Periodic Reporting for period 1 - T-Higgs (Phase-resolved THz-Higgs Spectroscopy on Superconductors)
Berichtszeitraum: 2023-03-01 bis 2025-08-31
The T-Higgs projects aims at the direct spectroscopy of the Higgs modes, the coherent order parameter oscillations in superconductors. We achieve this by driving Higgs modes with multicycle high-field THz pulses and tracing their response by a characteristic amplitude- and phase-resolved third harmonic generation (THG) signal. In this way we can perform a direct spectroscopy of the interplay of superconductivity with other orders in superconductors. In particular we are interested in the interplay with charge density wave (CDW) states that often appear in the vicinity of superconducting phases. As a time resolved technique the driven Higgs modes can also be used as transient probes of superconducting order, equivalent to a transient Meissner effect, and could potentially serve as novel criterion for non-equilibrium superconductivity. Furthermore, we aim to probe Higgs in the full momentum space, capture the symmetry of the order parameter, and apply it to current and upcoming topics in the field of superconductivity including light-induced or topological superconductivity.
We have established phase-resolved THz-THG probes at beamline and table-top experiments that allow for systematic studies of different superconductors. We focus on unconventional superconductivity in high-Tc cuprates for which Higgs modes are expected to show a rich spectrum and potential couplings to multiple orders based on the cuprate phase diagrams. Besides that, we also investigate other textbook examples for the interplay of superconductivity with CDWs. Experimentally we can successfully show the interplay of the Higgs mode with CDWs in these systems. It shows up as a Fano-like resonance in the amplitude and phase response of the THG signal. Most strikingly we can identify two distinguishable contributions to the THG directly in the time-domain signal and assign them to the corresponding orders. Here the interplay between the orders shows up as interference pattern in the time-domain. Analyzing the phase relation between the two THG contributions reveals the dynamical interplay between the orders. Further the phase relation/interference pattern can characterize the type of interaction (e.g. competing or coexisting) and show if upon external parameter changes (temperature, doping, magnetic field, THz-field strength) there is a change of the amplitude of the orders or a change in the coupling between the orders.
For light-induced states we have developed a pump-drive technique where the pump pulse induces non-equilibrium changes to the superconducting state and the drive uses the Higgs oscillations to characterize the transient state. Using methods from 2D spectroscopy on this scheme allows us to distinguish other excitations in the system from the transient Higgs response and thus revealing the intrinsic order parameter dynamics of the system.
Information on Higgs in different symmetry channels of the d-wave order parameter in cuprates we gain by populating the anti-Stokes channel that we read out with a time resolved Raman probe. That complements ongoing developments to reveal Higgs modes with full momentum resolution in future time resolved ARPES measurements.
We also merge ultrafast impulsive stimulated Raman probes with frequency domain Raman to identify local order dynamics of "puddles" that shape the global response of orders. This becomes important for incommensurate and coexisting orders. As first prime example we investigate such dynamics in NbSe2 where we find hybridisation of Higgs- and CDW order.
As new development we also probe collective Higgs-equivalent amplitude modes in excitonic insulators, a condensate of excitons. Here we started investigating novel materials systems that do not show CDW order or structural phase transitions that occur in parallel to the condensation.
As novel technical development we start using near field THz emitters to exfoliate thin flakes of samples right onto the emitter. This will allow spectroscopic probes on sub wavelength sized samples, near-field optical probes of in-plane superconducting plasmons, and in the future probes of twisted cuprates (then key road to topological superconductivity.)
For light-induced states we have developed a pump-drive technique where the pump pulse induces non-equilibrium changes to the superconducting state and the drive uses the Higgs oscillations to characterize the transient state. Using methods from 2D spectroscopy on this scheme allows us to distinguish other excitations in the system from the transient Higgs response and thus revealing the intrinsic order parameter dynamics of the system.
Information on Higgs in different symmetry channels of the d-wave order parameter in cuprates we gain by populating the anti-Stokes channel that we read out with a time resolved Raman probe. That complements ongoing developments to reveal Higgs modes with full momentum resolution in future time resolved ARPES measurements.
We also merge ultrafast impulsive stimulated Raman probes with frequency domain Raman to identify local order dynamics of "puddles" that shape the global response of orders. This becomes important for incommensurate and coexisting orders. As first prime example we investigate such dynamics in NbSe2 where we find hybridisation of Higgs- and CDW order.
As new development we also probe collective Higgs-equivalent amplitude modes in excitonic insulators, a condensate of excitons. Here we started investigating novel materials systems that do not show CDW order or structural phase transitions that occur in parallel to the condensation.
As novel technical development we start using near field THz emitters to exfoliate thin flakes of samples right onto the emitter. This will allow spectroscopic probes on sub wavelength sized samples, near-field optical probes of in-plane superconducting plasmons, and in the future probes of twisted cuprates (then key road to topological superconductivity.)
Pump-drive experiments, in which a short laser pump-pulse induces non-linear dynamics to a superconductor, not only allow us tracing the transient dynamics of a superconductor. By implementing the methods from 2D spectroscopy we can switch between the timeframe of the excitation pulse and the timeframe of the transient Higgs response. Doing so we can identify and separate out the dynamics of other excitations like quasiparticles that otherwise directly and indirectly (via screening) obscure the view on the intrinsic order parameter dynamics. Thus, for the transient states 2D pump-drive methods may become the most promising tool to identify intrinsically enhanced THG signals that are due to light-enhanced or light-induced superconductivity and as such potentially define a new criterion for non-equilibrium superconductivity.
Further, pump-drive schemes could become a versatile tool to be implemented in other probes within Higgs Spectroscopy, like the ARPES and Raman probes under development.
Observing very low frequency, over damped coherent oscillations in impulsive stimulated Raman probes may become a key observable for local structural puddles dynamics that shape the global order parameter dynamics.
Further, pump-drive schemes could become a versatile tool to be implemented in other probes within Higgs Spectroscopy, like the ARPES and Raman probes under development.
Observing very low frequency, over damped coherent oscillations in impulsive stimulated Raman probes may become a key observable for local structural puddles dynamics that shape the global order parameter dynamics.