首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 15 毫秒
1.
Bulk material of Nb3 (Ge0.8Nb0.2) with A15 structure and a superconducting transition temperature Tc of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in Tc up to 16.2 K. With Si ions only a Tc of 13 K was obtained, with Ar and O ions Tc remained below 9 K. From X-ray measurements carried out on high Tc Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant a0 of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in a0 is stronger than expected from the transition temperature of the implanted surface layer.  相似文献   

2.
The specific heat of A-15 Nb3Al.83Ge.21 with a transition temperature, Tc, of 20.0 K has been measured from 6 to 25 K. Values of the parameters derived from the data are γ=35.0±2 mJ/mole-K2; the Debye temperature, θD=278±5 K; the energy gap, △, normalized as 2△/kTc, is 4.9±.3, and the thermodynamic critical field, Hc(0), is 4700±300 Gauss.  相似文献   

3.
We have performed Extended X-ray Absorption Fine Structure (EXAFS) studies of the high-Tc ternary superconducting alloy Nb3Ge0.26Al0.74 above the Ge K-edge. The EXAFS indicates that the local environment surrounding the Ge atoms is characterized by a high degree of structural order, similar to that found in previous investigations of high-Tc Nb3Ge thin films. We also present direct “microscopic” structural evidence that this material is a solid solution of Nb3Ge and Nb3Al. No evidence of Nb3Ge microcrystals is observed.  相似文献   

4.
Heavy ion irradiation of A15 Mo3Ge with the low transition temperature Tc = 1.45 K raises Tc to ? 6 K. For the first time in A15 compounds a Tc degradation (≈ 3 K) after having passed through a maximum is observed until near 3.5 K the saturation value is reached. The effects on Tc are interpreted by variations of the deduced (H'c2, ?) density of states at the Fermi level. This picture consistently explains the Tc degradation of the high-Tc A15 compounds.  相似文献   

5.
The quadrupole 209Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi4Ge3O12 single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T 1 in pure and doped samples. Unlike T 1, the spin–spin relaxation time T 2 for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi4Ge3O12 with paramagnetic atoms strongly elongated T 2. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (~30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T 1 in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi4Ge3O12 crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.  相似文献   

6.
The superconducting transition temperature (Tc) of amorphous Nb3Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes Tc to initially decrease, Tc is found to increase under higher quasihydrostatic pressures. Tc(p) was also studied on an A-15 crystalline Nb3Ge sample obtained from the amorphous sample by annealing.  相似文献   

7.
The polarization switching in sinusoidal fields and the pyroelectric properties of Li2?x Na x Ge4O9 (0.2 ≤ x ≤ 0.3) crystals are measured in the temperature range T c ?T ≤ 40 K. The behavior of the P?E hysteresis loops with variations in temperature is investigated for crystals with phase transition temperatures T c < 300 K and T c > 300 K. It is shown that, for crystals with phase transition temperatures T c < 300 K, the temperature dependence of the hysteresis loop exhibits a behavior typical of crystals with second-order phase transitions. The crystals with phase transition temperatures T c > 300 K are characterized by double hysteresis loops in the temperature range T c ?T 1 ≈ 30 K. The correlation between the polarization properties and possible structural transformations of the Li2?x Na x Ge4O9 crystals due to the change in the concentration ratio of Na and Li ions is discussed.  相似文献   

8.
The effect of high energy neutron irradiation (E>1 MeV) on the superconducting transition temperature, Tc, of the A-15 compound Mo3Os is reported. Tc was found to decrease with increasing neutron dose, but at a rate considerably less than observed in other A-15 compounds composed of non-transition metals. The results lend support to the idea that the effect of ordering on Tc is smaller for A-15 compounds composed only of transition metals than those composed of transition and non-transition metals.  相似文献   

9.
The temperature dependence of the LO-TO splitting of the A-modes of Pb5Ge3O11 between room temperature and Tc=450K was determined from the Raman spectrum. The finite splitting of two A-modes above Tc leads to the conclusion that the space group of Pb5Ge3O11 in the paraelastic phase deviates from P6 which was determined by X-ray and other physical methods.  相似文献   

10.
Summary Dilute57Fe M?ssbauer-spectroscopy studies of RMn2X2 (X=Si and/or Ge, R=La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the following results: Fe in RMn2X2 does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields (∼100 kOe). The compounds LaMn2Si2, LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, NdMn2Ge2 and SmMn2Ge2, known to be ferromagnets withT c=300–350 K, are antiferromagnetically ordered above their correspondingT c. TherT N values extend from 385 K (SmMn2Ge2) to 470 K (LaMn2Si2). At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In RMn2Si2−x Ge x with intermediatex values, the Ge is much more dominant in determining the magnetic properties of the Mn sublattice. While PrMn2Si2 is an antiferromagnet (T N=365 K) and PrMn2Ge2 is a ferromagnetantiferromagnet (T c=328 K,T N=415 K), we find that in PrMn2SiGe, the magnetic behaviour is similar to that in pure PrMn2Ge2,T c=305 K andT N=395 K. Paper presented at ICAME-95, Rimini, 10–16 September 1995.  相似文献   

11.
Sputtered films of nominal composition Nb3Ge deposited onto hot substrates, of the type recently found by Gavaler to have superconductivity onsets at 22.3° K, have now been obtained with a maximum onset temperature of 23.2 ± 0.2° K and a transition width of ~ 1.2° K. The sputtering conditions necessary to achieve these results are discussed and suggest that the high Tc phase may be detrimentally affected by the presence of high energy particles present under normal sputtering conditions.  相似文献   

12.
The variations of the superconducting transition temperature (Tc) and of the lattice transformation temperature (TL) with composition are reported for the Chevrel-phase system CuxMo3S4 (1 ≤ × ≤ 2). A complex low temperature phase diagram was found, containing at least three distinct low temperature phases, each exhibiting its own characteristic Tc (11 K, 6.5 K, 4.5 K), TL (280 K - 125 K), and electronic transport behavior. Pressure-induced transformations between these three phases were observed at pressures below 25 kbar. Temperature-induced lattice distortion for compositions near Cu2Mo3S4 was found to increase the Tc above that (< 1.1 K) of the high temperature, higher symmetry rhombohedral phase. This is apparently the first time the latter behavior has been observed: in all previous studies of other materials, the opposite behavior was reported.  相似文献   

13.
Low temperature irradiation of thin films of Nb75Ge25 (Tc = 3.2 K) and Nb80Si20 (Tc = 4.7 K) with 20 MeV sulfur ions leads to an increase of Tc of about 0.5 K and a decrease of ? of about 1.5 to 3.5%. Annealing up to room temperature partly restores the initial values. Qualitatively the results can be explained by irradiation induced smearing of the structure factor, which is partially recovered by annealing.  相似文献   

14.
The magnetic and magnetocaloric properties of PrMn1.6Fe0.4Ge2around the ferromagnetic transitions T C inter ~ 230 K and T C Pr ~ 30 K have been investigated by magnetisation, 57Fe Mössbauer spectroscopy and electron paramagnetic resonance (EPR) measurements over the temperature range 5–300 K. The broad peaks in magnetic entropy around TC inter (intralayer antiferromagnetism of the Mn sublattice to canted ferromagnetism) and TC Pr (onset of ferromagnetic order of Pr sublattice in addition to ferromagnetically ordered Mn sublattice) are typical of second order transitions with maximum entropy values of -ΔS M ~ 2.0 J/kg K and -ΔS M ~ 2.2 J/kg K respectively for ΔB = 0–6 T. The EPR signal around T = 48 K of g value g ~ 0.8 is consistent with paramagnetic free ion Pr3?+?. Below TC Pr ~ 30 K the g value increases steadily to g ~ 2.5 at 8 K as saturation of the Pr3?+? ion is approached. The EPR measurements indicate additional effects in this system below T ~ 20 K with the appearance of EPR signals of low g value g ~ 0.6.  相似文献   

15.
I. I. Amelin 《JETP Letters》2002,76(3):185-188
Apparently, a two-dimensional CuO lattice is formed on the surface of copper oxide in the CuO-Cu interface. This lattice consists of Cu2+ and O1? ions, which form a narrow, partially filled two-dimensional band. In this case, local electron pairs (LEPs) can form in the oxygen subsystem as a result of the fulfillment of the Shubin-Vonsovskii conditions. A crude estimate of the formation temperature of LEPs gives T* ~ 10 4 K. At the concentration in the interface layer n~1.6×1020 cm?3 and the effective mass of carriers m* ~ m e, the onset temperature of Bose-Einstein condensation may take a value of T c ~ 1000 K. The estimate obtained for the temperature T c corresponds to the experimental value by an order of magnitude.  相似文献   

16.
Superconductivity and crystal structure data are reported for a series of ternary CuxMo3S4 samples with 0 ≤ x ≤ 1 which were prepared by anodic oxidation of Cu1.0Mo3S4. A phase with the composition Cu0.5Mo3S4 was found to have rhombohedral lattice parameters aR = 6.447(5) A? and αR = 93.74(4) ° and a superconducting critical temperature Tc = 5.8 K. The dependence of Tc on pressure for Cu0.5Mo3S4 has been measured between 0 and 19 kbar.  相似文献   

17.
The electrical properties of p-type Ge, Ge1−ySny, and Si0.09Ge0.882Sn0.028 samples grown on n-type Si substrates using ultra-high vacuum chemical vapor deposition have been investigated as a function of temperature. Degenerate parallel conducting layers were found in all Ge/Si, Ge1−ySny/Si, and Si0.09Ge0.882Sn0.028/Si samples, which are believed to be associated with dislocation defects at the interface produced by the lattice mismatch between the two materials. These degenerate conducting layers affect the electrical properties of all the thin epitaxial films. Additionally, temperature dependent Hall-effect measurements show that these materials exhibit a conductivity type change from p to n at around 370–435 K. The mobilities of these samples are generally lower than that of bulk Ge due to carrier scattering near the interface between the epitaxial layer and the Si substrate and also due to alloy scattering. Detailed behavior of temperature-dependent conductivity of these samples is also discussed.  相似文献   

18.
The compounds RRh2Ge2; and RRu2Ge2 were synthesized X-ray studies show that they have the expected ThCr2Si2; tetragonal-type structure Magnetization studies at 1 8–300 K, 151Eu Mossbauer studies at 4 l, 77 and 300 K and the crystallography studies show the following- All RRh2Ge2; like RRh2Si2; exhibit two magnetic phase transitions, one corresponding to the antiferromagnetic ordering of the local rare earth moments. TN = 8–90 K, the other corresponding to the itinerant electron ordering of the Rh sublattice. TM = 3–9 K The heavy rare earths in RRu2G2; order antiferromagnetically and undergo a spin-flop transition in a relatively low magnetic field, <10 kOe The light elements in RRu2Ge2; order in a ferromagnetic, somewhat unclear structure NdRu2Ge2, like NdRu2Si2, exhibits two peaks in the magnetization curves Again, the lower may correspond to itinerant electron ordenng or, alternatively, to spin reorientation of the rare earth sublattice Eu in both EuRh2Ge2 and EuRu2Ge2 is divalent, whereas Ce in both CeRh2Ge2 and CeRu2Ge2 is trivalent For all rare earths the ordenng transition in RRh2Ge2 is higher than in RRu2Ge2. This fact can be associated with the smaller R-R distances in RRh2Ge2 and/ or due to the stronger magnetic character of the Rh 4d conduction electrons Companson of the magnetic properties and 151Eu hyperfine interactions of Eu2+Rh2Ge2, Eu2+Ru2Ge2, Eu2+Rh2Si2 and Eu3+Ru2Si2 with all the other systems leads to the conclusion that the conduction electrons play the dominant role in determining the magnetic properties of these systems Crystal-field effects are also of considerable importance, since the Mossbauer studies yield for the second-order crystal-field parameter A02r24f〉 the huge values +385 and +282 K for EuRu2Ge2; and EuRh2Ge2, respectively The easy axis of magnetization in the Eu compounds is in the basal plane The large second-order crystal field predicts well the direction of the easy axis for all other rare earths No superconductivity has been observed in any of the compounds, down to 1 8 K A companson of the magnetic properties of the germanides with those of the silicides shows great similanties, the differences being accounted for by the different unit cell sizes and c/a ratios.  相似文献   

19.
The reflectance, the optical density, and the optical conductivity spectra of CaMn1 ? x Mo x O3 (x ≤ 0.07) polycrystals are studied over a wide spectral region. The substitution of Mo6+ ions for Mn4+ ions leads to a strong decrease in the first high-energy phonon band, which is related to vibrations in the MnO6 octahedron. The appearance of band-type charge carriers is detected in CaMn1 ? x Mo x O3 with x ≤ 0.04, and carrier localization is observed in CaMn0.93Mo0.07O3. This localization increases at T < 160 K, i.e., during the transition from the paramagnetic to the antiferromagnetic state with orbital ordering and a monoclinic structure. The optical conductivity spectrum of CaMn0.93Mo0.07O3 exhibits a strong shift of fundamental absorption bands, which is associated with a change in the band structure because of a decrease in the splitting of the 3d levels of Mn in the crystal field.  相似文献   

20.
Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa2Cu3Ox, high-Tc material doped with 1% Gd, and Rb1C60, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd3+ longitudinal relaxation time T1, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below Tc = 56 K. Atx = 6.95 (Tc = 90 K), the T1 behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb1C60, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号