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Register a new userStoichiometry, Spin Fluctuations, and Superconductivity in LaNiPO
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Superconductivity in LaNiPO is disrupted by small (~5%) amounts of non-stoichiometry on the lanthanum site, even though the electronic contribution to the heat capacity increases with increasing non-stoichiometry. All samples also exhibit specific heat anomalies consistent with the presence of ferromagnetic spin fluctuations (Tsf ~ 14 K). Comparison of layered nickel phosphide and nickel borocarbide superconductors reveals different structure-property correlations in the two families.
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Elucidating the microscopic origin of nematic order in iron-based superconducting materials is important because the interactions that drive nematic order may also mediate the Cooper pairing. Nematic order breaks fourfold rotational symmetry in the iron plane, which is believed to be driven by either orbital or spin degrees of freedom. However, as the nematic phase often develops at a temperature just above or coincides with a stripe magnetic phase transition, experimentally determining the dominant driving force of nematic order is difficult. Here, we use neutron scattering to study structurally the simplest iron-based superconductor FeSe, which displays a nematic (orthorhombic) phase transition at $T_s=90$ K, but does not order antiferromagnetically. Our data reveal substantial stripe spin fluctuations, which are coupled with orthorhombicity and are enhanced abruptly on cooling to below $T_s$. Moreover, a sharp spin resonance develops in the superconducting state, whose energy (~4 meV) is consistent with an electron boson coupling mode revealed by scanning tunneling spectroscopy, thereby suggesting a spin fluctuation-mediated sign-changing pairing symmetry. By normalizing the dynamic susceptibility into absolute units, we show that the magnetic spectral weight in FeSe is comparable to that of the iron arsenides. Our findings support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.
Superconductivity has its universal origin in the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc[1]. While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S=0 [2,3], they can also form parallel spin-triplet Cooper pairs with S=1 and an odd parity wavefunction[4-6], analogous to the equal spin pairing state in the superfluid 3He[7]. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation[8-11]. However, spin-triplet pairing is rare and has not been unambiguously identified in any solid state systems. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations[4-6], uranium based heavy-fermion materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity[12-14]. Indeed, UTe2, which has a Tc=1.6K [15,16], has been identified as a strong candidate for chiral spin-triplet topological superconductor near a FM instability[15-22], although the system also exhibits antiferromagnetic (AF) spin fluctuations[23,24]. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation at the Brillouin zone (BZ) boundary near AF order, analogous to the resonance seen in high-Tc copper oxide[25-27], iron-based[28,29], and heavy-fermion superconductors[30-32]. We find that the resonance in UTe2 occurs below Tc at an energy Er=7.9kBTc (kB is Boltzmanns constant) and at the expense of low-energy spin fluctuations. Since the resonance has only been found in spin-singlet superconductors near an AF instability[25-32], its discovery in UTe2 suggests that AF spin fluctuations can also induce spin-triplet pairing for superconductivity[33].
We present a high-pressure NMR study of the overdoped iron pnictide superconductor NaFe$_{0.94}$Co$_{0.06}$As. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate $1/^{75}T_1T$, first increase strongly with pressure but fall again at $P > P_{rm opt} =$ 2.2 GPa. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature $T_c$ shows a pressure-dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.
Identification of pairing mechanisms leading to the unconventional superconductivity realized in copper-oxide, heavy-fermions, and organic compounds is one of the most challenging issues in condensed-matter physics. Clear evidence for an electron-phonon mechanism in conventional superconductors is seen by the isotope effect on the superconducting transition temperatures $T_{rm SC}$, since isotopic substitution varies the phonon frequency without affecting the electronic states. In unconventional superconductors, magnetic fluctuations have been proposed to mediate superconductivity, and considerable efforts have been made to unravel relationships between normal-state magnetic fluctuations and superconductivity. Here, we show that characteristic experimental results on the ferromagnetic (FM) superconductor UCoGe ($T_{rm Curie} sim 2.5 $ K and $T_{rm SC} sim 0.6$ K) can be understood consistently within a scenario of the spin-triplet superconductivity induced by FM spin fluctuations. Temperature and angle dependencies of the upper critical magnetic field of the superconductivity ($H_{c2}$) are calculated on the basis of the above scenario by solving the Eliashberg equation. Calculated $H_{c2}$ well agrees with the characteristic experimental results observed in UCoGe. This is a first example that FM fluctuations are shown to be a pairing glue of superconductivity.
We report $^{57}$Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO$_{0.7}$ ($T_{c}=$ 28 K) enriched by $^{57}$Fe isotope. In the superconducting state, the spin component of $^{57}$Fe-Knight shift $^{57}K$ decreases almost to zero at low temperatures and the nuclear spin-lattice relaxation rate $^{57}(1/T_{1})$ exhibits a $T^{3}$-like dependence without the coherence peak just below $T_{c}$, which give firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below $T_c$ are consistently argued in terms of the extended s$_{pm}$-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease of $1/T_1T$ upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole ${bm q}$ space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antiferromagnetic interaction plays a vital role in mediating the Cooper pairing.
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