We study hybridization of collective plasmon and Carlson-Goldman-Artemenko-Volkov modes in a hybrid system, consisting of a two-dimensional layers of electron gas in the normal state and superconductor, coupled by long-range Coulomb forces. The interaction between these collective modes is not possible in a regular single-layer two-dimensional system since they exist in non-overlapping domains of dimensionless parameter $ωτ$, where $ω$ is the external electromagnetic field frequency and $τ$ is electron scattering time. Thus, in a single-layer structure, these modes are mutually exclusive. However, the coupling may become possible in a hybrid system consisting of two separated in space materials with different properties, in particular, the electron scattering time. We investigate the electromagnetic power absorption by the hybrid system and reveal the conditions necessary for the hybridization of collective modes.
We predict a strong enhancement of the critical temperature in a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor in the presence of a bosonic condensate of exciton-polaritons. The effect depends strongly on the ratio of the cutoff frequencies for phonon and exciton-polariton mediated BCS superconductivity, respectively. We also discuss a possible design of hybrid semiconductor-superconductor structures suitable for the experimental observation of such an effect.
Effects of a Collective Spin Resonance Mode on the STM Spectra of D-Wave Superconductors
在许多高温超导体中存在高能自旋共振模。受最近的扫描隧道显微镜（STM）实验的启发，我们在超导体Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+δ}$中研究了这种共振模式对局域态密度（LDOS）的影响. d波超导体中的电子与共振模之间的耦合在LDOS中产生高能峰，其显示围绕掺杂杂质的两个晶胞周期调制。这为我们提供了一种新的手段，不仅可以探测动态自旋集体模，而且可以研究它与电子激发的耦合。
By numerically solving the Bogoliubov-de Gennes equations for the single vortex state in a two-band superconductor, we demonstrate that the disparity between the healing lengths of two contributing condensates is strongly affected by the band Fermi velocities, even in the presence of the magnetic field and far beyond the regime of nearly zero Josephson-like coupling between bands. Changing the ratio of the band Fermi velocities alters the temperature dependence of the condensate lengths and significantly shifts parameters of the ``length-scales locking" regime at which the two characteristic l…
We analyze the paradigmatic competition between intra-band and cross-band Cooper-pair formation in two-band superconductors, neglected in most works to date. We derive the phase-sensitive gap equations and describe the crossover between the intraband-dominated and the crossband-dominated regimes, delimited by a ``gapless'' state. Experimental signatures of crosspairing comprise notable gap-splitting in the excitation spectrum, non-BCS behavior of gaps versus temperature, as well as changes in the pairing symmetry as a function of temperature. The consequences of these findings are illustrated …
Multiband superconductors can host collective excitations with marked differences with respect to their single-band counterpart. We first study the spectrum of collective amplitude fluctuations in a clean two-bands superconductor, showing that the spectral weight of the Higgs mode rapidly deviates from the naive extension of the single band case as the interband coupling is turned on. These results are then used to critically analyze the non-linear optical response in MgB$_2$, providing an explanation for the apparently contradictory results of recent experiments, pointing towards a selective …
Signature of Spin Collective Mode in Local Tunneling Spectra of a d-wave Superconductor
我们考虑了磁激发对d波超导体局域态密度的影响。磁化率在重整化$t-t'-J$模型内计算，并使用Polkovnikov { \it et al.}最初提出的极小模型考虑其对准粒子自能的影响。[cond-mat/0203176]。我们发现局域态密度沿$(π,0)$和$(π,π)$方向都具有周期性分量，并且随着准粒子能量的变化，相关波矢的大小也发生变化。与STM实验结果的比较表明，计算的LDOS调制与测量数据不一致。
We study the behavior of a topological Josephson junction in which two topological superconductors are coupled through a quantum dot. We focus on the case with the bulk superconducting gap being the largest energy scale. Two parameter regimes are investigated: a weak tunneling between the dot and the superconductors, with the dot near its charge degeneracy point, and a strong tunneling regime in which the transmission between the dot and the superconductors is nearly perfect. We show that in the former situation, the Andreev spectrum for each sector with fixed fermion parity consists of only t…
Time-Reversal Symmetry Breaking Superconducting State and Collective Modes in Kagome Superconductors
Capital Normal University（首都师范大学）；Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences（中国科学院大学理论科学研究所）；Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家研究中心）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）
In multiband superconductors, multiple collective modes exist associated with the multiple order parameters. Oscillations of the amplitude and the relative phase of the order parameters are called Higgs and Leggett modes, respectively. Recently, it has been suggested that nonmagnetic impurity scattering would enhance nonlinear coupling between the Higgs mode and an electromagnetic wave with a frequency located in the superconducting gap region, while its effect on the Leggett mode is still unresolved. Here, we theoretically investigated the nonlinear optical response of multiband Bardeen-Coope…
A theoretical methodology for exploring the conventional Bardeen-Cooper-Schrieffer (BCS) pairing instability for superconductivity from a correlated normal phase for all possible degrees of many-body correlation, has been developed. The Gutzwiller projection scheme with a correlation parameter was made use of in generating the BCS pairing state. A variational scheme was thereafter implemented, leading to a self-consistent equation for superconducting gap function. This equation shows explicit dependence of the gap function on the many body correlation parameter. This `pairing-gap' and the corr…
The high Tc superconductor FeTe0.55Se0.45 has recently been shown to support a surface state with topological character. Here we use low-energy laser-based ARPES with variable light polarization, including both linear and circular polarization, to re-examine the same material and the related FeTe0.7Se0.3, with larger Te concentration. In both cases we observe the presence of a surface state displaying linear dispersion in a cone-like configuration. The use of circular polarization confirms the presence of helical spin structure. These experimental studies are compared with theoretical studies …
Recent experiments with strong THz fields in both conventional and unconventional superconductors have clearly evidenced a marked third-harmonic generation below the superconducting temperature $T_c$. Its interpretation challenged substantial theoretical work aimed at establishing the relative efficiency of quasiparticle excitations and collective modes in triggering such a resonant response. Here we compute the non-linear current by implementing a time-dependent Bogoljubov de-Gennes approach, with the twofold aim to account non-perturbatively for the effect of local disorder, and to include t…
Collective modes in two dimensional topological superconductors are studied by an extended random phase approximation theory while considering the influence of vector field of light. In two situations, the s-wave superconductors without spin-orbit-coupling (SOC), and the hybrid semiconductor and s-wave superconductor layers with strong SOC, we get the analytical results for longitudinal modes which are found to be indeed gapless. Further more, the effective modes volumes can be calculated, the electric and magnetic fields can be expressed as the creation and annihilation operators of such mode…
Understanding the nature of Cooper pairs is essential to describe the properties of superconductors. The original proposal of Bardeen, Cooper, and Schrieffer (BCS) was based on electrons pairing with same energy and zero center-of-mass momentum. With the advent of new superconductors, different forms of pairing have been discussed. In particular, Cooper pairs with finite center-of-mass momentum have received large interest. Along with such finite-momentum pairs, pairing of electrons at different energies is also central to understanding some superconductors. Here, we investigate the interplay …
The hybridization of magnetism and superconductivity has been an intriguing playground for correlated electron systems, hosting various novel physical phenomena. Usually, localized d- or f-electrons are central to magnetism. In this study, by placing a PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecular monolayer on ultra-thin Pb films, we built a hybrid magnetism/superconductivity (M/SC) system consisting of only sp electronic levels. The magnetic moments reside in the unpaired molecular orbital originating from interfacial charge-transfers. We reported distinctive tunneling spect…
We explore ways in which the close proximity between graphene sheets and monolayers of 2D superconductors can lead to hybridization between their collective excitations. We consider heterostructures formed by combinations of graphene sheets and 2D superconductor monolayers. The broad range of energies in which the graphene plasmon can exist, together with its tunability, makes such heterostrucutres promising platforms for probing the many-body physics of superconductors. We show that the hybridization between the graphene plasmon and the Bardasis-Schrieffer mode of a 2D superconductor results …
Relaxation Dynamics of Superconducting/Normal Interfaces in the Intermediate State of a type-I superconductor
Low Temperature Physics Laboratory, College of Physics & Center of Quantum Materials and Devices, Chongqing University（重庆大学物理学院低温物理实验室与量子材料与器件中心）；Materials Genome Institute, Shanghai University（上海大学材料基因组研究院）
Nambu identity and collective modes in superconductors and superfluid $^3$He
集体模式在从超导体到超流体$^{3}$ He的各种不同的物理系统中表现出来。集体模是通过基于对称性破缺双阱势的Higgs-Anderson机制产生的。最近，在超导NbN和InO中，在强太赫兹激光场的存在下，集体模式进行了探索。在这两种情况下，发现了一个单一的集体模式，其振荡频率是超导能隙$Δ$的两倍。超流体$^{3}$他是各种集体模式的宿主。特别是，在超流$^{3}$ He B相，两个大规模的集体模式被发现与质量$\sqrt{{8}/{5}}Δ$和$\sqrt{{12}/{5}}Δ$。我们表明，对于这两种情况下的超导薄膜和超流$^{3}$他B相，集体模式满足南布身份，涉及到不同的集体模式的质量的能隙参数$Δ$。
