The electronic and phonon properties of the platinum pnictide superconductors $A$Pt$_3$P ($A$=Ca, Sr, and La) were studied using first-principles calculations. The spin-orbit coupling effect is significant in LaPt$_3$P but negligible in CaPt$_3$P and SrPt$_3$P, although they all share the same anti-pevroskite structure. Moreover, SrPt$_3$P has been demonstrated to exhibit an unexpected weak charge-density-wave(CDW) instability which is neither simply related to the Fermi-surface nesting nor to the momentum-dependent electron-phonon coupling alone. The instability is absent in CaPt$_3$P and can be quickly suppressed by the external pressure, accompanied with gradual decreases in the phonon softening and BCS $T_c$. Our results suggest SrPt$_3$P as a rare example where superconductivity is enhanced by the CDW fluctuations.
Density-functional calculations of the electronic structure and lattice dynamics of superconducting LaO$_{0.5}$F$_{0.5}$BiS$_{2}$: Evidence for an electron-phonon interaction near the charge-density-wave instability
本文用密度泛函方法研究了新发现的超导体LaO $_{0.5}$ F $_{0.5}$ BiS $_{2}$的电子结构、晶格动力学和电子-声子相互作用.在$\mathbf{k}$ =（$π$，$π$，0）处的一个强费米面嵌套表明接近电荷密度波不稳定性，并导致在这个与S原子面内位移相关的$\mathbf{k}$点处的虚谐声子。总能量分析只解决了一个浅的双势阱，防止出现静态长程有序。谐波和非谐波贡献的电子-声子耦合的评估，并给出了一个总的耦合常数$λ\simeq 0.85$提示这种材料是一个传统的超导体相反，结构相似的FeAs材料。
Employing the density-functional theory with local density approximation, we show that the fully hydrogenated monolayer-hexagonal boron nitride (H$_2$BN) has a direct-band gap of 2.96 eV in the blue-light region while the pristine $\textit{h}$-BN has a wider indirect-band gap of 4.78 eV. The hole-doped H$_2$BN is stable at low carrier density ($n$) but becomes dynamically unstable at higher $n$. We predict that it is a phonon-mediated superconductor with a transition temperature ($T_c$) which can reach $\sim$31 K at $n$ of $1.5\times$ $10^{14}$ holes cm$^{-2}$ near the lattice instability. The…
The representative cuprate, La$_{2-x}$M$_x$CuO$_4$, with M = Sr and $x = 1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in La$_{2-x}$Sr$_x$CuO$_4$, is found to be the ground state, where the structural phase transitions, HTT$\rightarrow$LTO$\rightarrow$LTLO, can be understood via phonon instability. While the La-O composition is identified to be responsible for…
The interplay between superconductivity and charge density waves (CDWs) under hole doping in cuprates is one of the central phenomena in condensed matter physics. Recently, CDWs are also observed in CaCuO$_2$-analogous nickelates RNiO$_2$ (R = La, Nd) but exhibit fundamentally different hole-doping-dependent behaviors compared to that in cuprates, raising a challenging question on its origin. In this article, we propose that electronic instability (EI) and moment-dependent electron-phonon coupling (MEPC), mainly contributed by Ni 3dx2-y2 and R 5dz2, respectively, may be the possible reasons fo…
We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa$_2$Cu$_3$O$_{7-δ}$ to study $B$$_{\rm 1g}$ phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at $T=T_{\rm B1g}$ above $T$$_{\rm c}$ due to softening, in addition to the one due to the onset of the superconductivity. $T$$_{\rm B1g}$ is clearly different from the pseudogap temperature with a maximum in the underdoped region. The region between $T$$_{\rm B1g}$ and $T$$_{\rm c}$ resembles that of superconducting fluctuation or charge density wave o…
Phonon softening is a ubiquitous phenomenon in condensed matter systems which is often associated with charge density wave (CDW) instabilities and anharmonicity. The interplay between phonon softening, CDW and superconductivity is a topic of intense debate. In this work, the effects of anomalous soft phonon instabilities on superconductivity are studied based on a recently developed theoretical framework that accounts for phonon damping and softening within the Migdal-Eliashberg theory. Model calculations show that the phonon softening in the form of a sharp dip in the phonon dispersion relati…
We present a minimal theory of superconductivity enhancement in ferroelectric-type materials. Simple expressions for the optical mode responsible for the soft mode transition are assumed. A key role is played by the anharmonic phonon damping which is modulated by an external control parameter (electron doping or mechanical strain) causing the appearance of the soft mode. It is shown that the enhancement in the superconducting critical temperature $T_{c}$ upon approaching the ferroelectric transition from either side is due to the Stokes electron-phonon scattering processes promoted by strong p…
A softening of phonon-dispersion has been observed experimentally in under-doped cuprate superconductors at the charge-density wave (CDW) ordering wave vector. Interestingly, the softening occurs below the superconducting (SC) transition temperature T$_{c}$, in contrast to the metallic systems, where the softening occurs usually below the CDW onset temperature T$_{\text{CDW}}$. An understanding of the `anomalous' nature of the phonon-softening and its connection to the pseudo-gap phase in under-doped cuprates remain open questions. Within a perturbative approach, we show that a complex interpl…
Using a microscopic model of lattice vibrations in the STO(001) substrate, an additional 50 mev longitudinal optical (LO) interface mode is identified. The soft mode propagating mainly in the first TiO2 layer ("O chains") has stronger electron - phonon coupling to electron gas in FeSe than a well known 100 mev hard mode. The coupling constant, critical temperature, replica band are calculated. Although there exists a forward in the electron - phonon scattering peak, it is clearly not as sharp as assumed in recent theories (delta function - like). The satellite is broad and its peak appears at …
We provide experimental and theoretical evidence for the realization of the Peierls-type structurally distorted state in the quasi-one-dimensional superconductor Sc$_3$CoC$_4$ by a phonon-softening mechanism. The transition from the high- to the final low-temperature phase below 80K proceeds via an extended intermediate temperature regime between 80K and 150K characterized by phonon-driven atom displacements. In support of the low-dimensional character of the title compound we find a highly anisotropic correlation-length of these dynamic distortions.
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
计算了无限层超导体CaCuO 2的平衡结构、能带、声子色散和s-和d-沟道电子-声子相互作用（EPI），其中CuO 2掺杂0.24个空穴。使用LDA和线性响应全电位LMTO方法。在平衡结构中，氧被发现轻微地弯曲出平面，因此，EF附近的能带的特征被发现与其他最佳掺杂的HTSC的那些相似。对于EPI，我们发现λ（s）=0.4，与以前的LDA计算YBa 2Cu 3 O 7。这支持了普遍的信念，即EPI机制本身不足以解释HTSC。发现λ（x^2-y^2）是正的，几乎和λ（s）一样大。这是令人惊讶的，并表明EPI可以增强其他一些d波配对机制。像在YBa 2Cu 3 O 7中，屈曲模式对EPI有显著贡献，尽管这些贡献与静态屈曲成比例，并且对于平坦平面将消失。这些数值结果可以理解从一个通用的紧束缚模型最初来自LDA带的YBa 2Cu 3 O 7。在未来，屈曲模式的非谐性的作用和自旋涨落的影响应进行调查。
The specific heat of two polymorphs of BaPd2As2 was measured. The ThCr2Si2-type polymorph (space group I4/mmm, $D_{4h}^{17}$, No. 139) is a previously reported superconductor with a transition temperature Tc $\simeq$ 3.5 K, while the CeMg2Si2-type polymorph (P4/mmm, $D_{4h}^{1}$, No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr2Si2-type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg2Si2-type. Moreover, a large specific-heat jump at Tc indicated that the superconductivity of ThCr2Si2-t…
Charge-density-wave (CDW) instability and pressure-induced superconductivity in bulk 1T-NbS2 are predicted theoretically by first-principles calculations. We reveal a CDW instability towards the formation of a stable commensurate CDW order, resulting in a sqart(13)*sqart(13) structural reconstruction featured with star-of-David clusters. The CDW phase exhibits one-dimensional metallic behavior with in-plane flat-band characteristics, and coexists with an orbital-density-wave order predominantly contributed by 4d_(z^2-r^2 ) orbital from the inner Nb atoms of the star-of-David cluster. By doubli…
We present neutron total scattering and density functional theory studies on quasi-one-dimensional superconducting K$_2$Cr$_3$As$_3$ revealing a frustrated structural instability. Our first principles calculations find a significant phonon instability which, under energy minimization, corresponds to a frustrated orthorhombic distortion. In diffraction studies we find large and temperature independent atomic displacement parameters which pair distribution analyses confirms and shows as resulting from highly localized orthorhombic distortions of the CrAs sublattice and coupled K displacements. T…
Recent experiments have tuned the monolayer 1T'-WTe2 to be superconducting by electrostatic gating. Here, we theoretically study the phonon-mediated superconductivity in monolayer 1T'-WTe2 via charge doping. We reveal that the emergence of soft-mode phonons with specific momentum is crucial to give rise to the superconductivity in electron-doping regime, whereas no such soft-mode phonons and no superconductivity emerge in hole-doping regime. We also find a superconducting dome, which can be attributed to the change of Fermi surface nesting condition as electron doping. By taking into account t…
BaNi$_2$As$_2$ is a nonmagnetic analogue of the iron pnictide superconductors, and in its tetragonal state exhibits an incommensurate charge-density-wave (IC-CDW) and a sizable elastoresistance. In this work, phonons in BaNi$_2$As$_2$ associated with the IC-CDW and uniform in-plane lattice distortions are investigated using high-resolution inelastic X-ray scattering. The in-plane transverse acoustic phonons reveal no softening at temperatures where the elastoresistance increases strongly, indicating the latter to be electronically driven. Systematic phonon measurements reveal the IC-CDW occurs…
Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. We determine the nematic correlation length and find that it sharply changes its $T$-dependence at $T_c$, revealing a strong connection between nematicity and s…
The coexistence of charge density wave and superconductivity in Pt-based layered superconductors : SrPt$_2$As$_2$ and LaPt$_2$Si$_2$
基于{ \it ab initio}密度泛函理论能带结构和声子计算，研究了SrPt $_2$ As $_2$和LaPt $_2$ Si $_2$中电荷密度波（CDW）与超导电性的共存现象.我们发现[As-Pt-As]层中Pt原子的局域分裂畸变在驱动SrPt $_2$ As $_2$中的五重超晶胞CDW不稳定性以及声子软化不稳定性中起着至关重要的作用。相反，LaPt $_2$ Si $_2$中的CDW和声子软化不稳定性在没有Pt原子分裂畸变的情况下发生，表明SrPt $_2$ As $_2$和LaPt $_2$ Si $_2$中CDW的驱动机制是不同的.我们发现，前者的CDW不稳定性来自费米面嵌套，而后者则来自鞍点散射。声子计算表明，在[{ \it X}-Pt-{ \it X}]层（{ \it X} = As或Si）中，CDW和超导电性共存.
Based on ab-initio calculated parameters, we apply a theoretical model on the iron-based BaFe2As2 superconductor that takes into account dramatic enhancements of the electron-phonon coupling of soft transverse phonons in the FeAs layers and antiferromagnetism. Our model is able to reproduce the Tc values of BaFe2As2 found under pressure in experiments. To calculate the Tc of the K-doped Ba1-xKxFe2As2 system as a function of the K content, we additionally consider the experimentally observed effective mass enhancements and Kondo temperatures in the strongly over-doped region (0.8 < x < 1), whic…
We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi$_2$As$_2$, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, $Q_{I-CDW}$. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around $Q_{I-CDW}$. The phonon continuously softens u…
Soft Phonon Charge-Density Wave Formation in the Kagome Metal KV$_3$Sb$_5$
Zhejiang University（浙江大学）；Hangzhou Normal University（杭州师范大学）；University of Science and Technology of China（中国科学技术大学）；Zhejiang University of Technology（浙江工业大学）；Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China（中国科学院微观尺度物理科学合肥国家研究中心，中国科学技术大学）
The quasi-skutterudite superconductors $A_3T_4$Sn$_{13}$ ($A$=Sr, Ca; $T$=Ir, Rh, Co) are highly tunable featuring a structural quantum critical point. We construct a temperature-lattice constant phase diagram for these isovalent compounds, establishing Ca$_{3}$Rh$_4$Sn$_{13}$ and Ca$_{3}$Co$_4$Sn$_{13}$ as members close to and far away from the structural quantum critical point, respectively. Deconvolution of the lattice specific heat and the electrical resistivity provide an approximate phonon density of states $F(ω)$ and the electron-phonon transport coupling function $α_{tr}^2F(ω)$ for Ca$…
Raman scattering near a d-wave Pomeranchuk instability
最近的运输和中子散射实验表明，在欠掺杂的铜氧化物的取向对称性破缺的动机，我们提出了一个理论研究的拉曼散射附近的d波波Pomeranchuk不稳定性（PI）。拉曼散射的d-波分量的电子和声子允许直接研究与PI相关的序参量波动。接近PI从正常状态，通过降低温度的中心峰出现在电子和，作为一个额外的低频功能，在声子散射。在低温超导态下，通过降低掺杂浓度来接近PI，中心峰被一个宽度和能量都大大减小、谱权增大的软模所取代。这些预测的低能量特征在拉曼散射可以确认在一个相当直接的方式存在的PI在高温铜酸盐超导体和Sr 3Ru 2 O 7。
