Project B7

Gregor Jotzu, Guido Meier, Alice Cantaluppi, Andrea Cavalleri, Daniele Pontiroli, Mauro Riccò, Arzhang Ardavan, and Moon-Sun Nam

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 T_c. 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 T_c, 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 three-dimensional isotropic system, where fluctuation effects are expected to be suppressed.

Phys. Rev. X 13, 021008 (2023)

A. Ribak, M. Buzzi, D. Nicoletti, R. Singla, Y. Liu, S. Nakata, B. Keimer, and A. Cavalleri

Coherent optical excitation of certain phonon modes in YBa₂Cu₃O_6+x has been shown to induce superconducting-like interlayer coherence at temperatures higher than T_c. Recent work has associated these phenomena to a parametric excitation and amplification of Josephson plasma polaritons, which are overdamped above Tc but are made coherent by the phonon drive. However, the dissipative response of uncondensed quasiparticles, which do not couple in the same way to the phonon drive, has not been addressed. Here, we investigate both the enhancement of the superfluid density, ωσ₂(ω), and the dissipative response of quasiparticles, σ1(ω), by systematically tuning the duration and energy of the mid-infrared pulse while keeping the peak field fixed. We find that the photoinduced superfluid density saturates to the zero-temperature equilibrium value for pulses made longer than the phonon dephasing time, while the dissipative component continues to grow with increasing pulse duration. We show that superfluid and dissipation remain uncoupled as long as the drive is on, and identify an optimal regime of pump pulse durations for which the superconducting response is maximum and dissipation is minimized.

Phys. Rev. B 107, 104508 (2023)

Jacqueline Bloch , Andrea Cavalleri , Victor Galitski, Mohammad Hafezi & Angel Rubio

An important goal of modern condensed-matter physics involves the search for states of matter with emergent properties and desirable functionalities. Although the tools for material design remain relatively limited, notable advances have been recently achieved by controlling interactions at heterointerfaces, precise alignment of low-dimensional materials and the use of extreme pressures. Here we highlight a paradigm based on controlling light–matter interactions, which provides a way to manipulate and synthesize strongly correlated quantum matter. We consider the case in which both electron–electron and electron–photon interactions are strong and give rise to a variety of phenomena. Photon-mediated superconductivity, cavity fractional quantum Hall physics and optically driven topological phenomena in low dimensions are among the frontiers discussed in this Perspective, which highlights a field that we term here ‘strongly correlated electron–photon science’.

Nature 606, 41–48 (2022)

D. Nicoletti, M. Buzzi, M. Fechner, P. E. Dolgirev, M. H. Michael, J. B. Curtis, E. Demler, G. D. Gu, A. Cavalleri

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 La_2-xBa_xCuO₄ 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 La_1.905Ba_0.095CuO4 and in L_a1.845Ba_0.155CuO₄. 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.

PNAS, 119 (39) e2211670119 (2022)

G. Homann, J. G. Cosme, and L. Mathey

We investigate the Meissner effect in a parametrically driven superconductor using a semiclassical U(1) lattice gauge theory. Specifically, we periodically drive the z-axis tunneling, which leads to an enhancement of the imaginary part of the z-axis conductivity at low frequencies if the driving frequency is blue-detuned from the plasma frequency. This has been proposed as a possible mechanism for light-enhanced interlayer transport in YBCO. In contrast to this enhancement of the conductivity, we find that the screening of magnetic fields is less effective than in equilibrium for blue-detuned driving, while it displays a tendency to be enhanced for red-detuned driving.

New J. Phys. 24, 113007 (2022)

Guido Homann, Jayson G. Cosme, and Ludwig Mathey

We demonstrate that parametric driving of suitable collective modes in cuprate superconductors results in a reflectivity R>1 for frequencies in the low terahertz regime. We propose to exploit this effect for the amplification of coherent terahertz radiation in a laserlike fashion. As an example, we consider the optical driving of Josephson plasma oscillations in a monolayer cuprate at a frequency that is blue-detuned from the Higgs frequency. Analogously, terahertz radiation can be amplified in a bilayer cuprate by driving a phonon resonance at a frequency slightly higher than the upper Josephson plasma frequency. We show this by simulating a driven-dissipative U(1) lattice gauge theory on a three-dimensional lattice, encoding a bilayer structure in the model parameters. We find a parametric amplification of terahertz radiation at zero and nonzero temperature.

Phys. Rev. Research 4, 013181 (2022)

D. Fu, D. Nicoletti, M. Fechner, M. Buzzi, G. D. Gu, and A. Cavalleri

Interlayer transport in high-T_C cuprates is mediated by superconducting tunneling across the C_uO₂ planes. For this reason, the terahertz frequency optical response is dominated by one or more Josephson plasma resonances and becomes highly nonlinear at fields for which the tunneling supercurrents approach their critical value I_C. These large terahertz nonlinearities are in fact a hallmark of superconducting transport. Surprisingly, however, they have been documented in La_2−xBa_xC_uO₄ (LBCO) also above T_C for doping values near x=1/8 and interpreted as an indication of superfluidity in the stripe phase. Here, electric-field-induced second harmonic is used to study the dynamics of time-dependent interlayer voltages when LBCO is driven with large-amplitude terahertz pulses, in search of other characteristic signatures of Josephson tunneling in the normal state. We show that this method is sensitive to the voltage anomalies associated with 2π Josephson phase slips, which near x=1/8 are observed both below and above T_C. These results document a regime of nonlinear transport that shares features of fluctuating stripes and superconducting phase dynamics.

Phys. Rev. B 105, L020502 (2022)

M. Buzzi, D. Nicoletti, S. Fava, G. Jotzu, K. Miyagawa, K. Kanoda, A. Henderson, T. Siegrist, J. A. Schlueter, M.-S. Nam, A. Ardavan, and A. Cavalleri

Resonant optical excitation of certain molecular vibrations in κ−(BEDT−TTF)₂Cu[N(CN)₂]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium T_c. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ−(BEDT−TTF)₂Cu[N(CN)₂]Cl and the metallic compound κ−(BEDT−TTF)₂Cu(NCS)₂. We find nonequilibrium photoinduced superconductivity only in κ−(BEDT−TTF)₂Cu[N(CN)₂]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.

Phys. Rev. Lett. 127, 197002 (2021)

Michele Buzzi, Gregor Jotzu, Andrea Cavalleri, J. Ignacio Cirac, Eugene A. Demler, Bertrand I. Halperin, Mikhail D. Lukin, Tao Shi, Yao Wang, and Daniel Podolsky

We propose a novel nonequilibrium phenomenon, through which a prompt quench from a metal to a transient superconducting state can induce large oscillations of the order parameter amplitude. We argue that this oscillating mode acts as a source of parametric amplification of the incident radiation. We report experimental results on optically driven K₃C₆₀ that are consistent with these predictions. The effect is found to disappear when the onset of the excitation becomes slower than the Higgs-mode period, consistent with the theory proposed here. These results open new possibilities for the use of collective modes in many-body systems to induce nonlinear optical effects.

Phys. Rev. X 11, 011055 (2021)

Guido Homann, Jayson G. Cosme, Junichi Okamoto, and Ludwig Mathey

We put forth a mechanism for enhancing the interlayer transport in cuprate superconductors, by optically driving plasmonic excitations along the c axis with a frequency that is blue-detuned from the Higgs frequency. The plasmonic excitations induce a collective oscillation of the Higgs field which induces a parametric enhancement of the superconducting response, as we demonstrate with a minimal analytical model. Furthermore, we perform simulations of a particle-hole symmetric U(1) lattice gauge theory and find good agreement with our analytical prediction. We map out the renormalization of the interlayer coupling as a function of the parameters of the optical field and demonstrate that the Higgs mode mediated enhancement can be larger than 50%.

Phys. Rev. B 103, 224503 (2021)

Marios H. Michael, Alexander von Hoegen, Michael Fechner, Michael Först, Andrea Cavalleri, and Eugene Demler

Nonlinear interactions between collective modes play a definitive role in far out of equilibrium dynamics of strongly correlated electron systems. Understanding and utilizing these interactions is crucial to photocontrol of quantum many-body states. One of the most surprising examples of strong mode coupling is the interaction between apical oxygen phonons and Josephson plasmons in bilayer YBa₂Cu₃O_6+x superconductors. Experiments by Hu et al. [Nat. Mater. 13, 705 (2014)] and Kaiser et al. [Phys. Rev. B 89, 184516 (2014)] showed that below _Tc, photoexcitation of phonons leads to enhancement and frequency shifts of Josephson plasmon edges, while above Tc, photoexcited phonons induce plasmon edges even when there are no discernible features in the equilibrium reflectivity spectrum. Recent experiments by von Hoegen et al. (arXiv:1911.08284) also observed parametric generation of Josephson plasmons from photoexcited phonons both below Tc and in the pseudogap phase. In this paper, we present a theoretical model of three-wave phonon-plasmon interaction arising from changes of the in-plane superfluid stiffness caused by the apical oxygen motion. Analysis of the parametric instability of plasmons based on this model gives frequencies of the most unstable plasmons that are in agreement with experimental observations. We also discuss how strong parametric excitation of Josephson plasmons can explain pump-induced changes in the terahertz reflectivity of YBa₂Cu₃O_6+x in the superconducting state, including frequency shifts and sharpening of Josephson plasmon edges, as well as appearance of a new peak around 2 THz. An interesting feature of this model is that overdamped Josephson plasmons do not give any discernible features in reflectivity in equilibrium, but can develop plasmon edges when parametrically excited. We suggest that this mechanism explains photoinduced plasmon edges in the pseudogap phase of YBa₂Cu₃O_6+x.

Phys. Rev. B 102, 174505 (2020)

Hongmin Gao, Frank Schlawin, Michele Buzzi, Andrea Cavalleri, Dieter Jaksch

We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO₃ and SrTiO₃.

Phys. Rev. Lett. 125, 053602 (2020)

M. Buzzi, D. Nicoletti, M. Fechner, N. Tancogne-Dejean, M. A. Sentef, A. Georges, T. Biesner, E. Uykur, M. Dressel, A. Henderson, T. Siegrist, J. A. Schlueter, K. Miyagawa, K. Kanoda, M.-S. Nam, A. Ardavan, J. Coulthard, J. Tindall, F. Schlawin, D. Jaksch, and A. Cavalleri

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ−(BEDT−TTF)₂Cu[N(CN)₂]Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T^∗≃50  K, far higher than the equilibrium transition temperature T_C=12.5  K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

Phys. Rev. X 10, 031028 (2020)

G. Homann, J. G. Cosme, and L. Mathey

We propose to induce a time-crystalline state in a high-T_c superconductor, by optically driving a sum resonance of the Higgs mode and a Josephson plasma mode. The generic cubic process that couples these fundamental excitations converts driving of the sum resonance into simultaneous resonant driving of both modes, resulting in an incommensurate subharmonic motion. We use a numerical implementation of a semiclassical driven-dissipative lattice gauge theory on a three-dimensional layered lattice, which models the geometry of cuprate superconductors, to demonstrate the robustness of this motion against thermal fluctuations. We demonstrate this light-induced time-crystalline phase for mono- and bilayer systems and show that this order can be detected for pulsed driving under realistic technological conditions.

Phys. Rev. Res. 2, 043214 (2020)

Ali Sanayei, Pascal Naidon, and Ludwig Mathey

We expand the Cooper problem by including a third electron in an otherwise empty band. We demonstrate the formation of a trimer state of two electrons above the Fermi sea and the third electron, for sufficiently strong interband attractive interaction. We show that the critical interaction strength is the lowest for small Fermi velocities, large masses of the additional electron, and large Debye energy. This trimer state competes with the formation of the two-electron Cooper pair, and can be created transiently via optical pumping.

Phys. Rev. Research 2, 013341 (2020)

B. Liu, M. Först, M. Fechner, D. Nicoletti, J. Porras, B. Keimer, A. Cavalleri

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature T_C, as evidenced by the terahertz-frequency optical properties in the nonequilibrium state. In YBa₂Cu₃O6_+x, this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La_2−xBa_xCuO₄, similar effects were reported also for excitation at near-infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date, it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to excite YBa₂Cu₃O_6.5 throughout the entire optical spectrum (3–750 THz). In the far-infrared region (3–24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drive c-axis apical oxygen atomic positions. For higher driving frequencies (25–750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of the CuO₂ planes, mediated either by a phonon or by charge transfer.

Physical Review X 10, 011053 (2020)

Yuto Ashida, Ataç İmamoğlu, Jérôme Faist, Dieter Jaksch, Andrea Cavalleri, and Eugene Demler

The light-matter interaction can be utilized to qualitatively alter physical properties of materials. Recent theoretical and experimental studies have explored this possibility of controlling matter by light based on driving many-body systems via strong classical electromagnetic radiation, leading to a time-dependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pump-probe setups with ultrashort laser pulses have so far been used to study transient light-induced modifications in materials. Here, we pursue yet another direction of controlling quantum matter by modifying quantum fluctuations of its electromagnetic environment. In contrast to earlier proposals on light-enhanced electron-electron interactions, we consider a dipolar quantum many-body system embedded in a cavity composed of metal mirrors and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum light-matter interaction. We analyze hybridization of different types of the fundamental excitations, including dipolar phonons, cavity photons, and plasmons in metal mirrors, arising from the cavity confinement in the regime of strong light-matter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energy-level structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavity-induced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradiant-type transition via the light-matter coupling without external pumping. We also discuss possible applications of the cavity-induced modifications in collective excitations to molecular materials and excitonic devices.

Phys. Rev. X 10, 041027 (2020)

M. Nuske, L. Broers, B. Schulte, G. Jotzu, S. A. Sato, A. Cavalleri, A. Rubio, J. W. McIver, L. Mathey

We demonstrate how the properties of light-induced electronic Floquet states in solids impact natural physical observables, such as transport properties, by capturing the environmental influence on the electrons. We include the environment as dissipative processes, such as inter-band decay and dephasing, often ignored in Floquet predictions. These dissipative processes determine the Floquet band occupations of the emergent steady state, by balancing out the optical driving force. In order to benchmark and illustrate our framework for Floquet physics in a realistic solid, we consider the light-induced Hall conductivity in graphene recently reported by J.~W.~McIver, et al., Nature Physics (2020). We show that the Hall conductivity is estimated by the Berry flux of the occupied states of the light-induced Floquet bands, in addition to the kinetic contribution given by the average band velocity. Hence, Floquet theory provides an interpretation of this Hall conductivity as a geometric-dissipative effect. We demonstrate this mechanism within a master equation formalism, and obtain good quantitative agreement with the experimentally measured Hall conductivity, underscoring the validity of this approach which establishes a broadly applicable framework for the understanding of ultrafast non-equilibrium dynamics in solids. 

Phys. Rev. Research 2, 043408 (2020)

Michele Buzzi, Michael Först and Andrea 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’.

Phil. Trans. R. Soc. A.377:20170478.20170478. (2019)

Optical excitation of stripe-ordered La₂−_xBa_xCuO₄ has been shown to transiently enhance superconducting tunneling between the CuO₂ planes. This effect was revealed by a blueshift, or by the appearance of a Josephson plasma resonance in the terahertz-frequency optical properties. Here, we show that this photoinduced state can be strengthened by the application of high external magnetic fields oriented along the c axis. For a 7 T field, we observe up to a tenfold enhancement in the transient interlayer phase correlation length, accompanied by a twofold increase in the relaxation time of the photoinduced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically enhanced interlayer coupling in La₂−_xBa_xCuO₄ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photoinduced state may emerge from activated tunneling between optically excited stripes in adjacent planes.

Physical Review Letters,121, 267003 (2018)

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 hypothesized previously, and rather extends all the way to room temperature.

Nature Physics, 14, 837–841 (2018)

A. von Hoegen, R. Mankowsky, M. Fechner, M. Först, A. Cavalleri

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 nonlinearities9 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.

Nature, 555, 79–82 (2018)

M. Buzzi, M. Först, R. Mankowsky, and A. Cavalleri

Optical pulses are routinely used to drive dynamic changes in the properties of solids. In quantum materials, many new phenomena have been discovered, including ultrafast transitions between electronic phases, switching of ferroic orders and non-equilibrium emergent behaviours, such as photoinduced superconductivity. Understanding the underlying non-equilibrium physics requires detailed measurements of multiple microscopic degrees of freedom at ultrafast time resolution. Femtosecond X-rays are key to this endeavour, as they can probe the dynamics of structural, electronic and magnetic degrees of freedom. Here, we review a series of representative experimental studies in which ultrashort X-ray pulses from free-electron lasers have been used, opening up new horizons for materials research.

Nature Reviews Materials, 3, pages 299–311 (2018)

W. Hu, D. Nicoletti, A. V. Boris, B. Keimer and A. Cavalleri

We report on an investigation of the redistribution of interlayer coherence in the trilayer cuprate Bi₂Sr₂Ca₂Cu₃O₁₀. The experiment is performed under the same apical-oxygen phonon excitation discussed in the past for the bilayer cuprate YBa₂Cu₃O_6.5. In Bi₂Sr₂Ca₂Cu₃O₁₀, we observe a similar spectral weight loss at the transverse plasma mode resonance as that seen in YBa₂Cu₃O_6.5. However, this feature is not accompanied by the light-enhanced interlayer coherence that was found in YBa₂Cu₃O_6+x, for which the transverse plasma mode is observed at equilibrium even in the normal state. These new observations offer an experimental perspective in the context of the physics of light-enhanced interlayer coupling in various cuprates.

Physical Reviev B, 95, 104508 (2017)

J. Okamoto, W. Hu, A. Cavalleri, L. Mathey

Recent pump-probe experiments reported an enhancement of superconducting transport along the c axis of underdoped YBa₂Cu₃O_6+δ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature T_c. This model reproduces the essential features of the enhancement measured below T_c. To reproduce the experimental results above T_c, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.

Phys. Rev. B 96, 144505 (2017)

D. Nicoletti, E. Casandruc, D. Fu, P. Giraldo-Gallo, I. Fisher, A. Cavalleri

We present measurements of transient photoconductivity in BaPb_1−xBi_xO₃ (BPBO)––a poorly understood material belonging to the bismuthate family, which has been coined “the other high-temperature superconductor.” The phase diagram of BPBO encompasses charge-density-wave (CDW) order in BaBiO₃ (x = 1), through superconductivity for intermediate compositions, to bad metal behavior in BaPbO₃ (x = 0). We present evidence for the coexistence of CDW order and superconductivity for underdoped compositions of BPBO––something that has been discussed previously, but never definitively established. These results are especially timely given that CDW correlations have recently been found in some underdoped cuprate superconductors, pointing toward a surprising commonality between some aspects of these materials. Our measurements also put energy scales on the associated charge order.

PNAS, 114,34, 9020–9025 (2017)

S. Aeschlimann, R. Krause, M. Chávez-Cervantes, H. Bromberger, R. Jago, E. Mali, A. Al-Temimy, C. Coletti, A. Cavalleri, I. Gierz

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.

Phys. Rev. B 96, 020301 (2017)

E. Pomarico, M. Mitrano, H. Bromberger, M. A. Sentef, A. Al-Temimy, C. Coletti, A. Stohr, S. Link, U. Starke, C. Cacho, R. Chapman, E. Springate, A. Cavalleri, I. Gierz

Electron-phonon coupling directly determines the stability of cooperative order in solids, including superconductivity, charge, and spin density waves. Therefore, the ability to enhance or reduce electron-phonon coupling by optical driving may open up new possibilities to steer materials' functionalities, potentially at high speeds. Here, we explore the response of bilayer graphene to dynamical modulation of the lattice, achieved by driving optically active in-plane bond stretching vibrations with femtosecond midinfrared pulses. The driven state is studied by two different ultrafast spectroscopic techniques. First, terahertz time-domain spectroscopy reveals that the Drude scattering rate decreases upon driving. Second, the relaxation rate of hot quasiparticles, as measured by time- and angle-resolved photoemission spectroscopy, increases. These two independent observations are quantitatively consistent with one another and can be explained by a transient threefold enhancement of the electron-phonon coupling constant. The findings reported here provide useful perspective for related experiments, which reported the enhancement of superconductivity in alkali-doped fullerites when a similar phonon mode was driven.

Phys. Rev. B 95, 024304 (2017)

S. Rajasekaran, E. Casandruc, Y. Laplace, D. Nicoletti, G. D. Gu, S. R. Clark, D. Jaksch & A. Cavalleri

Many applications in photonics require all-optical manipulation of plasma waves, which can concentrate electromagnetic energy on sub-wavelength length scales. This is difficult in metallic plasmas because of their small optical nonlinearities. Some layered superconductors support Josephson plasma waves, involving oscillatory tunnelling of the superfluid between capacitively coupled planes. Josephson plasma waves are also highly nonlinear, and exhibit striking phenomena such as cooperative emission of coherent terahertz radiation, superconductor–metal oscillations and soliton formation. Here, we show that terahertz Josephson plasma waves can be parametrically amplified through the cubic tunnelling nonlinearity in a cuprate superconductor. Parametric amplification is sensitive to the relative phase between pump and seed waves, and may be optimized to achieve squeezing of the order-parameter phase fluctuations or terahertz single-photon devices.

Nature Physics, 12, 1012–1016 (2016)

D. Nicoletti, A. Cavalleri

Strong optical pulses at mid-infrared and terahertz frequencies have recently emerged as powerful tools to manipulate and control the solid state and especially complex condensed matter systems with strongly correlated electrons. The recent developments in high-power sources in the 0.1–30 THz frequency range, both from table-top laser systems and from free-electron lasers, have provided access to excitations of molecules and solids, which can be stimulated at their resonance frequencies. Amongst these, we discuss free electrons in metals, superconducting gaps and Josephson plasmons in layered superconductors, and vibrational modes of the crystal lattice (phonons), as well as magnetic excitations. This review provides an overview and illustrative examples of how intense terahertz transients can be used to resonantly control matter, with particular focus on strongly correlated electron systems and high-temperature superconductors.

Advances in Optics and Photonics, 8, 3, pp. 401-464 (2016)

C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pröpper, T. Loew, J. Porras, B. Keimer, and A. Cavalleri

Optical excitation of apical oxygen vibrations in YBa₂Cu₃O_6+δ has been shown to enhance its c axis superconducting-phase rigidity, as evidenced by a transient blueshift of the equilibrium interbilayer Josephson plasma resonance. Surprisingly, a transient c axis plasma mode could also be induced above Tc by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of YBa₂Cu₃O_6+δ. However, despite the similarities between the transient plasma mode above Tc and the equilibrium Josephson plasmon, alternative explanations involving high-mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport and rather support transient superfluid tunneling as the origin of the light induced interlayer coupling in YBa₂Cu₃O_6+δ.

Physical Review B, 94, 224303 (2016)

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

The non-equilibrium control of emergent phenomena in solids is an important research frontier, encompassing effects such as the optical enhancement of superconductivity1. Nonlinear excitation2,3 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 correlations7,8, 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.

Nature, 530, 461 (2016)

Jun-ichi Okamoto, Andrea Cavalleri, and Ludwig Mathey

Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.

Phys. Rev. Lett. 117, 227001 (2016)

A. Cavalleri, I. Gierz

We review our recent time- and angle-resolved photoemission spectroscopy experiments, which measure the transient electronic structure of optically driven graphene. For pump photon energies in the near infrared (ℏω_pump=950 meV), we have discovered the formation of a population-inverted state near the Dirac point, which may be of interest for the design of THz lasing devices and optical amplifiers. At lower pump photon energies (ℏω_pump<400 meV), for which interband absorption is not possible in doped samples, we find evidence for free carrier absorption. In addition, when mid-infrared pulses are made resonant with an infrared-active in-plane phonon of bilayer graphene (ℏω_pump=200 meV), a transient enhancement of the electron-phonon coupling constant is observed, providing interesting perspective for experiments that report light-enhanced superconductivity in doped fullerites in which a similar lattice mode was excited. All the studies reviewed here have important implications for applications of graphene in optoelectronic devices and for the dynamical engineering of electronic properties with light.

Structural Dynamics 3, 051301 (2016)

E. Casandruc, D. Nicoletti, S. Rajasekaran, Y. Laplace, V. Khanna, G. D. Gu, J. P. Hill, and A. Cavalleri

We analyze the pump wavelength dependence for the photoinduced enhancement of interlayer coupling in La_1.885Ba_0.115CuO₄, which is promoted by optical melting of the stripe order. In the equilibrium superconducting state (T<TC=13K) in which stripes and superconductivity coexist, time-domain terahertz spectroscopy reveals a photoinduced blueshift of the Josephson plasma resonance after excitation with optical pulses polarized perpendicular to the CuO₂ planes. In the striped nonsuperconducting state (T_C<T<T_SO≃40K) a transient plasma resonance similar to that seen below T_C appears from a featureless equilibrium reflectivity. Most strikingly, both these effects become stronger upon tuning of the pump wavelength from the midinfrared to the visible, underscoring an unconventional competition between stripe order and superconductivity, which occurs on energy scales far above the ordering temperature.

Physical Review B, 91, 17 (2015)

R. Höppner, B. Zhu, T. Rexin, A. Cavalleri, and L. Mathey

We study the thermally fluctuating state of a bilayer cuprate superconductor under the periodic action of a staggered field oscillating at optical frequencies. This analysis distills essential elements of the recently discovered phenomenon of light-enhanced coherence in YBa₂Cu₃O_6+x, which was achieved by periodically driving infrared active apical oxygen distortions. The effect of a staggered periodic perturbation is studied using a Langevin and Fokker-Planck description of driven, coupled Josephson junctions, which represent two neighboring pairs of layers and their two plasmons. In a toy model including only two junctions, we demonstrate that the external driving leads to a suppression of phase fluctuations of the low-energy plasmon, an effect which is amplified via the resonance of the high-energy plasmon. When extending the modeling to the full layers, we find that this reduction becomes far more pronounced, with a striking suppression of the low-energy fluctuations, as visible in the power spectrum. We also find that this effect acts on the in-plane fluctuations, which are reduced on long length scales. All these findings provide a physical framework to describe light control in cuprates.

Phys. Rev. B 91, 104507 (2015)

Isabella Gierz, Matteo Mitrano, Hubertus Bromberger, Cephise Cacho, Richard Chapman, Emma Springate, Stefan Link, Ulrich Starke, Burkhard Sachs, Martin Eckstein, Tim O. Wehling, Mikhail I. Katsnelson, Alexander Lichtenstein, and Andrea Cavalleri

We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E_1u lattice vibration at 6.3  μm. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme-ultraviolet (XUV) pulses, we measure the response of the Dirac electrons near the K point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predict dramatic band structure changes when the E_1u vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.

Phys. Rev. Lett. 114, 125503 (2015)