arXiv subjects

T. Dietl

Publications and source records attributed to T. Dietl.

87 records · Page 5Linked to original sources

arXiv2002

Ising Quantum Hall Ferromagnet in Magnetically Doped Quantum Wells

We report on the observation of the Ising quantum Hall ferromagnet with Curie temperature $T_C$ as high as 2 K in a modulation-doped (Cd,Mn)Te heterostructure. In this system field-induced crossing of Landau levels occurs due to the giant spin-splitting effect. Magnetoresistance data, collected over a wide range of temperatures, magnetic fields, tilt angles, and electron densities, are discussed taking into account both Coulomb electron-electron interactions and s$-$d coupling to Mn spin fluctuations. The critical behavior of the resistance ``spikes'' at $T \to T_C$ corroborates theoretical suggestions that the ferromagnet is destroyed by domain excitations.

cond-mat.mes-hall↗

arXiv2002

Light and electric field control of ferromagnetism in magnetic quantum structures

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.

cond-mat.mtrl-sci↗

arXiv2002

Kinetic exchange vs. room temperature ferromagnetism in diluted magnetic semiconductors

Guided by the internal-reference rule and the known band offsets in III-V and II-VI diluted magnetic semiconductors, we discuss the feasibility of obtaining p-type conductivity, required for the carrier-induced ferromagnetism, as well as the cases for which the doping by shallow impurities may lead to the ferromagnetism driven by the double exchange. We consider the dependence of kinetic exchange on the p-d hybridization, on the electronic configurations of the magnetic ions, and on the energies of the charge transfer between the valence band of host materials and the magnetic ions. In the case of Mn-based II-VI compounds, the doping by acceptors is necessary for the hole-induced ferromagnetism. The latter is, however, possible without any doping for some of Mn-, Fe- or Co-based III-V magnetic semiconductors. In nitrides with Fe or Co carrier-induced ferromagnetism with T_C>300K is expected in the presence of acceptor doping.

cond-mat.mtrl-sci↗

arXiv2001

Magnetic domains in III-V magnetic semiconductors

Recent progress in theoretical understanding of magnetic anisotropy and stiffness in III-V magnetic semiconductors is exploited for predictions of magnetic domain characteristics and methods of their tuning. We evaluate the width and the energy of domain walls as well as the period of stripe domains in perpendicular films. The computed stripe width d = 1.1 um for Ga_0.957Mn_0.043As/In_0.16Ga_0.84As compares favorably to the experimental value 1.5 um, as determined by Shono et al. [Appl. Phys. Lett. 77, 1363 (2000)].

cond-mat.mtrl-sci↗

arXiv2001

Ferromagnetism of magnetic semiconductors--Zhang-Rice limit

It is suggested that p-d hybridization contributes significantly to the hole binding energy E_b of Mn acceptors in III-V compounds, leading in an extreme case to the formation of Zhang-Rice-like small magnetic polarons. The model explains both strong increase of E_b and evolution of Mn spin-resonance spectrum with the magnitude of valence-band offsets. For such a structure of Mn impurity in III-V materials, possible models accounting for the recently determined Curie temperature of about 940 K in a compensated Ga(0.91)Mn(0.09)N are discussed.

cond-mat.mtrl-sci↗

arXiv2001

Effects of Spin Polarization on Electron Transport in Modulation Doped Cd(1-x)Mn(x)Te/Cd(1-y)Mg(y)Te:I Heterostructures

We examine and identify magnetoresistance mechanisms in 2D system containing a sizable concentration of magnetic ions. We argue that some of these mechanisms can serve as a tool to measure spin polarization. Lack of spin degeneracy and enhanced localization make it possible to detect an additional QHE plateau associated with extended states floating-up in vanishing magnetic field.

cond-mat.mtrl-sci↗

arXiv2000

Carrier-induced ferromagnetism in p-Zn1-xMnxTe

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn1-xMnxTe epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

cond-mat.mtrl-sci↗

arXiv2000

Hole-mediated ferromagnetism in tetrahedrally coordinated semiconductors

A mean field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account: (i) strong spin-orbit and kp couplings in the valence band; (ii) the effect of strain upon the hole density-of-states, and (iii) the influence of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that theory describes the values of the Curie temperatures observed in the studied systems as well as explain the directions of the easy axis and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces unusual sign, magnitude, and temperature dependence of magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of novel ferromagnetic semiconductor systems are described.

cond-mat.mtrl-sci↗

arXiv1999

Weak localization in the 2D metallic regime of Si-MOS

The negative magnetoresistance due to weak localization is investigated in the two-dimensional metallic state of Si-MOS structures for high conductance values between 35 and 120 e^2/h. The extracted phase coherence time is equal to the momentum relaxation time at 10 K but nearly 100 times longer at the lowest temperature. Nevertheless, only weak logarithmic corrections to the conductivity are present in the investigated temperature and concentration range thus proving the absence of strong quantum effects due to electron-electron interaction. From saturation effects of the phase coherence time a lower boundary for spin-orbit scattering of about 200 ps is estimated.

cond-mat.mes-hall↗

arXiv1999

Carrier induced ferromagnetic interactions in p-doped Zn(1-x)MnxTe epilayers

p-type doping of molecular-beam-epitaxy grown layers of the diluted magnetic semiconductor Zn(1-x)MnxTe is achieved by using an active nitrogen cell. The strong interaction between the localized Mn spins and the holes deeply modifies the transport properties (metal-insulator transition, spin-dependent Hall effect). In spite of the weak localization of the carriers at low temperature, the holes clearly induce a ferromagnetic interaction between the localized spins, which is discussed as a function of Mn content and hole concentration.

cond-mat.mtrl-sci↗

arXiv1999

Magnetotransport properties of (Ga,Mn)As investigated at low temperature and high magnetic field

Magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As have been investigated. Measurements at low temperature (50 mK) and high magnetic field (<= 27 T) have been employed in order to determine the hole concentration p = 3.5x10^20 cm ^-3 of a metallic (Ga0.947Mn0.053)As layer. The analysis of the temperature and magnetic field dependencies of the resistivity in the paramagnetic region was performed with the use of the above value of p, which gave the magnitude of p-d exchange energy |N0beta | ~ 1.5 eV.

cond-mat.mtrl-sci↗

arXiv1999

Quantum ballistic transport in constrictions of n-PbTe

Conductance of submicron constrictions of PbTe:Bi was studied up to 8T and between 4.2K and 50mK. The structures were fabricated by electron beam lithography and chemical etching of high--electron mobility films grown by MBE on BaF_2. In the moderately strong magnetic fields perpendicular to the current, B>1T, the conductance shows accurate quantization in the units of 1e^2/h as a function of the side-gate voltage. In the absence of the field, a temperature-independent step structure, with an average step height approx. 1e^2/h, is observed. It is suggested that such a quantization may reflect the lifting of the Kramers degeneracy by the exchange interaction among the electrons, effective despite a large dielectric constant of bulk PbTe.

cond-mat.mes-hall↗

arXiv1998

Realistic Electron-Electron Interaction in a Quantum Wire

The form of an effective electron-electron interaction in a quantum wire with a large static dielectric constant is determined and the resulting properties of the electron liquid in such a one-dimensional system are described. The exchange and correlation energies are evaluated and a possibility of a paramagnetic-ferromagnetic phase transition in the ground state of such a system is discussed. Low-energy excitations are briefly described.

cond-mat.str-el↗

arXiv1998

Magnetoconductance noise and irreversibilities in submicron wires of spin-glass n-CdMnTe

Signatures of spin-glass freezing such as the appearance of 1/f conductance noise, the recovery of universal conductance fluctuations, aging, as well as magnetic and thermal irreversibilities are detected in mesoscopic wires of CdMnTe:I at millikelvin temperatures. Spectral characteristics of conductance time series are consistent with the droplet model of short-range spin-glasses.

cond-mat.mes-hall↗

arXiv1997

Interaction effects at the magnetic-field induced metal-insulator transition in Si/SiGe superlattices

A metal-insulator transition was induced by in-plane magnetic fields up to 27 T in homogeneously Sb-doped Si/SiGe superlattice structures. The localisation is not observed for perpendicular magnetic fields. A comparison with magnetoconductivity investigations in the weakly localised regime shows that the delocalising effect originates from the interaction-induced spin-triplet term in the particle-hole diffusion channel. It is expected that this term, possibly together with the singlet particle-particle contribution, is of general importance in disordered n-type Si bulk and heterostructures.

cond-mat.mes-hall↗