Inverse correlation between quasiparticle mass and T c in a cuprate high-T c superconductor

Abstract

Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors, and there have been suggestions that high-temperature superconductivity in copper-oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature T c is maximal in cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge-ordered phase. Recent experiments have shown a strong increase in quasiparticle mass m* in the cuprate YBa2Cu3O7-δ as optimal doping is approached, suggesting that quantum fluctuations of the charge-ordered phase may be responsible for the high-T c superconductivity. We have tested the robustness of this correlation between m* and T c by performing quantum oscillation studies on the stoichiometric compound YBa2Cu4O8 under hydrostatic pressure. In contrast to the results for YBa2Cu3O7-δ, we find that in YBa2Cu4O8, the mass decreases as T c increases under pressure. This inverse correlation between m* and T c suggests that quantum fluctuations of the charge order enhance m* but do not enhance T c.

Keywords: Cuprate superconductors; high pressure; quantum oscillations.

Fig. 1. Temperature-dependent magnetoresistance of Y124.

(A) c-axis resistivity at different temperatures, measured up to μ₀H = 67 T. The inset shows the derivative dρ_c/dH to emphasize the Shubnikov–de Haas QOs. (B) Temperature dependence of the QO amplitude. The red line shows a fit to the Lifshitz-Kosevich expression (see Materials and Methods) giving a quasiparticle mass m* = (1.80 ± 0.05) m_e (where m_e is the free electron mass).

Fig. 2. Magnetoresistance of Y124 at various pressures at T = 2.5 K.

Data for p = 0 are shown both before the pressure was applied and after it was removed.

Fig. 3. Oscillatory part of resistance versus field for different pressures at T = 2.5 K.

(A) Three curves for p = 0 GPa. (a) and (b) were measured outside the pressure cell: (a) in a 70-T coil and (b) in the same 60-T coil as the pressure cell measurements. The third curve is the result at p = 0 after depressurizing the cell. The arrows mark the position of a local maximum B_max in Δρ_c/ρ_c. The curves have been offset vertically for clarity. (B) Evolution of the QO frequency with pressure. For p = 0 (before pressurization), the frequency was taken from a direct fit to sin(2πF/B + φ), then the changes in the frequency as p is varied are inferred from B_max. Similar changes in F were also found by fitting each curve (as for p = 0) but at a higher noise level.

Fig. 4. Quasiparticle effective mass in Y124 under hydrostatic pressure.

(A) Amplitude of QOs as a function of temperature. The curves have been offset vertically for clarity. The solid lines are fits to the Lifshitz-Kosevich formula. The field windows used are given in table S1. (B) Variation of quasiparticle mass with pressure extracted from the fits. The dashed line is a guide to the eye. a.u., arbitrary units.

Fig. 5. Quasiparticle mass of Y124 compared to Y123 plotted versus T_c.

The top scale shows the hole doping level for Y123 (17). Data for Y123 are taken from Ramshaw et al. (15) and the references therein. The dashed line is a guide to the eye.