119 research outputs found
Effect of a magnetic field on the long-range magnetic order in insulating Nd2CuO4, nonsuperconducting and superconducting Nd1.85Ce0.15CuO4
We have measured the effect of a c-axis aligned magnetic field on the
long-range magnetic order of insulating Nd2CuO4, as-grown nonsuperconducting
and superconducting Nd1.85Ce0.15CuO4. On cooling from room temperature, Nd2CuO4
goes through a series of antiferromagnetic (AF) phase transitions with
different noncollinear spin structures. In all phases of Nd2CuO4, we find that
the applied c-axis field induces a canting of the AF order but does not alter
the basic zero-field noncollinear spin structures. Similar behavior is also
found in as-grown nonsuperconducting Nd1.85Ce0.15CuO4. These results contrast
dramatically with those of superconducting Nd1.85Ce0.15CuO4, where a c-axis
aligned magnetic field induces a static, anomalously conducting, long-range
ordered AF state. We confirm that the annealing process necessary to make
superconducting Nd1.85Ce0.15CuO4 also induces epitaxial, three-dimensional
long-range ordered cubic (Nd,Ce)2O3 as an impurity phase. In addition, the
annealing process makes a series of quasi two-dimensional superlattice
reflections associated with lattice distortions of Nd1.85Ce0.15CuO4 in the CuO2
plane. While the application of a magnetic field will induce a net moment in
the impurity phase, we determine its magnitude and eliminate this as a
possibility for the observed magnetic field-induced effect in superconducting
Nd1.85Ce0.15CuO4.Comment: 12 pages, 10 figures, to be published in Phys. Rev.
Magnetic impurity coupled to interacting conduction electrons
We consider a magnetic impurity which interacts by hybridization with a
system of weakly correlated electrons and determine the energy of the ground
state by means of an 1/N_f expansion. The correlations among the conduction
electrons are described by a Hubbard Hamiltonian and are treated to lowest
order in the interaction strength. We find that their effect on the Kondo
temperature, T_K, in the Kondo limit is twofold: First, the position of the
impurity level is shifted due to the reduction of charge fluctuations, which
reduces T_K. Secondly, the bare Kondo exchange coupling is enhanced as spin
fluctuations are enlarged. In total, T_K increases. Both corrections require
intermediate states beyond the standard Varma-Yafet ansatz. This shows that the
Hubbard interaction does not just provide quasiparticles, which hybridize with
the impurity, but also renormalizes the Kondo coupling.Comment: ReVTeX 19 pages, 3 uuenconded postscript figure
Interaction of a Magnetic Impurity with Strongly Correlated Conduction Electrons
We consider a magnetic impurity which interacts by hybridization with a
system of strongly correlated conduction electrons. The latter are described by
a Hubbard Hamiltonian. By means of a canconical transformation the charge
degrees of freedom of the magnetic impurity are eliminated. The resulting
effective Hamiltonian is investigated and various limiting cases
are considered. If the Hubbard interaction between the conduction electrons
is neglected reduces to a form obtained by the Schrieffer-Wolff
transformation, which is essentially the Kondo Hamiltonian. If is large and
the correlations are strong is changed. One modification concerns
the coefficient of the dominant exchange coupling of the magnetic impurity with
the nearest lattice site. When the system is hole doped, there is also an
antiferromagnetic coupling to the nearest neighbors of that site involving
additionally a hole. Furthermore, it is found that the magnetic impurity
attracts a hole. In the case of electron doping, double occupancies are
repelled by the impurity. In contrast to the hole-doped case, we find no
magnetic coupling which additionally involves a doubly occupied site.Comment: 16 pages, Revtex 3.
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
A layering model for superconductivity in the borocarbides
We propose a superlattice model to describe superconductivity in layered
materials, such as the borocarbide families with the chemical formul\ae\
BC and BC, with being (essentially) a rare earth, and a
transition metal. We assume a single band in which electrons feel a local
attractive interaction (negative Hubbard-) on sites representing the B
layers, while U=0 on sites representing the C layers; the multi-band
structure is taken into account minimally through a band offset . The
one-dimensional model is studied numerically through the calculation of the
charge gap, the Drude weight, and of the pairing correlation function. A
comparison with the available information on the nature of the electronic
ground state (metallic or superconducting) indicates that the model provides a
systematic parametrization of the whole borocarbide family.Comment: 4 figure
Field-Induced Uniform Antiferromagnetic Order Associated with Superconductivity in PrLaCeCuO
Strong correlation between field-induced antiferromagnetic (AF) order and
superconductivity is demonstrated for an electron-doped cuprate superconductor,
PrLaCeCuO (PLCCO). In addition to the specimen with
(which is close to the AF phase boundary, ), we show that
the one with ( K at zero field) also exhibits the
field-induced AF order with a reduced magnitude of the induced moment. The
uniform muon Knight shift at a low magnetic field ( Oe) indicates
that the AF order is not localized within the cores of flux lines, which is in
a marked contrast with theoretical prediction for hole-doped cuprates. The
presence of anomalous non-diagonal hyperfine coupling between muons and Pr ions
is also demonstrated in detail.Comment: 8 pages, 5 figures, to be published in J. Phys. Soc. Jp
Polaronic optical absorption in electron-doped and hole-doped cuprates
Polaronic features similar to those previously observed in the photoinduced
spectra of cuprates have been detected in the reflectivity spectra of
chemically doped parent compounds of high-critical-temperature superconductors,
both -type and -type. In NdCuO these features, whose
intensities depend both on doping and temperature, include local vibrational
modes in the far infrared and a broad band centered at 1000 cm.
The latter band is produced by the overtones of two (or three) local modes and
is well described in terms of a small-polaron model, with a binding energy of
about 500 cm. Most of the above infrared features are shown to survive
in the metallic phase of NdCeCu0, BiSrCuO, and
YBaCuO, where they appear as extra-Drude peaks. The occurrence
of polarons is attributed to local modes strongly coupled to carriers, as shown
by a comparison with tunneling results.Comment: File latex, 31 p., submitted to Physical Review B. Figures may be
faxed upon reques
Anisotropic Spin Hamiltonians due to Spin-Orbit and Coulomb Exchange Interactions
This paper contains the details of Phys. Rev. Lett. 73, 2919 (1994) and, to a
lesser extent, Phys. Rev. Lett. 72, 3710 (1994). We treat a Hubbard model which
includes all the 3d states of the Cu ions and the 2p states of the O ions. We
also include spin-orbit interactions, hopping between ground and excited
crystal field states of the Cu ions, and rather general Coulomb interactions.
Our analytic results for the spin Hamiltonian, H, are corroborated by numerical
evaluations of the energy splitting of the ground manifold for two holes on
either a pair of Cu ions or a Cu-O-Cu complex. In the tetragonal symmetry case
and for the model considered, we prove that H is rotationally invariant in the
absence of Coulomb exchange. When Coulomb exchange is present, each bond
Hamiltonian has full biaxial anisotropy, as expected for this symmetry. For
lower symmetry situations, the single bond spin Hamiltonian is anisotropic at
order t**6 for constant U and at order t**2 for nonconstant U. (Constant U
means that the Coulomb interaction between orbitals does not depend on which
orbitals are involved.)Comment: 50 pages, ILATEX Version 2.09 <13 Jun 1989
Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}
We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x},
which at x=1 exhibits a characteristic pseudogap temperature dependence,
confirming that Pr reduces the concentration of mobile holes in the CuO_{2}
planes. Our estimate of the rate of this counterdoping effect, obtained by
comparison with the shift in pure samples with reduced oxygen content, is found
insufficient to explain the observed reduction of T_c. From the temperature
dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment
and the local magnetic defect induced in the CuO_2 planes produce a long range
spin polarization in the planes, which is likely associated with the extra
reduction of T_c. We find a qualitatively different behaviour in the oxygen
depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T is nearly
the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This
difference may signal a weaker coupling of the Pr to the planes in the
underdoped compound, which might be linked with the larger Pr to CuO_2 plane
distance, and correspondingly weaker hybridization.Comment: 8 pages, 9 figures, accepted in Phys Rev
Periodic Anderson model with correlated conduction electrons
We investigate a periodic Anderson model with interacting conduction
electrons which are described by a Hubbard-type interaction of strength U_c.
Within dynamical mean-field theory the total Hamiltonian is mapped onto an
impurity model, which is solved by an extended non-crossing approximation. We
consider the particle-hole symmetric case at half-filling. Similar to the case
U_c=0, the low-energy behavior of the conduction electrons at high temperatures
is essentially unaffected by the f-electrons and for small U_c a quasiparticle
peak corresponding to the Hubbard model evolves first. These quasiparticles
screen the f-moments when the temperature is reduced further, and the system
turns into an insulator with a tiny gap and flat bands. The formation of the
quasiparticle peak is impeded by increasing either U_c or the c-f
hybridization. Nevertheless almost dispersionless bands emerge at low
temperature with an increased gap, even in the case of initially insulating
host electrons. The size of the gap in the one-particle spectral density at low
temperatures provides an estimate for the low-energy scale and increases as U_c
increases.Comment: 11 pages RevTeX with 13 ps figures, accepted by PR
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