The results of magnetoconductivity measurements in GaInAs quantum wells are presented. The observed magnetoconductivity appears due to the quantum interference, which lead to the weak localization effect. It is established that the details of the weak localization are controlled by the spin splitting of electron spectra. A theory is developed which takes into account both linear and cubic in electron wave vector terms in spin splitting, which arise due to the lack of inversion center in the crystal, as well as the linear terms which appear when the well itself is asymmetric. It is established that, unlike spin relaxation rate, contributions of different terms into magnetoconductivity are not additive. It is demonstrated that in the interval of electron densities under investigation ((0.98 − 1.85)·1012 cm −2 ) all three contribution are comparable and have to be taken into account to achieve a good agreement between the theory and experiment. The results obtained from comparison of the experiment and the theory have allowed us to determine what mechanisms dominate the spin relaxation in quantum wells and to improve the accuracy of determination of spin splitting parameters in A3B5 crystals and 2D structures. 73.20.Fz,73.70.Jt,71.20.Ej,72.20.My

We have analyzed the density of states of a two dlmens~onal electron gas in a GaAs-AIGaAs hetereostructure by measuMng the magnetocapacltance in magnetic f~elds up to 6 Tesla at temperatures below 10 K. The experlmental data are well described by a Gausslan-11ke density of states where the llnew~dth r ~s proportional to CB.

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We present magnetization measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation. A pump-probe technique is used with two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths of several nanoseconds to study the spin dynamics via time-resolved magnetization measurements using a Hall probe magnetometer. We find evidence for short spin-phonon relaxation times of the order of 1 µs. The temperature dependence of the spin-phonon relaxation time in our experiments is in good agreement with previously published theoretical results. We also established the presence of very short energy diffusion times, that act on a timescale of about 70 ns.Single molecular magnets (SMMs) are the novel class of materials, where identical iso-oriented magnetic molecules are regularly assembled in large crystals. Each of the molecules is built of superexchange-coupled magnetic metal ions; at low temperatures the coupling is so strong that the whole molecule can form a ground state described by a single net spin S.[1, 2, 3] By applying a magnetic field, the net (giant) spin of the molecule can be reversed; such uniaxial magnetic bistability is evidenced by hysteretic magnetization measurements. Quantum tunnelling of magnetization (QTM) though the magnetic anisotropy barrier E = DS 2 , where D is the uniaxial anisotropy parameter, is established by the presence of steps in hysteresis loops of SMMs at millikelvin temperatures. [4,5,6,7,8] At higher temperatures (few kelvin), the thermally activated relaxation can drive the molecule from the ground state spin orientation S over the barrier E to the opposite orientation −S. [1,2,3,4,5,6,7,8] Due to their unique quantum properties and magnetic bistability, SMMs are currently considered as promising candidates for a variety of exciting applications, such as high-density magnetic data storage, quantum computation and magnetoelectronics. [9,10,11,12] In order to develop these applications, it is important to be able to control the spin dynamics. For that purpose, we need to understand how the SMMs interact with their environment. In particular, it is essential to know the lifetimes of the exited spin states τ m , that define the spin-phonon relaxation time T 1 . The use of microwave radiation, that induces selective transitions from the ground state to the excited states, can provide a direct access to the spin dynamics. Such experiments by means of continuous-wave electron spin resonance (ESR) [2,13] and pulsed microwaves time-resolved magnetom- Time (ns) V min V Eq t 0.0 0.2 0.4 0.6 0.8 1.0 -30 -24 -18 -12 -6 m s = -1 0 m s = -9 m s = -8 m s = -7 Energy (K) µ 0 H (T) b) a) FIG. 1: (a) Zeeman diagram of the molecular magnet Fe8. For the lowest spin states the transitions for the microwave frequencies f = 118 GHz and f = 107 GHz occure at a magnetic field of µ0H ≈ 0.2 T. (b) Schematic view of a typical pump-probe experiment at T=2 K. Two microwave pulses p1 and p2 separated by a delay ∆t excite the spin system and the magnetization of the sa...

We present measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time resolved magnetization experiments using a Hall probe magnetometer. We investigate the relaxation behavior of magnetization after the microwave pulse. The analysis of the experimental data is performed in terms of different contributions to the magnetization after-pulse relaxation. We find that the phonon bottleneck with a characteristic relaxation time of ∼ 10 − 100 ms strongly affects the magnetization dynamics. In addition, the spatial effect of spin diffusion is evidenced by using samples of different sizes and different ways of the sample's irradiation with microwaves.

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Precision measurements of the vortex phase diagram in single crystals of the layered superconductor Bi2Sr2CaCu2O 8+δ in oblique magnetic fields confirm the existence of a second phase transition, in addition to the usual first order vortex lattice melting line Hm(T ). The transition has a strong first order character, is accompanied by strong hysteresis, and intersects the melting line in a tricritical point (H ⊥ m , H cr ). Its field dependence and the changing character of the melting line at the tricritical point strongly suggest that the ground state for magnetic fields closely aligned with the superconducting layers is a lattice of uniformly tilted vortex lines. [2,10,11,12]. This behavior in moderate H stops at a temperature dependent characteristic field H cr . Even though melting is still observed above H cr , the variation of H ⊥ m with increasing H becomes much weaker [11,12]. Several controversial interpretations of this changing behavior were proposed, such as layer decoupling [11], a commensurate transition [13], and a matching effect [14].In this Letter we focus on the high-temperature portion of the vortex phase diagram in single crystalline Bi 2 Sr 2 CaCu 2 O 8+δ in oblique fields, which can be established precisely using the well-defined discontinuity of the vortex density at the melting transition. We show that (H ⊥ m , H cr ) corresponds to a tricritical point in the vortex lattice phase diagram, where the melting crosses a novel transition from a composite lattice at low parallel fields, to another tilted lattice structure at high H . The experimental observation of large hysteresis suggests that this transition is strongly first order, consistent with recent predictions [15]. The identification of the vortex ground state at high parallel field as a tilted lattice structure resolves the open problem of the apparent anisotropy factor γ ef f , and allows one to determine the enhancement of H ⊥ m by magnetic coupling. We find the temperature dependence of γ ef f to be consistent with previous observations [16,17] and in quantitative agreement with the proposed model.