ABSTRACT The compound (NH3)xNaRb2C60 has been studied by SQUID magnetometry, and NMR. A double ir... more ABSTRACT The compound (NH3)xNaRb2C60 has been studied by SQUID magnetometry, and NMR. A double irradiation NMR experiment evidences a close distance between Na ions and ammonia, confirming the octahedral occupancy of this cluster which is usually observed in the ammonia intercalated fullerides. The Knight shift interaction of the ammonia protons with the conduction electrons is of pure anisotropic nature and is detected as a field dependent broadening of the NMR lines at low temperature. The recovery of the proton magnetisation follows a single exponential at all the investigated temperatures and fields. A Korringa behaviour is observed below 65 K and a clear Hebel–Slichter coherence peak is detected just below Tc. The Korringa relation does not hold and the Knight shift enhancement suggests the presence of a Coulomb repulsion.
This work aims at understanding the large reduction of superconducting critical temperature Tc ob... more This work aims at understanding the large reduction of superconducting critical temperature Tc observed in YSr2Cu306+x as compared to its YBa2Cu3O6+x counterpart (DeltaTc = -30 K). We report on a combined study of structural and electronic properties of RESr2Cu3O6+x (RE = Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa2Cu3O6+x RESr2Cu3O6+x is locally
The discovery of high-Tc superconductivity in F-doped RFeAsO (1111) was one of the most significa... more The discovery of high-Tc superconductivity in F-doped RFeAsO (1111) was one of the most significant breakthroughs in the field of condensed matter physics during the last decades. 1, 2 A striking worldwide interest has been indeed recently devoted to the synthesis of this class of materials, looking for suitable chemical substitutions of rare-earth (R) ions, transition metals (TM), and pnictogen elements (Pn) allowing one to enhance the critical temperature Tc, the highest value Tc≃ 55 K being currently reached in SmFeAsO1− xFx.
We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The be... more We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The behavior of the hyperfine constants and of the μSR linewidths have been investigated as a function of temperature; they have been correlated with the reorientational dynamics of the molecules, which have already been partially determined by means of Raman, NMR, and neutron scattering data. Our measurements
ABSTRACT Muons implanted in MnF2 and CoF2 are localized at low temperatures in an interstitial si... more ABSTRACT Muons implanted in MnF2 and CoF2 are localized at low temperatures in an interstitial site which exhibits octahedral symmetry. Measurements of the critical damping close toT N and the hyperfine field in the AF phase are reported.
Muonium adduct radicals have been studied in Norbornene and Bicyclooctene as a function of temper... more Muonium adduct radicals have been studied in Norbornene and Bicyclooctene as a function of temperature from the liquid phase down to the plastic phase and through the low temperature transition to a typical solid (brittle phase). The features of the radical hyperfine spectra are discussed. The temperature coefficient for the hyperfine constants is 0.1 MHz/K for the NBE exo isomer,
... References [1] R. De Renzi, G. GuidL P. Carretta. G. Calestani and SFJ Cox, Phys. Lett. A 135... more ... References [1] R. De Renzi, G. GuidL P. Carretta. G. Calestani and SFJ Cox, Phys. Lett. A 135 (1989) 132 [2] BX Yang, RF Kiefl, JH Brewer, JF Carolan, WN Hardy, R. Kadono, JR Kempton, SR Kreitzman, GM Luke, TM Riseman, D.LI. ...
A study is presented of endohedral muonium (Mu = p~+e-) in a functionalized fulleroid, the 6, 5 i... more A study is presented of endohedral muonium (Mu = p~+e-) in a functionalized fulleroid, the 6, 5 isomer of C61H 2. The temperature independence of the muonium depolarization rate implies that in the timescale of the p~SR experiment no isotropic reorientation regime sets in even at 280 K. The depolarization appears dominated by the interaction of Mu with methylene protons. No anisotropy of the hyperfine interaction was directly detected within experimental accuracy; an upper limit for the anisotropy can be fixed to 600 kHz by a transverse field measurement. 0009-2614/95/$09.50
We detected two spontaneous precession frequencies in antiferromagnetic pure LaMnO 3 , the end me... more We detected two spontaneous precession frequencies in antiferromagnetic pure LaMnO 3 , the end member of different families of charge-doped manganites. The muon site and local field orientation are identified for one frequency, which provides a measure of the staggered magnetization. We present additional zero and longitudinal field muon relaxation data on an LaMnO,,45 sample in which a new regime appears above an intermediate temperature T`, where regions of magnetic order coexist with domains of fluctuating moments. These results are discussed in terms of nucleation of the ordered phase in a superparamagnetic matrix.
We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear q... more We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear quadrupole resonance in orbitally ordered KCuF3. Our data show the presence of two inequivalent crystallographic sites with distinct orientations of the copper wave function, compatibly with the proposed orbital order for this compound. An unusual behaviour of the paramagnetic NQR signal at the Néel transition
We present139La NMR experiments on five powder samples of lanthanummanganites, with a Mn4+concent... more We present139La NMR experiments on five powder samples of lanthanummanganites, with a Mn4+concentration ranging from the antiferromagneticinsulator(AF) to the ferromagnetic-conducting (FM) region of the phasediagram. We measure a positive hyperfine couplingC=0.13T/|ìB . A signalfrom nuclei in a FM environment is present at all compositions, as evidencedby a hyperfine frequency in zero field experiments, by a positive hyperfineshift in NMR experiments
We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide the evidence that t... more We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to the normal metallic properties. We argue that this hole tagging calls for a subtler description of the correlated bands than the usual one. We also map
... Competition between Magnetic Order and Superconductivity in CeCu2 qsi2 ... TIME (micr osec) F... more ... Competition between Magnetic Order and Superconductivity in CeCu2 qsi2 ... TIME (micr osec) FIG. ... In the absence of a micro-scopic model fit function which adequately fits the data, we have used a generalized power law exp [—(At) ]which corresponds to an exponential with u ...
We present a study performed on several LaMnO >V compositions. Detailed analysis around x"0.09 de... more We present a study performed on several LaMnO >V compositions. Detailed analysis around x"0.09 detects phase separation between a higher transition, ¹ , and a lower one ¹ , , with coexisting #uctuating and magnetically ordered clusters (the Ne`el point, ¹ , , being the onset of an inhomogeneous non-collinear antiferromagnetic order detected by neutrons). The ordered clusters give rise to an extremely fast, static Kubo}Toyabe muon depolarization, while mixed static and dynamic local "elds are detected inside the #uctuating clusters. Below ¹ , precession frequency lines are resolved, despite the huge broadening introduced by localized Mn> ordered moments. A phase diagram is obtained and brie#y discussed in the context of electronic phase separation.
We present new muon spectroscopy data on a Fe3O4 single crystal, revealing different spin precess... more We present new muon spectroscopy data on a Fe3O4 single crystal, revealing different spin precession patterns in five distinct temperature ranges. A careful analysis of the local field and its straightforward modeling obtains surprisingly good agreement with experiments only if a very specific model of localized charges violating Anderson condition, and a correlated muon local dynamics are implemented. Muon evidence for fluctuations just above the Verwey temperature, precursor of the low temperature charge localized state, is provided.
Journal of the Chemical Society-Faraday Transactions I, 1985
... BY ALYSON HILL? ... Likewise, the assignment CH,C(OH)CH,Mu can also be rejected, both because... more ... BY ALYSON HILL? ... Likewise, the assignment CH,C(OH)CH,Mu can also be rejected, both because the enol form of acetone exists only in minute abundance and because its hyperfine constant should resemble that of CH,MucHOCOCH, (the muonium adduct of vinyl acetate ...
... Luigi Cristofolini and Roberto DeRenzi ... next to the attacked position; D, then lies approx... more ... Luigi Cristofolini and Roberto DeRenzi ... next to the attacked position; D, then lies approx-imately in the plane perpendicular to the benzene ring and containing the C-Mu bond." Let us call $J the angle between the long axis of C ~ O , 2, and 2: for the radicals B, C, and E, 4 z ...
Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature depende... more Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering
ABSTRACT The compound (NH3)xNaRb2C60 has been studied by SQUID magnetometry, and NMR. A double ir... more ABSTRACT The compound (NH3)xNaRb2C60 has been studied by SQUID magnetometry, and NMR. A double irradiation NMR experiment evidences a close distance between Na ions and ammonia, confirming the octahedral occupancy of this cluster which is usually observed in the ammonia intercalated fullerides. The Knight shift interaction of the ammonia protons with the conduction electrons is of pure anisotropic nature and is detected as a field dependent broadening of the NMR lines at low temperature. The recovery of the proton magnetisation follows a single exponential at all the investigated temperatures and fields. A Korringa behaviour is observed below 65 K and a clear Hebel–Slichter coherence peak is detected just below Tc. The Korringa relation does not hold and the Knight shift enhancement suggests the presence of a Coulomb repulsion.
This work aims at understanding the large reduction of superconducting critical temperature Tc ob... more This work aims at understanding the large reduction of superconducting critical temperature Tc observed in YSr2Cu306+x as compared to its YBa2Cu3O6+x counterpart (DeltaTc = -30 K). We report on a combined study of structural and electronic properties of RESr2Cu3O6+x (RE = Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa2Cu3O6+x RESr2Cu3O6+x is locally
The discovery of high-Tc superconductivity in F-doped RFeAsO (1111) was one of the most significa... more The discovery of high-Tc superconductivity in F-doped RFeAsO (1111) was one of the most significant breakthroughs in the field of condensed matter physics during the last decades. 1, 2 A striking worldwide interest has been indeed recently devoted to the synthesis of this class of materials, looking for suitable chemical substitutions of rare-earth (R) ions, transition metals (TM), and pnictogen elements (Pn) allowing one to enhance the critical temperature Tc, the highest value Tc≃ 55 K being currently reached in SmFeAsO1− xFx.
We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The be... more We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The behavior of the hyperfine constants and of the μSR linewidths have been investigated as a function of temperature; they have been correlated with the reorientational dynamics of the molecules, which have already been partially determined by means of Raman, NMR, and neutron scattering data. Our measurements
ABSTRACT Muons implanted in MnF2 and CoF2 are localized at low temperatures in an interstitial si... more ABSTRACT Muons implanted in MnF2 and CoF2 are localized at low temperatures in an interstitial site which exhibits octahedral symmetry. Measurements of the critical damping close toT N and the hyperfine field in the AF phase are reported.
Muonium adduct radicals have been studied in Norbornene and Bicyclooctene as a function of temper... more Muonium adduct radicals have been studied in Norbornene and Bicyclooctene as a function of temperature from the liquid phase down to the plastic phase and through the low temperature transition to a typical solid (brittle phase). The features of the radical hyperfine spectra are discussed. The temperature coefficient for the hyperfine constants is 0.1 MHz/K for the NBE exo isomer,
... References [1] R. De Renzi, G. GuidL P. Carretta. G. Calestani and SFJ Cox, Phys. Lett. A 135... more ... References [1] R. De Renzi, G. GuidL P. Carretta. G. Calestani and SFJ Cox, Phys. Lett. A 135 (1989) 132 [2] BX Yang, RF Kiefl, JH Brewer, JF Carolan, WN Hardy, R. Kadono, JR Kempton, SR Kreitzman, GM Luke, TM Riseman, D.LI. ...
A study is presented of endohedral muonium (Mu = p~+e-) in a functionalized fulleroid, the 6, 5 i... more A study is presented of endohedral muonium (Mu = p~+e-) in a functionalized fulleroid, the 6, 5 isomer of C61H 2. The temperature independence of the muonium depolarization rate implies that in the timescale of the p~SR experiment no isotropic reorientation regime sets in even at 280 K. The depolarization appears dominated by the interaction of Mu with methylene protons. No anisotropy of the hyperfine interaction was directly detected within experimental accuracy; an upper limit for the anisotropy can be fixed to 600 kHz by a transverse field measurement. 0009-2614/95/$09.50
We detected two spontaneous precession frequencies in antiferromagnetic pure LaMnO 3 , the end me... more We detected two spontaneous precession frequencies in antiferromagnetic pure LaMnO 3 , the end member of different families of charge-doped manganites. The muon site and local field orientation are identified for one frequency, which provides a measure of the staggered magnetization. We present additional zero and longitudinal field muon relaxation data on an LaMnO,,45 sample in which a new regime appears above an intermediate temperature T`, where regions of magnetic order coexist with domains of fluctuating moments. These results are discussed in terms of nucleation of the ordered phase in a superparamagnetic matrix.
We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear q... more We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear quadrupole resonance in orbitally ordered KCuF3. Our data show the presence of two inequivalent crystallographic sites with distinct orientations of the copper wave function, compatibly with the proposed orbital order for this compound. An unusual behaviour of the paramagnetic NQR signal at the Néel transition
We present139La NMR experiments on five powder samples of lanthanummanganites, with a Mn4+concent... more We present139La NMR experiments on five powder samples of lanthanummanganites, with a Mn4+concentration ranging from the antiferromagneticinsulator(AF) to the ferromagnetic-conducting (FM) region of the phasediagram. We measure a positive hyperfine couplingC=0.13T/|ìB . A signalfrom nuclei in a FM environment is present at all compositions, as evidencedby a hyperfine frequency in zero field experiments, by a positive hyperfineshift in NMR experiments
We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide the evidence that t... more We present results on heavily underdoped Y_1-xCa_xBa_2Cu_3O_6+y which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to the normal metallic properties. We argue that this hole tagging calls for a subtler description of the correlated bands than the usual one. We also map
... Competition between Magnetic Order and Superconductivity in CeCu2 qsi2 ... TIME (micr osec) F... more ... Competition between Magnetic Order and Superconductivity in CeCu2 qsi2 ... TIME (micr osec) FIG. ... In the absence of a micro-scopic model fit function which adequately fits the data, we have used a generalized power law exp [—(At) ]which corresponds to an exponential with u ...
We present a study performed on several LaMnO >V compositions. Detailed analysis around x"0.09 de... more We present a study performed on several LaMnO >V compositions. Detailed analysis around x"0.09 detects phase separation between a higher transition, ¹ , and a lower one ¹ , , with coexisting #uctuating and magnetically ordered clusters (the Ne`el point, ¹ , , being the onset of an inhomogeneous non-collinear antiferromagnetic order detected by neutrons). The ordered clusters give rise to an extremely fast, static Kubo}Toyabe muon depolarization, while mixed static and dynamic local "elds are detected inside the #uctuating clusters. Below ¹ , precession frequency lines are resolved, despite the huge broadening introduced by localized Mn> ordered moments. A phase diagram is obtained and brie#y discussed in the context of electronic phase separation.
We present new muon spectroscopy data on a Fe3O4 single crystal, revealing different spin precess... more We present new muon spectroscopy data on a Fe3O4 single crystal, revealing different spin precession patterns in five distinct temperature ranges. A careful analysis of the local field and its straightforward modeling obtains surprisingly good agreement with experiments only if a very specific model of localized charges violating Anderson condition, and a correlated muon local dynamics are implemented. Muon evidence for fluctuations just above the Verwey temperature, precursor of the low temperature charge localized state, is provided.
Journal of the Chemical Society-Faraday Transactions I, 1985
... BY ALYSON HILL? ... Likewise, the assignment CH,C(OH)CH,Mu can also be rejected, both because... more ... BY ALYSON HILL? ... Likewise, the assignment CH,C(OH)CH,Mu can also be rejected, both because the enol form of acetone exists only in minute abundance and because its hyperfine constant should resemble that of CH,MucHOCOCH, (the muonium adduct of vinyl acetate ...
... Luigi Cristofolini and Roberto DeRenzi ... next to the attacked position; D, then lies approx... more ... Luigi Cristofolini and Roberto DeRenzi ... next to the attacked position; D, then lies approx-imately in the plane perpendicular to the benzene ring and containing the C-Mu bond." Let us call $J the angle between the long axis of C ~ O , 2, and 2: for the radicals B, C, and E, 4 z ...
Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature depende... more Zero-field copper NMR measurements on YBa2Cu3O6 up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering
Uploads
Papers by Roberto Renzi