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Register a new userIsotope effect on the superconducting critical temperature of cuprates in the presence of charge order
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Using the large-$N$ limit of the $t$-$J$ model and allowing also for phonons and the electron-phonon interaction we study the isotope effect $alpha$ for coupling constants appropriate for YBCO. We find that $alpha$ has a minimum at optimal doping and increases strongly (slightly) towards the underdoped (overdoped) region. Using values for the electron phonon interaction from the local density approximation we get good agreement for $alpha$ as a function of $T_c$ and doping $delta$ with recent experimental data in YBCO. Our results strongly suggest that the large increase of $alpha$ in the underdoped region is (a) caused by the shift of electronic spectral density from low to high energies associated with a competing phase (in our case a charge density wave) and the formation of a gap, and (b) compatible with the small electron phonon coupling constants obtained from the local density approximation. We propose a similar explanation for the anomalous behavior of $alpha$ in Sr doped La$_2$CuO$_4$ near the doping 1/8.
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We reconsider the long-standing problem of the effect of spin fluctuations on the critical temperature and isotope effect in a phonon-mediated superconductor. Although the general physics of the interplay between phonons and paramagnons had been rather well understood, the existing approximate formulas fail to describe the correct behavior of $% T_{c}$ for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for $T_{c}$ which agrees well with exact numerical solutions of the Eliashberg equations for a broad range of parameters. Based on both numerical and analytical results, we predict a strong enhancement of the isotope effect when the frequencies of spin fluctuation and phonons are of the same order. This effect may have important consequences for near-magnetic superconductors such as MgCNi$_{3}$
Hole doped cuprates show a superconducting critical temperature $T_c$ which follows an universal dome-shaped behavior as function of doping. It is believed that the origin of superconductivity in cuprates is entangled with the physics of the pseudogap phase. An open discussion is whether the source of superconductivity is the same that causes the pseudogap properties. The $t$-$J$ model treated in large-N expansion shows $d$-wave superconductivity triggered by non-retarded interactions, and an instability of the paramagnetic state to a flux phase or $d$-wave charge density wave ($d$-CDW) state. In this paper we show that self-energy effects near $d$-CDW instability may lead to a dome-shaped behavior of $T_c$. In addition, it is also shown that these self-energy contributions may describe several properties observed in the pseudogap phase. In this picture, although fluctuations responsible for the pseudogap properties leads to a dome-shaped behavior, they are not involved in pairing which is mainly non-retarded.
Two principles govern the critical temperature for superconducting transitions: (1)~intrinsic strength of the pair coupling and (2)~effect of the many-body environment on the efficiency of that coupling. Most discussions take into account only the first but we argue that the properties of unconventional superconductors are governed more often by the second, through dynamical symmetry relating normal and superconducting states. Differentiating these effects is essential to charting a path to the highest-temperature superconductors.
In the course of seeking the microscopic mechanism of superconductivity in cuprate high temperature superconductors, the pseudogap phasetextemdash the very abnormal normal state on the hole-doped sidetextemdash has proven to be as big of a quandary as superconductivity itself. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for assessing the momentum-dependent phenomenology of the pseudogap, and recent technological developments have permitted a more detailed understanding. This report reviews recent progress in understanding the relationship between superconductivity and the pseudogap, the Fermi arc phenomena, and the relationship between charge order and pseudogap from the perspective of ARPES measurements.
Recently experiments on high critical temperature superconductors has shown that the doping levels and the superconducting gap are usually not uniform properties but strongly dependent on their positions inside a given sample. Local superconducting regions develop at the pseudogap temperature ($T^*$) and upon cooling, grow continuously. As one of the consequences a large diamagnetic signal above the critical temperature ($T_c$) has been measured by different groups. Here we apply a critical-state model for the magnetic response to the local superconducting domains between $T^*$ and $T_c$ and show that the resulting diamagnetic signal is in agreement with the experimental results.
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