We report on electrical measurements of the effective density of states in the ferromagnetic semiconductor material (Ga,Mn)As. By analyzing the conductivity correction due to enhanced electronelectron interaction the electrical diffusion constant was extracted for (Ga,Mn)As samples of different dimensionality. Using the Einstein relation allows to deduce the effective density of states of (Ga,Mn)As at the Fermi energy.PACS numbers: 75.50.Pp, The ferromagnetic semiconductor (Ga,Mn)As [1] has been studied intensely over the last decade and has become a model system for future spintronics applications [2,3]. With typical Mn-concentrations between 1 % and 15 % maximum Curie temperatures of up to ∼ 180 K have been reported [4,5]. Mn atoms on Ga-sites provide both holes and magnetic moments. For Mn concentrations larger than 1 % the impurity wavefunctions at the Fermi energy overlap and a metallic state forms. The ferromagnetic order between the magnetic moments of the Mn-ions is mediated by the delocalized holes [6]. A topic of current debate is whether the holes reside in an impurity band, detached and above the valence band or in the valence band [7]. A mean field picture based on the latter scenario allowed to predict, e.g. Curie temperature [6] or magnetocrystalline anisotropies [8] in (Ga,Mn)As correctly. On the other hand optical absorption experiments, carried out, e.g. in Ref. [9,10], suggest that even for high manganese concentrations of up to 7 % the Fermi energy stays in an impurity band, detached from the valence band, with a high effective hole mass of order ten free electron masses m e [9]. However, there is also indication that the impurity band and the valence band have completely merged as discussed in Ref. [7] and references therein. In the present letter we make use of the well known quantum mechanical conductivity correction due to electron-electron interaction (EEI) to extract the diffusion constant and hence the density of states at the Fermi energy, N (E F ). The electrically measured values of N (E F ) will be compared with recent theoretical calculations.In ferromagnetic (Ga,Mn)As the conductivity is decreasing with decreasing temperature below 10 K. This conductivity decrease can be explained by enhanced electron-electron interaction [11]. The effect of EEI arises from a modified screening of the Coulomb-potential due to the carriers' diffusive motion and depends on the dimensionality of the conductor [12]. As the conductivity decrease due to enhanced electron-electron interaction is depending on the electrical diffusion constant D, a detailed analysis of the conductivity decrease, different for different dimensionality, provides experimental access to the diffusion constant. Using the Einstein relation σ = N (E F )De 2 , with the conductivity σ, the effective density of states at Fermi's energy, N (E F ), can be determined.To investigate electron-electron interaction in quasi 1D, 2D and 3D systems we fabricated Hall-bar mesas (2D and 3D) and wire arrays (1D and crossover regime from 1D to ...
The fractional quantum Hall ͑FQH͒ effect is reported in a high mobility CdTe quantum well at millikelvin temperatures. Fully developed FQH states are observed at filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state for which the lowest energy excitation is not a spin flip. This can be accounted for by the relatively high intrinsic Zeeman energy in this single valley two-dimensional electron gas. FQH minima are also observed in the first excited ͑N =1͒ Landau level at filling factor 7/3 and 8/3 for intermediate temperatures. In contrast, the 5/2 FQH state remains absent down to T ϳ 10 mK. Interacting carriers in certain fractional quantum Hall ͑FQH͒ ground states can have opposite spins provided the Zeeman energy is sufficiently small. This is typically observed in GaAs-based two-dimensional electron gases ͑2DEGs͒, where an increase in the Zeeman energy induces a change in the spin polarization of the ground state from unpolarized to fully spin polarized. This transition has been reported for the FQH states at filling factor =4/ 3, =8/ 5, =2/ 3, or =2/ 5, 1-4 as well as in a GaAs 2D hole gas. 5 Subsequently, this behavior was elegantly interpreted within the composite fermions ͑CFs͒ model 6 for the FQH effect by invoking Zeeman energy-induced crossings between spinsplit composite fermion Landau levels ͑LLs͒, leading to possible changes in the spin configuration of the ground state. 7 More recently, the =4/ 3 FQH state was investigated in a strained Si quantum well, 8 where the associated resistance minimum was found to maintain its strength with increasing Zeeman energy, which was interpreted as the consequence of a spin-polarized ground state. The latter work addresses the interesting question of how the FQH effect manifests itself in a 2D system with an intrinsically larger Zeeman energy than in GaAs. However, the influence of the valley degeneracy inherent in Si is another degree of freedom that may also interfere with the FQH physics.In the present work, we study the evolution of FQH states under relatively high intrinsic Zeeman energy in a single valley electron system. This is made possible by investigating the FQH effect in a high-quality 2D electron gas in CdTe, a single valley, direct gap, semiconductor in which the bare electronic g factor is about four times larger than in GaAs. A fundamental asset of this system is the possibility to incorporate magnetic ions to form a so-called diluted magnetic semiconductor, which offers possible applications in the fields of spintronics and quantum computing. The transport measurements performed at millikelvin temperature reveal fully developed FQH states ͑i.e., zero longitudinal resistance and exact quantization of the Hall resistance͒ in the upper spin branch of the lowest ͑N =0͒ LL, which constitutes to our knowledge the first observation of the FQH effect in a II-VI semiconductor. Tilted magnetic fields experiments up to 28 T show no significant changes in the FQH gap both at filling factor 4/3 and 5/3, a behavior typical of spin-pola...
We studied phase-coherent phenomena in mesoscopic permalloy ͑Ni 81 Fe 19 ͒ samples by exploring lowtemperature transport. Both differential conductance as a function of bias voltage and magnetoconductance of individual wires display conductance fluctuations. Analysis of these fluctuations yields a phase coherence length of ϳ250 nm at 25 mK as well as a 1 / ͱ T temperature dependence. To suppress conductance fluctuations by ensemble averaging, we investigated low-temperature transport in wire arrays and extended permalloy films. In these samples we have measured conductance corrections which stem from electron-electron interaction; but attempts to detect signatures of weak localization were without success.
An error of a factor of 2 is displayed on the x axis of Fig. 2(d): It should read B total = 2 × 5 = 10 T instead of B total = 5 T. In the main text, page 3, paragraph 5, the sentence "For ν = 4/3(ν * CF = 2), these crossings occur for B total ∼ 3.4 T and B total ∼ 6.8 T, respectively," should be replaced with "For ν = 4/3(ν * CF = 2), these crossings occur for B total ∼ 6.8 T and B total ∼ 13.6 T, respectively. . . ."Also, the sentence "The same conclusions are drawn for the ν = 5/3(ν * CF = 1) FQH state, provided B total > 3 T" should be replaced with "The same conclusions are drawn for the ν = 5/3(ν * CF = 1) FQH state, provided B total > 6 T." None of the interpretations and conclusions of the paper are affected by these errors.
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