电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈     

Transport properties in the pseudobinary (Gd,Y) Al2 series

The temperature dependence of the electrical resistivity, the thermal conductivity and the thermopower of the cubic isostructural (Gd_xY_1–x)Al₂ series will be presented. The magnetic properties of this system are characterized by the existence of ferromagnetism for Gd concentrations x>0.3 while for low Gd contents cluster and spinglass behaviour is observed. The spin dependent scattering contribution to the transport phenomena has been obtained by comparing the experimental data of the magnetic compounds with those of the isostructural nonmagnetic YAl₂. For the ferromagnetic concentration range and forT>T _c (T _c =Curie temperature) we revealed a temperature independent contribution to the electrical resistivity, a contribution with a temperature variation of 1/T to the thermal resistivity and a linear temperature dependence of this part to the thermopower. These results are in good agreement with the temperature dependence calculated by solving the linearized Boltzmann equation for this type of scattering processes.     

Magnetic anomalies in Gd2PdSi3

The results of ac and dc magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd₂PdSi₃, establish that this compound orders magnetically below T_n = 20 K. Though the ordering appears to be of an antiferromagnetic-type, the paramagnetic Curie temperature is positive with the magnitude being nearly the same as that of T_N, suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below T_N Also considering the observation of Kondo-like characteristics above 21K earlier by us, the overall thermal, transport and magnetic behaviour of this compound is interesting.     

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu2Si2

The heavy fermion compound CeCu₂Si₂ is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ～ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T_c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T_c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E_F.     

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

The absolute thermopower of single phase YBa₂Cu₃O₇ and Y_0.8Er_0.2Ba₂Cu₃O₇ has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toT _c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.     

Transport properties in single phase superconductor Ba2YCu3O9-δ

The electrical resistivity and the thermopower are measured on the single phase superconductor Ba₂YCu₃O₉-δ (δ=2.1). The results indicate that the temperature dependences of the resistance and thermopower exhibit typical metallic behaviour, and the sample conducts via electrons at high temperatures. The behaviour of the thermopower can be described with Mott's semi-classical model. The specific heat of electrons in normal state has been estimated 780mJ/K·mole at 200K, i.e. γ=3.9mJ/K²·mole. Unusual phonon-drag effect is observed above the superconducting transition temperature T_c. Below T_c, the electrical resistivity and the thermopower all drop to zero corresponding to a superconducting ground state.     

The anomaly of the specific heat of La2-xSrxCuO4+δ near Tc

The specific heat of three polycristalline samples of La_2-xSr_xCuO_4+δ (x = 0.15, 0.15, 0.16) near T_c was measured with a resolution better than 0.1%. The anomaly at T_c is analysed for fluctuations for the first time. It is well described by a superposition of a mean field step and a contribution due to gaussian fluctuations. By a fit of a Lawrence-Doniach type of model to the specific heat near T_c we estimate ΔC_MF and the anisotropic coherence lengths ξ and ξ⊥ of the superconductor.     

Influence of a non-uniform current density profile on quasiparticle excitations and thermally activated flux creep in YBa2Cu3O7−σ

The transport current density, flowing radially from the center of a superconducting disk to its perimeter, in a so-called Corbino geometry, results in a double action on the vortex motion when the applied magnetic field is perpendicular to the disk’s plane. First, the depinned vortices are set into a nearly circular motion in the plane of the disk. Second, the non-uniform current density profile activates the intrinsic weak links, resulting in a non negligible proximity dominated quasiparticle contribution. In turn, these intrinsic junctions impede the circular motion of vortices giving rise to a proximity influenced thermally activated flux creep. This provides a simple technique to deconvolute the flux motion and the quasiparticle induced components of the total dissipative resistivity broadening below T _c . The case for a YBaCuO disk is hereby examined and measured in this context. It is shown that such a deconvolution can be made and theoretical laws for the field dependences are also obtained and confirm the data. Furthermore, the paraconductivity region just above T _c (B) appears to be dominated by the macroscopic fluctuations accompanying the vortex core motion.     

Anisotropic thermopower of (TMTSF)2PF6: 1D–2D cross over and SDW ordering

The thermopower of (TMTSF)₂PF₆ has been measured along the a-axis as well as along the b-axis. Marked anisotropy is seen in the whole temperature region studied. Close to 100 K, an anomaly is attributed to cross over from dominating one-dimensionality to dominating two-dimensionality. Precursor effects seen in S_a near T_c are attributed to SDW fluctuations. Well below T_c, both thermopower components exhibit typical semiconducting properties.     

Normal-state and superconducting properties of HfV2

We present experimental results for the low-temperature specific heat, electrical resistivity, elastic constants, ultrasonic attenuation and thermal conductivity of theC 15 Laves phase HfV₂ for both polycrystalline and single crystal samples. BelowT _c =9 K, the temperature dependence of the specific heat can roughly be approximated by aT ³ law over a large temperature range. This may be explained with strong coupling effects in consistence with the high value for C/T _c =1.9. Heat transport appears dominated by the known phonon contribution also belowT _c . There is no electronic contribution to the ultrasonic attenuation in HfV₂.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

Influence of oxygen deficiency on the thermoelectric power of Y1Ba2Cu3O7-x

In this paper we report measurements of the thermoelectric power on a series of Y₁Ba₂Cu₃O_7-x specimens with varying amounts of oxygen deficiency obtained by changing the cooling rate of the sintered specimens. The specimens have been characterized by X-ray diffraction measurements, electric resistivity and oxygen contents. The temperature variation of the thermopower reveals a peak just before the onset of superconducting transition. We examine possible theoretical explanations of this anomaly. In particular we argue that this anomaly is associated with the pairing fluctuations in the normal state close toT _c . We present some theoretical results in support of this conclusion.     

Resistivity,thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract

Resistance and thermopower measurements have been made on a series of compounds, YBa₂Cu₃- _x Zn _x O_7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below T_M , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (T_M ? T) ^1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of Å, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above T_c . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with T_c , and 2Δ/kT_c has an approximate value of 5.5.     

Oxygen defects relaxation in high-temperature YBa2Cu3O7 superconductors

The temperature dependences of normal state static magnet susceptibility χ(T) for high-temperature YBa₂Cu₃O₇ superconductors (T _c ≤ T ≤ 400 K) measured for the same sample before and after 22 years of storage are compared. It is shown that during longime storage of a maximally doped sample, its electronic system is more in equilibrium than it was in the initial state, while a similar underdoped sample decomposed completely. Comparing the χ(T) curves for YBa₂Cu₃O₇, we draw conclusions as to the nature of the Curie contribution and the fluctuation-induced contribution to the magnetic susceptibility of the investigated sample, and the experimental procedure for determining T _c.     

Elucidation of 3D-to-2D transformation by magnetization studies for Hc in Bi2Sr2CaCu2O8

New magnetization results (for Hc) on a single crystal of Bi2212 elucidating the transition from a one-component to a two- component response across the 3D-to-2D transformation are reported. The two-component response is identified to emanate from the 2D planes and interplanar links. The magnetization response above the transformation temperature T_s and very close to T_c is found to be irreversible in contrast to the reversible behavior so far believed in. A new phase diagram for the mixed state of layered superconductors near T_c is proposed.     

Thermal conductivity and thermal Hall effect in Bi- and Y-based high-T c superconductors

Measurements of the thermal conductivity (k_xx) and the thermal Hall effect (k_xy) in high magnetic fields in Y- and Bi-based high-T _c superconductors are presented. We describe the experimental technique and test measurements on a simple metal (niobium). In the high-T _c superconductors k_xx and k_xy increase below T _c and show a maximum in their temperature dependence. k_xx has contributions from phonons and quasiparticle (QP) excitations, whereas k_xy is purely electronic. The strong increase of k_xy below T _c gives direct evidence for a strong enhancement of the QP contribution to the heat current and thus for a strong increase of the QP mean free path. Using k_xy and the magnetic field dependence of k_xx we separate the electronic thermal conductivity ( k _xx ^el ) of the CuO ₂ -planes from the phononic thermal conductivity ( k _xx ^ph ). In YBa₂Cu₃O _{7 - δ} k _xx ^el shows a pronounced maximum in the superconducting state. This maximum is much weaker in Bi₂Sr₂CaCu₂O _{8 + δ} , due to stronger impurity scattering. The maximum of k _xx ^el is strongly suppressed by a magnetic field, which we attribute to the scattering of QPs on vortices. An additional magnetic field independent contribution to the maximum of k_xx occurs in YBa₂Cu₃O _{7 - δ} , reminiscent of the contribution of the CuO-chains, as determined from the anisotropy in untwined single crystals. Our data analysis reveals that below T _c as in the normal state a transport (τ) and a Hall ( ) relaxation time must be distinguished: The inelastic (i.e. temperature dependent) contribution to τ is strongly enhanced in the superconducting state, whereas displays the same temperature dependence as above T _c . We determine also the electronic thermal conductivity in the normal state from k_xy and the electrical Hall angle. It shows an unusual linear increase with temperature. Received 23 August 2000     

Effect of interlayer single-particle hoppings on the superconducting transition temperature

It is well known that the superconducting transition temperature of high-T _c cuprates depends on the number of CuO₂ planes in the unit cell. The multilayer structure implies the possibility of interlayer hopping. Under the assumption that the interlayer hopping can be specified by the parameter t _⊥(k) = t _⊥(cos(k _x ) − cos(k _y ))², the quasiparticle excitation spectrum for the bilayer cuprate in the superconducting state has been determined in the framework of the t − t′ − t″ − t _⊥ − J* model using the generalized mean-field approximation. It turns out that the interlayer hoppings does not create any additional mechanism of the Cooper paring and does not lead to an increase in T _c . The splitting of the upper Hubbard quasiparticle band attributed to the interlayer hoppings is manifested as two peaks in the doping dependence of the superconducting transition temperature at temperatures below the maximum T _c value for a single-layer cuprate. It has been found that antiferromagnetic interlayer correlations suppress the interlayer splitting. This probably leads to the common doping dependence of T _c for both single-layer and bilayer cuprates.     

Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr2O3

Abstract

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr₂O₃ around its phase transition at T_N = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn _sp ? 10^?3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around T_N . The temperature variation of the order parameter l(T) below T_N is well described by a power low l where τ^β = (T_N - T)/T_N and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to k_il_i terms in the dielectric permeability.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.