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Register a new userUpper critical field measurements up to 60 T in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields
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We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_(c2)(T) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.
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We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0) approximately 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H II ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively.
We report upper critical field B_c2(T) data for disordered (arsenic-deficient) LaO_0.9F_0.1FeAs_(1-delta) in a wide temperature and magnetic field range up to 47 T. Because of the large linear slope of Bc2 about -5.4 T/K to -6.6 T/K near Tc = 28.5 K the T-dependence of the in-plane Bc2(T) shows a flattening near 23 K above 30 T which points to Pauli-limited behavior with Bc2(0) about 63-68 T. Our results are discussed in terms of disorder effects within conventional and unconventional superconducting pairings.
By using a two-step method, we successfully synthesized the iron based new superconductor LaFeAsO_{0.9}F_{0.1-delta}$. The resistive transition curves under different magnetic fields were measured, leading to the determination of the upper critical field Hc2(T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 50 Tesla roughly. In addition, the Hall effect and magnetoresistance were measured in wide temperature region. A negative Hall coefficient R_H has been found, implying a dominant conduction mainly by electron-like charge carriers in this material. The charge carrier density determined at 100 K is about 9.8E20cm^{-3}, which is close to the cuprate superconductors. It is further found that the magnetoresistance does not follow Kohlers law. Meanwhile, the different temperature dependence behaviors of resistivity, Hall coefficient, and magnetoresistance have anomalous properties at about 230 K, which may be induced by some exotic scattering mechanism.
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 $H approx H_{c2}/3$.
We report the temperature dependencies of the upper critical fields $H_{ctext{2}}^{text{c}}(T)$ parallel to the c-axis and $H_{ctext{2}}^{text{ab}}(T)$ parallel to the ab-plane of single crystalline CaKFe$_4$As$_4$ inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 63 T. We show that the observed decrease of the anisotropy parameter $gamma(T)=H_{ctext{2}}^{text{ab}}/H_{ctext{2}}^{text{c}}$ from $simeq 2.5$ at $T_c$ to $simeq 1.5$ at 25 K can be explained by interplay of paramagnetic pairbreaking and orbital effects in a multiband theory of $H_{c2}$. The slopes of $dH_{ctext{2}}^{text{c}}/dTsimeq-4.4$ T/K and $dH_{ctext{2}}^{text{ab}}/dT simeq-10.9$ T/K at $T_c$ yield an electron mass anisotropy of $m_{ab}/m_csimeq 1/6$ and short coherence lengths $xi_csimeq 5.8,text{AA}$ and $xi_{ab}simeq 14.3,text{AA}$. The behavior of $H_{ctext{2}}(T)$ turns out to be similar to that of the optimal doped (Ba,K)Fe$_2$As$_2$, with $H_{ctext{2}}^{text{ab}}(0)$ extrapolating to $simeq 92$ T, well above the BCS paramagnetic limit.
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