Session F66: New Perspectives on Superconductivity in F- & D- Electron Systems -- From the Unconventional to the Topological

Unconventional superconductivity in UTe2

Spin triplet superconductivity was recently discovered at temperatures below 1.6 K in the correlated electron compound UTe₂ (1), with remarkably anisotropic and large upper critical field values, exceeding 35 T. Although it shares many characteristics with the ferromagnetic spin-triplet superconductors, UTe₂ does not order magnetically; instead, it exhibits properties of a quantum critical ferromagnet. The attendant strong magnetic fluctuations likely play an important role in establishing the superconducting pairing and protecting it against the typical destabilizing effects of high magnetic fields. Accumulating evidence supports the notion of a spin-triplet, nodal order parameter. Moreover, an unprecedented reentrant superconducting phase has been identified in UTe₂ at even higher magnetic fields, between 40 T and 65 T (2).

References:
(1) Science 365, 684 (2019)
(2) Nature Physics (2019), https://doi.org/10.1038/s41567-019-0670-x   

Manipulation of time reversal symmetry breaking superconductivity in Sr2RuO4 by uniaxial strain

Although the normal-state electronic structure of Sr₂RuO₄ is known with exceptional precision, even after two decades of research, the symmetry of it’s certainly unconventional superconducting state is currently under strong debate, e.g. the long time favoured spin-triplet p_x + i p_y state is ruled out by recent NMR experiments [1]. However, in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two-component order parameters. Probing Sr₂RuO₄ under uniaxial offers the possibility to lift the degeneracy between such components [2]. One key prediction for Sr₂RuO₄, a splitting of the superconducting and TRSB transitions under uniaxial stress has not been observed so far.
Here, we report results of muon spin relaxation (μSR) measurements on Sr₂RuO₄ placed under uniaxial stress. We observed a large stress-induced splitting between the onset temperatures of superconductivity and TRSB. Moreover, at high stress beyond the van Hove singularity [2], the TRSB phase is suppressed and a long-range ordered magnetic spin density wave phase is observed with an approx. 50 times stronger internal field.
We will discuss the uniaxial strain dependent electronic phase diagram and its implications for the symmetry of the superconducting order parameter in Sr₂RuO₄.




[1] A. Pustogow, et al., arXiv:1904.00047; K. Ishida et al., arXiv:1907.12236.
[2] C. Hicks, et al., Science 344, 283 (2014), M. E. Barber, et al., Phys. Rev. Lett. 120, 076602 (2018).
  

Knight Shift and Leading Superconducting Instability From Spin Fluctuations in Sr2RuO4

The chiral triplet pairing scenario proposed for Sr₂RuO₄ has been challenged by recent nuclear magnetic resonance (NMR) studies [A. Pustogow et al., arXiv:1904.00047 and K. Ishida et al., arXiv:1907.12236]. We perform a detailed theoretical study of spin-fluctuation mediated superconductivity guided by the spin-fluctuation spectrum measured from neutron scattering of this compound. Nodal even-parity solutions as well as odd-parity states with spins aligned predominantly out of the RuO₂ planes are found, both of which are compatible with the new data. The usual odd-parity state with spins primarily in the plane, the chiral k_x+ik_y, is difficult to stabilize and in contradiction to both NMR and neutron experiments. The presence of nodes in the spectral gap appears as a common feature for both even- and odd parity gaps. A surprising near-degeneracy of the nodal s′ and d_x²-y²-wave solutions suggests the possibility of a near-nodal time-reversal symmetry broken s′ +id_x²-y² pair state. Finally we discuss local signatures of such a state near nonmagnetic disorder, as well as the possibility of induced SDW in vicinity of impurities and under strain.   

Interplay of heavy fermion quantum criticality and unconventional superconductivity

According to the ‘Quantum Critical Paradigm’, antiferromagnetic (AF) quantum critical points (QCPs) in pristine heavy fermion metals cause emergent unconventional superconductivity (SC). This will be demonstrated for both CeCu₂Si₂ (CCS) and YbRh₂Si₂ (YRS) [M. Smidman et al., Phil. Mag. 98, 2930 (2018)]. CCS exhibits a 3D spin-density-wave QCP and was considered a d-wave superconductor until recently, when its specific heat was found to follow an exponential temperature dependence at low temperatures [S. Kittaka et al., Phys. Rev. Lett. 112, 067002 (2014)]. Based on atomic - substitution, neutron - scattering and penetration - depth results we show that CCS cannot be an (isotropic/anisotropic) s-wave superconductor but is best described by a model for a fully gapped two-band d-wave superconductor [G. M. Pang et al., Proc. Natl. Acad. Sci. USA 115, 5343 (2018); E. N. Nica et al., npj Quantum Materials 2, 24 (2017)]. YRS exhibits a magnetic-field induced partial-Mott AF QCP. For this material, no SC had been detected above 10 mK. However, magnetic and specific-heat measurements performed to about 1 mK revealed HF, i.e., unconventional, SC to develop at T_c = 2 mK. This is ascribed to a competition between nuclear-dominated AF hybrid order and the primary AF order of the 4f-electron spins by which the system is pushed towards its QCP [E. Schuberth et al., Science 351, 485 (2016)]. Our observations support the relevance of the Quantum Critical Paradigm, regardless of the microscopic origin of the AF instability.   

Pairing Tendencies, Orbital Selective Mott Phases, and Magnetic Block States in Multiorbital Models for Iron-Based Ladders and Chains

The discovery of superconductivity at high pressure in iron-based two-leg ladder 123 materials [1] established a new playground to better understand pairing tendencies in iron superconductors. Similarly as in Cu-oxide ladders, computational calculations using correlated electronic models can be performed with good accuracy in quasi-1D systems. Here, I will review our recent results for multiorbital Hubbard models varying the Hubbard and Hund couplings, and the electronic density. Clear indications of pair formation are found in lightly-doped ladders [2] and chains [3], and robust spin-singlet pair-pair correlations develop in those chains [3] (as in Cu-based ladders, we assume pressure leads to doping of the iron network [4]). An explanation for spin-singlet pairing based on an "orbital resonant valence bond" state is discussed for chains [5]. The magnetic properties in ladders and chains are also unexpectedly rich. An "orbital selective Mott phase" dominates in a wide range of parameters. In this regime, magnetic ``block'' states emerge, such as up-up-down-down patterns. We calculated the dynamical spin structure factor, finding a mixture of acoustic and optical modes [6] as in neutron experiments. Even more extended block states were recently discovered [7,8]. This complex behavior unveiled in models for 1D iron superconductors when studied accurately suggests that the physics of these materials could be far richer than anticipated.

[1] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); J. Ying et al., PRB 95, 241109(R) (2017).
[2] N. Patel et al., PRB 94, 075119 (2016)
[3] N. Patel et al., PRB 96, 024520 (2017)
[4] Y. Zhang et al., PRB 95, 115154 (2017); PRB 97, 045119 (2018)
[5] N. Patel et al., submitted
[6] J. Herbrych et al., Nat. Comm. 9, 3736 (2018)
[7] J. Herbrych et al., PRL 123, 027203 (2019)
[8] J. Herbrych et al., submitted