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In a variety of high-temperature superconductors the transition temperature ${T}_{\mathrm{c}}$ has been found to depend on the sample's detailed temperature and pressure history. ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ (Tl-2201) exhibits two distinct relaxation channels, one activated below 100 K and the other above 150 K, that have been attributed to oxygen ordering effects. In the present studies to 0.8 GPa we find that the activation energy for the low-temperature process, ${E}_{\mathrm{A}}^{\mathrm{LT}}(1 \mathrm{bar})\ensuremath{\approx}0.25$ eV, is independent of pressure, whereas the activation energy for the high-temperature process, ${E}_{\mathrm{A}}^{\mathrm{HT}}(1 \mathrm{bar})\ensuremath{\approx}0.75$ eV, appears to increase with pressure yielding an activation volume $\ensuremath{\sim}1.5 {\mathrm{cm}}^{3}/\mathrm{mol}.$ The activation volumes for the diffusion of oxygen defects in the high-temperature superconductors Tl-2201, ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (Y-123), and ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ (Hg-1201) are estimated using a simple hard-sphere model and found to be roughly half the experimental values. For Tl-2201 this comparison supports an indirect ``interstitialcy'' diffusion path for oxygen defects, whereas for Y-123 and Hg-1201 direct ``interstitial'' diffusion paths are indicated.

Detailed magnetic hysteresis measurements have been performed with the applied field parallel to the ab plane in single crystals of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6}$. Magnetization in the descending branch of the hysteresis loop is almost zero, indicating the presence of surface barriers which determine the penetration field of vortices. Magnetization with the field tilted slightly to the ab plane shows that the two components of magnetization due to ${\mathit{H}}_{\mathrm{\ensuremath{\parallel}}}$ and ${\mathit{H}}_{\mathrm{\ensuremath{\perp}}}$ act independently. The data present clear evidence of a lock-in transition in the compounds studied.

We present here the complete results of tight-binding calculations of the band structures and Fermi surfaces for the ${\mathrm{TlBa}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+3}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{\mathit{n}\mathrm{\ensuremath{-}}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+4}$ (n=1, 2, and 3) superconducting phases. We show the density of states (partial and total) and three-dimensional Fermi surfaces for each of these six phases. The single-Tl-O-layer Fermi surfaces contain noncylindrical features, and the Fermi surfaces of Tl 2:2:2:3, Tl 1:2:0:1, Tl 1:2:1:2, and Tl 1:2:2:3 show pockets corresponding to Tl-O band crossings at ${\mathit{E}}_{\mathit{F}}$.

We report magnetization studies on single crystals of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6}$ superconductors. For magnetic fields parallel to the c-axis configuration, fluctuations of vortices are observed in this layered compound. The characteristic temperature ${\mathit{T}}^{\mathrm{*}}$ where M is independent of applied field is about 84.2 K. Low-field magnetization measurements show ${\mathit{H}}_{\mathit{c}1}$=0 at ${\mathit{T}}_{\mathit{s}}$=86.5 K while the mean-field transition temperature ${\mathit{T}}_{\mathit{c}0}$=88.8 K. The experimental results strongly suggest the Tl compound is Josephson-coupled between superconducting layers, and the spontaneous excitations are thus the wandering vortex lines rather than pancakes.

We report direct evidence of surface barriers in single crystals of Tl[sub 2]Ba[sub 2]CuO[sub 6] superconductors. Magnetization is measured as a function of temperature with magnetic field applied parallel to the [ital c] axis. For small field, a characteristic linear increase in [ital M] is observed at an onset temperature [ital T][sub 1]. For higher magnetic field, a new feature at low [ital T] is observed in the [ital M]([ital T]) data. The flux penetration temperature is retarded to a higher temperature. The penetration field [ital H][sub [ital p]] at temperature below 50 K determined by the first linear rise in [ital M]([ital T]) can be fitted with the relation [ital H][sub [ital p]]([ital T])=[ital H][sub [ital c]]exp([minus][ital T]/[ital T][sub 0]), consistent with a model where two-dimensional pancakes penetrate the Bean-Livingston barrier by thermal activation. The low critical field [ital H][sub [ital c]1][sup [perpendicular]] determined from the bulk penetration of vortices is also presented.

The measured frequency shift in temperature of the ac susceptibility ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ single crystals is modeled by an Arrhenius-like expression to give flux-creep activation energies at various applied ac magnetic fields ranging from 15 to 200 Oe. The activation energies range from 288\ifmmode\pm\else\textpm\fi{}14 meV in an applied ac magnetic field of 15 Oe to 25\ifmmode\pm\else\textpm\fi{}3 meV in 200 Oe. Considerable flux creep is observed. It is estimated that ${\mathit{H}}_{\mathit{c}1}$ at a temperature of about 100 K is approximately 10 Oe. The results are interpreted in terms of Anderson flux-creep model and the activation-energy values reported here are similar to those deduced from magnetic-field-dependent resistivity measurements on the Tl-based single crystals, textured films, and bulk samples. The field dependence of the activation energies is observed to vary as 1/ \ensuremath{\surd}H . This low-field dependence is explained by a scaling argument where the vortex volume is approximated by ${\ensuremath{\xi}}^{2}$${\mathit{a}}_{0}$, with \ensuremath{\xi} the coherence length in the ab plane and ${\mathit{a}}_{0}$ the Abrikosov vortex-lattice spacing along the c axis of the crystal lattice.

We report magnetic-field-modulated microwave surface resistance in a single-crystal ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{BaCu}}_{2}$${\mathrm{O}}_{8}$ superconductor. At a fixed temperature, as the applied magnetic field is varied, the observed signal has two features: an intense sharp peak at low field and a broad maximum at high field. The peak field defined for the broad maximum does not have the same temperature dependence as the irreversiblity line determined from dc magnetization measurement. The results are analyzed on the basis of recent theoretical models. It is found that the flux-flow resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{f}}$\ensuremath{\propto}B for B${\mathit{B}}_{\mathrm{peak}}$ and ${\mathrm{\ensuremath{\rho}}}_{\mathit{f}}$=const for Bg${\mathit{B}}_{\mathrm{peak}}$. Scaling of the data at different T and B gives a universal function for the surface resistance.

The anisotropic thermal conductivity of Tl2Ba2CaCu2O8 and YBa2Cu3O7 single crystals have been measured. In contrast to some earlier measurements on YBa2Cu3O7, we found both an in-plane and an out-of-plane anomaly for these two compounds. We argue that phonon-phonon scattering may also play some role in the phenomenon.

The anisotropic resistivity and paraconductivity of Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} single crystals were measured. The in-plane and {ital c}-axis resistivities are, respectively, {similar to}7{times}10{sup {minus}4} and 10{sup {minus}1} {Omega} cm at room temperature. The {ital c}-axis resistivity decreases with decreasing temperature in the temperature range from room temperature down to the superconducting transition temperature, and the second derivative of {rho}{sub {ital c}} is negative, which is not characteristic of YBa{sub 2}Cu{sub 3}O{sub 7} and Bi{sub 2}Sr{sub 2.2}Ca{sub 0.8}Cu{sub 2}O{sub 8} single crystals, in which the second derivative is either positive or zero. The in-plane paraconductivity is obtained from the in-plane resistivity, and a method without the critical temperature as an adjustable parameter is employed to obtain the dimensionality. The data show that the superconductivity is two dimensional, but a crossover from two to three dimensions has also been observed near the transition temperature in some samples.

Spectroscopic ellipsometric measurements of thin polycrystalline ${(4\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})}_{y}{(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})}_{1\ensuremath{-}y}$ films reveal that there are important differences in optical properties and electronic structures between $\ensuremath{\alpha}$-phase $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ and Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. We report the optical functions of thin-film polycrystalline ${(4\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})}_{y}{(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})}_{1\ensuremath{-}y}$ and describe how they change depending on the degree of Cu deficiency. We find a reduction in the absorption strength in the spectral region of $1--3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. This reduction can be explained in terms of density of the Cu $3d$ states in $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$. Cu-poor $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ samples show an increase in band gap due to reduced $p\text{\ensuremath{-}}d$ interaction. We find that the reduction in the amplitudes of ${E}_{0}(A,B,C)$ transitions at the $\ensuremath{\Gamma}$ point and ${E}_{1}(A,B)$ transitions at the $N$ point are due to Cu deficiency. Local density approximation calculations with a modeled on-site self-interaction correction of the absorption coefficients of ${\mathrm{Cu}}_{8}{\mathrm{In}}_{8}{\mathrm{Se}}_{16}$ $(\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2})$, ${\mathrm{Cu}}_{5}{\mathrm{In}}_{9}{\mathrm{Se}}_{16}$ ($\mathrm{Cu}{\mathrm{In}}_{3}{\mathrm{Se}}_{5}$-like), and ${\mathrm{Cu}}_{2}{\mathrm{In}}_{10}{\mathrm{Se}}_{16}$ $(\mathrm{Cu}{\mathrm{In}}_{5}{\mathrm{Se}}_{8})$ are in good agreement with those of thin-film polycrystalline $\mathrm{Cu}\mathrm{In}{\mathrm{Se}}_{2}$ samples with 24.1, 15.6, and $9.1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Cu, respectively.

The anisotropic thermopower of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CuO}}_{6}$ single crystals has been measured between 80 and 300 K. Both ${\mathit{S}}_{\mathit{a}\mathit{b}}$ and ${\mathit{S}}_{\mathit{c}}$ vary linearly with temperature, but their slope signs are opposite. The results confirm again an earlier inference that the linear dependence of ${\mathit{S}}_{\mathit{a}\mathit{b}}$ on T with a negative slope is quite common to high-${\mathit{T}}_{\mathit{c}}$ superconductors.

We report the results of experiments carried out on single crystals of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ under nearly hydrostatic pressure conditions to 6 GPa. The critical temperature, ${\mathit{T}}_{\mathit{c}}$, is observed to possess a nonlinear pressure dependence. While the increase of ${\mathit{T}}_{\mathit{c}}$ with increasing oxygen concentration in ${\mathrm{YBa}}_{2}$${\mathrm{Ca}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is consistent with the observation of increasing ${\mathit{T}}_{\mathit{c}}$ with increasing pressure, the ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ system demonstrates an increasing ${\mathit{T}}_{\mathit{c}}$ with decreasing oxygen content, but has a positive ${\mathit{dT}}_{\mathit{c}}$/dP at low pressures. This behavior is clearly inconsistent with the picture applied to the Y-Ba-Cu-O system. In all the present measurements the curvature is downward and, in fact, for one sample it actually decreases. A theoretical explanation based upon the strong-coupling Eliashberg form of the BCS theory is put forth to explain this effect.

The electrical, magnetic, and optical properties of charge-transfer salts containing tetrathiafulvalene (TTF) and the pseudohalides, thiocyanate (SCN) and selenocyanate (SeCN), have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to a cation-radical-anion-radical system, such as tetrathiafulvalene tetracyanoquinodimethane (TTF) (TCNQ). Measurements of electrical conductivity, thermoelectric power, and optical reflectivity of single crystals of the nonstoichiometric salts (TTF)12(SCN)7 and (TTF)12(SeCN)7 show metal-like characteristics above 200 K (high-temperature region). The conductivities at room temperature are approximately 750 mho/cm comparable to those found in (TTF) (TCNQ), and increase with decreasing temperature down to approximately 200 K. The thermoelectric power at room temperature is small and positive (approximately 9 microvolts/deg K), and decreases linearly with decreasing temperature in this region (as expected for metal-like hole conduction along the TTF chains). The ESR intensity, however, decreases with decreasing temperature above 200 K. At 170 K a metal-nonmetal transition occurs, and the transport and magnetic properties below this temperature are characteristic of a semiconducting state.

A recent investigation in which Isett and Perez-Albuerne (1977) found that (tetrathiatetracene)2(iodine)3 is a stable organic metal down to 3.3 K is of considerable interest. In view of the diversity of suggestions made regarding this compound, measurements, were made of its electrical, magnetic, and optical properties. The results obtained indicate a metallic state at high temperatures, but also support a nonmetallic state at temperatures below 30 K. At 20-30 K, a metal-to-insulator phase transition occurs. This is indicated by the onset of a large and positive magnetoresistance, the leveling off in the temperature dependence of the ESR linewidth, and by recent derivative analysis of the electrical conductivity in this region. The investigated compound represents an interesting one-dimensional electronic system in which it may be possible to study the combined effects of disorder and interchain coupling on charge transport processes.

The drift mobilities of holes and electrons in very pure single crystals of naphthalene were experimentally determined by measurement of transit times of transient space-charge-limited currents. The room-temperature values are in general agreement with published data except for holes in the ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}}^{\ensuremath{'}}$ direction where a larger value of 0.99 ${\mathrm{cm}}^{2}$/V sec was measured. In the temperature range 220-300 \ifmmode^\circ\else\textdegree\fi{}K, the mobilities of holes and electrons were found to vary as ${T}^{\ensuremath{-}2.1}$ and ${T}^{\ensuremath{-}0.9}$, respectively, in the ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}}^{\ensuremath{'}}$ direction, while the mobility of holes was found to vary as ${T}^{\ensuremath{-}1.0}$ for the a crystal direction and as ${T}^{\ensuremath{-}0.8}$ for the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ crystal direction. While these results are consistent with band calculations based on the tight-binding approximation, the mobilities of electrons in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ crystal directions were found to be essentially independent of temperature, and hence cannot be accounted for by band theory. The experimental results are compared with predictions of the band and hopping models, and it appears that the transport mechanism is intermediate between these two modes of transport.

Mesures d'attenuation d'ultrasons sur des supraconducteurs a haute temperature a base de Thallium ayant une temperature de transition voisine de 110 K. Mesures effectuees entre 300 K et la temperature de l'Azote liquide (77 K). Observation de plusieurs pics accompagnes d'un pic supplementaire au voisinage de Tc. A la temperature critique, aucune chute aigue n'est observee avec la resolution experimentale disponible. L'attenuation croit avec l'abaissement de la temperature en dessous de Tc. Observation d'un comportement d'attenuation similaire dans les complexes a base de Y

Superconductivity at 20 K has been unambiguously observed in the Tl-Sr-Ca-Cu-O system. Superconductivity at 70 K was also observed in the same system. These observations may provide a new insight into the mechanism of oxide superconductivity.

We report measurements of the thermoelectric power of the high-temperature Tl-Ca-Ba-Cu-O superconductor. The data indicate a transition which is centered at 118 +- 3 K, in agreement with magnetic measurements and published resistivity results. The thermoelectric power is positive and, aside from the high transition temperature, is remarkably similar in magnitude and temperature dependence to that of the rare-earth-based oxide superconductors which have been reported previously.

Superconductivity at 80--90 K has been resistively and magnetically observed in the R-Tl-Sr-Ca-Cu-O systems (R represents rare earths including Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and excepting Ce) in which five metal elements all are required for the superconductivity. In particular, the Pr-Tl-Sr-Ca-Cu-O system and Tb-Tl-Sr-Ca-Cu-O system are the first superconducting systems above 77 K in which Pr or Tb are required for the superconductivity. We believe that even higher critical temperatures may be achieved by optimizing the starting composition and preparation conditions, and/or by further elemental substitutions.

Use of space-charge-limited current (SCLC) to observe defect annealing in neutron-irradiated silicon. From the observation of the trap-filled-limit voltage a deep-trap concentration in the irradiated unannealed crystals is found to be at least 1.4 x 10 to the 12th power per cu cm. Shallow traps are also observed. This deep-trap concentration decreases with anneal temperature up to 200 C, above which SCLC can no longer be observed. There is evidence that at least some of this decrease is due to the conversion of the deep traps to shallow traps.