126 research outputs found
Fermi pockets and correlation effects in underdoped YBa2Cu3O6.5
The detection of quantum oscillations in the electrical resistivity of
YBa2Cu3O6.5 provides direct evidence for the existence of Fermi surface pockets
in an underdoped cuprate. We present a theoretical study of the electronic
structure of YBa2Cu3O7-d (YBCO) aiming at establishing the nature of these
Fermi pockets, i.e. CuO2 plane versus CuO chain or BaO. We argue that electron
correlation effects, such as orbital-dependent band distortions and highly
anisotropic self-energy corrections, must be taken into account in order to
properly interpret the quantum oscillation experiments.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/YBCO_OrthoII_LDA.pd
Magnetic phase diagram of
Magnetometry, electrical transport, and neutron scattering measurements were
performed on single crystals of the Fe^{4+}-containing perovskite-related phase
Sr_3Fe_2O_7-x as a function of oxygen content. Although both the crystal
structure and electron configuration of this compound are closely similar to
those of well-studied ruthenates and manganates, it exhibits very different
physical properties. The fully-oxygenated compound (x=0) exhibits a
charge-disproportionation transition at T_D = 340 K, and an antiferromagnetic
transition at T_N = 115 K. For temperatures T \leq T_D, the material is a
small-gap insulator; the antiferromagnetic order is incommensurate, which
implies competing exchange interactions between the Fe^{4+} moments. The
fully-deoxygenated compound (x=1) is highly insulating, and its Fe^{3+} moments
exhibit commensurate antiferromagnetic order below T_N ~ 600 K. Compounds with
intermediate x exhibit different order with lower T_N, likely as a consequence
of frustrated exchange interactions between Fe^{3+} and Fe^{4+} sublattices. A
previous proposal that the magnetic transition temperature reaches zero is not
supported.Comment: 8 pages, 6 figure
Central mode and spin confinement near the boundary of the superconducting phase in YBa2Cu3O6.353 (Tc=18 K)
We have mapped the neutron scattering spin spectrum at low-energies in
YBa2Cu3O6.353 (Tc=18 K) where the doping ~0.06 is near the critical value
(pc=0.055) for superconductivity. No coexistence with long range ordered
antiferromagnetism is found. The spins fluctuate on two energy scales, one a
damped spin response with a ~2 meV relaxation rate and the other a central mode
with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum
mirrors that of a soft mode driving a central mode. Extremely short correlation
lengths, 42+-5 Angstrom in-plane and 8+-2 Angstrom along the c direction, and
isotropic spin orientations for the central mode indicate that the correlations
are subcritical with respect to any second order transition to Neel order. The
dynamics follows a model where damped spin fluctuations are coupled to the slow
fluctuations of regions with correlations shortened by the hole doping.Comment: 5 pages 4 figures. One figure revised and some text revision.
Accepted PRB Rapids February 14, 200
Crystallographic and superconducting properties of the fully-gapped noncentrosymmetric 5d-electron superconductors CaMSi3 (M=Ir, Pt)
We report crystallographic, specific heat, transport, and magnetic properties
of the recently discovered noncentrosymmetric 5d-electron superconductors
CaIrSi3 (Tc = 3.6 K) and CaPtSi3 (Tc = 2.3 K). The specific heat suggests that
these superconductors are fully gapped. The upper critical fields are less than
1 T, consistent with limitation by conventional orbital depairing. High,
non-Pauli-limited {\mu}0 Hc2 values, often taken as a key signature of novel
noncentrosymmetric physics, are not observed in these materials because the
high carrier masses required to suppress orbital depairing and reveal the
violated Pauli limit are not present.Comment: 8 pages, 8 figure
Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet SrFeO
We report a neutron scattering study of the magnetic order and dynamics of
the bilayer perovskite SrFeO, which exhibits a temperature-driven
metal-insulator transition at 340 K. We show that the Fe moments adopt
incommensurate spiral order below K and provide a
comprehensive description of the corresponding spin wave excitations. The
observed magnetic order and excitation spectra can be well understood in terms
of an effective spin Hamiltonian with interactions ranging up to third
nearest-neighbor pairs. The results indicate that the helical magnetism in
SrFeO results from competition between ferromagnetic
double-exchange and antiferromagnetic superexchange interactions whose
strengths become comparable near the metal-insulator transition. They thus
confirm a decades-old theoretical prediction and provide a firm experimental
basis for models of magnetic correlations in strongly correlated metals.Comment: PRL, in pres
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