Density-dependent thermopower oscillations in mesoscopic two-dimensional electron gases

Narayan, Vijay; Kogan, Eugene; Ford, Chris; Pepper, M.; Kaveh, M.; Griffiths, Jonathan; Jones, G. A. C.; Beere, Harvey E.; Ritchie, D. A.

doi:10.1088/1367-2630/16/8/085009

Cited by 9 publications

(17 citation statements)

References 20 publications

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“…Notice that in the simulation ρ also shows oscillations, which are not observed experimentally. A non-oscillating behaviour of R sheet (V g ) in the presence of oscillations in S(V g ) has been previously noticed in 2DEGs (GaAs/AlGaAs 42 43 and MoS 2 (ref. 36 )).…”

Section: Discussionmentioning

confidence: 55%

Giant oscillating thermopower at oxide interfaces

Pallecchi¹,

Telesio²,

et al. 2015

Nat Commun

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Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO3/SrTiO3 at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays high negative values of the order of 104–105μVK−1, oscillating at regular intervals as a function of the gate voltage. The huge thermopower magnitude can be attributed to the phonon-drag contribution, while the oscillations map the progressive depletion and the Fermi level descent across a dense array of localized states lying at the bottom of the Ti 3d conduction band. This study provides direct evidence of a localized Anderson tail in the two-dimensional electron liquid at the LaAlO3/SrTiO3 interface.

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Section: Discussionmentioning

confidence: 55%

Giant oscillating thermopower at oxide interfaces

Pallecchi¹,

Telesio²,

et al. 2015

Nat Commun

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“…In addition, it is observed that at a fixed T , decreasing |V g | also diminishes α, which is consistent with an Anderson-like transition to a metallic state [33,36]. However, the V g -dependence can be seen simply as a consequence of ξ >> L. In addition, the thermopower S of similar 2DEGs displays strong oscillations and even sign changes [22] which might have their origin in phase coherent transport [23]. Thus, there are several indications that electrons retain phase coherence over the length of the devices studied.…”

mentioning

confidence: 64%

“…[32], qualitative changes in β are not expected even in the presence of strong interactions, and this is consistent with our findings. It will be interesting to understand whether the recently observed strong enhancement in the magnitude of S measured in similar 2DEGs [17] or the observed violation of the Mott formula [17,22], where S and ρ were observed to oscillate asynchronously, reflect strong interaction effects or not. Lastly, we also wish to point out the similarities between our experimental results and the phenomenology of many-body delocalized phases in translationally-invariant 2D systems [33].…”

Section: Figures 2(a) and 2(b)mentioning

confidence: 98%

Observation of geometry-dependent conductivity in two-dimensional electron systems

et al. 2015

Self Cite

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We report electrical conductivity σ measurements on a range of two-dimensional electron gases (2DEGs) of varying linear extent. Intriguingly, at low temperatures (T ) and low carrier density (ns) we find the behavior to be consistent with σ ∼ L α , where L is the length of the 2DEG along the direction of transport. Importantly, such scale-dependent behavior is precisely in accordance with the scaling hypothesis of localization [Abrahams et al., Phys. Rev. Lett. 42, 673 (1979)] which dictates that in systems where the electronic wave function ξ is localized, σ is not a material-specific parameter, but depends on the system dimensions. From our data we are able to construct the "βfunction" ≡ (h/e 2 )d ln σ/d ln L and show this to be strongly consistent with theoretically predicted limiting values. These results suggest, remarkably, that the electrons in the studied 2DEGs preserve phase coherence over lengths ∼ 10 µm. This suggests the utility of the 2DEGs studied towards applications in quantum information as well as towards fundamental investigations into many-body localized phases.The scaling hypothesis of localization [1], formulated over thirty years ago, is a statement that the electrical conductivity σ is lengthscale-dependent in finite systems where the conduction electrons are short-ranged or localized. This can be understood by considering electronic states with localization length ξ in systems of different spatial extents: As depicted in Fig. 1(a), if ξ is greater than the linear extent of the system, then electrons are able to communicate across the system ends and there will be a finite conductance G even at T = 0 K. However, this conductance will decrease as the system size increases, ultimately vanishing for infinitely large systems. On the other hand, if the electronic states are extended, ξ → ∞, then even in the infinite system-size limit, G = 0. This intuitive picture is at the very heart of the scaling hypothesis which distinguishes between metallic and insulating states on the basis of the range of ξ: If the electronic states at the chemical potential µ are extended, then the system is a metal, but if they have a finite extent, the system is an insulator. In other words, the metallic state is defined by σ independent of system dimensions, whereas the insulating state is characterized by σ decaying with increasing system dimensions. This underlies the Anderson metal-to-insulator transition in which a "mobility-edge" in wave vector k-space demarcates short-ranged and long-ranged states [2].However, since the scaling hypothesis was put forward, to our knowledge there have been no experimental reports of length-dependent σ. In this paper, working with mesoscopic GaAs-based 2DEGs of varying linear extent L, we provide the experimental demonstration of σ-scaling consistent with the scaling hypothesis. We continuously tune ξ in the 2DEGs by applying a top-gate voltage V G and observe a crossover from a regime in which the electrical resistivity ρ ≡ 1/σ is independent of L to one where it is strongly dep...

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“…Here, the focus is not put on the creation of spin-polarized currents but rather on the dramatic changes seen in the junction thermopower when the orientation of the ferromagnetic electrodes is switched from the parallel to the antiparallel configuration. Finally, recent measurements of the charge thermopower in 2DES setups may indicate the presence of electronic correlations [48,49] or diffusive transport mechanisms [50,51].…”

Section: Introductionmentioning

confidence: 99%

Seebeck effects in two-dimensional spin transistors

2015

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We consider a spin-orbit-coupled two-dimensional electron system under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the generated voltage bias (charge Seebeck effect), spin bias (spin Seebeck effect) and magnetization-dependent thermopower (magneto-Seebeck effect) in the ballistic regime of transport at linear response. We find that the charge thermopower is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that it is always negative for normal leads. We carefully compare the exact results for the linear response coefficients and a Sommerfeld approximation. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient for parallel and antiparallel magnetization configuration. Remarkably, the thermopower can change its sign by tuning the Fermi energy. This effect disappears when the Rashba coupling is absent. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of a the Rashba potential.

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Density-dependent thermopower oscillations in mesoscopic two-dimensional electron gases

Cited by 9 publications

References 20 publications

Giant oscillating thermopower at oxide interfaces

Giant oscillating thermopower at oxide interfaces

Observation of geometry-dependent conductivity in two-dimensional electron systems

Seebeck effects in two-dimensional spin transistors

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