Temperature-and magnetic-field dependent measurements of the resistance of ultrathin superconducting TiN films are presented. The analysis of the temperature dependence of the zero field resistance indicates an underlying insulating behavior, when the contribution of Aslamasov-Larkin fluctuations is taken into account. This demonstrates the possibility of coexistence of the superconducting and insulating phases and of a direct transition from the one to the other. The scaling behavior of magnetic field data is in accordance with a superconductor-insulator transition (SIT) driven by quantum phase fluctuations in two-dimensional superconductor. The temperature dependence of the isomagnetic resistance data on the high-field side of the SIT has been analyzed and the presence of an insulating phase is confirmed. A transition from the insulating to a metallic phase is found at high magnetic fields, where the zero-temperature asymptotic value of the resistance being equal to h/e 2 .PACS numbers: 74.25. -q, 71.30.+h, 74.40.+k The interplay between superconductivity and localization is a phenomenon of fundamental interest, and the question of the nature of superconductivity and its evolution in two-dimensional disordered systems and a perpendicular magnetic field continues to receive a great deal of theoretical and experimental attention. Twodimensional systems are of special interest as two is the lower critical dimensions for both localization and superconductivity. Two ground states are expected to exist for bosons at T = 0: a superconductor with long-range phase coherence and an insulator in which the quantum mechanical correlated phase is disjointed. The zerotemperature superconductor-insulator transition (SIT) is driven purely by quantum fluctuations and is an example of a quantum phase transition [1]. The superconducting phase is considered to be a condensate of Cooper pairs with localized vortices, and the insulating phase is a condensate of vortices with localized Cooper pairs. Between these two states there is the only metallic phase point, and this metal has a bosonic nature as well. The theoretical description based on this assumption was suggested in [2]. At finite temperatures, a quantum phase transition is influenced by the thermal fluctuations, and according to the theory, (i) the film resistance R near the magnetic-field-induced SIT at low temperature T in the vicinity of the critical field B c is a function of one scaling variable δ = (B − B c )/T 1/νz , with the critical exponents ν and z being constants of order of unity, and (ii) at the transition point, the film resistance is of the order h/(2e) 2 ≈ 6.5 kΩ (the quantum resistance for Cooper pairs). Although much work has been done, and in many systems the scaling relations hold [3,4,5,6,7,8], the magnetic-field-induced SIT in disordered films remains a controversial subject, especially concerning the insulating phase and the bosonic conduction at B > B c . There is experimental evidence [7] that, despite the magnetoresistance being nonmonotonic, ...
The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10 6 cm 2 /Vs is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10 6 cm 2 /Vs does not prevent the formation of zero-resistance states in magnetic field in a two-dimensional system under the effect of microwave radiation. Current interest in studying the transport in twodimensional (2D) electron systems is related to the recent observation of resistance oscillations in magnetic field that arise in high-mobility GaAs/AlGaAs heterostructures under the effect of microwave radiation [1]. It was found that these oscillations are periodic in the inverse magnetic field (1/B) with a period determined by the ratio of the microwave radiation frequency to the cyclotron frequency. The photoresponse oscillations in magnetic field in a high-mobility 2D system (such oscillations were predicted more than 30 years ago [2]) fundamentally differed from the behavior of photoresponse in GaAs/AlGaAs heterostructures with lower mobilities [3]. The effect of microwave radiation on the magnetotransport in GaAs/AlGaAs heterostructures of moderate quality was found to manifest itself as a photoresistance peak caused by the heating of the 2D electron gas under the magnetoplasma resonance conditions [4]. Soon after the first experimental observation of the microwave radiation-induced resistance oscillations in magnetic field in high-mobility GaAs/AlGaAs heterostructures, it was shown that the minima of these oscillations may correspond to resistance values close to zero [5,6,7]. This unexpected experimental result initiated intensive theoretical studies of the aforementioned phenomenon [7,8,9,10,11,12,13,14,15,16]. However, despite the multitude of theoretical publications, the mechanisms responsible for the resistance oscillations under the effect of a microwave field in 2D systems with large filling factors remain open to discussion. The role of the mobility of charge carriers in the manifestation of microwaveinduced zero-resistance states arising in magnetic field in 2D systems also remains unclear. It is commonly believed that the mobility should exceed 3 × 10 6 cm 2 /Vs[17]. As for the experimental studies of the photoresponse to microwave radiation in 2D systems in classically strong magnetic fields, such studies, excluding a few of them [17,18,19,2...
In this study, we demonstrated experimentally and theoretically that oxygen
vacancies are responsible for the charge transport in HfO$_2$. Basing on the
model of phonon-assisted tunneling between traps, and assuming that the
electron traps are oxygen vacancies, good quantitative agreement between the
experimental and theoretical data of current-voltage characteristics were
achieved. The thermal trap energy of 1.25 eV in HfO$_2$ was determined based on
the charge transport experiments.Comment: 5 pages, 5 figure
The discovery of ferroelectric properties in hafnium oxide has brought back the interest in the ferroelectric non-volatile memory as a possible alternative for low power consumption electronic memories. As far as real hafnium oxide-based materials have defects like oxygen vacancies, their presence might affect the ferroelectric properties due to oxygen atom movements during repolarization processes. In this work, the transport experiments are combined with the modeling to study evolution of the oxygen vacancy concentration during the endurance and to determine the optimal defect density for a higher residual polarization in lanthanum-doped hafnium oxide.