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The Photoenhanced antinodal conductivity as a key to understand the phase diagram of High-Tc cuprates

Almost 30 years after the discovery of high-temperature superconductivity, the nature of the phase diagram of copper oxides, dominated by the ubiquitous presence of the pseudogap, is still subject of an intense debate. The pseudopgap state, which develops at small charge carrier concentrations (p) and temperatures (T), is characterized by an anisotropic gap in the density of states, even though none of the superconducting properties is present.
This exotic state exhibits several intriguing, yet poorly understood, features. In the last couple of years, the reported evidence of different kinds of ordering tendencies, e.g. charge ordering and exotic magnetic modes, boosted a huge effort to understand the nature of these symmetry-broken states. On the other hand, the most advanced theoretical tools for the simulation of the properties of strongly correlated materials (such as the Cluster Dynamical Mean Field Theory, CDMFT), suggest that the nature of the electronic excitations in the pseudogap phase could be ultimately regulated by the short-range Coulomb repulsion between two charges occupying the same lattice site. In this perspective, the symmetry-breaking states emerging in the pseudogap could be regarded as the consequence of a more general mechanism which tends to localize the charge carriers and decreases their ability to move into the lattice.
Our work adds an important piece of information to this puzzling story. The joint experimental and theoretical research activities were led by a team of Italian scientists (from the T-ReX Laboratory at Elettra, Trieste, the iLamp Laboratory at the Catholic University, Brescia, and the SISSA, Trieste), in collaboration with top-level international institutions (University of Duisburg, Germany; Université de Genève, Switzerland; University of Minnesota, USA and University of British Columbia, Canada). Taking advantage of state-of-the-art out-of-equilibrium spectroscopies, we revealed a peculiar property of these materials whose observation was precluded in equilibrium conditions. In cuprate superconductors the energy gap is anisotropic in k-space. The k-space point on the Fermi surface where the gap is almost zero is called node, while the point at the Brillouin zone edge, where the gap is maximal (~40 meV), is called antinode. In thermal equilibrium, the pseudogap-onset temperature, T*~250 K,  corresponds to an energy scale, kBT*~20 meV, which is smaller than the antinodal gap. Therefore equilibrium techniques, such as optics and transport measurements, can hardly access the properties of antinodal excitations, which are more subject to the effect of the short range Coulomb repulsion. Non-equilibrium spectroscopy can circumvent this problem: in a non-equilibrium approach, a high-energy (1.55 eV) optical pulse (the pump) is used to create a non-thermal distribution of antinodal excitations in the material, while a second pulse (the probe beam) is used to probe the nature of these excitations both via the optical or the photoelectronic properties.

 

Figure 1. The relative reflectivity variation δR/R(t) measured at the probe energy of 1.55 eV is reported as a function of the temperature (from 300 to 20 K) for three YBi2212 samples with different hole concentrations. The white lines are the δR/R(t) time traces at 110 K. In the right panel, the general phase diagram of cuprates is sketched. The pseudogap boundary T*neq (grey curve) is determined reporting the temperature at which a negative component in the δR/R(t) signal appears on YBi2212 (black dots) and underdoped Hg1201 (purple dots) samples. The grey circles indicate some of the temperatures at which the δR/R(t) data have been taken. The empty (full) circles correspond to a (non) zero negative signal in the δR/R(t) time traces. The green markers denote the critical temperature Tc of the samples. 

Figure 1 displays the time-resolved reflectivity variation δR/R(t) as a function of pump-and-probe delay t, for three Y-Bi2Sr2CaCuO8+δ samples having a different doping level p. An anomaly in the δR/R(t) signal, in the form of a negative component in the reflectivity signal, is the fingerprint of the pseudogap phase. The onset of this effect determines the T*(p) pseudogap line, as we show in the right panel of Fig. 1. By performing time-resolved optical measurements with a supercontinuum (white light) probe pulse, carried out at the T-ReX Laboratory at Elettra, we were able to address the microscopic origin of this pseudogap-related negative component in the reflectivity signal. This feature is related to a transient decrease of the electronic scattering rate, following the energy increase of the system after the interaction with the pump pulse. This is a non-trivial behavior, which can be revealed only by non-equilibrium spectroscopies. The TR-ARPES data add to the picture the information that the non-equilibrium population has a marked antinodal character. The outcome of our research project is that the antinodal excitations, which can be effectively excited in non-equilibrium experiments, exhibit an anomalous scattering rate which tends to decrease when the energy of the system is increased, either under the form of a thermodynamically-defined temperature or by an impulsive excitation.

The CDMFT solution of the 2D Hubbard Model shed new light into this exotic behavior: at equilibrium, the antinodal quasiparticles are strongly localized and they are subject to an extremely large number of scattering process, as a consequence of the short-range Coulomb repulsion between neighboring charge carriers. When energy is absorbed, the antinodal excitations evolve towards more delocalized and Bloch-like wavefunction, characterized by a reduced scattering rate. In other words, they became “more metallic”, and the conductivity of the material is “photo-enhanced”.
Our conclusion is that the pseudogap state is  a region in the phase diagram of copper oxides in which the short-range Coulomb repulsion between neighboring carriers leads to a dramatic k-space differentiation of the nature of the fundamental electronic excitations. In this scenario, the strongly-correlated ground state, characterized by an extremely low mobility of antinodal excitations, is inherently prone to stabilize ordering tendencies, whose characteristics depend on the details of Fermi surface of the considered cuprate compound.

 

This research was conducted by the following research teams:

F. Cilento1*, S. Dal Conte2,3, G. Coslovich4, S. Peli2,5, N. Nembrini2,5, S. Mor2, F. Banfi2,3, G. Ferrini2,3, H. Eisaki6, M.K. Chan7, C.J. Dorow7, M.J. Veit7, M. Greven7, D. van der Marel8, R. Comin9,10, A. Damascelli9,10,11, L. Rettig12, U. Bovensiepen12, M. Capone13, C. Giannetti2,3*, F. Parmigiani1,4.
 
1  Elettra – Sincrotrone Trieste S.C.p.A., T-ReX Laboratory, Trieste
2  Department of Physics, Università Cattolica del Sacro Cuore, Brescia
3  i-Lamp Laboratory, Università Cattolica del Sacro Cuore, Brescia
4  Department of Physics, Università degli Studi di Trieste, Trieste
5  Department of Physics, Università degli Studi di Milano, Milano
6  National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
7  School of Physics and Astronomy, University of Minnesota, Minneapolis, USA
8  Département de Physique de la Matière Condensée, Université de Genève, Switzerland
9  Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada
10  Quantum Matter Institute, University of British Columbia, Vancouver, Canada
11  Canadian Institute for Advanced Research, Toronto, Canada
12  Fakultaet für Physik and Zentrum für Nanointegration, Universitaet Duisburg-Essen, Germany
13  CNR-IOM Democritos National Simulation Center and SISSA, Trieste


Contact persons:
Federico Cilento:             federico.cilento@elettra.eu
Fulvio Parmigiani:            fulvio.parmigiani@elettra.eu


Reference

 F. Cilento, S. Dal Conte, G. Coslovich, S. Peli, N. Nembrini, S. Mor, F. Banfi, G. Ferrini, H. Eisaki, M. K. Chan, C. J. Dorow, M. J. Veit, M. Greven, D. van der Marel, R. Comin, A. Damascelli, L. Rettig, U. Bovensiepen, M. Capone, C. Giannetti and F. Parmigiani, “Photo-enhanced antinodal conductivity in the pseudogap state of high-Tcuprates”, Nat. Comm. 5, 4353 (2014), DOI: 10.1038/ncomms5353
Last Updated on Wednesday, 13 August 2014 09:08