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Tunable Multifractality in Quantum Matter

Alexander Petrović (Nanyang Technological University)
Wed, 11/03/2015 - 11:00am to 12:00pm
Physics Conference Room (S13-M01-11)
Vitor Pereira
Event Type: 


Fractals are scale-invariant spatial distributions which are ubiquitous in nature, governing the shapes of objects as diverse as snowflakes, trees and coastlines. A less well-known instance of fractal ordering occurs in strongly disordered materials, where electronic wavefunctions develop multifractal spatial distributions in the vicinity of the Anderson transition between extended and localised states. Such multifractal ordering has been predicted to enhance electron-electron interactions. In materials with instabilities to correlated electron phase formation (such as ferromagnets or superconductors), one may therefore envisage the possibility of tuning the quantum ground state via disorder.

In disordered superconductors, a large multifractal enhancement is expected in the pairing interaction (and hence the critical temperature Tc), provided that the Coulomb repulsion is weak. However, no such rise in Tc has ever been observed experimentally, due to the suppression of superconductivity by emergent granularity and a dynamically-augmented Coulomb repulsion in highly disordered materials. Using a range of experimental and numerical techniques (including electrical transport, magnetization, X-ray diffraction/scattering and density functional theory), we demonstrate that multifractal pairing enhancement does in fact occur in the quasi-one-dimensional superconductor Na2-δMo6Se6, due to the combination of random Na vacancy disorder with an intrinsically screened Coulomb repulsion. The pairing temperature Tons rises monotonically as the Anderson-Mott mobility edge is approached from the metallic side, in quantitative agreement with a multifractal enhancement model. Strikingly, Tons continues to rise in the localised phase after crossing the mobility edge, in accordance with theoretical predictions. The upper critical field Hc2 exceeds the weak-coupling Pauli limit by a factor of at least 4 in the localised regime, indicating a large increase in the superconducting gap energy.

Our results provide the first experimental perspective onto the unknown physics of correlated electron materials in the absence of Coulomb repulsion. We also show that the unique interplay between superconductivity and localisation in nanofilamentary materials renders them ideal building blocks for functional superconductors. Electron delocalisation drives an intrinsic stabilisation of phase fluctuations upon raising the temperature, magnetic field or electric current, in direct contrast to the behaviour of conventional homogeneous superconductors.

About the speaker

Alex graduated from Clare College, Cambridge, with a BA and MPhys in Experimental and Theoretical Physics, spending his final year working on muon spin relaxation and thermal hysteresis in underdoped cuprates. After moving to Geneva, Switzerland for his PhD, he spent the next 6 years building a helium-3 high-field scanning tunnelling microscope for use on unconventional superconductors. These labours eventually bore fruit, yielding the first real- space images of a vortex glass in a type-II superconductor, a multi-band scenario to explain the extremely high upper critical fields in Chevrel phases and a completely new type of vortex core. Since joining NTU in 2009, he has worked on a variety of projects including structural and magnetic phase transitions in magnetoelectric EuTiO3, ARPES in quasi-one-dimensional metals, phase emergence at the LaAlO3/SrTiO3 interface, manganite superlattices and the development of a 10mK 17T UHV local probe microscope inside a 350-ton floating laboratory.

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