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Signatures of enhanced superconducting phase coherence in cuprates

The capability to control material properties on short timescales is one of the key challenges of modern condensed matter physics. This possibility becomes even more attractive in the case of intriguing material phases, such as superconductivity. As a matter of fact, despite the evolution of non-equilibrium spectroscopies of the last two decades have increased our understanding of the physics of strongly correlated materials, after more than 30 years from its discovery, High Temperature Superconductivity is still discussed and a clear and unanimous explanation of the origin of the phenomenon is still lacking. Moreover, the understanding of the phenomena at the basis of this effects could affect several technological applications, from the need for fast digital circuits and for speeding up computer performances, to the detection of very low magnetic fields, with implication in geology (mineral exploration and earthquake prediction), medical sciences (neuron activity and magnetic resonance), oil prospecting and, of course, research.
We focused our research on cuprates, a class of materials known for displaying unconventional superconductivity at relatively temperatures, and on which various studies have shown the possibility of turning off (and, to some extent, on) superconductivity by ultrashort light pulses. In our work, we reveal that light pulses characterized by long wavelength (and a peculiar polarization) can induce, for a very short time interval (1-2 ps), a state displaying superconductivity even above the critical temperature, i.e. in conditions where superconductivity is not observed at equilibrium. 
In particular, we performed pump-probe experiments on optimally doped yttrium substituted Bi2212 (Bi2Sr2Y0.08Ca0.92Cu2O8+d), a material characterized by a superconducting phase below Tc=97 K, a pseudogap phase from Tc to T*=135 K and a strange metallic phase for higher temperatures. The sample has been excited by ultrashort mid-infrared pump pulses, with photon energy of 70 meV, about twice of the superconducting gap, that is the value of energy required for the lowest electronic excitation of the system at 0 K. The second fundamental parameter of the experiment is the pump polarization, which can be rotated along two different crystallographic directions, that is, parallel to the Copper-Copper (Cu-Cu) or Copper Oxygen (Cu-O) axis. The difference between two measurements at the same excitation photon energy, but different pump polarization, shows that low photon energy excitations polarized along the Cu-Cu axis are able to enhance the dynamical signal associated to superconductivity. The effect is not observed for higher photon energies (see Figure 1).
 

Figure 1.  Difference between the transient reflectivity due to Cu-Cu and Cu-O polarized pump in time and temperature, induced by excitations with (a) 70 and (b) 170 meV pump photon energies. The dashed lines highlight the critical temperature Tc

The experiments are complemented by an effective model, based on a Hamiltonian inspired by BCS theory (the one used to describe conventional superconductivity), accounting for the anisotropy of the superconducting gap in the reciprocal space, typical of non-conventional superconductors
The model ascribes the observed enhancement of the superconducting response to the possibility of increasing the phase coherence of the superconducting state in the material by applying a suitable electric field in specific directions of the copper-oxygen planes (see Figure 2 b and c). Such finding opens the route to the control of the onset of quantum coherence in complex materials through a properly designed electric field.
 

Figure 2.  Results of the effective model: (a) gap amplitude dynamics as a function of the excitation polarization (parallel to the Cu-Cu or Cu-O axis.) The increase of the signal associated to superconductivity due to low photon energy excitations polarized along the Cu-Cu axis is predicted by the model. (b) and (c) represents the phase of the pair operator expectation value, a quantity associated to the coherence of Cooper pairs, in the First Brillouin Zone for excitation polarized along the Cu-Cu and Cu-O axis respectively.


 

This research was conducted by the following research team:

Francesca Giusti1,2, Alexandre Marciniak1,2, Francesco Randi1,2, Giorgia Sparapassi1,2, Daniele Fausti1,2, Adolfo Avella3,4, Fabio Boschini5,6, Andrea Damascelli5,6, Hiroshi Eisaki7, Martin Greven8
 

Elettra Sincrotrone Trieste S.C.p.A., Basovizza Trieste, Italy
Department of Physics, Università degli Studi di Trieste, Trieste, Italy
CNR-SPIN, UOS di Salerno, I-84084 Fisciano (SA), Italy
Department of Chemistry, Princeton University, Princeton, New Jersey, USA
Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia, Canada
Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada
Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA



Contact persons:

Daniele Fausti, email: daniele.fausti@elettra.eu


Reference

Francesca Giusti, Alexandre Marciniak, Francesco Randi, Giorgia Sparapassi, Fabio Boschini, Hiroshi Eisaki, Martin Greven, Andrea Damascelli, Adolfo Avella, and Daniele Fausti, Signatures of Enhanced Superconducting Phase Coherence in Optimally Doped (Bi2Sr2Y0.08Ca0.92Cu2O8+d),Driven by Midinfrared Pulse Excitations, Phys. Rev. Lett. 122, 067002 (2019), https://doi.org/10.1103/PhysRevLett.122.067002

 
Last Updated on Thursday, 14 March 2019 12:21