Archiv der Kategorie: research

New publication in Nature Photonics: Generation of ultrafast magnetic steps for coherent control G. De Vecchi, G. Jotzu, M. Buzzi, S. Fava, T. Gebert, M. Fechner, A. Kimel, A. CavalleriNature Photonics, 19, 601–606 (2025) Abstract A long-standing challenge in ultrafast magnetism … Weiterlesen →

New publication in Nature Physics:

Probing inhomogeneous cuprate superconductivity by terahertz Josephson echo spectroscopy

A. Liu, D. Pavićević, M. H. Michael, A. G. Salvador, P. E. Dolgirev, M. Fechner, A. S. Disa, P. M. Lozano, Q. Li, G. D. Gu, E. Demler, A. Cavalleri
Nature Physics (2024)

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Abstract

Inhomogeneities crucially influence the properties of quantum materials, yet methods that can measure them remain limited and can access only a fraction of relevant observables. For example, local probes such as scanning tunnelling microscopy have documented that the electronic properties of cuprate superconductors are inhomogeneous over nanometre length scales. However, complementary techniques that can resolve higher-order correlations are needed to elucidate the nature of these inhomogeneities. Furthermore, local tunnelling probes are often effective only far below the critical temperature. Here we develop a two-dimensional terahertz spectroscopy method to measure Josephson plasmon echoes from an interlayer superconducting tunnelling resonance in a near-optimally doped cuprate. The technique allows us to study the multidimensional optical response of the interlayer Josephson coupling in the material and disentangle intrinsic lifetime broadening from extrinsic inhomogeneous broadening for interlayer superconducting tunnelling. We find that inhomogeneous broadening persists up to a substantial fraction of the critical temperature, above which this is overcome by the thermally increased lifetime broadening.

New publication in Nature:

Magnetic field expulsion in optically driven YBa2Cu3O6.48

S. Fava, G. De Vecchi, G. Jotzu, M. Buzzi, T. Gebert, Y. Liu., B. Keimer, A. Cavalleri
Nature 632, 75–80 (2024)

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Abstract

Coherent optical driving in quantum solids is emerging as a research frontier, with many reports of interesting non-equilibrium quantum phases and transient photo-induced functional phenomena such as ferroelectricity magnetism and superconductivity. In high-temperature cuprate superconductors, coherent driving of certain phonon modes has resulted in a transient state with superconductinglike optical properties, observed far above their transition temperature Tc and throughout the pseudogap phase . However, questions remain on the microscopic nature of this transient state and how to distinguish it from a non-superconducting state with enhanced carrier mobility. For example, it is not known whether cuprates driven in this fashion exhibit Meissner diamagnetism. Here we examine the timedependent magnetic field surrounding an optically driven YBa2Cu3O6.48 crystal by measuring Faraday rotation in a magneto-optic material placed in the vicinity of the sample. For a constant applied magnetic field and under the same driving conditions that result in superconducting-like optical properties , a transient diamagnetic response was observed. This response is comparable in size with that expected in an equilibrium type II superconductor of similar shape and size with a volume susceptibility χv of order −0.3. This value is incompatible with a photo-induced increase in mobility without superconductivity. Rather, it underscores the notion of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the drive.


New Publication in Physical Review B:

Ultrafast Raman thermometry in driven YBa2Cu3O6.48

T.-H. Chou, M. Först, M. Fechner, M. Henstridge, S. Roy, M. Buzzi, D. Nicoletti, Y. Liu, S. Nakata, B. Keimer and A. Cavalleri
Physical Reviews B 109, 195141 (2024)

Abstract:

Signatures of photoinduced superconductivity have been reported in cuprate materials subjected to a coherent phonon drive. A “cold” superfluid was extracted from the transient Terahertz conductivity and was seen to coexist with “hot” uncondensed quasiparticles, a hallmark of a driven-dissipative system of which the interplay between coherent and incoherent responses is not well understood. Here, time resolved spontaneous Raman scattering was used to probe the lattice temperature in the photoinduced superconducting state of YBa2Cu3O6.48. An increase in lattice temperature of up to 140 K was observed by measuring the time dependent Raman scattering intensity of an undriven “spectator” phonon mode. This value is consistent with the estimated increase in quasiparticle temperature measured under the same excitation conditions. These temperature changes provide quantitative information on the nature of the driven state and its decay, and may suggest a strategy to optimize this effect.


New publication in Nature Physics:

Resonant enhancement of photo-induced superconductivity in K3C60

E. Rowe, B. Yuan, M. Buzzi, G. Jotzu, Y. Zhu, M. Fechner, M. Först, B. Liu, D. Pontiroli, M. Riccò, A. Cavalleri

Nature Physics, published online 05. Oct. 2023

Abstract

Photo-excitation at terahertz and mid-infrared frequencies has emerged as an effective way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K3C60, a metastable zero-resistance phase was observed that has optical properties, nonlinear electrical transport and pressure dependencies compatible with non-equilibrium high-temperature superconductivity. Here we demonstrate a two-orders-of-magnitude increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies, a metastable superconducting-like phase is observed up to room temperature. The discovery of a dominant frequency scale sheds light on the microscopic mechanism underlying photo-induced superconductivity. It also indicates a path towards steady-state operation, limited at present by the availability of a suitable high-repetition-rate optical source at these frequencies.

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New publication in Physical Review X:

Superconducting fluctuations observed far above Tc in the isotropic superconductor K3C60

G. Jotzu, G. Meier, A. Cantaluppi, A. Cavalleri, D. Pontiroli, M. Riccò, A. Ardavan, M.-S. Nam
Physical Review X 13, 021008 (2023)

Abstract
Alkali-doped fullerides are strongly correlated organic superconductors that exhibit high transition temperatures, exceptionally large critical magnetic fields, and a number of other unusual properties. The
proximity to a Mott insulating phase is thought to be a crucial ingredient of the underlying physics and may also affect precursors of superconductivity in the normal state above Tc. We report on the observation of a sizable magneto-thermoelectric (Nernst) effect in the normal state of K3C60, which displays the characteristics of superconducting fluctuations. This nonquasiparticle Nernst effect emerges from an ordinary quasiparticle background below a temperature of 80 K, far above Tc =20 K. At the lowest fields and close to Tc, the scaling of the effect is captured by a model based on Gaussian fluctuations. The behavior at higher magnetic fields displays a symmetry between the magnetic length and the correlation length of the system. The temperature up to which we observe fluctuations is exceptionally high for a threedimensional isotropic system, where fluctuation effects are expected to be suppressed.

New publication in Nature Communications:

Periodic dynamics in superconductors induced by an impulsive optical quench

Pavel E. Dolgirev, Alfred Zong, Marios H. Michael, Jonathan B. Curtis, Daniel Podolsky, Andrea Cavalleri and Eugene Demler Nature Communications Physics 5, 234 (2022)

Abstract:

A number of experiments have evidenced signatures of enhanced superconducting correlations after photoexcitation. Initially, these experiments were interpreted as resulting from quasi-static changes in the Hamiltonian parameters, for example, due to lattice deformations or melting of competing phases. Yet, several recent observations indicate that these conjectures are either incorrect or do not capture all the observed phenomena, which include reflectivity exceeding unity, large shifts of Josephson plasmon edges, and appearance of new peaks in terahertz reflectivity. These observations can be explained from the perspective of a Floquet theory involving a periodic drive of system parameters, but the origin of the underlying oscillations remains unclear. In this paper, we demonstrate that following incoherent photoexcitation, long-lived oscillations are generally expected in superconductors with low-energy Josephson plasmons, such as in cuprates or fullerene superconductor K3C60. These oscillations arise from the parametric generation of plasmon pairs due to pump-induced perturbation of the superconducting order parameter. We show that this bi-plasmon response can persist even above the transition temperature as long as strong superconducting fluctuations are present. Our analysis offers a robust framework to understand light-induced superconducting behavior, and the predicted bi-plasmon oscillations can be directly detected using available experimental techniques.  

New publication in PNAS:

Coherent emission from surface Josephson plasmons in striped cuprates

D. Nicoletti, M. Buzzi, M. Fechner, P. E. Dolgirev, M. H. Michael, J. B. Curtis, E. Demler, G. D. Gu, A. Cavalleri
PNAS, 119 (39) e2211670119 (2022)

Abstract:

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence, and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La2-xBxCuO4 family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static but is activated when c-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La1.905Ba0.095CuO4 and in La1.845Ba0.155CuO4 . In this case, terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.

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New publication in PRX:

Amplification of Superconducting Fluctuations in Driven YBa2Cu3O6+x

A. von Hoegen, M. Fechner, M. Först, N. Taherian, E. Rowe, A. Ribak, J. Porras, B. Keimer, M. Michael, E. Demler, and A. Cavalleri
Physical Review X, 12, 031008 (2022) 

Abstract:

In cuprate high-Tc superconductors, resonant excitation of certain lattice vibrations has been shown to induce transient terahertz reflectivity features suggestive of nonequilibrium superconductivity above the critical temperature Tc. A microscopic mechanism for these observations is still lacking. Here, time-resolved measurements of scattering-angle- and polarization-dependent second-harmonic generation in driven YBa2Cu3O6+x reveal a three-order-of-magnitude amplification of a 2.5-THz electronic mode, which is unique because of its symmetry, momentum, and temperature dependence. A theory for amplification of finite-momentum Josephson plasma polaritons, which are assumed to be well formed below Tc but incoherent throughout the pseudogap phase, explains all these observations. A theoretical solution for the Fresnel-Floquet reflection that starts from the coherently oscillating Josephson plasma polaritons provides a possible mechanism for the nonequilibrium superconductorlike terahertz reflectivity reported earlier. Beyond the immediate case of cuprates, this work underscores the role of nonlinear mode mixing to amplify fluctuating modes above the transition temperature in a wide range of materials.