arXiv subjects

Bai Yang Wang

Publications and source records attributed to Bai Yang Wang.

29 records · Page 2Linked to original sources

arXiv2021

A Broken Translational Symmetry State in an Infinite-Layer Nickelate

A defining signature of strongly correlated electronic systems is the existence of competing phases with similar ground state energies, resulting in a rich phase diagram. While in the recently discovered nickelate superconductors, a high antiferromagnetic exchange energy has been reported, which implies the existence of strong electronic correlations, signatures of competing phases have not yet been observed. Here, we uncover a charge order (CO) in infinite-layer nickelates La1-xSrxNiO2 using resonant x-ray scattering across the Ni L-edge. In the parent compound, the CO arranges along the Ni-O bond direction with an incommensurate wave vector (0.344+/-0.002, 0) r.l.u., distinct from the stripe order in other nickelates which propagates along a direction 45 degree to the Ni-O bond. The CO resonance profile indicates that CO originates from the Ni 3d states and induces a parasitic charge modulation of La electrons. Upon doping, the CO diminishes and the ordering wave vector shifts toward a commensurate value of 1/3 r.l.u., indicating that the CO likely arises from strong correlation effects and not from Fermi surface nesting.

cond-mat.supr-con↗

arXiv2021

Insulator-to-Metal Crossover near the Edge of the Superconducting Dome in Nd$_{1-x}$Sr$_x$NiO$_2$

We report a systematic magnetotransport study of superconducting infinite-layer nickelate thin films Nd$_{1-x}$Sr$_x$NiO$_2$ with $0.15 \leq x \leq 0.225$. By suppressing superconductivity with out-of-plane magnetic fields up to 37.5 T, we find that the normal state resistivity of Nd$_{1-x}$Sr$_x$NiO$_2$ is characterized by a crossover from a metallic $T^2$-behavior to an insulating log(1/$T$)-behavior for all $x$ except $x = 0.225$, at which the resistivity is predominantly metallic. The log(1/$T$)-behavior is found to be robust against magnetic fields, inconsistent with scenarios involving localization or Kondo scattering, and points to an anomalous insulating state possibly driven by strong correlations. In the metallic state, we find no evidence for non-Fermi-liquid behavior arising from proximity to a putative quantum critical point located inside the superconducting dome.

cond-mat.supr-con↗

arXiv2021

Nickelate superconductivity without rare-earth magnetism: (La,Sr)NiO$_{2}$

The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-temperature transport ($x$ = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ~ 9 K at $x$ = 0.20. Additionally, we observe the unexpected indication of a superconducting ground state in undoped LaNiO$_{2}$, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)$_{1-x}$Sr$_{x}$NiO$_{2}$ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

cond-mat.supr-con↗

arXiv2021

Universal behavior of the bosonic metallic ground state in a two-dimensional superconductor

Anomalous metallic behavior, marked by a saturating finite resistivity much lower than the Drude estimate, has been observed in a wide range of two-dimensional superconductors. Utilizing the electrostatically gated LaAlO3/SrTiO3 interface as a versatile platform for superconductor-metal quantum phase transitions, we probe variations in the gate, magnetic field, and temperature to construct a phase diagram crossing from superconductor, anomalous metal, vortex liquid, to Drude metal states, combining longitudinal and Hall resistivity measurements. We find that the anomalous metal phases induced by gating and magnetic field, although differing in symmetry, are connected in the phase diagram and exhibit similar magnetic field response approaching zero temperature. Namely, within a finite regime of the anomalous metal state, the longitudinal resistivity linearly depends on field while the Hall resistivity diminishes, indicating an emergent particle-hole symmetry. The universal behavior highlights the uniqueness of the quantum bosonic metallic state, distinct from bosonic insulators and vortex liquids.

cond-mat.supr-con↗

arXiv2020

Phase Diagram of Infinite Layer Praseodymium Nickelate Pr$_{1-x}$Sr$_{x}$NiO$_2$ Thin Films

We report the phase diagram of infinite layer Pr$_{1-x}$Sr$_{x}$NiO$_2$ thin films synthesized via topotactic reduction from the perovskite precursor phase using CaH$_2$. Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between $x$ = 0.12 and 0.28, with a maximum superconducting transition temperature $T_{\rm{c}}$ of 14 K at $x$ = 0.18, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in Nd$_{1-x}$Sr$_{x}$NiO$_2$, a local $T_{\rm{c}}$ suppression near $x$ = 0.2 was not observed for the Pr$_{1-x}$Sr$_{x}$NiO$_2$ system. Normal state Hall effect measurements indicate mixed carrier contributions of both electrons and holes, and show a sign change in the Hall coefficient as functions of temperature and $x$, quite similar to that in Nd$_{1-x}$Sr$_{x}$NiO$_2$. Also similar is the observation of a minimum in the normal state resistivity associated with the superconducting compositions. These findings indicate an infinite layer nickelate phase diagram that is relatively insensitive to the rare-earth element, but suggest that disorder arising from the variations of the ionic radii on the rare-earth site affects the superconducting dome.

cond-mat.supr-con↗

arXiv2020

Isotropic Pauli-Limited Superconductivity in the Infinite Layer Nickelate Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$

The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.

cond-mat.supr-con↗

arXiv2020

A superconducting praseodymium nickelate with infinite layer structure

A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, epitaxially stabilized on SrTiO$_3$ substrates via topotactic reduction from the perovskite precursor phase. Here we present the synthesis and properties of PrNiO$_2$ thin films on SrTiO$_3$. Upon doping in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, we observe superconductivity with a transition temperature of 7-12 K, and robust critical current density at 2 K of 334 kA/cm$^2$. These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4$f$ configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO$_2$ planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

cond-mat.supr-con↗

arXiv2020

Superconducting Dome in Nd$_{1-x}$Sr$_x$NiO$_2$ Infinite Layer Films

We report the phase diagram of Nd$_{1-x}$Sr$_x$NiO$_2$ infinite layer thin films grown on SrTiO$_3$. A superconducting dome spanning $0.125 < x < 0.25$ is found, remarkably similar to cuprates, albeit over a narrower doping window. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the $\sim 1/x$ dependence observed for cuprates. This suggests the presence of both electron- and hole-like bands, consistent with band structure calculations.

cond-mat.supr-con↗

arXiv2020

Strain-Induced Room-Temperature Ferroelectricity in SrTiO$_3$ Membranes

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO$_3$ by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO$_3$ with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

cond-mat.mtrl-sci↗

arXiv2020

Aspects of the Synthesis of Thin Film Superconducting Infinite-Layer Nickelates

The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ calls for further investigation and optimization of the synthesis of this metastable infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO$_{3}$ (001) substrates, and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, it is essential that the precursor perovskite Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ thin film is stabilized with high crystallinity and no impurity phases; in particular, a Ruddlesden-Popper-type secondary phase is often observed. We have further investigated the evolution of the soft-chemistry topotactic reduction conditions to realize full transformation to the infinite-layer structure with no film decomposition or formation of other phases. We find that capping the nickelate film with a subsequent SrTiO$_{3}$ layer provides an epitaxial template to the top region of the nickelate film, much like the substrate. Thus, for currently optimized growth conditions, we can stabilize superconducting single-phase Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ (001) epitaxial thin films up to ~ 10 nm.

cond-mat.mtrl-sci↗

arXiv2020

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates

The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally-resolved electron energy loss spectroscopy to directly probe the Mott-Hubbard character of Nd$_{1-x}$Sr$_x$NiO$_2$. Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5$d$ states, and the systematic evolution of Ni 3$d$ hybridization and filling. These results clearly evidence the multiband nature of this system and the distinct electronic landscape for infinite-layer nickelates despite their formal similarity to the cuprates.

cond-mat.supr-con↗