Sergey Zhdanovich

Evidence for superconductivity in Li-decorated monolayer graphene

Significance Although superconductivity is well-known in intercalated bulk graphite, the ultimate goal of inducing superconductivity in single-layer graphene has not yet been achieved. We have here developed an experiment that combines ultralow-temperature (5 K) and ultrahigh-vacuum (10−11 torr) sample preparation with high-resolution angle-resolved photoemission spectroscopy (ARPES). We show that decorating monolayer graphene with a layer of lithium atoms enhances the electron–phonon coupling to the point where a superconducting state can be stabilized at low temperature. Measurements of the size of the superconducting gap by ARPES suggest a Tc of about 5.9 K. This result constitutes the first observation, to our knowledge, of superconductivity in monolayer graphene. Given the massive scientific and technological interest in graphene, our findings will have significant cross-disciplinary impact. Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron–phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131–134]. Although experiments have shown an adatom-induced enhancement of the electron–phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron–phonon coupling of up to λ≃0.58. On part of the graphene-derived π∗-band Fermi surface, we then observe the opening of a Δ≃0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with Tc≃5.9 K.

Population transfer between two quantum states by piecewise chirping of femtosecond pulses: theory and experiment.

We propose and experimentally demonstrate the method of population transfer by piecewise adiabatic passage between two quantum states. Coherent excitation of a two-level system with a train of ultrashort laser pulses is shown to reproduce the effect of an adiabatic passage, conventionally achieved with a single frequency-chirped pulse. By properly adjusting the amplitudes and phases of the pulses in the excitation pulse train, we achieve complete and robust population transfer to the target state. The piecewise nature of the process suggests a possibility for the selective population transfer in complex quantum systems.

Complete characterization of molecular vibration using frequency resolved gating

The authors propose a new approach to vibration spectroscopy based on the coherent anti-Stokes Raman scattering of broadband ultrashort laser pulses. The proposed method reveals both the amplitude and the phase of molecular vibrations by utilizing the cross-correlation frequency resolved optical gating (XFROG) technique. The spectrum of the anti-Stokes pulse is measured as a function of the time delay between the laser-induced molecular vibrations and a well characterized broadband femtosecond probe pulse. The iterative XFROG algorithm provides a simultaneous complete characterization of molecular vibrations both in frequency and time domains with high resolution. They demonstrate experimentally the feasibility of the proposed method and show one of its potential applications in disentangling the time behavior of a mixture of vibrationally excited molecules. The technique of femtosecond pulse shaping is used for further improvement of accuracy and stability against noise.

Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence

The possibility of driving phase transitions in low-density condensates through the loss of phase coherence alone has far-reaching implications for the study of quantum phases of matter. This has inspired the development of tools to control and explore the collective properties of condensate phases via phase fluctuations. Electrically gated oxide interfaces1,2, ultracold Fermi atoms3,4 and cuprate superconductors5,6, which are characterized by an intrinsically small phase stiffness, are paradigmatic examples where these tools are having a dramatic impact. Here we use light pulses shorter than the internal thermalization time to drive and probe the phase fragility of the Bi2Sr2CaCu2O8+δ cuprate superconductor, completely melting the superconducting condensate without affecting the pairing strength. The resulting ultrafast dynamics of phase fluctuations and charge excitations are captured and disentangled by time-resolved photoemission spectroscopy. This work demonstrates the dominant role of phase coherence in the superconductor-to-normal state phase transition and offers a benchmark for non-equilibrium spectroscopic investigations of the cuprate phase diagram.Pump–probe, time-resolved ARPES experiments with underdoped cuprates reveal the transient enhancement of the density of phase fluctuations, eventually leading to the collapse of superconductivity.

An XUV Source using a Femtosecond Enhancement Cavity for Photoemission Spectroscopy

Recent development of extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC) has enabled generation of high photon ux ( ̴ 1013-1014 photons/sec) in the XUV, at high repetition rates (> 50 MHz) and spanning the spectral region from 40 nm - 120 nm. Here we demonstrate the potential offered by this approach for angle-resolved photoemission spectroscopy by measuring the photoemission spectrum of Au using 8.3 and 25 eV photons with excellent resolution at rapid data rates.

Studying Correlated Electron Systems With a New Tunable (<25 eV) Tabletop XUV Source

We characterize a new table-top, tunable XUV source spanning 8 to 25 eV based on a femtosecond enhancement cavity. This source is designed to investigate correlated electron systems with angle and time resolved photoemission spectroscopy.