Temperature- and magnetic field dependence of the Hall effect in the heavy fermion system CeCu6

The temperature dependence of the Hall coefficient and the magnetic field dependence of the Hall resistivity of CeCu₆ have been determined in the temperature range 80 mK<T<10 K and in magnetic fields up to 10T. The Hall coefficientR _H shows a very strong temperature dependence with two extrema and a change of sign, and the Hall resistivity _xy has a strong field dependence with up to two changes of sign. The observed behavior can partially be explained by the field- and temperature dependence of the skew scattering contribution to the Hall coefficient.     

Nuclear spin conversion of methyl groups

A theory is presented for nuclear spin conversion of methyl groups, where the total spin of the three protons changes fromI=1/2 toI=3/2. The transition may be mediated by the magnetic dipolar interaction of the protons among themselves or with a nearby unpaired electron. In general the excess energy, i.e. the tunnelling energy , is transferred from the spin system to the lattice via the rotor-phonon coupling; for the case of an almost free rotor in the magnetic field of an unpaired electron spin, the direct coupling of the electron-proton interaction to the lattice motion may be the more efficient mechanism. AtT=0 the rate is found to be finite, at high temperatures it shows an Arrhenius behaviour. In the intermediate range, two different power laws may govern the temperature dependence, namely 1/T or 1/T ⁷; the latter is due to two-phonon scattering.     

Amplitude,wavevector and frequency dependence of magnetic fluctuations in YBa2Cu3O6+δ

Polarized neutrons and polarization analysis were used to determine the total amplitude of magnetic fluctuations in tetragonal YBa₂Cu₃O₆₊ for energies below 30 meV, both in the magnetically ordered state below and in the paramagnetic state above the Neél temperature. An energy analysis of the local magnetic correlation function (=S(r=0,)) was performed. The wavevector dependence of the energy integrated correlation function at high temperatures is dominated by local (i.e.q-independent) fluctuations. Total intensity, energy and wavevector dependence are substantially different from those of stable moments with quasi two-dimensional couplings. Magnetic amplitude fluctuation might be responsible for the observed behaviour.     

Transition of a homogeneous type II superconductor cylinder under the influence of axial current

We have calculated the magnetic field distribution and the resistance of a homogeneous type II superconductor cylinder at transition under the influence of axial current in zero external magnetic field. In this calculation we have expressed the magnetic field dependence on flow resistivity _f by two or more potential functions and have shown that the accuracy of this approximation is sufficient. For some values of the lower critical field and various temperatures, the magnetic field distribution and the resistance of the cylinder are calculated as a function of the applied electrical field and/or the applied current. The temperature dependence of the electric field at which, the normal state appears on the surface of the cylinder is also given.     

Electron drag in intermediate magnetic fields with low electron density

Electron drag between two two-dimensional electron systems has been measured in intermediate magnetic fields (/τ<ω_ck_BT) with a relatively low electron density. We explore, in this sample, the unusual increase of drag in intermediate magnetic fields which was well characterized by a nearly temperature independent B³ dependence. The anomalous behavior of electron drag observed in higher density samples is found to persist for low sample density.     

Rare Earth systems in a magnetic field: Coupling of elastic and magnetic properties

We study the mutual influence of magnetic and elastic properties of a Rare Earth system as an external magnetic field is applied. It is demonstrated that a magnetic phase transition should show up in the temperature dependence of certain sound velocities. Furthermore the magnetic field should induce a hybridization between the magnetic excitons and phonons. Although the theory is valid for any crystal field level scheme we use for an explicit example a singlet ₁ groundstate with a ₄ triplet first excited state.     

The spontaneous magnetization of a one-dimensional model of a ferromagnetic toroid in a magnetic field

The paper deals with the derivation of the dependence of the spontaneous magnetization on the temperature and the intensity of the magnetic field for a one-dimensional model of a ferromagnetic toroid. For a chain formed of N10H ^–1/2 particles this dependence has the form of a half-power law modified by exponential dependence on H/T for temperatures and intensities of the magnetic field for which H/T10.     

Two successive magnetic transitions in Y2Cu2O5 studied by μSR

Zero field muon spin rotation and magnetic susceptibility experiments on investigation of magnetic properties of cuprateY ₂Cu₂O₅ have been performed in the temperature range 4.2–30 K. Transverse fieldSR-experiments have been also carried out in order to obtain accurate information about transition temperature and to study the influence of the external magnetic field. Our data show that two magnetic phase transitions occur inY ₂Cu₂O₅ with lowering temperature. Upper Neel temperatureT _N=13 K is consistent with previous experimental data. We obtained the temperature dependence of the local magnetic field on the muonB (T) in the antiferromagnetic phase.B (T) reveals a peculiarity (some change of the slope) near the temperatureT _N=7.5 K, which can be interpreted as an additional magnetic phase transition caused by a change in magnetic ordering of the copper subsystem. Applying a small external magnetic field 50 Oe leads to smearing of the peculiarity inB (T) dependence.     

Magnetic-relaxation study in melt-textured ErBa2Cu3O7−δ ceramic material

We have studied the zero-field-cooled magnetic relaxation in the melt-textured ErBa₂Cu₃O_7– ceramic material. The magnetic relaxation rate dM/d (ln t) exhibits strong temperature and field dependence in the temperature range up to 80 K and the field range up to 5.5 T. The magnetic relaxation behavior observed in the melt-textured ErBa₂Cu₃O_7– ceramic sample is similar to that in single crystals. The magnetization to noise ratio is much improved, in particular, for temperatures approaching the superconducting transition temperature and for low magnetic fields. Several theoretical models were explored to fit the experimental data.     

Seebeck-effect in the mixed state of Y−Ba−Cu−O

We report on measurements of the Seebeck-effect, the Nernst-effect, and the magnetoresistance in the mixed state of ac-axis oriented expitaxial film of Y–Ba–Cu–O. In contrast to conventional superconductors we find a large Seebeck-coefficientS, which is comparable in magnitude to the Nernst-effect. The broadening of the super-conducting transitions of magnetoresistance and Seebeck-effect are rather similar with respect to (1) the temperature dependence, (2) the dependence on the direction between magnetic field and crystal axis and (3) the dependence on the direction between magnetic field and driving forces. The large Seebeck-effect has to be attributed to dissipation due to normal quasiparticle-excitations, since the vortex-contribution to the Seebeck-effect is by far too small to account for the observed magnitude ofS. It is argued that such a quasiparticle contribution to the dissipation is large in the high-T _c superconductors because of the small coherence lengths and thus the small vortex cores. Another possibility is that granularity leads to dissipation proportional to the normal state transport properties. The Seebeck-voltage depends on all dissipative processes other than vortex motion, whereas the Nernst-effect depends only on the vortex motion. Therefore by measurements of thermomagnetic effects the various dissipative properties may be separated.     

Thomas-Fermi theory for matter in a magnetic field as a limit of quantum mechanics

The Thomas-Fermi theory for electrons and fixed nuclei in a homogeneous magnetic field is shown to be a limit of quantum mechanics in the following sense: If the nuclear charges and the number of eletrons are multiplied by a, and the magnetic field by a ^4/3, then the ground-state energy, divided by a ^7/3, converges to the TF energy when a . A similar result holds also for the electronic density. This generalizes corresponding theorems for zero magnetic field due to Lieb, Simon, and Baumgartner.     

A Mathematical Representation of Quantizing the Motion of Relativistic Electrons in a Magnetic Field

It is shown that the quadratic component of the kinetic energy of continuous longitudinal motion of relativistic electrons in the external magnetic field is varied continuously between 0 and 2(2m _e c ²_B H) within each Landau energy level, undergoing an abrupt change at the boundaries of the levels. This results in the fact that in the quantum limit of a superstrong magnetic field where all electrons are at the zero Landau level, the maximum quadratic component of the kinetic energy of free longitudinal electron motion along the direction of the magnetic field is twice as high as the maximum quadratic component of the kinetic energy of its bound transverse motion.     

Spin-glass behavior of (La,Gd)B6 alloys

The low-frequency ac magnetic susceptibility of (La_1–x Gd _x )B₆ alloys (0.0035x0.0812) was measured between 0.04 and 10 K in very low and moderate applied magnetic fields up to 8 kOe. In addition, the low-field static susceptibility was measured between 4.2 and 200 K. At low temperatures the alloys exhibit the basic features of the spin glass state, that is sharp maxima and time dependent effects in the low-field ac susceptibility. The temperature and field dependence of the susceptibility follows a law of corresponding states forx0.01, which is characteristic for Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Effects from interactions between the Gd ions observed above the freezing temperature are consistent with a recent theory based on a virial expansion of the magnetic free energy for the RKKY model and yield |J|=0.04 eV for the magnitude of thes–f exchange integral. The same value is obtained from the initial susceptibility atT0 when compared with a molecular-field result. Above 1 K the initial susceptibility follows a single-impurity Curie law with the free-ion value of the magnetic moment of the Gd solute.     

Muon spin rotation in superconductors

By means of the muon spin rotation technique (⁺SR), the temperature dependence of the magnetic field inside the normal-conducting domains of high-purity tantalum crystals in the intermediate state has been measured in the temperature range 2.36K+SR. Possible applications of these findings to the study of long-range diffusion of positive muons at low temperatures are indicated.     

Influence of the concentration of Ca impurity on the magnetic threshold of the magnetoplastic effect in NaCl crystals

It is found experimentally that the threshold magnetic field B _c for the magnetoplastic effect, i.e., the field at which the depinning of dislocations from paramagnetic impurities in an external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C=(0.5–100) ppm. It is shown that the dependence B _c(C) exhibits a distinct tendency toward saturation. The physical interpretation of the observed dependence rests on the notion that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the impurity atoms changes according to a definite pattern. In particular, the average number of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to the previously observed increase of B _c with increasing temperature. The proposed interpretation is further supported by good correlation of the experimental dependence B _c(C) with the calculated function . Fiz. Tverd. Tela (St. Petersburg) 40, 81–84 (January 1998)     

Possible influence of two-band superconductivity on the behaviour of the second magnetization peak in La2−xSrxCuO4 single crystals

An anomalous temperature T dependence of the field H_on for the onset of the second magnetization peak (SMP) in La_1.81Sr_0.19CuO₄ single crystals (with the external magnetic field H oriented parallel to the c axis) is discussed. While the peak field H_p and the magnetization at H_p have a continuous variation with T, H_on exhibits a sudden decrease with increasing T for T  11 K–15 K. The analysis of the nature of the SMP in La_2−xSr_xCuO₄ single crystals in terms of a simple dynamic energy balance relation suggests that the observed behaviour could be related to the particular T dependence of the superfluid density in the case of two-band superconductivity, affecting the T variation of the elastic energy of the vortex system at low H.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

The Effect of a Magnetic Field on Photoinduced Electron Transfer in Solutions of Chlorophyll and Porphyrins

It is found that, with the action of photoexcitation and a variable magnetic field on halide-containing porphyrin solutions, a differential absorption is observed, whose spectra conform to the absorption spectra of cation radicals of pigments. It is shown that the effect increases linearly with an increase in the strength of a supplementary magnetic field (H) up to saturation (H _max 200 G). The scheme of the photophysical and photochemical deactivation of the energy of electronic excitation for chlorophyll-like pigments in the presence of CCl₄ is analyzed with allowance for the effect of the magnetic field on the intercombination conversion in an ion–radical pair. It is shown that solvate-separated ion radicals of the pigments in halide-containing solutions can be formed from the singlet state of tetrapyrrole molecules.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.     

Resonant magnon—optical phonon coupling in metamagnetic FeCl2·2H2O

The resonant coupling between magnons and optical phonons of symmetryB _g was observed in the Raman spectra at different magnetic fields in the antiferro-, ferri- and ferromagnetic phases of FeCl₂·2H₂O at 2K. The line positions of the coupled modes were calculated taking a field dependence of the coupling strength into account. Varying this strength and the frequencies of the uncoupled modes as parameters of this calculation a fit of the experimental data was obtained also at low fields where former interpretations were not satisfying. However, the field dependence of the coupling strength disagrees with a corresponding term of an appropriate microscopic coupling mechanism in which the local spins are disturbed by a modulation of the ctystal field via spin-orbit coupling. In accordance with the former results of the far infrared absorption measurements zero field splitting of the magnonlike mode was found to exist. In the AF-phase two magnonlike modes can be observed, but the Raman scattering cross section of one of these modes tends to zero at zero field corresponding to its vanishing phonon contributions responsible for the scattering cross sections of the coupled modes. The relationship between cross sections of the coupled modes and their phonon contributions was confirmed by analysing the field dependence of the mode intensities in the Raman spectra.A project of the Sonderforschungsbereich 65 Festkörperspektroskopie Darmstadt-Frankfurt, supported by the Deutsche Forschungsgemeinschaft