Search for novel mechanisms to increase the critical temperature of a superconductor

Lead Research Organisation: University of Cambridge
Department Name: Physics


In this proposal we investigate different aspects of superconductivity with the ultimate goal of finding novel ways - that can be tested experimentally - to increase substantially the critical temperature (Tc) of a superconductor/superfluid. Motivated by recent experimental advances in cold atom, manipulation of nanostructures and theoretical advances in high energy physics, we propose to achieve this goal by studying: 1) finite size effects in different models of high Tc superconductivity both theoretically and experimentally, 2) superconductivity in systems that do not thermalize, 3) superconductivity induced in systems with Efimov states (three particles bound states that occur in situations in which the two body interaction does not lead to bound states). In relation to 1) we aim a description, mostly analytical, of finite size effects in different mean field descriptions of high Tc superconductor. Then, for the models leading to a highest Tc's we plan to carry out a more refined theoretical analysis whose results can be used to describe superconductivity in realistic systems. Finally, in collaboration with experimentalists,we aim to chose the materials and parameters (size, grain shape...) most suitable for experimental studies, show experimentally that the critical temperature can be substantially (>15%) increased and propose technological applications. In relation to 2) we first provide a quantitative description of the stability of the equivalent of a Cooper's trimer in many body systems described by Efimov physics. Then we explore the feasibility of ground states based on a collection of Efimov states by using Monte Carlo techniques. If successful, we aim to describe quantitatively the resulting superconducting state andits stability to thermal fluctuations.In relation to 3) we first address the role of Anderson-Mott localization effects in the route to thermalization in a closed many body system by using exact diagonalization techniques, random matrix theory and the finite size scaling method. Based on these results we put forward a characterization of thermalization in closed many body systems. Finally we investigate superconductivity in systems that do not thermalize. Specifically we aim to identify the non-thermal quasiparticle distribution that enhances Tc the most.A fully theoretical/analytical descritption of these systems is challenging since many of them are strongly interacting. In high energy physics the Anti de Sitter (AdS) - conformal field theory (CFT) correspondence, provides, in certain cases a theoretical framework to tackle these problems. In relation with this problem we explore to what extent this technique provides a really quantitative description of quantum critical points and certain aspects of high temperature superconductivity.

Planned Impact

Who will benefit from this research outside the academic research community? 1. Companies developing nanoscience applications How? Our results about enhancement and control of superconductivity in nanograins are relevant in the design of superconducting nanocircuits and other nanodevices. What will be done to ensure that they benefit? We shall contact companies with interests in this area and shall present our findings, adapted to the audience, in conference and workshops focused on the exchange between the academic and commercial private sector. 2. Graduate Students How? My experience, leadership, teaching skills and method of work acquired in a leading US research university will directly enhance research excellence and promote competitiveness in UK. Students under my supervision will receive an excellent scientific training. They will be working on problems of broad and current interest for which it is more likely to achieve a greater impact. What will be done to ensure that they benefit? I will take one or two students in the early stages of the fellopwship. They will be working on hot topics. My top priority is that they developed critical thinking. Thus I plan frequent discussions in which we will try to understand a given topic but also to challenge the current view. 3. General public How? In the long run the results of enhancement of superconductivity can have an important impact in the general public. This proposal is a first step in the achievement of superconductivity at room temperature. Among other things such a techological/scientific breakthrough would lead to inexpensive, energy efficient, public transportation and electricity. What will be done to ensure that they benefit? Outreach activity such as the participation in science fair and talks about scientific and technical subjects with the potential to benefit the well-being of the general public.


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Description The work performed since the beginning of the project has resulted in the publication of twenty five papers
in refereed journal. All have been published in leading research journal in physics such as Physical Review, Physical Review Letters and Physical Review X. Five more, currently submitted, are
with referees. I have also been invited to present this research in fourteen international meetings and I have co organized three more.

Currently I am supervising two PhD students and several Master students. James Mayoh, my first PhD student, defended his thesis in 2015 and started a postdoc in Southampton University.

Research areas addressed since the beginning of the project and main results obtained so far:

I. Description and enhancement of superconductivity in nanoscale and low dimensional disordered superconductors.
We have put forward a fully quantitative formalism to describe finite size effects in clean nanoscale superconductors in the limit in which mean field theory is applicable [1]. This formalism has then been applied to study the potential enhancement of superconductivity by finite size effects in Iron Pnictides thin films [2]. We found that an increase of up to 30% in the critical temperature is feasible for sizes in which the mean field approach is correct. In [3] we develop a theoretical framework to describe deviations from mean field in nanoscale superconductors that included the combined effect
of thermal and quantum fluctuations. In collaboration with experimentalists we successfully applied this theory to describe the evolution of superconductivity in Pb nanograins [4,5].

In [6] we investigated the breaking of quasi-long range order in a strongly coupled, disordered, one-dimensional superconductor by using DMRG techniques. Our main finding is that even in 1d
superconductivity is robust to weak disorder. More importantly we identify a region of parameters close to the superconductor-insulator transition in which disorder enhance quasi long order. In [7] we study analytically finite size effects in strongly coupled nanoscale superconductors by using holographic techniques.
As a function of the coupling strength we identified the minimum size for superconductivity to exist. Moreover we showed analytically that in certain cases finite size effects can enhance the interaction that binds the electrons in a superconductor.

In [11,12] we have proposed a mechanisms to enhance superconductivity by nano-engineering of inhomogeneities in superconducting thin films. We have found that in weakly-coupled superconductors the critical temperature can be enhanced up to 300% with respect to its bulk value. Based on these theoretical findings, and in collaboration with experimentalists in the Materials Science Department of Cambridge University, we are currently exploring the possibility to submit a patent application.

II. Non equilibrium dynamics and the route of thermalization in strongly interacting, especially superconducting, systems.

In [8] we have investigated the route to thermalization in strongly coupled superconductors by using holographic techniques. We have identified a region of parameters in which the system does not thermalize after a quench. The role of disorder in the far from equilibrium dynamics of a strongly interacting system after a quench was investigated in [9] by exact diagonalization techniques. We
found that Anderson-Mott localization slows-down and eventually stops thermalization in a closed system initially at zero temperature. Moreover, for weaker disorder, we identified a novel route to thermalization characterized by power-law approach to thermal equilibrium.

In [10] we study the time evolution of a strongly coupled superconductor in a disordered potential after it is released from an atomic trap. We found that the dynamics around the insulator
transition is not universal. The time evolution is well described by a process of anomalous diffusion whose parameters depends strongly on the interaction strength and disorder. For instance the second
moment of the distribution function that controls the expansion velocity increases dramatically with time as the interaction increases. However a weaker disorder is enough to stop the expansion.

In [13] we have developed a formalism that describes the formation of a superfluid as the temperature is lowered smoothly. The resulting theory, published in a leading interdisciplinary journal (PRX) is a notable progress with respect to the textbook description based on the Kibble-Zurek scaling.


[1] 'BCS superconductivity in metallic nanograins: Finite-size corrections, low energy

excitations, and robustness of shell effects'

A. M. García-García, J. D. Urbina, E. Yuzbashyan, K. Richter and B. Altshuler, Phys. Rev.

B 83, 014510 (2011).

[2] 'Enhancement of the critical temperature in iron-pnictide superconductors by finite

size effects', A. M. García-García, M. A. N. Araujo, P. D. Sacramento, Phys. Rev. B 84,172502 (2011).

[3] 'Combined effect of thermal and quantum fluctuations in superconducting nanostructures: a path integral approach'
P. Ribeiro, A. M. García-García, Phys. Rev. Lett. 108, 097004 (2012).

[4] 'Experimental observation of thermal fluctuations in single superconducting Pb nanoparticles through tunneling measurements' I. Brihuega, A. M. García-García, P. Ribeiro, Miguel M. Ugeda, C. H. Michaelis, I. Brihuega and S. Bose, K. Kern, Phys. Rev. B 84, 104525 (2011) (Editor Suggestions)

[5] 'Quantifying fluctuations from the tunnelling differential conductance'

A. M. García-García, Pedro Ribeiro, J. Supercond. Nov. Magn. (2012), DOI:10.1007/s10948-012-1662-6

[6] 'Phase coherence in 1d Superconductivity by long-range hopping'
A. Lobos, M. Tezuka, A. M. García-García, arxiv:1212.6779,

[7] 'Holographic Description of Finite Size Effects in Strongly Coupled Superconductors'
A. M. García-García, J. E. Santos, B. Way, Phys. Rev. B 86, 064526 (2012).

[8] 'Quantum Quenches in Disordered Systems: Approach to Thermal Equilibrium without a Typical Relaxation Time'

E. Khatami, A. Relaño, M. Rigol, A. M. García-García, Phys. Rev. E 85, 050102 (2012) (Rapid Comm.)

[9] 'Testing the universality of the many body metal-insulator transition by time evolution of a disordered one-dimensional ultracold fermionic gas' M. Tezuka, A. M. García-García, Phys. Rev. A 85, 031602 (2012) (Rapid Comm.).

[10] 'Integrability, localization and lack of thermalization in holographic superconductivity'
Xin Gao, Antonio M. García-García, Hua Bi Zeng, Hai-Qing Zhang,
JHEP 1406 (2014) 019

[11] J. Mayoh, A. M. García-García, Phys. Rev. B 92, 174526 (2015).

[12] J. Mayoh, A. M. García-García, Phys. Rev. B 90, 134513 (2014).

[13] P .M. Chesler, A. M. Garcia-Garcia, H. Liu, Phys. Rev. X, 5 021015 (2015).
Exploitation Route The expected outcome this research, the increasing of the critical temperature of a superconductor,

are relevant for a more effcient energy production and distribution which are national priorities. More specifically a better understanding of the conditions for stronger superconductivity would have a beneficial impact in the UK economy as it would further stimulate the emergence of novel industries aimed, among other, to manufacture more efficient batteries, to upgrade the electrical grid or to provide energy at a lower cost. Moreover it will also reduce substantially the cost of technologies that require superconductivity, for instance medical devices based on nuclear magnetic resonance, as cooling expenses will be dramatically reduced. The training activities proposed are also of direct relevance for a better educational system, one of the main national priorities. Finally I also note that the research proposed fits well within the

EPSRC portfolio since the level of funding for the fields of superconductivity, materials for energy prediction and nanotechnology, all relevant for the proposal, not only are well represented but also its level of funding will be maintained in the near future. I envisage collaboration with companies developing nanoscience applications

How? Results about enhancement and control of superconductivity in nanostructures are relevant in the design of superconducting nanocircuits and other nanodevices. These devices have the potential to carry current without resistance up to temperatures higher than 100 K.

What will be done to ensure that they benefit? In a first stage I shall ask the Technology Transfer Office of Cambridge for advice and support to submit a patent request. Once it is granted I shall
contact companies with interests in this area. I shall also present our findings, adapted to the audience, in conferences and workshops focused on the exchange between the academic and the
commercial private sector
Sectors Electronics,Energy

Description My findings in nano superconductivity are an important step design novel superconducting devices with the potential to reduce power consumption. My findings in out of equilibrium superconductivity have the potential to unveil novel forms of superconductivity which could also be useful for technological applications. My teaching and mentoring activities are important for the training of new generations in problem solving. This knowledge is intended to be applicable in a broad spectrum of problems beyond physics.
First Year Of Impact 2009
Sector Education,Energy
Impact Types Cultural,Economic