Unconventional Oxygen Isotope Effects
is known that strong electron-phonon interactions can lead to the formation of
lattice polarons (quasiparticles dressed by lattice distortions) as a result
of the breakdown of the Migdal approximation. In the colossal
magnetoresistive (CMR) manganites, such quasiparticles are expected to exist.
A direct and clear-cut experimental technique for demonstrating the existence
of such quasiparticles in manganites is an observation of a giant oxygen
isotope shift of the ferromagnetic transition temperature (see Fig. 1) .
Such a novel isotope effect had never been observed before 1996 because
conventional theories of magnetism do not predict this isotope effect. This
important pioneering work not only shows that the nature of charge carriers
in manganites is of polaronic type, but also establishes a powerful
experimental technique to determine the strength of electron-phonon coupling
and the nature of charge carriers in strongly correlated oxide systems.
Following this original work, I have extensively studied various oxygen
isotope effects in manganites using different experimental techniques and
found many novel oxygen isotope effects in this system [2-4]. In particular,
the observed isotope effects on the transport properties in high-quality thin
films [3-4] provide crucial and quantitative experimental constraints on the
physics of manganites and the microscopic origin of colossal magnetoresistance.
Fig. 1 Temperature dependence of the normalized
magnetization of the 16O and 18O isotope samples of La0.80Ca0.20MnO3+y.
The Curie temperature shifts down by 21 K upon replacing 16O with 18O
isotope. This giant oxygen-isotope shift of the Curie temperature had never
been observed nor expected from conventional theories of ferromagnetism
before 1996. After .
Research on High Temperature Superconductors
correct microscopic theory for high-temperature superconductivity in cuprates
should be able to explain all the unusual physical properties
quantitatively and consistently. These include the pseudogap in the normal
state, novel isotope effects [5-9], dynamic charge and spin stripes, very
large supercarrier mass anisotropy, strongly anisotropic gap symmetry,
magnetic resonance peak, dip and hump features in angle-resolved
photoemission and tunneling spectra, as well as unusual optical properties.
After extensive experimental and theoretical studies of these properties
for many years, I can quantitatively explain these unusual results in a
consistent way [9-11].
tunneling spectrum for a slightly over-doped YBa2Cu3O7
(YBCO) crystal (left scale) together with the phonon density of states
obtained from inelastic neutron scattering (right scale). The vertical dashed
lines mark peak and/or shoulder features in the -d2I/dV2-like spectrum. If strong coupling to phonon modes
happens in YBCO, then the peak and/or shoulder features in the -d2I/dV2-like tunneling spectrum should line up with the
peak and/or shoulder features in the phonon density of states as well.
Indeed, nearly all 13 peak and/or shoulder features in -d2I/dV2-like spectrum match precisely with those in the
phonon density of states. The strong coupling features at 7.1 and 90.8 meV in
-d2I/dV2-like spectrum cannot compare with these neutron
data since the energy positions of these features are outside the energy
range of the neutron data. Nevertheless, the phonon peak at about 6.8 meV is
clearly seen in the high-resolution neutron data of Bi2Sr2CaCu2O8+y
(BSCCO). The feature at 90.8 meV should be the composite phonon energy of 7.1
and 83.7 meV since the sum of 7.1 and 83.7 meV is equal to 90.8 meV. This is
expected from the conventional strong-coupling theory. Such excellent
agreement between neutron and tunneling data provides clear evidence that the
bosonic modes mediating the electron pairing are phonons and that the
tunneling current of this junction is highly directional. After .
various unconventional isotope effects we have observed over last ten years
have provided direct and compelling evidence for strong electron-phonon
interactions and the existence of polarons or bipolarons (the breakdown of
the Migdel approximation) in cuprate superconductors, the role of
electron-phonon coupling in the pairing mechanism of high-temperature
superconductivity has been generally ignored. Only after 2001, have three
Nature papers by a Stanford group , a UC-Berkeley group , and a
Cornell group  also shown evidence for strong electron-phonon coupling in
cuprates from angle-resolved photoemission spectroscopy (ARPES) and scanning
tunneling microscopy (STM). These experiments show strong coupling to the
high-energy phonon modes. The recent optical experiments by a UCSB group 
also disprove magnetic origin of a bosonic mode mediating the pairing and they
thus attribute this mode to a phonon mode. My recent theoretical studies of
the pairing interactions and gap symmetry in cuprates have provided
compelling evidence for predominantly phonon-mediated pairing (see Fig. 2) 
and extended s-wave gap symmetry with eight line nodes (see Fig. 3)
. In particular, I show that low-energy phonon modes couple very strongly
to doped holes and may play a dominant role in the electron pairing. I have
also shown that high-temperature superconductivity in both cuprates and
bismuthates arises from Cooper pairing of polaronic charge carriers
Fig. 3 The high-resolution spectra of
the second derivative -d2ReS/dw2 of the real part of electron
self-energy S along the diagonal direction (right scale) and the -d2/dV2-like
tunneling spectrum for a slightly over-doped Bi2Sr2CaCu2O8+y
(BSCCO) crystal (left scale), Here w = EF−E−DD and DD is the diagonal superconducting gap. If we assign DD =7.0 meV for the
superconducting BSCCO, the peak features in -d2ReS/dw2
match precisely with in the tunneling spectrum and those in the phonon density
of states. This result rules out seemingly well accepted d-wave gap symmetry and strongly supports an extended
s-wave gap symmetry with eight
line nodes. After .
Research on CMR Materials
manganites, the CMR mechanism is still not clear although it is generally
accepted that the electron-phonon interaction plays an essential role in the
physics of this system. Further, the very nature of charge carriers in the
ferromagnetic state has not been clarified. Recently, we have shown
theoretically and experimentally that the low temperature metallic state of
doped manganites is not a conventional Fermi-liquid but has polaronic nature.
Even in the paramagnetic state, the nature of charge carriers is still under
intensive debate. Some theorists believe small polarons are the charge
carriers in the paramagnetic state and the others favor small bipolarons.
There are few experiments that can clearly distinguish between small polarons
and bipolarons because they behave similarly in most physical properties.
Studies of the oxygen-isotope effects on electrical and magnetic properties
may provide essential constraints on the nature of charge carriers in the
paramagnetic state. Another important issue to be clarified is the
microscopic origin of the intrinsic electronic inhomogeneity in some doped
manganites. The electronic inhomogeneity may be one of the key ingredients to
understand the microscopic mechanism of CMR.
1. G. M. Zhao, K. Conder, H. Keller, and K. A.
Muller, Nature 381, 676 (1996).
2. Oxygen isotope effects in manganites: Evidence
for(bi)polaronic charge carriers, G. M. Zhao, H. Keller, R. L. Greene, and K.
A.Muller, Physics of Manganites (Kluwer Academic/Plenum publisher,
NewYork, 1999) eds. T. A. Kaplan and S. D. Mahanti, page 221-241.
3. G. M. Zhao, Y. S. Wang, D. J. Kang, W. Prellier, M.
Rajeswari, H. Keller, T. Venkatesan, C. W. Chu, & R. L. Greene, Phys. Rev.
B (Rapid Communications) 62, R11 949 (2000).
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