http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

We report on a large electric-field response of quasi-two-dimensional electron gases generated at interfaces in epitaxial heterostructures grown from insulating oxides. These device structures are characterized by doping layers that are spatially separated from high-mobility quasi-two-dimensional electron gases and therefore present an oxide analog to semiconducting high-electron mobility transistors. By applying a gate voltage, the conductivity of the electron gases can be modulated through a quantum phase transition from an insulating to a metallic state.

http://science.sciencemag.org/content/sci/313/5795/1942.full.pdf

Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behaviour—including high-temperature superconductivity and colossal magnetoresistance—and potentially tune the phase transitions in such systems. Here we provide an overview of the achievements in this field and discuss the opportunities brought by the field-effect approach.

Electric field effect in correlated oxide systems

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

Gate-tunable phase transitions in thin flakes of 1T-TaS2

Magnetic-field-induced microwave absorption in sintered high-Tc copper oxide superconductors is found to be proportional to the surface area of samples and not to their volume, establishing that absorption takes place near the sample surface. The surface resistance and reactance increase with magnetic field as a result of energy loss through fluxons driven by superconducting currents. When the magnetic field is modulated, absorptive and dispersive signals arise from modulation of the complex surface impedance through processes induced by the changing magnetic fields.

https://iopscience.iop.org/article/10.1209/0295-5075/5/5/015/pdf

Microwave Magneto-Surface Impedance of High-Tc Superconductors

Experimental evidence is provided that the pinning force and the critical current density of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films can be controlled by electric fields. This novel effect is attributed to field-induced changes of the density of mobile charge carriers in the superconductor.

Influence of electric fields on pinning in YBa2Cu3O7- delta films.

Experimental evidence for a significant electric field effect in thin superconducting films of YBa2Cu3O7−δ is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×106 V/cm across insulating SrTiO3 barriers into thin cpitaxial YBa2Cu3O7−δ channel layers. The results demonstrate that with these structures the electrical resistivity aboveT
c (R=0) and the density of free carriers in the YBa2Cu3O7−δ films can be modified by 1–2% with gate voltages smaller than 50 V.

Electric field effect on superconducting YBa2Cu3O7?? films

We report on the formation of conducting phases in slightly doped La2CuO4 samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.

Percolative phase separation in La2CuO4+? and La2?x Sr x CuO4

Electric transport properties of sputtered YBa2Cu3O7−δ films were studied as a function of screw dislocation density, ranging from 5·107 cm−2 to 1.3·109 cm−2 as determined at the film surface. A correlation was found between the number of screw dislocations and the critical current density (J

 c
 ). Films with higher screw dislocation densities have higher critical current densities and a slower drop ofJ

 c
 as a function of applied magnetic fieldH.

/pdf/correlation-betweenj-c-and-screw-dislocation-density-in-m5oheq0x9h.pdf

K. A. Müller

Papers

Microwave Magneto-Surface Impedance of High-Tc Superconductors

Influence of electric fields on pinning in YBa2Cu3O7- delta films.

Electric field effect on superconducting YBa2Cu3O7?? films

Percolative phase separation in La2CuO4+? and La2?x Sr x CuO4

Correlation betweenJ c and screw dislocation density in sputtered YBa2Cu3O7-? films