6 research outputs found
Temperature-driven transition from a semiconductor to a topological insulator
We report on a temperature-induced transition from a conventional
semiconductor to a two-dimensional topological insulator investigated by means
of magnetotransport experiments on HgTe/CdTe quantum well structures. At low
temperatures, we are in the regime of the quantum spin Hall effect and observe
an ambipolar quantized Hall resistance by tuning the Fermi energy through the
bulk band gap. At room temperature, we find electron and hole conduction that
can be described by a classical two-carrier model. Above the onset of quantized
magnetotransport at low temperature, we observe a pronounced linear
magnetoresistance that develops from a classical quadratic low-field
magnetoresistance if electrons and holes coexist. Temperature-dependent bulk
band structure calculations predict a transition from a conventional
semiconductor to a topological insulator in the regime where the linear
magnetoresistance occurs.Comment: 7 pages, 6 figure
Dirac-screening stabilized surface-state transport in a topological insulator
We report magnetotransport studies on a gated strained HgTe device. This
material is a threedimensional topological insulator and exclusively shows
surface state transport. Remarkably, the Landau level dispersion and the
accuracy of the Hall quantization remain unchanged over a wide density range
(). This implies that
even at large carrier densities the transport is surface state dominated, where
bulk transport would have been expected to coexist already. Moreover, the
density dependence of the Dirac-type quantum Hall effect allows to identify the
contributions from the individual surfaces. A model can describe
the experiments, but only when assuming a steep band bending across the regions
where the topological surface states are contained. This steep potential
originates from the specific screening properties of Dirac systems and causes
the gate voltage to influence the position of the Dirac points rather than that
of the Fermi level.Comment: 12 pages 4 figure
Erkundung der Transporteigenschaften des dreidimensionalen Topologischen Isolators HgTe
In der vorliegenden Dissertation werden die Transporteigenschaften von verspannten HgTe-Volumenkristallen untersucht. Verspanntes HgTe stellt einen dreidimensionalen topologischen Isolator dar und ist zur Erkundung von topologischen OberflĂ€chenzustĂ€nden von speziellem Interesse, da es mit Hilfe von Molekularstrahlepitaxie in hoher KristallqualitĂ€t gewachsen werden kann. Die niedrige Defektdichte fĂŒhrt zu beachtlichen LadungstrĂ€gerbeweglichkeiten, die deutlich ĂŒber denen anderer topologischer Isolatoren liegen. Verspanntes HgTe hat jedoch eine kleine EnergielĂŒcke von ca. 20 meV. Deshalb ist es fĂŒr eine mögliche Verwendung des Materials ein wichtiger Aspekt, in welchem Parameterbereich OberflĂ€chentransport stattfindet. Um dieser Frage nachzugehen, werden die HgTe-Proben bei tiefen Temperaturen (T < 100 mK) und unter dem Einfluss hoher Magnetfelder in verschiedenen Orientierungen untersucht. Der Einfluss von Gate-Elektroden ober- und unterhalb der Struktur sowie von Deckschichten, die die OberflĂ€chen schĂŒtzen, wird diskutiert. Basierend auf einer Analyse des Quanten-Hall-Effekts wird gezeigt, dass der Transport in diesem Material von topologischen OberflĂ€chenzustĂ€nden dominiert ist. Die AbhĂ€ngigkeit der topologischen OberflĂ€chenzustĂ€nde von der Gate-Spannung wird dargestellt. Durch diese AbhĂ€ngigkeit ist es zum ersten Mal möglich, eine ungerade ganzzahlige Quanten-Hall-Plateau Sequenz nachzuweisen, die von den OberflĂ€chen senkrecht zum Magnetfeld stammt. Des Weiteren wird im Rahmen dieser Arbeit in Proben hoher OberflĂ€chenqualitĂ€t zum ersten Mal fĂŒr einen 3D TI der p-Typ QHE der OberflĂ€chenzustĂ€nde beobachtet. Aus der Gate-AbhĂ€ngigkeit der Messungen wird geschlossen, dass das Abschirmverhalten in 3D TIs nicht trivial ist. Die Transportdaten werden mit Hilfe von intuitiven theoretischen Modellen auf qualitative Weise analysiert.In the present thesis the transport properties of strained bulk HgTe devices are investigated. Strained HgTe forms a 3D TI and is of special interest for studying topological surface states, since it can be grown by MBE in high crystal quality. The low defect density leads to considerable mobility values, well above the mobilities of other TI materials. However, strained HgTe has a small band gap of ca. 20 meV. With respect to possible applications the question is important, under which conditions the surface transport occurs. To answer this question, the HgTe devices are investigated at dilution refrigerator temperatures (T<100 mK) in high magnetic fields of different orientation. The influence of top and back gate electrodes as well as surface protecting layers is discussed.
On the basis of an analysis of the quantum Hall behaviour it is shown that transport is dominated by the topological surface states in a surprisingly large parameter range. A dependence on the applied top gate voltage is presented for the topological surface states. It enables the first demonstration of an odd integer QHE sequence from the surfaces perpendicular to the magnetic field. Furthermore, the p-type QHE from the surface states is observed for the first time in any 3D TI. This is achieved in samples of high surface quality. It is concluded from the gate response that the screening behaviour in 3D TI devices is non-trivial. The transport data are qualitatively analysed by means of intuitive theoretical models