Juhn-Jong Lin

Charge density wave (CDW) resulting from a periodic distortion in the lattice creates orders beyond the original lattice. In 2 H − NbSe 2 , one of the layered transition metal dichalcogenides, the 3 × 3 charge order appears in two-dimensional (2D) layers. Although CDW can be observed by different techniques, the spatial distribution within a 2D layer has never been systematically visualized. Here, by using scanning tunneling microscopy and density functional theory, we monitored the evolution of CDW along the c axis and found a strong relation with the orbital charges. The results show that the appearance of the CDW varies while tuning the tunneling current and undergoes a transition from Se orbital charges to Nb orbital charges. The calculation of orbital charge distributions shows that both CDW intensity modulation and the transition are strongly intertwined with the distribution of Se 4 p orbitals and Nb 4 d orbitals.

Aluminum thin films are essential to the functionalities of electronic and quantum devices, where two-level systems (TLS) can degrade device performance. MBE-grown Al films may appeal to these applications due to their low TLS densities. We studied the energy distributions of TLS densities, g(E), in 10-nm-thick MBE-grown and electron-beam evaporated Al films through 1/f noise measurements between 80 and 360 K. At 300 K, the noise magnitudes in MBE-grown films are about three times lower than in the electron-beam evaporated films, corresponding to the g(E) values about ten times lower in the former than in the latter. Compared with previously established observations, we identified that the 1/f noise was generated by thermally activated TLS at grain boundaries.

Aluminum thin films are essential to the functionalities of electronic and quantum devices, where two-level systems (TLSs) can degrade device performance. Molecular beam epitaxy (MBE)-grown Al films may appeal to these applications due to their low TLS densities. We studied the energy distributions of TLS densities, g(E), in 10-nm-thick MBE-grown and electron-beam evaporated Al films through 1/f noise measurements between 80 and 360 K. At 300 K, the noise magnitudes in MBE-grown films are about three times lower than in the electron-beam evaporated films, corresponding to the g(E) values about ten times lower in the former than in the latter. Compared with previously established observations, we identified that the 1/f noise was generated by thermally activated TLS at grain boundaries.

Cobalt disilicide provides a promising nearly epitaxial superconducting material on silicon, which is compatible with high-density integrated circuit technology. We have characterized CoSi2 superconducting microwave cavities around 5.5 GHz for resonance frequency fluctuations at temperatures of 10–200 mK. Comparing with materials used in kinetic inductance detectors, we found relatively weak fluctuations (δfr/f0)2 following the spectral density A/fγ, with A ≃ 6 × 10−16 and γ slightly below 1 at an average number of photons of 30; the noise decreased with measurement power as 1/P1/2. The connection of the frequency fluctuations to two-level states and kinetic inductance fluctuations due to resistance noise is discussed.

Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al(111)⟨21¯1¯⟩∥sapphire(0001)[21¯1¯0]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness.

We have looked into cobalt disilicide (CoSi2) as a potential building block for superconducting quantum circuits. In order to achieve this, we annealed a thin layer of Co to create microwave cavities with thickness of d = 10–105 nm from CoSi2 embedded in the silicon substrate. The cavity properties were measured as a function of temperature and power. In the films measuring 10 and 25 nm, we find a significant kinetic inductance LK with a non-BCS power-law variation δLK ∝ T4.3±0.2 at low temperatures. The quality factor of the studied microwave resonances varied from 3 × 103 (d = 10 nm) to ∼5 × 104 (d = 105 nm) and increased as d(A − log d) with thickness, with two-level systems having very little effect. The power dependence of kinetic inductance was analyzed in terms of heat flow due to electron–phonon coupling, which was found to be stronger than estimated for heat relaxation by regular quasiparticles.

Granular metals offer tailorable electronic properties and play crucial roles in device and sensor applications. We have fabricated a series of nonmagnetic granular CoSi2 thin films and studied the Hall effect and transport properties. We observed a two orders of magnitude enhancement in the Hall coefficient in films fall slightly above the metal-insulator transition. This giant Hall effect (GHE) is ascribed to the local quantum-interference effect induced reduction of the charge carriers. Transmission electron microscopy images and transport properties indicate that our films form two dimensional granular arrays. The GHE may provide useful and sensitive applications.