587 research outputs found
Two mechanisms of pseudogap formation in Bi-2201: Evidence from the c-axis magnetoresistance
Measurements of the c-axis resistivity and magnetoresistance have been used
to investigate the pseudogap (PG) behavior in Bi_{2+z}Sr_{2-x-z}La_xCuO_y
(Bi-2201) crystals at various hole densities. While the PG opening temperature
T* increases with decreasing hole doping, the magnetic-field sensitivity of the
PG is found to have a very different trend: it appears at lower temperatures in
more underdoped samples and vanishes in non-superconducting samples. These data
suggest that besides the field-insensitive pseudogap emerging at T*, a distinct
one is formed above T_c as a precursor to superconductivity.Comment: 7 pages, 6 figures, accepted for publication in Europhysics Letters
(initially submitted to PRL on 14 June 2000
Crossover from a pseudogap state to a superconducting state
On the basis of our calculation we deduce that the particular electronic
structure of cuprate superconductors confines Cooper pairs to be firstly formed
in the antinodal region which is far from the Fermi surface, and these pairs
are incoherent and result in the pseudogap state. With the change of doping or
temperature, some pairs are formed in the nodal region which locates the Fermi
surface, and these pairs are coherent and lead to superconductivity. Thus the
coexistence of the pseudogap and the superconducting gap is explained when the
two kinds of gaps are not all on the Fermi surface. It is also shown that the
symmetry of the pseudogap and the superconducting gap are determined by the
electronic structure, and non-s wave symmetry gap favors the high-temperature
superconductivity. Why the high-temperature superconductivity occurs in the
metal region near the Mott metal-insulator transition is also explained.Comment: 7 pages, 2 figure
Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu(BO)$_2
We have studied spin excitation spectra in the Shastry-Sutherland model
compound SrCu(BO) in magnetic fields using far-infrared Fourier
spectroscopy. The transitions from the ground singlet state to the triplet
state at 24 cm and to several bound triplet states are induced by the
electric field component of the far-infrared light. To explain the light
absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM)
mechanism where light couples to a phonon mode, allowing the DM interaction.
Two optical phonons couple light to the singlet to triplet transition in
SrCu(BO). One is -polarized and creates an intra-dimer dynamic
DM along the c axis. The other is -polarized and creates an intra-dimer
dynamic DM interaction, it is in the plane and perpendicular to the
dimer axis. Singlet levels at 21.5 and 28.6 cm anti-cross with the first
triplet as is seen in far-infrared spectra. We used a cluster of two dimers
with a periodic boundary condition to perform a model calculation with scaled
intra- and inter-dimer exchange interactions. Two static DM interactions are
sufficient to describe the observed triplet state spectra. The static
inter-dimer DM in the c-direction cm splits the triplet state
sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87},
167205 (2001)]. The static intra-dimer DM in the plane (perpendicular to
the dimer axis) cm, allowed by the buckling of CuBO
planes, couples the triplet state to the 28.6 cm singlet as is seen from
the avoided crossing.Comment: 12 pages with 7 figures, some references correcte
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