Lara Benfatto's homepage


Non-linear THz driving of superconducting phase modes

February 2021

The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered superconducting cuprates, leading to a soft c-axis plasmon, whose energy if of order of 1 THz, and to a hard in-plane plasmon, whose energy (~100 THz) exceeds the THz range. So far, THz light pulses have been efficiently used to excite resonantly the out-of-plane Josephson plasma mode, as is typically shown by enhanced third-harmonic generation (THG) and marked pump-probe oscillations.  On the other end,  the high-energy in-plane plasma mode has been assumed to be insensitive to THz pumping. Nonetheless, recent experiments in cuprates show several effects that cannot be easily ascribed to the BCS response or to the Higgs mode. For example, the symmetry of the THG signal is not compatible with either mechanism, if one correctly includes the effects of disorder, as we have studied in  Phys. Rev. B 103, 014512 (2021). In a recent paper published in Nature Communications (Nat. Comm. 12, 752 (2021) we show that THz driving of both low-frequency and high-frequency plasma waves is possible via a general two-plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to marked differences in the thermal effects among the out-of-plane and in-plane response, consistently with the experiments. In particular our results link the observed survival of the in-plane THz non-linear driving above Tc to enhanced fluctuating effects in the phase stiffness in cuprates, paving the way to THz impulsive control of phase rigidity in unconventional superconductors.

THz driving of plasma

Left: Schematic of the mexican-hat potential of a superconductor. Light pulses can excite either a single Higgs mode (c) or two plasma mode with opposite momenta. In layered cuprates the phase stiffness is strongly anisotropic, with Js,c << Js,ab. The corresponding plasma excitations induced by a field polarized along c or along the ab planes occur at very different energies, and give rise to marked different behavior of the corresponding THG.

Non-linear transport: BKT or inhomogeneity?

August 2019

One of the hallmarks of the Berezinskii-Kosterlitz-Thouless (BKT) physics is the discontinuous jump of the superfluid stiffness Js at the transition temperature from a finite and universal value to zero. However, in real materials such a jump is usually replaced by a rapid and continuous downturn, that can be still ascribed to BKT physics once the low value of the vortex-core energy and a moderate sample inhomogeneity are taken into account, as our group demonstrated few years back by means of direct comparison between theoretical calculations  and the penetration-depth measurements by the group of P. Raycahudhuri at TIFR in Mumbai (see Phys. Rev. Lett. 107, 217003 (2011)). This effect can also be evinced by means of transport measurements thanks to the possibility of a large enough current to unbind vortex-antivortex pairs below Tc, generating an extra voltage that reflects in non-linear IV characteristics. The universal jump of Js at Tc should then reflects in an universal jump of the IV exponent a from a=3 right below Tc to a=1 right above it. What happens then when the Js jump is smeared by disorder? And what is the fate of the BKT signatures when the sample inhomogeneity occurs on mesoscopic length scales, making percolative effects more pronounced than BKT physics? In a recent work published in Phys. Rev. B 100, 064506 (2019) we demonstrated that while IV characteristics in thin NbN films represent a textbook example of BKT physics, the pronounced non linearity observed in STO-based interfaces do not seem to justify a BKT analysis. Rather, the observed IV characteristics can be well reproduced theoretically by modeling the effect of mesoscopic inhomogeneity of the superconducting state. Our results offer an alternative perspective on the spontaneous fragmentation of the superconducting background in confined two-dimensional systems.

Leggett mode controlled by light pulses

March 2019

The discovery of symmetry-broken phases that host multiple order parameters, such as multiband superconductors, has triggered an enormous interest in condensed matter physics. However, many challenges continue to hinder the fundamental understanding of how to control the collective modes corresponding to these multiple order parameters. In particular, the advent of THz spectroscopy with the use of very intense pulsed field paved the way to new protocols to detect these collective electronic excitations. In a recent paper published in Nature Physics we demonstrate that, in full analogy with phonons, Raman-active electronic collective modes can be manipulated by intense light pulses. By tuning a sum-frequency excitation process, we selectively trigger a collective mode that we identify with the one corresponding to the relative phase fluctuations between two superconducting order parameters—the so-called Leggett mode—in the multiband superconductor MgB2. The excellent comparison between experiments and theory is made possible but a step-to-step theoretical description of the full pump-probe protocols, that was lacking so far for these kind of experiments. On this respect, besides providing the first experimental evidence of the THz-induced excitation of the Leggett mode, we establishes a general protocol for the advanced control of Raman-active electronic modes in symmetry-broken quantum phases of matter.


Left: schematic of the two-photon absorption by the pump field in typical pump-probe experiments, where the probe signal is recorded at a fixed observation time tgate as a function of the pump-probe delay tpp between the pump and probe field. Right: experimental observation of the Leggett oscillations, compared with theoretical simulations

Hexatic phases in MoGe thin films

January 2019

According to the Berezinskii-Kosterlitz-Thouless theory, later refined by Halperin, Nelson and Young, the melting of a 2D solid crystal should happen via two subsequent transitions controlled by topological excitations. At the first transition thermally excited free dislocations proliferate in the lattice breaking the lattice rigidity but preserving its orientational order. At higher temperatures the emergence of isolated disclinations suppressed the remnant orientational order leading to a conventional, isotropic fluid. The melting of the vortex lattice in thin films of type II superconductors belong to the BKTHNY universality class. The intermediate phase is called hexatic since the orientationally-ordered liquid state is expected to have the hexagonal symmetry of the vortex lattice in the crystalline phase. In a recent paper published in Phys. Rev. Lett. we used a combination of transport measurements and STM imaging of the vortex lattice to demonstrate the existence of a hexatic phase in thin films of MoGe. Beside standard static characterization of the orientational liquid, we investigated its time evolution: by visualizing via STM the vortex lattice at regular time steps we proved that the vortexes in the hexatic fluid phase move, due to internal stress, along preferantial directions corresponding to hexgonal order.


(a) Field dependence of the resistivity and of the hexatic order parameter at 2K (b) Field dependence of the resistivity in the vortex-liquid phase at various temperature. (c) Resulting phase diagram of a-MoGe obtained by combining transport and  STS measurements of the vortex lattice.