Possible Eliashberg-Type Superconductivity Enhancement Effects in a Two-Band Superconductor MgB_{2} Driven by Narrow-Band THz Pulses.

We study THz-driven condensate dynamics in epitaxial thin films of MgB_{2}, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature-unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near ≈0.7 T_{c}, is attributed to THz-driven, long-lived, nonthermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair breaking.

Recent Development of Ultrafast Optical Characterizations for Quantum Materials.

The advent of intense ultrashort optical pulses spanning a frequency range from terahertz to the visible has opened a new era in the experimental investigation and manipulation of quantum materials. The generation of strong optical field in an ultrashort time scale enables the steering of quantum materials nonadiabatically, inducing novel phenomenon or creating new phases which may not have an equilibrium counterpart. Ultrafast time-resolved optical techniques have provided rich information and played an important role in characterization of the nonequilibrium and nonlinear properties of solid systems. Here, some of the recent progress of ultrafast optical techniques and their applications to the detection and manipulation of physical properties in selected quantum materials are reviewed. Specifically, the new development in the detection of the Higgs mode and photoinduced nonequilibrium response in the study of superconductors by time-resolved terahertz spectroscopy are discussed.

Unveiling the Degradation Mechanism of High-Temperature Superconductor Bi2Sr2CaCu2O8+δ in Water-Bearing Environments.

The physical properties of copper oxide high-temperature superconductors have been studied extensively, such as the band structure and doping effects of Bi2Sr2CaCu2O8+δ (Bi-2212). However, some chemical-related properties of these superconductors are rarely reported, such as their stability in water-bearing environments. Herein, we report experiments combined with ab initio calculations that address the effects of water in contact with Bi-2212. The evolution of Bi-2212 flakes with exposure to water for different time intervals was tested and characterized by optical microscopy (OM), atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), and electrical measurements. The thickness of Bi-2212 flakes is gradually decreased in water, and some thin flakes can be completely etched away after a few days. The stability of Bi-2212 in other solvents is also evaluated, including alcohol, acetone, HCl, and KOH. The morphology of Bi-2212 flakes is relatively stable in organic solvents. However, the flakes are etched relatively quick in HCl and KOH, especially in an acidic environment. Our results imply that hydrogen ions are primarily responsible for the deterioration of their properties. Both TEM and calculation results demonstrate that the atoms in the Bi-O plane are relatively stable when compared to the inner atoms in Sr-O, Ca-O, and Cu-O planes. This work contributes toward understanding the chemical stability of a Bi-2212 superconducting device in environmental medium, which is important for both fundamental studies and practical applications of copper oxide high-temperature superconductors.

Erratum: Observation of ultrahigh mobility surface states in a topological crystalline insulator by infrared spectroscopy.

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Observation of ultrahigh mobility surface states in a topological crystalline insulator by infrared spectroscopy.

Topological crystalline insulators possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here, we report infrared reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1-x Sn x Se in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm2 V-1 s-1, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.Probing optical and transport properties of the surface states in topological crystalline insulators remains a challenge. Here, Wang et al. demonstrate that the far-infrared conductivity of Pb1-x Sn x Se (x = 0.23-0.25) single crystals is dominated by the surface states where carriers show a high surface mobility of 40,000 cm2 V-1 s-1.

Effects of pressure and distortion on superconductivity in Tl2Ba2CaCu2O(8+δ).

The systematic evolution of the structural, vibrational, and superconducting properties of nearly optimally doped Tl2Ba2CaCu2O(8+δ) with pressure up to 30 GPa is studied by x-ray diffraction, Raman scattering, and magnetic susceptibility measurements. No phase transformation is observed in the studied pressure regime. The obtained lattice parameters and unit-cell volume continuously decrease with pressure by following the expected equation of state. The axial ratio of c/a exhibits an anomaly starting from 9 GPa. At such a pressure level, the deviation from the nonlinear variation of the phonon frequencies is detected. Both the above observations indicate the enhancement of the distortion upon compression. The superconducting transition temperature is found to exhibit a parabolic behavior with a maximum of 114 K around 7 GPa. We demonstrate that the interplay between the intrinsic pressure variables and distortion controls the superconductivity.

Design and synthesis of a new layered thermoelectric material LaPbBiS3O.

A new quinary oxysulfide LaPbBiS3O was designed and successfully synthesized via a solid-state reaction in a sealed evacuated quartz tube. This material, composed of stacked NaCl-like [M4S6] (where M = Pb, Bi) layers and fluorite-type [La2O2] layers, crystallizes in the tetragonal space group P4/nmm with a = 4.0982(1) Å, c = 19.7754(6) Å, and Z = 2. Electrical resistivity and Hall effect measurements demonstrate that it is a narrow gap semiconductor with an activation energy of ∼17 meV. The thermopower and the figure of merit at room temperature were measured to be -52 µV/K and 0.23, respectively, which makes LaPbBiS3O and its derivatives be promising for thermoelectric applications.

Persistent high-energy spin excitations in iron-pnictide superconductors.

Motivated by the premise that superconductivity in iron-based superconductors is unconventional and mediated by spin fluctuations, an intense research effort has been focused on characterizing the spin-excitation spectrum in the magnetically ordered parent phases of the Fe pnictides and chalcogenides. For these undoped materials, it is well established that the spin-excitation spectrum consists of sharp, highly dispersive magnons. The fate of these high-energy magnetic modes upon sizable doping with holes is hitherto unresolved. Here we demonstrate, using resonant inelastic X-ray scattering, that optimally hole-doped superconducting Ba(0.6)K(0.4)Fe(2)As(2) retains well-defined, dispersive high-energy modes of magnetic origin. These paramagnon modes are softer than, though as intense as, the magnons of undoped antiferromagnetic BaFe(2)As(2). The persistence of spin excitations well into the superconducting phase suggests that the spin fluctuations in Fe-pnictide superconductors originate from a distinctly correlated spin state. This connects Fe pnictides to cuprates, for which, in spite of fundamental electronic structure differences, similar paramagnons are present.