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Register a new userMagnetoresistance scaling, disorder, `hot spots and the origin of $T$-linear resistivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$
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The scaling of $H$-linear magnetoresistance in field and temperature was measured in under-doped (x = 0.19) and optimally-doped (x=0.31)~BaFe$_2$(As$_{1-x}$P$_x$)$_2$. We analyze the data based on an orbital model in the presence of strongly anisotropic quasiparticle spectra and scattering time due to antiferromagnetism. The magnetoresistance is dominated by the properties of small regions of the Fermi surface called `hot spots where antiferromagnetic excitations induce a large quasiparticle scattering rate. Approximate temperature-magnetic field scaling relations are derived and shown to be consistent with the experimental data. We argue that these results link the origin of linear-in-temperature resistivity to hot spots arising from an antiferromagnetic critical point, and magnetoresistance measurements provide a route to quantify this link.
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Theories of the strange metal, the parent state of many high temperature superconductors, invariably involve an important role for correlations in the spin and charge degrees of freedom. The most distinctive signature of this state in the charge transport sector is a resistance that varies linearly in temperature, but this phenomenon does not clearly point to one mechanism as temperature is a scalar quantity that influences every possible mechanism for momentum relaxation. In a previous work we identified an unusual scaling relationship between magnetic field and temperature in the in-plane resistivity of the unconventional superconductor BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, providing an opportunity to use the vector nature of the magnetic field to acquire additional clues about the mechanisms responsible for scattering in the strange metal state. Here we extend this work by investigating other components of the conductivity tensor under different orientations of the magnetic field. We find that the scaling phenomenon involves only the out-of-plane component of the magnetic field and is, strikingly, independent of the direction of the applied current. This suggests that the origin of the strange magnetotransport is in the action of the magnetic field on the correlated behavior of spin and charge degrees of freedom, rather than on the simple cyclotron motion of individual quasiparticles.
We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe$_2$As$_2$ single crystals. Both the $^{75}$As and $^{31}$P sites exhibit stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a universal phenomenon in the iron-based superconductors.
In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature $T_c$ is depressed at high concentrations ($xgtrsim$0.28), it shows an initial increase at lower $x$. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-$T_c$ family.
The interplay between superconductivity and Eu$ ^{2+}$ magnetic moments in EuFe$_2$(As$_{1-x}$P$_x$)$_2$ is studied by electrical resistivity measurements under hydrostatic pressure on $x=0.13$ and $x=0.18$ single crystals. We can map hydrostatic pressure to chemical pressure $x$ and show, that superconductivity is confined to a very narrow range $0.18leq x leq 0.23$ in the phase diagram, beyond which ferromagnetic (FM) Eu ordering suppresses superconductivity. The change from antiferro- to FM Eu ordering at the latter concentration coincides with a Lifshitz transition and the complete depression of iron magnetic order.
We investigate the in-plane anisotropy of Fe 3d orbitals occurring in a wide temperature and composition range of BaFe2(As1-xPx)2 system. By employing the angle-resolved photoemission spectroscopy, the lifting of degeneracy in dxz and dyz orbitals at the Brillouin zone corners can be obtained as a measure of the orbital anisotropy. In the underdoped regime, it starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. With increasing x, it well survives into the superconducting regime, but gradually gets suppressed and finally disappears around the non-superconducting transition (x = 0.7). The observed spontaneous in-plane orbital anisotropy, possibly coupled with anisotropic lattice and magnetic fluctuations, implies the rotational-symmetry broken electronic state working as the stage for the superconductivity in BaFe2(As1-xPx)2.
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