Margaret Murnane1,2
University of Colorado, Boulder1,KMLabs Inc.2
Margaret Murnane1,2
University of Colorado, Boulder1,KMLabs Inc.2
The macroscopic properties of solids, such as the electrical conductivity, greatly depend on the electronic states and their low-energy excitations near the Fermi level. In strongly-correlated quantum materials, many-body interactions between charges, phonons and spins can lead to the formation of a lower-energy ground state characterized by a pseudogap, rather than a sharp energy gap. Pseudogaps and the absence of a clear Fermi edge are found in a broad class of materials including cuprates, colossal magnetoresistance manganites and quasi-one-dimensional (quasi-1D) materials, where it’s origin has been a long standing and important puzzle.<br/>Many-body interactions occur on extremely short timescales: electron-electron interactions occur on femtoseconds (10<sup>-15 </sup>seconds) or faster, while phonons respond more slowly, within hundreds of femtoseconds. Traditionally, these interactions are probed by changing a material's temperature, pressure, or chemical composition and then measure its electrical properties to learn about the interactions. However, materials that host different interactions can exhibit very similar properties, making it challenging to pinpoint the exact nature of these interactions.<br/>In recent work we showed that by gently exciting a quantum material with an ultrafast laser pulse and probing it with extreme ultraviolet high harmonic pulses, we can measure not only the response times, but also see precisely how the electronic band structure changes. In combination, this makes it possible to identify the many body interactions underlying the ground state of a quantum material and thereby the nature of the pseudogap.[1]<br/>Specifically, we compared how the electrons in two different 1D materials responded after they were gently perturbed by light: (TaSe<sub>4</sub>)<sub>2</sub>I and Rb<sub>0.3</sub>MoO<sub>3</sub>, also known as rubidium blue bronze. Historically, both materials were thought to have a small insulating gap due to the coupling between electrons and phonons, called polarons. However, recent theory suggested that the insulating gap in such materials could be produced by polarons interacting to produce bipolarons. By tracking the energy and location of the excited electrons using time- and angle-resolved photoemission, we could see the signatures of bipolarons melting into single polarons in (TaSe<sub>4</sub>)<sub>2</sub>I.<br/>In contrast, electrons in Rb<sub>0.3</sub>MoO<sub>3</sub> respond and relax ten times faster (in ~60 femtoseconds) to light, clearly showing that interactions between electrons (Luttinger-liquid behavior) must be responsible for the insulating gap in that 1D material.<br/> More generally, ultrafast high harmonic probes provide an exquisite source of short wavelength light, with unprecedented control over the spectral, temporal, polarization and orbital angular momentum of the emitted waveforms, from the UV to the keV photon energy region. They enable powerful new tools for near-perfect x-ray imaging, for coherently manipulating quantum materials using light, and for extracting the functional transport, electronic, magnetic and mechanical properties of ultrathin films and nanosystems.[2-6]<br/>Zhang et al., Nano Letters <b>23</b>, 8392 (2023).<br/>Wang et al., Optica <b>10</b>, 1245 (2023).<br/>McBennett et al., Nano Letters <b>23</b>, 2129 (2023).<br/>Zhang et al., Structural Dynamics <b>9</b>, 014501 (2022).<br/>Tanksalvala et al., Science Advances <b>7</b>, eabd9667 (2021).<br/>Frazer et al., Physical Review Materials <b>4</b>, 073603 (2020).