Temperature and spectral dependences of persistent photoconductivity in YBa2Cu3Ox thin films

Temperature and spectral dependences of photoinduced changes of resistance were measured in YBa₂Cu₃O _x thin films with oxygen content ranging as 6.35 <x < 6.75. The absolute value of efficiency of initiation of photoinduced changes decreases with increase in oxygen content, but the position of peaks in the spectral dependence does not change with a change ofx. Temperature dependences of efficiency have an anomaly atT220 K, which is present in all the samples studied, and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependences of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in YBCO on both sides of the metal-insulator transition has a common origin.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Temperature and spectral dependences of persistent photoconductivity in YBa2Cu3O x thin films

Temperature and spectral dependences of photoinduced changes of resistance were measured in YBa₂Cu₃O _x thin films with oxygen content ranging as 6.35 <x < 6.75. The absolute value of efficiency of initiation of photoinduced changes decreases with increase in oxygen content, but the position of peaks in the spectral dependence does not change with a change ofx. Temperature dependences of efficiency have an anomaly atT∽220 K, which is present in all the samples studied, and correlates with anomalies observed by other experimental techniques. Qualitatively similar temperature and spectral dependences of efficiency for the samples in both the insulating and metallic phases may be considered as an indication that the persistent photoconductivity effect in YBCO on both sides of the metal-insulator transition has a common origin.     

Thickness Effect on Sheet Resistance in BSCCO Mixed Crystal Thin Film

In the Bi2212/Bi2201 mixed crystal thin films with same Bi2212 molar percentage of 70mol%, the superconducting transition takes place in the specimens thicker than about 44. SI transition is brought about by the domain size effect in this mixed crystal thin film. The film thickness also gives a remarkable influence on T _c in a dissipative state.     

Synthesis and electrical properties of cubic NaxWO3 thin films across the metal-insulator transition

Highly oriented (1 0 0) Na_xWO₃ thin films were fabricated in the composition range 0.1 ≤ x ≤ 0.46 by pulsed laser deposition technique. The films showed transition from metallic to insulating behaviour at a critical composition between x = 0.15 and 0.2. The pseudo-cubic symmetry of Na_xWO₃ thin films across the transition region is desirable for understanding the composition controlled metal-insulator transition in the absence of any structural phase transformation. The electrical transport properties exhibited by these films across the transition regime were investigated. While the resistivity varied as T² at low temperatures in the metallic regime, a variable range hopping conduction was observed for the insulating samples. For metallic compositions, a non-linear dependence of resistivity in temperature was also observed from 300 to 7 K, whose exponent varied with the composition of the film.     

Study of Josephson effects in microbridges of Bi-Sr-Ca-Cu-O films made by spray pyrolysis

Superconducting Bi-Sr-Ca-Cu-O (2212) films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. TheR-T curves of the films show metallic behaviour above the superconducting transition temperature.T _c (R=0) is observed between 80 and 85 K. Annealing temperature has a profound effect onT _c (R=0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films withc-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50 μm × 50 μm have been patterned photolithographically followed with chemical etching. The 1 V characteristics of the microbridges show Josephson effects due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

Strain effects and phase separation tendency in highly strained La0.7Ba0.3MnO3 thin films on LaAlO3 substrates

The magnetic and transport properties of epitaxial La_0.7Ba_0.3MnO₃ films on LaAlO₃ substrates have been investigated and compared with those on SrTiO₃ substrates. It is found that the ferromagnetic and metallic transition temperature of the highly compressive strained films on LaAlO₃ substrates decreases steadily with decreasing film thickness, while it changes little in the lightly strained films on SrTiO₃ substrates. The properties of the films on LaAlO₃ substrates are more sensitive to a post-annealing procedure. A tendency to phase separation, which induces a difference between the insulator-metal transition temperature T_MI and the Curie temperature T_C, is observed for the films with a medium oxygen annealing process. We argue that the phase separation is due to both the highly compressive strain and oxygen deficiency in the films.     

Growth parameter effect in superconducting YBa2Cu4O8 thin films by d.c. magnetron sputtering

High- T _c YBa₂Cu₄O₈ (124) thin films have been made by d.c. magnetron sputtering deposition on (100) MgO substrates. The effect of several processing variables, including the ratio of oxygen to argon, total pressure, and substrate temperature, on the superconducting properties of the thin films, were systematically investigated. The as-prepared films annealed in flowing oxygen at 800°C for 4 h under ambient pressure obtained nearly phase-pure 124 and exhibited superconducting onset transition at 75 K.     

Ultraviolet Triggered Switchable Mirrors

Trivalent rare earth hydrides stabilized in thin film form demonstrate spectacular optical and electronic properties. A thin film of YH_x or LaH_x can be transformed rapidly from metal to insulator, from shiny mirror to transparent window, simply by changing the surrounding hydrogen gas pressure or an electrolytic cell potential at room temperature (RT). At low temperatures, in‐situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination permits tuning through the T = 0 metal–insulator transition and reveals the important role played by strong electron correlations. We discuss the optical, electronic, magnetic, and structural properties of switchable mirrors from both the technological and scientific perspectives.     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Reactive sputtering of titanium and properties of titanium suboxide films for photochemical applications

The effect of oxygen partial pressure on the deposition rate, crystalline structure and optical absorption of thin film titanium suboxides prepared by the reactive sputtering of a titanium metal target is given. A wide variety of films ranging from metallic through semiconducting to dielectric specimens were deposited in a reproducible manner simply by controlling the oxygen content in the sputtering plasma atmosphere. In addition, polycrystalline stable semiconducting TiO₂ electrodes were deposited onto heated glass substrates. The spectral response was investigated; a main absorption edge of about 410 nm was obtained. The design of a special substrate table with a unique mask changer that allowed for the fabrication of different geometrical film patterns is also given.     

Tantalum Thin-Film Superconducting Transition Edge Thermometers

Sputter deposited Ta thin films make excellent superconducting transition edge temperature sensors. Depending on film thickness, their transition temperature, T_c, ranges from 4.4K to as least as low as 0.5K. A figure of merit of 50K^–1 is easily achieved. The films are mechanically extremely strong, and at room temperature show minimal aging. Using a simple heat treatment technique, T_c can be tuned to the desired operating range.     

Synthesis and Optical Studies of Superconducting MgB2 Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealed ex situ and rendered superconducting with T _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼10 min at 550°C. The optical absorption coefficient, α and photon energy, E followed a Tauc-type behavior, = _T (E - E_g )(\alpha E)^{1/2} = \beta _T (E - E_{\rm g} ). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameter β _T behaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Strain-Induced First-Order Magnetic Phase Transition in Epitaxial Sm<Subscript>0.35</Subscript>Pr<Subscript>0.15</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> Thin Film

Epitaxial Sm_0.35Pr_0.15Sr_0.5MnO₃ thin films were deposited on LaAlO₃ (LAO, (001)), SrTiO₃ (STO, (001)), and (La_0.18Sr_0.82)(Al_0.59Ta_0.41)O₃ (LSAT, (001)) single-crystalline substrates by using pulsed laser deposition technique. In order to examine the strain effect on electronic and magnetic properties, films were studied by X-ray diffraction, electrical resistivity, and dc magnetization measurements. The film grown on LAO substrate is under compressive strain, and it undergoes ferromagnetic → paramagnetic transition at Curie temperature (T _C) of ～ 165 K and metal → insulator transition at ～ 107 K. The films grown on STO and LSAT substrates are under tensile strain and have T _C of ～ 120 and 130 K, respectively, and show metal → insulator transition at ～ 145 and 137 K, respectively. At T < T _C, the zerofield and fieldcooled magnetization curves of all the films show a huge bifurcation. In the case of films on STO and LSAT substrates, hysteresis is also observed in fieldcooled cooling and warming magnetization vs. temperature measurement protocols at low magnetic field. All the signatures of the firstorder magnetic phase transition are absent in the case of film on LAO substrate. The occurrence and absence of firstorder magnetic phase transition in films on LAO, STO, and LSAT substrates, respectively, have been well explained through the substrateinduced film lattice strain.     

Optically induced changes and long-term relaxations of resistivity and critical temperature in He irradiated YBa2Cu3Ox

Thin films of YBa₂Cu₃O_x with superconducting critical temperatures (T_c) ranging from 58 K to 72 K were prepared by pulsed-laser deposition and subsequent irradiation with 75 keV He⁺ ions. Optical excitation of the films with a He-Ne laser resulted in a small resistivity reduction, when the experiment was carried out a short period of time after the ion irradiation. The value of T_c was essentially unchanged by the optical treatment. Further light excitation experiments did not show a significant effect on the resistivity of the samples. We conclude that ion-irradiated metallic YBa₂Cu₃O_x does not exhibit the persistent photoconductivity observed in oxygen-depleted samples with comparable critical temperatures. On the other hand we have observed a long-term relaxation of T_c towards larger values that takes place even when the samples are kept in the dark. Our results indicate that T_c may increase after ion irradiation to an asmptotic value about 20% over the initial value on a typical time scale of years. This observation may be important for possible applications of light-ion irradiated YBa₂Cu₃O_x.     

Electrical transport and semiconductor-semimetal transition in LaVO4

The electrical conductivity (), and thermoelectric power (S) (300 to 1200 K), magnetic susceptibility () (300 to 900 K) and DTA and TGA (300 to 1200 K) together with X-ray diffraction studies are reported. At room temperature the lattice is orthorhombic. The DTA study shows a broad peak around 1173 K indicating a possible structural phase transition of the compound. Practically no weight loss was observed in TGA from 300 to 1200 K. The electrical conductivity exhibits an exponential increase up to 855 K and between 855 and 885 K, it then jumps by a factor of 5×10⁴ and remains practically constant up to 1200 K, indicating a transition from semiconducting to the metallic state around 870 K. S values remain positive throughout the temperature range studied indicating electrons to be the main charge carrier. In the semiconducting state (300 to 855 K) the plot of S against T ^–1 is linear with a slope of 0.04 eV against the activation energy of 0.83 eV. It has been shown that conduction is due to hopping of holes from V⁴⁺ defect centres to V⁵⁺ normal sites. The defects exist due to a small oxygen deficiency. The semiconductor semi-metal transition appears due to the overlapping of vanadium 3d and oxygen 2p bands around 870 K.     

The Effect of Co Substitution on the Crystal Structure and Electrical Resistivity of (La0.85Ag0.15)MnO3 Compounds

The effect of Co substitution on the crystal structure and electrical transport properties of La_0.85Ag_0.15Mn_1−y Co _y O₃ compounds (0≥y≤0.50) has been studied. Structural transition from rhombohedral to orthorhombic symmetry has been observed with Co doping. The lattice parameters are found to increase with doping up to y=0.20, then it decreases. It is explained on the basis of transition from high spin state to low spin state of Co ions with increase in doping beyond y=0.20. Ferromagnetic (FM) metallic behavior with colossal magneto-resistivity has been observed up to y=0.10. However, for y≥0.15 compounds, the temperature dependence of resistivity ρ(T) follows semiconducting behavior. The electrical resistivity in the metallic region could be explained based on electron–electron and electron–magnon scattering mechanisms. The data in the semiconducting region could be explained based on the variable range hopping model for y=0.2 and adiabatic small polaron hopping model for y≥0.3.      

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.     

Structural,morphological, optoelectrical,linear, and non-linear optical properties of Ge10Se78Ag12 films

Studying the linear and non-linear optical properties is critical in terms of technological application, as it aids in developing the semiconducting materials for optoelectronic applications. Consequently, the present studies report the investigation of the influence of thermal annealing on the structural, morphology, linear, and non-linear optical properties of Ge₁₀Se₇₈Ag₁₂ thin films. X-ray diffraction analysis confirmed the amorphous state of Ge₁₀Se₇₈Ag₁₂ composition. The studied composition was annealed at a temperature between the glass transition and crystallization, and the annealing temperature T_an affected the number and intensity of crystalline phases. Some peaks disappeared at 383 K, indicating that this temperature represents a transition in the structure of the studied materials. The morphological changes caused by the thermal treatment were observed by the scanning electron microscopy (SEM). On the other hand, the linear and non-linear optical constants varied with T_an. The band gap was found to decrease from 1.70 to 1.43 eV and then increase to 1.91 eV with increasing the temperature from 363 to 573 K, confirming the presence of structural transition at 383 K. The optical and electrical conductivities were determined and found to vary with the temperature. The present results were analyzed and discussed.