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تسجيل مستخدم جديدThree energy scales characterizing the competing pseudogap state, the incoherent, and the coherent superconducting state in high-Tc cuprates
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We have studied the momentum dependence of the energy gap of Bi2(Sr,R)2CuOy by angleresolved photoemission spectroscopy (ARPES), particularly focusing on the difference between R=La and Eu. By comparing the gap function and characteristic temperatures between the two sets of samples, we show that there exist three distinct energy scales, {Delta}pg, {Delta}sc0, and {Delta}eff sc0, which correspond to T* (pseudogap temperature), Tonset (onset temperature of fluctuating superconductivity), and Tc (critical temperature of coherent superconductivity). The results not only support the existence of a pseudogap state below T* that competes with superconductivity but also the duality of competition and superconducting fluctuation at momenta around the antinode below Tonset.
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Large pulsed magnetic fields up to 60 Tesla are used to suppress the contribution of superconducting fluctuations (SCF) to the ab-plane conductivity above Tc in a series of YBa2Cu3O(6+x). These experiments allow us to determine the field Hc(T) and th
e temperature Tc above which the SCFs are fully suppressed. A careful investigation near optimal doping shows that Tc is higher than the pseudogap temperature T*, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. They can be accounted for by thermal fluctuations following the Ginzburg-Landau scheme for nearly optimally doped samples. A phase fluctuation contribution might be invoked for the most underdoped samples in a T range which increases when controlled disorder is introduced by electron irradiation. Quantitative analysis of the fluctuating magnetoconductance allows us to determine the critical field Hc2(0) which is found to be be quite similar to Hc(0) and to increase with hole doping. Studies of the incidence of disorder on both Tc and T* allow us to propose a three dimensional phase diagram including a disorder axis, which allows to explain most observations done in other cuprate families.
We explored by electronic Raman scattering the superconducting state of Bi-2212 single crystal by performing a fine tuned doping study. We found three distinct energy scales in A1g, B1g and B2g symmetries which show three distinct doping dependencies
. Above p=0.22 the three energies merge, below p=0.12, the A1g scale is no more detectable while the B1g and B2g scales become constant in energy. In between, the A1g and B1g scales increase monotonically with under-doping while the B2g one exhibits a maximum at p=0.16. The three superconducting energy scales appear to be an universal feature of hole-doped cuprates. We propose that the non trivial doping dependence of the three scales originates from Fermi surface topology changes and reveals competing orders inside the superconducting dome.
The discovery of the pseudogap in the cuprates created significant excitement amongst physicists as it was believed to be a signature of pairing, in some cases well above the room temperature. In this pre-formed pairs scenario, the formation of pairs
without quantum phase rigidity occurs below T*. These pairs condense and develop phase coherence only below Tc. In contrast, several recent experiments reported that the pseudogap and superconducting states are characterized by two different energy scales, pointing to a scenario, where the two compete. However a number of transport, magnetic, thermodynamic and tunneling spectroscopy experiments consistently detect a signature of phase-fluctuating superconductivity above leaving open the question of whether the pseudogap is caused by pair formation or not. Here we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region of the phase diagram commonly referred to as the pseudogap, two distinct states coexist: one that persists to an intermediate temperature Tpair and a second that extends up to T*. The first state is characterized by a doping independent scaling behavior and is due to pairing above Tc, but significantly below T*. The second state is the proper pseudogap - characterized by a checker board pattern in STM images, the absence of pair formation, and is likely linked to Mott physics of pristine CuO2 planes. Tpair has a universal value around 130-150K even for materials with very different Tc, likely setting limit on highest, attainable Tc in cuprates. The observed universal scaling behavior with respect to Tpair indicates a breakdown of the classical picture of phase fluctuations in the cuprates.
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Vladimir Z. Kresin Lawrence Berkeleyn National Laboratory
, University of California at Berkeley
2011
Doped cuprates are inhomogeneous superconductors. The concept of an intrinsic critical temperature, Tcintr.$equiv$ Tc*, whose value greatly exceeds that for the resistive Tcres.$equiv$ Tc, is supported by a number of experimental studies, including t
hose performed recently. These data are discussed in this review. The anomalous diamagnetism observed at Tcres.$equiv$ <T< Tc* is a manifestation of the presence of superconducting clusters embedded into a normal metallic matrix. The value of intrinsic critical temperature in some cuprates reaches a value which is close to room temperature. The a.c. properties of such inhomogeneous systems are discussed.
We derive analytic expressions for the critical temperatures of the superconducting (SC) and pseudogap (PG) transitions of the high-Tc cuprates as a function of doping. These are in excellent agreement with the experimental data both for single-layer
ed materials such as LSCO, Bi2201 and Hg1201 and multi-layered ones, such as Bi2212, Bi2223, Hg1212 and Hg1223. Optimal doping occurs when the chemical potential vanishes, thus leading to an universal expression for the optimal SC transition temperatures. This allows for the obtainment of a quantitative description of the growth of such temperatures with the number of layers, N, which accurately applies to the $Bi$, $Hg$ and $Tl$ families of cuprates. We study the pressure dependence of the SC transition temperatures, obtaining excellent agreement with the experimental data for different materials and dopings. These results are obtained from an effective Hamiltonian for the itinerant oxygen holes, which includes both the electric repulsion between them and their magnetic interactions with the localized copper ions. We show that the former interaction is responsible for the SC and the latter, for the PG phases, the phase diagram of cuprates resulting from the competition of both. The Hamiltonian is defined on a bipartite oxygen lattice, which results from the fact that only the $p_x$ and $p_y$ oxygen orbitals alternatively hybridize with the $3d$ copper orbitals. From this, we can provide an unified explanation for the $d_{x^2-y^2}$ symmetry of both the SC and PG order parameters and obtain the Fermi pockets observed in ARPES experiments.
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