We report on a comparative study of the electronic structure, phonon spectra, and superconducting properties for recently discovered superconducting hydrides, H3S and H3P. While the electronic structures of these two materials are similar, there are notable changes in the phonon spectra and electron-phonon coupling. The low-frequency bond-bending modes are softened in H3P and their coupling to the electrons at the Fermi surface is enhanced relative to H3S. Nevertheless, coupling to the high-frequency modes is reduced so the resulting calculated superconducting transition temperature is reduced from ~166 K in H3S to ~76 K in H3P.
Superconductivity of La$_3$Co$_4$Sn$_{13}$ and La$_3$Rh$_4$Sn$_{13}$: A comparative study
本文报道了方钴矿相关的LaPDMATHABMM Co $_4$ Sn $_{13}$和LaPDMATHABMM Rh $_4$ Sn $_{13}$超导体的电输运和热力学性质。对LaPDMATHABMM-Rh-$_4$-Sn $_{13}$施加外压，电阻超导临界温度Tc降低，而LaPDMATHABMM-Co-$_4$-Sn $_{13}$的临界温度随外压的增加而提高。正压力系数dTc/dP与La $_3$ Co $_4$ Sn $_{13}$中的微妙结构转变相关，并在晶格不稳定性的背景下进行了讨论。比热数据表明这两种化合物都是典型的BCS超导体。然而，La $_3$ Rh $_4$ Sn $_{13}$在较高温度下也表现出第二超导相，这是非均匀超导体的特征。计算了非均匀超导相的比热，与LaPDMATHABMM Rh $_4$ Sn $_{13}$的C（T）实验数据符合得很好.我们还发现，施加的压力减少了第二超导相。
Chromium (Cr) is a transition metal element with 3$d$ orbital electrons. In most compounds containing Cr, due to the correlation effect, twofold features of localization and itinerancy are expected. The localization gives rise to a magnetic moment, while the latter exhibits as the effective coherent weight for conductivity. Here we report the physical properties of Cr$_3$Ru compounds with bcc or A15 phases by using multiple experimental tools. The resistivity measurements show sharp superconducting transitions at $T_{\rm c}$ = 2.77 K and $T_{\rm c}$ = 3.37 K for the bcc and A15 structures. A h…
Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be…
Superconductivity emerges in $1T$-TiSe$_2$ when its charge density wave (CDW) order is suppressed by Cu intercalation or pressure. Since the CDW state is thought to be an excitonic insulator, an interesting question is whether the superconductivity is also mediated by the excitonic fluctuations. We investigated this question as to the nature of doping induced superconductivity in Cu$_x$TiSe$_2$ by asking if it is consistent with the phonon-mediated pairing. We employed the {\it ab initio} density functional theory and density functional perturbation theory to compute the electron-phonon coupli…
The BCS picture of superconductivity describes pairing between electrons originating from a single band. A generalization of this picture occurs in multi-band superconductors, where electrons from two or more bands contribute to superconductivity. The contributions of the different bands can result in an overall enhancement of the critical field and can lead to qualitative changes in the temperature dependence of the upper critical field when compared to the single-band case. While the role of orbital pair-breaking on the critical field of multi-band superconductors has been explored extensive…
We investigate theoretically the superconducting state of the undoped Fe-based superconductor ThFeAsN. Using input from $ab~initio$ calculations, we solve the Fermi-surface based, multichannel Eliashberg equations for Cooper-pair formation mediated by spin and charge fluctuations, and by the electron-phonon interaction (EPI). Our results reveal that spin fluctuations alone, when coupling only hole-like with electron-like energy bands, can account for a critical temperature $T_c$ up to $\sim7.5\,\mathrm{K}$ with an $s_{\pm}$-wave superconducting gap symmetry, which is a comparatively low $T_c$ …
We present a comparative study of magnetic excitations in the first two Ruddlesden-Popper members of the Hg-family of high-temperature superconducting cuprates, which are chemically nearly identical and have the highest critical temperature ($T_\mathrm{c}$) among all cuprate families. Our inelastic photon scattering experiments reveal that the two compounds' paramagnon spectra are nearly identical apart from an energy scale factor of $\sim130\%$ that matches the ratio of $T_\mathrm{c}$'s, as expected in magnetic Cooper pairing theories. By relating our observations to other cuprates, we infer …
Comparative analysis of tunneling magnetoresistance in low-$T_c$ Nb/AlAlOx/Nb and high-$T_c$ Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+δ}$ intrinsic Josephson junctions
本文详细地比较了传统的低$T_c$ Nb/AlAlOx/Nb结和稍过掺杂的Bi $_{2-y}$ Pb $_y$ Sr $_2$ CaCu $_2$ O $_{8+δ}$ [Bi（Pb）-2212]本征约瑟夫森结的磁隧穿特性，并进行了微观计算。据发现，这两种类型的路口表现在一个定性相似的方式。磁场和温度都以状态守恒的方式抑制超导性。这导致了隧道磁阻的特征符号从子带隙处的负值变为和带隙处的正值。我们推导出理论和实验验证的磁隧穿特性的标度律，并采用它们精确提取的上临界场$H_{c2}$。对于Nb，在$H_{c2}<H<H_{c3}$观察到一个扩展的表面超导电性区域。通过自洽分析不同偏压、耗散功率和不同台面尺寸下的磁隧穿数据，得到了Bi（Pb）-2212的参数，排除了自加热的影响。发现Bi（Pb）-2212的$H_{c2}(0)$为$\simeq 70$ T，并在$T\rightarrow T_c$处显著降低。在$T>T_c/2$处，子隙磁电阻的幅度呈指数级抑制，但在$T_c$以上，子隙磁电阻的幅度仍然为负，尽管非常小。这可能表明存在一个扩展的波动区域，然而，这并不破坏一般的二阶类型的相变在$T_c$。
Over the past six years, superconductivity at high temperatures has been reported in a variety of hydrogen-rich compounds under high pressure. That high-temperature superconductivity should exist in these materials is expected according to the conventional theory of superconductivity, as shown by detailed calculations. However here we argue that experimental observations rule out conventional superconductivity in these materials. Our results indicate that either these materials are unconventional superconductors of a novel kind, which we term `nonstandard superconductors', or alternatively tha…
The interplay between structural phase and electronic correlations has been an intriguing topic of research. An prominent example is the pressure-induced uncollapsed to collapsed tetragonal phase transition observed in CaFe$_2$As$_2$, which is accompanied with the emergence of superconductivity in the collapsed phase. Recently, a very similar structural phase transition was discovered in LaFe$_2$As$_2$, but in contrast to CaFe$_2$As$_2$, superconductivity was only observed in the uncollapsed phase, not the collapsed phase. Previous studies have attributed this puzzling observation to the diffe…
The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into questi…
Since the discovery of superconductivity at 200 K in H3S [1] similar or higher transition temperatures, Tcs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behaviour of a conventional phonon-mediated superconductor: ze…
The Meissner effect, magnetic field expulsion, is a hallmark of superconductivity. Associated with it, superconductors exclude applied magnetic fields. Recently Minkov et al. presented experimental results reportedly showing "definitive evidence of the Meissner effect" in sulfur hydride and lanthanum hydride under high pressure [1], and more recently Eremets et al. argued that "the arguments against superconductivity (in hydrides) can be either refuted or explained" [2]. Instead, we show here that the evidence presented in those papers does not support the case for superconductivity in these m…
