A. Furrer - Academia.edu (original) (raw)

Papers by A. Furrer

AIP Conference Proceedings, 1973

The crystalline electric field levels of intermetallic Laves phase compounds of type RxY1-xAl2 (R... more The crystalline electric field levels of intermetallic Laves phase compounds of type RxY1-xAl2 (R = Pr, Er, Tm) have been determined by neutron spectroscopy. The polycrystalline samples have been diluted with yttrium in order to reduce magnetic interactions between the rare earth ions and thereby reducing the widths of the crystal field transition lines. However, other relaxation mechanisms and the small crystal field splittings made it impossible to separate the crystal field transition peaks completely. The intensity arising from crystal field transitions has been identified by its temperature dependence as well as by comparison with corresponding ``non-magnetic'' YAl2 spectra. A least squares fitting procedure has been applied to the experimental data in order to obtain the best possible crystal field parameters. At temperatures T > 50 K and momentum transfers Q > 1 A˚-1 two-phonon processes have to be taken into account in analyzing the measured energy spectra. The resulting crystal field level schemes agree qualitatively with estimates based on a nearest neighbour point charge model and quantitatively with measured susceptibility data.

AIP Conference Proceedings, 1973

Because of the well-localized 4f moment, the exchange interaction in rare earth ionic compounds c... more Because of the well-localized 4f moment, the exchange interaction in rare earth ionic compounds can be described in terms of an effective field theory. It is shown that the effective field parameters are completely determined ``by combining the results obtained from neutron diffraction in the ordered state and from neutron crystal field spectroscopy in the paramagnetic state. This is exemplified for the cerium monopnictides in the molecular field approximation. To determine the molecular field in the ordered state we used the measured values of the magnetic moment1&ndash:2 at temperatures T gives rise to a fluctuating molecular field whose mean value is zero. Information about this effect (which we call the dynamical Zeeman effect) is contained either in the width or in the intensity of the crystal field transition lines depending on whether the crystal field interaction or the exchange interaction is dominant. The experimental susceptibility data3-5 are well accounted for by using our earlier crystal field6-7 and the present molecular field results without any further approximations (in a similar investigation performed by Wang and Cooper8 details about the magnetic structure and the crystal field splitting in most of the compounds were not yet available). In particular, the anomalous susceptibility behaviour in CeSb can be quantitatively explained.

Physica B, 2000

Inelastic neutron scattering has been employed to study the transitions between the lowest levels... more Inelastic neutron scattering has been employed to study the transitions between the lowest levels of the J-multiplet of the Ho> ions in Ho \V Ca V Ba Cu O W (x"0}0.3, y"6.3, 7) split by crystalline electric "led (CEF). The observed energy spectra exhibit a "ne structure which re#ects the behavior of the local hole density near the doping center. The results obtained are discussed in terms of the model of the CEF generated by the ordered planar charge structure.

Physica C: Superconductivity, 1996

... Program abstract (unpublished). [18] V. Bobrovskii. S. Verl..hovskii. A. Voronin. B. Goshchit... more ... Program abstract (unpublished). [18] V. Bobrovskii. S. Verl..hovskii. A. Voronin. B. Goshchitskii. V. Zubkov. T. O'yachkova. I. Zhdakhin. ... Physica B 192 (1993) 55. (20] M. Heinau. R. Baumann. B. Nick. M. Hartweg and L. Walz Z. Kristallogr. 209 (1994) 418. (21] JD Jorgensen. ...

Physica C: Superconductivity, 2000

Investigation of substitution effects for various sites in the ''1212'' perovskite-like systems w... more Investigation of substitution effects for various sites in the ''1212'' perovskite-like systems with a general formula Ž .

Physica B: Condensed Matter, 1997

We present a combined neutron and X-ray study of the effects of Ca doping in RBa2Cu3O7−δ (R = Y, ... more We present a combined neutron and X-ray study of the effects of Ca doping in RBa2Cu3O7−δ (R = Y, Er). The structural effects observed by diffraction methods are correlated with the observed changes in Tc and interpreted in terms of ion size and charge effects. We deduce from bond valence sum calculations that the partial substitution for R3+ (R = Y, Er) with Ca2+ causes overdoping of the CuO2 planes. Inelastic neutron scattering has been used to study the low-energy crystal-field (CEF) excitations of Er3+ in Er1−xCaxBa2Cu3O7−δ (0 ⩽ x ⩽ 0.2). Due to Ca doping the CEF transition around 10.8 meV decomposes into two individual transitions whose spectral weights depend distinctly on the Ca doping level. We conclude that these substructures occurring due to Ca doping originate from different local environments of Er3+ due to a phase separation in the overdoped region.

Physica B: Condensed Matter, 1997

ABSTRACT We present a study of the spin-wave excitations of Nd in Nd2-xCexCuO4 (x=0, 0.09, 0.15, ... more ABSTRACT We present a study of the spin-wave excitations of Nd in Nd2-xCexCuO4 (x=0, 0.09, 0.15, 0.18) by inelastic neutron scattering. Upon Ce-doping one observes a softening of the spin-wave branches. This is mainly explained by the decrease of the Cu-Nd exchange field at the Nd site, while the Nd-Nd exchange interactions remain almost unchanged. Consequently a high density of states of spin-wave excitations at low energies is created. The resulting calculated specific-heat shows a large gamma=C/T-value at low temperatures, which indicates that the novel heavy-fermion behavior in these compounds is mainly due to magnetic correlations of the Nd ions.

Physica B: Condensed Matter, 2004

To investigate the charge states in the CuO2 planes of the high-Tc superconductor HoBa2Cu3O6.95 i... more To investigate the charge states in the CuO2 planes of the high-Tc superconductor HoBa2Cu3O6.95 induced by the in-plane electron doping, we studied the crystal structure and the crystal field (CF) effects for Ho1−yThyBa2Cu3O6.95 (y≈0.07, 0.13) by neutron powder diffraction and inelastic neutron scattering methods, respectively. The Th-induced variation of the CF transitions is found to be compatible with an idea

Physica B: Condensed Matter, 1989

Abstract Quasi crystalline Pd 58.8 U 20.6 Si 20.6 has been investigated by elastic and inelastic ... more Abstract Quasi crystalline Pd 58.8 U 20.6 Si 20.6 has been investigated by elastic and inelastic neutron scattering (INS). In the energy loss range of 0–50 meV in the INS spectra we found a crystal-field transition at about 3 meV of considerable broadness which is in agreement with a model calculation with point charges located at the vertices of a slightly disturbed icosahedron.

Physica B: Condensed Matter, 2000

Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to ... more Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to be a necessity in characterizing unconventional quantum spin systems — in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S= 1 2 Cu 2+ compound KCuCl3. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet–triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl3 as a quantum spin liquid near to the dimer limit.

Journal of Magnetism and Magnetic Materials, 1976

Neutron inelastic scattering experiments have been performed in the antiferromagnet CeP in order ... more Neutron inelastic scattering experiments have been performed in the antiferromagnet CeP in order to determine the energy level splitting within the F5/2 ground state multiplet. The measured spectra have been analysed on the basis of a single-ion hamiltonian which takes into account crystal field, exchange and magnetoelastic interactions. The crystal field parameter B4 = (0.041 +/- 0.002) meV has been determined from measurements in the paramagnetic state at T = 15 K. The energy level splitting measured in the ordered state at T = 5.6 K yields the molecular field parameter lambda = (1.85 +/- 0.10) x 1025 Oe2/erg and the quadrupolar parameter A = (0.36 +/- 0.04) x 10-15 erg. These parameters give a good description of the zero-field magnetization.

Journal of Crystal Growth, 2005

ABSTRACT

Journal of Applied Physics, 2006

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by n... more The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10meV at 2.5K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb.

Journal of Applied Physics, 1994

The magnetic order of Pr ions in polycrystalline, tetragonal PrBa2Cu3O6.2 has been studied by neu... more The magnetic order of Pr ions in polycrystalline, tetragonal PrBa2Cu3O6.2 has been studied by neutron-diffraction measurements. The basic ordering consists of an antiferromagnetic arrangement with the propagation vector k=[1/2, 1/2, 1/2], but the magnetic ordering is imperfect due to finite correlation length ξc=(10±1) Å along the c axis. The Néel temperature is determined to be TN=(12±1) K, and the magnetic saturation moment of the Pr ions turns out to be (1.9±0.2) μB. The distance Pr-O(2) compared in several related perovskite-type Pr compounds correlates with the Néel temperature in accordance with the superexchange interaction.

Czechoslovak Journal of Physics, 1996

Inelastic neutron scattering experiments of the crystal-field (CEF) splitting of the R 3+ ions (R... more Inelastic neutron scattering experiments of the crystal-field (CEF) splitting of the R 3+ ions (R = Ho, Er, Tm) in the superconductor RNi 11 2 B 2 C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the thermodynamic magnetic properties of these systems. The calculated ordered magnetic moments are in agreement with magnetic neutron diffraction results, i.e. both the easy axis and the size of the zero-field moments are correctly reproduced. In addition, the entropy involved in the magnetic ordering obtained from specific heat measurements is in correspondence with the number of low-lying CEF states observed in this study. Our CEF parameter set also reproduces both the behavior and the anisotropy in M/H measured for single crystals.

Zeitschrift f�r Physik B Condensed Matter, 1994

Inelastic neutron scattering has been employed to study in detail the low-energy crystal-field (C... more Inelastic neutron scattering has been employed to study in detail the low-energy crystal-field (CEF) excitations of Ho 3+ in tetragonal antiferromagnetic (AF) Ho0.1Y0.9Ba2Cu30613. We observe a splitting of the lowest excited F 5 doublet at T-1.5 K. This splitting is explained as resulting from the exchange interaction with the AF subsystem of copper spins. The corresponding susceptibility at the Ho 3 + site has a symmetry lower than the tetragonal one of the CEF potential and, as a result, produces the splitting. The value of the coupling between Ho 3+ and Cu 2+ ions J~x=2.4 meV is derived from the experimentally observed value of the splitting AE=0.1 meV and parameters of the AF subsystem, known from the neutron scattering experiments. Some possible consequences of the rare-earth (RE)-copper exchange are discussed, including the interaction between RE ions via the virtual exchange by spin waves.

Chemical Physics Letters, 1984

ABSTRACT

Journal of Physics C: Solid State Physics, 1974

Inelastic neutron scattering experiments and susceptibility measurements have been carried out on... more Inelastic neutron scattering experiments and susceptibility measurements have been carried out on polycrystalline ErcY1-cAl2. A least-squares fitting procedure has been applied to the neutron data which favours four sets of crystal-field parameters. The results are compared with the measured susceptibility and other bulk magnetic properties. From this it is concluded that the crystal-field parameters x=-0.54 and W=-0.018 meV are the

Journal of Physics: Condensed Matter, 2003

... Two authors (MD and JR) acknowledge fruitful discussions with Bernard Delley, Manuel Richter,... more ... Two authors (MD and JR) acknowledge fruitful discussions with Bernard Delley, Manuel Richter,Peter Blaha, Helmut Eschrig and Karlheinz Schwarz as well as ... Phys.–Solid State 18 389 [2] Alekseev PA, Shitikov Yu L, Markova IA, Chistyakov OD, Savitskii EM and Kjems J 1982 ...

AIP Conference Proceedings, 1973

The crystalline electric field levels of intermetallic Laves phase compounds of type RxY1-xAl2 (R... more The crystalline electric field levels of intermetallic Laves phase compounds of type RxY1-xAl2 (R = Pr, Er, Tm) have been determined by neutron spectroscopy. The polycrystalline samples have been diluted with yttrium in order to reduce magnetic interactions between the rare earth ions and thereby reducing the widths of the crystal field transition lines. However, other relaxation mechanisms and the small crystal field splittings made it impossible to separate the crystal field transition peaks completely. The intensity arising from crystal field transitions has been identified by its temperature dependence as well as by comparison with corresponding ``non-magnetic'' YAl2 spectra. A least squares fitting procedure has been applied to the experimental data in order to obtain the best possible crystal field parameters. At temperatures T > 50 K and momentum transfers Q > 1 A˚-1 two-phonon processes have to be taken into account in analyzing the measured energy spectra. The resulting crystal field level schemes agree qualitatively with estimates based on a nearest neighbour point charge model and quantitatively with measured susceptibility data.

AIP Conference Proceedings, 1973

Because of the well-localized 4f moment, the exchange interaction in rare earth ionic compounds c... more Because of the well-localized 4f moment, the exchange interaction in rare earth ionic compounds can be described in terms of an effective field theory. It is shown that the effective field parameters are completely determined ``by combining the results obtained from neutron diffraction in the ordered state and from neutron crystal field spectroscopy in the paramagnetic state. This is exemplified for the cerium monopnictides in the molecular field approximation. To determine the molecular field in the ordered state we used the measured values of the magnetic moment1&ndash:2 at temperatures T gives rise to a fluctuating molecular field whose mean value is zero. Information about this effect (which we call the dynamical Zeeman effect) is contained either in the width or in the intensity of the crystal field transition lines depending on whether the crystal field interaction or the exchange interaction is dominant. The experimental susceptibility data3-5 are well accounted for by using our earlier crystal field6-7 and the present molecular field results without any further approximations (in a similar investigation performed by Wang and Cooper8 details about the magnetic structure and the crystal field splitting in most of the compounds were not yet available). In particular, the anomalous susceptibility behaviour in CeSb can be quantitatively explained.

Physica B, 2000

Inelastic neutron scattering has been employed to study the transitions between the lowest levels... more Inelastic neutron scattering has been employed to study the transitions between the lowest levels of the J-multiplet of the Ho> ions in Ho \V Ca V Ba Cu O W (x"0}0.3, y"6.3, 7) split by crystalline electric "led (CEF). The observed energy spectra exhibit a "ne structure which re#ects the behavior of the local hole density near the doping center. The results obtained are discussed in terms of the model of the CEF generated by the ordered planar charge structure.

Physica C: Superconductivity, 1996

... Program abstract (unpublished). [18] V. Bobrovskii. S. Verl..hovskii. A. Voronin. B. Goshchit... more ... Program abstract (unpublished). [18] V. Bobrovskii. S. Verl..hovskii. A. Voronin. B. Goshchitskii. V. Zubkov. T. O'yachkova. I. Zhdakhin. ... Physica B 192 (1993) 55. (20] M. Heinau. R. Baumann. B. Nick. M. Hartweg and L. Walz Z. Kristallogr. 209 (1994) 418. (21] JD Jorgensen. ...

Physica C: Superconductivity, 2000

Investigation of substitution effects for various sites in the ''1212'' perovskite-like systems w... more Investigation of substitution effects for various sites in the ''1212'' perovskite-like systems with a general formula Ž .

Physica B: Condensed Matter, 1997

We present a combined neutron and X-ray study of the effects of Ca doping in RBa2Cu3O7−δ (R = Y, ... more We present a combined neutron and X-ray study of the effects of Ca doping in RBa2Cu3O7−δ (R = Y, Er). The structural effects observed by diffraction methods are correlated with the observed changes in Tc and interpreted in terms of ion size and charge effects. We deduce from bond valence sum calculations that the partial substitution for R3+ (R = Y, Er) with Ca2+ causes overdoping of the CuO2 planes. Inelastic neutron scattering has been used to study the low-energy crystal-field (CEF) excitations of Er3+ in Er1−xCaxBa2Cu3O7−δ (0 ⩽ x ⩽ 0.2). Due to Ca doping the CEF transition around 10.8 meV decomposes into two individual transitions whose spectral weights depend distinctly on the Ca doping level. We conclude that these substructures occurring due to Ca doping originate from different local environments of Er3+ due to a phase separation in the overdoped region.

Physica B: Condensed Matter, 1997

ABSTRACT We present a study of the spin-wave excitations of Nd in Nd2-xCexCuO4 (x=0, 0.09, 0.15, ... more ABSTRACT We present a study of the spin-wave excitations of Nd in Nd2-xCexCuO4 (x=0, 0.09, 0.15, 0.18) by inelastic neutron scattering. Upon Ce-doping one observes a softening of the spin-wave branches. This is mainly explained by the decrease of the Cu-Nd exchange field at the Nd site, while the Nd-Nd exchange interactions remain almost unchanged. Consequently a high density of states of spin-wave excitations at low energies is created. The resulting calculated specific-heat shows a large gamma=C/T-value at low temperatures, which indicates that the novel heavy-fermion behavior in these compounds is mainly due to magnetic correlations of the Nd ions.

Physica B: Condensed Matter, 2004

To investigate the charge states in the CuO2 planes of the high-Tc superconductor HoBa2Cu3O6.95 i... more To investigate the charge states in the CuO2 planes of the high-Tc superconductor HoBa2Cu3O6.95 induced by the in-plane electron doping, we studied the crystal structure and the crystal field (CF) effects for Ho1−yThyBa2Cu3O6.95 (y≈0.07, 0.13) by neutron powder diffraction and inelastic neutron scattering methods, respectively. The Th-induced variation of the CF transitions is found to be compatible with an idea

Physica B: Condensed Matter, 1989

Abstract Quasi crystalline Pd 58.8 U 20.6 Si 20.6 has been investigated by elastic and inelastic ... more Abstract Quasi crystalline Pd 58.8 U 20.6 Si 20.6 has been investigated by elastic and inelastic neutron scattering (INS). In the energy loss range of 0–50 meV in the INS spectra we found a crystal-field transition at about 3 meV of considerable broadness which is in agreement with a model calculation with point charges located at the vertices of a slightly disturbed icosahedron.

Physica B: Condensed Matter, 2000

Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to ... more Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to be a necessity in characterizing unconventional quantum spin systems — in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S= 1 2 Cu 2+ compound KCuCl3. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet–triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl3 as a quantum spin liquid near to the dimer limit.

Journal of Magnetism and Magnetic Materials, 1976

Neutron inelastic scattering experiments have been performed in the antiferromagnet CeP in order ... more Neutron inelastic scattering experiments have been performed in the antiferromagnet CeP in order to determine the energy level splitting within the F5/2 ground state multiplet. The measured spectra have been analysed on the basis of a single-ion hamiltonian which takes into account crystal field, exchange and magnetoelastic interactions. The crystal field parameter B4 = (0.041 +/- 0.002) meV has been determined from measurements in the paramagnetic state at T = 15 K. The energy level splitting measured in the ordered state at T = 5.6 K yields the molecular field parameter lambda = (1.85 +/- 0.10) x 1025 Oe2/erg and the quadrupolar parameter A = (0.36 +/- 0.04) x 10-15 erg. These parameters give a good description of the zero-field magnetization.

Journal of Crystal Growth, 2005

ABSTRACT

Journal of Applied Physics, 2006

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by n... more The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10meV at 2.5K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb.

Journal of Applied Physics, 1994

The magnetic order of Pr ions in polycrystalline, tetragonal PrBa2Cu3O6.2 has been studied by neu... more The magnetic order of Pr ions in polycrystalline, tetragonal PrBa2Cu3O6.2 has been studied by neutron-diffraction measurements. The basic ordering consists of an antiferromagnetic arrangement with the propagation vector k=[1/2, 1/2, 1/2], but the magnetic ordering is imperfect due to finite correlation length ξc=(10±1) Å along the c axis. The Néel temperature is determined to be TN=(12±1) K, and the magnetic saturation moment of the Pr ions turns out to be (1.9±0.2) μB. The distance Pr-O(2) compared in several related perovskite-type Pr compounds correlates with the Néel temperature in accordance with the superexchange interaction.

Czechoslovak Journal of Physics, 1996

Inelastic neutron scattering experiments of the crystal-field (CEF) splitting of the R 3+ ions (R... more Inelastic neutron scattering experiments of the crystal-field (CEF) splitting of the R 3+ ions (R = Ho, Er, Tm) in the superconductor RNi 11 2 B 2 C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the thermodynamic magnetic properties of these systems. The calculated ordered magnetic moments are in agreement with magnetic neutron diffraction results, i.e. both the easy axis and the size of the zero-field moments are correctly reproduced. In addition, the entropy involved in the magnetic ordering obtained from specific heat measurements is in correspondence with the number of low-lying CEF states observed in this study. Our CEF parameter set also reproduces both the behavior and the anisotropy in M/H measured for single crystals.

Zeitschrift f�r Physik B Condensed Matter, 1994

Inelastic neutron scattering has been employed to study in detail the low-energy crystal-field (C... more Inelastic neutron scattering has been employed to study in detail the low-energy crystal-field (CEF) excitations of Ho 3+ in tetragonal antiferromagnetic (AF) Ho0.1Y0.9Ba2Cu30613. We observe a splitting of the lowest excited F 5 doublet at T-1.5 K. This splitting is explained as resulting from the exchange interaction with the AF subsystem of copper spins. The corresponding susceptibility at the Ho 3 + site has a symmetry lower than the tetragonal one of the CEF potential and, as a result, produces the splitting. The value of the coupling between Ho 3+ and Cu 2+ ions J~x=2.4 meV is derived from the experimentally observed value of the splitting AE=0.1 meV and parameters of the AF subsystem, known from the neutron scattering experiments. Some possible consequences of the rare-earth (RE)-copper exchange are discussed, including the interaction between RE ions via the virtual exchange by spin waves.

Chemical Physics Letters, 1984

ABSTRACT

Journal of Physics C: Solid State Physics, 1974

Inelastic neutron scattering experiments and susceptibility measurements have been carried out on... more Inelastic neutron scattering experiments and susceptibility measurements have been carried out on polycrystalline ErcY1-cAl2. A least-squares fitting procedure has been applied to the neutron data which favours four sets of crystal-field parameters. The results are compared with the measured susceptibility and other bulk magnetic properties. From this it is concluded that the crystal-field parameters x=-0.54 and W=-0.018 meV are the

Journal of Physics: Condensed Matter, 2003

... Two authors (MD and JR) acknowledge fruitful discussions with Bernard Delley, Manuel Richter,... more ... Two authors (MD and JR) acknowledge fruitful discussions with Bernard Delley, Manuel Richter,Peter Blaha, Helmut Eschrig and Karlheinz Schwarz as well as ... Phys.–Solid State 18 389 [2] Alekseev PA, Shitikov Yu L, Markova IA, Chistyakov OD, Savitskii EM and Kjems J 1982 ...