Session G27: Superconductivity:Low_Temp_2

Author not Attending

LaNiGa₂ is a centrosymmetric superconductor with orthorhombic crystal structure. Previous NMR measurements indicate that it breaks time-reversal symmetry due to the onset of spontaneous magnetic fields as it transitions into the superconducting phase, implying the possibility of spin-triplet pairing states. However, previous London penetration depth measurements agree with isotropic s-wave behavior down to 50 mK. One hint of non-BCS behavior comes from specific heat. Fits to a BCS form are successful over limited temperature ranges, but not over the entire range from 300 mK to Tc. We further investigate this discrepancy by using a dilution refrigerator to extend specific heat data to lower temperatures.   

Effect of uniaxial strain on the electronic structure of bulk NbReSi

Elemental rhenium (Re) was shown to exhibit time-reversal symmetry breaking in the superconducting (SC) state from a muon spin resonance experiment. Existence of nodes in the SC gap function, and its pairing nature remain elusive. Yet, this observation stimulated a search for time-reversal symmetry breaking and/or SC gap function with nodes in Re-rich compounds. Intriguingly, for some Re-included systems, time-reversal symmetry breaking has been observed in the SC state with isotropic gap functions. Recently, two crystalline forms of NbReSi such as hexagonal and orthorhombic structures in the SC state have been experimentally studied. For hexagonal NbReSi, the experimental data corroborates an upper critical field beyond the Pauli limit and multi-band SC gaps. Here we investigate, within first-principles methods, dominant orbitals and density of states at the Fermi level for the two forms of NbReSi with and without uniaxial strain, in order to have insights into the SC gap functions and their sensitivity to strain. We employ two first-principles methods including spin-orbit coupling such as solving Kohn-Sham equations and multiple scattering Green's function methods. We discuss the anisotropic nature of the band structure and compare with the experiments.   

Author not Attending

Significant enhancement of critical current density (J_c) by columnar defects (CDs) introduced by heavy-ion irradiation has been demonstrated in cuprate and iron-based superconductors (IBSs) [1,2]. Recently, we have discovered that J_c shows a broad peak (anomalous peak effect: APE) at around 1/3 of B_Φ (dose equivalent matching field) in IBSs with splayed CDs, [3]. Similar broad enhancement of J_c was also reported in a conventional superconductors 2H-NbSe₂ when tilted CDs are introduced at an angle from the c-axis [4]. The relationship between the two kinds of J_c enhancements induced by CDs in different superconductors with different configurations of CDs remains elusive. To clarify their relationship, we introduced splayed CDs into NbSe₂ and tilted CDs into IBSs and studied the evolution of APE as functions of the angle of CDs (θ_CD) and B_Φ.

It turns out that APE shows up in both of the above cases. Due to very large J_c and resulting large self-field, B_Φ dependence of APE in IBSs is difficult to follow. On the other hand, detailed B_Φ dependence of APE is clarified in NbSe₂. H_p/B_Φ increases more or less linearly at low B_Φ, and it shows a tendency of saturation at higher B_Φ before it is suppressed at very large B_Φ, possibly due to significant suppression of superconductivity in NbSe₂.

 

[1] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).

[2] Y. Nakajima et al., Phys. Rev. B 80, 012510 (2009).

[3] A. Park et al., Phys. Rev. B 97, 064516 (2018).

[4] S. Eley et al., Sci. Rep. 8, 13162 (2018).

    

Current-induced Self-organisation of Mixed Superconducting States

In the elemental superconductor Niobium we observe in the intermediate mixed state the coexistence of flux-free Meissner state domains and domains filled with vortices, called mixed state domains. Besides being a prominent example of superconducting vortex matter, the IMS can also act as a highly tunable model system for universal domain physics [1].

Vortices in a superconductor can be depinned using a transport current resulting in the flux flow state: For currents higher than the depinning current I_c vortices move orthogonal to the direction of the applied current. In contrast to the canonical mixed state, non-trivial ordering phenomena are expected in the flux flow regime of the IMS due to its highly heterogeneous domain structure. In a recent study [2] combining transport measurements and small-angle neutron scattering (SANS) on a Nb single crystal sample we could verify the existence of the IMS in the state of flux flow. Our main result is an observation of a transition from isotropic to anisotropic IMS scattering indicating that the IMS rearranges itself into a stripe superstructure in the flux flow regime. The stripe pattern is aligned perpendicular to the current direction along the motion of the vortices.

[1] M. Seul and D. Andelman 1995 Science 267(5197):476–483

[2] Xaver S Brems et al 2022 Supercond. Sci. Technol. 35 035003   

Imaging delocalized quasiparticles in the vortex state of NbSe2

In a superconducting vortex lattice, delocalized quasiparticles escape the vortex cores where the order parameter vanishes. Tracing these delocalized quasiparticles is essential in understanding quasiparticle poisoning of Majorana bound states in vortex cores of a topological superconductor. Here, we image delocalized quasiparticles around vortices in NbSe₂, for the first time, with atomic resolution by scanning tunneling noise microscopy. From shot-noise imaging, we extract spatial variation of the effective charge when tunneling into the vortex state, which can be understood by the Ginzburg-Landau model. Furthermore, we find that quasiparticle poisoning dominates when vortices are less than 4 times the coherence length apart.   

Effect of 2.5 MeV electron irradiation on vortex dynamics of Ca3(Ir,Rh)4Sn13 superconductor

Zero field London penetration depth, λ_L(T) and Campbell penetration depth, λ_C(T) were measured

in both pristine and electron-irradiated samples of single crystals Ca₃(Ir,Rh)₄Sn₁₃ using a tunnel diode resonator (TDR). The rate of T_C suppression and low-temperature London penetration depth were analyzed to understand the superconducting gap structure. The Campbell penetration depth was used to extract true (relaxation-free) critical current density as a function of temperature and magnetic field. The results and the constructed vortex phase diagram are compared between the pristine and irradiated samples, as well as with conventional measurements. The observed peak-effect, induced electron irradiation, implies its static origin and signifies a crossover in the pinning mechanism.   

Observation of the peak effect in Ga-doped Re3Ge7

Single crystals of intermetallic Re₃Ge_7-xGa_x have been grown and characterized by measuring magnetization, electrical resistivity, thermoelectric power, and specific heat. For Re₃Ge₇, a phase transition at T_s = 58.5 K is indicated by a kink in magnetic susceptibility and is confirmed by a λ-like anomaly in specific heat. The temperature dependence of the electrical resistivity follows a typical metallic behavior above T_s and sharply increases below T_s, showing a metal-to-insulator-like transition. By substitution Ga for Ge-site, Re₃Ge_7-xGa_x, superconductivity emerges upon suppressing the phase transition. In the superconducting state for x ~ 0.4, the peak effect has been investigated by critical current (J_c) measurements in applied magnetic fields (H). The transition from a weakly pinned region to a strongly pinned region is mapped out in the H-J_c phase diagram.   

Tunneling Spectroscopy of Gap Anisotropy in Niobium

The anistropic gap of pure superconducting Niobium has been calculated based on Eliashberg theory with the electron-phonon coupling and phonon density of states computed from density functional theory [1]. Quasiparticle tunneling experiments can in principle reveal the gap structure on the Niobium Fermi surfaces [2]. The tunneling conductance is a convolution of the angle-resolved local density of states on the Fermi surface, the angle-dependent transmission probability, and the derivative of the Fermi distribution function. NIS tunneling normal to different crystalline orientations allows one to determine the gap structure in the high-symmetry directions of the Fermi surface. The gap structure on different sheets can also be revealed if the difference in energy between the gaps for the same direction k/|k| is large compared to 3.5 k_BT. We report results for the tunneling conductance expected for a number of Niobium crystal orientations. Comparison with recent experimental measurements are reported.

[1] Mehdi Zarea, Hikaru Ueki, and J. A. Sauls. “Effects of anisotropy and disorder on the superconducting properties of Niobium,” arXiv:2201.07403 (2022).

[2] E.L. Wolf, Principles of electron tunneling spectroscopy, Oxford University Press (2012).   

Diamagnetic mechanism of non-reciprocal superconductivity in multilayered superconductors.

Non-reciprocal superconductivity has been a topic of great interest, motivated by the role of spin-orbit interactions on superconductor properties. Here we analyze critical current in a multilayer superconductor and show that formation of diamagnetic currents in the presence of magnetic field leads to non-reciprocity. This is a generic mechanism of non-reciprocity and should be present in all multilayered superconductors with interlayer spacing less than coherence length to preserve phase coherence between the layers. Above some critical current and magnetic field it is energetically favorable to form Josephson vortices, formation of vortices results in non-monotonic variation of non-reciprocal current contribution as a function of magnetic field and may result in its sign change. We report observation of non-reciprocal critical current in Al/InAs nanowires and show that magnetic field evolution of non-reciprocal critical current is consistent with the presence of diamagnetic currents and formation of Josephson vortices.   

Domain wall motion in a polycrystalline vortex lattice

We present transport measurements on superconductor-normal-superconductor (SNS) arrays studied in finite magnetic fields. By applying magnetic fields that correspond to an incommensurate filling factor (shifting the vortex lattice away from one that matches the SNS array), disorder within the system can be studied. By then driving vortex motion with applied current, we observed a two-step transition at the incommensurate fields, which demonstrates a transition from pinned vortices to lattice defect flow to lattice flow. This transport is suggestive of a polycrystalline structure with defects forming along the edges of the crystalline domain. These results inform the larger question of how disorder affects the vortex lattice.   

19F-NMR and magnetic susceptibility in LaO0.5F0.5BiS2

We report on the analysis of ¹⁹F-NMR parameters and complementary magnetic susceptibility in an ambient-pressure annealed powder of LaO_0.5F_0.5BiS₂ with a T_c of about 3K. Measurements were performed as functions of temperature T and applied magnetic field H. The ¹⁹F line position is dominated by the chemical and Knight shifts, which are typically T-independent. Nevertheless, we find a noticeable T-dependence of both the shift and linewidth above 10K and a stronger T-dependence of these parameters with decreasing T below 10K. The bulk magnetization can be fit reasonably well with a modified Brillouin function for dilute magnetic impurities. However, neither the linewidth nor the frequency shift responses to varying T can be attributed to random paramagnetic impurities. This suggests hidden local magnetic degrees of freedom as an alternative source of static fluctuations in the normal state. Ten Kelvin is the maximum T_c this material displays under pressure, or when samples of this material are pressure annealed. The decrease in lineshift, the linewidth increase, and the lineshape symmetry together suggest superconducting fluctuations throughout the sample, and not just in some regions, below 10K. A discussion of spin-lattice relaxation effects is presented to help with characterizing the situation.   

51V NMR studies on single crystal of A15 superconductor V3Si

The Martensitic transformation (MT) in A15 binary-alloy superconductor V₃Si is a second-order, displacive structural transition from cubic to tetragonal symmetry, at temperature T_m a few K above the superconducting transition temperature T_c = 17 K. Though studied extensively, the MT has not yet been conclusively linked with a transition to superconductivity, while V₃Si continues to be of current interest. Previous NMR studies on the MT in V₃Si have been on powder samples, and with little emphasis on temperature dependence during transformation. Here we study a high-quality single crystal, where quadrupolar splitting and Knight shift of NMR spectra for ⁵¹V allowed us to distinguish between transverse chains of V as a function of temperature. This capability has led to evidence of coexistence of transformed and untransformed phases over a few K below and above T_m [Gapud et al., Physica C, vol. 602, 1354137], as well as new details regarding the evolution of the Knight Shift and of the longitudinal relaxation time T1 across this transition, as will be reported and discussed.   

Role of spin orbital coupling in unconventional superconductivity

We have studied the superconducting properties of the ternary noncentrosymmetric superconductors TaXSi (X = Re, Ru), with the help of muon spin rotation/relaxation ($mu$SR) and density functional theory calculations. Our transverse-field $mu$SR measurements reveal isotropic s-wave superconductivity in TaReSi and multi-gap superconductivity with gap nodes in TaRuSi. Zero-field $mu$SR measurements, highly sensitive to very small magnetic fields find no evidence for spontaneous fields in the superconducting state of TaReSi, whereas we observe spontaneous fields which onset with superconductivity indicating broken time reversal symmetry (TRS) superconductivity in TaRuSi. Broken TRS in weakly coupled TaRuSi can be attributed to a non-unitary triplet pairing state, while in TaReSi, this state is suppressed due to strong anti-symmetric spin orbital coupling. Our results in TaXSi demonstrate that the strength of spin orbit coupling can be responsible for stabilizing unconventional superconductivity.   

NMR and NQR measurements in LaNiGa2

LaNiGa₂ is a layered centrosymmetric superconductor with an orthorhombic crystal structure. ??SR experiments have observed spontaneous magnetic fields at the onset of superconducting state, implying time-reversal symmetry breaking and suggesting the possibility of triplet pairing. We report on NQR measurements performed on ¹³⁹La down to dilution fridge temperatures showing a large shift to the spectrum in the superconducting state. We also report on continued normal state NMR measurements on ⁶⁹Ga determining the anisotropy of the electric field gradient at the two 69Ga sites. These measurements are shown to be consistent with band structure calculations.   

Thermodynamic properties of superconductors near magnetic quantum critical point

I will present the results of the thermodynamic manifestations of the quantum criticality in multiband unconventional superconductors. As a guiding example I will consider the scenario of magnetic quantum critical point in the model that captures superconductivity coexistence with the spin-density wave. I will show that in situations when superconducting order parameter has incidental nodes at isolated points, quantum magnetic fluctuations lead to the renormalization of the relative T-linear slope of the London penetration depth. This leads to the nonmonotonic dependence of the penetration depth as a function of doping and the concomitant peak structure across the quantum critical point. The theoretical analysis will be corroborated by making a comparison of the results with the recent experimental data from the low-temperature thermodynamic measurements at optimal composition in BaFe$_2$(As$_{1-x}$P$_x$)$_2$