Magnetic Field Enhanced Superconductivity in Epitaxial Thin Film WTe2.

In conventional superconductors an external magnetic field generally suppresses superconductivity. This results from a simple thermodynamic competition of the superconducting and magnetic free energies. In this study, we report the unconventional features in the superconducting epitaxial thin film tungsten telluride (WTe2). Measuring the electrical transport properties of Molecular Beam Epitaxy (MBE) grown WTe2 thin films with a high precision rotation stage, we map the upper critical field Hc2 at different temperatures T. We observe the superconducting transition temperature T c is enhanced by in-plane magnetic fields. The upper critical field Hc2 is observed to establish an unconventional non-monotonic dependence on temperature. We suggest that this unconventional feature is due to the lifting of inversion symmetry, which leads to the enhancement of Hc2 in Ising superconductors

Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa2S4 and NiGa2S4

Magnetic susceptibility and muon spin relaxation (muSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. The muSR data indicate a sharp onset of a frozen or nearly-frozen spin state at T* = 31(2) K, twice the spin-glass-like freezing temperature T_f = 16(1) K. The susceptibility becomes field dependent below T*, but no sharp anomaly is observed in any bulk property. A similar transition is observed in muSR data from the spin-1 isomorph NiGa2S4. In both compounds the dynamic muon spin relaxation rate lambda_d(T) above T* agrees well with a calculation of spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized classical regime of a 2D frustrated quantum antiferromagnet. There is no firm evidence for other mechanisms. At low temperatures lambda_d(T) becomes temperature independent in both compounds, indicating persistence of spin dynamics. Scaling of lambda_d(T) between the two compounds is observed from ~T_f to ~1.5T*. Although the muSR data by themselves cannot exclude a truly static spin component below T*, together with the susceptibility data they are consistent with a slowly-fluctuating "spin gel" regime between T_f and T*. Such a regime and the absence of a divergence in lambda_d(T) at T* are features of two unconventional mechanisms: (1) binding/unbinding of Z_2 vortex excitations, and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence of a sharp anomaly or history dependence at T* in the susceptibility of FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest transitions to low-temperature phases with unconventional dynamics.Comment: 13 pages, 6 figures, accepted for publication in Physical Review

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

p-Type doping represents a key step towards III-nitride (InN, GaN, AlN) optoelectronic devices. In the past, tremendous efforts have been devoted to obtaining high quality p-type III-nitrides, and extraordinary progress has been made in both materials and device aspects. In this article, we intend to discuss a small portion of these processes, focusing on the molecular beam epitaxy (MBE)-grown p-type InN and AlN—two bottleneck material systems that limit the development of III-nitride near-infrared and deep ultraviolet (UV) optoelectronic devices. We will show that by using MBE-grown nanowire structures, the long-lasting p-type doping challenges of InN and AlN can be largely addressed. New aspects of MBE growth of III-nitride nanostructures are also discussed

Molecular beam epitaxial growth, characterization, and nanophotonic device applications of InN nanowires on Si platform

Dislocation-free semiconductor nanowires are an extremely promising route towards compound semiconductor integration with silicon technology. However, precise control over nanowire doping, together with the surface charge properties, has remained a near-universal material challenge to date. In this regard, we have investigated the molecular beam epitaxial growth and the correlated surface electrical and optical properties of InN nanowires, a promising candidate for future ultrahigh-speed nanoscale electronic and photonic devices and systems, on Si platform.By dramatically improving the epitaxial growth process, intrinsic InN nanowires are achieved, for the first time, both within the bulk and on the non-polar InN surfaces. The near-surface Femi-level is measured to locate below the CBM, suggesting the absence of surface electron accumulation. Such intrinsic InN nanowires can possess an extremely low free carrier concentration of ~1e13 /cm3, as well as a close-to-theoretically-predicted electron mobility in the range of 8,000 to 12,000 cm2/V·s at room temperature. This result is in direct contrast to the universally observed 2DEG on the InN grown surfaces. Furthermore, the surface charge properties of InN nanowires, including the formation of 2DEG and the optical emission characteristics can be precisely tuned, for the first time, through the controlled n-type doping.More importantly, p-type doping into InN nanowires is also realized, for the first time. The presence of Mg-acceptors is clearly demonstrated by the PL spectra. Furthermore, p-type surface is observed from the XPS experiments, indicating the presence of free holes. Additionally, p-type conduction is directly measured by single nanowire field effect transistors.In the end of this thesis, InN nanowire p-i-n photodiodes are fabricated, with a light response up to the telecommunication wavelength range at low temperatures. This thesis work provides a vivid example, and paves the way for the rational “materials by design” development of silicon integrated InN-based device technology in the nanoscale.Les nanofils semi-conducteurs sans dislocations sont une voie très prometteuse vers l'intégration des semi-conducteurs composés avec la technologie silicium. Cependant, un contrôle précis de dopage des nanofils, ainsi que les propriétés de charge de surface, reste un défi universel à ce jour. À cet égard, nous avons étudié la croissance épitaxiale par faisceau moléculaire et les propriétés de surface corrélés électriques et optiques des nanofils de InN sur du substrat de silicium, qui ont émergé comme candidat prometteur pour l'avenir des dispositifs électroniques et photoniques à très haute vitesse et à échelle nanométriques.Pour la première fois, en améliorant le processus de croissance épitaxiale, InN intrinsèque est atteint, à la fois dans le volume et sur les surfaces non polaires de InN. Le niveau de Fermi à la surface est mesuré et localisée sous le CBM, ce qui suggère l'absence d'accumulation d'électrons en surface. Ces nanofils InN intrinsèques possédent une concentration de porteurs libres très faible ~1e13 /cm3, ainsi que d'une mobilité proche de le théoriquement prédite d'électrons entre 8000 à 12000 cm2/V·s à température ambiante. Ce résultat est en contraste direct avec les 2DEG observés sur les surfaces d'InN. En outre, les propriétés de charge de surface de nanofils InN, y compris la formation de 2DEG et les caractéristiques d'émission optiques, peut être réglé avec précision, pour la première fois, par l'intermédiaire du contrôle d'incorporation de dopants de type n.Plus important encore, dopage de type p dans les nanofils InN est également réalisé pour la première fois. La présence de niveaux d'énergie Mg-accepteur est démontrée par les spectres de PL. Dans ces nanofils dopés de Mg, il n'y a pas d'accumulation d'électrons de surface et le niveau de Fermi dans le volume est proche de la VBM, ce qui indique un matériau de type p.En fin, la jonction p-i-n basé sur des nanofils InN photodétecteurs qui peut être utilisé en mode photovoltaïque est démontrée, avec une réponse à la lumière jusqu'à la longueur d'onde des télécommunications à de basses températures. Ce travail de thèse fournit un exemple frappant, ainsi que prépare le terrain pour le développement "matériaux par conception" de la technologie des dispositifs en silicium intégrée à base InN à l'échelle nanométrique

Abnormal Photocurrent in Semiconductor p‐n Heterojunctions: Toward Multifunctional Photoelectrochemical‐Type Photonic Devices and Beyond

Abstract Semiconductor p‐n heterojunctions are important building blocks for modern electronic and photonic devices. Further combining semiconductor p‐n heterojunctions with light and electrolyte environment, interesting photoelectrochemical (PEC) phenomena can occur, which enriches the design principles of multifunctional devices. In fact, recent years have witnessed the emergence of PEC‐type photonic devices. For PEC‐type photonic devices, a key to realize multifunctionality is to control the photocurrent polarity of the photoelectrode. In this study, an abnormal photocurrent is reported from p‐InGaN/n‐GaN nanowire heterojunctions under a blue light illumination: although n‐GaN is transparent to the blue light (and thus optical absorption mainly occurs in p‐InGaN) and p‐InGaN in principle can only give negative PEC photocurrent, the detailed experiments show that positive PEC photocurrent can be generated from the p‐InGaN segment due to the existence of the built‐in electric field at the p‐n junction. This study shows a new route to control the photocurrent polarity in a semiconductor p‐n heterojunction photoelectrode. This unveiled role of the built‐in electric field is expected to impact the design of emerging PEC‐type photonic devices, as well as other novel photonic and electronic devices based on semiconductor nanowire p‐n heterojunctions