arXiv subjects

Ge He

Publications and source records attributed to Ge He.

34 records · Page 2Linked to original sources

arXiv2021

Magnetic field-tuned quantum criticality in optimally electron-doped cuprate thin films

Antiferromagnetic (AF) spin fluctuations are commonly believed to play a key role in electron pairing of cuprate superconductors. In electron-doped cuprates, it is still in paradox about the interplay among different electronic states in quantum perturbations, especially between superconducting and magnetic states. Here, we report a systematic transport study on cation-optimized La2-xCexCuO4 (x = 0.10) thin films in high magnetic fields. We find an AF quantum phase transition near 60 T, where the Hall number jumps from nH =-x to nH = 1-x, resembling the change of nH at the AF boundary (xAF = 0.14) tuned by Ce doping. In the AF region a spin dependent state manifesting anomalous positive magnetoresistance is observed, which is closely related to superconductivity. Once the AF state is suppressed by magnetic field, a polarized ferromagnetic state is predicted, reminiscent of the recently reported ferromagnetic state at the quantum endpoint of the superconducting dome by Ce doping. The magnetic field that drives phase transitions in a similar but distinct manner to doping thereby provides a unique perspective to understand the quantum criticality of electron-doped cuprates.

cond-mat.supr-con↗

arXiv2020

Raman Study of Cooper Pairing Instabilities in (Li$_{1-x}$Fe$_x$)OHFeSe

We studied the electronic Raman spectra of (Li$_{1-x}$Fe$_x$)OHFeSe as a function of light polarization and temperature. In the B$_{1g}$ spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well-resolved peaks below T$_c$. The nearly resolution-limited peak at 110 cm$^{-1}$ (13.6 meV) is identified as a collective mode. The peak at 190 cm$^{-1}$ (23.6 meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of (Li$_{1-x}$Fe$_x$)OHFeSe, the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in A$_{1g}$ and B$_{2g}$ symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides, this Letter demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.

cond-mat.supr-con↗

arXiv2020

Combinatorial Laser Molecular Beam Epitaxy System Integrated with Specialized Low-temperature Scanning Tunneling Microscopy

We present a newly developed facility, comprised of a combinatorial laser molecular beam epitaxy system and an in-situ scanning tunneling microscopy (STM). This facility aims at accelerating the materials research in a highly efficient way, by advanced high-throughput film synthesis techniques and subsequent fast characterization of surface morphology and electronic states. Compared with uniform films deposited by conventional methods, the so-called combinatorial thin films will be beneficial to determining the accurate phase diagrams of different materials due to the improved control of parameters such as chemical substitution and sample thickness resulting from a rotarymask method. A specially designed STM working under low-temperature and ultra-high vacuum conditions is optimized for the characterization of combinatorial thin films, in an XY coarse motion range of 15 mm $\times$ 15 mm and with sub-micrometer location precision. The overall configuration as well as some key aspects like sample holder design, scanner head, and sample/tip/target transfer mechanism are described in detail. The performance of the device is demonstrated by synthesizing high-quality superconducting FeSe thin films with gradient thickness, imaging surfaces of highly oriented pyrolytic graphite, Au (111), Bi2Sr2CaCu2O8+δ (BSCCO) and FeSe. In addition, we have also obtained clean noise spectra of tunneling junctions and the superconducting energy gap of BSCCO. The successful manufacturing of such a facility opens a new window for the next generation of equipment designed for experimental materials research.

physics.app-ph↗

arXiv2019

Non-Volatile Superconductivity in an Insulating Copper Oxide Induced via Ionic Liquid Gating

Manipulating the superconducting states of high-T_c cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Traditional methods are mostly based on a trial-and-error method that is difficult to implement and time consuming. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr_2CuO_{4\pmδ} film, based on two distinct mechanisms: 1) with positive electric fields, the film can be reversibly switched between non-superconducting and superconducting states, attributed to the carrier doping effect. 2) The film becomes more resistive by applying negative bias voltage up to -4 V, but strikingly, a non-volatile superconductivity is achieved once the gate voltage is removed. Such a persistent superconducting state represents a novel phenomenon in copper oxides, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unraveled by high-resolution scanning transmission electron microscope and in-situ x-ray diffraction experiments. The effective manipulation and mastering of volatile/non-volatile superconductivity in the same parent cuprate opens the door to more functionalities for superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-T_c superconductors.

cond-mat.supr-con↗

arXiv2019

Emergent superconductivity in single crystalline $\mathrm{MgTi}_2\mathrm{O}_4$ films via structural engineering

Spinel compounds have demonstrated rich functionalities but rarely shown superconductivity. Here, we report the emergence of superconductivity in the spinel $\mathrm{MgTi}_2\mathrm{O}_4$, known to be an insulator with a complicated order. The superconducting transition is achieved by engineering a superlattice of $\mathrm{MgTi}_2\mathrm{O}_4$ and $\mathrm{SrTiO}_3$. The onset transition temperature in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer can be tuned from 0 to 5 K in such geometry, concurrently with a stretched $c$-axis (from 8.51 to 8.53 Å) compared to the bulk material. Such a positive correlation without saturation suggests ample room for the further enhancement. Intriguingly, the superlattice exhibits isotropic upper critical field $H_{\mathrm{c}2}$ that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer is understood in combination with the electron energy loss spectra and the first-principles electronic structure calculations, which point to the birth of superconductivity in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer by preventing the Ti-Ti dimerization. Our discovery not only provides a platform to explore the interplay between the superconductivity and other exotic states, but also opens a new window to realize superconductivity in the spinel compounds as well as other titanium oxides.

cond-mat.supr-con↗

arXiv2019

Anomalies of upper critical field in the spinel superconductor LiTi$_2$O$_{4-δ}$

High-field electrical transport and point-contact tunneling spectroscopy were used to investigate superconducting properties of the unique spinel oxide, LiTi$_2$O$_{4-δ}$ films with various oxygen content. We find that the upper critical field $B_\mathrm{c2}$ gradually increases as more oxygen impurities are brought into the samples by carefully tuning the deposition atmosphere. It is striking that although the superconducting transition temperature and energy gap are almost unchanged, an astonishing isotropic $B_\mathrm{c2}$ up to $\sim$ 26 Tesla is observed in oxygen-rich sample, which is doubled compared to the anoxic sample and breaks the Pauli limit. Such anomalies of $B_\mathrm{c2}$ were rarely reported in other three dimensional superconductors. Combined with all the anomalies, three dimensional spin-orbit interaction induced by tiny oxygen impurities is naturally proposed to account for the remarkable enhancement of $B_\mathrm{c2}$ in oxygen-rich LiTi$_2$O$_{4-δ}$ films. Such mechanism could be general and therefore provides ideas for optimizing practical superconductors with higher $B_\mathrm{c2}$.

cond-mat.supr-con↗

arXiv2018

Tunable superconductivity in parent cuprate Pr$_{2}$CuO$_{4\pmδ}$ thin films

In this article, we studied the role of oxygen in Pr$_{2}$CuO$_{4\pmδ}$ thin films fabricated by polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr$_{2}$CuO$_{4\pmδ}$ samples were systematically investigated. It is found that with decreasing the oxygen content, the low-temperature Hall coefficient ($R_H$) and magnetoresistance change from negative to positive, similar to those with the increase of Ce-doped concentration in R$_{2-x}$Ce$_{x}$CuO$_{4}$ (R= La, Nd, Pr, Sm, Eu). In addition, $T_c$ versus $R_H$ for both Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$ and Pr$_{2}$CuO$_{4\pmδ}$ samples can coincide with each other. We conclude that the doped electrons induced by the oxygen removal are responsible for the superconductivity of $T^\prime$-phase parent compounds.

cond-mat.supr-con↗

arXiv2017

The effects of oxygen in spinel oxide Li1+xTi2-xO4-delta thin films

The evolution from superconducting LiTi2O4-delta to insulating Li4Ti5O12 thin films has been studied by precisely adjusting the oxygen pressure during the sample fabrication process. In the superconducting LiTi2O4-delta films, with the increase of oxygen pressure, the oxygen vacancies are filled, and the c-axis lattice constant decreases gradually. With the increase of the oxygen pressure to a certain critical value, the c-axis lattice constant becomes stable, which implies that the Li4Ti5O12 phase comes into being. The process of oxygen filling is manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The temperature of magnetoresistance changed from positive and negative shows a non-monotonous behavior with the increase of oxygen pressure. The theoretical explanation of the oxygen effects on the structure and superconductivity of LiTi2O4-delta has also been discussed in this work.

cond-mat.supr-con↗

arXiv2017

Promoting superconductivity in FeSe films via fine manipulation of crystal lattice

Stabilized FeSe thin films in ambient pressure with tunable superconductivity would be a promising candidate for superconducting electronic devices yet its superconducting transition temperature (Tc) is below 10 K in bulk materials. By carefully controlling the depositions on twelve kinds of substrates using pulsed laser deposition technique, high quality single crystalline FeSe samples were fabricated with full width of half maximum 0.515? in the rocking curve and clear four-fold symmetry in phi-scan from x-ray diractions. The films have a maximum Tc 15 K on the CaF2 substrate and do not show obvious decay in the air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the Tc becomes adjustable from 15 to < 2 K with quite narrow transition widths less than 2 K, and shows a positive relation with the out-of-plane (c-axis) lattice parameter of the films. However, there is no clear relation between the Tc and the surface atomic distance of the substrates. By reducing the thickness of the films, the Tc decreases and fades away in samples of less than 10 nm, suggesting that the strain effect is not responsible for the enhancement of Tc in our experiments.

cond-mat.supr-con↗

arXiv2017

Normal State Gap in Parent Cuprate Pr2CuO4+/-δ

We present a tunneling study on single crystalline parent cuprate thin films, i.e. a series of Pr2CuO4+/-δ(PCO) with tunable superconducting transition temperature. The zero-bias anomaly of differential conductance, well reported in the normal state of R2-xCexCuO4 (R = Pr, Nd, La) and named as normal state gap (NSG), is observed in the Ce-free samples. This NSG behaves quite robust against the magnetic field up to 16 T, but fades away with increasing the temperature. Most importantly, we find that the magnitude of the NSG becomes larger with increasing point-contact junction resistance on the superconducting films, which is further enhanced in the non-superconducting samples of more oxygen disorders. The origination of NSG can be understood in the framework of Altshuler-Aronov-Lee (AAL) theory, where the disorder-induced electron-electron interactions suppress the density of states and thereby result in a soft Coulomb gap.

cond-mat.supr-con↗

arXiv2017

Anisotropic electron-phonon coupling in the spinel oxide superconductor

Among hundreds of spinel oxides, LiTi2O4 (LTO) is the only one that exhibits superconductivity (Tc ~13 K). Although the general electron-phonon coupling is still the main mechanism for electron pairing in LTO, unconventional behaviors such as the anomalous magnetoresistance, anisotropic orbital/spin susceptibilities, etc. reveal that both the spin and the orbital interactions should also be considered for understanding the superconductivity. Here, we investigate tunneling spectra of [111]-, [110]- and [001]-oriented high quality LTO thin films. Several bosonic modes in tunneling spectra are observed in the [111]- and [110]-oriented films but not in [001]-oriented ones, and these modes still exist at T = 2Tc and beyond the upper critical field, which are confirmed as stemming from electron-phonon interaction by DFT calculations. These modes only appear in special surface orientations, indicating that the electron-phonon coupling in LTO system is highly anisotropic and may be enhanced by orbital-related state. The anisotropic electron-phonon coupling should be taken seriously in understanding the nature of LTO superconductivity.

cond-mat.supr-con↗

arXiv2016

A conducting nano-filament (CNF) network as a precursor to the origin of superconductivity in electron-doped copper oxides

Emergency of superconductivity at the instabilities of antiferromagnetism has been widely recognized in unconventional superconductors. In copper-oxide superconductors, spin fluctuations play a predominant role in electron pairing with electron dopants yet composite orders veil the nature of superconductivity for hole-doped family. However, in electron-doped copper oxide superconductors (cuprates) the AFM critical end point is still in controversy for different probes, demonstrating high sensitivity to oxygen content. Here, by carefully tuning the oxygen content, a systematic study of the Hall signal and magnetoresistivity up to 58 Tesla on LCCO thin films identifies two characteristic temperatures. The former is quite robust, whereas the latter becomes flexible with increasing magnetic field, thereby linking respectively to two- and three-dimensional AFM, evident from the multidimensional phase diagram as a function of oxygen and Ce dopants. A rigorous theoretical analysis of the presented data suggest the existence of conductive nano-filamentary structures that effectively corroborate all previously reported field studies. The new findings provide a uniquely consistent alternative picture in understanding the interactions between AFM and superconductivity in electron-doped cuprates and offer a consolidating interpretation to the pioneering scaling law in cuprates recently established by Bozovic et al. (Nature, 2016)

cond-mat.mes-hall↗

arXiv2016

Crystallographic dependent transport properties and oxygen issue in superconducting LiTi2O4 thin films

A systematic study is performed on the spinel oxide, i.e. LiTi2O4 thin films oriented along [111]-, [110]-, and [001]-directions, to reveal the crystallographic dependence of transport properties. With decreasing temperature, the LiTi2O4 displays roughly identical onset temperatures of twofold symmetry of in-plane angular dependent magnetoresistivity (AMR) (at ~ 100 K), crossover from negative- to positive- magnetoresistance (at ~ 50 K), and coherence length in the superconducting state. While, the twofold symmetry in AMR itself suggests anisotropic electron scattering. The superconducting transition temperature (Tc) seems insensitive to the lattice parameter. Moreover, the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) discloses that oxygen vacancies exist in the LiTi2O4 films. These oxygen vacancies cause the change of lattice but show little influence on superconductivity, differing from high-Tc cuprates where subtle variation of oxygen way lead to a significant change in superconductivity.

cond-mat.supr-con↗

arXiv2016

Transport anomalies and quantum criticality in electron-doped cuprate superconductors

Superconductivity research is like running a marathon. Three decades after the discovery of high-Tc cuprates, there have been mass data generated from transport measurements, which bring fruitful information. In this review, we give a brief summary of the intriguing phenomena reported in electron-doped cuprates from the aspect of electrical transport as well as the complementary thermal transport. We attempt to sort out common features of the electron-doped family, e.g. the strange metal, negative magnetoresistance, multiple sign reversals of Hall in mixed state, abnormal Nernst signal, complex quantum criticality. Most of them have been challenging the existing theories, nevertheless, a unified diagram certainly helps to approach the nature of electron-doped cuprates.

cond-mat.supr-con↗

arXiv2015

A close look at antiferromagnetism in multidimensional phase diagram of electron-doped copper oxide

Emergency of superconductivity at the instabilities of antiferromagnetism (AFM), spin/charge density waves has been widely recognized in unconventional superconductors. In copper-oxide superconductors, spin fluctuations play a predominant role in electron pairing with electron dopants yet composite orders veil the nature of superconductivity for hole-doped family. However, in electron-doped ones the ending point of AFM is still in controversy for different probes or its sensitivity to oxygen content. Here, by carefully tuning the oxygen content, a systematic study of Hall signal and magnetoresistivity up to 58 Tesla on optimally doped La2-xCexCuO4+-δ (x = 0.10) thin films identifies two characteristic temperatures at 62.5+-7.5 K and 25+-5 K. The former is quite robust whereas the latter becomes flexible with increasing magnetic field, thereby linked to two- and three-dimensional AFM, evident from the multidimensional phase diagram as a function of oxygen as well as Ce dopants. Consequently, the observation of extended AFM phase in contrast to μSR probe corroborates an elevated critical doping in field, providing an unambiguous picture to understand the interactions between AFM and superconductivity.

cond-mat.supr-con↗

arXiv2014

Anomalous magnetoresistance in the spinel superconductor LiTi2O4

Transition-metal oxides offer an opportunity to explore unconventional superconductors, where the superconductivity (SC) is often interrelated with novel phenomena such as spin/charge order, fluctuations, and Fermi surface instability (1-3). LiTi2O4 (LTO) is a unique compound in that it is the only known spinel oxide superconductor. In addition to electron-phonon coupling, electron-electron and spin fluctuation contributions have been suggested as playing important roles in the microscopic mechanism for its superconductivity (4-8). However, the lack of high quality single crystals has thus far prevented systematic investigation of their transport properties (9). Here, we report a careful study of transport and tunneling spectroscopy in epitaxial LTO thin films. In the superconducting state, the energy gap was found to decrease as a quadratic function of magnetic field. In the normal state, an unusual magnetoresistance (MR) was observed where it changes from anisotropic positive to isotropic negative as the temperature is increased. A constant charge carrier concentration without any abrupt change in lattice parameters as a function of temperature suggests that the isotropic MR stems from the suppression of spin scattering/fluctuations, while the anisotropic term originates from an orbital contribution. These observations point to an important role strong correlations play in this unique superconductor.

cond-mat.supr-con↗