Metal–insulator transition and superconductivity in Hg-doped BaPb0.75Bi0.25O3 (original) (raw)
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Metal-insulator transition and superconductivity in highly boron-doped nanocrystalline diamond films
physica status solidi (a), 2009
The low temperature electronic transport of highly boron-doped nanocrystalline diamond films is studied down to 300 mK. The films show superconducting properties with critical temperatures T c up to 2.1 K. The metal-insulator transition and superconductivity is driven by the dopant concentration and greatly influenced by the granularity in this system, as compared to highly boron-doped single crystal diamond. The critical boron concentration for the metal-insulator transition lies in the range from 2.3 × 10 20 cm −3 up to 2.9×10 20 cm −3 , as determined from transport measurements at low temperatures. Insulating nanocrystalline samples follow an Efros-Shklovskii type of temperature dependence for the conductivity up to room temperature, in contrast to Mott variable range hopping in the case of insulating single crystal diamond close to the metalinsulator transition.
Granular superconductivity in metallic and insulating nanocrystalline boron-doped diamond thin films
Journal of Physics D: Applied Physics, 2010
The low temperature electrical transport properties of nanocrystalline boron-doped diamond thin films (b-NCD) have been found to be strongly affected by the system's granularity. The important differences between the high and low temperature behavior are caused by the inhomogeneous nucleation of superconductivity in the samples. In this review we will discuss the experimental data obtained on several b-NCD thin films, which were studied by either varying their thickness or boron concentration. It will be shown that the low temperature properties are influenced by the b-NCD grain boundaries as well as by the appearance of an intrinsic granularity inside these granules. Moreover, superconducting effects have been found to be present even in insulating b-NCD films and are responsible for the negative magnetoresistance regime observed at low temperatures. On the other hand, the low temperature electrical transport properties of b-NCD films show important similarities with those observed for granular superconductors.
Nature of the superconductor–insulator transition in disordered superconductors
Nature, 2007
The interplay of superconductivity and disorder has intrigued scientists for several decades. Disorder is expected to enhance the electrical resistance of a system, whereas superconductivity is associated with a zero-resistance state. Although, superconductivity has been predicted to persist even in the presence of disorder 1 , experiments performed on thin films have demonstrated a transition from a superconducting to an insulating state with increasing disorder or magnetic field 2 . The nature of this transition is still under debate, and the subject has become even more relevant with the realization that high-
Theory of the magnetoresistance of disordered superconducting films
Physical Review B, 2006
Experiments on disordered superconducting thin films have revealed a magnetoresistance peak of several orders of magnitude. Starting from the disordered negative-U Hubbard model, we employ an ab initio approach that includes thermal fluctuations to calculate the resistance, and fully reproduces the experimental phenomenology. Maps of the microscopic current flow and local potential allow us to pinpoint the source of the magnetoresistance peak -the conducting weak links change from normal on the low-field side of the peak to superconducting on the high-field side. Finally, we formulate a simple one-dimensional model to demonstrate how small superconducting regions will act as weak links in such a disordered thin film. PACS numbers: 72.20.Dp, 71.10.Fd The interplay of disorder and superconductivity has been a subject of great interest and debate for many years . While the consequences of disorder are relatively well understood within BCS mean-field theory, the way disorder modifies the magnetic field and the temperature response of low-dimensional superconductors (SCRs), where the loss of superconductivity is due to phase fluctuations , remains an open question. This situation is underscored by recent puzzling experiments that demonstrated a huge magnetoresistance (MR) peak on the normal side of the SCR-insulator transition in thin films , the emergence of a "super-insulating" phase [6] in the same system, and the smearing of the transition at the two-dimensional SC interface formed between two insulating oxides [7]. One of the main reasons for our limited understanding of these experiments is that there is no microscopic theory that incorporates disorder, phase fluctuations and magnetic field. In this Letter we utilize a recently developed ab initio formalism [8] to address the puzzle of the MR peak. This tool allows us to calculate the conductance through a possibly disordered superconducting (SC) region, based on a microscopic model, using an approximation that takes into account the phase and amplitude fluctuations of the SC order parameter. Moreover, the detailed information provided by this approach on the local currents and chemical potentials (for details see ), in addition to the nature of the current flow (electrons or Cooper pairs) illuminates the microscopic physics behind the anomalous resistance peak.
Journal of Experimental and Theoretical Physics Letters, 2004
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 .
Journal of Experimental and Theoretical Physics, 2010
The transport properties (R(T) and R(H) dependences at various values of the transport current in magnetic fields up to 65 kOe) and low temperature heat capacity in magnetic fields up to 90 kOe of the BaPb 0.75 Bi 0.25 O 3 superconductor (T C ≈ 11.3 K) are investigated with the goal of clarifying the mechanisms determining the nonmonotonic behavior and hysteresis of its magnetoresistance R(H). The type of R(H) hys teretic dependences for BaPb 0.75 Bi 0.25 O 3 is analogous to that observed in granular high T c superconductors (HTSCs); however, unlike classical HTSC systems, the field width of the magnetoresistance hysteresis loop for polycrystalline BaPb 0.75 Bi 0.25 O 3 depends on the transport current. This means that although the mecha nisms responsible for the magnetoresistance hysteresis (the influence of the magnetic flux trapped in super conducting regions on the effective field in Josephson interlayers) are identical in these objects, the transport current in BaPb 0.75 Bi 0.25 O 3 may considerably affect the diamagnetic response of the superconductor. A con siderable effect of transport current on the field in which the R(H) dependences have a peak and exhibit hys terestic properties is observed. Such a behavior can be adequately interpreted using the model of the spatially inhomogeneous superconductor-insulator state proposed by Gorbatsevich et al. [JETP Lett. 52, 95 (1990)]. The nonmonotonic dependence of quantity C/T (C is the heat capacity) on the magnetic field discovered in the present study also agrees with the conclusions based on this model.
Metal-to-insulator transition and superconductivity in boron-doped diamond
Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, 2008
The experimental discovery of superconductivity in boron-doped diamond came as a major surprise to both the diamond and the superconducting materials communities. The main experimental results obtained since then on single-crystal diamond epilayers are reviewed and applied to calculations, and some open questions are identified. The critical doping of the metal-to-insulator transition (MIT) was found to coincide with that necessary for superconductivity to occur. Some of the critical exponents of the MIT were determined and superconducting diamond was found to follow a conventional type II behaviour in the dirty limit, with relatively high critical temperature values quite close to the doping-induced insulator-to-metal transition. This could indicate that on the metallic side both the electron-phonon coupling and the screening parameter depend on the boron concentration. In our view, doped diamond is a potential model system for the study of electronic phase transitions and a stimul...