arXiv subjects

E. Morenzoni

Publications and source records attributed to E. Morenzoni.

66 records · Page 4Linked to original sources

arXiv2009

Proximal magnetometry of monolayers of single molecule magnets on gold using polarized muons

The magnetic properties of a monolayer of Fe4 single molecule magnets grafted onto a Au (111) thin film have been investigated using low energy muon spin rotation. The properties of the monolayer are compared to bulk Fe4. We find that the magnetic properties in the monolayer are consistent with those measured in the bulk, strongly indicating that the single molecule magnet nature of Fe4 is preserved in a monolayer. However, differences in the temperature dependencies point to a small difference in their energy scale. We attribute this to a ~60% increase in the intramolecular magnetic interactions in the monolayer.

cond-mat.mes-hall↗

arXiv2008

The microscopic study of a single hydrogen-like impurity in semi-insulating GaAs

The charge dynamics of hydrogen-like centers formed by the implantation of energetic (4 MeV) muons in semi-insulating GaAs have been studied by muon spin resonance in electric fields. The results point to the significant role of deep hole traps in the compensation mechanism of GaAs. Electric-field-enhanced neutralization of deep electron and hole traps by muon-track-induced hot carriers results to an increase of the non-equilibrium carrier life-times. As a consequence, the muonium ($μ^+ + e^-$) center at the tetrahedral As site can capture the track's holes and therefore behaves like a donor.

cond-mat.mtrl-sci↗

arXiv2008

Depth dependent spin dynamics of canonical spin glass films: A low-energy muon spin rotation study

We have performed depth dependent muon spin rotation/relaxation studies of the dynamics of single layer films of {\it Au}Fe and {\it Cu}Mn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5-70 nm). A significant reduction in the muon spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5-10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to e.g. 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.

cond-mat.dis-nn↗

arXiv2007

Oxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-δ

Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-δas a function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with increasing x up to x\approx0.55, where superconductivity disappears. For decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where antiferromagnetic order and down to x\approx0.3 where spin-glass behavior vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.

cond-mat.supr-con↗

arXiv2007

Formation of hydrogen impurity states in silicon and insulators at low implantation energies

The formation of hydrogen-like muonium (Mu) has been studied as a function of implantation energy in intrinsic Si, thin films of condensed van der Waals gases (N2, Ne, Ar, Xe), fused and crystalline quartz and sapphire. By varying the initial energy of positive muons (mu+) between 1 and 30 keV the number of electron-hole pairs generated in the ionization track of the mu+ can be tuned between a few and several thousand. The results show the strong suppression of the formation of those Mu states that depend on the availability of excess electrons. This indicates, that the role of H-impurity states in determining electric properties of semiconductors and insulators depends on the way how atomic H is introduced into the material.

cond-mat.other↗

arXiv2006

Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping

The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic penetration depth λ_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-δ} and La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-δ} was studied by means of muon-spin rotation. A substantial OIE on λ_{ab}(0) with an OIE exponent β_O=-d\lnλ_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the oxygen isotope), and a small OIE on the transition temperature T_c with an OIE exponent α_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The observation of a substantial isotope effect on λ_{ab}(0), even in cuprates where the OIE on T_c is small, indicates that lattice effects play an important role in cuprate HTS.

cond-mat.supr-con↗

arXiv2005

Observation of non-exponential magnetic penetration profiles in the Meissner state - A manifestation of non-local effects in superconductors

Implanting fully polarized low energy muons on the nanometer scale beneath the surface of a superconductor in the Meissner state enabled us to probe the evanescent magnetic field profile B(z)(0<z<=200nm measured from the surface). All the investigated samples [Nb: kappa \simeq 0.7(2), Pb: kappa \simeq 0.6(1), Ta: kappa \simeq 0.5(2)] show clear deviations from the simple exponential B(z) expected in the London limit, thus revealing the non-local response of these superconductors. From a quantitative analysis within the Pippard and BCS models the London penetration depth lambda_L is extracted. In the case of Pb also the clean limit coherence length xi0 is obtained. Furthermore we find that the temperature dependence of the magnetic penetration depth follows closely the two-fluid expectation 1/lambda^2 \propto 1-(T/T_c)^4. While B(z) for Nb and Pb are rather well described within the Pippard and BCS models, for Ta this is only true to a lesser degree. We attribute this discrepancy to the fact that the superfluid density is decreased by approaching the surface on a length scale xi0. This effect, which is not taken self-consistently into account in the mentioned models, should be more pronounced in the lowest kappa regime consistently with our findings.

cond-mat.supr-con↗

arXiv2004

Oxygen-isotope effect on the in-plane penetration depth in cuprate superconductors

Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in cuprate high-temperature superconductors (HTS) are presented. First, the doping dependence of the OIE on the transition temperature T_c in various HTS is briefly discussed. It is observed that different cuprate families show a similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR, and magnetization studies of the total and site-selective OIE on lambda_{ab} are described in some detail. A substantial OIE on lambda_{ab} was observed in various cuprate families at all doping levels, suggesting that cuprate HTS are non-adiabatic superconductors. The experiments clearly demonstrate that the total OIE on T_c and lambda_{ab} arise from the oxygen sites within the superconducting CuO_2 planes, demonstrating that the phonon modes involving the movement of planar oxygen are dominantly coupled to the supercarriers. Finally, it is shown that the OIE on T_c and lambda_{ab} exhibit a relation that appears to be generic for different families of cuprate HTS. The observation of these unusual isotope effects implies that lattice effects play an essential role in cuprate HTS and have to be considered in any realistic model of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2003

Direct observation of non-local effects in a superconductor

We have used the technique of low energy muon spin rotation to measure the local magnetic field profile B(z) beneath the surface of a lead film maintained in the Meissner state (z depth from the surface, z <= 200 nm). The data unambiguously show that B(z) clearly deviates from an exponential law and represent the first direct, model independent proof for a non-local response in a superconductor.

cond-mat.supr-con↗

arXiv2003

Site-selective oxygen isotope effect on the magnetic field penetration depth in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-δ}$

We report site-selective oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) measurements on the in-plane penetration depth $λ_{ab} $ in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-δ}$, using the muon-spin rotation ($μ$SR) technique. A pronounced OIE on the transition temperature $T_c$ as well as on $λ_{ab}^{-2}(0)$ was observed, which mainly arises from the oxygen sites within the superconducting CuO$_2$ planes (100 % within error bar). The values of the corresponding relative isotope shifts were found to be $T_c/T_c= -3.7(4)%$ and $Δλ^{-2}_{ab}(0)/λ^{-2}_{ab}(0)= -6.2(1.0)%$. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

cond-mat.supr-con↗

arXiv2003

Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBa$_2$Cu$_3$O$_{7-δ}$

We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $λ_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ film using the novel low-energy muon-spin rotation technique. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $λ_{ab}$. A substantial isotope shift $Δλ_{ab}/λ_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^\ast$ is oxygen-isotope dependent with $Δm_{ab}^\ast/m_{ab}^\ast = 5.5(1.4)%$.

cond-mat.supr-con↗

arXiv2002

Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation

The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth $λ_{ab} (0)$ in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on $λ_{ab}^{-2}(0)$ was observed with a relative isotope shift of $Δλ^{-2}_{ab}/λ^{-2}_{ab}$=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{ab}^{\ast}$. The OIE exponents of T_{c} and of $λ_{ab}^{-2}(0)$ exhibit a relation that appears to be generic for cuprate superconductors.

cond-mat.supr-con↗