Low-frequency flux noise in current-carrying high critical temperature superconducting films

Single crystals of the spin dimer system Sr(3)Cr(2)O(8) have been grown for the first time. Magnetization, heat capacity, and magnetocaloric effect data up to 65 T reveal magnetic order between applied fields of H(c1) approximately 30.4 T and H(c2) approximately 62 T. This field-induced order persists up to T(c)(max) approximately 8 K at H approximately 44 T, the highest observed in any quantum magnet where H(c2) is experimentally accessible. We fit the temperature-field phase diagram boundary close to H(c1) using the expression T(c) = A(H-H(c1))(nu). The exponent nu = 0.65(2), obtained at temperatures much smaller than T(c)(max), is that of the 3D Bose-Einstein condensate (BEC) universality class. This finding strongly suggests that Sr(3)Cr(2)O(8) is a new realization of a triplon BEC where the universal regimes corresponding to both H(c1) and H(c2) are accessible at (4)He temperatures.

A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase, which can lead to a Hall voltage independent of the spin-orbit coupling within the material-a geometrical Hall effect. Here we show that the highly correlated metal uCu 5 possesses an unusually large controllable geometrical Hall effect at T < 1.2 K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20% of the ν = 1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in uCu 5 opens a new field of enquiry into the importance of the role of frustration in highly correlated electron materials.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S = 1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two-to quasi-one-dimensional while effectively maintaining the nearest-neighbour coupling strength.Coordination polymers are self-organising materials consisting of arrays of metal ions linked via molecular ligands, with non-coordinated counterions supplying charge neutrality. The choice of initial components permits a high level of control over the final product, enabling many different polymeric architectures to be obtained [1]. These materials provide a route to successful crystal engineering, and a number of functionalities are being actively studied, including gas storage [2-4], optoelectronic [5,6], ferroelectric [7,8] and magnetic properties [9-14].Although it is now possible to generate an assortment of disparate magnetic lattices using this method [15,16], true control of magnetic exchange interactions implies an ability to adjust selected parameters while keeping others constant. To this end, a series of coordination polymers based on Cu(II) ions bridged by pyrazine (C 4 H 4 N 2 ) molecules have proven to be highly versatile. In these systems it has been shown that it is possible to alter significantly the primary exchange energies via adjustment of the ligands [17] and the counterions [18, S9], or fine-tune the exchange by a few percent via isotopic substitution [20], all the while maintaining the same basic metal-pyrazine network. In this paper we demonstrate the power of this strategy by chemically engineering a reduction in the dimensionality of a magnetic system. After first designing a material based on coordinated planes of Cu(II), we adapt the recipe such that the ligand bridges are broken along a specific crystal direction, resulting in a chain-like compound. Because the ligand mediating the magnetic interactions in both cases is unchanged, the nearest-neighbour exchange energies of the two materials are found to be equal to each other to within 5%. The difference in numbers of nearest-neighbours, however, means that the strength of the combined exchange interactions acting on each magnetic ion in the quasitwo-dimensional material is twice that of its quasi-onedimensional cousin.Figs. 1(a) and (b) show the crystal structure of orthorhombic [Cu(pyz) 2 (pyO) 2 ](PF 6 ) 2 (where pyz = pyrazine and pyO = pyridine-N -oxide, C 5 H 5 NO) determined using single-crystal x-ray diffraction [21]. S = 1/2 Cu ions are linked by pyz molecules into nearly square planar ...