arXiv subjects

Yuecong Liu

Publications and source records attributed to Yuecong Liu.

7 recordsLinked to original sources

arXiv2025

Growth of (Cu,C)Ba$_{2}$Ca$_{2}$Cu$_3$O$_{9\pmδ}$ thin films on flexible Hastelloy tapes

The applications of superconducting cable or magnet require that the superconductors are made into wires or tapes. For cuprate superconductors, this is a big challenge because of the strong flux motion induced by high anisotropy, very short coherence length and strong thermal fluctuation, etc. One of the ways is to fabricate superconducting films on flexible metallic tapes with oxide buffer layers. The successful one so far is the REBa$_2$Cu$_3$O$_7$ (REBCO, RE=rare earth elements) films in tape form, as called the coated conductors. While the superconducting transition temperature of REBCO system is limited to about 90 K. Here we report the successful fabrication of another new non-toxic superconducting film, namely (Cu,C)Ba$_{2}$Ca$_{2}$Cu$_3$O$_{9\pmδ}$ on these flexible metallic tapes with LaMnO$_3$ and CeO$_2$ as the top layers. The onset superconducting transition occurs at 112 K and 110 K, and the zero-resistance transition temperatures are about 96 K and 98 K, respectively. The temperature dependent resistivity under magnetic fields in different directions reveal a relatively small anisotropy. Further optimization of the films will improve the zero resistance transition temperature, thus can also improve the characteristic properties for applications. Our results show that the (Cu,C)Ba$_{2}$Ca$_{2}$Cu$_3$O$_{9\pmδ}$ is a promising candidate material for the high power applications in liquid nitrogen temperature region.

cond-mat.supr-con↗

arXiv2024

Growth and characterization of the La$_{3}$Ni$_{2}$O$_{7-δ}$ thin films: dominant contribution of the $d_{x^{2}-y^{2}}$ orbital at ambient pressure

By using the pulsed-laser-ablation technique, we have successfully grown the La$_{3}$Ni$_{2}$O$_{7-δ}$ thin films with $c$-axis orientation perpendicular to the film surface. X-ray diffraction shows that the (00l) peaks can be well indexed to the La$_{3}$Ni$_{2}$O$_{7-δ}$ phase. Resistive measurements show that the samples can be tuned from weak insulating to metallic behavior through adjusting the growth conditions. Surprisingly, all curves of $ρ-T$ in the temperature region of 2$\sim$300~K do not show the anomalies corresponding to either the spin density wave or the charge density wave orders as seen in bulk samples. Hall effect measurements show a linear field dependence with the dominant hole charge carriers, but the Hall coefficient $R_{H}=ρ_{xy}/H$ exhibits strong temperature dependence. The magnetoresistance above about 50~K is positive but very weak, indicating a weakened or absence of multiband effect. However, a negative magnetoresistance is observed at low temperatures, which shows the delocalization effect by magnetic field. Detailed analysis on the magnetoresistance suggests that the delocalization effect at low temperatures is due to the Kondo-like effect, rather than the Anderson weak localization. Our transport results suggest that, the electronic conduction is fulfilled by the $d_{x^{2}-y^{2}}$ orbital with holes as the dominant charge carriers, while the interaction through Hund's coupling with the localized $d_{z^{2}}$ orbital plays an important role in the charge dynamics.

cond-mat.supr-con↗

arXiv2024

Decoupling between $d_{x^2-y^2}$ and $d_{z^2}$ orbitals in hole doped La$_3$Ni$_2$O$_7$

Through Sr and Ca doping to the La sites, we successfully obtained the hole doped La$_{3-x}$A$_x$Ni$_2$O$_7$ (A = Sr and Ca) thin films by using the pulsed-laser deposition technique. Temperature dependent resistivity shows an upturn at low temperatures, but some clear instabilities, either due to structure or the releasing of strain between the film and substrate, occur at high temperatures. After annealing the films under high pressure of oxygen atmosphere, the upturn at low temperatures is strongly suppressed; the high temperature instability is completely removed. Hall effect measurements show a clear hole-charge carrier behavior with the carrier density of an order of magnitude higher compared with the undoped films. Surprisingly, it is found that the Hall coefficient is almost temperature independent in the whole temperature region, indicating the absence of multiband effect and suggesting the decoupling of the $d_{x^2-y^2}$ and $d_{z^2}$ orbitals. This is contradicting to the rigid band picture of a bonding $d_{z^2}$ band just below the Fermi energy in the pristine sample.

cond-mat.supr-con↗

arXiv2024

Superconducting thin films of (Cu,C)Ba$_2$Ca$_2$Cu$_3$O$_{9+δ}$ with zero resistance transition temperature close to 100 K

High superconducting transition temperature is favorable for the applications of superconductors. Some cuprate superconductors have the transition temperatures above 100 K, such as the Hg- or Tl-based 1223 and 1234 phases, but many of them contain the toxic elements, like Hg and Tl. Meanwhile, the high anisotropy makes the vortices easy to move, thus the irreversibility magnetic field is very low in the liquid nitrogen temperature region. Here we report the successful synthesis of superconducting thin films (Cu,C)Ba$_{2}$Ca$_{2}$Cu$_3$O$_{9+δ}$ with the zero-resistance transition temperature reaching 99.7 K. The film shows a very good crystallinity with (00l) orientation. The superconducting transitions are rather sharp as revealed by both resistivity and magnetization measurements. Temperature dependent resistivity has been measured under different magnetic fields along c-axis and ab-plane, and the irreversibility lines have been achieved. The resistivity was also measured at different temperatures with the magnetic field rotated in the ac-plane, and the data can be nicely scaled by using the anisotropic Ginzburg-Landau model, yielding a temperature dependent anisotropy which varies from 17 at 110 K to 4 at 77 K. We also measured the magnetization-hysteresis-loops and calculated the critical current density through the Bean critical state model. The critical current density at 77 K is about 6$\times$10$^5$ A/cm$^2$ (zero field), thus the film may be a good candidate for the applications of superconducting cables or the high frequency superconducting filters.

cond-mat.supr-con↗

arXiv2024

Electronic correlations and partial gap in the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$

The discovery of superconductivity with a critical temperature of about 80~K in La$_{3}$Ni$_{2}$O$_{7}$ single crystals under pressure has received enormous attention. La$_{3}$Ni$_{2}$O$_{7}$ is not superconducting under ambient pressure but exhibits a transition at $T^{\ast} \simeq 115$~K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La$_{3}$Ni$_{2}$O$_{7}$ features strong electronic correlations which significantly reduce the electron's kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at $T^{\ast}$ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni-$d_{3z^{2}-r^{2}}$ orbital is removed due to the transition at $T^{\ast}$. Our experimental results provide pivotal information for understanding the transition at $T^{\ast}$ and superconductivity in La$_{3}$Ni$_{2}$O$_{7}$.

cond-mat.supr-con↗

arXiv2023

Absence of near-ambient superconductivity in LuH$_{2\pm\text{x}}$N$_y$

Recently near-ambient superconductivity was claimed in N-doped lutetium hydride (ref. 1). This induces a worldwide fanaticism about the dream of room temperature superconductivity under low pressures. By using a high pressure and high temperature synthesis technique, we have successfully obtained the nitrogen doped lutetium hydride (LuH$_{2\pm\text{x}}$N$_y$) with a dark-bluish color and a structure with the space group of $Fm\bar{3}m$ evidenced by x-ray diffraction. This structure is the same as that reported in ref. 1. The energy dispersive X-ray spectroscopy (EDS) confirmed the existence of nitrogen in some areas of the samples. At ambient pressure, we witness a kink of resistivity and magnetization at about 300 K, which may correspond to a rearrangement of hydrogen/nitrogen atoms, namely a structural transition. However, by applying a pressure from 1 GPa to 6 GPa, we have seen a progressively optimized metallic behavior without showing superconductivity down to 10 K. Temperature dependence of magnetization shows a roughly flat feature between 100 and 320 K, and the magnetization increases with magnetic field at 100 K, all these are not expected for superconductivity at 100 K. Thus, we conclude the absence of near-ambient superconductivity in this nitrogen-doped lutetium hydride under pressures below 6 GPa.

cond-mat.supr-con↗

arXiv2023

Pressure induced color change and evolution of metallic behavior in nitrogen-doped lutetium hydride

By applying pressures up to 42 GPa on the nitrogen-doped lutetium hydride (LuH$_{2\pm\text{x}}$N$_y$), we have found a gradual change of color from dark-blue to pink-violet in the pressure region of about 12 GPa to 21 GPa. The temperature dependence of resistivity under pressures up to 50.5 GPa shows progressively optimized metallic behavior with pressure. Interestingly, in the pressure region for the color change, a clear decrease of resistivity is observed with the increase of pressure, which is accompanied by a clear increase of the residual resistivity ratio (RRR). Fitting to the low temperature resistivity gives exponents of about 2, suggesting a Fermi liquid behavior in low temperature region. The general behavior in wide temperature region suggests that the electron-phonon scattering is still the dominant one. The magnetoresistance up to 9 tesla in the state under a pressure of 50.5 GPa shows an almost negligible effect, which suggests that the electric conduction in the pink-violet state is dominated by a single band. It is highly desired to have theoretical efforts in understanding the evolution of color and resistivity in this interesting system.

cond-mat.supr-con↗