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Single crystals of strontium barium niobate (SBN) solid solution, (Sr x Ba 1− x ) 1− y (Nb 2 O 5 ) y , 0.75⩾ x ⩾0.40, 0.5050⩾ y ⩾0.4975, were grown by the floating zone method. There are two series of the crystals. x varies and y is fixed at 0.5 in the first series of crystals. x is fixed at 0.61 and y varies in the second series. The typical size of the grown crystal is 5 mm in diameter and 70 mm in length. All crystals, regardless of the compositions, revealed the facet parallel to the c -plane and the clear striations. A part around the crystal can be easily broken due to hoop stress. The spontaneous growth-direction of SBN is 〈0 0 1〉. The congruent melting composition of SBN is revealed by employing a zone-melting method at the final stage of growth experiments. It is (Sr 0.61 Ba 0.39 ) 0.5015 (Nb 2 O 5 ) 0.4985 . The absorption edges of the crystals in both series are around 360 nm and are shifted by 10 nm toward blue in comparison with those of the crystals grown by CZ method.

Lithium tantalate (LiTaO 3 ) single crystals having different Li/Ta ratios were grown by using conventional and double crucible Czochralski method. The grown crystals appeared to have mole percent of Li 2 O ranging from 45.5 to 50 mol%. We analyzed the real composition with ICP method and measured the Curie temperatures (T c ) with DTA method. The relationship between T c and the mole percent of Li 2 O in LiTaO 3 crystals was derived. The transmission spectra and refractive indices were measured. The refractive indices measured by prism coupler were analyzed in terms of single-term Sellmeir parameters.

Strontium barium niobate (SBN, (Sr 0.61 Ba 0.39 ) 0.5015 (Nb 2 O 5 ) 0.4985 ) was grown by using Stepanov technique (ST) or double crucible Stepanov technique (DCST). DCST is a modified Stepanov technique in which the inner crucible is introduced to reduce the temperature fluctuation inside the die. The change of refractive indices was estimated from the interference fringe of the grown crystal. In addition, the change in Sr concentration was estimated by using the relationship between the composition and the extraordinary refractive index. The local variation in the refractive index of the crystal grown by DCST is less than or equal to 3×10 −5 and is 1/3 of the value of the crystal grown by ST. The corresponding fluctuation in Sr concentration for the crystal grown by DCST is 0.0003, which is 1/3 of the value of the crystal grown by ST.

Codoped lithium niobate of congruent and stoichiometric compositions had been successfully grown specifically for holographic data storage applications. The dopants we have selected are Mn and Ce and their role in the recording process is described. Growth methods, sample preparation, doping level measurements, holographic recording data as well as the results of optical characterization and photorefractive properties of the two-photon sensitivity are discussed. Compared to the previously grown crystals, the codoped LiNbO 3 shows a higher sensitivity and dynamic range for recording in red ( λ=632 nm ).

Near-stoichiometric LiNbO3 (LiNbO3, LiNbO3:Tb, and LiNbO3:Tb,Fe) crystals have exhibited a reversible photochromic effect at room temperature. Irradiation by UV light induced a visible absorption band extending from λ≈650 nm to the absorption edge, which could be bleached by subsequent illumination with visible light. Electron paramagnetic resonance (EPR) analysis at 15 K revealed an EPR signal of g≈2.0 appearing after UV irradiation with a simultaneous decrease in the Fe3+ signal intensity. This implies that the Fe3+ ions (natural impurities or dopants) act as electron traps and the induced absorption band is caused by the charge transfer from UV-sensitive absorption centers to Fe3+ ions via the conduction band. Using the induced absorption band, a high-sensitive holographic recording has been achieved with a continuous-wave green laser at λ≈532 nm. In addition, the fast response to UV light made it possible to quickly erase the stored information by UV illumination.

Photorefractive damage, photoconductivity, and photogalvanic currents of stoichiometry controlled LiNbO3 and LiTaO3 have been investigated. A nearly stoichiometric LiNbO3 single crystal shows a lower photorefractive damage resistance than congruent LiNbO3, however, crystals doped with a small amount of MgO (> 0.78 mol.%) exhibit no measurable photorefractive damage at 532 nm up to intensities of 2 MW/cm2. In the case of LiTaO3, it turned out that nearly stoichiometric crystals exhibit considerably high damage resistivity even without MgO doping. Green-induced infrared absorption (GRIIRA) in the crystals was also investigated by a photothermal technique. The GRIIRA in nearly stoichiometric LiNbO3 is found to be remarkably decreased by a small amount of MgO. GRIIRA in nearly stoichiometric LiTaO3 was strongly suppressed even without MgO doping.

We investigated the thermal conductivities of non-doped near-stoichiometric LiTaO 3 (SLT) and Mg (1 mol%)-doped near-stoichiometric LiTaO 3 (Mg:SLT) crystals along the X -, Y -, and Z -axes at room temperature. Those of non-doped congruent LiTaO 3 (CLT) crystal along the same axes were also estimated to investigate the effect of non-stoichiometric defects. The thermal conductivities were determined by measuring the thermal diffusivity using a laser-flash method and measuring the specific heat using a differential scanning calorimeter. Anisotropy of the thermal conductivities was found for all three crystals. That is, the thermal conductivities along the X -axis were the same as those along the Y -axis and less than those along the Z -axis. The thermal conductivities of the SLT crystal were the highest and were twice those of the CLT crystal. The thermal conductivities of the Mg:SLT crystal were slightly lower than those of the SLT crystal apparently due to the Mg-doping. We also investigated the effect of the difference in domain structure on thermal conductivity using as-grown Mg:SLT crystal with randomly distributed multi-domains and found that it did not affect the thermal conductivity along any axis. These findings regarding thermal conductivity should be useful for designing high-power laser applications using SLT and Mg:SLT crystals.

We report the dielectric anomaries at the phase transitions of Lu(Fe 2+ Fe 3+ )O 4 , which is considered as a charge and spin frustrated system. Dielectric constant shows a hump, just above the transition from the high temperature two-dimensional to the low temperature three-dimensional charge ordering. A few glassy behaviors were observed at this transition. The presence of spontaneous polarization in the low temperature phase was confirmed by the measurement of pyroelectricity. A step-wise increase of the dielectric constant with increasing temperature was observed between 150 K and 250 K as in other RFe 2 O 4 . These phenomena are explained by the presence of the polar domains with the ordering of Fe 2+ and Fe 3+ and the motions of the antiphase boundaries between them.

Undoped and MgO-doped LiNbO 3 single crystals with near-stoichiometric compositions were grown from K 2 O-doped melts and Li-rich melts by two different growth techniques. Near-stoichiometric LiNbO 3 single crystals, having large [Li]/[Nb] ratios of 0.988–0.993, showed smaller threshold concentrations of MgO than congruent crystals for the behavior of Curie temperature, OH absorption band and optical damage properties. We discuss the ideal defect structure models, and find that the Li-site vacancy model is consistent with the experimental results for MgO-doped near-stoichiometric LiNbO 3 single crystals.

Nondoped and MgO-doped LiNbO 3 single crystals of 30 mm in diameter and 25–30 mm in length were grown by the top-seeded solution growth method. The solution with [Li]/[Nb] ratio of 1 contained K 2 O component of 10.6 mol% (7 wt%). MgO of different concentrations was used to dope the solution. As-grown crystals were transparent, void-free and almost free from potassium contamination. The crystal grown without MgO-doping exhibited a Curie temperature of 1193°C, indicating that the crystal composition was considerably close to stoichiometry. The MgO-doped crystals grown from the above solution showed larger [Li]/[Nb] ratios (that is, closer to stoichiometry) than the crystals grown from the congruent composition melt doped with MgO of the same doping levels. The MgO-doped crystals with near stoichiometric composition showed a shift in absorption edge towards shorter wavelength, an increase in Curie temperature, and four orders of magnitude increase in photorefractive damage resistance compared to nondoped crystals grown from the congruent melt by the conventional Czochralski method.

With the aim of finding an optimum crucible for growth of Pr2−xCexCuO4 crystal, reactions between CuO flux and various crucible materials were studied. Among six different crucible materials, Pt, Al2O3, ZrO2, Y2O3, CeO2 and MgO, Pt crucible was suitable for using Ce free solution. For Ce doped solution, MgO crucible was better than other materials. The reactant between Pt crucible and CuO flux was identified as Pt2Cu. In the case of MgO crucible, Mg2CuOx and (Mg,Cu)Ox were produced as reactants. We found a method to restrain the reaction between the crucible and the solution. Using this method for growth of Ce doped crystal, we grew a crystal of size 30 mm∅×1.5 mm.

We have carried out electron tunnelling spectroscopy on single crystals with the use of a scanning tunnelling microscope at 4.2 K. A conductance peak structure was observed near zero bias voltage for the tunnelling along the Cu-O bonding direction. A clear superconducting gap was observed for the tunnelling along the Cu-Cu direction. Curves along the Cu-O and Cu-Cu directions are consistent with the -wave pairing symmetry which has line-nodes along the Cu-Cu direction, taking into account the periodic arrangement of surface oxygen atoms.

The ferroelectric domain wall thickness of a fluoride BaMgF4 single crystal was investigated by piezoresponse force microscopy. It was found that the domain wall thickness shows a strong spatial variation in the as-grown crystal and the polarization reversal process. The original wall thickness is greater (about two to seven times) than that switched by the tip fields of the atomic force microscope. A significantly narrower domain wall was obtained in the higher tip-field. The trapped defects at the domain wall play an important role in the spatial variation of the polarization width of 180° domain wall in the BaMgF4 single crystal. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferroelectric domain structures of (001)-oriented Pb(Zn1/3Nb2/3)O3–7%PbTiO3 (PZN-7%PT) single crystals were visualized and characterized by piezoresponse force microscopy (PFM). Locally regular domain configurations are found to be possibly related to the stable macroscopic properties in the PZN-7%PT single crystals. Nanoscale piezoresponse hysteresis loops measured by PFM tip revealed no evidence of local domain switching behavior in the PZN-7%PT single crystal. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoscale structures and physical properties are of fundamental interest, and thus variations in scanning probe microscopy techniques have been developed to understand nanoscale physical phenomena in basic research and potential technological applications. 1) Very recently, scanning near-field ultrasound holography has been developed that provides nanoscale-resolution images of the internal substructures of microelectronic materials and malaria parasites in red blood cells. 2) In the field of ferroelectrics, nanoscale domain structures of ferroelectric thin films, ceramics and single crystals can be imaged and controlled directly using piezoresponse force microscopy. 3–10) Besides, acoustic mode scanning force microscopy including atomic force acoustic microscopy, 11,12) ultrasonic force microscopy, 13–16) and scanning probe acoustic microscopy 17–21) have the advantage of visualizing nanoscale domain structures, elastic properties or subsurface configurations. In the present study, a type of alternating-force-modulated atomic force microscope (AFM) in the acoustic mode was developed by exciting acoustic waves on the piezoelectric transducer, which is fixed on a cantilever in the commercial AFM. This acoustic mode AFM was used to visualize the ferroelectric domains in BaTiO3 single crystals. An alternating force-modulated AFM, functioning in the acoustic mode, was set up by modifying a commercial AFM (SPA 400, SPI3800N, Seiko, Japan). A schematic sketch of the modified AFM is shown in Fig. 1. An external sinusoidal signal provided by a function generator (33120A, Agilent) was applied to the piezoelectric transducer attached to the cantilever, thereby inducing a periodical vibration in the cantilever/tip system. This modulated-tip vibration driven by the function generator emits a local stress field underneath the tip, resulting in an acoustic vibration inside the sample, which gives rise to a remarkable acoustic response of the ferroelectric microstructure due to physical phenomenal effects. The resultant acoustic signal carrying ferroelectric information was converted into an electrical signal through a piezoelectric lead zirconium titanate (PZT) ceramic transducer bonded onto the sample, and then fed to a lock-in amplifier (Model 7280 DSP, Signal Recovery Instrumentation) for detection. For the experiment, a 2-mmthick 90-mm-long Ti/Pt-coated silicon cantilever (Micro Mash, NSC12-B) with a spring constant of 14 N/m and a resonance frequency of 315 kHz was used. For the present study, we chose a 0.3-mm-thick (001)-oriented BaTiO3 single crystal. Figures 2(a) and 2(b) are a topography image and the corresponding acoustic image of a BaTiO3 single crystal, respectively, at a modulation frequency of 133.42 kHz. It can be clearly seen that the acoustic features are nearly the same as those of the topography image, indicating only slight variations in the surface microstructure after sample polishing. Figure 2(c) shows an acoustic image taken at a frequency of 1.419 MHz in the same scanning area. Unlike Fig. 2(b), a different acoustic response is clearly seen in Fig. 2(c), exhibiting remarkable acoustic contrasts in parallel strips about 300 nm in width, which are obviously the 90 ferroelectric domains in the BaTiO3 single crystal. The striped patterns are more distinguished in another large scanning area of 10 10 mm 2 measured at a frequency of 1.419 kHz, as shown in Fig. 3. The intercrossing domain phenomena shown in Fig. 3(b) reflect the domain state distribution at different depths throughout the specimen. Such phenomena were observed in Han and Cao’s studies of the averaged domain configurations using a polarized optical microscope, in which the domain configurations at various depths were exhibited in an optical image by adjusting the focus. 22)

A volume holographic filter operating in the reflection geometry at communication wavelengths was demonstrated in oxidized Fe-doped near-stoichiometric lithium niobate crystals, and the designed filter showed a narrow bandwidth of less than 0.2 nm and an express loss of more than 20 dB. An array of volume holographic filters was constructed in the same crystal with central wavelength separation of 0.5 nm.

Optical quality near stoichiometric lithium niobate (SLN) and Mn-doped SLN crystals have been grown by top-seeded solution growth from 55 mol% Li-rich solution. A uniform segregation of Mn throughout the crystal was obtained. Doping of Mn decreases the Curie temperature while the absorption edge shifts towards the longer wavelength region. The effect of Mn doping on the switching properties has been analyzed.

On the idea of material designing, we have succeeded in synthesizing Y 3 Ni 4 B 4 C 3 with a new crystal structure and Y 1 Ni 1 B 1 C 1 phases through phase search experiments. The Y 3 Ni 4 B 4 C 3 phase is a tetragonal layer-like structure that is built up of one half Y 1 Ni 2 B 2 C 1 unit and one full Y 1 Ni 1 B 1 C 1 unit stacked along the c -axis. For the Y 3 Ni 4 B 4 C 3 phase, the possible space group is I4 (No. 79), lattice parameters are a =0.355190(7) nm and c =2.56454(7) nm. For the Y 1 Ni 1 B 1 C 1 phase with a CeFeSi type structure, lattice parameters are a =0.35660(5) nm and c =0.75625(12) nm. The crystal structure of Y 3 Ni 4 B 4 C 3 corresponds to the T * phase of the cuprate high T c superconducting materials and m =3, n =2 in the homologous series (LnC) m (B–Ni 2 –B) n . By magnetic susceptibility measurements, superconducting transitions were observed at around 11.0 and 3.0 K. Although the Y 1 Ni 2 B 2 C 1 phase was included in the sample and the single phase was not obtained yet, it is considered that the Y 3 Ni 4 B 4 C 3 or the Y 1 Ni 1 B 1 C 1 phase show a superconducting transition.

Frequency and bandwidth of terahertz wave were simultaneously tuned by selectively choosing domain period in a fan-shaped periodically poled stoichiometric lithium tantalate crystal.

Simultaneous tunable terahertz wave generation with the frequency range (0.85 ~ 1.59 THz) and the bandwidth range (43 ~ 78 GHz) was demonstrated using a fan-shaped periodically poled stoichiometric lithium tantalate crystal.

Second harmonic UV (350nm) generation was demonstrated by periodically poled LaBGeO5 (PPLBGO) for the first time. PP structures were fabricated with 5.5μm periodicity. The phase-matching-wavelength agreed fairly well with the calculation from reported Sellmeier’s equation.

Changes of the dimensionality and Tc through iodine intercalation and oxidation have been studied in Bi2Sr2CaCu2O8+δ single crystals. Both intercalation and oxidation change the dimensionality of the physical properties from pseudo-two-dimensional to three-dimensional and decrease Tc. These effects are explained by the multilayer model, where a high-Tc cuprate is regarded as an alternating stack of CuO2 layers (superconducting layers) and block layers (normal layers), taking into account the proximity effect. The Tc-suppression by Cu-site substitution has also been studied for iodine-intercalated and pristine Bi2Sr2Ca(Cu1−zCoz)2O8+δ polycrystals with different δ-values.

Velocities and attenuations of longitudinal sound waves were measured at 100 MHz for a single crystal of non-stoichiometric ErFe 2 O 4-δ , along the and axes in the hexagonal frame, below 300 K. At room temperature, velocities were 5.22× 10 3 and 5.49×10 3 m/s along the and axes, respectively. These values correspond to the elastic constants of C 11 =1.94×10 12 dyne/cm 2 and C 33 =1.75×10 12 dyne/cm 2 . Slight softening was disclosed along the axis at around 240 K. Correspondingly, a large peak was observed in the attenuation. These anomalies are attributed to the emergence of two-dimensional spin ordering in this crystal at this temperature. On the contrary, no anomaly was detected along the axis at around 240 K, with rather high attenuation level almost independent of temperature. This absence of anomalies was discussed on the existence of three kinds of magnetic domains. Development of the correlation of Er spins was suggested below 30 K.

Nonvolatile holograms have been recorded in nondoped near-stoichiometric LiTaO3 crystals by use of a near infrared (IR) laser for writing and a ultraviolet beam as a gating source. This material exhibits good overall two-color holographic recording characteristics including a high sensitivity of 0.086 cm/J with a gating intensity of 1 W/cm2 at 350 nm, a large refractive index change of as much as 1×10−4, a high resistance to IR erasure, and a long hologram lifetime of five years. The sensitivity can be further enhanced using writing and gating beams of a shorter wavelength without sacrificing readout nonvolatility.

We have investigated microscale to nanoscale ferroelectric domain and surface engineering of a near-stoichiometric LiNbO3 crystal by using scanning force microscopy. The single crystals LiNbO3 fixed on metal substrates were polished to a 5 μm thickness. Artificial patterns of inverted-domain structures were fabricated in the samples, where polarization directions of the domains were switched by scanning the samples with a conductive cantilever while applying voltages. Furthermore, the negatively polarized surfaces in the patterns were preferentially etched in HF solution. As a result, cavity and mound-shaped surfaces were fabricated; these structures could be used to create functional templates and devices.

We report on the method that terahertz generation and detection were simultaneously performed via non-collinear optical parametric interaction in a nonlinear crystals using a Si-photodiode. The bandwidth manipulation between broad 3.3 THz and narrow 45GHz bandwidths was demonstrated at single quasi-phase-matching crystal using the position-dependence of the crystal.

Crystal structure and effects of Fe doping of a non-superconducting phase (B-phase) in the Bi-Sr-Cu-O system are examined by electron-diffraction and high-resolution transmission electron microscope measurements. The B phase has a long-range periodic structure closely related to the modulated structure of the superconducting phase and it crystallizes in a monoclinic superlattice with a s =0.54, b s =2.0, c s =2.5 nm and α =104°. The (BiO) 2 double layer is not a complete continuous sheet but suffers from a periodic displacement along the c -direction to form a terraced (BiO) 2 layer. Each BiO terrace consists of seven Bi atoms, and at every eighth lattice plane a vertical shear plane containing few Bi atoms is inserted. Fe doping has an effect on the periodicity of the terraced structure. Intergrowth of narrow (BiO) 2 terraces is induced in crystals doped with Fe.

We have performed two-color hologram multiplexing in stoichiometric LiNbO3:Tb,Fe, which exhibits a photochromic effect. One hundred angle-multiplexed holograms of nearly equivalent strength have been recorded from the colored state by use of 850 nm recording beams and a 436 nm gating light. The stored holograms were highly resistant against erasure during continuous readout. It was estimated that 80 million readouts with a bit rate of 1 Gbit/s is possible before the diffracted signal intensity decays to half of its initial value.

Nonvolatile holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of a two-color holographic recording method. An IR laser with wavelength of 778 nm is used for writing and a UV laser with wavelength of 350 nm is used for gating. Dependences of two-color sensitivity and M/♯ on gating and writing intensities are presented. Significant improvements of sensitivity, M/♯, and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. The obtained high two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is attributed to large light-induced absorption of small polarons.

We have investigated the ferroelectric domain structure formed in a Sr0.61Ba0.39Nb2O6 single crystal by cooling the crystal through the Curie point. Imaging the etched surface structure using a scanning force microscope (SFM) in both the topographic mode and the piezoresponse mode revealed that a multidomain structure of nanoscale islandlike domains was formed. The islandlike domains could be inverted by applying an appropriate voltage using a conductive SFM tip. Furthermore, a nanoscale periodically inverted-domain structure was artificially fabricated using the crystal which underwent poling treatment.

Undoped and MgO (0.5 and 1.0-mol%)-doped near-stoichiometric LiTaO3 (SLT) crystals were grown from off-congruent Li-rich solutions (Li~60 mol%) by the double-crucible Czochralski method using a continuous SLT ceramic grain charging system. Curie temperatures of the undoped and MgO (0.5 and 1.0-mol%)-doped SLT crystals are 688, 694 and 695°C, respectively. The ordinary and extraordinary refractive indices (no, ne) of these crystals were measured by the prism coupling technique in the wavelength range from 0.440 to 1.050 µm at room temperature, and the temperature-independent Sellmeier equations for each crystal were derived from the measured refractive index data. no of the SLT crystal was almost the same as that of a congruent-melt LiTaO3 (CLT) crystal, while ne of the SLT crystal was lower than that of the CLT crystal. ne was lower than no for the SLT crystal, similar to as in the case of the LiNbO3 crystal. The refractive indices of the SLT crystal, no and ne, were found to be almost independent of MgO concentration at the doping level of 0.5 and 1.0 mol%.

The optical damage resistances of near-stoichiometric LiNbO3 (LN) crystals doped with MgO were investigated at λ=532 nm. Their refractive indices were also measured in the wavelength range from 440 to 1050 nm, from which Sellmeier's equations were derived. While the congruent LN (CLN) crystal required 5 mol% MgO to suppress the optical damage, the near-stoichiometric LN (SLN) crystal doped with 1 mol% MgO showed no optical damage up to the input intensity of 2 MW/cm2. The ordinary refractive indices (no) of MgO-doped SLN crystals decreased with MgO doping concentration up to 4.6 mol%. In contrast, the extraordinary refractive indices (ne) of MgO-doped SLN crystals were found to be almost independent of MgO doping concentration up to 4.6 mol%. With respect to optical device design, it is advantageous that ne of MgO-doped SLN crystal is almost independent of MgO doping concentration up to 4.6 mol% and that the SLN crystal needs only 1 mol% MgO to suppress optical damage.

We present a more-developed method where narrow-band terahertz (THz) waves are simultaneously generated and detected at one quasi-phase matching crystal. For THz detection, we used a photodiode as a THz detector via parametric interaction between THz wave and weak probe beam, compare to the photoconductive antenna or electro-optic sampling methods.

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

Continuous tunable terahertz pulses were generated with a continuous grating-period change from 65 to 90 µm in periodically poled stoichiometric lithium tantalate crystal. Center frequency was tuned from 0.9 to 1.3 THz with as narrow as band-width of 21 GHz at crystal temperature of 130 K.

The electronic Raman scattering of both tetragonal ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ symmetries has been measured in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals as a function of the temperature. We observe that a redistribution of the electronic continuum spectrum below ${\mathit{T}}_{\mathit{c}}$ exhibits two aspects of the temperature dependence: One of them is associated with its energy scale, which has a significant temperature variation only just below ${\mathit{T}}_{\mathit{c}}$. This indicates an abrupt opening of the superconducting gap. The other is associated with the variation of the spectral weight, which shows a more gradual temperature dependence compared with the former. This indicates that a superconducting condensation of thermally excited quasiparticles occurs gradually with decreasing temperature. We also find that the ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ spectra exhibit a similar temperature dependence below ${\mathit{T}}_{\mathit{c}}$.

Tunneling spectroscopic measurement was carried out in the superconducting phase of Bi 2 Sr 2 CaCu 2 O 8 with use of a low temperature scanning tunneling microscope. Both the cleaved Bi-O surface and the lateral surface of a single crystal were investigated. The tunneling conductance shows an explicit superconducting gap structure at both surfaces. For the lateral surface the conductance curve is independent of the tip distance from the sample surface and gives directly the electronic density of states in the superconducting phase. The obtained gap structure is in good agreement with the BCS prediction, although it seems that the gap is slightly anisotropic. The obtained gap-ratio 2Δ/k B T c =6.1 indicates the strong-coupling nature of this superconductor. For the Bi-O surface, on the other hand, the normalized differential conductance increases with increasing the tip distance, especially at the outside of the superconducting gap as reported by Hasegawa et al . Such a dependence on the tip distance is attributed to series barriers of Bi-O, Sr-O layers and vacuum for the tunneling between the Cu-O layer and the tip metal.

Photorefractive damage, photoconductivities, and photogalvanic currents of LiNbO3 single crystals have been investigated as a function of [Li]/[Nb] and MgO concentrations. Near stoichiometric LiNbO3 single crystal, having large [Li]/[Nb] ratios of 0.993, shows lower photorefractive damage resistance than congruent LiNbO3. However, crystals doped with a small amount of MgO (>0.78 mol %) exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 2 MW/cm2. This remarkable decrease of photorefraction that was found can be attributed to the decrease of saturated space charge field results from a combination of the increased photoconductivity and the decreased photogalvanic current of the crystal.

We have found that Tb-doped near-stoichiometric LiNbO3 crystals have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV (λ≈650 nm) below it. Using this shallow trap as an intermediate energy state, two-color holographic recording has been carried out at λ=852 nm with a UV gating light at 313 nm. A few % of diffraction efficiency was achieved, and the measured two-color sensitivity was 0.01–0.02 cm/J. No reduction of diffraction efficiency was observed during 4 h continuous readout, and the bit-error-rate of constructed digital hologram was less than 10−5.

Tunable narrow band terahertz (THz) pulses were generated in periodically poled lithium niobate and lithium tantalite at 0.6 and 1.4 THz with bandwidth of 20 GHz. The generation THz frequency and intensity were strongly depending on temperature of crystal and pump laser spectral bandwidth.

The electrical resistivity, Hall effect, magnetoresistance and magnetic susceptibility of Ta 3 S 2 , Ta 2 S and Ta 6 S are reported for the first time. The Hall coefficient of Ta 3 S 2 changes its sign from positive to negative with increasing temperature. The Hall coefficient is pronouncedly dependent on magnetic field strength below 77 K. The magnetoresistance increases almost quadratically with the field strength below 77 K, and is still observed at 291 K. These results are interpreted in terms of two bands with the carrier densitiy 3.0×10 19 cm -3 of electrons and 7.9×10 17 cm -3 of holes. These results strongly suggest that Ta 3 S 2 is a semimetal with very slightly overlapping conduction and valence bands near the Fermi level. In contrast with Ta 3 S 2 , the compound Ta 2 S is a metal with hole conduction, while Ta 6 S is a metal with electron conduction.

Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2 Sr 2 CaCu 2 O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.

The anisotropic resistivity in the a − b plane and Hall coefficient have been measured in a single-crystal Bi 2 Sr 2 CaCU 2 O y (BSCCO ) and their temperature dependences have been analyzed by a two-band model involving holes and electrons. It is confirmed that the calculated values of the resistivities and Hall coefficient reproduce well the data and the carrier concentrations, mobilities and mean-free-paths obtained from the model are reasonable values in physics. It is found that the hole concentration is smaller than the electron one, but the hole mobility is larger than the electron one. The transport properties of BSCCO are discussed in terms of two types of the hole and electron carrier.

A 6H→3C transformation, which does not commonly occur at high temperatures, was observed in SiC. A 6H-SiC and TiN powder mixture was hot-pressed in a flowing Ar atmosphere. The starting 6H-SiC powder was completely transformed to 3C polytype above 2200°C, although 3C-SiC was not stable at this temperature. It was found that coexistence of N was essential for this 6H→3C transformation, and a small amount of Ti was detected in the transformed SiC grain. The results suggest that a solution of N or TiN in the SiC grain would transform the 6H-SiC to the 3C form.

The authors carried out a tunnelling spectroscopy measurement on a single crystal of superconducting oxide Bi2Sr2CaCu2O8- delta using a low temperature scanning tunnelling microscope. The superconducting gap parameter shows a spatial variation over a relatively small area ( approximately 10 nm). The authors obtained the energy gap parameter Delta (4.2 K)=26 meV, the most observable value, and correspondingly 2 Delta /kBTc=7, larger than the BCS value. The shape of the (dI/dV)-V curve suggests an anisotropy of the energy gap. Thermal smearing of the gap structure is observed below Tc. Temperature dependence of the gap parameter is consistent with BCS. Even above Tc the conductance curve is not flat.

We demonstrated tunable and narrow-band terahertz generation around l~3 THz via difference frequency generation (DFG) using femtosecond (fs) pulses in a periodically poled lithium niobate (PPLN) crystal and periodically poled stoichiometric lithium tantalate (PPSLT) crystal. Two different types of terahertz radiation, forward and backward, could be generated in the quasi-phase matching (QPM) structures by the DFG process. A broadband fs optical pulse contributes to the DFG process, which occurs between two different frequency components set of an ultrashort optical pulse within the frequency range and a terahertz frequency at the QPM structure. We observed a backward terahertz radiation at approximately 1.3 THz at PPLN crystal and the frequencies of terahertz radiations are easily controlled by varying the domain structures.