Raman scattering as a contactless room-temperature test of the quality of YBa2Cu3O7−x thin films
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Abstract:
The room-temperature Raman spectrum of YBa2Cu3O7−x is sensitive both to stoichiometry and to the transition from the semiconducting to the superconducting state as a function of oxygen annealing. In particular, it is shown that a good quality film with a Tc of ∼90 K and a sharp resistivity transition has not been produced if the frequency of the O(1) stretch along the c axis shifts from 500 cm−1 to lower frequencies. In this paper, two types of processing failures are illustrated: (1) lack of sufficient oxygen and (2) incorrect stoichiometry. No attempt has been made to predict Tc from the Raman data for less than optimum films.Keywords:
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We have observed the transition temperature of both the cubic and tetragonal phases of several high-temperature beta-W superconductors. The instability of the cubic lattice appears to be characteristic of high-temperature superconductors.
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That the superconducting transition in underdoped high temperature superconductors is controlled by phase ordering, implies that pairing is a local phenomenon that occurs on an intermediate length scale, and typically at a temperature above T~.. The discovery of local stripe order in the LSCO family of high temperature superconductors, along with the theoretical suggestion that such structures are a general feature of doped antiferromagnets lead us further to propose that in fact pairing occurs in the vicinity of an individual stripe. The transition to a superconducting state then follows at lower temperature due tO Josephson coupling between stripes. In this paper we review our microscopic model of high temperature pairing on individual stripes.
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The Cu-TiC composites with non-stoichiometric TiC have been tried to prepare by melt reaction method in this work. It is confirmed that the TiC with different stoichiometries can be successfully prepared by changing the Ti/C ratio in raw material. When there is much excessive Ti in Cu melts, sub-stoichiometric TiC, such as TiC0.6, will be synthesized. However, the sub-stoichiometric TiCx, is not formed by direct reaction between Ti and graphite. In Cu-TiC composites prepared by melt reaction method, near stoichiometric TiCy prefer to be firstly synthesized by reaction between Ti and graphite. Then near stoichiometric TiCy will further react with Ti in the melts to form sub-stoichiometric TiCx (x < y) when there is much excessive Ti.
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Superconducting thin films of YBa2Cu3O7−x have been successfully made by co-sputtering from the three targets in an Ar and O2 mixture atmosphere. After high-temperature anneal films are superconducting with an onset temperature at 90 K, a full transition temperature as high as 88 K, and a critical current density in excess of 1×105 A/cm2 at 78 K and 2×106 A/cm2 at 4.2 K. Highly oriented thin films have been obtained on SrTiO3 {100} substrates.
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The ultrasonic nonlinearity parameter beta of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7- delta / was measured from 300 to 77 K by using the second-harmonic generation technique. The ultrasonic nonlinearity parameter beta decreases with decreasing temperature very rapidly from room temperature to 200 K and vanishes around the transition temperature. The phenomenon can be explained by assuming a structural phase transition, a superconducting transition or a gain boundary change. The behavior is unique and different from that of single crystals.< >
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2 Te 3 NP 在空中被氧化并且在与中等激光力量加热下面形成一个肿块，是由双性人特征的拉曼线的出现揭示了 2 O 3 和 TeO 由 AFM 和版本的 2 和描述进一步上激光力量灼伤洞增加 NP 的表面。薄(厚度不到 20 nm ) 有与双性人不同的 stoichiometry 的 NP 2 Te 3 也被学习。从 non-stoichiometric NP 的拉曼线随双性人集中的增加与 stoichiometric 和显示特征变化的那些不同。有一样的厚度的薄 NP 但是不同 stoichiometries 表演与在光图象的底层相比的不同颜色对比。这显示在薄咬 NP 的光吸收系数强烈取决于他们的 stoichiometry。
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It is well known and it has been shown experimental evidence that the critical transition temperature Tc of any type of superconductor depends among other criteria on the crystal structure. Very good examples are thallium, mercury, lanthanum, tin and gallium. Also in high temperature superconductor (HTSC) cuprates a relation between the geometry of the crystal and the transition temperature has been found. This investigation of HTSC's with the model of a resonance effect between the de Broglie wavelength of paired current carrying particles and the dimension of the solid state structure forming a quantum well, has been suggested by the authors in a previous paper. In this model the dimension is given by the distance of the atoms providing the current carrying particles and at the same time the structure acts as a resonator stimulating a coherent phase transition from an electron or particle gas to a condensate.
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The potential of a new substrate, PrGaO3, for high Tc oxide superconducting films is investigated and results indicate it is the most suitable material studied so far. Single crystals were grown by the Czochralski method. The lattice mismatch between PrGaO3 and YBa2Cu3Ox superconductor was estimated to be only 0.02% at the elevated deposition temperature. The dielectric constant value was 24 at 10 kHz. The c-axis oriented YBa2Cu3Ox films were deposited onto PrGaO3 substrates by the laser ablation technique. Their superconducting transition temperature was as high as 90 K, even for a film thickness of only 500 Å.
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