Showing 1–50 of 95 results for author: Ahn, H

Isotopically enriched epitaxial CaWO$_{4}$ thin films for Er$^{3+}$ spin-photon quantum interfaces

Authors: Hanlin Tang, Kidae Shin, Ashwin K. Boddeti, Sebastian P. Horvath, Adam Turflinger, Joseph Alexander, Jeffrey A. Dhas, Zihua Zhu, Shuhang Pan, Jeff D. Thompson, Yingge Du, Frederick J. Walker, Charles H. Ahn

Abstract: Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time$^{4-6}$. The electron spin coherence is limited at high temperatures by paramagnetic impurities… ▽ More Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time$^{4-6}$. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3% $^{183}$W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er$^{3+}$ doped CaWO$_{4}$ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO$_{4}$ thin films using an isotopically purified $^{186}$WO$_{3}$ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. $^{183}$W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO$_{4}$ thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices. △ Less

Submitted 27 April, 2026; originally announced April 2026.

Finely Tunable Thermal Expansion of NiTi by Stress-Induced Martensitic Transformation and Thermomechanical Training

Authors: Won Seok Choi, Won-Seok Ko, Yejun Park, Edward L. Pang, Jong-Hoon Park, Hye-Hyun Ahn, Yuji Ikeda, Pyuck-Pa Choi, Blazej Grabowski

Abstract: Tailoring the thermal expansion of martensitic materials by crystallographic texture and anisotropic variation of lattice parameters is a promising route to a flexible design of thermally stable systems. NiTi alloys are prototype materials in this respect, with shape-memory and superelastic properties owing to their thermoelastic martensitic transformations. Here, we propose a method to realize fi… ▽ More Tailoring the thermal expansion of martensitic materials by crystallographic texture and anisotropic variation of lattice parameters is a promising route to a flexible design of thermally stable systems. NiTi alloys are prototype materials in this respect, with shape-memory and superelastic properties owing to their thermoelastic martensitic transformations. Here, we propose a method to realize finely tunable coefficients of thermal expansion (CTE) for the NiTi alloy based upon a special combination of mechanical and thermal training. We achieve a near-zero in-plane CTE that is smaller in value than that of the FeNi-based Invar alloy. Atomistic simulations and theoretical calculations guide the method design and clarify the underlying mechanisms of the relationship between the processing conditions, the microstructural evolution, and the thermal expansion behavior. The directions for further, finer adjustments of the CTE without constraints on the shape of the materials are indicated. △ Less

Submitted 20 February, 2026; originally announced February 2026.

Comments: 47 pages, 20 figures, 2 tables

Journal ref: Acta Materialia 302 (2026) 121623

Layer controlled orbital selective Mott transition in monolayer nickelate

Authors: Byungmin Sohn, Minjae Kim, Sangjae Lee, Wenzheng Wei, Juan Jiang, Fengmiao Li, Sergey Gorovikov, Marta Zonno, Tor Pedersen, Sergey Zhdanovich, Ying Liu, Huikai Cheng, Ke Zou, Yu He, Sohrab Ismail-Beigi, Frederick J. Walker, Charles H. Ahn

Abstract: Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy (ARPES), w… ▽ More Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy (ARPES), we report an orbital-selective decoherence of spectral density in the perovskite nickelate LaNiO3 towards the monolayer limit. The spectral weight of the dz2 band vanishes much faster than that of the dx2-y2 band as the thickness of the LaNiO3 layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory (DMFT) calculations show an orbital-selective Mott transition largely due to the localization of dz2 electrons along the c axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a dx2-y2-dz2 two-band model. △ Less

Submitted 23 September, 2025; originally announced September 2025.

Coexisting Kagome and Heavy Fermion Flat Bands in YbCr$_6$Ge$_6$

Authors: Hanoh Lee, Churlhi Lyi, Taehee Lee, Hyeonhui Na, Jinyoung Kim, Sangjae Lee, Younsik Kim, Anil Rajapitamahuni, Asish K. Kundu, Elio Vescovo, Byeong-Gyu Park, Changyoung Kim, Charles H. Ahn, Frederick J. Walker, Ji Seop Oh, Bo Gyu Jang, Youngkuk Kim, Byungmin Sohn, Tuson Park

Abstract: Flat bands, emergent in strongly correlated electron systems, stand at the frontier of condensed matter physics, providing fertile ground for unconventional quantum phases. Recent observations of dispersionless bands at the Fermi level in kagome lattice open the possibility of unifying the disjoint paradigms of topology and correlation-driven heavy fermion liquids. Here, we report the unprecedente… ▽ More Flat bands, emergent in strongly correlated electron systems, stand at the frontier of condensed matter physics, providing fertile ground for unconventional quantum phases. Recent observations of dispersionless bands at the Fermi level in kagome lattice open the possibility of unifying the disjoint paradigms of topology and correlation-driven heavy fermion liquids. Here, we report the unprecedented coexistence of these mechanisms in the layered kagome metal YbCr6Ge6. At high temperatures, an intrinsic kagome flat band-arising from the frustrated hopping on the kagome lattice-dominates the Fermi level. Upon cooling, localized Yb 4f-states hybridize with the topological kagome flat bands, transforming this state into the Kondo resonance states that are nearly dispersionless across the entire Brillouin zone. Crystalline symmetry forbids hybridization along specific high-symmetry lines, which stabilizes Dirac crossings of heavy-fermion character. Topological analysis of the resulting gaps reveals both trivial and nontrivial Z2 invariants, establishing the emergence of a Dirac-Kondo semimetal phase. Taken together, these results identify YbCr6Ge6 as a prototype of a topological heavy-fermion system and a platform where geometric frustration, strong correlations, and topology converge, with broad implications for correlated quantum matter. △ Less

Submitted 19 March, 2026; v1 submitted 5 September, 2025; originally announced September 2025.

Flat Midgap Topological Surface and Hypersurface Bands without Parameter Tuning

Authors: Keun Hyuk Ahn

Abstract: The Su-Schrieffer-Heeger model is extended to the three and higher dimensional systems. Nearly or absolutely flat midgap surface and hypersurface bands are predicted based on the topological analysis, which do not require fine tuning of parameters. By adding the on-site Coulomb interaction for the three dimensional systems, we computationally show that the large difference in the band widths betwe… ▽ More The Su-Schrieffer-Heeger model is extended to the three and higher dimensional systems. Nearly or absolutely flat midgap surface and hypersurface bands are predicted based on the topological analysis, which do not require fine tuning of parameters. By adding the on-site Coulomb interaction for the three dimensional systems, we computationally show that the large difference in the band widths between the surface and the bulk leads to the strongly correlated phenomena, specifically magnetism, confined only on the surface. Possible experimental realizations in solid state materials and metamaterials are discussed. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Comments: About 4 pages of main text (excluding the reference list) and 4 pages of supplemental material

THz carrier dynamics in $SrTiO_{3}/LaTiO_{3}$ interface two-dimensional electron gases

Authors: Ahana Bhattacharya, Andri Darmawan, Jeong Woo Han, Frederik Steinkamp, Nicholas S. Bingham, Ryan J. Suess, Stephan Winnerl, Markus E. Grune, Eric N. Jin, Frederick J. Walker, Charles H. Ahn, Rossitza Pentcheva, Martin Mittendorff

Abstract: A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like $SrTiO_{3}$ (STO) and $LaTiO_{3}$ (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect trans… ▽ More A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like $SrTiO_{3}$ (STO) and $LaTiO_{3}$ (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of $10^{12}$ $cm^2$ and 1000 $cm^2V^{-1}s^{-1}$, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 $m_{e}$. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Quantum critical electro-optic and piezo-electric nonlinearities

Authors: Christopher P. Anderson, Giovanni Scuri, Aaron Chan, Sungjun Eun, Alexander D. White, Geun Ho Ahn, Christine Jilly, Amir Safavi-Naeini, Kasper Van Gasse, Lu Li, Jelena Vučković

Abstract: Electro-optics, the tuning of optical properties of materials with electric fields, is key to a multitude of quantum and classical photonics applications. However, a major obstacle preventing many emerging use cases is inefficient modulation in cryogenic environments, as traditional tuning mechanisms degrade at low temperatures. Guided by the connection between phase transitions and nonlinearity,… ▽ More Electro-optics, the tuning of optical properties of materials with electric fields, is key to a multitude of quantum and classical photonics applications. However, a major obstacle preventing many emerging use cases is inefficient modulation in cryogenic environments, as traditional tuning mechanisms degrade at low temperatures. Guided by the connection between phase transitions and nonlinearity, we identify the quantum paraelectric perovskite SrTiO$_3$ (STO) as the strongest cryogenic electro-optic photonic material. As a result of the unique quantum paraelectric phase of STO, we demonstrate a dynamically tunable linear Pockels coefficient ($r_{33}$) exceeding 500 pm/V at $T=5$ K, and study its full temperature and bias dependence. We also measure an enhanced piezo-electric coefficient ($d_{33}$) above 90 pC/N. Both of these coefficients exceed all previously reported values for cryogenic materials, including lithium niobate ($r_{33}\approx24$ pm/V) and barium titanate ($r_{42}\approx170$ pm/V). Furthermore, by tuning STO towards \textit{quantum criticality} with oxygen isotope substitution we more than double the optical and piezo-electric nonlinearities, demonstrating a linear Pockels coefficient above 1100 pm/V. Our results probe the link between quantum phase transitions, dielectric susceptibility, and optical nonlinearities, unlocking opportunities in cryogenic optical and mechanical systems, and provide a framework for discovering new nonlinear materials. △ Less

Submitted 25 February, 2025; v1 submitted 20 February, 2025; originally announced February 2025.

Epitaxially defined Luttinger liquids on MoS$_2$ bicrystals

Authors: Bingchen Deng, Heonsu Ahn, Jue Wang, Gunho Moon, Ninad Dongre, Chao Lei, Giovanni Scuri, Jiho Sung, Elise Brutschea, Kenji Watanabe, Takashi Taniguchi, Fan Zhang, Moon-Ho Jo, Hongkun Park

Abstract: A mirror twin boundary (MTB) in a transition metal dichalcogenide (TMD) monolayer can host one-dimensional electron liquid of a topological nature with tunable interactions. Unfortunately, the electrical characterization of such boundaries has been challenging due to the paucity of samples with large enough size and high quality. Here, we report an epitaxial growth of monolayer molybdenum disulfid… ▽ More A mirror twin boundary (MTB) in a transition metal dichalcogenide (TMD) monolayer can host one-dimensional electron liquid of a topological nature with tunable interactions. Unfortunately, the electrical characterization of such boundaries has been challenging due to the paucity of samples with large enough size and high quality. Here, we report an epitaxial growth of monolayer molybdenum disulfide (MoS$_2$) bicrystals with well-isolated MTBs that are tens of micrometers long. Conductance measurements of these MTBs exhibit power-law behaviors as a function of temperature and bias voltage up to room temperature, consistent with electrons tunneling into a Luttinger liquid. Transport measurements of two distinct types of MTBs reveal the critical role of the atomic-scale defects. This study demonstrates that MTBs in TMD monolayers provide an exciting new platform for studying the interplay between electronic interactions and topology. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Journal ref: Phys. Rev. Lett. 134, 046301 (2025)

Seamless monolithic three-dimensional integration of single-crystalline films by growth

Authors: Ki Seok Kim, Seunghwan Seo, Junyoung Kwon, Doyoon Lee, Changhyun Kim, Jung-El Ryu, Jekyung Kim, Min-Kyu Song, Jun Min Suh, Hang-Gyo Jung, Youhwan Jo, Hogeun Ahn, Sangho Lee, Kyeongjae Cho, Jongwook Jeon, Minsu Seol, Jin-Hong Park, Sang Won Kim, Jeehwan Kim

Abstract: The demand for the three-dimensional (3D) integration of electronic components is on a steady rise. The through-silicon-via (TSV) technique emerges as the only viable method for integrating single-crystalline device components in a 3D format, despite encountering significant processing challenges. While monolithic 3D (M3D) integration schemes show promise, the seamless connection of single-crystal… ▽ More The demand for the three-dimensional (3D) integration of electronic components is on a steady rise. The through-silicon-via (TSV) technique emerges as the only viable method for integrating single-crystalline device components in a 3D format, despite encountering significant processing challenges. While monolithic 3D (M3D) integration schemes show promise, the seamless connection of single-crystalline semiconductors without intervening wafers has yet to be demonstrated. This challenge arises from the inherent difficulty of growing single crystals on amorphous or polycrystalline surfaces post the back-end-of-the-line process at low temperatures to preserve the underlying circuitry. Consequently, a practical growth-based solution for M3D of single crystals remains elusive. Here, we present a method for growing single-crystalline channel materials, specifically composed of transition metal dichalcogenides, on amorphous and polycrystalline surfaces at temperatures lower than 400 °C. Building on this developed technique, we demonstrate the seamless monolithic integration of vertical single-crystalline logic transistor arrays. This accomplishment leads to the development of unprecedented vertical CMOS arrays, thereby constructing vertical inverters. Ultimately, this achievement sets the stage to pave the way for M3D integration of various electronic and optoelectronic hardware in the form of single crystals. △ Less

Submitted 6 December, 2023; v1 submitted 5 December, 2023; originally announced December 2023.

Coexistence of Anomalous Hall Effect and Weak Net Magnetization in Collinear Antiferromagnet MnTe

Authors: K. P. Kluczyk, K. Gas, M. J. Grzybowski, P. Skupiński, M. A. Borysiewicz, T. Fąs, J. Suffczyński, J. Z. Domagala, K. Grasza, A. Mycielski, M. Baj, K. H. Ahn, K. Výborný, M. Sawicki, M. Gryglas-Borysiewicz

Abstract: Anomalous Hall effect (AHE) plays important role in the rapidly developing field of antiferromagnetic spintronics. It has been recently discussed that it can be a feature of not only uncompensated magnetic systems but also in altermagnetic materials. Hexagonal MnTe belongs to this appealing group of compounds exhibiting AHE and is commonly perceived as magnetically compensated. Here, we demonstrat… ▽ More Anomalous Hall effect (AHE) plays important role in the rapidly developing field of antiferromagnetic spintronics. It has been recently discussed that it can be a feature of not only uncompensated magnetic systems but also in altermagnetic materials. Hexagonal MnTe belongs to this appealing group of compounds exhibiting AHE and is commonly perceived as magnetically compensated. Here, we demonstrate that bulk form of MnTe exhibits small but detectable magnetic moment correlating with hysteretic behaviour of the AHE. We formulate a phenomenological model which explains how this feature allows to create a disbalance between states with opposite Néel vector and prevent the AHE signal from averaging out to zero. Moreover, we show how the dependence of AHE on the Néel vector arises on microscopical level and highlight the differences in Berry curvature between magnetically compensated and uncompensated systems. △ Less

Submitted 13 October, 2023; originally announced October 2023.

Journal ref: Physical Review B 110, 155201 (2024)

Microwave Spin Control of a Tin-Vacancy Qubit in Diamond

Authors: Eric I. Rosenthal, Christopher P. Anderson, Hannah C. Kleidermacher, Abigail J. Stein, Hope Lee, Jakob Grzesik, Giovanni Scuri, Alison E. Rugar, Daniel Riedel, Shahriar Aghaeimeibodi, Geun Ho Ahn, Kasper Van Gasse, Jelena Vuckovic

Abstract: The negatively charged tin-vacancy (SnV-) center in diamond is a promising solid-state qubit for applications in quantum networking due to its high quantum efficiency, strong zero phonon emission, and reduced sensitivity to electrical noise. The SnV- has a large spin-orbit coupling, which allows for long spin lifetimes at elevated temperatures, but unfortunately suppresses the magnetic dipole tran… ▽ More The negatively charged tin-vacancy (SnV-) center in diamond is a promising solid-state qubit for applications in quantum networking due to its high quantum efficiency, strong zero phonon emission, and reduced sensitivity to electrical noise. The SnV- has a large spin-orbit coupling, which allows for long spin lifetimes at elevated temperatures, but unfortunately suppresses the magnetic dipole transitions desired for quantum control. Here, by use of a naturally strained center, we overcome this limitation and achieve high-fidelity microwave spin control. We demonstrate a pi-pulse fidelity of up to 99.51+/0.03%$ and a Hahn-echo coherence time of T2echo = 170.0+/-2.8 microseconds, both the highest yet reported for SnV- platform. This performance comes without compromise to optical stability, and is demonstrated at 1.7 Kelvin where ample cooling power is available to mitigate drive induced heating. These results pave the way for SnV- spins to be used as a building block for future quantum technologies. △ Less

Submitted 30 August, 2023; v1 submitted 22 June, 2023; originally announced June 2023.

Comments: Final published version

Journal ref: Phys. Rev. X 13, 031022 (2023)

Photoluminescence Path Bifurcations by Spin Flip in Two-Dimensional CrPS4

Authors: Suhyeon Kim, Sangho Yoon, Hyo-Bin Ahn, Gangtae Jin, Hyesun Kim, Moon-Ho Jo, Changgu Lee, Jonghwan Kim, Sunmin Ryu

Abstract: Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters owing to long-range spin correlation and efficient transition between high and low-spin excited states of Cr3+ ions. In this study, we report on dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into f… ▽ More Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters owing to long-range spin correlation and efficient transition between high and low-spin excited states of Cr3+ ions. In this study, we report on dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing novel magneto-optic functions and devices. △ Less

Submitted 20 September, 2022; v1 submitted 8 June, 2022; originally announced June 2022.

Accessing power-law statistics under experimental constraints

Authors: Xavier Durang, Hyerim Ahn, Jae Youn Shim, Hye Yoon Park, Jae-Hyung Jeon

Abstract: Over the last decades, impressive progresses have been made in many experimental domains, e.g. microscopic techniques such as single-particle tracking, leading to plethoric amounts of data. In a large variety of systems, from natural to socio-economic, the analysis of these experimental data conducted us to conclude about the omnipresence of power-laws. For example, in living systems, we are used… ▽ More Over the last decades, impressive progresses have been made in many experimental domains, e.g. microscopic techniques such as single-particle tracking, leading to plethoric amounts of data. In a large variety of systems, from natural to socio-economic, the analysis of these experimental data conducted us to conclude about the omnipresence of power-laws. For example, in living systems, we are used to observing anomalous diffusion, e.g. in the motion of proteins within the cell. However, estimating the power-law exponents is challenging. Both technical constraints and experimental limitations affect the statistics of observed data. Here, we investigate in detail the influence of two essential constraints in the experiment, namely, the temporal-spatial resolution and the time-window of the experiment. We study how the observed distribution of an observable is modified by them and analytically derive the expression of the power-law distribution for the observed distribution through the scope of the experiment. We also apply our results on data from an experimental study of the transport of mRNA-protein complexes along dendrites. △ Less

Submitted 2 June, 2022; originally announced June 2022.

Comments: 9 pages, 7 figures

Heteroepitaxial control of Fermi liquid, Hund metal, and Mott insulator phases in the single-atomic-layer limit

Authors: Jeong Rae Kim, Byungmin Sohn, Hyeong Jun Lee, Sangmin Lee, Eun Kyo Ko, Sungsoo Hahn, Sangjae Lee, Younsik Kim, Donghan Kim, Hong Joon Kim, Youngdo Kim, Jaeseok Son, Charles H. Ahn, Frederick J. Walker, Ara Go, Miyoung Kim, Choong H. Kim, Changyoung Kim, Tae Won Noh

Abstract: Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenom… ▽ More Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenomena appear within a few atomic layers from the interface, thus limiting the application of available experimental probes. Here, we utilize atomic-scale epitaxy and photoemission spectroscopy to demonstrate the interface control of correlated electronic phases in atomic-scale ruthenate--titanate heterostructures. While bulk SrRuO$_3$ is a ferromagnetic metal, the heterointerfaces exclusively realize three distinct correlated phases in the single-atomic-layer limit. Our theory reveals that atomic-scale structural proximity effects lead to the emergence of Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO$_3$. These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. △ Less

Submitted 8 March, 2022; originally announced March 2022.

Comments: 25 pages, 5 main figures, 16 supplementary figures

Observation of Phase Controllable Majorana-like Bound States in Metamaterial-based Kitaev Chain Analogues

Authors: Kai Qian, David J. Apigo, Karmela Padavić, Keun Hyuk Ahn, Smitha Vishveshwara, Camelia Prodan

Abstract: We experimentally demonstrate that Majorana-like bound states (MLBSs) can occur in quasi-one-dimensional metamaterials, analogous to Majorana zero modes (MZM) in the Kitaev chain. In a mechanical spinner ladder system, we observe a topological phase transition and spectral-gap-protected edge MLBSs. We characterize the decaying and oscillatory nature of these MLBS pairs and their phase-dependent hy… ▽ More We experimentally demonstrate that Majorana-like bound states (MLBSs) can occur in quasi-one-dimensional metamaterials, analogous to Majorana zero modes (MZM) in the Kitaev chain. In a mechanical spinner ladder system, we observe a topological phase transition and spectral-gap-protected edge MLBSs. We characterize the decaying and oscillatory nature of these MLBS pairs and their phase-dependent hybridization. It is shown that the hybridization can be tuned to yield the analogue of parity switching in MZMs, a key element of topological qubits. We find strong agreements with theory. △ Less

Submitted 28 January, 2022; originally announced January 2022.

Tuning spin excitations in magnetic films by confinement

Authors: Jonathan Pelliciari, Sangjae Lee, Keith Gilmore, Jiemin Li, Yanhong Gu, Andi Barbour, Ignace Jarrige, Charles H. Ahn, Frederick J. Walker, Valentina Bisogni

Abstract: Spin excitations of magnetic thin films are the founding element for novel transport concepts in spintronics, magnonics, and magnetic devices in general. While spin dynamics have been extensively studied in bulk materials, their behaviour in mesoscopic films is less known due to experimental limitations. Here, we employ Resonant Inelastic X-Ray Scattering to investigate the spin excitation spectru… ▽ More Spin excitations of magnetic thin films are the founding element for novel transport concepts in spintronics, magnonics, and magnetic devices in general. While spin dynamics have been extensively studied in bulk materials, their behaviour in mesoscopic films is less known due to experimental limitations. Here, we employ Resonant Inelastic X-Ray Scattering to investigate the spin excitation spectrum in mesoscopic Fe films, from bulk-like down to 3 unit cells thick. In bulk-like samples, we find isotropic, dispersive ferromagnons consistent with the dispersion observed by neutron scattering in bulk single crystals. As the thickness is reduced, these ferromagnons survive and evolve anisotropically: renormalising to lower energies along the out-of-plane direction while retaining their dispersion in the in-plane direction. This thickness dependence is captured by simple Heisenberg model calculations accounting for the confinement in the out-of-plane direction through the loss of Fe bonds. Our findings highlight the effects of mesoscopic scaling on spin dynamics and identify thickness as a knob for fine-tuning and controlling magnetic properties in films. △ Less

Submitted 17 October, 2020; originally announced October 2020.

Comments: 17 pages, 4, figure, submitted

Journal ref: Nat. Mat. (2021)

Observation of Flat Frequency Bands at Open Edges and Antiphase Boundary Seams in Topological Mechanical Metamaterials

Authors: Kai Qian, Linghua Zhu, Keun Hyuk Ahn, Camelia Prodan

Abstract: Motivated by the recent theoretical studies on a two-dimensional (2D) chiral Hamiltonian based on the Su-Schrieffer-Heeger chains, we experimentally and computationally demonstrate that topological flat frequency bands can occur at open edges of 2D planar metamaterials and at antiphase boundary seams of ring-shaped or tubular metamaterials. Specifically, using mechanical systems made of magnetical… ▽ More Motivated by the recent theoretical studies on a two-dimensional (2D) chiral Hamiltonian based on the Su-Schrieffer-Heeger chains, we experimentally and computationally demonstrate that topological flat frequency bands can occur at open edges of 2D planar metamaterials and at antiphase boundary seams of ring-shaped or tubular metamaterials. Specifically, using mechanical systems made of magnetically coupled spinners, we reveal that the presence of the edge or seam bands that are flat in the entire projected reciprocal space follows the predictions based on topological winding numbers. The edge-to-edge distance sensitively controls the flatness of the edge bands and the localization of excitations. The analogue of the fractional charge state is also observed. Possible realizations of flat bands in a large class of metamaterials, including photonic crystals and electronic metamaterials, are discussed. △ Less

Submitted 24 November, 2020; v1 submitted 13 May, 2020; originally announced May 2020.

Journal ref: Phys. Rev. Lett. 125, 225501 (2020)

Effect of Low Oxygen Annealing on Photoelectrochemical Water Splitting Properties of $α$-Fe$_2$O$_3$

Authors: Yoichi Makimizu, Nhat Truong Nguyen, Hyo-Jin Ahn, JeongEun Yoo, Imgon Hwang, Stepan Kment, Patrik Schmuki

Abstract: Photoelectrochemical (PEC) water splitting is a promising method for conversing solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite ($α$-Fe$_2$O$_3$) is one of the most attractive materials for highly efficient charge carrier generation and collection due to its large specific surface area and shortening minority carrier diffusion length required to reach the… ▽ More Photoelectrochemical (PEC) water splitting is a promising method for conversing solar energy into chemical energy stored in the form of hydrogen. Nanostructured hematite ($α$-Fe$_2$O$_3$) is one of the most attractive materials for highly efficient charge carrier generation and collection due to its large specific surface area and shortening minority carrier diffusion length required to reach the surface. In the present work, PEC water splitting performance of $α$-Fe$_2$O$_3$ prepared by anodization of thin iron layers on an FTO glass and subsequent annealing in low O$_2$-Ar ambient with only 0.03% O$_2$ was investigated. The key finding is that annealing the anodic nanostructures with low oxygen concentration provides a strongly enhanced PEC performance compared with classic air annealing. The photocurrent of the former at 1.5 V vs. RHE results in 1.1 mA/cm2, being 11 times higher than that of the latter. The enhancement of the PEC performance for $α$-Fe$_2$O$_3$ annealed in low oxygen atmosphere can be attributed to controlled morphology, Sn doping, and introduction of oxygen vacancies, which contribute to the enhancement of the hole flux from the photogenerated site to the reactive surface and additionally lead to an enhanced hole transfer at the interface between the $α$-Fe$_2$O$_3$ and the electrolyte. From the obtained results, it is evident that low oxygen annealing is a surprisingly effective method of defect engineering and optimizing $α$-Fe$_2$O$_3$ electrodes for a maximized PEC water splitting performance. △ Less

Submitted 14 April, 2020; originally announced May 2020.

Stabilization of competing ferroelectric phases of HfO$_2$ under epitaxial strain

Authors: Yubo Qi, Sobhit Singh, Claudia Lau, Fei-Ting Huang, Xianghan Xu, Frederick J. Walker, Charles H. Ahn, Sang-Wook Cheong, Karin M. Rabe

Abstract: Hafnia (HfO$_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO$_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO$_2$ have attracted intensive research interest in recent years… ▽ More Hafnia (HfO$_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO$_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO$_2$ have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO$_2$ are used to discover that imposing an in-plane shear strain on the tetragonal phase induces a nonpolar to polar phase transition. This in-plane shear-induced polar phase is shown to be an epitaxial distortion of a known metastable ferroelectric $Pnm2_1$ phase of HfO$_2$. It is proposed that this ferroelectric $Pnm2_1$ phase can account for the recently observed ferroelectricity in the (111)-oriented HfO$_2$-based thin film [Nature Materials 17, 1095-1100 (2018)]. Further investigation of this second functional ferroelectric phase in HfO$_2$ could potentially improve the performances of HfO$_2$-based films in logic and memory devices. △ Less

Submitted 26 September, 2020; v1 submitted 23 January, 2020; originally announced January 2020.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 125, 257603 (2020)

Strain-engineering of Berry curvature dipole and valley magnetization in monolayer MoS$_2$

Authors: Joolee Son, Kyung-Han Kim, Y. H. Ahn, Hyun-Woo Lee, Jieun Lee

Abstract: The Berry curvature dipole is a physical quantity that is expected to allow various quantum geometrical phenomena in a range of solid-state systems. Monolayer transition metal dichalcogenides provide an exceptional platform to modulate and investigate the Berry curvature dipole through strain. Here we theoretically demonstrate and experimentally verify for monolayer MoS$_\rm{2}$ the generation of… ▽ More The Berry curvature dipole is a physical quantity that is expected to allow various quantum geometrical phenomena in a range of solid-state systems. Monolayer transition metal dichalcogenides provide an exceptional platform to modulate and investigate the Berry curvature dipole through strain. Here we theoretically demonstrate and experimentally verify for monolayer MoS$_\rm{2}$ the generation of valley orbital magnetization as a response to an in-plane electric field due to the Berry curvature dipole. The measured valley orbital magnetization shows excellent agreement with the calculated Berry curvature dipole which can be controlled by the magnitude and direction of strain. Our results show that the Berry curvature dipole acts as an effective magnetic field in current-carrying systems, providing a novel route to generate magnetization. △ Less

Submitted 2 July, 2019; v1 submitted 28 June, 2019; originally announced July 2019.

Comments: 5 pages, 5 figures

Role of remote interfacial phonons in the resistivity of graphene

Authors: Y. G. You, J. H. Ahn, B. H. Park, Y. Kwon, E. E. B. Campbell, S. H. Jhang

Abstract: The temperature ($\it T$) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates; HfO$_2$, SiO$_2$ and h-BN. The resistivity of graphene shows a linear $\it T$-dependence at low $\it T$ and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfa… ▽ More The temperature ($\it T$) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates; HfO$_2$, SiO$_2$ and h-BN. The resistivity of graphene shows a linear $\it T$-dependence at low $\it T$ and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfacial phonon scattering by surface optical phonons at the substrates. The use of an appropriate substrate can lead to a significant improvement in the charge transport of graphene. △ Less

Submitted 22 March, 2019; originally announced March 2019.

Journal ref: Appl. Phys. Lett. 115, 043104 (2019)

Suppressing Diffusion-Mediated Exciton Annihilation in 2D Semiconductors Using the Dielectric Environment

Authors: Aaron J. Goodman, Der-Hsien Lien, Geun Ho Ahn, Leo L. Spiegel, Matin Amani, Adam P. Willard, Ali Javey, William A. Tisdale

Abstract: Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton-exciton annihilation at notably low excitation densities (below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate constant for second-… ▽ More Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton-exciton annihilation at notably low excitation densities (below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate constant for second-order exciton-exciton annihilation, k_XX [cm2/s], is reduced by one or two orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in chemical-treated MoS2 to be D = 0.06 +/- 0.01 cm2/s, corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of τ = 20 ns, which is independent of the substrate. These results, together with numerical simulations, suggest that the effective exciton-exciton annihilation radius monotonically decreases with increasing refractive index of the underlying substrate. Exciton-exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities; the ability to tune these interactions via the dielectric environment is an important step toward more efficient optoelectronic technologies featuring atomically thin materials. △ Less

Submitted 2 November, 2018; originally announced November 2018.

Sr$_3$Ir$_2$O$_7$F$_2$: Topochemical conversion of a relativistic Mott state into a spin-orbit driven band insulator

Authors: Christi Peterson, Michael W. Swift, Zach Porter, Raphaele J. Clement, Guang Wu, G. H. Ahn, S. J. Moon, B. C. Chakoumakos, Jacob P. C. Ruff, Huibo Cao, Chris Van de Walle, Stephen D. Wilson

Abstract: The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic… ▽ More The topochemical transformation of single crystals of Sr$_3$Ir$_2$O$_7$ into Sr$_3$Ir$_2$O$_7$F$_2$ is reported via fluorine insertion. Characterization of the newly formed Sr$_3$Ir$_2$O$_7$F$_2$ phase shows a nearly complete oxidation of Ir$^{4+}$ cations into Ir$^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a half-filled J$_{eff}=1/2$ band into a nonmagnetic $J=0$ band insulator. First principles calculations reveal a remarkably flat insertion energy that locally drives the fluorination process to completion. Band structure calculations support the formation of a band insulator whose charge gap relies on the strong spin-orbit coupling inherent to the Ir metal ions of this compound. △ Less

Submitted 6 October, 2018; originally announced October 2018.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 98, 155128 (2018)

Control of hidden ground-state order in NdNiO$_3$ superlattices

Authors: Ankit S. Disa, Alexandru B. Georgescu, James L. Hart, Divine P. Kumah, Padraic Shafer, Elke Arenholz, Dario A. Arena, Sohrab Ismail-Beigi, Mitra L. Taheri, Frederick J. Walker, Charles H. Ahn

Abstract: The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures,… ▽ More The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO$_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases. △ Less

Submitted 20 September, 2018; originally announced September 2018.

Comments: 30 pages, 13 figures

Journal ref: Phys. Rev. Materials 1, 024410 (2017)

Flat Energy Bands within Antiphase and Twin Boundaries and at Open Edges in Topological Materials

Authors: Linghua Zhu, Emil Prodan, Keun Hyuk Ahn

Abstract: A model for two-dimensional electronic, photonic, and mechanical metamaterial systems is presented, which has flat one-dimensional zero-mode energy bands and stable localized states of a topological origin confined within twin boundaries, antiphase boundaries, and at open edges. Topological origins of these flat bands are analyzed for an electronic system as a specific example, using a two-dimensi… ▽ More A model for two-dimensional electronic, photonic, and mechanical metamaterial systems is presented, which has flat one-dimensional zero-mode energy bands and stable localized states of a topological origin confined within twin boundaries, antiphase boundaries, and at open edges. Topological origins of these flat bands are analyzed for an electronic system as a specific example, using a two-dimensional extension of the Su-Schrieffer-Heeger Hamiltonian with alternating shift of the chains. It is demonstrated that the slow group velocities of the localized flat band states are sensitively controlled by the distance between the boundaries and the propagation can be guided through designed paths of these boundaries. We also discuss how to realize this model in metamaterials. △ Less

Submitted 6 January, 2019; v1 submitted 27 July, 2018; originally announced July 2018.

Comments: 5 page main text, 2 page supplemental material

Journal ref: Phys. Rev. B 99, 041117 (2019)

Suppression of the Spectral Weight of Topological Surface States on the Nanoscale via Local Symmetry Breaking

Authors: Omur E. Dagdeviren, Subhasish Mandal, Ke Zou, Chao Zhou, Georg H. Simon, Frederick J. Walker, Charles H. Ahn, Udo D. Schwarz, Sohrab Ismail-Beigi, Eric I. Altman

Abstract: In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogenous, single-phase material. We show using scanning tunneling microscopy/spectroscopy that defects that break local symmetry of SnTe suppress… ▽ More In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogenous, single-phase material. We show using scanning tunneling microscopy/spectroscopy that defects that break local symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The finding highlights the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of topological surface states and can operate at practical temperatures. △ Less

Submitted 30 November, 2018; v1 submitted 1 April, 2018; originally announced April 2018.

Journal ref: Phys. Rev. Materials 2, 114205 (2018)

Photoinduced Nonequilibrium Dynamics in Charge Ordered Materials

Authors: Linghua Zhu, Tsezar F. Seman, Michel van Veenendaal, Keun Hyuk Ahn

Abstract: We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter a… ▽ More We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter and the periodic lattice distortion during CO melting, which becomes more energy efficient with lower photon energy. The cooling rate of the electron system correlates with the CO gap dynamics, responsible for an order of magnitude decrease in the cooling rate upon the gap reopening. We also find that the time-dependent frequency of coherent oscillation reflects the dynamics of the energy landscape, such as transition between single-well and double-well, which sensitively depends on the photon energy and the pump fluence. The results demonstrate the intricate nonequilibrium dynamics in CO materials. △ Less

Submitted 21 March, 2018; v1 submitted 7 August, 2017; originally announced August 2017.

Comments: 10 pages, 8 figures

Journal ref: Journal of Applied Physics 123, 115112 (2018)

Structural Distortions At Polar Manganite Interfaces

Authors: S. Koohfar, A. S. Disa, M. Marshall, F. J. Walker, C. H. Ahn, D. P. Kumah

Abstract: Electronic, lattice, and spin interactions at the interfaces between crystalline complex transition metal oxides can give rise to a wide range of functional electronic and magnetic phenomena not found in bulk. At hetero-interfaces, these interactions may be enhanced by combining oxides where the polarity changes at the interface. The physical structure between non-polar SrTiO$_3$ and polar La… ▽ More Electronic, lattice, and spin interactions at the interfaces between crystalline complex transition metal oxides can give rise to a wide range of functional electronic and magnetic phenomena not found in bulk. At hetero-interfaces, these interactions may be enhanced by combining oxides where the polarity changes at the interface. The physical structure between non-polar SrTiO$_3$ and polar La$_{1-x}$Sr$_x$MnO$_3$(x=0.2) is investigated using high resolution synchrotron x-ray diffraction to directly determine the role of structure in compensating the polar discontinuity. At both the oxide-oxide interface and vacuum-oxide interfaces, the lattice is found to expand and rumple along the growth direction. The SrTiO$_3$/La$_{1-x}$Sr$_x$MnO$_3$ interface also exhibits intermixing of La and Sr over a few unit cells. The results, hence, demonstrate that polar distortions and ionic intermixing coexist and both pathways play a significant role at interfaces with polar discontinuities. △ Less

Submitted 4 May, 2017; v1 submitted 17 April, 2017; originally announced April 2017.

Journal ref: Phys. Rev. B 96, 024108 (2017)

Experimental verification of orbital engineering at the atomic scale: charge transfer and symmetry breaking in nickelate heterostructures

Authors: Patrick J. Phillips, Paolo Longo, Alexandru B. Georgescu, Eiji Okunishi, Xue Rui, Ankit S. Disa, Fred Walker, Sohrab Ismail-Beigi, Charles H. Ahn, Robert F. Klie

Abstract: Epitaxial strain, layer confinement and inversion symmetry breaking have emerged as powerful new approaches to control the electronic and atomic-scale structural properties in complex metal oxides. Nickelate heterostructures, based on RENiO$_3$, where RE is a trivalent rare-earth cation, have been shown to be relevant model systems since the orbital occupancy, degeneracy, and, consequently, the el… ▽ More Epitaxial strain, layer confinement and inversion symmetry breaking have emerged as powerful new approaches to control the electronic and atomic-scale structural properties in complex metal oxides. Nickelate heterostructures, based on RENiO$_3$, where RE is a trivalent rare-earth cation, have been shown to be relevant model systems since the orbital occupancy, degeneracy, and, consequently, the electronic/magnetic properties can be altered as a function of epitaxial strain, layer thickness and superlattice structure. One such recent example is the tri-component LaTiO$_3$-LaNiO$_3$-LaAlO$_3$ superlattice, which exhibits charge transfer and orbital polarization as the result of its interfacial dipole electric field. A crucial step towards control of these parameters for future electronic and magnetic device applications is to develop an understanding of both the magnitude and range of the octahedral network's response towards interfacial strain and electric fields. An approach that provides atomic-scale resolution and sensitivity towards the local octahedral distortions and orbital occupancy is therefore required. Here, we employ atomic-resolution imaging coupled with electron spectroscopies and first principles theory to examine the role of interfacial charge transfer and symmetry breaking in a tricomponent nickelate superlattice system. We find that nearly complete charge transfer occurs between the LaTiO$_3$ and LaNiO$_3$ layers, resulting in a Ni$^{2+}$ valence state. We further demonstrate that this charge transfer is highly localized with a range of about 1 unit cell, within the LaNiO$_3$ layers. The results presented here provide important feedback to synthesis efforts aimed at stabilizing new electronic phases that are not accessible by conventional bulk or epitaxial film approaches. △ Less

Submitted 16 December, 2016; originally announced December 2016.

Journal ref: Phys. Rev. B 95, 205131 (2017)

The role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors

Authors: Ke Zou, Subhasish Mandal, Stephen Albright, Rui Peng, Yujia Pu, Divine Kumah, Claudia Lau, Georg Simon, Omur E. Dagdeviren, Xi He, Ivan Bozovic, Udo D. Schwarz, Eric I. Altman, Donglai Feng, Fred J. Walker, Sohrab Ismail-Beigi, Charles H. Ahn

Abstract: We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab in… ▽ More We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor. △ Less

Submitted 4 May, 2016; originally announced May 2016.

Comments: Physical Review B: Rapid Communications in Press

Journal ref: Phys. Rev. B 93, 180506(R) 2016

Orbital engineering in nickelate heterostructures driven by anisotropic oxygen hybridization rather than orbital energy levels

Authors: G. Fabbris, D. Meyers, J. Okamoto, J. Pelliciari, A. S. Disa, Y. Huang, Z. -Y. Chen, W. B. Wu, C. T. Chen, S. Ismail-Beigi, C. H. Ahn, F. J. Walker, D. J. Huang, T. Schmitt, M. P. M. Dean

Abstract: Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking $\mathrm{LaTiO_3-LaNiO_3-3x(LaAlO_3)}$, a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni $3d$ orbital energy levels, contradicting the often-invoked picture in which chan… ▽ More Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking $\mathrm{LaTiO_3-LaNiO_3-3x(LaAlO_3)}$, a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni $3d$ orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O $K$-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena. △ Less

Submitted 8 September, 2016; v1 submitted 29 March, 2016; originally announced March 2016.

Comments: Accepted for publication in Physical Review Letters. 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 117, 147401 (2016)

Linkage-length dependent structuring behaviour of bent-core molecules in helical nanostructures

Authors: Hanim Kim, Anna Zep, Seong Ho Ryu, Hyungju Ahn, Tae Joo Shin, Sang Bok Lee, Damian Pociecha, Ewa Gorecka, Dong Ki Yoon

Abstract: We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction… ▽ More We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction were used to elucidate the linkage length-dependent formation of HNFs. △ Less

Submitted 16 February, 2016; originally announced February 2016.

Comments: published in Soft Matter (2016). 17 pages in total, 5 figures, 2 figures in Supporting materials

Oxide 2D electron gases as a route for high carrier densities on (001) Si

Authors: Lior Kornblum, Eric N. Jin, Divine P. Kumah, Alexis T. Ernst, Christine C. Broadbridge, Charles H. Ahn, Fred J. Walker

Abstract: Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing… ▽ More Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing $GdTiO_3-SrTiO_3$ on silicon. Structural analysis confirms the epitaxial growth of heterostructures with abrupt interfaces and a high degree of crystallinity. Transport measurements show the conduction to be an interface effect, with $\sim 9\times 10^{13} \; cm^{-2}$ electrons per interface. Good agreement is demonstrated between the electronic behavior of structures grown on Si and on an oxide substrate, validating the robustness of this approach to bridge between lab-scale samples to a scalable, technologically relevant materials system. △ Less

Submitted 11 May, 2015; originally announced May 2015.

Comments: 10 pages, 4 figures

Reversible modulation of orbital occupations via an interface-induced state in metallic manganites

Authors: Hanghui Chen, Qiao Qiao, Matthew S. J. Marshall, Alexandru B. Georgescu, Ahmet Gulec, Patrick J. Phillips, Robert F. Klie, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi

Abstract: The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering inmetal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroele… ▽ More The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering inmetal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroelectric polarization on the nanoscale. The change in orbital occupation can be as large as 10%, greatly exceeding that of bulk manganites. This reversible orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use epitaxial thin film growth and scanning transmission electron microscopy to verify this key interfacial polar distortion and discuss the potential of reversible control of orbital polarization via nanoscale ferroelectrics. △ Less

Submitted 12 September, 2014; originally announced September 2014.

Comments: This version is to replace the old submission arXiv:1309.2976 (withdrawn). We have made substantial revision and improvement in the new version

Journal ref: Nano Lett. 14, 4965 (2014)

Synthesis of SnTe Nanoplates with {100} and {111} Surfaces

Authors: Jie Shen, Yeonwoong Jung, Ankit S. Disa, Fred J. Walker, Charles H. Ahn, Judy J. Cha

Abstract: SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morpholog… ▽ More SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morphology of SnTe such that only desired crystallographic surfaces are present. Here, we grow SnTe nanostructures using vapor-liquid-solid and vapor-solid growth mechanisms. Previously, SnTe nanowires and nanocrystals have been grown.1-4 In this report, we demonstrate synthesis of SnTe nanoplates with lateral dimensions spanning tens of microns and thicknesses of a hundred nanometers. The top and bottom surfaces are either (100) or (111), maximizing topological surface states on these surfaces. Magnetotransport on these SnTe nanoplates shows high bulk carrier density, consistent with bulk SnTe crystals arising due to defects such as Sn vacancies. In addition, we observe a structural phase transition in these nanoplates from the high temperature rock salt to low temperature rhombohedral structure. For nanoplates with very high carrier density, we observe a slight upturn in resistance at low temperatures, indicating electron-electron interactions. △ Less

Submitted 19 June, 2014; originally announced June 2014.

Comments: Nano Lett., Article ASAP,2014

Journal ref: Nano letters 14 (7), 4183-4188, 2014

Imaging ultrafast carrier transport in nanoscale devices using femtosecond photocurrent microscopy

Authors: B. H. Son, Jae-Ku Park, J. T. Hong, Ji-Yong. Park, Soonil. Lee, Y. H. Ahn

Abstract: One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time sca… ▽ More One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ultrafast carrier dynamics in nanoscale devices using a combination of scanning photocurrent microscopy and time- resolved pump-probe techniques. We investigate transit times of carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Carrier dynamics have been measured for various working conditions. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias, because of the increased field strength at the Schottky barrier. △ Less

Submitted 1 June, 2014; originally announced June 2014.

Pressure Dependent Structural Changes and Predicted Electrical Polarization in Perovskite RMnO3

Authors: T. Wu, T. A. Tyson, H. Chen, P. Gao, T. Yu, Z. Chen, Z. Liu, K. H. Ahn, X. Wang, S. -W. Cheong

Abstract: High pressure x-ray diffraction (XRD) measurements on RMnO3 (R=Dy, Ho and Lu) reveals that varying structural changes occurs for different R ions. Large lattice changes (orthorhombic strain) occur in DyMnO3 and HoMnO3 while the Jahn-Teller (JT) distortion remains stable. On the other hand, in LuMnO3, Mn-O bond distortions are observed in the region 4-8 GPa with the broad minimum in the JT distorti… ▽ More High pressure x-ray diffraction (XRD) measurements on RMnO3 (R=Dy, Ho and Lu) reveals that varying structural changes occurs for different R ions. Large lattice changes (orthorhombic strain) occur in DyMnO3 and HoMnO3 while the Jahn-Teller (JT) distortion remains stable. On the other hand, in LuMnO3, Mn-O bond distortions are observed in the region 4-8 GPa with the broad minimum in the JT distortion. High pressure IR measurements indicate that a phonon near 390 cm-1 corresponding to the complex motion of the Mn and O ions changes anomalously for LuMnO3. It softens in the 4-8 GPa region, which is consistent with the structural change in Mn-O bonds and then hardens at high pressures. By contrast, the phonons continuously harden with increasing pressure for DyMnO3 and HoMnO3. DFT calculations show that the E-phase LuMnO3 is the most stable phase up to the 10 GPa pressure examined. Simulations indicate that the distinct structural change under pressure in LuMnO3 can possibly be used to optimize the electric polarization by pressure/strain. △ Less

Submitted 31 March, 2014; originally announced March 2014.

Comments: 12 Figures

Nature of Structural Changes Near the Magnetic Ordering Temperature in Small-Ion Rare Earth Perovskites RMnO3

Authors: T. Yu, T. A. Tyson, H. Y. Chen, A. M. M. Abeykoon, Y. -S. Chen, K. H. Ahn

Abstract: Detailed structural measurements were conducted on a new perovskite, ScMnO3, and on orthorhombic LuMnO3. Complementary density functional theory (DFT) calculations were carried out, and predict that ScMnO3 possesses E-phase magnetic order at low temperature with displacements of the Mn sites (relative to the high temperature state) of ~0.07 Å, compared to ~ 0.04 Å predicted for LuMnO3. However, de… ▽ More Detailed structural measurements were conducted on a new perovskite, ScMnO3, and on orthorhombic LuMnO3. Complementary density functional theory (DFT) calculations were carried out, and predict that ScMnO3 possesses E-phase magnetic order at low temperature with displacements of the Mn sites (relative to the high temperature state) of ~0.07 Å, compared to ~ 0.04 Å predicted for LuMnO3. However, detailed local, intermediate and long-range structural measurements by x-ray pair distribution function analysis, single crystal x-ray diffraction and x-ray absorption spectroscopy, find no local or long- range distortions on crossing into the low temperature E-phase of the magnetically ordered state. The measurements place upper limits on any structural changes to be at most one order of magnitude lower than density functional theory predictions and suggest that this theoretical approach does not properly account for the spin-lattice coupling in these oxides and may possibly predict the incorrect magnetic order at low temperatures. The results suggest that the electronic contribution to the electrical polarization dominates and should be properly treated in theoretical models. △ Less

Submitted 19 March, 2014; originally announced March 2014.

Comments: 16 Figures

Active silicon integrated nanophotonics: ferroelectric BaTiO3 devices

Authors: Chi Xiong, Wolfram H. P. Pernice, Joseph H. Ngai, James W. Reiner, Divine Kumah, Frederick J. Walker, Charles H. Ahn, Hong X. Tang

Abstract: The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric Ba… ▽ More The integration of complex oxides on silicon presents opportunities to extend and enhance silicon technology with novel electronic, magnetic, and photonic properties. Among these materials, barium titanate (BaTiO3) is a particularly strong ferroelectric perovskite oxide with attractive dielectric and electro-optic properties. Here we demonstrate nanophotonic circuits incorporating ferroelectric BaTiO3 thin films on the ubiquitous silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline BaTiO3 directly on SOI and engineer integrated waveguide structures that simultaneously confine light and an RF electric field in the BaTiO3 layer. Using on-chip photonic interferometers, we extract a large effective Pockels coefficient of 213 plus minus 49 pm/V, a value more than six times larger than found in commercial optical modulators based on lithium niobate. The monolithically integrated BaTiO3 optical modulators show modulation bandwidth in the gigahertz regime, which is promising for broadband applications. △ Less

Submitted 4 February, 2014; v1 submitted 16 January, 2014; originally announced January 2014.

Journal ref: Nano Lett., 2014, 14 (3), pp 1419-1425

Hysteretic electrical transport in BaTiO$_3$/Ba$_{1-x}$Sr$_x$TiO$_3$/Ge heterostructures

Authors: J. H. Ngai, D. P. Kumah, C. H. Ahn, F. J. Walker

Abstract: We present electrical transport measurements of heterostructures comprised of BaTiO$_3$ and Ba$_{1-x}$Sr$_x$TiO$_3$ epitaxially grown on Ge. The Sr-alloying imparts compressive strain to the BaTiO$_3$, which enables the thermal expansion mismatch between BaTiO$_3$ and Ge to be overcome to achieve $c$-axis oriented growth. The conduction bands of BaTiO$_3$ and Ba$_{1-x}$Sr$_x$TiO$_3$ are nearly ali… ▽ More We present electrical transport measurements of heterostructures comprised of BaTiO$_3$ and Ba$_{1-x}$Sr$_x$TiO$_3$ epitaxially grown on Ge. The Sr-alloying imparts compressive strain to the BaTiO$_3$, which enables the thermal expansion mismatch between BaTiO$_3$ and Ge to be overcome to achieve $c$-axis oriented growth. The conduction bands of BaTiO$_3$ and Ba$_{1-x}$Sr$_x$TiO$_3$ are nearly aligned with the conduction band of Ge, which facilitates electron transport. Electrical transport measurements through the dielectric stack exhibit rectifying behavior and hysteresis, where the latter is consistent with ferroelectric switching. △ Less

Submitted 13 November, 2013; originally announced November 2013.

Ultrafast and widely tuneable vertical-external-cavity surface-emitting laser, mode-locked by a graphene-integrated distributed Bragg reflector

Authors: C. A. Zaugg, Z. Sun, V. J. Wittwer, D. Popa, S. Milana, T. Kulmala, R. S. Sundaram, M. Mangold, O. D. Sieber, M. Golling, Y. Lee, J. H. Ahn, A. C. Ferrari, U. Keller

Abstract: We report a versatile and cost-effective way of controlling the unsaturated loss, modulation depth and saturation fluence of graphene-based saturable absorbers (GSAs), by changing the thickness of a spacer between SLG and a high-reflection mirror. This allows us to modulate the electric field intensity enhancement at the GSA from 0 up to 400%, due to the interference of incident and reflected ligh… ▽ More We report a versatile and cost-effective way of controlling the unsaturated loss, modulation depth and saturation fluence of graphene-based saturable absorbers (GSAs), by changing the thickness of a spacer between SLG and a high-reflection mirror. This allows us to modulate the electric field intensity enhancement at the GSA from 0 up to 400%, due to the interference of incident and reflected light at the mirror. The unsaturated loss of the SLG-mirror-assembly can be reduced to$\sim$0. We use this to mode-lock a VECSEL from 935 to 981nm. This approach can be applied to integrate SLG into various optical components, such as output coupler mirrors, dispersive mirrors, dielectric coatings on gain materials. Conversely, it can also be used to increase absorption (up to 10%) in various graphene based photonics and optoelectronics devices, such as photodetectors. △ Less

Submitted 8 October, 2013; originally announced October 2013.

Journal ref: Optics Expr. 21, 31548 (2013)

Dynamical control of orbital occupations via a ferroelectric-induced polar state in metallic manganites

Authors: Hanghui Chen, Qiao Qiao, Matthew S. J. Marshall, Alexandru B. Georgescu, Ahmet Gulec, Patrick J. Phillips, Robert F. Klie, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi

Abstract: The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering in metal oxides. Here, we show how to dynamically modulate the orbital populations on Mn atoms at ferroelectric/manganite interfaces by switching the ferroelectric polarization. The change in orbital occu… ▽ More The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering in metal oxides. Here, we show how to dynamically modulate the orbital populations on Mn atoms at ferroelectric/manganite interfaces by switching the ferroelectric polarization. The change in orbital occupation can be as large as 10\%, greatly exceeding that of bulk manganites. This flippable orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use {\it ab initio} theory, epitaxial thin film growth, and scanning transmission electron microscopy to verify the predicted interfacial polar state and concomitant orbital splittings. △ Less

Submitted 14 September, 2014; v1 submitted 11 September, 2013; originally announced September 2013.

Comments: This paper has been withdrawn by the authors. A new version arXiv:1409.3672 replaces this old version

Modifying the Electronic Orbitals of Nickelate Heterostructures Via Structural Distortions

Authors: Hanghui Chen, Divine P. Kumah, Ankit S. Disa, Frederick J. Walker, Charles H. Ahn, Sohrab Ismail-Beigi

Abstract: We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are: (i) the construction of atomic-scale distortions… ▽ More We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are: (i) the construction of atomic-scale distortions about the Ni site via charge transfer and internal electric fields, and (ii) the use of three component (tri-component) superlattices to break inversion symmetry. We use {\it ab initio} calculations to implement the approach, with experimental verification of the critical structural motif that enables the design to succeed. △ Less

Submitted 11 September, 2013; originally announced September 2013.

Comments: 12 pages, 3 figures and 1 table

Journal ref: Phys. Rev. Lett. 110, 186402 (2013)

Momentum dependence in K-edge resonant inelastic x-ray scattering and its application to screening dynamics in CE-phase La$_{0.5}$Sr$_{1.5}$MnO$_4$

Authors: T. F. Seman, X. Liu, J. P. Hill, M. van Veenendaal, K. H. Ahn

Abstract: We present a formula for the calculation of K-edge resonant inelastic x-ray scattering on transition metal compounds, based on a local interaction between the valence shell electrons and the $1s$ core hole. Extending a previous result, we include explicit momentum dependence and a basis with multiple core-hole sites. We apply this formula to a single-layered charge, orbital and spin ordered mangan… ▽ More We present a formula for the calculation of K-edge resonant inelastic x-ray scattering on transition metal compounds, based on a local interaction between the valence shell electrons and the $1s$ core hole. Extending a previous result, we include explicit momentum dependence and a basis with multiple core-hole sites. We apply this formula to a single-layered charge, orbital and spin ordered manganite, La$_{0.5}$Sr$_{1.5}$MnO$_4$, and obtain good agreement with experimental data, in particular with regards to the large variation of the intensity with momentum. We find that the screening in La$_{0.5}$Sr$_{1.5}$MnO$_4$ is highly localized around the core-hole site and demonstrate the potential of K-edge resonant inelastic x-ray scattering as a probe of screening dynamics in materials. △ Less

Submitted 5 August, 2013; originally announced August 2013.

Journal ref: Physical Review B, Vol. 88, Issue 14, pages 144415(1)-144415(15) (2013)

Angular dependence of the Hall effect of lsmo films

Authors: Netanel Naftalis, Noam Haham, Jason Hoffman, Matthew S. J. Marshall, C. H. Ahn, Lior Klein

Abstract: We find that the Hall effect resistivity ($ρ_{xy}$) of thin films of \lsmo\ varies as a function of the angle $θ$ between the applied magnetic field and the film normal as $ρ_{xy}=a\cos θ+ b\cos 3θ$, where $|b|$ increases with increasing temperature and decreases with increasing magnetic field. We find that the angular dependence of the longitudinal resistivity and the magnetization cannot fully e… ▽ More We find that the Hall effect resistivity ($ρ_{xy}$) of thin films of \lsmo\ varies as a function of the angle $θ$ between the applied magnetic field and the film normal as $ρ_{xy}=a\cos θ+ b\cos 3θ$, where $|b|$ increases with increasing temperature and decreases with increasing magnetic field. We find that the angular dependence of the longitudinal resistivity and the magnetization cannot fully explain the surprising term $b$, suggesting it is a manifestation of an intrinsic transport property. △ Less

Submitted 6 November, 2012; originally announced November 2012.

Journal ref: Phys. Rev. B 86, 184402 (2012)

2μm Solid-State Laser Mode-locked By Single-Layer Graphene

Authors: A. A. Lagatsky, Z. Sun, T. S. Kulmala, R. S. Sundaram, S. Milana, F. Torrisi, O. L. Antipov, Y. Lee, J. H. Ahn, C. T. A. Brown, W. Sibbett, A. C. Ferrari

Abstract: We report a 2μm ultrafast solid-state Tm:Lu2O3 laser, mode-locked by single-layer graphene, generating transform-limited~410fs pulses, with a spectral width~11.1nm at 2067nm. The maximum average output power is 270mW, at a pulse repetition frequency of 110MHz. This is a convenient high-power transform-limited laser at 2μm for various applications, such as laser surgery and material processing. We report a 2μm ultrafast solid-state Tm:Lu2O3 laser, mode-locked by single-layer graphene, generating transform-limited~410fs pulses, with a spectral width~11.1nm at 2067nm. The maximum average output power is 270mW, at a pulse repetition frequency of 110MHz. This is a convenient high-power transform-limited laser at 2μm for various applications, such as laser surgery and material processing. △ Less

Submitted 25 October, 2012; originally announced October 2012.

Journal ref: Appl. Phys. Lett. 102, 013113 (2013)

Strongly momentum-dependent screening dynamics in La$_{0.5}$Sr$_{1.5}$MnO$_4$ observed with resonant inelastic x-ray scattering

Authors: X. Liu, T. F. Seman, K. H. Ahn, Michel van Veenendaal, D. Casa, D. Prabhakaran, A. T. Boothroyd, H. Ding, J. P. Hill

Abstract: We report strongly momentum-dependent local charge screening dynamics in CE-type charge, orbital, and spin ordered La$_{0.5}$Sr$_{1.5}$MnO$_4$, based on Mn K-edge resonant inelastic x-ray scattering data. Through a comparison with theoretical calculations, we show that the observed momentum dependence reflects highly localized, nearest neighbor screening of the transient local charge perturbation… ▽ More We report strongly momentum-dependent local charge screening dynamics in CE-type charge, orbital, and spin ordered La$_{0.5}$Sr$_{1.5}$MnO$_4$, based on Mn K-edge resonant inelastic x-ray scattering data. Through a comparison with theoretical calculations, we show that the observed momentum dependence reflects highly localized, nearest neighbor screening of the transient local charge perturbation in this compound with an exciton-like screening cloud, rather than delocalized screening. The size of the screening cloud is estmated to be about 0.4-0.5 interatomic distances. △ Less

Submitted 24 October, 2012; originally announced October 2012.

Shifting of surface plasmon resonance due to electromagnetic coupling between graphene and Au nanoparticles

Authors: Jing Niu, Young Jun Shin, Jaesung Son, Youngbin Lee, Jong Hyun Ahn, Hyunsoo Yang

Abstract: Shifting of the surface plasmon resonance wavelength induced by the variation of the thickness of insulating spacer between single layer graphene and Au nanoparticles is studied. The system demonstrates a blue shift of 29 nm as the thickness of the spacer layer increases from 0 to 15 nm. This is due to the electromagnetic coupling between the localized surface plasmons excited in the nanoparticles… ▽ More Shifting of the surface plasmon resonance wavelength induced by the variation of the thickness of insulating spacer between single layer graphene and Au nanoparticles is studied. The system demonstrates a blue shift of 29 nm as the thickness of the spacer layer increases from 0 to 15 nm. This is due to the electromagnetic coupling between the localized surface plasmons excited in the nanoparticles and the graphene film. The strength of the coupling decays exponentially with a decay length of d/R=0.36, where d is the spacer layer thickness and R is the diameter of the Au nanoparticles. The result agrees qualitatively well with the plasmon ruler equation. Interestingly, a further increment of the spacer layer thickness induces a red shift of 17 nm in the resonance wavelength and the shift saturates when the thickness of the spacer layer increases above 20 nm. △ Less

Submitted 23 August, 2012; originally announced August 2012.

Journal ref: Optics Express 20, 19690 (2012)

Strain-induced metal-insulator phase coexistence and stability in perovskite manganites

Authors: T. F. Seman, K. H. Ahn, T. Lookman, A. R. Bishop

Abstract: We present a detailed study of a model for strain-induced metal-insulator phase coexistence in perovskite manganites. Both nanoscale and mesoscale inhomogeneities are self-consistently described using atomic scale modes and their associated constraint equations. We also examine the stability of domain configurations against uniform and nonuniform modifications of domain walls. Our results show tha… ▽ More We present a detailed study of a model for strain-induced metal-insulator phase coexistence in perovskite manganites. Both nanoscale and mesoscale inhomogeneities are self-consistently described using atomic scale modes and their associated constraint equations. We also examine the stability of domain configurations against uniform and nonuniform modifications of domain walls. Our results show that the long range interactions between strain fields and the complex energy landscape with multiple metastable states play essential roles in stabilizing metal-insulator phase coexistence, as observed in perovskite manganites. We elaborate on the modes, constraint equations, energies, and energy gradients that form the basis of our simulation results. △ Less

Submitted 20 August, 2012; originally announced August 2012.

Comments: 16 figures

Effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites

Authors: T. F. Seman, K. H. Ahn, T. Lookman, A. Saxena, A. R. Bishop, P. B. Littlewood

Abstract: We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion w… ▽ More We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation between NdMnO3 and LaMnO3, and obtain the relations between distortions. We find good agreement between theoretical results and experimental data. △ Less

Submitted 10 August, 2012; originally announced August 2012.

Comments: 6 figures