共查询到20条相似文献,搜索用时 15 毫秒
1.
R. L. Lichti T. R. Adams D. W. Cooke R. S. Kwok C. Boekema J. C. Lam D. E. Farrell N. Bansal 《Hyperfine Interactions》1991,63(1-4):73-79
Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa2Cu3O7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration
depth anisotropy are observed forB‖c andB
c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux
pinning characteristics are suggested as the fundamental cause of the additional effects. 相似文献
2.
V. A. Gasparov L. Drigo A. Audouard D. L. Sun C. T. Lin S. L. Bud’ko P. C. Canfield F. Wolff-Fabris J. Wosnitza 《JETP Letters》2011,93(11):667-672
We report measurements of the temperature dependence of the radio frequency magnetic penetration depth in Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2 single crystals in pulsed magnetic fields up to 60 T. From our data, we construct an H-T-phase diagram for the inter-plane (H ‖ c) and in-plane (H ‖ ab) directions for both compounds. For both field orientations in Ba0.68K0.32Fe2As2 we find a concave curvature of the H
c2(T) lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism
we can describe H
c2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair breaking is not essential for Ba(Fe0.93Co0.07)2As2. For this electron-doped compound, the data support a H
c2(T) dependence that can be described by the Werthamer-Helfand-Hohenberg model for H ‖ ab and a two-gap behavior for H ‖ c. 相似文献
3.
V. P. D’yakonov V. I. Markovich V. L. Kovarskii A. V. Markovich M. Borowiec A. Endzheichak G. Szymczak 《Physics of the Solid State》1998,40(12):2017-2020
This paper presents the results of a study of the thermal properties of monoclinic single-crystal RbDy(WO4)2 at temperatures of 2–15 K and in magnetic fields up to 6 T. From the results of measurements of the heat capacity and thermograms,
two structural phase transitions are detected, at T
c1=4.9 K and T
c2=9.0 K. The transformation from the high-temperature phase to the low-temperature phase occurs via an intermediate phase.
The field dependences of the critical temperatures are found for various magnetic-field orientations. H-T phase diagrams are constructed for H‖a and H‖c. An anomalous increase (by almost an order of magnitude) of the relaxation time of the system, associated with structural
instability of the crystal lattice, is detected in the region of the structural phase transitions. A symmetry analysis is
carried out, and possible crystal structures of the low-temperature phase are indicated.
Fiz. Tverd. Tela (St. Petersburg) 40, 2221–2225 (December 1998) 相似文献
4.
Y. Kopelevich V. V. Lemanov S. Moehlecke J. H. S. Torres 《Physics of the Solid State》1999,41(12):1959-1962
Measurements of the basal-plane resistivity ρ
a(T,H) performed on highly oriented pyrolitic graphite, with magnetic field H∥c-axis in the temperature interval 2–300K and fields up to 8 T, provide evidence for the occurrence of both field-induced and
zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations.
Fiz. Tverd. Tela (St. Petersburg) 41, 2135–2138 (December 1999) 相似文献
5.
V. P. D’yakonov V. I. Markovich V. L. Kovarskii A. V. Markovich M. Borowiec A. Jedrzejczak H. Szymczak 《Physics of the Solid State》1998,40(4):691-698
The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO4)2 single crystal are investigated. It is established that a structural phase transition takes place at T
c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic
a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is
not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation
time (by almost an order of magnitude) following a thermal pulse is observed at T>T
c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition
in a magnetic field, and the magnetic-field dependence of T
c is investigated for various directions of the field.
Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998) 相似文献
6.
Results of an investigation of the path dependence of the critical current density J
c due to the plastic deformation of the flux line lattice in a weakly pinned YBa2Cu3O7-δ (YBCO) crystal for H‖c are reported. The procedure of minor hysteresis loops has been used to explore the path dependence of J
c and the metastability effects. Contrary to the behavior observed in low T
c systems. in YBCO it is found that at low temperatures, the multivaluedness in J
c(H) could persist beyond the notional peak field H
p, at which the anomalous variation in J
c(H) reaches its maximum value. 相似文献
7.
B. S. Dumesh M. I. Kurkin S. V. Petrov A. M. Tikhonov 《Journal of Experimental and Theoretical Physics》1999,88(6):1221-1228
The spectrum and intensities of NMR lines are investigated experimentally and theoretically for excitation by an alternating
magnetic field h‖ parallel to a static field H in the quasi-one-dimensional, six-sublattice antiferromagnet CsMnBr3. According to theory, two new NMR lines, which are not excited by a transverse magnetic field h
⊥, are observed near the phase transition from triangular to collinear structure (H=H
c
) [JETP 86, 197 (1998)].
Zh. éksp. Teor. Fiz. 115, 2228–2241 (June 1999) 相似文献
8.
A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect
to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals.
The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution.
Explicit calculations are presented in the limits |H| → |H
c2(T)|,T ∼ 0 andT →T
c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a
phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from
those of the closed FS. For example, an important parameter,h(T=0)=|Hc2(0)|/[-Tδ|H
c2
T|δT]T{s0} for the case of a FS open ink
z-direction with thek
z-bandwidth, 4t
3, very small compared to the Fermi energy,E
F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic
field dependence ofT
c and the temperature dependence of H
c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa2Cu3O7 used elsewhere, we find reasonable values for the upper critical fieldH
c2(0), the slope (dH
c2/dT)T
c0, anisotropic coherence lengths ζi(T=0),i=x, y, z, and (dT
c/d|H|)|H| → 0. 相似文献
9.
A comparative study of the thermomagnetic memory effects of J
c in two weakly pinned low T
c superconductors, Ca3Rh4Sn13 (CaRhSn) and Yb3Rh4Sn13 (YbRhSn), is presented. In both the systems, the peak effect (PE) phenomenon appears as an order-disorder transformation
through stepwise amorphization of the flux line lattice (FLL). However, in CaRhSn. we can witness another disorder-driven
transition (Bragg glass (BG) to a vortex glass (VG) in a distinct manner as in a single crystal of high T
c YBa2Cu3O7-δ for H‖c. 相似文献
10.
A. A. Zhukov G. K. Perkins A. D. Caplin H. Kupfer T. Wolf 《Czechoslovak Journal of Physics》1996,46(Z3):1773-1774
The magnetic moments parallel and perpendicular to thec-axis (measured simultaneously) have been studied as a function of direction of applied magnetic fieldH in twinned and de-twinned YBa2Cu3O7−δ single crystals. They show pronounced anomalies when the field direction approaches thec-axis. These allow clear identification of the angle ϕL at which vortices are locked into twin planes. Complete shielding of theH
ab field component (transverse Meissner effect) was observed in the locked state. For larger angles, up to ϕT, the vortices continue to be non-collinear with the applied field, but their direction deviates from the trapping plane.
ϕL exhibits a 1/H field-dependence, whereas ϕT shows only weak logarithmic variations withH. 相似文献
11.
The temperature R(T) and field R(H) dependences of the electrical resistance in the compound Eu0.7
A
0.3MnO3 (A=Ca, Sr) are investigated in the temperature range 4.2–200 K in magnetic fields up to 14 T. Above the antiferromagnetic transition
temperature T
a, the function R(T) is semiconducting in character. Application of a magnetic field H that exceeds a certain critical value H
c changes the character of the function R(T) for Eu0.7Sr0.3MnO3 to metallic (dR/dT>0). For T<T
a and H<H
c a jump in the resistance is observed indicating instability of the electronic state caused by competition between charge
and spin ordering of the Mn ions of different valences.
Fiz. Tverd. Tela (St. Petersburg) 40, 708–712 (April 1998) 相似文献
12.
A theory of μSR method is developed for uniaxial anisotropic high-T
c superconductors. In two extreme cases ofH
ext‖c
andH
ext⊥c
analytical formulas are obtained which makes it possible to determine from the position of van Hove singularities in the
Fourier-spectrum of muonium polarization, the type and parameters of the vortex lattice, such as λ
ab
(λ
c
). In caseH
ext≲H
c2 we obtained the shape of the Fourier-spectrum in analytical form and the simple method of determining the Ginsburg-Landau
parameter κ.
Convenient expressions for numerical calculations are obtained for the arbitrary orientation of the external field, and an
algorithm is provided to compute the mean fieldB, the vortex lattice parameters and the bulk field distribution in anisotropic superconductor. The Fourier spectrum of polarization
based on these calculations can be used to independently check the validity of the high-T
c parameters determination for “appropriate” orientations. 相似文献
13.
Ch. Neumann Ch. Heinzel P. Ziemann K. Fischer W. Gawalek 《Zeitschrift für Physik B Condensed Matter》1991,84(1):37-42
High quality flux grown and defect enriched peritectically grown YBa2Cu3O7–x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsH
ac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y2BaCuO5(211)-precipitates and microcracks act as strong pinning centers. ForH
ac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations. 相似文献
14.
M. E. Palistrant I. D. Chebotar V. A. Ursu 《Journal of Experimental and Theoretical Physics》2009,109(2):227-236
A theory describing the magnetic properties of a two-band superconductor with a varying charge carrier density is constructed.
The upper critical field H
c2(ab) parallel to the ab plane and field H
c2(c) parallel to the c axis are determined in the entire temperature range 0 < T < T
c
. A considerable increase in upper critical field H
c2(ab) as compared to H
c2(c) because of strong anisotropy of the system is detected. Anisotropy of coefficient γ
H
= H
c2(ab) / H
c2(c) is obtained as a function of temperature for pure MgB2 and as a function of the chemical potential in the case when Mg and B atoms are replaced with other chemical elements. A
correlation between the variation in the superconducting transition temperature upon an increase in the chemical potential
and critical magnetic fields H
c2(ab) and H
c2(c) is observed. The effect of doping on magnetic anisotropy is also determined. 相似文献
15.
G. I. Harus A. N. Ignatenkov N. K. Lerinman A. I. Ponomarev L. D. Sabirzyanova N. G. Shelushinina N. A. Babushkina L. M. Belova 《JETP Letters》1996,64(6):444-449
The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd1.82Ce0.18CuO4−δ
single crystal film (T
c
=6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B
c
2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect
in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account.
Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
16.
17.
Shoji Hirokawa 《Molecular physics》2013,111(5):1161-1176
The hamiltonian for a molecule enclosed in a cavity may be written H = H R(θ) + H T(r) + U RT(θ, r), where θ and r denote the molecular orientation and the displacement of the centre of mass. The crystalline potential seen by the guest molecule can be explicitly calculated on the basis of atom-atom and electrostatic multipolar interactions, by using an nth-order gradient formula in the spherical representation. We take a N 2 molecule encaged in β-quinol clathrate as our model system, and solve variationally the Schrödinger equation for H to clarify the translation-rotation coupling. Energy levels of H are obtained as a function of the height of the hindering potential for rotation, by modifying the strength of the electrostatic multipolar interaction. Three limiting cases of H R are important: (a) libration around the Z-axis, (b) free rotation, and (c) an oscillational rotation whose equilibrium orientation is perpendicular to the Z-axis. The constrained translation under consideration is an anharmonic and almost spherically symmetric oscillation. The Pauli principle must be applied to the eigenstate of H. Excepting the case (a), translational states of H are different for different nuclear spin species, and higher-order effects of U RT on these states are significant. For p-N2, some modes of motion interchange if the rotational motion changes from case (a) to case (c). If the constraining potential for translation is steeper, the effect of U RT will become smaller. The anharmonicity involved in H T is not negligible in discussing this effect. 相似文献
18.
The thermal conductivity (κ) of single crystals of tetragonal uniaxial antiferromagnets USb2 (T N = 202 K) and UBi2 (T N = 180.8 K) has been measured along the a-axis (κa ) over the temperature range from 0.5 to 300 K and along the c-axis (κc ) from 0.5 to 70 K. The as-grown samples have residual resistivity ratio (RRR) values of about 500–600 and 100–150 for UBi2 and USb2, respectively. The anisotropy of the thermal conductivity (κa (T)/κc (T) ~ 5) and the low-T Lorenz ratios are discussed in relation to Fermi surface topology for both compounds. 相似文献
19.
The anisotropy of the optical properties of a single crystal of the hexagonal manganite HoMnO3 has been investigated by spectroscopic ellipsometry in the spectral range 0.6–5.0 eV. It has been demonstrated that the optical
absorption edge for the polarization E ⊥ c is determined by the intense narrow transition O(2p) → Mn(3d) centered at 1.5 eV, whereas this transition for the polarization E ‖ c is strongly suppressed and shifted toward higher energies by 0.2 eV. It has been revealed that, at the temperature T = 293 K, the spectra for both polarizations E ‖ c and E ⊥ c exhibit a broad absorption band centered at ∼2.4 eV, which was earlier observed in nonlinear spectra during optical second
harmonic generation. 相似文献
20.
Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides
the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the
optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed
in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton
dispersion relationsk
⊥ (ω), andk
‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal
thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures
centred at the isotropic points. 相似文献