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Register a new userHigh performance temperature controller: application to the excess noise measurements of YBCO thermometers in the transition region
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Dedicated read-out electronics was developed for low impedance resistive thermometers. Using this high performance temperature controller, the temperature dependence of the excess noise of a YBa2Cu307-d (YBCO) sample in the superconducting transition was monitored as a function of the current bias. The noise could reach 3.10-8 K Hz-1/2 at 1 Hz, 5 mA bias and 90 K.
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We present results on ultra low noise YBa$_2$Cu$_3$O$_{7-delta}$ nano Superconducting QUantum Interference Devices (nanoSQUIDs). To realize such devices, we implemented high quality YBCO nanowires, working as weak links between two electrodes. We observe critical current modulation as a function of an externally applied magnetic field in the full temperature range below the transition temperature $T_C$. The white flux noise below 1 $mu Phi_0/sqrt{mathrm{Hz}}$ at T = 8 K makes our nanoSQUIDs very attractive for the detection of small spin systems.
The spectral energy gap is an important signature that defines states of quantum matter: insulators, density waves, and superconductors have very different gap structures. The momentum resolved nature of angle-resolved photoemission spectroscopy (ARPES) makes it a powerful tool to characterize spectral gaps. ARPES has been instrumental in establishing the anisotropic d-wave structure of the superconducting gap in high-transition temperature (Tc) cuprates, which is different from the conventional isotropic s-wave superconducting gap. Shortly afterwards, ARPES demonstrated that an anomalous gap above Tc, often termed the pseudogap, follows a similar anisotropy. The nature of this poorly understood pseudogap and its relationship with superconductivity has since become the focal point of research in the field. To address this issue, the momentum, temperature, doping, and materials dependence of spectral gaps have been extensively examined with significantly improved instrumentation and carefully matched experiments in recent years. This article overviews the current understanding and unresolved issues of the basic phenomenology of gap hierarchy. We show how ARPES has been sensitive to phase transitions, has distinguished between orders having distinct broken electronic symmetries, and has uncovered rich momentum and temperature dependent fingerprints reflecting an intertwined & competing relationship between the ordered states and superconductivity that results in multiple phenomenologically-distinct ground states inside the superconducting dome. These results provide us with microscopic insights into the cuprate phase diagram.
The pressure dependence of superconducting transition temperature $T_{rm c}$ has been investigated through the DC magnetic measurements for FeSe$_{0.8}$ and FeSe$_{1.0}$. For both samples, with increasing pressure $P$, the $T_{rm c}$$-$$P$ curve exhibits a two-step increase, showing a local maximum of $sim$11 K at $P$$sim$1.0 GPa and a rapid increase with an extremely large pressure coefficient for $P$$>$1.5 GPa. $T_{rm c}$ saturates at $sim$25 K (21 K) in FeSe$_{1.0}$ (FeSe$_{0.8}$) for $P$$>$3 GPa. A rapid decrease in superconducting volume fraction is observed with an increase in $T_{rm c}$ above 1.5 GPa, suggesting the presence of electronic inhomogeneity.
As established by scanning tunneling microscopy (STM) cleaved surfaces of the high temperature superconductor YBa$_2$Cu$_2$O$_{7-delta}$ develop charge density wave (CDW) modulations in the one-dimensional (1D) CuO chains. At the same time, no signatures of the CDW have been reported in the spectral function of the chain band previously studied by photoemission. We use soft X-ray angle resolved photoemission (SX-ARPES) to detect a chain-derived surface band that had not been detected in previous work. The $2k_textup{F}$ for the new surface band is found to be 0.55,AA$^{-1}$, which matches the wave vector of the CDW observed in direct space by STM. This reveals the relevance of the Fermi surface nesting for the formation of CDWs in the CuO chains in YBa$_2$Cu$_2$O$_{7-delta}$. In agreement with the short range nature of the CDW order the newly detected surface band exhibits a pseudogap, whose energy scale also corresponds to that observed by STM.
Recent photoemission data in the high temperature cuprate superconductor Bi2212 have been interpreted in terms of a sharp spectral peak with a temperature independent lifetime, whose weight strongly decreases upon heating. By a detailed analysis of the data, we are able to extract the temperature dependence of the electron self-energy, and demonstrate that this intepretation is misleading. Rather, the spectral peak loses its integrity above Tc due to a large reduction in the electron lifetime.
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