Transport coefficients and flux motion in Bi2Sr2CaCu2Ox single crystals

We present measurements of the electrical resistivity, thermal conductivity, and Hall, Nernst, and Seebeck effects in the mixed state of single crystalline Bi₂Sr₂CaCu₂O_x. It is shown that the sign of the Hall voltage changes twice as temperature decreases below T_c. From the Nernst effect we estimate the transport entropy S_φ to be about 10⁻¹⁰ erg/K cm. S_φ is equal to zero in the normal state, increases and passes through a maximum at the mixed state as expected. The temperature dependences of the thermoelectric power in magnetic fields are analogous to the resistive transition curves. These phenomena are discussed in terms of flux flow. The contribution of the flux flow to the thermal conductivity is estimated to be negligible. Lowering of the thermal conductivity at temperatures below T_c by a magnetic field is attributed to phonon scattering by the vortex lines.     

Magnetic behavior of 3d dopants in superconducting REBa2Cu3−xFexO7+δ (RE=Dy, Er)

Mössbauer studies on ⁵⁷Fe-doped superconducting REBa₂Cu₃O_7+δ (RE=Er, Dy) were made as a function of temperature for x=0.15 and 0.30. The magnetic behavior of the 3d dopants, which mainly occupy Cu(1) sites, undergoes antiferromagnetic ordering which is coexistent with superconductivity at low temperature. The dimensionality of the magnetic interaction changes from 2D to 3D when the rare earth changes from Er to Dy. the line-widths of the Mössbauer subspectra are characteristic of magnetic fluctuation behavior in the vicinity of a phase transition. Combining these results with those of Fe-doped Y-123 (pseudo 1D) and Gd (3D), the magnitude of the rare earth moments appears to be strongly correlated with the dimensionality of the magnetic interaction of Fe dopants in these compounds. However, the Mössbauer spectrum for ¹⁵⁵Gd in GdBa₂Cu_2.85Fe_0.15O_7+δ (T_{N(Fe) 14 K}) shows no magnetic order at 4.9 K.     

Quantum phenomena in glasses at low temperatures

Glasses exhibit surprising low-temperature properties caused by the tunneling motion of small atomic clusters. We report here on recent dielectric measurements on a glass with the components BaO–Al₂O₃–SiO₂. In contrast to expectation, below 100 mK the dielectric properties become sensitive to weak magnetic fields. In this temperature range dielectric constant and dielectric loss show an oscillatory behavior with increasing magnetic field. Below 6 mK a phase transition within the ensemble of tunneling systems is observed.     

Anomalous temperature dependence of resistivity in quasi-one-dimensional conductors in a strong magnetic field

We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (), when its effective dimensionality is one, and like a metal (), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor'kov and find that the theoretical results do not agree with the experimental observations. Received 20 October 1998     

Wall NMR in the weak ferromagnets YCrO3 and LuCrO3

Complex NMR spectra, consisting of a maximum of six resonance lines, were observed in the domain walls of YCrO₃ and LuCrO₃ and studied as a function of temperature and the in-plane magnetic field. The distribution of the NMR frequency across the domain wall was calculated taking into account the anisotropy of hyperfine fields and quadrupolar interactions. The calculated frequency distribution was used to simulate the shape of the NMR spectra in YCrO₃. The analysis of theoretical spectra in the centext of our experimental results and in particular the asymmetric behaviour of resonance lines in a magnetic field indicate the critical importance of the relaxation processes on the shape of NMR spectra. This conclusion was confirmed by an experiment in which the relaxation was simulated in YFeO₃. The NMR spectrum in LuCrO₃ can be explained by the same model.     

Dimensionality crossover upon magnetic saturation in Fe, Ni and Co

The magnetic saturation process of iron, nickel and cobalt single-crystal spheres is studied using neutron scattering in a vertical magnetic field. It is observed that upon magnetic saturation, the scattering intensities decrease instead of increasing. This indicates a decreasing coherent scattering with field. The spin precession around the field axis therefore can be assumed to be incoherent along directions transverse to the field. Comparison of the temperature dependence of the spontaneous magnetization measured by zero field NMR on the one hand and by the macroscopic magnetization on the other hand shows that Fe, Ni and Co are three-dimensional (3D) in the zero field ground state but one dimensional (1D) in the magnetically saturated state. The observed decrease in neutron scattering intensity is consistent with this conclusion. The change in dimensionality is associated with a crossover. Our neutron scattering study shows that the crossover occurs at a field that is smaller than the demagnetization field. The dimensionality crossover, therefore, is driven not by the field but by the associated forced magnetostriction.     

High field magnetoresistance of CeAl2 at low temperatures

We have measured the magnetoresistance ρ(H,T₀) of the magnetic Kondo lattice CeAl₂ at temperatures T₀ in the range 0.035 – 1.3 K, well below the Néel temperature T_N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H_M. The resistivity then levels out to a value which depends only weakly on field and temperature. For HM a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T₀) data obtained for the CePb₃ magnetic Kondo lattice, where the decrease in the resistivity at HH_M was considered earlier to be evidence of field induced superconductivity.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

铁基超导体Li0.8Fe0.2ODFeSe的红外光谱研究

测量和研究了铁基超导体Li_0.8Fe_0.2ODFeSe单晶的红外光学响应,发现室温下光电导率谱不存在Drude分量,载流子具有非相干输运行为.随着温度降低,Drude分量形成并不断变窄,同时在相应的反射率谱上出现清晰的等离子体边,表明散射率急剧降低.在最低温度,观察到超导能隙形成导致的光谱变化,光电导率谱在160 cm^-1以下受到显著压制.对比FeSe单晶的光谱数据,发现整体的光电导率谱型很相似,但自由载流子的谱重更低,揭示出样品具有更低的载流子浓度.另外还观察到温度变化诱导的谱重由低频向高频区域转移的现象,表明其存在强关联效应.     

SPIN-DEPENDENT INTERFACIAL TUNNELING AND MAGNETO RESISTANCE IN THE FAMILY OF GRANULAR PEROVSKITE La1-xSrxMnO*3

The magnetic transport properties in granular perovskite system La_1-xSr_xMnO^*₃ have been investigated. The spin-dependent inter facial tunneling and the corresponding giant magnetoresistance (GMR) effect have been observed in the whole temperature range below the Curie point T_c for the samp les with concentration x from 0.05 to 0.45. Theoretical analysis shows that the interfacial tunneling originates from the difference in magnetism between surfaces and cores, and the tunnel-type GMR stems from the field-induced change of interfacial magnetic order.     

Tb掺杂双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7的磁熵变和电输运性质

研究了Tb掺杂对双层锰氧化物La_(4/3)Sr_(5/3)Mn_2O_7磁熵变和电输运性质的影响.样品采用传统固相反应法制备,两样品的名义组分可以表示为(La_(1-x)Tb_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.025),磁场为7 T时的最大磁熵变?S_M分别为-4.60 J/(kg·K)和-4.18 J/(kg·K).比较后发现,Tb元素的掺杂使得最大磁熵变值减小,但同时增大了相对制冷温区.电性测量结果表明,x=0.025的样品在高温区的导电机制可以用小极化子模型解释,与母体三维变程跳跃模型不同;当温度降低至三维长程铁磁有序温度(T_c~(3D))附近时,掺杂样品发生金属绝缘相变;掺杂后样品在T_c~(3D)附近,磁电阻取得极大值(约为56%),表明是本征磁电阻效应.     

Spin texture and magnetoroton excitations at nu=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.     

Magnetic crystallization waves in He

The melting and growth of ³He crystals spin-polarized by an external magnetic field at temperatures below the Neel temperature are accompanied by spin supercurrents in addition to mass flows. In fields weak in comparison with exchange fields, the crystallization waves change in nature, because the spin currents begin to play a dominent role. The wave spectrum becomes linear with a velocity inversely proportional to the magnetic field. The attenuation of the waves at low enough temperatures is mainly due to the interaction of the moving crystal-liquid interface with thermal spin waves in the crystal. The waves could be weakly damped at temperatures below a few hundred microkelvins.     

Invar properties in the rare earth-3d transition metal alloys

The RCo₂ and R₂Fe₁₇ compounds (R = rare earth) exhibit Invar-like thermal expansion anomalies below their ordering temperatures. These spontaneous volume magnetostrictions are discussed by considering their magnetic properties. In RCo₂ compounds there is no intrisinc Co-moment is induced by the exchange and applied fields. The volume expansion anomaly is associated with the onset of the 3d magnetic moment. IN R₂Fe₁₇ compounds there is an intrisinc Fe- moment. Magnetic structures give evidence for positive and negative exchange interactions between Fe atoms which are strongly distance dependent. The thermal expansion anomaly is a result of this distance of the magnetic interactions.     

AC losses in multifilamentary Bi(2223) tapes with an interfilamentary resistive carbonate barrier

For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO₃ barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.     

Ultra-sharp metamagnetic transition in manganite Pr0.6Na0.4MnO3

We report the magnetic and electrical transport properties of manganite Pr_0.6Na_0.4MnO₃. At the temperature of 2 K, a field-induced steplike magnetization and resistivity transition are observed. The step transitions of magnetization and resistivity are shifted to higher fields as a result of field cooling, and transformed to a smooth broad one when the cooling field is higher than 20 kOe. Moreover, in a magnetic field slightly below the critical field, the magnetic and resistive relaxation exhibits a spontaneous step after a long incubation time when both the temperature and magnetic field are constant. Such steplike transitions are discussed in terms of a martensiticlike transformation associated with phase separation.     

High field magnetoresistance in Co–Al–O nanogranular films

We have studied magnetic and electrical transport properties of Co–Al–O nanogranular films. The films prepared by laser ablation consist of Co particles embedded in an insulating Al₂O₃ matrix. Different current–voltage characteristics are observed depending on Co concentration in the films. Tunneling magnetoresistance (TMR) shows an anomalous behavior in high magnetic fields at low temperatures. The presence of magnetic atoms dispersed in the insulator is a possible explanation of the observed anomaly of the TMR.     

Hopping conductivity and Coulomb correlations in 2D arrays of Ge/Si quantum dots

The temperature and magnetic-field dependences of the conductivity associated with hopping transport of holes over a 2D array of Ge/Si(001) quantum dots with various filling factors are studied experimentally. A transition from the Éfros-Shklovski? law for the temperature dependence of hopping conductivity to the Arrhenius law with an activation energy equal to 1.0–1.2 meV is observed upon a decrease in temperature. The activation energy for the low-temperature conductivity increases with the magnetic field and attains saturation in fields exceeding 4 T. It is found that the magnetoresistance in layers of quantum dots is essentially anisotropic: the conductivity decreases in an increasing magnetic field oriented perpendicularly to a quantum dot layer and increases in a magnetic field whose vector lies in the plane of the sample. The absolute values of magnetoresistance for transverse and longitudinal field orientations differ by two orders of magnitude. The experimental results are interpreted using the model of many-particle correlations of holes localized in quantum dots, which lead to the formation of electron polarons in a 2D disordered system.     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.