Laboratorium naukowe magnetotransportu przy niskich i ultra niskich temperaturach
A magnetotransport laboratory at low and ultra low temperatures
Dr. Grzegorz Tomaka - Laboratory Manager
Dr. Michał Marchewka
Dr Dariusz Żak
Mgr Paweł Śliż
The Magneto-transport Laboratory is equipped with a superconducting magnet system that generates a magnetic field up to 14 Tesla. With the use of 3He Cryogenic system it is possible to measure the magneto-transport phenomena in the temperature range from 0.3K to 300K. The electron transport study are performing in the low-dimensional structures based on the III-V and II-VI Compounds produced in the MBE Laboratory. In particular, next phenomena are observed: Quantum Hall Effect (IQHE), Shubnikov de Hassa Oscillation (SdH) and Magneto-phonon Resonance (MPR). Samples for measurements are made in the Laboratory of Lithography.
Electronic transport in quantum structures - single quantum wells (SQW), double quantum wells (DQW) and structures containing many quantum wells (MQW)
The research of such objects in our Laboratory is mainly about magneto-transport in strong magnetic fields. There are two types of electron-electron transport: parallel and perpendicular. Two oscillating effects - the Shubnikov-de Haas effect (SdH), the Hall effect quantum effect (IQHE), and the MPR combine to form comprehensive methods to determine 2DEG parameters in the studied structures. SdH and IQHE are observed at temperatures (0.3K - 6K) allowing them to determine 2DEG concentrations, electron mobility, Fermi levels, and Landau energy levels. MPR with phonon absorption is observed at temperatures above 77K, MPR peak positions in the magnetic field allow for energy calculations: Landau's levels, effective mass of electrons, and electron mobility at temperatures close to the operating conditions of instruments operating on the basis of the investigated structures.
- E.M. Sheregii, D. Ploch, M.Marchewka, G. Tomaka, A.Kolek, A, Stadler, K. Mleczko, W. Strusiński, A. Jasik, R. Jakiela, Low Temperature Physics, 30, 1146 (2004)
- G.Tomaka, E.M. Sheregii, T.Kąkol, W. Strupiński, A. Jasik and R.Jakiela, Charge carriers parameters in the conductive channels of HEMTs,Physica Status Solidi (a), 195, 127 (2003),
- G. Tomaka, E.M. Sheregii, T. Kąkol, W. Strupinski, R. Jakiela, A. Kolek, A. Stadler, K. Mleczko, Magnetotransport in single InGaAs quantum wells of different shapes, Cryst. Res. Tech., 38, 407 (2003).
In parallel and perpendicular magneto-transport in DQW, there are effects associated with tunneling:
• Splitting of symmetric and anti-symmetric electrons. DSAS-gap;
• DSAS gap is proportional to magnetic field B. Proportionality disappears in strong magnetic fields.
The experiments conducted for DQW revealed the effect of the oscillation of SdH with the perpendicular shape of the QW potential and the triangular. In order to explain the phenomenon, two Fermi-acid levels were introduced, which characterize the two electronic subsystems of symmetric and antimetallic states in DQW.
The presence of two Fermi levels means that the DQW can be divided into two subsystems with symmetrical and anti-symmetrical wave functions. You can treat these two subsystems as weakly interacting, because electron-electron interaction does not mix, meaning that they are described by their own decomposition functions and characterized by their own acid levels of Fermi energy.
MPR observations in DQWs of different QWs shapes and different electron concentration values in the 77K- 90K range in impulse magnetic fields up to 40T were also performed. Clear MPR-oscillations were observed for DQWs with perpendicular QWs, and in the case of triangular shape, a wide band consisting of a large number of peaks ranging from 5T to 25T was recorded. Four types of LO-phonons were used to interpret the MPR of the oscillation in DQW. In this way, each MPR-peak corresponds to a group of electron transitions. It is noticeable in the formation of MPR-peaks, the predominant role of InAs-like LO-phonons.
Also in the MPR in DQWs, the electronic statistics for the degenerate 2DEG are also important: electrons pass through the Fermi level. Shielding interchangeability is particularly important - shielding is removed in a strong magnetic field, resulting in a DSAS-gap increase until the quantum limit is reached. Once the quantum limit DSAS-gap remains constant, this is in line with the predictions of D. Huang and M.O. Monosrech model (Phys. Rev. B 54, 2044 (1996)).
- M. Marchewka, E.M. Sheregii, I. Tralle, D. Ploch, G. Tomaka, M. Furdak, A. Kolek, A. Stadler, K. Mleczko, D. Zak, W. Strupiński, A. Jasik, R. Jakiela, Magnetosp[ectroscopy of Double Quantum Wells, Physica E, 40, 894-904, (2008).
- M. Marchewka, E.M. Sheregii, I. Tralle, G. Tomaka, and D. Ploch, Weakly interacting Symmetric and antisymmetric states in bilayer Systems, Inter. J. of Moder. Phys. B, 21, 15181 (2007)
- D. Płoch, E.M. Sheregii, M. Marchewka, M. Woźny and G. Tomaka, Magnetophonon Resonance in Double Quantum Wells, Phys. Rev. B79,195434 (2009)
- M. Marchewka, E.M. Sheregii, I. Tralle, D. Płoch, A. Marcelli and M. Piccinini Optically detected symmetric and anti-symmetric states in DQW at room temperature, Phys. Rev. B80, 125316 (2009).
he study revealed the subtle structure of MPR peaks in MQW's parallel transport in GaAs / AlGaAs-heterostrukturch (ten GaAs-QW and eleven AlGaAs-barriers respectively).
The cleavage of electrons caused by tunneling in the coupled ten QW's was not manifested in the SdH and IQHE oscillations at low temperatures (1.6K), the more so it did not appear at temperatures above 77K where the MPR is observed.
The subtle structure of MPC peaks in MQW's observed is due to the presence of different QW phonons and barriers.
- E. M. Sheregii, Magnetophonon resonance as method of studying the vertical and parallel transport in superlattices and MQW structures. SPIE Proceedings series, v. 3725, Bellingham, Washington, 1998, pp. 134-143.
- G. Tomaka, J. Cebulski, E. M. Sheregii, W. Ściuk, W. Strupiriski, and L. Dobrzariski, Role of the thermal stress on the Magnetophonon peak structure in the parallel transport o GaAs/AIGaAs Multiple Quantum Wells, Acta Phys. Pol. A, 94, 597 (1998).
- G. Tomaka, J. Cebulski, E. M. Sheregii, W. Ściuk, W. Strupiriski, and L. Dobrzariski, Magnetophonon Resonance as method of controlling of the thermal stress in the multiple quantum wells, Material Science and Engeneering A288, 138 (2000).
- G. Tomaka, E.M. Sheregii, T. Kakol, Application of magnetophonon resoance to control the thermal stress in multiple quantum wells, Material Science and Engeneering, B 80, 173 (2001).
- Ploch, E Sheregii, M Marchewka and G Tomaka, Magnetophonon resonance in multimode lattices and two-dimensional structures (DQW), J.Phys.: Conf. Ser. 92, 012066 (2007).
- M.Zybert, M. Marchewka, G.Tomaka, E.M. Sheregii, Electron - electron interaction in Multiple quantum Wells Physica E 44,2056 (2012)
Doctoral dissertation Grzegorz Tomaki "Oscillatory effects in structures with many quantum wells" was awarded by the Minister of Science and Higher Education in 2009.
Topological Insulators based on semimetal Alloys HgCdTe
The results of the magneto-transport measurements (longitudinal magneto-resistance Rxx and the Hall resistance Rxy) over a wide interval of temperatures for several samples of Hg1−x CdxTe (x ≈ 0.13-0.15) grown by MBE is presented in this paper. An amazing temperature stability of the SdH-oscillation period and amplitude is observed in the entire temperature interval of measurements up to 50 K. Moreover, the quantum Hall effect (QHE) behaviour of the Hall resistance was shown in the same temperature interval. These peculiarities of the Rxx and Rxy for strained thin layers are interpreted using quantum Hall conductivity (QHC) on topologically protected surface states (TPSS). In the case of not strained layers it is assumed that the QHC on the TPSS contributes also to the conductance of the bulk samples.
Landau level index for the data of Fig. 2b, 3b and 4 plotted as a function of inverse magnetic field. The intercept of this plot for infinite magnetic field gives a value of -1/2 for samples AB9, B9, B4 and B6, which provides evidence that the observed IQHC can be well described by the two Dirac cones model. In the case of the B5 sample a spin splitted maxima of the SdH-oscillations are visible and the positions corresponded to the -N Landau index, are plotted what gives intercept ”0”. In the insert, the proposed energy band structure with TPSS is shown.
Proposed conception of the electron transport in the semimetal bulk Hg1-xCdxTe: the 2D-TPSS on parallel top and bottom surfaces (as well as on the side walls) form the metallic Berry phase (red colour) surround the bulk semimetal part (grey colour). As a result, metallic conductance on surfaces dominates the conductance of the entire sample.
The SdH oscillations and the IQHE for 4.2 K for 100 nm wide Hg0.865Cd0.135Te and b) the LL’s fan calculated for two Dirac cones for two interfaces; c) and d) represent the LL’s under the magnetic field region between 5 and 8T and 8 and 14 T respectively.
The experimental results on magneto-transport (QHC and SdH) obtained for the strained 100 nm thickness
Hg1−x CdxTe layer are interpreted on the basis of the 8x8 kp model and an advantage of the Hg1−xCdxTe as topological insulators is shown spectacularly: in comparison with pure HgTe the energy dispersion of semi-metallic HgCdTe is closer to linearity in the wider range of the momentum  what lead to an increase in the attractiveness of the Topological Insulator based on semimetal HgCdTe alloy for future applications.
- G. Tomaka, J. Grendysa, P. ´ Sli˙z, C. R. Becker, J. Polit, R. Wojnarowska, A. Stadler, and E. M. Sheregii, High-temperature stability of electron transport in semiconductors with strong spin-orbital interaction, PHYSICAL REVIEW B 93, 205419 (2016)
- M. Marchewka, J. Grendysa, D. Żak, G. Tomaka, P. Śliż and E.M. Sheregii, Massless Dirac fermions in semimetal HgCdTe, Solid State Communication, 250, 104-107 (2017)
- G. TOMAKA, J. GRENDYSA, M. MARCHEWKA, P. SLIŻ, C. R. BECKER, A. STADLER, and E.M. SHEREGII, Topological insulators based on the semi-metallic HgCdTe, Opto-Electronics Review 25(3), 188-197 · (2017)