We study theoretically how superlattices based on adatoms on surfaces of unconventional superconductors can be used to engineer novel pairing states that break time-reversal symmetry and exhibit non-trivial magnetic point symmetries. We illustrate this using a square-lattice Hubbard model with $d$-wave superconductivity and a subleading $s$-wave state as an example. An adatom superlattice with square-lattice symmetries is shown to stabilize an "orbital-altermagnetic superconductor'', a state that exhibits loop current patterns and associated orbital magnetic moments, which preserve superlattice translations but are odd under four-fold rotations. This state is further characterized by a non-zero Berry curvature quadrupole moment and, upon including spin-orbit coupling, by an altermagnetic spin splitting of the bands and non-trivial spin textures in the superlattice unit cell, with zero net spin moment.
Altermagnetic spin textures coupled to superconductors: Domain wall spin-triplet superconductivity and supercurrent-induced torques
Hearne Institute of Theoretical Physics, Department of Physics & Astronomy, Louisiana State University（路易斯安那州立大学物理与天文学系赫恩理论物理研究所）；Institute for Theoretical Physics III, University of Stuttgart（斯图加特大学理论物理第三研究所）
研究与传统 s 波超导体耦合的交变磁体中库珀对形式，考虑交变磁序参量空间不均匀性，发现超导性受交变磁畴影响，有三重态热点等，还存在超电流与交变磁序的相互作用，为相关研究提供独特机会。
In this work we will explore the properties of superconducting surfaces decorated by two-dimensional ferromagnetic adatom lattices. As discovered recently [Röntynen and Ojanen, Phys. Rev. Lett. \textbf{114}, 236803 (2015)], in the presence of a Rashba spin-orbit coupling these systems may support topological superconductivity with complex phase diagrams and high Chern numbers. We show how the long-range hopping nature of the effective low-energy theory generically gives rise to a phase diagram covered by a \emph{Chern mosaic} -- a rich pattern of topological phases with large Chern numbers. We…
We study the magnetic and electronic phases of a 1D magnetic adatom chain on a 2D superconductor. In particular, we confirm the existence of a `self-organized' 1D topologically non-trivial superconducting phase within the set of subgap Yu-Shiba-Rusinov (YSR) states formed along the magnetic chain. This phase is stabilized by incommensurate spiral correlations within the magnetic chain that arise from the competition between short-range ferromagnetic and long-range antiferromagnetic electron-induced exchange interactions, similar to a recent study for a 3D superconductor [M. Schecter et al. Phy…
Adsorption of one-third monolayer of Sn on an atomically-clean Si(111) substrate produces a two-dimensional triangular adatom lattice with one unpaired electron per site. This dilute adatom reconstruction is an antiferromagnetic Mott insulator; however, the system can be modulation-doped and metallized using heavily-doped p-type Si(111) substrates. Here, we show that the hole-doped dilute adatom layer on a degenerately doped p-type Si(111) wafer is superconducting with a critical temperature of 4.7 +- 0.3 K. While a phonon-mediated coupling scenario would be consistent with the observed TC, Mo…
A magnetic adatom chain, proximity coupled to a conventional superconductor with spin-orbit coupling, exhibits locally an odd-parity, spin-triplet pairing amplitude. We show that the singlet-triplet junction, thus formed, leads to a net spin accumulation in the near vicinity of the chain. The accumulated spins are polarized along the direction of the local $\mathbf{d}$-vector for triplet pairing and generate an enhanced persistent current flowing around the chain. The spin polarization and the "supercurrent" reverse their directions beyond a critical exchange coupling strength at which the sin…
Magnetic adatom chains on surfaces constitute fascinating quantum spin systems. Superconducting substrates suppress interactions with bulk electronic excitations but couple the adatom spins to a chain of subgap Yu-Shiba-Rusinov (YSR) quasiparticles. Using a scanning tunneling microscope, we investigate such correlated spin-fermion systems by constructing Fe chains adatom by adatom on superconducting NbSe$_2$. The adatoms couple entirely via the substrate, retaining their quantum spin nature. In dimers, we observe that the deepest YSR state undergoes a quantum phase transition due to Ruderman-K…
Inherent momentum-dependent gap structure of altermagnetic superconductors
Niels Bohr Institute, University of Copenhagen（尼尔斯·玻尔研究所，哥本哈根大学）；Department of Physics, University of Wisconsin–Milwaukee（威斯康星大学密尔沃基分校物理系）；Department of Physics and Astronomy, Uppsala University（乌普萨拉大学物理与天文学系）
Recent developments in the search for topological superconductivity have brought lattices of magnetic adatoms on a superconductor into intense focus. In this work we will study ferromagnetic chains of adatoms on superconducting surfaces with Rashba spin-orbit coupling. Generalising the deep-impurity approach employed extensively in previous works to arbitrary subgap energies, we formulate the theory of the subgap spectrum as a nonlinear matrix eigenvalue problem. We obtain an essentially analytical description of the subgap spectrum, allowing an efficient study of the topological properties. E…
Superconductivity as a Probe of Altermagnetism: Critical Temperature, Field, and Current
Centro de Física de Materiales (CFM-MPC) Centro Mixto CSIC-UPV/EHU（材料物理中心（CFM-MPC）CSIC-巴斯克国立大学混合中心）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）
Emergence of chiral $p$-wave and $d$-wave states in $g$-wave altermagnets
Asia Pacific Center for Theoretical Physics（亚太理论物理中心）；Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Indonesia（印度尼西亚大学数学与自然科学学院物理系）；Pohang University of Science and Technology（浦项科技大学）
Emergent superconductivity at 16.3 K in an altermagnetic candidate Na$_{2-x}$V$_2$Se$_2$O with broken inversion symmetry
Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences（中国科学院物理研究所及凝聚态物理北京国家实验室）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；College of Physics and Center of Quantum Materials and Devices, Chongqing University（重庆大学物理学院及量子材料与器件中心）；College of Physics and Electronic Engineering, Nanyang Normal University（南阳师范学院物理与电子工程学院）；School of Physics and Information Technology, Shaanxi Normal University（陕西师范大学物理学与信息技术学院）
Time-reversal symmetric topological superconductivity in Machida-Shibata lattices
I. Institute for Theoretical Physics, University of Hamburg（汉堡大学第一理论物理研究所）；Centre for Ultrafast Imaging（超快成像中心）；RIKEN Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS)（理化学研究所交叉理论与数学科学（iTHEMS））
We study odd-parity nematic superconductivity in doped topological insulators in presence of surface magnetic impurities. The peculiar surface subgap spectrum, characterized by a Majorana flatband, nodal cones and the surface states of the parent topological insulator, gives rise to an overall ferromagnetic RKKY interactions between the surface impurities. An additional coupling between the impurities and a preemptive chiral order parameter promote a surface time-reversal symmetry breaking solution at the surface of the system. We discuss the relevant scenarios and suggest to engineer surface …
Spin triplet pairing by suppressing altermagnetism
Center for Correlated Matter and School of Physics, Zhejiang University（浙江大学关联物质中心与物理学院）；Department of Physics, The Hong Kong University of Science and Technology（香港科技大学物理系）；School of Physics, Hangzhou Normal University（杭州师范大学物理学院）；Institute for Advanced Study in Physics, Zhejiang University（浙江大学物理高等研究院）
Pair-Density Wave from Doping an Altermagnetic Mott Insulator
Max Planck Institute for the Physics of Complex Systems（马克斯·普朗克复杂系统物理研究所）；Kavli Institute for Theoretical Sciences and School of Quantum, University of Chinese Academy of Sciences（中国科学院大学量子科学学院与理论科学 Kavli 研究所）
Two-dimensional transition metal carbides and nitrides (MXenes) are an emerging class of atomically-thin superconductors, whose characteristics are highly prone to tailoring by surface functionalization. Here we explore the use of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes, based on first-principles calculations combined with Eliashberg theory. We first demonstrate the stability of three different structural models of hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of over 30 K are obtained for hydrogenated Mo$_2$N and W$_2$N. Several m…
Abrikosov vortices in altermagnetic superconductors
Centro de Física de Materiales (CFM-MPC) Centro Mixto CSIC-UPV/EHU（材料物理中心（CFM-MPC）CSIC-巴斯克国立大学混合中心）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）
Graphene is the physical realization of many fundamental concepts and phenomena in solid state-physics, but in the long list of graphene remarkable properties, a fundamental block is missing: superconductivity. Making graphene superconducting is relevant as the easy manipulation of this material by nanolytographic techniques paves the way to nanosquids, one-electron superconductor-quantum dot devices, superconducting transistors at the nano-scale and cryogenic solid-state coolers. Here we explore the doping of graphene by adatoms coverage. We show that the occurrence of superconductivity depen…
Spin transport in a normal meta-altermagnetic superconducting nanowire junction
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；Hefei National Laboratory（合肥国家实验室）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）
We show that the surface of an $s$-wave superconductor decorated with a two-dimensional lattice of magnetic impurities can exhibit chiral topological superconductivity. If impurities order ferromagnetically and the superconducting surface supports a sufficiently strong Rashba-type spin-orbit coupling, Shiba sub-gap states at impurity locations can hybridize into Bogoliubov bands with non-vanishing, sometimes large, Chern number $C$. This topological superconductor supports $C$ chiral Majorana edge modes. We construct phase diagrams for model two-dimensional superconductors, accessing the dilut…
Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic $s$-wave superconductivity in epitaxial Rb$_3$C$_{60}$ films against merohedral …
Conditions for orbital selective altermagnetism in Sr$_2$RuO$_4$: tight binding model, similarities with cuprates and implications on superconductivity
Direction-selective triplet pairing and spin-edge locking in altermagnetic metals
South China Normal University（华南师范大学）；Guangdong Basic Research Center of Excellence for Structure and Fundamental Interactions of Matter（物质结构与基本相互作用广东省基础研究中心）；Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials（广东省量子工程与量子材料重点实验室）；Guangdong-Hong Kong Joint Laboratory of Quantum Matter（粤港澳量子物质联合实验室）；Frontier Research Institute for Physics（物理前沿研究院）
Altermagnetic-doping interplay as a route to enhanced d-wave pairing in the Hubbard model
School of Science, Harbin Institute of Technology, Shenzhen（哈尔滨工业大学深圳校区理学院）；Department of Physics, Jiangsu University（江苏大学物理学院）；Shenzhen Key Laboratory of Advanced Functional Carbon Materials Research and Comprehensive Application（深圳市先进功能碳材料研究及综合应用重点实验室）；Institute for Materials Research, Tohoku University（东北大学材料科学研究所）
Chains of magnetic adatoms on superconductors have been discussed as promising systems for realizing Majorana end states. Here, we show that dilute Yu-Shiba-Rusinov (YSR) chains are also a versatile platform for quantum magnetism and correlated electron dynamics, with widely adjustable spin values and couplings. Focusing on subgap excitations, we derive an extended $t-J$ model for dilute quantum YSR chains and use it to study the phase diagram as well as tunneling spectra. We explore the implications of quantum magnetism for the formation of a topological superconducting phase, contrasting it …
