Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity

Yang, Xu; Vaswani, Chirag; Sundahl, C.; Mootz, M.; Gagel, Pia; Luo, Liang; Kang, Jong Hon; Orth, Peter P.; Perakis, I. E.; Eom, Chang‐Beom; Wang, J.

doi:10.1038/s41563-018-0096-3

Cited by 82 publications

(75 citation statements)

References 34 publications

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“…At even larger peak electric fields, as in the case of SC Nb 3 Sn investigated in Ref. 45, the pump pulse destroys the SC condensate and one eventually access a perturbed nonequilibrium state lasting for long time scales, but oscillations due to SC resonances disappear.…”

Section: Schematic Of the Pump-probe Experimental Setup And Theorementioning

confidence: 86%

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

2019

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Time-resolved spectroscopies using intense THz pulses appear as a promising tool to address collective electronic excitations in condensed matter. In particular recent experiments showed the possibility to selectively excite collective modes emerging across a phase transition, as it is the case for superconducting and charge-density-wave (CDW) systems. One possible signature of these excitations is the emergence of coherent oscillations of the differential probe field in pump-probe protocols. While the analogy with the case of phonon modes suggests that the basic underlying mechanism should be a sum-frequency stimulated Raman process, a general theoretical scheme able to describe the experiments and to define the relevant optical quantity is still lacking. Here we provide this scheme by showing that coherent oscillations as a function of the pump-probe time delay can be linked to the convolution in the frequency domain between the squared pump field and a Raman-like non-linear optical kernel. This approach is applied and discussed in few paradigmatic examples: ordinary phonons in an insulator, and collective charge and Higgs fluctuations across a superconducting and a CDW transition. Our results not only account very well for the existing experimental data in a wide variety of systems, but they also offer an useful perspective to design future experiments in emerging materials.In the last decade, a significant advance in the investigation of complex systems has been gained thanks to the huge experimental progress in time-resolved spectroscopic techniques 1,2 . On very general grounds, the basic idea behind any pump-probe protocol is to first excite the system with a short and very intense electromagnetic pulse (pump), and then to monitor its relaxation towards equilibrium by using a secondary, weak pulse (probe) applied with a finite time delay with respect to the pump. This general protocol can then be implemented in several different ways, according to the nature of the spectroscopic measurement (angle-resolved photoemission, optical reflection or transmission, etc.) or to the wavelength of the pump/probe fields. However, in all cases one has to face with two phenomena which mark the difference with respect to ordinary equilibrium spectroscopies: (i) the use of an intense pulse triggers in general non-linear optical processes; (ii) the subsequent relaxation encodes by definition non-equilibrium phenomena on time scales which depend on the characteristics of the experiment and of the system under investigation. Due in part to these innovative aspects, many pump-probe protocols still lack a general theoretical framework able to connect the measured quantities to the material properties. More specifically, while Kubo linear-response theory 3 represents nowadays the standard theoretical tool needed to compute the optical response of any system to a weak external perturbation, an analogous protocol for timeresolved spectroscopies has not been established yet.The present work aims at filling in part this knowl- * mat...

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Section: Schematic Of the Pump-probe Experimental Setup And Theorementioning

confidence: 86%

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

2019

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“…Such THz photon driven coherence mechanism complements the spontaneous symmetry protection mechanism in the ground state. However, its observation requires intense THz pump and THz conductivity probe spectroscopy measurements [18][19][20][21][22] of the coherent phonon-dressed TPS, which has never been demonstrated so far.…”

Section: Introductionmentioning

confidence: 99%

Light control of surface–bulk coupling by terahertz vibrational coherence in a topological insulator

et al. 2020

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The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipationless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such a goal. Yet, surface-bulk coupling can lead to strong Dirac electron scattering with bulk carriers and phonons as well as impurities, assisted by such dissipative channel, which results in "topological breakdown". Here, we demonstrate that coherent lattice vibrations periodically driven by a single-cycle terahertz (THz) pulse can significantly suppress such dissipative channel in topological insulators. This is achieved by reducing the phase space in the bulk available for Dirac fermion scattering into during coherent lattice oscillations in Bi 2 Se 3 . This light-induced suppression manifests as a remarkable transition exclusively in surface transport, absent for bulk, above the THz electric fields for driving coherent phonons, which prolongs the surface transport lifetime. These results, together with simulations, identify the critical role of spin-orbit coupling for the "phase space contraction" mechanism that suppresses the surface-bulk coupling. Imposing vibrational quantum coherence into topological states of matter may become a universal light control principle for reinforcing the symmetry-protected helical transport.npj Quantum Materials (2020) 5:13 ; https://doi.

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“…From an experimental viewpoint, there appears to be a preference towards Method I, potentially due to its simplicity and the long relaxation of the excitations compared to the probe width (see e.g. [1,3,28]. A notable exception is Ref.…”

Section: Methodsmentioning

confidence: 99%

Theory of Time-Resolved Optical Conductivity of Superconductors: Comparing Two Methods for Its Evaluation

2019

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Time-resolved optical conductivity is an oft-used tool to interrogate quantum materials driven out of equilibrium. Theoretically calculating this observable is a complex topic with several approaches discussed in the literature. Using a non-equilibrium Keldysh formalism and a functional derivative approach to the conductivity, we present a comparison of two particular approaches to the calculation of the optical conductivity, and their distinguishing features, as applied to a pumped superconductor. The two methods are distinguished by the relative motion of the probe and gate times; either the probe or gate time is kept fixed while the other is swept. We find that both the methods result in same qualitative features of the time-resolved conductivity after pump is over. However, calculating the conductivity by keeping the gate fixed removes artifacts inherent to the other method. We provide software that, based on data for the first method, is able to construct the second approach.

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Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity

Cited by 82 publications

References 34 publications

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

Light control of surface–bulk coupling by terahertz vibrational coherence in a topological insulator

Theory of Time-Resolved Optical Conductivity of Superconductors: Comparing Two Methods for Its Evaluation

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