arXiv subjects

Z. Wilamowski

Publications and source records attributed to Z. Wilamowski.

13 recordsLinked to original sources

arXiv2012

Indirect Exchange Interaction in Fully Metal-Semiconductor Separated SWCNTs Revealed by ESR

The ESR response from highly metal-semiconductor(M-SC) separated SWCNTs for temperatures T between 0.39 and 200 K is characteristically different for the two systems. The signal originates from defect spins but interaction with free electrons leads to a larger line width for M tubes. The latter decreases with increasing T whereas it increases with T for SC tubes. The spins undergo a ferromagnetic phase transition below around 10 K. Indirect exchange is suggested to be responsible for the spin-spin interaction, supported by RKKY interaction in the case of M tubes. For SC tubes spin-lattice relaxation via an Orbach process is suggested to determine the line width.

cond-mat.mtrl-sci↗

arXiv2011

The RKKY coupling in diluted magnetic semiconductors

This paper is an attempt to modify the classic Ruderman-Kittel-Kasuya-Yosida (RKKY) model to allow the analysis of the magnetic resonance measurements. In our calculations, we follow the treatment of the original authors of the RKKY model but include the finite band splitting, Δ, as a phenomenological parameter. The RKKY exchange is not anymore of Heisenberg type and an anisotropy induced by the direction of carrier magnetization occurs.

cond-mat.mtrl-sci↗

arXiv2011

Spintronics in semiconductors

For the last years spin effects in semiconductors have been of great interest not only in the context of solid state physics, but also for their potential usage in technology. In this paper we give a short review of spintronic materials, in which electron spin as an additional degree of freedom is exploited. Afterwards, we discuss the properties of classic, non-magnetic semiconductors, where the efforts are put on enriching the traditional semiconductor technology engaging the electrical effects of spin effects. Various phenomena and scientific state of the art is highlighted.

cond-mat.mtrl-sci↗

arXiv2010

Rashba field in GaN

We discuss problem of Rashba field in bulk GaN and in GaN/AlGaN two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin-orbit coupling between bulk material and heterostructures. We observe coupled plasmon-cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin-orbit coupling in bulk material, not exceed 400 Gauss, alpha_BIA < 4*10^-13 eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/AlGaN heterostructures originates from properties of the interface.

cond-mat.mtrl-sci↗

arXiv2010

Properties of metal-insulator transition and electron spin relaxation in GaN:Si

We investigate properties of doping-induced metal-insulator transition in GaN:Si by means of electron spin resonance and Hall effect. While increasing the doping concentration, Si-related bands are formed below the bottom of the GaN conduction band. The D0 band of single-occupied Si donor sites is centered 27 meV below the bottom of the GaN conduction band, the D- band of double-occupied Si states at 2.7 meV below the bottom of the GaN conduction band. Strong damping of the magnetic moment occurs due to filling of the D- states at Si concentrations approaching the metal-insulator transition. Simultaneously, shortening of electron spin relaxation time due to limited electron lifetime in the single-occupied D0 band is observed. The metal-insulator transition occurs at the critical concentration of uncompensated donors equal to about 1.6 * 10^18 cm^-3. Electronic states in metallic samples beyond the metal-insulator transition demonstrate non-magnetic character of double-occupied states.

cond-mat.mtrl-sci↗

arXiv2010

Spin Dependent Joule Heating due to Rashba Coupling and Zitterbewegung

Investigating microwave absorption in asymmetric Si quantum wells in an external magnetic field, we discover a spin dependent component of Joule heating at spin resonance. We explain this effect in terms of Rashba spin-orbit coupling which results in a current induced spin precession and Zitterbewegung. Evidence is based on the observation of a specific dependence of the electron spin resonance line shape and its amplitude on the experimental geometry which in some range suggests a "negative" differential power absorption.

cond-mat.mtrl-sci↗

arXiv2003

Collective character of spin excitations in a system of Mn$^{2+}$ spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical an anomalously large Knight shift is observed. Our findings suggests the existence of a magnetic field induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [J. K{ö}nig and A. H. MacDonald, submitted Phys. Rev. Lett. (2002)]

cond-mat↗

arXiv2002

Magnetic polarons in weakly doped high-Tc superconductors

We consider a spin Hamiltonian describing $d$-$d$ exchange interactions between localized spins $d$ of a finite antiferromagnet as well as $p$-$d$ interactions between a conducting hole ($p$) and localized spins. The spin Hamiltonian is solved numerically with use of Lanczos method of diagonalization. We conclude that $p$-$d$ exchange interaction leads to localization of magnetic polarons. Quantum fluctuations of the antiferromagnet strengthen this effect and make the formation of polarons localized in one site possible even for weak $p$-$d$ coupling. Total energy calculations, including the kinetic energy, do not change essentially the phase diagram of magnetic polarons formation. For parameters reasonable for high-$T_c$ superconductors either a polaron localized on one lattice cell or a small ferron can form. For reasonable values of the dielectric function and $p$-$d$ coupling, the contributions of magnetic and phonon terms in the formation of a polaron in weakly doped high-$T_c$ materials are comparable.

cond-mat.supr-con↗

arXiv2001

The longitudinal spin relaxation of 2D electrons in Si/SiGe quantum wells in a magnetic field

The longitudinal spin relaxation time, T1, in a Si/SiGe quantum well is determined from the saturation of the ESR signal. We find values of a few microseconds. Investigations of T1 as a function of Fermi energy, concentration of scattering centers and of the momentum scattering time lead to the conclusion that for high electron mobility the spin relaxation is ruled by the Dyakonov-Perel (DP) mechanism while for low mobility the Elliott-Yaffet mechanism dominates. The DP relaxation is caused by Bychkov-Rashba coupling. Evaluation of the DP mechanism shows that 1/T1 for high electron mobility can be effectively reduced by an external magnetic field. The effect of the degenerate Fermi-Dirac statistics on the DP process is discussed.

cond-mat.soft↗

arXiv2001

Evaluation of the Bychkov- Rashba Field from the Spin Resonance of Electrons in a Si Quantum Well

From spin resonance of two-dimensional (2D) conduction electrons in a modulation doped SiGe/Si/SiGe quantum well structure we find a 2D anisotropy of both the line broadening (dephasing time) and the g-factor. We show that these can be explained consistently in terms of the Bychkov-Rashba (BR) field which here is the dominant coupling between electron motion and spin. We obtain a BR parameter of alpha = 1.1 e-12 eV cm - three orders of magnitude smaller as compared to III-V wells. Extrapolating for low electron concentrations we obtain a g-factor of the Si conduction band of 2.00073+/-0.00010.

cond-mat.mes-hall↗

arXiv2001

Single ion anisotropy of Mn doped GaAs measured by EPR

Electron paramagnetic resonance (EPR) study of MBE grown Mn doped GaAs is presented. The resolved fine structure allows us to evaluate the crystal field parameters of the spin Hamiltonian. The obtained cubic constant is a= -14.1 10^-4/cm. The axial field parameter, D, increases with Mn concentration, x, i.e., with the strain of (Ga,Mn)As layers. Extrapolation of D shows that the single ion anisotropy is the important contribution to the magnetic anisotropy which is observed in ferromagnetic layers with greater Mn concentrations. The analysis of the EPR linewidth shows that native defects of the concentration of 5 10^19/cm^3, but not the Mn ions, are the main origin of crystal field fluctuations.

cond-mat↗

arXiv2000

Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells

Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si quantum wells we determine the carrier-density-dependence of the magnetic susceptibility. Assuming weak interaction we evaluate the density of states at the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11 cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the mobility from q_TF yields good agreement with experimental values justifying the approach. The decrease in D(E_F) is explained by potential fluctuations which lead to tail states that make screening less efficient and - in a positive feedback - cause an increase of the potential fluctuations. Even in our high mobility samples the fluctuations exceed the electron-electron interaction leading to the formation of puddles of mobile carriers with at least 1 micrometer diameter.

cond-mat↗

arXiv1999

New type of antiferromagnetic polaron and bipolaron in HTc - superconductors

The possibility of formation of a new type of polaron based on the quantum aniferromagnet (AF) model is reported. We take into account exchange interactions between localized d-d spins of the AF, as well as the p-d interaction of the AF with p-carriers. The energy minimum is found when maximum charge density occurs on every second spin. The formation of such ``comb''-like polarons results from the damping of quantum fluctuations and the appearance of Van Vleck-like staggered magnetization. Such polarons tend to form pairs coupled by an AF ``glue''.

cond-mat.supr-con↗