arXiv subjects

H. Saadaoui

Publications and source records attributed to H. Saadaoui.

18 recordsLinked to original sources

arXiv2016

Bulk superconductivity at 84 K in the strongly overdoped regime of cuprates

By means of magnetization, specific heat and muon-spin relaxation measurements, we investigate high-pressure oxidized \mohpo, in which overdoping is achieved up to $p \sim 0.45$ hole/Cu, well beyond the $T_c - p$ superconducting dome of cuprates, where Fermi liquid behavior is expected. Surprisingly, we find bulk superconductivity with $T_c$=84 K and superfluid density similar to those of optimally doped Y123. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La214. These results point at an unusual high-$T_c$ phase with a large fraction of unpaired holes. Further experiments may assess the Fermi liquid properties of the present phase, which would put into question the paradigm that the high $T_c$ of cuprates originates from a non-Fermi liquid ground state.

cond-mat.supr-con↗

arXiv2016

Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO$_2$

Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO$_2$ is an intrinsic diluted magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2014

The Phase Diagram in Electron-Doped LCCO

Superconductors are a striking example of a quantum phenomenon in which electrons move coherently over macroscopic distances without scattering. The high-temperature superconducting oxides(cuprates) are the most studied class of superconductors, composed of two-dimensional CuO2 planes separated by other layers which control the electron concentration in the planes. A key unresolved issue in cuprates is the relationship between superconductivity and magnetism. In this paper, we report a sharp phase boundary of static three-dimensional magnetic order in the electron-doped superconductor La2-xCexCuO4-d where small changes in doping or depth from the surface switch the material from superconducting to magnetic. Using low-energy spin polarized muons, we find static magnetism disappears close to where superconductivity begins and well below the doping where dramatic changes in the transport properties are reported. These results indicate a higher degree of symmetry between the electron and hole-doped cuprates than previously thought.

cond-mat.supr-con↗

arXiv2014

Mott transition in granular aluminum

The presence of free spins in granular Al films is directly demonstrated by $μ$SR measurements. A Mott transition is observed by probing the increase of the spin-flip scattering rate of conduction electrons as the nano-size metallic grains are being progressively decoupled. Analysis of the magneto-resistance in terms of an effective Fermi energy shows that it becomes of the order of the grains electrostatic charging energy at a room temperature resistivity $ρ\approx 50,000~μΩ~cm$, at which a metal to insulator transition is known to exist. As this transition is approached the magneto-resistance exhibits a Heavy-Fermion like behavior, consistent with an increased electron effective mass.

cond-mat.str-el↗

arXiv2013

Absence of broken time reversal symmetry beneath the surface of YBCO films

We report the results of a search for spontaneous magnetism due to a time reversal symmetry breaking phase in the superconducting state of (110)-oriented YBCO films, expected near the surface in this geometry. Zero field and weak transverse field measurements performed using the low-energy muon spin rotation technique with muons implanted few nm inside optimally-doped YBCO-(110) films show no appearance of spontaneous magnetic fields below the superconducting temperature down to 2.9 K. Our results give an upper limit of ~0.02 mT for putative spontaneous internal fields.

cond-mat.supr-con↗

arXiv2013

μSR and NMR study of the superconducting Heusler compound YPd2Sn

We report on muon spin rotation/relaxation and $^{119}$Sn nuclear magnetic resonance (NMR) measurements to study the microscopic superconducting and magnetic properties of the Heusler compound with the highest superconducting transition temperature, \ypd\ ($T_c=5.4$ K). Measurements in the vortex state provide the temperature dependence of the effective magnetic penetration depth $λ(T)$ and the field dependence of the superconducting gap $Δ(0)$. The results are consistent with a very dirty s-wave BCS superconductor with a gap $Δ(0)=0.85(3)$ meV, $λ(0)= 212(1)$ nm, and a Ginzburg-Landau coherence length $ξ_{\mathrm{GL}}(0)\cong 23$ nm. In spite of its very dirty character, the effective density of condensed charge carriers is high compared to the normal state. The \mSR data in a broad range of applied fields are well reproduced by taking into account a field-related reduction of the effective superconducting gap. Zero-field \mSR measurements, sensitive to the possible presence of very small magnetic moments, do not show any indications of magnetism in this compound.

cond-mat.supr-con↗

arXiv2012

Nature of weak magnetism in SrTiO3/LaAlO3 multilayers

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.

cond-mat.mtrl-sci↗

arXiv2012

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

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+.

cond-mat.str-el↗

arXiv2011

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

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.

cond-mat.supr-con↗

arXiv2011

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

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.

cond-mat.mtrl-sci↗

arXiv2011

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

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.

cond-mat.supr-con↗

arXiv2010

Undulation instabilities in the meniscus of smectic membranes

Using optical microscopy, phase shifting interferometry and atomic force microscopy, we demonstrate the existence of undulated structures in the meniscus of ferroelectric smectic-C* films. The meniscus is characterized by a periodic undulation of the smectic-air interface, which manifests itself in a striped pattern. The instability disappears in the untilted smectic-A phase. The modulation amplitude and wavelength both depend on meniscus thickness. We study the temperature evolution of the structure and propose a simple model that accounts for the observed undulations.

cond-mat.soft↗

arXiv2010

Vortex lattice disorder in YBCO probed using Beta-NMR

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.

cond-mat.supr-con↗

arXiv2008

Local Magnetic Properties of a Monolayer of Mn12 Single Molecule Magnets

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.

cond-mat.mes-hall↗

arXiv2007

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

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.

cond-mat.str-el↗

arXiv2007

Beta-NMR of Isolated Lithium in Nearly Ferromagnetic Palladium

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.

cond-mat.mtrl-sci↗

arXiv2006

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

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.

cond-mat.stat-mech↗

arXiv2005

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

$β$-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.

cond-mat.str-el↗