Superconducting phase transitions in disordered NbTiN films

Abstract

The suppression of superconductivity in disordered systems is a fundamental problem of condensed matter physics. Here we investigate the superconducting niobium-titanium-nitride (Nb 1−x TixN) thin films grown by atomic layer deposition (ALD) where disorder is controlled by the slight tuning of the ALD process parameters. We observe the smooth crossover from the disorderdriven superconductor-normal metal transition (often reffered to as fermionic mechanism) to the case where bosonic mechanism dominates and increasing disorder leads to formation of metal with Cooper pairing. We show that, in 'moderately' disordered films, the transition to zero-resistance state occurs in a full agreement with the conventional theories of superconducting fluctuations and Berezinskii-Kosterlitz-Thouless transition. However, the 'critically' disordered films violate this accord showing low-temperature features possibly indicating the Bose metal phase. We show that it is the interrelation between film's sheet resistance in the maximum, Rmax, of the resistive curve R✷(T ) and Rq = h/4e 2 that distinguishes between these two behaviors. We reveal the characteristic features in magnetoresistance of the 'critically' disordered films with Rmax > Rq. 2/9

Disorder-induced 2D superconductivity in a NbTiN film grown on Si by ultrahigh-vacuum magneton sputtering

“…Instead of observing the direct transition between the superconducting state and the normal state, we observed an intermediate transition state, characterized by lower resistance values compared to the normal states. Such an intermediate state has been reported in literature [22], as a Bose Metal state. Such a state has been evidenced by properties such as a resistivity saturation as it approaches low temperatures, a giant positive magnetoresistance, Arrhenius like behavior of the resistance being proportional to exp (-1/k B T) and a lack of Berezinskii-Kosterlitz-Thouless (BKT) transition or T BKT = 0.…”

“…(b) A larger out-of-plane magnetic field 3.35 T to break zeroresistance superconducting state compared to the conventional disordered 2D superconducting system. Furthermore, the absence of the Bose metallic state implies a more robust disordered-induced 2D superconducting system compared to the size-induced disordered superconducting system [22].…”

“…Various attempts have been made to describe the effect of disorder on superconductivity [22]. Anomalous metallic states have been reported in certain systems [23][24][25][26][27].…”

Magnetoconductance and photoresponse properties of disordered NbTiN films

“…NbTiN is a potential alternative to Al for high-kinetic-inductance, low-loss microwave resonators [3,4] and a good candidate for Cooper-pair transistors [5]. Besides applied physics, NbTiN films are relevant for fundamental studies of disorder-driven effects in superconductors, e.g., superconductor-insulator [6] and Berezinskii-Kosterlitz-Thouless transitions [7].…”

Wafer-scale method for amorphizing superconducting thin films

“…5(c) by the green and blue lines, respectively. Other interesting materials include, e.g., ALD-grown NbTiN [36].…”