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Xue, Qi-Kun

Publications and source records attributed to Xue, Qi-Kun.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Vortex entropy and superconducting fluctuations in ultrathin underdoped Bi 2 Sr 2 CaCu 2 O 8+x superconductor

Vortices in superconductors can help identify emergent phenomena but certain fundamental aspects of vortices, such as their entropy, remain poorly understood. Here, we study the vortex entropy in underdoped Bi 2 Sr 2 CaCu 2 O 8+x by measuring both magneto-resistivity and Nernst effect on ultrathin flakes (≤2 unit-cell). We extract the London penetration depth from the magneto-transport measurements on samples with different doping levels. It reveals that the superfluid phase stiffness p s scales linearly with the superconducting transition temperature T c , down to the extremely underdoped case. On the same batch of ultrathin flakes, we measure the Nernst effect via on-chip thermometry. Together, we obtain the vortex entropy and find that it decays exponentially with T c or p s . We further analyze the Nernst signal above T c in the framework of Gaussian superconducting fluctuations. The combination of electrical and thermoelectric measurements in the two-dimensional limit provides fresh insight into high temperature superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Superionic fluoride gate dielectrics with low diffusion barrier for two-dimensional electronics

Exploration of new dielectrics with a large capacitive coupling is an essential topic in modern electronics when conventional dielectrics suffer from the leakage issue near the breakdown limit. Here, to address this looming challenge, we demonstrate that rare-earth metal fluorides with extremely low ion migration barriers can generally exhibit an excellent capacitive coupling over 20 μF cm -2 (with an equivalent oxide thickness of ~0.15 nm and a large effective dielectric constant near 30) and great compatibility with scalable device manufacturing processes. Such a static dielectric capability of superionic fluorides is exemplified by MoS 2 transistors exhibiting high on/off current ratios over 10 8 , ultralow subthreshold swing of 65 mV dec -1 and ultralow leakage current density of ~10 -6 A cm -2 . Therefore, the fluoride-gated logic inverters can achieve notably higher static voltage gain values (surpassing ~167) compared with a conventional dielectric. Furthermore, the application of fluoride gating enables the demonstration of NAND, NOR, AND and OR logic circuits with low static energy consumption. In particular, the superconductor–insulator transition at the clean-limit Bi 2 Sr 2 CaCu 2 O 8+δ can also be realized through fluoride gating. In conclusion, our findings highlight fluoride dielectrics as a pioneering platform for advanced electronic applications and for tailoring emergent electronic states in condensed matter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Persistent Josephson tunneling between Bi 2 Sr 2 CaCu 2 O 8 + x flakes twisted by 45 ° across the superconducting dome

There is a heated debate on the Josephson effect in twisted Bi 2 Sr 2 CaCu 2 O 8+x flakes. Recent experimental results suggest the presence of either anomalously isotropic pairing or exotic d+id-wave pairing, in addition to the commonly believed d-wave one. Here, we address this controversy by fabricating ultraclean junctions with uncompromised crystalline quality and stoichiometry at the junction interfaces. In the optimally doped regime, we obtain prominent Josephson coupling (2-4 mV) in multiple junctions with the twist angle of 45°, in sharp contrast to a recent report that shows two orders of magnitude suppression around 45° from the value at 0°. We further extend this study to the previously unexplored overdoped regime and observe pronounced Josephson tunneling at 45° together with Josephson diode effect up to 50 K. In conclusion, our work helps establish the persistent presence of an isotropic pairing component across the entire superconducting phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Observation of Coulomb blockade and Coulomb staircases in superconducting Pr 0.8 Sr 0.2 NiO 2 films

Motivated by the discovery of superconductivity in the infinite-layer nickelate family, we report an experimental endeavor to clean the surface of nickelate superconductor Pr 0.8 Sr 0.2 NiO 2 films by Ar + ion sputtering and subsequent annealing, and we study their electronic structures by cryogenic scanning tunneling microscopy and spectroscopy. The annealed surfaces are characterized by nano-sized clusters and Coulomb staircases with periodicity inversely proportional to the projected area of the nanoclusters, consistent with a double-barrier tunneling junction model. Moreover, the dynamical Coulomb blockade effects are observed and result in well-defined energy gaps around the Fermi level, which correlate closely with the specific configuration of the junctions. These Coulomb blockade-related phenomena provide an alternative plausible cause of the observed gap structure that should be considered in the spectroscopic understanding of nickelate superconductors with the nano-clustered surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Tuning superconductivity of Bi 2 Sr 2 CaCu 2 O 8+x by fluoride ion intercalation with LaF 3 gate dielectric

Here, we demonstrate fluorine intercalation into Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO) flakes, as thick as 100 nm, by back-gating a single crystalline LaF 3 substrate. Element-sensitive analysis not only confirms the presence of fluorine in BSCCO after gating but also reveals lateral diffusion of fluorine in BSCCO. Transport measurements further demonstrate the hole-doping effect of fluoride ions in BSCCO through effective modulation of the superconducting transitions. Our work introduces a distinct type of ions (F - ) that can be intercalated into materials via solid state back-gating. It broadens the toolbox of ion back-gating and is useful for addressing exotic phenomena in the heavily hole doped regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Little-Parks like oscillations in lightly doped cuprate superconductors

Understanding the rich and competing electronic orders in cuprate superconductors may provide important insight into the mechanism of high-temperature superconductivity. Here, by measuring Bi 2 Sr 2 CaCu 2 O 8+x in the extremely underdoped regime, we obtain evidence for a distinct type of ordering, which manifests itself as resistance oscillations at low magnetic fields (≤10 T) and at temperatures around the superconducting transition. By tuning the doping level p continuously, we reveal that these low-field oscillations occur only when p < 0.1. The oscillation amplitude increases with decreasing >p but the oscillation period stays almost constant. We show that these low-field oscillations can be well described by assuming a periodic superconducting structure with a mesh size of about 50 nm. Such a charge order, which is distinctly different from the well-established charge density wave and pair density wave, seems to be an unexpected piece of the puzzle on the correlated physics in cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers

Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (E V ), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ~90° rotation of the Brillouin zones that render holes’ momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.

2-dimensional systems↗
DOE OSTI2019

From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films

SnTe hosts ferroelectricity that competes with its weak nontrivial band topology: in the high-symmetry rocksalt structure—in which its intrinsic electric dipole is quenched—this material develops metallic surface bands, but in its rhombic ground-state configuration—which hosts a nonzero spontaneous electric dipole—the crystalline symmetry is lowered, and the presence of surface electronic bands is not guaranteed. In this work, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 atomic layers (ALs) are examined on freestanding samples, to which atomic layers were gradually added. Four-AL SnTe films are antiferroelectrically coupled, while thicker freestanding SnTe films are ferroelectrically coupled. The electronic band gap reduces its magnitude in going from 2 to 40 ALs, but it does not close due to the rhombic nature of the structure. Lastly, these results bridge the structure of SnTe films from the monolayer to the bulk.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Realizing an Epitaxial Decorated Stanene with an Insulating Bandgap

The exploration of intriguing topological quantum physics in stanene has attracted enormous interest but is challenged by lacking desirable material samples. The successful fabrication of monolayer stanene on PbTe(111) films with low-temperature molecular beam epitaxy and thorough characterizations of its atomic and electronic structures are reported here. In situ angle-resolved photoemission spectroscopy together with first-principles calculations identify two hole bands of p xy orbital with a spin-orbit coupling induced band splitting and meanwhile reveal an automatic passivation of p z orbital of stanene. Importantly, material properties are tuned by substrate engineering, realizing a decorated stanene sample with truly insulating bulk on Sr-doped PbTe. In conclusion, this finding paves a road for studies of stanene-based topological quantum effects and electronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Superconductivity in few-layer stanene

A single atomic slice of α-tin—stanene—has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α-tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. In situ angle-resolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Thus, our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetic quantum phase transition in Cr-doped Bi 2 (Se x Te 1-x ) 3 driven by the Stark effect

The interplay between magnetism and topology, as exemplified in the magnetic skyrmion systems, has emerged as a rich playground for finding novel quantum phenomena and applications in future information technology. Magnetic topological insulators (TI) have attracted much recent attention, especially after the experimental realization of quantum anomalous Hall effect. Future applications of magnetic TI hinge on the accurate manipulation of magnetism and topology by external perturbations, preferably with a gate electric field. In this work, we investigate the magneto transport properties of Cr doped Bi 2 (Se x Te 1-x ) 3 TI across the topological quantum critical point (QCP). We find that the external gate voltage has negligible effect on the magnetic order for samples far away from the topological QCP. However, for the sample near the QCP, we observe a ferromagnetic (FM) to paramagnetic (PM) phase transition driven by the gate electric field. Theoretical calculations show that a perpendicular electric field causes a shift of electronic energy levels due to the Stark effect, which induces a topological quantum phase transition and consequently a magnetic phase transition. Finally, the in situ electrical control of the topological and magnetic properties of TI shed important new lights on future topological electronic or spintronic device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Spin fluctuation induced linear magnetoresistance in ultrathin superconducting FeSe films

The discovery of high-temperature superconductivity in FeSe/STO has trigged great research interest to reveal a range of exotic physical phenomena in this novel material. Here we present a temperature dependent magnetotransport measurement for ultrathin FeSe/STO films with different thickness and protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is observed around the superconducting transition temperatures but absent otherwise. The experimental LMR can be reproduced by magnetotransport calculations based on a model of magnetic field dependent disorder induced by spin fluctuation. Thus, the observed LMR in coexistence with superconductivity provides the first magnetotransport signature for spin fluctuation around the superconducting transition region in ultrathin FeSe/STO films.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Stripes developed at the strong limit of nematicity in FeSe film

A single monolayer of iron selenide grown on strontium titanate shows an impressive enhancement of superconductivity compared with the bulk, as well as a novel Fermi surface topology, extreme two-dimensionality, and the possibility of phonon-enhanced electron pairing. For films thicker than one unit cell, however, the electronic structure is markedly different, with a drastically suppressed superconductivity and strong nematicity appearing. The physics driving this extraordinary dichotomy of superconducting behaviour is far from clear. In this paper, we use low-temperature scanning tunnelling microscopy to study multilayers of iron selenide grown by molecular beam epitaxy, and find a stripe-type charge ordering instability that develops beneath the nematic state. The charge ordering is visible and pinned in the vicinity of impurities. And as it emerges in the strong limit of nematicity, it suggests that a magnetic fluctuation with a rather small wavevector may be competing with the ordinary collinear antiferromagnetic ordering in multilayer films. Finally, the existence of stripes in iron-based superconductors, which resemble the stripe order in cuprates, not only suggests that electronic anisotropy and correlation are playing an important role, but also provides a platform for probing the complex interactions between nematicity, charge ordering, magnetism and superconductivity in high-temperature superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Discovery of robust in-plane ferroelectricity in atomic-thick SnTe

Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1–unit cell (UC) limit. The ferroelectric transition temperature T c of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

36 MATERIALS SCIENCE↗