Goran Karapetrov

Titanium nitride thin films are prepared using DC reactive magnetron sputtering at room temperature, with an RF bias applied to the substrate to systematically control film growth. By adjusting the substrate RF bias, the degree and length scale of disorder are tuned, leading to pronounced changes in film microstructure and superconducting properties. A dome-shaped dependence of the superconducting critical temperature, Tc, on RF bias is observed, accompanied by a minimum in the superconducting coherence length near the peak of the Tc dome. This correlated behavior reflects a controlled evolution from point-like disorder to a nanocrystalline microstructure that ultimately develops into well-ordered grains separated by electrically transparent grain boundaries. At intermediate RF bias, the films acquire an optimal grain size that enhances intragrain superconductivity while reducing the effective coherence length. This mesoscopic control of disorder and microstructure provides a pathway for engineering TiN films with superconducting properties that are desirable for applications requiring high normal-state sheet resistance in fully gapped superconductors, such as superconducting resonators and high-kinetic-inductance devices.

We report measurements of anisotropic triple-q charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1T-TiSe_{2} over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, T_{CDW}, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.

Transition metal dichalcogenides like 2H-NbSe2 exhibit remarkable electronic properties, but their performance is highly sensitive to defects, particularly selenium vacancies. Here, we demonstrate controlled migration of Se atoms from the bulk to the surface of NbSe2 single crystal, induced by heat, electron beam irradiation, and laser excitation. Using Raman spectroscopy, atomic force microscopy, and energy-dispersive x-ray spectroscopy, we identify the formation of nanosized (200–500 nm) amorphous selenium clusters on the surface, evidenced by a distinct Raman mode at 250 cm−1. Temperature-dependent studies reveal the thermally activated nature of this process, with an activation energy of 1.12 eV—significantly higher than in related dichalocogenide 1T-TiSe2, suggesting suppressed Se diffusion in NbSe2 at elevated temperatures. The results provide an estimate of the fabrication thermal budget for future electronic and optoelectronic devices utilizing NbSe2.

Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron–cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1−xHfx (x = 0 – 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x ∼ 0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying.

Superconducting nanowire detectors are a very prominent technology in fields that require single-photon detection with very high efficiency, such as quantum computing and communication or nanophotonics. Their high detection rates and unsurpassed timing precision would also make them ideal candidates for detector technology in other fields, such as nuclear and high energy physics. Typical collider experiments, however, operate in much more extreme conditions than most quantum optics setups. One of the main roadblocks for transforming superconducting nanowire photon detectors into superconducting nanowire particle detectors was the question of their performance in strong magnetic fields. In this work, I present the results of developing a novel thin film deposition method called Ion Beam Assisted Sputtering, which allowed me to grow superconducting Niobium Nitride films with upper critical fields as high as 32 T and study their microstructural and electromagnetic properties. As the nanowires are detectors with extremely small sizes, I also explore the nature of the suppression of the superconducting state in ultra-thin films. I propose two possible explanations for this phenomenon, one based on confinement of the BCS condensate, and the other on weak localization and competition between two strongly correlated electronic phases. Finally, I use this technology to fabricate superconducting nanowire single photon detectors capable of detection in fields as high as 5 T, currently the highest field at which nanowire detectors are known to operate. This result demonstrates the potential viability of the technology for nuclear and high energy physics, and is an important stepping stone to developing novel particle detectors.

The origin of the charge density wave (CDW) state in TiSe₂ has been under intense scrutiny since its discovery in 1976. Theoretical and experimental evidence of the origin of the CDW has been inconclusive, with findings in favor of either correlated electron effects such as the excitonic insulator mechanism, or conventional electron-phonon coupling mechanism, or the Jahn-Teller effect. The recent discovery of superconductivity upon application of pressure or doping with copper has renewed the interest in the material, as has the discovery of the chiral nature of the CDW, neither of which have an accepted theoretical explanation. We have used an ultrafast electron diffraction setup, that has been developed over the course of this thesis, to observe the ultrafast relaxation dynamics in thin (~50 nm) single crystals of TiSe₂. The dynamics are initiated by a 100 fs laser pump pulses that almost instantaneously excite the electronic subsystem and create a non-equilibrium state with highly excited electrons in the presence of cold ionic lattice. As the energetic electrons interact with ionic lattice of the crystal, the ultrafast electron diffraction method is used to monitor the electron-phonon relaxation dynamics. Both incoherent processes as well as the coherent breathing modes initiated along the van der Waals bonded layers are studied with time resolution of ~100 fs over several hundreds of picoseconds. The complex lattice dynamics are initiated by the photoinduced strain that originates from combination of thermoelastic effects due to lattice heating, displacive forces related to the sudden modification of electronic screening in the crystal, and non-thermal phase transitions initiated in the material. The above processes contribute to the complex relaxation dynamics observed through the changes in the instantaneous electron diffraction pattern. The dependence of the photoinduced strain on the pump laser fluence is monitored on picosecond timescale at low temperatures (TiSe₂ in CDW phase) and at room temperature (TiSe₂ in normal phase). The experimentally observed chirality of the CDW can be explained by the existence of a metastable chiral phase that can be accessed within a certain range of electronic temperatures. Our observed strain dynamics correlates with recent density functional theory (DFT) calculations. The DFT results reproduce the experimental findings without the need to invoke correlated electron effects such as the excitonic insulator condensate. Our results suggest that the chiral nature of the CDW as well as the observed non-thermal melting of the CDW under ultrafast laser excitation, could be explained solely through the modification of the electronic susceptibility at elevated temperatures. The findings in this work will stimulate further study of the chiral nature of charge ordering with possible future application of these phenomena in electronic devices and systems.