Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Low-dimensional Ge hole devices are promising systems with many potential applications such as hole spin qubits, Andreev spin qubits, Josephson junctions, and can serve as a basis for the realization of topological superconductivity. This vast array of potential uses for Ge largely stems from the exceptionally strong and controllable spin-orbit interaction (SOI), ultralong mean free paths, long coherence times, and CMOS compatibility. However, when brought into proximity with a superconductor (SC), metallization normally diminishes many useful properties of a semiconductor, for instance, typically reducing the $g$ factor and SOI energy, as well as renormalizing the effective mass. In this paper we consider metallization of a Ge nanowire (NW) in proximity to a SC, explicitly taking into account the 3D geometry of the NW. We find that proximitized Ge exhibits a unique phenomenology of metallization effects, where the 3D cross section plays a crucial role. For instance, in contrast to expectations, we find that SOI can be enhanced by strong coupling to the superconductor. We also show that the thickness of the NW plays a critical role in determining both the size of the proximity induced pairing potential and metallization effects, since the coupling between NW and SC strongly depends on the distance of the NW wave function from the interface with the SC. In the absence of electrostatic effects, we find that a sizable gap opens only in thin NWs ($d\lesssim 3$ nm). In thicker NWs, the wave function must be pushed closer to the SC by electrostatic effects in order to achieve a sizable proximity gap such that the required electrostatic field strength can simultaneously induce a strong SOI. The unique and sometimes beneficial nature of metallization effects in SC-Ge NW devices evinces them as ideal platforms for future applications in quantum information processing.

Abstract: We develop a model, which incorporates both intra- and intervalley scatterings to master equation, to explore exciton valley coherence in monolayer WS$_2$ subjected to magnetic field. For linearly polarized (LP) excitation accompanied with an initial coherence, our determined valley dynamics manifests the coherence decay being faster than the exciton population relaxation, and agrees with experimental data by Hao et al.[Nat. Phys. 12, 677 (2016)]. Further, we reveal that magnetic field may quench the electron-hole (e-h) exchange induced pure dephasing -- a crucial decoherence source -- as a result of lifting of valley degeneracy, allowing to magnetically regulate valley coherence. In particular, at low temperatures for which the exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is expected to attain ${\tau}_{\mathcal{C}}\sim 1$ ps, facilitating full control of qubits based on the valley pseudospin. For dark excitons, we demonstrate an emerging coherence even in the absence of initial coherent state, which has a long coherence time ($\sim 15$ ps) at low temperature. Our work provides an insight into tunable valley coherence and coherent valley control based on dark excitons.

Abstract: Bloch oscillations are a fundamental phenomenon linking the adiabatic transport of Cooper pairs to time. Here, we investigate synchronization of the Bloch oscillations in a strongly coupled system of sub-100 nm Al/AlOx/Al Josephson junctions in high-ohmic environment composed of highly inductive meanders of granulated aluminum and high-ohmic titanium microstrips. We observe a pronounced current mirror eff ect in the coupled junctions and demonstrate current plateaus, akin to the fi rst dual Shapiro step in microwave experiments. These fi ndings suggest that our circuit design holds promise for realizing protected Bloch oscillations and precise Shapiro steps of interest for current metrology.

Abstract: Light-emitting complex defects in silicon have been considered a potential platform for quantum technologies based on spin and photon degrees of freedom working at telecom wavelengths. Their integration in complex devices is still in its infancy, and it was mostly focused on light extraction and guiding. Here we address the control of the electronic states of carbon-related impurities (G-centers) via strain engineering. By embedding them in patches of silicon on insulator and topping them with SiN, symmetry breaking along [001] and [110] directions is demonstrated, resulting in a controlled splitting of the zero phonon line (ZPL), as accounted for by the piezospectroscopic theoretical framework. The splitting can be as large as 18 meV and it is finely tuned by selecting patch size or by moving in different positions on the patch. Some of the split, strained ZPLs are almost fully polarized and their overall intensity is enhanced up to 7 times with respect to the flat areas, whereas their recombination dynamics is slightly affected. Our technique can be extended to other impurities and Si-based devices such as suspended bridges, photonic crystal microcavities, Mie resonators, and integrated photonic circuits.

Abstract: Superconductor/semiconductor hybrid devices have attracted increasing interest in the past years. Superconducting electronics aims to complement semiconductor technology, while hybrid architectures are at the forefront of new ideas such as topological superconductivity and protected qubits. In this work, we engineer the induced superconductivity in two-dimensional germanium hole gas by varying the distance between the quantum well and the aluminum. We demonstrate a hard superconducting gap and realize an electrically and flux tunable superconducting diode using a superconducting quantum interference device (SQUID). This allows to tune the current phase relation (CPR), to a regime where single Cooper pair tunneling is suppressed, creating a $ \sin \left( 2 \varphi \right)$ CPR. Shapiro experiments complement this interpretation and the microwave drive allows to create a diode with $ \approx 100 \%$ efficiency. The reported results open up the path towards monolithic integration of spin qubit devices, microwave resonators and (protected) superconducting qubits on a silicon technology compatible platform.

Abstract: We investigate non-equilibrium transport properties of a quantum dot in the Coulomb blockade regime under the condition of negligible inelastic scattering during the dwelling time of the electrons in the dot. Using the quantum kinetic equation we show that the absence of thermalization leads to a double-step in the distribution function of electrons on the dot, provided that it is symmetrically coupled to the leads. This drastically changes nonlinear transport through the dot resulting in an additional (compared to the thermalized case) jump in the conductance at voltages close to the charging energy, which could serve as an experimental manifestation of the absence of thermalization.

Abstract: One of the most critical challenges in soaring the performance of perovskite solar cells is decreasing the density of trap states in the light-absorbing perovskite layer. These traps cause an increase in the recombination of charge carriers and decrease the efficiency of devices. One of the methods to study the trap density is space charge limited current (SCLC) analysis. For this purpose, some structures are needed with the ability to transport only electrons or holes. The trap density can be calculated by investigating the current-voltage diagram and finding the voltage corresponding to the slope change point. One of the challenges in these structures is using organic polymers like Spiro-OMeTAD, PEDOT: PSS, and PTAA as hole transport layers. They have problems like high acidity, lack of stability against moisture, low charge mobility, low conductivity, and high cost. In this work, a hole-only device structure is explained, made based on inorganic materials, which possesses high stability, a simple preparation method, and reasonable cost compared to conventional hole-only device structures. This structure is built by coating a nanostructured NiOx layer, perovskite, CIS, and Au on the ITO substrate. To investigate the performance of this structure, various perovskite layers were made at different experimental conditions, and their trap density was obtained by the proposed hole-only device structure. The analysis of the photovoltaic characteristics of cells revealed a clear correlation between the perovskite layer's trap density and the cells' performance. Our results show the introduced structure is a simple and stable structure that can be utilized in studying the trap density in perovskite layers to make more efficient cells.