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T. Y. Guan

Publications and source records attributed to T. Y. Guan.

13 recordsLinked to original sources
arXiv2024

Development of low-radon ultra-pure water for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory(JUNO) is a state-of-the-art liquid scintillator-based neutrino physics experiment under construction in South China. To reduce the background from external radioactivities, a water Cherenkov detector composed of 35~kton ultra-pure water and 2,400 20-inch photomultiplier tubes is developed. Even after specialized treatment, ultra-pure water still contains trace levels of radioactive elements that can contribute to the detector background. Among which $^{222}$Rn is particularly significant. To address this, an online radon removal system based on the JUNO prototype has been developed. By integrating micro-bubble generators to enhance degasser's radon removal efficiency, the radon concentration in water can be reduced to 1~mBq/m$^{3}$ level, meeting the stringent requirements of JUNO. Additionally, a highly sensitive online radon concentration measurement system capable of detecting concentrations $\sim$1~mBq/m$^3$ has been developed to monitor the radon concentration in water. In this paper, the details regarding both systems will be presented.

physics.ins-det↗
arXiv2024

Developing a $μ$Bq/m$^{3}$ level $^{226}$Ra concentration in water measurement system for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory (JUNO), a 20~kton multi-purpose low background Liquid Scintillator (LS) detector, was proposed primarily to determine the neutrino mass ordering. To suppress the radioactivity from the surrounding rocks and tag cosmic muons, the JUNO central detector is submerged in a Water Cherenkov Detector (WCD). In addition to being used in the WCD, ultrapure water is used in LS filling, for which the $^{226}$Ra concentration in water needs to be less than 50~$μ$Bq/m$^3$. To precisely measure the $^{226}$Ra concentration in water, a 6.0~$μ$Bq/m$^3$ $^{226}$Ra concentration in water measurement system has been developed. In this paper, the detail of the measurement system as well as the $^{226}$Ra concentration measurement result in regular EWII ultrapure water will be presented.

physics.ins-det↗
arXiv2023

System upgrade for $μ$Bq/m$^3$ level $^{222}$Rn concentration measurement

The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multipurpose underground liquid scintillator detector, was proposed for the determination of the neutrino mass hierarchy as primary physics goal. The central detector will be submerged in a water Cherenkov detector to lower the background from the environment and cosmic muons. Radon is one of the primary background sources. Nitrogen will be used in several sub-systems, and a highly sensitive radon detector has to be developed to measure its radon concentration. A system has been developed based on $^{222}$Rn enrichment of activated carbon and $^{222}$Rn detection based on the electrostatic collection. This paper presents the detail of a $μ$Bq/m$^3$ level $^{222}$Rn concentration measurement system and gives detailed information about how the adsorption coefficient was measured and how the temperature, flow rate, and $^{222}$Rn concentration affect the adsorption coefficient.

physics.ins-det↗
arXiv2023

Research of radon diffusion behavior in liquid scintillator

The background caused by radon and its daughters is an important background in the low background liquid scintillator (LS) detectors. The study of the diffusion behaviour of radon in the LS contributes to the analysis of the related background caused by radon. Methodologies and devices for measuring the diffusion coefficient and solubility of radon in materials are developed and described. The radon diffusion coefficient of the LS was measured for the first time and in addition the solubility coefficient was also obtained. In addition, the radon diffusion coefficient of the polyolefine film which is consistent with data in the literature was measured to verify the reliability of the diffusion device.

physics.ins-det↗
arXiv2013

Specific heat and thermal conductivity of ferromagnetic magnons in Yttrium Iron Garnet

The specific heat and thermal conductivity of the insulating ferrimagnet Y$_3$Fe$_5$O$_{12}$ (Yttrium Iron Garnet, YIG) single crystal were measured down to 50 mK. The ferromagnetic magnon specific heat $C$$_m$ shows a characteristic $T^{1.5}$ dependence down to 0.77 K. Below 0.77 K, a downward deviation is observed, which is attributed to the magnetic dipole-dipole interaction with typical magnitude of 10$^{-4}$ eV. The ferromagnetic magnon thermal conductivity $κ_m$ does not show the characteristic $T^2$ dependence below 0.8 K. To fit the $κ_m$ data, both magnetic defect scattering effect and dipole-dipole interaction are taken into account. These results complete our understanding of the thermodynamic and thermal transport properties of the low-lying ferromagnetic magnons.

cond-mat.str-el↗
arXiv2011

Field-induced quantum critical point and nodal superconductivity in the heavy-fermion superconductor Ce2PdIn8

The interplay between magnetism and superconductivity has been a central issue in unconventional superconductors. While the dynamic magnetism could be the source of electron pairing, the static magnetism is generally believed to compete with superconductivity. In this sense, the observation of Q phase, the coupled spin-density wave order and superconductivity, in the heavy-fermion superconductor CeCoIn5 is very puzzling. Whether this Q phase origins from the novel Fulde-Ferrel-Larkin-Ovchinnikov state is under hot debate. Here we report the resistivity and thermal conductivity study of a newly discovered heavy-fermion superconductor Ce2PdIn8 down to 50 mK. We find an unusual field-induced quantum critical point at the upper critical field Hc2 and unconventional nodal superconductivity in Ce2PdIn8. The jump of thermal conductivity k(H)/T near Hc2 suggests a first-order-like phase transition at low temperatures. These results mimic the features of the Q phase in CeCoIn5, implying that Ce2PdIn8 is another promising compound to investigate the exotic Q phase and FFLO state. The comparison between CeCoIn5 and Ce2PdIn8 may help to clarify the origin of the Q phase.

cond-mat.supr-con↗
arXiv2011

Unveiling the quantum critical point of an Ising chain

Quantum phase transitions occur at zero temperature upon variation of some nonthermal control parameters. The Ising chain in a transverse field is probably the most-studied model undergoing such a transition, from ferromagnetic to paramagnetic state1,2. This model can be exactly solved by using a Jordan-Wigner transformation, which transforms the spins into noninteracting spinless fermions1. At the quantum critical point, the magnetic excitations can carry arbitrarily low energy and dominate the low temperature properties. Here we report the unveiling of such quantum critical point in quasi-one-dimensional Ising ferromagnet CoNb2O6 by ultra-low-temperature thermal conductivity measurements. We find that in the paramagnetic state, phonons are scattered by the magnetic excitations above certain temperature Ts, which corresponds to a gap. As predicted by the theoretical model1, this gap linearly goes to zero with decreasing the transverse field, thus determining the quantum critical point of the Ising chain.

cond-mat.str-el↗
arXiv2010

Thermal conductivity of overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$ single crystal: Evidence for nodeless multiple superconducting gaps and interband interactions

The in-plane thermal conductivity $κ$ of overdoped FeAs-based superconductor BaFe$_{1.73}$Co$_{0.27}$As$_2$ ($T_c$ = 8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear term $κ_0/T$ is negligible, suggesting a nodeless superconducting gap in the $ab$-plane. In magnetic field, $κ_0/T$ increases rapidly, very different from that of conventional s-wave superconductors. This anomalous $κ_0/T(H)$ may reveal an exotic superconducting gap structure in overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$: the vanishing hole ($β$) pocket has a much larger gap than the electron ($γ$ and $δ$) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the {\it smaller} density of states has a {\it larger} gap.

cond-mat.supr-con↗
arXiv2010

Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of FeAs-based superconductor KFe$_2$As$_2$ single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior $ρ(T) \sim T^{1.5}$ at $H_{c_2}$ = 5 T, and the development of a Fermi liquid state with $ρ(T) \sim T^2$ when further increasing field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field $H_{c_2}$. In zero field there is a large residual linear term $κ_0/T$, and the field dependence of $κ_0/T$ mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe$_2$As$_2$ have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

cond-mat.supr-con↗
arXiv2009

Thermal conductivity study of KFe$_2$As$_2$ single crystal: clear evidence for unconventional superconducting gap with nodes

The in-plane resistivity $ρ$ and thermal conductivity $κ$ of extremely overdoped KFe$_2$As$_2$ ($T_c$ = 3.0 K) single crystal were studied. It is found that $ρ\sim T^{1.5}$ at low temperature, a typical non-Fermi liquid behavior of electrons scattered by antiferromagnetic spin fluctuations. In zero field, we observed a large residual linear term $κ_0/T$, about one third of the normal-state value. In low magnetic fields, $κ_0/T(H)$ increases very fast. Such a behavior of $κ_0/T$ mimics the d-wave cuprate superconductors, therefore provides clear evidence for nodes in the superconducting gap of KFe$_2$As$_2$. Based on the Fermi surface topology of KFe$_2$As$_2$, it is believed that the dominant intraband pairing via antiferromagnetic spin fluctuations results in the unconventional superconducting gap with nodes.

cond-mat.supr-con↗
arXiv2009

Nodeless superconducting gap in electron-doped BaFe$_{1.9}$Ni$_{0.1}$As$_2$ probed by quasiparticle heat transport

The in-plane thermal conductivity $κ$ of electron-doped iron-arsenide superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c$ = 20.3 K) single crystal was measured down to 70 mK. In zero field, the absence of a residual linear term $κ_0/T$ at $ T \to 0$ is strong evidence for nodeless superconducting gap. In magnetic field, $κ_0/T$ shows a slow field dependence up to $H$ = 14.5 T ($\approx$ 30% $H_{c_2}$). This is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments in BaFe$_{1.85}$Co$_{0.15}$As$_2$ ($T_c$ = 25.5 K), where isotropic superconducting gaps with similar size on hole and electron pockets were observed.

cond-mat.supr-con↗
arXiv2009

Multi-gap nodeless superconductivity in iron selenide FeSe$_x$: evident from quasiparticle heat transport

The in-plane thermal conductivity $κ$ of the iron selenide superconductor FeSe$_x$ ($T_c$ = 8.8 K) were measured down to 120 mK and up to 14.5 T ($\simeq 3/4 H_{c2}$). In zero field, the residual linear term $κ_0/T$ at $ T \to 0$ is only about 16 $μ$W K$^{-2}$ cm$^{-1}$, less than 4% of its normal state value. Such a small $κ_0/T$ does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of $κ_0/T$ in FeSe$_x$ is very similar to that in NbSe$_2$, a typical multi-gap s-wave superconductor. We consider our data as strong evidence for multi-gap nodeless superconductivity in FeSe$_x$. This kind of superconducting gap structure may be generic for all Fe-based superconductors.

cond-mat.supr-con↗