unconventional superconductivity
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2022 ◽  
Vol 7 (1) ◽  
Author(s):  
Gyanendra Singh ◽  
Claudio Guarcello ◽  
Edouard Lesne ◽  
Dag Winkler ◽  
Tord Claeson ◽  
...  

AbstractTwo-dimensional SrTiO3-based interfaces stand out among non-centrosymmetric superconductors due to their intricate interplay of gate-tunable Rashba spin-orbit coupling and multi-orbital electronic occupations, whose combination theoretically prefigures various forms of non-standard superconductivity. By employing superconducting transport measurements in nano-devices we present strong experimental indications of unconventional superconductivity in the LaAlO3/SrTiO3 interface. The central observations are the substantial anomalous enhancement of the critical current by small magnetic fields applied perpendicularly to the plane of electron motion, and the asymmetric response with respect to the magnetic field direction. These features cannot be accommodated within a scenario of canonical spin-singlet superconductivity. We demonstrate that the experimental observations can be described by a theoretical model based on the coexistence of Josephson channels with intrinsic phase shifts. Our results exclude a time-reversal symmetry breaking scenario and suggest the presence of anomalous pairing components that are compatible with inversion symmetry breaking and multi-orbital physics.


2021 ◽  
Author(s):  
◽  
William Doonan

<p>We have studied the nature of unconventional superconductivity in the rare-earth nitride (REN) samarium nitride (SmN) for the purposes of providing a deeper understanding of the mechanisms that lead to such a phenomenon in an already extremely interesting material. An approximate low energy model has been introduced for SmN based on previous bandstructure calculation and recent experimental results. This consists of the non-dispersive 4f band associated with the samarium ion crossing through the dispersive 5d band associated with the nitrogen ion. Due to large spin polarisation in the bandstructure we need only consider the majority-spin 5d and 4f bands which lead to an essentially spinless two band system. Starting from this two band system, we apply the k dot p method to it in order to create an effective model for the system. This effective model for the material acts as the platform from which we study the possible triplet superconducting pairing. Basing our pairing on the electron-phonon interaction we have postulated the existence of triplet pairing in the 5d band, from which we have successfully characterised the pair potential in this system through the self-consistency equation. The pair potential Delta_d could be solved analytically in a special case where the Fermi level was equal to the 4f band. In this case we find that above a threshold effective coupling strength the superconducting state is established and analytically known. In contrast to this result for the more general case where the Fermi level is different to the 4f band we numerically recover a solution that was exponential in the effective coupling strength which is similar to the pairing as we expect from the single band case. Analytic solutions in this case were not able to be found, however, we know that from our numerical investigations there will exist a solution for any effective coupling strength, contrasting with the special case where the pairing amplitude can disappear below a certain threshold. In conjunction to these results we also examined the situation where the 5d and 4f bands have hybridised together in order to search for unique pairing that may be resistant to disorder. By keeping the triplet pairing only in the 5d band, this translates to hybrid pairing between electrons in the two hybridised bands. Results from the hybridised bands system show a new singlet-like pairing Delta_S which is even in k and singlet in the hybridised band indices. Preliminary numerical results suggest that this pairing indeed exists and occurs only near the avoided crossing of the hybridised bands. The existence of such a pairing, originating from triplet pairing, has exciting implications for the robustness of the superconductivity in the presence of disorder and/or impurities.</p>


2021 ◽  
Author(s):  
◽  
William Doonan

<p>We have studied the nature of unconventional superconductivity in the rare-earth nitride (REN) samarium nitride (SmN) for the purposes of providing a deeper understanding of the mechanisms that lead to such a phenomenon in an already extremely interesting material. An approximate low energy model has been introduced for SmN based on previous bandstructure calculation and recent experimental results. This consists of the non-dispersive 4f band associated with the samarium ion crossing through the dispersive 5d band associated with the nitrogen ion. Due to large spin polarisation in the bandstructure we need only consider the majority-spin 5d and 4f bands which lead to an essentially spinless two band system. Starting from this two band system, we apply the k dot p method to it in order to create an effective model for the system. This effective model for the material acts as the platform from which we study the possible triplet superconducting pairing. Basing our pairing on the electron-phonon interaction we have postulated the existence of triplet pairing in the 5d band, from which we have successfully characterised the pair potential in this system through the self-consistency equation. The pair potential Delta_d could be solved analytically in a special case where the Fermi level was equal to the 4f band. In this case we find that above a threshold effective coupling strength the superconducting state is established and analytically known. In contrast to this result for the more general case where the Fermi level is different to the 4f band we numerically recover a solution that was exponential in the effective coupling strength which is similar to the pairing as we expect from the single band case. Analytic solutions in this case were not able to be found, however, we know that from our numerical investigations there will exist a solution for any effective coupling strength, contrasting with the special case where the pairing amplitude can disappear below a certain threshold. In conjunction to these results we also examined the situation where the 5d and 4f bands have hybridised together in order to search for unique pairing that may be resistant to disorder. By keeping the triplet pairing only in the 5d band, this translates to hybrid pairing between electrons in the two hybridised bands. Results from the hybridised bands system show a new singlet-like pairing Delta_S which is even in k and singlet in the hybridised band indices. Preliminary numerical results suggest that this pairing indeed exists and occurs only near the avoided crossing of the hybridised bands. The existence of such a pairing, originating from triplet pairing, has exciting implications for the robustness of the superconductivity in the presence of disorder and/or impurities.</p>


2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Lu Cao ◽  
Wenyao Liu ◽  
Geng Li ◽  
Guangyang Dai ◽  
Qi Zheng ◽  
...  

AbstractFor iron-based superconductors, the phase diagrams under pressure or strain exhibit emergent phenomena between unconventional superconductivity and other electronic orders, varying in different systems. As a stoichiometric superconductor, LiFeAs has no structure phase transitions or entangled electronic states, which manifests an ideal platform to explore the pressure or strain effect on unconventional superconductivity. Here, we observe two types of superconducting states controlled by orientations of local wrinkles on the surface of LiFeAs. Using scanning tunneling microscopy/spectroscopy, we find type-I wrinkles enlarge the superconducting gaps and enhance the transition temperature, whereas type-II wrinkles significantly suppress the superconducting gaps. The vortices on wrinkles show a C2 symmetry, indicating the strain effects on the wrinkles. By statistics, we find that the two types of wrinkles are categorized by their orientations. Our results demonstrate that the local strain effect with different directions can tune the superconducting order parameter of LiFeAs very differently, suggesting that the band shifting induced by directional pressure may play an important role in iron-based superconductivity.


Nature ◽  
2021 ◽  
Author(s):  
Myungchul Oh ◽  
Kevin P. Nuckolls ◽  
Dillon Wong ◽  
Ryan L. Lee ◽  
Xiaomeng Liu ◽  
...  

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Chong Liu ◽  
Hyungki Shin ◽  
Andrin Doll ◽  
Hsiang-Hsi Kung ◽  
Ryan P. Day ◽  
...  

AbstractSpin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO3, with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO3(001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO3. Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.


2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Sangmo Cheon ◽  
Ki Hoon Lee ◽  
Suk Bum Chung ◽  
Bohm-Jung Yang

AbstractRecently, unconventional superconductivity having a zero-bias conductance peak is reported in doped topological Dirac semimetal (DSM) with lattice distortion. Motivated by the experiments, we theoretically study the possible symmetry-lowering lattice distortions and their effects on the emergence of unconventional superconductivity in doped topological DSM. We find four types of symmetry-lowering lattice distortions that reproduce the crystal symmetries relevant to experiments from the group-theoretical analysis. Considering inter-orbital and intra-orbital electron density-density interactions, we calculate superconducting phase diagrams. We find that the lattice distortions can induce unconventional superconductivity hosting gapless surface Andreev bound states (SABS). Depending on the lattice distortions and superconducting pairing interactions, the unconventional inversion-odd-parity superconductivity can be either topological nodal superconductivity hosting a flat SABS or topological crystalline superconductivity hosting a gapless SABS. Remarkably, the lattice distortions increase the superconducting critical temperature, which is consistent with the experiments. Our work opens a pathway to explore and control pressure-induced topological superconductivity in doped topological semimetals.


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