SIMES Publications

Abstract

We report the synthesis of millimeter-sized freestanding single-crystal YBa2Cu3O7−x heterostructure membranes with nanoscale thickness. On a SrTiO3 substrate, a heteroepitaxial water-soluble sacrificial layer of Sr3Al2O6 followed by an encapsulated YBa2Cu3O7−x film is grown by pulsed laser deposition. A freestanding YBa2Cu3O7−x heterostructure membrane is obtained by etching the sacrificial layer. Its high crystalline quality is demonstrated via x-ray diffraction, electrical measurements, and scanning transmission electron microscopy.

Abstract

We demonstrate the synthesis of the high-mobility transparent conducting oxide La-doped BaSnO3 as freestanding nanomembranes. Membranes are fabricated by employing water-soluble sacrificial layers (Sr3Al2O6 and Ba3Al2O6 ) which are epitaxially grown on SrTiO3(001) substrates by pulsed laser deposition. We find a high crack density for membranes released using Sr3Al2O6 . However, the use of lattice-matched Ba3Al2O6 leads to large-area single crystal membranes with high electron mobilities. These results suggest the importance of lattice mismatch and internal strain within the sacrificial layer as important parameters for producing millimeter scale perovskite oxide nanomembranes with a high degree of crystallinity.

Abstract

Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO3 films grown epitaxially on SrTiO3 growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.

Abstract

We study the microscopic origins of photocurrent generation in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

Abstract

Time-resolved measurements of materials provide a wealth of information on quasiparticle dynamics, and have been the focus of optical studies for decades. In this paper, we develop a theory for explicitly evaluating time-resolved resonant inelastic x-ray scattering (tr-RIXS). We apply the theory to a noninteracting electronic system and reveal the particle-hole spectrum and its evolution during the pump pulse. With a high-frequency pump, the frequency and amplitude dependence analysis of the spectra agrees well with the steady-state assumptions and Floquet excitations. When the pump frequency is low, the spectrum extracts real-time dynamics of the particle-hole continuum in momentum space. These results verify the correctness of our theory and demonstrate the breadth of physical problems that tr-RIXS could shed light on.

Abstract

Using Ba(Fe0.975Co0.025)2As2 as an exemplar material exhibiting second-order electronic-nematic and antiferromagnetic transitions, we present measurements that reveal anomalies in the elastoresistance (∂ρij∂ɛkl) and elastocaloric effect (∂T∂ɛkl) at both phase transitions for induced strains ɛkl that do not share the symmetry of either order parameter. Both effects are understood to arise from the effect of strain on the transition temperatures; in the region close to the phase transitions this leads to (1) similarity between the strain and temperature derivatives of the resistivity and (2) similarity between the elastocaloric effect and the singular part of the specific heat. These mechanisms for elastoresistance and elastocaloric effect should be anticipated for any material in which mechanical deformation changes the transition temperature. Furthermore, these measurements provide evidence that the Fisher-Langer relation ρ(c)∝U(c) between the scattering from critical degrees of freedom and their energy density, respectively, holds near each of the transitions in the material studied under varying strain as it does for varying temperature.

Abstract

Fermi surface (FS) topology is a fundamental property of metals and superconductors. In electron-doped cuprate Nd2−xCexCuO4 (NCCO), an unexpected FS reconstruction has been observed in optimal- and overdoped regime (x = 0.15–0.17) by quantum oscillation measurements (QOM). This is all the more puzzling because neutron scattering suggests that the antiferromagnetic (AFM) long-range order, which is believed to reconstruct the FS, vanishes before x = 0.14. To reconcile the conflict, a widely discussed external magnetic-field–induced AFM long-range order in QOM explains the FS reconstruction as an extrinsic property. Here, we report angle-resolved photoemission (ARPES) evidence of FS reconstruction in optimal- and overdoped NCCO. The observed FSs are in quantitative agreement with QOM, suggesting an intrinsic FS reconstruction without field. This reconstructed FS, despite its importance as a basis to understand electron-doped cuprates, cannot be explained under the traditional scheme. Furthermore, the energy gap of the reconstruction decreases rapidly near x = 0.17 like an order parameter, echoing the quantum critical doping in transport. The totality of the data points to a mysterious order between x = 0.14 and 0.17, whose appearance favors the FS reconstruction and disappearance defines the quantum critical doping. A recent topological proposal provides an ansatz for its origin.

Abstract

Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

Abstract

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2−xIr1−ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with yupon cycling, we reveal that this structure–redox coupling arises from the local stabilization of short approximately 1.8 Å metal–oxygen π bonds and approximately 1.4 Å O–O dimers during oxygen redox, which occurs in Li2−xIr1−ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.

Abstract

We study the band structure of twinned and detwinned BaFe2As2 using angle-resolved photoemission spectroscopy. The combination of measurements in the ordered and normal states along four high-symmetry momentum directions Γ/Z−X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the dxz and dyzorbitals as well as folding due to magnetic order with a wave vector of (π,π,π). We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high-quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe2As2.