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Monotonic Increase of Upper Critical Field with Hole Doping in High-Tc Cuprate Bi2+xSr2-xCaCu2O8+d


Core Concepts
The upper critical field Hc2(0) of high-Tc cuprate Bi2+xSr2-xCaCu2O8+d increases monotonically with hole doping from the underdoped to the overdoped regime, in contrast to the parabolic doping dependence of the critical temperature Tc.
Abstract

The authors investigated the temperature (T) dependence of the irreversibility field Hirr(T) in high-critical-temperature cuprate Bi2+xSr2-xCa1-yYyCu2O8+δ (Bi-2212) single crystals over a wide range of hole doping level (p). Hirr(T) was evaluated by measuring the magnetization hysteresis loop. The value of Hirr(T) extrapolated to T = 0 K [Hirr(0)], is either equal to or sets the lower boundary for the upper critical field at T = 0 K [Hc2(0)].

The key findings are:

  • Tc shows a parabolic p-dependence with a peak at p = 0.16.
  • 0Hirr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, Tc = 24 K) to 209 T for the most overdoped sample (p = 0.200, Tc = 75 K).
  • The observed p-dependence of Hirr(0) in Bi-2212 is distinct from those in YBa2Cu3O7-δ and HgBa2CuO6+δ, in which a pronounced dip structure appears in the underdoped region. This suggests that the influence of the competing order in Bi-2212 is less prominent than that in the other two systems.
  • The present results qualitatively agree with Hc2(0) values evaluated from the specific heat measurements, but are higher than those estimated from the Nernst effect measurements.
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Stats
Tc = 24 K for the most underdoped sample (p = 0.065) Tc = 75 K for the most overdoped sample (p = 0.200) 0Hirr(0) = 19 T for the most underdoped sample (p = 0.065) 0Hirr(0) = 209 T for the most overdoped sample (p = 0.200)
Quotes
"Tc shows a parabolic p-dependence (peak at p = 0.16), whereas 0Hirr(0) increases monotonically with p by approximately one order of magnitude, from 19 T for the most underdoped sample (p = 0.065, Tc = 24 K) to 209 T for the most overdoped sample (p = 0.200, Tc = 75 K)." "The observed p-dependence of Hirr(0) in Bi-2212 is distinct from those in YBa2Cu3O7-δ and HgBa2CuO6+δ, in which a pronounced dip structure appears in the underdoped region."

Deeper Inquiries

What are the potential reasons for the distinct doping dependence of the upper critical field in Bi-2212 compared to YBa2Cu3O7-δ and HgBa2CuO6+δ?

The distinct doping dependence of the upper critical field (Hc2) in Bi-2212 compared to YBa2Cu3O7-δ (Y-123) and HgBa2CuO6+δ (Hg-1201) can be attributed to several factors. Firstly, the electronic structure and the nature of the superconducting state in Bi-2212 may differ significantly from those in Y-123 and Hg-1201. Bi-2212 exhibits a more pronounced two-dimensional layered structure, which can influence the behavior of vortices and the critical fields associated with them. Secondly, the presence of competing orders, such as charge order, plays a crucial role in determining the doping dependence of Hc2. In Y-123 and Hg-1201, pronounced dip structures in Hc2 are observed in the underdoped region, which are likely associated with the formation of charge orders that compete with superconductivity. In contrast, the study indicates that Bi-2212 does not exhibit such dip structures, suggesting that the influence of competing orders is less prominent in this material. This difference in the impact of competing orders can lead to a more monotonic increase of Hirr(0) with doping in Bi-2212, as opposed to the non-monotonic behavior seen in Y-123 and Hg-1201. Lastly, the specific interactions between the charge carriers and the lattice structure in Bi-2212 may also contribute to its unique doping dependence. The variations in the chemical composition and the resulting changes in the superconducting properties across different doping levels can further differentiate the behavior of Hc2 in these high-Tc cuprate superconductors.

How might the presence or absence of competing orders, such as charge order, affect the vortex phase diagram and the determination of the upper critical field in these high-Tc cuprate superconductors?

The presence or absence of competing orders, such as charge order, significantly affects the vortex phase diagram and the determination of the upper critical field (Hc2) in high-Tc cuprate superconductors. Competing orders can lead to the formation of a complex landscape in the phase diagram, where superconductivity is suppressed in certain doping regions due to the emergence of these orders. In materials like Y-123 and Hg-1201, the development of charge order is associated with a reduction in the upper critical field, leading to the observed dip structures in Hc2 as a function of doping. This indicates that the competing order effectively competes with the superconducting state, resulting in a lower Hc2 in the underdoped region. The presence of such competing orders can also alter the vortex dynamics, leading to different melting and irreversibility fields, which are critical for understanding the vortex phase diagram. Conversely, in Bi-2212, the absence of pronounced dip structures in Hirr(0) suggests that competing orders have a less significant impact on the superconducting state. This allows for a more straightforward relationship between doping and the upper critical field, as the lack of competing orders means that the superconducting properties can be more directly correlated with the doping level. Consequently, the vortex phase diagram for Bi-2212 may exhibit a more uniform increase in Hc2 with doping, reflecting a more stable superconducting state across a wider range of hole concentrations.

Could the anomalous vortex states observed at low temperatures and high magnetic fields in some cuprates provide insights into the true upper critical field in the overdoped regime of Bi-2212?

Yes, the anomalous vortex states observed at low temperatures and high magnetic fields in some cuprates could provide valuable insights into the true upper critical field (Hc2) in the overdoped regime of Bi-2212. These anomalous states, which include phenomena such as the second vortex solid phase and unconventional quantum vortex matter states, are believed to arise from the competition between superconductivity and other ordered phases, such as charge order. In the overdoped regime of Bi-2212, where the upper critical field continues to increase with doping, the presence of these anomalous vortex states may indicate that the true Hc2 could be significantly higher than what is measured under conventional conditions. The limited temperature and magnetic field ranges in standard experiments may not capture the full extent of the vortex dynamics and the associated critical fields. Furthermore, if these anomalous vortex states extend into regions above the estimated Hirr(0), it suggests that the actual upper critical field could be masked by the limitations of the experimental setup. Future studies utilizing pulsed magnetic fields and exploring a broader range of temperatures and doping levels could help elucidate the nature of these anomalous states and their relationship to the upper critical field in Bi-2212. This could lead to a more comprehensive understanding of the interplay between superconductivity and competing orders in high-Tc cuprates, particularly in the overdoped regime.
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