samir el shawish - Academia.edu (original) (raw)
Papers by samir el shawish
Fusion Engineering and Design
Fusion Engineering and Design
Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) ca... more Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the divertor water-cooling pipes due to high current density. A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt element between the target plate and the cassette body to divert electrical current towards more massive cassette body. Such a design solution is also considered as a diagnostic tool for measuring thermo currents (voltages) through a shunt resistor to control plasma detachment. In this study, a finite-element analysis is performed to provide a conservative upper value for the shunt resistance by investigating the impact of the electromagnetic disruption forces on the integrity of water-cooling pipes in the isolated-target design solution of the DEMO divertor. In the analysis, a maximum allowed pipe current is estimated assuming realistic material properties and magnetic fields, and true geometries of the divertor cassette and water-cooling pipes fixed to the cassette. The results show that local pipe yielding (representing damage initiation) develops for electrical currents larger than ∼3.5 kA, which sets a conservative (assuming extreme VDE halo currents) upper value for the shunt resistance to ∼40 µΩ.
The Proceedings of the International Conference on Nuclear Engineering (ICONE)
Crystals
Crystal plasticity finite element models have been extensively used to simulate various aspects o... more Crystal plasticity finite element models have been extensively used to simulate various aspects of polycrystalline deformations. A common weakness of practically all models lies in a relatively large number of constitutive modeling parameters that, in principle, would require dedicated measurements on proper length scales in order to perform reliable model calibration. It is important to realize that the obtained data at different scales should be properly accounted for in the models. In this work, a two-scale calibration procedure is proposed to identify (conventional) crystal plasticity model parameters on a grain scale from tensile test experiments performed on both single crystals and polycrystals. The need for proper adjustment of the polycrystalline tensile data is emphasized and demonstrated by subtracting the length scale effect, originating due to grain boundary strengthening, following the Hall-Petch relation. A small but representative volume element model of the microstructure is identified for fast and reliable identification of modeling parameters. Finally, a simple hardening model upgrade is proposed to incorporate the grain size effects in conventional crystal plasticity. The calibration strategy is demonstrated on tensile test measurements on 316L austenitic stainless steel obtained from the literature.
Journal of Nuclear Engineering and Radiation Science
Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment proc... more Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment procedure are employed in this paper to deliver more accurate statistics of predicted lives of pipes and their uncertainties under turbulent fluid mixing circumstances. The wide variety of synthetic fluid temperatures, generated with an improved spectral method, results in a set of estimated distributions of fatigue lives through linear one-dimensional (1D) heat diffusion, thermal stress estimates, and fatigue assessment codified rules. The results of the fatigue analysis indicate that, in order to avoid the inherent uncertainties due to comparatively short fluid temperature histories to the estimated fatigue lives, a conservative safe design against thermal fatigue could be attempted with the lower bounds of the predicted life distributions, such as the 5% probability life (5% of samples fail). The impact of the convection heat transfer coefficient on the predictions is also studied in a se...
Nuclear Engineering and Design, 2016
Physical Review B, 2008
The dc magnetization and the electron spin resonance (ESR) measurements have been performed on α-... more The dc magnetization and the electron spin resonance (ESR) measurements have been performed on α-NaMnO 2 polycrystalline sample, a quantum spin system on a frustrated two-dimensional (2D) triangular lattice with spatially anisotropic Heisenberg exchange. The ...
Physical Review B Condensed Matter and Materials Physics, Jun 1, 2006
We present an experimental and theoretical study of the magnetically frustrated spin system in pu... more We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu2Te2O5X2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coupled Cu2+ spins and (ii) coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that Cu2Te2O5X2 compounds are true three-dimensional systems of coupled spins. Susceptibility results are consistent with the existence of a singlet-triplet gap, whereas specific heat analysis shows that the singlet-triplet gap is filled with dense singletlike excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to a magnetic response of the system. Furthermore, measured specific heat data show excessive entropy when compared to the numerical results based on a pure spin system, which we attribute to the presence of phonons. Though Cu2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length ξ extends beyond the single tetrahedral cluster dimension, Cu2Te2O5X2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.
Phys Rev B, 2010
We study the electron spin resonance of low-dimensional spin systems at high temperature and test... more We study the electron spin resonance of low-dimensional spin systems at high temperature and test the Kubo-Tomita theory of exchange narrowing. In finite-size systems (molecular magnets), we found a double-peak resonance which strongly differs from the usual Lorentzian. For infinite systems, we have predictions for the linewidth and line shape as a function of the anisotropy strength. For this, we have used an interpolation between a nonperturbative calculation of the memory function at short times (exact diagonalization) and the hydrodynamic spin diffusion at long times. We show that the Dzyaloshinskii-Moriya anisotropies generally induce a much larger linewidth than the exchange anisotropies in two dimensions, contrary to the one-dimensional case.
Aps Meeting Abstracts, Mar 1, 2006
We have carried out high resolution time-of-flight neutron scattering measurements on a new high ... more We have carried out high resolution time-of-flight neutron scattering measurements on a new high quality single crystal of SrCu2-xMgx(BO3)2 with x = 0.1. These studies revealed the presence of new excitations within the singlet-triplet gap of this quasi-two dimensional, dilute, singlet ground state system. These new excitations showed little or no shift in energy with increasing applied magnetic field. In addition, we observe substantial broadening of the three triplet excitations in the dilute single crystal, as compared with pure SrCu2(BO3)2.^1 The triplet excitations in doped SrCu2-xMgx(BO3)2 therefore possess finite lifetimes at low temperatures in the range that can be measured with cold neutron spectroscopy. We have also calculated the dynamical spin structure factor using the zero temperature Lanczos method, and solving a Shastry-Sutherland model at zero and finite doping for different strengths of external magnetic field. This theory reproduces all the qualitative features observed in the experiments on SrCu2-xMgx(BO3)2. ^1 B.D. Gaulin et al., Phys. Rev. Lett., 93, 267202, 2004.
We report measurements of the specific heat of the quantum spin liquid system SrCu_2(BO_3)2 in co... more We report measurements of the specific heat of the quantum spin liquid system SrCu_2(BO_3)2 in continuous magnetic fields H of up to 33 T. The specific heat vs temperature at zero field shows an anomaly at 8 K, marking the opening of a gap in the spin singlet excitations. At fields H >= 12 T, we clearly see a second anomaly that shifts to lower temperatures as H is increased. We attribute its origin to single triplet excitations of the singlet dimer ground state. This conclusion is supported by calculations of the specific heat, which reproduce the experimental data, made using the finite temperature Lanczos method to solve a Shastry-Sutherland Hamiltonian including nearest and next-nearest neighbor Dzyaloshinsky-Moriya interactions. The parameters used to fit the data are the exchange constants J = 74 K and J'/J = 0.62, and the Dzyaloshinsky-Moriya coupling constants |D|=6.1 K, and |D'|=2.2 K.
Computational Materials Science, 2016
International Journal of Fatigue, 2016
Phys Rev Lett, 2006
High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with ... more High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with x 0:05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S 1=2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one-and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu 2 BO 3 2 . Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.
High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with ... more High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with x 0:05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S 1=2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one-and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu 2 BO 3 2 . Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.
The properties of the polaron and bipolaron are explored in the 1D Jahn-Teller model with dynamic... more The properties of the polaron and bipolaron are explored in the 1D Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity σ(ω). Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in σ(ω) between the two models are found and discussed. The influence of the electronphonon coupling and the electrostatic repulsion on the bipolaron binding energy, bipolaron masses, and correlation functions is investigated.
We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in t... more We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in the low energy magneto-optical absorption spectrum opens at low temperature. This previously unreported collective phenomenon arises at temperatures where the energy of the dominant exchange interaction exceeds the thermal energy. Simulations of the observed shifts in electron paramagnetic resonance spectral weight, which include spin anisotropy, reproduce this behavior yielding the magnitude of the spin anisotropy in these compounds. Comment: 4 pages (slightly run over) 4 figures
Physical Review B, 2003
The user has requested enhancement of the downloaded file. arXiv:cond-mat/0302211v1 [cond-mat.str... more The user has requested enhancement of the downloaded file. arXiv:cond-mat/0302211v1 [cond-mat.str-el] We show how to generalise the zero temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble, which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature, but whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.
Physical Review B, 2003
The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model... more The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity (). Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in () between the two models are found and discussed. The influence of the electron-phonon coupling and the electrostatic repulsion on the bipolaron binding energy, bipolaron masses, and correlation functions is investigated.
SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state ... more SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state and a realization of the Shastry-Suthrland model. One aspect of the study of SrCu2(BO3)2 for which there is a little information is the influence of impurities on the nature of the singlet ground state. There is much interest in such studies due to the remarkable
Fusion Engineering and Design
Fusion Engineering and Design
Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) ca... more Global plasma instabilities such as plasma disruptions and vertical displacement events (VDEs) can generate huge transient electrical currents in the conductive parts of the divertor. The interaction of the currents with the magnetic field induces Lorentz force loads. Particularly, the force induced by a VDE can impose a critical impact on the divertor water-cooling pipes due to high current density. A recently proposed isolated-target design solution for the DEMO divertor introduces a shunt element between the target plate and the cassette body to divert electrical current towards more massive cassette body. Such a design solution is also considered as a diagnostic tool for measuring thermo currents (voltages) through a shunt resistor to control plasma detachment. In this study, a finite-element analysis is performed to provide a conservative upper value for the shunt resistance by investigating the impact of the electromagnetic disruption forces on the integrity of water-cooling pipes in the isolated-target design solution of the DEMO divertor. In the analysis, a maximum allowed pipe current is estimated assuming realistic material properties and magnetic fields, and true geometries of the divertor cassette and water-cooling pipes fixed to the cassette. The results show that local pipe yielding (representing damage initiation) develops for electrical currents larger than ∼3.5 kA, which sets a conservative (assuming extreme VDE halo currents) upper value for the shunt resistance to ∼40 µΩ.
The Proceedings of the International Conference on Nuclear Engineering (ICONE)
Crystals
Crystal plasticity finite element models have been extensively used to simulate various aspects o... more Crystal plasticity finite element models have been extensively used to simulate various aspects of polycrystalline deformations. A common weakness of practically all models lies in a relatively large number of constitutive modeling parameters that, in principle, would require dedicated measurements on proper length scales in order to perform reliable model calibration. It is important to realize that the obtained data at different scales should be properly accounted for in the models. In this work, a two-scale calibration procedure is proposed to identify (conventional) crystal plasticity model parameters on a grain scale from tensile test experiments performed on both single crystals and polycrystals. The need for proper adjustment of the polycrystalline tensile data is emphasized and demonstrated by subtracting the length scale effect, originating due to grain boundary strengthening, following the Hall-Petch relation. A small but representative volume element model of the microstructure is identified for fast and reliable identification of modeling parameters. Finally, a simple hardening model upgrade is proposed to incorporate the grain size effects in conventional crystal plasticity. The calibration strategy is demonstrated on tensile test measurements on 316L austenitic stainless steel obtained from the literature.
Journal of Nuclear Engineering and Radiation Science
Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment proc... more Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment procedure are employed in this paper to deliver more accurate statistics of predicted lives of pipes and their uncertainties under turbulent fluid mixing circumstances. The wide variety of synthetic fluid temperatures, generated with an improved spectral method, results in a set of estimated distributions of fatigue lives through linear one-dimensional (1D) heat diffusion, thermal stress estimates, and fatigue assessment codified rules. The results of the fatigue analysis indicate that, in order to avoid the inherent uncertainties due to comparatively short fluid temperature histories to the estimated fatigue lives, a conservative safe design against thermal fatigue could be attempted with the lower bounds of the predicted life distributions, such as the 5% probability life (5% of samples fail). The impact of the convection heat transfer coefficient on the predictions is also studied in a se...
Nuclear Engineering and Design, 2016
Physical Review B, 2008
The dc magnetization and the electron spin resonance (ESR) measurements have been performed on α-... more The dc magnetization and the electron spin resonance (ESR) measurements have been performed on α-NaMnO 2 polycrystalline sample, a quantum spin system on a frustrated two-dimensional (2D) triangular lattice with spatially anisotropic Heisenberg exchange. The ...
Physical Review B Condensed Matter and Materials Physics, Jun 1, 2006
We present an experimental and theoretical study of the magnetically frustrated spin system in pu... more We present an experimental and theoretical study of the magnetically frustrated spin system in pure and substitutionally disordered compounds from the Cu2Te2O5X2 family of quantum magnets. Experimental magnetic susceptibilities and specific heats were analyzed simultaneously using models of (i) isolated tetrahedra of four antiferromagnetically coupled Cu2+ spins and (ii) coupled tetrahedra within one-dimensional chains, in both cases involving mean-field coupling to other chains. The results show that Cu2Te2O5X2 compounds are true three-dimensional systems of coupled spins. Susceptibility results are consistent with the existence of a singlet-triplet gap, whereas specific heat analysis shows that the singlet-triplet gap is filled with dense singletlike excitations that contribute to finite specific heat at temperatures far below the singlet-triplet gap, but do not contribute to a magnetic response of the system. Furthermore, measured specific heat data show excessive entropy when compared to the numerical results based on a pure spin system, which we attribute to the presence of phonons. Though Cu2+ spins are arranged in a geometrically frustrated tetrahedral antiferromagnetic configuration and spin correlation length ξ extends beyond the single tetrahedral cluster dimension, Cu2Te2O5X2 compounds do not exhibit ergodicity breaking at low temperatures, in contrast to the related geometrically frustrated kagomé and pyrochlore antiferromagnets.
Phys Rev B, 2010
We study the electron spin resonance of low-dimensional spin systems at high temperature and test... more We study the electron spin resonance of low-dimensional spin systems at high temperature and test the Kubo-Tomita theory of exchange narrowing. In finite-size systems (molecular magnets), we found a double-peak resonance which strongly differs from the usual Lorentzian. For infinite systems, we have predictions for the linewidth and line shape as a function of the anisotropy strength. For this, we have used an interpolation between a nonperturbative calculation of the memory function at short times (exact diagonalization) and the hydrodynamic spin diffusion at long times. We show that the Dzyaloshinskii-Moriya anisotropies generally induce a much larger linewidth than the exchange anisotropies in two dimensions, contrary to the one-dimensional case.
Aps Meeting Abstracts, Mar 1, 2006
We have carried out high resolution time-of-flight neutron scattering measurements on a new high ... more We have carried out high resolution time-of-flight neutron scattering measurements on a new high quality single crystal of SrCu2-xMgx(BO3)2 with x = 0.1. These studies revealed the presence of new excitations within the singlet-triplet gap of this quasi-two dimensional, dilute, singlet ground state system. These new excitations showed little or no shift in energy with increasing applied magnetic field. In addition, we observe substantial broadening of the three triplet excitations in the dilute single crystal, as compared with pure SrCu2(BO3)2.^1 The triplet excitations in doped SrCu2-xMgx(BO3)2 therefore possess finite lifetimes at low temperatures in the range that can be measured with cold neutron spectroscopy. We have also calculated the dynamical spin structure factor using the zero temperature Lanczos method, and solving a Shastry-Sutherland model at zero and finite doping for different strengths of external magnetic field. This theory reproduces all the qualitative features observed in the experiments on SrCu2-xMgx(BO3)2. ^1 B.D. Gaulin et al., Phys. Rev. Lett., 93, 267202, 2004.
We report measurements of the specific heat of the quantum spin liquid system SrCu_2(BO_3)2 in co... more We report measurements of the specific heat of the quantum spin liquid system SrCu_2(BO_3)2 in continuous magnetic fields H of up to 33 T. The specific heat vs temperature at zero field shows an anomaly at 8 K, marking the opening of a gap in the spin singlet excitations. At fields H >= 12 T, we clearly see a second anomaly that shifts to lower temperatures as H is increased. We attribute its origin to single triplet excitations of the singlet dimer ground state. This conclusion is supported by calculations of the specific heat, which reproduce the experimental data, made using the finite temperature Lanczos method to solve a Shastry-Sutherland Hamiltonian including nearest and next-nearest neighbor Dzyaloshinsky-Moriya interactions. The parameters used to fit the data are the exchange constants J = 74 K and J'/J = 0.62, and the Dzyaloshinsky-Moriya coupling constants |D|=6.1 K, and |D'|=2.2 K.
Computational Materials Science, 2016
International Journal of Fatigue, 2016
Phys Rev Lett, 2006
High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with ... more High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with x 0:05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S 1=2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one-and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu 2 BO 3 2 . Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.
High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with ... more High resolution neutron scattering measurements on a single crystal of SrCu 2ÿx Mg x BO 3 2 with x 0:05 reveal the presence of new spin excitations within the gap of this quasi-two-dimensional, singlet ground state system. The application of a magnetic field induces Zeeman-split states associated with S 1=2 unpaired spins which are antiferromagnetically correlated with the bulk singlet. Substantial broadening of both the one-and two-triplet excitations in the doped single crystal is observed, as compared with pure SrCu 2 BO 3 2 . Theoretical calculations using a variational algorithm and a single quenched magnetic vacancy on an infinite lattice are shown to qualitatively account for these effects.
The properties of the polaron and bipolaron are explored in the 1D Jahn-Teller model with dynamic... more The properties of the polaron and bipolaron are explored in the 1D Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity σ(ω). Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in σ(ω) between the two models are found and discussed. The influence of the electronphonon coupling and the electrostatic repulsion on the bipolaron binding energy, bipolaron masses, and correlation functions is investigated.
We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in t... more We report unexpected behaviour in a family of Cu spin- 1/2 systems, in which an apparent gap in the low energy magneto-optical absorption spectrum opens at low temperature. This previously unreported collective phenomenon arises at temperatures where the energy of the dominant exchange interaction exceeds the thermal energy. Simulations of the observed shifts in electron paramagnetic resonance spectral weight, which include spin anisotropy, reproduce this behavior yielding the magnitude of the spin anisotropy in these compounds. Comment: 4 pages (slightly run over) 4 figures
Physical Review B, 2003
The user has requested enhancement of the downloaded file. arXiv:cond-mat/0302211v1 [cond-mat.str... more The user has requested enhancement of the downloaded file. arXiv:cond-mat/0302211v1 [cond-mat.str-el] We show how to generalise the zero temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble, which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature, but whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.
Physical Review B, 2003
The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model... more The properties of the polaron and bipolaron are explored in the one-dimensional Jahn-Teller model with dynamical quantum phonons. The ground-state properties of the polaron and bipolaron are computed using a recently developed variational method. Dynamical properties of the ground state of a polaron are investigated by calculating the optical conductivity (). Our numerical results suggest that the Jahn-Teller and Holstein polarons are similar. However, in the strong-coupling regime qualitative differences in () between the two models are found and discussed. The influence of the electron-phonon coupling and the electrostatic repulsion on the bipolaron binding energy, bipolaron masses, and correlation functions is investigated.
SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state ... more SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state and a realization of the Shastry-Suthrland model. One aspect of the study of SrCu2(BO3)2 for which there is a little information is the influence of impurities on the nature of the singlet ground state. There is much interest in such studies due to the remarkable