A conventional superconducting state may be replaced by another dissipationless state hosting Cooper pairs with a finite momentum, leaving thermodynamic footprints for such a phase transition. Recently, a novel type of finite momentum pairing, so-called orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, has been proposed to occur in spin-orbit coupled superconductors such as bilayer $2mathrm{H-NbSe_{2}}$. So far, a thermodynamic demonstration, which is key for establishing this exotic phase, has been lacking. Here, we reveal a first-order quantum phase transition to the orbital FFLO state in tunneling spectroscopic measurements on multilayer $2mathrm{H-NbSe_{2}}$. The phase transition manifests itself as a sudden enhancement of the superconducting gap at an in-plane magnetic field $B_{//}$ well below the upper critical field. Furthermore, this transition shows prominent hysteresis by sweeping $B_{//}$ back and forth and quickly disappears once the magnetic field is tilted away from the sample plane by less than one degree. We obtain a comprehensive phase diagram for the orbital FFLO state and compare it with the theoretical calculation that takes into account the rearrangement of Josephson vortices. Our work elucidates the microscopic mechanism for the emergence of the orbital FFLO state.
{"title":"Spectroscopic evidence for a first-order transition to the orbital Fulde-Ferrell-Larkin-Ovchinnikov state","authors":"Zongzheng Cao, Menghan Liao, Hongyi Yan, Yuying Zhu, Liguo Zhang, Kenji Watanabe, Takashi Taniguchi, Alberto F. Morpurgo, Haiwen Liu, Qi-Kun Xue, Ding Zhang","doi":"arxiv-2409.00373","DOIUrl":"https://doi.org/arxiv-2409.00373","url":null,"abstract":"A conventional superconducting state may be replaced by another\u0000dissipationless state hosting Cooper pairs with a finite momentum, leaving\u0000thermodynamic footprints for such a phase transition. Recently, a novel type of\u0000finite momentum pairing, so-called orbital Fulde-Ferrell-Larkin-Ovchinnikov\u0000(FFLO) state, has been proposed to occur in spin-orbit coupled superconductors\u0000such as bilayer $2mathrm{H-NbSe_{2}}$. So far, a thermodynamic demonstration,\u0000which is key for establishing this exotic phase, has been lacking. Here, we\u0000reveal a first-order quantum phase transition to the orbital FFLO state in\u0000tunneling spectroscopic measurements on multilayer $2mathrm{H-NbSe_{2}}$. The\u0000phase transition manifests itself as a sudden enhancement of the\u0000superconducting gap at an in-plane magnetic field $B_{//}$ well below the upper\u0000critical field. Furthermore, this transition shows prominent hysteresis by\u0000sweeping $B_{//}$ back and forth and quickly disappears once the magnetic field\u0000is tilted away from the sample plane by less than one degree. We obtain a\u0000comprehensive phase diagram for the orbital FFLO state and compare it with the\u0000theoretical calculation that takes into account the rearrangement of Josephson\u0000vortices. Our work elucidates the microscopic mechanism for the emergence of\u0000the orbital FFLO state.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210010","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Rodrigo A. Fontenele, Natanael C. Costa, Thereza Paiva, Raimundo R. dos Santos
The attractive Hubbard model has become a model readily realizable with ultracold atoms on optical lattices. However, the superconducting (superfluid) critical temperatures, $T_c$'s, are still somewhat smaller than the lowest temperatures achieved in experiments. Here we consider two possible routes, generically called layering, to increase $T_c$: a bilayer and a simple cubic lattice, both with tunable hopping, $t_z$, between attractive Hubbard planes. We have performed minus-sign--free determinant quantum Monte Carlo simulations to calculate response functions such as pairing correlation functions, uniform spin susceptibility, and double occupancy, through which we map out some physical properties. We have found that by a judicious choice of fillings and intensity of on-site attraction, a bilayer can exhibit $T_c$'s between 1.5 and 1.7 times those of the single layer; for the simple-cubic lattice the enhancement can be 30% larger than the maximum for the single layer. We also check the accuracy of both a BCS-like estimate for $T_c$ in the attractive Hubbard model, as well as of an upper bound for $T_c$ based on the superfluid density.
{"title":"Increasing superconducting $T_c$ by layering in the attractive Hubbard model","authors":"Rodrigo A. Fontenele, Natanael C. Costa, Thereza Paiva, Raimundo R. dos Santos","doi":"arxiv-2408.17405","DOIUrl":"https://doi.org/arxiv-2408.17405","url":null,"abstract":"The attractive Hubbard model has become a model readily realizable with\u0000ultracold atoms on optical lattices. However, the superconducting (superfluid)\u0000critical temperatures, $T_c$'s, are still somewhat smaller than the lowest\u0000temperatures achieved in experiments. Here we consider two possible routes,\u0000generically called layering, to increase $T_c$: a bilayer and a simple cubic\u0000lattice, both with tunable hopping, $t_z$, between attractive Hubbard planes.\u0000We have performed minus-sign--free determinant quantum Monte Carlo simulations\u0000to calculate response functions such as pairing correlation functions, uniform\u0000spin susceptibility, and double occupancy, through which we map out some\u0000physical properties. We have found that by a judicious choice of fillings and\u0000intensity of on-site attraction, a bilayer can exhibit $T_c$'s between 1.5 and\u00001.7 times those of the single layer; for the simple-cubic lattice the\u0000enhancement can be 30% larger than the maximum for the single layer. We also\u0000check the accuracy of both a BCS-like estimate for $T_c$ in the attractive\u0000Hubbard model, as well as of an upper bound for $T_c$ based on the superfluid\u0000density.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210020","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chen-How Huang, Alejandro M. Lobos, Miguel A. Cazalilla
Quantum spin impurities coupled to superconductors are under intense investigation for their relevance to fundamental research as well as the prospects to engineer novel quantum phases of matter. Here we develop a large-$N$ mean-field theory of a strongly coupled spin-$tfrac{1}{2}$ quantum impurity in a conventional $s$-wave superconductor. The approach is benchmarked against Wilson's numerical renormalization group (NRG). While the large-$N$ method is not applicable in the weak-coupling regime where the Kondo temperature $T_K$ is smaller than the superconducting gap $Delta$, it performs very well in the strong coupling regime where $T_K gtrsim Delta$, thus allowing to obtain a reasonably accurate description of experimentally relevant quantities. The latter includes the energy of the Yu-Shiba-Rusinov subgap states, their spectral weight, as well as the local density of continuum states. The method provides a reliable analytical tool that complements other perturbative and non-perturbative methods, and can be extended to more complex impurity models for which NRG may be not easily applicable.
{"title":"A Large-$N$ Approach to Magnetic Impurities in Superconductors","authors":"Chen-How Huang, Alejandro M. Lobos, Miguel A. Cazalilla","doi":"arxiv-2408.17281","DOIUrl":"https://doi.org/arxiv-2408.17281","url":null,"abstract":"Quantum spin impurities coupled to superconductors are under intense\u0000investigation for their relevance to fundamental research as well as the\u0000prospects to engineer novel quantum phases of matter. Here we develop a\u0000large-$N$ mean-field theory of a strongly coupled spin-$tfrac{1}{2}$ quantum\u0000impurity in a conventional $s$-wave superconductor. The approach is benchmarked\u0000against Wilson's numerical renormalization group (NRG). While the large-$N$\u0000method is not applicable in the weak-coupling regime where the Kondo\u0000temperature $T_K$ is smaller than the superconducting gap $Delta$, it performs\u0000very well in the strong coupling regime where $T_K gtrsim Delta$, thus\u0000allowing to obtain a reasonably accurate description of experimentally relevant\u0000quantities. The latter includes the energy of the Yu-Shiba-Rusinov subgap\u0000states, their spectral weight, as well as the local density of continuum\u0000states. The method provides a reliable analytical tool that complements other\u0000perturbative and non-perturbative methods, and can be extended to more complex\u0000impurity models for which NRG may be not easily applicable.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210021","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hudson T. Horne, Collin M. Hugo, Brandon C. Reid, Daniel F. Santavicca
We report a systematic comparison of niobium nitride thin films deposited on oxidized silicon substrates by reactive DC magnetron sputtering and reactive high-power impulse magnetron sputtering (HiPIMS). After determining the nitrogen gas concentration that produces the highest superconducting critical temperature for each process, we characterize the dependence of the critical temperature on film thickness. The optimal nitrogen concentration is higher for HiPIMS than for DC sputtering, and HiPIMS produces higher critical temperatures for all thicknesses studied. We attribute this to the HiPIMS process enabling the films to get closer to optimal stoichiometry before beginning to form a hexagonal crystal phase that reduces the critical temperature, along with the extra kinetic energy in the HiPIMS process enabling greater adatom mobility and improving crystallinity. We also study the effects of an aluminum nitride buffer layer and substrate heating on the critical temperature.
{"title":"Optimization of Superconducting Niobium Nitride Thin Films via High-Power Impulse Magnetron Sputtering","authors":"Hudson T. Horne, Collin M. Hugo, Brandon C. Reid, Daniel F. Santavicca","doi":"arxiv-2408.17256","DOIUrl":"https://doi.org/arxiv-2408.17256","url":null,"abstract":"We report a systematic comparison of niobium nitride thin films deposited on\u0000oxidized silicon substrates by reactive DC magnetron sputtering and reactive\u0000high-power impulse magnetron sputtering (HiPIMS). After determining the\u0000nitrogen gas concentration that produces the highest superconducting critical\u0000temperature for each process, we characterize the dependence of the critical\u0000temperature on film thickness. The optimal nitrogen concentration is higher for\u0000HiPIMS than for DC sputtering, and HiPIMS produces higher critical temperatures\u0000for all thicknesses studied. We attribute this to the HiPIMS process enabling\u0000the films to get closer to optimal stoichiometry before beginning to form a\u0000hexagonal crystal phase that reduces the critical temperature, along with the\u0000extra kinetic energy in the HiPIMS process enabling greater adatom mobility and\u0000improving crystallinity. We also study the effects of an aluminum nitride\u0000buffer layer and substrate heating on the critical temperature.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210029","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We study non-Hermitian Josephson junctions formed by multiple superconductors and discover the emergence of exceptional points entirely determined by the interplay of the distinct superconducting phases and non-Hermiticity due to normal reservoirs. In particular, in Josephson junctions with three and four superconductors, we find stable lines and surfaces of exceptional points protected by non-Hermitian topology and highly tuneable by the superconducting phases. We also discover that, in Josephson junctions formed by laterally coupled superconductors, exceptional points can result from hybridized Andreev bound states and lead to the enhancement of supercurrents controlled by dissipation. Our work unveils the potential of multi-terminal Josephson junctions for realizing higher dimensional topological non-Hermitian superconducting phenomena.
{"title":"Non-Hermitian multiterminal phase-biased Josephson junctions","authors":"Jorge Cayao, Masatoshi Sato","doi":"arxiv-2408.17260","DOIUrl":"https://doi.org/arxiv-2408.17260","url":null,"abstract":"We study non-Hermitian Josephson junctions formed by multiple superconductors\u0000and discover the emergence of exceptional points entirely determined by the\u0000interplay of the distinct superconducting phases and non-Hermiticity due to\u0000normal reservoirs. In particular, in Josephson junctions with three and four\u0000superconductors, we find stable lines and surfaces of exceptional points\u0000protected by non-Hermitian topology and highly tuneable by the superconducting\u0000phases. We also discover that, in Josephson junctions formed by laterally\u0000coupled superconductors, exceptional points can result from hybridized Andreev\u0000bound states and lead to the enhancement of supercurrents controlled by\u0000dissipation. Our work unveils the potential of multi-terminal Josephson\u0000junctions for realizing higher dimensional topological non-Hermitian\u0000superconducting phenomena.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210022","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Superconductors can form ideal diodes carrying nondissipative supercurrents in the forward direction and dissipative currents in the backward direction. The Josephson diode has proven to be a promising design where the junction between the two superconductors comprises the weakest link and thus provides the dominant mechanism. We here propose a Josephson diode based on a single magnetic material with a conical spin structure. The helical spin rotation produces Rashba-like band splitting inversely proportional to the rotation period. Together with the Zeeman splitting caused by the time-reversal symmetry breaking of the noncoplanar spin texture, this results in a large diode efficiency close to the $0-pi$ transition of the magnetic Josephson junction.
{"title":"Nonreciprocal Josephson current through a conical magnet","authors":"Lina Johnsen Kamra, Liang Fu","doi":"arxiv-2409.00223","DOIUrl":"https://doi.org/arxiv-2409.00223","url":null,"abstract":"Superconductors can form ideal diodes carrying nondissipative supercurrents\u0000in the forward direction and dissipative currents in the backward direction.\u0000The Josephson diode has proven to be a promising design where the junction\u0000between the two superconductors comprises the weakest link and thus provides\u0000the dominant mechanism. We here propose a Josephson diode based on a single\u0000magnetic material with a conical spin structure. The helical spin rotation\u0000produces Rashba-like band splitting inversely proportional to the rotation\u0000period. Together with the Zeeman splitting caused by the time-reversal symmetry\u0000breaking of the noncoplanar spin texture, this results in a large diode\u0000efficiency close to the $0-pi$ transition of the magnetic Josephson junction.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210012","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We report the crystal structures and superconductivity (SC) of LaPt$_{x}$Si$_{2-x}$ ($0.5 leq x leq 1.0$) that are solid solutions of LaSi$_{2}$ and LaPtSi with centrosymmetric tetragonal ($I4_{1}/amd$, $D_{4h}^{19}$, #141) and non-centrosymmetric tetragonal ($I4_{1}md$, $C_{4v}^{11}$, #109) structures, respectively. It was found that at $0.86 leq x leq 1.00$, the non-centrosymmetric tetragonal symmetry is preserved, while partial disorder appears in alternating Pt and Si of the hyper-honeycomb network. The superconducting transition temperature $T_{rm c}$ was drastically reduced from 3.9 K to 1.5 K as $x$ varies from 1.0 to 0.86. Additionally, a hexagonal phase with an AlB$_{2}$-type structure ($P6/mmm$, $D_{6h}^{1}$, #191) has been discovered at $0.50 leq x leq 0.71$ with a honeycomb network of statistically distributed Pt and Si atoms. The hexagonal phase exhibited SC at $T_{rm c} = 0.38$ K. This system provides an opportunity to investigate the relationship between topological electronic states, SC, and disorders
{"title":"Structural Evolution from Hyper-Honeycomb to Honeycomb Networks and Superconductivity in LaPt$_x$Si$_{2-x}$","authors":"Sitaram Ramakrishnan, Tatsuya Yamakawa, Ryohei Oishi, Soichiro Yamane, Atsutoshi Ikeda, Masaki Kado, Yasuyuki Shimura, Toshiro Takabatake, Takahiro Onimaru, Yasuhiro Shibata, Arumugam Thamizhavel, Srinivasan Ramakrishnan, Shingo Yonezawa, Minoru Nohara","doi":"arxiv-2408.17033","DOIUrl":"https://doi.org/arxiv-2408.17033","url":null,"abstract":"We report the crystal structures and superconductivity (SC) of\u0000LaPt$_{x}$Si$_{2-x}$ ($0.5 leq x leq 1.0$) that are solid solutions of\u0000LaSi$_{2}$ and LaPtSi with centrosymmetric tetragonal ($I4_{1}/amd$,\u0000$D_{4h}^{19}$, #141) and non-centrosymmetric tetragonal ($I4_{1}md$,\u0000$C_{4v}^{11}$, #109) structures, respectively. It was found that at $0.86 leq\u0000x leq 1.00$, the non-centrosymmetric tetragonal symmetry is preserved, while\u0000partial disorder appears in alternating Pt and Si of the hyper-honeycomb\u0000network. The superconducting transition temperature $T_{rm c}$ was drastically\u0000reduced from 3.9 K to 1.5 K as $x$ varies from 1.0 to 0.86. Additionally, a\u0000hexagonal phase with an AlB$_{2}$-type structure ($P6/mmm$, $D_{6h}^{1}$,\u0000#191) has been discovered at $0.50 leq x leq 0.71$ with a honeycomb network\u0000of statistically distributed Pt and Si atoms. The hexagonal phase exhibited SC\u0000at $T_{rm c} = 0.38$ K. This system provides an opportunity to investigate the\u0000relationship between topological electronic states, SC, and disorders","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210015","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
S. Sinha, J. Lim, Z. Li, J. S. Kim, A. C. Hire, P. M. Dee, R. S. Kumar, D. Popov, R. J. Hemley, R. G. Hennig, P. J. Hirschfeld, G. R. Stewart, J. J. Hamlin
The recent surprising discovery of superconductivity with critical temperature $T_c$ = 32 K in MoB$_2$ above 70 GPa has led to the search for related materials that may superconduct at similarly high $T_c$ values and lower pressures. We have studied the superconducting and structural properties of Re$_{0.10}$Mo$_{0.90}$B$_2$ to 170 GPa. A structural phase transition from R3m to P6/mmm commences at 48 GPa, with the first signatures of superconductivity appearing above 44 GPa. The critical temperature is observed to increase with pressure. A complete resistive transition is observed only above 150 GPa, where the highest onset $T_c$ of 30 K is also achieved. Upon releasing pressure, the high pressure superconducting phase is found to be metastable. During unloading, a complete resistive superconducting transition is observed all the way down to 20 GPa (with onset $T_c sim 20$ K). Our results suggest that the P6/mmm structure is responsible for the observed superconductivity.
{"title":"Superconductivity in pressurized Re$_{0.10}$Mo$_{0.90}$B$_2$","authors":"S. Sinha, J. Lim, Z. Li, J. S. Kim, A. C. Hire, P. M. Dee, R. S. Kumar, D. Popov, R. J. Hemley, R. G. Hennig, P. J. Hirschfeld, G. R. Stewart, J. J. Hamlin","doi":"arxiv-2408.17416","DOIUrl":"https://doi.org/arxiv-2408.17416","url":null,"abstract":"The recent surprising discovery of superconductivity with critical\u0000temperature $T_c$ = 32 K in MoB$_2$ above 70 GPa has led to the search for\u0000related materials that may superconduct at similarly high $T_c$ values and\u0000lower pressures. We have studied the superconducting and structural properties\u0000of Re$_{0.10}$Mo$_{0.90}$B$_2$ to 170 GPa. A structural phase transition from\u0000R3m to P6/mmm commences at 48 GPa, with the first signatures of\u0000superconductivity appearing above 44 GPa. The critical temperature is observed\u0000to increase with pressure. A complete resistive transition is observed only\u0000above 150 GPa, where the highest onset $T_c$ of 30 K is also achieved. Upon\u0000releasing pressure, the high pressure superconducting phase is found to be\u0000metastable. During unloading, a complete resistive superconducting transition\u0000is observed all the way down to 20 GPa (with onset $T_c sim 20$ K). Our\u0000results suggest that the P6/mmm structure is responsible for the observed\u0000superconductivity.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210019","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Flavio Noronha, Askery Canabarro, Rafael Chaves, Rodrigo G. Pereira
Competition between magnetism and superconductivity can lead to unconventional and topological superconductivity. However, the experimental confirmation of the presence of Majorana edge states and unconventional pairing currently poses a major challenge. Here we consider a two-dimensional lattice model for a superconductor with spin-orbit coupling and exchange coupling to randomly distributed magnetic impurities. Depending on parameters of the model, this system may display topologically trivial or nontrivial edge states. We map out the phase diagram by computing the Bott index, a topological invariant defined in real space. We then use machine learning (ML) algorithms to predict the Bott index from the local density of states (LDOS) at zero energy, obtaining high-accuracy results. We also train ML models to predict the amplitude of odd-frequency pairing in the anomalous Green's function at zero energy. Once the ML models are trained using the LDOS, which is experimentally accessible via scanning tunneling spectroscopy, our method could be applied to predict the number of Majorana edge states and to estimate the magnitude of odd-frequency pairing in real materials.
磁性和超导性之间的竞争可能导致非常规和拓扑超导性。然而，实验证实马约拉纳边缘态和非常规配对的存在目前是一个重大挑战。在这里，我们考虑了一个具有自旋轨道耦合和随机分布磁性杂质交换耦合的超导体二维晶格模型。根据模型参数的不同，该系统可能显示拓扑上的琐碎边缘态或非琐碎边缘态。我们通过计算博特指数绘制出相图，博特指数是在实空间定义的拓扑不变量。然后，我们使用机器学习（ML）算法从零能量时的局部态密度（LDOS）预测 Bott 指数，获得了高精度的结果。我们还训练 ML 模型来预测零能量时反常格林函数中奇异频率配对的振幅。一旦使用 LDOS 训练出 ML 模型，我们的方法就可以应用于预测马约拉纳边沿态的数量和估计实际材料中奇频配对的幅度。
{"title":"Predicting topological invariants and unconventional superconducting pairing from density of states and machine learning","authors":"Flavio Noronha, Askery Canabarro, Rafael Chaves, Rodrigo G. Pereira","doi":"arxiv-2408.16499","DOIUrl":"https://doi.org/arxiv-2408.16499","url":null,"abstract":"Competition between magnetism and superconductivity can lead to\u0000unconventional and topological superconductivity. However, the experimental\u0000confirmation of the presence of Majorana edge states and unconventional pairing\u0000currently poses a major challenge. Here we consider a two-dimensional lattice\u0000model for a superconductor with spin-orbit coupling and exchange coupling to\u0000randomly distributed magnetic impurities. Depending on parameters of the model,\u0000this system may display topologically trivial or nontrivial edge states. We map\u0000out the phase diagram by computing the Bott index, a topological invariant\u0000defined in real space. We then use machine learning (ML) algorithms to predict\u0000the Bott index from the local density of states (LDOS) at zero energy,\u0000obtaining high-accuracy results. We also train ML models to predict the\u0000amplitude of odd-frequency pairing in the anomalous Green's function at zero\u0000energy. Once the ML models are trained using the LDOS, which is experimentally\u0000accessible via scanning tunneling spectroscopy, our method could be applied to\u0000predict the number of Majorana edge states and to estimate the magnitude of\u0000odd-frequency pairing in real materials.","PeriodicalId":501069,"journal":{"name":"arXiv - PHYS - Superconductivity","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210025","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jinwon Lee, Sanghun Lee, Andreas Kreisel, Jens Paaske, Brian M. Andersen, Koen M. Bastiaans, Damianos Chatzopoulos, Genda Gu, Doohee Cho, Milan P. Allan
The iron-based superconductor FeTe$_{0.55}$Se$_{0.45}$ is a peculiar material: it hosts a surface state with a Dirac dispersion, is a putative topological superconductor hosting Majorana modes in vortices, and has an unusually low Fermi energy. The superconducting state is generally thought to be characterized by three gaps in different bands, with the usual homogenous, spatially extended Bogoliubov excitations -- in this work, we uncover evidence that it is instead of a very different nature. Our scanning tunneling spectroscopy data shows several peaks in the density of states above a full gap, and by analyzing the spatial and junction-resistance dependence of the peaks, we conclude that the peaks above the first one are not coherence peaks from different bands. Instead, comparisons with our simulations indicate that they originate from generalized Shiba states that are spatially overlapping. This can lead to an amorphous state of Bogoliubov quasiparticles, reminiscent of impurity bands in semiconductors. We discuss the origin and implications of this new state.
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