We study the physical properties of a half-metallic ferromagnet$\mid$superconductor (HM$\mid$S) bilayer, allowing for an arbitrary bulk pairing symmetry of the superconductor and spin-dependent processes at the interface. In particular, we study how the possibility of unconventional pairing such as $p$- and d-wave and a spin-active interface influence the \textit{(i)} conductance spectra, \textit{(ii)} proximity effect, and \textit{(iii)} local density of states of such a bilayer. Our calculation is done both analytically and numerically in the ballistic limit, using both a continuum- and lattice-model. It is found that the spin-dependent phase-shifts occuring at the HM$\mid$S interface seriously influence all of the aforementioned phenomena. We explain our results in terms of Andreev reflection in the presence of a spin-active interface, allowing for both spin-filtering and spin-mixing processes. We demonstrate how the surface-bound states induced by the anisotropy of the superconducting order parameter at the HM$\mid$S interface are highly sensitive to these spin-dependent processes. Our results can be directly tested experimentally using STM-measurements and/or point-contact spectroscopy.
Unconventional superconductivity is usually associated with symmetry breaking in the system. Here we consider a simple setup consisting of a solid state material with conduction electrons and an applied surface acoustic wave (SAW), that can break time and spatial translation symmetries. We study the symmetries of the possible SAW-induced order parameters, showing that even-frequency spin-triplet odd-parity order parameter can occur. We suggest different methods of how to engineer the symmetries of the order parameters using SAWs and the applications of such setups.
We study the pairing symmetry of a doped Kane-Mele model on a honeycomb lattice with on-site Coulomb interaction. The pairing instability of Cooper pair is calculated based on the linearized Eliashberg equation within the random phase approximation (RPA). When the magnitude of the spin-orbit coupling is weak, even-frequency spin-singlet even-parity (ESE) pairing is dominant. On the other hand, with the increase of the spin-orbit coupling, we show that the even-frequency spin-triplet odd-parity (ETO) f-wave pairing exceeds ESE one. ETO f-wave pairing is supported by the longitudinal spin fluctu…
The pairing mechanism in iron-based superconductors is believed to be unconventional, i.e. not phonon-mediated. The achieved transition temperatures Tc in these superconductors are still significantly below those of some of the cuprates, with the exception of single layer FeSe films on SrTiO3 showing a Tc between 60 and 100 K, i.e. an order of magnitude larger than in bulk FeSe. This enormous increase of Tc demonstrates the potential of interface engineering for superconductivity, yet the underlying mechanism of Cooper pairing is not understood. Both conventional and unconventional mechanisms …
Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor
对于铁基高$T_c$超导体LaFeAsO $_{1-x}$ F $_x$，我们构造了一个极小模型，其中所有的五个Fe $d$带都参与其中.然后，我们研究了超导性的起源与五带随机相位近似通过求解Eliashberg方程。我们得出结论，所产生的自旋波动模式之间的嵌套断开费米口袋实现，基本上，一个扩展的s波配对，其中的间隙改变整个嵌套矢量的符号。
In a recent work by Schrodi $\textit{et al}$. [Phys. Rev. B. $\textbf{104}$, L140506 (2021)], the authors find an unconventional superconducting state with a sign-changing order parameter using the Migdal-Eliashberg theory, including the first vertex correction. This unconventional solution arises despite using an isotropic bare electron-phonon coupling in the Hamiltonian. We examine this claim using hybrid quantum Monte Carlo for a single-band Holstein model with a cuprate-like noninteracting band structure and identical parameters to Schrodi $\textit{et al}$.. Our Monte Carlo results for the…
Cooper pairs in two-dimensional unconventional superconductors with broken inversion symmetry are in a mixture of an even-parity spin-singlet pairing state with an odd-parity spin-triplet pairing state. We study the magnetic properties of the impurity bound states in such superconductors and find striking signatures in their spin polarization which allow to unambiguously discriminate a non-topological superconducting phase from a topological one. Moreover, we show how these properties, which could be measured using spin-polarized scanning tunneling microscopy (STM), also enable to determine th…
The origin of Cooper pairing in high-temperature superconductors, such as the copper-oxide and iron-based system, is still under debate. High transition temperatures together with unconventional pairing states support the picture of an electronic pairing glue in the cuprates, where superconductivity is mediated by collective bosonic excitations of the electron fluid. In other materials, most importantly iron based systems with only hole or only electron pockets, the microscopic origin is hotly debated. Scanning tunneling microscopy (STM) has been shown to be a powerful experimental probe to de…
Superconductors with unconventional pairings have been a fascinating subject of research, for which a central issue is to explore effects that can be used to characterize the pairing. The process of Andreev reflection--the reflection of an electron as a hole at a normal-mental-superconductor interface by transferring a Cooper pair into the superconductor--offers a basic mechanism to probe the pairing through transport. Here we predict that in Andreev reflection from unconventional superconductors, the reflected hole acquires an anomalous spatial shift normal to the plane of incidence, arising …
Unconventional Pairing in Doped Band Insulators on a Honeycomb Lattice : the Role of the Disconnected Fermi Surface and a Possible Application to Superconducting $β$-MNCl (M=Hf,Zr)
Probing Sign-Changing Order Parameters via Impurity States in unconventional superconductors: Implications for La$_3$Ni$_2$O$_7$ Superconductors with interlayer pairing
We study the temperature-dependent electronic B1g Raman response of a slightly under-doped single crystal HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ with a superconducting critical temperature Tc=122 K. Our main finding is that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at Tc. This result hints at an unconventional pairing mechanism, whereas spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to…
Noncentrosymmetric superconductors have sparked significant research interests due to their exciting properties, such as the admixture of spin-singlet and spin-triplet Cooper pairs. Here we report $μ$SR and thermodynamic measurements on the noncentrosymmetric superconductor La7Rh3 which indicate a fully established superconducting gap and spontaneous time-reversal-symmetry breaking at the onset of superconductivity. We show that our results pose severe constraints on any microscopic theory of superconductivity in the system. A symmetry analysis identifies ground states compatible with time-rev…
Non-magnetic impurities can lift the accidental degeneracy of unconventional pairing states, such as the $(d + i g)$-wave state recently proposed for Sr$_2$RuO$_4$. This type of effect would lead to a superconducting double transition upon impurity doping. In a model calculation it is shown how this behavior depends on material parameters and how it could be detected.
We study a two-orbital $t$-$J_1$-$J_2$ model, originally developed to describe iron-based superconductors at low energies, in the presence of bond disorder (via next-nearest-neighbor $J_2$-bond dilution). By using the Bogoliubov--de Gennes approach, we self-consistently calculate the local pairing amplitudes and the corresponding density of states, which demonstrate a change of dominant pairing symmetry from $s_\pm$ wave to $d$ wave when increasing disorder strength as long as $J_1\lesssim J_2$. Moreover, the combined pairing interaction and strong bond disorder lead to the formation of $s_\pm…
Spin fluctuations and unconventional pairing in KFe$_2$As$_2$
研究了重空穴掺杂终端材料KFe $_2$ As $_2$中自旋涨落与超导电性的关系。通过对由第一性原理计算得到的三维十轨道模型的布里渊区进行近似展开，构造了一个五轨道模型。应用无规相位近似，我们得到了自旋极化率，并求解了线性化的Eliashberg方程。实验观察到的不相称的自旋波动被理解为起源于带间相互作用，其中能带结构的多轨道性质导致在整个铁基超导体家族中的不对称性的电子-空穴。对于超导性，s波和d波配对被发现是在密切的竞争，其中在前者的间隙函数的符号变化是由无公度的自旋涨落驱动。对实验上观察到的KFe $_2$ As $_2$超导能隙中的节点提出了几种可能的解释。
The pairing mechanism in A$_3$C$_{60}$ is investigated by studying the properties of a three-orbital Hubbard model with antiferromagnetic Hund coupling in the normal and superconducting phase. Local orbital fluctuations are shown to be substantially enhanced in the superconducting state, with a fluctuation energy scale that matches the low-energy peak in the spectral weight of the order parameter. Our results demonstrate that local orbital fluctuations provide the pairing glue in strongly correlated fulleride superconductors and support the spin/orbital freezing theory of unconventional superc…
Fingerprints of unconventional pairing in superconductor-hole-gas heterostructures
Institute of Science and Technology Austria (ISTA)（奥地利科学技术研究所）；Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；Department of Physics, Norwegian University of Science and Technology（挪威科技大学物理系）
Unconventional pairing in the iron arsenide superconductors
我们用磁长程序作为工具来探测砷化铁超导体中的库珀对波函数。我们从理论上证明，只有当库珀对形成一个非常规的，符号变化的状态，反铁磁性和超导性可以共存于这些材料。在Ba（Fe $_{1-x}$ Co $_{x}$）$_{2}$ As $_{2}$中共存的观察则证明了该材料中的非常规配对。理论和中子衍射实验之间的详细协议，特别是不寻常的行为的磁序低于$T_{c}$，证明了我们的结论的鲁棒性。我们的研究结果强烈地表明，在砷化铁家族的所有成员中，超导性都是非常规的。
Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO$_{1-x}$F$_x$}
对于新发现的铁基高$T_c$超导体LaFeAsO $_{1-x}$ F $_x$，我们构造了一个极小模型，其中包含所有五个Fe $d$带是必要的.随机相位近似被应用到该模型中，以研究超导电性的起源。我们的结论是，多自旋涨落模式所产生的嵌套在断开的费米面实现了扩展的s波配对，而d波配对也可以是另一个候选者。
Measurement of the nonlinear Meissner effect in superconducting Nb films using a resonant microwave cavity: A probe of unconventional pairing symmetries
Recently synthesized Kagome compounds AV$_3$Sb$_5$ attract great attention due to the unusual coexistence of the topology, charge density wave and superconductivity. In this {\it Letter}, based on the band structures for CsV$_3$Sb$_5$ in pristine phase, we fit an effective 6-band model for the low-energy processes; utilizing the random phase approximation (RPA) on the effective minimal model, we obtain the momentum-resolved static spin susceptibility attributing the spin-fluctuation pairing mechanism, we find that the superconducting pairing strengths increase with the lift of the Coulomb corr…
Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models
Exotic quantum phase transitions in metals, such as the nematic and smectic states, were discovered one after another and found to be universal now. The emergence of unconventional density-wave order in frustrated kagome metal AV$_3$Sb$_5$ and its interplay with exotic superconductivity attract increasing attention. We reveal that the smectic bond-density-wave is naturally caused by the paramagnon interference mechanism, because strong scatterings among different van-Hove singularity points are induced. In addition, the fluctuations of the bond-order induce sizable "beyond-Migdal" pairing glue…
Motivated by recent observations of chiral-induced magnetization and spin-selective transport we studied the effect of chiral molecules on conventional BCS superconductors. By applying scanning tunneling spectroscopy, we demonstrate that the singlet-pairing s-wave order parameter of Nb is significantly altered upon adsorption of chiral polyalanine alpha-helix molecules on its surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of unconventional triplet-pairing components with either d-wave or p-wave symmetry, as…
Revealing Hidden Unconventional Pairing through Nonreciprocal Transport
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料科学中心）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Department of Physics, Hong Kong University of Science and Technology（香港科技大学物理系）；Department of Physics, The University of Tokyo（东京大学物理学系）；CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所理论物理国家重点实验室）；School of Physical Science and Technology, Soochow University（苏州大学物理科学与技术学院）；Institute for Advanced Study, Soochow University（苏州大学未来技术学院）；Interdisciplinary Center for Theoretical Physics and Information Sciences, Fudan University（复旦大学理论物理与信息科学交叉研究中心）；Hefei National Laboratory（合肥国家实验室）
Unconventional superconductivity is commonly linked to electronic pairing mechanisms, since it is believed that the conventional electron-phonon interaction (EPI) cannot cause sign-changing superconducting gap symmetries. Here, we show that this common understanding needs to be revised when one considers a more elaborate theory of electron-phonon superconductivity beyond standard approximations. We selfconsistently solve the full-bandwidth, anisotropic Eliashberg equations including vertex corrections beyond Migdal's approximation assuming the usual isotropic EPI for cuprate, Fe-based and heav…
We investigate the effects of Andreev bound states due to the unconventional pairing on the inverse proximity effect of ferromagnet/superconductor junctions. Utilizing quasiclassical Eilenberger theory, we obtain the magnetization penetrating into the superconductor. We show that in a wide parameter range the direction of the induced magnetization is determined by two factors: whether Andreev bound states are present at the junction interface and the sign of the spin-mixing angle. In particular, when Andreev bound states appear at the interface, the direction of the induced magnetization is op…
We report the finding of a novel pairing state in a newly discovered superconductor Na$_2$Cr$_3$As$_3$. This material has a noncentrosymmetric quasi-one-dimensional crystal structure and is superconducting at $T_{\mathrm{C}}\sim$ 8.0 K. We find that the magnetic penetration depth data suggests the presence of a nodal line $p_z$-wave pairing state with zero magnetic moment using transverse-field muon-spin rotation (TF-$μ$SR) measurements. The nodal gap observed in Na$_2$Cr$_3$As$_3$ compound is consistent with that observed in isostructural (K,Cs)$_2$Cr$_3$As$_3$ compounds using TF-$μ$SR measur…
