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Thin films, interfaces and 2D Materials

Crystal lattices with confined geometries, like thin films and interfaces, host plenty of novel phenomena and unique functionalities. Over the past decade, artificial material structure at nanoscale, such as heterointerfaces and monolayer 2D materials, is a major driving force toward the realization of novel physics in quantum materials. We are able to engineer a variety of crystal thin films and interfaces at atomically thin limits using molecular beam epitaxy (MBE).

Beyond novel techniques for quantum material synthesis, we investigate their properties by performing in-situ ARPES and STM measurements, giving us direct feedback on material qualities and novel electronic properties.

Enhanced superconductivity in monolayer FeSe

Interfacial Coupling

Bulk FeSe is an iron-based superconductor that has a maximum Tc of 8K. Surprisingly, the Tc of a single layer of FeSe film grown epitaxially on the SrTiO3 substrate is enhanced to over 55K. We perform in-situ synchrotron-based MBE/ARPES experiment to understand how the transition temperature can be enhanced from the bulk superconducting transition temperature (Tc) of 8K at this 1UC FeSe/STO interface. We found the origin of enhancement of two-fold: 1) additional electron doping through interface charge transfer and 2) interfacial mode coupling. Prominent replica bands in the photoemission spectra, separated by energy on the order of the phonon of the STO substrate, indicate the existence of electron-phonon coupling across the interface. The similarity of the replica bands and main bands imply that emission or absorption of a phonon by the electron occurs with very little momentum transfer (near q=0 scattering). This forward scattering could help boost the Tc even in superconductors whose order parameter has a sign change, as is the case in cuprates, and possibly the iron-based superconductors.

 

Substrates Effect

Since STO exhibits a wide range of unique properties, such as superconductivity and incipient ferroelectricity, it is challenging to clarify what plays the major role in the enhancement of Tc in the 1UC FeSe/STO system. In order to further explore the substrate effect, we study 1UC FeSe grown on the rutile TiO2 (100) surface. We found that 1UC FeSe/TiO2 and 1UC FeSe/STO have very similar electronic structure, superconducting gap size and Tc. The similarities between these two systems suggest that the dielectric constant, strain and crystal symmetry from the substrate likely do not play important roles in the enhancement of Tc. The study shows that the interfacial e-ph coupling is most likely to be the key reason for enhancing superconductivity in 1UC FeSe/STO systems.

 

Robust Doping Level

Charge transfer across an abrupt interface between two materials with different work functions is a common phenomenon in epitaxial thin films. In our recent study, monolayer FeSe is grown on top of LaTiO3(LTO)/STO heterostructure. Due to the lowered work function of LTO, additional charge transfer should be expected to occur between the 1UC FeSe and STO substrate with more LTO layers. Surprisingly, the electron density as measured by ARPES Fermi surface maps and superconducting gaps for 1UC FeSe/LTO/STO films remain nearly unchanged with different LTO thickness. The result shows that superconductivity in 1UC FeSe thin film is robust and is accompanied with an anchored “magic” doping level.

To understand the unique phenomena of “magic” doping level under the contest of iron-based superconductivity, we consider the possible interplay of first-order phase transition and superconductivity: Above a doping level of 0.11, an insulating phase may occur and form a phase separation between the superconducting phase and the insulation phase. As a result, only ~0.11 doping is visible by the ARPES experiment.

2D Materials

In recent years, advances in synthesis and characterization of 2D monolayer crystals have emerged as a new platform for experimental condensed matter physics. The bulk analog of 2D material is usually composed of atomic layers with out-of-plane weak van der Waals bonds. Only few high-quality 2D materials can be achieved by the mechanical exfoliation method. A typical example is the high mobility graphene, which is made by exfoliation from graphite. Other 2D materials, such as transition metal chalcogenides, can not be easily achieved by exfoliation from bulk crystal. Therefore, developing advanced monolayer material by vapor deposition method, such as MBE, is a crucial step to understanding the underlying physics of 2D material. In our group, we study various of new 2D materials with layer-by-layer growth of high-quality thin films combined with APRES measurement. Some of our works include measuring the transition from direct to indirect bandgap from bulk to monolayer MoSe2, and the observation of quantum spin Hall state in monolayer WTe2.

Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

(a) Crystal structure of MoSe2. (b) Theoretical band structures Along the Gamma-K direction of the monolayer MoSe2 film. (c) Experimental band structure, shown as second derivative of the data to enhance visibility. (d) Experimental band structure.

In 2014, Yi Zhang et al. measured the band gap in monolayer samples by using ARPES on high quality thin films of MoSe2. In the study, high quality MoSe2 is grown by Molecular Beam Epitaxy on bilayer graphene substrate, and then immediately transfer to in-situ ARPES. The work provides the first direct experimental evidence of the distinct transition in the band-structure for thin film samples with thickness ranging from one monolayer to multilayers. Studying films from one to eight layers, we found that the direct to indirect gap transition happens at one to two monolayers, consistent with theory.

Quantum spin Hall state in monolayer 1T’-WTe2

A quantum spin Hall insulator is a novel 2-dimentional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field. Quantum spin Hall state results from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling. In the study done by Shujie Tang et al., monolayer WTe2 is grown on bilayer graphene substrate, using a molecular beam epitaxy (MBE). Using ARPES and Scanning tunneling spectroscopy, we established evidence that monolayer 1T’-WTe2 is a new class of QSH insulator, with nontrivial band inversion, the opening of a 55meV bulk band gap, and a conducting edge state in contrast to the insulating bulk. Such findings provide a platform for studying QSH insulator in 2D transition metal dichalcogenides and for developing novel device applications. For further detail about this work, please go to Topological Insulators Section.