Probing Correlated States
Excitonic interactions in low-dimensional bilayers
Contact: Amina Ribeiro
Bilayers of 2D electron or hole systems are known to foster a plethora of new and often exotic states of matter. Putting 2D electron gases in close proximity to 2D hole gases should lead to even stronger interactions promoting excitonic phases. Additional interest in this material system stems from the close relationship of an “excitonic” device with those used for topological insulators. The width of the barrier limits the hybridization of the two charge layers and distinguishes the two phenomena. The excitonic behavior of InAs/GaSb combined quantum wells has been already studied before and indications of an excitonic insulating state [Du2017] have been found.
Using heterostructures consisting of p-doped GaSb and n-doped InAs layers, no voltage is needed to create a 2DHG/2DEG because the conduction band of InAs already lies below the valence band of GaSb. Using the same heterostructure and additionally separating both layers by undoped AlSb as a central barrier, small variations of the applied voltage will vary the charge concentration at the barrier region. This allows us to pursue capacitance and transport measurements since the charges are exchanged in the undoped zone. The measured capacitance contains the so-called quantum capacitance, which is proportional to the density of states of the two 2D layers and which contains information about the interactions between the charges.
In preliminary experiments we have already measured the capacitance signal across a 20 nm AlSb barrier between InAs and GaSb and find Landau-level related capacitance oscillations (see Fig. below). Varying the bias voltage leads to a systematic variation of the Landau level filling, demonstrating that the charge density varies.
References:
[Du2017] Evidence for a topological excitonic insulator in InAs/GaSb bilayers, Lingjie Du, Xinwei Li, Wenkai Lou, Gerard Sullivan, Kai Chang Junichiro, Rui-Rui Du, Nature Communications 8, 1971 (2017)
Electrical transport, magneto-transport, optical spectroscopy, NMR
Contact: Dr. Christian Reichl
The aim of this project is the study of correlated many-body states in high purity GaAs based semiconductor heterostructures. With electron mobilities having reached a new record high in recent years, a variety of exotic many-body ground states has been added to the spectrum of correlation physics. Most popular examples are those states, which are expected to possess non-Abelian statistics [DasSarma97, DasSarma05]. Understanding the nature of these many-body ground states solely by standard transport measurements is often not possible.
In this project, we will investigate highest-mobility electron systems by complementing standard magneto-transport measurements with optical spectroscopy [Skolnick87] and nuclear magnetic resonance (NMR) [Desrat02, Tiemann12] to determine their spin state and local properties [Hayakawa13] but also study novel correlated systems.
Investigating the excitation spectra of quasiparticles can strongly benefit from optical techniques, but is strongly limited by the sensitivity of the 2DEG density to the incident light field [Shields96]. We are pursuing three subprojects that can in principle minimize these effects of light sensitivity or even circumvent them completely. This subprojects build around embedding a 2DEG into a planar photonic micro-cavity [Smolka14], reduce the effect of DX-centers by applying a remote doping technique [Pantelides92] and optically inducing a 2DEG through an indirect transition via a pump laser [Rapaport01].
Due to the finite nuclear spin of Aluminum, Gallium and Arsenic the electron spins of a 2DEG will always interact with the nuclei of the confining semiconductor allowing the study spin state of a correlated state via a resistively-detected variant of NMR (RD-NMR) [Desrat02, Tiemann12]. In contrast to optical experiments, the technological requirements are not as challenging and conventional sample designs can be used.
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[Desrat02]: W. Desrat, D. K. Maude, M. Potemski, J. C. Portal, Z. R. Wasilewski, and G. Hill, Phys. Rev. Lett. 88, 256807 (2002).
[Tiemann12]: L. Tiemann, G. Gamez, N. Kumada, K. Muraki, Science 335, 828-831 (2012).
[Hayakawa13]: J. Hayakawa, K. Muraki, and G. Yusa, Nature Nanotechnology, 8(1) (2013).
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[Smolka14]: S. Smolka, W. Wuester, F. Haupt, S. Faelt, W. Wegscheider, and A. Imamoglu, Science 346, 332-335 (2014).
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[Desrat02]: W. Desrat, D. K. Maude, M. Potemski, J. C. Portal, Z. R. Wasilewski, and G. Hill, Phys. Rev. Lett. 88, 256807 (2002).
[Dmitriev12]: A. P. Dmitriev, I. V. Gornyi, and D. G. Polyakov, Phys. Rev. B 86, 245402 (2012).
[Deutschmann01]: R. A. Deutschmann, W. Wegscheider, M. Rother, M. Bichler, G. Abstreiter, Phys. Rev. Lett. 86, 1857-1860 (2001).
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[Vaezi14a]: A. Vaezi, Phys. Rev. X 4, 031009 (2014).
[Vaezi14b]: A. Vaezi, and M. Barkeshli, Phys. Rev. Lett. 113, 236804 (2014).