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Contemporary Physics

Photo-induced superconductivity

A. Cavalleri

Published Dec 11, 2017• 169 citations
Publisher

Abstract

Recent advances in laser technology have made it possible to generate of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example, to drive materials in a fashion similar to what done in the synthetic environment of optical lattices. One of the most interesting applications involves the creation of non-equilibrium phases with new emergent properties. Here, I discuss coherent control of the lattice to favour superconductivity at ‘ultra-high’ temperatures, sometimes far above the thermodynamic critical temperature Tc.

DOI
10.1080/00107514.2017.1406623

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Recent advances in laser technology have made it possible to generate of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example, to drive materials in a fashion similar to what done in the synthetic environment of optical lattices. One of the most interesting applications involves the creation of non-equilibrium phases with new emergent properties. Here, I discuss coherent control of the lattice to favour superconductivity at ‘ultra-high’ temperatures, sometimes far above the thermodynamic critical temperature Tc.
An exciting recent development is the ability to drive a quantum material into a highly excited, but still phase coherent state, using intense laser sources. Although such a state can be extremely short-lived, it can be long enough for detection using ultrafast spectroscopy. The authors here show how intense laser pulses can generate a new type of nonequilibrium superconducting phase. The authors demonstrate theoretically that the pump induces a ``twisted'' BCS state that subsequently evolves coherently after the cessation of the pulse. They show how the nonlinear coupling of the pump pulse light can induce coherent dynamics of $\mathrm{\ensuremath{\Delta}}(t)$, consistent with the experiment [R. Matsunaga et al. Phys. Rev. Lett. 111, 057002 (2013)]. Moreover, the authors show that more intense pump pulses can create a far-from-equilibrium phase of gapless superconductivity, originally predicted in the context of ultracold atomic gases. The terahertz pump is found to be much more efficient than the interaction quench at producing this gapless phase. These results open the door to engineering and observing new dynamical phases and phase transitions in quantum materials.
In this dissertation, we theoretically demonstrate laser light induced control and cooling over the order parameter of superconducting cuprates. We predict a non-equilibrium state with enhanced robustness against thermal fluctuations when pumped with coherent pulses. This might ultimately increase the critical temperature of the cuprate. In particular, we propose a parametric cooling scheme for a bilayer cuprate modelled phenomenologically as a stack of long intrinsic Josephson junctions. We identify a parameter regime, which according to the switching current distribution allows a reduction of the effective system temperature by approximately 25%. This cooling scheme suppresses the thermal excitation of Josephson vortices. This might lead to an indirect mechanism for controlling topological excitations in the phase of the superconducting order parameter. We further find that the strong driving of the cuprates can lead to parametric instabilities. To investigate the effect strong driving can have on the dissipative dynamics, we derive a FloquetMarkov master equation. We find that strong driving is a measure for probing the bath spectral density of otherwise inaccessible condensed matter systems.
Controlling and stabilising collective phases of many-body quantum systems is a problem of deep fundamental and technological interest. In this thesis, we perform a theoretical investigation on how useful quantum phases may be engineered in strongly correlated fermionic lattice systems, especially through periodic driving. We first compute the phase diagram of the one-dimensional t–J model with the addition of non-standard pair hopping terms. We show that at dilute fillings these terms enhance superconductivity while, counter- intuitively, suppressing it at large fillings. We argue that this is due to dynamical constraints originating from the fact that local pairs cannot overlap. We conjecture that these constraints may play a more significant role in the physics of two-dimensional systems where the t–J model is studied as a candidate model of high-Tc superconductivity. We begin to investigate these dynamical constraints on a ladder geometry. We then study the fermionic Hubbard model under periodic driving. We show that the driving induces a strong and robust singlet-pairing effect consistent with a superconducting state. This could provide a new mechanism for light-induced superconductivity in some classes of strongly cor- related materials. We show using Floquet theory that the dynamics of the driven Hubbard model are described precisely by the t–J model studied in the previous section. As the driven Hubbard model is also implementable with ultracold fermions in an optical lattice, our results could lead to their use as a quantum simulator for a broad class of candidate models for high-Tc superconductivity.
Thanks to growing capibilities in modulating the properties of materials by selectively driving a specific degree of freedom, along with recent developments in terahertz science, it has been possible to excite and probe the Josephson mode in cuprate superconductors. It is believed that in these materials, phase fluctuations in the superconducting order parameter, which are manifested by Josephson plasmonic excitations can be important in determining the critical temperature. Current efforts in finding novel ways of enhancing superconductivity are geared towards developing ways of using coherent control of this phase to reduce thermal fluctuations, hence improving the effective critical temperature in such materials. With this in view, we propose a 'Doppler' Cooling scheme for layered superconductors having a bilayer structure and conduct a feasibility study to assess their applicability to such systems. By modelling a bilayer cuprate unit-cell as two coupled Josephson junctions, we find that it is possible to induce transitions between and interlayer and higher energy intralayer plasmonic mode via driving from an externally applied electromagnetic field only when the junction is current biased. Our study shows that the physics of the system allow such a scheme to be implemented. The low recoil imparted by photons at terahertz frequencies enables us to apply the same framework as for cooling of atomic systems. The relatively long lifetimes (∼ 0.1 ns) against spontaneous decay estimated imply that cooling is not likely to be limited by inherent features of the scheme but will rather depend on other factors, such as the competition between cooling and rethermalisation. These factors need to be further studied to assess the experimental viability of the proposed scheme.