共查询到20条相似文献,搜索用时 31 毫秒
1.
R.M. Mueller U. Köbler K. Fischer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):207-216
For the ferromagnets EuS and GdMg, in which fourth-order exchange interactions (i.e. biquadratic, three-spin and four-spin interactions) have been identified, the deviation of the spontaneous magnetization
with respect to the T
=0 value is shown to follow a T2 law instead of the famous T3/2 law expected for a Heisenberg ferromagnet. Moreover, the observed T2 law holds for temperatures as large as 0.8TC and the extrapolated magnetization value for does not conform to ferromagnetic saturation. This is because the fourth-order exchange interactions generate a second order-parameter
which is assumed to govern the order of the transverse moment components. These moment components have a finite expectation
value for at the expense of the Heisenberg order parameter. Like the spontaneous magnetization, the critical field curves B
c
(
T
) of the metamagnet EuSe and the antiferromagnet EuTe also start decreasing with a T2 term for . It is argued that the T2 law is a consequence of the fourth-order exchange interactions. This is shown experimentally by a study of the critical field
curves [0pt] pertinent to the longitudinal (Heisenberg) order-parameter in the diamagnetically diluted antiferromagnets EuxSr1-xTe. In this solid solution series a particular composition of x
c
=0.85 exists at which the different fourth-order interaction processes compensate each other in the high temperature average. As
a consequence, an EuxSr1-xTe sample with x
=0.85 meets the requirements of a Heisenberg antiferromagnet at least if a quantity is considered for which the high-temperature
average over all fourth-order interactions is decisive. This seems to be the case for the critical field curve [0pt] which
gives the phase boundary to the paramagnetic phase. In fact, a crossover from a T2 to a T3/2 law is observed for [0pt] on approaching xc. This, we believe, shows the frequently observed T2 law is caused by the fourth-order interactions.
Received 23 July 1998 and Received in final form 12 October 1998 相似文献
2.
Abstract Electrical resistance measurements have been performed on samples of Hg1-xFexSe and Hg1-xMnxTe for × ≤ 0.30 as a function of pressure to 7.0 GPa. Both materials crystallize in the B3 structure at atmospheric pressure and room temperature and belong to the class of materials known as diluted magnetic semiconductors. At elevated pressures, HgTe undergoes a phase change from the B3 to B9 phase which is accompanied by an increase in the electrical resistivity of several orders of magnitude. The results reported here are that increasing × in both materials serves to increase the B3-B9 transition pressure. The implication being that partial replacement of the Hg-bonds by Fe-Se and Mn-Te, respectively, increases the stability range of the B3 phase 相似文献
3.
M. Abbate J.A. Guevara S.L. Cuffini Y.P. Mascarenhas E. Morikawa 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(2):203-208
We studied the changes in the electronic structure of SrTi1-xRuxO3 across the metal-insulator transition. The parent compound, SrTiO3, is a well known diamagnetic insulator; whereas the doped compound, SrTi1-xRuxO3, becomes a ferromagnetic metal above x
C
= 0.35. The techniques used in the study were photoemission (PES) and O 1
s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model
calculations. The PES and XAS spectra of SrTi1-xRuxO3 show the Ru 4
d bands growing in the band gap of SrTiO3 . The analysis in terms of the Hubbard model indicates that the Ti 3
d and Ru 4
d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals
embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major
role in the transition.
Received 10 September 2001 相似文献
4.
In2(Se1-xTex)3 polycrystalline films were prepared by a dual-source thermal evaporation technique. The depositions onto glass and SnO2-coated glass substrates were carried out in a vacuum chamber and followed by an annealing in neutral ambient (Ar or N2). The structural, morphological and compositional studies of the films were made by X-ray diffraction, energy-dispersive
X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman scattering and optical transmission.
Optimum conditions are investigated for the formation of the ternary compound In2(Se1-xTex)3 in order to tune the band gap by changing the Te concentration. The film properties as a function of Te amount are discussed.
It is shown that single-phase, textured and homogeneous layers of In2(Se1-xTex)3 can be grown with x≤0.2 at optimal deposition and heat treatment conditions. For x≅0.17 these films showed an energy band
gap of about 1.45 eV and an electrical conductivity at room temperature six orders of magnitude higher than that of the binary
γ-In2Se3 thin films.
Received: 9 July 1999 / Accepted: 25 November 1999 / Published online: 13 July 2000 相似文献
5.
W. Boujelben A. Cheikh-Rouhou J.C. Joubert 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):419-423
Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite
oxides Pr0.7Sr0.3-x
xMnO3 ( 0 ?
x
? 0.3) and Pr0.7-x
xSr0.3MnO3 ( 0 ?
x
? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties
in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite
structure and R
c space group for x
? 0.2 in strontium deficient samples and for x
? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations
show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements
versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium
deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient
ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for
x
= 0 to 90 K for x
= 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x
= 0 to 315 for x
= 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ?
x
? 0.1) in the strontium lacunar ones when the temperature decreases.
Received 12 April 2000 and Received in final form 8 January 2001 相似文献
6.
L. Zhang W. L. Zhong C. L. Wang Y. P. Peng Y. G. Wang 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):565-573
The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution BaxSr1-xTiO3 system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation
frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals
and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of
new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the
experimental results. For BaxSr1-xTiO3 system, the decrease of xcauses the decrease of the barrier height.
Received 3 August 1998 and Received in final form 22 November 1998 相似文献
7.
8.
X-ray diffraction, transmission electron microscopy and a laser-flash method were used to investigate the order–disorder transition and thermal conductivity of (Yb x Nd1? x )2Zr2O7 (0 ≤ x ≤ 1.00) solid solutions. The structures were found to be pyrochlore-type for 0 ≤ x ≤ 0.25, defect fluorite for 0.45 ≤ x ≤ 1.00 and a mixture of these at 0.30 ≤ x ≤ 0.40. The thermal conductivities of (Yb x Nd1? x )2Zr2O7 first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbNdZr2O7 has the lowest thermal conductivity due to the reduced cation mean free path at the compositional combination of equal molar Yb3+ and Nd3+ cations. 相似文献
9.
H. Chang N.X. Chen J.K. Liang G.H. Rao 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):55-60
The structural stability of La2Co17-xMx (M = Mn, Mo, Nb, Ti, V, Al, Cr, Ni and Si) based on the interatomic potential has been studied. The calculated site preference
of the third element M is found to be the 6c site, which is in agreement with the experiments. In the calculations, if the
crystal cohesive energy of La2Co16Mn is taken as the highest one in the crystallization of La2Co17-xMx, the lowest content x of the third element M (M = Mn, Mo, Nb, Ti and V) required to stabilize La2Co17-xMx, is near that found in the experiments. The differences of the cell parameters between the calculated and the experimental
values are less than 0.4%. The differences of the atomic parameters for Co (or M) between the calculated and the experimental
values are about or even smaller than 1%, and that of La is about 3%. Because the energies of La(Co1-xAlx)13 are lower than those of La2(Co1-xAlx)17, La2(Co1-xAlx)17 could not be formed in the experiments. In the calculations, with either a range of deformation of the structure or the reconstruction
of the initial structure La2Co17 from LaCo5, the same results including the cohesive energy curves and the crystallographic parameters can be retrieved after the action
of the interatomic potentials.
Received 1st November 2002 / Received in final form 17 February 2003 Published online 23 May 2003
RID="a"
ID="a"e-mail: hchang@aphy.iphy.ac.cn 相似文献
10.
O. Toulemonde F. Studer A. Barnabé A. Maignan C. Martin B. Raveau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):159-167
The XAS study at Cr, Co, Ni and Mn K-edges was performed for the doped CMR manganites Ln0.5Ca0.5Mn1-xBxO3 with Ln=La, Nd, Sm and B= Cr, Co, Ni (), on the samples that were studied previously for their ferromagnetic-metallic to antiferromagnetic-insulator transition.
We observed that the formal charges of the doping elements are Ni2+, Co2+ and Cr3+. It is also evidenced that the average formal charge of the manganese is increased after doping, in agreement with the charge
compensation keeping “O3” stoichiometry. These results suggest that the doping elements participate directly to the band structure.
Received: 9 January 1998 / Received in final form: 6 April 1998 / Accepted: 7 April 1998 相似文献
11.
J. Souletie Y. Tabata T. Taniguchi Y. Miyako 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):43-46
The usual scaling equations at a phase transition, employed out of their usual validity range, with Tc a negative constant, fit properties observed in correlated electron systems with unmatched accuracy. We illustrate this behavior
with our data in Ce(Ru1-xRhx)2Si2 for x
= 0.4 and 0.5 and comment these results and what they imply physically.
Received 22 June 1998 and Received in final form 29 September 1998 相似文献
12.
A. Takahashi H. Shiba 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):413-417
The optical absorption in ferromagnetic metal La1-xSrxMnO3 is anomalous; it has a wide-range absorption up to about 1 eV even at zero temperature. Since 3d electrons in La1-xSrxMnO3 partially fill doubly degenerate eg orbitals, the orbital degrees of freedom are crucial to understand this metallic system. We argue that the interband transition
within eg orbitals is important in the optical absorption. The optical spectrum is modified also by the inter-orbital Coulomb interaction.
We have examined perturbatively the effect of the Coulomb interaction on the spectrum. Available experiments are discussed
by comparing with the present results.
Received: 13 February 1998 / Accepted: 17 March 1998 相似文献
13.
A. Nossov J. Pierre J. Beille V. Vassiliev B. Slobodin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):467-472
The influence of hydrostatic pressure on the transport properties of (La1-xRx)0.67Ca0.33MnO3 (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature TC under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation
law, with a characteristic temperature T0 decreasing with pressure. The variation of TC with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La1-xRx)0.67Sr0.33MnO3 manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering
by the magnetic moment of the rare earth.
Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998 相似文献
14.
Materials with the formula Yb2-xAlxMo3O12 (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8) were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffraction, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3012 solid solutions crystallize in a single monoclinic phase for 1.7 〈 x 〈 2.0 and in a single orthorhombic phase for 0.0 〈 x 〈 0,4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhonlbic phase transition temperature of A12Mo3012 can be reduced by partial substitution of A13+ by Yb3+, and the Yb2-zAlxMo3012 (0.0 〈 x 〈 2.0) materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of A13+ for Yb3+ in Yb2Mo3012 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be -9.1 x 10-6/K, -5.5 x 10-6/K, 5.74 x 10-6/K, and 9.5 x 10 6/K for Ybl.sAlo.2(MoO4)3, Yb1.6Alo.4(MoO4)3, Ybo.4All.6(Mo04)3, and Ybo.2Al1.8(MoO4)3, respectively. 相似文献
15.
Band-edge optical spectra of Cd1−x
FexTe solid solutions differ substantially from those of undoped CdTe. The pattern of the change in photoluminescence spectra
with increasing Fe concentration is connected with a change in radiative recombination channels.
Fiz. Tverd. Tela (St. Petersburg) 40, 897–899 (May 1998) 相似文献
16.
I. Souza Azevedo E. Baggio Saitovitch M. Hücker B. Büchner H. Micklitz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):313-315
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K >
T
> 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.
Received: 23 June 1998 / Accepted: 14 July 1998 相似文献
17.
E.F. Wassermann B. Rellinghaus T. Roessel J. Kästner W. Pepperhoff 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):361-365
We measured the thermal expansion and the specific heat of TixFe100-x alloys with x
= 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn2) like TiFe2, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat cmag. For fixed composition and c
mag
(
T
) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from
moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of
atomic volume and moment for TiFe2 reveals.
Received: 26 January 1998 / Accepted: 17 April 1998 相似文献
18.
R. Turan B. Aslan O. Nur M.Y.A. Yousif M. Willander 《Applied Physics A: Materials Science & Processing》2001,72(5):587-593
We have studied the effect of the strain relaxation on the band-edge alignments in a Pt/p-Si1-xGex Schottky junction with x=0.14 by internal photoemission spectroscopy and current–voltage measurements. We have shown that
the variations in the band-edge alignments can be observed directly by measuring the optical and electrical properties of
a simple Schottky junction. The strain in the Si1-xGex layer has been partially relaxed by thermal treatments at two different temperatures. The degree of relaxation and other
structural changes have been determined by a high-resolution X-ray diffractometer. Both optical and electrical techniques
have shown that the barrier height of the Pt/Si0.86Ge0.14 junction increases with the amount of relaxation in the Si1-xGex layer. This shows that the valence-band edge of the Si1-xGex layer moves away from the Fermi level of the Pt/Si1-xGex junction. The band-edge movement results from the increase in the band gap of the Si1-xGex layer after the strain relaxation. This result agrees with the theoretical predictions for the strain-induced effects on
the Si1-xGex band structure.
Received: 18 October 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001 相似文献
19.
Abstract High-pressure X-ray diffraction using synchrotron radiation has been performed on UP1-x -Sx (X=0.1; 0.25; 0.4) up to 53 GPa UP1-x Sx is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP1-x Sx with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP1-x Sx 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur. 相似文献
20.
U. Köbler A. Hoser H.A. Graf M.-T. Fernandez-Diaz K. Fischer T. Brückel 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):217-224
We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection
intensities in the diamagnetically diluted antiferromagnetic compounds EuxSr1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently.
These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the
order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter.
The observed MnO-type diffraction intensities originate in the sublattice magnetizations, , of both order parameters. Due to the different composition dependencies for biquadratic interaction processes and three-spin interaction processes , the ferromagnetic three-spin interactions dominate for x
>
x
c
=0.85, while for x
<0.85 the antiferromagnetic biquadratic interactions dominate. Associated with this sign change in the fourth-order interaction
sum the transverse order parameter changes from the antiferromagnetic MnO type for x
<0.85 to ferromagnetic for x
>0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at x
c
=0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x
>0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain
this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic
superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting obeys a T2 law until a temperature as large as 0.75TN irrespective of the nature of the transverse order parameter. The T2 law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin
dynamics of both completely. From the linear composition dependence of the normalized T2 coefficient the existence of three-spin interactions is again confirmed.
Received 23 July 1998 and Received in final form 12 October 1998 相似文献