Virtual and Real Bosons in Unconventional Superconductors

超导性是由虚拟玻色子交换引起的电子之间吸引人的相互作用引起的。在常规超导体中，相关的玻色子是声子，而高温或非常规超导体通常表现出过高的转变温度，并具有足够的非平凡对状态，从而导致声子交换。在许多高温超导体中，磁有序相和超导相之间存在竞争，这暗示着磁激发是玻色胶。除了虚声子的交换，超热电子还可以非弹性地散射并产生真正的玻色子。当用隧道光谱研究超导体时，隧道光谱中包含了实玻色子和虚玻色子的信号。对于声子介导的超导体，通常在平面结中忽略了超导态的真实声子发射，而电子-声子耦合可以通过BCS准粒子密度的重整化来确定(Mc米兰方法)。在正常态下，非弹性隧道谱可以忽略电子态密度的重整化，但可以探测到真实声子的发射。在最近的一项工作中，我们证明了当使用STM进行隧道光谱时，在超导态下必须同时考虑实(非弹性)声子发射和虚(弹性)声子发射，而在正常态下只能探测到实声子。虽然这使常规超导体中电声耦合的确定变得复杂，但这一发现对确定非常规超导体中的电子-声子耦合非常有用。在这里，可以在正常状态下检测到磁激励。在超导态，能隙的形成大大改变了电子态的密度，而磁激发谱则发生了改变，因为它代表了费米面附近电子气的低能多体激发。该项目的关键思想是在非传统超导体的STM谱中分离出弹性和非弹性隧穿贡献，并利用这一新信息来识别与这些实验和其他现有观测相一致的自然配对相互作用。

Superconductivity is caused by an attractive interaction between electrons that is mediated by virtual Boson exchange. In conventional superconductors, the relevant Bosons are phonons, while high temperature or unconventional superconductors often show too high transition temperatures and have sufficiently non-trivial pairing states to be caused by phonon exchange. In many of the high temperature superconductors a competition between magnetically ordered and superconducting phases exist hinting for magnetic excitations as the bosonic glue. Besides the exchange of virtual phonons, hot electrons can also inelastically scatter and create real bosons. When investigating superconductors with tunneling spectroscopy, signatures of both real and virtual bosons are contained in the tunneling spectra. For phonon mediated superconductors, typically the emission of real phonons in the superconducting state has been neglected in planar junctions and the electron-phonon coupling could be determined from the renormalization of the BCS quasi-particle density of states (McMilan method). In the normal state, renormalization of the electronic density of states can be neglected but emission of real phonons can be detected with inelastic tunneling spectroscopy. In a recent work, we showed that when tunneling spectroscopy is carried out using STM, both real (inelastic) and virtual (elastic) phonon emission have to be taken into account in the superconducting state, while only real phonons can be detected in the normal state. While this complicates the determination of the electron-phonon coupling in conventional superconductors, this finding can be very useful to determine the coupling in unconventional superconductors. Here, magnetic excitations can be detected in the normal state. In the superconducting state, both the electronic density of states is drastically modified by the gap formation and the magnetic excitation spectrum is altered as it represents low energy many body excitation of the electron gas near the Fermi surface. The key idea of the project is to disentangle the elastic and inelastic tunneling contributions in STM-spectra of unconventional superconductors and to utilize this new information to identify a natural pairing interaction that is consistent with these experiments and other existing observations.