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Laser cooling of high temperature superconductors

Anastasia S. D. Dietrich

Published Jan 1, 2019• 0 citations
Publisher

Abstract

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.

DOI
10.5287/ora-xk4r28amx

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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.
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.
We propose and analyze a scheme for parametrically cooling bilayer cuprates based on the selective driving of a c-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time-dependent. We show how modulation at the difference frequency between the intraand interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching c-axis transport from a superconducting to resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing non-equilibrium superconductivity even above Tc, provided a finite pair density survives between the bilayers out of equilibrium.
Studies on out-of-equilibrium dynamics have paved a way to realize a new state of matter. Superconductor-like properties above room temperatures recently suggested to be in copper oxides achieved by selectively exciting vibrational phonon modes by laser have inspired studies on an alternative and general strategy to be pursued for high-temperature superconductivity. We show that the superconductivity can be enhanced by irradiating laser to correlated electron systems owing to two mechanisms: First, the effective attractive interaction of carriers is enhanced by the dynamical localization mechanism, which drives the system into strong coupling regions. Second, the irradiation allows reaching uniform and enhanced superconductivity dynamically stabilized without deteriorating into equilibrium inhomogeneities that suppress superconductivity. The dynamical superconductivity is subject to the Higgs oscillations during and after the irradiation. Our finding sheds light on a way to enhance superconductivity that is inaccessible in equilibrium in strongly correlated electron systems.
Controlling the states of quantum systems is a subject of major interest for both fundamental science and technological development. In this thesis, we carry out theoretical studies on how this can be achieved using light, both of classical and of quantum mechanical nature. We study a driven cavity system, where the driving field is classical and the cavity field is quantum mechanical. We demonstrate how virtual scattering of driving photons inside the cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that both the strength and the sign of the interactions can be tuned by the driving parameters and that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, driving-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results open up a new avenue for engineering quantum materials on-demand and the possibility of studying the effect of long-range interactions in condensed matter systems. Following that, we take a more general approach to investigate the effect of long-range density-density electron attractive interactions on a superconductor. We show that the low-lying excitations of a 2D conventional superconductor can be significantly altered by these long-range interactions. Using BCS theory, we find that these interactions combine non-linearly with the intrinsic local attractions of the superconductor to increase the Bogoliubov quasiparticle excitation energies, thus enlarging the superconducting gap. Moreover, we show how the long-range nature of the driven-cavity-induced attractions qualitatively changes the collective excitations of the superconductor. Specifically, they lead to the appearance of additional collective excitations of the excitonic modes. They further push the Higgs mode "in gap": it lies below the Bogoliubov quasiparticle continuum such that it cannot decay through the quasiparticle excitations.
Cuprates are d-wave superconductors which exhibit a rich phase diagram: they are characterized by superconducting fluctuations even above the critical temperature, and thermal disorder can reduce or suppress the phase coherence. However, photoexcitation can have the opposite effect: recent experiments have shown an increasing phase coherence in optimally doped BSCCO with mid-infrared driving. Time-resolved terahertz spectroscopies are powerful techniques to excite and probe the non-equilibrium states of superconductors, directly addressing collective modes, such as amplitude (Higgs) oscillations. In this work, we calculate the full time evolution of the current generated by a cuprate with a quench-drive spectroscopy setup. Analyzing the response in Fourier space with respect to both the real time and the quench-drive delay time, we look for the signature of a transient modulation of higher harmonics, as well as the Higgs mode, in order to characterize the ground state phase. In particular, this approach can provide a smoking gun for induced or increased phase coherence when applied to the pseudogap phase. These results can pave the way for future experimental schemes to characterize and study superconductors alongside incoherent phases and phase transitions, including induced and transient superconductivity.
By irradiating with a single ultrafast laser pulse a superconducting electrode of a Josephson junction, it is possible to drive the quasiparticles (qp's) distribution strongly out of equilibrium. The behavior of the Josephson device can, thus, be modified on a fast time scale, shorter than the qp's relaxation time. This could be very useful, in that it allows fast control of Josephson charge qubits and, in general, of all Josephson devices. If the energy released to the top layer contact S1S1S1 of the junction is of the order of $\ensuremath{\sim}\ensuremath{\mu}J$, the coherence is not degradated because the perturbation is very fast. Within the framework of the quasiclassical Keldysh Green's function theory, we find that the order parameter of S1S1S1 decreases. We study the perturbed dynamics of the junction, when the current bias is close to the critical current, by integrating numerically its classical equation of motion. The optical ultrafast pulse can produce switchings of the junction from the Josephson state to the voltage state. The switches can be controlled by tuning the laser light intensity, the pulse duration, and the bias current of the Josephson junction.