Showing 1–46 of 46 results for author: Ellis, D

Crystallinity Evolution of MOCVD-Grown $β$-Ga$_2$O$_3$ Films Probed by In Situ HT-XRD under Different Reactor Heights

Authors: Imteaz Rahaman, Botong Li, Bobby G. Duersch, Hunter D. Ellis, Kathy Anderson, Kai Fu

Abstract: The crystallinity of $β$-Ga$_2$O$_3$ thin films grown by metal-organic chemical vapor deposition (MOCVD) is strongly influenced by reactor design and the resulting growth environment. In this work, we investigate the role of reactor height on the crystallinity evolution of MOCVD-grown $β$-Ga$_2$O$_3$ films by directly comparing long- and short-chamber showerhead configurations. Structural evolutio… ▽ More The crystallinity of $β$-Ga$_2$O$_3$ thin films grown by metal-organic chemical vapor deposition (MOCVD) is strongly influenced by reactor design and the resulting growth environment. In this work, we investigate the role of reactor height on the crystallinity evolution of MOCVD-grown $β$-Ga$_2$O$_3$ films by directly comparing long- and short-chamber showerhead configurations. Structural evolution was probed by in situ high-temperature X-ray diffraction (HT-XRD) as the MOCVD-grown films were heated from 25~$^\circ$C to 1100~$^\circ$C. Temperature-dependent XRD reveals a consistent redshift of the $β$-Ga$_2$O$_3$~($-201$) reflection after HT-XRD heating and subsequent cooling to room temperature for both reactor geometries, indicating a similar thermally driven strain response. Quantitative rocking-curve analysis shows a non-monotonic temperature dependence of the ($-201$) full width at half maximum (FWHM), with minimum values of approximately 2.03$^\circ$ and 2.72$^\circ$ for the short- and long-chamber films, respectively, reflecting differences in mosaic alignment established during growth. Atomic force microscopy further shows that short-chamber-grown films exhibit smoother surfaces, with root-mean-square roughness values of approximately 7.7~nm before and 7.3~nm after HT-XRD heating, compared to 19.3~nm and 12.3~nm, respectively, for long-chamber-grown films. Overall, these results indicate that reactor height influences the initial crystalline and morphological templates of $β$-Ga$_2$O$_3$ films and modulates their elevated-temperature structural response, providing practical insights for optimizing MOCVD reactor design for high-quality $β$-Ga$_2$O$_3$ growth. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 20 pages, 5 figures

Dosimetry for Proton Therapy Using a β-Ga$_2$O$_3$ Metal-Semiconductor-Metal Detector with Low-Noise Amplification

Authors: Hunter D. Ellis, Ajayvarman Mallapillai, Jared Miller, Imteaz Rahaman, Botong Li, Vikren Sarkar, Kai Fu

Abstract: Intensity-modulated proton therapy (IMPT) employs proton radiation rather than conventional X-rays to treat cancerous tumors. This approach offers significant advantages by minimizing the radiation exposure of surrounding healthy tissue, leading to improved patient outcomes and reduced side effects compared to traditional X-ray therapy. To ensure patient safety, each treatment plan must be experim… ▽ More Intensity-modulated proton therapy (IMPT) employs proton radiation rather than conventional X-rays to treat cancerous tumors. This approach offers significant advantages by minimizing the radiation exposure of surrounding healthy tissue, leading to improved patient outcomes and reduced side effects compared to traditional X-ray therapy. To ensure patient safety, each treatment plan must be experimentally validated before clinical implementation. However, current dosimetry devices face limitations in performing angled beam measurements and obtaining multi-depth assessments, both of which are essential for verifying IMPT treatment plans. In this study, the performance of a β-Ga$_2$O$_3$-based metal-semiconductor-metal (MSM) detector with a low-noise amplifier is studied and evaluated under various proton radiation doses and energy levels delivered by a MEVION S250i proton accelerator. The detector performance is also compared with that of an ionization chamber. The β-Ga$_2$O$_3$ detector exhibits a linear response with proton dose for single-spot irradiations, and its response to varying proton energies closely matches both the ion chamber data and simulated dose distributions. These findings highlight the potential of β-Ga$_2$O$_3$-based detectors as robust dosimetry devices for IMPT applications. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Comments: 7 pages, 4 figures

High thermal conductivity of rutile-GeO$_2$ films grown by MOCVD: $52.9~\mathrm{W\,m^{-1}\,K^{-1}}$

Authors: Imteaz Rahaman, Michael E. Liao, Ziqi Wang, Eugene Y. Kwon, Rui Sun, Botong Li, Hunter D. Ellis, Bobby G. Duersch, Dali Sun, Jun Liu, Mark S. Goorsky, Michael A. Scarpulla, Kai Fu

Abstract: Rutile germanium dioxide (r-GeO2) has recently emerged as a promising ultrawide-bandgap (UWBG) semiconductor owing to its wide bandgap (~4.4-5.1 eV), ambipolar doping potential, and high theoretical thermal conductivity. However, experimental data on the thermal conductivity of r-GeO2 epitaxial layers have not been reported, primarily due to challenges in phase control and surface roughness. Here,… ▽ More Rutile germanium dioxide (r-GeO2) has recently emerged as a promising ultrawide-bandgap (UWBG) semiconductor owing to its wide bandgap (~4.4-5.1 eV), ambipolar doping potential, and high theoretical thermal conductivity. However, experimental data on the thermal conductivity of r-GeO2 epitaxial layers have not been reported, primarily due to challenges in phase control and surface roughness. Here, we report a high thermal conductivity of 52.9 +/- 6.6 W m^-1 K^-1 for high-quality (002) r-GeO2 films grown by metal-organic chemical vapor deposition (MOCVD) and characterized using time-domain thermoreflectance (TDTR). The phase control was achieved through a seed-driven stepwise crystallization (SDSC) approach, and the surface roughness was significantly reduced from 76 nm to 16 nm (locally as low as 1 A) via chemical mechanical polishing (CMP). These results highlight the promise of r-GeO2 as a UWBG oxide platform for power electronics applications. △ Less

Submitted 31 October, 2025; originally announced October 2025.

Comments: 17 pages, 4 figures

$β$-Ga$_2$O$_3$--Based Radiation Detector for Proton Therapy

Authors: Hunter D. Ellis, Imteaz Rahaman, Apostoli Hillas, Botong Li, Vikren Sarkar, Kai Fu

Abstract: Intensity modulated proton therapy (IMPT) is an advanced cancer treatment modality that offers significant advantages over conventional X-ray therapies, particularly in its ability to minimize radiation dose beyond the tumor target. This reduction in unnecessary irradiation exposure significantly lowers the risk to surrounding healthy tissue and reduces side effects compared to conventional X-ray… ▽ More Intensity modulated proton therapy (IMPT) is an advanced cancer treatment modality that offers significant advantages over conventional X-ray therapies, particularly in its ability to minimize radiation dose beyond the tumor target. This reduction in unnecessary irradiation exposure significantly lowers the risk to surrounding healthy tissue and reduces side effects compared to conventional X-ray treatments. However, due to the high complexity of IMPT plans, each plan must be independently validated to ensure the safety and efficacy of the radiation exposure to the patient. While ion chambers are currently used for this purpose, their limitations-particularly in angled-beam measurements and multi-depth assessments-hinder their effectiveness. Silicon-based detectors, commonly used in X-ray therapy, are unsuitable for IMPT due to their rapid degradation under proton irradiation. In this study, a $β$-Ga$_2$O$_3$-based metal-semiconductor-metal (MSM) detector was evaluated and compared with a commercial ion chamber using a MEVION S250i proton accelerator. The $β$-Ga$_2$O$_3$ detector demonstrated reliable detection of single-pulse proton doses as low as 0.26 MU and exhibited a linear charge-to-dose relationship across a wide range of irradiation conditions. Furthermore, its measurement variability was comparable to that of the ion chamber, with improved sensitivity observed at higher bias voltages. These results highlight the strong potential of $β$-Ga$_2$O$_3$ as a radiation-hard detector material for accurate dose verification in IMPT. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Comments: 16 page, 5 figures

Study of Optical Properties of MOCVD-Grown Rutile GeO2 Films

Authors: Imteaz Rahaman, Anthony Bolda, Botong Li, Hunter D. Ellis, Kai Fu

Abstract: Rutile germanium dioxide (r-GeO$_2$) is a promising ultra-wide bandgap (UWBG) semiconductor, offering a high theoretical Baliga figure of merit, potential for p-type doping, and favorable thermal and electrical properties. In this work, we present a comprehensive optical investigation of crystalline r-GeO$_2$ thin films grown on r-TiO$_2$ (001) substrates via metal-organic chemical vapor depositio… ▽ More Rutile germanium dioxide (r-GeO$_2$) is a promising ultra-wide bandgap (UWBG) semiconductor, offering a high theoretical Baliga figure of merit, potential for p-type doping, and favorable thermal and electrical properties. In this work, we present a comprehensive optical investigation of crystalline r-GeO$_2$ thin films grown on r-TiO$_2$ (001) substrates via metal-organic chemical vapor deposition (MOCVD). Cathodoluminescence (CL) spectroscopy reveals broad visible emissions with distinct peaks near 470~nm and 520~nm. CL mapping indicates enhanced emission intensity in regions with larger crystalline domains, highlighting the correlation between domain size and optical quality. X-ray photoelectron spectroscopy (XPS) confirms the presence of Ge$^{4+}$ oxidation state and provides a bandgap estimation of $\sim$4.75~eV based on valence band and secondary electron cutoff analysis. UV--Vis transmittance measurements show a sharp absorption edge near 250--260~nm, corresponding to an optical bandgap in the range of 4.81--5.0~eV. These findings offer valuable insights into the defect-related emission behavior and band-edge characteristics of r-GeO$_2$, reinforcing its potential for future applications in power electronics and deep-ultraviolet optoelectronic devices. △ Less

Submitted 2 August, 2025; originally announced August 2025.

Comments: 18 pages, 5 figures

Phase Competition and Rutile Phase Stabilization of Growing GeO2 Films by MOCVD

Authors: Imteaz Rahaman, Botong Li, Hunter D. Ellis, Kathy Anderson, Feng Liu, Michael A. Scarpulla, Kai Fu

Abstract: Rutile germanium dioxide (r-GeO2) is an ultra-wide bandgap semiconductor with potential for ambipolar doping, making it a promising candidate for next-generation power electronics and optoelectronics. Growth of phase-pure r-GeO2 films by vapor phase techniques like metalorganic chemical vapor deposition (MOCVD) is challenging because of polymorphic competition from amorphous and quartz GeO2. Here,… ▽ More Rutile germanium dioxide (r-GeO2) is an ultra-wide bandgap semiconductor with potential for ambipolar doping, making it a promising candidate for next-generation power electronics and optoelectronics. Growth of phase-pure r-GeO2 films by vapor phase techniques like metalorganic chemical vapor deposition (MOCVD) is challenging because of polymorphic competition from amorphous and quartz GeO2. Here, we introduce seed-driven stepwise crystallization (SDSC) as a segmented growth strategy for obtaining r-GeO2 films on r-TiO2 (001) substrate. SDSC divides the growth into repeated cycles of film deposition and cooling-heating ramps, which suppress the non-rutile phases. We discuss the underlying mechanisms of phase selection during SDSC growth. We demonstrate continuous, phase-pure, partially epitaxial r-GeO2 (001) films exhibiting x-ray rocking curves with a FWHM of 597 arcsec. SDSC-based growth provides a generalizable pathway for selective vapor-phase growth of metastable or unstable phases, offering new opportunities for phase-selective thin-film engineering. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Carbon-Nanotube/$β$-Ga$_2$O$_3$ Heterojunction PIN Diodes

Authors: Hunter D. Ellis, Botong Li, Haoyu Xie, Jichao Fan, Imteaz Rahaman, Weilu Gao, Kai Fu

Abstract: $β$-Ga$_2$O$_3… ▽ More $β$-Ga$_2$O$_3$ is gaining attention as a promising semiconductor for next-generation high-power, high-efficiency, and high-temperature electronic devices, thanks to its exceptional material properties. However, challenges such as the lack of viable p-type doping have hindered its full potential, particularly in the development of ambipolar devices. This work introduces a novel heterojunction diode (HD) that combines p-type carbon nanotubes (CNTs) with i/n-type $β$-Ga$_2$O$_3$ to overcome these limitations. For the first time, a CNT/$β$-Ga$_2$O$_3$ hetero-p-n-junction diode is fabricated. Compared to a traditional Schottky barrier diode (SBD) with the same $β$-Ga$_2$O$_3$ epilayer, the CNT/$β$-Ga$_2$O$_3$ HD demonstrates significant improvements, including a higher rectifying ratio ($1.2 \times 10^{11}$), a larger turn-on voltage (1.96 V), a drastically reduced leakage current at temperatures up to 300 °C, and a 26.7% increase in breakdown voltage. Notably, the CNT/$β$-Ga$_2$O$_3$ HD exhibits a low ideality factor of 1.02, signifying an ideal interface between the materials. These results underline the potential of CNT/$β$-Ga$_2$O$_3$ heterojunctions for electronic applications, offering a promising solution to current limitations in $β$-Ga$_2$O$_3$-based devices. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Comments: 17 pages, 5 figures

Seed-Driven Stepwise Crystallization (SDSC) for Growing Rutile GeO2 Films via MOCVD

Authors: Imteaz Rahaman, Botong Li, Bobby Duersch, Hunter D. Ellis, Kai Fu

Abstract: Germanium dioxide (r-GeO2) is an emerging new ultrawide bandgap (UWBG) semiconductor with significant potential for power electronics, thanks to its large-size substrate compatibility and ambipolar doping capability. However, phase segregation during metal-organic chemical vapor deposition (MOCVD) on substrates like r-TiO2 has posed a significant barrier to achieving high-quality films. Convention… ▽ More Germanium dioxide (r-GeO2) is an emerging new ultrawide bandgap (UWBG) semiconductor with significant potential for power electronics, thanks to its large-size substrate compatibility and ambipolar doping capability. However, phase segregation during metal-organic chemical vapor deposition (MOCVD) on substrates like r-TiO2 has posed a significant barrier to achieving high-quality films. Conventional optimization of growth parameters has been found so far not very insufficient in film coverage and film quality. To address this, a seed-driven stepwise crystallization (SDSC) growth approach was employed in this study, featuring multiple sequential deposition steps on a pre-templated substrate enriched with r-GeO2 seeds. The process began with an initial 180-minute deposition to establish r-GeO2 nucleation seeds, followed by a sequence of shorter deposition steps (90, 60, 60, 60, 60, and 60 minutes). This stepwise growth strategy progressively increased the crystalline coverage to 57.4%, 77.49%, 79.73%, 93.27%, 99.17%, and ultimately 100%. Concurrently, the crystalline quality improved substantially, evidenced by a ~30% reduction in the Full Width at Half Maximum (FWHM) of X-ray diffraction rocking curves. These findings demonstrate the potential of the SDSC approach for overcoming phase segregation and achieving high-quality, large-area r-GeO2 films. △ Less

Submitted 10 June, 2025; v1 submitted 26 December, 2024; originally announced December 2024.

Comments: 17 pages, 6 figures, 1 Table

A TEM Study of MOCVD-Grown Rutile GeO2 Films

Authors: Imteaz Rahaman, Botong Li, Hunter D. Ellis, Brian Roy Van Devener, Randy C Polson, Kai Fu

Abstract: Ultrawide bandgap (UWBG) semiconductors are promising for next-generation power electronics, largely attributed to their substantial bandgap and exceptional breakdown electric field. Rutile GeO2 (r-GeO2) emerges as a promising alternative, particularly because of its ambipolar dopability. However, research on r-GeO2 is still in its infancy, and further investigation into its structural properties… ▽ More Ultrawide bandgap (UWBG) semiconductors are promising for next-generation power electronics, largely attributed to their substantial bandgap and exceptional breakdown electric field. Rutile GeO2 (r-GeO2) emerges as a promising alternative, particularly because of its ambipolar dopability. However, research on r-GeO2 is still in its infancy, and further investigation into its structural properties is essential for enhancing epilayer quality. In our previous work, we identified distinct surface morphologies; square-patterned and smooth regions of epitaxial r-GeO2 films grown on r-TiO2 (001) substrates using metal-organic chemical vapor deposition (MOCVD).This research employs transmission electron microscopy (TEM) to investigate the structural characteristics of the material. The findings indicate that the square-patterned regions are crystalline, whereas the smooth regions exhibit amorphous properties. The measured lattice spacing in the (110) plane is 0.324 nm, slightly exceeding the theoretical value of 0.312 nm. This discrepancy suggests the presence of tensile strain in the r-GeO2 film, resulting from lattice mismatch or thermal expansion differences with the substrate. We also observed a threading dislocation density of 1.83*10^9 cm-2, consisting of 11.76% screw-type, 29.41% edge-type, 55.89% mixed-type dislocations, and 2.94% planar defects. These findings offer valuable insights into the growth mechanisms and defect characteristics of r-GeO2. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Comments: 19 pages, 5 figures

Epitaxial Growth of Rutile GeO$_2$ via MOCVD

Authors: Imteaz Rahaman, Bobby Duersch, Hunter D. Ellis, Michael A. Scarpulla, Kai Fu

Abstract: Rutile Germanium Dioxide (r-GeO$_2$) has been identified as an ultrawide bandgap (UWBG) semiconductor recently, featuring a bandgap of 4.68 eV, comparable to Ga$_2$O$_3$ but offering bipolar dopability, higher electron mobility, higher thermal conductivity, and higher Baliga's figure of merit (BFOM).These superior properties position GeO$_2$ as a promising material for various semiconductor applic… ▽ More Rutile Germanium Dioxide (r-GeO$_2$) has been identified as an ultrawide bandgap (UWBG) semiconductor recently, featuring a bandgap of 4.68 eV, comparable to Ga$_2$O$_3$ but offering bipolar dopability, higher electron mobility, higher thermal conductivity, and higher Baliga's figure of merit (BFOM).These superior properties position GeO$_2$ as a promising material for various semiconductor applications. However, the epitaxial growth of r-GeO$_2$, particularly in its most advantageous rutile polymorph, is still at an early stage. This work explores the growth of r-GeO$_2$ using metal-organic chemical vapor deposition (MOCVD) on an r-TiO$_2$ (001) substrate, utilizing tetraethyl germane (TEGe) as the precursor. Our investigations reveal that higher growth temperatures significantly enhance crystalline quality, achieving a full width at half maximum (FWHM) of 0.181 degree at 925 degree C, compared to 0.54 degree at 840 degree C and amorphous structures at 725 degree C. Additionally, we found that longer growth durations increase surface roughness due to the formation of faceted crystals. Meanwhile, adjusting the susceptor rotation speed from 300 RPM to 170 RPM plays a crucial role in optimizing crystalline quality, effectively reducing surface roughness by approximately 15 times. This study offers a foundational guide for optimizing MOCVD growth conditions of r-GeO$_2$ films, emphasizing the crucial need for precise control over deposition temperature and rotation speed to enhance adatom mobility and effectively minimize the boundary layer thickness. △ Less

Submitted 2 July, 2024; originally announced July 2024.

Comments: 20 pages, 5 figures, 3 tables

Growth of GeO2 on R-plane and C-plane Sapphires by MOCVD

Authors: Imteaz Rahaman, Hunter D. Ellis, Kathy Anderson, Michael A. Scarpulla, Kai Fu

Abstract: Rutile Germanium Dioxide (GeO2) has been recently theoretically identified as an ultrawide bandgap (UWBG) semiconductor with bandgap 4.68 eV similar to Ga2O3 but having bipolar dopability and ~2x higher electron mobility, Baliga figure of merit (BFOM) and thermal conductivity than Ga2O3. Bulk crystal growth is rapidly moving towards making large sized native substrates available. These outstanding… ▽ More Rutile Germanium Dioxide (GeO2) has been recently theoretically identified as an ultrawide bandgap (UWBG) semiconductor with bandgap 4.68 eV similar to Ga2O3 but having bipolar dopability and ~2x higher electron mobility, Baliga figure of merit (BFOM) and thermal conductivity than Ga2O3. Bulk crystal growth is rapidly moving towards making large sized native substrates available. These outstanding material properties position GeO2 as a highly attractive UWBG semiconductor for various applications. However, the epitaxial growth in the most advantageous polymorph (rutile), ensuring controlled phase, pristine surface/interface quality, precise microstructure, and optimal functional properties, is still in its infancy. In this work, we explored growth of GeO2 by metal-organic chemical vapor deposition (MOCVD) on both C- and R-plane sapphire. Utilizing tetramethylgermane (TMGe) as a precursor, we have investigated the influences of different parameters on the film properties, including growth temperature, chamber pressure, TMGe flow rate, oxygen flow rate, shroud gas flow rate, and rotation speed. The total pressure emerged as a crucial parameter while growth attempts at low total pressure resulted in no films for a wide range of temperatures, precursor flow rate, argon flow rates, and susceptor rotation rate. A phase diagram, derived from our experimental findings, delineates the growth windows for GeO2 films on sapphire substrates. This study serves as a pioneering guide for the MOCVD growth of GeO2 films. △ Less

Submitted 13 May, 2024; originally announced May 2024.

Comments: 22 pages, 14 Figures

Robust Diamond/\b{eta}-Ga2O3 Hetero-p-n-junction Via Mechanically Integrating Their Building Blocks

Authors: Imteaz Rahaman, Hunter D. Ellis, Kai Fu

Abstract: We report a novel approach for crafting robust diamond/\b{eta}-Ga2O3 hetero-p-n-junctions through the mechanical integration of their bulk materials. This resulting heterojunction, with a turn-on voltage of ~2.7 V at room temperature, exhibits resilient electrical performance across a temperature spectrum up to 125°C, displaying minimal hysteresis-measuring as low as 0.2 V at room temperature and… ▽ More We report a novel approach for crafting robust diamond/\b{eta}-Ga2O3 hetero-p-n-junctions through the mechanical integration of their bulk materials. This resulting heterojunction, with a turn-on voltage of ~2.7 V at room temperature, exhibits resilient electrical performance across a temperature spectrum up to 125°C, displaying minimal hysteresis-measuring as low as 0.2 V at room temperature and below 0.7 V at 125°C. Remarkably, the ideality factor achieves a record low value of 1.28, setting a new benchmark for diamond/ \b{eta}-Ga2O3 heterojunctions. The rectification ratio reaches over 10^8 at different temperatures. This effortlessly fabricated and remarkably resilient diamond/Ga2O3 hetero-p-n-junction pioneers a novel pathway for the exploration and fabrication of heterojunctions for ultra-wide bandgap semiconductors with substantial lattice mismatch and different thermal expansion coefficients. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 13 pages, 4 figures, 1 table, journal and this draft has been submitted to 'Applied Physics Letters'

Electronic Excitations of Hematite Heteroepitaxial Films Measured by Resonant Inelastic X-Ray Scattering at the Fe L-edge

Authors: David S. Ellis, Ru-Pan Wang, Deniz Wong, Jason K. Cooper, Christian Schulz, Yi-De Chuang, Yifat Piekner, Daniel A. Grave, Markus Schleuning, Dennis Friedrich, Frank M. F. de Groot, Avner Rothschild

Abstract: Resonant Inelastic X-Ray Scattering (RIXS) spectra of hematite were measured at the Fe L3-edge for heteroepitaxial thin films which were undoped and doped with 1% Ti, Sn or Zn, in the energy loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers (q), conducted at both low temperature (T=14K) and room temperature. While we can not rule… ▽ More Resonant Inelastic X-Ray Scattering (RIXS) spectra of hematite were measured at the Fe L3-edge for heteroepitaxial thin films which were undoped and doped with 1% Ti, Sn or Zn, in the energy loss range in excess of 1 eV to study electronic transitions. The spectra were measured for several momentum transfers (q), conducted at both low temperature (T=14K) and room temperature. While we can not rule out dispersive features possibly owing to propagating excitations, the coarse envelopes of the general spectra did not appreciably change shape with q, implying that the bulk of the observed L-edge RIXS intensity originates from (mostly) non-dispersive ligand field (LF) excitations. Summing the RIXS spectra over q and comparing the results at T=14 K to those at T=300 K, revealed pronounced temperature effects, including an intensity change and energy shift of the 1.4 eV peak, a broadband intensity increase of the 3-4 eV range, and higher energy features. The q-summed spectra and their temperature dependences are virtually identical for nearly all of the samples with different dopants, save for the temperature dependence of the Ti-doped sample's spectrum, which we attribute to being affected by a large number of free charge carriers. Comparing with magnetization measurements for different temperatures and dopings likewise did not show a clear correlation between the RIXS spectra and the magnetic ordering states. To clarify the excited states, we performed spin multiplet calculations which were in excellent agreement with the RIXS spectra over a wide energy range and provide detailed electronic descriptions of the excited states. The implications of these findings to the photoconversion efficiency of hematite photoanodes is discussed. △ Less

Submitted 15 February, 2022; originally announced February 2022.

Comments: 11 pages, 5 figures, and .pdf supplementary file

Journal ref: Phys. Rev. B 105, 075101 (2022)

The rust challenge -- On the correlations between electronic structure, excited state dynamics and photoelectrochemical performance of hematite photoanodes for solar water splitting

Authors: Daniel A. Grave, Natav Yatom, David S. Ellis, Maytal Caspary Toroker, Avner Rothschild

Abstract: In recent years, hematite potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics and charge transfer phenomena, an… ▽ More In recent years, hematite potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics. △ Less

Submitted 8 December, 2020; originally announced December 2020.

Journal ref: Advanced Materials 2018

A tuneable telecom-wavelength entangled light emitting diode

Authors: Z. -H. Xiang, J. Huwer, J. Skiba-Szymanska, R. M. Stevenson, D. J. P. Ellis, I. Farrer, M. B. Ward, D. A. Ritchie, A. J. Shields

Abstract: Entangled light emitting diodes based on semiconductor quantum dots are promising devices for security sensitive quantum network applications, thanks to their natural lack of multi photon-pair generation. Apart from telecom wavelength emission, network integrability of these sources ideally requires electrical operation for deployment in compact systems in the field. For multiplexing of entangled… ▽ More Entangled light emitting diodes based on semiconductor quantum dots are promising devices for security sensitive quantum network applications, thanks to their natural lack of multi photon-pair generation. Apart from telecom wavelength emission, network integrability of these sources ideally requires electrical operation for deployment in compact systems in the field. For multiplexing of entangled photons with classical data traffic, emission in the telecom O-band and tuneability to the nearest wavelength channel in compliance with coarse wavelength division multiplexing standards (20 nm channel spacing) is highly desirable. Here we show the first fully electrically operated telecom entangled light emitting diode with wavelength tuneability of more than 25nm, deployed in an installed fiber network. With the source tuned to 1310.00 nm, we demonstrate multiplexing of true single entangled photons with classical data traffic and achieve entanglement fidelities above 95% on an installed fiber in a city. △ Less

Submitted 26 September, 2019; originally announced September 2019.

Comments: 15 pages, 7 figures

Journal ref: Commun Phys 3, 121 (2020)

Bulk-sensitive Imaging of Twin Domains in La$_{2-x}$Sr$_x$CuO$_4$ under Uniaxial Pressure

Authors: Xin Yu Zheng, Renfei Feng, D. S. Ellis, Young-June Kim

Abstract: We report our study of twin domains in $La_{2-x}Sr_x CuO_4$ under uniaxial pressure. Using bulk-sensitive x-ray microdiffraction in Laue geometry, we image the distribution of twin domains at room temperature. When compressive uniaxial pressure is applied along one of the in-plane crystallographic axes, the domain population changes dramatically. We observe that the twin domain with shorter lattic… ▽ More We report our study of twin domains in $La_{2-x}Sr_x CuO_4$ under uniaxial pressure. Using bulk-sensitive x-ray microdiffraction in Laue geometry, we image the distribution of twin domains at room temperature. When compressive uniaxial pressure is applied along one of the in-plane crystallographic axes, the domain population changes dramatically. We observe that the twin domain with shorter lattice parameter along the direction of pressure is unstable under compression, and disappears completely with only moderate pressure. On the other hand, application of tensile pressure changes the domain structure only slightly, demonstrating the asymmetric response of the sample to uniaxial pressure. Our observations suggest that a crystal's response to uniaxial pressure is complex and could deviate easily from the linear-response regime. △ Less

Submitted 3 August, 2018; originally announced August 2018.

Comments: 5 pages, 4 figures, to appear in App. Phys. Lett

Journal ref: Appl. Phys. Lett. 113, 071906 (2018)

Controllable photonic time-bin qubits from a quantum dot

Authors: J. P. Lee, L. M. Wells, B. Villa, S. Kalliakos, R. M. Stevenson, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Bennett, A. J. Shields

Abstract: Photonic time bin qubits are well suited to transmission via optical fibres and waveguide circuits. The states take the form $\frac{1}{\sqrt{2}}(α\ket{0} + e^{iφ}β\ket{1})$, with $\ket{0}$ and $\ket{1}$ referring to the early and late time bin respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot we demonstrate complete con… ▽ More Photonic time bin qubits are well suited to transmission via optical fibres and waveguide circuits. The states take the form $\frac{1}{\sqrt{2}}(α\ket{0} + e^{iφ}β\ket{1})$, with $\ket{0}$ and $\ket{1}$ referring to the early and late time bin respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot we demonstrate complete control over the phase, $φ$. We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence. Finally, we encode different qubits in different energies of the Raman scattered light, demonstrating wavelength division multiplexing at the single photon level. △ Less

Submitted 2 April, 2018; originally announced April 2018.

Journal ref: Phys. Rev. X 8, 021078 (2018)

Low lying magnetic states of the mixed valence cobalt ludwigite

Authors: M. Matos, J. Terra, D. E. Ellis

Abstract: There are two interpretations offered for the different structural and magnetic properties of the mixed valence homo-metallic ludwigites, Co3O2BO3 and Fe3O2BO3. One of them associates the physical behavior to charge ordering processes among the cations, as is well known in simpler oxides. The other attributes the effects to local pairwise magnetic interactions. Recently first principles calculatio… ▽ More There are two interpretations offered for the different structural and magnetic properties of the mixed valence homo-metallic ludwigites, Co3O2BO3 and Fe3O2BO3. One of them associates the physical behavior to charge ordering processes among the cations, as is well known in simpler oxides. The other attributes the effects to local pairwise magnetic interactions. Recently first principles calculations in the iron ludwigite have shown that the structural cation dimerization is due to the formation of strong magnetic dyads supporting the second model. Here we confirm the dominance of magnetic interactions to explain the absence of dimerization in the cobalt compound. Density functional non-collinear spin calculations are carried out on Co3O2BO3 to determine its low temperature magnetic order. Low spin is found on tri-valent cobalt sites, thus preventing the formation of the ferromagnetic dyad, the mechanism which favors dimerization in Fe3O2BO3. We conclude that the difference between high spin Fe3+ and low spin Co3+ pairwise interactions is responsible for the observed differences between the two compounds. The pairwise magnetic interactions also explain the difference between the existence of low temperature bulk AF state in the Fe ludwigite and its absence in the Co material. △ Less

Submitted 17 May, 2019; v1 submitted 27 February, 2018; originally announced February 2018.

Comments: 17 pages, 2 Tables, 3 Figures

Magnetic states at the surface of alpha-Fe2O3 thin films doped with Ti, Zn or Sn

Authors: David S. Ellis, Eugen Weschke, Asaf Kay, Daniel A. Grave, Kirtiman Deo Malviya, Hadar Mor, Frank M. F. de Groot, Hen Dotan, Avner Rothschild

Abstract: The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped (T_M~200 K) and Sn-doped (T_M~300 K) cases, while Zn and Ti-doped samples were always in the high and low temperature phases, respectively. In contrast to… ▽ More The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped (T_M~200 K) and Sn-doped (T_M~300 K) cases, while Zn and Ti-doped samples were always in the high and low temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions Sn4+ and Ti4+, for which T_M dramatically decreases, these dopants substantially increase T_M in thin films, far exceeding the bulk values. The normalized Fe LII-edge dichroism for T<T_M does not strongly depend on doping or temperature, except for an apparent increase of the peak amplitudes for T<100 K. We observed magnetic field-induced inversions of the dichroism peaks. By applying a magnetic field of 6.5 T on the Ti-doped sample, a transition into the T>T_M state was achieved. The temperature dependence of the critical field for the Sn-doped sample was characterized in detail. It was demonstrated the sample-to-sample variations of the Fe LIII-edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-depedent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures. △ Less

Submitted 28 September, 2017; originally announced September 2017.

Comments: 8 pages, 8 figures

Journal ref: Physical Review B 96, 094426 (2017)

Surface Acoustic wave modulation of a coherently driven quantum dot in a pillar microcavity

Authors: B. Villa, A. J. Bennett, D. J. P. Ellis, J. P. Lee, J. Skiba-Szymanska, T. A. Mitchell, J. P. Griffiths, I. Farrer, D. A. Ritchie, C. J. B. Ford, A. J. Shields

Abstract: We report the efficient coherent photon scattering from a semiconductor quantum dot embedded in a pillar microcavity. We show that a surface acoustic wave can periodically modulate the energy levels of the quantum dot, but has a negligible effect on the cavity mode. The scattered narrow-band laser is converted to a pulsed single-photon stream, displaying an anti-bunching dip characteristic of sing… ▽ More We report the efficient coherent photon scattering from a semiconductor quantum dot embedded in a pillar microcavity. We show that a surface acoustic wave can periodically modulate the energy levels of the quantum dot, but has a negligible effect on the cavity mode. The scattered narrow-band laser is converted to a pulsed single-photon stream, displaying an anti-bunching dip characteristic of single-photon emission. Multiple phonon sidebands are resolved in the emission spectrum, due to the absorption and emission of vibrational quanta in each scattering event. △ Less

Submitted 4 July, 2017; originally announced July 2017.

Comments: To be published in Applied Physics Letters

Journal ref: Applied Physics Letters, vol. 111, no. 1, p. 011103, Jul. 2017

Electrically driven and electrically tunable quantum light sources

Authors: J. P. Lee, E. Murray, A. J. Bennett, D. J. P. Ellis, C. Dangel, I. Farrer, P. Spencer, D. A. Ritchie, A. J. Shields

Abstract: Compact and electrically controllable on-chip sources of indistinguishable photons are desirable for the development of integrated quantum technologies. We demonstrate that two quantum dot light emitting diodes (LEDs) in close proximity on a single chip can function as a tunable, all-electric quantum light source. Light emitted by an electrically excited driving LED is used to excite quantum dots… ▽ More Compact and electrically controllable on-chip sources of indistinguishable photons are desirable for the development of integrated quantum technologies. We demonstrate that two quantum dot light emitting diodes (LEDs) in close proximity on a single chip can function as a tunable, all-electric quantum light source. Light emitted by an electrically excited driving LED is used to excite quantum dots the neighbouring diode. The wavelength of the quantum dot emission from the neighbouring driven diode is tuned via the quantum confined Stark effect. We also show that we can electrically tune the fine structure splitting. △ Less

Submitted 16 January, 2017; originally announced January 2017.

Comments: 4 pages

Journal ref: Appl. Phys. Lett. 110, 071102 (2017)

Opening a nodal gap by fluctuating spin-density-wave in lightly doped La$_{2-x}$Sr$_x$CuO$_4$

Authors: Itzik Kapon, David S. Ellis, Gil Drachuck, Galina Bazalitski, Eugen Weschke, Enrico Schierle, Jörg Strempfer, Christof Niedermayer, Amit Keren

Abstract: We investigate whether the spin or charge degrees of freedom are responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on La$_{2-x}$Sr$_x$CuO$_4$, which is on the edge of the antiferromagnetic phase. We found that fluctuating incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass dispersion exists e… ▽ More We investigate whether the spin or charge degrees of freedom are responsible for the nodal gap in underdoped cuprates by performing inelastic neutron scattering and x-ray diffraction measurements on La$_{2-x}$Sr$_x$CuO$_4$, which is on the edge of the antiferromagnetic phase. We found that fluctuating incommensurate spin-density-wave (SDW) with a the bottom part of an hourglass dispersion exists even in this magnetic sample. The strongest component of these fluctuations diminishes at the same temperature where the nodal gap opens. X-ray scattering measurements on the same crystal show no signature of charge-density-wave (CDW). Therefore, we suggest that the nodal gap in the electronic band of this cuprate opens due to fluctuating SDW with no contribution from CDW. △ Less

Submitted 20 December, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 95, 104512 (2017)

Flexible superconducting Nb transmission lines on thin film polyimide for quantum computing applications

Authors: David B. Tuckerman, Michael C. Hamilton, David J. Reilly, Rujun Bai, George A. Hernandez, John M. Hornibrook, John A. Sellers, Charles D. Ellis

Abstract: We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film Nb/polyimide… ▽ More We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film Nb/polyimide ribbon cables provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the low-temperature stages (from ~ 4 K down to ~ 20 mK) of a dilution refrigerator. S-parameters are presented for 2-metal layer Nb microstrip structures with lengths ranging up to 550 mm. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides are remarkably low at deep cryogenic temperatures, typically 100$\times$ smaller than corresponding room temperature values. This enables fairly long-distance transmission of microwave signals without excessive attenuation and permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2$\times$10$^{-5}$ at 20 mK. Our fabrication techniques could be extended to more complex structures such as multiconductor, multi-layer stripline or rectangular coax, and integrated attenuators and thermalization structures. △ Less

Submitted 14 June, 2016; originally announced June 2016.

Comments: 22 pages, 10 figures (included)

Journal ref: Supercond. Sci. Technol. 29, 084007 (2016)

A semiconductor photon-sorter

Authors: A. J. Bennett, J. P. Lee, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Shields

Abstract: Photons do not interact directly with each other, but conditional control of one beam by another can be achieved with non-linear optical media at high field intensities. It is exceedingly difficult to reach such intensities at the single photon level but proposals have been made to obtain effective interactions by scattering photons from single transitions. We report here effective interactions be… ▽ More Photons do not interact directly with each other, but conditional control of one beam by another can be achieved with non-linear optical media at high field intensities. It is exceedingly difficult to reach such intensities at the single photon level but proposals have been made to obtain effective interactions by scattering photons from single transitions. We report here effective interactions between photons created using a quantum dot weakly coupled to a cavity. We show that a passive single-photon non-linearity can modify the counting statistics of a Poissonian beam, sorting the photons in number. This is used to create strong correlations between detection events and sort polarisation correlated photons from an uncorrelated stream using a single spin. These results pave the way for optical switches operated by single quanta of light. △ Less

Submitted 20 May, 2016; originally announced May 2016.

Comments: Based on the initial journal submission. 11 pages, 4 figures

Neutron Scattering Studies of Spin-Phonon Hybridization and Superconducting Spin-Gaps in High Temperature Superconductor $La_{2-x}(Sr,Ba)_{x}CuO_{4}$

Authors: J. J. Wagman, J. P. Carlo, J. Gaudet, G. Van Gastel, D. L. Abernathy, M. B. Stone, G. E. Granroth, A. I. Koleshnikov, A. T. Savici, Y. J. Kim, H. Zhang, D. Ellis, Y. Zhao, L. Clark, A. B. Kallin, E. Mazurek, H. A. Dabkowska, B. D. Gaulin

Abstract: We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic or… ▽ More We present time-of-fight neutron-scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO) with 0 $\leq$ x $\leq$ 0.095 and $La_{2-x}Sr_{x}CuO_{4}$ (LSCO) with x = 0.08 and 0.11. This range of dopings spans much of the phase diagram relevant to high temperature cuprate superconductivity, ranging from insulating, three dimensional (3D) commensurate long range antiferromagnetic order, for x $\leq$ 0.02, to two dimensional (2D) incommensurate antiferromagnetism co-existing with superconductivity for x $\geq$ 0.05. Previous work on lightly doped LBCO with x = 0.035 showed a clear resonant enhancement of the inelastic scattering coincident with the low energy crossings of the highly dispersive spin excitations and quasi-2D optic phonons. The present work extends these measurements across the phase diagram and shows this enhancement to be a common feature to this family of layered quantum magnets. Furthermore we show that the low temperature, low energy magnetic spectral weight is substantially larger for samples with non-superconducting ground states relative to any of the samples with superconducting ground states. Spin gaps, suppression of low energy magnetic spectral weight as a function of decreasing temperature, are observed in both superconducting LBCO and LSCO samples, consistent with previous observations for superconducting LSCO. △ Less

Submitted 29 September, 2015; originally announced September 2015.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 93, 094416 (2016)

Ramsey Interference in a Multi-level Quantum System

Authors: J. P. Lee, A. J. Bennett, J. Skiba-Szymanska, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Shields

Abstract: We report Ramsey interference in the excitonic population of a negatively charged quantum dot revealing the coherence of the state in the limit where radiative decay is dominant. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing… ▽ More We report Ramsey interference in the excitonic population of a negatively charged quantum dot revealing the coherence of the state in the limit where radiative decay is dominant. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing the laser detuning to reveal 2, 3 and 4 level system behaviour. We show that under finite field the phase-sensitive control of two optical pulses from a single laser can be used to prepare both population and spin qubits simultaneously. △ Less

Submitted 24 August, 2015; originally announced August 2015.

Journal ref: Phys. Rev. B 93, 085407 (2016)

Correlation of the Superconducting Critical Temperature with Spin and Orbital Excitation Energies In (Ca{x}La{1-x})(Ba{1.75-x}La{0.25+x})Cu{3}O{y} as Measured by Resonant Inelastic X-ray Scattering

Authors: David Shai Ellis, Yao-Bo Huang, Paul Olalde-Velasco, Marcus Dantz, Jonanthan Pelliciari, Gil Drachuck, Rinat Ofer, Galina Bazalitsky, Jorge Berger, Thorsten Schmitt, Amit Keren

Abstract: Electronic spin and orbital (dd) excitation spectra of (Ca{x}La{1-x})(Ba{1.75-x}La{0.25+x})Cu{3}O{y} samples are measured by resonant inelastic x-ray scattering (RIXS). In this compound, Tc of samples with identical hole dopings is strongly affected by the Ca/Ba substitution x due to subtle variations in the lattice constants, while crystal symmetry and disorder as measured by line-widths are x in… ▽ More Electronic spin and orbital (dd) excitation spectra of (Ca{x}La{1-x})(Ba{1.75-x}La{0.25+x})Cu{3}O{y} samples are measured by resonant inelastic x-ray scattering (RIXS). In this compound, Tc of samples with identical hole dopings is strongly affected by the Ca/Ba substitution x due to subtle variations in the lattice constants, while crystal symmetry and disorder as measured by line-widths are x independent. We examine two extreme values of x and two extreme values of hole-doping content y corresponding to antiferromagnetic and superconducting states. The x dependence of the spin mode energies is approximately the same for both the antiferromagnetic and superconducting samples. This clearly demonstrates that RIXS is sensitive to J even in doped samples. A positive correlation between the superexchange J and the maximum of Tc at optimal doping Tc^{max} is observed. We also measured the x dependence of the d_{xy} -> d_{x^2-y^2} and d_{xz/yz} -> d_{x^2-y^2} orbital splittings. We infer that the effect of the unresolved d_{3z^2-r^2} -> d_{x^2-y^2} excitation on Tc^{max} is much smaller than the effect of J. There appears to be dispersion in the d_{xy} -> d_{x^2-y^2} peak of up to 0.05 eV. Our fitting of the peaks furthermore indicates an asymmetric dispersion for the d_{xz/yz} -> d_{x^2-y^2} excitation. A peak at ~0.8 eV is also observed, and attributed to a dd excitation in the chain layer. △ Less

Submitted 9 August, 2015; originally announced August 2015.

Comments: 12 pages, 13 figures, and a supplementary .pdf file. To be published in Physical Review B

Cavity-enhanced coherent light scattering from a quantum dot

Authors: A. J. Bennett, J. P. Lee, D. J. P. Ellis, T. Meany, E. Murray, F. Floether, J. P. Griffths, I. Farrer, D. A. Ritchie, A. J. Shields

Abstract: Resonant excitation of atoms and ions in macroscopic cavities has lead to exceptional control over quanta of light. Translating these advantages into the solid state with emitters in microcavities promises revolutionary quantum technologies in information processing and metrology. Key is resonant optical reading and writing from the emitter-cavity system. However, it has been widely expected that… ▽ More Resonant excitation of atoms and ions in macroscopic cavities has lead to exceptional control over quanta of light. Translating these advantages into the solid state with emitters in microcavities promises revolutionary quantum technologies in information processing and metrology. Key is resonant optical reading and writing from the emitter-cavity system. However, it has been widely expected that the reflection of a resonant laser from a micro-fabricated wavelength-sized cavity would dominate any quantum signal. Here we demonstrate coherent photon scattering from a quantum dot in a micro-pillar. The cavity is shown to enhance the fraction of light which is resonantly scattered towards unity, generating anti-bunched indistinguishable photons a factor of 16 beyond the time-bandwidth limit, even when the transition is near saturation. Finally, deterministic excitation is used to create 2-photon N00N states with which we make super-resolving phase measurements in a photonic circuit. △ Less

Submitted 7 August, 2015; originally announced August 2015.

Journal ref: Science Advances 2(4) 2016

Ultrafast electrical control of a resonantly driven single photon source

Authors: Y. Cao, A. J. Bennett, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Shields

Abstract: We demonstrate generation of a pulsed stream of electrically triggered single photons in resonance fluorescence, by applying high frequency electrical pulses to a single quantum dot in a p-i-n diode under resonant laser excitation. Single photon emission was verifed, with the probability of multiple photon emission reduced to 2.8%. We show that despite the presence of charge noise in the emission… ▽ More We demonstrate generation of a pulsed stream of electrically triggered single photons in resonance fluorescence, by applying high frequency electrical pulses to a single quantum dot in a p-i-n diode under resonant laser excitation. Single photon emission was verifed, with the probability of multiple photon emission reduced to 2.8%. We show that despite the presence of charge noise in the emission spectrum of the dot, resonant excitation acts as a filter to generate narrow bandwidth photons. △ Less

Submitted 22 July, 2015; originally announced July 2015.

Journal ref: Appl. Phys. Lett. 105, 051112 (2014)

Non-complicated EuTiO3 Structure

Authors: David S. Ellis, Hiroshi Uchiyama, Satoshi Tsutsui, Kunihisa Sugimoto, Kenichi Kato, Alfred Q. R. Baron

Abstract: A recently published paper [arXiv:1206.5417 ]showed strong incommensurate diffraction peaks in EuTiO3 around zone boundary R-points. We wish to convey that in our samples, and, it seems, at least another case from the literature, these peaks are absent. A recently published paper [arXiv:1206.5417 ]showed strong incommensurate diffraction peaks in EuTiO3 around zone boundary R-points. We wish to convey that in our samples, and, it seems, at least another case from the literature, these peaks are absent. △ Less

Submitted 21 February, 2013; originally announced February 2013.

Comments: 1-2 pages 1 figure

Phonon Softening and Dispersion in EuTiO3

Authors: David S. Ellis, Hiroshi Uchiyama, Satoshi Tsutsui, Kunihisa Sugimoto, Kenichi Kato, Daisuke Ishikawa, Alfred Q. R. Baron

Abstract: We measured phonon dispersion in single crystal EuTiO$_3$ using inelastic x-ray scattering. A structural transition to an antiferrodistortive phase was found at a critical temperature $T_0$=287$\pm$1 K using powder and single-crystal x-ray diffraction. Clear softening of the zone boundary \emph{R}-point \textbf{q}=(0.5 0.5 0.5) acoustic phonon shows this to be a displacive transition. The mode ene… ▽ More We measured phonon dispersion in single crystal EuTiO$_3$ using inelastic x-ray scattering. A structural transition to an antiferrodistortive phase was found at a critical temperature $T_0$=287$\pm$1 K using powder and single-crystal x-ray diffraction. Clear softening of the zone boundary \emph{R}-point \textbf{q}=(0.5 0.5 0.5) acoustic phonon shows this to be a displacive transition. The mode energy plotted against reduced temperature could be seen to nearly overlap that of $\rm SrTiO_3$, suggesting a universal scaling relation. Phonon dispersion was measured along $Γ$-$X$ (0 0 0)$\rightarrow$(0.5 0 0). Mode eigenvectors were obtained from a shell model consistent with the \textbf{q}-dependence of intensity and energy, which also showed that the dispersion is nominally the same as in $\rm SrTiO_3$ at room temperature, but corrected for mass. The lowest energy optical mode, determined to be of Slater character, softens approximately linearly with temperature until the 70-100 K range where the softening stops, and at low temperature, the mode disperses linearly near the zone center. △ Less

Submitted 12 November, 2012; v1 submitted 6 October, 2012; originally announced October 2012.

Comments: 5 pages, 4 figures

Spin-Split Conduction band in \eb\ and Tuning of Half-Metallicity with External Stimuli

Authors: Jungho Kim, Wei Ku, Chi-Cheng Lee, D. S. Ellis, B. K. Cho, A. H. Said, Yu. Shvyd'ko, Young-June Kim

Abstract: We report Eu L3-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB6. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic ordering temperature and follows the magnetic order parameter. This spectral feature is attributed to the inter-site excitation from the local 4f orbital to… ▽ More We report Eu L3-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB6. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic ordering temperature and follows the magnetic order parameter. This spectral feature is attributed to the inter-site excitation from the local 4f orbital to the spin-split 5d orbital on the neighboring site, illustrating the essential role of exchange splitting of the conducting electrons. Based on our density functional theory calculations, the RIXS data suggest that EuB6 at low temperature can be consistently described with a semi-metallic electronic structure with incomplete spin-polarization. We propose routes to achieve half-metallicity in EuB6, which utilize the strong tunability of the electronic structure against gate voltage, strain, and magnetic field. △ Less

Submitted 6 July, 2011; originally announced July 2011.

Comments: 5 pages, 4 figures

Electronic Structure of Doped Lanthanum Cuprates Studied with Resonant Inelastic X-Ray Scattering

Authors: D. S. Ellis, Jungho Kim, Harry Zhang, J. P. Hill, Genda Gu, Seiki Komiya, Yoichi Ando, D. Casa, T. Gog, Young-June Kim

Abstract: We report a comprehensive Cu $K$-edge RIXS investigation of $\rm La_{2-x}Sr_xCuO_4$ (LSCO) for 0$\leq$x$\leq$0.35, stripe-ordered $\rm La_{1.875}Ba_{0.125}CuO_4$ (LBCO), and $\rm La_{2}Cu_{0.96}Ni_{0.04}O_4$ (LCNO) crystals. The RIXS spectra measured at three high-symmetry momentum transfer (\textbf{q}) positions are compared as a function of doping and for the different dopants. The spectra in t… ▽ More We report a comprehensive Cu $K$-edge RIXS investigation of $\rm La_{2-x}Sr_xCuO_4$ (LSCO) for 0$\leq$x$\leq$0.35, stripe-ordered $\rm La_{1.875}Ba_{0.125}CuO_4$ (LBCO), and $\rm La_{2}Cu_{0.96}Ni_{0.04}O_4$ (LCNO) crystals. The RIXS spectra measured at three high-symmetry momentum transfer (\textbf{q}) positions are compared as a function of doping and for the different dopants. The spectra in the energy range 1-6 eV can be described with three broad peaks, which evolve systematically with increased doping. The most systematic trend was observed for \textbf{q}=($π$, 0) corresponding to the zone boundary. As hole doping increased, the spectral weight transfer from high energies to low energies is nearly linear with \emph{x} at this \textbf{q}. We interpret the peaks as interband transitions in the context of existing band models for this system, assigning them to Zhang-Rice band$\rightarrow$upper Hubbard band, lower-lying band$\rightarrow$upper Hubbard band, and lower-lying band$\rightarrow$Zhang-Rice band transitions. The spectrum of stripe-ordered LBCO was also measured, and found to be identical to the correspondingly doped LSCO, except for a relative enhancement of the near-infrared peak intensity around 1.5-1.7 eV. The temperature dependence of this near-infrared peak in LBCO was more pronounced than for other parts of the spectrum, continuously decreasing in intensity as the temperature was raised from 25 K to 300 K. Finally, we find that 4\% Ni substitution in the Cu site has a similar effect on the spectra as does Sr substitution in the La site. △ Less

Submitted 15 January, 2011; originally announced January 2011.

Comments: 10 pages (according to my pdf), 8 figures. Was accepted for publication in PRB. When I did the preview in the previous step, somehow figure 2 looked very broken into pieces ?

Magnetic nature of the 500 meV peak in $\rm La_{2-x}Sr_{x}CuO_4$ observed with resonant inelastic x-ray scattering at the Cu $K$-edge

Authors: D. S. Ellis, Jungho Kim, J. P. Hill, S. Wakimoto, R. J. Birgeneau, Y. Shvyd'ko, D. Casa, T. Gog, K. Ishii, K. Ikeuchi, A. Paramekanti, Young-June Kim

Abstract: We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at ${\bf q}=(π,0)$ in resonant inelastic x-ray scattering (RIXS) experiments on $\rm La_2CuO_4$. The intensity of this peak persists above the Néel temperature (T$_{N}$=320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with t… ▽ More We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at ${\bf q}=(π,0)$ in resonant inelastic x-ray scattering (RIXS) experiments on $\rm La_2CuO_4$. The intensity of this peak persists above the Néel temperature (T$_{N}$=320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon $\rm B_{1g}$ Raman peak in $\rm La_2CuO_4$, and with suitable rescaling, agrees with the Raman peak shifts in $\rm EuBa_2Cu_3O_6$ and $\rm K_2NiF_4$. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation. △ Less

Submitted 7 February, 2010; originally announced February 2010.

Comments: 13 pages, 9 figures

Measurement of x-ray absorption spectra of overdoped high-temperature cuprate superconductors: Inapplicability of the single-band Hubbard model

Authors: D. C. Peets, D. G. Hawthorn, K. M. Shen, Young-June Kim, D. S. Ellis, H. Zhang, Seiki Komiya, Yoichi Ando, G. A. Sawatzky, Ruixing Liang, D. A. Bonn, W. N. Hardy

Abstract: X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_{6+delta} (Tl-2201) and La_{2-x}Sr_xCuO_{4+delta} (LSCO) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepeak gains less intensity tha… ▽ More X-ray absorption spectra on the overdoped high-temperature superconductors Tl_2Ba_2CuO_{6+delta} (Tl-2201) and La_{2-x}Sr_xCuO_{4+delta} (LSCO) reveal a striking departure in the electronic structure from that of the underdoped regime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygen K edge, while the lowest-energy prepeak gains less intensity than expected above p ~ 0.21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates. △ Less

Submitted 8 May, 2009; originally announced May 2009.

Comments: 4 pages, 2 figures

Journal ref: Phys. Rev. Lett. 103, 087402 (2009)

Random Hamiltonian in thermal equilibrium

Authors: Dorje C. Brody, David C. P. Ellis, Darryl D. Holm

Abstract: A framework for the investigation of disordered quantum systems in thermal equilibrium is proposed. The approach is based on a dynamical model--which consists of a combination of a double-bracket gradient flow and a uniform Brownian fluctuation--that `equilibrates' the Hamiltonian into a canonical distribution. The resulting equilibrium state is used to calculate quenched and annealed averages o… ▽ More A framework for the investigation of disordered quantum systems in thermal equilibrium is proposed. The approach is based on a dynamical model--which consists of a combination of a double-bracket gradient flow and a uniform Brownian fluctuation--that `equilibrates' the Hamiltonian into a canonical distribution. The resulting equilibrium state is used to calculate quenched and annealed averages of quantum observables. △ Less

Submitted 14 January, 2009; originally announced January 2009.

Comments: 8 pages, 4 figures. To appear in DICE 2008 conference proceedings

Journal ref: J. Phys.: Conf. Ser. (2009) 174, 012041

Comparison of Resonant Inelastic X-Ray Scattering Spectra and Dielectric Loss Functions in Copper Oxides

Authors: Jungho Kim, D. S. Ellis, H. Zhang, J. P. Hill, F. C. Chou, T. Gog, D. Casa, Young-June Kim

Abstract: We report empirical comparisons of Cu K-edge indirect resonant inelastic x-ray scattering (RIXS) spectra, taken at the Brillouin zone center, with optical dielectric loss functions measured in a number of copper oxides. The RIXS data are obtained for Bi$_2$CuO$_4$, CuGeO$_3$, Sr$_2$Cu$_3$O$_4$Cl$_2$, La$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$, and analyzed by considering both incident and scattered… ▽ More We report empirical comparisons of Cu K-edge indirect resonant inelastic x-ray scattering (RIXS) spectra, taken at the Brillouin zone center, with optical dielectric loss functions measured in a number of copper oxides. The RIXS data are obtained for Bi$_2$CuO$_4$, CuGeO$_3$, Sr$_2$Cu$_3$O$_4$Cl$_2$, La$_2$CuO$_4$, and Sr$_2$CuO$_2$Cl$_2$, and analyzed by considering both incident and scattered photon resonances. An incident-energy-independent response function is then extracted. The dielectric loss functions, measured with spectroscopic ellipsometry, agree well with this RIXS response, especially in Bi$_2$CuO$_4$ and CuGeO$_3$. △ Less

Submitted 9 November, 2008; originally announced November 2008.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. B 79, 094525 (2009)

Hamiltonian statistical mechanics

Authors: Dorje C. Brody, David C. P. Ellis, Darryl D. Holm

Abstract: A framework for statistical-mechanical analysis of quantum Hamiltonians is introduced. The approach is based upon a gradient flow equation in the space of Hamiltonians such that the eigenvectors of the initial Hamiltonian evolve toward those of the reference Hamiltonian. The nonlinear double-bracket equation governing the flow is such that the eigenvalues of the initial Hamiltonian remain unpert… ▽ More A framework for statistical-mechanical analysis of quantum Hamiltonians is introduced. The approach is based upon a gradient flow equation in the space of Hamiltonians such that the eigenvectors of the initial Hamiltonian evolve toward those of the reference Hamiltonian. The nonlinear double-bracket equation governing the flow is such that the eigenvalues of the initial Hamiltonian remain unperturbed. The space of Hamiltonians is foliated by compact invariant subspaces, which permits the construction of statistical distributions over the Hamiltonians. In two dimensions, an explicit dynamical model is introduced, wherein the density function on the space of Hamiltonians approaches an equilibrium state characterised by the canonical ensemble. This is used to compute quenched and annealed averages of quantum observables. △ Less

Submitted 15 September, 2008; v1 submitted 18 August, 2008; originally announced August 2008.

Comments: 8 pages, 2 figures, references added

Journal ref: J. Phys. A: Math. Theor. (2008) 41, 502002

Cavity-enhanced radiative emission rate in a single-photon-emitting diode operating at 0.5 GHz

Authors: David J. P. Ellis, Anthony J. Bennett, Samuel J. Dewhurst, Christine A. Nicoll, David A. Ritchie, Andrew J. Shields

Abstract: We report the observation of a Purcell enhancement in the electroluminescence decay rate of a single quantum dot, embedded in a microcavity light-emitting-diode structure. Lateral confinement of the optical mode was achieved using an annulus of low-refractive-index aluminium oxide, formed by wet oxidation. The same layer acts as a current aperture, reducing the active area of the device without… ▽ More We report the observation of a Purcell enhancement in the electroluminescence decay rate of a single quantum dot, embedded in a microcavity light-emitting-diode structure. Lateral confinement of the optical mode was achieved using an annulus of low-refractive-index aluminium oxide, formed by wet oxidation. The same layer acts as a current aperture, reducing the active area of the device without impeding the electrical properties of the p-i-n diode. This allowed single photon electroluminescence to be demonstrated at repetition rates up to 0.5 GHz. △ Less

Submitted 4 April, 2008; originally announced April 2008.

Comments: 11 pages, 4 Figures. To be published in New Journal of Physics

Observation of a 500meV Collective Mode in La$_{2-x}$Sr$_x$CuO$_4$ and Nd$_2$CuO$_4$

Authors: J. P. Hill, G. Blumberg, Young-June Kim, D. Ellis, S. Wakimoto, R. J. Birgeneau, Seiki Komiya, Yoichi Ando, B. Liang, R. L. Greene, D. Casa, T. Gog

Abstract: Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La$_{2-x}$Sr$_x$CuO$_4$ at 500 meV. The mode is peaked around the ($π$,0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr c… ▽ More Utilizing resonant inelastic x-ray scattering, we report a previously unobserved mode in the excitation spectrum of La$_{2-x}$Sr$_x$CuO$_4$ at 500 meV. The mode is peaked around the ($π$,0) point in reciprocal space and is observed to soften, and broaden, away from this point. Samples with x=0, 0.01, 0.05, and 0.17 were studied. The new mode is found to be rapidly suppressed with increasing Sr content and is absent at $x$=0.17, where it is replaced by a continuum of excitations. The peak is only observed when the incident x-ray polarization is normal to the CuO planes and is also present in Nd$_2$CuO$_4$. We suggest possible explanations for this excitation. △ Less

Submitted 20 September, 2007; originally announced September 2007.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. Lett. 100, 097001 (2008).

Oxide-apertured microcavity single-photon emitting diode

Authors: D. J. P. Ellis, A. J. Bennett, A. J. Shields, P. Atkinson, D. A. Ritchie

Abstract: We have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet-oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity struc… ▽ More We have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet-oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, we demonstrate a single-photon emitting diode where current may be injected into a single dot. △ Less

Submitted 13 September, 2007; originally announced September 2007.

Journal ref: Appl. Phys. Lett. 90, 233514 (2007)

Observation of the Purcell effect in high-index-contrast micropillar

Authors: A. J. Bennett, D. J. P. Ellis, A. J. Shields, P. Atkinson, I. Farrer, D. A. Ritchie

Abstract: We have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and Aluminium Oxide. Compared to the more widely studied GaAs/AlAs micropillars these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. We have studied a number of samples containing a low density of InGaAs/GaAs self assembled quantum dots in a cavi… ▽ More We have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and Aluminium Oxide. Compared to the more widely studied GaAs/AlAs micropillars these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. We have studied a number of samples containing a low density of InGaAs/GaAs self assembled quantum dots in a cavity and here report observation of a three fold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect. △ Less

Submitted 13 September, 2007; originally announced September 2007.

Journal ref: Appl. Phys. Lett. 90, 191911 (2007)

Charge-transfer exciton in La2CuO4 probed with resonant inelastic x-ray scattering

Authors: D. S. Ellis, J. P. Hill, S. Wakimoto, R. J. Birgeneau, D. Casa, T. Gog, Young-June Kim

Abstract: We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features w… ▽ More We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation. △ Less

Submitted 11 September, 2007; originally announced September 2007.

Comments: 5 pages, 4 figures

Electrically addressing a single self-assembled quantum dot

Authors: D. J. P. Ellis, A. J. Bennett, P. Atkinson, D. A. Ritchie, A. J. Shields

Abstract: We report on the use of an aperture in an aluminum oxide layer to restrict current injection into a single self-assembled InAs quantum dot, from an ensemble of such dots within a large mesa. The insulating aperture is formed through the wet-oxidation of a layer of AlAs. Under photoluminescence we observe that only one quantum dot in the ensemble exhibits a Stark shift, and that the same single d… ▽ More We report on the use of an aperture in an aluminum oxide layer to restrict current injection into a single self-assembled InAs quantum dot, from an ensemble of such dots within a large mesa. The insulating aperture is formed through the wet-oxidation of a layer of AlAs. Under photoluminescence we observe that only one quantum dot in the ensemble exhibits a Stark shift, and that the same single dot is visible under electroluminescence. Autocorrelation measurements performed on the electroluminescence confirm that we are observing emission from a single quantum dot. △ Less

Submitted 6 December, 2006; originally announced December 2006.

Comments: 12 pages, 3 figures

Journal ref: Appl. Phys. Lett. 88, 133509 (2006)

Single-photon-emitting diodes: a review

Authors: A. J. Bennett, P. Atkinson, P. See, M. B. Ward, R. M. Stevenson, Z. L. Yuan, D. C. Unitt, D. J. P. Ellis, K. Cooper, D. A. Ritchie, A. J. Shields

Abstract: Compact and reliable sources of non-classical light could find many applications in emerging technologies such as quantum cryptography, quantum imaging and also in fundamental tests of quantum physics. Single self-assembled quantum dots have been widely studied for this reason, but the vast majority of reported work has been limited to optically excited sources. Here we discuss the progress made… ▽ More Compact and reliable sources of non-classical light could find many applications in emerging technologies such as quantum cryptography, quantum imaging and also in fundamental tests of quantum physics. Single self-assembled quantum dots have been widely studied for this reason, but the vast majority of reported work has been limited to optically excited sources. Here we discuss the progress made so far, and prospects for, electrically driven single-photon-emitting diodes (SPEDs). △ Less

Submitted 31 October, 2006; originally announced October 2006.

Comments: 13 pages, 10 figures

Journal ref: Phys. Stat. Sol. (b) 243, No 14, p3730 (2006)

Damped Soft Phonons and Diffuse Scattering in 40PMN-60PT

Authors: C. Stock, D. Ellis, I. P. Swainson, Guangyong Xu, H. Hiraka, Z. Zhong, H. Luo, X. Zhao, D. Viehland, R. J. Birgeneau, G. Shirane

Abstract: Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-60% PbTiO$_{3}$ (PMN-60PT) with pure Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) and PbTiO$_{3}$ (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN… ▽ More Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-60% PbTiO$_{3}$ (PMN-60PT) with pure Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) and PbTiO$_{3}$ (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and PZN. PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unit cell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortion is long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present. Even though polar nanoregions are absent, the soft optic mode is short-lived for wavevectors near the zone-centre. Therefore, PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder which is the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zone centre and not due to the formation of short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of the PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates with increasing doping of PbTiO$_{3}$. △ Less

Submitted 20 March, 2006; originally announced March 2006.

Comments: 13 pages, 13 figures

Journal ref: Phys. Rev. B., 73, 064107 (2006)