Phase diagrams on composition-spread FeyTe1-xSex films.

FeyTe1-xSex, an archetypical iron-based high-temperature superconductor with a simple structure but rich physical properties, has attracted lots of attention because the two end compositions, Se content x = 0 and 1, exhibit antiferromagnetism and nematicity, respectively, making it an ideal candidate for studying their interactions with superconductivity. However, what is clearly lacking to date is a complete phase diagram of FeyTe1-xSex as functions of its chemical compositions since phase separation usually occurs from x âˆ¼ 0.6 to 0.9 in bulk crystals. Moreover, fine control of its composition is experimentally challenging because both Te and Se are volatile elements. Here we establish a complete phase diagram of FeyTe1-xSex, achieved by high-throughput film synthesis and characterization techniques. An advanced combinatorial synthesis process enables us to fabricate an epitaxial composition-spread FeyTe1-xSex film encompassing the entire Se content x from 0 to 1 on a single piece of CaF2 substrate. The micro-region composition analysis and X-ray diffraction show a successful continuous tuning of chemical compositions and lattice parameters, respectively. The micro-scale pattern technique allows the mapping of electrical transport properties as a function of relative Se content with an unprecedented resolution of 0.0074. Combining with the spin patterns in literature, we build a detailed phase diagram that can unify the electronic and magnetic properties of FeyTe1-xSex. Our composition-spread FeyTe1-xSex films, overcoming the challenges of phase separation and precise control of chemical compositions, provide an ideal platform for studying the relationship between superconductivity and magnetism.

Combinatorial laser molecular beam epitaxy system integrated with specialized low-temperature scanning tunneling microscopy.

We present a newly developed facility comprising a combinatorial laser molecular beam epitaxy system and an in situ scanning tunneling microscope (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 in 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 rotary-mask method. A specially designed STM working under low-temperature and ultrahigh vacuum conditions is optimized for the characterization of combinatorial thin films in an XY coarse motion range of 15 mm × 15 mm with submicrometer location precision. The overall configuration and some key aspects such as the 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 and imaging surfaces of highly oriented pyrolytic graphite, Au (111), Bi2Sr2CaCu2O8+δ (BSCCO), and FeSe. In addition, we also have 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 equipment designed for experimental materials research.

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating.

Manipulating the superconducting states of high transition temperature (high-Tc) cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr2CuO4±Î´ (PCO) films, based on two distinct mechanisms. Firstly, with positive electric fields, the film can be reversibly switched between superconducting and non-superconducting states, attributed to the carrier doping effect. Secondly, 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 phenomenon represents a distinctive route of manipulating superconductivity in PCO, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments. The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-Tc superconductors.

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

The evolution from superconducting LiTi2O4-δ to insulating Li4Ti5O12 thin films has been studied by precisely tuning the oxygen pressure in the sample fabrication process. In superconducting LiTi2O4-δ films, with the increase of oxygen pressure, the oxygen vacancies are filled gradually and the c-axis lattice constant decreases. When the oxygen pressure increases to a certain critical value, the c-axis lattice constant becomes stable, which implies that the sample has been completely converted to Li4Ti5O12 phase. The two processes can be manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The transition temperature (T ch ) of magnetoresistance from the positive to the negative shows a nonmonotonic behavior, i.e. first decrease and then increase, with the increase of oxygen pressure. We suggest that the decrease Tch can be attributed to the suppressing of orbital-related state, and the inhomogeneous phase separated regions contribute positive MR and thereby lead to the reverse relation between Tch and oxygen pressure.

Tunable critical temperature for superconductivity in FeSe thin films by pulsed laser deposition.

Stabilized FeSe thin films in ambient pressure with tunable superconducting critical temperature would be a promising candidate for superconducting electronic devices. By carefully controlling the depositions on twelve kinds of substrates using a pulsed laser deposition technique single crystalline FeSe thin films were fabricated. The high quality of the thin films was confirmed by X-ray diffraction with a full width at half maximum of 0.515° in the rocking curve and clear four-fold symmetry in φ-scan. The films have a maximum T c ~ 15 K on the CaF2 substrate and were stable in ambient conditions air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the superconducting critical temperature becomes adjustable below 15 K with quite narrow transition width less than 2 K. These FeSe thin films deposited on different substrates are optimized respectively. The Tc of these optimized films show a relation with the out-of-plane (c-axis) lattice parameter of the FeSe films.

Healing Effect of Controlled Anti-Electromigration on Conventional and High-Tc Superconducting Nanowires.

The electromigration process has the potential capability to move atoms one by one when properly controlled. It is therefore an appealing tool to tune the cross section of monoatomic compounds with ultimate resolution or, in the case of polyatomic compounds, to change the stoichiometry with the same atomic precision. As demonstrated here, a combination of electromigration and anti-electromigration can be used to reversibly displace atoms with a high degree of control. This enables a fine adjustment of the superconducting properties of Al weak links, whereas in Nb the diffusion of atoms leads to a more irreversible process. In a superconductor with a complex unit cell (La2-x Cex CuO4 ), the electromigration process acts selectively on the oxygen atoms with no apparent modification of the structure. This allows to adjust the doping of this compound and switch from a superconducting to an insulating state in a nearly reversible fashion. In addition, the conditions needed to replace feedback controlled electromigration by a simpler technique of electropulsing are discussed. These findings have a direct practical application as a method to explore the dependence of the characteristic parameters on the exact oxygen content and pave the way for a reversible control of local properties of nanowires.

Evolution of electronic states in n-type copper oxide superconductor via electric double layer gating.

The occurrence of electrons and holes in n-type copper oxides has been achieved by chemical doping, pressure, and/or deoxygenation. However, the observed electronic properties are blurred by the concomitant effects such as change of lattice structure, disorder, etc. Here, we report on successful tuning the electronic band structure of n-type Pr2-xCexCuO4 (x = 0.15) ultrathin films, via the electric double layer transistor technique. Abnormal transport properties, such as multiple sign reversals of Hall resistivity in normal and mixed states, have been revealed within an electrostatic field in range of -2 V to + 2 V, as well as varying the temperature and magnetic field. In the mixed state, the intrinsic anomalous Hall conductivity invokes the contribution of both electron and hole-bands as well as the energy dependent density of states near the Fermi level. The two-band model can also describe the normal state transport properties well, whereas the carrier concentrations of electrons and holes are always enhanced or depressed simultaneously in electric fields. This is in contrast to the scenario of Fermi surface reconstruction by antiferromagnetism, where an anti-correlation is commonly expected.