共查询到20条相似文献,搜索用时 31 毫秒
1.
L. Seetha Lakshmi K. Dörr K. Nenkov A. Handstein V. S. Sastry K.-H. Müller 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(2):115-121
We report the charge state modification effects at the Mn site on the ground
state properties of colossal magnetoresistive manganites. Ta5+
substitution results in an appreciable increase in the lattice parameters
and unit cell volume due to increased Mn3+ concentration. The
ferromagnetic-metallic ground state modifies to a cluster glass insulator
for
. The reduction in the transition temperatures with
increasing x is ∼39 K/at.%. Besides the modification of majority carrier concentration due to increased Mn3+ concentration and enhanced local structural effects, the local
electrostatic potential of the substituent seems to contribute to the
unusually strong reduction of the transition temperatures of the compounds.
Thermo magnetic irreversibility just below Curie temperature (Tc),
non-saturation of magnetization, two distinct magnetic transitions in ac
susceptibility in an appropriate static field: close to Tc and other at
low temperature (the spin freezing temperature (Tg)) and non-stationary
dynamics with a characteristic maximum in the magnetic viscosity close to
Tg confirm a cluster glass state for
. These results
find additional support from a linear low temperature magnetic specific heat
of x = 0.10 with a characteristic broad maximum close to Tg. 相似文献
2.
G.?Urbanik T.?H?nke C.?Hess B.?Büchner A.?Ciszewski V.?Hinkov C. T.?Lin B.?Keimer 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(4):483-489
We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7-
δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of
this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the
c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in
a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional
step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that
such fractional steps connect surfaces which differ significantly in their tunneling conductance. 相似文献
3.
A. Kondrat J. E. Hamann-Borrero N. Leps M. Kosmala O. Schumann A. K?hler J. Werner G. Behr M. Braden R. Klingeler B. Büchner C. Hess 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,70(4):461-468
We have prepared the newly discovered Fe-based superconducting material LaO1-xFxFeAs (0 ≤ x ≤ 0.2) in polycrystalline form and have investigated the samples by means of structural, thermodynamic and transport
measurements. Our investigations reveal a non superconducting phase at 0 ≤ x 0.04 which for x = 0 is characterized by a structural transition towards an orthorhombic distortion at Ts ≈ 160 K and antiferromagnetic spin order at TN ≈ 138 K. Both transitions lead to strong anomalies in various transport properties as well as in magnetization and in specific
heat. Remarkably, the transition temperatures are only weakly doping dependent up to x ≈ 0.04. However, the transitions are
abruptly suppressed at x ≥ 0.05 in favour of a superconducting phase with a critical temperature Tc
20 K. Upon further increasing the F-doping Tc increases up to a maximum of Tc = 26.8 K at x = 0.1 which is followed by a decrease down to Tc ≈ 10 K at x ≥ 0.15. 相似文献
4.
N. Watanabe N. Koshizuka 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(1):29-33
We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y
1-y
Ca
y
Ba
2
Cu
4
O
8
system (
y
= 0 - 0.15). The phonon anomalies above T
c
of the frequencies are observed for the out-of-phase O(2)-O(3) A
g
and O(4) A
g
modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors
associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba
phonon frequency at in the undoped samples and its energy increases with Ca doping.
Received 9 June 1999 相似文献
5.
N. V. Anshukova B. M. Bulychev A. I. Golovashkin L. I. Ivanova A. A. Minakov A. P. Rusakov 《Physics of the Solid State》2003,45(7):1207-1212
The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB2 were measured at low temperatures and in the neighborhood of T c . In addition to the well-known superconducting transition at T c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB2 was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB2 of a second group of carriers and its transition to the superconducting state at Tc2≈10?12 K. 相似文献
6.
N. V. Anshukova B. M. Bulychev A. I. Golovashkin L. I. Ivanova I. B. Krynetskii A. A. Minakov A. P. Rusakov 《Journal of Experimental and Theoretical Physics》2003,97(1):70-77
We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB2) in the vicinity of Tc and at lower temperatures. It was established that MgB2, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at Tc≈40 K, MgB2 exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB2 and to an increase in the density of the Bose condensate corresponding to these carriers at Tc2≈10–12 K. 相似文献
7.
We examine by molecular dynamics simulations the relaxation of polymer-solvent mixtures close to the glass transition. The
simulations employ a coarse-grained model in which polymers are represented by bead-spring chains and solvent particles by
monomers. The interaction parameters between polymer and solvent are adjusted such that mixing is favored. We find that the
mixtures have one glass transition temperature T
g or critical temperature T
c of mode-coupling theory (MCT). Both T
g and T
c (> T
g decrease with increasing solvent concentration . The decrease is linear for the concentrations studied (up to = 25%. Above T
c we explore the structure and relaxation of the polymer-solvent mixtures on cooling. We find that, if the polymer solution
is compared to the pure polymer melt at the same T, local spatial correlations on the length scale of the first peak of the static structure factor S(q) are reduced. This difference between melt and solution is largely removed when comparing the S(q) of both systems at similar distance to the respective T
c. Near T
c we investigate dynamic correlation functions, such as the incoherent intermediate scattering function (t), mean-square displacements of the monomers and solvent particles, two non-Gaussian parameters, and the probability distribution
P(ln r;t) of the logarithm of single-particle displacements. In accordance with MCT we find, for instance, that (t) obeys the time-temperature superposition principle and has relaxation times which are compatible with a power law increase close (but not too close) to T
c. In divergence to MCT, however, the increase of depends on the wavelength q, small q values having weaker increase than large ones. This decoupling of local and large-length scale relaxation could be related
to the emergence of dynamic heterogeneity at low T. In the time window of the relaxation an analysis of P(ln r;t) reveals a double-peak structure close to T
c. The first peak correponds to “slow” particles (monomer or solvent) which have not moved much farther than 10% of their diameter
in time t, whereas the second occurs at distances of the order of the particle diameter. These “fast” particles have succeeded in leaving
their nearest-neighbor cage in time t. The simulation thus demonstrates that large fluctuations in particle mobility accompany the final structural relaxation
of the cold polymer solution in the vicinity of the extrapolated T
c. 相似文献
8.
A. E. Karkin S. V. Naumov B. N. Goshchitskii A. M. Balbashov 《Journal of Experimental and Theoretical Physics》2005,100(6):1142-1152
The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr2RuO4 single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρa) and that along the c axis (ρc), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (H ∥ a and H ∥ c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρa(T) and ρc(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (~10m e , where m e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρa and ρc. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρa, c(T) ∝ 1/T type. 相似文献
9.
L. I. Yakovenkova L. E. Kar’kina O. A. Elkina 《Bulletin of the Russian Academy of Sciences: Physics》2009,73(7):935-938
The transmission electron microscopy was used to examine the dislocation structure of intermetallic Ti3Al after deformation at temperatures T = 1073–1273 K. It is established that its microstructure contains mobile 2c + a and superdislocations. Possible models describing the destruction of barriers associated with 2c + a superdislocations in pyramidal planes are discussed using the results of computer simulation of the superdislocation core structure in Ti3Al. 相似文献
10.
For a given crystal structure, say body-centred-cubic, the many-body Hamiltonian H in which nuclear and electron motions are to be treated from the outset on the same footing, has parameters, for the elements, which can be classified as (i) atomic mass M, (ii) atomic number Z, characterizing the external potential in which electrons move, and (iii) bcc lattice spacing, or equivalently one can utilize atomic volume,
. Since the thermodynamic quantities can be determined from H, we conclude that T
c
, the superconducting transition temperature, when it is non-zero, may be formally expressed as T
c
=
. One piece of evidence in support is that, in an atomic number vs. atomic volume graph, the superconducting elements lie in a well defined region. Two other relevant points are that (a) T
c
is related by BCS theory, though not simply, to the Debye temperature, which in turn is calculable from the elastic constants C
11, C
12, and C
44, the atomic weight and the atomic volume, and (b) T
c
for five bcc transition metals is linear in the Cauchy deviation C
* = (C
12 - C
44 )/(C
12 + C
44 ). Finally, via elastic constants, mass density and atomic volume, a correlation between C
* and the Debye temperature is established for the five bcc transition elements.Received: 13 May 2004, Published online: 23 July 2004PACS:
74.62.-c Transition temperature variations - 74.70.Ad Metals; alloys and binary compounds 相似文献
11.
The anionic conductivity of HoF3 single crystals with a β-YF3 structure (orthorhombic crystal system, space group Pnma) is investigated over a wide range of temperatures (323–1073 K). The unit cell parameters of HoF3 crystals are as follows: a=0.6384±0.0009 nm, b=0.6844±0.0009 nm, and c=0.4356±0.0005 nm. It is revealed that the conductivity anisotropy of the HoF3 crystals is insignificant over the entire temperature range covered. The crossover from one mechanism of ion transfer to another mechanism is observed near the critical temperature Tc≈620 K. The activation enthalpy of electrical conduction is found to be ΔH1=0.744 eV at T<Tc and ΔH2=0.43 eV at T>Tc. The fluorine vacancies are the most probable charge carriers in HoF3 crystals. The fluorine ionic conductivities at temperatures of 323, 500, and 1073 K are equal to 5×10?10, 5×10?6, and 2×10?3 S cm?1, respectively. 相似文献
12.
The parameters of the long-wavelength exciton band for Rb2CdI4 films are investigated in the temperature range 90–410 K. It is found that the Rb2CdI4 films undergo a sequence of phase transitions at temperatures Tc1=380 K (paraphase → incommensurate phase), Tc2=290 K (incommensurate phase → ferroelastic phase I), and Tc3 = 210 K (ferroelastic phase I → ferroelastic phase II). The parameters of the exciton band (such as the spectral position and the half-width) measured during heating and cooling of the Rb2CdI4 film differ significantly. This is especially true for the incommensurate phase. Upon heating of the incommensurate phase, the domain boundaries become frozen, whereas the cooling of this phase is accompanied by the generation of solitons and their pinning, which, in turn, results in a first-order phase transition at the temperature Tc2. It is revealed that the oscillator strength of the exciton band anomalously increases in the range of existence of commensurate phase I (Tc3<-T<-Tc2) due to ordering of the Rb2CdI4 crystal lattice. 相似文献
13.
A. Pashkin K. Thirunavukkuarasu Y.-L. Mathis W. Kang C. A. Kuntscher 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(4):285-290
We present measurements of the infrared response of the
quasi-one-dimensional organic conductor (TMTSF)2FSO3 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 FSO3 anion is
proposed, in order to explain the anomalous behavior of this new
mode. 相似文献
14.
M. van Leeuwen 《The European Physical Journal C - Particles and Fields》2009,61(4):569-574
We present selected recent results of multi-hadron correlation measurements in azimuth and pseudorapidity at intermediate
and high p
T
in Au+Au collisions at
, from the STAR experiment at RHIC. At intermediate p
T
, measurements are presented that attempt to determine the origin of the associated near-side (small Δφ) yield at large pseudo-rapidity difference Δη that is found to be present in heavy ion collisions. In addition, results are reported on new multi-hadron correlation measures
at high-p
T
that use di-hadron triggers and multi-hadron cluster triggers with the goal to constrain the underlying jet kinematics better
than in the existing measurements of inclusive spectra and di-hadron correlations. 相似文献
15.
In high temperature (high T
c
) cuprate superconductors the gap in the electronic density of states is not fully filled at T
c
; it evolves into a partial (pseudo)gap that survives way beyond T
c
, challenging the conventional views. We have investigated the pseudogap phenomenon in the field-temperature (H-T) diagram of Bi2Sr2CaCu2O8 + y
over a wide range of hole doping (
). Using interlayer tunneling transport in magnetic fields up to 60 T to probe the density-of states (DOS) depletion at low excitation energies we mapped the pseudogap closing field H
pg
. We found that H
pg
and the pseudogap onset temperature T
* are related via a Zeeman relation
, irrespective of whether the magnetic field is applied along the c-axis or parallel to CuO2 planes. In contrast to large anisotropy of the superconducting state, the field anisotropy of H
pg
is due solely to the g-factor. Our findings indicate that the pseudogap is of singlet-spin origin, consistent with models based on doped Mott insulator.Received: 2 February 2004, Published online: 10 August 2004PACS:
74.25.Dw Superconductivity phase diagrams - 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.) - 74.72.Hs Bi-based cuprates 相似文献
16.
G. M. Abramova A. M. Vorotynov G. A. Petrakovskii N. I. Kiselev D. A. Velikanov A. F. Bovina R. F. Al’mukhametov R. A. Yakshibaev É. V. Gabitov 《Physics of the Solid State》2004,46(12):2225-2228
Electrical, resonant, and magnetic properties of intercalated copper chromium disulfide CuCrS2 are studied. It is established that CuCrS2 is an antiferromagnetic semiconductor with Néel temperature TN=40.7 K and an effective magnetic moment of 4.3µB. Anomalies in the electrical, magnetic, and resonant properties of CuCrS2 are found at Tc=110 K, which suggest an electron transition accompanied by alteration of the valences of the 3d-metal ions. 相似文献
17.
We discuss one of the most prominent features of the very recent preliminary elliptic flow data of J/ψ-mesons from the PHENIX Collaboration (PHENIX Collaboration (C. Silvestre), arXiv:0806.0475 [nucl-ex]). Even within the
rather large error bars of the measured data a negative elliptic flow parameter (v2) for J/ψ in the range of p
T = 0.5-2.5 GeV/c is visible. We argue that this negative elliptic flow at intermediate pT is a clear and qualitative signature for the collectivity of charm quarks produced in nucleus-nucleus reactions at RHIC.
Within a parton recombination approach we show that a negative elliptic flow puts a lower limit on the collective transverse
velocity of heavy quarks. The numerical value of the transverse flow velocity for charm quarks that is necessary to reproduce the data is (charm) ∼ 0.55-0.6c and therefore compatible with the flow of light quarks. 相似文献
18.
Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < T c (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state T c < T < 400 K for optimally oxygen-doped samples of YBa2Cu3O6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties. 相似文献
19.
K. Terasaki 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):676-678
Although assigning D
s0
+(2317) to the I
3 = 0 component
of iso-triplet four-quark mesons is favored by experiments, its neutral and doubly charged partners have not yet been observed.
It is discussed why they were not observed in inclusive e
+
e
-→cˉ experiment and that they can be observed in B decays. 相似文献
20.
Results from RHIC have shown that there is an enhanced baryon/meson ratio in the intermediate transverse momentum range (2<pT<6 GeV/c) in Au+Au collisions at both =130 and 200 GeV. This was initially demonstrated by measurements of the p̄/π- ratio which was then extended in pT by the Λ/K0
S ratio. The data were successfully described by models utilising different hadronization mechanisms: those having recombination
of quarks and others having an interplay between flow, jet quenching and incorporating baryon junction loops. The strange
particle data from the first Au+Au run at =200 GeV gave tantalising hints that the observed enhancement of baryons compared to mesons was diminished by a pT of 6 GeV/c, but a lack of statistics in this range made a definitive statement impossible. Here we present an extended analysis
of identified strange baryons and mesons in Au+Au collisions at =200 GeV using data obtained by the STAR experiment from the 2004 running period. The increase in statistics extends the
measurement of Λ hyperons out to at least 7 GeV/c and K0
S mesons out to 9 GeV/c. This data allows us to place limits on the range where in-vacuum fragmentation functions are applicable
and the effect of baryon dominance is reduced. We also discuss the prospects for making these measurements using multiply-strange
baryons and mesons (Ω and ϕ). 相似文献