Publications by Andrea Cavalleri


Measuring non-equilibrium dynamics in complex solids with ultrashort X-ray pulses.

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences 377 (2019) 20170478-

M Buzzi, M Först, A Cavalleri

Strong interactions between electrons give rise to the complexity of quantum materials, which exhibit exotic functional properties and extreme susceptibility to external perturbations. A growing research trend involves the study of these materials away from equilibrium, especially in cases in which the stimulation with optical pulses can coherently enhance cooperative orders. Time-resolved X-ray probes are integral to this type of research, as they can be used to track atomic and electronic structures as they evolve on ultrafast timescales. Here, we review a series of recent experiments where femtosecond X-ray diffraction was used to measure dynamics of complex solids. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.


Pressure tuning of light-induced superconductivity in K3C60

Nature Physics Nature Publishing Group 14 (2018) 837–841-

A Cantaluppi, M Buzzi, G Jotzu, D Nicoletti, M Mitrano, D Pontiroli, M Riccò, A Perucchi, P Di Pietro, A Cavalleri

Optical excitation at terahertz frequencies has emerged as an effective means to dynamically manipulate complex materials. In the molecular solid K 3 C 60 , short mid-infrared pulses transform the high-temperature metal into a non-equilibrium state with the optical properties of a superconductor. Here we tune this effect with hydrostatic pressure and find that the superconducting-like features gradually disappear at around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K 3 C 60 , in which a larger electronic bandwidth induced by pressure is also detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesized previously, and rather extends all the way to room temperature.


Probing optically silent superfluid stripes in cuprates

Science American Association for the Advancement of Science 359 (2018) 575-579

S Rajasekaran, J Okamoto, L Mathey, V Thampy, M Fechner, GD Gu, A Cavalleri

Unconventional superconductivity in the cuprates coexists with other types of electronic order. However, some of these orders are invisible to most experimental probes because of their symmetry. For example, the possible existence of superfluid stripes is not easily validated with linear optics, because the stripe alignment causes interlayer superconducting tunneling to vanish on average. Here we show that this frustration is removed in the nonlinear optical response. A giant terahertz third harmonic, characteristic of nonlinear Josephson tunneling, is observed in La1.885Ba0.115CuO4 above the transition temperature Tc = 13 kelvin and up to the charge-ordering temperature Tco = 55 kelvin. We model these results by hypothesizing the presence of a pair density wave condensate, in which nonlinear mixing of optically silent tunneling modes drives large dipole-carrying supercurrents.


Probing the interatomic potential of solids with strong-field nonlinear phononics

Nature Nature Research 555 (2018) 79-82

A Von Hoegen, A Cavalleri, R Mankowsky, M Fechner, M Först

Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy. However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies. For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions. So far it has only been possible to exploit second-order phonon nonlinearities at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A1 (transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.


Pressure tuning of light-induced superconductivity in K3C60.

Nature physics 14 (2018) 837-841

A Cantaluppi, M Buzzi, G Jotzu, D Nicoletti, M Mitrano, D Pontiroli, M Riccò, A Perucchi, P Di Pietro, A Cavalleri

Optical excitation at terahertz frequencies has emerged as an effective means to dynamically manipulate complex materials. In the molecular solid K3C60, short mid-infrared pulses transform the high-temperature metal into a non-equilibrium state with the optical properties of a superconductor. Here we tune this effect with hydrostatic pressure and find that the superconducting-like features gradually disappear at around 0.3 GPa. Reduction with pressure underscores the similarity with the equilibrium superconducting phase of K3C60, in which a larger electronic bandwidth induced by pressure is also detrimental for pairing. Crucially, our observation excludes alternative interpretations based on a high-mobility metallic phase. The pressure dependence also suggests that transient, incipient superconductivity occurs far above the 150 K hypothesised previously, and rather extends all the way to room temperature.


An effective magnetic field from optically driven phonons

Nature Physics Springer Nature 13 (2016) 132-136

TF Nova, A Cartella, A Cantaluppi, M Först, RV Mikhaylovskiy, D Bossini, AV Kimel, R Merlin, A Cavalleri

Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator-metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO 3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research-for example, as a way to affect the topology of electronic phases.


Possible light-induced superconductivity in K3C60 at high temperature

Nature Nature Publishing Group 530 (2016) 461–464-

M Mitrano, A Cantaluppi, D Nicoletti, S Kaiser, A Perucchi, S Lupi, P Di Pietro, D Pontiroli, M Riccò, SRJF Clark, D Jaksch, A Cavalleri

The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects such as the optical enhancement of superconductivity. Nonlinear excitation of certain phonons in bilayer copper oxides was recently shown to induce superconducting-like optical properties at temperatures far greater than the superconducting transition temperature, Tc (refs 4, 5, 6). This effect was accompanied by the disruption of competing charge-density-wave correlations, which explained some but not all of the experimental results. Here we report a similar phenomenon in a very different compound, K3C60. By exciting metallic K3C60 with mid-infrared optical pulses, we induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. These same signatures are observed at equilibrium when cooling metallic K3C60 below Tc (20 kelvin). Although optical techniques alone cannot unequivocally identify non-equilibrium high-temperature superconductivity, we propose this as a possible explanation of our results.


Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>6.5</inf>

Nature 516 (2014) 71-73

R Mankowsky, A Subedi, M Först, SO Mariager, M Chollet, HT Lemke, JS Robinson, JM Glownia, MP Minitti, A Frano, M Fechner, NA Spaldin, T Loew, B Keimer, A Georges, A Cavalleri

© 2014 Macmillan Publishers Limited. All rights reserved.Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures1-3. In complex oxides, this method has been used to melt electronic order4-6, drive insulator-to-metal transitions7 and induce superconductivity8. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa2Cu3O6+x (refs 9, 10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-O2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the dx2-y2 character of the in-plane electronic structure is likely to favour superconductivity.


Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5.

Nature 516 (2014) 71-73

R Mankowsky, A Subedi, M Först, SO Mariager, M Chollet, HT Lemke, JS Robinson, JM Glownia, MP Minitti, A Frano, M Fechner, NA Spaldin, T Loew, B Keimer, A Georges, A Cavalleri

Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures. In complex oxides, this method has been used to melt electronic order, drive insulator-to-metal transitions and induce superconductivity. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa2Cu3O6+x (refs 9, 10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-O2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.


Bi-directional ultrafast electric-field gating of interlayer charge transport in a cuprate superconductor

Nature Photonics 5 (2011) 485-488

A Dienst, MC Hoffmann, D Fausti, JC Petersen, S Pyon, T Takayama, H Takagi, A Cavalleri

In cuprate superconductors, tunnelling between planes makes three-dimensional superconductive transport possible. However, the interlayer tunnelling amplitude is reduced when an order-parameter-phase gradient between planes is established. As such, interlayer superconductivity along the c-axis can be weakened if a strong electric field is applied along the c-axis. In this Letter, we use high-field single-cycle terahertz pulses to gate interlayer coupling in La1.84Sr0.16CuO4. We induce ultrafast oscillations between superconducting and resistive states and switch the plasmon response on and off, without reducing the density of Cooper pairs. In-plane superconductivity remains unperturbed, revealing a non-equilibrium state in which the dimensionality of the superconductivity is time-dependent. The gating frequency is determined by the electric field strength. Non-dissipative, bi-directional gating of superconductivity is of interest for device applications in ultrafast nanoelectronics and represents an example of how nonlinear terahertz physics can benefit nanoplasmonics and active metamaterials. © 2011 Macmillan Publishers Limited. All rights reserved.


Quantum interference between charge excitation paths in a solid-state Mott insulator

Nature Physics 7 (2011) 114-118

S Wall, D Brida, SR Clark, HP Ehrke, D Jaksch, A Ardavan, S Bonora, H Uemura, Y Takahashi, T Hasegawa, H Okamoto, G Cerullo, A Cavalleri

Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast 1,2 timescale. So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations 3-5 , dominate. However, the underlying quantum dynamics of bareg electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices 6,7 where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F 2 TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25THz. Time-dependent simulations of the Mottg Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holong doublon pairs. © 2011 Macmillan Publishers Limited. All rights reserved.


Nonlinear phononics as an ultrafast route to lattice control

Nature Physics 7 (2011) 854-856

M Först, C Manzoni, S Kaiser, Y Tomioka, Y Tokura, R Merlin, A Cavalleri

Two types of coupling between electromagnetic radiation and a crystal lattice have so far been identified experimentally. The first is the direct coupling of light to infrared-active vibrations carrying an electric dipole. The second is indirect, involving electron-phonon coupling and occurring through excitation of the electronic system; stimulated Raman scattering is one example. A third path, ionic Raman scattering (IRS; refs4,5), was proposed 40 years ago. It was posited that excitation of an infrared-active phonon could serve as the intermediate state for Raman scattering, a process that relies on lattice anharmonicities rather than electron-phonon interactions. Here, we report an experimental demonstration of IRS using femtosecond excitation and coherent detection of the lattice response. We show how this mechanism is relevant to ultrafast optical control in solids: a rectified phonon field can exert a directional force onto the crystal, inducing an abrupt displacement of the atoms from their equilibrium positions. IRS opens up a new direction for the optical control of solids in their electronic ground state, different from carrier excitation. © 2011 Macmillan Publishers Limited. All rights reserved.


Light-induced superconductivity in a stripe-ordered cuprate

Science 331 (2011) 189-191

D Fausti, RI Tobey, N Dean, S Kaiser, A Dienst, MC Hoffmann, S Pyon, T Takayama, H Takagi, A Cavalleri

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional "striped" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO 4, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.


Ultrafast coupling between light, coherent lattice vibrations, and the magnetic structure of semicovalent LaMnO(3).

Phys Rev Lett 103 (2009) 097402-

S Wall, D Prabhakaran, AT Boothroyd, A Cavalleri

Coherent lattice vibrations are excited and probed with pulses of 10 fs duration in LaMnO(3). The measured frequencies correspond to those of Jahn-Teller stretching and of out-of phase rotations of the oxygen octahedra. Surprisingly, the amplitude and damping rate of both modes exhibit a sharp discontinuity at the Néel temperature, highlighting nontrivial coupling between light, lattice, and magnetic structure. We explain this effect by applying the Goodenough-Kanamori rules to the excited state of LaMnO(3), and note that charge transfer can invert the sign of the semicovalent exchange interaction, which in turn perturbs the equilibrium bond lengths.


Enhanced photosusceptibility near T(c) for the light-induced insulator-to-metal phase transition in vanadium dioxide (vol 99, art no 226401, 2007)

PHYSICAL REVIEW LETTERS 100 (2008) ARTN 019906

DJ Hilton, RP Prasankumar, S Fourmaux, A Cavalleri, D Brassard, MA El Khakani, JC Kieffer, AJ Taylor, RD Averitt


Optical switching in v O2 films by below-gap excitation

Applied Physics Letters 92 (2008)

M Rini, Z Hao, RW Schoenlein, C Giannetti, F Parmigiani, S Fourmaux, JC Kieffer, A Fujimori, M Onoda, S Wall, A Cavalleri

We study the photoinduced insulator-metal transition in V O2, correlating its threshold and dynamics with excitation wavelength. In single crystals, switching can only be induced with photon energies above the 670 meV gap. This contrasts with the case of polycrystalline films, where formation of the metallic state can be initiated also with photon energies as low as 180 meV, which are well below the bandgap. Perfection of this process may become conducive to schemes for optical switches, limiters, and detectors operating at room temperature in the mid-infrared. © 2008 American Institute of Physics.


Ultrafast single-shot diffraction imaging of nanoscale dynamics

Nature Photonics 2 (2008) 415-419

A Barty, S Boutet, MJ Bogan, S Hau-Riege, S Marchesini, K Sokolowski-Tinten, N Stojanovic, R Tobey, H Ehrke, A Cavalleri, S Düsterer, M Frank, S Bajt, BW Woods, MM Seibert, J Hajdu, R Treusch, HN Chapman

The transient nanoscale dynamics of materials on femtosecond to picosecond timescales is of great interest in the study of condensed phase dynamics such as crack formation, phase separation and nucleation, and rapid fluctuations in the liquid state or in biologically relevant environments. The ability to take images in a single shot is the key to studying non-repetitive behaviour mechanisms, a capability that is of great importance in many of these problems. Using coherent diffraction imaging with femtosecond X-ray free-electron-laser pulses we capture time-series snapshots of a solid as it evolves on the ultrafast timescale. Artificial structures imprinted on a Si 3 N 4 window are excited with an optical laser and undergo laser ablation, which is imaged with a spatial resolution of 50nm and a temporal resolution of 10ps. By using the shortest available free-electron-laser wavelengths and proven synchronization methods this technique could be extended to spatial resolutions of a few nanometres and temporal resolutions of a few tens of femtoseconds. This experiment opens the door to a new regime of time-resolved experiments in mesoscopic dynamics. © 2008 Macmillan Publishers Limited. All rights reserved.


Chemistry. All at once.

Science 318 (2007) 755-756

A Cavalleri


Enhanced photosusceptibility near Tc for the light-induced insulator-to-metal phase transition in vanadium dioxide.

Phys Rev Lett 99 (2007) 226401-

DJ Hilton, RP Prasankumar, S Fourmaux, A Cavalleri, D Brassard, MA El Khakani, JC Kieffer, AJ Taylor, RD Averitt

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.


Control of the electronic phase of a manganite by mode-selective vibrational excitation.

Nature 449 (2007) 72-74

M Rini, R Tobey, N Dean, J Itatani, Y Tomioka, Y Tokura, RW Schoenlein, A Cavalleri

Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 71 meV (17 THz). A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced and current-driven phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn-O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap. Phase control by coherent manipulation of selected metal-oxygen phonons should find extensive application in other complex solids--notably in copper oxide superconductors, in which the role of Cu-O vibrations on the electronic properties is currently controversial.

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