24 research outputs found
Complex and strongly anisotropic magnetism in the pure spin system EuRh2Si2
In divalent Eu systems, the 4f local moment has a pure spin state J=S=7/2.
Although the absence of orbital moment precludes crystal electric field
effects, we report a sizeable magnetic anisotropy in single crystals of
EuRh2Si2. We observed a surprisingly complex magnetic behavior with three
succesive phase transitions. The Eu2+ moments order in a likely
amplitude-modulated structure below 24.5K, undergoing a further transition to a
structure that is possibly of the equal moment type, and a first order
transition at lower temperatures, presumably into a spin spiral structure. The
sharp metamagnetic transition observed at low fields applied perpendicular to
the hard axis is consistent with a change from a spiral to a fan structure.
These magnetic structures are presumably formed by ferromagnetic planes
perpendicular to the c axis, stacked antiferromagnetically along c but not of
type I, at least just below the ordering temperature. Since EuRh2Si2 is
isoelectronic to EuFe2As2, our results are also relevant for the complex
Eu-magnetism observed there, especially for the transition from an
antiferromagnetic to a ferromagnetic state observed in EuFe2P2 upon
substituting As by P.Comment: submitted to Journal of Physics: Condensed Matte
Charge, lattice and magnetism across the valence crossover in EuIrSi single crystals
We present a detailed study of the temperature evolution of the crystal
structure, specific heat, magnetic susceptibility and resistivity of single
crystals of the paradigmatic valence-fluctuating compound EuIrSi. A
comparison to stable-valent isostructural compounds EuCoSi (with
Eu), and EuRhSi, (with Eu) reveals an anomalously large
thermal expansion indicative of the lattice softening associated to valence
fluctuations. A marked broad peak at temperatures around 65-75 K is observed in
specific heat, susceptibility and the derivative of resistivity, as thermal
energy becomes large enough to excite Eu into a divalent state, which localizes
one f electron and increases scattering of conduction electrons. In addition,
the intermediate valence at low temperatures manifests in a moderately
renormalized electron mass, with enhanced values of the Sommerfeld coefficient
in the specific heat and a Fermi-liquid-like dependence of resistivity at low
temperatures. The high residual magnetic susceptibility is mainly ascribed to a
Van Vleck contribution. Although the intermediate/fluctuating valence duality
is to some extent represented in the interconfiguration fluctuation model
commonly used to analyze data on valence-fluctuating systems, we show that this
model cannot describe the different physical properties of EuIrSi with
a single set of parameters.Comment: 12 pages, 4 figures, 1 tabl
Multiband superconductivity with unexpected deficiency of nodal quasiparticles in CeCu2Si2
Superconductivity in the heavy-fermion compound CeCu2Si2 is a prototypical
example of Cooper pairs formed by strongly correlated electrons. For more than
30 years, it has been believed to arise from nodal d-wave pairing mediated by a
magnetic glue. Here, we report a detailed study of the specific heat and
magnetization at low temperatures for a high-quality single crystal.
Unexpectedly, the specific-heat measurements exhibit exponential decay with a
two-gap feature in its temperature dependence, along with a linear dependence
as a function of magnetic field and the absence of oscillations in the field
angle, reminiscent of multiband full-gap superconductivity. In addition, we
find anomalous behavior at high fields, attributed to a strong Pauli
paramagnetic effect. A low quasiparticle density of states at low energies with
a multiband Fermi-surface topology would open a new door into electron pairing
in CeCu2Si2.Comment: 5 pages, 4 figures (main text) + 5 pages, 6 figures (supplemental
material), published in Phys. Rev. Let
Valence effect on the thermopower of Eu systems
We investigated the thermoelectric transport properties of EuNi2P2 and
EuIr2Si2 in order to evaluate the relevance of Kondo interaction and valence
fluctuations in these materials. While the thermal conductivities behave
conventionally, the thermopower curves exhibit large values with pronounced
maxima as typically observed in Ce- and Yb-based heavy-fermion materials.
However, neither the positions of these maxima nor the absolute thermopower
values at low temperature are in line with the heavy-fermion scenario and the
moderately enhanced effective charge carrier masses. Instead, we may relate the
thermopower in our materials to the temperature-dependent Eu valence by taking
into account changes in the chemical potential. Our analysis confirms that
valence fluctuations play an important role in EuNi2P2 and EuIr2Si2.Comment: 7 pages, 3 figure
Thermodynamic study of gap structure and pair-breaking effect by magnetic field in the heavy-fermion superconductor CeCu2Si2
This paper presents the results of specific-heat and magnetization
measurements, in particular their field-orientation dependence, on the first
discovered heavy-fermion superconductor CeCuSi (
K). We discuss the superconducting gap structure and the origin of the
anomalous pair-breaking phenomena, leading e.g., to the suppression of the
upper critical field , found in the high-field region. The data
show that the anomalous pair breaking becomes prominent below about 0.15 K in
any field direction, but occurs closer to for . The
presence of this anomaly is confirmed by the fact that the specific-heat and
magnetization data satisfy standard thermodynamic relations. Concerning the gap
structure, field-angle dependences of the low-temperature specific heat within
the and planes do not show any evidence for gap nodes. From
microscopic calculations in the framework of a two-band full-gap model, the
power-law-like temperature dependences of and , reminiscent of nodal
superconductivity, have been reproduced reasonably. These facts further support
multiband full-gap superconductivity in CeCuSi.Comment: 10 pages, 8 figures, published in Phys. Rev.
Fingerprint of dynamical charge/spin correlations in the tunneling spectra of colossal magnetoresistive manganites
We present temperature-dependent scanning tunneling spectroscopy measurements
on () films with different degrees of biaxial
strain. A depletion in normalized conductance around the Fermi level is
observed both above and below the insulator-to-metal transition temperature
, for weakly as well as highly-strained films. This pseudogap-like
depletion globally narrows on cooling. The zero-bias conductance decreases on
cooling in the insulating phase, reaches a minimum close to and
increases on cooling in the metallic phase, following the trend of macroscopic
conductivity. These results support a recently proposed scenario in which
dynamical short-range antiferromagnetic/charge order correlations play a
preeminent role in the transport properties of colossal magnetoresistive
manganites [R. Yu \textit{et al}., Phys. Rev. B \textbf{77}, 214434 (2008)].Comment: 9 pages, 4 figure
TSFZ Growth of Eu-Substituted Large-Size LSCO Crystals
The travelling solvent floating zone (TSFZ) growth of Eu-substituted LSCO (La1.81−xEuxSr0.19CuO4, with nominal x = 0 ÷ 0.4) single crystals was systematically explored for the first time. The substitution of La with Eu considerably decreased the decomposition temperature. Optimal growth parameters were found to be: oxygen pressure 9.0–9.5 bars; Eu-free CuO-poor solvent (66 mol% CuO) with a molar ratio of La2O3:SrCO3:CuO = 4:4.5:16.5 and growth rate 0.6 mm/hour. The obtained single crystals were characterized with optical polarized microscopy, X-ray diffraction and energy-dispersive X-ray spectroscopy analysis. The solubility of Eu in LSCO appeared to be limited to x~0.36–0.38 under the used conditions. The substitution of La3+ with smaller Eu3+ ions led to a structural transition from tetragonal with space group I4/mmm for La1.81Sr0.19CuO4 (x = 0) to orthorhombic with space group Fmmm for La1.81−xSr0.19EuxCuO4 (x = 0.2, 0.3, 0.4), and to a substantial shrinking of the c-axis from 13.2446 Å (x = 0.0) to 13.1257 Å (x = 0.4). Such structural changes were accompanied by a dramatic decrease in the superconducting critical temperature, Tc, from 29.5 K for x = 0 to 13.8 K for 0.2. For x ≥ 0.3, no superconductivity was detected down to 4 K
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Flux Growth and Characterization of Bulk InVO4 Crystals
The flux growth of InVO4 bulk single crystals has been explored for the first time. The reported eutectic composition at a ratio of V2O5:InVO4 = 1:1 could not be used as a self-flux since no sign of melting was observed up to 1100 °C. Crystals of InVO4 of typical size 0.5 × 1 × 7 mm3 were obtained using copper pyrovanadate (Cu2V2O7) as a flux, using Pt crucibles. X-ray powder diffraction confirmed the orthorhombic Cmcm structure. Rests of the flux material were observed on the sample surface, with occasional traces of Pt indicating some level of reaction with the crucible. X-ray absorption spectroscopy showed that oxidation states of indium and vanadium ions are +3 and +5, respectively. The size and high quality of the obtained InVO4 crystals makes them excellent candidates for further study of their physical properties
Superconducting gap and vortex lattice of the heavy fermion compound CeCu_2Si_2
The order parameter and pairing mechanism for superconductivity in heavy
fermion compounds are still poorly understood. Scanning tunneling microscopy
and spectroscopy at ultra-low temperatures can yield important information
about the superconducting order parameter and the gap structure. Here, we study
the first heavy fermion superconductor, CeCu_2Si_2. Our data show the
superconducting gap which is not fully formed and exhibits features that point
to a multi-gap order parameter. Spatial mapping of the zero bias conductance in
magnetic field reveals the vortex lattice, which allows us to unequivocally
link the observed conductance gap to superconductivity in CeCu_2Si_2. The
vortex lattice is found to be predominantly triangular with distortions at
fields close to \sim 0.7 H_{c2}.Comment: 6 pages, 4 figures, revised version accepted for publication in PR
Divalent EuRh2Si2 as a reference for the Luttinger theorem and antiferromagnetism in trivalent heavy-fermion YbRh2Si2
Application of the Luttinger theorem to the Kondo lattice YbRh2Si2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh2Si2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh2Si2 has a large FS essentially similar to the one seen in YbRh2Si2 down to 1 K. In EuRh2Si2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh2Si2 indicate that the formation of the AFM state in YbRh2Si2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system