791 research outputs found
Electronic phase diagram of LaBaSrCuO
We performed systematic measurements of magnetic susceptibility on single
crystals of LaBaSrCuO. The dependence of the
superconducting transition temperature on Sr-concentration demonstrates a
step-like pattern upon doping at {\it x}0.08 as the crystal structure
changes from low-temperature tetragonal (LTT) to low-temperature orthorhombic
(LTO) phase at low temperature. Upon cooling, an anomalous upturn in the
susceptibility was observed at the structural phase transition between the
LTT-LTO phases under the magnetic field parallel to {\it c}-axis.Comment: 6 pages, 4 figures, Proceeding paper of the Stripes2000 conference in
Roma, Ital
NMR relaxation time around a vortex in stripe superconductors
Site-dependent NMR relaxation time is calculated in the vortex
state using the Bogoliubov-de Gennes theory, taking account of possible
"field-induced stripe'' states in which the magnetism arises locally around a
vortex core in d-wave superconductivity. The recently observed huge enhancement
below at a core site in TlBaCuO is
explained. The field-induced stripe picture explains consistently other
relevant STM and neutron experiments.Comment: 4 pages, 4 figure
Neutron Scattering Study of Temperature-Concentration Phase Diagram of (Cu1-xMgx)GeO3
In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the
spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF)
phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low
doping region. For higher x, however, the SP state disappears and only the AF
state remains. These features are common for all the doped CuGeO3 systems so
far studied, indicating the existence of universal T-x phase diagram. Recently,
Masuda et al. carried out comprehensive magnetic susceptibility (chi)
measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled
significantly better than the Zn doped systems. They found that TN suddenly
jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a
drop in chi corresponding to the SP ordering also disappears at x>xc. They thus
concluded that there is a compositional phase boundary between two distinct
magnetic phases. To clarify the nature of two phases, we performed
neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various
x. Analysis of the data at fixed temperature points as a function of doping
concentration has revealed sudden changes of order parameters at the critical
concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic
increase of the AF moment takes place at xc. The spin-Peierls order parameter
delta associated with lattice dimerization shows a precipitous decrease at all
temperature below Tsp. However, it goes to zero above xc only at the low
temperature limit.Comment: 9 pages, 9 figure
Antiferromagnetism and hole pair checkerboard in the vortex state of high Tc superconductors
We propose a microscopic state for the vortex phase of BSCO superconductors.
Around the vortex core or above H_{c2}, the d wave hole pairs form a
checkerboard localized in the commensurate antiferromagnetic background. We
discuss this theory in connection with recent STM experiments.Comment: Final versio
Absence of magnetic field effect on static magnetic order in electron-doped superconductor Nd_{1.86}Ce_{0.14}CuO_4
Neutron-scattering experiments were performed to study the magnetic field
effect on the electron-doped cuprate superconductor Nd_{1.86}Ce_{0.14}CuO_4,
which shows the coexistence of magnetic order and superconductivity. The (1/2
3/2 0) magnetic Bragg intensity, which mainly originates from the order of both
the Cu and Nd moments at low temperatures, shows no magnetic field dependence
when the field is applied perpendicular to the CuO_{2} plane up to 10 T above
the upper critical field. This result is significantly different from that
reported for the hole-doped cuprate superconductors, in which the quasi-static
magnetic order is noticeably enhanced under a magnetic field.Comment: 4 pages, 3 figure
Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88LaCe0.12CuO4
We use neutron scattering to demonstrate that electron-doped superconducting
Pr0.88LaCe0.12CuO4 in the underdoped regime is electronically phase separated
in the ground state, showing the coexistence of a superconducting phase with a
three-dimensional antiferromagnetically ordered phase and a
quasi-two-dimensional spin density wave modulation. The Neel temperature of
both antiferromagnetic phases decreases linearly with increasing
superconducting transition temperature (Tc) and vanishes when optimal
superconductivity is achieved. These results indicate that the electron-doped
copper oxides are close to a quantum critical point, where the delicate
energetic balance between different competing states leads to microscopic
heterogeneity.Comment: 14 pages, 4 figures, accepted to Phys. Rev. B as a rapid
communicatio
Spin ordering quantum transitions of superconductors in a magnetic field
We argue that recent neutron scattering measurements by Lake et. al. (Science
291, 1759 (2001)) of the spin excitation spectrum of LSCO in a magnetic field
can be understood in terms of proximity to a phase with co-existing
superconductivity and spin density wave order. We present a general theory for
such quantum transitions, and argue that their low energy spin fluctuations are
controlled by a singular correction from the superflow kinetic energy, acting
in the region outside the vortex cores. We propose numerous experimental tests
of our theory.Comment: 4 pages, 3 eps fig
High-T_{c} Superconductors with AF Order: Limitations on Spin-Fluctuation Pairing Mechanism
The very intriguing antagonistic interplay of antiferromagnetism (AF) and
superconductivity (SC), recently discovered in high-temperature
superconductors, is studied in the framework of a microscopic theory. We
explain the surprisingly large increase of the magnetic Bragg peak intensity
at in the magnetic field at low
temperatures in . Good agreement
with experimental results is found. The theory predicts large anisotropy of the
relative intensity %, i.e.
. The quantum (T=0) phase
diagram at H=0 is constructed. The theory also predicts: (i) the magnetic field
induced AF order in the SC state; (ii) small value for the spin-fluctuation
coupling constant . The latter gives very small SC
critical temperature , thus questioning the spin-fluctuation
mechanism of pairing in HTS oxides.Comment: Linguistic changes, improved readabilty, changed titl
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