We report on fluctuation magnetoconductivity and magnetic irreversibility of Y 1−x Pr x Ba 2 Cu 3 O 7−␦ single crystals and a polycrystalline sample. Although our samples are all single phase ͑orthorhombic͒ and the single crystals show no sign of structural inhomogeneity, all the samples exhibit two close and sharp genuine superconducting transitions. On the other hand, while the resistive transition of the polycrystalline sample exhibits in addition a coherence transition characteristic of a granular superconductor and the magnetic irreversibility displays the signature of the intergrain flux dynamics, the single crystals show no sign of these features. In view of these facts, we conclude that the well characterized split superconducting transition must result from a peculiar phase separation related with oxygen doping.
The extraordinary Hall effect coefficients Rs(T ) of the canonical spin glass alloys AuF e8at% and AuM n8at% and of an archetype reentrant system AuF e18at% were measured as functions of temperature. The data show a critical cusp-like term superimposed on a smooth background for the spin glasses, and change of sign with temperature for the reentrant. The results can be interpreted consistently by invoking a chiral Hall effect contribution as proposed by Kawamura.Spin glasses have been intensively studied for more than thirty years, as paradigms for the statistical mechanics of the whole vast family of complex systems. However the origin the spin freezing in the canonical spin glasses, dilute alloys such as AuF e or CuM n where the spins are Heisenberg, has long remained an enigma. Experiments and in particular critical exponent measurements [1] have shown definitive evidence for a non-zero ordering temperature, while numerical work on Heisenberg spin glasses in dimension three indicated that there was an Edwards-Anderson type of ordering only at zero temperature [2] though recent simulations contradict this [3]. For vector spin glasses there exists a chiral order parameter in addition to the Edwards-Anderson spin parameter and there were early suggestions that chirality might play a role in vector spin glass ordering [4]. Kawamura and coworkers [5,6,7] have made concrete large scale numerical investigations of the chiral driven ordering mechanism which they postulated to explain freezing in Heisenberg spin glasses. In real samples the spin and chiral order are linked through Dzaloshinsky-Moriya (DM) random anisotropy terms which are inevitably present. Magnetic torque experiments on a range of spin glasses have shown in-field transition lines up to high applied fields [8], in excellent agreement with extensive simulations on Heisenberg systems submitted simultaneously to anisotropy and field, where both chiral and spin ordering were monitored [6,9]. The robustness of order under applied fields is a clear indication that chirality is a primary ingredient of spin glass ordering in vector systems. The torque measurements were interpreted using the chiral ordering scenario.In this context it would be of considerable interest to have a complementary direct observation of chirality in the spin glasses. Chirality is a "hidden" parameter and no technique was known with which to monitor it experimentally, until Kawamura proposed that the Hall effect should present a direct signature of the chiral susceptibility [10]. On the chiral scenario the extraordinary Hall signal (linked to the sample magnetization rather than the magnetic field) should have a critical term due to chirality. The key point is whether there is critical behaviour of the Hall coefficient R s (T ) at the glass temperature T g , and accompanying non-linear effects. R s is defined as the ratio of the Hall signal to the magnetization, once corrections have been made for the ordinary Hall term. Careful Hall experiments on spin glasses have been reported [11...
The Hall effect has been studied in a series of AuFe samples in the re-entrant concentration range, as well as in part of the spin glass range. An anomalous Hall contribution linked to the tilting of the local spins can be identified, confirming theoretical predictions of a novel topological Hall term induced when chirality is present. This effect can be understood in terms of Aharonov-Bohm-like intrinsic current loops arising from successive scatterings by canted local spins. The experimental measurements indicate that the chiral signal persists, meaning scattering within the nanoscopic loops remains coherent, up to temperatures of the order of 150K.
We present macroscopic experimental evidence for field-induced microscopic quantum fluctuations in different hole-and electron-type cuprate superconductors with varying doping levels and numbers of CuO 2 layers per unit cell. The significant suppression of the zero-temperature in-plane magnetic irreversibility field relative to the paramagnetic field in all cuprate superconductors suggests strong quantum fluctuations due to the proximity of the cuprates to quantum criticality. DOI: 10.1103/PhysRevB.76.140506 PACS number͑s͒: 74.25.Dw, 74.25.Op, 74.40.ϩk, 74.72.Ϫh High-temperature superconducting cuprates are extreme type-II superconductors that exhibit strong thermal, disorder, and quantum fluctuations in their vortex states.1-9 While much research has focused on the macroscopic vortex dynamics of cuprate superconductors with phenomenological descriptions, [1][2][3][4][5]7 little effort has been made to address the microscopic physical origin of their extreme type-II nature. 9 Given that competing orders ͑COs͒ can exist in the ground state of these doped Mott insulators besides superconductivity ͑SC͒, 9-15 the occurrence of quantum criticality may be expected. 11,13,16 The proximity to quantum criticality and the existence of COs can significantly affect the low-energy excitations of the cuprates due to strong quantum fluctuations 8,9 and the redistribution of quasiparticle spectral weight among SC and COs. 9,17,18 Indeed, empirically the low-energy excitations of cuprate superconductors appear to be unconventional, exhibiting intriguing properties unaccounted for by conventional Bogoliubov quasiparticles.9,17-19 Moreover, external variables such as temperature ͑T͒ and applied magnetic field ͑H͒ can vary the interplay of SC and COs, such as inducing or enhancing 20,21 the COs at the price of more rapid suppression of SC, thereby leading to weakened superconducting stiffness and strong thermal and field-induced fluctuations.1-3 On the other hand, the quasi-two-dimensional nature of the cuprates can also lead to quantum criticality in the limit of decoupling of CuO 2 planes. 6 In this work we demonstrate experimental evidence from macroscopic magnetization measurements for field-induced quantum fluctuations among a wide variety of cuprate superconductors with different microscopic variables such as the doping level ͑␦͒ of holes or electrons, and the number of CuO 2 layers per unit cell ͑n͒. 22 We suggest that the manifestation of strong fieldinduced quantum fluctuations is consistent with a scenario that all cuprates are in close proximity to a quantum critical point ͑QCP͒. 6To investigate the effect of quantum fluctuations on the vortex dynamics of cuprate superconductors, our strategy involves studying the vortex phase diagram at T → 0 to minimize the effect of thermal fluctuations, and applying magnetic field parallel to the CuO 2 planes ͑H ʈ ab͒ to minimize the effect of random point disorder. The rationale for having H ʈ ab is that the intrinsic pinning effect of layered CuO 2 planes generally dominates ov...
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