Our experimental and theoretical study of the non-crystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As is aimed at exploring basic physical aspects of this relativistic transport effect. The non-crystalline AMR reflects anisotropic lifetimes of the holes due to polarized Mn impurities while the crystalline AMR is associated with valence band warping. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential. We develop experimental methods directly yielding the non-crystalline and crystalline AMR components which are then independently analyzed. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. We discuss generic implications of our experimental and theoretical findings including predictions for non-crystalline AMR sign reversals in dilute moment systems. Anisotropic magnetoresistance (AMR) is a response of carriers in magnetic materials to changes of the magnetization orientation. Despite its importance in magnetic recording technologies the understanding of the microscopic physics of this spin-orbit (SO) coupling induced effect is relatively poor. Phenomenologically, AMR has a non-crystalline component, arising from the lower symmetry for a specific current direction, and crystalline components arising from the crystal symmetries [1,2]. In ferromagnetic metals, values for these coefficients can be obtained by numerical ab initio transport calculations [3], but these have no clear connection to the standard physical model of transport arising from spin dependent scattering of current carrying low mass s-states into heavymass d-states [4]. Experimentally, the non-crystalline and, the typically much weaker, crystalline AMR components in metals have been indirectly extracted from fitting the total AMR angular dependences [2].Among the remarkable AMR features of (Ga,Mn)As ferromagnetic semiconductors are the opposite sign of the non-crystalline component (compared to most metal ferromagnets) and the crystalline terms reflecting the rich magnetocrystalline anisotropies [5,6,7,8,9,10,11]. Microscopic numerical simulations [6,12] consistently describe the sign and magnitudes of the non-crystalline AMR and capture the more subtle crystalline terms associated with e.g. growth-induced strain [8,12]. As in metals, however, the basic microscopic physics of the AMR still needs to be elucidated which is the aim of the work presented here.Theoretically, we separate the non-crystalline and crystalline components by turning off and on band warping and match numerical microscopic simulations with model analytical results. This provides the physical interpretation of the origin of AMR, and of the sign of the noncrystalline term in particular. Experimentally, we obtain direct and independent access to the non-crystalline and crystalli...
We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible substitutional MnGa dopings. The growth and post-growth annealing procedures were optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the midinfrared absorption spectra whose position exhibits a prevailing blue-shift for increasing Mn-doping. In the visible range, a peak in the magnetic circular dichroism blue shifts with increasing Mndoping. These observed trends confirm that disorder-broadened valence band states provide a better one-particle representation for the electronic structure of high-doped (Ga,Mn)As with metallic conduction than an energy spectrum assuming the Fermi level pinned in a narrow impurity band.PACS numbers: 74.20. Mn, 74.25.Nf, 74.72.Bk, 74.76.Bz The discovery of ferromagnetism in (Ga,Mn)As above 100 K [1] opened an attractive prospect for exploring the physics of magnetic phenomena in doped semiconductors and for developing advanced concepts for spintronics. Assessment of a wide range of magnetic and transport properties of the material [2][3][4] showed that in ferromagnetic (Ga,Mn)As with Mn dopings x > 1%, disorderbroadened and shifted host Bloch bands represent a useful one-particle basis for describing this mixed-crystal degenerate semiconductor. The common kinetic-exchange model implementation of this valence band theory and the more microscopic tight-binding Anderson model or ab-initio density functional theory can all be shown [5] to be mutually consistent on the level of atomic and orbital resolved band structure. The main utility of valence band theories have been in providing a qualitative and often semi-quantitative description of phenomena originating from the exchange split and spin-orbit coupled electronic structure and in assisting the development of prototype spintronic devices [4]. Other basic physical properties of (Ga,Mn)As, namely those reflecting the vicinity of the metal-insulator transition and localization and electronelectron interaction effects, remain to be fully understood and require to go beyond the commonly employed perturbative or disorder averaged Bloch-band theories.In the insulator non-magnetic regime (x 1%), the system is readily described by localized Fermi level states residing inside a narrow impurity band separated from the valence band by an energy gap of magnitude close to the isolated Mn Ga impurity binding energy. Recently, a debate has been stirred by proposals, based in particular on optical spectroscopy measurements [6], that the narrow impurity band persists in high-doped (Ga,Mn)As with metallic conduction. Several phenomenological variants of the impurity band model have been proposed for the high-doped regime [6][7][8][9][10] which are mutually inconsistent from the perspective of the assumed atomic orbital nature of the impurity band states [5]. Further theoretical inconsistencies arise when recreating ...
We have found that Fermi contours of a two-dimensional electron gas at GaAs/AlxGa1−xAs interface deviate from a standard circular shape under the combined influence of an approximately triangular confining potential and the strong in-plane magnetic field. The distortion of a Fermi contour manifests itself through an increase of the electron effective cyclotron mass which has been measured by the cyclotron resonance in the far-infrared transmission spectra and by the thermal damping of Shubnikov-de Haas oscillations in tilted magnetic fields with an in-plane component up to 5 T. The observed increase of the cyclotron effective mass reaches almost 5 % of its zero field value which is in good agreement with results of a self-consistent calculation. 72.40, 72.20
This article is devoted to the problem of the validity of the reciprocity theorem in high-temperature superconductors (HTSC). The violation of the reciprocity theorem in zero external magnetic fields has been studied. Experimental data obtained for two different superconducting materials: BiSrCaCuO and YBaCuO are presented. Results show that the basic form of the reciprocity theorem (without consideration of any additional anisotropy) is not valid near the critical temperature. We assume that the reciprocity theorem breaking is connected with the existence of an extraordinary transverse electric field originated from additional anisotropy and more general form of the reciprocity relations should be valid. However, the origin of this anisotropy is not clear yet. We suggest that the vortex-antivortex dynamics model taking into account vortex guiding can be responsible for the observed effect. Also the explanation based on weak P and T symmetry breaking in HTSC which is supported by the observation of the spontaneous magnetisation can not be excluded.Comment: 15 pages, 11 figure
Longitudinal and transverse voltages have been measured in zero external magnetic fields. In close vicinity of the superconducting transition nonzero transverse voltage has been observed while far away from Tc, both above and below no such voltage has been detected. The value of the transverse resistivity depends on the value of the transport current. Several models have been discussed taking into account also the penetration of self field due to the applied transport current. It seems that observed results can be explained using the Kosterlitz-Thouless model as a result of an unpairing of vortex-antivortex pairs created below Tkt due to fluctuations. At Tkt free vortices and antivortices are created and can contribute to a dissipation of energy. Their movement should also be responsible for the observed nonzero transverse voltage.Comment: 3 pages in Latex, 3 figs.ep
Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow us to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent field calculations of the electronic structure.
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