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Register a new userCorrelation between linear resistivity and Tc in organic and pnictide superconductors
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Nicolas Doiron-Leyraud
Publication date
2009
fields
Physics
and research's language is
English
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A linear temperature dependence of the electrical resistivity as T -> 0 is the hallmark of quantum criticality in heavy-fermion metals and the archetypal normal-state property of high-Tc superconductors, yet in both cases it remains unexplained. We report a linear resistivity on the border of spin-density-wave order in the organic superconductor (TMTSF)2X (X = PF6, ClO4), whose strength scales with the superconducting temperature Tc. This scaling, also present in the pnictide superconductors, reveals an intimate connection between linear-T scattering and pairing, shown by renormalization group theory to arise from antiferromagnetic fluctuations, enhanced by the interference of superconducting correlations. Our results suggest that linear resistivity in general may be a consequence of such interference and pairing in overdoped high-Tc cuprates is driven by antiferromagnetic fluctuations, as in organic and pnictide superconductors.
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We show that the zero field normal-state resistivity above Tc for various levels of electron doping - both for LaO1-xFxFeAs (La-1111) and SmO1-xFxFeAs (Sm-1111) members of the 1111-iron-pnictide superconductor family - can be scaled in a broad temperature range from 20 to 300 K onto single curves for underdoped La-1111 (x=0.05-0.075), for optimally and overdoped La-1111 (x=0.1-0.2) and for underdoped Sm-1111 (x=0.06-0.1) compounds. The scaling was performed using the energy scale {Delta}, the resistivity {rho}_{Delta} and the residual resistivity {rho}_0 as scaling parameters as well as by applying a recently proposed model-independent scaling method (H. G. Luo, Y. H. Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have been calculated and the compositional variation of {Delta} has been determined. The observed scaling behaviour for {rho}(T) is interpreted as an indication of a common mechanism which dominates the scattering of the charge carriers in underdoped La-1111, in optimally and overdoped La-1111 and in underdoped Sm-1111 compounds..
The quasi-1D organic Bechgaard salt (TMTSF)$_2$PF$_6$ displays spin-density-wave (SDW) order and superconductivity in close proximity in the temperature-pressure phase diagram. We have measured its normal-state electrical resistivity $rho_a(T)$ as a function of temperature and pressure, in the $T to 0$ limit. At the critical pressure where SDW order disappears, $rho_a(T) propto T$ down to the lowest measured temperature (0.1 K). With increasing pressure, $rho_a(T)$ acquires a curvature that is well described by $rho_a(T) = rho_0 + AT + BT^2$, where the strength of the linear term, measured by the $A$ coefficient, is found to scale with the superconducting transition temperature $T_c$. This correlation between $A$ and $T_c$ strongly suggests that scattering and pairing in (TMTSF)$_2$PF$_6$ have a common origin, most likely rooted in the antiferromagnetic spin fluctuations associated with SDW order. Analysis of published resistivity data on the iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ reveals a detailed similarity with (TMTSF)$_2$PF$_6$, suggesting that antiferromagnetic fluctuations play a similar role in the pnictides.
Close to a zero temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of the electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors and there have been suggestions that high temperature superconductivity in the copper oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature $T_c$ is maximum in the cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge ordered phase. Recent experiments have shown a strong increase in quasiparticle mass $m^*$ in the cuprate YBa$_2$Cu$_3$O$_{7-delta}$ as optimal doping is approached suggesting that quantum fluctuations of the charge ordered phase may be responsible for the high-$T_c$ superconductivity. We have tested the robustness of this correlation between $m^*$ and $T_c$ by performing quantum oscillation studies on the stoichiometric compound YBa$_2$Cu$_4$O$_8$ under hydrostatic pressure. In contrast to the results for YBa$_2$Cu$_3$O$_{7-delta}$, we find that in YBa$_2$Cu$_4$O$_8$ the mass decreases as $T_c$ increases under pressure. This inverse correlation between $m^*$ and $T_c$ suggests that quantum fluctuations of the charge order enhance $m^*$ but do not enhance $T_c$.
Superconducting condensation energy $U_0^{int}$ has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that $U_0^{int}propto T_c^n$ with $n$ = 3 to 4, which is in sharp contrast to the simple BCS prediction $U_0^{BCS}=1/2N_FDelta_s^2$ with $N_F$ the quasiparticle density of states at the Fermi energy, $Delta_s$ the superconducting gap. A similar correlation holds if we compute the condensation energy through $U_0^{cal}=3gamma_n^{eff}Delta_s^2/4pi^2k_B^2$ with $gamma_n^{eff}$ the effective normal state electronic specific heat coefficient. This indicates a general relationship $gamma_n^{eff} propto T_c^m$ with $m$ = 1 to 2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.
We synthesized the five batches of the samples of the novel P3 type superconductor, Na$_{x}$(H$_{3}$O)$_{y}$CoO$_{2}cdot y$H$_{2}$O, by the soft chemical process starting from $alpha$-NaCoO$_{2}$. The chemical and structural properties varied rather widely from batch to batch, with a result that $T_{c}$ varied from 4.6 K to 3.2 K. The magnetic susceptibility above $T_{c}$ shows upturn at low temperature as in the case of the P2 phase. The $T_{c}$ seems to be well correlated to the lattice parameters.
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