Magnetic-Field Tuning of Light-Induced Superconductivity in Striped La2−xBaxCuO4

Abstract

Optical excitation of stripe-ordered La2-xBaxCuO4 has been shown to transiently enhance superconducting tunneling between the CuO2 planes. This effect was revealed by a blueshift, or by the appearance of a Josephson Plasma Resonance in the terahertz-frequency optical properties. Here, we show that this photo-induced state can be strengthened by the application of high external magnetic fields oriented along the c-axis. For a 7-Tesla field, we observe up to a ten-fold enhancement in the transient interlayer phase correlation length, accompanied by a two-fold increase in the relaxation time of the photo-induced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically-enhanced interlayer coupling inLa2-xBaxCuO4 does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photo-induced state may emerge from activated tunneling between optically-excited stripes in adjacent planes.Charge and spin ordered phases are found throughout wide regions of the phase diagrams of high-TC cuprates [1,2,3,4,5,6,7]. These orders tend to compete with the superconducting state, reducing its coherence. A well-studied case is that of single-layer compounds, in which the doped holes arrange themselves in one-dimensional charge stripes separated by regions of oppositely phased antiferromagnetic order in the CuO2 planes. In La2-xBaxCuO4 (LBCO), for example, stripes completely suppress superconductivity at x = 1/8 doping [8,9], and coexist with it at lower and higher doping values (see phase diagram in Fig. 1(a)).Recent experimental evidence [10,11] suggests that the individual striped planes may be made up of a spatially modulated superfluid, a so-called "pair-density-wave" (PDW), in which the interlayer Josephson tunneling is frustrated by symmetry (see 90° stacking in inset of Fig. 1(a)) [12,13,14].Pressure and magnetic fields have been used to tune the striped state in LBCO and in related compounds. In particular, hydrostatic pressures of few GPa were shown to increase TC [15] and to partially suppress charge order [16]. On the other hand, magnetic fields H ≲ 10 T were shown to amplify the effect of dynamical layer decoupling, leading to a reduction of interlayer tunneling [17,18] and the stabilization of charge-and spin-order [19,20].More recently, optical excitation with femtosecond laser pulses has emerged as a means to drive the interplay between stripes and superconductivity [21], enhancing one or the other transiently. Excitation of either the in-plane Cu-O stretching mode in nonsuperconducting La1.675Eu0.2Sr0.125CuO4 (LESCO1/8) [22,23], or of La2-xBaxCuO4 with high-energy (1.5 eV) optical pulses [24,25], were both shown to enhance interlayer tunneling [26].

Colloquium: Nonthermal pathways to ultrafast control in quantum materials

“…In recent years, the field has seen an evolution from the early measurements of transient absorption and reflectivity (Chemla and Shah, 2000;Elsayed-Ali et al, 1987;Koshihara et al, 1990;Miyano et al, 1997;Schoenlein et al, 1987;Tsen, 2001) into a multifaceted set of techniques, where the particular details of experiments depend on the targeted subsystem and dynamics (Averitt and Taylor, 2002;Orenstein, 2012). Some of the most advantageous capabilities of these approaches are: (i) direct detection of transient changes in the electronic joint density of states via frequency-resolved measurements of the transient complex optical conductivity from the THz to the extreme ultraviolet range (Baldini et al, 2020;Jager et al, 2017;Sie et al, 2015;Siegrist et al, 2019), (ii) simultaneous measurements of the dynamics of different subsystems by combining multiple detection schemes , including transient non-linear optical processes (Mahmood et al, 2021;Sala et al, 2016;Woerner et al, 2013) and polarization rotations sensitive to changes in magnetic orders (Beaurepaire et al, 1996;Kimel et al, 2020;Kirilyuk et al, 2010;Němec et al, 2018;Schlauderer et al, 2019;Walowski and Münzenberg, 2016), (iii) the integrability with other external stimuli, for example magnetic fields and hydrostatic pressure (Cantaluppi et al, 2018;Mitrano et al, 2014;Nicoletti et al, 2018;Trigo et al, 2012), and (iv) the ability to modulate and control the optical pulse to gain real-space information (Gedik et al, 2003;Mahmood et al, 2018;Torchinsky et al, 2014). These techniques have enabled the observation of a wide range of phenomena in the time domain including quasiparticle relaxation dynamics, electron-boson coupling strengths, gap magnitudes, photoexcited order parameters and collective mode oscillations …”

Ultrafast renormalization of the onsite Coulomb repulsion in a cuprate superconductor

“…First, it provides a possible key to interpret the recent observation of light-enhanced superconductivity in cuprates [86][87][88][89][90], which still lacks a comprehensive microscopic understanding. A 140-meV reduction of the Hubbard U will close the CT gap by 70 meV, or 3 % of its equilibrium value.…”

Photoinduced Nonequilibrium Response in Underdoped YBa2Cu3O6+xZhang1, Wang2, Xiang3 et al. 2020Phys. Rev. X14080View full textAdd to dashboardCite

“…Indeed, a loss of low-frequency spectral weight in σ 1 (ω) has always been observed in cuprate superconductors across the superconducting transitions in the equilibrium state, including measurements polarized along the c-axis 28 . However, this was not the case for the above mentioned works on laser-induced response in normal state 5,6,14,18,[25][26][27] . In those works, when a 1/ω dependence or an upward increase with decreasing frequency was identified in σ 2 (ω), the low-frequency σ 1 (ω) does not drop compared with the values in the static state.…”

“…According to this equation, the 1/ω dependence in σ 2 (ω) could be taken as an indication of superconductivity, and the 4πω · σ 2 (ω) at zero frequency limit could be used to estimate the superconducting condensate or London penetration depth. Recently, 1/ω dependence observed in transient σ 2 (ω) was widely taken to be another evidence for the photo-induced transient superconductivity in literature 5,6,14,18,[25][26][27] . We would like to remark that the observation of 1/ω dependence in σ 2 (ω) within a very limited range of frequency is not a sufficient condition for the identification of superconductivity.…”