- and 2H- refer to different polytypes (crystal structures). The CDW transitions occur at 473 K in 1T-TaSe2 and at 117 K in 2H-TaSe2, respectively. Notice that 1T-TaSe2 has a higher resistivity below its transition, whereas the resistivity of 2H-TaSe2 decreases at its transition.

Except for 2H-NbS2, the CDW phenomenon occurs in all polytypes of all members of the class MX2, where M = V, Nb, or Ta, and X = S or Se.[90] Of these materials, only 2H-NbS2 does not show a large Tc enhancement with pressure. Since NbS2 is also the only material which does not have a CDW transition, its comparatively small dTc/dP ( only 5× 10-6 K/bar) is evidence that CDW suppression is the cause of the dTc/dP in the other MX2 materials.[90,217] A host of experiments show that perturbations (such as pressure, hydrogen, and impurities) tend to rapidly increase Tc up to the point where the CDW is suppressed, whereas they affect Tc only slowly above the CDW transition. For example, NbSe2 has a large dTc/dP (4.95 × 10 -5 K/bar) for pressures below 35 kbar, the pressure where the CDW is completely suppressed. Above 35 kbar, the slope is only dTc/dP = 2.8 × 10-6 K/bar, even smaller than that in NbS2.[217] Obviously this is no coincidence.

The thrust of many of the superconductivity experiments on the TMDC's is that there is a competition between the superconducting and CDW transitions, so that preventing one condensed state encourages the other one. This line of reasoning says that once the CDW is completely suppressed by a perturbation, a material reaches its intrinsic magnitude of Tc.[91] By ``intrinsic'', one means the Tc value that would be expected from the usual theories given thetaD, Lambdaep, etc. Further application of the perturbation may then either increase or decrease Tc, depending on the properties of the individual superconductor.

The reason that CDW's and superconductivity compete with one another is quite simple. In the BCS theory of superconductivity,[16] the condensation energy of the superconducting state is 1/2 N(0) Deltas2, where Deltas is the superconducting energy gap. In a similar mean-field theory of the charge-density wave transition, the condensation energy is also proportional to N(0) DeltaCDW2 (with some additional multiplicative factors),[204] where DeltaCDW is now the CDW energy gap. The juxtaposition of these two expressions for the condensation energy immediately shows why the two phases tend not to coexist: they both need to create a gap at the Fermi surface. Once the Fermi surface has a gap above it, any further phase transition driven by an instability of the Fermi sea is completely suppressed.

In real metals, a CDW transition tends to gap only a portion of the Fermi surface, leaving some condensation energy available for use in a superconducting transition. Thus a high-temperature CDW transition tends merely to lower Tc and not to altogether prevent superconductivity. Bilbro and McMillan found a relationship between the amount that the superconducting transition temperature is lowered by a higher-temperature CDW phase transition and the amount of Fermi surface which is removed in that transition.[23] The relationship is:


where Tc is the CDW-suppressed superconducting transition temperature, Tc0 is the intrinsic transition temperature, TCDW is the CDW onset temperature, and N1/N is the fraction of the density-of-states removed in the CDW transition. Fuller, Chaikin, and Ong[91] obtained N1/N as a function of pressure P in NbSe3 by measuring the size of the resistivity discontinuity at TCDW(P). The result is N1/N = (0.6 - 0.18p), where p is the pressure in kbar. Fuller and coauthors[91] used these numbers in conjunction with Eqn. gif to fit their Tc(P) data, and found good agreement at low pressures.

This agreement is convincing evidence that CDW-suppression of superconductivity is indeed due to the competition of the two condensed phases for the Fermi surface. Any perturbation of the materials that tends to suppress the CDW, such as pressure, hydrogenation, or intercalation, will therefore also tend to increase Tc. The enhancement of Tc by hydrogenation in the TMDC's obviously is due to a very different mechanism than that which has been put forward for the transition metals and the alkali-metal GIC's. Since hydrogen first increases and then depresses Tc in C8K, one might wonder whether CDW's might also play a role. This possibility is discussed below in connection with the results on the KHg-GIC's. Before relating the outcome of the hydrogenation experiments on the KHg-GIC's, the details of the experiments are briefly described.

next up previous contents
Next: Experimental Methods: Hydrogenation Up: Hydrogenation and Superconductivity Previous: Superconductivity in the (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995