Superconducting Thin Films

Objectives: The recent impressive progress in the MBE growth of high-temperature single layer superconductors in oxide heterostructures opens exciting possibilities for novel sensors (superconducting hot electron nano-bolometers) that may lead to potential breakthroughs in modern THz sensing and IR fiber communications technologies. This project pursues complex research of nonequilibrium effects in these novel La₂CuO₄ /La_2-xSr_xCuO₄ heterostructures to establish the foundation for sensing and communication technologies based on unique superconducting nanomaterials with ultra-small electron heat capacity.

Intellectual Merit: The research plan includes comprehensive, fundamental investigations of the transport properties of the superconducting interface, focusing on critical issues related to the nonequilibrium electron and phonon effects in low-dimensional superconductors and heterostructures. By providing the needed fundamental and technological bases, this program includes design and development of advanced optoelectronic devices, such as ultra-fast detectors, THz mixers, single IR photon counters with outstanding sensitivity.

Broader Impact: The proposed devices have a number of important applications in THZ environmental and industrial monitoring, astrophysics, homeland security, and medicine. If successful, the proposed program will have a strong impact on optical communication and networking, quantum imaging and metrology, quantum optical computing, bio-photonics, and single-molecule spectroscopy. In addition, this research will provide a strong platform for the training and education of students of minority institutions in important areas of growth and characterization of new materials, research and development of novel nanoscaled devices.

Background and basic sample characteristics

The La1.59Sr0.41CuO4 / La1.72Sr0.28CuO4 (LSCO) novel heterostructures are made of atomically thin layers of the LSCO compounds. Typical profile of studied samples is presented below. The sample contains three atomic layers of superconducting LSCO(superconductor, S) located between two sets of 10 atomic layers of overdoped LSCO material (metal, M). The dependence of the resistance of the sample on the temperature is presented in the right figure below. The inset to the figure shows the first derivative of the resistance vs temperature at varying magnetic fields. With decreasing temperature the resistance decreases,

demonstrating a transition to the superconducting state. The transition depends on the magnetic field. These dependencies are taken at small electric current, in the linear regime at which the voltage is proportional to the current. This regime obeys Ohm's Law.

Nonlinear VI Characteristics

Nonlinear properties of the heterostructures or, in other words, deviation from Ohm's Law are the subject of current research. The dependence of the electric field on the current density is shown in figure below. The dependence exhibits nonlinear behavior at zero magnetic filed. With increasing magnetic field the sample resistance and the non-Ohmic deviations are reduced.

Differential Resistance

As shown in the figure below at small electric currents the variation of the differential resistivity is proportional to the square of the current. We introduce a coefficient

to describe the slope of the linear dependence at small currents.

Thus the describes the nonlinear response of the system.

Temperature dependence of

The two figures below show the temperature dependence of the nonlinear response of different samples. For the studied samples, the dependence shows a

maximum inside the superconducting transition region. At high temperatures the coefficient

decreases with increasing temperature. This behavior is found even when samples are in the normal state. This is shown in the right figure below at T>20K.

Berezinski-Kosterlitz-Thouless (BKT) transitions

Ultrathin cuprates superconducting films have been studied intensively with numerous attempts to observe the Berezinski-Kosterlitz-Thouless transition. Still, the physics of the superconducting transition is not completely understood even for the linear dc response. It is considered to be well established that at the superconducting transition the major part of the resistance drop is controlled by a vortex motion, occurring below temperature T0C at which a superconducting condensate is formed. In this regime the nonlinearity of current-voltage characteristics

is due to the current induced dissociation of the vortices-antivortex pairs. According to the BKT theory, is equal to three at the critical temperature associated with the BKT transition. At temperatures above the critical temperature, with the increase of the sample resistance the electron heating effects become important.The heating originates in vortex cores and may lead to a formation of vortex lines (vortex train) subsequently evolving in resistive domains.

To understand the origin of the observed nonlinearity we apply the heating

model to the whole set of data obtained in the experiments. Experiments show that near the normal state, at which the resistance is practically independent on the magnetic field, the obtained data are in a reasonable agreement with the homogeneous heating model. At lower resistance (and temperatures) a significant disagreement between the model and the experiment is found indicating the dominant contribution of a different nonlinear mechanism.

Thermal Conductance

In the heating model, the electric current warms up the electrons and increases the electron temperature. For small overheating, the increase in temperature is proportional to the power absorbed by electrons per unit area. In a stationary state, the increase of the electron temperature is inversely proportional to the electronic heat capacity and to the coupling of the electron system to the thermal bath (i.e., the phonons).

The coupling is controlled by the electron-phonon scattering time. Comparison of the experimental data with the heating model yields the thermal conductance. The dependence of g on temperature for the two samples is presented below. At high temperatures the thermal conductance follow

dependence. The obtained dependence is in agreement with the expected dependence of the electron-phonon interaction via deformational potential. The electron-phonon scattering time is found to be significantly smaller than that observed in usual metals. In accordance with the theory the time should be significantly small in the studied oxide structures due to very small carrier density. The obtained results indicate strong potential of the oxide superconducting heterojunctions for applications.