Orbital Two-channel Kondo Effect Research Articles

Emergence of orbital two-channel Kondo effect in epitaxial TiN thin films

The continual scaledown and stringent defect control in electronic devices are opening up frontiers in Kondo effect-a powerful testbed for probing magnetic impurity coupling to a continuum of electronic states. In disordered ferromagnets, degenerate quantum states like two-level systems (TLS) with a pseudo-half spin can couple with the continuum to mimic Kondo physics, creating the orbital two-channel Kondo (2CK) effect below the characteristic Kondo temperature (TK). By employing vacuum-annealing to inject ∼12 at.% nitrogen vacancies (VN) in TiN1-x films, we generated room-temperature ferromagnetic ordering with 13.6 emu g−1 saturation magnetization. The transport characteristics exhibited non-Fermi-liquid (NFL) behavior below TK of 20.2 K. The orbital 2CK effect is signified by a clear transition from logarithmic (T < T0∼40 K), to square-root dependence (T < TK) and subsequent deviation from it (T < TD∼10 K) in resistivity upturn behavior in three distinct low-temperature regimes. By controlling VN, we establish the necessity of ferromagnetism and ultrafast tunneling centers for generating orbital 2CK effect and NFL behavior in disordered metallic systems, extending their scope in non-magnetic nanomaterials.

Low-temperature resistivity anomaly and weak spin disorder in Co2MnGa epitaxial thin films

The topological material of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnGa}$ with potentially high spin polarization has attracted more and more attention, in the fields of fundamental science and spintronic applications. Here we report the electrical- and magnetotransport behaviors in epitaxial $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnGa}$ thin films. An angle-independent negative linear magnetoresistance and a negative anisotropy magnetoresistance are observed in the whole temperature range from 3 to 300 K, indicating the existence of weak spin disorder and a high spin polarization. Intriguingly, with decreasing temperature an anomalous resistivity upturn that exhibits $\ensuremath{-}{T}^{1/2}$ dependence is observed, and this phenomenon is robust under magnetic fields. Further analysis demonstrates that the three-dimensional enhanced electron-electron interaction effect is responsible for these phenomena rather than the orbital two-channel Kondo effect or other quantum interference effects, which provides deeper insight into the transport behavior of the $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnGa}$ materials.

Influence of orbital two-channel Kondo effect on anomalous Hall effect in ferrimagnetic composites of LaNiO3 and CoFe2O4

In this report, we have investigated the magnetoresistance (MR) and Hall effect of the ferrimagnetic composites containing LaNiO3 and CoFe2O4 (CFO) (with CFO content 15% and 20%) which exhibit orbital two-channel Kondo (2CK) effect and therefore pronounced resistivity upturn at low temperature. Both composites manifest a negative to positive crossover in MR with increasing temperature. The MR is described by the Khosla and Fisher model of spin fluctuations scattering of conduction electrons and the two-band theory based on hybridized p-d sub-bands. The Hall resistivity of the composites consists of both ordinary and anomalous part. The negative sign of the ordinary Hall coefficient suggests electrons as the dominating charge carriers. The coefficient of anomalous Hall resistivity () follows the scaling relation () with longitudinal resistivity () at high temperature above the resistivity upturn. However, at low temperature shows non-monotonous behaviour and deviates from the scaling relation where orbital 2CK effect takes place. More detailed study below the resistivity upturn of the composite with 20% CFO reveals that this deviation occurs around the Kondo temperature. This breakdown of scaling relation around the Kondo temperature indicates the possible influence of orbital 2CK on the anomalous Hall effect.

Tuning spin one channel to exotic orbital two-channel Kondo effect in ferrimagnetic composites of LaNiO3 and CoFe2O4

We report the tuning from spin one channel to orbital two-channel Kondo (2CK) effect by varying CoFe2O4 (CFO) content in the composites with LaNiO3 (LNO) along with the presence of ferrimagnetism. Although there is no signature of resistivity upturn in the case of pure LNO, all the composites exhibit a distinct upturn in the temperature range of 30–80 K. For composites with lower percentage of CFO (10%), the electron spin plays the key role in the emergence of resistivity upturn which is affected by external magnetic field. On the other hand, when the CFO content is increased (⩾15%), the upturn shows strong robustness against high magnetic field (⩽14 T) and a crossover in temperature variation from to T1/2 at the Kondo temperature, indicating the appearance of orbital 2CK effect. The orbital 2CK effect originates due to the scattering of conduction electrons from the structural two-level systems which is created at the interfaces between the two phases (LNO and CFO) of different crystal structures as well as inside the crystal planes. The specific heat data at low temperature (⩽40 K), deviates from the usual linear temperature variation of the electronic contribution. With higher CFO content it shows more deviation which also indicates the increasing amount of two-level system. A negative magnetoresistance (MR) is observed at low temperature (<30 K) for composites containing both lower (10%) and higher percentage (15%) of CFO. We have analyzed the negative MR using Khosla and Fisher semi-empirical model based on spin dependent scattering of conduction electrons from localized spins.

Electrical Transport of Perpendicularly Magnetized L10-MnGa and MnAl Films

Ferromagnetic films of [Formula: see text]-ordered MnGa and MnAl that exhibit giant perpendicular magnetic anisotropy and great controllability in the magnetism and structural disorders show promising applications not only in magnetic recording, permanent magnets and spintronics, but also in controllable studies of disorder-relevant electrical transport phenomena. In this paper, we review the intriguing experimental observations of the orbital two-channel Kondo effect and anomalous Hall effect in [Formula: see text]-ordered MnGa and MnAl thin films with perpendicular magnetic anisotropy. We also give a perspective with regards to the future technological and fundamental applications of these perpendicularly magnetized Mn-based binary alloy films.

Anomalous resistivity upturn in epitaxial L21-Co2MnAl films

Despite of the great scientific and technology interest, highly ordered full-Heusler L21-Co2MnAl films have remained a big challenge in terms of the availability and the electrical transport. Here we report the controllable growth and the intriguing transport behavior of epitaxial L21-Co2MnAl films, which exhibit a low-temperature (T) resistivity upturn with a pronounced T1/2 dependence, a robust independence of magnetic fields, and a close relevance to structural disorder. The resistivity upturn turns out to be qualitatively contradictory to weak localization, particle-particle channel electron-electron interaction (EEI), and orbital two-channel Kondo effect, leaving a three-dimensional particle-hole channel EEI the most likely physical source. Our result highlights a considerable tunability of the structural and electronic disorder of magnetic films by varying growth temperature, affording unprecedented insights into the origin of the resistivity upturn.

Observation of orbital two-channel Kondo effect in a ferromagnetic L10-MnGa film.

The experimental existence and stability of the fixed point of the two-channel Kondo (2CK) effect displaying exotic non-Fermi liquid physics have been buried in persistent confusion despite the intensive theoretical and experimental efforts in past three decades. Here we report an experimental realization of the two-level system resonant scattering-induced orbital 2CK effect in a ferromagnetic L10-MnGa film, which is signified by a magnetic field-independent resistivity upturn that has a logarithmic and a square-root temperature dependence beyond and below the Kondo temperature of ~14.5 K, respectively. Our results not only evidence the robust existence of orbital 2CK effect even in the presence of strong magnetic fields and long-range ferromagnetic ordering, but also extend the scope of 2CK host materials from nonmagnetic nanoscale point contacts to diffusive conductors of disordered alloys.

Orbital two-channel Kondo effect in epitaxial ferromagnetic L10-MnAl films

The orbital two-channel Kondo effect displaying exotic non-Fermi liquid behaviour arises in the intricate scenario of two conduction electrons compensating a pseudo-spin-1/2 impurity of two-level system. Despite extensive efforts for several decades, no material system has been clearly identified to exhibit all three transport regimes characteristic of the two-channel Kondo effect in the same sample, leaving the interpretation of the experimental results a subject of debate. Here we present a transport study suggestive of a robust orbital two-channel Kondo effect in epitaxial ferromagnetic L10-MnAl films, as evidenced by a magnetic field-independent resistivity upturn with a clear transition from logarithmic- to square-root temperature dependence and deviation from it in three distinct temperature regimes. Our results also provide an experimental indication of the presence of two-channel Kondo physics in a ferromagnet, pointing to considerable robustness of the orbital two-channel Kondo effect even in the presence of spin polarization of the conduction electrons.