arXiv subjects

A. Cavalleri

Publications and source records attributed to A. Cavalleri.

At least 91 records · Page 5Linked to original sources

arXiv2014

Redistribution of phase fluctuations in a periodically driven cuprate superconductor

We study the thermally fluctuating state of a bi-layer 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 YBa$_2$Cu$_3$O$_{6+x}$, 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 onto the in-plane fluctuations, which are reduced on long length scales. All these findings provide a physical framework to describe light control in cuprates.

cond-mat.supr-con↗

arXiv2014

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

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 achieved when 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.

cond-mat.supr-con↗

arXiv2014

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

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), interbilayer 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.

cond-mat.supr-con↗

arXiv2014

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

We report on a photo-induced transient state of YBa2Cu2O6+x 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. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by hypothesizing that the light re-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 10^4 cm^2/(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 non-equilibrium physics and open new prospects for optical control of complex solids.

cond-mat.supr-con↗

arXiv2013

The Gravitational Universe

The last century has seen enormous progress in our understanding of the Universe. We know the life cycles of stars, the structure of galaxies, the remnants of the big bang, and have a general understanding of how the Universe evolved. We have come remarkably far using electromagnetic radiation as our tool for observing the Universe. However, gravity is the engine behind many of the processes in the Universe, and much of its action is dark. Opening a gravitational window on the Universe will let us go further than any alternative. Gravity has its own messenger: Gravitational waves, ripples in the fabric of spacetime. They travel essentially undisturbed and let us peer deep into the formation of the first seed black holes, exploring redshifts as large as z ~ 20, prior to the epoch of cosmic re-ionisation. Exquisite and unprecedented measurements of black hole masses and spins will make it possible to trace the history of black holes across all stages of galaxy evolution, and at the same time constrain any deviation from the Kerr metric of General Relativity. eLISA will be the first ever mission to study the entire Universe with gravitational waves. eLISA is an all-sky monitor and will offer a wide view of a dynamic cosmos using gravitational waves as new and unique messengers to unveil The Gravitational Universe. It provides the closest ever view of the early processes at TeV energies, has guaranteed sources in the form of verification binaries in the Milky Way, and can probe the entire Universe, from its smallest scales around singularities and black holes, all the way to cosmological dimensions.

astro-ph.CO↗

arXiv2013

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

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) in the Extreme Ultraviolet (EUV) to measure the time- and momentum-dependent electronic structure of photo-excited K0.3MoO3. Prompt depletion of the Charge Density Wave (CDW) condensate launches coherent oscillations of the amplitude mode, observed as a 1.7-THz-frequency modulation of the bonding band position. In contrast, the anti-bonding band oscillates at about half this frequency. We attribute these oscillations to coherent excitation of phasons via parametric amplification of phase fluctuations.

cond-mat.str-el↗

arXiv2012

Deconstructing the Hubbard Hamiltonian by Ultrafast Quantum Modulation Spectroscopy in Solid-state Mott Insulators

Most available theories for correlated electron transport are based on the Hubbard Hamiltonian. In this effective theory, renormalized hopping and interaction parameters only implicitly incorporate the coupling of correlated charge carriers to microscopic degrees of freedom. Unfortunately, no spectroscopy can individually probe such renormalizations, limiting the applicability of Hubbard models. We show here that the role of each individual degree of freedom can be made explicit by using a new experimental technique, which we term 'quantum modulation spectroscopy' and we demonstrate here in the one-dimensional Mott insulator ET-F2TCNQ. We explore the role on the charge hopping of two localized molecular modes, which we drive with a mid infrared optical pulse. Sidebands appear in the modulated optical spectrum, and their linshape is fitted with a model based on the dynamic Hubbard Hamiltonian. A striking asymmetry between the renormalization of doublons and holons is revealed. The concept of quantum modulation spectroscopy can be used to systematically deconstruct Hubbard Hamiltonians in many materials, exposing the role of any mode, electronic or magnetic, that can be driven to large amplitude with a light field.

cond-mat.str-el↗

arXiv2012

The interaction between stray electrostatic fields and a charged free-falling test mass

We present an experimental analysis of force noise caused by stray electrostatic fields acting on a charged test mass inside a conducting enclosure, a key problem for precise gravitational experiments. Measurement of the average field that couples to test mass charge, and its fluctuations, is performed with two independent torsion pendulum techniques, including direct measurement of the forces caused by a change in electrostatic charge. We analyze the problem with an improved electrostatic model that, coupled with the experimental data, also indicates how to correctly measure and null the stray field that interacts with test mass charge. Our measurements allow a conservative upper limit on acceleration noise, of 2 fm/s$^2$\rthz\ for frequencies above 0.1 mHz, for the interaction between stray fields and charge in the LISA gravitational wave mission.

gr-qc↗

arXiv2011

Ultrafast strain engineering in complex oxide heterostructures

We report on ultrafast optical experiments in which femtosecond mid-infrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO3 epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

cond-mat.str-el↗

arXiv2011

Brownian force noise from molecular collisions and the sensitivity of advanced gravitational wave observatories

We present an analysis of Brownian force noise from residual gas damping of reference test masses as a fundamental sensitivity limit in small force experiments. The resulting acceleration noise increases significantly when the distance of the test mass to the surrounding experimental apparatus is smaller than the dimension of the test mass itself. For the Advanced LIGO interferometric gravitational wave observatory, where the relevant test mass is a suspended 340 mm diameter cylindrical end mirror, the force noise power is increased by roughly a factor 40 by the presence of a similarly shaped reaction mass at a nominal separation of 5 mm. The force noise, of order 20 fN\rthz\ for $2 \times 10^{-6}$ Pa of residual H$_2$ gas, rivals quantum optical fluctuations as the dominant noise source between 10 and 30 Hz. We present here a numerical and analytical analysis for the gas damping force noise for Advanced LIGO, backed up by experimental evidence from several recent measurements. Finally, we discuss the impact of residual gas damping on the gravitational wave sensitivity and possible mitigation strategies.

gr-qc↗

arXiv2011

Increased Brownian force noise from molecular impacts in a constrained volume

We report on residual gas damping of the motion of a macroscopic test mass enclosed in a nearby housing in the molecular flow regime. The damping coefficient, and thus the associated thermal force noise, is found to increase significantly when the distance between test mass and surrounding walls is smaller than the test mass itself. The effect has been investigated with two torsion pendulums of different geometry and has been modelled in a numerical simulation whose predictions are in good agreement with the measurements. Relevant to a wide variety of small-force experiments, the residual-gas force noise power for the test masses in the LISA gravitational wave observatory is roughly a factor 15 larger than in an infinite gas volume, though still compatible with the target acceleration noise of 3 fm s^-2 Hz^-1/2 at the foreseen pressure below 10^-6 Pa.

gr-qc↗

arXiv2011

Driving magnetic order in a manganite by ultrafast lattice excitation

Optical control of magnetism, of interest for high-speed data processing and storage, has only been demonstrated with near-infrared excitation to date. However, in absorbing materials, such high photon energies can lead to significant dissipation, making switch back times long and miniaturization challenging. In manganites, magnetism is directly coupled to the lattice, as evidenced by the response to external and chemical pressure, or to ferroelectric polarization. Here, femtosecond mid-infrared pulses are used to excite the lattice in La0.5Sr1.5MnO4 and the dynamics of electronic order are measured by femtosecond resonant soft x-ray scattering with an x-ray free electron laser. We observe that magnetic and orbital orders are reduced by excitation of the lattice. This process, which occurs within few picoseconds, is interpreted as relaxation of the complex charge-orbital-spin structure following a displacive exchange quench - a prompt shift in the equilibrium value of the magnetic and orbital order parameters after the lattice has been distorted. A microscopic picture of the underlying unidirectional lattice displacement is proposed, based on nonlinear rectification of the directly-excited vibrational field, as analyzed in the specific lattice symmetry of La0.5Sr1.5MnO4. Control of magnetism through ultrafast lattice excitation has important analogies to the multiferroic effect and may serve as a new paradigm for high-speed optomagnetism.

cond-mat.str-el↗

arXiv2011

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

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

cond-mat.supr-con↗

arXiv2011

Probing photo-induced melting of antiferromagnetic order in La0.5Sr1.5MnO4 by ultrafast resonant soft X-ray diffraction

Photo-excitation in complex oxides1 transfers charge across semicovalent bonds, drastically perturbing spin and orbital orders2. Light may then be used in compounds like magnetoresistive manganites to control magnetism on nanometre lengthscales and ultrafast timescales. Here, we show how ultrafast resonant soft x-ray diffraction can separately probe the photo-induced dynamics of spin and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only partially reduced. Cluster calculations aid our interpretation by considering different magnetically ordered states accessible after photo-excitation. Nonthermal coupling between light and magnetism emerges as a primary aspect of photo-induced phase transitions in manganites.

cond-mat.str-el↗

arXiv2010

Gas damping force noise on a macroscopic test body in an infinite gas reservoir

We present a simple analysis of the force noise associated with the mechanical damping of the motion of a test body surrounded by a large volume of rarefied gas. The calculation is performed considering the momentum imparted by inelastic collisions against the sides of a cubic test mass, and for other geometries for which the force noise could be an experimental limitation. In addition to arriving at an accurated estimate, by two alternative methods, we discuss the limits of the applicability of this analysis to realistic experimental configurations in which a test body is surrounded by residual gas inside an enclosure that is only slightly larger than the test body itself.

physics.class-ph↗

arXiv2007

Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide

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

cond-mat.str-el↗

arXiv2007

Thermal gradient-induced forces on geodetic reference masses for LISA

The low frequency sensitivity of space-borne gravitational wave observatories will depend critically on the geodetic purity of the trajectories of orbiting test masses. Fluctuations in the temperature difference across the enclosure surrounding the free-falling test mass can produce noisy forces through several processes, including the radiometric effect, radiation pressure, and outgassing. We present here a detailed experimental investigation of thermal gradient-induced forces for the LISA gravitational wave mission and the LISA Pathfinder, employing high resolution torsion pendulum measurements of the torque on a LISA-like test mass suspended inside a prototype of the LISA gravitational reference sensor that will surround the test mass in orbit. The measurement campaign, accompanied by numerical simulations of the radiometric and radiation pressure effects, allows a more accurate and representative characterization of thermal-gradient forces in the specific geometry and environment relevant to LISA free-fall. The pressure dependence of the measured torques allows clear identification of the radiometric effect, in quantitative agreement with the model developed. In the limit of zero gas pressure, the measurements are most likely dominated by outgassing, but at a low level that does not threaten the LISA sensitivity goals.

gr-qc↗

arXiv2006

Torsion pendulum facility for direct force measurements of LISA GRS related disturbances

A four mass torsion pendulum facility for testing of the LISA GRS is under development in Trento. With a LISA-like test mass suspended off-axis with respect to the pendulum fiber, the facility allows for a direct measurement of surface force disturbances arising in the GRS. We present here results with a prototype pendulum integrated with very large-gap sensors, which allows an estimate of the intrinsic pendulum noise floor in the absence of sensor related force noise. The apparatus has shown a torque noise near to its mechanical thermal noise limit, and would allow to place upper limits on GRS related disturbances with a best sensitivity of 300 fN/Hz^(1/2) at 1mHz, a factor 50 from the LISA goal. Also, we discuss the characterization of the gravity gradient noise, one environmental noise source that could limit the apparatus performances, and report on the status of development of the facility.

gr-qc↗