SIMES Publications

Abstract

Quantum-well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum-well states in fractured single crystals of CeCoIn₅. We confirm that confinement occurs by showing that these states’ binding energies are photon-energy independent and are well described with a phase accumulation model, commonly applied to quantum-well states in thin films. This indicates that atomically flat thin films can be formed by fracturing hard single crystals. For the two samples studied, our observations are explained by free-standing flakes with thicknesses of 206 and 101 Å. We extend our analysis to extract bulk properties of CeCoIn₅. Specifically, we obtain the dispersion of a three-dimensional band near the zone center along in-plane and out-of-plane momenta. We establish part of its Fermi surface, which corresponds to a hole pocket centered at Γ. We also reveal a change of its dispersion with temperature, a signature that may be caused by the Kondo hybridization.

Abstract

Time- and angle-resolved photoemission spectroscopy is a powerful probe of electronic band structures out of equilibrium. Tuning time and energy resolution to suit a particular scientific question has become an increasingly important experimental consideration. Many instruments use cascaded frequency doubling in nonlinear crystals to generate the required ultraviolet probe pulses. We demonstrate how calculations clarify the relationship between laser bandwidth and nonlinear crystal thickness contributing to experimental resolutions and place intrinsic limits on the achievable time-bandwidth product. Experimentally, we tune time and energy resolution by varying the thickness of nonlinear β-BaB₂O₄ crystals for frequency upconversion, providing a flexible experiment design. We achieve time resolutions of 58–103 fs and corresponding energy resolutions of 55–27 meV. We propose a method to select crystal thickness based on desired experimental resolutions.

Abstract

Copper oxide high-T_C superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity^1,2,3,4. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order^{5,6,7,8,9,10,11,12,13,14} in nearly optimally doped Bi₂Sr₂CaCu₂O_8+δ. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.

Abstract

We study high-harmonic generation in two-dimensional electron systems with Rashba and Dresselhaus spin-orbit coupling and derive harmonic generation selection rules with the help of group theory. Based on the band structures of these minimal models and explicit simulations we reveal how the spin-orbit parameters control the cutoff energy in the high-harmonic spectrum. We also show that the magnetic field and polarization dependence of this spectrum provides information on the magnitude of the Rashba and Dresselhaus spin-orbit coupling parameters. The shape of the Fermi surface can be deduced at least qualitatively and if only one type of spin-orbit coupling is present, the coupling strength can be determined.

Abstract

We present a comprehensive study of the frequency-dependent sensitivity for measurements of the AC elastocaloric effect by applying both exactly soluble models and numerical methods to the oscillating heat flow problem. These models reproduce the finer details of the thermal transfer functions observed in experiments, considering here representative data for single-crystal Ba(Fe_1−xCo_x)₂As₂. Based on our results, we propose a set of practical guidelines for experimentalists using this technique. This work establishes a baseline against which the frequency response of the AC elastocaloric technique can be compared and provides intuitive explanations of the detailed structure observed in experiments.

Abstract

We investigate the hypothesis of dynamic tetragonal domains occurring in cubic CH₃NH₃PbI₃using high-resolution neutron spectroscopy to study a fully deuterated single crystal. The R-point scattering above the 327.5 K cubic-tetragonal phase transition is always resolution limited in energy and therefore inconsistent with dynamic-domain predictions. This behavior is instead consistent with the central peak phenomenon observed in other perovskites. The scattering may originate from small, static, tetragonal-phase domains nucleating about crystal defects, and the temperature dependence demonstrates the transition is first order.

Abstract

The rare-earth tritellurides (RTe₃, where R=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y) form a charge density wave state consisting of a single unidirectional charge density wave for lighter R, with a second unidirectional charge density wave, perpendicular and in addition to the first, also present at low temperatures for heavier R. We present a quantum oscillation study in magnetic fields up to 65 T that compares the single charge density wave state with the double charge density wave state both above and below the magnetic breakdown field of the second charge density wave. In the double charge density wave state it is observed that there remain several small, light pockets, with the largest occupying around 0.5% of the Brillouin zone. By applying magnetic fields above the independently determined magnetic breakown field, the quantum oscillation frequencies of the single charge density wave state are recovered, as expected in a magnetic breakdown scenario. Measurements of the electronic effective mass do not show any divergence or significant increase on the pockets of Fermi surface observed here as the putative quantum phase transition between the single and the double charge density wave states is approached.

Abstract

Cryogenic electron microscopy (cryo-EM) was the basis for the 2017 Nobel Prize in Chemistry for its profound impact on the field of structural biology by freezing and stabilizing fragile biomolecules for near atomic-resolution imaging in their native states. Beyond life science, the development of cryo-EM for the physical sciences may offer access to previously inaccessible length scales for materials characterization in systems that would otherwise be too sensitive for high-resolution electron microscopy and spectroscopy. Weakly bonded and reactive materials that typically degrade under electron irradiation and environmental exposure can potentially be stabilized by cryo-EM, opening up exciting opportunities to address many central questions in materials science. New discoveries and fundamental breakthroughs in understanding are likely to follow. In this Perspective, we identify six major areas in materials science that may benefit from the interdisciplinary application of cryo-EM: (1) batteries, (2) soft polymers, (3) metal−organic frameworks, (4) perovskite solar cells, (5) electrocatalysts, and (6) quantum materials. We highlight long-standing questions in each of these areas that cryo-EM can potentially address, which would firmly establish the powerful tool’s broad scope and utility beyond biology.

Abstract

Lithium metal is a promising anode to provide high energy density for next-generation batteries. However, it has not been implemented due to its low cycling efficiency, which results from the formation of an unstable solid electrolyte interphase (SEI). The SEIs formed with traditional liquid electrolytes are heterogeneous and easy to crack during cycling, thus resulting in the formation of dendritic and dead Li, and further devastating the electrode performance. To solve these issues, efforts have been made to replace natural SEIs with artificial SEIs (ASEIs). Here, we discuss critical design principles of ASEIs based on the understanding of SEI failure mechanisms. Three key principles for a successful ASEI are identified: (1) mechanical stability, which can be either high strength or adaptivity, (2) spatially uniform Li⁺ transport with moderate conductivity and even single-ion conduction, and (3) chemical passivation to mitigate Li-electrolyte parasitic reactions. Selected examples of recently developed ASEIs are categorized and elaborated. Finally, future directions are given for ASEI designs.

Abstract

We report in situ structural measurements of shock‐compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X‐ray diffraction to laser‐driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X‐ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post‐perovskite have been indirectly reported in microsecond‐scale plate‐impact experiments. Therefore, we interpret the high‐pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post‐perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO₃ on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments.