المؤلفون: Śnieżek, D., Wróbel, Jarosław, Kojdecki, M., Śliwa, C., Schreyeck, S., Brunner, K., Molenkamp, L. W., Karczewski, G., Wróbel, Jerzy

المصدر: Phys. Rev. B 107, 045103 (2023)

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics

الوصف: Measurements of magnetotransport in SnTe/PbTe heterojunctions grown by the MBE technique on (001) undoped CdTe substrates were performed. At low magnetic fields, quantum corrections to conductivity were observed that may be attributed to the presence of topological states at the junction interface. For a sample with 5 nm thick SnTe layer, the data analysis suggests that midgap states are actually gapped. However, the phase coherence effects in 10 nm and 20 nm SnTe/PbTe samples are fully explained assuming existence of gapless Dirac cones. Magnetotransport at higher magnetic fields is described in the framework of mobility spectrum analysis (MSA). We demonstrate that the electron- and hole-like peaks observed simultaneously for all SnTe/PbTe heterojunctions may originate from the concave and convex parts of the energy isosurface for topological states -- and not from the existence of quasiparticles both carrying negative and positive charges. This interpretation is supported by numerical calculations of conductivity tensor components for gapless (100) Dirac cones, performed within a classical model and based on the solutions of Boltzmann transport equation. Our approach shows the feasibility of MSA in application to magnetotransport measurements on topological matter.

الوصول الحر: http://arxiv.org/abs/2208.05262Test

المؤلفون: Scheffler, L., Werther, J., Gas, K., Schumacher, C., Gould, C., Sawicki, M., Kleinlein, J., Molenkamp, L. W.

مصطلحات موضوعية: Condensed Matter - Materials Science

الوصف: A detailed study of the influence of molecular beam epitaxial growth conditions on the structural and magnetic characteristics of CuMnSb films on lattice matched GaSb is presented. For a set of nine 40~nm thick layers, the Mn and Sb fluxes are varied to produce material with different elemental compositions. It is found that the layers grown under a relative Mn to Sb flux ratio of $\Phi_{\text{Mn}}$/$\Phi_{\text{Sb}}=1.24\pm0.02$ are closest to the stoichiometric composition, for which the N\'{e}el temperature ($T_\text{N}$) attains its maximum values. Mn-related structural defects are believed to be the driving contribution to changes in the vertical lattice parameter. Having established the optimum growth conditions, a second set of samples with CuMnSb layer thickness varied from 5 to 510 nm is fabricated. We show that for sufficiently large thicknesses, the magnetic characteristics ($T_\text{N}\simeq62$ K, Curie-Weiss temperature $\Theta_\text{CW} = -100$ K) of the stoichiometric layers do correspond to the parameters reported for bulk samples. On the other hand, we observe a reduction of $T_\text{N}$ as a function of the CuMnSb thickness for our thinnest layers. All findings reported here are of particular relevance for studies aiming at the demonstration of N\'{e}el vector switching and detection in this noncentrosymmetric antiferromagnet, which have been recently proposed.
Comment: The following article has been accepted by Applied Physics Letters. After it is published, it will be found at https://doi.org/10.1063/5.0092828Test

الوصول الحر: http://arxiv.org/abs/2206.13258Test

المؤلفون: Liu, N., Schreyeck, S., Fijalkowski, K. M., Kamp, M., Brunner, K., Gould, C., Molenkamp, L. W.

مصطلحات موضوعية: Condensed Matter - Materials Science

الوصف: MnBi2Te4 has recently been predicted and shown to be a magnetic topological insulator with intrinsic antiferromagnetic order. However, it remains a challenge to grow stoichiometric MnBi2Te4 films by molecular beam epitaxy (MBE) and to observe pure antiferromagnetic order by magnetometry. We report on a detailed study of MnBi2Te4 films grown on Si(111) by MBE with elemental sources. Films of about 100 nm thickness are analyzed in stoichiometric, structural, magnetic and magnetotransport properties with high accuracy. High-quality MnBi2Te4 films with nearly perfect septuple-layer structure are realized and structural defects typical for epitaxial van-der-Waals layers are analyzed. The films reveal antiferromagnetic order with a Neel temperature of 19 K, a spin-flop transition at a magnetic field of 2.5 T and a resistivity of 1.6 mOhm cm. These values are comparable to that of bulk MnBi2Te4 crystals. Our results provide an important basis for realizing and identifying single-phase MnBi2Te4 films with antiferromagnetic order grown by MBE.

الوصول الحر: http://arxiv.org/abs/2111.13960Test

المؤلفون: Fijalkowski, K. M., Liu, N., Hartl, M., Winnerlein, M., Mandal, P., Coschizza, A., Fothergill, A., Grauer, S., Schreyeck, S., Brunner, K., Greiter, M., Thomale, R., Gould, C., Molenkamp, L. W.

المصدر: Phys. Rev. B 103, 235111 (2021)

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Other Condensed Matter, Condensed Matter - Strongly Correlated Electrons

الوصف: In recent attempts to observe axion electrodynamics, much effort has focused on trilayer heterostructures of magnetic topological insulators, and in particular on the examination of a so-called zero Hall plateau, which has misguidedly been overstated as direct evidence of an axion insulator state. We investigate the general notion of axion insulators, which by definition must contain a nontrivial volume to host the axion term. We conduct a detailed magneto-transport analysis of Chern insulators comprised of a single magnetic topological insulator layer of varying thickness as well as trilayer structures, for samples optimized to yield a perfectly quantized anomalous Hall effect. Our analysis gives evidence for a topological magneto-electric effect quantized in units of e$^2$/2h, allowing us to identify signatures of axion electrodynamics. Our observations may provide direct experimental access to electrodynamic properties of the universe beyond the traditional Maxwell equations, and challenge the hitherto proclaimed exclusive link between the observation of a zero Hall plateau and an axion insulator.

الوصول الحر: http://arxiv.org/abs/2105.09608Test

المؤلفون: Železný, J., Fang, Z., Olejník, K., Patchett, J., Gerhard, F., Gould, C., Molenkamp, L. W., Gomez-Olivella, C., Zemen, J., Tichý, T., Jungwirth, T., Ciccarelli, C.

المصدر: Phys. Rev. B 104, 054429 (2021)

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science

الوصف: Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced non-equilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetiresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field -- current indices which complicates its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb.
Comment: 29 pages, 13 figures

الوصول الحر: http://arxiv.org/abs/2102.12838Test

المؤلفون: Fuchs, C., Hofer, M., Fürst, L., Shamim, S., Kießling, T., Buhmann, H., Molenkamp, L. W.

المصدر: Journal of Applied Physics; 11/7/2023, Vol. 134 Issue 17, p1-10, 10p

مصطلحات موضوعية: SEMICONDUCTOR devices, TEMPERATURE effect, LOW temperatures, OHMIC resistance, QUANTUM wells, QUANTUM transitions

مستخلص: We present a setup for fast, low-bias (≤ 1 mV) DC transport measurements with μs time resolution for high ohmic resistance (≈ 20 k Ω) semiconducting samples. We discuss the circuitry and instrumentation for the measurement approach that can be applied to any kind of semiconductor device or (gated) two-dimensional material and demonstrate the main measurement artifacts in typical measurements by means of circuit simulation. Based on the latter, we present a simple two-step protocol for eliminating the measurement artifacts reliably. We demonstrate the technique by measuring the transitions between quantum Hall plateaus in the HgTe quantum wells and resolve plateaus as short-lived as 100 μs. [ABSTRACT FROM AUTHOR]

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المؤلفون: Böttcher, J., Tutschku, C., Molenkamp, L. W., Hankiewicz, E. M.

المصدر: Phys. Rev. Lett. 123, 226602 (2019)

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics, High Energy Physics - Theory

الوصف: Recent experimental progress in condensed matter physics enables the observation of signatures of the parity anomaly in two-dimensional Dirac-like materials. Using effective field theories and analyzing band structures in external out-of-plane magnetic fields (orbital fields), we show that topological properties of quantum anomalous Hall (QAH) insulators are related to the parity anomaly. We demonstrate that the QAH phase survives in orbital fields, violates the Onsager relation, and can be therefore distinguished from a quantum Hall (QH) phase. As a fingerprint of the QAH phase in increasing orbital fields, we predict a transition from a quantized Hall plateau with $\sigma_\mathrm{xy}= -\mathrm{e}^2/\mathrm{h}$ to a not perfectly quantized plateau, caused by scattering processes between counterpropagating QH and QAH edge states. This transition can be especially important in paramagnetic QAH insulators, such as (Hg,Mn)Te/CdTe quantum wells, in which exchange interaction and orbital fields compete.
Comment: 15 pages and 10 figures A few minor typos have been corrected, e.g., units in Eq. (4) have been corrected

الوصول الحر: http://arxiv.org/abs/1901.05425Test

المؤلفون: Kessel, M., Lunczer, L., Tarakina, N. V., Brüne, C., Buhmann, H., Molenkamp, L. W.

مصطلحات موضوعية: Condensed Matter - Materials Science

الوصف: We investigate the crystal properties of CdTe nanowires overgrown with HgTe. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) confirm, that the growth results in a high ensemble uniformity and that the individual heterostructures are single-crystalline, respectively. We use high-resolution X-ray diffraction (HRXRD) to investigate strain, caused by the small lattice mismatch between the two materials. We find that both CdTe and HgTe show changes in lattice constant compared to the respective bulk lattice constants. The measurements reveal a complex strain pattern with signatures of both uniaxial and shear strains present in the overgrown nanowires.

الوصول الحر: http://arxiv.org/abs/1812.01248Test

المؤلفون: Khouri, T., Pezzini, S., Bendias, M., Leubner, P., Zeitler, U., Hussey, N. E., Buhmann, H., Molenkamp, L. W., Titov, M., Wiedmann, S.

المصدر: Phys. Rev. B 99, 075303 (2019)

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics

الوصف: In this study we have measured the magnetoresistance response of inverted HgTe quantum wells in the presence of a large parallel magnetic field up to 33 T is applied. We show that in quantum wells with inverted band structure a monotonically decreasing magnetoresistance is observed when a magnetic field up to order 10 T is applied parallel to the quantum well plane. This feature is accompanied by a vanishing of non-locality and is consistent with a predicted modification of the energy spectrum that becomes gapless at a critical in-plane field $B_{c}$. Magnetic fields in excess of $B_c$ allow us to investigate the evolution of the magnetoresistance in this field-induced semi-metallic region beyond the known regime. After an initial saturation phase in the presumably gapless phase, we observe a strong upturn of the longitudinal resistance. A small residual Hall signal picked up in non-local measurements suggests that this feature is likely a bulk phenomenon and caused by the semi-metallicity of the sample. Theoretical calculations indeed support that the origin of these features is classical and a power law upturn of the resistance can be expected due to the specifics of two-carrier transport in thin (semi-)metallic samples subjected to large magnetic fields.

الوصول الحر: http://arxiv.org/abs/1808.02268Test

المؤلفون: Budewitz, A., Bendias, K., Leubner, P., Khouri, T., Shamim, S., Wiedmann, S., Buhmann, H., Molenkamp, L. W.

مصطلحات موضوعية: Condensed Matter - Mesoscale and Nanoscale Physics

الوصف: Magnetotransport measurements are presented on paramagnetic (Hg,Mn)Te quantum wells (QWs) with an inverted band structure. Gate-voltage controlled density dependent measurements reveal an unusual behavior in the transition regime from n- to p-type conductance: A very small magnetic field of approximately 70 mT is sufficient to induce a transition into the nu = -1 quantum Hall state, which extends up to at least 10 Tesla. The onset field value remains constant for a unexpectedly wide gate-voltage range. Based on temperature and angle-dependent magnetic field measurements we show that the unusual behavior results from the realization of the quantum anomalous Hall state in these magnetically doped QWs.

الوصول الحر: http://arxiv.org/abs/1706.05789Test