We have performed extensive zero field µSR experiments on pure YBa2Cu3O6+y and diluted Yrare-earth substituted Y0.92Eu0.08Ba2Cu3O6+y and Y0.925Nd0.075Ba2Cu3O6+y at light hole-doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment m(h, T ) fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the re-entrant regime (0 ≤ h ≤ 0.056) and in the so-called Cluster Spin Glass state coexisting with superconductivity (0.056 < h 0.08). We argue a common magnetic ground state for these two doping regions and dub it frozen antiferromagnet. Conversely either frustration or finite-size effects prevail in the thermally activated antiferromagnetic state, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at hc = 0.056(2).

An array of superconducting islands placed on a normal metal film offers a tunable realization of nanopatterned superconductivity. This system enables investigation of the nature of competing vortex states and phase transitions between them. A square array creates the eggcrate potential in which magnetic field-induced vortices are frozen into a vortex insulator. We observed a vortex insulator-vortex metal transition driven by the applied electric current and determined critical exponents that coincided with those for thermodynamic liquid-gas transition. Our findings offer a comprehensive description of dynamic critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

We have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2−ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (Tc ∼32 K), phase separation temperature (Tps ∼520 K) and iron-vacancy order temperature (Tvo ∼580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below ∼300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides a clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the Tps and Tc is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KxFe2−ySe2.

While it is known that the nature and the arrangement of defects in complex oxides have an impact on the material functionalities, little is known about control of superconductivity by oxygen interstitial organization in cuprates. Here we report direct compelling evidence for the control of T c by manipulation of the superconducting granular networks of nanoscale puddles, made of ordered oxygen stripes, in a single crystal of YBa 2 Cu 3 O 6.5 + y with average formal hole doping p close to 1/8. Upon thermal treatments we were able to switch from a first network of oxygen defect striped puddles with OVIII modulation (q OVIII (a*) = (h + 3/8, k, 0) and q OVIII (a*) = (h + 5/8, k, 0)) to a second network characterized by OXVI modulation (q OXVI (a*) = (h + 7/16, k, 0) and qox-VI Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.(a*) = (h + 9/16, k, 0)) and finally to a third network with puddles of OV periodicity (q OV (a*) = (4/10, 1, 0) and q OV (a*) = (6/10, 1, 0)). We map the microscopic spatial evolution of the out of plane OVIII, OXVI and OV puddle nanosize distribution via scanning micro-diffraction measurements. In particular, we calculated the number of oxygen chains (n) and the charge density (hole concentration p) inside each puddle, analyzing areas of 160 × 80 μm 2 , and recording 12 800 diffraction patterns to reconstruct each spatial map. The high spatial inhomogeneity shown by all the reconstructed spatial maps reflects the intrinsic granular structure that characterizes cuprates and iron chalcogenides, disclosing the presence of several complex networks of coexisting superconducting domains with different lattice modulations, charge densities and gaps as in the proposed multi-gap scenario called superstripes.

Despite intensive research a physical explanation of high Tc superconductors remains elusive. One reason for this is that these materials have generally a very complex structure making useless theoretical models for a homogeneous system. Little is known on the control of the critical temperature by the space disposition of defects because of lack of suitable experimental probes. X-ray diffraction and neutron scattering experiments used to investigate y oxygen dopants in YBa2Cu3O6+y lack of spatial resolution. Here we report the spatial imaging of dopants distribution inhomogeneity in YBa2Cu3O6.67 using scanning nano X-ray diffraction. By changing the X-ray beam size from 1 micron to 300 nm of diameter, the lattice inhomogeneity increases. The ordered oxygen puddles size distribution vary between 6–8 nm using 1 × 1 μm2 beam, while it is between 5–12 nm with a fat tail using the 300 × 300 nm2 beam. The increased inhomogeneity at the nanoscale points toward a network of superconducting puddles made of ordered oxygen interstitials.

Magnetization, AC susceptibility and µSR measurements have been performed in neutral phthalocyaninato lanthanide ([LnPc2] 0 ) single molecule magnets in order to determine the low-energy levels structure and to compare the low-frequency spin excitations probed by means of macroscopic techniques, such as AC susceptibility, with the ones explored by means of techniques of microscopic character, such as µSR. Both techniques show a high temperature thermally activated regime for the spin dynamics and a low temperature tunneling one. While in the activated regime the correlation times for the spin fluctuations estimated by AC susceptibility and µSR basically agree, clear discrepancies are found in the tunneling regime. In particular, µSR probes a faster dynamics with respect to AC susceptibility. It is argued that the tunneling dynamics probed by µSR involves fluctuations which do not yield a net change in the macroscopic magnetization probed by AC susceptibiliy. Finally resistivity measurements in [TbPc2] 0 crystals show a high temperature nearly metallic behaviour and a low temperature activated behaviour.

The pyrochlore compounds Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 show an exotic form of magnetism called the spin ice state, resulting from the interplay between geometrical frustration and ferromagnetic coupling. A fascinating feature of this state is the appearance of magnetic monopoles as emergent excitations above the degenerate ground state. Over the past years, strong effort has been devoted to the investigation of these monopoles and other properties of the spin ice state in bulk crystals. A tantalising prospect is to incorporate spin ice materials into devices for spintronics and devices that can manipulate the magnetic monopoles. This would require the availability of spin ice thin films. Here, we report the fabrication of Ho 2 Ti 2 O 7 thin films using pulsed laser deposition. These films not only show a high crystalline quality, but also exhibit the hallmarks of a spin ice: a pronounced magnetic anisotropy and an intermediate plateau in the magnetisation along the [111] crystal direction.The magnetic Ho 3+ ions in Ho 2 Ti 2 O 7 form a lattice of corner-sharing tetrahedra in the pyrochlore structure and interact via ferromagnetic coupling 1-7 . Their magnetic moments are Ising-like due to the crystal field anisotropy and are aligned along the set of ‹111› axes. The resulting degenerate ground state of each tetrahedron has two holmium spins pointing inwards and two holmium spins pointing outwards. This is the so-called "ice rule", from the analogy with the H-O bond lengths in solid water. A local breaking of the ice rule, due to the flipping of one holmium spin shared between two neighbouring tetrahedra, results in a 3-in-1-out and a 1-in-3-out configuration. This effectively creates a positive and a negative magnetic charge in the adjacent tetrahedra 8 , which can be regarded as a monopole-antimonopole pair 9-10 . Such pairs can dissociate and the individual monopoles can move away from each other by flipping a chain of spins along their route. This process takes place without further violating the ice rule, so that the energy cost for the monopoles to be brought to infinity stays finite, being linked to the energy required for the first excitation. Signatures of emergent magnetic monopoles in bulk spin ice crystals have been observed by several groups [11][12][13][14][15] . The magnetic counterparts of two fundamental effects in electronics have also been demonstrated: a basic capacitor effect for magnetic charges 16 and the introduction of magnetic defects hindering the monopole flow, similar to residual defectinduced resistance for electrons 17 . Further effort has been dedicated to tune the monopole chemical potential in a range where mutual interaction plays a role, to mimic electronic correlations in a purely magnetic Coulomb gas 18 . In conjunction with all the mentioned experiments, the term "magnetricity" is coined to describe the flow of magnetic charges as the equivalent of electricity.However, until now all experiments on spin ice materials have been performed on bulk crystals. The possibility of ma...

The cleanest way to observe a dynamic Mott insulator-to-metal transition (DMT) without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. Here, we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT ) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as the thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of a nonequilibrium drive is to generate an effective temperature and hence the transition belonging in the thermal universality class. DOI: 10.1103/PhysRevB.97.020504 A Mott insulator [1][2][3] arising from the concurrent action of the electron-electron correlations and electron trapping by a periodic atomic potential is an exemplary manifestation of many-body quantum physics [4][5][6][7][8]. A remarkable correspondence between the quantum mechanics in a D-dimensional system and the classical statistical mechanics of a D + 1-dimensional system [9] leads to the conjecture about its classical counterpart, a vortex Mott insulator that would form in a type II superconductor if the density of the superconducting vortices matches the density of the pinning sites [10,11]. Experimentally, the vortex Mott insulator was claimed in the studies of the vortex matching effect in Ref. [12], and was conclusively evidenced in Ref. [13] by measurements of the compressibility of the vortex system localized by periodic surface holes. The implications of the existence of the vortex Mott state are far reaching. First and foremost, the Mottness embraces not only the quantum but classical realm, thus offering a perfect laboratory to study quantum many-body physics by exploring classical vortex systems.An enabling discovery of the current-driven vortex Mott insulator-to-metal transition in a proximity array [14] provided the first tangible example of a dynamic Mott transition having settled the vortex quantum mechanical mapping on a firm experimental basis. That the revealed nonequilibrium critical behavior with respect to the nonequilibrium drive is the same as that of a conventional thermal Mott transition with respect to temperature raises a largely open class of questions. Among these is a central issue in condensed matter physics: the generalization of a thermodynamic phase transition to nonequilibrium conditions. There have been tantalizing reports that in systems where tuning parameters such as temperature, pressure, or magnetic field alter the symmetry, the nonequilibrium drive generates an effective temperature an...