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Register a new userAccurate characterization of tip-induced potential using electron interferometry
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Using the tip of a scanning probe microscope as a local electrostatic gate gives access to real space information on electrostatics as well as charge transport at the nanoscale, provided that the tip-induced electrostatic potential is well known. Here, we focus on the accurate characterization of the tip potential, in a regime where the tip locally depletes a two-dimensional electron gas (2DEG) hosted in a semiconductor heterostructure. Scanning the tip in the vicinity of a quantum point contact defined in the 2DEG, we observe Fabry-Perot interference fringes at low temperature in maps of the device conductance. We exploit the evolution of these fringes with the tip voltage to measure the change in depletion radius by electron interferometry. We find that a semi-classical finite-element self-consistent model taking into account the conical shape of the tip reaches a faithful correspondence with the experimental data.
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We present a detailed experimental study on the electrostatic interaction between a quantum dot and the metallic tip of a scanning force microscope. Our method allowed us to quantitatively map the tip-induced potential and to determine the spatial dependence of the tips lever arm with high resolution. We find that two parts of the tip-induced potential can be distinguished, one that depends on the voltage applied to the tip and one that is independent of this voltage. The first part is due to the metallic tip while we interpret the second part as the effect of a charged dielectric particle on the tip. In the measurements of the lever arm we find fine structure that depends on which quantum state we study. The results are discussed in view of scanning gate experiments where the tip is used as a movable gate to study nanostructures.
Due to their high energy, hot electrons in quantum Hall edge states can be considered as single particles that have the potential to be used for quantum optics-like experiments. Unlike photons, however, electrons typically undergo scattering processes in transport, which results in a loss of coherence and limits their ability to show quantum-coherent behaviour. Here we study theoretically the decoherence mechanisms of hot electrons in a Mach-Zehnder interferometer, and highlight the role played by both acoustic and optical phonon emission. We discuss optimal choices of experimental parameters and show that high visibilities of $gtrsim 85%$ are achievable in hot-electron devices over relatively long distances of 10 $mu$m. We also discuss energy filtration techniques to remove decoherent electrons and show that this can increase visibilities to over $95%$. This represents an improvement over Fermi-level electron quantum optics, and suggests hot-electron charge pumps as a platform for realising quantum-coherent nanoelectronic devices.
Cryogenic low noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in applications such as radio astronomy, deep space communications, and quantum computing, and the physical mechanisms governing the microwave noise figure are therefore of practical interest. In particular, the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. Here, we report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using a Schottky thermometry method. At temperatures $sim 20$ K, we observe a nonlinear trend of thermal resistance versus power that is consistent with heat dissipation by phonon radiation. Based on this finding, we consider heat transport by phonon radiation at the low-noise bias and liquid helium temperatures and estimate that the thermal noise from the gate is several times larger than previously assumed owing to self-heating. We conclude that without improvements in thermal management, self-heating results in a practical lower limit for microwave noise figure of HEMTs at cryogenic temperatures.
We have developed a novel method for crystalline hydrogenation of graphene on the nanoscale. Molecular hydrogen was physisorbed at 5 K onto pristine graphene islands grown on Cu(111) in ultrahigh vacuum. Field emission local to the tip of a scanning tunneling microscope dissociates H$_2$ and results in hydrogenated graphene. At lower coverage, isolated point defects are found on the graphene and are attributed to chemisorbed H on top and bottom surfaces. Repeated H$_2$ exposure and field emission yielded patches and then complete coverage of a crystalline $sqrt{3}$ $times$ $sqrt{3}$ R30{deg} phase, as well as less densely packed 3 $times$ 3 and 4 $times$ 4 structures. The hydrogenation can be reversed by imaging with higher bias voltage.
It is widely believed that topological superconductivity, a hitherto elusive phase of quantum matter, can be achieved by inducing superconductivity in topological materials. In search of such topological superconductors, certain topological insulators (like, Bi$_2$Se$_3$) were successfully turned into superconductors by metal-ion (Cu, Pd, Sr, Nb etc. ) intercalation. Superconductivity could be induced in topological materials through applying pressure as well. for example, a pressure-induced superconducting phase was found in the topological insulator Bi$_2$Se$_3$. However, in all such cases, no conclusive signature of topological superconductivity was found. In this review, we will discuss about another novel way of inducing superconductivity in a non-superconducting topological material -- by creating a mesoscopic interface on the material with a non-superconducting, normal metallic tip where the mesoscopic interface becomes superconducting. Such a phase is now known as a tip-induced superconducting (TISC) phase. This was first seen in 2014 on Cd$_3$As$_2$ at IISER Mohali, India. Following that, a large number of other topological materials were shown to display TISC. Since the TISC phase emerges only at a confined region under a mesoscopic point contact, traditional bulk tools for characterizing superconductivity cannot be employed to detect/confirm such a phase. On the other hand, such a point contact geometry is ideal for probing the possible existence of a temperature and magnetic field dependent superconducting energy gap and a temperature and magnetic field dependent critical current. We will review the details of the experimental signatures that can be used to prove the existence of superconductivity even when the text-book tests for detecting superconductivity cannot be performed. Then, we will review different systems where a TISC phase could be realized.
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