Honeycomb oxide heterostructure: a new platform for Kitaev quantum spin liquid

Kitaev quantum spin liquid, massively quantum entangled states, is so scarce in nature that searching for new candidate systems remains a great challenge. Honeycomb heterostructure could be a promising route to realize and utilize such an exotic quantum phase by providing additional controllability of Hamiltonian and device compatibility, respectively. Here, we provide epitaxial honeycomb oxide thin film Na3Co2SbO6, a candidate of Kitaev quantum spin liquid proposed recently. We found a spin glass and antiferromagnetic ground states depending on Na stoichiometry, signifying not only the importance of Na vacancy control but also strong frustration in Na3Co2SbO6. Despite its classical ground state, the field-dependent magnetic susceptibility shows remarkable scaling collapse with a single critical exponent, which can be interpreted as evidence of quantum criticality. Its electronic ground state and derived spin Hamiltonian from spectroscopies are consistent with the predicted Kitaev model. Our work provides a unique route to the realization and utilization of Kitaev quantum spin liquid

Terahertz spectroscopy of high temperature superconductors and their photonic applications

Development of the ultrashort pulse lasers made it possible to realize the terahertz (THz) generation and detection in a broad frequency range between 0.1 and 30 THz. Extensive studies of the superconducting materials have been performed from basic sciences to the photonic applications because the THz frequencies span the gap in the electromagnetic spectrum between the electronic and the photonic technologies. Especially, the ultrashort pulses allow us to monitor the non-equilibrium dynamics of the superconductors and develop the ultrafast switching. Starting from a brief overview of the cuprate superconductor, we review the basic optical properties and photonic applications of the superconductors in a wide range of THz frequencies. It should be noted that numerous equilibrium and non-equilibrium THz spectroscopic measurements of the high temperature superconductors demonstrated the existence of the superconducting gap and its recovery dynamics and also terahertz electromagnetic wave modulation combined with the concept of metamaterials. These superconducting metamaterials will pave the way for better understanding of the new material properties and giving new ideas in application aspects

Nematic Fluctuations in Optimally Doped BaFe1.87Co0.13As2 Observed in Photoinduced Reflectivity Change

The photoinduced reflectivity change of optimally doped BaFe1.87Co0.13As2 is investigated. It is observed that the nematic signal in the photoinduced reflectivity change continues to increase as temperature decreases, reflecting its fluctuating nature. However, fluence-dependent measurements show saturation of the nematic signal at higher pump fluence, a typical behavior of a static order parameter. Temperature-dependent evolution of the saturation and the characteristic relaxation time of the nematic signal suggest that the nematicity of a spin and/or orbital origin develops at lower temperature on top of the nematicity coupled to the lattice that dominates the higher temperature nematic response. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Terahertz time domain spectroscopy of GdBCO superconducting thin films

We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K

Terahertz transmission properties of GdBCO superconductor thin films

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Plasmon nanogap-enhanced transition temperature of terahertz active device based on superconductors

We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K. Those thinner superconducting films are suitable to develop various THz applications combined with plasmonic nanostructures to enhance light-matter interactions. Furthermore, this combined system would create opportunities in studying electrodynamics of Cooper pairs inside the limited volume underneath the plasmonic structures and enhancing a transition temperature of the active terahertz switching device based on the superconducting thin films