Heterostructures and artificial atoms from quantum matter

 
Heterostructures of quantum matter are particularly interesting systems. Quantum matter is defined by quantum effects that generate phenomena that surpass incoherent or mean-field behavior, and often are collective and emergent. Canonical quantum materials are the heavy-fermion compounds, the high-Tc cuprates, the iridates, correlated organic compounds, and the iron pnictides.

Unprecedented effects can occur if quantum materials are stacked, packed in quantum wells, brought into contact at interfaces, or altered by control parameters such as strain and electric fields imposed by gates. The phenomena thus induced are unforeseeable in their breadth and complexity.

In a recent review article, we provide an overview of this young scientific field and shed light on the enormous possibilities it may generate [1].

 

<p style="text-align: justify;"><strong>Fig. 1</strong>: Epitaxially grown quantum-matter heterostructures are already ubiquitous today.  A wide variety of quantum materials can nowadays be stacked epitaxially, often with single unit cell thickness control. Due to emergent quantum effects, these heterostructures can provide functionalities not achievable in the bulk compounds. The figure shows a series of transmission-electron-microscopy images of heterostructures taken from the literature [4-8].  More examples can be found in [1]. </p>

Fig. 1: Epitaxially grown quantum-matter heterostructures are already ubiquitous today.  A wide variety of quantum materials can nowadays be stacked epitaxially, often with single unit cell thickness control. Due to emergent quantum effects, these heterostructures can provide functionalities not achievable in the bulk compounds. The figure shows a series of transmission-electron-microscopy images of heterostructures taken from the literature [4-8].  More examples can be found in [1]. 

 
In a further review, we consider zero-dimensional electron systems, also known as quantum dots, from quantum matter [2].

The electron system of a quantum dot is described by a coherent many-body wave function with one macroscopic phase. Because of the shell structure of the electron states and their coherency across the dots, quantum dots are appropriately described as ‘artificial atoms’. We anticipate remarkable scientific advances and possibly important applications of these artificial atoms made from functional quantum materials.

With our recent experimental progress in patterning and contacting of oxide two-dimensional electron systems [3], we are now able to explore the surprising properties of complex-oxide artificial atoms and of molecules or even of solids assembled from them.

 

<p style="text-align: justify;"><strong>Fig. 2</strong>: Dependence of the size quantization energy and the charging energy of thin quantum dots as a function of dot diameter. For a large range of dot sizes, the energy scale is readily dominated by correlation energies. In contrast, the size quantization energy and the charging energy of quantum dots based on standard semiconductors are 1–2 orders of magnitude larger (not plotted) and are therefore more dominant dot properties. The inset shows an illustration of an artificial atom built from a correlated material. The drawing shows the top view of a quantum dot formed by a perovskite, its BO<sub>6</sub> octahedra depicted in gray. The colored wave pattern in the background symbolizes the coherent many-electron wave function formed by the correlated system.</p>

Fig. 2: Dependence of the size quantization energy and the charging energy of thin quantum dots as a function of dot diameter. For a large range of dot sizes, the energy scale is readily dominated by correlation energies. In contrast, the size quantization energy and the charging energy of quantum dots based on standard semiconductors are 1–2 orders of magnitude larger (not plotted) and are therefore more dominant dot properties. The inset shows an illustration of an artificial atom built from a correlated material. The drawing shows the top view of a quantum dot formed by a perovskite, its BO6 octahedra depicted in gray. The colored wave pattern in the background symbolizes the coherent many-electron wave function formed by the correlated system.

 

 
References:

  1. H. Boschker, J. Mannhart - Accepted in Ann. Rev. Cond. Mat. Article / cond-mat / arXiv: 1607.07239 (2016).
  2. J. Mannhart, H. Boschker, T. Kopp, R. Valenti Rep. Prog. Phys. 79, 084508 (2016).
  3. C. Woltmann, T. Harada, H. Boschker, V. Srot, P.A. van Aken, H. Klauk, J. Mannhart  Phys. Rev. Appl. 4, 064003 (2015).

  4. S.J. Haigh, A. Gholinia, R. Jalil, S. Romani, L. Britnell, et al. Nat. Mater. 11, 764 (2012).
  5. A.K. Yadav, C.T. Nelson, S.L. Hsu, Z. Hong, J.D. Clarkson, et al. Nature 530, 198 (2016).
  6. C.R. Hughes, T. Harada, T. Asaba, R. Ashoori, A.V. Boris, H. Hilgenkamp, M.E. Holtz, L. Li, J. Mannhart, D.A. Muller, X. Remshaw Wang, D.G. Schlom, A. Soukiassian, H. Boschker - Ferromagnetism and Conductivity in Atomically Thin SrRuO3. Not yet published.  
  7. M. Ziese, I. Vrejoiu Phys. Status Solidi 7, 243 (2013).
  8. H.N. Lee, H.M. Christen, M.F. Chisholm, C.M. Rouleau, D.H. Lowndes Nature 433, 395 (2005).
 
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