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ABSTRACT: We report an angle-dependent study of the magnetic torque $\tau(\theta)$
within the vortex state of single-crystalline LaO$_{0.9}$F$_{0.1}$FeAs and
SmO$_{0.9}$F$_{0.1}$FeAs as a function of both temperature $T$ and magnetic
field $H$. Sharp peaks are observed at a critical angle $\theta_c$ at either
side of $\theta=90^{\circ}$, where $\theta$ is the angle between $H$ and the
inter-planar \emph{c}-axis. $\theta_c$ is interpreted as the critical depinning
angle where the vortex lattice, pinned and locked by the intrinsic planar
structure, unlocks and acquires a component perpendicular to the planes. We
observe a series of smaller replica peaks as a function of $\theta$ and as
$\theta$ is swept away from the planar direction. These suggest
commensurability effects between the period of the vortex lattice and the
inter-planar distance leading to additional kinked vortex configurations.
03/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Single crystals of MgCNi$_3$, with areas sized up to 1 mm$^{2}$, were grown
by the self flux method using a cubic anvil high pressure technique. The first
critical field \textit{H$_{c1}$}, determined from a zero temperature
extrapolation, is around 18 mT. Using the tunnel - diode resonator technique,
the London penetration depth was measured with no applied \textit{dc} field and
the Campbell penetration depth was measured with the external \textit{dc}
fields up to 9T for two different sample orientations with respect to the
direction of applied magnetic field. The absolute value of the London
penetration depth, $\lambda(0) = 245 \pm 10$ nm was determined from the
thermodynamic Rutgers formula. The superfluid density,
$\rho_s=(\lambda(0)/\lambda(T))^2$ was found to follow the clean isotropic
\textit{s}-wave behavior predicted by the weak - coupling BCS theory in the
whole temperature range. The low - temperature behavior of the London
penetration depth fits the BCS analytic form as well and produces close to the
weak - coupling value of $\Delta (0)/k_BT_c = 1.71$. The temperature dependence
of the upper critical field, \textit{$H_{c2}$}, was found to be isotropic with
a slope at \textit{T$_c$} of -2.63 T/K and \textit{H$_{c2}$}(0) $\approx$ 12.3
T at zero temperature. The Campbell penetration depth probes the vortex lattice
response in the mixed state and is sensitive to the details of the pinning
potential. For MgCNi$_3$, an irreversible feature has been observed in the TDR
response when the sample is field-cooled and warmed versus zero-field-cooled
and warmed. This feature possesses a non-monotonic field dependence and has
commonly been referred to as the peak effect and is most likely related to a
field - dependent non - parabolic pinning potential.
01/2013;
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ABSTRACT: Hydrostatic pressure Raman measurements have been carried out on the SmFeAsO
series of oxypnictides with varying amount of doping (F substitution for O and
Co for Fe) and transition temperature (T_{c}), in order to investigate lattice
modifications and their connection to doping and superconductivity. Synchrotron
XRD data on some of these compounds indicated that at low doping the lattice
constants vary smoothly with pressure, but with increasing F concentration
there is a deviation from the normal equation of state and these effects are
related with modifications in the superconducting FeAs4 tetrahedra. The
hydrostatic pressure Raman measurements show that the A1g mode of the rare
earth atom for the superconducting compounds deviates from the linear pressure
dependence at the same pressures where the XRD results indicate
pressure-induced lattice anomalies. A similar anomaly is found for the As
phonon of same symmetry. As in cuprates, the effect is diminished in the
undoped compounds and it is not related with the F substitution being present
in the Sm(Fe_{1-x}Co_{x})AsO as well. The calculated Gr\"uneisen parameter for
the Sm phonon ({\gamma \approx}1.5) is very similar to the corresponding values
of cuprates and it does not vary with doping. For the Fe mode it has higher
value ({\gamma \approx}1.8) than for As ({\gamma \approx}1) indicating a more
anharmonic phonon.
10/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with
lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high
pressure. The crystals are of good structural quality and become
superconducting when O is partially substituted by F (PrFeAsO1-xFx and
NdFeAsO1-xFx) or when Fe is substituted by Co (SmFe1-xCoxAsO). From
magnetization measurements, we estimate the temperature dependence and
anisotropy of the upper critical field and the critical current density of
underdoped PrFeAsO0.7F0.3 crystal with Tc = 25 K. Single crystals of
SmFe1-xCoxAsO with maximal Tc up to 16.3 K for x = 0.08 were grown for the
first time. From transport and magnetic measurements we estimate the critical
fields and their anisotropy, and find these superconducting properties to be
quite comparable to the ones in SmFeAsO1-xFx with a much higher Tc of = 50 K.
The magnetically measured critical current densities are as high as 109 A/m2 at
2 K up to 7 T, with indication of the usual fishtail effect. The upper critical
field estimated from resistivity measurements is anisotropic with slopes of
-8.7 T/K (H // ab-plane) and -1.7 T/K (H // c-axis). This anisotropy (= 5) is
similar to that in other Ln1111 crystals with various higher Tc s.
08/2012;
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[show abstract]
[hide abstract]
ABSTRACT: The magnetic phase diagram of EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ was
investigated by means of magnetization and muon-spin rotation studies as a
function of chemical (isovalent substitution of As by P) and hydrostatic
pressure. The magnetic phase diagrams of the magnetic ordering of the Eu and Fe
spins with respect to P content and hydrostatic pressure are determined and
discussed. The present investigations reveal that the magnetic coupling between
the Eu and the Fe sublattices strongly depends on chemical and hydrostatic
pressure. It is found that chemical and hydrostatic pressure have a similar
effect on the Eu and Fe magnetic order.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in
undoped and near-optimally doped SmFeAsO1−x
F
x
iron-pnictide superconductor single crystals (SC).
In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition
is observed. From the relaxation time in the normal state, significantly above T
SDW, the second moment of the Eliashberg function is determined to be λ〈ω
2〉≈135meV2. Below T
SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening
of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2ΔSDW/k
B
T
SDW=7±3, at4.2K.
In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in
the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent
gaps.
KeywordsOptical femtosecond spectroscopy–Iron-pnictide superconductors–Spin density wave–Electron-phonon coupling
Journal of Superconductivity and Novel Magnetism 04/2012; 24(1):489-493. · 0.65 Impact Factor
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[show abstract]
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ABSTRACT: We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor
with T
c=49.5K by means of femtosecond spectroscopy. Multiple relaxation processes are evident, with distinct superconducting state
dynamics and a clear “pseudogap”-like feature with an onset above 180K indicating the existence of a temperature-independent
gap of magnitude Δ
PG=61±9meV above and below T
c. The fluence and temperature dependence of the superconducting state dynamics shows similar behaviour to the cuprate superconductors
with the superconducting-condensate vaporization energy of E
va/k
B≈1.5 K per Fe ion.
Journal of Superconductivity and Novel Magnetism 04/2012; 22(6):575-578. · 0.65 Impact Factor
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R. Khasanov,
S. Sanna,
G. Prando,
Z. Shermadini,
M. Bendele,
A. Amato,
P. Carretta,
R De Renzi,
J. Karpinski, S. Katrych,
H. Luetkens,
N. D. Zhigadlo
[show abstract]
[hide abstract]
ABSTRACT: The interplay between magnetism and superconductivity in LaFeAsO_0.945F_0.055
was studied as a function of hydrostatic pressure up to p~2.4GPa by means of
muon-spin rotation (\muSR) and magnetization measurements. The application of
pressure leads to a substantial decrease of the magnetic ordering temperature
T_N and a reduction of the magnetic phase volume and, at the same time, to a
strong increase of the superconducting transition temperature T_c and the
diamagnetic susceptibility. From the volume sensitive \muSR measurements it can
be concluded that the superconducting and the magnetic areas which coexist in
the same sample are inclined towards spatial separation and compete for phase
volume as a function of pressure.
Physical Review B 09/2011; 84:100501. · 3.69 Impact Factor
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N. D. Zhigadlo, S. Katrych,
M. Bendele,
P. J. W. Moll,
M. Tortello,
S. Weyeneth,
V. Yu. Pomjakushin,
J. Kanter,
R. Puzniak,
Z. Bukowski,
H. Keller,
R. S. Gonnelli,
R. Khasanov,
J. Karpinski,
B. Batlogg
[show abstract]
[hide abstract]
ABSTRACT: Polycrystalline samples and single crystals of SmFeAs1-xPxO1-y were
synthesized and grown employing different synthesis methods and annealing
conditions. Depending on the phosphorus and oxygen content, the samples are
either magnetic or superconducting. In the fully oxygenated compounds the main
impact of phosphorus substitution is to suppress the N\'eel temperature TN of
the spin density wave (SDW) state, and to strongly reduce the local magnetic
field in the SDW state, as deduced from muon spin rotation measurements. On the
other hand the superconducting state is observed in the oxygen deficient
samples only after heat treatment under high pressure. Oxygen deficiency as a
result of synthesis at high pressure brings the Sm-O layer closer to the
superconducting As/P-Fe-As/P block and provides additional electron transfer.
Interestingly, the structural modifications in response to this variation of
the electron count are significantly different when phosphorus is partly
substituting arsenic. Point contact spectra are well described with two
superconducting gaps. Magnetic and resistance measurements on single crystals
indicate an in-plane magnetic penetration depth of 200 nm and an anisotropy of
the upper critical field slope of 4-5. PACS number(s): 74.70.Xa, 74.62.Bf,
74.25.-q, 81.20.-n
07/2011;
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[show abstract]
[hide abstract]
ABSTRACT: To probe manifestations of multiband superconductivity in oxypnictides, we
measured the angular dependence of the magnetic torque $\tau(\theta)$ in the
mixed state of LaO$_{0.9}$F$_{0.1}$FeAs single crystals as a function of
temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic
contribution of the Fe ions is properly treated in order to extract the
effective mass anisotropy parameter $\gamma=(m_c/m_{ab})^{1/2}$ from
$\tau(\theta)$. We show that $\gamma$ depends strongly on both $T$ and $H$,
reaching a maximum value of $\sim$ 10 followed by a decrease towards values
close to 1 as $T$ is lowered. The observed field dependencies of the London
penetration depth $\lambda_{ab}$ and $\gamma$ suggest the onset of suppression
of a superconducing gap at $H \approx H_{c2}/3$.
06/2011;
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A. Alfonsov,
F. Murányi,
V. Kataev,
G. Lang,
N. Leps,
L. Wang,
R. Klingeler,
A. Kondrat,
C. Hess,
S. Wurmehl,
A. Köhler,
G. Behr,
S. Hampel,
M. Deutschmann, S. Katrych,
N. D. Zhigadlo,
Z. Bukowski,
J. Karpinski,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: We report a detailed investigation of GdO1-xFxFeAs (x=0, 0.07, and 0.14) samples by means of high-field and high-frequency electron spin resonance (HF-ESR) together with measurements of thermodynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K, whereas both doped samples do not show such order and are superconducting with Tc=20 K (x=0.07) and Tc=45 K (x=0.14). The Gd3+ HF-ESR reveals an appreciable exchange coupling between Gd and Fe moments, through which the static magnetic order is clearly seen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolution of the magnetism in the FeAs planes upon F doping. It is found that in both superconducting samples, where the Fe long-range order is absent, there are short-range, static on the ESR time scale magnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicative of the ground states’ coexistence.
Phys. Rev. B. 03/2011; 83(9).
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S. Weyeneth,
P. J. W. Moll,
R. Puzniak,
K. Ninios,
F. F. Balakirev,
R. D. McDonald,
H. B. Chan,
N. D. Zhigadlo, S. Katrych,
Z. Bukowski,
J. Karpinski,
H. Keller,
B. Batlogg,
L. Balicas
[show abstract]
[hide abstract]
ABSTRACT: The low-temperature antiferromagnetic state of the Sm-ions in both
nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single
crystals was studied by magnetic torque, magnetization, and magnetoresistance
measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We
uncover in both compounds a distinct rearrangement of the antiferromagnetically
ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed
magnetic fields, as a sharp change in the sign of the magnetic torque, which is
sensitive to the magnetic anisotropy and hence to the magnetic moment in the
$ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization
for magnetic fields perpendicular to the conducting planes. This rearrangement
of magnetic ordering in $35-40$~T is essentially temperature independent and
points towards a canted or a partially polarized magnetic state in high
magnetic fields. However, the observed value for the saturation moment above
this rearrangement, suggests that the complete suppression of the
antiferromagnetism related to the Sm-moments would require fields in excess of
60~T. Such a large field value is particularly remarkable when compared to the
relatively small N\'{e}el temperature $T_{\rm N}\simeq5$~K, suggesting very
anisotropic magnetic exchange couplings. At the transition, magnetoresistivity
measurements show a crossover from positive to negative field-dependence,
indicating that the charge carriers in the FeAs planes are sensitive to the
magnetic configuration of the rare-earth elements. This is indicates a finite
magnetic/electronic coupling between the SmO and the FeAs layers which are
likely to mediate the exchange interactions leading to the long range
antiferromagnetic order of the Sm ions.
02/2011;
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ABSTRACT: The superconducting and magnetic properties of FeySe0.25Te0.75 single crystals (0.9≤y≤1.1) were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y<1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y≥1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the FeySe1−xTex can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.
Phys. Rev. B. 12/2010; 82(21).
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Z. Guguchia,
J. Roos,
A. Shengelaya, S. Katrych,
Z. Bukowski,
S. Weyeneth,
F. Murányi,
S. Strässle,
A. Maisuradze,
J. Karpinski,
H. Keller
[show abstract]
[hide abstract]
ABSTRACT: A combination of x-ray diffraction, magnetization, and 75As nuclear magnetic
resonance (NMR) experiments were performed on single-crystal EuFe1.9Co0.1As2.
The strength of the hyperfine interaction between the 75As nuclei and the
Eu^(2+) 4f states suggests a strong coupling between the Eu^(2+) moments and
the Fe1.9Co0.1As2 layers. Such a strong interlayer coupling may be due to an
indirect exchange interaction between the localized Eu^(2+) 4f moments,
mediated by the Fe 3d conduction electrons. Magnetic susceptibility as well as
75As-NMR measurements reveal a decrease of the SDW transition temperature to
T_SDW = 120 K as a result of Co doping. A change of the slope in the
temperature dependence of the NMR frequency of the 75As lower-satellite line
was observed at 225 K. At the same temperature also a change of the satellite
line shape was found. These changes of the NMR spectra may be caused by the
formation of a nematic phase below 225 K in EuFe1.9Co0.1As2.
10/2010;
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A. Alfonsov,
F. Murányi,
V. Kataev,
G. Lang,
N. Leps,
L. Wang,
R. Klingeler,
A. Kondrat,
C. Hess,
S. Wurmehl,
A. Köhler,
G. Behr,
S Hampel,
M. Deutschmann, S. Katrych,
N. D. Zhigadlo,
Z. Bukowski,
J. Karpinski,
B. Büchner
[show abstract]
[hide abstract]
ABSTRACT: We report a detailed investigation of GdO$_{1-x}$F$_{x}$FeAs (x = 0, 0.07 and
0.14) samples by means of high-field/high-frequency electron spin resonance
(HF-ESR) together with measurements of thermodynamic and transport properties.
The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K,
whereas both doped samples do not show such order and are superconducting with
T$_c$ = 20 K (x = 0.07) and T$_c$ = 45 K (x = 0.14). The Gd$^{3+}$ HF-ESR
reveals an appreciable exchange coupling between Gd and Fe moments, through
which the static magnetic order is clearly seen in the parent compound. Owing
to this coupling, HF-ESR can probe sensitively the evolution of the magnetism
in the FeAs planes upon F doping. It is found that in both superconducting
samples, where the Fe long-range order is absent, there are short-range, static
on the ESR time scale magnetic correlations between Fe spins. Their occurrence
on a large doping scale may be indicative of the ground states' coexistence.
10/2010;
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[show abstract]
[hide abstract]
ABSTRACT: The superconducting and magnetic properties of Fe$_{y}$Se$_{0.25}$Te$_{0.75}$ single crystals ($0.9\leq y \leq1.1$) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with $y<1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\geq 1$, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe$_{y}$Se$_{1-x}$Te$_{x}$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.
10/2010;
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[show abstract]
[hide abstract]
ABSTRACT: We probe the local quasiparticles density of states in micron-sized SmFeAsO(1-x)F(x) single crystals by means of scanning tunnelling spectroscopy. Spectral features resemble those of cuprates, particularly a dip-hump-like structure developed at energies larger than the gap that can be ascribed to the coupling of quasiparticles to a collective mode, quite likely a resonant spin mode. The energy of the collective mode revealed in our study decreases when the pairing strength increases. Our findings support spin-fluctuation-mediated pairing in pnictides.
Physical Review Letters 10/2010; 105(16):167005. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newly discovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of the Fermi surface associated with their antiferromagnetic and superconducting phases, which is essential for understanding their magnetism, pairing symmetry and superconducting mechanism. Calculations 4 and surface-sensitive measurements 5,6,7 provided early guidance, but lead to contradictory results, generating a need for a direct experimental probe of their bulk Fermi surface. In antiferromagnetic LaFeAsO1-xFx 8,9 we observe a complex pattern in the Fourier spectrum of the oscillatory component superimposed onto the magnetic torque signal revealing a reconstructed Fermi surface, whose geometry is not fully described by band structure calculations. Surprisingly, several of the same frequencies, or Fermi surface cross-sectional areas, are also observed in superconducting LaFeAsO1-xFx (with a superconducting transition temperature Tc ~ 15 K). Although one could attribute this to inhomogeneous F doping, the corresponding effective masses are largely enhanced with respect to those of the antiferromagnetic compound. Instead, this implies the microscopic coexistence of superconductivity and antiferromagnetism on the same Fermi surface in the underdoped region of the phase diagram of the LaFeAsO1-xFx series. Thus, the dHvA-effect reveals a more complex Fermi surface topography than that predicted by band structure calculations4 upon which the currently proposed superconducting pairing scenarios10,11,12,13 are based, which could be at the origin of their higher Tcs when compared to their phosphide analogs. Comment: 22 pages, supplementary information inclosed
09/2010;
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S. Weyeneth,
M. Bendele,
R. Puzniak,
F. Murányi,
A. Bussmann-Holder,
N. D. Zhigadlo, S. Katrych,
Z. Bukowski,
J. Karpinski,
A. Shengelaya,
R. Khasanov,
H. Keller
[show abstract]
[hide abstract]
ABSTRACT: The magnetic-field dependence of the in-plane magnetic penetration depth λab for optimally doped SmFeAsO1−xFy was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density ρs∝λab− 2 as the field is increased to 1.4 T. This behaviour of ρs is analysed within a two-band model with self-consistently derived coupled gaps and ρs=ρs1+ρs2, where ρs1 related to the larger gap is field independent, and ρs2 related to the smaller gap is strongly suppressed with increasing field.
EPL (Europhysics Letters) 09/2010; 91(4):47005. · 2.17 Impact Factor
