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Recently, we published a paper entitled “Copper migration and surface oxidation of CuxBi2Se3 in ambient pressure environments”

https://iopscience.iop.org/article/10.1088/2515-7639/ac93b5

This is our first foray into ambient pressure x-ray photoelectron spectroscopy (XPS), and quantitative analysis of XPS data, which would not have been possible without collaboration with Slavo Nemsak and Lorenz Fitting of the Advanced Lightsource. We also collaborated with several UCD faculty and their students for growing (Curro), intercalating (Koski), and characterizing (Taufour) the specimens.

Intercalation–adding new atoms or molecules between the layers of layered materials–allows to introduce new functionality in various ways.  In topological insulators such as Bi2Se3, intercalation is a popular way to introduce superconductivity or magnetism, and copper intercalation is one way to achieve superconductivity.  It turns out, that intercalated atoms are not always static, and can sometimes move around due to external factors (perhaps the best example of this is the Li-ion batteries in all our devices, where moving intercalated Li in and out is the entire point).  This fact has not been appreciated in Cu-intercalated Bi2Se3, which is a potential topological superconductor.  We found that when CuxBi2Se3 is exposed to oxygen, there are chemical changes in the near-surface region including Cu migration and depletion of selenium concomitant with the development of a bismuth oxide layer.  This is important because 1) specimens and devices constructed from CuxBi2Se3 may be exposed to air during fabrication or storage which can change their near-surface chemistry 2) near-surface chemistry is particularly critical for these materials because the participation of the surface states in superconductivity determines if these are topological superconductors or not.

See also, this ALS science brief: https://als.lbl.gov/copper-migrates-to-surface-of-topological-insulator-in-air/

Congratulations to Adam, Matt, and the entire collaboration team!

Our paper came out recently, entitled “Electronic structure and topology across $T_c$ in the magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{S}_2$”

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.155115

Co3Sn2S2 is a ferromagnet below Tc=177K, and the time reversal symmetry breaking in the magnetic phase stabilizes Weyl points.  Earlier studies have established this material to be a Weyl semimetal in its ground state, but our recent paper is the first published work through Tc.  We observe spectroscopic evidence of a topological phase transition across Tc (somewhat expected), but surprisingly also see flatter bands above Tc, suggesting enhanced electronic correlation effects above Tc.  This work was a collaboration between our group, as well as the Savrasov (theory) and Taufour (synthesis) groups at UC Davis.  Congratulations to Antonio, Seva, Adam, Sudheer, and Zio!

Our new paper is out, “Nanosecond dynamics in intrinsic topological insulator ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Se}}_3$ revealed by time-resolved optical reflectivity”

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.L020301

Bi2Se3 is unique among 3D topological insulators (TIs) in its larger bulk band gap, isotropic surface state, and isolated Dirac point, which uniquely lend it to optoelectronic phenomena including exciton condensation.  Optical excitations are made in the bulk, but until now, there has been limited characterization of photoexcited bulk carriers in insulating Bi2Se3 at low temperature.  We show three orders of magnitude slowing of bulk carrier decay rates when Bi2Se3 is made insulating with Sb-doping, and show that the primary relaxation mechanism is via radiative recombination across the bulk band gap, demonstrating previously unreported optical phenomena in a canonical material.

Our new paper is out, “Three interaction energy scales in the single-layer high-$T_c$ cuprate $\mathrm{Hg}{\mathrm{Ba}}_2\mathrm{Cu}{O}_{4+\delta }$”

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.205109

Congratulations to Sudheer, Antonio, Jayita, and all our collaborators!

Summary: Cuprate high-temperature superconductors have the highest transition temperatures (Tc) at ambient conditions, but the microscopic mechanism of superconductivity and other mysterious electronic phases in these materials remains unresolved.  These materials share a common structural unit of a CuO2 plane, and different cuprate compounds differ from one another in the number of nearby CuO2  planes (single-layer vs multiple-layer) and in the chemistry of the layers separating CuO2  planes or blocks of CuO2  planes from one another.

Among single-layer cuprates, the Tc varies between 40K and 100K, but the origin of this difference is not understood.  The mechanism of producing or enhancing superconductivity can be revealed through interactions between electrons (which pair up into Cooper pairs in the superconducting state) and collective excitations of the atomic lattice, spins, etc (which can serve as the ‘pairing glue’); similar statements can be made about other emergent electronic phases.  In cuprate high-temperature superconductors, the electronic properties tend to be different for electrons moving in different directions relative to the atomic bonds in the CuO$_2$  planes, which necessitates a momentum-resolved probe such as angle-resolved photoemission spectroscopy (ARPES). Notably, it is only in the lower-Tc single-layer cuprates that these interactions between electrons and collective excitations have been well-characterized by ARPES.

We present the first comprehensive ARPES study of $\mathrm{Hg}{\mathrm{Ba}}_2\mathrm{Cu}{O}_{4+\delta }$ (Hg1201), a single-layer cuprate which reaches a Tc of 98K.  We identify three different interactions between electrons and collective excitations, which may play a role in enhancing superconductivity and explaining the mysterious electronic phase above Tc called the pseudogap.  This allows us to draw connections to existing studies of cuprates to establish which electronic phenomena are universal and which are materials- or technique-dependent.  Hg1201 has yielded important insights from multiple experimental techniques, and the comprehensive nature of the present ARPES work makes it a starting point for establishing a cohesive multi-technique narrative about this prototype material.

Two phase transitions driven by surface electron doping in ${\mathrm{WTe}}_2$

Phys. Rev. B 102, 121110(R) (2020)

Materials’ structure and function are intimately connected, whereby one material can yield multiple emergent electronic phases when it assumes different crystal structures.  We drive a structural phase transition with continuous electron doping, image the effects on electronic band structure, and provide microscopic understanding of this process via first principles calculations, hereby demonstrating a new pathway for structural phase transitions in multifunctional 2D materials such as WTe₂.  This is a paradigm material for realizing giant magnetoresistance, topological states, and Majorana Fermions, which are affected by fine-tuning of electronic occupation. We demonstrate how this fine tuning can stabilize unexpected structural changes.

Congratulations to Antonio!

https://science.sciencemag.org/content/369/6505/775.full

TMDs, and cuprates, and TIs oh my!

Observation of long excitation lifetime in photoexcited Sb-doped Bi2Se3 nanoplatelets

Three interaction energy scales in single-layer high-Tc cuprate HgBa2CuO4+δ

Phase transitions via electron doping in WTe2

Our angle-resolved photoemission (ARPES) system was installed last week!  More photos here

The following commentary, written with my postdoc advisor Nuh Gedik, is available online: https://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4273.html

This commentary is part of an upcoming issue of Nature Physics, focusing on quantum materials, and it discusses the contributions that the experimental technique of angle-resolved photoemission spectroscopy (ARPES) has made to this research area. Quantum materials exhibit a panoply of phenomena, and this diverse class of materials are linked together via a shared paradigm of emergence–the idea that the aggregate many-electron properties of materials cannot be derived from the behavior of a single electron (i.e. more is different). Trying to understand or predict these emergent phenomena drives basic research in this area, but in the future, the fruits of this research may have applications in materials for energy production/harnessing and next-generation electronics.

ARPES directly measures how electrons move in crystalline solids, and can thus pinpoint emergent electronic phenomena with high precision. The present commentary discusses how ARPES experiments have illuminated important physics in three classes of quantum materials (cuprate high temperature superconductors, iron-based high temperature superconductors, topological insulators), and how the challenges presented by these materials have in turn driven the improvement of ARPES experimental technology. One of these state-of-the-art ARPES instruments will be installed in my lab in the next several months!

I am excited to start as an assistant professor in the physics department at the University of California Davis, and I am building a lab focused on the growth and study of novel quantum materials, particularly unconventional and high temperature superconductors, correlated electron systems, topological materials, and energy materials.  This lab will use both angle-resolved photoemission spectroscopy (ARPES) and time resolved techniques for elucidating the electronic structure and dynamics of quantum materials such as unconventional superconductors, materials with topological surface state, and 2D materials.   These advanced materials hold promise for revealing new emergent phenomena, elucidating interactions in many body systems, and enabling tomorrow’s electronics and energy resources.