Showing 1–41 of 41 results for author: Chow, K

Operation of a high-frequency, phase-slip qubit

Authors: Cheeranjeev Purmessur, Kaicheung Chow, Bernard van Heck, Angela Kou

Abstract: Aluminum-based Josephson junctions are currently the main sources of nonlinearity for control and manipulation of superconducting qubits. A constriction-based junction provides an alternative source of nonlinearity that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation through the use of superconductors with larger energy gaps. Junctions ma… ▽ More Aluminum-based Josephson junctions are currently the main sources of nonlinearity for control and manipulation of superconducting qubits. A constriction-based junction provides an alternative source of nonlinearity that promises new types of protected qubits and the possibility of high-temperature and high-frequency operation through the use of superconductors with larger energy gaps. Junctions made from such superconductors have been challenging to incorporate into superconducting qubits because of difficulty controlling junction parameters and have had extremely low lifetimes, which limited their utility. Here we demonstrate that junctions made using titanium nitride (TiN) are a promising and controllable qubit platform. We use TiN junctions to build superconducting qubits based on quantum phase slips through the junction. We operate the qubit at zero flux where the qubit frequency (~17 GHz) is mainly determined by the inductance of the qubit. We perform readout and coherent control of the superconducting qubit, and measure qubit lifetimes >60 $μ$s. Finally, we demonstrate operation of the qubit at temperatures exceeding 300 mK. Our results add the TiN-based junction as a tool for superconducting quantum information processing and opens avenues for new classes of superconducting qubits. △ Less

Submitted 9 July, 2026; v1 submitted 10 February, 2025; originally announced February 2025.

Journal ref: Nat Commun 16, 11456 (2025)

Probing Near-Field Thermal Emission of Localized Surface Phonons from Silicon Carbide Nanopillars

Authors: Saman Zare, Ramin Pouria, Philippe K. Chow, Tom Tiwald, Carl P. Tripp, Sheila Edalatpour

Abstract: Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic materials is a promising mechanism for tuning the spectrum of near-field thermal radiation. Previous studies have theoretically shown that thermal emission of LSPhs results in narrow-band peaks in the near-field spectra, whose spectral locations can be modulated by changing the dimensions of the nanostructure.… ▽ More Thermal emission of localized surface phonons (LSPhs) from nanostructures of polaritonic materials is a promising mechanism for tuning the spectrum of near-field thermal radiation. Previous studies have theoretically shown that thermal emission of LSPhs results in narrow-band peaks in the near-field spectra, whose spectral locations can be modulated by changing the dimensions of the nanostructure. However, near-field thermal emission of LSPhs has not been experimentally explored yet. In this study, we measure the spectrum of near-field thermal radiation from arrays of 6H-silicon carbide (6H-SiC) nanopillars using an internal-reflection-element based spectroscopy technique. We present an experimental demonstration of thermal emission of the transverse dipole, quadrupole, and octupole, as well as longitudinal monopole from 6H-SiC nanopillars at a near-field distance from the array. We show that the spectral location of the longitudinal monopole and transverse dipole are significantly affected by the near-field coupling between neighboring nanopillars as well as the intercoupling of the nanopillars and the substrate. We also experimentally demonstrate that the spectrum of near-field thermal radiation from 6H-SiC nanopillar arrays can be tuned by varying the dimensions of the nanopillars, providing an opportunity for designing emitters with tailored near-field thermal radiation. △ Less

Submitted 2 February, 2023; originally announced February 2023.

An Environmentally Stable and Lead-Free Chalcogenide Perovskite

Authors: Tushar Gupta, Debjit Ghoshal, Anthony Yoshimura, Swastik Basu, Philippe K. Chow, Aniruddha S. Lakhnot, Juhi Pandey, Jeffrey M. Warrender, Harry Efstathiadis, Ajay Soni, Eric Osei-Agyemang, Ganesh Balasubramanian, Shengbai Zhang, Su-Fei Shi, Toh-Ming Lu, Vincent Meunier, Nikhil Koratkar

Abstract: Organic-inorganic halide perovskites are intrinsically unstable when exposed to moisture and/or light. Additionally, the presence of lead in many perovskites raises toxicity concerns. Herein is reported a thin film of BaZrS3, a lead-free chalcogenide perovskite. Photoluminescence and X-ray diffraction measurements show that BaZrS3 is far more stable than methylammonium lead iodide (MAPbI3) in mois… ▽ More Organic-inorganic halide perovskites are intrinsically unstable when exposed to moisture and/or light. Additionally, the presence of lead in many perovskites raises toxicity concerns. Herein is reported a thin film of BaZrS3, a lead-free chalcogenide perovskite. Photoluminescence and X-ray diffraction measurements show that BaZrS3 is far more stable than methylammonium lead iodide (MAPbI3) in moist environments. Moisture- and light-induced degradations in BaZrS3 and MAPbI3 are compared by using simulations and calculations based on density functional theory. The simulations reveal drastically slower degradation in BaZrS3 due to two factors - weak interaction with water, and very low rates of ion migration. BaZrS3 photo-detecting devices with photo-responsivity of ~46.5 mA W-1 are also reported. The devices retain ~60% of their initial photo-response after 4 weeks in ambient conditions. Similar MAPbI3 devices degrade rapidly and show ~95% decrease in photo-responsivity in just 4 days. The findings establish the superior stability of BaZrS3 and strengthen the case for its use in optoelectronics. New possibilities for thermoelectric energy conversion using these materials are also demonstrated. △ Less

Submitted 15 December, 2019; originally announced December 2019.

Journal ref: Adv.Funct.Mater. 30 (2020) 2001387

Direct observation of hole carrier density profiles and their light induced manipulation at the surface of Ge

Authors: T. Prokscha, K. H. Chow, Z. Salman, E. Stilp, A. Suter

Abstract: We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable mu… ▽ More We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable muon spin polarization. By tuning the energy of the low-energy $μ^+$ between 1 keV and 20 keV the near-surface region between 10 nm and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the $10^{17}$ cm$^{-3}$ range, which is opposite to the properties of cleaved Ge surfaces. By illumination with light the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength $λ$ this change can be persistent ($λ= 405, 457$ nm) or non-persistent ($λ= 635$ nm) at temperatures $< 270$ K. This difference is attributed to the different kinetic energies of the photo-electrons. Photo-electrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contact-less local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies. △ Less

Submitted 21 July, 2020; v1 submitted 25 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. Applied 14, 014098 (2020)

Ionic and electronic properties of the topological insulator Bi$_2$Te$_2$Se investigated using $β$-detected nuclear magnetic relaxation and resonance of $^8$Li

Authors: Ryan M. L. McFadden, Aris Chatzichristos, Kim H. Chow, David L. Cortie, Martin H. Dehn, Derek Fujimoto, Masrur D. Hossain, Huiwen Ji, Victoria L. Karner, Robert F. Kiefl, C. D. Philip Levy, Ruohong Li, Iain McKenzie, Gerald D. Morris, Oren Ofer, Matthew R. Pearson, Monika Stachura, Robert J. Cava, W. Andrew MacFarlane

Abstract: We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $β$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface.… ▽ More We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $β$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $τ_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin. △ Less

Submitted 14 February, 2019; v1 submitted 17 October, 2018; originally announced October 2018.

Comments: 17 pages, 12 figures, submitted to Phys. Rev. B

Journal ref: Phys. Rev. B 99, 125201 (2019)

Exact States in Waveguides With Periodically Modulated Nonlinearity

Authors: E. Ding, H. N. Chan, K. W. Chow, K. Nakkeeran, B. A. Malomed

Abstract: We introduce a one-dimensional model based on the nonlinear Schrodinger/Gross-Pitaevskii equation where the local nonlinearity is subject to spatially periodic modulation in terms of the Jacobi dn function, with three free parameters including the period, amplitude, and internal form-factor. An exact periodic solution is found for each set of parameters and, which is more important for physical re… ▽ More We introduce a one-dimensional model based on the nonlinear Schrodinger/Gross-Pitaevskii equation where the local nonlinearity is subject to spatially periodic modulation in terms of the Jacobi dn function, with three free parameters including the period, amplitude, and internal form-factor. An exact periodic solution is found for each set of parameters and, which is more important for physical realizations, we solve the inverse problem and predict the period and amplitude of the modulation that yields a particular exact spatially periodic state. Numerical stability analysis demonstrates that the periodic states become modulationally unstable for large periods, and regain stability in the limit of an infinite period, which corresponds to a bright soliton pinned to a localized nonlinearity-modulation pattern. Exact dark-bright soliton complex in a coupled system with a localized modulation structure is also briefly considered . The system can be realized in planar optical waveguides and cigar-shaped atomic Bose-Einstein condensates. △ Less

Submitted 7 November, 2017; originally announced November 2017.

Comments: EPL, in press

Microscopic Dynamics of Li$^{+}$ in Rutile TiO$_{2}$ Revealed by $^{8}$Li $β$-detected NMR

Authors: Ryan M. L. McFadden, Terry J. Buck, Aris Chatzichristos, Chia-Chin Chen, David L. Cortie, Kim H. Chow, Martin H. Dehn, Victoria L. Karner, Dimitrios Koumoulis, C. D. Philip Levy, Chilin Li, Iain McKenzie, Rotraut Merkle, Gerald D. Morris, Matthew R. Pearson, Zaher Salman, Dominik Samuelis, Monika Stachura, Jiyu Xiao, Joachim Maier, Robert F. Kiefl, W. Andrew MacFarlane

Abstract: We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $β$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) \times 10^{10}$ s$^{-1}$. We suggest th… ▽ More We report measurements of the dynamics of isolated $^{8}$Li$^{+}$ in single crystal rutile TiO$_{2}$ using $β$-detected NMR. From spin-lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K; and one at higher temperatures. At low temperature, the activation barrier is $26.8(6)$ meV with prefactor $1.23(5) \times 10^{10}$ s$^{-1}$. We suggest this is unrelated to Li$^{+}$ motion, and rather is a consequence of electron polarons in the vicinity of the implanted $^{8}$Li$^{+}$ that are known to become mobile in this temperature range. Above 100 K, Li$^{+}$ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of $0.32(2)$ eV and $1.0(5) \times 10^{16}$ s$^{-1}$, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li$^{+}$ concentration does not limit the diffusivity even up to high concentrations, but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li$^{+}$ and polaron motion. △ Less

Submitted 19 September, 2017; originally announced September 2017.

Comments: 14 pages, 9 figures

Journal ref: Chem. Mater. 29, 10187 (2017)

Nanoscale Phase Change Memory with Graphene Ribbon Electrodes

Authors: Ashkan Behnam, Feng Xiong, Andrea Cappelli, Ning C. Wang, Enrique A. Carrion, Sungduk Hong, Yuan Dai, Austin S. Lyons, Edmond K. Chow, Enrico Piccinini, Carlo Jacoboni, Eric Pop

Abstract: Phase change memory (PCM) devices are known to reduce in power consumption as the bit volume and contact area of their electrodes are scaled down. Here, we demonstrate two types of low-power PCM devices with lateral graphene ribbon electrodes: one in which the graphene is patterned into narrow nanoribbons and the other where the phase change material is patterned into nanoribbons. The sharp graphe… ▽ More Phase change memory (PCM) devices are known to reduce in power consumption as the bit volume and contact area of their electrodes are scaled down. Here, we demonstrate two types of low-power PCM devices with lateral graphene ribbon electrodes: one in which the graphene is patterned into narrow nanoribbons and the other where the phase change material is patterned into nanoribbons. The sharp graphene "edge" contacts enable switching with threshold voltages as low as ~3 V, low programming currents (<1 μA SET, <10 μA RESET) and ON/OFF ratios >100. Large-scale fabrication with graphene grown by chemical vapor deposition also enables the study of heterogeneous integration and that of variability for such nanomaterials and devices. △ Less

Submitted 20 August, 2015; originally announced August 2015.

Comments: submitted to Applied Physics Letters

Photo-induced persistent inversion of germanium in a 200-nm-deep surface region

Authors: T. Prokscha, K. H. Chow, E. Stilp, A. Suter, H. Luetkens, E. Morenzoni, G. J. Nieuwenhuys, Z. Salman, R. Scheuermann

Abstract: The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion wit… ▽ More The controlled manipulation of the charge carrier concentration in nanometer thin layers is the basis of current semiconductor technology and of fundamental importance for device applications. Here we show that it is possible to induce a persistent inversion from n- to p-type in a 200-nm-thick surface layer of a germanium wafer by illumination with white and blue light. We induce the inversion with a half-life of ~12 hours at a temperature of 220 K which disappears above 280 K. The photo-induced inversion is absent for a sample with a 20-nm-thick gold capping layer providing a Schottky barrier at the interface. This indicates that charge accumulation at the surface is essential to explain the observed inversion. The contactless change of carrier concentration is potentially interesting for device applications in opto-electronics where the gate electrode and gate oxide could be replaced by the semiconductor surface. △ Less

Submitted 16 December, 2013; originally announced December 2013.

Journal ref: Scientific Reports 3, 2569 (2013)

Social Judgment Theory Based Model On Opinion Formation, Polarization And Evolution

Authors: H. F. Chau, C. Y. Wong, F. K. Chow, C. -H. F. Fung

Abstract: The dynamical origin of opinion polarization in the real world is an interesting topic physical scientists may help to understand. To properly model the dynamics, the theory must be fully compatible with findings by social psychologists on microscopic opinion change. Here we introduce a generic model of opinion formation with homogeneous agents based on the well-known social judgment theory in soc… ▽ More The dynamical origin of opinion polarization in the real world is an interesting topic physical scientists may help to understand. To properly model the dynamics, the theory must be fully compatible with findings by social psychologists on microscopic opinion change. Here we introduce a generic model of opinion formation with homogeneous agents based on the well-known social judgment theory in social psychology by extending a similar model proposed by Jager and Amblard. The agents' opinions will eventually cluster around extreme and/or moderate opinions forming three phases in a two-dimensional parameter space that describes the microscopic opinion response of the agents. The dynamics of this model can be qualitatively understood by mean-field analysis. More importantly, first-order phase transition in opinion distribution is observed by evolving the system under a slow change in the system parameters, showing that punctuated equilibria in public opinion can occur even in a fully connected social network. △ Less

Submitted 26 July, 2014; v1 submitted 9 August, 2013; originally announced August 2013.

Comments: 9 pages, 5 figures together with an addition 24 videos available on request, extensively revised, to appear in Physica A

Journal ref: Physica A 415, 133-140 (2014)

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers

Authors: Z. Salman, O. Ofer, M. Radovic, H. Hao, K. H. Chow, M. D. Hossain, C. D. P. Levy, W. A. MacFarlane, G. M. Morris, L. Patthey, M. R. Pearson, H. Saadaoui, T. Schmitt, D. Wang, R. F. Kiefl

Abstract: We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3 using beta-detected nuclear magnetic resonance. The spin lattice relaxation rate of 8 Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO3 exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temper… ▽ More We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3 using beta-detected nuclear magnetic resonance. The spin lattice relaxation rate of 8 Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO3 exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO3/SrTiO3 interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ~4-6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (~0.002 muB) in the two samples with a larger LaAlO3 spacer thickness. △ Less

Submitted 21 November, 2012; originally announced November 2012.

Comments: 5 pages, 3 figures. Accepted for publication in PRL

Journal ref: Phys. Rev. Lett. 109, 257207 (2012)

Transport in Nanoribbon Interconnects Obtained from Graphene Grown by Chemical Vapor Deposition

Authors: Ashkan Behnam, Austin S. Lyons, Myung-Ho Bae, Edmond K. Chow, Sharnali Islam, Christopher M. Neumann, Eric Pop

Abstract: We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm2/V/s, comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices s… ▽ More We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm2/V/s, comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ~2 x 10^9 A/cm2, the highest reported for nanoscale CVD graphene interconnects. At temperatures below ~5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges. △ Less

Submitted 2 September, 2012; originally announced September 2012.

Comments: Nano Letters (2012); supplement available online

Journal ref: Nano Letters vol. 12, pp. 4424-4430 (2012)

$β$ NMR study of Isolated $^8$Li in the enhanced paramagnet Platinum

Authors: Oren Ofer, K. H. Chow, I. Fan, M. Egilmez, T. J. Parolin, M. D. Hossain, J. Jung, Z. Salman, R. F. Kiefl, C. D. P. Levy, G. D. Morris, M. R. Pearson, H. Saadaoui, Q. Song, D. Wang, W. A. MacFarlane

Abstract: We report β detected nuclear magnetic resonance (βNMR) measurements of 8Li+ implanted into high purity Pt. The frequency of the 8Li βNMR resonance and the spin-lattice relaxation rates 1/T1 were measured at temperatures ranging from 3 to 300 K. Remarkably, both the spin-lattice relaxation rate and the Knight shift K depend linearly on temperature T although the bulk susceptibility does not. K is f… ▽ More We report β detected nuclear magnetic resonance (βNMR) measurements of 8Li+ implanted into high purity Pt. The frequency of the 8Li βNMR resonance and the spin-lattice relaxation rates 1/T1 were measured at temperatures ranging from 3 to 300 K. Remarkably, both the spin-lattice relaxation rate and the Knight shift K depend linearly on temperature T although the bulk susceptibility does not. K is found to scale with the Curie-Weiss dependence of the Pt susceptibility extrapolated to low temperatures. This is attributed to a defect response of the enhanced paramagnetism of Pt, i.e. the presence of the interstitial Li+ locally relieves the tendency for the Curie-Weiss susceptibility to saturate at low T . We propose that the low temperature saturation in \c{hi} of Pt may be related to an interband coupling between the s and d bands that is disrupted locally by the presence of the Li+. △ Less

Submitted 19 August, 2012; originally announced August 2012.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 86, 064419 (2012)

Observation of slow order parameter fluctuations in superconducting films using beta-detected NMR

Authors: E. Morenzoni, H. Saadaoui, D. Wang, M. Horisberger, E. Kirk, W. A. MacFarlane, G. D. Morris, K. H. Chow, M. D. Hossain, C. P. Levy, T. J. Parolin, M. R. Pearson, Q. Song, R. F. Kiefl

Abstract: We report beta-NMR investigations of polarized 8Li implanted in thin Pb and Ag/Nb films. At the critical superconducting temperature, we observe a singular peak in the spin relaxation rate in small longitudinal magnetic fields, which is attributed to fluctuations in the superconducting order parameter. However, the peak is more than an order of magnitude larger than the prediction based on the enh… ▽ More We report beta-NMR investigations of polarized 8Li implanted in thin Pb and Ag/Nb films. At the critical superconducting temperature, we observe a singular peak in the spin relaxation rate in small longitudinal magnetic fields, which is attributed to fluctuations in the superconducting order parameter. However, the peak is more than an order of magnitude larger than the prediction based on the enhancement of the dynamic electron spin susceptibility by superconducting fluctuations and reflects the presence of unexpected slow fluctuations. Furthermore the fluctuations are rapidly suppressed in a small magnetic field, which may explain why they have not been observed previously with conventional NMR or NQR. △ Less

Submitted 21 September, 2011; originally announced September 2011.

Comments: 5 pages, 2 figures

Journal ref: Physical Review B 85, 220501(R) (2012)

Depth Dependence of the Structural Phase Transition of SrTiO_3 Studied with β-NMR and Grazing Incidence X-ray Diffraction

Authors: Z. Salman, M. Smadella, W. A. MacFarlane, B. D. Patterson, P. R. Willmott, K. H. Chow, M. D. Hossain, H. Saadaoui, D. Wang, R. F. Kiefl

Abstract: We present an investigation of the near-surface tetragonal phase transition in SrTiO3, using the complementary techniques of beta-detected nuclear magnetic resonance and grazing-incidence X-ray diffraction. The results show a clear depth dependence of the phase transition on scales of a few microns. The measurements support a model in which there are tetragonal domains forming in the sample at tem… ▽ More We present an investigation of the near-surface tetragonal phase transition in SrTiO3, using the complementary techniques of beta-detected nuclear magnetic resonance and grazing-incidence X-ray diffraction. The results show a clear depth dependence of the phase transition on scales of a few microns. The measurements support a model in which there are tetragonal domains forming in the sample at temperatures much higher than the bulk phase transition temperature. Moreover, we find that these domains tend to form at higher temperatures preferentially near the free surface of the crystal. The details of the tetragonal domain formation and their depth/lateral dependencies are discussed. △ Less

Submitted 5 May, 2011; originally announced May 2011.

Comments: Accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 83, 224112 (2011)

Search for broken time-reversal symmetry near the surface of YBCO films using Beta-NMR

Authors: H. Saadaoui, G. D. Morris, Z. Salman, Q. Song, K. H. Chow, M. D. Hossain, C. D. P. Levy, T. J. Parolin, M. R. Pearson, M. Smadella, D. Wang, L. H. Greene, P. J. Hentges, R. F. Kiefl, W. A. MacFarlane

Abstract: Weak spontaneous magnetic fields are observed near the surface of YBCO films using Beta-detected Nuclear Magnetic Resonance. Below Tc, the magnetic field distribution in a silver film evaporated onto the superconductor shows additional line broadening, indicating the appearance of small disordered magnetic fields. The line broadening increases linearly with a weak external magnetic field applied p… ▽ More Weak spontaneous magnetic fields are observed near the surface of YBCO films using Beta-detected Nuclear Magnetic Resonance. Below Tc, the magnetic field distribution in a silver film evaporated onto the superconductor shows additional line broadening, indicating the appearance of small disordered magnetic fields. The line broadening increases linearly with a weak external magnetic field applied parallel to the surface, and is depth-independent up to 45 nm from the Ag/YBCO interface. The magnitude of the line broadening at 10 K extrapolated to zero applied field is less than 0.2 G, and is close to nuclear dipolar broadening in the Ag. This indicates that any fields due to broken time-reversal symmetry are less than 0.2 G. △ Less

Submitted 5 January, 2011; originally announced January 2011.

Journal ref: Phys. Rev. B 83, 054504 (2011)

Exact solutions for periodic and solitary matter waves in nonlinear lattices

Authors: C. H. Tsang, Boris A. Malomed, K. W. Chow

Abstract: We produce three vast classes of exact periodic and soliton solutions to the one-dimensional Gross-Pitaevskii equation (GPE) with the pseudopotential in the form of a nonlinear lattice (NL), induced by a spatially periodic modulation of the local nonlinearity. It is well known that NLs in Bose-Einstein condensates (BECs) may be created by means of the Feshbach-resonance technique. The model may al… ▽ More We produce three vast classes of exact periodic and soliton solutions to the one-dimensional Gross-Pitaevskii equation (GPE) with the pseudopotential in the form of a nonlinear lattice (NL), induced by a spatially periodic modulation of the local nonlinearity. It is well known that NLs in Bose-Einstein condensates (BECs) may be created by means of the Feshbach-resonance technique. The model may also include linear potentials with the same periodicity. The NL modulation function, the linear potential (if any), and the corresponding exact solutions are expressed in terms of the Jacobi's elliptic functions of three types, cn, dn, and sn, which give rise to the three different classes of the solutions. The potentials and associated solutions are parameterized by two free constants and an additional sign parameter in the absence of the linear potential. In the presence of the latter, the solution families feature two additional free parameters. The families include both sign-constant and sign-changing NLs. Density maxima of the solutions may coincide with either minima or maxima of the periodic pseudopotential. The solutions reduce to solitons in the limit of the infinite period. The stability of the solutions is tested via direct simulations of the GPE. As a result, stability regions are identified for the periodic solutions and solitons. The periodic patterns of the cn type, and the respective limit-form solutions in the form of bright solitons, may be stable both in the absence and presence of the linear potential. The stability of the two other solution classes, of the dn and sn types, is only possible with the linear potential. △ Less

Submitted 11 December, 2010; originally announced December 2010.

Comments: to be published in a special issue of Discrete and Continuous Dynamical Systems, Series S, Volume 4, Number 5 (2011)

The magnetic structure of the $zigzag$ chain family Na$_{x}$Ca$_{1-x}$V$_2$O$_4$ determined by muon-spin rotation

Authors: Oren Ofer, Yutaka Ikedo, Tatsuo Goko, Martin Mansson, Jun Sugiyama, Eduardo J. Ansaldo, Jess H. Brewer, Kim H. Chow, Hiroya Sakurai

Abstract: We present muon-spin rotation measurements on polycrystalline samples of the complete family of the antiferromagnetic (AF) $zigzag$ chain compounds, Na$_x$Ca$_{1-x}$V$_2$O$_4$. In this family, we explore the magnetic properties from the metallic NaV$_2$O$_4$ to the insulating CaV$_2$O$_4$. We find a critical $x_c(\sim0.833)$ which separates the low and high Na-concentration dependent transition te… ▽ More We present muon-spin rotation measurements on polycrystalline samples of the complete family of the antiferromagnetic (AF) $zigzag$ chain compounds, Na$_x$Ca$_{1-x}$V$_2$O$_4$. In this family, we explore the magnetic properties from the metallic NaV$_2$O$_4$ to the insulating CaV$_2$O$_4$. We find a critical $x_c(\sim0.833)$ which separates the low and high Na-concentration dependent transition temperature and its magnetic ground state. In the $x<x_c$ compounds, the magnetic ordered phase is characterized by a single homogenous phase and the formation of incommensurate spin-density-wave order. Whereas in the $x>x_c$ compounds, multiple sub-phases appear with temperature and $x$. Based on the muon data obtained in zero external magnetic field, a careful dipolar field simulation was able to reproduce the muon behavior and indicates a modulated helical incommensurate spin structure of the metallic AF phase. The incommensurate modulation period obtained by the simulation agrees with that determined by neutron diffraction. △ Less

Submitted 11 August, 2010; originally announced August 2010.

Comments: 7 pages, 7 figures, accepted for publication in PRB

The magnetic phase of the perovskite CaCrO$_3$ studied with $μ^{+}$SR

Authors: Oren Ofer, Jun Sugiyama, Martin Månsson, Kim H. Chow, Eduardo J. Ansaldo, Jess H. Brewer, Masahiko Isobe, Yutaka Ueda

Abstract: We investigated the magnetic phase of the perovskite CaCrO$_3$ by using the muon spin relaxation technique accompanied by susceptibility measurements. A thermal hysteresis loop is identified with a width of about 1 K at the transition temperature. Within the time scale of the muon lifetime, a static antiferromagnetic order is revealed with distinct multiple internal fields which are experienced i… ▽ More We investigated the magnetic phase of the perovskite CaCrO$_3$ by using the muon spin relaxation technique accompanied by susceptibility measurements. A thermal hysteresis loop is identified with a width of about 1 K at the transition temperature. Within the time scale of the muon lifetime, a static antiferromagnetic order is revealed with distinct multiple internal fields which are experienced in the muon interstitial sites below the phase-transition temperature, $T_N=90 K$. Above $T_N$, lattice deformations are indicated by transverse-field muon-spin rotation and relaxation suggesting a magneto-elastic mechanism. △ Less

Submitted 27 April, 2010; originally announced April 2010.

Comments: 5 pages, 4 figures. Accepted for publication in PRB

Vortex lattice disorder in YBCO probed using Beta-NMR

Authors: H. Saadaoui, W. A. MacFarlane, Z. Salman, G. D. Morris, Q. Song, K. H. Chow, M. D. Hossain, C. D. P. Levy, A. I. Mansour, T. J. Parolin, M. R. Pearson, M. Smadella, D. Wang, R. F. Kiefl

Abstract: Beta-NMR has been used to study vortex lattice disorder near the surface of the high-Tc superconductor YBCO. The magnetic field distribution from the vortex lattice was detected by implanting a low energy beam of highly polarized 8Li into a thin overlayer of silver on optimally doped, twinned and detwinned YBCO samples. The resonance in Ag broadens significantly below the transition temperature… ▽ More Beta-NMR has been used to study vortex lattice disorder near the surface of the high-Tc superconductor YBCO. The magnetic field distribution from the vortex lattice was detected by implanting a low energy beam of highly polarized 8Li into a thin overlayer of silver on optimally doped, twinned and detwinned YBCO samples. The resonance in Ag broadens significantly below the transition temperature Tc as expected from the emerging field lines of the vortex lattice in YBCO. However, the lineshape is more symmetric and the dependence on the applied magnetic field is much weaker than expected from an ideal vortex lattice, indicating that the vortex density varies across the face of the sample, likely due to pinning at twin boundaries. At low temperatures the broadening from such disorder does not scale with the superfluid density. △ Less

Submitted 5 March, 2010; originally announced March 2010.

Comments: 10 pages, 6 figures

Journal ref: Phys. Rev. B 80, 224503 (2009)

Persistent Spin Dynamics in the $S=1/2$ V$_{15}$ Molecular Nano-Magnet

Authors: Z. Salman, R. F. Kiefl, K. H. Chow, W. A. MacFarlane, T. Keeler, T. Parolin, S. Tabbara, D. Wang

Abstract: We present muon spin lattice relaxation measurements in the V15 spin 1/2 molecular nano-magnet. We find that the relaxation rate in low magnetic fields (<5 kG) is temperature independent below ~10 K, implying that the molecular spin is dynamically fluctuating down to 12 mK. These measurements show that the fluctuation time increases as the temperature is decreased and saturates at a value of ~6… ▽ More We present muon spin lattice relaxation measurements in the V15 spin 1/2 molecular nano-magnet. We find that the relaxation rate in low magnetic fields (<5 kG) is temperature independent below ~10 K, implying that the molecular spin is dynamically fluctuating down to 12 mK. These measurements show that the fluctuation time increases as the temperature is decreased and saturates at a value of ~6 nsec at low temperatures. The fluctuations are attributed to V15 molecular spin dynamics perpendicular to the applied magnetic field direction, induced by coupling between the molecular spin and nuclear spin bath in the system. △ Less

Submitted 14 May, 2008; originally announced May 2008.

Comments: Accepted for publication in Phys. Rev. B, 5 pages, 5 figure

Journal ref: Phys. Rev. B 77, 214415 (2008)

Local Magnetic Properties of a Monolayer of Mn12 Single Molecule Magnets

Authors: Z. Salman, K. H. Chow, R. I. Miller, A. Morello, T. J. Parolin, M. D. Hossain, T. A. Keeler, C. D. P. Levy, W. A. MacFarlane, G. D. Morris, H. Saadaoui, D. Wang, R. Sessoli, G. G. Condorelli, R. F. Kiefl

Abstract: The magnetic properties of a monolayer of Mn12 single molecule magnets grafted onto a Si substrate have been investigated using depth-controlled $β$-detected nuclear magnetic resonance. A low energy beam of spin polarized radioactive 8Li was used to probe the local static magnetic field distribution near the Mn12 monolayer in the Si substrate. The resonance linewidth varies strongly as a functio… ▽ More The magnetic properties of a monolayer of Mn12 single molecule magnets grafted onto a Si substrate have been investigated using depth-controlled $β$-detected nuclear magnetic resonance. A low energy beam of spin polarized radioactive 8Li was used to probe the local static magnetic field distribution near the Mn12 monolayer in the Si substrate. The resonance linewidth varies strongly as a function of implantation depth as a result of the magnetic dipolar fields generated by the Mn12 electronic magnetic moments. The temperature dependence of the linewidth indicates that the magnetic properties of the Mn12 moments in this low dimensional configuration differ from bulk Mn12. △ Less

Submitted 30 April, 2008; originally announced April 2008.

Comments: 6 pages, 4 figures

Journal ref: Nano Lett., 7 (6), 1551 -1555, 2007

Hyperfine Fields in an Ag/Fe Multilayer Film Investigated with 8Li beta-Detected Nuclear Magnetic Resonance

Authors: T. A. Keeler, Z. Salman, K. H. Chow, B. Heinrich, M. D. Hossian, B. Kardasz, R. F. Kiefl, S. R. Kreitzman, C. D. P. Levy, W. A. MacFarlane, O. Mosendz, T. J. Parolin, M. R. Pearson, D. Wang

Abstract: Low energy $β$-detected nuclear magnetic resonance ($β$-NMR) was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 Å)/Ag(200 Å)/Fe(140 Å) (001) magnetic multilayer (MML) grown on GaAs. The resonance lineshape in the Ag layer shows dramatic broadening compared to intrinsic Ag. This broadening is attributed to large induced… ▽ More Low energy $β$-detected nuclear magnetic resonance ($β$-NMR) was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 Å)/Ag(200 Å)/Fe(140 Å) (001) magnetic multilayer (MML) grown on GaAs. The resonance lineshape in the Ag layer shows dramatic broadening compared to intrinsic Ag. This broadening is attributed to large induced magnetic fields in this layer by the magnetic Fe layer. We find that the induced hyperfine field in the Ag follows a power law decay away from the Ag/Fe interface with power $-1.93(8)$, and a field extrapolated to $0.23(5)$ T at the interface. △ Less

Submitted 27 March, 2008; originally announced March 2008.

Comments: 5 pages, 4 figure. To be published in Phys. Rev. B

Journal ref: Phys. Rev. B 77, 144429 (2008)

Giant Vortices Below the Surface of NbSe$_2$ Detected Using Low Energy $β$-NMR

Authors: Z. Salman, D. Wang, K. H. Chow, M. D. Hossain, S. R. Kreitzman, T. A. Keeler, C. D. P. Levy, W. A. MacFarlane, R. I. Miller, G. D. Morris, T. J. Parolin, H. Saadaoui, M. Smadella, R. F. Kiefl

Abstract: A low energy radioactive beam of polarized $^8$Li has been used to observe the vortex lattice near the surface of superconducting NbSe$_2$. The inhomogeneous magnetic field distribution associated with the vortex lattice was measured using depth-resolved $β$-detected NMR. Below $T_c$ one observes the characteristic lineshape for a triangular vortex lattice which depends on the magnetic penetrati… ▽ More A low energy radioactive beam of polarized $^8$Li has been used to observe the vortex lattice near the surface of superconducting NbSe$_2$. The inhomogeneous magnetic field distribution associated with the vortex lattice was measured using depth-resolved $β$-detected NMR. Below $T_c$ one observes the characteristic lineshape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with magnetic field. In particular in a low field of 10.8 mT the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed. △ Less

Submitted 7 March, 2007; originally announced March 2007.

Comments: 5 pages, 3 figures. Submitted to Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 98, 167001 (2007)

Beta-NMR of Isolated Lithium in Nearly Ferromagnetic Palladium

Authors: T. J. Parolin, Z. Salman, J. Chakhalian, Q. Song, K. H. Chow, M. D. Hossain, T. A. Keeler, R. F. Kiefl, S. R. Kreitzman, C. D. P. Levy, R. I. Miller, G. D. Morris, M. R. Pearson, H. Saadaoui, D. Wang, W. A. MacFarlane

Abstract: The temperature dependence of the frequency shift and spin-lattice relaxation rate of isolated, nonmagnetic Li-8 impurities implanted in a nearly ferromagnetic host (Pd) are measured by means of beta-detected nuclear magnetic resonance (b-NMR). The shift is negative, very large and increases monotonically with decreasing T in proportion to the bulk susceptibility of Pd for T > T*~ 100 K. Below T… ▽ More The temperature dependence of the frequency shift and spin-lattice relaxation rate of isolated, nonmagnetic Li-8 impurities implanted in a nearly ferromagnetic host (Pd) are measured by means of beta-detected nuclear magnetic resonance (b-NMR). The shift is negative, very large and increases monotonically with decreasing T in proportion to the bulk susceptibility of Pd for T > T*~ 100 K. Below T*, an additional shift occurs which we attribute to the response of Pd to the defect. The relaxation rate is much slower than expected for the large shift and is linear with T below T*, showing no sign of additional relaxation mechanisms associated with the defect. △ Less

Submitted 16 January, 2007; originally announced January 2007.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett. 98, 047601 (2007)

$β$-NMR of Isolated $^{8}$Li$^{+}$ Implanted into a Thin Copper Film

Authors: Z. Salman, A. I. Mansour, K. H. Chow, M. Beaudoin, I. Fan, J. Jung, T. A. Keeler, R. F. Kiefl, C. D. P. Levy, R. C. Ma, G. D. Morris, T. J. Parolin, D. Wang, W. A. MacFarlane

Abstract: Depth-controlled $β$-NMR was used to study highly spin-polarized $^8$Li in a Cu film of thickness 100 nm deposited onto a MgO substrate. The positive Knight Shifts and spin relaxation data show that $^8$Li occupies two sites at low temperatures, assigned to be the substitutional ($S$) and octahedral ($O$) interstitial sites. Between 50 to 100 K, there is a site change from $O$ to $S$. The temper… ▽ More Depth-controlled $β$-NMR was used to study highly spin-polarized $^8$Li in a Cu film of thickness 100 nm deposited onto a MgO substrate. The positive Knight Shifts and spin relaxation data show that $^8$Li occupies two sites at low temperatures, assigned to be the substitutional ($S$) and octahedral ($O$) interstitial sites. Between 50 to 100 K, there is a site change from $O$ to $S$. The temperature dependence of the Knight shifts and spin-lattice relaxation rates at high temperatures, i.e. when all the Li are in the $S$ site, is consistent with the Korringa Law for a simple metal. △ Less

Submitted 29 December, 2006; originally announced January 2007.

Comments: Accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 75, 073405 (2007)

Incommensurate magnetic order in Ag$_{2}$NiO$_{2}$

Authors: J. Sugiyama, Y. Ikedo, K. Mukai, J. H. Brewer, E. J. Ansaldo, G. D. Morris, K. H. Chow, H. Yoshida, Z. Hiroi

Abstract: The nature of the magnetic transition of the half-filled triangular antiferromagnet Ag$_{2}$NiO$_2$ with $T_{\rm N}$=56K was studied with positive muon-spin-rotation and relaxation ($μ^+$SR) spectroscopy. Zero field $μ^+$SR measurements indicate the existence of a static internal magnetic field at temperatures below $T_{\rm N}$. Two components with slightly different precession frequencies and w… ▽ More The nature of the magnetic transition of the half-filled triangular antiferromagnet Ag$_{2}$NiO$_2$ with $T_{\rm N}$=56K was studied with positive muon-spin-rotation and relaxation ($μ^+$SR) spectroscopy. Zero field $μ^+$SR measurements indicate the existence of a static internal magnetic field at temperatures below $T_{\rm N}$. Two components with slightly different precession frequencies and wide internal-field distributions suggest the formation of an incommensurate antiferromagnetic order below 56 K. This implies that the antifrerromagnetic interaction is predominant in the NiO$_2$ plane in contrast to the case of the related compound NaNiO$_2$. An additional transition was found at $\sim$22 K by both $μ^+$SR and susceptibility measurements. It was also clarified that the transition at $\sim$260 K observed in the susceptibility of Ag$_{2}$NiO$_{2}$ is induced by a purely structural transition. △ Less

Submitted 9 June, 2006; originally announced June 2006.

Comments: 6 pages, 6 figures, Phys. Rev. B 74, in press

Muon spin relaxation study of the magnetism in unilluminated Prussian Blue analogue photomagnets

Authors: Z. Salman, T. J. Parolin, K. H. Chow, T. A. Keeler, R. I. Miller, D. Wang, W. A. MacFarlane

Abstract: We present longitudinal field muon spin relaxation ($μ$SR) measurements in the unilluminated state of the photo-sensitive molecular magnetic Co-Fe Prussian blue analogues M$_{1-2x}$Co$_{1+x}$[Fe(CN)$_6$]$\cdot z$ H$_2$O, where M=K and Rb with $x=0.4$ and $\simeq 0.17$, respectively. These results are compared to those obtained in the $x=0.5$ stoichiometric limit, Co$_{1.5}$[Fe(CN)$_6$]$\cdot 6$… ▽ More We present longitudinal field muon spin relaxation ($μ$SR) measurements in the unilluminated state of the photo-sensitive molecular magnetic Co-Fe Prussian blue analogues M$_{1-2x}$Co$_{1+x}$[Fe(CN)$_6$]$\cdot z$ H$_2$O, where M=K and Rb with $x=0.4$ and $\simeq 0.17$, respectively. These results are compared to those obtained in the $x=0.5$ stoichiometric limit, Co$_{1.5}$[Fe(CN)$_6$]$\cdot 6$ H$_2$O, which is not photo-sensitive. We find evidence for correlation between the range of magnetic ordering and the value of $x$ in the unilluminated state which can be explained using a site percolation model. △ Less

Submitted 7 June, 2006; v1 submitted 29 November, 2005; originally announced November 2005.

Comments: 7 pages, 12 figures

Journal ref: Phys. Rev. B 73, 174427 (2006)

Near-Surface Structural Phase Transition of SrTiO$_3$ Studied with Zero-Field $β$-Detected Nuclear Spin Relaxation and Resonance

Authors: Z. Salman, R. F. Kiefl, K. H. Chow, M. D. Hossain, T. A. Keeler, S. R. Kreitzman, C. D. P. Levy, R. I. Miller, T. J. Parolin, M. R. Pearson, H. Saadaoui, J. D. Schultz, M. Smadella, D. Wang, W. A. MacFarlane

Abstract: We demonstrate that zero-field $β$-detected nuclear quadrupole resonance and spin relaxation of low energy $^8$Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO$_3$ single crystal occurs at $T_c \sim 150$ K, i.e., 45 K higher than $T^{bulk}_c$, and that the tetragonal domains formed below $T_c$ are ra… ▽ More We demonstrate that zero-field $β$-detected nuclear quadrupole resonance and spin relaxation of low energy $^8$Li can be used as a sensitive local probe of structural phase transitions near a surface. We find that the transition near the surface of a SrTiO$_3$ single crystal occurs at $T_c \sim 150$ K, i.e., 45 K higher than $T^{bulk}_c$, and that the tetragonal domains formed below $T_c$ are randomly oriented. △ Less

Submitted 7 June, 2006; v1 submitted 21 November, 2005; originally announced November 2005.

Comments: 4 pages, 5 figures

Journal ref: Phys. Rev. Lett. 96, 147601 (2006)

$β$-Detected NMR of $^8$Li in the Normal State of 2H-NbSe$_2$

Authors: D. Wang, M. D. Hossain, Z. Salman, D. Arseneau, K. H. Chow, S. Daviel, T. A. Keeler, R. F. Kiefl, S. R. Kreitzman, C. D. P. Levy, G. D. Morris, R. I. Miller, W. A. MacFarlane, T. J. Parolin, H. Saadaoui

Abstract: $β$-NMR of isolated $^8$Li has been investigated in the normal state of 2H-NbSe$_2$. In a high magnetic field of 3T a single resonance is observed with a Gaussian line width of 3.5 kHz. The line shape varies weakly as function of magnetic field and temperature but has a strong orientation dependence. The nuclear electric quadrupole splitting is unresolved implying that the electric field gradien… ▽ More $β$-NMR of isolated $^8$Li has been investigated in the normal state of 2H-NbSe$_2$. In a high magnetic field of 3T a single resonance is observed with a Gaussian line width of 3.5 kHz. The line shape varies weakly as function of magnetic field and temperature but has a strong orientation dependence. The nuclear electric quadrupole splitting is unresolved implying that the electric field gradients are 10-100 times smaller than in other non-cubic crystals. The nuclear spin relaxation rate is also anomalously small but varies linearly with temperature as expected for Korringa relaxation in a metal. These results suggest that Li adopts an interstitial position between the weakly coupled NbSe$_2$ layers and away from the conduction band. △ Less

Submitted 14 November, 2005; originally announced November 2005.

Comments: accepted for publication in Physica B

Journal ref: Physica B, 374-375, 239 (2006)

Beta-Detected NQR in Zero Field with a Low Energy Beam of \nuc{8}{Li}$^+$

Authors: Z. Salman, R. F. Kiefl, K. H. Chow, W. A. MacFarlane, D. J. Arseneau, S. Daviel, C. D. P. Levy, Y. Maeno, R. Poutissou

Abstract: Beta-detected nuclear quadrupole resonances (\bnqr) at zero field are observed using a beam of low energy highly polarized radioactive \nuc{8}{Li}$^+$. The resonances were detected in SrTiO$_3$, Al$_2$O$_3$ and Sr$_2$RuO$_4$ single crystals by monitoring the beta-decay anisotropy as a function of a small audio frequency magnetic field. The resonances show clearly that \nuc{8}{Li} occupies one si… ▽ More Beta-detected nuclear quadrupole resonances (\bnqr) at zero field are observed using a beam of low energy highly polarized radioactive \nuc{8}{Li}$^+$. The resonances were detected in SrTiO$_3$, Al$_2$O$_3$ and Sr$_2$RuO$_4$ single crystals by monitoring the beta-decay anisotropy as a function of a small audio frequency magnetic field. The resonances show clearly that \nuc{8}{Li} occupies one site with non-cubic symmetry in SrTiO$_3, two in Al$_2$O$_3$ and three sites in Sr$_2$RuO$_4$. The resonance amplitude and width are surprisingly large compared to the values expected from transitions between the $|\pm 2> \leftrightarrow |\pm 1>$ spin states, indicating a significant mixing between the $|\pm m>$ quadrupolar split levels. △ Less

Submitted 14 November, 2005; originally announced November 2005.

Comments: accepted for publication in Physica B

Journal ref: Physica B, 374-375, 468 (2006)

Multiplpe Choice Minority Game With Different Publicly Known Histories

Authors: H. F. Chau, F. K. Chow, K. H. Ho, W. C. Man

Abstract: In the standard Minority Game, players use historical minority choices as the sole public information to pick one out of the two alternatives. However, publishing historical minority choices is not the only way to present global system information to players when more than two alternatives are available. Thus, it is instructive to study the dynamics and cooperative behaviors of this extended gam… ▽ More In the standard Minority Game, players use historical minority choices as the sole public information to pick one out of the two alternatives. However, publishing historical minority choices is not the only way to present global system information to players when more than two alternatives are available. Thus, it is instructive to study the dynamics and cooperative behaviors of this extended game as a function of the global information provided. We numerically find that although the system dynamics depends on the kind of public information given to the players, the degree of cooperation follows the same trend as that of the standard Minority Game. We also explain most of our findings by the crowd-anticrowd theory. △ Less

Submitted 5 September, 2005; v1 submitted 20 April, 2005; originally announced April 2005.

Comments: Extensively revised, to appear in New J Phys, 7 pages with 4 figures

Comment on `Generating functional analysis of Minority Games with real market histories'

Authors: K. H. Ho, W. C. Man, F. K. Chow, H. F. Chau

Abstract: Retracted after receiving a detailed clarification from Coolen Retracted after receiving a detailed clarification from Coolen △ Less

Submitted 18 March, 2005; v1 submitted 14 March, 2005; originally announced March 2005.

Comments: retracted after receiving clarifications from Coolen

Memory is relevant in the symmetric phase of the minority game

Authors: K. H. Ho, W. C. Man, F. K. Chow, H. F. Chau

Abstract: Minority game is a simple-mined econophysical model capturing the cooperative behavior among selfish players. Previous investigations, which were based on numerical simulations up to about 100 players for a certain parameter $α$ in the range $0.1 \lesssim α\lesssim 1$, suggested that memory is irrelevant to the cooperative behavior of the minority game in the so-called symmetric phase. Here usin… ▽ More Minority game is a simple-mined econophysical model capturing the cooperative behavior among selfish players. Previous investigations, which were based on numerical simulations up to about 100 players for a certain parameter $α$ in the range $0.1 \lesssim α\lesssim 1$, suggested that memory is irrelevant to the cooperative behavior of the minority game in the so-called symmetric phase. Here using a large scale numerical simulation up to about 3000 players in the parameter range $0.01 \lesssim α\lesssim 1$, we show that the mean variance of the attendance in the minority game actually depends on the memory in the symmetric phase. We explain such dependence in the framework of crowd-anticrowd theory. Our findings conclude that one should not overlook the feedback mechanism buried under the correlation in the history time series in the study of minority game. △ Less

Submitted 25 April, 2005; v1 submitted 22 November, 2004; originally announced November 2004.

Comments: 5 pages, 4 figures, to appear in PRE

Study of the Wealth Inequality in the Minority Game

Authors: K. H. Ho, F. K. Chow, H. F. Chau

Abstract: To demonstrate the usefulness of physical approaches for the study of realistic economic systems, we investigate the inequality of players' wealth in one of the most extensively studied econophysical models, namely, the minority game (MG). We gauge the wealth inequality of players in the MG by a well-known measure in economics known as the modified Gini index. From our numerical results, we conc… ▽ More To demonstrate the usefulness of physical approaches for the study of realistic economic systems, we investigate the inequality of players' wealth in one of the most extensively studied econophysical models, namely, the minority game (MG). We gauge the wealth inequality of players in the MG by a well-known measure in economics known as the modified Gini index. From our numerical results, we conclude that the wealth inequality in the MG is very severe near the point of maximum cooperation among players, where the diversity of the strategy space is approximately equal to the number of strategies at play. In other words, the optimal cooperation between players comes hand in hand with severe wealth inequality. We also show that our numerical results in the asymmetric phase of the MG can be reproduced semi-analytically using a replica method. △ Less

Submitted 23 September, 2004; v1 submitted 16 February, 2004; originally announced February 2004.

Comments: 9 pages in revtex 4 style with 3 figures; minor revision with a change of title; to appear in PRE

Muonium as a shallow center in GaN

Authors: K. Shimomura, R. Kadono, K. Ohishi, M. Mizuta, M. Saito, K. H. Chow, B. Hitti, R. L. Lichti

Abstract: A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14… ▽ More A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN. △ Less

Submitted 5 February, 2004; originally announced February 2004.

Comments: 4 figures, to be published in Phys. Rev. Letters

Journal ref: Phys. Rev. Lett. 92, 135505 (2004)

Minority Game With Peer Pressure

Authors: H. F. Chau, F. K. Chow, K. H. Ho

Abstract: To study the interplay between global market choice and local peer pressure, we construct a minority-game-like econophysical model. In this so-called networked minority game model, every selfish player uses both the historical minority choice of the population and the historical choice of one's neighbors in an unbiased manner to make decision. Results of numerical simulation show that the level… ▽ More To study the interplay between global market choice and local peer pressure, we construct a minority-game-like econophysical model. In this so-called networked minority game model, every selfish player uses both the historical minority choice of the population and the historical choice of one's neighbors in an unbiased manner to make decision. Results of numerical simulation show that the level of cooperation in the networked minority game differs remarkably from the original minority game as well as the prediction of the crowd-anticrowd theory. We argue that the deviation from the crowd-anticrowd theory is due to the negligence of the effect of a four point correlation function in the effective Hamiltonian of the system. △ Less

Submitted 22 July, 2003; originally announced July 2003.

Comments: 10 pages, 3 figures in revtex 4.0

Multichoice Minority Game: Dynamics and Global Cooperation

Authors: F. K. Chow, H. F. Chau

Abstract: In the original two-choice minority game (MG), selfish players cooperate with each other even though direct communication is not allowed. Moreover, there is a periodic dynamics in the MG whenever the strategy space size is much smaller than the number of strategies at play. Do these phenomena persist if every player has $N_c > 2$ choices where all player's strategies are picked from a reduced st… ▽ More In the original two-choice minority game (MG), selfish players cooperate with each other even though direct communication is not allowed. Moreover, there is a periodic dynamics in the MG whenever the strategy space size is much smaller than the number of strategies at play. Do these phenomena persist if every player has $N_c > 2$ choices where all player's strategies are picked from a reduced strategy space? We answer this question by studying a multichoice minority game model known as MG($N_c$,$|{\mathbb S}|$). Numerical simulation shows that these two models have very similar global cooperative behaviors. Nevertheless, unlike in the MG, periodic dynamics does not always appear in the MG($N_c$,$|{\mathbb S}|$) even when the strategy space size is much smaller than the number of strategies at play. △ Less

Submitted 13 May, 2003; v1 submitted 28 October, 2002; originally announced October 2002.

Comments: revtex4, 19 pages with 12 figures; extensively rewritten, more justification added

How To Attain Maximum Profit In Minority Game?

Authors: H. F. Chau, F. K. Chow

Abstract: What is the physical origin of player cooperation in minority game? And how to obtain maximum global wealth in minority game? We answer the above questions by studying a variant of minority game from which players choose among $N_c$ alternatives according to strategies picked from a restricted set of strategy space. Our numerical experiment concludes that player cooperation is the result of a su… ▽ More What is the physical origin of player cooperation in minority game? And how to obtain maximum global wealth in minority game? We answer the above questions by studying a variant of minority game from which players choose among $N_c$ alternatives according to strategies picked from a restricted set of strategy space. Our numerical experiment concludes that player cooperation is the result of a suitable size of sampling in the available strategy space. Hence, the overall performance of the game can be improved by suitably adjusting the strategy space size. △ Less

Submitted 31 December, 2001; originally announced December 2001.

Comments: 4 pages in revtex 4 style

Multiple Choice Minority Game

Authors: F. K. Chow, H. F. Chau

Abstract: Minority game is a model of heterogeneous players who think inductively. In this game, each player chooses one out of two alternatives every turn and those who end up in the minority side wins. It is instructive to extend the minority game by allowing players to choose one out of many alternatives. Nevertheless, such an extension is not straight-forward due to the difficulties in finding a set o… ▽ More Minority game is a model of heterogeneous players who think inductively. In this game, each player chooses one out of two alternatives every turn and those who end up in the minority side wins. It is instructive to extend the minority game by allowing players to choose one out of many alternatives. Nevertheless, such an extension is not straight-forward due to the difficulties in finding a set of reasonable, unbiased and computationally feasible strategies. Here, we propose a variation of the minority game where every player has more than two options. Results of numerical simulations agree with the expectation that our multiple choices minority game exhibits similar behavior as the original two-choice minority game. △ Less

Submitted 15 February, 2002; v1 submitted 10 September, 2001; originally announced September 2001.

Comments: Extensively rewritten, basic conclusion remains unchanged, 8 pages in revtex 4.0

Muon Spin Relaxation Investigation of the Spin Dynamics of Geometrically Frustrated Antiferromagnets Y2Mo2O7 and Tb2Mo2O7 Static Critical Behavior of the Spin-Freezing

Authors: S. R. Dunsiger, R. F. Kiefl, K. H. Chow, B. D. Gaulin, M. J. P. Gingras, J. E. Greedan, A. Keren, K. Kojima, G. M. Luke, W. A. MacFarlane, N. P. Raju, J. E. Sonier, Y. J. Uemura, W. D. Wu

Abstract: The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF=22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residu… ▽ More The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF=22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy. △ Less

Submitted 2 December, 1996; v1 submitted 27 November, 1996; originally announced November 1996.

Comments: Latex file, 3 postscript figures (included)

Journal ref: Phys. Rev. B, {54}, 9019 (1996)