Research
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Summary of Research Thrusts
Broadly speaking, there are three main areas of research being pursued at present: Mott metal-insulator transition and high-temperature superconductivity; equipment development; and spectroscopy with photons.
1. Our hypothesis is that the Mott metal-insulator transition occurs in hydrogen-terminated diamond (HTD). We aim to investigate this phenomenon in HTD with near-field infrared nanospectroscopy. Another project in this area is on doping copper and sulfur into lead apatite to induce a metal-insulator transition and possibly high-temperature superconductivity.
2. Equipment development: in experimental physics, it is important to develop experimental capabilities that allow spectroscopy measurements under previously unexplored conditions that are challenging to attain.
a) We have developed in-house, cryogenic capabilities for infrared transmission and reflectance measurements with the Bruker 80v spectrometer and for ellipsometry measurements with the Woollam variable angle spectrometer. Many phase transitions occur below room temperature and require measurements down to liquid He-4 temperatures in ultra-high vacuum.
b) We have developed micro-ellipsometry in the spectral range between 0.6 eV and 6.5 eV with a spot-size of about 100 μm.
c) We have invented a current-sustained plasma lamp for intense infrared radiation for scattering-type scanning near-field infrared microscopy. See our published article in Optics Express 25, 20421-20430 (2017). This optical technique allows ultra-broadband nano-spectroscopy measurements with ~ 10 nm spatial resolution. A U.S. Patent was granted for this invention (Inventors: M. M. Qazilbash and D. J. Lahneman, “Infrared light generating system”, U.S. Patent No. 9,934,927. Issued: April 2018). We have optimized the plasma lamp for more efficient data acquisition by using a near-infrared laser to sustain the plasma (see our paper "Hyperspectral infrared imaging of surface phonon-polaritons in SrTiO3”, Physical Review B 104, 235433 (2021).
3. Spectroscopy with infrared, visible and ultraviolet photons in condensed phases of matter. In principle, photon spectroscopy can be used to measure most known materials. Presently, there is considerable interest in the study of solids in which there are strong interactions between a large number of quantum enitities. Specifically, strong electron-electron interactions in the presence of orbital, spin, and lattice degrees of freedom lead to emergent phenomena. Broadly speaking, emergence is linked to self-organization and phase transitions in many-particle systems. Because photons strongly couple to charged particles, they can be used to probe the motion of electrons and lattice dynamics, thereby providing insight into the interactions that lead to emergent phenomena in complex materials.
Highlights of past research results
1. The metal-insulator transition in vanadium dioxide
An interesting application of broadband infrared nano-spectroscopy is the investigation of the strongly correlated metal that exists in the heterogeneous state in the vicinity of the metal-insulator transition (MIT) in vanadium dioxide (VO2). VO2 undergoes the MIT at Tc ~ 340 K accompanied by a structural transformation from a monoclinic (M1) unit cell in the low-temperature insulating phase to tetragonal (rutile) symmetry in the metallic phase. VO2 has been studied extensively for two main reasons. First, the driving mechanism of the MIT has remained controversial and many experimental and theoretical studies have focused on understanding it. The controversy is mainly related to the relative importance of the electron-electron and electron-phonon interactions in VO2. Second, VO2 has immense potential for applications because the phase transition occurs only 40 K above room temperature and involves dramatic changes in transport, infrared, and optical properties. Moreover, the phase transition can also be triggered electrically and optically leading to new electronic and optical devices. The MIT in VO2 has been studied previously by far-field infrared and THz techniques. In past work, we were able to combine far-field infrared spectroscopy with near-field infrared imaging to obtain the charge dynamics of the emergent metallic state near the MIT. The evolution of the thermally induced MIT in a thin film of VO2 was imaged with a near-field infrared microscope. Infrared images obtained at closely-spaced temperature intervals showed that the VO2 film underwent a percolative transition with increasing temperature across the MIT as shown in the figure below.
4 μm x 4 μm size images of the infrared near-field scattering amplitude obtained by scattering scanning near-field optical microscope (s-SNOM) operating at the infrared frequency ω = 930 cm-1. These images are displayed for representative temperatures in the insulator-to-metal transition regime of VO2 to show percolation in progress with increasing temperature. The metallic regions (light blue, green and red colors) give higher scattering near-field amplitude compared to the insulating phase (dark blue color). The color bar represents the variation in the scattering amplitude in relative units. Adapted from M. M. Qazilbash et al., Science 318, 1750 (2007).
Our recent work (PRB 97, 085146 (2018)) shows that stochasticity at nanometer length scales is completely suppressed in the thermally driven MIT in sputtered, polycrystalline VO2 films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way (see figure below). The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in VO2 and similar phase-change materials.
Near-field infrared amplitude images of the same region at different temperatures are displayed. Higher infrared amplitude corresponds to metallic regions, while lower signals correspond to insulating regions. The signals are normalized to the average signal of the completely insulating 329-K image (not shown). (a) and (b) show separate heating runs. (c) and (d) show separate cooling runs.
2. The metal-insulator transition in manganites
Manganites (oxides of manganese with perovskite crystal structure) are best known for colossal magnetoresistive effects . However, in addition to the magnetic phase transitions, manganites also exhibit metal-insulator transitions. The metal-insulator transitions in some manganites (e.g. La0.67Sa0.33MnO3) are linked to the ferromagnetic transition. Charge, spin, orbital, and lattice degrees of freedom are active in the manganites leading to a multiplicity of phases and competing ground states.
We have performed near-field infrared microscopy on a thin La0.67Sa0.33MnO3 (LSMO) film near the phase transition regime. In a recently published work (Journal of Physics: Condensed Matter 30 025602 (2018)) , we find that the near-field infrared data is consistent with the bulk of the LSMO film undergoing the thermally-driven nonpercolative second-order transition from a metallic, ferromagnetic phase to an insulating, paramagnetic phase. We find persistent infrared contrast on the nanoscale that is independent of temperature and which we attribute to two novel phases with different conductivities coexisting in the vicinity of the film-substrate interface. These two coexisting phases at the film-substrate interface do not undergo the metal–insulator transition (MIT) and hence are different from the metallic, ferromagnetic and insulating, paramagnetic phases in the bulk of the film.
Near-field infrared amplitude scans of the La0.67Sr0.33MnO3 film at different temperatures. Scans are carried out in the same spatial area of 2.84 μm ×2.84 μm at different temperatures between 296 K and 330 K during a heating run. All scans are displayed using a common color scale in units of volts. The photodetector outputs a voltage signal that is a measure of the near-field infrared amplitude.
3. Infrared spectroscopy of the iron-based superconducting and spin-density wave systems
The iron-based pnictides/chalcogenides are a large new class of materials that exhibit high temperature superconductivity. Unlike the cuprates in which the parent material is a Mott insulator, in the 122-iron-arsenides for example, the ground state of the parent material is an anti-ferromagnetically ordered metal. At high temperatures, the parent compound RFe2As2 (R = Ba, Sr, Ca) is a tetragonal, paramagnetic metal with spin correlations. Upon cooling, it undergoes nearly simultaneous structural and magnetic phase transitions at Tc ~ 150 K to an orthorhombic, anti-ferromagnetically ordered metal. Upon electron doping with cobalt and hole doping with potassium in BaFe2As2, for example, the magnetic ordering and structural instability are suppressed, leading to a paramagnetic metallic phase that is susceptible to high temperature superconductivity.
Rare-earth doped CaFe2As2 undergoes several phase transitions. For high enough Nd and Pr substituents, the anti-ferromagnetic order is quenched. The resulting tetragonal, paramagnetic phase undergoes a first-order structural collapse at ~ 70 K and upon further cooling exhibits superconductivity at ~ 45 K, the highest superconducting transition temperature in the 122-iron-arsenide system albeit with a small volume fraction. The Ca1-xLaxFe2As2 material shows a different behavior compared to the Pr- and Nd-doped samples. In Ca1-xLaxFe2As2, the anti-ferromagnetism is quenched with increasing La doping but no structural change to a collapsed tetragonal (cT) phase occurs. We have investigated these exotic rare-earth doped iron-arsenide materials to gain insight into the occurrence of the cT phase transition. Please refer to our published work in PRB 94, 064514 (2016).
In a recent work (PRB 98, 224505 (2018)), we discover that the superconductor BaFe1.9Pt0.1As2 has fully gapped (nodeless) Fermi surfaces, and we observe the strong-coupling electron-boson interaction features in the infrared absorption spectra (see figure below). Through modeling with the Eliashberg function based on Eliashberg theory, we obtain a good quantitative description of the energy gaps and the strong-coupling features. The full Eliashberg equations are solved to check the self-consistency of the electron-boson coupling spectrum, the largest energy gap, and the transition temperature (Tc). Our experimental data and analysis provide compelling evidence that superconductivity in BaFe1.9Pt0.1As2 is induced by the coupling of electrons to a low-energy bosonic mode that does not originate solely from phonons.
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