Engineering of conduction and valence band in semiconductors can make charges move only along flat plane. These are called two-dimensional (2D) electron electron systems, and routinely achieved in various systems such as the Silicon-MOSFET or GaAs/AlGaAs heterostructures. The 2D electrons also act as an excellent platform to verify many fundamental theorems of quantum physics, at the same time discovering some new ones. Our work in such systems aim to investigate the effects of Coulomb interaction on the charge and spin transport in such low-dimensional systems at ultra-low temperatures that has been predicted to result in several exotic quantum states of charge.
Solid state emulation of Mott-Hubbard physics in two dimensions: Planar doping of silicon abd Germanium with phosphorus
(Devices fabricated at the University of New South Wales, Sydney, Australia)
Si:P and Ge:P δ-layers are a new class of two-dimensional electron system, where electron transport occurs in an impurity band that is intrinsically half-filled. This is because these systems are formed by sandwiching an atomically thin electrically conducting layer of P atoms in Si or Ge matrix, where each P atom contributes exactly one electron to the impurity band.
From the application perspective, continued investment in Si and Ge based devices reflects its deep rooted impact on the present day electronics. In this respect, our investigation reveals extremely low noise in Si:P δ-layers, one of the lowest values ever reported for doped semiconductors. Phosphorous doped Si systems being one of the standard materials for the semiconductor industry, the observation of ultra-low noise characteristics is extremely important and desirable.
Additionally, these devices offer the possibility of investigating the novel and exotic phenomena that arises when the Fermi lies at or close to the centre of the band. Bulk crystals of Si doped with P has been one of the classic systems to emulate the Hubbard model. Being confined to two dimensions, localization and quantum interference are inherent to these devices at low temperatures. Instigation of these provides valuable information about the nature of transport in the system. The devices exhibit weak localization and universal conductance fluctuations (UCF), but there exists an anomalous suppression of UCF for devices with low doping densities.
Coulomb Phenomena at Low Dimensions:
New many-body phases in localized 2D systems:
Ever since the first two-dimensional electron systems (2DES) were realized, they have been used extensively to investigate various theories and concepts of charge localization. The scaling theory of localization prohibits extended electronic states in two dimensions (2D) at absolute zero in presence of disorder. While there has been extensive theoretical work on the possibility of a delocalization caused by electron-electron interactions, conclusive experimental evidence of such an effect has not been observed. On the contrary, the insulating phase in 2D at low temperatures has proven robust, in particular in the case of strong localization, where the resistivity is much larger than h/e2.
For strong disorder, insulating variable-range hopping transport has been observed, with interaction effects only leading to a modification of the single-particle density of states. In low but finite disorder an interaction driven localization mechanism has been suggested in form of pinned charge-density waves. However, no deviation from the insulating nature of transport has been reported in potential realizations of such phases, nor is it expected theoretically. The intermediate regime where disorder and interaction effects are equally important, is very challenging to study theoretically and experimentally and is still not well understood. Our work represents an attempt to close this gap.
We have observed direct experimental evidence that the insulating phase of a disordered, yet strongly interacting two-dimensional electron system (2DES) becomes unstable at low temperatures. As the temperature decreases, a transition from insulating to metal-like transport behavior is observed, which persists even when the resistivity of the system greatly exceeds the quantum of resistivity h/e2. The results have been achieved by measuring transport on a mesoscopic length-scale while systematically varying the strength of disorder.
The Coulomb interaction in a quantum degenerate system, expressed by the ratio rS of the inter-particle Coulomb repulsion energy to the Fermi energy, determines the ground state and elementary excitations when rS >> 1. In 2D, possibility of various forms of charge density wave ground states, including Wigner crystalline, striped or bubble phases, has attracted significant theoretical and experimental attention, even though a convincing detection of such phases with conventional transport experiments remains inconclusive. Our research is directed in investigating these possibilities as well.
Selected Publication(s):
1. M. Baenninger, A. Ghosh, M. Pepper, H. E. Beere, I. Farrer and D. A. Ritchie, Physical Review Letters 100, 016805 (2008);
2. Koushik R et al., Physical Review B (2012).
Nowadays, nanostructures such as quantum dots (a tiny puddle of charge) and quantum wires (appears when the puddle is pulled to form a one-dimensional line of charge) can be created with nano-fabrication processes on band-engineered semiconductor heterostructures. These nanostructures are extremely interesting systems, where various intricate aspects of quantum mechanics, such as quantum entanglement, fractional excitations ( Tomonaga-Luttinger liquid) etc. can be investigated in a controllable manner. This has also seen semiconductor quantum dots being proposed as a quantum bit, or “qubit”, in the next generation quantum computation and information technology. Our work is directed toward detection and measurement of quantum entanglement in quantum dot nanostructures that form the very basis of quantum information processing.
Selected Publication(s):
1. S. Goswami, C. Siegert, M. Baenninger, M. Pepper, I. Farrer, D. A. Ritchie, and A. Ghosh, Physical Review Letters 103, 026602 (2009).
2. C. Siegert, Arindam Ghosh, M. Pepper, I. Farrer, D. A. Ritchie, D. Anderson and G. A. C. Jones, Physical Review B (Rapid Comm.) 78, 081302(R) (2008); (Selected as Editor’s selection.)
3. Christoph Siegert, Arindam Ghosh, Michael Pepper, Ian Farrer, David A. Ritchie, Nature Physics 3, 315 (2007).
Spin-effects in semiconductor meso or nanoscale systems
The exchange energy component of Coulomb interaction leads to a number fascinating effects involving the electron spin in nanostructures. The ferromagnet-like spin-alignment in quantum dots and wires has attracted immense attention worldwide. The spin-blockade effect, analogous to the Coulomb blockade, reflects a spin-conservation rule in transport through dots. Another form of exchange between the lead-electrons and that within a quantum dot, has led to controlled simulation of the Kondo-effect, which is the basic interaction in the field of magnetism. These developments have opened up a new class of experimental and theoretical investigation in non-local spin control, generation or injection.
Embedding magnetic moments into semiconductor heterostructures offers a tunable access to various forms of magnetic ordering and phase transitions in low-dimensional electron systems. In general, the moments are introduced artificially, by either doping with ferromagnetic atoms, or electrostatically confining odd-electron quantum dots. Recently, we have reported experimental evidence of an independent, and intrinsic, source of localized spins in high-mobility GaAs/AlGaAs heterostructures with large setback distance (≈ 80nm) in modulation doping. Measurements reveal a quasi-regular distribution of the spins in the delocalized Fermi sea, and a mutual interaction via the Ruderman–Kittel–Kasuya–Yosida
(RKKY) indirect exchange below 100 milliKelvin. We have shown that a simple model on the basis of the fluctuations in background potential on the host two-dimensional electron system can explain the observed results quantitatively, which suggests a ‘disorder templated’ microscopic origin of the localized moments.
We have also shown recently that thermal properties, such as the Seebeck coefficient, of these magnetically engineered systems can be very large, and can exceed the same in non-magnetic 2D electron systems by nearly hundred times. These results open up a field of electro-thermal research in semiconductor heterostructures, and allows us great flexibility in designing new semiconductor-based meta-materials with exotic magnetic and thermal properties.