Development Of In Situ Transport Measurement System And Study On Transport Properties Of Selenide Thin Films

We have designed and built the world's first in situ low-temperature transport measurement system,in which molecular beam epitaxy(MBE),scanning tunneling microscope(STM),electrode deposition,mechanical scratching isolation and micro-fourpoint probe(M4PP)transport measurement are combined in an ultra-high vacuum chamber.This system is capable of growing high-quality thin films by MBE and performing in situ STM characterization.More importantly,transport measurements ranging from 5 K to room temperature can be carried out by in situ electrode evaporation and M4PP contacting electrodes.By using the system,we systematically investigated the superconducting properties of FeSe films grown on SrTiO3(FeSe/STO).In addition,we have also grown high-quality NiAs-type CrSe films on SrTiO3(111)substrates by MBE and studied their physical properties.Iron-based superconductor is a new kind of unconventional high temperature superconducting material,among which the binary compound FeSe has the simplest lattice structure and is an ideal material to study the mechanism of high temperature superconductivity.In 2012,our group grew monolayer FeSe films on SrTiO3 substrate and found that its superconducting gap was as large as 20 meV,ten times bigger than that of the bulk FeSe.However,it is very difficult to measure the intrinsic properties of the monolayer films by ex situ transport measurement because the additional films deposited for protecting the monolayer FeSe films from air contamination during sample transfer can induce side effects into the FeSe films.With this homebuilt in situ measurement system,we investigated the FeSe/SrTiO3 and found that its superconducting zero resistance transition temperature TcZero could reach 32 K,the highest TcZero reported so far.The influence of electrode deposition,film thickness and FeTe protecting layer on the superconductivity were also investigated.The work not only deepens the understanding of superconductivity in FeSe/SrTiO3,but also verifies the excellent performance of our in situ transport measurement system.We have systematically studied the CrSe films grown on SrTiO3(111).The structural,magnetic and transport properties of CrSe films are investigated.We found a noncollinear magnetic structure at low temperature in CrSe films.Paramagnetic to ferromagnetic phase transition occurs at 70 K,and the non-collinear magnetic structure develops at about 50 K.Transport measurement results reveal "hump" features in resistivity which are similar to topological Hall effect at low temperature.Sign reversal of anomalous Hall coefficient is also observed.The electronic properties can be controlled by temperature,gate voltage and post-annealing.The unusual transport properties of magnetic CrSe thin films indicate that CrSe may be a good candidate material for spintronics devices.