Effect of inert gas on pulsed laser deposition of rare earth doped fluoride thin films

The realization of crystalline sub micrometric films of Nd³⁺:LiYF₄ fluorides on pure LiYF₄ substrates by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration. Both low and high laser ablation fluency regimes were checked. The fundamental contribution of He inside a vacuum chamber to obtain crystalline Nd³⁺:LiYF₄ films was demonstrated. The optical characteristics of the films were analyzed via recording the emission spectra following IR excitation, polarized both with E || and E ⊥ to the substrate c-axis. Lifetime measurements of the fundamental Nd³⁺ ion transition in the films were also performed. All the results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal under the same experimental conditions. The scanning electron microscope pictures of the depositions gave indications about the film surface morphology.     

Observation of Non-Classical Rotational Inertia in Solid 4He Confined in Porous Gold

No Heading Recent torsional oscillator measurements on solid ⁴He confined in Vycor glass at 62 bars show supersolid response, an abrupt drop in rotational moment of inertia, at 175 mK¹. We have investigated the pore-size dependence of the supersolid behavior by confining solid ⁴He in a different porous host, porous gold, of a considerably larger pore diameter. When solid ⁴He in porous gold is cooled below 0.2 K a sharp drop in the resonant period is found. The supersolid response exhibits a strong dependence on the amplitude of oscillation.PACS numbers: 67.80–s. 67.80 Mg., 0.5.70.Fh, 05.30.Jp     

Study of Solid 4He in Two Dimensions

Defects are believed to play a fundamental role in the supersolid state of ⁴He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at zero temperature of the properties of solid ⁴He in presence of many vacancies, up to 30 in two dimensions (2D). In all studied cases the crystalline order is stable at least as long as the concentration of vacancies is below?2.5?%. In the 2D system for a small number, n _v , of vacancies such defects can be identified in the crystalline lattice and are strongly correlated with an attractive interaction. On the contrary when n _v ?10 vacancies in the relaxed system disappear and in their place one finds dislocations and a revival of the Bose-Einstein condensation. Thus, should zero-point motion defects be present in solid ⁴He, such defects would be dislocations and not vacancies, at least in 2D. In order to avoid using periodic boundary conditions we have studied the exact ground state of solid ⁴He confined in a circular region by an external potential. We find that defects tend to be localized in an interfacial region of width of about 15 ?. Our computation allows to put as upper bound limit to zero-point defects the concentration 3×10^?3 in the 2D system close to melting density.     

In Pursuit of the Elusive Supersolid

The excitement following the initial report of supersolid behavior for ⁴He embedded in porous Vycor glass has been tempered by the realization that many of the early supersolid observations were contaminated by effects arising from an anomaly in the elastic properties of solid ⁴He. In an attempt to separate dynamic elastic effects from a true supersolid signal, we employed a torsional oscillator with two eigen-frequencies to study the ⁴He-Vycor system. We found that frequency-dependent elastic signals can entirely account for the observed period shift signals. Although, we conclude that supersolid does not exist for the ⁴He-Vycor case, the question of its presence in bulk samples remains open. In our current experiments we apply the two-frequency test to bulk samples of solid ⁴He. Again we find a frequency-dependent contribution arising from elastic effects; however, in some cases we also find a small frequency-independent contribution, which may indicate the existence of a remnant supersolid phase. Given the history of this subject such results must be treated with caution.     

NMR in pure3He films on a Nuclepore substrate

The properties of³He films on a Nuclepore substrate have been measured by pulsed NMR at a Larmor frequency of 10 MHz between 1.3 and 4.2 K. The³He film thickness was varied from 0.14 to 2 layers. The spin-spin relaxation timeT ₂ agrees well with previous measurements of³He films on Mylar and Vycor glass at low temperatures. The spin-lattice relaxation timeT ₁ for submonolayer films shows a strong temperature dependence consistent with a thermally activated process. This behavior has not previously been observed on amorphous substrates. The spin diffusion coefficient was measured for the thickest films at 4.2 and 2.6 K and found to be consistent with free atom motion of the³He in the vapor. In thin films or at low temperatures, the diffusion was too small to be observed. The magnetic coupling between the³He nuclei in a film and the protons in the Nuclepore substrate was determined from the effect of the³He on the proton-lattice relaxation time. It is about 100 times weaker than the interaction between³He and the fluorine nuclei in a Teflon substrate.     

Preparation of Li and Er codoped ZnO thin films and their photoluminescence

Li-Er codoped ZnO thin films have been prepared on Si(100) substrates by pulsed laser deposition (PLD). Both the as-grown and post-annealed films exhibit good crystalline quality with preferred c-axis orientation. After post-annealing at 850 °C, the photoluminescence (PL) related to intra-4f shell of Er³⁺ can be clearly observed. The Li-Er codoped ZnO film shows higher intensity of PL around 1.54 μm than the Er monodoped ZnO film. The behavior is attributed to the lowering of the symmetry of the crystal field around Er³⁺ ions by introducing Li⁺ into ZnO lattice, which is also confirmed by Raman scattering spectra.     

Effect of Impurities on Supersolid Condensate: A Ginzburg–Landau Approach

Basing our arguments on a wavefunction that contains both positional and superfluid order, we propose a Ginzburg–Landau functional for a supersolid with the two order parameters necessary to describe such a phase: density n _B (r) and supersolid order parameter ψ _V . We argue that adding lighter. ³He atoms to a ⁴He supersolid produces attractive regions for vacancies, leading to patches of higher T _c. On the other hand, the supersolid stiffness decreases in this granular state with increased ³He disorder. Both effects are linear in ³He concentration.     

Quantum lattice gas and the existence of a supersolid

We discuss, within mean field theory, the possible phase diagrams of a quantum lattice gas model (as considered by Matsuda and Tsuneto atT=0) with particular reference to the existence of a supersolid phase, displaying both long-range diagonal (or crystalline) order and long-range off-diagonal order (as characteristic of a superfluid). For parameter ranges which provide a reasonable representation of the known phase diagram of ⁴ He, we show that a supersolid phase may appear between the usual solid and superfluid phases. Such a phase might not, however, extend down to absolute zero.     

An evaluation of metal and semiconductor films formed by ionized-cluster beam deposition

The mechanism of film formation and the properties of films deposited by the ionized-cluster beam technique were investigated. In this technique, strong adhesion of the film to the substrate and good crystalline deposition are expected.A high adhesive strength of over 100 kg cm^-2 for Cu films on glass substrates was obtained in the experiments. The migration of adatoms consisting of ionized and neutral clusters was observed on the substrte surface. This effect, called the migration effect, can be considered to characterize film formation by cluster beam deposition and to produce good crystalline films. Si single crystals were obtained on Si substrates. A p-n junction photodiode was fabricated by the deposition of n-type Si on a p-type substrate. The diode shows improved spectral sensitivity in the UV region compared with that of commercially available solar cells.     

Crystallinity and surface roughness dependent photoluminescence of Y1 − xGdxVO4:Eu thin films grown on Si (100) substrate

Gd-substitution dependency on the photoluminescence in YVO₄:Eu³⁺ films grown on Si (100) substrates have been investigated by analyzing the crystalline phase and surface morphology of the films. The substitution of Gd induced not only the change of crystallinity but also the surface roughness of the films. The change of the preferred orientation in the films can be explained on the basis of the lattice mismatch between the film and Si (100) substrate. Also, the surface roughness of the films shows the similar behavior to the grain size as a function of Gd amounts. The photoluminescence (PL) intensity obtained from the Y_1 − xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL intensity is more dependent on the surface roughness than the crystallinity of films. In particular, the incorporation of Gd into the YVO₄ lattice remarkably enhanced the intensity of PL and the highest emission intensity of Y_0.57Gd_0.40Eu_0.03VO₄ film was 3.3 times higher than that of YVO₄:Eu³⁺ film.     

The Phase of Submonolayer 4He Films Near Monolayer Completion

We study the state of ⁴He films physisorbed to general Lennard–Jones type substrates at coverages near monolayer completion as a function of the range C₃ and well-depth D of the substrate-helium potential. By examining the liquid-state energetics as well as the coverage dependence of the third sound speed and roton energies in the two-parameter space (C₃,D), we can estimate the position of the boundary between those strongly interacting substrates wherein the ⁴He film essentially forms a quasi two-dimensional solid prior to second layer formation and weaker interacting substrates for which the ⁴He film remains mobile and superfluid. Our approach utilizes a combination of information from both variational calculations and also correlated basis function theory to examine in detail the excitation structure in the monolayer liquid as a function of film coverage and substrate potential.      

Effect of inert gas on pulsed laser deposition of rare earth doped fluoride thin films

The realization of crystalline sub micrometric films of Nd³⁺:LiYF₄ fluorides on pure LiYF₄ substrates by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration. Both low and high laser ablation fluency regimes were checked. The fundamental contribution of He inside a vacuum chamber to obtain crystalline Nd³⁺:LiYF₄ films was demonstrated. The optical characteristics of the films were analyzed via recording the emission spectra following IR excitation, polarized both with E || and E to the substrate c-axis. Lifetime measurements of the fundamental Nd³⁺ ion transition in the films were also performed. All the results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal under the same experimental conditions. The scanning electron microscope pictures of the depositions gave indications about the film surface morphology.     

Metal-insulator transition in epitaxial films of LaMnO3 manganites grown by magnetron sputtering

We have studied thin films of LaMnO₃ manganite grown by RF magnetron sputtering at high pressure on crystalline substrates with cubic symmetry. It is established that these films exhibit a metal-insulator transition, whereas LaMnO₃ grown on orthorhombic substrates remains in a dielectric state. The parameters of the metal-insulator transition have been studied as dependent on the level and symmetry of mechanical stresses that arise during the epitaxial growth of LaMnO₃ films on various substrates. The resistance of LaMnO₃ films grown on SrTiO₃ substrates has been studied as a function of the film thickness. It is found that the presence of excess oxygen due to substitution in the cation system can significantly influence the Mn⁴⁺/Mn³⁺ ion ratio in the film and thus lead to the appearance of the metal-insulator transition.     

Nuclear magnetic susceptibility of3He adsorbed on graphite below 4.2 K

The nuclear magnetic susceptibility of³He films adsorbed on graphite substrates was measured by means of pulsed NMR techniques at 10 and 20 MHz. Submonolayer, monolayer, and multilayer coverages were examined, as well as one monolayer of³He mixed with various amounts of⁴He. The temperature range of measurements extended from 0.35 to 4.2 K. For pure³He and for³He mixed with⁴He the nuclear magnetic susceptibility displays departures from Curie's law. The observed behavior in pure³He layers can be fitted to an ideal-Fermi-gas curve with an effective degeneracy temperature of 0.15 K. The nuclear magnetic susceptibility of³He–⁴He films is also found to fall below the values predicted by Curie's law as the temperature is lowered, but the results suggest that in this case the film is not uniform, possibly as a result of a phase separation in the film at low temperatures.Work supported in part by the National Science Foundation and a Navy Equipment Loan contract.     

Nonlinear optical properties of thin iron films grown on MgO (100) by pulsed laser deposition

Thin films of iron were fabricated on MgO (100) substrates using pulsed laser deposition technique. The crystalline property and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. It was found that the morphology of the Fe film deposited at room temperature was characteristic of continuous metal film. Increasing the deposition temperature caused the smooth Fe film to break up, and nanoscale Fe islands were formed. We performed the z-scan measurements to study the third-order optical nonlinearity of the continuous approximately 9-nm-thick iron film. The results show that the ultrathin iron film exhibits large nonlinear refractive coefficient, n₂=7.09×10⁻⁵ cm²/kW, and nonlinear absorption coefficient, β=−5.52×10⁻³ (cm/W), at the wavelength of 532 nm.     

Structure of thin Mo-Ni films

Mixed films (6 μm thick) of the binary transition metal alloy system Mo_xNi_100−x (4 < x < 90) were deposited by r.f. sputtering at room temperature onto glass substrates. The composition and homogeneity of the films were tested. The structure of the thin films was determined by X-ray techniques. Depending on the film composition, a crystalline phase, an amorphous phase or a mixed phase of amorphous and crystalline structure is observed.     

Formation of β-FeSi2 thin films on non-silicon substrates

β-FeSi₂ films were prepared on non-silicon substrates by sputtering. The crystalline growth, stress induced cracks and adhesive ability to the substrate were investigated on substrate temperature and thermal expansion coefficient of substrate materials. It was found that crack formation in β-FeSi₂ films was dependent on the thermal expansion coefficients of CaF₂, MgO and quartz glass insulating materials. High-density cracks were observed from β-FeSi₂ films on CaF₂ and quartz glass substrates with large difference of the thermal expansion coefficient between β-FeSi₂ film and substrate materials, and it was crack-free on MgO substrate with a thermal expansion coefficient close to that of β-FeSi₂ films. Polycrystalline β-FeSi₂ films grew on Mo, Ta, W, Fe and stainless steel (SS) substrates at low substrate temperature around 400 °C. There was no α-FeSi₂ phase confirmed in the films. All the films had continuous structures without noticeable cracks even though they have different thermal expansion coefficients. Capacity-voltage measurements showed that β-FeSi₂ films formed on SS substrates has n-type conductivity, with residual carrier concentrations of about 1.3∼6.4 × 10¹⁸ cm^− 3. Auger electron spectroscopy depth profile measurements identified homogeneous distribution of Fe and Si atoms in the film region, but with a large interface region between the film and the substrate.     

3He Effect on 2D Superfluidity in 3He–4He Mixture Films on Planar Gold

There have been a number of experiments exploring the nature of 2D superfluidity and the configuration of ³He–⁴He mixture films on various substrates. To date, a possible film-structure at T=0 is that of a simple layer model, ³He/superfluid ⁴He/solid-like ⁴He/substrate, in which the submonolayer superfluidity is strongly affected by the coverage of the ³He overlayer. Yet the mechanism is not been fully understood. In this paper, we report a QCM study at 60 MHz for the ³He effect on the superfluidity of mixture films on flat gold, mainly focusing on the anomalous depletion of the temperature dependence of the superfluid density σ _s. In the measurements, we kept the ³He coverage constant (n ₃= 0, 3.6, 7.2, 19.0, 57.2, or 92.8 μmol/m²) and then incrementally added ⁴He. We observed the evolution of the ³He effect on σ _s(T) with increasing ³He coverage; this depletion of σ _s(T) rapidly increases and then saturates near n ₃～1 layer. From the analysis of the linear-temperature region in the plot of the dissipation peak temperature T _p as a function of the superfluid ⁴He coverage n _4s and comparison with previous studies on Mylar and porous gold, we found a universal function for the strength of the ³He effect for all substrates.     

Preliminary Measurement of the Critical Casimir Effect near the Tricritical Point in 3He–4He Mixture Films

We report a preliminary measurement of the critical Casimir effect in ³ He- ⁴ He mixture films near the tricritical point. Whereas we had found that the pure ⁴ He film adsorbed on Cu substrates thins due to the critical Casimir force near the lambda transition, in our present experiments we find that the adsorbed mixture film thickens near the tricritical point. This difference in behavior most likely reflects the difference in the universality class of these two phase transitions and the difference in boundary conditions that the order parameter must satisfy at liquid-substrate and liquid-vapor interfaces.     

Anomalous Response of 4He Confined in Nanoporous Media to Torsional Oscillation

The existence of “Non-Classical Rotational Inertia (NCRI)” in solid ⁴He below 0.2?K has been controversial and interpreted by a number of different theories. We report on torsional oscillator measurements for ⁴He in a nanoporous Gelsil glass, which has a network of nanopores with 3.5?nm in diameter. In addition to the usual “low-T NCRI” with an onset temperature 0.15?K, we find a larger decrease in rotational moment of inertia in a broad range of temperature from 0.2 to 1.9?K. This “high-T inertial anomaly” is accompanied with multiple dissipation peaks, but has no dependence on torsional oscillation velocity unlike the low-T NCRI. Since the high-T anomaly is observed also in confined liquid states, it originates in amorphous solid ⁴He layer near the pore wall. Our result shows that different types of supersolid—like phenomena, i.e. inertial anomalies, can coexist in a single ⁴He sample, even with genuine superfluidity of liquid ⁴He.