Session D36: DMP Past Chair's Symposium: Tools That Explore Materials Physics

Honey, I Shrunk the Synchrotron: Electron Microscopy in Condensed Matter Physics

In this talk, I will present our recent experimental results using the latest generation of monochromated aberration-corrected scanning and transmission electron microscopes (STEM). Four examples of how one can reveal the magnetic ordering and orbital ordering of materials with atomic size electron probes will be shown. We have also used the principle of detailed balance to morph the electron microscope in a primary thermometer and revealed the anharmonic behavior of materials at the nanoscale. Our efforts in studying the phonon dispersion of materials under the presence of different isotopic species will be also mentioned. Finally, current and future limitations in the experiments and requirements to reveal the magnetic moment (orbital and spin), charge ordering, crystal field splitting, spin-orbit-coupling, optical dichroism, and other physical phenomena associated with broken symmetries will be discussed.   

Ultrafast Electron Calorimetry: Uncovering New Light-Induced Phases in Magnetic and 2D Materials

We have developed a powerful new method called ultrafast electron calorimetry that can uncover hidden
phases in magnetic and charge density wave materials. By using time and angle-resolved photoemission
spectroscopy to measure the dynamic electron temperature and full band structure as the laser excitation
is varied, one can clearly identify when changes in state or couplings occur in a material. Recently we
used this approach to coherently manipulate the structure, electron-phonon couplings, and state of the 2D
charge density wave (CDW) material 1T-TaSe 2 . Instead of the two material phases normally accessible
using temperature tuning (CDW and normal phases), ultrafast laser excitation expands the phase diagram
to include at least two new phases: a new metastable CDW state, and an inverted CDW state. Moreover,
the electron temperature, bandwidth, band gap and band shift are all modulated by the CDW breathing
mode, and this modulation changes in phase as the material enters a new state. In addition to the ability to
map the state of a material, we can use pulse sequences to coherently guide a material from one phase to
another.
Shi et al., Science Advances 5, eaav4449 (2019).
Zhang et al., http://arxiv.org/abs/1906.09545
Tengdin et al., Science Advances 4, 9744 (2018).
Tengdin et al., Science Advances, In press, (2020).   

Combining STM, AFM, and Magnetotransport Measurements for In-Operando Studies of Quantum Materials*

Research in new quantum materials require multi-mode measurements spanning length scales, correlations of atomic scale variables with macroscopic functions, and with ultimate spectral resolution only obtainable at ultra-low temperatures. In this talk I describe a multi-mode instrument achieving µeV tunneling resolution with in-operando measurement capabilities of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and magnetotransport inside a dilution refrigerator operating at 10 mK. I illustrate the capabilities of this new instrument in the study of quantum Hall edge states in graphene devices. The edge states, a set of alternating compressible and incompressible strips, are formed at the electrostatic pn junction boundary geometrically defining the Hall bar. To comprehensively characterize these microscopic objects, we apply all capabilities of the new instrument using modalities of AFM, STM, and magnetotransport measurements at mK temperatures. The Kelvin probe force microscopy (KPFM) mode of AFM detects the chemical potential transitions when Landau levels are being filled or emptied as a function of back gate potential and show the same fidelity for Landau level spectroscopy as STS measurements. In particular, symmetry breaking states can be resolved at filling factors ν = ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. With KPFM we can map the dispersion of the Landau levels across the quantum Hall edge boundary as a function of density and spatial position, including resolving the ν = ±1 edge modes. The microscopic properties of quantum Hall edge states can now be correlated with macroscopic magnetotransport measurements.

*In collaboration with S. Kim, J. Schwenk, D. Walkup, F. Ghahari, M. Slot, Y. Zeng, C. Dean, J. Berwanger, F. Giessibl, K. Watanabe, T. Taniguchi, and N. Zhitenev.   

Capturing In Operando Electronic Structure of Microscopic 2D Materials

Angle Resolved Photoemission Spectroscopy (ARPES) is a premier technique for determining the electronic structure of correlated and topological materials. Recently, we and several other groups have adapted ARPES to submicron dimensions through the development of nano-scale scanning X-ray beams, creating so-called nanoARPES endstations at synchrotrons around the world. This exciting technique now enables the in operando measurement of electronic structure for materials in applied fields and currents, as well as the ability to probe the effect of screening and strain on many body physics in confined geometries . In this talk, I will present recent results in which nanoARPES is applied to 2D materials and heterostructures at length scales below 200 nm. These include few layer graphene and transition metal dichalcogenide / graphene heterostructures. Work done in collaboration with Professors T. Heinz (Stanford U.), J. Katoch (CMU), S Ulstrup, J. Miwa, and Ph. Hofmann (Aarhus U.).   

Nanoscale quantum sensing of quantum materials through a single spin magnetometer

We introduce single spin magnetometry based on nitrogen-vacancy (NV) centers in diamond as a new measurement platform to locally probe the magnetic properties of quantum materials with a nanoscale spatial resolution. Utilizing this measurement platform, we have demonstrated control and local measurements of spin chemical potential in a magnetic insulator yttrium iron garnet (YIG) at the ambient condition, which opens up new possibilities for nanoscale imaging of spin-related phenomena. In additional, we investigate the temperature-dependent microwave response of an exfoliated high-T_c superconductor Bi₂Sr₂CaCu₂O_8+x (BSCCO) flake, from which the superconducting phase transition and penetration depth can be obtained in a non-invasive fashion. Our results demonstrate the unique capability enabled by NV spin magnetometry in exploring the exotic local spin and charge behavior of emergent material systems.