For a most up to date cross-sectional view of our research activities, please check out our Publications page!
EXPERIMENTAL TECHNIQUES
Angle-Resolved Photoemission Spectroscopy (ARPES)
Based on the photoelectric effect, the ARPES technique directly images the energy- and momentum-resolved electronic states of matter. At the fundamental level, ARPES is the most direct probe for unveiling the electronic structure of a material, which is the basis for governing the macroscopic properties of a material. Importantly and furthermore, ARPES can also reveal many-body effects on the electronic states, originating from electron-lattice coupling, electron-electron interactions, and other emergent orders. For further reading, refer to reviews (RMP2021, ARCMP2012) on the technique of ARPES and its application on quantum materials. Our group carries out ARPES measurements both in our home lab in Brockman Hall as well as by traveling around the world to synchrotron facilities such as the Advanced Light Source, Stanford Synchrotron Radiational Light Source, National Synchrotron Light Source-II, Canadian Light Source, and others. Our group is interested in developing and utilizing in-situ tuning of quantum materials for the ARPES technique to better understand their emergent phenomena.
Uniaxial Strain
In-situ tunable uniaxial strain directly couples to the lattice symmetry. Initially motivated by the discovery of the enigmatic nematic order in the iron-based superconductors, clamp-type of strain devices were developed that could squeeze on the samples during measurement such that all the structural domains could be aligned within the beamspot of the ARPES light source, giving us access to measure intrinsic electronic structure of the nematic phase. Since then, tunable strain devices have been developed using piezo-electric stacks where the application of a bias voltage could be used to squeeze or pull on the sample directly at the measurement position, enabling not only detwinning capability but also the capability to do nematic susceptibility type of measurements on the electronic structure.
Recent Publications:
- Y. Guo et al. Phys. Rev. B 108, L081104 (2023)
- J. Huang, Y. Guo, and M. Yi. Synchrotron Radiation News 36, 30 (2023)
Magneto-ARPES
A new capability our group has been working on recently is magneto-ARPES, which allows ARPES measurements in the presence of a small but finite magnetic field. Traditionally, magnetic fields are carefully shielded by ARPES chambers due to the adverse effect they have on the trajectory of photoelectrons. Hence magnetic field as a common tuning knob for manipulating emergent phases in quantum materials for many experimental probes has been excluded for ARPES measurements. Recently, we have established the capability to perform ARPES under an in-situ tunable finite magnetic field, and have started to understand extrinsic effects on the photoelectrons such that we can start to perform magneto-ARPES measurements in regimes where intrinsic effects of field on quantum materials can be manifested.
Recent Publications:
Molecular Beam Epitaxy (MBE)
Molecular beam epitaxy (MBE) is a way to synthesize single crystalline thin films layer by layer. It utilizes co-evaporation of raw materials unto a crystalline substrate, on which thin films of desired stoichiometry form. Our lab has a MBE system connected in vacuo to our ARPES analysis chamber. This enables us to grow thin films and directly transfer to the ARPES system for measurement of the electronic structure without breaking vacuum.
Recent Publications:
QUANTUM MATERIALS
Quantum materials is an emerging field that umbrellas a wide range of fascinating materials where the collective behaviors are much more than the defining characters of the individual constituents. This broad field expands a vast range of materials that exhibit properties arising from strongly interacting electrons and or from topology. Our group is interested in probing materials such as high temperature superconductors, heavy fermion systems, charge density wave systems, topological materials, transition metal dichalcogenides, etc. For further reading, refer to the special issue of Nature Physics and Nature Materials on Quantum Materials. Below is a selection of material classes that we are working on.
Geometrically Frustrated Lattices
Correlation effects are manifested in materials where the electron kinetic energy is small compared to electron interaction energy scale. Such physics can be manifested in crystalline materials where the geometry of the lattice structure induces a destructive interference of the electronic wavefunction. This quantum interference effect results in so called “flat electronic bands,” which has been theoretically predicted to induce exotic many-body effects. In 2D, this can be realized in kagome lattices. In 3D, this can be realized in pyrochlore lattices.
Recent Publications:
- J. Huang et al. npj Quantum Materials 9, 71 (2024)
- J. Huang et al. Nat. Phys. 20, 603 (2024) (news&views; news release; SSRL highlight)
- S. Cheng et al. npj Quantum Materials 9, 14 (2024)
- X. Teng and J. S. Oh et al. Nat. Phys. 814 (2023) (news release)
- X. Teng et al. Nature 609, 490 (2022) (news release; ALS Science highlight)
Unconventional Superconductivity
Mott physics is where the electron correlation effect is so strong such that otherwise metallic compounds would open up a Mott gap at the chemical potential separated by lower and upper Hubbard bands. In multi-orbital materials, such kind of strong correlation effects could manifest in an orbital-dependent way. Such physics is revealed in the iron-based superconductor family. In particular, we find an evolution between a correlated metallic phase to an orbital-selective Mott phase by tuning the ratio of Te and Se in the Fe(Te,Se) superconductor family. As a result, a Fermi surface topological change is observed purely due to the Mott localization of the dxy orbital, in absence of any symmetry breaking orders.
Recent Publications:
2D Magnetism
The unique properties of 2D magnets, such as their ultrathin nature, high tunability, and potential for integration with other 2D materials, make them ideal candidates for spintronic technologies. Not only be suitable for applications, vdW magnets are top candidates for studying the interplay of dimensionality, magnetism, topology, lattice, electronic correlation effect. In our group, we are studying the electronic states of various vdW magnets, includes CrGeTe3, CrSiTe3, Fe3GeTe2, Fe3GaTe2 and Fe5GeTe2. Check out our recent publications, as well as related new releases.
Recent Publications:
- H. Wu et al. Nat. Comm. 15, 2739 (2024) (news release)
- H. Wu et al. Phys. Rev. B 109, 104410 (2024)
- H. Wu et al. Phys. Rev. B 109, 045416 (2024)
Topological materials
The past 20 years of developments in condensed matter have witnessed a revolution sparked by the realization of topology in electronic systems. The topological band structure directly visualized by ARPES on the surface or bulk of quantum materials not only provides a solid basis for promising material applications such as constructing next-generation spintronics and valleytronics, but also links to particles originally proposed in high energy physics and beyond. These topological quasiparticles, meanwhile, encode deeper mathematical concepts studied in geometry and topology. Our group’s recent efforts on topological materials have fostered the understanding of weak topological insulating phases in the bismuth halide systems, demonstrated the new topological phase of Kramers nodal line metal, and explored charge order induced topological quasiparticles. Further details can be found in the recent publications.
Recent Publications:
- J. Huang et al. Phys. Rev. X. 11, 031042 (2021)
- H. Wu et al. npj Quantum Materials 7, 31 (2022)
- Y. Zhang et al. Commun. Phys. 6, 134 (2023)
- Y. Zhang et al. Phys. Rev. B. 108, 155121 (2023)
- J. S. Oh et al. Phys. Rev. B. 108, L201104 (2023)
Sponsors
We are grateful to the following funding agencies for sponsoring our research:
