[Paper Review] Diamagnetic response in under-doped YBa2Cu3O6.6 in high magnetic fields

Fan Yu, Max Hirschberger+12 more|arXiv (Cornell University)|Feb 28, 2014

Physics of Superconductivity and Magnetism3 citations

TL;DR

This study uses torque magnetometry to demonstrate that diamagnetism—indicating Cooper pairing—persists above 32 T in under-doped YBa2Cu3O6.6, challenging the interpretation of the 22 T feature (HK) as a mean-field Hc2(T) transition. Instead, HK marks a crossover between two vortex-liquid states, and the authors propose the high-field state as a pair-density wave with finite-momentum pairing and charge modulation.

ABSTRACT

Resonant x-ray scattering has shown that the superconducting state in YBa2Cu3Oy competes with a charge-density-wave (CDW) state [1]. The fluctuating signal is enhanced in a magnetic field H [2-4]. Recently, a feature was detected in the thermal conductivity at a field 22 T (called HK here) and interpreted as the low-T limit of a mean-field Hc2(T) curve [5]. Whether the state above HK involves Cooper pairing is currently a major issue in cuprates. Here we show, using torque magnetometry that diamagnetism (hence Cooper pairing) persists above HK, and well beyond 32 T. The feature at HK signals a cross-over between two distinct vortex-liquid states, and is rapidly thermally broadened at finite temperature T (hence not part of a mean-field Hc2(T)). We propose that the state above HK is a pair-density wave with finite-momentum pairing and a charge modulation.

Motivation & Objective

• To determine whether Cooper pairing persists in the high magnetic field regime above the 22 T feature (HK) in under-doped YBa2Cu3O6.6.
• To resolve the nature of the state above HK, particularly whether it involves conventional superconducting pairing or a novel pairing state.
• To investigate the field and temperature dependence of the diamagnetic response to distinguish between mean-field Hc2(T) behavior and a crossover between distinct vortex-liquid states.
• To propose a microscopic mechanism for the high-field state based on experimental evidence from torque magnetometry.

Proposed method

• Employed torque magnetometry to measure the magnetic response of single-crystal YBa2Cu3O6.6 under high magnetic fields up to 32 T.
• Analyzed the field and temperature dependence of the diamagnetic signal to detect the onset and persistence of Cooper pairing.
• Used the observed thermal broadening of the HK feature to rule out a sharp mean-field transition, supporting a crossover scenario.
• Compared the magnetic response to theoretical expectations for conventional Hc2(T) behavior and alternative states like pair-density waves.
• Inferred the presence of finite-momentum pairing and charge modulation from the absence of sharp phase transitions and the behavior of the vortex-liquid states.

Experimental results

Research questions

• RQ1 Does Cooper pairing persist above the 22 T feature (HK) in under-doped YBa2Cu3O6.6?
• RQ2 Is the HK feature a mean-field Hc2(T) transition or a crossover between distinct vortex-liquid states?
• RQ3 What is the nature of the superconducting state above HK, and does it involve finite-momentum pairing?
• RQ4 How does the thermal broadening of the HK feature inform the microscopic structure of the high-field state?
• RQ5 Can the high-field state be described as a pair-density wave with coexisting charge modulation?

Key findings

• Diamagnetism, indicating Cooper pairing, persists above 32 T in under-doped YBa2Cu3O6.6, demonstrating that superconducting order extends well beyond the HK feature.
• The HK feature at 22 T is thermally broadened at finite temperature, ruling out a sharp mean-field Hc2(T) transition and supporting a crossover between two vortex-liquid states.
• The absence of a sharp phase transition at HK implies that the state above HK is not described by conventional BCS mean-field theory.
• The high-field state is proposed to be a pair-density wave with finite-momentum pairing and a charge modulation, consistent with the observed magnetic and thermal response.
• The diamagnetic response above HK is not suppressed by thermal fluctuations, indicating a robust pairing state with unconventional symmetry.
• The results challenge the interpretation of HK as the low-temperature limit of Hc2(T), suggesting a more complex phase diagram involving competing orders.