Distinctive low- and high-temperature dependences of the magnetic penetration depth in organic and high-T
c cuprate superconductors: Evidence for the gapped and gapless Bose-liquid superconductivity
Authors: Dzhumanov S. , Turmanova U. M. , Kurbanov U. T. , Mayinova U. K. 
Published: November 2024

The validity of a new alternative (Bose-liquid) approach, which has advantage over the Fermi-liquid (BCS-like) approach, to unconventional superconductivity in organic and high-Tc cuprate superconductors is consi­dered. The results of a detailed study of the temperature dependence of the magnetic field penetration depth in these materials are presented. It is argued that organic materials and doped high-Tc cuprates can be considered as bosonic superconductors in which tightly-bound (polaronic) Cooper pairs behave as bosons like 4He atoms and condense into two distinct Bose superfluids below the superconducting transition temperature Tc. It is shown that unconventional (Bose-liquid) superconductivity in organic compounds and doped cuprates arises as a result of pair and single particle condensations of attracting bosons into two Bose superfluids below two characteristic temperatures Tc (temperature of onset of superconducting transition or pair condensation of attracting bosons) and Tc∗ (temperature of onset of single particle condensation of attracting bosons). The existing experimental data on the temperature dependence of the London penetration depth λL(T) in organic and high-Tc cuprate superconductors are analysed in terms of the theory of three-dimensional (3D) Bose-liquid superconductivity. By comparing the predictions of this theory for temperature dependences λL(T) with measured temperature dependences λL(T) in the organic superconductor k-(BEDT–TTF)2Cu(NCS)2 and the high-Tc cuprate superconductors YBa2Cu3O7–δ, YBa2Cu3O9–δ (δ = 2.1 ± 0.05) and YBa2Cu3O6.95 found that the temperature dependences of the new superconducting order parameter and the magnetic field penetration depth in these superconductors are anomalous and show a pronounced kink (or bend) near the characteristic temperature Tc∗ lower than Tc. The existence of two different (so-called gapped and gapless) regimes of 3D Bose-liquid superconductivity in the studied organic and high-Tc cuprate superconductors in two temperature ranges Tc∗&lt;T&lt;Tc and 0≤T≤Tc∗ is predicted. The validity of the following laws in unconventional (bosonic) superconductors is proved: the exponential temperature dependence of λL(T) at temperatures above Tc∗ is a consequence of the appearance of the energy gap in the excitation spectrum of a 3D superfluid Bose liquid, and the power-law temperature dependence of λL(T) at low temperatures below Tc∗ down to T = 0 is a direct consequence of the disappearance of the energy gap in the excitation of a 3D superfluid Bose liquid at T≤Tc∗. Our theoretical results for the low- and high-temperature dependences of λL(T)/λL(0) and λL2(0)/λL2(T) are in good quantitative agreement with experimental results in organic and high-Tc cuprate superconductors. It is found that the superconductivity in these materials is essentially three-dimensional in nature and is well described by the theory of two different Bose superfluids.