arXiv subjects

L. E. Chow

Publications and source records attributed to L. E. Chow.

6 recordsLinked to original sources

arXiv2024

Angular dependence of hump-shape Hall Effects for distinguishing between Karplus-Luttinger and Geometrical Origins

Among the vast magnetic heterostructures explored in Condensed Matter Physics, two contrasting interpretations of the hump-shaped Hall Effects remain ambiguous and debated, namely, the overlap of two opposite-signed Karplus-Luttinger Hall loops associated with inhomogeneous collinear domains with perpendicular anisotropy, or the Geometrical/Topological Hall Effect emanated from hexagnal close-packed lattice of Skyrmion ground state with smoothly varying non-collinear moments. Their similarity in topology implies difficulty in discrimination via magnetic imaging. Here, this ambiguity is overcome and clarified by the divergence exponent of hump peak fields extracted from Hall measurements with magnetic field rotation on several heterostructures. Their difference in sensitivity to in-plane fields reveals that the former mechanism involves higher uniaxial anisotropy than the latter, departing from the Skyrmion ground state regime by the Ginzburg-Landau framework of triple-q spin-wave superposition. Numerous material systems can be summarized into a single curve of divergence exponent versus the collinear quality factor, bridging the crossover of the two mentioned mechanisms.

cond-mat.str-el↗

arXiv2023

Pairing symmetry in infinite-layer nickelate superconductor

The superconducting infinite-layer nickelate family has risen as a promising platform for revealing the mechanism of high-temperature superconductivity. However, its challenging material synthesis has obscured effort in understanding the nature of its ground state and low-lying excitations, which is a prerequisite for identifying the origin of the Cooper pairing in high-temperature superconductors. In particular, the superconducting gap symmetry of nickelates has hardly been investigated and remains controversial. Here, we report the pairing symmetry of the infinite-layer nickelates determined by London penetration depth measurements in neodymium-based (Nd,Sr)NiO$_2$ and lanthanide-based (La,Ca)NiO$_2$ thin films of high crystallinity. A rare-earth-specific order parameter is observed. While the lanthanide nickelates follow dirty line-node behaviour, the neodymium-counterpart exhibits nodeless order parameters such as the $(d+is)$ wave. In contrast to the cuprates, our results suggest that the superconducting order parameter in nickelates is beyond a single $d_(x^2-y^2 )$-wave gap. Furthermore, the superfluid density shows a long tail near the superconducting transition temperature which is consistent with the emergence of a two-dimensional to three-dimensional crossover in the superconducting state. These observations challenge the early theoretical framework and propel further experimental and theoretical interests in the pairing nature of the infinite-layer nickelate family.

cond-mat.supr-con↗

arXiv2022

Pauli-limit violation in lanthanide infinite-layer nickelate superconductors

Superconductivity can be destroyed by a magnetic field with an upper bound known as the Pauli-limit in spin-singlet superconductors. Almost all the discovered superconductors are spin-singlet, with the highest transition temperature $T_c$ at ambient pressure achieved in the cuprate family. The closest cuprate analogue is the recently discovered infinite-layer nickelate, which hosts substantial structural and electronic similarity to the cuprate. A previous magnetotransport study on Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ has observed an isotropic Pauli-limited upper critical field. Here, we report a large violation (>2 times) of Pauli-limit in every crystallographic directions in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ regardless of the doping $x$. Such a large violation of the Pauli-limit in all directions in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ is unexpected and unlikely accounted by a Fulde Ferrell-Larkin-Ovchinnikov (FFLO)-state, strong spin-orbit-coupling, strong-coupling or a large pseudogap. On the other hand, in agreement with the previous report, we observe a Pauli-limiting critical field in Nd$_{1-x}$Sr$_x$NiO$_2$ and the superconducting anisotropy decreases as doping increases, suggesting a spin-singlet pairing. Therefore, superconductivity in La$_{1-x}$(Ca/Sr)$_x$NiO$_2$ could be driven by a non-spin-singlet Cooper pairing mechanism with an attractive high-$T_c$ at 10 K, an order of magnitude higher than the known spin triplet superconductors, favourably extending the application of spin-triplet superconductivity in topological matter, non-dissipative spintronics, and quantum computing.

cond-mat.supr-con↗

arXiv2022

Superconductivity in infinite-layer nickelate La$_{1-x}$Ca$_x$NiO$_2$ thin films

We report the observation of superconductivity in infinite-layer Ca-doped LaNiO$_2$ (La$_{1-x}$Ca$_x$NiO$_2$) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1-xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15<x<0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient $R_H$ changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the $R_H$-sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation in order to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize the significant role of lattice correlation on the multiband structures of the infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2022

Observation of perfect diamagnetism and interfacial effect on the electronic structures in Nd0.8Sr0.2NiO2 superconducting infinite layers

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

cond-mat.supr-con↗

arXiv2021

Charge and spin order dichotomy in NdNiO$_2$ driven by SrTiO$_3$ capping layer

Superconductivity in infinite-layer nickelates holds exciting analogies with that of cuprates, with similar structures and $3d$-electron count. Using resonant inelastic x-ray scattering (RIXS) we studied electronic and magnetic excitations and charge density correlations in Nd$_{1-x}$Sr$_{x}$NiO$_2$ thin films with and without an SrTiO$_3$ capping layer. We observe dispersing magnons only in the capped samples, progressively dampened at higher doping. In addition, we detect an elastic resonant scattering peak in the uncapped $x=0$ compound at wave vector (1/3,0), remindful of the charge order signal in hole doped cuprates. The peak weakens at $x=0.05$ and disappears in the superconducting $x=0.20$ film. The uncapped samples also present a higher degree of Ni$3d$-Nd$5d$ hybridization and a smaller anisotropy of the Ni$3d$ occupation with respect to the capped samples. The role of the capping on the possible hydrogen incorporation or on other mechanisms responsible for the electronic reconstruction far from the interface remains to be understood.

cond-mat.supr-con↗