308 research outputs found
Interplay of antiferromagnetism, ferromagnetism and superconductivity in EuFe_2(As_1-xP_x)_2 single crystals
We report a systematic study on the influence of antiferromagnetic and
ferromagnetic phases of Eu^2+ moments on the superconducting phase upon doping
the As site by isovalent P, which acts as chemical pressure on EuFe_2As_2. Bulk
superconductivity with transition temperatures of 22 K and 28 K are observed
for x=0.16 and 0.20 samples respectively. The Eu ions order
antiferromagnetically for x=0.22
whereupon the Eu ions order ferromagnetically. Density functional theory based
calculations reproduce the observed experimental findings consistently. We
discuss in detail the coexistence of superconductivity and magnetism in a tiny
region of the phase space and comment on the competition of ferromagnetism and
superconductivity in the title compound.Comment: 6 pages, 5 figures, 1 tabl
Unique phase diagram with narrow superconducting dome in EuFe(AsP) due to Eu local magnetic moments
The interplay between superconductivity and Eu magnetic moments in
EuFe(AsP) is studied by electrical resistivity measurements
under hydrostatic pressure on and single crystals. We can map
hydrostatic pressure to chemical pressure and show, that superconductivity
is confined to a very narrow range in the phase diagram,
beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The
change from antiferro- to FM Eu ordering at the latter concentration coincides
with a Lifshitz transition and the complete depression of iron magnetic order.Comment: 4 page
Optical investigations of the chemical pressurized EuFe2(As1-xPx)2: an s-wave superconductor with strong interband interaction
Superconducting EuFe2(As0.82P0.18)2 single crystals are investigated by
infrared spectroscopy in a wide frequency range. Below Tc=28K a superconducting
gap forms at 2\Delta_{0} = 9.5 meV = 3.8 k_B T_c causing the reflectivity to
sharply rise to unity at low frequency. In the range of the gap the optical
conductivity can be perfectly described by BCS theory with an -wave gap and
no nodes. From our analysis of the temperature dependent conductivity and
spectral weight at T>T_c, we deduce an increased interband coupling between
hole- and electron-sheets on the Fermi surface when approaches T_c
Anisotropy, disorder, and superconductivity in CeCu2Si2 under high pressure
Resistivity measurements were carried out up to 8 GPa on single crystal and
polycrystalline samples of CeCu2Si2 from differing sources in the homogeneity
range. The anisotropic response to current direction and small uniaxial
stresses was explored, taking advantage of the quasi-hydrostatic environment of
the Bridgman anvil cell. It was found that both the superconducting transition
temperature Tc and the normal state properties are very sensitive to uniaxial
stress, which leads to a shift of the valence instability pressure Pv and a
small but significant change in Tc for different orientations with respect to
the tetragonal c-axis. Coexistence of superconductivity and residual
resistivity close to the Ioffe-Regel limit around 5 GPa provides a compelling
argument for the existence of a valence-fluctuation mediated pairing
interaction at high pressure in CeCu2Si2.Comment: 12 pages, 7 figure
Experimental Quantification of Entanglement Through Heat Capacity
A new experimental realization of heat capacity as an entanglement witness
(EW) is reported. Entanglement properties of a low dimensional quantum spin
system are investigated by heat capacity measurements performed down to very
low temperatures (400mK), for various applied magnetic field values. The
experimentally extracted results for the value of heat capacity at zero field
matches perfectly with the theoretical estimates of entanglement from model
Hamiltonians. The studied sample is a spin antiferromagnetic
system which shows clear signature of quantum phase transition (QPT) at very
low temperatures when the heat capacity is varied as a function of fields at a
fixed temperature. The variation of entanglement as a function of field is then
explored in the vicinity of the quantum phase transition to capture the sudden
loss of entanglement.Comment: 8 pages, 6 figures, To be published in NJ
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
Magnetic and superconducting properties on S-type single-crystal CeCuSi probed by Cu nuclear magnetic resonance and nuclear quadrupole resonance
We have performed Cu nuclear magnetic resonance/nuclear quadrupole
resonance measurements to investigate the magnetic and superconducting (SC)
properties on a "superconductivity dominant" (-type) single crystal of
CeCuSi. Although the development of antiferromagnetic (AFM)
fluctuations down to 1~K indicated that the AFM criticality was close, Korringa
behavior was observed below 0.8~K, and no magnetic anomaly was observed above
0.6 K. These behaviors were expected in -type
CeCuSi. The temperature dependence of the nuclear spin-lattice
relaxation rate at zero field was almost identical to that in the
previous polycrystalline samples down to 130~mK, but the temperature dependence
deviated downward below 120~mK. In fact, in the SC state could be
fitted with the two-gap -wave rather than the two-gap -wave
model down to 90~mK. Under magnetic fields, the spin susceptibility in both
directions clearly decreased below , indicative of the formation of
spin singlet pairing. The residual part of the spin susceptibility was
understood by the field-induced residual density of states evaluated from
, which was ascribed to the effect of the vortex cores. No magnetic
anomaly was observed above the upper critical field , but the
development of AFM fluctuations was observed, indicating that superconductivity
was realized in strong AFM fluctuations.Comment: 10 pages, 8 figure
Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR
We report bulk superconductivity (SC) in
EuSr(FeCo)As single crystals by
means of electrical resistivity, magnetic susceptibility, and specific heat
measurements with 20 K with an antiferromagnetic
(AFM) ordering of Eu moments at 2.0 K in
zero field. As NMR experiments have been performed in the two external
field directions (H) and (H). As-NMR spectra are analyzed in
terms of first order quadrupolar interaction. Spin-lattice relaxation rates
(1/) follow a law in the temperature range 4.2-15 K. There is no
signature of Hebel-Slichter coherence peak just below the SC transition
indicating a non s-wave or s type of superconductivity. The increase of
1/ with lowering the temperature in the range 160-18 K following
law reflecting 2D AFM spin fluctuations
Signatures of quantum criticality in hole-doped and chemically pressurized EuFe_2As_2 single crystals
We study the effect of hole-doping and chemical pressure (isovalent doping)
in single crystals of KEuFeAs and
EuFe(AsP), respectively, by measurements of the
thermopower, , and electrical resistivity, . The evolution of
upon doping indicates drastic changes of the electronic configuration at
critical values and , respectively,
as the spin-density-wave transition is completely suppressed and
superconductivity (SC) emerges. For the case of chemical pressure, the
comparison with published ARPES measurements indicates a Lifshitz transition at
. The temperature dependences and
observed in the normal state above the SC transition
suggest quantum criticality in both systems.Comment: PRB accepte
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