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[show abstract]
[hide abstract]
ABSTRACT: The synthesis and crystallography, magnetic susceptibility chi, magnetization
M, specific heat Cp, in-plane electrical resistivity rho and in-plane magnetic
penetration depth measurements are reported for single crystals of APd2As2 (A =
Ca, Sr, Ba) versus temperature T and magnetic field H. The crystals were grown
using PdAs self-flux. CaPd2As2 and SrPd2As2 crystallize in a collapsed
body-centered tetragonal ThCr2Si2-type structure (I4/mmm), whereas BaPd2As2
crystallizes in the primitive tetragonal CeMg2Si2-type structure (P4/mmm), in
agreement with literature data. The rho(T) data exhibit metallic behavior for
all three compounds. Bulk superconductivity is reported for CaPd2As2 and
SrPd2As2 below Tc = 1.27 and 0.92 K, respectively, whereas only a trace of
superconductivity is found in BaPd2As2. No other phase transitions were
observed. The chi(T) and M(H) data reveal anisotropic diamagnetism in the
normal state, with chi_c > chi_ab for CaPd2As2 and BaPd2As2, and chi_c < chi_ab
for SrPd2As2. The normal and superconducting state data indicate that CaPd2As2
and SrPd2As2 are conventional type-II nodeless s-wave electron-phonon
superconductors. The electronic superconducting-state heat capacity data for
CaPd2As2, which has an extremely sharp heat capacity jump at Tc, are analyzed
using our recent elaboration of the alpha-model of the BCS theory of
superconductivity, which indicates that the s-wave gap in this compound is
anisotropic in momentum space.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Measurements of temperature-dependent in-plane resistivity, $\rho(T)$, were
used to determine the upper critical field and its anisotropy in high quality
single crystals of stoichiometric iron arsenide superconductor KFe$_2$As$_2$.
The crystals were characterized by residual resistivity ratio,
$\rho(300K)/\rho(0)$ up to 3000 and resistive transition midpoint temperature,
$T_c$=3.8 K, significantly higher than in previous studies on the same
material. We find increased $H_{c2}(T)$ for both directions of the magnetic
field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$
scaling is not expected for orbital limiting mechanism of the upper critical
field in clean materials.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We report on the infrared studies of the interlayer charge dynamics of a prototypical pnictide superconductor Ba(Fe_{0.926}Co_{0.074})_{2}As_{2}. We succeeded in probing the intrinsic interlayer response by performing infrared experiments on the crystals with a cleaved ac surface. Our experiments identify the coexistence of the suppression of the electronic spectral weight and the development of a coherent Drude-like response in the normal state. The formation of the interlayer condensate is clearly observed in the superconducting state and appears to be linked to coherent contribution to the normal-state conductivity.
Physical Review Letters 03/2013; 110(9):097003. · 7.37 Impact Factor
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N. W. Salovich,
H. Kim,
A. K. Ghosh,
R. W. Giannetta,
W. Kwok,
U. Welp,
B. Shen,
S. Zhu,
H. -H. Wen, M. A. Tanatar,
R. Prozorov
[show abstract]
[hide abstract]
ABSTRACT: The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2
crystals, with and without columnar defects produced by 1.4 GeV 208Pb
irradiation. The low temperature behavior of unirradiated samples was
consistent with a fully gapped superconducting state with a minimum energy gap
delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation
levels corresponding to mean column-column separations of 32 nm and 22 nm. At
very high irradiation levels (column-column separation of 10 nm) a T^2 power
law was observed below Tc/3, most likely due to elevated scattering. Neither
the location nor the sharpness of the superconducting transition was affected
by irradiation. The data provides evidence for an s+/- pairing state.
02/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent inter-plane resistivity, $\rho _c(T)$, was measured for
the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a broad
isoelectron phosphorus substitution range from $x$=0 to $x$=0.60, from
non-superconducting parent compound to heavily overdoped superconducting
composition with $T_c\approx 10~K$. The features due to structural and magnetic
transitions are clearly resolved in $\rho _c(T)$ of the underdoped crystals. A
characteristic maximum in $\rho _c(T)$, found in the parent BaFe$_2$As$_2$ at
around 200~K, moves rapidly with phosphorus substitution to high temperatures.
At the optimal doping, the inter-plane resistivity shows $T$-linear temperature
dependence without any cross-over anomalies, similar to the previously reported
in-plane resistivity. This observation is in stark contrast with dissimilar
temperature dependences found at optimal doping in electron-doped
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Our finding suggests that despite similar
values of the resistivity and its anisotropy, the temperature dependent
transport in the normal state is very different in electron and isoelectron
doped compounds. Similar temperature dependence of both in-plane and
inter-plane resistivities, in which the dominant contributions are coming from
different parts of the Fermi surface, suggests that scattering is the same on
the whole Fermi surface. Since magnetic fluctuations are expected to be much
stronger on the quasi-nested sheets, this observation may point to the
importance of the inter-orbital scattering between different sheets.
02/2013;
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[show abstract]
[hide abstract]
ABSTRACT: In-plane resistivity measurements as a function of temperature, magnetic
field and its orientation with respect to the crystallographic $ab-$plane were
used to study the upper critical field, $H_{c2}$, of two overdoped compositions
of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and
$x$=0.072. Measurements were performed using precise alignment (with accuracy
less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The
dependence of the $H_{c2}$ on angle $\theta$ between the field and the
$ab$-plane was measured in isothermal conditions in a broad temperature range.
We found that the shape of $H_{c2}(\theta)$, clearly deviates from
Ginzburg-Landau functional form.
01/2013;
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S. Yeninas, M. A. Tanatar,
J. Murphy,
C. P. Strehlow,
O. E. Ayala-Valenzuela,
R. D. McDonald,
U. Welp,
W. K. Kwok,
T. Kobayashi,
S. Miyasaka,
S. Tajima,
R. Prozorov
[show abstract]
[hide abstract]
ABSTRACT: The upper critical field $H_{c2}$ of optimally doped iron-based
superconductor SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x$ = 0.35, $T_c$ = 25 K)
was measured as a function of temperature down to 1.6 K for two principal
directions of magnetic field $H \parallel c$ and $H \parallel a$. Measurements
were performed in pulsed magnetic fields up to 65 T using a tunnel-diode
resonator technique on as-grown and heavy-ion irradiated single crystals, with
columnar defect density corresponding to a matching field $B\phi$ = 25 T. The
$H_{c2,c}(T)$ is close to $T$-linear, while clear saturation is observed for
$H_{c2,a}(T)$, leading to a strongly temperature dependent anisotropy parameter
$\gamma$. The linear shape of $H_{c2,c}(T)$ is very similar to that observed in
nodal KFe$_2$As$_2$ but very different from full-gap LiFeAs. Irradiation does
not introduce any additional features on $H_{c2}(T)$ line corresponding to the
matching field. Instead, it suppresses uniformly both $T_c$ and $H_{c2}$,
keeping their ratio constant.
01/2013;
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[show abstract]
[hide abstract]
ABSTRACT: The in-plane magnetic penetration depth, $\lambda_m(T)$, was measured in
single crystals of SrPd$_2$Ge$_2$ superconductor in a dilution refrigerator
down to T=60 mK and in magnetic fields up to $H_{dc} = 1$ T by using a tunnel
diode resonator. The London penetration depth, $\lambda$, saturates
exponentially approaching $T\rightarrow 0$ indicating fully gapped
superconductivity. The thermodynamic Rutgers formula was used to estimate
$\lambda(0) = 426$ nm which was used to calculate the superfluid density,
$\rho_s(T)=\lambda^2(0)/\lambda^2(T)$. Analysis of $\rho_s(T)$ in the full
temperature range shows that it is best described by a single - gap behavior,
perhaps with somewhat stronger coupling. In a magnetic field, the measured
penetration depth is given by the Campbell penetration depth which was used to
calculate the theoretical critical current density $j_c$. For $H \le 0.45$ T,
the strongest pinning is achieved not at the lowest, but at some intermediate
temperature, probably due to matching effect between temperature - dependent
coherence length and relevant pinning lengthscale. Finally, we find a
compelling evidence for surface superconductivity. Combining all measurements,
the entire $H$-$T$ phase diagram of SrPd$_2$Ge$_2$ is constructed with an
estimated $H_{c2}(0)=0.4817$ T.
01/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent London penetration depth, $\lambda(T)$, was measured in
optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K,
RRR=$\rho(300K)/\rho(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4)
single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor.
Annealing increases the RRR and decreases the absolute value of the London
penetration depth from $\lambda(0) = 300 \pm 10$ nm in as-grown sample to
$\lambda(0) = 275 \pm 10$ nm. At low temperatures, $\lambda(T) \sim T$
indicating superconducting gap with line nodes. Analysis of the full -
temperature range superfluid density is consistent with the line nodes, but
differs from the simple single - gap $d-$wave. The observed behavior is very
similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently
substituted pnictides are inherently different from the charge - doped
materials.
01/2013;
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Physical Review B. 01/2013; 87(1):014506.
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[show abstract]
[hide abstract]
ABSTRACT: Single crystals of (Ca1-xLax)10(Pt3As8)(Fe2As2)5 (x = 0 to 0.182)
superconductors have been grown and characterized by X-ray, microprobe,
transport and thermodynamic measurements. Features in the magnetic
susceptibility, specific heat and two kinks in the derivative of the electrical
resistivity around 100 K in the x = 0 compound support the existence of
decoupled structural and magnetic phase transitions. With La doping, the
structural/magnetic phase transitions are suppressed and a half-dome of
superconductivity with a maximal Tc around 26 K is observed in the
temperature-concentration phase diagram.
12/2012;
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E. D. Mun,
S. L. Bud'ko,
C. Martin,
H. Kim, M. A. Tanatar,
J. -H. Park,
T. Murphy,
G. M. Schmiedeshoff,
N. Dilley,
R. Prozorov,
P. C. Canfield
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present the systematic measurements of the temperature and
magnetic field dependences of the thermodynamic and transport properties of the
Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic
fields up to 140 kOe to address the possible existence of a field-tuned quantum
critical point. Measurements of magnetic field and temperature dependent
resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric
power indicate that the AFM order can be suppressed by applied magnetic field
of $H_{c}$ $\sim$ 4 kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of
its low temperature states emerges: (I) AFM state, characterized by spin
density wave (SDW) like feature, which can be suppressed to $T$ = 0 by the
relatively small magnetic field of $H_{c}$ $\sim$ 4\,kOe, (II) field induced
anomalous state in which the electrical resistivity follows $\Delta\rho(T)
\propto T^{1.5}$ between $H_{c}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL)
state in which $\Delta\rho(T) \propto T^{2}$ for $H \geq$ 8 kOe. Regions I and
II are separated at $T$ = 0 by what appears to be a quantum critical point.
Whereas region III appears to be a FL associated with the hybridized 4$f$
states of Yb, region II may be a manifestation of a spin liquid state.
11/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent resistivity is studied in single crystals of
iron-arsenide superconductor Na$_{1-\delta}$Fe$_{1-x}$Co$_x$As for electrical
current directions along, $\rho_a (T)$, and transverse, $\rho_c (T)$, to the
Fe-As layers. Doping with Co increases stability of this compound to reaction
with the environment and suppresses numerous features in both $\rho_a(T)$ and
$\rho_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $\rho_a (T)$
with $x$ follows a universal trend observed in other pnictide superconductors,
exhibiting a $T$-linear temperature dependence close to the optimal doping and
development of $T^2$ dependence upon further doping. $\rho_c (T)$ in parent
compound shows a non - monotonic behavior with a crossover from non-metallic
resistivity increase on cooling from room temperature down to $\sim$ 80 K to a
metallic decrease below this temperature. Both $\rho_a (T)$ and $\rho_c (T)$
show several correlated crossover - like features at $T>$ 80 K. Despite a
general trend towards more metallic behavior of inter - plane resistivity in
Co-doped samples, the temperature of the crossover from insulating to metallic
behavior (80 K) does not change much with doping.
08/2012;
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J. -Ph. Reid,
A. Juneau-Fecteau,
R. T. Gordon,
S. Rene de Cotret,
N. Doiron-Leyraud,
X. G. Luo,
H. Shakeripour,
J. Chang, M. A. Tanatar,
H. Kim, [......],
T. Saito,
H. Fukazawa,
Y. Kohori,
K. Kihou,
C. H. Lee,
A. Iyo,
H. Eisaki,
B. Shen,
H. -H. Wen,
Louis Taillefer
[show abstract]
[hide abstract]
ABSTRACT: The nature of the pairing state in iron-based superconductors is the subject
of much debate. Here we argue that in one material, the stoichiometric iron
pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state,
characterized by symmetry-imposed vertical line nodes in the superconducting
gap. This evidence is reviewed, with a focus on thermal conductivity and the
strong impact of impurity scattering on the critical temperature Tc. We then
compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the
pairing symmetry is s-wave and the Tc is ten times higher. The transition from
d-wave to s-wave within the same crystal structure provides a rare opportunity
to investigate the connection between band structure and pairing mechanism. We
also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which
the pairing symmetry is probably not d-wave, but more likely s-wave with
accidental line nodes.
07/2012;
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S. J. Moon,
A. A. Schafgans,
S. Kasahara,
T. Shibauchi,
T. Terashima,
Y. Matsuda, M. A. Tanatar,
R. Prozorov,
A. Thaler,
P. C. Canfield,
A. S. Sefat,
D. Mandrus,
D. N. Basov
[show abstract]
[hide abstract]
ABSTRACT: We report on infrared studies of charge dynamics in a prototypical pnictide
system: the BaFe2As2 family. Our experiments have identified hallmarks of the
pseudogap state in the BaFe2As2 system that mirror the spectroscopic
manifestations of the pseudogap in the cuprates. The magnitude of the infrared
pseudogap is in accord with that of the spin-density-wave gap of the parent
compound. By monitoring the superconducting gap of both P- and Co-doped
compounds, we find that the infrared pseudogap is unrelated to
superconductivity. The appearance of the pseudogap is found to correlate with
the evolution of the antiferromagnetic fluctuations associated with the
spin-density-wave instability. The strong-coupling analysis of infrared data
further reveals the interdependence between the magnetism and the pseudogap in
the iron pnictides.
07/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We reply to the Comment by T. Klein, P. Rodi\`{e}re and C. Marcenat
(arXiv:1205.2566), on our paper, "Precision global measurements of London
penetration depth in FeTe$_{0.58}$Se$_{0.42}$", Phys. Rev. B 84, 174502 (2011).
Our work was motivated by Klein \emph{et. al}, "Thermodynamic phase diagram of
Fe(Se$_{0.5}$Te$_{0.5}$) single crystals in fields up to 28 Tesla", Phys. Rev.
B 82, 184506 (2010). In their paper, Klein et al. have attributed a factor of
five difference in the value of the London penetration depth obtained from
their tunnel diode resonator (TDR) measurements and calculated from the "field
of first penetration" to the surface roughness, although they have not verified
it experimentally. In our paper, we have studied the effects of deliberately
introduced surface roughness and found that its effects are minor and cannot be
responsible for the difference of such magnitude. Instead, we suggest that the
value of the "field of first penetration" measured with Hall - arrays cannot be
used to extract a true lower critical field due to several reasons outlined in
our Reply. We emphasize that the accuracy of the calibration procedure of the
TDR technique has been carefully verified in several prior studies and our work
on FeTe$_{0.58}$Se$_{0.42}$ further confirms it. We show that in their Comment,
Klein \emph{et. al} use wrong arguments of the universal behavior of the
superfluid density in the gapless limit, because it is inapplicable for the
multi - band superconductors. We also discuss the applicability of the clean -
limit $\gamma-$ model and the influence of the disorder on the obtained
results.
07/2012;
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S J Moon,
A A Schafgans,
S Kasahara,
T Shibauchi,
T Terashima,
Y Matsuda, M A Tanatar,
R Prozorov,
A Thaler,
P C Canfield,
A S Sefat,
D Mandrus,
D N Basov
[show abstract]
[hide abstract]
ABSTRACT: We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe_{2}As_{2} family. Our experiments have identified hallmarks of the pseudogap state in the BaFe_{2}As_{2} system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.
Physical Review Letters 07/2012; 109(2):027006. · 7.37 Impact Factor
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K Hashimoto,
K Cho,
T Shibauchi,
S Kasahara,
Y Mizukami,
R Katsumata,
Y Tsuruhara,
T Terashima,
H Ikeda, M A Tanatar,
H Kitano,
N Salovich,
R W Giannetta,
P Walmsley,
A Carrington,
R Prozorov,
Y Matsuda
[show abstract]
[hide abstract]
ABSTRACT: In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λ(L). In two-dimensional systems, this ratio n/m* (~1/λ(L)(2)) determines the effective Fermi temperature, T(F). We report a sharp peak in the x-dependence of λ(L) at zero temperature in clean samples of BaFe(2)(As(1)(-x)P(x))(2) at the optimum composition x = 0.30, where the superconducting transition temperature T(c) reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T(c)/T(F) at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
Science 06/2012; 336(6088):1554-7. · 31.20 Impact Factor
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E. Hassinger,
G. Gredat,
F. Valade,
S. Rene de Cotret,
A. Juneau-Fecteau,
J. -Ph. Reid,
H. Kim, M. A. Tanatar,
R. Prozorov,
B. Shen,
H. -H. Wen,
N. Doiron-Leyraud,
Louis Taillefer
[show abstract]
[hide abstract]
ABSTRACT: The electrical resistivity rho of the iron-arsenide superconductor
Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four
underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic
ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases
linearly with pressure. At pressures above around 1.0 GPa, a second sharp
anomaly is detected at a lower temperature T_0, which rises with pressure. We
attribute this second anomaly to the onset of a phase that causes a
reconstruction of the Fermi surface. This new phase expands with increasing x
and it competes with superconductivity. We discuss the possibility that a
second spin-density wave orders at T_0, with a Q vector distinct from that of
the spin-density wave that sets in at T_N.
05/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We present evidence of type I superconductivity in YbSb2 single crystals,
from DC and AC magnetization, heat capacity and resistivity measurements. The
critical temperature and critical field are determined to be $T_c\approx$ 1.3 K
and $H_c\approx$ 55 Oe. A small Ginzburg-Landau parameter \kappa = 0.05,
together with typical magnetization isotherms of type I superconductors, small
critical field values, a strong Differential Paramagnetic Effect (DPE) signal,
and a field-induced change from second to first order phase transition, confirm
the type I nature of the superconductivity in YbSb2. A possible second
superconducting state is observed in the radiofrequency (RF) susceptibility
measurements, with $T_{c}^{(2)}\approx$ 0.41 K and $H_{c}^{(2)}\approx$ 430 Oe.
02/2012;
