91 research outputs found
A theory of the electric quadrupole contribution to resonant x-ray scattering: Application to multipole ordering phases in Ce_{1-x}La_{x}B_{6}
We study the electric quadrupole (E2) contribution to resonant x-ray
scattering (RXS). Under the assumption that the rotational invariance is
preserved in the Hamiltonian describing the intermediate state of scattering,
we derive a useful expression for the RXS amplitude. One of the advantages the
derived expression possesses is the full information of the energy dependence,
lacking in all the previous studies using the fast collision approximation. The
expression is also helpful to classify the spectra into multipole order
parameters which are brought about. The expression is suitable to investigate
the RXS spectra in the localized f electron systems. We demonstrate the
usefulness of the formula by calculating the RXS spectra at the Ce L_{2,3}
edges in Ce_{1-x}La_{x}B_{6} on the basis of the formula. We obtain the spectra
as a function of energy in agreement with the experiment of
Ce_{0.7}La_{0.3}B_{6}. Analyzing the azimuthal angle dependence, we find the
sixfold symmetry in the \sigma-\sigma' channel and the threefold onein the
\sigma-\pi' channel not only in the antiferrooctupole (AFO) ordering phase but
also in the antiferroquadrupole (AFQ) ordering phase, which behavior depends
strongly on the domain distribution. The sixfold symmetry in the AFQ phase
arises from the simultaneously induced hexadecapole order. Although the AFO
order is plausible for phase IV in Ce_{1-x}La_{x}B_{6}, the possibility of the
AFQ order may not be ruled out on the basis of azimuthal angle dependence
alone.Comment: 12 pages, 6 figure
Superconductivity induced by spark erosion in ZrZn2
We show that the superconductivity observed recently in the weak itinerant
ferromagnet ZrZn2 [C. Pfleiderer et al., Nature (London) 412, 58 (2001)] is due
to remnants of a superconducting layer induced by spark erosion. Results of
resistivity, susceptibility, specific heat and surface analysis measurements on
high-quality ZrZn2 crystals show that cutting by spark erosion leaves a
superconducting surface layer. The resistive superconducting transition is
destroyed by chemically etching a layer of 5 microns from the sample. No
signature of superconductivity is observed in rho(T) of etched samples at the
lowest current density measured, J=675 Am-2, and at T < 45 mK. EDX analysis
shows that spark-eroded surfaces are strongly Zn depleted. The simplest
explanation of our results is that the superconductivity results from an alloy
with higher Zr content than ZrZn2.Comment: Final published versio
Pressure Induced Changes in the Antiferromagnetic Superconductor YbPd2Sn
Low temperature ac magnetic susceptibility measurements of the coexistent
antiferromagnetic superconductor YbPd2Sn have been made in hydrostatic
pressures < 74 kbar in moissanite anvil cells. The superconducting transition
temperature is forced to T(SC) = 0 K at a pressure of 58 kbar. The initial
suppression of the superconducting transition temperature is corroborated by
lower hydrostatic pressure (p < 16 kbar) four point resisitivity measurements,
made in a piston cylinder pressure cell. At ambient pressure, in a modest
magnetic field of ~ 500 G, this compound displays reentrant superconducting
behaviour. This reentrant superconductivity is suppressed to lower temperature
and lower magnetic field as pressure is increased. The antiferromagnetic
ordering temperature, which was measured at T(N) = 0.12 K at ambient pressure
is enhanced, to reach T(N) = 0.58 K at p = 74 kbar. The reasons for the
coexistence of superconductivity and antiferromagnetism is discussed in the
light of these and previous findings. Also considered is why superconductivity
on the border of long range magnetic order is so much rarer in Yb compounds
than in Ce compounds. The presence of a new transition visible by ac magnetic
susceptibility under pressure and in magnetic fields greater than 1.5 kG is
suggested.Comment: 5 pages, 6 figure
The dual nature of 5f electrons and origin of heavy fermions in U compounds
We develop a theory for the electronic excitations in UPt which is based
on the localization of two of the electrons. The remaining electron is
delocalized and acquires a large effective mass by inducing intra-atomic
excitations of the localized ones. The measured deHaas-vanAlphen frequencies of
the heavy quasiparticles are explained as well as their anisotropic heavy mass.
A model calculation for a small cluster reveals why only the largest of the
different hopping matrix elements is operative causing the electrons in
other orbitals to localize.Comment: 6 pages, 3 figure
Pressure dependence of the magnetization of URu2Si2
The ground state of URu2Si2 changes from so-called hidden order (HO) to
large-moment antiferromagnetism (LMAF) upon applying hydrostatic pressure in
excess of 14 kbar. We report the dc-magnetization M(B,T,p) of URu2Si2 for
magnetic fields B up to 12 T, temperatures T in the range 2 to 100 K, and
pressure p up to 17 kbar. Remarkably, characteristic scales such as the
coherence temperature T*, the transition temperature T0, and the anisotropy in
the magnetization depend only weakly on the applied pressure. However, the
discontinuity in dM/dT at T0, which measures the magnetocaloric effect,
decreases nearly 50 % upon applying 17 kbar for M and B parallel to the
tetragonal c-axis, while it increases 15-fold for the a-axis. Our findings
suggest that the HO and LMAF phases have an astonishing degree of similarity in
their physical properties, but a key difference is the magnetocaloric effect
near T0 in the basal plane
The Fermi surface and f-valence electron count of UPt3
Combining old and new de Haas-van Alphen (dHvA) and magnetoresistance data,
we arrive at a detailed picture of the Fermi surface of the heavy fermion
superconductor UPt3. Our work was partially motivated by a new proposal that
two 5f valence electrons per formula unit in UPt3 are localized by correlation
effects -- agreement with previous dHvA measurements of the Fermi surface was
invoked in its support. Comprehensive comparison with our new observations
shows that this 'partially localized' model fails to predict the existence of a
major sheet of the Fermi surface, and is therefore less compatible with
experiment than the originally proposed 'fully itinerant' model of the
electronic structure of UPt3. In support of this conclusion, we offer a more
complete analysis of the fully itinerant band structure calculation, where we
find a number of previously unrecognized extremal orbits on the Fermi surface.Comment: 23 pages, 12 figures, latex, iopart clas
Triple-q octupolar ordering in NpO_2
We report the results of resonant X-ray scattering experiments performed at
the Np M_4,5 edges in NpO_2. Below T_0 = 25 K, the development of long-range
order of Np electric quadrupoles is revealed by the growth of superlattice
Bragg peaks. The electronic transition is not accompanied by any measurable
crystallographic distortion, either internal or external, so the symmetry of
the system remains cubic. The polarization and azimuthal dependence of the
intensity of the resonant peaks is well reproduced assuming Templeton
scattering from a triple-q longitudinal antiferroquadrupolar structure.
Electric quadrupole order in NpO_2 could be driven by the ordering at T_0 of
magnetic octupoles of Gamma_5 symmetry, splitting the Np ground state quartet
and leading to a singlet ground state with zero dipole magnetic moment.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Lett. v2: resubmitted
after referee report
Superconducting transition temperatures and coherence length in non s-wave pairing materials correlated with spin-fluctuation mediated interaction
Following earlier work on electron or hole liquids flowing through assemblies
with magnetic fluctuations, we have recently exposed a marked correlation of
the superconducting temperature Tc, for non s-wave pairing materials, with
coherence length xi and effective mass m*. The very recent study of Abanov et
al. [Europhys. Lett. 54, 488 (2001)] and the prior investigation of Monthoux
and Lonzarich [Phys. Rev. B 59, 14598 (1999)] have each focussed on the concept
of a spin-fluctuation temperature T_sf, which again is intimately related to
Tc. For the d-wave pairing via antiferromagnetic spin fluctuations in the
cuprates, these studies are brought into close contact with our own work, and
the result is that k_B T_sf ~ hbar^2 / m* xi^2. This demonstrates that xi is
also determined by such antiferromagnetic spin-fluctuation mediated pair
interaction. The coherence length in units of the lattice spacing is then
essentially given in the cuprates as the square root of the ratio of two
characteristic energies, namely: the kinetic energy of localization of a charge
carrier of mass m* in a specified magnetic correlation length to the hopping
energy. The quasi-2D ruthenate Sr_2RuO_4, with Tc ~ 1.3 K, has p-wave
spin-triplet pairing and so is also briefly discussed here.Comment: Accepted for publication in Phys. Rev.
Local Moment Formation in the Periodic Anderson Model with Superconducting Correlations
We study local moment formation in the presence of superconducting
correlations among the f-electrons in the periodic Anderson model. Local
moments form if the Coulomb interaction U>U_cr. We find that U_cr is
considerably stronger in the presence of superconducting correlations than in
the non-superconducting system. Our study is done for various values of the
f-level energy and electronic density. The smallest critical U_cr values occur
for the case where the number of f- electrons per site is equal to one. In the
presence of d-wave superconducting correlations we find that local moment
formation presents a quantum phase transition as function of pressure. This
quantum phase transition separates a region where local moments and d-wave
superconductivity coexist from another region characterized by a
superconducting ground state with no local moments. We discuss the possible
relevance of these results to experimental studies of the competition between
magnetic order and superconductivity in CeCu_2Si_2.Comment: 4 pages. accepted for publication in Phys. Rev.
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