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1.
Previous work on Ba 2TiGe 2O 8 crystals has shown an unusual low-temperature (~ 223 K on cooling, ~ 273 K on heating) phase transition. Precession x-ray photographs on Ba 2TiGe 2O 8 single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase. 相似文献
2.
We studied the changes in the electronic structure of SrTi 1-xRu xO 3 across the metal-insulator transition. The parent compound, SrTiO 3, is a well known diamagnetic insulator; whereas the doped compound, SrTi 1-xRu xO 3, 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 SrTi 1-xRu xO 3 show the Ru 4
d bands growing in the band gap of SrTiO 3 . 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
3.
A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2 ↦hcp-1 in pure C 70 and in C 70-rich mixed crystal C
70(1 - x)
C 60x. With the specific anisotropic pseudospin interactions adapted to the C 70 crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the
model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases.
The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment
shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C
70(1 - x)
C 60x are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a
range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement
with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis.
Received 20 April 2001 and Received in final form 26 September 2001 相似文献
4.
The Rhodes-Wohlfarth parameter extended to ferroelectrics by Tokunaga [J. Phys. Soc. Jap. 57, 4275 (1988)] is here analyzed within the model. It is shown that it can be directly related with the displacive degree of the transition as described by the ratio
C
/
E
0
, between the non-local coupling, C, driving the transition and the depth of the energy well, E 0, associated with the distorted structure. However, the Rhodes-Wohlfarth parameter becomes asymptotically constant as C
/
E
0
decreases, i.e. for systems closer to the order disorder limit. Under this viewpoint, the very limited range of values observed for this
experimental parameter is explained and is shown that, in general, it can only assess quantitatively the character of the
transition in rather displacive cases. The argument can be generalized to more complex systems, and when applied to well known
materials, a rough estimation of the displacive degree and the relevant microscopic energetic parameters in rather displacive
ferroelectrics is possible.
Received 23 December 1998 and Received in final form 4 May 1999 相似文献
5.
Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite
oxides Pr 0.7Sr 0.3-x
xMnO 3 ( 0 ?
x
? 0.3) and Pr 0.7-x
xSr 0.3MnO 3 ( 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.
The superconducting and magnetic properties of Nb/Pd 1-xFe x/Nb triple layers with constant Nb layer thickness d Nb=200 ? and different interlayer thicknesses 3 ?≤ d PdFe ≤ ? are investigated. The thickness dependence of the magnetization and of the superconducting transition temperature shows
that for small iron concentration x the Pd 1-xFe x layer is likely to be in the paramagnetic state for very thin films whereas ferromagnetic order is established for x ≥ 0.13.
The parallel critical field Bc2||( T){B_{c2||}}(T) exhibits a transition from two-dimensional (2D) behavior where the Nb films are coupled across the interlayer, towards a
2D behavior of decoupled Nb films with increasing d PdFeand/or x. This transition allows a determination of the penetration depth x F{\xi _F} of Cooper pairs into the Pd 1-xFe x layer as a function of x. For samples with a ferromagnetic interlayer x F{\xi _F} is found to be independent of x. 相似文献
7.
Photofragmentation of Fe(H 2O) n
+
clusters ( n
=1-9) is investigated at three different wavelengths, 532, 355 and 266 nm. Two fragmentation pathways are observed depending essentially
on the photon energy, but also on the parent size n. The fragmentation products belong to two ion families, Fe(H 2O) m
+
and FeOH(H 2O) m
+
, which correspond to dehydration and intracluster dehydrogenation reactions respectively. The ion yields are studied as a
function of the laser fluence in order to determine the number of photons implied in the photofragmentation process. This
allows us to estimate that the D[(H 2O) n-1Fe +-(H 2O)] bond energy is ranging between 0.44 eV and 0.55 eV for .Photon absorption cross sections are also derived from the fluence experiments, and two different behaviors are observed:
i) At 355 nm, far away from any transition, progressive solvation of the metal ion results in an increasing absorption cross section from n
=2 to n
=9. This can be attributed to a forbidden transition of bare , which becomes progressively allowed because of the interaction with more and more water ligands. ii) At 266 nm, close to
several allowed transitions of bare , a distinct maximum is observed for the absorption of ion. It may be attributed to a change in the spin multiplicity when switching from and on one hand to Fe(H 2O) on the other.
Received: 11 November 1997 / Revised: 18 February 1998 / Accepted: 22 April 1998 相似文献
8.
The phase front during the 218 K transition in KD 2PO 4 crystals under a thermal gradient perpendicular to the c ferroelectric axis is observed to have a factory-roof shape. This shape is studied versus the magnitude of G e in samples cut with faces in (100), (010), (001) planes or in (
0), (110), (001) ones. A geometric approach as well as the calculation of the elastic-strain energy caused by lattice misfits
along the phase front demonstrate the incoherent interface nature of the phase front. Furthemore, the results and their interpretation
allow to predict the sign of the lattice deformation u
xx
( > 0).
Received 25 April 2002 Published online 29 November 2002 相似文献
9.
We measured fluorescence lifetimes and fluorescence spectra (excitation and emission) of tryptophan residues of α 1-acid glycoprotein (three Trp residues) and β-lactoglobulin (two Trp residues) in absence and presence of 450 μM progesterone.
Progesterone binds only to α 1-acid glycoprotein. In absence of progesterone, each of the two proteins displays three fluorescence lifetimes. Addition of
progesterone induces a partial inhibition of the S
o
→
1
L
a
transition without affecting fluorescence lifetimes. The same experiments performed in presence of denatured proteins in
6 M guanidine show that addition of progesterone inhibits partially the S
o
→
1
L
a
transition and its peak is 15 nm shifted to the red compared to that obtained for native proteins. However, the S
o
→
1
L
b
transition position peak is not affected by protein denaturation. Thus, the tertiary structure of the protein plays an important
role by modulating the tryptophan electronic transitions. Fluorescence emission decay recorded in absence and presence of
progesterone yields three fluorescence lifetimes whether proteins are denatured or not. Thus, protein tertiary structure is
not responsible for the presence of three fluorescence lifetimes. These characterize tryptophan substructures reached at the
excited states and which population (pre-exponential values) depend on the tryptophan residues interaction with their microenvironment(s)
and thus on the global conformation of the protein. 相似文献
10.
The oxygen deficient cobaltites LnBaCo 2O 5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T
N
∼340 K and at T
CO
∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions
(spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co 2+ and Co 3+ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by
the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also
confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements.
From the refined saturated magnetic moment values only, ∼3.7
and ∼2.7
, the electronic configuration of the Co ions in LnBaCo 2O 5 remains conjectural. Two pictures, with Co 3+ ions either in intermediate spin state ( t
5
2g
e
1
g
) or in high spin state ( t
4
2g
e
2
g
), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the
qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo 2O 5 (Ln = Tb, Dy, Ho).
Received 13 December 2000 相似文献
11.
A Cr 3+ EPR study of lithium heptagermanate crystals, Li 2Ge 7O 15 (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG
have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the
vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T
C
as a result of emergence of two structurally nonequivalent positions for Cr 3+ ions.
Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998) 相似文献
12.
ABSTRACTThe almost multiferroic perovskite EuTiO 3 (ETO) has been prepared as films on substrates of SrTiO 3. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at TN = 5.1 K and a structural transition at TS = 282 K . The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5 eV and larger than observed before for any bulk and thin film samples. 相似文献
13.
We present measurements of the infrared response of the
quasi-one-dimensional organic conductor (TMTSF) 2FSO 3 along
( E
) and perpendicular ( E
) to the
stacking axis as a function of temperature. Above the
metal-insulator transition related to the anion ordering the
optical conductivity spectra show a Drude-like response. Below the
transition an energy gap of about 1500 cm -1 (185 meV) opens,
leading to the corresponding charge transfer band in the optical
conductivity spectra. The analysis of the infrared-active
vibrations gives evidence for the long-range crystal structure
modulation below the transition temperature and for the
short-range order fluctuations of the lattice modulation above the
transition temperature. We also report about a new infrared mode
at around 710 cm-1 with a peculiar temperature behavior, which
has so far not been observed in any other (TMTSF) 2X salt
showing a metal-insulator transition. A qualitative model based on
the coupling between the TMTSF molecule vibration and the
reorientation of electrical dipole moment of the FSO 3 anion is
proposed, in order to explain the anomalous behavior of this new
mode. 相似文献
14.
An alternative model has been proposed for the phase transition from cubic argentite ß-Ag2S to monoclinic acanthite α-Ag2S in silver sulfide as a disorder–order transition. It has been shown that, as the temperature decreases below the transition temperature Ttrans, S atoms equiprobably occupying the sites of the body centered cubic (bcc) nonmetal sublattice of argentite are concentrated at four sites of the monoclinic nonmetal sublattice, whereas the other sites remain vacant. A disorder-order transition channel including three superstructure vectors of k9 and k4 stars has been determined. The distribution function of sulfur atoms in monoclinic acanthite α-Ag2S has been calculated. It has been shown that displacements of sulfur atoms distort the bcc nonmetal sublattice of argentite, forming a monoclinic lattice, where silver atoms are spaced by quite large distances and occupy their crystallographic positions with a probability of 1. The region of allowed values of the long-range order parameters η9 and η4 for the model monoclinic ordered phase α-Ag2S has been determined. 相似文献
15.
It is found that YBa 2Cu 3O 6+x
samples subjected to a load absorb oxygen. The phenomenon is associated with a load-induced shift in the phase transition
point. In addition, this shift increases the chemical potential of the oxygen atoms in the loaded sample and is a reason for
the increased compressibility of the material near the phase transition.
Fiz. Tverd. Tela (St. Petersburg) 39, 240–242 (February, 1997) 相似文献
16.
Polycrystalline two-layered perovskite La 2.5-xK 0.5+xMn 2O
7 + δ
( 0 <
x
< 0.5) samples have been prepared by a modified sol-gel method and their magnetoresistance and magnetocaloric effects have been
studied. A large deviation between the metal-insulator (MI) transition temperature ( T
ρ
) and the magnetic transition temperature (T C) is observed. Large magnetoresistance (MR) effects with Δρ/ρ
of ∼40% at 12 kOe are obtained in wide temperature ranges. The maximum of the magnetic entropy change peaks at its Curie temperature
(T C), far above its MI transition temperature ( T
ρ
). The large magnetic entropy change ( ∼1.4 J/kg.K) is obtained in the sample La 2.5-xK 0.5+xMn 2O
7 + δ
( x
= 0.35) upon 10 kOe applied magnetic field.
Received 2 May 2002 / Received in final form 1st October 2002 Published online 19 December 2002
RID="a"
ID="a"e-mail: wzhong@ufp.nju.edu.cn 相似文献
17.
The temperature-induced spin crossover of iron(II) in the [
Fe
(
tpa
)(
NCS
)
2
] complex has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS), extended X-ray absorption
fine structure (EXAFS) spectroscopy, conventional M?ssbauer spectroscopy (MS) and by measurements of the magnetic susceptibility
(SQUID). The various measurements consistently show that the transition is complete and abrupt and exhibits a hysteresis between
102 and 110 K. The dependence of the hyperfine parameters of the high-spin (HS) and of the low-spin (LS) phase on temperature
is gradual while the effective thickness (determined by the Lamb-M?ssbauer factor f
LM
) shows a step at the transition temperature. This step could be identified clearly because the effective thickness is measured
directly by NFS. The Lamb-M?ssbauer factor, the Debye temperature and the mean-square displacement of iron(II) could be determined
for the HS and for the LS phase. When comparing the NIS data with the results from density functional theory (DFT), the Fe-N
stretching vibrations of both LS and HS phases could be unambiguously identified and the f
LM
could be factorized for both phases into a lattice and a molecular part. The structural information from EXAFS and DFT geometry
optimization are in reasonable agreement.
Received 19 June 2001 相似文献
18.
Raman spectra are presented for the single bonded dimeric fullerene (C
60
-
)
2
and compared to optical spectra and Raman spectra of the isostructural and isoelectronic heterofullerene (C 59N) 2. The spectra of both materials exhibit strong correlations with respect to splitting, line position, and line intensity.
This holds for non resonant excitation with blue and green lasers as well as for the strong resonances observed with red lasers.
The latter observation is consistent with a downshift for the electronic transition energies as compared to C 60. The absorption edge of thin films of (C 59N) 2 was found at 1.4 eV. The three intercage modes were observed at 82, 103, and 111, and at 88, 98, and 106 cm -1 for (C 59N) 2 and (C
60
-
)
2
, respectively. A surprising difference was found for the position of the pentagonal pinch modes in the two materials as they
were observed at 1461 and at 1451 cm -1, for (C 59N) 2 and (C
60
-
)
2
, respectively. This is interpreted as a consequence of some characteristic differences in the electronic structure of the
two compounds.
Received 25 January 2000 and Received in final form 10 April 2000 相似文献
19.
CuGeO 3 exhibits a Spin-Peierls (SP) transition, at T
SP
= 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which
can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering
shows that in this 1D direction the correlation length follows the “universal” behaviour expected for the thermal fluctuations
of a real order parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field temperature,
T
SP
MF
, of about 60 K and invalidates any mean field scenario for the SP transition of CuGeO 3. As T
SP
MF
is as high as 4
T
SP
we propose that the 3D-SP order is achieved by the interchain coupling between 1D solitons which form below about 16-20 K.
CuGeO 3 being in the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of CuGeO 3 originate from the X-ray scattering on a very broad damped critical response of lower frequency than the “critical” phonon
modes. From the quantitative analysis of the 1D fluctuations we also estimate the microscopic parameters of the SP chain.
These parameters allow to locate CuGeO 3 close to the quantum critical boundary separating the gapped SP ground state to the ungapped anti-ferromagnetic ground state.
The vicinity of a quantum critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the importance
of the interchain coupling in the physics of CuGeO 3. Finally we compare these findings with those obtained for the organic SP systems (BCPTTF) 2PF 6, (TMTTF) 2PF 6 and MEM(TCNQ) 2. From a similar analysis of the pretransitional lattice fluctuations it is found that (BCPTTF) 2PF 6 and (TMTTF) 2PF 6 are located on the SP gapped classical-quantum boundary and are in the adiabatic regime where the fluctuations lead to the
formation of a pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ) 2 inside the SP quantum phase around the crossover line between the adiabatic and non-adiabatic regimes.
Received 13 September 2000 and Received in final form 6 February 2001 相似文献
20.
The magnetic properties of ferroborate single crystals with substituted compositions Nd 1 − x
Dy
x
Fe 3(BO 3) 4 ( x = 0.15, 0.25) with competing exchange Nd-Fe and Dy-Fe interactions are investigated. For each composition, we observed a
spontaneous spin-reorientation transition from the easy-axis to the easy-plane state and step anomalies on the magnetization
curves for the spin-flop transition induced by a magnetic field B | c. The measured parameters and effects are interpreted using a unified theoretical approach based on the molecular field approximation
and on calculations performed in the crystal-field model for the rare-earth ion. The experimental temperature dependences
of the initial magnetic susceptibility from T = 2 K to T = 300 K, anomalies on the magnetization curves for B | c in fields up to 1.8 T, and their evolution with temperature, as well as temperature and field dependences of magnetization
in fields up to 9 T are described. In the interpretation of experimental data, the crystal-field parameters in trigonal symmetry
for the rare-earth subsystem are determined, as well as the parameters of Nd-Fe and Dy-Fe exchange interactions. 相似文献
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