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Register a new userSpin-orbit coupling effects on the electronic properties of the pressure-induced superconductor CrAs
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We present the effects of spin-orbit coupling on the low-energy bands and Fermi surface of the recently discovered pressure-induced superconductor CrAs. We apply the Lowdin down-folding procedure to a tight-binding hamiltonian that includes the intrinsic spin-orbit interaction, originating from the Cr 3d electrons as well as from As 4p ones. Our results indicate that As contributions have negligible effects, whereas the modifications to the band structure and the Fermi surface can be mainly ascribed to the Cr contribution. We show that the inclusion of the spin-orbit interaction allows for a selective removal of the band degeneracy due to the crystal symmetries, along specific high symmetry lines. Such release of the band degeneracy naturally determines a reconstruction of the Fermi surface, including the possibility of changing the number of pockets.
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The recently synthesized ThFeAsN iron-pnictide superconductor exhibits a $T_c$ of 30 K, the highest of the 1111-type series in absence of chemical doping. To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the ${}^{75}$As Knight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multi-band $s^{pm}$-wave gap symmetry in the dirty limit, while the gap-to-$T_c$ ratio $Delta/k_mathrm{B}T_c$ hints at a strong-coupling scenario. Pressure modulates the geometrical parameters, thus reducing $T_c$, as well as $T_m$, the temperature where magnetic-relaxation rates are maximized, both at the same rate of approximately -1.1 K/GPa. This decrease of $T_c$ with pressure is consistent with band-structure calculations, which relate it to the deformation of the Fe 3$d_{z^2}$ orbitals.
A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P4/nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T = 50 K) and tetragonal (T = 190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a Bulk modulus around 31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the c-axis. Key bond angles of the SeFe4 pyramids and FeSe4 tetrahedra are also determined as a function of pressure.
The recent discovery of pressure induced superconductivity in the binary helimagnet CrAs has attracted much attention. How superconductivity emerges from the magnetic state and what is the mechanism of the superconducting pairing are two important issues which need to be resolved. In the present work, the suppression of magnetism and the occurrence of superconductivity in CrAs as a function of pressure ($p$) were studied by means of muon spin rotation. The magnetism remains bulk up to $psimeq3.5$~kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at $psimeq$7~kbar. At 3.5 kbar superconductivity abruptly appears with its maximum $T_c simeq 1.2$~K which decreases upon increasing the pressure. In the intermediate pressure region ($3.5lesssim plesssim 7$~kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature ($T_c$) and of the superfluid density ($rho_s$). A scaling of $rho_s$ with $T_c^{3.2}$ as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs.
The orthorhombic uranium dichalcogenide UTe$_2$ displays superconductivity below 1.7 K, with the anomalous feature of retaining 50$%$ of normal state (ungapped) carriers, according to heat capacity data from two groups. Incoherent transport that crosses over from above 50 K toward a low temperature, Kondo lattice Fermi liquid regime indicates strong magnetic fluctuations and the need to include correlation effects in theoretical modeling. We report density functional theory plus Hubbard U (DFT+U) results for UTe$_2$ to provide a platform for modeling its unusual behavior, focusing on ferromagnetic (FM, time reversal breaking) long range correlations along the ${hat a}$ axis as established by magnetization measurements and confirmed by our calculations. States near the Fermi level are dominated by the $j=frac{5}{2}$ configuration, with the $j_z=pmfrac{1}{2}$ sectors being effectively degenerate and half-filled. Unlike the small-gap insulating nonmagnetic electronic spectrum, the FM Fermi surfaces are large (strongly metallic) and display low dimensional features, reminiscent of the FM superconductor UGe$_2$.
We report $^{75}$As-NMR results for CrAs under pressure, which shows superconductivity adjoining a helimagnetically ordered state. We successfully evaluated the Knight shift from the spectrum, which is strongly affected by the quadrupole interaction. The Knight shift shows the remarkable feature that the uniform spin susceptibility increases toward low temperatures in the paramagnetic state. This is in sharp contrast to CrAs at ambient pressure, and also to cuprates and Fe pnictides, where antiferromagnetic correlations are dominant. Superconductivity emerges in CrAs under unique magnetic correlations, which probably originate in the three-dimensional zigzag structure of its nonsymmorphic symmetry.
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