107 research outputs found
Far-infrared optical conductivity of CeCu2Si2
Journal ref.: J. Phys.: Condens. Matter 25, 065602 (2013): We investigated
the optical reflectivity of the heavy-fermion metal CeCu2Si2 in the energy
range 3 meV - 30 eV for temperatures between 4K - 300K. The results for the
charge dynamics indicate a behavior that is expected for the formation of a
coherent heavy quasiparticle state: Upon cooling the spectra of the optical
conductivity indicate a narrowing of the coherent response. Below temperatures
of 30 K a considerable suppression of conductivity evolves below a peak
structure at 13 meV. We assign this gap-like feature to strong electron
correlations due to the 4f-conduction electron hybridization.Comment: 7 pages, 3 figure
Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model
A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined
theoretical and experimental study. Despite a dominant intradimer coupling J1,
sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K.
The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural
beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in
alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a
honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in
the coupling regime of these isostructural compounds is governed by the nature
of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a
Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the
superexchange is realized via O-O edges of the tetrahedron. Implications for a
broad range of systems are discussed.Comment: 8 pages, 5 figures, 1 table; discussion extende
Ferromagnetism and superconductivity in P-doped CeFeAsO
We report on superconductivity in CeFeAs1-xPxO and the possible coexistence
with Ce- ferromagnetism (FM) in a small homogeneity range around x = 30% with
ordering temperatures of T_SC = T_C = 4K. The antiferromagnetic (AFM) ordering
temperature of Fe at this critical concentration is suppressed to T^N_Fe ~ 40K
and does not shift to lower temperatures with further increase of the P
concentration. Therefore, a quantum-critical-point scenario with T^N_Fe -> 0K
which is widely discussed for the iron based superconductors can be excluded
for this alloy series. Surprisingly, thermal expansion and X-ray powder
diffraction indicate the absence of an orthorhombic distortion despite clear
evidence for short range AFM Fe-ordering from muon-spin-rotation measurements.
Furthermore, we discovered the formation of a sharp electron spin resonance
signal unambiguously connected with the emergence of FM ordering.Comment: 5 pages, 4 figures, published in Phys. Rev. B (Rapid Communication,
Editors suggestion
Structure and magnetism of Cr2BP3O12: Towards the quantum-classical crossover in a spin-3/2 alternating chain
Magnetic properties of the spin-3/2 Heisenberg system Cr2BP3O12 are
investigated by magnetic susceptibility chi(T) measurements, electron spin
resonance, neutron diffraction, and density functional theory (DFT)
calculations, as well as classical and quantum Monte Carlo (MC) simulations.
The broad maximum of chi(T) at 85K and the antiferromagnetic Weiss temperature
of 139 K indicate low-dimensional magnetic behavior. Below TN = 28 K, Cr2BP3O12
is antiferromagnetically ordered with the k = 0 propagation vector and an
ordered moment of 2.5 muB/Cr. DFT calculations, including DFT+U and hybrid
functionals, yield a microscopic model of spin chains with alternating
nearest-neighbor couplings J1 and J1' . The chains are coupled by two
inequivalent interchain exchanges of similar strength (~1-2 K), but different
sign (antiferromagnetic and ferromagnetic). The resulting spin lattice is
quasi-one-dimensional and not frustrated. Quantum MC simulations show excellent
agreement with the experimental data for the parameters J1 ~= 50 K and J1'/J1
~= 0.5. Therefore, Cr2BP3O12 is close to the gapless critical point (J1'/J1 =
0.41) of the spin-3/2 bond-alternating Heisenberg chain. The applicability
limits of the classical approximation are addressed by quantum and classical MC
simulations. Implications for a wide range of low-dimensional S = 3/2 materials
are discussed.Comment: Published version: 13 pages, 7 figures, 5 tables + Supplementary
informatio
ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties
Below the Kondo temperature the heavy Fermion compound YbRhSi
shows a well defined Electron Spin Resonance (ESR) with local Yb
properties. We report a detailed analysis of the ESR intensity which gives
information on the number of ESR active centers relative to the ESR of well
localized Yb in YPd:Yb. The ESR lineshape is investigated regarding
contributions from itinerant centers. From the ESR of monoisotopic
YbRhSi we could exclude unresolved hyperfine contributions
to the lineshape.Comment: 3 Figure
Spin dynamics of observed by Electron Spin Resonance
Below the Kondo temperature electron spin resonance (ESR) usually
is not observable from the Kondo-ion itself because the characteristic spin
fluctuation energy results in a huge width of the ESR line. The heavy fermion
metal YbRhSi seems to be an exceptional case where definite ESR
spectra show characteristic properties of the Kondo-ion Yb well
\textit{below} . We found that the spin dynamics of
YbRhSi, as determined by its ESR relaxation, is spatially
characterized by an anisotropy of the zero temperature residual relaxation
only.Comment: Presented at NanoRes 2004, Kazan; 4 pages, 3 Figure
Field dependence of the Eu2+ spin relaxation in EuFe(2-x)CoxAs2
The layered compound EuFe2As2 is an interesting model system to investigate
the effects of well defined local Eu2+ 4f states on the itinerant electronic
and magnetic properties of the FeAs layers. To address this subject, we
investigated the series EuFe2-xCoxAs2 (0.1 <= x <=0.75) by electron spin
resonance (ESR) of Eu2+ to probe the spin dynamics of the itinerant subsystem.
We relate the results to dc-susceptibility measurements and band structure
calculations. As a consequence of the weak coupling between the local and
itinerant subsystems we found that the spin relaxation is well understood in
terms of the exchange coupling among the local Eu2+ spins. A pronounced field
dependence of the Eu2+ spin relaxation demonstrates the direct influence of
magnetic fluctuations at the Fe2-xCoxAs2 layers.Comment: 13 pages, 5 figure
Anisotropic optical conductivity of the putative Kondo insulator CeRuSn
Kondo insulators and in particular their non-cubic representatives have
remained poorly understood. Here we report on the development of an anisotropic
energy pseudogap in the tetragonal compound CeRuSn employing optical
reflectivity measurements in broad frequency and temperature ranges, and local
density approximation plus dynamical mean field theory calculations. The
calculations provide evidence for a Kondo insulator-like response within the
plane and a more metallic response along the c axis and qualitatively
reproduce the experimental observations, helping to identify their origin
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