1,720,968 research outputs found
3d-electron Heisenberg pyrochlore Mn2Sb2O7
Get PDFIn frustrated magnetic systems, geometric constraints or the competition among interactions introduce extremely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momenta, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn2Sb2O7 is an extremely rare Heisenberg pyrochlore system with directionally unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5 K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins’ interactions. The striking similarities to 3d transition-metal pyrochlores with unquenched angular momenta suggests that the low spin-orbit coupling in the 3d block makes Heisenberg pyrochlores far more accessible than previously imagined. ©2016 American Physical Society1431sciescopu
Upper critical field of the noncentrosymmetric superconductor BiPd
No full textThe superconducting parameters and upper critical field of the noncentrosymmetric superconductor BiPd have proven contentious. This material is of particular interest because it is a rare example of a 4f -electron-free noncentrosymmetric superconductor of which crystals may be grown and cleaved, enabling surface-sensitive spectroscopies. Here, using bulk probes augmented by tunneling data on defects, we establish that the lower of the previously reported upper critical fields corresponds to the bulk transition. The material behaves as a nearly weak-coupled BCS s-wave superconductor, and we report its superconducting parameters as drawn from the bulk upper critical field. Possible reasons behind the order-of-magnitude discrepancy in the reported upper critical fields are discussed. (c) 2016 American Physical Society1661sciescopu
Dirac surface states and nature of superconductivity in Noncentrosymmetric BiPd
Get PDFIn non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin-orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen-Cooper-Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization. ©2015 Macmillan Publishers Limited. All rights reserved131261sciescopu
Signatures of new d-wave vortex physics in overdoped Tl2Ba2CuO6+x revealed by TF-μ+SR
Get PDFThe spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature of superconductors; in Type-II superconductors above the lower critical field, this screening takes the form of a lattice of magnetic flux vortices. Using implanted spin-1/2 positive muons, one can measure the vortex lattice field distribution through the spin precession and deduce key parameters of the superconducting ground state, and thereby fundamental properties of the superconducting pairing. Muon spin rotation/relaxation (μSR) experiments have indeed revealed much interesting physics in the underdoped cuprates, where superconductivity is closely related to, or coexistent with, disordered or fluctuating magnetic and charge excitations. Such complications should be absent in overdoped cuprates, which are believed to exhibit conventional Fermi liquid behaviour. These first transverse field (TF)-μ+SR experiments on heavily-overdoped single crystals reveal a superfluid density exhibiting a clear inflection point near 0.5Tc, with a striking doping-independent scaling. This reflects hitherto unrecognized physics intrinsic to d-wave vortices, evidently generic to the cuprates, and may offer fundamentally new insights into their still-mysterious superconductivity. © 2015, Nature Publishing Group. All rights reserved1211sciescopu
Nodeless superconductivity in the noncentrosymmetric superconductor BiPd
No full textWe report low-temperature thermal conductivity measurements on high-quality single crystalline α-BiPd, a noncentrosymmetric superconductor with T c ≃ 3.8 K. In zero magnetic field, the residual linear term κ 0/T is absent, indicating a full gap and suggesting dominant singlet pairing. Consistent with this picture, κ 0(H)/T shows weak field dependence in low field, but grows relatively fast in fields above 0.03 T (≃1/5H c2). Implications for the material's disputed high-field behavior are discussed, and constraints are placed on the previously proposed scenarios. © 2016 IOP Publishing Ltd1441sciescopu
Electronic structure of Li2 RuO3 studied by LDA and LDA+DMFT calculations and soft x-ray spectroscopy
Get PDFThe electronic structure of Li2RuO3 was investigated using x-ray emission and absorption spectroscopy and by theoretical calculations employing two approaches: the local density approximation (LDA) and a combination of LDA with the cluster extension of dynamical mean-field theory (LDA+DMFT). The evolution of the spectral properties with the strength of electronic correlations is analyzed. We show that for moderate values of on-site Coulomb repulsion U and intra-atomic Hund's rule exchange JH,Li2RuO3 is in an orbital-selective strongly correlated state in the sense that a part of the t2g manifold (i.e., xz/yz) behaves as local atomic orbitals susceptible to Hubbard correlations, while the remaining (xy) orbitals must be described as bond-centered molecular orbitals. Both theoretical approaches succeed in explaining the x-ray data, and a comparison of the theoretical and experimental spectra provides a reasonable estimate of the possible correlation strength (U) and Hund's coupling (JH) in Li2RuO3. © 2015 American Physical Society1771sciescopu
Magnetic transitions in the chiral armchair-kagome system Mn2Sb2 O7
Get PDFThe competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn2Sb2O7forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn2Sb2O7exhibits two distinct magnetic phase transitions, at 11.1 and 14.2 K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multicomponent order parameter. Although previously reported in the P3121 space group, Mn2Sb2O7actually crystallizes in P2, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional armchair-kagome lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems. © 2017 American Physical Society3
High-resolution structure studies and magnetoelectric coupling of relaxor multiferroic Pb(Fe0.5Nb0.5)O3
Get PDFPb(Fe0.5Nb0.5)O3 (PFN), one of the few relaxor multiferroic systems, has a G-type antiferromagnetic transition
at TN = 143K and a ferroelectric transition at TC = 385 K. By using high-resolution neutron-diffraction
experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range
structures of PFN: (i) a clear sign of short-range structural correlation above TC, (ii) no sign of the negative
thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric
coupling below TN. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder
between Fe3+ and Nb5+ atoms.We argue that this disorder gives rise to short-range structural correlations arising
from O disorder in addition to Pb displacement.7811Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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