arXiv subjects

Agnieszka Wołoś

Publications and source records attributed to Agnieszka Wołoś.

4 recordsLinked to original sources

arXiv2025

Self-organization mechanism in Bridgman-grown MnBi2Te4/(Bi2Te3)n: influence on layer sequence and magnetic properties

The growth of high-quality magnetic topological insulator crystals by the Bridgman method remains challenging due to thermodynamic limitations inherent to this technique. Nevertheless, this approach continues to provide bulk materials with significantly reduced free carrier concentrations compared to epitaxial methods. Here, we investigate the Inverted Vertical Bridgman growth of MnBi2Te4/(Bi2Te3)n crystals, with particular emphasis on the structural ordering of MnBi2Te4 septuple layers within the Bi2Te3 quintuple-layer matrix and its influence on magnetic properties. Through a detailed analysis of growth dynamics, we identify four distinct stages, including a turbulent flow regime promoting pure MnBi2Te4 phase, rapid MnTe precipitation reducing Mn content in the melt, a stationary growth phase supporting ordered stacking of septuple and quintuple layers, and a final stage marked by flow cessation and defect formation. We demonstrate that septuple layer spacing is inversely correlated with MnTe supersaturation due to the diffusion-limited incorporation in stationary growth phase. Magnetic characterization reveals antiferromagnetic ordering in pure MnBi2Te4 phase and in MnBi2Te4/(Bi2Te3)n heterostructure, with ferromagnetism emerging for wider septuple layer spacing. We determine critical temperatures for observed antiferromagnetic and ferromagnetic phase transitions and magnetic anisotropy constants for ferromagnetic samples. Our findings highlight key growth parameters governing magnetic and structural quality, offering a pathway to scalable synthesis of layered topological insulators with tunable magnetic properties.

cond-mat.mtrl-sci↗

arXiv2017

Surface-enhanced Raman scattering of graphene caused by self-induced nanogating by GaN nanowire array

A constant height of gallium nitride (GaN) nanowires with graphene deposited on them is shown to have a strong enhancement of Raman scattering, whilst variable height nanowires fail to give such an enhancement. Scanning electron microscopy reveals a smooth graphene surface which is present when the GaN nanowires are uniform, whereas graphene on nanowires with substantial height differences is observed to be pierced and stretched by the uppermost nanowires. The energy shifts of the characteristic Raman bands confirms that these differences in the nanowire height has a significant impact on the local graphene strain and the carrier concentration. The images obtained by Kelvin probe force microscopy show clearly that the carrier concentration in graphene is modulated by the nanowire substrate and dependent on the nanowire density. Therefore, the observed surface enhanced Raman scattering for graphene deposited on GaN nanowires of comparable height is triggered by self-induced nano-gating to the graphene. However, no clear correlation of the enhancement with the strain or the carrier concentration of graphene was discovered.

cond-mat.mes-hall↗

arXiv2016

Stable topological insulators achieved using high energy electron beams

Topological insulators are transformative quantum solids with immune-to-disorder metallic surface states having Dirac band structure. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift ($\sim 2.5$ MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap, and reach the charge neutrality point (CNP). Controlling the beam fluence we tune bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional (2D) character on the order of ten conductance quanta $G_0 =e^2/h$, and reveals, both in Bi$_2$Te$_3$ and Bi$_2$Se$_3$, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size.

cond-mat.mtrl-sci↗

arXiv2015

Enhanced Raman scattering and weak localization in graphene deposited on GaN nanowires

The influence of GaN nanowires on the optical and electrical properties of graphene deposited on them was studied using Raman spectroscopy and microwave induced electron transport method. It was found that interaction with the nanowires induces spectral changes as well as large enhancement of Raman scattering intensity. Surprisingly, the smallest enhancement (about 30-fold) was observed for the defect induced D' process and the highest intensity increase (over 50-fold) was found for the 2D transition. The observed energy shifts of the G and 2D bands allowed to determine carrier concentration fluctuations induced by GaN nanowires. Comparison of Raman scattering spatial intensity maps and the images obtained using scanning electron microscope led to conclusion that vertically aligned GaN nanowires induce a homogenous strain, substantial spatial modulation of carrier concentration in graphene and unexpected homogenous distribution of defects created by interaction with nanowires. The analysis of the D and D' peak intensity ratio showed that interaction with nanowires also changes the probability of scattering on different types of defects. The Raman studies were correlated with weak localization effect measured using microwave induced contactless electron transport. Temperature dependence of weak localization signal showed electron-electron scattering as a main decoherence mechanism with additional, temperature independent scattering reducing coherence length. We attributed it to the interaction of electrons in graphene with charges present on the top of nanowires due to spontaneous and piezoelectric polarization of GaN. Thus, nanowires act as antennas and generate enhanced near field which can explain the observed large enhancement of Raman scattering intensity.

cond-mat.mes-hall↗