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S Lee, C Tarantini,
P Gao,
J Jiang,
J D Weiss,
F Kametani,
C M Folkman,
Y Zhang,
X Q Pan,
E E Hellstrom,
D C Larbalestier,
C B Eom
[show abstract]
[hide abstract]
ABSTRACT: Significant progress has been achieved in fabricating high-quality bulk and thin-film iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high-field applications, large critical current densities (Jc) and irreversibility fields (Hirr) are indispensable along all crystal directions. On the other hand, the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. Here we show that artificially engineered undoped Ba-122/Co-doped Ba-122 compositionally modulated superlattices produce ab-aligned nanoparticle arrays. These layer and self-assemble along c-axis-aligned defects, and combine to produce very large Jc and Hirr enhancements over a wide angular range. We also demonstrate a structurally modulated SrTiO3(STO)/Co-doped Ba-122 superlattice with sharp interfaces. Success in superlattice fabrication involving pnictides will aid the progress of heterostructured systems exhibiting new interfacial phenomena and device applications.
Nature Material 05/2013; 12:392-396. · 32.84 Impact Factor
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I. Pallecchi,
V. Ferrando,
E. Galleani D’Agliano,
D. Marré,
M. Monni,
M. Putti, C. Tarantini,
F. Gatti,
H. U. Aebersold,
E. Lehmann,
X. X. Xi,
E. G. Haanappel,
C. Ferdeghini
[show abstract]
[hide abstract]
ABSTRACT: In this paper we present normal-state magnetoresistivity data of magnesium diboride epitaxial thin films with different levels of disorder, measured at 42 K in magnetic fields up to 45 T. Disorder was introduced in a controlled way either by means of neutron irradiation or by carbon doping. From a quantitative analysis of the magnetoresistivity curves with the magnetic field either parallel or perpendicular to the plane of the film, we extract the ratio of the scattering times in π and σ bands. We demonstrate that the undoped unirradiated thin film has π scattering times smaller than σ ones, i.e., a shorter mean free path for π charge carriers; upon irradiation, both bands become increasingly more disordered; eventually the highly irradiated sample (neutron fluence ∼7.7×1017 cm−2) and the C-doped sample have comparable scattering times in the two types of bands. This description of the effect of disorder in the two kinds of bands on transport is consistent with the residual resistivity values and with the temperature dependence of the resistivity.
Physical Review B 04/2013; 72(18):184512. · 3.69 Impact Factor
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ABSTRACT: We report on the superior vortex pinning of single and multilayer
Ba(Fe1-xCox)2As2 thin films with self-assembled c-axis and artificially
introduced ab-plane pins. Ba(Fe1-xCox)2As2 can accept a very high density of
pins (15-20 vol%) without Tc suppression. The matching field is greater than 12
T, producing a significant enhancement of the critical current density Jc, an
almost isotropic Jc (Theta,20T) > 10^5 A/cm2, and global pinning force density
Fp of about 50 GN/m^3. This scenario strongly differs from the high temperature
cuprates where the addition of pins without Tc suppression is limited to 2-4
vol%, leading to small HIrr enhancements and improved Jc only below 3-5 Tesla.
11/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Magnetometry measurements in high quality LiFeAs single-crystals reveal a
change in the sign of the magnetic hysteresis in the vicinity of the upper
critical field $H_{c2}$, from a clear diamagnetic response dominated by the
pinning of vortices, to a considerably smaller net hysteretic response of
opposite sign, which \emph{disappears} at $H_{c2}$. If the diamagnetic response
at high fields results from pinned vortices and associated screening
super-currents, this sign change must result from currents circulating in the
opposite sense, which give rise to a small field-dependent magnetic moment
\emph{below} $H_{c2}$. This behavior seems to be extremely sensitive to the
sample quality or stoichiometry, as we have observed it only in a few fresh
crystals, which also display the de Haas van Alphen-effect. We provide
arguments against the surface superconductivity, the flux compression, and the
random $\pi$ junction scenarios, which have been previously put forward to
explain a paramagnetic Meissner effect, below the lower critical field
$H_{c1}$. The observed anomalous hysteresis at high fields will be compatible
with the existence of chiral gap wave-functions, which possess a field
dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate
how a $(d_{x^2 - y^2} + id_{xy})$ or a $(p_x+ip_y)$ chiral superconducting
component can be stabilized in the mixed state of $s_{\pm}$ superconductor, due
to the combined effects of the magnetic field and the presence of competing
pairing channels. The realization of a particular chiral pairing depends on the
microscopic details of the strengths of the competing pairing channels.
08/2012;
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[show abstract]
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ABSTRACT: The K- and Co-doped BaFe2As2 (Ba-122) superconducting compounds are
potentially useful for applications because they have upper critical fields
(Hc2) of well over 50 T, Hc2 anisotropy Gamma < 2, and thin film critical
current densities exceeding 1 MAcm-2 at 4.2 K. However, thin-film bicrystals of
Co-doped Ba-122 clearly exhibit weak link behavior for [001] tilt
misorientations of more than about 5 degrees, suggesting that textured
substrates would be needed for applications, as in the cuprates. Here we
present a contrary and very much more positive result in which untextured
polycrystalline (Ba0.6K0.4)Fe2As2 bulks and round wires with high grain
boundary density have transport critical current densities well over 0.1 MAcm-2
(SF, 4.2 K), more than 10 times higher than that of any other ferropnictide
wire. The enhanced grain connectivity is ascribed to their much improved phase
purity and to the enhanced vortex stiffness of this low-anisotropy compound
(Gamma ~ 1-2) compared to YBa2Cu3O7-x (Gamma ~ 5).
Nature Material 04/2012; 11(8). · 32.84 Impact Factor
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[show abstract]
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ABSTRACT: The anisotropic irreversibility field B$_{Irr}$ of two $YBa_2Cu_3O_{7-x}$
thin films doped with additional rare earth (RE)=(Gd,Y) and Zr and containing
strong correlated pins (splayed BaZrO$_{3}$ nanorods, and $RE_2O_3$
nanoprecipitates), has been measured over a very broad range up to 45T at
temperatures 56 K<T<$T_c$. We found that the experimental angular dependence of
$B_{Irr}(\theta)$ does not follow the mass anisotropy scaling
$B_{Irr}(\theta)=B_{Irr}(0)(cos^2\theta+\gamma^{-2}sin^2\theta)^{-1/2}$, where
$\gamma=(m_c/m_{ab})^{1/2}=5-6$ for the RE-doped $YBa_2Cu_3O_{7-x}$ (REBCO)
crystals, m$_{ab}$ and m$_{c}$ are the effective masses along the ab plane and
the c-axis, respectively, and $\theta$ is the angle between B and the c-axis.
For B parallel to the ab-planes and to the c-axis correlated pinning strongly
enhances B$_{Irr}$, while at intermediate angles, $B_{Irr}(\theta)$ follows the
scaling behavior
$B_{Irr}(\theta)\propto(cos^2\theta+\gamma_{RP}^2sin^2\theta)^{1/2}$ with the
effective anisotropy factor $\gamma_{RP}\approx3$ significantly smaller than
the mass anisotropy would suggest. In spite of the strong effects of c-axis
BaZrO$_{3}$ nanorods, we found even greater enhancements of B$_{Irr}$ for
fields along the ab-planes than for fields parallel to the c-axis, as well as
different temperature dependences of the correlated pinning contributions to
B$_{Irr}$ for B//ab and B//c. Our results show that the dense and strong pins,
which can now be incorporated into REBCO thin films in a controlled way, exert
major and diverse effects on the measured vortex pinning anisotropy and the
irreversibility field over wide ranges of B and T. In particular, we show that
the relative contribution of correlated pinning to B$_{Irr}$ for B//c increases
as the temperature increases due to the suppression of thermal fluctuations of
vortices by splayed distribution of BaZrO$_{3}$ nanorods.
01/2012;
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C. Tarantini,
M. Putti,
A. Gurevich,
Y. Shen,
R.K. Singh,
J.M. Rowell,
N. Newman,
D.C. Larbalestier,
P. Cheng,
Y. Jia,
and H-H Wen
Physical Review Letters 01/2012; 104(087002). · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report measurements of Hc2(T) up to 85 Tesla on Ba1-xKxAs2Fe2 single
crystals and FeSe1-xTex films tuned by doping and strain. We observed an Hc2
enhancement by nearly 25 T at 30 K for the optimally-doped Ba1-xKxAs2Fe2 as
compared to the previous results and extraordinarily high slopes dHc2/dT =
250-500 T/K near Tc in FeSe1-xTex indicating an almost complete suppression of
the orbital pair-breaking. Theoretical analysis of Hc2(T) in FeSe1-xTex and the
optimally doped Ba1-xKxAs2Fe2 predicts an inhomogeneous
Fulde-Ferrel-Larkin-Ovchinnikov state for H//ab and T < 3-10 K, and shows that
Hc2 in multiband Fe based superconductor can be enhanced by doping and strain
much more effectively than by the conventional way of increasing disorder.
08/2011;
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ABSTRACT: This article gives a brief comparative overview of the upper critical fields Hc2(T)—the magnetic fields above which superconductivity disappears—of MgB2- and Fe-based superconductors. We discuss manifestations of multiband superconductivity, Pauli pair-breaking, and pairing symmetry in the shapes of Hc2(T) curves, and the ways of tuning Hc2(T) by doping and by impurity scattering. We show that the effective route to high Hc2 is by disordering in MgB2 and by doping-assisted tuning of the Fermi surface in Fe-based superconductors. These effects allow extremely high Hc2 values in both material classes that well exceed those found in Nb-based superconductors, opening up new opportunities for high-field applications.
MRS Bulletin. 07/2011; 36(08):626 - 630.
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S. Lee,
J. Jiang,
J.D. Weiss,
C.W. Bark, C. Tarantini,
M.D. Biegalski,
A. Polyanskii,
Y. Zhang,
C.T. Nelson,
X.Q. Pan,
E.E. Hellstrom,
D.C. Larbalestier,
C.B. Eom
[show abstract]
[hide abstract]
ABSTRACT: In our previous report, we demonstrated the growth of high-quality epitaxial Co-doped Ba(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>2</sub>As<sub>2</sub> (Ba-122) thin films using single crystalline SrTiO<sub>3</sub> (STO) templates on (La,Sr)(Al,Ta)O<sub>3</sub> (LSAT) substrates with pulsed laser deposition (PLD). Here, we report the dependence of the structural and superconducting properties of Ba-122 thin films on STO templates that were 2-150 unit cell (u.c.) thick. We obtained genuine epitaxial thin films with high crystalline quality and excellent superconducting properties for templates above a critical thickness, which was 50 u.c. for our growth condition. Although the quality of Ba-122 is excellent above the critical thickness, the best crystalline quality and superconducting properties were obtained on 100 u.c.-thick STO templates. The best-quality Ba-122 thin films show an on-set transition temperature (T<sub>c</sub>, <sub>on-set</sub>) as high as 22.8 K, a transition width (ΔT<sub>c</sub>) as narrow as 1.3 K, and critical current density (J<sub>c</sub>) as high as 3 MA/cm<sup>2</sup> .
IEEE Transactions on Appiled Superconductivity 07/2011; · 1.04 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The Fe-based superconductors (FBS) are an important new class of
superconducting materials. As with any new superconductor with a high
transition temperature and upper critical field, there is a need to establish
what their applications potential might be. Applications require high critical
current densities, so the usefulness of any new superconductor is determined
both by the capability to develop strong vortex pinning and by the absence or
ability to overcome any strong current-limiting mechanisms of which grain
boundaries in the cuprates are a cautionary example. In this review we first
consider the positive role that grain boundary properties play in the metallic,
low temperature superconductors and then review the theoretical background and
current experimental data relating to the properties of grain boundaries in FBS
polycrystals, bi-crystal thin films, and wires. Based on this evidence, we
conclude that grain boundaries in FBS are weak linked in a qualitatively
similar way to grain boundaries in the cuprate superconductors, but also that
the effects are a little less marked. Initial experiments with the textured
substrates used for cuprate coated conductors show similar benefit for the
critical current density of FBS thin films too. We also note that the
particular richness of the pairing symmetry and the multiband parent state in
FBS may provide opportunities for grain boundary modification as a better
understanding of their pairing state and grain boundary properties are
developed.
Reports on Progress in Physics 01/2011; 74:124511. · 14.72 Impact Factor
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C. Tarantini,
S. Lee,
Y. Zhang,
J. Jiang,
C. W. Bark,
J. D. Weiss,
A. Polyanskii,
C. T. Nelson,
H. W. Jang,
C. M. Folkman,
S. H. Baek,
X. Q. Pan,
A. Gurevich,
E. E. Hellstrom,
C. B. Eom,
D. C. Larbalestier
[show abstract]
[hide abstract]
ABSTRACT: We report the field and angular dependences of J<sub>c</sub> of truly epitaxial Co-doped BaFe <sub>2</sub> As <sub>2</sub> thin films grown on SrTiO <sub>3</sub>/( La , Sr )( Al , Ta ) O <sub>3</sub> with different SrTiO <sub>3</sub> template thicknesses. The films show J<sub>c</sub> comparable to single crystals and a maximum pinning force F<sub>p</sub>(0.6 T <sub> c </sub>)>5 GN / m <sup>3</sup> at H /H<sub> irr </sub>∼0.5 indicative of strong high-field vortex pinning. Due to the strong correlated c -axis pinning, J<sub>c</sub> for field along the c -axis exceeds J<sub>c</sub> for H ||ab plane, inverting the expectation of the H<sub>c2</sub> anisotropy. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.
Applied Physics Letters 05/2010; · 3.84 Impact Factor
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A. Perucchi,
D. Nicoletti,
M. Ortolani,
C. Marini,
R. Sopracase,
S. Lupi,
U. Schade,
M. Putti,
I. Pallecchi, C. Tarantini,
M. Ferretti,
C. Ferdeghini,
M. Monni,
F. Bernardini,
S. Massidda,
P. Dore
[show abstract]
[hide abstract]
ABSTRACT: The possibility of multiband conductivity and multigap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2Δ/kTc values close to 1.8 and 3.8.
Physical Review B 03/2010; 81(9):092509. · 3.69 Impact Factor
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C Tarantini,
M Putti,
A Gurevich,
Y Shen,
R K Singh,
J M Rowell,
N Newman,
D C Larbalestier,
Peng Cheng,
Ying Jia,
Hai-Hu Wen
[show abstract]
[hide abstract]
ABSTRACT: We report the effect of alpha-particle irradiation on the reduction of the critical temperature T{c} of a NdFeAs(OF) single crystal. Our data indicate that irradiation defects cause both nonmagnetic and magnetic scattering, resulting in the Kondo-like excess resistance Delta rho(T) proportional to lnT over 2 decades in temperatures above T{c}. The critical density of magnetic irradiation defects which suppresses T{c} is found to be much higher than those for cuprates and multiband BCS superconductors. We suggest that such anomalously weak pair breaking by irradiation defects indicates that magnetic scattering in pnictides is coupled with pairing interactions mediated by spin fluctuations.
Physical Review Letters 02/2010; 104(8):087002. · 7.37 Impact Factor
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S Lee,
J Jiang,
Y Zhang,
C W Bark,
J D Weiss, C Tarantini,
C T Nelson,
H W Jang,
C M Folkman,
S H Baek,
A Polyanskii,
D Abraimov,
A Yamamoto,
J W Park,
X Q Pan,
E E Hellstrom,
D C Larbalestier,
C B Eom
[show abstract]
[hide abstract]
ABSTRACT: Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.5 K, where rho=resistivity), a small transition width (DeltaT(c)=1.3 K), a superior critical current density J(c) of 4.5 MA cm(-2) (4.2 K) and strong c-axis flux pinning. Implementing SrTiO(3) or BaTiO(3) templates to match the alkaline-earth layer in the Ba-122 with the alkaline-earth/oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multifunctional single-crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin-film overlayers.
Nature Material 02/2010; 9(5):397-402. · 32.84 Impact Factor
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C. Tarantini,
S. Lee,
Y. Zhang,
J. Jiang,
C. W. Bark,
J. D. Weiss,
C. T. Nelson,
H. W. Jang,
C. M. Folkman,
S. H. Baek,
X. Q. Pan,
A. Gurevich,
E. E. Hellstrom,
C. B. Eom,
and D. C. Larbalestier
Applied Physics Letters 01/2010; 96:142510. · 3.84 Impact Factor
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S Lee,
J Jiang,
C T Nelson,
C. W. Bark,
J. D. Weiss, C. Tarantini,
H W Jang,
C M Folkman,
S H Baek,
A. Polyanskii,
D. Abraimov,
A Yamamoto,
Y Zhang,
X. Q. Pan,
E. E. Hellstrom,
D. C. Larbalestier,
C. B. Eom
[show abstract]
[hide abstract]
ABSTRACT: Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties, control grain boundaries and strain effects, and evaluate device applications. So far superconducting properties of ferropnictide thin films appear compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report novel template engineering using single-crystal intermediate layers of (001) SrTiO3 and BaTiO3 grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe2As2 with high transition temperature (zero resistivity Tc of 21.5K), small transition widths (delta Tc = 1.3K), superior Jc of 4.5 MA/cm2 (4.2K, self field) and strong c-axis flux pinning. Implementing SrTiO3 or BaTiO3 templates to match the alkaline earth layer in the Ba-122 with the alkaline earth-oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multi-functional single crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin film overlayers.
10/2009;
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[show abstract]
[hide abstract]
ABSTRACT: We have investigated the evolution of the low-temperature specific heat anomaly (TN=5.4 K in zero field) in polycrystalline SmFeAsO samples with magnetic fields up to 35 T. The anomaly remains very sharp up to 16 T and becomes rounded with little shift in temperature at higher fields. Doped (superconducting) SmFeAsO0.85F0.15 sample shows a similar behavior up to 16 T. The initial slope of the critical field dBc/dT is 160 T/K for undoped SmFeAsO and 70 T/K for doped SmFeAsO0.85F0.15 with Bc(T) defined at the peak of the specific heat anomaly. The insensitivity to the application of an external magnetic field is unique to Sm and is not observed in CeFeAsO whose anomaly shifts with initial slope dBc/dT=5.7 T/K. We argue that SmFeAsO(F) presents an unprecedented case of spin reorientation at the antiferromagnetic transition.
Phys. Rev. B. 09/2009; 80(21).
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S Lee,
J Jiang,
J. D. Weiss,
C M Folkman,
C. W. Bark, C. Tarantini,
A Xu,
D. Abraimov,
A. Polyanskii,
C T Nelson,
Y Zhang,
S H Baek,
H W Jang,
A Yamamoto,
F Kametani,
X. Q. Pan,
E. E. Hellstrom,
A. Gurevich,
C. B. Eom,
D. C. Larbalestier
[show abstract]
[hide abstract]
ABSTRACT: We show that despite the low anisotropy, strong vortex pinning and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1-xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1-xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry. Comment: To appear in Applied Physics Letters
07/2009;
