News archive

New publication in PNAS:

Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3

D. Nicoletti, E. Casandruc, D. Fu, P. Giraldo-Gallo, I. Fisher, A. Cavalleri
PNAS, Early Edition 08.Aug. 2017

Abstract:
Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle interrelation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights by tracking the relaxation kinetics following excitation at frequencies related to the broken-symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1−xBixO3––a material for which superconductivity is “adjacent” to a competing CDW phase––after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below TC = 7 K), a similar response was also found immediately above TC. This observation evidences the presence of a robust gap up to T40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behavior was observed in the same material forT40 K. Here, the photoconductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature-dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above TC in underdoped BaPb1−xBixO3, and provide information on the dynamical interplay between superconductivity and charge order.


 

New publication in PRB:

Enhancement of superexchange pairing in the periodically driven Hubbard model

J. R. Coulthard, S. R. Clark, S. Al-Assam, A. Cavalleri, and D. Jaksch
Physical Review B, 96, 085104 (2017)

Abstract:
Recent experiments performed on cuprates and alkali-doped fullerides have demonstrated that key signatures of superconductivity can be induced above the equilibrium critical temperature by optical modulation. These observations in disparate physical systems may indicate a general underlying mechanism. Multiple theories have been proposed, but these either consider specific features, such as competing instabilities, or focus on conventional BCS-type superconductivity. Here we show that periodic driving can enhance electron pairing in strongly correlated systems. Focusing on the strongly repulsive limit of the doped Hubbard model, we investigate in-gap, spatially inhomogeneous, on-site modulations. We demonstrate that such modulations substantially reduce electronic hopping, while simultaneously sustaining superexchange interactions and pair hopping via driving-induced virtual charge excitations. We calculate real-time dynamics for the one-dimensional case, starting from zero- and finite-temperature initial states, and we show that enhanced singlet-pair correlations emerge quickly and robustly in the out-of-equilibrium many-body state. Our results reveal a fundamental pairing mechanism that might underpin optically induced superconductivity in some strongly correlated quantum materials.


 

Fellowship:

Reimar Lüst Fellowship for Roman Mankowsky

For his dissertation, "Nonlinear phononics and structural control of strongly correlated materials", Roman Mankowsky has been awarded with the Max Planck Society's Reimar Lüst Fellowship at the ‘General Meeting’ of the Max Planck Society in Weimar.

With the two year postdoctoral fellowship, Roman will continue his research on the control of ferroelectrics on femtosecond time scales in the quantum condensed matter group in Hamburg.


 

New publication in PRL:

Nonlinear Electron-Phonon Coupling in Doped Manganites

V. Esposito, M. Fechner, R. Mankowsky, H. Lemke, M. Chollet, J. M. Glownia, M. Nakamura, M. Kawasaki, Y. Tokura, U. Staub, P. Beaud, and M. Först
Physical Review Letters, 118, 247601

Abstract:
We employ time-resolved resonant x-ray diffraction to study the melting of charge order and the associated insulator-to-metal transition in the doped manganite Pr0.5Ca0.5MnO3 after resonant excitation of a high-frequency infrared-active lattice mode. We find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density-functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drives these dynamics, highlighting a new avenue of nonlinear phonon control.


 

New publication in PRL:

Ultrafast Reversal of the Ferroelectric Polarization

R. Mankowsky, A. von Hoegen, M. Först, A. Cavalleri
Physical Review Letters, 118, 197601 (2017) [Editors suggestion]

Abstract:
We report on the demonstration of ultrafast optical reversal of the ferroelectric polarization in LiNbO3. Rather than driving the ferroelectric mode directly, we couple to it indirectly by resonant excitation of an auxiliary high-frequency phonon mode with femtosecond midinfrared pulses. Because of strong anharmonic coupling between these modes, the atoms are directionally displaced along the ferroelectric mode and the polarization is transiently reversed, as revealed by time-resolved, phase-sensitive, second-harmonic generation. This reversal can be induced in both directions, a key prerequisite for practical applications.


 

Welcome to

Gregor Jotzu

Welcome to Gregor Jotzu  who joined as Postdoc.


 

Defense:

Congratulations to Eliza Casandruc

Congratulatiuons to Eliza who successfully defended her dissertation 'Nonlinear Optical Control of Josephson Coupling in
Cuprates' (Univ. Hamburg) on March 15th.


 

New publication in PRL:

Dynamical stability limit for the charge density wave in K0.3MoO3

R. Mankowsky, B. Liu, S. Rajasekaran, H. Liu, D. Mou, X. J. Zhou, R. Merlin, M. Först and A. Cavalleri
Physical Review Letters, 118, 116402 (2017)

Abstract:
We study the response of the one-dimensional charge density wave in K0.3MoO3 to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.


 

New publication in PRB:

Optical melting of the transverse Josephson plasmon: a comparison between bilayer and trilayer cuprates

W. Hu, D. Nicoletti, A. V. Boris, B. Keimer and A. Cavalleri
Physical Reviev B, 95, 104508 (2017)

Abstract:
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 YBa2Cu3O6.5. In Bi2Sr2Ca2Cu3O10, we observe a similar spectral weight loss at the transverse plasma mode resonance as that seen in YBa2Cu3O6.5. However, this feature is not accompanied by the light-enhanced interlayer coherence that was found in YBa2Cu3O6+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.


 

New publication in PRL:

Multiple supersonic phase fronts launched at a complex-oxide hetero-interface

M. Först, K.R. Beyerlein, R. Mankowsky, W. Hu, G. Mattoni, S. Catalano, M. Gibert, O. Yefanov, J.N. Clark, A. Frano, J.M. Glownia, M. Chollet, H. Lemke, B. Moser, S.P. Collins, S.S. Dhesi, A.D. Caviglia, J.-M. Triscone, and A. Cavalleri
Physical Review Letters, 118, 027401 (2017)

Abstract:
Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a nonequilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved nonresonant and resonant x-ray diffraction to clarify the underlying physics and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in NdNiO3, when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO3 substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the NdNiO3 film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex-oxide heterointerfaces.


 

Welcome to

Kenneth Beyerlein

Welcome to Kenneth Beyerlein, who joins our group as Postdoc.


 

New publication in Optics Letters:

Generation of narrowband, high-intensity, carrier-envelope phase-stable pulses tunable between 4 and 18  THz

B. Liu, H. Bromberger, A. Cartella, T. Gebert, M. Först, and A. Cavalleri
Optics Letters, 42, 1, pp. 129-131 (2017) [Editors' pick]

Abstract:
We report on the generation of high-energy (1.9 μJ) far-infrared pulses tunable between 4 and 18 THz frequency. Emphasis is placed on tunability and on minimizing the bandwidth of these pulses to less than 1 THz, as achieved by difference-frequency mixing of two linearly chirped near-infrared pulses in the organic nonlinear crystal DSTMS. As the two near-infrared pulses are derived from amplification of the same white light continuum, their carrier envelope phase fluctuations are mutually correlated, and hence the difference-frequency THz field exhibits absolute phase stability. This source opens up new possibilities for the control of condensed matter and chemical systems by selective excitation of low-energy modes in a frequency range that has, to date, been difficult to access.


 

New publication in PRB:

Dynamical decoherence of the light induced inter layer coupling in YBa2Cu3O6+δ

C. R. Hunt, D. Nicoletti, S. Kaiser, D. Pröpper, T. Loew, J. Porras, B. Keimer, A. Cavalleri
Physical Review B 93, 224522 (2016)

Abstract:

Optical excitation of apical oxygen vibrations in YBa2Cu3O6+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 YBa2Cu3O6+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 thetransient 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 transientsuperfluid tunneling as the origin of the light induced interlayer coupling in YBa2Cu3O6+δ.

 

Honor:

Andrea Cavalleri elected AAAS Fellow

Andrea Cavalleri was elected as an AAAS Fellow “for pioneering contributions to the development and use of ultrafast and ultra-intense laser fields to create and probe photo induced phase transitions in correlated electron materials.”

 

The American Association for the Advancement of Science is the world’s largest general scientific society and publisher of the journal Science. AAAS was founded in 1848 and includes nearly 250 affiliated societies and academies of science, serving 10 million individuals. The non-profit AAAS is open to all and fulfills its mission to “advance science and serve society” through initiatives in science policy, international programs, science education, public engagement, and more.


 

Defense:

Congratulations to Roman Mankowsky

On October 25th Roman successfully defended his dissertation 'Nonlinear phononics and structural control of strongly correlated materials' (Univ. Hamburg) with summa cum laude.

→ Thesis 


 

New publication in Nature Physics:

An effective magnetic field from optically driven phonons

T. F. Nova, A. Cartella, A. Cantaluppi, M. Först, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel, R. Merlin and A. Cavalleri
Nature Physics, Advance Online Publication (October 24, 2016)

Abstract:

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 ErFeO3. 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.
 

→ Press releases


 

Welcome to

Ankit Disa, Michael Fechner and David Fu

Welcome to Ankit Disa
who joined our group as Postdoc,
Michael Fechner who joined our group as Scientist
and welcome to David (Dawei) Fu who joined our group as PhD-Student from IMPRS-UFAST.

 

New publication in Nature Physics:

Parametric amplification of a superconducting plasma wave

S. Rajasekaran, E. Casandruc, Y. Laplace, D. Nicoletti, G. D. Gu, S. R. Clark, D. Jaksch & A. Cavalleri
Nature Physics, 12, 1012–1016 (2016), published online - 11 July 2016

Abstract:
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.

Press releases


 

Welcome to

Sam Moore, Young-Mi Park and Benedikt Schulte

Welcome to Sam Moore,
he will be our guest for a year;
welcome to Young-Mi Park
who joined us as PostDoc

and welcome to Benedikt Schulte
our new PhD-Student.
   

 

 

 


 

New publication in PRB:

Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order

V. Khanna, R. Mankowsky, M. Petrich, H. Bromberger, S. A. Cavill, E. Möhr-Vorobeva, D. Nicoletti, Y. Laplace, G. D. Gu, J. P. Hill, M. Först, A. Cavalleri and S. S. Dhesi
Physical Review B 93, 224522 (2016)

Abstract:
We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of ∼0.5 mJ/cm2. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure.


 

New publication in Nature Materials:

Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4

M. P. M. Dean, Y. Cao, X. Liu, S.Wall, D. Zhu, R. Mankowsky, V. Thampy, X. M. Chen, J. G. Vale, D. Casa, Jungho Kim, A. H. Said, P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama, M. Yabashi, Y. Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först and J. P. Hill
Nature Materials,15, 601–605 (2016)

Abstract:
Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4.We find that the nonequilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photocarriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a fewhundred picoseconds.The marked dierence in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

→ Press releases and more


 

New publication in PRB:

Broadband THz spectroscopy of the insulator-metal transition driven by coherent lattice deformation at the SmNiO3/LaAlO3 interface

W. Hu, S. Catalano, M. Gibert, J.-M. Triscone, A. Cavalleri
Physical Reviews B, 93, 161107(R)(2016)

Abstract:
We investigate the nonequilibrium insulator-metal transition driven in a SmNiO3 thin film by coherent optical excitation of the LaAlO3 substrate lattice. By probing the transient optical properties over a broad frequency range (100 - 800 cm-1), we analyze both the time-dependent metallic plasma and the infrared optical phonon line shapes. We show that the light-induced metallic phase in SmNiO3 has the same carrier density as the equilibrium metallic phase. We also report that the LaAlO3 substrate acts as a transducer only at the earlier time delays, as the vibrations are driven coherently. No long-lived structural rearrangement takes place in the substrate. Finally, we show that the transient insulator-metal transition occurs both below and above the Néel temperature. We conclude that the supersonic melting of magnetic order measured with ultrafast x rays is not the driving force of the formation of the metallic phase. We posit that the insulator-metal transition may origin from the rearrangement of ordered charges at the interface propagating into the film.


 

New publication in PRB:

Proposed cavity Josephson plasmonics with complex-oxide heterostructures

Y. Laplace, S. Fernandez-Pena, S. Gariglio, J.-M. Triscone, A. Cavalleri
Physical Review B, 93, 075152 (2016)

Abstract:
We discuss how complex-oxide heterostructures that include high-Tc superconducting cuprates can be used to realize an array of submillimeter cavities that support Josephson plasmon polaritons. These cavities have severalattractive features for new types of light-matter interaction studies and we show that they promote “ultrastrong”coupling between THz frequency radiation and Josephson plasmons. Cavity electrodynamics of Josephsonplasmons allows us to manipulate the superconducting order-parameter phase coherence. As an example, wediscuss how it could be used to cool superconducting phase fluctuations with light.


 

New publication in Nature:

Possible light-induced superconductivity in K3C60 at high temperature

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
Nature, 530, 461–464 (2016)

Abstract:
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. 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.

Read more     ⇒ Press releases + more


 

Welcome to

Michele Buzzi

Welcome to Michele Buzzi, who joined our group on a fellowship of the Swiss National Science Foundation.


 

Welcome to

Alexander von Hoegen

Welcome to Alexander von Hoegen, who joined our group as PhD student in Hamburg.


 

Award:

Dannie Heineman Prize 2015 to Andrea Cavalleri

The Dannie Heineman Prize 2015 of the Minna-James-Heineman Foundation Hanover goes to Prof. Andrea Cavalleri in recognition of his research on light-induced phase transitions in strongly correlated electron systems.

Cavalleri has been awarded for his outstanding scientific work during the public anniversary of the Göttingen Academy of Sciences and Humanities on November 14, 2015.

At the same ceremony, Melanie Schnell, leader of the Max Planck Research Group Structure and Dynamics of Cold and Controlled Molecules, has been awarded the Academy Prize for Chemistry 2015 in recognition of her pioneering development of new methods to study the rotational spectra of state-selected molecules in extremely cold conditions.

 Since 1961, the Göttingen Academy of Sciences and Humanities every two years awards the Dannie Heineman Prize to a scientist who has recently published an outstanding paper on new and significant developments in science. The prize has an international character, i.e. candidates from all over the world are considered. Mainly research papers by younger researchers in the field of natural sciences (mathematics, physics, chemistry, biology) are considered; however, also work from the humanities may be awarded.

Photos: Adrienne Lochte, Akademie der Wissenschaften zu Göttingen (AdWGö)

 


 

Fellowship:

Humbold Research Fellowship awarded to James McIver


James McIver has been awarded fellowship of the Alexander von Humboldt Foundation to support his research in the Quantum Condensed Matter Dynamics group.

James McIver received a PhD in physics from Harvard University in 2014 working in Nuh Gedik’s research group at the Massachusetts Institute of Technology. In December 2014 he joined Andrea Cavalleri’s research group Quantum Condensed Matter Dynamics as a postdoc. The Humboldt Fellowship will support James' research plan on the ultrafast manipulation of topological order. This is important because it provides the foundation for the development of topological transistors that utilize switchable topological conduction channels and potentially other exotic phenomena for high-speed next-generation circuitry.

The Alexander von Humboldt Foundation provides its prestigious Research Fellowships to support promising young scientists with above average qualifications in their early career. The fellowships allow international scientists to carry out long-term research projects in Germany. Applicants are free in the choice of their topic of research and their academic host.


 

Welcome to

Thomas Gebert

  Welcome to Thomas Gebert, who joined our group as a beamline scientist in Hamburg.


 

New publication in PRL:

THz-Frequency Modulation of the Hubbard U in an Organic Mott Insulator

R. Singla, G. Cotugno, S. Kaiser, M. Först, M. Mitrano, H. Y. Liu, A. Cartella, C. Manzoni, H. Okamoto, T. Hasegawa, S. R. Clark, D. Jaksch, and A. Cavalleri
Physical Review Letters 115, 187401 (2015)

Abstract:
We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator. We find that the Mott gap, which is probed resonantly with 10 fs laser pulses, oscillates with the pump field. This observation reveals that molecular excitations can coherently perturb the electronic on-site interactions (Hubbard U) by changing the local orbital wave function. The gap oscillates at twice the frequency of the vibrational mode, indicating that the molecular distortions couple quadratically to the local charge density.


 

New publication in PRL:

Tracking Primary Thermalization Events in Graphene with Photoemission at Extreme Time Scales

I. Gierz, F. Calegari, S. Aeschlimann, M. Chávez Cervantes, C. Cacho, R. T. Chapman, E. Springate, S. Link, U. Starke, C. R. Ast, and A. Cavalleri
Physical Review Letters 115, 086803 (2015)

Abstract:
Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is, instead, predicted to be dominant at the earliest time delays. Here, > 8 fs extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time-and angle-resolved photoemission spectroscopy. Over a time window of approximately 25 fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme time scales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for petahertz electronics.


 

Welcome to

Matthias Budden

Welcome to Matthias Budden, who joined our group as PhD student in Hamburg.


 

Two successful defenses in less than 24 hours:

Congratulations to Matteo Mitrano and Rashmi Singla

Matteo successfully defended his dissertation 'Nonequilibrium Emergent Phenomena in Organic Molecular Solids' (Univ. Hamburg) on July 15th.

Roughly 16 hours later, Rashmi passed the defense for her dissertation 'Measuring correlated electron dynamics on few femtoseconds time scale' (Univ. Hamburg).

→ Matteo's PhD-Thesis, → Rashmi's PhD-Thesis


 

Welcome to

Vincent Kemlin

Welcome to Vincent Kemlin, he joined our group as PostDoc in Hamburg.


 

New publication in Nature Materials:

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

M. Först, A.D. Caviglia, R. Scherwitzl, R. Mankowsky, P. Zubko, V. Khanna, H. Bromberger, S.B. Wilkins, Y.-D. Chuang, W.S. Lee, W.F. Schlotter, J.J. Turner, G.L. Dakovski, M.P. Minitti, J. Robinson, S.R. Clark, D. Jaksch, J.-M. Triscone, J.P. Hill, S.S. Dhesi, A. Cavalleri
Nature Materials,14, 883-888 (2015) (online 06. July 2015)

Abstract:
Static strain in complex oxide heterostructureshas been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.


 

Welcome to

Falk-Ulrich Stein

Welcome to Falk-Ulrich Stein, he joined our group as PostDoc in Hamburg.      

 

New publication in PRB:

Wavelength-dependent optical enhancement of superconducting interlayer coupling in La1.885Ba0.115CuO4

E. Casandruc, D. Nicoletti, S. Rajasekaran, Y. Laplace, V. Khanna, G. D. Gu, J. P. Hill, and A. Cavalleri
Physical Review B, 91, 17 (2015)

Abstract:
We analyze the pump wavelength dependence for the photoinduced enhancement of interlayer coupling in
La1.885Ba0.115CuO4, which is promoted by optical melting of the stripe order. In the equilibrium superconducting
state (T <TC = 13 K) 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 (TC < T < TSO≃40 K) a transient
plasma resonance similar to that seen belowTC 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.


 

Welcome to

Enrico Pomarico

Welcome to Enrico Pomarico, he joined our group as PostDoc in Hamburg.  

 

 

 

 

New publication in PRL:

Proposed Parametric Cooling of Bilayer Cuprate Superconductors by Terahertz Excitation

S. J. Denny, S. R. Clark, Y. Laplace, A. Cavalleri, and D. Jaksch
Physical Review Letters 114, 137001 (2015)

⇒ See also APS Physics: Synopsis: Making Superconductors Sturdier

Abstract:
We propose and analyze a scheme for parametrically cooling bilayer cuprates based on the selective driving of a c-axis vibrational mode. The scheme exploits the vibration as a transducer making the Josephson plasma frequencies time dependent. We show how modulation at the difference frequency between the intrabilayer and interbilayer plasmon substantially suppresses interbilayer phase fluctuations, responsible for switching c-axis transport from a superconducting to a resistive state. Our calculations indicate that this may provide a viable mechanism for stabilizing nonequilibrium superconductivity even above Tc, provided a finite pair density survives between the bilayers out of equilibrium.


 

New publication in PRB:

Coherent modulation of the YBa2Cu3O6+x atomic structure by displacive stimulated ionic Raman scattering

R. Mankowsky, M. Först, T. Loew, J. Porras, B. Keimer, and A. Cavalleri
Physical Review Phys. Rev. B 91, 094308 (2015)

Abstract:
We discuss the mechanism of coherent phonon generation by stimulated ionic Raman scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of YBa2Cu3O6+x to the resonant excitation of apical oxygen motions at 20 THz by midinfrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric (Ag) Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.


 

Andrea Cavalleri elected Fellow of the Institute of Physics (IOP)

 

New publication in PRL:

Phonon-Pump Extreme-Ultraviolet-Photoemission Probe in Graphene: Anomalous Heating of Dirac Carriers by Lattice Deformation

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, Andrea Cavalleri
Physical Review Letters, 114, 125503 (2015)

Abstract:
We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E1u 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 E1u vibration is driven, which we use to explain the anomalous dynamics observed in the experiment.


 

New publication in PRB:

Two distinct kinetic regimes for the relaxation of light-induced superconductivity in La1.675Eu0.2Sr0.125CuO4

C. R. Hunt, D. Nicoletti, S. Kaiser, T. Takayama, H. Takagi, and A. Cavalleri
Phys. Rev. B 91, 020505(R) (2015)

Abstract:
We address the kinetic competition between charge striped order and superconductivity in La1.675Eu0.2Sr0.125CuO4. Ultrafast optical excitation is tuned to a midinfrared vibrational resonance that destroys charge order and promptly establishes transient coherent interlayer coupling in this material. This effect is evidenced by the appearance of a longitudinal plasma mode reminiscent of a Josephson plasma resonance. We find that coherent interlayer coupling can be generated up to the charge-order transition TCO ≈ 80K, far above the equilibrium superconducting transition temperature of any single layer cuprate. Two key observations are extracted from the relaxation kinetics of the interlayer coupling. First, the plasma mode relaxes through a collapse of its coherence length and not its density. Second, two distinct kinetic regimes are observed for this relaxation, above and below spin-order transition TSO ≈ 25K. In particular, the temperature-independent relaxation rate observed below TSO is anomalous and suggests coexistence of superconductivity and stripes rather than competition. Both observations support arguments that a low temperature coherent stripe (or pair density wave) phase suppresses c-axis tunneling by disruptive interference rather than by depleting the condensate.


 

Fellowship:

Miller Research Fellowship at UC Berkeley for Cassi Hunt

Congratulations to Cassi Hunt who was awarded a Miller Research Fellowship which will allow her to pursue a three-years' postdoctoral research stay at the University of California in Berkeley.

At the moment, she is finishing her doctorate at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in the group of Andrea Cavalleri.

Miller Research Fellowships are intended for exceptional young scientists of great promise who have recently been awarded, or who are about to be awarded, the doctoral degree. About eight to ten Fellowships in the area of nature science are granted each year by the Miller Institute for Basic Research in Science at the University of California in Berkeley.

⇒ more information on the Fellowship


 

Award:

Max Born Prize 2015 goes to Andrea Cavalleri.

For his "time-resolved measurements of photoinduced phase transitions in correlated electronic materials" Prof. Dr. Andrea Cavalleri, director of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), is awarded the prestigious Max Born Prize. The prize of the German Physical Society (DPG) and the British Institute of Physics (IOP) commemorates the work of the physicist Max Born. Since 1973, the two organizations jointly award the annual prize to a British and a German researcher in alternation. A the DPG’s spring meeting next year, Prof. Dr. Andrea Cavalleri will be given 3,000 euro prize money, the Max-Born-commemorative medal and the certificate with the dedication "For the most valuable and recent scientific contributions to physics".

⇒ Press release of the MPSD  ⇒ News at Merton College Oxford

 


 

New publication in Nature:

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

R. Mankowsky, A. Subedi, M. Först, S. O. Mariager, M. Chollet, H. T. Lemke, J. S. Robinson, J. M. Glownia, M. P. Minitti, A. Frano, M. Fechner, N. A. Spaldin, T. Loew, B. Keimer, A. Georges & A. Cavalleri

Nature 516 , 71–73 (2014)

⇒ MPG Press release   ⇒ SLAC Press release

Abstract:

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 . 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.

 


 

Welcome to

James McIver

Welcome to James McIver, he joined our group as PostDoc in Hamburg.  

 

New publication in PRB:

Femtosecond x rays link melting of charge-density wave correlations and light-enhanced coherent transport in YBa2Cu3O6.6

M. Först, A. Frano, S. Kaiser, R. Mankowsky, C. R. Hunt, J. J. Turner, G. L. Dakovski, M. P. Minitti, J. Robinson, T. Loew, M. Le Tacon, B. Keimer, J. P. Hill, A. Cavalleri, and S. S. Dhesi

Physical Review B, 90, 90, 184514 (2014)

Abstract:
We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of
charge-density wave correlations in underdoped YBa2Cu3O6.6. We find that when coherent interlayer transport
is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge-density
wave order is melted. These results indicate that charge ordering and superconductivity may be competing up
to the charge ordering transition temperature, with the latter becoming a hidden phase that is accessible only by
nonlinear phonon excitation.

→ related science


 

Our 1st PhD from the University of Hamburg

Congratulations to Giovanni Cotugno

Giovanni passed the defense for his PhD in theoretical physics research about "Tuning and vibrational modulation of strongly correlated organic salts".

→ Giovannis PhD-Thesis


 

EMRS fall metting in Warsaw:

Best student presentation award for Roman Mankowsky

In recognition of the best student presentation in Symposium V (Functional Perovskite Systems) Roman Mankowsky was honored with a 'best student presentation award' 2014 at the Fall meeting of the Europian Material Research Society (EMRS) in Warsaw.

EMRS website


 

New publication in PRB:

Optically induced superconductivity in striped La2−xBaxCuO4 by polarization-selective excitation in the near infrared

D. Nicoletti , E. Casandruc, Y. Laplace, V. Khanna, C. R. Hunt, S. Kaiser, S. S. Dhesi, G. D. Gu, J. P. Hill, A. Cavalleri

Physical Review B, 90, 100503(R), (2014)

Abstract:
We show that superconducting interlayer coupling, which coexists with and is depressed by stripe order in
La1.885Ba0.115CuO4, can be enhanced by excitation with near-infrared laser pulses. For temperatures lower than
Tc =13 K, we observe a blue shift of the equilibrium Josephson plasma resonance, detected by terahertz-frequency
reflectivity measurements. Key to this measurement is the ability to probe the optical properties at frequencies
as low as 150 GHz, detecting the weak interlayer coupling strengths. For T >Tc a similar plasma resonance,
absent at equilibrium, is induced up to the spin-ordering temperature TSO  40 K. These effects are reminiscent
but qualitatively different from the light-induced superconductivity observed by resonant phonon excitation in
La1.675Eu0.2Sr0.125CuO6.5. Importantly, enhancement of the below-Tc interlayer coupling and its appearance above
Tc are preferentially achievedwhen the near-infrared pump light is polarized perpendicular to the superconducting
planes, likely due to more effective melting of stripe order and the less effective excitation of quasiparticles from
the Cooper pair condensate when compared to in-plane excitation.

→ related science


 

Welcome to

Biaolong Liu and Christopher Ott

Welcome to Biaolong Liu, he joined our group as PhD-student in Hamburg.  

 

 

Welcome to Christopher Ott who joins us as lab technician in Hamburg.

 

New publication in PRB:

Theory of nonlinear phononics for coherent light control of solids

Alaska Subedi, Andrea Cavalleri, Antoine Georges
Physical Review B, 89, 220301(R) (2014)  

Abstract:
We present a microscopic theory for ultrafast control of solids with high-intensity terahertz frequency optical pulses. When resonant with selected infrared-active vibrations, these pulses transiently modify the crystal structure and lead to new collective electronic properties. The theory predicts the dynamical path taken by the crystal lattice using first-principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. Two classes of dynamics are identified. In the perturbative regime, displacements along the normal mode coordinate of symmetry-preserving Raman active modes can be achieved by cubic anharmonicities. This explains the light-induced insulator-to-metal transition reported experimentally in manganites. We predict a regime in which ultrafast instabilities that break crystal symmetry can be induced. This nonperturbative effect involves a quartic anharmonic coupling and occurs above a critical threshold, below which the nonlinear dynamics of the driven mode displays softening and dynamical stabilization.

→ Related Science


 

New publication in PRB:

Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ

S. Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri
Phys. Rev. B 89, 184516 (2014)  

Abstract:
We report on a photoinduced transient state of YBa2Cu3O6+δ in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature Tc, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low-frequency imaginary conductivity σ2(ω). Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive σ2(ω) that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu3O6.45 and YBa2Cu3O6.5. The data above Tc can be fitted by hypothesizing that the light establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 104 cm2 / V sec at 100 K, with a density of charge carriers similar to the below-Tc superfluid density. Our results are indicative of highly unconventional nonequilibrium physics and open new prospects for optical control of complex solids.

→ Related Science


 

New publication in Nature Materials:

Optically enhanced coherent transport in YBa2Cu3O6.5 by ultrafast redistribution of interlayer coupling

W. Hu, S. Kaiser, D. Nicoletti, C. R. Hunt, I. Gierz, M. C. Hoffmann, M. Le Tacon, T. Loew, B. Keimer & A. Cavalleri
Nature Materials, 13, 705–711 (2014)  
⇒ News & Views by N. Peter Armitage

Abstract:
Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amplitude apical oxygen distortions in the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional electronic properties. Below the superconducting transition temperature (Tc = 50 K) inter-bilayer coherence is transiently enhanced at the expense of intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar effect is observed up to room temperature, with transient inter-bilayer coherence emerging from the incoherent ground state and similar transfer of spectral weight from high to low frequency. These observations are compatible with previous reports of an inhomogeneous normal state that retains important properties of a superconductor, in which light may be melting competing orders or dynamically synchronizing the interlayer phase. The transient redistribution of coherence discussed here could lead to new strategies to enhance superconductivity in steady state.

→ Press releases   → Related Science


 

New publication in PRL:

Melting of Charge Stripes in Vibrationally Driven La1.875Ba0.125CuO4: Assessing the Respective Roles of Electronic and Lattice Order in Frustrated Superconductors

M. Först, R. I. Tobey, H. Bromberger, S. B. Wilkins, V. Khanna, A. D. Caviglia, Y.-D. Chuang, W. S. Lee, W. F. Schlotter, J. J. Turner, M. P. Minitti, O. Krupin, Z. J. Xu, J. S. Wen, G. D. Gu, S. S. Dhesi, A. Cavalleri, and J. P. Hill
Physical Review Letters, 112, 157002  

Abstract:
We report femtosecond resonant soft x-ray diffraction measurements of the dynamics of the charge order and of the crystal lattice in nonsuperconducting, stripe-ordered La1.875Ba0.125CuO4. Excitation of the in-plane Cu-O stretching phonon with a midinfrared pulse has been previously shown to induce a transient superconducting state in the closely related compound La1.675Eu0.2Sr0.125CuO4. In La1.875Ba0.125CuO4, we find that the charge stripe order melts promptly on a subpicosecond time scale. Surprisingly, the low temperature tetragonal (LTT) distortion is only weakly reduced, reacting on significantly longer time scales that do not correlate with light-induced superconductivity. This experiment suggests that charge modulations alone, and not the LTT distortion, prevent superconductivity in equilibrium.

→ Press releases   → Related Science


 

New publication in PRL:

Pressure-Dependent Relaxation in the Photoexcited Mott Insulator ET-F2 TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates

M. Mitrano, G. Cotugno, S.R. Clark, R. Singla, S. Kaiser, J. Stähler, R. Beyer, M. Dressel, L. Baldassarre, D. Nicoletti, A. Perucchi, T. Hasegawa, H. Okamoto, D. Jaksch and A. Cavalleri
Physical Review Letters, 112, 117801

Abstract:
We measure the ultrafast recombination of photoexcited quasiparticles (holon-doublon pairs) in the one dimensional Mott insulator ET-F2 TCNQ as a function of external pressure, which is used to tune the electronic structure. At each pressure value, we first fit the static optical properties and extract the electronic bandwidth t and the intersite correlation energy V. We then measure the recombination times as a function of pressure, and we correlate them with the corresponding microscopic parameters. We find that the recombination times scale differently than for metals and semiconductors. A fit to our data based on the time-dependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the Mott-Hubbard exciton dictates the efficiency of the recombination.

→ Related Methods & Experiment


 

New publication in Optics Letters:

Pulse shaping in the mid-infrared by a deformable mirror

Andrea Cartella, Stefano Bonora, Michael Först, Giulio Cerullo, Andrea Cavalleri, and Cristian Manzoni
Optics Letters, Vol. 39, Issue 6, pp. 1485-1488 (2014)

Abstract:
We present a pulse-shaping scheme operating in the mid-infrared (MIR) wavelength range up to 20 μm. The spectral phase is controlled by a specially designed large stroke 32-actuator deformable mirror in a grating-based configuration. We demonstrate the shaper capability of compressing the MIR pulses, imparting parabolic and third-order spectral phases and splitting the spectral content to create two independent pulses.

→ Related Methods & Experiment


 

New publication in Scientific Reports:

Optical Properties of a Vibrationally Modulated Solid State Mott Insulator

S. Kaiser, S. R. Clark, D. Nicoletti, G. Cotugno, R. I. Tobey, N. Dean, S. Lupi, H. Okamoto, T. Hasegawa, D. Jaksch & A. Cavalleri
Scientific Reports, 4, Article number: 3823 (2013)  

Abstract:
Optical pulses at THz and mid-infrared frequencies tuned to specific vibrational resonances modulate the lattice along chosen normal mode coordinates. In this way, solids can be switched between competing electronic phases and new states are created. Here, we use vibrational modulation to make electronic interactions (Hubbard-U) in Mott-insulator time dependent. Mid-infrared optical pulses excite localized molecular vibrations in ET-F2TCNQ, a prototypical one-dimensional Mott-insulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A red-shifted charge-transfer resonance is observed, consistent with a time-averaged reduction of the electronic correlation strength U. Secondly, a sideband manifold inside of the Mott-gap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantum-modulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holon-doublon coupling to the molecules and that electron double-occupancies strongly squeeze the vibrational mode.

→ Related Science


 

New publication in PRB:

Photoinduced melting of magnetic order in the correlated electron insulator NdNiO3

A. D. Caviglia, M. Först, R. Scherwitzl, V. Khanna, H. Bromberger, R. Mankowsky, R. Singla, Y.-D. Chuang, W. S. Lee, O. Krupin, W. F. Schlotter, J. J. Turner, G. L. Dakovski, M. P. Minitti, J. Robinson, V. Scagnoli, S. B.Wilkins, S. A. Cavill,, M. Gibert, S. Gariglio, P. Zubko, J.-M. Triscone, J. P. Hill, S. S. Dhesi, and A. Cavalleri
Physical Review B, 88, 220401(R) (2013)  

Abstract:
Using ultrafast resonant soft x-ray diffraction, we demonstrate photoinduced melting of antiferromagnetic order in the correlated electron insulator NdNiO3. Time-dependent analysis of the resonant diffraction spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore, we demonstrate that the magnetic ordering on the Ni and Nd sites, which are locked together in equilibrium, become decoupled during this nonthermal process.


 

New publication in Nature Materials:

Snapshots of non-equilibrium Dirac carrier distributions in graphene.

Isabella Gierz, Jesse C. Petersen, Matteo Mitrano, Cephise Cacho, Edmond Turcu, Emma Springate, Alexander Stöhr, Axel Köhler, Ulrich Starke, Andrea Cavalleri
Nature Materials, advance online 06. Oct. 2013
 

files/qcmd-theme/img_slider/slider-slgraph-sm.jpgAbstract:
The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

 

 

Q-MAC Start:

The ERC Synergy Grant research project Frontiers in Quantum Materials’ Control (Q-MAC) has started on October 1, 2013.

The four research teams that will collaborate in this project are led by Prof. Andrea Cavalleri (speaker) from the Max Planck Institute for the Structure and Dynamics of Matter and the University of Oxford, Prof. Dieter Jaksch from University of Oxford, Prof. Jean Marc Triscone from the University of Geneva and Prof. Antoine Georges from the École Polytechnique, College de France and University of Geneva.

For this six years’ project, the team was awarded about 10 million Euros by the European Research Council.

The overarching goal of the project is to combine materials design, coherent optical methods and multiple theoretical approaches to control ordered states of strongly correlated electron materials, also referred to as “quantum” or “complex” materials.

One of the most important goals is to optimize superconductivity at higher temperatures than achieved so far, possibly even at room temperature.

Complex-oxide heterostructures and strain engineering at interfaces will be used to control the equilibrium electronic properties of complex solids. Secondly, coherent optical control of crystal lattices with strong field THz transients will be used to dynamically enhance selected quantum properties at the expense of other competing orders. A combination of femtosecond optical and X-ray experiments with Free Electron Lasers, material design and time dependent real-materials simulations will bring the project close to the goal. The most ambitious goal will be to develop laser-cooling techniques to reduce quantum phase fluctuations between planes of cuprate superconductors.

⇒ ERC Press release

⇒ Q-MAC Project page


 

Isabella Gierz became leader of the newly founded

Otto Hahn Group for Ultrafast Electron Dynamics

We are interested in the response of the electronic structure to strong electronic and vibrational excitation and in a control of the electronic properties with tailored femtosecond light pulses. Special emphasis lies on the dynamics of massless Dirac particles in graphene, on one-dimensional metallic wires with strong electronic correlations, and on the control of the electron spin with light in low-dimensional systems with a strong spin-orbit interaction.

Group homepage


 

Alice Cantaluppi joins the group in Hamburg

 

New publication in PRB:

Photoinduced Melting of the Orbital Order in La0.5Sr1.5MnO4 with 4-femtosecond laser pulses

R. Singla, A. Simoncig, M. Först, D. Prabhakaran, A.L. Cavalieri, and A. Cavalleri
Physical Review B, 88, 075107 (2013)  

Abstract:
By measuring time-dependent optical birefringence with 4-femtosecond (fs) laser pulses, we determine the time scale for photoinduced melting of orbital order in the single-layer manganite La0.5Sr1.5MnO4. Such high time resolution is required to distinguish atomic motions that control the Jahn-Teller distortion from even faster electronic rearrangements. The experiment reveals an 18 fs bottleneck for the loss of orbital order, corresponding to about one-quarter period of the in-plane Jahn-Teller mode. Furthermore, we observe coherent oscillations of this Jahn-Teller mode. Both their amplitude and the birefringence drop exhibit a threshold in their fluence dependence, indicating cooperativity in the lattice response.

→ Related Science


 

New publication in Solid State Communications:

Displacive lattice excitation through nonlinear phononics viewed by femtosecond X-ray diffraction

M. Först, R. Mankowsky, H. Bromberger, D.M. Fritz, H. Lemke, D. Zhu, M. Chollet, Y. Tomioka, Y. Tokura, R. Merlin, J.P. Hill, S.L. Johnson, A. Cavalleri
Solid State Communications, 169 24–27 (2013)  

Abstract:
The nonlinear lattice dynamics of La0.7Sr0.3MnO3, as initiated by strong mid-infrared femtosecond pulses made resonant with a specific lattice vibration, are measured with ultrafast X-ray diffraction at the LCLS free electron laser. Our experiments show that large amplitude excitation of an infrared-active stretching mode leads also to a displacive motion along the coordinate of a second, anharmonically coupled, Raman mode. This rectification of the vibrational field is described within the framework of the Ionic Raman Scattering theory and explains how direct lattice excitation in the nonlinear regime can induce a structural phase transition.

→ Related Science


 

New publication in PRB:

Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet angle-resolved photoemission spectroscopy

H. Y. Liu, I. Gierz, J. C. Petersen, S. Kaiser, A. Simoncig, A. L. Cavalieri, C. Cacho, I. C. E. Turcu, E. Springate, F. Frassetto, L. Poletto, S. S. Dhesi, Z.-A. Xu, T. Cuk, R. Merlin, and A. Cavalleri
Physical Review B, 88, 045104 (2013)  

Abstract:
We use time- and angle-resolved photoemission spectroscopy in the extreme ultraviolet to measure the time- and momentum-dependent electronic structures of photoexcited K0.3MoO3. Prompt depletion of the charge-density wave condensate launches coherent oscillations of the amplitude mode, observed as a 1.7-THz-frequency modulation of the bonding band position. In contrast, the antibonding band oscillates at about half this frequency. We attribute these oscillations to coherent excitation of phasons via parametric amplification of phase fluctuations.

→ Related Science


 

Luca Piovani and Tobia F. Nova join the group in Hamburg

 

New publication in Nature Materials:

Optical excitation of Josephson plasma solitons in a cuprate superconductor

A. Dienst, E. Casandruc, D. Fausti, L. Zhang, M. Eckstein, M. Hoffmann, V. Khanna, N. Dean, M. Gensch, S. Winnerl, W. Seidel, S. Pyon, T. Takayama, H. Takagi & A. Cavalleri
Nature Materials, 12, 535-541 (2013)
 

Abstract:
Josephson plasma waves are linear electromagnetic modes that propagate along the planes of cuprate superconductors, sustained by interlayer tunnelling supercurrents. For strong electromagnetic fields, as the supercurrents approach the critical value, the electrodynamics become highly nonlinear. Josephson plasma solitons (JPSs) are breather excitations predicted in this regime, bound vortex–antivortex pairs that propagate coherently without dispersion. We experimentally demonstrate the excitation of a JPS in La1.84Sr0.16CuO4, using intense narrowband radiation from an infrared free-electron laser tuned to the 2-THz Josephson plasma resonance. The JPS becomes observable as it causes a transparency window in the opaque spectral region immediately below the plasma resonance. Optical control of magnetic-flux-carrying solitons may lead to new applications in terahertz-frequency plasmonics, in information storage and transport and in the manipulation of high-Tc superconductivity.


 

M. Zoubeir Emambokus joins the group in Oxford

 

Faton Krasniqi and Srivats Rajasekaran join the group in Hamburg

 

News archive